Synthesis of Iron Chalcogenide Nanocrystals and Deposition of Thin Films from Single Source Precursors

A thesis submitted to the University of Manchester for the Degree of Doctor of Philosophy in the Faculty of Engineering and Physical Sciences

2013

Masood Akhtar

School of Chemistry, The University of Manchester, Oxford Road Manchester, M13 9PL

Table of contents

Table of contents ...... 2

List of Figures ...... 9

List of Tables ...... 16

Abbreviations ...... 17

Abstract ...... 20

Declaration ...... 21

Dedication ...... 23

Acknowledgement ...... 24

Chapter 1 ...... 26

General Introduction ...... 26

1.1 Summary ...... 27

1.2 Classification of solids ...... 28

1.3 Semiconductors ...... 28

1.3.1 Semiconductor nanoparticles ...... 31

1.3.2 Applications of semiconductors ...... 32

1.4 Synthesis of nanoparticles ...... 33

1.4.1 Methods for the preparation of semiconductor nanoparticles ...... 35

1.4.2 Single-Molecular precursor method ...... 36

1.5 Thin films semiconductor materials ...... 38

1.5.1 Chemical Vapor Deposition (CVD) ...... 39

1.5.2 Types of chemical vapour deposition ...... 39

1.5.3 The process of chemical vapour deposition ...... 41

1.5.4 Precursors for chemical vapour deposition ...... 43

1.5.5 Issues in using conventional precursors for CVD ...... 43

1.5.6 Advantages of single source precursors ...... 44

1.6 Iron chalcogenides ...... 44

1.7 Iron sulfide ...... 45

1.8 Synthesis of iron sulfide nanocrystals ...... 47

1.9 Single source precursor route ...... 47

1.9.1 Dialkyl and mixed alkyl dithiocarbamatoiron(III) complexes ...... 47

1.9.2 Other single source Precursors ...... 48

1.9.3 Thiosemicarbazone complexes of iron(II)...... 49

1.10 Iron sulfide thin films ...... 51

1.11 Single source precursor ...... 52

1.11.1 Dialkyl and mixed alkyl dithiocarbamatoiron(III) complexes ...... 52

1.11.2 Thiobiuret iron(III) complexes ...... 52

1.12 Synthesis of iron sulfide nanoparticles and thin films from dual Source53

1.13 Iron selenide ...... 57

1.13.1 Iron selenide nanoparticles ...... 57

1.13.2 Methods for the preparation of iron selenide nanocrystals ...... 58

1.13.3 Single source precursor route ...... 60

1.13.4 Imidodiselenodiphosphinatoiron(II) complexes ...... 60

1.13.5 Magnetic properties of iron selenide ...... 60

1.14 Iron telluride nanoparticles ...... 61

1.14.1 Methods for the Preparation of iron telluride nanocrystals ...... 61

1.15 Conclusion ...... 62

1.16 References ...... 62

Chapter 2 ...... 72

Deposition of iron sulfide nanocrystals and thin films from tris(dialkyldithiocarbamato)iron(III) complexes ...... 72

2.1 Summary ...... 73

2.2 Introduction ...... 74

2.3. Precursor synthesis ...... 77

i 2.3.1 Synthesis of [Fe(S2CNEt Pr)3] (1) ...... 77

2.3.2 Synthesis of [Fe(S2CN(Hex)2)3] (2) ...... 78

2.3.3 Synthesis of [Fe(S2CNEtMe)3] (3) ...... 78

2.3.4 Synthesis of [Fe(S2CN(Et)2)3] (4) ...... 78

2.4 Synthesis of iron sulfide nanocrystals ...... 79

2.5 Results and discussion ...... 79

i 2.5.1 X-ray single crystal structure of [Fe(S2CNEt Pr)3] (1) ...... 79

2.5.2 X-ray single crystal structure of [Fe(S2CNEtMe)3] (3) ...... 79

2.5.3 Thermogravimetric analysis (TGA) ...... 83

2.5.4 Powder X-ray diffraction of iron sulfide nanocrystals ...... 84

2.5.5 Transmission electron microscopy of iron sulfide nanocrystals ...... 87

2.5.6 The effect of surfactants ...... 91

2.5.7 The magnetic properties of iron sulfide nanocrystals...... 92

2.6 Iron sulfide thin films ...... 98

i 2.6.1 The deposition of iron sulfide thin films from [Fe(S2CNEt Pr)3] (1) 98

2.6.2 The deposition of iron sulfide thin films from [Fe(S2CN(Hex)2)3] (2) ...... 101

2.6.3 The deposition of iron sulfide thin films from [Fe(S2CNEtMe)3] (3) ...... 102

2.6.4 The deposition of iron sulfide thin films from [Fe(S2CN(Et)2)3] (4) ...... 105

2.7 Conclusion ...... 107

2.8 References ...... 110

Chapter 3 ...... 115 4

The synthesis of iron sulfide nanocrystals from tris(O-alkylxanthato)iron(III) complexes ...... 115

3.1 Summary ...... 116

3.2 Introduction ...... 117

3.3 Precursor synthesis ...... 119

3.3.1 Synthesis of Fe(S2COMe)3 (1) ...... 119

3.3.2 Synthesis of Fe(S2COEt)3 (2) ...... 120

i 3.3.3 Synthesis of Fe(S2CO Pr)3 (3) ...... 120

i 3.3.4 Synthesis of Fe(S2CO Bu)3 (4) ...... 120

3.4 Synthesis of iron sulfide nanocrystals ...... 121

3.5 Results and discussion ...... 121

3.5.1 The X-ray single crystal structure of [iPrOC(S)S-S(S) COiPr] ...... 121

3.5.2 Thermogravimetric analysis (TGA) ...... 123

3.5.3 Powder X-ray diffraction of iron sulfide nanocrystals ...... 124

3.5.4 Transmission electron microscopy of iron sulfide nanocrystals ..... 127

3.5.5 The effect of capping agents...... 130

3.5.6 The magnetic properties of iron sulfide nanocrystals...... 136

3.6 Conclusion ...... 140

3.7 References ...... 141

Chapter 4 ...... 145

Deposition of iron selenide nanocrystals and thin films from tris(N,N-diethyl- N’-naphthoylselenoureato)iron(III) ...... 145

4.1 Summary ...... 146

4.2 Introduction ...... 147

4.3 Experimental ...... 148

4.3.1 Synthesis of ligand [(C10H7CONHCSeN(C2H5)2)] ...... 148

4.3.2 Synthesis of [Fe(C10H7CONCSeN(C2H5)2)3] ...... 148

4.3.3 Synthesis of iron selenide nanocrystals ...... 149

4.3.4 Deposition of thin films by Aerosol Assisted Chemicl Vapour Deposition (AAACVD) method ...... 149

4.4 Results and discussion ...... 150

4.4.1 X–ray single crystal structure of [Fe(C10H7CONCSeN(C2H5)2)3] .. 150

4.4.2 Thermogravimetric analysis of [Fe(C10H7CONCSeN(C2H5)2)3] .... 152

4.4.3 Powder X-ray diffraction for iron selenide (FeSe2) nanocrystals ... 153

4.4.4 Transmission electron microscopy of iron selenide nanocrystals ... 154

4.4.5 Effect of the capping agent ...... 155

4.4.6 The magnetic properties of iron selenide nanocrystals ...... 156

4.5 Iron selenide thin films ...... 158

4.6 Conclusion ...... 160

4.7 References ...... 161

Chapter 5 ...... 163

Synthesis of iron selenide nanocrystals and thin films from bis(tetra-iso- propyldiselenoimido-diphosphinato)iron(II) and bis(tetra-phenyl- diselenoimidodiphosphinato)iron(II) complexes...... 163

5.1 Summary ...... 164

5.2 Introduction ...... 165

5.3 Experimental ...... 166

5.3.1 Synthesis of ligands ...... 166

5.3.2 Synthesis of imido(tetradiisopropyldiselnodiphosphanate) ligand i [(SeP Pr2)2N] (1) ...... 167

5.3.3 Synthesis of ligand imido(tetradiphenyldiselnodiphosphanate) ligand

[SePPh2)2N] (2) ...... 167

5.3.4 Synthesis of complex bis(tetraisopropyldiselenoimidodiphosphinato)- i iron(II) [Fe{(SeP Pr2)2N}2] (3) ...... 167

5.3.5 Synthesis of complex bis(tetraphenyldiselenoimidodiphosphinato)-

iron(II) [Fe{(SePPh2)2N}2] (4) ...... 168

5.3.6 Synthesis of iron selenide nanocrystals ...... 168

5.3.7 Deposition of thin films ...... 168

5.4 Results and discussion ...... 169

5.4.1 X-ray single crystal structure of [Fe{(SePPh2)2N}2] (4) ...... 170

5.4.2 X-ray single crystal structure [(SePPh2)2N)-O-(SePPh2)2N)] ...... 171

5.4.3 Thermogravimetric analysis ...... 174

5.4.4 Powder X-ray diffraction for iron selenide nanocrystals ...... 174

5.4.5 Transmission electron microscopy of iron selenide nanocrystals ... 176

5.4.6 The effect of surfactants ...... 179

5.5. Deposition of iron selenide thin films from bis(tetraiso- i propyldiselenoimidodiphosphinato)iron(II) [Fe{(SeP Pr2)2N}2] (3) ...... 180

5.5.1 Powder X-ray diffraction of iron selenide thin films ...... 180

5.5.2 Scanning electron microscopy of iron selenide thin films ...... 181

5.5.3 Interferometer microscopy of iron selenide thin films ...... 184

5.6 Deposition of iron selenide thin films from bis(tetraphenyl-

diselenoimidodiphosphinato)iron(II) [Fe{(SePPh2)2N}2] ...... 184

5.6.1 Scanning electron microscopy of iron selenide thin films ...... 185

5.6.2 Atomic force microscopy ...... 186

5.7 Conclusion ...... 187

5.8 References ...... 188

Chapter 6 ...... 190

General Experimental ...... 190

6.1 Chemicals ...... 191 7

6.2 Synthesis of iron sulfide nanocrystals ...... 191

6.3 Deposition of iron sulfide and iron selenide thin films by AACVD method ...... 191

6.4 Characterization methods ...... 192

6.5 Elemental analysis ...... 192

6.6 Thermogravimetric analysis ...... 193

6.7 Magnetic measurements ...... 193

6.8 X-Ray crystallography ...... 193

6.9 Powder X-ray Diffraction (p-XRD) ...... 194

6.10 Transmission Electron Microscopy (TEM) ...... 194

6.11 Scanning Electron Microscopy (SEM) ...... 195

6.12 Energy dispersive X-ray spectrometry ...... 195

6.13 AFM and Interferometer microscopy analysis ...... 195

6.14 References ...... 196

Chapter 7 ...... 197

Conclusion and Future Work ...... 197

7.1 Conclusion ...... 198

7.2 Future Work ...... 201

Appendix ...... 202

List of Publications ...... 203

Words: 42,193

Characters (no spaces): 230,825

Characters (with spaces): 272,422

List of Figures Chapter 1 1.1 Bandgap diagrams of conductor, semiconductor and insulator 29 1.2 Bandgap diagrams of n-type and p-type semiconductors 30 1.3 E-k diagram illustrating (A) Photon absorption in a direct band gap 31 semiconductor (B) Photon absorption in an indirect band gap semiconductor 1.4 A schematic diagram of bottom-up and top-down techniques 34 1.5 Schematic diagram of an AACVD kit 42 1.6 Schematic representation of CVD processes 42 1.7 Schematic (isothermal) free energy-composition diagram for the iron 46 sulfide minerals 1.8 Shows different phases and morphology of iron sulfide nanostrutures 51 from single source precursors Chapter 2 i 2.1 X-ray Single crystal structure of [Fe(S2CNEt Pr)3] (1) 80

2.2 X-ray Single crystal structures of [Fe(S2CNEtMe)3] (3) 81 i 2.3 Thermogravimetric analysis of complexes [Fe(S2CNEt Pr)3](1) 83

[Fe(S2CN(Hex)2)3] (2), [Fe(S2CNEtMe)3] (3) and [Fe(S2CN(Et)2)3] (4) at heating rate of 10 ˚C/min. under nitrogen, flow rate of nitrogen was 100 ml /minute i 2.4 p-XRD pattern for greigite (Fe3S4) nanocrystals from [Fe(S2CNEt Pr)3] 84 (1) in oleylamine at (a) 170 (b) 230 (c) 300 °C. The * shows pyrrhotite (FeS) phase

2.5 p-XRD pattern for greigite (Fe3S4) nanocrystals in oleylamine from 85

[Fe(S2CN(Hex)2)3] (2) at (a) 170 (b) 230 and (c) 300 °C. The * shows pyrrhotite (FeS) phase

2.6 p-XRD pattern for greigite (Fe3S4) nanocrystals in oleylamine from 85

[Fe(S2CNEtMe)3] (3) at (a) 170 (b) 230 and (c) 300 °C. The * shows pyrrhotite (FeS) phase

2.7 p-XRD pattern for greigite (Fe3S4) nanocrysal from [Fe(S2CN(Et)2)3] (4) 86 in oleylamine at (a) 170 (b) 230 and (c) 300 °C

2.8 Fe3S4 (greigite) and FeS (pyrrhotite) nanocrystals nanocrystals from 87 complex (1) at different temperatures in oleylamine (a) 170 °C (b) 230 °C (c) 300 °C and (d) SAED pattern of the nanocrystals grown at 300 °C from complex (1)

2.9 TEM and HRTEM of Fe3S4 (greigite) and FeS (pyrrhotite) nanocrystals 88 from complex (1) at 300 °C showing (a) hexagonal and cubic shape particles, inset shows small particles lying on a large particle and (b) lattice fringes showing d-spacing

2.10 TEM images of Fe3S4 (greigite) nanocrystals from complex (2) at (a) 170 88 °C and (b) 230 °C in oleylamine

2.11 TEM images of Fe3S4 (greigite) nanocrystals from complex (3) at (a) 230 88 °C and (b) 300 °C in oleylamine

2.12 TEM images of Fe3S4 (greigite) nanocrystals from complex (4) in 89 oleylamine at (a, b) 230 °C and (c, d) 300 °C. The inset in (b ane e) shows different shape of nanocrystals at 230 °C. HRTEM (f) showing lattice fringes with a d-spacing 5.2 Å corresponding to the (111) reflections of greigite 2.13 Graph shows the relative peak intensity of pyrrhotite (FeS) peak at 2- 89

theta 33.92 (101 plane) and greigite (Fe3S4) at different temperatures i from three different precursors [Fe(S2CNEt Pr)3] (1), [Fe(S2CN(Hex)2)3]

(2) and [Fe(S2CNEtMe)3] (3) respectively

2.14 p-XRD pattern of iron sulfide nanocrystals grown from [Fe(S2CN 91 i Et Pr)3] (1) at 300 °C in (a) hexadecylamine and (b) octadecene. *

shows the pyrrhotite (FeS) phases, + shows pyrrhotite (Fe7S8) and #

shows greigite (Fe3S4) phase respectively 2.15 Nanocrystals from complex (1) at 300 °C in different capping agent (a) 91 hexadecylamine, and (b) octadecene 2.16 Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves for 93 iron sulfide nanocrystals obtained from complex (1) by thermolysis in (a) oleylamine at 300 °C and (b) in HDA 2.17 Room temperature magnetic hysteresis loops of (a) complex (1) and iron 93 sulfide nanocrystals grown by thermolysis of (1) in oleylamine at (b) 230 °C and (c) 300 °C 2.18 Room temperature magnetic hysteresis loops of iron sulfide nanocrystals 94 obtained from (1) in (a) HDA and (b) ODE at 300 °C 2.19 Magnetic hysteresis loops at 5 K of iron sulfide nanocrystals obtained by 95 thermolysis of complex (1) in (a) oleylamine at 300 °C and (b) in HDA at 300 °C 2.20 Room temperature magnetic hysteresis loops of (a) complex (2) and (b) 97 iron sulfide nanocrystals obtained from (2) in oleylamine at 170 °C 2.21 Room temperature magnetic hysteresis loops of (a) complex (3) and (b) 97 iron sulfide nanocrystals obtained from (3) in oleylamine at 300 °C

2.22 (a) Shows the standard patterns p-XRD of iron sulfide (pyrite FeS1.96 99 (ICDD No: 01-073-8127)) and thin films deposited from complex i [Fe(S2CNEt Pr)3] (1) onto silicon at (b) 350, (c) 400, (d) 450 °C. The (+) symbol denotes the marcasite

2.23 SEM images of iron sulfide (pyrite FeS1.96) films deposited from 100 i complex [Fe(S2CNEt Pr)3] (1) onto silicon at (a) 350, (b) 400, (c) 450 °C, (d) elemental mapping at 400 °C 2.24 (a) 3D AFM images of iron sulfide thin film and (b) shows average 100 roughness and Rms roughness (c) Shows TEM images of scratched thin film and (d) shows the SAED pattern of the thin films from precursor i [Fe(S2CNEt Pr)3] (1) at 400 °C

2.25 (a) Shows the standard patterns p-XRD of iron sulfide (pyrite FeS1.96 101 (ICDD No: 01-073-8127)), and iron sulfide thin films deposited from

Fe(S2CN(Hex)2)3] (2) onto silicon at (b) 350, (c) 400 and (d) 450 °C. The asterisk symbol (*) denotes the pyrrhotite phase. The major diffraction peaks could be indexed as (200), (220), (201), (311) and (023) planes of

cubic pyrite (FeS1.96) and (*) shows pyrrhotite-IT (Fe0.95S1.05)

2.26 SEM images of the films deposited from complex Fe(S2CN(Hex)2)3] (2) 102 at (a) 350 (b) 400 (c) 450 °C and (d) elemental mapping at 350 °C (e) 3D AFM images of iron sulfide thin film and (f) shows average roughness and Rms roughness at 400 °C

2.27 (a) Shows the standard patterns p-XRD of iron sulphide (pyrite FeS1.96 103 (ICDD No: 01-073-8127)), (b) thin films deposited from complex

[Fe(S2CNEtMe)3] (3) on silicon substrate at 350 °C shows pyrites, (c) 400 °C shows mixture of pyrite and pyrrhotite ( ) and (d) 450 °C mostly pyrites. Whereas (+) shows marcasite

2.28 SEM images of iron sulfide thin films from precursor [Fe(S2CNEtMe)3] 104 (3) deposited at (a) 350, (b) 400, (c) 450 °C and (d) HRTEM at 400 °C showing d-spacing (3.12(Å)) corresponding to (111) plan of pyrite phase, (e) 3D AFM image of iron sulfide thin film and (f) shows average roughness and Rms roughness of thin films from precursor (3) at 400 °C

2.29 (a) Shows the p-XRD standard patterns of iron sulfide (pyrite FeS1.96 105 (ICDD No: 01-073-8127)), and iron sulfide thin films deposited from

complex [Fe(S2CN(Et)2)3] (4) (b) at 350 °C shows pyrites with (+) minor marcasite (c) 400 °C, hexagonal pyrrhotite-IT and (d) 450 °C hexagonal pyrrhotite-IT 2.30 SEM images of iron sulfide thin films deposited from precursor (4) at (a) 106 350, (b) 400 and (c) 450 °C. Inset (b) shows the SAED pattern (d) elemental mapping at 400 °C (e) 3D AFM images of iron sulfide thin films and (f) shows average roughness and Rms roughness from precursor (4) at 400 °C 2.31 Phases of iron sulfide nanocrystals grown by thermolysis from precursors 109 (1) – (3). The relative amount of each phase is represented as the height of the cylinder. These are approximated based on the powder XRD results. Compared to the relative thermodynamic stabilities of the various phases of iron sulfides (the z axis) and the phases after Vaughan and Lennie 2.32 The main phases of iron sulfide thin films from precursors (1)–(4). The 110 relative amount of each phase is represented as the height of the cylinder. These are approximated based on the p-XRD results. Compared to the relative thermodynamic stabilities of the various phases of iron sulfides (the z-axis) and the phases after O’Brien and co-workers and Vaughan and Lennie Chapter 3 3.1 X-ray single crystal structure of [iPrOC(S)S-S(S)COiPr] 122

3.2 Thermogravimetric analysis of complexes [Fe(S2COMe)3] (1), 124

i i [Fe(S2COEt)3] (2), [Fe(S2CO Pr)3] (3) and [Fe(S2CO Bu)3] (4) at heating rate of 10 °C/min. under nitrogen, flow rate 100 ml minute-1

3.3 p-XRD pattern for predominantly greigite (Fe3S4) nanocrystals in 125

oleylamine from [Fe(S2COMe)3] (1), at (a) 170 (b) 230 and (c) 300 °C . The diffraction peaks for (220), (311), (400), (511) and (440) planes of

greigite (Fe3S4) are dominant. (#) shows the pyrrhotite (FeS) phase

3.4 p-XRD pattern for greigite (Fe3S4) nanocrystals in oleylamine from 125

[Fe(S2COEt)3] (2) at (a) 170 (b) 230 and (c) 300 °C. The diffraction

peaks for (220), (311), (400), (511) and (440) planes of greigite (Fe3S4) are dominant. The * shows pyrite and # pyrrhotite (FeS) phases respectively

3.5 p-XRD pattern for greigite (Fe3S4) nanocrystals in oleylamine from 126 i [Fe(S2CO Pr)3] (3) at (a) 170 (b) 230 and (c) 300 °C. The peaks for (220),

(311), (400), (511) and (440) planes are of greigite (Fe3S4) phase. The + shows marcasite phase

3.6 p-XRD pattern for greigite (Fe3S4) nanocrystals in oleylamine from 127 i [Fe(S2CO Bu)3] (4) at (a) 170 (b) 230 and (c) 300 °C. The peaks for

(220), (311), (400), (511) and (440) of planes are of greigite (Fe3S4) phase. The * shows pyrite and # pyrrhotite (FeS) phases

3.7 TEM images of iron sulfide nanocrystals obtained from [Fe(S2COMe)3] 128 (1), in oleylamine at (a) 230 °C and (b) 300 °C 3.8 TEM images of iron sulfide nanoparticles obtained from thermolysis of 128

[Fe(S2COEt)3] (2), in oleylamine at 170 °C. (a) shows the different shapes of nanocrystals, (b) HRTEM image showing lattice fringes with a d-spacing of 2.98 Å corresponding to the (311) plane reflection of greigite

3.9 TEM images of iron sulfide nanocrystals obtained from [Fe(S2COEt)3] 129 (2) in oleylamine at (a, b) 230 °C and (c) 300 °C. The insets in (c) show different shape of nanocrystals at 300 °C, whereas (d) shows the SAED pattern of the nanocrystals i 3.10 TEM images of iron sulfide nanocrystals obtained from [Fe(S2CO Pr)3] 129 (3), in oleylamine at (a, b) 230 °C , (c) 300 °C and (d) showing lattice fringes with a d-spacing of 5.72 Å corresponding to the reflection of (111) of greigite phase i 3.11 TEM images of iron sulfide nanocrystals obtained from [Fe(S2CO Bu)3] 130 (4), in oleylamine (a) at 170 °C, (b) 230 °C and (c) HRTEM at 230 °C showing lattice fringes with a d-spacing of 3.50 (Å) corresponding to the (220) reflection plane of greigite phase (d) TEM image of nanocrystals produced at 300 °C 3.12 p-XRD pattern from complexes (1-4) in hexadecylamine at 230 °C for 131 1hour reaction time, (a) and (b) show pattern for pyrite phase obtained from complexes (1,2), (c) shows the pattern for greigite phase obtained from complex (3) and (d) shows a pattern for pyrrhotite phase obtained from complex (4)

3.13 Nanocrystals obtained from (a) complex (1) in hexadecylamin at 230 °C, 132 insets shows individual nanocrystals, (b) TEM images from complex (2) in hexadecylamine, insets shows sheets like crystallites, (c) shows the crystallites with different morphology obtained from complex (3) in hexadecylamine at 230 °C and (d) shows nanocrystals obtained from complex (4) in hexadecylamine at 230 °C 3.14 p-XRD (a) from complex (1), (b) from complex (2), (c) from complex (3) 132 and (d) from complex (4) in octadecylamine at 230 °C after 1 hour. The (*) shows the pyrite phase and (#) shows pyrrhotite phase 3.15 TEM images of the nanocrystals obtained from complex (1-4) in 133 octadecylamine at 230 °C after 1 hour (a) from (1), (b) from (2), (c) from (3) and (d) from (4) 3.16 p-XRD (a) from complex (1). (b) from complex (2), (c) from complex (3) 134 and (d) from complex (4) in octadecylamine at 230 °C in 3 hours reaction time. The (*) shows pyrite and (#) shows pyrrhotite phase 3.17 Nanocrystals obtained from (a) complex (2) in octadecylamine after 3 134 hours at 230 °C, (b) shows the lattice fringes correspond to greigite phase obtained from complex (2), whereas (c) shows nanocrystals from complex (3) in octadecylamine at 230 °C in 3 hours reaction time 3.18 (a) ZCF and FC (b) at 5 K, 50 K and at 300 K from iron sulfide (greigite 137 phase dominant with pyrrhotite phase) nanocrystals obtained from

[Fe(S2COMe)3] (1) in oleylamine at 230 °C 3.19 (a) ZFC/FC magnetisation at 100 Oe applied field (b) Field dependence 138 of the magnetisation at 5, 50 and 300 K from iron sulfide (greigite phase) i obtained from the complex [Fe(S2CO Pr)3] (3) at 300 °C 3.20 (a) ZFC and cold field magnetization curves (b) Room temperature 139 magnetic hysteresis loops of iron sulfide (greigite phase) nanocrystals at i 5K, 50 K and 300K obtained from [Fe(S2CO But.)3] (4) in oleylamine at 230°C Chapter 4

4.1 X-ray Single crystal structure of [Fe(C10H7CONCSeN(C2H5)2)3] showing 151 molecules 1 (a) and molecule 2 (b)

4.2 Thermogravimetric analysis of [Fe(C10H7CONHCSeN(C2H5)2)3] at 153 heating rate of 10˚C /min under nitrogen, flow rate of nitrogen 100 ml/minute

4.3 p-XRD pattern for ferroselite (FeSe2) nanocrystals in oleylamine at (a) 153 190 and (b) 240 and (c) 290 °C respectively. Whereas *shows different

phase of ferroselite (FeSe2) 4.4 Growth of nanocrystals in oleylamine at (a) 190 °C (b) lattice fringes 154 showing d-spacing at 190 °C (c), (d) growth of nanocrystals at 240 °C and (e), (f) at 290 °C 4.5 p-XRD pattern for iron selenide nanocrystals obtained from the 155

thermolysis of complex [Fe(C10H7CONCSeN(C2H5)2)3] in (a) dodecanthiol, (b) oleylamine and (c) in the mixture of dodecanthiol and oleylamine at 240 °C for 30 minutes reaction time. The symbol (*) shows

ferrosilite ((FeSe2) (ICDD No: 00-012-0290)) phase 13

4.6 TEM images of iron selenide nanocrystals obtained from the thermolysis 156

of complex [Fe(C10H7CONCSeN(C2H5)2)3] in (a) dodecanthiol and (b) in the mixture of dodecanthiol and oleylamine (1:1) at 240 °C for 30 minutes reaction time 4.7 Field-dependence of the magnetization at 300 K of iron selenide 157 nanocrystals obtained by thermolysis of complex in oleylamine at 190, 240 and 290 °C. Inset: the reminiscence of a hysteresis loop associated to the minor ferromagnetic phase, formed at 240 °C 4.8 Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves for 157 iron selenide nanocrystals obtained by thermolysis in oleylamine at 190, 240 and 290 °C 4.9 (a) The p-XRD pattern for iron selenide thin film deposited at 625 °C on 159 silicon substrate (b) SEM images of the same film (c) roughness of the film (d) AFM 3D images of the surface roughness of thin film (e) SEM showingn thickness of the film. 4.10 The elemental mapping of thin film of (a) iron, (b) selenium, (c) shows 159 the distribution of iron and selenium and (d) shows the EDX analysis of thin film Chapter 5

5.1 X-ray single crystal structure of [Fe{(SePPh2)2N}2] 171

5.2 X-ray single crystal structures of [(SePPh2)2N)-O-(SePPh2)2N)] 172 i 5.3 Thermogravimetric analysis of complexes (a) [Fe{(SeP Pr2)2N}2] and (b) 174

[Fe{(SePPh2)2N}2]

5.4 p-XRD pattern for ferroselite (FeSe2) nanocrystals in oleylamine from 175 i complex [Fe{(SeP Pr2)2N}2] at (a) 190, (b) 240 and (c) 290 °C

respectively. Whereas *shows phase of ferroselite (FeSe2) phase

5.5 p-XRD pattern for ferroselite (FeSe2) (ICDD No: 00-021-0432) 176

nanocrystals in oleylamine from complex ([Fe{(SePPh2)2N}2]) at (a) 190, (b) 240 and (c) 290 °C respectively. Whereas * denotes to another phase

of ferroselite (FeSe2) (ICDD No: 01-074-0247) phase i 5.6 FeSe2 nanocrystals from complex [Fe{(SeP Pr2)2N}2] (3) in oleylamine at 177 (a) 190, (b) 240 and (c) 290 °C. HRTEM (d) showing lattice fringes with a d-spacing of 3.69 Å corresponding to the (110) plane reflection of

orthorhombic ferroselite (FeSe2) (ICDD No: 00-021-0432) phase produced at 290 °C 5.7 Growth of nanocrystals in oleylamine from complex (4) 178

[Fe{(SePPh2)2N}2] at (a) 190, (b) 240 and (c) 290 °C, (d) HRTEM showing lattice fringes with a d-spacing of 2.56 Å corresponding to the

(111) reflection plane of orthorhombic ferroselite (FeSe2) (ICDD No: 00- 021-0432) phase obtained at 290 °C 5.8 p-XRD pattern for iron selenide nanocrystals (a) from complex 179 i [Fe{(SeP Pr2)2N}2] and (b) from complex [Fe{(SePPh2)2N}2] in hexadecylamine at 240 °C respectively

5.9 TEM images of iron selenide nanocrystals (a and b) from complex 180 i [Fe{(SeP Pr2)2N}2] and (c) from complex [Fe{(SePPh2)2N}2] in hexadecylamine at 240 °C i 5.10 p-XRD pattern for thin films deposited from complex [Fe{(SeP Pr2)2N}2] 181 at (a) 500, (b) 550 and (c) 600 °C respectively. The main phase is iron

selenide (Fe7Se8) ICDD No: 01-071-0586 new no. 04-007-1554),

whereas FeSe2 ((#) ICDD No: 00-012-0291) and FeSe2 ((+) ICDD No: 01-074-0247) are the minor phases obtained 5.11 SEM images of iron selenide thin films deposited from complex 182 i [Fe{(SeP Pr2 )2 N}3 at (a) 500, (b) 550 and (c) 600 °C respectively. The insets shows SEM images at higher magnification 5.12 EDX analysis of iron selenide thin films deposited at (a) 500, (b) 550 and 182 i (c) 600 °C from complex [Fe{(SeP Pr2)2N}2] on silicon substrate 5.13 Elemental mapping of iron selenide thin films deposited from complex 183 i [Fe{(SeP Pr2)2N}2] at (a) 500, (b) 550 and (c) 600 °C respectively 5.14 Thickness of iron selenide thin films deposited at 600 °C on silicon 183 i substrate from complex [Fe{(SeP Pr2)2N}2] (3) 5.15 3D interferometer images of iron selenide thin films deposited at (a) 500, 184 (b), 550 and (c) 600 °C on silicon substrate from complex i [Fe{(SeP Pr2)2N}2] (3) 5.16 SEM images of iron selenide thin films deposited from complex 185

[Fe{(SePPh2)2N}2] at (a) 500, (b) 550 and (c) 600 °C on to silicon substrate 5.17 Thickness of iron selenide thin films deposited at 600 °C on silicon 186

substrate from complex [Fe{(SePPh2)2N}2] (4) 5.18 3D AFM images of iron selenide thin films deposited from complex 186

[Fe{(SePPh2)2N}3] at (a) 500, (b) 550 and (c) 600 °C respectively 5.19 Histograms showing average roughness and Rms roughnes of the iron 187

selenide thin films deposited from complex ([Fe{(SePPh2)2N}2]) (4) at (a) 500, (b) 550 and (c) 600 °C onto the silicon substrate Chapter 6 6.1 Aerosol Assisted Chemical Vapour Deposition (AACVD) Apparatus 192

List of Tables 1.1 Classification of materials based on Resistivity, energy gap (Eg) 28 and carrier density (n) at room temperature 1.2 Properties of some common semiconductors 32 1.3 Main applications of semiconductors 33 1.4 The different single source precursor producing different phases 50 of iron sulfide with different morphology 2.1 Structural refinement data for complex (1) and complex(3) 82 obtained from X-ray single crystallography 2.2 Phases of iron sulfide and morphology of the nanocrystals 90 produced from different single source precursors 2.3 Phases and morphology of the crystalilites of iron sulfide thin 107 films 3.1 Structural refinement data for [iPrOC(S)S-S(S)COiPr] 123 3.2 Summary of phases and morphology of iron sulfide nanocrystals 135 obtained from all four precursors at different reaction conditions

4.1 Structural refinement data for [Fe(C10H7CONCSeN(C2H5)2)3.0.5- 152

(C7H8)]

5.1 Structural refinement data for [(SePPh2)2N)-O-(SePPh2)2N)] and 173

[Fe{(SePPh2)2N}2]

Abbreviations AACVD Aerosol-Assisted Chemical Vapour Deposition CTAB Cetyl trimethylammonium bromide EDS Energy Dispersive X-ray Spectroscopy EG Ethylene Glycol EN Ethylenediamine HDA Hexadecylamine LACVD Laser-Assisted Chemical Vapour Deposition LED Light Emitting Diodes LP-CVD Low-Pressure Chemical Vapour Deposition MOCVD Metal-Organic Chemical Vapour Deposition

NH3 Ammonia OA Oleic Acid ODE 1-Octadecene OM Oleylamine PACVD Photo-Assisted Chemical Vapour Deposition PECVD Plasma-Enhanced Chemical Vapour Deposition PVs Photovoltaic devices TBDS Tert-Butyl Disulfide TEM Transmission Electron Microscopy THF Tetrahydrofuran TOP Tri-n-Octylphosphine TOPO Tri-n-Octylphosphine Oxide AFM Atomic Force Microscopy CVD Chemical Vapour Deposition

DCM Dichloromethane DETA Diethylenetriamine DIW Deionized Water

dtc diethyldithiocarbamate e.g exempli gratia

EDAX Energy Dispersive Analysis X-Ray en ethylenediamine Et Ethyl et al et aliae eV Electron Volt FC Field-Cooled HRTEM High-Resolution Transmission Electron Microscopy iBu Isobutyl ICDD International Centre for Diffraction Data iPr Isopropyl KI Keck Interferometer Me Methyl Mr Remnant Magnetization Ms Magnetization Saturation NCs Nanocrystals Nm Nanometer

NMR Nuclear Magnetic Resonance OMVPE Organo Metallic Vapour Phase Epitaxy phen 1,10-phenanthroline p-XRD Powder X-ray Diffraction

Rms Root mean square

SAED Selected Area Electron Diffraction

SEM Scanning Electron Microscope SQUID Superconducting Quantum Interferance Device SSPs Single Source Precursors TB Blocking Temperature

TGA Thermogravimetric Analysis

XPS X-ray Photoelectron Spectroscopy ZFC Zero-Field-Cooled

G4.NH2 Generation 4 terminated with amino DSNPs Dendrimer-stabilized nanoparticles

Abstract Recently there is growing interest for the production of cheap and nontoxic colloidal nanomaterials or thin films for photovoltaic applications. Iron chalcogenides are cheapest materials available for solar cell applications. The work presented here involve the synthesis of iron chalcogenide nanocrystals by colloidal methods and the deposition of thin films by aerosol assisted chemical vapour deposition (AACVD) method from single source precursors. In addition, a comprehensive literature review of iron chlacogenide nanocrystals and thin films is presented.

Several new iron complexes belonging to thiocarbamato, xanthato, selenoureato and imidodiphosphinato, have been synthesised. Tris(dialkyldithiocarbamato)- i iron(III) complexes of general formula [Fe(S2CNRR’)3] where R = Et, R’ = Pr; R, R’ = Hex; R = Me, R’ = Et and R, R’ = Et and tris(O-alkylxanthato)iron(III) i i complexes of general formula [Fe(S2COR)3] where R = Me, Et, pr and Bu have i been synthesised. The X-ray single crystal structure of [Fe(S2CNEt Pr)3], i i Fe(S2CNEtMe)3 and [ PrOC(S)S-S(S)CO Pr] were determined. Iron complexes were used as a single source precursors for the deposition of iron sulfide nanocrystals by thermolysis in oleylamine, hexadecylamine and octadecene and thin films on silicon substrate at different temperatures. The complexes show typical paramagnetic behaviours whereas the iron sulfide nanocrystals produced show ferromagnetic behaviour. The greigite and pyrrhotite phases with hexagonal and cubic morphology were obtained by thermolysis. Pyrite and pyrrhotite phases were dominant in thin films.

The complex tris(N,N-diethyl-N’-naphthoylselenoureato)iron(III) and its X-ray single crystal structure is also reported. Long rod like nanocrystals of orthorhombic ferroselite (FeSe2) obtained by thermolysis in oleylamine, dodecanthiol and in the mixture of oleylamine and dodecanthiol at 190, 240, and 290 °C. Paramagnetic behaviour was found under magnetic measurement of iron selenide nanocrystals. A very thin film of iron selenide (FeSe) phase was deposited on silicon substrate at 625 °C by AACVD method.

i The complexes [Fe{(SeP Pr2)2N}2] and [Fe{(SePPh2)2N}2] were synthesised. The X-ray single crystal structure of [Fe{(SePPh2)2N}2] and [(SePPh2)2N)-O- (SePPh2)2N)] were also reported. A mixture of orthorhombic ferroselite (FeSe2) with rod and plate-like crystallites was obtained from the thermolysis of these complexes in oleylamine and hexadecylamine at 190, 240 and 290 °C temperatures. Also the mixed phases of iron selenide (Fe7Se8 and FeSe2) thin films having rod and sheet-like morphology were deposited at different deposition temperature (500, 550 and 600 °C) onto silicon substrates from these complexes.

Declaration I hereby declare that no portion of the work referred to in the thesis has been submitted in support of an application for another degree or qualification of the University of Manchester or any other University or other Institute of Learning.

……………………..

Masood Akhtar

Copyright 1. The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the “Copyright”) and s/he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes.

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Dedication

To parents, wife and children:

Ahsan, Anum and Anaya

Acknowledgement First and foremost, I praise to God, the Almighty, who has made it possible for me to research the world of science and specifically chemistry so that I may pursue my passion and my dreams. I have also been supported by many good people to whom I would like to express my deepest gratitude.

I am sincerely grateful to my supervisor, Prof. Paul O’Brien, for giving me the opportunity to research at the University of Manchester; he has guided me throughout my research. He has been always kind and helpful. I am also thankful to EPSRC for their financial support.

Dr Muhammad Azad Malik has been instrumental in my success and has encouraged me during this long journey to complete PhD. Dr. Malik’s valuable advice, guidance and trust during my study made it possible for me to complete this goal. Also many thanks for his patience, help and guidance for writing papers and thesis. I learnt not only chemistry but also lots of other things from different aspects of life from his company.

I am grateful to all the staff in the School of Chemistry for their support and especially to all technical staff in School of Materials: Ms Judith Shackleton, Mr Gary, Chris Wilkins, Mike Faulkner, Alan Harvey and Andrew Forrest for their assistance on the XRD, SEM, TEM and AFM. I am also thankful to Dr. James Raftrey and Dr. Madeleine Helliwell who helped me with the single crystals structure analysis. I am grateful for the help extended to me by Dr. Floriana Tuna with the magnetic measurement and helping while drafting papers. I am thankful to Dr. Michael Ward University of Leeds for TEM analysis.

I greatly acknowledge good company and support of my all, present and past, members of the POB group for their help during my study. I am also thankful to Dr. Karthick, Dr. Dyo, Khadijt, Dr.Yousf and Hanan. My special thanks to Dr. Javeed and Dr. Ahmad for TEM.

Without material and spiritual support of my parents this journey I could not have completed my PhD; I am forever grateful for their kindness. I am grateful

to my immediate family: my brothers, sisters, and brothers and sisters-in-laws, for their assistance in numerous ways.

My lovely children, Ahsan, Anum and Anaya’s love and affection was a great source of motivation for me, whilst patience and understandings of my wife has been a great strength; without their support and encouragements I could not have finished this work.

Masood Akhtar

Chapter 1

General Introduction

1.1 Summary In this chapter, a general discussion about semiconductors and band structure is described. The different methods for the synthesis of nanocrystals and deposition of thin films and applications of semiconductors and nanocrystals is described. In addition, a literature review regarding iron chlacogenide nanocrystals and thin films is presented.

1.2 Classification of solids Solid materials can be classified on the basis of electrical conductivity into three categories: conductor, semiconductor, and insulator.1,2 A conductor is a material which has much lower resistance to the flow of electrical current e.g copper (Cu), zinc (Zn) silver (Ag) and sodium (Na). All these have partially filled outermost band and electrons in this band are free to move when an electric field is applied. As compared to conductors insulators offer more resistance to the flow of electric current. In this material, all the electrons are present in the valence band and the conduction band is empty. Due to a large band gap it is hard to promote an electron from the valence band to the conduction band. A semiconductor shows resistance to the flow of electric current in between insulators and conductors. They have completely filled valence band which is separated by a small gap from an empty conduction band. Electrons can easily obtain enough energy to move from the valence band to the conduction band. Their electrical conductivity is sensitive to temperature, illumination and magnetic field. The classification of inorganic solids on the basis of resistivity, energy gap and carrier density are given in Table 1.1.3

Table 1.1 Classification of materials based on Resistivity, energy gap (Eg) and carrier density (n) at room temperature.3

-3 Type of Solid Resistivity (Ohm- Eg (eV) n (cm ) Examples cm) Conductor 10-5 to 10-6 ------1022 Copper, Silver, Tungsten -2 9 17 Semiconductor 10 to 10 0

12 22 Insulator < 10 to 10 4  Eg << 1 Rubber, Sulfur

1.3 Semiconductors A semiconductor is a material, which exhibits electrical conductivity somewhere between that of a conductor and an insulator. Semiconductors and insulators almost have same resistance in the absence of an electric field. Semiconductors have a smaller band gap which dictates electrical properties. Semiconductors 28

have a variety of applications in electronic devices ranging from computers to cellular phones. In bulk semiconductor material, at low temperature, the lower energy level (valence band) is filled with electrons while the higher energy level (conduction band) is empty (Figure 1.1).

Conduction band Conduction band Conduction band

Fermi energy

Band gap Energy

Valence band Valence band Valence band

Conductor Semi conductor Insulator

Figure 1.1 Bandgap diagrams of conductor, semiconductor and insulator.

The band gap is the energy difference between the two bands. The materials with a band gap (Eg) between 0.3 and 3.8 eV are considered as semiconductors. At higher temperature, electrons in the valence band gain energy and can be promoted to the higher-energy conduction band leaving a positive ‘hole’ in the valence band. The number of holes which the semiconductor can produce controls the conductivity, essentially. Intrinsic semiconductors (pure semiconductor) e.g silicon, germanium and gallium are characterised by an equal number of charge carriers’ i.e. the number of excited electrons and number of holes are equal. The electrical properties of pure semiconductors can be modified by doping process where impurities are introduced to the pure semiconductor. Each impurity atom adds an electron or a hole which allows the free passage of charge.

Semiconductors are divided in to two types (n-types and p-types) depending on the nature of introduced impurity. Semiconductors with more electrons in the doping region are called n-type semiconductor and the semiconductor with more holes in the doping region are called p-type semiconductor. In n-type 29

semiconductor, the Fermi level lies close to the conduction band edge, while in the p-type semiconductor Fermi level lies closer to the valence band edge (Figure 1.2).

Conduction band Conduction band

Ed ~ 0.01 eV ) Donor level

Eg ~ 1 eV Energy(E Acceptor level

E a ~ 0.01 eV

Valence band Valence band

n-type p-type

Figure 1.2. Bandgap diagrams of n-type and p-type semiconductors.

Semiconductors can be divided into direct band gap semiconductors and indirect band gap semiconductors on the basis of their optical properties; in direct band gap semiconductors the transition from conduction band to valence band involves no change in momentum. This is due to the fact that the minimum energy of the conduction band lies directly above the maximum energy of the valence band in momentum space.

Hence, for direct band gap materials, the excess energy of the electron-hole recombination can be taken away as a heat or as a photon of light. Conversely, in indirect band gap semiconductors the electron goes from the bottom of the conduction band to the top of the valence band. It also undergoes a significant change in momentum (Figure 1.3). This change is due to shifting of minimum energy in the conduction band to the valence band. Indirect band gap semiconductors are inefficient at emitting light. In nanocrystalline semiconductors, two factors are responsible for the change in behaviour of semiconductor from the bulky materials. The first factor is high surface/volume

ratio and other is the size of the particles.4 The band gap of semiconductor is dependent on its size. When there is an increase in the band gap, the optical spectra of nanocrystalline semiconductors show a hypsochromic shift.5

Conduction edge band gap

Eg hυ = Eg Eg hυ = Eg + E

Valence edge band gap

K= 0 K= 0 A B Figure 1.3. E-k diagram illustrating (A) Photon absorption in a direct band gap semiconductor (B) Photon absorption in an indirect band gap semiconductor.

1.3.1 Semiconductor nanoparticles Over the past three decade nanoparticles of semiconductor materials have been given considerable attention for their applications in device technologies. Nanoparticles show very special chemical and physical properties due to the quantum confinement effect. The surface properties have significant effects on their structural and optical properties because of the high surface-to-volume ratio of nanoparticles. Initially the synthesis of nanomaterials was performed merely to understand the physical properties of these materials and develop smaller, highly uniform and lower dimensionality materials (quantum dots), where the quantum-confinement effects are large. Later on interest in these materials shifted to the development of a reliable and reproducible method for large-scale synthesis. Some common properties of semiconductors are given in Table 1.2.

Table 1.2. Properties of some common semiconductors.6

Semiconductor Structure Lattice Band-gap Type Parameters (Å) (eV)

IV-IV

Ge Cubic 5.646 0.66 Indirect

Si Cubic 5.437 1.12 Indirect

II-VI

ZnS Cubic, 5.420, 3.820, 3.68, 3.91 Direct Hexagonal 6.260

CdS Cubic, 5.832, 4.140, 2.42, 2.51 Direct Hexagonal 6.710

ZnSe Cubic, 5.669, 4.000, 2.70 Direct Hexagonal 6.540

CdSe Cubic, 4.300, 7.010 1.70,1.75 Direct Hexagonal

CdTe Cubic 6.482 1.56 Direct

III-V

GaP Cubic 5.451 2.26 Indirect

GaAs Cubic 5.653 1.42 Direct

InP Cubic 5.869 1.35 Direct

InAs Cubic 6.058 0.36 Direct

AlAs Cubic 5.661 2.16 Indirect

1.3.2 Applications of semiconductors The elemental semiconductor (Si, Ge) are used for the production of transistors and integrated circuits, while the compound semiconductors used for solar cells, light emitting diodes (LED), optical detectors, thermal imaging device, environmental sensors. The main applications of semiconductors are summarized in Table 1.3.

Table 1.3. Main applications of semiconductors.7

Semiconductor Applications

Si, Ge Diodes, transistors, integrated circuits

GaAs Solar cells, light emitting diode (LEDs)

GaAs/AlGaAs Hetero-structure laser, solar cells, high electron mobility transistors, hetero-junction bipolar transistors

GaP Red LEDs, photocathode’s

GaAsP Red LEDs

InGaP Red LEDs

AlGaInP Yellow/green LEDs

InGaP/AlGaInP Red laser pointer

GaSb/AlGaSb Thermal imaging device, environmental sensors

GaN Blue LEDs

InGaN Green LEDs

InP Gunn diodes, weather radar devices

InP/InGaAs Detector in optical fibre technology

ZnS (Ag doped) Blue phosphor for TV cathode ray tube

ZnS (Mn doped) Thin film electroluminescent displays (TFELs)

ZnSe, ZnSSe Blue/green LEDs and lasers

ZnSe, ZnS Interference coating for optical components

ZnCdS Solar cells

ZnCdS (Cu doped) Green phosphor in TV cathode ray tubes

CdS/CdTe Infrared detector

CdHgTe Infrared detector, thermal imaging system

1.4 Synthesis of nanoparticles Nanoparticles can be synthesized by different methods, in which two methods are very common. The first method known as the ‘top-down’ approach and the other is known as the ‘bottom-up’ approach. The top-down approach generally involves etching or molding of materials into smaller components. In this 33

method techniques such as precision engineering and lithography are involved. Over the past 30 years, these techniques have been used to develop and refine the semiconductor industry. Traditionally this approach has been used in making parts for electronics goods and computers. In bottom-up approach the structures are built, atom-by-atom or molecule-by-molecule.

To obtain a desired structure, large numbers of atoms, molecules or particles are used or created by chemical synthesis, and then arranged through naturally occurring processes. Diagrams which illustrate both approaches are shown in Figure 1.4.

Figure 1.4. A schematic diagram of bottom-up and top-down techniques.

There are several chemical methods which are used to produce nanoparticles from atoms based on transformations in solution e.g. chemical vapour deposition (CVD), laser pyrolysis, plasma or flame spraying synthesis, atomic, or molecular condensation and sol-gel processing. These chemical processes depend on the availability of suitable/appropriate ‘metal-organic’ precursors. Another bottom-up technique is self-assembly technique in which atoms or molecules arrange themselves into nanoscale structures by chemical or physical interactions between their units. The best examples of self-assembly process are the formation of salt crystals and snowflakes.

For thousands of years, self-assembly has occurred in nature but its use in industry is relatively new. In this technique, environmental and economic interest are involved, through which materials or product form themselves, using less energy and creating less waste. Positional assembly is the last bottom-up technique, in which atoms, molecules or clusters are manipulated and positioned one-by-one. It is currently not suitable as an atomic-scale industrial process because it is extremely laborious.

1.4.1 Methods for the preparation of semiconductor nanoparticles For the preparation of semiconductor nanoparticlematerials a wide range of synthetic methods have been reported. There is still a problem with the reproducible preparation of materials, which is required for technological application. For the production of semiconductor nanoparticles, the chemical methods can be divided into four categories, the colloidal route, growth-in- confined matrices, gas-phase synthesis and the organometallic route.8

In the colloidal method precipitation reaction is carried out in a homogeneous solution in the presence of stabilizers.8 The stabilizers role is to prevent agglomeration and further growth of the colloids, by keeping them in solution. This is one of efficient method for the preparation of semiconductor nanoparticles. However, the problems with reproducibility are still there as instability and poor crystallinity of the materials, because it used low temperature for synthesis.

Synthesis in confined matrices is used to control dispersity by growth in defined cavities. This resulted in the extension of studies to grow nanoscale materials in zeolites,9,10 layered solids,11 molecular sieves,12,13 micelles (including reverse micelles),14 gels,15 and polymers.16,17 The preparations of clusters and particles of semiconductor metal sulfides in porous silica-pillared layered phosphates,11 12 Fe2O3 nanoparticles in mesoporous silicate, Zn4X and Cd4X clusters (X = S, Se) in an aluminate or borate cage,13 are the best examples of confined matrices synthesis.

Semiconductor nanoparticles preparation via gas phase synthesis involves atmospheric or low-pressure evaporation of either powders or preformed semiconductor, or the co-evaporation of the two elemental components. By using these techniques, the results are deposition of particles with larger size distributions, ranging from 10-200 nm. Furthermore, in this method the problem of particle aggregation due to the absence of a surface passivating (capping) agent is still there.

The problems related with the colloidal method for the synthesis of nanoparticles can be overcome by an alternative method, which makes use of organometallic or metal organic compounds under anaerobic conditions. Bawendi et al.18 pioneered this method, by which good quality, monodispersed, highly crystalline nanoparticles of CdSe, CdS, and CdTe were synthesized. By using this method, a solution of chalcogen source and dimethylcadmium were mixed in tri-n-octylphosphine (TOP) and injected into hot tri-n-octylphosphine oxide (TOPO) at temperatures ranging from 120 °C – 250 °C. TOPO is a high boiling-point coordinating solvent which stabilizes the nanocrystalline colloidal dispersions and electronically passivates the semiconductor surface. This capping agent can be replaced by other groups such as pyridine, 4-picoline, tris(2-ethylhexyl)phosphate and 4-(trifluoromethyl)thiophenol. However, the use of TOP/TOPO combination proves the most efficient passivating system known currently.

1.4.2 Single-Molecular precursor method Although Bawendi’s method,18 is good for the preparation of fine-quality nanoparticles, toxic and pyrophoric chemical such as Cd(CH3)2 have been a problem related to this method. Another method, utilising single source precursors, has been introduced to avoid the use of hazardous compounds. The use of single-source precursors for the synthesis of semiconductor nanoparticles 19 was first reported by Steigerwald et al. where [Cd(Se(C6H5))]2 or [Cd(SePh)2]

[Et2PCH2CH2PEt2] were used as a single-source precursors to prepare CdSe nanoparticles in refluxing pyridine. These single-molecular precursors contain both metal and non-metal (chalcogenide) elements within the same molecule.

Therefore, they have many advantages for the one-step synthesis of nanoparticles containing those elements. Later on, the synthesis of sulfide and selenide nanocrystals from the thermolysis of single-source precursors was reported from several groups. The use of single-source precursors has a number of advantages over other existing methods.20 For example,

(a) The synthesis of nanoparticles from single source routes, avoid the need for volatile, toxic and/or pyrophoric precursors.

(b) Some nanoparticles of group II-VI are air sensitive. All synthesis of precursors is carried out under inert conditions, with the resulting precursors being air and moisture stable.

(c) In single source routes only one involatile precursor is involved. So purification of this precursor is easier than that of two or more volatile precursors. Hence there is less chance of impurities being introduced into the nanoparticles.

Single-source precursors were originally designed for chemical vapour deposition (CVD) of thin films. For example, ZnS film has been deposited from 21 [Zn(S2CNEt2)2]2 a single source precursor in vapour phase. However, this compound has low volatility for use in conventional metal-organic chemical vapour deposition (MOCVD) processes. O’Brien et al.22 used new n i bis(dialkyldithiocarbamates) of zinc, [Zn(S2CNMeR)2]2 (R = Et, Pr, Pr and n Bu). These precursors are more volatile than the [Zn(S2CNEt2)2]2 precursor in CVD of polycrystalline ZnS films. This precursor gave high quality relatively , monodisperse nanoparticles. The precursor of the type M[E2CNRR ]2 were used in a novel one pot synthesis of quantum dot of II-VI semiconductors. O’Brien et al.23 prepared CdS and CdSe nanoparticles by thermolysis of diethyldithio- or diethyldiseleno-carbamato complexes of cadmium; [Cd(S2CN(C2H5)2)2]2,

[Cd(Se2CN(C2H5)2)2]2 and [RCdE2CN(C2H5)2]2, R = Me (methyl), Np (neopentyl) and E = S, Se, in TOPO. The air stable single molecule precursors, 24 bis(methylhexyldithiocarbamato)cadmium(II), [Cd(S2CNMeHex)2]; a 25 cadmium complex of dithiobiurea, [Cd(NH2CSNH)2Cl2]; cadmium

26 ethylxanthate, [Cd(C2H5OCS2)2]; and a thiosemicarbazide complex of 27 cadmium, [Cd(NH2CSNHNH2)2Cl2], were used to synthesized CdS nanoparticles by thermolysis in hot TOPO to give TOPO capped CdS nanoparticles. All nanoparticles showed spherical morphologies except CdS 28 from [Cd(NH2CSNHNH2)2Cl2] which showed rod-like CdS particles. Li et 29 al. prepared rod shaped CdS nanoparticles by thermolysing [Cd(C2H5OCS2)2] in hexadecylamine (HDA) at 260 °C. Revaprasadu et al.30 reported the synthesis of quantum dots of the II-VI semiconductors ME (M = Zn or Cd and E = S or Se) with spherical shapes. They used bis[methyl(n-hexyl)di-thio or - seleno]carbamato [M(E2CNMeHex)2, E = S or Se] complexes as a precursor.

Sreekumari et al.28 suggested that the nature of the precursor can influence the size and shape of the nanoparticles as well as the quality of the nanocrystals produced. They studied the effects of single molecule precursors on the synthesis of CdS nanoparticles.

Li et al.29 reported that the formation of anisotropic CdS nanocrystals related to high monomer concentration in the reaction environment to shift the reaction far from thermodynamic equilibrium and give rod-like nanocrystals, while if there is a low monomer concentration, the reaction is close to thermodynamic equilibrium, which favours isotropic growth resulting in the formation of spherical particles.

1.5 Thin films semiconductor materials Thin films, like nanoparticles, exhibit unique properties that cannot be observed in bulk materials. Synthesis of nanomaterials (thin films and nanoparticles) with precisely controlled size and shape has attracted great deal of research interest in last two decades. Thin films can be deposited on a substrate by chemical and physical methods, e.g: MOCVD, sputtering, laser ablation etc. Following steps are involved in the formation of thin films.

1. The formation of thin films from any deposition techniques starts with a random nucleation process followed by further nucleation and growth stages.

2. The various deposition conditions, such as growth rate, growth temperature, nature of the precursor and substrate are responsible for nucleation and growth stage.

3. The characterstics of the thin films such as film structure, defects and stress depends on the deposition conditions at nucleation stage.

4. The deposition condition is also responsible for the crystal phase and orientation of the films.

The unique material properties of the thin films are resulting from the atomic growth process, size effects, including quantum size effects. The thin films are characterized by the thickness, crystalline orientation and multilayer aspects. The different deposition techniques for the deposition of thin films are outlined below.

1.5.1 Chemical Vapor Deposition (CVD) Chemical Vapour Deposition (CVD) involves the deposition of solid thin film on a selected substrate from the precursors in vapour form. It is a process used to produce high purity and quality and performance thin film materials. In a typical CVD process, decomposition of volatile precursor material occurs and gives solid deposition of thin films onto the substrate.

1.5.2 Types of chemical vapour deposition A number of different forms of CVD are currently in use. These forms are different by means of chemical reaction started (activation process) and condition of that process. Metal organic chemical vapor deposition (MOCVD) including aerosol-assisted chemical vapor deposition (AACVD) and low- pressure chemical vapor deposition (LP-CVD), photo-assisted chemical vapor deposition (PACVD), plasma-enhanced chemical vapor deposition (PECVD), and laser-assisted chemical vapor deposition (LACVD) are various variants to the conventional CVD process.

1.5.2.1 Metal Organic Chemical Vapor Deposition (MOCVD) MOCVD is primarly used for deposition of crystalline thin films of semiconductors and related materials. MOCVD is based on metal organic precursors. It is a process in which pyrolysis of vapor-phase mixture of various chemical reagents (i.e., precursors) onto substrate to form thin solid films of materials. For last 30 years the deposition of a wide range of semiconductor materials has been obtained by MOCVD. It is now extensively used for the production of solar cells, injection lasers, light-emitting diodes (LEDs), photodetectors, hetero-junction bipolar transistors, quantum-well lasers, and various other electronic and optoelectronic devices.

1.5.2.2 Aerosol-Assisted Chemical Vapor Deposition (AACVD) AACVD is the process in which a liquid/gas aerosol is generated ultrasonically and is responsible for the transportation of precursors to the substrate. This technique is suitable for use with in volatile precursors. It is not necessary that precursors should be volatile for AACVD, but at least they should be merely soluble in any solvent so that their aerosol can be obtained by using any suitable technique like ultrasonic modulation. AACVD is useful technique to deposit coatings, films, and other related structures from thermally unstable or the involatile precursors. High-class CVD products can be produced at a lower cost by employing this method. Some key advantages of the AACVD, as compared to other CVD techniques, are as follows:

 Broader choice of chemical precursors and their availability for obtaining high quality thin films at lower cost;

 The simple mass transportation of precursors to the reactor by means of aerosols;

 Deposition rate can be controlled through adjusting the mass transport of precursors;

 The reaction environment is more flexible ranging from low pressure to atmospheric pressure;

In a typical CVD procedure, the precursor is fully dissolved in a solvent, e.g. toluene, tetrahydrofuran (THF) or acetonitrile. The higher boiling point of 40

toluene makes it the preferred choice over tetrahydrofuran (THF) and acetonitrile. A controlled stream of carrier gas (nitrogen or argon) flows through the precursor/solvent flask, which is placed in an ultrasonic modulator to generate micron-sized aerosol droplets. These aerosols are transported by the carrier gas into the hot reactor, where the precursor is decomposed and produces a thin film of material.

AACVD technique also has disadvantages which are as follows:

 Limited solubility of precursor in a solvent;

 A large volume of the solvent is used for solubility which increase the amounts of flammable vapor in the reactor chamber that could potentially ignite.

All AACVD experiments for the deposition of thin films were performed using an improvised AA-CVD Kit. Figure 1.5 illustrates schematic diagram of the AACVD kit used in this work.

1.5.3 The process of chemical vapour deposition The process of the deposition of thin films by CVD involves the thermally induced reaction of the precursors on a heated surface. There are several steps involved in this process, which is given below:

1. Mass transport of reagents to the deposition zone.

2. Gas phase reactions in the boundary layer to produce film precursors and by- products.

3. Mass transport of film precursor to surface.

4. Adsorption of film precursor on surface.

5. Surface diffusion of precursor to growth site.

6. Surface chemical reactions lead to film deposition and by-product de-sorption

7. Mass transport of by-product out reaction zone.

A schematic diagram (Figure 1.6) illustrating all the steps involved in the process of CVD.

AACVD KIT

Heater Carrier gas

in To Trap Traptrap

Substrate

Humidifier

Figure 1.5. Schematic diagram of an AACVD kit.

Figure 1.6. Schematic representation of CVD processes.

1.5.4 Precursors for chemical vapour deposition The choice of the molecular precursors is very important for CVD processes. The nature of the precursor affects the nature and morphology of the product. Suitable precursors for CVD must have some of the following characteristics:

 The suitable precursor should be readily volatile at lower temperature than its decomposition temperature; the thermal stability of a precursor is an important factor in determining its suitability.

 The purity of the precursor plays an important role in the preparation of high quality pure thin films without contamination; there should be clean decomposition without residual impurities .

 It should be compatible with other precursors or solvent during the growth of complex materials.

 Precursors should be non-toxic and environment friendly.

 It should be easily available and cost effective for its preparation.

1.5.5 Issues in using conventional precursors for CVD Some issues are involved in the uses of conventional precursors for the CVD process including:

 To control and maintain the stoichiometry of the thin films, a large imbalance between the mole ratios of the precursors in the feed gas is needed with conventional precursors;

 The high toxicity of hydride gases such as H2S, H2Se, NH3, PH3, SiH4 and AsH3 and highly pyrophoric nature of metal alkyls;

 The reactivity of metal alkyls in moisture and air may leads to the formation of alkoxides and contaminate the thin films;

 Prereactions in MOCVD reactor within the reactant molecules affect the morphology and stoichiometries of the final product.

1.5.6 Advantages of single source precursors The use of single source precursor for CVD has some advantages which are explained as follows

 The uses of the single source precursors minimize the exclusion of toxic gases and also the pre-reactions may be limited.

 It is easy to maintain the stoichiometries of metal and ligand compositions by using single source precursor.

 Control of the coordination numbers at the metal by the formation of adducts or aggregates decrease the sensitivity towards the air and moisture.

It is also easy to control the decomposition temperature by designing the precursor to get the thin film at low temperature.

1.6 Iron chalcogenides Iron chalcogenides are important, because of their potential use as an absorber in solar cells. Among these iron chalcogenides compounds, pyrite (FeS2) and iron selenide (FeSe) are attractive materials with suitable band gaps for potential applications in solar cells. It has been used as a mineral detector in radio receivers, and is still used by 'crystal radio' hobbyists. Pyrite has been proposed as an abundant, inexpensive material in low cost photovoltaic solar panels. It is also used to make marcasite jewellery.31-35

In the last few years a considerable interest has developing in the synthesis and characterization of iron containing nanocrystals;36-38 they have potential applications in; information storage devices, catalysis, biomedicine, imaging, and sensors.39 In the literature the synthesis of iron oxide nanomaterials is well documented, but the preparation of iron chalcogenides [FexEy (E = S, Se or Te)] has remained relatively unexplored.40-43 This omission may be due to their complex crystal structures and physical properties.44

Iron selenide (FeSe) is an attractive material having a direct band gap of 1.23 eV.34,35 Only few iron selenide compounds are known so far. For example FeSe crystallizes in the tetragonal and hexagonal space groups, while FeSe2 crystallizes in the cubic structure and in the orthorhombic marcasite-type structure. The properties of bulk FeSe crystals have been reported with Fe3Se4 45,46 and Fe7Se8 phases. The phase diagram of iron selenides shows similarities with the NiAs-like crystal structure. Over a certain composition range at room temperature, this structure may extend at least from 51 to 59 % selenium. The phase exists within this range as a hexagonal NiAs-like structure with composition Fe Se (H-phase), and as a monoclinic, with composition FeSe (M- phase).31,32

The iron telluride compound has the simplest crystal structure and exhibits antiferromagnetic ordering around 70 K and does not show superconductivity. Iron telluride is non-superconducting under normal conditions. There are some indications that it might become a super conductor under the external 47,48 pressure. Amorphous FeTe2 exhibits strong antiferromagnetic properties 48,49 while crystalline FeTe2 shows 3D antiferromagnetic ordering at ~100 K.

1.7 Iron sulfide The iron sulfide system has a complex phase diagram with several phases. The phases of iron sulfide are: pyrite (cubic-FeS2), marcasite (calcium chloride structure-FeS2), pyrrhotite-1T (Fe1-xS), pyrrhotite-4M (Fe7S8) or Fe9S10, greigite 50-53 (cubic spinel-Fe3S4), troilite-2H (FeS), and mackinawite (Fe1+xS). Pyrite is a semiconductor material and has potential application as an absorber material in thin film solar cells due to its suitable band gap (Eg= 0.8–0.95 eV), high absorption coefficient (~105 M-1 cm−1) and its low toxicity.54

The thermodynamic stability of different phases of iron sulfide is shown in the Figure 1.7. Vaughan et al.55 measured the values of free energy and suggested that only pyrite and troilite are truly stable phases. Dashed lines are used to show the relationship involving the intermediate pyrrohotite which represent an increased stability over the vacancy disordered solid solution but not the most stable form.

mackinawite

-60 - amorphous FeS disordered solid solution -80 -

-100 - greigite

(Fe)

1 1 -

-120 - monoclinic pyrrhotite

troilite K J mol J K

f marcasite -140 - G Fe S (6C)

∆ 11 12

(11C) Fe9S10 (5C) -160 - Fe10S11 pyrite

FeS FeS2 Composition Figure 1.7 Schematic (isothermal) free energy-composition diagram for the iron sulfide minerals.51,55

Stoichiometric iron sulfide (FeS) adopts the trollite structure, with antiferromagnetic properties at room temperature and undergoes transition into an NiAs-type above 120 °C, which includes the pyrrhotites ((Fe1−xS) and 56,57 (Fe7S8)). Due to an iron vacancy, many pyrrhotites (Fe1−xS) give a range of compositions with superstructures and interesting magnetic and/or electrical 58-60 properties. The non-stoichiometric Fe1−xS shows different morphologies including whiskers,61 nanowires,62 and U-shaped microslots.63 The magnetic and electrical properties of iron sulfides are dependent on the stoichiometric ratio between iron and sulfur.60,64,65 Iron sulfides with different Fe:S ratios, ranging from 0.5 to 1.05, are present in nature. Among the natural iron sulfide minerals, mackinawite is paramagnetic; pyrite and marcasite are diamagnetic; pyrrhotite and greigite are ferromagnetic; and troilite is antiferromagnetic.55,66

The optical properties of FeS2 crystallites were investigated with ambient temperature absorption spectroscopy. The FeS2 crystallites possess a well- defined, broad optical absorption located at 1420 nm (0.87 eV). Electrochemical performance of the as-prepared cubic FeS2 crystallites in the cell configuration of Li/FeS2 was also evaluated. The discharge capacity of cubic FeS2 crystallites 46

-1 can reach about 756 mA h g , which is close to that of natural FeS2 cell (750 -1 67 mA h/g) or synthetic FeS2 cell (775 mA h g ).

1.8 Synthesis of iron sulfide nanocrystals Several methods have been reported for the synthesis of iron sulfide nanocrystals including thermolysis of metal complexes (single source precursor) in amines.

1.9 Single source precursor route Several single source precursors have been used to grow iron sulfide nanoparticles.

1.9.1 Dialkyl and mixed alkyl dithiocarbamatoiron(III) complexes Chen, et al.67used tris(diethyldithiocarbamato)iron(III) complex as a single source precursor to synthesised cubic FeS2 crystallites. In a typical reaction tris(diethyldithiocarbamato)iron(III) complex was added to the distilled water in a Teflon-lined stainless steel autoclave. Autoclave was sealed and maintained at 180 °C for 12 hours. The precipitate was filtered off, washed with distilled water and ethanol and dried under vacuum for 6 hour at 50 °C. Cubic FeS2 crystallites with an average diameter of 500 nm were obtained.

Two-dimensional (2D) magnetic pyrrhotite (Fe7S8) and greigite (Fe3S4) nanosheets were synthesised by thermolysing single-source precursors such as

[Fe(dtc)2(phen)] (phen = 1,10-phenanthroline; dtc = diethyldithiocarbamate) and 68 [Fe(dtc)3] in oleylamine. Under optimized reaction temperature of 280 °C, both monoclinic Fe7S8 and cubic Fe3S4 obtained presented hexagonal sheet structure with sizes of 500–800 nm and 100–500 nm, respectively. Both complexes [(Fe(dtc)3] and [Fe(dtc)2(phen)] were also thermolysed in different combinations of capping agents such as oleic acid (OA), oleylamine (OM) and

1-octadecene (ODE). The complex [(Fe(dtc)3] produced cubic greigite (Fe3S4) nanoparticles with size ranging from 30 to 100 nm when thermolysed in a mixture of OM and ODE (20:20) or OA, OM and ODE (10:10:20)) at 280 °C.42

[Fe(dtc)2(phen)] thermolysed in a mixture of OM and ODE (20:20 mmol)

solvent system produced hexagonal pyrrhotite (Fe7S8) nanoplates with width ranging from 500 nm to 1 µm and the thickness from 20 to 55 nm. HRTEM shows single craystaline nature of the nanoplates. The same complex produced

FeSx nanoribons of several micron long and 15 nm wide, when thermolysed in a mixture of OA, OM and ODE (10:10:20 mmol).42

1.9.2 Other single source Precursors 1.9.2.1 Bis(tetra-n-butylammonium)tetrakis[benzenethiolato-µ3-sulfido- iron] O’Brien et al.40 used the cubane Fe-S cluster bis(tetra-n-butylammonium)- n tetrakis[benzenethiolato-µ3-sulfido-iron] [N Bu4]2[Fe4S4(SPh)4] in octylamine and dodecylamine at 180 and 220 °C respectively. Decomposition in octylamine produced pyrrohotite (Fe7S8) spherical nanoparticles with an average diameter of 5.6 nm whereas the decomposition in dodecylamine produced greigite (Fe3S4) nanoparticles with size ranging from 2.5 to 4.5 nm.

1.9.2.2 Tris(diethydithiophosphato)iron(III) complex 69 Alivisatos et al. used tris(diethydithiophosphato)iron(III) [Fe(S2P(Et2O)2)3] complex, as single source precursor in the surfactant hexadecyltrimethyl- ammoniumbromide (CTAB) in a Teflon lined stainless steel acid digestion bomb at 200 °C. The cubic pyrite (FeS2) single phase was obtained. The reaction temperature, pH value, precursor, and surfactant have been found to play important roles in the control of material purity. Transmission electron microscopy (TEM) analysis showed large quasi-cubic agglomerations of nanocrystals with an average size of over 100 nm.

1.9.2.3 Poly sulfide iron(N-methylimidazole(N-MeIm))[Fe(N-MeIm)6]S8 70 Beal et al. reported the synthesis of greigite (Fe3S4) and pyrrhotite (Fe1-xS) in oleylamine by using poly sulfide iron(N-methylimidazole (N-MeIm)) [Fe(N-

MeIm)6]S8 complex as a single source precursor. At 300 °C thermal decomposition of [Fe(N-MeIm)6]S8 in oleylamine by using hot injection method followed by rapid cooling at room temperature produced the cubic spinal greigite (Fe3S4). These crystallites were relatively small size with different

morphology. Energy dispersive x-ray spectroscopy (EDS) measurement shows the composition of Fe:S is 47:53 which was greater than the expected ratio of

43:57. The NiAs-type hexagonal pyrrhotite (Fe1-xS) crystallites of different morphology was produced from the reaction of [Fe(N-MeIm)6]S8 in olylamine at 300 °C for 4 hours. The EDS analysis shows Fe:S ratio 1:1. It was also observed that during the reaction time less than 4 hours a mixture of greigite

(Fe3S4) and pyrrhotite (Fe1-xS) obtained which indicates that greigite is an intermediate stage which later on can be converted in to thermally more stable pyrrhotite (Fe1-xS) phase.

1.9.2.4 Thiobiuret iron(III) complexes O’Brien et al.71 used iron(III) complex of 1,1,5,5-tetra-iso-propyl-2-thiobiuret as single source precursor for the synthesis of iron sulfide nanoparticles, by thermolysis in hot oleylamine, octadecene, and dodecanethiol. Several combinations of different injection solvents and capping agents were used in the reaction mixture. The shape and the phase of the material were controlled by changing the solvent, capping agent, and growth temperature or precursor concentration. The thermolysis of iron complex in oleylamine/oleylamine produced Fe7S8 nanoparticles with different morphologies (spherical, hexagonal plates and nanowires) depending on the growth temperature and precursor concentration.

1.9.3 Thiosemicarbazone complexes of iron(II)

Thiosemicarbazone complexes of Iron(II), Fe(cinnamtscz)2 and

Fe(cinnamtsczH)2Cl2 (where cinnamtsczH=cinnamaldehyde thiosemicarbazone) were pyrolysed in furnace for 3 hours at 540 and 460 °C respectively.31

Decomposition of Fe(cinnamtscz)2 results in the formation of triclinic FeS2 with average size 52 nm and the decomposition of Fe(cinnamtsczH)2Cl2 gave hexagonal FeS nanoparticles with average size of 21 nm. The nanoparticles obtained from the both precursors have spherical plate like morphologies.

EDAX analysis shows that the Fe:S ratio for FeS2 is Fe1.03S1.97 and for FeS was almost 1:1. Fe(cinnamtsczH)2Cl2 was refluxed in ethylene glycol under nitrogen for 3 hours to give a viscous black solution. Addition of methanol produced a 49

black residue which was annealed at 350 °C under nitrogen for 1 hour. Analysis of the material showed to be hexagonal FeS nanoparticles with average diameter of 18 nm.72 Tabll 1.4 show the different phases and morphology of iron sulfide nanostruture from single source precursors. The morphology of the nanocrystals were also shown in Figure 1.8.

Table 1.4 The different single source precursor producing different phases of iron sulfide with different morphology.

Precursors Capping Phase Morphology Ref. agent

Diethyldithiocarbamato- Distilled cubic FeS2, Cubic, 67 iron(III) water, Oleylamine greigite Hexagonal (Fe3S4)

bis(tetra-n- Octylamine Pyrrohotite Spherical 40 butylammonium)tetrakis[ (Fe7S8) benzenethiolato-mu- Dodecylamine greigite sulfido-iron] (Fe3S4)

Fe(Ddtc)2(Phen) Oleylamine (Fe7S8) Hexagonal

Iron(III) diethyl CTAB FeS2 Quasi-cubic 67 dithiophosphate

[Fe(N-MeIm)6]S8 Oleylamine greigite 70 (Fe3S4) and pyrrhotite (Fe1-xS)

1,1,5,5-tetra-iso-propyl-2- Oleylamine Fe7S8 Spherical, 71 thiobiuret Hexagonal, Nanowires

Iron(II) Fe(cinnamtscz)2 pyrolysis triclinic FeS2, Spherical 72 and Fe(cinnamtsczH)2Cl2 hexagonal plates FeS

Figure 1.8. Shows different phases and morphology of iron sulfide nanostrutures from single source precursors.40,67,69,71

Spherical and rod shape nanocrystals of FeS were synthesised by the decomposition of Fe(ACDA)3 [HACDA=2-aminocyclopenten-1-dithiocar- boxylic acid] complex in ethylenediamine (EN), ethylene glycol (EG) or ammonia (NH3). Optical measurement showed band gaps of 3.13 eV, 3.02 eV and 2.75 eV from FeS nanoparticles synthesized using EG, EN and NH3 respectively. Photocatalytic activity of FeS nanoparticles was caried out by methylene blue degradation experiment and showed better performance than 73 commercial TiO2.

1.10 Iron sulfide thin films The iron sulfide thin films have been prepared by atmospheric- or low-pressure 74-77 metal-organic chemical vapour deposition (AP or LP MOCVD; FeS2).

Schleigh and Chang deposited FeS2 thin films by using iron pentacarbonyl 74 [Fe(CO)5], hydrogen sulfide, and tert-butyl sulfide as precursors by LPCVD. 78,79 Other techniques include sulfurization of iron oxides to FeS2, ion beam and 80 81 reactive sputtering (FeS2), plasma assisted sulfurization of iron (FeS2), flash 82 83 evaporation (FeS2), vacuum thermal evaporation (FeS2), vapour transport 84 85 (FeS2), and chemical spray pyrolysis (FeS2).

1.11 Single source precursor There have been a very limited number of iron complexes employed as single- source precursors for the deposition of iron sulfide as Fe1+xS, FeS2, and Fe1-xS thin films, which include dithiocarbamato complexes [Fe(S2CNRR’)3] (R,R’ = Et,Et, Me,iPr)86 and the sulfur-bridged binuclear iron carbonyl complex 87 [Fe2(CO)6(μ-S2)].

1.11.1 Dialkyl and mixed alkyl dithiocarbamatoiron(III) complexes

Thin films of FeS2 was deposited by LP-MOCVD and AACVD between 350 i and 500 °C from [Fe(S2CNnBu2)3] and [Fe(S2CNMe Pr)3] as a single source i precursor. The complex Fe(S2CNMe Pr)3 produced a good quality thin films of

FeS2 by AACVD method. However the FeS2 thin films deposited from 86 [Fe(S2CNnBu2)3] gave amorphous XRD patterns.

Highly textured magnetic Fe1-xS nanorods were deposited on silicon by chemical vapour deposition using the single source precursor N,N- diethyldithiocarbamatoiron(III). The magnetic behaviour of the as deposited

Fe1-xS nanorods shows ferrimagnetism. The magnetic profiling of the iron-rich

Fe1-xS rods grown on silicon revealed a curie temperature of 360 °C; the in- plane:out-of-plane magnetic saturation ratio is 4:1 with a saturation magnetization of 19.7emu g-1.50

1.11.2 Thiobiuret iron(III) complexes Ramasamy et al.43 used iron thiobiuret complexes with 1, 1, 5, 5,-tetraalkyl-2- thiobiurete (alkyl = methyl, ethyl, and isopropyl) as a single source precursor for the deposition of iron sulfide (FeS and FeS2) thin films by Aerosol-Assisted

Chemical Vapour Deposition (AACVD). Different iron sulfide phases including

FeS hexagonal troilite and cubic pyrite (FeS2) and tetragonal pyrrhotite (Fe1-xS) were deposited depending on the nature of precursor and the deposition temperature.

1.12 Synthesis of iron sulfide nanoparticles and thin films from dual Source

One-dimensional nanostructures of iron di-sulfide (cubic pyrite FeS2) were synthesized by solvothermal process at low temperature.88 For the synthesis of

1-D FeS2 nanostructures a closed Teflon-lined stainless steel chamber was used.

In this chamber an appropriate amount of FeSO4.7H2O or FeCl3 or

Fe(NO3)3.9H2O and thiourea (tu,NH2CSNH2) with molar ratio 1:4 were taken in this chamber and then add ethylenedimine)en,NH2CH2CH2NH2) up to 80% of its volume and stir it for 30 minutes. The closed chamber was kept in a box furnace at the desired temperature for 12 hours. After cooling down to room temperature, the resulting black precipitate was filtered off and washed with ethanol. Black powder was obtained from black precipitate when dried under vacuum at 120 °C for 6 hours.

The morphology of the FeS2 nanostructures was controlled by the anions and the iron source, temperature and the molar concentrations of the precursors. The short nanorods of FeS2 with lengths of 500 nm and diameters between 40-100 nm were produced when FeSO4.7H2O used as the iron source. For the same experiment when FeCl3 was used as iron source, large FeS2 nanowires with some micro-rods were obtained. Uniform nanowires with diameter ranging 40-

60 nm and length up to 10 µm were produced when Fe(NO3)3.9H2O was used as the iron source. To study the effect of temperature, Fe(NO3)3.9H2O was used as iron source by keeping the other parameter same. Mixture of nanorods, nanoparticles and nanowires were obtained at the temperature 150 °C. At the temperature 180 °C uniform nanowires with circular cross-section and square cross-section was produced whereas at 210 °C nanoribbons were obtained. The molar concentration also plays important role in the morphology of the nanoparticles. For instance when half concentration was used, the nanowires 53

with only circular cross-section were obtained and when twice molar concentration was used, nanowires with rectangular cross-sections and almost ribbon like nanostructures were obtained.88

Deposition of FeS nanoparticles onto mesoporous silica gel microparticles was carried out by using two approaches: (A) direct coating of {FeS-

G4.NH2}DSNPs (Generation 4 terminated with amino, Dendrimer-stabilized nanoparticles) onto silica particles and (B) using G4.NH2 (Generation 4 terminated with amino) coated silica particles to incorporate Fe2+ ions for the subsequent formation of FeS nanoparticles.89

Iron sulfide 3D-Fe3S4 flower-like microsphere was synthesized via a simple biomolecule-assisted hydrothermal process.90 A calculated amount of L-cysteine

(C3H7NS) dissolved in deionised water and mixed with the aqueous solution of

FeSO4 under constant and vigorous stirring. Then the resulting solutions were transferred in to a Teflon-lined autoclave and heat it for 24 hours at 200 °C. Then it was cooled to room temperature and washed with deionized water and ethanol and dried in a vacuum oven at 60 °C for 2 hours.90 The magnetic properties show that the product is ferromagnetic. It was also observed that the

Fe3S4 flower-like microspheres could store hydrogen electrochemically and discharge capacity of 214 mAh g-1was measured without any activation under normal atmospheric conditions at room temperature.90

Cubic greigite (Fe3S4) nanoparticles were synthesized in a mixed solvent system 80 using ethylene glycol and water (EG+H2O) in a 3:1 ratio. Different composition of mixed solvent EG and H2O produced greigite nanosheets and nanoparticles. Both of them shows ferromagnetic behaviours; however the saturation magnetization of Fe3S4 nanoparticles was higher than that of Fe3S4 nanosheets because of difference in sizes and shapes.91

92 Korgel et al. synthesized iron sulfide (FeS2) Pyrite-phase nanocrystals form solvent-based dispersions, or “solar paint,” to fabricate photovoltaic devices (PVs). These nanocrystals were sprayed onto substrates as absorber layers in devices with several different architectures, including Schottky barriers,

heterojunctions, and organic/inorganic hybrid architectures, to explore their viability as a photovoltaic material. None of the devices exhibited PV response. The electrical conductivity of the nanocrystal films was about 4 to 5 S/cm. The lack of PV response appears to derive from the highly conductive surface- related defects in pyrite.

93 Jin et al. reported a single crystalline cubic pyrite (FeS2) nanowires which was synthesized via thermal sulfidation of steel foil for the first time. A solution of sulfur in oleylamine was injected into a vessel containing FeCl2 and oleylamine at 220 °C. Nanocrystals were isolated after cooling to room temperature by adding toluene and ethanol. Isolated nanocubes were ~150 nm, whereas dendrites were 40 nm. The sizes of both nanocubes and nanodendrites can be increased by increasing the reaction time. The pyrite nanowires have the length greater than 2 μm with 4−10 nm diameter. Electrical transport measurements showed the pyrite nanowires to be highly p-doped, with an average resistivity of 0.18 ± 0.09 Ω cm and carrier concentrations on the order of 1021 cm−3.

94 Meester et al. prepared iron disulfide (FeS2) using iron(III) acetylacetonate

[Fe(acac)3], tert-butyl disulfide, and hydrogen. Thin films of pyrite have been prepared by LP-MOCVD from iron pentacarbonyl (Fe(CO)5) and di-tert-butyl disulfide (TBDS) on Si, Gap, and ZnS substrates. Pure pyrite thin film was deposited at temperature 450-500 °C. The films were polycrystalline with grain size ranging 1-10 µm depending on the substrate orientation, growth rate and substrate temperature.

95 Puthussery et al. synthesized of FeS2 nanocrystals inks. The synthesis involves the injection of sulfur in diphenyl ether solution into a vessel containing FeCl2 and octadecylamine at 220 °C. These nanocrystals have oblate and spheroidal morphology with 5- 20 nm size. Small size (>100 nm) colloidal iron sulfide nanocrystal inks were also prepared directly from natural pyrite dust without any chemical conversion, which are highly stable and monodispersed.95

Collodial iron pyrite (FeS2) nanocrystal inks for thin films were prepared by injecting a solution of sulfur dissolved in diphenyl ether in to a solution of FeCl2

in octadecylamine at 220 °C.95 Most of the nanocrystals have a doughnut like appearance with depression or holes in their centres. Thin films of pyrite nanocrystals were also deposited on various substrate (glass, quartz and silicon) using layer-by-lyer dip-coating from chloroform solution.

FeS2 thin films were synthesized by sulfuration, under vacuum, of amorphous iron oxide thin films pre-deposited by spray pyrolysis of FeCl3.6H2O (0.03M) based aqueous solution onto glass substrate heated at 350 °C. At optimum sulfuration temperature (450 °C) for 6 hours, black green layers having granular structure and high molar absorption coefficient (~5.104 cm-1) were produced.79

Nanosheet films of FeS and FeS2 were synthesised on iron substrates through one-step hydrothermal treatment of iron foil and sulfur powder in the presence and absence of hydrazine. A piece of iron foil and sulfur powder were placed in a Teflon-lined autoclave and then added 15 ml of deionized water or aqueous hydrazine. The autoclave was heated for 12 hours at 160 °C and then cooled to room temperature. The iron foil was taken out washed with ethanol and dried. The iron sufide nanosheets synthesised in the presence of hydrazine were identified as pyrrhotite (FeS) and iron sulfide nanosheets obtained in the absence of hydrazine was identified as pyrite (FeS2). These nanosheets films were used as novel photocathodes in tandem solar cells with dye-sensitized TiO2 nanorods film as the corresponding photoanodes. The results showed that the iron sulfide nanosheets films are attractive photocathodes for tandem dye-sensitized solar cells.96

Iron sulfide nanocrystal (Fe3S4) thin films were deposited on Silicon substrate at room temperature in adc-magnetron sputter chamber. The films were annealed at different temperatures (350, 450 and 550 °C) in a tube furnace for 2 hours.

The thermal annealing convert the Fe3S4 into Fe7S8 and Fe3O4 at low 94 temperatures and single phase of Fe3O4 at higher temperature.

Bi et al.98 reported pure pyrite phase nanocrystals. The iron precursor was prepared by mixing FeCl2, oleylamine and trioctylphosphine oxide (TOPO) at 170 °C. A solution of sulfur in oleylamine was injected into iron precursor

solution at 220 °C. Monodisperse nanocubes with sizes ranging from 60 to 200 nm, controlled by varying the amount of TOPO were produced in 2 hours reaction time. Thin films of 400 nm thickness with ± 25 nm roughnesses were prepared by dip-coating method. Films showed optical band gaps of 0.93 eV (indirect) and 1.38 eV (direct) with high molar absorption efficiency (~ 2 × 105 cm-1). Nanoparticles prepared using TOPO as a capping agent showed improved stability which was linked to the attractive charge interactions between FeS2 and

TOPO to provide better surface passivation of FeS2 nanocrystals.

99 Li et al. synthesized FeS2 nanocrystals of cubes and dendrite shape.

Nanoplates of FeS2 were grown by injecting an organometallic precursor

(Fe(CO)5) into a solution containing oleylamine and sulfur. Nanoplates with lateral size of 150 nm and thickness around 30 nm were isolated from this method.100 The irregular shaped plates were mainly comprised of hexagonal shaped crystallites.

101 Beal et al. synthesized Fe3S4 spherical nanoparticles with size of 6.5 ± 0.5 nm by injecting sulfur in oleylamine into a vessel containing Fe(acac)2 and hexadecylamine at 300 °C. Magnetic properties of these Fe3S4 nanoparticles were compared to magnetic properties of similar size Fe3O4 nanocrystals. Fe3S4 nanoparticles showed saturation magnetization of 12 emu/g at 10 K and blocking temperature around ~ 50 K.

1.13 Iron selenide FeSe has many different phases: tetragonal phase α–FeSe with PbO-structure, a NiAs-type β-phase with a wide range of homogeneity showing a transformation from hexagonal to monoclinic symmetry and FeSe2 phase that has the orthorhombic marcasite structure.102 The most studied of these compounds are the hexagonal Fe7Se8, which is a ferrimagnetic with a Curie temperature at about

125 K, and monoclinic Fe3Se4.

1.13.1 Iron selenide nanoparticles In recent years iron selenide (FeSe) has been given much attention, because of its conductive, optical, electronic, and magnetic properties. The conductive 57

properties of iron selenide depend on the composition of these materials. Iron selenide can be semiconductors, or even superconductors with characterization of ferro/ferrimagnetic metals.103-105

1.13.2 Methods for the preparation of iron selenide nanocrystals Iron selenide can be synthesized by the elemental reaction in evacuated tubes at elevated temperature.106,107 It also can be synthesized by reaction of aqueous 108,109 metal salt solutions with toxic gas H2Se. There is another method i.e mechanical alloying for preparation of iron selenides.110,111 However all these methods are complicated and usually give impure phases. Secondly, in all these methods, a large amount of energy is required, and is also a time consuming method. The other problem related with these techniques is the difference in the melting points of constituents iron (1535 °C) and selenium (217 °C). It becomes difficult to get iron selenide compounds with desired stoichiometry by these techniques.

FeSe2 nanorods and FeTe2 nanocrystallites have been synthesized by hydrothermal co-reduction method using N2H4.H2O as the reductant. An aqueous solutin of FeCl3.6H2O, Na2SeO3, hydrazinehydrate (N2H4.H2O) in distilled water under constant stirring. The mixture was heated at 140 °C for 12 hours in a stainless Teflon-lined autoclave. The black product was filtered off, cooled and washed with distilled water and ethanol. This product was dried under vacuum at 50 °C for 6 hours. The p-XRD analysis showed orthorhombic 112 FeSe2.

Iron selenide nanocrystals with hexagonal (H) and monoclinic (M) NiAs-like structure were synthesized using a one-pot thermal decomposition of ferrous chloride and selenium powder in oleylamine.113 Iron selenide (FeSe) was electrodeposited on to indium doped tin oxide coated conducting glass (ITO) substrate at various bath temperature from 30 to 90 °C in an aqueous electrolytic bath containing FeSO4 and SeO2.

Stoichiometric films of iron selenide with well-defined composition were obtained by adjusting the deposition parameter. Morphological study showed

homogeneity with uniform grains for film deposited at higher bath 114 temperature. Iron selenide FeSex (x=0.80, 0.84,0.88, 0.92) thin films were prepared by a pulsed laser deposition method on different substrates such as

(SrTiO3(001), (STO), (La,Sr)(Al,Ta)O3(001)(LSAT), and LaAlO3(001)(LAO) substrate. All of the thin films show a single phase and c-axis oriented epitaxial growth and are super conductor.115

Selenium deficient regular square FeSex nanoflakes with tetragonal PbO-type phase and have been grown from ferrous chloride and selenium trioctyl phosphine. For this synthesis a simple solution-based route in the co-solvent of oleylamine and oleic acid was adopted. The addition of the organic acid, i.e. oleic acid, at the correct time during heating of the starting materials, is of great importance to the yield of the tetragonal PbO-type phase. Interestingly, the chemical composition and the edge length of the single crystalline FeSex nanoflakes can be tailored by using an organic diol which can act as a reducing agent and a ligand as well.116

Fe7Se8 polyhedra nanorods with high index facets and Fe7Se8 nanorods can be selectively synthesized by a solvothermal reaction in a mixed solvent of diethylenetriamine (DETA) and deionized water (DIW).117 In a typical experiment FeSO4.7H2O and Na2SeO3 were added to a mixed solution with a volume ratio of organic amine and water 1:2 under stirring. This solution was sealed in Teflon-lined autoclave and maintained at temperature 140 °C for 12 hours. After cooled to room temperature naturally, the brick-red material was collected and washed with distilled water and ethanol and dried under vacuume at 60 °C for 6 hours. This gave the Fe7Se8 polyhedra whereas the Fe7Se8 nanorods were obtained by changing the solvent ratio ftom 1:2 to 1:4. The polyhedra crystals are bounded by two {001} and twelve {012} facets. Magnetic measurements indicate that the Fe7Se8 polyhedra and nanorods show a weak ferromagnetic ordering at room temperature. A new photoluminescence 117 emission at 403 nm from the Fe7Se8 nanocrystals has been observed.

Iron Selenide Fe3Se4 nanostructure have been synthesized by a one-pot high- temperature organic-solution-phase methiod.82 The size of these nanostructures 59

can be tuned from 50 to 500 nm, and their shapes can be varied from nanosheets and nanocacti, to nanoplates. These nanostructures exhibit hard magnetic properties, with giant coercivity values reaching 40 KOe at 10 K and 40 KOe at room temperature.125 The magnetic properties can be easily tunable by doping and substituting Fe ions by other transition-metal elements such as Co.118

1.13.3 Single source precursor route Only few complexes have been used as a single source precursor for the synthesis of iron selenide nanocrystals and thin films so far.

1.13.4 Imidodiselenodiphosphinatoiron(II) complexes

Iron complexes of imidodiselenodiphosphinato [Fe{(SePPh2)2N}2], and imidothioselenodiphosphinato [Fe{(SePPh2NPPh2S)2N}2] have been synthesized and used as single source precursors.119 Pyrolysis of complexes

[Fe{(SePPh2)2N}2] and [Fe{(SePPh2NPPh2S)2N}2] deposited orthorhombic iron selenide (FeSe2). SEM images of the deposited FeSe2 indicated polygonal crystallites with increased sizes. XPS measurements showed the surface oxidation of the materials in which iron and selenium were bounded to oxygen.

Magnetic measurements revealed the ferromagnetic behaviour of the FeSe2 powders.83,119 SEM analysis of the deposited material at 500 °C revealed the morphology to be randomly dispersed polygonal crystallites. At the higher deposition temperature (550 °C), similar morphology was also observed with the only difference being that the particles were of bigger sizes.

1.13.5 Magnetic properties of iron selenide

All of the Fe3Se4 nanocacti, nanosheets, and nanoplatelets exhibit hard magnetic properties. The coercivity of nanocacti reaches 3.4 kOe at room temperature, which rises more than 10-fold to about 36 kOe at 10 K. There seems to be a paramagnetic component, since the hysteresis loops do not reach saturation even at a field of 90 kOe. This suggests that either the anisotropy field is much larger than 90 kOe or the spins in the system are noncollinear. The nanosheets have a lower coercivity of 29 kOe at 10 K, while the nanoplatelets show higher coercivity values of 40 kOe at 10 K and 4.0 kOe at room temperature. The

different coercivity values may originate from the difference in grain sizes and stoichiometry.119

1.14 Iron telluride nanoparticles As compared to the iron sulfide and iron selenide the iron telluride remained unexplored. FeTe2 with pyrite or marcasite type structure shows 3D magnetic 120 ordering and semiconductivity. The iron-based compound FeTe0.8S0.2 exhibit superconductivity if soaked in red wine due to presence of tartaric acid in red wine. Tetragonal iron telluride Fe1+xTe is the parent member of a new family of superconductors FeTe 1−y Sey. In contrast to the superconducting FeSe the parent member is non-superconducting and possesses a large magnetic moment on iron ion that for higher Fe excess displays incommensurate spin density wave type of magnetism.121-123

1.14.1 Methods for the Preparation of iron telluride nanocrystals Traditionally,iron telluride is prepared directly by mixing the elements in sealed tubes124,125 at high temperatures and high pressures. New synthetic methods such as organometallic vapor phase epitaxy (OMVPE) and metal organic chemical vapor deposition (MOCVD) routes have been used for the synthesis of 126-128 126 FeTe2. Bochmann et al. reported the synthesis of tellurium complex t [M{Bu 2P(E)NR}2] and also used as precursor for the gas-phase deposition of 129 FeTe2 films.

130 Zhang et al. reported a aqueous route to prepare nanocrystalline FeTe2 with an orthorhombic phase through a reaction between an alkaline aqueous solution obtained by dissolving elemental tellurium and iron(II) complex 130 Na2[Fe(EDTA)] aqueous solution at 140 °C. A tellurium alkaline aqueous solution was directly used as a tellurium source to avoid the use of H2Te and

K2Te2. As compared with synthesis of H2Te and K2Te2, this method is convenient to obtained iron(II) complex Na2[Fe(EDTA)] instead of iron(II) ion which reduced the amount of by product.

Marcasite FeTe2 nanorods have been prepared through a hydrothermal method in alkaline aqueous solution at the temperature range of 120-170 °C. The p- 61

XRD shows the orthorhombic FeTe2. Iron(II) complexes instead of iron(II) ion greatly reduced the amount of byproduct.131

FeTe and FeTe2 were also prepared by a low-temperature solution chemistry method. HDA and TOPO were heated with Fe(CO)5 at 160 °C and then injected tellurium dissolved in TOP. The iron and and tellurium ratio should be 20:1 to form pure FeTe and for FeTe2 large amount of tellurium required. The solution was heated up to 250 °C for 1 hour and cooled to 80 °C and then added to hexane/ethanol followed by centrifugation. The product was washed further with dichloromethane to remove residue and dried under vacuume. FeTe products are two-dimensional single crystals nanosheets with thickness of 2-3 nm and edge length ranging from 200 nm to several micrometers. FeTe2 form as a mixture of nanosheets and one-dimensional sheet-derived nanostructurs.132

1.15 Conclusion For the growing interest in nanoparticles and thin films based solar energy generation it is important to find cheap, nontoxic and environmentally friendly materials. Iron sulfide is the cheapest and non toxic material for photovoltaic cells. We have reviewed the synthetic methods and properties of iron chalcogenides nanoparticles and thin films in this chapter.

1.16 References 1. J. Singh, Optical properties of condensed matter and applications, John Wiley & Sons Ltd. 2006.

2. L. E. Brus, J. Chem. Phys., 1984, 80, 4033.

3. H. T. Grahn, Introduction to semiconductor physics; World Scientific: Singapore; London, 1999.

4. T. Trindad, P. O’Brien, N. L. Pickett, Chem. Mater., 2001, 13, 3843.

5. M. A. Malik, P. O'Brien, Top. Orgnomet. Chem., 2005, 9, 173.

6. N. P. Lieske, J. phys. chem. Solids, 1984, 45, 821.

7. A. C. Jones, P. O'Brien, CVD of compound semiconductors: precursor synthesis, development and applications; Germany, Cambridge, 1997.

8. M. A. Malik, P. O’Brien, N. Revaprasadu, Precursor Routes to Semiconductor Quantum Dots. Taylor & Francis, 2005, 180, 689.

9. M. M. Garcia, H. Villavicencio, M. Hernandez-Velez, O. Sanchez, J. M. Martinez-Duart, Mater. Sci. Eng., C15, 2001, 101.

10. Y. Wang, N. Herron, J. Phys. Chem., 1987, 91, 257.

11. T. Cassagneau, G. B. Hix, D. J. Jones, P. Maireless-Torres, M. Romari, J. Roziere, J. Mater. Chem., 1994, 4, 189.

12. T. Abe, Y. T., T. Uematsu, M. Iwamoto, J. Chem. Soc., Chem. Commun., 1995, 1617.

13. G. Blasse, G. J. D., M. E. Brenchley, M. T. Weller, Chem. Phys. Lett., 1995, 234, 177.

14. C. Petit, T. K. J., F. Billoudet, M. P. Pileni, Langmuir.,1994, 10, 4446.

15. K. M. Choi, K. J. Shea, J. Phys. Chem., 1994, 98, 3207.

16. R. Tassoni, R. R. S., Chem. Mater., 1994, 6, 744.

17. M. Moffitt, A. E., Chem. Mater., 1995, 7, 1178.

18. C. B. Murray, D. J. Norris, M. G. Bawendi, J. Am. Chem. Soc.,1993, 115, 8706.

19. J. G. Brennan, T. S., P. J. Carroll, S. M. Stuczynski, L. E. Brus, M. L. Steigerwald, J. Am. Chem. Soc., 1989, 111, 4141.

20. N. L. Pickett, P. O’Brien, The Chem. Rec., 2001, 1, 467.

21. D. M. Frigo, O. F. Z. K., P. O'Brien, J. Cryst. Growth, 1989, 96, 989.

22. M. Motevalli, P. O’Brien, J. R. Walsh, I. M. Watson, Polyhedron, 1996, 15, 2801.

23. T. Trindade, P. O’Brien, J. Mater. Chem., 1996, 6, 343.

24. M. A. Malik, N. Revaprasadu, P. O’Brien, Chem. Mater., 2001, 13, 913.

25. P. S. Nair, T. R., N. Radhakrishnan, G. A. Kolawole, J. Mater. Chem., 2001, 11, 1555.

26. P. S. Nair, T. R., N. Revaprasadu, G. A. Kolawole, P. O’Brien. J. Mater. Chem., 2002, 12, 2722.

27. P. S. Nair, T. R., N. Revaprasadu, G. A. Kolawole, P. O’Brien, J. Chem. Soc. Chem. Commun., 2002, 564.

28. P. S. Nair, T. R., N. Revaprasadu , P. O’Brien, Mater. Sci. and Tech., 2005, 21, 237.

29. Y. Li, X. L., C. Yung, Y. Li, J. Mater. Chem., 2003, 13, 2641.

30. B. Ludolph, M.A. Malik, P. O'Brien, N. Revaprasadu, J. Chem. Soc. Chem. Commun., 1998, 1849.

31. P. Terzieff, K. L. Komarek, Monats Chem., 1978, 109, 651.

32. P. Terzieff, K. L. Komarek, Monats Chem., 1978, 109, 1037.

33. A. Ennaoui, S. Fiechter, C. Pettenkofer, N. Alonso-Vante, K. Buker, M. Bronold, C. Hopfner, H. Tributsch, Solar Energy Mater. Solar Cells. 1993, 29, 289.

34. N. Hamdadou, A. Khelil, M. Morsli1, J. C. Bern, J. Phys. D: Appl. Phys., 2006, 39, 1042.

35. S. M. Pawar, A. V. Moholkar, U. B. Suryavanshi, K. Y. Rajpure, C. H. Bhosale, Sol Energy Mater. Sol. Cells, 2007, 91, 560.

36. J. Park, K. An, Y. Hwang, J. G. Park, H. J. Noh, J. Y. Kim, J. H. Park, N. M. Hwang and T. Hyeon, Nat. Mater., 2004, 3, 891.

37. Y. Hou, Z. Xu and S. Sun, Angew. Chem., Int. Ed., 2007, 46, 6329.

38. B. H. Zeng and S. Sun, Adv. Funct. Mater., 2008, 18, 391.

39. Y. W. Hun and S. J. W. Cheon, Acc. Chem. Res., 2008, 41, 179.

40. (a) P. V. Vanitha and P. O’Brien, J. Am. Chem. Soc., 2008, 130, 17256. (b) Christou, G.; Garner, C. D. J. Chem. Soc., Dalton Trans. 1979, 1093.(c) Que, L.; Holm, R. H.; Mortenson, L. E. J. Am. Chem. Soc. 1975, 97,463. (d) Rao, P. V.; Holm, R. H. Chem. ReV. 2004, 104, 527.

41. W. H. Feng, Z. Liang, S. Weidong, S. Shuyan, L. Yonngqian, w. Song and Z. Hongjie, Dalton Trans., 2009, 9246.

42. Y. Zhang, Y. Du, H. Xu and Q. Wang, CrystEngComm, 2010, 12, 3658.

43. K. Ramasamy, M. A. Malik, M. Helliwell, F. Tuna and P. O’Brien, Inorg. Chem., 2010, 49(18), 8495.

44. H. Y. Lai and C. J. Chen, J. Cryst. Growth, 2009, 311, 4698.

45. T. Kamimura, K. Kamigaki, T. Hirone, K. Sato, J. Phys. Soc. Japan., 1967, 22, 1235.

46. A. F. Andreson, Acta Chem. Scand., 1968, 22, 827.

47. Y. Mizuguchi, F. Tomioka, S. Tsuda, T. Yamaguchi, Y. Takano, Physica, 2009, 469, 1027.

48. J. H. Zhang, B. Wu, C.J. O’Connor, W.B. Simmons, J. Appl. Phys. 1993, 73, 5718.

49. J. Llewellyn, T. Smith, Proc. Phys. Soc. 1959, 29, 203.

50. J. M. Soon, L.Y. Goh, and K.P. Loh, Appl. Phys. Lett. 2007, 91, 084105.

51. D. J. Vaughan,A. R. Lennie, Sci. Prog. (Edinburgh), 1991, 75, 371.

52. J. B. Goodenough, Mater. Res. Bull.1978, 13, 1305.

53. C. N. R. Rao, K. P. R. Pisharody. Prog.Solid State Chem. 1975, 10, 207.

54. (a) V. Bessergenev, J. Phys.: Condens. Matter, 2004, 16, S531–S552. (b) C. Steinhagen , T. B. Harvey , C. J. Stolle , J. Harris , B. A. Korgel , J. Phys. Chem. Lett., 2012, 3 (17), 2352. (c) H. A. Macpherson, C. R. Stoldt, ACS Nano, 2012, 6 (10), 8940. (d) A. Kirkeminde, R. Scott, S. Ren, Nanoscale, 2012, 4, 7649.

55. D. J. Vaughan and J. R. Craig, Mineral Chemistry of Metal Sulfides, Cambridge University Press, Cambridge, UK, 1978.

56. J. R. Gosselin, M. G. Townsend, and R. J. Tremblay, Solid State Commun., 1976, 19, 799.

57. S. Kar and S. Chaushuri, Mater. Lett., 2005, 59, 289.

58. H. Nakazawa and N. Morimoto, Mater. Res. Bull.1971, 6, 345.

59. J. L. Horwood, M. G. Townsend, and A. H. Webster, J. Solid State Chem., 1976, 17, 35.

60. F. Li and F. Franzen, J. Solid State Chem. 1996, 126, 108.

61. M. J. Almond, H. Redman, and D. A. Rice, J. Mater. Chem., 2002, 10, 2842.

62. M. Nath, A. Choudhury, A. Kundu, and C. N. R. Rao, Adv. Mater. Weinheim,Ger, 2000, 15, 2098.

63. X. Ma, F. Xu, X. Wang, Y. Du, L. Chen, and Z. Zhang, J. Cryst. Growth, 2005, 277, 314.

64. J. C. Ward, Rev. Pure Appl. Chem., 1970, 20, 175.

65. D. Rickard and G. W. Luther, III, Chem. Rev., 2007, 107, 514.

66. E. Makovicky, Rev. Mineral. Geochem., 2006, 61, 7.

67. X. Chen, Z .Wang, X. Wang, J. Wan, J. Liu, Y. Qian, Inorg.Chem. 2005, 44, 951.

68. W. Han and M. Y. Gao, Cryst. Growth Des., 2008, 8, 1023. 66

69. C. Wadia, Y. Wu, S. Gul, S. K. Volkman, J. Guo and A. P. Alivisatos, Chem. Mater., 2009, 21, 2568.

70. J. H. beal, P. G. Etchegoin and R. D tilley, J. phy Chem. C2010, 114. 3817.

71. S. D. Disale and S. S. Garje, Adv. Sci. Lett., 2010, 3(1), 80.

72. A. L. Abdelhady, M. A. Malik, P. O’Brien, F. Tuna, J. Phys. Chem., C, 2012, 116 (3), 2253.

73. S. K. Maji, A. K. Dutta, P. Biswas, D. N. Srivatava, P. Paul, a. Mondal, B. Adhikary, Appl. Catalysis A: Gen, 2012, 419, 170.

74. D. M. Schleigh,H. S. W. Chang, J. Cryst. Growth, 1991, 112, 737.

75. B. Thomas, C. Hoepfner, K. Ellmer, S. Fiechter, H. Tributsch, J. Cryst. Growth, 1995, 146, 630.

76. B. Thomas, T. Cibik, C. Hoepfner, D. Diesner, G. Ehlers, S. Fiechter, K. Ellmer, J. Mater. Sci., 1998, 9, 61.

77. B. Meester, L. Reijnen, A. Goossens, J. Schoonman, J. Phys.(Paris) 1999, 9, Pr8–613.

78. G. Smestad, E. Ennaoui, S. Fiechter, H. Tributsch, W. K. Hofman, M. Birkholz, Sol. Energy Mater, 1990, 20, 149.

79. B. Ouertani, J. Ouerfelli, M. Saadoun, B. Bessais, H. Ezzaouia, Bernede, J. C. Mater. Charact., 2005, 54, 431.

80. M. Birkholz, D. Lichtenberger, C. Hoepfner, S. Fiechter, Sol. Energy Mater. Sol. Cells, 1992, 27, 243.

81. S. Bausch, B. Sailer, H. Keppner, G. Willeke, E. Bucher, G. Frommeyer, Appl. Phys. Lett. 1990, 25, 57.

82. I. J. Ferrer, C. Sanchez, J. Appl. Phys. 1991, 70, 2641.

83. B. Rezig, H. Dalma, M. Kanzai, Renewable Energy, 1992, 2, 125. 67

84. A. Ennaoui, G. Schlichtlorel, S. Fiechter, H. Tributsch, Sol. Energy Mater. Sol. Cells, 1992, 25, 169.

85. G. Smestad, A. Da Silva, H.Tributsch, S. Fiechter, M. Kunst, N. Meziani, M. Birkholz, Sol. Energy Mater. 1989, 18, 299.

86. P. O’Brien, D. J. Otway, J. Park, H. Mater. Res. Soc. Symp. Proc., 2000, 606, 133.

87. S. G. Shyu, J. S. Wu, C. C. Wu, S. H. Chuang, K. M. Chi, Inorg. Chim. Acta, 2002, 334, 276.

88. S. Kar, S. Chaudhuri, Chem. Phys. Lett. 2004, 398, 22.

89. X. Shi, K. Sun, L. P. Balogh, J. R. Baker, Jr. Nanotechnology, 2006, 17, 4554.

90. F. Cao, W. Hu, L. Zhou, W. Shi, S. Song, Y. Lei, S. Wang, H. Zhang, Dalton Trans, 2009, 9246.

91. Z. J. Zhang, X. Y. Chen, J. alloys and comp., 2009, 488, 339.

92. C. Steinhagen, T. B. Harvey, C. J. Stolle, J. Harris, and B. A. Korgel, J. Phys. Chem. Lett., 2012, 3, 2352.

93. M. C. Acevedo, M. S. Faber, Y. Tan, R. J. Hamers, and S. Jin, Nano Lett., 2012, 12, 1977.

94. B. Meester, L. Reijnen, A. Goossens, J. Schoonman, Chem. Vap. Deposition, 2000, 6, 121.

95. (a) J. Puthussery, S. Seefeld, N. Berry, M. Gibbs, and M. Law, J. Am. Chem. Soc., 2011, 13, 716. (b) L. Ding, X. Fan, X. Sun, J. Du, Z. Liu and C. Tao, RSC Adv., 2013, 3, 4539.

96. Y. Hu, Z. Zheng, H. Jia, Y. Tang, and L. Zhang, J. Phy. Chem. C 2008., 112, 13037.

97. H. F. Liu, A. Huang and D Z Chi, J. Phy. D: Appl. Phys., 2010, 43, 455405.

98. Y. Bi, Y. Yuan, C. L. Exstrom, S. A. Darveau, J. Huang, Nano Lett., 2011, 11, 4953.

99. W. Li, M. Doblinger, A. Vaneski, A. L. Rogach, F. Jackel, J. Feldmann, J. Mater. Chem., 2011, 21, 17946.

100. A. Kirkeminde, B. A. Ruzicka, R. Wang, S. Puna, H. Zhao, S. Ren. ACS Appl. Mater. Interfaces. 2012, 4, 1174.

101. J. H. L., S. Prabakar, N. Gaston, G. B. The, P. G. Etchegoin, G. Williams, R. D. Tilley, Chem Mater., 2011, 23, 2514.

102. F. C. Hsu, J. Y. Luo, K.W. Yeh, T. K. Chen, T. W. Huang, P. M. Wu, Y. C. Lee, Y. L. Huang, Y.Y. Chu, D. C. Yan, M. Ken Wu, www.pnas.org/cgi/doi/10.1073/pnas.0807325105, 2008, 105(38), 14262.

103. X. J. Wu, D. Z. Shen, Z. Z. Zhang, J. Y. Zhang, K. W. Liu, B. H. Li, Y. M. Lu, B. Yao, D. X. Zhao, B. S. Li, C. X. Shan, X. W. Fan, H. J. Liu, C. L. Yang, Appl. Phys. Lett., 2007, 90,112105-1.

104. X. J. Wu, Z. Z. Zhang, J. Y. Zhang, B. H. Li, Z. G. Ju, Y. M. Lu, B. S. Li, and D. Z. Shen, J. Appl. Phys., 2008, 103,113501-1.

105. F. C. Hsu, J. Y. Luo, K. W. Yeh, T. K.Chen, T. W. Huang, P. M.Wu, Y. C. Lee, Y. L. Huang, Y. Y. Chu, D. C. Yan, M. K. Wu, Proc. Nat. Acad. Sci., 2008, 105, 14262.

106. P. Terzieff, K. L. Komarek, Monats Chem., 1978, 109, 651.

107. P. Terzieff, K. L. Komarek, Monats Chem., 1978, 109, 1037.

108. X. J. Wu, D. Z. Shen, Z. Z. Zhang, J. Y. Zhang, K. W. Liu, B. H. Li, Y. M. Lu, B. Yao, D. X. Zhao, B. S. Li, C. X. Shan, X. W. Fan, H. J. Liu, C. L.Yang, Appl. Phys. Lett., 2007, 90, 112105-1.

109. X. J. Wu, Z. Z. Zhang, J. Y. Zhang, B. H. Li , Z. G . Ju, Y. M. Lu, B. S. Li, D. Z. Shen., J. Appl. Phys., 2008, 103, 113501-1.

110. C. E. M. Campos, J. C. de Lima, T. A. Grandi, K. D. Machado, V. Drago, P. S. Pizani., J. Magn. Magn. Mater., 2004. 270, 89.

111. C. E. M. Campos, V. Drago, J. C. de Lima, T. A. Grandi, K. D. Machado, M. R. Silva, J. Magn. Magn. Mater., 2004, 269, 6.

112. A. Liu, X. Chen, Z. Zhang, Y. Jiang, C. Shi, Solid State Communications, 2006, 138, 538.

113. C. R. Lin, Y.J. Siao, S.Z. Lu, and C. Gau, IEEE TRANSACTION ON MAGNETICS, 2009, 45(10).

114. S.Thanikaikarasan, T. Mahalingam, K. Sundaram, A. Kathalingam, Y. D. Kim, T. Kim, Vacuum, 2009, 83, 1066.

115. Y. Han, W.Y. Li, L. X. Cao, S. Zhang and B. R. Zhao, J. Phys.:Condens. Matter, 2009, 21, 235702.

116. L. Chen, H. Zhan, X. Yang, Z. Sun, J. Zhang, X. C. Liang, M. Wu, and J. Fang, CrystEngComm, 2010, 12, 4386.

117. M. R. Gao, Z. Y. Lin, J. Jiang, H.B. Yao, Y.M. Lu, Q. Gao, W.T. Yao, and S. H. Yu, Chem, Eur. J, 2011, 17, 5068.

118. H. Zhang, G. Long, D. Li, R. Sabirianov, H. Zeng, Chem, Mater. 2011, 23, 3769.

119. T. T. Oyetunde, PhD thesis, university of Manchester, 2011.

120. A. Wold, K. Dwight, Solid State Chemistry, 1993, 171.

121. W. Bao, Y. Qiu, Q. Huang, M. A. Green, P. Zajdel, M. R. Fitzsimmons, M. Zhernenkov, S. Chang, Minghu Fang, B. Qian, E. K. Vehstedt, Jinhu Yang, H. M. Pham, L. Spinu, Z. Q. Mao, Phys. Rev. Lett., 2009, 102, 247001.

122. F. C. Hsu, J. Y. Luo, K. W. Yeh, T. K. Chen, T. W. Huang, P. M. Wu, Y. C. Lee, Y. L. Huang, Y. Y. Chu, D. C. Yan, M. K. Wu, Proc. Natl. Acad. Sci., 2009, 105, 14262.

123. T. M. McQueen, Q. Huang, V. Ksenofontov, C. Felser, Q. Xu, H. W. Zandbergen, Y. S. Hor, J. Allred, A. J. Williams, D. Qu, J. Checkelsky, N. P. Ong, R. J. Cava, Phys. Rev., 2009, 79, 014522.

124. T. A. Bitter, R. J. Bouchard, W. H. Cloud, P.C. Donohue, W.J. Siemons, Inorg. Chem.,1968, 7, 2208.

125. J. Mikkelson, A. Wold, J. Solid St. Chem., 1971, 3, 39.

126. M. Bochmann, Chem. Vap. Deposition, 1996, 2, 85.

127. M. L. Steigerwald, Chem. Mater., 1989. 1, 52.

128. M. L. Steigerwald, T. Siegrist, S.M. Stuczynski, Chem. Mater., 1992, 114, 3155.

129. X. J. Song, M. Bochmann, J. Chem. Soc., Dalton Trans., 1997, 2689.

130. W. Zhang, Y. Cheng, J. Zhan, W. Yu, L. Yang, L. Chen, Y. Qian, Materials Science and Engineering B79. 2001, 244.

131. W. Zhang, Z. Yang, J. Zhan, L. Yang, W. Yu, G. Zhou, Y. Qian, Materials lett., 2001, 47, 367.

132. K. D. Oyler, X. Ke, I.T. Sines, P. Schiffer, and R. E. Schaak, Chem. Mater., 2009, 21, 3655.

Chapter 2

Deposition of iron sulfide nanocrystals and thin films from tris(dialkyldithiocarbamato)iron(III) complexes

2.1 Summary Symmetrical and unsymmetrical dithiocarbamato complexes of iron(III) with i the general fomula [Fe(S2CNRR’)3] where R = Et, R’ = Pr (1); R, R’ = Hex (2); R = Me, R = Et (3) R, R’ = Et (4); have been used as single source precursors to synthesize iron sulfide nanocrystals by thermolysis in oleylamine, hexadecyleamine and octadecene at different temperatures. The nanocrystals obtained were studied by p-XRD and TEM and magnetic measurements. Nanocrystals of iron sulfide with greigite, pyrrhotite and mixed phase were grown at different thermolysis temperatures from each precursor. These complexes were also used as single source precursors for the deposition of iron sulfide thin films by Aerosol Assisted Chemical Vapour Deposition (AACVD) method. The thin films obtained were studied by p-XRD, AFM, SEM and TEM.

The precursors with shorter alkyl chain length required higher temperature for decomposition. Symmetrical alkyl groups with longer chain alkyl groups gave pure greigite phase at lower thermolysis temperature of 170 °C but a mixture of greigite and pyrrhotite at higher temperatures. The unsymmetrical alkyl groups gave mixed phase (greigite and pyrrhotite) iron sulfide nanocrystals at all temperatures.

The unsymmetrical complexes deposited the mixed phases (pyrite and marcasite) at all deposition temperatures except the complex (3) which deposited pyrite and pyrrhotite at 400 °C. The symmtrical complex (2) with longer alkyl groups produced a mixture of pyrite and pyrrhotite phases at 350 and 450 °C but pyrite and mackinawite at 400 °C whereas the complex (4) with shorter alkyl groups deposited a mixture of pyrite and marcasite at 350 °C but a pure pyrrhotite phase at 400 and 450 °C.

2.2 Introduction There is a growing demand for non-toxic, abundant, and cheap materials even with lower efficiencies for photovoltaic applications. Most of the materials used so far are either toxic or non-abundant such as cadmium, lead, indium and selenium, which mean that these materials cannot contribute significantly to a large scale future sustainable energy needs. Recent estimates of the annual electricity potential as well as material extraction costs and environmental friendliness lead to the identification of materials that could be used in photovoltaic applications.1 The most promising materials including copper and iron sulfides.

Iron sulfides are an interesting class of materials with many different forms, which includes: pyrite (cubic-FeS2), marcasite (calcium chloride structure-FeS2), troilite (FeS), mackinawite (Fe1+xS), pyrrhotite (Fe1-xS, Fe7S8), smythite 2,3 (hexagonal-Fe3S4) and greigite (cubic spinel-Fe3S4). Iron sulfides exhibit a wide range of properties, from the semiconducting, nanomagnetic pyrite (FeS2) 4,5 to ferromagnetic Fe3S4. Various types of iron sulfides with different Fe:S ratio, ranging from 0.5 to 1.05 are found in nature. The magnetic and electrical properties of iron sulfides are dependent on the stoichiometric ratio between iron and sulfur.6,7 Among the natural iron sulfide minerals, mackinawite is paramagnetic; pyrite and marcasite are diamagnetic; pyrrhotite and greigite are ferromagnetic; and troilite is antiferromagnetic.3,8,9

Considerable interest has developed in the synthesis and characterization of iron-containing nanocrystals,10-12 due to their potential applications in: information storage devices, catalysis, biomedicine, imaging, and sensors.13 Whilst the synthesis of iron oxide nanomaterials is well documented, the preparation of iron chalcogenides [Fe(S, Se, Te)] nanomaterials has remained unexplored.14-18

Pyrite is an interesting phase of iron sulfide because of its potential application as an absorber material for thin film solar cells due to its band gap (Eg = 0.8– 0.95 eV), high molar absorption coefficient (105 cm−1) and perceived low toxicity.19 74

In the Fe-S system the stoichiometric ratio of iron:sulfur is important in determining the structure. Stoichiometric iron sulfide (FeS) adopts the trollite structure, with antiferromagnetic properties at room temperature and undergoes transition into the NiAs-type structure above 120 °C, which includes the 20, 21 pyrrhotites (Fe1−xS) and Fe7S8. Due to Fe vacancy ordering many pyrrhotites

(Fe1−xS) give a range of compositions with superstructures and interesting magnetic and electrical properties.22,23 The magnetic behaviour of the pyrrhotites 24 is very sensitive to changes in composition. FexS samples with x = 0.87–0.88 are Weiss type ferromagnetic. The non-stoichiometric Fe1−xS shows different morphologies including whiskers,25 nanowires,26 and U-shaped microslots.26, 27

Iron sulfide synthesis is challenging as small stoichiometry variations lead to large changes in the structural, chemical, and physical properties. O’Brien et al. prepared pyrrhotite Fe7S8 nanocrystals from a single source cubane Fe-S cluster by thermolysis in octylamine at 180 °C and greigite Fe3S4 in dodecylamine at 14 200 °C. Feng et al. prepared uniform 3D Fe3S4 flower-like microspheres by a hydrothermal reaction using L-cysteine as a source of sulfur and as a coordinating ligand.15 Pyrrhotite nanosheets and nanorods also have been prepared by high temperature approach in Keck Interferometer (KI) flux using sulfur and iron powders as the starting materials.28 Intracellular pyrrhotite and greigite nanocrystals are synthesised by some magnetotactic bacteria.29-31 Rao et al. synthesized the nanowires of iron sulfides such as Fe7S8 and Fe1−xS by the thermal decomposition of Fe1−xS(en)0.5 (en: ethylenediamine) in 26 ethylenediamine. Alivisatos et al. reported the preparation of pyrite FeS2 nanocrystals by thermolysing the single source precursor tris(diethyldithiophosphato)iron(III) [Fe(S2P(EtO)2)3] in hexadecyltrimethyl- ammonium bromide (CTAB) at 200 °C in a Teflon-lined stainless steel acid digestion bomb.32 p-XRD studies on the materials prepared showed the deposition of cubic pyrite FeS2without any noticeable impurity peaks from orthorhombic marcasite FeS2 or hexagonal troilite FeS. Greigite Fe3S4 and pyrrhotite Fe7S8 nanosheets were prepared by Gao et al. from tris(diethyldithiocarbamato)iron(III) (Fe(S2CNEt2)3) and bis(diethyldithiocarbamato)Fe(II):1,10-phenanthroline [Fe(S2CNEt2)2(phen)] by thermolysis in 75

olelylamine at 240-320 °C.33 Wang et al. reported the synthesis of various iron sulfide nanostructures including nanocrystals, nanoribbons and nanoplates using the same complexes in a mixed solvent system of oleic acid(OA)/oleylamine(OM) and 1-octadecene (ODE).16 Most recently O’Brien et al. used a series of iron(III) thioburet complexes as single source precursors for the synthesis of iron sulfide thin films by aerosol assisted chemical vapour (AACVD) method.17 Different iron sulfide phases including FeS hexagonal troilite and cubic pyrite FeS2 and tetragonal pyrrhotite Fe1-xS were deposited depending on the precursor and the deposition temperature.17

Iron sulfide thin films have been prepared by atmospheric- or low-pressure 34-37 metal-organic chemical vapour deposition (AP or LP MOCVD; FeS2).

Schleigh and Chang deposited FeS2 thin films by using iron pentacarbonyl 34 [Fe(CO)5], hydrogen sulfide, and tert-butyl sulfide as precursors by LPCVD. 38,39 Other techniques include sulfurization of iron oxides to FeS2, ion beam and 40 41 reactive sputtering (FeS2), plasma assisted sulfurization of iron (FeS2), flash 42 43 evaporation (FeS2), vacuum thermal evaporation (FeS2), vapour transport 44 45 (FeS2), and chemical spray pyrolysis (FeS2). Different phases of iron sulfide nanoparticles were produced by high-energy mechanical milling combined with 27,46 47 mechanochemical processing for FeS and FeS2, dendrimer-stabilized FeS, 48 solvothermal synthesis of Fe3S4, sulfur-reducing bacteria for Fe1-xS and 49,50 51 Fe3S4, laser pyrolysis of iron complexes for FeS, polymer-stabilized wet 52 53 chemical synthesis of FeS, reverse micelles for FeS2, and the decomposition 54 of single-source precursors for FeS2. Meester et al. prepared iron disulfide

(FeS2) using iron(III) acetylacetonate [Fe(acac)3], tert-butyl disulfide, and hydrogen.55

There have been a very limited number of iron complexes employed as single- source precursors for the deposition of iron sulfide as Fe1+xS, FeS2, and Fe1-xS thin films, which include dithiocarbamato complexes [Fe(S2CNRR’)3] (R,R’ = Et,Et,1 Me,iPr)56 and the sulfur-bridged binuclear iron carbonyl complex 57 [Fe2(CO)6(μ-S2)]. Puthussery et al. prepared colloidal pyrite NCs by injecting a

solution of sulfur dissolved in diphenyl ether into a solution of FeCl2 in octadecylamine at 220 °C and stirring for several hours.58

In this chapter we describe the use of symmetrical and unsymmetrical iron(III) complexes of dialkyldithiocarbamates as single source precursors to synthesise iron sulfide nanocrystals by thermolysis in oleylamine, octadecene and hexadecylamineand iron sulfide thin films by aerosol-assisted chemical vapour deposition (AACVD) method. The phases, size, and shape of iron sulfide nanostructures were systematically characterized by means of powder X-ray diffraction (p-XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and the relationship of iron sulfide phases with the type of precursor was investigated. Furthermore, superconducting quantum interference device (SQUID) magnetometry was used to determine the magnetic properties of these materials.

2.3. Precursor synthesis Dialkyldithiocarbamatoiron(III) compounds were prepared by adopting literature methods reported previously.59,60 A brief description of each synthesis is given below.

i 2.3.1 Synthesis of [Fe(S2CNEt Pr)3] (1) N-Ethyl-isopropyl amine (2g, 22.9 mmol) and sodium hydroxide (0.917g, 22.9 mmol) were mixed in 100 ml of methanol in a beaker with stirrer. The mixture was cooled to about 0 °C in an ice bath while stirring. A solution of carbon disulfide (1.74g, 22.9 mmol) in methanol (10 ml) was slowly added to this mixture while vigorously stirring. The mixture was allowed to warm at room temperature after addition of all carbon disulfide. At this stage a freshly prepared solution of iron chloride (FeCl3) (1.24g, 7.6 mmol) in methanol (50 ml) was added drop wise to the reaction mixture by dropping funnel. A black precipitate started to form which was filtered after the completion of reaction. The crude product was recrystallized from toluene at room temperature to give i shiny black needle of [Fe(S2CNEt Pr)3]. Yield 2.5g (59 %), mpt: decomposition between 232-247 °C by changing its color from bright black to yellow brownish

powder, Elemental analysis: Found: C, 38.9; H, 6.3; N, 7.0; S, 33.5; Fe, 9.7 %. -1 Calc: C, 39.8 ; H, 6.7 ; N, 7.7; S, 35.5; Fe, 10.3 %. IR (vmax /cm ): 2969(w), 1472(s), 1422(s), 1336(s), 1291(s), 1195(s), 1048(s), 880(s), Mass (m/z): 542 i (molecular ion peak), 380 (Fe(S2CNEt Pr)2), 319 (Fe(S2CNC3H4)), 162 i (S2CNEt Pr) and 130 (S2CNC3H4).

2.3.2 Synthesis of [Fe(S2CN(Hex)2)3] (2) Compound (2) was prepared by the same method using dihexylamine. The crude product was recrystallized from chloroform at room temperature.Yield 2.0 g (68 %), mpt: 174 ˚C. Elemental analysis: Found: C, 55.1; H, 9.7; N, 4.9; S, 21.9; Fe, -1 6.7 %. Calc: C, 55.9; H, 9.3; N, 5.0; S, 22.9; Fe, 6.6 %. IR (vmax /cm ): 2921(w), 1492(s), 1422(s), 1195(s), 1024 (s), 979(s), 758(s), 609(s), Mass (m/z): 835

(molecular ion peak), 576 (Fe(S2CNC12H26)2), 316 (Fe(S2CNC12H26)), 231 (Fe

(S2CNC6H13) and 228 (C13H26NS).

2.3.3 Synthesis of [Fe(S2CNEtMe)3] (3) Compound (3) was prepared as above using N-ethylmethyl amine. The crude black product was recrystallized from tetrahydrofuran at room temperature to give shiny black needle of Fe(S2CNEtMe)3. Yield 3.5g (68%), mpt: The sample decomposed at 223 °C. Elemental analysis: Found: C, 31.0; H, 5.1; N, 8.9; S, - 41.6; Fe, 11.7 %. Calc: C, 31.4; H, 5.2; N, 9.1; S, 41.6; Fe, 12.1 %. IR (vmax /cm 1): 2921(w), 1493(s), 1422(s), 1362(s), 1195 (s), 1101(s), 724(s), 609(s), Mass

(m/z):458 (molecular ion peak) 324 (Fe(S2CNEtMe)2, 295 (Fe(S2CN)2(C4H11)) and 190 (Fe(S2CNC3H8)).

2.3.4 Synthesis of [Fe(S2CN(Et)2)3] (4) A mixture 5 g (22.2 mmol) of N,N-diethyl dithiocarbamide sodium salt in 50 ml of water were stirred in a beaker for 15 minutes. A freshly prepared solution of iron chloride (FeCl3) (1.19g, 7.3 mmol) in methanol (60 ml) was added drop wise to the reaction mixture by dropping funnel. Black precipitate was formed which was filtered. Yield 3.40 g (92 %), mpt: 255 ˚C. Elemental analysis: Found: C, 36.2; H, 5.9; N, 7.9; S, 36.8; Fe, 10.3 %. Calc: C, 35.9; H, 6.4; N, 8.3; -1 S, 38.4; Fe, 11.1 %. IR (vmax /cm ): 2974(w), 1484(s), 1372(s), 1207(s), 1132

(s), 1073(s), 912(s), 783(s), Mass (m/z): 500 (molecular ion peak), 444

(C2H5CN), 352 (S2NC5H10) and 180 (FeSNC5H10)

2.4 Synthesis of iron sulfide nanocrystals

Iron sulfide nanocrystals were prepared by decomposition of the [Fe(S2CNR2)3] complexes in oleylamine. In a typical reaction, 15 ml of oleylamine was refluxed under vacuum at 90 °C for 30 min, then it purged with nitrogen gas for 30 min. at same temperature. Then, 0.5g (0.92 mmol) of precursor was added into hot oleylamine, and the reaction temperature was slowly increased to the desired temperature (170, 230 and 300 °C). The temperature was maintained for one hour and the mixture was allowed to cool at room temperature. Addition of 30 ml of methanol produced a black precipitate which was separated by centrifugation. The black residue was washed twice with methanol and redispersed in toluene or hexane for further characterisations.

2.5 Results and discussion i 2.5.1 X-ray single crystal structure of [Fe(S2CNEt Pr)3] (1) i The X-ray single crystal structure of [Fe(S2CNEt Pr)3] (1) (Figure 2.1) is monoclinic with space group P2(1)/n. The structure is based on a monomeric molecule in which iron atom is bonded to six sulfur atoms, two from each dithiocarbamato ligand. The geometry about the iron centre is octahedral. Fe-S bond lengths range from 2.300(9) to 2.349(9) Å which are considerably shorter i than those observed by us for [Fe(SON(CN Pr2)2)3] (2.418(8) Å) or 17 [Fe(SON(CNMe2)2)3] (2.421(5) Å). The structural refinement data are given in the Table 2.1 and selected bond lengths and angles given in caption to Figure 2.1 and

2.5.2 X-ray single crystal structure of [Fe(S2CNEtMe)3] (3)

The structure of the complex Fe(S2CNEtMe)3 (3) (Figure 2.2) is based on a monomer in which iron atom is attached to six sulfur atom from three dithiocarbamato ligands. The structure contains half a molecule of THF as crystallisation solvent. The geometry on iron is distorted octahedral. Fe-S bond lengths range from 2.288(6), from the ligand disordered at the ethyl/methyl 79

sites, to 2.310(8) Å which are slightly shorter than those observed for complex i (1) but significantly shorter than those observed for [Fe(SON(CN Pr2)2)3] 17 (2.418(8) Å) or [Fe(SON(CNMe2)2)3] (2.421(5) Å). The structural refinement data are given in Table 2.1 and selected bond lengths and angles given in caption to Figure 2.2

i Figure 2.1. X-ray Single crystal structure of [Fe(S2CNEt Pr)3] (1), Selected bond lengths (Å) and bond angles (°): Fe(1)-S(1) 2.3143(9), Fe(1)-S(2) 2.3147(9), Fe(1)-S(3) 2.3018(9), Fe(1)-S(4) 2.3126(9), Fe(1)-S(5) 2.3002(9), Fe(1)-S(6) 2.3485(9), S(5)- Fe(1)-S(3) 100.04(3), S(5)-Fe(1)-S(4) 87.97(3), S(3)-Fe(1)-S(4) 74.95(3), S(5)-Fe(1)- S(1) 94.96(3), S(3)-Fe(1)-S(1) 161.67(4), S(4), Fe(1)-S(1) 95.25(3), S(5)-Fe(1)-S(2) 166.41(4), S(3)-Fe(1)-S(2) 91.48(3), S(4)-Fe(1)-S(2) 102.08(3), S(1)-Fe(1)-S(2) 75.21(3), S(5)-Fe(1)-S(6) 74.45(3), S(3)-Fe(1)-S(6) 95.29(3), S(4)-Fe(1)-S(6) 158.27(3), S(1)-Fe(1)-S(6) 98.85(30), S(2)-Fe(1)-S(6) 97.47(3).

Figure 2.2. X-ray Single crystal structure of [Fe(S2CNEtMe)3] (3), Selected bond lengths (Å) and bond angles (°): Fe(1)-S(1) 2.2876(16), Fe(1)-S(2) 2.2966(17), Fe(1)- S(3) 2.3098(18), Fe(1)-S(4) 2.2975(17), Fe(1)-S(5) 2.3075(17), Fe(1)-S(6) 2.3041(17), S(1)-Fe(1)-S(2) 75.77(6), S(1)-Fe(1)-S(4) 162.81 (7), S(2)-Fe(1)-S(4) 94.70(6), S(1)- Fe(1)-S(6) 95.23(6), S(2)-Fe(1)-S(6) 163.97(7), S(4), Fe(1)-S(6) 97.11(6), S(1)-Fe(1)- S(5) 101.27(6), S(2)-Fe(1)-S(5) 92.83(6), S(4)-Fe(1)-S(5) 93.37(6), S(6)-Fe(1)-S(5) 75.72(5), S(1)-Fe(1)-S(3) 91.58(6), S(2)-Fe(1)-S(3) 99.00(6), S(4)-Fe(1)-S(3) 75.59(6), S(6)-Fe(1)-S(3) 94.42(6), S(5)-Fe(1) S(3) 164.36(6).

The crystallographic refinement parameters data for the crystal structure of complexes (1) (CCDC reference numbers 776129) and (3) (CCDC reference numbers 776131) are given in Table 2.1.

Table 2.1. Structural refinement data for complex (1) and complex (3) obtained from X-ray single crystallography.

Empirical formula C18 H36 Fe N3 S6 (1) C14 H28 Fe N3 O0.50 S6 (3)

Formula weight 542.71 494.60

Crystal system, space Monoclinic, P2(1)/n Triclinic, P-1 group

Unit cell dimesnions a = 14.7078(11) Å, b = a = 7.974(2) Å, b = 11.999(3) Å, 10.1581(8) Å, c = c = 12.112(3) Å, alpha = 17.8800(14) Å, alpha = 76.157(5) deg., 90 deg., beta = beta = 79.844(5) deg., 105.7890(10) deg., gamma = 83.699(5) deg. gamma = 90 deg.

Volume 2570.5(3) A3 1104.8(5) A3

Z, Calculated density 4, 1.402 Mg/m3 2, 1.487 Mg/m3

Absorption coefficient 1.085 mm-1 1.256 mm-1

Crystal size 0.32 x 0.25 x 0.20 mm 0.35 x 0.35 x 0.13 mm

Theta range for data 1.60 to 28.26 deg. 2.18 to 25.02 deg. collection

Data / restraints / 6047 / 0 / 262 3824 / 54 / 263 parameters

Goodness-of-fit on 1.139 1.049 F^2

Final R indices R1 = 0.0539, wR2 = R1 = 0.0618, wR2 = 0.1455 [I>2sigma(I)] 0.1175

R indices (all data) R1 = 0.0644, wR2 = R1 = 0.0891, wR2 = 0.1589 0.1236

Largest diff. peak and 1.797 and -0.316 e.A-3 1.078 and -0.645 e.A-3 hole

Reflections collected / 21485 / 6047 [R(int) = 5695 / 3824 [R(int) = 0.0312] unique 0.0497]

Limiting indices -19<=h<=18, -9<=h<=9,

-12<=k<=13, -11<=k<=14,

-23<=l<=23 -13<=l<=14

2.5.3 Thermogravimetric analysis (TGA) i TGA of the complexes [Fe(S2CNEt Pr)3] (1), [Fe(S2CN(Hex)2)3] (2),

[Fe(S2CNEtMe)3] (3) and [Fe(S2CN(Et)2)3] (4) indicates single step decomposition with a rapid weight loss between 220-300 °C, for i [Fe(S2CNEt Pr)3] (1), 170-270 °C for [Fe(S2CN(Hex)2)3] (2), 220-300 °C for

[Fe(S2CNEtMe)3] (3) and for [Fe(S2CN(Et)2)3] (4) 220-300 °C respectively.

The solid decomposition residue amounts to 33 % for complex (1) which is higher than the calculated value 22 % for FeS. And the solid decomposition residue amounts to 10 % for complex (2) which is close to the calculated value (11%) for FeS. Complex (3) gives 21% of residue which is slightly higher than the calculated value 19 % for FeS. The final residue amounts 17.5 % for complex (4) which is less than the calculated value 21 % for FeS. The TGA results are shown in Figure 2.3.

4 100 3 2 1 80

Massof residue (%) 20

0 0 100 200 300 400 500 Temperature (°C )

i Figure 2.3. Thermogravimetric analysis of complexes [Fe(S2CNEt Pr)3] (1)

[Fe(S2CN(Hex)2)3] (2), [Fe(S2CNEtMe)3] (3) and [Fe(S2CN(Et)2)3] (4) at heating rate of 10 ˚C/min. under nitrogen, flow rate of nitrogen was 100 ml /minute.

2.5.4 Powder X-ray diffraction of iron sulfide nanocrystals The p-XRD pattern of the nanocrystals obtained at 170, 230 and 300 °C from i [Fe(S2CNEt Pr)3] (1) correspond to a mixture of the cubic greigite (Fe3S4) (ICDD No: 00-016-0713) and hexagonal pyrrhotite (FeS) (ICDD No: 00-002- 1241) (Figure 2.4).

(311) * (440) *(400) (220) (511) * (111) (c)

(b)

Intensity(a.u)

(a)

10 20 30 40 50 60 70 80

2-Theta (degree)

i Figure 2.4. p-XRD pattern for greigite (Fe3S4) nanocrystals from [Fe(S2CNEt Pr)3] (1) in oleylamine at (a) 170 (b) 230 (c) 300 °C . The * shows pyrrhotite (FeS) phase.

The intensity of the peaks for pyrrhotite (FeS) varied depending upon the growth temperature. At the lowest temperature (170 °C) the diffraction peaks for

(111), (220), (311), (400), (511) and (440) planes of greigite (Fe3S4) were dominant with very low intensity peaks for pyrrhotite (FeS). At the higher growth temperatures of 230 and 300 °C the peak intensity for pyrrhotite (FeS) increased. The p-XRD pattern obtained for the samples from thermolysis of

[Fe(S2CN(hex)2)3] (2) at different temperatures is shown in Figure 2.5. At a growth temperature of 170 °C and almost pure phase of greigite (Fe3S4) was obtained but as the growth temperature increased from 170 to 230 to 300 °C, the intensity of the peaks for pyrrhotite (FeS) increased in a similar way to those observed in case of complex (1). Complex [Fe(S2CNEtMe)3] (3) showed similar behaviour at 230 and 300 °C but no product was produced at a growth 84

temperature of 170 °C (Figure 2.6). The p-XRD pattern obtained for the samples from thermolysis of [Fe(S2CN(Et)2)3] (4) at different temperatures is shown in Figure 2.7.

(311) *

(440) *(400) (220) (511) * (111) (c)

(b) Intensity(a.u)

(a)

10 20 30 40 50 60 70 80 2-Theta (degree)

Figure 2.5. p-XRD pattern for greigite (Fe3S4) nanocrystals in oleylamine from

[Fe(S2CN(Hex)2)3] (2) at (a) 170 (b) 230 and (c) 300 °C. The * shows pyrrhotite (FeS) phase.

(311)

(400) (440) (220) * * (511) (111) * (444) (c)

Intensity(a.u) (b)

(a)

10 20 30 40 50 60 70 80 2-Theta (degree)

Figure 2.6. p-XRD pattern for greigite (Fe3S4) nanocrystals in oleylamine from

[Fe(S2CNEtMe)3] (3) at (a) 170 (b) 230 and (c) 300 °C . The * shows pyrrhotite (FeS) phase.

Complex [Fe(S2CN(Et)2)3] (4) gave no product at a growth temperature of 170 °C (Figure 2.7) but at the growth temperatures of 230 and 300 °C, gave almost pure greigite (Fe3S4) phase. The diffraction peaks for (111), (220), (311), (400),

(511) and (440) planes of greigite (Fe3S4) were dominant.

The iron sulfide phase deposited depended on the growth temperature as well as on the nature of the precursor. Complex (1) gives a mixture of cubic greigite and hexagonal pyrrhotite at all temperatures whereas complex (2) gives a pure greigite (Fe3S4) at 170 °C and complex (3) does not produce any product at 170 °C.

(311) (111) (220) (400) (440) ) (511)

a.u ( (c)

(b) Intensity

(a)

10 20 30 40 50 60 70 80 2- Theta (degree)

Figure 2.7. p-XRD pattern for greigite (Fe3S4) nanocrystal from [Fe(S2CN(Et)2)3] (4) in oleylamine at (a) 170 (b) 230 and (c) 300 °C.

The greigite (Fe3S4) phase was dominant in all three complexes at lower temperatures but the ratio of greigite to pyrrhotite decreases as the growth temperature increases. Conversely the complex (4) does not produce any product at 170 °C and gives a pure greigite (Fe3S4) at 230 °C and 300 °C.

2.5.5 Transmission electron microscopy of iron sulfide nanocrystals TEM images show the iron sulfide predominantly hexagonal shape nanocrystals with some cubic crystallites (Figures 2.8 and 2.9 (a)).

(a) (b)

100 nm 100 nm

Figure 2.8. Fe3S4 (greigite) and FeS (pyrrhotite) nanocrystals from complex (1) at different temperatures in oleylamine (a) 170 °C (b) 230 °C (c) 300 °C and (d) SAED pattern of the nanocrystals grown at 300 °C from complex (1).

The SAED pattern shows the crystalline nature (single crystal) of the nanocrystals (Figure 2.8(d)) with d-spacing of 3.0 Å corresponding to the greigite phase of Fe3S4. HRTEM shows the size range of the crystallites between 100-200 nm (Figure 2.9 (a)). The d-spacing calculated from the lattice fringes (Figure 2.9 (b)) matches with that of SAED pattern.

TEM images of iron sulfide (greigite (Fe3S4)) nanocrystals from

[Fe(S2CN(Et)2)3] (4) in oleylamine at (a) ,(b) at 230 and (c), (d) at 300 °C. The inset in figure 2.12(b and e) shows the different shapes of nanocrystals at 230

˚C. HRTEM of iron sulfide (greigite (Fe3S4)) nanocrystals from

[Fe(S2CN(Et)2)3] (4) in oleylamine at 230 °C showing lattice fringes with a d- spacing of 5.2 Å corresponding to the (111) reflection of greigite iron sulfide phase.

Figure 2.9. TEM and HRTEM of Fe3S4 (greigite) and FeS (pyrrhotite) nanocrystals from complex (1) at 300 °C showing (a) hexagonal and cubic shape particles, inset shows small particles lying on a large particle and (b) lattice fringes showing d-spacing.

(a) (b)

100 nm 100 nm

Figure 2.10. TEM images of Fe3S4 (greigite) nanocrystals from complex (2) at (a) 170 °C and (b) 230 °C in oleylamine.

Figure 2.11. TEM images of Fe3S4 (greigite) nanocrystals from complex (3) at (a) 230 °C and (b) 300 °C in oleylamine.

Figure 2.12. TEM images of Fe3S4 (greigite) nanocrystals from complex (4) in oleylamine at (a, b) 230 °C and (c, d) 300 °C. The inset in (b ane e) shows different shape of nanocrystals at 230 °C. HRTEM (f) showing lattice fringes with a d-spacing 5.2 Å corresponding to the (111) reflections of greigite.

100 100

80 80

60 60

40 40

(a.u) intensity Relative Pyrrhotite (1) 20 Pyrrhotite (2) 20 Pyrrhotite (3) 0 0 160 180 200 220 240 260 280 300 320 Temperature (oC ) Figure 2.13. Graph showing the relative peak intensity of pyrrhotite (FeS) peak at 2- theta 33.92 (101 plane) and greigite (Fe3S4) at different temperatures from three i different precursors [Fe(S2CNEt Pr)3] (1), [Fe(S2CN(Hex)2)3] (2) and [Fe(S2CNEtMe)3] (3) respectively.

i Figure 2.13 shows that when we use [Fe(S2CNEt Pr)3] (1) precursor the amount of pyrrohitite increases with increasing temperature. These results are consistent with the pyrrhotite phase being more stable at high temperature. When

[Fe(S2CN(Hex)2)3] (2) was used at 170 °C pure greigite (Fe3S4) was deposited. At a growth temperature 230 °C and 300 °C the amount of pyrrotite increases.

Another interesting result shows that when [Fe(S2CNEtMe)3] (3) is used, there is no deposition of material at 170 °C, but at 230 °C to 300 °C the amount of pyrrhotite increases with increasing the temperature. The different phases of iron sulfide nanocrystals with different morphologies produced from symmetrical and unsymmetrical dithiocarbamato complexes of iron(III) are given in Table 2.2.

Table 2.2. Phases of iron sulfide and morphology of the nanocrystals produced from different single source precursors.

Compounds Temp. Phase Morphology (°C)

i [Fe(S2CNEt Pr)3] 170 greigite + pyrrhotite hexagonal + cubic 230 greigite + pyrrhotite hexagonal + cubic

300 greigite + pyrrhotite hexagonal + cubic

[Fe(S2CN(Hex)2)3] 170 greigite hexagonal + cubic 230 greigite + pyrrhotite hexagonal + cubic

300 greigite + pyrrhotite hexagonal + cubic

[Fe(S2CNEtMe)3] 170 no product hexagonal + cubic 230 greigite + pyrrhotite hexagonal + cubic

300 greigite + pyrrhotite hexagonal + cubic

[Fe(S2CN(Et)2)3] 170 no product hexagonal + cubic 230 greigite hexagonal + cubic

300 greigite hexagonal + cubic

2.5.6 The effect of surfactants i Complex (1) [Fe(S2CNEt Pr)3] was also thermolysed in octadecene and hexadecylamine at 300 °C to see if the capping agent had any effect on the nature of the growth of iron sulfide phases. The p-XRD patterns of the nanocrystals produced with the different capping agents is shown in Figure 2.14. The pXRD pattern of the nanocrystals produced from the thermolysis in hexadecylamine shows (Figure 2.14 (a)) a mixture of pyrrhotite (*) and greigite (#) phases.

* * *

+ *

(b) #

Intensity(a.u) # #

(a)

30 40 50 60 2-Theta (degree)

i Figure 2.14. p-XRD pattern of iron sulfide nanocrystals grown from [Fe(S2CN Et Pr)3] (1) at 300 °C in (a) hexadecylamine and (b) octadecene. * shows the pyrrhotite (FeS) phases, + shows pyrrhotite (Fe7S8) and # shows greigite (Fe3S4) phase respectively.

Figure 2.15. Nanocrystals from complex (1) at 300 °C in different capping agent (a) hexadecylamine, and (b) octadecene.

Interestingly the thermolysis in octadecene (Figure 2.14 (b)) produced pyrrhotite (*) (FeS) as the main product with a weak intensity peak for pyrrhotite (+)

(Fe7S8). This result is in contrast to that observed from the thermolysis in oleylamine where the greigite phase was dominant at all temperatures. TEM images show the morphology as hexagonal shaped crystallites (Figure 2.15 (a)) obtained from HDA and nanosheets from octadecene (Figure 2.15 (b)).

2.5.7 The magnetic properties of iron sulfide nanocrystals The precursors (1–3) show typical paramagnetic behaviours (i.e. the room temperature magnetization (M) increases linearly with the increase of the magnetic field (H); see Figures 2.17(a), 2.20(a) and 2.21(a) ) whereas all Fe1-xS nanocrystals prepared from the thermolysis of (1), (2) or (3) at various temperatures, show ferromagnetic behaviour with magnetization well saturated under applied magnetic fields of 20 kOe (1 Oe = 10-4 T) or even weaker.

It is well known that the magnetic properties of magnetic nanocrystals are governed by the anisotropy energy that constrains the nanocrystals magnetization to align along a specific direction known as the easy axis. The maximum anisotropy energy for a nanocrystal depends on the product of its volume, V, and the anisotropy constant, K.61 Above the so-called blocking temperature, TB, the thermal energy is sufficient to surmount the anisotropy energy barrier and the nanocrystals magnetization is free to align in arbitrary directions. Thus, for an assembly of such nanocrystals, the net magnetization measured in the absence of an applied magnetic field (i.e. the remnant magnetization of the sample) will average out to zero and the particles are said to be in superparamagnetic state.62 An estimate of the blocking temperature is typically obtained from peaks in the field cooled (FC) and zero-field cooled (ZFC) magnetization curves recorded at very small magnetic fields. The experimental ZFC/FC curves of iron sulfide nanocrystals obtained by thermolysis of complex (1) in oleylamine at 300 °C are presented in Figure 2.16 (a). In this case no peaks can be discerned in the ZFC/FC curves below 300 K, suggesting that the blocking temperature of these nanocrystals is well above room temperature. Indeed, the magnetization versus field measurements at room

temperature reveal the presence of a hysteresis loop with a coercive field (Hc) (i.e. the magnetic field needed for the magnetization to return to zero) of ca. 300

Oe and a remnant magnetization (Mr) of 6.5 emu/g (i.e. the magnetization retained by nanocrystals when the magnetic field is switched off) (Figure 2.17 (a)), indicating that these nanocrystals are already in the blocking regime at 300 K. Their magnetization appears to saturate at 10 kOe applied magnetic field.

The magnetization saturation (Ms) value at room temperature is 18.9 emu/g.

Figure 2.16. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves for iron sulfide nanocrystals obtained from complex (1) by thermolysis in (a) oleylamine at 300 °C and (b) in HDA.

Figure 2.17. Room temperature magnetic hysteresis loops of (a) complex (1) and iron sulfide nanocrystals grown by thermolysis of (1) in oleylamine at (b) 230 °C and (c) 300 °C.

We have also investigated the magnetic behaviour at 300 K of the iron sulfide nanocrystals obtained from thermolysis of (1) in oleylamine at 230 °C (Figure 2.17 (b)). As observed, the magnetization versus field associated to these nanocrystals also approaches saturation at 10 kOe applied magnetic field.

However, the saturation magnetization (Ms) value of the iron sulfide nanocrystals grown at 300 °C (Ms = 18.9 emu/g) is significantly higher than that of the nanocrystals obtained at 230 °C (Ms = 4.1 emu/g). Cycling the magnetic field between -70 - and +70 kOe gave rise to hysteresis loops in both cases, but the coercivity (Hc) and the remanence-to-saturation ratio R = Mr/Ms for the nanocrystals grown at 230 °C (Hc = 600 Oe; R = 0.44 at 300 K) are well above those of the nanocrystals grown at 300 °C (Hc = 300 Oe; R = 0.34 at 300 K). This may suggest larger anisotropy energy for the former.

Figure 2.18. Room temperature magnetic hysteresis loops of iron sulfide nanocrystals obtained from (1) in (a) HDA and (b) ODE at 300 °C.

Figure 2.18 shows that there are significant differences between the magnetic properties of the Fe1-xS nanocrystals obtained by thermolysis at 300 °C of complex (1) in hexadecylamine (HDA) and those obtained from octadecene (ODE) in identical thermal conditions. While thermolysis in HDA produces ferromagnetic nano-crystallites with magnetization well saturated at 10 kOe (MS = 9.2 emu/g; Figure 2.18(a)), thermolysis in ODE generates nanosheets whose magnetic behaviour at 300 K is predominantly paramagnetic (Figure 2.18 (b) 94

and the inset of Figure 2.18). Although the magnetization value at 30 kOe applied magnetic field is very small (M = 1.08 emu/g), the magnetization versus field shows irreversibility between -30 and 30 kOe. The coercivity and remamant magnetization values are 110 Oe and 2.1 emu/g for the nanocrystals thermolysed in HDA and 1200 Oe and 0.08 emu/g for those thermolysed in ODE respectively.

Figure 2.19. Magnetic hysteresis loops at 5 K of iron sulfide nanocrystals obtained by thermolysis of complex (1) in (a) oleylamine at 300 °C and (b) in HDA at 300 °C.

It is worth noting that the coercivity and the Mr/Ms ratio for the nanocrystals of (1) annealed in HDA at 300 °C (Figure 2.18 (a)) are much smaller than those of the nanocrystals grown from oleylamine at 300 °C (Figure 2.17 (a)). The shapes of the two hysteresis loops are also significantly different. These differences cannot be fully assigned to small differences in crystallinity and particles size

(as proved by p-XRD and TEM). These Fe1-xS nanocrystals have different compositions of FeS and Fe3S4, which results in different Fe:S ratios. Below TB the iron vacancies, which are likely to be different in the two compounds, start to order ferro- or ferrimagnetically.

It is possible that the blocking temperatures of the two compounds are different. To test this hypothesis we have recorded magnetization hysteresis loops at 5 K (Figure 2.19), where it is supposed that both samples are well with in their blocking regimes, and also monitored the evolution of their magnetization over 95

the temperature range 5 – 300 K, in both zero-field-cooled (ZFC) and field cooled (FC) regimes, using a magnetic field of 100 Oe (Figure 2.16). Interestingly, close inspection of the ZFC/FC magnetization curves of the sample obtained from HDA reveals the opening of a maximum just about room temperature, suggesting that they approach the superparamagnetic state near room temperature, while those obtained from oleylamine are well in the blocking regime. Comparison of the hystersis loops recorded at 5 K (Figure 2.19) reveals similar magnetic behaviour for both samples. The saturation magnetization of the iron sulfide nanocrystals grown from oleylamine (Ms =

22.0 emu/g) is higher than that of the nanocrystals obtained from HDA (Ms =

10.9 emu/g). However, the coercive field and the remanant-to-saturation Mr/Ms ratio are slightly smaller for the iron sulfide nanocrystals obtained from oleylamine (Hc = 940 Oe, R = 3.1/10.9 = 0.44), as compared to nanocrystals obtained from HDA (Hc = 940 Oe, R = 9.7/22.1 = 0.47). This clearly indicates higher anisotropy energy for the latter. The fact that Ms value is significantly higher for the nanocrystals obtained from oleylamine at 300 °C indicates that these materials have higher crystallinity, in good agreement with the XRD and TEM data. It is also worth noting that the R values of these compounds are close to the expected R = 0.5 value for noninteracting, randomly oriented particles with uniaxial symmetry. Deviations from the non interating value are expected, and these high R values preclude the existence of significant interparticle interactions.63

Both hysteresis loops almost reach saturation at 10 kOe applied field and reveal a ferromagnetic behaviour with characteristic Ms, Hc and Mr values of 18.2 emu/g, 280 Oe and 6.9 emu/g for the nanocrystals obtained from (2) at 170 °C (Figure 2.20(b)) and 15.8 emu/g, 120 Oe and 3.4 emu/g from thermolysis of (3) at 300 °C (Figure 2.21 (b)) respectively.

It is evident from the magnetic measurements that the magnetization curves are well saturated after the application of a magnetic field higher than 10 kOe. The difference in the saturation magnetization of the samples is mainly due to the different compositions in Fe3S4 and FeS, but a series of other intrinsic factors

such as: size, shape, crystallinity, crystal defects are also contributing to the observed magnetic behaviour.

Figure 2.20. Room temperature magnetic hysteresis loops of (a) complex (2) and (b) iron sulfide nanocrystals obtained from (2) in oleylamine at 170 °C.

Figure 2.21. Room temperature magnetic hysteresis loops of (a) complex (3) and (b) iron sulfide nanocrystals obtained from (3) in oleylamine at 300 °C.

Figures 2.20 (b) and 2.21 (b) display the magnetic hysteresis curves of the iron sulfide nanocrystals obtained from complexes (2) and (3) by thermolysis at 170 and 300 °C respectively, in oleylamine.

On the other hand, pyrrhotite Fe1-xS samples rarely consist of only one structural type because at temperatures below 350 ○C the iron vacancies in the Fe-S system start to order producing a series of superstructures, which can display properties as varied as: antiferromagnetic, paramagnetic, ferromagnetic, and ferrimagnetic. Any small contribution from these phases could impact on the magnetic properties. Defects at the particle surface can also influence the magnetic properties.

2.6 Iron sulfide thin films All four complexes were used to deposit thin films onto silicon substrates by the Aerosol-Assisted Chemical Vapour Deposition (AACVD) method.

i 2.6.1 The deposition of iron sulfide thin films from [Fe(S2CNEt Pr)3] (1) Deposition was carried out at substrate temperature from 350 to 450 °C with argon flow rate at of 160 sccm on silicon substrates. Reflective dark brown uniform films were deposited at 350 and 450 °C and black uniform films were obtained at 400 °C.

The p-XRD pattern of the as deposited films at 350 - 450 °C (Figure 2.22) show films of cubic pyrite (FeS1.96 (ICDD No: 01-073-8127)) with a smaller amount of marcasite (FeS2 (ICDD No: 04-003-2016)). Diffraction pattern of cubic pyrite

(FeS1.96) planes of (111), (200), (210), (220), (311) and (023) were dominant at all deposition temperature with only a minor peak at 54.63(2θ) corresponding to marcasite.

The SEM images (Figure 2.23 (a)) shows the sheets and rod like crystallites (size 2–5µm) from the deposition at 350 °C. At deposition temperatures of 400 °C sheets like crystallites (Figure 2.23 (b)) are deposited. Sheets and cube like crystallites (Figure 2.23(c)) were formed at 450 °C. EDX analysis shows that the films are composed of iron: sulfur ratio 50:50 (350 °C), 50:49 (400 °C), 78:21

(450 °C). The Fe:S ratio is 1:1 in the films deposited at 350 and 400 °C. The elemental mapping (Figure 2.23 (d)) shows the uniform distribution of iron and sulfur.

The surface topography of the films analyzed by 3D AFM image (Figure 2.24 (a)) of films deposited at 400 °C shows well interconnected crystallites. The average roughness value ranged 147.1 nm (Figure 2.24 (b)). TEM images (Figure 2.24 (c)) of the scratched films shows irregular shaped crystallites and SAED pattern (Figure 2.24 (d)) shows single crystalline nature of the crystallites.

(200)

(023) (220) (311) (d) (111) (210) (211) + (c)

Intensity (a.u) Intensity

(b) (a) 20 25 30 35 40 45 50 55 60 65 2- Theta (degree)

Figure 2.22. (a) shows the standard patterns p-XRD of iron sulfide (pyrite FeS1.96 i (ICDD No: 01-073-8127)) and thin films deposited from complex [Fe(S2CNEt Pr)3] (1) onto silicon at (b) 350, (c) 400, (d) 450 °C. The (+) symbol denotes the marcasite.

(a) (b)

20μm 50μm

10μm 20μm

Figure 2.23. SEM images of iron sulfide (pyrite FeS1.96) films deposited from complex i [Fe(S2CNEt Pr)3] (1) onto silicon at (a) 350, (b) 400, (c) 450 °C, (d) elemental mapping at 400 °C.

(a) (b)

20nm

Figure 2.24. (a) 3D AFM images of iron sulfide thin film and (b) shows average roughness and Rms roughness (c) Shows TEM images of scratched thin film and (d) i shows the SAED pattern of the thin films from precursor [Fe(S2CNEt Pr)3] (1) at 400 °C.

100

2.6.2 The deposition of iron sulfide thin films from [Fe(S2CN(Hex)2)3] (2) The reflective black uniform films were deposited at 350, 400 and 450 °C. The p-XRD pattern of the as deposited films at 350 and 450 °C (Figure 2.25 (b) and

2.25 (d)) show cubic pyrite (FeS1.96(ICDD No: 01-073-8127)) as the predominant phase with impurities of hexagonal pyrrhotite-IT (Fe0.95S1.05 (ICDD No: 01-075-0600)). Films produced at 400 °C mainly consist of cubic pyrite

(FeS1.96(ICDD No: 01-073-8127)) (Figure 2.25 (c)) phase with a smaller amount of tetragonal mackinawite (FeS (ICDD No: 04-003-6935)) phase.

SEM images of films deposited at 350 and 400 °C show sheet-like crystallites (Figure 2.26 (a) and 2.26 (b)) with size ranging from 3-5 µm. Flower like clusters were deposited (Figure 2.26 (c)) at 450 °C with size ranging from 5-10 μm. EDX analysis shows that the films are composed of iron: sulfur ratios 65:35 (350 and 400 °C), and 53:44 (450 °C).

(023) (200)

(220) (311) (d) + * * *

(c)

(a.u) Intensity (b)

(a) 20 25 30 35 40 45 50 55 60 65 2- Theta (degree)

Figure 2.25. (a) Shows the standard patterns p-XRD of iron sulfide (pyrite FeS1.96

(ICDD No: 01-073-8127)), and iron sulfide thin films deposited from Fe(S2CN(Hex)2)3] (2) onto silicon at (b) 350, (c) 400 and (d) 450 °C. The asterisk symbol (*) denotes the pyrrhotite phase. The major diffraction peaks could be indexed as (200), (220), (201),

(311) and (023) planes of cubic pyrite (FeS1.96) and (*) shows pyrrhotite-IT (Fe0.95S1.05).

101

(a) (b)

5μm 20μm

5μm

20μm 5μm

(e) (f)

Figure 2.26. SEM images of the films deposited from complex Fe(S2CN(Hex)2)3] (2) at (a) 350 (b) 400 (c) 450 °C and (d) elemental mapping at 350 °C (e) 3D AFM images of iron sulfide thin film and (f) shows average roughness and Rms roughness at 400 °C.

The elemental mapping (Figure 2.26 (d)) of the image shows the uniform distribution of iron and sulfur. SAED pattern of the films grown at 400 °C show the characteristic of single crystallites (Figure 2.26 (b) inset). The 3D AFM image (Figure 2.26 (e)) shows the growth of closely packed crystallites onto a silicon substrate at 400 °C with an average roughness of 52.74 nm (Figure 2.26 (f)).

2.6.3 The deposition of iron sulfide thin films from [Fe(S2CNEtMe)3] (3) The p-XRD pattern of the as deposited films at 350 °C (Figure 2.27 (b)) correspond to the cubic pyrite (FeS1.96 (ICDD No: 01-073-8127)) phase and at

400 °C (Figure 2.27 (c)) a mixture of cubic pyrite (FeS1.96) and hexagonal 102

pyrrhotite-IT (Fe0.95S1.05) whereas the films deposited at 450 °C (Figure 2.27

(d)) consist of cubic pyrite (FeS1.96). All three samples show a weak diffraction peak at 54.63(2θ) corresponding to marcasite.

The SEM images from the films deposited at 350 °C on silicon substrate from complex [Fe(S2CNEtMe)3] (3) show clusters of hexagonal plates with the size range of 3-6 µm. The size of individual plates range from 0.5-1 µm (Figure 2.28 (a) and inset in 2.28 (a)).

(200)

(023) (d) (220) (311)

(c)

* * * * Intensity (a.u) Intensity

(b)

(a) 20 25 30 35 40 45 50 55 60 65 2-Theta (degree)

Figure 2.27. (a) Shows the standard patterns p-XRD of iron sulfide (pyrite FeS1.96

(ICDD No: 01-073-8127)), (b) thin films deposited from complex [Fe(S2CNEtMe)3] (3) on silicon substrate at 350 °C shows pyrites, (c) 400 °C shows mixture of pyrite and pyrrhotite ( ) and (d) 450 °C mostly pyrites. Whereas (+) shows marcasite.

Films deposited at growth temperature 400 and 450 °C show (Figure 2.28 (b)) and 2.28 (c)) sheet-like crystallites with an average lengths of 3 µm. EDX analysis showed that the films are composed of iron:sulfur ratio 65:35 (350 °C), 53:44 (400 °C), 64:35 (450 °C).

The high-resolution transmission electron microscopy (HR-TEM) images of the scratched films deposited from complex [Fe(S2CNEtMe)3] (3) at growth temperature 400 °C show the lattice fringes (Figure 2.28 (d)) with a d-spacing of 103

3.12Å corresponding to the (111) reflection of cubic pyrite phase. The surface topography of the films deposited at 400 °C analyzed by a 3D AFM image (Figure 2.28 (e)) show globular crystallites with an average roughness value of 136.7nm (Figure 2.28 (f)).

(a) (b)

1 μm

5μm 5μm

10μm 5nm

(e) (f)

Figure 2.28. SEM images of iron sulfide thin films from [Fe(S2CNEtMe)3] (3) deposited at (a) 350, (b) 400, (c) 450 °C and (d) HRTEM at 400 °C showing d-spacing (3.12(Å )) corresponding to (111) plan of pyrite phase, (e) 3D AFM image of iron sulfide thin film and (f) shows average roughness and Rms roughness of thin films from precursor (3) at 400 °C.

104

2.6.4 The deposition of iron sulfide thin films from [Fe(S2CN(Et)2)3] (4) The p-XRD patterns of the as deposited films at 350 °C (Figure 2.29 (b)) show the cubic pyrite (FeS1.96(ICDD No: 01-073-8127)) phase with major diffraction peaks (200), (220), (311), and (023). The weak diffraction peak at 54.63(2θ) correspond to marcasite phase. The p-XRD patterns of the as deposited films at 400 and 450 °C (Figure 2.29 (c) and 2.29 (d)) show pure hexagonal pyrrhotite-

IT (Fe0.95S1.05 (ICDD No: 01-075-0600)). The major diffraction peaks for hexagonal pyrrhotite-IT could be indexed to (100), (101), (102) and (110) planes.

(200)

(d) (100) (101) (102) (110)

(c)

(023)

(a.u) Intensity (311) (220) (b) + (a)

20 25 30 35 40 45 50 55 60 65 2- Theta (degree)

Figure 2.29. (a) Shows the standard patterns p-XRD of iron sulfide (pyrite FeS1.96 (ICDD No: 01-073-8127)), and iron sulfide thin films deposited from complex

[Fe(S2CN(Et)2)3] (4) (b) at 350 °C shows pyrites with (+) minor marcasite (c) 400 °C, hexagonal pyrrhotite-IT and (d) 450 °C hexagonal pyrrhotite-IT.

SEM images of films deposited at 350, 400 and 450 °C show sheet-like crystallites (Figure 2.30 (a), 2.30 (b) and 2.30 (c)) with an average size of 1-5 µm. The EDX analysis of the films show the composition of iron:sulfur as 61:39 (350 and 450 °C), 56:44 (400 °C). TEM images (Figure 2.30 (b) inset) from the

105

scratched sample of films grown at 400 °C shows the characteristic of single crystallites.

(a) (b)

5μm 5μm

50μm 6μm

(e) (f)

Figure 2.30. SEM images of iron sulfide thin films deposited from precursor (4) at (a) 350, (b) 400 and (c) 450 °C. Inset (b) shows the SAED pattern (d) elemental mapping at 400 °C (e) 3D AFM images of iron sulfide thin films and (f) shows average roughness and Rms roughness from precursor (4) at 400 °C.

The elemental mapping of the films (Figure 2.30 (d)) show almost equal distribution of iron and sulfur. The AFM image of the films deposited at 400 °C (Figure 2.30 (e)) shows the growth of closely packed crystallites onto a silicon substrate with an average roughness (Figure 2.30 (f)) of 7.48 nm.

The phases and morphology of the nanocrystallites thin films deposited at different temperatures are given in the Table 2.3. 106

Table 2.3. Phases and morphology of the crystallites of iron sulfide thin films.

Complex Temp.°C Phase Morphology

i [Fe(S2CNEt Pr)3] 350 pyrite + marcasite sheet + rod 400 pyrite + marcasite sheet

450 pyrite + mercasite sheet + cube

[Fe(S2CN(Hex)2)3] 350 Pyrite + pyrrhotite sheet 400 Pyrite + mackinawite sheet

450 Pyrite + pyrrhotite flower

[Fe(S2CNEtMe)3] 350 pyrite + marcasite plate 400 pyrite+marcasite+ pyrrhotite sheet

450 pyrite + marcasite sheet

[Fe(S2CN(Et)2)3] 350 pyrite + marcasite sheet 400 pyrrhotite sheet

450 pyrrhotite sheet

2.7 Conclusion Iron sulfide minerals are featured with significant temperature-induced composition- and phase-transformations. Our study involved the use of different thermolysis temperatures (170, 230, 300 °C) and also their different precursors ((1), (2), (3) (4)). The relative stabilities of various phases of iron sulfide are shown in Figure 2.31 using a plot similar to that developed by Vaughan and Lennie.17,64 The height of the pyramid on the negative z axis represents the free energy of formation of each phase. The solid line represents the thermodynamic 64 stability and connects the stable phases FeS (troilite) and FeS2 (pyrite).

The p-XRD pattern showed predominantly the greigite (Fe3S4) phase with some low intensity peaks for pyrrhotite (FeS) at lower temperatures. The amounts of pyrrhotite (FeS) increased by increasing the growth temperatures for complexes (1-3). These results are consistent with those reported in the literature.64 The complex (4) gives pure greigite (Fe3S4) at both 230 and 300 °C.

107

Complex (1) gives a mixture of cubic greigite and hexagonal pyrrhotite at all temperatures whereas complex (2) give a pure greigite (Fe3S4) at 170 °C and complex (3) does not produce any product at 170 °C. Complex (4) gives the pure greigite (Fe3S4) phase at both 230 and 300 °C. Greigite (Fe3S4) was dominant from all three complexes at lower temperatures but the ratio of greigite to pyrrhotite decreased as the growth temperature increased.

The effect of solvent on the phase was investigated by using octadecene and i hexadecylamine at 300 °C for complex [Fe(S2CNEt Pr)3] (1). The p-XRD pattern of the nanocrystals produced from the thermolysis in hexadecylamine show a mixture of pyrrhotite and greigite phase whereas thermolysis in octadecene produced the pyrrhotite (FeS) as main product with a weak intensity peak for pyrrhotite (Fe7S8). It is interesting to note that thermolysis in oleylamine gave the greigite phase as dominant at all temperatures.

Magnetic measurements for the complexes (1), (2) and (3) show typical paramagnetic behaviours whereas the iron sulfide nanoparticles produced from all three complexes show ferromagnetic behaviour.

Iron sulfide thin films study involved the use of different deposition temperatures of 350, 400, 450 °C by using precursors (1), (2), (3) and (4). The relative stabilities of various phases of iron sulfide are shown in Figure 2.32 using a plot similar to that developed by O’Brien and co-workers and Vaughan and Lennin.17,64 The height of the pyramid on the negative z axis represents the free energy of formation of each phase. The solid line represents the thermodynamic stability and connects the stable phases FeS (troilite) and FeS2 (pyrite).

108

100

-50

-100

-150

-200

= Complex (1) = Complex (2) = Complex (3)

Figure 2.31. Phases of iron sulfide nanocrystals grown by thermolysis from precursors (1) –(3). The relative amount of each phase is represented as the height of the cylinder. These are approximated based on the p-XRD results. Compared to the relative thermodynamic stabilities of the various phases of iron sulfides (the z axis) and the phases after Vaughan and Lennie. 17,64

The symmetrical tris(dialkyldithiocarbamato)iron(III) with the shorter alkyl groups (4) gave the pure pyrrhotite phase at the higher growth temperatures (400, 450 °C). The same precursor produced pure greigite phase nanocrystals from the thermolysis in oleylamine. Pyrite phase is dominant in all the samples obtained from the unsymmetrical complexes whereas the pyrrhotite phase is dominant in the samples deposited from symmetrical complexes.

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Figure 2.32. The main phases of iron sulfide thin films from precursors (1)–(4). The relative amount of each phase is represented as the height of the cylinder. These are approximated based on the p-XRD results. Compared to the relative thermodynamic stabilities of the various phases of iron sulfides (the z-axis) and the phases after O’Brien and co-workers and Vaughan and Lennie.17,64

2.8 References 1. C. Wadia, A. P. Alivisatos, D. M. Kammen, Environmental Science & Technology, 2009, 43, 2072.

2. J. B. Goodenough, Mater. Res. Bull., 1978, 13, 1305.

3. C.N. R. Rao, K. P. R. Pisharody, Prog. Solid State Chem., 1976, 10, 207.

4. H. Wang, I. Salveson, Phase Trans., 2005, 78, 547.

5. C. I. Pearce, R. A. D .Pattrick, D. J. Vaughan, Rev. Min. Geochem., 2006, 61, 127.

6. J. C. Ward, ReV. Pure Appl. Chem., 1970, 20, 175.

7. D. Rickard, G. W.Luther, III, Chem. Rev., 2007, 107, 514.

110

8. E. Makovicky, ReV. Mineral Geoche., 2006, 61, 7.

9. D. J. Vaughan, J. R. Craig, Mineral Chemistry of Metal Sulfides. Cambridge University Press: Cambridge, U.K.,

10. J. Park, K. An, Y. Hwang, J. G. Park, H. J. Noh, J. Y. Kim, J. H. Park, N. M. Hwang, T. Hyeon, Nat. Mater., 2004, 3, 891.

11. Y. Hou, Z. Xu, S. Sun, Angew. Chem. Int. Ed., 2007, 46, 6329.

12. B. H. Zeng, S. Sun , Adv. Funct. Mater., 2008, 18, 391.

13. Y. W. Hun, S. J. W. Cheon, Acc. Chem. Res., 2008, 41, 179.

14. P. V. Vanitha, P. O’Brien, J. Am. Chem. Soc., 2008, 130, 17256.

15. W. H. Feng, Z. Liang, S. Weidong, S. Shuyan, L. Yonngqian, w. Song, Z. Hongjie, Dalton Trans., 2009, 9246.

16. Yejun Zhang, Yaping Du, Huarui Xu and Qiangbin Wang, Cryst. Eng.Comm., 2010, 12, 3658

17. K. Ramasamy, M. A. Malik, M. Helliwell, F. Tuna, P. O’Brien, Inorg. Chem., 2010, 49(18), 8495

18. H. Y. Lai, C. J. Chen, J. Cryst. Growth, 2009, 311, 4698.

19. V. Bessergenev, J. Phys.: Condens. Matter, 2004, 16, S531–S552.

20. J. R. Gosselin, M. G. Townsend, and R. J. Tremblay, Solid State Commun., 1976, 19, 799.

21. S. Kar and S. Chaushuri, Mater. Lett., 2005, 59, 289.

22. H. Nakazawa and N. Morimoto, Mater. Res. Bull., 1971, 6, 345.

23. J. L. Horwood, M. G. Townsend, and A. H. Webster, J. Solid State Chem., 1976, 17, 35.

24. F. Li and F. Franzen, J. Solid State Chem., 1996, 126, 108.

111

25. M. J. Almond, H. Redman, and D. A. Rice, J. Mater. Chem., 2002, 10, 2842.

26. M. Nath, A. Choudhury, A. Kundu, and C. N. R. Rao, Adv. Mater. Weinheim,Ger, 2000, 15, 2098.

27. X. Ma, F. Xu, X. Wang, Y. Du, L. Chen, and Z. Zhang, J. Cryst. Growth, 2005, 277, 314.

28. X. Kong, T. Lou, Y. J. Li, J. Alloys Cmpd., 2005, 390, 236.

29. M. Posfai, P. B. Buseck, D. A. Bazylinski, R. B. Frankle, Am. Minerol., 1998, 83, 1469.

30. S. Mann, N. H. C. Sparks, R. B. Frankel, D. A. Bazylinski, H. W. Jannasch, Nature, 1990, 342, 258.

31. D. A. Bazylinski, B. R. Haywood, S. Mann, R. B Frankel, Nature, 1993, 366, 218.

32. C. Wadia, Y. Wu, S. Gul, S. K. Volkman, J. Guo and A. P. Alivisatos, Chem. Mater., 2009, 21, 2568.

33. W. Han, M. Y. Gao, Cryst. Growth Des., 2008, 8, 1023.

34. D. M. Schleigh, H. S. W. Chang, J. Cryst. Growth, 1991, 112, 737.

35. B. Thomas, C. Hoepfner, K. Ellmer, S. Fiechter, H. Tributsch, J. Cryst. Growth, 1995, 146, 630.

36. B. Thomas, T. Cibik, C. Hoepfner, D. Diesner, G. Ehlers, S. Fiechter, K. Ellmer, J. Mater. Sci., 1998, 9, 61.

37. B. Meester, L. Reijnen, A. Goossens, J. Schoonman, J. Phys.(Paris) 1999, 9, Pr8–613.

38. G. Smestad, E. Ennaoui, S. Fiechter, H. Tributsch, W. K. Hofman, M. Birkholz, Sol. Energy Mater, 1990, 20, 149.

112

39. B. Ouertani, J. Ouerfelli, M. Saadoun, B. Bessais, H. Ezzaouia, Bernede, J. C. Mater. Charact.,2005, 54, 431.

40. M. Birkholz, D. Lichtenberger, C. Hoepfner, S. Fiechter, Sol. Energy Mater. Sol. Cells, 1992, 27, 243.

41. S. Bausch, B. Sailer, H. Keppner, G. Willeke, E. Bucher, G. Frommeyer, Appl. Phys. Lett. 1990, 25, 57.

42. I. J. Ferrer, C. Sanchez, J. Appl. Phys. 1991, 70, 2641.

43. B. Rezig, H. Dalma, M. Kanzai, Renewable Energy, 1992, 2, 125.

44. A. Ennaoui, G. Schlichtlorel, S. Fiechter, H. Tributsch, Sol. Energy Mater. Sol. Cells, 1992, 25, 169.

45. G. Smestad, A. Da Silva, H. Tributsch, S. Fiechter, M. Kunst, N. Meziani, M. Birkholz, Sol. Energy Mater. 1989, 18, 299.

46. P. P. Chin, J. Ding, J. B. Yi. B. H. Liu, J. Alloys Compd. 2005, 390, 255.

47. X. Shi, K. Sun, L. P. Balogh, Baker, JR. Jr., Nanotechnology 2006,17, 4554.

48. F. Cao, W. Hu, L. Zhou, W. Shi, S. Song, Y. Lei, S. Wang, H. Zhang, Dalton Trans, 2009, 9246.

49. J. H. P. Watson, B. A. Cressey, A. P. Roberts, D. C. Ellwood, J. M. Charnock, A. K. Soper, J. Magn. Magn. Mater. 2000, 214, 13.

50. J. H. P. Watson, D. C. Ellwood, A. K. Soper Charnock, J. M.J. Magn. Magn. Mater., 1999, 203, 69.

51. X. X. Bi, P. C. Eklund, Mater. Res. Soc. Symp. Proc., 1993, 286, 161.

52. K. M. Paknikar, V. Nagpal, A. V. Pethkar, Rajwade, J. M. Sci. Tecnol. Adv. Mater. 2005, 6, 370.

53. J. P. Wilcoxon, P. P. Newcomer, G. A. Samara, Solid State Commun. 1996, 98, 581. 113

54. X. Chen, Z .Wang, X. Wang, J. Wan, J. Liu, Y. Qian, Inorg. Chem. 2005, 44, 951.

55. B. Meester, L. Reijnen, A. Goossens, J. Schoonman, Chem. Vap. Deposition, 2000, 6, 121.

56. P. O’Brien, D. J. Otway, J. Park, H. Mater. Res. Soc. Symp. Proc. ,2000, 606, 133.

57. S. G. Shyu, J. S. Wu, C. C. Wu, S. H. Chuang, K. M. Chi, Inorg. Chim. Acta, 2002, 334, 276.

58. J. Puthussery, S. Seefeld, N. Berry, M. Gibbs, and M. Law, J. Am. Chem. Soc., 2011, 13, 716.

59. M. A. Malik, N. Revaprasadu, P. O’Brien, Chem. Mater., 2001, 13, 913.

60. P. S. Nair, T. R., N. Radhakrishnan, G. A. Kolawole, J. Mater. Chem., 2001, 11, 1555.

61. G. F. Goya, T. S. Berquó, F. C. Fonseca, M. P. Morales, J. App. Phys., 2003, 94, 3520.

62. V. S. Coker, N. D. Telling, G. van der Laan, R. A. D. Patrick, C. I. Pearce, E. Arenholz, F. Tuna, R. E. P. Winpenny, J. R. Lloyd, ACS Nano, 2009, 3, 1922.

63. G. Hadjipanayis, D. J. Sellmyer, B. Brandt, Phys. Rev. B, 1981, 23, 3349.

64. D. J. Vaughan, A. R. Lennie, Sci. Prog. (Edinburgh) 1991, 75, 371.

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Chapter 3

The synthesis of iron sulfide nanocrystals from tris(O-alkylxanthato)iron(III) complexes

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3.1 Summary i i Tris(O-alkylxanthato)iron(III) [Fe(S2COR], where (R = Me, Et, Pr and Bu) complexes have been synthesiszed, characterised and used as single source precursors for the synthesis of iron sulfide nanocrystals by thermolysis in oleylamine, hexadecylamine and octadecylamine at 170, 230 and 300 °C temperatures. The morphology of the crystallites was strongly affected by the reaction conditions as well as the type of precursor used.

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3.2 Introduction Iron sulfide nanocrystals are being investigated because of their potential application in: imaging, catalysis, sensors and information storage devices.1-4 Particularly The magnetic nanocrystals of iron sulfide are of importance due to their potential uses in magnetic data storage devices and as magnetic resonance imaging contrast agents.5-7 Iron sulfides have several phases including pyrite

(cubic-FeS2), greigite (cubic spinel-Fe3S4), marcasite (calcium chloride _ structure-FeS2), pyrrhotite-1T (Fe1 xS), pyrrhotite-4M (Fe7S8), (Fe9S10), troilite- 8-11 2H (FeS), and mackinawite (Fe1+ x S). The pyrite phase among these phases have particular interest as it has potential applications as an absorber material for solar cells. Pyrite has a high absorption coefficient (~105 cm−1) and band gap (Eg = 0.8–0.95 eV) suitable for photovolatic applications. Pyrite is a non-toxic and also a cheap material.12

Stoichiometric iron sulfide (FeS) adopts the trollite structure, which have antiferromagnetic properties at room temperature. It also undergoes a transition 13,14 into a NiAs-type structure at higher temperatures. Many pyrrhotites Fe1−xS phases with different compositions are known which exhibit superstructures and have interesting electrical and magnetic properties.15,16 The magnetic behaviour 17 of the pyrrhotites is very sensitive to changes in composition. FexS samples with x = 0.87–0.88 are Weiss-type ferromagnetic materials due to unbalanced ferromagnetic coupling. On the other hand nonstochiometric Fe1−xS shows a wide variety of interesting morphologies, including whiskers like grains,18 nanowires,19 U-shaped microslots19 and make interesting ferromagnetic semiconductors.19,20

Thin films of iron sulfide have been prepared by different techniques, which include atmospheric- or low-pressure metal-organic chemical vapour deposition 21-24 25,26 (AP-or LP-MOCVD), Sulfurization of iron oxides (FeS2), ion beam and 27 28 reactive sputtering (FeS2), plasma assisted sulfurization of iron (FeS2), flash 29 30 evaporation (FeS2), vacuum thermal evaporation (FeS2). vapour transport 31 32 8 (FeS2), and chemical spray pyrolysis (FeS2). Soon et al. reported iron-rich iron sulfide crystals (Fe1+xS) by using iron(III) diethyldithiocarbamate as a

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precursor by chemical vapour deposition (CVD) method. These iron-rich iron sulphide crystals (Fe1+xS) show interesting shape anisotropy as well as ferromagnetic properties.

33 Meester et al. reported the synthesis of iron disulfide (FeS2) by using iron(III) acetylacetonate [Fe(acac)3], tert-butyl disulfide, and hydrogen. Single-source precursors including dithiocarbamatoiron(III) complexes [Fe(S2CNRR’)3] (R, R’ = Et, Et, Me, iPr)34 and the sulfur-bridged binuclear iron carbonyl complex 35 [Fe2(CO)6(μ-S2)] have been used for the deposition of iron sulfide as Fe1+xS, 36 FeS2, and Fe1−x S thin films. O’Brien et al. reported the synthesis cubane-type Fe–S cluster and thermolysed it in octylamine at 180 °C to give pyrrhotite

(Fe7S8) and in dodecylamine at 200 °C to give greigite (Fe3S4) nanocrystals.

37 Gao et al. used tris(diethyldithiocarbamato)iron(III) (Fe(S2CNEt2)3) and bis(diethyldithiocarbamato)Fe(II):1,10-phenanthroline [Fe(S2CNEt2)2(phen) to prepare nanosheets of Greigite (Fe3S4) and pyrrhotite (Fe7S8) by thermolysis in oleylamine at 240–320 °C. Iron sulfide thin films were prepared from a series of iron(III) thiobiurets complexes by Aerosol Assisted Chemical Vapour Deposition (AACVD) method.38

Law et al.39 prepared colloidal pyrite nanocrystals by injecting a solution of sulfur dissolved in diphenyl ether into a solution of FeCl2 in octadecylamine at

220 °C. Pyrite (FeS2) nanocrystals of pure phase were synthesised from iron– oleylamine complexes with sulfur in oleylamine. The nanodendrites or nanocubes were obtained by adjusting the iron-oleylamine concentration.40

Our group reported the synthesis of greigite, pyrrhotite and mixed phases iron sulfide nanocrystals at different thermolysis temperatures, from symmetrical- and un symmetrical- dialkyldithiocarbamatoiron(III) complexes. The unsymmetrical alkyl groups gave mixed phases (greigite and pyrrhotite), whereas symmetrical alkyl groups with longer chain alkyl groups gave pure greigite phase at lower thermolysis temperature of 170 °C.41 We also used iron(III) complex of 1,1,5,5-tetra-iso-propyl-2-thiobiuret as a single-source precursors for the synthesis of iron sulfide nanoparticles in hot oleylamine.42

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Thermolysis of the iron complexes in oleylamine/oleylamine produced Fe7S8 nanoparticles with different morphologies (spherical, rods, or plates), depending on the growth temperature and precursor concentration.42 Most recently we reported the use of symmetrical- and un symmetrical- dialkyldithiocarbamato- iron(III) complexes as single source precursors and produced iron sulfide thin films by Aerosol Assisted Chemical Vapour Deposition (AACVD) method.43

In this chapter we report the use of tris(O-alkylxanthato)iron(III) complexes as single source precursors to synthesise iron sulfide nanocrystals in oleylamine, hexadecylamine and octadecylamine and investigate the effect of different reaction conditions on the shape and size of the nanocrystals.

3.3 Precursor synthesis The tris(O-alkylxanthato)iron(III) complexes were prepared by adapting literature methods reported previously.5 A brief description of each synthesis is given below.

3.3.1 Synthesis of Fe(S2COMe)3 (1) Methanol (10 g, 312.5 mmol) and sodium hydroxide (12.50 g, 312.5mmol) were mixed with water (30 ml) in a beaker and stirred it for 2 hours. The mixture was cooled in an ice bath to 0 oC and carbon disulfide (23.56 g, 312.5mmol) in diethyl ether (20 ml) was added drop wise into the reaction mixture while vigorously stirring. The mixture was then allowed to return to room temperature.

A freshly prepared solution of iron chloride (FeCl3) (16.76 g, 103.3 mmol) in water was added drop wise to the reaction mixture at room temperature by dropping funnel. A black precipitate started to form which was filtered after the completion of reaction. The crude product was washed with water to give a pure product. Yield 10.8g (28%), m.pt. 253-255 oC, Elemental analysis: Found: C, 18.8; H, 2.9; S, 50.4; Fe, 13.5 %; Calc. C, 19.1; H, 2.4; S, 51.0; Fe, 14.8%. IR -1 (vmax /cm ): 2937(w), 1435(s), 1229(s), 1162(s), 1027 (s), 939(s), Mass (m/z):

355 (molecular ion peak), 270 (S2COCH3), 226 (Fe(S2COH3) and 164

(S2COCH3)2.

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3.3.2 Synthesis of Fe(S2COEt)3 (2) Ethanol (excess) and sodium hydride (5.22 g, 217.4 mmol) were mixed in a beaker and stirred for 2 hours. The mixture was cooled to about 0 °C in an ice bath while stirring. Carbon disulfide (16.55 g, 217.4 mmol) was then added to the reaction mixture while vigorously stirring. The mixture was allowed to warm at room temperature. A freshly prepared solution of iron chloride (FeCl3) (11.75 g, 72.43 mmol) in water (50 ml) was added drop wise to the reaction mixture by dropping funnel. A black precipitate started to form which was filtered after the completion of reaction. The crude product was washed with water and dried under vacuum. Yield 8.7g (83.4%), m.pt. 117-118 °C. Elemental Analysis: Found C, 23.6; H, 2.9; S, 43.5; Fe, 15.30. Calc. C, 25.8; H, -1 3.6; S, 45.8; Fe, 13.3. IR (vmax /cm ): 2979(w), 1742(s), 1440(s), 1366(s), 1237 (s), 1106(s), 922(s), 842(s), Mass (m/z):419 (molecular ion peak) 298

(S2COC2H5), 210 (FeS) and 270 (C2H5).

i 3.3.3 Synthesis of Fe(S2CO Pr)3 (3) Complex (3) was prepared using ipropanol by the same method as we used for compound (2). The crude product was washed with water. Yield 10.76 g (78%), m. pt. 56-57 oC. Elemental analysis: Found C, 31.83; H, 4.13; S, 43.52; Fe -1 8.12%, Calc. C, 31.23; H, 4.59; S, 41.69; Fe 12.10%. IR (vmax /cm ): 2970(w), 1738(s), 1455(s), 1354(s), 1258(s), 1097(s), 961(s), 864(s), Mass (m/z): 461

(molecular ion peak) and 219 (S2COC3H7)2.

i 3.3.4 Synthesis of Fe(S2CO Bu)3 (4) Complex (4) was prepared by the same method as compound (2) was prepared using ibutanol. The crude product was washed with water.Yield, 21.80g (77%), m. pt. 116-127 oC. Elemental analysis: Found C, 32.5; H, 4.9; S, 36.3; Fe, 11.8, -1 Calc. C, 35; H, 5.4; S, 38.2; Fe, 11.1 %. IR (vmax /cm ): 2979(w), 1738(s), 1448(s), 1348(s), 1264(s), 1144(s), 1080(s), 897(s), 793(s), Mass (m/z):503

(molecular ion peak) 447 (Fe), 298 (Fe(S2COC4H9)) and 242 (C4H9).

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3.4 Synthesis of iron sulfide nanocrystals In a typical reaction, 15 ml of oleylamine was refluxed under vacuum at 90 °C for 30 minutes before being purged in nitrogen gas for 30 minutes. The precursor (0.5g, 1.32mmol) was then added into hot oleylamine, and the reaction temperature slowly increased to a desired point (170, 230 and 300 °C). The temperature was maintained for one hour and the mixture was allowed to cool to room temperature. An addition of 30 ml of methanol produced a black precipitate which was separated by centrifugation. The black residue was washed twice by methanol and redispersed in toluene or hexane for further characterisation.

3.5 Results and discussion The four complexes (1-4) are dark brown solids.The reason for the elemental analysis not matching 100% to the calculated values of the corresponding complexes may be due the decomposition of the complexes at lower temperatures to give black iron sulfide hence the dark colour of the samples. The elemental analysis could not be improved even after recrystallizing them several times. In fact the recrystallized samples showed analysis worse than the non-recrystallized samples.

3.5.1 The X-ray single crystal structure of [iPrOC(S)S-S(S) COiPr] The recrystallization of the complex tris(O-ipropylxanthato)iron(III) (3) from hot acetone gave a colorless crystalline product, which was identified as a dimerised ligand (Figure 3.1). The crystal structure (Figure 3.1) shows the fusion of two ligands by the formation of S-S (disulfide) linkage between S2 and S3. This type of sulfur bond linkage (sulfur bridge) is common in naturally occurring compounds. Disulfide bonds play an important role in the folding and stability of some proteins, usually proteins secreted to the extracellular medium. The disulfide bond length is (2.05 Å) is similar to that found in literature.44 The refinement data is given in Table 3.1

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Figure 3.1. X-ray single crystal structure of [iPrOC(S)S-S(S)COiPr], Selected Bond lengths (Å) and angles (°): C(4)-O(1) 1.309(2), C(4)-S(1) 1.632(2), C(4)-S(2) 1.761(2), C(5)-O(2) 1.312(2), C(5)-S(4) 1.636(2), C(5)-S(3) 1.766(2), C(6)-O(2) 1.482(2), S(2)- S(3) 2.0509(7), O(1)-C(4)-S(1) 129.53(16), O(1)-C(4)-S(2) 115.00(14), S(1)-C(4)-S(2) 115.43(12), O(2)-C(5)-S(4) 129.73(15), O(2)-C(5)-S(3) 114.39(14), S(4)-C(5)-S(3) 115.88(12).

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Table 3.1. Structural refinement data for [iPrOC(S)S-S(S)COiPr].

Empirical formula C8 H14 O2 S4

Formula weight 270.43

Crystal system, space group Monoclinic, P2(1)/c

Unit cell dimesnions a = 9.6106(10) Å, alpha = 90 deg.,

b = 12.0863(13) Å , beta = 110.558(2) deg.,

c = 12.000(13) Å gamma = 90 deg.

Volume 1305.1(2) A3

Z, Calculated density 4, 1.376 Mg/m3

Absorption coefficient 0.703 mm-1

Crystal size 0.30 x 0.22 x 0.20 mm

Theta range for data collection 2.26 to 28.25 deg.

Data / restraints / parameters 3053 / 0 / 131

Goodness-of-fit on F2 0.856

Final R indices [I>2sigma(I)] R1 = 0.0277, wR2 = 0.0758

R indices (all data) R1 = 0.0282, wR2 = 0.0763

Largest diff. peak and hole 0.570 and -0.160 e.A-3

Reflections collected / unique 25337 / 1987 [R(int) = 0.0437]

Limiting indices -12<=h<=12, -15<=k<=15, -12<=l<=15

3.5.2 Thermogravimetric analysis (TGA) TGA of all the complexes ((1)-(4)) show two step decomposition with a rapid weight loss between 92-162-230 °C for [Fe(S2COMe)3] (1), 104-189-247 °C for i [Fe(S2COEt)3] (2), 92-178-238 °C for [Fe(S2CO Pr)3] (3) and, 58-200-252°C for i [Fe(S2CO But.)3] (4) respectively.

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100

(4) 40 (1) (2)

residue(%) of Mass

20 (3)

0 0 100 200 300 400 500 600 Temperature (°C)

Figure 3.2. Thermogravimetric analysis of complexes [Fe(S2COMe)3] (1), i i [Fe(S2COEt)3] (2), [Fe(S2CO Pr)3] (3) and [Fe(S2CO Bu)3] (4) at heating rate of 10 °C/min. under nitrogen, flow rate 100 ml minute-1.

The solid decomposition residue amounts to 30.8 % for complex (1) which is close to the calculated value 31.8 % for FeS2. Also the solid decomposition residue amounts to 31 % for complex (2) which is also close to the calculated value (28.64%) for FeS2. Complex (3) gives 15% of residue which is slightly lower than the calculated value 19 % for FeS. The final residue amounts 48 % for complex (4) which is considerably less than the calculated value 59 % for

Fe3S4. The TGA results are shown in Figure 3.2.

3.5.3 Powder X-ray diffraction of iron sulfide nanocrystals

The p-XRD patterns of the nanocrystals obtained from [Fe(S2COMe)3] (1) at different growth temperatures are shown in Figure 3.3. The growth at 170 °C produced a pure greigite phase (Fe3S4), (ICDD NO: 00-016-0713) with very low intensity peaks (Figure 3.3 (a)).

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(311)

(400) (440) (220) (#) (511) (c) (#) (533)(444)

Intensity(a.u) (b) (a)

20 30 40 50 60 70 2-Theta(degree)

Figure 3.3. p-XRD pattern for predominantly greigite (Fe3S4) nanocrystals in oleylamine from [Fe(S2COMe)3] (1), at (a) 170 (b) 230 and (c) 300 °C. The diffraction peaks for (220), (311), (400), (511) and (440) planes of greigite (Fe3S4) are dominant. The symbol (#) shows the pyrrhotite (FeS) phase.

(311)

(220) (c) * # (400) * # (440) * (511) # * (b)

Intensity(a.u) (a)

20 30 40 50 60 70 2-Theta(degree)

Figure 3.4. p-XRD pattern for greigite (Fe3S4) nanocrystals in oleylamine from

[Fe(S2COEt)3] (2) at (a) 170 (b) 230 and (c) 300 °C. The diffraction peaks for (220),

(311), (400), (511) and (440) planes of greigite (Fe3S4) are dominant. The symbol * and # shows pyrite and pyrrhotite (FeS) phases respectively.

125

The p-XRD pattern of the nanocrystals obtained at temperatures of 230 and

300 °C from [Fe(S2COMe)3] (1), correspond to a mixture of the cubic greigite

(Fe3S4) (ICDD NO: 00-016-0713) and hexagonal pyrrhotite (FeS) phase (ICDD No: 00-002-1241) (Figure 3.3(b,c)).

The p-XRD patterns obtained for the samples from thermolysis of complex (2) at different temperatures are shown in Figure 3.4. At 170 °C (Figure 3.4 (a)) pure greigite (Fe3S4) was obtained but as the growth temperature was increased from 170 to 230, then to 300 °C, a mixture of different phases of iron sulfide was obtained. At 230 °C (Figure 3.4 (b)) greigite and pyrite was obtained. Whereas at the higher growth temperature 300 °C greigite, pyrite (*) and pyrrhotite (#) were obtained (Figure 3.4 (c)). This observation is consistent with the thermodynamic stability of mixtures of pyrite and pyrrhotite over greigite.9,43

The p-XRD (Figure 3.5 (a)) pattern of the product obtained from thermolysis of complex (3) at 170 °C showed no peak which indicates the amorphous nature of the material produced. Thermolysis at 230 °C produced almost pure phase greigite (Figure 3.5 (b)), so did the thermolysis at 300 °C (Figure 3.5 (c)), with an additional peak at 2-theta 44º corresponding to the marcasite (+) phase of iron sulfide.

(311)

(440) (400) (220) + (511) (c)

(b)

(a) Intensity(a.u)

20 30 40 50 60 70 2-Theta(degree)

Figure 3.5. p-XRD pattern for greigite (Fe3S4) nanocrystals in oleylamine from i [Fe(S2CO Pr)3] (3) at (a) 170 (b) 230 and (c) 300 °C. The peaks for (220), (311), (400),

(511) and (440) planes are of greigite (Fe3S4) phase. The symbol + shows marcasite phase. 126

(311)

* (220) (400) (440) * * # (511) (c) * # # * (533) (b)

Intensity(a.u)

(a)

20 30 40 50 60 2-Theta(degree)

Figure 3.6. p-XRD pattern for greigite (Fe3S4) nanocrystals in oleylamine from i [Fe(S2CO Bu)3] (4) at (a) 170 (b) 230 and (c) 300 °C. The peaks for (220), (311), (400),

(511) and (440) of planes are of greigite (Fe3S4) phase. The symbol * shows pyrite and # pyrrhotite (FeS) phases.

The complex (4) shows similar behaviour to the complex (2), giving the pure phase of greigite at a growth temperature of 170 and 230 °C (Figure 6 (a, b)) but at the growth temperature of 300 °C, a mixture of greigite, pyrite (*) and pyrrhotite (#) phases were obtained (Figure 3.6 (c)). The diffraction peaks (Figure 3.6 (c)) for (220), (311), (400), (422), (511) and (440) planes of greigite

(Fe3S4) were dominant as compared to other phases.

3.5.4 Transmission electron microscopy of iron sulfide nanocrystals TEM images of the iron sulfide nanocrystal from (1) in oleylamine at 230 and 300 °C are shown in Figure 3.7 (a,b). The nanocrystals obtained show mixed morphologies (polyhedra) at all temperatures. The inset in (Figure 3.7 (b)) shows a single hexagonal nanocrystal obtained at 300 °C from complex (1). The approximate size of the crystallites obtained from complex (1) in oleylamine ranging from 30 nm to 124 nm in length and 21 nm to 52 nm in width.

The iron sulfide nanocrystals from [Fe(S2COEt)3] (2) at 170 °C also show different sizes and shapes (Figure 3.8 (a). These nanocrystals are relatively small in size as compared to the nanocrystals obtained from complex (1). For the nanocrystal the size ranging from 14 nm to 75 nm in length and from 12 to 48 127

nm in width (approximately). HRTEM images (Figure 3.8 (b)) show clear lattice fringes with a d-spacing of 2.98 Å corresponding to (311) reflection plane of greigite phase.

Figure 3.7. TEM images of iron sulfide nanocrystals obtained from [Fe(S2COMe)3] (1), in oleylamine at (a) 230 °C and (b) 300 °C.

(a) (b)

50 nm 50 nm 5 nm

Figure 3.8. TEM images of iron sulfide nanoparticles obtained from thermolysis of

Figure 3.9 (a) shows the TEM image of iron sulfide nanocrystals obtained with mixed morphologies from (2) at 230 °C in oleylamine. Lattice fringes in HRTEM image (Figure 3.9 (b)) with d-spacing 5.72Å correspond to (111) plane of greigite phase. Almost similar shapes of nanocrystals are obtained at 300 °C (Figure 3.9 (c)). The SAED pattern (Figure 3.9 (d)) shows that the nanocrystals are highly crystalline material and single crystal in nature.

The TEM images of iron sulfide nanocrystals from complex (3) in oleylamine at 230 °C and 300 °C are shown in (Figure 3.10 (a, b and c). Relatively large size nanocrystals were produced by complex (3). The size of the nanocrystal ranges

128

from 50 nm to 200 nm in length and from 40 nm to 120 nm in width. Figure 3.10 (d) shows the lattice fringes of the crystallites with a d-spacing 5.72 Å corresponding to (111) reflection plane of greigite.

Figure 3.9. TEM images of iron sulfide nanocrystals obtained from [Fe(S2COEt)3] (2) in oleylamine at (a, b) 230 °C and (c) 300 °C. The insets in (c) show different shape of nanocrystals at 300 °C, whereas (d) shows the SAED pattern of the nanocrystals.

i Figure 3.10. TEM images of iron sulfide nanocrystals obtained from [Fe(S2CO Pr)3] (3), in oleylamine at (a, b) 230 °C, (c) 300 °C and (d) showing lattice fringes with a d- spacing of 5.72 Å corresponding to the reflection of (111) of greigite phase.

129

Figure 3.11. shows the TEM images of iron sulfide nanocrystals obtained from i [Fe(S2CO Bu)3] (4), in oleylamine (a) at 170 °C , (b) 230 °C (c) HRTEM at 230 °C showing lattice fringes with a d-spacing of 3.50 (Å) corresponding to the (220) reflection plane of greigite phase (d) TEM image of nanocrystals produced at 300 °C.

Figure 3.11 (a, b) shows the TEM images of nanocrystals grown at at 170 and 230 °C from complex (4). The HRTEM (Figure 3.11 (c)) at 230 °C showing lattice fringes with a d-spacing of 3.50 Å corresponding to the (220) reflection of greigite phase. TEM image of nanocrystals at 300 °C (Figure 3.11 (d)) shows the different shapes (sheets and pyramids) of crystallites. At lower temperature of 170 °C Complex (4) produced large size (139 nm in length and 65 nm in width) crystallites (Figure 8 (a)). Small size crystallites were produced at higher temperature of 300 °C (Figure 8 (d)). For these nanocrystals, size ranging from 25 nm to 75 nm in length and 12 to 38 in width. The broad peaks in p-XRD pattern at higher temperature also support the evidence of small size crystallites.

3.5.5 The effect of capping agents Complexes (1-4) were also thermolysed in hexadecylamine and octadecylamine at 230 °C to see if the capping agent had any effect on the nature of the growth of iron sulfide phases. Significant differences were noticed from the thermolysis of the complexes (1-4) in hexadecylamine at 230 °C for 1 hour. 130

The diffraction pattern for complexes (1) and (2) shows peaks for (111), (200), (210), (211), (220) and (311) planes corresponding to pure pyrite phase (Figure 3.12 (a and b) (ICDD No. 00-042-1340)) at 230 °C in hexadecylamine. Thermolysis of complex (3) in hexadecylamine at 230 °C produced pure greigite (ICDD No. 00-016-0713) phase (Figure 12 (c)) whereas the complex (4) produced pure pyrrhotite phase (ICDD No.00-002-1241) (Figure 3.12 (d)).

(100) (101) (102) (110) (d)

(311) (400) (440)

(511) (c)

(200) (210) (111) (211) (220) (311)

(b) Intensity (a.u) (a)

25 30 35 40 45 50 55 60

2-Theta(degree)

Figure 3.12. p-XRD pattern from complexes (1-4) in hexadecylamine at 230 °C for 1hour reaction time, (a) and (b) show pattern for pyrite phase obtained from complexes (1-2), (c) shows the pattern for greigite phase obtained from complex (3) and (d) shows a pattern for pyrrhotite phase obtained from complex (4).

TEM images of the nanocrystals obtained from complex (1) in hexadecylamine at 230 °C show a mixture of irregular and rectangular crystallites (inset Figure 3.13 (a)). The complex (2) in hexadecylamine at 230 °C in 1 hour reaction time produced plate like crystallites (Figure 3.13 (b)). The insets in (Figure 3.13 (b)) show a sheet like crystallites. Interesting morphology was observed from the nanocrystals obtained from complex (3) in hexadecylamine at 230 °C in 1 hour reaction time. The (Figure 3.13 (c)) shows a sheets, cubes and trigonal shape crystallites. Sheets and cube like crystallites (Figure 3.13 (d)) were obtained

131

from the thromolysis of complex (4) in hexadecylamine at 230 °C in 1 hour reaction time.

Figure 3.13. Nanocrystals obtained from (a) complex (1) in hexadecylamin at 230 °C, insets shows individual nanocrystals, (b) TEM images from complex (2) in hexadecylamine, insets shows sheets like crystallites, (c) shows the crystallites with different morphology obtained from complex (3) in hexadecylamine at 230 °C and (d) shows nanocrystals obtained from complex (4) in hexadecylamine at 230 °C.

(100) (101) (102)

(110) (311) (d) (*) (400) (511) (440) (c) (220) (311)(*) (#) (*) (*) (#) (511) (440) (b) Intensity(a.u) (311) (220) (*) (400) (511) (440) (a)

20 25 30 35 40 45 50 55 60

2-Theta(degree)

Figure 3.14. p-XRD (a) from complex (1), (b) from complex (2), (c) from complex (3) and (d) from complex (4) in octadecylamine at 230 °C after 1 hour. The (*) shows the pyrite phase and (#) shows pyrrhotite phase.

The p-XRD pattern of the nanocrystals produced from the thermolysis of complexes (1-4) in octadecylamine at 230 °C show that the complex (1) 132

produced predominantly greigite phase (ICDD No. 00-016-0713) (Figure 3.14 (a)) with traces of pyrite (*). Complex (2) produced a mixture of greigite, pyrite and pyrrhotite phases (Figure 3.14 (b)). Complex (3) showed (Figure 3.14 (c)) similar behaviour to complex (1) whereas complex (4) produced a pure pyrrhotite phase (ICDD No. 00-002-1241)

Figure 3.15. TEM images of the nanocrystals obtained from complex (1-4) in octadecylamine at 230 °C after 1 hour (a) from (1), (b) from (2), (c) from (3) and (d) from (4).

The TEM images of the nanocrystals obtained from complexes (1-4) in octadecylamine after 1 hour reaction time (Figure 3.15 (a-d)) show no well- defined shapes of the crystallites. Law et al.39 reported that longer growth times in octadecylamine produced pure phase material but our experiments for 3 hour growth time showed no change in the phase of material. The only difference observed with extending reaction times was the observation of larger crystallites and hence the sharper p-XRD peaks (Figure 3.16(a-d)). TEM images in Figure 3.17 show the larger size of crystallites as compared to those produced after 1 hour. Table 3.2 shows the summary of the different phases of iron sulfide

133

nanocrystals produced from all four complexes under various reaction conditions.

(102) (100) (101)

(110) (d) (*) (*) (c) (*) (311) (*) (#) (*) (#) (511) (440) (*)

(b) Intensity(a.u) (311) (220) (*) (400) (511) (440) (*) (a)

20 25 30 35 40 45 50 55 60 2-Theta (degree)

Figure 3.16. p-XRD (a) from complex (1), (b) from complex (2), (c) from complex (3) and (d) from complex (4) in octadecylamine at 230 °C in 3 hours reaction time. The (*) shows pyrite and (#) shows pyrrhotite phase.

Figure 3.17. Nanocrystals obtained from (a) complex (2) in octadecylamine after 3 hours at 230 °C, (b) shows the lattice fringes correspond to greigite phase obtained from complex (2), whereas (c) shows nanocrystals from complex (3) in octadecylamine at 230 °C in 3 hours reaction time.

134

Table 3.2 Summary of phases and morphology of iron sulfide nanocrystals obtained from all four precursors at different reaction conditions.

Complex Capping Temp. (°C) Phase Morphology of agent crystallites

[Fe(S2COMe)3] oleylamine 170 greigite hexagonal, cubic oleylamine 230 greigite + pyrrhotite and trigonal oleylamine 300 greigite + pyrrhotite hexadecyla- 230 pyrite rectangular mine octadecyla- 230 greigite + pyrite irregular mine

[Fe(S2COEt)3] oleylamine 170 greigite hexagonal and oleylamine 230 greigite + pyrite trigonal oleylamine 300 greigite + pyrite + pyrrhotite hexadecyla- 230 pyrite plates mine octadecyla- 230 greigite + pyrite + irregular mine pyrrhotite i [Fe(S2CO Pr)3] oleylamine 170 amorphous material hexagonal, cubic oleylamine 230 greigite and trigonal oleylamine 300 greigite + marcasite sheets, cubes, hexadecyla- 230 greigite and trigonal mine octadecyla- 230 greigite + pyrite irregular mine i [Fe(S2CO Bu)3] oleylamine 170 greigite sheets, cubes, oleylamine 230 greigite hexagonal and oleylamine 300 greigite + pyrite + trigonal pyrrhotite hexadecyla- 230 pyrrhotite sheets and cubes mine octadecyla- 230 pyrrhotite irregular mine

135

3.5.6 The magnetic properties of iron sulfide nanocrystals Changes in magnetisation across the samples were measured by SQUID magnetometry and are illustrated in Figures 3.18–3.20. Magnetic characterisation reveals that all iron sulfide nanocrystals prepared from the thermolysis of complexes (1), (3) and (4) at various temperatures show room temperature magnetic hystersis loops with magnetization well saturated under applied magnetic fields of 10-15 kOe (1 Oe = 10-4 T).

It is known that magnetic properties of nanocrystals are governed by the anisotropy energy that constrains the nanocrystals magnetisation to align along a specific direction known as the easy axis. The temperature above which thermal energy inside a magnetic system is high enough to enable the free alignment of the magnetisation in arbitrary directions is called the blocking temperature (TB), and is typically obtained from peaks in the zero-field cooled (ZFC) and field- cooled (FC) magnetisation curves recorded at very small magnetic fields. The blocking temperature is graphically determined to be the point at which the gradient of the ZFC curve approaches zero. It can be seen that no peaks can be discerned in the ZFC/FC magnetisation curves recorded at 100 Oe for nanocrystals generated from (1), (3) and (4) (Figures 3.17 (a), 3.18 (a) and 3.19 (a)), suggesting that the blocking temperature of these nanocrystals is well above the room temperature. Indeed, the field dependence of the magnetisation reveals hysteresis loops at 5, 50 and 300 K (Figures 3.18 (b), 3.19 (b) and 3.20 (b)) for all nanocrystals, suggesting that all of them are already in their blocking regime at room temperature. In the case of the iron sulfide nanocrystals obtained by thermolysis of complex (1) in oleylamine at 230 °C, a magnetic hystersis curve with a coercive field (Hc) (i.e the magnetic field needed for the magnetization to return to zero) of ca. 230 Oe at 300 K, increasing to 520 Oe at

50 K and to 590 Oe at 5 K, and a remnant magnetization (Mr) (i.e. the magnetization retained by nanocrystals when the magnetic field is switched off) of 7.85 emu g-1 at 300 K, 12.53 emu g-1 at 50 K and 13.39 emu g-1 at 5 K is obtained. This result is in contrast with our previous results on nanocrystals of 39 Fe3S4 and Fe7S8, which shows super paramagnetism at room temperature.

136

(a)

(b)

Figure 3.18. (a) ZCF and FC (b) at 5 K, 50 K and at 300 K from iron sulfide (greigite phase dominant with pyrrhotite phase) nanocrystals obtained from [Fe(S2COMe)3] (1) in oleylamine at 230 °C.

The remanent magnetisation of 7.85 emu g-1 at room temperature, is significantly larger than that reported in literature for Fe1-xS (pyrrhotite) nanopillars (2.5 emu g-1 at 300 K)5, and slightly larger than our reported value of Mr = 6.5 emu g-1 (300 K).39 This difference may originate from the difference in the shape of the nanocrystals and small variation in their FeS content.

137

(a)

(b)

Figure 3.19. (a) ZFC/FC magnetisation at 100 Oe applied field (b) Field dependence of the magnetisation at 5, 50 and 300 K from iron sulfide (greigite phase) obtained from i the complex [Fe(S2CO Pr)3] (3) at 300 °C.

34 Comparison to pyrrhotite Fe7S8 and greigite Fe3S4 nanosheets prepared from

Fe(S2CNEt2)2(Phen) and Fe(S2CNEt2)3 in oleylamine shows that the nanocrystals reported here have a significantly larger magnetisation saturation -1 -1 -1 (MS = 35.8 emu g at 300 K; 37.6 emu.g at 5 K) as compared to 18.9 emu g for the former. The remanence-to-saturation ratio R = Mr/Ms (R = 0.22-0.36) is also larger, suggesting an either larger anisotropy energy or a higher degree of crystallinity.

We have also investigated the magnetic behaviour at different temperature (5, 50 and 300 K) of the iron sulfide nanocrystals obtained from thermolysis of

138

complex (3) in oleylamine at 230 °C (Figure. 3.19 (b)). The iron sulfide nanocrystals grown at 230 °C from complex (3) shows magnetic hystersis loops with Ms = 34.46 (at 5 K), 34.52 (at 50 K) and 32.44 emu g-1 (at 300 K), respectively.

The remanence to saturation ratio (R = Mr/Ms = 0.43 (14.78/34.46) , 0.40 (13.9 / 34.52) and 0.30 (9.65 / 32.44) at 5, 50 and 300 K, while the coercive field increases on cooling from 334 Oe at 300 K to 680 Oe at 50 K and 770 Oe at 5 K. Improved magnetic properties were also obtained for nanocrystals obtained from complex (4) in oleylamine at 230 °C (Figure 3.20 (b)).

(a)

(b)

Figure 3.20. (a) ZFC and cold field magnetization curves (b) Room temperature magnetic hysteresis loops of iron sulfide (greigite phase) nanocrystals at 5K, 50 K and i 300K obtained from [Fe(S2CO But.)3] (4) in oleylamine at 230 °C. 139

Their magnetization tends to saturate under an applied magnetic field of 10 kOe or so (Ms = 34.40 (at 5 K), 34.27 (at 50 K) and 32.20 (at 300 K). Cycling the sample between -40 and 40 kOe magnetic field gives rise to a hysteresis loop at all temperatures, with a coercive field of 580, 450 and 180 Oe, and a remanent magnetisation of 12.1 (R = 0.35), 10.3 (R = 0.30) and 6.0 emu g-1 (R = 0.19), for temperature of 5, 50 and 300 K respectively.

3.6 Conclusion Tris(O-alkylxanthato)iron(III) complexes were used as a single source precursor in oleylamine for the synthesis of iron sulfide nanocrystals at different deposition temperature. The p-XRD of the nanocrystals obtained at 230 and

300 °C from [Fe(S2COMe)3] (1) correspond to a mixture of the cubic greigite

(Fe3S4) and hexgonal pyrrhotite (FeS). The p-XRD of the nanocrystals obtained for the samples from thermolysis of complex [Fe(S2COEt)3] (2) at different temperatures shows that at growth temperature of 170 °C pure phase of cubic greigite (Fe3S4) was obtained but as the growth temperature increased from 170 to 230 to 300 °C, the mixture of different phases of iron sulfide obtained. At 230 °C greigite and pyrite phases were obtained. At the higher growth temperature of 300 °C greigite, pyrite and pyrrhotite phases were obtained. The i complex [Fe(S2CO Pr)3] (3) gave no product at lower temperature (170 °C) and gave pure greigite at 230 °C. At 300 °C greigite dominates. The complex i [Fe(S2CO Bu)3] (4) shows similar behaviour to the complex (2) giving the pure greigite at 170 and 230 °C but at temperature 300 °C gave the mixed phases of gregite, pyrite and pyrrhotite.

Significant differences were noticed in the phase of iron sulfide nanocrystals by changing the capping agent. When hexadecylamine was used as capping agent, complexes (1) and (2) gave the pure pyrite phase at 230 °C whereas complex (3) gave pure greigite phase and complex (4) gave phase pure pyrrhotite under similar reaction conditions. Thermolysis of complexes (1) and (3) in octadecylamine produced greigite as a main product with the traces of pyrite phase at 230 °C. Complex (2) produced a mixture of greigite, pyrite and pyrrhotite and complex (4) gave pure pyrrhotite phase. 140

The nanocrystals obtained by thermolysis of complexes showed random shapes with a wide range of size distribution as reported previously.42 Large size (size ranging from 14 to 139 nm in length and 12 to 65 nm in width) nanocrystals were synthesised in oleylamine whereas smaller nanocrystals 12 to 31 nm length 7 to 26 nm width) were obtained from hexadecylamine. The random shaped nanocrystals tend to be less stable than those with the spherical shape as observed previously.45

3.7 References 1. J. Park, K. An, Y. Hwang, J. G. Park, H. J. Noh, J. Y. Kim, J. H. Park, N. M. Hwang and T. Hyeon, Nat. Mater., 2004, 3, 891.

2. Y. Hou, Z. Xu and S. Sun, Angew. Chem., Int. Ed., 2007, 46, 6329.

3. B. H. Zeng and S. Sun, Adv. Funct. Mater., 2008, 18, 391.

4. Y. W. Hun and S. J. W. Cheon, Acc. Chem. Res., 2008, 41, 179.

5. S. Sun, C. B. Murray, D. Weller, L. Folks, A. Moser, Science, 2000, 287, 1989.

6. S. Cheong, P. Ferguson, K. W. Feindel, I. F. Hermans, P. T. Callaghan, C. Meyer, A. Slocombe, C. H. Su, F. Y. Cheng, C. S. Yeh, B. Ingham, M. F. Toney, R. D. Tilley, Angew. Chem. Int. Ed., 2011, 50, 4206.

7. D. A, J. Herman, P. Ferguson, S. Cheong, I. F. Hermans, B. J. Ruck, K. M. Allan, S. Prabakar, J. L. Spencer, C. D. Lendrum, R. D. Tilley, Chem. Commun., 2011, 47, 9221.

8. J. M. Soon, L. Y. Goh, K. P. Loh, Y. L. Foo, L. Ming, J. Ding, Appl. Phys. Lett., 2007, 91, 084105.

9. D. J.Vaughan, A. R. Lennie, Sci. Prog. (Edinburgh), 1991, 75, 371.

10 J. B. Goodenough, Mater. Res. Bull.,1978, 13, 1305.

11. C. N. R. Rao, K. P. R Pisharody. Prog. Solid State Chem., 1975, 10, 207.

141

12. V. Bessergenev J. Phys.: Condens. Matter, 2004, 16, S531.

13. J. R. Gosselin, M. G. Townsend, R. J. Tremblay, Solid State Commun., 1976, 19, 799.

14. S. Kar and S. Chaushuri, Mater. Lett.,2005, 59, 289.

15. H. Nakazawa and N. Morimoto, Mater. Res. Bull., 1971, 6, 345.

16. J. L. Horwood, M. G. Townsend, A. H. Webster, J. Solid State Chem., 1976, 17, 35.

17. F. Li and F. Franzen, J. Solid State Chem. 1996, 126, 108.

18. M. J. Almond, H. Redman, D. A. Rice, J. Mater. Chem. 2002, 10, 2842.

19. M. Nath, A. Choudhury, A. Kundu, C. N. R. Rao, Adv. Mater. Weinheim, Ger. 2000, 15, 2098.

20. X. Ma, F. Xu, X. Wang, Y. Du, L. Chen, Z. Zhang, J. Cryst. Growt., 2005, 277, 314.

21. D. M. Schleigh, H. S. W. Chang, J. Cryst. Growth, 1991, 112, 737.

22. B.Thomas, C. Hoepfner, K. Ellmer, S. Fiechter, H. Tributsch, J. Cryst. Growth, 1995, 146, 630.

23. B. Thomas, T. Cibik, C Hoepfner, D. Diesner., G. Ehlers, S. Fiechter, K. Ellmer, J. Mater. Sci. 1998, 9, 61.

24. B. Meester, L. Reijnen, A. Goossens, J. Schoonman, J. Phys.(Paris), 1999, 9, Pr8–613.

25. G. Smestad, E. Ennaoui, S. Fiechter, H. Tributsch , W. K. Hofman, M. Birkholz, Sol. Energy Mater. 1990, 20, 149.

26. B. Ouertani, J. Ouerfelli,, M. Saadoun, B. Bessais, H. Ezzaouia, Bernede, J. C. Mater. Charact., 2005, 54, 431.

142

27. M. Birkholz, D. Lichtenberger, C. Hoepfner, S. Fiechter, Sol. Energy Mater. Sol. Cells, 1992, 27, 243.

28. S. Bausch, B. Sailer, H. Keppner, G. Willeke, E. Bucher, G. Frommeyer, Appl. Phys. Lett., 1990, 25, 57.

29. I. J. Ferrer, C. Sanchez, J. Appl. Phys., 1991, 70, 2641.

30. B. Rezig, H. Dalma, M. Kanzai, Renewable Energy, 1992, 2, 125.

31. A. Ennaoui, G. Schlichtlorel, S. Fiechter, H. Tributsch, Sol. Energy Mater. Sol. Cells,1992, 25, 169.

32. G. Smestad, A. Da Silva, H.Tributsch, S. Fiechter, M. Kunst, N. Meziani, M. Birkholz, Sol. Energy Mater., 1989, 18, 299.

33. B. Meester, L. Reijnen, A. Goossens, J. Schoonman, Chem. Vap. Deposition, 2000, 6, 121.

34. P. O’Brien, D. J. Otway, J. Park, H. Mater. Res. Soc. Symp. Proc., 2000, 606, 133.

35. S. G. Shyu, J. S Wu, C. C. Wu, S. H. Chuang, K. M. Chi, Inorg.Chim.Acta, 2002, 334, 276.

36. P. V. Vanitha, P. O’Brien, J. Am. Chem. Soc., 2008, 130, 17256.

37. W. Han, M. Gao, j. Cryst. Growth Des., 2008, 8, 1023.

38. K. Ramasamy, M. A. Malik, M. Helliwell, F. Tuna, P. O’Brien, Inorg. Chem., 2010, 49(18), 8495.

39. J. Puthussery, S. Seefeld, N. Berry, M. Gibbs, M. Law, J. Am. Chem. Soc., 2011, 133, 716.

40. W. Li , M. Doblinger , A. Vaneski , A. L. Rogach , F. Jackel, J. Feldmann , J. Mater. Chem., 2011, 21, 17946.

41. M. Akhtar, J. Akhtar, M. A. Malik, P. O’Brien, F. Tuna, J. Raftery, M. Helliwell, J. Mater. Chem., 2011, 21(26), 9737. 143

42. A. L. Abdelhady, M. A. Malik, P. O’Brien, F. Tuna, J. Phys. Chem., C, 2012, 116 (3), 2253.

43. M. Akhtar, A. L. Abdelhady, M. A. Malik, P. O'Brien, J. Cryst. Growt., 2012, 346, 106.

44. C. S. Sevier, C. A. Kaiser, Nat. Rev. Molecular and Cellular Biology, 2002, 3 (11), 836.

45. X. Peng, Adv. Mater. 2003, 15, 459.

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Chapter 4

Deposition of iron selenide nanocrystals and thin films from tris(N,N-diethyl-N’- naphthoylselenoureato)iron(III)

145

4.1 Summary The complex tris(N,N-diethyl-N’-naphthoylselenoureato)iron(III) was synthesised and its X-ray single crystal structure determined. Thermolysis of this complex in oleylamine and dodecanthiol at 190, 240 and 290 °C temperatures produced nanocrystals of FeSe2 whereas Aerosol Assisted Chemical Vapour Deposition (AACVD) produced FeSe thin films onto silicon substrates. The nanocrystals and thin films produced were characterised by powder X-ray Diffraction (p-XRD), Scanning Electron Microscopy (SEM), Atomic Force microscopy (AFM), and Transmission Electron Microscopy (TEM).

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4.2 Introduction Iron selenides can be semiconductors, or superconductors with the characteristics of ferro/ferrimagnetic metals.1,2 Iron selenide has attractive conductive, optical, electronic, and magnetic properties with a direct band gap of 1.23 eV.3-5 The conductive properties of iron selenides depend on the composition of the material. Only a few iron selenides are known so far. FeSe crystallises in tetragonal and hexagonal forms, whilst FeSe2 crystallises in cubic and orthorhombic (marcasite-type) structures. The properties of bulk iron 6,7 selenide crystals have been reported for Fe3Se4 and Fe7Se8 phases. Iron selenides has a similar phase diagram to the NiAs system. The structure depends on the composition of Se, which can range from 51 to 59 % at room temperature. These structures may be hexagonal, NiAs-like structures with composition

Fe7Se8 (H-phase), and as a monoclinic structure of the same phase with 8-10 composition Fe3Se4 (M-phase).

Iron selenide has been synthesised by the reaction of aqueous metal salt 11,12 13,14 solutions with H2Se gas and by mechanical alloying. Iron selenide (H and M phase) nanocrystals have been prepared from FeCl2 and Se powder in oleylamine at 200 °C.10 The difference in melting points of iron (1535 °C) and selenium (217 °C) make it difficult to obtain the required stoichiometries.

Tetragonal β-FeSex nanoflakes were also synthesised from FeCl2 (in olylamine) 15 16 and TOP-Se with oleic acid at 320 °C. Nanoplates of β-FeSex been synthesised from FeCl2 and Se powder in glycol. Polyhedra, Fe7Se8 and Fe7Se8 17 nanorods were prepared from FeSO4.7H2O and Na2SeO3 in a mixture of diethylenetriamine and deionized water by a solvothermal method. Magnetic measurements showed that these nanocrystals are strongly coupled.

In this chapter we report the synthesis of iron selenide nanocrystals by thermolysis in oleylamine, dodecanthiol and also in the mixture of oleylamine and dodecanthiol. Thin films were also deposited by Aerosol Assisted Chemical Vapour Deposition (AACVD) using tris(N,N-diethyl-N’-naphthoylselenoureato)iron(III) as single source precursor. To our knowledge this is the first

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time a metal-organic precursor has been used for the synthesis of iron selenide nanocrystals or thin films.

4.3 Experimental All synthesis was performed under an inert atmosphere using standard Schlenk techniques. All reagents were purchased from Sigma-Aldrich and used as received. Solvents were distilled prior to use.

4.3.1 Synthesis of ligand [(C10H7CONHCSeN(C2H5)2)] The ligand was prepared by a modification of the method reported in the literature.18,19 Briefly an acetone solution of napthoyl chloride (4.61g, 24.2 mmol in 30 ml) was added into an acetone solution of KSeCN (3.5g, 24.2 mmol in 30 ml) in a 250 ml two neck flask at room temperature under nitrogen. The color of the contents in the flask changed from white to greenish-yellow. The mixture was stirred for 5 minutes to ensure completion of the reaction. Addition of a di-ethylamine (2.5 ml, 24.2 mmol in 10 ml acetone) solution into the mixture resulted in change of color from green to orange red. The reaction mixture was stirred for another 5 minutes at room temperature and then 50 ml of anhydrous ether was added and the flask was left undisturbed for 5 minutes. The

[C10H7CONHCSeN(C2H5)2] was extracted into ether layer which was separated by a separating funnel. The ether solution was evaporated and gave red solid residue which was re-crystallized from ethanol to give yellow cubic crystals of

[C10H7CONHCSeN(C2H5)2]. Yield, 33%; Elemental analysis: Found: C, 57.60, H, 5.31; N, 8.29 Calc: C, 56.97; H, 5.44; N, 8.40 % IR: 3023 υ(C–H), 1753 -1 1 υ(C=O), 1198 υ(C=Se), 1361 υ(C-N) cm . H NMR (δ CDCl3, 400 MHz) (400 MHz; CDCl3):1.42 (t, 3H), 1.30 (t, 3H), 3.6 (q, 2H), 4.18 (q, 2H), 7.85(m, Ar- 4H), 7.56 (p, Ar-3H), 8.5 (s, NH), Mass (m/z): 333 (100%).

4.3.2 Synthesis of [Fe(C10H7CONCSeN(C2H5)2)3]

The ligand [(C10H7CONHCSeN(C2H5)2)] (2g, 6 mmol) was dissolved in ethanol (50 ml) and warmed on rotary evaporator until dissolution. The solution of iron(III) acetate (0.323g, 1.6 mmol) in 30 ml distilled water was added to the hot solution by dropping funnel. Then a solution of sodium acetate (0.323g, 2.3) 148

mmol in 5 ml distilled water was added to the above solution and stirred for 1 hour. After 1 hour 100 ml of distilled water was added to the reaction and the reaction stored in the fridge for 1 hour. The solid product was separated by filtration and washed with an excess of water and recrystallized a mixture of toluene and Hexane. Yield: 3.26 g, mpt: 226 °C. Elemental analysis: Found: C,

54.5; H, 4.8; N, 7.8; Fe, 4.4, Calc: C, 54.6; H, 5.1; N, 7.9; Fe, 5.2 %. IR (vmax /cm-1): 1628(s), 1486(s), 1407(s), 1352(s), 1188(s), 1076(s), Mass (m/z): 444, 381, 190, and 158.

4.3.3 Synthesis of iron selenide nanocrystals Iron selenide nanocrystals were obtained by the thermolysis of

[Fe(C10H7CONCSeN-(C2H5)2)3] in oleylamine and dodecanthiol. In a typical experiment 10 ml of oleylamine in a three necked round bottom flask was heated at 90 °C for 10 minutes under vacuum and then purged with nitrogen gas for 5 minutes. 0.3g of [Fe(C10H7CONCSeN(C2H5)2)3] complex was added directly in to the oleylamine and the reaction temperature was slowly increased to 290 °C. After maintaining this temperature for 5 minutes the heating was stopped. The mixture was allowed to cool at room temperature. Addition of 20 ml acetone produced a black precipitate which was centrifuged, washed twice with acetone to remove any excess ligand. The black nanocrystals were suspended in toluene for further investigation.

4.3.4 Deposition of thin films by Aerosol Assisted Chemicl Vapour Deposition (AAACVD) method In a typical deposition, 0.25g (0.23 mmol) of the precursor

([Fe(C10H7CONCSeN(C2H5)2)3]) was dissolved in 20 ml of THF in a two- necked 100 ml round-bottomed flask. This flask was connected with a gas inlet that allowed the carrier gas (argon) to pass into the solution to aid the transport of aerosol. This flask was also connected to the reactor tube by a piece of reinforced tubing. The argon flow rate was controlled by a Platon flow gauge. Five silicon substrates were placed inside the reactor tube, which is placed in a Carbolite furnace. The precursor solution in a round-bottomed flask was kept in a water bath above the piezoelectric modulator of a Pifco ultrasonic humidifier 149

(model 1077). The aerosol droplets of the precursor generated were transferred into the hot wall zone of the reactor by a carrier gas. Both the solvent and precursor was evaporated, and the precursor vapor reached the heated substrate surface where only a very thin film was deposited at 625 °C. There was no deposition at lower or higher temperatures.

4.4 Results and discussion

4.4.1 X–ray single crystal structure of [Fe(C10H7CONCSeN(C2H5)2)3] Single-crystal X-ray structure shows that there are two molecules in the asymmetric unit, together with a toluene solvent molecule (Figure 4.1).

The structure contains two independent molecules, each is a tris chelate facial isomer; but there are significant differences in the conformation of the ethyl group. Both are characterized by {Se3, O3} coordination of the iron(III) centre. Each molecule is paired with its enantiomorph conforming to the centrosymmetric P1- space group.

The bond lengths of Fe-O range from 1.987(8) to 2.032(8) Å. The Fe-Se bond lengths range from 2.518(2) to 2.555(2) Å, which are larger than those reported for Fe-S, as expected due to the larger ionic radius of selenium as compared to sulfur.20 The single-crystal X-ray structural (CCDC reference number 776130) refinement data are given in Table 4.1.

150

(a)

(b)

Figure 4.1. The X-ray single crystal structure of [Fe(C10H7CONCSeN(C2H5)2)3] showing molecules 1 (a) and 2 (b). Selected bond lengths (Å) and bond angles (°); Molecule 1: Fe(1)- O(3) 1.987(8), Fe(1)- O(2) 1.998(8), Fe(1)- O(1) 2.016(8), Fe(1)- Se(3) 2.524(2), Fe(1)- Se(1) 2.540(2), Fe(1)- Se(2) 2.543(2), O3-Fe1-O2 88.0(3), O3- Fe1-O1 93.5(3), O2-Fe1-O1 86.4(3), O3-Fe1-Se3 86.3(2), O2-Fe1-Se3 100.3(2), O1- Fe1-Se3 173.3(2), O3-Fe1-Se1 95.9(2), O2-Fe1-Se1 171.8(2), O1-Fe1-Se1 86.2(2), Se3-Fe1-Se1 87.15(7), O3-Fe1-Se2 166.9(2), O2-Fe1-Se2 86.9(2), O1-Fe1-Se2 98.2(2), Se3-Fe1-Se2 82.76(6), Se1-Fe1-Se2 90.67(6). Molecule 2: Fe(2)-O(6) 1.992(8),Fe(2)- O(4) 2.012(8), Fe(2)-O(5) 2.032(8), Fe(2)-Se(6) 2.518(2), Fe(2)-Se(5), 2.530(2), Fe(2)- Se(4) 2.555(2), O(6)-Fe(2)-O(4) 88.3(3), O(6)-Fe(2)-O(5) 95.1(3), O(4)-Fe(2)-O(5) 85.3(3), O(6)-Fe(2)-Se(6) 85.8(2), O(4)-Fe(2)-Se(6) 101.3(2), O(5)-Fe(2)-Se(6) 173.3(2), O(6)-Fe(2)-Se(5) 96.0(2O(4)-Fe(2)-Se(5) 171.2(2), O(5)-Fe(2)-Se(5) 86.6(2), Se(6)-Fe(2)-Se(5) 86.75(7), O(6)-Fe(2)-Se(4) 166.3(3); O(4)-Fe(2)-Se(4) 86.6(2), O(5)- Fe(2)-Se(4) 97.2(2), Se(6)-Fe(2)-Se(4) 82.71(6), Se(5)-Fe(2)-Se(4) 90.88(6).

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Table 4.1 Crystal data for [Fe(C10H7CONCSeN(C2H5)2)3.0.5(C7H8)]

Empirical formula C51.50 H55 Fe N6 O3 Se3 Formula weight 1098.74

Crystal system, space group Triclinic, P-1

Unit cell dimesnions a = 10.242(5) Å alpha = 84.455(7) deg. b = 19.254(9) Å beta = 79.483(8) deg. c = 24.700(12) Å gamma = 89.850(7) deg.

Volume 4766(4) A3

Z, Calculated density 4, 1.531 Mg/m3

Absorption coefficient 2.657 mm-1

Crystal size 0.55 x 0.25 x 0.10 mm

Theta range for data collection 2.04 to 25.03 deg.

Data / restraints / parameters 16578 / 1125 / 1176

Goodness-of-fit on F^2 1.115

Final R indices [I>2sigma(I)] R1 = 0.1038, wR2 = 0.2402

R indices (all data) R1 = 0.1218, wR2 = 0.2490

Largest diff. peak and hole 5.064 and -3.636 e.A-3

Reflections collected / unique 16578 / 16578 [R(int) = 0.0000]

Limiting indices -12<=h<=12, -22<=k<=22, -29<=l<=29

4.4.2 Thermogravimetric analysis of [Fe(C10H7CONCSeN(C2H5)2)3] Thermogravimetric analysis of the complex shows two steps decomposition (Figure 4.2) with rapid weight loss observed between the temperature ranges 160-260 °C and 260-400 °C. The final residue value (8.6%) is considerably smaller to that of the calculated value (12.78%) for FeSe.

152

100

Mass of residue(%) of Mass 20

0 100 200 300 400 500 600 700 Temperature (oC)

Figure 4.2. Thermogravimetric analysis of [Fe(C10H7CONHCSeN(C2H5)2)3] at heating rate of 10˚C /min under nitrogen, flow rate of nitrogen 100 ml/minute.

4.4.3 Powder X-ray diffraction for iron selenide (FeSe2) nanocrystals

The p-XRD pattern of the FeSe2 nanocrystals obtained from the thermolysis of

[Fe(C10H7CONCSeN(C2H5)2)3] in oleylamine at 190, 240 and 290 °C (Figure

4.3 (a- c)) growth temperatures show orthorhombic ferroselite (FeSe2) (ICDD No: 00-021-0432) with small quantity of another orthorhombic ferroselite

(FeSe2) (ICDD No: 00-012-0290).

(111) (120)

(101) (211) (110) (011) * (031) (122)

* (c)

(a.u) Intensity (b)

(a)

20 30 40 50 60 70 2-Theta (degree)

Figure 4.3. p-XRD pattern for ferroselite (FeSe2) nanocrystals in oleylamine at (a) 190 and (b) 240 and (c) 290 °C respectively. Whereas the symbol * shows different phase of ferroselite (FeSe2).

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The differection peaks for (110), (101), (111), (120), (211), (031) and (122) planes of orthorhombic ferroselite (FeSe2) (ICDD No: 00-021-0432) were dominant.

The p-XRD pattern show that ferroselite (FeSe2) nanocrystals are grown at all temperatures but the intensity of the peaks increased significantly for the nanocrystals obtained at highest temperature of 290 °C, indicating the growth of larger crystallites is promoted at higher temperatures.

4.4.4 Transmission electron microscopy of iron selenide nanocrystals

TEM images of the ferroselite (FeSe2) nanocrystals show the growth of overlapping thin sheets at 190 °C (Figure 4.4 (a)) whereas cluster of long rod- shaped crystallites are formed at higher temperature of 240 °C (Figure 4.4 (c)).

Figure 4.4. Growth of nanocrystals in oleylamine at (a) 190 °C (b) lattice fringes showing d-spacing at 190 °C (c), (d) growth of nanocrystals at 240 °C and (e), (f) at 290 °C. 154

HRTEM image of nanoparticles deposited at 240 °C, as long rods, (Figure 4.4 (d). Figure 4.4 (b) shows in the HRTEM images with clear lattice fringes of a crystal obtained at 190 °C with d-spacing 2.56 Å corresponding to (111) reflection of ferroselite (FeSe2).

TEM analysis for the sample prepared at 290 °C shows large cuboid crystallites (ca 300 nm) (Figure 4.4 (e) and 4.4 (f)). The growth of larger size and shaped crystals at higher temperatures may be due to higher growth rate as well as the thermodynamic or kinetic stability of the different shapes.21-23

4.4.5 Effect of the capping agent To study the effect of different capping agents on the growth of nanocrystals, the complex [Fe(C10H7CONCSeN(C2H5)2)3] was also thermolysed in dodecanthiol and in the mixture of oleylamine and dodecanthiol in 1:1 ratio at 240 °C for 30 minutes. The p-XRD pattern of the nanocrystals produced from the thermolysis of this complex in dodecanthiol shows (Figure 4.5 (a)) a mixture of iron selenide (Fe7Se8), (ICDD No: 01-071-0586) and ferrosilite (FeSe2), (ICDD No: 00-012-0290).

(111) (110) (101) (120) (011) (211) * (031) (c)

(a.u) Intensit * (b)

(a)

20 30 40 50 60 70 2-Theta (degree)

Figure 4.5. p-XRD pattern for iron selenide nanocrystals obtained from the thermolysis of complex [Fe(C10H7CONCSeN(C2H5)2)3] in (a) dodecanthiol, (b) oleylamine and (c) in the mixture of dodecanthiol and oleylamine at 240 °C for 30 minutes reaction time.

The symbol * shows ferrosilite ((FeSe2) (ICDD No: 00-012-0290)) phase.

155

Thermolysis in oleylamine (Figure 4.5 (b)) and in the mixture of oleylamine and dodecanthiol (Figure 4.5 (c)) produced ferrosilite (FeSe2) (ICDD No: 00-021-

0432) with the traces of ferrosilite (FeSe2) (ICDD No: 00-012-0290) respectively.

TEM images analysis (Figure 4.6 (a)) obtained from the thermolysis in dodecanthiol shows rod and long plate-like crystallites. The cluster of long rods (Figure 4.6 (b)) were obtained when thermolysed in the mixture of dodecanethiol and oleylamine.

Figure 4.6. TEM images of iron selenide nanocrystals obtained from the thermolysis of complex [Fe(C10H7CONCSeN(C2H5)2)3] in (a) dodecanthiol and (b) in the mixture of dodecanthiol and oleylamine (1:1) at 240 °C for 30 minutes reaction time.

4.4.6 The magnetic properties of iron selenide nanocrystals

The magnetic properties FeSe2 nanocrystals prepared at 190, 240 and 290 °C was investigated at 300 K as plotted in Figure 4.6. In all cases, the magnetization increases linearly on varying the magnetic field from 2 to 70 kOe, without showing any sign of saturation, which indicates the paramagnetic behaviour of the nanocrystals.

This is in agreement with the formation of paramagnetic FeSe2 nanocrystals in all cases. For small magnetic fields ranging from -2 to 2 kOe, a reminescence of a magnetic hysteresis loop is observed (Hc = 200 Oe), which is more pronounced for the nanocrystals formed at 240 and 290 °C (see the inset of

156

Figure 4.7). This could suggest that a minor ferrimagnetic phase (i.e. Fe7Se8 or

Fe3O4) is present in these samples.

o FeSFeSeat2 at 190 190 C°C 0.4 2 o FeSFeSeat2 at 240 240 C°C 2 o ) FeSFeSeat2 at 290 290 C°C

0.2 2 emu/g ( 0.0

0.01

-0.2

0.00

Magnetization -0.01 -0.4 T = 300 K -2 -1 0 1 2

-80 -60 -40 -20 0 20 40 60 80

Magnetic Field (kOe)

Figure 4.7. Field-dependence of the magnetization at 300 K of iron selenide nanocrystals obtained by thermolysis of complex in oleylamine at 190, 240 and 290 °C. Inset: the reminiscence of a hysteresis loop associated to the minor ferromagnetic phase, formed at 240 °C.

0.010 ZFC, FC, FeSe at 190 oC 2 ZFC, FC, FeSe at 240 oC 2 ) 0.008 ZFC, FC, FeSe at 290 oC

2 emu/g ( 0.006

0.004

H = 100 Oe

Magnetization 0.002

0.000 0 50 100 150 200 250 300 Temperature (K)

Figure 4.8. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves for iron selenide nanocrystals obtained by thermolysis in oleylamine at 190, 240 and 290 °C.

157

To verify this hypothesis, the thermal variation of the magentization was recorded in both zero-field cooled (ZFC) and field-cooled (FC) regimes with an applied magnetic field of 100 Oe (Figure 4.7). As anticipated, the magnetization value decreases continously with decreasing temperature and the FC and ZFC magnetization curves almost super impose one another, confirming that paramagnetic FeSe2 is the dominant phase in all these samples.

The small irreversibility of the ZFC and FC curves of the samples generated at 240 and 290 °C is consistent with the presence of a small ferrimagnetic impurity, as described above.

4.5 Iron selenide thin films Thin films of iron selenide were deposited at 625 °C on silicon substrates from a

THF solution of [Fe(C10H7CONCSeN(C2H5)2)3] by AACVD method. The films obtained at 625 °C were very thin which exhibits a broad p-XRD pattern seen in Figure 4.9 (a).There was no deposition at temperatures lower than 625 °C.

The p-XRD pattern shows the deposition of FeSe phase (ICDD No: 01-073- 6936). The broad peaks in the p-XRD pattern indicate the small size of crystallites which is confirmed by the SEM image (Figure 4.9 (b)). SEM micrograph shows almost uniform films consisting of ca 100 nm spherical particles.

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Figure 4.9. (a) The p-XRD pattern for iron selenide thin film deposited at 625 °C on silicon substrate (b) SEM images of the same film (c) roughness of the film (d) AFM 3D images of the surface roughness of thin film (e) SEM showingn thickness of the film.

The surface topography of the films analyzed in a 2D AFM image (Figure 4.9 (c, d)) show well-interconnected crystallites with an average roughness value of 11.30 nm. The thickness of the films (Figure 4.9 (e)) shows two different values of 151 nm and 161 nm. So the average thickness of the films is 156 nm.

159

Figure 4.10. The elemental mapping of thin film of (a) iron, (b) selenium, (c) shows the distribution of iron and selenium and (d) shows the EDX analysis of thin film.

The elemental mapping (Figure 4.10 (a-c)) images of the iron selenide thin films show an excess of elemental selenium. EDX analysis (Figure 4.10 (d)) shows that the film is composed of iron selenide with a Fe:Se ratio close to 1:4 but it was surprising to note that there were no peaks in p-XRD pattern matching to elemental selenium or any other phase of iron selenide. This may be due to the films being too thin for any diffraction. Repeated attempts to deposit a thicker film using higher concentration of precursor and a longer deposition time were unsuccessful. The films were deposited only on silicon, no deposition was obtained on glass substrates. Definitive results can only be obtained from a thicker film using other techniques such as ICP and XPS analysis.

4.6 Conclusion Mixed phases of iron selenide were obtained using tris(N,N-diethyl-N’- naphthoylselenoureato)iron(III) as a single source precursor by thermolysis in oleylamine and dodecanthiol at 190, 240, and 290 °C. At all the deposition temperatures, the similar p-XRD results obtained. Thin film deposition was 160

carried out by AACVD method. A very thin film was deposited at 625 °C. No deposition occured below or above this temperature. The p-XRD results showed iron selenide (FeSe). The size of the iron selenide crystallites is very small. Elemental maping showed an excess of selenium.

4.7 References 1. J. Park, K. An, Y. Hwang, J. G. Park, H. J. Noh, J. Y. Kim, J. H. Park, N. M. Hwang, T. Hyeon, Nat. Mater., 2004, 3, 891.

2. Y. Hou, Z. Xu, S. Sun, Angew. Chem. Int. Ed., 2007, 46, 6329.

3. S. V. Gaponenko, Optical Properties of Seemiconductor Nanocrystals, Cambridge University Press: 1998.

4. D L. Leslie-Pelecy, R. D. Rieke, Chem. Mater., 1996, 8, 1770.

5. J. Y. Ying, Chem. Eng. Sci., 2006, 61, 1540.

6. B. H. Zeng, S. Sun , Adv. Funct. Mater., 2008, 18, 391.

7. Y. W. Hun, S. J. W. Cheon, Acc. Chem.Res., 2008, 41, 179.

8. P. Terzieff, K. L. Komarek, Monats Chem., 1978,109, 651.

9. P. Terzieff, K. L. Komarek, Monats Chem., 1978, 109, 1037,

10. C. Lin, Y. Siao, S. Lu, C. Gau, Magnetics, IEEE Transactionson, 2009, 45 (10), 4275

11. H. Y. Lai, C. J. Chen, J. Cryst. Growth, 2009, 311,4698.

12. H. Wang, I. Salveson, Phase Trans., 2005, 78, 547.

13. C. I. Pearce, R. A. D .Pattrick, D. J. Vaughan, Rev. Min.Geochem., 2006, 61,127.

14. J. C. Ward, ReV. Pure Appl. Chem., 1970, 20, 175.

15. L. Chen, H. Zhan, X. Yang, Z. Sun, J. Zhang, D. Xu, C. Liang, M. Wu and J. Fang. CrystEngComm, 2010, 12, 4386. 161

16. L. Chen, X. Yang, X. Fu, C. Wang, C. Liang, and M. Wu, Eur. J. Inorg. Chem., 2011, 2098.

17. M. Gao, Z. Lin, J. Jiang, H.Yao, Y. Lu, Q. Gao, W. Yao and S. Yu, Chem. Eur. J. 2011, 17, 5068.

18. I. B. Douglass, J. Am. Chem. Soc., 1937, 59, 740.

19. I. B. Douglass, F. B. Dains, J. Am. Chem. Soc., 1934, 56, 1408.

20. M. Akhtar. J. Akhtar, M. A. Malik, P. O’Brien, F. Tuna, J. Raftery, M. Helliwell, J. Mater. Chem., 2011, 21, 9737.

21. B. Qin, H. Chen, H. Liang, L. Fu, X. Liu, X. Qiu, S. Liu, R. Song, Z. Tang, J. Am. Chem. Soc., 2010, 132 (9), 2886.

22. Y. Xia, T. D. Nguyen, M. Yang, B. Lee, A. Santos, P. Podsiadlo, Z. Tang, S. C. Glotzer, N. A. Kotov, Nat. Nanotech. 2011, 6, 580.

23. Y. Gao, Z. Tang, Small, 2011, 7 (15), 2133.

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Chapter 5

Synthesis of iron selenide nanocrystals and thin films from bis(tetra-iso-propyldiselenoimidodiphosphinato)iron(II) and bis(tetra-phenyl- diselenoimidodiphosphinato)iron(II) complexes.

163

5.1 Summary Mixed phases of iron selenide nanocrystals were obtained from the thermolysis of bis(tetraisopropyldiselenoimidodiphosphinato)iron(II) and bis(tetraphenyldiselenoimidodiphosphinato)iron(II) complexes in oleylamine and hexadecylamine at various temperatures. These complexes were also used for the deposition of thin films by Aerosol Assisted Chemical Vapour Deposition (AACVD). Thermolysis of these complexes in oleylamine produced the rod and plate-like crystallites. Also the mixed phases of iron selenide thin films were deposited at different deposition temperature on silicon substrates from these complexes. Thin films from complex (3) produced rod and sheet-like crystallites whereas the complex (4) produced spherical crystallites.

164

5.2 Introduction In recent years iron selenides has received a lot of attention, because of their conductive, optical, electronic, and magnetic properties. Iron selenide could be semiconductors, or even superconductors with characteristics of ferro/ferrimagnetic metals. The conductive properties of iron selenide depend on the composition and phase.1-3

Iron selenide has been synthesized by different methods including the elemental reaction in evacuated tubes at elevated temperature,4,5 by reaction of aqueous 6,7 8,9 metal salt solutions with toxic gas H2Se, and mechanical alloying. However all these methods required large amount of energy and are time consuming. Also it is hard to get pure phase material using these methods.

Hexagonal (H) and monoclinic (M) nanocrystals of iron selenide with NiAs-like structure were synthesized in one-pot by thermal decomposition of ferrous chloride and selenium powder in oleylamine.10 Gao et al.11 reported the solvothermal preparation of Fe7Se8 polyhedra and Fe7Se8 nanorods in a mixed solvent of diethylenetriamine (DETA) and deionized water (DIW). The magnetic measurements of Fe7Se8 polyhedra and nanorods show a weak ferromagnetic ordering at room temperature. Iron selenide (FeSe2) nanorods also have been synthesized by hydrothermal co-reduction method using N2H4.H2O as the reductant.12

13 Zeng et al. prepared iron selenide (Fe3Se4) nanostructures by a one-pot high- temperature organic-solution-phase method. The size of these nanostructures can be tuned from 50 to 500 nm, and their shapes can be varied from nanosheets and annocacti, to nanoplates. These nanostructures exhibit hard magnetic properties, with large coercivity values reaching 40 KOe at 10 K and 40KOe at room temperature. The magnetic properties can be easily tunable by doping and substituting Fe ions by other transition-metal elements such as Co.13

We have14 synthesised tris(N,N-diethyl-N’-naphthoylselenoureato)iron(III) complex and its X-ray single crystal structure determined. Thermolysis of this complex in oleylamine at different temperatures (190, 240 and 290 °C)

165

produced nanocrystals of FeSe2 whereas Aerosol Assisted Chemical Vapour Deposition produced FeSe thin films on silicon substrates.

15 Fang et al. reported regular square FeSex nanoflakes with tetragonal PbO-type phase from ferrous chloride and selenium trioctyl phosphine. Iron selenide (FeSe) were electrodeposited at various bath temperature ranging from 30 to 90 °C on to indium doped tin oxide coated conducting glass (ITO) substrate in an aqueous electrolytic bath containing FeSO4 and SeO2. By adjusting the deposition parameter a stoichiometric films with well-defined composition were obtained. Morphological studied showed homogeneity with uniform grains for 16 film deposited at higher bath temperature. Iron selenide FeSex (x=0.80, 0.84,0.88, 0.92) thin films were also prepared by a pulsed laser deposition method on different substrate (SrTio3(001), (STO), (La,Sr)(Al,Ta)O3(001) (LSAT), and LaAlO3(001) (LAO) substrate. All of the thin films show a single phase and c-axis oriented eptixial growth and are super conductor.17

This chapter contains the synthesis of bis(tetraisopropyldiselenoimidodiphosphinato)iron(II) (3) and bis(tetraphenyldiselenoimidodiphosphinato)iron(II) (4) complexes and their use as a single source precursors to prepare iron selenide nanocrystals. These complexes were also used for the deposition of thin films by Aerosol Assisted Chemical Vapour Deposition (AACVD).

5.3 Experimental All synthesis was performed under an inert atmosphere of dry nitrogen using standard Schlenk techniques. All reagents were purchased from Sigma-Aldrich and used as received. Solvents were distilled prior to use.

5.3.1 Synthesis of ligands The preparation of the ligand was carried out by a modification of the method reported in literature.18

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5.3.2 Synthesis of imido(tetradiisopropyldiselnodiphosphanate) ligand i [(SeP Pr2)2N] (1) A solution of diisopropylphosphine chloride (10 ml 62.84 mmol) dissolved in toluene (25 ml) was added dropwise into the solution of hexamethyldisilazane (6.5 ml 31.42 mmol) in toluene (25 ml) in dry toluene in 30 minutes at 50 °C. The reaction mixture was heated at 50 °C whilst stirring for 3 hours. After 3 hours add 4.96 g (62.84 mmol) of selenium powder and increase the temperature to 100 °C and reflux for 6 hours. The reaction mixture was cooled overnight and filtered. Washed with DCM and diethyl ether for several time. The product was recrystallized in DCM. Yield: 6.9g (39%), mpt: 172-184 °C. Elemental analysis:

Calculated for C12H28NP2Se2: C, 35.39; H, 7.18; N, 3.44; P, 15.21 %. Found: C, 35.42; H, 7.57; N, 3.35; P, 15.13 %.

5.3.3 Synthesis of ligand imido(tetradiphenyldiselnodiphosphanate) ligand

[SePPh2)2N] (2) The same procedure was adopted for the preparation of diphenyldiselnodiphosphanate ligand as we used for the preparation diisopropyldiselnodiphosphanate ligand. Yield: 6.3 g (43%), mpt: 213-221 °C.

Elemental analysis: Calculated for C24H21NP2Se2: C, 53.04; H, 3.90; N, 2.58; P, 11.40 %. Found: C, 53.11; H, 3.59; N, 2.54; P, 11.32 %.

5.3.4 Synthesis of complex bis(tetraisopropyldiselenoimidodiphosphinato)- i iron(II) [Fe{(SeP Pr2)2N}2] (3) Imido(tetradiisopropyldiselnodiphosphanate) ligand (5.0g, 3.92mmol) and sodium methoxide (0.06g, 1.11mmol) was dissolved in dry methanol (30 ml). The mixture was stirred under nitrogen for 20 minutes at room temperature. A methanolic solution of FeCl3 (0.063 g, 0.388mmol in 20 ml methanol) was added dropwise in to the above solution. After stirring for 2 hours the resulting solution was filtered and recrystallized in chloroform. mpt: 132-140 °C. Elemental analysis: Calc: C, 33.17; H, 6.45; N, 3.22; P, 14.27; Fe, 6.4, Found: C, -1 32.77; H, 6.63; N, 3.15; P, 13.60; Fe, 6.6, %. IR (vmax /cm ): 2961(s), 1459(s), 1382(s), 1226(s), 1026(s), 883(s), 761(s), 624(s), Mass (m/z): 724, 404, and 360.

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5.3.5 Synthesis of complex bis(tetraphenyldiselenoimidodiphosphinato)- iron(II) [Fe{(SePPh2)2N}2] (4) A similar procedure was adopted for the synthesis of complex (4) as used in the synthesis of complex (3). The deep brown crude product was recrystallized from chloroform to give brownish crystals in a pale yellow solvent. The solvent was separated by decanting. The slow evaporation of the decant solvent gave white transparent crystals which were determined by X-ray crystallography. The brownish crystals were analysed by elemental analysis, IR, Mass spectroscopy and X-ray crystallography. mpt: 218-232 °C. Elemental analysis: Calc: C, 50.50; H, 3.50; N, 2.4; P, 11.86; Fe, 4.9 Found: C, 49.31; H, 3.11; N, 2.33; P, 10.35; Fe, -1 4.9, %. IR (vmax /cm ): 2983(s), 1434(s), 1322(s), 1100(s), 914(s), 736(s), 618(s), Mass (m/z): 862, 480, and 400.

5.3.6 Synthesis of iron selenide nanocrystals Iron selenide nanocrystals were prepared by thermolysis of imido complexes in oleylamine. In a typical experiment, 10 ml of oleylamine was heated at 90 °C for 10 minutes under vacuum in a three necked round bottom flask and then purged with nitrogen gas for 5 minutes. 0.3g of (0.23 mmol) complex was added directly in to the hot oleylamine and the reaction temperature was slowly increased to 290 °C. After maintaining temperature for 5 minutes the heating was stopped and the mixture was allowed to cool at room temperature. Addition of 20 ml acetone produced a black precipitate which was centrifuged, washed twice with acetone to remove any excess ligand. Black nanocrystals were resuspended in toluene for further investigation.

5.3.7 Deposition of thin films The complexes (3) and (4) were used as a single source precursor for the depositin of iron selenide thin films. The deposition was carried out by AACVD method on to the silicon substrate at 500, 550 and 600 °C.

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5.4 Results and discussion The complex (4) was synthesized by the reaction of anhydrous iron(III) chloride with the corresponding ligand in dry methanol. The crude product was recrystallized from chloroform to give brownish cubic crystals in a pale yellow solvent which were separated by decanting the solvent. After 24 hours the decant solution gave transparent white crystals by slow evaporation at room temperature. The first crop of brownish crystals, also the major product was identified as bis(tetraphenyldiselenoimidodiphosphinato)iron(II) by elemental analysis and X-ray single crystal structure determination. The second crop of white transparent crystals a minor product was identified as (SePPh2)2N)-O-

(SePPh2)2N) by elemental analysis and X-ray structure. Both products were unexpected. The reaction of one mole of iron(III) with three moles of ligand was expected to result in the formation of a tris(tetraphenyldiselenoimidodiphosphinato)iron(III) complex but instead it gave a bis(tetraphenyldiselenoimidodiphosphinato)iron(II) complex with iron(II) by reducing iron(III). This result confirms the results obtained by Oyetunde.19 He did not mention or isolated the second product which gives better insight in to the reaction mechanism. The isolation and identification of the second product gave a clue to the reaction mechanism. We think that oxygen reacts with the ligands in presence of iron(III) to give RP-O-PR and 2Se2 species while reducing iron(III) to iron(II) complex as shown in the Equation 5.1

(5.1)

i Both reactions (reaction of [(SeP Pr2)2N] and [SePPh2)2N] with Fe(III) chloride) always produced iron(II) complexes. A literature search also revealed another similar iron(II) structure20 with the corresponding sulfur ligand containing methyl groups. We can almost generalise this behaviour of the ligand based i upon above observation. We believe that the reaction of [(SP Pr2)2N] and

[SPPh2)2N] with iron(III) will also result in the formation of iron(II) complexes.

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5.4.1 X-ray single crystal structure of [Fe{(SePPh2)2N}2] (4) The structure (Figure 5.1) is based on a monomer where the geometry around iron is close to tetrahedral. Each iron is bonded to four selenium atoms from the two ligands. Iron(II) has a wide range of coordination geometries and spin-states. At the high spin state iron(II) has 4 unpaired electrons with the d6 configuration 4 2 which consist of t2g eg in octahedral geometry, and with a coordination number of four, which makes it possible to have a tetrahedral geometry. However at the 6 octahedral low spin states, iron(II) is diamagnetic consisting of t2g , with no unpaired electron and a coordination number of six and being able to have an octahedral geometry.

In terms of spin states, both iron(II) and iron(III) have two choices for the spin state based on how the splitting energy (Δ) compares with the electron pairing energy (Δ). More electron-pairing energy is needed to put two electrons into the same lower orbital, highest value of spin (‘high spin’) states occurs than the splitting energy to raise one electron into an upper orbital. As a result the complex has the highest possible number of electrons.

The ligands are diagonally across the cube faces from each other in tetrahedral complexes. The t2 orbitals point closer to the ligands and are therefore more unfavourable places for electrons to occupy as compared to the eg.

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Figure 5.1. X-ray single crystal structure of [Fe{(SePPh2)2N}2], Selected bond lengths (Å) and bond angles (°): Fe(1)-Se(2) 2.6369(7), Fe(1)-Se(1) 2.6736(8), N(1)-P(1) 1.578(6), N(1)-P(2) 1.583(6), N(2)-P(4) 1.595(5), N(2)-P(3) 1.593(5), P(1)-Se(1) 2.1669(18), P(2)-Se(2) 2.1800(18), P(3)-Se(3), 2.1720(16), P(4)-Se(4) 2.1717(16), Se(2)-Fe(1)-Se(1) 88.38(3), Se(3)-Fe(2)-Se(4) 88.61(2), P(1)-Se(1)-Fe(1) 88.28(5), P(2)-Se(2)-Fe(1) 95.91(5).

5.4.2 X-ray single crystal structure [(SePPh2)2N)-O-(SePPh2)2N)] The structure (Figure 5.2) shows two ligands attached with oxygen atom via phosphorus atoms. The bond lengths between the oxygen and phosphorus atom are 1.61Å which is close to those reported in phosphates.21 There are no unusual bond lengths or angles in this structure to discuss but the determination of this structure helped to understand the mechanism of this reaction. The crystallographic refinement data for both crystal structures are given in Table 5.1.

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Figure 5.2. X-ray single crystal structure of [(SePPh2)2N)-O-(SePPh2)2N)], Selected Bond lengths (Å) and angles (°): N(1)-P(2) 1.5517(12), N(1)-P(1) 1.6200(12), P(1)- Se(1) 2.1165(4), P(2)-O(1) 1.6175(6), P(2)-N(1)-P(1) 135.32(8), N(1)-P(1)-C(7) 104.81(6), N(1)-P(1)-C(1) 104.68(6), C(7)-P(1)-C(1) 104.29(6), N(1)-P(1)-Se(1) 118.28(4), C(7)-P(1)-Se(1) 111.55(5), C(1)-P(1)-Se(1) 112.02(5), N(1)-P(2)-O(1) 115.18(5), N(1)-P(2)-C(13) 107.09(6), O(1)-P(2)-C(13) 106.68(6), N(1)-P(2)-C(19) 116.66(6), O(1)-P(2)-C(19) 99.82(6), C(13)-P(2)-C(19) 110.98(6).

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Table 5.1. Structural refinement data for [(SePPh2)2N)-O-(SePPh2)2N)] and

[Fe{(SePPh2)2N}2] .

Empirical formula C49H42.28Cl1.45N2.28OP4Se2 C48 H40 Fe N2 P4 Se4

Formula weight 1015.48 1140.39

Crystal system, space Monoclinic, C2/c Triclinic, P-1 group

Unit cell dimesnions a = 10.7461(2) Å, a = 10.0919(5) Å,

b = 20.0351(4) Å, b = 12.9751(6) Å,

c = 21.4593(4) Å, c = 18.0953(8) Å,

alpha = 90 deg., alpha = 89.753(3) deg., beta = 82.344(2) deg., beta = 92.3890(10) deg., gamma = 77.988(2) deg. gamma = 90 deg.

Volume 4616.15(15) A3 2296.25(19) A3

Z, Calculated density 4, 1.461 Mg/m3 2, 1.649 Mg/m3

Absorption coefficient 4.395 mm-1 7.857 mm-1

Crystal size 0.22 x 0.19 x 0.17 mm 0.17x 0.15 x 0.13 mm

Theta range for data 4.12 to 72.39 deg. 4.52 to 68.54 deg. collection

Data / restraints / 4468 / 2 / 273 7985 / 0 / 535 parameters

Goodness-of-fit on F^2 1.063 1.565

Final R indices R1 = 0.0283, wR2 = 0.0737 R1 = 0.0589, wR2 = 0.1703 [I>2sigma(I)]

R indices (all data) R1 = 0.0317, wR2 = 0.0757 R1 = 0.0661, wR2 = 0.1781

Largest diff. peak and 0.926 and -0.743 e.A-3 5.157 and -0.839 e.A-3 hole

Reflections collected / 14593 / 4468 [R(int) = 14524 / 7985 [R(int) = unique 0.0315] 0.0376]

Limiting indices -13<=h<=13, -20<=k<=24, -10<=h<=12, -14<=k<=15, - -26<=l<=26 21<=l<=21

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5.4.3 Thermogravimetric analysis Two step decomposition (Figure 5.3 (a)) with rapid weight loss between 200 and 360 °C and 360 and 460 °C were observed from thermogravimetric analysis of the complex bis(tetraisopropyldiselenoimidodiphosphinato)iron(II). The final residue value (10.2%) is close to the calculated value (10.6%) for FeSe. A single step decomposition (Figure 5.3(b)) with major loss between 250 and 610 °C was observed from complex bis(tetradiphenyldiselenoimidodiphosphinato)iron(II).

100

Mass of residue(%) of Mass (b)

(a) 0 0 100 200 300 400 500 600 Temperature (°C )

i Figure.5.3 Thermogravimetric analysis of complexes (a) [Fe{(SeP Pr2)2N}2] and (b)

[Fe{(SePPh2)2N}2].

The final residue value (22.1%) is considerably higher than the calculated value

(12.7%) for FeSe2 and also significantly lower than the calculated value (28.63%) for Fe3Se4.

5.4.4 Powder X-ray diffraction for iron selenide nanocrystals i The nanocrystals produced by the thermolysis of complexes [Fe{(SeP Pr2)2N}2] and [Fe{(SePPh2)2N}2] in oleylamine at various temperatures (190, 240 and 290 °C) were analysed by p-XRD, TEM and HRTEM. The p-XRD pattern (Figure 5.4 (a, b and c)) of the nanocrystals obtained at (a) 190, (b) 240 and (c) 290 °C from bis(tetraisopropyldiselenoimidodiphosphinato)iron(II) (3) indicates a mixture of orthorhombic ferroselite (FeSe2) (ICDD No: 00-021-0432) with

174

small quantity of another orthorhombic ferroselite (FeSe2) (ICDD No: 01-074- 0247). At all growth temperatures the diffraction peaks for (110), (101), (111),

(120), (211), (031), and (122) planes of orthorhombic ferroselite (FeSe2) (ICDD No: 00-021-0432) were dominant.

(111) (120)

(110) (101) (211)

* (031) * (122) (c)

Intensity(a.u) (b)

(a)

20 30 40 50 60 70 80 2- Theta (degree)

Figure 5.4. p-XRD pattern for ferroselite (FeSe2) nanocrystals in oleylamine from i complex [Fe{(SeP Pr2)2N}2 at (a) 190, (b) 240 and (c) 290 °C respectively. Whereas symbol * shows anothe ferroselite (FeSe2) phase.

A very low intensity peaks correspond to the orthorhombic ferroselite ((*) (FeSe2) (ICDD No: 01-074-0247)) was observed. The intensity of the peaks (Figure 5.4) increases as the reaction temperature rises, which shows the temperature dependence of nanocrystals formation. At the highest temperature (290 °C) the intensity of the diffraction peaks were high which confirms the growth of crystallites is temperature dependent.

The p-XRD pattern for the nanocrystals obtained from the thermolysis of complex ([Fe{(SePPh2)2N}2]) (4) in oleylamine at different growth temperatures (190, 240 and 290 °C) shows the similar behaviour to the complex (3). In the

Figure 5.5 (a-c) show a mixture of orthorhombic ferroselite (FeSe2) (ICDD No:

00-021-0432) with small quantity of another orthorhombic ferroselite (FeSe2) (ICDD No: 01-074-0247).

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(111) (120)

(101) (211)

*(031) (110) (122) (c) *

Intensity(a.u) (b)

(a)

20 30 40 50 60 70 80 2- Theta (degree)

Figure 5.5. p-XRD pattern for ferroselite (FeSe2) (ICDD No: 00-021-0432) nanocrystals in oleylamine from complex ([Fe{(SePPh2)2N}2]) at (a) 190, (b) 240 and

5.4.5 Transmission electron microscopy of iron selenide nanocrystals

TEM analysis show rod and plates-like morphology of the ferroselite (FeSe2) nanocrystals produced from the thermolysis of complexes in oleylamine at different growth temperatures (190, 240 and 290 °C). The TEM micrographs (Figure 5.6 (a)) of the iron selenide nanocrystals obtained from complex i [Fe{(SeP Pr2)2N}2] (3) in oleylamine at 190 °C show the growth of rod-like crystallites. At the temperature of 240 °C flower like cluster (Figure 5.6 (b)) of long rods were formed whereas at higher growth temperature 290 °C plate-like crystallites (Figure 5.6 (c)) were produced. TEM micrographs also confirm the formation of highly crystalline material at higher temperature. The HRTEM images of iron selenide nanocrystals show the lattice fringes (Figure 5.6 (d)) with a d-spacing of 3.69 Å corresponding to the (110) reflection plane of orthorhombic ferroselite (FeSe2) (ICDD No: 00-021-0432) phase.

176

i Figure 5.6. FeSe2 nanocrystals from complex [Fe{(SeP Pr2)2N}2] (1) in oleylamine at (a) 190, (b) 240 and (c) 290 °C. HRTEM (d) showing lattice fringes with a d-spacing of

3.69 Å corresponding to the (110) plane reflection of orthorhombic ferroselite (FeSe2) (ICDD No: 00-021-0432) phase produced at 290 °C.

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Figure 5.7. Growth of nanocrystals in oleylamine from complex (4)

([Fe{(SePPh2)2N}2]) at (a) 190, (b) 240 and (c) 290 °C, (d) HRTEM showing lattice fringes with a d-spacing of 2.56 Å corresponding to the (111) reflection plane of orthorhombic ferroselite (FeSe2) (ICDD No: 00-021-0432) phase obtained at 290 °C.

Figure 5.7 (a-c) show the plate-like crystallites of iron selenide obtained from complex ([Fe{(SePPh2)2N}2]) (4) in oleylamine at (a) 190, (b) 240 and (c) 290 respectively. The TEM images of iron selenide nanocrystals obtained at higher temperature (Figure 5.7 (c)) are more crystalline in nature as compared to the nanocrystals obtained at lower temperature. This is also a supporting evidence of the temperature dependence of iron selenide nanocrystals formation. The HRTEM images (Figure 5.7 (d)) of plate-like nanocrystals show the lattice fringes. The d-spacing was measured from HRTEM micrographs as 2.56 Å corresponding to the (111) reflection plane of orthorhombic ferroselite (FeSe2) (ICDD No: 00-021-0432) phase.

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5.4.6 The effect of surfactants To study the effect of capping agents on the growth of iron selenide phases and morphology, these complexes were thermolysed in hexadecylamine at 240 °C. The p-XRD pattern (Figure 5.8 (a, and b)) of the nanocrystals produced from the thermolysis of complexes (3) and (4) respectively in hexadecylamine show a mixture of orthorhombic ferroselite (FeSe2) (ICDD No: 00-021-0432) and orthorhombic ferroselite (*) (FeSe2) (ICDD No: 01-074-0247). From the p-XRD pattern it is clear that there was no change in the phases of iron selenide by the change of the capping agent.

(111)

(120)

(101) (211)

(110) *(031) (122) (b)

Intensity(a.u)

(a)

20 30 40 50 60 70 80 2- Theta(degree)

Figure 5.8. p-XRD pattern for iron selenide nanocrystals (a) from complex i [Fe{(SeP Pr2)2N}2] and (b) from complex [Fe{(SePPh2)2N}2] in hexadecylamine at 240 °C respectively.

TEM images (Figure 5.9 (a, b)) of the iron selenide nanocrystals produced from i complex [Fe{(SeP Pr2)2N}2] in hexadecylamine at 240 °C show the starfish like cluster of long rods. Whereas plate-like crystallites (Figure 5.9 (c)) of iron selenide were obtained from complex [Fe{(SePPh2)2N}2] in hexadecylamine at 240 °C. The nanocrystals produced in hexadecylamine are more crystalline with well defined shapes.

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Figure 5.9. TEM images of iron selenide nanocrystals (a and b) from complex i [Fe{(SeP Pr2)2N}2] and (c) from complex [Fe{(SePPh2)2N}2] in hexadecylamine at 240 °C.

5.5. Deposition of iron selenide thin films from bis(tetraiso- i propyldiselenoimidodiphosphinato)iron(II) [Fe{(SeP Pr2)2N}2] (3) Deposition was carried out onto silicon substrates at temperatures from 500 to 600 °C with an argon carrier gas flow rate of 160 sccm. The reflective dark brown films were deposited at 600 °C whereas dark brown films were deposited at 550 and 500 °C.

5.5.1 Powder X-ray diffraction of iron selenide thin films The p-XRD Pattern of the deposited films from bis(tetraisopropyldiselenoimidodiphosphinato)iron(II) complexes at 500 (Figure 5.10 (a)), 550 (Figure 5.10 (b) and 600 °C (Figure 5.10 (c) shows the mixture of iron selenide ((Fe7Se8) (ICDD 180

No: 01-071-0586) new no. 04-007-1554), FeSe2 ((# ) ICDD No: 00-012-0291)) and FeSe2 ((+) ICDD No: 01-074-0247) phases. The intensity of the peaks in p- XRD patterns for the films deposited at 500 °C (Figure 5.10 (a)) and 550 °C are weak as compared to the film deposited at higher temperature of 600 °C. The p-

XRD pattern for the films deposited at 600 °C indicates iron selenide ((Fe7Se8) ICDD No: 01-071-0586), as a main phase with major diffraction peaks appearing for (114), (016), (302), and (037) planes. The broad peaks in the pattern indicate the small size of the crystallites.

(#)

(016) (114) (302) (+)

(037) (c) Intensity(a.u)

(b) (#) (a)

20 30 40 50 60 70 2-Theta (degree)

i Figure 5.10. p-XRD pattern for thin films deposited from complex [Fe{(SeP Pr2)2N}2] at (a) 500, (b) 550 and (c) 600 °C respectively. The main phase is iron selenide (Fe7Se8)

ICDD No: 01-071-0586 new no. 04-007-1554), whereas FeSe2 ((#) ICDD No: 00-012-

0291) and FeSe2 ((+) ICDD No: 01-074-0247) are the minor phases obtained.

5.5.2 Scanning electron microscopy of iron selenide thin films The SEM images of films deposited at 500 and 600 °C shows the sheet-like crystallites (Figure 5.11 (a) and (c)) whereas rod-like (Figure 5.11 (b)) crystallites were obtained at 550 °C. The insets in the Figure 5.11 (a and c) clearly shows the sheet-like crystallites with size ranging from 5-8 μm. Small rod-like crystallites with size ranging from 1-5 μm was obtained at 550 °C and shown as inset in Figure 5.11 (b).

181

Figure 5.11. SEM images of iron selenide thin films deposited from complex i [Fe{(SeP Pr2)2N}2] at (a) 500, (b) 550 and (c) 600 °C respectively. The insets show SEM images at higher magnification.

The EDAX analysis (Figure 5.12 (a-c)) of the films show the composition of iron:selenium ratio as 72:28 (500 °C), 75:25 (550 °C) and 74:26 (600 °C) which is not consistent with expected stoichiometry for FeSe2. This may be due to the reason that EDAX analysis uses a very small area of the thin films. These particular areas appears to be iron rich.

Figure 5.12. EDX analysis of iron selenide thin films deposited at (a) 500, (b) 550 and i (c) 600 °C from complex [Fe{(SeP Pr2)2N}2] on silicon substrate. 182

The elemental mapping images of iron selenide thin films deposited at 500 and 550 °C (Figure 5.13 (a and b)) indicates the uniform distribution of iron and selenium whereas the thin film deposited at 600 °C (Figure 5.13 (c)) was enriched with iron.

Figure 5.13. Elemental mapping of iron selenide thin films deposited from complex i [Fe{(SeP Pr2)2N}2] at (a) 500, (b) 550 and (c) 600 °C respectively.

The thickness of thin film (Figure 5.14 (a and b)) of iron selenide deposited i complex [Fe{(SeP Pr2)2N}2] (3) at 600°C shows the thickness ranging from 12 to 18μm. This is due to the large size of the crystallites deposited.

Figure 5.14. Thickness of iron selenide thin films deposited at 600 °C on silicon i substrate from complex [Fe{(SeP Pr2)2N}2] (3). 183

5.5.3 Interferometer microscopy of iron selenide thin films i The surface topography of the films deposited from complex [Fe{(SeP Pr2)2N}2]

(3) were analysed by Interferometer image as the AFM did not produced good images. The Interferometer images of the thin films (Figure 5.15 (a, b and c)) reveals the loosely packed crystallites. The average roughness values are 3.5 μm for the films deposited at 500 and 550 C whereas 3.2 μm for the film deposited at 600 °C.

Figure 5.15. 3D interferometer images of iron selenide thin films deposited at (a) 500, i (b), 550 and (c) 600 °C on silicon substrate from complex [Fe{(SeP Pr2)2N}2] (3).

5.6 Deposition of iron selenide thin films from bis(tetraphenyldiselenoimidodiphosphinato)iron(II) [Fe{(SePPh2)2N}2]

The films deposited from the complex [Fe{(SePPh2)2N}2] (4) at different deposition temperatures (500, 550 and 600 °C) were very thin. The p-XRD pattern could not be obtained for these very thin films.

184

5.6.1 Scanning electron microscopy of iron selenide thin films SEM images of thin films (Figure 5.16 (a, b and c)) deposited at 500, 550 and 600 °C show almost uniform films of spherical particles of ca 100 nm size. The iron:selenium ratio for the deposited films were analysed by EDAX analysis, which shows iron:selenium ratio as 58:42 (500 °C), 69:31 (550 °C), and 79:21 (600 °C) respectively. These results were also obtained from a small area of the thin films rather than the whole films and shows unexpected results.

Figure 5.16. SEM images of iron selenide thin films deposited from complex

([Fe{(SePPh2 )2 N}2]) at (a) 500, (b) 550 and (c) 600 °C onto silicon substrate.

The thickness of thin film (Figure 5.17) of iron selenide deposited complex

[Fe{(SePPh2)2N}3] (4) at 600 °C shows the thick ness of 600 nm. This indicates that the deposited films are uniform and well packed as we already have shown in the SEM images.

185

Figure 5.17. Thickness of iron selenide thin films deposited at 600 °C on silicon substrate from complex [Fe{(SePPh2)2N}2]) (4).

5.6.2 Atomic force microscopy 3D AFM images (Figure 5.18 (a, b and c)) were used to analyse the surface topography of the films.

Figure 5.18. 3D AFM images of iron selenide thin films deposited from complex

([Fe{(SePPh2)2N}2]) at (a) 500, (b) 550 and (c) 600 °C respectively. 186

These images show the growth of closely packed particles on to silicon substrates at 500, 550 and 600 °C. The average roughness values (Figure 5.19 (a, b and c)) are 182 nm for the film deposited at 500 °C, 26 nm for the film deposited at 550 °C and 8 nm for the film deposited at 600 °C.

Figure 5.19. Histograms showing average roughness and Rms roughnes of the iron selenide thin films deposited from complex [Fe{(SePPh2)2N}2] (4) at (a) 500, (b) 550 and (c) 600 °C onto the silicon substrate.

5.7 Conclusion Mixed phase of iron selenide nanocrystals were obtained from the thermolysis of imido complexes in oleylamine at various temperatures. Thermolysis of these

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complexes in oleylamine produced the rod and plate-like crystalites. Also the mixed phases of iron selenide thin films were deposited at different deposition temperature on silicon substrates from these complexes. On the other hand the deposition of iron selenide thin films from complex (3) produced rod and sheet- like crystallites whereas the complex (4) produced spherical crystallites.

5.8 References 1. X. J. Wu, D. Z. Shen, Z. Z. Zhang, J. Y. Zhang, K. W. Liu, B. H. Li, Y. M. Lu, B. Yao, D. X. Zhao, B. S. Li, C. X. Shan, X. W. Fan, H. J. Liu, C. L. Yang, Appl. Phys. Lett., 2007, 90,112105-1.

2. X. J. Wu, Z. Z. Zhang, J. Y. Zhang, B. H. Li, Z. G. Ju, Y. M. Lu, B. S. Li, and D. Z. Shen, J. Appl. Phys., 2008, 103,113501-1.

3. F. C. Hsu, J. Y. Luo, K. W. Yeh, T. K.Chen, T. W. Huang, P. M.Wu, Y. C. Lee, Y. L. Huang, Y. Y. Chu, D. C. Yan, M. K. Wu, Proc. Nat. Acad. Sci., 2008, 105, 14262.

4. P. Terzieff, K. L. Komarek, Monats Chem., 1978, 109, 651.

5. P. Terzieff, K. L. Komarek, Monats Chem., 1978, 109, 1037.

6. X. J. Wu, D. Z. Shen, Z. Z. Zhang, J. Y. Zhang, K. W. Liu, B. H. Li, Y. M. Lu, B. Yao, D. X. Zhao, B. S. Li, C. X. Shan, X. W. Fan, H. J. Liu, C. L.Yang, Appl. Phys. Lett., 2007, 90, 112105-1.

7. X. J. Wu, Z. Z. Zhang, J. Y. Zhang, B. H. Li , Z. G . Ju, Y. M. Lu, B. S. Li, D. Z. Shen., J. Appl. Phys., 2008, 103, 113501-1.

8. C. E. M. Campos, J. C. de Lima, T. A. Grandi, K. D. Machado, V. Drago, P. S. Pizani., J. Magn. Magn. Mater., 2004. 270, 89.

9. C. E. M. Campos, V. Drago, J. C. de Lima, T. A. Grandi, K. D. Machado, M. R. Silva, J. Magn. Magn. Mater., 2004, 269, 6.

10. C. R. Lin, Y.J. Siao, S.Z. Lu, and C. Gau, IEEE TRANSACTION ON MAGNETICS, 2009, 45(10).

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11. M. R. Gao, Z. Y. Lin, J. Jiang, H.B. Yao, Y.M. Lu, Q. Gao, W.T. Yao, and S. H. Yu, Chem, Eur. J, 2011, 17, 5068.

12. A. Liu, X. Chen, Z. Zhang, Y. Jiang, C. Shi , Solid State Communications, 2006, 138, 538.

13. H. Zhang, G. Long, D. Li, R. Sabirianov, and H. Zeng, Chem, Mater. 2011, 23, 3769.

14. M. Akhtar, J. Akhtar, M. A. Malik, F. Tuna, M. Helliwell, P. O’Brien, J. Mater. Chem., 2012, 22, 14970.

15. L. Chen, H. Zhan, X. Yang, Z. Sun, J. Zhang, X. C. Liang, M. Wu, and J. Fang, CrystEngComm, 2010, 12, 4386.

16. S. Thanikaikarasan, T. Mahalingam, K. Sundaram, A. Kathalingam, Y. D. Kim, T. Kim, Vacuum, 2009, 83, 1066.

17. Y. Han, W.Y. Li, L. X. Cao, S. Zhang and B. R. Zhao, J. Phys.:Condens. Matter, 2009, 21, 235702.

18. P. Bhattacharyya, J. Novosad, J. Phillips, A. M. Z. Slawin, D. J. Williams, J. D. Woolins, J. Chem. Soc. Dalton Trans., 1995, 1, 1607.

19. T. T. Oyetunde, PhD thesis, University of Manchester, 2011.

20. M. R. Churchill, J. Wormald, Inorg. Chem., 1971, 10(8), 1778.

21. B. Gamoke, D. Neff, J. Simons, J. Phys. Chem.A, 2009, 113, 5677.

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Chapter 6

General Experimental

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6.1 Chemicals N-Ethyl-isopropyl, dihexylamine, N-ethylmethyl amine, N,N-diethyldithio- carbamide sodium, oleylamine, hexadecylamine, octadecylamine, di-ethylamine, sodium hydroxide, sodium hydride, sodium acetate, sodium methoxide, carbon disulfide, napthoyl chloride, Potassium selenocyanate, iron(III) acetate, iron(III) chloride diisopropylphosphine chloride, hexamethyldisilazane, selenium powder, toluene, tetrahydrofuran, hexane, diethyl ether, anhydrous ether, methanol, ethanol, ipropanol, ibutanol, acetone, octadecene, distilled water, dodecanthiol and dichloromethane.

6.2 Synthesis of iron sulfide nanocrystals Iron sulfide and iron selenide nanocrystals were prepared by thermolysis of single source precursors in oleylamine, hexadecylamine, octadecene, octadecanol and dodecanthiol at different growth temperatures.The detailed synthetic procedure for both nanocrystals were described in preceding chapters.

6.3 Deposition of iron sulfide and iron selenide thin films by AACVD method Thin films of iron sulfide and iron selenide were deposited on silicon substrate at different deposition temperatures. In a typical deposition a weighed quantity of the precursor was dissolved in toluene in a two-necked 100 ml round-bottom flask. The flask was connected with a gas inlet and a reactor by a piece of reinforced tubing. The gas inlet allowed the carrier gas (argon) to pass into the solution to aid the transport of the aerosol. The gas (argon) flow rate was controlled by a Platon flow gauge. About six silicon substrates (approx. 1 x 2 cm) were placed inside the reactor tube for the depositon of thin films. The reactor tube was placed in a carbolite furnace. The round-bottom flask containing precursor solution was kept in a water bath above the ultrasonic humidifier (Model No. 1077).

The carrier gas transferred the aerosol droplets of the precursor solution into the hot reactor. On the hot surface of the substrate thin films deposited by thermally induced reaction. AACVD appratus is shown in the Figure 6.1. 191

Figure 6.1. Aerosol Assisted Chemical Vapour Deposition (AACVD) Apparatus.

6.4 Characterization methods Mass spectra were recorded on a Micromass platform II instrument by atmospheric pressure chemical ionization (APCI) method. Mass spectra were recorded on a Kratos concept 1S instrument.

Infrared spectra were recorded on a Specac single reflectance ATR instrument (4000–400 cm-1, resolution 4 cm-1).

NMR spectra were obtained on chloroform-D solution using Bruker AC300 FTNMR instrument. 1H NMR spectra were referenced to the solvent signal and the chemical shifts are reported relative to Me4Si.

Melting points were recorded on the Barloworld SMP10 Melting Point Apparatus.

6.5 Elemental analysis Elemental analysis was perfomed by microanalysis section in the school of chemistry. The analysis of C, H, N and S was carried out on a Carlo Erba EA 1108 elemental analyser which is calibrated with standard reference material. A tin container was used to place the sample and container was dropped in to

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furnace at 1000 °C. The oxygen was ejected from the furnace. The sample starts decomposing and forming carbon dioxide, water vapour, nitrogen and sulfur dioxide. The amount of each element present in the sample was determined by gas chromatography. For metal analysis a weighed quantity of the precursor was added in a tube with an acid. The mixture containing tube was heated to the temperature at which the sample is digested and decomposed by the acid. The residue was transferred to volumetric flask containing water. This solution is analysed by Fisons Horizon ICP-OES which measures the concentration of each element in the solution.

6.6 Thermogravimetric analysis Thermogravimetric analysis (TGA) measurements were performed by school of chemistry microanalysis team. TGA were carried out by a Seiko SSC/S200 model under a heating rate of 10 °C min-1 under nitrogen.

6.7 Magnetic measurements Magnetic measurement was performed by using a Quantum Design MPMS-XL SQUID magnetometer equipped with a 7 T magnet. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves were recorded over 5 – 300 K temperature range with an applied magnetic field of 100 Oe.

6.8 X-Ray crystallography Single-crystal X-ray diffraction data for the complex were collected using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) on a Bruker APEX diffractometer. The structures were solved by direct methods and refined by full-matrix least squares1 on F2. All non-H atoms were refined anisotropically. Hydrogen atoms were included in calculated positions, assigned isotropic thermal parameters and allowed to ride on their parent carbon atoms. All calculations were carried out using the SHELXTL package.2

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6.9 Powder X-ray Diffraction (p-XRD) p-XRD diffraction studies were performed on a Bruker AXS D8 diffractometer using Cu-Kα radiation. The samples were mounted flat and scanned between 20 to 80o in a step size of 0.05 with a various count rate depending on the nature of the sample. The diffraction patterns obtained from the sample were then compared with the patterns in the ICDD index. Powder x-ray Diffraction (p- XRD) is a powerful technique used to identify the crystalline materials. This technique also used for the identification of different phases present in materials. It also used to measure the structural properties including strain state, epitaxy, preferred orientation, grain size, phase composition and defect structure of these phases.

The p-XRD technique is a nondestructive technique, which makes it ideal for structure studies. The basic principle of X-ray diffraction is based on Bragg’s equation. It shows that the angle of incidence could be calculated in terms of the path difference between a ray reflected by one plane and that reflected by the next plane after it in the lattice. By comparing the positions and intensities of the diffraction peaks with the known crystalline materials present in the data base, the synthesised material can be identified. In addition, different phases in a sample can also be identified and quantified. The observed line broadening in the p-XRD pattern can be used to estimate the average particle size.

6.10 Transmission Electron Microscopy (TEM) TEM images were collected on Philips CM200 transmission electron microscope using an accelerating voltage 200 kV. TEM samples were prepared by evaporating a drop of a dilute suspension of the sample in toluene or hexane on carbon coated copper grid. The excess solvent was allowed to dry completely at room temperature. Some images were also collected on Tecnai microscope using accelerating voltage of 300kV.

The transmission electron microscope (TEM) is essential tool for the structural imaging of nanoparticles. TEM was used to obtain structural information of nanocrystals. In addition, TEM also provides the means to obtain a diffraction

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pattern from a small specimen area, Selected Area Diffraction (SAD) pattern. The SAD patterns are often used to determine the Bravais lattice and lattice parameters of crystalline materials.

6.11 Scanning Electron Microscopy (SEM) SEM was carried out using Philips XL30 FEG SEM. Energy dispersive X-ray analysis (EDAX) was performed using DX4. All samples were carbon coated using Edward’s coating system E306A before SEM analysis.

SEM techniques give three-dimension images of the object with nanoscale resolution, which can be easily interpreted. Also it can monitor the growth of thin films and nanostructures.

The basic principle of SEM is that the finely focused electron beam is scanned across a specimen to produce a signal; the intensity of signal depends on the shape, chemical composition and crystal arrangement of nanoparticlesl/nanocrystals.

6.12 Energy dispersive X-ray spectrometry EDX analysis used to identify and quantify the elements in samples on micron scale. It also can be used to identify the chemical composition and elemental mapping of the sample. EDX has limitations, it cannot distinguish ionic from non-ionic, isotopic species, and elements with atomic numbers less than carbon (Z<5).

6.13 AFM and Interferometer microscopy analysis The atomic force microscopy (AFM) analysis of the thin films was carried using a Veeco CP2 instrument. AFM analysis gave the 2D and 3D images of the thin films surface which used to explain the surface topography of thin films. The Interferometer microscopy analysis was carried by using MicroXam100 instrument. It was used for the surface mapping of very thin films.

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6.14 References 1. Sheldrick, G. M. SHELXS-97 and SHELXL-97, University of Göttingen,

Germany, 1997.

2. Bruker, SHELXTL Version 6.12, Bruker AXS Inc., Madison, Wisconsin, USA, 2001.

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Chapter 7

Conclusion and Future Work

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7.1 Conclusion The first chapter of this thesis presents a review of the methods for the preparation of iron chalcogenide nanoparticles and thin films and also their electrical and magnetic properties.

The second chapter involves the synthesis of several iron thiocarbamato i complexes including [Fe(S2CNEt Pr)3], [Fe(S2CN(Hex)2)3], [Fe(S2CNEtMe)3] and [Fe(S2CN(Et)2)3] and their characterization by elemental analysis, mass spectrometry, infrared spectroscopy and thermogravemetric analysis. The X-ray i single crystal structure of [Fe(S2CNEt Pr)3] is based on a monomeric molecule in which iron atom is bonded to six sulfur atoms, two from each dithiocarbamato ligand. The geometry about the iron centre is octahedral. The structure of the complex Fe(S2CNEtMe)3 contains half a molecule of THF as crystallisation solvent and the geometry on iron is distorted octahedral. Thermolysis of these complexes in olelyleamine, hexadecyleamine and octadecene at different temperatures (170, 230, 300 °C) produced iron sulfide nanocrystals. Nanocrystals of iron sulfide with greigite, pyrrhotite and mixed phases were grown at different thermolysis temperatures from each precursor. Deposition temperature as well as the capping agent were important factors in controlling the phase and shape of the deposited material. Iron sulfide thin films were also deposited from these complexes on silicon substrates at temperatures of 350, 400, 450 °C by using the AACVD method. The unsymmetrical i complexes [Fe(S2CNEt Pr)3] and [Fe(S2CNEtMe)3] deposited thin films with mixture of pyrite, pyrrhotite and marcasite at all growth temperatures. The symmetrical [Fe(S2CN(Hex)2)3] complex with longer alkyl groups gave a mixture of pyrite and pyrrhotite at 350 and 450 °C. The same complex at 400 °C produced a different mixture (pyrite and mackinawite). The symmetrical complex with the shorter alkyl groups [Fe(S2CN(Et)2)3] gave the pure pyrrhotite phase at the higher growth temperatures (400, 450 °C). Pyrite phase is dominant in all the samples obtained from the unsymmetrical complexes whereas the pyrrhotite phase is dominant in the samples deposited from symmetrical complexes. SEM images of the thin films show the sheets, rods, cubic, hexagonal plates and flower like cluster morphology. Closely packed crystallites 198

surface topography with uniform distribution of iron and sulfur was observed by AFM and elemental mapping analysis.

The third chapter describes the synthesis of iron sulfide nanocrystals from tris(O-alkylxanthato)iron(III) complexes. The p-XRD pattern of the nanocrystals obtained from the thermolysis of [Fe(S2COMe)3] in oleylamine at

230 and 300 °C correspond to a mixture of the cubic greigite (Fe3S4) and hexgonal pyrrhotite (FeS). The pure phase of cubic greigite (Fe3S4) was obtained from complex [Fe(S2COEt)3] at 170 °C but as the growth temperature increased from 170 to 230 to 300 °C, the mixture of different phases (greigite, pyrite and pyrrhotite) of iron sulfide produced. At lower temperature (170 °C) i no product was obtained from the complex [Fe(S2CO Pr)3] but pure greigite phase dominates at higher temperatures of 230 and 300 °C. The complex i [Fe(S2CO Bu)3] giving the pure greigite at 170 and 230 °C but at temperature 300 °C gave the mixed phases of gregite, pyrite and pyrrhotite. Thermolysis of complexes [Fe(S2COMe)3] and [Fe(S2COEt)3] in hexadecylamine gave the pure i pyrite phase at 230 °C whereas complex [Fe(S2CO Pr)3] gave pure greigite i phase and complex [Fe(S2CO Bu)3] gave phase pure pyrrhotite under similar i reaction conditions. The complexes [Fe(S2COMe)3] and [Fe(S2CO Pr)3] in octadecylamine produced greigite as a main product with the traces of pyrite phase at 230 °C. Complex [Fe(S2COEt)3] produced a mixture of greigite, pyrite i and pyrrhotite and complex [Fe(S2CO Bu)3] gave pure pyrrhotite phase. The nanocrystals obtained show the mixed morphologies (polyhedra) at all temperatures. Magnetic characterisation reveals that all iron sulfide nanocrystals show room temperature magnetic hystersis loops with magnetization well saturated under applied magnetic fields of 10-15 kOe (1 Oe = 10-4 T).

Fourth chapter reports the synthesis and X-ray single crystal structure of complex tris(N,N-diethyl-N’-naphthoylselenoureato)iron(III) and its use as single source precursor for the deposition of iron selenide thin films and the synthesis of nanoparticles. The structure of this complex contains two independent molecules, each is a tris chelate facial isomer; but there are significant differences in the conformation of the ethyl groups. Each molecule is

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paired with its enantiomorph conforming to the centrosymmetric P1- space group. Thermolysis of the complex in oleylamine at 190, 240, and 290 °C produced the nanocrystals of orthorhombic ferroselite (FeSe2). Ferroselite

(FeSe2) phases were also obtained when the complex was thermolysed in dodecanthiol and in the mixture of oleylamine and dodecanthiol in 1:1 ratio at 240 °C for 30 minutes. Long rod like morphology was observed under TEM analysis. Magnetic measurement showed the paramagnetic behaviour of iron selenide nanocrystals. A very thin film of iron selenide was deposited on a silicon by AACVD method substrate at high temperature (625 °C) only. The p- XRD pattern of these films corresponded to iron selenide (FeSe) phase. The SEM image showed the deposition of nanocrystalline FeSe and the elemental maping showed an excess of selenium.

i Chapter five describes the synthesis of [Fe{(SeP Pr2)2N}2] and

[Fe{(SePPh2)2N}2] complexes. These complexes were synthesized by the reaction of anhydrous iron(III) chloride with the corresponding ligand in dry methanol and recrystallize in chloroform. The reaction of one mole of iron(III) with three moles of ligand was expected to result in the formation of a tris(tetraphenyldiselenoimidodiphosphinato)iron(III) complex but instead it gave a bis(tetraphenyldiselenoimidodiphosphinato)iron(II) complex with iron(II) by reducing iron(III). The complex bis(tetraphenyldiselenoimidodiphosphinato)- iron(II) obtained as the first crop of brownish crystals, the second crop of white transparent crystals a minor product was identified as (SePPh2)2N)-O-

(SePPh2)2N) by elemental analysis and X-ray structure.

Mixed phase with rod and plate-like crystalites of iron selenide nanocrystals was obtained from the thermolysis of these complexes in oleylamine and hexadecylamine at 190, 240 and 290 °C temperatures. The p-XRD pattern shows a mixture of orthorhombic ferroselite (FeSe2) with small quantity of another orthorhombic ferroselite (FeSe2) at all the deposition temperatures. Also the mixed phases of iron selenide thin films were deposited at different deposition temperature (500, 550 and 600 °C) onto silicon substrates from these i complexes. The p-XRD Pattern of the deposited films from [Fe{(SeP Pr2)2N}2]

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(3) and [Fe{(SePPh2)2N}2] (4) at 500, 550 and 600 °C shows the mixture of iron selenide ((Fe7Se8 and FeSe2) The intensity of the peaks in p-XRD patterns for the films deposited at 500 °C and 550 °C are very small as compared to the film deposited at higher temperature of 600 °C. SEM images shows the rod and i sheet-like crystallites of thin films deposited from complex [Fe{(SeP Pr2)2N}2] and [Fe{(SePPh2)2N}2] respectively.

7.2 Future Work Future work should focus on the following

1) Synthesis of single source precursors for iron telluride.

2) A proper synthetic investigation of imidodithio/diseleno-phosphinato complexes of iron. I suggest the synthesis of imidodithiophosphinato complexes as the work presented in this thesis dealt only with diseleno complexes.

3) The synthesis of dopped greigite nanocrystals. The doped samples of Co, Ni and Zn have already been prepared and being investigated by X-ray magnetic circular dichroism (XMCD), TEM and magnetic studies. The work in this field is ongoing and has not been presented in this thesis.

4) Preparation of iron complexes with long alkyl chains to be used for the deposition of self-capping iron chalcogenide materials.

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Appendix

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List of Publications 1. Masood Akhtar, Javeed Akhter, M. Azad Malik, Paul O’Brien, Floriana Tuna, James Raftery and Madeleine Helliwell, “Deposition of iron sulfide nanocrystals from single source precursors”, Journal of Material Chemistry, 2011, 21, 9737

2. Masood Akhtar, Ahmed Lutfi Abdelhady, M. Azad Malik, Paul O’Brien, “Deposition of iron sulfide thin films by AACVD from single source precursors”, Journal of Crystal Growth, 346(2012),106–112

3. Masood Akhtar, Javeed Akhtar, Mohammad Azad Malik, Floriana Tuna, Madeleine Helliwell and Paul O’Brien, “Deposition of iron selenide nanocrystals and thin films from tris(N,N-diethyl-N’- naphthoylselenoureato)iron(III)”, Journal of Material Chemistry, 2012, 22, 14970

4. Javeed Akhtar, Masood Akhtar, Mohammad Azad Malik, Paul O’Brien, and James Raftery, “A Single-Source Precursor Route to Unusual PbSe Nanostructures by a Solution−Liquid−Solid Method”, Journal of American Chemical Society, 2012, 134, 2485−2487

5. Masood Akhtar, Mohammad Azad Malik, Floriana Tuna and Paul O’Brien, “The synthesis of iron sulfide nanocrystals from tris-(O- alkylxanthato)iron(III) complexes”, J. Mater. Chem. A, 2013, 1 (31), 8766 - 8774

6. Masood Akhtar, Mohammad Azad Malik and Paul O’Brien, “Routes to Iron Chalcogenide Thin films and Nanoparticles” (Review) ready for submission.

7. Masood Akhtar, Ahmed Lutfi Abdelhady, M. Azad Malik and Paul O’Brien “The use of iron thiobiuret complexes as a single source precursor for the synthesis of iron sulfide nanoparticles” First draft ready.

8. Masood Akhtar, Mohammad Azad Malik and Paul O’Brien, “Synthesis of iron selenide nanocrystals and thin films from bis(tetra-iso- 203

propyldiselenoimidodiphosphinato)iron(II) and bis(tetra-phenyl- diselenoimidodiphosphinato)iron(II) complexes”, manuscript ready.

9. Masood Akhtar, Mohammad Azad Malik and Paul O’Brien, “Simple route for the synthesis of iron chalcogenides nanocrystals from dual source”, Experimental work completed.

10. Masood Akhtar, Mohammad Azad Malik and Paul O’Brien, “Synthesis of iron sulfide nanocrystal from a Cubane-Type Fe-S Cluster with different morphology”, Experimental work completed.

11. Masood Akhtar, Mohammad Azad Malik and Paul O’Brien, “Synthesis of iron sulfide nanocrystals from iron dithiophosphinate complexes” Experimental work underway.

12. Masood Akhtar, Mohammad Azad Malik and Paul O’Brien, “Synthesis of lead sulfide nanoparticles from bis-(O-alkylxanthato)Lead(II) complexes” Experimental work underway.

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