Covalent Functionalization of Germanane

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

Shishi Jiang

Graduate Program in Chemistry

The Ohio State University

2016

Dissertation Committee:

Dr. Joshua E. Goldberger, Advisor

Dr. Anne Co

Dr. Patrick Woodward

Shishi Jiang

2016

Abstract

The group IV graphane analogues represent a new family of single atom-thick

two-dimensioanl materials, in which the structure, stability and properties can be tailored

for covalent ligand termination. These materials have attracted considerable interest as

electronic, optoelectronic and topological materials. Here, we have established the

synthesis, properties and surface functionalization of germananes. For the first time, we

have reported gram-scale, millimeter-size hydrogen- and methyl-terminated germanane

(GeH and GeCH3), via the topotactic deintercaltion of CaGe2 with HCl and CH3I,

respectively. While the surface is oxidizing over time, the bulk of both GeH and GeCH3

are resilient to air for several weeks. Compared with GeH’s band gap at ~1.6 eV, the band gap of GeCH3 is increased by ~0.1 eV to 1.7 eV, producing a strong band edge photoluminescence with little layer dependence. Additionally, the thermal stability is

o o improved from GeH at 75 C to GeCH3 at 250 C. Furthermore, germanane samples with

various ratios between methyl and hydrogen termination (Ge(CH3)xH1-x) were created

from a co-deintercaltion of CaGe2 in the mixture of HCl and CH3I solution. The structure

and property of Ge(CH3)xH1-x changes pseudo-linearly with the x value, in a similar

fashion as Vegard’s law. Finally, this alkylation method was successfully extended to

various organohalides, producing a library of germanane terminated with different

ii organic ligands, which allows us to understand the influence of ligand size and electronegativity on electronic structure. Ligands that have a greater size and are more electronegative will expand the Ge-Ge bond length, thereby lowering the band gap.

Simply by changing the identity of the organic ligand, the band gap of germanane can be tuned by ~15%, highlighting the power of surface chemistry to manipulate the properties of single-atom thick materials.

iii

Acknowledgments

I would like to express my deepest appreciation to my advisor, Dr. Joshua

Goldberger, for his intelligence, motivation and patience as a mentor and for offering me the opportunity to work on an interesting and exciting project. His enthusiasm and persistence in research has inspired me and will continue beyond my graduate study. I also would like to thank Dr. Anne Co and Dr. Patrick Woodward for serving on my committee and for being encouraging and supportive.

I hold many thanks to my collaborates, Dr. Leonard Brillson, Dr. David McComb,

Dr. Ezekiel Johnston-Halperin, Dr. Ronald Kawakami, Dr. Wolfgang Windl and their graduate students for all the stimulating meetings we had, the feedback I received and for helping me to develop the background in physics and material science.

My sincere thanks extend to everyone in Goldberger’s group, previously and currently, for the inspiring discussions, the late night hangout in the lab and all the fun we have had. I also want to thank all my dear friends scattering all over the world, for all the great conversations we had during the up and downs.

Last but not least, my special thanks goes to my big family, especially my mother, and my boyfriend, for your endless support, love and caring.

Vita

2004-2007 ...... Wenling High School, China

2007-2011 ...... B.S. with Honors, Physical Science; B.S., Chemistry

University of Science and Technology of China, China

Publications

1. E. Bianco, S. Butler, S. Jiang, O. Restrepo, W. Windl, J. E. Goldberger, “Stability

and Exfoliation of Germanane: A Germanium Graphane Analogue.” ACS Nano 7,

4414-4421 (2013).

2. S. Jiang, S. Butler, E. Bianco, O. Restrepo, W. Windl, J. E. Goldberger,

“Improving the Stability and Optoelectronic Properties of Germanane via One-

Step Covalent Methyl-Termination.” Nat. Commun. 5, 3389 (2014).

3. S. Jiang, E. Bianco, J. E. Goldberger, “The Structure and Amorphization of

Germanane.” J. Mater. Chem. C 2, 3185-3188 (2014).

4. Y. Ma, Y. Chen, Y. Ma, S. Jiang, J. E. Goldberger, T. Vogt, Y. Lae, “Inter- and

Intra-Layer Compression of Germanane.” J. Phys. Chem. C 118, 28196-28201,

(2014).

5. M. Arguilla, S. Jiang, B. Chitara, J. E. Goldberger, “Synthesis and Stability of

Two-Dimensional Ge/Sn Graphane Alloys.” Chem. Mater. 26, 6941-6945 (2014).

6. S. Jiang,‡ M. Arguilla,‡ N. Cultrara,‡ J. E. Goldberger, “Covalently-Controlled

Properties by Design in Group IV Graphane Analogues.” Acc. Chem. Res. 48,

144-151 (2015).

7. J. R Young, B. Chitara, N. D. Cultrara, M. Q. Arguilla, S. Jiang, F. Fan, E.

Johnston- Halperin, J. E. Goldberger, “Water Activated Doping and Transport in

Multilayered Germanane Crystals.” J. Phys.: Condens. Matter, 3, 034001 (2016).

Fields of Study

Major Field: Chemistry

Table of Contents

Abstract ...... ii

Acknowledgments...... iv

Vita ...... v

Table of Contents ...... vii

List of Tables ...... xi

List of Figures ...... xii

Chapter 1: Introduction ...... 1

1.1 Two-Dimensional Materials ...... 2

1.1.1 Materials from van der Waals Solids ...... 2

1.1.2 MXenes ...... 11

1.1.3 Germanene, Silicene and Stanene ...... 12

1.2 Covalent-Functionalization of 2D Materials ...... 15

1.3 Functionalization on Si/Ge (111) Surface ...... 20

1.4 Zintl Phase and Deintercalation Chemistry ...... 23

1.5 References for Chapter1 ...... 26

Chapter 2: Hydrogen-Terminated Germanane (GeH) ...... 41 vii

2.1 Introduction ...... 41

2.2 Experimental ...... 42

2.2.1 Materials ...... 42

2.2.2 Sample Synthesis and Preparation ...... 43

2.2.3 Instrumentation ...... 44

2.3 Results and Discussion ...... 45

2.3.1 Structural Characterization ...... 45

2.3.2 Optical Properties and Band Structure ...... 56

2.3.3 Air Stability ...... 61

2.3.4 Thermal Stability ...... 62

2.4 Conclusion ...... 70

2.5 References for Chapter 2 ...... 70

Chapter 3: Methyl-Terminated Germanane (GeCH3) ...... 76

3.1 Introduction ...... 76

3.2 Experimental ...... 77

3.2.1 Materials ...... 77

3.2.2 Sample Synthesis and Preparation ...... 77

3.2.3 Instrumentation ...... 79

3.3 Results and Discussion ...... 80

viii

3.3.1 Structural Characterization ...... 80

3.3.2 Optical Properties and Band Structure ...... 85

3.3.3 Thermal Stability ...... 91

3.4 Conclusion ...... 92

3.5 References for Chapter 3 ...... 93

Chapter 4: Enhanced Methyl Coverage and Stability in GeCH3 via an Optimized Air-Free

Route ...... 96

4.1 Introduction ...... 96

4.2 Experimental ...... 97

4.2.1 Materials ...... 97

4.2.2 Sample Synthesis and Preparation ...... 97

4.2.3 Instrumentation ...... 98

4.3 Results and Discussion ...... 99

4.3.1 Air Stability of GeCH3 Synthesized from Bi-Phase Method ...... 99

4.3.2 Reaction Optimization ...... 108

4.3.3 Characterization of GeCH3 from the Optimized Method ...... 110

4.3.4 Air and Thermal Stability of GeCH3 from the Optimized Method ...... 113

4.4 Conclusion ...... 116

4.5 References for Chapter 4 ...... 116

Chapter 5: Interplay of Ligand Strain and Electronegativity in Tuning the Electronic

Structure of Covalently Functionalized Germanane ...... 119

5.1 Introduction ...... 119

5.2 Experimental ...... 120

5.2.1 Materials ...... 120

5.2.2 Sample Synthesis and Preparation ...... 120

5.2.3 Instrumentation ...... 122

5.3 Results and Discussion ...... 123

5.3.1 Full Coverage Ligands...... 123

5.3.2 Larger Ligands ...... 129

5.3.3 Ge(CH3)xH1-x ...... 132

5.3.4 Ligand Effect on Electronic Structure ...... 136

5.4 Conclusion ...... 140

5.5 References for Chapter 5 ...... 141

Chapter 6: Conclusion and Outlook ...... 145

Bibliography ...... 147

List of Tables

Table 2.1 Synthesis conditions for different GeH ...... 44

Table 2.2 Two-phase refined result of GeH...... 51

13 Table 3.1 Comparison of vibrational modes in GeCH3, GeCD3 and Ge CH3 ...... 84

Table 4.1 Water to CaGe2 ratio vs. reaction status ...... 109

Table 5.1 Vibrational modes in GeCH2CH=CH2 ...... 125

Table 5.2 Vibrational modes in GeCH2OCH3 ...... 125

Table 5.3 Fraction of organic functionalization in GeRxH1-x...... 126

Table 5.4 Interlayer spacing between germanium layers in GeRxH1-x...... 128

Table 5.5 a-lattice parameter of GeRxH1-x ...... 128

Table 5.6 Raman shift of four nearly full functionalized germanane...... 129

Table 5.7 Relationship between bond angle, bond length and a-lattice parameter...... 129

Table 5.8 Reaction condition and resulting CH3 coverage in Ge(CH3)xH1-x...... 133

List of Figures

Figure 1.1 Crystal structure of layered van der Waals solids ...... 4

Figure 1.2 Visualization of graphene via microscopy ...... 6

Figure 1.3 Layer-dependent band structure, PL and absorption in MoS2 ...... 9

Figure 1.4 Structure of MAX phases and the corresponding MXenes ...... 12

Figure 1.5 Crystal structure of puckered silicene, germanene and stanene ...... 13

Figure 1.6 STM of silicene on Ag (111) surface ...... 15

Figure 1.7 Routes of covalent functionalization on graphene ...... 17

Figure 1.8 Calculated band structure in functionalized silicene ...... 19

Figure 1.9 Calculated band gap and lattice constant in functionalized stanene ...... 20

Figure 1.10 Functionalization on Si/Ge (111) surface via wet chemistry ...... 22

Figure 1.11 Scheme of topotactic deintercalation...... 24

Figure 2.1 Topotactic synthesis of GeH ...... 47

Figure 2.2 PDF of Ge and GeH ...... 49

Figure 2.3 Refined PDF results of GeH ...... 51

Figure 2.4 FTIR of GeH...... 52

Figure 2.5 Raman of GeH and Ge ...... 53

Figure 2.6 XPS of Ge 2p in GeD and Ge (111) wafer ...... 54

Figure 2.7 TEM of GeH ...... 56

xii

Figure 2.8 DRA and band structure of GeH ...... 58

Figure 2.9 Absorption spectrum of GeH fitted to different band structures ...... 59

Figure 2.10 Air stability of GeH ...... 62

Figure 2.11 Thermal stability of GeH ...... 64

Figure 2.12 FTIR of GeH in the far IR range...... 65

Figure 2.13 Temperature-dependent PDFs...... 67

Figure 2.14 FTIR of GeH made from acetic acid...... 68

Figure 2.15 Temperature-dependent DRA of different GeH samples ...... 69

Figure 3.1 Picture of bi-phase CH3I/H2O reaction setup...... 78

Figure 3.2 The one-step topotactic transformation of CaGe2 into GeCH3 crystals...... 82

Figure 3.3 FTIR of GeCH3 with different isotopes...... 84

Figure 3.4 Ge 2p3/2 XPS spectra for GeD and GeCH3 ...... 85

Figure 3.5 The optical properties and band structure of GeCH3...... 88

Figure 3.6 AFM images and height profiles of exfoliated GeCH3 flakes ...... 89

Figure 3.7 The enhanced thermal stability in GeCH3 ...... 92

Figure 4.1 FTIR of bi-phase GeCH3 and GeCD3 collected before and after HCl treatment, along with after HCl treatment and exposure to air for a day ...... 101

Figure 4.2 Survey and Ge 2p3/2 XPS of bi-phase GeCH3 collected before and after HCl treatment, along with after HCl treatment and exposure to air for a day ...... 102

Figure 4.3 Time-dependent FTIR of as-obtained bi-phase GeCH3 ...... 104

Figure 4.4 Cl 2p XPS spectra and FTIR (far IR range) of bi-phase GeCH3 collected before and after HCl treatment...... 104

xiii

Figure 4.5 Reactions happened in bi-phase GeCH3 under different conditions ...... 106

Figure 4.6 PL of bi-phase GeCH3 collected before and after HCl treatment, along with after HCl treatment and exposure to air for a day ...... 107

Figure 4. 7 FTIR and XRD of uncomplete reaction between CaGe2 and CH3I with four equivalents of water ...... 110

Figure 4.8 FTIR and Ge 2p3/2 XPS comparison between two GeCH3 samples ...... 112

Figure 4.9 DRA and PL comparison between two GeCH3 samples ...... 113

Figure 4.10 Air stability of GeCH3 obtained from air-free method ...... 114

Figure 4.11 Thermal stability of two GeCH3 samples ...... 115

Figure 5.1 Characterization of four nearly full functionalized GeR ...... 124

Figure 5.2 XRD of GeR (R = alkyls) ...... 131

Figure 5.3 FTIR of GeR (R = alkyls) ...... 132

Figure 5.4 TGA of Ge(CH3)xH1-x...... 133

Figure 5.5 XRD of Ge(CH3)xH1-x ...... 135

Figure 5.6 Raman and DRA of Ge(CH3)xH1-x ...... 136

Figure 5.7 Ligand effect on band gap ...... 138

Figure 5.8 Raman shift and band gap correlation in all GeRxH1-x ...... 140

xiv

Chapter 1: Introduction

Silicon and germanium, the workhorse materials of the semiconductor industry,

are the most important and ubiquitous materials of the current era. Still, the neverending push toward device miniaturization calls for decreasing the size of electronic channels.

The past decade has witnessed the creation of two-dimensional (2D), or single-atom-thick

materials, which have led to the discovery of new phenomena and properties that are not

present in the parent three-dimensional (3D) materials.1-4 Furthermore, the creation of

single-atom thick derivatives of silicon and germanium oriented in a honeycomb

arrangement are predicted to have direct band gaps compared to the indirect gap in the

bulk of silicon and germanium, opening up light-emission based applications for these materials.5 Consequently, the creation of stable, robust single-atom thick 2D sheets of these semiconductor materials would have enormous implications in the electronics and optoelectronic industries.

The focus of this dissertation involves the synthesis, properties, and applications of functionalized germanium Graphane analogues. In this chapter, we first summarized the development of other 2D materials and their functionalization. Then we describe previous studies on the functionalization chemistry on Si/Ge (111) surfaces and the topotactic deintercalation chemistry that has been used to synthesize 2D silicon and

germanium derivatives.

1.1 Two-Dimensional Materials

Historically, 2D materials are substances with a thickness of a few nanometers or

less, where electrons and heat can transport freely in the 2D plane but restricted in the

third direction. Here, by 2D materials, we specifically refer to materials in which the

atomic organization and bond strength along two-dimensions are similar and much

stronger than along a third dimension. In this section, we discuss 2D materials in three

categories: 1) materials whose crystal structure features neural, single-atom-thick or

polyhedral-thick layers of atoms with covalent or ionic bonding along two dimensions and van der Waals bonding along the third, and single-to few layers can be prepared via exfoliation; 2) materials that are synthesized from the chemical etching or deintercalation of a 3D ionic crystals like MXenes; 3) materials that can only be grown via the deposition of single-atom thick films on surfaces, such as group IV graphene analogues.

1.1.1 Materials from van der Waals Solids

The general physical approach to obtain a 2D material is through the exfoliation of layered van der Waals solids,1, 6 whose crystal structures are connected via strong covalent or ionic bonding within the 2D plane to form single-atom-thick or polyhedral- thick layers, whereas the layers are held together via van der Waals interaction in the third out of plane direction (Figure 1.1). Currently, the most well studied 2D materials are the single layers of graphite, hexagonal boron nitride (hBN), black phosphorus (BP) and layered transition metal dichalcogenides (LTMDs). Single layers of graphite (graphene) and hBN consist of single-atom-thick sheets of sp2-hybridized carbon (or boron and

nitrogen in hBN) in an extended honeycomb network (Figure 1.1a,b). While the single

layer BP, phosphorene, also consists of linked six-atom rings, those rings are puckered

due to the sp3 hybridization of phosphorus atoms (Figure 1.1c).7 Additionally, phosphorene has two different edges, armchair and zigzag edges, featuring structural anisotropy. LTMDs refers to MX2, where M = Ti, Zr, Hf, V, Nb, Ta, Re and X = S, Se,

Te. The structure of LTMDs is comprised of hexagonally packed MX6 octahedrons (for

d0, d3, and some d1 metals) or trigonal prisms (for d1 and d2 metals) share edges with their

six nearest MX6 neighbors within each layer to form 1T and 2H crystal structures,

respectively (Figure 1.1d).8

Figure 1. 1 Crystal structure of layered van der Waals solids: (a) graphite, (b) hexagonal boron nitride, (c) black phosphorus [Reprinted with permission from ref 73.

Copyright 2014 American Chemical Society.] and (d) layered transition metal dichalcogenides (MX2, X = yellow spheres) with octahedral and trigonal prismatic coordination in 1T and 2H crystal structure, respectively [Reprinted with permission from

Though the exfoliation of LTMDs can be traced back to 1960s-1990s, when

materials like MoS2 and NbSe2 were successfully exfoliated into single- or few-layers and characterized to some extent,9-13 the extensive research in 2D materials only started after the isolation of graphene in 2004.1 Part of the reason is that the techniques back in 4

1960s are limited and with the well-developed modern characterization methods, it is possible to locate, identify and characterize these extremely thin 2D materials nowadays.14, 15 Optical microscopy and fluorescence quenching microscopy (FQM) are

powerful techniques used to locate single and fewer layer flakes. To visualize 2D

materials under optical microscopy, it is usually exfoliated or transferred onto a SiO2/Si

1, 6 substrates with ideal dielectric (SiO2) thickness. Here, the color of the dielectric-coated

wafer depends on an interference effect from reflection off of the two surfaces of the

dielectric. Single- and few-layer flakes on the surface of the dielectric modify the

interference and create a color contrast between the flake and the substrate (Figure

1.2b).15 FQM uses an organic fluorophore whose fluorescence is quenched in the

presence of single- or few-layer sheets via a fluorescence resonance energy transfer

mechanism (Figure 1.2c).16 However, the above microscopy methods cannot conclusively

tell if a flake is single, double or few layers. The most direct technique to determine the

layer thickness is atomic force microscopy (AFM) (Figure 1.2a). Nevertheless, due to the difference between tip-substrate and tip-sample interaction, the sample height cannot be measured accurately in a single layer.17 This issued can be solved via the measurement of the folded edges or the steps in the exfoliated sample, where both interactions are between tip and sample.1, 6

Figure 1. 2 Visualization of graphene via microscopy. (a) AFM, (b) optical microscopy, and (c) FQM of single- and fewer-layer graphene, all scale bars are 10 μm.

Raman spectroscopy can be used to fingerprint the layer-thickness according to the change in peak position, width and intensity, allowing unambiguous and high- throughput identification of the number of layers, which is very useful in this emerging research area.18-22 Furthermore, systematically study in the Raman spectroscopy in the

past decade has shown that besides determining the number of layers, it can also be used

to determine the orientation of layers, the quality and types of edge, and the effects of

perturbations, such as electric and magnetic fields, strain, doping, disorder and functional

groups.23

To further understand the structure and morphology of 2D materials, higher

resolution microscopy is necessary. Transmission electron microscopy (TEM) and

aberration-corrected scanning transmission electron microscopy (STEM) can provide

detailed information on the crystallinity, layer sizes, interlayer interaction, stacking and

defects of these ultrathin flakes.24-33 Furthermore, since electronic states in graphene can

be directly addressed by a metallic tip,34 scanning tunneling microscopy (STM) not only

can measure the morphology and electronic structure of graphene,35, 36 but also can

manipulate single atoms at specific points in order to build and characterize

nanostructures.

In addition to the ultra-thin thickness, the intense interest in 2D materials is

mainly due to the discovery of new and unique phenomena that is not present in their

parent 3D materials. In single-layer graphene's band structure, the linear dispersion at the

K point and massless Dirac fermions2 gives rise to novel properties like ambipolar field effect,1 high carrier mobility (~200,000 cm2V-1s-1),37, 38 leading to the detection of single

molecule39 and the observation of room-temperature quantum Hall effect40, 41,

respectively. Studies have also shown that graphene has other unique properties like high

thermal conductivity ( 5000 W/mK),42 exceptional mechanicals strength,43 and even

proton transport44. Graphene∼ -based materials have been proposed for a host of

applications ranging from transparent electrodes for displays and solar cells45-48 to

composite materials49, barrister transistors,50 chemical sensors,51, 52 and high-speed and

radio frequency logic devices53.

LTMDs provide a large family of 2D materials, whose electronic properties can range from metals to semiconductors to insulators, exhibiting interesting phenomena like

indirect to direct transition, excitonic effect, charge density waves and

superconductivity.54 Some of the semiconducting LTMDs, for example, when M = Mo,

W, feature an indirect to direct band gap transition upon exfoliation into single layers

(Figure 1.3a,b).3, 4 Comparing the photoluminescence spectra of those single layers to double-, multi-layer flakes and bulk crystals, not only the intensity is several orders of magnitude stronger, the band gap emission energy also blue shifts (Figure 1.3c-e).3, 4 This

sizable direct band gap (1-2 eV)3, 4, 55 at single layer and recently reported nearly 100%

photoluminescence quantum yield56 allows their applications in electronics and

57 optoelectronics. Field-effect transistor (FET) devices built from monolayer MoS2 have shown moderate carrier mobility (200 cm2 V-1 s-1) and high on/off ratio (108).58 Device application of LTMDs has also been established in photodetectors,59 solar cells,60, 61 light-

emitting diodes,62 lasing,63 and photocatalysts64-66.

Figure 1. 3 Layer-dependent band structure, PL and absorption in MoS2. (a,b)

American Chemical Society.] (c) PL of monolayer and bilayer MoS2. Inset, layer-

dependent quantum yield for 1-6 layers in log scale. (d) Absorption spectra (left axis)

normalized by the layer number and the corresponding PL spectra (right axis) normalized by peak A. (e) Layer-dependent band gap energy of 1-6 layers of MoS2. The band gap

values were inferred from the energy of the indirect gap PL peak I and the direct gap PL

peak A for 2-6 layers and monolayer, respectively. As a reference, the (indirect) band gap

energy of bulk MoS2 is shown as dashed line. [(c-e) Reprinted with permission from

American Physical Society.] 9

A single layer of hBN is known as “white graphene” due to its similar structure as

graphene and its wide band gap67 (5.97 eV). Because of its atomically smooth surface,68

wide band gap and small lattice mismatch with graphite,69 hBN was found to be one of

the best dielectric substrate for graphene to maximize graphene’s electronic

performance.70 It has also been used as the dielectric layer in other 2D materials to

protect fragile single layer materials due to its flatness, high air, thermal and chemical

stability.71

BP was reported to have high mobility and a small band gap (0.3 eV) back to

1960s.72 It was only recently that phosphorene was successfully exfoliated.73 Due to BP’s

structural anisotropy, phosphorene’s electronic properties also differ in these two

directions. The electron effective mass, for example, is several times lighter in the

armchair direction than in the zigzag direction, and a strong angular-dependent transport

was reported in few-layer BP.73, 74 Unlike semiconducting LTMDs, in which direct band

gap is only observed in monolayers,3, 4 the bandgap of BP is expected to be direct for all

thicknesses,73 a significant benefit for optoelectronic applications. The direct band gap is also layer-dependent, increasing from the bulk (0.3 eV) to single layer (1.5 eV),73 which are just the values between those of graphene and the semiconducting LTMDs. As a result, semiconductors with the band gap at nearly the entire range between 0 and 2 eV can now be provided with layered materials. Few-layer BP based FETs are reported to

4 5 2 have an on/off ratios of ~10 -10 and room temperature mobilities up to ~ 200-1000 cm

V-1 s-1,73-75 making it a promising candidate for high-performance electronic and optoelectronic device applications. Compared with graphene and MoS2, phosphorene is

air sensitive, which will degrade under ambient condition in hours.76, 77 This degradation

77 71 can be suppressed by encapsulated with AlOx atomic overlayers or hBN or even by

dipping in solvents like NMP to form a protecting layer over the surface78.

1.1.2 MXenes

In the past five years, a new and large family of 2D materials, MXenes, was

discovered.79 MXenes are produced by selective etching of the A element from the MAX

80 (Mn+1AXn) phases (Figure 1.4), where “M” is an early transition metal (Ti, Nb, Mo,

etc.), “A” is a main group IIIA or IVA element (Al, Si, Sn, etc.), “X” is carbon or/and

n- nitrogen, and n = 1-3. The MAX phases consists of [M6X] octahedrons that share edges with each other to form layered sheets, which are connected via M-A metallic bonding81

between the layers (Figure 1.4, top). The strong interlayer interaction makes it nearly

impossible to get single- or few-layer thick materials via mechanical exfoliation.

However, due to the fact that the bonding between M-X is much stronger than that between M-A, and A is a relative reactive species, the A layers can be selectively etched with aqueous HF at room temperature without disrupting the M–X bonds.82 This process

is always aided with sonication to help delaminate the layers to obtain MXene phases, or

MX layers that are terminated with –OH, –H, –O– and/or –F ligands.82 Currently, about nine MXenes have been produced by this method and other etchants like NH4F and LiF

with HCl were also reported.83, 84 Scanning and transmission electron microscopy have

been used to study the crystallinity and structure of MXenes.81

Figure 1. 4 Structure of MAX phases and the corresponding MXenes. [Reprinted

Weinheim.]

The high electronic conductivities83 and the hydrophilicity due the surface ligands

indicates that MXenes would be promising candidates for many applications such as

hydrogen storage media,85 catalysts support,86 conductive reinforcement additives,

electrochemical energy storage like batteries87-89 and electrochemical capacitors.90, 91.

1.1.3 Germanene, Silicene and Stanene

As the counterpart of graphene for silicon, germanium and tin; silicene, germanene and stanene were predicted to be structurally stable and have novel and

fantastic physical properties akin to graphene such as linear energy dispersion near the K

point, and nontrivial topological properties.92-94 While sharing similar properties with

graphene, they have the advantage of being more easily integrated into the current

semiconductor industry. Compared with graphene, the larger Si-Si/Ge-Ge/Sn-Sn interatomic distance weakens the π-π overlaps, resulting in a buckled structure with sp2-

sp3 hybridization (Figure 1.5).95 The weak π-π bonding makes these materials incredibly

reactive, and consequently they preferentially a fourth bond to stabilize the sp3

hybridization.

Figure 1. 5 Crystal structure of puckered silicene, germanene and stanene. (a) and

(b) are side and top view, respectively. Atoms with different colors (red and yellow) are

not in the same plane, but are of the same element. [Reprinted with permission from

American Physical Society.] 13

However, silicon, germanium and β-tin, all preferentially crystallize into the diamond crystal structure type. Therefore, the synthesis of group IV graphene analogues

can only be accomplished by deposition under ultrahigh vacuum conditions. It was

96 97 demonstrated that silicene could be deposited on Ag (111) (Figure 1.6), ZrB2 (0001)

or Ir (111)98 surfaces. Similar methods were used to grow germanene on Pt (111)99 and

100 101 Au (111) surfaces; and stanene on Bi2Te3 (111) surface. Unfortunately, all of them

are grown on metal substrates, which will mask the 2D materials’ delicate electrical properties, making it impossible to check the theoretical predictions of strange quantum

effects. Another problem of these 2D materials is their chemical instability.102 Because

these single layers can react with the air rapidly, the transfer of them into an insulating substrate or even direct measurement of electronic properties remains challenging. So far,

103 there is only one experimental silicene device report, where Al2O3 was used to

encapsulate silicene together with the native Ag (111) substrate, and it was found that

silicene has the Dirac-like ambipolar charge transport as predicted and a carrier mobility of 100 cm2 V-1 s-1.

Figure 1. 6 STM of silicene on Ag (111) surface. STM image of (a) clean Ag (111) surface, and (b) silicene on Ag (111). (c) Simulation of silicene on Ag (111) to describe the observed STM in (a) and (b). [Reprinted with permission from http://dx.doi.org/10.1103/PhysRevLett.108.155501, ref 96. Copyright 2012 by the

American Physical Society.]

1.2 Covalent-Functionalization of 2D Materials

Since a 2D material is entirely made up of surface, its properties and reactivity are profoundly dependent on the substrate, its local electronic environment, and mechanical deformations. For example, simply by changing or modifying the supporting dielectric

substrate can dramatically change graphene’s carrier mobility and/or reactivity.70, 104 The

sensitivity to the environment is also why 2D materials can usually be used as sensors,44,

45 though lacking selectivity. However, through chemical functionalization of 2D materials, we would not only develop a synthetic handle to tune the electronic structure of 2D materials via the identity of terminating substituent, but could also provide the

approach to highly selective sensing application like the detection of biological agents.

Graphene and hBN are the two most chemical and thermal stable materials among

all 2D materials. To functionalize them, relatively high energy is needed to break the sp2

3 hybridization to form sp hybridization. H2 under a hot tungsten filament and F2 at high

temperature (600 oC) were used to terminate graphene with –H and –F, respectively.105-107

Though patterned hydrogen adsorption on graphene opens a band gap of around 450 meV

near the Fermi level,108 half or full functionalization will disrupt the excellent carrier

mobility in graphene, resulting in an insulator with a band gap of 3-5 eV.105-107 Graphene

can also be functionalized with reactive intermediates like diazonium salts,109 benzyol

peroxide,110 nitrenes,111 carbenes112 and arynes113 (Figure 1.7). Those chemical

modification will open a band gap in graphene, change its carrier mobility and improve

its solubility and processability.114 However, it is hard to control the percentage of

functionalization due to the chemicals’ high reactivity and graphene’s region dependent

reactivity.115, 116 Similarly, hBN can be funtionalized with nitrenes and hydroxyl.117, 118

Figure 1. 7 Routes of covalent functionalization on graphene. [Reprinted with

LTMDs are consist of neutral layers and seem to lack the possibility for chemical

functionalization if pristine. Nevertheless, chemical exfoliation can introduce sulfur

vacancy into MoS2 and the functionalization can be achieved on these vacancies by

119 ligand conjugation of thiol. Another route is to introduce negative charges on MoS2 via chemical exfoliation and then use strong electrophiles like oragnohalides or diazonium salts to quench the charge via covalent functionalization of organic ligands to sulfur.120

However, both methods can only produce a low percentage of ligand coverage and it is

also hard to control the coverage. The preparation of MXenens will always introduce

covalently bound surface ligands to the transition metal (M) to offset the etching of main

group element (A).79 However, it is always a mixture of ligands and it is hard to control

and quantify the percentage of each ligand, making it hard to study the ligand effect on

the properties on MXenens.

Hydrogen-terminated silicane (SiH) is predicted to have an indirect gap at 2.9 eV,

while termination with different ligands converts it into a material with a direct gap

between 2.1 to 2.5 eV (Figure 1.8).5 In heavier elements, novel physical properties like

topological insulators have been predicted when germanium or tin is functionalized with

specific ligands (Figure 1.9),121, 122 opening the door for novel lateral heterostructures

between topological and conventional states. Experimentally, no direct covalent

functionalization has been reported in silicene, germanene or stanene. However, functionalized silicene derivatives have been reported123 and will be summarized in the

last section of this chapter and functionalized germanene derivatives will be covered in

details in this dissertation.

Figure 1. 8 Calculated band structure in functionalized silicene. Silicene was

American Chemical Society.]

Figure 1. 9 Calculated band gap and lattice constant in functionalized stanene.

[Reprinted with permission from http://dx.doi.org/10.1103/PhysRevLett.111.136804, ref

1.3 Functionalization on Si/Ge (111) Surface

Though there is no experimental study on the direct functionalization of silicene

or germanene, the functionalization chemistry on Si/Ge (111) surfaces has been well established (Figure 1.10), where they can be passivated with hydrogen or organic ligands

to improve the stability.124, 125 Upon exposure to air, the surface of Si/Ge (111) substrates

will oxidize immediately. Fortunately, after treating with degassed 40% aqueous NH4F or

aqueous HF for several minutes, surface oxides will be removed and flat hydrogen

terminated Si (111)126, 127 and more rough hydrogen-terminated Ge(111)128 surfaces can

be achieved, respectively. Subsequently, halogenation on Si (111) can be achieved via

radical reaction by treating H-Si (111) with halogen sources like PCl5, Cl2, CCl3Br or N-

bromosuccinimide, under high temperature (80-100 oC) with benzoyl peroxide as initiator

or under UV light.129-131 Besides the fact that these similar halogenation methods can be

applied to Ge (111),132, 133 atomic flat Cl-Ge (111) can also be obtained through a mild approach by treating the oxidized Ge (111) wafer with 10% aqueous HCl for 10 mins.134

There are various approaches to attaching organic ligands to Si/Ge (111) surfaces,

depending on the starting precursors (Figure 1.10). Hydrosilylation or hydrogermylation is the reaction between H-Si/Ge (111) and alkene or alkyne under conditions like high temperature (>150 oC), UV light or the addition of radical initiator or catalysts. During

these reactions, the unsaturated bond is inserted into Si/Ge-H bond, producing alkyl and

alkenyl terminated Si/Ge (111).135-139 Direct reaction between H-Si (111) and Grignard

reagent is also reported while the same chemistry is not working for H-Ge (111).138

Another approach is to react halogen-terminated Si/Ge (111) with strong organic base like Grignard reagent or organolithium.129, 140, 141 While hydrosilylation/hydrogermylation

is a quick reaction that can be completed within a couple of hours,

halogenation/alkylation can allow the attachment of small ligands like methyl or

acetylene which would give higher surface coverage compared with bulky ligands.132, 142,

143 Furthermore, thiols can also be covalently grafted to Ge (111) surface through the

reaction between H-Ge (111) and thiols.144

Figure 1. 10 Functionalization on Si/Ge (111) surface via wet chemistry. Reaction

conditions are highlighted in green.

While H-Si (111) and Br-Si (111) surfaces are stable in air for several minutes,131,

145 H-Ge (111) surface will oxidize immediately upon exposure to air while Cl-Ge (111)

surface are reported to oxidize after an hour of air exposure.146 Though not air-stable over

long periods, these hydrogen- or halogen-terminated surfaces indeed serve as useful

precursors for the organic functionalization reactions. The organic ligand terminated

Si/Ge (111) surfaces are remarkably air stable with alkyl-S-Ge (111)144 stable for up to 12

hours and alkyl-Si/Ge (111) stable from several days to weeks in air or from 30 miniutes to several hours in boiling water or organic solvents.129, 133, 136, 141 The organic

functionalized surfaces are also thermally stable to some extent with C-Ge/Si bonds more

thermally stable than S-Ge bond. Alkyl-S-Ge (111) is stable up to 177 oC and complete

desorption is found when heated to 277 oC.144 While alkyl-Ge (111) surface is thermally

stable up to 200-400 oC,133, 147 alkyl-Si (111) is stable at 300-350 oC.129, 148 From all the

alkyl-Ge (111) reactions, only halogenation/alkylation and neat thermal

hydrogermylation yield well-ordered monolayer surface and enhanced stability.139, 141

1.4 Zintl Phase and Deintercalation Chemistry

The synthesis of one-dimensional inorganic polymers and single rings of

polysilanes, polygermanes, and polystannanes from small molecules has been well

149-151 developed from the reaction between X-(R2M)2-X and alkali metals or between X-

(R2M)2-X and R3MLi, where X refers to halogens, R referes to organic ligands and M

refers to Si/Ge/Sn. To date, there have been no synthetic routes for preparing group IV

graphane analogues from small molecule precursors due to the lack of a mechanism for

controlling growth in two dimensions.

However, the layered Zintl phases, CaSi2, CaGe2, and BaSn2, are comprised of

− − − puckered honeycomb [Si ]n, [Ge ]n, and [Sn ]n graphane-like layers, held together by the

group II [M2+] cations (Figure 1.11, left). Consequently, the preparation of germanium

graphane analogues relies on developing soft chemical processes that can topotactically deintercalate the M2+ cations while maintaining the structure and covalently terminating

the anionic group IV layers (Figure 1.11). Furthermore, those puckered honeycomb

frameworks are structurally analogous to the Si/Ge (111) surfaces. In principle, the

established surface functionalization chemistry149, 156, 164, 181, 182 on Si/Ge (111) surfaces

can be used to modify the ligand if the topotactic deintercalation can produce either

hydrogen- or halogen-terminated frameworks.

Figure 1. 11 Scheme of topotactic deintercalation. Blue: group IVA elements, yellow:

alkali metals, red: functional ligands.

The topotactic deintercalation of CaSi2 using HCl can be traced back to Wöhler in

the 1860s and Kautsky in the 1920s, and the structure and properties were partially

152-159 resolved in the 1980s and 1990s. It was reported that siloxane (Si6H3(OH)3)

preferentially forms at temperatures greater than 0 °C and layered polysilane

152, 153, 157 (Si6H6−x(OH)1−x (x < 1)) forms at -30 °C. These structures are silicon graphane analogues, or silicanes, with –H and –OH or mostly –H terminal substituents, respectively. Compared with the indirect band gap of crystalline silicon (1.1 eV), siloxene has a direct band gap at around 2.4 eV with strong photoluminescence (PL).153

Using these layered polysilane (Si6H6−x(OH)1−x (x < 1)) as precursors, the same surface

functionalization chemistry developed on Si/Ge (111) surfaces applied to synthesize

organic ligand terminated silicanes,160-162 or silicon graphane analogues, in the past

decade with approaches like halogenation/alkylation, hydrosilylation, and even direct reaction between Grignard reagents and Si6H6−x(OH)1−x, featuring PL ranging from 2.7 to

2.9 eV.

However, the topotactic deintercalation of CaSi2 in aqueous HCl readily produces

partially OH-terminated SiHx(OH)1−x due to the significantly stronger Si−O bond (800

kJ/mol) compared with the Si−H bond (300 kJ/mol)163. This ambiguity in surface

functionalization convolutes efforts to correlate the effects of surface functionalization on

the optoelectronic properties of these single-atom thick semiconductors. In contrast, the difference of bond strength is much smaller between Ge−O (660 kJ/mol) and Ge−H (320 kJ/mol)163 and furthermore, any native germanium oxide or hydroxide termination is

readily dissolved in aqueous HCl, thereby producing pure germanane (GeH). Indeed it

was reported by Brandt and Stutzmann that CaGe2 thin films grown on germanium wafers can be topotactically deintercalated to form GeH, with little surface oxidation.164

There are no reports on the further functionalization of hydrogen terminated germanane

(GeH). We attempted to apply functionalization chemistry developed from Si/Ge (111) surfaces to GeH and found that although halogens like bromine could be grafted to form

Ge-Br bond, the radical reaction will amorphize the germanium framework. Furthermore, as will be discussed in the next chapter, GeH will thermally amorphize at temperatures higher than 75 oC and makes reactions that require high temperature not applicable. Thus,

new routes toward functionalization of germanane need to be developed.

In this dissertation, we highlight the synthesis and stability study of both the

hydrogen- and methyl-terminated germanium graphane analogues. We also show that the

methylation chemistry can be extended to various organic functional groups. Finally, the influence of the identity of surface-terminating ligand and the percentage of organic functionalization on the electronic structure of these 2D materials are established.

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Chapter 2: Hydrogen-Terminated Germanane (GeH)

This chapter is reprinted with permission from ACS Nano, 2013, 7, 4414-442.

This chapter is also reproduced by permission of The Royal Society of Chemistry from

J. Mater. Chem. C, 2014, 2, 3185-3188

2.1 Introduction

There has been considerable excitement about the use of two-dimensional (2D) van der Waals materials in optoelectronic applications such as photodetectors and solar cells, due to the change in their electronic structure when exfoliated into single layers.1-8

Similarly, significant changes in electronic structure are expected when three-

dimensional semiconductors such as Si and Ge are converted into their 2D analogues.9-11

Layered silicon and germanium solids have been synthesized from the topotactic

12-14 deintercalation of layered Zintl phases such as CaSi2 and CaGe2. While numerous

studies have shown that the silicon lattice oxidizes immediately in air,12, 15 there is only

one report on GeH thin films grown on Ge (111) substrate and the air and thermal

stability is not rigorously characterized,13 which is an essential prerequisite for any

application. Furthermore, compared with the previous thin film GeH, the preparation of

41 bulk, multilayered crystals greatly facilitates the characterization of elemental composition, structure, and properties using conventional solid-state methods.

Herein, we have synthesized for the first time, millimeter-scale crystals of a hydrogen-terminated germanium multilayered graphane analogue (germanane, GeH) from the topotactic deintercalation of CaGe2. GeH has a layered structure and features a band gap of 1.59 eV. The surface layer of GeH only slowly oxidizes in air over the span of 5 months, while the underlying layers are resilient to oxidation. The GeH is thermally stable up to 75 oC; however, above this temperature amorphization and dehydrogenation begin to occur. Additionally, by preparing GeH via the deintercalation with different acids, we confirmed that the low temperature amorphization is intrinsic to the GeH structure. Finally, we performed high-level theory calculations of the electronic structure, predicting a direct band gap of 1.53 eV and an electron mobility five times higher than that of bulk Ge.

2.2 Experimental

2.2.1 Materials

Germanium powders and calcium turnings were obtained from ACROS Organics and used without any further purification. Aqueous HCl and acetic acid were obtained from Fisher Scientific and used without any further purification. Methanol, aqueous HBr and HI were obtained from Sigma-Aldrich and used without further purification. All water used here was purified with Milli-Q Reference from Millipore.

2.2.2 Sample Synthesis and Preparation

To synthesize calcium germanide (CaGe2), calcium turnings were cut into small

pieces right before the reaction. Around 1g of germanium and 290 mg of calcium, with

the stoichiometric of 1:1.05, were loaded into a feet long quartz tube inside a glovebox.

The tube was then connected to a Schlenk line, evacuated and sealed under vacuum. In a

tube furnace, the sealed quartz tube was heated up to 950 oC at the ramping rate of 10 oC/min and stayed at 950 oC for 18 hrs, followed by slowly cooling down to 400 oC at the

rate of 5 oC/hr and then to room temperature at the rate of 40 oC/hr.

To get GeH flakes, CaGe2 crystals were put in an extraction thimble, fully

immersed in concentrated aqueous HCl in a beaker, with a stir bar at the bottom of the

thimble. The reaction was running at -40 oC for around a week. For purification, GeH

flakes were washed with MilliQ-water followed by methanol, and then dried on a

Schlenk line.

To study the thermal stability and amorphization of GeH, different versions of

GeH were made by replacing the proton source, concentrated aqueous HCl, with

concentrated aqueous HBr, HI, acetic acid and dilute HCl. The reaction temperature

ranged from -40 oC to room temperature while the reaction time varies from 5 to 14 days

(Table 2.1).

Acid (Kα) Temperature (oC) Time (days) Conc. HCl (106) -40 8 Conc. HBr (109) -10 6 Conc. HI (1010) -40 11 Conc. HCl (106) 25 5 Acetic acid (1.7 x 10-5) 25 14 1.6 M HCl (106) 25 10 Table 2. 1 Synthesis conditions for different GeH.

For thermal stability study, GeH flakes were annealed every 25 oC from 75 to 200

o C for 4 hours, respectively, under the flowing of 5% H2 in Ar, and then cooled down to

room temperature for characterizations. This process is repeated for all the GeH samples

prepared with various acid.

2.2.3 Instrumentation

X-ray diffraction (XRD) was measured using Bruker D8 powder X-ray

diffractometer. The room temperature XRD patterns of CaGe2 and GeH were collected

under capillary mode, while the temperature-dependent XRD patterns were collected under flat plate mode. Fourier transform infrared spectroscopy (FTIR) were collected on a Perkin-ElmerFrontier Dual-Range FIR/MidIR spectrometer that was loaded in an Ar- filled glovebox. The Raman spectra were collected using 633 nm (He-Ne red laser) illumination on a Renishaw InVia Raman equipped with a CCD detector. X-ray photoelectron spectroscopy (XPS) was collected using a Kratos Axis Ultra X-ray photoelectron spectrometer equipped with a monochromated (Al) X-ray gun. The Ar ion was used to etch the top layer of GeH, and the etch rate was calibrated using SiO2.

Transmission electron microscopy (TEM) was collected on CM200 from Philips. Diffuse

reflectance absorption (DRA) was collected using a PerkinElmer Lambda 950 UV/Vis

spectrometer, with a diffuse reflectance integrating sphere attachment. X-ray

fluorescence (XRF) measurements were performed using an Olympus DELTA hand-held

X-ray fluorescence analyzer. Thermogravimetric analysis (TGA) was performed using a

Q-500 thermogravimetric analyzer. Samples were analyzed from room temperature to

o o 375 C at a ramping rate of 10 C/min under the flowing of N2 at the rate of 50 ml/min.

The pair distribution function (PDF) measurements were performed at the 11-ID-

B beamline at the Advanced Photon Source at Argonne National Laboratory using high-

energy X-ray radiation (58–60 keV). For the sample preparation, the GeH flakes were

finely ground and loaded into a kapton capillary tube, which was sealed with epoxy at

both ends. During the measurement, a 2D image plate camera16 was used with a sample

to detector distance of 188.248 mm, as calibrated by using a CeO2 standard and Fit 2D

software.17 The scattering from the kapton tube was measured independently and subtracted as a background. Fit 2D software was used to integrate the raw data to get

S(Q).17 The real space PDF, or G(r), was obtained using PDFgetX3 software.18 The

PDFgui program was used to refine the crystal structures.19 For temperature-dependent

PDF, variable temperature data was collected by ramping the temperature from room temperature to 250 oC at a rate of 0.5 oC/ min. Each data set was collected over a time

span of 3 minutes and a temperature range of 1.5 oC.

2.3 Results and Discussion

2.3.1 Structural Characterization

o After annealing at 950 C, highly crystalized CaGe2 was formed in the β-phase, confirmed with XRD (Figure 2.1b, d). This β-CaGe2 has six germanium layers per unit

cell (R-3m) with the lattice parameter of a = 3.987 Å, c = 30.0582 Å (c/6 = 5.097 Å).

After reacting with HCl, the big CaGe2 crystal trunks were automatically exfoliated into

flakes, yielding crystallites of GeH that are 2-3 mm in diameter and <100 μm in thickness

(Figure 2.1c). XRD pattern (Figure 2.1e) of GeH confirms that it can be fit to a 2H unit

cell (2 GeH layers per hexagonal c-unit cell spacing) with a = 3.88 Å and c = 11.0 Å (5.5

Å per layer). However, with recent synchrotron diffraction data, we found that a 6R unit cell (6 GeH layers per hexagonal c-unit cell spacing) with a = 3.97 Å and c = 33.1 Å (5.5

Å per layer) gives out better refinement results with the diffraction pattern. Compared to the original CaGe2, the hydrogen-terminated germanane is slightly contracted in the a-

direction but expanded in the c-direction due to the replacement of Ca2+ with two Ge-H

bonds between each layer. These lattice parameters do not correspond to any of the

previously reported allotropes of germanium.20, 21 The narrower full-width-half-maximum

(FWHM) of the (100) (or (012) in 6R) and (110) diffraction reflections ( 0.4o in 2θ)

compared to the (002), (011), and (112) (or (006), (104) and (116) in 6R)∼ peaks ( 1.3o in

2θ) indicates that there is a significant amount of disorder along the c-axis, which∼ is common in layered materials. This interlayer disorder and the lack of more peaks precludes Rietveld structural refinement with the in-house XRD pattern.

Figure 2. 1 Topotactic synthesis of GeH. (a) Scheme of topotactic deintercalation of

CaGe2 into GeH. (b,c) Optical images and (d,e) XRD of CaGe2 (b,d) and GeH (c,e) crystals with select crystals on graph paper with a 1mm grid.

In contrast to XRD, which measures the averaged long range structure, PDF treats

Bragg and diffuse scattering on an equal basis, and is used to unravel the short and medium range atomic arrangement in various materials from amorphous to crystalline.22-

27 Thus, it is a really powerful method to solve the structure of materials with varying degrees of disorder, like the GeH here. In the measurements, the PDF gives us the probability of finding an atom at a given distance r from another atom. In other words, it measures the atom-atom distance distribution.

The PDFs of crystalline Ge and GeH at room temperature directly confirms the honeycomb 2D network of germanium atoms in GeH. The structural models of GeH and crystalline Ge are shown in Figure 2.2 a and b, respectively. They both consist of hexagonal, puckered sp3 layers of germanium atoms with similar Ge–Ge bond lengths and Ge–Ge–Ge bond angles (Figure 2.2c and highlighted in red boxes in Figure 2.2a,b).

Compared with the G(r) of crystalline Ge, GeH has systematic absent interactions at 5.66,

7.35, and 8.95 Å (Figure 2.2d) that arise in 3D crystalline Ge. In Ge, these peaks correspond to Ge-Ge pairs between atoms in different (111) layers (Figure 2.2b, arrows).

All other peaks can be indexed to the Ge-Ge pairs within a single Ge (111) plane (Figure

2.2c). The distance of the Ge atoms on a particular colored ring from the central Ge atom in Figure 2.2c corresponds to the same color peak in the PDF of Figure 2.2d. The large interlayer disorder of GeH prevents the observation of scattering between any interlayer

Ge–Ge pairs.

Figure 2. 2 PDF of Ge and GeH. Structural models of germanane (a) and crystalline germanium (b), both of which have a hexagonal, sp3-puckered layer of germanium atoms

((c), boxes in (a) and (b)). (c) A single (111) plane of crystalline germanium, representing a single layer of GeH. The distance of the germanium atoms on a certain colored ring from the central germanium atom corresponds to the same color peak in (d). (d) PDFs of

GeH and Ge, data are truncated at 10 Å to highlight the differences. The starred peaks correspond to the Ge–Ge pairs arrowed in (b).

This obtained room temperature PDF data was refined to elucidate the structure of

GeH (Figure 2.3). Since the sample that analyzed in the PDF had trace amounts of crystalline Ge, the data was refined with two different phases. The first phase was

nanocrystalline GeH with a 2H P63mc space group, anisotropic thermal factors and a

refined spherical envelope function. Since the only difference between 2H and 6R unit

cell is the stacking order and as mentioned above, only in plane Ge-Ge pairs were

observed in the PDF of GeH, there should be no difference in using P63mc or R-3m space

group for the first phase. The second phase was crystalline Ge with an Fd-3m space

group, isotropic thermal factors, and no spherical envelope function. The refinement that

incorporates both phases can accurately account for all observed peaks, and produce a

decent fit from 1-30 Å with an Rw value of 0.16 (Figure 2.3 and Table 2.2). The

percentage of crystalline Ge impurity was refined to ~5%. In GeH, a large out-of-plane

2 thermal factor (U33) of around 0.6 Å was observed for the Ge atoms, compared to the in-

2 plane thermal factors (U11 = U22) of 0.02 Å , reflecting a large distribution of interlayer

thicknesses. Other models of the 2D germanane framework such as a Cmca framework

that resembles the hydrogenated orthorhombic black phosphorus structure,28 or the

previous reported Ge allotropes20, 21 do not give reasonable fits to the room temperature

PDF data. However, we would like to point out that the large interlayer thermal factor

(U33) possibly indicates that a larger box model containing multiple Ge layers may more accurately model the distribution of the interlayer spacing and give us a more reasonable thermal factor.

Figure 2. 3 Refined PDF results of GeH. The blue and red curves are the experimental and calculated data, respectively, while the green curve is the difference between them.

Inset is a model of GeH structure with the calculated bond length and bond angle.

GeH Ge Space group P63mc Fd-3m a (Å) 3.986(9) 5.648(8) c (Å) 11.03(36) Atomic position of Ge(1) 0, 0, 0.0177(31) 0, 0, 0 Atomic position of Ge(2) 2/3, 1/3, 0.09 2 Ge(1) U11 = U22 (Å ) 0.02(6) 0.0062(36) 2 Ge(1) U33 (Å ) 0.62(38) 2 Ge(2) U11 = U22 (Å ) 0.02(6) 2 Ge(2) U33 (Å ) 0.57(42) Percentage 94.6(2.0) Sp diameter (Å) 28.0(4.3) NA Rw 0.16 Table 2. 2 Two-phase refined result of GeH.

To further confirm hydrogen termination, we performed FTIR, Raman

spectroscopy and XPS on the germanane product. Transmission mode FTIR (Figure 2.4) of samples ground up and pressed into KBr pellets shows extremely strong Ge-H stretching at ~2000 cm-1 and multiple wagging modes at 570, 507, and 475 cm-1.

Additionally, weak vibrational modes at 770 and 825 cm-1 are also observed. These two

vibrations also occur in the spectra of amorphous Ge0.7:H0.3 thin films and have been

assigned by M. Cardona et al. to originate from bond-bending Ge-H2 modes from nearest

29, 30 neighbor Ge atoms. Thus, we hypothesize that these vibrations correspond to Ge-H2

bond-bending modes from neighboring Ge atoms at the edges of each crystalline

germanane sheet and/or to Ge-H2 bonds within the lattice arising from Ge vacancies. We

do not observe the presence of the broad, intense Ge-O-Ge and Ge-O vibrational modes

that occur between 800 and 1000 cm-1.31

Figure 2. 4 FTIR of GeH. 52

From Raman spectroscopy (Figure 2.5), the main Ge-Ge stretch in GeH occurs at

-1 -1 302 cm , which is slightly blue-shifted compared to the 297 cm E2 Raman mode for

crystalline germanium. In addition, a second vibrational mode emerges at 228 cm-1. We performed ab initio calculations of the Γ-point phonon modes in GeH using Perdew-

Burke-Ernzerhof (PBE) functionals as implemented in VASP.32, 33 These calculations

predict the presence of Ge-based A1 and E2 Raman modes (assuming a C6v point group)

that occur at 223 and 289 cm-1, respectively, which are in good agreement with the

observed Raman modes. The symmetries of the vibrational modes are shown in the inset

of Figure 2.5.

Figure 2. 5 Raman of GeH (blue) and Ge (red). The middle inset highlights the

difference in energy of the E2 peak between GeH and Ge while the right insets are schematic illustrations of the A1 and E2 vibrational modes.

XPS measurements are also indicative of a single germanium oxidation state. XPS

analysis of the Ge 2p3/2 peak for GeH shows a single peak at 1217.8 eV, which is

+ 0 indicative of Ge . A shift in the Ge 2p3/2 peak energy from Ge (1217.0 eV) is expected

since hydrogen is more electronegative than germanium (Figure 2.6). A control Ge (111)

wafer with surface oxide shows a mixture of germanium oxidation states ranging from

Ge+ (1217.0 eV) to Ge2+ (1218.9 eV) to Ge4+ (1221.4 eV).

Figure 2. 6 XPS of Ge 2p in GeD (blue) and Ge (111) wafer.

TEM analysis indicates the product has a layered morphology with individual

layers having less contrast than the 10 nm lacey carbon support grid (Figure 2.7a,b). The

energy dispersive X-ray spectrum has a strong Ge signal, an absence of Ca and O signals,

and the presence of trace amounts of Cl. The Cl:Ge ratio was estimated to be 2:98 (Figure 54

2.7d). Figure 2.7c is an electron diffraction pattern taken orthogonal to the layers, showing a hexagonal arrangement of diffraction peaks that occur in the a- and b- directions. These data further confirm that the crystallinity of the germanium layered framework is preserved upon HCl treatment, and there is a strong registration in the stacking between each layer. The GeH electron diffraction pattern can be indexed to a simple hexagonal unit cell with a = b ≈ 3.87 Å, assuming a [001] zone axis.

Figure 2. 7 TEM of GeH. (a) Low-magnification and (b) magnified TEM micrograph of

GeH platelets. (c) Electron diffraction pattern of platelets collected down the 0001 zone axis. (d) Energy dispersive X-ray spectroscopy of the GeH sheets.

2.3.2 Optical Properties and Band Structure

The optical properties of germanane were investigated by DRA. This silver-black material has a broad absorption over visible wavelengths, and a linear approximation of the absorption edge suggests a band gap of approximately 1.59 eV (Figure 2.8a). The

Tauc/Davis-Mott expression for materials with 2D densities of states predicts that the

absorbance A(ħω) at photon energy ħω near the band edge would be a step function with

a discontinuity in absorbance at the band gap if the band gap was direct allowed. If the

band gap was indirect allowed, the absorbance would be proportional to ħω – Eg' ± Ep,

where Eg' is the indirect gap and Ep is the energy of a particular phonon mode. However,

it has been experimentally established that the Tauc/Davis-Mott approximations of absorption cannot unambiguously determine the transition mechanism for fundamental absorption for bulk materials with 2D densities of states.34-36 We modeled the absorbance assuming direct-allowed, direct-forbidden, indirect-allowed, and indirect-forbidden gaps using both 2D and 3D densities of states (Figure 2.9). All of these plots estimated fundamental gaps ranging from 1.48 to 1.60 eV. These analyses are complicated by a

broad Urbach edge at the lower end of the absorption tail, which is often indicative of a

large doping concentration or disorder. The presence of photoluminescence is often a

stronger test of a direct band gap. Previously reported studies of GeH thin films proposed

that GeH is a direct band gap material with a fundamental absorption gap at 1.8 eV based

on photothermal deflection spectroscopy and photoluminescence that occurs at 0.45 eV

lower, or 1.35 eV.37 We did not observe any photoluminescence from 1.1 to 1.8 eV when

exciting from 1.38 to 1.96 eV at temperatures ranging from 14 to 300 K. This lack of

photoluminescence and linear slope in our samples might suggest that germanane has an

indirect band gap. However, the lack of photoluminescence alone is not sufficient

evidence of an indirect gap. A direct band gap material could lack photoluminescence if

there is a large concentration of nonradiative defect states or impurities in the sample or if

the material possesses unique surface or edge states. The presence of any of these can

quench photoluminescence and also contribute to the observed bowed Urbach edge.

Therefore, further optimization of the growth and etching chemistry will be necessary

before dismissing the potential existence of a direct band gap. Also, we propose that more

direct measurements, such as angle-resolved photoemission spectroscopy, as well as

additional temperature-dependent absorption studies are necessary to completely

conclude whether germanane has a direct or indirect band gap, especially since our theory

predicts GeH to have a direct band gap.

Figure 2. 8 DRA and band structure of GeH. (a) DRA spectrum of GeH plotted as hνα

vs photon energy highlighting a 1.59 eV band gap. (b) Electronic band structure of an

isolated single layer of GeH calculated using HSE-06 theory including spin-orbit coupling predicting a 1.56 eV direct band gap. The hole and electron effective masses for each extrema are indicated in red.

Figure 2. 9 Absorption spectrum of GeH fitted to different band structures. The

fitting is according to Tauc/Davis-Mott expressions of 2D densities of states and 3D

densities of states and a 37.5 phonon vibration (deduced via the 300 cm-1 Raman shift) was determined. 59

Band structure calculations suggest that germanane is a direct band gap material

both as isolated layers and in the crystal structure having two layers per unit cell. We

used the density functional theory (DFT) code VASP32, 33 to optimize the geometry and

calculate the band structure of isolated single layer and two-layer unit cell GeH. The interactions between cores and electrons were described for relaxation by projector augmented wave (PAW) pseudopotentials38 within the PBE exchange-correlation

function39, 40 with a plane-wave cuto energy of 600 eV. van der Waals interactions

between the layers were included usingﬀ the DFT-D2 method by Grimme.41 For the two-

layer structure, the unit cell was modeled as a P63mc unit cell with relaxed lattice

parameters of a = 4.05 Å and c = 10.56 Å, thus having a 5.3 Å layer spacing. For the

isolated single-layer structure, the calculations were performed in a unit cell with 20 Å of

additional vacuum between GeH layers. To obtain an accurate description of the band

gap in this system, the hybrid HSE0642-44 exchange-correlation function was used. With

this function we obtain a direct gap at the Γ point of 1.56 eV for an isolated layer (Figure

2.8b) and 1.53 eV for the two layer unit cell, which is in excellent agreement with the

observed experimental band gap. The calculated band gap for the two-layer unit cell at

the A point of the Brillouin zone is ~1.77 eV. The difference in energy between the

conduction band minimum at the M point and the valence band maximum at Γ is 2.48

and 2.33 eV for an isolated layer and two-layer unit cell, respectively. In both cases, spin-

orbit splitting at the Γ valence band maximum is 0.2 eV.

Additionally, the effective masses of the conduction and valence bands at each

extremum were calculated for the isolated single layer and are shown in Figure 2.8b. In

bulk crystalline germanium, the conduction band minima occur in the four equivalent

* valleys at the L <111> point, which have much higher effective mass (meL = 1.64) than

* 45 the conduction band valleys at Γ (meΓ = 0.041). However, since GeH can be thought of

as hydrogen-terminated isolated (111) sheets of germanium, we are effectively

eliminating the L wavevector in the Brillouin zone.45 We also calculated from first

principles46 the phonon-limited electronic mobility for an isolated single layer, obtaining

a high mobility of 18,195 cm2 V-1 s-1. This five times increase in electron mobility from

bulk Ge (3,900 cm2 V-1 s-1) is consistent with the reduced electron effective mass in GeH.

2.3.3 Air Stability

The potential utility of germanane for any optoelectronic or sensing device

strongly hinges on its air and temperature stability. Some previous reports state that

hydrogen-terminated Ge (111) surfaces having the same atomic configuration as GeH are resistant to oxidation when the Ge surface has minimal defects, although some debate remains.31, 47, 48 Because FTIR spectroscopy is a sensitive probe of the presence of Ge-O

vibrational modes at 800-1000 cm-1, we exposed the GeH samples in air for a series of

times and the FTIR was measured under the attenuated total reflectance (ATR) mode.

Virtually no change from 800 cm-1 to 1000 cm-1 was observed after 60 days’ air exposure,

thus proving that the bulk of GeH resists oxidation (Figure 2.10a). Additionally, time- dependent XPS was performed to probe changes in the Ge oxidation state of the surface after exposing these layered GeH crystals to air (Figure 2.10b), and the percentage of each germanium oxidation state for all spectra was calculated by applying a standard

Gaussian fit. After 1 month of exposure to air, a Ge2+/3+ shoulder emerges at ~1219.3 eV

(19.5% Ge2+/3+). This peak becomes more intense after 5 months of air exposure (29.7%

Ge2+/3+). After Ar etching the top 0.5 nm (<1 layer), the Ge2+/3+ almost completely disappears with 10.1% Ge2+/3+ remaining. Together, the XPS and FTIR suggest that only the surface of GeH becomes oxidized over time, while the bulk is resilient to oxidization.

Figure 2. 10 Air stability of GeH. (a) Time-dependent FTIR of a GeH flake after exposure to ambient atmosphere for up to 60 days, collected via reflection mode, highlighting minimal changes in the relative intensity of the Ge-H to Ge-O vibrations. (b)

Time-dependent Ge 2p XPS spectra of germanane immediately after exposure to atmosphere, after 1 day, 1 month, 5 month and after 5 months, followed by Ar etching by

0.5 nm.

2.3.4 Thermal Stability

The temperature stability of germanane was probed via various methods like

TGA, DRA, XRD, and Raman. It is shown that DRA is the most sensitive method to detect any thermal amorphization. TGA shows a ~1.1% mass loss at 200-250 oC, which 62

is close to the expected mass loss of 1 equivalent of hydrogen in GeH, as well as a 1.7%

mass loss of that occurs between 320 and 355 oC (Figure 2.11a). This second mass loss likely corresponds to the loss of Cl (3.6% molar). ATR-FTIR measurement in the far IR range indicates that partial germanium is bonded to chlorine (Figure 2.12) due to the presence of Ge-Cl stretching vibrations at ~375 cm-1.49 XRF analysis further supports

this, as there is approximately an order of magnitude decrease in the chlorine intensity after annealing at 375 oC. Furthermore, it has been reported in previous temperature-

programmed desorption studies that Cl desorbs off of germanium at temperatures ranging

from 300 to 350 oC.50 However, there is a significant change in the absorption spectrum

when annealing at temperatures above 75 oC. The absorption onset, as detected by DRA,

red shifts by 0.06 eV upon annealing at 75 oC (Figure 2.11b). The absorption profile

continues to red-shift with higher temperature annealing until 250 oC, when the

absorption onset (0.58 eV) goes below that of bulk germanium (0.67 eV). Previously

studies have reported that amorphous Ge thin films have band gaps lower than that of

bulk germanium (0.50 vs 0.67 eV)51 and amorphous hydrogenated germanium films have larger band gaps (1.1 eV)52. There is no obvious change in the XRD patterns (Figure

2.11c) until 150 oC, at which point the c-axis decreases from 11.04 Å to 10.70 Å and the

FWHM of this (002) reflection decreases from 1.3o to 0.8o in 2θ. The diffraction pattern

shows complete amorphization upon annealing at 175 oC. Raman spectroscopy shows a

consistent decrease in the intensity of both out of plane (A1) and in plane (E2) modes as a

function of annealing temperature (Figure 2.11d). After 175 oC, there is ~2 order

magnitude decrease in the Raman scattering intensity of both modes. Taken together, this

63 suggests that amorphization occurs at temperatures (~75 oC) well below that of dehydrogenation temperature (200-250 oC).

Figure 2. 11 Thermal stability of GeH. (a) TGA analysis of GeH. (b) DRA spectra, (c)

XRD patterns, and (d) Raman spectra of GeH measured after four hours’ annealing at various temperatures under 5% H2 in Ar. In (c) the starred peaks correspond to reflections of an internal Ge standard, and the dashed line is drawn to guide the eye.

Figure 2. 12 FTIR of GeH in the far IR range.

Since PDF measurements does not require periodic lattice and can detect amorphization features, the temperature-dependent PDF measurements can provide more evidence of the amorphization process than XRD. Upon annealing, the scattering between GeH's in-plane Ge-Ge pairs decreases gradually, which coincides with an increase in scattering of Ge-Ge pairs at 5.7 Å, 7.4 Å, and 9.0 Å (Figure 2.13a). These distances correspond to the interlayer Ge–Ge interactions that occur in crystalline Ge and not in GeH. Both the drop off in GeH's Ge–Ge scattering, and the emergence of these new scattering peaks, start at temperatures between 75 and 100 oC, and start to change more significantly between 100 and 125 oC (Figure 2.12a, inset). A more direct signature of amorphization is the disappearance of scattering peaks at high-r values. However, the 65

5% crystalline Ge impurity prevents us from comparing the relative changes in the high-r

and low-r range of the raw data.22, 53 Therefore, we subtracted the partial PDF pattern of crystalline Ge from the temperature dependent PDF data by rescaling the room temperature crystalline Ge PDF to match the amplitude of features above 28 Å of the

variable temperature PDF.27 The Ge-subtracted difference curves at room temperature

and 205 oC are plotted for comparison (Figure 2.13b). Only scattering features at low-r

(1–15 Å) remain in 205 oC PDF, indicative of amorphization. All together, the

temperature dependent PDF confirms that germanane amorphization occurs at

significantly lower temperatures than the 200–250 oC dehydrogenation temperature.

Figure 2. 13 Temperature-dependent PDFs. (a) Temperature-dependent PDF of GeH

from room temperature (purple) to 250 oC (red). The data are truncated at 10 Å to

highlight the differences. Inset is the ratio of the intensity of different peaks (arrowed) to

the peak at 4 Å. The color of the arrows correspond to the color of the curve in the inset.

(d) Difference curve of PDF at room temperature and 205 oC after subtracting crystalline

Ge phase, representing the true features coming from GeH.

To study if the low temperature amorphization is due to the presence of residual

trace percentage of Ge-Cl in GeH, we synthesized different versions of GeH with

different acids, at different temperatures. Though a few percent of halides are always

present in samples prepared from HX (X = Cl, Br, I), no carbonyl vibrational mode was

detect in the FTIR measurement in the range of 1600-1800 cm-1 (Figure 2.14). Since

DRA is the most sensitive method to detect the thermal amporphization, temperature-

dependent DRA measurements were performed on all the GeH samples. A similar

temperature dependent red shift in the DRA spectra, with a 75 oC amorphization onset,

was observed for all of them, regardless of acid treatment conditions (Figure 2.15). This evidence nullifies the hypothesis that low-temperature amorphization is caused by Ge-Cl, and confirms that GeH intrinsically amorphizes upon annealing, independent of the chemical identity of trace impurities.

Figure 2. 14 FTIR of GeH made from acetic acid.

Figure 2. 15 Temperature-dependent DRA of different GeH samples. The GeH samples were synthesized from concentrated HCl at -40 oC (a), acetic acid at room temperature (b), concentrated HBr at -10 oC (c), concentrated HI at -40 oC (d), concentrated HCl at room temperature (e), and 1.6 M HCl at room temperature (f). 69

2.4 Conclusion

In summary, we have created gram-scale, millimeter-sized crystallites of

hydrogen-terminated germanane (GeH) and have characterized for the first time their

long-term resistance to oxidation and thermal stability, a necessary prerequisite for any

practical application. PDF measurements were also used to directly derive of the

hexagonal two-dimensional puckered graphane-like framework of germanane. The

temperature-dependent PDF data directly confirms that germanane amorphizes upon the

increasing of temperature. Additionally, we have confirmed that the thermally-induced

amorphization onset is inherent to the germanane material and cannot be attributed to the

identity of a particular impurity. Theory predicts that GeH has a direct band gap of 1.56

eV with low effective masses, thus strongly increasing the already high carrier mobilities

found in Ge without the penalty of the low bulk gap. This notion of creating

dimensionally reduced molecular-scale “allotropes” of materials with fundamentally

different and potentially transformative properties compared to the bulk can be clearly

expanded beyond carbon.

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43. Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. [Erratum to Document Cited in CA139:042043]. J. Chem. Phys. 2006, 124, 219906.

44. Paier, J.; Marsman, M.; Hummer, K.; Kresse, G.; Gerber, I. C.; Angyan, J. G. Screened Hybrid Density Functionals Applied to Solids. J. Chem. Phys. 2006, 124, 154709.

45. Madelung, O.; Schulz, M.; Weiss, H., Eds. Landolt-Bornstein. Berlin: Springer; 1987.

46. Restrepo, O. D.; Varga, K.; Pantelides, S. T. First-Principles Calculations of Electron Mobilities in Silicon: Phonon and Coulomb Scattering. Appl. Phys. Lett. 2009, 94, 212103.

47. Deegan, T.; Hughes, G. An X-ray Photoelectron Spectroscopy Study of the HF Etching of Native Oxides on Ge (111) and Ge (100) Surfaces. Appl. Surf. Sci. 1998, 123, 66-70.

48. Bodlaki, D.; Yamamoto, H.; Waldeck, D.; Borguet, E. Ambient Stability of Chemically Passivated Germanium Interfaces. Surf. Sci. 2003, 543, 63-74.

49. Behnke, F. W.; Tamborski, C. The Infrared Spectra and Vibrational Assignments for Some Organometallic Compounds: DTIC Document; 1962.

50. Holman, Z. C.; Kortshagen, U. R. Nanocrystal Inks without Ligands: Stable Colloids of Bare Germanium Nanocrystals. Nano Lett. 2011, 11, 2133-2136.

51. Donovan, T.; Spicer, W.; Bennett, J. Evidence for a Sharp Absorption Edge in Amorphous Ge. Phys. Rev. Lett. 1969, 22, 1058.

52. Chambouleyron, I.; Graeff, C.; Zanatta, A.; Fajardo, F.; Mulato, M.; Campomanes, R.; et al. The Perspectives of Hydrogenated Amorphous Germanium as an Electronic Material. Phys. Status Solidi B 1995, 192, 241-251.

53. White, C. E.; Provis, J. L.; Gordon, L. E.; Riley, D. P.; Proffen, T.; van Deventer, J. S. J. Effect of Temperature on the Local Structure of Kaolinite Intercalated with Potassium Acetate. Chem. Mater. 2010, 23, 188-199.

Chapter 3: Methyl-Terminated Germanane (GeCH3)

This chapter is reproduced with permission from Nat. Commun. 2014, 5, 3389

3.1 Introduction

Recent research on two-dimensional (2D) materials has shown the significant role of the immediate environment on the properties and reactivity of these van der Waals layers.1-5 This suggests, on the other hand, the intriguing possibility of manipulating the

properties of single-atom thick materials by covalent termination with rationally designed

substituents.6, 7 For example, a quantum spin Hall state with a surface-tunable spin-orbit

gap has been recently predicted for 2D Sn graphene analogues that are terminated with

halides but not with hydrogen,8 opening the door for novel lateral heterostructures

between topological and conventional states. Furthermore, researchers have recently

developed layered van der Waals systems in which each surface atom requires a covalent

ligand to become coordinatively saturated, such as MXenes, the amine coordinated II–VI

chalcogenides, and the group IV graphene analogues such as silicane or germanane.6, 9-13

Because the valence/conduction bands of these materials are comprised of metal–metal or

metal–anion bonding within the 2D plane,14 rather than the p-bands such as that in

graphene, these systems offer the possibility to tune the entire electronic properties on the

basis of the identity and electron withdrawing capability of the substituent, without

completely disrupting the relevant electronic states. Therefore, developing a general

synthetic approach to graft different organic ligands to the 2D layers will help to develop

the understanding of ligand influence on properties beside theory predictions.

In this chapter, we demonstrate a facile, one-step metathesis approach that

directly converts CaGe2 crystals into mm-sized crystals of methyl-terminated germanane

(GeCH3). Replacing –H termination in GeH with –CH3 increases the band gap by ~ 0.1

eV to 1.7 eV, and produces band edge fluorescence with a quantum yield of ~ 0.2%, with

little dependence on layer thickness. Furthermore, the thermal stability of GeCH3 has

been increased to 250 oC compared with 75 oC for GeH. This one-step metathesis approach should be applicable for accessing new families of two-dimensional van der

Waals lattices that feature precise organic terminations and with enhanced optoelectronic properties.

3.2 Experimental

3.2.1 Materials

Iodomethane and isopropanol were obtained from Sigma-Aldrich and used without further purification. Aqueous HCl was obtained from Fisher Scientific and used without any further purification. Water used here was purified with Milli-Q Reference from Millipore.

3.2.2 Sample Synthesis and Preparation

To synthesize GeCH3, the CaGe2 crystals were loaded into an extraction thimble,

fully immersed in iodomethane, with a separated distilled water phase outside in the

77 beaker and stir bar at the bottom of the extraction thimble (Figure 3.1). The reaction was running at room temperature for around a week. After the reaction, those exfoliated flakes were rinsed with isopropanol, concentrated HCl (aq) followed by isopropanol. The sample was then dried on a Schlenk line at room temperature.

Figure 3. 1 Picture of bi-phase CH3I/H2O reaction setup. The CaGe2 crystals are loaded into an extraction thimble, fully immersed in CH3I, while the byproduct, CaI2, is transferred into the H2O region in the beaker.

For all thermal stability study, the room temperature sample was annealed at different temperatures under the flow of 5% H2 in Ar, then cooled down and characterized at room temperature. 78

To study the photoluminescence (PL) intensity of GeCH3 flakes with different

thickness. The bulk GeCH3 flakes were exfoliated onto 285 nm SiO2/Si substrate with

kapton tape. The tape residue was cleaned with acetone and then with isopropanol,

followed by a N2 blow dry to clean the residue solvent on the substrate.

3.2.3 Instrumentation

Powder X-ray diffraction (XRD) (Bruker D8 powder X-ray diffractometer,

Rigaku MiniFlexII X-Ray diffractometer) and single-crystal XRD (Nonius Kappa CCD

diffractometer) were used to study the structure of GeCH3. Fourier transform infrared

spectroscopy (FTIR) measurements were collected on a Perkin-Elmer Frontier Dual-

Range FIR/MidIR spectrometer that was loaded in an Ar-filled glovebox. X-ray photoelectron spectroscopy (XPS) was collected using a Kratos Axis Ultra X-ray photoelectron spectrometer equipped with a monochromated (Al) X-ray gun. The atomic force microscopy (AFM) images were collected on a Bruker AXS Dimension Icon

Atomic/Magnetic Force Microscope with Scan Asyst. Diffuse reflectance absorption

(DRA) (Perkin-Elmer Lambda950 UV/Vis Spectrometer) and PL (Cary Eclipse

Fluorescence Spectrophotometer) measurements were conducted to study the optical properties of the bulk solid crystals. In the PL measurements, the excitation wavelength was set to 380 nm, the excitation and emission slit widths were set to 20 nm and 5 nm, respectively. The absolute quantum yield of the solid samples was measured with the

Quanta-phi (HORIBA Scientific) assembled in Fluorolog (HORIBA Scientific).

The temperature-dependent and the thickness-dependent PL measurements were collected on exfoliated flakes, using a Renishaw InVia Raman equipped with a CCD

79 detector upon excitation of a 633 nm HeNe laser at a power density of ~24mW/cm2, with a laser spot size of ~2 mm diameter. The thicknesses of these flakes were measured by

AFM to identify exfoliated flakes that had regions of relatively uniform thickness larger than the excitation spot size. The weighted average height from the AFM measurement was used to determine the thickness. For the temperature-dependent PL, exfoliated flakes were annealed at different temperatures under the flow of 5% H2 in Ar in situ, and their

PL was recollected on the same flake after cooling down to room temperature. The same trend was observed for three different exfoliated flakes. Thermogravimetric Analysis

(TGA) using Q-500 thermogravimetric analyzer, was collected at the ramping rate of 10 o C/min and under the flowing of N2 at the rate of 50 ml/min. Elemental Analysis

(Atlantic Microlab Inc) of the C/H ratio was collected to determine the ratio of CH3- termination to H-termination.

3.3 Results and Discussion

3.3.1 Structural Characterization

Since our trials on the functionalization of GeH turned out to be unsuccessful due to the amorphization of GeH upon radical reaction, we looked back into the starting material, CaGe2, and tried to do methylation directly on it with methylation reagent like

- CH3I. We hypothesized that the anionic Ge on the surface and edges of the crystals could react topotactically in an SN2 or metathesis-like fashion with CH3I, to form a Ge-CH3 bond along with CaI2 byproduct. The expansion of the lattice would allow the precursors to diffuse inward and the reaction would proceed to completion. However, in our initial experiments with pure CH3I, only the surface layers of CaGe2 had reacted, likely due to

the low solubility of CaI2. Instead, we developed a bi-phase CH3I/H2O solvent reaction in

which CaGe2 crystals are fully immersed in CH3I, while CaI2 is transferred into the H2O

layer (Figure 3.1). After rinsing in concentrated HCl to remove any trace residual CaI2,

and then isopropanol, the reaction yields crystals of GeCH3 that are ~1mm in diameter

and ~100 mm in thickness (Figure 3.2b). Single crystal X-ray diffraction analysis (Figure

3.2c) shows that each one of these crystallites is a single crystal having a hexagonal

spacing of a = 3.96 Å; however, the interlayer turbostratic disorder and curvature of these

crystallites preclude determination of the c axis spacing. Similar to GeH, the broad and

small number of peaks in the in-house XRD makes it impossible to distinguish the

structure between a 2H P63mc and a 6R R-3mc unit cell (Figure 3.2d). From the analysis

of the XRD pattern, GeCH3 crystals can be fit to either a 2H unit cell with a = 3.97 Å and

c = 17.26 Å (8.63 Å per layer) or 6R unit cell with a = 3.99 Å and c = 51.78 Å (8.63 Å

per layer). This corresponds to an expansion in the a-direction and a 3.1 Å increase in

interlayer spacing compared to GeH,6 which has the interlayer spacing of 5.50 Å. This

3.1 Å increase is close to twice the difference between the Ge-C bond length (1.95 Å) and

the Ge-H bond length (1.52 Å) plus twice the difference between the van der Waals radii

15 of -CH3 (2.0 Å) and -H (1.2 Å), further indicating substitution of the -H substituent with a -CH3 substituent.

Figure 3. 2 The one-step topotactic transformation of CaGe2 into GeCH3 crystals. (a)

Schematic illustration of conversion of CaGe2 (left) into GeCH3 (right). (b) Optical

images of GeCH3 crystals with select crystals on graph paper with a 1mm grid. (c)

Single-crystal XRD pattern of GeCH3 collected down the [001] zone axis. (d) Powder

XRD patterns of GeH (blue) and GeCH3 (red). The starred peaks correspond to

diffraction reflections of an internal Ge standard. The dotted line highlights the changes

in the 100 reflections between GeH and GeCH3.

FTIR spectroscopy further confirms the –CH3 surface termination in GeCH3 and that our sample is free of residual oxide (Figure 3.3). In GeCH3, the major Ge-H

stretching frequency6, 16-19 at ~2,000 cm-1 is almost completely gone and replaced by a

-1 Ge-C stretch that occurs at 573 cm . The other major modes that are observed in GeCH3

-1 correspond to –CH3 stretching at 2,907 and 2,974 cm , –CH3 bending at 1,403 and 1,237

-1 -1 19, 20 cm and –CH3 rocking at 778 cm . The residual amount of Ge-H stretching suggests

that there exists a small percentage of –H termination, and elemental analysis suggests

that 90% ± 10% of the germanium atoms are terminated with –CH3. This residual –H

could result either from the minor solubility of H2O in CH3I, or during the washing

process. To further confirm the identity of each vibrational mode, we also created

13 13 -1 Ge CH3 and GeCD3. In Ge CH3, the Ge-C stretch shifts down to 558 cm , and the other

-1 –CH3 vibrational modes slightly decrease by 1–10 cm (Table 3.1). There is a much

more significant change in the vibrational energies of GeCD3, as the –CD3 stretching

-1 modes are shifted to 2,240 and 2,116 cm , the –CD3 bending mode decreases to 1,024

-1 -1 and 954 cm , the –CD3 rocking mode decreases to 584 cm , and the Ge-CD3 stretch

-1 19 -1 decreases to 530 cm (Table 3.1). As the 778 cm –CH3 rocking mode can possibly

mask the existence of any residual Ge-O-Ge or Ge-O vibrational modes which normally

-1 16, 18 occur from 800–1,000 cm , the shift of this rocking mode in GeCD3 allows

elucidation of any residual Ge–O–Ge or Ge–O. The only vibrational modes observed in

-1 this region for GeCD3 are the 770 and 830 cm vibrations that correspond to bond-

6, 17, 18 bending Ge-H2 modes from nearest neighbor Ge atoms at the crystal edges.

Figure 3. 3 FTIR of GeCH3 with different isotopes. FTIR spectra of GeH (i), GeCH3

13 (ii), Ge CH3 (iii) and GeCD3 (iv). The intensity of the four spectra are all multiplied by

0.5 in the range of 400–900 cm-1.

* 13 Vibrational Modes GeCH3 Ge CH3 GeCD3 -CH3 stretch 2974, 2907 2972, 2902 2240, 2116 -CH3 bend 1403, 1237 1402, 1230 1024, 954 -CH3 rock 778 764 584 Ge-CH3 stretch 573 558 530 13 Table 3. 1 Comparison of vibrational modes in GeCH3, GeCD3 and Ge CH3.

13 *: Here, C refers to either C or C, H refers to either H or D depending on the isotope listed. The units for all vibrations are in cm-1.

XPS measurements indicate a single Germanium +1 oxidation state (Figure 3.4),

further suggesting that GeO2 and other surface oxides are not present. The Ge 2p3/2 peak occurs at 1,217.5 eV, which is slightly shifted compared with the observed 1,217.8 eV peak in GeH, but consistent with CH3-terminated Ge (111). This slight shift to lower XPS binding energy is consistent with previously observed XPS spectra of –H or –CH3

21 terminated silicon surfaces due to the more electron donating nature of –CH3 compared

with –H.

Figure 3. 4 Ge 2p3/2 XPS spectra for GeD and GeCH3.

3.3.2 Optical Properties and Band Structure

The absorption and fluorescence measurements of GeCH3 are consistent with that

of a direct band gap semiconductor. GeCH3 has strong PL emission centered at 1.7 eV 85

(730nm (red)), which is close to the observed DRA onset at 1.69 eV (Figure 3.5a). This

corresponds to a 0.1 eV increase in band edge compared with GeH (1.59 eV) (Figure

3.5b). Band structure calculations for the measured structure using the hybrid HSE0622-24 exchange-correlation functional, confirm this band gap, and predict that the two-layer

unit cell has a direct band gap of 1.82 eV (Figure 3.5c). The red PL can be easily detected

by eye under UV illumination in both solid-state samples and in suspension in isopropanol (Figure 3.5d). The full-width at half maximum (FWHM) of the fluorescence emission is ~250 meV. The absolute quantum yield of the solid flakes was measured to be 0.23%. This represents a minimum bound of the quantum yield, due to the difficulty in correcting for self-absorption in solid-state measurements. The FWHM and quantum yield values are close to those observed in exfoliated single-layer MoS2, which are 50–

150 meV, and 0.4–0.5%,25, 26 respectively. We hypothesize that even further

improvements in the quantum yield could arise with better control over the degree of –

CH3 versus –H functionalization. In MoS2 and many other metal dichalcogenides, a direct

band gap is only observed when exfoliated down to single layers,27 making the

preparation of large-area single layers a necessary and challenging requirement before

optoelectronic devices can be fabricated. In contrast, we observe that the PL emission

intensity of exfoliated samples is linearly proportional to the number of layers from 13–

65 layers (Figure 3.5e, 3.6). The band edge emission does not depend on layer thickness,

at least with 13 layers and above, reflecting the relatively weak electronic coupling and

orbital overlap of the conduction and valence bands in neighboring layers. This is in

contrast to the overlap of neighboring C 2pz orbitals in graphite, as well as the S 3pz

27 orbitals at the G point in MoS2. Finally, the intense PL contrasts with our observations on our previously reported crystals of GeH, of which we have yet to observe any band edge PL.6 Taken together, this data shows that the nature of the covalently modifiable

surface ligand can tune the optoelectronic properties of these materials.

Figure 3. 5 The optical properties and band structure of GeCH3. (a) DRA (red) and

PL (blue) of GeCH3. (b) DRA spectra of GeH (blue) and GeCH3 (red) plotted in

Kubelka-Munk function versus photon energy. (c) Electronic band structure of a bilayer

GeCH3 unit cell calculated using the hybrid HSE06 theory including spin-orbit coupling with experimental lattice parameters (3.96 Å), predicting a 1.82 eV direct band gap. The hole and electron effective masses for each extrema are indicated in red. (d) Images of

GeCH3 photoluminescence of crystals (left) and in suspension in isopropanol (right), upon illumination with a handheld 365 nm light. Scale bars, 1 cm. (e) PL intensity of exfoliated GeCH3 thin flakes having average thicknesses ranging from 13-65 nm. Inset is the raw PL spectra of flakes with the thickness of 13 nm (wine), 27nm (violet), 29nm

(orange), 31 nm (olive), 49nm (magenta) and 65nm (blue). 88

Continued

Figure 3. 6 AFM images and height profiles of exfoliated GeCH3 flakes. The scale bar in d is in 1μm, and all other scale bars are in 2μm. 89

Figure 3.6 continued

3.3.3 Thermal Stability

We previously observed that GeH begins to amorphize upon annealing at 75 oC, is

completely amorphous above 175 oC and starts to dehydrogenate between 200 oC and

o 6 250 C. In contrast, GeCH3 has considerably enhanced thermal stability. According to

TGA, a transition occurs starting at ~300 oC (Figure. 3.7a) that corresponds to the expected mass loss for approximately 90% CH3-termination, which is in excellent agreement with our elemental analysis. We have previously found that DRA is a much more sensitive probe of the degree of amorphization than any other technique, due to the reduced band gap of amorphous germanium.6 In the temperature-dependent DRA, there

is virtually no shift in band edge emission up to 200 oC, whereas after annealing at 250 oC

and 300 oC, the band edge red-shifts by 0.06 and 0.10 eV, respectively (Figure 3.7b). The

powder XRD pattern also shows negligible changes after annealing up to 250 oC, but it is

almost completely amorphous after annealing at 300 oC (Figure. 3.7c). The intensity of

photoluminescence emission also started to decrease after annealing at 250 oC (Figure

3.7d). These techniques collectively demonstrate that GeCH3 begins to amorphize at 250

oC. Considering the lack of any PL in our previous studies on GeH, the enhanced stability

upon methyl-termination is likely necessary to realize semiconductor properties such as

band edge photoluminescence that are often disrupted by defect states.

Figure 3. 7 The enhanced thermal stability in GeCH3. (a) Thermogravimetric analysis of GeCH3. (b,c) DRA spectra (b) and powder XRD patterns (c) of bulk GeCH3 after annealing in 5% H2 in Ar for four hours at various temperatures. (d) Photoluminescence spectra of a single exfoliated GeCH3 flake using the same annealing procedure.

3.4 Conclusion

In summary, we have created for the first time GeCH3, a covalently modified direct band gap germanane, via a one-step topotactic metathesis reaction of CaGe2 crystals with CH3I. We have shown that covalent methyl surface termination not only

increases the band gap by 0.1 eV, but also enhances the thermal stability compared with

GeH. The photoluminescence quantum yield is on the same order of magnitude as other

single-layer metal chalcogenides, but does not have the stringent single-layer requirement to observe such band edge emission, making these materials intriguing building blocks for future optoelectronic devices. This topotactic metathesis reaction can be extended to create new families of organic-terminated van der Waals materials from other solid-state crystal structures. Dimensionally reduced atomic-scale derivatives can have dramatically different properties than their parent analogue, which can be tuned with the nature of the surface substituent.

3.5 References for Chapter 3

1. Wang, Q. H.; Jin, Z.; Kim, K. K.; Hilmer, A. J.; Paulus, G. L. C.; Shih, C.-J.; et al. Understanding and Controlling the Substrate Effect on Graphene Electron- Transfer Chemistry via Reactivity Imprint Lithography. Nature Chem. 2012, 4, 724-732.

2. Du, X.; Skachko, I.; Barker, A.; Andrei, E. Y. Approaching Ballistic Transport in Suspended Graphene. Nat. Nanotechnol. 2008, 3, 491-495.

3. Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; et al. Boron Nitride Substrates for High-Quality Graphene Electronics. Nat. Nanotechnol. 2010, 5, 722-726.

4. Yokota, K.; Takai, K.; Enoki, T. Carrier Control of Graphene Driven by the Proximity Effect of Functionalized Self-Assembled Monolayers. Nano Lett. 2011, 11, 3669-3675.

5. Tongay, S.; Zhou, J.; Ataca, C.; Liu, J.; Kang, J. S.; Matthews, T. S.; et al. Broad- Range Modulation of Light Emission in Two-Dimensional Semiconductors by Molecular Physisorption Gating. Nano Lett. 2013, 13, 2831-2836.

6. Bianco, E.; Butler, S.; Jiang, S.; Restrepo, O. D.; Windl, W.; Goldberger, J. E. Stability and Exfoliation of Germanane: a Germanium Graphane Analogue. ACS Nano 2013, 7, 4414-4421.

7. Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutiérrez, H. R.; et al. Progress, Challenges, and Opportunities in Two-dimensional Materials Beyond Graphene. ACS Nano 2013, 7, 2898-2926.

8. Xu, Y.; Yan, B.; Zhang, H.-J.; Wang, J.; Xu, G.; Tang, P.; et al. Large-Gap Quantum Spin Hall Insulators in Tin Films. Phys. Rev. Lett. 2013, 111, 136804.

9. Mashtalir, O.; Naguib, M.; Mochalin, V. N.; Dall’Agnese, Y.; Heon, M.; Barsoum, M. W.; et al. Intercalation and Delamination of Layered Carbides and Carbonitrides. Nature Commun. 2013, 4, 1716.

10. Lukatskaya, M. R.; Mashtalir, O.; Ren, C. E.; Dall’Agnese, Y.; Rozier, P.; Taberna, P. L.; et al. Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide. Science 2013, 341, 1502-1505.

11. Roushan, M.; Zhang, X.; Li, J. Solution-Processable White-Light-Emitting Hybrid Semiconductor Bulk Materials with High Photoluminescence Quantum Efficiency. Angew. Chem. Int. Ed. 2012, 51, 436-439.

12. Dahn, J. R.; Way, B. M.; Fuller, E.; Tse, J. S. Structure of Siloxene and Layered Polysilane (Si6H6). Phys. Rev. B 1993, 48, 17872-17877.

13. Okamoto, H.; Kumai, Y.; Sugiyama, Y.; Mitsuoka, T.; Nakanishi, K.; Ohta, T.; et al. Silicon Nanosheets and Their Self-Assembled Regular Stacking Structure. J. Am. Chem. Soc. 2010, 132, 2710-2718.

14. Restrepo, O. D.; Mishra, R.; Goldberger, J. E.; Windl, W. Tunable Gaps and Enhanced Mobilities in Strain-Engineered Silicane. J. Appl. Phys. 2014, 115, -.

15. Pauling, L. The nature of chemical bond, 3rd edn. Cornell University Press: Ithaca, New York, 1960.

16. Vogg, G.; Brandt, M. S.; Stutzmann, M. Polygermyne—A Prototype System for Layered Germanium Polymers. Adv. Mater. 2000, 12, 1278-1281.

17. Cardona, M. Vibrational Spectra of Hydrogen in Silicon and Germanium. Phys. Status Solidi B 1983, 118, 463-481.

18. Rivillon, S.; Chabal, Y. J.; Amy, F.; Kahn, A. Hydrogen Passivation of Germanium (100) Surface Using Wet Chemical Preparation. Appl. Phys. Lett. 2005, 87, 253101.

19. Griffiths, J. E. Infrared Spectra of Methylgermane, Methyl-D3-Germane, and Methylgermane-D3. J. Chem. Phys. 1963, 38, 2879-2891.

20. Knapp, D.; Brunschwig, B. S.; Lewis, N. S. Transmission Infrared Spectra of CH3–, CD3–, and C10H21–Ge (111) Surfaces. J. Phys. Chem. C 2011, 115, 16389- 16397.

21. Nemanick, E. J.; Hurley, P. T.; Brunschwig, B. S.; Lewis, N. S. Chemical and Electrical Passivation of Silicon (111) Surfaces through Functionalization with Sterically Hindered Alkyl Groups. J. Phys. Chem. B 2006, 110, 14800-14808.

22. Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 8207-8215.

23. Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Erratum: “Hybrid Functionals Based on a Screened Coulomb potential” [J. Chem. Phys.118, 8207 (2003)]. J. Chem. Phys. 2006, 124, 219906.

24. Paier, J.; Marsman, M.; Hummer, K.; Kresse, G.; Gerber, I. C.; Ángyán, J. G. Screened Hybrid Density Functionals Applied to Solids. J. Chem. Phys. 2006, 124, 154709.

25. Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805.

26. Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M. Photoluminescence from Chemically Exfoliated MoS2. Nano Lett. 2011, 11, 5111-5116.

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Chapter 4: Enhanced Methyl Coverage and Stability in GeCH3 via an Optimized

Air-Free Route

4.1 Introduction

In the previous two chapters, we have reported the synthesis of two different

ligand-terminated germanium graphane analogues: hydrogen-terminated germanane

(GeH) and methyl-terminated germanane (GeCH3), and found that GeCH3 is not only

more thermal stable than GeH, but also exhibits strong photoluminescence (PL) and

1, 2 features a higher band gap. However, the reported GeCH3 has around 10% hydrogen

termination and its air stability is not studied yet. Since 2D materials are very sensitive to

the immediate environment,3-8 a small difference like the increase of methyl to hydrogen

ratio can potentially further improve the optoelectronic properties in GeCH3. For

instance, simply by treating MoS2 in organic superacid, its quantum yield increased

dramatically from below 1% to 95%.9 Furthermore, the previous reaction is a bi-phase

reaction, which is exposed to air and water, making it hard to extend the reaction to other

very air and/or water sensitive organohalides with potentially interesting functional

groups.

To these ends, in this chapter, we first studied the air stability of GeCH3

synthesized from the previous bi-phase method and found that it is very air sensitive due

to the introduced Ge-Cl bond during the HCl treatment step. Then, we developed an

optimized air-free route to achieve higher methyl to hydrogen ratio in GeCH3 (~5%

hydrogen). Additionally, we found that at least six equivalents of water is needed during

the reaction. Finally we have shown that GeCH3 prepared from this air-free method has

improved air and thermal stability, with the bulk sample resilient to air for at least 50

days and the methyl desorption temperature increased by around 75 oC.

4.2 Experimental

4.2.1 Materials

Iodomethane and isopropanol were obtained from Sigma-Aldrich and was used

without further purification. Aqueous HCl was obtained from Fisher Scientific and used

without any further purification. Water used here was purified with Milli-Q Reference

from Millipore. Acetonitrile was also obtained from Sigma-Aldrich and was distilled before use. To distill acetonitrile, CaH2 was first put into acetonitrile and let sit for at

least an hour, then the solvent was distilled into dried molecular sieve at atmosphere

press. The distilled solvent was then bubbled with Ar gas for 20 mins.

4.2.2 Sample Synthesis and Preparation

In the bi-phase method, the CaGe2 crystals were loaded into an extraction thimble,

fully immersed in iodomethane, with a separated distilled water phase outside in the

beaker and stir bar at the bottom of the extraction thimble. The reaction was running at

room temperature for around a week. After the reaction, those exfoliated flakes were

rinsed with isopropanol, concentrated HCl (aq) followed by isopropanol. The sample was

then dried on a Schlenk line at room temperature.

In the optimized synthetic route, CaGe2 crystals were first loaded into an air-free

flask inside the glovebox. The flask was then connected to the Schlenk line, where all the

joints are evacuated and filled with Ar for three times. Liquid reagents including

iodomethane, acetonitrile and water were then added into the flask under flowing Ar. In a

typical reaction, the molar ratios of CaGe2:CH3I:H2O were controlled at 1:30:10 while

CH3CN was used as a solvent. The reaction was run at room temperature for around a week. Reactions with different CaGe2 to water molecular ratios were also conducted with

the same number of equivalents of iodomethane and acetonitrile, as this ratio was found to be critical for the reaction to initialize and to complete. For purification, the suspension was washed with distilled acetonitrile inside a glovebag and then dried on a Schlenk line.

For thermogravimetric analysis (TGA) and elemental analysis, samples were evacuated at

80 oC for three hours on a Schlenk line to get rid of any absorbed water or solvent.

4.2.3 Instrumentation

Powder X-ray diffraction (XRD) (Bruker D8 powder X-ray diffractometer,

Rigaku MiniFlexII X-Ray diffractometer) was used to study the structure of GeCH3.

Fourier transform infrared spectroscopy (FTIR) measurements were collected on a

Perkin-Elmer Frontier Dual-Range FIR/MidIR spectrometer that was loaded in an Ar-

filled glovebox. X-ray photoelectron spectroscopy (XPS) was collected using a Kratos

Axis Ultra X-ray photoelectron spectrometer equipped with a monochromated (Al) X-ray

gun and Ar ion was used for etching. Diffuse reflectance absorption (DRA) measurements (Perkin-Elmer Lambda950 UV/Vis Spectrometer) and photoluminescence

(PL) (Cary Eclipse Fluorescence Spectrophotometer) measurements were conducted to

study the optical properties of the bulk solid crystals. In the photoluminescence (PL)

measurements, the excitation wavelength was set to 380 nm, the excitation and emission

slit widths were set to 20 nm and 5 nm, respectively. TGA (Q-500 thermogravimetric

o analyzer) was collected under flowing N2 with the rate at 10 C/min. Elemental Analysis

(Galbraith Laboratories, Inc.) of the C/H ratio was collected to determine the ratio of

CH3-termination to H-termination in the products.

4.3 Results and Discussion

4.3.1 Air Stability of GeCH3 Synthesized from Bi-Phase Method

10 The starting material, CaGe2, is well known to be very air sensitive. Though it

was protected from air and water with CH3I during the previous bi-phase reaction, a

small amount of water and oxygen can always dissolve into the CH3I layer and oxidize the structure. Consequently, we found that the as-obtained GeCH3 is already oxidized,

-1 with features like a shoulder on the –CH3 rocking mode in the range of 800-1000 cm in the FTIR spectrum (Figure 4.1a), indicating Ge-O bonding11. HCl treatment was then followed to wash away residual germanium oxides (GeOx). After this HCl treatment, the

Ge-O stretching shoulder disappears, indicating that the GeOx are gone. However, since

-1 the Ge-O stretching mode is so close to the strong –CH3 rocking at 770 cm , it is

ambiguous to determine the presence of Ge-O vibration just based on the appearance of a

shoulder peak. To separate the Ge-O stretching mode from –CH3 rocking modes and

clearly demonstrate the appearance and disappearance of GeOx, we collected the same

series of FTIR spectra with GeCD3 (Figure 4.1b). In the as-obtained GeCD3, it is obvious

that germanium is partially oxidized with a broad Ge-O stretching mode between 800 and

1000 cm-1. After HCl washing, this single broad peak is gone and replaced with a doublet

1 which can be assigned to a Ge-H2 bending mode as previous reported in GeH. However, after exposure to air for a day, the Ge-O stretching mode appears again in both GeCH3 and GeCD3 (Figure 4.1).

100

Figure 4. 1 FTIR of bi-phase GeCH3 (a) and GeCD3 (b) collected before (blue) and

after HCl treatment (red), along with after HCl treatment and exposure to air for a day (green).

101

The surface oxidization states of both the before and after HCl treatment samples

were then studied by XPS (Figure 4.2). The as-obtained bi-phase GeCH3 shows a mixture of different oxidation states of Ge+ (1217.6 eV) and Ge2+/3+ (1219.4 eV), indicating that

the surface of these flakes are oxidized (Figure 4.2b). After HCl treatment, only a single

peak was observed at 1217.5 eV, which is indicative of Ge+, confirming that the surface

GeOx are removed with HCl (Figure 4.2b). Additionally, upon exposure to air for three

days, a Ge2+/3+ shoulder emerges at 1219.9 eV with a percentage of 35.6%, which was

calculated by applying a standard Gaussian∼ fit. Consequently, although oxygen-free

GeCH3 is obtained after HCl treatment, both the surface and the bulk of GeCH3 are not

stable in air over time, surprisingly in contrast to the previous studies on alkyl-terminated

Ge (111) surfaces which are reported to be stable in air for several days.12, 13

Figure 4. 2 (a) Survey and (b) Ge 2p3/2 XPS of bi-phase GeCH3 collected before

(blue) and after HCl treatment (red), along with after HCl treatment and exposure

to air for a day (green).

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To further understand the cause of the oxidization, we exposed the as-obtained bi-

phase GeCH3 to air for various time periods and collected the FTIR on them. It was found that the Ge-O stretching shoulder did not change over a time of 26 days (Figure

4.3), indicating that Ge-C bonds are inert to air. Both the survey and Cl 2p XPS spectra

indicate that Cl element was introduced into the sample during the HCl treatment (Figure

4.2a, 4.4a). To confirm that the Cl is bond to Ge, we then collected the FTIR spectrum of

GeCH3 before and after HCl treatment in the far IR range and found the emergence of

Ge-Cl stretching vibrational mode in the sample after HCl treatment at around 375 cm-1

(Figure 4.4b), which is in agreement with the previous reported Ge-Cl vibrational mode.14 Furthermore, the immersing of oxidized Ge (111) wafer into aqueous HCl is

known as a standard approach to obtain Cl-terminated Ge (111) surface.15

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Figure 4. 3 Time-dependent FTIR of as-obtained bi-phase GeCH3.

Figure 4. 4 (a) Cl 2p XPS spectra and (b) FTIR (far IR range) of bi-phase GeCH3 collected before (blue) and after HCl treatment (red).

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We have summarized the reaction scheme that happened on bi-phase GeCH3 in

Figure 4.5. During the topotactic deintercalation, due to the water sensitivity of CaGe2,

the bi-phase reaction (Figure 4.5, top right) will produce the as-obtained GeCH3 with

partial hydroxide termination (Ge(CH3)1-x(OH)x) , which will not further oxidize in air

since the Ge-C bond is air stable. The HCl washing successfully replaces those hydroxide

ligands with chlorine, resulting in Ge(CH3)1-xClx. However, the Ge-Cl bonds make those post-HCl samples air sensitive. Upon exposure to air, Ge-Cl is oxidized, yielding

Ge(CH3)1-x(OH)x again.

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Figure 4. 5 Reactions happened in bi-phase GeCH3 under different conditions. In these crystal structures, yellow, blue, black, gray, red and green spheres refer to calcium, germanium, carbon, hydrogen, oxygen, and chlorine atoms, respectively.

Besides FTIR and XPS, the observed PL in GeCH3 is also very sensitive to oxidization (Figure 4.6). The as-obtained bi-phase GeCH3 has a PL at 1.90 eV with a shoulder at around 1.76 eV, both of which are much higher than the reported band gap

106

2 absorption of GeCH3 at around 1.7 eV. After the HCl treatment, a single asymmetric PL peak was observer at 1.7 eV with a full-width-at-half-maximum (FWHM) of ~250 meV.

However, after one day’s air exposure, PL blue shifts back to 1.87 eV with a shoulder at

1.76 eV, in the similar positions with the one before HCl treatment. Combining with the observed changes in FTIR and XPS, we can conclude that the PL of GeCH3 is also

dependent on the oxidization states of germanium as PL blue shifts when germanium

oxidizes.

Figure 4. 6 PL of bi-phase GeCH3 collected before (blue) and after HCl treatment

(red), along with after HCl treatment and exposure to air for a day (green).

107

4.3.2 Reaction Optimization

With the above conclusions in mind, we hypothesized that an air and water free

synthetic route would produce oxygen-free GeCH3, which would be air stable since no

Ge-Cl will present. Additionally, to get higher methyl coverage, the hydrogen source, water, should be avoided or at least minimized. Towards these ends, we first tried to run the reaction both air and water free using distilled acetonitrile as the solvent.

Unfortunately, no reaction was observed after a week with no appearance of –CH3

vibrational modes in FTIR and only CaGe2 diffraction observed in the XRD pattern.

While keeping the reaction air-free, we then started to adjust the water to CaGe2 ratio and

found that water is actually essential during the reaction (Table 4.1). With 0-2 equivalents of water to CaGe2, no sign of reaction was observed after a week. When the amount of

water was increased to 4 equivalents, the methylation starts to happen with the emergence

of –CH3 vibrational modes and Ge-C stretching mode in FTIR (Figure 4.7a).

Additionally, a new peak in the XRD pattern was observed at around 8 Å with a d- spacing of 1 nm (Figure 4.7b), the same interlayer spacing in the bi-phase GeCH3 before

HCl treatment. However, with such small amount of water, the reaction is not complete

even after a month, which is confirmed by the residual CaGe2 phase in XRD (Figure

4.7b). With a larger amount of water (H2O:CaGe2 ≥ 6), the reaction was complete in a

week, similar to the reaction time needed in the bi-phase method where water was used as

the solvent. To find out why water is necessary during the reaction, we first tested the

solubility of the by-product, CaI2, in the distilled acetonitrile and found that more than five times of the produced CaI2 could be dissolved in the amount of acetonitrile that was

108

added into the reaction. Therefore, the solubility of CaI2 in acetonitrile is not the

limitation of the reaction. The next hypothesis is that water is needed to coordinate with

2+ 2+ - the Ca ions between the layers to weaken the ionic bonding between Ca ions and Ge

framework, and to move the Ca2+ ions out from the lattices. The hypothesis also explains

why at least 6 equivalents of water are needed since the most common coordination

number of water with Ca2+ was found to be 6-8.16 Additionally, we performed reactions replacing the water with other Ca2+ chelating molecules including EDTA

(ethylenediaminetetraacetic acid) and crown ether (18-crown-6). However, the CaGe2

remained completely unreacted after a week.

H2O:CaGe2 ratio Reaction Status 0 No reaction after a week. 2 No reaction after a week. 4 Reacted to some extent, but reaction is not complete in a month. 6 Reaction is complete in a week. 10 Reaction is complete in a week. Table 4. 1 Water to CaGe2 ratio vs. reaction status.

109

Figure 4. 7 (a) FTIR and (b) XRD of uncomplete reaction between CaGe2 and CH3I with four equivalents of water. The starred peaks in (b) are from CaGe2 phase while the dotted peaks are from GeCH3 phase. The unlabeled peak is from an internal germanium standard.

4.3.3 Characterization of GeCH3 from the Optimized Method

Spectroscopy like FTIR, XPS, DRA and PL were collected on the as-obtained

GeCH3 from this optimized method and compared with previous HCl treated GeCH3

(Figure 4.8, 4.9). All spectra show similar features between both samples, with no Ge-O stretching mode observed in FTIR, a single Ge+ peak appeared at 1217.5 eV in the XPS and the same absorption onset and PL peak position, confirming that GeCH3 synthesized from this air-free method is oxygen-free and behaves similarly with the bi-phase GeCH3.

Nevertheless, those spectra vary from each other in some details. In FTIR, weaker Ge-H

110

-1 vibration (~2000 cm ) intensity was observed in the air-free GeCH3 (Figure 4.8a),

suggesting a higher methyl to hydrogen termination ratio. This is confirmed by elemental

analysis, which yields a methyl coverage of 95±3%. Higher methyl coverage is expected since less hydrogen source is present during the reaction. In the optical property measurements, air-free GeCH3 has a narrower PL peak (FWHM = 180 meV) and a

sharper slope (or more close to the ideal step function of absorption predicted by Tauc-

Davis Mott estimates) in the absorption curve (Figure 4.9), both of which are indicative

of better sample quality with less defects. The improvement in sample quality is also

anticipated since two process that can potentially introduce defects, the oxidization of

CaGe2 upon air exposure and the later HCl treatment, are eliminated in the air-free

method. To conclude, with this optimized air-free method, oxygen-free GeCH3 could be

obtained without HCl treatment and it has higher methyl coverage and more narrow

FWHM of photoluminescence.

111

Figure 4. 8 (a) FTIR and (b) Ge 2p3/2 XPS comparison between two GeCH3 samples.

GeCH3 samples synthesized from bi-phase method after HCl treatment are shown in red

(dashed line) while those from the air-free method are shown in green (solid line). 112

Figure 4. 9 DRA and PL comparison between two GeCH3 samples. GeCH3 samples

synthesized from bi-phase method after HCl treatment are shown in red (dashed line)

while those from the air-free method are shown in green (solid line).

4.3.4 Air and Thermal Stability of GeCH3 from the Optimized Method

The potential utility of methyl-terminated germananes for any functional device

strongly hinges on their air and thermal stability. Previous reports have shown that alkyl-

terminated Ge (111) surfaces are stable in air for several days12, 13 and the bulk GeH is

resistant to oxidization of at least two months1. Here, we use FTIR and XPS to probe the

presence of Ge-O in the bulk and on the surface of GeCH3 prepared from this optimized air-free method, respectively. After exposure to air for 50 days, No change was observed

113

in the range of 800-1000 cm-1 (Figure 4.10a), where Ge-O stretching vibrational mode

11 falls, thus proving that the bulk of GeCH3 resists oxidation. XPS was measured on

pristine sample and after exposure to air for 3 days, 10 days, and 10 days plus etching, respectively (Figure 4.10b), and the percentage of the germanium oxidation state for all spectra was calculated by applying a standard Gaussian fit. After 3 days’ exposure to air, a Ge2+/3+ shoulder emerges at 1219.3 eV (28.3% Ge2+/3+), which becomes more intense

after 10 days of air exposure (44.2∼ % Ge2+/3+). However, after Ar etching the top layer, the

Ge2+/3+ almost completely disappears with 19.0% remaining. Together, the XPS and

FTIR suggest that only the surface of GeCH3 becomes oxidized over time while the bulk

is resilient to air, similar to what was observed in GeH1.

Figure 4. 10 Air stability of GeCH3 obtained from air-free method. (a) FTIR and (b)

Ge 2p3/2 XPS of GeCH3 after exposed to air for various time periods.

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The thermal stability of GeCH3 is also improved with this optimized method. We

collected the TGA of GeCH3 synthesized from previous bi-phase method and the optimized air-free method and took the derivatives (DTA, differential thermal analysis)

of both spectra (Figure 4.11). From the DTA spectra, it is apparent that the peak of

methyl desorption increased from 345 oC in the bi-phase method to 420 oC in the air-free

method. The latter desorption temperature is very close to the recent report on atomic flat

o 13 CH3-terminated Ge (111), where methyl desorption was found between 400-450 C.

Furthermore, the percentage weight loss in TGA also increased from bi-phase GeCH3 to

air-free GeCH3, suggesting that 95% of Ge atoms are terminated with –CH3 in the air- free method, matching perfectly with the elemental analysis result.

Figure 4. 11 Thermal stability of two GeCH3 samples. TGA (top, left axis) and DTA

(bottom, right axis) of GeCH3 synthesized from bi-phase (red) and air-free (green)

methods, respectively. 115

4.4 Conclusion

In summary, we have studied the air stability of GeCH3 produced from the bi- phase method and found that it is the Ge-Cl bond that is introduced during the HCl treatment that makes GeCH3 air sensitive. Furthermore, the methylation method has been

optimized to an air-free approach and water is found to be essential for the topotactic

deintercalation reaction. The resulting GeCH3 using this new method not only shows higher methyl coverage and more narrow FWHM of photoluminescence, also exhibits

greater air and thermal stability. Since there exists a large library of numerous

commercially available organohalides, this optimized route can be extended to attach

different organic ligands to germanane, or sensing applications where selective sensors

could be obtained with specific ligand. More importantly, comparison between

germanane grafted with different ligands will allow for systematic understanding of the

effects of ligand identity on material properties, which will be discussed in the next

chapter.

4.5 References for Chapter 4

1. Bianco, E.; Butler, S.; Jiang, S.; Restrepo, O. D.; Windl, W.; Goldberger, J. E. Stability and Exfoliation of Germanane: a Germanium Graphane Analogue. ACS Nano 2013, 7, 4414-4421.

2. Jiang, S.; Butler, S.; Bianco, E.; Restrepo, O. D.; Windl, W.; Goldberger, J. E. Improving the Stability and Optical Properties of Germanane via One-step Covalent Methyl-Termination. Nat. Commun. 2014, 5, 3389.

3. Wang, Q. H.; Jin, Z.; Kim, K. K.; Hilmer, A. J.; Paulus, G. L. C.; Shih, C.-J.; et al. Understanding and Controlling the Substrate Effect on Graphene Electron- Transfer Chemistry via Reactivity Imprint Lithography. Nature Chem. 2012, 4, 724-732. 116

4. Du, X.; Skachko, I.; Barker, A.; Andrei, E. Y. Approaching Ballistic Transport in Suspended Graphene. Nat. Nanotechnol. 2008, 3, 491-495.

5. Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; et al. Boron Nitride Substrates for High-Quality Graphene Electronics. Nat. Nanotechnol. 2010, 5, 722-726.

6. Yokota, K.; Takai, K.; Enoki, T. Carrier Control of Graphene Driven by the Proximity Effect of Functionalized Self-Assembled Monolayers. Nano Lett. 2011, 11, 3669-3675.

7. Tongay, S.; Zhou, J.; Ataca, C.; Liu, J.; Kang, J. S.; Matthews, T. S.; et al. Broad- Range Modulation of Light Emission in Two-Dimensional Semiconductors by Molecular Physisorption Gating. Nano Lett. 2013, 13, 2831-2836.

8. Xu, Y.; Yan, B.; Zhang, H.-J.; Wang, J.; Xu, G.; Tang, P.; et al. Large-Gap Quantum Spin Hall Insulators in Tin Films. Phys. Rev. Lett. 2013, 111, 136804.

9. Amani, M.; Lien, D.-H.; Kiriya, D.; Xiao, J.; Azcatl, A.; Noh, J.; et al. Near- Unity Photoluminescence Quantum Yield in MoS2. Science 2015, 350, 1065- 1068.

10. Vogg, G.; Meyer, L. J.-P.; Miesner, C.; Brandt, M. S.; Stutzmann, M. Polygermanosilyne Calcium Hydroxide Intercalation Compounds Formed by Topotactic Transformation of Ca(Si1-xGex)2 Alloy Zintl Phases in Ambient Atmosphere. Monatsh. Chem. 2001, 132, 1125-1135.

11. Rivillon, S.; Chabal, Y. J.; Amy, F.; Kahn, A. Hydrogen Passivation of Germanium (100) Surface Using Wet Chemical Preparation. Appl. Phys. Lett. 2005, 87, 253101.

12. He, J.; Lu, Z.-H.; Mitchell, S. A.; Wayner, D. D. Self-Assembly of Alkyl Monolayers on Ge (111)1a. J. Am. Chem. Soc. 1998, 120, 2660-2661.

13. Wong, K. T.; Kim, Y.-G.; Soriaga, M. P.; Brunschwig, B. S.; Lewis, N. S. Synthesis and Characterization of Atomically Flat Methyl-Terminated Ge (111) Surfaces. J. Am. Chem. Soc. 2015, 137, 9006-9014.

14. Behnke, F. W.; Tamborski, C. The Infrared Spectra and Vibrational Assignments for Some Organometallic Compounds: DTIC Document; 1962.

15. Lu, Z. Air-Stable Cl-Terminated Ge (111). Appl. Phys. Lett. 1996, 68, 520-522.

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16. Katz, A. K.; Glusker, J. P.; Beebe, S. A.; Bock, C. W. Calcium Ion Coordination: a Comparison with that of Beryllium, Magnesium, and Zinc. J. Am. Chem. Soc. 1996, 118, 5752-5763.

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Chapter 5: Interplay of Ligand Strain and Electronegativity in Tuning the

Electronic Structure of Covalently Functionalized Germanane

5.1 Introduction

As the covalent functionalization of numerous families of 2D materials including

the group IVA graphane analogues,1-9 the transition metal dichalcogenides,10-12 and the

MXene phases,13-15 is starting to be established experimentally and theoretically, a systematic understanding about how and to what extent the electronic structure of a 2D material can be tailored by the identity of the ligand is essential before surface functionalization chemistry can be utilized effectively to rationally tailor properties.

Germanane, on the other hand, is an ideal model system for probing this ligand effect. In this single atom-thick puckered honeycomb framework, covalently bonded chemical groups will directly couple to the orbitals comprising conduction and valence bands, facilitating the tuning of electronic structure. Furthermore, the arrangement of Ge atoms in these 2D materials is identical to that of a Ge (111) surface, which has a well- established history of surface functionalization for comparison.16-18 Experimentally, we

have developed a general route for creating multilayered crystals of organic terminated

germanane in the last chapter.

Herein, through the preparation of a library of ligand-functionalized germanane

crystals, we establish the influence of ligand size and electronegativity on electronic

119

structure. We show that nearly uniform ligand termination can only be achieved with

small enough functional groups, with larger ligands resulting in partial hydrogen-

termination. When there is a homogeneous distribution of different ligands, the band gaps

and Raman shifts are dictated by their relative stoichiometry in a pseudo-linear fashion similar to Vegard’s law. Ligands that have a greater size and are more electronegative will expand the Ge-Ge bond length, thereby lowering the band gap. Simply by changing the identity of the organic ligand, the band gap of germanane can be tuned by ~ 15%, highlighting the power of surface chemistry to manipulate the properties of single-atom thick materials.

5.2 Experimental

5.2.1 Materials

All the organohalides were obtained from either Sigma or Oakwood and were used without further purification. Acetonitrile was also obtained from Sigma and was distilled before use. To distill acetonitrile, CaH2 was first put into acetonitrile and let sit

for at least an hour, then the solvent was distilled into dried molecular sieve at 150-200

oC, at atmosphere pressure. The distilled solvent was then bubbled with argon for 20

minutes. Aqueous HCl was obtained from Fisher Scientific and used without any further

purification. Water was purified with Milli-Q Reference from Millipore.

5.2.2 Sample Synthesis and Preparation

To get various GeR (R: organic ligand), CaGe2 crystals were first loaded into an

air-free flask inside the glovebox. The flask was then connected to the Schlenk line, where all the joints are evacuated and back filled with argon for three times. Liquids like

120

organohalides, acetonitrile and water were then added into the flask under the flowing of

argon. The molecular ratio of CaGe2 to organohalides to water were all controlled at

1:30:10. The reactions were running at room temperature for one to two weeks,

depending on the ligand. For purification, the suspension was washed with distilled

acetonitrile inside a glovebag and then dried on a Schlenk line, followed by HCl

treatment to wash out the salts or solvents that were trapped between the layers. During

the HCl treatment, the sample was washed with HCl for three times, followed by

acetonitrile or dichloromethane for three times to remove HCl, then the washed sample

was dried on a Schlenk line.

To get Ge(CH3)xH1-x, CaGe2 crystals were first loaded into an air-free flask inside the glovebox. The flask was then connected to the Schlenk line, where all the joints are evacuated and back filled with argon for three times. Liquid reagents like organohalides, acetonitrile and diluted HCl were then added into the flask under the flowing of argon.

The concentration of HCl was controlled here to control the ratio of –H to –CH3

termination. The reaction was running at room temperature for around one week. For

purification, the suspension was washed with distilled acetonitrile inside a glovebag and

then dried on a Schlenk line, followed by HCl washing to remove the salts and small

molecules trapped between the layers. During this HCl treatment, the sample was washed

with HCl for three times, followed by acetonitrile for three times to remove HCl, then the

washed sample was dried on a Schlenk line.

121

For thermogravimetric analysis (TGA) and elemental analysis, samples were evacuated at 80 oC for three hours on a schlenk line to get rid of any absorbed water or solvent.

5.2.3 Instrumentation

Powder X-ray diffraction (XRD) (Bruker D8 powder X-ray diffractometer,

Rigaku MiniFlexII X-Ray diffractometer) was used to study the structure of GeR and

Ge(CH3)xH1-x samples. Fourier transform infrared spectroscopy (FTIR) were collected on

a Perkin-Elmer Frontier Dual-Range FIR/MidIR spectrometer that was loaded in an Ar-

filled glovebox. Diffuse reflectance absorption (DRA) measurements (Perkin-Elmer

Lambda950 UV/Vis Spectrometer, OceanOptics USB4000) was conducted to study the

band gap of the bulk solid crystals. TGA (Q-500 thermogravimetric analyzer) was

o collected in flowing N2 at 10 C/min. Elemental Analysis (Galbraith Laboratories, Inc.)

of the C/H ratio was collected to determine the ratio of CH3-termination to H-termination

for all the GeR.

Raman spectra were collected using a Renishaw InVia Raman equipped with a

CCD detector upon excitation using a 633-nm HeNe laser. During the measurement, the photoluminescence is so strong that it will saturate the detector with laser power above 3

μW/cm2 on a bare sample. Below this laser power, no Raman vibrational modes were

observed on GeH or GeCH3. We sealed the sample in a thick capillary and run the Raman

measurements on samples inside the capillary and was able to excite the sample with a

higher laser power density of ~24 mW/cm2 and vibrational modes was observed in all

GeRs. The capillary here absorbed partial fluorescence and helped to increase the

122 maximum of the laser power that could be applied before saturating the detector. At least ten spectra were collected on each sample at different spots. Each spectrum was fitted into a function with a mix of 70% Lorentz function and 30% Gaussian function, along with an asymmetric factor. The average and standard deviation of the peak positions found in the ten spectra were reported.

5.3 Results and Discussion

5.3.1 Full Coverage Ligands

The first group of organic ligands –CH3, –CH2OCH3, and –CH2CH=CH2 can be grafted onto nearly every single germanium atom in the germanane framework. In the

FTIR spectra of these materials, an intense Ge-C stretching mode occurs at ~ 570-590 cm-1 for these different spectra while the major Ge-H stretching frequency at 2000 cm-1 is almost completely gone (Figure 5.1a). As grown, no Ge-O stretching vibrations from

-1 800-1000 cm are observed, indicating a lack of oxidation. The GeCH3 and GeH FTIR spectra are fully consistent with our previous isotopic labeling studies.3 Other spectroscopic signatures indicate successful grafting of these ligands onto the framework,

-1 -1 such as the -C=C- bending modes at 1626 cm , the =CH2 stretch at 3075 cm , the trans

-1 -1 =CH wagging at 990 cm , and the =CH2 wagging at 908 cm for –CH2CH=CH2, and all the other vibrational modes are consistent with the spectroscopic fingerprints of these functional groups (Table 5.1, 5.2). The degree of functionalization in GeRxH1-x, was calculated from the C:H ratio measured via elemental analysis (Table 5.3). For these three ligands, x ranges from 0.95(2)-1.05(7), indicative of nearly complete termination with the organic ligand.

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Figure 5. 1 Characterization of four nearly full functionalized GeR. (a) FTIR, (b)

XRD and (c) Raman of GeCH3 (i, red), GeH (ii, blue), GeCH2CH=CH2 (iii, olive) and

GeCH2OCH3 (iv, orange). The inset in (c) is the scheme of the E2 vibrational mode of the

germanium framework. The starred peaks in (a) refers to Ge-C bond and those in (b) is coming from the internal Ge standard. All the dash lines are drawn for eye guide.

124

Frequency (cm-1) Vibrational Modes 3075 =CH2 str. 3000 =CH2 str. / =CH str. 2950 -CH2- str. 2914 -CH2- str. 2849 -CH2- str. 2006 Ge-H str. 1819 =CH2 wag overtone 1626 C=C str. Left shoulder of 1417 -CH2- def. 1417 =CH2 def. 1385 =CH2 def. / -CH2- wag 1297 =CH- rock 1150 -CH2- twist 1059 C-C str. 990 “trans” =CH wag 908 =CH2 wag 828 Ge-H2 bend 762 -CH2- rock 590 “cis” =CH wag/ Ge-C str. 516 “cis” =CH wag/ Ge-C str. 470 “cis” =CH wag Table 5. 1 Vibrational modes in GeCH2CH=CH2

Frequency (cm-1) Assignment 2973, 2919, 2883, 2847 C-H str. 2808 -CH3 def. overtone 1456, 1437 -C-H def. 1418,1390, 1368 -CH2- wag 1256 -CH2- twist 1210 -CH3 rock 1148, 1084, 1004, 920 COC str. 779 -CH2- rock 572 Ge-C str. Table 5. 2 Vibrational modes in GeCH2OCH3

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-R ligand C (wt %) C (wt Residual fraction of Expected in GeRxH1-x %) mass organic residual Mass (wt %) coverage x (wt %) -CH3 13.02(5) 3.34(1) 83.64(6) 0.95(2) 83.6(3) -CH2CH3 16.2(4) 3.7(2) 80.1(6) 0.69(8) 78(2) -(CH2)2CH3 15.6(3) 3.59(8) 80.8(4) 0.50(5) 78(2) -(CH2)3CH3 11.8(3) 2.71(4) 85.5(3) 0.339(14) 78.4(7) -(CH2)6CH3 34.1(1.4) 6.4(2) 59.5(1.6) 0.60(9) 55(4) -CH(CH3)CH2CH3 6.68(5) 2.09(14) 91.2(2) 0.14(2) 88.8(1.2) CH2I 4.47(12) 1.01(3) 94.52(15) 0.591(8) 94.4(1.3) CH2CF3 4.06(4) 1.00(9) 94.94(13) 0.21(2) 93(3) CH2OCH3 20.06(8) 4.0(3) 75.9(4) 1.05(7) 75(7) CH2CH=CH2 31.4(2) 4.29(16) 64.3(4) 1.02(3) 63.3(7) CH2C6H5 18.6(2) 2.1(2) 79.3(4) 0.29(7) 73(5) * Table 5. 3 Fraction of organic functionalization (x ) in GeRxH1-x.

* x was calculated from elemental analysis measurements, assuming excess equivalents

of –H are terminated on the germanane framework. For allyl and dimethyl ether

termination, that have slightly higher C:H ratios than expected for full coverage, x was

calculated by the molecular ratio between ligand numbers calculated from carbon and

hydrogen, respectively.

The change in lattice parameters after deintercalation are also indicative of

covalent termination. The starting material, β-CaGe2 crystallizes into a 6-layer R-3m crystal structure. After covalent termination with –H/-CH3, peak broadening and small

number (3-5) of reflections in the XRD make it impossible to unambiguously assign the resulting layered structures between a 2H 2-layer hexagonal unit cell or a 6R 6-layer

Rhombohedral unit cell. Functionalization with other ligands requires a further reduction of P1 triclinic unit cell symmetry. No matter what the resulting space group is, the first

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diffraction reflection corresponds to the interlayer spacing. The interlayer spacing increases with ligand length, and is within 1-2 Å of the value expected if each ligand is fully extended and oriented parallel to the c-axis (Table 5.4). The a-lattice parameter of the honeycomb framework was determined from second major reflection, and calculated assuming it was either a 2H (100) reflection or a 6R (012) reflection, and increases from

GeH < GeCH2CH=CH2 < GeCH3 < GeCH2OCH3 (Figure 5.1b, Table 5.5). However, in

the Raman spectra, the shift of the E2 mode, corresponding to the in-plane Ge-Ge

framework vibration, increases from GeCH3 > GeH > GeCH2CH=CH2 > GeCH2OCH3

(Figure 5.1c, Table 5.6). The energy of the Raman shift is not correlated to the a-lattice

parameter, due to the possibility for the 2D framework to buckle and produce different

Ge-Ge-Ge bond angles. For example, by keeping the Ge-Ge bond length at a constant

2.43 Å, changing the Ge-Ge-Ge bond angle by just 3 degrees from 109o to 106o can result

in a change in a-lattice parameter that varies from 3.96 to 3.88 Å (Table 5.7).

Consequently, the Raman energy of the E2 vibrational mode serves as a much more direct

readout of the Ge-Ge bond length when comparing different ligands.

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Ligand Experimental* (Å) Expected interlayer spacing* (Å) H 5.5 NA CH3 8.6 8.0 CH2CH3 10.4 9.0 (CH2)2CH3 13.1 12.0 (CH2)3CH3 13.8 13.0 (CH2)6CH3 18.0 20.1 CH(CH3)CH2CH3 13.0 12.0 CH2I 10.6 9.7 CH2OCH3 10.9 11.7 CH2CH=CH2 12.2 11.3 CH2C6H5 12.9 13.1 Table 5. 4 Interlayer spacing between germanium layers in GeRxH1-x.

* Experimental values are determined from X-ray diffraction. Expected values are

calculated from the difference between Ge-H and Ge-C bond plus the height difference

between hydrogen and other ligands, assuming the ligands are fully extended and

oriented parallel to the c-axis.

Ligand 2H (P63mc) (Å) 6R (R-3m) (Å) CH3 3.96(3) 3.99(3) CH2CH3 3.93(4) 3.95(4) (CH2)2CH3 3.94(4) 3.96(4) (CH2)3CH3 3.92(6) 3.94(6) (CH2)6CH3 3.92(5) 3.93(5) CH(CH3)CH2CH3 3.89(5) 3.90(5) CH2I 3.92(8) 3.93(8) CH2OCH3 4.00(7) 4.03(7) CH2CH=CH2 3.94(7) 3.96(7) CH2C6H5 3.91(5) 3.93(5) Table 5. 5 a-lattice parameter of GeRxH1-x. The results were calculated from two-layer

(P63mc) or six-layer (R-3m) per unit cell, separately. The uncertainty was calculated from

FWHM of the (100) peak in P63mc or (012) in R-3m.

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Ligand Raman Shift (cm-1) H 301.6(2) CH3 303.3(3) CH2OCH3 299.4(5) CH2CH=CH2 300.6(4) Table 5. 6 Raman shift of four nearly full functionalized germanane.

d (Å) θ (o) a (Å) 2.43 109 3.96 2.43 106 3.88 2.45 109 3.99 2.45 106 3.91 Table 5. 7 Relationship* between bond angle, bond length and a-lattice parameter.

* From the structure, the relationship can be expressed as cosθ=1-a2/2d2, where θ is the

Ge-Ge-Ge bond angle, a is the a-lattice parameter, and d is the Ge-Ge bond length.

5.3.2 Larger Ligands

Many other ligand functionalized germanane materials were prepared to further

probe the effect of ligand sterics. When functionalized with linear primary alkyl ligands

that are –CH2CH3 or larger, the a-lattice parameter decreases with increasing chain

length, and decreases even further with the bulkier secondary-butyl ligand (Figure 5.2,

Table 5.5). The a-lattice parameter represents the distance between neighboring ligands

on a single side of the framework. This shrinking of the lattice parameter results from

partial hydrogenation of the framework, which is confirmed by the presence of Ge-H vibrational modes that appear in the FTIR as intense Ge-H stretches at around 2000 cm-1

(Figure 5.3). From elemental analysis measurements of the C:H ratio, the degree of

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organic functionalization of GeRxH1-x, ranges from x = 0.14(2)-0.69(8) (Table 5.3).

Partial organic functionalization with these larger ligands is consistent with previous

studies on the density of organic functionalization on Ge (111) and Si (111) surfaces. It has been reported that complete functionalization of the atop Si/Ge atoms occurs with –

18-21 CH3, –C≡CH, and –C≡CCH3. The internuclear distance between the atop Si and Ge

sites are 3.8 Å and 4.0 Å, respectively, for unreconstructed (111) surfaces, whereas these

three organic ligands have van der Waals radii in the in-plane direction that are equal to or smaller than methyl, which is 2.0 Å22. However, only partial hydrogen-termination on

these (111) surfaces is attained with bulkier ligands. On Si (111) surfaces, a maximum of

85% of ethyl ligands can be grafted onto the surface, due to the close non-bonding H---H

interaction that occurs between hydrogen atoms on neighboring ligands.23 Thus, it is

more likely that the size of the atom or functional group at the β position, not the size of

the entire ligand, is critical to determine if it can be fully grafted onto the framework or

not. Comparing ligands like –CH3, –CH2CH3, –CH2CH=CH2 and –CH2OCH3, though the

size of the ligands increases from –CH3 < –CH2CH3 < –CH2OCH3 < –CH2CH=CH2, the

size of functional groups at β position increases from H in –CH3 < O in –CH2OCH3 < CH

in –CH2CH=CH2 < CH3 in –CH2CH3. Since ethyl has the largest functional group at β

position, it is too bulkier to fit into the germanium framework and partial hydrogenation

are introduced to compensate the strain.

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Figure 5. 2 XRD of GeR (R = alkyls). The starred peak is coming from the internal Ge standard and the dashed line is drawn to guide the eyes.

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Figure 5. 3 FTIR of GeR (R = alkyls).

5.3.3 Ge(CH3)xH1-x

With larger ligands, partial hydrogenation of the germanane framework is unavoidable due to ligand size effects. Therefore to understand the influence of how homogeneously mixed ligand terminations affect the structure and properties of the framework, we synthesized Ge(CH3)xH1-x (0

5.4). The loading molar ratios of CaGe2 to CH3I to HCl to water and the resulting x values were summarized in Table 5.8.

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Figure 5. 4 TGA of Ge(CH3)xH1-x.

+ CaGe2:H :H2O:CH3I x 1:0:10:30 0.95 1:0.27:10:30 0.73 1:0.54:10:30 0.47 Table 5. 8 Reaction condition and resulting CH3 coverage (x) in Ge(CH3)xH1-x.

The XRD patterns (Figure 5.5a) of each product show that surface functionalization occurs homogeneously throughout the sample, as there is a single reflection for the interlayer spacing in each sample, with a full width half-maximum

o o ranging between 1 to 1.5 2θ, for Ge(CH3)0.95H0.05 to GeH, respectively. In contrast,

when high concentrations of HCl were used as the proton source, two reflections that

corresponding to the interlayer spacing were observed, due to the faster deintercalation

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kinetics of HCl. Comparing the XRD of each sample, there is a consistent and near-linear

decrease in the a-lattice parameter with decreasing fractional coverage of –CH3 (Figure

5.5b). However, there is only a small decrease in the interlayer spacing from 8.6 Å to 8.4

Å for, Ge(CH3)0.95H0.05 to Ge(CH3)0.4H0.6, followed by a sharp 3 Å drop with GeH. This

4 is consistent with previous observations on Ge1-xSnxH1-x(OH)x graphane alloys that when

>4% of the framework atoms are terminated with larger –OH ligands, the interlayer spacing is dictated by the interactions between the –OH ligands on neighboring layers.

Additionally, both the energy of the E2 Raman mode, and the absorption onset decreases

pseudolinearly with increasing % -H, albeit with some bowing (Figure 5.6). Together this

clearly demonstrates that the relative ratio of different surface functional groups on a 2D

framework will influence the lattice constant, the vibrational and electronic properties, in

a fashion analogous to Vegard’s Law for binary alloys of semiconductors.

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Figure 5. 5 XRD of Ge(CH3)xH1-x. In (a), the starred peak is coming from an internal Ge

standard and the dashed line is drawn to guide the eyes. In (b), the a-lattice parameter

(blue) is calculated from (012) reflection assuming a 6R unit cell while the interlayer spacing (red) is calculated from the first reflection in (a).

135

Figure 5. 6 Raman and DRA of Ge(CH3)xH1-x. The dash line in (a) is drawn to guide the eyes while those in (c) highlights the absorption onset energy at the crossing points.

5.3.4 Ligand Effect on Electronic Structure

The identity of the ligand and the percent of functionalization both significantly influence the electronic and vibrational properties. First, to elucidate how ligand identity affects the properties, we compare the four germanane lattices that have nearly uniform ligand termination (–H, –CH3, –CH2OCH3, and –CH2CH=CH2). According to numerous

136

theoretical simulations on group IV graphane analogs, such as the silicane derivatives,24

the Si-Si bond length is the most significant structural feature of the framework in tuning

the band gap, with the band gap inversely proportional to bond length. Consequently both

the ligand size and the ligand electronegativity can change the bond length and band gap.

Larger ligands are expected to strain the Ge-framework in a tensile fashion, thereby

providing a lower band gap. Ligands with greater electronegativity are expected and have

been calculated to withdraw electron density out of the framework and lower the band

6, 9 gap. The size of these four ligands decreases from –CH2CH=CH2 > –CH2OCH3 > –

CH3 > –H (Figure 5.7a). The ligand electronegativity can be estimated by the field/inductive component (F) of the Hammett constants. Going from greater to smaller

ligand electronegativity, the Hammett constant decreases from –CH2OCH3 > –H > –

CH3 > –CH2CH=CH2 (Figure 5.7b). The band gap of the germanane lattice

functionalized with these four ligands was determined using the absorption onset (Figure

5.7c). The absorption onset increases from GeCH2OCH3 (1.45 eV) < GeCH2CH=CH2

(1.55 eV) < GeH (1.57 eV) < GeCH3 (1.66 eV). The trend of band gap with different ligands is consistent with the trend of the E2 Raman vibrational mode, which confirms

previous calculations that band gap is inversely proportional to Ge-Ge bond length. This

observed band gap value increases with decreasing ligand electronegativity, with the

exception of GeCH2CH=CH2. The allyl substituent is the most sterically bulky ligand of

these four, and consequently strains the lattice with complete termination, resulting in a

lower band gap than GeH. It is clear that neither the size nor the electronegativity of the

ligand alone determines the band gap of the functionalized germanane framework.

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Figure 5. 7 Ligand effect on band gap. (a) Relative size of the four ligands decreasing

from left to right. (b) Relative electronegativity of the four ligands decreasing from left to

right. (c) DRA of the GeR terminated with these four ligands increasing from

GeCH2OCH3 (organe) to GeCH2CH=CH2 (olive) to GeH (blue) to GeCH3 (red).

138

This influence of ligand identity and band gap holds to be true even for ligand

chemistries that produce partially functionalized germanane lattices. The E2 Raman mode

energy versus the band gap determined via absorption onset of 10 different ligand functionalized germanane frameworks are plotted in Figure 5.8. Again, the band gap increases proportionally with the E2 Raman shift, further confirming the relationship

between Ge-Ge bond length and band gap. Generally, alkyl-functionalized germanane lattices have band gap energies that are larger than GeH, due to the electron-donating nature of these ligands. However, GeCH3 has the highest energy band gap and E2 Raman

mode among these alkyl ligands, as partial hydrogen functionalization limits the electron-

donating influence of the ligand. With electron-withdrawing ligands such as -CH2I, and –

CH2OCH3, the band gap of the functionalized germanane lattice is lower than GeH.

Ligands that have low coverage densities (GeRxH1-x; x<0.3) typically have band gaps that are relatively close to GeH. Overall, the band gap of germanane can change by 15% from

1.45 eV in GeCH2OCH3 to 1.66 eV in GeCH3.

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Figure 5. 8 Raman shift and band gap correlation in all GeRxH1-x.

5.4 Conclusion

In conclusion, through the synthesis and characterization of germanane frameworks functionalized with a large array of ligands, we have shown how to rationally

tune the electronic structure of a 2D material via the chemical identity of a covalently

bound ligand. Generally, ligands that are more electron-withdrawing and have greater

steric bulk will expand the Ge-Ge framework and lower the band gap, with complete ligand coverage. When functionalized with mixtures of different ligands, the band gaps and Raman shifts are dictated by the relative stoichiometry of both ligands, akin to

Vegard’s law for solid solutions. We thus have provided important, systematic guidelines for the rational tuning of electronic structure of single-atom thick layered materials via ligand chemistry, which should be applicable to different families of functionalizable 2D 140

materials including the transition metal dichalcogenide and MXene lattices.10-15 As the multitude of exciting predictions of topological quantum spin Hall effects in the group IV graphane analogues require either appreciable tensile strain or strongly electronegative ligands to achieve band inversion,6-8 this work establishes the experimental limits to

which the Ge-Ge bond lengths can be strained via surface chemistry. While the band gap

decrease with the ligands shown here may not be large enough to enable experimental

realization of quantum spin Hall state in germanane, the expanded tin lattice may be able

to better accommodate ligands. The ability to covalently tune the band gap of germanane

may prove especially useful for investigating additional phenomena, for example, the

creation of lateral heterostructures by patterning multiple kinds of surface chemical

groups on a single atomic sheet, or for studying excitonic behavior. Similar to how strain

is used to enhance the mobility of SiGe channels in modern transistors,25 or how

“chemical pressure” – the substitution of ions in a solid-state lattice with similar charge

but different sizes – is used to optimize correlated electronic behavior such as

superconductivity or charge density waves,26-28 we show that surface functionalization

chemistry is a powerful approach for manipulating the electronic structure of atomically

thin 2D materials.

5.5 References for Chapter 5

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2. Bianco, E.; Butler, S.; Jiang, S.; Restrepo, O. D.; Windl, W.; Goldberger, J. E. Stability and Exfoliation of Germanane: a Germanium Graphane Analogue. ACS Nano 2013, 7, 4414-4421.

3. Jiang, S.; Butler, S.; Bianco, E.; Restrepo, O. D.; Windl, W.; Goldberger, J. E. Improving the Stability and Optical Properties of Germanane via One-Step Covalent Methyl-Termination. Nat. Commun. 2014, 5, 3389.

4. Arguilla, M. Q.; Jiang, S.; Chitara, B.; Goldberger, J. E. Synthesis and Stability of Two-Dimensional Ge/Sn Graphane Alloys. Chem. Mater. 2014, 26, 6941-6946.

5. Zhu, F.-f.; Chen, W.-j.; Xu, Y.; Gao, C.-l.; Guan, D.-d.; Liu, C.-h.; et al. Epitaxial Growth of Two-Dimensional Stanene. Nat. Mater. 2015, 14, 1020-1025.

6. Xu, Y.; Yan, B. H.; Zhang, H. J.; Wang, J.; Xu, G.; Tang, P. Z.; et al. Large-gap quantum spin Hall insulators in tin films. Phys. Rev. Lett. 2013, 111, 136804.

7. Si, C.; Liu, J.; Xu, Y.; Wu, J.; Gu, B.-L.; Duan, W. Functionalized Germanene as a Prototype of Large-Gap Two-Dimensional Topological Insulators. Phys. Rev. B 2014, 89, 115429.

8. Ma, Y.; Dai, Y.; Wei, W.; Huang, B.; Whangbo, M.-H. Strain-induced quantum spin Hall effect in methyl-substituted germanane GeCH3. Sci. Rep. 2014, 4, 7297.

9. Butler, S. Z.; Hollen, S. M.; Cao, L.; Cui, Y.; Gupta, J. A.; Gutiérrez, H. R.; et al. Progress, Challenges, and Opportunities in Two-dimensional Materials Beyond Graphene. ACS Nano 2013, 7, 2898-2926.

10. Knirsch, K. C.; Berner, N. C.; Nerl, H. C.; Cucinotta, C. S.; Gholamvand, Z.; McEvoy, N.; et al. Basal-plane functionalization of chemically exfoliated molybdenum disulfide by diazonium salts. ACS Nano 2015, 9, 6018-6030.

11. Voiry, D.; Goswami, A.; Kappera, R.; Castro e Silva, C. d. C.; Kaplan, D.; Fujita, T.; et al. Covalent functionalization of monolayered transition metal dichalcogenides by phase engineering. Nat. Chem. 2015, 7, 45-49.

12. Chou, S. S.; De, M.; Kim, J.; Byun, S.; Dykstra, C.; Yu, J.; et al. Ligand Conjugation of Chemically Exfoliated MoS2. J. Am. Chem. Soc. 2013, 135, 4584- 4587.

13. Mashtalir, O.; Naguib, M.; Mochalin, V. N.; Dall’Agnese, Y.; Heon, M.; Barsoum, M. W.; et al. Intercalation and Delamination of Layered Carbides and Carbonitrides. Nat. Commun. 2013, 4, 1716.

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14. Naguib, M.; Kurtoglu, M.; Presser, V.; Lu, J.; Niu, J.; Heon, M.; et al. Two- Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2. Adv. Mater. 2011, 23, 4248-4253.

15. Lukatskaya, M. R.; Mashtalir, O.; Ren, C. E.; Dall’Agnese, Y.; Rozier, P.; Taberna, P. L.; et al. Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide. Science 2013, 341, 1502-1505.

16. He, J. L.; Lu, Z. H.; Mitchell, S. A.; Wayner, D. D. M. Self-Assembly of Alkyl Monolayers on Ge(111)1a. J. Am. Chem. Soc. 1998, 120, 2660-2661.

17. Buriak, J. M. Organometallic Chemistry on Silicon and Germanium Surfaces. Chem. Rev. 2002, 102, 1271-1308.

18. Knapp, D.; Brunschwig, B. S.; Lewis, N. S. Chemical, Electronic, and Electrical Properties of Alkylated Ge(111) Surfaces. J. Phys. Chem. C 2010, 114, 12300- 12307.

19. Wong, K. T.; Kim, Y.-G.; Soriaga, M. P.; Brunschwig, B. S.; Lewis, N. S. Synthesis and Characterization of Atomically Flat Methyl-Terminated Ge (111) Surfaces. J. Am. Chem. Soc. 2015, 137, 9006-9014.

20. Yu, H.; Webb, L. J.; Ries, R. S.; Solares, S. D.; Goddard, W. A.; Heath, J. R.; et al. Low-temperature STM images of methyl-terminated Si (111) surfaces. J. Phys. Chem. B 2005, 109, 671-674.

21. Hurley, P. T.; Nemanick, E. J.; Brunschwig, B. S.; Lewis, N. S. Covalent Attachment of Acetylene and Methylacetylene Functionality to Si (111) Surfaces: Scaffolds for Organic Surface Functionalization While Retaining Si-C Passivation of Si (111) Surface Sites. J. Am. Chem. Soc. 2006, 128, 9990-9991.

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23. Yu, H.; Webb, L. J.; Solares, S. D.; Cao, P.; Goddard, W. A.; Heath, J. R.; et al. Scanning tunneling microscopy of ethylated Si (111) surfaces prepared by a chlorination/alkylation process. J. Phys. Chem. B 2006, 110, 23898-23903.

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27. Hegger, H.; Petrovic, C.; Moshopoulou, E.; Hundley, M.; Sarrao, J.; Fisk, Z.; et al. Pressure-Induced Superconductivity in Quasi-2D CeRhIn5. Phys. Rev. Lett. 2000, 84, 4986.

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Chapter 6: Conclusion and Outlook

We have introduced a new family of two-dimensional (2D) materials, group IV graphane analogues. These germanium graphane analogues are real materials that can be synthesized as robust single crystals in gram-scale quantities and exfoliated into single layers. The structure, optical properties, air and thermal stability of these materials have been extensively characterized. Furthermore, the methylation method has been successfully extended to graft various organic ligands onto germanane. Through comparing the structures and properties of germananes terminated with 10 different ligands, a systematical understanding has been established on how and to what extend can the electronic structure of germanane be tailored via surface functionalization, providing a guideline to manipulate the electronic structure of 2D materials.

In principle, this novel functionalization method can be further applied to produce other group IV, silicon and tin, graphane analogues. The silicon graphane analogue is predicted to have a high mobility with sizable band gap and can be seamless integrated into the current semiconductor industry. The tin graphane analogue is particularly interesting because it is predicted to be a topological insulator when functionalized with specific ligands. Furthermore, the creation of lateral heterostructures by patterning

145 multiple kinds of surface chemical group on a single atomic sheet could pave the avenue to study additional phenomena.

Compared with all other 2D van der Waals solids, these group IV graphane analogues offer the capability for covalent surface termination, providing a versatile handle for tailoring the structure, stability, and electronic properties in single-atom-thick materials. Such exquisite control over the material properties makes these systems attractive candidates for a multitude of applications such as sensing, optoelectronics, and thermoelectrics and also offers the potential for new physical phenomena such as the quantum spin Hall effect. Overall, this new class of 2D materials is posed to have a great impact not only in the traditional sectors of nanoscience but also in opening up a new research paradigm in covalently controlled properties by design.

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Bibliography

Chapter 1