SUNLIGHT-EXCITED INFRARED LONG PERSISTENCE OF

2+ 3+ 3+ 3+ SRAL2O4: EU , DY , ER / ND NANOPHOSPHORS

by

NAIYIN YU

(Under the Direction of Zhengwei Pan)

ABSTRACT

Strontium aluminate co-doped with divalent , trivalent dysprosium ions

2+ 3+ (SrAl2O4: Eu , Dy ) shows a green demonstrated long persistence lasting for up to 15 hours under sunlight excitation. The research presented in this thesis focus on the Er3+/ Nd3+ activated

2+ infrared (IR) long persistence due to the persistent energy transfer process in SrAl2O4: Eu ,

Dy3+, Er3+/ Nd3+ nanophosphor system. The XRD, SEM, EDX and optical spectra analysis method are employed to characterize the synthesized by combustion route. The

2+ 3+ 3+ efficient energy transfer process from sensitizer Eu to the Er / Nd in SrAl2O4 host system is investigated. Meanwhile, the long persistence luminescence in IR region is obtained.

To our knowledge, there is no related material system regarding sunlight-excited infrared long persistence nanophosphor reported.

2+ 3+ 3+ 3+ INDEX WORDS: SrAl2O4: Eu , Dy , Er / Nd , Infrared (IR) long persistence, Persistent

energy transfer

SUNLIGHT-EXCITED INFRARED LONG PERSISTENCE OF

2+ 3+ 3+ 3+ SRAL2O4: EU , DY , ER / ND NANOPHOSPHORS

by

NAIYIN YU

B.S., Shanghai Jiao Tong University, China, 2006

A Thesis Submitted to the Graduate Faculty of The University of Georgia in Partial Fulfillment

of the Requirements for the Degree

MASTER OF SCIENCE

ATHENS, GEORGIA

2008

© 2008

NAIYIN YU

All Rights Reserved

SUNLIGHT-EXCITED INFRARED LONG PERSISTENCE OF

2+ 3+ 3+ 3+ SRAL2O4: EU , DY , ER / ND NANOPHOSPHORS

by

NAIYIN YU

Major Professor: Zhengwei Pan

Committee: Loris Magnani Heinz-Bernd Schüttler

Electronic Version Approved:

Maureen Grasso Dean of the Graduate School The University of Georgia August 2008 iv

ACKNOWLEDGEMENTS

Thanks to Dr. Zhengwei Pan who gave me all the support and inspiration to finish this thesis. Thanks to Dr. Loris Magnani and Dr. Heinz-Bernd Schüttler who served as my committee members. Thanks also to all the other group members, Yiying Lu, Yanjun Chuang and especially

Dr. Feng Liu and Dr. Zhanjun Gu for their support during my two years research. v

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ...... ⅳ

LIST OF TABLES...... ⅶ

LIST OF FIGURES...... ⅷ

CHAPTER

1 INTRODUCTION...... 1

REFERENCES ...... 3

2 BACKGROUND REVIEW...... 4

2.1 LONG PERSISTENT ...... 4

2+ 3+ 2.2 MECHANISMS FOR THE OF SRAL2O4: EU , DY ...... 5

2.3 RARE EARTH ELEMENTS ...... 8

2.4 ENERGY TRANSFER IN INORGANIC SYSTEM BETWEEN RE IONS ...... 10

REFERENCES ...... 11

3 SYNTHESIS AND CHARACTERIZATION OF SRAL2O4 HOST NANOPHOSPHORS ...... 12

3.1 PREPARATION OF RE-DOPED SRAL2O4 NANOPHOSPHORS ...... 12

3.2 CHARACTERIZATION METHODS ...... 14

REFERENCES ...... 17

2+ 2+ 3+ 4 SRAL2O4: EU AND SRAL2O4: EU , DY OPTICAL PROPERTIES AND LONG

PERSISTENCE PHENOMENON...... 18

4.1 STRUCTURE OF SRAL2O4 SYNTHESIZED BY COMBUSTION METHOD ...... 18 vi

2+ 2+ 3+ 4.2 OPTICAL SPECTRA ANALYSIS OF SRAL2O4: EU / EU , DY SYSTEM ...... 24

2+ 3+ 4.3 LONG PERSISTENCE LUMINESCENCE IN SRAL2O4: EU / DY

NANOPHOSPHORS ...... 30

4.4 CONCLUSIONS ...... 34

REFERENCES ...... 35

2+ 3+ 3+ 3+ 5 IR LONG PERSISTENCE IN SRAL2O4: EU , DY , ER / ND NANOPHOSPHORS

THROUGH PERSISTENT ENERGY TRANSFER ...... 36

2+ 3+ 5.1 ENERGY TRANSFER BETWEEN EU AND ER IONS IN SRAL2O4...... 36

2+ 3+ 3+ 5.2 IR LONG PERSISTENCE IN SRAL2O4: EU , DY , ER NANOPHOSPHOR ...... 41

2+ 3+ 3+ 5.3 IR LONG PERSISTENCE IN SRAL2O4: EU , DY , ND NANOPHOSPHOR

THROUGH ENERGY TRANSFER ...... 42

5.4 METHODS FOR LONG PERSISTENCE PROPERTIES IMPROVEMENT ...... 45

5.5 CONCLUSIONS ...... 48

REFERENCES ...... 50

6 CONCLUSIONS ...... 51 vii

LIST OF TABLES

Page

Table 4.1: Unit cell parameters for SrAl2O4 nanocrystallince ...... 20

2+ 3+ Table 4.2: Decay times of SrAl2O4: 0.8 % Eu , x % Dy Nanophosphors ...... 33

2+ 3+ 3+ Table 5.1: Decay times of SrAl2O4: 0.8 % Eu , 1.0 % Dy , x % Er Nanophosphors at

1530 nm...... 42

viii

LIST OF FIGURES

Page

2+ Figure 4.1: (a) XRD pattern of SrAl2O4: Eu ; (b) Standard SrAl2O4 XRD, JCPDS No. 34-379

………………………………………………………………………………………...19

Figure 4.2: SEM images of SrAl2O4 with the magnifications of 15000 (a) and 30000 (b)...... 21

Figure 4.3: EDX spectra of SrAl2O4, (a) whole SEM image with a magnification of 60000, (b)

EDX energy spectra, (c) SrK, (d) AlL, (e) OK, (f) EuM energy distributions ...... 22

2+ Figure 4.4: (a) Absorption and excitation spectra from SrAl2O4: Eu nanophosphor, (b)

2+ 3+ Absorption and excitation spectra from SrAl2O4: Eu , Dy nanophosphor ...... 23

2+ Figure 4.5: The influence of emission intensities from various Eu concentrations in SrAl2O4

host nanophosphors, λex = 350 nm. Inset is the relation between the 515 nm emission

intensities and Eu2+ concentrations...... 25

2+ Figure 4.6: Excitation (Exc.) and emission (Em.) spectra of SrAl2O4: Eu nanophosphors at

77 K (a) and 300 K (b)...... 26

Figure 4.7: Synchronous emission spectra and schematic energy levels diagram at 77 K from

2+ SrAl2O4: Eu nanophosphor...... 29

2+ 2+ 3+ Figure 4.8: Excitation (Exc.) and emission (Em.) spectra of SrAl2O4: Eu / Eu , Dy

nanophosphors...... 31

2+ 3+ Figure 4.9: Decay curves of green long persistence from SrAl2O4: Eu , Dy nanophosphors .. 32

2+ 2+ Figure 5.1: 1530 nm IR excitation (Exc.) and emission (Em.) spectra from SrAl2O4: Eu / Eu ,

Er3+ nanophosphors...... 37 ix

2+ 3+ Figure 5.2: Schematic diagram of energy transfer between Eu and Er ions in SrAl2O4

nanophosphors...... 39

2+ 3+ Figure 5.3: 1530 nm IR excitation (Exc.) and emission (Em.) spectra of SrAl2O4: Eu , Er

nanophosphors, Exc. and Em. are normalized at 519 nm...... 40

Figure 5.4: Schematic energy level diagram and optical spectra of Nd3+ ion...... 43

2+ Figure 5.5: 1059 nm IR excitation (Exc.) and emission (Em.) spectra from SrAl2O4: 0.8% Eu ,

1.0% Dy3+, x% Nd3+ nanophosphors, Exc. and Em. are normalized at 582 nm ...... 44

2+ 3+ Figure 5.6: 1530 nm IR excitation (Exc.) and emission (Em.) spectra from SrAl2O4: Eu , Dy ,

3+ Er nanophosphors additive with various amount of H3BO3 ...... 46

Figure 5.7: XRD patterns of (a) 20 % H3BO3 additive SrAl2O4 nanophosphor, (b) 800˚C

produced SrAl2O4 nanophosphor...... 47

Figure 5.8: 1530 nm IR excitation (Exc.) and emission (Em.) spectra of SrAl2O4 Nanophosphors

produced from different ignition temperatures...... 49

CHAPTER 1

INTRODUCTION

2+ 3+ Green long persistent phosphor aluminate, SrAl2O4 activated by Eu , Dy ions have long been of interest for its high quantum efficiency, long afterglow without radiation and good chemical stability [1, 2]. Up to now many efforts have been made on the study of infrared (IR) long persistence of SrAl2O4 host phosphor, while little extension is investigated.

As we known, Er3+ and Nd3+ are two most popular rare earth IR emission ions. The Er3+ ion exhibits IR eye-safe laser emission at 1530 nm, included in so-called S-band: 1460 nm -1530 nm, which is very important in the optical communication because the S-band represents the low-loss optical communication window in silica fiber. Another interested IR emission ion, Nd3+ ion has been known and exploited as laser amplification in the glass host for years. All types of

Nd-doped laser sources use fluorophosphates glass and can be operate at 1050 — 1070 nm for widespread commercial applications. Broad IR emission is also located at another low-loss communication window, and has great potential in biomedical application. Here, it is innovative

3+ 3+ 2+ 3+ to introduce Er / Nd ion into SrAl2O4: Eu , Dy system in order to obtain the IR long persistent luminescence.

Combustion synthesis route holds promise for the preparation of nanophsphors. Since the late 1980’s, combustion synthesis has been investigated as a method to produce homogeneous, crystalline, fine nano-oxide powders [3]. This synthesis method produces rapid, exothermic, self- sustaining reactions resulting from the appropriate combination of oxidizers and an organic fuel.

1 All SrAl2O4 related nanophosphors in this thesis are fabricated by combustion method.

The objectives of this thesis involve the preparation, characterization and analysis of the structure and long persistence mechanism through optical spectra from Eu2+, Dy3+, Er3+ / Nd3+ ions co-doped SrAl2O4 host nanophosphor. The thesis is organized as follows:

Chapter 2 reviews the fundamental concepts and mechanism on long persistent phosphors.

The luminescent properties of rare earth (RE) ions are also introduced.

Chapter 3 is concerned with the synthesis and characterization of RE doped SrAl2O4 nanophosphors. The combustion synthesis process is used to prepare the samples.

2+ Chapter 4 shows the structure and optical analysis results obtained from SrAl2O4: Eu ,

3+ 2+ Dy nanophsphors. A new way to illustrate the occupied sites of Eu ions in SrAl2O4 host is investigated.

2+ Chapter 5 focuses on the IR long persistence through energy transfer in SrAl2O4: Eu ,

Dy3+, Nd3+ nanophosphors. The sunlight excited IR long persistence is obtained in this system.

Meanwhile, more methods to improve the long persistence properties are followed.

Finally, the conclusions of this thesis are presented in Chapter 6.

2 REFERENCES

[1] T. Matsuzawa, Y. Aoki, J. Electrochem. Soc. 143 (1996) 2670

[2] C. Chang, D. Mao, J. Alloy. Compd. 348 (2003) 224

[3] L. R. Pederson, W. Weber, Mater. Lett. 10 (1990) 437

3 CHAPTER 2

BACKGROUND REVIEW

2.1 LONG PERSISTENT PHOSPHORS

In general, luminescence can be categorized into fluorescence and phosphorescence.

Fluorescence emission takes place simultaneously with the absorption of radiation and stops shortly after the radiation ceases. Phosphorescence is used to describe the process in which luminescence is proceeded by the population residing in a meta-stable state, such as, capture of electrons by traps in semiconductors. In this session, we are going to talk about long persistent phosphorescence and its mechanism.

Phosphorescence can be classified by its lifetime. Very short persistent phosphorescence has a lifetime of the same order of magnitude as the lifetime of the excited state. Normally, it is no longer than a few milliseconds and it is due to shallow traps. Short persistent phosphorescence lasts for seconds and it generally becomes noticeable to the human eye.

Persistent phosphorescence and long persistent phosphorescence have lifetimes in minutes or hours, respectively. For practical visual applications, the persistence time is defined as the time for the afterglow emission intensity to decay to 0.032 mcd/m2, which is approximately 100 times the limit of human perception with dark-adapted eye.

4 The mechanism for long persistent phosphorescence can be explained in terms of a simplified three-level diagram including a ground state, an excited state, and a meta-stable trapping state for the active electrons. Phosphorescence lifetimes are usually longer than the lifetime of the excited state and depend on the trap depth and trapping-detrapping mechanism.

Fluorescence, on the other hand, is usually based on a two-level electron transition mechanism, which includes a ground state and an excited state only. The decay time of fluorescence depends on the transition strength between the two states.

2+ 3+ 2.2 MECHANISMS FOR THE PHOSPHORESCENCE OF SRAL2O4: EU , DY

So far, four different mechanisms have been proposed to explain the phosphorescence of

2+ co-doped SrAl2O4: Eu system. In this section, we describe mechanisms but also introduce their shorting comings. The trapping dynamic of long persistence is usually complicated and still need to be further studied.

(a) Hole trapping mechanism [1].

2+ This mechanism relies largely on the photoconductivity study of power SrAl2O4: Eu sample, which showed UV irradiation induces a hole-type photoconductivity and hence suggested the existence of a hole trapping [1]. It is suggested that the holes originate from the capture of electrons from the valence band (VB) by Eu2+ ions, and the codopants Dy3+ trap these holes, and that the return of trapped hole to Eu2+ ions sites for delayed emission is triggered by thermally activated electron promotion from the top of the VB top to Dy4+ ions. This hole trapping model makes two crucial assumptions: the ground state of Eu2+ ion isclose in energy to the top of VVB, and Eu2+ ion excited by irradiation becomes and excited Eu+ ions upon electron

5 capture. While it is highly unlikely that the species Eu+ and Dy4+ ion are generated under a UV or visible excitation. Furthermore, electronic band structure calculations carried out for SrAl2O4:

Eu2+ system show that the Eu2+ d-block bands lies just below the conduction band (CB) bottom.

This finding is consistent with preliminary XPS measurements, which locate the 4f-block levels of Eu2+ at approximately 3 eV above the VB top [2]. Thus, these observations definitely do not support the mechanism of hole trapping model.

(b) Oxygen vacancy mechanism [3].

It still retain the hole trapping process suggested from the photoconductivity experiments and assumed the occurrence of energy transfer to Eu2+ ions and proposed that the UV excitation promotes an electron from the VB to a discrete level of unknown origin, while the hole created in the VB is trapped by an alkaline earth vacancy level VSr [3]. The thermal energy allow the transfer of the electron from the discrete level of unknown origin to an oxygen vacancy level VO.

From which a recombination takes place toward the VSr level. The energy released is then transferred to Eu2+ ions, which is excited an then is de-excited instantaneously. In this mechanism, the role of Dy3+ codoping is considered to increase the number of cation vacancies.

2+ EPR measurements of SrAl2O4: Eu sample show that the concentration of decreases during the

UV excitation and increases when the excitation is stopped until the extinction of phosphorescence. This finding evidences that Eu2+ participates in the trapping process and, hence, does not support the idea of energy transfer to Eu2+ after the trapping, suggested in this mechanism.

(c) Cation vacancy mechanism [4].

It is similar with the last explanation in invoking the electron excitation into a discrete level of unknown origin, the trapping of the holes created in the VB at cation vacancy levels and

6 the occurrence of energy transfer to Eu2+. However, it is assumed that the thermal energy enables the detrapping of the holes to the valence band, and then their recombination with the electrons present in the discrete levels of unknown origin [4]. As for the role of the codopant Dy3+, this mechanism assumed that UV radiation would initiate the heteronuclear inter-valence charge transfer between Eu2+ and Dy3+into Eu3+ and Dy2+, which is believed to delay the 520 nm long

2+ 3+ persistence because the reverse reaction is thermally activated. While for SrAl2O4: Eu , Dy system, Dy3+ is difficult to support because the lifetime of Dy2+ is certainly too short to explain the occurrence of phosphorescence lasting several hours. In addition, cation vacancy trapping mechanism still disobeys with the EPR measurements results of SrAl2O4 sample.

(d) Charge transfer mechanism [2].

This mechanism relies on the facts that d orbitals of Eu2+ are located near the CB bottom

2+ of SrAl2O4, that the Eu concentration decreases under UV excitation, and that phosphor samples always have a small amount of Eu3+ ions [2]. Under UV irradiation, electrons are promoted from the occupied 4f levels of Eu2+ to the empty 5d levels and from the VB top to the unoccupied 4f levels of residual Eu3+. The electrons promoted to the 5d levels can be trapped at

3+ the VO defects located in the vicinity of the photo-generated Eu cations, while the holes created

2+ in the VB can be trapped at the VSr levels. Due to these trapping processes, Eu ion is oxidized to Eu3+, while residual Eu3+ is reduced to Eu2+. The thermal energy at ambient temperature causes the detrapping of the trapped electrons directly to the 5d levels of Eu3+, hence leading to

3+ the green phosphorescence. However, the inherence of Eu ions in SrAl2O4 system is still a controversy.

7 2.3 RARE EARTH ELEMENTS

The terms RE and lanthanides are used for describing elements in lanthanide series, actinides series as well as yttrium. RE elements possess a number of similar chemical properties and their electronic structure of the RE elements is in the form of 4fn5s25p65d06s2, which are all stable in valence state of +3, with the electronic configuration as 4fn-15s25p65d06s2. In RE ions the 4fn electron shell is shielded by 5s and 5p electrons outside, so that they are screened from the fields of surroundings. Therefore, the energy levels of the 4f electrons are not much affected by the crystal environment, and remain almost the same in various host materials. The screening effect of the outer shells, 3s and 5p, also lead to a weak coupling between the 4f electrons and the crystal lattice, i.e., the electron-phonon coupling is weak. As a consequence, the spectral lines of 4f-4f transitions are not accompanied by a broad phonon side band and are characteristically sharp and well defined [5].

The electronic transitions in 4fn configuration are dominated by electric dipole or magnetic dipole interactions. In free RE ions, the electric dipole 4f-4f transitions are parity forbidden because of the selection rule. However, in a crystal field with an odd parity component, orbitals of different parity can be mixed and the electric dipole transitions are greatly enhanced.

The luminescence from such transitions depends strongly on the site symmetry of the host crystallization. On the other hand, the magnetic dipole transitions are parity allowed and not affected significantly by the local site symmetry. Due to the nature of the 4f-4f transitions, the transition probability is small compared with the normal electric dipole-allowed transitions. The excitation of the ions through 4f-4f transitions is usually not efficient, and the emission is weak, with a relatively long radiation lifetime in the range of milliseconds.

8 The 4fn eigenfunctions of RE ions can be expressed in a different basis set, commonly

2S+1 denoted in terms of Russell-Saunders states LJ, which would be also used in this thesis. For

RE ions, the azimuth quantum number of the 4f shell is 3 and corresponding seven-fold orbitals in it. In the ground state, J = L-S when the shell is less than half filled; otherwise, J = L+S.

The luminescence from Eu2+ is very different from that of Eu3+. The electronic configuration of Eu2+ is 4f7, identical to that of Gd3+. However, as the number of charge in the core of Eu2+ is one fewer than for Gd3+, the separation of the levels is smaller in Eu2+. The lowest

6 4 -1 excited state of 4f levels is the PJ (J = 7/2, 5/2, and 3/2) manifold, which is about 2.8×10 cm

8 above the ground state, S7/2.

2+ 6 1 6 In a free Eu ion, the 4f 5d level is not much higher than the PJ. It contains two portions with difference only in the electron spin direction; the octet, in which the spin of all the

7 electrons is parallel, and the sextet, in which the spin of all the six 4f electrons is parallel but that of the 5d electron is anti-parallel to the 4f ones. The sextet is slightly lower in energy and its

6 1 7 transition to the ground state in spin-forbidden. Therefore the lifetime of this 4f 5d to 4f transition is long, in the range of 10-5-10-6 s, though it is an electric dipole allowed transition.

The 4f65d1 state of Eu2+ ion is sensitive to the crystal field. It is split into sublevels and its lowest sublevel is of a lower energy, depending on the strength of the crystal field. The splitting is of different degrees such that the lowest 4f65d1 sublevel can be varied from above to well

6 2+ below the PJ manifold. Among a weak crystal field environment, the 4f-4f transitions of Eu ion takes place, as observed in SrAlF5 [6], SrBe2Si2O7 [7] etc. On the other hand, a strong crystal field makes the 4f65d1 sublevel the lowest excited state, the luminescence from these sublevel is varied in color from blue to red, with the increase of the crystal field strength. For example, the

9 center of the emission band in SrAl12O19 is at 395 nm and 460 nm in Sr2Al6O11 [8], 520 nm in

SrAl2O4 [9], 625 nm in SrO [10], respectively.

Since the 4f65d1 to 4f7 transition is electron dipole allowed, the transition probability is very high, about 3 orders higher than that of magnetic dipole allowed transition and of transitions due to state mixing as in Eu3+. The d electron couples with the crystal lattice strongly resulting in a broad phonon side band in the absorption and emission spectra. These are the essential characteristics of Eu2+ in crystals.

2.4 ENERGY TRANSFER IN INORGANIC SYSTEM BETWEEN RE IONS

Disregarding transfer by charge carriers, there remain two different mechanisms for transfer from an absorbing center A to a luminescent center B.

(a) Radiactive transfer, depending on how efficiently the B luminescent is excited by the

A emission. In inorganic systems the absorption strength of B is usually small, so that

transfer of this type is not often encountered.

(b) Nonradiactive transfer associated with resonance between A and B. Two tpypes of

interaction are of importance electronic multipole and exchange interaction. In the

former case the transfer probability is proportional, among other things, to the optical

strength of the transitions of A and B involved in the transfer process; in the latter

case it is proportional to the wavefunction overlap of A and B.

An energy transfer usually occurs between two different RE ions. Recently, we found another efficient energy transfer case from absorbing divalent europium to trivalent erbium ions in SrAl2O4 host material.

10 REFERENCES

[1] T. Matsuzawa, Y. Aokim, N. Takeuchi, J. Electrochem. Soc. 143 (1996) 2670

[2] F. Clabau Thesis, Université de Nantes, Nantes, France, 2005

[3] T. Aitasalo, P. Deren, J. Solid State Chem. 171 (2003) 114

[4] C. Beauger, Thesis, Université de Nice, Nice, France, 1999

[5] B. Henderson and G.F. Imbusch, Optical Spectroscopy of Inorganic Solids, Clarendon Press,

Oxford, 1989

[6] R.A. Hews, and M.V. Hoffman, J. Lumin. 3 (1970) 261

[7] J. Berstegen, J. Lumin. 9 (1974) 297

[8] G. Blasse, Sl Wanmaker, J. Electochem. Soc. 115 (1968) 673

[9] G. Blasse, Philips Res. Rep. 23 (1968) 201

[10] N. Yamashita, J. Lumin. 59 (1993) 195

11 CHAPTER 3

SYNTHESIS AND CHARACTERIZATION OF SRAL2O4 HOST NANOPHOSPHORS

3.1 PREPARATION OF RE -DOPED SRAL2O4 NANOPHOSPHORS

The combustion synthesis method is used to prepare SrAl2O4 host nanophosphors in this thesis. This method results in low-density voluminous mass in contrast to a solid lump by conventional solid-state reaction method. Also, the combustion process achieves the homogenous incorporation of and large-scale production of nanophosphor in a short interval of time. For SrAl2O4, as an oxide phosphor, the advantages of combustion synthesis are in its ability to produce well-crystallized, fine particle size powders rapidly without extensive high temperature annealing and mechanical separation steps. Oxidizing atmosphere prevails in the combustion process.

Incorporation of europium in divalent state invariably requires a reducing atmosphere.

Lots of methods have been proposed and developed to reduce Eu3+ to Eu2+. The most common

3+ scheme is to calcine the Eu -contained material in a reducing atmosphere, such as H2 or CO [1,

2]. Another way is to irradiate the Eu3+ -contained sample with high-energy rays, like UV, or x- rays [3, 4]. While a reduction phenomenon of Eu3+ to Eu2+ was observed for the first time when

3+ - Eu ions were doped into an AlO4 tetrahedron-containing compounds in an oxidizing atmosphere of air by high temperature solid-state reaction [5]. XRD patterns and PL spectra

12 were used to confirm the compound structure, respectively and detected the simultaneous existence of both divalent and trivalent europium ions. The abnormal Eu3+ to Eu2+ reduction process is explained by a charge compensation model. Lately, this reduction process is realized even when the sample is prepared in an oxidizing atmosphere, e.g. pure O2 [6]. Four conditions are necessary to realize the reduction of Eu3+  Eu2+ in solid-state compounds with high temperature, oxidizing synthesis environment. These are: (1) the host compounds contain no oxidizing ions; (2) the Eu3+ ion substitutes for the divalent cation in the hosts; (3) the substituted cation has similar radius to the divalent Eu ion; and (4) the host compound has an

- - - appropriate structure, which is composed of the tetrahedral anion groups, e.g. BO4 , SO4 , PO4 ,

- - SiO4 , and AlO4 [7]. The combustion synthesis method satisfies all the above four conditions in the preparation of SrAl2O4 and its RE ions doped phosphors.

In this experiment, Sr(NO3)2, Al(NO3)3 together with Eu(NO3)3, Dy(NO3)3 and Er(NO3)3

/ Nd(NO3)3 are mixed and dissolved with de-ioned water, at the stoichiometric ratio of Sr(1-

2+ 3+ 3+ 3+ x)Al2O4: x (Eu , Dy , Er / Nd ). The x refers to the atom ratio hereafter. H3BO3, as a flux along with the combustible agent, glycine and carbonhydrazide is added to the mixture. The aqueous mixture is heated at 100 ˚C, and after the redox mixture is turn to gel condition, the temperature is rapidly increased to 500 ˚C to ignite the reactants.

13 3.2 CHARACTERIZATION METHODS

3.2.1 X-ray Diffraction (XRD) Analysis

The phase purity and homogeneity of the combustion powder product is investigated by

XRD technique. The XRD profiles are taken by X’Pert PRO diffractometer using Cu Kα radiation. Scherrer Formula, Dp = 0.94 λ / β cosθ is used to calculate the crystal size, where λ (Å) is the x-ray source wavelength, 1.542 Å; β (degrees) is the full-width at half maximum (FWHM); 2θ (degrees) is the Bragg angle. Though one drawback of the above simple method is that it works only if both stress-related and instrument-related broadening are negligible in comparison with particle size effects, our particle sizes that are in the 10 - 100 nm range which is often met with this condition. The average sizes calculated by application of

Scherrer’s equation is under 50 nm.

3.2.2 Scanning Electron Microscopy (SEM)

A FEI inspect F field emission gun (FEG) scanning electron microscope, SEM is used to examine the size distribution and the morphology of SrAl2O4 host nanophosphors.

3.2.3 Energy Dispersive X-ray (EDX) Analysis

An EDX spectrum normally displays peaks corresponding to the energy level for which the most x-ray has been received. Each of these peaks is unique to atom, and therefore

14 corresponds to a single element. Usually, the higher a peak in a spectrum, the more concentrated the element is in the specimen. The qualitative calibration and mapping are also used to analysis elements distributions in the certain sample.

3.2.4 Optical Spectroscopy

The photoluminescence (PL) spectra of SrAl2O4 host nanophosphors are recorded using a

Jobin Yvon Fluorolog-3, iHR-320 spectrophotometer with a xenon lamp as the source of excitation. The detection range of this spectrophotometer covers from 250 nm to 1550 nm.

Three types of optical spectroscopic techniques were employed in this thesis. (a)

Excitation spectra, an excitation spectrum gives a varied exciting light and the intensity of the emitted light at a fixed emission wavelength is measured as a function the excitation wavelength.

The excitation spectrum gives information on the positions of excited states, similar to the absorption spectrum does. (b) Emission spectra, an emission spectrum provides information on the spectral distribution of the light emitted by a sample, which is fed into the emission monochromator, and the emission intensity is recorded as a function of wavelength. (c)

Synchronous scanning spectra, a synchronous scan bases on synchronous scanned excitation and emission monochromators with a wavelength shift that enables one to acquire the narrowest and least overlapping bands. The synchronous spectrum is a superposition of excitation and emission spectra. This method has two advantages over common methods. First, the synchronous spectrum has a much smaller half-width. Second, it is recorded at the edges of excitation and emission bands. Therefore, even small variations in absorption or/and emission bands can significantly affect the synchronous band.

15 To measure the PL, the light of the Xenon arc lamp is dispersed into monochromatic radiation using dual spectrometer system where the excitation and emission intensities are both calibrated.

The PL of long persistent phosphors is quite different from non-persistent phosphors.

Photo charging process has been found due to the trapping processes for long persistent phosphors. We therefore prefer to use strong excitation intensity and slow scanning speed when measure the excitation spectrum of a long persistent phosphor. In this way, it would quickly saturate the traps and also good to keep all collected data from the correct zone.

16 REFERENCES

[1] S. Poort, Chem. Mater. 7 (1995) 1547

[2] G. Blasse, Philips Res. Rep. 23 (1968) 189

[3] R. Danby, J. Phys. C: Solid State Phys. 21 (1988) 485

[4] A. Lakshmanan, Tomita, Phys. Status Solide a 173 (2003) 503

[5] J.A. Duggy, M.D. Ingram, J. Non-Cryst. Solids 21 (1976) 373

[6] P. Dorenbos, Chem. Mater. 17 (2005) 6452

[7] M. Peng, J. Lumin. 127 (2007) 735

17 CHAPTER 4

2+ 2+ 3+ SRAL2O4: EU AND SRAL2O4: EU , DY OPTICAL PROPERTIES AND LONG

PERSISTENCE PHENOMENON

4.1 STRUCTURE OF SRAL2O4 SYNTHESIZED BY COMBUSTION METHOD

In order to determine the and establish chemical nature of the combustion products, XRD study is carried out to explore the compositions of SrAl2O4 and its doped samples. The measurements are taken using 45 kV tube voltage and 40 mA tube current.

2+ The XRD patterns of SrAl2O4: 1 % Eu and SrAl2O4 standard XRD patterns in JCPDS (NO. 34-

379) are shown in Fig. 4.1 (a) and 4.1 (b) respectively. The peaks of (0 0 1), (-2 1 1), (2 2 0), (2 1

1) and (0 3 1) planes, which characterize SrAl2O4 crystal in a monoclinic lattice [1] are obtained

2+ in SrAl2O4: 1 % Eu sample. This confirms that the major phase present in the phosphors is

2+ monoclinic SrAl2O4. The incorporated Eu ions have little effect on crystal distortion, no additional peak is observed. By applying Scherrer formula to the FWHM of the diffraction peaks, we calculate that the crystal size of SrAl2O4 is in the range of 17 nm to 27 nm. In addition, the refined crystallographic unit cell parameters are obtained and listed in Table 4.1. The values of both samples are roughly matched with SrAl2O4 standard values given in JCPDS (NO. 34-379).

A decrescent trend of unit cell parameter values in nanocrystals could be clearly

18

2+ Fig. 4.1 (a) XRD pattern of SrAl2O4: Eu ; (b) Standard SrAl2O4 XRD, JCPDS No. 34-379.

19 2+ 3+ observed. SrAl2O4: Eu , Dy nanophosphor is also well crystalline and its XRD pattern shows

2+ the same case as that of SrAl2O4: Eu .

Table 4.1 Unit cell parameters for SrAl2O4 nanocrystalline

Sample a (Å) b (Å) c (Å) β(˚) Cell volume (Å3)

JCPDS Std No. 34-379 8.427 8.814 5.159 93.45 382.75

SrAl2O4 nanophosphor 8.406 8.789 5.141 93.24 379.21

2+ Fig. 4.2 is SEM surface micrograph of SrAl2O4: 1 % Eu nanophosphor dissolved in the deionized water. The uniform phosphors from combustion route are observed in 2 µm range and well crystalline, which consists with the results from XRD analysis. Instead of the particle like shape happened on all previous combustion-synthesized nanomaterials, SrAl2O4 nanophosphor take an unusual needle-like shape. The reason is unknown, but these needle like phosphors are very useful to apply in the constructed materials to detect the damage of bridge or the high buildings, and may also be used as the decorative materials for the hotel, the plaza.

2+ The uniform distribution of Eu ions in SrAl2O4 host nanophosphor is shown in Fig. 4.3.

EDX energy microananlysis is employed to approach elements contribution in the sample. The radiation from CK is also detected due to the residual fuel during the reaction.

2+ 2+ 3+ The absorption spectra of SrAl2O4: 1 % Eu and SrAl2O4: 1 % Eu , 1 % Dy at room temperature (RT) are shown in Fig. 4.4 (a) and 4.4 (b) (black line) respectively. These two spectra have similar profiles and the edges are at about the same wavelength. Both of them show strong absorption in near UV region. The intense peak at 328 nm corresponds to the dipole allowed f-d transition of Eu2+ ion [2]. It is clear from the figure that the excitation peaks (red line

20

Fig. 4.2 SEM images of SrAl2O4 with the magnifications of 15000 (a) and

30000 (b).

21

Fig. 4.3 EDX spectra of SrAl2O4, (a) whole SEM image with a magnification of

60000, (b) EDX energy spectra, (c) SrK, (d) AlL, (e) OK, (f) EuM energy distributions.

22

2+ 2+ 3+ (a) SrAl2O4:Eu (b) SrAl2O4:Eu , Dy

Absorption Absorption Excitation Excitation (a.u.) (a.u.)

Intensity Intensity

300 350 400 450 500 300 350 400 450 500 Wavelen gth (nm) Wavelen gth (nm)

2+ Fig. 4.4 (a) Absorption and excitation spectra from SrAl2O4: Eu nanophosphor, (b)

2+ 3+ Absorption and excitation spectra from SrAl2O4: Eu , Dy nanophosphor.

23 in Fig. 4.4） maximum for them shift slightly towards shorter wavelength as compared with other micro-size samples which are around 343 nm [3]. This shift may be due to the quantum size effect of the nanophosphors. The size reduction should have widened the band gap and hence, they need higher energy radiations to be excited [4]. Since the absorption transitions of

Dy3+ are two orders weaker than that of Eu2+, no sharp peak, corresponding to Dy3+, observed in

2+ 3+ 3+ SrAl2O4: Eu , Dy sample compared with Dy free-sample.

2+ 2+ 3+ 4.2 OPTICAL SPECTRA ANALYSIS OF SRAL2O4: EU / EU , DY SYSTEM

2+ 4.2.1 Spectra of SrAl2O4: x % Eu at RT

2+ Fig.4.5 shows the emission spectra of SrAl2O4: x % Eu (x= 0.1, 0.5, 0.8, 1.0, 1.5, 2.0,

5.0) at RT. For all samples, brightest luminescence is centered at around 515 nm. The strongest emission intensity occurs in the sample with Eu2+ concentration of x = 1.0. Concentration quenching occurs when x is larger than 1.0%. The emission peak is known as the characteristic electronic transition of Eu2+ ions between its 4f65d1  4f7 levels [5]. The emission spectra

2+ together with the absorption spectra above of SrAl2O4: Eu samples verify the spontaneous reduction of Eu3+ ions in the presence of aluminate oxides. This phenomenon is classified as microscopic basicity in which the electron donating ability of oxygen surrounding the RE ions is affected by the Al3+ ions. The lowered basicity in Al3+ together with the low reduction potential of Eu3+  Eu2+ may account for the spontaneous reduction of Eu3+ by the electron ejected from the defect center at high temperature [6].

24

Fig. 4.5 The influence of emission intensities from various Eu2+ concentrations in

SrAl2O4 host nanophosphors, λex = 350 nm. Inset is the relation between the 515 nm emission intensities and Eu2+ concentrations.

25

2+ Fig. 4.6 Excitation (Exc.) and emission (Em.) spectra of SrAl2O4: Eu nanophosphors at 77 K(a) and 300 K (b).

26 2+ 4.2.2 Spectra of SrAl2O4: Eu at 77 K

2+ Fig. 4.6 shows the emission and excitation spectra of SrAl2O4: Eu at 77 K (a) and 300 K

(b). The emission spectrum at low temperature contains two bands; besides 515 nm board band appears at 300 K, a shorter one, located at 445 nm is observed at the same time. The high-energy emission is somewhat lower in intensity. The 77 K excitation spectra are also wavelength dependent. The 445 nm excitation spectrum ends at 420 nm, whereas the one at 515 nm has a tail followed after 420 nm and finally terminates at 460 nm. Previous literatures have reported this low temperature character [2]; at 300 K, efficient energy transfer occurs between Eu2+ ions two emission bands, the high-energy band is emerged with the broad hand at 515 nm. While Fig. 4.6

(b) shows that, the 300 K excitation spectrum monitored at 515 nm begins with the similar profile as that at 77K, but instantly drops down to the edge ended at 420 nm and no prior mentioned tail is observed. The range of the missing tail matches well with that of high-energy emission band covered at 77K. No overlap exists between the low energy band absorption and high-energy band emission, which disagrees with the energy transfer explanation proposed before by others [2].

It is well known that SrAl2O4 crystallizes into a monoclinic structure with space group

P21 and contains 4 formula units for a total of 28 atoms and all of the constituent atoms occupy

2a sites according to the Wyckoff notation [7]. The monoclinic structure is constructed by corner-shared AlO4- tetrahedrons forming zigzag strings, and Sr2+ ions penetrate to the opening of the structure to compensate the electric charge of AlO4-. Sr2+ ions are identified into two inequivalent sites; site I, high energy band and site II, low energy band, with low symmetry and coordinated by 8 oxygen ions and Al3+ by 4 oxygen ions. Eu2+ ions can equally replace Sr2+ at

27 these two sites and experience different crystal field, which result the different degree of splitting for the 4f65d1 electron level. The two fluorescence bands observed in Fig. 4.6 are corresponding to the two sites of Eu2+ [8]. Meanwhile, the platean-like absorption band is the overlapping result of two absorption bands from these two europium sites. In the later part of this chapter, we are

2+ going to investigate energy position of Eu ions in SrAl2O4 host phosphor in a new way.

2+ The schematic synchronous emission spectra diagram at 77 k from SrAl2O4: Eu with

various wavelength steps are shown in Fig. 4.7. For small scanning step Δλ, e.g. Δλ (nm) = 30, only the high-energy band, located at 445 nm is effectively excited. Low emission broad band of

Eu2+ activated sample is observed as the monitored steps increasing, and becomes comparable

with the high energy one. For Δλ ≥160 nm, it is hardly to observe the emission peak centered at

445 nm. The position of emission spectrum for Eu2+ is due to the transitions between the ground state and the crystal field components of the 4f65d1 excited state configuration. Hence, these two broad emission peaks correspond to two Eu2+ ions centered in different sites, I and II.

Although the 4f7 electrons in ground state of Eu2+ are not sensitive to lattice environment due to the shielding function of the electrons in the inner shell, the excited 4f65d1 configurations may couple strongly to the lattice. Consequently, the mixed states of 4f and 5d will be split by the crystal field, which may lead to the various shifts of the emission peak. Therefore, in addition to covalency, the size of the cation and the strength of the crystal field also influence the Eu2+ emission. The Sr2+ and Eu2+ ions are very similar in their ionic sizes, 1.21 Å and 1.20 Å, respectively. When occupied by Eu2+ ions, the Sr2+ sites will have quite similar local distortions and the influence on the SrAl2O4 crystal structure is small. But the Eu-O distances are different in two sites. So the structure will be more distorted in the high energy site occupied by Eu2+ ions.

Anamorphic crystal lattices result when the surroundings of Eu2+ are changed and so the

28

Fig. 4.7 Synchronous emission spectra and schematic energy levels diagram at 77 K

2+ from SrAl2O4: Eu nanophosphor.

29 emission wavelengths change correspondingly. The synchronous emission spectra also imply the different spaces between excitation and emission band under the crystal field effect. The repellant from cations nearby is weaker in low energy band due to the larger 4f-5d energy band splitting. It will lower its energy and therefore result in the emission at low energy.

2+ In SrAl2O4: Eu , the high energy broad emission band is also sensitive to temperature as well as concentration quenching. At higher temperature, the emission at 445 nm is almost quenched and only the longer wavelength emission is observed. When the concentration of Eu2+ is small enough, i.e. 0.01%, the emission intensities from 445nm and 515 nm are almost the same. As discussed above, we suggest the 445 nm and 515 nm emission bands do originate from the 4f65d1  4f7 transition of Eu2+ ions located at two different crystallographic strontium sites.

Whereas, the high energy band emission is sensitive to the temperature and concentration quenching, the energy transfer between two sites are hard to investigate from this thesis.

2+ 3+ 4.3 LONG PERSISTENCE LUMINESCENCE IN SRAL2O4: EU , DY NANOPHOSPHORS

2+ 3+ SrAl2O4: Eu , Dy is well known as a very long and bright persistent phosphor. Fig. 4.8

2+ 2+ exhibits the excitation and emission spectra of SrAl2O4: 0.8 % Eu and SrAl2O4: 0.8 % Eu , 1.0

% Dy3+ nanophosphors. Both of these two emission spectra are excited by 340 nm. They have similar profiles except that there is a 5 nm red shift from Eu2+, Dy3+ co-doped sample. And the emission intensity is also weakened because of quenching by Dy3+ ions trapping. Fig. 4.9 shows

2+ 3+ SrAl2O4: 0.8 % Eu , x % Dy (x= 0, 1.0, 1.6, 2.0) samples decay curves. To illustrate the slow decay at the long time, a double logarithm plot is applied.

30

2+ 2+ 3+ Fig. 4.8 Excitation (Exc.) and emission (Em.) spectra of SrAl2O4: Eu / Eu , Dy nanophosphors.

31

2+ 3+ Fig. 4.9 Decay curves of green long persistence from SrAl2O4: Eu , Dy nanophosphors.

32 It is well documented that the afterglow time decay behavior may follow a first order I =

n I0 exp [-αt], and the general order I = I0 (1+ γ t) kinetics behavior [9]. We represented the long persistent behavior of the synthesized phosphors by using Lorentzian fit, the decay curve of

SrAl2O4 samples were by simulated by the following equation to understand the long persistent behavior of the phosphor.

I = I0 + A1 exp (-t)/ τ1 +A2 exp (-t)/ τ2 ……………….. (1)

Where I represent the phosphorescence intensity, I0, A1, A2 are the constants, t is the time, and τ1,

τ2 are the decay constants of the phosphors.

Significant and varied values of τ1 and τ2 clearly indicate the concentration of shallow and deeper traps, respectively [10]. The higher initial intensity can be attributed to the presence of sufficient numbers of shallow traps, while longer decay times are due to the deeper trap density. However, it is emphasized that mere fitting of data may not be a physical evidence of only two trapping levels of different energies. There may be more than two trapping levels as suggested in Refs [11]. But, they can always be averaged into two types of trapping levels, i.e., shallow and deep.

2+ 3+ Table 4.2 lists the parameters generated from (1) for SrAl2O4: 0.8 % Eu , x % Dy (x=

0, 0.5, 1.0, 1.6, 2.0) samples decay curves. The best Dy3+ ion concentrations for green long

2+ persistence from SrAl2O4: 0.8 % Eu system is around 1.6 %.

2+ 3+ Table 4.2 decay times of SrAl2O4: 0.8 % Eu , x % Dy nanophosphors

Concentration of Dy3+ ion, x % 0.5 1.0 1.6 2.0

τ1 (s) 13.4 16.4 21.4 13.0

τ2 (s) 343.1 357.7 423.6 408.4

33 4.4 CONCLUSIONS

2+ The optical properties in two different sites of SrAl2O4: Eu nanophosphor is investigated by synchronous emission spectra. We suggest that high energy emission is more sensitive to the temperature and concentration quenching. The current results cannot show the energy transfer process between these two sites at RT which still need further discussion.

34 REFERENCES

[1] J. Hanawalt, Adv. X-ray Anal. 20 (1997) 63

[2] M. Zaitoun, T. Yamazaki, J. Lumin. 78 (1998) 63

[3] Z. Fu, S. Zhang, J. Solid State Chem., 178 (2005) 230

[4] R. Bhargava, D. Gallagher, Phys. Rev. Lett. 72 (1994) 416

[5] G. Blasse, A. Bril, Philips Technical Review 31 (1970) 10, pp. 304–334

[6] J.A. Duggy, M.D. Ingram, J. Non-Cryst. Solids 21 (1976) 373

[7] V. Schulze, Z. Anorg. Allg. Chem. 475 (1981) 205

[8] S.H.M. Poort, W.P. Blodpoel, and G. Blasse, Chem. Mater. 7 (1995) 1547

[9] S. Shionoya, W.M. Yen, Phosphor Handbook, CRC Press, NY, 1999

[10] H. Chander, D. Haranath, J. Cryst. Growth 271 (2004) 307

[11] D. Jia, Opt. Mater. 22 (2003) 65

35 CHAPTER 5

2+ 3+ 3+ 3+ IR LONG PERSISTENCE IN SRAL2O4: EU , DY , ER / ND NANOPHOSPHORS

THROUGH PERSISTENCE ENERGY TRANSFER

2+ 3+ It has been introduced above that the co-doped SrAl2O4: Eu , Dy shows a green demonstrated persistence lasting for up to 15 hours. In this chapter, we will employ the optical spectra analysis method to investigate the IR long afterglow due to the persistent energy transfer

2+ 3+ 3+ from Eu to Er / Nd ions in SrAl2O4 host nanophosphor.

2+ 3+ 5.1 ENERGY TRANSFER BETWEEN EU AND ER IONS IN SRAL2O4

2+ Fig. 5.1 shows the emission and excitation spectra in IR region of SrAl2O4: 1.0 % Eu ,

3+ 3+ 1.0 % Er and SrAl2O4: 1.0 % Er samples synthesized by combustion method. All relevant internal 4f  4f electron transitions of Er3+ ions in the range of 290 – 700 nm are observed and

3+ identified from the excitation spectrum of SrAl2O4: Er sample. The intense blue green

2 4 excitation centered at around 519 nm is clearly observed, which corresponds to the H11/2  I15/2

4 transitions. A red broad band centered at 649 nm is also observed and associated with the F9/2 

4 3+ I15/2 transitions. As there is no intrinsic excitation energy level at 340 nm for Er ions, it is hard to observe the IR emission in Eu2+ ions free sample (as shown in Fig. 5.1, black dash curve). In contrast, in the Eu2+-Er3+ co-doped system, an intense green luminescence

36

2+ 2+ 3+ Fig. 5.1 1530 nm IR excitation (Exc.) and emission (Em.) spectra from SrAl2O4: Eu / Eu , Er nanophosphors.

37 can be obtained under sunlight irradiation due to the 4f65d→4f7 allowed transitions of the Eu2+ ions. Part of the energy of green photons can be given to the Er3+ ions by energy transfer process.

3+ 4 4 An IR emission band centered at 1530 nm from Er ions I13/2 → I15/2 transitions can thus be obtained. An obvious overlap between the emission spectra Eu2+ and the absorption of Er3+ can be seen from Fig. 5.1 (inset). The excitation spectrum of Eu2+- Er3+ co-doped sample suggests that the excitation is dominated by the Eu2+ excitation band in UV-VIS region, which results the efficient IR emissions from the Er3+ ions at 1530 nm, i.e. the energy transfer between the Eu2+ and Er3+ ions in this system is effective. Fig. 5.2 is the schematic diagram of the Eu2+ and Er3+ ions energy levels.

To investigate the energy transfer efficiency between Eu2+ and Er3+ ions, we focus on the

2+ 3+ samples with the same ratios of Eu and that of Dy ions in SrAl2O4 host materials. Fig.5.3

2+ 3+ shows the emission and excitation spectra at 1530 nm from SrAl2O4: 0.8 % Eu , 1.0 % Dy , x

% Er3+ samples produced by combustion synthesis method, where x = 0.5, 0.8, 1.2, 1.6, 2.0. The

2 spectra are normalized at 519 nm, which is identified as the intrinsic absorption by H11/2 state from Er3+ ions. The relative intensities at UV excitation band and emissions set at 1530 nm suggest the energy transfer efficiency is related to concentrations of Er3+ ions. In the beginning,

3+ 3+ 2 when the amount of doped Er ion is small, more and more Er ions occupied at H11/2 state could accept the electrons transferred from Eu2+ ions as the number of Er3+ ion rising, the energy transition ratio is increasing. The maximum relative IR emission intensity is observed from the sample with x = 1.6 %. The concentration quenching happens when the Er3+ ions incorporated amount is larger than 1.6 % mol, which results in the lowered relatively intensities. XRD patterns also suggest when x is larger than 1.6 %, SrO phases become precipitate in this system. The variations in 649 nm excitation intensities show the opposite trend compared with

38

2+ 3+ Fig. 5.2 Schematic diagram of energy transfer between Eu and Er ions in SrAl2O4 nanophosphors.

39

IR Exc. IR Em. 2+ 3+ SrAl2O4: 0.8% Eu , x% Er

x = 0.5 x = 0.8 x = 1.2 (a.u.) x = 1.6 x = 2.0

Intensity

300 450 600 1400 1500 1600 Wavelength (nm)

Fig. 5.3 1530 nm IR excitation (Exc.) and emission (Em.) spectra of SrAl2O4:

Eu2+, Er3+ nanophosphors, Exc. and Em. are normalized at 519 nm.

40 1530 nm radiation, which verifies a cross relaxation (CR) process in this system. Such CR

3+ 2 process leads the energy conversion of one Er ion in H11/2 state with another one in the ground

4 4 state up converts into I13/2 state under coulomb effect, without any radiation. Therefore, the I13/2

4  I15/2 transitions are enhanced due to the CR process; on the other hand, less number of

2 4 electrons could relax from H11/2 to F9/2 state, the associated excitation by 650 nm is thus weakened for 1530 nm emission. This is another evidence for the efficient energy transfer

2+ 3+ 2+ 3+ process from sensitizer Eu to the activator Er ions in SrAl2O4: Eu , Er system.

2+ 3+ 3+ 5.2 IR LONG PERSISTENCE IN SRAL2O4: EU , DY , ER NANOPHOSPHOR

The VIS long persistence due to the energy transfer has been reported [1]. In SrAl2O4 host nanophosphor, the intense IR emission due to the energy transfer process from Eu2+ to Er3+ ions

3+ 2+ 3+ has been suggested above. Hence, Er ion activated SrAl2O4: Eu , Dy system shows great potential as an IR long persistence material.

2+ 3+ 3+ Table 5.1 shows the 1530 nm IR lifetime in SrAl2O4: 0.8 % Eu , 1.0 % Dy , x % Er (x =

0.2, 0.5, 0.8, 1.0, 1.2, 1.4, 1.6, 2.0) samples synthesized by combustion method. The decay times from the above SrAl2O4 host nanophophors are obtained by Lorentzian fitting method, I = I0 +

A1 exp (-t)/ τ1 +A2 exp (-t)/ τ2. We found the decay times change in the similar trends as that for energy transfer efficiency due to the varied amount of incorporated Er3+ ions. One possible reason is that single Er3+ ions activated sample also shows relatively weak afterglow [2], although similar charge defects should be created in substituting Sr2+ by Er3+ as is the case of doping Eu2+ ions. At first, the concentration of Dy3+ ions together with that of Er3+ ions are too low to form enough traps or defects in the matrix materials. The depths and relative densities of

41 3+ the traps in SrAl2O4 tend to saturate as the concentration of Er ions increasing, and the duration of 1530 nm lifetime is prolonged. However, if the doped amount of Er3+ is too large, it may result in concentration quenching and may lower the luminescent effect at the same time.

2+ 3+ 3+ Table 5.1 Decay times of SrAl2O4: 0.8 % Eu , 1.0 % Dy , x % Er nanophosphors at 1530 nm

Concentration of Er3+, x 0.2 0.5 0.8 1.0 1.2 1.4 1.6 2.0

τ1 (s) 7.71 10.29 14.33 21.12 21.76 40.66 22.01 29.78

τ2 (s) 78.94 91.89 115.97 133.24 124.11 203.35 168.60 148.71

2+ 3+ 3+ 5.3 IR LONG PERSISTENCE IN SRAL2O4: EU , DY , ND NANOPHOSPHOR THROUGH ENERGY

TRANSFER

2+ 3+ 3+ Similar processes are investigated in SrAl2O4: Eu , Dy , Nd system. The long persistence at a broad band at 881 nm, a strong intensity band at 1059 nm and another band at

1361 nm is observed respectively. Here we take 1059 nm IR long persistence as an example.

Schematic energy level diagram of Nd3+ ion and its optical spectra are showed in Fig. 5.4. The

2+ 3+ 3+ SrAl2O4: 0.8 % Eu , 1.0 % Dy , x % Nd (x = 0.2, 0.4, 0.6, 1.0) are excited by 380 nm to avoid Nd3+ ion intrinsic absorptions. The excitation and emission spectra shown in Fig. 5.5 are normalized at 582 nm. The maximum energy transfer ratio appears when the concentration of

Nd3+ ion is around 0.4 %, which is not as high as the case in Er3+ ion activated system. One possible reason for this is because of Nd3+ ion low concentration quenching at 1059 nm luminescence [3].

42

Fig. 5.4 Schematic energy level diagram and optical spectra of Nd3+ ion.

43

2+ Fig. 5.5 1059 nm IR excitation (Exc.) and emission (Em.) spectra from SrAl2O4: 0.8% Eu ,

1.0% Dy3+, x% Nd3+ nanophosphors, Exc. and Em. are normalized at 582 nm.

44 5.4 METHODS FOR LONG PERSISTENCE PROPERTIES IMPROVEMENT

5.4.1 Effects of Additive with H3BO3

Improvements in brightness and persistence are further achieved by doping the SrAl2O4:

Eu2+, Dy3+ lattice with divalent ions like Mg2+ and Zn2+ or monovalent ions such as K+ and Na+.

This type of compensation reduces the charge defects induced by substitution of the trivalent RE ion sites into alkaline earth metal ion sites, Sr2+ sites, within the aluminate [4]. Besides, the persistence property can also be enhanced by the addition of B2O3 in solid-state reaction synthesis method [5]. The B2O3 acts as an inert high temperature solvent, or flux to facilitate the grain growth of strontium aluminate, which increases the penetration of trap centers in the material. Here we introduce H3BO3 as the flux in combustion synthesis method. Fig. 5.6 shows

2+ 3+ the spectra from SrAl2O4: Eu , Er system added with various amount of H3BO3. 10% H3BO3 affiliated sample has a blue shift compared with H3BO3 free sample, which indicates the decreased size resultants by adding H3BO3. 10% H3BO3 does not affect the phase stability of

2+ 3+ 3+ 3+ SrAl2O4: Eu , Dy , Er system; in contrast, B can reduce the forming temperature of SrAl2O4 and form fusible borate to speed the reacting materials diffusion, which results a smaller dimension products [6]. However, with increasing H3BO3 content, the radiation intensity largely weakened, the XRD spectrum also shows the changes. In Fig. 5.7 (a), SrAl2O4 as well as SrB2O4 phases appear in the 20 % H3BO3 addition sample.

45

2+ 3+ Fig. 5.6 1530 nm IR excitation (Exc.) and emission (Em.) spectra from SrAl2O4: Eu , Dy ,

3+ Er nanophosphors additive with various amount of H3BO3.

46

Fig. 5.7 XRD patterns of (a) 20 % H3BO3 additive SrAl2O4 nanophosphor,

(b) 800˚C produced SrAl2O4 nanophosphor.

47 5.4.2 High Temperature Ignition

The exothermicity of the combustion reaction is due to the oxidation of the fuel, e.g. carbohydrazide, glycine, to N2 and H2O. However exothermicity of these combustion precursors is not high enough to sustain combustion and an external heat source is needed for the completion of the reaction. A higher ignition temperature can shorten the time required for the reaction and prevent the probabilities of nitrates self-decomposition. The various ignited temperature samples are also prepared through combustion synthesis method to investigate this process. As it is shown in Fig. 5.8, the most efficient energy transfer between Eu2+ and Er3+ occurs when the environment temperature is 700oC. Fig. 5.7 (b) suggests that an excessive external heat does not always promote in the forming of monoclinic SrAl2O4 crystalline. Because phase transitions happen between the monoclinic and hexagonal SrAl2O4 crystals when the reaction temperature is higher than 700oC [4]. Fig. 5.8 verifies by lowered 1530 nm emission intensities from the sample prepared at 800oC.

5.5 CONCLUSIONS

2+ 3+ 3+ 3+ A novel sunlight activated IR long persistent system, SrAl2O4: Eu , Dy , Er / Nd is investigated by combustion synthesis method. This kind of persistent phosphor can be excited by

UV-VIS region, with the IR persistent luminescence lifetime up to several hours.

48

IR Exc. IR Em. 2+ 3+ 3+ SrAl2O4: Eu , Dy , Er Ignited at Different Temperatures

600 oC 700 oC o (a.u.) 800 C

Intensity

300 400 500 600 1400 1500 1600 Wavelength (nm)

Fig. 5.8 1530 nm IR excitation (Exc.) and emission (Em.) spectra of SrAl2O4

Nanophosphors produced from different ignition temperatures.

49 REFERENCES

[1] W. Yen, D. Jia, US Patent 6953536 (2005)

[2] E. Nakazawa, T. Mochida, J. Lumin. 72 (1997) 236

[3] J. Stroud, App. Opt. 7 (1968) 751

[4] T. Matsuzawa, Y. Aoki, J. Electrochem. Soc. 143 (1996) 2670

[5] W. M. Yen, W. Jia, Phosphors with long persistent green phosphorescence, US Patent

6267911 B1 (July 31, 2001)

[6] A. Nag, T. Kutty, J. Alloy. & Compd. 354 (2003) 221

50 CHAPTER 6

CONCLUSIONS

The novel IR long persistence phosphor has great promising use in materials tracking, ink-jet printing tags, combat ID- urban flight, novel sensors and forensic imaging, etc. In this

2+ 3+ 3+ 3+ thesis we claim that the SrAl2O4: Eu , Dy , Er / Nd co-doped system exhibits IR long persistence by sunlight irradiation with hours life time is observed. The formation of combustion prepared RE doped SrAl2O4 nanophosphors is confirmed by XRD analysis. This homogeneous single phase of monoclinic SrAl2O4 based material is also convenient to charge into various shapes. It is a new report for related sunlight-excited infrared- long persistence nanophosphor

2+ 3+ 3+ 3+ reported. Combustion synthesized SrAl2O4: Eu , Dy , Er / Nd nanophosphor has great potential as witness material. Further exploration is to increase the energy transfer ratio between

2+ 3+ 3+ Eu and Er / Nd and long persistent last time in SrAl2O4 system.

51