Tổng hợp vật liệu huỳnh quang ba màu sử dụng trong các đèn huỳnh quang compact

Tổng hợp vật liệu huỳnh quang ba màu sử dụng trong các đèn huỳnh quang compact Tổng hợp vật liệu huỳnh quang ba màu sử dụng trong các đèn huỳnh quang compact Tổng hợp vật liệu huỳnh quang ba màu sử dụng trong các đèn huỳnh quang compact luận văn tốt nghiệp,luận văn thạc sĩ, luận văn cao học, luận văn đại học, luận án tiến sĩ, đồ án tốt nghiệp luận văn tốt nghiệp,luận văn thạc sĩ, luận văn cao học, luận văn đại học, luận án tiến sĩ, đồ án tốt nghiệp

MATERIALS SCIENCE

BUI VAN HAO

SYNTHESIS OF TRICOLOR PHOSPHORS USED IN FLUORESCENT AND COMPACT FLUORESCENT LAMPS

MASTER THESIS OF MATERIALS SCIENCE

BATCH ITIMS - 2006

Supervisor: Assoc Prof Dr Pham Thanh Huy

VI ỆN ĐÀO TẠO QUỐC TẾ VỀ KHOA HỌC VẬT LIỆU

BÙI VĂN HÀO

TỔNG HỢP VẬT LIỆU HUỲNH QUANG BA MÀU SỬ DỤNG TRONG CÁC ĐÈN HUỲNH QUANG VÀ HUỲNH QUANG

Firstly, I am deeply thankful my supervisor, Assoc Prof Dr Pham Thanh Huy for his direct guidance and financial support during my study He has believed and created the best conditions for my research

I would like to thank and give my best regards to the directorate and all the teachers, scientists of ITIMS who have brought me much knowledge through interesting lessons and opportunities in future

I further thank to Dr Tran Ngoc Khiem, Dr Trinh Xuan Anh, Dr Le Anh Tuan, MSc Tran Trong An, MSc Phi Van Luong, Nguyen Manh Cuong, Duong Thanh Tung and other members of Photonics and Optoelectronics Lab for their kind help during my research

I would also like to thank to the directors of Quy Nhon University for the permission and financial support for my study

Finally, I am deeply thankful my family and all my friends who have helped, encouraged and supported me at any time

Hanoi, September 2008 Master student: Bui Van Hao

Contents

Introduction 1

Chapter 1: An overview of phosphors used in lighting devices 3

1.1 Early phosphors for fluorescent lamps 3

1.2 Tricolor phosphors for luminescent lamps based on rare earth activators 7

1.2.1 Blue emitting phosphors 9

1.2.2 Green emitting phosphors 11

1.2.3 Red emitting phosphors 12

1.3 Phosphor synthesis methods 13

1.3.1 Solid state reaction method 13

1.3.2 Combustion synthesis 13

1.3.3 Spray pyrolysis 13

1.3.4 Sol-gel method 15

Chapter 2: Experimental 18

2.1 Preparation of blue-emitting phosphor BaMgAl10O17:Eu2+ by sol – gel method 18

2.2 Preparation of green emitting phosphors (LaGd)PO4:Tb3+ by sol – gel method 20

2.3 Preparation of red emitting phosphors Y2O3:Eu3+ by sol – gel method 21

2.4 Material characterization and analysis 23

2.4.1 Infrared spectroscopy 23

2.4.2 X-ray diffraction techniques 26

2.4.5 Photoluminescence spectroscopy - Luminescence and

excitation spectra 33

Chapter 3: Results and discussion 35

3.1 Structure and luminescent properties of BaMgAl10O17:Eu2+ blue emitting phosphor 35

3.1.1 Crystalline structure of the powders 35

3.1.2 Morphology of the phosphor 39

3.1.3 Photoluminescent spectra of the phosphors 40

3.2 Structure and photoluminescent properties of (LaGd)PO4:Tb3+ green emitting phosphor 46

3.2.1 Crystalline structure of the phosphor 46

3.2.2 Morphology of the phosphor 48

3.2.3 Photoluminescent properties of the phosphors 49

3.3 Structure and photoluminescent properties of Y2O3:Eu3+ red emitting phosphor 52

3.3.1 Formation of Y 2 O 3 :Eu 3+ phosphor using citric acid as chelating agent in the sintering process 52

3.3.2 Morphology of Y 2 O 3 :Eu 3+ phosphor 54

3.3.3 Luminescent properties of the phosphors 55

3.3.4 Effect of the ratio of metal ions to acid on particle size and luminescent properties of the phosphors 63

Conclusions 65

INTRODUCTION

In modern society, people rely heavily on rare earth based devices In almost any office, light is produced by fluorescent lamps in which UV radiation from mercury plasma is converted into visible light by rare-earth based phosphors In addition, in our information society, vast amounts of information are read from displays, in most cases CRTs, in which rare-earth materials are applied

After more than 50 years of extensive research on the luminescent materials applied in lamps and displays, compounds are obtained with almost ideal physical properties: very high energy efficiencies for cathoderay phosphors and very high UV absorptions and quantum efficiencies for lamp phosphors In fluorescent lamps with a very high color rendering index (CRI), three or more phosphors are applied In Philips high-quality fluorescent lamps with the so-called 'Color 80' phosphor blend, BaMgAl10O17:Eu (BAM), (Ce,Gd,Tb)MgB5O10 (CBT) and Y2O3:Eu (YOX) are used as blue-, green- and red-emitting phosphors, respectively In these materials, rare-earth ions are applied for absorption of the UV radiation and the generation of visible light

The phosphor price contributes considerably to the price of the lamp (tens of percent) Therefore, cost price reduction is a very important topic in research on phosphors to be applied in fluorescent lamps An obvious way to reduce the cost price of the phosphors is to optimize the preparation of the phosphors or to reduce the amount of phosphor applied in the lamp, e.g by reducing the grain size of the phosphors

In this thesis, synthesis processes and luminescent properties of tricolor phosphors are presented The phosphors we aim to prepare are BaMgAl10O17:Eu2+, (La,Gd)PO4:Tb3+ and Y2O3:Eu3+ corresponding to blue-, green- and red-emitting phosphors, respectively Sol – gel is chosen as the method to synthesize the phosphors

BATCH ITIMS – 2006

Title of Master Thesis:

“SYNTHESIS OF TRICOLOR PHOSPHORS USED IN

FLUORESCENT AND COMPACT FLUORESCENT LAMPS”

Author: Bui Van Hao

Supervisor: Assoc Prof Dr Pham Thanh Huy

by X-ray diffraction (XRD) technique Morphology and particle size of the phosphors are observed by scanning electron microscope (SEM) and transmission electron microscope (TEM) Photoluminescence measurements are performed using a continuous wave He–Cd laser (325 nm) as the excitation source

Tên luận văn:

Tác giả: Bùi Văn Hào

Người hướng dẫn: PGS TS Phạm Thành Huy

Y2O3:Eu3+ tương ứng với các màu phát quang là xanh lam, xanh lục và đỏ Các vật liệu ban đầu bao gồm các ôxít đất hiếm và các muối kim loại Axít nitric được dùng để hoà tan các ôxít thành các dung dịch muối Axít citric và axít tactaric được dùng làm tác nhân tạo sol Một số yếu tố ảnh hưởng đến cấu trúc và tính chất phát quang của các bột phát quang như nồng độ pha tạp, nhiệt độ thiêu kết được khảo sát Cấu trúc tinh thế của bột nhận được được khảo sát nhờ kỹ thuật nhiễu xạ tia X Hình thái và kích thước hạt được quan sát bằng công nghệ hiển vi điện tử quét (SEM) và hiển vi điện tử truyền qua (TEM) Các phép đo phổ huỳnh quang của bột phát quang được thực hiện với nguồn kích thích laser He-Cd ở bước sóng 325 nm

Chapter 1

AN OVERVIEW OF PHOSPHORS USED IN

LIGHTING DEVICES

1.1 Early phosphors for fluorescent lamps

A fluorescent lamp is filled with a noble gas at a pressure of 400 Pa, containing 0.8 Pa mercury Under the excitation of electrons accelerated by electric field inside the tube, mercury atoms are excited In the process of returning to the ground state, they emit radiations which are mainly in ultraviolet region About 85% of emitted radiation is at 254 nm and 12% at

185 nm The rest 3% is at longer wavelength ultraviolet and visible region (297, 302, 313, 334, 365, 405, 408, 436, 546, 576 and 579 nm) The lamp phosphors should convert 254 and 185 nm radiations into visible light The luminescent activators of the phosphors absorb these radiations and emit other wavelengths in visible region when they return to ground state Requirements for the practical use of phosphors in fluorescent lamps are high emission efficacy under excitation with 254 nm radiation and physical and chemical stability in the rare gas/mercury discharge space

In the early time of luminescent lighting (1938-1948), a mixture of two phosphors was used, namely MgWO4 and (ZnBe)2SiO4:Mn2+ While the former has a broad bluish-white emission band with a maximum at 480 nm (figure 1.1) and can be efficiently excited by short wavelength ultraviolet region, the latter gives an emission spectrum that covers the green to red part

of visible spectrum (figure 1.2)

Fig 1.1 Emission spectra of MgWO 4 [1.1]

From figure 2, we can see that the emission band of Zn2SiO4:Mn2+ (A) is narrow but that of (Zn,Be)2SiO4:Mn2+ (B) is broad In beryllium-free

Zn2SiO4:Mn2+, the emission transition in Mn2+ ion is 4T1→6A1 This emission

is relatively narrow because the transition occurs within 3d5 electron configuration

Fig 1.2 Emission spectra of Zn 2 SiO 4 :Mn 2+ (A) and (ZnBe) 2 SiO 4 :Mn 2+ (B) [1]

500 600 700

Wavelength (nm) Wavelength (nm)

When a part of Zn2+ ions is replaced by Be2+ ions, effect of the crystal field on Mn2+ will vary from ion to ion depending on what kind of the neighboring metal ions This is because of the large difference between ionic radii of Zn2+ and Be2+ (0.60 and 0.27 angstrom, respectively) For this reason, the emission band of (Zn,Be)2SiO4:Mn2+ is broader than that of beryllium-free

Zn2SiO4:Mn2+ The presence of beryllium ions increases the effect of crystal field on Mn2+ ions Therefore, the emission shifts to longer wavelength

A disadvantage of this Mn2+ phosphor is its unstableness in lamp It is easy to be decomposed under ultraviolet radiation In addition, beryllium is very toxic and later not acceptable for application In 1948, these phosphors were superseded by one phosphor with blue and orange emission (figure 1.3), viz Sb3+ and Mn2+ activated calcium halophosphate

Fig 1.3 Emission spectrum of Ca 5 (PO 4 ) 3 (Cl,F):Sb 3+ , Mn 2+

under the excitation of mercury radiation [2]

Table 1.1 Explanation of the origin of the individual peaks in figure 1.3

A serious drawback of halophosphate lamps is the fact that it is impossible to obtain simultaneously high brightness and color rendering index (CRI) If the brightness is high (efficacy ~ 80 lm/W), the CRI value is about

60 The CRI value can be reach at 90, but then the brightness decreases (~50 lm/W) [3]

Koedam and Opstelten predicted that a luminescent lamp with an efficacy of 100 lm/W and a CRI of 80–85 can be reached by combining three phosphors that have narrow emission band centered around 450, 550 and 610

nm A few years later, such a lamp was realized using rare earth activated phosphors This type of lamp is known as tricolor lamp

A typical "cool white" fluorescent lamp utilizing two rare earth doped phosphors, Tb3+, Ce3+:LaPO4 for green and blue emission and Eu3+:Y2O3 for red Figure 1.4 shows emission spectrum of this kind of phosphor under the excitation of mercury radiations

Fig 1.4 Emission spectrum of the phosphors using Tb 3+ , Ce 3+ :LaPO 4 for

green and blue emission and Eu 3+ :Y 2 O 3 for red emission [2]

1.2 Tricolor phosphors for luminescent lamps based on rare earth activators

The rare-earth elements consist of the 15 lanthanides from La (atomic number 57) to Lu (atomic number 71), Sc (atomic number 21), and Y (atomic number 39) The electronic configurations of trivalent rare-earth ions in the ground states are shown in Table 1.2 As shown in the table, Sc3+ is equivalent

to Ar, Y3+ to Kr, and La3+ to Xe in electronic configuration The lanthanides from Ce3+ to Lu3+ have one to fourteen 4f electrons added to their inner shell

configuration, which is equivalent to Xe Ions with no 4f electrons, i.e., Sc3+,

Y3+, La3+, and Lu3+, have no electronic energy levels that can induce excitation and luminescence processes in or near the visible region In contrast, the ions from Ce3+ to Yb3+, which have partially filled 4f orbitals, have energy levels characteristic of each ion and show a variety of luminescence properties around the visible region Many of these ions can be used as luminescent ions in phosphors, mostly by replacing Y3+, Gd3+, La3+, and Lu3+ in various compound crystals

Table 1.2 Electronic configurations of trivalent rare earth ions in the ground

state [4]

The 4f electronic energy levels of lanthanide ions are characteristic of each ion The levels are not affected much by the environment because 4f

electrons are shielded from external electric fields by the outer 5s2 and 5p6

electrons This feature is in strong contrast with transition metal ions, whose

3d electrons, located in an outer orbit, are heavily affected by the environmental or crystal electric field The characteristic energy levels of 4f

electrons of trivalent lanthanide ions have been precisely investigated by Dieke and co-workers The results are shown in figure 1.5, which is known as

a Dieke diagram

1.2.1 Blue emitting phosphors

The highest intensity of the blue emitting phosphors corresponds to the emission maximum at the wavelength of 450 nm At this wavelength, the color rendering index is of order of 90 Higher color rendering indices are is feasible when the blue emission is at 480 nm However, this occurs at the expense of lamp efficacy Since tricolor lamps aim the combination of both high emission intensity and good color rendition, only phosphors emitting in the region 440 and 460 are interested in practical applications [4]

The materials most used in fluorescent lamps nowadays that satisfy these requirements are Eu2+ - activated phosphors The commonly used hosts are barium based aluminates BaMgAl10O17, BaMg2Al16O27 and BaAl12O19 Other host materials used are CaAl2O4, M2MgSi2O7 Among these materials, BaMgAl10O17 is the most popular in use BaMgAl10O17:Eu2+ blue-emitting phosphors have been widely applied in many devices such as cathode ray tubes (CRT), plasma display panels (PDP) and field emission display (FED) The emission peak at 450 nm of the phosphors corresponds to the transition from the 4f65d excited state to the 4f7 ground state of Eu2+ (figure 1.6)

Fig 1.5 Dieke diagram [4]

The electronic configuration of Eu2+ is 4f7 Eu2+ usually gives broad-band

emission due to f-d transitions The wavelength positions of the emission

bands depend very much on hosts, changing from the near- UV to the red

This dependence is interpreted as due to the crystal field splitting of the 5d

level, as shown schematically in Figure 1.6 With increasing crystal field strength, the emission bands shift to longer wavelength The luminescence

peak energy of the 5d-4f transitions of Eu2+ are affected most by crystal parameters

Fig 1.6 Schematic diagram of the energies of 4f 7 and 4f 6 5d 1 levels in Eu 2+ influenced by crystal field ∆ and emission spectra of Eu 2+ in BaMgAl 10 O 17 and related compounds using 254 nm excitation at 300K [4]

- - -: Ba 0.95 Eu 0.05 MgAl 10 O 17 ; — - — - —: Ba 0.825 Eu 0.05 Mg 0.5 Al 10.5 O 17.125

— — —: Ba 0.75 Eu 0.05 Mg 0.2 Al 10.8 O 17.2 ———: Ba 0.70 Eu 0.05 Al 11 O 17.25

1.2.2 Green emitting phosphors

The green emitting ion in the lamp is Tb3+ Popularly used green phosphors are LaPO4:Ce3+,Tb3+, MgAl11O19:Ce3+,Tb3+, (La,Gd)PO4:Tb3+ In these materials, cerium and gadolinium are used as sensitizers The characteristic luminescence is due to 5D4→7FJ transitions of Tb3+

Fig 1.7 Emission spectra of green emitting phosphors using Ce 3+ ((a) and

(c)) and Gd 3+ as sensitizers [1]

1.2.3 Red emitting phosphors

Y2O3:Eu3+ is the most popular red emitting phosphor used in fluorescent lamps because of its high quantum efficiency, and especially, it can be effectively excited by 254 nm radiation, the strongest emission of mercury vapor The emission peak of this phosphor is located at about 612 nm corresponding to the transition between 5D0 and 7F2 energy levels of Eu3+ ion

Fig 1.8 Excitation and emission spectra of the phosphor [6]

1.3 Phosphor synthesis methods

1.3.1 Solid state reaction method

Almost all phosphors are synthesized by solid-state reactions between raw materials at high temperatures Figure 1.9 shows the general concept of the synthesis process First, the high-purity materials of the host crystal, activators, and fluxes are blended, mixed, and then fired in a container As the product obtained by firing is more or less sintered, it is crushed, milled, and then sorted to remove coarse and excessively crushed particles In some cases, the product undergoes surface treatments

1.3.2 Combustion synthesis

Beginning in the late 1980’s, combustion synthesis has been investigated

as a method to produce homogeneous, crystalline, fine oxide powders The method produces rapid, exothermic, self-sustaining reactions resulting from the appropriate combination of oxidizers (e.g., metal nitrates, ammonium nitrate, or ammonium perchlorate) and an organic fuel (e.g., urea, carbohydrazide, or glycine) For combustion to occur, it is necessary that a large amount of heat be released during the formation of the products

1.3.3 Spray pyrolysis

Spray pyrolysis is an aerosol process commonly used to form a wide variety of materials in powder form including metals, metal oxides, ceramics, superconductors, fullerenes and nanostructured materials from the gas flow

SP offers specific advantages over conventional material processing techniques (gas-to-particle conversion processes, liquid or solid-state processing followed by milling), such as a higher purity of the produced

powders, a better uniformity in chemical composition, a narrower size distribution, a better regularity in shape and the synthesis of multicomponent materials Another advantage is the relative simplicity of the process which allows easy scale-up However, challenges still exist for SP, e.g to increase production rates, to better understand the influence of the operating conditions

to control particle size, shape and internal morphology (filled or hollow particles)

Fig 1.9 Phosphor synthesis process by solid state reaction method [4]

The sizes of the processed particles are most often micronic and micronic (50 nm 5 µm) This technology has been used for many years in the material, chemical and food industries It consists of five main steps: (i) generation of a spray from a liquid precursor by an appropriate droplet

Classification (Sedimentation, elutriation, sieving)

Coarse crushing (Crusher, ball mill)

Final product Sieving

(Surface

treatment)

Washing

Synthesis (firing)

generator, (ii) spray transport by an air flow during which solvent evaporation occurs then concomitant solute precipitation when the solubility limit is exceeded inside droplets, (iii) thermolysis of the precipitated particles at higher temperatures to form micro/nano-porous particles, (iv) intra-particulate sintering to form dense particles and (v) finally, extraction of the particles

1.3.4 Sol-gel method

Phosphors used for most emissive display devices are in the form of powders The quality of the displays depends on the nature of the powders used; fine and uniform powders with good crystallinity are generally preferred This is especially true in low-voltage applications In these devices, lower energy electrons do not penetrate into phosphor grains very deeply, and

in order to maintain efficiency the size of phosphor grains has to be reduced

to reflect this fact The grain size of phosphors prepared via solid-state chemical reactions depends on the temperature and the length of the sintering process Lower temperature and shorter sintering periods give rise to smaller grain size particles, but both the crystallinity and grain uniformity are poor if the treatment parameters are such as not to allow the chemical reaction to be completed

To resolve this problem, wet methods of preparation are often used The sol–gel method of phosphor preparation is regarded as a wet method A kind

of metalorganic compound, known as alkoxides of metals, is used as precursors These metalorganic alkoxides either are in liquid form or are soluble in certain organic solvents Through the use of the appropriate reagents, the processes hydrolysis and condensation can be induced to produce homogeneous gels from the mixture of alkoxides To obtain powder

or ceramic samples, the gels can be baked, sintered, and powderized as in other traditional methods

In this work, we have used sol – gel method to synthesize the phosphors because of its advantages:

(a) High homogeneity of the chemical composition of the materials produced occurs Molecule-level-homogeneous multi-component materials can be obtained Because of the better homogeneity, contributions to the optical spectra of these materials from inhomogeneous sources are generally expected to be smaller than those encountered in unordered systems

(b) High uniformity of doping ions distribution exists No local concentration quenching will occur because of impurity clustering High doping concentration becomes possible

(c) Processing temperature can be very low This allows the doping of fragile organic and biological molecules into porous inorganic materials and the fabrication of organic–inorganic hybrid materials

(d) The microstructure (porosity and size of pores) of the materials can

be controlled Nanoscaled uniform pores can be obtained at intermediate processing temperature while high density materials can be produced with higher annealing temperature

(e) The sol–gel procedures produce little unintentional contamination For example, no milling and grinding, processes known to contaminate samples, are needed In cases where phosphor powders are prepared by the sol–gel method, powderizing may be used and trace of foreign particles can

be mix in This "contamination" does not enter into the lattice and will not affect the intrinsic optical properties of the phosphor

The technique has the following disadvantages:

(a) The drying and annealing processes have to be slow and deliberate; otherwise cracks and striations will appear in the samples

(b) It is difficult to completely remove the residual hydroxyls from the sol–gel materials To get rid of these organic groups, samples have to be annealed above 1000°C and this may produce undesirable side effects

Chapter 2

EXPERIMENTAL

In this research, based on our ready instruments, we have chosen sol-gel method to synthesize tricolor phosphors Details about initial materials and experimental procedures are described as following:

2.1 Preparation of blue-emitting phosphor BaMgAl 10 O 17 :Eu 2+ by sol – gel method

2.1.1 Starting materials

• Aluminium nitrate Al(NO3)3.9H2O 98.5%, AR

• Magnesium oxide MgO 98%, AR

• Barium nitrate Ba(NO3)2 99%, Merck

Fig 2.1 Synthesis process of BaMgAl 10 O 17 :Eu 3+ via sol-gel technique

The solutions were then mixed together and constantly stirred to obtain a transparent solution In the next step, citric acid solution was introduced and stirring was still kept on In the process of sol formation, the temperature of the solution was increased up to 80 oC Further heating at 100 oC led to the formation of water Evaporation of water made the sol turned into transparent

Al(NO3)3.9H2O, Ba(NO3)2 Eu2O3, MgO + HNO3

Transparent solution

Stirring

Transparent solution

Stirring Citric acid

gel and an extremely viscous resin was formed At this time, further heating at

120 oC caused the combustion of the gel and a yellow white powder was formed, called dry gel The dry gel was sintered at different temperatures and finally calcinated in reducing atmosphere (5% H2 in N2) to obtain the blue-emitting phosphors

2.2 Preparation of green emitting phosphors (LaGd)PO 4 :Tb 3+ by sol – gel method

Stoichiometric amounts of rare earth oxides La2O3, Gd2O3 and Tb4O7

were dissolved in diluted nitric acid and then were mixed with a water-ethanol solution containing citric acid as chelating agent (NH4)2HPO4 was added into the former clear solution The mixture was then heated at 70 oC for 2 hours to obtain a transparent sol Further heating at 100 oC led to the formation of transparent gel and when increased temperature up to 120 oC, the gel turned in

after sintering the dry gel at different temperatures The synthesis process can

be summarized as in figure 2.2

Fig 2.2 Synthesis process of (LaGd)PO 4 :Tb green-emitting phosphors

2.3 Preparation of red emitting phosphors Y 2 O 3 :Eu 3+ by sol – gel method

Sol formation

Heating at 70 oC for 2 hours

Fig 2.3 Synthesis process of Y 2 O 3 :Eu 3+ red-emitting phosphors

We have used citric acid and tartaric acid as chelating agents to fabricate red emitting phosphors Citric acid consists of one – OH group and three – COOH groups while tartaric acid contains two – OH groups and two – COOH groups This difference plays an important role in sol – gel method which hydrolysis and condensation reaction rate are the key factors of sol quality

[7], [8] To synthesize microphosphors, pH has been kept at lowest (~ 0.5 – 1) At low pH, condensation and agglomeration of particles occur simultaneously, so that the obtained phosphors have grain size in microscale

In contrast, nanophosphors have been synthesized at higher pH At high pH, condensation reaction dominates and agglomeration is negligible [7], [8], [9] Experimentally, we have seen that nanoparticles can be obtained by using tartaric acid as chelating agent and using citric acid is easier to get microphosphors This may be attributed to the difference in the number of (– OH) and (– COOH) groups contained in these acids

2.4 Material characterization and analysis

The IR spectra of the gels powders were measured by using Nicolet 6700 FT-IR Spectrometer Crystallite structure of the phosphors was studied with Brucker D8-Advance XRD meter using Cu Kα radiation The morphology and particle size of the phosphors were observed by using ESEM FEI, Hitachi S-

4800 Field Emission Scanning Electron Microscope (FESEM) and JEOL JEM-1010 Transmission Electron Microscope (TEM) Photoluminescence measurements were performed using a continuous wave He–Cd laser (325 nm) as the excitation source

2.4.1 Infrared spectroscopy

Infrared spectroscopy (IR spectroscopy) is the subset of spectroscopy that deals with the infrared region of the electromagnetic spectrum It covers a range of techniques, the most common being a form of absorption spectroscopy As with all spectroscopic techniques, it can be used to identify compounds or investigate sample composition

The infrared portion of the electromagnetic spectrum is divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum The far-infrared, approximately 400-10 cm-1 (1000–30 μm), lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy The mid-infrared, approximately 4000-400 cm-1 (30–1.4 μm) may be used to study the fundamental vibrations and associated rotational-vibrational structure The higher energy near-IR, approximately 14000-4000 cm-1 (1.4–0.8 μm) can excite overtone or harmonic vibrations The names and classifications of these subregions are merely conventions They are neither strict divisions nor based on exact molecular or electromagnetic properties

Infrared spectroscopy exploits the fact that molecules have specific frequencies at which they rotate or vibrate corresponding to discrete energy levels These resonant frequencies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms and, by the associated vibronic coupling In order for a vibrational mode in a molecule to

be IR active, it must be associated with changes in the permanent dipole In particular, in the Born-Oppenheimer and harmonic approximations, i.e when the molecular Hamiltonian corresponding to the electronic ground state can be approximated by a harmonic oscillator in the neighborhood of the equilibrium molecular geometry, the resonant frequencies are determined by the normal modes corresponding to the molecular electronic ground state potential energy surface Nevertheless, the resonant frequencies can be in a first approach related to the strength of the bond, and the mass of the atoms at either end of it Thus, the frequency of the vibrations can be associated with a particular bond type

Simple diatomic molecules have only one bond, which may stretch More complex molecules have many bonds, and vibrations can be conjugated, leading to infrared absorptions at characteristic frequencies that may be related to chemical groups For example, the atoms in a CH2 group, commonly found in organic compounds can vibrate in six different ways: symmetrical and antisymmetrical stretching, scissoring, rocking, wagging and twisting:

Fig 2.4 Vibrating modes of molecular [10]

The infrared spectrum of a sample is collected by passing a beam of infrared light through the sample Examination of the transmitted light reveals how much energy was absorbed at each wavelength This can be done with a monochromatic beam, which changes in wavelength over time, or by using a Fourier transform instrument to measure all wavelengths at once From this, a transmittance or absorbance spectrum can be produced, showing at which IR

Symmetrical

stretching

Antisymmetrical stretching

Scissoring

wavelengths the sample absorbs Analysis of these absorption characteristics reveals details about the molecular structure of the sample

Fig 2.5 Nicolet 6700 FT-IR spectrometer

2.4.2 X-ray diffraction techniques

X-ray diffraction (XRD) is a versatile, non-destructive technique that reveals detailed information about the chemical composition and crystallographic structure of natural and manufactured materials

The three-dimensional structure of materials is defined by regular, repeating planes of atoms that form a crystal lattice When a focused X-ray beam interacts with these planes of atoms, part of the beam is transmitted, part is absorbed by the sample, part is refracted and scattered, and part is diffracted Diffraction of an X-ray beam by a crystalline solid is analogous to

diffraction of light by droplets of water, producing the familiar rainbow rays are diffracted by each mineral differently, depending on what atoms make up the crystal lattice and how these atoms are arranged

X-Fig 2.6 A schematic picture of X-Ray diffraction from atomic planes

In X-ray powder diffractometry, X-rays are generated within a sealed tube that is under vacuum A current is applied that heats a filament within the tube, the higher the current the greater the number of electrons emitted from the filament This generation of electrons is analogous to the production of electrons in a television picture tube A high voltage, typically 15-60 kilovolts, is applied within the tube This high voltage accelerates the electrons, which then hit a target, commonly made of copper When these electrons hit the target, X-rays are produced The wavelength of these X-rays

is characteristic of that target These X-rays are collimated and directed onto

the sample, which has been ground to a fine powder A detector detects the ray signal; the signal is then processed either by a microprocessor or electronically, converting the signal to a count rate Changing the angle between the X-ray source, the sample, and the detector at a controlled rate between preset limits is an X-ray scan

X-When an X-ray beam hits a sample and is diffracted, we can measure the distances between the planes of the atoms constitute the sample by applying Bragg's Law:

θ

where the integer n is the order of the diffracted beam, λ is the wavelength of

the incident X-ray beam, d is the distance between adjacent planes of atoms

(the d-spacings), and q is the angle of incidence of the X-ray beam Since we

know λ and we can measure θ, we can calculate the d-spacing The geometry

of an XRD unit is designed to accommodate this measurement The

characteristic set of d-spacing generated in a typical X-ray scan provides a

unique "fingerprint" of the materials present in the sample When properly interpreted, by comparison with standard reference patterns and measurements, this "fingerprint" allows for identification of the material

Fig 2.7 Phillips XRD X-pert IMS

2.4.3 Scanning electron microscopy (SEM)

Fig 2.8 ESEM FEI