A NOVEL CHEMICAL APPROACH TO FABRICATE

A NOVEL CHEMICAL APPROACH TO FABRICATE ZnO NANOSTRUCTURES Thesis submitted in partial fulfillment of the requirements for the award of the degree of MASTER OF TECHNOLOGY by

SAROJ KUMAR PATRA (06PH6213) Under the supervision of

Dr. P. ROYCHAUDHURI

DEPARTMENT OF PHYSICS & METEOROLOGY INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR

Department of Physics and Meteorology Indian Institute of Technology Kharagpur Kharagpur –721302, India

CERTIFICATE This is to certify that the thesis entitled “A NOVEL CHEMICAL APPROACH TO FABRICATE

ZnO

NANOSTRUCTURES” is

submitted in partial fulfillment of the requirements for the award of degree of Master of Technology in Solid State Technology at the Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur. It is a faithful record of bona-fide research work carried by Mr. Saroj Kumar Patra (Roll No. 06PH6213) under my supervision and guidance. It is further certified that no part of thesis has been submitted to any other University or Institute for the award of any other Degree or Diploma to the best of my knowledge..

Dr. Partha Roy Chaudhuri Department of Physics & Meteorology IIT Kharagpur

Acknowledgement

With deep sense of gratitude, I wish to express my sincere thanks to my guides Dr. Partha Roy Chaudhuri of The Department of Physics and Meteorology, Indian Institute Of Technology, Kharagpur, for his excellent guidance and support throughout the work. I received very useful and excellent academic training from him, which has stood in good stead while writing this thesis. His unique inimitable style has left an indelible impression on me. His constant encouragement, help and review of the entire work during the course of the investigation are invaluable. Nevertheless, it helped me acquire and develop some of the skills and intricacies of good independent research

I must express my deep appreciation and gratitude to Mr. Avijit Ghosh and Mr. Pijus Kanti Samanta of Physics department for their constant and selfless support at every stage of my project work. Without their support and contributions, this project would have been difficult to complete.

My sincere thanks to all the faculty members, research scholars and friends for their encouragement and kind help extended toward me.

Last but not the least I am very thankful to my batch mates and all my hall mates for their immense cooperation without whom my stay here would not have been memorable.

Finally, I would like to share this moment of happiness with my family members for whom I do what I do.

Saroj Kumar Patra

Abstract

A wet-chemical route was successfully employed to synthesize ZnO nanorods and Flower-like bundle of Zinc Oxide (ZnO) nanosheets. The synthesized nanostructure was structurally characterized using X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM). The results shows a preferential growth by the selective capping of the different nonpolar crystal planes in the ZnO nanocrystals while using hexamine (chelating agent for this growth) there by inducing a preferred shape of the crystals. The optical property was studied using photoluminescence and UV-visible spectroscopy, FTIR spectroscopy which shows a strong visible emission due to deep level defect states’ emission. Thus this fabricated material will be very useful in visible light emitting and nanophotonic devices.

CONTENTS Certificate Acknowledgement Abstract

CHAPTER – 1 : INTRODUCTION ……………………………….……01 1.1 Crystal Structure ………………………………………………………………..…04 1.2 Lattice Parameters ………………………………………………………………....06 1.3 Electronic band Structure ……………………………………………………...….08

CHAPTER 2 : REVIEW OF CONTEMPORARY WORKS ……..….10 CHAPTER 3 : DIFERENT METHODS FOR THE GROWTH OF ZnO NANOSTRUCTURES …………………………………...…….15 3.1 Bulk Growth ………………………………………………………………….…….15 3.1.1 Hydrothermal method …………………………………………….…….15 3.1.2 Vapour transport ……………………………………………………...…16 3.1.3 Melt growth …………………………………………………………..…..17 3.2 Substrate growth ……………………………………………………………..…….17 3.3 Chemical Vapour Deposition ………………………………………………….…..18 3.4 Sputtering …………………………………………………………………….…….19 3.5 Chemical Route …………………………………………………………………….19

CHAPTER 4 : CHARACTERISATIONTECHNIQUES ………..…….20 4.1 X-Ray Diffraction ……………………………………………………………..……21 4.2 Field Emission Scanning Electron Microscopy ……………………….………….22 4.3 Transmission Electron Microscopy ……………………………………………….25 4.4 Photoluminescence Spectroscopy ………………………………………...……….27

4.5 Raman Spectroscopy …………………………………………………………..…..31 4.6 FTIR Spectroscopy ……………………………………………………………..….33 4.7 UV/VIS Spectroscopy …………………………………………………………..….35

CHAPTER 5 : SYNTHESIS OF ZnO NANOSTRUCTURES BY CHEMICAL ROUTE ……………………………………………..…37 5. FABRICATION ……………………………………………………………….…….37 5.1 Synthesis of ZnO nanorods by chemical route ………………………………...…37 5.2 Reactions mechanism …………………………………………………………...….37 5.3 Synthesis of ZnO nanorods using Hexamine ……………………………………..38

CHAPTER 6 : RESULTS & DISCUSSION ……………………………41 6.1. ZnO NANORODS …………………………………………………………...…….42 6.1.1. Structural Characterization (X-Ray Diffraction) ……………………..42 6.1.2. Surface Morphology (FESEM) ……………………………………...….43 6.1.3. Photoluminescence ………………………………………………………46 6.1.4. Raman Spectra ………………………………………………………..…51 6.1.5. UV-Visible Spectroscopy ………………………………………………..53 6.1.6. FTIR Spectroscopy ………………………………………………..…….55 6.1.7. Conclusions ………………………………………………………......…..55 6.2. FLOWER-LIKE BUNDLE OF ZnO NANOSHEETS (EFFECT OF HEXAMINE ON THE ZnO NANOSTRUCTURE) ……………….….56 6.2.1. X-Ray Diffraction …………………………………………………….….56 6.2.2. Field Emission Scanning Electron Microscopy ………………………..59 6.2.3. Photoluminescence Spectroscopy ………………………………………62 6.2.4. UV-visible spectroscopy ……………………………………………..…..64 6.2.5. Conclusions ………………………………………………………...…….66

SCOPE FOR FUTURE WORK …………………………………………67 REFERENCE …………………………………………………………….68

Chapter 1

INTRODUCTION

CHAPTER – 1 INTRODUCTION 1.1 Crystal Structure 1.2 Lattice Parameters 1.3 Electronic band Structure

1. INTRODUCTION The word “nano”, just a fraction that indicates one billionth of a unit quantity until recently; however, the same is redefining the understanding of matter at an extraordinary pace every day. Noble prize winning inventions of bucky balls and carbon fullerene structures, first electron microscope image of the carbon microtubules, later called as carbon nanotubes (CNTs), followed by the invention of inorganic fullerenes and anisotropic nanostructures can be termed as major breakthroughs in the field of nanoscience and technology. Synthesis of size and shape controlled nanostructures (triangles, cubes, tubes, wires, rods, fibers, etc.), their self-assembly, properties and possible applications are under rigorous research. Realizing the importance of nanotechnology, state of the art technology centers with excellent processing, characterization and device fabrication facilities are being developed. The significantly different physical properties of these nanostructured materials have been ascribed to their characteristic structural features in between the isolated atoms and the bulk macroscopic materials. ‘‘Quantum confinement’’, the most popular term in the nano-world, is essentially due to the changes in the atomic structure as a result of the direct influence of the ultra-small length scale on the energy band 1

Chapter 1

INTRODUCTION

structure. The exceptional electronic, mechanical, optical and magnetic properties of the nanoscale materials can all be attributed to the changes in the total energy and structure of the system. In the free electron model the energies of the electronic states and the spacing between energy levels, both vary as a function of 1/L2, with L being the dimension in that direction. At nanoscale dimensions the normally collective electronic properties of the solid become severely distorted and the electrons at this length scale tend to follow the ‘‘particle in a box’’ model, might often require higher order calculations to account for band structure. The electronic states are more like those found in the localized molecular bonds than the macroscopic solids. The main implication of such confinement is the change in the system total energy; and hence the overall thermodynamic stability. The chemical reactivity, being a function of the system structure and the occupation of the outermost energy levels, will be significantly affected at such a length scale, causing a corresponding change in the physical properties [1] Lattice and vibrational parameters as well as optical properties and processes in ZnO were extensively studied many decades ago. Growth methods such as chemical vapor transport, vapor phase growth, hydrothermal growth which also had the additional motivation of doping with Li in an effort to obtain p-type material, high quality platelets and so on have been investigated. ZnO bulk crystals have been grown by a number of methods, as has been reviewed recently and large-size ZnO substrates are available. High quality ZnO films can be grown at temperatures below 700 °C. There have also been a number of reports on laser emission from ZnO-based structures at room temperature (RT) and beyond. Several experiments confirmed that ZnO is very resistive to high energy radiation making it a very suitable candidate for space applications. ZnO

2

Chapter 1

INTRODUCTION

is easily etched in all acids and alkalis, and this provides an opportunity for fabrication of devices with considerable ease. The present renaissance is based on the possibility to grow epitaxial layers, quantum wells, nanorods and related objects or quantum dots and on the hope towards: ¾ a material for blue/UV optoelectronics, including light emitting or even laser diodes in addition to (or instead of) the GaN-based structures, ¾ a radiation-hard material for electronic devices in a corresponding environment, ¾ a material for electronic circuits that is transparent in the visible and/or usable at elevated temperature ¾ a diluted- or ferro- magnetic material, when doped with Co, Mn, Fe, V, etc., for semiconductor spintronics, ¾ a transparent highly conducting oxide when doped with Al, Ga, In, etc., as a cheaper alternative to ITO. For several of the above-mentioned applications a stable, high, and reproducible p-doping is obligatory. Though progress has been made in this crucial field this aspect still forms a major problem. The emphasis of the present very active period of ZnO research is essentially on the same topics as before, but including nanostructures, new growth and doping techniques and focusing more on application- related aspects. In the following we will consider first structure, growth techniques, characterization and some of the fundamental properties of ZnO nanostructures of reduced dimensionality with emphasis on reviewing of the structural and optic properties. The future plan is to get exciting nanostructures modifying different experimental parameters and study of their properties for noble application in the field of Photonics 3

Chapter 1

INTRODUCTION

1.1 CRYSTAL STRUCTURE Most of the group-II-VI binary compound semiconductors crystallize in either cubic zinc-blende or hexagonal wurtzite structure where each anion is surrounded by four cations at the corners of a tetrahedron, and vice versa. This tetrahedral coordination is typical of sp3 covalent bonding, but these materials also have a substantial ionic character. ZnO is a II-VI compound semiconductor whose ionicity resides at the borderline between covalent and ionic semiconductor. The crystal structures shared by ZnO are wurtzite B4, zinc blende B3, and rock salt B1, as schematically shown in Fig. 1.1. At ambient conditions, the thermodynamically stable phase is wurtzite. The zincblende ZnO structure can be stabilized only by growth on cubic substrates, and the rock salt NaCl structure may be obtained at relatively high pressures.

Fig 1.1. Stick and ball representation of ZnO crystal structures: (a) cubic rock salt (B1), (b) cubic zinc blende (B3), and (c) hexagonal wurtzite (B4). Shaded gray and black spheres denote Zn and O atoms, respectively.

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Chapter 1

INTRODUCTION Zinc oxide hexagonal wurtzite-type structure has a polar hexagonal axis,

the c-axis, chosen to be parallel to z. The primitive translation vectors a and b lay in the x–y plane, are of equal length, and include an angle of 120°, while c is parallel to the zaxis. The point group is in the various notations 6mm or C6v, the space group P63mc or C46v. One zinc ion is surrounded tetrahedral by four oxygen ions and vice versa. The primitive unit cell contains two formula units of ZnO. The values of the primitive translation vectors are at room temperature a = b ≈ 0.3249 nm and c ≈ 0.5206 nm. The ratio c/a of the elementary translation vectors deviates with values around 1.602 slightly from the ideal value c/a = 8/3 = 1.633. In contrast to other II–VI semiconductors, which exist both in the cubic zincblende and the hexagonal wurtzite-type structures (like ZnS, which gave the name to both structures) ZnO crystallizes with great preference in the wurtzite-type structure. The two latter structures both form a face-centered cubic lattice (FCC), however with different arrangements of the atoms within the unit cell, i.e. different bases. The ground-state total energy of ZnO in wurtzite, zincblende, and rock salt structures has been calculated as a function of unit-cell volume using a first-principles periodic Hartree-Fock (HF) linear combination of atomic orbitals (LCAO) theory by Jaffee and Hess. [2] The total-energy data versus volume for the three phases are shown in Fig. 1.2 along with the fits to the empirical functional form of the third order Murnaghan equation, which is used to calculate the derived structural properties,

5

Chapter 1

INTRODUCTION

FIG. 1.2. Total energy vs volume (both per ZnO f.u.) for the three phases: zinc blende (squares), wurtzite (diamonds), and rock salt (circles). The zero of energy is the sum of the total energy of an isolated Zn and an isolated O atom.

Where E0, V0, and B0 are the total energy, volume per ZnO and bulk modulus at zero pressure P, respectively, and B = dB/dP is assumed to be constant. 1.2 LATTICE PARAMETERS: The lattice parameters of a semiconductor usually depend on the following factors: ¾ Free electron concentration acting via deformation potential of a conduction band minimum occupied by these electrons ¾ Concentration of foreign atoms and defects and their difference of ionic radii with respect to the substituted matrix ion ¾ External strains ( for example, those induced by substrate) ¾ Temperature

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Chapter 1

INTRODUCTION The lattice parameters of any crystalline material are commonly and most

accurately measured by high resolution x-ray diffraction (HRXRD) by using the Bond method [3] for a set of symmetrical and asymmetrical reflections. For the wurtzite ZnO, lattice constants at room temperature determined by various experimental measurements and theoretical calculations are in good agreement. The lattice constants mostly range from 3.2475 to 3.2501 Å for the a parameter and from 5.2042 to 5.2075 Å for the c parameter. The c/a ratio vary in a slightly wider range, from 1.593 to 1.6035 .The deviation from that of the ideal wurtzite crystal is probably due to lattice stability and ionicity. It has been reported that free charge is the dominant factor responsible for expanding the lattice proportional to the deformation potential of the conduction-band minimum and inversely proportional to the carrier density and bulk modulus. The point defects such as zinc antisites, oxygen vacancies, and extended defects, such as threading dislocations, also increase the lattice constant. For the zinc-blende polytype of ZnO, the calculated lattice constants based on a modern ab initio technique are predicted to be 4.60 and 4.619 Å. Ashrafi et al. [4] characterized the zinc-blende phase of ZnO films grown by plasma assisted metalorganic molecular beam epitaxy using reflection high-energy electron-diffraction (RHEED), x-ray diffraction (XRD), transmission electron microscope (TEM), and atomic-force microscope (AFM) measurements. A high-pressure phase transition from the wurtzite to the rocksalt structure decreases the lattice constant down to the range of 4.271–4.294 Å. The experimental values obtained by x-ray diffraction are in close agreement. The predicted lattice parameters of 4.058–4.316 Å using various calculation techniques, such as the HF, are about 5% smaller or larger than the experimental values. 7

Chapter 1

INTRODUCTION

1.3 ELECTRONIC BAND STRUCTURE The band structure of a given semiconductor is pivotal in determining its potential utility. Consequently, an accurate knowledge of the band structure is critical if the semiconductor in question is to be incorporated in the family of materials considered for device applications. Several theoretical approaches of varying degrees of complexity have been employed to calculate the band structure of ZnO for its wurzite, zinc-blende, and rocksalt polytypes. Besides, a number of experimental data have been published regarding the band structure of the electronic states of wurtzite ZnO. X-ray- or UV reflection/absorption or emission techniques have conventionally been used to measure the electronic core levels in solids. These methods basically measure the energy difference by inducing transitions between electronic levels (for example, transitions from the upper valence-band states to the upper conduction-band states, and from the lower valence-band states) or by exciting collective modes (for example, the upper core states to the lower edge of the conduction band and to excitations of plasmons). Another important method for the investigation of the energy region is based on the photoelectric effect extended to the x-ray region, namely, photoelectron spectroscopy (PES). The peaks in emission spectrum correspond to electron emission from a core level without inelastic scattering, which is usually accompanied by a far-less-intense tail region in the spectrum. More recently, angle-resolved photoelectron spectroscopy (ARPES) technique has started to be used. This technique together with synchrotron radiation excitation has been recognized as a powerful tool that enables experimental bulk and surface electronic bandstructure determinations under the assumptions of k conservation and single nearly-freeelectron-like final band. 8

Chapter 1

INTRODUCTION Since ZnO is a direct gap semiconductor with the global extrema of the

uppermost valence and the lowest conduction bands (VB and CB, respectively) at the same point in the Brillouin zone, namely at k = 0, i.e. at the Г-point, we are mainly interested in this region. The lowest CB is formed from the empty 4s states of Zn2+ or the anti-bonding sp3 hybrid states. A typical representation of the band structure looks as follows [5]:

Fig 1.3 Band Diagram

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Chapter 2

REVIEW OF CONTEMPORARY WORKS

CHAPTER 2

REVIEW OF CONTEMPORARY WORKS The research on ZnO started around in the 1930s due to its prospects in optoelectronics applications owing to its direct wide band gap Eg 3.3 eV at 300 K.[8] Some optoelectronic applications of ZnO overlap with that of GaN, another wide-gap semiconductor Eg 3.4 eV at 300 K which is widely used for production of green, blueultraviolet, and white light-emitting devices [9].. But the research peaked around the end of the 1970s and the beginning of the 1980s. The field is fuelled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics application Then the interest faded away, partly because it was not possible to dope ZnO both n- and p-type, which is an indispensable prerequisite for applications of ZnO in optoelectronics, partly because the interest moved to structures of reduced dimensionality, like quantum wells, which were at that time almost exclusively based on the III–V system GaAs/Al1–yGayAs. The emphasis of ZnO research at that time was essentially on bulk samples covering topics like growth, doping, transport, deep centers, band structure, excitons, bulk- and surface-polaritons, luminescence, high excitation or many-particle effects and lasing. Zinc oxide nanostructures have been fabricated and proven versatility and compatibility in numerous applications. ZnO nanostructures were synthesized in the form of nanorods, nanowires, nanotubes, nanobelts, nanocombs, nanosprings, nanorings, nanobows and nanopropellers, etc. The wide interest in ZnO has resulted from the

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Chapter 2


following fundamental characteristic features with potential applications in electronic, structural and bio-materials: ¾ Direct band gap semiconductor (3.37 eV) ¾ Large exciton binding energy (60 meV) ¾ Near UV emission and transparent conductivity ¾ Piezoelectric property resulting from its non-centrosymmetric structure ¾ Bio-safe and bio-compatible Though research on ZnO has started many decades ago but in the current technological arena, many academic and government laboratories, companies and NGO”s across the globe are deeply engaged to exploit the various nano form of ZnO and its application to the emerging technology. A few of them are:Hui Zhang‘s [10] research group at State Key Laboratory of Silicon Materials, Zhejiang University, People’s Republic of China has interests lie at the ZnO nanowires fabricated by a simple chemical sol–gel process. The diameters of the ZnO nanowires were very uniform, at about 60 nm. Sang Sub Kim’s [11] research group at Chonbuk National University, South Korea has interests lie at the synthesis of aligned ZnO nanocolumns synthesized by catalyst-free metal organic chemical vapor deposition (MOCVD) on various substrates including Al2O3 (0001).In particular, ZnO nanocolumns grown on GaN buffered Al2O3 (0001) has shown an excellent alignment in both the vertical and the horizontal direction. In spite of different sizes and alignments of ZnO nanocolumns depending on the substrates or other processing parameters employed, individual nanocolumns are of defect free single-crystalline nature and of high optical quality. Field effect transistors

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were fabricated using individual ZnO nanocolumns to characterize their electrical properties as well as to test a potential of their use in nanoscale electronic devices. A Chrissanthopoulos’s [12] research group at University of Patras, Greece is working on growth of ZnO nanostructures on carbon nanotubes by thermal evaporation. It was concluded from their experiment that the main building block of the observed morphologies were the nanorod whose self-assembling resulted in various structures such as polypods and nano hedgehogs, depending on various factors as well as the location of the ZnO–CNT junction. Minlin Zhang’s [13] research group at Department of Material Science and Chemical Engineering, Harbin Engineering University, China is involved in preparation of ZnO nanorods through wet chemical method. They found the structure of the grown ZnO were wurtzite structure which have blue emission at 466 nm and green– yellow emission at 542 nm. The formation of nano structures at different temperatures was shown schematically as in the Fig 2.1

Fig.2.1. Schematic illustration of the formation of ZnO nanorods at different temperatures in the presence of PEG.

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Chapter 2

REVIEW OF CONTEMPORARY WORKS Hyung-Shik Shin’s [14] research group at Chonbuk National University,

Republic of Korea is studying Low temperature solution synthesis and characterization flower-shaped ZnO nanostructures composed of hexagonal ZnO nanorods The IR spectrum showed the standard peak of zinc oxide at 523 cm-1. Raman scattering exhibited a sharp and strong E2 mode at 437 cm-1 which further confirmed the good crystallinity and wurtzite hexagonal phase of the grown nanostructures. The photoelectron spectroscopic measurement showed the presence of Zn, O, C, zinc acetate and Na. The binding energy ca. 1021.2 eV (Zn 2p3/2) and 1044.3 eV (Zn 2p1/2), are found very close to the standard bulk ZnO binding energy values. The O 1s peak is found centered at 531.4 eV with a shoulder at 529.8 eV. Room-temperature photoluminescence (PL) demonstrated a strong and dominated peak at 381 nm with a suppressed and broad green emission at 515 nm. X W Sun’s [15] research group at Nanyang Technological University, Singapore is interested in growth of tubular ZnO in aqueous solution based hydrothermal decomposition. The also proposed possible mechanism of tubular ZnO formation. Their results showed that ZnO microtubes originated from an ageing process from ZnO microrods at a lower temperature (compared to the temperature when hydrothermal deposition of ZnO microrods was dominant) due to the preferential chemical dissolution of the metastable Zn-rich (0001) polar surfaces. Xueliang Qiao’s [16] research group at Huazhong University of Science and Technology, P.R. China is involved in studying the chemical route to prepare ZnO nanoparticles. Their results revealed that with the increase of the reaction temperature, the morphology of particles seems to change from rod-like to short prism-like form. The

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Chapter 2


photoluminescence band was at near 380 nm and no other bands were observable, which proved to exhibit high optical property and might have potential application in nanoscale optoelectronic devices.

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Chapter 3

DIFERENT METHODS FOR THE GROWTH

CHAPTER 3 DIFERENT METHODS FOR THE GROWTH OF ZnO NANOSTRUCTURES 3.1 Bulk Growth 3.1.1 Hydrothermal method 3.1.2 Vapour transport 3.1.3 Melt growth 3.2 Substrate growth 3.3 Chemical Vapour Deposition 3.4 Sputtering 3.5 Chemical Route

3. DIFERENT METHODS FOR THE GROWTH OF ZnO NANOSTRUCTURES There are various methods of synthesis of ZnO either in the form of bulk nanocrystal or in the thin film form, including bulk growth, substrate growth, chemical vapour deposition, sputtering and chemical route etc. [6],[7] 3.1 Bulk Growth Growth of bulk ZnO crystals is mainly carried out by three methods: hydrothermal, vapor phase and melt growth. 3.1.1 Hydrothermal method The hydrothermal method uses ZnO single-crystal seeds (suspended by Pt wire), and sintered ZnO strings together with a KOH (3 mol/ l) and LiOH (1 mol/ l)

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Chapter 3


aqueous solution are used as a nutrient. The seeds and the nutrient are placed into a Pt crucible. This crucible is sealed by welding and placed in an autoclave. The autoclave is then placed into a two-zone vertical furnace. ZnO is transferred from the nutrient in the higher-temperature zone to the seeds in the lower-temperature zone. The seeds grow to bulk ingots about 10 mm in size after 2 weeks. The growth temperature is 300–400 °C at a pressure between 70 and 100 MPa. A Pt inner container is used for preventing impurity incorporation from the aqueous solution. The crystal shapes depend on the precursor and the solution basicity and on the shapes of seed crystals. The crystal color is nonuniform because of the anisotropic crystal growth in which the growth rate of each sector depends on orientation. When used as substrates for epitaxy, proper surface preparation is necessary to evaluate the quality of hydrothermally grown ZnO. 3.1.2 Vapour transport A method which produces very high quality bulk ZnO wafers is based on vapor transport. In this method, the reaction takes place in a nearly closed horizontal tube. Pure ZnO powder used as the ZnO source is placed at the hot end of the tube which is kept at about 1150 °C. The material is transported to the cooler end of the tube, maintained at about 1100 °C, by using H2 as a carrier gas. A carrier gas is necessary because the vapor pressures of O and Zn are quite low over ZnO at these temperatures. The likely reaction in the hot zone is ZnO(s) +H2 (g) →Zn (g) +H2O (g). At the cooler end, ZnO is formed by the reverse reaction, assisted by a single-crystal seed. To maintain the proper stoichiometry, a small amount of water vapor is added. Growth time of 150– 175 h provided 2-inch.-diameter crystals of about 1 cm in thickness. Vapor transport

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Chapter 3


using chlorine and carbon as transporting agents has been used to achieve ZnO crystal growth at moderate temperature of 950–1000 °C. 3.1.3 Melt growth Another method for producing bulk ZnO is that of melt growth. The melt method is based on a pressurized induction melting apparatus (see Fig.). The melt is contained in a cooled crucible. Zinc oxide powder is used as the starting material. The heat source used during the melting operation is radio frequency (rf) energy, induction heating. The rf energy produces joule heating until the ZnO is molten at about 1900 °C. Once the molten state is attained, the crucible is slowly lowered away from the heated zone to allow crystallization of the melt. 3.2 Substrate growth In order to reduce the strains and dislocation density in epitaxial ZnO and related films, closely lattice matched substrates are favored for growth. Sapphire substrates are commonly used for ZnO heteroepitaxial growth, primarily on the (0001) orientation. In addition, ZnO and related oxides have been grown on Si, SiC, GaAs, CaF2 and ScAlMgO4. The ZnO layers have been grown on sapphire by using a variety of growth techniques, including PLD. In the pulsed-laser deposition (PLD) method, high-power laser pulses are used to evaporate material from a target surface such that the stoichiometry of the material is preserved in the interaction. As a result, a supersonic jet of particles (plume) is directed normal to the target surface. The plume expands away from the target with a strong forward directed velocity distribution of different particles. The ablated species condense on the substrate placed opposite to the target. A schematic diagram of the

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typical PLD system is shown in Fig... The main advantages of PLD are its ability to create high-energy source particles, permitting high quality film growth at low substrate temperatures, typically ranging from 200 to 800 °C, its simple experimental setup, and operation in high ambient gas pressures in the 10−5–10−1-Torr range. For the growth of ZnO by PLD technique, usually UV excimer lasers (KrF: 248 nm and ArF: 193 nm) and Nd: yttrium aluminum garnet (YAG) pulsed lasers (355 nm) are used for ablation of the ZnO target in an oxygen environment. In some cases, Cu-vapor laser emitting at 510–578 nm is also used for the same purpose. Cylindrical ZnO tablets made from pressed ZnO powder are usually used as targets. Single-crystal ZnO has been used to grow highquality ZnO thin films very recently. A pure Zn metal is used only in rare cases. The properties of the grown ZnO films depend mainly on the substrate temperature, ambient oxygen pressure, and laser intensity. 3.3 Chemical Vapour Deposition Among other growth methods, chemical-vapor deposition (CVD) technology is particularly interesting not only because it gives rise to high-quality films but also because it is applicable to large-scale production. This technique is widely used in the fabrication of epitaxial films toward various GaN-based optoelectronic devices, and similar trend might be expected for future applications of ZnO. There are several modifications of this method depending on precursors used. In the CVD method, ZnO deposition occurs as a result of chemical reactions of vapor-phase precursors on the substrate, which are delivered into the growth zone by the carrier gas. The reactions take place in a reactor where a necessary temperature profile is created in the gas flow direction.

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3.4 Sputtering One of the most popular growth techniques for early ZnO investigations was sputtering (dc sputtering, rf magnetron sputtering, and reactive sputtering). As compared to sol gel and chemical-vapor deposition the magnetron sputtering was a preferred method because of its low cost, simplicity, and low operating temperature. ZnO films grow at a certain substrate temperature by sputtering from a high-purity ZnO target using a rf magnetron sputter system. The growth is usually carried out in the growth ambient with O2/Ar+O2 ratios ranging from 0 to 1 at a pressure of 10−3–10−2 Torr. O2 serves as the reactive gas and Ar acts as the sputtering enhancing gas. ZnO can also be grown by dc sputtering from a Zn target in an Ar+O2 gas mixture. The rf power applied to the plasma is tuned to regulate the sputtering yield rate from the ZnO target. For these experiments, the target is presputtered for 5–15 min before the actual deposition begins to remove any contamination on the target surface, make the system stable, and reach optimum condition. 3.5 Chemical Route Nanocrystalline ZnO particles can be prepared by a novel template free aqueous solution based chemical route from zinc nitrate hexahydrate without any requirement of calcinations step at high temperature. Aqueous solution of zinc nitrate and sodium hydroxide are mixed and stirred using a magnetic stirrer. The resultant precipitate is filtered and dried to get the ZnO nanocrystals of different size and shape.

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Chapter 4

CHARACTERISATION TECHNIQUES

CHAPTER 4 CHARACTERISATION TECHNIQUES

4.1 X-Ray Diffraction 4.2 Field Emission Scanning Electron Microscopy 4.3 Transmission Electron Microscopy 4.4 Photoluminescence Spectroscopy 4.5 Raman Spectroscopy 4.6 FTIR Spectroscopy 4.7 UV/VIS Spectroscopy

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4.1 X-Ray Diffraction X-ray diffraction is a powerful tool for materials characterization as well as for detailed structural elucidation. As the physical properties of solid (e.g., electrical, optical, magnetic etc.) depend on atomic arrangements of materials, determination of the crystal structure is an indispensable part of the structural and chemical characterization of materials. X-ray patterns are used to establish the atomic arrangements of the materials because of the fact that the lattice parameter, d (spacing between different planes) is of the order of x-ray wavelength. Further, X-ray diffraction method can be used to distinguish crystalline materials from nonocrystalline (amorphous) materials. The structure identification is made from the x-ray diffraction pattern analysis and comparing it with the internationally recognized database containing the reference pattern (JCPDS). From X-ray diffraction pattern we can obtain the following information:(i) To judge formation of a particular material system. (ii) Unit cell structure, lattice parameters, miller indices. (iii)Types of phases present in the material (iv) Estimation of crystalline/amorphous content in the sample. (v) Evaluation of the average crystalline size from the width of the peak in a particular phase pattern. Large crystal size gives rise to sharp peaks, while the peak width increases with decreasing crystal size. (vi) An analysis of structural distortion arising as a result of variation in d-spacing caused by the strain, thermal distortion.

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Determination of crystal size: The X-ray diffraction analysis has been the most popular method for the estimation of crystallite size in nanomaterials and therefore, has been extensively used in the present work. The evaluation of crystallite sizes in the nanometer range warrants careful analytical skills. The broadening of the Bragg peaks is ascribed to the development of the crystallite refinement and internal stain. To size broadening and stain broadening, the full width at half maximum (FWHM) of the Brag peaks as a function of the diffraction angle is analysed. Crystallite size of the deposits is calculated by the Xray diffraction (XRD) peak broadening. The diffraction patterns are obtained using Cu Kα radiation at a scan rate of 10/min. The full width half maxima (FWHM) of the diffraction peaks were estimated by pseudo-Voigt curve fitting. After subtracting the instrumental line broadening, which was estimated using quartz and silicon standards, the grain size can be estimated the Scherrer equation D=

0.9λ β cos θ

Where λ is wave length of X-ray, β is FWHM in radian, θ is peak angle.

4.2 Field Emission Scanning Electron Microscopy This is one of the widely used instruments in material research laboratories. In this technique, electrons are used instead of light waves to see the microstructure of surface of a specimen. However since electrons are excited to high energy (keV), so wavelength of electron waves are quite small and resolution is quite

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high. The electromagnetic lenses used in it are not a part of image formation system, but just helps to focus the electron beam on specimen surface. This gives two of the major benefits of SEM: range of magnification and depth of field in the image, giving three dimensional information of image. In a typical SEM, electrons are thermionically emitted from a tungsten or lanthanum hexaboride (LaB6) cathode and are accelerated towards an anode; alternatively, electrons can be emitted via field emission (FE). Tungsten is used because it has the highest melting point and lowest vapour pressure of all metals, thereby allowing it to be heated for electron emission. The electron beam, which typically has an energy ranging from a few hundred eV to 100 keV, is focused by one or two condenser lenses into a beam with a very fine focal spot sized 1 nm to 5 nm. The beam passes through pairs of scanning coils in the objective lens, which deflect the beam horizontally and vertically so that it scans in a raster fashion over a rectangular area of the sample surface. When the primary electron beam interacts with the sample, the electrons lose energy by repeated scattering and absorption within a teardrop-shaped volume of the specimen known as the interaction volume, which extends from less than 100 nm to around 5 µm into the surface, The size of the interaction volume depends on the beam accelerating voltage, the atomic number of the specimen and the specimen's density. The energy exchange between the electron beam and the sample results in the emission of electrons and electromagnetic radiation which can be detected to produce an image, as described below.

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CHARACTERISATION TECHNIQUES The most common imaging mode monitors low energy (