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Chapter- 3SYNTHESIS AND CHARACTERIZATION OF PURE AND TIN-DOPED ZnO NANOCRYSTALLINE (BULK) MATERIALS 3.0 INTRODUCTION Recently, zinc oxide (ZnO) has attracted much attention within the scientific community as a ‘future material’. This is however, somewhat of a misnomer, as ZnO has been widely studied since 1935, with much of our current industry and day-to-day lives critically reliant upon this compound. The renewed interest in this material has arisen out of the development of growth technologies for the fabrication of high quality single crystals and epitaxial thin films/layers, allowing for the realization of ZnO-based electronic and optoelectronic devices [1]. ZnO, a II-VI semiconductor with a direct wide band gap of 3.35 eV at room temperature and large exciton binding energy of 60 meV [2], is one of the most promising materials for the fabrication of optoelectronics devices [3] operating in the blue and ultraviolet (UV) regions and gas sensing applications. It has a wide range of technological applications including transparent conducting electrodes for solar cells, flat panel displays, surface acoustic devices, chemical and biological sensors and UV lasers. Controlled synthesis of semiconductor nanostructures in terms of size and shape has strong motivation to researchers because their properties can be controlled by shape and size. Novel applications can be investigated and are dependent of their structural properties. As the morphology of nano-materials is one of the key factors that affect their properties. ZnO is a versatile functional nanomaterial with novel morphologies. It has a rich family of nanostructures [4] as shown in figure 3.1 and 3.2, such as nanotubes, nanowires, nanorods, nanobelts, nanocables, nanosheets, nanotetrapods, nanomultipods, nanoflowers, nanoneedles, shuttle-like, combs-like, nanorings and nanoribbons which can be fabricated by different techniques. Figure 3.1 A collection of nanostructures of ZnO synthesized under controlled conditions by thermal evaporation of solid powders. Most of the structures presented can be produced with 100% purity.Figure. 3.2 Seamless single-crystal nanorings of ZnO. (a) Structure model of ZnO, showing the ±(0001) polar surfaces. (b-e) Proposed growth process and corresponding experimental results showing the initiation and formation of the single-crystal nanoring via the self-coiling of a polar nanobelt. The nanoring is initiated by folding a nanobelt into a loop with overlapped ends as a result of long-range electrostatic interactions among the polar charges; the short-range chemical bonding stabilizes the coiled ring structure; and the spontaneous self-coiling of the nanobelt is driven by minimization of the energy contributed by polar charges, surface area, and elastic deformation. (f) SEM images of the as-synthesized, single-crystal ZnO nanoring. (g) The ‘slinky’ growth model of the nanoring. (h) The charge model of an α-helix protein, in analogy to the charge model of the nanobelt during the self-coiling process. During the past few years, attention has also been focused on the research field of one-dimensional (1D) nanostructure materials, such as nanowires and nanorods, because of their fundamental importance and the wide range of potential applications for nanodevices [5-6]. In the present investigation, ZnO nanocrystals were synthesized using co-precipitation method to realize the size controllable growth of ZnO. A number of reaction conditions for example solvents, precursors, acidity and basicity were used to synthesize ZnO nanocrystal with difference morphologies. The effects of the reaction conditions on the final products were systematically investigated. Important parameters related to the physical properties of ZnO are tabulated in Table 3.1. It should be noted that still there exists uncertainty in some of these values like hole mobility, thermal conductivity etc.3.1 GENERAL ASPECTS OF ZINC OXIDE AND ITS APPLICATIONS 3.1.1 CRYSTAL STRUCTURE OF ZINC OXIDE (ZO) Most of the group II–VI binary compound semiconductors crystallize in either cubic zinc blende or hexagonal wurtzite (Wz) 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 nature, but these materials also have a substantial ionic character that tends to increase the bandgap beyond the one expected from the covalent bonding. ZnO is a II–VI compound semiconductor whose ionicity resides at the borderline between the covalent and ionic semiconductors. The crystal structures shared by ZnO are wurtzite (B4), zinc blende(B3), and rocksalt (or Rochelle salt) (B1) as schematically shown in Figure 3.3. B1, B3, and B4 denote the Strukturbericht designations for the three phases. Under ambient conditions, the thermodynamically stable phase is that of wurtzite symmetry. The zinc blende ZnO structure can be stabilized only by growth on cubic substrates, and the rocksalt or Rochelle salt (NaCl) structure may be obtained at relatively high pressures, as in the case of GaN. At ambient pressure and temperature, ZnO crystallizes in the wurtzite (B4 type) structure, which has a hexagonal unit cell with two lattice parameters a and c in the ratio of c/a= (8/3)1/2 =1.633 (in an ideal wurtzite structure) and belongs to the space group C46v in the Schoenflies notation and P63mc in the Hermann–Mauguin notation. symmetry. A schematic representation of the wurtzitic ZnO structure is shown in figure 3.4. Table 1.1 Different parameters of physical properties of ZnO Materials. PROPERTIES VALUES Molecular formula ZnO Molar mass 81.408 g/mol Appearance White solid Odor Odorless Stable phase Wurtzite Density 5.606 g/cm3 Melting point 1950oC (decomposes) Boiling point 1950oC (decomposes) Solubility in water 0.16mg/100ml (30oC) Lattice parameters a=b c a/c 3.249 ? 5.206 ? 1.602 Lattice Hexagonal Space Group P63mc Band gap 3.3 eV (direct) Refractive Index 2.008-2.029 Thermal conductivity 1-1.2 W cm?-?1?K?–?1 Exciton binding Energy 60 meV Electron effective mass 0.24 Electron Hall Mobility 200 cm2/Vs Hole effective mass 0.59 Hole Hall Mobility 5-50 cm2/Vs Figure 3.3 Stick-and-ball representation of ZnO crystal structures:(a) cubic rocksalt (B1), (b) cubic zinc blende (B3), and (c) hexagonal wurtzite (B4). Shaded gray and black spheres denote Zn and O atoms, respectively. Figure 3.4 Schematic representation of a wurtzitic ZnO structure with lattice constants a in the basal plane and c in the basal direction, u parameter, which is expressed as the bond length or the nearest-neighbor distance b divided by c (0.375 in ideal crystal), a and b (109.47 in ideal crystal) bond angles, and three types of second-nearest-neighbor distances b’1, b’2, and b’3. The structure is composed of two interpenetrating hexagonal close packed (hcp) sub-lattices, each of which consists of one type of atom displaced with respect to each other along the threefold c-axis by the amount of u=3/8=0.375 (in an ideal wurtzite structure) in fractional coordinates. The internal parameter u is defined as the length of the bond parallel to the c-axis (anion–cation bond length or the nearest-neighbor distance) divided by the c lattice parameter. The basal plane lattice parameter (the edge length of the basal plane hexagon) is universally depicted by a; the axial lattice parameter (unit cell height), perpendicular to the basal plane, is universally described by c. Each sub-lattice includes four atoms per unit cell, and every atom of one kind (group II atom) is surrounded by four atoms of the other kind (group VI), or vice versa, which are coordinated at the edges of a tetrahedron. The crystallographic vectors of wurtzite are a’=a(1/2, 31/2/2, 0); b’= a(1/2, -31/2/2, 0) and c’=a(0,0, c/a). In Cartesian coordinates, the basis atoms are (0, 0, 0), (0, 0, uc), a(1/2, 31/2/6, c/2a) and a(1/2, 31/2/6, [u+1/2]c/a). Also this hexagonal lattice is characterized by two interconnecting sublattices of Zn2+ and O2?, such that each Zn ion is surrounded by tetrahedra of O ions, and vice-versa. This tetrahedral coordination gives rise to polar symmetry along the hexagonal axis. This polarity is responsible for a number of the properties of ZnO, including its piezoelectricity and spontaneous polarization, and is also a key factor in crystal growth, etching and defect generation. The four most common face terminations of wurtzite ZnO are the polar Zn terminated (0001) and O terminated (0001) faces (c-axis oriented), and the non-polar (1120) (a-axis) and (1010) faces which both contain an equal number of Zn and O atoms. The polar faces are known to posses different chemical and physical properties, and the O-terminated face possess a slightly different electronic structure to the other three faces [7]. Additionally, the polar surfaces and the (1010) surface are found to be stable, however the (1120) face is less stable and generally has a higher level of surface roughness than its counterparts. The (0001) plane is basal. Aside from causing the inherent polarity in the ZnO crystal, the tetrahedral coordination of this compound is also a common indicator of sp3 covalent bonding. However, the Zn-O bond also possesses very strong ionic character and thus ZnO lies on the borderline between being classed as a covalent and ionic compound, with an iconicity of fi = 0.616 on the Phillips iconicity scale [8]. The lattice parameters of the hexagonal unit cell are a = 3.2495 ? and c = 5.2069 ?, and the density is 5.605 gcm-3[9].In an ideals wurtzite crystal, the axial ratio c/a and the u parameter (which is a measure of the amount by which each atom is displaced with respect to the next along the c-axis) are correlated by the relationship u.c/a = (3/8)1/2, where c/a = (8/3)1/2 and u= 3/8 for an ideal crystal. ZnO crystal deviate from this ideal arrangement by changing both of these values. This deviation occurs such that the tetrahedral distance is kept roughly constant in the lattice. Experimentally, for wurtzite ZnO, the real values of u and c/a were determined in the range u = 0.3817-0.3856 and c/a = 1.593-1.6035[10-12]. In addition to the wurtzite phase, ZnO is also known to crystallize in the cubic zincblende and rocksalt (NaCl) structures, which are illustrated in figure 3.2. Sphalerite structure, which is known as the zincblende structure, is stable only by the growth on cubic structure [13-15], whilst the rocksalt structure is a high-pressure metastable phase forming at ~ 10GPa, and cannot be epitaxially stabilized [16]. Theoretical calculations indicate that a fourth phase, cubic cesium chloride, may be possible at extremely high temperatures, however, this phase has yet to be experimentally observed [17]. The wurtzite structure (Figure 3.1) differs from sphalerite structure in being derived from an expanded hcp anion (O2-) array rather than a ccp array, but as in sphalerite the cations (Zn2+) occupy one type of tetrahedral hole. This structure has (4, 4)-coordination as same as sphalerite structure. The local symmetries of cations and anions are identical towards their nearest neighbours in wurtzite and sphalerite but differ at second - nearest neighbours.3.1.2 DEFECTS AND IMPURITIES IN ZINC OXIDE Zinc oxide crystal has native point defect which greatly affects its optical and electrical properties. These defects create electronic states in the band gap which influence its optical emission properties. The as grown ZnO crystal has always found to be n-type. It has been shown theoretically that both Oxygen vacancy VO and Zinc interstitial ZnI have high formation energies in n-type ZnO and they are deep level donors [18].Thus it is considered that neither VO nor ZnI exists in measurable quantity. Van de Walle has proposed that hydrogen H is a dominant background donor in ZnO that were exposed to H during growth [19]. Group III elements Al, Ga and In are donor impurities to ZnO that can substitute Zn upto concentration greater than 1020 cm-3. The search for high conductivity p-type ZnO still remains an active area of research. It has been predicted theoretically that Li substitituted Zn, LiZn and Na substituted Zn NaZn creates shallow acceptor levels, but neither produces high-conductivity p-type ZnO [20]. N,P, As and Sb have been used as acceptors to produce n-type ZnO [21], where it is reported that Zn-vacancy in ZnO acts as defect- type acceptor.3.1.3 ELECTRONIC STRUCTURE/PROPERTIES OF ZINC OXIDE ZnO has a relatively large direct band gap of ~3.3 eV at room temperature; therefore, pure ZnO is colorless and transparent. The electronic band structure of ZnO has been calculated by a number of groups [22–28]. The results of a band structure calculation using the Local Density Approx- imation (LDA) and incorporating atomic self-interaction corrected pseudopotentials (SIC-PP) to accurately account for the Zn 3d electrons, is shown in figure 3.5 [28]. The band structure is shown along high symmetry lines in the hexagonal Brillouin zone. Both the valence band maxima and the lowest conduction band minima occur at the point k = 0 indicating that ZnO is a direct band gap semiconductor. The bottom 10 bands (occurring around ?9 eV) correspond to Zn 3d levels. The next 6 bands from ?5 eV to 0 eV correspond to O 2p bonding states. The first two conduction band states are strongly Zn localized and correspond to empty Zn 3s levels. The higher conduction bands (not illustrated here) are free-electron-like. The O 2s bands (also not illustrated here) associated with core-like energy states, occur around ?20 eV. The band gap as determined from this calculation is 3.77 eV. This correlates reasonably well with the experimental value of 3.4 eV, and is much closer than the value obtained from standard LDA calculations, which tend to underestimate the band gap by ～3 eV due to its failure in accurately modeling the Zn 3d electrons. In addition to calculations for the band structure of bulk ZnO, Ivanov and Pollmann have also carried out an extensive study on the electronic structure of the surfaces of wurtzite ZnO [27]. Using the empirical tight-binding method (ETBM) to determine a Hamiltonian for the bulk states, the scattering theoretical method was applied to determine the nature of the surface states. The calculated data was found to be in very good agreement with experimental data obtained from electron energy loss spectroscopy (EELS) and ultra- violet photoelectron spectroscopy (UPS). Figure 3.5 The LDA band structure of bulk wurtzite ZnO calculated using dominant atomic self-interaction-corrected pseudopotentials (SIC-PP). This method is much more efficient at treating the d –bands than the standard LDA method. [Reprinted with permission from D. Vogel, P. Krüger and J. Pollmann, Phys.Rev. B 52, R14316 (1995). Copyright 1995 by the American Physical Society] Figure 3.6 shows the wave-vector-resolved local density of states (LDOSs) on the first three layers of the (0001)-Zn (left panel) and (0001)-O (right panel) surfaces, for the, M and K points of the surface Brillouin zone. The bulk LDOS (calculated using the ETBM) is given by the dashed lines. Surface induced positive changes to the LDOS are shown as hatched. No surface states are present in the band gap, the Zn surface shows an increase in back bonds (denoted by B in figure 3.6) and anti-back bonds (denoted by A) surface states, while the O face simply shows an increase in P resonances and states. This result suggests that the Zn face possesses more covalent character, arising from the Zn 4s–O 2p states, whilst the O face is more ionic. Experimentally, the ZnO valence band splits into three band states, A, B and C by spin-orbit and crystal-field splitting. This splitting is schematically illustrated in figure 3.7. The A and C subbands are known to posses Г7 symmetry, whilst the middle band, B, has Г9 symmetry [29]. The band gap has temperature dependence up to 300 K given by the relationship: Eg(T)= Eg (T=0) 5.05×10-4T2900-T …………………….(3.1)These properties, combined with the lattice dynamics of ZnO give rise to interesting optical properties. Advantages associated with a large band gap include higher breakdown voltages, ability to sustain large electric fields, lower electronic noise, and high-temperature and high-power operation. The bandgap of ZnO can further be tuned from ~3–4 eV by its alloying with magnesium oxide or cadmium oxide. Most ZnO has n-type character, even in the absence of intentional doping. Nonstoichiometry is typically the origin of n-type character, but the subject remains controversial. An alternative explanation has been proposed, based on theoretical calculations, that unintentional substitutional hydrogen impurities are responsible. Controllable n-type doping is easily achieved by substituting Zn with group-III elements such as Al, Ga, In or by substituting oxygen with group-VII elements chlorine or iodine. Reliable p-type doping of ZnO remains difficult. This problem originates from low solubility of p-type dopants and their compensation by abundant n-type impurities. This problem is observed with GaN and ZnSe. Measurement of p-type in "intrinsically" n-type material is complicated by the inhomogeneity of samples. Current limitations to p-doping do not limit electronic and optoelectronic applications of ZnO, which usually require Figure 3.6 Wave-vector-resolved LDOS’s on the first three layers of the (0001)-Zn (left panel) and (0001?)-O (right panel) surfaces. The bulk LDOS is given by the dashed lines and surface induced positive changes to the LDOS are shown as hatched. The letters A, B, P and S represent anti-back bonds, back bonds, P resonances and S resonances respectively. Figure 3.7 Schematic diagram representing the crystal-field and spin-orbit splitting of the valence band of ZnO into 3 subband states A, B and C at 4.2 Kjunctions of n-type and p-type material. Known p-type dopants include group-I elements Li, Na, K; group-V elements N, P and As; as well as copper and silver. However, many of these form deep acceptors and do not produce significant p-type conduction at room temperature.3.1.4 MECHANICAL AND THERMAL PROPERTIES The mechanical properties of materials involve various concepts such as hardness, stiffness, and piezoelectric constants, Young’s and bulk modulus, and yield strength. The solids are deformed under the effect of external forces and the deformation is described by the physical quantity strain. The internal mechanical force system that resists the deformation and tends to return the solid to its undeformed initial state is described by the physical quantity stress. Within the elastic limit, where a complete recoverability from strain is achieved with removal of stress, stress σ is proportional to strain ε . ZnO is a relatively soft material with approximate hardness of 4.5 on the Mohs scale. Its elastic constants are smaller than those of relevant III-V semiconductors, such as GaN. The high heat capacity and heat conductivity, low thermal expansion and high melting temperature of ZnO are beneficial for ceramics. Among the tetrahedrally bonded semiconductors, it has been stated that ZnO has the highest piezoelectric tensor or at least one comparable to that of GaN and AlN. This property makes it a technologically important material for many piezoelectrical applications, which require a large electromechanical coupling.3.1.5 ELECTRICAL PROPERTIES OF ZnO As a direct and large bandgap material, ZnO is attracting much attention for a variety of electronic and optoelectronic applications. Advantages associated with a large bandgap include high-temperature and high-power operation, lower noise generation, higher breakdown voltages, and ability to sustain large electric fields. The electron transport in semiconductors can be considered for low and high electric fields. (i). At sufficiently low electric fields, the energy gained by the electrons from the applied electric field is small compared to the thermal energy of electrons and therefore the energy distribution of electrons is unaffected by such a low electric field. Because the scattering rates determining the electron mobility depend on the electron distribution law function, electron mobility remains independent of the applied electric field, and Ohm’slaw is obeyed. (ii). When the electric field is increased to a point where the energy gained by electrons from the external field is no longer negligible compared to the thermal energy of the electron, the electron distribution function changes significantly from its equilibrium value. These electrons become hot electrons characterized by an electron temperature larger than the lattice temperature. Furthermore, as the dimensions of the device are decreased to submicron range, transient transport occurs when there is minimal or no energy loss to the lattice during a short and critical period of time, such as during transport under the gate of a field effect transistor or through the base of bipolar transistor. The transient transport is characterized by the onset of ballistic or velocity overshoot phenomenon. Because the electron drift velocity is higher than its steady-state value, one can design a device operating at frequencies exceeding those expected from linear scaling of dimensions.3.1.6 APPLICATIONS OF ZnO NANOMATERIALS The applications of zinc oxide powder are numerous, and the principal ones are summarized below. Most applications exploit the reactivity of the oxide as a precursor to other zinc compounds. For material science applications, zinc oxide has high refractive index, good thermal, binding, antibacterial and UV-protection properties. Consequently, it is added into various materials and products, including plastics, ceramics, glass, cement, rubber, lubricants, paints, ointments, adhesive, sealants, pigments, foods, batteries, ferrites, fire retardants, etc.Electronics ZnO has wide direct band gap (3.37 eV or 375 nm at room temperature). Therefore, its most common potential applications are in laser diodes and light emitting diodes (LEDs). Some optoelectronic applications of ZnO overlap with that of GaN, which has a similar bandgap (~3.4 eV at room temperature). Compared to GaN, ZnO has a larger exciton binding energy (~60 meV, 2.4 times of the room temperature thermal energy), which results in bright room-temperature emission from ZnO. Other properties of ZnO favorable for electronic applications include its stability to high-energy radiation and to wet chemical etching. Radiation resistance makes ZnO a suitable candidate for space applications. The pointed tips of ZnO nanorods result in a strong enhancement of an electric field. Therefore, they can be used as field emitters. Aluminium-doped ZnO layers are used as transparent electrodes. The constituents Zn and Al are much cheaper and less poisonous compared to the generally used indium tin oxide (ITO). One application which has begun to be commercially available is the use of ZnO as the front contact for solar cells or of liquid crystal displays. Transparent thin-film transistors (TTFT) can be produced with ZnO. As field-effect transistors, they even may not need a p–n junction, thus avoiding the ptype doping problem of ZnO. Some of the field-effect transistors even use ZnO nanorods as conducting channels. SpintronicsZnO has also been considered for spintronics applications: if doped with 1-10% of magnetic ions (Mn, Fe, Co, V, etc.), ZnO could become ferromagnetic, even at room temperature. Such room temperature ferromagnetism in ZnO:Mn has been observed, but it is not clear yet whether it originates from the matrix itself or from Mn-containing precipitates. PiezoelectricityThe piezoelectricity in textile fibers coated in ZnO have been shown capable of "self-powering nanosystems" with everyday mechanical stress generated by wind or body movements. In 2008 the Center for Nanostructure Characterization at the Georgia Institute of Technology reported producing an electricity generating device (called flexible charge pump generator) delivering alternating current by stretching and releasing zinc oxide wires. This mini-generator creates an oscillating voltage up to 45 millivolts, converting close to seven percent of the applied mechanical energy into electricity. Theresearchers used wires having the lengths of 0.2-0.3 mm and diameters of three to fivemicrometers, but the device could be scaled down to nanometer size. Sensors Zinc oxide nanorod sensors are devices detecting changes in electrical current passing through zinc oxide nanowires due to adsorption of gas molecules. Selectivity to hydrogen gas was achieved by sputtering Pd clusters on the nanorod surface. The addition of Pd appears to be effective in the catalytic dissociation of hydrogen molecules into atomic hydrogen, increasing the sensitivity of the sensor device. The sensor detects hydrogen concentrations down to 10 parts per million at room temperature, whereas there is no response to oxygen. ZnO has high biocompatibility and fast electron transfer kinetics. Such features advocate the use of this material as a biomimic membrane to immobilize and modify biomolecules. Cigarette Filters Zinc oxide is a constituent of cigarette filters for removal of selected components from tobacco smoke. A filter consisting of charcoal impregnated with zinc oxide and iron oxide removes significant amounts of HCN and H2S from tobacco smoke without affecting its flavor. Rubbers Manufacture About 50% of ZnO use is in rubber industry. Zinc oxide activates vulcanization, which otherwise may not occur at all. Zinc oxide and stearic acid are ingredients in the commercial manufacture of rubber goods. A mixture of these two compounds allows a quicker and more controllable rubber cure. ZnO is also an important additive to the rubber of car tyres. Vulcanization catalysts are derived from zinc oxide, and it considerably improves the thermal conductivity, which is crucial to dissipate the heat produced by the deformation when the tyre rolls. ZnO additive also protect rubber from fungi (see medical applications) and UV light. Concrete Industry Zinc oxide is widely used for concrete manufacturing. Addition of ZnO improves the processing time and the resistance of concrete against water.Medical Zinc oxide as a mixture with about 0.5% iron (III) oxide (Fe2O3) is called calamine and is used in calamine lotion. There are also two minerals, zincite and hemimorphite, which have been called calamine historically. When mixed with eugenol, a chelate, zinc oxide eugenol is formed which has restorative and prosthodontic applications in dentistry. Reflecting the basic properties of ZnO, fine particles of the oxide have deodorizing and antibacterial action and for that reason are added into various materials including cotton fabric, rubber, food packaging, etc. Enhanced antibacterial action of fine particles compared to bulk material is not intrinsic to ZnO and is observed for other materials, such as silver. Zinc oxide is a component of barrier cream used in nappy rash or diaper rash. It is also a component in tape (called "zinc oxide tape") used by athletes as abandage to prevent soft tissue damage during workouts. Food Additive Zinc oxide is added to many food products, e.g., breakfast cereals, as a source of zinc, a necessary nutrient. (Other cereals may contain zinc sulfate for the same purpose.) Some prepackaged foods also include trace amounts of ZnO even if it is not intended as a nutrient. Pigment Zinc white is used as a pigment in paints and is more opaque than lithopone, but less opaque than titanium dioxide. It is also used in coatings for paper. Chinese white is a special grade of zinc white used in artists' pigments. Because it reflects both UVA and UVB rays of ultraviolet light, zinc oxide can be used in ointments, creams, and lotions to protect against sunburn and other damage to the skin caused by ultraviolet light. It is the broadest spectrum UVA and UVB absorber that is approved for use as a sunscreen by the FDA, and is completely photo stable. It is also a main ingredient of mineral makeup. Coatings Paints containing zinc oxide powder have long been utilized as anticorrosive coatings for various metals. They are especially effective for galvanized Iron. The latter is difficult to protect because its reactivity with organic coatings leads to brittleness and lack of adhesion. Zinc oxide paints however, retain their flexibility and adherence on such surfaces for many years. ZnO highly n-type doped with Al, Ga or nitrogen is transparent and conductive (transparency ~90%, lowest resistivity ~10?4?cm). ZnO: Al coatings are being used for energy-saving or heat-protecting windows. The coating lets the visible part of the spectrum in but either reflects the infrared (IR) radiation back into the room (energy saving) or does not let the IR radiation into the room (heat protection), depending on which side of the window has the coating. Various plastics, such as poly (ethylene-naphthalate) (PEN), can be protected by applying zinc oxide coating. The coating reduces the diffusion of oxygen with PEN. Zinc oxide layers can also be used on polycarbonate (PC) in outdoor applications. The coating protects PC form solar radiation and decreases the oxidation rate and photo-yellowing of PC. LITERATURE SURVEY Over the past decade, tremendous efforts have been made to synthesize nanoscaled or microscaled ZnO crystals. Up to now, ZnO nanostructures with various sizes and morphologies have been successfully synthesized and reported in the literature [4]. Besides, as is well known, impurity doping in semiconductors with selective elementsgreatly affects the basic physical properties, such as the electrical, optical, and magnetic properties, which are crucial for their practical application, and the doping effect has attracted extraordinary attention. Recently, various doped ZnO nanostructures with different elements (e.g., Al, As, In, Sn, Mg, and Sb) have been achieved [12-17]. Preetam Singh et al. [30] reported the preparation of ZnO nanopowder by ultrasonic mist chemical vapor deposition (UM–CVD) system. This is a promising method for large area deposition at low temperature inspite of being simple, inexpensive and safe. The high temperature X-ray diffraction (XRD) of the powder showed prominent (100), (002) and (101) reflections among which (101) are of highest intensity. With increase in temperature, a systematic shift in peak positions towards lower 2θ values has been observed, which may be due to change in lattice parameters. Temperature dependence of lattice constants under vacuum shows linear increase in their values. The synthesized powder exhibited the estimated direct bandgap (Eg) of 3?43 eV. It has been reported that annealing temperature in the range of 400-650 0C, affects the crystallography, particle size and thermo-power of bulk ZnO [31]. The small change in lattice constants of a and c (lattice constants a =b=3.2469 ? increase to 3.2488 ?, c=5.2049 ? slightly decrease to 5.2031 ?). The Zn-O bond length was related with ZnO unit cell views of the direction approximately parallel to O2-and Zn2+. The powder of bulk ZnO exhibited good distribution of particles after being annealed below 600 0C and covered with the nanoparticles, while other portions retained the smooth morphology. The particle sizes increased from 73.50 to 79.67 nm with increase in annealing temperatures from 400 oC to 650 0C. The bulk ZnO has highest thermo power of -92.99 ?VK-1 at room temperature for annealing temperature of 550 0C and indicating that the behavior of the n-type thermoelectric material. S. Suwanboon et al. [32] reported the synthesis of nanocrystalline ZnO powder by precipitation method by using zinc acetate dihydrate and PVP as starting materials. The indexing of the XRD pattern of calcined powders in air at 600 0C for 1 hour, reveals the hexagonal structure with the smallest crystallite size of ~ 44.76 nm, and lattice parameters a and c of 0.3249 and 0.5204 nm, respectively at 3x10-4 M PVP. The SEM images showed that the morphology has been changed from plate-like to small rod shape when adding PVP to solutions and the morphology has tended to be monosized at higher PVP concentration. The smallest grain sizes of ZnO powders were ~ 130 nm at 3x10-4 M PVP. The optical band gap of ZnO powders in this study varied between 3.218-3.229 eV. The zinc oxide whiskers were synthesized by Fan xi-mei et al. [33] using the equilibrium gas expanding method at the temperature of 700 0C with metallic zinc as the main raw material without any catalysts. The results showed that the as-grown samples are composed of uniform tetrapod-like ZnO whiskers. The length and diameter of the arms of the tetrapod-like ZnO whiskers increase obviously with the increase of the growth time. The strong single ultraviolet (UV) emission centering 385?391 nm without any accompanying deep-level emission is observed in the room temperature photoluminescence (PL) spectra of the whiskers. The intensity of UV emission increases markedly with the increase of growth time. Mansi Dhingra et al. [34] s reported the preparation of ZnO nanoparticles, by the sol–gel method and the powder pressed in the form of pellets were used for gas sensing. The hybrid Zinc oxide/polyaniline (ZnO/PANI) structure was obtained by the addition of PANI on the surface of ZnO. The UV–VIS absorption of the modified pellets showed band edge at 363 nm corresponding to ZnO, while a change in the absorption peaks for PANI was observed. The possible interaction between Zn2+ of ZnO and NH-group of PANI was confirmed using Raman spectroscopy studies. The results reveal that the hybrid structures exhibit much higher sensitivity to NH3 gas at room temperature than pure ZnO, which is sensitive to NH3 gas at higher temperatures. This enhancement has been attributed due to the creation of active sites on the ZnO surface due to the presence of PANI. Y. H. Shin et al. [35] have grown the high-quality single-crystalline ZnO by using chemical vapor transport and the photoluminescence (PL) measurements were performed on as-grown, hydrogenated and hydro-genated and annealed n-type ZnO bulk samples in order to investigate the origins of their yellow and green emission bands. After hydrogenation, the defect-related peak at 2.10 eV was no longer present in the PL spectrum at room temperature, the peak intensity at 2.43 eV was unchanged and the intensity of the emission peak at 3.27 eV was strongly increased. These results indicate that the yellow band emission is due to oxygen vacancies because the emission peak at 2.10 eV disappears when these vacancies are passivated by hydrogen atoms. The emission peak at 2.43 eV originates from complexes between oxygen vacancies and other crystal defects. The peak at 3.274 eV is related to shallow donor impurities due to hydrogen donors. The quantum dots structure of ZnO prepared by wet chemical method has been reported by H. Zhou et al. [36]. By annealing treatment at 150 0C –500 0C, the effect of the change in the structure of the dots on their luminescence properties has also been studied. The surface of the as-prepared dots is passivated by a thin layer of Zn(OH)2 , thus, the dots consist of a ZnO/Zn(OH)2 core-shell structure. The weak excitonic transition of ZnO quantum dots is strongly correlated with the presence of the surface shell of Zn(OH)2. When Zn(OH)2 is present, the excitonic transition is quenched. The synthesis and characterization of n-type ZnO nanomaterial and its application as temperature sensor has been reported Richa Srivastava et al. [37]. The ZnO nanomaterial has been synthesized by flash heating the oxalate at 450 0C for 15 min. The oxalate produced by a conventional co-precipitation method is pressed in the form of pellet and then it is used as a sensing element. The variations in resistance of sensing pellet at different temperatures were recorded. The relative resistance was decreased linearly with increasing temperatures over the range, 120 0C – 260 0C. The activation energy of ZnO calculated from Arrhenius plot was found 1.12 eV. Temperature response in terms of the relative variation, ΔR, of sensor resistance to a given temperature was measured. Scanning electron micrograph of the sensing element has been studied. Pellet of the ZnO is comprised of nanorods of varying diameters and different lengths. Diameter of ZnO nanorods varies from 75 to 300 nm. X-ray diffraction pattern of the sensing element reveal their nano-crystalline nature. Optical characterization of the sensing material was carried out by UV-visible spectrophotometer. By UV-Vis spectra, the estimated value of band gap of ZnO was found 4.7 eV. 3.3 GENERAL METHODS OF SAMPLE PREPARATION (IN BULK) Bulk zinc oxide (ZnO) nanostructures have been synthesized using various approaches such as electrodepositions, oxidation process, chemical reduction, vacuumevaporation, hydrothermal method, vapour transport and already reported in literature.Some of these techniques have been discussed and described briefly below. 3.3.1 ELECTRODEPOSITION Electrochemical deposition is a very powerful technique for achieving uniform and large area synthesis of ZnO nanostructures, because it exerts a strong external driving force to make the reactions take place, even if they are non-spontaneous. The growth of ZnO nanostructures can occur on a general substrate, flat or curved, without any seeds, as long as the substrate is conductive. Also, under such an external electric field, better nanowire alignment and stronger adhesion to the substrate have been observed. The ZnO nanowire growth was observed at only the cathode of a D.C. power source, and at both electrodes for an A.C. power source. Most importantly, electrodeposition has been shown to be an effective way of doping ZnO nanowires by adding different ingredients into the reaction solution. For electrodeposition, a standard three-electrode setup is typically used, with a saturated Ag/AgCl electrode as the reference electrode and Pt as the counter-electrode. The anode, where growth usually takes place, is placed parallel to the cathode in the deposition solution. The electrical bias throughout the reaction system is controlled by a constant voltage source to maintain a constant driving force to the reaction, or by a constant current source to keep a constant reaction rate. Sheng Xu et al. used a ZnCl2 and KCl mixed solution electrolyte to grow vertically aligned ZnO nanowire arrays on a SnO2 glass substrate. During the growth, O2 was continuously bubbled through the solution in order to keep a relatively high level of O2 dissolved in the solution, which was necessary for the growth of high quality ZnO nanowires. Reduction of O2 at the cathode provides a source of OH?, which is required to coordinate with Zn2+ and then undergo dehydration to form ZnO. It was found that the dimensions of ZnO nanowires could be controlled from 25 to 80 nm by the varying the ZnCl2 concentration.3.3.2 OXIDATION PROCESS The simplest way to obtain ZnO consists in oxidizing a zinc sheet in an atmospherecontaining oxygen. Obviously it necessary to choose the temperature and the oxygen partial pressure of the oxidation in the region of stability of ZnO. Experimentally it was found that copper, like almost all metals, oxidizes in the presence of oxygen gas in the form of a uniform film of oxide. Two regimes are usually observed. The first one is when zinc is oxidized at pressure below the ZnO dissociation pressure in this case asingle layer of ZnO is formed over the zinc. 3.3.3 CHEMICAL REDUCTION In contrast to microemulsion systems, nanoparticles are synthesized in one phase in which the metal salt was initially dissolved. This method is a simple one-pot solution-phase method for synthesis of a variety of metal nanoparticles, including zinc nanocrystals. Zinc salt and reducing agent are injected in the same solvent in the presence of a stabilizer. The reaction temperature and additives are the factors affecting on the shape of zinc nanoparticles. 3.3.4 VACUUM VAPOR DEPOSITION (VVD) The Zn and Cd nanowires were prepared by evaporating Zn grains and Cd grains onto an Si substrate under vacuum without any catalyst. Commercial grains (Beijing Chemical Factory) of Zn or Cd had their oxide layer removed with dilute HCl solution, washed with ethanol, and dried under vacuum. The substrate was cleaned with ethanol and dried in air as well. The cleaned substrate was placed with the Zn or Cd grains in a pyrex glass tube, which was subsequently evacuated at room temperature, to a level of the order of 10?2 Torr. For deposition of Zn nanowires, the glass tube was heated rapidly in a tube furnace from room temperature to 300 0C at a temperature increasing rate of 14 0C min?1 and then to 350 0C at a rate of 10 0C min?1 and this temperature was maintained for 20 min. For deposition of Cd nanowires, the raised temperature program is from room temperature to 250 0C at a temperature increasing rate of 12 0C min?1 and then to 300 0C at a rate of 10 0C min?1 and this temperature was maintained for 20 min. For deposition of Cd nanowires, the raised temperature program is from room temperature to 250 0C at a temperature increasing rate of 12 0C min?1 and then to 300 0C at a rate of 10 0C min?1 and this temperature was maintained for 20 min.3.3.5 HYDRTHERMAL METHOD The ZnO nanoparticles by hydrothermal method, in this method the stock solutions of Zn(CH3COO)2.2H2O (0.1 M) was prepared in 50ml methanol under stirring. To this stock solution 25ml of NaOH (varying from 0.2 M to 0.5 M) solution prepared in methanol was added under continuous stirring in order to get the pH value of reactants between 8 and 11. These solutions was transferred into teflon lined sealed stainless steel autoclaves and maintained at various temperature in the range of 100 – 200 0C for 6 and 12 hrs under autogenous pressure. It was then allowed to cool naturally to room temperature. After the reaction was complete, the resulting white solid products were washed with methanol, filterednand then dried in air in a laboratory oven at 60 0C.3.3.6 VAPOR 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 0C. The material is transported to the cooler end of the tube, maintained at about 1100 0C, 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 using chlorine and carbon as transporting agents has been used to achieve ZnO crystal growth at moderate temperature of 950–1000 0C.3.3.7 MELT GROWTH Another method for producing bulk ZnO is that of melt growth. The melt method is based on a pressurized induction melting apparatus. 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 (r.f.) energy, induction heating. The r.f. energy produces joule heating until the ZnO is molten at about 1900 0C. Once the molten state is attained, the crucible is slowly lowered away from the heated zone to allow crystallization of the melt. PRESENT INVESTIGATION The present study focuses on the synthesis of pure and Sn-doped ZnO nanopowders by co-precipitation method and the effect of reaction temperatures, concentration of the precursors and time of growth on its properties. This synthesis method of doped ZnO powders has many advantages such as (1) powders with nanometer- size can be obtained by this method, (2) the reaction is carried out under moderate conditions, (3) powders with different morphologies by adjusting the reaction conditions and (4) the as-prepared powders have different properties from that of the bulk.3.4.1 SYNTHESIS/PREPARATION OF PURE AND Sn-DOPED ZINC OXIDE In the present investigation the pure and Sn-doped ZnO nanomaterials in bulk have been synthesized by co-precipitation method. All the chemical reagents used in present investigations were of analytical grade and used without any further purification. In a typical procedure, 0.1 M solution of Zinc acetate dehydrate (Merck purity > 98 %) [Zn(CH3COO2)?2H2O] was first dissolved in methanol and double distilled water in the volume ratio 3:1, respectively. Further appropriate wt. % of Tin chloride pentahydrate (SnCl4?5H2O, Sigma- Aldrich purity > 98 % ) was added into starting solution for tin doping with continuous stirring until a homogeneous solution with pH value of reactants between 8 and 10 was obtained. A few drops of acetic acid were added to improve the clarity of solution. The Sn/Zn ratio was kept 0, 5, 10 and 15 wt. %. The 1 M Sodium hydroxide (NaOH, Merck purity > 97 %) solution was dissolved in base precursor solution. The white precipitates were obtained and were then vigorously stirred at room temperature for 5 hour. This white precipitate was washed with double distilled water, filtered and dried at 300 0C for 10 hours in oven. The dried powder was thoroughly mixed and ground for at least two hours, then shaped into pellets (10 mm dia & 2mm thick) and finally sintered at 700 0C for 12 hours.3.4.2 STRUCTURAL STUDIES3.4.2 (a) Phase Identification and Determination of Lattice ParametersThe phase identification and lattice parameters of ZnO have been investigated over many decades. The lattice parameters of a semiconductor usually depend on the following factors: (i) free electron concentration acting via deformation potential of a conduction band minimum occupied by these electrons, (ii) concentration of foreign atoms and defects and their difference of ionic radii with respect to the substituted matrix ion, (iii) external strains (e.g., those induced by substrate), and (iv) temperature. The lattice parameters of any crystalline material are commonly and most accurately measured by high resolution X-ray diffraction (HRXRD) using the Bond method [31] for a set of symmetrical and asymmetrical reflections. For phase identification/gross structural characterization of the as synthesized zinc oxide material (in form of pellet), the most appropriate technique i.e. X-ray diffraction was employed. The X-ray diffraction was carried out through wide-angle Philips (X’ Pert PRO, Model PW 3040, at Indian Institute of Technology Kanpur (IIT-K)) powder diffractometer having CuKα radiation. The pellets were mounted by a cello tape on the specimen holder with the X-ray beam incident on the flat smooth surface of the pellet. Proper care was taken in mounting the sample, so that any error due to misorientation may not creep in the measurements. The diffractometer was first calibrated by monitoring theta (2θ) values from a standard silicon sample. The diffractograms were recorded with scan speed 20/minute (0.030/s) and step of (0.20) in range of 20-800. A large number of specimens, synthesized under different conditions were explored through XRD. Lattice constant and crystal structure have been usually measured by X–ray powder diffraction (XRD) using Cu Kα radiation in θ/2θ mode [38]. Figure 3.8 shows the representative powder x-ray diffraction patterns of pure ZnO and Sn-doped ZnO with various Sn doping concentration (5,10 & 15 wt.%). The presence of reflections such as (100), (002), (101), (102), (110), (103), (200), (112), (201) and (202) has been detected with considerable intensities. Quantitative analysis of powder x-ray diffraction patterns revealed that the as synthesized doped and undoped ZnO powder consists of pure single phase crystalline hexagonal wurtzite phase of Zinc oxide (JCPDS card no. 89 -1379) which belongs to the space group P63mc. No other reflection peaks from impurities, such as other oxides of Sn or Zn are detected, indicating high purity of the product of Sn-doped ZnO. The lattice parameters of Sn-doped ZnO have been calculated using high angle XRD lines such as (200), (112) and (201) shown in Figure 3.8. The variation of calculated lattice parameters of Sn-doped ZnO with dopant ratio of [Sn]/[Zn] equal to 0, 5, 10, 15 by wt.% are shown in Figure 3.9. A small decrease in the a-lattice parameter of the hexagonal unit cell has Figure 3.8 The x-ray diffraction (XRD) patterns of nanocrystalline pure and Sn doped ZnO powders prepared by co-precipitation method. Figure 3.9 The variation of lattice parameters, calculated from X-ray diffraction data of the pure and Sn-doped ZnO nanocrystalline materials. been observed with increasing Sn content. This may possibly occur due to the strong covalent bonding in a-b plane and difference in ionic radii of Zn+2(0.74 ?) and Sn+4(0.72 ?) ions. However, the c-lattice parameter increases due to the weaker covalent bonding along c-axis (since the c-parameter (4.592 ?) is larger than a-parameter (3.407 ?)) in a-c plane. It is also perceptible from the XRD peak (101) shown in Figure 3.10, that the undoped as well as doped Zinc oxide powder grows along the orientation of (101) with different crystallite size.3.4.2(b) Grain Size DeterminationThe crystallite size was calculated from x-ray diffraction data using the Debye-Scherrer formula; Dhkl = 0.9λ/β cosθ …………………………………… (3.2)where λ is the x-ray wavelength (1.5418 ? for CuKα), ? is the Bragg angle and β is the full width at half its maximum intensity (FWHM) of the most intense diffraction peak (101).The calculated crystallite size of doped ZnO as a function of Sn doping concentrations is shown in table 3.2. From the table it is observed that the crystallite size decreases with increasing Sn concentration in ZnO. The minimum crystallite size of 292 ? is found for 15 wt% Sn-doped ZnO. This is due to the lesser ionic radius of Sn+4 (RSn+4 3.4.2 (c) Surface Morphological StudiesThe scanning electron microscope is one of the most useful and versatile instruments for the investigation of surface topography, microstructutral features, etc. The principle involved in imaging is to make use of the scattered secondary electrons when a finely focused electron beam impinges on the surface of the specimen. The electrons are produced by a thermal emission source, such as heated tungsten filament, or by using field emission cathode. To create SEM image, the incident electron beam is scanned in a raster pattern across the sample surface. Secondary electrons are produced due to the interaction of the primary electron beam. The emitted electrons are detected at each position in the scanned area by an electron detector. Intensity of the emitted electron signal is displayed as brightness on a cathode ray tube. There are two modes of imaging: one is by using Secondary Electrons and the other is by using Backscattering Electrons. Secondary electron imaging provides high resolution imaging of fine surface morphology and for this, the samples must be electrically conductive. ZEISS (at IIT Kanpur) scanning Figure 3.10 XRD patterns of (101) plane of nanocrystalline pure and Sn doped ZnO powders prepared by co-precipitation method. Table 3.2 The variation of bandgap of the Pure and Sn-doped ZnO powder with different Sn concentration.Sn/Zn dopant concentration in ZnOAverage crystallite size D( ?) 0 wt% 307 5 wt% 304 10 wt. % 302 15 wt.% 292 electron Microscope was used for recording surface image of ZnO pellets in the present work. A very thin layer of gold was coated over the ZnO samples to obtain conductivity without significantly affecting surface morphology. The surface morphological examination with Field emission Scanning Electron Microscopy (FESEM) shown in figure 3.11 revealed the fact that the particles (group of grains) are closely packed and pores/voids between the grains are very few and pores/voids between the grains are decreases with increasing the Sn dopent concentration as shown in figure 3.11 (b) to 3.10 (d). It may also be noted that the particle sizes observed by FESEM (figure 4a-d) are higher as compared with that calculated from the XRD data. This is due the fact that the XRD technique provide the average mean crystallite size of grains/crystallites (single crystals) while FESEM shows the particles which are agglomeration of many grains. The XRD and FESEM data can be reconciled by the fact that smaller primary particles have a large surface free energy and would, therefore, tend to agglomeration faster and grow into larger grains. 3.4.2 (d) Structural / Microstructural Characterization Explored Through Transmission Electron Microscope (TEM)Like the other physical properties of solids e.g. mechanical, electrical, magnetic, thermal properties, surface reactivity etc., the ‘microstructural changes’ are found to significantly affect the physical properties too. The term ‘Microstructure’ refers to the assemblages of lattice defects in solids, which are commonly, classified as point, line, surface and volume defects. For developing new semiconductor oxides with improved properties, an understanding of microstructure-properties correlation is of fundamental importance. The structural features of semiconducting metal oxides (SMO’s) are especially important since the semiconducting properties are known to be crucially dependent on the microstructure, phase composition, phase transformation, order-disorder transition, etc. The X-ray and neutron diffraction techniques are widely used to examine the average i.e. gross structural features of semiconducting oxide and other related phases. However, these techniques are not useful in gauging the microstructural features of the materials. In the case of new semiconducting oxide materials, there are several features, which need yet another technique that can probe the material with regard to the local structures (up to about 50-100 ?). Transmission Electron Microscopy is capable of characterizing the Figure 3.11 Field Emission Scanning Electron Microscopic (FESEM) images of (a) pure ZnO and (b) 5% Sn-doped ZnO pellets, sintered at 7000C. Figure 3.10 Field Emission Scanning Electron Microscopic (FESEM) images of (c) 10% Sn doped ZnO and (d) 15% Sn doped ZnO pellets, sintered at 7000C.local structural characteristics for example local phases and can also provide information about the chemical composition of the local regions. This technique has been extensively applied to the study of all semiconducting oxide materials.In present investigations, we have used the transmission electron microscopic technique (Tecnai 20 G2 TEM, at CSR, Indore) widely to investigate the microstructural characteristics of the pure and doped ZnO nanomaterials. In this chapter, the results obtained on the Sn-doped ZnO have been described and discussed. Specimen Preparation TechniquesIn order to observe fine high-resolution images, it is necessary to prepare thin samples without introducing contamination or defects. For this purpose, it is important to select an appropriate specimen preparation method for each material, and to find an optimum condition for each method. There are various specimen preparation techniques for high-resolution transmission electron microscopy listed below.CrushingElectropolishingUltramicrotomyIon MillingFocused Ion Beam (FIB)Vacuum EvaporationOut of these techniques crushing method is used in present investigation. In this technique, a specimen is usually crushed with an agate mortar and agate pestle. The flakes, which are obtained, are suspended in organic solvent such as methanol (CH3OH), and dispersed with by stirring with a glass stick. Finally, the solvent containing the specimen flakes is dripped onto a microgrid (carbon coated and 200 mesh) on a filter paper. Since this is the simplest method, and it is also possible to find thin regions of a few nanometers thickness with little contamination on the surface, it is quite useful for high-resolution electron microscopy. However, since grain boundaries are rather fragile, it is usually difficult to observe them in specimens prepared by this method. In order to explore furthermore the structural/ microstructural of the pure and Sn-doped ZnO nanomaterial, the transmission electron microscopic (TEM) technique has been employed in both the imaging and diffraction modes. The TEM investigations reveals that the as synthesized bulk Sn-doped ZnO with increasing Sn-dopant concentration from 0 to 15% have many nanostructures of different shapes e.g. nanospheres (≈50 nm) and nanorods (≈100 to 200 nm). Typical transmission electron micrograph of a nanospheres & nanorods are shown in figures 3.11 (a, c, e, g) and their corresponding selected area electron diffraction (SAED) patterns are shown in figures 3.11 (b, d, f, h) respectively. The SAED patterns (circles) from nanorods and nanospheres have been indexed to a face hexagonal wurtzite system with lattice parameter a=b=0.3249 nm and c=0.5206 nm. These tallies quit well the lattice parameter of ZnO showing that the Sn occupy the Zn-sites in the lattice structure. It also seems that these nanoshperes and nanorods having the dimension of 70-150 nm, are evolved due to the agglomeration of the very small nanoparticles.3.4.3 OPTICAL PROPERTIESZinc oxide is generally transparent to visible light but strongly absorbs ultra violet light below 3655 A. The absorption is typically stronger than other white pigments. The optical absorption spectra of the as synthesized pure and Sn-doped ZnO powders with various Sn-dopant concentration (0 to 15 wt %) as a function of wavelength were recorded using dual beam ultraviolet–visible (UV–VIS-NIR) spectrometer (Cary 50) in the wavelength range between 200 and 800 nm at room temperature at IIT-Kanpur, India and are shown in figure 3.12. It is evident clearly from the figure 3.12 that the as synthesized nanomaterials have low absorbance in the visible/near infrared region while the absorbance is high in the ultraviolet region. The bandgap of pure and Sn-doped ZnO powder have been determined by ultraviolet (UV) absorption spectra. The results are in good agreement with the reports by other investigators [39-41]. The absorption coefficient ‘α’ was found to follow the Tauc relation [42-44]; α = Ao(h- Eg)n / h ..………….(3.3)where Ao is a constant which is related to the effective masses associated with the bands and Eg is the bandgap energy, h is the photon energy, α is the absorption coefficient, n is a constant which is 1/2 for direct bandgap material and n is 2 for indirect bandgap material. As our material was direct bandgap, So we put in equation (ii), n= ?; α = Ao(h- Eg)1/2 / h, ……………(3.4) 3067055080 Figure 3.11 (a) Transmission Electron micrographs (TEM) of pure zinc oxide showing the direct images of nanoparticles and (b) their corresponding selected area electron diffraction (SAED) patterns. 306705129540 Figure 3.11 (c) & (d) Transmission Electron micrographs (TEM) of 5% Sn- doped zinc oxide showing the direct images of nanoparticles and their corresponding selected area electron diffraction (SAED) patterns. 193675173990 Figure 3.11 (e) & (f) Transmission Electron micrographs (TEM) of 10% Sn- doped zinc oxide showing the direct images of nanoparticles and their corresponding selected area electron diffraction (SAED) patterns.193675264795 Figure 3.11 (g) Transmission Electron micrographs (TEM) of 15% Sn-doped zinc oxide showing the direct images of nanoparticles and (h) their corresponding selected area electron diffraction (SAED) patterns. Figure 3.12 The Optical Absorbance spectra of pure and Sn-doped ZnO powders The extrapolation of straight line to (αh)2 = 0 axis gives the value of energy of bandgap. Plots of (αh)2 vs. the photon energy h for powder of varying Sn-doping concentrations are shown in figure 3.13. Linearity of the plots indicates that the material is of direct bandgap nature. Extrapolation of linear portion of the graph to the energy axis at α= 0 gives the optical bandgap of about 3.35, 3.39, 3.49 & 3.42, at pure and 5 wt.% , 10 wt.% and 15 wt.% Sn-doped ZnO powder respectively shown in table 3.3. From it is clear that the optical band gap increased or shifted to higher energy (blue shift) with increasing Sn doping concentration. This blue-shift behavior can explain by the modification of the band structure, i.e., narrowing of both the valence and conduction bands. The change in band gap can be attributed due to the Burstein-Moss band gap widening and band gap narrowing due to electron-electron and electron-impurity scattering [45].3.4.4 ELECTRICAL PROPERTIES The fundamental study of the electrical properties of ZnO nanostructures is crucial for developing their future applications in nanoelectronics. In order to study the effect of Sn-doping on the conductivity as well as the conduction mechanism in the Zinc oxide semiconductor, the electrical resistivity of all pelletized pure and tin doped zinc oxide with different Sn contents were measured by collinear four probe method at room temperature and it is graphically reported as a function of the dopant (Sn) concentration in Figure 3.14. Pure ZnO has a very high resistivity of the order of 1.27x103 Ω cm. It is remarked that the resistivity of Sn-doped ZnO (SZO) decreased considerably as the tin concentration increased, with the sample containing 10 wt % Sn, showing the lowest resistivity of 2.86x101 Ω cm. However, with further increase of the dopant (Sn) concentration from 15 wt%, the resistivity started to increase significantly. Reports in the literature indicate that the most widely accepted explanation for this effect is that tin play the role of an effective donor in ZnO layers, when a small amount of Sn is introduced in the precursor solution of ZnO. It can be further explained by the substitution/introduction of Sn4+ into the Zn2+ sites, generating free electrons. 3.4.5 CONCLUSIONS The pure and tin doped zinc oxide were prepared by co-precipitation method. The analysis of x-ray diffraction patterns revealed that as synthesised doped and undoped Figure 3.13 Evolution of the (αhυ)2 vs. hυ curves of pure and Sn-doped ZnO powders prepared from 0.1 M Zn(CH3COO)2.2H2O. Table 3.3 The variation of Average crystallite size of the Pure and Sn- doped ZnO powder with different Sn concentration.Sn/Zn dopant concentration in ZnOBandgap (eV) 0 wt% 3.3500 5 wt% 3.3955 10 wt. % 3.4948 15 wt.% 3.4246ZnO materials are pure crystalline hexagonal wurtzite phase of Zinc oxide. However, a small decrease in the lattice parameters has been observed with increasing Sn content. This possibly occurs due to the difference in ionic radii of Zn+2(0.74 ?) and Sn+4(0.72 ?) ions. Surface morphological examination with FESEM revealed the fact that the grains are closely packed and pores between the grains are very few. The formation of ZnO nanoparticles / nanorods were also confirmed by transmission electron microscopy (TEM) and selected area electron diffraction (SAED) studies. The average particle size have been found to be about 70-150 nm. The optical bandgap of Sn-doped ZnO nanomaterials were obtained from optical absorption spectra by UV-Vis absorption spectroscopy. 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