Chapter 1 INTRODUCTION

Transcription

1 Chapter 1 INTRODUCTION Nanotechnology refers to a field related to the control of matter on an atomic and molecular scale. Generally, nanotechnology deals with structures of the size typically 100 nanometers or smaller, and involves developing materials or devices below that size. In the last two decades, an enormous progress has been reported in the synthesis and characterization of nanomaterials for device applications. Nanostructured materials frequently show behavior, which is intermediate between that of a macroscopic solid and that of an atomic or molecular system. Semiconductor nanostructures such as quantum wells, quantum dots, nanowires, nanotubes, nanobelts are attractive not only for their fundamental physical properties but also for their potential applications in quantum devices [1-4]. These materials are of great interest because they show unique properties related to their small size and large surface-to-volume ratio [5]. In particular, semiconductor nanostructures have potential applications in nanoelectronics [6], nanosensors [4] and nano-photonic [7] devices. 1.1 Semiconductor Nanostructures Semiconductor nanostructures have a small length scale in one, two or three dimensions with the optical, electrical and physical properties significantly different from their bulk counterpart. There have been increasing interests in the synthesis, characterization, and applications of such low-dimensional structures over the last decade. In general, they can be classified as zero dimensional, like quantum dots or nanoparticles; one dimensional like nanotubes, nanowires, nanobelts; and two dimensional like quantum wells. These nanostructures have drawn increasing attention over the last decade since they provide a means to create artificial potentials for electrons and holes in semiconductors, at length scales comparable to or smaller than the de Broglie wavelength. Modern epitaxial techniques have made possible atomic scale features in the growth or vertical direction, and the advent of nanolithography has made feasible the fabrication of sub-100 nm in 1

2 lateral dimensions. Using these quantum confinement characteristics, novel electronic and photonic devices with unique but tailored properties have become feasible with additional degrees of freedom in design. Semiconductor quantum dots are frequently used in light emitting diodes (UV to far IR regions) [8,9], photovoltaic applications [10,11], memory devices [12,13], tagging biological molecules or species [14,15] etc. Onedimensional nanostructures such as nanowires and nanotubes are attractive for semiconductor lasers [4,16], high mobility field effect transistor (FET) [17,18], nanosensors [19] etc. On the other hand, two-dimensional nanostructures have gained potential interest in quantum well infrared photo detectors [20] and quantum well lasers [21]. 1.2 Density of States of Low Dimensional Structures To calculate various optical properties such as the rate of absorption or emission and transport of carriers in low dimensional structures, one needs to know the number of available states per unit energy, i.e. density of states (DOS). The density of states describes the available electronic states in the bands and their dependence on energies. For the bulk case, the density of states in terms of energy is given by where respectively. 1 2m g( E) 3D de = 2 2 2π h 3 / 2 E 1 2 de, (1.1) m, h and E are the mass of electron, reduced Planck constant and electron energy, In two dimensional structures, such as the quantum well, the density of states is given by m g( E) 2 D de = de (1.2) 2 πh It is significant to note that the 2-D density of states does not depend on energy. Taking into account the other energy levels in a quantum well, the density of states can be expressed as a staircase like function given by [22]: m g( E) 2D de = H ( E Ei ) de 2, (1.3) π h i 2

3 where H E E ) is the Heaviside function. It takes the value of zero when E is less ( i than E and 1, when E is equal to or greater than E. E is the i -th energy level within i the quantum well. For one dimensional structure, the density of states per unit volume at energy E is given by 1 m ( E) 1 de = 2 π h 1/ 2 1 1/ E g D 2 Using more than the first energy level, the density of state function becomes 1/ 2 i de i (1.4) 1 2m ni H ( E Ei ) g( E) 1D de = de, (1.5) 2 i E 1/ 2 π h ( E ) where is the degeneracy factor. For quantum structures with dimension lower than 2, n i it is possible for the same energy level to occur for more than one arrangement of confined states. To account for this, a second factor i n i (E) is introduced. In a 0-D structure, all the available states exist only at discrete energies described and can be represented by a delta function. In real quantum dots, however, the size distribution leads to a broadening of this line function. g( E) 3D DOS g( E) 1D DOS BULK Energy ( E ) QUANTUM WIRE Energy ( E ) g( E) 2D DOS g( E) 0D DOS QUANTUM WELL Energy ( E ) QUANTUM DOT Energy ( E ) Figure 1.1: Density of states vs energy for a bulk material (3D), quantum well (2D), quantum wire (1D) and quantum dot (0D). 3

4 The plot of density of states with energy for bulk (3D), two-dimensional (2D), onedimensional (1D) and zero-dimensional (0D) systems is schematically shown in figure Excitonic Bohr Radius: Strong and Weak Confinement Excitonic Bohr radius is a useful parameter to study the quantum confinement in semiconductor nanostructures. When a hole remains localized at the lattice site and a detached electron remains in its neighborhood, they will be under Coulombic interaction and can become bound to form a hydrogen-like atom. An exciton results from the binding of the electron with the hole, having slightly less energy than the unbound electron and hole. The excitonic Bohr radius ( a a B B ) of a material can be expressed as 2 h ε 1 1 = + 2 * * πe me mh where ε is the permittivity of the material, m * e and (1.6) m * h are the effective mass of electron and hole, respectively. For many semiconductors, the masses of the electron and hole are generally in the range 0.1 to 3, where m is the free electron mass. For m0 m0 0 example, the excitonic Bohr radius of CdS, GaAs and Ge is estimated to be ~ 3 nm, 14.7 nm and 24.3 nm, respectively [23-25]. When the size of a semiconductor nanoparticle approaches to exciton Bohr radius, the quantum confinement plays a dominant role. Two limiting region of confinement can be identified on the basis of the ratio of the dimension of the nanostructure (D) to the exciton Bohr radius, namely a weak-confinement regime with D a B (but not D ab ) and a strong confinement regime for D ab. An extended limit D a B corresponds to no confinement. Under weak-confinement condition the exciton can undergo unrestricted translational motion, just as in the bulk material, but for strong confinement this translational motion becomes restricted. There is an increase in the spatial overlap of the electron and hole wave function with decreasing particle size resulting in enhanced electron-hole interaction. As a result, the energy splitting becomes greater between the radiative and nonradiative exciton states. An optical index of the confinement is the blue shift (shift to higher energies) of the optical absorption edge and the exciton energy with decreasing nanoparticle size. a B 4

5 1.4 Properties and Applications of II-VI Semiconductors II-VI semiconductors are based on one element from group- II and another from group- VI of periodic table, each type being bonded to four nearest neighbors of the other type. The increased amount of charge from group-vi to group-ii atoms tends to cause the bonding to be more ionic than in the case of III-V semiconductors. II-VI semiconductors can be created in ternary and quaternary forms, much like the III-V semiconductors. Representative II-VI semiconductors are ZnO, ZnS, ZnSe, ZnTe; CdS, CdSe, CdTe and HgS, HgSe, HgTe etc. In all II-VI semiconductors, radiative recombination take place due to their direct band gap nature. Most of the II-VI semiconductors have moderate to wide optical band gap in the range 1.5 ev to 3.6 ev. However, HgSe and HgTe are narrow band gap semiconductors and behave like semimetal [26]. Crystal structure, optical band gap, dielectric constant, effective mass and carrier mobility of important II- VI semiconductors are listed in the table-1. Table-1: Different parameters for II-VI semiconductors at 300 K [Ref. 27]. II-VI semi cond uctor s Crystal structure Bulk band gap E (ev) g Dielectric constant (static) ε r 0 Effective mass Mobility Electron Hole Electron Hole ( m ) ( m0 ) ( μ n ) ( μ p ) cm 2 /V.s cm 2 /V.s ZnS Zincblende Wurtzite ZnO Wurtzite ZnSe Zincblende ZnTe Zincblende CdS Zincblende Wurtzite CdSe Wurtzite CdTe Zincblende

6 II-VI semiconductors are unipolar in nature i.e. they are either p-type or n-type. They are attractive due to their size tunable optical absorption and emission properties in a large spectral regime [28,29]. They have attracted much attention due to nanoscale material synthesis in different shapes and sizes using bottom-up approach and their interesting optical and electrical behavior. Figure 1.2 shows the reported optical absorption and emission spectra of CdSe quantum dots of different diameters. It is observed that the whole visible spectrum (400 nm-700 nm) can be absorbed by simply varying the Q-dot diameter, which is useful for applications in optical devices. Figure 1.2: Room temperature absorption and photoluminescence spectra of CdSe quantum dots. [After C. B. Murray, Ref. 30] Yan et al. [31] reported self-organized, highly ordered comb-like single crystalline ZnO nanostructures with diameter ranging from 10 nm to 300 nm, using a chemical vapor transport and condensation system. Under optical excitation, each individual ZnO nanowire served as a Fabry-Perot optical cavity, and together they acted as a highly ordered nanowire ultraviolet laser array. The lasing behavior was evident by the appearance of sharp cavity modes. 6

7 CdS and CdSe nanostructures have plenty of applications in several optical devices. CdSe nanostructures have been found to be attractive for hybrid [32] and inorganic heterojunction [33] based solar cells. (a) 40 nm 40 nm (c) Figure 1.3: TEM micrographs of (a) CdSe and (b) CdTe nanocrystals used for solar cells. (c) Normalized photoaction spectrum of a typical bilayer device. (d) I-V characteristics of the device with AM1.5G power conversion efficiency 2.9%. [After Gur et al., Ref. 33] (b) (d) Gur et al. [33] fabricated air stable all inorganic heterojunction solar cells by sequentially spin-casting films of CdTe and CdSe on indium tin oxide (ITO) glass coated with alumina. Figures 1.3(a) and (b) show transmission electron micrographs (TEM) of CdSe and CdTe nanocrystals used for device fabrication. A typical photoaction spectrum (Figure 1.3c) showed external quantum efficiency approaching 70%. Figure 1.3(d) represents I-V characteristics of an optimized device, which employed a Ca top contact capped with Al. Under simulated illumination, the cell exhibited an AM1.5G power conversion efficiency of 2.9%, with short circuit current (Isc) 13.2 ma/cm 2, open circuit 7

8 voltage (V oc ) 0.45 V, and fill factor (FF) CdSe quantum dots have also prospective application in LED with different colors [34]. CdS nanostructures exhibited potential applications in photodetector [35], CdTe based solar cell [36], light emitting diodes, laser etc. ZnS, ZnSe, ZnTe nanostructures have several applications in optical devices. 1.5 Properties of Bulk Cadmium Sulfide (a) Figure 1.4: Schematic model of crystal structure of bulk CdS (a) hawleyite (cubic) and (b) greenockite (hexagonal) structure [37,38]. Cadmium sulfide is a direct band gap semiconductor with bulk optical band gap ~2.42 ev in the visible region. It possesses two types of crystal structures (i) cubic zinc blende and (ii) hexagonal wurtzite. The schematic models of crystal structure of bulk CdS are shown in the figure 1.4. The effective mass of electron ( ) and holes ( ) in CdS is 0.21 m and 0.80 m, respectively [27]. The electron and hole mobility of bulk CdS is μe ~ * m e (b) * mh 0 cm 2 /V.s and μ ~ 15 cm 2 /V.s, respectively. CdS is basically a unipolar semiconductor h with n-type conductivity due to sulfur deficiency. The energy band diagram (E vs k) of bulk CdS in direction parallel and perpendicular to the c-axis is depicted in figure 1.5. The energy gap and valance band splitting energies measured at ~0 K are indicated. 8

9 k (10 7 cm -1 ) Figure 1.5: Energy band diagram of bulk CdS. [After D. Long, Ref. 39] 1.6 A Brief Review of Work on CdS Nanostructures D Nanostructures Zero dimensional CdS nanostructures, called quantum dots (Q-dots), are usually synthesized in colloidal solution routes or grown on solid substrates. In the colloidal approach, precursors of the material are reacted in the presence of a stabilizing agent that restricts the growth of nucleated particles to keep it within the excitonic Bohr radius estimated by equation 1.6. There are plenty of reports on the growth of CdS Q-dots or nanocrystals. An aqueous Cd (II) salts can be mixed with anionic or Lewis basic polymers such as sodium polyphosphate or polyamines [40 42], and the subsequent addition of a sulfide source produces CdS nanoparticles that are in the size range of 1 nm to 10 nm. Size tuning is possible by controlling relative concentrations and the rate of addition. By varying the crystallite size, the optical band gap of CdS quantum dots can be tuned in a broad spectral energy range from E g ~ 2.4 ev to 4 ev at room temperature [43]. A simplest three-dimensional confinement model based on effective-mass approximation predicts the energy shift of CdS quantum dots, Δ E as [44] 9

10 2 2 h π 1 ΔE = 2 2R m * e 1 + m * h 1.786e εr E * RY, (1.7) where R is the cluster radius, and E 4 = e 1 + is the effective Rydberg 2 2 * 2ε h m * e mh * 1 RY energy. Although the effective-mass approximation theory predicts the variation of optical band gap with crystallite size, the tight-binding approximation closely matches with the experimental data [45]. Figure 1.6 shows a comparison of the experimental data with theoretical models for the optical band gap of small CdS clusters as a function of cluster diameter. Figure 1.6: Variation of exciton transition energy with cluster diameter of CdS. [After Wang et al., Ref. 46] Figure 1.7: PL spectra of 4.5 nm CdS quantum dots. [After Spanhel et al., Ref. 40] The onset of light absorption for quantum dots varies as a function of size, as described earlier. Ideally, the corresponding light emission from quantum dots should follow the trend shown in figure 1.2. However, the nature of the quantum dot surface is critical for photoluminescence measurements. Figure 1.7 shows the emission spectra of unpassivated CdS QDs of diameter 4.5 nm, stabilized by sodium polyphosphate in an aqua solution of M cadmium [40]. The bottom purple spectrum is as-prepared, while the rest of the spectra are for samples activated with increasing Cd(II) in basic solution. It can be observed that the emission spectra radically change by an activation step using additional Cd(II) in the basic aqueous solution without changing the particle size. Presumably, the 10

11 trap states are either being filled or energetically move closer to the band edges by this simple chemical treatment. This kind of behavior is useful for chemical sensing with quantum dots. There are some reports of chemical sensing of small molecules and ions with quantum dots via analyte-induced changes in photoluminescence [47 49]. Particle size and surface treatment can yield a range of emission colors easily visible to the naked eye. The influence of the surface on photoluminescence can be understood in terms of the trap states described in figure 1.8. These trap states are caused by defects, such as vacancies, local lattice mismatches, dangling bonds at the surface. Figure 1.8: Schematic energy band diagram of CdS bulk and nanostructures. [After L. Brus, Ref. 28] The excited electrons or holes can be trapped by these defect states with reduced radiative recombination of carriers. For quantum dots, the surface passivation has most frequently been achieved by coating with a higher band gap semiconductor quantum shell [50-52]. Rossetti et al. [53] synthesized small CdS crystallites (~3 nm) exhibiting a large blue shift (0.8 ev) in absorption edge, which was explained by the effect of quantum confinement of electrons and holes in a small volume. Sapra et al. [54] synthesized water-soluble 11

12 cysteine ester passivated CdS nanocrystals of average size 2.0 nm by one-pot solution phase technique. This passivation quenched the deep trapped surface states and caused the nanoclusters to emit in the blue region, which otherwise emitted in the yellow orange region. The result established the possibility of using CdS nanoclusters as fluorescent biological probes. Nakamura et al. [55] observed bright electroluminescence from selfassembled CdS quantum dots introduced into the p-n junction of ZnSe and the emission peak wavelength was tuned by altering the dimension D Nanostructures CdS 1D nanostructures, like nanowires, nanorods, nanotubes, nanobelts and nanoribbons can be synthesized using both chemical as well as physical process. Among the chemical methods, template-assisted growth, solvothermal and hydrothermal synthesis are attractive for nanowire and nanotube fabrication. Template assisted growth is one of the convenient and low cost processes for nanowire and nanotube synthesis with controlled length and variable diameter. Porous alumina templates have been frequently used for the growth of 1D CdS nanostructures due to their well-arranged, hexagonal uniform porous structures. Porous alumina has numerous advantages over other templates, due to its insulating nature, hardness and optical transparency in visible region (band gap of alumina ~ 6 ev to 8 ev). Routkevitch et al. [56] synthesized CdS nanowires in porous alumina templates with length up to 1 μm and diameter as small as 9 nm by an ac electrochemical deposition process. Xu et al. [57] fabricated single crystalline hexagonal CdS nanowires of uniform diameter with size 10 nm-100 nm. There are several reports for the growth of CdS nanowires and nanotubes on alumina templates using chemical bath deposition process (CBD) [58,59], microwave irradiation method [60], sol-gel process [61] and chemical vapor deposition (CVD) [62] etc. However, high purity, single crystalline one dimensional CdS nanostructures, such as nanowires, nanobelts and nanoribbon can be fabricated in a physical process by the evaporation of CdS powder or organometallic precursors. Vapor-liquid-solid (VLS) and vapor-solid (VS) methods have also been proposed for the growth of 1D nanostructures [63-65]. In the methods, the nanowire diameter, length and arrangement on a substrate are governed by the size of used metal catalysts (such as Au). Barrelet et al. [66] reported the growth of CdS 12

13 nanowires by a nanocluster-catalyzed vapor-liquid-solid mechanism using a single-source molecular precursor Cd(S2CNEt 2) 2 and fabricated high yield single-crystal nanowires with controlled diameters (10 nm-20 nm) and excellent optical properties. Figure 1.9 shows the typical high resolution TEM micrograph of a CdS nanowire of diameter 9 nm synthesized by nanocluster-catalyzed vapor-liquid-solid process. Gao et al. [67] synthesized CdS nanobelts with rectangular cross section by evaporating CdS powder. Figure 1.9: Typical TEM image of single crystalline CdS nanowires of diameter 9 nm. [After Barrelet et al., Ref. 66] Figure 1.10: SEM micrograph of CdS nanobelts of thickness 30 nm-60 nm. [After Gao et al., Ref. 67] Figure 1.10 shows scanning electron microscopy (SEM) image of CdS nanobelts of thickness 30 nm-60 nm with several micrometers length grown on Au coated Si substrate at 650 C for 1 hour. Optical and electrical properties of CdS 1D nanostructures were reported by several researchers. Hoang et al. [68] studied temperature dependent PL for several single CdS nanowires from 5 K to 295 K using microphotoluminescence spectroscopy measurement. At low temperatures, both near band edge (NBE) and spatially localized defect-related PL were observed in many nanowires. Figure 1.11 shows the temperature dependent PL spectra of a single nanowire. As the temperature was raised, the defect states rapidly quenched at varying rates leaving the NBE PL which dominated up to room temperature. All PL lines from the nanowires closely followed the temperature dependent band edge, similar to those observed in bulk CdS. 13

14 (a) Figure 1.11: Temperature dependent PL spectra of a single CdS nanowire. [After Hoang et al., Ref. 68] (b) 5 μm Figure 1.12: (a) Schematic showing the cross-section of CdS nanowire device structure. (b) Optical micrograph of the device (top) and electroluminescence image from the device at room temperature (bottom) (c) Emission intensity vs injection current. [After Duan et al., Ref. 4] Duan et al. [4] fabricated electrically driven laser using CdS nanowires of diameter 80 nm-200 nm. Devices were fabricated by assembling CdS nanowires on heavily doped p-si on insulator substrates (> cm -3 ; 500 nm thick), followed by electron-beam lithography and electron-beam evaporation of 60 nm 80 nm aluminum oxide, 40 nm Ti and 200 nm Au. One end of the nanowire was left uncovered for emission output from the device. The schematic cross section of the nanowire device is (c) 14

15 shown in figure 1.12(a). The micrograph and electroluminescence image of the nanowire laser are shown in figure 1.12 (b). Figure 1.12 (c) shows the emission intensity vs injection current of the nanowire laser. The intensity increased rapidly above about 200 ma, which corresponds to the onset of lasing. (a) Figure 1.13: (a) Current density vs voltage characteristics of CdS photovoltaic device under dark (black curve) and illumination (red curve). (b) Spectral response measurement showing an external quantum efficiency approaching 40%. [After Wu et al., Ref. 69] Wu et al. [69] demonstrated solution-processed all inorganic solar cell making a heterojunction between copper sulfide nanocrystals and CdS nanorods. The device was fabricated on ITO coated glass as well as plastic substrates. The best device performance was achieved up to 1.6% power conversion efficiency with an incident radiation intensity 100 mw/cm 2, at 25 o C, and AM = 1.5 G (Figure 1.13) D Nanostructures 2-D CdS nanostructures such as quantum well (QW) can be fabricated using both physical and chemical process. Hetterich et al. [70] had grown highly strained ultrathin CdS quantum wells embedded in a ZnS matrix using molecular-beam epitaxy (MBE). Eychmuller et al. [71] synthesized quantum dots in a quantum well (QDQW) using a three-layered structure consisting of size-quantized CdS particles acting as the core and a complete layer of HgS on the surface of the core and again CdS as the outermost shell. In this structure, colloidal QDQW nanocrystals have a lower band gap layer (eg. HgS) (b) 15

16 sandwiched between a higher band gap core (CdS) and an outer CdS shell. The spherical QDQW nanostructure combined the promising characteristics of both two-dimensional planar QW and zero-dimensional colloidal QDs. Xu et al. [72] observed that such colloidal QDQWs can be used as a strong gain medium for observing lasing action, and the threshold current density was much lower than those in other laser systems. 1.7 Polymer Nanocomposites for Hybrid Devices The surface properties of semiconductor nanostructures play a crucial role in optical emission. A lot of experiments had been carried out to synthesize surface passivated nanostructures using a core-shell structure or encapsulated with some host materials. Polymer nanocomposites are most attractive in this respect. Polymer nanocomposites can be fabricated by either in situ chemical synthesis in polymer matrix or dispersing nanomaterials in a suitable polymer. Such hybrid nanostructures have attracted a lot of research interests due to their enhanced stability, as polymer matrix prevents both oxidation and coalescence. Sun et al. [73] demonstrated a strategy for the in situ synthesis of CdS polymer nanocomposites. They patterned 2D and 3D micro/nanostructures of CdS polymer nanocomposites, combining photopolymerization via a laser four-beam interference technique with in situ synthesis of CdS nanoparticles in the patterned polymer matrix. The morphology and size dependent optical properties of CdS nanoparticles in polymer matrices were investigated. Woggon et al. [74] synthesized CdS nanocrystals in polyvinyl alcohol matrix in a weak confinement regime. The estimated radius of the nanoparticles was found to be nm, where a is the Bohr radius in CdS. 7a B B The electro-optical properties of CdS embedded in PVA matrix were also investigated. Semiconductor nanocomposites-based devices have several advantages for application over fully organic-based ones due to their improved long-term stability. The other distinct feature of the polymer nanocomposites is the extremely high interfacial area between the nanocomposites and the polymer matrix, which can be useful in sensor applications. Semiconductor nanocomposites have major applications in hybrid solar cell, light emitting diodes and nanocrystals memory devices. The bottleneck of organic solar cells is the extremely low electron mobilities. The presence of a second material in hybrid nanostructure provides an interface for charge transfer. However, charge transfer rates 16

17 can be remarkably fast in case of organics that are chemically bound to semiconductor nanocrystals. An investigation on CdS-polymer nanocomposites for hybrid solar cell was carried out by Wang et al. [75]. The photovoltaic device was fabricated by suitable dispersion of multi-armed CdS nanorods in a conjugated polymer poly[2-methoxy-5-(2- ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) solution. The best device performance was achieved with a power conversion efficiency of 1.17% under AM 1.5 illumination. (a) Figure 1.14: Solar cell characteristics of CdSe nanocrystals in polymer matrix (a) EQEs of 7 nm diameter nanorods with lengths 7 nm, 30 nm, and 60 nm, with intensity mw/cm 2 at 515 nm. (b) Solar cell characteristics of 7 nm by 60 nm nanorod. [After Huynh et al., Ref. 32] Huynh et al. [32] demonstrated that CdSe nanorods can be used to fabricate readily processed and efficient hybrid solar cells together with polymers. The optical band gap of CdSe was tuned by altering the nanorod radius to optimize the overlap between the absorption spectrum of the cell and the solar emission spectrum. In a typical device structure, CdSe nanorods of diameter 7 nm and length 60 nm and conjugated polymer poly-3(hexylthiophene) were used. The external quantum efficiency of over 54% and power conversion efficiency of 6.9% under 0.1 mw/cm 2 illumination at 515 nm were demonstrated (Figure 1.14). Under 1.5 A.M. solar condition, 1.7% power conversion efficiency was obtained. The potential use of pure polymer light-emitting diodes is also ultimately limited by their low quantum efficiency as well as by their poor stability due to (b) 17

18 oxygen. In this respect, polymer nanocomposite based LEDs have several advantages. Sun et al. [76] observed pure white-light emission from CdTe nanocrystal polymer composites. Three-dimensionally confined nanoparticles embedded in organic layers were investigated extensively for their promising applications in nonvolatile flash memory devices with nanoscale floating gates. Li et al. [77] observed the memory effect in capacitors consisting of a blend of core/shell-type CdSe/ZnS nanoparticles and a conducting polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) sandwiched between a metal electrode and indium tin oxide coated glass substrates. 1.8 CdS Nanostructures on 1D Templates An attractive approach for nanostructure fabrication is the organization of functional semiconductor nanostructures on one dimensional templates such as nanowire, nanotube and nanobelt surfaces. Such one dimensional heterojunction nanostructures have attracted substantial research efforts for their possible application in photoelectrochemical cells [78], light-emitting diodes [79], electrochromic devices [80] and sensor systems [81]. Hayden et al. [82] fabricated n-type CdS shell on p-type silicon nanowires by pulsed laser deposition for LED application. Cao et al. [83] reported a simple room-temperature chemical reduction route to coat multi-walled CNTs with a uniform layer of CdS. They investigated the electronic properties of composite CNT/CdS core-shell nanowires by surface photovoltage spectroscopy. Lee et al. [84] synthesized CdS quantum dots on vertically aligned ZnO nanorod surfaces using a chemical bath deposition method for semiconductor sensitized solar cell application. Gao et al. [85] demonstrated enhanced ethanol-sensing behavior of CdS nanoparticle grown on SnO 2 nanobelt surfaces via a simple sonochemical approach. Thus the study of semiconductor nanostructure heterojunctions are attractive for both fundamental aspects as well as nanodevice applications. 18

19 1.9 Organization of the Thesis From the foregoing discussions, it is clear that the study of CdS nanostructures is a very promising field of research for their applications in nano-scale electronic and optical devices. More studies on the various aspects of synthesis, characteristics and device applications of CdS nanostructures are needed for fundamental and applied research. The present dissertation contains six chapters with a focus on some of the above issues of CdS nanostructures. Chapter-2 reports the fabrication of hexagonal well-arranged porous alumina templates of pore diameter varying from 22 nm- 175 nm. The growth of CdS nanowires in porous alumina templates using a two-cell chemical and electrochemical methods have been described in details. The structural and optical properties of grown nanowires have been investigated. Temperature dependent growth and characteristics of junction-like CdS nanostructures in polyvinyl alcohol (PVA) matrix are presented in chapter-3. CdS nanostructures have been deposited on p-si and ITO-coated glass substrates at 70 o C, 80 o C, 85 o C and 90 o C by a chemical bath deposition process using CdCl 2, and (NH 2) 2CS as precursors. CdS-PVA nanocomposites films have been characterized in details using XRD, Raman and XPS analyses. The optical absorption and emission properties of nanocomposites samples have also been studied. The charge storage behavior of CdS- PVA nanocomposites/conducting polymer poly [2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylene-vinylene] heterostructure has exhibited memory characteristics in a floating gate structure. Negative resistance behavior in current-voltage characteristics due to tunneling and superior dielectric properties of nanocomposites are discussed in this chapter. Chapter-4 reports the synthesis of quantum confined CdS nanoparticles on multiwalled carbon nanotube (MWCNT) surfaces. Optoelectronic properties of these nanostructures have been investigated. The dielectric property of MWCNT-CdS nanostructures embedded in PVA matrix is discussed. The dc and ac conduction characteristics of MWCNT-CdS/PVA nanocomposites are explained by the variable range hopping and random free-energy barrier models. 19

20 Chapter-5 deals with the growth of CdS nanostructures on Ge nanowire templates. The core-shell Ge/CdS radial nanowire heterostructures have been fabricated by a combination of VLS and chemical bath process. The structural properties have been investigated using XRD and Raman analyses. I-V characteristics and photosensing behaviors of Ge/CdS heterojunction devices are discussed in this chapter. Chapter-6 summarizes the conclusion of the present study and recommendations for future investigations Contributions of the Thesis The study in this dissertation has demonstrated the potential of one-dimensional CdS nanostructures and nanocomposites for their possible applications in charge storage, nano-capacitor, photovoltaic and photodetector devices. 20