Synthesis and Reactivity in Inorganic, Metal-Organic

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Cadmium Sulfide One-Dimensional Nanostructures: Synthesis, Characterization and Application

Soumitra Kar a; Subhadra Chaudhuri a a Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, India Online Publication Date: 01 February 2006 To cite this Article: Kar, Soumitra and Chaudhuri, Subhadra (2006) 'Cadmium Sulfide One-Dimensional Nanostructures: Synthesis, Characterization and Application', Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 36:3, 289 - 312 To link to this article: DOI: 10.1080/15533170600596055 URL: http://dx.doi.org/10.1080/15533170600596055

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Synthesis and Reactivity in Inorganic, Metal-Organic and Nano-Metal Chemistry, 36:289–312, 2006 Copyright # 2006 Taylor & Francis Group, LLC ISSN: 0094-5714 print/1532-2440 online DOI: 10.1080/15533170600596055

Cadmium Sulfide One-Dimensional Nanostructures: Synthesis, Characterization and Application Soumitra Kar and Subhadra Chaudhuri Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, India

With the current need for the advancement of the nanotechnology and nanoscience, it is a challenge for the scientific community to develop technologically important nanomaterials with tailored size and shape with a view to enhance or tune their fundamental physical and chemical properties, which could be useful for the fabrication of nano-scale devices. Cadmium sulfide is a widely studied material for its wide range of applications in field of photonics, optoelectronics and photovoltaics etc. Keeping in mind the recent interest of the 1-D nanomaterials this article reviews different aspects of the synthesis, characterization and application of CdS 1-D nanomaterials. Different synthesis techniques ranging from simple wet chemical route, solvothermal routes, catalytic and non-catalytic thermal evaporation routes etc. are discussed in detail for the fabrication of various kinds of 1-D CdS nanostructures. Fundamental properties such as optical absorption, photoluminescence, Raman spectroscopic studies, electrical properties, and photoconducting properties are also discussed here. Applicability of these various 1-D nanoforms of CdS in the fields of laser, thin film transistor, photonic circuit element, etc. are discussed. Keywords

cadmium sulfide, one-dimensional, nanostructures, solvothermal process, VLS technique, VS technique, photoluminescence, Raman spectra, photoconductivity, laser

INTRODUCTION CdS is an important II-VI semiconducting material used for different applications in optoelectronics such as nonlinear optics, flat panel displays, light emitting diodes, lasers, thin film transistors, etc.[1 – 7] Bulk CdS has a direct band gap of 2.4 eV at 300 K; it is possible to obtain bulk CdS in both wurtzite and zinc blende structures with the wurtzite phase being more stable. For bulk CdS, the excitonic binding energy is around 28 meV. Consequently, the excitons are stable only at a low temperature.[8] It finds wide application

Received 15 January 2006; accepted 18 January 2006. Address correspondence to Subhadra Chaudhuri, Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700 032, India. E-mail: [email protected] or [email protected]

as a commercial photovoltaic material, such as the window material in heterojunction solar cells.[9,10] In recent years, nanocrystalline CdS attracted much attention because of their properties in the nanoforms differ significantly from those of their bulk counterparts.[11] Nanoparticles of CdS are by far the most studied system among all the semiconducting nanoparticles.[12] The typical Bohr exciton diameter of CdS is around 5.8 nm; consequently, CdS nanoparticles in the size range of 1 –6 nm show sizable quantum confinement effects.[13] On the other hand, when crystallite radius becomes comparable to or less than the Bohr exciton radius, there is a considerable enhancement of the exciton binding energy.[14] Since the first discovery of carbon nanotubes, one-dimensional (1-D) semiconductor materials such as nanorods, nanowires, nanotubes and nanobelts/nanoribbons have attracted extensive interest because of their fundamental physical, chemical, optical, electrical and magnetic properties, which are distinct from their bulk counterparts, and also due to their potential applications in nano-scale devices.[15 – 32] It is well known that 1-D nanostructures can play an important role both as interconnect and functional units in fabricating electronic, optoelectronic, electrochemical and electromechanical devices with nanoscale dimension. A report has recently been published claiming that “Nanowires, nanorods, nanowhiskers, it does not mater what you call them, they are the hottest property in nanotechnology” (Nature 419 (2002) 553). In general, nanotechnology can be understood as a technology of design, fabrication and applications of nanostructures and nanomaterials. Nanotechnology also includes fundamental understanding of the physical properties and phenomena of nanomaterials and nanostructures. Studies of fundamental relationships between physical properties and materials dimensions in the nanometer scale are of much importance in nanoscience. Thus, in order to explore the novel physical properties and realize potential applications of nanostructures, the ability to fabricate and tune the size, shape of the nanomaterials is the first corner stone in nanotechnology. Thus keeping in mind the recent interest of nanotechnology and the importance of CdS in different electronic and

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optoelectronic applications, different aspects of the fabrication, basic characterization and applications of one-dimensional (1D) CdS nanostructures have been discussed in this review article. SYNTHESIS OF 1-D CdS NANOFORMS The basic concept behind the 1-D nanostructures formation is the controlled and restricted crystal growth. Crystal growth means, the evolution of a solid from a vapor, liquid or solid phase resulting into nucleation and subsequently resulting into crystal growth. When the concentration of the elementary species or building blocks of a solid becomes sufficient, they assemble together to form a small cluster, which act as the nucleation center or seed. For the successful fabrication of 1-D nanoforms the building blocks need to be supplied at a controlled rate so that the resulting crystal have a homogeneous composition and uniform morphology. The basic need for the growth of the 1-D nanoforms is to prevent the growth of the seeds in two dimensions. This could be achieved by using various templates, use of appropriate capping agents to kinetically control the growth rates of various facets of the seed, use of supersaturation condensation to modify the growth habit of the seed, introducing a liquid-solid interface to reduce the symmetry of a seed etc.[27] A variety of synthesis techniques, such as, template-directed synthesis,[33 – 35] vapor-solid growth,[36 – 38] vapor-liquid-solid (VLS) growth,[28,39 – 41] solution-liquid-solid (SLS) growth,[42] solvothermal process[43 – 46] were reported so far to synthesize various metals,[47 – 50] semiconducting oxides,[38,51 – 55] [56 – 59] [60 – 68] nitrides and sulfides in different 1-D nanoforms. Solvothermal process has been employed[44,69 – 73] by several groups to fabricate CdS nanoparticles and nanorods. CdS nanowires were prepared by template assisted electrochemical routes[33,35] and sol-gel technique,[34] polymer assisted solvothermal process,[74] vapor-solid (VS) method,[63] metal catalyzed vapor-liquid-solid (VLS) techniques,[62,75] and so forth. Synthesis of CdS Nanorods by Solvothermal Process Solvothermal synthesis utilizes a solvent under pressure and temperature above its critical point to increase the solubility of the solid reactants and speed up the reactions. In a typical procedure, the precursors and a solvent were placed in an autoclave, which was then placed at an elevated temperature and pressure to speed up the reaction to generate nanoforms. The major advantage of this approach is that most of the materials can be made soluble in a solvent by heating and pressurizing it close to its critical point. The solvent properties such as polarity, ability to donate or accept lone pair of electrons, softness, self-cohesiveness and viscosity will strongly influence the solubility and transport behavior of the ions involved in the heterogeneous liquid-solid reactions. The solvent polarity is generally used to describe the overall solvation ability of a solvent, which influences the interactions

between the solvent and the solute molecules or ions.[76] In the solvothermal process, Cd source such as Cd powder, CdSO4, CdCl2, Cd(NO3)2 etc. along with sulfur source such as sulfur powder or thiourea or Na2S etc. taken in proper molar ratios were added to a Teflon-lined autoclave filled with ethylenediamine (NH2CSNH2) upto a certain percentage of volume fraction. Generally, sulfur sources were used in excess to avoid sulfur deficiency and achieve high yield. The Cd source reacts with the bidantate ligand ethylenediamine molecules to form an ethylenediamine-Cd complex [Cd(en)2þ 2 ]. Thiourea decomposes at lower temperature under basic environment to produce S22. Thus, the Cd-en complex reacts with the S source to produce CdS lamellar products along with its surface absorbed en molecules. With time and temperature, the folds within these lamellar products agglomerated together and finally broke up to form needle like fragments. Finally, these fragments grew to form CdS nanorods. The en molecules decomposed at higher temperatures, leaving behind the pure CdS nanorods in the form of powders. These powder precipitates were collected, washed in water and alcohol and subsequently dried in vacuum at 40– 100 8C for 4 –6 h. These CdS nanorods were found to possess hexagonal wurtzite structure. Figure 1a shows the XRD pattern of the CdS nanorods with all the diffraction peaks corresponding to the hexagonal

FIG. 1. (a) XRD pattern of the CdS nanorods revealing their hexagonal wurtzite phase, and (b) EDAX pattern of the same sample reveals the chemical purity of the sample.


SYNTHESIS OF CdS NANOSTRUCTURES

˚ and wurtzite phase of CdS with lattice constants a ¼ 4.14 A ˚ c ¼ 6.72 A. These match well with those in the JCPDS Card (Joint Committee on Powder Diffraction Standards, Card no. 41-1049). The chemical composition and the stoichiometry of the CdS nanorods were investigated through EDAX. Figure 1b shows the EDAX spectra recorded on a bunch of nanorods revealing their chemical purity. Elemental analysis reveals the atomic percentages of Cd and S to be 51.2 and 48.8 respectively, which is consistent with the stoichiometric CdS within experimental error. Figure 2 shows the TEM and HRTEM images of the CdS nanorods synthesized at 180 8C with Cd(NO3)2 and thiourea as the precursors.[73] Figure 2a revealed that the diameters of the CdS nanorods were 25 nm and their lengths were 800 nm. One representative HRTEM image of a single CdS nanorod is shown in Figure 2b and the corresponding SAED pattern is shown in the Figure 2c. Both the patterns revealed that these CdS nanorods were perfectly single crystalline having hexagonal wurtzite structure. The measured spacing of the lattice ˚ , which corresponds fringes in the HRTEM image was 3.36 A to the (002) plane of the wurtzite CdS. This (002) direction was also the growth direction of the CdS nanoribbons and

FIG. 2. (a) TEM image of the CdS nanorods, (b) HRTEM image of one CdS nanorod and (c) SAED pattern of the CdS nanorod. [Reprinted with permission from Ref. 73: J. Nanosci. Nanotech. 2006, 6, 771– 776].

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this was also confirmed from the SAED pattern. (100) and (101) lattice planes corresponding to the hexagonal wurtzite CdS are also indicated in the SAED pattern. Another interesting observation was that the aspect ratio of the CdS nanorods steadily increased with temperature. When the synthesis temperature was 140 8C, the diameters of the CdS rods were 12 nm with lengths within 100– 200 nm. Whereas when the temperature was increased to 200 8C the diameter of the nanorods were 35 nm and lengths 1400 nm. The TEM images shown in Figure 3 revealed that the precursors were also influencing factors of the aspect ratio of the CdS nanorods. These results suggested that Cd(NO3)2.4H2O and thiourea were the best combination for obtaining larger CdS nanorods. The increase in the aspect ratio of the CdS nanorods with the temperature could be explained by the fact that the high temperature helps the precursors to dissociate easily resulting in the steady formation of the nanorods. The reason behind the formation of CdS nanorods with different aspect ratios using different precursors could be similar to those discussed earlier for the nanoparticles. It was also noticed that the aspect ratios of the nanorods increased significantly with the increase of the filling fraction of the Teflon vessel. This could be due to the fact that the increase in the filling fraction increases the internal pressure of the closed reaction chamber, which might have helped the precursors to

FIG. 3. TEM image of the CdS nanorods prepared with different conditions: (a) Precursors: Cd(NO3)2 . 4H2O and thiourea, T ¼ 140 8C, (b) Precursors: Cd(NO3)2 . 4H2O and thiourea, T ¼ 200 8C, (c) Precursors: cadmium acetate and thiourea, T ¼ 200 8C, (d) Precursors: CdCl2 and thiourea, T ¼ 200 8C, and (f) Precursors: Cd(NO3)2 . 4H2O and Na2S, T ¼ 200 8C.


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dissociate easily. Another factor responsible could be that with the increase in the amount of solvents the precursors dissociate more easily and reacts more steadily. The aspect ratios of the CdS 1-D nanoforms could be increased significantly by a polymer assisted solvothermal process.[74] In this process the Cd source was dispersed in polyacrylamide gel and subsequently the dried gel along with the sulfur source were transferred to a Teflon lined autoclave filled with ethylenediamine. The closed autoclave was then placed at temperature between 170– 200 8C for 4 –10 days. Inspite of the increase in the aspect ratio the crystal structure and phase of these CdS nanowires remain hexagonal. It was believed that the ethylenediamine molecules were absorbed within the Cd2þ ion containing polymer matrix to form a gel with many small pores. When sufficient solvent was absorbed these pores were connected with each other to form continuous channels. The lateral growth of CdS crystals was confined within these channels of the polymer matrix and the axial growth along the (001) direction was favorable. Synthesis of Multi-Armed CdS Nanorods For the fundamental understanding of the structural aspects of the 1-D nanoforms the synthesis and study of the complex 1-D nanoforms such as two-, three- and four-armed CdS nanorods are important. Jun et al.[77] reported the formation of multi-armed CdS nanorods by thermal decomposition of an air-stable, single-source molecular precursor, Cd(S2CNEt2)2, in hexadecylamine (HDA). They reported that at relatively higher temperatures straight nanorods were produced whereas at lower temperatures multi-armed CdS nanorods were favored. Under high-temperature conditions (300 8C), 1-D nanorod formation with purely wurtzitephase of CdS was obtained from wurtzite-phased nucleation seeds. In general, the (00-1) face of the wurtzite crystalline form is more reactive than other faces, and under the high growth rate regime, the formation of rods is favored over that of spherical-shaped nanocrystals.[78 – 80] Higher precursor concentrations also generate nucleation seeds with a larger average size, and subsequent growth steps result in nanorods with larger size and relatively constant aspect ratio. As the growth temperature decreases, two different phases of nucleation seeds coexist in certain temperature regimes (180 8C), and at lower temperature range (120 8C) zinc blende-phased seeds dominate (Figure 4).[81] Different growth rates between the crystallographic surfaces resulted in bi-, tri-, or tetrapods from zinc blende-phased seeds. Tetrapods are formed at the mildest growth condition of 120 8C, where the formation of the four arms proceeds evenly on the four different f111g surfaces of CdS zinc blende core to form (001) faces of the wurtzite-phased arms. Bi- or tripods are obtained when the growth becomes relatively fast at higher temperature (180 8C). It is possible that once the formation of the two or three reactive (00-1) wurtzite faces on the f111g faces of the zinc blende core occurs, the growth rate on the (00-1)

FIG. 4. (a) TEM image and SAED pattern of the multiarmed CdS nanorods [reprinted with permission from Ref. 82 Adv. Mater. 2002, 14, 1537]. (b) Schematic representation of the growth of CdS tetrapod nanorod, the fourth arm being perpendicular to the page. (c) HRTEM image of a bipod- (B) pencilshape. It is clearly seen that wurtzite arms with (00-1) direction grown out of f111g faces of zinc blende core or end [Reprinted with permission from Ref. 77. J. Am. Chem. Soc. 2001, 123, 5150].

face is fast compared to that of the remaining f111g surface(s) of the zinc blende core because the resulting (00-1) faces have more surface area and defects during crystal growth. Finally, under fast growth conditions (i.e., 300 8C), wurtzite seeds are favored, and only 1-D nanorods are formed. Multi-armed CdS nanorods were also prepared by solvothermal approaches.[82 – 84] Gao et al.[82] have reported a simple surfactant (dodecylthiol)-ligand (ethylenediamine) coassisted solvothermal approach to synthesize multi-armed CdS nanorods (Figure 7). The growth mechanism was quite similar to that of Jun et al.[77] i.e., nucleation of cubic seed and subsequent formation of wurtzite structured arms over it. Chen et al.[83] have also synthesized CdS multi-armed nanorods by ethylenediamine assisted solvothermal recrystallization technique. In this technique, previously prepared CdS nanocrystals undergo a solvothermal synthetic route in ethylenediamine. With increase in the recrystallization temperature, the shapes of the nanorods changes from straight to tetrapod shaped forms. This was attributed to the temperature dependence of the solubility of the CdS nanocrystals, i.e., at lower temperature only some part of the CdS nanocrystals dissolved in the solvent whereas at relatively higher temperature CdS nanocrystals dissolve easily and enough energy ensures the growth of CdS tetrapods. All these CdS multi-armed nanorods were single crystalline with hexagonal wurtzite structure.



Template-Assisted Chemical Synthesis of CdS Nanowires Template-assisted synthesis has been a common approach to prepare aligned nanowires. As the nanowires grow, the nanopores of the template, typically anodized aluminum oxide (AAO), are filled. Because the nanopores, perpendicular to the AAO membrane surface, are uniform in diameter and hexagonally packed, the nanowires embedded in the template form highly ordered and vertically aligned nanowire arrays.[85] Electrochemical synthesis using a template is one of the most efficient methods for the growth of nanowires because the growth occurs exclusively in the direction perpendicular to the substrate surface.[86] CdS nanowires were synthesized by AC electrodeposition in porous AAO template.[87,88] CdS nanowires with tunable diameters were synthesized by a electrochemically induced deposition in the pores of an AAO template from an acidic chemical bath containing cadmium chloride and thioacetamide.[33] Xu et al.[33] prepared AAO templates of pore sizes of about 8, 20, and 90 nm by potentiostatic anodization of high purity aluminium plates (0.15 mm thick) in aqueous solutions of 14% H2SO4, and 4% and 2% oxalic acid respectively, at 0 –20 8C. After anodization, the remaining aluminium was etched by a 20% HCl-0.1 M CuCl2 mixed solution. Then the barrier layer was dissolved using 20% H2SO4. Finally, a silver film was deposited by vacuum evaporation onto one surface of the template membrane to provide a conductive contact. The deposition of CdS was performed potentiostatically at a potential value 20.65 V referred to the saturated calomel electrode (SCE) in a three-electrode configuration in a glass cell at 70 8C by immersing the cell in a water bath for 5– 8 h. A platinum plate, SCE and Ag/AAO substrate were used as the counter, reference and working electrode respectively. The electrolyte solution consisted of 0.05 M CdCl2 and 0.10 M thioacetamide. In order to avoid erosion of the template, the initial pH of the solution was adjusted to 4.3 by adding appropriate amounts of HCl. After the deposition, the AAO was washed ultrasonically in water and finally dried in air at room temperature. AAO was completely removed from the Ag/AAO/CdS samples by mounting on a glass using epoxy resin and then dissolving the AAO template in 1 M NaOH at 25 8C and finally washed in water. However, after the template was removed, the embedded arrays of nanowires with a high aspect ratio normally collapse into an entangled mass due to the surface tension force exerted on the nanowires during the evaporation of the liquids.[89 – 91] This makes template-assisted synthesis greatly limited when vertically aligned nanowire arrays are desired for devices. To avoid the liquid-gas interface, and thus to eliminate surface tension force during the evaporation of the liquids, a supercritical drying approach was used by Liang et al.[35] They have electrodeposited CdS nanowires into the nanopores of the AAO templates decorated with a conductive Au back layer. After deposition, AAO was completely

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removed by dissolving in 1 M NaOH aqueous solution keeping the sample in the solution. The nanowire samples were then quickly transferred to a autoclave filled with ethanol. The ethanol in the pores was replaced by liquid CO2 (Tc ¼ 31.1 8C and Pc ¼ 73.8 bar). After the autoclave was filled with liquid CO2 and the residual ethanol was at or below the 10 ppm level, the temperature of the autoclave was increased to 45 8C in 2 h, reaching a pressure of about 100 bar. The samples were maintained at 100 bar and 45 8C for 3 h. Finally, CO2 underwent a transformation from the supercritical phase to the gas phase by decreasing the pressure to atmospheric pressure at the same temperature. By using the drying process in supercritical CO2, they successfully tamed the surface tension force in all steps, including AAO dissolution, ethanol-exchange, and phase transformation of CO2 from liquid to supercritical and to gas state. Figure 5 displays the SEM images of the samples by the supercritical CO2 drying process. It can be seen that large-area, noncollapsed, and vertically aligned nanowire arrays are formed. These nanowire arrays do not aggregate, and the nanowires keep their hexagonally packed pattern after the removal of the template as if they were still embedded in the template (Figure 5b, c). The side view image (Figure 5d) indicates that the AAO has been completely dissolved, and the nanowires are vertically aligned on the Au film substrate, which ensures a conductive contact between the nanowires and the substrate. However the structural studies revealed that large numbers of stacking faults and twin segments were present within the CdS nanowires. CdS nanowires were also fabricated within AAO template by a sol-gel approach.[34] They have prepared CdS nanowires by immersing the alumina membrane with pore diameter 100 nm in 20 mL of 4 mM Na2S aqueous solution containing 1.15 M mercapto ethanol

FIG. 5. SEM images of the aligned CdS nanowire arrays synthesized by electrochemical deposition in AAO template and subsequently dried in supercritical CO2: (a) lower magnification, (b and c) higher magnification, and (d) side view. [Reprinted with permission from Ref 35: J. Am. Chem. Soc. 2004, 126, 16338.]


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(HSCH2CH2OH), as a protective agent, and 30 mM sodium hexametaphosphate (HMP), as a stabilizing agent, to modify the alumina pore wall for 10 min. Then, to this solution 20 mL of 4 mM cadmium chloride solution was added. All solutions were purged with nitrogen prior to and during the reaction process. The reaction process lasts ca. 5 h at room temperature. The CdS particles were deposited on the pore walls and on both sides of the membrane. After 5 h, the membrane was removed from the sol and dried in air for 30 min at ca. 75 8C and the particles of CdS on both faces of the membranes were removed by polishing with alumina powder. The composite membranes were placed in a tube furnace for annealing under argon atmosphere, and the temperature was increased to 80 8C for 30 min, 150 8C for 30 min, and 500 8C for 2 h, then decreased back down to room temperature. The array of single-crystal CdS nanowires within the pores of the template membrane was obtained. It well known that alumina is a Lewis acid with the acidity center on Al3þ ; the S2+ ions will preferably attach to the alumina wall, at the reaction sites with Cd2þ, and induce the deposition of colloidal CdS particles on the alumina wall. It is important to note that deposition does not work when the alumina template is immersed in the CdCl2 solution first, and the CdS mainly precipitates on the surfaces of the alumina membrane. HSCH2CH2OH is used as a protecting agent absorbed on the surface of the CdS colloidal particles to make the CdS particles very fine. Zhang et al.[92] have reported a new room-temperature approach to fabricate aligned CdS nanowire bundles in water system using a simple 1-D inorganic coordination polymer (KCd(SCN)3) as soft template. Synthesis by the Vapor-Solid (VS) Method The vapor-solid (VS) growth is nothing but an evaporation condensation process. This is one kind of spontaneous growth process, where the growth of 1-D nanoforms is driven by the decrease in Gibbs free energy, which arises from either recrystallization or decrease in supersaturation. The 1-D nanoforms grown by this process are generally single crystalline with very few imperfections. The formation of the 1-D nanostructure via VS process is due to the anisotropic growth. Several mechanisms may be responsible for the anisotropic growth such as, different growth rates at different facets in a crystal, presence of imperfection or defects in a specific crystal directions or preferential accumulations of impurities on specific facets. Sears[36] was the first to explain the growth of mercury whisker by anisotropic growth in 1955. CdS nanowires[63] and nanobelts/nanoribbons[1,3,93] were synthesized via this VS technique. Ye et al.[63] first reported the synthesis of CdS nanowires by vapor solid process. For the growth of the CdS nanowires, they have used CdS powder as the precursor. CdS powder, used as the precursor, was prepared through chemical route by adding aqueous Na2S solution dropwise to the aqueous solution of

Cd(NO3)2 . 4H2O with vigorous stirring and Ar gas was bubbled through the aqueous solution of Cd(NO3)2 . 4H2O to prevent oxidation. The yellow precipitate thus obtained, was washed successively in deionized water and absolute ethanol and it was then dried in vacuum at room temperature. These CdS powder was thermally evaporated in a horizontal tube furnace at 900 8C in a flowing Ar atmosphere for different periods of time. The CdS vapor species got deposited on a Si substrate placed 10 cm away from the source, forming micrometer-sized spheroids (Figure 6). With increasing deposition time short nanorods were originated from these spheroids (Figure 6) and ultimately resulted into long nanowires

FIG. 6. SEM images of CdS nanowires produced by a non-catalytic VS method. Images were taken after a heating times of (a) 45 min, when the nanowires are typically several micrometers in length; (b) 60 min, when the nanowires are tens of micrometers in length; and (c) 90 min, when the nanowires are hundreds of micrometers in length. [Reprinted with permission from Ref. 63: J. Phys. Chem. B 2002, 106, 10338.]



(Figure 6). Duan et al.[1] prepared CdS nanoribbons by using a vacuum vapour transport approach. Specifically, a small amount of CdS powder (100 mg) was transferred into one end of the vacuum tube and it was sealed. The vacuum tube was heated in such a way that the end with CdS powder was maintained at 900 8C, while the other end was kept at 50 8C lower temperature. Within 2 hours, most of the CdS was transported to the cooler end and was deposited on the tube wall. The resulting materials are predominantly nanoribbons having thicknesses of 30 –150 nm, widths of 0.5 –5 mm, and lengths of 10 –200 mm. Where as Chen et al.[93] have prepared CdS nanoribbons in a horizontal tube furnace by thermally evaporating commercially available CdS powders at 1100 8C in flowing Ar atmosphere. In the VS method the source materials vaporize to the molecular level with stoichiometric cation-anion molecules which condense on the substrate and the molecules arrange themselves in such a way that the proper local charge balance and the structural symmetry is maintained resulting in a nucleation center. With further intake of the molecules, the surfaces that have lower energy e.g., side surfaces starts to form and the low energy surfaces tend to be flat. As the molecules have higher mobility at the higher growth temperature, the newly arrived molecules tend to diffuse to the rough growth front resulting in the increase in the surface area, which leads to the formation of the nanoribbons. The rough surface at the growth front leads to the rapid intake of the incoming molecules resulting in the rapid formation of the nanoribbons. The newly arrived molecules can stick to the rough growth front or the side surfaces. But, the smooth side surfaces and the high mobilities prevent them from remaining on the surface. The molecules randomly diffuse on the surface and finally accumulate at the lower energy growth sites. In the VS process it is very typical that the width of the 1-D nanoforms decrease towards their growing end. Thus at higher growth temperature or at higher vapor concentration generally CdS nanoribbons/nanobelts were preferred over the nanowires. The 1-D nanostructures formed by the VS technique were single crystalline with hexagonal wurtzite crystal structure. Synthesis by Vapor-Liquid-Solid (VLS) Method For the VLS process an experimental set up similar to that of the VS process is used. In the VLS process,[28,41,94,95] a second material commonly known as the catalyst is intentionally introduced to direct and confine the crystal growth onto a specific orientation and within a confined area. The catalyst forms a liquid droplet by itself or by alloying with the growth material, which acts as the trap of growth species. Enriched growth materials in the catalyst droplets subsequently precipitate upon supersaturation at the surface of the substrate resulting in the 1-D growth. Wagner et al.[94,95] first proposed the VLS theory in 1964 to explain the experimental results and observations in the growth of Si nanowire. So in the VLS

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process, the presence of the catalyst is always necessary and a liquid droplet is always found at the tip of the nanowire. Schematic representation of the VLS model is presented in Figure 7 on the basis of the in situ TEM observations of the Ge nanowire growth by VLS process.[41] VLS process is the most widely used technique to synthesize CdS nanowires[62,75,96] and nanoribbons/nanobelts.[2,64,97 – 101] Wang et al.[62] first reported fabrication of CdS nanowires by a gold catalyst assisted VLS approach by thermally evaporating CdS powders in a flowing Ar atmosphere at 800 8C over an Au-coated Si substrate. Barrelet et al.[75] synthesized CdS nanowires from a single source molecular precursor (Cd(S2CNEt2)2) at 750– 850 8C. Dong et al.[64] reported the fabrication of CdS nanobelts by an Au catalyzed VLS approach on tungsten substrate. Catalytic VLS approach have been utilized by several groups to synthesize CdS nanowires and nanobelts/nanoribbons using CdS powders in a flowing Ar atmosphere over an Au-coated substrate within 750 – 1000 8C. It was observed that several parameters such as temperature, flow rate of the carrier Ar gas, position of the substrates etc. play important roles in defining the morphology of the 1-D CdS nanoforms. Figure 8 shows the SEM images of the products produced at different temperatures over the Si wafer

FIG. 7. (A) Schematic view of the VLS process, (B) In situ TEM images recorded during the Ge nanowire growth revealing the role of the catalyst particle in the nanowire growth by VLS process [Fig. B reprinted with permission from Ref. 41: J. Am. Chem. Soc. 2001, 123, 3165].


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FIG. 8. SEM image of the CdS nanostructures produced on the Si wafer placed directly over the source powder at different temperatures: (a) 800 8C, (b) 900 8C, (c) 950 8C and (d) 1000 8C. Insets in images (b) and (c) reveals the presence of Au nanoparticles at the tip of the nanowires and nanoribbons. Inset in (d) shows multiarmed CdS nanorods. [Reprinted with permission from Ref. 141: J. Phys. Chem. B 2006, 110, 4542–4547].

placed directly over the quartz boat containing the CdS powder. Figure 8a shows the products obtained at 800 8C, which consisted of only a few nanowires. Figure 8b shows the formation of large quantities of nanowires at 900 8C. The diameters of these nanowires were 50 nm and lengths 1 mm. The image shown in the inset of Figure 8b reveals the presence of the Au nanoparticles at the tip of the nanowires. The composition of the particles was confirmed from EDAX studies. Further increase of temperature to 950 8C produced large number of nanoribbons (Figure 8c) having widths ranging from 200 to 250 nm and lengths of the order of a few micrometers. The Au nanoparticles were also detected at the tip of the CdS nanoribbons as can be seen in the SEM image shown in the inset of Figure 8c. At 1000 8C temperature, thick nanorods with diameters 200– 400 nm and lengths 2 mm were observed (Figure 8d). Along with these nanorods a few multi-armed nanostructures were also produced as displayed in the inset of Figure 8d. Thus, from the SEM observations it was indicated that increase in the synthesis temperature increases the dimensionality of the CdS nanostructures. Lower temperature favors the formation of CdS nanowires whereas nanoribbons were formed at relatively high temperature. But excessive high temperature favors the formation of sub-micrometer column structures. Lots of interesting 1-D nanostructures were formed on the Si substrates placed towards the down stream edge as can be seen from the SEM images in Figure 9. SEM images in Figures 9a and 9b show the products obtained on the Si substrates at a distance 6 cm from the source powder at temperatures 900 8C and 1000 8C respectively. Figure 9a shows that short column structures were formed perpendicular to the surface of the

FIG. 9. SEM images of the CdS nanowires produced at different substrate positions towards the downstream side and at different temperatures: (a) 6 cm away from the source at 900 8C, (b) 6 cm away from the source at 1000 8C, (c,d) 1 cm away from the source at 950 8C, and (e, f ) 1 cm away from the source at 1000 8C. The inset in (b) reveals the formation of pearl necklace type nanowire of CdS. [Reprinted with permission from Ref. 141: J. Phys. Chem. B 2006, 110, 4542–4547].

substrates. Figure 9b reveals the formation of network like CdS nanowires. A few junction points of the branched network structures are highlighted in the figure by drawing circles around them. Along with these network like nanowires a few pearl necklace type nanowires were also detected as revealed by the inset of Figure 9b. The SEM image in the inset of Figure 9b revealed the presence of spherical nanoparticles in the nanowires at regular intervals giving it the precise look of necklace. Figures 9c and 9d reveals the formation of ultra long uniform nanowires on the Si substrates placed next to the CdS powder source at 950 8C. These nanowires have diameters within 40– 60 nm and lengths 50 mm. When the temperature was increased to 1000 8C, CdS nanowires were still observed (Figure 9e and 9f) at the same position i.e., over the substrates next to the source. But, the diameters of the nanowires were slightly increased varying within 50 – 100 nm. Magnified SEM images revealed the presence of a few helical like nanowires as can be seen in Figure 9f. All these CdS nanoforms were non-aligned with some degree of orientation being observed on the sample obtained at a distance of 6 cm at 900 8C. The results discussed above indicated that at the same temperature, say 950 8C, CdS



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nanoribbons were observed when the substrate was directly clipped over the source while CdS nanowires were obtained when the substrate was placed next to the quartz boat containing CdS powder. The temperature was same for both positions of the substrates. These results indicated that low but steady vapor concentrations of the CdS favors the formation of nanowires. So, the basic requirement for obtaining the desired nanowire arrays was to ensure slow but steady flow of the CdS molecular vapors perpendicular to the Au-coated Si substrate for a brief period of time. For this purpose a quartz container of the CdS source powder was specially designed that served the dual purpose of the substrate holder as well as the container for the source CdS powder. The quartz boat was centrally covered with two open horizontal channels on both sides of it. The boat was filled with CdS powder and the Au coated Si wafer was placed on one of the open channels with the Au layer facing the CdS powder from a distance 3 mm. Almost total area of one of the open channels was covered by the wafer keeping two narrow slit like opening on both side of it for the exit of the carrier Ar gas from the quartz boat. The carrier Ar gas entered through the other opening at high temperature and carried the CdS molecular vapors to the side having

the Si wafer where it strikes the wafer steadily before coming out of the boat through the small openings on both sides of the Si wafer. Figures 10a and 10b show the SEM images of the products after 45 min of deposition, revealing the formation of well aligned CdS nanowire arrays. Cross-sectional view of the nanowire arrays is shown in the inset of Figure 10b. This image indicates the presence of spherical particle at the tip of the nanowires. All the nanowires were almost identical in length and diameter, which is important for the fabrication of devices. The diameters of the nanowires were about 60 nm and length 1 mm. The SEM images also indicate the crosssections of the nanowires to be circular. For the purpose of understanding the growth mechanism, the experiment was performed for 15 min and the resultant products are shown in Figure 10c. It was observed that the morphologies and sizes of the CdS nanoribbons depend on the synthesis temperature as well as the flow rate of Ar gas. Figure 11 shows the SEM image of the product obtained at 900 8C. When the flow rate of Ar gas was 200 cm3/min, sword like nanoribbons were found lying all over the Si substrate. The inset shows a closer view of these CdS nanoribbons revealing the presence of Au

FIG. 10. SEM image of the CdS nanowire arrays deposited at 900 8C (a) low magnification image and (b) high magnification image. Cross-sectional view of the CdS nanowire arrays is shown in the inset of Figure (b). The image in (c) reveals the initial stage of the CdS nanowires arrays formation. [Reprinted with permission from Ref. 141: J. Phys. Chem. B 2006, 110, 4542–4547].

FIG. 11. SEM image of the CdS nanoribbons synthesized at 900 8C with different Ar flow rates, (a) 200 cm3/min and (b, c) 800 cm3/min. The images (a) and (b) represents the CdS nanoribbons formed on the surface of the Si wafer whereas (c) shows the CdS nanoribbons produced at the downstream edge of the substrate. Insets in all the three figures show the closer view of these nanoribbons. [Reprinted with permission from Ref. 101: J. Phys. Chem. B 2005, 109, 19134–19138].


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nanoparticles at their tips. The composition of the nanoparticles present at the tips of the nanoribbons was also studied through EDAX and unlike the results obtained from the body of the nanoribbons where only Cd and S were detected, presence of Au along with Cd and S was confirmed. Lengths of the CdS nanoribbons were 5 mm whereas widths of these nanoribbons at their bases were 200 nm which decreased gradually towards their tips. The diameters of the Au nanoparticles were 50 nm. Instead of the uniform distribution of the nanoribbons (as mentioned above), randomly distributed star like assemblies of CdS nanoribbons were observed with an increased Ar flow rate (800 cm3/min) when all other experimental conditions were kept unchanged (Figure 11b). Magnified SEM image shown in the inset of Figure 11b revealed that these star like assemblies were composed of several sword like CdS nanoribbons. The sizes and shapes of these CdS nanoribbons were similar to those obtained in the previous case. At the same experimental condition, longer nanoribbons (length 10 mm) were also observed in large population at the downstream edge of the Si substrate (Figure 11c). A closure view of these CdS nanoribbons also indicates the presence of Au nanoparticles at their tips (inset of Figure 11c). Figure 12a shows the SEM image of the nanoribbons obtained at 950 8C with Ar flow rate 200 cm3/min. The nanoribbons obtained in this condition were a few hundred micrometers in length and their widths varied within 200 –300 nm. The widths of these nanoribbons were uniform throughout their lengths, except at the growth front, where they were again sword like. Au nanoparticles were also observed at the tips of these nanoribbons as revealed by the SEM image shown in the inset of Figure 12a. Figure 12b shows the magnified SEM image of the CdS

FIG. 12. SEM image of the CdS nanoribbons synthesized at 950 8C with different Ar flow rates: (a and b) 200 cm3/min. and (c and d) 800 cm3/min. The image shown in the inset of (a) reveals the presence of Au particles at the tips of the CdS nanoribbons. The arrow in the magnified image shown in (b) indicates the thickness of a CdS nanoribbon. The image in (d) reveals the wavy nature of the ribbons produced at higher Ar flow rate. [Reprinted with permission from Ref. 101: J. Phys. Chem. B 2005, 109, 19134–19138].

nanoribbons from which the thicknesses of the CdS nanoribbons could be estimated. The average thickness of the CdS nanoribbons was 50 nm which was of the order of the diameter of the Au nanoparticles. In fact, the thicknesses of the CdS nanoribbons could be controlled by controlling the sizes of the Au nanodroplets. Star like CdS nanoribbon assemblies were also observed at 950 8C (Figure 12c) when the flow rate of Ar gas was increased to 800 cm3/min. These nanoribbons were not straight and smooth throughout their lengths but they were wavy towards their growing end, which was evident from the magnified SEM image of a single CdS nanoribbon (Figure 12d). The morphology and the crystal structure of the CdS nanostructures were further confirmed from the transmission electron microscopic (TEM) images. Figure 13a shows the TEM images of the CdS nanowires produced at 950 8C. TEM image of a single nanoribbon is shown in Figure 13b. The ripple-like contrasts observed on the CdS nanowires and nanoribbons were due to the strains caused by the bending of the nanoforms. Figures 13c –d show the base tip area of a CdS nanowire, which was collected from the CdS nanowire arrays. The presence of a spherical Au nanoparticles at the tip of the CdS nanowire was confirmed from Figure 13d. One representative HRTEM image of a single CdS nanowire is shown in Figure 13e and the corresponding SAED pattern is shown in the inset. Both the patterns reveal that these CdS nanowires are perfectly single crystalline having hexagonal wurtzite structure. The measured spacing of the lattice fringes in the ˚ , which corresponds to the (002) HRTEM image was 3.36 A plane of the wurtzite CdS. This (002) direction was also the growth direction of the CdS nanowires and this was also confirmed from the SAED pattern. The (100) lattice plane corresponding to the hexagonal wurtzite CdS is also indicated in the SAED pattern. Figure 13f shows the TEM image of the sample corresponding to the SEM image shown in Figure 10c. This image reveals the presence of a few dark contrast particles along with a light contrast elongated structures. The EDAX measurements facility available with the TEM indicated that the dark nanoparticles were mainly consisted of Au whereas the elongated light portion was pure CdS. These two SEM (Figure 1c) and TEM (Figure 13f) images showed the initial nucleation stage of the nanowire from the Au nanoparticles. TEM images shown in Figure 14a reveal the presence of the Au nanoparticles at the front of the CdS nanoribbons. The TEM image shown in the Figure 14b reveals the narrowing width and the rough side surfaces towards the growth front of the CdS nanoribbons. In VLS process a thin layer of metal catalyst such as Au is deposited on the substrate, which at high temperature breaks up to form liquid nanodroplets. These metal droplets absorb the incoming source molecular vapor and finally, upon supersaturation, solid nanostructures starts appearing with the metal nanoparticles at their tips. So the characteristic of the VLS growth process is the existence of the metal nanoparticles at



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FIG. 13. TEM images of the CdS nanoforms: (a) CdS nanowires produced at 950 8C over the substrates placed 1 cm away from the source, (b) CdS nanoribbon, (c) Base region of a CdS nanowire from the nanowire array sample, (d) Tip of the above nanowire revealing the presence of Au nanoparticle at its tip. (e) HRTEM image of a single CdS nanowire along with the corresponding SAED pattern in the inset. (f) TEM image shows the initial nucleation stage of the CdS nanowires. [Reprinted with permission from Ref. 141: J. Phys. Chem. B 2006, 110, 4542–4547].

the tip of the nanostructures. In the VS method the source materials vaporize to the molecular level with stoichiometric cation-anion molecules which condensed to the substrate and the molecule will be arranged in such a way that the proper local charge balance and the structural symmetry is maintained resulting in a nucleation center. With further intake of the molecules, the surfaces that have lower energy, e.g., side surfaces, start to form and the low energy surfaces tend to be flat. As the molecules have higher mobility at the high growth temperature, the newly arrived molecules tend to diffuse to the rough growth front resulting in the increase in the surface area, which leads to the formation of the nanoribbons. The rough surface at the growth front leads to the rapid intake of the incoming molecules resulting in the rapid formation of nanoribbons. The newly arrived molecules can stick to the rough growth front or the side surfaces. But the smooth side surfaces and the high mobility prevent them from remaining on the surface. The molecules randomly diffuse on the

surface and finally accumulate at the lower energy growth sites. Thus, the general indication of the VS growth process is the decrease of the diameter/width of the nanostructures towards their growth end. On the basis of the SEM and TEM observations by the authors’ group the existing growth models for better understanding of the growth process of CdS nanoforms and the resulting schematic view of the growth mechanism is shown in Figure 15. We believe that for the uniform nanowires and the nanowire arrays, VLS process was the main growth mechanism whereas for the CdS nanoribbons and other types of nanowires: network-like, pearl necklace-like and helical-like nanowires, VLS as well as VS processes were equally responsible for the evolution of the morphology. The rectangular cross-section of the CdS nanoribbons suggests that the conventional VLS process alone cannot explain the growth mechanism. Thus, at high CdS vapor concentrations, the CdS vapor species were not only absorbed by the Au liquid droplets but they were also


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FIG. 14. TEM image of the CdS nanoribbon; (a) Tip containing the Au nanoparticles and (b) the rough growth front indicating the role of vapor-solid (VS) mechanism. [Reprinted with permission from Ref. 101: J. Phys. Chem. B 2005, 109, 19134–19138].

deposited below the Au nanodroplets, whose portion of the newly formed nanostructures were still in their restructuring stage (see Figure 14b). As a result, the widths of the nanostructures increased and by the influence of VS method, CdS nanoribbons originated. Since the growth of CdS nanoribbons is initiated by the Au catalyst nanoparticles via VLS process, control of the sizes of the Au nanoparticles may be useful to control the thicknesses of the CdS nanoribbons. At the lowest possible temperature favoring the CdS nanowire formation i.e., at 900 8C, the slow but steady supply of the CdS vapors surrounding the Au liquid nanodroplets ensures the formation of the nanowires perpendicular to the substrates. For the fabrication of CdS nanowire arrays, the role of the quartz boat containing the CdS powder was most crucial. In the same experimental setup and at the experimental conditions, a

FIG. 15. Schematic view of the formation of different CdS nanoforms. [Reprinted with permission from Ref. 141: J. Phys. Chem. B 2006, 110, 4542–4547].

simple change in the design of the quartz boat utilized as the source and substrate holder, transformed the non-aligned CdS nanowires to aligned nanowire arrays. Formation of the network like and pearl necklace like nanowires (Figure 9b) at a distance 6 cm from the source at 1000 8C could also be due to the influence of the VS process. The temperature of the substrate region was 850 8C. The nanowires were initially formed by the VLS technique. At high temperatures, the CdS powder evaporates rapidly and as a result, CdS vapor concentration surrounding the substrates becomes also high. The nanowire formation was initiated by the Au catalyzed VLS technique. But as the temperature was relatively low, mobility of the newly arrived molecules were also low and as a result those CdS species deposited on the side wall of the nanowires could not move to the growth front and instead they remain on the wall of the nanowires giving rise to the non-uniform diameters of the nanowires. This might result in some rough patch or kink on the nanowire surface, which acts as the energetically favored site for the newly arrived vapor species to be absorbed on it, and this gives rise to the branched or network like structures. The pearl necklace formation could be explained by the fact that when the diameter of the Au particle was small, the growth due to the VLS process was fast and due to the lower mobility, the molecules absorbed on the surface could not move rapidly i.e., they were lagging behind the Au mediated growth resulting into the formation of flattening or pearl-like structure formation at regular intervals. The helical like structure formation as shown in 9e and 9f could be due to the same reason as discussed above. Synthesis of CdS Nanotubes With increasing applications of carbon nanotubes, inorganic semiconductor nanotubes are also important systems to study. Rao et al.[102] first reported CdSe and CdS nanotubes by a surfactant assisted synthesis route. In a typical preparation of CdS nanotubes, a suspension of a fine powder of cadmium oxide (10 mmol) was prepared in 20 ml of Triton 100-X (24 mmol). An aqueous solution of thioacetamide in basic medium was added dropwise under constant stirring, to the suspension at 40 8C in an argon atmosphere. The resulting mixture was refluxed for 12 h and left overnight. The product was washed repeatedly with cyclohexane and diethyl ether and dried. When AOT was used as a surfactant, the procedure was as follows. To a solution of AOT (4.5 mmol) in 25 ml cyclohexane, cadmium oxide (10 mmol) was added and stirred rigorously to form a good suspension. The rest of the procedure was similar to that with Triton 100-X. These nanotubes were polycrystalline in nature with hexagonal wurtzite phase. Xiong et al.[103] have produced CdS nanotubes and nanowires via an in situ micelle –template –interface reaction (ISMTIR) route by adjusting the concentration of surfactant. The whole reaction system is made up of SDS rod-like micelles as the template, CS2 as the oil phase and sulfur



source, NH3 as the attacking and coordination agent, and CdCl2 in water. It was found that SDS rod-like micelles were excellent templates for the growth of nanotubes and nanowires of small size. Due to a diameter of 10– 20 nm of the micelles, the CdS nanotubes formed have outer diameters of 15 nm on average and wall thicknesses of ca. 5 nm, which is within the exciton diameter of bulk CdS. Porous anodic aluminum oxide (AAO) membranes were utilized by several workers[104 – 107] to synthesize CdS nanotubes via different routes. Peng et al.[104] produced CdS nanotube arrays of up to 60 nm in length and 100 nm in outer diameter within the pores of the PAO templates using molecular anchor templating synthesis method. Zhou et al.[105] produced CdS nanotubes by two steps. First, pure metal Cd nanowires were electrodeposited inside the nanochannels of anodic alumina membrane (AAO), and then the outer walls of the Cd nanowires were sulfurized under an atmosphere of sulfur to form a CdS layer under controlled conditions; the (CdS þ Cd) nanowires were formed in situ. Secondly, the CdS nanotubes were formed by removal of the Cd, together with AAM, upon dissolution in 0.5 HCl for 1 hour. Zhang et al.[106] synthesized CdS nanotubes within the AAO template by reacting 0.02 M cadmium chloride, 0.05 M thiourea and 0.02 M sodium citrate in an aqueous solution using sodium citrate as a complexing agent. The pH of the solution was adjusted to 11.5 using NH3 . H2O, and the bath temperature was 80 8C. The chemical deposition lasted for 10 and 30 min respectively, and then the two samples were

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obtained and washed with deionized water. After drying at 60 8C, the two samples were annealed in N2 atmosphere at 350 8C for 1 h. AAO templates were also utilized to fabricate CdS nanotube arrays by CVD approach using cadmium bis (diethyldithiocarbamate) [Cd(S2CNEt2)2] as a single-source molecular precursor.[107] SEM images of the top and crosssectional view of the CdS nanotube arrays produced by the CVD approach are shown in Figure 16 a – b. Corresponding TEM and SAED pattern (Figure 16c – d) reveals their polycrystalline nature. A template free, surfactant free ultrasonic irradiation method have been utilized by Shao et al.[108] to fabricate CdS nanotubes. But all these nanowires fabricated by the above mentioned processes were either polycrystalline or very poor crystalline. Recently Hu et al.[109] have reported the fabrication of wurtzite-type CdS and CdSe nanotubes via a Sn nanowire-templated route under thermal annealing. The tubes are structurally uniform and defect-free single crystals. For the synthesis of CdS nanotubes, a mixture of CdS, SnO, SnO2, and activated carbon powders was put into a graphite crucible, which was then placed at the center of a quartz tube was set horizontally inside a high temperature resistance furnace. Several strip-like graphite wafers were placed downstream inside the tube to act as the substrate for the material growth. The tube was then sealed and the furnace was heated at a rate of 10 8C/min to 600 8C. This temperature was kept for 1.0 h, then increased to 1150 8C and remained stable over 4.0 h. During the whole synthesis, a pure N2 flow at a flow rate of 500 ml/min was kept introducing through the quartz

FIG. 16. SEM images of the as-prepared CdS nanotubes (a) Top-view of well-aligned CdS nanotubes after removing the surface cover layer. (b) Lateral view of CdS nanotube arrays. (c) TEM image and (d) SAED pattern of the CdS nanotubes revealing their polycrystalline nature. [Reprinted with permission from Ref. 107: Sol. State Commun. 2005, 133, 19].


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tube and the pressure within the tube was maintained at 1 atm. Yellow-colored products were deposited on the substrates and inner wall of the quartz tube after the furnace was cooled to room temperature. TEM observations revealed Sn-filled CdS nanotubes in the product. Most of the nanotubes were several micrometers long and displayed pin-like structure (showing tapering along their axes) with a large Sn ball at the tip; the diameters and wall thicknesses gradually decrease along the structure axes, typically, from 250– 350 and 80– 100 nm at the bottoms, to 50 –100 and 20 –40 nm at the tips. A minor fraction of the tubes has uniform diameters and wall thicknesses throughout their entire lengths; the diameters and wall thicknesses are 250– 350 and 80 –100 nm, respectively. As shown in Figure 17a, a tube may be sealed at both ends and continuously filled with Sn in the hollow cavities except for a short section near a tip end. Figures 17b and 17c show magnified framed areas in Figure 17a, clearly displaying a tube (lighter contrast) and a Sn filled region (darker contrast). As shown in Figure 17d, this tube is open at both ends, and the Sn filling occupies almost the entire hollow cavity, forming a Sn– CdS coaxial cable. An electron diffraction pattern (topleft inset) taken from the hollow cavity (bottom-right inset) was indexed as the (100) zone axis of a wurtzite CdS single crystal. A HRTEM image, Figure 17e, confirmed the structural uniformity of this single-crystalline tube; the lattice fringes of the (100) and (001) planes with a d spacing of 0.36 and 0.67 nm, respectively, can be clearly seen, and the tube’s axis direction, e.g., the growth direction, was parallel to the (001) crystallographic orientation of a wurtzite CdS crystal.

FIG. 17. (a), (b), (c), and (d) TEM images of the Sn-filled CdS tubes [insets in (d): top-left ED pattern taken from the bottom-right tube], (e) HRTEM image of the tube shown in (d). [Reprinted with permission from Ref. 109: Appl. Phys. Lett. 2005, 87, 113107].

Synthesis of Doped/Complex Ternary 1-D CdS Nanoforms Substitutional doping of semiconductors with paramagnetic transition-metal ions can produce magnetic materials called dilute magnetic semiconductors (DMSs).[110] The interesting magnetic and magneto-optical properties of DMSs, which arise from spin-exchange interactions between the dopant ions and the semiconductor charge carriers (sp-d exchange interactions), have been the focus of intense efforts for conventional planar semiconductor structures because of the possibility of utilizing these materials for semiconductor spin based electronics or spintronics.[111,112] Much less attention has been paid to doping of nanostructures with paramagnetic metal ions, although, clear evidence for cobalt and manganese doped nanocrystals of different group II-VI materials have been obtained.[113 – 115] Chen et al.[116] have fabricated hexagonally ordered arrays of quantum wires of the diluted magnetic semiconductor Cd12xMnxS (0 , x , 0:2) by incorporating the semiconductor alloy into the ordered wire-like pores of mesoporous silica hosts (MCM-41). Wang et al.[117,118] have reported a hydrothermal approach to synthesize Cd12xMnxS nanorods. This approach was quite similar to those of CdS nanorod synthesis via solvothermal route. Here Mn salt was also added along with the Cd and S source in appropriate molar ratios and a mixed solution of water and ethylenediamine was used as the solvent. Radovanovic et al.[119] have reported an Mn-doped semiconducting nanowires by a catalytic growth route. Mn-doped nanowires were prepared by adapting metal nanocluster-catalyzed chemical vapor deposition (CVD) methods for II-VI[75] nanowire growth. The synthesis of Mn-doped CdS (Mn:CdS) and ZnS (Mn:ZnS) nanowires was carried out using a core/shell methodology. First, CdS and ZnS nanowires were synthesized using gold nanocluster-catalyzed vapor-liquid-solid (VLS) growth with single-source[Cd(S2CNEt2)2 and Zn(S2CNEt2)2] precursors, respectively.[75] Second, shell layers were grown at lower temperature using either a mixture of Cd(S2CNEt2)2 and Mn2(CO)10 (molar ratio: n(Cd)/n(Mn)) 10/1) or Zn(S2CNEt2)2 and MnCO3 (molar ratio: n(Zn)/n(Mn)) 5/1) as precursors. The shells were grown at 400 8C for Mn:CdS and at 500 8C for Mn:ZnS nanowires. Doping concentrations were systematically varied by changing the Mn/Cd or Mn/Zn precursor ratio and the shell thickness. Following shell growth, the nanowires were annealed at ca. 250 8C for 2 h. In the past few years, extensive attention has been devoted to tune the band gap of the nanowires through the quantum confinement effect.[120 – 122] However, progress in this field shows the ability to generate nanowires with diameters in a limited range, in which the quantum confinement effect is not prominent. On the other hand, the diameter tuning of electronic, optical, and magnetic properties may cause problems in many applications. For example, the formation of the intrinsic Fabry-Perot cavity for nanolaser of individual nanowires requires its diameter to be larger than a critical value.[24,123]



Alloyed II-VI semiconductors such as ternary CdS12xSex and Cd12xZnxS nanowires were reported to have continuous tuning of the band gap through composition modulation. CdS12xSex nanowires were prepared by a solvothermal approach[124,125] by using Cd(NO3)2 . 4H2O, CS(NH2)2, Se and N2H4 . H2O as reactants in ethylenediamine solvent at 120– 200 8C. Liang et al.[126] have synthesized CdS12xSex nanowires by a template assisted electrochemical route. Pan et al.[127] have synthesized CdSxSe1-x (0 , x , 1) nanobelts by physical evaporation of commercial-grade CdS and CdSe in the presence of an Au catalyst at 900 8C. Cd12xZnxS nanowires were also prepared by a hydrothermal approach using solvothermally synthesized CdS nanorods as template.[128] Cd12xZnxS nanoribbons of variable composition (0 , x , 1) were also prepared by combining a laser ablation of CdS with thermal evaporation of ZnS in presence of Au catalyst.[129] Other ternary semiconductors such as CdIn2S4 nanowires were also synthesized by a solvothermal route.[130] Other Synthesis Processes All the 1-D CdS nanostructures discussed above are of hexagonal wurtzite type. There are very few reports on the synthesis of cubic CdS nanorods. Small cubic rods were prepared by chemical routes.[131,132] Long CdS nanowires with typical widths of 70 – 80 nm and length ranging from several micrometers to tens of micrometers can be obtained by the in situ growth on substrate of a carboxylic acidfunctionalized self-assembled monolayer (SAM), which formed from a rigid aromatic molecule on mica surfaces.[133] CdS nanorods, nanowires also prepared by sonochemical[134] microwave irradiation,[135] microemulsion[136] based approaches etc. Pearl necklace type CdS nanofibres were prepared by organogel templates.[137] More importantly there are also few reports on the synthesis of core-shell type CdS nanowires. Preparation of coaxial (core-shell) CdS –ZnS and Cd12xZnxS– ZnS nanowires has been achieved via a one-step metal-organic chemical vapor deposition (MOCVD) process with co-fed single-source precursors of CdS and ZnS. Singlesource precursors of CdS and ZnS of sufficient reactivity difference were prepared and paired up to form coaxial nanostructures in a one-step process.[138,139] A CdS core-carbon nanotube shell type 1-D nanostructures were also reported by Cao et al.[140] BASIC CHARACTERIZATION OF THE CdS 1-D NANOFORMS CdS 1-D nanoforms were characterized by different techniques to explore their physical and chemical properties and also to study the novelty of these nanoforms and to test their applicability in device fabrication. The as deposited samples are generally first characterized by the X-ray diffractometer in order to identify the phase and crystal quality of the products. The chemical compositions and the stoichiometry of the samples are tested by either energy dispersive analysis

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of X-ray (EDAX) or X-ray photoelectron spectroscopy (XPS) studies. These studies are specially required to measure the amount of the dopant element in the doped CdS nanoforms and also to determine the compositions of the complex ternary semiconductors such as Cd12xZnxS or CdS12xSex nanostructures to verify the band gap tunability of these compound semiconductors. Morphologies and dimensions of the nanoforms are determined from the scanning electron micrographs (SEM) or transmission electron micrographs (TEM). For the SEM studies, the as deposited samples are directly transferred to the SEM chamber without disturbing the original nature of the products. For the TEM studies the nanoforms are dispersed in alcohol and subsequently deposited on the thin carbon-film-coated Cu/Ni grids. High-resolution TEM (HRTEM) images and the selected area diffraction pattern (SAED) were necessary to identify the crystalline quality, crystal structure, defect properties etc. From the HRTEM images one can distinguish the hexagonal and cubic phases of the 1-D CdS nanoforms and the presence of defects such as stacking faults, twining etc. But, here in this review article we shall only discuss those characterizations of the 1-D CdS nanoforms, which are generally useful to explore the fundamental physical and chemical properties. Optical Absorption Property For the optical absorption/transmission spectroscopic measurements CdS 1-D nanoforms are ultrasonically dispersed in ethanol or any other suitable solvent and subsequently the optical spectra were measured at room temperature. Optical absorption spectra for the CdS nanowires and nanoribbons were found to be quite similar to each other. Figure 18a shows[141] the optical absorption spectra of the CdS nanowires. The figure shows sharp excitonic nature of the optical absorption spectrum. The band edge determined from the peak position of the first derivative of the absorption spectrum was at 514 nm i.e., at 2.41 eV. This was the bulk bands gap of CdS. Similar absorption spectra were also reported by other groups for 1-D CdS nanoforms.[73,74] The sharp optical absorption edge reveals excellent crystal quality of the CdS nanostructures. For cubic CdS nanorods the absorption peak was observed at 490 nm. But due to the unavailability of cubic bulk phase of CdS, this value could not be compared with the bulk value.[133] Whereas, Xiong et al.[103] have reported the absorption peak at 459 nm for CdS nanotubes due to the quantum confinement effect caused by the thinness of the tube wall. Also, the band gap tunability of the 1-D CdS12xSex ternary semiconductors could be verified from the optical absorption spectra.[125,126] Photoluminescence (PL) Study In PL, a material gains energy by absorbing light at some wavelength by promoting an electron from a low to higher energy level. After a characteristic lifetime in the excited state, electrons will return to the ground state by radiative or


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FIG. 18. (a) Room temperature optical absorption spectrum of the CdS nanowires [Reprinted with permission from Ref. 142: J. Phys. Chem. B 2006, 110, 4542–4547]. (b) UV-vis absorption spectra of a series of CdS12xSex nanowires embedded in the AAO membranes. [Reprinted with permission from Ref. 127: J. Phys. Chem. B 2005, 109, 7120].

non-radiative transition. The spectral distribution and the time dependence of the emission are related to the electronic transition probabilities within the sample, and can be utilized to provide qualitative information about the chemical composition, structure, impurities, kinetic process and energy transfer. Sensitivity of this PL technique is very high and the presence of very low concentrations of materials can be detected. The photoluminescence (PL) properties of the CdS nanostructures corresponding to the radiative transitions may be measured at room temperature with a view to further assess their quality. Figure 19a shows the room temperature PL spectra recorded with 400 nm excitation.[141] Although from all the CdS nanostructures intense green emissions were observed, maximum intensity was observed for the CdS nanowire arrays. The intensities were almost identical for the CdS nanowires and nanoribbons. Peak position of the green emission for the nanowire arrays was at 520 nm and for the

FIG. 19. (a) Room temperature PL spectrum of the CdS nanowire array (i) and nonaligned CdS nanowires (ii) [Reprinted with permission from Ref. 142: J. Phys. Chem. B 2006, 110, 4542–4547]. (b) Low temperature PL spectrum of a single Mn doped CdS nanowire. [Reprinted with permission from Ref. 119: Nano Lett. 2005, 5, 1407]. (c)The normalized PL spectra of the obtained CdSxSe12x nanobelts excited with a He-Cd laser (325 nm). Curves (a) and (i) are the PL spectra for CdS and CdSe nanobelts, respectively, and curves (b)–(h) are for the CdSxSe1-x nanobelts collected at 650, 670, 695, 720, 745, 770, and 800 8C, respectively. The data above the spectra show the relative concentrations of S and Se in the corresponding sample. [Reprinted with permission from Ref. 128: J. Am. Chem. Soc. 2005, 127, 15692].

other structures this was at 525 nm. This peak corresponds to the near band edge emission. Similar band edge emission was also reported on CdS nanowires[75] and nanoribbons.[2,101,142] Size dependent blue shift was reported in the quantum sized cubic CdS nanorods.[131] Red emission was also reported due to the trapped luminescence from CdS nanowires.[62] The absence of the emissions from the surface and deep states associated with the defects and impurities demonstrate that



these single-crystal CdS nanowire arrays possess high quality optical properties and should be excellent building block for photonic devices. In spite of the absence of any prominent defect-related PL peaks, the existence of some low intensity peaks just below the band edge could not be ruled out, as the radiative centers are always present surrounding the surfaces of the nanoribbons owing to their high surface-to-volume ratio. The fluctuation in the surface charge density and local fields result in the inhomogeneous broadening of the excitonic peaks.[142] The influence of the surface is revealed from the broad asymmetric emission towards the higher wavelength and these emissions originated from the surface donor acceptor pair (DAP) recombinations.[142] Zhan et al.[74] have reported a luminescence peak at 380 nm from the polymer assisted solvothermally synthesized CdS nanowires, which may be attributed to a higher level transition in CdS crystallites. It has been reported that this kind of band-edge luminescence arises from the recombination of excitons and/or trapped electron-hole pairs.[143] Photoluminescence studies of the doped CdS 1-D nanoforms are of much more interest. Radovanovic et al.[119] reported the photoluminescence properties of single Mn doped CdS nanowires. PL spectrum recorded on a single Mn:CdS nanowire at 5 K (Figure 19b) excited at 395 nm exhibited two distinct features centered at 495 and 570 nm. The relatively narrow peak at 495 nm corresponds to the CdS nanowire band-edge emission. A comparison of PL spectra recorded from similarly sized Mn:CdS and CdS nanowires under the same conditions shows that the presence of Mn dopant ions leads to a partial quenching of the CdS band edge emission, which is consistent with the energy transfer to Mn ions in the lattice (see below). Significantly, the good agreement of the second feature centered at 570 nm with PL data from single-crystal and nanocrystalline Mn:CdS[144 – 146] enables the assignment of this peak to the Mn2þ 4T1 to 6A1 d-d ligand-field transition. This spectral feature is characteristic of isolated Mn2þ in a quasi-tetrahedral site.[147] Wang et al.[117,118] have reported little red-shift in the room temperature PL spectra for the CdMnS nanorods. They have synthesized the material hydrothermally but proper explanations were not given for the unexpected results. Pan et al.[127] have showed that all the CdSxSe12x nanobelts have a strong single PL emission band near their band-edges, and these spectral peaks could shift from 508 nm (for pure CdS) to 705 nm (for pure CdSe) (Figure 19c). The spectral shift of the band-edge emission of the alloyed nanobelts with the deposition temperature should come from the variety of their band gap energy, due to the different relative composition of Se or S. At the same time, the continuous shift of the PL bands for the obtained nanobelts with their compositions gives further evidences for the formation of the alloyed CdSxSe12x nanobelts via intermixing of the wide band gap of CdS (2.48 eV) and the narrow band gap of CdSe (1.77 eV), rather than the formation of the independent CdS and CdSe nanobelts. Furthermore, it is worth

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noting that the bandedge emission of these ternary CdSxSe12x nanobelts is very strong at room temperature, indicating their potential applications in adjustable nano/micro optoelectronic devices in the visible region. Raman Spectroscopic Study Raman spectroscopy is a powerful tool for the investigation of doping concentration, lattice defect identification and crystal orientation properties of the materials. Raman spectra for all the CdS nanostructures were more or less identical and one such representative spectrum is shown in Figure 20. The Raman peaks were analogous to the pure crystalline CdS.[88,148] The peaks at 305, 609, 915, and 1215 cm21 corresponds to the first-order (1-LO), second-order (2-LO), thirdorder (3-LO) and fourth-order (4-LO) longitudinal optical phonon bands of CdS respectively. Chen et al.[93] have used the Raman spectra to investigate the crystal quality of their synthesized CdS nanoribbons. The typical Raman spectrum of a single nanoribbon (at 301 and 605 cm21) was similar to that of the bulk crystal. However, the spectrum measured on some selected sites of the single nanoribbon was found to be shifted towards lower values (299 and 600 cm21), and appeared to be asymmetric. The Raman shift of the LO mode to a lower frequency was tentatively attributed to the mechanical stress in the nanoribbon, since it has a single crystalline structure, and the smallest size was about 50 nm. Therefore, the phonon confinement effect was plausibly precluded in the experiment. The shifts probably resulted from the intrinsic stress accumulated during the CVD process, because the Raman scattering is highly sensitive to the stress. Moreover, the peak asymmetry is known to occur due to the stacking faults. All the aforementioned aspects lead to the conclusion that a single CdS nanoribbon has a crystalline structure quality with a few local defects. Electrical Properties Long et al.[149] reported the temperature dependent conductivity and current – voltage curve of a single CdS nanowire,

FIG. 20. Raman spectra of one representative CdS nanowire sample [Ref. 141: J. Phys. Chem. B 2006, 110, 4542–4547].


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which was synthesized by a simple aqueous chemical growth method. A pair of platinum microleads was fabricated on the single CdS nanowire by focused ion-beam deposition. The room-temperature conductivity and the band gap of the single CdS wire were 0.82 V21cm21 and 0.055 eV, respectively. This value was much larger than those of some CdS films prepared by chemical bath deposition.[150] The reason could be due to a small quantity of Ga ion doping in the course of fabricating Pt microleads by using the FIB system. Figure 21 shows the temperature dependence of resistance and I – V curve of the single CdS nanowire. It was found that the resistance of the nanowire increases exponentially with decreasing temperature, namely, the temperature dependence follows the thermal-activation model flnR(T) T21g. They have showed that doping has significantly lowered the activation energy and increased the conductivity of the single CdS nanowire. Figure 21b shows the I – V curve of the CdS wire at room temperature. The curve is linear and exhibits ohmic behavior in the low-voltage regime, because at low electric fields the carriers are thermally activated. However, at high electric fields, a nonlinear I –V curve is observed, because the number of carriers deviates appreciably from the thermal equilibrium value due to the fact that the field-generated carriers increase by field emission from trap centers.

FIG. 21. (a) Temperature dependence of resistance of the single CdS nanowire at room temperature. (b) I– V curve of the single CdS nanowire at room temperature. [Reprinted with permission from Ref. 150: Appl. Phys. Lett. 2005, 86, 153102.].

Photoelectric Effect The photodetection or so called optical switching properties of 1D nanostructures are of particular interest because an optical gating can work as an alternative to electrical gating for devices used in memory storages and logic circuits, where the performance critically relies on binary switching.[151 – 153] Recently, photodetectors and optical switches have been investigated in several 1D nanostructures, such as InP nanowires,[151] ZnO nanowires,[152] and In2O3 nanowires.[153] Gao et al.[99] have studied the photoconductive properties of the CdS nanobelts. Figure 22a shows the typical current – voltage (I–V ) curves of a CdS nanobelt photoconductor measured in the dark and upon white-light illumination with different powers. It can be seen that CdS nanobelts are highly insulating in the dark but show a pronounced increase in

FIG. 22. (a) I– V curves of a CdS nanobelt photoconductor measured in the dark (solid circle) and upon white-light illumination with different powers (5 W: Open circle; 10 W: Open triangle; 20 W: Open square). Inset shows a schematic diagram of the nanobelt photoconductor. (b) Reversible switching of a CdS nanobelt photoconductor between low and high conductance states when the white light (10 W) was turn on and off. The bias on the nanobelt is 5 V. [Reprinted with permission from Ref. 99: Appl. Phys. Lett. 2005, 86, 173105].



conductance of up to four orders of magnitude upon exposure to white light. This behavior is consistent with an increase in the number of free carriers in the semiconductor under illumination, as expected from the optical properties of both bulk and nanoscale CdS.[154,155] Figure 22b shows the reversible switching of the nanobelt photoconductor between low and high conductance states when the white light was turn on and off. It can be seen that the response time of such device is very fast, with typical rise time of 1 s and decay time of 3 s. The characteristics of the photoconductive CdS nanobelts suggest that they are good candidates for optoelectronic switches, with the dark insulating state as “OFF” and the light-exposed conducting state as “ON”. The photoconductivity mechanism of polycrystalline CdS films has been studied previously, of which a complex process of electron-hole generation, trapping, and recombination within CdS is involved.[154,155] A common feature of metal chalcogenide materials, the presence of oxygen on nanobelt surface is notable.[154] The oxygen molecules adsorb on the nanobelt surface as negatively charged ions by capturing free electrons from the n-type CdS, thereby creating a depletion layer with low conductivity near the nanobelt surface.[152,153] Since the thickness of the nanobelts is small, usually a few tens of nm, the nanobelts are probably almost depleted of carriers,[156] leading to a high resistance in the dark state. In the illuminated state, the absorption of light with energy greater than the band gap will generate not only electrons in the conduction band that increase free carrier densities and consequent conductivity of CdS nanobelts, but also holes with an equal density that can recombine with adsorbed/chemisorbed oxygen ions at the belt surface and discharge O2 molecules. Thus, the trapped electrons are released into the conduction band and also increase the conductivity of the nanobelt. This photoinduced conductivity changes allow them to reversibly switch the nanobelts between OFF and ON states (Figure 21b), which is an optical gating phenomenon analogous to the commonly used electrical gating.[152,153] This study showed the possibility of creating highly sensitive nanobelt photodetectors and optical switches by exploring the photoconductive properties of CdS nanobelts. Similar photoelectric effect was also reported in CdS nanoribbons by Chen et al.[93] and in CdS hemispherical nanowires by Li et al.[157] APPLICATIONS OF 1-D CdS NANOFORMS Fabrication of nanoscale devices with the desired nanoforms of potentially important materials is the ultimate goal of nantechnology. Semiconducting CdS have wide applications in the field of optoelectronic and photovoltaic devices. 1-D nanoforms of CdS also finds interesting applications in emerging nano-scale device fabrications. Laser Duan et al.[7] have first reported about the single CdS nanowire based laser. In general, a nanowire will function as

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a single-mode optical waveguide. In the case of CdS nanowires the minimum diameter needed to support a single mode is of the order of 70 nm. If the ends of the nanowire are cleaved, they can function as two reflecting mirrors that define Fabry – Perot optical cavity. Pan et al.[100] have showed the waveguide properties of the CdS nanoribbons. The waveguide properties of the CdS nanoribbons were spectrally detected with the use of near-field scanning optical microscopy. They have demonstrated that the locally produced band-edge photoluminescence can be effectively transported several hundred micrometers away along a single CdS nanoribbon, and the emission bands showed a significant red shift with respect to increasing traveling distances. A band-tail mechanism was proposed to describe this waveguide behavior in CdS nanoribbons. Liu et al.[2] have detected lasing in CdS nanoribbons upon optical pumping. The mechanism of lasing in single CdS nanowires was elucidated by temperature-dependent spectroscopic studies and studies by Agarwal et al.[32] showed that the mechanism was exciton-based: exciton-exciton scattering from 4.2 to 75 K and exciton-LO scattering at higher temperatures. They have proposed that the excitonic mechanism lasing could be enhanced through of nanowire laser cavities based on radial nanowire heterostructures. Specifically, small diameter nanowires coated by a larger band-gap material might be used efficient nanoscale lasing structures, where the small diameter nanowire provides a low threshold active medium due to exciton confinement, and the outer shell would facilitate waveguiding within a small modal volume optical cavity. p-n Junction Diode The high performance of inorganic materials combined with the diversity of organic materials makes the inorganic-organic hybrid device attractive for optoelectronics. It was recently reported that the charge conduction channel might be crucial for the performance of hybrid devices.[158] As the first step toward elucidating the inherent charge conduction channel of the vertically aligned CdS nanowires, Liang et al.[35] have evaporated a p-type organic layer of N,N0 -bis(naphthalene1-yl)-N,N0 -bis(phenyl)benzidine (NBP) onto the vertically aligned nanowire array prepared by the supercritical drying process to form a nanowire/NBP junction. The asymmetrical I– V curve displayed in Figure 23 clearly shows a rectification effect[159] of a good p-n junction. Thin Film Transistor Duan et al.[1] have taken the nanomaterial-enabled electronics in a new direction by exploiting nanomaterials not for the next generation of nanoelectronics, but for highperformance macroelectronics. Si nanowires and /or CdS nanoribbons were ensemble into oriented nanowire thin films to yield a novel electronic substrate; this substrate was processed using standard methods to produce NW-TFTs with conducting channels formed by multiple parallel single-crystal


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Absorption spectrum of the composite layer was the same as the superposition of the absorption spectrum of each individual material. The photoluminescence signal from MEH-PPV.lm was reduced as a result of the mixing. The energy conversion efficiency of MEH-PPV solar cell improved from 0.0012% to about 0.60% when combined with the vertically aligned CdS nanorods.

FIG. 23. I –V curve of an inorganic nanowire/organic NPB hybrid device. The inset shows the configuration of the device. The forward bias corresponds to a positive potential applied on the semitransparent Au electrode. The electrode area is 1 mm2. [Reprinted with permission from Ref. 35: J. Am. Chem. Soc. 2004, 126, 16338].

NW paths. In such NW-TFTs, charges travel from source to drain within single crystals, thus ensuring high carrier mobility. Solar Cell Among various types of organic solar cells, the organic – inorganic hybrid solar cell is one of the most promising type since it not only has a large interface area where excitons, the bound electron –hole pairs, may effectively dissociate but also has two separate channels for efficient electron and hole transport, i.e., the semiconductor nanorods and the polymer layers, respectively. Further improvement of hybrid solar cell was reported by Alivisatos et al. combining CdSe nanorods with poly(3-hexylthiophene).[159] Their devices showed an open circuit voltage of 0.7 V, a short circuit current of 5.7 mA, a fill factor of 0.4 and a power conversion efficiency of 1.7% under the air mass 1.5 condition. Compared to the earlier hybrid devices, their cells have the advantage of the improved optical absorption in the visible range by the semiconductor nanorods. The photovoltaic performance of the devices may still be improved if the nanorods are aligned vertically between the two electrodes to minimize the carrier transport paths and if various nanorod materials are tested for optimization. However, both the fabrication and the alignment of nanorods with different materials are difficult tasks. Electroplating on porous alumina membrane should be a relatively easy process by which to obtain aligned nanorods with various materials.[160] Kang et al.[161] have reported the optoelectronic properties of hybrid solar cells investigated by mixing cadmium sulfide nanorods with a conjugated polymer, poly[2-methoxy-5(20 -ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV). CdS nanorods were grown vertically on Ti substrates by electrochemical deposition through a porous alumina template.

Photonic Circuit Element Integrated photonics have the potential to overcome limitations of speed and power dissipation being faced in silicon based electronics, and thereby enable applications from advanced communications to revolutionary computing systems.[162 – 164] Central to progress in this area has been the development of materials and structures, including photonic crystals (PCs)[165] and plasmon waveguides,[164,166] that can guide and manipulate light in increasingly complex ways. Precisely defined defects can produce waveguides in PCs that enable light to be guided through sharp bends,[163] although the length scale of these structures is still on the order of the wavelength of light. Light has been transported in much smaller structures using nanoscale plasmon waveguides consisting of metal nanoparticles;[164,166] however, these have shown substantial losses. Submicron wires or nanowires can also function as waveguides,[167,168] yet work[168] suggests that bends will require curvatures larger than PC- and plasmon-based approaches due to the exponential dependence of loss (attenuation coefficient) on the radius of curvature in dielectric waveguide bends.[167] Resonant structures[168] and integrated mirrors[169] can improve the transmission at sharp bends in dielectric waveguides, but it remains unclear whether passive waveguides[1,3] could be used to manipulate light as required for integrated photonics. Barrelet et al.[172] have reported an approach for guiding and manipulating light on sub-wavelength scales using active nanowire waveguides and devices. Quantitative studies of cadmium sulfide (CdS) nanowire structures show that light propagation takes place with only moderate losses through sharp and even acute angle bends. In addition, measurements demonstrate that efficient injection into and modulation of light through nanowire waveguides are achievable in active devices. The ability to inject, guide, and manipulate light on a subwavelength scale using nanowire components that can be assembled into integrated structures represents a promising pathway towards integrated nanoscale photonic systems. Optical Switching The photodetection or so called optical switching properties of 1-D nanostructures are of particular interest because an optical gating can work as an alternative to electrical gating for devices used in memory storages and logic circuits, where the performance critically relies on binary switching.[151 – 153] The photoconductivity studies of the 1-D CdS



nanoforms discussed earlier indicates the applicability of the 1-D CdS nanoforms for optical switching applications. CONCLUSIONS The importance of the 1-D CdS nanostructures such as nanorods, nanowires, nanobelts/nanoribbons or nanotubes in the advancement of nanoscience and nanotechnology has been discussed here. A variety of synthesis approaches have been utilized by the researchers to synthesize various types of 1-D CdS nanoforms. It was noted that wet chemical approaches such as solvothermal/hydrothermal routes is particularly suitable to get bulk quantity of nanoforms in the powder form. Template assisted electrochemical deposition process is a widely utilized route to achieve aligned nanowire arrays. VLS and VS process are ideal for getting ultra long single crystalline CdS nanowires and nanobelts/ nanoribbons. Optical, electrical, photovoltaic properties of these 1-D nanoforms of CdS showed encouraging results. Applicability of these 1-D nanoforms was already tested in the fields of photonics, electronics, optoelectronics and photovoltaics. REFERENCE 1. Duan, X.; Niu, C.; Sahi, V.; Chen, J.; Parce, J. W.; Empedocles, S.; Goldman, J. L. High-performance thin-film transistors using semiconductor nanowires and nanoribbons. Nature 2003, 425, 274– 278. 2. Liu, Y. K.; Zapien, J. A.; Geng, C. Y.; Shan, Y. Y.; Lee, C. S.; Lifshitz, Y.; Lee, S. T. High-quality CdS nanoribbons with lacing cavity. Appl. Phys. Lett. 2004, 85, 3241– 3243. 3. Zhang, J.; Jiang, F.; Zhang, L. Fabrication of single-crystalline semiconductor CdS nanobelts by vapor transport. J. Phys. Chem. B 2004, 108, 7002–7005. 4. Agata, M.; Kurase, H.; Hayashi, S.; Yamamoto, K. Photoluminescence spectra of gas evaporated CdS films microcrystals. Solid State Commun. 1990, 76, 1061– 1065. 5. Ullrich, B.; Bagnall, D. M.; Sakai, H.; Segawa, Y. Photoluminescence properties of thin CdS films on glass formed by laser ablation. Solid State Commun. 1999, 109, 757– 760. 6. Artemyev, M. V.; Sperling, V.; Woggon, U. Electroluminescence in thin solid films of closely packed CdS nanocrystals. J. Appl. Phys. 1997, 81, 6975 –6977. 7. Duan, X. F.; Huang, Y.; Agarwal, R.; Lieber, C. M. Singlenanowire electrically driven lasers. Nature 2003, 421, 241–245. 8. Springer Series in Solid State Sciences 60. In Excitonic Processes in Solids; Ueta, M., Kanzaki, H., Kobayashi, K., Toyozawa, Y., Hanamura, E., Eds.; Springer: Berlin, 1986. 9. Danaher, W. L.; Lyons, L. E.; Morris, G. C. Some properties of thin films of chemically deposited cadmium sulphide. Sol. Energy Mater. 1985, 12, 137– 148. 10. Livingstonet, F. M.; Tsang, W. M.; Barlow, A. J.; Rue, R. M.; De, La.; Duncan, W. Si/CdS heterojunction solar cells. J. Phys. D: Appl. Phys. 1977, 10, 1959– 1964. 11. Nanda, J.; Kuruvilla, B. A.; Sarma, D. D. Photoelectron spectroscopic study of CdS nanocrystallites. Phys. Rev. B 1999, 59, 7473– 7479.

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