Electrochemical Synthesis and Charge Transport Properties of CdS

Jun 22, 2010 - Silambarasan Arjunan , Helen P. Kavitha , S. Ponnusamy , Navaneethan Mani , Yasuhiro Hayakawa. Journal of Materials Science: Materials ...
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J. Phys. Chem. C 2010, 114, 11911–11917

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Electrochemical Synthesis and Charge Transport Properties of CdS Nanocrystalline Thin Films with a Conifer-like Structure Ya-Xiong Nan,†,‡,§ Fei Chen,† Li-Gong Yang,*,‡ and Hong-Zheng Chen*,†,‡ State Key Laboratory of Silicon Materials, MOE Key Laboratory of Macromolecule Synthesis and Functionalization & Department of Polymer Science and Engineering, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, Zhejiang-California International Nanosystems Institute, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China, and State Key Laboratory for Mechanical BehaVior of Materials, Xi’an JiaoTong UniVersity, Xi’an 710049, People’s Republic of China ReceiVed: April 6, 2010; ReVised Manuscript ReceiVed: June 10, 2010

Well-defined intersectional CdS nanocrystalline thin films with a conifer-like morphology were synthesized by a facile electrochemical process. The morphology, structure, and phase composition of CdS nanostructures were examined by X-ray diffraction, field emission scanning electron microscopy, atomic force microscopy, and energy-dispersive X-ray spectroscopy. The electrochemical deposition conditions influencing the synthesis of these conifer-like CdS nanocrystals, such as deposition temperature, deposition time, current density, and concentrations of the precursors, were studied systematically. The orientation-transformation mechanism of the growth of conifer-like CdS nanocrystals was proposed based on the results, which is beneficial for the shape-controlled synthesis of other shaped nanostructures. The charge transport properties of these thin films before and after annealing treatment were also studied via the space-charge-limited conduction model, from which the total density of trapping states and the electron mobility in CdS nanocrystalline thin films were estimated. Introduction The synthesis of inorganic nanocrystals, especially semiconducting nanostructures, with controllable sizes, shapes, and crystallization has attracted noticeable research interest because they are potential building blocks for advanced materials1 and optoelectronic applications in light-emitting devices, photodetectors, solar cells,2-4 and other devices. During the past decade, nanoparticles and one-dimensional (1D) nanostructures, such as nanowires, nanobelts, nanotubes, and nanorods,5-8 have been the focus of scientific research. Compared with dispersed nanoparticles and 1D nanostructures, the thin-film-based nanostructures9,10 may be employed as building blocks to more practical large-scale optoelectronic devices. Though CdTe, CdSe, TiO2, and ZnO thin films were investigated a lot,11-14 more attention still should be paid to the better controllability of growth of the thin films with surface nanostructures. Cadmium sulfide (CdS), as one of the most important II-VI group semiconductors with a direct band gap of 2.4 eV, has wide applications in optoelectronic devices.15-18 Various methods have been developed for the synthesis of 1D nanostructured CdS, including the microwave-solvothermal method, hydrothermal route, surfactant-ligand coassisted solvothermal synthesis, and colloidal micellar approach.19-23 Nevertheless, most of CdS nanomaterials prepared from these methods are dispersed and have less controllability. So far, film-based CdS nanostructures have been investigated by a few groups by means of * To whom correspondence should be addressed. E-mail: hzchen@ zju.edu.cn (H.-Z.C.), [email protected] (L.-G.Y.). † State Key Laboratory of Silicon Materials, MOE Key Laboratory of Macromolecule Synthesis and Functionalization & Department of Polymer Science and Engineering, Zhejiang University. ‡ Zhejiang-California International Nanosystems Institute, Zhejiang University. § Xi’an JiaoTong University.

vacuum evaporation,24 chemical vapor deposition,25 chemical bath deposition,26 and electrodeposition.16 Among these approaches, electrochemical deposition is much more promising because it can meet the industrial requirements for large-scale production with a low cost. As to the charge transport of CdS, the electrical properties of CdS polycrystalline films and the defect effects were investigated during the 1970-1980s.27-29 Recently, the research on the charge transport of CdS focused on the electrical properties of dispersed CdS nanostructures30-32 and CdS thin films synthesized by chemical bath deposition and thermal evaporation33-36 and on the charge transport properties of CdS nanostructures/polymer semiconductors (or inorganic semiconductors, such as CdTe, TiO2, InP, etc.) hybrid heterojunctions.37-39 However, there have been few investigations on the charge transport properties of CdS nanocrystalline thin films with a complex surface morphology and nanostructures. In this paper, we report the synthesis of CdS nanocrystalline thin films with a conifer-like morphology by a facile electrochemical deposition approach. The reaction conditions influencing the synthesis of these conifer-like CdS nanocrystals were studied in detail. The growth mechanism for the conifer-like CdS nanocrystals was proposed and discussed. The electrical properties of these surface nanostructured thin films were also measured, and based on the correlation between the morphology and the electrical properties, the transport characteristics of these films were investigated. Experimental Section Synthesis and Characterization of CdS Nanocrystalline Thin Films. The general synthesis of conifer-like CdS nanocrystalline thin films by electrodeposition can be described as below. The etched indium-tin oxide (ITO) glass, as the working

10.1021/jp103085n  2010 American Chemical Society Published on Web 06/22/2010

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Figure 1. (a-c) FESEM images of conifer-like CdS nanocrystalline thin films at different locations and magnifications. The inset in (c) is the high-magnification image. (d) The geometric distribution of the CdS nanoconifers with an intercross angle of ∼60°. (e) Photograph of the asprepared CdS nanocrystalline thin film (yellow region) on an etched ITO substrate. (f) EDX spectrum of the corresponding conifer-like CdS nanocrystals. (deposition conditions: 0.10 g of CdCl2 · 2.5H2O and 0.10 g of sulfur powder at 120 °C and 0.08 mA · cm-2 for 45 min.)

electrode, is immersed vertically into the electrolyte with CdCl2 · 2.5H2O and sulfur powder as precursors and a dimethylsulfoxide (DMSO)-water mixture as a component solvent, followed by heating to 120 °C and keeping this temperature for 45 min. In a typical process, the electrolyte was prepared by dissolving CdCl2 · 2.5H2O (0.10 g) and the sublimated elemental sulfur (0.10 g) in a mixture containing 90 mL of DMSO and 20 mL of deionized water. All the chemicals were reagent grade without further treatments. The electrodeposition was carried out at a constant current density (0.08 mA · cm-2) and 120 °C for 45 min. After deposition, the film was rinsed with hot DMSO solvent and dried at 80 °C. The synthesized CdS thin films were characterized by X-ray diffraction (XRD) patterns on a Rigaku D/max-2550PC X-ray diffractometer with Cu KR radiation (λ ) 1.5406 Å). The energy-dispersive X-ray spectroscopy (EDX) spectrum was recorded on a Horiba 7593-H EDX spectroscope. Field emission scanning electron microscopy (FESEM) images were obtained on a Hitachi-4800 instrument. Atomic force microscopy (AFM) images were acquired on a Veeco-diMultiMode scanning probe microscope. The UV-vis absorption spectrum was recorded on a CARY 100Bio UV-visible spectrophotometer. Charge Transport Property Characterization of CdS Nanocrystalline Thin Films. To study the charge transport properties of the above-prepared CdS nanocrystalline thin films, ITO/CdS/Al sandwich structured devices were fabricated by depositing an ∼55 nm layer of aluminum as a cathode on the

annealed CdS films by thermal evaporation under 7.5 × 10-7 Torr. The active area for the device was 0.03 cm2. The annealing process was carried out in a nitrogen atmosphere at 380 and 450 °C for 2 h. For a comparison, the carrier transport within ITO/CdS/Al devices made of the as-prepared CdS thin films were also studied. The thickness of the CdS thin film was measured on an AMBIOS XP-1 high-resolution surface profiler. The current-voltage (I-V) characteristics of the ITO/CdS/Al devices were measured under dark conditions in air on an Agilent 4155C semiconductor parameter analyzer. Results and Discussion Synthesis of CdS Nanocrystalline Thin Films with a Conifer-like Morphology. Figure 1a-c shows the typical FESEM images of the as-prepared CdS nanocrystalline thin films obtained by an electrochemical deposition approach. The SEM images of the thin films at different locations and magnifications reveal that the whole ITO glass is fully covered with highly intersectional conifer-like CdS nanocrystals. The geometric distribution of these conifer-like nanostructures (nanoconifers) is optimized in two-dimensional (2D) directions with a high density (Figure 1b). The average length and width of the nanoconifers are about 650 and 250 nm, respectively, with the average intercross angle between two contact nanoconifers being approximately 60° (Figure 1c,d). Figure 1e shows the photograph of the obtained conifer-like CdS nanocrystalline

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Figure 2. Two-dimensional AFM images of the as-prepared CdS nanocrystals synthesized with 0.10 g of CdCl2 · 2.5H2O and 0.10 g of sulfur powder at 120 °C and 0.08 mA · cm-2 for (a) 45 and (b) 15 min. Panels (c, d) and (e, f) are the corresponding 3D AFM images and cross-sectional SEM images of (a) and (b), respectively.

thin films (yellow region) on the etched ITO substrate with an area of 13 mm × 6 mm determined by the dimensions of the counter electrode of platinum foil. The corresponding EDX spectrum shown in Figure 1f indicates the presence of Cd and S elements with a ratio of 1:1.04, in agreement with the stoichiometric composition of CdS. The observed signals of Si and Cl elements are from the ITO glass substrate and the remaining electrolyte, respectively. We also characterized the three-dimensional (3D) structures of the as-prepared conifer-like CdS nanocrystals by AFM, shown in Figure 2. The AFM images further confirm that the sample really presents an intersectional conifer-like structure (Figure 2a) and shows that the surface of the thin film is not smooth enough (Figure 2c). By measuring the typical sample, we find that the nanoconifer is about 650 nm in length and 250 nm in width, in accord with the SEM results. Meanwhile, the average surface roughness is about 270 nm, whereas the total thickness of the CdS film is ∼540 nm. It means that there is a 200-300 nm layer under the conifer-like top layer, which would be the seed layer for nanoconifers. Simultaneously, the obvious perpendicularly oriented growth of CdS nanocrystals shown in Figure 2e illuminates that this underlayer is a totally unconifer-

like structure and indicates that some changes have happened during the formation of the conifer-like CdS nanocrystals. Figure 2b,d,f shows the AFM images and the cross-sectional SEM images of the as-synthesized CdS thin film prepared with 0.10 g of CdCl2 · 2.5H2O and 0.10 g of sulfur powder at 120 °C and 0.08 mA · cm-2 for 15 min, compared with the typical deposition conditions mentioned above, which clearly uncovered the formation of CdS nanoparticles (∼100 nm in diameter) continuously instead of nanoconifers during a shorter deposition time. This thin film with the surface roughness of ∼50 nm and thickness of ∼200 nm must play the role of seed layer. In view of the evidently morphological difference between the two CdS nanocrystalline thin films discussed above, the XRD patterns of the CdS thin films deposited for 15, 30, 45, and 60 min were taken in order to investigate the structure transformation during the growth of nanoconifers (Figure 3). The XRD patterns reveal that all the diffraction peaks of the four samples can be indexed as the hexagonal wurtzite CdS except some peaks attributed to the ITO glass, which are consistent with the literature data (JCPDS card, File No. 411049). More importantly, it is observed that the intensity of the (100) plane diffraction peak gradually becomes stronger relative

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Figure 3. XRD patterns of CdS nanocrystalline thin films prepared with 0.10 g of CdCl2 · 2.5H2O and 0.10 g of sulfur powder at 120 °C and 0.08 mA · cm-2 for 15, 30, 45, and 60 min.

Figure 4. SEM images of CdS nanocrystalline thin films prepared with 0.20 g of CdCl2 · 2.5H2O and 0.20 g of sulfur powder at 0.08 mA · cm-2 at (a) 90, (b) 100, (c) 110, and (d) 120 °C for 60 min.

to that of the (002) plane diffraction with increasing the deposition time. The relatively strong and narrow (002) diffraction peak observed in the 15 min-deposited sample without nanoconifers indicates that CdS nanocrystals grow preferentially oriented along the [001] direction. Considering that the direction of ion motion during electrodeposition is perpendicular to the ITO substrate, the (002) plane should be parallel to the substrate; in other words, the oriented growth is perpendicular to the ITO substrate in the beginning. With the progress of the deposition, the gradually intensified (100) diffraction peak, in this case, can be attributed to the preferential oriented growth along the [100] direction. Observing that the (100) plane is vertical to the (002) plane, it is interesting to find that the preferential growth orientation of the nanocrystals changes obviously from the [001] direction perpendicular to the substrate at first to the [100] direction parallel to the substrate with the appearance of nanoconifers. Influence Factors of Conifer-like CdS Nanocrystals. To better understand the formation process of these electrodeposited conifer-like CdS thin films, the effect of deposition parameters, including the deposition temperature, deposition time, current density, and the concentrations of CdCl2 · 2.5H2O and sulfur powder,onthegrowthofnanoconifersisinvestigatedsystematically. Figure 4 shows the SEM images of CdS samples prepared at different temperatures with other identical conditions. It is found that no CdS nanoconifers are formed when the deposition happens at 90 °C, with the relatively smooth surface and small grains only. Scalelike CdS nanocrystals with the loomed intercross angle of about 60° are observed when the deposition

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Figure 5. SEM images of CdS nanocrystalline thin films prepared with 0.20 g of CdCl2 · 2.5H2O and 0.20 g of sulfur powder at 110 °C and 0.08 mA · cm-2 for (a) 15, (b) 30, (c) 45, and (d) 60 min. The insets in (a) and (d) are the corresponding cross-sectional views.

temperature increases to 100 °C. If the temperature increases to 110 °C, CdS nanoconifers are formed clearly, though the aspect ratio is low (∼2:1) due to the unintegrality of the growth of nanoconifers. As the deposition is carried out at 120 °C, welldefined intersectional CdS nanoconifers are grown, accompanied by the aspect ratio increasing to ∼3:1. While the deposition temperature increases to a higher state, the deposited film partly flakes off the substrate because of the weakening of the adhesion between the film and ITO glass due to the higher thermal stress. These observations indicate that the temperature of 120 °C is the optimal reaction condition for CdS nanoconifer growth during electrodeposition. Figure 5 presents the morphology evolution of the as-prepared CdS films from nanoparticles to nanoconifers for different deposition times. When the CdS thin film was deposited for 15 min, a relatively smooth CdS nanocrystalline film with a perpendicular growth orientation was grown continuously (Figure 5a). After 15 min, scalelike CdS nanocrystals were formed sparsely from the fine nucleuses (Figure 5b). If the deposition lasted for 45 min, highly intersectional CdS nanoconifers were presented closely (Figure 5c). No obvious variety was observed between 45 and 60 min-deposited films with the exception of the thickness (Figure 5d and its inset). According to the above-mentioned processes, the formation of CdS nanoconifers can be divided into three steps. First, perpendicularly oriented CdS nanocrystals (inset of Figure 5a) are deposited on the ITO glass continuously with a top view of particles (Figure 5a), which play the role of seed layer. Next, CdS grains with a diameter of 30-40 nm begin to nucleate on this seed layer, and then reunite to form the rudiments of the nanoconifers. Finally, conifer-like CdS nanocrystals grow upon those nanoconifers and the geometric distribution appeared gradually. Figure 6 exhibits the SEM images of CdS samples synthesized at different current densities. The CdS films deposited at 0.04 mA · cm-2 correspond to the above-mentioned second step of the formation of CdS nanoconifers (Figure 6a). Conifer-like CdS nanocrystals grow obviously with the current density of 0.06 mA · cm-2 (Figure 6b). When the deposition current density increases to 0.08 mA · cm-2, the configuration of the nanoconifers is developed integrally. However, when the current density is increased to 0.10 mA · cm-2, the nanoconifers become compressed in the third dimension. Thus, the current density of 0.08 mA · cm-2 should be a critical point.

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Figure 6. SEM images of CdS nanocrystalline thin films prepared with 0.20 g of CdCl2 · 2.5H2O and 0.20 g of sulfur powder at 110 °C at (a) 0.04, (b) 0.06, (c) 0.08, and (d) 0.10 mA · cm-2 for 60 min.

Figure 7. SEM images of CdS nanocrystalline thin films prepared with both CdCl2 · 2.5H2O and sulfur powder using (a) 0.05, (b) 0.10, (c) 0.15, and (d) 0.20 g at 110 °C and 0.08 mA · cm-2 for 60 min. The inset in (a) is the high-magnification view.

Taking into account eq 1

j ) nqVs

(1)

(where j is the current density, n is the number of charged ions, q is the number of charge per ion (e.g., 2e for Cd2+), V is the ion velocity, and s is the sectional area of the ion channel), then the aforesaid morphology change caused by the increase of current density can be related to V, the ion velocity. Cd and S atoms can diffuse adequately at the ITO glass to form the close-packed (001) plane, namely, oriented growth perpendicular to the substrate, due to the low ion velocity at a weak current density. Contrarily, the atoms construct the unclose-packed (100) plane. The formation of the compressed CdS nanoconifers results from the dominant deposition on the (100) plane instead of the (001) plane, that is, oriented growth parallel to the ITO substrate. Moreover, the crack observed in Figure 6c can be attributed to the residual stress owing to lattice mismatch at the interface of CdS and ITO (the hexagonal cell of CdS with cell constants a ) 4.141 Å and c ) 6.720 Å; the cubic cell of In2O3 with a ) 8.762 Å).40 On the other hand, the cracking is thought to result from the film shrinkage as a result of the remaining DMSO loss during drying.41 It is found that changing the concentrations of CdCl2 · 2.5H2O and the sulfur powder also changes the morphology of CdS nanoconifers greatly (Figure 7). On increasing the precursors’ concentrations from 0.05 to 0.20 g, the configuration of the conifer-like CdS nanocrystalline thin films becomes worse generally (Figure 7a-d). On the assumption that the CdCl2 · 2.5H2O and sulfur powder concentrations are constant at an arbitrary interval, ∆t, eq 1 is still applicable during the deposition. At the constant current density of 0.08 mA · cm-2, the ion velocity V decreases with increasing the concentrations of the reactants, n, resulting in the attenuation of the nanoconifers due to the dominant perpendicularly oriented growth to the substrate, in accord with the effect of the current density mentioned above. Additionally, at the reactants concentrations of 0.05 g, the CdS grains begin to grow abnormally as a result of the transition of reaction from a electrodeposition to a hydrothermal process due to the Cd and S atoms being exhausted or the reaction tending to equilibrium prematurely (Figure 7a). The highmagnification SEM image presents that the growth of the

Figure 8. Schematic illustrations of the growth process of CdS nanocrystalline thin films with a conifer-like structure.

nanoconifers is alright at the abnormal region, indeed, consistent with the above discussion (inset in Figure 7a). Growth Mechanism of Conifer-like CdS Thin Films. The growth mechanism for CdS nanocrystals with a conifer-like structure during the electrochemical deposition is schematically proposed in Figure 8. In the beginning, the motion velocity of Cd and S atoms from electrolyte to ITO is slow because of the abundance of these atoms, based on eq 1 with a constant current density. Hence, those atoms can diffuse adequately to form the close-packed (001) plane of their own to complete the oriented growth of CdS nanocrystals perpendicular to the ITO substrate. These [001] oriented nanocrystals are thought to adjust the reactants’ concentrations as the seed layer. With the decrease of the reactants’ concentrations continually, the velocity of those atoms increases, causing the formation of an unclose-packed (100) plane. This perpendicular transformation of the oriented direction is accompanied by the fine CdS grains reuniting to turn into nanoconifers (Figure 5b,c). Furthermore, the intercross angle of ∼60° observed between the two contact nanoconifers can be attributed to the hexagonal distribution of each Cd or S atomic layer in the wurtzite structure. On the other hand, it is thought to result from the lowest spatial resistance and energy required. UV-Wisible Spectra of Conifer-like CdS Nanocrystalline Thin Films. Figure 9 shows the room-temperature absorption spectra of the CdS thin films prepared with different deposition times. The gradual increase of the absorption intensity from 15 to 45 min of deposition without an obvious variety from 45 to 60 min of deposition is observed. Thickness variety is not the exclusive reason for this phenomenon because it varies clearly with the increase of the deposition time from 15 to 60 min. As suggested in Figure 5, when the deposition time increases from 15 to 45 min, the intensity increase can be ascribed to the

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Figure 9. Absorption spectra of CdS nanocrystalline thin films prepared with 0.10 g of CdCl2 · 2.5H2O and 0.10 g of sulfur powder at 120 °C and 0.08 mA · cm-2 for 15, 30, 45, and 60 min.

Figure 10. Typical I-V characteristics of ITO/CdS/Al devices in logarithmic scale. The CdS thin films were prepared with 0.10 g of CdCl2 · 2.5H2O and 0.10 g of sulfur powder at 120 °C and 0.08 mA · cm-2 for 15, 30, 45, and 60 min.

gradual transformation of the dominant oriented growth from the (001) plane with a narrow interplanar spacing of 3.3419 Å to the (100) plane with a wide interplanar spacing of 3.5658 Å. However, the slight change of absorption intensity between 45 and 60 min-deposited films is thought to result from the complete growth of the nanoconifers. Besides, with increasing the deposition time from 15 to 60 min, an obvious red shift of the absorption edge from 500 to 550 nm and the enhanced below-gap absorption can be observed in Figure 9. The red shift of the absorption edge demonstrates that the shrinkage of the band gap with the film growth, which could be understood by the formation of more CdS nanoconifers in varied growth directions, as discussed above. The main mechanism responsible for the below-gap absorption might be the stoichiometric relationship between Cd2+ and S2- varied for longer deposition times when CdS nanocrystals were growing from the electrolyte. Therefore, with the film growing, more defects are formed and the optical transition involving these midgap levels will be enhanced.

Electrical Properties of CdS Nanocrystalline Thin Films. The charge transport characteristics of CdS nanocrystalline thin films were characterized by unipolar space-charge-limited (SCL) conduction. It is known that the shape of the SCL I-V relationship depends on the position of the quasi-Fermi level relative to the detailed distribution of local states near the collecting electrode. For spatially homogeneous samples, the gradient of trap concentration along the direction vertical to the sample surface could be ignored, and the charge carrier mobility could be estimated from the transition voltage where the charge conduction mode in samples transits from the Ohmic to SCL regime.42 Additionally, the density of local states could be obtained by the den Boer method.43 However, CdS nanocrystalline thin films in our investigation are spatially inhomogeneous, according to the above discussion. For this case, traps have a distribution not only of energies but also along the film thickness. The density of local states can then be obtained by the differential scheme proposed by Sworakowski.44 Typical I-V curves of the devices with the ITO/CdS/Al configuration are given in Figure 10, from which the evolution tendency of the total density of localized trapping states, Nt, and the electron mobility, µe, of the thin films for different growth periods can be traced. The electrical properties of the as-prepared CdS thin films are summarized in the left part of Table 1. With the increase of the deposition time, Nt decreases gradually owing to the uniform distribution of intracrystalline impurities of Cl and the decrease of the grain boundaries with the growth of the grains, leading to the increasing electron mobility, except the mobility of the 60 min-deposited film due to that the point defects of Cd or S vacancy resulted from the mismatched stoichiometric relationship with the longer deposition time. To study the annealing effects on the transport characteristics of CdS nanocrystalline thin films, the samples synthesized with 0.10 g of CdCl2 · 2.5H2O and 0.10 g of sulfur powder at 0.08 mA · cm-2 and 120 °C for 60 min were heated to 380 and 450 °C in a nitrogen atmosphere for 2 h, in view of the phase transformation in CdS at ∼300 °C.45 The right part of Table 1 shows the electrical properties of the annealed CdS films. As we expected, Nt decreases gradually with increasing the annealing temperature, while µe is proportional to the annealing temperature. This is due to the volatilization or the uniform distribution of the impurities (Cl and the excessive S, etc.), which reduces the defects. Moreover, the phase transformation of CdS to a totally wurtzite structure plays a great role in the variety of Nt and µe. Compared with the unannealed CdS thin films, the Nt of the 450 °C annealed sample decreases to 3.54 × 1015 cm-3, which is lower than the reported value of 8.9 × 1015 cm-3 extracted from thermal-vacuum evaporated CdS films,36 whereas the µe increases only up to 0.0077 cm2 · V-1 · s-1, much smaller than the value reported for thermal-vacuum evaporated CdS films (0.094 cm2 · V-1 · s-1).36 It is worthy of noting that the thickness of the samples investigated here is only several hundred

TABLE 1: Electrical Properties of the As-Prepared and the Annealed CdS Thin Films as-prepared CdS thin filmsa deposition time (min) Nt (× 1015 cm-3) µe (× 10-3 cm2 · V-1 · s-1)

15 111 1.1

30 27.8 2.8

annealed CdS thin filmsb 45 10.8 6.6

60 5.6 5.2

annealing temperature (°C) Nt (× 1015 cm-3) µe (× 10-3 cm2 · V-1 · s-1)

RTc 3.93 5.4

380 3.65 5.6

450 3.54 7.7

a These unannealed CdS thin films were prepared with 0.20 g of CdCl2 · 2.5H2O and 0.20 g of sulfur powder at 110 °C and 0.08 mA · cm-2 for 15, 30, 45, and 60 min. b These annealed CdS thin films were prepared under the same conditions with 0.10 g of CdCl2 · 2.5H2O and 0.10 g of sulfur powder at 120 °C and 0.08 mA · cm-2 for 60 min. c RT ) room temperature.

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nanometers, whereas the typical thickness of samples prepared by vacuum evaporation is several micrometers. In addition to the influence of primary defects estimated above, the influences of the scattering and grain boundaries on the electron transport characteristics of our CdS samples have to be considered in view of the finite thickness effect. With the assumption of the noninteracting and diffusive scattering mechanisms, the electron mobility µe could be written as27 -1 -1 -1 µ-1 e ) µb + µg + µs

(2)

where µb, µg, and µs refer to bulk, grain boundary, and surface effects on the total mobility, respectively. Here, we have included the trapping effect and other scattering mechanisms in µb. In our electrodeposited CdS films, the transformation of the growth orientation of nanocrystals causes more grain boundaries existing along the direction of charge carrier transport than those in thermally deposited CdS films with a well-oriented columnar microstructure in the thickness direction. Thus, the µg-1 effect in our CdS films is significant due to the stronger scattering of grain boundaries even though the total density of localized trapping states is smaller than that of thermally evaporated CdS films.36 Meanwhile, the effect of surface scattering on the mobility has to be considered when the film thickness can compare with the mean surface-scattering length of carriers.27 In our case, the growth process for CdS films indicates the conifer-like layer forms on the top of a seeding layer with perpendicularly oriented nanostructures. Because the crystal axes of these conifer-like nanocrystals are perpendicular to the thickness direction, the electrons will be strongly scattered by the surface of this top layer before they are collected by the electrode. Assuming that the mean surface-scattering length of CdS is about 100 nm and the surface-scattering effect of the seeding layer could be ignored, the mobility will be approximately reduced to a half due to the enhanced surface scattering of the conifer-like top layer with a thickness of about 270 nm.27 Therefore, according to eq 2, µe is prominently lower in the investigated CdS samples even though the trap density is not so high. Conclusions In summary, well-defined intersectional CdS nanocrystals with a conifer-like structure are prepared by a facile electrodeposition method. On the basis of the systematic investigation on the deposition conditions, such as deposition temperature, deposition time, current density, and concentrations of the precursors, in detail, we found that the ion motion velocity dominated the variety of the morphology during the electrochemical process and proposed an orientation-transformation mechanism for these nanoconifers to explain the perpendicular transformation of the oriented growth from the [001] to the [100] direction. The absorption study revealed that the absorption range broadened with the appearance of CdS nanoconifers. Meanwhile, the total density of trapping states and electron mobility of 3.54 × 1015 cm-3 and 0.0077 cm2 · V-1 · s-1, respectively, were obtained with the SCLC model. The results demonstrate that this controllable electrochemical process provides a potential simple way for the construction of other nanostructures. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 50703035, 50990063, and 51011130028) and the developing

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