Solvothermal Synthesis of High-Aspect Ratio Alloy Semiconductor

Feb 5, 2009 - E-mail: [email protected] (S.K.); [email protected] (S.S)., † ... Template-Free Synthesis of Nanostructured CdxZn1–xS with Tun...
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J. Phys. Chem. C 2009, 113, 3617–3624

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Solvothermal Synthesis of High-Aspect Ratio Alloy Semiconductor Nanowires: Cd1-xZnxS, a Case Study Subhajit Biswas,† Soumitra Kar,*,† Swadeshmukul Santra,*,†,‡,§ Y. Jompol,† M. Arif,† and Saiful I. Khondaker†,| NanoScience Technology Center, Department of Chemistry, Biomolecular Science Center, and Department of Physics, UniVersity of Central Florida, Orlando, Florida 32826 ReceiVed: NoVember 19, 2008; ReVised Manuscript ReceiVed: December 15, 2008

High-aspect ratio Cd1-xZnxS (x ) 0-1) alloy semiconductor nanowires are reported here for the first time using an ethylenediamine (en)-assisted solvothermal approach. The composition of the en-water (en-w) mixed solvent plays a crucial role in determining the morphology and crystalline phase of the alloy nanowires in the middle range of x (i.e., when x approaches 0.5). A phase transformation from hexagonal to cubic was observed for the middle range of x in an en-dominated solvent (en/w, 5:1). Nanowires with hexagonal phases were formed for the entire range of x with increasing water content of the solvent system (en/w, 2:1). The aspect ratio of the nanowires was found to be dependent on the x value as well as solvent composition. Alloy nanowires exhibit photoresponse properties when illuminated under white light. The synthesis technique is modified to synthesize core-shell Cd1-xZnxS/ZnS nanowires. Enhancement in photoluminescence efficiency is observed with a ZnS shell over the alloy nanowires. 1. Introduction Nanowires (NWs) have attracted intensive experimental and theoretical interest as a result of their novel fundamental properties and potential applications in nanoscale devices.1-10 The II-VI binary semiconductor NWs possess unique electronic and optical properties and are useful in novel nanoscale devices such as light-emitting diodes,11 single-electron transistors,12 and thin-film field-effect transistors.13 The properties of semiconductor NWs are highly size dependent,14 and thus precise control of their size, i.e., diameter and aspect ratio (length/diameter), is of critical importance. Band gap energy of binary semiconductor nanostructures is an important parameter for electronic and optoelectronic applications. The optical band gap of the semiconductor nanostructures can be tuned by changing their size. However, tuning the diameter and aspect ratio of onedimensional (1D) nanostructures is extremely challenging. Specifically, tuning the diameter of 1D nanostructures within the Bohr diameter range to achieve desired band gap tunability is critical. However, engineering the composition of the material is an alternative option to tuning the band gap of 1D nanostructures for specific applications. Semiconducting alloy NWs with variable elementary composition provide an extra degree of freedom in band gap engineering. Recent advances in synthesis techniques have led to the exploration of ternary alloy NWs with tunable band gaps.15,16 CdS (Eg ) 2.42 eV at room temperature) and ZnS (Eg ) 3.6 eV at room temperature) are important members of the II-VI semiconductors. The 1D nanostructures of these materials are widely used in the fields of sensors, lasers, waveguides, transistors, solar cells, etc.4,17-23 Ternary alloy nanostructures * To whom correspondence should be addressed. Fax: 1-407-882-2819. Telephone: 1-407-882-2848. E-mail: [email protected] (S.K.); [email protected] (S.S). † NanoScience Technology Center. ‡ Department of Chemistry. § Biomolecular Science Center. | Department of Physics.

having the composition of Cd1-xZnxS (x ) 0.0-1.0, i.e., in between CdS and ZnS) are expected to provide electronic band gap tunability.24 In comparison to CdS, Cd1-xZnxS with a larger band gap increases the short circuit current in solar cell devices.24 Because of this wide band gap feature, Cd1-xZnxS is a promising material for optoelectronic applications in the UV-visible spectral region. Furthermore, the lattice constant of Cd1-xZnxS is well matched to common substrates such as GaAs, GaP, or Si,15,25 which is favorable for electronic device fabrication. Cd1-xZnxS thin films have been widely used as window material for heterojunction solar cells.26,27 Because of the growing interest of 1D nanostructures in optoelectronics, such morphologies of Cd1-xZnxS are of especial interest. Recently, there have been a few reports on the synthesis of ternary alloy Cd1-xZnxS NWs using a high-temperature-based thermal evaporation process.28-32 However, these high-temperature processes require special equipment and skills, which make chemical synthesis much more desirable. There are very few reports on the chemical synthesis of high-aspect ratio alloy semiconductor NWs. Recently, Zhang et al. reported the synthesis of Zn-doped CdS nanorods from a single precursor via a solvothermal technique.33 High-aspect ratio Cd1- xZnxS NWs with x ) 0.0-1.0 have not been reported so far via chemical routes. Thus, it is highly desirable and challenging to synthesize high-aspect ratio alloy Cd1-xZnxS NWs via chemical routes. Moreover, in order to improve the performance of these 1D nanostructures in devices, it is important that the efficiency of 1D semiconductor nanostructures is increased. Because surface defects are prominent due to a large surface to volume ratio, the efficiency of nanostructures and hence their performance in nanodevices could be improved by reducing surface defects.34-38 Thus it is also important to fabricate core-shell NW heterostructures via simple synthesis techniques. In this article, we have addressed the above-mentioned dual challenges: (i) synthesis of high-aspect ratio alloy semiconductor NWs via simple chemical routes and (ii) fabricating a core-shell heterostructure keeping the aspect ratio of the NWs high via

10.1021/jp810177a CCC: $40.75  2009 American Chemical Society Published on Web 02/05/2009

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simple manipulation of the technique used to synthesize core NWs. To the best of our knowledge, this is the first report of the synthesis of high-aspect ratio Cd1-xZnxS NWs for the entire range of x ) 0.0-1.0 via a solvothermal route. Preliminary studies indicate that alloy NWs possess significant electrical transport properties. In addition, we have also demonstrated the fabrication of high-aspect ratio Cd1-xZnxS/ZnS core-shell NWs by simple manipulation of the solvothermal process. 2. Experimental Section For the synthesis of Cd1-xZnxS NWs, a Teflon-lined stainless steel cylindrical closed chamber with a 40 mL capacity was used. All the chemicals were of analytical grade and were used without any further purification. An appropriate amount of zinc acetate [(CH3COO)2Zn, 2H2O], cadmium acetate [(CH3COO)2Cd, 4H2O], and thiourea (tu, NH2CSNH2) were placed in the Teflon-lined chamber, which was then filled with an ethylenediamine (en, NH2CH2CH2NH2) and water mixture with different volume ratios (en/water, 5:1 and 2:1) up to 80% of its volume. The zinc-cadmium and sulfur sources were used in 1:3 molar ratios. After the contents has been stirred for a few minutes, the closed chamber was placed inside a preheated oven at 175 °C for 8 h and then cooled to room temperature. The resulting precipitates were filtered off and washed several times in water and ethanol. The final product was dried in vacuum at room temperature for 6 h to get the final powder product. The core-shell NW growth was carried out in a similar experimental setup. In a typical procedure, cadmium acetate, zinc acetate, and thiourea (1:3 molar ratios) were used with appropriate amounts of en. The closed chamber was placed inside a preheated oven at 175 °C, and the reaction was continued for 6 h. In the next step, a mixed aqueous solution of zinc acetate dihydrate (2.6 molar times that of the initial cationic source) and thiourea (1:3 molar ratio) was added, and the reaction was continued for an additional 4 h. Upon normal cooling, the product was washed several times in water and ethanol and finally dried in vacuum for characterization. The products were analyzed using a X-ray diffractometer (XRD) with Cu KR radiation, and the compositional analysis was done using energy dispersive X-ray spectrometry (EDAX). Microstructures and crystal structures of the nanorods were obtained by using a transmission electron microscopic (TEM) and high-resolution TEM (HRTEM) studies. Photoluminescence (PL) measurements were carried out at room temperature using 300 nm as the excitation wavelength with a luminescence spectrometer. Electrical transport properties were also measured by designing a simple device structure. The detail is discussed in the text. 3. Results and Discussion In this article, we have synthesized two types of high-aspect ratio NW systems: (A) Cd1-xZnxS alloy and (B) Cd1-xZnxS/ ZnS core-shell coaxial heterostructures. In order to maintain the ease and clarity of the presentation, the results are discussed as separate sections in the manuscript. 3.1. Cd1-xZnxS Alloy NWs. From our previous studies on the solvothermal synthesis of CdS and ZnS nanostructures,39-42 we have observed that en guided the formation of 1D nanostructures. The bivalent metal ions (Cd2+ or Zn2+) interact with the lone pair of electrons of nitrogen atoms in en. A subsequent reaction of en-ligated metal ions with S2- ions forms a 2D complex of CdS · 0.5en or ZnS · 0.5en.43 This complex possesses an organic-inorganic lamellar structure with inorganic CdS or ZnS layers separated by organic en spacers.43 As the reaction

Figure 1. X-ray diffraction pattern of Cd1-xZnxS alloy NWs synthesized with a 5:1 volume ratio of en and water.

proceeds, the complex dissociates, collapsing the lamellar structure. As a result, needle-shaped CdS or ZnS templates are formed. Under normal solvothermal conditions, the complex (such as CdS · 0.5en) and binary sulfide (such as CdS) phase remain in equilibrium; this favors en-assisted recrystallization of the templates. Over time, the templates lead to the formation of well-faceted nanorods or nanowires via en-assisted recrystallization of the templates and/or Ostwald ripening.42 It was also observed that the coordination between the Zn ion and en was stronger than that between Cd ions and en. Elimination of the en from the ZnS · 0.5en complex could be enhanced by increasing the thermal energy or by introducing a foreign noncoordinating solvent such as water.42 On the basis of our earlier works, we have selected a mixed solvent system of en and water in different volumetric compositions to synthesize Cd1-xZnxS NWs. The crystal structure and phase of the powder products were investigated through XRD. Figure 1 shows the XRD patterns of the Cd1-xZnxS NWs synthesized with a 5:1 volume ratio of en and water. The XRD pattern revealed the formation of pure hexagonal wurtzite CdS when only cadmium acetate was used as the cationic source. With the incorporation of Zn sources (up to x ) 0.1) along with the cadmium sources, a gradual XRD peak shift toward larger diffraction angles was observed, indicating the formation of a Cd1-xZnxS phase. Interestingly, when more of the zinc source (x ) 0.25 - 0.65) was administered, the XRD pattern revealed the formation of a cubic crystal structure instead of a hexagonal phase. These cubic XRD patterns also showed characteristic peak shifts toward higher diffraction angles for higher zinc values. More interestingly, when the x value approaches 1 (x ) 0.75-0.95), the XRD pattern again transformed to those corresponding to the wurtzite phase. Thus, the XRD pattern indicated a phase transformation from wurtzite to cubic for intermediate x values. Now, when only the zinc source was used, the XRD pattern reveals the formation of a complex product with no specific peaks corresponding to the crystalline phases of ZnS. This complex product was reported earlier as the ZnS · 0.5en complex.43 The differences in the coordination ability of en with Zn2+ and Cd2+ ions and some other kinetic factors related to the solvothermal growth might be the reason for the difference. 43 In order to obtain phase pure ZnS, the en to water volume ration was increased to 2:1,

High-Aspect Ratio Alloy Semiconductor Nanowires

Figure 2. X-ray diffraction pattern of Cd1-xZnxS alloy NWs synthesized with a 2:1 volume ratio of en and water.

and the whole series of the Cd1-xZnxS nanostructures was synthesized again with this solvent composition. XRD patterns of the products obtained with the new solvent composition are shown in Figure 2. Figure 2 reveals the formation of wurtzite crystals for the entire range of x (i.e., x ) 0.0-1.0) with a gradual shift in peak position toward higher degrees with increasing x value. This peak shift signifies the semiconductor alloying with tailored elementary composition. The gradual XRD peak shift to larger diffraction angles correspondes to the increase in x; the Zn content may also rule out the phase separation or separated nucleation of CdS and ZnS nanostructures.44 Incorporation of Zn2+ ions with smaller ionic radii than Cd2+ ions in the CdS lattice reduced the lattice constant and hence the peak shift toward a larger diffraction angle. The interesting part of the XRD study is the observation a phase transformation from hexagonal to cubic for the intermediate range of x (x value ) 0.25-0.65) in en-rich solvent. As there is a high order of lattice mismatch between ZnS and CdS lattice, the substitution has forced a small change in the site symmetry and that might be the reason behind the phase transformation with a sufficient amount of both ions present in the Cd1-xZnxS alloy lattice structure.40 Also, the difference between the solubility of ZnS and CdS in en can cause a change in the kinetic condition and hence a transformation to the much more stable cubic state. Normally, the wurtzite phases of ZnS and CdS are stable at high temperature. But, the organic solventassisted solvothermal process is able to produce the wurtzite phase at relatively low temperatures. In our previous work on ZnS,39 we observed a similar hexagonal to cubic phase transformation with higher en concentration. This could be attributed to the formation of intermediate complex structures (ZnS · 0.5en) and the subsequent decomposition of the complex to ZnS. As already mentioned, the presence of water in the solvent could trigger steady decomposition of the complex, favoring the formation of wurtzite phase. So the different site symmetry in the Cd1-xZnxS lattice and rate of decomposition of the complex directed the growth of cubic Cd1-xZnxS in an en-rich solvent. Both the Zn and S atoms in wurtzite and sphalerite are four coordinated. So the phase transformation only requires a partial atomic rearrangement. Wurtzite has the simple hexagonal close-packed stacking order of ABABAB along the [001] direction with a resultant base of (001), and sphalerite has the cubic close-packed stacking order of ABCABC along the [111] direction with the characteristic crystallographic facet

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3619 of (111). On transformation from wurtzite to sphalerite, the (001) plane of wurtzite converts directly to the (111) plane of sphalerite.45 The change in the lattice parameter with x could be explained using Vegard’s law analysis, which is an empirical law that relates the substitution of a guest ion into the host lattice with the experimentally observed degree of lattice change due to the increasing defect ion concentration. Statistical substitution into a lattice site is predicted to lead to a lattice contraction for smaller ions and a lattice expansion for larger ions. Isolation of the defect ion at only surface sites or at interstitial sites will result in insignificant lattice shifts arising primarily from strain effects. This law has been found to be in good agreement with the experimental results obtained for Cd1-xZnxS thin films.46 We have measured the composition of the alloy nanostructures from the recorded XRD pattern. Vegard’s law relationship can be written as cx ) cCdS + (cZnS - cCdS)x, where cCdS, cZnS, and cx are c-axis lattice constants of hexagonal CdS, ZnS, and Cd1-xZnxS. respectively. As all the nanostructures synthesized with 2:1 en/w were crystallized with hexagonal structures, we have calculated the lattice constant values and compositions for the samples synthesized with an en/water ratio of 2:1. The results are summarized in Table 1. These results indicate that incorporation of Zn into the CdS lattice is much more favorable than the reverse process. When we have added equal amounts of Cd and Zn sources during synthesis, Cd was the primary component in the alloy lattice. Also, with the addition of 25 atom % of Zn or Cd, the actual amount present in the samples was found to be 17 and 7 atom %, respectively, confirming the fact that Zn is much easier to incorporate into the lattice. Mobility of the cations is the determining factor for the rate of substitution. The smaller radius and lighter mass of Zn (rZn2+ ) 74 pm and mZn ) 65.4 amu) would have a higher mobility than the larger radius and heavier mass of Cd (rCd2+ ) 92 pm and mCd ) 112.4 amu). Representative XPS spectra of the sample Cd0.75Zn0.25S synthesized with solvent composition 5:1 en/w are shown in Figure 3. The survey XPS spectrum (Figure 3a) of the product shows the presence of Zn, Cd, and S along with O and C. The presence of very small amounts of C and O2 in the spectrum is due to the carbon tape used for measurement and the adsorbed gaseous molecules such as CO2 and H2O, respectively. Highresolution XPS spectra of Zn 2p3/2, Cd 3d, and S 2p3 are shown in panels b-d of of Figures 3, respectively. The Zn 2p and Cd 3d peaks at binding energies 1023 eV (Zn 2p3/2), 405 eV (Cd 3d5), and 413 eV (Cd 3d3) were attributed to the Cd1-xZnxS molecular environment. The atomic ratio of Cd, Zn, and S determined from the XPS study is 0.81:0.19:1. The morphologies of the above products are investigated with transmission electron microscopy. Figure 4 depicts the TEM images of the samples synthesized with solvent composition 5:1 en/w. Figure 4a shows the two-dimensional sheetlike feature of the ZnS · 0.5en complex. Subsequent TEM images reveal that the incorporation of the Cd source along with the Zn source could transform the sheetlike features to nanowires even at the same solvent composition. When 5 atom % of a Cd source was introduced along with the Zn source, bunches of NWs were observed along with the well-separated NWs (Figure 4b). Further increase in the amount of the Cd percentage (10 and 25 atom %) formed well-separated NWs (Figure 4c,d). TEM images indicated that not only can the incorporation of a foreign noncoordinating solvent influence the decomposition of the complex but also relatively weak coordinating cations could perform the same role. TEM images shown in panels e and f of

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TABLE 1: Lattice Constants and Compositions of the Cd1-xZnxS nanostructuresa calculated from XRD lattice parameter (Å)

sample Cd0.9Zn0.1S Cd0.75Zn0.25S Cd0.5Zn0.5S Cd0.25Zn0.75S Cd0.1Zn0.9S a

a a a a a

) ) ) ) )

4.09, 4.07, 4.04, 3.85, 3.83,

c c c c c

) ) ) ) )

6.68 6.64 6.58 6.29 6.27

atomic % added during synthesis

atomic % determined from XRD

Cd)90,Zn)10 Cd)75,Zn)25 Cd)50,Zn)50 Cd)25,Zn)75 Cd)10,Zn)90

Cd)93,Zn)7 Cd)83,Zn)17 Cd)71,Zn)29 Cd) 7,Zn)93 Cd) 5,Zn)95

Synthesized with an en/w volume ratio of 2:1.

Figure 3. Representative XPS spectra of Cd0.75Zn0.25S NWs: (a) survey spectrum, (b) high-resolution Zn 2p3 peak, (c) Cd 3d spectrum, and (d) S 2p3 spectrum.

Figure 4 represent the samples with x ) 0.5 and 0.25, i.e., cubic Cd1-xZnxS nanostructures. These images show the presence of a few undefined crystals along with nanorods. Diameters of the nanorods varied within 10-20 nm. TEM images shown in panels g and h of Figure 4 representing Cd0.9Zn0.1S and CdS, respectively, reveal the formation of uniform NWs. Thus the TEM studies indicated the presence of undefined crystals along with the NWs in the samples having cubic crystal structures only. TEM images of samples synthesized with a 2:1 en/w solvent composition are depicted in Figure 5. Interestingly, only NWs were observed for the entire range of x with the 2:1 en/w solvent composition. The comparative study of the TEM images shown in Figures 4 and 5 reveals the formation of uniform NWs for x g 0.25 in the relatively water-rich solvent, whereas the en-rich solvent produced uniform NWs for x values within the range of 0.0-0.1. Panels a-f of Figure 5 show TEM images for samples with x ) 1, 0.75, 0.5, 0.25, 0.1, and 0.0, respectively.

These results indicated that the presence of water helps to reduce the strong coordination between Zn and en inducing steady decomposition of the complex and hence favoring the formation of uniform NWs. On the other hand, coordination between Cd and en was weak compared to that between Zn and en. This weak coordination became weaker in the presence of more water, which is responsible for nonuniform NW growth. From the XRD pattern (Figure 1), it was observed that the intensities of two peaks of wurtzite (100) and (101) are reduced significantly with the incorporation of Zn into the CdS lattice, and the intensity becomes extremely weak; in contrast, the intensity of the (002) sphalerite peak increases. Simultaneously, the new peaks show an abrupt and very noticeable broadening. These changes are interpreted as a result of a collapse of the NW structure occurring on the wurtzite-to-sphalerite phase transformation. In sphalerite ZnS or CdS, the S2- anions form a face-centered cubic arrangement with Zn2+ and Cd2+ cations

High-Aspect Ratio Alloy Semiconductor Nanowires

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Figure 5. TEM images of Cd1-xZnxS samples synthesized in an en and water mixture with a 2:1 volume ratio for different x values of (a) 1, (b) 0.9, (c) 0.75, (d) 0.5, (e) 0.1, and (f) 0.0.

Figure 4. TEM images of Cd1-xZnxS samples synthesized in an en and water mixture with a 5:1 volume ratio for different x values of (a) 1, (b) 0.95, (c) 0.9, (d) 0.75, (e) 0.50, (f) 0.35, (g) 0.1, and (h) 0.

occupying half the tetrahedral holes (or vice versa); while in wurtzite [space group P63mc (No. 186)], the S2- anions form a hexagonal close-packed lattice along the crystallographic c-direction with the Zn2+ and Cd2+ cations occupying half of the tetrahedral holes. The hexagonal ZnS nanocrystals, exhibiting significantly 1D-oriented growth, are consistent with their anisotropic wurtzite structure because of the unique structural feature of the (001 h) facet and the existence of a 63-screw axis along the c-direction (Donnay-Harker law).47 On the other hand, because of the high symmetry of the sphalerite structure, cubic ZnS nanocrystals are formed in a granular shape even at the initial growth step with a much higher monomer concentration than that in bulk solution. Important crystal facets are equivalent to {111} in general.48 Granular ZnS nanocrystals are produced because a spherelike shape shows the lowest Gibbs energy as compared to that of any other shape.49 The surface energy difference associated with different crystallographic planes holds a general sequence as γ{111} < γ{100} ) γ{001} < γ{110} in cubic symmetry nanocrystals. The high-energy {110} surface is mostly observed in NWs, but its instability is often observed by the formation of spherical clusters in terms of atom sublimation.50 It is well-known that nucleating sphalerite crystals show predominantly the (111) plane that results directly from the (001) plane of the wurtzite phase. In addition, (111) and (100) represent the two largest d-spacing planes, so both of them unambiguously dominate the surface area of the sphalerite when no specific crystal direction growth otherwise exists.50 For cubic nanostructures, the impeding effect of en was giving rise to the formation of a one-dimensional structure,

whereas Ostwald ripening was taking effect for the growth of spherical crystals. The XRD pattern shown in Figure 1 revealed a phase transformation from wurtzite to sphalerite for 0.25 e x e 0.65. TEM images shown in Figure 4 reveal the formation of large crystallites along with the nanowires for the cubic samples lying within the range of 0.25 e x e 0.65. The correlation between the phase transformation and formation of large crystallites has been discussed in the previous paragraph. Because of crystal symmetry, large crystallites are favored over 1D structures in the cubic phase. Thus, it is interesting to investigate the crystal structure of the nanowires accompanying large crystallites in the cubic samples. In order to study the crystal structure of individual NWs, high-resolution TEM (HRTEM) images were recorded from individual NWs. Representative HRTEM images are shown in Figure 6. Figure 6a shows the HRTEM image of one CdS NW revealing a staking fault-free single crystalline structure. The image in the inset shows the magnified view of a selected region of the HRTEM image revealing a perfect wurtzite crystal feature. Measured lattice spacing was 3.35 Å for the lattice planes perpendicular to the axis of the nanowire, representing the (002) lattice plane of wurtzite CdS. The growth direction of the CdS nanowires was [002]. Figure 6b shows the HRTEM image of a ZnS NW exhibiting its perfect single crystalline wurtzite crystal structure. A portion of the HRTEM image is magnified and shown in the inset to exhibit the wurtzite crystal structure of the ZnS NWs. Measured lattice spacing was 3.12 Å for the lattice planes perpendicular to the axis of the nanowire, representing the (002) lattice plane of wurtzite ZnS. The growth direction of the ZnS nanowires was also [002]. Figure 6c shows the HRTEM image of a Cd0.50Zn0.50S NW synthesized with a 2:1 en/w solvent composition. The image along with the magnified portion shown in the inset confirms the perfect wurtzite structure of the NW. The inset also shows the schematic diagram of the wurtzite staking of the atoms in a unit cell. Figure 6d exhibits the HRTEM image of a

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Figure 7. I -V characteristics of a Cd0.25Zn0.75S sample measured with and without white light illumination. Inset shows a schematic of the device after drop casting. CdZnS nanowires formed a network and were in contact with the two electrodes. Figure 6. HRTEM images of wurtzite (a) CdS, (b) ZnS, (c) wurtzite Cd0.50Zn0.50S NW, and (d) cubic Cd0.50Zn0.50S NW. Selected portions of all HRTEM images are magnified and shown in their respective insets with a view to reveal the crystal structure of the NWs.

Cd0.50Zn0.50S NW synthesized with a 5:1 en/w solvent composition. The HRTEM image along with the magnified portion shown in the inset clearly indicates the cubic sphalerite structure of the NWs. The inset also shows the schematic representation of the cubic unit cell. The HRTEM image (not shown here) of the large crystallites also confirmed their cubic crystal structure. As we have already mentioned, because CdxZn1-xS thin films were studied as solar cell materials, it is important to investigate the photoresponse properties of our alloy NWs. In order to investigate the photoresponse property, we have measured the electrical transport properties of representative CdxZn1-xS NWs. The devices used for electrical transport measurements in this work were fabricated on heavily doped n-type Si substrates with a 250 nm SiO2 capped layer. Source and drain electrodes with a channel length l ) 3 µm and width w ) 200 µm were defined using standard photolithography (LOR 3A/Shipley 1813). After developing in CD26, 5 nm of chromium (Cr) and 50 nm of gold (Au) were thermally evaporated onto the substrate, and finally the sample was lifted off in acetone. Drop casting a Cd1-xZnxS solution on a Si-SiO2 substrate containing the electrodes resulted in an array of NWs bridging between electrodes. The inset of Figure 6 shows a schematic of the device. The room temperature dc current-voltage (I-V) characteristic curves were measured in a probe station using a Keithley 6517a electrometer for generating the voltage and reading the current both in the dark and under illumination. White light from Oriel 96000 solar simulator was used for illumination. Figure 7 shows the I-V curves for one of our films containing Cd0.25Zn0.75S. In the dark, the current is less than 10 pA for a voltage of up to 30 V. When the sample was illuminated under white light, there was a significant increase of conducting carriers and hence a large enhancement of current at a positive bias. The current is 76 nA under 120 W illumination and 108 nA under 150 W illumination at +20 V bias. This is an increase of more than three orders of magnitude compared to that of the dark current, suggesting that CdZnS nanowires are applicable to optoelectronic and photovoltaic devices. A detailed study of the electronic transport of the device will be presented elsewhere. 3.2. Core-Shell Cd1-xZnxS/ZnS NWs. We further utilized our expertise in the synthesis of dopant-based core-shell CdS: Mn/ZnS NW42 to synthesize excitation band gap tunable core-shell CdxZn1-xS/ZnS NWs. As it is easier to incorporate

Figure 8. (a) TEM image of the representative Cd0.75Zn0.25S/ZnS core-shell alloy NWs. (b) HRTEM image of one selected Cd0.75Zn0.25S/ ZnS NW, showing distinct core and shell parts. Magnified portion of the tip part of the core-shell nanowire is shown in the inset exhibiting clear core-shell interfaces. (c) XRD patterns for the representative alloy NWs and their core-shell counterparts. (d) Room-temperature PL spectra of representative alloy NWs and their core-shell counterparts.

Zn2+ ions into the CdS lattice because of a higher mobility of Zn2+ ions, in this report, we have mainly focused on the zincrich Cd1-xZnxS/ZnS alloy NWs. The idea was to simply add Zn2+ and S2- aqueous sources to the needle-shaped Cd1-xZnxS templates and allow the solvothermal process to continue for an additional 4 h. As expected, the ZnS layer grew radially over the high-energy surface of the Cd1-xZnxS templates. A TEM image of a representative Cd0.75Zn0.25S/ZnS NW sample is shown in Figure 8a. The diameters of the NWs were in the range of 10-20 nm. Figure 8b shows a HRTEM image of a single Cd1-xZnxS/ZnS NW, exhibiting a well-defined core-shell nanostructure. The TEM micrograph showed two distinct regions, a crystalline core and a relatively low-contrast crystalline ZnS shell that uniformly surrounded the central Cd1-xZnxS core. Lattice spacing of the shell part is ∼3.31 Å, representing the (100) lattice plane of ZnS. As shown in Figure 8b, a clear boundary was observed between the core and the shell. The key to the fabrication of the ZnS shell layer is the high-energy

High-Aspect Ratio Alloy Semiconductor Nanowires surface of the Cd1-xZnxS core template as mentioned in our earlier report.42 The ZnS shell thickness is 2-3 nm. To further support the core-shell structure of the NWs, we performed XRD, XPS, EDAX, and PL as described below. The XRD pattern (Figure 8c) of the alloy core-shell NWs clearly indicated the presence of wurtzite ZnS along with wurtzite Cd1-xZnxS with different x values. Also, the XRD pattern confirmed the formation of the wurtzite phase of Cd0.75Zn0.25S/ZnS NWs. The peaks were well in accordance with the peaks of the bare Cd0.75Zn0.25S NWs and shifted to the higher 2θ angle from the CdS-ZnS core-shell NWs. The lattice parameters for the core-shell alloy NW sample were calculated to be a ) 4.10 Å and c ) 6.62 Å, which were shifted from the bulk values of CdS and are matched well with the bare Cd0.75Zn0.25S NWs. This indicates the added Zn2+ precursors for the shell part did not participate in the growth of core alloy NWs. The x values for the core-shell alloy NWs were calculated using Vegard’s law and found to be x ) 0.08 and x ) 0.21 for Cd0.9Zn0.1S/ZnS and Cd0.75Zn0.25S/ZnS, respectively. Because the diameters of the well-faceted NWs were less than 10 nm, all the XRD peaks showed characteristic broadening except the peak corresponding to the growth direction for the NWs. Because the Cd1-xZnxS NWs were very long compared to their diameter, as expected, a narrow XRD peak at 26.5° was found. This peak corresponded to the growth direction of the NWs, that is, [002], that was consistent with the HRTEM (Figure 8b). The core-shell NW composition was further characterized by using EDAX and XPS studies. The XPS study reveals more Zn than that of the EDAX study revealing the presence of more Zn surrounding the external surface of the NWs indicating the presence of ZnS outer shell. This result is expected as XPS is more surface sensitive than EDAX. The results are consistent with our earlier report.42 The formation of the core-shell nanostructure was also supported by room-temperature photoluminescence (PL) studies of Cd1-xZnxS/ZnS CSNWs. PL properties of Cd1-xZnxS/ZnS alloy NWs as well as Cd1-xZnxS NWs are presented in Figure 8d. The PL spectrum of the Cd1-xZnxS NWs showed a green emission band corresponding to band edge emission.51,52 The broad asymmetric tail of the luminescence peak could be attributed to the large number of surface states formed.51,52 It could be noted that the PL spectra of the Cd0.9Zn0.1S NWs are slighty red shifted (∼11 nm) compared to that of the CdS nanowires and nanoribbons (emission at ∼ 515 nm) reported earlier.51,52 Because zinc and cadmium ions have approximately a 10% difference in ionic radius, an initial addition of 10% Zn could introduce lattice strain. Subsequent release of lattice strain from the crystals could lead to the formation of defect states such as surface states. These surface states could lead to the initial red shift of the PL spectra (526 nm). On the other hand, the addition of 25% Zn leads to the formation of a CdZnS alloy phase, which has a larger band gap. Thus blue shift of the PL spectra of the Cd0.75Zn0.25S (emission at ∼506 nm) could be attributed to the band gap increase due to alloying. The PL spectrum of the core-shell alloy NWs exhibited relatively strong green emission bands, which also exhibited blue shift with alloying. The relatively sharp and strong emission indicates the presence of fewer surface states being responsible for photodegradation and luminescence quenching.53 Thus, the PL study also supports the formation of a core-shell structure of Cd1-xZnxS/ZnS NWs. 4. Conclusion In conclusion, we have demonstrated that semiconductor alloying can be done in 1D nanostructures by using a solvo-

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3623 thermal route. Solvent composition in a mixed solvent system of en and water is crucial in determining the crystal structure and morphology of the alloy NWs. The same synthesis process could be extended to synthesize Cd1-xZnxS/ZnS alloy-based core-shell NWs by proper manipulation of the protocol. The ratio of Zn and Cd in the alloy NWs is another factor that determined the phase and dimension of the NWs. Preliminary electronic transport studies indicated the photoresponse behavior of the alloy NWs. Thus, the photoresponse properties of the alloy NWs in combination with our capability to produce core-shell structures indicate that the prospect is good for these materials being used in future photovoltaic devices. Acknowledgment. This work has been partly supported by the National Science Foundation (NSF-NIRT Grant EEC056560 and NSF CBET-63016011). S.I.K. also acknowledges NSF Career Award 0748091 and NSF Grant 0801924 for financial assistance. References and Notes (1) Acharya, S.; Patla, I.; Kost, J.; Efrima, S.; Golan, Y. J. Am. Chem. Soc. 2006, 128, 9294. (2) Kar, S.; Chaudhuri, S. Chem. Phys. Lett. 2004, 398, 22. (3) Kar, S.; Chaudhuri, S. J. Phys. Chem. B 2005, 109, 3298. (4) Kar, S.; Chaudhuri, S. Synth. React. Inorg. Met.-Org. and NanoMet. Chem. 2006, 36, 289. (5) Kar, S.; Dev, A.; Chaudhuri, S. J. Phys. Chem. B 2006, 110, 17848. (6) Kar, S.; Pal, B. N.; Chaudhuri, S.; Chakravorty, D. J. Phys. Chem. B 2006, 110, 4605. (7) Rao, C. N. R.; Deepak, F. L.; Gundiah, G.; Govindaraj, A. Prog. Solid State Chem. 2003, 31, 5. (8) Rao, C. N. R.; Satishkumar, B. C.; Govindaraj, A.; Nath, M. ChemPhysChem 2001, 2, 78. (9) Zeiri, L.; Patla, I.; Acharya, S.; Golan, Y.; Efrima, S. J. Phys. Chem. C 2007, 111, 11843. (10) Wong, E. W.; Sheehan, P. E.; Lieber, C. M. Science 1997, 277, 1971. (11) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (12) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699. (13) Ridley, B. A.; Nivi, B.; Jacobson, J. M. Science 1999, 286, 746. (14) Alivisatos, A. P. Science 1996, 271, 933. (15) Bailey, R. E.; Nie, S. M. J. Am. Chem. Soc. 2003, 125, 7100. (16) Zhong, X. H.; Feng, Y. Y.; Knoll, W.; Han, M. Y. J. Am. Chem. Soc. 2003, 125, 13559. (17) Duan, X. F.; Niu, C. M.; Sahi, V.; Chen, J.; Parce, J. W.; Empedocles, S.; Goldman, J. L. Nature 2003, 425, 274. (18) Zhang, J.; Jiang, F. H.; Zhang, L. D. J. Phys. Chem. B 2004, 108, 7002. (19) Lin, Y. F.; Song, J.; Ding, Y.; Lu, S. Y.; Wang, Z. L. AdV. Mater. 2008, 20, 3127. (20) Hsu, Y. J.; Lu, S. Y.; Lin, Y. F. Chem. Mater. 2008, 20, 2854. (21) Lin, Y. F.; Song, J.; Ding, Y.; Lu, S. Y.; Wang, Z. L. Appl. Phys. Lett. 2008, 93, 242503. (22) Lin, Y. F.; Hsu, Y. J.; Lu, S. Y.; Chen, K. T.; Tseng, T. Y. J. Phys. Chem. C 2007, 111, 13418. (23) Lin, Y. F.; Hsu, Y. J.; Lu, S. Y.; Kung, S. C. Chem. Commun. 2006, 2391. (24) Rincon, M. E.; Martinez, M. W.; Miranda-Hernandez, M. Sol. Energy Mater. Sol. Cells 2003, 77, 25. (25) Li, Y. C.; Ye, M. F.; Yang, C. H.; Li, X. H.; Li, Y. F. AdV. Funct. Mater. 2005, 15, 433. (26) Kuroyanagi, A. Thin Solid Films 1994, 249, 91. (27) Wu, B. J.; Cheng, H.; Guha, S.; Haase, M. A.; Depuydt, J. M.; Meishaugen, G.; Qiu, J. Appl. Phys. Lett. 1993, 63, 2935. (28) Liu, Y. K.; Zapien, J. A.; Shan, Y. Y.; Geng, C. Y.; Lee, C. S.; Lee, S. T. AdV. Mater. 2005, 17, 1372. (29) Lui, T. Y.; Zapien, J. A.; Tang, H.; Ma, D. D. D.; Liu, Y. K.; Lee, C. S.; Lee, S. T.; Shi, S. L.; Xu, S. J. Nanotechnology 2006, 17, 5935. (30) Zhai, T. Y.; Gu, Z. J.; Yang, W. S.; Zhang, X. Z.; Huang, J.; Zhao, Y. S.; Yu, D. P.; Fu, H. B.; Ma, Y.; Yao, J. N. Nanotechnology 2006, 17, 4644. (31) Hsu, Y. J.; Lu, S. Y.; Lin, Y. F. AdV. Funct. Mater. 2005, 15, 1350. (32) Hsu, Y. J.; Lu, S. Y. Chem. Commun. 2004, 2102. (33) Zhang, Y. C.; Chen, W. W.; Hu, X. Y. Cryst. Growth Des. 2007, 7, 580.

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