Controllable Fabrication of High-Quality 6-Fold Symmetry-Branched

May 29, 2008 - The ZnS nanowires were synthesized in first step and were then used as templates for the following growth of 6-fold symmetrical CdS ...
0 downloads 0 Views 1MB Size
Download PDF
J. Phys. Chem. C 2008, 112, 9253–9260

9253

Controllable Fabrication of High-Quality 6-Fold Symmetry-Branched CdS Nanostructures with ZnS Nanowires as Templates Weichang Zhou, Anlian Pan, Yun Li, Guozhang Dai, Qiang Wan, Qinglin Zhang, and Bingsuo Zou* Micronano Research Center, Hunan UniVersity, Changsha, 410082, China ReceiVed: January 21, 2008; ReVised Manuscript ReceiVed: March 11, 2008

Absolute 6-fold symmetrically branched CdS nanostructures were heteroepitaxially grown with a simple twostep evaporation method. The ZnS nanowires were synthesized in first step and were then used as templates for the following growth of 6-fold symmetrical CdS nanobelts or nanowire at varied temperature in the second step. The orientation relationship between ZnS nanowires and CdS nanobelts was investigated to confirm the effect of crystalline substrates on the epitaxial (1D directional) growth process of branch nanostructures. The photoluminescence spectra of as-synthesized nanostructures displayed a dominant and sharp band edge emission band, only accompanied with a weak and broad trap-state emission band under high excitation at room temperature. The present approach of epitaxial growth of symmetrical nanostructures from presynthesized template nanostructures may supply new opportunities for high-quality complex nanostructure growth, which may find new applications in the researches and devices of such nanostructures. 1. Introduction Low dimensional semiconductor nanostructures are emerging as attractive building blocks for the assembly of electronic and optoelectronic device systems.1–4 II-VI semiconductor materials are of particular interest primarily because of their wide range of optoelectronic properties and easy fabrication. The spectrum of energy band gaps of II-VI semiconductor ranges from nearinfrared to ultraviolet. Single crystalline ZnS and CdS or alloy ZnxCd1-xS nanowires or nanobelts have been synthesized and demonstrated attractive optical properties, such as nanoscale lasers,5–9 optical waveguides,10 photosensors,11 and avalanche photodetectors12 in the past few years, which make them promising candidates for nanophotonics applications. In experimentals researcheres already found that the semiconductor heterostructure give better performance in properties than the homostructures.13 Therefore people hope to find way to design some devices with those nanoelements of varied structures and go advance in their applications. Hence the self-assembled structures of organic or inorganic units have attracted much attention, but the nonchemistry connection between units limited their cooperative and/or collective physical responses because of the multiboundary of electronic states. Because of the structural, environmental, and compositional factors, only a small number of buliding blocks can be produced, so the fabrication of multicomponent and multifunctional building blocks with high-quality nanowires or nanobelts is still important and difficult task so far. Recently, single-crystal branched nanowire heterostructures14 or complex 1-D hierarchical nanowire arrays heteroepitaxial growth on nanoribbons15 have initiated new attentions for their rich architectures. Synthetic methods have been studied to assemble three-dimensional branched and hyperbranched structures using a variety of materials and techniques including selfassembled dendritic growth of nanowires16,17 and growth of multibranched nanowire structures via sequential seeding of catalysts.18,19 A variety of novel hierarchical nanostructures with * To whom correspondence should be addressed.

6-, 4-, and 2-fold symmetries of ZnO-In2O3 have been successfully grown by a vapor transport and condensation technique.20 Wang et al.21 have even reported the formation mechanism of 6-fold nanopropeller arrays of ZnO by a two-step hightemperature solid-vapor process, using Sn as catalyst. Jung et al. reported the ragged branched heterostructure of ZnS/CdS via metal-organic chemical vapor deposition (MOCVD) technique.14 It is surprising that these reports seldom cover the optical responses of these nanostructures, which is the dominant quality evaluation factor for their applications. In our reproduction of those nanostrucutres, all of them can give deeptrap photoluminescence (PL) at low excitation, which reflected that there are defects or impurities in these nanostructures. Such results limited their applications in photonic applications. How to avoid defects and/or impurities in the nanostructure should be an important subject in its growth. Here we report the hierarchical CdS nanowires or nanobelts are grown symmetrically from ZnS nanowire backbones by a simple thermal vapor transport and condensation technique. These hierarchical nanostructures have basic 6-fold symmetries. The optical spectral study indicates very good luminescence properties. The feasibility of forming symmetrical complex nanostructures opens a door to design more complex three-dimensional building blocks and exploit their unique properties. This new combination of technologies represents a significant step in the pursuit of new functional nanoscale materials and devices, which may find applications in a variety of fields such as luminescence, laser, field emission, photovoltaics, nanophotonics, and multifunctional applications. 2. Experimental Section To avoid the impurity introduction, a multistep physical evaporation growth technique is proposed to synthesize highquality 6-fold symmetrical branched CdS nanostructures. The first step is to prepare ZnS nanowires. Appropriate amounts of commercial grade ZnS powder were placed onto a ceramic boat at the center of a quartz tube, which was placed into a horizontal tube furnace. Next to the ceramic boat, several pieces of silicon

10.1021/jp800599e CCC: $40.75  2008 American Chemical Society Published on Web 05/29/2008

9254 J. Phys. Chem. C, Vol. 112, No. 25, 2008 slice coated with 5-nm Au nanoparticles film were placed downstream of the gas flow. High-purity Ar/H2 was injected into the quartz tube with a constant flow rate (100 SCCM) to eliminate the O2 inside before heating. The flow rate of the Ar/ H2 carrier gas was adjusted to 10 SCCM after 90 min, and the furnace was rapidly heated to 1100 °C and maintained for about 90 min without changing any conditions. After the furnace was cooled down, white products were deposited on the surface of the silicon slices located at the temperature region of 400-500 °C during reaction. In the following step, the silicon slices covered with ZnS nanowires were precovered with 1-nm Au nanofilm as the secondary catalyst and placed back into the furnace at different location for growing new nanobelts or nanowires. The same processing as that of preparing ZnS nanowires is repeated in the furnace, finally, the final products were grown on the silicon slice downstream where the temperature is in the range of 400-550 °C, by introducing CdS powder as precursor instead of ZnS at the region in the tube furnace where the temperature is 760 °C under Ar/H2 flow of 10 SCCM. The morphology and structure of the as-grown nanostructures were examined with field emission scanning electron microscopy (FE-SEM, JSM-6700F) and X-ray powder diffraction (XRD, using a D/max 5000 utilizing CuKR radiation). The substratebound nanostructures were sonicated in ethanol and then deposited on carbon-coated copper grids for transmission electron microscope (TEM, JEM-3010) characterization operating at 300 kV. The PL was investigated on Alpha SNOM (WITec, Germany) system at room temperature using Ar+ laser with the wavelength of 488 nm as the excitation source. 3. Results and Discussion Parts a and b of Figure 1 show the SEM images of ZnS nanowires that serve as template deposited on Si substrate at low and medium magnifications, respectively. It is observed that these nanowires are straight with the diameter in the range of 80-120 nm and uniform along the axis and length up to several tens of micrometers. A black dot (Au nanoparticle) located at tip of every nanowires is observed, which indicates that the nanowire growth obeys the vapor-liquid-solid (VLS) mechanism.22 The inset in Figure 1b shows the end cross section of a representative nanowire after the Au dot is removed, where a pit in the center is still left. A 6-fold faceted end of ZnS nanowire can be seen from its silhouette, which is the crucial to the next growth into 6-fold symmetrical CdS nanostructures. The structure of the as-grown ZnS nanowires is determined by XRD (see Figure 1c). All of the diffraction peaks can be assigned to the hexagonal wurtzite ZnS structure with lattice constants of a ) 3.82098 Å and c ) 6.2573 Å (JCPDS card 36-1450), consistent with the standard data file well. Further structural analysis of ZnS nanowires are performed by means of transmission electron microscopy (TEM, JEM-3010) operating at 300 kV. Figure 1d is a typical TEM image of a single nanowire, of which the diameter is about 100 nm, and the inset shows its selected area electron diffraction (SAED) pattern. The SAED demonstrates the single-crystal quality of the nanowire and can be indexed to a hexagonal wurtzite structure with lattice parameters a ) 3.84 Å and c ) 6.35 Å. The reciprocal lattice peaks determined from SAED can be indexed to wurtzite ZnS structure showing that the nanowire growth along the [0001] direction. The corresponding HRTEM lattice image (Figure 1e) further confirms the single-crystalline structure. The measured lattice stripe spacing of 0.31 nm corresponds to the interplanar distance of (0002) planes as known from bulk ZnS crystal.

Zhou et al. To enable the growth of high-quality branched heteronanostructure, a thin layer of Au nanocatalyst was deposited by physical evaporation onto the surface of as-grown ZnS nanowires on Si substrate. In ref 14 Jung et al. have reported the heterostructures of ZnS/CdS via a MOCVD technique by using Au particle catalysts. Their Au particles are formed from the solution adsorption after chemical reduction and therein thermal treatment, so their final products showed sparse and ragged nanobranches, which is easy for HRTEM observation. In our technique, the evaporated Au can be densely deposited on the surface of ZnS nanowires(see Supporting Information Figure S1), which may be an advantage for later growth. Then the branched nanostructures were grown at the location Au particles on the surface of ZnS nanowires in the same tube furnace by using CdS powder as precursors but not organic Cd complex. The growth temperature was adjusted to obtain good-quality CdS nanowires or nanobelts on the ZnS backbone nanowires. The high temperature around 550 °C clearly supports the formation of CdS nanobelts and below that lead to the nanowires. The density of branch on the backbone can be controlled to an extent by varying (1) the amount of Au particles used to seed the branch growth,23 (2) the temperature, and (3) the carrier gas flow rate of CdS precursors, the temperature can change the size of catalyst droplet on the surface of ZnS nanowire backbone and determine the later growth mechanism. The morphologies of 6-fold symmetrical branched nanostructures are shown in Figure 2. Figure 2a shows that the branched nanostructures are separately grown on Si substrate in large scale. Figure 2b shows a single 6-fold symmetrical branched nanostructure at higher magnification, in which the Au nanoparticles are clearly seen at the tips of the swordlike branched CdS nanobelts. It is apparent that just the VLS mechanism cannot completely account for the formation of CdS nanobelts, since their transverse sizes become larger during growth. The formation of swordlike CdS nanobelts should relate itself to the VLS + vapor-solid (VS) mechanism since the alloy droplets particle, which is the symbol of VLS mechanism. Therefore, a VS process24 may also contribute to the formation of these swordlike CdS nanobelts. That is to say, when the newly formed CdS nanorods are growing in the longitudinal direction via the VLS process, some CdS vapor could epitaxially deposit onto the side surfaces of the rods via the VS process under the suitable high temperature and secondary growth priority of the early formed nanorods, finally leading to the formation of CdS nanobelts. This two-process mechanism is well studied on the growth of CdSSe25 and CdSe26 nanobelts. By reduction of growth temperature or placement of the ZnS/Si substrate at a slightly lower temperature region, 6-fold symmetrically branched CdS nanowires but not nanobelts on the surface of ZnS nanowires (parts c and d of Figure 2) could be obtained. The branched CdS nanowires are long enough to connect with others and form a linked network; moreover, the nanowires are much denser than the nanobelts. Whether in nanobelts or in nanowires, the angles between adjacent branches in the perpendicular to the ZnS nanowire approach 60° and these nanobranches are epitaxially growth along six directions and queued in the c axis direction. The growth of 6-fold symmetrically branched CdS nanowires with controlled branch density, together with 6-fold symmetrical branched CdS nanobelts, further suggests that our approach should be quite general and thereby can provide access to symmetrically branched nanostructures with a wider range of compositions. In contrast to the observation by Jung et al., all the branches in our experiments have 90° angles to the c axis of ZnS nanowire after careful examination. This proves

High-Quality 6-Fold Symmetry-Branched CdS Nanostructures

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9255

Figure 1. Characterization of ZnS nanowires: (a) low-magnification SEM image; (b) medium-magnification SEM image; (c) XRD of as-prepared ZnS nanowires; (d) typical TEM image of a single nanowire; the inset shows its selected area electron diffraction (SAED) pattern; (e) the corresponding HRTEM lattice image of part d.

the high quality of ZnS nanowires with perfect surface of 100, because the appearance of 60° branches in their results indicates the structural defects with the 012 facet on the ZnS surface. Such defects occur in the MOCVD technique with the excess gas out of Zn and Cd complex decomposition. This drawback, therefore, cannot produce highquality nanostructures. This phenomenon reflected reversely the preponderance of evaporation technique. Figure 3 shows the XRD of the as-prepared products, all the diffraction peaks can be assigned to CdS crystal (JCPDS card 41-1049). We can not observe the diffraction peaks of ZnS

backbone nanowires after a definite time growth, but in the early stage of growth the XRD pattern of alloyed ZnxCd1-xS can be observed (see Supporting Information Figure S2). The reason for not ZnS XRD may be that the backbone ZnS nanowires could be transformed to CdS during reaction by Cd ions pervasion into ZnS and substitute for Zn ions in H2 reductive atmosphere, which may help to the partial decomposition of ZnS surface. Further, the ZnS backbone was covered by the thick CdS nanobelt layer and the covered thickness is over 1 µm. The limited penetration depth of X-ray and minor ZnS amount diminished the XRD signal of inside ZnS backbone.

9256 J. Phys. Chem. C, Vol. 112, No. 25, 2008

Zhou et al.

Figure 2. SEM of 6-fold symmetrically branched nanostructures: (a) low-magnification image of CdS nanobelts; (b) high-magnification image showing a single representative 6-fold branched CdS nanobelts; (c) low-magnification image of CdS nanowires; (d) high-magnification image of CdS nanowires.

Figure 3. XRD diffraction pattern of 6-fold symmetrical branched CdS nanostructures: (a) the products of half-deposition time; (b-c) the products of full deposition time.

TEM is used to further characterize the microstructures and crystallinity of the branch nanostructures and understand the growth mechanism. Actually Jung et al. already carefully studied the junction structure of ZnS/CdS with sparse branches, in which the junction are composed of Zn1-xCdxS alloy, the composition where far from junction is still ZnS. In our system, however, neighboring branches are closely queued on six directions on the surface of ZnS due to the dense and uniform Au particle adsorption, so probably no ZnS could be detected from the current energy-dispersive X-ray spectroscopy (EDS) and XRD techniques directly. We need to check carefully the situation in our products. We selected a symmetrically branched CdS

nanobelt sample as an example. Figure 4a is the low-magnification TEM image of the symmetrical branched nanostructure, in which CdS nanobelts grow around the backbone ZnS nanowire. Figure 4b is the high-resolution TEM (HRTEM) image of the branched CdS nanobelt, showing the single-crystal wurtzite structure. The insets in parts a and b of Figure 4 are the SAED of backbone and branch, respectively, which confirm the single crystal of both backbone and branched nanostructures. The reciprocal lattice peaks determined from SAED can be indexed to wurtzite CdS structure. Because of the projection nature of the TEM image and multiple arms around and coating the backbone nanowire, HRTEM images of backbone nanowire are always difficult to obtain due to their large thickness along the electron beam direction. A HRTEM image (see Figure 4c) of junction region between backbone and branch shows that backbone end was also covered with a single crystal CdS layer. Parts d and e of Figure 4 are the EDS of backbone and branched, respectively, which show that the components of backbone and branched are ZnxCd1-xS and CdS, respectively, which is because the CdS vapor cover on the surface of ZnS nanowire and some Cd ions intercalate gradually into ZnS lattice to form the alloy. Therefore we can conclude that the junction surface region is made of single crystal CdS, the inner region is ZnxCd1-xS. Because of the limited penetration of the electron beam, we can not observed ZnS structure directly, which is certain present in the core of backbone even minor amount at definite growth time due to the gradual intercalation process. The growth mechanism of 6-fold symmetrical branched nanostructure can be proposed according to the ZnS and CdS lattice orientations in a schematic diagram of the orientation relationship of epitaxial CdS nanobelts on ZnS nanowire substrate as determined by SEM and TEM observations. It is still necessary to know the CdS growths initiate at the arris or

High-Quality 6-Fold Symmetry-Branched CdS Nanostructures

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9257

Figure 4. TEM of 6-fold symmetry branched nanostructures: (a) a typical TEM image of a single branched nanostructure; (b) the corresponding HRTEM lattice image of part a, the insets in parts a and b show its SAED pattern of backbone and branch, respectively; (c) HRTEM image of junction region between backbone and branch; (d-e) EDS of backbone and branch, respectively; (f) Schematic of the proposed growth of 6-fold symmetrical branched nanostructures on backbone nanowires with different orientations and a basic cell of sword-like CdS nanobelts revealing the crystallographic relations of the lattice planes and directions involved in epitaxial growth.

at the facet. Both the Au seeding particles and the lattice surface of the crystalline ZnS nanowires play a dominant role in the epitaxial growth of CdS nanobelts along its preferential orientations. Deposition and crystallization of CdS on the surface of single-crystal ZnS surface take place when the concentration of CdS vapor reaches supersaturation in the liquid alloy droplet

formed by Au catalyst. The VLS growth process offer an elastic boundary condition for epitaxial growth dislocation free 1D branch nanostructures while impossible for epitaxial growth defect-free 2D film configuration. Because of the surface tension, the alloyed droplets are mainly located at the central location of the side facet separately but not the arris of nanowire, so the

9258 J. Phys. Chem. C, Vol. 112, No. 25, 2008

Figure 5. PL of 6-fold symmetry branched nanostructures using Ar+ 488 nm as excitation light source at room temperature.

branches all should grow near the central line of the long side facet. The growth direction of ZnS nanowires in our sample is [0001] and is enclosed by {21j1j0}; this result is similar to ZnO nanowires27 and ZnS nanobelts.28 The orientation of the substrate surface is to a large extent reproduced and transferred to the growth direction of branched nanostructures during the catalystassisted VLS growth process and the growth direction of branched CdS nanobelts is [1000] and [0001], which are perpendicular and parallel to the long axis of ZnS nanowires, respectively. The enclosed facets of CdS nanobelts are ((0001), ((0100), ((21j00), respectively. Therefore, {21j1j0} facets of the ZnS backbone nanowires are parallel to the growth axis ((21j00) planes of branched CdS nanobelts and ((0001) planes of CdS are parallel to ((0001) planes of ZnS as proved in Figure 4f. The [0001] direction of ZnS nanowires is perpendicular to the [1000] growth direction of CdS nanobelts, which is consistent with the observed result that 90° angle between the branch and the backbone at the junction region. The hexagonal symmetry of six side facets of ZnS nanowire backbone should contribute in part to the CdS growth into 6-fold symmetrical branched structures. It is clearly that the lattice of wide side of CdS nanobelt close to ZnS favors the (0001) of ZnS nanowire because their c axis mismatch (6.9%) is smaller than that (7.7%) of the a axes, although lattice mismatch along c axis over 5%, which block the continuum growth to nanoplate on the ZnS surface and therein cause alloyed growth. Matthews and Blakeslee29 described a model for misfit dislocations formed in planar heterostructures, which shows that it is possible to create perfect, dislocation-free heterojunctions from nanowires below a critical radius, even from materials with a large lattice mismatch, that is, the lattice misfit can be accommodated at a given size. By the way, the CdS nanowires form at lower temperature densely on the surface of ZnS nanowire, its growth follows the absolute VLS mechanism, because the c axis of CdS nanowire is perpendicular to that of the long axis of ZnS nanowire. This indicates that the growth direction of branched CdS nanowires may be [1000], which is consistent with the growth direction of branched CdS nanobelts. The optical properties of as-grown 6-fold symmetrical branched nanostructures are investigated by PL measurement using Ar+ laser (488nm) as excitation light at room temperature. In Figure 5, a sharp and strong luminescence emission peak is seen at 502 nm and gradually redshifts to 520 nm when increasing laser power; at the same time a broad and weak band range from 673 to 800 nm, centered at 716 nm, was only observed under high excitation. The emissions from 502 to 520 nm are assigned to band-edge luminescence arose from the recombination of excitons and/or shallow trapped electron-hole

Zhou et al. pairs in CdS nanostructures. This result reflected the high quality of as-prepared nanostructures for no deeptrap emission could be observed. The absence of emission from ZnS and ZnCdS is because their bandgap is larger than CdS, the exciton recombination occurs at the lowest level in a semiconductor heterostructure. Such band edge emission will not be detected in the nanostrucutres by the growth techniques in refs 14 and 21 because defects or impurities may be present in their nanostructures. There are two causes for the spectral redshift with laser power. One is the multelement alloyed structure, which may produce many carriers in nanostructure. Higher excitation will produce more carrier-carrier scattering and lead to the electron-hole plasma at a shreshold power, so redshift occurs. Another is the light transport; the higher the excitation, the longer the emitted light may transport, longer transportation lead to more electron-phonon coupling and therein emission redshift. Usually, two emission bands are observed for CdS nanostructures: excitonic and trapped state luminescence, respectively.30 The trap emission is mainly due to nonstoichiometric sites or defects of CdS and the excess of sulfur or cadmium vacancies at the interface, which is well-known to quench radiative recombination in the band gap and not due to the low crystallinity.31 The trap-state emissions related to surface defects have also been observed in the case of CdS thin films.32 The broad red luminescence band may arise from transitions of electrons trapped at surface states to the valence band of CdS crystals. In this complex system the trapped state emission may be caused by the thermal electronic effect in a carrier-rich system, which could be realized at high excitation. It is reasonable that there are minor lattice and surface defects near the surface and hetrojunction in our CdS nanostructures with the ZnS nanowires as template. Figure 6 is the PL spatial profile of the sample in Figure 2a obtained from a Vitec confocal optical microscopy, from which we can see many spots in line of the emission on the ends of CdS nanobelts under high excitation. Figure 6a show the emission spots at varied altitudes at high excitation, and Figure 6b gives the green emission spots with the same altitude at the same location as Figure 6a. At low excitation, only one large spot near excitation location is observed. Although the emitted spots are not regularly distributed due to different altitudes, this result indicates the exciton propagate in this high-quality nanostructures and produce emission spots at many branches on the backbones. The pure CdS nanowires or nanobelts usually emit light at about 503 nm under photoexcitation.7,10 In this ZnS/CdS nanostructures, the emission band at 502 nm or slightly less under weak laser excitation, should originate from the interface of ZnS/CdS, because the CdS:Zn layer at the interface may possess slightly larger bandgap and larger radiative recombination efficiency than individual CdS. When the laser power increase, the amount of photoinduced excitons will become larger, and can move longer distance to the other ends of the CdS nanowires or nanobelts, hence the emission band will shift to lower energy and become broad after propagate (see Figure 5). Of course the formation of electron-hole plasma also can produce above redshift of the luminescence. Compared with the growth processes of ZnS/CdS nanostructures by MOCVD14 and ZnO nanopropeller by graphite reduction and Sn catalysis,21 we found the evaporation technique in this report shows several significant advantages although they are all catalysts assisted VLS growth at the start. One, the Au catalysts obtained by evaporation can be uniformly distributed on the surface of preformed nanowires, which are beneficial for the growth of periodical and symmetrical nanostructures;

High-Quality 6-Fold Symmetry-Branched CdS Nanostructures

J. Phys. Chem. C, Vol. 112, No. 25, 2008 9259 surface hexagonal symmetrically. Such structures may find even important applications in the future devices. 4. Conclusion In summary, a simple two-step evaporation technique is used to produce high-quality 6-fold symmetrical branched CdS nanostructures heteroepitaxially, grown with single-crystal ZnS nanowires as template. The orientation relationship between ZnS nanowires and CdS nanobelts demonstrates the importance of crystalline substrates on the epitaxial growth process of nanostructures and the orientation of the substrate surface is to a large extent reproduced and transferred to the growth direction of branched nanostructures because the lattice mismatch can be accommodated under a given radius during VLS growth 1D nanostructures. PL spectra for the as-synthesized nanostructures display a dominant emission at 502s520 nm due to band-edge emission of CdS with varied excitation power. The present approach to epitaxial growth of high-quality complex nanostructures from presynthesized nanostructures materials may be a prevailing way to fabricate more complex three-dimensional structures and devices exploiting the unique properties of nanostructures, find applications in a variety of fields such as nanophotonics, field emission, photovoltaics, and multifunctional nanocomposites. Acknowledgment. We thank the financial supports of NSFC of China (Term no.90606001 and 90406024), National 973 project (2002CB713802) of MOST of China.

Figure 6. (a) Emission spots of nanostructure at all altitudes at high excitation; (b) green emission spots with the same altitude at the same location as part a. The red circle is the excitation spot.

the Au solution capped nanowire may adsorb Cl- and water droplet; drying it at low temperature would lead to particle aggregation, i.e., produce unavoidiable irregular or nonuniform particle distribution under sequential thermal processing. Moreover the adsorbed concentration is not easy to control, pH and chemical species all may influence the adsorption. These characteristics are in facts present in the solution experiments. The aggregated Au particles cannot produce symmetrically CdS nanostructure (see Supporting Information FIgure S3). In our system the ZnS nanowires can be used for CdS growth after Au evaporation without thermal treatment to reduce the chance of Au aggregation. Two, the chemical intermediates in abovementioned examples may disturb the secondary growth. For example, the adsorbed Cl- and water, usually present, may decompose into HCl and corrode the nanowire, which may produce defects or pits on the surface of ZnS nanowire. The additional CdS branch grow at 120° to the nanowire may reflect the existence of surface defect on the original ZnS nanowire.14 The multistep structures on the surface of ZnO nanopropeller20 should be related to the addition of Sn atoms, which can also be vaporized and infix ZnO lattice, change the growth direction and habit of ZnO, even may form new materials like ZnO:Sn or ZnSnO3 nanowires at definite condition.33 Our above analysis is based on the related experimental results. In our clean evaporation technique, no extravaporized species is present in the chamber to be interposed into the growth, so the clean and smooth nanowires/nanobelts can form on the ZnS nanowire

Supporting Information Available: SEM of Au-adsorbed ZnS nanowires after 700 °C thermal treatment, XRD of intermediate product of CdS/ZnS nanowires, and TEM image of CdS/ZnS nanostructure under catalyst with thermal-treated catalyst. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Nanowires and Nanobelts, Vol. I: Metal and emiconductor Nanowires & Vol. II: Nanowires and Nanobelts of Functional Materials; Wang, Z. L., Ed.; Kluwer Academic:New York, 2003. (2) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, H. Q. AdV. Mater. 2003, 15, 353. (3) (a) Patolsky, F.; Lieber, C. M. Mater. Today 2005, 8, 20. (b) Li, Y.; Qian, F.; Xiang, J.; Lieber, C. M. Mater. Today 2006, 9, 18. (4) Thelander, C.; Agarwal, P.; Brongersma, S.; Eymery, J.; Feiner, L. F.; Forchel, A.; Scheffler, M.; Riess, W.; Ohlsson, B. J.; Go¨sele, U.; Samuelson, L. Mater. Today 2006, 9, 28. (5) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241. (6) Agarwal, R.; Barrelet, C. J.; Lieber, C. M. Nano Lett. 2005, 5, 917. (7) Pan, A. L.; Liu, R. B.; Yang, Q.; Zhu, Y. C.; Yang, G. Z.; Zou, B. S.; Chen, K. Q. J. Phys. Chem. B 2005, 109, 24268. (8) Liu, Y. K.; Zapien, J. A.; Shan, Y. Y.; Geng, C. Y.; Lee, S. T. AdV. Mater. 2005, 17, 1372. (9) Zapien, J. A.; Jiang, Y.; Meng, X. M.; Chen, W.; Au, F. C. K.; Lifshitz, Y.; Lee, S. T. Appl. Phys. Lett. 2004, 84, 1189. (10) Pan, A. L.; Liu, D.; Liu, R. B.; Wang, F. F.; Zhu, X.; Zou, B. S. Small 2005, 1, 980. (11) Jie, J. S.; Zhang, W. J.; Jiang, Y.; Meng, X. M.; Li, Y. Q.; Lee, S. T. Nano Lett. 2006, 9, 1887. (12) Hayden, O.; Agarwal, R.; Lieber, C. M. Nat. Mater. 2006, 5, 352. (13) Ilan Gur, Neil A. ; Fromer, M. L.; Geier, A.; Alivisatos, P. Science 2005, 310, 462–465. (14) Jung, Y. W.; Ko, D. K.; Agarwal, R. Nano Lett. 2007, 7, 264. (15) Li, Y. Q.; Tang, J. X.; Wang, H.; Zapien, J. A.; Shan, Y. Y.; Lee, S. T. Appl. Phys. Lett. 2007, 90, 093127. (16) Zhou, J.; Ding, Y.; Deng, S. Z.; Gong, L.; Xu, N. S.; Wang, Z. L. AdV. Mater. 2005, 17, 2107. (17) May, S. J.; Zheng, J. G.; Wessels, B. W.; Lauhon, L. J. AdV. Mater. 2005, 17, 598.

9260 J. Phys. Chem. C, Vol. 112, No. 25, 2008 (18) Wang, D.; Qian, F.; Yang, C.; Zhong, Z. H.; Lieber, C. M. Nano Lett. 2004, 4, 871. (19) Dick, K. A.; Deppert, K.; Larsson, M. W.; Martensson, T.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Nat. Mater. 2004, 3, 380. (20) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287. (21) Gao, P. X.; Wang, Z. L. Appl. Phys. Lett. 2004, 84, 2883. (22) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (23) Wang, D. L.; Qian, F.; Yang, C.; Zhong, Z. H.; Lieber, C. M. Nano Lett. 2004, 4, 871. (24) Brenner, S. S.; Sears, G. W. Acta Mater. 1956, 4, 268. (25) Pan, A. L.; Yang, H.; Yu, R. C.; Zou, B. S. Nanotechnology 2006, 17, 1083. (26) Venugopal, R.; Lin, P.; Liu, C.; Chen, Y. J. Am. Chem. Soc. 2005, 127, 11262.

Zhou et al. (27) Gao, P. X.; Ding, Y.; Wang, Z. L. Nano Lett. 2003, 3, 1315. (28) Ma, C.; Moore, D.; Li, J.; Wang, Z. L. AdV. Mater. 2003, 15, 228. (29) Matthews, J. W.; Blakeslee, A. E. J. Cryst. Growth 1974, 27, 118. (30) (a) Nell, M.; Marohn, J.; Mclendon, G. J. Phys. Chem. 1990, 94, 4359. (b) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (31) Pinna, N.; Weiss, K.; Urban, J.; Pileni, M. P. AdV. Mater. 2001, 13, 261. (32) (a) Bouchenaki, C.; Ullrich, B.; Zielinger, J. P. J. Lumin. 1991, 48. (b) Feldman, B. J.; Duisman, J. A. Appl. Phys. Lett. 1981, 37, 1092. (33) Xue, X. Y.; Feng, P.; Wang, Y. G.; Wang, T. H. Appl. Phys. Lett. 2007, 91, 022111.

JP800599E