ZnO Nano-Heterostructure

Apr 22, 2012 - STXM data were analyzed using the aXis2000 software package, which allows for detailed interactive processing of the images and fitting...
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Tracking the Interface of an Individual ZnS/ZnO NanoHeterostructure Zhiqiang Wang,†,§ Jian Wang,‡ Tsun-Kong Sham,*,† and Shaoguang Yang§ †

Department of Chemistry, University of Western Ontario, London, Canada N6A 5B7 Canadian Light Source Inc., University of Saskatchewan, Saskatoon, Canada S7N 0X4 § Nanjing National Laboratory of Microstructures and School of Physics, Nanjing University, Nanjing, China 210093 ‡

S Supporting Information *

ABSTRACT: Understanding and controlling the electronic structure in semiconductor heterostructures is of foremost importance in order to achieve their promise in broad applications. Here we report the chemical mapping and the electronic structure of a single ZnS nanobelt/ZnO nanorod heterostructure by scanning transmission X-ray microscopy. It is the first time that nanoscaled spectroscopy across the interface between the ZnS and ZnO nanocomponents is studied in detail. Nano-X-ray absorption near edge structure at both O K-edge and Zn L3,2-edge indicates that ZnO was anchored underneath the side surfaces of ZnS nanobelt. The threshold shift of the edge energy across the ZnS/ZnO interface illustrates an upward movement of the unoccupied states of O character in the conduction band of ZnO and a downward movement of the unoccupied states of S character in the conduction band of ZnS, revealing the effect of the interface on band alignments of the ZnS/ZnO nanocomposite.

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INTRODUCTION One-dimensional (1D) semiconducting nanostructures have attracted extraordinary attention in the past two decades due to their unique properties and potential applications as building blocks for nanodevices, such as lasers,1,2 sensors,3,4 transistors,5 generators,6 and photodetectors.7−11 Recently, 1D nanoheterostructures consisting of chemically distinct components are attracting increasing interest because of the possibility of tuning their chemical, electronic, and optical properties at a wider range12−26 and performing diverse functionalities within a single nanostructure.27,28 Motivated by these prospects, significant progress has been made in the synthesis of various axial,12,14,17,29 radial,30−34 and branched nano-heterostructures,15,18,35,36 which provides a testing ground to study the fundamental effect of different components and their interface on the optical and electronic properties of nano-heterostructures. As important II−VI semiconductors, ZnO and ZnS with wide bandgap energies (3.37 and 3.7 eV at room temperature, respectively) have a wide range of applications including flat panel displays, sensors, lasers, transducers, and photovoltaic devices.37−42 ZnO/ZnS nano-heterostructures, such as nanocables,30,43,44 nanosaws,45 nanorings,46 and biaxial nanobelts,47−50 have been synthesized via solution and vapor phase techniques. Most recently, high-quality hierarchical ZnS nanobelt/ZnO nanorod heterostructures have been prepared by a simple two-step vapor-phase transport method.41 The electronic structures and optical properties of these ZnS/ZnO nano-heterostructures exhibit some amazing properties.47,52,53 However, there are few reports about these properties of a © 2012 American Chemical Society

single ZnS/ZnO nano-heterostructure, especially its morphology, chemistry, and band alignment relating to the individual ZnS and ZnO component as well as their interface. Physical properties of a semiconductor heterostructure primarily depend on the relative alignment of the conduction and valence band edges of the components. Therefore, understanding and controlling the electronic structure in ZnS/ZnO nanocomposite is of foremost importance. To achieve this goal, the capability to image the local chemical component and electronic structure variations within an individual ZnS/ZnO nano-heterostructure across the interface is crucial; such information is apparently absent until now. Using the brilliant, undulator-based and polarizationcontrolled third-generation synchrotron sources, scanning transmission X-ray microscopy (STXM) provides an excellent combination of chemical speciation (X-ray absorption near edge structures, XANES) and microscopy with a spectral resolution and a spatial resolution of 0.05 eV and of 30 nm, respectively. STXM has been successfully applied to investigate the electronic structure and surface defects of individual carbon nanotube (CNT), a single catalyst under relevant operation conditions, and the chemical imaging of RuO2 supported on CNT as well as multilayer of reduced-graphene oxide.54−57 Here, we report STXM chemical images and the electronic structure of a single ZnS nanobelt/ZnO nanorod heterostructure across the ZnS and ZnO interface. Specifically, the Received: February 8, 2012 Revised: April 20, 2012 Published: April 22, 2012 10375

dx.doi.org/10.1021/jp301289x | J. Phys. Chem. C 2012, 116, 10375−10381

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XANES at Zn L3,2-, S L3,2-, and O K-edges were used to image the ZnS and ZnO component in an individual ZnS/ZnO nanocomposite composed of a ZnS nanobelt attached by ZnO nanorods. It is the first time that the nanoscaled spectroscopy across the interface between the ZnS and ZnO nanocomponents was studied in detail. This study reveals the morphology of the interface and its effect on the position of the valence band and the conduction band of ZnS and ZnO.

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EXPERIMENTAL SECTION The synthesis of the hierarchical ZnS nanobelt/ZnO nanorod heterostructures has been reported elsewhere.51 Briefly, the synthesis was carried out in a horizontal furnace via a two-step vapor-phase transport method. ZnS nanobelts were first prepared by thermal evaporation of ZnS powder. A Si wafer coated with a thin layer of gold (∼10 nm) was used as the substrate. After the evaporation, a layer of white powder was found on the surface of the Si wafer. In the following step, a 1:1 mixture of ZnO and graphite powder (in molar ratio) was placed in the center of another quartz tube. The Si wafer, already deposited with a layer of white powder of ZnS, was used as the substrate. High purity N2 was used as the carrier gas. The reaction was carried out at 900 °C for 60 min. After the reaction, a light yellow product was found on the substrate. The morphologies and microstructures of the products were characterized by scanning electron microscopy (SEM; PhilipsXL30) and transmission electron microscopy (TEM; Philips Tecnai F20). STXM measurement was conducted at the SM beamline of the Canadian Light Source (CLS), which is equipped with a 25 nm outermost-zone zone plate (CXRO, Berkeley Lab). The diffraction-limited spatial resolution for this zone plate is 30 nm. Image sequence (stack) scans over a range of photon energies were acquired for the same sample region at the Zn L3,2-, S L3,2-, and O K-edge. STXM data were analyzed using the aXis2000 software package, which allows for detailed interactive processing of the images and fitting of the X-ray absorption spectra. More details of the sample preparation and STXM measurement can be found in the Supporting Information.

Figure 1. SEM, TEM, and EDX images of the as-synthesized ZnS nanobelt/ZnO nanorod heterostructures.51 (a) SEM image of the product, which is comprised of a large amount of hierarchical nanostructures. The inset of (a) is a high-magnification SEM image showing that the nanorod branches grew in a row on the side surfaces of the nanobelt. Scale bars in (a) and its inset are 10 and 1 μm, respectively. (b) TEM image of a single ZnS/ZnO nano-heterostructure. (c) EDX image of the ZnS/ZnO nano-heterostructure, showing the elemental profiles for Zn, O, and S along the red line displayed in (b). (d) HRTEM image of the ZnS nanobelt/ZnO nanorod heterostructure. The inset of (d) is the corresponding SAED pattern.

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RESULTS AND DISCUSSION Figure 1a shows the SEM images of the as-synthesized product, which consists of a large amount of hierarchical nanostructures. The beltlike backbone is 1−2 μm in width and hundreds of micrometers in length. The rodlike branches with a diameter of 100−160 nm and a length of 220−330 nm grew and stood in a row on the side surfaces of the backbone. Figure 1b displays the TEM image of an individual hierarchical nanostructure. The nanorod branches stand vertically on the side of the nanobelt backbone. The diameter and length of the nanorods are in the range of 37−100 and 70−250 nm, respectively. Energy disperse X-ray spectroscopy (EDX) line profile analysis is presented in Figure 1c, revealing that the nanobelt backbone is composed of Zn and S and the nanorod Zn and O. The ZnO branch is significantly thicker than the ZnS nanoribbon. A highresolution TEM image is shown in Figure 1d. The two parts with light and dark contrast come from the nanobelt backbone and the nanorod branch of the hierarchical nanostructures shown in Figure 1b, respectively. Both the backbone and the branch are single-crystalline with clear lattice fringes. The lattice spacing of 0.33 and 0.31 nm in light contrast region corresponds to (100) and (002) planes of hexagonal ZnS,

and that of 0.28 and 0.26 nm in dark contrast region corresponds to (100) and (002) planes of hexagonal ZnO. It is noted that the ZnS and ZnO component have the same orientation along [210] and [001] directions, respectively. The STXM image of a single hierarchical ZnS nanobelt/ZnO nanorod heterostructure recorded at the Zn L3-edge (∼1027 eV) is shown in Figure 2a. The nanorods with light contrast were grown on the side surface of the nanobelt. The morphology and dimensions of the ZnS/ZnO nano-heterostructures obtained by STXM agree well with that by SEM. The chemical mapping displayed in Figure 2b indicates that the nanobelt is ZnS and the nanorods are ZnO, consistent with the TEM and EDX results. However, it should be noted that STXM has the unique ability to distinguish the Zn signal between ZnS and ZnO (Figure S1a,b) because they are clearly discernible in their XANES (Figure S2a), while EDX can only 10376

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colors and shapes across the interface between ZnS and ZnO are shown in Figure 4b,c. In the region of the nanorod, all XANES show same features as that of pure ZnO nanostructures (Figure 4b). In the region of the nanobelt, most of the XANES display pure ZnS feature (Figure 4c). However, as the region is approaching the interface (e.g., the dark blue rectangular ROI labeled “ZnS-R7” in the nanobelt in Figure 4a), the XANES (curve “ZnS-R7” in Figure 4c) exhibits features from both ZnS and ZnO. It indicates that oxygen or ZnO was anchored into the subsurface of the side surfaces of the ZnS nanobelt during the second step of the synthesis. Across the interface region (the ROI labeled “Interface” and “ZnO-R8” in Figure 4a, XANES labeled “Interface” in Figure 4c, and “ZnO-R8” in Figure 4b), the XANES emerges to become that of ZnO. This behavior anchors the seeding for the continuing growth of the ZnO nanorod. At the S L3,2-edge, the nano-XANES across the interface between the ZnS nanobelt and the ZnO nanorod reveals similar behavior. Figure 4e,f shows the corresponding S L3,2-edge nanoXANES across the ZnS/ZnO interface (Figure 4d shows the same ZnS/ZnO region as that in Figure 4a). Again, despite the weaker signal due to a lower cross section comparing to the Zn L- and O K-edge, one can still see that the S L-edge signal begins to weaken noticeably at the boarder (XANES labeled “ZnS-R6”, “Interface”, and “ZnO-R6”) but still well within the nanobelt before crossing the apparent interface between the belt and the rod. No S signal was found in the nanorod region. At the O K-edge, the nano-XANES across the interface provides much clearer evidence. Figure 4h,i displays the evolution of the O K-edge nano-XANES across the ZnO/ ZnS interface (Figure 4g shows the same region as that in Figure 4a,d). It is evident that oxygen is present within the boarder of the ZnS nanobelt (XANES labeled “ZnS-R6” and “Interface”), and the XANES feature is clearly ZnO-like within the sampling region. Hence, the data from both Zn L3,2 and O K-edge confirm that ZnO exists at the subsurface of the ZnS nanobelt where ZnO nanorod is attached. On the basis of the above analysis, the distribution of Zn, S, and O in the hierarchical nanostructure is obtained. First, Zn exists in all the areas of the hierarchical nanostructure. Second, S distributes uniformly in the nanobelt backbone except in the region in the vicinity of the interface. Finally, O is found not only in the secondary branched nanorods but also inside the subsurface of the nanobelt backbone, anchoring the growth of the ZnO nanorod on the side of the ZnS nanobelt. Since STXM can determine the absolute thickness of the sample based on X-ray absorption, the thickness distribution of the hierarchical ZnS nanobelt/ZnO nanorod heterostructure has been obtained and is shown in Figure 5a,b. The thickness was obtained from stack fitting with the quantitavely scaled reference spectra of 1 nm thickness (Figure S5). The average thickness of the ZnS nanobelt and ZnO nanorod is determined to be about 63 and 120−150 nm, respectively (±10%). This result confirms that ZnO grows as thicker rods than the thickness of the nanobelt, which is very uniform (∼63 nm) along the belt. For the ZnO nanorods, the thickness at the bottom region (near the interface) of the nanorod is determined to be in the range of 55−65 nm, similar to that of the ZnS nanobelt. For the ZnO submicrometer domains on the surface of ZnS nanobelt, their thickness is determined to be 47−71 nm. Based on the thickness distribution, a model of ZnS/ZnO nano-heterostructures is proposed and shown in Figure 5c,

Figure 2. STXM and chemical mapping images of an individual ZnS nanobelt/ZnO nanorod heterostructure. (a) STXM image of a single ZnS/ZnO nano-heterostructure at Zn L3-edge. Scale bar in (a) is 1 μm. (b) Chemical mapping image of a single ZnS/ZnO nanoheterostructure at Zn L3-edge (green: ZnS; red: ZnO).

determine the presence of the elements, not their chemical identity. We have obtained XANES at the Zn L3,2-edge in a number of regions of interest (ROIs) as marked in Figure 3a,b (the

Figure 3. STXM image and XANES of a single ZnS nanobelt/ZnO nanorod heterostructure. (a) STXM image of a single ZnS/ZnO nanoheterostructure at Zn L3,2-edge. Scale bar in (a) is 1 μm. (b) Isolated XANES of each marked area displayed in (a).

marked ROIs are symbolic, whose size error is ±10% of the actual mask size). It can be seen that the XANES from the ROIs on the nanobelt backbone share the same features as that from the pure ZnS nanobelt, while the XANES from the branched nanorods display ZnO characteritics. The XANES selected from the ROI marked “O3” represents the early stage of the ZnO nanorod growth and exhibits features from both ZnS and ZnO, while the ROI marked “O4” represents a fully grown ZnO nanorod on ZnS nanobelt. Now, let us look closely at the spectral features across the interface between ZnS and ZnO component in ROI “O4” as shown in Figure 4a; similar to the line scan in EDX, the XANES selected from the nanoscaled ROIs (about 240 × 50−380 × 70 nm2) denoted with different 10377

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Figure 4. High-magnification STXM images and nano-XANES of a branched ZnO nanorod on the ZnS nanobelt. (a, d, g) High-magnification STXM images of a 1.7 μm × 1.7 μm region (as marked in Figure 2a) at Zn L-edge (1027 eV) (a), S L-edge (165 eV) (d), and O K-edge (537 eV) (g), respectively. The selected regions of interest (ROIs), which have an area of 240 × 50−380 × 70 nm2 (a), 250 × 50−380 × 80 nm2 (d), and 300 × 75 nm2 (g) (error: ± 10% of the actual mask size), are rectangular and elliptical shaped in ZnS and ZnO regions, respectively. The black hollow rectangular ROI is located in the interface region. Scale bars in (a, d, g) are 500 nm. STXM images without ROI masks are shown in Figure S4. (b, c, e, f, h, i) Corresponding nano-XANES across the interface between ZnS and ZnO at Zn L-edge (b, c), S L-edge (e, f), and O K-edge (h, i), respectively. The nano-XANES from the ZnS, ZnO, and ZnS/ZnO interface are labeled “ZnS-Ri”, “ZnO-Ri” (i = 1, 2, 3, ...), and “Interface”, respectively. The color of the spectrum corresponds to the color labeled ROI.

where the initial anchoring of ZnO inside the ZnS nanobelt initiates the growth of the ZnO nanorod. It is well-known that an important characteristic of the wurtzite-structured ZnS (or ZnO) is polar surface.58 The (001) and (00−1) surfaces of wurtzite ZnS (or ZnO) are terminated with Zn and S (or O), respectively, resulting in positively and negatively charged polar surfaces. Positively charged Zn terminated (001) surface is chemically active. It is conceivable that during the second growth step ZnO precursor was absorbed preferablly on the (001) surface of ZnS nanobelt. Because of the large lattice mismatch between ZnS and ZnO (∼15%), it is hard to directly epitaxially grow ZnO on the (001) surface of ZnS. At high temperature, the ZnS/ZnO mixture would be first formed at the kink or step sites on the ZnS (001) surface, which reduced the effect of the large lattice mismatch between ZnS and ZnO and facilitated the growth of ZnO on ZnS nanobelt. Because of the polar surface, ZnO preferablly grew along [001] direction on the (001) surface of ZnS nanobelt during the second growth step.

It is also informative to observe the trend of the edge (threshold) energy (E0) across the interface. The threshold can be tracked as the point of inflection of the rising edge or the peak of the whiteline resonance. In either case, the position of the E0 arisies from the transition from Zn L3, O K, and S L3,2 electrons to the bottom of the conduction band with Zn d/s, O p, and S d/s character, respectively. Thus, the movement in E0 can track the movement of the bottom of the conduction band. The E0 can be obtained by taking the maximum of the first derivative of the nano-XANES across the interface between ZnS and ZnO at the Zn L3,2-, O K-, and S L3,2-edge. The observed E0 shifts are summarized in Figure 6, which shows an upward/downward movement when approaching the ZnS/ ZnO interface. Understandably, these values are with relatively large uncertainty, especially in the case of the S L3,2-edge result. Nonetheless, the trend is qualitatively interesting in that the O K-edge result indicates an upward movement of the unoccupied states of O character in the conduction band of ZnO (Figure 6a), whereas the S L3,2-edge result shows a downward 10378

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Figure 5. Thickness distribution and schematic illustration of the ZnS nanobelt/ZnO nanorod heterostructure. (a, b) Thickness distribution of the nanobelt and nanorods at Zn L3,2-edge. Scale bars in (a) and (b) are 2 μm. The vertical bar tracks the thickness. The average thickness of the ZnS nanobelt and ZnO nanorod is determined to be about 63 and 120−150 nm, respectively (±10%). (c) Schematic illustration of the ZnS nanobelt/ZnO nanorod heterostructrue. The diagram shows the 3D image (with top and side views) of a single ZnS nanobelt/ZnO nanorod heterostructure, where the green and red represent ZnS and ZnO, respectively.

movement of the unoccupied states of S character in the conduction band of ZnS (Figure 6b). On the basis of E0 shifts in the ZnS/ZnO interface, the conduction band minimum (CBM) energy of ZnS is lower than its natural band energy, while that of ZnO is higher (Figure 6d). This situation can be compared with a recent calculation using ZnO/ZnS core/shell nanowire, in which the strained CBM energies are both lower than the natural band energies for both ZnO and ZnS.52 More definite conclusion awaits further experiment and calculation. With the 25 nm outer-zone zone plate, the diffraction-limited spatial resolution of STXM is ∼30 nm. Based on TEM result, the size of the ZnS/ZnO interface region is much smaller than that of the selected ROIs in Figure 4 and the resolution limit. The ROIs at the interface region (black hollow rectangle in Figure 4a,d,g) include not only ZnS/ZnO interface but also some pure ZnS and ZnO region. Therefore, it is challenging to directly probe the chemical information on the interface. However, a progressive scan across the interface can still

Figure 6. Edge energy (E0) threshold across the interface between ZnS and ZnO and schematic diagram of the valence band and conduction band alignments for ZnS/ZnO nano-heterostructures. (a, b, c) Edge energy (E0) threshold across the ZnS/ZnO interface at (a) O K-edge, (b) S L3,2-edge, and (c) Zn L3,2-edge. The x-axis in (a), (b), and (c) is defined as the distance between the left side surface of ZnS nanobelt and the nanoscaled region of interest in Figure 4a, 4d, and 4g, respectively. The ZnO-Ri and ZnS-Ri (i = 1, 2, 3, ...) in (a), (b), and (c) correspond to that in Figure 4a−c, 4d−f, and 4g−i, respectively. (d) Schematic diagram of the valence band and conduction band alignments for ZnS nanobelt/ZnO nanorod heterostructures. The solid lines show the calculated natural band alignments for ZnS and ZnO, and the dashed lines show the strained band alignments.52

provide some qualitative information, such as the threshold energy shifts across the interface. We worked near the resolution limit of STXM; however, the XANES spectrum from each nanosized ROI is of high enough quality compared 10379

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University of Western Ontario is supported by NSERC, CFI, OIT, and CRC (TKS). CLS is supported by CFI, NSERC, CHIR, NRC, and the University of Saskatchewan. Research at Nanjing University is supported by NSFC (61176087).

to that from bulk sample. The nano-XANES across the interface can provide reliable information. Therefore, the observed effects are not washed out by the limitation of spatial resolution. On the other hand, the effects are neither influenced by side maxima of the point spread function (PSF). If the side maxima of PSF affected the STXM images and XANES spectra, sulfur and oxygen signals would be found at the dark yellow elliptic ROI labeled “ZnO-R6” in the ZnO region (Figure 4d) and dark yellow rectangular ROI labeled “ZnS-R6” in the ZnS region (Figure 4g), respectively. However, only oxygen signal (curve “ZnS-R6” in Figure 4i) was found at the dark yellow rectangular ROI in ZnS region, and no sulfur signal (curve “ZnO-R6” in Figure 4e) was found at the dark yellow elliptic ROI in ZnO region. At the present time, the spatial resolution of routine STXM is still not high enough to directly probe the chemical information on the interface in this kind of heterostructures with nanometer resolution. We argue, however, that as a new technique emerges, it is a process from its infancy to maturity and we are analyzing the data within its limitation at the present time. Recently, the best spatial resolution of STXM imaging has been demonstrated at 10 nm with a 17 nm outermost-zone zone plate at an appropriate energy, i.e., Fe L-edge.59 Although it is getting increasingly challenging, the spatial resolution of STXM will continue to advance as the lithography technology improves and the size of features that can be manufactured decreases, and new approaches to circumvent the diffraction limitation become available. When the possible resolution of STXM reaches sub-10 nm, a direct chemical analysis of the interface in the heterostructures in question by STXM will be achievable.

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(1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897−1899. (2) Duan, X. F.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241−245. (3) Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K. J.; Dai, H. J. Science 2000, 287, 622−625. (4) Cui, Y.; Wei, Q. Q.; Park, H. K.; Lieber, C. M. Science 2001, 293, 1289−1292. (5) Bachtold, A.; Hadley, P.; Nakanishi, T.; Dekker, C. Science 2001, 294, 1317−1320. (6) Wang, Z. L.; Song, J. H. Science 2006, 312, 242−246. (7) Fang, X. S.; Hu, L. F.; Huo, K. F.; Gao, B.; Zhao, L. J.; Liao, M. Y.; Chu, P. K.; Bando, Y.; Golberg, D. Adv. Funct. Mater. 2011, 21, 3907−3915. (8) Menzel, A.; Subannajui, K.; Guder, F.; Moser, D.; Paul, O.; Zacharisa, M. Adv. Funct. Mater. 2011, 21, 4342−4348. (9) Fang, X. S.; Xiong, S. L.; Zhai, T. Y.; Bando, Y.; Liao, M. Y.; Gautam, U. K.; Koide, Y.; Zhang, X. G.; Qian, Y. T.; Golberg, D. Adv. Mater. 2009, 21, 5016−5021. (10) Bai, S.; Wu, W. W.; Qin, Y.; Cui, N. Y.; Bayerl, D. J.; Wang, X. D. Adv. Funct. Mater. 2011, 21, 4464−4469. (11) Fang, X. S.; Bando, Y.; Liao, M. Y.; Zhai, T. Y.; Gautam, U. K.; Li, L.; Koide, Y.; Golberg, D. Adv. Funct. Mater. 2010, 20, 500−508. (12) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617−620. (13) Lauhon, L. J.; Gudiksen, M. S.; Wang, C. L.; Lieber, C. M. Nature 2002, 420, 57−61. (14) Wu, Y. Y.; Fan, R.; Yang, P. D. Nano Lett. 2002, 2, 83−86. (15) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287−1291. (16) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J. B.; Wang, L. W.; Alivisatos, A. P. Nature 2004, 430, 190−195. (17) Robinson, R. D.; Sadtler, B.; Demchenko, D. O.; Erdonmez, C. K.; Wang, L. W.; Alivisatos, A. P. Science 2007, 317, 355−358. (18) Cheng, C. W.; Liu, B.; Yang, H. Y.; Zhou, W. W.; Sun, L.; Chen, R.; Yu, S. F.; Zhang, J. X.; Gong, H.; Sun, H. D.; Fan, H. J. ACS Nano 2009, 3, 3069−3076. (19) Fang, X. S.; Bando, Y.; Gautam, U. K.; Zhai, T. Y.; Gradecak, S.; Golberg, D. J. Mater. Chem. 2009, 19, 5683−5689. (20) Shen, G. Z.; Ye, C. H.; Golberg, D.; Hu, J. Q.; Bando, Y. Appl. Phys. Lett. 2007, 90, 73115. (21) Shen, G. Z.; Chen, P. C.; Bando, Y.; Golberg, D.; Zhou, C. W. Chem. Mater. 2008, 20, 6779−6783. (22) Shen, G. Z.; Chen, D.; Zhou, C. W. Chem. Mater. 2008, 20, 3788−3790. (23) Bierman, M.; Jin, S. Energy Environ. Sci. 2009, 2, 1050−1059. (24) Shen, G. Z.; Chen, D. Sci. Adv. Mater. 2009, 1, 213−226. (25) Jiang, Z.; Qing, Q.; Xie, P.; Gao, R. X.; Lieber, C. M. Nano Lett. 2012, 12, 1711−1716. (26) Kempa, T. J.; Cahoon, J. F.; Kim, S. K.; Day, R. W.; Bell, D. C.; Park, H. G.; Lieber, C. M. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 1407−1412. (27) He, R. R.; Law, M.; Fan, R.; Kim, F.; Yang, P. D. Nano Lett. 2002, 2, 1109−1112. (28) Law, M.; Zhang, X. F.; Yu, R.; Kuykendall, T.; Yang, P. D. Small 2005, 1, 858−865. (29) Wen, C. Y.; Reuter, M. C.; Bruley, J.; Tersoff, J.; Kodambaka, S.; Stach, E. A.; Ross, F. M. Science 2009, 326, 1247−1250. (30) Wang, X. D.; Gao, P. X.; Li, J.; Summers, C. J.; Wang, Z. L. Adv. Mater. 2002, 14, 1732−1735. (31) Han, S.; Li, C.; Liu, Z. Q.; Lei, B.; Zhang, D. H.; Jin, W.; Liu, X. L.; Tang, T.; Zhou, C. W. Nano Lett. 2004, 4, 1241−1246.

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CONCLUSIONS In summary, the chemical mapping and electronic structure of an individual ZnS nanobelt/ZnO nanorod heterostructure have been obtained by STXM. It is the first time that nanoscaled spectroscopy across the interface between the ZnS and ZnO nanocomponents was studied in detail. The results clearly reveal the morphology of the interface and its effect on the band alignments of the ZnS/ZnO nanocomposite.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details of sample preparation and STXM measurement, calculation of thickness, chemical mapping images, XANES of ZnS nanobelt and ZnO nanorod, STXM images and micro-XANES at O K- and S L-edge, highmagnification STXM images of a branched ZnO nanorod on the ZnS nanobelt, and reference spectra of 1 nm thick ZnS and ZnO. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Ph (519) 661-2111 ext 86341; Fax (519) 661-3022; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Jiangfeng Gong for his TEM technical assistance and Mr. Linda Wu for CAD drawing. Research at the 10380

dx.doi.org/10.1021/jp301289x | J. Phys. Chem. C 2012, 116, 10375−10381

The Journal of Physical Chemistry C

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(32) Zhang, D. H.; Liu, Z. Q.; Han, S.; Li, C.; Lei, B.; Stewart, M. P.; Tour, J. M.; Zhou, C. W. Nano Lett. 2004, 4, 2151−2155. (33) Kuo, C. H.; Hua, T. E.; Huang, M. H. J. Am. Chem. Soc. 2009, 131, 17871−17878. (34) Sun, X. H.; Sham, T. K.; Rosenberg, R. A.; Shenoy, G. K. J. Phys. Chem. C 2007, 111, 8475−8482. (35) Dick, K. A.; Deppert, K.; Larsson, M. W.; Martensson, T.; Seifert, W.; Wallenberg, L. R.; Samuelson, L. Nat. Mater. 2004, 3, 380−384. (36) Gao, J.; Lebedev, O. I.; Turner, S.; Li, Y. F.; Lu, Y. H.; Feng, Y. P.; Boullay, P.; Prellier, W.; Tendeloo, G. V.; Wu, T. Nano Lett. 2012, 12, 275−280. (37) Wang, X. D.; Song, J. H.; Wang, Z. L. J. Mater. Chem. 2007, 17, 711−720. (38) Fang, X. S.; Wu, L. M.; Hu, L. F. Adv. Mater. 2011, 23, 585− 598. (39) Fang, X. S.; Zhai, T. Y.; Gautam, U. K.; Li, L.; Wu, L. M.; Bando, Y.; Golberg, D. Prog. Mater. Sci. 2011, 56, 175−287. (40) Gullapalli, H.; Vemuru, V. S. M.; Kumar, A.; Botello-Mendez, A.; Vajtai, R.; Terrones, M.; Nagarajaiah, S.; Ajayan, P. M. Small 2010, 6, 1641−1646. (41) Zhou, W. B.; Baneyx, F. ACS Nano 2011, 5, 8013−8018. (42) Fang, X. S.; Bando, Y.; Liao, M. Y.; Gautam, U. K.; Zhi, C. Y.; Dierre, B.; Liu, B. D.; Zhai, T. Y.; Sekiguchi, T.; Koide, Y.; et al. Adv. Mater. 2009, 21, 2034−2039. (43) Zhang, H.; Yang, D. R.; Ma, X. Y.; Que, D. L. Nanotechnology 2005, 16, 2721−2725. (44) Panda, S. K.; Dev, A.; Chaudhuri, S. J. Phys. Chem. C 2007, 111, 5039−5043. (45) Shen, G. Z.; Chen, D.; Lee, C. J. J. Phys. Chem. B 2006, 110, 15689−15693. (46) Wu, X.; Jiang, P.; Ding, Y.; Cai, W.; Xie, S. S.; Wang, Z. L. Adv. Mater. 2007, 19, 2319−2323. (47) Yan, J.; Fang, X. S.; Zhang, L. D.; Bando, Y.; Gautam, U. K.; Dierre, B.; Sekiguchi, T.; Golberg, D. Nano Lett. 2008, 8, 2794−2799. (48) Wang, Z. Q.; Liu, X. D.; Gong, J. F.; Huang, H. B.; Gu, S. L.; Yang, S. G. Cryst. Growth Des. 2008, 8, 3911−3913. (49) Fan, X.; Zhang, M. L.; Shafiq, I.; Zhang, W. J.; Lee, C. S.; Lee, S. T. Adv. Mater. 2009, 21, 2393−2396. (50) Murphy, M. W.; Kim, P. S. G.; Zhou, X.; Zhou, J.; Coulliard, M.; Botton, G. A.; Sham, T. K. J. Phys. Chem. C 2009, 113, 4755−4757. (51) Wang, Z. Q.; Liu, Z.; Gong, J. F.; Wang, S.; Yang, S. G. CrystEngComm 2011, 13, 6774−6779. (52) Schrier, J.; Demchenko, D. O.; Wang, L. W. Nano Lett. 2007, 7, 2377−2382. (53) Murphy, M. W.; Zhou, X. T.; Ko, J. Y. P.; Zhou, J. G.; Heigl, F.; Sham, T. K. J. Chem. Phys. 2009, 130, 084707. (54) de Smit, E.; Swart, I.; Creemer, J. F.; Hoveling, G. H.; Gilles, M. K.; Tyliszczak, T.; Kooyman, P. J.; Zandbergen, H. W.; Morin, C.; Weckhuysen, B. M.; et al. Nature 2008, 456, 222−225. (55) Zhou, J. G.; Wang, J.; Fang, H. T.; Wu, C. X.; Cutler, J. N.; Sham, T. K. Chem. Commun. 2010, 46, 2778−2780. (56) Felten, A.; Gillon, X.; Gulas, M.; Pireaux, J. J.; Ke, X. X.; Van Tendeloo, G.; Bittencourt, C.; Najafi, E.; Hitchcock, A. P. ACS Nano 2010, 4, 4431−4436. (57) Zhou, J. G.; Wang, J.; Sun, C. L.; Maley, J. M.; Sammynaiken, R.; Sham, T. K.; Pong, W. F. J. Mater. Chem. 2011, 21, 14622−14630. (58) Wang, H. T.; Wu, T. J. Mater. Chem. 2011, 21, 15095−15099. (59) Chao, W.; Fischer, P.; Tyliszczak, T.; Rekawa, S.; Anderson, E.; Naulleau, P. Opt. Express 2012, 20, 9777−9783.

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dx.doi.org/10.1021/jp301289x | J. Phys. Chem. C 2012, 116, 10375−10381