ZnS Nanowire Heterostructures

Photoluminescence properties of the heterostructures, different from pure .... element sulfur species on the surface, or interstitial states associate...
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J. Phys. Chem. B 2006, 110, 15689-15693

15689

Hierarchical Saw-like ZnO Nanobelt/ZnS Nanowire Heterostructures Induced by Polar Surfaces Guozhen Shen,*,† Di Chen,‡ and Cheol Jin Lee§ Nanoscale Materials Center, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, Photocatalytic Materials Center, National Institute for Materials Science (NIMS), Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan, and Department of Electronics Engineering, Korea UniVersity, 5-1 Anam-Dong, Seongbuk-Gu, Seoul 136-701, Korea ReceiVed: May 17, 2006; In Final Form: June 18, 2006

Saw-like nanostructures composed of single-crystalline ZnO nanobelts and single-crystalline ZnS nanowires have been successfully synthesized by a vapor-solid process. Several techniques, including scanning electron microscope, transmission electron microscopy, and photoluminescence spectroscopy, were used to investigate the structures, morphology, and photoluminescence properties of the products. Due to the similar crystal habits of wurtzite ZnO and ZnS with chemically active Zn-terminated (0001) and chemically inactive O-terminated (or S-terminated) (0001h) polar surfaces, hierarchical saw-like nanostructures were considered to be formed by the initiation of a chemically active Zn-terminated ZnO (0001) polar surface. Photoluminescence properties of the heterostructures, different from pure ZnO nanobelts or ZnS nanowires, were also studied at room temperature.

1. Introduction Semiconductor heterostructures with modulated composition and/or doping enable passivation of interfaces and the generation of devices with diverse functions.1 One-dimensional (1D) nanosized semiconductor materials with well-defined heterostructures have attracted much research attention in recent years due to their unique optoelectronic properties and potential applications in nanodevice fabrication.2-5 Various kinds of 1D heterostructures have already been reported including superlattice nanowires, core-sheath coaxial nanowires, nanotypes, metal insulator nanocables, bi-coaxial nanowires, semiconductormetal nanocalbes, metal-semiconductor nanowire heterojunctions, etc.2-28 Recently, new hierarchical heterostructures, in which the major cores and the branches consist of different materials, have attracted considerable attention with respect to the realization of multicomponent system functional electronic devices. For example, Ren et al. synthesized hierarchical In2O3 (stem) and ZnO (branch) nanowire heterostructures with 6-, 4-, and 2-fold symmetries.29,30 Park’s group systematically synthesized various hierarchical heterostructures built up of ZnO (branch) and other semiconductors (stem).31 Hu and Zhang reported the synthesis of hierarchical Si (stem) and SiO2 (branch) nanowire heterostructures.32,33 We synthesized hierarchical ZnS (stem) and SiO2 (branch) heterostructures.34 Zhu et al. reported the preparation of ZnS/BN heterostructures.35 In this study, we report on the synthesis of saw-like nanostructures composed of ZnO nanobelts (stem) and ZnS nanowires (branch) by a simple thermal evaporation process. The formation of saw-like heterostructures is induced by the polar surfaces of wurtzite ZnO and ZnS. * To whom correspondence should be addressed. E-mail: shen.guozhen@ nims.go.jp. Fax: +81-29-851-6280. † Nanoscale Materials Center, National Institute for Materials Science. ‡ Photocatalytic Materials Center, National Institute for Materials Science. § Korea University.

SCHEME 1: Schematic Diagram of the Two-HeatingZone Tube Furnace Utilized in the Experiments

2. Experimental Section The synthesis of saw-like ZnO nanobelt/ZnS nanowires heterostructures is based on atmospheric pressure vapor phase evaporation of zinc powder and SnS powder in a two-heatingzone horizontal tube furnace36,37 as shown in Scheme 1. In this experiment, silicon wafer was used as the substrate for the deposition of nanostructures. In a typical process, zinc powder and SnS powder with a molar ratio of 1:1 were put into two ceramic boats separately in the two-heating-zone system, where SnS powder was placed in the first heating zone upstream and zinc powder in the second heating zone downstream as shown in Scheme 1. Prior to heating the reactor, the quartz tube was purged with high-purity argon for 10 min. After that both the heating zones of the furnace were heated to 550 °C from room temperature in 20 min and then the first heating zone with SnS powder was continuously heated to 1100 °C while the second heating zone was kept at 550 °C. After the temperature of the first heating zone reached 1100 °C, the whole furnace was kept at the reaction temperatures for an additional 30 min and then cooled to room temperature naturally. After reaction, waxlike product was found deposited on the substrate. The structure of the products was checked with X-ray diffraction (XRD, RINT 2200HF). The morphology of the

10.1021/jp0630119 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/25/2006

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Figure 1. XRD pattern of the synthesized product.

Figure 2. (a, b) Low-magnification and (c-e) high-magnification SEM images of the obtained product, showing the saw-like morphology.

products was analyzed by scanning electron microscope (SEM, JSM-6700F). The morphology and microstructure analysis were conducted by using high-resolution transmission electron microscopy (HRTEM, JTM-3000F) with an energy-dispersive X-ray spectrometer (EDS). The photoluminescence measurements were conducted at room temperatures by using a HeCd laser line of 325 nm as the excitation source. 3. Results and Discussion Figure 1 shows the XRD pattern of the synthesized products. The pattern obviously shows two sets of diffraction peaks. Those marked with “ZO” can be indexed to hexagonal wurtzite ZnO (JCPDS file: 36-1451). The others marked with “ZS” can be indexed to hexagonal wurtzite ZnS (JCPDS file: 36-1450). No peaks of other crystalline impurities are detected under XRD measurement, indicating the formation of pure ZnS and ZnO phases. The product morphology was examined by SEM, as shown in Figure 2. Low-magnification SEM images (Figure 2a,b) reveal that the product consists of numerous one-dimensional saw-like nanostructures, which have one side flat and the other side with small teeth.38,39 Typical saw-like nanostructures have diameters of ∼100 nm and lengths ranging from several tens to hundreds of micrometers. The saw-like morphology, straight or highly curved, is clearly seen from the high-magnification SEM images in Figure 2c,d. The Side-view image in Figure 2e clearly shows that the whole saw-like nanostructure has a rectangular cross-section, indicating the formation of saw-like

Figure 3. (a) TEM images of the saw-like nanostructures. (b, c) EDS spectra taken from the parts framed in part a. (d) TEM image of a single saw-like nanostructure. (e) Typical SAED pattern showing two sets of diffraction spots of wurtzite structured ZnO and ZnS.

nanobelt structures. The thickness of the nanobelts is about 50 nm as indicated by the SEM image. A low-magnification TEM image of the saw structures is shown in Figure 3a. The one-sided teeth structure of the saws is clearly seen. Energy-dispersive X-ray spectra (EDS) generated with an electron nanoprobe (∼20 nm) were collected from the two areas along the saw structure: the teeth part and the belt part (as marked with circles in Figure 3a); these spectra are shown in Figure 3b,c. The results show that the teeth consist of Zn and S with an atomic ratio of ∼1:1, indicating the formation of pure ZnS structures (Figure 3b), while the belt part consists of only Zn and O with an atomic ratio of ∼1:1, revealing that the belt part is pure ZnO structure (Figure 3c). Thus the EDS results indicate the formation of ZnO nanoribbion (stem), ZnS nanowires (branch), heterostructures. Figure 3d shows the TEM image of a single saw-like ZnO/ZnS heterostructure. Its corresponding selected area electron diffraction (SAED) pattern is shown in Figure 3e. The diffraction spots in this pattern can be indexed as the diffraction along the [21h1h0] zone axis of both ZnO (space group P63mc, a ) b ) 3.249 Å, c ) 5.206 Å) and ZnS (space group P63mc, a ) b ) 3.820 Å, c ) 6.257 Å). The SAED pattern shows that the ZnO and ZnS have a crystalline orientation relationship as follows:

[21h1h0]ZnO//[21h1h0]ZnS

(1)

(0001)ZnO//(0001)ZnS

(2)

(011h0)ZnO//(011h0)ZnS

(3)

ZnO Nanobelt/ZnS Nanowire Heterostructures

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Figure 5. The projected structure of wurtzite structured ZnE (E ) O, S) along the a-axis, showing the {0001} polar surfaces. (b) Structural model of ZnE showing the stacking of tetrahedrons.

Figure 6. Raman spectrum of the ZnO/ZnS heterostructures. Figure 4. (a) TEM image of a typical ZnO-ZnS heterostructure and its corresponding SAED pattern. (b) HRTEM image of the branched ZnS nanowire. (c) HRTEM image taken from the ZnO-ZnS interface. (d) HRTEM image of the ZnO nanobelt.

Figure 4a shows the high-magnification TEM image of another saw-like heterostructure. From this image, it can be seen that the ZnO nanobelt has a width of about 50 nm and the ZnS nanowires have lengths of 50-150 nm. The diffraction spots in the SAED pattern inset of the heterostructure also can be indexed to two sets of crystals for ZnO and ZnS, with a similar relationship as discussed above. Detailed microstructures of the ZnO/ZnS heterostructures were carried out with high-resolution TEM. Figure 4b shows the lattice-resolved HRTEM image taken from the ZnS nanowire framed in Figure 4a. The observed lattice fringe is separated by the 0.63 nm distance corresponding to the (0001) lattice plane spacing of wurtzite ZnS, indicating that the preferred growth direction of the ZnS nanowire is along the [0001] direction. Figure 4c shows the HRTEM image of the interface between ZnO and ZnS (framed in Figure 4a). The clearly resolved fringes are 0.31 nm for wurtzite ZnS and 0.26 nm for wurtzite ZnO, corresponding to the (0002) lattice plane spacing of wurtzite ZnS and ZnO, respectively. The interface between the ZnO and ZnS fragments is homogeneous and atomically sharp. The HRTEM image of the ZnO nanobelt is shown in Figure 4d. The resolved lattice fringe is 0.52 nm, corresponding to the (0001) spacing of wurtzite ZnO. Combined with the SAED pattern in Figure 4a, it can be seen that the ZnO nanobelt is a single crystal with the growth along the [011h0] direction. Saw-like heterostructures synthesized presently are formed via a two-step growth process: the first step is the fast growth

of ZnO nanobelts along the [011h0] direction at low temperature (550 °C) by thermal oxidation of zinc powder; the second step is the slow decomposition of SnS to Sn and S and then the reaction of Zn and S to subsequent growth of ZnS nanowires along the (0001) of ZnO, which can be expressed as the reactions SnS f Sn + S and Zn +S f ZnS. Our results have shown only saw-like heterosturctures with ZnS nanowires grown along one side of the ZnO nanobelts and they have a crystalline orientation relationship as [21h1h0]ZnO//21h1h0]ZnS, (0001)ZnO// (0001)ZnS, and (011h0)ZnO//(011h0)ZnS. We deduce the possible mechanism as follows. Structurally, wurtzite ZnO and ZnS crystals can be described schematically as a number of alternating planes composed of 4-fold tetrahedral-coordinated O2- (or S2-) and Zn2+ ions, stacked alternatively along the c axis39-42 (Figure 5a,b). The oppositely charged ions produce positively charged (0001)-Zn and negatively charged (0001h)-O [or (0001h)-S] polar surfaces (Figure 5a). Studies have found that the Znterminated (0001) surface is chemically active, while the O-terminated (or S-terminated) surface is chemically inactive, resulting in the growth of saw-like structures. It is thought that the formation of saw-like ZnO/ZnS heterostructures here is also induced by the polar surfaces. In the first step, ZnO nanobelts are formed. The positively charged (0001)-Zn surface along the ZnO nanobelts is chemically active and tiny Zn clusters may exist at the growth front. In the second step, these Zn clusters react with sulfur and initiate the growth of ZnS teeth, forming the saw-like ZnO nanobelt/ZnS nanowires heterostructures. The optical properties of the as-synthesized heterostructures were investigated with Raman and photoluminescence (PL) spectra. Figure 6 shows the Raman spectrum of a typical product. The strong peak at 520 cm-1 arises from the silicon substrate used for the sample deposition. Besides, four other

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Shen et al. nanowires, these two shoulder emissions should be correlated with the perfect epitaxial heterostructures in the ZnO/ZnS heterostructures, which are similar to a previous report on ZnO/ SnO2 heterostructures.5 A more detailed mechanism of the origination of the green emission in the as-synthesized ZnO/ ZnS heterostructures is required and is still under investigation. 4. Conclusion

Figure 7. Room temperature PL spectra of ZnO nanobelts, ZnO nanobelt/ZnS nanowire heterostructures (HS), and ZnS nanowires.

peaks located at 216, 348, 436, and 570 cm-1 were observed from the spectrum. These peaks can be associated to ZnO and ZnS, respectively. The peak at 216 cm-1 is due to a first-order LA model of ZnS according to the literature.43,44 The peak near 348 cm-1 is obviously due to the longitudinal optical (LO) phonon peak of ZnS.43,45 The other two peaks at 436 and 570 cm-1 can be assigned to the E2 and A1 (LO) modes, respectively, according to previous reports.46,47 In the Raman spectrum, the vibrational peaks of both ZnO and ZnS are observed, which also confirm the formation of ZnO/ZnS heterostructures. Figure 7 shows the room temperature PL spectra of the ZnO/ ZnS heterostructures as well as the ZnO nanobelts, and ZnS nanowires. ZnO nanobelts used here were synthesized by the thermal evaporation process as we described before and ZnS nanowires were synthesized by thermal evaporation of ZnS powders. As shown in Figure 6, pure ZnO nanowires show a UV emission at 380 nm (corresponding to the near band edge peak of ZnO) and a symmetric green emission (coming from the recombination of holes with electrons occupying the singly ionized oxygen vacancy within the ZnO nanobelts),48 while ZnS nanowires only show a broad emission at 560 nm (coming from some self-activated centers, vacancy states, element sulfur species on the surface, or interstitial states associated with the ZnS nanowires as in previous reports).49,50 For the PL spectrum of the saw-like ZnO/ZnS heterostructures, a strong, wide, asymmetric green emission centered at 495 nm as well as a weak ultraviolet (UV) emission at 380 nm are observed. It is quite different from the ZnO nanobelts or ZnS nanowires. Compared the PL spectrum of the synthesized ZnO/ZnS heterostructures with the spectra of ZnO nanobelts and ZnS nanowires, it can been deduced that the UV emission at 380 nm corresponds to the near band edge peak of the stem ZnO nanobelt that is responsible for the recombination of free excitons through an exciton-exciton collision process.48-50 As for the green emission centered at 495 nm, it obviously resulted from the superposition of the green emission band of both the ZnO nanobelt (stem) and the ZnS nanowires (branch) within the saw-like heterostructures. Thus, the asymmetric green emission is related to the intrinsic defect structures, such as the singly ionized charge state of specific defects, oxygen vacancies originating from the oxygen deficiency, element sulfur speices on the surfaces, and the special interface structures.49-53 Besides the contribution of individual ZnO and ZnS to the green emission, another two shoulder peaks at ∼600 and ∼650 nm were also observed for the synthesized heterostructures. As there are no similar emissions from pure ZnO nanobelts or pure ZnS

In this paper, by using a simple thermal evaporation method, saw-like ZnO nanobelt/ZnS nanowires heterostructures, consisting of ZnO nanobelts as the stem and teeth-like ZnS nanowires as the branches, have been successfully synthesized. The ZnO nanobelt and ZnS nanowires within a single saw-like heterostructure have a crystalline orientation relationship as [21h1h0]ZnO// 21h1h0]ZnS, (0001)ZnO//(0001)ZnS, and (011h0)ZnO//(011h0)ZnS. The saw-like heterostructures are formed by the initiation of a chemically active Zn-terminated ZnO (0001) polar surface based on similar crystal habits of wurtzite ZnO and ZnS structures. The study here may inspire interest in exploring other complicated nanostructures and their potential applications in laser, gas sensor, photocatalysis, and nano devices. References and Notes (1) Sze, S. M. Physics of Semiconductor DeVices; Wiley-Interscience: New York, 1981. (2) Lauhon, L. J.; Gudiksen, M. S.; Wang, D.; Lieber, C. M. Nature 2002, 420, 57. (3) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (4) Lauhon, L. J.; Gudiksen, M. S.; Lieber, C. M. Philos. Trans. R. Soc. London A 2004, 362, 1247. (5) Kuang, Q.; Jiang, Z. Y.; Xie, Z. X.; Lin, S. C.; Lin, Z. W.; Xie, S. Y.; Huang, R. B.; Zheng, L. S. J. Am. Chem. Soc. 2005, 127, 11777. (6) Cui, Y.; Lieber, C. M. Science 2001, 291, 851. (7) Wu, Y.; Fan, R.; Yang, P. D. Nano Lett. 2002, 2, 83. (8) Bjork, M. T.; Ohlsson, B. J.; Sass, T.; Persson, A. I.; Thelander, C.; Magnusson, K.; Deppert, M. H.; Wallenberg, L.; Samulson, L. Nano Lett. 2002, 2, 87. (9) Manna, L.; Scher, E. C.; Li, L. S.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 7136. (10) Wang, X. D.; Song, J. H.; Li, P.; Ryou, J. H.; Dupuis, R. D.; Summers, C. J.; Wang, Z. L. J. Am. Chem. Soc. 2005, 127, 7920. (11) Ye, C. H.; Zhang, L. D.; Fang, X. S.; Wang, Y.; Yan, P.; Zhao, J. AdV. Mater. 2004, 16, 1019. (12) Li, Q.; Wang, C. R. J. Am. Chem. Soc. 2003, 125, 9892. (13) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Nature 2004, 430, 61. (14) Li, Q.; Wang, C. R. Appl. Phys. Lett. 2003, 82, 1398. (15) Hsu, Y. J.; Lu, S. Y. Chem. Commun. 2004, 2102. (16) Bu, W. B.; Hua, Z.; Chen, H. R.; Shi, J. L. J. Phys. Chem. B 2005, 109, 14461. (17) Maynor, B. W.; Li, J. Y.; Lu, C. G.; Liu, J. J. Am. Chem. Soc. 2004, 126, 6409. (18) Hu, J. Q.; Bando, Y.; Liu, Z. W.; Sekiguchi, T.; Golberg, D.; Zhan, J. H. J. Am. Chem. Soc. 2003, 125, 11306. (19) Fu, L.; Liu, Z. M.; Liu, Y. Q.; Han, B. X.; Wang, J. Q.; Hu, P.; Cao, L. C.; Zhu, D. B. J. Phys. Chem. B 2004, 108, 13074. (20) Wang, C. R.; Wang, J.; Li, Q.; Yi, G. C. AdV. Funct. Mater. 2005, 15, 1471. (21) Jung, S. W.; Park, W. I.; Yi, G. C.; Kim, M. AdV. Mater. 2003, 15, 1358. (22) Park, W. I.; Yi, G. C.; Kim, M.; Pennycook, S. J. AdV. Mater. 2003, 15, 526. (23) Zhan, J. H.; Bando, Y.; Hu, J. Q.; Sekiguchi, T.; Golberg, D. AdV. Mater. 2005, 17, 225. (24) 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. (25) Lei, B.; Li, C.; Zhang, D. H.; Han, S.; Zhou, C. W. J. Phys. Chem. B 2005, 109, 18799. (26) Zhan, J. H.; Bando, Y.; Hu, J. Q.; Liu, Z. W.; Yin, L. W.; Golberg, D. Angew. Chem., Int. Ed. 2005, 44, 2140. (27) Li, Y. B.; Bando, Y.; Golberg, D. AdV. Mater. 2004, 16, 93. (28) Hu, J. Q.; Bando, Y.; Zhan, J. H.; Golberg, D. AdV. Mater. 2005, 17, 1964. (29) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287.

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