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Growth Modes of ZnS Nanostructures on the Different Substrates Haiping Tang,†,‡ Bong Jun Kwon,† Jinwoong Kim,† and Ji-Yong Park*,† Department of Physics and DiVision of Energy Systems Research, Ajou UniVersity, Suwon, 443-749, Korea, and Department of Mechanical and Electrical Engineering, Baoji UniVersity of Arts and Sciences, Baoji, Shannxi, 721007, People’s Republic of China ReceiVed: September 30, 2010; ReVised Manuscript ReceiVed: October 25, 2010
ZnS nanostructures including nanorods are synthesized on Si substrates with and without a SiO2 overlayer using a vapor transport method with a Au layer as a catalyst. Growth modes of ZnS nanostructures grown on both substrates are characterized with scanning electron microscopy (with elemental analysis), X-ray diffraction, and luminescence measurements. These investigations suggest that different growth mechanisms work for ZnS nanostructures depending on the substrates. A vapor-liquid-solid (VLS) mechanism is favored for ZnS nanostructures on a Si substrate, while a vapor-solid (VS) mechanism is favored for a Si/SiO2 substrate. The origin of the differences is discussed in terms of differences in the interaction of Au with substrates and different catalytic activities as a result. 1. Introduction There is mounting interest in wide-band-gap semiconductors with possible applications in optoelectronic devices, energy conversion devices, and transparent conducting electrodes. While wide-band-gap semiconductors based on III-V compounds have already found many applications in optoelectronics devices such as light-emitting diodes, ones based on II-VI are mostly still under development and have been actively researched in recent years.1,2 Among many II-VI semiconducting materials, zincbased materials such as ZnO have recently garnered much interest along with their nanostructures.3,4 As a family of zincbased semiconducting materials, ZnS is a direct wide-band-gap II-VI semiconductor with a high refractive index and significant transmittance in the visible range of the electromagnetic spectrum with a band gap in the range of 3.5-3.9 eV at 300 K, which is larger than that of ZnO. ZnS has been also investigated for a number of applications such as flat panel displays, fieldemission devices, light-emitting diodes, and phosphors.5-7 In recent years, there have been also efforts to synthesize ZnS nanostructures, which may extend their possible applications for photodetectors, sensors, and more with large surface-volume ratio and better crystal quality as in the case of ZnO. Various methods have been employed to synthesize ZnS nanostructures, especially quasi-one-dimensional (1D) structures such as nanowires and nanorods. Growth methods such as laser ablation,8 vapor transport with9-13 or without14,15 catalyst, carbothermal chemical vapor deposition,16 electrochemical deposition,17-19 and solvothermal method have been used.20-22 In many cases, the synthesized ZnS nanowires are found to have wurtzite structures10-12,14-21 although reports of cubic phase9,13 or mixtures8 can also be found. A vapor-liquid-solid (VLS) mechanism seems to be favored for high temperature growth in most cases.8-12,16 Reported luminescence spectra typically consist of weak or no band gap emission with stronger defect emissions. However, reports of successful synthesis of onedimensional nanostructures of ZnS are still limited and growth * To whom correspondence should be addressed,
[email protected]. † Ajou University. ‡ Baoji University of Arts and Sciences.
mechanisms are not well understood, especially with different growth conditions. In this work, ZnS nanowires are successfully synthesized on Si substrates both with and without a SiO2 overlayer using ZnS powder as the source and a gold layer as a catalyst. Detailed analyses are performed on individual ZnS nanowires as well as the bulk samples. Different growth modes depending on the substrates are identified, and the possible origin of the differences is discussed. 2. Experimental Methods The synthesis of ZnS nanowires is carried out in a horizontal quartz tube (75 mm in diameter) furnace using the vapor phase transport method23 with an inner quartz tube which contains the source and the substrates. A small quartz tube (800 mm in length and 20 mm in diameter) with both ends open is loaded with ZnS powder (Aldrich, 99.99%) at the upstream and substrates such as Si(100) or Si(100)/SiO2 (200 nm) at the downstream. With a dual tube setup, high concentrations of Zn and S vapors can be maintained only around the substrates without contaminating the rest of the outer tube. Both substrates are cleaned using a standard wafer cleaning procedure and then coated with a thin gold layer (1-5 nm) as the catalyst by e-beam evaporation. The Au coated substrates are located downstream of the small tube, 40 mm away from the source. Then the small quartz tube is inserted into the furnace. Before heating, the furnace is purged with high-purity nitrogen gas in order to remove residual oxygen in the furnace. The source is heated to approximately 1100 °C with a heating rate of 50 °C min-1 under a flow of high-purity nitrogen as carrier gas at the rate of 130-140 sccm (standard cubic centimeters per minute). The system is typically maintained for 2 h for the growth. The temperature of the substrate reaches 900-950 °C during the growth. After evaporation and deposition, the quartz tube is drawn out of the furnace when it has cooled down to about 60 °C. This procedure produces a light yellow layer on the two kinds of substrates. The morphologies of the samples are examined using a field emission scanning electron microscope (HITACHI, S4800 FESEM) equipped with a energy dispersive X-ray spectrometer
10.1021/jp109394c 2010 American Chemical Society Published on Web 11/11/2010
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Figure 1. (a) Low and (b) high magnifications of top view SEM images of the ZnS nanowires on Si/SiO2 substrates. (c) Low and (d) high magnifications of cross-sectional SEM images of the ZnS nanowires on Si/SiO2 substrates. Insets: (c) a high magnification image of individual nanowires; (d) an EDS spectrum of underlying layer.
(EDS). The X-ray diffraction (XRD) patterns are acquired by a Rigaku system using graphite monochromated Cu KR (λ ) 0.1541 nm) radiation. Cathodoluminescence (CL) spectra are obtained at room temperature with a Gatan MonoCL3+ system equipped with a high-sensitivity photomultiplier tube (PMT) attached to an SEM (HITACHI, S-4300SE). Acceleration voltage of 10 kV is used for all the CL measurements presented in this paper. 3. Results and Discussion The general morphology of the ZnS nanowires on the Si(100)/ SiO2 substrate is shown in Figure 1a, which shows that the ZnS nanostructures mostly consist of nanowires and do not have a vertical alignment but take random growth directions on the substrate. Inspections of the magnified images such as in panels b-d of Figure 1 show that the diameters of individual ZnS nanowires are in the range of 100-300 nm, while lengths can be up to several tens of micrometers. The individual nanowires are rather straight and do not contain nanoparticles at the ends as can be seen in the inset of Figure 1c. Cross-sectional images of the sample as shown in panels c and d of Figure 1 indicate the existence of a few-micrometer-thick and continuous layer between nanowires and the substrate. The elemental analysis of the layer underneath nanowires using EDS as shown in the inset of Figure 1d indicates the underlying layer is ZnS with weight ratio of almost 1:1. The ZnS nanostructures synthesized with the same growth condition but on Si(100) substrate without SiO2 overlayer show markedly different growth morphologies as shown in Figure 2. Figure 2a shows that the ZnS nanostructures in this case consist of wormlike or zigzag nanorods and small number of nanowires. The magnified image in Figure 2b shows that the wormlike nanorods are short and irregular, and the diameter of the nanowires is about 100 nm. The irregular-shaped nanorods are similar to ones reported before.13 Figure 2c shows the crosssectional image of the sample. Although the nanowires were sparse, they have a vertical alignment on the substrate without
the continuous ZnS layer on the substrate unlike Figure 1. The length of the wires is mostly several micrometers. The magnified cross-sectional image as shown in Figure 2d show Au particles at the ends of these nanostructures, different from the case of ZnS nanowires grown on Si/SiO2 substrates as in Figure 1. Figure 3 shows the XRD patterns of the ZnS nanostructures on the two kinds of substrates. All the diffraction peaks can be well indexed to the hexagonal phase of ZnS reported in the JCPDS card (No. 36-1451). The results indicate that the ZnS samples mostly consist of wurtzite structures and no characteristic peaks are observed for other impurities, such as Zn and S. ZnS is known to take two crystal structures in bulk. Zinc blende structure is known to be a stable phase at low temperature while wurtzite structure is stable at higher temperature in bulk. Wurtzite phase has larger band gap energy then zinc blende. However, most ZnS nanostructures from literature seem to prefer wurtzite structures even for the low temperature growth,10-12,14-21 and it is attributed to the favorable energetics involving facets on the surfaces of nanowires.24,25 In Figure 3, the relative intensities of the peaks for the nanowires on Si/SiO2 substrates are higher than those on Si and the fwhm (full width at halfmaximum) on Si/SiO2 substrates is narrower than that on Si, indicating that the crystallinity of the ZnS nanowires on Si/ SiO2 substrates is better than that on Si. By comparing growth morphologies in Figure 1 and Figure 2, the ZnS nanostructures are found to follow different growth modes depending on the substrates. The thick ZnS underlayer and the lack of nanoparticles at the ends of nanowires in the case of Si/SiO2 substrate indicate a vapor-solid (VS) growth mechanism.26 On the other hand, the nanoparticles at the ends of nanostructures as seen in Figure 2 suggest VLS growth mechanism27 for Si substrate. In the VS mechanism, a ZnS layer is first formed on the substrate. Once a condition favorable for anisotropic growth is met due to competition between interactions among species and with the substrate, one-dimensional growth starts. Then, subsequent adsorption of the species occurs on the crystallographic planes such that anisotropic growth
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Figure 2. (a) Low and (b) high magnifications of top view SEM images of the ZnS nanostructures on Si substrates. (c) Low and (d) high magnifications of cross-sectional SEM images of the ZnS nanostructures on Si substrates.
Figure 3. XRD spectra of the ZnS nanostructures on Si/SiO2 substrates (upper curve) and Si substrates (lower curve).
proceeds exposing a plane with the lowest free energy.25 In VLS growth, catalytic particles are first saturated with source materials (in this case, Zn) and nanostructures start to grow from the particles as source materials start to nucleate. The same growth conditions with the same Au catalyst layer (evaporated at the same time) for the growth are employed on two substrates for this experiment. Therefore, the substrate is most likely responsible for the different growth modes observed here. In order to elucidate the role of substrates for the growth of ZnS nanostructures, the evolutions of Au catalyst layers on both substrates with temperatures are compared. First, the same amount of Au is deposited on both Si and Si/SiO2 substrates with e-beam evaporation at the same time. Both substrates are subsequently annealed in the furnace under the same gas flow as for the growth but without a ZnS source at the same time. The changes in morphologies of the Au layer after annealing at several different temperatures are checked and compared with SEM. One example for the case of a 5 nm thick Au layer (typically used for ZnS growth) after 500 °C annealing is shown in Figure 4. Similar morphologies are found for a higher annealing temperature of 700 °C at which ZnS nanowires already start to form. Continuous layers of Au break down and
clusters form on both substrates as diffusion of Au atoms is facilitated at high temperature. However, the morphologies of clusters are quite different depending on the substrates. Rather homogeneous Au clusters with diameter 20-40 nm are found on the Si/SiO2 substrate while clusters formed on the Si substrate are irregular in shape and size as can be seen in Figure 4. Small clusters and bigger structures connected by a few small clusters seem to coexist on the Si substrate. Observed difference can be attributed to the different interaction of Au with the substrates. On Si/SiO2 substrate, the interaction between Au and the substrate is weak such that clustering among Au atoms is favored. On the Si substrate, however, Au can form a low temperature eutectic alloy with Si at ∼360 °C as temperature goes up.28 Therefore, the nanoclusters found in the Si substrate are thought to be Au/Si alloy while pure Au clusters are formed on the Si/SiO2 substrate. Then the observed difference in the growth modes can be attributed to the difference in the solubility of Zn vapor in nanoclusters of pure Au and Au/Si alloys. While Zn forms a eutectic alloy with Au at ∼950 K and has quite large solubility in Au even at room temperature,29,30 there is negligible solubility of Zn in Si at the reaction temperature of the current experiment.31,32 Therefore, the solubility of Zn on Au/Si alloy nanoparticles is expected to be much reduced compared to the case of Zn on pure Au nanoparticles. When the solubility of Zn is high on Au nanoparticles, a large amount of Zn can be incorporated into Au. Then the excess Zn can fast react with S vapor in the furnace, forming a continuous ZnS layer which acts as a seeding layer for the subsequent growth of nanowires following VS growth as can be seen in Figure 1 for the case of Si/SiO2 substrate. In this case, Au nanoparticles just promote the adsorption of ZnS layer without directly catalyzing the growth of ZnS nanowires. On the other hand, limited solubility of Zn in Au/Si alloy particles can be a preferable condition for the VLS mechanism33 without forming a continuous ZnS layer. With limited solubility, nanoparticles are quickly supersaturated with only a small amount of Zn. Then Zn starts to nucleate and reacts with S to form one-dimensional structures. In this case, Au/Si alloy particles directly catalyze
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Figure 4. SEM images of Au nanoclusters on (a) Si and (b) Si/SiO2 substrates after undergoing annealing up to 500 °C.
Figure 5. An SEM image and corresponding EDS spectrum of ZnS nanowires transferred to a Si substrate.
the growth of ZnS nanostructures. Two kinds of nanostructures as seen in the case of Si substrates in Figure 2 can be also explained by alloying of Si into Au clusters. Figure 4a shows that the nanoclusters on Si substrates are not as uniform in shape and size as on Si/SiO2 substrate. This can be due to the different amount of Au/Si alloying as cluster size varies or fluctuates during annealing. The solubility of Zn may be affected by the composition ratio between Si and Au in the clusters, which offers different growth conditions such as the temperature at which supersaturation of Zn happens for VLS growth. Different kinds of nanostructures, therefore, can be formed as a result of different microscopic growth conditions. The characteristics of individual ZnS nanowires grown on Si/SiO2 are further investigated with EDS and CL. Unlike the secondary electron emission as used for the imaging, the excitation volumes for EDS and CL are quite big so that it is difficult to isolate signals from individual nanostructures. Instead, we transferred as-grown ZnS nanowires onto a Si substrate so that individual nanowires are lying on the substrate. Then individual nanowires are located with SEM, and EDS and CL spectra are obtained in these individual, isolated nanowires to eliminate background signals originating from the underlying layer and/or other types of nanostructures. Figure 5 shows an example of EDS characterizations of ZnS nanowires transferred to a Si substrate after they were synthesized on a Si/SiO2 substrate. No peaks other than Zn, S, and Si (from substrate) are visible in the spectrum, confirming that this nanowire is ZnS with weight ratio around 1:1. CL is also carried out to study the luminescent characteristics in the nanostructures as presented in Figure 6. The CL spectrum shows a weak band gap emission and a strong green emission centered at about 343 and 453 nm, respectively. The emission at 343 nm corresponds to a band gap energy of 3.6 eV, which is similar to the bulk case. More strong emission at 453 nm is typically observed in nanostructures, which can be attributed to vacancies, interstitial states, or surface recombinations as in previous reports.8,11,14,20
Figure 6. CL spectrum of the ZnS nanowires measured at room temperature.
4. Conclusions We have synthesized ZnS nanostructures using a vapor transport method with Au as catalyst. The growth morphologies of ZnS nanostructures on Si and Si/SiO2 are found to be significantly different. Different kinds of catalytic nanoclusters formed on the substrates are attributed to the differences in the growth of ZnS nanostructures. This work demonstrates that the interaction between the catalyst and substrate can play an important role in the growth of nanostructures. Acknowledgment. This work was supported by the National Research Foundation of Korea (NRF), by the ministry of Education, Science and Technology (2009-0094049, 313-20072-C00266, Nano R&D program), and by the grant of ‘The Ajou University Excellence Research Program’ in 2010. References and Notes (1) Springer Handbook of Electronic and Photonic Materials; Kasap, S., Capper, P., Eds.; Springer: New York, 2006. (2) Wide Bandgap Semiconductors: Fundamental Properties and Modern Photonic and Electronic DeVices; Takahashi, K., Yoshikawa, A., Sandhu, A., Eds.; Springer: Berlin, 2007. (3) Ozgur, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc, H. J. Appl. Phys. 2005, 98, 041301. (4) Klingshirn, C. ChemPhysChem 2007, 8, 782. (5) Falcony, C.; Garcia, M.; Ortiz, A.; Alonso, J. C. J. Appl. Phys. 1992, 72, 1525. (6) Aslam, F.; Binks, D. J.; Rahn, M. D.; West, D. P.; O’Brien, P.; Pickett, N.; Daniels, S. J. Chem. Phys. 2005, 122, 184713. (7) Fang, X.; Bando, Y.; Shen, G.; Ye, C.; Gautam, U.; Costa, P.; Zhi, C.; Tang, C.; Golberg, D. AdV. Mater. 2007, 19, 2593. (8) Jiang, Y.; Meng, X.-M.; Liu, J.; Hong, Z.-R.; Lee, C.-S.; Lee, S.T. AdV. Mater. 2003, 15, 1195. (9) Meng, X. M.; Liu, J.; Jiang, Y.; Chen, W. W.; Lee, C. S.; Bello, I.; Lee, S. T. Chem. Phys. Lett. 2003, 382, 434.
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