Ga-Catalyzed Growth and Optical Properties of Ternary Si-ZnS

Dec 3, 2008 - Synopsis. Bulk-quantity polycrystalline ternary alloyed Si-ZnS nanowires have been successfully synthesized by one-step thermal evaporat...
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Ga-Catalyzed Growth and Optical Properties of Ternary Si-ZnS Nanowires Yiqing Chen,* Qingtao Zhou, Xinhua Zhang, Yong Su, Chong Jia, Qiang Li, and Weihai Kong

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 728–731

School of Materials Science and Engineering, Hefei UniVersity of Technology, Hefei, Anhui 230009, People’s Republic of China ReceiVed January 30, 2008; ReVised Manuscript ReceiVed September 20, 2008

ABSTRACT: Bulk-quantity polycrystalline ternary alloyed Si-ZnS nanowires have been successfully synthesized by one-step thermal evaporation of a mixed powder of ZnS and SiO, using metallic gallium as a catalyst. The morphology and structure of the assynthesized ternary alloyed Si-ZnS nanowires are characterized by using X-ray diffraction, scanning electron microscopy, and highresolution transmission electron microscopy. The observations reveal that the ternary alloyed Si-ZnS nanowires have the same structure with cubic ZnS or Si. X-ray energy dispersive spectrometer analysis indicates that there is remarkable Si/Zn/S variation along the Si-ZnS nanowire. The room-temperature photoluminescence spectrum shows that the as-synthesized Si-ZnS nanowires have two emission peaks at 355 and 685 nm, and feature the superimposed optical properties of ZnS and Si.

1. Introduction One-dimensional (1D) semiconductor nanowires have attracted much attention because of their novel properties that make them potentially ideal functional components for nanometerscale electronics and optoelectronics.1-3 Over the past decade, a variety of synthetic methods have been developed to produce a number of semiconductor nanowires such as Si,4 ZnO,5 SnO2,6 and GaN.7 However, the research of 1D semiconductor nanostructures is mainly focused on elemental and binary semiconductors. It has been shown that materials with ternary alloyed nanostructures may offer more unique properties than the corresponding plain and binary compounds.8-14 Zinc sulfide, as an important II-VI group semiconductor compound, shows a wide band gap of 3.5-3.7 and 3.7-3.8 eV for sphalerite and wurtzite ZnS, and plays an important role in optoelectronic applications.15-18 1D ZnS nanostructures have been widely investigated for their potential optoelectronic applications.19-22 On the other hand, silicon nanowires have also attracted considerable attention because of their peculiar electronic and optical properties and the central part that silicon plays in the semiconductor industry.23,24 Recently, Bando’s group has synthesized side-by-side ZnS-Si nanowires by two different routes:25,26 (1) The two-stage thermal evaporation of the mixed SiO and ZnS powders under a precise temperature control, in which Si nanowires were first formed from the disproportionation of SiO powder, and then ZnS nanowires were grown on the Si nanowire substrates via a thermal evaporation of ZnS powder. (2) The one-stage thermal evaporation of the mixed Si and ZnS powders with a small amount of tin sulfide as an additive. During this process, metallic tin particles were used to catalyze the growth of ZnS/Si side-by-side 1D heterostructures (composite nanowires) via the well-known vaporliquid-solid (VLS) mechanism. The interface between the ZnS side fragment and Si side fragment is homogeneous and atomically sharp, due to the similar crystal structures and very close lattice constants of cubic ZnS and Si (ZnS: a ) 0.5406 nm, JCPDS Card No. 05-0566; Si: a ) 0.5430 nm, JCPDS Card No. 27-1402). * Corresponding author. Phone: +86-551-290-1365. Fax: +86-551-290-1362. E-mail: [email protected].

Herein, different from the reported ZnS-Si ZnS/Si side-byside 1D heterostructures (composite nanostructures), we demonstrate polycrystalline ternary alloyed Si-ZnS nanowires with a cubic structure which have been successfully synthesized by one-step thermal evaporation of a mixed powder of ZnS, SiO, and Ga. In the synthesis process, Ga particles served as a catalyst for the VLS growth of the ternary alloyed Si-ZnS nanowires. Moreover, the optical properties of the ternary alloyed Si-ZnS nanowires have also been investigated by photoluminescence (PL).

2. Experimental Section The ternary alloyed Si-ZnS nanowires were synthesized in a horizontal tube furnace. A ceramic boat containing a mixed powder of ZnS (1 g, 99.99%), SiO (1 g, 99.99%), and Ga (0.1 g, 99.99%) was located in the central heating zone of the alumina tube in the furnace, and a silicon wafer was placed downstream to collect the products. The distance between the source materials and the Si wafer was about 15 cm. After evacuation of the alumina tube to 0.1 Torr, a carrier gas of high-pure Ar was introduced into the tube at a constant rate of 60 sccm, and then the furnace was rapidly heated to 1150 °C, and kept at this temperature for 1 h. After the reaction was terminated and the furnace was cooled to room temperature, the silicon wafer was found to have been deposited with brown products. The collected products were characterized by using X-ray diffraction (XRD, D/MAX-RB), field-emission scanning electron microscopy (FESEM, JEOL-JSM-6700F), and a transmission electron microscope (TEM, JEOL-2010) equipped with an X-ray energy dispersive spectrometer (EDS). The photoluminescence (PL) spectrum was obtained by a steady-state spectrofluorometer (FLUOROLOG-3-TAU) with a Xe lamp as the excitation light source at room temperature.

3. Results and Discussion Figure 1a shows a representative FESEM image of the assynthesized products. It can be observed that the sample is composed of large quantities of long wire-like nanostructures with a typical length up to several tens of micrometers. A typical high-magnification FESEM image of the nanowires is shown in Figure 1b, which clearly reveals that each nanowire has a uniform diameter ranging from 200 to 400 nm and a rough surface. XRD was used to obtain the structure of the products, and the pattern is shown in Figure 2a. All of the peaks in the pattern

10.1021/cg800112d CCC: $40.75  2009 American Chemical Society Published on Web 12/03/2008

Properties of Ternary Si-ZnS Nanowires

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Figure 2. (a) XRD pattern taken from the synthesized products. (b) EDS spectrum of the ternary alloyed Si-ZnS nanowires.

Figure 1. FESEM images of the as-synthesized ternary alloyed SiZnS nanowires: (a) low and (b) high magnification.

can be indexed as the diamond-like (cubic) Si phase (JCPDS Card No. 27-1402) or the zinc blende (cubic) ZnS phase (JCPDS Card No. 05-0566). The lattice constant obtained by refinement of the XRD data for the products is a ) 0.5447 nm. No peaks from SiS2 or other impurities were detected. EDS attached in the FESEM system was used to obtain the compositions of the products (Figure 2b). It reveals that the products are composed of the elements Si, Zn, and S, and shows that the general atomic ratio of Si/Zn/S is about 1:1:1. Detailed microstructure and composition information of the as-synthesized nanowires was further characterized by TEM. Figure 3a shows a TEM image of a typical nanowire with a diameter of about 200 nm. A catalyst particle can be obviously observed at the tip of the nanowire, which has been confirmed by EDS (Figure 3c) as pure Ga. The corresponding selected area electron diffraction (SAED) pattern (Figure 3b) indicates that the nanowire appeared to be polycrystalline, and the diffraction rings in the SAED pattern corresponding to (111), (220), and (311) crystal plane of the cubic structural ZnS or Si as seen. Further HRTEM studies provide detailed structural information of the nanowire. Figure 4a-c displays the HRTEM images taken from different areas along the nanowire shown in Figure 3a. The interplanar spacing is about 0.31 nm, which corresponds to the (111) plane of the cubic structure of ZnS or Si. The insets show the corresponding SAED patterns, which confirm that the nanowire has the same structure with cubic ZnS or Si. Some twin planes exist across the nanowire (Figure 4c), and this structure is obviously reflected in the SAED pattern, as shown in the inset of Figure 4c. In fact, twin planes are often

Figure 3. (a) Typical TEM image of one ternary alloyed Si-ZnS nanowire. (b) SAED pattern taken from the nanowire. (c) EDS spectrum of the catalyst particle.

found in the nanowires with cubic structure.27-29 As observed in the EDS spectra of Figure 4d-f, there is remarkable Si/Zn/S variation along the nanowire. The peaks of Cu and C are generated from the copper grid, and the trace amount of oxygen originates from unavoidable oxygen adsorption on the surface due to exposure to air during sample processing.25,30 On the basis of the results of the characterization, especially since the nanowires have inhomogeneous distribution in the composition, it is reasonable that the products are ternary alloyed Si-ZnS nanowires, which have the same crystal structure with Si or cubic ZnS. Vapor-solid (VS) and VLS mechanisms have been widely used to explain the formation of 1D structures.31,32 In our case, the as-grown alloyed Si-ZnS nanowires are capped with Ga droplets, which is the key feature of the VLS growth mechanism. At the given high temperature, the liquid Ga in the ceramic

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Figure 4. (a-c) and (d-f) HRTEM images and EDS spectra (Cu and C peaks are caused by the surface of the copper grids in the TEM measurement) from various detection position of the ternary alloyed Si-ZnS nanowire shown in Figure 3a.

boat is evaporated and transferred to the low temperature zone, where a large quantity of liquid Ga particles is formed due to the surface tension. At the same time, the starting materials may undergo the following main reactions:26,33

ZnS(s) f Zn(v) + 1/2S2

(1)

2SiO(s) f Si(v) + SiO2

(2)

The vapor involved Zn, S2, and Si is also transferred to the low temperature zone by the Ar carrier gas, and continuously dissolves into liquid Ga droplets, which catalyze the VLS gowth of ternary alloyed Si-ZnS nanowires. Because the cubic ZnS and Si have the similar crystal structures and very close lattice constants, Bando’s group has obtained the side-by-side ZnS/Si nanowires through thermal coevaporation of ZnS and Si powders using metallic tin as the catalyst by VLS,26 or through two-stage thermal evaporation of the mixed SiO and ZnS powders under a precise temperature control,25 while we have synthesized the ternary alloyed SiZnS nanowires by thermal evaporation of a mixed powder of ZnS and SiO using Ga as catalyst. Why were different products obtained by similar thermal evaporation processes? The exact reason is uncertain now. Because there is remarkable Si/Zn/S variation along the as-synthesized ternary alloyed Si-ZnS nanowires (Figure 4d-f), we believe that the asymmetrical vapor supply involved Zn, S2, and Si may play a key role in the formation of the ternary alloyed Si-ZnS nanowires. The PL is used to further investigate the optical properties of the as-synthesized ternary alloyed Si-ZnS nanowires. The room-temperature PL spectrum is shown in Figure 5, using a Xe lamp at 280 nm as the excitation source. Two emission bands around 355 nm (3.49 eV) and 685 nm (1.81 eV) can been found

Figure 5. Room-temperature PL spectrum of the ternary alloyed SiZnS nanowire using a Xe lamp with an excitation wavelength of 280 nm.

in the PL spectrum. It is interesting that the ternary alloyed SiZnS nanowires feature the superimposed optical properties of ZnS and Si. The strong UV emission peak at 355 nm may be attributed to the band edge luminescence of cubic ZnS, and exhibits a red shift. It has been documented that the emission peak at 335 nm is attributed to the band edge luminescence of ZnS.34 In fact, there are many ZnS(Si) alloyed domains and Si(ZnS) domains in the polycrystalline Si-ZnS nanowires. Therefore, much interface exists between the ZnS(Si) alloyed domain and Si(ZnS) domain, resulting in much internal stress.

Properties of Ternary Si-ZnS Nanowires

The internal stress can modify the electrical and optical properties, and cause the red shift.35 The red shift of emission peak of the band edge luminescence of cubic ZnS may be attributed to the compressive stress in the ternary alloyed SiZnS nanowires. For the weak emission peak at 685 nm, it have been observed in the porous silicon,36,37 silicon nanowires,38 the ZnS-Si composite nanowires,25,26 and ZnSe/Si bicoaxial nanowire heterostructures.30 They attributed the emissions at 685 nm to the quantum effect of nanoscale Si. In our case, the inhomogeneous distribution in the composition of the ternary alloyed Si-ZnS nanowires may lead to the decrease of Si domains, which results in a relatively weaker emission peak at 685 nm.

4. Conclusions In conclusion, via the VLS growth mechanism, polycrystalline ternary alloyed Si-ZnS nanowires have been successfully synthesized by one-step thermal evaporation of a mixed powder of ZnS and SiO, using metallic gallium as a catalyst. XRD, HRTEM, and EDS studies indicate that the ternary alloyed SiZnS nanowires have the same structure with cubic ZnS or Si, and inhomogeneous distribution in the composition along the nanowires. The room-temperature PL spectrum shows that the as-synthesized ternary alloyed Si-ZnS nanowires have two emission peaks at 355 and 685 nm, and feature the superimposed optical properties of ZnS and Si. Acknowledgment. This work was financially supported by the Natural Science Foundation of China (NSFC, No.20671027) and by the Innovation Foundation of Science and Technology for Postgraduates of Hefei University of Technology.

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