Temperature-Controlled Growth of Silicon-Based Nanostructures by Thermal Evaporation of SiO Powders Z. W. Pan, † Z. R. Dai, † L. Xu, ‡ S. T. Lee, ‡, * and Z. L. Wang* ,†,§ School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, Center of Super-Diamond and AdVanced Film, Department of Physics and Materials Science, City UniVersity of Hong Kong, Kowloon, Hong Kong ReceiVed: NoVember 20, 2000; In Final Form: January 17, 2001 Silicon-based nanostructures with different morphologies, sizes, compositions, and microstructures were grown on Si wafers by thermal evaporation of SiO powders at 1350 °C for 5 h under 300 Torr of a flowing gas mixture of 5% H 2- Ar at a flow rate of 50 standard cubic centimeters per minute (sccm). The SiO powders and Si wafers were placed inside an alumina tube, which was heated by a tube furnace. The local temperature inside the tube was carefully calibrated by a thermal couple. After evaporation, Si-containing products with different colors and appearances were formed on the surfaces of the Si wafers over a wide temperature range of 890-1320 °C and a long distance of ∼85 mm. Basing on the colors and appearances of the products, five distinct zones, which corresponding to different temperature ranges, were clearly identified from the highest temperature of 1320 °C to the lowest temperature of 890 °C. They are zone I (1250-1320 °C), zone II (1230-1250 °C), zone III (1180-1230 °C), zone IV (930-1180 °C), and zone V (890-930 °C). The deposited products were systematically studied by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results show that, besides Si nanowires, many other kinds of Si-based nanostructures such as octopuslike, pinlike, tadpolelike, and chainlike structures were also formed. The temperature distribution inside the alumina tube was found to play a dominant role on the formation of these structures. It is demonstrated that a control over the growth temperature can precisely control the morphologies and intrinsic structures of the silicon-based nanomaterials. This is an important step toward design and control of nanostructures. The growth mechanisms of these products were briefly discussed. 1. Introduction Silicon-based nanoscale materials have attracted much at- tention in recent years for their valuable semiconducting, mechanical, and optical properties, as well as their potential applications in mesoscopic research and nanodevices. They are, for example, considered as candidates for one-dimensional quantum transistors, composites, and light-emitting diodes. 1 Consequently, a great deal of effort has been made in fabricating Si-based nanostructures, especially Si nanowires. Several tech- niques have been developed to produce Si nanowires, including lithography and etching, 2-4 scanning tunneling microscopy, 5,6 vapor-liquid-solid (VLS) growth, 7-10 laser ablation of metal- containing Si target 11-14 or metal-free Si/SiO 2 target, 11,15 and thermal evaporation of Si-SiO 2 mixture 11,16,17 or SiO pow- ders. 11,18 Among these techniques, the thermal evaporation technique developed by Lee et al. 11 is of particular interest and has attracted much attention in recent years due to its low cost and ease of manufacture. By using SiO powders as the source material, this technique can be used to easily produce a large quantity of high-purity (no metal contamination), ultralong (in millimeters), and uniform-sized (a few nanometers to tens of nanometers in diameter) Si nanowires. The quality of the Si nanowires produced by this technique is comparable to those produced by traditional VLS 7-10 and laser ablation. 11-15 In thermal evaporation, oxides were found to play a dominant role in the nucleation and growth of Si nanowires. The growth mechanism of Si nanowires from thermal evaporation of SiO powders, however, is not fully understood. Thus, more detailed and systematic experimental investigations are required. In this paper, we systematically investigate nanomaterials produced from thermal evaporation of SiO powders as a function of the local temperature. Our results show that, besides Si nanowires, many other kinds of Si-based nanostructures such as octopuslike, pinlike, tadpolelike, and chainlike structures were also formed at different temperature zones. Our experiments show that control of the temperature can also control the morphology, size, crystallization, and composition of the Si- based nanostructures. 2. Experimental Method The apparatus used for thermal evaporation of SiO powders is schematically shown in Figure 1. An alumina tube was mounted inside a horizontal tube furnace. A 3-5 g sample of 99.9% pure SiO powder (from Aldrich) was placed in an alumina crucible and located at the center of the alumina tube. Several striplike Si wafers (60 mm in length and 10 mm in width) were placed one by one on a long alumina plate (15 cm in length and 20 mm in width) to act as the deposition substrates for the grown materials. SiO is dark brown and SiO 2 is transparent white. The position of the alumina plate inside the * Corresponding authors. E-mail: zhong.wang@mse.gatech.edu (ZLW), and APANNALE@cityu.edu.hk (STL). † Georgia Institute of Technology. ‡ City University of Hong Kong. § School of Materials Science and Engineering and School of Chemistry and Biochemistry, Georgia Institute of Technology. 2507 J. Phys. Chem. B 2001, 105, 2507-2514 10.1021/jp004253q CCC: $20.00 © 2001 American Chemical Society Published on Web 03/09/2001