Ultrafine and uniform silicon nanowires grown with zeolites

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Ultrafine and uniform silicon nanowires grown with zeolites C.P. Li a , X.H. Sun a,b , N.B. Wong a,b , C.S. Lee a , S.T. Lee a, * , Boon K. Teo c,1 a Department of Physics and Materials Science, Center of Super-Diamond and Advanced Films, The City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR, China b Department of Biology and Chemistry, The City University of Hong Kong, Hong Kong SAR, China c Department of Chemistry, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, IL 60607, USA Received 21 June 2002; in final form 21 August 2002 Abstract Ultrafine and uniform silicon nanowires (SiNWs), with a Si crystalline core of 1–5 nm (average 3 nm) in diameter and a SiO 2 outer layer of 10–20 nm thick, were synthesized by the oxide-assisted growth method via the dispropor- tionation of thermally evaporated SiO using zeolite as a template/precursor. From transmission and secondary electron microscopic characterizations, we deduced that the zeolite acted to limit the lateral growth of the Si crystalline core and supply the excess oxide to form the thick oxide outer layer. The ultrafine SiNWs exhibited strong photoluminescence that peaked at 720 nm. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction Since the discovery of Si whiskers [1], silicon nanowires (SiNWs) have attracted much attention in mesoscopic research and device applications, as well as in the fundamental research because of their highly interesting optical and electrical properties [2–14]. The metal-catalyst vapor–liq- uid–solid (VLS) reaction has been used to grow SiNWs of different diameters [2]. Other growth methods and/or strategies include the oxide-as- sisted growth [3–7]. It is obvious that the control of the diameter and uniformity of SiNWs is a crucial factor in the design and fabrication of nanoscale devices. In this Letter, we report a new method for the preparation of very fine (1–5 nm) and uniform SiNWs using zeolites as templates and/or precursors. SiNWs were prepared by thermal evaporation of pure SiO powder (Aldrich, 325 mesh, 99.9%) at 1250 °C in an evacuated alumina tube. The zeolite (Zeolite Y, a mixture of SiO 2 ,Al 2 O 3 , and Na 2 O) substrate was packed was held by quartz wool in an inner alumina tube, through which the carrier gas exited. The carrier gas consisted of 95% Ar and 5% H 2 with a flow rate of 50 SCCM (standard cubic cm per min) was forced to flow through the zeolites and the whole system was kept at a pres- sure of 400 mbar. The zeolite substrate changed from a white powder to small green pallets. The www.elsevier.com/locate/cplett Chemical Physics Letters 365 (2002) 22–26 * Corresponding author. Fax: +852-27844696. E-mail addresses: apannale@cityu.edu.hk (S.T. Lee), boonkteo@uic.edu (B.K. Teo). 1 Also corresponding author. 0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 3 7 5 - 1 products were first examined with a scanning electron microscope (SEM) (Philips XL 30 FEG), which was equipped with energy dispersive X-ray spectroscopy (EDS). The SiNWs samples from the surface of the beads were dispersed onto ÔholeyÕ carbon TEM grids. The nanostructure of the samples were then characterized by high-resolu- tion transmission electron microscope (HRTEM) (Philips CM200 at 200 kV). A micro-Raman spectrometer (Renishaw 2000 micro-Raman spec- trometer) was used to characterize the PL prop- erties of the sample at room temperature. The 514.5 nm emission from argon ion laser was used to excite the luminescence. Fig. 1. (a) A typical SEM image of the SiNWs. (b) A zoom-out image of (a). C.P. Li et al. / Chemical Physics Letters 365 (2002) 22–26 23 Fig. 1a is an overview of the SEM image of the SiNWs. A large quantity of SiNWs was found on the surface of the zeolite pallet. In the zoom-out image (Fig. 1b), we observed that the SiNWs were attached to the surface of the zeolite pallet. EDS results show that SiNWs are composed of mainly Si, O, and a small amount of Al. The small amount of Al came from zeolite and provides strong evi- dence for the proposed growth mechanism to be described later. Fig. 2 is the TEM image of a single SiNW. In the TEM results, we found each SiNW has a very fine crystalline silicon core and a thick amorphous silicon dioxide outer layer. The diam- eters of the Si cores range from 1 to 5 nm, with the dominant diameter of 3 nm. These Si cores are very fine and uniform in diameter throughout the entire length (1 micron or longer) of each wire. The diameter of the amorphous SiO 2 layer of the SiNWs ranges from 20 to 40 nm and is also quite uniform throughout the entire length of the wires. A central Si core of 1.3 nm in diameter and a relative thick SiO 2 outer layer of 20 nm in diameter were observed in our samples. Assuming a Si–Si bond length of 0.235 nm, this fine nanowire of 1.3 nm in diameter contains only six to seven silicon atoms across the short dimension. To the best of our knowledge, this is the finest SiNW synthesized to date. The amorphous oxide surface of this wire is quite rough. This phenomenon can be found in other SiNWs with the Si core less than 2 nm in diameter. Due to the very fine SiNWs, the selected area electron diffraction (SAED) revealed only the amorphous structure of the outside SiO 2 layer, as well as the carbon film in background. The inset of Fig. 2 shows a HRTEM image of the same SiNW. It confirms that the core is crystalline silicon with 3.1  AA d-spacing. The SiO 2 layer has a thickness of 12 nm on both sides of the center SiNW. The overall diameter of the nanowire is 30 nm. Fig. 3 shows a proposed mechanism for the formation of these very fine and uniform SiNWs. The growth of the SiNWs is similar to the previ- ously described oxide-assisted growth mechanism [15]. The SiO powders were firstly sublimated at 1250 °C and formed nanoclusters in the vapor phase. In the present experiment, the zeolites were positioned downstream from the SiO starting material where the temperature was about 930 °C. The SiO nanoclusters in the vapor subsequently deposited on the surface of zeolites and some dif- fused into the channels of the zeolites as shown schematically in Fig. 3a. At that temperature re- gime, SiO nanoclusters disproportionated to form Si and SiO 2 and resulted in the precipitation of silicon nanoparticles (the nuclei of Si nanowires) surrounded by shells of silicon oxide. In the channels of zeolites, the nucleation process was limited by the openings of the channels and a large quantity of SiO 2 in zeolites retarded the dispro- portionation of SiO. Therefore, the core of SiNWs Fig. 2. The TEM image of a typical single SiNW with a Si core diameter of 3 nm covered with a SiO 2 layer of 28 nm. The inset is the HRTEM image of the same SiNW. Fig. 3. Proposed growth mechanism for the very fine and uniform SiNWs. 24 C.P. Li et al. / Chemical Physics Letters 365 (2002) 22–26 was limited to 1–3 nm in diameter at the nucle- ation stage, while the zeolite supplied additional silicon oxide to form the shell of the SiNW, re- sulting in an oxide layer much thicker than that in normal SiNWs, as depicted in Fig. 3b. The finding of Al in SiNWs provides strong evidence that the oxide layer of SiNWs comes partly from the zeo- lite. Because the silicon oxide outer layer plays a key role in the growth process of SiNWs, the thicker oxide layer limited the lateral growth of the Si nucleation core. At this point, the Ôoxide-as- sistedÕ growth process became operative, with the ÔoxideÕ being primarily supplied Ôin situÕ by the zeolites. The increased SiO 2 local concentration (from the zeolite) at the Si–SiO 2 interface again limits the growth of the wires to larger diameter. The net result is a very fine (1–5 nm in diameter) and uniform SiNW sheathed by a thick and uni- form oxide layer (20–40 nm in diameter) and each SiNW has a ÔrootÕ in the zeolite, as shown in Fig. 3c. This growth process is consistent with the SEM results (see Fig. 1b) showing that the SiNWs were attached to the surface of zeolites. The photoluminescence (PL) of these very fine SiNWs was measured at room temperature. Very weak PL intensity was obtained from the normal SiNW sample of 20–50 nm in diameter (curve a in Fig. 4). The PL peak centers at around 600 nm. However, the very fine SiNW samples reported in this Letter exhibited very strong (at least one order of magnitude higher) photoluminescence in the PL measurement (curve b Fig. 4). The PL peak centers around 720 nm. The strong PL intensity probably arises from the quantum size effect of ultrafine Si core (<5 nm in diameter) in association with the interface between the ultrafine silicon core and the sheathing silicon oxide layer [16]. In summary, we have demonstrated that zeolite can be used as a template/precursor to grow very fine and uniform SiNWs via the disproportiona- tion reaction of SiO by thermal evaporation. The diameter of the Si core ranges from 1 to 5 nm with an average of 3 nm sheathed by a thick and uni- form oxide layer of 20–40 nm in diameter. The SiNWs show unusually strong photoluminescence. Acknowledgements The authors would like to dedicate this Letter to Mrs. Anna Lee in her memory. This work was supported in part by the Research Grants Council of Hong Kong (CityU 1063/01P) and the Strategic Research Grants of the City University of Hong Kong (No. 7001175) as well as by a grant from the National Science Foundation, USA (to B.K. Teo). B.K. 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[16] In a separate experiment, we measured the photolumines- cence (PL) of Al doped (5–10%) as-prepared SiNWs of 20 nm in diameter and observed no significant enhancement in the PL over undoped SiNWs, suggesting that the-order-of magnitude enhancement in the PL of our sample doesnÕt arise primarily from the Al in the silicon oxide layer of SiNWs prepared from zeolites. 26 C.P. Li et al. / Chemical Physics Letters 365 (2002) 22–26 . Ultrafine and uniform silicon nanowires grown with zeolites C.P. Li a , X.H. Sun a,b , N.B. Wong a,b , C.S August 2002 Abstract Ultrafine and uniform silicon nanowires (SiNWs), with a Si crystalline core of 1–5 nm (average 3 nm) in diameter and a SiO 2 outer layer