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Đây là một bài báo khoa học về dây nano silic trong lĩnh vực nghiên cứu công nghệ nano dành cho những người nghiên cứu sâu về vật lý và khoa học vật liệu.Tài liệu có thể dùng tham khảo cho sinh viên các nghành vật lý và công nghệ có đam mê về khoa học
Morphology and growth mechanism study of self-assembled silicon nanowires synthesized by thermal evaporation Z. Zhang 1 , X.H. Fan, L. Xu, C.S. Lee, S.T. Lee * Center of Super-Diamond and Advanced Films & Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, People's Republic of China Received 18 December 2000; in ®nal form 8 January 2001 Abstract Silicon nanowires (SiNWs) grown from `sun¯ower-seed'- and `mushroom'-shaped particles have been observed by electron microscopies. The SiNWs were synthesized by thermal evaporation of SiO powders without any metal cata- lysts. The SiNWs grown on the sun¯ower-seed-shaped particles had sub-branches of SiNWs terminated by Si bulbs. The SiNWs on the mushroom-shaped particles were densely and uniformly distributed on the surface of the mushroom cone. The growth history suggests that these SiNWs were formed by nucleation which originated from the surface of amorphous SiO particle matrixes via phase separation and precipitation followed by growth through oxide-assisted vapor±soild reaction. Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction The electronic, magnetic, optical and chemical properties of nano-materials can be very dierent from their bulk counterparts and depend sensi- tively on their size, shape and composition. For example, bulk silicon is very good in electronic but poor in light emission properties at room temper- ature, because of its indirect band gap of $1.1 eV and a small exciton binding energy ($15 eV). In contrast, silicon nanowires (SiNWs) of a few na- nometers in diameter have shown unusual photo- luminescence and Raman spectra [1±4], implying a strong quantum size con®nement eect which re- laxes the k-selection rule to overcome the indirect nature of optical transition. In addition, lithium doping of SiNWs [5] has a promising application in energy storage as advanced battery cell materi- als. The size, shape and structure of SiNWs depend sensitively on their composition, as well as the temperature and other parameters of the synthetic process. The experimental results of thermal evaporation synthesis have shown that SiNWs are rich in morphology in dierent deposition tem- perature regions under the same process condition. The variation of morphology not only exists in the diameter distribution from 10±100 nm, but also in the diversity of shapes from octopus-like, chain- like, spring-like, tadpole-like, to single wires with a constant diameter [6±9]. Though the formation and structure characterization of those SiNWs of dierent morphologies have been extensively 30 March 2001 Chemical Physics Letters 337 (2001) 18±24 www.elsevier.nl/locate/cplett * Corresponding author. Fax: +852-2784-4696. E-mail address: apannale@cityu.edu.hk (S.T. Lee). 1 On leave from Beijing Laboratory of Electron Microscopy, Center of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 2724,100080, Beijing, China. 0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2614(01)00183-X investigated recently by transmission electron mi- croscopy (TEM), the growth mechanism related to various morphologies remains an open question. One of the reasons is that since SiNW specimens for TEM examination are normally chosen and removed from speci®c regions of a substrate where they have been deposited, the growth history re- lated to the as-deposited position cannot be traced to the whole deposition-collecting-substrate re- gions under dierent temperatures. Accordingly, we apply scanning electron microscopy (SEM) to investigate the morphology variation of SiNWs directly at their original deposited sites and carried out chemical analysis by X-ray energy dispersion spectroscopy (XEDS). Although SEM cannot provide the high spatial resolution images of the SiNW structure achievable by TEM, it neverthe- less can allow a systematic study of the growth process in dierent temperature zones directly at the original deposition locations. Thus far, oven-laser ablation [10±13] and ther- mal evaporation [14±17] are two methods nor- mally used for the synthesis of SiNWs. The key point in these two methods is the formation of a sucient amount of silicon atoms and/or silicon oxide clusters in gas phase from the target powders of silicon or silicon oxide by laser ablation or high temperature evaporation. A growth mechanism for these two cases is the vapor±liquid±solid (VLS) model, in which a metal catalyst (Ni or Fe) liquid droplet plays the central role in dissolving/ab- sorbing the vapor-phase silicon atoms and/or sili- con oxide clusters. When the Ni(Fe)Si 2 droplet reaches supersaturation after dissolving sucient silicon atoms from the gas phase, precipitation of silicon nanowires from the droplet can be induced. Based on a systematic analysis of the growth mechanism of semi-conductor nanowires, Lee et al. [18] proposed an oxide-assisted model by which the SiNWs were formed in two steps. Firstly, amorphous nanoparticles of SiO are nucleated on the surface of SiO matrix particles, followed by phase separation and successive silicon precipita- tion due to the disproportionation or oxidation± reduction reaction of amorphous SiO in the tem- perature range 950±1250°C. Secondly, the amor- phous nanoparticles of SiO at the tips of SiNWs continuously absorb Si±O clusters from the gas phase, which ensures the continual phase separa- tion and precipitation and results in the formation of SiNWs made of a Si core and a SiO 2 sheath. The one-dimensional growth of SiNWs is facili- tated by the SiO 2 sheath that con®nes the lateral growth of SiNWs, and the high absorptivity for Si±O clusters of the molten SiO nanoparticle at the tip as a growth front [18]. The semi-liquid nature of the SiO tip is due to melting point lowering induced by the nanosize eect associated with the SiO nanoparticle. Actually, phase separation and/ or precipitation of both silicon and SiO 2 crystal- lites in nanometer size have been well documented in the literature [19±21]. In the present Letter we report a systematic SEM study of SiNW growth history related to the morphology and growth temperature. We show that the formation of SiNWs is indeed closely re- lated to the amorphous SiO particles, and the growth and self-assembling process of SiNWs on the surface of SiO particles strongly support the oxide-assisted growth model of SiNWs with vari- ous morphologies [18]. 2. Experimental To study the growth history and related mech- anism, silicon substrate wafers used as SiNWs collectors were arranged in the temperature re- gions ranging 600±1350°C within the furnace. The relationship of SiNW morphology to dierent temperature zones on the substrate was analyzed with SEM (Philips SEM FEG Model XL30). The possible impact of chemical composition on the morphology of SiNWs was investigated by using X-ray energy dispersion spectroscopy (XEDS) at- tached to the SEM. The thermal evaporation condition for SiNWs synthesis was the same as that reported previously [8] but with a growth time of 10 min. The short synthesis time was used so as to study the initial stage of SiNW growth. 3. Results and discussions Fig. 1 shows the SEM images of SiNWs and particles on the Si substrate at a temperature of Z. Zhang et al. / Chemical Physics Letters 337 (2001) 18±24 19 around 1000°C. The composition of the particles was con®rmed by XEDS to be Si m O n with a m/n ratio close to 1:1. Careful microstructure exam- ination shows that the Si substrate serves only as a collector for SiNW deposition having no other relationship to SiNWs. A characteristic feature of Fig. 1 is that the SiNWs are grown either radially outward from the particle (Fig. 1a) or in some preferred directions (Fig. 1b). The SEM images reveal a close relationship between the SiNWs and SiO particles in that all visible SiO particles are always connected to SiNWs, as evident from Fig. 1b. Another noticeable feature is that many more SiNWs appear on the surface of larger particles, as is clear from Fig. 1a at higher mag- ni®cation. The relationship between the SiNWs and SiO particles can be seen more clearly from Fig. 2a, which shows that SiNWs are grown directly from the surface of a SiO particle. In addition, each SiNW has a nanoparticle at its tip while its root is connected to a hole on the particle surface. Fig. 2b is an SEM image from the same region in Fig. 1a but with a high magni®cation, which shows clearly the initial growth stage of SiNWs from the surface Fig. 2. (a) and (b) An initial growth stage of SiNWs can be found from fresh SiO particles and (c) `C' indicates a piece unfolded from the left side of the ®gure. Fig. 1. SEM images of SiNWs deposited on Si at about 1000°C. (a) SiNWs are connected and grown radially outward from the surface of the particle: and (b) some SiNWs show preferred growth directions from dierent sized particles. 20 Z. Zhang et al. / Chemical Physics Letters 337 (2001) 18±24 of SiO particles. The initial growth stage of SiNWs can be further visualized from Fig. 2c. The clean surface (marked by letter C) was produced by unfolding from a grown surface layer as shown in Fig. 2a,b, thus it remains relatively clean as it re- ceived little exposure to the growth ambient. On the other hand, the covered layer is full of SiNWs, while the curved edge of the unfolded layer (de- noted by an arrowhead in Fig. 2c) is also relatively free of SiNWs. In accordance with previous TEM studies [12,18,22], the above SEM results provide further experimental evidence that the nucleation sites of SiNWs are on the surface of a SiO matrix, where the amorphous SiO nanoparticles are formed, and successive phase separation (or disproportiona- tion) and precipitation occur under suitable tem- perature and chemical composition. Due to the morphological sensitivity of SiNWs to the growth temperature and composition, many types of SiNWs can be formed, rendering the de- termination of the growth mechanism of SiNWs more dicult. Fig. 3a is a low-magni®cation SEM image of SiNWs deposited on the Si substrate at a temperature about 1200°C. A characteristic fea- ture of this image is that there are some sun¯ower- seed-shaped particles, with the size of about 0:2 Â 0:5mm 2 . In the vicinity of this type of par- ticle, there are also some small particles with ir- regular shapes. These particles are connected by many relatively straight SiNWs of uniform diam- eter as strands, as shown in Fig 3b. Images at higher magni®cation in Fig. 3c show that the surface of the particles is fully covered with self- assembled SiNWs. A careful examination of the image at still higher magni®cation (Fig. 3d) shows Fig. 3. (a) Self-assembled SiNWs are grown on the surface of sun¯ower-seed-shaped SiO particles; (b) straight SiNWs are found connecting these particles; (c) SiNWs have characteristic sub-branches of SiNWs; and (d) silicon bulbs are found at the tips of the SiNWs sub-branches. Z. Zhang et al. / Chemical Physics Letters 337 (2001) 18±24 21 the self-assembled SiNWs to have many sub- branches of SiNWs grown out from their surface. The sub-branches of SiNWs are all terminated by Si bulbs, making them dierent from those SiNWs reported before [5,6]. The average diameter of the main branch of SiNWs is in the range 40±60 nm, and that of the sub-branch of SiNWs is 30±40 nm. The bulb on the tip of the sub-branch is less than 100 nm in diameter. The sub-branch SiNW is thin and normally curved at the region close to its connecting point with the main branch of SiNWs, and its diameter increases gradually to form the bulb at its tip. HREM results reveal that the main- and sub-branches of SiNWs are clothed with a SiO 2 out-layer of a few nm in thickness. HREM and electron diraction results show that the bulbs also have a silicon core and an amorphous SiO 2 outer layer. A systematic XEDS analysis shows that the sun¯ower-seed-shaped particles are com- posed of silicon and oxide only, while the oxygen content varies from the top to bottom of the par- ticles. The oxygen content decreases from 54 at.% at the head of the seed particle (denoted by letter H), to 45 at.% at the middle (noted as M), and down to 34 at.% around the bottom of the parti- cles (marked as B). The oxygen content of the particles implies that the SiNWs were nucleated and grown from the sun¯ower-seed-shaped SiO x particles. This again supports the oxide-assisted growth model of SiNWs. The dependence of SiNW morphology on temperature can also be extracted from the fea- tures revealed in the temperature region of 1180°C, where particles with the mushroom shape are observed on top of silicon substrates, as shown in Fig. 4a. SEM image at higher magni®- Fig. 4. (a) SiNWs covered mushroom-shaped SiO particles deposited at 1180°C, (b) a mushroom-shaped particle, (c) SiNWs on the smooth cone surface of a mushroom-shaped particle, and (d) many straight and parallel SiNWs connecting a mushroom-shaped particle and another particle nearby (possibly a piece broken o from the former). 22 Z. Zhang et al. / Chemical Physics Letters 337 (2001) 18±24 cation (Fig. 4b) shows that these particles have a cone shape pussy surface and the top surface of the cone is rather rough. Fig. 4c is the SEM im- age of the mushroom-shaped particles at still higher magni®cation, which shows clearly that the surface is full of self-assembled SiNWs 30±40 nm in diameter. SiNWs at the top surface of the mushroom particles are smaller in diameter. Al- though HREM studies con®rm these curved nanowires to be SiNWs, XEDS analysis reveals an obvious change in oxygen content from 54 at.% at the bottom to 27 at.% at the top surface of the mushroom particles. That is the Si/O ratio changes from about 1:1 to 3:1. There are two possibilities for the higher oxygen signals by XEDS at the bottom (denoted by letter B) and middle (marked as H) than that at the top (T) parts shown in Fig. 4b. Firstly, though the outer shell of SiNWs on the cone surface at regions B and H has the same chemical content as that in the region T, the inner part underneath the outer shell of the former has a higher oxygen content, thus giving rise to an XEDS signal rich in oxygen. A second possibility is that the SiNWs on the surface of the mushroom particles indeed have a dierent oxygen content. Fig. 4d shows that a mushroom-shaped particle and a piece nearby (apparently separated from the former) are connected by many straight SiNWs. The string-like SiNWs connecting the two particles may be understood as follows. While the SiNWs grown directly and self-assembled on the surface of the particles are highly curved, those SiNWs connecting two particles are constrained to be- come straight and aligned in parallel. From the above experimental results, we conclude that SiNWs attached to the surface of two particles are not formed individually in the gas phase ®rst and then deposited on the surface of the particles. Instead, they are nucleated di- rectly from the surface of the SiO particles or matrix, and grown continuously by phase sepa- ration (or disproportionation) and precipitation from the nano-sized amorphous particles on the tip, which serves as a growth front. The pre- cipitation-induced SiO 2 outer layer prevents fur- ther lateral growth, thus favoring the growth of SiNWs along one dimension. All these results agree well with the oxide-assisted growth model of SiNWs [18]. 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