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
Si and SiO x nanostructures formed via thermal evaporation Yong-jun Chen * , Jian-bao Li, Jin-hui Dai Department of Materials Science and Engineering, State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China Received 18 April 2001; in ®nal form 13 June 2001 Abstract Various Si and SiO x x 1 to 2 nanostructures were formed via a thermal evaporation method of heating pure silicon powder at 1373 K under Ar ¯ow. An alkali-treated quartz glass plate coating with catalyst precursor of a FeNO 3 3 aqueous solution was used as substrate. The product exhibited morphologies of ®st-capped SiO x ®bers (Si-core), tree-like SiO x nano®bers and tadpole-like SiO x nano®bers in dierent areas of the substrate. The dierent local temperature gradient, concentration of silicon vapor and silicon oxide vapor, and also the substrate surface condition were suggested to be responsible for the versatile morphologies of the products. Ó 2001 Elsevier Science B.V. All rights reserved. 1. Introduction Silicon and silica nanostructures have attracted considerable attention because of their unique properties and promising application in meso- scopic research, nanodevices, opto-electronics devices [1±4]. For instance, the brightness of blue light emitted by mesoporous silica ®bers is hun- dred times than that produced by porous silicon [5], making silica ®bers attractive for use as high- intensity light sources [6,7], near-®eld optical mi- croscopy probes and hosts to lasting materials and waveguides [8]. Previous researchers have reported the synthesis of crystalline silicon nano- wires with an out-layer of amorphous silicon oxide [5,9±11]. SiO 1:4 nanowires [12] and amor- phous silica nanowires [13] have also been pre- pared recently. It is of interest to note that silicon oxide may form some novel morphology such as silica `nano¯ower' [14], radial patterns of car- bonated silica ®bers [15], silica nanowire `bundles' and silica `nanobrushes' [16] under dierent con- ditions. In this Letter, some interesting self-as- sembled Si and SiO x nanostructures were formed, which consist of ®st-capped SiO x ®bers (Si-core), tree-like SiO x nano®bers and tadpole-like SiO x nano®bers. 2. Experimental A conventional tube furnace holding an alu- mina tube (24 Â 800 mm 2 ) was employed to syn- thesize the nanostructures. An alumina boat loaded with pure silicon powder using as silicon source was placed in the middle part of the tube. Another alumina boat holding a catalyst-coated substrate (20 Â 10 Â 1:5mm 3 ) was placed next to the ®rst boat with a distance of 10 mm on the downstream side of the ¯owing argon. This set-up is similar to that reported in [17]. The catalyst- coated substrate was prepared as follows. A quartz 31 August 2001 Chemical Physics Letters 344 (2001) 450±456 www.elsevier.com/locate/cplett * Corresponding author. Fax: + 86-10-6278-2753. E-mail address: chyj99@mails.tsinghua.edu.cn (Y j. Chen). 0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 00742-4 glass substrate was ultrasonically cleaned in dis- tilled water, followed by etching in a heated 6M NaOH solution ($353 K) for 15 min. After leaching in distilled water and ethanol and drying in air, the substrate was quickly dipped in a 0.005M ferric nitrate FeNO 3 3 aqueous solution to obtain a thin catalyst ®lm. Before heating, the furnace was ¯ushed with pure argon ¯ow (150 sccm) for 15 min. It was then heated to 1373 K at a rate of 20 K/min and held for 30 min, then cooled to room temperature un- der an constant argon ¯ow (30 sccm). A thin layer of brown substance was found depositing on the substrate. As-grown samples were characterized by FE-SEM (JSM-6301F, 5±20 kV), TEM (JEM- 200CX, 200 kV), HRTEM (JEM-2010F, 200 kV) and EDS(X) attached with SEM and HRTEM (Link ISIS-300), respectively. Fig. 1. (a) Zone distribution diagram of the substrate. SEM images of the products are shown in b±j. (b) Web-like ®bers formed in zone A. (c) A closer view showing a ®st-like object-capped ®ber. (d) A tree-like object consists of a Si rod and SiO x nano®bers generated in zone B. (e) The product consists of numerous tadpole-like objects created in zone C. (f) Magni®ed image of the samples presented on the left side of e. (g) Magni®ed image of the samples presented on the right side of e. (h) Product generated in another area of zone C. (i) A magni®ed image of h. (j) Products presented in another area of zone C. Y. Chen et al. / Chemical Physics Letters 344 (2001) 450±456 451 3. Results SEM observation shows that the as-grown samples consist of ®brous substance and exhibit dierent morphologies in dierent zones of the substrate. Generally the products can be divided into three types: ®st-capped web-like ®bers, tree- like ®bers and tadpole-like ®bers. And the sub- strate was correspondingly divided into three zones (indicated as A, B and C), as shown in Fig. 1a. Fig. 1b shows that the product in Zone A consists of web-like ®bers with diameters of 40±250 nm and lengths of tens to hundreds of micrometers. Careful observation shows that, in most cases, each ®ber has a ®st-like object attached at one end, which looks like a long arm with a ®ngers-clenched hand (Fig. 1c). EDS reveals the composition of these ®bers and ®st-like objects are Fig. 1. (Continued) 452 Y. Chen et al. / Chemical Physics Letters 344 (2001) 450±456 SiO x x 1 to 2. The product in zone B is shown in Fig. 1d, taking the shape of a tree. EDS indi- cates that it consists of a Si stalk ($3 lm length, 380 nm diameter) and SiO x nano®bers branches ($3 lm length and 70±230 nm diameter). While, the product in zone C is in the form of tadpoles with dierent length (Fig. 1e). Figs. 1f and 1g show that the tadpoles on the left and right side of zone C have lengths of 15±30 and 30±70 lm, respec- tively. At the same time, the ®bers in each `tadpole' coalesced together and formed a bundle (referred to as `nano®ber-bundle' hereafter) with disordered growth direction. Generally, a Si rod links a SiO x nano®ber-bundle and a small particle (Fe catalyst by EDS), which can be seen clearly from Fig. 1h (as can also be observed vaguely in Figs. 1f and 1g). The higher-magni®ed images further con- ®rmed this structure, that is, numerous SiO x nano®bers extruded from a single Si rod (Figs. 1i and 1j). A very long nano®ber with diameter of $50 nm formed in zone A is partly shown in Fig. 2a. The selected area electron diraction (SAED) pattern (inset the Fig. 2a) indicates that it is a crystalline Si nano®ber. As shown in Fig. 2b, TEM shows that the product in zone C exhibits a structure of branched SiO x nano®bers grown from a Si rod, which agrees with the SEM result mentioned above. However, the number of nano®ber is re- duced due to the later processing, e.g., the prepa- ration of TEM sample. The higher-magni®ed image (Fig. 2c) clearly shows a diversity of con- trast between rod and nano®bers, which reveals the dierence of composition between them (Si rod Fig. 2. TEM images of (a) a Si nano®ber formed in zone A. The inset is the SAED pattern, indicating it is a crystalline Si nano®ber. (b) A sample formed in zone C, showing branched SiO x nano®bers grown from a Si rod. (c) Magni®ed image of b, showing apparent bubbles formed next to the tip of Si rod. (d) The nano®bers in the middle part of a nano®ber-bundle formed in zone C. The inset is the SAED pattern, indicating they are amorphous. Y. Chen et al. / Chemical Physics Letters 344 (2001) 450±456 453 and SiO x nano®bers). Fig. 2d shows that the nano®bers in the middle part of a nano®ber-bun- dle are self-aligned and have typical diameters of 20 nm. The SAED pattern (inset the Fig. 2d) proved that they are amorphous SiO x nano®bers. HRTEM image (Fig. 3a) further indicates that the Si nano®ber in zone A has a crystalline Si-core and an amorphous SiO x outer shell. The lattice distance of the crystalline core is measured to be 0.31 nm, which is equal to the spacing of the {1 1 1} planes of Si. EDX also indicates that the core and the outer shell contain Si and SiO x , re- spectively. It seems that the result of EDX is dif- ferent from that of EDS. However, they agree well with each other because EDS (attached with SEM) could merely analyze the composition of the sample surface (SiO x shell), whereas EDX (at- tached with HRTEM) can further analyze the composition from surface to the deep body (from SiO x shell to Si-core). The nano®bers of the Fig. 3. HRTEM images of (a) a Si nano®ber formed in zone A, showing a structure of crystalline Si-core sheathed with amorphous SiO x layer. (b) A SiO x nano®ber in the middle of a nano®ber-bundle formed in zone C, indicating an amorphous state. 454 Y. Chen et al. / Chemical Physics Letters 344 (2001) 450±456 nano®ber-bundles in zone C, however, are com- pletely non-crystalline SiO x nano®bers, which are proved by EDX and HRTEM (Fig. 3b) and also agree with the result of SAED. 4. Discussion The various structures of the products re¯ect the dierence of growth condition in dierent zones (zone A, B, C). Firstly, the dierent tem- perature gradient from zone A to C is responsible for the diversity of ®ber-diameter by in¯uencing the nano®ber growth rate. In our experiments, the order of temperature gradient is zone A > zone B > zone C. Since the largest temperature gradi- ent in zone A results in the highest nano®ber growth rate, relative thick ®bers are developed (as seen in Fig. 1b). While the smallest temperature gradient in zone C causes the formation of relative thin nano®bers. In zone B, due to an intermediate temperature gradient, a moderate growth rate re- sults in the intermediate diameter of the product. Secondly, the competitive growth between Si ®bers (or rods) and SiO x nano®bers causes a dif- ferent structures of the product. Here, silicon oxide vapor is probably generated by the reaction of silicon vapor and the silica substrate at high tem- perature, which is consistent with the supposition by Zhu et al. [6,14]. However, Yu et al. [5] assumed that amorphous state formed might be related to the low temperature and short reaction time. However, the real reason is not very clear up to now. In zone A, the highest concentration of sili- con vapor generated due to the shortest distance from silicon source. On the other side, the con- centration of silicon oxide vapor is quite small because the silicon oxide vapor generated in this area may mainly ¯ow o the outlet by Ar ¯ow. Therefore, as mentioned above, Si sub-micrometer ®bers sheathed with a thin layer of amorphous SiO x formed. In zone C, however, the concentra- tion of silicon vapor is quite low due to the con- sumption in zones A and B, while the concentration of silicon oxide vapor is quite high due to the additional accumulation of that gener- ated in zones A and B. Therefore, a structure of numerous SiO x nano®ber attaching to a short and thin Si rod is developed. The product in zone B presents an intermediate morphology, namely a tree-like structure consisting of a Si rod and SiO x ®bers. Thirdly, the surface condition of the substrate may also have an important eect on the structure of the product. As depicted in Fig. 4, the treat- ment with alkali solution (NaOH) made the sur- face of the quartz substrate rough and porous (Fig. 4a). Hence, major catalyst aggregated within these holes (Fig. 4b) and only minor catalyst re- mained on the planar surface. In zone A, the highest concentration of the silicon vapor results in the long web-like ®bers. However, the catalyst resided on the surface has a smaller size and that aggregated within holes has a larger size, which leads to the thin and thick diameter ®bers, re- spectively (as seen in Fig. 1b). In zone C, the smaller concentration of silicon vapor caused h (d) (c) (b) (a) Fig. 4. A schematic model for Si rods and SiO x nano®bers growth. (a) Surface condition of a treated quartz substrate. (b) Si rods grown in holes via a VLS process. (c) SiO x nano®bers grown from Si rods. (d) Lodging of Si rods and SiO x nano®bers with disordered orientation. Sometimes Si rod has a small catalyst plate attached. Y. Chen et al. / Chemical Physics Letters 344 (2001) 450±456 455 fewer silicon rods. Due to the `con®ne eect' of the holes, only after these Si rods protruded out of the holes can the SiO x nano®bers formed at the up ends of Si rods. Vapor±liquid±solid (VLS) [12,13], solid±liquid± solid (SLS) [18], oxide-assisted (OA) [19] etc., were used to explain the growth mechanism of silicon nanowires and silica nanowires. In our experi- ments, Si ®bers sheathed with SiO x shells formed in zone A should grow via an OA model because each ®ber is generally attached by a SiO x particle (®st-like object), which is similar to the result of Zhang et al. [20]. Si rods formed in zone C should grow via a VLS mechanism (Fig. 4b), which can be veri®ed by the apparent bubbles formed next to the tips of Si rods (Fig. 2c). EDS also reveals the aggregation of catalyst Fe at the tips of Si rods. However, the growth of SiO x nano®bers seems to be dominated by a vapor±solid (VS) process (Fig. 4c), because the catalyst aggregated at the tip of Si rod seems to be merely involved in the nucleation and initial ®ber growth. Subsequently, nano®bers grew by absorbing the growth units from silicon oxide vapor and the growth process no longer in- volved liquid phase. Yet, the reason of a small object attaching to the other end of Si rod is not clear. We suppose that the growth of the products (in zone C) within holes is similar to the growth of trees within pits (Fig. 4c). Once the length of the nano®ber-bundles reaches to a certain value, the nano®ber-bundles fall down when they are sub- jected to some unbalanced force such as Ar ¯ow, gravity force, thermal shock and etc. (Fig. 4d). Moreover, the falling directions are at random. Therefore, Si rod attached by a small object is analogous to the tree-root attached by soil parti- cle; except the soil particle is substituted by the small catalyst particle remained within holes. 5. Conclusion Si and SiO x nanostructures of ®st-capped ®bers, tree-like and tadpole-like objects were generated by heating pure silicon powder at 1373 K under Ar ¯ow. SEM, TEM, HRTEM and EDS(X) reveal that the dierent local temperature gradient, con- centration of silicon vapor and silicon oxide vapor in dierent areas result in the versatile structures of Si and SiO x . 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Si and SiO x nanostructures formed via thermal evaporation Yong-jun Chen * , Jian-bao Li, Jin-hui Dai Department of Materials Science and Engineering,. Ceramics and Fine Processing, Tsinghua University, Beijing 100084, China Received 18 April 2001; in ®nal form 13 June 2001 Abstract Various Si and SiO x x