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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
Synthesis and photoluminescence property of silicon carbon nanowires synthesized by the thermal evaporation method Enlei Zhang a , Yuanhong Tang a,b, Ã , Yong Zhang a , Chi Guo a a College of Materials Science and Engineering, Hunan University, Changsha 410082, People’s Republic of China b Powder Metallurgy Research Institute, Central South University, Changsha 410083, People’s Republic of China article info Article history: Received 13 November 2008 Received in revised form 18 November 2008 Accepted 18 November 2008 Available online 27 November 2008 PACS: 81.07.Bc 81.40.Tv Keywords: Nanostructures Crystal growth Electron microscopy Optical properties abstract The purity of b -SiC nanowires is raised obviously by using an ordered nanoporous anodic aluminum oxide template by the thermal evaporation method without any m etal catalyst. The microstructures were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and high-resolution transmission electron microscopy. The results show that the synthesized products mainly consist of nanowires, which are single-crystalline b -SiC with diameters of about 50 nm and tens of micrometers long. The nanowires axes lie along the /111S direction and possess a high density of planar defects. The b -SiC nanowires exhibit the strong photoluminescence peaks at wavelength 400 nm, which is significantly shifted to the blue compared with the reported luminescence results of SiC materials. The blueshift may be ascribed to morphology, quantum size confinement effects of the nanomaterials and abundant structure defects that existed in the nanowires. Finally, the growth mechanism of SiC nanowires and the effect of anodic aluminum oxide template are also analyzed and discussed. & 2008 Elsevier B.V. All rights reserved. 1. Introduction Recently, one-dimensional structures such as wires, rods, belts and tubes have become the focus of intensive research because of their unique applications in functional materials and the fabrica- tions of the nanoscale devices [1–3]. As an important wide band- gap semiconductor with high electron mobility, SiC nanowires would be favorable for applications in high-temperature, high- power and high-frequency nanoscale devices [4]. Recent results [5] show that the elasticity and strength of SiC nanowires are considerably greater than those of SiC whiskers and bulk SiC. A variety of methods on the synthesis of SiC nanowires have been developed, including laser ablation [6,7], chemical vapor deposition via silicon precursor [8–11], physical evaporation, hydrothermal method [12,13] and catalyst-assisted vapor liquid solid mechanism [14]. However, these products are available at the cost of either high pure and expensive carbon nanotube or the hazardous and easily explosive silicon (carbon) precursor of SiH 4 or SiCl 4 (CH 4 ). In addition, the synthesized products were of low yield and with much SiC bulk. Thus, large-scale synthesis of pure b -SiC nanowires still remains a challenge to be considered for the above-mentioned disadvantage. In this work, we have developed a simple method for synthesizing large-scale pure b -SiC nanowires by heat-activated carbon with SiO powders using anodic aluminum oxide (AAO) template without any metal catalyst. SiO powders cannot react with activated carbon directly because of AAO template. The synthesized SiC nanowires were of high yield without much bulk. The synthesized nanowires consist about 50 nm diameter core wrapped with an amorphous SiO 2 sheath. The crystal growth direction /111S is clearly observed. Photoluminescence spec- trum centered at 400 nm is referred to the SiC nanowires. Based on an analysis of experimental conditions, a growth mechanism and the effect of AAO template are proposed to explain the formation of pure SiC nanowires. 2. Experimental 2.1. Fabrication process of the AAO template The AAO template was prepared by a two-step aluminum anodic oxidation process similar to that previously described, to obtain a uniform pore structure [15,16]. Prior to anodization, the high-purity aluminum thin sheets (99.99%) were annealed at 600 1C for 2 h, rinsed in distilled water and then electropolished to ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physe Physica E 1386-9477/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2008.11.004 Ã Corresponding author at: College of Materials Science and Engineering, Hunan University, Changsha 410082, People’s Republic of China. Tel./fax: +86 7318821778. E-mail address: yhtang2000@163.com (Y.H. Tang). Physica E 41 (2009) 655–659 achieve a smooth surface. Subsequently, the aluminum samples were anodized in 0.3 M oxalic acid (40 V, 17 1C, 6 h and Al sheet as an anode). In the first step, anodized layer was removed by etching in a mixture of phosphoric acid and chromic acid at 60 1C for 12 h. During the second step, the samples rinsed in distilled water and oxalic acid were anodized again in 0.3 M oxalic acid (40 V, 16 1C, 10 h and Al sheet as an anode). After the second-step anodization, the unwanted aluminum matrix was dissolved in saturated CuCl 2 solution at room temperature. Finally, the template was rinsed with distilled water and immersed in 5% phosphoric acid for about 30 min at room temperature to adjust the pore diameter and remove the barrier layer at the bottom of nanoholes. 2.2. Synthesis of SiC nanowires The preparation apparatus for synthesis of SiC nanowires is a conventional furnace with horizontal alumina tube. Solid SiO powders (1 g, purity 99.9%) were placed in a graphite crucible and covered with an AAO template. The activated carbon (2 g) was placed on the AAO template. Then, the crucible was covered with a graphite lid, placed in the hot zone inside the alumina tube, as shown in Fig. 1. The chamber was flushed with high purity of Ar (40 sccm) to eliminate O 2 by means of rotary vacuum pump for many times. Afterwards, the furnace was rapidly heated from room temperature to 1400 1C at a heating rate of 10 1C/min and maintained for reaction for 2 h in atmosphere pressure. The sample was taken out when it was cooled down to room temperature, and the AAO template surface was deposited with thick layer of light-blue fluffylike products. Morphology and crystal lattice of the samples were observed by field-emission transmission electron microscopy (TEM, JEOJ JSM-5600LV) and high-resolution transmission electron micro- scopy (HR-TEM, JEOL JEM-3010). The crystalline structure was analyzed by X-ray diffraction (XRD, Semens D5000). The possible chemical composition of as-grown products was investigated by energy-dispersive X-ray spectroscopy (EDS) attached to the TEM. The IR measurement was completed on a WQF-410 spectrometer with a resolution of 0.65 cm À1 . Phtotoluminescence (PL, Hitatch F-4500) spectroscopy measurement was performed with xenon lamp under 354 nm as the excitation source at room temperature. 3. Results and discussion A typical SEM image of the nanoporous AAO template surface is shown in Fig. 2. The image shows the nanopores on the surface with an average pore diameter of about 50 nm, which are connected to each other to form the nanonetwork. Fig. 3a ARTICLE IN PRESS Fig. 1. Schematic illustration of synthesized b-SiC nanowires. Fig. 2. SEM image of the nanoporous AAO template surface. Fig. 3. SEM image of b-SiC nanowires synthesized by thermal evaporation: (a) with the AAO template and (b) without the AAO template. E.L. Zhang et al. / Physica E 41 (2009) 655–659656 displays SEM image of SiC nanowires synthesized using the AAO template by thermal evaporation. It reveals that large quantities of randomly distributed wire-like products have been obtained. The nanowires are of uniform diameter of about 50 nm and lengths up to tens of micrometers. In addition, it is very important to put the AAO template over SiO powders. To test this, we removed the AAO template to repeat the above process, and much more bulk was found, as shown in Fig. 3b. We think that silica source could not react with activated carbon directly because of using AAO template, and the template made the concentration of the SiO vapor increase. Thus, overgrowth of the nanowires became possible by using AAO template. The X-ray diffraction pattern for the obtained sample is shown in Fig. 4. As can be seen from the pattern, the major diffraction peak can be indexed as the (111), (2 0 0), (2 2 0), (311) and (2 2 2) reflections of cubic b -SiC (unit cell parameter a ¼ 0.4389 nm). These values are almost identical to the known values for b -SiC (JCPDS Card no. 73-1665). The internal structure of SiC nanowires was characterized by TEM. Fig. 5a displays a typical TEM image of the SiC nanowires, revealing that the periphery of SiC nanowires is very clean and straight. It also shows that the SiC nanowires possess a high density of planar defects, stacking faults which are perpendicular to the wires axes, similar to the already reported results [17–19]. With regard to energetic consideration, the formation of stacking faults during the growth of SiC nanowires is favorable due to the contribution of stacking faults themselves with lower energy. By HR-TEM image (Fig. 6) observation, we have found that nanowires have a crystalcore and an amorphous sheath with thickness about 2 nm. The SiO 2 sheath could be easily removed by etching in hydrofluoric acid (HF). The thickness of the SiO 2 sheath could be controlled by changing the etching time. Fig. 6 also shows that the spacing of lattice fringes is 0.25 nm, corresponding to the {111} plane spacing, and also indicates that nanowire grows along /111S direction. The fast Fourier transform (FFT, inset of Fig. 6) indicates that the nanowires only possess /111S crystal orienta- tion. The possible chemical composition of the sample was analyzed through the EDS data recorded from several pure nanowires (Fig. 5b). The presence of peaks demonstrates that the nanowires are composed of Si, C and small amount of O. It is found that the molecular ratio of Si/C/O of the nanowires is about 3:2:2, which corresponds well to the standard SiC and SiO 2 . The small quantity of oxygen may come from the resident oxide layer. The IR spectrum of the as-synthesized SiC nanowire samples (Fig. 7) also shows b -SiC characteristic absorption band at 791 cm À1 and SiO 2 characteristic absorption bands at 470 and 1000 cm À1 [13,20]. The SiO 2 characteristic absorption peaks are quite intense possibly due to the SiO 2 outer layers of the SiC nanowires; this confirms the composition of nanowires. To investigate PL properties of the synthesized b -SiC nano- wires, the corresponding measurement was carried out at room temperature and a PL spectrum (Fig. 8) was obtained. When excited with light from a xenon source (excitation wavelength 354 nm), the nanowires have an emission band between 330 and 600 nm. It is clear that a strong peak centered at 400 nm is observed. Compared with previously reported luminescence from the bulk [21], film [22] and nanowire [23] of SiC, the emission peak for b -SiC nanowires is obviously shifted to the blue. The emergence of the peak with a blueshift is due to the existence of oxygen defects in the amorphous layer, the special rough core– shell interface and the morphology effects such as stacking faults in the nanowires’ core [24]. It also may be attributed to the quantum confinement effect because of the small size [23,25]. Clearly, no metal catalyst was employed during the whole procedure. Thus, the growth mechanism may not follow the previously reported vapor–liquid–solid (VLS) model. On the basis of experiments, we suggest a possible growth model for b -SiC nanowires. The chemical reaction equations during the process can be described as in the following. ARTICLE IN PRESS 700 600 500 400 300 200 100 0 40 60 80 2θ (°) Intensity (a.u.) β-Sic (200) β-Sic (220) β-Sic (111) β-Sic (311) β-Sic (222) Fig. 4. XRD pattern of the as-received samples. Fig. 5. (a) TEM image of b-SiC nanowires. (b) The EDS spectrum of b-SiC nanowire. E.L. Zhang et al. / Physica E 41 (2009) 655–659 657 When the furnace is heated to a high temperature, SiO vapor is generated, and SiO gas pressure can be maintained much higher in the graphite crucible. Hence, under very high SiO partial pressure the disproportionation reaction of gaseous SiO into Si and SiO 2 can take place according to the reaction (1). It was found that if the concentration of the vapor was high, overgrowth of the nanowires became possible [26]. According to the oxide-assisted growth mechanism, silica decomposed from SiO is believed to play an important role, significantly enhancing the nucleation and one- dimensional growth of Si nanowires, which are clothed with a SiO 2 sheath 2SiOðgÞ!SiðsÞþSiO 2 ðsÞ (1) where s and g in the brackets refer to solid and gas state, respectively. The reaction temperature being 1400 1C, the Si nanowires with a SiO 2 sheath as templates would react with activated carbon to form SiC nanowires according to the following reactions (2) and (3): SiðsÞþCðsÞ!SiCðsÞ (2) SiO 2 ðsÞþ3CðsÞ!SiCðsÞþ2COðgÞ (3) In fact, reaction (3) proceeds through two stages in which a gaseous intermediate SiO gas is generated according to the following reaction (4). Once CO is formed, SiO maybe produced according to reaction (5): SiO 2 ðsÞþ2CðsÞ!SiOðgÞþCOðgÞ (4) SiO 2 ðsÞþCOðgÞ!SiOðgÞþCO 2 ðgÞ (5) The SiO vapor formed in above steps subsequently reacts further with carbon and CO according to the following reaction: SiOðgÞþ2CðsÞ!SiCðsÞþCOðgÞ (6) SiOðgÞþ3COðgÞ!SiCðsÞþ2CO 2 ðgÞ (7) According to thermodynamics calculation for reactions (6) and (7), the standard free energy changes are approximately À77.4 and À39.2 kJ/mol at 1400 1C, respectively. Therefore, both reactions should proceed. The generated CO 2 vapor can be taken into reaction (8) leading to the formation of CO vapor. CO 2 ðgÞþCðsÞ!2COðgÞ (8) During the cooling stage, reaction (9) can occur, SiOðgÞþCOðgÞ!SiCðsÞþ2SiO 2 ðsÞ (9) Since SiC has much higher melting point than SiO 2 , the solidification of SiC occurs faster than that of SiO 2 and the amorphous, viscous SiO 2 may enclose the crystalline SiC nano- wires [27,28]. This reaction leads to the decrease in enthalpy and Gibbs energy at temperature below 900 1C. As compared with reactions (2) and (3), this reaction is thermodynamically favor- able, and produces large mounts of SiC/SiO 2 composite nanowires. ARTICLE IN PRESS Fig. 6. HR-TEM image of b-SiC nanowire. The inset is the corresponding fast Fourier transform (FFT). Fig. 7. IR spectrum of the as-synthesized SiC nanowires sample. Fig. 8. Room-temperature PL spectrum of b-SiC nanowires. E.L. Zhang et al. / Physica E 41 (2009) 655–659658 4. Conclusions In summary, scales of pure crystalline b -SiC nanowires with diameters about 50 nm were synthesized using AAO template by direct thermal evaporation without any metal catalyst at high temperature. The as-synthesized products mainly consist of b -SiC nanowires. By means of XRD, SEM, EDS, IR and TEM (HR-TEM), b -SiC nanowires have been characterized and discussed in detail. The growth direction of nanowires lies along the /111S direction. 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