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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
Silicon quantum-wires arrays synthesized by chemical vapor deposition and its micro-structural properties M. Lu a , M.K. Li a , L.B. Kong a , X.Y. Guo b , H.L. Li a, * a Department of Chemistry, Lanzhou University, Lanzhou 730000, China b Laboratory of Special Functional Materials, Henan University, Kaifeng 475001, China Received 16 July 2002; in final form 1 May 2003 Abstract Well-aligned arrays of silicon nanowires (SiNWs) have been synthesized by a chemical vapor deposition (CVD) template method without catalyst. The micro-structures of the SiNWs were studied by high-resolution transmission electron microscopy (HRTEM). Selected-area electron diffraction (SAED) and X-ray diffraction (XRD) indicate that each nanowire is essentially a single crystal and has a different orientation in an array. According to VLS mechanism, the growth of SiNWs without catalyst is related to the structure of template. The superior field emission behavior is believed to result from the oriented growth and the sharp tips of SiNWs. Ó 2003 Elsevier Science B.V. All rights reserved. 1. Introduction Quantum wires of silicon as a special form of crystalline silicon have stimulated much interest because of their low dimension and quantum- confinement effect [1–4]. It has been suggested that they may be used for developing one-dimensional (1D) quantum wires, high-speed field effect tran- sistors and light-emitting devices with extremely low power consumption. In order to be capable of being incorporated effectively into devices, these applications usually require controlled orientation and size of the grown nanostructure. To date, sil- icon nanowires (SiNWs) have been successfully synthesized by different methods, such as laser ablation, thermal evaporation and lithography [2–8]. However, SiNWs produced by most of these methods are of random orientation, self-gathering and twisting each other, which restrict their ap- plications in nanoelectronic. Recently, a-SiNWs were controlled grown directly on a Si substrate via a solid–liquid–solid mechanism [9]. Liu et al. [10] have prepared vertically aligned a-SiNWs on a large scale on silicon oxide substrate by thermal chemical vapor deposition (CVD). Metal catalyst is an essential element in these methods, which is required for the nucleation and growth of SiNWs. In this Letter, for the first time, we employed alumina template to prepare well-aligned SiNWs arrays by CVD without catalyst. The size and shape of SiNWs can be readily controlled by the template and may vary over a wide range Chemical Physics Letters 374 (2003) 542–547 www.elsevier.com/locate/cplett * Corresponding author. Fax: +86-931-891-2582. E-mail address: lihl@lzu.edu.cn (H.L. Li). 0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00747-4 according to the template used [11–14]. More im- portantly, there are three advantages. First, tem- plate-confined growth demonstrated to be an efficient approach to the production of highly or- dered and isolated nanowires arrays over large areas. Second, it becomes possible to extend the traditional ideas of using catalyst for the growth of SiNWs. Third, compared with a high density of defects near the tip of SiNWs prepared by other methods, our present SiNWs with sharp tips and perfect lattices might be promising materials for future nanoprobes and superior field emitters. 2. Experimental Alumina template was prepared by anodic ox- idation of electropolished aluminum plate at a cell voltage of 20 V in 0.5 M phosphoric acid at 25 °C for 1.5 h [14,15]. After anodization, the alumina membrane was separated from aluminum sub- strate using the voltage-decreasing method [16]. Finally, the membrane was rinsed thoroughly with distilled water and dried in a pure nitrogen flow. Subsequently, the membrane was placed in a quartz boat and then inserted into the center of a 4-cm-long quartz tube reactor heated by tungsten filament. Atmosphere in the reactor was pumped with a mechanical vacuum pump. A flow of H 2 (10 ml/min) and Ar (30 ml/min) was purged for 0.5 h before the reactor was heated to reaction temperature, 900 °C. Then a flow of SiH 4 was in- troduced at the same rate with H 2 (10 ml/min) for 1 h. After deposition, the SiH 4 and H 2 flows were turned off, and the sample was cooled to room temperature in an Ar atmosphere. Before TEM characterization, the deposits on one side of the alumina membrane were removed by polishing with alumina power. The SiNWs were released from the template in 6 M NaOH for 24 h, and then thoroughly washed with distilled water. Conventional TEM analysis and high-resolution transmission electron microscope (HRTEM) were both performed on a JEOL-2010 microscope at 200 kV equipped with link-ISIS energy dispersive spectroscopy (EDS) elemental composition ana- lyzer. SEM image of SiNWs was obtained as fol- lowers: the sample was glued (using epoxy) to a metallic support with the cross-section up and then immersed into 6 M NaOH solution for 20 min in order to dissolve alumina. The sample was then sputtered with $10 nm of Au prior to imaging (JSM-5600LV electron microscope). For lower angle X-ray diffraction (XRD) study, the silicon films on both surfaces of the alumina membrane were removed. The sample was transferred on to the standard silicon supporter and the spectrum was obtained using a D/MAX-2400 X-ray dif- fractometer. The field emission measurements were carried out in a vacuum chamber at a pressure of about 10 À7 Torr at room temperature. The sample was used as the cathode, while a copper sheet polished served as an anode. The distance between the an- ode and the sample (cathode) surface was con- trolled by the thickness of a mica spacer containing a hole ($1mm 2 ) in the center. Voltages up to 3 kV were applied to the anode and the emission current was detected with a micro-amp- erometer. 3. Results and discussion Fig. 1a shows the cross-sectional SEM image of the SiNWs array after dissolving alumina. One can see that the nanowires produced are very straight and form a well-aligned array, indicating there are few defects and little growth stress. As would be expected, the nanowires are of a uniform diameter of about 50 nm, which is consistent with the pore diameter of the alumina template. A sharp tip is found at the end of each nanowire, which could be useful to a field emitter. Meanwhile, there is a silicon surface film at the bottom of the SiNWs, which is always presented in other nanomaterials synthesized by template method [14–18]. Fig. 1b shows the SiNWs are still parallel with each other when alumina is partially dissolved, maintaining their orientation within the template. The EDS spectra collected from the middle part of the SiNWs arrays (Fig. 1c) show the presence of sili- con in addition to Al and O. C and Cu are at- tributed to the copper micro-grid with carbon film used to support a sample in TEM measurement. The presence of trace of P confirms our previous M. Lu et al. / Chemical Physics Letters 374 (2003) 542–547 543 finding that anion ions of anodizing electrolytes are incorporated in the formation of alumina template during the anodization procedure [19]. A single SiNW is shown in Fig. 2a, in which alumina is dissolved completely. The selected-area electron diffraction (SAED) pattern taken from this SiNW is show in Fig. 2b. From this photo, it can be seen that the diffraction spots are clear and organized in an almost precise hexagon or paral- lelogram, indicating that the diamond lattice structure of bulk Si is also preserved in the SiNWs. According to the geometry analyses of electron diffraction, the cubic indices of the diffraction spots in the electron diffraction pattern are de- marcated. The similar results were obtained on the other SiNWs, indicating that each single SiNW is a single crystal. In order to reveal the micro-structure of SiNWs in detail, the HRTEM was employed to investigate them at atomic scale. The incident electron beam is parallel to the [1 1 0] zone axis. The HRTEM image in Fig. 3a shows the representative micro-structural characteristics of individual SiNWs. It is clear that the straight SiNWs has smooth surface and the Fig. 1. (a) SEM image of SiNWs arrays, (b) TEM image of SiNWs arrays, (c) EDS spectrum of SiNWs arrays. Fig. 2. (a) TEM image of a single SiNW, (b) SAED pattern of the single SiNW, the inset data are the cubic indices of the diffractive spots. 544 M. Lu et al. / Chemical Physics Letters 374 (2003) 542–547 change in diameter along its length is seldom ob- served. The SiNWs are virtually defect-free and show no kink, dislocation and small angle bound- aries because of the periodic change growth direc- tion along the length of the SiNWs. The growth plane is one of the (1 1 1) planes and the fast growth direction is along the [)2 1 1] axis of the SiNWs. The (1 1 1) plane family, which is the densest packed plane with the lowest surface energy in silicon structure, hence is important for SiNWs nucleation and growth. When the (1 1 1) planes are parallel to the axes of the nanowires, the system energy is reduced significantly. In this case, the (1 1 1) planes aligned parallel to the growth axes. Fig. 3a also shows that the tip of the SiNWs is sharp and has a perfect lattice structure. This is in contrast with the previous SiNWs that are round and contain a high density of stacking faults and micro-twins. Obviously, the sharp tip suggests a distinctly different formation mechanism com- pared with the previous works. Fig. 3b shows the image of the silicon film de- posited on the surface of the template, which has rough surface. The interplanar spacing between visible fringer is 0.28 nm, corresponding to the (0 0 2) plane of silicon. It is visible that the (0 0 2) lattice fringes are not continuous due to the dis- location, micro-twins and stacking faults indicated by the arrow. The film can be modeled as con- sisting of segments with different orientation but with a similar structure as the SiNWs. We also notice a thin film amorphous layer exists around the Si film, which is identified to be amorphous silicon oxide resulting from surface oxidation. The XRD spectrum of the SiNWs arrays in Fig. 4 contains seven peaks, which are identified to match well with the (1 1 1), (2 2 0), (3 1 1), (4 0 0), (3 3 1), (4 2 2) and (5 1 1) diffraction peaks of the diamond lattice structure of bulk silicon through the PCPDFWIN software. The other unmarked peaks are assigned to the alumina. Calculated from the interplanar spacing of the most intense (1 1 1) peak (d ¼ 0:3147 nm), the lattice parameter of the SiNWs can be obtained as a SiNWs ¼ 0.5451 nm, which is 0.387% larger than the standard value a Si ¼ 0.5430 nm for bulk silicon, revealing there is a slight lattice expansion and distortion in the SiNWs structure. From the XRD results, the array of SiNWs appears to be polycrystalline structure, which seems contradicting to the above results of SAED. This can be understood that the statistical results obtained by the XRD pattern and the dif- fraction patterns of different whiskers indicate different orientations. The growth of SiNWs catalyzed by metal parti- cles is usually considered to be a vapor–liquid–solid Fig. 3. Typical HRTEM images of SiNWs (a) and Si film de- posited on the surface of SiNWs arrays. Fig. 4. XRD spectrum of SiNWs arrays. M. Lu et al. / Chemical Physics Letters 374 (2003) 542–547 545 mechanism. However, it is clear that the conven- tional VLS mechanism based on the role of cata- lyst [20–22] could not explain the growth of SiNWs, because we did not use any catalyst in the deposition. We propose the following mechanism. During the early stage of the CVD process, most of the silicon atoms were originated from the dis- sociation of silane at high temperature. There are a large of Lewis acid nature of surface sites in amorphous and transition alumina and these sites have the intrinsic catalytic activity of transition alumina in front of the decomposition of silane [23]. Therefore it is possible that the internal channel surface within alumina has a catalytic behavior in addition to its template effect. Most silicon atoms in vapor phase will be deposited on the walls of alumina nanochannels. The high density of dangling bonds at the surface of atomic Si will lead to the bonding each other between silicon atoms and a continuous diffusion into the channels of alumina. Further condensation will then produce SiNWs in the channels of alumina. Furthermore, the carrier gas Ar will collide with the pore surface and the atomic Si has absorbed and exchange energy and momentum with the atom, causing overcooling at the surface. Because the precipitation, nucleation and growth of SiNWs always occurred at the area near the cold fringer, such an overcooling is important for providing temperature gradient used as an external driving force for nanowire growth. However the reason for the formation of sharp tip on the SiNWs is not clear yet, further work is required to understand this phenomenon. ZhangÕs has demonstrated SiNWs emitters dis- play attractive field emission properties, which may be exploited for practical applications [24]. Fig. 5 illustrates the curve of current versus voltage (I–V curve) for SiNWs arrays, revealing the ro- bustness of the emission process from the emitter. The turn-on field for electron emission, defined as the macroscopic fields needed to produce a current density of 0.01 mA/cm 2 ,is$14 V/lm. The I–V data analyzed by the Fowler–Nordheim theory [25] is presented in the inset of Fig. 5. As can be seen, almost straight line is obtained, indicating that the field emission from SiNWs is a barrier tunneling quantum mechanical process. The superior field emission behavior is believed to originate from the sharp tips and oriented growth of SiNWs. 4. 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Silicon quantum-wires arrays synthesized by chemical vapor deposition and its micro-structural properties M. Lu a , M.K. Li a ,. form 1 May 2003 Abstract Well-aligned arrays of silicon nanowires (SiNWs) have been synthesized by a chemical vapor deposition (CVD) template method without