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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
Physica E 31 (2006) 218–223 Photoluminescence and growth mechanism of amorphous silica nanowires by vapor phase transport Y. Yang a , B.K. Tay a , X.W. Sun a,Ã , H.M. Fan b , Z.X. Shen c a School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore b Department of Physics, National University of Singapore, Blk S12, 2 Science Drive 3, Singapore 117542, Singapore c School of Physical and Mathematical Sciences, Nanyang Technological University, Block 5, 1 Nanyang Walk, Singapore 637616, Singapore Received 23 November 2005; received in revised form 13 December 2005; accepted 20 December 2005 Available online 21 February 2006 Abstract Amorphous silica [SiO x ð1oxo2Þ] nanowires were fabricated on silicon substrate in an acidic environment by heating the mixture of ZnCl 2 , and VO 2 powders at 1100 1C. The length of SiO x nanowires ranges from micrometers to centimeters, with uniform diameters of 10–500 nm depending on substrate temperature. Room-temperature photoluminescence spectra of the SiO x nanowires showed two strong luminescence peaks in the red and green region, respectively. The photoluminescence was suggested to originate from nonbridging oxygen hole center (red band), and hydrogen-related species in the structure of SiO x (green band). The study on chemical reactions and growth of the SiO x nanowires revealed the formation process of silica nanowires in acidic environment was closely related to the vapor–solid–liquid mechanism. r 2006 Elsevier B.V. All rights reserved. PACS: 66.66.Fn; 6.37.Hk; 78.55.Hx Keywords: Silica nanowires; Photoluminescence; Vapor phase transport 1. Introduction One-dimensional (1D) structures with nanometer dia- meters, such as nanotubes and nanowires, are ideal vehicles for testing and understanding fundamental concepts about dimensionality and size effect in, for example, optical, electrical, and mechanical properties. Their applications range from probing tips in microscopy to interconnect in nanoelectronics [1]. The synthesis of 1D nanostructures is of fundamental importance to nanotechnology. Nanowires are particularly interesting as they offer the opportunity to investigate electrical and thermal transport processes in size-confined systems, with the possibility of providing a deep under- standing of physics at the nano-scale. Silicon and silica nanostructures have attracted considerable attention because of their potential applications in light-emitting devices compatible to CMOS technology. Amorphous silica nanowires (SiONWs) are promising 1D luminescence materials. The photoluminescence (PL) band of bulk silica or silica films has a peak within 1.6–7.0 eV [2–4] from both experimental measurements and theoretical calculations. Yu et al. [5] have pointed out the potential applications of silica nanowires in high-resolution optical heads of scanning near-field optical microscope or nanointerconnec- tions in future integrated optical devices. Much research interest has recently been directed to synthesize these materials by various approaches, to understand their growth mechanism and to realize their controlled growth on planar substrates. Various approaches, for instance, vapor phase trans port [6], bio-mimetic strategies [7,8], excimer laser ablation [9], physical and thermal chemical evaporation [10–16], carbothermal reduction [17], thermal chemical vapor deposition [18], thermal oxidation [19] and solution method [20,21] have been emp loyed to fabricate the nanostructured silica with different morphologies including ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2005.12.159 Ã Corresponding author. Tel.: +65 67905369; fax: +65 67920415. E-mail address: exwsun@ntu.edu.sg (X.W. Sun). silica ‘‘nanoflowers’’ [22], radial patterns of carbonated silica fibers [10], silica nanowire ‘‘braids’’ [6], ‘‘bundles’’ and silica ‘‘nanobrushes’’ [23]. In the vapor transport process, catalyst such as In 2 O 3 , Al, Ni, Ga and Sn, and transport gases such as O 2 , and H 2 , are often introduced into the reaction. In this paper, we shall report a catalyst- assisted synthesis of amorphous silica nanowires by vapor phase transport method in an acidic environment without transport gas. 2. Experiment Nanostructural SiONWs have been prepared by a simple vapor-phase transport method using high-temperature tube furnace, which has been reported in our previous work [24]. In brief, the mixture of hydrous zinc chloride (ZnCl 2 Á nH 2 O) and vanadium dioxide (VO 2 ) powder was placed at the end of a slender one-end sealed quartz tube. A p-type silicon trip with (1 0 0) orientation was also inserted into the quartz tube at downstream area with lower temperature (1090–700 1C) as the source of silicon as well as substrate. Then the small qua rtz tube was placed into a bigger quartz tube and pushed into the tube furnace. The furnace was heated from room temperature to 1100 1C and kept at this temperature for 30 min. When temperature reached 975 1C, an extra mechanical pump was used to collect the white fog due to the hydrolyzation and oxidation of ZnCl 2 , and maintain a pressure of 2–0.3 Pa in the tube. The morphology, size, and crystal structure, of the SiONWs were determined using a cold cathode field emission scanning electron microscope (SEM) from Jeol (model JSM-6340F), and a transmission electron micro- scope (TEM) from Jeol (model JEM-2010F) too. The chemical composition analysis was carried out using energy dispersive X-ray spectroscopy (EDX) which was attached to the SEM. PL measurements were carried out at room temperature using a Micro-PL system from Renishaw. The excitation line used was 325 nm and the average power was 10 mW. Laser beam was focused into a spot diameter below 1 mm on the specimen in the PL measurement. 3. Results and discussion Figs. 1(a) and (b) show the SEM images of the nanowires with high aspect ratio (length/diameter). Fig. 1(c) shows the photograph of the sample, with positions (a) and (b) labeled, the SEM images in Figs. 1(a) and (b) were taken, respectively. It can be seen from Fig. 1(c) that bulk quantity of cotton-like nanostructures was formed. The cotton-like SiONWs were formed on silicon wafer with temperature ranged from 1050 to 720 1C due to a temperature gradient. The average diameter of SiO x nanowires varied from 10 to o100 nm when the substrate temperature decreased from 1050 to 1000 1C (region A which is near the powder source), Fig. 1(a), and it suddenly increased from o100 to 500 nm when the substrate temperature decreased from 1000 to 720 1C (region B which is further away from the source), Fig. 1(b). Fig. 1(a) is a typical SEM image of product found in region A. It can be seen that the products synthes ized consist of a large number of curved SiO x nanowires with length of a few tens of micrometers. Fig. 1(b) shows that the products in region B generally align in one direction with length up to a few hundreds of micrometers. The diameters measured are around 200 nm. Some SiONW nanowires grown in 1000–720 1C region were up to 1 cm in length, which was observed in SEM by tracking the nanowires in their growth direction. Similar cotton-like nanowires has been obtained by Lee et al. [25] grown on TiN/Ni/Si and TiN/Ni/SiO2 substrates. ARTICLE IN PRESS Fig. 1. Low magnification SEM images of the amorphous silica nanowires deposited at two different temperature regions: (a) region A of 1050–1000 1C, 1020 1C (the average diameter is 30–50 nm); (b) region B of 1000–720 1C, 900 1C (the average diameter is around 200 nm); (c) photo of a typical sample growing on a silicon trip. Y. Yang et al. / Physica E 31 (2006) 218–223 219 Fig. 2(a) shows a TEM image of the SiO x nanowires obtained near 1000 1C temperature region. The inset is the corresponding selected-area electron diffraction (SAED) pattern recorded from the nanowires. The as-deposited SiONWs is of amorphous phase, indicated by the highly diffusive SAED ring pattern. We can see that, the nanowires are remarkably clean and smooth. Fig. 2(b) shows the high-magnification TEM image of a catalyst tip, and Figs. 2(c) and (d) are the corresponding SAED and high-resolution TEM image of the catalyst, respectively. The catalyst is crystallized and surrounded by amorphous silica, indicating that the growth process of amorphous silica nanowires is catalyst-assisted. EDX was applied to examine the chemical composition of the as-grown nanowires. Fig. 3(a) shows the EDX spectrum of the round tip on top of a nanowire (Fig. 2(b)), and Fig. 3(b) shows the EDX spectrum from long bundles of tangled silica nanowires in Fig. 1(b), where the nanowires are so long that almost no tip was in the area examined. Analyzing Figs. 3(a) and (b) using the EDX spectrometer’s own computer program, the chemical compositions are 48.77 at% of O, 41.87 at% of Si, 8.66 at% of V, 0.74 at% of Zn, and no Cl, for the catalyst tip in Fig. 2(b), and 60.28 at% of O, 39.40 at% of Si, 0.32 at% of V, and no Zn or Cl, for the long nanowires in Fig. 1(b). Thus, it is confirmed that, V acted as a catalyst in the growth of silica nanowires; however, most of Zn (ZnO x or ZnCl 2 ) with a low-melting point, was probably pumped out from the tube. It is worth mentioning that, the composition obtained here serves only as an evidence for our argument, as in general, there is about 1% error in the composition analysis from EDX. Fig. 4 shows the room-temperature PL spectra of the as- grown SiONW sample obtained under excitation of 325 nm (3.8 eV) light from He–Cd laser. For the first time, both distinct PL bands corresponding to red and green light emissions are observed aroun d 770 nm (1.61 eV) and 550 nm (2.25 eV), respectively. By fitting the curves by two Gaussian functions, two peak energies E red (red ARTICLE IN PRESS Fig. 2. (a) TEM image (the inset shows the SAED pattern) of the as- grown amorphous SiO x nanowires in the temperature region of $ 1000 1C; (b) TEM image of a catalyst tip, and the corresponding (c) SAED and (d) high-resolution TEM image. Fig. 3. EDX spectra for (a) the catalyst tip in Fig. 2(b), and (b) tangled long silica nanowires in Fig. 1(b). 400 500 600 700 800 90 0 0 1000 2000 3000 4000 5000 6000 7000 8000 E red E green 500 nm 100 nm 50 nm PL intensity (a.u.) Wavelen g th ( nm ) Fig. 4. Photoluminescence spectra recorded at room temperature from SiONWs with the diameters of $50, $100, and $500 nm, respectively. Y. Yang et al. / Physica E 31 (2006) 218–223220 emission band) and E green (green emission band) can be obtained, although the fitting was quite subjective. The results are tabulated in Table 1 . The red emission band has a relatively constant intensity, and a red-shift of about 18 nm, while the green emission band becomes weaker compared to the red ones, and shows a blue-shift of about 14 nm, as the average diameter of nanowires increases from $50 to $ 500 nm, corresponding to SEM observations. It is worth mentioning that, an average diameter of 50 nm is too thick to show any distinct quantum effect on PL. Thus the relatively small shift in emission peaks should not be directly related to size reduction. There are several nanostructure defects related to the PL of the SiO x system. The red emission band at 1.61 eV is attributed to bulk non-bridged oxygen hole center (NBOHC), which is denoted as Si2O [26,27]. The NBOHC induced band was observed in the oxygen-rich silica and in the high –OH content silica. For our oxygen- deficient but –OH rich sample, it is possible for the NBOHC ð Si2O Þ to be induced by the high-energy photon (3.8 eV) excitation in our PL measurement: Si2 O2O2Si ! 2ð Si2O Þ or Si2 OH ! Si2O þ H: The red emission band properties observed in our PL spectra are similar to those of surface-oxidized silicon nanocrystals, and mesoporous silica [26], without the exhibition of green emission band. The green emission band at 2.25 eV can be attributed to hydrogen-related species in the composites of SiONWs [28]. Defect concentration in SiONWs is related primarily to the high surface area and the complex chemistry that occurs during growth. Thus the PL intensity is highly related to surface area and inverse proportional to nanowire diameters as observed in Fig. 4. Considering the width of the red ($150 nm full-width half-maximum (FWHM)) and green ($100 nm FWHM) emissions in Fig. 4, the maximum peak shifts for red (18 nm) and green (14 nm) emissions for nanowires with different diameters are rather small (Table 1 and Fig. 4). Obviously, the large widths of red and green emissions indicate a large range of energy transitions, and the emission peak should corre- spond to the dominant transition [29]. At the moment, we cannot identify a direct link between the shift and the nanowire diameter. However, we speculate that the peak shifts for nanowires with different diameter are due to the fabrication temperature, at which these nanowires grow. The temperature directly affects the chemical reactions (reaction rates) during nanowire formation, resulting in various defects with varied concentration. According to Liu et al. [30] , the VO 2 has a weak and broad emission band near 600 nm. However, PL from catalyst can be ignored since it is not the major component in the area examined (Fig. 3(b)). The well accepted vapor–liquid–solid (VLS) mechanism is responsible for the catalyst-assisted amorphous SiO x nanowires growth in our experiment [5,31]. The key factor in VLS is the formation of liquid droplets due to adding a liquid forming agent. Due to the existence of a temperature gradient downstream the quartz tube, liquid droplets were formed from vapor phase, from the reaction happened in quartz tube. However, there was no extra silicon source besides silicon substrate; thus, we suggest that the formation of SiO x is related to the reaction of ZnCl 2 and silicon. Vadadium or VO x evaporated from VO 2 acts as catalyst for silica nanowire to grow by VLS growth mechanism. The reactions between VO 2 and oxidants (i.e., O 2 and Cl 2 ) were not considered because the VO 2 was not oxidized to higher oxidation state (V 5+ ) indicated by the color of this oxide on substrate where nanowires could be found. Further investigation using V 2 O 5 instead of VO 2 revealed that neither catalyst tip nor silica nanowires could be found; with the absence of VO 2 , ZnO nanowires were found growing on Si substrates, which is consistent with our previous work [32]. Thus, the catalyst should be related to V 4+ compound. As we know, ZnCl 2 is highly hygroscopic. The powder used in our experiment was actually ZnCl 2 Á nH 2 O, which behaves as a mild Lewis acid, with a pH value of around 4 [33]. It is hydrolyzed to an oxychloride when hydrated forms are heated. Among the solid reactant, ZnCl 2 has the lowest melting and boiling points (275 and 756 1C, respectively) [34]. When the temperature at the source approached 800 1C or higher, the tube was heavily filled with white fog. The porous silicon substrate after experi- ments, suggests that, when the furnace is heated from room temperature to 1100 1C, Si, O 2 (residue in air), and ZnCl 2 containing moisture may react in a complex way. Experi- ments without ZnCl 2 did not produce any silica nanowire, indicating that the ZnCl 2 must be a source of silicon wafer etchant. In accordance with the SEM, TEM and EDX data, the main reactions that could produce SiONWs are as follows: Hydrolyzation of ZnCl 2 Á nH 2 O [35] 2ZnCl 2 ðlÞþH 2 OðlÞ!Zn 2 OCl 2 ðgÞþ2HClðgÞ. Oxidation of ZnCl 2 in the melt [36] ZnCl 2 ðlÞþ1=2O 2 ðgÞ!ZnOClðgÞþ1=2Cl 2 ðgÞ and ZnCl 2 ðlÞþx=2O 2 ðgÞ!ZnO x ðlÞþCl 2 ðgÞ. ARTICLE IN PRESS Table 1 Peak emission wavelength of silica nanowires with an average diameter of 50, 100, and 500 nm, respectively, obtained by fitting the curves in Fig. 4 Diameter (nm) Peak wavelength of E red (nm) Peak wavelength of E green (nm) 50 755.0 569.0 100 768.0 567.7 500 773.7 555.2 Y. Yang et al. / Physica E 31 (2006) 218–223 221 Adsorption on substrate [37] Cl; Cl 2 þ Si ! Si surf À xCl þ 1=2H 2 ðgÞ; HCl: 8 > < > : Chemical reaction Si À xCl ! SiCl xðadsÞ . Product desorption SiCl xðadsÞ ! SiCl x ðgÞ. Oxidation SiCl x ðgÞþx=2O 2 ðgÞ!SiO x ðgÞþx=2Cl 2 , 2ZnðgÞþxO 2 ðgÞ!2ZnO x ðgÞ. Hence, the white fog may contain SiO x ,Cl 2 , HC l, ZnO x , ZnCl 2 , and zinc oxychloride, with little amount of VO 2 . Since vapor pressure of VO 2 is low even at high temperature, and the melting point of VO 2 is 1967 1C [33],VO 2 is most likely alloyed with Si or SiO x to form liquid drops at higher temperature. The bond enthalpies in gaseous diatomic species of Si–O bond, Zn–O bond, and Zn–Cl bond are 799.6713.4, 15974, and 228.9719.7 kJ/ mol [38], respectively; and the lattice energy in a thermo- chemical cycle of SiO 2 , ZnO, and ZnCl 2 are calculated to be 13125, 3971, and 2734 kJ/mol [39], respectively; while SiCl 4 in vapor pha se at high temperature is known to be unstable. It is much easier to break the Si–Cl and Zn–Cl bonds than the Si–O and Zn–O bonds; i.e. it is more easily to form SiO 2 than the rest compounds during reactions. From our experiments, pumping was necessary for synth- esis of SiONWs with co ntrolled amount of zinc and chloride residue. By keep pumping the furnace tube, small liquids of ZnO x and zinc oxychloride vapor could be sucked out with the high substrate temperature ranging from 1050 to 720 1C. Meanwhile, gases containing Cl 2 , and HCl could be sucked out as well to avoid excessive etching of substrate. 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