Preparation of silica nanowires using porous silicon as Si source

Preparation of silica nanowires using porous silicon as Si source

Applied Surface Science 258 (2011) 1470–1473 Contents lists available at SciVerse ScienceDirect Applied Surface Science journal homepage: www.elsevi...

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Applied Surface Science 258 (2011) 1470–1473

Contents lists available at SciVerse ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Preparation of silica nanowires using porous silicon as Si source Yi Liang, Bai Xue, Yang Yumeng, Nie Eryong, Liu Donglai, Sun Congli, Feng Huanhuan, Xu Jingjing, Chen Yu, Jin Yong, Jiao Zhifeng, Sun Xiaosong ∗ School of Materials Science and Engineering, Sichuan University Chengdu 610064, Sichuan, PR China

a r t i c l e

i n f o

Article history: Received 3 June 2011 Received in revised form 21 September 2011 Accepted 25 September 2011 Available online 1 October 2011 Keywords: Silica nanowires Porous silicon Catalyst-free Stress-driven

a b s t r a c t This very paper is focusing on the preparation of silica nano-wires via annealing porous silicon wafer at 1200 ◦ C in H2 atmosphere and without the assistant metal catalysts. X-ray diffraction, X-ray energy dispersion spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy and selected area diffraction technology have been employed for characterizing the structures, the morphology and the chemical components of the nano-wires prepared, respectively. It is found that the diameter and the length of the nano-wires were about 100 nm and tens micron, respectively. Meanwhile, it is also necessary to be pointed out that silica NWs only formed in the cracks of porous wafers, where the stress induced both by the electro-chemical etching procedure for the porous silicon preparation and nanowires growth procedure is believed to be lower than that at the center of the island. Therefore, a stress-driven mechanism for the NWs growth model is proposed to explain these findings. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Since the discovery of carbon nano-tubes (CNTs) [1], scientific and technological community have shown unprecedented enthusiasms in the studies on one dimensional nano-materials such as NTs, nanowires (NWs) and nano-rods (NRs). As an important optical material, amorphous silica has been intensively and extensively investigated for a long time, due to its unique physical and chemical stability and efficient photoluminescence emission [2,3]. Recently, silica nano-structures have also stimulated considerable interests because of the potentials for biological and environmental sensing device manufactory owing a great deal to their large specific surface area, the extraordinary physical and chemical stability, and the well-established protocols for coating silica with bioselective coatings [4,5] as well. For instance, it has been reported that silica NWs can be doped [6,7] or coated with functional materials [8] to extend their functionality to the application areas of micro-catalysis and functional micro-photonics. So far, many attempts have been made to synthesize silica NWs, and therefore, various growth mechanisms have been proposed in accordance with the methods employed. One commonly employed method of preparing silica NWs is the vapor–liquid–solid (VLS) method, proposed by Wagner and Ellis in 1964 [9], by which the metal catalyst droplets act as a site for vapor-phase adsorption of Si atoms. Subsequent adsorption of Si atoms onto the metal droplets results in a super-saturating liquid alloy, acting as the nucleation

∗ Corresponding author. Tel.: +86 028 85471569; fax: +86 028 85416050. E-mail address: [email protected] (S. Xiaosong). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.09.109

of the solid semiconductor and then ensuring NWs growth at the liquid metal/semiconductor interface. According to the VLS mechanism, many attempts of using different metal catalysts, such as Ga [10,11], Ni [12], Sn [13], In–Ni [14] or different Si sources have also been demonstrated. From view point of applications, it is worthy to be noted that the residual metal catalysts might be one kind of contaminations that would form the deep-level defects in silicon based materials, and, thus, would give rise to the obstruction on the application of silicon based nano-materials prepared by the ways mentioned above. In this case, developing the catalyst-free technique is, hence, of much importance [15]. However, compare with those papers dealing with the catalyst-based methods, there are a few papers that devote to the catalyst-free method [16,15,17,18]. Hence, the mechanism could be simply summarized as that Si wafers [16,15], SiO2 powder [17], and a mixture of Si and SiO2 powder [18] are used as the Si source. Meanwhile, until now, there are few reports on the fabrication of silica NWs by using PS wafers as the Si source [19]. And the oxygen come from either the O2 gas flow [16] or the source materials (e.g., SiO2 [17,18]) or even might be attributed to the residue O2 gas in the chamber or in the carrier gas other than O2 [15]. The growth mechanism is believed to be either oxide assistant model or stress-driven model [20]. Thus, the further studies on the growth mechanisms for the catalyst-free method are still necessary, yet. In this paper, we demonstrate a simple method to synthesis silica NWs. In this way, by simply annealing PS wafer and no additional metal used, the silica NWs can be produced. It is found that comparing with the previously reported catalyst assistant methods, this process presents many interesting profits that conventional catalyst assistant VLS methods do not have, i.e., (i) metal

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contamination could be eliminated since metal catalyst is not needed. (ii) PS wafer is used as the only Si source, so that the toxic precursor gases such as SiH4 or SiCl4 can be avoid. (iii) Silica NWs are directly grown on PS wafer; consequently there is no need to transfer the produced nano-materials for further device manufactory. A growth mechanism corresponding to these advantages will be proposed later in this paper. 2. Experimental The growth of silica NWs was carried out in a horizontal tube furnace system with a digital gas flow controller. In order to prepare the PS buffer on the p-type (1 0 0) Si wafers (10  cm), the electrochemical anodic etching procedure was conducted at 30 mA/cm2 for 30 min in the aqueous solution of HF (40%) and C2 H5 OH (99.7%) with a ratio of 1:1 in volume at room temperature (for details please see the reference [21,22]. The prepared PS wafers were then placed onto a quartz plate that would be transferred into the quartz tube of the furnace connected to a mechanical pump. For the silica NWs growth, the quartz tube was firstly evacuated at 10 m Torr at room temperature and H2 (purity: 99.99%, 30 sccm) was then introduced into the furnace tube. By adjusting the pumping speed, the pressure in the tube was maintained at 380 Torr. And then, the furnace was heated up to 1200 ◦ C rapidly and maintained at this temperature for 10 min. After annealing procedure, the power supply was turned off and the furnace was cooled down naturally to room temperature in H2 atmosphere. The as-grown products were characterized by scanning electron microscope (SEM, JEOL JSM-5900LV), high-resolution transmission electron microscope (HR-TEM, Tecnai G2 F20) and atomic force microscope (AFM, SPI-4000). The selected area diffraction (SAED) was also conducted with the attached equipment on HR-TEM. The chemical composition investigation was carried out with the X-ray energy dispersion spectroscope (EDS) attachment on the SEM. 3. Results and discussion Fig. 1 presents the SEM image of the as-prepared PS sample, which gives the typical morphology of the as-prepared PS sample and is similar to the result reported previously [23]. As can be seen in Fig. 1(a), there are many islands with an area of hundreds ␮m2 , and hence the separation of the islands is estimated to be several ␮m. A cross sectional SEM image is presented in Fig. 1(b), which tells that the thickness of the PS wafer was about 30 ␮m. Meanwhile, it is worthy to be pointed out that the wall of the crack is rough, and some tube-like structure can be found there. Fig. 1(c) is an AFM image, from which it can be found that there are many hillocks on the surface of island, and the height of the hillock is about 40 nm. A drastic change took place during the annealing procedure. Fig. 2 presents the SEM images of the annealed PS sample. As shown in Fig. 2(a) and (b), abundance NWs with typical diameter of 100 nm and lengths of 10 ␮m can be clearly seen in the crack of the annealed PS wafer. Prokes and Arnold [24] has reported the growth of germanium NWs by similar annealing PS wafer, where, however, it could be found that the NWs grew both in the crack and on the surface of the islands. This feature is different from our results mentioned above, which should be paid close attention and be believed to be related to the growth mechanism that will be discussed later. In order to determine the composition of the grown silica NWs, the energy dispersive X-ray spectroscope (EDS) equipped on the SEM was employed. For this purpose, in order to eliminate the interferences of PS, the silica NWs were scratched out from the PS wafers firstly and, then, dispersed in alcohol (GR) by ultrasonic oscillation. After ultrasonic treatment, the alcohol was dropped onto the Cu

Fig. 1. SEM images of (a) the plain view and (b) the angled view of the silicon wafers after anodic etching. There are a lot of separated islands of the area of hundreds ␮m2 on the surface of the wafer. The separation of the islands is about 1 ␮m. A tube-like structure can be found on the wall of the crack. The AFM image, (c), tells that there are a lot of hillocks of the height of 100 nm on the surface of the island edge.

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Fig. 2. (a) Low and (b) high magnification SEM images of as-prepared silica NWs grown in the crack of the PS wafer. The diameter of the silica NWs ranges from 50 to 150 nm.

Fig. 4. TEM bright field image (a) and (b) high resolution image of the silica NWs. The SAED pattern (the inset in Fig. 4(b)) indicates that the silica NWs are of amorphous structure.

Fig. 3. SEM image of the silica NWs dispersed on the Cu grid. The inset is the EDS of the silica NWs, from which we can estimate that the atomic ratio of oxygen to silicon is nearly 2:1.

grids for both EDS and TEM characterizations. Fig. 3 presents SEM image of the silica NWs dispersed on the Cu grid. The inset of Fig. 3 illustrates the results of EDS, which suggests that the ratio of silicon to oxygen is nearly 2:1 in the as-prepared silica NWs. Further study

on the silica NWs was carried out with HR-TEM and its equipped SAED. According to the images shown in Fig. 4, it is believed that these silica NWs grown on the PS are of smooth surface and amorphous structure. It cannot be done the growth of silica NWs without the silicon and oxygen. No doubt, Si comes from the PS wafer. But two issues are still worth to be mentioned. One is that due to the large specific area of porous structure, there is more potential silicon atom “providers” than a bare silicon wafer. The other one is proposed by Bratu and Hofer [25], that is, with increasing temperature, a surprising increase in the sticking coefficient of hydrogen occurs on a Si surface. The sticking coefficient increase is as large as three orders higher while the temperature between 580 K and 1050 K, and it would be even higher if there are defects or cracks on the surface. The enhancement of the adsorption of hydrogen onto the

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it can be seen that most of the silica NWs stood perpendicularly to the PS wafer. The nano-wire alignment due to overcrowding effect is somewhat similar to the production of aligned carbon nanotubes [33,34]. 4. Conclusion

Fig. 5. A cross-sectional SEM image of the annealed PS wafers, from which it can be seen that the silica NWs were confined with the tube-like structure and, therefore, were nearly grown perpendicularly to the PS wafer.

In summary, by annealing PS wafers at high temperature in a H2 atmosphere, the silica NWs with diameters of 50–150 nm and length of 10 ␮m have been synthesized. Metal catalysts and additional silicon-based materials are not necessary, and thus the present work provides not only simple but catalyst-free method to synthesize SiO2 NWs. The growth of Silica NWs is most likely to obey the stress-driven mechanism. Since no metal catalyst was involved, the product would be free from metal catalyst contamination. The probable mechanism has been discussed and the induced stress may play an important role in the growth of NWs. Acknowledgement

surface of Si wafer allows a much greater surface diffusion of silicon atom [26]. Meanwhile, the oxide layer of the PS wafer probably plays the role of oxygen provider. Because the PS surface is Si–Hx terminated and metastable, it will commence oxidation in air under visible light in a few minutes [27–29]. The residual oxygen in the reaction chamber, or the leakage of heating system [30] might also be probable other oxygen sources. The NWs could not form without the introduction of H2 to the furnace. A controlled experiment has been done by using N2 as the growth ambient atmosphere from which we have found that few silica NWs was grown in the crack. As mentioned above, the silica NWs would only grow in the cracks. The cross-sectional image of the as-PS island edge shows a multilayered rough structure there, which is believed to be caused by the internal stress, usually a compressive stress [24]. Thus, it is reasonable to be deduced that during the anodic etching process, the surface of Si wafer would accumulate more and more stress. As soon as the stress reaches the critical value the surface would break into small pieces to relax the stress, and hence the crack pattern forms, thus rendering them stress-free regions. This leads to a stress gradient between stress-free crack and stressful regions, the center of islands. It is believed that the effect of the gradient is negligible at room temperature. During the annealing procedure, however, as the temperature was raised at 1200 ◦ C, the influence of the stress gradient would become significant as the thermal expansion coefficient of Si is 4.6 × 10−5 ◦ C at 1200 ◦ C [31] one order of magnitude higher than that of SiO2 [24], the main component of PS. The enhanced stress-gradient might cause the surface diffusion of silicon and oxygen into the crack areas [20,24,32]. According to the H2 activation of surface diffusion model mentioned above, a huge influx of silicon and oxygen atom from islands to the cracks results in the accumulation of Si and O in these regions. An interesting topic is why the amorphous material formed in this annealing procedure is in the form of 1D silica NWs, just like those grown in the catalyst-assistant process via VLS mechanism. As proposed above, during annealing procedure, a great deal of silicon and oxygen atoms will diffuse to the crack at high temperature, where Si atom will react with O atom to form numerous SiO2 nano-particles (NPs) through homogeneous nucleation. The development of silica NPs would be confined by the tube-like structures and this confine effect is believed to lead the growth of silica NWs in the crack. Thus the dense SiO2 NPs would lead to the concurrent growth of congested SiO2 NWs. The tube-like structure would confine the development of NWs predominantly in the vertical direction. As a result, silica NWs emerged as aligned bundles perpendicular to the PS wafer surface. Fig. 5 is a low-magnified cross-sectional SEM image of the annealed PS wafers, from which

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