An alternative route for the synthesis of silicon nanowires via porous anodic alumina masks
© Márquez et al; licensee Springer. 2011
Received: 4 April 2011
Accepted: 17 August 2011
Published: 17 August 2011
Amorphous Si nanowires have been directly synthesized by a thermal processing of Si substrates. This method involves the deposition of an anodic aluminum oxide mask on a crystalline Si (100) substrate. Fe, Au, and Pt thin films with thicknesses of ca. 30 nm deposited on the anodic aluminum oxide-Si substrates have been used as catalysts. During the thermal treatment of the samples, thin films of the metal catalysts are transformed in small nanoparticles incorporated within the pore structure of the anodic aluminum oxide mask, directly in contact with the Si substrate. These homogeneously distributed metal nanoparticles are responsible for the growth of Si nanowires with regular diameter by a simple heating process at 800°C in an Ar-H2 atmosphere and without an additional Si source. The synthesized Si nanowires have been characterized by field emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman.
One-dimensional semiconductor nanostructures have recently attracted intense research attention due to their novel physical properties [1–5], including electrical, magnetic, optical, and mechanical, and their potential for device applications in chemical and biological sensors, optoelectronic, transistors, etc. [6–8]. All these properties and potential applications can be modulated by controlling the chemical composition and the dimensionality of the nanowires, during the synthesis process . Different methods have been used to synthesize Si nanowires (Si NWs) such as vapor-liquid-solid (VLS) process [10–12], laser ablation , chemical vapor deposition [14, 15] or even thermal evaporation [16, 17]. Electrodeposition techniques are an interesting alternative for nanowires growth due to the low cost and simplicity of the process [18–20]. This methodology uses a porous structure, which acts as a template, whose pores are electrochemically filled with the material of interest. This technique, however, has many technical problems to obtain nanowires with high aspect ratio.
In this study, we present an alternative procedure to those previously reported for the synthesis of nanowires. A porous structure (anodic aluminum oxide membrane) acts as an efficient template during the synthesis, controlling the dimensionality of the Si NWs. This methodology is based on the use of a porous membrane on which the catalyst is deposited. The use of silicon substrates as source for the Si NWs growth has recently been reported . Nevertheless, in our study, the treatment temperature is clearly lower, the reaction time is reduced, the diameter of the Si NWs is regular and dependent on the synthesis parameters and the length of the nanowires is adjustable, controlling the growth time . In this procedure, the diameter of the Si NWs can be related to the size of metal nanoparticles, whose dimensionality is adjustable by controlling the temperature, thickness of deposited material, and pore diameter of anodic alumina membrane used in the process . In summary, it is noteworthy that the originality of this process lies in using the same substrate where the catalyst is deposited, as source of silicon, avoiding the use of complex systems with silicon-based vapor, together with a template that allow us to obtain silicon nanowires with regular dimensions.
Preparation of the anodic aluminum oxide templates
The synthesis of highly ordered porous alumina templates has been described elsewhere [23–28]. High-purity (99.999%) aluminum sheets, used as starting material, were degreased by using a mixture of HF, HNO3, HCl, and water (1:10:20:69,%v/v) and by ultrasonication in acetone. After that, the aluminum sheets were annealed under nitrogen atmosphere at 400°C for 3 h to remove mechanical stresses. Next, the aluminum foils were electropolished in a perchloric acid-ethanol solution (1:4, v/v) at 2°C. The anodization of the aluminum foils was made in two steps. The first anodization step was carried out using a constant voltage source (40 V) in a 0.3 M oxalic acid solution for 24 h and at a temperature around 1°C, then the oxide layer was removed by using a mixture of chromic and phosphoric acids at 30°C. The second anodization step was carried out for 3 h under identical conditions to the first anodization step. Afterwards, a saturated HgCl2 solution was used to dissolve the aluminum metal. Next, the barrier layer of the bottom part was removed and the pore diameter was widened by dipping the membrane in a 5 wt.% H3PO4 solution at 35°C for 20 min. The thickness of the free-standing porous alumina membrane was measured by field emission scanning electron microscopy (FESEM) to be 10 μm with a pore diameter of ca. 60 nm.
This anodic aluminum oxide (AAO) membrane was directly supported on a silicon (100) wafer. Other more compact Si substrates (Si (110) or Si (111)) are not able to generate any growth. The Si used in the growth process of nanowires is obtained from thermally generated defects on the surface of Si (100). These defects can be observed subsequently to the synthesis of Si NWs, as small cracks on the substrate, with loss of material. This Si is extracted from the single crystal and used in the growth of the Si NWs.
The adherence of the AAO template on the silicon substrate is produced by van der Waals forces and it can be substantially improved by wetting the AAO membrane in propan-2-ol/ethanol (2:1, v/v) mixture. After that, the template supported on the Si (100) was dried at 60°C overnight.
Deposition of the catalyst on the AAO-Si sample
Thermal treatment and growth of Si NWs
Initially, 1,000 mL min-1 of a mixture of hydrogen and argon (1:7 v/v) was flowed during the heating ramp (25°C min-1). When a temperature of 800°C was reached, samples were maintained in these conditions for 30 min. Finally, the flow of argon was readjusted to 1,000 mL min-1 and hydrogen was stopped. After that, the cooling ramp was set at 20°C min-1 under flowing argon during 5 h.
The morphology and microstructure of the Si NWs grown over AAO templates were analyzed by FESEM (Philips, FEG-XL30S, 20 kV, Philips Electronic Instruments Co., Chicago, IL, USA) and by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-3000F, JEOL, Tokyo, Japan). Raman spectra were also recorded using a confocal Raman microscope (Renishaw RM2000, Renishaw plc, Wotton-under-Edge, UK) equipped with a laser source at 514 nm, a Leica microscope, and an electrically refrigerated CCD camera. The spectral resolution was set at 5 cm-1, laser power employed was less than 5 mW and the acquisition time was around 2 min.
HRTEM samples were prepared by dispersing the synthesized Si NWs in an ultrasound bath with ethanol followed by homogenization and placing 5 μL of this solution onto a copper grid coated with a lacy carbon film.
X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI 3027 system, by using the Mg Ka (1,253.6 eV) radiation of a twin anode in the constant analyzer energy mode with a pass energy of 50 eV.
Results and discussion
Electron diffraction experiments on the Si NWs observed by TEM did not result in a diffraction pattern, evidencing the amorphous nature of this material. Upon closer inspection of the HRTEM images of the metal nanoparticles (inset of Figure 7b), it can be observed that the ordered fringes are demonstrating the crystalline nature of the metal particles generated during the melting process of the catalysts through the mask. On the other hand, EDXS measurements confirmed the composition of individual Si NWs to consist of silicon and oxygen (see the inset of Figure 7c). The oxygen signal is due to the presence of silicon oxides, possibly located on the surface.
The results obtained by XPS and EDX indicate that the Si NWs are constituted by Si0, SiO2, and substoichiometric silicon oxides (SiO x ). Moreover, studies of electron diffraction by TEM reveal that the Si NWs are amorphous in nature. Possibly, Si NWs are composed of a Si0 core surrounded by a silicon oxide shell. Different studies on the synthesis of amorphous silica nanowires consider that the explanation for the amorphous nanowires production is the growth temperature. In fact, when temperature is not high enough, recrystallization is not produced and, in our case, we have used a constant growth temperature of 800°C.
The peak at ca. 485 cm-1 (m) can be justified as due to the bond Si-O of amorphous SiO2 or also to substoichiometric oxides. The Raman peak at ca. 584 cm-1 (m) has been assigned to Si-O-Si bending of silicon oxides. The broad peak at 931 cm-1 is due to the stretching mode of amorphous Si-Si (vibration that is also observed at 512 cm-1). Finally, the Figure 9 shows three peaks at ca. 678 (w), 798 (m), and 860 cm-1 (w), that have been associated to the stretching mode of Si-O.
In the present work, we have used AAO masks to synthesize Si NWs on Si (100) substrates, by using Fe, Au, and Pt as catalysts. In this approach, the Si (100) substrate acted as both silicon source and growth substrate, allowing the synthesis of Si NWs with regular dimensions.
The growth mechanism corresponds to a VLS process. In this mechanism, the growth happens when silicon from the Si (100) substrate diffuses into the alloy puddle, favoring the melting of Si into the alloy .
The diameter of the nanowires ranged from ca. 30-50 nm, with an average size of ca. 40 nm and was related to the pore size of the AAO mask. HRTEM revealed the amorphous nature of the Si NWs, possibly due to the low growth temperature used during the synthesis. EDX, XPS, and Raman have shown that they are composed of Si0 and silicon oxides (SiO2-SiO x ) possibly forming a Si0 core surrounded by a silicon oxide shell. Nevertheless, further research is needed to clarify this point.
The authors gratefully recognize the financial support provided by MEC through the grants MAT2006-08158, MAT2007-66476-C02-02, MAT2010- 19804 and European Community FP6-029192. Financial supports from US Department of Energy through the Massey Chair project at University of Turabo and from the National Science Foundation through the contract CHE-0959334 are also acknowledged. One of us (TC) thanks the economical support from MICROLAN S.A. The "Servicio Interdepartamental de Investigación (SIdI)" from Universidad Autónoma de Madrid and "Centro de Microscopía Luis Bru" from Universidad Complutense de Madrid are acknowledged for the use of the HRTEM and FESEM facilities.
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