Large-scale synthesis, characterization and photoluminescence properties of amorphous silica nanowires by thermal evaporation of silicon monoxide

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Abstract

A single step non-catalytic process based on thermal evaporation of silicon monoxide has been established for large-scale synthesis of silica nanowires. Scanning electron microscopy, high-resolution transmission electron microscopy equipped with energy dispersive X-ray spectrometry (EDAX), X-ray diffractometry were used to characterize the morphology and structure of the material. The as-synthesized nanowires had amorphous structures with diameters in the range 30–100 nm and hundreds of micrometers in length. The EDAX analysis revealed that the nanowires consisted of mainly two elements Si and O in an atomic ratio of approximately 1:2 corresponding to silicon dioxide. Photoluminescence spectra of the silica nanowires showed strong blue emission around 393 nm. Nucleation and growth of silica nanowires has been discussed on the basis of tiny oxide cluster formation that acts as nucleation centers for the nanowires growth.

Introduction

During past two decades, a lot of attention has been paid to the growth and characterization of one-dimensional (1-D) nanostructures such as nanotubes, nanowires, nanobelts because of their distinctive structure, unique properties and applications [1], [2], [3]. Silicon-based nanostructures have attracted significant attention due to their potential applications in electronics and opto-electronic devices [4]. For example, silicon oxide (SiOx) nanowires show intensive blue light emission, which may be a candidate material for high-resolution optical heads of scanning near-field optical microscopes, nanointerconnection integrated optical devices, low-dimensional wave-guides, etc. [5], [6], [7]. Several methods such as laser ablation [5], [8], thermal evaporation [7], [9], carbothermal reduction or carbon-assisted growth [6], [10], [11], direct thermal oxidation of Si wafers [12], [13], [14] have been used to synthesize SiOx nanowires. However, most of these methods employ metal catalysts such as Au [7], [8], [11], [14], Ni [14], [15], Fe [5], [6], Co [16], Ga [17], [18], Cu [19], Sn [20] to assist the synthesis process and consequently, the nanowires have significant presence of embedded residual metallic impurities that may affect their properties. In the recent past, non-catalytic growth of silica nanowires via carbothermal reduction of metal oxides such as MgO, CuO, WO3 has also been reported. Despite considerable experimental efforts, the growth mechanism of silica nanowires is not well understood and indeed no consensus about the growth mechanisms has been achieved. One school of thoughts believes that vapor–liquid–solid (VLS) [5], [6], [21] or solid–liquid–solid (SLS) [13], [15] processes are the possible mechanisms in catalyst-assisted growth of amorphous SiOx (a-SiOx) nanowires. Other school suggested different chemical reactions and sequences for the a-SiOx nanowires formation [10], [11], [17] to explain their experimental results. Recently, Aharonovich and Lifshitz [22] found that metal catalyst is essential for SiOx nanowires growth and proposed an alternative catalyst-assisted mechanism based on preferential adsorption of SiOx on the catalyst droplet without penetration into it.

In this paper, we report large-scale synthesis of pure silica (SiO2) nanowires by a non-catalytic approach based on thermal evaporation of SiO under argon ambient with traces of oxygen. The process is simple yet elegant and involves only single process step wherein the SiO vapors are transported from hot zone (∼1200 °C) to downstream low-temperature zone where they are allowed to condense on a substrate. The structure and photoluminescence (PL) property of the as-deposited material has been investigated and the growth of SiO2 nanowires is discussed on the basis of tiny SiO2 cluster formation via direct reaction of SiO vapors with O2 that subsequently acts as nucleation center for SiO2 nanowire growth.

Section snippets

Experimental

The growth process was carried out in a conventional three-zone horizontal quartz tube furnace, the schematic of which is shown in Fig. 1. The requisite amount of source material, i.e., silicon monoxide (SiO) granules (purity ∼99.9%; Pure Tech Inc., New York, USA) was kept in an alumina boat that was placed in the center of the quartz tube. Ultrasonically cleaned silicon strips of 2×2 cm2 with and without Ni film were placed downstream in the lower temperature zone of the furnace on an alumina

Microstructural analysis

A low-magnification SEM micrograph of the as-deposited wool-like thick white film on Ni-coated silicon wafer and its magnified view is shown in Fig. 2(a) and (b), respectively, where high density of 1-D nanostructures in the form of wires having hundreds of micrometers length is clearly seen. Fig. 2(a) also reveals that several layers of nanowires were deposited one over another and total thickness of the film is estimated to be more than 100 μm. Fig. 2(b) gives an idea of nanowires diameter

Conclusions

Amorphous silicon dioxide nanowires of several hundred microns in length and tens of nanometers in diameter have been synthesized in bulk by a non-catalytic single step process using thermal evaporation of silicon monoxide under argon atmosphere with traces of oxygen. The nanowires were free from metal contaminations and showed blue photoluminescence at room temperature. It is proposed that in-situ formation of SiO2 vapors via reaction of SiO vapors with O2 leads to the formation of SiO2

Acknowledgements

The authors wish to thank Ms. Manisha and Dr. S.K. Halder for XRD measurements of the samples and the Director, NPL for his permission to publish this work.

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