In the present study, we have demonstrated that arrays of vertically aligned Si nanowire were successfully produced on (001)Si substrates by using the PS nanosphere lithography combined with the Au-assisted selective chemical etching process. The diameter of the Si nanowire produced was very uniform and observed to be approximately 120 nm. Based on the analyses of the TEM image and the corresponding SAED patterns, it can be concluded that the Si nanowires produced have a single-crystalline nature and formed along the [001] direction.
In order to further study the oxidation mechanism of Si nanowires, Si nanowires with diameters of 120 nm and 60 nm of silicon were prepared for a series of different temperature and time on thermal oxidation. The oxidation kinetics of Si nanowires with different diameter were investigated by TEM. The radius core of Si and the thickness of oxide shell were found to decrease and increase with oxidation temperature and time. In addition, the oxidation rate of 60-nm-diameter Si nanowires is faster than that of blank Si and 120-nm-dianeter silicon nanowires due to the stress effects. The thickness of outer SiO2 shell was found to increase parabolically with oxidation time, indicating that the growth of SiO2 shell is diffusion-controlled. By measuring the growth rate of SiO2 shell at different temperatures, the activation energies for the growth of SiO2 shells on 120-nm-diameter and 60-nm-diameter Si nanowires were determined to be about 65.4 kJ/mol, and 62.7 kJ/mol, respectively.
For the gas sensing experiments, blank-Si wafer and periodic Si nanowire arrays, were used as the gas sensor in this study. Their gas sensing properties towards acetone and ammonia were investigated at room temperature. Whether exposed to acetone or ammonia the sensitivity of the Si nanowires sensor is much higher than that of the blank-Si sensor. In this study, the gas sensitivity of the Si nanowires sensor reaches as high as 14% for 660 ppm acetone and 900% for 9 ppm ammonia. The enhanced sensing performances of the Si nanowires sensor can be attributed to its high surface-to-volume ratio.
