Influence of Gas Sort on the Nucleation Region Width of Si Nanocrystal Grains Prepared by Pulsed Laser Ablation

We have calculated the nucleation region (NR) location of Si nanocrystal grains prepared by pulsed laser ablation (PLA) with fluence of 4？J/cm2 in 10 Pa gas at room temperature, and ambient gases were He, Ne, and Ar, respectively. Results of calculation indicated that NR width in Ne gas was narrowest, while it was widest in He gas. Maximum mean size of grains deposited on substrates under ablated spot, which were placed horizontally, was the smallest in Ne gas. It would be attribute to more effective energy transfer during the process of collision when atomic mass of Si and ambient gas Ne are more close to each other. In this work, an additional gas flow with the same element as ambient gas was introduced, which is vertical to the plume axis at different lateral positions above ablated spot. 1. Introduction Nanocrystal Si grain is the starting point for preparing nanostructured materials and devices on account of its unique physical properties compared with bulk Si [1–3]. During the past several decades, preparation technology of nanometer materials became increasingly mature [4–8]. Among the methods of experimental and theoretical studies about Si grains, pulsed laser ablation (PLA) is one of the most commonly used methods due to its unique advantages such as rapid thermogenic speed, high atoms concentration, and small surface contamination. However, so far, there is still a great difficulty to obtain ideal size of Si grains through PLA; the most main reason is the lack of knowledge about the mechanism of nucleation and growth processes in ambient gas, which is important for both technological application and fundamental research. On the study of nucleation of Si nanocrystal grains by PLA method, Muramoto et al. [9, 10], Geohegan et al. [11], Makimura et al. [12], and so forth have verified grains formed during ablated Si atoms transmission in inert gas by different methods. But they did not mention the nucleation condition, formation position, and growth process. In 2005, Fu et al. [13] brought forward the conception of nucleation region for the first time; the viewpoint considered that grains should nucleate and grow up simultaneously through collision between ablated atoms and background gas in the NR after nanosecond laser ablated Si crystal target, and its width decided the mean size and size consistency of grains; thus, finding out the effect degree of different parameters on the NR width becomes a key problem. In this work, Si nanocrystal grains were prepared by PLA through introducing a vertical gas flow at three different positions relative to