Nucleation And Growth Of Silicon Nanoparticles Prepared By Pulse Laser Ablation

For silicon nanoparticles prepared by pulsed lase deposition technology,the main reason to hinder controllable preparation with the size and size-distribution is that the mechanism of the nucleation and growth is not clarified.At the same time,the intrinsic cause of the velocity splitting phenomenon of the ablation particles that interact with the nucleation growth process is also inconclusive.A new theory model for the nucleation and growth of silicon nanoparticles was build via Monte Carlo simulation,the results of nanocrystalline silicon size distribution obtained by this model were in good agreement with the experimental data.The velocity splitting of ablated particles and its essential causes,the nucleation and growth process were studied use the model,the applicability of this model to europium material was also tried.The main results are described as follows:1.According to the physical properties of crystalline silicon and the results of previous studies,it is reasonable to assume that the velocity range of the ablation particle required for nucleation growth is 1000³000 m/s,the density threshold is 7.2×10¹⁹ m^-3,the atoms contained in the stable nucleus was 6.This model was used to study the size distribution of silicon nanoparticles in low pressure environment,and the results were consistent with the experimental data.The size distribution of silicon particles prepared by different laser energy ablation was calculated,and the results show that it were consistent with the experimental data too.2.Research on the splitting phenomenon of velocity distribution showed that the velocity splitting is because of the extrusion pressure on particle clusters exceeds its velocity plasticity,and the stretching after the extrusion results in the gradual disappearance of the splitting phenomenon;The change of free path and the introduction of nucleation growth process destroyed the original balance of force and plasticity,and changed the characteristic time of splitting;the pressure threshold of velocity splitting in the ambient gas of helium?He?,neon?Ne?and argon?Ar?was[10 Pa⁴000 Pa],[5 Pa³000 Pa]and[1 Pa¹500 Pa],respectively.3.The research on the nucleation and growth process showed that the ambient gas pressure and the substrate have important influence on nucleation and growth times and nanoparticles size.With the increasing of the ambient gas pressure,the peak and the duration of nucleation and growth first increased and then decreased.Combined with the drag model,the supersaturation range for nucleation and growth was calculated.The results showed that:The nucleation times and growth times increased first and then decreased with the increase of the supersaturation;The growth times is more than nucleation times when the supersaturation is 0²600,while it's less than nucleation times when the supersaturation is 2600³0000;The degree of supersaturation is large?>20000?when the ablated particles just left from Knudsen Layer,but the nucleation and growth times is not too much,mainly because the ablation particles temperature is too high to converate;4.The nucleation growth model of Si nanocrystals prepared by pulse laser deposition was extended to europium target,and the nucleation growth model of europium nanocrystals prepared by pulse laser deposition was build.The model was used to simulate the velocity splitting of europium,and the results were in good agreement with the experimental data.