Study On Preparation Technics And Microstructure Of Nano-Silicon

When the structure of silicon transforms to nano-scale, the decrease of particle size will lead to surface and interface increasing, crystal lattice distortion, and the number and kinds of defects changing, which results in low dimension effect, surface effect, quantum confinement effect and quantum transportation feature. It is important to study the internal rule between microstructure and the particle size and its influence mechanism to the properties of nano-materials systematically, which has crucial theoretic and practical meaning for realizing the manipulation and assembling of single molecule and nano-component.In this study we designed and made the preparation equipment of nano-silicon powder by LICVD (laser induced chemical vapor deposition), and introduced the designing thought of key parts. And have prepared crystal nano-silicon particles with the diameter ranging from 30 to 50 nra.Laser energy value of threshold with different gas component and flux were studied, and the reason of different threshold values was analyzed. In this study, we found that with the content of SiH₄ increasing, decomposing gas demanded less laser energy value of threshold. If the content of SiH₄ kept constant, the laser energy value of threshold that decomposed SiH₄ increased linearly at first, but when the gas flux exceeded 100ml/min, the laser energy value of threshold increased slowly because the coaxial Ar could not "suppress" the reaction flame which leaded to radial dilation of reaction gases and increasing the effective absorption to laser energy.The microstructure of nano-silicon under different reaction gas flux was studied. We found that the prepared nano-silicon particles changed from crystal to amorphous structure with the increase of reaction gas flux. The research to laser power density indicated that the non-crystal components and defects in the grainsdecreased gradually with the increase of laser power density.When the reaction gas flux was a constant, the coaxial protecting gas had an optimal flux value. When the flux of the coaxial Ar was equivalent to the reaction gas at the nozzle, we got the nano-silicon with the most homogeneous microstructure.On the condition of the equipment parameters, the optimal technology parameters for preparing nano-silicon with uniform dimension were as follows: the air pressure of reaction room was 80Kpa; the laser power density was more than 700W/cm2; the reaction gas flux is 40ml/min; the content of SitLt was 20%; the coaxial Ar flux was 480ml/min. Under this condition, we could get uniformly crystal nano-silicon particles with diameter ranging from 30 to 60 nm.The thermodynamics of SiH* decomposition was analyzed. The driving force of thermodynamics for decomposition increased with the increase of temperature.The analysis to dynamics of nucleation and growth indicated that at the beginning of nucleation and growth the main growing mode of nano-crystal was laminar. Then with the increase of super-cooling degree of crystallization the growth of nano-crystal depended on defects. The nano-crystal might grow with jumping mode because of the disturbance of the reaction gases. The crystal orientation of nano-silicon is <112> at the lower reaction gas flow and <111> at the higher flow.During the preparation of nano-silicon, the surface conditions of nano-silicon particles were different because the saturation of H atoms to dangling bonds of nano-silicon surface differed. The surface of nano-silicon changed from rough to smooth with the increase of SiRj content.The existence of surface dangling bonds often resulted in nano-silicon particles conglutinating with each other to form chain or ring. Nano-crystal often emerged between two particles.The study of defects in nano-silicon indicated that in the microstructure the...