Preparation And Properties Of Freestanding Porous Silicon And Its Composite Materials

In recent years, as a kind of important semiconductor material, silicon has been in an important position in many fields due to its excellent performance. Due to the excellent properties of silicon,porous silicon was first prepared in this thesis and was further used to fabricate a supercapacitor.Porous silicon with different porosity was successfully prepared by anodic oxidation method.Scanning electron microscope(SEM) and photoluminescence spectrometer were used to analyze basic properties of porous silicon and studied the formation mechanism of porous silicon. The result of experiment showed that holes on the surface of the porous silicon were uniform distribution with a size of around 50 nm and the cross-section of as-prepared porous silicon has a lath-shaped structure.The porosity and thickness of porous silicon layer increased with the increase of corrosion current density and corrosion time. With the increase of the porosity of porous silicon, peak was blue shifted from 650 nm to 580 nm.For further utilization of porous silicon layer on the basis of preparation of porous silicon, multistep anodic oxidation method was used to prepare freestanding porous silicon and a controlled preparation of freestanding porous silicon was realized. Then the effect of corrosion current density and time on stripping of the porous silicon was studied. At the same time, the effects on the porosity,thickness, reflectance and photoluminescence property of porous silicon were also analyzed. The result showed that the preparation of freestanding porous silicon required to controlling the current density in a specific range. If the current was too large, thin layer would be cracked. If the current was too small, thin layer could not be stripped off. There were nonlinear changes between the porosity of freestanding porous silicon thin layer and the corrosion current density/time, and linear changes between the thickness of porous silicon and the corrosion current density/time. Using ultraviolet spectrometer to study the reflectivity of the freestanding porous silicon with different parameters, the reflectivity decreases with the increase of corrosion current density/time. The minimum value is 4.8%and 6.3%, respectively. In order to make full use of freestanding porous silicon thin layer, we filled the holes of the porous silicon with CdS nanoparticles by the electrophoretic method, to improve photoluminescence properties of freestanding porous silicon.In order to use freestanding porous silicon in the supercapacitor, the freestanding porous silicon/Zn O composite material was prepared on the basis of preparing the freestanding porous silicon with two step anodic oxidation method. Then we developed a new method for synthesis ofcomposite material with ZnO nanoparticles in freestanding porous silicon using vacuum filtration and hydro-thermal approach for supercapacitor. The morphology, composition and luminescence properties of the samples were characterized by scanning electron microscope, X-ray energy dispersive spectrum analysis and photoluminence spectrum analysis. The results showed that ZnO nanoparticles existed in the freestanding porous silicon and porous silicon/ZnO composite material formed successfully. Moreover, the electrochemical performance of the composite material with ZnO nanoparticles in freestanding porous silicon as a supercapacitor was investigated by cyclic voltamerty,electrochemical impendance spectrum test and galvanostatic charge/discharge. The specific capacitances can reach 15.7F/g, with an increase of 120 times compared with that of the freestanding porous silicon.