Preparation Of Two-dimensional Si Phononic Crystal And Study On Its Properties

Considering the increasingly serious energy crisis and environmental problems,the thermoelectric materials become a new direction in the field of materials.Nanostructure of ordered porous silicon not only has the good features of nano-material but also can realize the thermal phonon scattering which can effectively reduce lattice thermal conductivity of Si materials for promoting its thermoelectric properties.Based on noble-metal-assisted chemical etching method,we prepare ordered porous silicon using SiO2-template.To realize the order of porous silicon,firstly,we prepared three-dimensional SiO2 nanosphere templates,two-dimensional SiO2 nanosphere templates and two-dimensional Ag@SiO2 templates.Detailed contents and results are as follows:(1)Preparation of three-dimensional SiO2 nanosphere array:SiO2 nanospheres were prepared using a modified Stober method.The three-dimensional nanosphere array was obtained through self-organization by a vertical deposition.Sintering makes the nano-array more regular than the as-deposited state,and we obtained the three-dimensional hole-bridge structure by etching the sintered array using HF.We find that the hole-bridge structure gives the largest magnetic signal at room temperature(Mmax=0.016 emu/cm3).Oxygen vacancies after sintering and surface defects from the HF-etching are thought to be responsible in large measure for the ferromagnetism.(2)Preparation of two-dimensional SiO2 nanosphere templates:We obtained two-dimensional SiO2 nanosphere templates by changing precursor solution volume fraction,controlling temperature and humidity.In addition,we have also found that sintering removed the alkenes and intermediate in incomplete reaction in SiO2 nanospheres,making the SiO2 nanospheres more stable.(3)Preparation of Ag@SiO2 nanospheres and the two-dimensional ordered Ag@SiO2 templates:We prepared Ag@SiO2 nanospheres with 2-4 nm Ag nanoparticles.The morphology,magnetic and photoluminescence properties of Ag@SiO2 nanospheres were measured.Two-dimensional ordered Ag@SiO2 templates were prepared by referring to the conditions in preparing two-dimensional SiO2 nanosphere templates.(4)Preparation of ordered porous silicon:Based on the catalytic effect of noble metal in etching process of ordered porous silicon,we modified the surface of monodisperse SiO2 nanospheres with Ag.Then Ag@SiO2 were two-dimensional regularly arranged on the surface of silicon by vertical deposition method The templates was made more regular by sintering This effect results in fabricating ordered holes and ordered porous silicon successfully.Compared with disordered porous Si,thermal conductivity of the ordered porous silicon was lower.