First-principle study on optical properties of spherical and cylindrical hydrogen-passivated Si nanoparticles with different sizes

To investigate the size dependence of the optical properties of the hydrogen-passivated Si nanoparticles (Hp-SiNPs), the energy bands and optical dielectric functions for two types of nanostructures, that is, the spherical Hp-SiNPs (SHp-SiNPs) with various diameters and the cylindrical Hp-SiNPs (CHp-SiNPs) with the same cross-section and various heights are calculated by using CASTEP software package based on density functional theory under generalised gradient approximation. The results show that for both SHp-SiNPs and CHp-SiNPs, the bandgaps increase, and the optical absorption edges of imaginary part of dielectric functions demonstrate blue-shifts as the sizes of Hp-SiNPs are reduced, which are consistent with the previous experimental data and theoretical conclusions. For CHp-SiNPs, the optics dielectric function behaves anisotropically owing to the geometrical anisotropy.