Calculation of structural and optical properties of silicon quantum dots: tuneable absorption energy and negative electron affinity

The structural and optical properties of hydrogen-terminated silicon quantum dots were investigated by using the Gaussian 09 program. The models of silicon quantum dots assume that the silicon atoms are covalently bonded in diamond crystal structure, with quantum dot diameters varying from 0.8 to 1.6 nanometers. The bonds of silicon atoms at the surface were terminated by hydrogen atoms. The calculations of optimized structures and ground state electronic properties for the quantum dots have been performed using Hartree-Fock with 6-31G* basis set, and the exited states were then calculated by using time-dependent density functional theory (TDDFT) with (B3LYP) hybrid functional. The results show that the hydrogen-terminated silicon quantum dots have tuneable absorption energy, depending on particle size, and the larger particles have lower absorption energy. The calculated UV-VIS spectrum results show that with the quantum dots diameter changing from 0.8 nm to 1 nm to 1.2 nm the absorption peak moves from 5.23 eV (238 nm) to 4.68 eV (264 nm) to 4.03 eV (308 nm). Moreover, the hydrogen-capped silicon nanocrystals also show negative electron affinity (NEA).