The Effect of Alkyl Termination on the Optical and Electronic Properties of Silicon Nanoparticles

In this study, we use a combination of (time-dependent) density functional theory and many-body perturbation theory methods to study the impact of alkyl termination on the optical and electronic properties of silicon nanoparticles (SiNPs), as well as the effect of increasing particle size. A comparative study of hydrogen and methyl-terminated SiNPs reveals that replacing hydrogen atoms with methyl groups results in a reduction of the fundamental gap, optical gap, and exciton binding energy. The effect of replacing hydrogen by methyl diminishes with increasing size of the silicon core of the particles, which can be attributed to the decreasing surface-to-volume ratio. Larger hydrogen-terminated SiNPs, therefore, serve as increasingly accurate models for alkyl-terminated SiNPs. The size of the lowest energy excited-state, exciton, increases when replacing (more of the) hydrogen atoms with methyl groups for a given silicon core size, suggesting that the exciton delocalises onto the methyl groups. Analysis of the relevant natural transition orbitals confirms that both the excited electron and hole components of the exciton partially delocalise on to the methyl groups, with increased delocalisation in the case of the excited electron. The reduced fundamental and optical gaps and exciton binding energy in methyl-terminated SiNPs, and probably by extension alkyl terminated SiNPS in general, are likely due to the electron-donating nature of methyl groups combined with exciton delocalisation.