The optoelectronic properties of group IV nanoparticles.

We use time-dependent density functional theory and many-body perturbation theory to compare and contrast the electronic and optical properties of hydrogen capped carbon, silicon and germanium group IV nanoparticles. Hydrogen-capped silicon nanoparticles are model systems for quantum confinement and hence it is an interesting question if carbon and germanium nanoparticles similarly behave as archetypes of quantum confinement. We find that for several properties, such as the trends in the fundamental and optical gaps with particle size, all group IV systems behave similarly. However, there are also other properties, such as the variation of the electron affinity with particle size and the electronic character of the low-energy excitons and frontier quasiparticle states, for which the carbon particles behave fundamentally differently from their silicon and germanium counterparts and deviate from the idealised cartoon picture of quantum confinement. We speculate that the fact that hydrogen is more electropositive than carbon, and more electronegative than silicon and germanium, combined with the large gap of the carbon nanoparticles, pushes the unoccupied C-H σ-bonds into the gap, which then pins the lowest unoccupied quasiparticle state and excited electron component of the excitons on the surface.