Optical absorption of diamond nanocrystals fromab initiodensity-functional calculations

Abstract

Absorption spectrum of small nanodiamonds, i.e., diamondoids has been recently measured exhibiting features that are not understood. Previous calculations, even beyond standard density-functional theory (DFT), failed to obtain the experimental optical gaps $\left(E_{\text{g}}\right)$ of diamondoids. We show that all-electron time-dependent DFT (TD-DFT) calculations including hybrid functional in the TD-DFT kernel are able to provide quantitatively accurate results. Our calculations demonstrate that Rydberg transitions are characteristic even for relatively large nanodiamonds resulting in low $E_{\text{g}}$. The nonmonotonic size dependence of $E_{\text{g}}$ is explained by symmetry considerations.