Quantum confinement effects on optical transitions in nanodiamonds containing nitrogen vacancies

Colored nitrogen-vacancy (NV) centers in nanosize diamonds (d~5 nm) are promising probe materials because their optical transitions are sensitive to mechanical, vibrational, and spin changes in the surroundings. Here, a linear response time-dependent density functional theory approach is used to describe the optical transitions in several NV-doped diamond quantum dots (QDs) in order to investigate size effects on the absorption spectra. By computing the full optical spectrum up to band-to-band transitions, we analyze both the localized ``pinned'' midgap and the charge-transfer excitations for an isolated reduced NV center. Subband charge-transfer excitations are shown to be size dependent, involving the excitation of the dopant sp³ electrons to the diamond conduction band. Additionally, the NV-doped systems exhibit characteristic sp³-sp³ excitations whose experimental energies are reproduced well and do not depend on QD size. However, the NV position and global cluster symmetry can affect the amount of the energy splitting of the vertical excitation energies of the midgap transitions.