Influence of confinement and surface functionalization effects on fluorescent properties of the impurity–vacancy complexes in ultrasmall nanodiamonds

We have done a thorough ab-initio research of vacancy–impurity complexes (including nitrogen, silicon and germanium) in 0.65 nm thick diamond nanocrystal and compared the results with those obtained for the complexes, located in diamond nanofilms of the same thickness and bulk crystals. While we consider only hydrogen-terminated and bare surfaces of nanoparticles and nanofilms , we are able to investigate the influence of confinement effects onto formation energy and electron structure of the impurity levels in the vicinity of band gap. It was found, that formation energy of these centers in nanoobjects (nanofilms and nanoparticles) is 2 eV less than that of bulk crystals. This implies preferable formation of these centers during growth close to the surface or in ultrasmall nanodiamonds. Confinement effects also significantly affect the band structure of nanoobjects. Beside the well-known effect of the band gap widening, they also lead to “popping up” of filled impurity levels over the valence band maximum. For negatively charged NV− center, located in nanodiamond, this results in interference of the upper, partially filled impurity level with the surface states, bringing about the splitting of this impurity level. We predict, that this effect would manifest itself in intermittent luminescence of NV− doped ultrasmall nanodiamonds with size below 5 nm. •We have undertaken a thorough ab-initio investigation of vacancy–impurity complexes in ultrasmall diamond nanoobjects (nanofilms and nanoparticles).•It was found that for all defect centers located in nanoobjects formation energy drops by 2 eV in comparison to the one observed in bulk crystal.•The confinement effects also significantly influence band structure of nanoobjects.•For negatively charged NV− center located in nanodiamond this results in the splitting of this impurity level.