Effect of Surface Passivation on Nanodiamond Crystallinity

Diamonds approaching the nanoscale have the potential for use as probe materials as their optical properties can be sensitive to optical/electric fields, mechanical stress/pressure, and the configuration of nuclear spins. The surface of nanodiamonds impacts their optical properties and sensing capabilities, and examining the nanodiamond surface through X-ray absorption can give insights into molecular surface structures. Here, quantum dot models with varying amounts of surface carbon passivation are prepared, optimized, and compared. The loss of the diamond sp(3) lattice is examined by investigating the bond length and tetrahedral character of the carbons comprising nanodiamonds for the appearance of aromatic sp(2) surface domains. Electronic transitions in the carbon K-edge region, using the energy-specific time-dependent density functional theory method, as well as vibrational spectra are computed from the optimized models. The surface reorganization is shown to affect the electronic characteristics of the nanodiamond. As a result, there is a distinct absorption peak in the carbon K-edge region, along with stretching modes in the vibrational spectra, that can be correlated to the nature of the surface hybridization of the nanodiamond.