Tunable optical property and zero-field splitting of transition metal adatom-graphene quantum dot systems

Abstract

Graphene quantum dots (GQDs) with transition metal adatoms were analysed using a first principles approach aimed at finding novel optical materials. The optical properties of the systems that constitute GQDs with a size of 0.4-1.1 nm and five transition metals (Cr, Mo, Pd, Pt, and W) were investigated by time-dependent density functional theory. The HOMO-LUMO gaps, absorption spectra, zero-field splittings, and intersystem-crossing gaps are the main results. Unusual quantum dot size dependencies were found in the energy levels and absorption spectra that stem from extra confinement induced by the transition metal adatoms. Due to the molecular orbital hybridization between GQDs and transition metal adatoms, the major absorption peaks in the visible range could be achieved with a smaller diameter of graphene quantum dots that are relatively free from the influence of deformations. In addition, transition metal-induced magnetic effects were also confirmed by large zero-field splittings, which are negligible in pristine GQDs. The major absorption peaks are located near each red (1.59-1.98 eV), green (2.19-2.38 eV), and blue-violet (2.48-3.26 eV) spectral range, covering the entire visible spectrum, whereas the zero-field splitting mostly covers terahertz regions. These results suggest that the transition metal adatom-GQD systems are strong candidates for optical emission and absorption materials with excellent tunability over a wide spectral range.