Tuning the optoelectronic properties of graphene quantum dots by BN-ring doping: A density functional theory study

Graphene monolayer is a material with zero band gap, because of which its applications in optoelectronics are limited. The question arises, can we modify the optoelectronic properties of graphene by doping it with other atoms? Synthesis of 2D monolayer of graphene doped with hetero-atoms such as boron and nitrogen, and a few computational studies of their structural and electronic properties were previously reported. In this work, we aim to answer this question for graphene quantum dots (GQDs) by replacing their carbon rings with $(BN)_3$ (borazine) hexagonal rings. We have studied in detail the geometry, electronic structure, and optical absorption spectra of fourteen different borazine-ring doped diamond-shaped GQDs using first-principles density functional theory (DFT). These BN-GQDs differ in the location, orientation, and the number of borazine rings. We computed their optical absorption spectra using time-dependent DFT (TDDFT) and examined: (a) for single-ring doped BN-GQDs the influence of ring location on optical properties, and (b) for double-ring doped systems, the influence of location, mutual distance and orientation of the rings on their absorption spectra. Frontier molecular orbitals are studied in detail to understand the nature of low-lying optical excitations. We also performed a group-theoretic analysis of the influence of their reduced symmetries on their optical properties. Our results indicate that BN-ring doping can achieve significant control over the optical properties of GQDs. The comparison of the optical absorption spectra of the BN-GQDs with the parent GQD shows remarkable spectral broadening with optical gap spanning over infrared to visible region. Thus, systematic BN-ring doping provides easy tunability of the electronic and optical properties of BN-GQDs, which is very promising for optoelectronic applications.