Graphene quantum dots: Theory and experiment

The current paper presents a consolidated view on empirical and virtual realities related to photonics of graphene quantum dots (GQDs). Empirically, the term is related to liquid and/or solidified dispersions of stacked nanosize framed graphene sheets/molecules, formed in due course of the circumference polyderivatization thus retaining sp2 electron configuration of carbon atoms in basal plane. Predominantly, the framed molecules are presented by reduced graphene oxides (rGOs), largely varied by the framing chemical composition, size and shape. Placed in different surrounding, the molecules tend to form fractal structures thus still more strengthening inhomogeneity of the final GQDs. This size-content-shape-fractal inhomogeneity is the main factor governing the peculiarity of empirical spectral properties of GQDs. Virtually, GQDs photonics is attributed to individual framed molecules leaving the fractal inhomogeneity aside while focusing on chemical content only. However, a new important factor is met on the way that is related to open-shell character of electronic systems of framed graphene molecules. The feature is discussed in the paper by analyzing computed absorption spectra of a set of model molecules involving both framed sp2 graphene and 'bulk' sp3 graphene molecules, the latter resulted from the chemical modification occurred not only at the bare sheet circumference but at its basal plane as well. UHF ground states and ZINDO/S excited states of the molecules were analyzed. The UHF-ZINDO/S combination is well coherent in the case of 'bulk' closed-shell molecules for which UHF and RHF ground state results are identical. In the case of open-shell molecules, the incoherence of the UHF and ZINDO/S approaches is revealed in a considerable decreasing of the HOMO-LUMO energy gap provided by the restricted character of ZINDO/S formalism. The feature leads to drastic unrealistic red shift of absorption spectra, strongly contradicting experimental reality relevant to GQDs. Since GQDs photonics is a largely extending area, the development of feasible computational tools to describe excited states of large openshell molecules is highly requested. © 2017 Advanced Study Center Co. Ltd.