  The combination of wide-band gap semiconductors like zinc oxide (ZnO) and
graphene quantum dots (GQDs) is a promising strategy to tune optoelectronic
properties of GQDs and to develop new functionalities. Here we report on a
theoretical design of not-yet-synthesized hybrid materials composed of ZnO
clusters surrounded by carbon moieties, hereinafter referred to as ZnO-embedded
graphene quantum dots. Their structure and light absorption properties are
presented, with an in-depth analysis of the nature of the photoexcited states.
The stability of the (ZnO)nC96-2n system with n=1, 3, 4, 7, 12 and 27 is
investigated by estimating cohesive energy and performing vibrational mode
analysis. A strong dependence of the structural and optoelectronic properties
of the hybrid material on the amount of ZnO pairs is revealed and discussed. A
strong light absorption and unexpected enhancement of Raman modes related to
the vibrations in carbon moiety are observed for highly symmetric (ZnO)27C42
system that makes it an ideal study subject. Complementary excited state
analysis, charge density difference (CDD) analysis and interfragment charge
transfer analysis enabled reaching deep insights into the nature of the excited
states. A dominating contribution of doubly degenerate locally excited states
in broadband light absorption by (ZnO)27C42 is identified. The present results
are helpful to elucidate the nature of the fundamental internal mechanisms
underlying the light absorption in ZnO-embedded graphene quantum dots, thereby
providing a scientific background for future experimental study of
low-dimensional metal-oxygen-carbon materials family.
