Endohedral Two-Dimensional Fullerene Networks: A First-Principles Study of Electronic and Optical Properties

The quasi-hexagonal phase of the two-dimensional fullerene network (qHPC60), recently synthesized, has emerged as a stable carbon-based material with distinct structural and electronic features. In this work, we employed density functional theory (DFT) calculations within the PBE functional to investigate the electronic and optical properties of its endohedral derivatives. The encapsulation of nitrogen, cerium, and strontium atoms inside fullerene cages was systematically analyzed at different concentrations. Our results show that nitrogen encapsulation preserves the semiconducting character of pristine qHPC60 and introduces localized intragap states with potential relevance for discrete optical emission, whereas cerium and strontium encapsulation induces a metallic behavior associated with intraband states near the conduction edge. These modifications induce a red shift of the absorption onset into the visible spectrum, accompanied by enhanced refractive and absorptive responses. The main electronic features identified at full encapsulation are preserved at intermediate fillings, with Ce and Sr reverting to a semiconducting behavior at the lowest filling examined. Overall, the findings highlight impurity-endowed qHPC60 as a promising platform for optoelectronic and light-harvesting applications.