Endohedral Derivatives of the Recently Synthesized Two-Dimensional Fullerene Networks: Electronic and Optical Insights from First-Principles Calculations

Abstract

The quasi-hexagonal phase of the two-dimensional fullerene network (qHPC$_{60}$), 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 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 encapsulation preserves the semiconducting backbone of pristine qHPC$_{60}$ while introducing localized electronic states that alter the bandgap and enable new transition channels. Nitrogen encapsulation produces intragap states with potential relevance for discrete optical emission, whereas cerium and strontium generate 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 robustness of the electronic structure under reduced concentrations indicates that the fully encapsulated limit adequately represents the system. Overall, the findings highlight impurity-endowed qHPC$_{60}$ as a promising platform for optoelectronic and light-harvesting applications.

Figures (6)

• Figure 1: Structures of qHPC$_{60}$ and endohedral variations. Pristine qHPC$_{60}$ (a), encapsulation with N (b), Ce (c), and Sr (d), and encapsulation fractions of 100% (e), 75% (f), 50% (g), and 25% (h). Here, X denotes a generic encapsulated atom (N, Ce, or Sr).
• Figure 2: Electronic band structures for pristine qHPC$_{60}$ (a), N@EqHPC$_{60}$ (b), Ce@EqHPC$_{60}$ (c), and Sr@EqHPC$_{60}$ (d).
• Figure 3: Electronic densities of states for pristine qHPC$_{60}$ (a), N@EqHPC$_{60}$ (b), Ce@EqHPC$_{60}$ (c), and Sr@EqHPC$_{60}$ (d). Energies are aligned to $E_\mathrm{F}=0$. The shaded region indicates the bandgap.
• Figure 4: Electronic band structures of endohedral qHPC$_{60}$ systems at different filling concentrations. Panels (a-c) correspond to N@EqHPC$_{60}$, (d-f) to Ce@EqHPC$_{60}$, and (g-i) to Sr@EqHPC$_{60}$, with filling values of 75%, 50%, and 25%, respectively.
• Figure 5: Simulated absorption spectra ($\alpha$), refractive index ($\eta$), and reflectivity ($R$) as a function of photon energy for pristine qHPC$_{60}$ (a,e,i), N@EqHPC$_{60}$ (b,f,j), Ce@EqHPC$_{60}$ (c,g,k), and Sr@EqHPC$_{60}$ (d,h,l). Solid, dashed, and dotted lines correspond to E$\parallel$X, E$\parallel$Y, and E$\parallel$Z polarization directions, respectively. Colored bands indicate the visible spectrum range.