Tuning the electronic and optical properties of impurity-engineered two-dimensional graphullerene half-semiconductors

A novel material consisting of a monolayer of ${\mathrm{C}}_{60}$ buckyballs with hexagonal symmetry has recently been observed experimentally, named graphullerene. In this study, we present a comprehensive ab initio theoretical analysis of the electronic and optical properties of both pristine and impurity-engineered monolayer graphullerene using spin-dependent density functional theory (spin-DFT). Our findings reveal that graphullerene is a direct band gap semiconductor with a band gap of approximately 1.5 eV at the $\mathrm{\ensuremath{\Gamma}}$ point, agreeing well with experimental data. Notably, we demonstrate that adding impurities, in particular substitutional nitrogen, substitutional boron, or adsorbent hydrogen, to graphullerene results in the formation of spin-dependent deep donor and deep acceptor levels, thereby giving rise to a variety of half-semiconductors. Using an Ising-type Hamiltonian, we attribute the spin-dependent impurity band splittings to a strong exchange interaction of about 0.6 eV for N, 0.8 eV for B, and 1.0 eV for H between the impurity/adatom spin and the total angular momentum of the electrons in the surrounding carbon atoms in graphullerene. All the impurities exhibit a magnetic moment of approximately ${\ensuremath{\mu}}_{B}$ per impurity. This impurity engineering enables the tuning of spin-polarized exciton properties in graphullerene, with spin-dependent band gap energies ranging from 0.43 eV ($\ensuremath{\lambda}\ensuremath{\sim}2.9\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{m}$) to 1.5 eV ($\ensuremath{\lambda}\ensuremath{\sim}820$ nm), covering the near-infrared (NIR) and short-wavelength infrared (SWIR) regimes. Our results suggest that both pristine and impurity-engineered graphullerene have significant potential for the development of carbon-based 2D semiconductor spintronic and opto-spintronic devices.