Electronic, vibrational, and optical properties of fullerene-S₈ co-crystals

Sulfur and fullerenes are well-known materials that have received significant attention over many years and fullerene-S₈ co-crystals have been reported recently. Here, via density functional theory (DFT) calculations, we shed light on the electronic, vibrational, and optical properties of the C₆₀-2S₈ and C₇₀-2S₈ co-crystals. In both co-crystals, the holes and electrons are characterized by very small effective masses that are comparable to those derived in fullerene single crystals. Interestingly, the S₈ molecules are found not to contribute to charge transport as the calculations show that both types of carriers move over the networks formed by the fullerene molecules. Calculations of the excited electronic states point to the formation of charge-transfer states, where electrons are transferred from fullerene molecules to S₈ molecules, i.e., the fullerenes act in these co-crystals as electron donors. However, the energies of these charge-transfer states in both C₆₀-2S₈ and C₇₀-2S₈ are higher than those of several excited states localized on C₆₀ or C₇₀; therefore, the charge-transfer states play no role in the low-energy part of the absorption spectrum. In agreement with experimental data, the calculations also show that the fullerene-S₈ inter-molecular interactions are very weak in the ground state; as a result, the infrared (IR) spectra of the co-crystals represent a simple superposition of the spectra of S₈ and fullerene molecules. Since the spectra are largely silent in the long-wave IR region (800-1250 cm⁻¹), these fullerene-S₈ co-crystals are potential candidates for thermal IR imaging applications.