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

Abstract

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. © 2023 The Royal Society of Chemistry.