Controlling the gap of fullerene microcrystals by applying pressure: Role of many-body effects

We characterize the optical properties of C60 fullerene microcrystals as a function of hydrostatic pressure. Calculations were done using first-principles many-body theories based on evaluating electronic energy levels in the GW approximation. We compute electronic excited states in the crystal by diagonalizing the Bethe-Salpeter equation (BSE). Our results confirm the existence of bound excitons in the crystal. Both the electronic gap and optical gap decrease continuously and non-linearly as pressure of up to 6 GPa is applied. As a result, the absorption spectrum shows strong redshift. We also observe that 'negative' pressure shows the opposite behavior: the gaps increase and the optical spectrum shifts toward the blue end of the spectrum. Negative pressure can be realized by adding cubane (C8H8) or other molecules with similar size to the interstitials of the microcrystal. For the moderate lattice distortions studied here, we have found that the optical properties of fullerene microcrystals with intercalated cubane are similar to the ones of an expanded undoped microcrystal. Based on these findings, we propose doped C60 as active element in piezo-optical devices.