Monoclinic and orthorhombic cysteine crystals are small gap insulators

The electronic band structure of both the monoclinic and orthorhombic cysteine polymorphs is investigated using density functional theory (DFT) calculations in the generalized gradient approximation (GGA). Besides, orthorhombic cysteine crystals were grown, and their optical absorption was measured, being estimated an energy gap of E g exp = 4.68 eV for recrystallized cysteine powder. The existence of a small secondary optical absorption structure for the recrystallized sample around 4.4 eV suggests the existence of defect levels inside the band gap. The calculated valence and conduction bands are very flat for both cysteine polymorphs, suggesting that they are insulators.