Structural and Optoelectronic Properties of the α-, β-, and γ-Glycine Polymorphs and the Glycine Dihydrate Crystal: A DFT Study

Density functional calculations were performed to study the properties of the three main glycine solid-state polymorphs alpha, beta, and gamma and the glycine dehydrate (GDH) crystal. Optimized unit cell geometries, Kohn-Sham electron energy bands, electron densities of states, population charges, carrier effective masses, optical absorption, and complex dielectric functions were obtained for each glycine system using a GGA functional plus the TS dispersion correction, leading to lattice parameters very close to the experimental values. The theoretical fundamental gaps of all glycine crystals are indirect and near 5 eV. The carrier effective masses are anisotropic and especially heavy (similar to 26 free electron masses at most) for holes in the case of the GDH crystal. The optical absorption and the dielectric function are very sensitive to the polarization of light in all glycine-based crystals.