Anhydrous proline crystals: Structural optimization, optoelectronic properties, effective masses and Frenkel exciton energy

The anhydrous orthorhombic L-proline crystal P2(1)2(1)2(1) (Z = 4) was studied using density functional theory (DFT) calculations to obtain its structural, electronic and optical properties. Optical absorption measurements were carried out as well, and the resulting absorption spectrum was compared with theoretical results. It is shown that pure LDA and GGA functionals fail to obtain lattice parameters close to experiment, being required the inclusion of a dispersion correction scheme to achieve good results. A complex electronic structure with eight very close indirect band gaps starting at 4.87 eV with a 12 meV energy range is hinted by the simulations. Applying the Delta-sol scheme to correct the DFT band gap, a theoretical gap estimate of 5.50 eV was obtained, which is very close to the experimental value of 5.54 eV extracted from the optical absorption experimental data. Calculated effective masses of electrons and holes in anhydrous proline are very anisotropic and large, as well as the complex dielectric function and the optical absorption. This is likely due to the way proline molecules inside the crystal form permanent electron dipoles interacting through hydrogen bonds and van der Waals forces. Furthermore, the Frenkel exciton binding energy in proline was experimentally and theoretically found to be about 1.3 eV.