L-Serine Anhydrous Crystals: Structural, Electronic, and Optical Properties by First-Principles Calculations, and Optical Absorption Measurement

The X-ray diffraction data of l-serine anhydrous crystals was taken into account to initialize the total energy minimization process of their unit cell through density functional theory (DFT) computations, which were performed within both the local density and generalized gradient approximations with dispersion, LDA, and GGA+D, respectively. The calculated lattice parameters are in good agreement with the experimental results for the dispersion corrected generalized gradient approximation functional, with a unit cell volume larger by only about 0.32%; the Mulliken and Hirschfield charges show the zwitterionic state of the L-serine molecules in the DFT converged crystals. The electronic (band structure, density of states) and optical absorption properties were calculated to explain the light absorption of the L-serine anhydrous crystalline powder we have measured at room temperature. The optical absorption related to transitions between the top of the valence band and the bottom of the conduction band involves O-2p valence states and H-1s conduction states. The LDA (4.74 eV) and GGA+D (4.75 eV) estimated energy gaps are about 1 eV below the estimated value from optical absorption measurements (5.90 eV). Small values were obtained for the electron effective masses, which are almost isotropic, whereas large anisotropic values were found for hole effective masses, suggesting that the L-serine anhydrous crystal behaves like an n-type wide gap semiconductor. Different dielectric function profiles obtained for some of the most important symmetry directions also demonstrate the optical anisotropy of L-serine anhydrous crystals.