Optical absorption and electronic band structure first-principles calculations ofα-glycine crystals

Light absorption of $\ensuremath{\alpha}$-glycine crystals grown by slow evaporation at room temperature was measured, indicating a $5.11\ifmmode\pm\else\textpm\fi{}0.02\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ energy band gap. Structural, electronic, and optical absorption properties of $\ensuremath{\alpha}$-glycine crystals were obtained by first-principles quantum mechanical calculations using density functional theory within the generalized gradient approximation in order to understand this result. To take into account the contribution of core electrons, ultrasoft and norm-conserving pseudopotentials, as well as an all electron approach were considered to compute the electronic density of states and band structure of $\ensuremath{\alpha}$-glycine crystals. They exhibit three indirect energy band gaps and one direct $\ensuremath{\Gamma}\text{\ensuremath{-}}\ensuremath{\Gamma}$ energy gap around $4.95\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The optical absorption related to transitions between the top of the valence band and the bottom of the conduction band involves $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ valence states and $\mathrm{C},\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ conduction states, with the carboxyl group contributing significantly to the origin of the energy band gap. The calculated optical absorption is highly dependent on the polarization of the incident radiation due to the spatial arrangement of the dipolar glycine molecules; in the case of a polycrystalline sample, the first-principles calculated optical absorption is in good agreement with the measurement when a rigid energy shift is applied.