Produção Nacional
Optical absorption and DFT calculations in L -aspartic acid anhydrous crystals: Charge carrier effective masses point to semiconducting behavior
2012; American Physical Society; Volume: 86; Issue: 19 Linguagem: Inglês
10.1103/physrevb.86.195201
ISSN1550-235X
AutoresA. M. Silva, B. P. Silva, F. A. M. Sales, V. N. Freire, E. Moreira, U. L. Fulco, E.L. Albuquerque, F. F. Maia, E. W. S. Caetano,
Tópico(s)Solid-state spectroscopy and crystallography
ResumoDensity functional theory (DFT) computations within the local-density approximation and generalized gradient approximation in pure form and with dispersion correction (GGA+D) were carried out to investigate the structural, electronic, and optical properties of $L$-aspartic acid anhydrous crystals. The electronic (band structure and density of states) and optical absorption properties were used to interpret the light absorption measurements we have performed in $L$-aspartic acid anhydrous crystalline powder at room temperature. We show the important role of the layered spatial disposition of $L$-aspartic acid molecules in anhydrous $L$-aspartic crystals to explain the observed electronic and optical properties. There is good agreement between the GGA+D calculated and experimental lattice parameters, with ($\ensuremath{\Delta}a$, $\ensuremath{\Delta}b$, $\ensuremath{\Delta}c$) deviations of ($0.029,\ensuremath{-}0.023,\ensuremath{-}0.024$) (units in \AA{}). Mulliken [J. Chem. Phys. 23, 1833 (1955)] and Hirshfeld [Theor. Chim. Acta 44, 129 (1977)] population analyses were also performed to assess the degree of charge polarization in the zwitterion state of the $L$-aspartic acid molecules in the DFT converged crystal. The lowest-energy optical absorption peaks related to transitions between the top of the valence band and the bottom of the conduction band involve O 2$p$ valence states and C 1$p$ and O 2$p$ conduction states, with the carboxyl and COOH lateral chain group contributing significantly to the energy band gap. Among the calculated band gaps, the lowest GGA+D (4.49-eV) gap is smaller than the experimental estimate of 5.02 eV, as obtained by optical absorption. Such a wide-band-gap energy together with the small carrier effective masses estimated from band curvatures allows us to suggest that an $L$-aspartic acid anhydrous crystal can behave as a wide-gap semiconductor. A comparison of effective masses among directions parallel and perpendicular to the $L$-aspartic molecules layers reveals that charge transport must be favored in the former case. Finally, we also show that there is a strong optical anisotropy in the dielectric function of $L$-aspartic acid anhydrous crystals.
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