Departamento de Bioquímica, Universidade Federal do Rio Grande do Sul, 90035-003 Porto Alegre-RS, Brazil.
Universidade Estadual do Piauí, 64260-000 Piripiri-Pi, Brazil.
J Chem Phys. 2014 Mar 28;140(12):124511. doi: 10.1063/1.4869179.
Results of optical absorption measurements are presented together with calculated structural, electronic, and optical properties for the anhydrous monoclinic L-asparagine crystal. Density functional theory (DFT) within the generalized gradient approximation (GGA) including dispersion effects (TS, Grimme) was employed to perform the calculations. The optical absorption measurements revealed that the anhydrous monoclinic L-asparagine crystal is a wide band gap material with 4.95 eV main gap energy. DFT-GGA+TS simulations, on the other hand, produced structural parameters in very good agreement with X-ray data. The lattice parameter differences Δa, Δb, Δc between theory and experiment were as small as 0.020, 0.051, and 0.022 Å, respectively. The calculated band gap energy is smaller than the experimental data by about 15%, with a 4.23 eV indirect band gap corresponding to Z → Γ and Z → β transitions. Three other indirect band gaps of 4.30 eV, 4.32 eV, and 4.36 eV are assigned to α3 → Γ, α1 → Γ, and α2 → Γ transitions, respectively. Δ-sol computations, on the other hand, predict a main band gap of 5.00 eV, just 50 meV above the experimental value. Electronic wavefunctions mainly originating from O 2p-carboxyl, C 2p-side chain, and C 2p-carboxyl orbitals contribute most significantly to the highest valence and lowest conduction energy bands, respectively. By varying the lattice parameters from their converged equilibrium values, we show that the unit cell is less stiff along the b direction than for the a and c directions. Effective mass calculations suggest that hole transport behavior is more anisotropic than electron transport, but the mass values allow for some charge mobility except along a direction perpendicular to the molecular layers of L-asparagine which form the crystal, so anhydrous monoclinic L-asparagine crystals could behave as wide gap semiconductors. Finally, the calculations point to a high degree of optical anisotropy for the absorption and complex dielectric function, with more structured curves for incident light polarized along the 100 and 101 directions.
呈现了无水单斜 L-天冬酰胺晶体的光学吸收测量结果以及计算得到的结构、电子和光学性质。采用密度泛函理论（DFT）在广义梯度近似（GGA）内包括色散效应（TS，Grimme）来进行计算。光学吸收测量表明，无水单斜 L-天冬酰胺晶体是一种宽带隙材料，主带隙能量为 4.95 eV。另一方面，DFT-GGA+TS 模拟产生的结构参数与 X 射线数据非常吻合。理论与实验之间的晶格参数差异 Δa、Δb、Δc 分别小至 0.020、0.051 和 0.022 Å。计算得到的带隙能比实验数据小约 15%，具有对应于 Z→Γ 和 Z→β 跃迁的 4.23 eV 间接带隙。另外三个间接带隙分别为 4.30 eV、4.32 eV 和 4.36 eV，分别对应于 α3→Γ、α1→Γ 和 α2→Γ 跃迁。另一方面，Δ-sol 计算预测主带隙为 5.00 eV，仅比实验值高 50 meV。电子波函数主要源于 O 2p-羧基、C 2p-侧链和 C 2p-羧基轨道，分别对最高价和最低导带能量贡献最大。通过改变晶格参数使其偏离其收敛的平衡值，我们表明单元沿着 b 方向比沿着 a 和 c 方向更柔软。有效质量计算表明，空穴输运行为比电子输运行为更各向异性，但质量值允许一定的电荷迁移，除了沿着垂直于形成晶体的 L-天冬酰胺分子层的方向以外，因此无水单斜 L-天冬酰胺晶体可能表现为宽隙半导体。最后，计算表明吸收和复介电函数具有高度的各向异性，对于沿 100 和 101 方向极化的入射光，曲线更具结构。
