Search

Using a combination of the density functional theory and tight-binding calculations, we study electronic and optical properties of asymmetric chevron-type graphene nanoribbons recently synthesized with atomic precision. We demonstrate that the low-energy optical selection rules in such infinite chiral ribbons are more reminiscent of those for zigzag rather than for armchair ribbons. It is also shown that, starting from about 25 nm long ribbons, the low-energy absorption and therefore the selection rules of infinitely long ribbons are well reproduced in the finite cluster approach. Hence, ribbons longer than 25 nm (about 28 unit cells) can be treated as infinitely long ones.

We present ab initio quantum calculation of the optical properties of formamide in vapor phase and in water solution. We employ time dependent density functional theory for the isolated molecule and many-body perturbation theory methods for the system in solution. An average over several molecular dynamics snapshots is performed to take into account the disorder of the liquid. We find that the excited state properties of the gas-phase formamide are strongly modified by the presence of the water solvent: the geometry of the molecule is distorted and the electronic and optical properties are severely modified. The important interaction among the formamide and the water molecules forces us to use fully quantum methods for the calculation of the excited state properties of this system. The excitonic wave function is localized both on the solute and on part of the solvent.

We modify the analytic linear tetrahedron method to calculate the optical properties of surfaces. As st test case, the reflectance anisotropy of InP(110) is calculated within the density-functional theory in the local-density approximation. The convergence with respect to the number of k points and the choice of the triangles is extensively discussed. The resulting spectra are interpreted and compared with experimental results.

We compute the optical properties of the (110) surface of gallium arsenide within the first-principles density-functional theory local-density approximation scheme, using norm-conserving pseudopotentials. Starting from the surface electronic structure calculation, we analyze the imaginary part of the theoretical dielectric function, separating surface and bulk contributions. The effects of the nonlocality of the pseudopotential are studied, by working both in the transverse gauge (neglecting them) and in the longitudinal gauge (where they are automatically included). The two calculations, although giving different dielectric functions, yield the same reflectance anisotropy, which compares well with experimental data and with previous theoretical results.