Using a density-functional framework, we investigate the vibrational spectra of vitreous ${\text{SiO}}_2$ to determine to what extent these spectra provide information about the medium-range structure of the oxide network. We carry out a comparative study involving three model structures, which all feature a nondefective network of corner-sharing tetrahedra but differ through their Si-O-Si bond-angle distributions and ring statistics. We first address the results of typical diffraction probes. Fair agreement with experiment is achieved for the total neutron and total x-ray structure factors of all models, indicating limited sensitivity of these structure factors to the medium-range structure. The same consideration also applies to the Si-O and O-O partial structure factors. At variance, the Si-Si partial structure factor is found to be highly sensitive to the Si-O-Si bond-angle distribution. We then address typical vibrational spectra, such as the inelastic neutron spectrum, the infrared spectra, and the Raman spectra. For the inelastic neutron spectrum and the infrared spectra, the comparison with experiment is fair for all models, indicating poor sensitivity to the structural arrangement of tetrahedra. The only noticeable exception is the feature at $\sim 800{\text{cm}}^{-1}$ which shifts to higher frequencies with decreasing Si-O-Si angles. At variance, the Raman spectra are shown to be very informative about the medium-range organization of the network through their sensitivity to the concentrations of three-membered and four-membered rings. Our study indicates that the considered experimental data are globally consistent with a medium-range structure characterized by an average Si-O-Si bond angle of $148°$ and with small-ring concentrations as derived from the intensities of the experimental Raman defect lines. To describe the infrared and Raman couplings, our work also introduces parametric models which reproduce well the spectra calculated from first principles.

We analyze the principal vibrational spectra of vitreous ${\mathrm{GeO}}_2$ and derive therefrom structural properties referring to length scales beyond the basic tetrahedral unit. We generate a model structure that yields a neutron structure factor in accord with experiment. The inelastic-neutron, the infrared, and the Raman spectra, calculated within a density-functional approach, also agree with respective experimental spectra. The accord for the Raman spectrum supports a Ge-O-Ge angle distribution centered at 135°. The Raman feature $X_2$ is found to result from vibrations in three-membered rings, and therefore constitutes a distinctive characteristic of the medium-range structure.

The electronic energy levels of some representative isolated and oxide-supported organic dyes, relevant for photovoltaic applications, are investigated using many-body perturbation theory within the GW approximation. We consider a set of all-organic dyes (denominated L0, L2, L3, and L4) featuring the same donor and anchor groups and differing for the linker moieties. We first calculate the energy levels of the isolated molecules, thus allowing us to address the effects of the different linker groups, and resulting in good agreement with photo-electron spectroscopic and electrochemical data. We then consider the L0 dye adsorbed on the (101) surface of anatase-TiO₂. We find a density of occupied states in agreement with experimental photo-electron data. The HOMO-LUMO energy gap of the L0 dye is found to be reduced by ∼1 eV upon adsorption. Our results validate the reliability of GW calculations for photovoltaic applications and point to their potential as a powerful tool for the screening and rational design of new components of electrochemical solar cells.

We report on a first-principles investigation of the structural and vibrational properties of vitreous germania $\left(v\text{−}\mathrm{Ge}{\mathrm{O}}_2\right)$. Our work focuses on a periodic model structure of 168 atoms, but three smaller models are also studied for comparison. We first carry out a detailed structural analysis both in real and reciprocal spaces. Our study comprises the partial pair correlation functions, the angular distributions, the total neutron correlation function, the neutron and $x$-ray total structure factors, and the Faber-Ziman and Bhatia-Thornthon partial structure factors. We find overall good agreement with available experimental data. We then obtain the vibrational frequencies and eigenmodes. We analyze the vibrational density of states in terms of Ge and O motions, and further in terms of rocking, bending, and stretching contributions. The inelastic neutron spectrum is found to differ only marginally from the vibrational density of states. Using a methodology based on the application of finite electric fields, we derive dynamical Born charge tensors and Raman coupling tensors. For the infrared spectra, we calculate the real and imaginary parts of the dielectric function, including the high-frequency and static dielectric constants. The Raman spectra are shown to be sensitive to the medium-range structure and support an average Ge-O-Ge angle of 135°. We identify the shoulder $X_2$ as a signature of breathing O vibrations in three-membered rings. Four-membered rings are found to contribute to the main Raman peak. We advance an interpretation for the shoulder $X_1$ in terms of delocalized bond-bending modes. We derive bond polarizability parameters from the calculated Raman coupling tensors and demonstrate their level of reliability in reproducing the spectra. The calculated vibrational spectra all show good agreement with the respective experimental spectra.

Using a density-functional scheme, we investigate the electronic, structural, and vibrational properties of amorphous silicon nitride. Through a Car-Parrinello molecular-dynamics simulation, we generate a model structure formed mainly by a network of ${\text{SiN}}_4$ tetrahedra a large fraction of which are edge sharing. Only a small fraction of atoms are overcoordinated and undercoordinated. First, the structural properties such as angular distributions, atomic arrangements in first-neighbor shells, the neutron total structure factor, the radial distribution function, and pair-correlation functions are examined. Next, the electronic properties are analyzed by considering the quasiparticle density of states which is calculated through the $GW$ method. Good agreement is found with experimental data when available. Successively, we focus on a range of vibrational spectra. First, the vibrational density of states is analyzed in terms of its decomposition into N and Si contributions. Then, we investigate the Born effective charge tensors, the high-frequency, and static dielectric constants and calculate the real and imaginary parts of the dielectric function in the infrared. Therefrom we obtain the infrared-absorption spectrum and the refractive index that are found to be in accord with experimental measurements. Moreover, we address the Raman spectrum which is compared with available experimental data. Electronic structure and vibrational properties of the point defects present in our model are also discussed. Density-functional and $GW$ schemes appear to be appropriate for modeling materials based on silicon nitride. In particular, our modeling of silicon nitride achieved a successful level of comparison with experiments. This allows us to infer that $a{\text{-Si}}_3{\text{N}}_4$ features a high content of edge-sharing tetrahedra, which are absent in the crystalline phases of silicon nitride at ambient conditions.