Structural and vibrational properties of wurtzite ZnO with oxygen-deficient defects: ab initio and potential-based calculations.

The structural and vibrational properties of the ZnO wurtzite phase with oxygen vacancies in different charged states are studied using first-principles and potential-based methods. The calculations based on density-functional theory are performed to determine the atomic configurations around defects. The DFT results are discussed and compared with those obtained using the static lattice method in the traditional shell model. Both computational approaches predict the same character of crystal lattice relaxation around oxygen vacancies. The phonon local symmetrized densities of states are calculated using the Green function method. The frequencies of localized vibrations of various symmetry types induced by oxygen vacancies in neutral and positively charged states are determined. The calculation results allow estimating the effect of oxygen vacancies on the formation of the intense Raman peak.

Defect structure and vibrational states in Eu-doped cubic gadolinium oxide.

We consider an effect of trivalent Eu impurities occupying two different crystallographic positions in cubic gadolinium oxide on its lattice structure and phonon spectrum. The numerical methods employed in the study take into account the shell model to describe interatomic interactions. Using cluster approach, the equilibrium lattice structures and phonon local symmetrized densities of states are calculated. The frequencies of resonant vibrations of various symmetry types induced by europium ions are determined using the Green function method. The participation of ions located around Eu impurities in the symmetrical resonant vibrations is discussed. The calculation results show that europium ions in two non-equivalent structural sites are responsible for the formation of the strongest Raman peak associated with the resonant vibrations at oxygen ions in doped Gd2O3.

Carbon Bond Breaking under Ar+-Ion Irradiation in Dependence on sp Hybridization: Car-Parrinello, Ehrenfest, and Classical Dynamics Study.

The modeling of low-temperature plasma synthesis of low-dimensional carbon coatings remains a challenge, since the long-time spans must be simulated for structural relaxation. A proper theoretical method should be chosen to address possible charge-transfer processes. Considering the possibility of linear-chained carbon (LCC) synthesis simulation, a numerical study of the C-C bond breaking under slow argon-ion irradiation is performed for the model molecules of 2,2,3,3-tetramethylbutane (sp3 hybridization), 1,2,3-butatriene (sp2), and 2-butyne (sp). Threshold energies of bond breaking are calculated for the carbon atoms in all three hybridizations. Three levels of theory are applied, and the results are compared with experiment. Based on the accuracy of the values obtained, an approach is proposed for modeling the ion-assisted plasma synthesis.

Energy band gaps and excited states in Si QD/SiO x /R y O z (R = Si, Al, Zr) suboxide superlattices.

X-ray and optical spectroscopies were applied in order to study the band structure and electronic excitations of the SiO x /R y O z (R = Si, Al, Zr) suboxide superlattices. The complementary x-ray emission and absorption measurements allow for the band gap values for the SiO x layers to be established, which are found to have almost no dependency on the cation type R. It is determined that, after annealing, the stoichiometric factor x remains near 1.8 in all the systems under study, implying that the silicon quantum dot synthesis reaction is not fully completed. It is shown that the SiO x /Al2O3 multilayer contains octahedral structural motifs (SiO6) usually found in stishovite, whereas SiO x /SiO2 and SiO x /ZrO2 demonstrate an electronic structure similar to conventional silica. The intrinsic electronic excited states are examined by means of synchrotron-excited photoluminescence spectroscopy. Low-energy UV-excited luminescence of SiO x layers is found to have the same spectrum in all of the studied structures, while VUV-excited spectra strongly depend on the cation R. In these measurements, manifestations of 'slow' exciton-mediated and 'fast' defect-related luminescence are distinguished using nanosecond time resolution. It is shown that both mobile and bounded excitons appear in the suboxide layer under 6.2 eV and 5.8 eV irradiation and then relax radiatively through the triplet-singlet transition of the neighbouring oxygen-deficient centers. The complete picture of the optical excitation and relaxation processes in these materials is illustrated in a general diagram depicting electronic states.