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.