Gotthard Seifert

An accurate total energy calculation is essential in materials computation. To date, many tight-binding (TB) approaches based on parameterized hopping can produce electronic structures comparable to those obtained using first-principles calculations. However, TB approaches still have limited applicability for determining material properties derived from the total energy. That is, the predictive power of the TB total energy is impaired by an inaccurate evaluation of the repulsive energy. The complexity associated with the parametrization of TB repulsive potentials is the weak link in this evaluation. In this study, we propose a new method for obtaining the pairwise TB repulsive potential for crystalline materials by employing the Chen-Möbius inversion theorem. We show that the TB-based phonon dispersions, calculated using the resulting repulsive potential, compare well with those obtained by first-principles …

We elucidate the flexoelectricity of materials in the high strain gradient regime, of which the underlying mechanism is less understood. By using the generalized Bloch theorem, we uncover a strong flexoelectric-like effect in bent thinfilms of Si and Ge due to a high strain gradient-induced insulator-to-metal transition. We show that an unusual type-II band alignment is formed between the compressed and elongated sides of the bent film, resulting in a spatial separation of electron and hole. Therefore, upon the insulator-to-metal transition, electrons transfer from the compressed side to the elongated side to reach the thermodynamic equilibrium, leading to pronounced polarization along the film thickness dimension. The obtained transverse flexoelectric coefficients are unexpectedly high, with a quadratic dependence on the film thickness. This new mechanism is extendable to other semiconductor materials with moderate energy gaps. Our findings have important implications for the future applications of flexoelectricity in semiconductor materials.

The pertechnetate ion TcVIIO4– is a nuclear fission product whose major issue is the high mobility in the environment. Experimentally, it is well known that Fe3O4 can reduce TcVIIO4– to TcIV species and retain such products quickly and completely, but the exact nature of the redox process and products is not completely understood. Therefore, we investigated the chemistry of TcVIIO4– and TcIV species at the Fe3O4(001) surface through a hybrid DFT functional (HSE06) method. We studied a possible initiation step of the TcVII reduction process. The interaction of the TcVIIO4– ion with the magnetite surface leads to the formation of a reduced TcVI species without any change in the Tc coordination sphere through an electron transfer that is favored by the magnetite surfaces with a higher FeII content. Furthermore, we explored various model structures for the immobilized TcIV final products. TcIV can be incorporated …

We elucidate the flexoelectricity of semiconductors in the high strain gradient regime, the underlying mechanism of which is less understood. By using the generalized Bloch theorem, we uncover a strong flexoelectric-like effect in bent thinfilms of Si and Ge due to a high-strain-gradient-induced band gap closure. We show that an unusual type-II band alignment is formed between the compressed and elongated sides of the bent film. Therefore, upon the band gap closure, electrons transfer from the compressed side to the elongated side to reach the thermodynamic equilibrium, leading to a pronounced change of polarization along the film thickness dimension. The obtained transverse flexoelectric coefficients are unexpectedly high with a quadratic dependence on the film thickness. This new mechanism is extendable to other semiconductor materials with moderate energy gaps. Our findings have important …

The structural prerequisites are investigated, which make star-shaped molecules suitable precursors for the formation of columnar liquid-crystalline phases. Electronic structure calculations on smaller mesogens show that not all conformers exhibit the atomistic structure, the stability against distortion, and additional dipole moments, which favour columnar stacking. For the presently studied compounds with short terminating alkyl chains, the calculations indicate that the steric factor becomes dominant with increasing star size. Thus, the optimised geometric structures were employed to generate a simplified mathematical model of the structures, which accounts only for the steric interaction in the larger stars. With the help of these diagrams, the most common conformers of star-shaped molecules can be derived in a systematic fashion.

A novel route is reported for the processing of nanocomposites consisting of multi-walled carbon nanotubes (MWNT) embedded in amorphous SiO2. SiO2 is used for the sintered composite matrix and SiOx either for the unsintered or for the interface region with the MWNT. The material has been characterized by high-resolution transmission electron microscopy, high-resolution electron energy loss spectroscopy and thermal gravimetric analysis. Based on our observations, theoretical simulations were performed which were based on tight binding calculations. Models are proposed which acount for a stable SiOx/tube interface.

More than 150 different structural defects in periodic molybdenum disulfide (Mo S 2) monolayers were investigated systematically for the present study by density-functional calculations. All defects were chosen to be triangular, but of different size and distance from one another. Molecular-dynamics simulations proved the defect stability against temperature effects. The energetic properties of the defective Mo S 2 monolayers––especially the density of states––can be viewed as a combination of edge and bulk properties with the defect size being of dominating influence. A dependence on the defect-edge termination was present only in the very smallest defects. The definition of a localization parameter helps to characterize the grade of localization of the states. Finally, a strong influence of the defect distance on the electronic properties could not be observed, except for very small distances.

Fewest-switches surface hopping (FSSH) is employed in order to investigate the nonadiabatic excited-state dynamics of thiophene and related compounds and hence to establish a connection between the electronic system, the critical points in configuration space and the deactivation dynamics. The potential-energy surfaces of the studied molecules were calculated with complete active space self-consistent field and time-dependent density-functional theory. They are analyzed thoroughly to locate and optimize minimum-energy conical intersections, which are essential to the dynamics of the system. The influence of decoherence on the dynamics is examined by employing different decoherence schemes. We find that irrespective of the employed decoherence algorithm, the population dynamics of thiophene give results which are sound with the expectations grounded on the analysis of the potential …