Publications

Surface passivation by hydrogenated amorphous silicon nitride (a-SiN x :H) is determined by the combined effect of two mechanisms: so-called chemical passivation by reducing the density of interface states (D it) and field-effect passivation as a result of the number of fixed charges (Q f) at the interface with Si. These are related to the K defect center: *Si-N3. Defects near the interface (in both Si as in a-SiN x :H), modeled by force field Molecular Dynamics (MD) and ab initio Density Functional Theory (DFT), can be related to Q f and D it measured experimentally using CV-MIS (Capacitance–Voltage Metal–Insulator–Semiconductor). The compositional build up at the interface as is determined by HRTEM (High Resolution Transmission Electron Microscopy) and modeled by MD corresponds to each other; a gradual change from Si to the bulk a-SiN x :H composition in the first 2nm of the a-SiN x :H layer. At the c-Si side a highly distorted layer (about 1–3nm) caused by the insertion of N and/or H is found. The insertion and adhesion of N into and at the Si surface is called nitridation and can be altered by using a NH3 plasma prior to a-SiN x :H deposition. HRTEM image analysis shows that by varying the nitridation of the Si surface the amount and penetration depth of N inside the Si surface is altered. Using MD modeling, it is shown that this process changes the amount of K-centers at the surface, which explains the variation in Q f and D it that is found experimentally. Ab initio DFT studies of a-SiN x :H (x=1.17) show that K-centers and Si atoms in distorted configuration, are the dominating defects resulting in a higher D it. For lower x (x=1) the D it caused by K-centers increases, which is observed experimentally too.

The reduction of a thin surface oxide on the Rh(1 1 1) surface by CO is studied in situ by photoemission spectroscopy, scanning tunneling microscopy, and density functional theory. CO molecules are found not to adsorb on the surface oxide at a sample temperature of 100 K, in contrast to on the clean and chemisorbed oxygen covered surface. Despite this behavior, the surface oxide may still be reduced by CO, albeit in a significantly different fashion as compared to the reduction of a phase containing only chemisorbed on surface oxygen. The experimental observations combined with theoretical considerations concerning the stability of the surface oxide, result in a model of the reduction process at these pressures suggesting that the surface oxide behaves as a source of oxygen for the CO-oxidation reaction.

Standard density functionals without van der Waals interactions yield an unsatisfactory description of ice phases, specifically, high density phases occurring under pressure are too unstable compared to the common low density phase Ih observed at ambient conditions. Although the description is improved by using functionals that include van der Waals interactions, the errors in relative volumes remain sizable. Here we assess the random phase approximation (RPA) for the correlation energy and compare our results to experimental data as well as diffusion Monte Carlo data for ice. The RPA yields a very balanced description for all considered phases, approaching the accuracy of diffusion Monte Carlo in relative energies and volumes. This opens a route towards a concise description of molecular water phases on surfaces and in cavities.

An approach is proposed to evaluate the average velocities of adsorbate molecules in one-dimensional nanopore, when quantum tunneling between neighboring potential minima leads to nonzero velocity. The approach is used to calculate the hydrogen isotope molecule (H2, D2, T2) velocities in ultrathin carbon single-wall nanotubes (SWNT) (3,3) and (6,0). It is shown that the isotope mass difference leads to large differences of the quantum tunneling value and large differences of the average molecule velocities, especially inside the tube (6,0). It is shown that different tube chirality leads to drastically different velocities of adsorbate molecules, even if the diameters of both nanotubes do not differ significantly.

The adsorption dynamics of H2 on Pd(111) alloyed with V has been studied using molecular beam experiments (MB) and ab initio density functional theory (DFT) calculations. The experiments show that dissociation on pure Pd(111) is dominated by dynamical steering and therefore shows strong rotational hindering. The alloy surface exhibits a sticking probability independent of rotational excitation and an isotropic distribution of inelastically scattered molecules, attributed to a dynamical precursor. DFT calculations of the potential energy surface and ab initio simulations of scattering events demonstrate that the different behavior of Pd and Pd/V surfaces arises from the existence of a flat physisorption state.

The adsorption and dissociation of molecular water on the defective (001) surface of galena (PbS) have been investigated using both periodic and embedded cluster electronic structure methods. The results show that in the periodic calculations, the energies are particularly sensitive to the thickness of the simulated slab. The predicted activation and reaction energies from the different approaches are in good agreement given the different levels of theory used, and show that water will easily dissociate at a surface step site, with the product structure being the more stable in such an environment.

We show that a combination of the Generalized Davidson method and harmonic Ritz values (called harmonic Davidson) is well-suited for solving large interior eigenvalue problems using a plane wave basis. The algorithm enables us to calculate impurity and band edge states for systems of 100,000 atoms in about one day on 32 cores. We demonstrate the capabilities of the method by calculating the electronic states of a large GaAs quantum dot embedded in an AlAs matrix.

No abstract is provided for this article.

We present a systematic investigation of the role and importance of excitonic effects on the optical properties of transitions metal oxide perovskites. A representative set of fourteen compounds has been selected, including 3$d$ (SrTiO$_3$, LaScO$_3$, LaTiO$_3$, LaVO$_3$, LaCrO$_3$, LaMnO$_3$, LaFeO$_3$ and SrMnO$_3$), 4$d$ (SrZrO$_3$, SrTcO$_3$ and Ca$_2$RuO$_4$) and 5$d$ (SrHfO$_3$, KTaO$_3$ and NaOsO$_3$) perovskites, covering a band gap ranging from 0.1 eV to 6.1 eV and exhibiting different electronic, structural and magnetic properties. Optical conductivities and optical transitions including electron-hole interactions are calculated through the solution of the Bethe-Salpeter equation (BSE) with quasi-particle energies evaluated by single-shot $G_0W_0$ approximation. The exciton binding energies are computed by means of a model-BSE (mBSE), carefully benchmarked against the full BSE method, in order to obtain well-converged results in terms of k-point sampling. The predicted results are compared with available measured data, with an overall satisfactory agreement between theory and experiment.

Abstract The GW approximation represents the state-of-the-art ab-initio method for computing excited-state properties. Its execution requires control over a larger number of parameters, and therefore, its application in high-throughput studies is hindered by the complex and time-consuming convergence process across a multidimensional parameter space. To address these challenges, we develop a fully-automated open-source workflow for G0W0 calculations within the AiiDA framework and the projector augmented wave (PAW) method. The workflow is based on an efficient estimation of the errors in the quasi-particle (QP) energies due to basis-set truncation and ultra-soft PAW potentials norm violation, which allows a reduction in the dimensionality of the parameter space and avoids the need for multidimensional convergence searches. Protocol validation is conducted through a systematic comparison against established experimental and state-of-the-art GW data. To demonstrate the effectiveness of the approach, we construct a database of QP energies for a dataset of over 320 bulk structures.

We demonstrate the general ability of an extreme softening of non-normconserving Vanderbilt pseudopotentials. We discuss the important case of first-row elements and elements containing core d electrons. Explicitly, such extremely soft pseudopotentials are constructed for carbon, hydrogen, nitrogen, and gallium, the d electrons of which have to be treated as valence electrons. Using a plane-wave basis with cutoff energies between 13 and 16 Ry we observe no loss of accuracy compared to very hard and accurate pseudopotentials working at cutoff energies between 60 and 110 Ry.

Single wall carbon nanotubes with diameter 0.4 nm grown in the channels of AlPO4-5 crystals were studied by Raman spectroscopy. Up to 21 different laser lines were used to characterize vibrational properties. Spectra depend strongly on the energy of the laser line used for excitation. It was found that only two types of nanotubes with different chiralities, (5,0) and (4,2), are responsible for the spectra observed. The frequencies of the radial breathing modes were reliable assigned. A strong response was also observed for frequencies ∼1250 cm−1. The positions of two such peaks assigned to the (5,0) do not depend on the laser energy whereas only one such a peak was observed for the (4,2) nanotube. Its frequency shifts with the laser energy like the D line of large diameter nanotubes, but the rate of the shift is only a half of the value known for the latter. We did not observe a radial breathing mode of the (3,3) tube even though a laser line close to the predicted resonance energy was used. On the other hand, we detected a possible presence of the (5,1) tube in the spectrum excited with the UV laser line.