Patrick Vogt

Atomic Structures of Silicene Layers Grown on Ag(111): Scanning Tunneling Microscopy and Noncontact Atomic Force Microscopy Observations

Silicene, the considered equivalent of graphene for silicon, has been recently synthesized on Ag(111) surfaces. Following the tremendous success of graphene, silicene might further widen the horizon of two-dimensional materials with new allotropes artificially created. Due to stronger spin-orbit coupling, lower group symmetry and different chemistry compared to graphene, silicene presents many new interesting features. Here, we focus on very important aspects of silicene layers on Ag(111): First, we present scanning tunneling microscopy (STM) and non-contact Atomic Force Microscopy (nc-AFM) observations of the major structures of single layer and bi-layer silicene in epitaxy with Ag(111). For the (3 × 3) reconstructed first silicene layer nc-AFM represents the same lateral arrangement of silicene atoms as STM and therefore provides a timely experimental confirmation of the current picture of the atomic silicene structure. Furthermore, both nc-AFM and STM give a unifying interpretation of the second layer (√3 × √3)R ± 30° structure. Finally, we give support to the conjectured possible existence of less stable, ~2% stressed, (√7 × √7)R ± 19.1° rotated silicene domains in the first layer.

Cu(I)-catalyzed sulfoximination

Abstract The reaction of PhINTs with sulfoxides in the presence of catalytic amounts of CuOTf afforded the corresponding N-tosylsulfoximines in high yield. The use of enantiomerically pure sulfoxides allowed stereoselective access to N-tosylsulfoximines with complete retention of configuration at sulfur.

The fate of the 2√3 × 2√3R(30°) silicene phase on Ag(111)

Silicon atoms deposited on Ag(111) produce various single layer silicene sheets with different buckling patterns and periodicities. Low temperature scanning tunneling microscopy reveals that one of the silicene sheets, the hypothetical √7 × √7 silicene structure, on 2√3 × 2√3 Ag(111), is inherently highly defective and displays no long-range order. Moreover, Auger and photoelectron spectroscopy measurements reveal its sudden death, to end, in a dynamic fating process at ∼300 °C. This result clarifies the real nature of the 2√3 × 2√3R(30°) silicene phase and thus helps to understand the diversity of the silicene sheets grown on Ag(111).

Molecular simulation of the vapour–liquid phase coexistence of neon and argon using ab initio potentials

Gibbs ensemble simulations using ab initio intermolecular potentials are reported for the vapour–liquid phase coexistence of neon and argon. For neon two different quantum chemical ab initio potentials of well-known quality are used to investigate the effect of the quality of pair interactions. In addition calculations are also reported for neon using a potential that includes three-body interactions. For argon, simulations are compared with results obtained from NPH-ensemble molecular dynamics simulations. It is found that the results of a perfect pair potential must occur outside the experimental temperature–density phase envelope. Therefore, if a perfect pair potential is used, many-body interactions and quantum effects must be considered to obtain good agreement with experiment.

How approximate is the experimental evaluation of quadrupole coupling constants in liquids? A novel computational study

Model calculations to investigate the deuteron quadrupolar relaxation in liquid water are performed. Techniques not amenable to experiment, such as switching on and off the intermolecular or intramolecular electric field gradients and simulating rigid liquid water, give insight into the microscopic effects leading to relaxation. In experimental studies it is usually assumed that the deuteron quadrupolar relaxation is governed largely by the reorientational motion of an average electric field gradient, and the error in this assumption is readily extracted from the model calculations. As expected, this error is significant for deuterons in hydrogen bonds. These model calculations should provide a guide to better understanding of quadrupolar relaxation and experimental evaluation of relaxation.

Electric field gradients are highly pair-additive

The electric field gradients at the central deuterons in one hundred clusters consisting of heavy water molecules, previously calculated in a supermolecular approach including many-body effects, is calculated assuming pair-additivity. Excellent pair-additivity of the electric field gradient is found for the water clusters. This result is confirmed for one hundred clusters extracted from a water-DMSO mixture. The use of pair-additivity results in substantial computer time savings in the quantum chemical calculation of the electric field gradients for nuclei in liquid systems using the so-called cluster approach, and hence their quadrupole coupling constants. It also permits the simulation of quadrupolar relaxation times from electric field gradient hypersurfaces obtained within the pair-additive approximation.

Phase formation and strain relaxation of Ga2O3 on c-plane and a-plane sapphire substrates as studied by synchrotron-based x-ray diffraction

Heteroepitaxial Ga2O3 was deposited on c-plane and a-plane oriented sapphire by plasma-assisted molecular beam epitaxy and probed by ex-situ and in-situ synchrotron-based x-ray diffraction. The investigation on c-plane sapphire determined a critical thickness of around 33 A, at which the monoclinic β-phase forms on top of the hexagonal α-phase. A 143 A thick single phase α-Ga2O3 was observed on a-plane sapphire, much thicker than the α-Ga2O3 on c-plane sapphire. The α-Ga2O3 relaxed very fast in the first 30 A in both out-of-plane and in-plane directions as measured by the in-situ study.

Kinetics versus thermodynamics of the metal incorporation in molecular beam epitaxy of (InxGa1−x)2O3

We present a detailed study of the reaction kinetics and thermodynamics of the plasma-assisted oxide molecular beam epitaxy of the ternary compound (InxGa1−x)2O3 for 0 ≤ x ≤ 1. We measured the growth rate of the alloy in situ by laser reflectrometry as a function of growth temperature TG for different metal-to-oxygen flux ratios rMe, and nominal In concentrations xnom in the metal flux. We determined ex situ the In and Ga concentrations in the grown film by energy dispersive X-ray spectroscopy. The measured In concentration x shows a strong dependence on the growth parameters TG, rMe, and xnom whereas growth on different co-loaded substrates shows that in the macroscopic regime of ∼μm3 x does neither depend on the detailed layer crystallinity nor on crystal orientation. The data unveil that, in presence of In, Ga incorporation is kinetically limited by Ga2O desorption the same way as during Ga2O 3 growth. In contrast, In incorporation during ternary growth is thermodynamically suppressed by the presence o...

Electrophilic surface sites as precondition for the chemisorption of pyrrole on GaAs(001) surfaces

We report how the presence of electrophilic surface sites influences the adsorption mechanism of pyrrole on GaAs(001) surfaces. For this purpose, we have investigated the adsorption behavior of pyrrole on different GaAs(001) reconstructions with different stoichiometries and thus different surface chemistries. The interfaces were characterized by x-ray photoelectron spectroscopy, scanning tunneling microscopy, and by reflectance anisotropy spectroscopy in a spectral range between 1.5 and 5 eV. On the As-rich c(4 × 4) reconstruction that exhibits only nucleophilic surface sites, pyrrole was found to physisorb on the surface without any significant modification of the structural and electronic properties of the surface. On the Ga-rich GaAs(001)-(4 × 2)/(6 × 6) reconstructions which exhibit nucleophilic as well as electrophilic surface sites, pyrrole was found to form stable covalent bonds mainly to the electrophilic (charge deficient) Ga atoms of the surface. These results clearly demonstrate that the existence of electrophilic surface sites is a crucial precondition for the chemisorption of pyrrole on GaAs(001) surfaces.

A relation between the systematic errors of a quantum chemical potential and of fluid properties calculated from it

Abstract For five ab initio potentials applied in simulations it was observed that the pressure vs. temperature curves at constant density are shifted parallel to the experimental curves. This allows extrapolation of the simulation results to the values that would have been obtained with exact potentials. An explanation based on model calculations is given, showing that this effect results from a systematic compensation for the changes induced by variation in the potential depth and equilibrium distance.