Ultrathin films of Ge on the Si(100)2 × 1 surface

The Ge/Si(100)2 × 1 interface was investigated by means of Auger electron spectroscopy, low‐energy electron diffraction, thermal desorption spectroscopy, and work function measurements, in the regime of a few monolayers. The results show that growth of Ge at room temperature forms a thermally stable amorphous interface without significant intermixing and interdiffusion into the substrate, for annealing up to ~1100 K. Therefore, the Ge‐Si interaction most likely takes place at the outmost silicon atomic plane. The charge transfer between Ge and Si seems to be negligible, indicating a rather covalent bonding. Regarding the Ge overlayer morphology, the growth mode depends on the substrate temperature during deposition, in accordance with the literature. Stronger annealing of the germanium covered substrate (>1100 K) causes desorption of not only Ge adatoms, but also SiGe and Ge₂ species. This is probably due to a thermal Ge‐Si interdiffusion. In that case, deeper silicon planes participate in the Ge‐Si interaction. Above 1200 K, a new Ge superstructure (4 × 4)R45^o was observed. Based on that symmetry, an atomic model is proposed, where Ge adatom pairs interact with free silicon dangling bonds.     

Vibrational spectroscopy for glycine adsorbed on silicon clusters: Harmonic and anharmonic calculations for models of the Si(1 0 0)-2 × 1 surface

The vibrational spectroscopy of a glycine molecule adsorbed on a silicon surface is studied computationally, using different clusters as models for the surface. Harmonic frequencies are computed using density functional theory (DFT) with the B3LYP functional. Anharmonic frequency calculations are carried out using vibrational self-consistent field (VSCF) algorithms on an improved PM3 potential energy surface. The results are compared with experiments on Glycine@Si(1 0 0)-2 × 1.

The main findings are: (1) Agreement of the computed frequencies with experiment improves with cluster size. (2) The anharmonic calculations are generally in better agreement with experiment than the harmonic ones. The improvements due to anharmonicity are most significant for hydrogenic stretching. (3) An important part of the anharmonic effects is due to anharmonic coupling between different normal modes of the system. (4) The anharmonic coupling between glycine vibrational modes is much larger than the anharmonic coupling between glycine and “phonon” (cluster) modes.

Implications of the results for surface vibrational spectroscopy are discussed.     

Addition reactions of nitrones on the reconstructed C(1 0 0)-2 × 1 surfaces

In this paper, the reactions of nitrone, N-methyl nitrone, N-phenyl nitrone and their hydroxylamine tautomers (vinyl-hydroxylamine, N-methyl-vinyl-hydroxylamine and N-phenyl-vinyl-hydroxylamine) on the reconstructed C(1 0 0)-2 × 1 surface have been investigated using hybrid density functional theory (B3LYP), Møller–Plesset second-order perturbation (MP2) and multi-configuration complete-active-space self-consistent-field (CASSCF) methods. The calculations showed that all the nitrones can react with the surface “dimer” via facile 1,3-dipolar cycloaddition with small activation barriers (less than 12.0 kJ/mol at B3LYP/6-31g(d) level). The [2+2] cycloaddition of hydroxylamine tautomers on the C(1 0 0) surface follows a diradical mechanism. Hydroxylamine tautomers first form diradical intermediates with the reconstructed C(1 0 0)-2 × 1 surface by overcoming a large activation barrier of 50–60 kJ/mol (B3LYP), then generate [2+2] cycloaddition products via diradical transition states with negligible activation barriers. The surface reactions result in hydroxyl or amino-terminated diamond surfaces, which offers new opportunity for further modifications.     

Formation of Au nanocluster ordered array on Si(111)‐7 × 7 surface

We report a successful approach in fabricating well shape two‐dimensional Au nanocluster hexagonal array on an Si(111)‐7 × 7 surface. The size of Au hexagons is about 2.5 × 2.5 nm². The easiness of preparing large scale Au hexagonal array makes this approach usable to fabricate a wide range of other metal nanocluster arrays. Further deposition of small amount of Au nanoclusters on the Au hexagonal array surface will prefer to occupy the holes of the hexagonal array at room temperature. Finally, a reasonable structure of formatting the hexagonal Au nanocluster array on the Si(111)‐7 × 7 surface was proposed. Copyright © 2015 John Wiley & Sons, Ltd.     

Impurity and vacancy clustering at the Σ3(1 1 1)[1 −1 0] grain boundary in strontium titanate

The interaction of selected point defects and the Σ3(1 1 1)[1 −1 0] symmetrical twin boundary in SrTiO₃ is investigated by density-functional band-structure calculations. The pristine boundary is SrO₃-terminated and exhibits electronic properties which are comparable with the pure bulk phase. Both the mirror-symmetric twin and some laterally shifted structures have low grain boundary energies, thus they may coexist in a real crystal if external stress is applied. With varying chemical potential of the electrons, the TiO₃ sublattice of the pristine boundary is destabilised by both electron enrichment and electron depletion. These results may explain, why different translation states can be observed by electron microscopy within the same bicrystal.Substitutional doping of the Ti columns next to the boundary plane with formally 3+ cations yields a contraction compared with the pure boundary. The same result, accompanied by more pronounced local geometry changes, is obtained for the translation state in the presence of O vacancies. Neutral O vacancies in the boundary plane are the most stable species followed by positively charged O vacancies in the neighbouring Sr/O plane parallel to the boundary. The Fermi level is shifted either upward to metal-derived states by substitutional doping or downward into the O-2p manifold for O vacancies. These findings may rationalise both the observed low, mainly electronic, grain boundary conductivity of SrTiO₃ boundaries and the changes in local potential found at the grain boundary by tunnelling and force microscopy experiments.     

Atomic arrangement of Al-induced clusters on Si(0 0 1) surface at high temperature

The atomic structure of the Al-induced clusters on Si(0 0 1) surface formed by the annealing of 0.5 ML Al/Si(0 0 1) at 500 °C has been studied using coaxial impact collision ion scattering spectroscopy (CAICISS). CAICISS results proposed that the Al atoms occupy the cave site (T4 site) and off-centered T4 site. To determine the structure of the Al-induced clusters definitely, classical ion-scattering trajectory simulations using scattering and recoiling imaging code (SARIC) have been performed for the recently proposed most possible four different cluster models (Bunk, Zotov, Kotlyar, and Zavodinsky model). Our CAICISS spectra and simulation results show that the Bunk model is the best plausible one among the models. As the results of the simulations, it is found that Al–Si dimers has been oriented on the topmost layer of the Si(0 0 1) surface with a bonding length (Δz) of 1.00 ± 0.05 Å.     

A 2D Supramolecular Network Based on Aromatic π···π Stacking Interactions

Using ferrocenecarboxylic acid (FcCOOH) as organometallic ligand in the synthesis of heterometallic complexes led to the isolation of the compound [(FcCOO)Cu(bpy)₂](BF₄) · bpy · CH₃OH. It was characterized by IR spectroscopy, EA, powder XRD, UV, and TGA measurements. Single‐crystal X‐ray structural analysis revealed that a unique 2D supramolecular network purely formed by aromatic π ··· π stacking interactions was observed, namely, {[(FcCOO)Cu(bpy)₂](BF₄) · bpy · CH₃OH}_∞ ( 1 ). The solid UV/Vis diffuse reflectance spectrum revealed the optical energy gap of 1 to be 3.54 eV, which is dramatically blue shifted compared with the value of ferrocene. Experimental results of thermal analysis and electrochemical analysis show that 1 has good thermal and better electrochemical stability.     

Adsorption of alkali metals on Ge(0 0 1)(2 × 1) surface

Ab initio total energy calculations have been performed for Na, K and Rb adsorption on Ge(0 0 1)(2 × 1) surface. It was found that the adsorption site of AM is AM size dependent. Structural analysis showed that the Ge–Ge dimer bond becomes stronger with increasing AM size. As the coverage increases from 0.5 to 1 ML it turns out that no depolarization effect occurs upon Na adsorption, while this effect becomes more important with increasing AM size. We also found that for all adsorption systems investigated the germanium surface is metallic and semiconducting for the coverage of 0.5 and 1 ML, respectively.     

Insight into the π‐hole···π‐electrons tetrel bonds between F2ZO (Z = C,Si, Ge) and unsaturated hydrocarbons

The intermolecular π‐hole···π‐electrons interactions between F₂ZO (Z = C, Si, Ge) molecules and unsaturated hydrocarbons including acetylene, ethylene, 1,3‐butadiene and benzene were constructed to reveal the differences of tetrel bonds forming by carbon and heavier tetrel atoms. The ab initio computation in association with topological analysis of electron density, natural bond orbital, and energy decomposition analysis demonstrate that the strength of Si···π and Ge···π tetrel bonds is much stronger than that of C···π tetrel bonds. The Si···π and Ge···π tetrel bonds exhibit covalent or partially covalent interaction nature, while the weak C···π tetrel bonds display the hallmarks of noncovalent interaction, the electrostatic interaction is the primary influencing factor. The Si···π and Ge···π interactions are determined by both the σ‐ and π‐electron densities, while the C···π interactions are dominated mainly by the π‐electron densities. The π‐hole···π‐electrons tetrel bonds are dominated by electrostatic interaction, and polarization has a comparable contribution in the Si···π and Ge···π tetrel bonds.     

Mechanisms for H2O decomposition on the Si(111)‐7 × 7 surface: A DFT cluster model study

The mechanisms for the complete decomposition of water molecules on the Si (111)‐7 × 7 surface were investigated theoretically. The reaction pathways for dissociation of four water molecules over the adatom and rest atom sites were calculated using the density functional theory (DFT) in conjunction with the B3LYP functional. The calculated results demonstrated that the initial O? H bond dissociation from the first H₂O to form the adsorbed OH species is more preferential on the adatom site (Si_a) than the rest atom site (Si_r) of Si (111)‐7 × 7. Four water molecules dissociate successively over the adatom site, backbonds of adatoms which are saturated by OH species can reasonably be the place of insertion of oxygen atoms, yielding a tetrahedral SiO₄ structure with one on top and three inserted oxygen atoms. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Specifically (π → π*)-Induced Cyclohexenone Reactions 6-Benzylbicyclo [4.4.0]dec-1-en-3-ones

6-Benzylbicyclo [4.4.0]dec-1-en-3-one ( 9 ) and the 2-methyl homologue ( 10 ) underwent a (γ → α )-1, 3-benzyl shift to the β,γ-unsaturated ketones 21 and 22 , respectively, when excited in the π π* absorption band. The quantum yield was ca. 0.1 at 254 nm for the formation of both products in alkane solvents. These reactions occur specifically from the S₂(π, π*) state in competition with its decay to the S₁(n, π*) and T states. The triplet reaction of 9 , initiated by n → π* irradiation and by sensitization, was a double-bond shift to 20 , whereas no identifiable product was observed from 10 under these conditions. Direct and acetone-sensitized irradiations of 21 and 22 resulted in oxadi-π-methane rearrangements to mixtures of syn- and anti- 30 and syn- and anti- 31 , respectively.