Bistability of single 1,5 cyclooctadiene molecules on Si(001)

The adsorption and current-induced bistability of single 1,5 cyclooctadiene molecules on Si(001) were studied in ultrahigh vacuum by low-temperature scanning tunneling microscopy (STM). After a dosage of ≈0.05 L at room temperature followed by cooling to the measuring temperature of 7 K, we find that the cyclic alkene molecule preferably adsorbs in the bridge structure with both C=C double bonds reacting with two adjacent Si dimers via [2+2] cycloaddition reaction. The time-dependent current measured upon tunneling through the adsorbed molecule at fixed STM tip height displays a switching between two current levels with the same mean residence time in each level. Higher bias and/or reduced tip height—and therefore higher current—increase the switching rate, suggesting that the reversible switching is due to inelastic electron tunneling. The observed bistability is interpreted as a dynamic interconversion between two degenerate conformations of the adsorbed molecule.     

The effect of Ti on the wetting of CaF<Subscript>2</Subscript> substrate by In–Ti and Ga–Ti alloys. Ab-initio consideration

CaF₂ is a thermodynamically stable, non-reactive compound, displaying a relatively high contact angle with pure liquid metal melts. A remarkable decrease of this contact angle takes place when small amounts of Ti are added to liquid In (a decrease from 125 to 20°) or to liquid Ga (from 118 to 60°). Thermodynamic analysis indicates that this feature cannot be attributed to chemical reactions between the substrate and the melt. It was suggested that the reason for this behavior may be a preferential titanium adsorption from the liquid In–Ti or Ga–Ti solution at the substrate surface. In order to understand the nature of the In–Ti or Ga–Ti bonding in the vicinity of the CaF₂ surface, the adsorption energy of 0.5 monolayer of In and Ga was computed for three different surface conditions: (i) clean CaF₂(111), (ii) CaF₂(111) with In or Ga adatoms, and (iii) CaF₂(111) with Ti adatoms. The differences in adsorption energies for these configurations are related to the electron charge redistribution in the vicinity of the interface, and to the density of states of the electronic subsystems. It was found that the adsorption energy of In or Ga increases due to the lateral interactions with the adatoms. According to the analysis, a strong lateral interaction exists between Ti adatoms and Me, while relatively weak interaction exists between Me and Me adatoms. The difference of the lateral interactions was considered in order to explain the improvement of the wetting of CaF₂ substrate by Ti alloying of In and Ga.     

An anchoring strategy for photoswitchable biosensor technology: azobenzene-modified SAMs on Si(111)

A mild and efficient procedure has been developed to obtain covalently attached self-assembled monolayers (SAMs) on Si(111) with photochromic azobenzene head-groups. Starting from neat or diluted carboxylic acid functionalized monolayers on-chip coupling reactions were applied to attach hydroxyl-functionalized azobenzene units to the SAMs by ester bond formation. The modified surfaces were characterized by high-resolution X-ray photoelectron spectroscopy (XPS), transmission Fourier transform infrared spectroscopy (FT-IR), and contact angle measurements. Reversible cis ↔ trans isomerizations of photoswitchable SAMs were monitored by wettability measurements. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users. Dedicated with respect and compliments to Professor Helmut Schwarz on the occasion of his 65th birthday.     

Controlled deflection of cold atomic clouds and of Bose-Einstein condensates

We present a detailed, realistic proposal and analysis of the implementation of a cold atom deflector using time-dependent far off-resonance optical guides. An analytical model and numerical simulations are used to illustrate its characteristics when applied to both non-degenerate atomic ensembles and to Bose-Einstein condensates. Using for all relevant parameters values that are achieved with present technology, we show that it is possible to deflect almost entirely an ensemble of ⁸⁷Rb atoms falling in the gravity field. We discuss the limits of this proposal, and illustrate its robustness against non-adiabatic transitions.     

The O(6) description of the nuclear rotation

A recently developed algebraic version of the collective model has shown that a full range of Bohr model calculations can be executed in bases that range continuously from those of a spherical vibrator to a beta-vibrational Wilets-Jean limit. Thus, the establishment of close relationships between the algebraic structure of this model and the IBM is of special importance because one can learn from the complementary perspectives they afford. In this paper, we show by calculations that the familiar rotor-gamma vibrational spectra of the Bohr model can be obtained in the IBM by the addition of a scalar cubic in the quadrupole moment operators, of the type considered recently by Van Isacker, to an O(6) Hamiltonian. Simple fits of the low-lying spectra, electromagnetic transition rates and moments of inertia of the ground and gamma bands of¹⁶²Dy and¹⁶⁸Er are presented.     

Toward a description of reactions of CO and H2 on Fe surfaces based on SCF hybrid AOs and electronegativity equalization

Maximum localization hybrids and electronegativity equalization have been used to obtain a simple reliable SCF picture of directional and charge-transfer effects determining the state and reactivity of CO adsorbed on iron. The hybridization procedure has been applied to H₂ adsorbed on FeCO, so as to describe the evolution of the complex toward a final geometry, which corresponds to a possible chemisorbed =CH−OH species, with the CO bond strongly inclined toward the surface. Abbiamo usato ibridi di massima localizzazione e la condizione di uguaglianza delle elettronegatività per ottenere una descrizione semplice e al tempo stesso affidabile degli effetti direzionali e di trasferimento di carica che determinano lo stato e la reattività del CO adsorbito su ferro. Il procedimento di ibridazione è stato applicato a H₂ adsorbito su FeCO in modo tale da descrivere l'evoluzione del complesso verso una geometria finale, la quale corrisponde ad una possibile specie chemisorbita =CH−OH, in cui il legame CO è fortemente inclinato verso la superficie. Используются смешанные состояния с максимальной локализацией и уравнивание электроотрицательности для получения простой SCF картины для ориентационных эффектов и эффектов переноса заряда, которые определяют состояние и реактивность адсорбированного CO на железе. Применяется процедура гибридизации к адсорбированной молекуле H₂ на FeCO. Описывается эволюция комплекса в сторону конечной геометрии, которая соответствует возможной группе =CH−OH с связью CO, которая сильно наклонена по отношению к поверхности.     

Formation of extended straight molecular chains by pairing of thymine molecules on the Ag–Si(111) surface

Here, we report on the assembly of thymine molecules into extended straight chains of single and multiple dimer rows on a Si(111)/Ag R30° surface. Using variable temperature scanning tunneling microscopy, we follow the nucleation process and formation of self-assembled structures. Submonolayer coverages at 120 K are disordered and exhibit a high density of thymine dimers. Upon annealing the dimers assemble into extended dimer chains along three equivalent high-symmetry surface directions. At low coverages single dimer rows are favored. At increased coverage chains with multiple dimer rows are observed, with a preference to an even multiplicity. We show that a complex cross-talk between H-bond thymine–thymine interactions and commensurization of dimer chains to the Ag/Si surface leads to this specific layout.     

Stranski–Krastanov growth of InGaN quantum dots emitting in green spectra

Self-assembled InGaN quantum dots (QDs) were grown on GaN templates by metalorganic chemical vapor deposition. 2D–3D growth mode transition through Stranski–Krastanov mode was observed via atomic force microscopy. The critical thickness for In_0.67Ga_0.33N QDs was determined to be four monolayers. The effects of growth temperature, deposition thickness, and V/III ratio on QD formation were examined. The capping of InGaN QDs with GaN was analyzed. Optimized InGaN quantum dots emitted in green spectra at room temperature.     

Magnetically trapped atoms for compact atomic clocks

We investigate the hyperfine transition of magnetically trapped non-condensed atoms. The two principal frequency shifts, the second order Zeeman effect and the mean field interaction are considered. Analytic models of the mean frequency and its trap induced spread are developed. Comparisons with existing experiments evaluate the role of the atoms’ oscillatory motion. The analytic model proves to be equivalent to existing Monte Carlo simulations. The formulae provide a simple tool for optimising the design of a new experiment. Applied to the two-photon transition |F=1,m _F =−1〉→|F=2,m _F =1〉 in ⁸⁷Rb and the conditions of a typical atom chip experiment, a line spread as small as 11 mHz is predicted giving a quality factor of 10¹². The system is promising for application in precision instruments such as compact atomic clocks.     

Hydrogen associative desorption from Ru(1010)

The associative desorption of hydrogen from the Ru(10 0) surface has been studied on the atomic level by means of density-functional calculations with various exchange-correlation functionals and kinetic Monte Carlo simulations. The simulations reproduce forming structures of the layers and main features of TPD spectra for a wide range of hydrogen coverages. In particular, it has been shown that the decrease of binding energies with coverage due to lateral repulsion is correlated with the appearance of low-temperature peaks in the spectra.     

Quantum states of rigid linear rotors confined in a slit pore: quantum sieving of hydrogen isotopes and extreme one dimensional confinement

The quantum states of molecular hydrogen isotopes confined in carbon slit pores of varying width are calculated by direct diagonalization of the single particle Hamiltonian, evaluated with a model potential. The results are used to calculate the zero-pressure isotopic selectivity that can be expected from adsorption on these nanostructures, and the contribution from the rotational degrees of freedom is analysed in detail. It is shown that the role of the rotational degrees of freedom is far from negligible, resulting in values of the isotopic separation 8 to 50 times higher than those calculated using spherically symmetric models for the hydrogen molecule. It is also shown that if the pores are narrow enough, the molecular axis in the ground state is found preferentially aligned along the slit planes, and the conditions when this happens will be analysed in detail. This behaviour, termed “extreme one dimensional confinement”, is analogous to a similar behaviour observed in carbon nanotubes.     

Photoelectron diffraction: from phenomenological demonstration to practical tool

The potential of photoelectron diffraction—exploiting the coherent interference of directly-emitted and elastically scattered components of the photoelectron wavefield emitted from a core level of a surface atom to obtain structural information—was first appreciated in the 1970s. The first demonstrations of the effect were published towards the end of that decade, but the method has now entered the mainstream armoury of surface structure determination. This short review has two objectives: First, to outline the way that the idea emerged and the way this evolved in my own collaboration with Neville Smith and his colleagues at Bell Labs in the early years: Second, to provide some insight into the current state-of-the art in application of (scanned-energy mode) photoelectron diffraction to address two key issue in quantitative surface structure determination, namely, complexity and precision. In this regard a particularly powerful aspect of photoelectron diffraction is its elemental and chemical-state specificity.     

Transition between corrugated metal films and?split-ring-resonator arrays

We fabricate split-ring-resonator arrays via direct laser writing of polymers, followed by atomic-layer deposition of titania, chemical vapor deposition of silver, and focused-ion-beam milling. While structures like that have been fabricated previously by other means, our approach here allows for a direct comparison with the optical properties of corrugated metal surfaces, which are fabricated along the same lines. This comparison reveals substantial differences regarding the magnetic metamaterial properties. In particular, we find that the optical response of the corrugated metal surfaces is due to a higher-order magnetic resonance, whereas that of the split-ring resonators stems from their fundamental magnetic resonance.     

Energy dependence of the isotopic effect in the (n,p) reaction on the germanium isotopes

Cross sections for^70,72,73,74Ge(n, p)^70,72,73,74Ga,⁷⁰Ge(n, 2n)⁶⁹Ge,⁷²Ge (n,)⁶⁹Zn ^m and⁷⁴Ge(n, )⁷¹Zn ^m reactions are measured in the energy range from 13.0 to 16.6 MeV by the activation method using Ge(Li) detector-ray spectroscopy and compared with predictions of the reaction model incorporating preequilibrium and equilibrium emission mechanisms to interpret the energy dependence of the isotopic effect occuring in the (n, p) reaction. The fitted single-particle state-density parametersg, determined here for the germaniums are discussed together with theg-values found previously for the Se, Zr and Pd isotopic chains. A validity of the consistency condition between the precompound and compound models, which relatesg to the experimental level-density parametera viaa= ² g/6 is demonstrated.     

Charge transfer and polarization screening in organic thin films: phthalocyanines on Au(100)

Core hole screening effects at organic/metal interfaces were studied by core level X-ray photoemission spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), and valence band ultraviolet photoemission spectroscopy (UPS). The comparison of energetic shifts in XPS and XAES enables the estimation of electronic relaxation energy (screening ability). Magnesium phthalocyanine (MgPc) and zinc phthalocyanine (ZnPc) evaporated on single crystalline Au(100) were used as model molecules. Two different features in the Mg KLL spectra can be clearly separated for (sub-)monolayer coverages, while only minor changes of the shape of Mg 1s are observed. Applying a dielectric continuum model, the major screening mechanism cannot be described sufficiently by polarization screening due to mirror charges, significant contributions by charge transfer screening have to be considered. In contrast, small screening effects in the bulk material can be explained by surface polarization.     

Square-pyramidal iron coordination modules as potential spin switches for the chemisorption on gold

Octahedral iron(II) complexes of a unique pyridine-derived tetrapodal pentadentate polyamine ligand, 2,6-C₅H₃N(CMe[CH₂NH₂]₂)₂, show temperature-dependent spin crossover (SCO) depending on the nature of a sixth monodentate ligand L (imidazol or pyridine derivative). For L = 1-methylimidazol, the redox behaviour of the complex, as determined by cyclic voltammetry, suggests an accompanying ligand exchange. Pyridine-4-thiol and the disulphides: 4-(2-methyldisulphanyl)pyridine, 4-(2-hexadecyldisulphanyl)pyridine and 1,2-bis([pyridine-4-yl]methyl)disulphane, were studied as mono-dentate ligands L, with a view to enable chemisorption of iron(II) complexes on a gold surface. In the case of pyridine-4-thiol, the participation of the thiolate functional group in iron coordination is difficult to suppress, whereas the disulphides enter into yet unrecorded redox chemistry with iron(II), yielding a di-iron(III) complex containing a persulphide bridge (S ₂²⁻).     

Formation of surface stabilized Si nanocrystal by pulsed laser ablation in hydrogen gas

We prepared silicon nanocrystallites by pulsed laser ablation (PLA) of a Si target in hydrogen background gas. A mixture of hydrogen and helium was used as a background gas and the hydrogen partial pressure was varied. The deposited nanocrystal-film system shows a hierarchical structure composed of surface hydrogenated silicon nanocrystallites as the primary structure and aggregates of the nanocrystallites as the secondary structure. The size of the primary particles was not sensitive to the hydrogen partial pressure, while the porosity of the secondary structure constituted by the aggregation of the primary particles increased with increasing hydrogen partial pressure. This indicates that the surface is stabilized and that aggregation of the primary structure is depressed by surface hydrogenation. The optical gap energy of the deposits shifted to higher energy with increasing hydrogen partial pressure due to the formation of well-isolated nanocrystallites by surface stabilization. These results indicate that PLA in hydrogen gas is a promising technique to prepare surface stabilized and controlled silicon nanocrystallites.     

Spherically symmetric non-commutative space: d=4

In order to find a non-commutative analog of Schwarzschild or Schwarzschild–de Sitter black hole we investigate spherically symmetric spaces generated by four non-commutative coordinates in the frame formalism. We present two solutions which, however, do not possess the prescribed commutative limit. Our analysis indicates that the appropriate non-commutative space might be found as a subspace of a higher-dimensional space.     

Absorption measurements in InSe:Ho single crystal under an electric field

InSe:Ho single crystal was grown by Bridgman-Stockberger method. Electric field effects on the absorption measurements have been investigated as a function of temperature in InSe:Ho single crystal. The absorption edge shifted towards longer wavelengths and a decrease of intensity in absorption spectra occurred under an electric field of 7.5 kV/cm. Using absorption measurements, steepness parameter and Urbach energy were calculated under electric field. Applied electric field caused an increase in the Urbach energy. At 10 K and 320 K, the first exciton energies were calculated as 1.322 and 1.301 eV for zero voltage and 1.245 and 1.232 eV for applied electric field, respectively.