Chemisorption of CO on differently prepared Cu(111) surfaces

The adsorption of CO on Cu(111) has been studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), electron energy loss spectroscopy (EELS), work function measurements and thermal desorption spectroscopy. Two LEED overlayers of CO on Cu(111) have been found: $\text{√3 × √3}\text{R}\text{30°}$ and $\text{√}\text{7}\text{3}\text{× √}\text{7}\text{3}\text{R}\text{49.1°}$. Two different heats of adsorption were derived from thermal desorption spectra: 44.2 and 35.1 kj/mole. The isosteric heat of adsorption evaluated from work function measurements corresponds to the thermal desorption results. Energy losses due to CO adsorption have been found by means of EELS at 4.7, 7.7, and 13.8 eV.     

Pulse-width influence on the laser-induced structuring of CaF2 (111)

We have investigated the morphology of CaF₂ (111) irradiated by 780 nm laser pulses of varying pulse width (200 fs-8 ns) with fluences above the damage threshold. Large differences can be observed which we relate to the mechanisms and dynamics of defect production in this wide band gap material. The best defined and most controllable ablation is obtained for laser pulse widths of a few picoseconds. For nanosecond and femtosecond pulses strong fracturing of the crystal is observed with damage outside the laser irradiated zone. This has a thermal origin for nanosecond pulses but a non-thermal origin for pulse widths below approximately 1 ps.     

Type B epitaxy of Ge on CaF2(111) surface

It was known experimentally that type B orientation, which is rotated 180° about the [111] axis, dominated the heteroepitaxial growth of Ge(111) on a CaF₂(111) substrate at an elevated temperature. We performed first principles calculations using density functional theory to determine the energetics of the Ge(111)/CaF₂(111) interface and found that the type B orientation of the Ge film is most likely a result of a direct bonding between Ge atoms and Ca²⁺ at the CaF₂ surface with the top F^? layer depleted. Our theoretical prediction is supported by our X-ray diffraction experiments on {111} < 121> biaxially textured Ge/CaF₂ samples.     

Preferential sputtering induced stress domains and mesoscopic phase separation on CaF2(111)

We report the formation of novel mesoscopic two-dimensional bubble structures coexisting with nanometer-scale, two monolayer high, meandering islands of characteristic separation approximately 11 nm on CaF2(111) surfaces exposed to glancing incidence ion beam irradiation (4 degrees, 4.5 keV Ar+). The island and bubble structures can be explained, respectively, by nanoscale stress domain related local self-ordering of a single calcium adlayer on phase-separated F- (fluorine ion) and F-center (color center) terminated mesoscopic domains.     

Laser ablation condensation of alpha-PbO(2)-type TiO(2)

A high-pressure phase of TiO(2) with an alpha-PbO(2)-type structure has been synthesized via very energetic Nd-YAG laser pulse irradiation of oxygen-purged Ti target. The nanometer-size alpha-PbO(2)-type particles were (11;0), (010), and (001) faceted but the larger ones were spherical. The combined effects of rapid heating and cooling, the nanophase effect, and dense surfaces account for the formation of coherently strained alpha-PbO(2) particles. The refined cell volume indicated a considerable residual stress to stabilize the dense structure to ambient condition.     

Laser induced charge transfer from Ni to CO adsorbed on (111) and (110) surfaces

Ni(110) and (111) surfaces covered with up to one monolayer of CO were irradiated with the light of a dye laser in the photon energy range 2.0 to 3.4 eV. Two-photon photoemission was observed when the laser light was focussed. Upon defocussing a signal was measured which did not depend on the potential of the sample and showed a linear intensity dependence. It is caused by electrons transferred from the Ni substrate into adsorbate states. The signal vanishes for photon energies below 2 eV. This shows that the adsorbate state lies at most 2 eV above the fermi level. The lifetime τ of the electrons in the adsorbate states is estimated to be 10^?10 < τ < 10^?7 s. No fluorescence in the photon energy range above 1 eV could be detected.     

Atomic force microscope studies of CuCl island formation on CaF2(111) substrates

We have grown by molecular beam epitaxy (MBE) CuCl thin films at various thicknesses and substrate temperatures on CaF₂(111) substrates. Atomic force microscope (AFM) topographs reveal that islanding is the dominant growth mechanism. Quantitative analysis of the AFM data enabled us to determine the amount of the substrate remaining exposed after the deposition as well as the total amount of CuCl deposited. We calculated the reciprocal-space height correlation function, 〈 | h(q, t) |each of our films and compared them to the predictions of the shadowing growth theory, which enabled us to extract the important kinetic parameter of surface diffusion length for the growth condition of each of the four films.     

Origin of Degradation of the CaF2/BaF2 Buffer Layers on Si(111)

Doklady Physics - The growth temperature dependences of the capacitance–voltage characteristics and chemical stability of the CaF2/BaF2 buffer layers formed on Si by molecular beam epitaxy...     

Nature of versatile chemisorption on TiC(111) and TiN(111) surfaces

Density-functional calculations on the polar TiX(111) (X = C, N) surfaces show (i) for clean surfaces, strong Ti3d-derived surface resonances (SR’s) at the Fermi level and X2p-derived SR’s deep in the upper valence band and (ii) for adatoms in periods 1-3, pyramidic trends in atomic adsorption energies, peaking at oxygen (9 eV). A concerted-coupling model, where adatom states couple to both kinds of SR’s in a concerted way, describes the adsorption. The chemisorption versatility and the general nature of the model indicate ramifications and predictive abilities in, e.g., growth and catalysis.     

TCAD simulation of tunneling leakage current in CaF2/Si(111) MIS structures

We introduce a simulation technique suitable to model the tunneling leakage current in the metal(polySi)/CaF₂/Si(111) MIS structures using TCAD simulators Minimos-NT and ViennaSHE. The simulations are performed using the real physical parameters of the CaF₂/Si tunnel barrier. The results obtained for the case of near-equilibrium carrier transport are in a good agreement with experimental data and also with the simulation results yielded by our reference physical model. The obtained non-equilibrium hot-electron tunnel leakages in the hypothetical transistors with CaF₂ as a gate dielectric are comparable to those in the structures with silicon dioxide. Being an important step forward for the device application of calcium fluorite, this work opens the possibility of simulating the characteristics of different silicon-based systems with crystalline insulators.