The chemistry of La on the Si(001) surface

This paper reports state-of-the-art electronic structure calculations of La adsorption on the Si(001) surface. We predict La chains in the low coverage limit, which condense in a stable phase at a coverage of 1/5 monolayer. At 1/3 monolayer we predict a chemically rather inert, stable phase. La changes its oxidation state from La(3+) at lower coverages to La(2+) at coverages beyond 1/3 monolayer. In the latter oxidation state, one electron resides in a state with a considerable contribution from La-d and f states.Comment: 10 pages, 13 figures, 3 table

An investigation of the interaction of N2O with the Si(111)-7 × 7 surface using AES and optical reflectometry; A comparison with O2

At 300 K, N2O decomposes into N2, leaving behind atomic oxygen at the Si(111)¿7 × 7 surface. Decomposition at two different sites is proposed, having the overall initial reaction probability: s(0) = (6.7 ± 0.7) × 106. SiOx(x not, vert, similar 1) bonds are predominantly formed, saturation occurring at monolayer coverage. This oxygen monolayer appears to completely prevent further oxygen uptake by additional N2O or O2 exposures, in contrast with the adsorption behaviour of O2 on Si(111)-7 × 7, which exhibits slow sorption beyond one monolayer

RSA Study of Dimers

The first theoretical study of a dimer's adsorption process at homogenous surface is presented. By using the RSA algorithm, we show example monolayers, discuss estimations of random jamming coverage and measure the surface blocking function, which could be used for calculating real systems kinetics. We also found the correlation function for generated coverages and analyzed orientational ordering inside the adsorbed monolayer. Results were compared with theoretical and experimental data. Keywords: adsorption, RSA, monolayer, colloid particlesComment: 13 pages, 13 figure

Acetylene on Si(100) from first principles: adsorption geometries, equilibrium coverages and thermal decomposition

Adsorption of acetylene on Si(100) is studied from first principles. We find that, among a number of possible adsorption configurations, the lowest-energy structure is a ``bridge'' configuration, where the C$_2$H$_2$ molecule is bonded to two Si atoms. Instead, ``pedestal'' configurations, recently proposed as the lowest-energy structures, are found to be much higher in energy and, therefore, can represent only metastable adsorption sites. We have calculated the surface formation energies for two different saturation coverages, namely 0.5 and 1 monolayer, both observed in experiments. We find that although, in general, the full monolayer coverage is favored, a narrow range of temperatures exists in which the 0.5 monolayer coverage is the most stable one, where the acetylene molecules are adsorbed in a $2\times 2$ structure. This result disagrees with the conclusions of a recent study and represents a possible explanation of apparently controversial experimental findings. The crucial role played by the use of a gradient-corrected density functional is discussed. Finally, we study thermal decomposition of acetylene adsorbed on Si(100) by means of finite-temperature Molecular Dynamics, and we observe an unexpected behavior of dehydrogenated acetylene molecules.Comment: 8 pages, 3 figures (submitted to J. Chem. Phy

Structural studies of phosphorus induced dimers on Si(001)

Renewed focus on the P-Si system due to its potential application in quantum computing and self-directed growth of molecular wires, has led us to study structural changes induced by P upon placement on Si(001)-$p(2\times 1)$. Using first-principles density functional theory (DFT) based pseudopotential method, we have performed calculations for P-Si(001) system, starting from an isolated P atom on the surface, and systematically increasing the coverage up to a full monolayer. An isolated P atom can favorably be placed on an {\bf M} site between two atoms of adjacent Si dimers belonging to the same Si dimer row. But being incorporated in the surface is even more energetically beneficial due to the participation of the {\bf M} site as a receptor for the ejected Si. Our calculations show that up to 1/8 monolayer coverage, hetero-dimer structure resulting from replacement of surface Si atoms with P is energetically favorable. Recently observed zig-zag features in STM are found to be consistent with this replacement process. As coverage increases, the hetero-dimers give way to P-P ortho-dimers on the Si dimer rows. This behavior is similar to that of Si-Si d-dimers but are to be contrasted with the Al-Al dimers, which are found between adjacent Si dimers rows and in a para-dimer arrangement. Unlike Al-Si system P-Si does not show any para to ortho transition. For both systems, the surface reconstruction is lifted at about one monolayer coverage. These calculations help us in understanding the experimental data obtained using scanning tunneling microscope.Comment: To appear in PR

Electron Transfer Reaction Through an Adsorbed Layer

We consider electron transfer from a redox to an electrode through and adsorbed intermediate. The formalism is developed to cover all regimes of coverage factor, from lone adsorbate to monolayer regime. The randomness in the distribution of adsorbates is handled using coherent potential approximation. We give current-overpotential profile for all coverage regimes. We explictly analyse the low and high coverage regimes by supplementing with DOS profile for adsorbate in both weakly coupled and strongly coupled sector. The prominence of bonding and anti-bonding states in the strongly coupled adsorbates at low coverage gives rise to saddle point behaviour in current-overpotential profile. We were able to recover the marcus inverted region at low coverage and the traditional direct electron transfer behaviour at high coverage

Gestufte Oberflächen und Quasieindimensionale Strukturen - Photoemission an 3d Metallen auf W(110)

The coverage dependent electronic structure of Cu and Co an vicinal W(110) surfaces has been investigated with angle resolved photoelectron spectroscopy. To prepare the quasi-one-dimensional Cu and Co systems the method of step edge decoration of the vicinal W(110) surfaces has been used. The vicinal surfaces with step edges in (110), (100) and (111) direction has been investigated using LEED. From the characteristic spot splitting a terrace width of 11 atom rows was determined. The band structures of the flat and the vicinal surfaces have indicated that the step edges have no bearing an the bulk band structure at k$_{\parallel}$=0. But the surface band structure shows a different dispersion and different energy positions of surface states. An analysis of the W 4$\int_{\frac{\tau}{2}}$ core level spectra has resulted in an additional contribution of the step edges in the spectra of the vicinal surfaces wich a surface core level shift between 120 and 150meV. A Cu and Co coverage dependent investigation of the core levels shows that there is no Co induced surface reconstruction and up to 0.15 monolayer no Cu induced surface reconstruction. In the range of 0.15 and 0.3 monolayer Cu the surface peak shifts to higher binding energies. This is probably a result of a surface reconstruction of the W substrat. In the core level spectra wich Co coverage the intensity of the surface peak decreases linear with Co coverage and the intensity of a new contribution, the interface structure, increases with Co coverage. With Co respectively Cu coverage the contribution of the step edge shifts to lower respectively higher binding energies. This can be attributed to a charge transfer between the adsorbat and the substrat in different directions. For coverages of 0.1 monolayer Co respectively Cu electronic states near the fermi edge has been observed. These states show a dispersion with k$_{\parallel}$ along the step edges but they show no dispersion perpendicular to the step edges. This is the first evidence of a quasi-one-dimensional structure (Co respectively Cu quantum wire) an vicinal W(110) surfaces

Theoretical study of O adlayers on Ru(0001)

Recent experiments performed at high pressures indicate that ruthenium can support unusually high concentrations of oxygen at the surface. To investigate the structure and stability of high coverage oxygen structures, we performed density functional theory calculations, within the generalized gradient approximation, for O adlayers on Ru(0001) from low coverage up to a full monolayer. We achieve quantitative agreement with previous low energy electron diffraction intensity analyses for the (2x2) and (2x1) phases and predict that an O adlayer with a (1x1) periodicity and coverage of 1 monolayer can form on Ru(0001), where the O adatoms occupy hcp-hollow sites.Comment: RevTeX, 6 pages, 4 figure