Pd layers on a W(100) surface

Pd films from zero to several monolayers in thickness on a W(100) surface are studied by AES, LEED, work function change (A0) measurements and TDS. Similar to other metals on W, the adsorption and annealing behaviour differs drastically from that on the W(110) surface but resembles the behaviour of other metals on the W(100) and Mo(100) surface.     

Desorption kinetics and directional dependence of the surface diffusion of potassium on stepped W(100) surfaces

Using a surface ionisation ion microscope the desorption parameters and the diffusion constant of potassium were measured on stepped W(100) surfaces. The activation energy of ionic desorption as well as the corresponding prefactor do not depend on the step density; the mean adsorption lifetime τ can be expressed as τ=1.6×10^?14s exp(2.44 eV/kT).Whereas the surface diffusion of potassium on “flat” W(100) and on W(S)-[9(100)×(110)] was found to be isotropic, on W(S)- [5(100)×(110)] and W(S)-[3(100)×(110)] it occurs preferentially parallel to the step direction. The diffusion constant D_∥ for this direction has roughly the same value for all investigated surfaces: D_∥=7.8×10^?2 cm²s^?1 exp(?0.42 eV/kT). For the direction perpendicular to the steps D⊥ depends on the step density, whereby the activation energy as well as the prefactor increase with increasing step density.     

LEED study of the stepped surface of vicinal Si (100)

Periodic stepped surfaces prepared on vicinal silicon (100) by ion bombardment and annealing have been studied by LEED. An unusual aspect of the diffraction patterns from [01̄1] zone samples was the presence of fractional order multiplets with half the splitting of the integral order doublets. An explanation is given, based on the superposition of diffraction from three distinct domain types. These differ in the orientations of their dangling bonds and the arrangements of successive terraces. The simpler patterns from [001] zone samples evidenced just two reconstruction domains. All non-integral order beams disappeared when samples were exposed to ionized hydrogen. This facilitated both the identification of certain lattice distortions, and the determination of step heights, which were found to be two atomic layers in all cases.     

Hydrogen-induced reconstruction on W(100) and Mo(100) by surface infrared spectroscopy

Infrared absorption and low-energy electron-diffraction measurements of H adsorbed on W(100) and Mo(100) show that on each surface, distinct wavenumbers characterize the H-substrate stretching modes associated with the different long-range structures of the complicated T-θ phase diagram. Hydrogen is bonded at a two-fold bridge site at all temperatures and coverages investigated and the wavenumber of the symmetric stretch mode, v₁, is determined by the local geometry, i.e. the substrate dimer length. Analysis of the coverage dependence of the v₁ wavenumber shows that, at low coverages (θ ≲0.3), the effective H-H interactions are very different for the two substrates, leading to a uniform H layer on W(100) and to island formation on Mo(100). In general, the phase transitions are continuous on W(100), with regions of intermediate structures, and first order on Mo(100), with regions of coexisting phases.     

Oxygen adsorption on stepped Pd(100) surfaces

We use scanning tunneling microscopy (STM) and high-resolution core-level spectroscopy (XPS) measurements to study the initial oxidation of vicinal Pd(100) surfaces exhibiting close-packed (111) steps. The XPS data analysis is supported by detailed surface-core level shift calculations based on density-functional theory. Both STM images and the XPS spectra are found to be perfectly consistent with a characteristic zigzag O decoration of the Pd steps predicted by a preceding cluster-expansion based theoretical study [Y. Zhang and K. Reuter, Chem. Phys. Lett. 465, 303 (2008)]. Continued oxygen uptake leads to the additional stabilization of a p(2 × 2)-O overlayer on the Pd(100) terraces, and ultimately to step bunching with the resulting large Pd(100) terraces covered by the PdO(101) surface oxide.     

Growth of germanium thin films on the W(100) surface

The growth of Ge thin films on the surface of a textured predominantly (100)-oriented tungsten ribbon is studied by thermal desorption spectrometry at different substrate temperatures over a wide range of coverages. The mechanism of growth of the Ge films at T = 300 K is similar to a layer-by-layer mechanism. For T > 300 K, the films grow through the Stranski-Krastanov mechanism, according to which the completion of the monolayer coverage is followed by the formation of three-dimensional crystallites; as a result, the desorption kinetics changes. For small coverages (i.e., in the absence of lateral interactions), the activation energy of Ge desorption from W(100) is E = 4.9 ± 0.2 eV. In a monolayer, this activation energy decreases to E = 3.9 ± 0.2 eV due to the repulsive lateral interactions. The energy of pairwise lateral interactions is determined to be ω = 0.3 eV.     

Distinct binding states in adsorption on a homogeneous surface: Te on W(100)

The adsorption of Te on a W(100) surface is studied by thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and work function change (Δ?) measurements. Three distinct binding states are observed in the first monolayer corresponding the coverages from 0 to $\text{1}\text{2}$ monolayers (ML), $\text{1}\text{2}$ to $\text{2}\text{3}$ ML and$\text{2}\text{3}$ to 1 ML. Within each state a coverage dependence of the desorption parameters is found. The three binding states are discussed in terms of heterogeneity induced by lateral interactions and in terms of inherently different adsorption sites.     

Site-selective adsorption of xenon on a stepped Ru(0001) surface

The adsorption of xenon has been studied with UV photoemission (UPS), flash desorption (TDS) and work function measurements on differently conditioned Ru(0001) surfaces at 100 K and at pressures up to 3 × 10^?5 Torr. Low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) served to ascertain the surface perfectness. On a perfect Ru(0001) surface only one Xe adsorption state is observed, which is characterized by$\text{Xe}\text{5}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$,$\text{1}\text{2}$ electron binding energies of 5.40 and 6.65 eV, an adsorption energy of E_ad≈ 5 kcal/mole and dipole moment of μ'_T ≈ 0.25 D. On a stepped-kinked Ru(0001) surface, the terrace-width, the step-height and step-orientation of which are well characterized with LEED, however, two coexisting xenon adsorption states are distinguishable by an unprecedented separation in$\text{Xe}\text{5}\text{p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$,$\text{1}\text{2}$ electron binding energies of 800 meV, by their different UPS intensities and line shapes, by their difference in adsorption energy ofΔE_ad ≈ 3 kcal/mole and finally by their strongly deviating dipole moments of μ_S = 1.0 D and μ_T = 0.34 D. The two xenon states (which are also observed on a slightly sputtered surface) are identified as corresponding to xenon atoms being adsorbed at step and terrace sites, respectively. Their relative concentrations as deduced from the UPS intensities quantitatively correlate with the abundance of step and terrace sites of the ideal TLK surface structure model as derived from LEED. Furthermore, ledge-sites and kink-sites are distinguishable via E_ad. The E_ad heterogeneity on the stepped-kinked Ru(0001) surface is interpreted in terms of different coordination and/or different charge-transfer-bonding at the various surface sites. The enormous increase in Xe 5p electron binding energy of 0.8 eV for Xe atoms at step sites is interpreted as a pure surface dipole potential shift. —The observed effects suggest selective xenon adsorption as a tool for local surface structure determination.