The oxidation of Pd(111) leads to an incommensurate surface oxide, which was studied by the use of scanning tunneling microscopy, surface x-ray diffraction, high resolution core level spectroscopy, and density functional calculations. A combination of these methods reveals a two-dimensional structure having no resemblance to bulk oxides of Pd. Our study also demonstrates how the atomic arrangement of a nontrivial incommensurate surface can be solved by molecular dynamics in a case where experimental techniques alone give no solution.

We present a structural determination of the surface reconstruction of the Sm(0001) surface using surface x-ray diffraction, scanning tunneling microscopy, and ab initio calculations. The reconstruction is associated with a large (22%) expansion of the atomic radius for the top monolayer surface Sm atoms. The mechanism driving the surface reconstruction in Sm is unique among all elements and is connected to the strong correlations of the 4f electrons in Sm and the intermediate valence observed in certain Sm compounds. The atoms constituting the top monolayer of Sm(0001) have vastly different chemical properties compared to the layer underneath and behave as if they were an adsorbate of a different chemical species.

The adsorption of 2.4 Langmuir oxygen on V(I 10) induces a c(6 x 2) reconstruction with an oxygen coverage of 0.5 ML. Its structure was determined using STM, quantitative LEED and ab initio density functional calculations in combination with molecular dynamics. Driven by the strong vanadium-oxygen bonding, the vanadium atoms at the surface are significantly rearranged compared to their bulk positions. The reconstructed geometry offers threefold and fourfold coordinated hollow sites, which are partially occupied by oxygen. The large set of structural data derived from LEED I V analysis (R-Pc = 0.11) and ab initio calculations, as well as the experimental and simulated STM images agree well. Additionally the structure of clean V(I 10) was determined. (C) 2002 Elsevier Science B.V. All rights reserved.

The initial stages of Pd thin film growth on clean and C- and O-precovered V(100) surfaces at room temperature (RT) and 200degreesC have been studied by means of scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). In the presence of C and O, the adsorption of Pd atoms in the clean four-fold hollow sites of the V surface is strongly preferred. Upon deposition of Pd in the submonolayer range, it was possible to stabilize single Pd adatoms and small clusters at RT. Annealing such a surface at 200degreesC leads to the formation of rectangular Pd islands (size approximate to40 x 40 Angstrom(2)) and to the compression of the initial C and 0 adlayer in the areas between the Pd. For higher Pd coverages we found that oxygen acts as an anti-surfactant, shifting the onset of second layer growth dramatically. The reason for the change of the growth mode of Pd on V(100) from Stranski-Krastanov to a Volmer-Weber-type growth in the presence of oxygen can be found in a higher free energy of the film/substrate interface compared to the clean surface. (C) 2001 Elsevier Science B.V. All rights reserved.

The interaction of oxygen with Pd particles, vapor deposited onto a thin alumina film grown on a NiAl(1 1 0) substrate, was studied by STM, AES, LEED, XPS, TPD and molecular beam techniques. The results show that O2 exposure at 400–500 K strongly influences the oxide support. We suggest that the oxygen atoms formed by dissociation on the Pd surface can diffuse through the alumina film and react with the NiAl substrate underneath the Pd particles, thus increasing the thickness of the oxide film. The surface oxygen inhibits hydrogen adsorption, and readily reacts with CO at 300–500 K. For large and crystalline Pd particles, the system exhibits adsorption–desorption properties which are very similar to those of the Pd(1 1 1) single crystal surface. The molecular beam and TPD experiments reveal that, at low coverage, CO adsorbs slightly stronger on the smaller Pd particles, with an adsorption energy difference of ≈5–7 kJ mol−1 for 1 and 3–5 nm Pd particles studied.

Carbon adsorption on V(100) was studied by both experimental methods and density functional theory. At low carbon coverages of ΘC=0.18 ML and oxygen below the experimental detection limit, measured scanning tunneling microscopy (STM) images show both areas of local c(2×2) structure and 〈010〉 oriented rows of C atoms. At higher coverages of ΘC=0.41 ML, mainly 〈010〉 oriented C rows with some local p(1×2) patterns are formed. The observed c(2×2) pattern is attributed to the presence of oxygen, since a mixture of carbon and oxygen favors the c(2×2) superstructure according to both the STM and the ab initio results. The calculations show that for ΘC=0.50 ML the p(1×2) structure is more stable than c(2×2) by 0.13 eV per adsorbed atom. From the ab initio results it is predicted that p(1×2) changes into c(2×2) at a mixed coverage of about ΘC≈0.37 ML and ΘO≈0.13 ML. The geometry of the c(2×2) structure was determined using quantitative low energy electron diffraction showing good agreement with the ab initio data. Also the simulated STM images agree well with the experimental STM data.

Scanning tunneling spectroscopy measurements on clean V(001), carbon-covered V(001) and the oxygen-induced V(001) (1×5) reconstruction are reported. The clean V(001) surface shows a strong surface state 0.03 eV below the Fermi level. Isolated impurities shift the surface state 0.05 eV upwards in energy and broaden the peak observed in dI/dV. No significant influence of monoatomic steps on the surface state could be observed. For tunneling resistances down to about 1 MΩ the surface state is unaffected by the tip of the scanning tunneling microscope. A surface state is detected around +0.75 eV in small c(2×2) patches which are observed at higher carbon (and oxygen) coverages. The oxygen induced (1×5) reconstruction of V(001) shows a peak at similar energy (+0.63 eV) in the areas with O and C atoms in fourfold hollow sites and a peak around +0.91 eV above the rows of bridge-site oxygen. Ab initio band structure calculations confirm the existence of a surface state of dz2 symmetry with an energy close to that observed experimentally on clean V(001). This agreement provides strong evidence that the V(001) surface is not magnetic (at least at room temperature) as predicted by the calculations. We also compare the experimentally observed peak shifts on the carbon and oxygen covered surfaces with calculational results for carbon-covered geometries.

The adsorption of 1 L oxygen on the V(100) surface at 200°C leads to a (1×5) reconstruction. The structure of this surface was determined by scanning tunneling microscopy (STM), low energy electron diffraction (quantitative LEED) and density functional theory calculations. The STM images show dark lines every fifth vanadium lattice constant. Between these (1×5) lines, oxygen resides in four-fold coordinated hollow sites with a coverage of ≈70%. From the LEED analysis (Pendry R-factor=0.17) it was found that in the dark lines oxygen adsorbs in bridge sites, in agreement with the ab initio calculations. The driving force behind this reconstruction is surface stress induced by the vanadium–oxygen bonds.

The first scanning tunneling spectroscopy measurements on V(001) are reported. A strong surface state is detected which is very sensitive to the presence of segregated carbon impurities. The surface state energy shifts from 0.03 eV below the Fermi level at clean areas towards higher energies (up to approximately 0.2 eV) at contaminated areas. Because of the negative dispersion of this state, the upward shift cannot be described in a simple confinement picture. Rather, the surface state energy is governed by vanadium surface s- d interactions which are altered by carbon coverage.

Adsorption and desorption of oxygen on Pd(111) were studied by high-flux molecular beam adsorption, LEED, TDS and scanning tunnelling microscopy (STM) between 300 and 623 K sample temperature for oxygen coverages ΘO up to 1 ML. While adsorption below ΘO=0.25 is precursor mediated and proceeds without changes of the Pd substrate, it is activated for ΘO>0.25 and induces a massive change of the surface structure. STM reveals the formation of a new surface phase which consists of islands with a local oxygen coverage of 1 ML but less Pd atoms than the bulk-terminated (111) layer. Its formation and decay require activated mass transport of Pd and O atoms over mesoscopic distances. Due to island growth of this phase the oxygen sticking decreases linearly between ΘO=0.25 and 1 ML. For ΘO>0.25 ML the TPD rate maxima are shifted towards higher temperature with increasing initial coverage, indicating autocatalytic desorption kinetics. Desorption occurs preferentially from a dilute chemisorbed phase on Pd(111) terraces, with the islands of the high oxygen-density phase acting as a reservoir for O. The experimental TPD data can be well described by a simple mathematical model considering phase equilibrium during desorption.

We have studied the submonolayer growth of vanadium on the Pd(111) surface at different substrate temperatures. By means of low-energy ion spectroscopy (LEIS), Auger electron spectroscopy (AES) scanning tunnelling microscopy (STM), X-ray photoelectron diffraction (XPD) and ab initio local density functional calculation we find that, at room temperature, deposited vanadium atoms substitute surface palladium atoms. In addition, islands are formed on the surface that consist mostly of the substituted palladium atoms. At higher temperatures vanadium diffuses into subsurface layers and, at a temperature of 300°C, only a small amount of vanadium is observed in the topmost layer. STM revealed the formation of a (3×3)R30° superstructure and XPD measurements demonstrated that this structure is due to vanadium atoms incorporated in the second layer. This finding is confirmed by ab initio calculations. A model for the (3×3)R30° structure based on the experiments and the ab initio calculations is given.

Adsorption of hydrogen and deuterium on V(111) is governed by dynamical steering at low molecular energies whereas at high beam energies direct adsorption is observed. Strong rotational effects and clear isotope effects in the adsorption dynamics of H2 and D2 can be seen. At elevated surface temperatures hydrogen dissolves in the bulk; the absorption coefficient is strongly dependent on surface temperature. For CO a large fraction dissociates upon adsorption. At the clean surface an intrinsic precursor facilitates adsorption and at finite coverages an extrinsic precursor leads to a sticking coefficient independent of coverage. Oxygen can adsorb up to a saturation coverage of 3.8 monolayers followed by surface oxidation. For all three gases calibration procedures for surface coverages are presented as well as quantitative values for the sticking coefficients.