L. Hammer

The (0001)-surface of 6HSiC: morphology, composition and structure

Abstract The morphology, composition and structure of wet chemically prepared 6HSiC(0001) samples were investigated immediately after introduction into vacuum. Scanning tunneling microscopy displayed large atomically flat areas on the surface. The step structure found is correlated to the sample periodicity normal to the surface. Most step heights are multiples of half the vertical unit cell length. Low-energy electron diffraction (LEED) revealed good surface order with bulk-like lateral periodicity. From high-resolution electron energy loss spectroscopy the saturation of dangling bonds with hydroxyl species could be determined. This termination is responsible for an electron beam sensitivity found in LEED. Upon annealing the oxygen is removed and carbon-carbon bonds develop on the surface as demonstrated by Auger electron spectroscopy. This new structure is ordered in a (√3 × √3)R30° periodicity.

Strong relaxations at the Cr2O3(0001) surface as determined via low-energy electron diffraction and molecular dynamics simulations

Abstract The surface structure of Cr 2 O 3 (0001) was investigated by quantitative low-energy electron diffraction and molecular dynamic simulations. In qualitative agreement with each other, both methods indicate strong vertical relaxations at and near the surface. These relaxations are concomitant with a charge reduction and depolarization, which stabilize the surface, yielding energies close to those found for non-polar oxide surfaces with non-divergent surface potentials. The lateral arrangement of oxygen atoms is identical to that in the bulk, i.e. there are no lateral distortions to accomodate the strong interlayer relaxations. The latter extend extend deep into the surface, with the experimentally determined changes of the first four interlayer distances being −38%, −21%, −25% and +11% with respect to the unrelaxed bulk values.

Subsurface cation vacancy stabilization of the magnetite (001) surface

Iron oxides play an increasingly prominent role in heterogeneous catalysis, hydrogen production, spintronics, and drug delivery. The surface or material interface can be performance-limiting in these applications, so it is vital to determine accurate atomic-scale structures for iron oxides and understand why they form. Using a combination of quantitative low-energy electron diffraction, scanning tunneling microscopy, and density functional theory calculations, we show that an ordered array of subsurface iron vacancies and interstitials underlies the well-known (2×2)R45° reconstruction of Fe3O4(001). This hitherto unobserved stabilization mechanism occurs because the iron oxides prefer to redistribute cations in the lattice in response to oxidizing or reducing environments. Many other metal oxides also achieve stoichiometry variation in this way, so such surface structures are likely commonplace. The surface reconstruction of magnetite is explained more accurately with the inclusion of subsurface cation vacancies. [Also see Perspective by Chambers] Stabilization of the surfaces of magnetite Accurate structures of iron oxide surfaces are important for understanding their role in catalysis, and, for oxides such as magnetite, applications in magnetism and spin physics. The accepted low-energy electron diffraction (LEED) structure for the surface of magnetite, in which the bulk surface termination undergoes an undulating distortion, has a relatively poor agreement with experiment. Bliem et al. show that the LEED structure is much more accurately described by a structure that includes subsurface cation vacancies and occupation of interstitial sites (see the Perspective by Chambers). Such cation redistribution occurs in many metal oxides and may play a role in their surface structures. Science, this issue p. 1215; see also p. 1186

A LEED investigation of clean and oxygen covered Rh(100)

The clean and oxygen covered Rh(100) surface is investigated by LEED intensity measurements and calculations. For the clean surface an almost unrelaxed surface with a slight first layer expansion (0.5 ± 2%) results. Oxygen is made to adsorb at low temperatures whereby two superstructures develop. A (2 × 2) structure at coverage θ = 14 is followed by a (2 × 2) p 2gg structure at θ = 12. The symmetry of the latter pattern and the inspection of the intensities of the corresponding fractional order spots indicate towards a weak adsorbate induced surface reconstruction. The (2 × 2) structure, however, develops on the unreconstructed surface. For this phase a quasidynamical intensity analysis followed by a full dynamical refinement identifies oxygen as adsorbed in hollow sites. The adsorbate-substrate layer distance is 0.95 ± 0.04 A corresponding to an oxygen radius of 0.78 ± 0.02 A. The expansion of the first substrate layer distance found for the clean surface is reversed to a contraction of the same value. Experimental and calculated spectra agree well as described by a Pendry R-factor of 0.27.

Oxygen diffusion through thin Pt films on Si(100)

We have investigated the diffusion of oxygen through evaporated platinum films on Si(100) upon exposure to air using substrates covered with Pt films of spatially and continuously varying thickness (0–500 Å). Film compositions and morphologies before and after silicidation were characterized by modified crater edge profiling using scanning Auger microscopy, energy-dispersive X-ray microanalysis, scanning tunneling microscopy, and transmission electron microscopy. We find that oxygen diffuses through a Pt layer of up to 170 Å forming an oxide at the interface. In this thickness range, silicide formation during annealing is inhibited and is eventually stopped by the development of a continuous oxide layer. Since the platinum film consists of a continuous layer of nanometer-size crystallites, grain boundary diffusion of oxygen is the most probable way for oxygen incorporation. The diffusion constant is of the order of 10−19 cm2/s with the precise value depending on the film morphology.

Oxidation of low-index FeAl surfaces

Abstract The oxidation of the (100), (110) and (111) surfaces of the intermetallic compound FeAl has been investigated using LEED and XPS. On all three surfaces, oxidation at room temperature leads to the formation of an amorphous oxide film on top of an Al-depleted interlayer. The film growth can be divided into two regions of differing kinetics, i.e. the initial formation of a closed oxide film and a subsequent thickening. In the first region, the oxygen-uptake rate varies significantly with surface orientation, while in the thickening regime the uptake is the same for all surfaces. The maximum thickness as well as the composition of the oxide films were found to depend on the initial oxidation rate. At higher oxidation temperatures, ordered oxide films of around 5–8 A in thickness are formed, very similar to those observed on NiAl. Photoemission spectra from these ordered phases showed evidence for Al atoms in two different chemical environments, i.e. the well-known oxide species in the interior of the film and an additional species present at the oxide/alloy interface.

Erratum to: “Strong relaxations a the Cr2O3(0001) surface as determined via low-energy electron diffraction and molecular dynamics simulations” [Surf. Sci. 372 (1997) L291]

In the LEED intensity analysis, covered by the Erlangen group, the Debye temperature of oxygen was erroneously chosen to be far too low. The fit of all layer Debye temperatures to the experimental data, which has now been carried out in addition and i n parallel to the structural parameters, changes the interlayer relaxation values. This is in contrast to general experience, according to which the consideration of thermal vibrations leaves the structure more or less unchanged but improves the quality of the theory experiment fit. The latter is also true in the present case with the best-fit Pendry R-factor decreasing from 0.32 to 0.26. Using the same procedure of analysis as in the original paper, the best fit develops for O o = 6 0 0 K, OCr--630 K and the interlayer distance relaxations (starting with the top layer) result in a = 6 0 % , b = 3%, c = 2 1 % and d = +6%. In addition to the original work, the next interlayer distance between Oand Cr-layers was also varied and was found to relax by e = +2%. The new values confirm our earlier message that the relax-

LEED structure analysis of p(√3 × √3 R30°-O/Ni(111)

Abstract The p(√3 × √3)R30 ° structure formed by oxygen adsorption on Ni(111) at coverage θ = 0.33 was investigated by LEED intensity measurements and their full dynamical analysis. Oxygen is found to reside on threefold coordinated fcc hollow sites at height 1.08 ± 0.02 A above the substrate with a Ni-O bondlength of 1.80 ± 0.02 A. Compared to the unrelaxed clean surface, which was also analysed, the topmost nickel layer spacing is slightly expanded to 2.05 ± 0.02 A (bulk value: 2.03 A) whereas the second to third interlayer spacing is practically unrelaxed (2.02 ± 0.02 A). A lateral reconstruction of the first substrate layer as found for the p(2 × 2) phase can be ruled out, i.e., lateral atomic shifts must be smaller than the error limit of 0.03 A. Vertical buckling in the first substrate layer is excluded by symmetry. This means that the reconstruction of the p(2 × 2) phase is lifted in the transition p(2 × 2)→ p(√3 × √3)R30°, i.e., by a coverage increase from θ = 0.25 to θ = 0.33. The quality and reliability of the structure determinations are mirrored by a Pendry R -factor of R = 0.16 for both the clean and oxygen-covered surface.

Ordered phases of C2H2 and C2H2 on the Ni(111) face

Combined LEED and EELS measurements have been performed to study the ordering behaviour of C2H2 and C2H4 on Ni(111). Each system forms three different phases of long-range order [(2 × 2), (3 × 3)R30°, (23 × 3)R30° for C2H2; c(4 sx 2), (2 × 2), (31122) for C2H4] depending on temperature and exposure. EELS spectra indicate that each series of structure phases consists of the same surface species. LEED intensity spectra taken from both the (2 × 2) phases show strong similarities which leads to the qualitative conclusion that C2H2 and C2H4 both prefer the same adsorption site. A kinematical evaluation of superstructure spots of the non-primitive phases (23 × 3)R30° and (31122) together with an antiphase-splitting observation leads us to suggest bridge positions as adsorption sites for the carbon atoms of the molecules.

Hydrogen on W(110): an adsorption structure revisited

Abstract We present a quantitative study of the atomic structure of the clean and H-covered W(110) surface employing an analysis of lowenergy electron diffraction (LEED) as well as density functional theory (DFT) calculations. Our results give no evidence of a noteworthy reconstruction of the W(110) surface upon H-adsorption, and thus discard the widely accepted model of a H-induced lateral shift of the top layer based on earlier LEED data. Moreover, we offer a reinterpretation of the latter which goes along without such a surface reconstruction. In detail, we find good agreement between the LEED analysis and the DFT calculations on a small contraction of the first interlayer distance by about 3% for the clean surface, which is reduced to half this size at full H coverage. Hydrogen itself is found to be adsorbed in quasi-threefold coordinated hollow sites at a height of about 1.20 A above the first substrate layer.