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Abstracts:

Phys. Rev. Lett. 71(7) 1059-1062 (1993)

Experimental Evidence for Anticorrugating Effects in He-Metal Interactions at Surfaces

K.H. Rieder, G. Parschau, and B. Burg

We show that systematic differences occur in the corrugations of the systems Ni(110)c(2x4)H, and Rh(110)(1x3)H and (1x2)H, when determined from He and Ne diffraction: Whereas the hills along the H-free close-packed metal rows correspond to underlying metal atoms in all corrugations deduced from Ne diffraction, they are shifted to short bridge sites in all He-derived corrugations. This assignment is possible, as the H adatoms mark threefold hollow sites on both (110) substrates. These observations provide strong experimental support for anticorrugating effects in He-metal surface interactions as proposed by Annett and Haydock [Phys. Rev. Lett. 53, 838 (1984)].

Surf. Sci. 297, 162-166 (1993)

A He-scattering study of the hydrogen-induced "streaky phase" of Ni(11O)

D. Farías, I. Eckert, B. Burg and K.H. Rieder

The structure of the (1x2) "streaky phase" formed upon adsorption of deuterium on Ni(110) was studied using He scattering. The presence of a pronounced rainbow pattern in the in-plane-spectra observed with the beam incident along [001] signals unambiguously that the "streaky phase" is of the missing-row structure type. The corrugation of the (1x2) troughs could be determined as approx. 1.55 Å, which represents an increase of 0.3 Å with respect to the estimated corrugation of a clean Ni-missing-row surface; this observation suggests that the deuterium atoms occupy sites near the ridges of the missing-row troughs. From out-of-plane measurements of the remnant (2x1) phase, the fraction of unreconstructed Ni domains coexisting with the missing-row structure has been estimated as 10%.

Surf. Rev. and Lett. 1(1) 51-65 (1994)

Surface structural research with atom beam diffraction: Helium versus Neon

K.H. RIEDER

A brief review on surface structural studies with He and Ne atom beam diffraction is presented. It is shown that for scattering from clean and hydrogen-covered metal surfaces with Ne, systematically larger corrugation amplitudes are obtained so that charge density contours closer to the surface atom cores are scanned. This leads to an enhanced sensitivity towards details of atomic arrangements at surfaces and sometimes surface bonding aspects can also be revealed. Most importantly, however, Ne seems to give more faithful pictures of surface atomic arrangements than He, probably due to larger influences of anticorrugating effects in He-metal surface interactions.

Phys. Rev. Lett. 73(23), 3125-3128 (1994)

Water Adsorption on Perfect CaF2(111) Studied with He Scattering: Experimental Evidence for Ordering of Nanoclusters

Annemarie Lehmann, Gregor König, and Karl-Heinz Rieder

Elastic and inelastic He-atom scattering were used to nondestructively investigate the water adsorption on UHV cleaved CaF2(111) surfaces. Below 145 K a p(4x4) adsorbate structure was observed. The corrugation function deduced from the diffraction intensities directly shows the formation of a two-dimensional periodic structure from water nanoclusters. Using the observed lateral extension and employing the Bernal-Fowler-Pauling rules, we arrive at six H2O molecules per cluster. This surprising result is confirmed by the observation of a very low energetic Einstein mode at approx. 6 meV.

J. Phys.: Condens. Matter 6, 9519-1950 (1994)

REVIEW ARTICLE

The intrinsic stress of polycrystalline and epitaxial thin metal films

R. Koch

It is well known that thin films develop large intrinsic stress during their preparation. The intrinsic stress either originates from strained regions within the films (grain boundaries, dislocations, voids, impurities, etc) or at the film/substrate (lattice mismatch, different thermal expansion, etc) and film/vacuum interfaces (surface stress, adsorption, etc) or is due to dynamic processes (recrystallization, interdiffusion, etc). Since the magnitude of most of these stress contributions is directly related to film morphology, important structural information can be extracted from measurements of the intrinsic stress. This article presents a thorough discussion of today's understanding of the growth of thin films and reviews the related atomistic mechanisms responsible for intrinsic stress. On the basis of these ideas recent experimental results on the intrinsic stress of UHV deposited polycrystalline and epitaxial thin metal films are discussed. Depending on the respective growth mode of the films - Volmer-Weber, Stranski-Krastanov and Frank-Van der Merwe modes - characteristic stress behaviours are observed. In situ intrinsic stress measurements are therefore a promising new technique to gain additional insight into film growth.

Surf. Sci. 331-333, 1430-1434 (1995)

He atom scattering studies of CaF2 (111): surface properties and phonons

Gregor König, Annemarie Lehmann, K.H. Rieder

CaF2 is important for many practical applications. Various optical devices are made from fluoride or are coated with fluoride films. Epitaxially grown CaF2 is considered to be an appropriate insulator in microelectronics. The bulk cubic crystal is well characterized and understood, but the knowledge about the surface properties, as stability, defects and adsorption behaviour, in fact is less extensive. Therefore, He atom scattering studies of the CaF2 surface were performed. The preparation was done by cleavage in UHV. Diffraction spectra show a well ordered surface of a slightly better quality than polished and annealed ones. Time-of-flight measurements were made at primary energies between 21 and 26 meV. As a result, the dispersional relation of the pseudo Rayleigh mode was obtained throughout the entire surface Brillouin zone in reasonable agreement with recent calculations.

Chem. Phys. Lett. 235, 65-68 (1995)

He-diffraction study of defect correlated water adsorption on the CaF2 (111) surface

Annemarie Lehmann, Gregor König, K.H. Rieder

Water adsorption on the CaF2 (111) cleavage plane was investigated by He-beam scattering after disturbing the surface with electron beams (with energies of 100, 300 and 500 eV) and ion beams. It is shown that different types of defects are produced, which sensitively influence both the He-diffraction intensities and the background scattering. Also, the adsorption behaviour of water on CaF2 (111) is drastically changed. Water adsorption is observed even at room temperature due to electron induced defects. However, if the surface is sputtered with 500 eV Ne+ ions, active defect sites are not produced and water adsorption does not occur up to 3 X 103 L.

Surface Science 331-333, 150-155 (1995)

He-scattering studies on the adsorption of oxygen on Rh(311)

D. Farías, H. Tröger, K.H. Rieder

The adsorption of oxygen on Rh (311) has been investigated by means of He scattering. The formation of ordered (2x1) and (1x3) structures was observed. (The notation refers to the centered rectangular unit cell of the substrate.) Ordered oxygen structures could only be prepared by annealing oxygen pre-exposed surfaces to at least 450 K. Both oxygen structures are stable up to 900 K. For the (2x1) phase evidence is presented that suggests that the oxygen atoms are adsorbed along every second close-packed row. Oxygen removal from the (1x3) phase by hydrogen at 430 K leads to a c(1x3) structure, which is metastable and reverts to the clean c(1x1) surface above 500 K. The presence of a pronounced rainbow pattern for the c(1x3) signals unambiguously that its structure is of the missing-row type. This suggests that oxygen induces a surface reconstruction in the (1x3) phase, whereby at least one out of three close-packed rows is missing and the oxygen atoms fill the resulting deep troughs.

Surface Science 331-333, 57-61 (1995)

He-diffraction studies of hydrogen chemisorption on rhodium(311)

R. Apel, D. Farías, H. Tröger, K.H. Rieder

The (311) surfaces of fcc materials are of special interest as they offer, due to the existence of (111) and (100) microfacets, a large variety of differently coordinated adsorption sites. He atom beam diffraction experiments were performed on Rh(311) in order to investigate the formation of ordered chemisorption structures of hydrogen. The observation of c(1x3), p(1x1) and p(1x1 )rec phases is in agreement with previous LEED studies. However, before the formation of the c(1x3), a c(1x1) phase was identified with He diffraction, which requires a coverage of 1 ML. Therefore, the initial sticking coefficient of hydrogen is practically unity and the saturation H-coverage reached in the p(1x1)rec phase is 2-3 ML, much larger than anticipated.

Surf. Rev. and Lett. 2(2) 153-157 (1995)

Chemisorption site determination of hydrogen in the system c(1x1)H/Rh(311) employing He-diffraction

R. Apel, D. Farías and K.H. Rieder

He-beam diffraction data evaluated for clean Rh(311) yield a best-fit corrugation, which shows a pronounced corrugation amplitude along the (100)-microfacets and a practically vanishing amplitude along the (111)-microfacets in agreement with expectation, but in contrast to Ni(311) where the reverse situation was found. Hydrogen adsorption leads to a sequence of c(1x1), c(1x3), p(1x1), c(1x3), and p(1x1)rec phases. The fact that the c(1x1) phase was observed with He-diffraction, but not in previous LEED studies, again proves the exceptional sensitivity of He scattering for adsorbed hydrogen. Surface charge density calculations were performed to reproduce the shape and amplitude of the best-fit corrugation function of the c(1x1)H-phase. In this way we found that the H atoms occupy places between the topmost bridge sites and the adjacent threefold hollow sites on the (111)-microfacets; the H-Rh bond length amounts to 1.92 ± 0.1 Å.

Chem. Phys. Lett. 233, 569-574 (1995)

Adsorption of CO on Ni(311). Investigations with HREELS

P. Schilbe, S. Siebentritt, K.H. Rieder

HREELS investigations of CO adsorption on the Ni(311) surface are presented. The results show a striking similarity to CO adsorption on the Ni(110) surface which proves that the crucial structural features that govern the adsorption behavior are the close packed rows. In addition to the on top and bridge sites occupied at the Ni(110) surface there is evidence that threefold adsorption sites are occupied as well on the Ni(311) surface at saturation at low temperatures. Our results also support the reliability of the frequency rule of CO adsorption at on top and bridge sites if one takes into account the coverage dependence of the frequencies. Coadsorption with hydrogen drastically changes the adsorption behavior of CO.

phys. stat. sol. (b) 192, 313-324 (1995)

Building Nanostructures by Controlled Manipulation of Single Atoms and Molecules with the Scanning Tunneling Microscope

G. Meyer, B. Neu, and K.H. Rieder

Our present experiences in manipulating single atoms and molecules on solid surfaces by using the tip of a scanning tunneling microscope (STM) are reviewed. The construction and performance characteristics of our temperature variable STM is shortly described, which allows investigations in the temperature range between 15 and 300 K. As example for horizontal manipulation, in which the adatoms or admolecules are pulled along the surface by the STM tip, and the formation of man-made surface structures, the formation of the letters 'F' and 'U', which form the logo of our university is presented. The adspecies used in these experiments are CO molecules and the substrate a Cu(211) surface. An example of vertical manipulation, in which an adparticle is picked up by the tip, carried to another place and released back to the surface, concerns Xe on the same substrate. The latter technique allows also to vary the chemical composition of the tip apex in a well-defined manner. Consequences in imaging the surface with tips of different compositions are presented and discussed. Finally, STM observations of ordered high density phases of CO on the Cu(211) substrate are discussed.

Appl. Phys. A 60, 343-345 (1995)

Rapid communication

Controlled lateral manipulation of single molecules with the scanning tunneling microscope

G. Meyer, B. Neu, K.H. Rieder

We report on the first successful lateral manipulation of molecules and controlled formation of nanostructures with the Scanning Tunneling Microscope (STM) at temperatures above 4 K as used by Eigler and collaborators. Among the first structures, we built the letters F and U forming the logo of our university at 30 K with CO molecules on a Cu(211) substrate. Our method to manipulate the molecules is analogous to that employed successfully up to now only by Eigler and co-workers. First experiences concerning the manipulation of the CO molecules on the highly anisotropic substrate are presented and the crucial role of the tip composition in imaging the CO molecules is demonstrated.

Appl. Phys. Lett., in print (1996)

Morphology of strained epitaxial Fe films on Cu(311)

S. Fölsch, B.-Ch. Choi, and K.H. Rieder

The heteroepitaxial growth of Fe on the asymmetrical Cu(311) surface leads to (211)-oriented strained bcc films with a highly anisotropic morphology. In the initial stage the evolution of pseudomorphic Fe islands is observed: anisotropic lateral stress induces the formation of a regular array of Fe patches with a periodic separation. With proceeding growth the overlayer transforms to bcc structure by means of dislocations introduced along the direction of dominant stress. The topography of the resulting bcc film corresponds to a regular faceted structure whose periodicity is determined by the arrangement of pseudomorphic islands formed in the initial stage of growth.

Phys. Rev. B, submitted (1996)

Ultrathin epitaxial Iron films on a highly asymmetrical substrate: Fe/Cu(311)

S. Fölsch, B.-Ch. Choi, and K.H. Rieder

The structure and morphology of ultrathin epitaxial Fe films on Cu(311) is examined in situ by spot profile analyzing-low energy electron diffraction (SPALEED). Cu(311) is a highly asymmetrical substrate which is characterized by a trough-like surface structure composed of uniaxially arranged close-packed Cu atom rows. Deposition at 140 K initially leads to the formation of pseudomorphic Fe. The constraint to lower dominant stress along the close-packed atom rows forces the formation of pseudomorphic patches with a quasi-periodic separation of 35 Å. With proceeding growth these patches act as nucleation sites for 3D islands which gradually relax via a Pitsch transformation towards a strained bcc structure. Coalescense occurs at approx. 5 ML and produces a periodically faceted growth front of the overlayer. The resulting surface topography corresponds to a regular up-and-down staircase and remains unchanged for coverages > 5 ML: the periodic facet separation measures 35 Å, 7-8 layers contribute to the growth front. As a consequence of coalescense the film locks into a lattice arrangement which is predominantly dictated by the substrate and is likely to correspond to an fcc-like structure. The periodic facetting of the film surface, on the other hand, enables that partial relaxation towards strained bcc structure persists in the top layers of the film.

Surf. Sci. 352-354, 155-160 (1996)

He-diffraction studies of hydrogen adsorption on Pd(311)

D. Farías, H. Tröger, M. Patting, K.H. Rieder

The formation of ordered structures of hydrogen on Pd(311) at a substrate temperature of 110 K has been investigated with low-temperature He beams. We found, before the completion of a saturated c(1x1) phase, the formation of three low-coverage (2x1) phases (the notation refers to the centered rectangular unit cell of the substrate). The first (2x1)H structure is completed after an exposure of just 0.09 L, suggesting - with an initial sticking coefficient near unity - a coverage of 0.25 ML where hydrogen atoms adsorb along every second close-packed row. Upon additional H2 exposure, this phase evolves into the (2x1)2H phase, which reaches its optimum order after 0.20 L and shows a glide symmetry plane along [2-3-3]. Progressive filling of the close-packed rows leads to the appearance of a (2x1)3H phase and finally to the formation of the c(1x1). Diffraction spectra for this phase show only slight changes upon H2 exposure in the range 0.5-10 L. The existence of three low-coverage hydrogen overlayers for coverages < 1 ML suggests an interaction mechanism between hydrogen atoms which is very different to the one present on Pd(110). Possible structural models for the reported phases are discussed.

Surface Review and Letters (1996) - accepted

Surface phonon dispersion in graphite and its modification under interaction with lanthanum

S. Siebentritt, R. Pues, A. M. Shikin, K.H. Rieder

Using high resolution electron energy loss spectroscopy (HREELS) the surface phonon dispersion of graphite has been determined in the GK-direction over the whole energy range and the whole Brillouin zone. By depositing lanthanum and annealing we prepared a GIC-like phase which grows on top of an intermediate carbide. The phonon dispersion of this phase shows the same modes as graphite, but the optical ones are softened and the acoustical ones are stiffened. This is described within a Born-von Karman model. The evolution of the phonon dispersion gives a first hint that the GIC-like phase may develop in two stages: first a monolayer graphite on top of the carbide and then the very thin GIC layer.

Phys. Rev. B (1996) - submitted

Surface Phonon Dispersion in Graphite and in a Lanthanum Graphite-Intercalation-Compound

S. Siebentritt, R. Pues, K.H. Rieder

Using high resolution electron energy loss spectroscopy (HREELS) the surface phonon dispersion of graphite has been determined in the GK-direction over the whole energy range and the whole Brillouin zone. Born-von Karman model calculations are used to describe the dispersion relations. An unexpected result is the splitting of the ZA and ZO mode at the K- point. Following a previously introduced procedure to form in situ rare earth graphite intercalation compounds (GIC) which for lanthanum results in an intermediate carbide phase we prepared this carbidic phase and the final GIClike phase. The carbide shows five dispersionless features which may be attributed to Einstein modes of graphite islands. The phonon dispersion of the final phase shows the same modes as graphite shifted in energy: softening of the optical and stiffening of the acoustical phonons occurs. This is described within a Born-von Karman model by weakening the next neighbor interaction and strengthening the second next neighbor interaction. The evolution of the phonon dispersion gives a first hint that the GIClike phase may develop in two stages: first a monolayer graphite on top of the carbide and then the very thin GIC layer.

Surf. Sci. 360 157-170 (1996)

Adsorption of Hydrogen and of Oxygen on an open metal surface - HREELS investigation at Ni(311)

P. Schilbe, S. Siebentritt, R. Pues, K.H. Rieder

The adsorption behavior of hydrogen and oxygen on the stepped Ni(311) surface has been investigated by HREELS. A series of metastable phases is found for hydrogen adsorption at low temperatures with a succession of different adsorption sites indicated by the following loss peaks: 55meV and 149meV for the threefold site, shifting with higher coverage to 65meV and 155meV, respectively; 40meV and 90meV for the fourfold site, shifting to 35meV and 85meV with coverage; and 110meV and 124meV for an additional site between close packed rows. Room temperature adsorption of hydrogen leads to the reconstruction of the surface with occupation of threefold and fourfold sites, represented by loss peaks at 60meV and 145meV for the threefold site and 74meV for the fourfols site. Oxygen is most likely initially adsorbed on a bridge site (loss peak at 66meV). The stepped surface is already oxidized at very low exposures to oxygen, as seen by the characteristic vibration for oxide islands at 55meV and later on by the Fuchs-Kliewer mode of NiO at 68meV.

Phys. Rev. Lett. 77, 2113 (1996)

Scanning Tunneling Microscopy Manipulation of Native Substrate Atoms:
A New Way to Obtain Registry Information on Foreign Adsorbates

G. Meyer, S. Zöphel, K. H. Rieder

We demonstrate successful lateral manipulation of host atoms on Cu(211): In a controlled manner we are able to release Cu atoms from upper terraces, move them on the lower terrace to form dimers and trimers and adhere them to the terrace edge again. We apply this novel ability to deduce information on adsorption sites of foreign species: By bringing Cu atoms close to CO molecules we determine the location of the adsorbate. The sites of C2H4 and Pb are then found via coadsorption with CO.

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(Last modified: 15 November 1996)