[Back to First Page]
[Back to Publications]
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.
[Back to First Page]
[Back to Publications]
(Last modified: 15 November 1996)