[show abstract][hide abstract] ABSTRACT: A quasi-1D system is prepared using the Pt(110) surface
as a template. The electronic surface resonance structure is studied by
angle-resolved photoemission spectroscopy for the clean surface as well as
for different Bromine coverages. A Fermi surface mapping reveals saddle
points at the Fermi level in the interior of the surface Brillouin zone.
Correspondingly, a maximum in the static response function χ(q, 0) at
the connecting vector q is expected. With 1/2Gx < q < 2/3Gx
one observes indeed a 3-fold periodicity around defects and a 2-fold
periodicity at low temperature for ΘBr = 0.5 ML. Cooling of a
defect-free c(2×2)-Br/Pt(110) preparation counter-intuitively
results in a loss of long-range order. Motivated by DFT calculations this is
attributed to an anomalous order-order phase transition into the (2×1) phase accompanied by intense, strongly anisotropic fluctuations within a
temperature range of ~200 K. The peculiar behaviour is rationalised in
terms of a competition between inter-adsorbate repulsion and an adsorbate
triggered 2kF interaction in the substrate.
Physics of Condensed Matter 08/2013; · 1.28 Impact Factor
[show abstract][hide abstract] ABSTRACT: Adsorption and coadsorption of carbon monoxide and oxygen on different types of Au clusters on R(15 × 3)C/W(110) and R(15 × 12)C/W(110), respectively, are studied with respect to the catalytic behavior for oxidation of CO as well as of surface carbon. Carburization of the W(110) surface results in a weakening of the adsorption bond for molecularly adsorbed CO. Dissociation of carbon monoxide, which occurs on W(110), is reduced on the low-carbon coverage R(15 × 12) surface and completely suppressed on the carbon-saturated R(15 × 3) phase. Deposition of gold results in a blocking of adsorption sites for molecularly adsorbed CO and reopening of the dissociation channel. Probably the latter is associated with the existence of double-layer gold clusters and islands. At room temperature the gold clusters on both carburized templates are stable in CO atmosphere as shown by in-situ STM measurements. In contrast, exposure to oxygen alters the clusters on the R(15 × 12) surface, implying dissociation of oxygen not only on the substrate but also on or in immediate vicinity of the gold clusters. On the Au-free carburized templates oxygen adsorbs dissociatively and is released as CO at temperatures beyond 800 K due to reaction with carbon atoms from the templates. Deposition of gold enhances the desorption rate of the formed CO at the low-temperature end of the recombinative CO desorption range, indicating a promoting effect of gold for oxidation of surface carbon. In contrast, low-temperature CO oxidation catalyzed by the deposited Au clusters is not observed. Two reasons could be identified: (1) weakly bound CO with desorption temperatures between 100 and 200 K (as reported for other related systems) is not observed, and (2) oxygen atoms are bonded too strongly to the templates.
The Journal of Physical Chemistry C 08/2013; 117(33):17024-17032. · 4.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: Quasi-critical fluctuations occur close to critical points or close to continuous phase transitions. In three-dimensional systems, precision tuning is required to access the fluctuation regime. Lowering the dimensionality enhances the parameter space for quasi-critical fluctuations considerably. This enables one to make use of novel properties emerging in fluctuating systems, such as giant susceptibilities, Casimir forces or novel quasi-particle interactions. Examples are discussed ranging from simple metal-adsorbate systems to unconventional superconductivity in iron-based superconductors.
Journal of Nanoparticle Research 05/2013; 15(5):1407. · 2.18 Impact Factor
[show abstract][hide abstract] ABSTRACT: Reflectance difference spectroscopy (RDS) is applied to follow in situ the preparation of clean and carburized W(1 1 0) surfaces and to study the temperature-induced transition between the R(15 × 3) and R(15 × 12) carbon/tungsten surface phases. RDS data for this transition are compared to data obtained from Auger-electron spectroscopy and low-energy electron diffraction. All techniques reveal that this transition, occurring around 1870 K, is reversible with a small hysteresis, indicating a first-order-like behaviour. The present results also prove a high surface sensitivity of RDS, which is attributed to the excitation of electronic p-like surface resonances of W(1 1 0).
[show abstract][hide abstract] ABSTRACT: Noble metal nanostructures of Au, Ag and Cu were prepared on two types of carbon-modified W(110) surfaces-R(15 × 12) and R(15 × 3)-and investigated by means of scanning tunneling microscopy. For all deposited metals qualitatively the same behaviour is observed: On the R(15 × 12)-template always isotropic clusters are formed. In contrast, on the R(15 × 3)-substrate the anisotropy of the nanostructures can be tuned from clusters at low temperatures via thin nanowires to thicker nanobars at high deposition temperatures. At intermediate temperatures on the R(15 × 3) the anisotropic Au nanowires arrange themselves into straight lines along domain boundaries induced by deposition of the Au metal. Similarities and differences to Au nanostructures as recently reported by Varykhalov et al. [A. Varykhalov, O. Rader, W. Gudat. Physical Review B 77, 035412 (2008).] are discussed.
[show abstract][hide abstract] ABSTRACT: The time-dependent degradation of the oxygen exchange kinetics of the solid oxide fuel cell cathode material La0.58Sr0.4Co0.2Fe0.8O3 − δ (LSCF) is investigated at 600 °C. Special emphasis is placed on systematic long-term dc-conductivity relaxation measurements (t > 1000 h) in dry as well as in humidified atmospheres in order to obtain representative trends for the application of LSCF in intermediate-temperature SOFCs. The determination of the chemical surface exchange coefficient kchem of oxygen is combined with investigations of the elemental surface compositions and depth profiles of fresh and degraded samples by X-ray photoelectron spectroscopy (XPS), providing further insight into the mechanisms of degradation. The slow decrease of kchem by a factor of 2 during exposure of the sample to a dry O2–Ar reference atmosphere for 1000 h at 600 °C can be ascribed to an enrichment of La and Sr in correlation with an elevated oxygen concentration within about 30–35 nm depth. The interpretation of the XPS core level spectra indicates the formation of SrO and La2O3 secondary phases in this zone. The subsequent treatment in a humidified atmosphere for 1000 h results in a pronounced initial decrease of kchem by an additional factor of 10, followed by a time dependent decay of about 15% kh− 1. A Sr-rich silicate layer of about 10 nm thickness is identified by XPS as the major cause of the degradation in humidified atmosphere. The evidence of Si-poisoning over the whole sample surface could also be confirmed by post-test SEM analysis. In addition, indications of a re-structuring of the sample surface during the degradation are shown. These results indicate, that with LSCF as a cathode in ambient (humid) air in SOFC stacks containing various Si-sources, such as glass or glass-ceramic seals, and thermal insulation materials a significant decrease of the surface oxygen exchange coefficient can occur, even at temperatures as low as 600 °C. In order to prevent a severe Si-induced degradation, dry air should be used as an oxidant. However, even in dry atmosphere a minor decrease of kchem can occur during long-term operation due to changes in the relative cation and oxygen content at the surface.
Solid State Ionics 06/2011; 191(1):61–67. · 2.05 Impact Factor
[show abstract][hide abstract] ABSTRACT: Although the freezing of aqueous solutions is important for nature and different branches of science and freeze-applications, our understanding of the freezing process is not complete. For example, numerous measurements of micrometer-scaled (NH(4))(2)SO(4)/H(2)O droplets report one freezing event below the eutectic point. However, measurements of larger millimeter-scaled droplets reveal two freezing events: the freezing out of ice and subsequent freezing of a residual freeze-concentrated solution. To resolve this apparent contradiction we performed numerous calorimetric measurements which indicate that the freezing of a residual solution of millimeter-scaled 5-38 wt% (NH(4))(2)SO(4) droplets occurs mainly between ∼ 210 and 225 K. We also find that micrometer-scaled droplets produce one freezing event which is within or in the vicinity of the ∼ 210-225 K region. This fact and the analysis of thermograms suggest that the residual solution of micrometer-scaled droplets may partly crystallize simultaneously with ice and partly transform to glass at T(g)≈172 K. Our results suggest for the first time that the size of (NH(4))(2)SO(4)/H(2)O droplets may affect the number of freezing events below the eutectic point.
[show abstract][hide abstract] ABSTRACT: Variable-temperature scanning tunnelling microscopy is used to study an order-order phase transition in a virtually defect-free quasi–one-dimensional surface system. The phase transition is driven by competing electronic interactions. The phase diagram is captured by a modified Landau formalism containing a coupling term between two different subsystems. The extra term has the effect of a spontaneously generated field which drives the phase transition. The proposed formalism applies to a variety of problems, where competing interactions produce sometimes counter-intuitive ordering phenomena.
[show abstract][hide abstract] ABSTRACT: The degradation of the solid oxide fuel cell cathode material
with respect to the oxygen exchange kinetics in
-containing atmospheres has been investigated at
. The material exhibits excellent stability in dry
atmosphere over 1000 h. No appreciable degradation has been observed after exposure to a
-rich atmosphere, but a decrease in the chemical surface exchange coefficient of oxygen by 1 order of magnitude has been found
after 1000 h under moist conditions (30% relative humidity). Post-test X-ray photoelectron spectroscopy depth analysis showed
significant changes in the Nd:Ni cation ratio within the topmost 100 nm of the
-degraded sample. It is concluded that the degradation mechanism involves hydroxides as intermediate species even if the predominant
species in the degraded surface is
Journal of The Electrochemical Society. 10/2010; 157(11):B1537-B1541.
[show abstract][hide abstract] ABSTRACT: The aim of this study is to investigate the influence of wettability on direct and protein-mediated cell-surface interactions. Nanocrystalline diamond (NCD) is chosen as a cell growth substrate, because it offers the possibility to adjust surface properties like roughness or chemical termination, thus realizing hydrophilic or hydrophobic surfaces. Cell adhesion is studied on NCD films with a defined roughness but different surface chemistries (hydrogen, fluorine, and oxygen terminations) using two different cell lines (LLC-PK1 and PANC-1). They are cultured with and without the addition of fetal bovine serum (FBS) to the growth medium in order to discriminate between direct and mediated cell adhesion. For both cell lines, the cells are not able to adhere to hydrophobic surfaces without the addition of FBS to the growth medium, whereas they can attach in the presence of FBS. Hydrophilic surfaces enable cell attachment with and without the addition of FBS to the medium.
[show abstract][hide abstract] ABSTRACT: Various oxidation techniques (plasma-beam, sulfo-chromic acid, UV-ozone, heating in air) were applied to single-crystalline (111) and (100) diamond surfaces as well as nanocrystalline diamond (NCD) films and analyzed by X-ray photoelectron spectroscopy (XPS) with respect to oxygen content and type of carbon–oxygen groups formed upon oxidation. Due to their increased surface, NCD films show a significantly higher oxygen uptake as compared to their single-crystal counterparts. No marked differences were observed between the different oxidation techniques. For all oxidation techniques used, several carbon–oxygen groups are simultaneously present on the surface. The relative fraction of singly-oxidized carbon atoms (attributed to isolated ether or epoxy-like groups) generally decreases slightly with increasing oxygen content, but always remains the dominating species.
Diamond and Related Materials 01/2010; 19(5):474-478. · 1.71 Impact Factor
[show abstract][hide abstract] ABSTRACT: When adsorbed on the strongly anisotropic Pt(110) surface Br forms a sequence of (n × 1) structures. In the present study we investigate the (4 × 1) structure by scanning tunneling microscopy, quantitative low-energy electron diffraction and density-functional calculations. We show that the optimal structural model contains essentially the same adsorption sites as the (3 × 1) structure, but with a different preference. The positions of the substrate atom are consistent with a frozen surface phonon of fourfold periodicity, suggesting that the phase diagram can be understood on the basis of a tunable charge density wave (Swamy et al 2001 Phys. Rev. B 86 1299). The structure could also be explained by assuming short-range interactions only, but evidence is presented that adsorbate-adsorbate interactions mediated by quasi-one-dimensional surface resonances play a major role in both cases.
[show abstract][hide abstract] ABSTRACT: The interaction of Cl with Pt(110) was studied in UHV by scanning tunneling microscopy, low-energy electron diffraction, temperature-programmed desorption, and DFT calculations. Up to half a monolayer (ML) of Cl forms an adsorbate structure. Compression to higher local coverages leads to erosion of Pt atoms from the top layer and formation of PtCl(4) pentamers. Annealing results in healing of the Pt defects and formation of a long-range-ordered PtCl(4)/Cl/Pt(110) adlayer that is remarkably similar to the adlayer structure found after deposition of [PtCl(4)](2-) from an electrolyte onto Au(100). Coadsorption of 0.5 ML of Cl with CO initiates PtCl(4) formation, but no volatile compounds are formed under these conditions. In similar experiments with Br, the original Pt surface is left intact, even if Br is compressed to local coverages of 0.75 ML, in agreement with the expected lower corrosion activity of Br.
Journal of the American Chemical Society 03/2009; 131(8):2827-9. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Silver nanoclusters arranged in quasi-one-dimensional chains with a nearest-neighbor cluster-cluster distance of 1.4 nm were prepared on the R(15×12)-C/W(110) surface. The silver cluster chains form local thermodynamic equilibrium structures. Interactions between neighboring clusters are addressed by investigating the length distributions of silver cluster chains with scanning tunneling microscopy. Comparison with theoretical expectations derived from a one-dimensional Ising model yields evidence for a slightly repulsive interaction energy of about 20–30 meV.
[show abstract][hide abstract] ABSTRACT: The structure and morphology of mixed GaOx/WOx thin film systems prepared by either sequential or simultaneous oxide deposition has been studied by Analytical (High-Resolution) Transmission Electron Microscopy (TEM), Selected Area Electron Diffraction (SAED), X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM). GaOx and WOx were prepared by thermal evaporation of the corresponding oxides (β-Ga2O3 and WO3, respectively) in 10−2Pa oxygen onto a NaCl substrate, which was held at temperatures between 298 and 580K. The film morphology of differently prepared mixed GaOx/WOx films not only differs significantly from those of the pure oxides, but also strongly depends on the preparation conditions and the order of deposition. Upon co-deposition of GaOx/WOx films onto NaCl(001) at temperatures above 580K a gallia morphology was observed, which is strikingly different from hitherto reported pure GaOx films, i.e., growth of small-diameter (20–25nm) sub-stoichiometric gallia dendrites. The dendritic growth is apparently mediated by the GaOx/NaCl(001) contact area rather than being an intrinsic feature of the three-dimensional mixed-oxide bulk. In particular, neither sequentially deposited GaOx/WOx films nor co-deposited films on top of a pre-deposited WOx interlayer exhibited this peculiar morphology.
Materials Chemistry and Physics - MATER CHEM PHYS. 01/2009; 116(1):175-182.
[show abstract][hide abstract] ABSTRACT: Quasi-one-dimensionally ordered chains of silver and cobalt clusters are
grown on the R(15×12)-C/W(110) template and investigated by
scanning-tunneling microscopy. Both Ag and Co nucleate at the same area
within the large template unit cell. We attribute this area to the
carbon-poor part of the unit cell. Clusters exhibit a narrow size
distribution, peaking at cluster sizes of seven to eight atoms. The
strong preference of this cluster size is attributed primarily to the
size of favorable adsorption areas in the R(15×12) unit cell.
For cobalt atoms the adsorption strength seems to be more homogeneous
across the unit cell than for Ag, allowing also growth of small clusters
on less favorable regions of the unit cell at low temperatures.
[show abstract][hide abstract] ABSTRACT: Nanocrystalline diamond (NCD) films and nanoparticulate diamond powder (DP) are the two main representatives of diamond at the nanoscale. This study was designed to investigate the suitability of these biomaterials as cell growth supports and to determine surface characteristic properties best suited to cell attachment and proliferation. Surface topography, chemical termination and wetting properties of NCD- and DP-coated borosilicate glass substrates were correlated to attachment, proliferation and differentially regulated gene expression of human renal epithelial cells (HK-2 cell line) cultured on these surfaces. Hydrogen-terminated NCD (NCD-H) surfaces were shown to inhibit cell attachment, which indicates that the lack of functional polar groups prevents adherent cells from settling on a surface, whether nanostructured or not. In contrast to NCD-H, oxygen-terminated NCD (NCD-O) as well as DP surfaces demonstrated improved cell attachment, as compared to borosilicate glass, which is a commonly used material for cell growth supports. NCD-O not only revealed an increased cell attachment, but also a markedly increased proliferation rate. Finally, none of the investigated surface modifications appeared to cause adverse cellular reactions or markedly alter cellular phenotype.
[show abstract][hide abstract] ABSTRACT: Gallium oxide thin films were prepared by thermal evaporation and deposition of Ga2O3 on NaCl(001) cleavage planes at varying substrate temperatures, oxygen pressures and deposition rates. The structure of the so-prepared thin films was checked by Transmission Electron Microscopy and Selected Area Diffraction and also characterized by X-ray Photoelectron Spectroscopy and Atomic Force Microscopy, both in the as-deposited state and after different oxidative and reductive treatments. The substrate temperature proved to be most crucial for the structure of the gallium oxide films, ranging from low-contrast amorphous structures at low substrate temperatures (298 K) to nanosphere structures at higher temperatures (580 K). The stability of the films was found to be mainly determined by the interaction of substrate temperature and deposition rate. Crystalline β-Ga2O3 structures were obtained after oxidative, reductive and annealing treatments at and beyond 773 K suggesting that the crystallization is mainly a thermal annealing effect.
Thin Solid Films 06/2008; 516(15):4742-4749. · 1.60 Impact Factor