A perfectly stoichiometric and flat CeO2(111) surface on a bulk-like ceria film.

In surface science and model catalysis, cerium oxide (ceria) is mostly grown as an ultra-thin film on a metal substrate in the ultra-high vacuum to understand fundamental mechanisms involved in diverse surface chemistry processes. However, such ultra-thin films do not have the contribution of a bulk ceria underneath, which is currently discussed to have a high impact on in particular surface redox processes. Here, we present a fully oxidized ceria thick film (180 nm) with a perfectly stoichiometric CeO2(111) surface exhibiting exceptionally large, atomically flat terraces. The film is well-suited for ceria model studies as well as a perfect substitute for CeO2 bulk material.

Synovial C-reactive protein as a marker for chronic periprosthetic infection in total hip arthroplasty.

The aim of this study was to assess the role of synovial C-reactive protein (CRP) in the diagnosis of chronic periprosthetic hip infection. We prospectively collected synovial fluid from 89 patients undergoing revision hip arthroplasty and measured synovial CRP, serum CRP, erythrocyte sedimentation rate (ESR), synovial white blood cell (WBC) count and synovial percentages of polymorphonuclear neutrophils (PMN). Patients were classified as septic or aseptic by means of clinical, microbiological, serum and synovial fluid findings. The high viscosity of the synovial fluid precluded the analyses in nine patients permitting the results in 80 patients to be studied. There was a significant difference in synovial CRP levels between the septic (n = 21) and the aseptic (n = 59) cohort. According to the receiver operating characteristic curve, a synovial CRP threshold of 2.5 mg/l had a sensitivity of 95.5% and specificity of 93.3%. The area under the curve was 0.96. Compared with serum CRP and ESR, synovial CRP showed a high diagnostic value. According to these preliminary results, synovial CRP may be a useful parameter in diagnosing chronic periprosthetic hip infection.

Routes to rupture and folding of graphene on rough 6H-SiC(0001) and their identification.

Twisted few layer graphene (FLG) is highly attractive from an application point of view, due to its extraordinary electronic properties. In order to study its properties, we demonstrate and discuss three different routes to in situ create and identify (twisted) FLG. Single layer graphene (SLG) sheets mechanically exfoliated under ambient conditions on 6H-SiC(0001) are modified by (i) swift heavy ion (SHI) irradiation, (ii) by a force microscope tip and (iii) by severe heating. The resulting surface topography and the surface potential are investigated with non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM). SHI irradiation results in rupture of the SLG sheets, thereby creating foldings and bilayer graphene (BLG). Applying the other modification methods creates enlarged (twisted) graphene foldings that show rupture along preferential edges of zigzag and armchair type. Peeling at a folding over an edge different from a low index crystallographic direction can result in twisted BLG, showing a similar height as Bernal (or AA-stacked) BLG in NC-AFM images. The rotational stacking can be identified by a significant contrast in the local contact potential difference (LCPD) measured by KPFM.

Structural transitions of epitaxial ceria films on Si(111).

The structural changes of a (111) oriented CeO2 film grown on a Si(111) substrate covered with a hex-Pr2O3(0001) interface layer due to post deposition annealing are investigated. X-ray photoelectron spectroscopy measurements revealing the near surface stoichiometry show that the film reduces continuously upon extended heat treatment. The film is not homogeneously reduced since several coexisting crystalline ceria phases are stabilized due to subsequent annealing at different temperatures as revealed by high resolution low energy electron diffraction and X-ray diffraction. The electron diffraction measurements show that after annealing at 660 °C the ι-phase (Ce7O12) is formed at the surface which exhibits a (√7 × âˆš7)R19.1° structure. Furthermore, a (√27 × âˆš27)R30° surface structure with a stoichiometry close to Ce2O3 is stabilized after annealing at 860 °C which cannot be attributed to any bulk phase of ceria stable at room temperature. In addition, it is shown that the fully reduced ceria (Ce2O3) film exhibits a bixbyite structure. Polycrystalline silicate (CeSi(x)O(y)) and crystalline silicide (CeSi1.67) are formed at 850 °C and detected at the surface after annealing above 900 °C.

Concept for support and heating of plate-like samples in the ultra-high vacuum.

We present the concept for a sample holder designed for mounting and heating of plate-like samples that is based on a clamping mechanism for easy handling. The clamping mechanism consists of a U-shaped bracket encompassing the sample support plate from the rear. Two spring wires are fixed in the walls of the bracket spanning the sample to secure it with only two point contacts. This enables the sample to freely expand or contract during heating and cooling. To accommodate for a large variety in sample size, shape, and quality, we introduce two designs differing in the generation of the clamping force: One pressing the sample against the spring wires, the other one pulling the spring wires onto the sample. Both designs yield an automatically even alignment of the sample during the mounting process to achieve an even load distribution and reliable fixation specifically for brittle samples. For high temperature treatment, the sample holders are enhanced by a resistive heating plate. As only the sample and a small fraction of the sample holder are heated, temperatures of 1300 °C are reached with only 8 W heating power. The sample support and heating components are mounted on a 11 mm × 13 mm base plate with a handle that can be transferred between the sample entry stage, the preparation stage, and surface science experiments in the ultra-high vacuum system.

Morphology and nanostructure of CeO2(111) surfaces of single crystals and Si(111) supported ceria films.

The surface morphology of CeO(2)(111) single crystals and silicon supported ceria films is investigated by non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM) for various annealing conditions. Annealing bulk samples at 1100 K results in small terraces with rounded ledges and steps with predominantly one O-Ce-O triple layer height while annealing at 1200 K produces well-ordered straight step edges in a hexagonal motif and step bunching. The morphology and topographic details of films are similar, however, films are destroyed upon heating them above 1100 K. KPFM images exhibit uniform terraces on a single crystal surface when the crystal is slowly cooled down, whereas rapid cooling results in a significant inhomogeneity of the surface potential. For films exhibiting large terraces, significant inhomogeneity in the KPFM signal is found even for best possible preparation conditions. Applying X-ray photoelectron spectroscopy (XPS), we find a significant contamination of the bulk ceria sample with fluorine while a possible fluorine contamination of the ceria film is below the XPS detection threshold. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) reveals an accumulation of fluorine within the first 5 nm below the surface of the bulk sample and a small concentration throughout the crystal.

Versatile system for the temperature-controlled preparation of oxide crystal surfaces.

We present a versatile system for the preparation of oxide crystal surfaces in the ultra-high vacuum (UHV) at temperatures up to 1300 K. Thermal treatment is accomplished by direct current heating of a tantalum foil in contact with the oxide sample. The sample temperature is measured by a thermocouple at a position close to the crystal and its reading is calibrated against the surface temperature determined by a second thermocouple temporarily attached to the surface. The design of the sample holder is based on a transferable plate originally developed for a commercial UHV scanning probe microscope. The system is, however, also suitable for the use with electron spectroscopy or electron diffraction based surface analytical techniques. We present results for the high-temperature preparation of CeO(2)(111) surfaces with atomically flat terraces exhibiting perfect atomic order and cleanliness as revealed by non-contact atomic force microscopy (NC-AFM) imaging. NC-AFM imaging is, furthermore, used to demonstrate the temperature-controlled aggregation of gold atoms on the CeO(2)(111) surface and their evaporation at high temperatures.

Manipulation of individual water molecules on CeO2(111).

Water molecules adsorbed on the CeO(2)(111) surface are investigated by non-contact atomic force microscopy (NC-AFM) at several tip-sample temperatures ranging between 10 and 300 K. Depending on the strength of the tip-surface interaction, they appear as triangular protrusions extended over three surface oxygen atoms or as small pits at hollow sites. During NC-AFM imaging with the tip being close to the surface, occasionally the transfer of molecules between tip and surface or the tip-induced lateral displacement of water molecules to equivalent surface lattice sites is observed. We report how this situation can be exploited to produce controlled lateral manipulations. A protocol to manipulate the water molecules between pre-defined neighbouring equivalent adsorption sites of the regular lattice as well as across a surface oxygen vacancy is demonstrated.

Ordering of monodisperse Ni nanoclusters by templating on high-temperature reconstructed alpha-Al2O3(0001).

We demonstrate that the characteristic [Formula in text] reconstructed surface of alpha-alumina (Al(2)O(3)) acts as a nanotemplate for the growth of well-ordered monodisperse arrays of Ni nanoclusters. Due to the insulating nature of the substrate we use dynamic scanning force microscopy operated in the non-contact mode (NC-AFM) to characterize the nanotemplate, to examine the size and distribution of metallic clusters on the surface and to investigate their position with respect to the surface atomic structure. The present NC-AFM results for the interaction of Ni with alpha-Al(2)O(3) are supported by density functional theory (DFT) calculations. The ability of alpha-Al(2)O(3)(0001) to act as a nanotemplate is attributed to a spatially modulated affinity towards the accommodation of Ni into the top layer by substituting the surface Al atoms at certain sites on the [Formula in text] reconstructed surface formed by high-temperature annealing. The insulating template, demonstrated for Al(2)O(3), may be a generally attractive system for the study of nanostructures which need to be isolated from a conducting bulk.

Atomic resolution non-contact atomic force microscopy of clean metal oxide surfaces.

In the last two decades the atomic force microscope (AFM) has become the premier tool for topographical analysis of surface structures at the nanometre scale. In its ultimately sensitive implementation, namely dynamic scanning force microscopy (SFM) operated in the so-called non-contact mode (NC-AFM), this technique yields genuine atomic resolution and offers a unique tool for real space atomic-scale studies of surfaces, nanoparticles as well as thin films, single atoms and molecules on surfaces irrespective of the substrate being electrically conducting or non-conducting. Recent advances in NC-AFM have paved the way for groundbreaking atomic level insight into insulator surfaces, specifically in the most important field of metal oxides. NC-AFM imaging now strongly contributes to our understanding of the surface structure, chemical composition, defects, polarity and reactivity of metal oxide surfaces and related physical and chemical surface processes. Here we review the latest advancements in the field of NC-AFM applied to the fundamental atomic resolution studies of clean single crystal metal oxide surfaces with special focus on the representative materials Al(2)O(3)(0001), TiO(2)(110), ZnO(1000) and CeO(2)(111).

Concept for support and cleavage of brittle crystals.

We report on sample holders for crystals to be cleaved for the preparation of surfaces with large atomically flat terraces. The concept for mounting sample crystals is based on a vicelike clamping mechanism to securely hold the crystal in position while reducing the risk of fragmentation. Sample holders based on this concept and made of suitable materials allow preparation and cleavage of crystals in the ultrahigh vacuum at high or low temperatures. To cleave the crystal, we employ a scalpel blade mounted on a wobble stick to generate a highly localized stress field initiating the cleavage process. The sample holders are used for experiments of highest resolution scanning force microscopy, however, the concept can be transferred to any other system where cleavage faces of crystals are of interest. Exemplarily, scanning force microscopy results demonstrate that (111) cleavage faces of CaF2 crystals can be prepared with steps only a few F-Ca-F triple-layers high and atomically flat terraces extending over areas of several microm2.

Improvement of a dynamic scanning force microscope for highest resolution imaging in ultrahigh vacuum.

We report on a modification of a commercial scanning force microscope (Omicron UHV AFM/STM) operated in noncontact mode (NC-AFM) at room temperature in ultrahigh vacuum yielding a decrease in the spectral noise density from 2757 to 272 fm/Hz. The major part of the noise reduction is achieved by an exchange of the originally installed light emitting diode by a laser diode placed outside the vacuum, where the light is coupled into the ultrahigh vacuum chamber via an optical fiber. The setup is further improved by the use of preamplifiers having a bandpass characteristics tailored to the cantilever resonance frequency. The enhanced signal to noise ratio is demonstrated by a comparison of atomic resolution images on CeO(2)(111) obtained before and after the modification.

How flat is an air-cleaved mica surface?

Muscovite mica is an important mineral that has become a standard substrate, due to its easy cleavage along the {001} planes, revealing a very flat surface that is compatible with many biological materials. Here we study mica surfaces by dynamic atomic force microscopy (AFM) operated in the non-contact mode (NC-AFM) under ultra-high vacuum (UHV) conditions. Surfaces produced by cleaving in UHV cannot be imaged with NC-AFM due to large surface charges; however, cleavage in air yields much less surface charge and allows for NC-AFM imaging. We present highly resolved NC-AFM images of air-cleaved mica surfaces revealing a rough morphology originating from a high density of nanometre-sized particles. Among these particles, we find regularly shaped structures indicating the growth of crystallites on the surface. The contamination layer cannot be removed by degassing in UHV; even prolonged heating at a temperature of 560 K under UHV conditions does not yield an atomically flat surface.

Nanotemplate with holes: ultrathin alumina on Ni3Al(111).

We have determined the structure of the ultrathin (sqrt[67] x sqrt[67])R12.2 degrees aluminum oxide on Ni3Al(111) by a combination of scanning tunneling microscopy and density functional theory. In addition to other local defects, the main structural feature of the unit cell is a 0.4-nm-diameter hole reaching down to the metal substrate. Understanding the structure and metal growth on this oxide allows us to use it as a template for growing highly regular arrays of nanoparticles.

Lateral manipulation of atomic size defects on the CaF(2)(111) surface.

Atomic scale manipulation on insulating surfaces is one of the great challenges of non-contact atomic force microscopy. Here we demonstrate lateral manipulation of defects occupying single ionic sites on a calcium fluoride (111)-surface. Defects stem from the interaction of the residual gas with the surface. The process of surface degradation is briefly discussed. Manipulation is performed over a wide range of path lengths ranging from tens of nanometres down to a few lattice constants. We introduce a simple manipulation protocol based on line-by-line scanning of a surface region containing defects to be manipulated, and record tip-surface distance and cantilever resonance frequency detuning as a function of the manipulation pathway in real time. We suggest a hopping model to describe manipulation where the tip-defect interaction is governed by repulsive forces.

Imaging the atomic arrangements on the high-temperature reconstructed alpha-Al2O3(0001) surface.

Alumina is a technologically important oxide crystal because of its use as a catalyst and as a substrate for microelectronic applications. A precise knowledge of its surface atomic structure is a prerequisite for understanding and controlling the physical processes involved in many of its applications. Here we use a dynamic scanning force microscopy technique to image directly the atomic structure of the high-temperature phase of the alpha-Al2O3(0001) surface. Evidence for a surface reconstruction appears as a grid of protrusions that represent a rhombic unit cell, and we confirm that the arrangement of atoms is in the form of surface domains with hexagonal atomic order at the centre and disorder at the periphery. We show that, on exposing the surface to water and hydrogen, this surface structure is important in the formation of hydroxide clusters. These clusters appear as a regular pattern of rings that can be explained by self-organization processes involving cluster-surface and cluster-cluster interactions. Alumina has long been regarded as the definitive test for atomic-resolution force microscopy of insulators so the whole class of insulating oxides should now open for direct atomic-scale surface investigations.

Unambiguous interpretation of atomically resolved force microscopy images of an insulator.

The (111) surface of CaF2 was imaged with dynamic mode scanning force microscopy and modeled using atomistic simulation. Both experiment and theory showed a clear triangular contrast pattern in images, and theory demonstrated that the contrast pattern is due to the interaction of a positive electrostatic potential tip with fluorine ions in the two topmost surface layers. We find a good agreement of position and relative height of scan line features between theory and experiment and thus establish for the first time an unambiguous identification of sublattices of an insulator imaged by force microscopy.

Absorption and thermal conductivity of oxide thin films measured by photothermal displacement and reflectance methods.

Photothermal reflectance and photothermal displacement measurements of optical absorption and thermal conductivity are reported for electron-beam-(EB) deposited and ion-plated (IP) thin films of TiO(2), Ta(2)O(5), and ZrO(2). Of the particular set of samples investigated, the EB films have higher absorption than the IP films. The absorption of the EB samples decreases over a period of ~ 90 min on irradiations with an Ar-ion laser of 488-nm wavelength. By contrast, the absorption of the IP samples changes insignificantly or not at all. Photothermal displacement area scans of coating surfaces yield lower defect densities for the IP samples compared with the EB samples for all three oxide materials. The feasibility and limitations of photothermal measurements for thin-film optical and thermal characterizations are discussed.