A Computational Study of Cluster Dynamics in Structural Lubricity: Role of Cluster Rotation

We present a computational study of sliding between gold clusters and a highly oriented pyrolytic graphite substrate, a material system that exhibits ultra-low friction due to structural lubricity. By means of molecular dynamics, it is found that clusters may undergo spontaneous rotations during manipulation as a result of elastic instability, leading to attenuated friction due to enhanced interfacial incommensurability. In the case of a free cluster, shear stresses exhibit a non-monotonic dependency on the strength of the tip-cluster interaction, whereby rigid clusters experience nearly constant shear stresses. Finally, it is shown that the suppression of the translational degrees of freedom of a cluster's outermost-layer can partially annihilate out-of-plane phonon vibrations, which leads to a reduction of energy dissipation that is in compliance with Stokesian damping. It is projected that the physical insight attained by the study presented here will result in enhanced control and interpretation of manipulation experiments at structurally lubric contacts

Probing three-dimensional surface force fields with atomic resolution: Measurement strategies, limitations, and artifact reduction

Noncontact atomic force microscopy (NC-AFM) is being increasingly used to measure the interaction force between an atomically sharp probe tip and surfaces of interest, as a function of the three spatial dimensions, with picometer and piconewton accuracy. Since the results of such measurements may be affected by piezo nonlinearities, thermal and electronic drift, tip asymmetries, and elastic deformation of the tip apex, these effects need to be considered during image interpretation. In this paper, we analyze their impact on the acquired data, compare different methods to record atomic-resolution surface force fields, and determine the approaches that suffer the least from the associated artifacts. The related discussion underscores the idea that since force fields recorded by using NC-AFM always reflect the properties of both the sample and the probe tip, efforts to reduce unwanted effects of the tip on recorded data are indispensable for the extraction of detailed information about the atomicscale properties of the surface. © 2012 Baykara et al

Thermophysical property measurements in electromagnetic levitators

Proper measurements of thermophysical properties of hot levitated liquid drops require the following: accurate temperature measurement (brightness measurement, emissivity measurement); precise drop shape measurements with submillisecond time resolution (density determination, rotational and vibrational shape information); precise control of drop shape (high symmetry variable gap levitators); accurate energy transfer measurements (direct measurements of energy transfer rates for defined gas flows over samples with quantitative measurements of energy transfer rates for defined flows over samples with known shapes); and precise measurements of repetitive sample motions (rapid repetitive shape measurements, frequency measurements with reflected laser light, measurements in the levitator and as a freely falling drop). Recent advances in coil design and control of sample rotation in an electromagnetic levitator are discussed with respect to the above requirements

Advanced atomic force microscopy techniques

Cataloged from PDF version of article.Although its conceptual approach is as simple as the technique used in record players already introduced in the 19th century, the invention of the atomic force microscope (AFM) in 1986 by Binnig, Quate, and Gerber was a milestone for nanotechnology. The scanning tunneling microscope (STM), introduced some years earlier, had already achieved atomic resolution, but is limited to conductive surfaces. Since its operational principle is based on the detection of the forces acting between tip and sample, this restriction does not exist for the AFM. Consequently, atomic force microscopy quickly became the standard tool for nanometer-scale imaging of all types of surfaces in all environments. True atomic resolution was first achieved in the 1990s. The most convincing results, however, were restricted to the so-called noncontact mode in vacuum for a long time, but recent technical developments overcame this limitation, and atomic-resolution imaging is now also a standard in liquids. Beyond pushing the resolution limit to the picometer range, the invention of the AFM triggered the development of a growing number of new scanning probe methods and approaches, ranging from an expansion of the properties that can be mapped to the active manipulation of surfaces and small particles. Practically every month, reports on the growing capabilities of AFMs appear. Nearly every physical effect that influences the tip–sample interaction has been used to improve existing modes and to develop new ones. For example, many recently presented techniques include the excitation of higher cantilever oscillation modes; it is amazing in how many ways the shaking of a simple cantilever can improve our knowledge about the tip–sample interaction. Another direction is high-speed atomic force microscopy, which is one of the eminent challenges that need to be solved in order to allow the in situ observation of biological processes. Data acquisition times have already reached the millisecond range, enabling the visualization of the dynamic behavior of biological molecules and cells. Other recent accomplishments include imaging of organic molecules with unprecedented resolution, full three-dimensional mapping of surface force fields, and the imaging and discrimination of individual chemical bonds. The development of advanced techniques is the focus of this Thematic Series, following the Thematic Series “Scanning probe microscopy and related techniques” edited by Ernst Meyer and the Thematic Series “Noncontact atomic force microscopy” edited by Udo Schwarz. The articles that are part of the series demonstrate that, despite its 25 years of history, the AFM is still far from reaching its limits, and today’s developments are far-reaching. As the number of research groups utilizing advanced atomic force microscopy techniques increases with each passing year, the technical improvements, data-acquisition approaches, analysis procedures, user friendliness, and application areas of the technique further diversify. With this Thematic Series, it is our intention to stimulate these improvements. We thank all authors for contributing their excellent work to this series. Furthermore, we acknowledge all referees for their promptly provided reports keeping the publication times short and attractive for contributors. Finally, we are grateful to the open access policy of the Beilstein Journal of Nanotechnology providing the ground for unrestricted discussions on advanced atomic force microscopy techniques. Thilo Glatzel, Hendrik Hölscher, Thomas Schimmel, Mehmet Z. Baykara, Udo D. Schwarz and Ricardo Garcia December 201

Probing three-dimensional surfaces force fields with atomic resolution: Measurement strategies, limitations, and artifact reduction

Cataloged from PDF version of article.Noncontact atomic force microscopy (NC-AFM) is being increasingly used to measure the interaction force between an atomically sharp probe tip and surfaces of interest, as a function of the three spatial dimensions, with picometer and piconewton accuracy. Since the results of such measurements may be affected by piezo nonlinearities, thermal and electronic drift, tip asymmetries, and elastic deformation of the tip apex, these effects need to be considered during image interpretation. In this paper, we analyze their impact on the acquired data, compare different methods to record atomic-resolution surface force fields, and determine the approaches that suffer the least from the associated artifacts. The related discussion underscores the idea that since force fields recorded by using NC-AFM always reflect the properties of both the sample and the probe tip, efforts to reduce unwanted effects of the tip on recorded data are indispensable for the extraction of detailed information about the atomic-scale properties of the surface

Exploring atomic-scale lateral forces in the attractive regime: a case study on graphite (0001)

Cataloged from PDF version of article.A non-contact atomic force microscopy-based method has been used to map the static lateral forces exerted on an atomically sharp Pt/Ir probe tip by a graphite surface. With measurements carried out at low temperatures and in the attractive regime, where the atomic sharpness of the tip can be maintained over extended time periods, the method allows the quantification and directional analysis of lateral forces with piconewton and picometer resolution as a function of both the in-plane tip position and the vertical tip-sample distance, without limitations due to a finite contact area or to stick-slip-related sudden jumps of tip apex atoms. After reviewing the measurement principle, the data obtained in this case study are utilized to illustrate the unique insight that the method offers. In particular, the local lateral forces that are expected to determine frictional resistance in the attractive regime are found to depend linearly on the normal force for small tip-sample distances

2000 Families: identifying the research potential of an origins - of migration study

Despite extensive recent advances in the empirical and theoretical study of migration, certain critical areas in the analysis of European migration remain relatively underdeveloped both theoretically and empirically. Specifically, we lack studies that both incorporate an origin comparison and trace processes of intergenerational transmission across migrants over multiple generations and incorporating family migration trajectories. This paper outlines the development, data and design of such a study, the 2000 Families study, framed within a theoretical perspective of ?dissimilation? from origins and over generations. We term the study an origins-of-migration study, in that it captures the country of origin, the family origins and potentially the originating causes of migration processes and outcomes. The resulting data comprised nearly 2,000 migrant and non-migrant Turkish families with members across three or more generations, covering. 50,000 individuals. We reflect on the potential of this study for migration research

Understanding Scanning Tunneling Microscopy Contrast Mechanisms on Metal Oxides: A Case Study

Cataloged from PDF version of article.A comprehensive analysis of contrast formation mechanisms in scanning tunneling microscopy (STM) experiments on a metal oxide surface is presented with the oxygen-induced (2√2 √2)R45 missing row reconstruction of the Cu(100) surface as a model system. Density functional theory and electronic transport calculations were combined to simulate the STM imaging behavior of pure and oxygen-contaminated metal tips with structurally and chemically different apexes while systematically varying bias voltage and tip sample distance. The resulting multiparameter database of computed images was used to conduct an extensive comparison with experimental data. Excellent agreement was attained for a large number of cases, suggesting that the assumed model tips reproduce most of the commonly encountered contrast-determining effects. Specifically, we find that depending on the bias voltage polarity, copper-terminated tips allow selective imaging of two structurally distinct surface Cu sites, while oxygenterminated tips show complex contrasts with pronounced asymmetry and tip sample distance dependence. Considering the structural and chemical stability of the tips reveals that the copper-terminated apexes tend to react with surface oxygen at small tip sample distances. In contrast, oxygenterminated tips are considerably more stable, allowing exclusive surface oxygen imaging at small tip sample distances. Our results provide a conclusive understanding of fundamental STM imaging mechanisms, thereby providing guidelines for experimentalists to achieve chemically selective imaging by properly selecting imaging parameters