Browsing by Author "Schwarz, U. D."

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Open Access
3D Force field spectroscopy
(Springer, Cham, 2015) Baykara, Mehmet Z.; Schwarz, U. D.; Morita, S.; Giessibl, F. J.; Meyer, E.; Wiesendanger, R.
With recent advances in instrumentation and experimental methodology, noncontact atomic force microscopy is now being frequently used to measure the atomic-scale interactions acting between a sharp probe tip and surfaces of interest as a function of three spatial dimensions, via the method of three-dimensional atomic force microscopy (3D-AFM). In this chapter, we discuss the different data collection and processing approaches taken towards this goal while highlighting the associated advantages and disadvantages in terms of correct interpretation of results. Additionally, common sources of artifacts in 3D-AFM measurements, including thermal drift, piezo nonlinearities, and tip-related issues such as asymmetry and elasticity are considered. Finally, the combination of 3D-AFM with simultaneous scanning tunneling microscopy (STM) is illustrated on surface-oxidized Cu(100). We conclude the chapter by an outlook regarding the future development of the 3D-AFM method.
Open Access
Advanced atomic force microscopy techniques
(Beilstein - Institut zur Foerderung der Chemischen Wissenschaften, 2012-12-21) Glatzel, T.; Holscher, H.; Schimmel, T.; Baykara, M. Z.; Schwarz, U. D.; Garcia, R.
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 2012
Open Access
Atom-specific forces and defect identification on surface-oxidized Cu ( 100 ) with combined 3D-AFM and STM measurements
(American Physical Society, 2013) Baykara, M. Z.; Todorović, M.; Mönig, H.; Schwendemann, T. C.; Ünverdi, Ö.; Rodrigo, L.; Altman, E. I.; Pérez, R.; Schwarz, U. D.
The influence of defects on the local structural, electronic, and chemical properties of a surface oxide on Cu(100) were investigated using atomic resolution three-dimensional force mapping combined with tunneling current measurements and ab initio density functional theory. Results reveal that the maximum attractive force between tip and sample occurs above the oxygen atoms; theory indicates that the tip, in this case, terminates in a Cu atom. Meanwhile, simultaneously acquired tunneling current images emphasize the positions of Cu atoms, thereby, providing species-selective contrast in the two complementary data channels. One immediate outcome is that defects due to the displacement of surface copper are exposed in the current maps, even though force maps only reflect a well-ordered oxygen sublattice. The exact nature of the defects is confirmed by the simulations, which also reveal that the arrangement of the oxygen atoms is not disrupted by the copper displacement. In addition, the experimental force maps uncover a position-dependent modulation of the attractive forces between the surface oxygen and the copper-terminated tips, which is found to reflect the surface's inhomogeneous chemical and structural environment. As a consequence, the demonstrated method has the potential to directly probe how defects affect surface chemical interactions. © 2013 American Physical Society.
Open Access
Atomic force microscopy: Methods and applications
(Elsevier, 2017) Baykara, Mehmet Z.; Schwarz, U. D.; Lindon, J.; Tranter, G. E.; Koppenaal, D.
This chapter provides an overview of atomic force microscopy, covering the fundamental aspects of the associated instrumentation and methodology as well as representative results from the literature highlighting a variety of application areas. In particular, atomic-resolution imaging and spectroscopy capabilities are emphasized, in addition to applications in biology, nanotribology and catalysis research. Finally, an outlook on emerging aspects and future prospects of atomic force microscopy is provided.
Open Access
Exploring atomic-scale lateral forces in the attractive regime: a case study on graphite (0001)
(Institute of Physics Publishing, 2012) Baykara, M. Z.; Schwendemann, T. C.; Albers, B. J.; Pilet, N.; Mönig, H.; Altman, E. I.; Schwarz, U. D.
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. © 2012 IOP Publishing Ltd.
Open Access
Low-temperature scanning probe microscopy
(Springer, 2017) Baykara, Mehmet Z.; Morgenstern, M.; Schwarz, A.; Schwarz, U. D.; Bhushan, B.
This chapter is dedicated to scanning probe microscopy (SPM ) operated at cryogenic temperatures, where the more fundamental aspects of phenomena important in the fields of nanoscience and nanotechnology can be investigated with high sensitivity under well-defined conditions. In general, scanning probe techniques allow the measurement of physical properties down to the nanometer scale. Some techniques, such as scanning tunneling microscopy (STM ) and scanning force microscopy (SFM ), even go down to the atomic scale. Various properties are accessible. Most importantly, one can image the arrangement of atoms on conducting surfaces by STM and on insulating samples by SFM. However, electronic states (scanning tunneling spectroscopy), force interaction between different atoms (scanning force spectroscopy), magnetic domains (magnetic force microscopy), magnetic exchange interactions (magnetic exchange force microscopy and spectroscopy), local capacitance (scanning capacitance microscopy), local contact potential differences (Kelvin probe force microscopy), local temperature (scanning thermal microscopy), and local light-induced excitations (scanning near-field microscopy) can also be measured with high spatial resolution, among others. In addition, some modern techniques even allow the controlled manipulation of individual atoms/molecules and the visualization of the internal structure of individual molecules. Moreover, combined STM/SFM experiments are now possible, mainly thanks to the advent of tuning forks as sensing elements in low-temperature (LT) SPM systems. Probably the most important advantage associated with the low-temperature operation of scanning probes is that it leads to a significantly better signal-to-noise ratio than measuring at room temperature. This is why many researchers work below 100 K. However, there are also physical reasons to use low-temperature equipment. For example, visualizing the internal structure of molecules with SFM or the utilization of scanning tunneling spectroscopy with high energy resolution can only be realized at low temperatures. Moreover, some physical effects such as superconductivity or the Kondo effect are restricted to low temperatures. Here, we describe the advantages of low-temperature scanning probe operation and the basics of related instrumentation. Additionally, some of the important results achieved by low-temperature scanning probe microscopy are summarized. We first focus on the STM, giving examples of atomic manipulation and the analysis of electronic properties in different material arrangements, among others. Afterwards, we describe results obtained by SFM, reporting on atomic-scale and submolecular imaging, as well as three-dimensional (3-D ) force spectroscopy. Results obtained with the method of Kelvin probe force microscopy (KPFM ) that is used to study variations in local contact potential difference (LCPD ) are briefly discussed. Magnetic force microscopy (MFM ), magnetic exchange force microscopy (MExFM ), and magnetic resonance force microscopy (MRFM ) are also introduced. Although the presented selection of results is far from complete, we feel that it gives an adequate impression of the fascinating possibilities of low-temperature scanning probe instruments. In this chapter low temperatures are defined as lower than about 100 K and are normally achieved by cooling with liquid nitrogen or liquid helium. Applications in which SPMs are operated close to 0∘C are not covered in this chapter.
Open Access
Noncontact atomic force microscopy II
(Beilstein-Institut Zur Forderung der Chemischen Wissenschaften, 2014) Baykara, M. Z.; Schwarz, U. D.
Open Access
Noncontact atomic force microscopy III
(Beilstein-Institut Zur Forderung der Chemischen Wissenschaften, 2016-06) Baykara, M. Z.; Schwarz, U. D.
Open Access
Noncontact atomic force microscopy: an emerging tool for fundamental catalysis research
(American Chemical Society, 2015) Altman, E. I.; Baykara, M. Z.; Schwarz, U. D.
ConspectusAlthough atomic force microscopy (AFM) was rapidly adopted as a routine surface imaging apparatus after its introduction in 1986, it has not been widely used in catalysis research. The reason is that common AFM operating modes do not provide the atomic resolution required to follow catalytic processes; rather the more complex noncontact (NC) mode is needed. Thus, scanning tunneling microscopy has been the principal tool for atomic scale catalysis research. In this Account, recent developments in NC-AFM will be presented that offer significant advantages for gaining a complete atomic level view of catalysis.The main advantage of NC-AFM is that the image contrast is due to the very short-range chemical forces that are of interest in catalysis. This motivated our development of 3D-AFM, a method that yields quantitative atomic resolution images of the potential energy surfaces that govern how molecules approach, stick, diffuse, and rebound from surfaces. A variation of 3D-AFM allows the determination of forces required to push atoms and molecules on surfaces, from which diffusion barriers and variations in adsorption strength may be obtained. Pushing molecules towards each other provides access to intermolecular interaction between reaction partners. Following reaction, NC-AFM with CO-terminated tips yields textbook images of intramolecular structure that can be used to identify reaction intermediates and products.Because NC-AFM and STM contrast mechanisms are distinct, combining the two methods can produce unique insight. It is demonstrated for surface-oxidized Cu(100) that simultaneous 3D-AFM/STM yields resolution of both the Cu and O atoms. Moreover, atomic defects in the Cu sublattice lead to variations in the reactivity of the neighboring O atoms. It is shown that NC-AFM also allows a straightforward imaging of work function variations which has been used to identify defect charge states on catalytic surfaces and to map charge transfer within an individual molecule.These advances highlight the potential for NC-AFM-based methods to become the cornerstone upon which a quantitative atomic scale view of each step of a catalytic process may be gained. Realizing this potential will rely on two breakthroughs: (1) development of robust methods for tip functionalization and (2) simplification of NC-AFM instrumentation and control schemes. Quartz force sensors may offer paths forward in both cases. They allow any material with an atomic asperity to be used as a tip, opening the door to a wide range of surface functionalization chemistry. In addition, they do not suffer from the instabilities that motivated the initial adoption of complex control strategies that are still used today.
Open Access
Nonuniform friction-area dependency for antimony oxide surfaces sliding on graphite
(American Physical Society, 2013) Ritter, C.; Baykara, M. Z.; Stegemann, B.; Heyde, M.; Rademann, K.; Schroers, J.; Schwarz, U. D.
We present frictional measurements involving controlled lateral manipulation of antimony nanoparticles on graphite featuring atomically smooth particle-substrate interfaces via tapping- and contact-mode atomic force microscopy. As expected from earlier studies, the power required for lateral manipulation as well as the frictional forces recorded during the manipulation events exhibit a linear dependence on the contact area over a wide size range from 2000 nm2 to 120 000 nm2. However, we observe a significant and abrupt increase in frictional force and dissipated power per contact area at a value of about 20 000 nm2, coinciding with a phase transition from amorphous to crystalline within the antimony particles. Our results suggest that variations in the structural arrangement and stoichiometry of antimony oxide at the interface between the particles and the substrate may be responsible for the observed effect. © 2013 American Physical Society.
Open Access
Probing three-dimensional surface force fields with atomic resolution: measurement strategies, limitations, and artifact reduction
(Beilstein - Institut zur Foerderung der Chemischen Wissenschaften, 2012) Baykara, M. Z.; Dagdeviren, O. E.; Schwendemann, T. C.; Mönig, H.; Altman, E. I.; Schwarz, U. D.
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.
Open Access
Simultaneous measurement of multiple independent atomic-scale interactions using scanning probe microscopy: data interpretation and the effect of cross-talk
(American Chemical Society, 2015) Baykara, M. Z.; Todorović, M.; Mönig, H.; Schwendemann, T. C.; Rodrigo, L.; Altman, E. I.; Pérez, R.; Schwarz, U. D.
In high-resolution scanning probe microscopy, it is becoming increasingly common to simultaneously record multiple channels representing different tip-sample interactions to collect complementary information about the sample surface. A popular choice involves simultaneous scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM) measurements, which are thought to reflect the chemical and electronic properties of the sample surface. With surface-oxidized Cu(100) as an example, we investigate whether atomic-scale information on chemical interactions can be reliably extracted from frequency shift maps obtained while using the tunneling current as the feedback parameter. Ab initio calculations of interaction forces between specific tip apexes and the surface are utilized to compare experiments with theoretical expectations. The examination reveals that constant-current operation may induce a noticeable influence of topography-feedback-induced cross-talk on the frequency shift data, resulting in misleading interpretations of local chemical interactions on the surface. Consequently, the need to apply methods such as 3D-AFM is emphasized when accurate conclusions about both the local charge density near the Fermi level, as provided by the STM channel, and the site-specific strength of tip-sample interactions (NC-AFM channel) are desired. We conclude by generalizing to the case where multiple atomic-scale interactions are being probed while only one of them is kept constant.
Open Access
Three-dimensional interaction force and tunneling current spectroscopy of point defects on rutile TiO2(110)
(American Institute of Physics Inc., 2016) Baykara, M. Z.; Mönig, H.; Schwendemann, T. C.; Ünverdi, Ö.; Altman, E. I.; Schwarz, U. D.
The extent to which point defects affect the local chemical reactivity and electronic properties of an oxide surface was evaluated with picometer resolution in all three spatial dimensions using simultaneous atomic force/scanning tunneling microscopy measurements performed on the (110) face of rutile TiO2. Oxygen atoms were imaged as protrusions in both data channels, corresponding to a rarely observed imaging mode for this prototypical metal oxide surface. Three-dimensional spectroscopy of interaction forces and tunneling currents was performed on individual surface and subsurface defects as a function of tip-sample distance. An interstitial defect assigned to a subsurface hydrogen atom is found to have a distinct effect on the local density of electronic states on the surface, but no detectable influence on the tip-sample interaction force. Meanwhile, spectroscopic data acquired on an oxygen vacancy highlight the role of the probe tip in chemical reactivity measurements.
Open Access
Understanding scanning tunneling microscopy contrast mechanisms on metal oxides: a case study
(American Chemical Society, 2013) Mönig, H.; Todorović, M.; Baykara, M. Z.; Schwendemann, T. C.; Rodrigo, L.; Altman, E. I.; Pérez, R.; Schwarz, U. D.
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 oxygen-terminated 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, oxygen-terminated 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. © 2013 American Chemical Society.