Tip-sample Force Research Articles

Effect of Inhibitor Modifications on Initial Localized Dealloying of Metallic Alloy Surfaces

Despite a substantial number of reports on corrosion testing is available, only a few projects aim to understand the basic processes driving materials degradation. A special type of corrosion, localized dealloying, takes place on metallic alloy surfaces modified with self-assembled monolayers (SAMs) of organic inhibitors[1-2]. Corrosion initiation at the nanoscale can be addressed by controlling the spatial distribution and molecular organization of inhibitor-molecule SAMs at nano and micro length scales. We use micro-contact printing on model ultra-flat gold surfaces and obtain well-defined complex organic layers following a multi-step approach [3]. This approach was applied on locally atomically flat Cu3Au (100) surfaces to study initial localized dealloying which results in the initiation of localized dealloying being strongly dependent on the nanoscale surface morphology (Figure 1) [4]. In addition, different organic inhibitors containing -SH (thiol end) films were subsequently applied on polycrystalline Cu and CuZn alloys to analyse inhibitor film stabilities. Force Spectroscopy of Atomic Force Microscopy (FS-AFM) provides a stability status of such inhibitor layers by analysing AFM tip sample force interaction on inhibitor-modified metallic surfaces.

The effect of glycerin solution density and viscosity on vibration amplitude of oblique different piezoelectric MC near the surface in 3D modeling

The surface topography in nanoscale is one of the most important applications of AFM. The analysis of piezoelectric microcantilevers vibration behavior is essential to improve the AFM performance. To this end, one of the appropriate methods to simulate the dynamic behavior of microcantilever (MC) is a numerical solution with FEM in the 3D modeling using COMSOL software. The present study aims to simulate different geometries of the four-layered AFM piezoelectric MCs in 2D and 3D modeling in a liquid medium using COMSOL software. The 3D simulation was done in a spherical container using FSI domain in COMSOL. In 2D modeling by applying Hamilton’s Principle based on Euler–Bernoulli Beam theory, the governing motion equation was derived and discretized with FEM. In this mode, the hydrodynamic force was assumed with a string of spheres. The effect of this force along with the squeezed-film force was considered on MC equations. The effect of fluid density and viscosity on the MC vibrations that immersed in different glycerin solutions was investigated in 2D and 3D modes and the results were compared with the experimental results. The frequencies and time responses of MC close to the surface were obtained considering tip-sample forces. The surface topography of MCs different geometries were compared in the liquid medium and the comparison was done in both tapping and non-contact mode. Various types of surface roughness were considered in the topography for MC different geometries. Also, the effect of geometric dimensions on the surface topography was investigated. In liquid medium, MC is installed at an oblique position to avoid damaging the MC due to the squeezed-film force in the vicinity of MC surface. Finally, the effect of MC’s angle on surface topography and time response of the system was investigated.

On the origin of amplitude reduction mechanism in tapping mode atomic force microscopy

The origin of amplitude reduction in Tapping Mode Atomic Force Microscopy (TM-AFM) is typically attributed to the shift in resonance frequency of the cantilever due to the nonlinear tip-sample interactions. In this paper, we present a different insight into the same problem which, besides explaining the amplitude reduction mechanism, provides a simple reasoning for the relationship between tip-sample interactions and operation parameters (amplitude and frequency). The proposed formulation, which attributes the amplitude reduction to an interference between the tip-sample and dither force, only deals with the linear part of the system; however, it fully agrees with experimental results and numerical solutions of the full nonlinear model of TM-AFM.

SPM Imaging of DNA, Redox Proteins and Bacteria Spores Under in-Situ Conditions

Scanning probe microscopy (SPM) can be successfully used in biological investigations1. Scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) are two main SPM techniques and they are capable of imaging biological objects at molecular level under in situ conditions. With this advantage, STM and AFM are applied to study not only the topographical structures of bio-samples in their natural environment, but also their electrochemical properties at liquid-solid interfaces. The electrochemical properties of biomolecules are usually studied by integral electrochemical methods, like cyclic voltammetry. The degree of surface coverage, the surface orientation of enzymes and the activity of a single enzyme remain a question of dispute, however, electrochemical SPM techniques can bring clarification. Electrochemical scanning tunnelling microscopy (EC-STM) and electrochemical atomic force microscopy (EC-AFM) provide visual topological information of biomolecules adsorbed on the electrode surface at single-molecular level. Some experimental issues, such as the requirement of the sample being conductive, potential damage to samples caused by the tunnelling current or tip-sample force, may limit their application. Scanning electrochemical potential microscopy (SECPM), a technique that measures the surface potential distribution at zero current, can be a promising alternative. SECPM shares most of the hardware setup with EC-STM except for the current amplifier being substituted by a potentiometer. It is easy to switch between SECPM and EC-STM to compare the topographical profile of the same area by mapping electron density and surface potential2. In our study, SPM techniques are used to study various biological objects, from nanometer-sized DNA and redox proteins to micrometer-sized bacteria spores. Since high-resolution images of DNA origami nanostructures can be achieved by using AFM3, we use both EC-AFM and SECPM techniques to investigate the morphology and potential-dependent adsorption behaviour of DNA scaffold and origamis4 on the modified HOPG surface. We found DNA scaffold can adsorb on HOPG in a certain potential range, and the adsorption structure changes with the applied electrochemical potential. Horseradish peroxidase (HRP), a redox enzyme, was studied using both, EC-STM and SECPM under in situ conditions5. HRP molecules were adsorbed on highly oriented pyrolytic graphite (HOPG) surfaces and imaged in phosphate buffered saline. We found that SECPM shows a better resolution than EC-STM in imaging this biomolecule. SECPM is also capable of measuring the surface charge distribution of the adsorbed biomolecules at the electrochemical interfaces. AFM in liquid was used to investigate the morphology and cellular process of Streptomycetes venezuelae both the wild type and the mutant6. To immobilize the bacteria on the surface for imaging, we use SiC patterned by photolithography with hole arrays (1.5×1.5×0.6 µm) to trap the bacteria and then measure under the physiological condition. The designed hole arrays trap the bacteria without causing any denaturation and due to the high electronic conductivity, they can serve as the working electrode under electrochemical conditions for EC-AFM and SECPM studies. EC-AFM was applied to image the morphology of the bacterial spore under different potential, and we found the morphology of the spore didn’t change in the wide potential window. (1) Choi, E.; Kim, A.; Son, H.; Pyo, S. G. J. Nanosci. Nanotechnol. 2014, 14 (1), 924. (2) Herpich, M.; Friedl, J.; Stimming, U. In Surface Science Tools for Nanomaterials Characterization; Springer Berlin Heidelberg: Berlin, Heidelberg, 2015; pp 1–67. (3) Bald, I.; Keller, A. Molecules 2014, 19 (9), 13803. (4) Kozyra, J.; Ceccarelli, A.; Torelli, E.; Lopiccolo, A.; Gu, J.-Y.; Fellermann, H.; Stimming, U.; Krasnogor, N. ACS Synth. Biol. 2017, 6 (7), 1140. (5) Baier, C.; Stimming, U. Angew. Chem. Int. Ed. Engl. 2009, 48 (30), 5542. (6) Santos-Beneit, F.; Gu, J.-Y.; Stimming, U.; Errington, J. Heliyon. Accepted

Imaging of viscoelastic soft matter with small indentation using higher eigenmodes in single-eigenmode amplitude-modulation atomic force microscopy.

In this short paper we explore the use of higher eigenmodes in single-eigenmode amplitude-modulation atomic force microscopy (AFM) for the small-indentation imaging of soft viscoelastic materials. In viscoelastic materials, whose response depends on the deformation rate, the tip–sample forces generated as a result of sample deformation increase as the tip velocity increases. Since the eigenfrequencies in a cantilever increase with eigenmode order, and since higher oscillation frequencies lead to higher tip velocities for a given amplitude (in viscoelastic materials), the sample indentation can in some cases be reduced by using higher eigenmodes of the cantilever. This effect competes with the lower sensitivity of higher eigenmodes, due to their larger force constant, which for elastic materials leads to greater indentation for similar amplitudes, compared with lower eigenmodes. We offer a short theoretical discussion of the key underlying concepts, along with numerical simulations and experiments to illustrate a simple recipe for imaging soft viscoelastic matter with reduced indentation.

Enhanced mechanical properties of the high-resolution EUVL patterns of hybrid photoresists containing hexafluoroantimonate

The present study investigates the mechanical properties of hybrid photoresists in the context of their pattern-collapse behaviors. The mechanical properties such as the DMT (Derjaguin, Muller, Toropov) modulus and tip-sample adhesion forces of the high-resolution patterns obtained from two hybrid EUV photo-resists, 1.5% & 2.15%-MAPDSA-MAPDST bearing hexafluoroantimonate and triflate counter ion moieties have been investigated using peak force quantitative nano-mechanical mapping (PF-QNM) technique. The mechanical properties of the well resolved high-resolution 90–20 nm (L/5S) line patterns, 20–32 nm (L/2S-L/5S) lines patterns and nano-features such as line-elbow connections have been investigated and analyzed against the differences in their SbF6− composition. For the 1.5%- MAPDSA-MAPDST resist case the DMT modulus and tip-sample adhesion forces are found strong dependence on the resist line width and line spacing, as compared to 2.15%- MAPDSA-MAPDST resist. There is a significant improvement in the modulus value of 5 GPa for 2.15%-MAPDSA-MAPDST resist in contrary to the ~2.7 GPa for the 1.5%-MAPDSA-MAPDST resist for the 20 nm (L/2S) Line patterns. Similarly, the tip sample adhesion forces on resist surface are also found dependent on patterns aspect ratio as well as on SbF6− content in the resist composition. These studies revealed that an increase of the SbF6− content in the resist formulation, imparts cascading effects to the mechanical properties of their high-resolution nanopatterns, which in turns helps to reduce the pattern collapse resulting in superior patterning performances.

Multi-frequency Atomic Force Microscopy based on enhanced internal resonance of an inner-paddled cantilever

In this work, we study the performance of a new cantilever design during multi-frequency tapping mode Atomic Force Microscopy (AFM). The system consists of a base cantilever with an inner paddle which, under harmonic excitation, vibrates like a system of linearly coupled oscillators engaging simultaneously a lower, in-phase and a higher, out-of-phase resonant mode. The cantilever is designed so that the 2nd mode frequency (i.e., the out-of-phase eigenfrequency) coincides with an integer multiple of the fundamental mode frequency, providing the necessary conditions for realization of internal resonance. During tapping mode, the nonlinear tip-sample force activates the internal resonance and thereby amplifies the out-of-phase resonant mode. We study energy transfer from the lower to the higher mode for different cantilever designs in order to optimize the internal resonance and the amplification of the high-frequency component of the measured response. Numerical and experimental examination of the inner-paddled cantilever performance confirms that a 1:2 internal resonance is optimal. AFM scans of cyanobacteria are taken, and the higher mode observables (amplitude and phase) along with the phase of the lower mode reveal differences in branch selection caused by variations in the stiffness that are not detected by the topographical data. In a computational simulation of an AFM scan across a flat sample with stratified stiffness, we show that the amplitude of the higher mode is significantly more sensitive to variations in Young’s modulus than the phase of the lower mode. Further, unlike for a conventional cantilever, we show that the phase of the lower mode indeed depends on the sample’s Young’s modulus in the absence of a dissipative component in the tip-sample interaction force.

Studying Nanoscale Mechanics of Single Molecules and Cells with Atomic Force Microscopy

This talk will present non-invasive mechanical approaches to high-resolution biological imaging developed in our group. We have demonstrated atomic force microscopy based imaging of chemical groups within protein complexes with Angstrom scale resolution and determined physiologically relevant intracellular forces in adherent cells. These approaches rely on sensitive measurements of tip-sample forces in liquid environment, and take advantage of the information encoded in force distance curves that can be chemically specific. In particular, the high-resolution cell stiffness images we obtained on mouse fibroblasts allowed us to develop and test a mechanical model of the cell that suggests that nanoscale stiffness patterns of cells are directly governed by intracellular forces. These patterns can be used to determined intracellular forces like cortex tension, plasma membrane tension, and the tension across actin filament bundles.

Review: Tip-based vibrational spectroscopy for nanoscale analysis of emerging energy materials

Vibrational spectroscopy is one of the key instrumentations that provide non-invasive investigation of structural and chemical composition for both organic and inorganic materials. However, diffraction of light fundamentally limits the spatial resolution of far-field vibrational spectroscopy to roughly half the wavelength. In this article, we thoroughly review the integration of atomic force microscopy (AFM) with vibrational spectroscopy to enable the nanoscale characterization of emerging energy materials, which has not been possible with far-field optical techniques. The discussed methods utilize the AFM tip as a nanoscopic tool to extract spatially resolved electronic or molecular vibrational resonance spectra of a sample illuminated by a visible or infrared (IR) light source. The absorption of light by electrons or individual functional groups within molecules leads to changes in the sample’s thermal response, optical scattering, and atomic force interactions, all of which can be readily probed by an AFM tip. For example, photothermal induced resonance (PTIR) spectroscopy methods measure a sample’s local thermal expansion or temperature rise. Therefore, they use the AFM tip as a thermal detector to directly relate absorbed IR light to the thermal response of a sample. Optical scattering methods based on scanning near-field optical microscopy (SNOM) correlate the spectrum of scattered near-field light with molecular vibrational modes. More recently, photo-induced force microscopy (PiFM) has been developed to measure the change of the optical force gradient due to the light absorption by molecular vibrational resonances using AFM’s superb sensitivity in detecting tip-sample force interactions. Such recent efforts successfully breech the diffraction limit of light to provide nanoscale spatial resolution of vibrational spectroscopy, which will become a critical technique for characterizing novel energy materials.

Breaking the Time Barrier in Kelvin Probe Force Microscopy: Fast Free Force Reconstruction Using the G-Mode Platform.

Atomic force microscopy (AFM) offers unparalleled insight into structure and material functionality across nanometer length scales. However, the spatial resolution afforded by the AFM tip is counterpoised by slow detection speeds compared to other common microscopy techniques (e.g., optical, scanning electron microscopy, etc.). In this work, we develop an ultrafast AFM imaging approach allowing direct reconstruction of the tip-sample forces with ∼3 order of magnitude higher time resolution than is achievable using standard AFM detection methods. Fast free force recovery (F3R) overcomes the widely viewed temporal bottleneck in AFM, that is, the mechanical bandwidth of the cantilever, enabling time-resolved imaging at sub-bandwidth speeds. We demonstrate quantitative recovery of electrostatic forces with ∼10 μs temporal resolution, free from influences of the cantilever ring-down. We further apply the F3R method to Kelvin probe force microscopy (KPFM) measurements. F3R-KPFM is an open loop imaging approach (i.e., no bias feedback), allowing ultrafast surface potential measurements (e.g., <20 μs) to be performed at regular KPFM scan speeds. F3R-KPFM is demonstrated for exploration of ion migration in organometallic halide perovskite materials and shown to allow spatiotemporal imaging of positively charged ion migration under applied electric field, as well as subsequent formation of accumulated charges at the perovskite/electrode interface. In this work, we demonstrate quantitative F3R-KPFM measurements-however, we fully expect the F3R approach to be valid for all modes of noncontact AFM operation, including noninvasive probing of ultrafast electrical and magnetic dynamics.

Minimizing tip-sample forces and enhancing sensitivity in atomic force microscopy with dynamically compliant cantilevers

Due to the harmonic motion of the cantilever in Tapping Mode Atomic Force Microscopy, it is seemingly impossible to estimate the tip-sample interactions from the motion of the cantilever. Not directly observing the interaction force, it is possible to damage the surface or the tip by applying an excessive mechanical load. The tip-sample interactions scale with the effective stiffness of the probe. Thus, the reduction of the mechanical load is usually limited by the manufacturability of low stiffness probes. However, the one-to-one relationship between spring constant and applied force only holds when higher modes of the cantilever are not excited. In this paper, it is shown that, by passively tuning higher modes of the cantilever, it is possible to reduce the peak repulsive force. These tuned probes can be dynamically more compliant than conventional probes with the same static spring constant. Both theoretical and experimental results show that a proper tuning of dynamic modes of cantilevers reduces the contact load and increases the sensitivity considerably. Moreover, due to the contribution of higher modes, the tuned cantilevers provide more information on the tip-sample interaction. This extra information from the higher harmonics can be used for mapping and possibly identification of material properties of samples.

Quantitative atomic force microscopy

A variety of atomic force microscopy (AFM) modes is employed in the field of surface science. The most prominent AFM modes include the amplitude modulation (AM) and the frequency modulation (FM) mode. Over the years, different ways for analyzing data acquired with different AFM modes have been developed, where each analysis is usually based on mode-specific assumptions and approximations. Checking the validity of the seemingly different approximations employed in the various analysis methods can be a tedious task. Moreover, a straightforward comparison of data analyzed with different methods can, therefore, be challenging. Here, we combine the existing evaluation methods which have been separately developed for the different AFM modes and present a unifying set of three equations. These three AFM equations allow for a straightforward analysis of AFM data within the harmonic approximation, regardless of the AFM mode. The three AFM equations provide the three and only pieces of information about the tip-sample force available within the harmonic approximation. We demonstrate the generality of our approach by quantitatively analyzing three-dimensional AFM data obtained in both the AM and FM mode.

3D simulation of AFM non-uniform piezoelectric micro-cantilever with various geometries subjected to the tip-sample forces

Atomic force microscope (AFM) is a powerful instrument for investigation of surface topography at different workspaces. It is important to understand the dynamic behavior of AFM to improve its performance. 3D numerical method is suitable in order to simulate experimental conditions. This paper has investigated modeling and dynamic simulation of rectangular, Dagger and V-shaped geometries of AFM piezoelectric micro-cantilever (MC) with two electrode layers in the air environment. For a better understanding of the system dynamic, multi-layer MC dynamic equation has been derived. Euler-Bernoulli beam theory has been used for modeling the AFM cantilever. Hamilton’s principle has been used for the MC modeling and the finite element method (FEM) has been applied for its discretization. In 3D, with respect to the tip-sample forces piezoelectric MC has been simulated via the COMSOL software. The frequency and time responses have been also investigated. The topographies have been performed on different surfaces with various roughness’s types in the tapping and non-contact mode. The results of these two methods have been compared with experimental results. Moreover, the effects of MC geometrical parameters on surfaces topography and frequency responses have been studied and optimal dimensions of topographies have been obtained for each of the beam geometries. Simulations of various tip geometries have been performed in order to examine the effects of tip dimensions on the frequency and time responses. Furthermore, the effect of tip displacement on the frequency response has been investigated for different MC lengths.

Distinguishing between Mechanical and Electrostatic Interaction in Single Pass Multi Frequency Electrostatic Force Microscopy Measurements on a Molecular Material.

Single-pass electrostatic force microscopy is postulated as one of the most advanced techniques in terms of spatial resolution and fastness in data acquisition for the study of electrostatic phenomena at the nanoscale. However, crosstalk anomalies, in which mechanical interactions combine with tip-sample electrostatic forces, are still a major issue to overcome, specifically in soft and biological samples. In this paper we propose a novel method based on bimodal-atomic force microscopy to distinguish mechanical crosstalk from electrostatic images. The method is based in the comparison of bimodal AFM images with electrostatic ones, where pure mechanical interaction can be discerned from a mixture of mechanical and electrostatic interactions. The proposed method is optimized and demonstrated using a supramolecular charge transfer material. Finally, the method is used as a tool to depict different crosstalk levels in tetrathiafulvalene-based (TTF) assemblies, discerning between electrical and mechanical interactions. This kind of observation is important for obtaining accurate descriptions of charge distribution in samples made from organic and molecular layers and materials.

Photoinduced Force Mapping of Plasmonic Nanostructures.

The ability to image the optical near-fields of nanoscale structures, map their morphology, and concurrently obtain spectroscopic information, all with high spatiotemporal resolution, is a highly sought-after technique in nanophotonics. As a step toward this goal, we demonstrate the mapping of electromagnetic forces between a nanoscale tip and an optically excited sample consisting of plasmonic nanostructures with an imaging platform based on atomic force microscopy. We present the first detailed joint experimental-theoretical study of this type of photoinduced force microscopy. We show that the enhancement of near-field optical forces in gold disk dimers and nanorods follows the expected plasmonic field enhancements with strong polarization sensitivity. We then introduce a new way to evaluate optically induced tip-sample forces by simulating realistic geometries of the tip and sample. We decompose the calculated forces into in-plane and out-of-plane components and compare the calculated and measured force enhancements in the fabricated plasmonic structures. Finally, we show the usefulness of photoinduced force mapping for characterizing the heterogeneity of near-field enhancements in precisely e-beam fabricated nominally alike nanostructures - a capability of widespread interest for precise nanomanufacturing, SERS, and photocatalysis applications.

Vectorial scanning force microscopy using a nanowire sensor.

Self-assembled nanowire (NW) crystals can be grown into nearly defect-free nanomechanical resonators with exceptional properties, including small motional mass, high resonant frequency and low dissipation. Furthermore, by virtue of slight asymmetries in geometry, a NW's flexural modes are split into doublets oscillating along orthogonal axes. These characteristics make bottom-up grown NWs extremely sensitive vectorial force sensors. Here, taking advantage of its adaptability as a scanning probe, we use a single NW to image a sample surface. By monitoring the frequency shift and direction of oscillation of both modes as we scan above the surface, we construct a map of all spatial tip-sample force derivatives in the plane. Finally, we use the NW to image electric force fields distinguishing between forces arising from the NW charge and polarizability. This universally applicable technique enables a form of atomic force microscopy particularly suited to mapping the size and direction of weak tip-sample forces.

Development of low temperature atomic force microscopy with an optical beam deflection system capable of simultaneously detecting the lateral and vertical forces.

The atomic force microscopy (AFM) is a very important tool for imaging and investigating the complex force interactions on sample surfaces with high spatial resolution. In the AFM, two types of detection systems of the tip-sample interaction forces have been used: an optical detection system and an electrical detection system. In optical detection systems, such as optical beam deflection system or optical fiber interferometer system, both the lateral and the vertical tip-sample forces can be measured simultaneously. In electrical detection systems, such as qPlus or Kolibri sensors, either the lateral or vertical forces can be measured. Simultaneous measurement of the lateral and vertical interaction forces effectively allows investigation of force interactions because the force is a vector with magnitude and direction. In this study, we developed a low-temperature, frequency-modulation AFM using an optical beam deflection system to simultaneously measure the vertical and lateral forces. In this system, the heat sources, such as a laser diode and a current-to-voltage converter, for measuring the photocurrent of the four-segmented photodiode are located outside the observation chamber to avoid a temperature increase of the AFM unit. The focused optical beam is three-dimensionally adjustable on the back side of the cantilever. We demonstrate low-noise displacement measurement of the cantilever and successful atomic resolution imaging using the vertical and lateral forces at low temperatures.

Noncontact atomic force microscopy III.

No other method has opened the door to progress in nanoscience and nanotechnology as much as the introduction of scanning probe methods did in the 1980s, since they offer a way to visualize the nanoworld. For maximum impact, however, the ability to image and manipulate individual atoms is the key. Initially, scanning tunneling microscopy was the only scanning-probe-based method that was able to achieve this resolution. Atomic force microscopy (AFM), on the other hand, was quickly developed into a versatile tool with applications ranging from materials characterization in ultrahigh vacuum and nanofabrication under ambient conditions, to biological studies in liquids, but its resolution was limited to the nanometer scale. The reason for this restriction resulted from the fact that the resolution in probe microscopy scales with the sharpness of the tip. In conventional AFM operational modes, a tip that is located at the end of a leaf spring (the so-called cantilever) is either dragged over the surface in permanent contact or gently taps the surface while vibrating, and, whichever mode is used, tips quickly blunt through either permanent or intermittent contact. Maintaining the atomic sharpness of an initially atomically sharp tip requires that the tip never touches the surface. But how can the tip know that the surface is there if it is not allowed to touch? This problem was solved in the 1990s through the realization that the attractive forces acting on the tip when it is in close proximity to the sample affect the resonance frequency of the cantilever even though it is not in actual contact with the surface. Noncontact atomic force microscopy (NC-AFM) makes use of this effect by tracking the shift of the cantilever resonance frequency due to the force field of the surface without ever establishing physical contact between the tip and sample. Much to the astonishment of many, changes induced by individual atoms turned out to induce frequency shifts that are large enough to be detected, and thus atomic-scale imaging with AFM became a reality. Since the beginnings, almost two decades ago, NC-AFM has evolved into a powerful method that is able not just to image surfaces, but also to quantify tip–sample forces and interaction potentials as well as to manipulate individual atoms on conductors, semiconductors, and insulators alike. For the community to keep track of the rapid development in the field, a series of annual international conferences, starting in Osaka, Japan in 1998, has been established. The most recent conference from this series was held in Lindau, Germany, from September 18–22, 2011. Once again, substantial progress was presented; NC-AFM is now able to quantitatively map three-dimensional force fields of surfaces with atomic resolution in ultrahigh vacuum as well as in liquids, and methodological developments add more information to the measurements, for example, through the driving of higher cantilever harmonics or the recording of tunneling currents. For this Thematic Series of the Beilstein Journal of Nanotechnology, many of the presenters from the Lindau conference agreed to submit contributions in order to assemble a series that showcases the present state of the art in the field. I would like to thank all authors who have contributed their excellent original work to this series, all referees whose promptly provided reports have provided valuable suggestions for further improvements while keeping the publication times short, and the entire NC-AFM community for supporting the open access policy of the Beilstein Journal of Nanotechnology. Udo D. Schwarz New Haven, February 2012

Synergy of Resonant Oscillatory Modes in Atomic Force Microscopy of Polymers

ABSTRACTThe set of oscillatory resonance AFM modes is expanded with frequency modulation mode and frequency imaging in amplitude modulation mode. The backgrounds of these modes are discussed and their capabilities are compared on the practical examples. The data show how these techniques complement the amplitude modulation with phase imaging. The frequency imaging enhances the compositional mapping of heterogeneous samples. Frequency modulation mode provides a superior capability in imaging at low tip-sample forces.

Investigation of the range of validity of the pairwise summation method applied to the calculation of the surface roughness correction to the van der Waals force

It has long been recognized that stochastic surface roughness can considerably change the van der Waals (vdW) force between interacting surfaces and particles. However, few analytical expressions for the vdW force between rough surfaces have been presented in the literature. Because they have been derived using perturbative methods or the proximity force approximation the expressions are valid when the roughness correction is small and for a limited range of roughness parameters and surface separation. In this work, a nonperturbative approach, the effective density method (EDM) is proposed to circumvent some of these limitations. The method simplifies the calculations of the roughness correction based on pairwise summation (PWS), and allows us to derive simple expressions for the vdW force and energy between two semispaces covered with stochastic rough surfaces. Because the range of applicability of PWS and, therefore, of our results, are not known a priori, we compare the predictions based on the EDM with those based on the multilayer effective medium model, whose range of validity can be defined more properly and which is valid when the roughness correction is comparatively large. We conclude that the PWS can be used for roughness characterized by a correlation length of the order of its rms amplitude, when this amplitude is of the order of or smaller than a few nanometers, and only for typically insulating materials such as silicon dioxide, silicon nitride, diamond, and certain glasses, polymers and ceramics. The results are relevant for the correct modeling of systems where the vdW force can play a significant role such as micro and nanodevices, for the calculation of the tip-sample force in atomic force microscopy, and in problems involving adhesion.