Table of contents

With the ongoing miniaturization of devices and controlled nanostructuring of materials, the importance of atomic-scale information on surfaces and surface properties is growing continuously. The astonishing progress in nanoscience and nanotechnology that took place during the last two decades was in many ways related to recent progress in high-resolution imaging techniques such as scanning tunnelling microscopy and transmission electron microscopy. Since the mid-1990s, non-contact atomic force microscopy (NC-AFM) performed in ultrahigh vacuum has evolved as an alternative technique that achieves atomic resolution, but without the restriction to conducting surfaces of the previously established techniques. Advances of the rapidly developing field of NC-AFM are discussed at annual conferences as part of a series that started in 1998 in Osaka, Japan. This special issue of Nanotechnology is a compilation of original work presented at the 7th International Conference on Non-contact Atomic Force Microscopy that took place in Seattle, USA, 12–15 September 2004.

Over the years, the conference grew in size and scope. Atomic resolution imaging of oxides and semiconductors remains an issue. Noticeable new developments have been presented in this regard such as, e.g., the demonstrated ability to manipulate individual atoms. Additionally, the investigation of individual molecules, clusters, and organic materials gains more and more attention. In this context, considerable effort is undertaken to transfer the NC-AFM principle based on frequency modulation to applications in air and liquids with the goal of enabling high-resolution surface studies of biological material in native environments, as well as to reduce the experimental complexity, which so far involves the availability of (costly) vacuum systems. Force spectroscopy methods continue to be improved and are applied to topics such as the imaging of the three-dimensional force field as a function of the distance with atomic resolution, the investigation of near-surface electronic states, the quantification of adhesion forces, and the lateral mapping of surface potentials. The origin of energy dissipation, which is closely related to an in-depth understanding of tip-surface interactions and imaging mechanisms, was the subject of an ongoing discussion and addressed by various theoretical, computational, and experimental contributions.

A characteristic of the NC-AFM conference series is the lively and friendly atmosphere, which year after year stimulates scientific discussions between the participants. This time, the programme included 5 invited talks, 84 contributed presentations, and 113 participants; furthermore, three educational lectures were given as part of a pre-conference workshop targeted at NC-AFM newcomers, which was attended by 30 participants. I would like to thank the members of the international steering committee and the programme committee for their continued effort in organizing the meeting. Special thanks go to the chair of the programme and local organizing committees S Fain and the conference manager J Kvamme for making the meeting a success.

Financial support is acknowledged from the corporate sponsors MikroMasch USA, Nanonis GmbH, Nanosurf AG, Omicron Nanotechnology, PSIA, Inc., and RHK Technology, as well as from the institutional sponsors National Science Foundation and PNNL/UW Joint Institute for Nanoscience. Finally, I would like to express my gratitude to everyone who participated in assembling this special issue including the authors, the reviewers, and, in particular, the excellent and experienced journal team from the Institute of Physics Publishing headed by Nina Couzin, for devoting their time and efforts so that we could make this issue a useful representation of the progress in NC-AFM while maintaining our tight publication schedule.

In conclusion, I would like to mention that the Seattle conference was the first one of the NC-AFM series that took place in the USA. As such, it was part of a series of recent activities within the USA, which will help in establishing a strong domestic NC-AFM community.

The surface photovoltage (SPV) of a structured semiconductor surface is deduced via detection of the contact potential difference measured with Kelvin probe force microscopy (KPFM). Our setup is based on a quantitative KPFM method complemented with modulated laser illumination in order to measure SPV. A high lateral resolution and quantitative values for the SPV are obtained by operating the KPFM in the so-called frequency modulation (FM) mode which is advantageous compared to the amplitude sensitive (AM) mode as proven by our simulations. In contrast to similar studies based on scanning tunnelling microscopy, KPFM offers the clear advantage that there is virtually no electric DC field between tip and sample and, therefore, the SPV is not affected by the presence of the tip.

We use atomistic simulation to study how the contrast pattern in dynamic force microscopy depends on the structure of the tip. Using a NaCl island on the NaCl(001) surface as a template, we consider several different NaCl tip configurations, with and without OH contamination. We find that the most significant changes in images are produced by very soft tips, which demonstrate large relaxations due to tip–surface interaction. Harder tips produce qualitatively similar images regardless of the atomic configuration presented to the surface. Contamination of the tip by hydroxyl groups reduces the strength of the interaction, and, in certain configurations, can produce asymmetries in the image even on the flat terrace of the island.

Dynamic force spectroscopy experiments were conducted with a silicon tip on graphite in ultrahigh vacuum. Spectroscopy curves were acquired in the constant-amplitude as well as in the constant-excitation mode. From both modes we extract quantitative force and energy dissipation curves. We show that the calculated tip–sample interaction curves are independent of the operational mode. This proves that quantitative dynamic force spectroscopy is also possible in the constant-excitation mode.

The surface of an anatase titanium dioxide (TiO₂) film grown on LaAlO₃(001) was observed using noncontact atomic force microscopy (NC-AFM). After cleaning with cycles of argon ion sputtering and annealing in vacuum, (1 × 4)- and (1 × 5)-reconstructed terraces appeared. In addition to the terraces, the sputter-annealed surface included many agglomerations. The presence of Tiⁿ⁺ () shown by x-ray photoelectron spectroscopy indicates that the agglomerations are due to TiO_x () species. Limited replenishment of oxygen by the substrate and the low diffusivity of the Tiⁿ⁺ to the substrate are possible causes for the generation of the TiO_x.

Surfaces of polymerized 2,4-hexadiyne-1,6 bis(p-toluene-sulphonate) (poly-PTS) single crystals were investigated on a molecular scale by noncontact atomic force microscopy (NC-AFM). Molecularly resolved images of the surface structures, the energy dissipation and the surface potential (SP) distribution were successfully obtained. The possible origins of energy dissipation are discussed in terms of the relationship with the molecular fluctuation. Furthermore, to ascertain the origin of the SP distribution, the obtained SP image was compared with the calculated local charges on the individual atoms forming the molecule.

The imaging mechanism for both NC-AFM and STM at atomic resolution can be considered as a process of making and breaking of chemical bonds between the tip and the sample. Based on that concept, a universal relation between tunnelling conductance G and attractive atomic force F is established, which can be verified experimentally. This further implies an experimentally verifiable relation between the threshold resistance R and diffusion activation energy E in atom manipulation.

The spectroscopy mode of the electric force microscope (EFM) was used to study the variation of the local electric force gradients above CVD-grown thin films of iridium oxide field emitters. The spacing, radius, and morphology of the nanoprotrusions were observed to change significantly within the sample. In this research, we present the first EFM study of the electric force gradients above the iridium oxide (IrO₂) nano-arrays. The electric force gradients are measured with the amplitude change of the EFM cantilever vibration operated in air using the lift-mode technique. Regions in the array with nanorods of largest spacing yield the largest force gradients, but a relatively large non-uniformity of the force gradients is observed. However, relatively homogeneous force gradients were obtained in areas with nanorods of smallest radius and densest population (flower-cluster zone).

Cerium dioxide is an insulating material that is important in various fields of catalysis and often used as an oxygen reservoir. The (111) surface of CeO₂ is catalytically active and we study this surface in its various oxidation states by dynamic force microscopy, an analysis that may eventually help to establish structure–function relationships for catalysis. We present highly resolved images and atomic details of surface features on CeO₂(111) like terrace structures, step edges, kinks and hexagonally shaped pits that are naturally formed during surface preparation by sputtering and annealing cycles. We find that such structures can be well imaged with atomic resolution and that they exhibit morphological characteristics that are significantly different in details from those of cleaved CaF₂(111), a surface with exactly the same crystallographic structure and very similar lattice constants and ionic radii. In this study, we continuously monitor the shape of the tip apex and demonstrate that high quality atomic resolution imaging is possible with one tip, although the tip apex is drastically changed by wear during 45 h of scanning.

The nanoscale specificity of interaction measurements and additional imaging capability of the atomic force microscope make it an ideal technique for measuring solvation shells in a variety of liquids next to a range of materials. Unfortunately, the widespread use of atomic force microscopy for the measurement of solvation shells has been limited by uncertainties over the dimensions, composition and durability of the tip during the measurements, and problems associated with quantitative force calibration of the most sensitive dynamic measurement techniques. We address both these issues by the combined use of carbon nanotube high aspect ratio probes and quantifying the highly sensitive frequency modulation (FM) detection technique using a recently developed analytical method. Due to the excellent reproducibility of the measurement technique, additional information regarding solvation shell size as a function of proximity to the surface has been obtained for two very different liquids. Further, it has been possible to identify differences between chemical and geometrical effects in the chosen systems.

Contact-type scanning nonlinear dielectric microscopy (SNDM) can measure the distribution of linear and nonlinear dielectric properties of insulators with sub-nanometre resolution. SNDM, however, has a tip-abrasion problem, and this problem is one of the obstacles in obtaining a high-quality image.

In this paper, in order to improve SNDM, we propose a non-contact scanning nonlinear dielectric microscopy (NC-SNDM) which focuses on the nonlinear dielectric signal for controlling the non-contact condition. With the NC-SNDM technique, topographic and SNDM images were obtained successfully. Moreover, from the calculation results, it was confirmed that NC-SNDM has atomic-order sensitivity to the gap between the tip and the specimen.

We performed an ab initio study of tip–sample interactions between an SiH₃ tip and an InAs(110) surface when the tip is on top of As and In atoms. The calculated force curves and the corresponding normalized frequency shift curves exhibit discontinuities. We analysed these jumps in the force curves from the point of view of the structural changes of the InAs(110) surface due to its interaction with the tip and compared them with those obtained for an Si (111) 7 × 7 surface. The simulation of a complete NC-AFM image shows reasonable agreement with the experiment.

Using a simplified yet realistic strain-relaxation model, we analyse facet-resolved non-contact AFM (NC-AFM) data of self-assembled strained germanium islands grown on Si(100) at 650 °C and extract estimates of the nucleation barrier and pyramid-to-dome shape transition barrier. We estimate the shape transition barrier of Ge dots as , and the nucleation barrier as  eV.

The restatoms of the Ge(111)-c (2 × 8) surface are imaged by noncontact atomic force microscopy (NC-AFM) at room temperature. In the NC-AFM measurements on this surface, there are two types of topographic contrast images depending on the tip apex structure and/or composition. In one of them only the adatoms are imaged, while in the other both the adatom and the restatom are imaged. In some of the images in the latter case, the distinction between adatoms and restatoms is unclear due to a special configuration and/or composition of the tip apex. A small number of Sn atoms embedded in the surface allows the discrimination between the adatom and the restatom.

An investigation of the mechanical properties of single wall carbon nanotubes (SWNT) fixed at a tip apex was performed using a frequency modulation-atomic force microscope (FM-AFM). The FM-AFM method allows the measurement of conservative and non-conservative forces separately and unambiguously. The FM-AFM analysis provides information that aids the understanding of the effects of the interaction between the free SWNT end and the surface: the resonant frequency shifts provide information on the effective SWNT spring constant, while the damping signal gives information on the type of contact between the tube and the surface. The variation of the damping signal as a function of the tip surface distance shows that the additional energy loss produced by the interaction between the tube and the surface is mostly due to an adhesion hysteresis. As a result, the increase of the damping signal is correlated to the existence of intermittent contact situations. The whole variations show how the contact between the free SWNT end and the surface modifies the elastic response of the tube.

Using model systems consisting of ionic tips interacting with ionic surfaces, we have performed non-equilibrium molecular dynamics simulations of atomic scale imaging in non-contact atomic force microscopy at close approach to the surface, where structurally reversible instabilities consisting of surface atom jumps can lead to a hysteresis in the tip force. This results in dissipated energies comparable to those observed experimentally for similar systems. The development of reversible instabilities is dependent on the nature of the tip and surface, the tip lateral position, distance of closest approach and system temperature. For the first time, we present constant height dissipation images of the MgO(001) surface imaged with an anion terminated MgO tip and the NaCl(001) surface imaged with an anion terminated NaCl tip which show different contrast mechanisms. We also find much larger dissipation energies at low coordinated surface sites. When an MgO tip is brought close to the NaCl(001) surface at low temperatures, a surface atom is permanently pulled away from the surface on retraction; however, we find that the system returns to its original configuration leading to stable imaging at higher temperatures.

Frequency modulation atomic force microscopy (FM-AFM) has been modified to operate in a liquid environment within an atomic force microscope specifically designed for investigating biological samples. We demonstrate the applicability of FM-AFM to biological samples using the spectroscopy mode to measure the unbinding forces of a single receptor–ligand (biotin–avidin) interaction. We show that quantitative adhesion force measurements can only be obtained provided certain modifications are made to the existing theory, which is used to convert the detected frequency shifts to an interaction force. Quantitative force measurements revealed that the unbinding forces for the biotin–avidin interaction were greater than those reported in previous studies. This finding was due to the use of high average tip velocities, which were calculated to be two orders of magnitude greater than those typically used in unbinding receptor–ligand experiments. This study therefore highlights the potential use of FM-AFM to study a range of biological systems, including living cells and/or single biomolecule interactions.

By applying scanning nonlinear dielectric microscopy (SNDM), we identified the position of electrons/holes existing in the gate SiO₂–Si₃N₄–SiO₂ (ONO) film of the metal-oxide-nitride-oxide-semiconductor (MONOS) type Flash memory. The electrons were detected in the Si₃N₄ part of the ONO film. The holes, on the other hand, were found in the Si₃N₄ film as well as in the bottom SiO₂ film.

Use of the atomic force microscope (AFM) in quantitative force measurements inherently requires a theoretical framework enabling conversion of the observed deflection properties of the cantilever to an interaction force. In this paper, the theoretical foundations of using frequency modulation atomic force microscopy (FM-AFM) in quantitative force measurements are examined and rigorously elucidated, with consideration being given to both 'conservative' and 'dissipative' interactions. This includes a detailed discussion of the underlying assumptions involved in such quantitative force measurements, the presentation of globally valid explicit formulae for evaluation of so-called 'conservative' and 'dissipative' forces, discussion of the origin of these forces, and analysis of the applicability of FM-AFM to quantitative force measurements in liquid.

Based on topographic images of the GaAs(110) surface obtained by non-contact atomic force microscopy (NC-AFM) with different tip–sample distances, we discuss the tip–sample distance dependence of the cantilever vibration amplitude and the tip-induced surface Ga atom relaxation. In the case of a tip which reveals only As atoms as one kind of protrusion in the NC-AFM image, the damping of the cantilever vibration amplitude is small, and the frequency shift decreases gradually with decreasing tip–sample distance. On the other type of tip with which the bright and darker protrusions corresponding to both As and Ga atoms are observed, a large damping of cantilever vibration amplitude is measured with decreasing tip–sample distance. The frequency shift curve measured with this tip has a singular point. From this frequency shift curve, we conclude that the force acting between this type of tip and the sample surface is a hysteretic force. In this tip case, tip-induced surface relaxation of the topmost Ga atoms occurs. There is a relationship between the damping of the cantilever vibration amplitude and the tip-induced surface atom relaxation; that is, the energy dissipation due to the relaxation becomes remarkable.

The SrTiO₃(100) surface as a substrate for supramolecular structuring was studied using frequency-modulated non-contact atomic force microscopy (FM-NCAFM). After appropriate etching and heating treatments, unit-cell steps with atomically flat and smooth surfaces were successfully obtained. These surfaces were found to be quite stable under UHV conditions as long as the temperature was kept below 600 K. Specially tailored porphyrin-based molecules were deposited on the substrates as a trial case to control their diffusion process on the substrate. Strong suppression of their mobility on the surface was found by FM-NCAFM observations.

Experimental results on the lateral manipulation of single atoms at semiconductor surfaces using non-contact atomic force microscopy (NC-AFM) are presented. These experiments prove that deposited adsorbates on top of a surface, as well as intrinsic adatoms of semiconductor surfaces, are suitable for being manipulated using the short-range interaction force acting between the outermost atoms of a semiconductor tip and the atoms at the surface. The analysis of the data from some of the experiments presented here indicates a pulling process of the tip on the manipulated atoms. The atom-by-atom creation, at room temperature, of patterns composed by a few inherent atoms of a heterogeneous surface is also presented.

The (111) surface of CaF₂ has been intensively studied with large-amplitude frequency-modulation atomic force microscopy, and the atomic contrast formation is now well understood. It has been shown that the apparent contrast patterns obtained with a polar tip strongly depend on the tip terminating ion, and three sub-lattices of anions and cations can be imaged. Here, we study the details of atomic contrast formation on CaF₂(111) with small-amplitude force microscopy utilizing the qPlus sensor that has been shown to provide the utmost resolution at high scanning stability. Step edges resulting from cleaving crystals in situ in the ultra-high vacuum appear as very sharp structures, and on flat terraces the atomic corrugation is seen in high clarity even for large area scans. The atomic structure is also not lost when scanning across triple layer step edges. High-resolution scans of small surface areas yield contrast features of anion- and cation sub-lattices with unprecedented resolution. These contrast patterns are related to previously reported theoretical results.

Electrostatic force microscopy at cryogenic temperatures is used to probe the electrostatic interaction of a conductive atomic force microscopy tip and electronic charges trapped in localized states in an insulating layer on a semiconductor. Measurement of the frequency shift of the cantilever as a function of tip–sample bias voltage shows discrete peaks at certain voltages when the tip is located near trap centres. These discrete changes in frequency are attributed to one by one filling of individual electronic states when the quantized energies traverse the substrate conduction band Fermi energy as the tip–sample voltage is increased. Theoretical analysis of the experiment suggests that such a measurement of the cantilever frequency shift as a function of bias voltage can be interpreted as an AC force measurement, from which spectroscopic information about the location and energy of localized states can be deduced. Experimental results from the study of a sample with InAs quantum dots as trap centres are presented.

Atomic scale data obtained with atomic force microscopy (AFM) in the dynamic mode on graphite and a single-walled carbon nanotube (SWNT) are compared with simulations to relate the observed contrast to the real surface structure. The complete surface unit cell, and particularly the carbon site asymmetry on graphite, can be resolved in the attractive non-contact regime as well as in the weakly repulsive intermittent contact regime. On an SWNT, no carbon site asymmetry is present and the contrast above neighbouring carbon atoms in the attractive non-contact regime is symmetric. The experimental results can be qualitatively reproduced using a Lennard-Jones potential and a graphene layer with and without carbon site asymmetry to model the tip–sample interaction.