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Direct nanoscale and atomic resolution imaging is a key issue in nanoscience and nanotechnology. The invention of the dynamic force microscope in the early 1990s was an important step forward in this direction as this instrument provides a universal tool for measuring the topography and many other physical and chemical properties of surfaces at the nanoscale. Operation in the so-called non-contact mode now allows direct atomic resolution imaging of electrically insulating surfaces and nanostructures which has been an unsolved problem during the first decade of nanotechnology. Today, we face a most rapid development of the technique and an extension of its capabilities far beyond imaging; atomically resolved force spectroscopy provides information about local binding properties and researchers now develop sophisticated schemes of force controlled atomic manipulation with the tip of the force microscope.

Progress in the field of non-contact force microscopy is discussed at the annually held NC-AFM conferences that are part of a series started in 1998 with a meeting in Osaka, Japan. The 6th International Conference on Non-contact Atomic Force Microscopy took place in Dingle, Ireland, from 31 August to 3 September 2003 and this special issue is a compilation of the original publications of work presented at this meeting. The papers published here well reflect recent achievements, current trends and some of the challenging new directions in non-contact force microscopy that have been discussed during the most stimulating conference days in Dingle. Fundamental aspects of forces and dissipation relevant in imaging and spectroscopy have been covered by experimental and theoretical contributions yielding a more detailed understanding of tip--surface interaction in force microscopy. Novel and improved imaging and spectroscopy techniques have been introduced that either improve the performance of force microscopy or pave the way towards new functionalities and applications. With regard to studies on the specific systems investigated, there was a strong emphasis on oxides and ionics, as well as on organic systems.

Following previous pioneering work in uncovering the atomic structure of insulating oxides with force microscopy, it was shown in the meeting that this important class of materials is now accessible for a quantitative atomic scale surface characterization. Single organic molecules and ordered organic layers are building blocks for functional nanostructures currently developed in many laboratories for applications in molecular electronics and sensor technologies. The Dingle conference impressively demonstrated that dynamic force microscopy is ready for its application as an analytical tool for these promising future nanotechnologies.

The meeting was a great success scientifically and participants enjoyed the beauty of the conference site. I would like to thank all members of the international steering committee, the programme committee and the co-chairs, J Pethica, A Shluger and G Thornton, for their efforts in preparing the meeting.

The members of the local organising committee, J Ballentine-Armstrong, G Cross, S Dunne, S Jarvis and Ö Özer, kept the meeting running smoothly and created a very pleasant atmosphere. The generous financial support from Science Foundation Ireland (SFI), is greatly appreciated; SFI is dramatically raising the profile of Irish science. I would also like to express my sincere gratitude to N Couzin and the journal team from Institute of Physics Publishing for their editorial management and perfect co-operation in the preparation of this special issue.

We present a number of energetically favourable adsorption sites of C₆₀ on the Si(001) surface using a DFT method. This will form the first step of theoretical simulations of the manipulation of C₆₀ molecules on the silicon surface by non-contact atomic force microscopy (NC-AFM). Calculations of adsorption energies and geometries performed within the local density approximation (LDA) agree with previous LDA studies. However, we find that the binding energies are significantly reduced (by at least half) if the generalized gradient approximation (GGA) is employed.

The large corrugation amplitudes in scanning tunnelling microscope (STM) images of metal surfaces have been commonly attributed to the action of forces between the tip and the sample. We have investigated the Cu(100) surface using a high-resolution non-contact atomic force microscope/scanning tunnelling microscope (nc-AFM/STM) in UHV. Force gradient and STM topography images were acquired simultaneously using constant tunnelling current feedback. Force gradient images showed atomic resolution whereas STM scans exhibited almost no contrast, corresponding to a flat tip trajectory during scans. The corrugation height in force gradient images was found to increase as the set tunnelling current was increased. Force gradient and tunnel current were directly measured as a function of separation, to determine the operating conditions during imaging. The STM operation regime is found to lie between the minimum of the stiffness curve and the start of repulsive force.

Alkali chloride thin films on Au(111) are investigated under ultrahigh vacuum conditions at room temperature by noncontact atomic force microscopy in combination with Kelvin probe force microscopy. Sample preparation is carried out in situ and optimized in order to achieve a sub-monolayer coverage showing extended alkali chloride island formation. The local surface potential for LiCl, NaCl, KCl and RbCl thin films on Au(111) was directly determined and compared to values of the bare Au(111) substrate. Hence, the absolute change in the local work function ΔΦ was evaluated. We observe a linear dependence between ΔΦ and the ionic radius of the mentioned alkali chloride thin films.

We use highly oriented pyrolytic graphite (HOPG) with a submonolayer coverage of C₆₀ and monitor the step height from C₆₀ to HOPG as a function of dc bias for non-contact atomic force microscopy (NC-AFM). We find a strong dependence of the step height on dc bias between tip and sample surface. The step height is modified by uncompensated electrostatic forces, resulting from the work function difference of  meV between C₆₀ and HOPG, as determined by Kelvin probe force microscopy (KPFM) measurements. In comparison, steps from C₆₀ to C₆₀ or from HOPG to HOPG show no dependence on dc bias. This effect is well described by simulations of the electrostatic interaction between tip and sample. The results clearly demonstrate the influence of uncompensated electrostatic forces on height measurements in NC-AFM, and show that correct height determination requires the use of KPFM with active control of the bias voltage.

A mixed self-assembled monolayer (SAM) of two kinds of thiolate with a hydrogen- or a chlorine-terminated bicyclo[2.2.2]octane moiety was formed on Au(111). The terminal groups (X =  H and Cl) on the bicyclo[2.2.2]octane moieties were identified with noncontact atomic force microscopy (NC-AFM) using a Si tip with a native oxide layer.

Amplitude resonance curves of a driven dynamic force microscope cantilever interacting with a Cu(100) substrate in ultra high vacuum are derived and analysed, extending a previous study on Al₂O₃(0001) (Polesel-Maris et al 2003 Nanotechnology14 1036). It is shown that the charges that are trapped on the oxidized n⁺-doped silicon tip give rise to long-range electrostatic forces that dominate the van der Waals forces, due to the metallic nature of the substrate. These electrostatic forces cannot be compensated by the usual procedure consisting of applying a constant bias voltage between the tip and the sample. Indeed, the trapped charge does not remain constant on the timescale of the experiment due to charge leakage across the tip oxide layer. Removal of this oxide by electron field emission solves this problem and allows the access of a pure van der Waals tip–substrate interaction regime.

We investigated alkanethiol phase-separated self-assembled monolayers (PS-SAMs) using the Kelvin probe force microscope (KFM) combined with the non-contact atomic force microscope (NC-AFM). PS-SAMs were composed of two species of different chain-length alkanethiol. The surface potential (SP) measurements showed that the value of SP was linearly increased as a function of the chain length and that the gradient was about 9 mV per CH₂ unit. In addition, we succeeded in obtaining nanometre-scale variations of the SP, which were discussed in the context of the surface structures.

A possible mechanism of atomic scale dissipation in non-contact atomic force microscopy (NC-AFM) is investigated using a non-equilibrium classical molecular dynamics simulation method with stochastic boundary conditions. We find that for simple flat crystal surfaces, such as the MgO(001) which does not possess intrinsic surface soft vibrational modes, dissipation effects can be explained by tip induced soft modes associated with well-known atomic instabilities produced by the approaching tip. Extrapolating results of calculations performed for a succession of tip oscillation frequencies, we suggest that dissipation energies can be obtained via this mechanism which are of the same order of magnitude of experimentally observed contrasts.

Non-contact atomic force microscopy (NC-AFM) was used to study thin films of C₆₀ on Au(111). After observing the Au(111) reconstruction, 2–3 monolayers of C₆₀ were deposited onto the Au surface. The close-packed C₆₀ surface was imaged by NC-AFM with molecular resolution. Enhanced corrugation and a stretching of the C₆₀ lattice were observed at step edges. Based on a calculation of the force required to displace an edge molecule, it is proposed that the edge effects are a result of tip-induced displacements of edge molecules. While imaging small clusters of C₆₀, some molecules were removed, leading to structural rearrangements of the clusters.

Using realistic molecular dynamics simulations on the MgO(001) surface we show that the friction coefficient and dissipation energies due to the stochastic friction force mechanism are much smaller than observed experimentally in close agreement with previous analytical models. Therefore, the method is generalized by including a single soft vibrational mode at the surface. Using methods of non-equilibrium statistical mechanics, we derive an equation of motion for the tip interacting with the soft mode. It is shown that this method enables the simultaneous description of both stochastic friction and adhesion.

The adsorption and reaction of methanol (CH₃OH) and nitrogen dioxide (NO₂) on CeO₂(111) surfaces in different oxidation states were studied by noncontact atomic force microscopy (NC-AFM). After CH₃OH exposure, surface oxygen defects without reconstruction of the surrounding oxygen atoms were healed by methoxy species (CH₃O). Under a CH₃OH atmosphere, a heterogeneous feature in the reactivity of surface oxygen atoms to CH₃OH was observed by successive NC-AFM observation: new line defects of oxygen were produced as a result of a chemical event. Successive NC-AFM measurements also revealed that oxygen vacancies on a slightly reduced CeO₂(111) surface were healed by exposure to NO₂. The contrast of healed oxygen defects in NC-AFM images was similar to that of other surface oxygen atoms. This indicates that dissociative adsorption of NO₂ occurred at the defects. Nitrogen trioxide (NO₃) was also observed on a slightly reduced CeO₂(111) surface after NO₂ exposure. On a CeO₂(111) surface without surface oxygen defects, neither adsorption of NO₂ nor formation of NO₃ was observed. The reaction mechanisms for the formation of line defects and NO₃ are discussed briefly.

The mechanism of atomic-scale image formation in non-contact AFM on metallic surfaces is analysed using total-energy pseudopotential calculations. Depending on the tip–apex configuration, we find two different imaging modes. Both clean and metal contaminated Si tips provide atomic resolution arising from the very strong covalent tip–sample interaction, in striking similarity with the imaging mechanism found on semiconductor surfaces. A completely new mechanism, reversible short-range electrostatic imaging, due to subtle charge-transfer interactions is identified for oxidized Si tips. Contrary to the strong covalent-bond imaging, this new mechanism causes only negligible surface perturbation and can account for recent experimental results.

In this study we use first principle methods to study the interaction of three different tip models with a CaF₂(111) surface. The tip models are based on a 'realistic' approach to tip simulation and consist of a pure silicon tip, an oxygen terminated silicon tip and a water contaminated silica (SiO₂) tip. We find that each tip demonstrates characteristic features which should be observable in experiments and hence provide signatures which would aid in the identification of tip and surface species.

Silicon cantilevers covered with dodecyl monolayers anchored via silicon–carbon covalent bonds were prepared by a wet process and used for non-contact atomic force microscopy (NC-AFM) of TiO₂(110)-(1 × 1) surfaces. Clear images of atomic rows on atomically flat terraces were observed with the dodecyl-coated samples when they were biased around 2.0 V with respect to the cantilevers. The bias voltage required to give clear images for alkyl-coated cantilevers was higher than that for uncoated ones. Since the cantilevers are thermally and chemically stable, they are applicable to various force microscopy to distinguish chemical species on surfaces.

Micropatterning techniques using vacuum ultra-violet light at 172 nm in wavelength and an ultra-violet laser at 244 nm were applied to organosilane self-assembled monolayers (SAMs) formed on Si substrates covered with a 2 nm thick oxide layer. These SAMs were prepared from precursors, that is, octadecyltrimethoxysilane (ODS) or p-chloromethylphenyltrimethoxysilane (CMPhS). Polar functional groups such as COOH were generated through photochemical oxidation of the SAMs. Furthermore, the SAMs were simultaneously etched and finally removed from the photoirradiated area. Surface potentials of the photoirradiated areas at various irradiation dose rates were measured by Kelvin-probe force microscopy (KFM) using the unirradiated area on each of the photopatterned samples as a reference. Changes in the surface chemical composition of the SAMs could be clearly and sensitively measured by KFM through the difference in surface potential between the irradiated and unirradiated SAMs.

Single-molecule imaging at nanometre resolution using noncontact AFM has been applied to image a protein complex on mica using single-wall carbon nanotube (CNT) technology. Hydrated RNA polymerase (RNAP)–DNA complex was imaged using constant-amplitude tapping mode AFM. CNT tips (1–2 nm diameter) prepared by chemical vapour phase deposition were mounted onto standard Pt/Ir coated AFM tips. The effectiveness of the CNT tip scanning was tested by imaging the profile of an RNAP–DNA complex. The results demonstrate the merits of high-resolution carbon nanotube nc-AFM scanning and suggest that CNT tips may provide a general tip–surface reference standard for defining tip–surface interactions on molecular-bonded organic surfaces.

In frequency modulation atomic force microscopy, the stiffness, quality factor and oscillation amplitude of the cantilever are important parameters. While the first atomic resolution results were obtained with amplitudes of a few hundred ångstrom, it has subsequently been shown that smaller amplitudes should result in a better signal-to-noise ratio and an increased sensitivity to the short-range components of the tip–sample interaction. Stable oscillation at small amplitudes is possible if the product of stiffness and amplitude and the energy stored in the oscillating cantilever are large enough. For small amplitudes, stability can be achieved by using stiff cantilevers. Here, we discuss the physical requirements for small amplitude operation and present design criteria and technical details of the qPlus sensor, a self-sensing cantilever with large stiffness that allows small amplitude operation.

To control the interaction between tetrakis(3,5 di-t-butylphenyl)porphyrin (TBPP) molecules and substrates, 5-(4-methylthiophenyl)-10,15,20-tris(3,5-di-t-butylphenyl)porphyrin (MSTBPP) molecules were tailored; one of the four di-t-butylphenyl groups of TBPP molecules was replaced by a methylthiophenyl group. By means of non-contact atomic force microscopy (NC-AFM) operated in the force feedback control mode, the conformation of MSTBPP molecules deposited on an Au(111) substrate was investigated. It was found that the methylthiophenyl group introduced into the TBPP molecules enhanced the attractive interaction between the molecule and the substrate. This interaction seems to improve the spatial resolution of NC-AFM.

The adsorption of two kinds of porphyrin (Cu-TBPP) and perylene (PTCDA) derived organic molecules deposited on KBr and Al₂O₃ surfaces has been studied by non-contact force microscopy in ultra-high vacuum, our goal being the assembly of ordered molecular arrangements on insulating surfaces at room temperature. On a Cu(100) surface, well ordered islands of Cu-TBPP molecules were successfully imaged. On KBr and Al₂O₃ surfaces, it was found that the same molecules aggregate in small clusters at step edges, rather than forming ordered monolayers. First measurements with PTCDA on KBr show that nanometre-scale rectangular pits in the surface can act as traps to confine small molecular assemblies.

Tip–sample interactions are at the origin of atomic resolution in non-contact atomic force microscopy (NC-AFM) but also make it difficult to understand the meaning of atomic features observed as NC-AFM images. (110) surfaces of III–V semiconductors such as GaAs or InP have relaxed (1 × 1) surfaces with both anions and cations of different dangling bond states. Since these atoms are located at the surface where an atom attached to the tip apex can reach to interact with them and show significant relaxation via tip–sample interaction, NC-AFM images of the surface are expected to strongly depend upon the atomic species or the electronic state of the topmost atom at the tip apex. We have taken a number of NC-AFM images of GaAs(110) surfaces with a Si tip by means of room-temperature ultra-high vacuum NC-AFM and categorized them into two types of image which have different tip–sample distance dependencies. In comparison with a theoretical prediction of tip–surface interaction on the GaAs(110) surface, we have found that one of these images is due to a Si atom attached to the apex which reveals the tip–sample interaction with As and Ga surface atoms as individual peaks in the NC-AFM image.

The dynamic characteristics of a tip oscillating in the nc-AFM mode in close vicinity to a Cu(100)-surface are investigated by means of phase variation experiments in the constant amplitude mode. The change of the quality factor upon approaching the surface deduced from both frequency shift and excitation versus phase curves yield to consistent values. The optimum phase is found to be independent of distance. The dependence of the quality factor on distance is related to 'true' damping, because artefacts related to phase misadjustment can be excluded. The experimental results, as well as on-resonance measurements at different bias voltages on an Al(111) surface, are compared to Joule dissipation and to a model of dissipation in which long-range forces lead to viscoelastic deformations.

We have studied the interactions responsible for the contrast formation in nc-AFM imaging of the α-alumina(0001) surface using the oxide and Si dangling bond tip models. The full image simulation performed using the rigid oxide tip and common cantilever parameters suggests the possibility of atomic and chemical resolution on the plane surface in a narrow range of experimental parameters. The interaction of both the oxide and Si dangling bond tips with the surface is stronger at the centres of triangles formed by three oxygen ions. This makes the image interpretation for the Si dangling bond tip unique—the bright spots should correspond to the centres of O triangles and centres of dark spots are located above the surface Al ions. In the case of the oxide tip, the image pattern depends on the tip polarity and is different, but easily recognizable. However, only Al ions or triangles formed by three oxygen ions could be resolved in the simulated images.