Single-atom Tip Research Articles

Atomic-scale friction image of graphite in atomic-force microscopy.

We theoretically investigated the image of atomic-scale friction of graphite in atomic-force microscopy (AFM), based on numerical simulation for a static model. We performed systematic calculations of lateral force images of AFM aiming to clarify the effects of cantilever stiffness, scan direction, anisotropy of the cantilever, and surface deformation. The simulation is performed for a simple atomistic model with a single-atom tip connected with the cantilever spring scanned on a monolayer graphite surface. The process in which the conservative lateral force becomes a nonconservative frictional force is clarified. ``Stick regions'' of the tip atom are also discussed in relation to the cantilever stiffness. Calculated frictional-force image patterns are in good agreement with experimental ones. We also find the supercell frictional-force images and discuss their mechanisms. \textcopyright{} 1996 The American Physical Society.

Simulations of the Effects of Tip Apex Geometries on Atomic Force Microscopy Images

Simulation works on the effects of tip apex geometries on atomic force microscopy (AFM) images were examined. Tips and samples employed in those simulations were mostly made of a single component. Short-range interatomic potentials such as Lennard-Jones and Morse were used. With these potentials, it was found that a single atom tip (a tip with an atom protruding at its apex) is necessary for obtaining true atomic resolution. In many cases flat tip geometries (tips with multiple atoms at their apexes) produce various images that do not correspond to the surface atom arrangements, which may lead to various faulty AFM image interpretations.

Simulation of tip - sample interaction in the atomic force microscope

Recent simulations of the interaction between planar surfaces and model atomic force microscope (AFM) tips have suggested that there are conditions under which the tip may become unstable and `avalanche' toward the sample surface. Here we investigate via computer simulation the stability of a variety of model AFM tip configurations with respect to the avalanche transition for a number of fcc metals. We perform Monte Carlo simulations at room temperature using the equivalent-crystal theory (ECT) of Smith and Banerjea. Results are compared with recent experimental results as well as with our earlier work on the avalanche of parallel planar surfaces. Our results on a model single-atom tip are in excellent agreement with recent experiments on tunnelling through mechanically controlled break junctions.

Effect of the tip structure on atomic-force microscopy.

Theoretical simulation of atomic-force microscope (AFM) images is performed with a model of a single-atom tip or two kinds of multiple-atom diamond tips scanned on a graphite substrate surface. Several fundamental effects on AFM images are systematically investigated. First, it is made clear how the AFM images and the force distributions change as the load varies. Then, for the multiple-atom tips, the effects of the tip orientation and the tip apex structure, including the bond length, are examined. The characteristics of the AFM images, such as their detailed microscopic pattern, the symmetry, and the corrugation amplitude, depend strongly on these effects. In the cluster models, the interatomic potential within the tip and the surface is assumed to be harmonic, and that between the tip and the surface is taken as being of the Lennard-Jones type. It is clearly shown that AFM images reflect not merely the geometrical structure of the surface, but also various microscopic properties of the tip and the surface.

Effects of tip size and asymmetry on scanning tunneling microscope topographs

We investigated broadening and skewing of scanning tunneling microscope topographs caused by tip size and symmetry. Model images were generated numerically by convoluting a gaussian response function for the tip with an idealized graphite image. Atomic-scale features were retained even for a 9.4 Å FWHM tip function, which demonstrated that single-atom tips are not required to obtain atomic resolution topographs. Model tips with an elliptical radial cross section produced many of the familiar skewed images of graphite. Such response functions could also be used to remove the skew from an experimental topograph, and thus provide an approximation for the shape of the physical tip.

Site-dependent electronic effects, forces, and deformations in scanning tunneling microscopy of flat metal surfaces.

Interactions with a single-atom tip become important a few angstroms prior to adhesive contact to a metallic sample. The collapse of the barrier produces a connecting channel; directed atomiclike electronic states form at its ends, resulting in an overall attraction. The wave functions of states near the Fermi level nevertheless remain evanescent. These effects are found to be strongly site dependent even on a very flat metal surface, and may explain the atomic resolution achieved in scanning tunneling microscopy of such surfaces. Deformation of the tip cannot explain such observations.

Wave aspects of electron and ion emission from point sources

Electrons emitted from single-atom tips are analysed in a model with the tip as a round tubelike constriction. The source creates two-dimensional minimum wavepackets whose temporal evolution agrees well with observed beam divergences. With few-atom tips interference effects may arise due to the electron's non localized emission from several atoms. The phase space density of emitted electrons is around 10−2 h−3, and electron-electron correlation experiments become feasible. In ion field emission mode observed beam divergences are dominated by thermal broadening and other effects rather than quantum mechanical principles. Applications include electron interferometers of lower energy and wider beam separation than hitherto in use, as well as proton and possibly other ion interferometers. Highly focused low energy beams could also be produced.

Atomic theory of scanning tunneling microscopy.

We present a quantitative analysis of the modifications of the scanning-tunneling-microscopy images due to the local perturbations of the electronic states induced by the tip in close proximity to the sample surface. Using an empirical tight-binding method, we have calculated the electronic states of a prototype tip-sample system consisting of a single-atom tip and the graphite surface, as a function of the tip-sample distance. We find that as the tip approaches the sample, their states start to interact and become laterally confined in the vicinity of the tip at small tip-sample separation. These states influence the tunneling phenomenon by connecting the tip and sample surface electronically. The effect of the tip-induced localized states is discussed, and the expression for the tunneling current is reformulated by incorporating the tip-induced states. Calculations using this expression show that the corrugation amplitude obtained from scanning tunneling microscopy is enhanced and deviates from the proportionality to the local density of states of the bare sample at the Fermi level evaluated at the center of the tip.

Simple theory for the atomic-force microscope with a comparison of theoretical and experimental images of graphite.

In this theory the atom-atom interactions within the sample are modeled with use of a force-constant matrix that is valid in the harmonic approximation. The tip-atom-sample interactions are modeled by a Lennard-Jones potential. Images for single-atom tips that are calculated from this theory, in which the atoms are allowed to relax, are similar to previous theoretical images that were calculated from a rigid-sample model. The asymmetric images produced by experiment could be reproduced theoretically with double-atom tips. All calculations were performed using a commercially available desk-top computer.

Spin-polarized electron microscopy

A current scientific challenge with many ramifications for magnetic technology is to image magnetic microstructure with the highest possible spatial resolution in order to observe magnetic domains or even spin configurations within a domain wall. Ultimately one can envision imaging the magnetic moment of individual atoms which would also make possible the observation of antiferromagnetic structures. The measurement of the spin polarization of secondary electrons generated by a finely focussed (unpolarized) scanning electron microscope (SEM) beam to obtain high-resolution magnetization images is presented. An alternative measurement, using a spin-polarized incident beam in an SEM, has many difficulties which are discussed. To measure spin configurations with higher spatial resolution, the possibility of introducing electron spin polarization in scanning field-emission and tunneling microscopy is considered. The measurement of the spin polarization of secondary electrons generated by a specially prepared single-atom scanning field-emission tip looks promising. High-resolution imaging of spin configurations in scanning tunneling microscopy appears possible if the tip itself is a source of spin-polarized electrons. The potential advantages and unsolved problems involved in using a ferromagnetic tip or an optically pumped semiconductor tip are described.