The near-field scanning thermal microscope.

We report on the design, characterization, and performance of a near-field scanning thermal microscope capable to detect thermal heat currents mediated by evanescent thermal electromagnetic fields close to the surface of a sample. The instrument operates in ultrahigh vacuum and retains its scanning tunneling microscope functionality, so that its miniature, micropipette-based thermocouple sensor can be positioned with high accuracy. Heat currents on the order of 10(-7) W are registered in z spectroscopy at distances from the sample ranging from 1 to about 30 nm. In addition, the device provides detailed thermographic images of a sample's surface.

Surface structure. Subatomic resolution force microscopy reveals internal structure and adsorption sites of small iron clusters.

Clusters built from individual iron atoms adsorbed on surfaces (adatoms) were investigated by atomic force microscopy (AFM) with subatomic resolution. Single copper and iron adatoms appeared as toroidal structures and multiatom clusters as connected structures, showing each individual atom as a torus. For single adatoms, the toroidal shape of the AFM image depends on the bonding symmetry of the adatom to the underlying structure [twofold for copper on copper(110) and threefold for iron on copper(111)]. Density functional theory calculations support the experimental data. The findings correct our previous work, in which multiple minima in the AFM signal were interpreted as a reflection of the orientation of a single front atom, and suggest that dual and triple minima in the force signal are caused by dimer and trimer tips, respectively.

The influence of chemical bonding configuration on atomic identification by force spectroscopy.

The force between two atoms depends not only on their chemical species and distance, but also on the configuration of their chemical bonds to other atoms. This strongly affects atomic force spectroscopy, in which the force between the tip of an atomic force microscope and a sample is measured as a function of distance. We show that the short-range forces between tip and sample atoms depend strongly on the configuration of the tip, to the point of preventing atom identification with a poorly defined tip. Our solution is to control the tip apex before using it for spectroscopy. We demonstrate a method by which a CO molecule on Cu can be used to characterize the tip. In combination with gentle pokes, this can be used to engineer a specific tip apex. This CO Front atom Identification (COFI) method allows us to use a well-defined tip to conduct force spectroscopy.

Revealing the angular symmetry of chemical bonds by atomic force microscopy.

We have measured the angular dependence of chemical bonding forces between a carbon monoxide molecule that is adsorbed to a copper surface and the terminal atom of the metallic tip of a combined scanning tunneling microscope and atomic force microscope. We provide tomographic maps of force and current as a function of distance that revealed the emergence of strongly directional chemical bonds as tip and sample approach. The force maps show pronounced single, dual, or triple minima depending on the orientation of the tip atom, whereas tunneling current maps showed a single minimum for all three tip conditions. We introduce an angular dependent model for the bonding energy that maps the observed experimental data for all observed orientations and distances.

Analysis of force-deconvolution methods in frequency-modulation atomic force microscopy.

In frequency-modulation atomic force microscopy the direct observable is the frequency shift of an oscillating cantilever in a force field. This frequency shift is not a direct measure of the actual force, and thus, to obtain the force, deconvolution methods are necessary. Two prominent methods proposed by Sader and Jarvis (Sader-Jarvis method) and Giessibl (matrix method) are investigated with respect to the deconvolution quality. Both methods show a nontrivial dependence of the deconvolution quality on the oscillation amplitude. The matrix method exhibits spikelike features originating from a numerical artifact. By interpolation of the data, the spikelike features can be circumvented. The Sader-Jarvis method has a continuous amplitude dependence showing two minima and one maximum, which is an inherent property of the deconvolution algorithm. The optimal deconvolution depends on the ratio of the amplitude and the characteristic decay length of the force for the Sader-Jarvis method. However, the matrix method generally provides the higher deconvolution quality.