W. Allers

A scanning force microscope with atomic resolution in ultrahigh vacuum and at low temperatures

We present a new design of a scanning force microscope (SFM) for operation at low temperatures in an ultrahigh vacuum (UHV) system. The SFM features an all-fiber interferometer detection mechanism and can be used for contact as well as for noncontact measurements. Cooling is performed in a UHV compatible liquid helium bath cryostat. The design allows in situ cantilever and sample exchange at room temperature; the subsequent transport of the microscope into the cryostat is done by a specially designed transfer mechanism. Atomic resolution images acquired at various temperatures down to 10 K in contact as well as in noncontact mode are shown to demonstrate the performance of the microscope.

Dynamic scanning force microscopy at low temperatures on a van der Waals surface: graphite (0001)

The (0001) surface of highly oriented pyrolytic graphite is studied by scanning force microscopy in both contact and dynamic mode. Low temperatures were necessary for the dynamic mode measurements in order to achieve the required signal to noise ratio. At 22 K, atomic scale structures with 2.46 A periodicity and trigonal symmetry of the individual maxima were obtained in both modes. Since graphite exhibits a van der Waals surface in good approximation, this result shows that comparatively weak forces of van der Waals type are sufficient for successful imaging in the dynamic mode on the atomic scale. However, since the positions of the observed maxima correspond to the ones found by scanning tunneling microscopy and contact scanning force microscopy, but not to the positions of the carbon atoms, it also opens new questions on the imaging mechanism in the dynamic mode.

Simultaneous imaging of the In and As sublattice on InAs(110)-(1 × 1) with dynamic scanning force microscopy

Abstract Distance-dependent dynamic scanning force microscopy (SFM) measurements of InAs(110)-(1×1) acquired in ultrahigh vacuum at low temperatures are presented. On this surface, the atoms of the As sublattice are lifted by 80 pm with respect to the In sublattice and terminate the surface. Thus, since in most dynamic SFM images only protrusions with the periodicity of one sublattice are observed, these protrusions are correlated with the positions of the As atoms. However, under certain conditions, an additional contrast is visible which can be attributed to an interaction between the foremost tip atoms and the In atoms. Possible contrast mechanisms are discussed in terms of tip–sample distance and tip structure.

Dynamic force microscopy with atomic resolution at low temperatures

Abstract In this paper, we review some of the most important results obtained with our low-temperature force microscope operated in ultrahigh vacuum. In particular, we stress the resolution capabilities on the atomic scale. After describing some recent modifications of our earlier published setup, we first compare quasi-atomic resolution in the contact mode with true-atomic resolution in the non-contact mode on graphite. On xenon, we demonstrate that weak Van der Waals interactions are sufficient to achieve atomic resolution. Thereafter, atomic scale contrast with ferromagnetic tips on nickel oxide, an insulating antiferromagnet, is discussed with respect to recent theoretical calculations regarding the detection of exchange forces. Finally, tip-induced relaxation is visualized by imaging a point defect on indium arsenide at different tip–sample distances.

Scanning and friction-force microscopy of thin C60 films on GeS(001)

The surface morphology of thin C60 films grown epitaxially under ultra-high vacuum conditions on layered GeS(001) substrates has been studied by scanning force microscopy. The individual C60 layers were imaged down to molecular resolution. The growth mechanism was found to be of layer-by-layer type at the initial stages of growth, but seems to be very sensitive to the substrate temperature. The tribological properties of these films have been probed simultaneously by means of lateral force microscopy. The frictional coefficient of the C60 layers was determined to be significantly smaller than the frictional coefficient of the GeS substrate. This demonstrates that well-ordered C60 films can have even better lubricating properties than a layered material.

Friction in the Low-Load Regime: Studies on the Pressure and Direction Dependence of Frictional Forces by Means of Friction Force Microscopy

The ability of friction force microscopy to give insight into frictional processes on the molecular and atomic scale is exemplified by two different experiments. In the first example, friction force microscopy is employed on a GeS(001) sample covered with islands of epitaxially grown layers of C60 molecules. These measurements help analysing the nature of the dependence of the frictional force F f on the normal force F n . The markedly different frictional behavior on both materials results in a flip of the contrast in the acquired friction force maps with increasing F n . The second example focuses on triglycine sulfate (010) cleavage faces. Their asymmetric surface potential causes a highly direction-dependent frictional force when investigated with the friction force microscope. Both examples are discussed in detail and compared with the theory.

Nanomechanical investigations and modifications of thin films based on scanning force methods

We have studied the nanomechanical properties of thin films epitaxially grown on GeS(001) substrates by scanning force methods. The local frictional coefficient derived for islands was found to be significantly smaller than on the GeS(001) substrate demonstrating that well ordered films can lower the frictional force even compared with a layered material. In the second part of our study, we have used a scanning force microscope (SFM) for nanomechanical modification of a variety of thin film substrates including high- superconductors and thin metallic films on insulating substrates. A combination of photolithography and SFM-based nanofabrication allowed to link the nanoscopic to the macroscopic world and to perform transport measurements on the nanofabricated structures.

Investigation of the Mechanics of Nanocontacts Using a Vibrating Cantilever Technique

A vibrating cantilever technique is presented, which allows the continuous measurement of the tip-sample interaction force F int(z) in the contact as well as in the non-contact region as a function of the tip-sample distance z. The method relies on the measurement of the frequency difference Δf = f — f0 between the eigenfrequency f 0 of the free cantilever and the actual resonance frequency f of the cantilever, which is influenced by the tip-sample interaction potential.

Dynamic Scanning Force Microscopy at Low Temperatures

In this paper, we review the design and various applications of a low temperature scanning force microscope for ultrahigh vacuum. It has been adopted for dynamic mode measurements, a powerful method to image surfaces with a resolution similar to scanning tunneling microscopy, but without the limitation to conducting materials. With this instrument, we have studied semiconducting (InAs), conducting (HOPG) and insulating samples (xenon thin film). Finally, we discuss a new experimental method to determine the tip-sample interaction with high accuracy.