Ch. Loppacher

Ultrathin films of NaCl on Cu(111) : a LEED and dynamic force microscopy study

Ultrathin films of NaCl on Cu(111) have been studied with low-energy electron diffraction (LEED) and Dynamic force microscopy (DFM). The orientation and the lattice constant of the films are revealed by LEED while DFM allows a real space view on their growth modes. The ability of the DFM to image local mechanical surface properties is demonstrated at a substrate step which is covered by a continuous NaCl film.

Ferroelectric domain characterisation and manipulation : A challenge for scanning probe microscopy

Domain writing and reading on the nanometer scale is addressed with scanning force microscopy (SFM) Compared to other scanning probe methods, SFM provides broad possibilities for the on-line data controlling. i.e. three-dimensional mapping of polarisation distribution, differentiation between polarisation and topography, nanoscale domain switching of domains with a 60 nm diameter, recording of nanoscale hysteresis loops, phase transition mapping. domain wall imaging with 9 nm resolution, atomic resolution of ferroelectric surfaces, etc. All these issues are reported in this paper. The challenging result of such a concerted investigation is the possibility of using SFM for nanoscale domain writing and reading with nanometer resolution. Fig. 1 illustrates such an example where line shaped c - domains are purposely written into a ferroelectric Barium-Titanate single crystal with a 400 nm line-width. With this figure we highly appreciate and honour the work of Bob Newnham passing our best nano-wishes for his future.

Friction on the atomic scale: An ultrahigh vacuum atomic force microscopy study on ionic crystals

We performed atomic force microscopy in ultrahigh vacuum on the ionic crystal of KBr(001). The morphology and the tribological properties of this cleavage face are characterized and discussed. The local friction coefficient was extracted by means of the two‐dimensional histogram technique. For loads below 3 nN a linear behavior was found between normal and lateral forces yielding a friction coefficient of less than 0.04. In this load regime, wearless friction is observed. For higher loads, the friction coefficient increases to a value of about 0.7–1.2. The corresponding topography images reveal the typical onset of wear. On the atomic scale a periodicity of 4.7 A was found which corresponds to the distance of equally charged ions on the KBr(001) surface. On this scale, the lateral force map exhibits the typical stick‐slip phenomenon which is discussed in terms of a novel theoretical approach.

Carbon nanotubes as tips in non-contact SFM

Abstract The demand for sharp and stable tips suggests the use of carbon nanotubes as probing tips in scanning force microscopy. Here, we report a comparison of the long-range forces of conventional tips and nanotube tips, topographical images of various surfaces, such as Cu(111), Si(111)7×7 and NaCl(100), as well as images of a bundle of multiwalled nanotubes, which was deposited by severe tip crashing. It is found that the long-range forces of carbon nanotube probing tips are reduced and that they are more resistant to wear than conventional silicon tips

Phase variation experiments in non-contact dynamic force microscopy using phase locked loop techniques

This work presents constant amplitude Dynamic Force microscopy (DFM) measurements under ultra-high vacuum conditions performed with home-built digital electronics based on the principle of phase locked loop (PLL) techniques. In DFM so-called topography is often measured in constant frequency shift (Delta f) mode. This study describes the influence of phase shifts on constant af imaging. Therefore, phase variation experiments were acquired, leading to information about the cantilever resonance behaviour close to the surface. As sample, an evaporated thin film of NaCl on a Cu(111) substrate was chosen in order to obtain a heterogeneous system with clean Cu and NaCl areas. The atomic structure of both materials was resolved, which is the first time true atomic resolution was obtained on a metal. Large apparent topography variations are observed on this heterogeneous sample when changing the phase between the excitation and oscillation of the cantilever end. Such artefacts can be explained by comparison with phase variation experiments

Surface morphology, chemical contrast, and ferroelectric domains in TGS bulk single crystals differentiated with UHV non-contact force microscopy

Ferroelectric bulk single crystals of tri-glycine sulphate (TGS) have been investigated in ultra-high vacuum (UHV) using dynamic force microscopy (DFM) in the non-contact (nc) mode. Bath chemical contrast from different sub-unit cell cleavage steps, and ferroelectric domains were differentiated by recording the variation in interaction force affecting the excitation amplitude A(exc) applied to the piezo shaker in constant amplitude DFM. No chemical difference was found for steps measuring half the unit cell height b, in contrast to b/4-steps where sulphate ions change the local short-range chemical forces. By varying the bias voltage applied to the TGS counter electrode, the sign of bound surface charge within each ferroelectric domain was determined. Domain walls separating regions with antiparallel polarisation vectors are resolved down to a 9 nm domain wall width. Furthermore, we achieved atomic resolution with nc-DFM on cleaved TGS samples indicating the monoclinic unit cell at the ferroelectric sample surface with a = 1.0 +/- 0.05 nm, c = 0.55 +/- 0.05 nm, and beta = 107 +/- 3 degrees.

Aspects of dynamic force microscopy on NaCl/Cu(111): resolution, tip–sample interactions and cantilever oscillation characteristics

Ultrathin films of NaCl on Cu(111) have been studied using a dynamic force microscope. We present images with atomic resolution at step sites on the NaCl films, Force spectroscopy measurements of the tip-sample interaction on NaCl-covered areas and the Cu substrate are analysed with respect to electrostatic, van der Waals and short-range contributions, The interaction contrast between NaCl and Cu is shown to be reflected in the oscillation characteristics of the cantilever, First results of a resonance analysis with the help of a novel digital oscillation control system are presented.

Using higher flexural modes in non-contact force microscopy

The oscillation characteristics of higher flexural modes of a rectangular microfabricated silicon cantilever have been studied in ultra-high vacuum (UHV) for a free cantilever and for a typical situation in non-contact force microscopy. The results are discussed with respect to the use of such modes in dynamic force microscopy (DFM) and local dissipation measurements.

Fuzzy controlled feedback applied to a combined scanning tunneling and force microscope

A feedback mechanism based on fuzzy logic has been applied to operate a combined atomic force microscope (AFM)/scanning tunneling microscope (STM), which is able to measure the resonance frequency shift Delta f of the cantilever-type spring and the mean tunneling current >(I)over bar (t)< simultaneously. Using a decision making logic, the microscope can be scanned over a heterogeneous surface without tip crash. On the conductive parts of the sample, the STM mode is preferred, whereas the noncontact (nc)-AFM mode is used on the poorly conductive parts of the surface. The transition from the STM mode to nc-AFM mode is performed smoothly with the fuzzy logic feedback

Dynamic force microscopy across steps on the Si(111)-(7 × 7) surface

Abstract Force microscopy in atomic resolution with an oscillating tip has been performed across monatomic steps of the Si(111)-(7×7) surface using the tunnelling current or frequency shift as the feedback parameter. The contrast of simultaneously recorded images in both feedback modes is discussed. A significant difference between tip–sample interactions on the upper and lower terrace close to a step is analyzed in detail by means of Kelvin-type measurements. No contact potential variation across the step is found. A simple model for the force contrast is suggested which takes into account the different effective interaction areas or volumes on the upper and the lower terrace.