M. Bammerlin

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.

Nanotribology: an UHV-SFM study on thin films of C60 and AgBr

We performed scanning force microscopy (SFM) in ultrahigh vacuum (UHV) on C60 and AgBr thin films deposited on NaCl(001) substrates. The morphology of the initial growth stage and the nanotribological properties of these thin films are characterized and discussed. A novel experimental approach is presented where local friction coefficients are determined: the lateral (frictional) forces are measured as a function of normal load, controlled by an external ramp generator. The local friction coefficient can be extracted by means of the two-dimensional histogram technique. In the low load regime, friction coefficients of 0.15 +/- 0.02, 0.33 +/- 0.07 and > 0.03 were found between probing SiOx tip and C60, AgBr and NaCl, respectively. The two-dimensional histogram reveals significant details about the force regime of wear-less friction and the initial stage of wear on these thin films.

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.

Ultrahigh vacuum atomic force microscopy : true atomic resolution

In this note we report the first observation of salient features of the Si(111)(7 x 7) reconstructed surface across monatomic steps by dynamic atomic force microscopy (AFM) in ultrahigh vacuum (UHV). Simultaneous measurements of the resonance frequency shift Delta f of the Si cantilever and of the mean tunneling current (I) over bar(t) from the cleaned Si tip indicate a restricted range for stable imaging with true atomic resolution. The corresponding characteristics vs. distance reveal why feedback control via Delta f is problematic, whereas it is as successful as in conventional STM via (I) over bar(t). Furthermore, local dissipation (energy loss of 10(-14) W) through individual atoms is observed and explained by the coupling of the surface atoms to phonons.

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.

Ferroelectric domains and material contrast down to a 5 nm lateral resolution on uniaxial ferroelectric triglycine sulphate crystals

We operate dynamic force microscopy (DFM) in non-contact mode under ultrahigh vacuum conditions in order to differentiate local chemical inhomogeneities from ferroelectric domains in bulk single crystals of triglycine sulphate (TGS), Individual layers of both mixed glycine and sulphate, and pure glycine ions are resolved resulting in a lateral material contrast of 5 nm and a sub-unit cell vertical contrast. The clue information is deduced when analysing the excitation amplitude A(exc) applied to the piezo shaker in constant amplitude DFM which is strongly affected either by the short-range chemical force or the longer range electrostatic contribution. The latter arises from the bound surface charge within individual ferroelectric domains and results in a long-range tip-sample interaction. From variation of the bias voltage applied to the TGS counter electrode we deduce the sign of the bound surface charge within each ferroelectric domain. Hence, domain walls that separate regions of antiparallel polarization are resolved down to a 9 nm domain wall width. Copyright (C) 1999 John Wiley & Sons, Ltd.

Molecular impurities at the NaCl(100) surface observed by scanning force microscopy

Abstract We have studied cleavage faces of NaCl crystals containing (OCN)− molecules with a scanning force microscope in ultra-high vacuum. The dopant content in the crystals was determined using infrared spectroscopy. The molecular impurities appear as nm-sized elevations in the scanning force micrographs. Their position at cleavage steps is discussed in terms of both diffusion and step formation during cleavage. Although the atomic periodicity of the NaCl lattice is imaged, high-resolution investigations of impurities are limited by a convolution between the tip apex and the elongated molecules sticking out of the surface.

New Developments in Scanning Probe Microscopy

Four topics will be treated in this article: 1) One of the central questions of contact force microscopy is the determination of contact area. Imaging of well-defined structures is one way to estimate the size of the contact. Recently, continuum elasticity models were used to describe the nanometer-sized contact. The lateral contact stiffness method was found to be particularly interesting, because it is rather independent of the selected formalism. 2) Progress has been made with non-contact force microscopy, where true atomic resolution has been achieved. It is found that the instrument has to be operated at similar tip-sample distances as in STM. On Si(111)7×7, the strongest attraction is found for the adatoms, which is in agreement with theoretical models. The contrast at step sites is found to be influenced by short-range chemical forces and long-range electrostatic or van der Waals forces. Spectroscopic methods were used to investigate the frequency shifts between the upper and lower terrace. Local variations of the contact potential are found to be below the detection limit, whereas variations of electrostatic forces due to changes in the interaction volume are found to be predominant. 3) Some artifacts of scanning probe microscopy are briefly discussed. The tip artifact, where the sample topography is convoluted with the tip geometry is the most common artifact. The second artifact is related to laser beam interference between the sample surface and the rear side of the cantilever, which can cause interference patterns in laser beam deflection force microscopy images. 4) The application of AFM-based technology to the construction of chemical and physical sensors is found to be extremely successful. Micro-machined cantilevers are the central part of these sensors. After the chemieal treatment of the cantilevers (active or functional probes), the conditions of the surface film are monitored with a sensitive deflection sensor. The reaction with environmental gases leads to changes of surface stress (stress mode) or mass changes, which are detected as a resonanee frequency shift (frequency mode). Using the bimetallic effect, small heat changes can be translated into cantilever deflections (calorimeter mode).