Alexis Baratoff

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.

Cu-TBPP and PTCDA molecules on insulating surfaces studied by ultra-high-vacuum non-contact AFM

The adsorption of two kinds of porphyrin (Cu-TBPP) and perylene (PTCDA) derived organic molecules deposited on KBr and Al2O3 surfaces has been studied by non-contact force microscopy in ultra-high vacuum, our goal being the assembly of ordered molecular arrangements on insulating surfaces at room temperature. On a Cu(100) surface, well ordered islands of Cu-TBPP molecules were successfully imaged. On KBr and Al2O3 surfaces, it was found that the same molecules aggregate in small clusters at step edges, rather than forming ordered monolayers. First measurements with PTCDA on KBr show that nanometre-scale rectangular pits in the surface can act as traps to confine small molecular assemblies.

Molecular dynamics simulations of dynamic force microscopy : applications to the Si(111)-7 X 7 surface

Abstract Molecular dynamics simulations have been performed to understand true atomic resolution, which has been observed on the Si(111)-7×7 surface by dynamic force microscopy (DFM) in ultra high vacuum (UHV). Stable atomic-scale contrast is reproduced in simulations at constant mean height above a critical tip–sample separation when monitoring the interaction force between tip and sample. Missing or additional adatoms can be recognized in such scans, although they are less well resolved than native adatoms. The resonance frequency shift, as well as arbitrary scans, e.g., at constant force, can be computed from a series of force–distance characteristics. By means of dynamic simulations, we show how energy losses induced by interaction with an oscillating tip can be monitored, and that they occur even in the non-contact range.

Quantifying the atomic-level mechanics of single long physisorbed molecular chains

Significance Mechanical properties of biopolymers such as DNA and proteins have been studied to understand the details of complex processes in living systems via systematic statistical analyses of repeated measurements. However, the mechanical behavior of a single molecular chain pulled off a surface has never been investigated with atomic-scale resolution. Herein, we present such a study on in situ polymerized fluorene chains by pulling individual chains with the tip of an atomic force microscope at 4.8 K. The measured variations of the force gradient provide detailed insights into the detachment process of fluorene units and the role of near-incommensurability with the substrate structure. Individual in situ polymerized fluorene chains 10–100 nm long linked by C–C bonds are pulled vertically from an Au(111) substrate by the tip of a low-temperature atomic force microscope. The conformation of the selected chains is imaged before and after manipulation using scanning tunneling microscopy. The measured force gradient shows strong and periodic variations that correspond to the step-by-step detachment of individual fluorene repeat units. These variations persist at constant intensity until the entire polymer is completely removed from the surface. Calculations based on an extended Frenkel–Kontorova model reproduce the periodicity and magnitude of these features and allow us to relate them to the detachment force and desorption energy of the repeat units. The adsorbed part of the polymer slides easily along the surface during the pulling process, leading to only small oscillations as a result of the high stiffness of the fluorenes and of their length mismatch with respect to the substrate surface structure. A significant lateral force also is caused by the sequential detachment of individual units. The gained insight into the molecule–surface interactions during sliding and pulling should aid the design of mechanoresponsive nanosystems and devices.

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

Multiscale approach for simulations of Kelvin Probe force microscopy with atomic resolution

The distance dependence and atomic-scale contrast recently observed in nominal contact potential difference (CPD) signals simultaneously recorded by the Kelvin probe force microscopy (KPFM) using non-contact atomic force microscopy is addressed theoretically. In particular, we consider probing an insulating surface where the applied bias voltage affects electrostatic forces acting on the atomic scale. Our approach is a multiscale one. First, the electrostatics of the macroscopic tip-cantilever-sample system is treated, both analytically and numerically. Then the resulting electric field under the tip apex is inserted into a series of density functional theory calculations for a realistic neutral but reactive silicon nano-scale tip interacting with a NaCl(001) sample. Theoretical expressions for amplitude modulation (AM) and frequency modulation (FM) KPFM signals and for the corresponding local contact potential differences (LCPD) are obtained and evaluated for several tip oscillation amplitudes A up to 10 nm. For A = 0.01 nm, the computed LCPD contrast is proportional to the slope of the atomistic force versus bias in the AM mode and to its derivative with respect to the tip-sample separation in the FM mode. Being essentially constant over a few Volts, this slope is the basic quantity which determines variations of the atomic-scale LCPD contrast. Already above A = 0.1 nm, the LCPD contrasts in both modes exhibit almost the same spatial dependence as the slope. As the most basic quantity, the slope is shown to be approximately expressed in terms of intrinsic charge distribution and dipole moment and their variation due to the chemical interactions. The slope is also influenced by the macroscopic bodies. As a second part, we introduce a method to measure the distances between atomic configurations which is useful when seeking the tip-apex structures. The broad application of this method includes conformational search and machine-learning based interatomic potentials.

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

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.

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.

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.