M. Salmeron

CALIBRATION OF FRICTIONAL FORCES IN ATOMIC FORCE MICROSCOPY

The atomic force microscope can provide information on the atomic‐level frictional properties of surfaces, but reproducible quantitative measurements are difficult to obtain. Parameters that are either unknown or difficult to precisely measure include the normal and lateral cantilever force constants (particularly with microfabricated cantilevers), the tip height, the deflection sensor response, and the tip structure and composition at the tip‐surface contact. We present an in situ experimental procedure to determine the response of a cantilever to lateral forces in terms of its normal force response. This procedure is quite general. It will work with any type of deflection sensor and does not require the knowledge or direct measurement of the lever dimensions or the tip height. In addition, the shape of the tip apex can be determined. We also discuss a number of specific issues related to force and friction measurements using optical lever deflection sensing. We present experimental results on the latera...

Lateral stiffness: A new nanomechanical measurement for the determination of shear strengths with friction force microscopy

We present a technique to measure the lateral stiffness of the nanometer-sized contact formed between a friction force microscope tip and a sample surface. Since the lateral stiffness of an elastic contact is proportional to the contact radius, this measurement can be used to study the relationship between friction, load, and contact area. As an example, we measure the lateral stiffness of the contact between a silicon nitride tip and muscovite mica in a humid atmosphere (55% relative humidity) as a function of load. Comparison with friction measurements confirms that friction is proportional to contact area and allows determination of the shear strength.

Measurement of interfacial shear (friction) with an ultrahigh vacuum atomic force microscope

We have studied the variation of frictional force with externally applied load for a Pt‐coated atomic force microscope tip in contact with the surface of mica cleaved in ultrahigh vacuum. At low loads, the frictional force varies with load in almost exact proportion to the area of contact as predicted by the Johnson–Kendall–Roberts (JKR) theory [K. L. Johnson, K. Kendall, and A. D. Roberts, Proc. R. Soc. London Ser. A 324, 301 (1971)] of elastic adhesive contacts. The friction‐load relation for a deliberately modified tip shape was proportional to an extended JKR model that predicts the area‐load relation for nonparabolic tips. The tip shape was determined experimentally with a tip imaging technique and was consistent with the predicted friction behavior. This demonstrates that the frictional force is proportional to the area of contact between the tip and sample. Using the JKR/extended JKR model, interfacial surface energies and shear strengths can be estimated.

Dissociative hydrogen adsorption on palladium requires aggregates of three or more vacancies

During reaction, a catalyst surface usually interacts with a constantly fluctuating mix of reactants, products, ‘spectators’ that do not participate in the reaction, and species that either promote or inhibit the activity of the catalyst. How molecules adsorb and dissociate under such dynamic conditions is often poorly understood. For example, the dissociative adsorption of the diatomic molecule H2—a central step in many industrially important catalytic processes—is generally assumed to require at least two adjacent and empty atomic adsorption sites (or vacancies). The creation of active sites for H2 dissociation will thus involve the formation of individual vacancies and their subsequent diffusion and aggregation, with the coupling between these events determining the activity of the catalyst surface. But even though active sites are the central component of most reaction models, the processes controlling their formation, and hence the activity of a catalyst surface, have never been captured experimentally. Here we report scanning tunnelling microscopy observations of the transient formation of active sites for the dissociative adsorption of H2 molecules on a palladium (111) surface. We find, contrary to conventional thinking, that two-vacancy sites seem inactive, and that aggregates of three or more hydrogen vacancies are required for efficient H2 dissociation.

ATOMIC-SCALE FRICTION AND WEAR OF MICA

The frictional behavior of mica surfaces with silicon nitride tips has been investigated systematically with the AFM as a function of load, tip geometry, mica lattice orientation and humidity. Frictional forces are found to be proportional to loads between 10 and 80 nN. The friction coefficient is quite reproducible for different samples, tip radii, scanning speed and direction. At low loads, however, a non-linear behavior of the friction versus load is observed. At high (> 70%) relative humidity and in water, friction is reduced. Repeated scanning of mica surfaces shows layer-by-layer wear processes.

Structure, composition and chemisorption studies of thin ordered iron oxide films on platinum (111)

Abstract We report the epitaxial growth of iron oxide overlayers on a platinum (111) single crystal, in the monolayer and the multilayer regimes. Different LEED structures have been observed for each of these regimes. At one monolayer coverage iron oxide forms an incommensurate overlayer with 10% longer unit cell vectors than those of clean platinum. At coverages of 3 to 10 monolayers, iron oxide forms a new structure, which is (2×2) with respect to that formed at one monolayer. AES and XPS show that the iron in the oxide overlayer is in the 2+ oxidation state and from ISS studies it was determined that both iron and oxygen are present in the outermost atomic layer in about equal amounts. Water adsorption on the iron oxide films was studied by TPD. It was found that the desorption spectrum is very sensitive to the presence of various types of defects on the oxide.

Subsurface impurities in Pd(111) studied by scanning tunneling microscopy

Low concentrations of three distinct impurity species beneath the Pd(111) surface are studied by STM. The subsurface impurities are distinguished by their image contrast, diffusion properties, and interactions with adsorbed molecules. Isolated subsurface impurities appear at low gap resistance (<≈ MΩ) as three-fold symmetric modulations of the Pd 1×1 surface corrugation. One impurity type is found to occupy substitutional sites in the layer below the surface. Based on Auger spectroscopy this species is identified as sulfur. The other two species are found to occupy octahedral interstitial sites immediately below the surface layer. Two-dimensional diffusion of the interstitial impurities occurs below room temperature. The onset temperature for diffusion is lowered dramatically in the presence of surface adsorbates. Quantitative measures of the diffusion barriers are consistent with surface facilitated diffusion of interstitial oxygen and carbon atoms. The mobile impurities interact with adsorbed atoms and ...

Tip-Surface Forces During Imaging by Scanning Tunneling Microscopy

The effect of compressive and shear forces between tip and surface during the operation of the scanning tunneling microscope (STM) is illustrated with examples obtained both in air and vacuum environments. We show that at typical gap resistances used in STM (≤20 GΩ) these forces can have significant effects. Compressive or repulsive forces give rise to anomalous topographic corrugations (elastic deformations) as well as to permanent damage (inelastic or plastic deformation). These forces also cause the anomalously low values obtained in measurements of the tunneling barrier height. The effects of shear forces when imaging weakly bound material will also be demonstrated.

Effective tip radius in electrostatic force microscopy

A method to determine the effective electrostatic tip radius of arbitrarily shaped conducting tips in atomic force microscopy is presented. The method is based on the finding that for conductive samples, the electrostatic force can be separated into two contributions: one from a constant background that depends only on the macroscopic shape of the tip (cone or pyramid and cantilever), and another that depends only on the radius of curvature of the tip apex. Based on a simple theoretical expression derived from the generalized image charge method, we show that the tip radius can be directly determined from experimental force-distance characteristics. For irregular tip shapes, we show that the measured tip radius is the average of two principal curvatures, in agreement with tip shape images obtained by scanning electron microscopy.

Coadsorption and interactions of O and H on Pd(111)

The interactions between oxygen and hydrogen coadsorbed on Pd(1 1 1) have been studied by scanning tunneling microscopy (STM) in the temperature range from 25 to 230 K. Atomic oxygen without coadsorbed hydrogen forms a (2×2) structure in a continuous layer or in the form of islands. Individual oxygen atoms are also observed between the islands at low temperature. Coadsorbed hydrogen modifies the STM image contrast and enhances the diffusion of the isolated oxygen atoms. Above 120 K hydrogen modifies the structure of the oxygen islands, transforming them into rows of oxygen atom pairs at nearest neighbor distances. Above 150 K, hydrogen causes all (2×2)-O islands to convert into a (√3×√3) structure, which is stable up to 210 K. Above 210 K the (√3×√3) structure transforms back to (2×2) due to dissolution of the surface hydrogen into the bulk. During these transformations the number of oxygen atoms on the surface remains unchanged. Above 220 K the oxygen population decreases by reaction with dissolved hydrogen to form H2O which desorbs from surface.