Ma Lantz

Lateral stiffness of the tip and tip-sample contact in frictional force microscopy

In atomic force and frictional force microscopy, quantitative interpretation of lateral stiffness at the tip-sample contact requires a detailed understanding of all factors contributing to the frictional force as measured in a typical experiment. We used a scanning transmission electron microscope to image and determine the geometry of the tip apex of a variety of atomic force microscope cantilevers. On the basis of this measured structure, we then used finite element analysis to model the lateral stiffness of the tip and found that the tip stiffness is often smaller than the lateral stiffness of the cantilever. Furthermore, we analyzed the stiffness of the tip sample contact and found that for sharp tips the contact stiffness can also be comparable to the lateral stiffness of the cantilever. If these two effects are ignored, significant errors can result in the calculation of lateral forces. We demonstrated the effects of lateral tip and contact stiffness experimentally and used the measurements to calcu...

FORCE MICROSCOPY IMAGING IN LIQUIDS USING AC TECHNIQUES

Two ac techniques for imaging under liquids using atomic force microscopy are investigated. In the first method, the sample is oscillated with a sinusoidal displacement, whereas in the second method, the cantilever is oscillated with a sinusoidal magnetic force. Both techniques are successful for topographic imaging under liquids with the tip in repulsive contact with the sample. Of the two methods, the cantilever driven technique is found to be less noisy. In addition to topographic imaging, noncontact magnetic force imaging under liquid is demonstrated.

Characterization of tips for conducting atomic force microscopy in ultrahigh vacuum

We have investigated the reliability of a variety of metal coated and semiconductor tips for use in conducting atomic force microscopy experiments in an ultrahigh vacuum (UHV) environment. In order to obtain reliable conduction data we find it necessary to first clean the tips using a short argon ion sputter. Scanning transmission electron microscopy is used to image tips after the conductivity experiments and found to be very useful for assessing tip wear and interpreting conductivity data. Tip reliability is found to be strongly dependent on the sample and the experimental conditions. Wear and contamination of the tip are found to be severe problems which are related to the tip-sample adhesion. We illustrate these effects and highlight some of the common reliability problems which we encountered using specific examples. In general, we find that metal coated tips are not reliable enough to obtain repeatable data, especially if lateral forces are exerted on the tip. Homogeneous semiconductor tips, once cl...

Measurement of forces during the modification of C60 islands

Abstract A UHV atomic force microscope with a conducting tip is used to measure the tip–sample conductance as a function of the applied force on well-ordered, monolayer islands of C60 on Cu(111). By imaging the sample before and after each force–distance experiment, it was possible to investigate the forces required for the removal of individual C60 molecules from the islands. The removal of C60 occurs near defects or edges of the C60 islands and requires an applied force of 5–20xa0nN, which corresponds to applied pressures of order 1xa0GPa. In addition, it was possible to investigate the strength of the C60 film on the molecular scale. It was found that the mechanical stiffness of a C60 molecule is of order 6xa0N/m and the islands appear to undergo a reversible yield process at an applied pressure of around 1.2xa0GPa.

AFM at liquid-solid interfaces

An introduction to experimental aspects of atomic force microscopy specific to the liquid-solid interface is given. Emphasis is given to the measurement of forces (e.g. normal, friction, material compliance, the liquid hydrodynamics) acting on the tip in liquids. Specific examples which are discussed include solvation forces and their relation to local lubrication, and the measurement of single asperity contacts in simple liquids. Some general problems which remain outstanding are highlighted.