Project XIII – Extreme Nanotribology: Experiments and Modelling on the atomic scale related to dissipation force microscopy and tunneling microscopy

Contributors: A. Shluger, R. Garcia, R. Erlandsson, J. Pethica, E. Meyer

The extreme nano-tribology, where only several atoms can be involved in initial stages of energy dissipation, is achieved in Non-Contact AFM. To consider dissipation mechanisms on this scale one should establish their relation to local vibrational surface properties. This requires extremely sensitive experimental methods and a theory based on non-equilibrium statistical consideration of the interaction of the tip with vibrating surface atoms. As has been demonstrated in recent studies, this effect is crucial as surface atoms never establish complete equilibrium with the moving tip. Two dissipation mechanisms associated with surface vibrations are currently considered: (i) a friction force acting on the tip is determined by the response of the phonons associated with the surface atoms; (ii) surface atoms may experience instabilities at the closest approach of the tip which may lead to formation of local soft vibrational modes at the surface interacting strongly with the tip and resulting in a adhesion hysteresis phenomenon. Soft vibrational modes associated with a molecule at the end of a functionalised tips tip can also be formed. This project should allow us to elucidate relative contributions of these mechanisms and to achieve better understanding of the energy dissipation at nano-contact of different surfaces.

Non-linear effects in force microscopy

We are interested in understanding how the dynamics of a nanometer-size object is affected by the non-linear character of the physical interactions involved.

3d-Modelling of force microscopy

Force microscopes are key tools to explore the nanometer- scale world. However, most of the understanding of the dynamics of a force microscope is based on simplified models where the cantilever-tip system is assumed to be a point-mass object. A Theory will be developed to achieve a more realistic (3D) description of the cantilever-tip ensemble. Experiments will also performed in air and liquids to verify how and when the 3D character of the probe is relevant to interpret the physical properties of nanocontacts or nanostructures.

Experiments with combined force and tunneling microscopy

Scanning Tunneling Microscopy (STM) has during the last two decades evolved as one of the foremost techniques for atomic scale surface characterization. Since the invention of the STM, it has been obvious that force effects over the tunnel junction plays an important role during tunnel imaging, but very few detailed investigations have been performed as STM instruments normally lack the capability to measure the interaction force. Combined STM/AFM instruments offer the possibility to address the fundamental question about the relationship between the tunneling current and short-range forces, such as metallic adhesive forces.

Furthermore, a combination of atomically resolved AFM/STM imaging and laterally resolved force measurements appears feasable for future experiments. The sensitivity of the equipment makes it potentially possible to measure chemical interaction forces of individual bonds. This kind of measurements requires the combination of atomically resolved imaging with high lateral stability and drift compensation. These very demanding measurements are among the ultimate goals of the work we would like to pursue.