Quantifying the phononic contribution to friction at the single-atom scale

• Applicant: Privatdozent Dr. Alfred John Weymouth
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 444750204

On a global scale, 20% of the energy produced is used to overcome friction. Despite the importance of friction, it is most often explained with empirical formulas. To move beyond this, towards the future goal of predicting frictional behavior, a description of the energy dissipation pathways is required starting from basic physical principles. One of these pathways is energy dissipation via phonons. Even without material wear, phonons transfer energy away from the sliding surfaces. One method to study the effect of phononic friction is to compare the frictional properties of surfaces terminated in hydrogen to surfaces terminated with deuterium. As hydrogen (H) and deuterium (D) are very similar apart from their mass, differences in friction are attributed to differences in energy dissipation via phonons. In vacuum, clean and atomically-flat silicon surfaces can be prepared and saturated with H or D. Although a difference in friction between these two surfaces was reported, there is an ongoing debate in the literature as to whether this difference is due to phononic differences or due to a different number of unsaturated surface bonds. This is because no experiment has both studied the surface and measured lateral forces with atomic resolution. In this project, I apply lateral force microscopy to this outstanding question and perform measurements on two sample systems to determine the energy dissipation as an atomically-precise tip moves laterally over H- and D-terminated surfaces. With a low-temperature non-contact lateral force microscope, both the surface and tip apex can be characterized at the single atom level. By measuring the damping of the lateral tip oscillation over individual atoms we will characterize this energy dissipation precisely. These measurements will allow us not only to answer the ongoing question of phononic differences versus a dangling bond density but moreover, by applying this state-of-the-art atomic force microscopy technique, to characterize phononic friction at the atomic scale.

• DFG Programme: Research Grants