Nanoscale friction: Distorted by the tip.

When two macroscopic bodies in contact slide, energy is dissipated. However, at the nanoscale, tiny amounts of energy can also be dissipated when the sliding bodies are not in contact with each other. Such non-contact form of friction — the origin of which has been associated with van der Waals (vdW) forces — has been measured with highly sensitive atomic force microscopy (AFM). In a friction force microscope, the tip scans over a surface and provides the well-characterized conditions needed to determine the relations between the applied load, the lateral friction force and the energy dissipated during sliding. Reporting in Nature Materials, Marcin Kisiel and colleagues now show that dynamic AFM is able to detect the energy dissipated in the creation of tip-induced local distortions in the electronic spatial distribution associated with the charge density wave (CDW) of the low-temperature ground state of NbSe2 — a layered dichalchogenide1. At the microscopic level, friction force microscopy has revealed that the energy dissipation during contact sliding is associated with stick–slip motion: chemical bonds at the contact region are stretched (stick), and eventually broken and re-formed (slip) as the tip moves laterally2. The mechanical energy stored during the stick stage is released during the slip stage, converted into atomic vibrations (phonons), and finally dissipated as heat. However, extending these studies to the single-chemical-bond level requires the atomic force microscope to be operated in the so-called dynamic mode3. In this case, the tip mounted at the end of a micrometresized cantilever oscillates vertically — that is, in the direction normal to the surface — to minimize and control the contact area. The tip–sample interaction can then be detected by the changes induced in the oscillation amplitude and frequency that characterize the cantilever dynamics, and the energy dissipated can be monitored through the energy that has to be added to keep the oscillation amplitude of the cantilever constant. In dynamic operation mode, AFM can also detect the elusive, non-contact friction arising from vdW interactions, which is mediated by the long-range electromagnetic fields created by thermal and quantum fluctuations of the electronic density and, in some cases, by static surface charges arising from material inhomogeneities or a bias voltage4. Yet sensing non-contact friction requires ultrasoft cantilevers oscillating like a pendulum over the surface. In fact, such an AFM design has recently NANOSCALE FRICTION