One of the most fundamental questions in tribology concerns the area dependence of friction at the nanoscale. Here, experiments are presented where the frictional resistance of nanoparticles is measured by pushing them with the tip of an atomic force microscope. We find two coexisting frictional states: While some particles show finite friction increasing linearly with the interface areas of up to 310,000 nm 2 , other particles assume a state of frictionless sliding. The results further suggest a link between the degree of surface contamination and the occurrence of this duality.

Antimony nanoparticles grown on highly oriented pyrolytic graphite and molybdenum disulfide were used as a model system to investigate the contact-area dependence of frictional forces. This system allows one to accurately determine both the interface structure and the effective contact area. Controlled translation of the antimony nanoparticles ͑areas between 10 000 and 110 000 nm 2 ͒ was induced by the action of the oscillating tip in a dynamic force microscope. During manipulation, the power dissipated due to tip-sample interactions was recorded. We found that the threshold value of the power dissipation needed for translation depends linearly on the contact area between the antimony particles and the substrate. Assuming a linear relationship between dissipated power and frictional forces implies a direct proportionality between friction and contact area. Particles about 10 000 nm 2 in size, however, were found to show dissipation close to zero. To explain the observed behavior, we suggest that structural lubricity might be the reason for the low dissipation in the small particles, while elastic multistabilities might dominate energy dissipation in the larger particles.

Nanometer scale metallic particles have been manipulated on an atomically flat graphite surface by atomic force microscopy techniques and quantitative information on interfacial friction was extracted from the lateral manipulation of these nanoparticles. Similar to conventional friction force microscopy, the particle-surface interfacial friction was extracted from the torsional signal of the cantilever during the particle pushing process. As a model system, we chose antimony particles with diameters between 50 and 500nm grown on a highly oriented pyrolytic graphite substrate. Three different manipulation strategies have been developed, which either enable the defined manipulation of individual nanoparticles or can be utilized to gather data on a larger number of particles found within a particular scan area, allowing for fast and statistically significant data collection. While the manipulation strategies are demonstrated here for operation under vacuum conditions, extensive testing indicated that the proposed methods are likewise suited for ambient environments. Since these techniques can be applied to a large variety of chemically and structurally different material combinations as well as a large range of particle sizes, our results indicate a viable route to solve many recent issues in the field of nanoscale friction, such as the influence of contact size and interface crystallinity.

We report on the application of a home-built scanning force microscope for translation and in-plane rotation of nanometer-sized latex spheres by dynamic surface modification (DSM). This technique is based on the increment of the amplitude of the oscillating voltage applied at the dither piezo that drives the cantilever vibrations in the dynamic mode of scanning force microscopy. Thus, it is easily possible to switch between the imaging mode and DSM mode, enabling the direct manipulation of nanostructures under ambient conditions with high precision. The main advantage of our technique compared to earlier methods is that it operates with an active feedback loop, allowing a steady manipulation of nanostructures independent of the surface corrugation or sample tilt. Such controlled translations of latex spheres enable us in addition to study their properties regarding friction, adhesion, and cohesion. By translating latex spheres of different sizes (radii between 50 and 110 nm), it was found that the force needed to move a particle depends on its dimensions. Finally, the lateral translation of an intentionally marked sphere gives evidence that sliding is preferred over rolling.