André Schirmeisen

Frictional duality observed during nanoparticle sliding.

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.

Temperature dependence of point contact friction on silicon

Point contact friction and adhesion between a silicon tip and an untreated silicon(111) wafer are measured as a function of sample temperature in ultrahigh vacuum by friction force microscopy. While the friction coefficient changes drastically in the temperature range from 50K to room temperature, and shows a reproducible maximum near 100K, the simultaneously recorded adhesion shows much less temperature dependence. Interestingly, the velocity dependence of friction shows a logarithmic increase below 150K although it is nearly constant above 150K. This peculiar behavior has profound consequences for tribological properties of devices manufactured from silicon.

Interfacial friction obtained by lateral manipulation of nanoparticles using atomic force microscopy techniques

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 pro...

Principles of atomic friction: from sticking atoms to superlubric sliding

Tribology—the science of friction, wear and lubrication—is of great importance for all technical applications where moving bodies are in contact. Nonetheless, little progress has been made in finding an exact atomistic description of friction since Amontons proposed his empirical macroscopic laws over three centuries ago. The advent of new experimental tools such as the friction force microscope, however, enabled the investigation of frictional forces occurring at well-defined contacts down to the atomic scale. This research field has been established as nanotribology. In the present article, we review our current understanding of the principles of atomic-scale friction based on recent experiments using friction force microscopy.

Transition from static to kinetic friction of metallic nanoparticles

Nanometer scale metallic islands were manipulated on a graphite surface by placing the tip of an atomic force microscope on top of the particles. Above a certain lateral force threshold particle sliding is observed, which allows us to quantify the transition from static to kinetic friction. This transition shows hysteretic character in the force domain and is characterized by a constant ratio of kinetic versus static friction of one half.

Measuring the Friction of Nanoparticles: A New Route towards a Better Understanding of Nanoscale Friction

Tribology--the science of friction, wear, and lubrication--is of considerable importance for all technical applications where moving bodies are in contact. Nonetheless, little progress has been made in finding an exact atomistic description of friction since Amontons proposed his empirical macroscopic laws over three centuries ago. The advent of new experimental tools, such as the friction force microscope, however, has enabled the investigation of frictional forces down to the atomic scale. Recently, this tool has been used to measure the friction of nanoparticles sliding over flat surfaces, thereby enabling a much larger range of material combinations and interface contact areas to be studied. These advances offer new insight into the atomistic concepts of friction.

Reproducible attachment of micrometer sized particles to atomic force microscopy cantilevers

We present a method, which allows attaching micrometer sized spheres to an atomic force microscope cantilever in a reproducible manner. Spheres of different size with a minimum amount of glue are attached to a predefined position on the cantilever. This is performed by using an optical microscope and a laser-pulled micropipette, which guarantees nondestructive handling of the delicate cantilever beams. The method employs a simple setup consisting of a stereomicroscope and a micromanipulator. Images of the modified cantilevers were taken with a scanning electron microscope to clarify the position of the glued spheres on the cantilever. Electron dispersive x-ray analysis reveals that the surface of the microsphere is not covered with the glue, except at the contact area to the cantilever.

Growth and topography study of MnBi films

We deposited (Mn/Bi)x bilayers (x=1,2,3) on clean glass substrates at room temperature and at 200 °C without using a protective SiOx layer in order to investigate the interaction between preparation conditions, homogeneity, granularity, and topography. During annealing at temperatures between 280 and 320 °C, no protective SiOx layer on top of the (Mn/Bi)x bilayers influences the formation of MnBi crystallites, i.e., the granularity. The coercive field of the resulting MnBi films is enhanced reaching values of up to 1.25 T. The large coercive fields indicate a single‐domain MnBi crystallite size of only 0.2–0.3 μm, which is favorable for a possible application as a magneto‐optic storage material.

Dynamic force spectroscopy using the constant-excitation and constant-amplitude modes

Dynamic force spectroscopy experiments were conducted with a silicon tip on graphite in ultrahigh vacuum. Spectroscopy curves were acquired in the constant-amplitude as well as in the constant-excitation mode. From both modes we extract quantitative force and energy dissipation curves. We show that the calculated tip–sample interaction curves are independent of the operational mode. This proves that quantitative dynamic force spectroscopy is also possible in the constant-excitation mode.

Metallic adhesion and tunnelling at the atomic scale

We simultaneously measured the distance dependence of the force and the tunnelling current between a W(111) tip and a Au(111) sample in an ultrahigh vacuum at T = 150 K. The tip was characterized by field ion microscopy. Even at atomically close contact no evidence of structural instabilities was found. The scaling of the force curves show an unexpectedly long distance scaling parameter of λ = 0.2 nm. We conclude that not only the apex atoms contribute to the adhesion forces, but the first three layers play an almost equal role. Using a model that correlates the force and the tunnelling current, we are able to extract the tip density of states. Possible reasons for the long scaling length are discussed.