Atomic-scale friction : a scanning probe study on crystalline surfaces

Abstract

Friction is one of the most fascinating and yet elusive phenomena in physics. Everyday life cannot be imagined in the absence of friction. It allows us to walk, to climb stairs, to sit in a chair, to stop a car, to handle tools. In technological applications friction is the evil of all motion and huge amounts of money are spent annually on energetic and mechanical losses due to friction and wear. Friction is also responsible for natural disasters like earthquakes, landslides and avalanches. Along the centuries mankind has tried to understand and control friction but in spite of the huge volume of experimental knowledge with remarkable technical applications, very little is known about the fundamental, elementary processes taking place on the atomic level at the interface between sliding surfaces. The contact between two apparently flat solids consists in reality of a very large number of micro-protrusions or asperities belonging to both contacting surfaces. Studying and understanding the processes responsible for the occurrence of friction at the buried interface of a single-asperity became the tasks of a new but rapidly expanding science called nanotribology. In the present work we have used a variable-temperature ultra-high vacuum atomic force microscope (VT-UHV-AFM) to investigate the frictional properties, namely the stick-slip behavior, of (100) and (111) crystalline diamond and (001) sodium chloride surfaces. While diamond is a technologically important material and a perfect candidate for an ideal friction experiment, NaCl is well established as a representative model, standard surface for nanotribological investigations. In order to properly simulate the interface with a single asperity at the nanoscale, sharp AFM-tips are used on atomically flat surfaces. The ultra-high vacuum conditions are an essential ingredient for well-defined and reproducible experiments. A hard, stable and sharp AFM-tip termination is of the essence for friction measurements. Therefore, using a hot-filament assisted chemical vapor deposition (HF-CVD) of diamond, a method of growing individual good quality diamond crystallites at the apexes of standard Si AFM tips was demonstrated; the resulting tips showed sharpness, hardness, stability and reliable behavior during friction measurements. We have investigated the atomic-scale friction behaviour between standard silicon nitride tips and diamond-coated tips and a specially prepared hydrogen-terminated (100) diamond sample by means of ultra-high vacuum atomic force microscopy. Tunneling experiments revealed a very flat surface and the typical atomic features (dimers) of a (2x1) surface reconstruction of the hydrogen-terminated (100) diamond sample. When using a diamond-terminated tip, for the first time atomically resolved topography, normal force error signal and lateral force maps are simultaneously obtained and perpendicular domains, hydrogen atomic positions and atomic steps between domains could be observed. This was attributed to a very sharp tip, namely one hydrogen atom-terminated tip, describing a stick-slip movement in two orthogonal directions and causing a dynamic rearrangement of the surface atoms; these results were consistent with an ab-initio electronic structure calculation which reveals the fact that the repulsive interaction between the apex H-atom at the tip and H-atoms at the surface is the essential factor that governs the atomic stick-slip behaviour Similar experiments were carried out in UHV on a (111) crystalline diamond surface with standard silicon nitride and diamond-coated tips. The friction measurements with the same diamond-coated tip on this surface led to atomic resolution: the measured periodicity is consistent with the one of the individual hydrogen atoms of the diamond surface. This reconfirmed the use of an atomically sharp tip and, similar to the results on the (100) diamond surface, we believe that the repulsion between the last H-atom of the tip and the H-atoms of the surface termination is the most important ingredient controlling the complicated two-dimensional atomic scale stick-slip behavior observed experimentally. Finally, atomic-scale friction between a silicon tip and the atomically flat (001) NaCl surface was investigated in ultra-high vacuum at various sample temperatures in the interval from 25 to 300 K. The temperature dependence of measured average friction forces is found to be in good qualitative agreement with theoretical models that consider a thermally-activated discontinuous tip movement during scanning and predict higher friction forces at low temperature. Higher mean friction values observed for two temperature values were attributed to possible changes in the tip apex configuration.