Nanometer scale friction and wear on self-assembled monolayers investigated by atomic force microscopy

Nanometer scale tribology investigates the phenomena of adhesion, friction and wear at the molecular level. It is driven by a basic research interest to unveil the fundamental nature of the processes responsible for energy dissipation in friction and the modification of the interface by wear. Furthermore, for modern technologies, such as hard disk drives and micro-electromechanical systems (MEMS), it is crucial to understand the role of monolayer thick lubricants, i.e. to correlate the mechanical properties with the molecular structure of these layers. Atomic force microscopy (AFM) is used to investigate wear on muscovite mica and friction on selfassembled monolayers on the nanometer scale. Two basic mechanisms for energy dissipation are revealed. First, on muscovite mica, the production of surface defects, i.e. the rupture of Si-O bonds at the surface, although not yet visible in contact AFM images, has a noticeable contribution to friction. This contribution can be explained with a model based on the stressinduced enhancement of the rate of thermal defect production. Besides, the accumulation of these defects leads to the creation of˚ deep holes, which have not been observed with AFM A before. Second, on monolayer islands of alkylsilane molecules, self-assembled in an all-trans, upright configuration on mica, a correlation between discrete structural changes and stepwise changes of the frictional properties, occurring at specific threshold loads, is determined. A model of discrete molecular tilts explains the stepwise decrease in height accompanied by an increase in friction observed for increasing applied load and allows to quantify the increase in frictional force: Meta-stable states are dictated by the zigzag skeletal structure of the carbon atoms and the requirement of optimal packing density. Pressure-induced transitions between meta-stable states of interlocked molecules are also observed for alkyl thiol monolayer islands on gold. Measurements on specific thiol monolayers, containing a photo-polymerizable diacetylene group within the alkyl chain, show that frictional properties can be manipulated by internal molecular scaffolding. Upon polymerization a linear polymer backbone forms, which covalently links neighboring molecules, preventing the relative motion of the molecules necessary for tilting. Thus the typical stepwise increase in friction with increasing load observed for unpolymerized monolayers vanishes upon polymerization.