CPM Seminar

The response of ultrasmall volumes coming into contact has long been of interest to those concerned with friction, lubrication and adhesion. With the advent of new microelectomechanical systems, this field is likely to expand in importance over the coming years. Such issues are also important for understanding atomic force microscope imaging mechanisms and establishing experimental methods for utilizing tip-sample interaction regions which provide maximum resolution. In addition, measurement of the strong short range force-distance relation between a tip and surface on a highly localized lateral scale may provide a means of atomic or molecular 'fingerprinting'. For the detailed investigation of tip-surface interactions we have designed and built force controlled Atomic Force Microscopes (AFMs) for use in air, liquid and ultra high vacuum (UHV). The force control technique provides quantitative information on the nature of tip-surface interactions by applying magnetic forces directly to magnetic material attached behind the AFM tip via a current carrying coil. Oscillating the applied force to the lever and measuring the resulting displacement amplitude gives a continuous measurement of the absolute force gradient or contact stiffness. This sensitive measurement has enabled us to measure solvation shells with liquid systems. In addition the application of static forces can be extremely useful. One immediate technical advantage is the low hysteresis and creep characteristics of the motion controlled by magnetic forces as opposed to piezo displacements. Once in contact we can perform force controlled nanoindentation enabling large forces to be applied even with very compliant levers. We have also removed mechanical instabilities of the lever using magnetic force feedback. This has enabled us to do force spectroscopy at hitherto inaccessible tip-sample separations.

Measured force-displacement relations can give a quantitative insight into a wide variety of phenomena. However, the tip-flat interaction at the nanometer scale may be strongly affected by surface local properties and by adsorbates, so that the tip displacement is controlled by a rather wide variety of possible forces and reactions. This often makes it difficult to interpret results, particularly when taken outside rigorously controlled environments such as UHV. The effect of the measurement technique on the measurement itself also needs to be explored particularly with reference to large cantilever oscillations and similar conditions under which standard models may not be appropriate.

To understand the overall force-displacement response, which is what is observed experimentally, some means is required of combining the components due to the individual physical processes present. Summing these forces fully self-consistently, with the exact elastic shape deformation of the surfaces, is extremely complex. This problem is discussed and we propose a simplfied analysis which nevertheless demonstrates the essential features of the possible responses, and allows an understanding of the main interactions which control a given load range and geometry.

Wednesday, October 29th, 15:30
Ernest Rutherford Physics Building, room 114