Direct measurement of interatomic force gradients using an ultra-low amplitude atomic force microscope

<div class="line" id="line-9"> Interatomic force gradients between a W tip and a 7x7 reconstructed Si(111) surface were measured using an off-resonance, ultra-low amplitude AFM technique. The amplitudes used were less than 1 &ring;A (peak-to-peak) which allowed direct measurement of the interaction force gradients as a function of separation. The force gradient curves are shown to consist of an attractive van-der-Waals part and shortrange attractive and repulsive interactions. The van-der-Waals background can be subtracted, leaving a short range interaction with an energy parameter of 1.9&ndash;3.4 eV and an interaction length scale of 0.54&ndash;1.26 &ring;A characteristic of a single atomic bond. This correlates well with our observation of single atom resolved force gradient images. In general, the interaction is reversible up to the zero-intercept of the force gradient (inflection point of the energy). Beyond this point hysteresis tends to be observed and the onset of inelastic deformation can be clearly discerned. An analysis of the atomic scale contact gives reasonable values for the interfacial energy, yield strength, and the energy per atom needed to initiate plastic deformation.</div>