Investigation of contact interactions of (001) MgO: molecular dynamics simulations studies

Molecular dynamics simulations have been performed to investigate surface contact interactions between small MgO probes (blocks of cross-section n x n atoms, where n = 2, 4, 6, 8 and 10), and the MgO (001) surface (periodic MgO slabs) at 300 K. The contact behaviour was monitored by measuring the force on the probe and the interplanar spacings at different vertical distances between the probe and slab. A mechanistic description of the probe's approach has thus been obtained. In all cases, a characteristic instability 'jump' was observed, where the probe was attracted towards the surface, signalled by large atomic displacements and an increased attractive force. It was found that the normal force per atom experienced by a probe decreases as the area increases and could be characterised by a logarithmic exponent, q = 0.12 (corresponding simulations for NaCl have yielded a similar value). Hysteresis occurred in every case after the jump and a rich variety of phenomena were observed, such as 'neck' formation, accompanied by atomic dislocations, and systematic removal of atoms from the surface, as the probe was withdrawn.