Transient adhesion and conductance phenomena in initial nanoscale mechanical contacts between dissimilar metals

TL;DR

This study investigates how initial nanoscale contacts between dissimilar metals exhibit transient adhesion and conductance changes due to tip wetting, with these properties stabilizing after multiple indentations.

Contribution

It provides a detailed experimental analysis of transient phenomena in metal contacts and introduces a contact mechanics model explaining the decay of adhesion and conductance.

Findings

Initial conductance and adhesion are higher than steady-state values.

Conductance and adhesion decrease exponentially with indentation cycles.

Surface composition influences mechanical and electrical contact properties.

Abstract

We report on transient adhesion and conductance phenomena associated with tip wetting in mechanical contacts produced by the indentation of a clean W(111) tip into a Au(111) surface. A combination of atomic force microscopy and scanning tunneling microscopy was used to carry out indentation and to image residual impressions in ultra-high vacuum. The ~7 nm radii tips used in these experiments were prepared and characterized by field ion microscopy in the same instrument. The very first indentations of the tungsten tips show larger conductance and pull-off adhesive forces than subsequent indentations. After ~30 indentations to a depth of ~1.7 nm, the maximum conductance and adhesion forces reach steady-state values approximately 12x and 6x smaller than their initial value. Indentation of W(111) tips into Cu(100) was also performed to investigate the universality of tip wetting phenomena with a different substrate. We propose a model from contact mechanics considerations which quantitatively reproduces the observed decay rate of the conductance and adhesion drops with a 1/e decay constant of 9-14 indentation cycles. The results show that the surface composition of an indenting tip plays an important role in defining the mechanical and electrical properties of indentation contacts.