Oct 24, 2013
Interface atoms decide nanoadhesion and conductance
Indentation is a widely used and relatively simple way to test the mechanical properties of materials and the technique is being extended to ever-shrinking length scales. While computer models of atomic scale mechanical contacts are getting better and better at telling us how atoms rearrange in a material when it is indented, it still remains very challenging to acquire experimental indentation data at the nanoscale and verify what goes on in simulations. Researchers at McGill University in Montreal, Canada, used a combined field ion and scanning probe microscope to create an atomically defined indentation system that allows for nanoscale contact between metals. In recent work, they studied the effects of nanoadhesion and electronic conductance when a sharp tungsten needle indents the flat surface of a gold single crystal.
When the two clean metals in the set up (a tungsten needle and gold surface) meet for the first time, they cling to each other with great adhesive force. As a result, a large amount of mechanical energy must be applied to separate the metals. The separation process is accompanied by some gold atoms sticking to the tungsten needle and altering its surface composition. Surprisingly, after a number of repeated contacts (a few dozen), it becomes much easier – about six times easier – to separate the materials.
The scientists propose that surface diffusion of adatoms – single atoms whizzing around on the surface of materials – might help mediate the separation. In fact, this kind of "self healing" mediated by adatoms has already been seen in studies of other types of phenomena, such as friction.
In the McGill group’s experiments, when the mobile gold atoms ‘wet’ the surface of the tungsten needle, the electronic conductance through the metal nanocontact also dramatically reduces. The order-of-magnitude drop in conductance stems from the increased number of interfaces between different crystalline configurations, causing electrons to bounce around at the junction rather than zip right through.
We believe that this work may help guide the design of more reliable nanoscale switches in the future – for example, ones that open and close without sticking and that conduct electrons with ease.
More information can be found in the journal Nanotechnology 24 475704
About the author
William Paul completed his PhD in physics at McGill University in the Nanoscience & SPM Group. This research was performed with the assistance of postdoc Dr. David Oliver, research associate Dr Yoichi Miyahara, and Prof. Peter Grütter. When not tending an ultrahigh vacuum system, Dr. Paul can be found at a piano, soldering electronics or brewing a variety of beers and ciders. He gratefully acknowledges NSERC for generous support in the form of a Vanier Canada Graduate Scholarship during his PhD studies. Dr. Paul can now be found studying nanoscale magnetism in the lab of Andreas Heinrich at IBM Research – Almaden.