Nanoscale contacts are ubiquitous in probe-based metrology, nanomanufacturing, nanodevices, and nanoparticle applications. The morphology and chemistry of these contacts control their interfacial properties, which range from electrical and thermal transport in devices, to resolution in measuring or patterning applications, to adhesion and friction in dynamic contacts. Because of the difficulty of direct observation, these contacts are typically described using models, such as contact mechanics or transport models, and material parameters, such as the adhesion energy or electrical contact resistance. In this body of work, we directly investigated the performance of nanoscale contacts through in situ experiments performed inside of a transmission electron microscope. Three case studies will be described. First, the contact area was directly observed for nanoprobes during contact and loading. Results demonstrated that the size of the contact significantly exceeded predictions of conventional models, due to effects of reversible plasticity and of hysteretic behavior upon unloading. Second, the electrical contact resistance was measured between platinum nanowires. The electrical current across the contact was more than an order of magnitude lower than expected from transport theories, due to disruption by insulating surface layers. Finally, the adhesion strength was investigated for technologically relevant interfaces. The measurements of load-dependent adhesion undermined the use of classic models (like that of Johnson-Kendall-Roberts), which are used in thousands of scientific publications per year. The physical origin of the deviation was revealed to be a transition from crack-like gradual separation to uniform decohesion. Taken together, these three case studies demonstrate the importance of accounting for atomic-scale processes when describing and predicting the behavior of nanoscale contacts.
Tevis D. B. Jacobs is a William Kepler Whiteford Faculty Fellow and Associate Professor in the Department of Mechanical Engineering and Materials Science at the University of Pittsburgh. He received his Ph.D. in Materials Science and Engineering from the University of Pennsylvania, where he also did his post-doctoral work. He is a recipient of the Early Career award from the Society of Tribologists and Lubrication Engineers, and the CAREER award from the National Science Foundation.