Quantitative measurement of contact area and electron transport across platinum nanocontacts for scanning probe microscopy and electrical nanodevices

Abstract

Conductive modes of atomic force microscopy are widely used to characterize the electronic properties of materials, and in such measurements, contact size is typically determined from current flow. Conversely, in nanodevice applications, the current flow is predicted from the estimated contact size. In both cases, it is very common to relate the contact size and current flow using well-established ballistic electron transport theory. Here we performed 19 electromechanical tests of platinum nanocontacts with in situ transmission electron microscopy to measure contact size and conductance. We also used molecular dynamics simulations of matched nanocontacts to investigate the nature of contact on the atomic scale. Together, these tests show that the ballistic transport equations under-predict the contact size by more than an order of magnitude. The measurements suggest that the low conductance of the contact cannot be explained by the scattering of electrons at defects nor by patchy contact due to surface roughness; instead, the lower-than-expected contact conductance is attributed to approximately a monolayer of insulating surface species on the platinum. Surprisingly, the low conductance persists throughout loading and even after significant sliding of the contact in vacuum. We apply tunneling theory and extract best-fit barrier parameters that describe the properties of this surface layer. The implications of this investigation are that electron transport in device-relevant platinum nanocontacts can be significantly limited by the presence and persistence of surface species, resulting in current flow that is better described by tunneling theory than ballistic electron transport, even for cleaned pure-platinum surfaces and even after loading and sliding in vacuum.

Insights into tribology from in situ nanoscale experiments

“…Separate in situ TEM adhesion experiments demonstrated the critical role that surface chemistry plays in the deformation of platinum nanocontacts. Liquid-like diffusive behavior is not observed in selfmated platinum nanocontacts, 85 likely due to the presence of surface monolayers of oxygen or carbonaceous material (Figure 6c-d), 86 which is consistent with atomistic simulations. 87 These surface layers prevent the motion of the surface steps that are required for diffusive plasticity.…”

“…(The scale bars are 5 nm in length.) However, subsurface defect-based plasticity is observed85 in platinum nanocontacts (c), and real-time electrical measurements86 are consistent with the presence of atomic-scale surface layers (d), which may disrupt surface diffusion. (a, b) were reproduced with permission from Reference 80, © 2014 Nature Publishing Group.…”

“…Surprisingly, these surface layers are not removed by mechanical means, even with loading and sliding in vacuum. 86 Subsurface dislocation processes in metal contacts Dislocation-mediated processes in the subsurface region of a contact are critical for understanding deformation and energy dissipation in metals in sliding contact. 88 The movement and self-organization of dislocations leads to a dynamic and complex subsurface microstructure.…”

Matching Atomistic Simulations and In Situ Experiments to Investigate the Mechanics of Nanoscale Contact

“…Contact tests were performed inside of a transmission electron microscope (2100F, JEOL, Tokyo, Japan) operated at 200 keV with the use of an in situ nanoindenter (PI-95 Picoindenter, Hysitron Inc., Minneapolis, MN). The experimental setup is similar to that of [37][38][39] and is shown in Fig. 1(a,b).…”

Insights into dynamic sliding contacts from conductive atomic force microscopy

“…15 Several CAFM investigations have focused on the relationship between current transfer and contact area, both of which should increase with pressure. 7,[16][17][18][19] However, some of these studies showed that, for very small contacts ($50 nm or less), the expected current-contact area relationship does not always hold true. 16,17,20,21 One explanation for this lack of correlation is that trace contamination of the contact can inhibit conduction 22 such that the magnitude of the electrical current does not reect the size of the contact.…”