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

Ultrahigh strength and shear-assisted separation of sliding nanocontacts studied in situ

The behavior of materials in sliding contact is challenging to determine since the interface is normally hidden from view. Using a custom microfabricated device, we conduct in situ, ultrahigh vacuum transmission electron microscope measurements of crystalline silver nanocontacts under combined tension and shear, permitting simultaneous observation of contact forces and contact width. While silver classically exhibits substantial sliding-induced plastic junction growth, the nanocontacts exhibit only limited plastic deformation despite high applied stresses. This difference arises from the nanocontacts’ high strength, as we find the von Mises stresses at yield points approach the ideal strength of silver. We attribute this to the nanocontacts’ nearly defect-free nature and small size. The contacts also separate unstably, with pull-off forces well below classical predictions for rupture under pure tension. This strongly indicates that shearing reduces nanoscale pull-off forces, predicted theoretically at the continuum level, but not directly observed before.

To understand and predict friction, it is crucial to observe sliding at the nanoscale to uncover the mechanisms at play. Here, the authors show that nano-contacts exhibit strength near the ideal limit, and find that pull-off forces predicted by continuum models are reduced by shearing.

Improving the reliability of conductive atomic force microscopy-based electrical contact resistance measurements

Electrical contact resistance (ECR) measurements performed via conductive atomic force microscopy (C-AFM) suffer from poor reliability and reproducibility. These issues are due to a number of factors, including sample roughness, contamination via adsorbates, changes in environmental conditions such as humidity and temperature, as well as deformation of the tip apex caused by contact pressures and/or Joule heating. Consequently, ECR may vary dramatically from measurement to measurement even on a single sample tested with the same instrument. Here we present an approach aimed at improving the reliability of such measurements by addressing multiple sources of variability. In particular, we perform current-voltage spectroscopy on atomically flat terraces of highly oriented pyrolytic graphite (HOPG) under an inert nitrogen atmosphere and at controlled temperatures. The sample is annealed before the measurements to desorb adsorbates, and conductive diamond tips are used to limit tip apex deformation. These precautions lead to measured ECR values that follow a Gaussian distribution with significantly smaller standard deviation than those obtained under conventional measurement conditions. The key factor leading to this improvement is identified as the switch from ambient conditions to a dry nitrogen atmosphere. Despite these improvements, spontaneous changes in ECR are observed during measurements performed over several minutes. However, it is shown that such variations can be suppressed by applying a higher normal load.

Simulations of the effect of an oxide on contact area measurements from conductive atomic force microscopy

Nanoscale contact area in conductive atomic force microscopy can be determined by analyzing current flow using electron transport theories. However, it is recognized that native oxides on the conductive tip will reduce current flow, thus degrading the accuracy of the measured contact area. To quantify the adverse effect of an oxide on contact area measurements, we use molecular dynamics simulations of an oxide-coated platinum tip and a crystalline platinum substrate, where both the contact size and conductance can be inferred from the positions of atoms in the interface. We develop a method to approximate conductance based on the distance between atoms in platinum channels across the contact. Then, the contact area calculated from conductance using ballistic transport and tunneling theories is compared to that obtained using the known positions of atoms in the contact. The difference is small for very thin (<0.1 nm) or very thick (>1.0 nm) oxides, where ballistic transport and tunneling theories work well; however, the difference is significant for oxides between these limits, which is expected to be the case for platinum in many practical applications.