However, for both basic science and technological motivations, we sometimes want to apply electrodes on small scales onto materials where damage from traditional patterning methods is unavoidable and can have severe consequences for the resulting measurements. For example, this work used electrodes patterned onto PDMS, a soft silicone rubber. The elastomer-supported electrodes were then laminated (reversibly!) onto the surface of a single crystal of rubrene, a small molecule organic semiconductor. Conventional lithography onto such a fragile van der Waals crystal is basically impossible, but with this approach the investigators were able to make nice transistor devices to study intrinsic charge transport in the material.
One issue with PDMS as a substrate is that it is very squishy with a large thermal expansion coefficient. Sometimes that can be useful (read this - it's very clever), but it means that it's very difficult to put truly nanoscale electrodes onto PDMS and have them survive without distortion, wrinkling, cracking of metal layers, etc. PDMS also really can't be used at temperatures much below ambient. A more rigid substrate that is really flat would be great, with the idea that one could do sophisticated fab of electrode patterns, and then "flip" the electrode substrate into contact with the material of interest, which could remain untouched or unblemished by lithographic processes.
In this recent preprint, a collaboration between the Gervais group at McGill and the CINT at Sandia, the investigators used a rigid sapphire (Al2O3) substrate to support patterned Au electrodes separated by a sub-micron gap. They then flipped this onto completely unpatterned (except for large Ohmic contacts far away) GaAs/AlGaAs heterostructures. With this arrangement, cleverly designed to remain in intimate contact even when the device is cooled to sub-Kelvin temperatures, they are able to make a quantum point contact while in principle maintaining the highest possible charge mobility of the underlying semiconductor. It's very cool, though making truly intimate contact between two rigid substrates over mm-scale areas is very challenging - the surfaces have to be very clean, and very flat! This configuration, while not implementable for too many device designs, is nonetheless of great potential use for expanding the kinds of materials we can probe with nanoscale electrode arrangements.