The EGaIn Technique

We use a method to contact SAMs electrically safely: the metal surface on which the SAMs are formed serves as the bottom-electrode and the SAM is contacted with a eutectic alloy of Gallium and Indium (EGaIn) that has a thin, highly conductive surface layer of conductive GaOx of 0.7 nm); this alloy is non-toxic and has non-Newtonian properties so it can be shaped and forms stable features in microchannels (unlike Hg for instance).

This technique has the advantage over solid metal top-electrodes that it does not destroy, or penetrate the SAMs (despite their thicknesses of about 2 nm), and gives metal-SAM-metal structures that are well-defined. The Figure shows a series of photographs that show (from left to right) the formation of cone-shaped tips of a EGaIn. By just pulling a mico-needle (i) out of a drop of a eutectic alloy of Ga and In (EGaIn away, two head-to-head cone-shaped EGaIn structures can be formed (ii) until the two cones separate. The cone-shaped tip of EGaIn can be brought in contact with the target surface supporting a SAM by simply lowering the microneedle using the same micromanipulator (iii) until contact with the SAM is made (iv). This so-called EGaIn technique was developed in the group of Whitesides, recently we have expanded this technique and form routinely EGaIn top-electrode stabilized in microfluidic channels making it possible to study the mechanisms of charge transport as a function of temperature, integrate the EGaIn technique with optics, and study the junctions with (potentiodynamic) impedance spectroscopy.

As can be seen from our Nanofabrication section, we took the EGaIn technique to the next level by designing reusable soft top electrodes. We showed that arrays of junctions can be formed by fabricating the top electrode shown in the following schematic (see Nanoscale 20157, 19547-19556)

We followed the process of EGaIn filling into the fabricated top electrode through optical imaging.

Fig 1: (a) An optical micrograph of a cross-section of a top-electrode showing the through-hole before it was filled with the liquid-metal. The inset shows a SEM image of one of the pillar-line structures of the mold. Scale bar: 50 μm. (b) Optical image (viewed through the ITO) of one of the though-holes filled with GaOx/EGaIn. (c) Photograph of a complete device (top view).