Is it possible to connect metal electrodes to the two ends of a molecule?  The ability to form metal–molecule–metal (MMM) junctions has been one of the primary goals of researchers working in the field of ‘molecular electronics’ because these junctions allow for the fundamental measurement of charge transport through molecules.  Molecules are small, synthetically tunable, potentially inexpensive, and therefore have the potential to be useful as components in molecular–scale electronic devices.  

A challenge in this field is establishing a reliable method to connect metal electrodes to a molecule of interest.  One approach to this problem has been to sandwich a monolayer of molecules between two flat electrodes to perform ‘ensemble’ measurements through a layer of many identical molecules.  The first step towards making these measurements is the formation of a self-assembled monolayer (SAM) of molecules on a metallic surface (typically atomically–flat gold).  A SAM is an appealing model system because it forms spontaneously when the metal is exposed to certain types of molecules.  The resulting SAM is a densely–packed layer of aligned molecules that resemble the bristles on a brush or the vertically–oriented fibers on a carpet.  The underlying metallic substrate serves as one of the electrodes since the SAM bonds to the metal.  The ability to create a conformal electrode on top of the SAM without damaging this fragile layer of molecules remains a challenge.  Current methods of forming the top electrode (e.g., liquid metal droplets, physical vapor deposition, conducting polymer films) have the drawback of either having a low yield (due to electrical shorting) or that the data cannot be interpreted in a straightforward manner.

This paper by Niskala and You describes a simple way to make reliable metallic contacts on top of a SAM at room temperature.  The approach uses a soft, teflon–like stamp to transfer thin pads of metal (gold, cobalt, nickel) to form metallic contacts on top of the SAM.  Metal is first deposited onto the stamp and then transferred onto the SAM.  The low tensile modulus allows the stamp to conform to the substrate and the low surface energy allows the metal to transfer easily to the SAM.  Transfer of the metal is also facilitated by the use of a dithiol SAM; that is, a SAM with thiols on both ends of the molecules.  The metal pads on top of the SAM are contacted electrically with the tip of a conducting atomic force microscope to measure charge transport through the SAM.

This approach has the advantage of producing MMM junctions on a SAM at room temperature with a high yield.  It also represents a significant step towards scalability since many small (200 x 200 nm) junctions can be formed with a single stamp.  The disadvantage of this approach is that it requires the use of a conducting probe AFM to make the measurements, which is tedious for laboratory studies.  Regardless, the approach is very promising for making reliable MMM junctions and represents an important step towards realizing integrated devices.