A popular area of research in molecular electronics is the formation of reproducible and stable metal electrodes on organic layers; however, forming these contacts atop self-assembled monolayers (SAMs) is not trivial since SAMs tend be fragile and easily destroyed. Current methods that are successful in making intimate contact to monolayers are typically employed as nonpermanent, analytical techniques and have not yet been suitable for the creation of commercial devices.
As published in JACS, the You Group has developed the methodology to form metal-molecule-metal junctions by employing low surface energy perfluoropolyether (PFPE) stamps in the soft lithographic process, nanotransfer printing (nTP). They show that by using PFPE, thin films of Au, Ni, and Co can be transferred in tightly compact nanoarray junctions onto thiol terminated SAMs formed on a variety of metal thin film electrodes. In addition, the junctions formed are reproducible and promising for commercial applications boasting promising electrical characteristics. The formed junctions provide both an analytical test bed to probe electrical characteristics of various self-assembled monolayers sandwiched between metal electrodes as well as a promising step toward forming permanent and functional metal-molecule-metal junctions.
Self-assembled monolayers (SAMs) using aryl or alkyl thiol chemistries have been well documented and characterized on inert metal surfaces such as gold, platinum, and silver. However, self-assembly of aryl/alkyl thiols on pristine, oxide-free transition metal surfaces, such as Ni, Co, and Fe has been met with limited success mainly due to the presence of a native oxide which inhibits the formation of direct sulfur-metal linkages.
Work published in JACS by the You Group demonstrates a convenient method of direct SAM-metal formation on Ni, Co, and Fe thin film surfaces by employing an electroreduction step in basic conditions under inert atmosphere. They report SAM coverage with both thiol and isocyanide attachment, verifying the removal of the undesired oxide layer by x-ray photo-electron spectroscopy. By a combination of cyclic voltammetry, surface IR, and contact angle studies they show the SAM density on Ni, Co, and Fe by this electroreduction method to be comparable to that of their noble metal counterparts.