Spectroscopic Tracking of Molecular Transport Junctions Generated by Using Click Chemistry

The development of efficient methods for constructing molecular transport junctions (MTJs) with the capability to spectroscopically identify molecules assembled within the junctions continues to challenge the field of molecular electronics.[1,2] Most of the current work in MTJ fabrication relies primarily on ex situ syntheses of molecular wires (e.g. dithiolated molecules) followed by subsequent insertion of the molecules into the gap devices.[3] The problems associated with this approach are: 1) the difficulty involved in synthesizing long molecular wires with thiols on both ends because of the low stability and synthetic yields of these molecules, and 2) complications in bridging the electrodes because of a strong tendency of such molecular wires to aggregate.[4] In addition, the small junction sizes (normally only several nanometers in width) often prohibit the use of routine spectroscopic tools to identify the contents within MTJs. Therefore, a modular method for in situ synthesis of molecular wires to bridge nanogaps[4,5] that allows spectroscopic tracking of the assembly process merits development. Herein, we report a new method to fabricate MTJs using the alkyne-azide “click reaction” within nanogaps fabricated by On-Wire Lithography (OWL), while using surface enhanced Raman scattering (SERS) to characterize the assembly processes within the gaps. This strategy for forming MTJs proceeds in high yields, and, as a result of the accessible functional group requirements of click chemistry, is a modular approach that can be used to form MTJs comprised of different molecular components. Additionally, this approach is well suited for studying transport properties of various molecular architectures because the resulting triazole formed by reacting the alkyne and azide groups retains the conjugation required for the electronic transport.