Molecular Electronics DOI: 10.1002/anie.200806028 Spectroscopic Tracking of Molecular Transport Junctions Generated by Using Click Chemistry** Xiaodong Chen, AdamB. Braunschweig, Michael J. Wiester, Sina Yeganeh, Mark A. Ratner,* and Chad A. Mirkin* The development of efficient methods for the construction of molecular transport junctions (MTJs) and the ability 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 synthesiz- ing long molecular wires with thiol groups on both ends because of the low solubility and reaction 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 wide) often prohibit the use of routine spectroscopic tools to identify the contents of MTJs. Therefore, a modular method for the in situ synthesis of molecular wires to bridge nanogaps, [4, 5] which allows spectro- scopic tracking of the assembly process, merits development. Herein, we report a new method for the fabrication of MTJs by using the alkyne–azide “click reaction” within nanogaps fabricated by on-wire lithography (OWL), while using sur- face-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 mod- ular approach that can be used to form MTJs comprising different molecular components. Additionally, this approach is well-suited for studying the transport properties of various molecular architectures because the resulting triazole formed by the reaction of the alkyne and azide groups retains the conjugation required for electronic transport. OWL is an electrochemistry-based nanofabrication tech- nique used to prepare a wide variety of nanowire-based structures (e.g., nanogaps and disk arrays) with control over composition and morphology. [6] The obtained structures have been used for prototyping nanostructured materials with advanced functions in the context of molecular electronics [6–8] and SERS. [9–11] OWL allows the preparation of gaps with feature-size control down to 2 nm, which makes them promising testbeds for the fabrication of MTJs. [7] The characteristics of OWL-fabricated nanogaps include high- throughputs and tunable, molecular-sized features. Herein, we use click chemistry for the in situ modular synthesis of molecular wires within the OWL-fabricated nanogaps. Click chemistry is a synthetic approach popularized by Sharpless and co-workers that involves reactions that proceed quickly, with high yields and specificity, under mild conditions. [12] An advantage of forming molecular wires by using the click methodology within the OWL-fabricated nanogaps is that in situ fabrication within a confined space (nanogap) is challenging for other existing testbeds, such as scanning probes [13–15] and wire crossing, [16] because these techniques are not easily solution-processable. On the other hand, mechan- ical break-junction techniques [17, 18] provide only limited control over gap size in comparison with nanogaps formed by OWL. In this study, the copper(I)-catalyzed 1,3-dipolar Huisgen cycloaddition reaction (click reaction) between azide and alkyne groups (Figure 1c) is utilized as a model reaction for preparing molecular wires within OWL-fabricated nano- gaps to form MTJs. The general scheme for bridging the nanogaps by using click chemistry for MTJ fabrication is shown in Figure 1 a. In a typical experiment, 4-ethynyl-1-thioacetylbenzene (1) [19] is first assembled into a monolayer on the surfaces of the Figure 1. a) Schematic illustration of click chemistry within the nano- gaps. b) Molecules used in this study. c) The alkyne–azide click reaction. [*] Dr. X. Chen, [+] Dr. A. B. Braunschweig, [+] M. J. Wiester, [+] S. Yeganeh, Prof. M. A. Ratner, Prof. C. A. Mirkin Department of Chemistry and International Institute for Nanotechnology, Northwestern University 2145 Sheridan Road, Evanston, IL 60208 (USA) E-mail: ratner@northwestern.edu chadnano@northwestern.edu [ + ] These authors contributed equally to this work. [**] C.A.M. acknowledges support from the NUNSF-NSEC. and for an NSSEF Fellowship from the Department of Defense. A.B.B. is grateful for an NIH Postdoctoral Fellowship. S.Y. thanks the ONR for an NDSEG fellowship. M.A.R. acknowledges funding from the MRSEC at NU. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200806028. Communications 5178  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2009, 48, 5178 –5181