Wet chemical synthesis of soluble gold nanogaps.

A central challenge in molecular electronics is to create electrode pairs separated by only a few nanometers that can accommodate a single molecule of interest to be optically or electrically characterized while residing in the gap. Current techniques for nanogap fabrication are largely based on top-down approaches and often rely on subsequent deposition of molecules into the nanogap. In such an approach, the molecule may bridge the gap differently with each experiment due to variations at the metal-molecule interface. Conversely, chemists can readily synthesize gold nanorods (AuNRs) in aqueous solution. Through controlled end-to-end assembly of the AuNRs into dimers or chains, facilitated via target molecules, they can be used as electrical contacts. In this way, the preparation of AuNR-molecule-AuNR junctions by wet chemical methods may afford a large number of identical devices with little variation in the interface between molecule and electrode (AuNR). In this Account, we highlight recent progress in using chemically synthesized AuNRs as building blocks for molecular electronic applications. We outline the general synthesis and properties of AuNRs and describe the aqueous growth of dimeric AuNR structures from an insulating molecule linked to AuNR precursors (gold seeds). Conjugated, electronically active molecules are typically not soluble under the conditions required for the bottom-up growth of AuNRs. Therefore, we present a strategy that utilizes host-guest chemistry in order to make such π-systems compatible with the AuNR growth procedure. In order to electrically characterize the AuNR-molecule-AuNR constructs, we must transfer them onto a substrate and contact external electrodes. We discuss the implications of using electron-beam lithography for making this contact. In addition, we introduce a novel fabrication approach in which we can grow AuNR nanogap electrodes in situ on prepatterned substrates, thus circumventing post-processing steps that potentially damage the nanogap environment. Due to the inherent optical properties of AuNRs, electromagnetic field enhancement in the nanogaps lets us spectroscopically characterize the molecules via surface-enhanced Raman scattering. We discuss the incorporation of oligopeptides functionalized with acetylene units having uniquely identifiable vibrational modes. This acetylene moiety allows chemical reactions to be performed in the gaps via click chemistry, and the oligopeptide linking platform opens for integration of larger biological components.