Preparation of nascent molecular electronic devices from gold nanoparticles and terminal alkyne functionalised monolayer films

A metal-molecule-GNP assembly has been fabricated using an acetylene-terminated phenylene-ethynylene molecular monolayer, namely 4-((4-((4-ethynylphenyOethynyl)phenyl)ethynyl)benzoic acid (HOPEA), sandwiched between a gold substrate bottom electrode and gold nanoparticle (GNP) top contact electrode. In the first stage of the fabrication process, a monolayer of directionally oriented (carboxylate-to-gold) HOPEA was formed onto the bottom electrode using the Langmuir-Blodgett (LB) technique. In the second stage, the gold-substrate supported monolayer was incubated in a solution of gold nanoparticles (GNPs), which resulted in covalent attachment of the GNPs on top of the film via an alkynyl carbon-Au sigma-bond thereby creating the metallic top electrode. Adsorption of the GNPs to the organic LB film was confirmed by both UV-vis absorption spectroscopy and X-ray photoemission spectroscopy (XPS), whilst the contact angle showed changes in the physical properties of the film surface as a result of top-coating of the LB film with the GNPs. Importantly, surface-enhanced Raman scattering (SERS) confirmed the covalent attachment of the metal particles to the LB film by formation of Au-C sigma-bonds via a heterolytic cleavage of the alkyne C-H bond. Electrical properties of these nascent metal-molecule-GNP assemblies were determined from I-V curves recorded with a conductive-AFM in the Peak Force Tunneling AFM (PF-TUNA (TM)) mode. The I-V curves obtained from these structures rule out the formation of any significant number of short-circuits due to GNP penetration through the monolayer, suggesting that this strategy of self-assembly of GNPs to alkyne-terminated monolayers is an effective 'soft' procedure for the fabrication of molecular junctions without damaging the organic layer.