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

Abstract

A metal–molecule–GNP assembly has been fabricated using an acetylene-terminated phenylene–ethynylene molecular monolayer, namely 4-((4-((4-ethynylphenyl)ethynyl)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 σ-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 σ-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™) 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.