Dynamics of CO2-Plasticized Electron Transport in Au Nanoparticle Films:  Opposing Effects of Tunneling Distance and Local Site Mobility

The electron-transport properties of unlinked and linked solid-state films of very small “Au38” nanoparticles (monolayer protected clusters, or MPCs) remarkably change in opposing directions upon contact with increasing pressures of CO2 gas (0∼6.6 MPa). Electronic conductivities (σEL) of dropcast, unlinked Au MPC films increase up to 31-fold with increasing CO2 pressure, while σEL of dithiol-linked Au MPC films decreases with increasing CO2 pressure. In conductivity by electron hopping between electron donor and acceptor Au MPC cores, the organic protecting ligands serve as a solvent shell whose properties influence the dynamics of electron transport. Exposure of Au MPC films to CO2 and consequent swelling by CO2 (or organic vapor) sorption into the monolayers changes two key factors: core edge-to-edge electron tunneling distances (d HOP) and a previously underappreciated effect of dimension and/or frequency of local core thermal motions. Swelling induces increases in both, but depending on which factor is dominant, the net σEL can increase or decrease. In unlinked films under increasing CO2 pressure, increased thermal motions of Au MPC cores and their monolayers enhance electron-hopping rates more than swelling-induced increases in d HOP decrease them. The net effect is increasing σEL (i.e., increased local mobility negates increased average tunneling distances). In contrast, in dithiol-linked films local thermal motions are constrained by the dithiol linker between Au MPC cores, leaving CO2 sorption-induced swelling and increase in d HOP as the more dominant factor; σEL now decreases with increasing CO2 pressure.