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 (σ_EL) of dropcast, unlinked Au MPC films increase up to 31-fold with increasing CO₂ pressure, while σ_EL 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 σ_EL 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 σ_EL (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; σ_EL now decreases with increasing CO₂ pressure.