Electron Hopping through Films of Arenethiolate Monolayer-Protected Gold Clusters

Abstract

Electron hopping in films of arenethiolate (benzylthiolate, phenylethylthiolate, phenylbutanethiolate, and cresolthiolate) monolayer-protected cluster molecules (MPCs) is investigated through measurements of solid-state electronic conductivity. Electron donor−acceptor hopping rates between the Au cores of arenethiolate MPCs exceed those of previously studied solid-state alkanethiolate MPC films, but the electronic coupling term, β = 0.8 Å-1, is nearly the same. Rate constants range from 108 to 1011 s-1 across the series of arenethiolate MPCs; for the case of cresolthiolate, the rate corresponds to a single molecule resistance of ∼107 Ω/cresolthiolate ligand. The 4−8 kJ/mol activation-barrier energies for electron hopping are generally in line with the Marcus theory prediction. The low barrier energies and large rate constants arise from a combination of the low dielectric medium environment of the reactants (the MPC cores) and the partly aromatic tunneling bridges. The sharp increase in film conductivity upon substituting arenethiolate ligands for >50% of the hexanethiolates on a hexanethiolate-protected MPC suggests a percolation effect.

Thioacetic-Acid Capped PbS Quantum Dot Solids Exhibiting Thermally Activated Charge Hopping Transport

“…The results of the first-order electron hopping rate constant, k ET ) of the arenethiolate capped-Au nanoparticle thin films. 58 This result supports the idea that the remaining long insulating OA, even after the ligand exchange process, limits the electron transfer in PbS-TAA QD solids.…”

Vapor Sorption and Electrical Response of Au‐Nanoparticle– Dendrimer Composites

“…Attempts have been made to explain E a with the "outer sphere" reorganization energy known from Marcus theory. [38] In another approach, E a was related to the classical Coulomb charging energy, which is required for transferring an electron from one electrically neutral particle to another. [5] In fact, experimental data for E a compare remarkably well with the expected Coulomb charging energies.…”

Single‐Electron Transfer in Nanoparticle Solids

“…In scanning tunneling spectroscopic (STS) studies of isolated particles, [1][2][3] the resulting currentpotential (I-V) profile generally exhibits a Coulomb blockade in the central region, beyond which a Coulomb staircase (single-electron transfer; SET) may be identified. Such unique characteristics are the fundamental basis for the development of single-electron transistors.[4] By contrast, in studies of nanoparticle ensembles that form (sub)micrometer-thick solid films, [5][6][7][8] typically only linear (Ohmic) I-V behavior is observed, especially at a relatively high voltage bias, because of rampant structural defects within these particle solids that facilitate interparticle charge transfer (e.g., percolation effects). Fundamentally, the collective conductivity properties of organized assemblies of particles are found to be determined not only by the particle chemical structure (core size, shape, and surface ligands), but by the specific chemical environments and interparticle interactions as well.…”

“…The fact that we observed peaks instead of a staircase (as in STM measurements) can be ascribed to electron diffusion along the particle ensembles. Earlier studies of nanoparticles dispersed in polymer matrices and thick nanoparticle films [5,24] have shown that the charge migration (and hence conductance) is essentially limited by electron diffusion within the nanoparticle solids, akin to the case of redox-active polymer melts. Thus, the firstorder electron-hopping rate constant (k ET , s -1 ) can be calculated from the electron-diffusion coefficient (D EH ) (assuming a square lattice for the nanoparticle arrays, which is a rather reasonable approximation according to the STM measurements, see below) [5,24] [25] i P nFAD…”

“…At lower temperatures where SET is absent, k ET can also be evaluated by the ensemble specific conductivity (r), [5] …”