Measuring single electron charging energy in self-assembled single nanoparticle devices: Coulomb blockade threshold vs. Arrhenius energy

Single-nanoparticle (NP) devices formed by self-assembling NPs onto alkanedithiol-functionalized break junctions exhibit Coulomb blockade (CB) conductance suppressions at low temperatures. We have studied temperature dependence of conductance inside the CB region and find \textit{multiple }activation energies (\textit{Ea}): A small \textit{Ea }at low temperatures, and a larger \textit{Ea }at high temperatures. The small \textit{Ea }is independent of NP size and is attributed to an energy state located at the metal--molecule contact. The larger \textit{Ea }scales with NP size and is attributed to single electron charging energy of the NPs. Importantly, we observe a significant ($\sim $5--100 fold) discrepancy between values of charging energies obtained from CB voltage thresholds and \textit{Ea}. To account for the discrepancy, we propose a model in which electrons are temporarily localized at the energy states near the metal--molecule interface and lose energy. The proposed model is supported by ultraviolet photoelectron spectroscopy of alkanedithiol monolayers on gold which indicates a presence of energy states close to the Fermi level of gold likely arising from gold--thiolate bonds. A suitably modified Orthodox theory successfully describes our measurements.