Single-electron charging features of larger, dodecanethiol-protected gold nanoclusters: electrochemical and scanning tunneling microscopy studies

Abstract

In this report, we demonstrate the single-electron charging features of larger-sized (ca. 3.72 nm) Au nanoclusters protected with dodecanethiol [approximate composition, Au₁₄₁₅(RS)₃₂₈] using combined electrochemical and scanning tunneling microscopic (STM) studies. In particular, these nanoclusters show a highly populated single-electron charging peak in voltammetric experiments, where the calculated capacitance is in good agreement with the experimentally obtained value of 1.6 aF. In comparison to the voltammetric studies, STM measurements over a single Au particle on the highly oriented pyrolytic graphite surface reveal nonlinear current-voltage (I-V) characteristics with a large central gap, signifying single-electron-transfer features. The I-V results demonstrate a clear Coulomb blockade effect with a central gap of around 0.2 eV, which is in good agreement with the orthodox theory for the double barrier tunnel junction system. The standard heterogeneous electron-transfer rate constant estimated from impedance measurements is found to be of 7.97 × 10⁻⁶ cm·s⁻¹, suggesting that the process is very sluggish. Furthermore, diffusion coefficient (D_c) values calculated from chronoamperometry and impedance measurements are in good agreement with theoretically calculated values using the modified Stokes-Einstein equation. The electron-transfer rate constant estimated from cyclic voltammograms of adsorbed monolayer protected Au nanoclusters is found to be about 2 s⁻¹, which is slower than that reported for its smaller analogues.