Quantised double layer charging of monolayer-protected clusters in a room temperature ionic liquid

We report, for the first time, symmetric (versus cluster potential of zero charge) charging of narrowly dispersed monolayer-protected Au145 cluster films in a room temperature ionic liquid. Near-reversible voltammetric features are found, indicating up to nine electrochemically accessible charge states with assigned values between −4 and +5. The zero charge state has a stability range of 276 mV, very similar as found in solution-phase voltammetry. All other charge states are more narrowly spaced, indicating a sudden increase in cluster capacitance. We ascribe this effect to an abrupt change in local dielectric constant on insertion of one or more counter ions, following departure from zero charge, with a possible contribution of coupled electron–ion transfer. Our findings open perspectives of potential technological importance involving nanocrystal-based device-like behaviour.