Electronic Conductivity of Solid-State, Mixed-Valent, Monolayer-Protected Au Clusters

Abstract

Electronic conductivity, σ EL, in solid-state films of alkanethiolate monolayer protected Au clusters (Au MPCs) occurs by a bimolecular, electron self-exchange reaction, whose rate constant is controlled by (a) the core-to-core tunneling of electronic charge along alkanethiolate chains and (b) the mixed valency of the MPC cores (e.g., a mixture of cores with different electronic charges). The tunneling mechanism is demonstrated by an exponential relation between the electronic conductivity of Au 309 (C n ) 92 MPCs (average composition) and n, the alkanethiolate chainlength, which varies from 4 to 16. The electron tunneling coefficient β n = 1.2/CH 2 or, after accounting for alkanethiolate chain interdigitation, β dis = 0.8 Å - 1 . Quantized electrochemical double layer charging of low polydispersity Au 140 (C6) 53 MPCs was used to prepare solutions containing well-defined mixtures of MPC core electronic charges (such as MPC 0 mixed with MPC 1+ ). Electronic conductivities of mixed-valent, solid-state Au 140 (C6) 53 MPC films cast from such solutions are proportional to the concentration product [MPC 0 ][MPC 1+ ], and give a MPC 0/1+ electron self-exchange rate constant of ca. 10 10 M - 1 s - 1 .