Monolayer-protected Au Clusters Research Articles

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

Electronic conductivity, σEL, 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 Au309(Cn)92 MPCs (average composition) and n, the alkanethiolate chainlength, which varies from 4 to 16. The electron tunneling coefficient βn = 1.2/CH2 or, after accounting for alkanethiolate chain interdigitation, βdis = 0.8 A-1. Quantized electrochemical double layer charging of low polydispersity Au140(C6)53 MPCs was used to prepare solutions containing well-defined mixtures of MPC core electronic charges (such as MPC0 mixed with MPC1+). Electronic conductivities of mixed-valent, solid-state Au140(C6)53 MPC films ...

Monolayer-Protected Cluster Growth Dynamics

With the objective of better understanding the Brust synthesis reaction, this paper examines the evolution of the core sizes of hexanethiolate monolayer-protected Au clusters (MPCs) in a typical synthesis reaction mixture, at time intervals over the course of 125 h. Transmission electron microscopy shows that the average MPC core diameter gradually increases over the first 60 h of reaction and then remains largely unchanged afterward at ∼3.0 nm. Differential pulse voltammetry of purified MPC aliquots removed from the synthesis reaction exhibit quantized double-layer (QDL) charging peaks. QDL charging peaks have been previously shown to be a strong function of MPC core size and dispersity and reveal (i) the presence of several discernible core sizes in each sample and (ii) an increase in cluster capacitance (CCLU) with longer reaction times, consistent with the electron microscopy results.

The Monolayer Thickness Dependence of Quantized Double-Layer Capacitances of Monolayer-Protected Gold Clusters

This report describes how the electrochemical double-layer capacitances of nanometer-sized alkanethiolate monolayer-protected Au clusters (MPCs) dissolved in electrolyte solution depend on the alkanethiolate chain length (C4 to C16). The double-layer capacitances of individual MPCs (C(CLU)) are sufficiently small (sub-attoFarad, aF) that their metal core potentials change by >0.1 V increments for single electron transfers at the electrode/solution interface. Thus, the current peaks observed are termed "quantized double layer charging peaks", and their spacing on the potential axis varies with C(CLU). Differential pulse voltammetric measurements of C(CLU) in solutions of core-size-fractionated (i.e., monodisperse) MPCs are compared to a simple theoretical model, which considers the capacitance as governed by the thickness of a dielectric material (the monolayer, whose chain length is varied) between concentric spheres of conductors (the Au core and the electrolyte solution). The experimental results fit the simple model remarkably well. The prominent differential pulse voltammetric charging peaks additionally establish this method, along with high-resolution transmission electron microscopy and laser ionization-desorption mass spectrometry, as a tool for evaluating the degree of monodispersity of MPC preparations. We additionally report on a new tactic for the preparation of monodisperse MPCs with hexanethiolate monolayers.

Quantized Capacitance Charging of Monolayer-Protected Au Clusters

A theoretical model based on electrostatic interactions is developed to account for the formal potentials of current peaks observed in differential pulse voltammetry of solutions of 10 different nanometer-sized alkylthiolate and arylthiolate monolayer-protected gold clusters. The current peaks arise from successive, quantized (single-electron) capacitative charging of ensembles of individual cluster cores (i.e., electrochemical ensemble Coulomb staircase charging). Experimental peak potentials for a given cluster change roughly linearly with changes in its core charge state, as predicted by the theory, and the sub-attofarad capacitances (CCLU) of individual clusters obtained from the slopes of such plots agree with those estimated from a simple concentric sphere capacitor model. The charging of clusters with very small cores becomes redox molecule-like, indicating as reported recently, the emergence of HOMO−LUMO energy gaps. Because the quantized charging currents of the clusters are diffusion controlled,...