Electronic interactions between gold nanoclusters in constrained geometries

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The interactions between gold nanoclusters dispersed in decane $({\mathrm{C}}_{10}{\mathrm{H}}_{22})$ when confined in narrow gaps of a few nanometers were investigated. The diameter of the gold core was $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The clusters were capped off with alkanethiol molecules of different lengths (${\mathrm{C}}_{6}\mathrm{S}$ and ${\mathrm{C}}_{15}\mathrm{S}$) to prevent metallic contact with each other. By applying force to the gap walls, the cluster suspension was squeezed out of the gap in a nearly continuous way. In the case of the shorter ligand ${\mathrm{C}}_{6}\mathrm{S}$, the width of the minimum gap at the highest applied force was $7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ ($\ensuremath{\sim}3$ cluster diameters). In contrast with the continuous decrease in thickness, the gap capacitance exhibited stepwise increases, which are interpreted as Mott type insulator-to-metal transition of aggregates of clusters under pressure when the average distance between the metallic cores becomes less than $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. With longer ligands, ${\mathrm{C}}_{15}\mathrm{S}$, the thickness of the minimum gap was $12--15\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. In that case, no discontinuous steps in thickness or capacitance were observed, showing that the longer ligand effectively prevents exchange interactions between the metallic cores of the nanoclusters.