Bundling of Nanowires Induced by Unbound Ligand

We report on the dilution-induced agglomeration of ultrathin gold nanowires (AuNWs) into regular bundles. Wires with a metal core diameter of 1.6–1.7 nm surrounded by a ligand shell of oleylamine formed stable colloids in n-hexane and cyclohexane. Dilution with pure solvent induced the self-assembly into bundles with a regular, hexagonal cross-section. Small-angle X-ray scattering and thermogravimetric analysis indicated that bundles formed only if the ligand shell was sufficiently sparse. Dilution with pure solvent shifts the chemical equilibrium and reduces the ligand density, thus enabling agglomeration. We show that agglomeration is driven not by van der Waals forces but by the depletion forces of linearly shaped molecules. Linear solvent molecules or small amounts of unbound ligand align normal to the nanowire if the ligand shell is sparse. The resulting reduction in entropy creates a driving force for the AuNWs to bundle such that the low-entropy domains overlap and the overall entropy is increased. Dilution-induced nanowire bundling is thus explained as a combined effect of ligand desorption and destabilization by depletion.