Chain Length and Solvent Control over the Electronic Properties of Alkanethiolate-Protected Gold Nanoparticles at the Molecule-to-Metal Transition

Alkanethiolate protected gold nanoparticles are one of the most widely used systems in modern science and technology, where the emergent electronic properties of the gold core are valued for use in applications such as plasmonic solar cells, photocatalysis, and photothermal heating. Though choice in alkane chain length is not often discussed as a way in which to control the electronic properties of these nanoparticles, we show that the chain length of the alkyl tail exerts clear control over the electronic properties of the gold core, as determined by conduction electron spin resonance spectroscopy. The control exerted by chain length is reported on by changes to the g-factor of the metallic electrons, which we can relate to the average surface potential on the gold core. We propose that the surface potential is modulated by direct charge donation from the ligand to the metal, resulting from the formation of a chemical bond. Furthermore, the degree of charge transfer is controlled by differences between the dielectric constant of the medium and the ligand shell. Together, these observations are used to construct a simple electrostatic model that provides a framework for understanding how surface chemistry can be used to modulate the electronic properties of gold nanoparticles.