Plasmons: Chemical Bonding Coupling Induced Surface Plasmon Resonance Splitting in Self-Assembled Gold Nanoparticles

Abstract

Recently, the effect of π-bonding electrons conjugating to a gold surface has been predicated by first principles. In this paper, we observe surface plasmon resonance splitting due to the covalence and π-bonding electrons conjugated effect. When surfactantless Au NPs are linked on the glass substrate by sulfur, the SPR peak is split from one mode (638 nm) into two modes at 615 and 643 nm. The binding energy of 3s electrons for sulfur atoms has a huge red-shift in XPS peaks to confirm the π-bonding electrons conjugated effect. Furthermore, a proposed classical coupling dielectric function model, adding the gold−sulfur covalence bonding effect and conjugated π-bond electrons from sulfur to gold into the Drude model, is achieved to explain the mechanism of the SPR split phenomenon. In the extreme case, when the Au surface atoms are all covered by thiol sulfur atoms, or not covered at all by thiols, the SPR peak will not split. We found 3.28% of the Au atoms are bound by sulfur atoms and 92% of the conjugated electrons of the sulfur atoms transfer to Au electronic gas. The coverage rate of the sulfur atoms bonded to the surface of the Au atoms is about 25%, and this can cause the SPR splitting.