A Chemist's View on Electronic and Steric Effects of Surface Ligands on Plasmonic Metal Nanostructures

ConspectusPlasmonic metal nanostructures have been extensively developed over the past few decades because of their ability to confine light within the surfaces and manipulate strong light-matter interactions. The light energy stored by plasmonic nanomaterials in the form of surface plasmons can be utilized to initiate chemical reactions, so-called plasmon-induced catalysis, which stresses the importance of understanding the surface chemistry of the plasmonic materials. Nevertheless, only physical interpretation of plasmonic behaviors has been a dominant theme, largely excluding chemical intuitions that facilitate understanding of plasmonic systems from molecular perspectives. To overcome and address the lack of this complementary understanding based on molecular viewpoints, in this Account we provide a new concept encompassing the well-developed physics of plasmonics and the corresponding surface chemistry while reviewing and discussing related references. Inspired by Roald Hoffmann's descriptions of solid-state surfaces based on the molecular orbital picture, we treat molecular interfaces of plasmonic metal nanostructures as a series of metal-ligand complexes. Accordingly, the effects of the surface ligands can be described by bisecting them into electronic and steric contributions to the systems. By exploration of the quality of orbital overlaps and the symmetry of the plasmonic systems, electronic effects of surface ligands on localized surface plasmon resonances (LSPRs), surface diffusion rates, and hot-carrier transfer mechanisms are investigated. Specifically, the propensity of ligands to donate electrons in a σ-bonding manner can change the LSPR by shifting the density of states near the Fermi level, whereas other types of ligands donating or accepting electrons in a π-bonding manner modulate surface diffusion rates by affecting the metal-metal bond strength. In addition, the formation of metal-ligand bonds facilitates direct hot-carrier transfer by forming a sort of molecular orbital between a plasmonic structure and ligands. Furthermore, effects of steric environments are discussed in terms of ligand-ligand and ligand-surface nonbonding interactions. The steric hindrance allows for controlling the accessibility of the surrounding chemical species toward the metal surface by modulating the packing density of ligands and generating repulsive interactions with the surface atoms. This unconventional approach of considering the plasmonic system as a delocalized molecular entity could establish a basis for integrating chemical intuition with physical phenomena. Our chemist's outlook on a molecular interface of the plasmonic surface can provide insights and avenues for the design and development of more exquisite plasmonic catalysts with regio- and enantioselectivities as well as advanced sensors with unprecedented chemical controllability and specificity.