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

Conspectus Plasmonic metal nanostructures have been extensivelydevelopedover the past few decades because of their ability to confine lightwithin the surfaces and manipulate strong light-matter interactions.The light energy stored by plasmonic nanomaterials in the form ofsurface plasmons can be utilized to initiate chemical reactions, so-calledplasmon-induced catalysis, which stresses the importance of understandingthe surface chemistry of the plasmonic materials. Nevertheless, onlyphysical interpretation of plasmonic behaviors has been a dominanttheme, largely excluding chemical intuitions that facilitate understandingof plasmonic systems from molecular perspectives. To overcome andaddress the lack of this complementary understanding based on molecularviewpoints, in this Account we provide a new concept encompassingthe well-developed physics of plasmonics and the corresponding surfacechemistry while reviewing and discussing related references. Inspiredby Roald Hoffmann's descriptions of solid-state surfaces basedon the molecular orbital picture, we treat molecular interfaces ofplasmonic metal nanostructures as a series of metal-ligandcomplexes. Accordingly, the effects of the surface ligands can bedescribed by bisecting them into electronic and steric contributionsto the systems. By exploration of the quality of orbital overlapsand the symmetry of the plasmonic systems, electronic effects of surfaceligands on localized surface plasmon resonances (LSPRs), surface diffusionrates, and hot-carrier transfer mechanisms are investigated. Specifically,the propensity of ligands to donate electrons in a & sigma;-bondingmanner can change the LSPR by shifting the density of states nearthe Fermi level, whereas other types of ligands donating or acceptingelectrons in a & pi;-bonding manner modulate surface diffusion ratesby affecting the metal-metal bond strength. In addition, theformation of metal-ligand bonds facilitates direct hot-carriertransfer by forming a sort of molecular orbital between a plasmonicstructure and ligands. Furthermore, effects of steric environmentsare discussed in terms of ligand-ligand and ligand-surfacenonbonding interactions. The steric hindrance allows for controllingthe accessibility of the surrounding chemical species toward the metalsurface by modulating the packing density of ligands and generatingrepulsive interactions with the surface atoms. This unconventionalapproach of considering the plasmonic system as a delocalized molecularentity could establish a basis for integrating chemical intuitionwith physical phenomena. Our chemist's outlook on a molecularinterface of the plasmonic surface can provide insights and avenuesfor the design and development of more exquisite plasmonic catalystswith regio- and enantioselectivities as well as advanced sensors withunprecedented chemical controllability and specificity.