Localized surface plasmon resonance (LSPR) properties of noble metal (Au and Ag) nanostructures have long been an exciting field of research interest as unique optical properties can be understood based on light-matter interaction at the nanometer scale. Tuning of LSPR properties of noble metal nanostructures has been historically performed by changing their size, shape, and local dielectric environment. However, programmable tuning of LSPR properties of Au and Ag nanostructures through surface chemistry modification is still lacking. Here, we present that functionalizing Au triangular nanoprisms (Au TNPs) with para-substituted thiocinnamic acid, covalently attached onto the metal via Au-S self-assembled monolayer (SAM) forming a metal-organic ligand hybrid interface, causes a dramatic red shift in the LSPR dipole peak position (~115 nm) of TNPs. Our single particle LSPR characterizations show that the magnitude of LSPR shifts depends on the para-substitution (CH3, OCH3, H, NO2, and CF3) where electron denating groups (CH3, OCH3) provide more red-shift than electron withdrawing groups (NO2, and CF3). We hypothesize that the highly conjugated thiocinnamic acids influence the Au free electrons either by removing or injecting electrons into TNPs through metal-organic ligand hybrid interface resulting in an unprecedentedly large dipole peak position shift of TNPs. Additionally, ultraviolet spectroscopy characterizations also show a strong influence on the Au work function depending on the para-substitution of the ligands. Our time-dependent density functional theory (TDDFT) calculations show that an appropriate energy level alignment between the molecular orbitals of para-substituted thiocinnamic acid and the Au Fermi energy at the metal-organic hybrid interface dictates the magnitude of the LSPR shifts. We expect that our unique finding can help in guiding the future optimization of metal-organic ligand hybrid nanostructures for diverse optoelectronic applications.

Tuning Au nanostructure localized surface plasmon resonance properties through highly conjugated aromatic ligand modification

ACS Fall 2021

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