Inorganic–Organic Interfacial Electronic Effects in Ligand-Passivated WO_3–x Nanoplatelets Induce Tunable Plasmonic Properties for Smart Windows

Transition-metal oxide (TMO) nanocrystals (NCs), displaying localized surface plasmon resonance (LSPR) properties, are an emerging class of nanomaterials due to their high stability, high earth abundance, and wide range of spectral responses covering the near-to-far infrared region of the solar spectrum. Although surface passivating ligands are ubiquitous to colloidal NC-based research, the role of ligands, specifically the impact of their chemical structure on the dielectric and LSPR properties of TMO NC films, has not been investigated in detail. Here, we report for the first time the chemical effects at the metal–ligand (inorganic–organic) interfaces influencing the optical constants and LSPR properties of thin films comprising highly oxygen-deficient, sub-stoichiometric, LSPR-active tungsten oxide (WO3–x) nanoplatelets (NPLs). We studied ligands with two different types of binding head groups, aromatic conjugation, and short and long hydrocarbon chains. Using density functional theory calculations, we determine that the changes in the interfacial dipole moments and polarizability control the permittivity at the interface, resulting in the alteration of dielectric and LSPR properties of ligand-passivated NPL in thin nanocrystalline films. The photochromic properties of WO3–x NPL passivated with different ligands in thin films have also been investigated to highlight the impact of interfacial permittivity caused by the chemical structures of passivating ligands. Taken together, this study provides a fundamental understanding of emerging properties at the metal–ligand interface that could be further optimized for energy efficiency in smart windows.