Broadband Tunable Mid-infrared Plasmon Resonances in Cadmium Oxide Nanocrystals Induced by Size-Dependent Nonstoichiometry

A central theme of nanocrystal (NC) research involves synthesis of dimension-controlled NCs and studyof size-dependent scaling laws governing their optical, electrical, magnetic, and thermodynamic properties. Here, we describe the synthesis of monodisperse CdO NCs that exhibit high quality-factor (up to 5.5) mid-infrared (MIR) localized surface plasmon resonances (LSPR) and elucidate the inverse scaling relationship between carrier concentration and NC size. The LSPR wavelength is readily tunable between 2.4 and ∼6.0 μm by controlling the size of CdO NCs. Structural and spectroscopic characterization provide strong evidence that free electrons primarily originate from self-doping due to NC surface-induced nonstoichiometry. The ability to probe and to control NC stoichiometry and intrinsic defects will pave the way toward predictive synthesis of doped NCs with desirable LSPR characteristics.