Semiconductor nanocrystals are key materials for achieving localized surface plasmon resonance (LSPR) excitation in the extended spectral ranges beyond visible light, which are critical wavelengths for chemical sensing, infrared detection, and telecommunications. Unlike metal nanoparticles which are already widely exploited in plasmonics, little is known about the near-field behavior of semiconductor nanocrystals. Near-field interactions are expected to vary greatly with nanocrystal carrier density and mobility, in addition to properties such as nanocrystal size, shape, and composition. Here we demonstrate near-field coupling between anisotropic disk-shaped nanocrystals composed of Cu2-xS, a degenerately doped semiconductor whose electronic properties can be modulated by Cu content. Assembling colloidal nanocrystals into mono- and multilayer films generates dipole-dipole LSPR coupling between neighboring nanodisks. We investigate nanodisks of varying crystal phases (Cu1.96S, Cu7.2S4, and CuS) and find that nanodisk orientation produces a dramatic change in the magnitude and polarization direction of the localized field generated by LSPR excitation. This study demonstrates the potential of semiconductor nanocrystals for the realization of low-cost, active, and tunable building blocks for infrared plasmonics and for the investigation of light-matter interactions at the nanoscale.

Nanocrystals composed of CuS that exhibit semimetallic behavior are capable of supporting localized surface plasmon resonances in the near-infrared wavelengths. A major challenge in utilizing these nanocrystals for plasmonic applications is the ability to accurately control their nanoscale morphology and chemical composition, both of which are known to affect plasmon wavelength and amplitude. Here, we investigate the important role of halide ions in controlling the solvothermal synthesis of colloidal CuS nanocrystals. When oleylamine is used as a stabilizing surfactant, we find that the addition of halide ions to the reaction mixture instigates nanocrystal shape focusing in a process similar to Ostwald ripening, from nanodisks to faceted triangular prisms. We demonstrate that this shape focusing is likely to occur from the competition between CuS surface binding interactions with oleylamine ligands and halide ions and is the strongest for I − ions. We also demonstrate that this shape focusing, in combination with seedmediated growth, can be used to generate colloidal dispersions of CuS nanoprisms with narrow size distributions.

Copper sulfide nanocrystals support localized surface plasmon resonances in the near-infrared wavelengths and have significant potential as active plasmonic nanomaterials due to the tunability of this optical response. While numerous strategies exist for synthesizing copper sulfide nanocrystals, few methods result in nanocrystals with both controlled morphological shapes and crystallinity. Here, we synthesize and characterize ultrathin (<5 nm) CuS nanosheets that are formed by solventless thermolysis, utilizing Cu alkanethiolates as single-source precursors. Layered Cu alkanethiolate precursors adopt a highly ordered structure which can be further stabilized in the presence of Cl and also serve to template the formation of nanosheets. We show that, in the absence of Cl, only isotropic and disk-like CuS nanocrystals form. These findings offer further insight into the use of layered metal-organic single-source precursors as templates for anisotropic nanocrystal growth.