Effects of carrier density and shape on the localized surface plasmon resonances of Cu _2-xS Nanodisks

A major challenge in the synthesis of plasmonic semiconductor nanocrystals is the ability to control localized surface plasmon resonance (LSPR) properties by varying the size, shape, and carrier density of the nanocrystal. For example, copper sulfide (Cu _2-xS) nanodisks possess two distinct LSPR modes that occur in the infrared range. Here, we demonstrate that the wavelengths of these LSPR modes can be modulated by independently varying the aspect ratio of the disk and the overall carrier density of Cu _2-xS. These variables can be controlled during nanocrystal growth by carrying out thermolysis of a copper-thiolate precursor under a specific gas environment. Our results show that during thermolysis, the presence of oxygen enhances the growth rate of crystalline Cu _2-xS nanodisks and the formation of Cu vacancies that contribute to free carrier concentration. By carrying out thermolysis under a nitrogen environment, we are able to tune the aspect ratio of nanodisks independent of Cu vacancy formation. Using these methods to carefully control nanodisk size and carrier density, we demonstrate that nanodisks achieve a critical carrier density beyond which the nanocrystals undergo an irreversible phase change, placing a limit on LSPR wavelength tuning in these doped semiconductor nanocrystals.