through either self-doping or extrinsic doping. This free carrier concentration determines the frequency and extinction of the plasmon resonance. Semiconductors generally have at least an order lower carrier concentration compared to metals. As a consequence of this, plasmon resonance in semiconductors appears in near infrared (NIR) or terahertz (THz) region in comparison to metals, whose surface plasma frequency lies in the UV-Vis region. Most of the semiconductors exhibiting free carrier oscillation are either metal oxides or chalcogenides. Many studies have been conducted to understand the origin, modulation, and application of plasmon resonance in these materials. One of the relatively less explored material in this regard is the non-stoichiometric copper phosphide (Cu 3-x P). Recent study by Crovetto et al reports that the bulk Cu 3-x P thin films are p-type semimetal and suggest the possibility of formation of p-type semiconducting phase in Cu 3-x P nanocrystals due quantum confinement effect in nanosize regime. Regardless of the nature of Cu 3-x P, that is whether if its a semimetal (thin film) or semiconductor (nanocrystal), this material exhibit a broad absorption peak in the NIR region. In nanoparticles, this absorption arise from localised surface plasmon resonance on contrary to assigning this as an interband transition in thin films. The origin of the plasmon resonance in Cu 3-x P nanocrystals is identical to the well studied plasmonic copper chalcogenides. The excess hole concentration generated as result of Cu vacancies induces the plasmon resonance in all these nanocrystals.