In summary, this work investigates the tunability of the broad infrared peak in Cu 3-x P nanocrystals which is attributed as the plasmon resonance of holes. The frequency shift of infrared peak in accordance with the refractive index of the solvent medium do confirms the dominant plasmonic nature of this peak. Therefore, the hole concentration was used as a knob to tune the plasmon peak. Experiments suggest that post synthetic ligands treatment indeed modulate the plasmon frequency not only by mere coordination to the surface but also etching the surface. This was evident from the decomposition of Cu during the oxidation and from Cu 2+ to Cu + to Cu 0 in the case of reduction . We further extended this approach for tuning the carrier concentration by applying electrochemical bias where anodic potential increased the hole carrier density and blueshifted the plasmon peak.