For colloidal nanocrystals, the stabilising ligands can tune the carrier density on the nanocrystal. Magnitude of this tuning depends on the strength of the bonding interaction of donor moiety on the ligand. To investigate the effect of ligation on the plasmon resonance , three different types ligands were added to the system. In addition, the alkyl chain length of these ligands (octadecyl group) was kept similar so that refractive index changes around the nanocrystal is relatively invariable and similar electronic effect on the binding groups. It should be noted that the as synthesized nanocrystals were passivated by oleylamine. Figure shows the changes in plasmon resonance following the addition of equimolar concentration of 1-octadecylphosphonic acid, 1-octadecanethiol, and stearic acid to the Cu 3-x P nanocrystals dispersed in tetrachloroethylene. From the plots, it's clear that all ligands resulted in the blue shift of the spectra hinting at the withdrawal of electron density from the nanocrystal. In the case of ligands with oxygen donors, the strong electron-withdrawing phosphonate group showed a relatively large shift (35 nm) in comparison to the carboxylate group (10 nm) as shown in Figure a,b. In the case of thiol treatment, there was a 40 nm blueshift of the plasmon band with simultaneous increment in the extinction (Figure ). Interestingly, ligation of the thiol group has been shown to redshift the plasmon band in copper chalcogenide nanocrystals. Being a well known hole acceptor (depending on the band alignment), one would expect thiols to induce a similar redshift of plasmon in Cu 3-x P. Exposure to thiol induced a change in color of the dark Cu 3-x P solution to nearly transparent as well the for-mation of a white precipitate when treatment was done in hexane. The former observation is a clear indication of the dissolution of Cu 3-x P and the latter is the by-product of this dissolution process (Copper(I) thiolate complex). This is very similar to the etching of Cu nanoparticles by thiol resulting in the white precipitate. In addition, etching of Cu 2-x S was observed following treatment of phosphines with a redshift in the plasmon band. This arise as a result of the abstraction of sulfur from the lattice. Based on these, we confirm that the observed blueshift is a consequence of the etching of Cu Compared to ligand treatment, a more reversible method for tuning the plasmon band is through electrochemical charging. Electrochemical tuning of carrier concentration and simultaneous monitoring of plasmon resonance were carried out using a spectroelectrochemical setup. Intercalation of Li + ions into Cu 3-x P lattice and subsequent transformation has been reported in the literature. We expect such a shift in the plasmon band could be due to such adsorption/pre-intercalation event on the surface . Hence nanocrystal charging was carried out in a more robust TBAPF 6 electrolyte medium. Holes (electrons) were injected by applying an anodic (cathodic) potential. Injecting holes should increase the hole concentration and results in a blueshift of the plasmon peak. Opposite effect is expected while injecting electron. CV and corresponding plasmon spectra. In, Figure , we have also plotted the peak position vs the potential. As the electrode is swept in the anodic direction (0 V to 1.5 V ), there was a well-defined oxidation peak around 1 V in the forward scan with the onset of blueshift of the plasmon peak. The correlation of data in Figure a & c confirms that the oxidation peak in CV indeed corresponds to the oxidation of Cu + in Cu 3-x P. Upon reversal of this scan (1.5 V to 0 V), no voltammetric peaks were observed. However, the spectra continue to blueshift. This suggests that hole injection still proceeds in this regime but at a relatively slow hole transfer kinetics as evidenced by the low anodic current. At the end of the sweep on the anodic side (0 V to 1.5 V and then back to 0 V), the plasmon peak shifted about 85 nm (64 meV). Similarly, upon sweeping in forward cathodic direction (0 V to -1.5 V), the plasmon peak started to redshift, and a negative current was measured. A small and less pronounced peak was also observed in the voltammogram during this scan. Interestingly, the plasmon peak position remains unchanged during this peak potential. This suggests that this small CV peak should be originating from some other charge transfer event (Probably reduction of trace oxygen) which not related to the plasmon shift . However, raising the electrode Fermi level beyond -1 V, there was increasing the cathodic current and redshift in the plasmon peak. Overall, there is a redshift of 40 nm during the entire cathodic sweep (0 to -1.5 V and then back to 0 V). By the end of the CV cycle, the plasmon peak didn't recover to its initial position. There is a hysteresis associated with the plasmon position during the reductive charge transfer. In the following discussion, we provide a possible explanation for this behaviour. Despite this, the potential sweep demonstrates the tuning of plasmon resonance by applied potential.