To check the reversibility of plasmon modulation and long-term stability of these nanocrystals under charging, alternating cathodic and anodic potential steps were applied. The long equilibration time for the potential step (300 s) ensures a steady state population of charged Cu 3-x P nanocrystals, unlike relatively fast CV. Figure a shows the plasmon spectra during two cycles of cathodic (-1.5 V) and anodic (1.5 V) steps. During reduction, the plasmon peak redshifted ≈ 40 nm, and extinction decreased. Oxidation following this induced the opposite effect on plasmon, i.e., blueshift and increment in the extinction. Here also, the plasmon shift during the anodic potential (oxidation) was larger than during the cathodic potential (reduction). This along with the hysteresis during CV arises as a result of higher Cu + concentration available for oxidation in comparison to the Cu 2+ available for reduction in the nanocrystal, which agrees with the non-stoichiometric nature of Cu 3-x P lattice.