The average diameters of CdS NCs were estimated based on the excitonic absorption peak in the steady-state absorption spectra, while those of Cu-doped CdS NCs were estimated by the peak width of XRD patterns. The atomic ratio of Cu per Cd ranged from 0-12%, as confirmed by X-ray fluorescence spectroscopy. Cu doping did not introduce XRD peaks related to impurities or optical absorption tails in the longer wavelength, indicating the absence of the CuxS domain. The synthesized aqueous solutions of MPA-capped CdS and Cu-CdS NCs exhibited absorption below 450-500 nm, depending on the size and composition (Supplementary Fig. ). Cu doping slightly shifted the absorption spectra to a longer wavelength, probably because of the formation of midgap states dominated by Cu(3d) above the valence band.