MPA-capped CdS and Cu-doped CdS NCs were synthesized by following a previous study. Specifically, 0.5 mmol of CdCl2 was dissolved in 99 mL of deionized water. For Cu-doping, CuCl2•2H2O was also added to the solution. Then 1.25 mmol of MPA was added to the solution, and the pH of the solution was adjusted to 10-11 with 2.0 M NaOH aqueous solution. After N2 bubbling for 30 min, 1.0 mL of 0.37 M Na2S aqueous solution was quickly injected into the solution at 100°C and the solution was refluxed for 4 h to form the CdS NCs. The reaction time was varied to control the particle size. The obtained CdS NCs and Cu-doped CdS NCs were precipitated by methanol, the solution was centrifuged (8500 rpm, 5 min), and then the precipitate was dried in a vacuum.