XRD patterns were recorded by an X-ray diffractometer (Rigaku Ultima IV). Organic functional groups on the surface of NCs were analyzed by an FTIR spectrometer (HORIBA FT-720 combined with an attenuated total reflection setup, resolution: 2 cm -1 ) and JASCO FT/IR-6100. Steady-state absorption spectra were measured using UV-3600 (SHIMADZU), and the emission spectra were measured using FP-6500 (JASCO) and RF-6000 (SHIMADZU). X-ray fluorescence (XRF) measurements were performed using Supermini200 (Rigaku).