Lanthanum-doped ZnO quantum dots with greatly enhanced fluorescent quantum yield

The lanthanum-doped ZnO quantum dots (QDs) are synthesized by a modified sol-gel method under atmospheric conditions. The as-prepared quantum dots are characterized by X-ray powder diffraction, energy dispersion spectrum analysis and high resolution transmission electron microscopy. The optical properties of the products are studied by ultra-violet spectroscopy and fluorescent spectroscopy. The results show that the doped quantum dots exhibit greatly enhanced luminescent properties and their quantum yield centered around 495 nm is increased from 30.5% for un-doped ZnO QDs already improved by silane surface modification to 77.9% for La-doped ZnO QDs at a proper La-doping content, which is the highest reported so far for the green-emitting ZnO QDs. Positron annihilation spectroscopy is employed to probe the vacancy-type defects of ZnO QDs. Finally, anti-counterfeiting inks are prepared by incorporating La-doped ZnO QDs into the transparent oil and their possible potential applications are explored.