High-quality violet-blue emitting ZnxCd1-xS/ZnS core/shell quantum dots (QDs) are synthesized by a new method, called nucleation at low temperature/shell growth at high temperature. The resulting nearly monodisperse ZnxCd1-xS/ZnS core/shell QDs have high PL quantum yield (near to 100%), high color purity (FWHM) <25 nm), good color tunability in the violet-blue optical window from 400 to 470 nm, and good chemical/photochemical stability. More importantly, the new well-established protocols are easy to apply to large-scale synthesis; around 37 g ZnxCd1-xS/ZnS core/shell QDs can be easily synthesized in one batch reaction. Highly efficient deep-blue quantum dot-based light-emitting diodes (QD-LEDs) are demonstrated by employing the ZnxCd1-xS/ZnS core/shell QDs as emitters. The bright and efficient QD-LEDs show a maximum luminance up to 4100 cd m(-2), and peak external quantum efficiency (EQE) of 3.8%, corresponding to 1.13 cd A(-1) in luminous efficiency. Such high value of the peak EQE can be comparable with OLED technology. These results signify a remarkable progress, not only in the synthesis of high-quality QDs but also in QD-LEDs that offer a practicle platform for the realization of QD-based violet-blue display and lighting.