Cd-free core–shell nanocrystals (ZnSe_1−xTe_x/ZnS, 0⩽x⩽0.5) emitting in the pure blue region were prepared using an aqueous colloidal method followed by post-preparative ultraviolet irradiation. The photoluminescence (PL) efficiency and peak wavelength were maximized (40%, 448 nm) through a heat treatment of the nanocrystals during irradiation in a Zn²⁺-containing thiol solution. Because of the small lattice mismatch between ZnSe_1−xTe_x and ZnS, post-preparative irradiation at high temperature resulted in the formation of a thick ZnS shell (ca. 1.6 nm) around the core (ca. 2.2 nm in diameter) without deterioration of PL efficiency. Surface substitution of Te by S and size-selective precipitation of nanocrystals before ultraviolet irradiation resulted in intense PL. Quantum mechanical calculations show that the wave function of the electron of the exciton in the Te-containing core extends well into the ZnS shell. The calculations also reveal that a thick shell can confine the electrons inside the particles and thereby improve the PL efficiency and stability against the pH of the solution. Nanocrystals that had been post-preparatively irradiated showed good stability in solution and in a glass matrix even after months of storage in air.