Synthesis and Optical Properties of Europium‐Doped ZnS: Long‐Lasting Phosphorescence from Aligned Nanowires

Quasi‐aligned Eu 2+ ‐doped wurtzite ZnS nanowires on Au‐coated Si wafers have been successfully synthesized by a vapor deposition method under a weakly reducing atmosphere. Compared with the undoped counterpart, incorporation of the dopant gives a modulated composition and crystal structure, which leads to a preferred growth of the nanowires along the [01 $\bar 1$ 0] direction and a high density of defects in the nanowire hosts. The ion doping causes intense fluorescence and persistent phosphorescence in ZnS nanowires. The dopant Eu 2+ ions form an isoelectronic acceptor level and yield a high density of bound excitons, which contribute to the appearance of the radiative recombination emission of the bound excitons and resonant Raman scattering at higher pumping intensity. Co‐dopant Cl – ions can serve not only as donors, producing a donor–acceptor pair transition with the Eu 2+ acceptor level, but can also form trap levels together with other defects, capture the photoionization electrons of Eu 2+ , and yield long‐lasting (about 4 min), green phosphorescence. With decreasing synthesis time, the existence of more surface states in the nanowires forms a higher density of trap centers and changes the crystal‐field strength around Eu 2+ . As a result, not only have an enhanced Eu 2+ 4f 6 5d 1 –4f 7 intra‐ion transition and a prolonged afterglow time been more effectively observed (by decreasing the nanowires' diameters), but also the Eu 2+ related emissions are shifted to shorter wavelengths.