Fabrication of assembled-spheres YVO4:(Ln3+, Bi3+) towards optically tunable emission

This work initiated a study on the preparation of assembled-spheres YVO4:(Ln(3+), Bi3+) (Ln = Eu, Sm, Dy, Ho), in which Bi3+ co-doping of a lower content, aims to achieve emission tailored for many applications by supressing the serious changes in host structures when higher concentrations of Bi3+ are involved. The formation of assembled-spheres is beneficial from the simultaneous dissolution of the starting Bi and Ln salts with the use of ethylene glycol. As a result, all assembled-spheres are featured by low surface absorbents and little interior lattice defects, which decreased the nonradiative rate to give a luminescence performance superior to other morphologies like cereal-like architectures. Further, four paths were applied to achieve the tunable multicolour-emission in these assembled-spheres, which include: (i) selecting the dopant Ln(3+) with characteristic emissions; (ii) varying the concentration of dopant Ln(3+); (iii) co-doping Ln(3+)/Bi3+ with dual emissions from Bi3+ and Ln(3+); and (iv) adjusting the excitation wavelength. These tunable multicolour-emissions were demonstrated to be the consequence of the energy transfer process from the VO43- group to Ln(3+) in YVO4:Ln(3+) or those from Bi3+ and the VO43- group to Ln(3+) in the presence of lower Bi3+ content, as followed by a surprising red-shift of the absorption edge and extension of the luminescence lifetimes of Ln(3+). The diverse approaches found in this work for emission tailoring are fundamentally important and may provide a general way to achieve multicolour-tunable emission for many applications.