Power-dependent upconversion luminescence properties of self-sensitized Er₂WO₆ phosphor

A Yb³⁺ free self-sensitized Er₂WO₆ phosphor has been synthesized via a solid-state reaction method. The phosphor material, Er₂WO₆, has a monoclinic crystal structure with space group P2/c (13). The deconvoluted high-resolution X-ray photoelectron spectra of all the core elements in the Er₂WO₆ phosphor material were explored. The highly resolved absorption peaks in the ultra-violet, visible and near-infra-red (NIR) regions of the diffuse reflectance spectrum were due to the Stark-splitting of the 4f energy levels of the Er³⁺ ions. Under 980 nm NIR laser excitation, the Er₂WO₆ phosphor showed an intense up-converted red emission at 677 nm due to the ⁴F_9/2→⁴I_15/2 transitions of the Er³⁺ ions. The cross-relaxation and resonance energy transfer process involved in the key intermediate ⁴F_3/2 and ⁴F_5/2 levels of the Er³⁺ and their role in generating red emissions were investigated. The laser pump power versus upconversion intensity plot showed a slope with an n value <1 and the possible reasons behind this behavior were investigated. The photoluminescence properties of the Er₂WO₆ phosphor in the visible and NIR region were further analyzed. The potential application of the phosphor as a marker in latent fingerprint detection was also evaluated.