Multicolor emission of CdNb₂O₆:Eu³⁺, Er³⁺ and CdNb₂O₆:Eu³⁺, Er³⁺, Yb³⁺ phosphors with dual excitation and dual emission in UV to IR for latent fingerprint applications

Rare earth (RE³⁺)-doped inorganic oxides are promising materials for advanced photonic applications due to their remarkable optical properties and multicolor emission capabilities under ultraviolet (UV) or infrared (IR) excitation. These properties make them suitable for use in displays, white light-emitting diodes (LEDs), solar energy devices, and biological labeling. In this study, a series of CdNb₂O₆-based phosphors doped with Eu³⁺, Er³⁺, and Yb³⁺ ions—specifically CdNb₂O₆:xEu³⁺, yEr³⁺ and CdNb₂O₆:xEu³⁺, yEr³⁺, zYb³⁺ (x = 0.5–5 mol%, y = 2 mol%, z = 2 mol%)—were successfully synthesized using the molten salt method. X-ray diffraction (XRD) analysis confirmed a single-phase orthorhombic columbite structure in all compositions up to 5 mol% Eu³⁺ doping. Scanning electron microscopy (SEM) revealed a noticeable reduction in grain size with increased Eu³⁺ concentration, especially in the Yb³⁺ co-doped samples. Photoluminescence analysis showed strong red and green emissions from Eu³⁺ and Er³⁺, respectively, under 464 nm and 378 nm excitation wavelengths. An increase in Eu³⁺ content enhanced red emission while diminishing green emission, indicating tunable chromatic output. Color coordinates (CIE) under UV light further supported the emission adjustability from green to red. Additionally, enhanced ⁴I_13/2 transitions in Yb³⁺-co-doped samples indicated effective energy absorption at the ⁴I_11/2 level near 980 nm, highlighting the host's suitability for IR-responsive applications. Notably, the synthesized phosphors demonstrated strong potential for latent fingerprint (LFP) detection, enabling clear visualization of intricate fingerprint patterns essential for forensic investigations. These findings underscore the multifunctionality and practical relevance of CdNb₂O₆-based RE³⁺-doped phosphors in optical and security-related technologies.