Synthesis of Li+-ion activated NaYF4: Er3+/Yb3+ phosphors via a modified solid-state process for latent fingerprint detection

Abstract Li+-ion codoped NaYF4: Er3+/Yb3+ phosphors (β-NaYF4) with a hexagonal structure were synthesized via a modified solid-state route. High-speed planetary ball milling and solid-liquid mixing were simultaneously used to overcome the drawbacks of high synthesis temperatures in conventional routes. A pure β-NaYF4 phase was obtained through calcination at 600 °C for 3 h. Increases in the codoping content of Li+ ion caused a slight shift in X-ray diffraction peak positions toward high angles owing to the distortion of the local crystal field. Field emission scanning electron microscope images showed agglomerated spherical particles of approximately 0.7 µm with narrow size distribution. The upconversion properties of β-NaYF4 codoped with Li+-ion were explored. Two emission bands in the green regions (520 nm and 545 nm) and one emission band in the red region (615 nm) were observed owing to the 2H11/2→4I15/2, 4S3/2→4I15/2, and 4F9/2→4I15/2 transitions of Er3+, respectively. Codoping with 6 mol% Li+ increased the upconversion intensity by three times, which was explained using the energy level diagram. The present phosphors with improved upconversion properties were utilized for latent fingerprint detection on smooth surfaces of regularly used polymer sheets, glass substrates, and compact discs. Using the present phosphors, the base elements with three-level features, such as sharp ridges, valleys, ridge flow, bifurcation, ridge shapes, and dots, were observed on all hydrophilic and hydrophobic surfaces. The prepared phosphors exhibited promising characteristics to detect the features of fingerprint impression for individual identification in forensic applications.