Strongly Fluorescent Blue-Emitting La2O3: Bi3+ Phosphor for Latent Fingerprint Detection

Blue-emitting bismuth-doped lanthanum oxide (La₂O₃: Bi³⁺) with various concentrations of Bi was synthesized using the sol-gel combustion method and used for visualization of latent fingerprints (LFPs). An X-ray diffraction (XRD) study revealed the hexagonal structure of the phosphors and total incorporation of the bismuth in the La₂O₃ matrix. Field Emission Scanning Electron Microscopy (FE-SEM) and Fourier Transform Infrared Spectroscopy (FTIR) were used to study the morphology and the relative vibrations of the synthesized samples. Photoluminescence (PL) studies showed strong blue emission around 460 nm due to the ³P₁ → ¹S₀ transition. Clear bright-blue fingerprint images were obtained with the powder dusting method on various surfaces like aluminum, compact discs, glass, wood and marble. A first evaluation of these images indicated a clear visualization of all three levels of details and a very high contrast ranging from 0.41 on marble to 0.90 on aluminum. As a further step, we used an algorithm for extracting fingerprint minutiae with which we succeeded in detecting all three levels of fingerprint details and even the most difficult ones, like open and closed pores. According to these analyses, La₂O₃: Bi phosphor is demonstrated to be an effective blue fluorescent powder for excellent visualization of latent fingerprints.