Narrowband Far-Red Emitting Sr ₂ ZnAl ₂₂ O ₃₆ :Mn ⁴⁺ Spinel Phosphor for Integrated Horticultural LED, Optical Thermometry, and Forensic Fingerprint Imaging

Abstract

A fundamental trade-off between photophysical performance and host stability continues to constrain practical deployment of Mn 4+ -doped phosphors, even as their narrow-band far-red emission (650–730 nm) attracts growing interest across lighting, sensing, and security applications. Fluoride hosts such as K 2 SiF 6:Mn 4+ achieve IQE > 90%, yet moisture-induced hydrolytic degradation remains a critical reliability barrier. Oxide alternatives, though thermally stable, are encumbered by broad spectral output (fwhm > 50 nm), incomplete octahedral site occupancy, accelerated nonradiative decay, and suppressed IQE below 50%. Moreover, phosphors simultaneously satisfying thermometric relative sensitivity ( S r ) > 0.3% K –1 and Michelson contrast >0.8 in latent fingerprint (LFP) detection have not been demonstrated within a single composition. To address these issues, Mn 4+ -activated Sr 2 ZnAl 22 O 36 (SZAO:Mn 4+ ) was synthesized via controlled solid-state calcination (1400 °C, 6 h), yielding phase-pure material (>99%) with Mn 4+ preferentially substituting distorted AlO 6 octahedral sites. UV-blue excitation produces narrow-band far-red emission at 657 nm (IQE ∼ 63%), attributed to spin-forbidden 2 E g → 4 A 2g transitions. Spectral congruence with phytochrome P r and P fr absorption supports application in phosphor-converted LEDs for controlled environment crop production. Thermally resolved photoluminescence measurements yield a quenching activation energy ( E a ) of 0.61 eV, and thermoluminescence glow-curve deconvolution confirms multiple discrete trap-state populations modulating carrier recombination kinetics. Fluorescence intensity ratio analysis affords noncontact optical thermometry at S r of 0.50% K –1 . Under UV excitation, SZAO:Mn 4+ resolves level 3 ridge minutiae across chemically and texturally diverse substrates (Michelson contrast = 0.92), demonstrating forensic-grade LFP discrimination. Taken together, these findings position SZAO:Mn 4+ as a structurally robust, host-versatile platform integrating far-red phosphor conversion, luminescence thermometry, and LFP detection.