Comparative investigation on carbon dots and chloride fluxes modified ZnAl2O4:Cr3+ nanophosphors for temperature sensing, cutting-edge forensic, anti-counterfeiting and w-LEDs applications

A novel Cr³⁺ doped ZnAl₂O₄ is synthesized by grafting carbon dots (CDs) and chloride fluxes (NaCl, KCl, NH₄Cl) through the solid-state reaction method. Upon excitation at 530 nm wavelength, the Cr³⁺ ions in the ZAO NPs emit red light due to the influence of a strong crystal field. This red emission arises from the electric dipole ²E_g→⁴A_2g of the Cr³⁺ ions. Among the chloride fluxes examined, NH₄Cl had a significant impact on the PL intensity. Notably, when CDs are incorporated into the ZAO:7Cr³⁺/NH₄Cl NPs, a remarkable increase of 17.25-folds in photoluminescence (PL) intensity is observed. This enhancement is substantially higher than that of CDs alone (11.23-folds) and NH4Cl alone (3.82-folds). The increased PL intensity is attributed to the Förster resonance energy transfer (FRET) mechanism. Furthermore, the addition of CDs (5 wt.%) enhanced the colour purity (CP) of ZAO:7Cr³⁺/NH₄Cl NCs to 99.8%, achieving a high Internal quantum efficiency (I_QE) of 93.4 %. Notably, the CDs (5 wt.%)@ZAO:7Cr³⁺/NH₄Cl NCs maintained 92.43 % of their emission intensity even at 420 K, demonstrating exceptional thermal stability compared to ZAO:7Cr³⁺ NPs. Moreover, the NPs holds promise for optical thermometry, boasting a high relative sensitivity (S_r) of 6.74 × 10^–2 %K^-1 across a temperature range spanning from 300 to 480 K. Moreover, the latent fingerprints (LFPs) developed using CDs(5 wt.%)@ZAO:7Cr³⁺/NH₄Cl NCs demonstrate remarkable selectivity and high contrast. Under UV irradiation, the structural features of LFPs at levels I–III are clearly visible. In addition, the strong red fluorescence emitted by CDs(5 wt.%)@ZAO:7Cr³⁺/NH₄Cl NCs makes them suitable for applications in AC. CDs(5 wt.%)@ZAO:7Cr³⁺ NCs exhibit significant potential for w-LED fabrication, achieving a correlated colour temperature (CCT) of 5517 K, a CIE coordinate of (0.332, 0.331), and a high colour rendering index (CRI) of R_a = 94, outperforming ZAO:7Cr³⁺/NH₄Cl NPs. Overall results clearly demonstrates that, CDs(5 wt.%)@ZAO:7Cr³⁺/NH₄Cl NCs are effective for applications in optical thermometry, dactyloscopy, w-LEDs, and AC.