• Structural and photophysical investigation of luminescent red color emissive ternary europium complexes have been carried out experimentally and theoretically. • The foremost purpose of investigation is to study the impact of substituted neutral ligands on optoelectronic properties of complexes. • Photoluminescence emission spectra specify that fluorinated β-diketone ligand have efficiently sensitized Eu(III) ion via antenna effect. • Red color emanating synthesized materials could found good significance in solid state electronics and OLEDs. The photophysical and optoelectronic characteristics of a series of ternary europium complexes having general formula [Eu(TFNB) 3 L] were estimated in detail. The synthesized octa coordinated complexes have different heteroaromatic ancillary ligand (L) i.e., 1,10-phenanthroline (Phen) derivatives. The synthesized heteroleptic europium complexes were thoroughly characterized by various techniques. Photoluminescence emission spectra specify that β-diketone ligand has efficiently sensitized Eu (III) ion through antenna effect which results in excellent luminescent intensity and long decay time. Most intense peak accredited to 5 D 0 → 7 F 2 is accountable for red luminescence of complexes. By investigating the PL emission data, CIE color coordinates were calculated which indicates that subsequent complexes give emission in red region. The Judd-Ofelt parameters of trivalent europium complexes were calculated and also derived theoretically via computational methods. The higher value of hypersensitive electric dipole Ω 2 intensity parameter displays the existence of highly polarizable chemical environment around central metal ion. Red light emanating materials might have found good significance in solid state electronics, flat panel devices and OLEDs. 

Influence of coordinating environment on photophysical properties of UV excited sharp red emitting material: Judd Ofelt analysis

Non-cytotoxic upconversion nanocrystals are preferred candidates because they offer exceptional advantages for numerous applications, ranging from optical thermometry to bioimaging/biomedical applications. In this report, we demonstrate the luminescence characteristics and practical utility of a multifunctional upconversion nanophosphor based on Yb3+/Er3+:La2(WO4)3 (LWO) flakes. Strong upconversion green emission was observed from 6-mol % Er3+-doped LWO nanophosphor flakes excited by a 980 nm laser. We further enhanced the upconversion emission considerably by co-doping LWO nanophosphors with Yb3+/Er3+ to exploit energy migration from Yb3+ to Er3+ ions. The exceptional improvement in upconversion green and near-infrared emission was achieved by Yb3+ ion co-doping up to 6 mol %; beyond 6 mol %, emission intensities remarkably dropped due to concentration quenching. Photometric parameters were evaluated with and without Yb3+ ion-doped LWO nanophosphors, which exhibited a high green color purity of 95.6%, to elucidate their energy transfer mechanism. In addition, temperature-dependent upconversion emission trends were evaluated by analyzing the fluorescence intensity ratio, exhibiting higher temperature sensitivity than that previously reported. This suggests the applicability of our proposed nanophosphors to optical thermometry. As for bioimaging applications, the non-cytotoxicity of the optimized nanophosphor was confirmed based on distinct fluorescence images of a normal fibroblast cell line (L929). Furthermore, we demonstrated the strong cytotoxicity of nanophosphors against human colon cancer (HCT-116) cells. Based on the results, non-cytotoxic Yb3+(6 mol %)/Er3+ (6 mol %):LWO upconversion nanophosphor flakes are expected to be exceptional candidates owing to their extensive suitability to the fields of upconversion lasers, optical thermometry, and biomedical and anticancer applications. The results indicate the potential of upconversion materials in the effective execution of multiple strategic applications.

Biocompatible Yb3+/Er3+ Co-activated La2(WO4)3 Upconversion Nanophosphors for Optical Thermometry, Biofluorescent, and Anticancer Agents.

The present article reports the optical absorption and upconversion (UC) studies of 1.0 mol% Er3+/2.0 mol% Yb3+ doped/codoped glasses prepared by melt-quenching technique. The elements present and the composition of the prepared glass have been confirmed from XPS and XRF analysis respectively. Judd-Ofelt intensity parameters have been calculated using the absorption spectrum which is further utilized to predict the nature of Er_O bond, the transition probabilities, branching ratios and radiative lifetimes. The CIE study shows non-colour tunable and highly pure green emission (94.2%). The temperature-dependent UC emission spectra of the 2.0 mol% Yb3+ sensitized glass have been recorded at three different pump power densities to establish a reliable FIR based temperature scale. Furthermore, the Arrhenius fitting of the temperature-dependent spectra reveals low thermal quenching of green luminescence in the codoped glass.

Fluorescence intensity ratio technique and its reliability

The ability of rare-earth-doped ferroelectric oxides to achieve outstanding upconversion (UC) performances under NIR irradiation despite possessing intrinsic electric properties drives researchers all over the globe to work in this field. The structural and spectroscopic characteristics of the Bi4Ti3O12 phosphor integrated with Er3+, Yb3+, and Al3+ have been thoroughly investigated in this study. The considerable increase in UC emission ∼three times caused by the addition of Al3+ ions has been observed and discussed. The processes connected with the UC emission related to the pump power variation have been realized using the rate law equation. Aside from having high sensitivity of 0.011 K−1 at room temperature, the prepared phosphor possesses excellent thermal stability, i.e., it retains ∼73% of its initial intensity with the addition of Al3+ ions.

Tailoring of upconversion luminescence of Al3+ engineered titanate phosphor for non-invasive thermometry

Influence of Fe2O3 content on the optical features and radiation shielding efficiency of CaO-Na2O-B2O3 glass system

This work mainly reports the optical thermometry based on frequency upconversion and the infrared luminescence of Er3+-Yb3+ codoped TeO2-B2O3 (TBO) glasses prepared via melt-quenching technique. The amorphous nature of the prepared glass samples has been confirmed from the XRD analysis. FTIR analysis of the prepared glasses shows the existence of possible bands along with the phonon energy ∼670 cm−1. Judd-Ofelt analysis has been performed on the basis of absorption spectra and various radiative parameters calculated. The order of the intensity parameters (Ω6 > Ω2 > Ω4) indicates ionic nature of the Er-O bond which is further confirmed by the naphelauxetic ratio, covalency and bonding parameter. The enhancement of about 155 times and 327 times in the green and red UC emission intensity of the codoped glass as compared to the Er3+ doped TBO glass has been reported. The absorption cross-section around 980 nm in the codoped glass is found to be ∼43 times larger as compared to the 4I15/2 → 4I11/2 absorption transition in the Er3+ doped glass. The temperature sensing study of the codoped glass with high ‘Ω6’ value has been performed and low value of the absolute sensitivity has been explained on the basis of Judd-Ofelt parameters. The figure of merit for gain bandwidth (9216.81 × 10−27 cm3) & maximum gain coefficient (2.27 cm−1) corresponding to the infrared luminescence peaking at ∼1.5 μm and spectroscopic quality factor (0.28) have been evaluated.

Optical thermometry and broad infrared luminescence in highly sensitized TBO glass

This research article reports the non-contact optical thermometric study using the UC emission intensity ratio of thermally and non-thermally coupled levels. Tm3+/Er3+/Yb3+ codoped TeO2-WO3-Bi2O3 (TWBi) glass have been synthesized using melt-quenching technique. After analyzing the optical absorption and UC emission spectra at room temperature, the temperature-dependent study of UC emission spectra has been performed in the 303–523 K range. First, the optical temperature sensing using the luminescence intensity ratio of thermally coupled levels of Er3+ ion has been studied. The maximum value of absolute and relative sensitivities has been found to be ∼0.0088 K−1 at 483 K and ∼0.0120 K−1 at 303 K, respectively. Furthermore, the luminescence intensity ratio approach has been employed to study non-contact temperature sensing in non-thermally coupled levels. While employing the ratiometric approach involving non-thermally coupled levels, a simultaneous improvement in absolute and relative sensitivities has been witnessed. There, the maximum value of absolute and relative sensitivities has been found as high as ∼0.7143 K−1 at 523 K and ∼0.0249 K−1 at 483 K, respectively. It has been found that the relative sensitivity in the case of non-thermally coupled levels (NTCLs) clearly dominates the relative sensitivity of TCLs.

Non-contact optical thermometry via non-thermally coupled levels in upconverting glass

Bismuth modified Er3+/Yb3+: TeO2-WO3 (TWBi) glass system containing silver metallic nanoparticles have been synthesized via melt-quenching method. Optical absorption spectra have been used for optical bandgap and Judd-Ofelt analysis. Judd-Ofelt intensity parameters show ionic type of bonding between the activator and ligand ions. The HR-TEM analysis of the glass containing silver confirms the presence of silver metallic nano-particles (AgNPs) with average particle size ∼ 42 nm at 30 h heat-treatment. The improvement observed in the UC emission intensity of Er3+ ions has been elucidated by considering the energy transfer processes and crystal field modification. Temperature sensing ability and luminescence stability of Er3+/Yb3+: TWBi glass have been explored with and without the incorporation of AgNPs. It has been found that the upconverting glass with AgNPs have high absolute sensitivity, relative sensitivity and luminescence stability with respect to temperature as compared to the upconverting glass without AgNPs. 

Vishab Kesarwani

Papers

Optical thermometry and broad infrared luminescence in highly sensitized TBO glass

Fluorescence intensity ratio technique and its reliability

Non-contact optical thermometry via non-thermally coupled levels in upconverting glass

Impact of AgNPs on the optical thermometry and stability of bismuth modified tellurium-tungstate upconverting glass

Blue frequency upconversion emission and optical thermometry of Ca3(PO4)2: Tm3+/Yb3+