Browsing by Author "P.K. Vishwakarma"
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PublicationArticle Confidence intervals for the reliability characteristics via different estimation methods for the power Lindley model(Gnedenko Forum, 2022) Abhimanyu S. Yadav; P.K. Vishwakarma; H.S. Bakouch; Upendra Kumar; S. ChauhanIn this article, classical and Bayes interval estimation procedures have been discussed for the reliability characteristics, namely mean time to system failure, reliability function, and hazard function for the power Lindley model and its special case. In the classical part, maximum likelihood estimation, maximum product spacing estimation are discussed to estimate the reliability characteristics. Since the computation of the exact confidence intervals for the reliability characteristics is not directly possible, then, using the large sample theory, the asymptotic confidence interval is constructed using the above-mentioned classical estimation methods. Further, the bootstrap (standard-boot, percentile-boot, students t-boot) confidence intervals are also obtained. Next, Bayes estimators are derived with a gamma prior using squared error loss function and linex loss function. The Bayes credible intervals for the same characteristics are constructed using simulated posterior samples. The obtained estimators are evaluated by the Monte Carlo simulation study in terms of mean square error, average width, and coverage probabilities. A real-life example has also been illustrated for the application purpose. © 2022 Reliability: Theory and Applications. All rights reserved.PublicationArticle Engineered ultra-luminous La/YPO4:Eu3+ nanophosphors for advanced security ink and latent fingerprint (LFPs) detection(Elsevier Ltd, 2024) Prince Kumar; P.K. Vishwakarma; S.B. Rai; A. BahadurDesigning versatile NPs for multifarious applications is crucial for achieving an array of criterion in realm, including improving efficiency, inexpensiveness, flexibility, least power consumption, etc. Facile hydrothermal treatment of La/YPO4:Eu3+ NPs yields composite phase growth signifying even-sized NPs with the requisite elemental composition. Relative CTB predominates in LaPO4:Eu3+; while luminescence of YPO4:Eu3+ is notably higher when stimulated by atomic lines. Nevertheless, La/YPO4:Eu3+ NPs exhibit adequate luminescence under both CTB and 394 nm (7F0→5L6) excitations, evincing intrinsic QY (63.91 %) with relatively large lifetime (∼6 ms) of 5D0 states, substantiated by J-O analysis. Designed NPs is readily cling to friction ridge skin residues demonstrating the efficient LFPs development through the ‘powder dusting approach’ on various substrate. NPs conjugated PVC gold medium and aqueous PVA were effectively used for security encryptions using the screen-printing technique. Reddish-orange emission with different excitation probabilities resulting in high QY emphasizes them in a class of futuristic versatile materials with potential applications in security ink, LFP development, LEDs, and display devices. © 2024 Elsevier B.V.PublicationArticle Enhanced green up/down-conversion emissions through phase transformation in Ho3+/Yb3+ co-doped Y2O3:ZrO2 phosphors in presence of Na+ ions(Elsevier B.V., 2023) P.K. Vishwakarma; S.B. Rai; A. BahadurThis study reports the intense green upconversion and downconversion emissions from Ho3+/Yb3+/Na+ co-doped Y2O3:ZrO2 phosphor prepared by traditional solid-state reaction technique for fixed concentration of Ho3+/Yb3+ ions and different concentrations of Na+ ion. The phosphor samples exhibit a multiphase crystalline structure in the absence of Na+ ions. However, with co-doping of Na+ ions (at 20 mol%), it almost turns into Y4Zr3O12 with single rhombohedral phase. When Na + ions are present in the sample, the SEM image reveals a growth in particle size. The UV–Vis–NIR measurements show an enhancement in the absorbance of doped REs ions (Ho3+/Yb3+) on Na+ co-doping. The Ho3+/Yb3+ co-doped Y2O3:ZrO2 phosphor shows an intense green upconversion (UC) emission at 549 nm due to Ho3+ ions on the excitation with 980 nm radiation. In the presence of Na + ions, a modification of the crystallinity, growth in crystallite size (particle size) and an increase in the asymmetry of the crystalline structure around the REs ions led to an enhancement in the intensity of the UC emission almost about ∼8.5 times. The Log-Log plot demonstrates the involvement of two photons in the UC process. The lifetime of the 5F4/5S2 level decreases with increasing the concentration of Na + ion. Thus, the prepared phosphor could be an excellent candidate for source of green light, finger printing, optoelectronic devices etc. © 2023PublicationArticle Eu3+ activated gadolinium oxide quantum dots: Biocompatible red luminescent phosphor for photonic, latent fingerprint imaging, and anticancer therapy(Elsevier Inc., 2024) K. Naveen Kumar; L. Vijayalakshmi; P.K. Vishwakarma; K. Saijyothi; Jiseok Lim; Masoom Raza SiddiquiIn this study, we employed a straightforward chemical reduction method to synthesize Gd2O3 quantum dots (QDs) doped with Eu3+ ions, yielding a remarkable red emission. X-ray diffraction (XRD) analysis confirmed the cubic structure of both pristine and Eu3+ doped Gd2O3 QDs. Characterization of particle size and elemental composition was accomplished through Transmission Electron Microscope (TEM) and X-ray photoelectron spectroscopy (XPS), respectively. Photoluminescence analysis under UV excitation revealed a blue emission at 436 nm for the undoped Gd2O3 QDs. Furthermore, room temperature photoluminescence analysis was conducted on the undoped Gd2O3 quantum dots (QDs) with a UV excitation source. While Gd2O3 QDs emitted a blue light at 436 nm under 396 nm illumination, the Eu3+ doped Gd2O3 QDs exhibited a striking red luminescence under the same 394 nm wavelength. The red luminescence was notably enhanced in Gd2O3 QDs containing 0.5 mol% Eu3+ ions, although concentration quenching was observed when the concentration exceeded this level. A transfer of energy from pristine Gd2O3 QDs to Eu3+ ions was observed, leading to color purity exceeding 80.8% in the optimized sample. Notably, the 0.5 mol% Eu3+-doped Gd2O3 QD phosphor displayed non-cytotoxicity on C2C12 muscle myoblast cells while demonstrating remarkable cytotoxicity against MDA-MB-231 human breast cancer cells. This versatile material holds great promise for applications in latent fingerprinting, security coding, labeling, anticancer therapies, bio-imaging, and the advancement of lanthanide-doped QD phosphors for a wide range of photonic and biomedical applications. © 2024 Elsevier Inc.PublicationArticle Europium-Doped lanthanum oxide quantum dots: A promising quantum dots for latent fingerprint detection and photonic applications with remarkable red luminescence and biocompatibility(Elsevier Ltd, 2024) K. Naveen Kumar; L. Vijayalakshmi; P.K. Vishwakarma; K. Saijyothi; Jiseok Lim; Masoom Raza SiddiquiWe employed a facile chemical reduction method to synthesize La2O3 quantum dots (QDs) doped with Eu3+ ions, resulting in a vivid red emission. X-ray diffraction (XRD) confirmed a cubic structure for both undoped and Eu3+ ions-doped La2O3 QDs. Transmission Electron Microscope (TEM) and X-ray photoelectron spectroscopy (XPS) assessed particle size and elemental composition, respectively. Photoluminescence analysis revealed blue emission at 432 nm for undoped La2O3 QDs under UV excitation and enhanced red luminescence for Eu3+-doped QDs, peaking at 394 nm excitation with an optimal Eu3+ ion content of 0.5 mol%. Colorimetric analysis demonstrated over 90 % color purity for the optimized sample. Lifetime studies probed red emission dynamics. The optimized QDs were employed for latent fingerprint (LFP) detection via powder dusting, showing high sensitivity, selectivity, and reliability. Cytotoxicity testing on mouse muscle myoblast cell lines (C2C12) confirmed noncytotoxicity and biocompatibility, endorsing their potential in various applications like latent fingerprinting, bioimaging, labeling, and security coding, thereby advancing lanthanide-doped QD phosphor development. © 2024 Elsevier LtdPublicationArticle Intense red and green emissions from Ho3+/Yb3+ co-doped Sodium Gadolinium Molybdate Nano-phosphor: Effect of calcination temperature and Intrinsic optical bistability(Elsevier Ltd, 2021) P.K. Vishwakarma; S.B. Rai; A. BahadurWe present the structural and luminescent properties of Ho3+/Yb3+ co-doped Gd0.5Na0.5MoO4 nano-phosphor synthesized through the facile co-precipitation method at different calcination temperatures. The X-ray diffraction (XRD) patterns of the samples show the pure tetragonal phase formation. The upconversion (UC) spectra recorded on excitation with 980 nm laser show the intense emission bands at 545 nm (green) and 659 nm (red) in which red emission is more intense than green emission. The intense red is suitable for bio-imaging. Under 452 nm excitation, the intense green downconversion (DC) emission is observed. It is observed that with increasing the calcination temperature, the UC/DC emission intensity increases. One of the interesting features explored is the observation of intrinsic optical bistability (IOB). This material may be applicable in various fields such as biological cell imaging, phototherapy, optical switching, display devices, solar cells, etc. © 2020 Elsevier LtdPublicationArticle Large enhancement in upconverted green emission intensity from Ho 3+ /Yb 3+ co-doped Y 2 Ti 2 O 7 phosphor in the presence of Zn 2+(Elsevier Ltd, 2019) P.K. Vishwakarma; A. Bahadur; Akanksha Maurya; S.B. RaiThe Ho 3+ , Yb 3+ co-doped Y 2 Ti 2 O 7 phosphor samples have been synthesized through solid state reaction method in the absence and presence of Zn 2+ ions. The X-ray diffraction (XRD) analysis show that the phase of the crystal is face centered cubic. The zinc free phosphor shows weak upconversion emission in green region due to 5 S 2 , 5 F 4 → 5 I 8 transition of Ho 3+ ion on excitation with 980 nm radiation. The emission intensity is enhanced significantly in the presence of Zn 2+ ions due to increase in crystallinity, particle size and asymmetry in the crystal field. We are reporting ˜92 times enhancement in the green emission intensity for 30 mol% co-doping of Zn 2+ ions. The lifetime of 5 S 2 level of Ho 3+ ion increases by increasing the concentration of Zn 2+ ions. The intense green emission in this material may be suitable for display and other fluorescence based devices. © 2019 Elsevier LtdPublicationArticle Low temperature optical sensor based on non-thermally coupled level of Ho3+ and defect level of Zn2+ in Yb3+:Y2Ti2O7 phosphor(Elsevier Ltd, 2020) P.K. Vishwakarma; P.K. Shahi; S.B. Rai; A. BahadurIn this paper, non-contact optical low temperature sensing behavior is explored between non-thermally coupled level of Ho3+ and the defect level of Zn2+ ion on doping in Yb3+:Y2Ti2O7 phosphor for the first time. The Ho3+/Zn2+ co-doped phosphor prepared by solid state reaction method demonstrates a temperature dependent behavior of emission intensity below the room temperature (25 °C). Under 450 nm (diode laser) excitation, the fluorescence at 552 nm (Green) and 754 nm (NIR) from Ho3+ ions and at 713 nm (NIR) from defect level of Zn2+ ions has been observed and studied at different temperatures in between 300K-100K. An enhancement in the emission intensity of defect band along with Ho3+ ions emission bands is observed on lowering the temperature. The ratio of emission intensity of peaks from Ho3+ level and defect level is considered for temperature sensing by polynomial fitting in FIR technique. The emission intensity of defect band increases with a larger rate than Ho3+ ions emission bands which increase the temperature sensor sensitivity. This paper opens a new field to use defect emission as a low temperature sensor. Thus, the present phosphor may be used as a low temperature sensing device. © 2020 Elsevier LtdPublicationArticle Optical properties of Er3+, Yb3+ co-doped calcium zirconate phosphor and temperature sensing efficiency: Effect of alkali ions (Li+, Na+ and K+)(Elsevier Ltd, 2018) A. Maurya; A. Bahadur; A. Dwivedi; A.K. Choudhary; T.P. Yadav; P.K. Vishwakarma; S.B. RaiThe Er3+/Yb3+ co-doped calcium zirconate phosphors have been synthesized through solid state reaction method. The structural characterizations confirm the crystalline nature of the phosphor. The Er3+, Yb3+ co-doped phosphor emits intense green emission centered at 523 and 544 nm due to the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions, respectively and weak red emission at 662 nm due to 4F9/2 → 4I15/2 transition of Er3+ on excitation with 976 nm radiation. The emission intensity of the green band dominates over the red band. When the alkali ions (Li+, Na+, K+) are added in the Er3+/Yb3+ co-doped phosphor, the emission intensities of bands enhance significantly. The fluorescence intensity ratio correspond to 523 and 544 nm peaks at different temperatures have been obtained which confirm the temperature sensing ability of the samples. In presence of alkali ions, the temperature sensor sensitivity increases and is maximum for K+ co-doped phosphor. It is found that the energy gap between 2H11/2 and 4S3/2 levels increases with crystallinity in order of K+>Na+>Li+> without alkali ion. Thus, the Li+, Na+ and K+ incorporated Er3+, Yb3+ co-doped calcium zirconate phosphor may be useful for photonic devices and temperature sensor. © 2018 Elsevier LtdPublicationArticle Structural and optical properties of composite phosphor Thx(MoO4)2/Bi2(1-x)Mo2O9; (1≥x≥0) doped with Ho3+/Yb3+ as a multifunctional materials(Elsevier Ltd, 2023) P.K. Vishwakarma; S.B. Rai; A. BahadurComposite phosphor's, Thx(MoO4)2/Bi2(1-x)Mo2O9:2Ho3+/5Yb3+ have been synthesized cumulatively by facile co-precipitation process in ethylene glycol (EG) medium and subsequently placed for calcinations. Rietveld refinement of XRD patterns and FTIR analysis confirms the phase formation with a systematic variation of individual phases in dual-phase composite system [Th(MoO4)2-Orthorhombic; β-Bi2Mo2O9-Mononclinic]. Phase-dependent UV-Vis absorption of REs ions (Ho3+/Yb3+) has been observed and the difference between the optical bandgap of both phases leads to show the different aspect of the efficacy of optical properties. PL emission spectra of these phosphors show a strong green, relatively weak red and NIR emissions under λexci= 450 nm. The Yb3+ ions in the phosphor act as sensitizer and responsible for commencing the efficient UC emission, which shows nearly identical emission behavior under 980 nm excitation. Drastic changes in emission intensity (UC/DC) are observed with a relative dual-phase variation. Basically, the different crystal field scenarios around REs ions pertaining to both phases are responsible for the changes in emission intensity. The properties like dual-mode emission, phase dependent emission, color tuning properties, make them multifarious, that can be used for biological cell imaging, phototherapy, optical switching, display devices, solar cells, LEDs, etc. © 2023 Elsevier Ltd