Browsing by Author "Abhishek Dwivedi"

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Dual mode (upconversion and downshifting) behavior of Ho3+/Yb3+/Bi3+ co-doped YTaO4 phosphor and its application as a security ink
(Elsevier Ltd, 2020) Abhishek Roy; Abhishek Dwivedi; H. Mishra; Devendra Kumar; S.B. Rai
The Ho3+/Yb3+ co-doped YTaO4 phosphor has been synthesized in absence and presence of Bi3+ ion using high temperature solid state reaction method. X-ray diffraction (XRD) patterns confirm the pure phase formation of the synthesized materials. The vibrational behavior of the samples is studied by Fourier Transform Infrared (FTIR) spectroscopy, which suggests the phonon frequency of the material about 625 cm−1. The direct optical band gap of the synthesized phosphor has been calculated using UV–Vis absorption spectrum and the value is found to be 4.80 eV. However, the value of band gap is reduced in presence of Bi3+ ions (4.26 eV). Ho3+/Yb3+ co-doped YTaO4 phosphor shows intense green and weak red upconversion emissions on 980 nm excitation. Ho3+/Yb3+ co-doped YTaO4 phosphor sample also shows intense green and weak red emissions on 450 nm excitation. The downshifting emission is enhanced two to three times in presence of Bi3+ due to metal to metal charge transfer self activation of the host. However, the UC emission intensity almost remains unchanged even in presence of Bi3+. Bi3+ ion also shows an intense broad blue emission on UV excitation. The Ho3+/Yb3+ co-doped YTaO4 phosphor sample has been used for security ink applications in green region via 980 nm excitations and in blue region in presence of Bi3+ via UV (365 nm) excitation. Thus, the dual mode emission of this phosphor is applicable for security ink applications in two different regions. This material also permits one to get an intense tunable radiation from 350 to 650 nm on excitation with 342 nm. There are very few dye lasers, which emit intense tunable radiation in this wavelength region in such a wide wavelength range. © 2019 Elsevier B.V.
Effect of host on the radiative (upconversion emission) as well as non-radiative relaxation (laser induced optical heating) in Tm3+/Yb3+ co-doped phosphors
(Elsevier B.V., 2020) Abhishek Dwivedi; Devendra Kumar; S.B. Rai; A.K. Rai
Tm3+/Yb3+ co-doped YVO4 (YVO:Tm, Yb); GdVO4 (GVO:Tm, Yb); Y2O3 (YO:Tm, Yb); Gd2O3 (GdO:Tm, Yb) and CaAl2O4 (CAO:Tm, Yb) phosphor samples have been synthesized using high temperature Solid State Reaction method. X-ray diffraction measurements confirm the pure phase formation of all the five phosphors. Fourier Transform Infrared (FTIR) measurements give information about the different vibrational groups and also the phonon frequency of the host materials. The radiative as well as non-radiative phenomena are observed in all phosphors on excitation with 980 nm laser radiation. Radiative process gives upconversion (UC) emission in Visible and NIR regions due to Tm3+ ion. On the other hand the non-radiative processes generate heat in the material known as laser induced optical heating. The Upconversion (UC) emission behavior of all the phosphors has been investigated on 980 nm excitation which gives intense blue, weak red and prominent NIR emissions. The overall UC emission intensity is maximum in Tm3+/Yb3+ co-doped GdVO4 phosphor as compared to the other four. The crystalline behavior, surface morphology and vibrational behavior play important role in emissive properties. Laser induced optical heating is observed in all the phosphors which show an increase in heating in the phosphors with the increase in pump power. The optical heating is affected by phonon frequencies of the phosphor materials. In the present work, heating is more prominent in the case of vanadate host (a self-activated host) which has phonon frequency near about 800 cm−1. Overall, the UC emission and laser induced optical heating is dominant in self-activated vanadate host as compared to the other oxide hosts. In between vanadate hosts, GVO:Tm, Yb shows better UC emission as well as laser induced optical heating. © 2020 Elsevier B.V.
Effect of Synthesis Techniques on the Optical Properties of Ho 3+ /Yb 3+ Co-doped YVO 4 Phosphor: A Comparative Study
(American Chemical Society, 2019) Abhishek Dwivedi; Ekta Rai; Devendra Kumar; Shyam B. Rai
The Ho 3+ /Yb 3+ -codoped YVO 4 phosphors have been synthesized by three different techniques (viz., solution combustion, sol-gel, and solid-state reaction techniques). X-ray diffraction patterns confirm the formation of a pure phase in the samples synthesized by all of the three methods; however, the average crystallite sizes in the three cases are different. The crystallite size increases if they are heated to higher temperature. The particle sizes are measured by scanning electron microscopy, which shows an increase in particle size with increasing the calcination temperature. The vibrational behavior of all of the three synthesized phosphor samples is studied by the Fourier transform infrared (FTIR) technique. The UV-vis absorption measurements give a large number of bands in all of the three samples prepared by three different methods. The upconversion (UC) emissions in all three samples have been monitored using a 980 nm diode laser. It gives an intense red emission in all of the three samples. Upconversion emission intensity is more prominent in the phosphor sample synthesized by the sol-gel technique and heated at 1473 K. The enhancement in UC emission intensity is well understood by the shape and size of the particles and also confirmed by the FTIR and UV-vis measurements. It is interesting to note that whereas UC measurements give red and weak green emissions, downshifting (DS) measurements show intense green, weak red, and broad blue emissions on UV excitation (323 nm). The DS behavior shows the same characteristics of the enhancement in overall emission. Overall, the phosphor sample synthesized by the sol-gel method gives better results in upconversion and downshifting behaviors when heated at 1473 K. © 2019 American Chemical Society.
Enhanced multimodal behaviour of Tm3+/Yb3+ co-doped YTaO4 ceramic material through Bi3+ activation and sensitization: Application as a spectral converter
(Elsevier Ltd, 2020) Abhishek Roy; Abhishek Dwivedi; Devendra Kumar; H. Mishra; S.B. Rai
The multimodal [Upconversion (UC), Downshifting (DS) and Quantum cutting (QC)] behaviour of Tm3+/Yb3+ co-doped YTaO4 ceramic material is investigated extensively. The samples are prepared by solid-state reaction method at two different temperatures (at 1573 and 1723 K) and their structural and optical properties are compared with each other. The effect of synthesis temperature as well as doping, on crystal phase was analysed via X-ray diffraction measurements. Particles shape and size are investigated through scanning electron microscopy images. The structural behaviour is further verified through vibrational structural study of material via FTIR measurements. This shows similar structural changes as observed in XRD patterns of the samples prepared at two different temperatures. The optical band gap of YTaO4 is nearly 4.8 eV at both the temperatures. Intense blue (476 nm), red (648 nm) and NIR (802 nm) peaks are observed in UC emission spectra of Tm3+/Yb3+ co-doped YTaO4 ceramic on excitation with 980 nm. The blue UC emission is suitable for display devices. The UC emission intensity increases significantly when sample is prepared at higher temperature (1723 K). Addition of Bi3+ ion further enhances the overall UC emission intensity due to change in environment around the activator ion. On excitation with UV radiation (λExc = 284 nm), YTaO4: 1 Tm3+ produces intense DS emission at 454 nm (blue region) due to energy transfer from charge transfer band of [TaO4]3- to Tm3+ ion. The sample synthesized at 1723 K shows better DS emission. One interesting result is that addition of Bi3+ ion enhances the DS emission due to formation of metal-metal charge transfer band. We have also observed the quantum cutting phenomenon in which self-activated blue photon is converted into two or more NIR photons. The efficiency of quantum cutting emission increases significantly in presence of Bi3+ ion and shows strong, intense NIR emission through CET. Thus, the multimodal behaviour of Tm3+/Yb3+/Bi3+ doped YTaO4 makes them a suitable candidate for a spectral converter that can be used in display and solar cell devices. © 2020 Elsevier Ltd and Techna Group S.r.l.
Host dependent upconversion, color tunability and laser induced optical heating via NIR excitation in Ho3+, Yb3+ doped YXO4 (X = V, Nb, Ta) phosphor materials: Application as a security tool and optical heater
(Elsevier Ltd, 2022) Abhishek Roy; Abhishek Dwivedi; H. Mishra; S.B. Rai
This work reports a comparative upconcversion (UC) emission study of the Ho3+, Yb3+co-doped in pure YVO4, YTaO4, YNbO4 and in their mixed (YTa1-xVxO4, YTa1-yNbyO4, YNb1-zVzO4, where x, y, z= 0, 0.25, 0.50 and 0.75, 1 mol) phosphor materials synthesized via high temperature solid state reaction method at 1473 K. The pure and the mixed phosphors have been structurally analysed through X-ray diffraction technique. The phonon frequencies and the surface morphologies are investigated through Fourier transform Infrared (FTIR) and scanning electron microscope (SEM) techniques for pure and mixed compositions. Under NIR (980 nm) excitation, YNbO4:Ho3+,Yb3+ and YTaO4:Ho3+,Yb3+phosphors emit intense green alongwith weak red and very weak NIR UC emissions, whereas the YVO4:Ho3+,Yb3+ phosphor exhibits strong red emission with weak green and NIR UC emissions. The UC emissions in the three cases is explained using energy level diagram and the phonon frequency of the host matrix. The pure YNbO4 host shows comparatively efficient UC emission than the other two due to its large particle size and low phonon frequency. Therefore, the YNbO4:Ho3+,Yb3+ serves as an excellent infrared to green upconverter and useful in display devices. On the other hand, the color tunability from green to orange/red UC emissions is observed on varying the vanadate (V) concentration in the mixed compositions (in YTa1-xVxO4 and YNb1-zVzO4). The variation of ‘V’ in YTa1-xVxO4:Ho3+,Yb3+ and in YNb1-zVzO4:Ho3+,Yb3+intensify the vibrational bands which promotes the non-radiative relaxation and hence tune the upconversion emission color as well as the intensity. The UC emission intensity in YTa1-xVxO4:Ho3+,Yb3+is maximum for x= 0.25. For higher concentrations of V, the green to orange/red color tunable emission is observed. Thus, the mixing of ‘V’ acts as sensitizer as well as modulater in YTa1-xVxO4:Ho3+,Yb3+ phosphor. The colortunable emission in mixed compositions with high color purity makes it promising for security and display applications. The mixing of different compositions of NbO4 and VO4 give different proportions of red and green emissions. On the other hand, mixing of NbO4 and TaO4 give intense green color. Such type of materials are also very useful in security ink applications. The laser induced optical heating is found maximum in case of pure YVO4:Ho3+,Yb3+ phosphor. The temperature of sample YVO4:Ho3+,Yb3+ increases upto 353 K at power density 107.59 W/cm2. Therefore, the YVO4:Ho3+,Yb3+ can be used as an optical heater in medicals such as physiotherapy, cancer and wound treatments etc. © 2021
Monochromatic NIR UC emission in Tm3+/Yb3+ co-doped GdVO4 phosphor: The effect of the Bi3+ ion concentration and pump power of a diode laser
(OSA - The Optical Society, 2018) Abhishek Dwivedi; Devendra Kumar; Shyam Bahadur Rai
A Tm3∕Yb3∕Bi3 co-doped GdVO4 phosphor sample has been synthesized using a high-temperature solid-state reaction technique. The X-ray diffraction patterns reveal pure phase formation and crystalline behavior of the synthesized samples. Intense blue and NIR upconversion emissions have been observed upon excitation with a 980 nm diode laser. It is found that the addition of the Bi3 ion to the phosphor reduces the intensity of blue emission and enhances the NIR emission intensity to the extent that NIR emission is nearly monochromatic [I NIR∕IBlue ∼ 14]. This ratio is further improved up to 70 times (almost monochromatic) by varying the pump power of the diode laser. Thus, this material can be used as a cheap source of monochromatic NIR emission at 800 nm for bioimaging. © 2018 Optical Society of America
Study of near-infrared induced color tunability and optical bistability in Ho3+/Yb3+ codoped YV0.75Ta0.25O4 phosphor
(Elsevier B.V., 2021) Abhishek Roy; Abhishek Dwivedi; Santosh Kumar; Hirdyesh Mishra; A.K. Rai; S.B. Rai
This work reports the green to red color tunable upconversion emission in a newly synthesized Ho3+/Yb3+ codoped YV0.75Ta0.25O4 phosphor, prepared by solid-state reaction (SSR) method. The crystal phase formation has been confirmed by X-Ray diffraction analysis. The Fourier transform infrared (FTIR) spectrum show bands at different frequencies due to V-O and Ta-O, and found to play a vital role in relaxation (radiative and non-radiative) processes. The synthesized phosphor sample show intense upconversion (UC) emission in green (∼539 nm), red (∼659 nm) and in near infrared (NIR∼ 754 nm) regions on 980 nm laser excitation. These bands appear as a result of (5F4+5S2→5I8), (5F5→5I8) and (5F4+5S2→5I7) transitions of Ho3+ ion, respectively. The tunability in color from green to orange-red is observed on varying the pump power of NIR laser (980 nm excitation). The intrinsic optical bistability is observed in both, green & red upconversion bands with the increase and decrease in power of NIR source. The Ln I-Ln P plots suggest that green & red UC emissions are due to two photon absorption at low laser power. A decrease in the slope values at higher pump powers confirms the laser induced optical heating (LIOH) in the material. The pump power based color tunable emission and optical bistability can be utilized in security detection, optical memory, optical color coding, color switching and in various advanced applications. © 2021 Elsevier B.V.
The impact of pure and mixed self-activated YXO4 phosphor materials (X=V, Nb and Ta) on downshifting and quantum cutting emission behaviours of Ln3+ doped (Ln3+=Ho3+ and Yb3+) ions
(Elsevier Ltd, 2023) Abhishek Roy; Abhishek Dwivedi; H. Mishra; S.B. Rai
In this work, the downconversion (DC) behaviour of Ho3+ ions doped in pure YVO4, YNbO4, YTaO4 hosts and their intermixed (i.e. Ta/V, Ta/Nb and Nb/V) compositions have been investigated and compared. The samples were synthesized by solid-state reaction technique at 1473 K. The UV–Vis absorption and photoluminescence behaviour of 1 mol % Ho3+ doped YVO4, YNbO4 and YTaO4 as well as their intermixed compositions have been studied. The DS green emission of Ho3+ ion in vanadate host (λexc = 326 nm) is 5 times stronger than in niobate host (λexc = 262 nm) due to efficient energy transfer between [VO4]3- and Ho3+ ion. The downshifting (DS) emissions are also investigated on direct excitation of Ho3+ ion in YVO4, YNbO4 and YTaO4 hosts on excitation with 450/456 nm (5I8→5G6 transition of Ho3+ ion) wavelength. It is found that the overall DS emission intensity is dominant in YNbO4:Ho3+ phosphor due to low phonon frequency and larger particle size of YNbO4. The Ho3+ doped in mixed hosts viz. YTa1-xVxO4, YTa1-yNbyO4, YNb1-zVzO4 (where x, y and z = 0, 0.25, 0.50, 0.75 and 1.0) phosphors also emit intense green DS emission along with their violet-blue band emission on ultraviolet (UV) excitation. The mixing of Ta/V and Ta/Nb enhances the blue emission of host matrix on UV excitation. The green emission is found to increase with ‘V’ and ‘Nb’ concentrations in YTa1-xVxO4:0.01Ho3+, YTa1-yNbyO4:0.01Ho3+, respectively. Further, we also investigated quantum cutting (QC) in YNb1-zVzO4:5 mol% Yb3+ (where z = 0, 0.25, 0.50, 0.75, 1.0) phosphors on UV excitation (λexc = 262 and 326 nm). The intensity of QC NIR emission is found to increase with the incorporation of ‘V’ in YNb1-zVzO4 phosphors. The strongest QC emission is observed in pure YVO4 host as compared to pure YNbO4 as well as other mixed matrices. The QC efficiency of NIR emission for YVO4:5 Yb has been calculated and found to 129%. Therefore, the prepared phosphor material is highly applicable to increase the efficiency of Si- solar cell by QC NIR emission. © 2023 Elsevier Ltd and Techna Group S.r.l.
Tm3+, Yb3+ activated ANbO4 (A = Y, Gd, La) phosphors: A comparative study of optical properties (downshifting and upconversion emission) and laser induced heating effect
(Institute of Physics Publishing, 2017) Abhishek Dwivedi; Kavita Mishra; S.B. Rai
A comparative study of optical properties and laser induced heating effects in Tm3+, Yb3+ doped YNbO4, GdNbO4 and LaNbO4 phosphors is presented in this work. The phosphors were structurally characterized by x-ray diffraction and scanning electron microscopy measurements. The vibrational structures of the phosphors were studied using FTIR measurements. The optical band gaps (E g), calculated from the Wood and Tauc plot, are found to be 3.78, 4.50 and 3.27 eV for YNbO4, GdNbO4 and LaNbO4, respectively. The luminescence property (downshifting (DS) and upconversion (UC)) was studied in the powder and the pellet forms of the phosphor samples. The DS emission of Tm3+ doped ANbO4 phosphors (λ ex = 265 nm) consists of broad blue emission due to (NbO4)3- group overlapped with sharp peaks due to f-f transition of Tm3+ ion with most prominent emission one in the case of YNbO4 phosphor. The DS emission is comparatively more intense in the pellet form. The NIR excited UC emission spectra of Tm3+, Yb3+ co-doped ANbO4 phosphors contain intense blue and NIR emissions due to the Tm3+ ion. Contrary to the DS study, the best UC result is found for LaNbO4 phosphor in pellet form. Further, the laser induced heating effect in UC emission with respect to laser pump power and irradiation time has also been studied in Tm3+, and Yb3+ co-doped ANbO4 phosphors. It was found to be more effective in the case of the YNbO4 host where the heating effect is more prominent in the powder sample. We discuss the mechanisms involved in these observations in detail. © 2016 IOP Publishing Ltd.
Two steps enhancement of dual mode (UC and DS) behaviour of Ho3+/Yb3+ and Tm3+/Yb3+ co-doped GdVO4 phosphors: Improvement in spectral and color purity
(Elsevier Ltd, 2021) Kartikey Dharendra; Abhishek Roy; Abhishek Dwivedi; Awadhesh Kumar Rai; Shyam Bahadur Rai; Devendra Kumar
In the present work, the dual mode (Upconversion and Downshifting) emission behavior of Tm3+/Yb3+ and Ho3+/Yb3+ co-doped GdVO4 phosphors synthesized via high temperature solid state reaction technique have been studied. The pure phase formation has been confirmed by X-rays diffraction (XRD) studies. The crystalline nature has been found to improve on calcining the phosphors at 1473 K and also by incorporation of TiO2. The surface particles’ shape and size have been studied by SEM technique. The vibrational behaviour of the vanadate host has been investigated by Fourier Transform Infrared (FTIR) measurements. The UV–vis absorption spectra show the formation of charge transfer band (CTB) in UV region due to (VO4)3 group. The non-linear upconversion (UC) spectra show intense blue and NIR emission in GdVO4: Tm3+/Yb3+ phosphor on excitation with 980 nm radiation, whereas intense green and red UC emissions are observed in GdVO4: Ho3+/Yb3+ phosphor. The downshifting (DS) behaviour has been investigated in both the phosphors which show the self activation behaviour of GdVO4 host along with some sharp peaks in blue and green regions due to f-f transitions in Tm3+ and Ho3+ ions. The dual mode (UC and DS) emission has been further improved on heating the samples at higher temperatures and also by incorporation of TiO2. The color of the phosphor samples in the UC emission spectra has been visualized by CIE diagram. The color purity enhances in both the phosphors by increasing the calcination temperature and also by addition of TiO2. The spectral purity has been also calculated for the UC emission spectra and its also shows the same trend. Such phosphors are applicable for the display devices in blue, green and red regions via dual mode processes. © 2021 Elsevier Ltd