High purity blue photoluminescence in thulium activated α-Na3Y(VO4)2 nanocrystals via host sensitization

Journal Pre-proof High purity blue photoluminescence in thulium activated α-Na3Y(VO4)2 nanocrystals via host sensitization Linju Ann Jacob, S. Sisira, Kamal P. Mani, Kukku Thomas, Dinu Alexander, P.R. Biju, N.V. Unnikrishnan, Cyriac Joseph PII:

S0022-2313(19)31537-6

DOI:

https://doi.org/10.1016/j.jlumin.2020.117169

Reference:

LUMIN 117169

To appear in:

Journal of Luminescence

Received Date: 3 August 2019 Revised Date:

25 February 2020

Accepted Date: 27 February 2020

Please cite this article as: L.A. Jacob, S. Sisira, K.P. Mani, K. Thomas, D. Alexander, P.R. Biju, N.V. Unnikrishnan, C. Joseph, High purity blue photoluminescence in thulium activated α-Na3Y(VO4)2 nanocrystals via host sensitization, Journal of Luminescence (2020), doi: https://doi.org/10.1016/ j.jlumin.2020.117169. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

High purity blue photoluminescence in thulium activated α-Na3Y(VO4)2 nanocrystals via host sensitization Linju Ann Jacob, Sisira S, Kamal P Mani, Kukku Thomas, Dinu Alexander, Biju P R, N V Unnikrishnan, Cyriac Joseph* School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam- 686560, India *Corresponding author. Tel.: +91 04812731043, E-mail: [email protected] Abstract A novel blue emitting glaserite type α-Na3Y(VO4)2 nanocrystals substituted with the trivalent thulium ions were synthesized for the first time via citrate based solution combustion method. The monoclinic structure and well dispersed spherical morphology of the synthesized nanocrystals were confirmed by XRD, TEM, HRTEM and SAED analysis. Host lattice emits broad band blue color and it originates from the VO43-. The presence of a very intense and broad charge transfer band in excitation spectra of α-Na3Y(VO4)2:Tm3+ monitored under 474 nm demonstrates the efficient energy transfer from VO43- to Tm3+ in α-Na3Y(VO4)2:Tm3+. Photoluminescence studies of Tm3+ activated α-Na3Y(VO4)2 samples show blue emission under excitation wavelength 310 nm. The optimal doping concentration of Tm3+ ions in α-Na3Y(VO4)2 phosphors is found to be around 1.5 mol % and the major mechanism for concentration quenching is the dipole - dipole interaction. The quantum yield for the sample with optimum concentration is measured to be 18.16%. The CIE chromaticity color coordinates were calculated and it is very much closer to the NTSC standards with high color purity. The lifetime of the thulium characteristic blue emission is found to be in the order of 0.1 ms. All the results clearly indicate that pure α-Na3Y(VO4)2 as well as rare earth Tm3+ activated α-Na3Y(VO4)2 nanocrystals can be used as potential candidates in the field of optoelectronics, especially for the fabrication of near-UV based white LEDs, bio-imaging and labeling etc. Keywords: Vanadate; Solution combustion method; Photoluminescence; Blue emission

1. Introduction Rare earth elements, owing to their unique electronic configuration exhibit a wide range of intriguing properties hence find immense applications particularly in the area of solid state lighting. Generation of white light is the hot spot in the field of solid state lighting due to its attractive merits of energy saving, long life time, faster response as well as ecofriendly nature as compared with conventional incandescent and fluorescent lamps [1–3]. Presently, the most common method for realization of W-LED are based on a blue emitting InGaN LED chip with a yellow emitting phosphor (YAG: Ce3+) or by employing a near UVLED coated with multi-phased phosphor emitting blue, green and red light. However these methods do not meet the optimum requirements of white light LEDs due to challenges like separate preparation of phosphors, matching of the particle sizes of individual phosphor materials with one another to avoid agglomeration and difficulty in the homogenous mixing of them in appropriate ratio to obtain the final product. Another excellent option to generate efficient white emission is to fabricate a single phase white emitting phosphor pumped by UV or near UV-LED chips in order to achieve good color rendering index, better reproducibility, lower manufacturing cost and a simpler fabrication process. Energy transfer plays a critical role in attaining white light emission in single phase phosphor via host sensitization as well as by co-doping sensitizer and activator into the same host matrix. It is well known that vanadate compounds are potential host for single phase white light generation due to its strong ultraviolet absorption capacity, effective transfer of the absorbed energy to the dopant ions and wide color tuning by the proper selection of dopant ions [4]. Vanadate based materials also possess interesting electronic and optical properties, excellent chemical stability and optical quality. Yttrium vanadate has been widely accepted as potential host and sensitizer for rare earth ions especially in single phase white light generation [5].

Alkali metal rare earth double orthovanadates (A3RE(VO4)2) nanocrystals particularly α-Na3Y(VO4)2 have attracted considerable attention on account of their wide range of optical applications [6–9]. It is reported that Na3Y(VO4)2 exist in three polymorphs: a low temperature monoclinic α-phase, a high temperature hexagonal β-phase variety and γ-phase obtained by the rapid quenching of the β-phase[10]. Different kinds of cations will bring slight distortion to the tetrahedral structure of the vanadate anion without significant changes in the structure. The changes in the crystal structure as well as the nature of the symmetry of the lanthanide ion sites have an effective role on the luminescence properties of rare earth based nanophosphors and is crucial in the design of new phosphors. In addition, the site symmetry of the Y3+ ion in α-Na3Y(VO4)2 is C2v and the Yttrium (Y) atom occupies slightly distorted octahedral site which is of lower symmetry in comparison with the symmetry of yttrium (D2d) in YVO4 [11,12]. It is obvious that these factors highly influences the intensity of rare earth emission corresponds to electric dipole transitions and brings out α-Na3Y(VO4)2 as an excellent host lattice for all rare earth elements. Europium doped α-Na3Y(VO4)2 synthesized by conventional solid state reaction method has been reported as an excellent luminescent material by Zhang et al [11]. However the solid state method requires a high reaction temperature, long reaction time, tedious milling process and agglomerated and irregular morphology is an inherent drawback which limits the luminescence performance and applications of the phosphor [13]. This can be overcome by employing citrate based solution combustion method which is comparatively a better technique as it ensures the formation of uniform particles with high yield at moderate temperature. This method is particularly useful in the synthesis of ultra-fine oxide powders because the starting raw materials are homogeneously mixed in liquid phases [13]. Among various trivalent rare earth ions, thulium has gained significant attention due to its blue emission and the relative simplicity of its energy levels. However,

photoluminescence study of thulium activated inorganic phosphors possessing good performance is seldom and is the motivation behind the selection of thulium in this study. Also the electric dipole transition corresponding to the blue emission can be dominant in the case of thulium activated α-Na3Y(VO4)2 owing to the structure. To our best knowledge photoluminescence study of thulium activated α-Na3Y(VO4)2 has not been investigated so far. In the present work, a detailed photoluminescence study of thulium doped α-Na3Y(VO4)2 synthesized via citric acid based solution combustion method and energy transfer mechanism between vanadate and Tm3+ were studied for various doping concentration of Tm3+ ions. 2. Experimental methods A series of thulium doped α-Na3Y(VO4)2 nanocrystals (α-Na3Y1-xTmx(VO4)2, where x=0, 0.005, 0.01, 0.015, 0.02, 0.03, 0.04, 0.05) were prepared via low temperature citrate based solution combustion method. Analytical grade ammonium vanadate (NH4VO3), sodium hydroxide (NaOH), citric acid monohydrate (C6H8O7.H2O) and high purity yttrium nitrate hexahydate (Y(NO3)3.6H2O (99.9%)) and thulium nitrate pentahydrate (Tm(NO3)3.5H2O (99.99%)) were used as the starting materials. 0.1M Yttrium nitrate and citric acid were dissolved in 10 ml de-ionised water under magnetic stirring. The molar ratio of total metal cations to citric acid was fixed at 1:4. 10 ml aqueous solutions of 0.2M NH4VO3 and 0.3M NaOH were mixed together and heated at 70oC under stirring for 1 h and allowed to cool. The solution containing yttrium nitrate and citric acid was added slowly to this under stirring resulting a homogenous transparent yellowish orange solution. It is then covered with a polyethylene cap and heated at 80oC under stirring. During heating, color of the solution turned into blackish green and then into blue and the color change is more likely due to partial reduction of Vanadium (V) by citric acid. After removing the cap, the solution is allowed to evaporate by heating at 120oC until the pH value of the solution reaches 7. This solution is transferred into a crucible and rapidly heated at

500oC for two hours in a muffle furnace and then allowed to cool to room temperature. A white fluffy ultra fine nanocrystals of Na3Y(VO4)2 were formed. A series of thulium activated Na3Y(VO4)2 (Na3Y1-xTmx(VO4)2, where x=0.005, 0.01, 0.015, 0.02, 0.03, 0.04, 0.05) were prepared by replacing x molar fraction of yttrium nitrate by thulium nitrate in the above procedure. The formation of phosphors occurs according to the following equation. 2NH4VO3 + 3NaOH + xTm(NO3)3+ (1-x)Y(NO3)3 → Na3Y1-xTmx(VO4)2 + 2NH4NO3 +HNO3 +H2O Thermal decomposition of NH4NO3 takes place in the temperature range 230oC to 260oC. Decomposition of citric acid and NH4NO3 results in huge enthalpy change which leads to exothermic reaction and thus along with citric acid NH4NO3 will also act as fuel for the combustion process. 3. Characterization techniques Crystalline nature and phase purity of the as-synthesized samples were examined by recording X-ray powder diffraction pattern using X’Pert PRO PANalytical X-ray powder diffractometer with Cu-Kα radiation (λ=1.5460Å) in the 2θ range 10˚ to 60˚. BRUKER 66 V Fourier transform infrared spectrometer was used to identify the various functional groups of the grown samples in the range 400–4000 cm-1. The morphology of the samples was examined by Transmission Electron Microscopic observations on a JEOL JEM 2100 Transmission Electron Microscope. Thermogravimetric analysis was carried out on Shimadzu thermal analyzer DT-40. The elemental mapping and EDS spectra of the sample were recorded using JSM- 6490LA scanning electron microscope. Perkin Elmer Lambda25 UV– Vis spectrometer was used to record the absorption spectra of the samples in the wavelength range 200–800 nm. Photoluminescence analysis was carried out by Horiba Jobin-Yvon

Fluoromax-4 spectrofluorometer and the quantum yield of the sample was accessed with a Quanta- φ F-3029 integrating sphere attachment. 4. Results and Discussion 4.1. Structural characterization X-ray powder diffraction patterns of α-Na3Y1-xTmx(VO4)2, where x=0, 0.005, 0.01, 0.015, 0.02, 0.03, 0.04 nanocrystals are presented in figure 1. The well defined diffraction peaks of the sample are assigned to that of pure monoclinic Na3Y(VO4)2 with space group P21/n(14)

according to the ICDD file No.055-0797. Absence of diffraction peaks

corresponding to other phases indicates that introduction of Tm3+ does not alter the crystal structure of the host matrix and Tm3+ ions are successfully incorporated into the host lattice. No additional peaks within the XRD pattern revealed the absence of unreacted residue and impurity in the sample. This result certifies that citrate based solution combustion method is an efficient way for preparing high quality Na3Y(VO4)2 nanocrystals. The lattice parameters of the synthesized samples were calculated using Unit Cell Win software and compared with ICDD data and are given in table 1.

Fig.1: XRD patterns of α-Na3Y1-xTmx(VO4)2 nanocrystals

The crystal structure of α-Na3Y(VO4)2 comprises of two octahedra YO6 and NaO6 and isolated VO43- tetrahedra [10,11]. The site symmetry of the Y3+ ion in α-Na3Y(VO4)2 is C2v and also the yttrium (Y) atom occupies slightly distorted octahedral site with coordination number 6. Since Tm3+ replaces the crystallographic site of yttrium ion, the slight difference in the ionic radius of Y3+ ion (0.90 Å ) and Tm3+ ion (0.87 Å) results in slight decrease in the lattice parameters with doping concentration [14] and plays a prominent role in the emission of Tm3+ ion in α-Na3Y(VO4)2 host lattice. Table 1: The unit cell parameters of α-Na3Y1-xTmx(VO4)2 nanocrystals Lattice parameters

Cell volume

x

a (Å)

b (Å)

c (Å)

β

(Å)3

ICDD

7.2311

9.7648

5.4995

92.9861

387.583

0

7.2297

9.7607

5.4995

93.0053

387.557

0.005

7.2194

9.7461

5.4895

93.0342

385.709

0.01

7.2216

9.7429

5.4875

93.0469

385.558

0.015

7.2192

9.7448

5.4843

93.0622

385.263

0.02

7.2116

9.7408

5.4831

93.0063

384.640

0.03

7.2078

9.7321

5.4773

93.0626

383.648

0.04

7.1987

9.7336

5.4760

93.0393

383.140

0.05

7.1798

9.7312

5.4810

93.0088

382.998

The crystallite size (D) of the powder sample can be calculated from the Scherrer formula as D= Kλ/βCosθ, where K is the shape factor (0.9), λ is the wavelength of the X ray source (1.5406Å), θ is the diffraction angle and β is the full width at half maximum [15]. The estimated crystallite size of the nanocrystals is obtained in the range of 20-30 nm. Elemental mapping and energy dispersive X-ray analysis (EDX) were also performed to determine the chemical composition of the as-prepared thulium doped α-Na3Y(VO4)2

nanocrystals. Figure 2(a) illustrates the EDX spectrum of a representative sample α-Na3Y0.985Tm0.015(VO4)2 and the results confirmed the presence of signals of Sodium (Na), Yttrium (Y), Vanadium (V), Oxygen (O), Thulium (Tm) composition of the nanocrystals. No other impurity element is present in the EDX spectrum and it reveals the purity of the sample and is in consistent with the XRD analysis. The elemental mapping images shown in figure 2 (b) revealed that the elements were homogeneously distributed in the matrix. The corresponding SEM image is given in the inset of figure 2(a).

Fig. 2: (a) EDX spectrum (b) Elemental mapping of α-Na3Y0.985Tm0.015(VO4)2 FTIR spectra of as synthesized α-Na3Y1-xTmx(VO4)2 where x= 0.005, 0.01, 0.015, 0.02 nanocrystals were recorded in order to analyze the vibrational transitions of the samples and spectra are displayed in figure 3. The bands observed in the 960-400 cm-1 region of spectra are due to internal vibrations of the VO43- ion. Isolated VO43- ion occupies tetrahehral

Td symmetry and four normal modes are expected for the isolated VO43- ion. These are ν1 (symmetric stretching) and ν3 (asymmetric stretching) modes of O-V-O, ν2 (symmetric bending) and ν4 (asymmetric bending) modes of V-O-V vibrations respectively where ν3 and ν4 are Raman and IR active and ν1 and ν2 are only Raman active in Td symmetry [16].

Fig. 3: FTIR spectrum of α-Na3Y1-xTmx(VO4)2 nanocrystals However in α-Na3Y(VO4)2 molecule, VO43- occupies a lower symmetry C1 [10]. Due to this all the normal modes should be IR and Raman active. Moreover, the degeneracy of stretching ν3 and deformation (ν2 and ν4) should be lifted. The band corresponding to 903 cm1

is assigned to ν1 modes. The strong bands were observed at 723 cm-1, 820 cm-1, 958 cm-1 are

identified as ν3 mode. The strong bands at 411 cm-1 corresponds to ν2 mode of VO43-. The existence of these bands in the IR spectra proved the formation of α-Na3Y(VO4)2 as concluded from the XRD patterns. Very weak peaks present at 3275 cm -1, 2323 cm-1, 1439 cm-1, 1738 cm-1 are assigned to the stretching and bending modes of OH vibrational modes [17]. The less intense peak at 1368 cm-1 is due to the anti-symmetric stretching vibrations of carbonate group. These peaks are insignificant when compared with the strong characteristic peaks of VO43- and indicating the formation of α-Na3Y1-xTmx(VO4)2 with high purity.

The phase purity of the synthesized sample is further ascertained by the thermogravimetric analysis of both the precursor sample and α-Na3Y1-xTmx(VO4)2 nanocrystals obtained after annealing at 500oC. Figure 4 depicts the thermal response of the pre-annealed precursor sample and that of the synthesized nanocrystals. The precursor sample undergoes a rapid weight loss of nearly 65% in different stages up to 500oC due to the expulsion organic components and then marked a small stable region indicating the formation of α-Na3Y1-xTmx(VO4)2. In contrast to this, there is no considerable weight loss occurred for the annealed sample up to 570oC and indicating the phase purity and thermal stability of the α-Na3Y1-xTmx(VO4)2 nanocrystals prepared by the citrate based solution combustion method. On further heating the sample undergoes weight loss indicating the onset of decomposition.

Fig. 4: TG curves of precursor sample and sample annealed at 500oC The morphology of the as-prepared α-Na3Y0.985Tm0.015(VO4)2 nanocrystal were investigated by Transmission electron microscopy (TEM), High-resolution transmission electron microscopy (HRTEM) and Selected area electron diffraction (SAED) observations. TEM image shown in figure 5(a) represent the overall characteristics of the synthesized nanphosphors. The sample consists of spherical particles with size in the range 10-40 nm, majority of which are agglomerated. The higher magnification image given in figure 5(b)

show a region with well dispersed particles with comparable size. Citric acid plays multiple role in the growth of α-Na3Y(VO4)2 nanocrystals. Citrate ions not only act as a fuel but also act as chelating agent due to the presence of carboxylic and hydroxyl functional groups. It is reported that excessive addition of citric acid facilitates the formation of perfect spherical nanocrystals of uniform size [18]. The presence of very small particles in the TEM image is due to the rupture of the carbon film over the copper grid. This is evidenced from EDS spectrum of the sample given in figure 5(e), which shows an additional peak of carbon also.

Fig. 5 : (a) & (b) TEM images, (c) HRTEM image (d) SAED pattern and (e) EDS spectrum of α-Na3Y0.985Tm0.015(VO4)2 nanocrystals

The well resolved fringes in the HRTEM image of α-Na3Y0.985Tm0.015(VO4)2 as shown in figure 5(c) revealed highly crystalline nature of the sample. The d spacing were found to be 2.79 Å, 2.92 Å, 3.92 Å in accordance with that of the higher intensity planes (031), ( 20), (111) in ICDD file No.055-0797 of monoclinic α-Na3Y(VO4)2 sample respectively. The selected area electron diffraction pattern (SAED) of α-Na3Y0.985Tm0.015(VO4)2 depicted in

figure 5(d) exhibit bright diffraction spots superimposed on concentric diffused rings which identified the planes (221), (031), (111), (101) corresponding to the d spacing 2.520 Å, 2.803 Å, 3.901 Å, 4.283 Å respectively of the monoclinic α-Na3Y(VO4)2 sample. 4.2 Optical characterization

Fig. 6: UV-Vis absorption spectrum of α-Na3Y1-xTmx(VO4)2, nanocrystals The optical absorption of pure α-Na3Y(VO4)2 and α-Na3Y1-xTmx(VO4)2 where x=0.005, 0.01, 0.015 nanocrystals are examined by UV-visible spectroscopy in the range 200 nm to 700 nm and the spectra are as shown in figure 6. Intense and broad absorption band from 240 nm to 370 nm can be ascribed to the charge transfer from the oxygen 2p orbital to the central vanadium 3d orbital of (VO4)3- group present in the host lattice [19]. However any absorption peaks of Tm3+ were not detected due to the low doping concentration of Tm3+ ions as well as the more intense charge transfer band of (VO4)3- ion. The intense broad band attributed to the charge transfer band of VO43- observed in the UV-Vis absorption spectra indicates the possibility of sensitization of Tm3+ via host. A detailed study of photoluminescence excitation and emission characteristics of the as-prepared pure α-Na3Y(VO4)2 nanocrystals and thulium activated α-Na3Y(VO4)2

nanocrystals were carried out in order to explore the potential of the new nanophosphor. Figure 7(a) illustrates the excitation spectrum of pure α-Na3Y(VO4)2 monitored at 438 nm emission which showed a broad and intense band with the maximum centered around 310 nm. This is due to the charge transfer from the oxygen ligands to the central vanadium atom inside the [VO4]3- ion. On the basis of molecular orbital theory, excitation centered at 272 nm and 310 nm correspond to electric or magnetic dipole allowed transitions from the ground state 1A1 to 1T1 and 1T2 excited states of VO43- ion, respectively as observed in the absorption spectrum of the pure sample. The emission spectra of pure α-Na3Y(VO4)2 sample in figure 7(b) showed a broad band from 350 nm to 575 nm centered around 438 nm [20]. The emission bands are ascribed to the overlapping of transitions from 3T2 and 3T1 to 1A1 in the VO43- distorted tetrahedra .

Fig. 7: Photoluminescence (a) excitation and (b) emission spectrum of pure α-Na3Y(VO4)2 In order to investigate the effects of Tm3+ on luminescence properties of αNa3Y(VO4)2, excitation and emission spectra of a series of α-Na3Y1-xTmx(VO4)2 where x= 0.005, 0.01, 0.015, 0.02, 0.03, 0.04, 0.05 nanocrystals were investigated. Figure 8 illustrates the excitation spectra of α-Na3Y1-xTmx(VO4)2 where x= 0.005, 0.01, 0.015, 0.02, 0.03, 0.04, 0.05 nanocrystals monitored at 474 nm emission. The PL excitation spectra of all the samples exhibited an intense and broad band in the range of 230-345 nm with a maximum peak

around 310 nm which is attributed to a charge transfer from the oxygen ligands to the central vanadium atom inside the VO43- ion of host lattice [21–23]. Moreover, the direct f-f excitation peaks of Tm3+ are very weak compared to charge transfer band as well as due to the low doping level of Tm3+. Hence it is evident that there is an efficient energy transfer from the vanadate group to Tm3+ ions nonradiatively which confirms the existence of sensitization of Tm3+ via host.

Fig. 8: Photoluminescence excitation spectrum of α-Na3Y1-xTmx(VO4)2 The PL emission spectra of α-Na3Y1-xTmx(VO4)2 where x = 0.005, 0.01, 0.015, 0.02, 0.03, 0.04, 0.05 monitored under 310 nm excitation. As depicted in figure 9, the emission spectra of all samples showed intense and sharp blue emission peaks centered at 474 nm which correspond to 1G4 → 3H6 electric dipole transition and a weak red emission centered at 647 nm which attributed to 1G4 → 3F4 magnetic dipole transition, respectively. Here, the intensity of blue emission 1G4 → 3H6 is much stronger than red emission 1G4 → 3F4 indicating that Tm3+ are located in an asymmetric cation environment which is consistent with a symmetry C2v without an inversion center. This result corroborates that thulium doped αNa3Y(VO4)2 emerge as a promising blue emitting nanophosphor for UV excitable single phased white light generation.

According to Judd-O-felt theory [24,25] the intensity of transition between different J numbers will depend on the symmetry of the local environment of the thulium ion. In bulk αNa3Y(VO4)2 the Tm3+ occupies a site with no inversion symmetry. In this case the selection rule for electric dipole transitions are allowed only if |∆| = 2, 4 or 6 and is highly sensitive to the local environment of the Tm3+ ions. The J-O theory states that the magnetic dipole transition are allowed if |∆| = 0 or 1 but not J = 0 → J’= 0 regardless of the environment.

Fig. 9: Photoluminescence emission spectrum of α-Na3Y1-xTmx(VO4)2 monitored at excitation wavelength of 310 nm

The intensity of the red emission (1G4 → 3F4) is scarcely affected by Tm3+ concentration, whereas the intensity of blue emission (1G4 → 3H6) strongly depends on the Tm3+ concentration. In general, the intensity ratio of electric dipole (ED) to magnetic dipole (MD) transition has been employed to estimate the symmetry of the local environment of the rare earth ions. Here, the intensity of blue emission (ED) is very much larger compared with that of the red emission (MD) which suggests that Tm3+ ions mainly occupy the non-centrosymmetric lattice site whereas magnetic dipole transition is permanently allowed and insensitive to the centrosymmetry. It is observed that the emission intensity increases with increase in doping concentration of Tm3+ upto x = 0.015 and then decreases with further

addition of Tm3+ ion. The quantum yield for the sample with optimum concentration is measured to be 18.16%. Figure 10 shows variation of PL emission intensities of α-Na3Y1-xTmx(VO4)2 nanocrystals for different doping concentrations at 474 nm monitored under excitation wavelength of 311 nm. This is due to the occurrence of cross-relaxation between two Tm3+ ions ( 1G4 + 3H6 → 3F2 + 3F4) and ( 1G4 + 3H6 → 3H4 + 3H5) and are designated as cross relaxations CR1, CR2 respectively [26].

Fig. 10: Variation of PL emission intensities of α-Na3Y1-xTmx(VO4)2 nanophosphors for different doping concentrations Based on the photoluminescence spectra of α-Na3Y1-xTmx(VO4)2, the energy transfer mechanism from VO43- to Tm3+ has been deduced using energy level diagram [27] as shown in figure 11. Since the excited energy level of VO43- is at 33,000 cm-1, the host lattice has energy to populate the upper emitting level 1D2 of Tm3+. More energy from 1D2 state rapidly relaxed to the lower emitting level 1G4 via non-radiative transition and finally relaxed to lower states through radiative transitions. As the doping concentration of Tm3+ increases, Tm3+ ions comes closer and closer and 1G4 get depopulated via above proposed cross relaxations which results in concentration quenching.

Fig.11: Simplified energy level diagram of VO43- and Tm3+ in thulium doped α-Na3Y(VO4)2 The quenching of fluorescence from the 1G4 state is attributed to energy transfer from an excited Tm3+ ion to a nearby unexcited Tm3+ ion, via cross-relaxation process. The energy gap about 6200 cm-1 between the 1G4 state and next lower-lying 3F2 state is nearly equal to seven highest phonon energy of the VO43- ions, available in the α-Na3Y(VO4)2 host lattice. Therefore, in the case of Tm3+ ion in the α-Na3Y(VO4)2 host, the multi-phonon relaxation is negligible. The energy migration among thulium ions may be due to (a) multipole-multipole interaction (b) exchange interaction. The critical distance Rc below 5 Å favours concentration quenching due to exchange interaction and Rc above 5 Å indicates multipolemultipole interaction. According to Blasse equation [28], the critical distance between Tm3+ ions is calculated as

 3V  RC = 2   4πNxc 

1/ 3

where V is the unit cell volume (387.8 Å3), xc is the optimum concentration (0.015 mol) and N is the number of Tm3+ in the unit cell (N= 2). Since Rc is found to be 29.95 Å which is much greater than 5 Å, concentration quenching of Tm3+ may be attributed to multipolemultipole interaction. The interaction type and quenching mechanism can be further analyzed using Dexter’s formula [29]. log (I/C) =K - (θ/3) log (C) where C is the doping concentration of RE ions, I is the emission intensity, K is a Dexter’s constant and θ represents the interaction type constant of multipolar between RE ions. θ = 6, 8, 10 correspond to electric dipole-dipole (d-d), dipole-quadrupole (d-q) and quadrupolequadrupole (q-q) interactions respectively.

Fig.12: The plot of log (I/C) vs. log C at λem = 474 nm The relationship between the luminescence intensity and doping concentration are given by the curve (log (I/C) - log (C)) as is shown in figure 12 and the plot is linearly fitted with a slope of -1.929 and the θ value is found to be 5.787 which is close to 6. This result indicates that the electric dipole-dipole interaction is preferred for the non-radiative energy migration among the Tm3+ ions in α-Na3Y1-xTmx(VO4)2 nanocrystals.

Fig. 13: The decay curve of α-Na3Y1-xTmx(VO4)2 nanocrystals (a) host emission (438 nm) (b) Tm3+ emission (474 nm) monitored at 310 nm excitation The decay curve of α-Na3Y1-xTmx(VO4)2 nanocrystals (x=0.01, 0.015 and 0.04) were recorded for excitation at 310 nm and monitoring the host emission (438 nm) and are displayed in figure 13(a). The decay curves of the host emission show double exponential behavior with short lifetime values 0.473 µs, 0.453 µs and 0.408 µs and long life time values 12.15 µs, 11.28 µs and 9.13 µs for Tm3+ concentrations x = 0.01, 0.015 and 0.04 respectively. The average lifetimes of the host emission are calculated as 10.59 µs, 9.88 µs and 7.56 µs for Tm3+ concentrations x=0.010, 0.015 and 0.040 respectively. The average lifetime values of host emission rapidly decreases with the increase in concentration of Tm3+ ions. The double exponential nature of the decay curves and decrease in lifetime of host emission with thulium content reveal that there is a non-radiative energy transfer from host to Tm3+ ions. The decay curves of Tm3+ emission at 474 nm under 310 nm excitation for samples with different thulium concentrations are shown in figure 13(b). The curves show a single exponential behavior and the lifetime value of thulium emission are 0.119 ms, 0.099 ms and 0.066 ms for Tm3+ concentrations x = 0.01, 0.015 and 0.04 respectively. The decrease in the lifetime value of thulium emission with concentration is due to the possible luminescence quenching due to the cross relaxation channels as mentioned in the energy level diagram.

The colorimetric performance of the α-Na3Y1-xTmx(VO4)2 nanocrystal where x= 0.005, 0.01, 0.015, 0.02, 0.03, 0.04, 0.05 can be evaluated by calculating the color coordinates for the sample using intensity of PL emission spectra ranging from 430-700 nm at excitation wavelength of 310 nm. The quality of the luminescence emission was further assessed by the value of color purity and is estimated by the following expression

Color purity =

(  ) (  ) (  ) (  )

, where ( ,  ) is the CIE chromaticity coordinate of

the sample, ( ,  ) is the CIE chromaticity coordinate of the white illumination, and ( ,  ) represents the CIE chromaticity coordinate of the dominant wavelength [30]. Table 2: The CIE Chromaticity coordinates of α-Na3Y1-xTmx(VO4)2 nanocrystals Doping concentration (x)

CIE chromaticity coordinates

Colour purity (%)

x

y

0.005

0.1513

0.1098

84.6

0.01

0.1519

0.1069

84.8

0.015

0.1634

0.1175

80.0

0.02

0.1660

0.1273

77.8

0.03

0.1645

0.1253

78.5

0.04

0.1699

0.1260

75.4

0.05

0.1675

0.1296

77.1

Fig. 14: The CIE Chromaticity coordinates of α-Na3Y1-xTmx(VO4)2 nanocrystals where x = 0, 0.005, 0.01, 0.015, 0.02, 0.03, 0.04 and 0.05. The inset corresponds to the digitial photograph of α-Na3Y0.985Tm0.015(VO4)2 nanocrystals and (b) enlarged view of the region containing CIE coordinates The CIE 1931 color matching functions are employed in calculating the color coordinates. The CIE chromaticity coordinates of α-Na3Y1-xTmx(VO4)2 nanocrystals where x= 0.005, 0.01, 0.015, 0.02, 0.03, 0.04, 0.05 are shown in the Table 2 and are indicated in the CIE chromaticity coordinate diagram as shown in the figure 14. The representative digital luminescence photographs of α-Na3Y0.985Tm0.015(VO4)2 nanocrystals is displayed in the inset. An enlarged view of the region containing CIE coordinates is given as Fig. 14(b) for better clarity. The obtained color coordinates lie within the blue region and the small change in color coordinates with doping concentration is due to the gradual increase in the degree of crystal distortion. The CIE Chromaticity co-ordinates x = 0.15 and y = 0.10 for the concentration value x= 0.015, are closer to the National Television Standard Committee (NTSC) standard values (x = 0.14 and y = 0.08). 5. Conclusion In summary, a series of thulium doped α-Na3Y(VO4)2 have been prepared successfully via citrate based solution combustion method. The XRD results revealed the pure monoclinic phase of the sample and TEM images depicted highly crystalline nature and excellent

uniform spherical morphology of the as-prepared nanocrystals. The chemical composition of the samples was confirmed from the EDX spectra of the sample. An intense and broad absorption band in the UV-Vis absorption spectra as well as in the photoluminescence excitation spectra indicate the efficient energy transfer from VO43- to Tm3+ in thulium doped α-Na3Y(VO4)2. The intense and sharp blue emission located at 474 nm is observed under the near UV excitation (310 nm). The optimal doping concentration of Tm3+ is about 1.5 mol % and the main concentration quenching mechanism is determined to be the dipole-dipole interaction. The CIE chromaticity coordinates of the α-Na3Y(VO4)2:Tm3+ is closer to National Television Standards Committee. The lifetime of Tm3+ characteristic blue emission is found to be of the order 0.1 ms and decay analysis of host emission monitored at blue emission also confirms the efficient energy transfer from vanadate to thulium ions. Thus, the as-prepared novel blue phosphors are promising candidate for the near UV excitable W-LEDs and display applications. Acknowledgments The author Linju Ann Jacob is thankful to UGC, Govt. of India for the financial assistance through the scheme of faculty development program (FDP). Sisira S is thankful to Human Resource Development Group, Council of Scientific & Industrial Research, Govt. of India for CSIR- JRF fellowship. Dinu Alexander is thankful to University Grants Commission, Govt. of India for the award of RFSMS fellowship. Kukku Thomas is thankful to UGC, Govt. of India for MANF fellowship. The authors acknowledge Govt. of India and Department of Science and Technology, Govt. of India for the financial assistance through SAP-DRS (No.F.530/12/DRS/2009 (SAP-1)) and DST-PURSE (SR/S9/Z23/2010/22(C,G)) programs, respectively. The authors acknowledge MoU-DAE-BRNS Project (No.2009/34/36/ BRNS/3174), Department of Physics, St. Thomas College Palai, India for extending the experimental facility.

Declarations of interest None References [1]

A.R. Dhobale, M. Mohapatra, V. Natarajan, S. V. Godbole, Synthesis and photoluminescence investigations of the white light emitting phosphor, vanadate garnet, Ca2NaMg2V3O12 co-doped with Dy and Sm, J. Lumin. 132 (2012) 293–298. doi:10.1016/j.jlumin.2011.09.004.

[2]

L. Li, X. Liu, H.M. Noh, B.K. Moon, B.C. Choi, J.H. Jeong, Photoluminescence of rare earth ions coactivated Ca9Y(VO4)7 with cold, natural and warm white emission, Mater. Chem. Phys. 158 (2015) 18–30. doi:10.1016/j.matchemphys.2015.03.029.

[3]

Y. Zhang, W. Gong, J. Yu, Y. Lin, G. Ning, Tunable white-light emission via energy transfer in single-phase LiGd(WO4)2:Re3+ (Re = Tm, Tb, Dy, Eu) phosphors for UVexcited WLEDs, RSC Adv. 5 (2015) 96272–96280. doi:10.1039/c5ra19345a.

[4]

W.T. Draai, G. Blasse, Energy transfer from vanadate to rare‐earth ions in calcium sulphate, Phys. Status Solidi. 21 (1974) 569–579. doi:10.1002/pssa.2210210221.

[5]

P. Huang, D. Chen, Y. Wang, Host-sensitized multicolor tunable luminescence of lanthanide ion doped one-dimensional YVO4 nano-crystals, J. Alloys Compd. 509 (2011) 3375–3381. doi:10.1016/j.jallcom.2010.12.069.

[6]

M.M. Kimani, J.W. Kolis, Synthesis and luminescence studies of a novel white Dy:K and yellow emitting phosphor Dy,Bi:K3Y(VO4)2 with potential application in white light emitting diodes, J. Lumin. 145 (2014) 492–497. doi:10.1016/j.jlumin.2013.07.054. 3Y(VO4)2

[7]

A. Duke John David, G.S. Muhammad, V. Sivakumar, Synthesis and photoluminescence properties of Sm3+substituted glaserite-type orthovanadates K3Y[VO4]2 with monoclinic structure, J. Lumin. 177 (2016) 104–110. doi:10.1016/j.jlumin.2016.04.025.

[8]

S.K. Hussain, T.T.H. Giang, J.S. Yu, UV excitation band induced novel Na3Gd(VO4)2:RE3+ (RE3+ = Eu3+ or Dy3+ or Sm3+) double vanadate phosphors for solid-state lightning applications, J. Alloys Compd. 739 (2018) 218–226. doi:10.1016/j.jallcom.2017.12.200.

[9]

Y. Wang, X. Liu, Y. Li, L. Jing, Synthesis and photoluminescence of multicolor tunable K3Gd(VO4)2:Eu3+ phosphors, Elsevier B.V, 2015. doi:10.1016/j.jallcom.2015.09.060.

[10]

B.Y.R. Salmon, C. Parent, G.L.E. Flem, M. Vlasse, The Crystal Structure of the Sodium Erbium Orthovanadate α-Na3Er(VO4)2, Acta Cryst. (1976). B32, 2799-2802. doi:10.1107/S0567740876008911

[11] Q. Zhang, Y. Hu, G. Ju, S. Zhang, F. Xue, Photoluminescence of a novel Na3Y(VO4)2:Eu3+ red phosphor for near ultraviolet light emitting diodes application, J. Mater. Sci. Mater. Electron. 28 (2017) 2529–2537. doi:10.1007/s10854-016-5827-0. [12] K.Y. Kim, S.J. Yoon, K. Park, Synthesis and photoluminescence properties of redemitting Ca3-3x/2(VO4)2:xEu3+ phosphors, J. Lumin. 160 (2015) 78–84.

doi:10.1016/j.jlumin.2014.11.046. [13] S.A. Miller, H.H. Caspers, H.E. Rast, Lattice vibrations of Yttrium Vanadate, Phys. Rev. 168 (1968) 964–969. doi:10.1103/PhysRev.168.964. [14] K. Thomas, D. Alexander, S. Sisira, P.R. Biju, N. V. Unnikrishnan, M.A. Ittyachen, C. Joseph, NUV/blue LED excitable intense green emitting terbium doped lanthanum molybdate nanophosphors for white LED applications, J. Mater. Sci. Mater. Electron. 28 (2017) 17702–17709. doi:10.1007/s10854-017-7708-6. [15] A.L. Patterson, The scherrer formula for X-ray particle size determination, Phys. Rev. 56 (1939) 978–982. doi:10.1103/PhysRev.56.978. [16] M. Sobczyk, Optical properties of α-Nd3+: Na3Y(VO 4)2 single crystals-Potential laser materials, Opt. Mater. (Amst). 35 (2013) 852–859. doi:10.1016/j.optmat.2012.10.043. [17] B.V. Rao, K. Jang, H. Sueb, S. Yi, J. Jeong, Synthesis and photoluminescence characterization of RE 3 + ( = Eu 3 + , Dy 3 + ) -activated Ca 3 La ( VO 4 ) 3 phosphors for white light-emitting diodes, J. Alloys Compd. 496 (2010) 251–255. doi:10.1016/j.jallcom.2009.12.175. [18] G. Du, X. Kan, Y. Han, Z. Sun, W. Guo, Citric acid assisted hydrothermal synthesis of microparticles of ErPO4:Dy 3+, Mater. Lett. 79 (2012) 55–57. doi:10.1016/j.matlet.2012.03.086. [19] X. Huang, H. Guo, Synthesis and photoluminescence properties of Eu3+-activated LiCa3ZnV3O12 white-emitting phosphors, RSC Adv. 8 (2018) 17132–17138. doi:10.1039/c8ra03075h. [20] C. Hsu, R.C. Powell, Energy transfer in europium doped yttrium vanadate crystals, J. Lumin. 10 (1975) 273–293. doi:10.1016/0022-2313(75)90051-4. [21] T. V. Gavrilović, D.J. Jovanović, K. Smits, M.D. Dramićanin, Multicolor upconversion luminescence of GdVO4:Ln3+/Yb3+ (Ln3+ = Ho3+, Er3+, Tm3+, Ho3+/Er3+/Tm3+) nanorods, Dye. Pigment. 126 (2016) 1–7. doi:10.1016/j.dyepig.2015.11.005. [22] H. Zhang, X. Fu, S. Niu, G. Sun, Q. Xin, Photoluminescence of YVO4:Tm phosphor prepared by a polymerizable complex method, Solid State Commun. 132 (2004) 527– 531. doi:10.1016/j.ssc.2004.09.008. [23] S.A. Mohitkar, G. Kalpana, K. Vidyasagar, Syntheses, structure and rare earth metal photoluminescence of new and known isostructural A2Mo4Sb2O18 (A=Ce, Pr, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) compounds, J. Solid State Chem. 184 (2011) 735– 740. doi:10.1016/j.jssc.2011.01.008. [24] B.R. Judd, Optical Absorption Intensities of Rare-Earth Ions, Phys. Rev. 197 (1962). 750-761 [25] G.S. Ofelt, Intensities of Crystal Spectra of Rare‐Earth Ions, J. Chem. Phys. 37 (2005) 511–520. doi:10.1063/1.1701366. [26] O. Silvestre, M.C. Pujol, M. Rico, F. Güell, M. Aguiló, F. Díaz, Thulium doped monoclinic KLu(WO4)2single crystals: Growth and spectroscopy, Appl. Phys. B Lasers Opt. 87 (2007) 707–716. doi:10.1007/s00340-007-2664-0. [27] W.T. Carnall, P.R. Fields, K. Rajnak, Electronic energy levels of the trivalent lanthanide aquo ions. IV. Eu8+, J. Chem. Phys. 49 (1968) 4424–4442. doi:10.1063/1.1669893.

[28] G. Blasse, Energy transfer in oxidic phosphors, Phys. Lett. A. 28 (1968) 444–445. doi:10.1016/0375-9601(68)90486-6. [29] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21 (1953) 836–850. doi:10.1063/1.1699044. [30] E.W. Barrera, C. Cascales, M.C. Pujol, K.H. Park, S.B. Choi, F. Rotermund, J.J. Carvajal, X. Mateos, M. Aguiló, F. Díaz, Synthesis of Tm:Lu 2 O 3 nanocrystals for phosphor blue applications, Phys. Procedia. 8 (2010) 142–150. doi:10.1016/j.phpro.2010.10.025.

Highlights • Tm3+ doped α-Na3Y(VO4)2 nanocrystals were studied for the first time • Uniform spherical morphology via citrate based solution combustion method • High purity blue emission in thulium activated nanocrystal by host sensitization • CIE coordinates close to the NTSC standards.

School of Pure & Applied Physics Mahatma Gandhi University Priyadarsini hills Kottayam-686560, Kerala, India

Dr. Cyriac Joseph Assistant professor

06-12-2019

I certify that the submitted manuscript is the report of our original research work and is not submitted elsewhere for publication and all authors have seen and approved the final version of the manuscript. Thanking you Yours sincerely

Dr. Cyriac Joseph

School of Pure & Applied Physics Mahatma Gandhi University Priyadarsini hills Kottayam-686560, Kerala, India

Dr. Cyriac Joseph Assistant professor

06-12-2019

Declaration of interests

The authors declare that they have no conflict of interests that could have influenced the work reported in this paper.

Dr. Cyriac Joseph