Observation of intermediate bands in Eu3+ doped YPO4 host: Li+ ioneffect and blue to pink light emitterAbdul Kareem Parchur, Amresh Ishawar Prasad, Shyam Bahadur Rai, Raghvendra Tewari, Ranjan Kumar Sahu et al. Citation: AIP Advances 2, 032119 (2012); doi: 10.1063/1.4739504 View online: http://dx.doi.org/10.1063/1.4739504 View Table of Contents: http://aipadvances.aip.org/resource/1/AAIDBI/v2/i3 Published by the American Institute of Physics. Related ArticlesBeam profile indicator for swift heavy ions using phosphor afterglow AIP Advances 2, 032116 (2012) Analysis of the radiative lifetime of Pr3+ d-f emission J. Appl. Phys. 112, 013536 (2012) Mn-activated K2ZrF6 and Na2ZrF6 phosphors: Sharp red and oscillatory blue-green emissions J. Appl. Phys. 112, 013506 (2012) Photoluminescence investigation of the indirect band gap and shallow impurities in icosahedral B12As2 J. Appl. Phys. 112, 013508 (2012) Strong blue light emission from a-SiNx:O films via localized surface plasmon enhancement Appl. Phys. Lett. 101, 013106 (2012) Additional information on AIP AdvancesJournal Homepage: http://aipadvances.aip.org Journal Information: http://aipadvances.aip.org/about/journal Top downloads: http://aipadvances.aip.org/most_downloaded Information for Authors: http://aipadvances.aip.org/authors

AIP ADVANCES 2, 032119 (2012)

Observation of intermediate bands in Eu3+ doped YPO4host: Li+ ion effect and blue to pink light emitter

Abdul Kareem Parchur,1 Amresh Ishawar Prasad,2 Shyam Bahadur Rai,1

Raghvendra Tewari,3 Ranjan Kumar Sahu,4 Gunadhor Singh Okram,5

Ram Asaray Singh,6 and Raghumani Singh Ningthoujam2,a

1Department of Physics, Banaras Hindu University, Varanasi 221 005, India2Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India3Material Science Division, Bhabha Atomic Research Centre, Mumbai 400 085, India4CSIR-National Metallurgical Laboratory, Jamshedpur 831 007, India5UGC-DAE Consortium for Scientific Research, Indore 452 001, India6Department of Physics, Dr. Hari Singh Gour University, Sagar 470 003, India

(Received 27 May 2012; accepted 13 July 2012; published online 20 July 2012)

This article explores the tuning of blue to pink colour generation from Li+ ion co-doped YPO4:5Eu nanoparticles prepared by polyol method at ∼100-120 ◦C withethylene glycol (EG) as a capping agent. Interaction of EG molecules capped on thesurface of the nanoparticles and/or created oxygen vacancies induces formation ofintermediate/mid gap bands in the host structure, which is supported by UV-Visibleabsorption data. Strong blue and pink colors can be observed in the cases of as-prepared and 500 ◦C annealed samples, respectively. Co-doping of Li+ enhances theemission intensities of intermediate band as well as Eu3+. On annealing as-preparedsample to 500 ◦C, the intermediate band emission intensity decreases, whereas Eu3+

emission intensity increases suggesting increase of extent of energy transfer fromthe intermediate band to Eu3+ on annealing. Emission intensity ratio of electric tomagnetic dipole transitions of Eu3+ can be varied by changing excitation wavelength.The X-ray photoelectron spectroscopy (XPS) study of as-prepared samples confirmsthe presence of oxygen vacancies and Eu3+ but absence of Eu2+. Dispersed particlesin ethanol and polymer film show the strong blue color, suggesting that these materialswill be useful as probes in life science and also in light emitting device applications.Copyright 2012 Author(s). This article is distributed under a Creative CommonsAttribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4739504]

I. INTRODUCTION

Eu3+ doped oxide, vanadate, molybdate, tungstate and phosphate compounds are the excellentred emitting phosphors due to their thermal and chemical stability.1–8 The great interest in dopingof trivalent lanthanide ions in host arises from their unique intra-configurational f-f transitions,which occur as sharp and intense emission lines. They are extensively used in plasma displaypanels (PDPs), field emission displays (FEDs), cathode ray tubes (CRTs), fluorescent lamps andlaser devices, etc.9–11 However the production of phosphor with uniform particle size distribution,small particle size, shape control and easy dispersion in polar solvents (like ethanol, methanol) arechallenging area. Moreover, decrease in particle size results more surface dangling bonds which canabsorb OH- and CO3

2- from the surrounding environment such as aqueous medium or atmosphereduring wet chemical synthesis route. Such small particles can be dispersible in polar medium tosome extent. If capping agent is added to such particles, the extent of dispersion in medium withlong duration increases. On other hand, variation of luminescence intensity is also dependent on

aAuthors to whom correspondence should be addressed, Phone: +91-22-2559321 and Fax: +91-22-25505151;E-mail: rsn@barc.gov.in (R. S. Ningthoujam)

2158-3226/2012/2(3)/032119/17 C© Author(s) 20122, 032119-1

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type of capping agents. i.e., Capping agent with long chain hydrocarbon having functional group-O-H, -COOH (e. g. ethylene di-ammine tetra-acetate (EDTA), citric acid) can reduce luminescenceintensity; whereas capping agent with short chain hydrocarbon (e. g. ethylene glycol (EG)) improvesluminescence as compared to those prepared in water medium.12 Recently, Eu3+ doped CaMoO4,GdPO4 and Tb3+ doped CaMoO4 are successively prepared by polyol method.13–15 Also, some othermethods such as hydrothermal and Pechini’s methods are used for the preparation of LaF3 and Y2O3

nanoparticles.16, 17

Among the oxide phosphors, YPO4 has a large indirect band gap (∼8.6 eV), high dielectricconstant (∼7-10), optically isotropic with a refractive index (∼1.72), high melting point (∼1600◦C) and small phonon energy (∼1080 cm-1).18–22 Such small vibration energy is a good choice ofthe host materials and will allow for effective radiative transitions between electronic energy levelsof the rare earth ions in YPO4 host. YPO4 has tetragonal structure with space group I41/amd (D19

4h ,zircon type, Z = 4) and the Y3+ ion occupies D2d site symmetry. The Y3+ ion is coordinated to the 8oxygen atoms to form dodecahedron and the PO4 tetrahedrons are isolated from each other in such amanner -YO8-PO4-YO8-PO4-. YO8 group has two different types of Y-O bonds; whereas PO4 grouphas one type of P-O bond. The bond length of four Y-O bonds is ∼2.313 Å and remaining four bondshave ∼2.374 Å.22 It was reported that Y-O bond length changes whereas P-O bond length almostremains the same on annealing YPO4 nanoparticles at higher temperatures.23

In past few years, Ningthoujam and his co-workers24 has examined the luminescence propertiesof Ln3+ ion doped YPO4 nanoparticles. It was found that the presence of water molecules up to 800 ◦Cin Ce3+ co-doped YPO4:Eu was observed and thus the luminescence intensity was quenched sig-nificantly. There are many research articles available to show the enhancement of luminescence inEu3+ doped YPO4 nanophosphor by co-doping Li+ and Bi3+ appreciably, in which the intensity ofmagnetic dipole transition is predominant over electric dipole transition.2, 25, 26 But in some cases,the intensity of electric dipole transition is predominant over magnetic dipole transition in Eu3+

doped (Y/La)PO4/YPO4.nH2O, which is expected from the theoretical view on site symmetry ofY3+ where Eu3+ ions occupy.27–32

Existence of intermediate bands observed in YPO4 system has not been discussed much inliterature. This understanding will be useful in transfer process from such intermediate bands toEu3+. Generation of light emitting diodes (LED) in green and red regions has been reported.However, material with efficient blue emitter is challenging to LED applications. Could the blueemitter be produced from YPO4:Eu3+?

In this study, we have prepared 5 at.% Eu3+ doped YPO4 (YPO4:5Eu) and Li+ co-dopedYPO4:5Eu at relatively low temperature of ∼100-120 ◦C for 1 h using polyol route where EGmolecules act as capping agent as well as reaction medium. Their detail crystal structure and lumi-nescence are studied. Variation in luminescence intensities of magnetic and electric dipole transitionswith different concentrations of Li+ and heat treatment is observed. Interestingly, intermediate bandsbetween band gap of YPO4 are observed. Variation in color can be observed in this study by modi-fication of surface and particle size/annealing temperature.

II. EXPERIMENTAL SECTION

A. Materials and synthesis

The nanoparticles of 5 at.% Eu3+ doped YPO4 (YPO4:5Eu) and Li+ (Li+ = 3, 5, 7 and 10 at.%)co-doped YPO4:5Eu are prepared at low temperature (∼100-120 ◦C) for 1 h in ethylene glycol (EG).The starting materials for Y3+, PO4

3-, Eu3+, Li+ are yttrium oxide (Y2O3, 99.99%, Sigma Aldrich),ammonium dihydrogen phosphate (NH4H2PO4, 99.999%, Sigma Aldrich), europium oxide (Eu2O3,99.9%, Sigma Aldrich) and lithium hydroxide (LiOH, 99.99%, Sigma Aldrich), respectively. Intypical synthesis procedure of 5 at.% Li+ and 5 at.% Eu3+ doped YPO4 nanoparticles, 1 g of Y2O3,0.086 g of Eu2O3 and 0.012 g of LiOH were dissolved together in concentrated nitric acid (HNO3)in a 250 ml two necked round bottom flask and were heated at 80 ◦C with addition of double distilledwater at least five times in order to remove the excess acid. To this 1.13 g of NH4H2PO4 dissolvedin 10 ml of double distilled water and 100 ml of ethylene glycol (EG) were added. The solution

032119-3 Parchur et al. AIP Advances 2, 032119 (2012)

was stirred for 2 h and followed by 1 h sonication for uniform mixing. The reaction mixture washeated at ∼100-120 ◦C for 1 h under refluxing condition until the white precipitation is completed.EG molecules act as a solvent as well as capping agent during the reaction for YPO4 nanoparticles.When the nucleation starts, surrounding EG molecules cap smaller particles and thus, particle growthis slow. The agglomeration among the particles is hindered. Dielectric medium of reaction is alsohelp in controlling particle size. The precipitate so obtained was washed two times by centrifugationin ethanol to remove the excess of EG, and then dried at room temperature for four days. Finally,the as-prepared sample is divided into two parts: one part of the sample was annealed at 500 ◦Cin ambient atmosphere at the rate of 2 ◦C/min for 4 h. During this, organic capping agent (EG) isremoved as CO2 and H2O gaseous due to burning. In addition, only uncapped particles remain andgrowth of particles occurs.

B. Material characterization

The crystal structure of the material was identified by PW 1071 Philips powder X-ray diffrac-tometer (XRD) with Ni filtered Cu-kα (1.5405 Å) radiation at 30 kV and 20 mA. All patterns wererecorded over the angular range 10 ≤ 2θ /deg ≤ 70 with a step size of �2θ = 0.02◦. The Scher-rer relation was used to calculate the average crystallite size from XRD spectrum. The relation isexpressed as follows:

D = 0.89λ

βhklCosθ(1)

where λ is the wavelength of the X-ray and βhkl the full width at half maximum (FWHM) of peak ofthe XRD pattern.33 The contribution of instrument to FWHM is removed by using standard Si.13

Transmission electron microscopy (TEM) image of samples was recorded using a JEOL at anacceleration voltage of 200 kV. For TEM measurement, the samples were grinded and mixed togetherwith EG and dispersed particles could be achieved under ultrasonic vibration for 1 h. A drop ofthe dispersed particles was put over the carbon coated copper grid and evaporated to dryness in theambient atmosphere. The high resolution TEM (HRTEM) images were recorded at 300 keV usingFEI Titan Microscopy. Infrared (IR) spectrum was measured with a FTIR spectrometer (BomemMB 102) with a resolution of 1 cm-1. The sample was mixed with KBr (Sigma Aldrich, 99.99%) in1:5 ratio and pellet was prepared. Such pellet was used to record the spectra. The photoluminescence(PL) spectra of these powder phosphors were recorded using Hitachi F-4500 spectrometer with a150 W Xe lamp as a source at the spectral resolution of 3 nm. All the measurements were carriedout at room temperature. PL decay was recorded with Edinburgh instrument F920 equipped withNd-YAG laser pumped optical parameters oscillator (OPO) having a pulse width of 10 ns andrepetition frequency of 10 Hz as the excitation source. Chemical bonding energies of ions in samplewere measured using X-ray photoelectron spectroscopy (XPS) SPECS, Germany (Mg Kα X-raysource, hv = 1253.6 eV). UV-Visible absorption spectra were recorded using Simadzu UV 310PCspectrophotometer.

III. RESULTS AND DISCUSSION

A. XRD study

The XRD patterns of as-prepared and 500 ◦C annealed 0-10 at.% Li+ co-doped YPO4:5Eusamples are matched with JCPDS card no: 11-0254. The patterns show that samples have singlephase and exhibit tetragonal structure of YPO4 having space group I41/amd. Figure 1 shows the typicalXRD pattern of 10 at.% Li+ co-doped YPO4:5Eu annealed at 500 ◦C. The average crystallite sizesof 10 at.% Li+ co-doped YPO4:5Eu as-prepared and 500 ◦C annealed samples are found to be ∼18and 42 nm, respectively. The samples show high crystalline nature and intensity increases with Li+

ion concentration. Also annealing at 500 ◦C confirms the increase in crystalline nature.

032119-4 Parchur et al. AIP Advances 2, 032119 (2012)

FIG. 1. Typical XRD pattern of 500 ◦C annealed sample of 10 at.% Li+ co-doped YPO4:5Eu.

FIG. 2. FTIR spectra of as-prepared and 500 ◦C annealed samples of 5 at.% Li+ co-doped YPO4:5Eu.

B. FTIR study

Figure 2 shows the FTIR spectra of 5 at.% Li+ co-doped YPO4:5Eu nanoparticles of as-preparedand 500 ◦C annealed samples. Both show similar pattern in peaks except at ∼883 and 1384 cm-1 inthe case of as-prepared sample. The peaks at ∼538 and 630 cm-1 correspond to the bending vibrationsof PO4

3- (termed as v4 region). The strong bands centered at 1043 and 1084 cm-1 correspond tostretching vibrations of PO4

3- (termed as v3 region).34, 35 The vibration intensity of v3 region is foundto be 18% stronger than that of v4 region. The integrated intensities of both v3 and v4 are found toincrease with annealing temperature. We do not find any extra phosphorus containing groups suchas P2O7

4- which is generally observed at ∼1265 cm-1.36 The intensities of peaks correspondingto bending and stretching vibrations of O-H/H2O centered at ∼1644 and 3400 cm-1 decrease withannealing temperature. In the case of as-prepared sample, the weak peaks due to stretching vibrationsof H-C-H group of EG are found at 2880 and 2948 cm-1 and the peaks centered at ∼883 and 1384 cm-1

correspond to NO3- group which may be originated from the presence of HNO3 which is added

in reaction.37 The OH- ions on the surface of particle act as source of quencher to luminescenceintensity and the luminescence intensity can be significantly enhanced after removing OH- ionsby heat treatment (discussed later). The peak at ∼2349 cm-1 corresponds to asymmetric stretchingvibration of CO2, which arises from absorption over particle or IR lamp passing through air medium.

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FIG. 3. (a) TEM and (b) HRTEM images of as-prepared sample and (c) HRTEM image of 500 ◦C annealed sample of 5at.% Li+ co-doped YPO4:5Eu. (d) SAED pattern of (a).

C. TEM study

Transmission electron microscope (TEM) and High resolution TEM (HRTEM) are used tocharacterize the morphology and structure of as-prepared and 500 ◦C annealed 5 at.% Li+ co-dopedYPO4:5Eu samples. Figures 3(a) and 3(b) show TEM and HRTEM images of as-prepared sample.The well-resolved lattice fringes of as-prepared sample having inter-planar distance (2.79 Å) areobserved and are assigned to (211) plane of tetragonal system after Fast-Fourier-Transform (FFT).Figure 3(c) shows lattice fringes of its 500 ◦C annealed sample. Two types of lattice fringes havinginter-planar spacing of 3.43 and 2.82 Å corresponding to the (200) and (211) planes of tetragonalphase of YPO4 are found. The selected area electron diffraction pattern of as-prepared sample isshown in Figure 3(d). The high crystallinity is confirmed by the rings as shown in Figure 3(d).The assignment of the rings is shown in figure itself. Spherical particle sizes of 40 and 70 nm areobserved from as-prepared and 500 ◦C annealed samples, respectively.

D. XPS study

To study the Li+ ion effect on chemical binding energies of ions in YPO4:5Eu compound, XPSmeasurements of as-prepared 0 and 10 at.% Li+ co-doped YPO4:5Eu samples are studied (Fig. 4).The XPS spectrum in Fig. 4(a) shows the peak corresponding to Li (1s) having core binding energy(BE) ∼44.5 eV for 10 at.% Li+ co-doped YPO4:5Eu sample. Figure 4(b) shows the peaks at ∼156.17and 156.82 eV corresponding to Y(3d3/2) and ∼131.18 and 131.95 eV corresponding to Eu2+(4d3/2)

032119-6 Parchur et al. AIP Advances 2, 032119 (2012)

FIG. 4. XPS spectra of as-prepared samples of 0 and 10 at.% Li+ co-doped YPO4:5Eu. Peaks corresponding to the corebinding energies of individual elements are shown in (a)-(e). Binding energy of Eu2+(4d5/2) is missing whereas that ofEu3+(4d5/2) is observed after the expansion (c).

for 0 and 10 at.% Li+ co-doped YPO4:5Eu samples, respectively. Notably, the peak for P(2p) fallson 131-132 eV. It is difficult to distinguish Eu2+(4d3/2) and P(2p). When the spectra in range of125-144 eV is expanded (Fig. 4(c)), we observe a small peak at ∼141.1 eV which corresponds toEu3+(4d3/2) and no peak corresponding to Eu2+(4d5/2) is observed at ∼127.1 eV.24 This confirms thehigh probability of Eu3+ present in samples. Figure 4(d) shows the XPS spectra of Y(2p1/2) and itscorresponding peaks at ∼299.4, 300.1 eV for 0 and 10 at.% Li+ YPO4:5Eu samples are observed.The O(1s) peaks for 0 and 10 at.% Li+ YPO4:5Eu samples are found to be at 529.1 and 529.8 eV,are observed (Fig. 4(e)). From this study, BE of individual ions increases on Li+ co-doping. Thissuggests the improvement of crystallinity on Li+ co-doping (i.e., defect decreases). Shifting of BEsignifies that possibility of change in positive charge and/or chemical environment around Y3+/Eu3+.XPS study confirms that the Li+ co-doping changes the chemical environment around Y3+/Eu3+.It is to be noted that there is asymmetric nature of O(1s) peak at the higher energy site (i.e., humpat 530.5 eV), which is signature of oxygen vacancy in lattice.38 Deconvolution of peak (O(1s)) ofas-prepared 0 and 10 at.% Li+ co-doped YPO4:5Eu samples using Lorentzian distribution function

032119-7 Parchur et al. AIP Advances 2, 032119 (2012)

FIG. 5. Excitation spectra of Li+ (0, 3, 5, 7 and 10 at.%) co-doped YPO4:5Eu nanoparticles: (a) as-prepared and (b) 500 ◦Cannealed samples (monitoring emission at 594 nm).

is shown in Figs. S1(a) and S1(b).39 Here, symbol ‘S’ refers to Supplemental Material. Two peaks(∼ 529 and 530 eV) could be fitted well.

E. Luminescence study

1. Excitation study

Figure 5(a) shows excitation spectra of as-prepared samples of Li+ (0, 3, 5, 7 and 10 at.%)co-doped YPO4:5Eu by monitoring the emission wavelength at 594 nm. The excitation spectrumconsists of strong absorption band between 225-280 nm with center at ∼255 nm and full widthat half maxima (FWHM) ∼25 nm, which can be assigned to the charge transfer from O2- toEu3+ (Eu-O CT). There are sharp absorption peaks at 366, 386 and 399 nm, which correspondto 7F0,1→5D4, 7F0,1→5G1,5L7 and 7F0→5L6 of Eu3+, respectively.3, 24 The absorption intensity of7F0→5L6 transition of Eu3+ ion at 399 nm (FWHM ∼6 nm) is 2.2 times stronger than Eu-O CTabsorption indicating a weak energy transfer from Eu-O CT band to Eu3+.40, 41 Figure 5(b) shows theexcitation spectra of 500 ◦C annealed samples by monitoring the emission wavelength at ∼594 nm.Peaks are similar to as-prepared samples. However, Eu-O CT band is found at ∼241 nm and itsintensity is less than that of as-prepared sample. The blue shift of Eu-O CT band with respectto as-prepared sample indicates increase of ionicity with annealing. In pure Eu2O3,42 there is noabsorption peak at ∼340-350 nm and also intensity of Eu-O CT band is more than that of Eu3+

(399 nm). In this study, absorption intensity in ∼340-350 nm is very high and also intensity of Eu-O

032119-8 Parchur et al. AIP Advances 2, 032119 (2012)

FIG. 6. Luminescence spectra of YPO4:5Eu: as-prepared and 500 ◦C annealed samples at different excitation wavelengths,375 nm filter is used for 240-350 nm excitations and no filter for 399 nm excitation.

CT band is much less than that of Eu3+ (399 nm). It means that there is host/intermediate bandabsorption. However, pure YPO4 has band gap of 8.6 eV (145 nm), which is more than the energy ofEu-O CT band. Large absorption band in ∼300-500 nm in excitation spectra (Fig. 5) indicates thatthere are intermediate bands/localized levels between the band gap of YPO4. Similar observationswere reported in Eu3+/Li co-doped LaPO4 and ZnO.29, 43 In overall, Li+ doping improves absorptionintensity.

2. Emission study

We have recorded the emission spectra of Li+ (0, 3, 5, 7 and 10 at.%) co-doped YPO4:5Eu atdifferent excitation wavelengths: 240-300 nm, 340, 350 and 399 nm. Samples with Li+ = 0, 5 and 10at.% are shown in Figs. 6–8 and remaining samples are shown in Figures S2 and S3.39 We have used375 nm filter for recording emission spectra in all excitations except 399 nm (no filter is used). In theemission spectra of YPO4:5Eu (Fig. 6), the broad emission intensity in 400-550 nm with maximumat 430 nm in case of as-prepared sample and at 460 nm in case of 500 ◦C annealed sample whenexcited at λ ≥ 270 nm are observed. This broad peak emission is related to host or intermediatebands. The red-shift in emission peak is related to increase of crystallite size on annealing. Theintensity of broad emission increases with increasing excitation wavelength from 240 to 399 nmand even is much more than that of Eu3+ emission at 593 and 615 nm (magnetic 5D0→7F1 andelectric 5D0→7F2 dipole transitions, respectively). It is expected that luminescence intensity of Eu3+

should be more on excitation at 240-250 nm (Eu-O CT band) as compared to that at 399 nm because

032119-9 Parchur et al. AIP Advances 2, 032119 (2012)

FIG. 7. Luminescence spectra of 5 at.% Li+ co-doped YPO4:5Eu: as-prepared and 500 ◦C annealed samples at differentexcitation wavelengths, 375 nm filter is used for 240-350 nm excitations and no filter for 399 nm excitation.

of energy transfer from Eu-O CT band to Eu3+, but this is opposite to this observation. It meansthat there are intermediate absorption bands between band gap of YPO4. This may be related tointeraction with absorbed gases (CO3

2-, H2O) or capping agent (EG) in case of as-prepared sampleor carbon remained in case of 500 ◦C annealed sample or defects present in lattice. The emissionintensities after excitation at 340, 350 and 399 nm in case of as-prepared sample are very high andfound to saturate, whereas 500 ◦C annealed sample shows the saturation at 399 nm excitation. In thecase of 500 ◦C annealed sample, the host/intermediate bands emission intensity decreases and thebroadening of emission peak in ∼400-570 nm occurs as compared to as-prepared sample. This maybe related to the extent of decrease of defect or absorbed gases or capping agent (EG) on annealingand thus it enhances energy transfer rate from host/intermediate band to Eu3+.

When Li+ is co-doped in to YPO4:5Eu, the intensity related to host/intermediate bands increasesup to Li+ = 3 at.% and then decreases with further increase of Li+ in case of as-prepared samples(Figs. 7 and 8 and Figs. S2 and S3).39 In case of 500 ◦C, this intensity remains almost unchangedexcept 10 at.% of Li+ where intensity is found to be high. In all Li+ co-doping systems, the 500 ◦Cannealed samples show lower luminescence intensity related to host/intermediate as compared toas-prepared sample. This trend is similar to that without Li+ co-doping. Overall, the peak maximumcorresponding to the host/intermediate shifts to the higher wavelength by ∼10-20 nm with Li+

co-doping up to 10 at.%. This may be related to the increase in crystallite/particle size on annealing.In addition to strong host/intermediate band emission, the electric and magnetic dipole transi-

tions of Eu3+ at 593 and 615 nm are observed. Their intensity variations with excitation wavelengthsas well as Li+ concentrations are found. These are clearly shown in Figs. S4–S13.39 Emission

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FIG. 8. Luminescence spectra of 10 at.% Li+ co-doped YPO4:5Eu: as-prepared and 500 ◦C annealed samples at differentexcitation wavelengths, 375 nm filter is used for 240-350 nm excitations and no filter for 399 nm excitation.

intensities at different excitation wavelengths are compared by fitting the area under electric andmagnetic dipole transitions centered at 593 and 615 nm, respectively using Gaussian distributionfunction:

I = IB +n∑

i=1

Ai

wi√

π/2e− 2(λ−λi )2

w2i (2)

where I is the observed intensity, IB the background intensity, wi the width at half maximum intensityof the curve and Ai area under the curve. λ is wavelength and λi is the mean wavelength valuecorresponding to the transition. During calculations of peak areas under electric and magnetic dipoletransitions the wavelength range 580-630 nm is used. The asymmetric environment of europiumion (Eu3+) in host lattice can be calculated by intensity ratio of the electric (5D0 → 7F2) tomagnetic (5D0→ 7F1) dipole transitions. This is known as asymmetric ratio represented by A21,where subscripts ‘2’ and ‘1’ refer transitions of 5D0 to 7Fj, j = 2 and 1, respectively. The A21 isdefined as

A21 =

630∫

600

I2 dλ

600∫

580

I1 dλ

(3)

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TABLE I. Asymmetric ratio (A21) values of as-prepared and 500 ◦C annealed samples of Li+ (0, 3, 5, 7 and 10 at.%)co-doped YPO4:5Eu under 240, 250, 260, 270, 280, 290, 300, 340, 350 and 399 nm excitations.

Sample A21 calculated from different excitation wavelengths (nm) atat.% Li+ 240 250 260 270 280 290 300 340 350 399

As-prepared 0 0.61 0.66 0.68 0.69 1.20 1.11 1.07 0.69 1.08 0.643 0.47 0.47 0.48 0.47 0.96 0.83 0.83 0.48 0.93 0.465 0.56 0.59 0.60 0.61 1.05 1.02 1.09 0.60 0.98 0.687 0.54 0.67 0.65 0.63 1.12 1.12 1.08 0.65 1.13 0.7210 0.56 0.60 0.62 0.60 1.30 0.98 0.95 0.60 1.01 0.65

500 ◦C 0 0.90 1.00 1.05 1.09 1.07 1.02 1.02 0.99 0.96 0.883 1.03 1.04 1.07 1.12 1.12 1.11 1.08 0.97 0.98 0.955 0.88 1.04 1.06 1.06 1.09 1.03 1.02 1.00 0.97 0.967 1.07 1.21 1.34 1.28 1.33 1.21 1.31 1.19 1.19 1.1110 1.00 1.16 1.30 1.30 1.30 1.28 1.28 1.24 1.20 1.05

FIG. 9. Variation of (a) A2 (intensity of electric dipole transition, 5D0 → 7F2), (b) w2 (its FWHM, nm) and (c) A21 (asymmetricratio) for Li+ (0, 3, 5, 7 and 10 at.%) co-doped YPO4:5Eu: as-prepared and 500 ◦C annealed samples under 399 nm excitation.

A21 values for as-prepared samples at different excitation wavelengths are calculated. Table I givesthe A21 values of all as-prepared and 500 ◦C annealed samples at different excitation wavelengths.

In case of as-prepared samples, luminescence intensity of Eu3+ increases with Li+ up to 3at.% and then decreases for excitations at 240-270, 340 and 399 nm. Figures 9(a)–9(c) show thevariation of intensity of electrical dipole transition (A2), its FWHM and asymmetric ratio (A21) withLi+ at 399 nm excitation. A2 increases with Li+ up to 3 at.% and decreases with further increaseof Li+. Its FWHM decreases from ∼8.1 to 7.8 nm as Li+ concentration increases up to 10 at.%.A21 value is found to be less than 1.0, indicating higher intensity for magnetic dipole transition overthat for electric dipole transition. It is to be noted that A21 is found to be ≥1.0 at 280-300 and 350nm excitations (Table I). When we see site symmetry of Eu3+ in YPO4, it should be D2d whichhas asymmetric environment with two different Eu-O bond lengths.22, 23, 44 In such view, the electric

032119-12 Parchur et al. AIP Advances 2, 032119 (2012)

dipole transition should be more than the magnetic dipole transition (i. e., A21 ≥ 1.0). Variationin intensities of these two dipole transitions at different excitation wavelengths was not discussedin the previous studies to the best of authors’ knowledge. This may be related to the interactionof incoming excitation light with Eu3+ environment (nearest environment (O) and second nearestenvironment (PO4). The interaction parameter will depend on incoming wavelength and intensity.This revelation after analysis is one of present findings.

In case of 500 ◦C annealed samples, variation of luminescence intensity with Li+ is similarto as-prepared samples, but their luminescence intensity and FWHM are higher than as-preparedsamples for 399 nm excitation (Figs. 9(a) and 9(b)). Increase in luminescence intensity is related todecrease of non-radiative rate from surface dangling bonds and capping ligand (EG) on annealing.A21 is found to close to 1.0 at all excitation wavelengths and such behavior is different from as-prepared samples. Highest A21 is found to be ∼1.34 for 7 at.% Li+ co-doped YPO4:5Eu under 260and 280 nm for 500 ◦C annealed sample. Its value varies between ∼0.9-4.0 in some glass systemsand ∼7-10 in CaMoO4 hosts.3, 45, 46

Sohn et al.27 reported the effect of excitation wavelengths at ∼147 nm (host excitation) and254 nm (Eu-O CT excitation)) on emission intensity and A21 of Eu3+ doped YPO4. At 254 nmexcitation, A21 is 1:1, whereas this becomes 1:0.8 at ∼147 nm excitation. Ningthoujam and his co-workers47 have examined the luminescence properties of Eu3+ ion doped YPO4 nanoparticles cappedby ethylenediamine tetraacetic acid (EDTA). It is found that the excitation wavelength significantlyinfluences the emission intensity and A21 due to the presence of capped layer on the surface ofnanoparticles. Recently, Li and his coworkers29 studied white light emission from oleic acid cappedLaPO4:Eu3+ nanorods. Due to the formation of intermediate state/mid-gap states as a consequence ofthe interaction of chemical bonding of oleic acid to LaPO4:Eu3+, the color coordinates from Red toWhite under different excitation wavelengths (270-395 nm) were obtained. Guo and his coworkers31

found the higher luminescence intensity at electric dipole transition than that at magnetic dipoletransition in glass-ceramic having Eu3+ doped YPO4 nano crystals. In addition to Eu3+ they foundthe presence of Eu2+ in glass matrix which results broad emission peak centered at ∼436 nmdue to the 5d-4f transition of Eu2+. Conversion from Eu3+ to Eu2+ is related to the reductionprocess occurred from fluoride used in the preparation of glass-ceramic. Ray and his coworkers32

prepared YPO4:Eu nanorods and nanoparticles, which have dominant in electric dipole transitionthan magnetic dipole transition. From above literatures, variation of luminescence intensities ofelectric and magnetic dipole transitions depends on sample characteristics. Now, we have a questionfor origin of broad luminescence observed in ∼400-500 nm whether it is from the host or due toEu2+ present in the sample. In our study, the probability of reduction of Eu3+ to Eu2+ is very lowsince there is no fluoride precursor during preparation. Also, our XPS study (Fig. 4) confirms the nopeak corresponding to Eu2+ in the sample spectrum. This suggests that the observed luminescencein ∼400-500 nm is related to the presence of intermediate bands within host, which arises fromoxygen defect in lattice (from XPS study) or capping ligand interaction with surface of particles oradsorbed gases over surface of particles (from IR study).

Figures 10(a) and 10(b) show the photographs of 5 at.% Li+ co-doped YPO4:5Eu as-preparedand 500 ◦C annealed nanoparticles under the Nd-YAG laser excitation at 266 nm (power ∼ 0.3 W atfocusing spot). As-prepared sample shows strong blue color whereas 500 ◦C annealed sample showspink colour. When we see emission spectrum for 260-270 nm excitation (Fig. 7, in which μW poweris used), it is expected that blue-green colour should be instead of pink colour. In our opinion, thehigh power of laser makes electric and magnetic dipole transition intensities more prominent overhost emission for 500 ◦C sample (pink colour).

The presence of EG molecules capped on surface of as-prepared Li+ co-doped YPO4:5Euparticles are useful to easy dispersion of nanoparticles in polar solvents like ethanol and polyvinylalcohol (PVA). For a typical dispersion, 10 mg of as-prepared 5 at.% Li+ co-doped YPO4:5Eu isdispersed in 5 ml of ethanol followed by ultrasonication. Figure 10(c) shows photograph of re-dispersed as-prepared 5 at.% Li+ co-doped YPO4:5Eu, before and after exposure of 266 nm laserexcitation. Further, to make a thin film of 5 at.% Li+ co-doped YPO4:5Eu, 10 mg of as-preparedsample is mixed with 2.5 ml of distilled water. To that, 1 g of PVA and 2.5 ml of ethanol are added.The solution is ultrasonicated for 30 min to make uniform dispersion. This solution was placed

032119-13 Parchur et al. AIP Advances 2, 032119 (2012)

FIG. 10. Photographs of 5 at.% Li+ co-doped YPO4:5Eu: (a) as-prepared, (b) 500 ◦C annealed nanoparticles, (c) as-preparednanoparticles re-dispersion in ethanol and (d) as-prepared nanoparticles re-dispersion in PVA thin film before and after266 nm laser excitation. All photographs are recorded using Nikon Coolpix P500 digital camera.

over poly petri dish. It is kept for 4 days at room temperature for drying. In this way polymerfilms of 5 at.% Li+ co-doped YPO4:5Eu having thickness ∼0.2-0.3 mm and ∼10 cm diameter areprepared. The film shows very bright blue under 266 nm laser excitation (Fig. 10(d)). Both dispersedparticles in the ethanol and the film show blue colour with the CIE (Commission internationale del’Eclairage) chromaticity co-ordinates (0.17, 0.17). Li et al.29 reported strong blue emission of oleicacid capped Eu3+ doped LaPO4 having CIE values around (0.22, 0.13) under 380 nm excitation.Uniform brightness of the thin film confirms the homogeneous distribution of particles in PVAmatrix. It is found that peak positions of electric and magnetic dipole transitions are unaffectedafter dispersion of nanoparticles in PVA polymer matrix (which is not shown here). The emissionintensity of as-prepared nanoparticles re-dispersed in ethanol and PVA matrix is slightly less thanthe powder sample. This is due to the presence of less no of Eu3+ ions per unit volume of dispersion.This film will be useful in the development of optoelectronic devises.

Figure 11 shows the schematic diagram of energy levels in Li+ co-doped YPO4:5Eu. Band gapof YPO4 (8.6 eV = 145 nm) is more than Eu-O charge transfer band (4.8-5.1 eV = 240-260 nm)and their band edges are crossing each other.18 Because of defect on surface or capping ligandson particles, there is possibility to have intermediate bands, which is also supported by UV-Visibleabsorption measurement (Fig. 12). Absorption band extends up to 500 nm. Such intermediate bandabsorption helps in enhancement of luminescence intensity by energy transfer (ET) to Eu3+. Similartype of results are shown in case of LaPO4 nanoparticles.29 After excitation at ∼280-399 nm, theelectrons from the valence band go to excited state which falls on intermediate bands/mid gap statesand holes are created at the valence band. After the removing the excitation source, the electronscomes to the valence band (ground state). During this, the electron-hole recombination takes placeresulting to emission at blue to pink colour regions. In addition, emission due to Eu3+ (5D0 → 7F1

and 5D0 → 7F2) could be observed due to energy transfer from intermediate bands or Eu-O CT bandto the excited state of Eu3+.

032119-14 Parchur et al. AIP Advances 2, 032119 (2012)

FIG. 11. Schematic diagram for energy transfer process among the Eu-O, intermediate bands and Eu3+ state in YPO4:5Eu.

FIG. 12. UV-Visible absorption spectra of as-prepared and 500 ◦C annealed 5 at.% Li+ co-doped YPO4:5Eu nanoparticles.

F. Luminescence Decay Study

The luminescence decay curves of the level 5D0 (593 nm) of Eu3+ for as-prepared and500 ◦C annealed samples of Li+ (0, 3 and 10 at.%) co-doped YPO4:5Eu have been shown inFigs. 13(a)–13(c). Excitation wavelength is fixed at 464 nm from Nd-YAG laser. The decay curvesare not well fitted by mono-exponential equation (I = I0 exp(t/τ )). The typical mono-exponential curvefitting to data of 3 at.% Li+ doped YPO4:5Eu annealed at 500 ◦C with χ2 = 3.195 is shown inFigure S14 and parameters obtained after fitting is given in Table S1.39 The fitting behavior can beclearly understood by plotting ln(I) vs. t, which is shown in Fig. S14 (inset).39 The fitted straightline does not match with decay data points. This suggests that environment of Eu3+ ions in latticeare not same in different positions.

All decay data are fitted by using bi-exponential decay equation, which is expressed as

I = I1e(−t/τ1) + I2e(−t/τ2), (4)

032119-15 Parchur et al. AIP Advances 2, 032119 (2012)

FIG. 13. Lifetime decay spectra of as-prepared and 500 ◦C annealed Li+ doped YPO4:5Eu nanoparticles of (a) Li+ = 0,(b) Li+ = 3 and (c) Li+ = 10 at.% under 464 nm laser excitation. Emission is monitored at 590 nm. (d) Bi-exponential fittingto luminescence decay data of 3 at.% Li+ doped 500 ◦C annealed sample and fitting parameters are shown in the figure itself.The y-axis in (a)-(c) are represented on log scale.

where I1 and I2 are the intensities at different time intervals and τ 1 and τ 2 their correspondinglifetimes. The bi-exponential fitting to luminescence decay data of 500 ◦C annealed 3 at.% Li+

co-doped YPO4:5Eu nanoparticles is shown in Fig. 13(d). The fitting parameters are given in figureitself. The average lifetime can be calculated using the equation,

τav = I1τ21 + I2τ

22

I1τ1 + I2τ2(5)

The parameters obtained after bi-exponential equation fitted to data are given in Table II. In caseof as-prepared samples, the lifetime of Eu3+ increases from 1.05 to 1.26 ms with increasing Li+

concentration from 0 to 3 at.% and decreases with further increase of Li+ (i.e. 10 at.% Li+, wherelifetime is 1.09 ms). This behavior is similar to variation in luminescence intensity with Li+ (Fig. 9).In case of 500 ◦C annealed samples, lifetime value of 1.52 ms is observed for Li+ = 0 and 3 at.%and 1.85 ms for Li+ = 10 at.%. The lifetime value increases on annealing. The bi-exponential decaysuggests the availability of Eu3+ ions on surface (τ 1) and core (τ 2) of particles. On annealing, thepercentage of τ 1 decreases and that of τ 2 increases. This indicates a decrease of non-radiative ratefrom surface of particles on annealing.

IV. CONCLUSIONS

Li+ (Li+ = 0, 3, 5, 7 and 10 at.%) co-doped YPO4:5Eu nanoparticles are prepared usingpolyol method at reaction temperature 100-120 ◦C. As-prepared samples are annealed to 500 ◦C toremove the presence of dangling bonds on the surface of nanoparticles. In the case of as-prepared

032119-16 Parchur et al. AIP Advances 2, 032119 (2012)

TABLE II. Parameters obtained after bi-exponential decay fit to data of as-prepared (ASP) and 500 ◦C annealed samples(500 ◦C). Excitation and emission wavelengths are fixed at 464 and 594 nm, respectively. Nd-YAG laser source is used.

Li+ I1 τ 1 I2 τ 2 τ ava

Sample (at.%) (%) (μs) (%) (μs) (μs) χ2b

ASP 0 81 572 19 1728 1052 0.972500 ◦C 79 473 21 2337 1520 1.456ASP 3 87 540 13 2380 1259 1.287500 ◦C 81 518 19 2436 1520 1.484ASP 10 85 547 15 1966 1094 1.166500 ◦C 82 486 18 2887 1855 1.598

aτav = I1 τ21 + I2 τ2

2I1τ1 + I2τ2

.bχ2 = ∑

k wk2[Xk − Fk ]2/n, χ2 is goodness of fitting, wk is weighting factor for data points (wk = 1/

√Fk ), Xk is the

calculated lifetime and Fk is the measured lifetime data.

samples, the formation of intermediate bands/mid gap states is found. The intensities of boththe intermediate bands (400-500 nm) and Eu3+ (500-700 nm) emission of the samples increasewith Li+ up to 3 at.% and then decrease with increasing Li+ in the case of as-prepared samples.However, significant change has not been observed in intermediate band emission intensities for500 ◦C annealed samples with/without co-doping of Li+ and its emission intensity is slightly lessthan as-prepared sample except 10 at.% Li+ co-doping. However the intensity of Eu3+ emissionis significantly enhanced on annealing at 500 ◦C. The intensities of intermediate band and Eu3+

emission are significantly influenced by the excitation wavelengths (240-399 nm). Blue emission isnot due to the Eu2+ confirmed by XPS study and this is assigned to intermediate bands within hostband gap where electron-hole recombination takes place after excitation. We observe core bindingenergy with a small peak at ∼141.1 eV in XPS spectra which corresponds to Eu3+(4d3/2) and nopeak corresponding to Eu2+(4d5/2) is observed at ∼127.1 eV. This confirms the high probabilityof Eu3+ present in samples. The as-prepared nanoparticles are re-dispersed in ethanol and PVAthin film is prepared form the re-dispersed particles. The film shows uniform bright blue. The CIEchromaticity coordinates of as-prepared samples are close to (0.17, 0.17). They can be potentialcandidate for life science activity and LEDs applications. The emission intensity ratio of electric tomagnetic dipole transitions varies with the excitation wavelength. In case of as-prepared samples,the lifetime of Eu3+ (5D0) increases from 1.05 to 1.26 ms with increasing Li+ concentration up to3 at.% and decreases with Li+ (i.e. 10 at.% Li+) where lifetime is ∼1.09 ms and such behavior issimilar to variation in luminescence intensity. In case of 500 ◦C annealed samples, lifetime valuesare found to be ∼1.52 and 1.85 ms for 0 and 10 at.% Li+ co-doping, respectively. This workdemonstrates the preparation of high quality luminescence material by introducing Li+ ion, whichhelps in enhancement of luminescence intensity of Eu3+.

ACKNOWLEDGMENT

This work has been sponsored by the University Grants Commission (UGC) under the D.S. Kothari Postdoctoral Fellowship Scheme (No.F.4-2/2006(BSR)/13-309/2008(BSR)) to Dr. A.K. Parchur. Authors thank Dr. T. Mukherjee, Chemistry Group; Dr. D. Das and Dr. R. K. Vatsa,Chemistry Division, BARC for their support and encouragement during this work.

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