Luminescence behavior of YVO4:Dy3+ phosphors with enhanced photoluminescence on co-doping Bi3+ ions

Journal of Alloys and Compounds 583 (2014) 259–266

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Luminescence behavior of YVO4:Dy3+ phosphors with enhanced photoluminescence on co-doping Bi3+ ions Ch. Victory Devi a, Ganggam Phaomei b, N. Yaiphaba c, N. Rajmuhon Singh a,⇑ a

Department of Chemistry, Manipur University, Canchipur 795003, Imphal, India Berhampur University, Berhampur 760002, Orissa, India c D.M. College of Science, Imphal 795002, India b

a r t i c l e

i n f o

Article history: Received 24 June 2013 Received in revised form 5 August 2013 Accepted 24 August 2013 Available online 31 August 2013 Keywords: Pure tetragonal phase Bi3+ Sensitizer Energy transfer Multi-color tunable

a b s t r a c t YVO4:Dy3+, Bi3+ phosphors were prepared successfully by co-precipitation method using ethylene glycol as a capping agent as well as reaction medium at a relatively low temperature of 150 °C. XRD patterns of all samples are well crystallized to a pure tetragonal phase of YVO4. The unit cell volume increases linearly with the Bi3+ ion concentrations suggesting the quantitative incorporation of Bi3+ ions in lattice sites of Y3+ in YVO4 matrix. On introduction of Bi3+ ions, the VO43 excitation band shifts to longer wavelengths. Energy transfer from V5+–Bi3+ CT state to Dy3+ was observed, giving rise to the characteristic visible emissions from Dy3+. The luminous intensity of the electric dipole transition as well as magnetic dipole transitions of Dy3+are enhanced significantly on co-doping of Bi3+ ions. The optimum concentration of Dy3+ for highest luminescence is found to be 1 at.% Bi3+ and the luminescence intensity increases on annealing samples at 500 °C. Furthermore, the emission colors of YVO4:5Dy3+ can be tuned from greenish blue to near white by changing the excitation wavelength and then to yellowish white on codoping Bi3+ ions, making the phosphors a great potential for applications in fluorescent lamps and field emission displays (FEDs). Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Rare earth-doped luminescent materials have been attracting much attention during past decades owing to their wide applications in the field of illuminations and displays, such as fluorescent lamps, cathode-ray tubes, field emission displays and in developing of white light-emitting diodes (LEDs) [1–9]. However, the foremost requirement for good lamp phosphors is high efficiency under excitation with ultraviolet radiation and excellent chemical stability, long lifetime, etc. [10]. Recently, most of the investigations have been carried out by co-doping with either rare earth or transition metal ions in different matrices in order to attain higher luminescent efficiency via sensitization/energy transfer as well as to achieve multi-color emissions from single phosphors. Tian et al. [11] reported the emission intensity of BAZr(BO3)3:Eu3+ can be improved by co-doping Al3+. Further, Tian group have shown the enhanced photoluminescence of YVO4:Eu3+ by co-doping large-sized divalent ions, such as Sr2+, Ba2+ and Pb2+ ion [12]. Others reported in the literature for novel applications are CaWO4:Eu3+, Tb3+; YVO4:Bi3+, Eu3+; (Y, Gd)VO4: Eu3+, Er3+; CaMoO4:Eu3+, Li+, Si4+; YPO4:Dy3+, Bi3+, etc. [13–17]. However, most of the phosphors co-doped with large divalent ⇑ Corresponding author. Tel.: +91 09436080780; fax: +91 3852435145. E-mail address: [email protected] (N. Rajmuhon Singh). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.08.139

ions are usually synthesized by solid state reaction, which can result in the increase of particle size as well as impure phases in samples. Therefore, how to obtain the pure phase of the phosphors co-doped with large divalent or trivalent ions is the key in successfully achieving brighter phosphors. In the present study we have synthesized YVO4:Dy3+ phosphors by co-doping with Bi3+ ions by simple co-precipitation method using ethylene glycol as a capping agent. YVO4:Dy3+ is considered as potential phosphors due to its high absorption and luminescence efficiencies in ultraviolet (UV) region. Its emission color is close to white owing to blue (4F9/2 ? 6H15/2 and yellow (4F9/2 ? 6H13/2) emissions of Dy3+ [18–20]. YVO4 is also an excellent host lattice for rare-earth ions due to their efficient energy transfer from vanadate ions to activators leading to intense different color emissions [21,22]. The vanadate groups have strong absorption in the ultraviolet (UV) region and can generate blue emission bands themselves. It has a zircon type tetragonal structure with space group I41/amd. It composes of {VO4}3 tetrahedron in which V5+ ion is tetrahedrally co-ordinated and distorted dodecahedral (eightfold coordination in which Y3+ ion is linked with eight neighboring oxygen ions) [23,24]. Because of this properties, lanthanide ions could be firmly coordinated by eight O2 in YVO4 when it substitutes for Y3+. Bi3+ is considered as effective sensitizers because 6s2 ? 6s6p excitations of Bi3+ can be used to harvest the near-UV light [25,26].

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Based on these above reasons, the incorporation of Bi3+ ions may be favorable for photoluminescence of YVO4:Dy3+ in nearUV region. So, we attempt to synthesize Bi3+ co-doped YVO4:5Dy3+ phosphors via ethylene glycol route with multicolor emission. The luminescence properties were studied in detail. In presence of Bi3+ ions, emission intensities of YVO4:5Dy3+ are greatly enhanced. 2. Experimental 2.1. Sample preparation The 5 at.% Dy3+ doped YVO4 and Bi3+ (0.5, 1, 3, 5,7 and 10 at.%) co-doped nanoparticles are prepared at low temperature of 150 °C using ethylene glycol as a capping agent. The material sources used are Y2(CO3)3, Dy(CH3COO)3, NH4VO3 and Bi(NO3)3. In a typical preparation of 1 at.% Bi3+ co-doped YVO4:5Dy3+ nanoparticles, 0.25 g of Y2(CO3)3, 0.025 g of Dy(CH3COO)3 and 0.0072 g of Bi(NO3)3 were dissolved together in concentrated nitric acid (HNO3) in a 250 ml two necked round bottom flask and were heated with addition of double distilled water at least five times in order to remove the excess acid. To this, 0.1738 g of NH4VO3 and 25 ml of ethylene glycol (EG) were added. The solution was stirred for uniform mixing and NaOH was added as a pH regulating agent. The reaction mixture was then heated at 120–150 °C for 3 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 YVO4 nanoparticles. When the nucleation starts, surrounding EG molecules cap smaller particles and thus, particle growth is slow. The agglomeration among the particles is hindered. The precipitate so obtained was allowed to cooled at room temperature and then washed three times by centrifugation in ethanol and then acetone to remove the excess of EG, and then dried at room temperature for 4 days. Finally, the asprepared samples were annealed at 500 °C for 3 h and then their photoluminescence properties were studied. Fig. 1. XRD pattern of as-prepared and 500 °C annealed 5 at.% Bi3+ co-doped YVO4:5Dy3+.

2.2. Characterization All samples were characterized by X-ray powder diffraction (XRD) data using PANalytical powder diffractometer(X’Pert PRO) with Cu Ka (1.5405 Å) radiation with Ni filter. Scanning electron microscope (SEM) images were taken by using FEI Quanta 250 equipped with an energy dispersive X-ray spectroscopy. Fourier transform infrared (FTIR) spectra were measured on a Shimadzu (8400S) using a KBr pellet technique. The photoluminescence spectra and decay curves were measured by Perkin Elmer (LS-55) instrument with a Xe discharge lamp, equivalent to 20 kW for 8 ls duration, as the excitation source having pulse width at half height < 10 ls. All the measurements were performed at room temperature.

3. Results and discussions 3.1. XRD study Fig. 1 shows the XRD pattern of as-prepared and 500 °C heated samples of 5 at.% Bi co-doped YVO4:Dy3+. The result of XRD indicates that the samples are well crystallized and all the diffraction peaks are well indexed with the tetragonal phase of YVO4 with space group I41/amd141 (JCPDS: 17-0341). No other phase of impurity is detected indicating the samples are well crystallized with a pure single tetragonal phase of YVO4. The crystallite size is calculated from intense diffraction peak (2 0 0) using Debye’s formula

D ¼ 0:9k=bcosh

Fig. 2. Plot of unit cell volume vs. Bi3+ concentrations in as-prepared samples.

ð1Þ

where k is the X-ray wavelength used, h is the diffraction angle of the diffraction peak (2 0 0), and b is defined as the half width after subtracting instrumental broadening. The average crystallite size for the as-prepared samples varies in the range of 24–37 nm. On annealing the samples at 500 °C, the peak intensity of the samples is increased significantly and peak widths are slightly decreased. The sharpened diffracted peaks and increased intensity indicate improvement in crystallinity of the nanoparticles. The crystallize size of 5 at.% Bi3+ co-doped YVO4:5Dy3+ is found to be 35 nm and 80 nm for as-prepared and 500 °C heated samples respectively. The unit cell volume increases almost linearly with Bi3+ ions as expected by Vegards law (Fig. 2). Such increase is understandable to

the ionic radii of Y3+ (0.088 nm) is slightly lower than Dy3+ (0.091 nm) and Bi3+ (0.096). It is believed that a pure single phase was successfully developed through our preparation procedure. The energy dispersive X-ray (EDX) spectrum was taken to confirm the chemical composition of the prepared samples. EDAX pattern of YVO4:Dy3+ revealed the presence of Y, O, V and Dy3+ only, a pure YVO4:Dy3+ phase. And the EDAX spectra of 5Bi3+ co-doped YVO4:Dy3+ as presented in Fig. 3b exhibits not only Y, V, Dy3+ signals but also the Bi which indicates the incorporation of Bi3+ in the YVO4:Dy3+ nanocrystals. In addition, neither Na or c signals were detected in the EDAX spectrum taken for the Bi3+ co-doped YVO4 which means that the product is free from NaOH and carbonates.

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261

Fig. 3. EDX pattern of (a) YVO4:Dy3+ and (b) YVO4:Dy3+, 5Bi3+.

3.2. SEM study

Fig. 4. SEM image of (a) YVO4:Dy3+ and (b) YVO4:Dy3+, 5Bi3+ phosphors.

SEM image is used to study the morphology of the prepared samples. Typical SEM image of YVO4:Dy3+ and 5Bi3+ co-doped YVO4:Dy3+ phosphors were shown in Fig. 4. The particles tend to be seriously agglomerated in the Dy3+ activated phosphors. However, the particles size grew somewhat and agglomeration seems to be decreased on co-doping the Bi3+ ions resulting a cobblestone like shaped with diameter 90 nm.

3.3. FT-IR study Fig. 5 shows the IR spectra of as-prepared and 500 °C annealed samples. The as-prepared sample shows peak at 452, 826, 1022, 1375, 1500, 1644, 2941 and 3314 cm1. The broad peak around 3314 cm1 and weak band at 1651 cm1 are attributed to OAH stretching vibration and HAOAH bending vibration, respectively [27]. These two bands are completely different from that observed from coordinated water. So, the bands may be due to the water absorbed on the surface of the samples. The bands at 2875 cm1 and 2947 cm1 are assigned to CH2 stretching vibrations of EG molecules present on the surface of the nanoparticles [28]. Peak around 1022 and 1369 cm1 are related to V@O and CAO bonds, respectively [29]. The strong peak at 826 cm1 is attributed to VAO absorption from VO3 [30]. And the peak at 452 cm1 is related 4 to YAO band [31]. This confirms the formation of YVO4 nanocrystalline agreeing well with result of XRD. On annealing the samples at 500 °C, the peak around 2872–2948 cm1 disappeared and the absorption bands of OAH stretching also decreases that is, the presence of organic impurity such as OAH, CH2 in the samples

Fig. 5. FT-IR spectra of as-prepared and 500 °C annealed 5 at.% Bi3+ co-doped YVO4:5Dy3+ samples.

are reduced thereby increasing the degree of crystallinity of the samples. 3.4. Photoluminescence study It is well known that YVO4:Ln phosphors exhibit red, orange– red and green emission for Ln = Eu3+, Sm3+ and Dy3+ under short

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wavelength UV radiation. So, we synthesized Bi3+ co-doped YVO4:5Dy3+ phosphors via ethylene glycol route and further heated to 500 °C to study their luminescence behavior. The Dy3+ activated YVO4 phosphor shows a strong absorption spectra at 284 nm in UV–vis spectrum, attributing to the charge transfer from oxygen ligand to central vanadium atom within the VO3 4 (Fig. S1) [32]. This band corresponds to transition from the 1A2 (1T1) ground state to 1A1 (1E) and 1E (1T2) excited states of VO3 4 ions [33]. The excitation spectra (Fig. 6) obtained on monitoring emission at 575 nm consists of a broad band ranging from 220 to 350 nm with a maximum 280 nm and shoulder at 310 nm. It is well known that 310 excitation is attributed to the charge transfer from oxygen ligands to the central vanadium atom inside the VO3 absorption. 4 The short wavelength 280 nm excitation is assigned to the overlap of VO3 absorption and charge transfer transition between Dy3+ 4 and O2 [23,24]. The appearance of a broad excitation band confirms the emission of Dy3+occurs via energy transfers from excited VO3 4 . Beyond this, weaker excitation band also appears in longer wavelength originating from 4f to 4f transitions of Dy3+ ions. On introduction of Bi3+ ions into YVO4:5Dy3+ phosphor their excitation spectra were enhanced significantly and band edge was shifted to longer wavelength from 310 nm to 315 nm. The red shift of the charge transfer band absorption band of YVO4:5Dy3+ by co-doping Bi3+ ions is due to extra absorption of involving the BiAO component in addition to VAO charge transfer bands [34]. This increase seems to be monotonic only up to 1 at.% Bi3+ when less than half of the yttrium ions are substituted by Bi3+ ions. For substitutions exceeding 1 at.% Bi3+, the long wavelength CAT

band diminishes in intensity and disappears completely when the concentration of Bi3+ ion is 10 at.%. However, the absorption in the short-wavelength (240–275 nm) seems to be increase gradually (Fig. 6a). So, the excitation spectra of the samples with low bismuth content can be explained by the fact that Bi3+ ions are present in a more symmetrical (zircon-type) environment, and hence the 6s2 electrons are on top of the 2p level of O2 as a result of which the excitation band shifts to longer wavelengths. Beyond a certain limit on increasing bismuth content we observe a drastic reduction in the long wavelength absorption in the excitation spectra, even though the samples are still within the composition regime of the zircon phase. This may be due to the formation of Bi3+ aggregates which may act as trapping centers and dissipate absorbed energy non-radiatively, instead of transferring to Dy3+. And the wavelength corresponding to DyAO CT decreases from 280.36 nm to 277.15 nm with Bi3+ ions (Fig. 6c). The blue shift of the CT band with Bi3+ ion concentration suggests the increase in the ionicity of the DyAO bond. This result suggests that the sensitization effect of Bi3+ ion on Dy3+ varies with Bi3+ concentration. In case of 500 °C annealed samples, the excitation intensity is enhanced significantly and more broaden peak around 200– 313 nm is observed as compared to as-prepared samples. The shoulder peak around 311 nm is shifted to longer wavelength 316 nm and became more prominent in case of annealed samples. This indicates sufficient transfer of energy from Bi3+ to VO3 4 and then to Dy3+. Fig. 7 shows the emission spectra of Bi3+ co-doped YVO4:5Dy3+ 3+ (Bi = 0, 0.5, 1, 3, 5, 7 and 10 at.%) as prepared samples by excita-

(a)

(b)

280.5

(c)

Dy-O CTpeak position (nm)

280.0

ASP/575 nm

279.5 279.0 278.5 278.0 277.5 277.0 0

2

4

at.% Bi

6

8

10

3+

Fig. 6. (a) Excitation spectra of Bi3+ co-doped YVO4:5Dy3+ (Bi3+ = 0, 0.5, 1, 3, 5, 7 and 10 at.%): (a) as-prepared samples, (b) 500 °C annealed samples, and (c) change in position of DyAO CT band with Bi3+ concentration up to 10 at.% (monitoring emission at 575 nm).

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263

Fig. 7. Emission spectra of Bi3+ co-doped YVO4:5Dy3+: (a) and (b) as-prepared and 500 °C annealed samples under excitation of 280 nm and 310 nm, (c) expanded emission spectra of 500 °C annealed samples under 280 excitation.

  . The emission spectra of tion at 280 (DyAO) and 310 nm VO3 4 Bi3+ co-doped YVO4:5Dy3+ under 280 nm and 310 nm excitation show similar behavior. Excitation into the Dy3+ AO2 CT and V5+AO2 CT absorption band at 280 nm and 310 nm yields mainly the characteristic emission of Dy3+ corresponding to electric 4 F9/2 ? 6H13/2 and magnetic dipole 4F9/2 ? 6H15/2 transitions together with the weak emission of V5+–Bi3+ CT [Fig. 6b] indicating the ET from V5+–Bi3+ CT state to Dy3+ ion. However, a small broad peak around 410–430 nm appear in the emission spectra of YVO4:5Dy3+, Bi3+ under excitation of 310 nm. This emission is related to host emission suggesting the inefficient transfer of en3+ ergy from VO3 4 to Dy . On annealing the samples at 500 °C, the intensity of band at 410–430 nm under 310 nm excitation disappear due to decrease of lattice defects and thus energy transfer 3+ from host/VO3 increases and Dy3+ emission intensity are 4 to Dy correspondingly enhanced. The emission intensities of 4F9/2 ? 6HJ (J = 13/2 and 15/2) for as prepared samples are more under 280 nm excitation than that recorded after 310 nm excitation (Fig. S2). This indicates efficient energy transfer from both V5+–Bi3+ and DyAO absorption band to Dy3+. The change in luminescence intensity of Bi3+ co-doped YVO4: 5Dy3+ nanoparticles with Bi3+ ions has been studied by calculating

the integrated area under emission peaks of 4F9/2 ? 6H13/2 and 6 H15/2 after fitting the Gaussian distribution function:

I ¼ I0 þ

2 X i¼1

wi

2 Ai 2 pffiffiffiffiffiffiffiffi ffi e2ðkkci Þ =wi p=2

ð2Þ

where I is the observed intensity, I0 is the background intensity, wi the FWHM of the curve, Ai the area under the curve, k the wavelength and kci the mean value corresponding to the transition. The fitting were carried out for 4F9/2 ? 6H13/2 and 6H15/2 in between 460 and 600 nm. A typical fitting for 3 at.% Bi3+ co-doped YVO4:5Dy3+ heated at 500 °C under 280 nm excitation are shown in Fig. S3. The emission intensity of Bi3+ co-doped YVO4:5Dy3+ phosphors increases with increasing Bi3+ ions up to 1 at.% Bi3+ ions. The increase in the emission intensity of Dy3+ with Bi3+ content is due to the energy transfer from Bi3+ to Dy3+. Increasing Bi3+ content weaken the emission intensities of Dy3+, probably due to the energy loss resulting from the electron migration among Bi3+ ions to the quenching sites. Thus, the optimum concentration for highest luminescence intensity for YVO4:5Dy3+, Bi3+ is found at 1 at.% Bi3+ which is clearly shown in fig. 8. Fig. 8 shows the variation of integrated area, FWHM (w) and asymmetric ratio (A21) of electric

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Fig. 8. (a) Variation of intensity of electric dipole transition, 4F9/2 ? 6H13/2 (A2), (b) its w (FWHM) and (c) A21 (asymmetric ratio) for Bi3+ (0, 0.5, 1, 3, 5, 7 and 10 at.%) co-doped YVO4:5Dy3+: as-prepared and 500 °C annealed samples under 280 nm excitation.

dipole transition 4F9/2 ? 6H13/2 for all as-prepared and 500 °C annealed samples under 280 nm excitation. The calculated values of A21 of magnetic dipole transitions for as-prepared and 500 °C annealed samples at 280 nm and 310 nm excitation wavelength are given in Table 1. Asymmetric ratio (A21) is defined as the ratio of intensity of electric dipole transition to that of magnetic dipole transition. This ratio is used for understanding the symmetry of Dy3+ in YVO4 and is represented as A21:

Table 1 A21 values of Bi3+ co-doped YVO4:5Dy3+ (Bi3+ = 0, 0.5, 1, 3, 5, 7 and 10 at.%) under 280 nm and 310 nm excitation. Bi3+ (at.%)

A21 calculated from 280 and 310 nm 280 (nm)

310 (nm)

As-prepared

0 0.5 1 3 5 7 10

2.00 1.65 1.85 1.88 1.87 1.84 1.73

1.84 1.74 1.83 1.97 1.83 1.85 1.77

500 °C

0 0.5 1 3 5 7 10

2.08 2.17 2.24 2.28 2.30 2.34 2.27

1.88 2.01 2.13 2.21 2.08 1.98 1.99

R 590

A21 ¼ R560 500 460

I2 dk I1 dk

ð3Þ

where I1 and I2 represent the respective integrated intensity of F9/2 ? 6H13/2 and 6H15/2 transitions of Dy3+, respectively. In case of as-prepared samples, A2 increases with Bi3+ up to 1 at.% and decreases with further increase of Bi3+. Its FWHM decreases from 8.15 to 7.81 nm as Bi3+ concentration increases up to 10 at.%. A21 is found to be greater than unity indicating the higher intensity for electric dipole transition over magnetic dipole transition. That is, Dy3+ occupies a site with D2d symmetry without inversion centre in the host of YVO4 and is very sensitive to local symmetry [35]. However, in the present case, the magnetic dipole transition is also enhanced accordingly with Bi3+ ion concentrations. It is worthwhile to mention here that the influence of Bi3+ ion on yellow-to-blue emission intensity ratio (A21) is negligible. This may be attributed to the fact that Bi3+ ion has little effect on the local site symmetry of Dy3+ ion and Dy3+AO2 interaction when Bi3+ ions are substituted into Y3+ sites in the YVO4 system owing to the same ionic valence and approximate ionic radius of Bi3+ (0.096), Dy3+ (0.091 nm) and Y3+ (0.088 nm). Variation in intensities of these two dipole transition at 280 nm and 310 nm excitation may be related to the interaction of incoming excitation light with Dy3+ environment (nearest environment (O) and second nearest   environment VO3 . The variation of integrated area and FWHM 4 4

(w) of electric dipole transition 4F9/2 ? 6H13/2 and magnetic dipole F9/2 ? 6H15/2 for all as-prepared and 500 °C annealed samples under 280 nm and 310 nm excitation are shown in Figs. S4–S5.

4

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Fig. 9. The CIE chromaticity of YVO4:5Dy3+, 1Bi3+ phosphors under 280 nm excitation.

From the above results and analysis, the emission color can be tuned by varying the excitation wavelength. Usually CIE coordinates reflect this factor. Thus CIE coordinates of YVO4:Dy3+ is x = 0.330, y = 0.357 when excited at a shorter wavelength of 280 nm. When extending the excitation wavelength to 310 nm their emission color changes from greenish blue to near white and its CIE coordinates is x = 0.320, 0.335. So, CIE clearly indicates that multicolor tunable emission was accomplished by just using different excitation wavelength. On co-doping Bi3+ ions into YVO4:5Dy3+ the CIE coordinates are shifted towards pure yellow. That is, color changes from greenish blue to pure yellow with CIE coordinates (0.387, 0.409) for as-prepared 1 at.% Bi3+ co-doped nanoparticles. Fig. 9 shows that Bi3+ co-doped YVO4:5Dy3+ phosphors emit pure yellow light. However, the influence of co-doping of Bi3+ ions in YVO4:5Dy3+ system on CIE chromaticity coordinates is negligible. For 5 at.% Bi3+ co-doped particles, the CIE coordinates is (0.389, 0.410). This can be understandable as the intraconfigurational 4f–4f emission transitions of Dy3+ ions are slightly affected by crystal field environment. 3.5. Decay lifetime study

In case of 500 °C annealed samples, the emission intensity and FWHM are higher as compared to as-prepared samples for both 280 nm and 310 nm excitation. Intensity increases up to 3 at.% Bi3+ and then decreases. Increase in luminescence intensity is due to decrease of non-radiative rate from surface dangling bonds and capping agent (EG) on annealing leading to improvement in crystallize size of the samples.

(a)

The luminescence decay curve of 4F9/2 level of Dy3+ have been measured for as-prepared and 500 °C annealed samples of Bi3+ (0, 0.5, 1, 3, 5, 7 and 10 at.%) co-doped YVO4:Dy3+. The emission and excitation wavelength used are 575 nm and 280 nm respectively. Decay curve are not well fitted by mono-exponential curve fitting. All decay data (Fig. 10) are now well fitted by bi-exponential decay equation, which is expressed as

(b) 600 Intensity (arb. units)

500

λexc=280 nm and λem=575 nm 3+

400

3+

0

YVO4:5Dy ,Bi (500 C) 3 at.% Bi 1 7 0

300

3+

200 100

0.5

1.0

1.5

2.0

Decay time (ms)

(c)

Fig. 10. (a) Luminescence decay spectra of as-prepared and 500 °C annealed Bi3+ co-doped YVO4:5Dy3+ nanoparticles at 280 excitation, (b) Bi-exponential fitting to luminescence decay data of 500 °C annealed 3 at.% co-doped YVO4:5Dy3+ nanoparticles.

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I ¼ I1 et=s1 þ I2 et=s2

ð4Þ

where I1 and I2 are the intensities at different time intervals and s1 and s2 their corresponding lifetimes. The average lifetime can be calculated using

sav ¼

I1 s21 þ I2 s22 I 1 s1 þ I 2 s2

ð5Þ

Usually bi-exponential decay is observed when energy transfer takes place from donor/host to activator [36]. So, energy is transfer from Bi3+ to Dy3+ in the present study. Bi-exponential fitting of 500 °C heated 1 at.% Bi3+ co-doped YVO4:5Dy3+ is shown in Fig. 10 Ó. The average decay lifetime is found to be increased with Bi3+ ions up to 1 at.% for as-prepared samples and then decreases on further increasing Bi3+ ions agreeing well with the variation of luminescence intensity with Bi3+ ions. The average lifetime value of asprepared 5 at.% Dy3+ doped YVO4 is found to be 0.212 ms. This value is in accord with previous report of 0.209 ms given for YVO4:Dy3+ [37]. For Bi3+ co-doping samples, the average lifetime are found to be 0.725, 0.225 and 0.022 ms for 1, 3 and 7 at.% Bi3+. These values are slightly longer than the Dy3+-doped one. In case of 500 °C heated samples, the decay lifetime increases up to 3 at.% Bi3+. For 500 °C heated samples, the average lifetime calculated are found to be 0.334, 0.737, 1.38 and 0.528 ms for 0, 1, 3 and 7 at.% Bi3+. The longer lifetime is related to the increase of particle size and decreases of non-radiative rate from the surface of particles on annealing the samples. 4. Conclusion Bi3+ co-doped YVO4:5Dy3+ nanoparticles were prepared successfully by co-precipitation method via ethylene glycol route. All samples are well crystallized into a pure tetragonal phase of YVO4. With Bi3+ co-doping, the VO3 absorption shifts to longer 4 wavelength showing the efficient transfer of energy from Bi3+ to Dy3+. ET from V5+–Bi3+ CT state to Dy3+ was observed, giving rise to the characteristic visible emissions from Dy3+. The emission intensity of Dy3+ is significantly improved on co-doping Bi3+ ions. On co-doping of Bi3+ ions the CIE coordinates changes from greenish blue to yellow white and the emission color can be tuned from greenish blue to near white by just changing the excitation wavelength. Such tuning of emission color would have a great potential application in white LEDs. Acknowledgement Authors thank Department of Science and Technology (DST), New Delhi, Govt. of India for providing financial support.

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