Synthesis and photoluminescence properties of a novel Sr₂CeO₄:Dy³⁺ nanophosphor with enhanced brightness by Li⁺ co-doping

Abstract

A series of Dy³⁺ (0.5–9 mol%) and Li⁺ (0.5–3 mol%) co-doped strontium cerate (Sr₂CeO₄) nanopowders are synthesized by low temperature solution combustion synthesis. The effects of Li⁺ doping on the crystal structure, chemical composition, surface morphology and photoluminescence properties are investigated. The X-ray diffraction results confirm that all the samples calcined at 900 °C show the pure orthorhombic (Pbam) phase. Scanning electron microscopy analysis reveals that the particles adopt irregular morphology and the porous nature of the product. Room temperature photoluminescence results indicate that the phosphor can be effectively excited by near UV radiation (290 to 390 nm) which results in the blue (484 nm) and yellow (575 nm) emission. Furthermore, PL emission intensity and wavelength are highly dependent on the concentration of Li⁺ doping. The emission intensity is enhanced by ∼3 fold with Li⁺ doping. White light is achieved by merely varying dopant concentration. The colour purity of the phosphor is confirmed by CIE co-ordinates (x = 0.298, y = 0.360). The study demonstrates a simple and efficient method for the synthesis of novel nanophosphors with enhanced white emission.

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1. Introduction

Over the last decade extensive attention has been given to the research areas related to white light emission due to their potential application in modern lighting systems. These systems are preferred replacements for classical incandescent and fluorescent lamps, due to their high efficiencies.^1–3 The most widely used method to fabricate white light emitting devices (LEDs) involves the use of phosphors as light transducers to convert near-ultraviolet (NUV) or blue light to white light.^4–6 NUV phosphor converted LEDs are preferred in several applications due to their excellent optical stability, high color tolerance and high conversion efficiency.^7–9 Therefore, phosphors that are efficiently excited by near-UV light are desirable.

Rare earth (RE) elements when doped into a suitable inorganic host can act as an effective luminophore. In fact RE doped phosphors based light emitting devices have already been used in various applications.^3–6 Rare earths act as efficient emitters both in trivalent and divalent states owing to inner f–f and f–d transitions respectively. This fact has been explored in order to engineer the absorption and emission spectrum of practical phosphors.¹⁰ Generally, the trivalent rare earth ions like Eu³⁺, Sm³⁺and Dy³⁺ having f–f transitions result in sharp emission peaks, since the electron–lattice interaction is very weak for inner electrons. Whereas f–d transitions of divalent rare earth ions such as Eu²⁺ results in broad emission peak. This is due to the participation of the d-electrons in chemical bonding. It may be noted that in divalent RE ions, the (4f)ⁿ to (4f)ⁿ⁻¹(5d)¹ transitions involve the promotion of a single electron from the 4f orbitals to an empty 5d orbital.^11,12

Phosphors employing trivalent Dysprosium ions (Dy³⁺) have been widely studied due to the potential applications in white light emission. This is because Dy³⁺ has intense blue (484 nm) and yellow (575 nm) emission lines.^13–16 However these Dy³⁺ transitions are very sensitive to the chemical environment surrounding the luminophore. Hence, the ratios of the two emission bands are different in different hosts. Therefore, it is possible to modulate the emitted light by suitably choosing host or varying the composition of the host, for the realization of white light emission.^15–17 Hence the choice of host is an important criterion to harness the rich luminescence characteristics of the Dy³⁺ luminophore.

Among the inorganic hosts studied so far, oxide-based phosphors have received the most attention due to their ease of synthesis, stability, and the nontoxic nature.^17–21 Therefore oxide-based phosphors are likely to emerge as a preferred choice for making display devices and solid state lighting. Danielson et al.²² have reported a novel blue luminescent material Sr₂CeO₄, prepared using a combinatorial material synthesis technique. Since then, Sr₂CeO₄ has attracted much attention of the researchers due to several advantages, like lack of radioactive elements, non-toxicity, higher chemical and thermal stability.^23–25 Apart from these advantages, Sr₂CeO₄ also possesses some desirable qualities such as (i) ligands-to-metal charge transfer (CT) transition of Ce⁴⁺at ∼340 nm, (ii) efficient energy transfer that can occur from the Ce⁴⁺–O²⁻ CT state to the trivalent rare earth in Sr₂CeO₄:RE³⁺,²³ (iii) high efficiency for absorbing UV radiation and (iv) the large band gap and covalent bond energy prove its capability for using as a phosphor material.²⁴ The Sr₂CeO₄ blue luminescence is believed to originate from a ligand-to-metal (O²⁻ → Ce⁴⁺) charge transfer mechanism. This process occurs between the lower coordination number terminal oxygen atoms associated with the low dimensional structure, in combination with an adjacent Ce⁴⁺ centre in Sr₂CeO₄ structure. The emission is fairly broad (FWHM ∼100 nm) at the blue region (450–500 nm), which makes this material a potential candidate for phosphor applications.^23–26

Recent research reports reveal that the doping of RE ions along with the alkali metal ions (Li⁺, K⁺ and Na⁺) emerged as an effective way to enhance the photoluminescence (PL) brightness and emission characteristics of the nanophosphors.^27–30 With the removal of the charge unbalance problem with a less defective crystal structure, alkali metal ions not only enhance the efficiency of trivalent RE ions, but also play a crucial role to control emission colors of phosphor materials. This technique has been employed on several systems, like CaMO₄:Dy³⁺ (M = W, Mo),²⁷ Y₂Zr₂O₇:Dy³⁺,²⁸ YBO₃:Eu³⁺,²⁹ YPO₄:Eu³⁺,³⁰ etc. to achieve impressive luminescence properties. But so far, to the best of our knowledge, there are no studies on the white light emission from Sr₂CeO₄:Dy³⁺ nanophosphors with Li⁺ co-doping. In this paper we report the tuning of white light in Sr₂CeO₄:Dy³⁺, Li⁺ nanophosphor synthesized by solution combustion route. Here we also report the quality of white light using CIE coordinates and correlated color temperature (CCT).

2. Experimental

2.1. Materials and synthesis

All the chemicals used are of analytical grade purity, and are used without further purification. Strontium nitrate (Sr(NO₃)₂·9H₂O; 99.99%, Merck Ltd.), Cerium nitrate (Ce(NO₃)₂·6H₂O; 99.99%, Sigma Aldrich Ltd.) and Dysprosium nitrate (Dy(NO₃)₃·6H₂O; 99.99%, Sigma Aldrich Ltd.) are mixed in suitable stoichiometric ratios to get products with chemical formula Sr_2−xDy_xCeO₄ (x = 0.005–0.09). Oxalydihydrazide (C₂H₆N₄O₂) is prepared by using hydrazine hydrate and diethyloxalate as reported in the literature³¹ and is used as fuel.

The fuel:oxidizer ratio is calculated based on oxidizing and reducing valencies of the reactants by setting F/O = 1, as reported in our earlier report.³² An aqueous solution containing stoichiometric amounts of reactants is taken in a cylindrical pyrex petri dish of approximately 300 ml capacity and introduced into a muffle furnace maintained at a temperature of 500 ± 10 °C. The reaction mixture undergoes thermal dehydration and auto-ignites with liberation of gaseous products such as oxides of nitrogen and carbon.³¹ The combustion flame propagates throughout the reaction mixture. The heat of reaction is sufficient for the decomposition of the redox mixture and to form the desired product. The whole process is completed in less than 5 min and a highly porous white Sr₂CeO₄:Dy³⁺ nanopowders were obtained. Similar procedure is followed to prepare different concentrations of Li⁺ co-doped Sr_2−xDy_xCeO₄:Li_y (x = 0.075, y = 0.005 to 0.03) phosphors, (lithium nitrate (LiNO₃), 99.99%, Merck Ltd. is used as source of Li⁺). All the samples after synthesis are calcined at 900 °C for 3 h to improve the crystallinity.

2.2. Characterization

The powder X-ray diffraction studies are carried out using Shimadzu 7000 powder X-ray diffractometer with Cu K_α radiation (λ = 1.541 Å). Fourier Transform Infra-red spectroscopy (FTIR) spectrum is taken using a Perkin-Elmer Rx1 instrument. The morphology and structure of the samples are inspected using a Hitachi-3000 scanning electron microscope (SEM). For performing SEM, a layer of Au is deposited on the samples using a sputtering deposition set up. This is necessary to get clear SEM images since the samples are insulating. Transmission electron microscopy (TEM) analysis and High resolution TEM (HRTEM) are performed on a Hitachi H-8100 (accelerating voltage up to 200 kV, LaB₆ filament) equipped with energy dispersive spectroscopy (EDS) (Kevex Sigma TM Quasar, USA). The photoluminescence (PL) measurements are carried out using a Jobin Yvon spectrofluorometer (Fluorolog-3, HORIBA) equipped with a 450-W xenon lamp as the excitation source.

3. Results and discussion

3.1 Powder X-ray diffraction analysis (PXRD)

Fig. 1 and 2 shows the XRD patterns of Sr₂CeO₄:Dy³⁺ and Sr₂CeO₄:Dy³⁺:Li⁺ nanoparticles. The XRD patterns fits well with the orthorhombic crystal structure with JCPDS 50-0511 with Pbam space group. No additional peaks for other phases have been found in both the patterns of Sr_2−xCeO₄:Dy_x and Sr_2−yDy_xCeO₄:Li_y. This indicates that dopants ions were effectively doped into the Sr₂CeO₄ host lattices. Since the ionic radii of Sr²⁺ and Li⁺ are 0.072 nm and 0.068 nm (for octahedral co-ordination) respectively, Li⁺ easily substitutes Sr²⁺ in the Sr₂CeO₄ lattice. However, a careful observation of the XRD patterns in Fig. 2(b) reveals that peaks are shifted slightly towards higher 2θ values with the increase in concentration of Li⁺. Shrinking of unit cell caused due to the substitution of larger Sr²⁺ by smaller Li⁺ is responsible for this higher angle shift. It is interesting to note that in contrast with the other XRD peaks, when Li⁺ concentration is x = 0.03, the pattern shifts towards lower angle. The difference in the peak shift to higher/lower angle can be explained as follows. When the concentration of Li⁺ ions is below x = 0.03, Li⁺ ions occupy substitutional sites (Sr²⁺). However, when x > 0.07, of Li⁺ ions begin to take interstitial sites resulting in the host lattice expansion. Hence beyond a critical co-doping concentration, the trends in the peak shift reverses. The calculated values of the lattice parameter (a), crystallite size (D) and FWHM of the (130) peak are presented in Table 1. The crystallite size of all the synthesized samples lies within the range 19–27 nm, which confirms the formation of Sr₂CeO₄ crystals in the nanosized region. With Li incorporation, we observe a reduction in the FWHM, and a concomitant increase in the sharpness of the (130) peak. This indicates that Li⁺ acts as a self-promoting agent for the enhanced crystallization of the lattice.³³

Fig. 1 XRD of Sr_2−xDy_xCeO₄ (x = 0.005–0.09) nanophosphor.

Fig. 2 (a) XRD patterns of Sr_1.9925−yDy_0.0075Li_yCeO₄ (y = 0.005–0.03) nanophosphor. (b) (130) peak shift patterns as Li concentration is increased.

Table 1 Estimated lattice parameter and particle size of Sr_2-xDy_xCeO₄(x = 0.005–0.09) and Sr_1.9925-yDy_0.0075Li_yCeO₄ (y = 0.005–0.03) nanophosphor

Dy^3+ Conc.	Lattice parameter (Å)			V Å^3	D nm	Li^+Conc.	Lattice parameter (Å)			V Å^3	D nm
							a	b	c
a	b	c
0.005	6.10	1.05	3.59	230.0	28.0	0.005	6.10	1.05	3.58	228.0	27.0
0.0075	6.10	1.05	3.60	230.0	24.0	0.0075	6.10	1.050	3.60	228.5	27.0
0.01	6.06	1.05	3.60	229.5	23.0	0.01	6.10	1.05	3.60	229.0	32.0
0.03	6.06	1.05	3.60	229.5	18.0	0.02	6.10	1.04	3.58	229.0	27.0
0.05	6.06	1.05	3.60	229.2	22.0	0.03	6.10	1.04	3.58	227.0	25.0
0.07	6.05	1.05	3.61	229.0	19.0
0.09	6.05	1.05	3.66	229.0	19.0

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3.2 Scanning electron microscopy (SEM)

SEM micrographs of Sr₂CeO₄:Dy³⁺ and Li⁺ co-doped Sr₂CeO₄:Dy³⁺, nanophosphors are shown in Fig. 3(a–d) respectively. It is observed from the micrographs that some bigger agglomerates are spherical, but the primary particles are irregular in shape. Agglomerates made of these irregular particles range in size between few microns to a few tens of microns. Minimization of surface energy is the most likely reason for this observed agglomeration of nanoparticle.³⁴ Typical of combustion derived products, all the samples exhibit large number of pores and voids in their structure. This can be attributed to large amount of gases escaping out of the reaction mixture during the combustion.³⁵ Interestingly in all the samples some agglomerates assume spherical shape of varying sizes (∼1–3 μm), which is very unusual for combustion derived products.

Fig. 3 SEM micrographs of (a, b) Sr_2-xDy_xCeO₄ (x = 0.0075) and (c, d) Sr_1.9925−yDy_0.0075Li_yCeO₄ (y = 0.01) nanophosphor.

3.3 Fourier transform infrared spectroscopy (FTIR)

In order to confirm the phase formation and purity of the samples, FTIR analysis is carried out. Fig. 4(a) and (b) shows the FTIR spectra of Sr₂CeO₄:Dy³⁺, and Sr₂CeO₄:Dy³⁺, Li⁺ nanophosphor respectively. The dominant band corresponding to CO₃²⁻ from the precursor appeared around 1457 cm⁻¹ in all the products. On the other hand, the peak at 3465 cm⁻¹ is attributed to the physically adsorbed H₂O.³⁶ Bands ranging from 400–900 cm⁻¹ originates from stretching and bending modes of metal–oxygen (M–O) groups. Two distinct bands were observed at 440 and 861 cm⁻¹ which originates from stretching vibrations of Ce–O in octahedral coordination state (CeO₆ octahedral units).³⁷ Since there is no discernible change in the position of peaks, it is reasonable to believe that the dopant has occupied the position of Sr²⁺ ions in crystal.

Fig. 4 FTIR spectra of (a) Sr_1.9925Dy_0.0075CeO₄ and (b) Sr_1.9925Dy_0.0075Li_0.01CeO₄ nanophosphor.

3.4 Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM) is carried out on the Sr₂CeO₄:Dy³⁺, Li⁺ nanophosphor samples. TEM, selected area electron diffraction pattern (SAED) and high-resolution TEM (HRTEM) micrographs of the nanophosphor are shown in Fig. 5(a–c). Fig. 5(a) clearly indicates that the particles are nearly spherical and elongated with diameters ∼10 nm. In addition, we can observe some larger particles with diameters ∼30 nm. Typical of combustion derived products, all the particles are highly agglomerated. The diffraction pattern (Fig. 5(b)) revealed a polycrystalline composition with four clearly distinguishable spotty but fully closed rings, which are indexed according to the Sr₂CeO₄ orthorhombic structure. The lattice fringe observed (Fig. 5(c)) correspond to an interplanar distance of 0.3 nm; this is comparable to the interplanar distance of the (130) plane planes in Sr₂CeO₄.²²

Fig. 5 (a) TEM (b) SAED pattern (c) HRTEM of Sr_1.9925Dy_0.0075CeO₄ nanophosphor.

3.5 Photoluminescence studies (PL)

The photoluminescence characteristics of the Sr₂CeO₄:Dy³⁺ and Li⁺ co-doped Sr₂CeO₄:Dy³⁺nanophosphors are investigated by PL excitation and emission spectra. PL excitation spectra of Sr_2−xDy_xCeO₄(x = 0.005–0.09) and Sr_1.9925−yDy_0.0075Li_yCeO₄ (y = 0.005–0.03) nanophosphors are shown in Fig. 6(a) and (b). This excitation spectrum is recorded by monitoring the emission of Dy³⁺ at 575 nm. The excitation spectra consist of broad bands and some sharp narrow peaks. The broad bands consist of two peaks centred at 290 and 340 nm; this is attributed to the charge transfer band (CTB) caused by an electron transfer from the O²⁻ (2p) orbital to the empty states of the 4f⁶ configuration of Ce⁴⁺ (Ce⁴⁺–O²⁻ transition).³⁸ The peak at 290 nm originated due to transition from terminal oxygen (O_t) to Ce⁴⁺. The peak at 340 nm is ascribed to transition from equatorial oxygen (O) to Ce⁴⁺.³⁸ Apart from this broad CTB, the excitation spectra contain several other sharp peaks observable in the longer wavelength region between 350 to 480 nm. These sharp features are due to intra-configuration f–f transitions of the Dy³⁺ dopant ions. In general, the direct excitation of the Dy³⁺ ground state to the higher levels in the 4f configuration results in the electronic transitions (⁶H_15/2 → ⁶P_3/2) at 326 nm, (⁶H_15/2 → ⁶P_7/2) at 352 nm, (⁶H_15/2 → ⁶P_5/2) at 366 nm, (⁶H_15/2 → ⁴I_13/2) at 385 nm, (⁶H_15/2 → ⁴G_11/2) at 424 nm, (⁶H_15/2 → ⁴I_15/2) at 453 nm and (⁶H_15/2 → ⁴F_9/2) at 475 nm.³⁹ However we could observe sharp peaks only at 385 and 424 nm. This might be because the other expected peaks (expected at 326, 352 and 366 nm) are buried within the broad CTB of host lattice.

Fig. 6 PL excitation spectra of (a) Sr_2−xDy_xCeO₄ (x = 0.005–0.09) and (b) Sr_1.9925-yDy_0.0075Li_yCeO₄ (y = 0.005–0.03) nanophosphor monitored at emission wavelength of 575 nm.

Photoluminescence emission spectra of the Sr_2-xDy_xCeO₄ (x = 0.005–0.09) and Sr_1.9925-yDy_0.0075Li_yCeO₄ (y = 0.005–0.03) nanophosphors, excited by 424 nm wavelength, are shown in Fig. 7(a) and (b), respectively. The emission spectra of all the phosphors (Sr₂CeO₄:Dy³⁺ and Li⁺ co-doped Sr₂CeO₄ : Dy³⁺) consist of a broad band ranging from 400 to 500 nm corresponding to host emission. Along with this host emission there is overlap of Dy³⁺ emission at 488 nm. Besides this there is a prominent emission peak at 575 nm, and a weak feature is observed at 662 nm. The emission peaks of Dy³⁺ at 488 nm, 575 nm and 662 nm can be assigned to the transitions from the⁴F_9/2 to the ⁶H_15/2, ⁶H_13/2 and ⁶H_11/2 states, respectively.³⁷ Among these emissions, the yellow band (575 nm) and the blue band (488 nm) are the predominant transitions. It is well known that ⁴F_9/2 → ⁶H_15/2 lines originate from magnetic dipole transitions, while the ⁴F_9/2 → ⁶H_3/2 lines originate from electric dipole transitions. The magnetic dipole transition is independent of the symmetry and the site occupied by Dy³⁺ ions in the host. But the electric dipole transition is very sensitive to the crystal field, and the emission intensity is strongly affected by the site symmetry of the dopant ion.⁴⁰ The site symmetry of Dy³⁺ can be determined by taking the ratio of emission intensities of both magnetic dipole and electric dipole transition (i.e. I₅₇₅/I₄₈₈). In the emission spectrum of Dy³⁺ ions, the yellow emission (⁴F_9/2 → ⁶H_13/2) is often dominant when the Dy³⁺ ion is located in the low symmetry local site without inversion center and the blue emission (⁴F_9/2 → ⁶H_15/2) is stronger than yellow one when it is located at a high-symmetry local site with an inversion center.^41,42 Fig. 8 shows the plot of ratio of transition intensities with dopant concentration. The value is found to decrease with increasing dopant concentration. This indicates that the Dy³⁺located in the low symmetry local site without inversion center.⁴³

Fig. 7 PL emission spectra of (a) Sr_2-xDy_xCeO₄(x = 0.005–0.09) and (b) Sr_1.9925−yDy_0.0075Li_yCeO₄ (y = 0.005–0.03) nanophosphor excited at 424 nm.

Fig. 8 Variation of the relative intensities (I_R) with Dy³⁺ concentrations. I_R is the ratio of the intensities of the 575 nm magnetic dipole transition line, and the 488 nm electric dipole transition line (i.e. I₅₇₅/I₄₈₈).

In order to investigate the difference in emission spectra resulting from various excitations, the PL emission spectra of Sr₂CeO₄:Dy³⁺ and Li⁺ co-doped Sr₂CeO₄:Dy³⁺ was recorded at 290 nm (host excitation) and 424 nm (direct excitation). Fig. 9 shows emission spectra upon excitation at 290 nm. It can be seen that the emission spectra have the similar properties to that of samples excited at 424 nm except for variation in PL intensities. This is true for all Sr₂CeO₄:Dy³⁺ and Li⁺ co-doped Sr₂CeO₄:Dy³⁺ with different doping concentrations. Additionally, it is worth noting that in the PL emission spectra the broad blue emission band due to the host emission decreases with the increase of Dy³⁺doping concentration. This is indicating energy transfer from the host to the Dy³⁺ ions as dopant content increases. This clearly indicates that Sr₂CeO₄ is a very efficient host for Dy³⁺.

Fig. 9 PL emission spectra of (a) Sr_2−xDy_xCeO₄ (x = 0.005–0.09) and (b) Sr_1.9925−yDy_0.0075Li_yCeO₄ (y = 0.005–0.03) nanophosphor excited at 290 nm.

Fig. 10 is the schematic energy-level diagram showing the excitation and emission mechanism of Sr₂CeO₄:Dy³⁺ phosphors. Upon host excitation at 290 nm, the photon energy is absorbed by host leading to the charge transfer of O²⁻ → Ce⁴⁺; this energy is transferred to 4f shell of Dy³⁺. Thus the electrons of Dy³⁺ at ⁴F_9/2 excited state can populate both from non-radiative charge transfer feeding and non-radiative transitions from higher excited states. Then the characteristic emissions at 488, 575 and 662 nm correspond to the⁴F_9/2 → ⁶H_j (j = 15/2, 13/2, 11/2) transitions, respectively.¹⁶ While the direct excitation of Dy³⁺ ions inducing the transitions from ground state to the metastable excited states. Part of the excited electrons in these metastable excited levels are repopulated into ⁴F_9/2 level by the multiphonon assisted non-radiative transitions. The transitions from ⁴F_9/2 to ⁶H_j (j = 15/2, 13/2, 11/2) are radiative and produce the emission lines of 488, 575 and 662 nm, respectively.

Fig. 10 Schematic diagram illustrating the energy and charge transfer pathways, and electronic transitions in Sr_2−xDy_xCeO₄⁺.

Fig. 11(a) and (b) shows the variation of PL intensity with Dy³⁺, and Li⁺ co-doped Sr₂CeO₄ nanophosphors respectively. It can be seen, upon excitation at 290 nm and 424 nm, the intensity (I) vs. dopant (x) curves shows similar trend. That is the emission intensity initially increases quickly with Dy³⁺ doping concentration and reaches a maximum value at x = 0.0075; subsequently it decreases due to the concentration quenching. It is found that for both host excitation and direct excitation the optimum Li⁺ concentration is y = 0.01 for a fixed Dy³⁺ concentration, x = 0.0075.

Fig. 11 Emission intensity as a function of Dy³⁺ concentrations in Sr_2−xDy_xCeO₄ (x = 0.0025–0.09). Inset shows variation of emission intensity of Sr_1.9925-yDy_0.0075Li_yCeO₄ as a function of Li concentration (y = 0.005–0.03). The nanophosphor is monitored at both 290 and 424 nm excitation wavelengths.

The type of interaction for concentration quenching can be estimated by Dexter theory.⁴⁴ The emission intensity per activator ion is given by the equation.

where x is the activator concentration, Q is the exchange interaction, where Q = 6, 8 or 10 for electric dipole–dipole (D–D), electric dipole–quadrupole (D–Q), or electric quadrupole–quadrupole (Q–Q) interactions respectively.⁴⁵ K and β are the constants for a given host lattice. Fig. 12(a) and (b) shows the plot of log[Dy³⁺] vs. log I/Dy³⁺ for excitation wavelength at 424 and 290 nm respectively. The trend of the graph at 350 nm is found to be linear with the slope −1.849. Using eqn (1) the Q value obtained is 6 indicating dipole–dipole interactions is the key mechanism for the concentration quenching of Dy³⁺ emission in Sr₂CeO₄ host. However, when the sample is excited at 290 nm the slope value obtained is −1.523 where concentration quenching is due to energy transfer.

Fig. 12 Plot of log[Dy³⁺] vs. log(I/[Dy³⁺]). The slope of the straight line fit indicates that (a) dipole–dipole interaction, and (b) energy transfer are the causes for the concentration quenching in Dy³⁺ ions, when excitation wavelengths are 424 and 290 nm respectively.

3.6 Influence of Li⁺ co-doping: charge compensation

The evolution of the emission spectra after Li⁺ co-doping upon 280 nm and 350 nm are shown Fig. 7(b) and 9(b) respectively. For each sample, Dy³⁺ concentration is fixed at x = 0.0075 and Li⁺ concentration is varied from x = 0.005–0.03. As expected, co-doping Li⁺ does not have any influence on position of the emission peaks. However, the emission intensity of Sr₂CeO₄:Dy³⁺, Li⁺ increases considerably when compared to the Sr₂CeO₄:Dy³⁺ nanophosphor. This improvement of PL emission intensity with Li⁺ codoping can be attributed to the charge compensation phenomenon.^27,28

The charge compensation demands removal of three Sr²⁺ ions to introduce two rare earth (RE³⁺) ions for effective lattice substitution, this might restrict the RE³⁺ concentration, causing an increase in concentration of cation vacancies.³⁷ Hence it becomes difficult to maintain the charge neutrality in the RE doped phosphors. General outer electronic configuration of alkali metals is ns¹, and they acquire mono-positive charge after losing their electron. Therefore, these alkali metals can be used for charge compensation in Dy³⁺ in Sr₂CeO₄ phosphors. Charge compensation is achieved by replacing two Sr²⁺ ions by one Dy³⁺ and one Li⁺ ion. Lattice distortion and non-radiative transitions will appear if more Li⁺ ions are added, which reduces the emission intensity.

3.7 Commission Internationale de l'Eclairage (CIE) coordinates of Sr₂CeO₄:Dy³⁺

For practical lighting applications, colour quality is specified in terms of 1931 CIE chromatic color coordinates. This scheme is based on the known spectral response of the human eye to the 3 primary colors: red, green and blue.⁴⁶ In general the colour of any light sources can be represented on the (x,y) coordinates in this colour space. These colour coordinates are one of the most important parameters for evaluating phosphors performance. Fig. 13 shows the CIE 1931 chromaticity diagram for Sr₂CeO₄:Dy³⁺(x = 0.005–0.09) phosphors. The calculated CIE color coordinates for all the samples are summarized in Fig. 13 (inset). As can be seen, all the samples fall into the scope of white light emission. Ideal white light with CIE (x, y) = (0.298, 0.360) and correlated color temperature CCT = 6896 K are realized when the concentration of Dy³⁺ (x = 0.03). Hence this material shows promise for a white light source to meet the needs of the illustrated applications.

Fig. 13 CIE 1931 colour coordinates of Sr_2−xDy_xCeO₄(x = 0.0025–0.09) nanophosphors as a function of x.

4. Conclusion

We have successfully demonstrated a simple, low temperature energy saving route to synthesize Dy³⁺ doped and Dy³⁺, Li⁺ co-doped Sr₂CeO₄ nanophosphors. A wide range of Dy³⁺ and Li⁺ concentrations have been explored. The resulting nanophosphors exhibit a bright fluorescent yellow emission at 574 nm (⁴F_9/2 → ⁶H_13/2) and blue emission at 488 nm (⁴F_9/2 → ⁶H_15/2). Furthermore, we have examined the comparative photoluminescence (PL) of the Dy³⁺ doped and Dy³⁺-Li⁺ co-doped Sr₂CeO₄ nanophosphors. These studies clearly show that Li⁺ co-doping enhances the emission intensity of the phosphors. Furthermore Li incorporation also improves the crystallinity of the phosphors. We suggest that the luminescence improvement observed is due to charge compensation expected in systems with Li co-activators. White light emission was achieved by tuning yellow to blue ratio, which was done by varying the Dy³⁺ dopant concentration. The quality of emitted white light was evaluated using CIE 1931 chromatic color coordinates.

Acknowledgements

DLM is grateful to the Management and Principal of Shridevi Institute of Engineering & Technology, Tumkur, for their constant support and encouragement.

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