Concentration and penetration depth dependent tunable emissions from Eu³⁺ co-doped SrY₂O₄:Dy³⁺ nanocrystalline phosphor

Abstract

A series of Dy³⁺ ion single-doped and Dy³⁺/Eu³⁺ ion co-doped white-light emitting SrY₂O₄ nanocrystalline phosphors were synthesized by a citrate sol–gel method. After the samples were annealed at 1300 °C, the X-ray diffraction patterns confirmed their orthorhombic structure. The particles were closely-packed and their optical properties were monitored by photoluminescence spectroscopy. The Dy³⁺ ions acted as luminescent centers and substituted Y³⁺ ions in the SrY₂O₄ host lattice, where they are located in C_s sites, and the characteristic emission of Dy³⁺ ions (⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_13/2 transitions) with intense yellow emission band at 578 nm occurred. When the Eu³⁺ ions were co-doped into the SrY₂O₄:Dy³⁺ (1 mol%) nanocrystalline phosphor, white-light emission was observed under excitation at 381 nm. The energy transfer between Eu³⁺ and Dy³⁺ ions was calculated and the chromaticity coordinates were also presented. The cathodoluminescence (CL) spectra confirmed that the penetration depth is inversely proportional to the total atomic weight and atomic number of the compound. From CL analysis, the emission intensities of Dy³⁺ ion single-doped and Dy³⁺/Eu³⁺ co-doped SrY₂O₄ nanocrystalline phosphors increased continuously upon increasing both the accelerating voltage from 1 to 5 kV and the filament current from 30 to 47 μA.

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1. Introduction

Warm white emission with good color rendition is essential for phosphor-converted white light-emitting diodes (WLEDs) aimed at indoor illuminations.^1,2 To obtain warm WLEDs for general illumination, rare-earth (RE) ion doped metal oxide nanocrystalline phosphors have attracted much attention due to the optical, chemical, and electronic properties resulting from the trivalent state of their 4f electrons.³ RE oxides are the most stable oxides and are used in the fields of luminescent devices, optical transmission, biochemical probes, medical diagnosis, etc.⁴

Recently, the luminescence properties of RE³⁺ ions activated SrY₂O₄ host lattice have received increasing interest due to potential applications in the development of advanced lighting and display technology.⁵ SrY₂O₄ has an ordered CaFe₂O₄ structure, and is composed of two types of yttrium sites, i.e., one is an almost undistorted octahedral site and the other is a substantially distorted site, and also one strontium that occupies the bicapped trigonal prismatic site.^6–8 Typically, the RE³⁺ ions activated phosphors can be divided into two classes, where one class displays broad band emission due to the 5d–4f transition (Eu²⁺, Ce³⁺) and the other class displays narrow band emission owing to the transition between the 4f levels (Eu³⁺, Tb³⁺, Gd³⁺, Yb³⁺, Sm³⁺, Tm³⁺, etc.). In general, phosphors doped with Eu³⁺ ions emit pure red or reddish orange light due to the intraconfigurational f–f transitions of Eu³⁺ ions, from the ⁵D₀ excited state to ⁷F_J (J = 1,2,3,4,5) levels of 4f⁶. Likewise, dysprosium (Dy³⁺) exhibits solid-state luminescence in a variety of lattices because it mainly consists of two fine bands in the blue and yellow regions corresponding to the (⁴F_9/2 → ⁶H_15/2) and (⁴F_9/2 → ⁶H_13/2) transitions, respectively.^9,10 By proper adjustment of yellow-to-blue intensity ratio (Y/B) values, there is an opportunity to obtain near-white light emission.¹¹ However, the Dy³⁺ single-doped SrY₂O₄ phosphor suffers from the lack of a red component. In order to compensate for the red component, Eu³⁺ ions were introduced into the SrY₂O₄ host lattice.

Classically, phosphors have been prepared by traditional solid-state reaction (SSR) methods, but this procedure usually requires high temperature, time consuming heating, and consequent grinding processes. Due to insufficient mixing and low reactivity of the raw materials, several impurity phases may co-exist in the resultant product, and also the particles attained by this SSR process are in the micrometer range.¹² However, nowadays, there are many wet-chemical techniques available such as co-precipitation, hydrothermal, solvothermal, combustion, sol–gel, and spray-pyrolysis methods. Among them, the sol–gel process has gained considerable attention due to its unique features, resulting in the preparation of ultra-fine uniform ceramic powders with excellent purity and at a relatively low reaction temperature.¹³

In this work, we studied the Eu³⁺ ion co-activated SrY₂O₄:Dy³⁺ nanocrystalline phosphor using a citrate sol–gel method. The structural and morphological properties were studied by X-ray diffraction (XRD) patterns, scanning electronic microscopy (SEM), and transition electronic microscopy (TEM) investigations. The energy transfer efficiency from Dy³⁺ to Eu³⁺ ions was discussed based on the photoluminescence (PL) emission intensities. From the cathodoluminescence (CL) spectra, for the first time, we showed the relationship between atomic weight and penetration depth.

2. Experimental details

SrY₂O₄:Dy³⁺/Eu³⁺ nanocrystalline phosphors were prepared via the citrate sol–gel method by taking stoichiometric amounts of high-purity grade strontium nitrate [Sr(NO₃)₂], yttrium nitrate hexahydrate [Y(NO₃)₃·6H₂O], dysprosium nitrate pentahydrate [Dy(NO₃)₃·5H₂O], europium nitrate pentahydrate [Eu(NO₃)₃·5H₂O], and citric acid [HOC(COOH)(CH₂COOH)₂]. Solution one was prepared by dissolving 1 mM of strontium nitrate and 2 mM of citric acid in de-ionized (DI) water, and solution two was prepared by dissolving 2(1 − x) mM of yttrium nitrate, 2x mM of RE (Eu³⁺/Dy³⁺) nitrate and 4 mM of citric acid in DI water in another beaker. Initially, the two solutions were stirred individually by a magnetic stirrer for 30 min and after that the two solutions were mixed and stirred until the formation of homogeneous solution. The mixture was then heated on a hot plate with continued magnetic stirring and the solution temperature was maintained at 80 °C. The beaker was closed with a cap for 12 h, and then the cap was removed. After opening the cap, the solution was evaporated within 1 h and a yellowish wet gel was produced. The gel was dried at 120 °C, which yields the porous solid matrices called xerogel. This precursor decomposed to give black-colored flakes of extremely fine particle size on further heating at 500 °C for 5 h. The resulting sample was further annealed for characterization.

XRD patterns of the samples were recorded on a Mac Science (M18XHF-SRA) X-ray powder diffractometer with CuKα = 1.5406 Å. The morphology of the samples was examined by field-emission SEM (FE-SEM, JEOL JSM-6700) and field-emission TEM (FE-TEM, JEOL JEM-2100F) images. The room-temperature PL spectra were recorded on a Photon Technology International (PTI, USA) fluorimeter with a Xe-arc lamp of 60 W power. The CL properties were measured by a Gatan (UK) MonoCL3 system attached to the SEM (Hitachi S-4300 SE).

3. Results and discussion

The composition and phase purity of the SrY₂O₄:Dy³⁺/Eu³⁺ nanocrystalline phosphor samples were confirmed by XRD patterns. Fig. 1(a) shows the XRD patterns of SrY₂O₄:Dy³⁺ (1 mol%)/Eu³⁺ (1 mol%) (i.e., 1Dy³⁺/1Eu³⁺) individually doped and co-doped samples annealed at 1300 °C. All the diffraction peaks were well assigned to the pure orthorhombic phase of SrY₂O₄ and they were exactly matched to the standard values of SY given in the JCPDS card (No. 32-1272) with space group Pnam (62), indicating that the obtained samples are of a single phase and the co-doped ions (Eu³⁺ and Dy³⁺) have no impact on the phase purity of the host lattice. Generally, the crystallite size can be assessed by the Scherrer equation, D_hkl = kλ/β cos θ, where D is the average crystallite size, λ is the X-ray wavelength (1.5406 Å), β is the full width at half maximum (FWHM) and θ is the diffraction angle of an observed peak. The strongest diffraction peaks were used to calculate the crystallite sizes of SrY₂O₄ phosphor annealed at 1300 °C, which yielded an average value of about 72.5 nm. Fig. 1(b) shows the SEM and TEM images of the SrY₂O₄ nanocrystalline phosphors annealed at 1300 °C. From these images, it is clear that the particles are closely packed. As we know that closely packed particles help to prevent the scattering of light, consequently we can obtain more efficient output light.¹⁴ The selected-area electron diffraction (SAED) pattern of the sample is also shown in the inset of Fig. 1(b), and it displays regularly arranged diffraction spots, indicating that the SrY₂O₄ nanocrystalline phosphor is of a single crystalline nature.

Fig. 1 (a) XRD patterns of 1Dy³⁺/1Eu³⁺ individually doped and co-doped SrY₂O₄ nanocrystalline phosphors annealed at 1300 °C and (b) SEM image of SrY₂O₄:1Dy³⁺ sample (insets show the corresponding TEM and SAED patterns).

Fig. 2(a) shows the PL excitation (PLE) spectra of SrY₂O₄:Dy³⁺ phosphors as a function of Dy³⁺ ion concentration by monitoring the emission wavelength at 578 nm, which corresponds to the electronic transition (⁴F_9/2 → ⁶H_13/2). The excitation spectra of Dy³⁺ consist only of narrow f–f transition lines with low oscillator strength (10⁻⁶) due to their forbidden nature according to the parity selection rule from 300 to 500 nm,¹⁵ and it also includes the host absorption band (HAB) which occurs due to the efficient energy transfer from the host to Dy³⁺ ions.¹⁰ Typically, the charge transfer state (CTS) occurred between 257 and 307 nm. The f–f transitions are assigned to the electronic transitions (⁶H_15/2 → ⁶P_3/2) at 325 nm, (⁶H_15/2 → ⁶P_7/2) at 351 nm, (⁶H_15/2 → ⁶P_5/2) at 365 nm, (⁶H_15/2 → ⁴I_13/2) at 387 nm, (⁶H_15/2 → ⁴G_11/2) at 426 nm, and (⁶H_15/2 → ⁴I_15/2) at 451 nm for Dy³⁺ ions. It is noticeable that due to the change of the Dy³⁺ polarization effect on the surrounding O²⁻ ions in the host lattice, the intensity of the HAB decreased and the intensity of CTS increased. Furthermore, upon increasing the Dy³⁺ ion concentration, the intensity of the characteristic Dy³⁺ excitation bands increased up to 1 mol% and then decreased due to the concentration quenching.

Fig. 2 (a) PLE and (b) PL emission spectra of SrY₂O₄:Dy³⁺ phosphors as a function of Dy³⁺ ion concentration by monitoring the emission wavelength at 578 nm and the excitation wavelength at 351 nm, respectively.

Fig. 2(b) shows the PL emission spectra of SrY₂O₄:Dy³⁺ phosphors with different concentrations of Dy³⁺ ions by monitoring the excitation wavelength at 351 nm. The PL spectra revealed three emission peaks at 488 nm (blue), 578 nm (yellow) and 678 nm (red) corresponding to the (⁴F_9/2 → ⁶H_15/2), (⁴F_9/2 → ⁶H_13/2) and (⁴F_9/2 → ⁶H_11/2) transitions, respectively. Among these transitions, (⁴F_9/2 → ⁶H_13/2) at 578 nm is the hypersensitive (forced electric dipole) transition with Δj = 2, which dominates the other two, while the transition (⁴F_9/2 → ⁶H_15/2) at 488 nm, which has lower intensity, is the magnetic dipole transition. It is obvious that the hypersensitive (forced electric dipole) transition is strongly influenced by the external surrounding environment and the magnetic dipole transition is insensitive to the crystal field strength around the Dy³⁺ ions.¹⁶ Generally, the intensity ratio of the ED to MD transitions is used as a measure to understand the symmetry of the local environment around the trivalent 4f⁵ ions. In this case of a SrY₂O₄ host lattice, the ED transition of the Dy³⁺ ions is more intense than the MD transition, indicating that the Dy³⁺ ions occupy the low symmetry sites. Thus, the yellow emission is often dominant in the emission spectrum.⁷ As the concentration of Dy³⁺ ions increased from 0.5 to 4 mol%, the emission intensities gradually increased and reached the maximum value at 1 mol%. With a further increase of Dy³⁺ ion concentration, the emission intensity decreased remarkably due to concentration quenching. The unique phenomena for the concentration quenching of Dy³⁺ is the cross-relaxation mechanism between the neighboring Dy³⁺ ions. The energy of the (⁴F_9/2 → ⁶H_11/2 + ⁶H_9/2) transition matches that of the (⁶H_15/2 → ⁶F_11/2 + ⁶H_9/2) transition. Therefore, upon increasing the Dy³⁺ ion concentration, the resonance energy transfer between the neighboring Dy³⁺ ions through cross-relaxation is more frequent.¹⁷

The PLE spectra of the SrY₂O₄:1Dy³⁺/1Eu³⁺ co-activated nanocrystalline phosphor, obtained by monitoring with emission wavelengths at both 578 and 615 nm corresponding to the (⁴F_9/2 → ⁶H_13/2) and (⁵D₀ → ⁷F₂) transitions of Dy³⁺ and Eu³⁺ ions, respectively, are shown in Fig. 3(a). Fundamentally, the emission properties of phosphor materials strongly depend on the synthetic route, size of the particles, and concentration of activator ions. The particle sizes of Eu³⁺ and Dy³⁺ individually doped and co-doped SrY₂O₄ samples were identical because they were synthesized under similar conditions. Thus, this allows a comparison of the emission properties of SrY₂O₄:Dy³⁺ phosphor particles co-doped with different concentrations of Eu³⁺ ions. From these two excitation spectra, we noticed that both the spectra consist of a broad excitation band with band maxima at 271 nm in the shorter wavelength region, and some sharp excitation peaks in the longer wavelength region which occurred due to the f–f transitions of Dy³⁺ and Eu³⁺ ions. They are assigned to the electronic transitions of (⁶H_15/2 → ⁶P_3/2) at 325 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 393 nm, (⁶H_15/2 → ⁴G_11/2) at 423 nm, and (⁶H_15/2 → ⁴I_15/2) at 453 nm for Dy³⁺ ions, and of (⁷F₀ → ⁵D₄) at 362 nm, (⁷F₀ → ⁵L₆) at 393 nm, and (⁷F₀ → ⁵D₂) at 465 nm for Eu³⁺ ions. These peaks exhibited greater intensity by monitoring with the yellow emission wavelength (578 nm) than by monitoring with the red emission (615 nm) wavelength. This occurs when the tail of the (⁴F_15/2 → ⁶H_13/2) emission of Dy³⁺ ions is almost overlapped with the (⁵D₀ → ⁷F_(0,1)) emission of Eu³⁺ ions and the virtually suppressed emission of Dy³⁺ ions is overlapped with the strong (⁵D₀ → ⁷F₂) emission of Eu³⁺ ions.¹⁸ These spectral features well signify that efficient energy transfer occurred between Dy³⁺ and Eu³⁺ ions in the co-doped sample.

Fig. 3 (a) PLE spectra of 1Dy³⁺/1Eu³⁺ co-doped SrY₂O₄ nanocrystalline phosphor by monitoring with emission wavelengths at both 578 nm (yellow) and 615 nm (red), (b) Gaussian fitting curves for SrY₂O₄:1Dy³⁺/1Eu³⁺ sample by monitoring at 578 nm, and (c) PL emission spectra of 1Dy³⁺/Eu³⁺ co-doped SrY₂O₄ nanocrystalline phosphors at different Eu³⁺ ion concentrations upon exciting at 381 nm.

A broad excitation band of the SrY₂O₄:1Dy³⁺/1Eu³⁺ co-activated nanocrystalline phosphor was obtained due to the overlapping of three excitation peaks as shown in Fig. 3(b). For clear understanding, the broad band was deconvoluted into three bands using Gaussian fitting. The band centered at 232 nm is called the HAB and this band occurred due to Dy³⁺ ions. The remaining two bands with band maxima located at 264 and 275 nm are the CTBs of Eu³⁺ ions. The SrY₂O₄ host lattice contains two Y sites, and the Eu³⁺ ions occupy these two Y sites, so there exists two CTBs in the excitation spectrum of co-doped samples. This figure confirms the presence of both Dy³⁺ and Eu³⁺ ions in the co-doped sample and also confirms that there is efficient energy transfer from Dy³⁺ to Eu³⁺ ions.

Fig. 3(c) shows the PL emission spectra of SrY₂O₄:Dy³⁺/Eu³⁺ nanocrystalline phosphor as a function of Eu³⁺ ion concentration for a fixed 1 mol% of Dy³⁺ ion, measured by exciting at 381 nm. Instead of 274 nm, we used 381 nm as the excitation wavelength because the Dy³⁺ and Eu³⁺ excitation efficiency was nearly equal and also useful for color rendering properties at this wavelength. From the PL emission spectra, various emission peaks originating from ⁵D_J (J = 0,1,2,3) levels of Eu³⁺ ions, corresponding to the (⁵D₃ → ⁷F₂) transition at 429 nm, (⁵D₃ → ⁷F₃) transition at 448 nm, (⁵D₂ → ⁷F₀) transition at 466 nm, (⁵D₁ → ⁷F₁) transition at 532 nm, (⁵D₀ → ⁷F₀) transition at 578 nm, (⁵D₀ → ⁷F₁) transition at 590 nm, (⁵D₀ → ⁷F₂) transition at 614 nm, and (⁵D₀ → ⁷F₃) transition at 650 nm were observed.¹⁹ The blue and yellow emission bands of Dy³⁺ ions were split into doublets. The blue emission band corresponding to the (⁴F_9/2 → ⁶H_15/2) transition was split into two peaks occurring at 466 and 490 nm, and the yellow emission band corresponding to the (⁴F_9/2 → ⁶H_13/2) transition was split into two intense peaks occurring at 578 and 590 nm. On the contrary, the (⁴F_9/2 → ⁶H_11/2) transition occurred at 650 nm (red region). Upon increasing the Eu³⁺ ion concentration, the emission peak intensities of ⁵D₀ increased considerably earlier than ⁵D_1,2,3 energy levels because the populations at these ⁵D_1,2,3 levels are easily relaxed to the ⁵D₀ level through multi-phonon relaxations.³ This occurs because the energy gaps between the neighboring ⁵D_J levels are very small and they can easily be bridged by phonons. Hence, these results reveal that the SrY₂O₄ host lattice has relatively low phonon energy.¹³

At equal concentrations of Eu³⁺ and Dy³⁺ ions (at 1 : 1 ratio), the intensities of the blue band at 490 nm, yellow band at 578 nm, and red band at 614 nm were almost equal, but when the Eu³⁺ ion concentration increased (i.e., 1 : 2, 1 : 3 ratios), the emission peak intensity corresponding to the hypersensitive electric dipole transition (⁵D₀ → ⁷F₂) of Eu³⁺ ions at 614 nm exhibited a systematic enhancement. This could occur due to the energy transfer from Dy³⁺ to Eu³⁺ ions, whereas the energy of the ⁴F_9/2 level of Dy³⁺ was somewhat higher than that of ⁵D₀ level of Eu³⁺. Therefore, the energy transfer is possible due to the phonon-aided non-radiative relaxation from the ⁴F_9/2 level of Dy³⁺ to the ⁵D₀ level of Eu³⁺ ions.^20,21 Furthermore, the emission peak occurring at 590 nm also increased due to the overlapping of the tail of the (⁴F_9/2 → ⁶H_13/2) emission of the Dy³⁺ ions with the (⁵D₀ → ⁷D₁) emission of Eu³⁺ ions. The energy transfer efficiency from Dy³⁺ to Eu³⁺ ions was calculated by the following equation of , where I_so and I_s are the integrating emission intensities of the hypersensitive transition (⁴F_9/2 → ⁶H_13/2) of Dy³⁺ at 578 nm in the absence and presence of Eu³⁺ ions, respectively. The calculated energy transfer efficiencies are 35.28, 48.19, and 67.34 for 1Dy³⁺/1Eu³⁺, 1Dy³⁺/2Eu³⁺, and 1Dy³⁺/3Eu³⁺, respectively.

The CL spectrum of the Dy³⁺ ion activated SrY₂O₄:1Dy³⁺ nanocrystalline phosphor at constant accelerating voltage (5 kV) and filament current (51 μA) is shown in Fig. 4(a). The CL spectrum consists of a broad blue emission band at 490 nm and a yellow emission band at 580 nm corresponding to the (⁴F_9/2 → ⁶H_15/2) and (⁴F_9/2 → ⁶H_13/2) transitions, respectively. The CL spectrum is similar to the PL spectrum, apart from the intensity. Fig. 4(b) and (c) show the CL intensities of the SrY₂O₄:1Dy³⁺ nanocrystalline phosphor as functions of accelerating voltage and filament current, respectively. When the filament current was fixed at 51 μA, and the accelerating voltage was increased from 1 to 5 kV, the CL intensity also increased, as can be seen in Fig. 4(b). Likewise, when the accelerating voltage was fixed at 5 kV, the CL intensity increased upon increasing the filament current from 30 to 51 μA. The increase in CL intensity with an increase in electron energy and filament current is attributed to the deeper penetration of electrons into the phosphor body and the large electron beam current density.¹⁵ The electron penetration depth can be assessed by the empirical formula of L = 250 (A/ρ) (E/Z^1/2)ⁿ, where n = 1.2/(1 − 0.29 log₁₀ Z) and A is the atomic weight, ρ is the bulk density, Z is the atomic number, and E is the accelerating voltage.^22,23 The calculated atomic weight, atomic number, penetration depth, and Commission Internationale De l'Eclairage (CIE) coordinates of Dy³⁺ ion single-doped SrY₂O₄ nanocrystalline phosphors are summarized in Table 1.

Fig. 4 (a) CL spectrum of the SrY₂O₄:1Dy³⁺ nanocrystalline phosphor at 5 kV and 51 μA, and CL intensities of the SrY₂O₄:1Dy³⁺ nanocrystalline phosphor as a function of (b) accelerating voltage and (c) filament current.

Table 1 Calculated atomic weight (A), atomic number (Z), penetration depth (L), and CIE chromaticity coordinates of Eu³⁺ co-doped SrY₂O₄:1Dy³⁺ nanocrystalline phosphors

	Atomic weight (A)	Atomic number (Z)	Penetration depth (L, Å)			CIE chromaticity coordinates (x, y)
			1 kV	3 kV	5 kV	1 kV	3 kV	5 kV
1Dy	330.901	147.22	4.81	167.45	872.75	(0.399, 0.400)	(0.403, 0.401)	(0.397, 0.396)
1Dy/1Eu	332.163	149.02	4.58	161.75	848.84	(0.418, 0.364)	(0.469, 0.378)	(0.461, 0.378)
1Dy/2Eu	333.424	149.50	4.53	160.72	844.94			(0.507, 0.379)
1Dy/2Eu	334.685	149.98	4.48	159.69	841.05			(0.529, 0.382)

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Fig. 5(a) shows the CL spectra of Eu³⁺ ion co-activated SrY₂O₄:1Dy³⁺ nanocrystalline phosphors. The CL spectra were similar to the PL spectra. From the CL spectra, three main intense peaks were observed in the blue, yellow, and red regions. When increasing the Eu³⁺ ion concentration, the total atomic weight and atomic number of the compound increased, thus leading to a decrease in penetration depth. The calculated penetration depths as a function of Eu³⁺ ion concentration and accelerating voltage are shown in Table 1. Generally, in PL, based on the acceptor concentration, the energy transfer occurrs between the co-doped elements from the higher excited states of the donor to the next lower excited level of the acceptor by selecting a suitable excitation wavelength. However, in CL, the energy transfer process might occur at very high dopant concentrations but sometimes it will not happen because the emission intensity most probably depends upon the penetration depth of electrons. It is well known that high-energy electrons in the Eu³⁺ luminescent centers are easily excited and give efficient emission as compared to other RE ions. Here, the penetration depth effect on the Dy³⁺ emission was clearly observed. As the penetration depth decreased with increasing atomic weight, the CL intensity of Dy³⁺ ions decreased. Particularly, this can be observed in the blue region compared to the yellow region because the (⁴F_9/2 → ⁶H_13/2) transition of Dy³⁺ ions overlaps with the (⁵D₀ → ⁷F₀) transition of Eu³⁺ ions. This is the first report on penetration depth based emission intensities instead of energy transfer in the co-doped samples. The reason behind this aspect is that the mechanism of CL is different from that of PL. In PL, ultraviolet and/or visible light is used to excite luminescent centers with energy of only 4–6 eV, while under electron-beam irradiation during the CL, the luminescent centers of phosphors are excited by either direct or indirect excitations because the energy of fast electrons under acceleration from an anode voltage can be tuned from a few thousand to thousands of electron volts.²⁴ Hence, the excitation energy on individual ions is much larger in CL than in PL. Furthermore, the CL intensity of 1Eu³⁺ ion co-doped SrY₂O₄:1Dy³⁺ samples increased upon increasing both the accelerating voltage from 1 to 5 kV, and the filament current from 30 to 47 μA due to the increased penetration depth. The increased emission intensities in the three (blue, yellow, and red) regions were shown in Fig. 5(b) and (c).

Fig. 5 (a) CL spectra of Eu³⁺ ion co-activated SrY₂O₄:1Dy³⁺ nanocrystalline phosphors, and CL intensities of SrY₂O₄:1Dy³⁺/1Eu³⁺ co-doped samples as a function of (b) accelerating voltage and (c) filament current at 490 nm (blue), 578 nm (yellow), and 616 nm (red) wavelengths.

The CIE chromaticity coordinates of Dy³⁺ ion single-doped and Eu³⁺/Dy³⁺ ion co-doped SrY₂O₄ nanocrystalline phosphors as a function of Eu³⁺ ion concentration were calculated and presented in Fig. 6. All these phosphors exhibited excellent CIE coordinates that are present in the white emission region. The Dy³⁺ single-doped SrY₂O₄ showed chromaticity coordinates in the cool white region (0.2835, 0.3034), and by incorporating the Eu³⁺ ions, the chromaticity coordinates were shifted from the cool white region to the warm white color region due to the stronger Eu³⁺ doping effect. The calculated chromaticity coordinates were (0.2977, 0.2823), (0.3379, 0.2922), and (0.3745, 0.3021) for SrY₂O₄:1Dy³⁺/1Eu³⁺, SrY₂O₄:1Dy³⁺/2Eu³⁺, and SrY₂O₄:1Dy³⁺/3Eu³⁺ phosphors, respectively. From the CL results, the measured chromaticity coordinates of Eu³⁺ ion co-doped samples were (0.4609, 0.3782), (0.5066, 0.3796), and (0.5293, 0.3823) for SrY₂O₄:1Dy³⁺/1Eu³⁺, SrY₂O₄:1Dy³⁺/2Eu³⁺, and SrY₂O₄:1Dy³⁺/3Eu³⁺ phosphors, respectively. Additionally, by varying the accelerating voltage for the equally doped case (1Dy³⁺/1Eu³⁺), the chromaticity coordinates of (0.4182, 0.3636), (0.4689, 0.3778), and (0.4609, 0.3782) were obtained for 1, 3, and 5 kV, respectively. Similarly, by varying the filament current, the obtained coordinates were (0.4676, 0.3805), (0.468, 0.3818), (0.4657, 0.3803), and (0.4644, 0.3806) for 30, 34, 42, and 47 μA, respectively. For Dy³⁺ ions single-doped samples, by varying accelerating voltage, the obtained coordinates were (0.3997, 0.4), (0.4031, 0.4018), and (0.3971, 0.3956) for 1, 3, and 5 kV, respectively. At the filament currents of 30 and 42 μA, the chromaticity coordinates of (0.3994, 0.3973) and (0.3982, 0.3959) were obtained. When the accelerating voltage and filament current were fixed at 5 kV and 51 μA, the obtained chromaticity coordinates were (0.3971, 0.3956).

Fig. 6 CIE chromaticity coordinates of Dy³⁺ single-doped and Dy³⁺/Eu³⁺ co-doped SrY₂O₄ nanocrystalline phosphors [(1) 1Dy³⁺, (2) 1Dy³⁺/1Eu³⁺, (3) 1Dy³⁺/2Eu³⁺, and (4) 1Dy³⁺/3Eu³⁺].

4. Conclusion

The Dy³⁺ ion single-doped and Dy³⁺/Eu³⁺ ion co-doped SrY₂O₄ nanocrystalline phosphors were successfully synthesized by the citrate sol–gel method. The SEM and TEM measurements revealed that the particles are closely packed with a single crystalline nature and the XRD patterns confirmed their pure orthorhombic structure. The Dy³⁺ ion single-doped SrY₂O₄ nanocrystalline phosphor emitted cool white light, and the PL spectra showed that the yellow emission band (⁴F_9/2 → ⁶H_13/2) was more dominant than the blue emission band (⁴F_9/2 → ⁶H_15/2), which demonstrates that the Dy³⁺ ions occupied the low symmetry sites, and the optimum concentration was found at 1 mol%. The PL spectra of Dy³⁺/Eu³⁺ co-doped SrY₂O₄ samples exhibited warm white light emission upon increasing the Eu³⁺ ion concentration and the energy transfer efficiency between Dy³⁺ and Eu³⁺ ions was calculated. In CL analysis, no saturated luminescence was observed for Dy³⁺ ion single-doped and Dy³⁺/Eu³⁺ co-doped SrY₂O₄ nanocrystalline phosphors when increasing both the accelerating voltage from 1 to 5 kV and the filament current from 30 to 47 μA. From these results, SrY₂O₄:Dy³⁺/Eu³⁺ co-doped nanocrystalline phosphors might be promising materials to obtain natural white light for optical display systems and indoor applications.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2013-010037).

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