Optoelectronic characterization of Eu³⁺ doped MLa₂O₄ (M = Sr, Ca, Mg) nanophosphors for display devices

Abstract

Eu³⁺ doped MLa₂O₄ (M = Mg, Ca, Sr) nanophosphors were synthesized by a rapid facile gel combustion route. Luminescence properties of these prepared nanophosphors were analyzed by their excitation and emission spectra. The excitation spectrum consisted of some peaks in the 350–410 nm range due to the f–f transitions. The emission spectra of prepared nanophosphors had transitions of Eu³⁺ ions i.e. ⁵D₀ → ⁷F₀ (580 nm), ⁵D₀ → ⁷F₁ (594–596 nm), ⁵D₀ → ⁷F₂ (614–618, 628–629 nm), and ⁵D₀ → ⁷F₃ (650–651 nm). The main emission peak was observed at 614–618 nm of ⁵D₀→⁷F₂ transitions of Eu³⁺ ions. The enhancement in optical properties was observed when materials were reheated at higher temperatures. The nanostructural morphology was confirmed with scanning as well as transmission electron microscopy. The prepared materials were having size in the range of 10–50 nm. X-ray powder diffraction (XRD) technique was used to determine the crystal structure and phase of the prepared phosphor materials. XRD measurements revealed that the crystallinity of MLa₂O₄ materials increased with increasing the sintering temperature. The prepared materials had bright red emitting optical properties that could be suitably applied in various display devices.

Keywords:

• nanophosphor
• MLa_(2-z)O₄:zEu³⁺
• f-f transitions
• luminescence
• combustion technique

Public Interest Statement

Europium-doped mixed oxide has received particular attention due to their efficient optical properties. These materials have bright red emission under the UV light source. Generally all the prime colors are important for the generation of the full color. This prepared series of compounds emit red luminescence (614–618 nm) that could be proficiently used for the generation of the white light for the various optoelectronic display applications.

1. Introduction

Technologically, rare earth-doped oxides nanophosphors are the strong motivators for the research due to their outstanding thermochemical and optoelectronic properties (Gouteron, Michel, Lejus, & Zarembowitch, 1981; Morais, Scalvi, Cavalheiro, Tabata, & Oliveira, 2008). The electronic structure of trivalent europium ion when used as an activator has a non-degenerate ground state ⁷F₀ which is well separated from the first excited ⁵D₀ multiplet (Venkatramu et al., 2010), provide unique properties to various host lattices (Garcı́a-Hipólito et al., 2003; Joffin, Dexpert-Ghys, Verelst, Baret, & Garcia, 2005; Quan, Wang, & Lin, 2005). The emission spectrum of Eu³⁺ ion is generally not affected by the influence of the surrounding due to the shielding effect of 5s, 5p electrons (Zhang, Lü, Xiu, & Wang, 2007). Hence, due to this excellent property of Eu³⁺ ion, it is used as an activator for the synthesis of red phosphor materials having outstanding properties (Cong et al., 2008; Davolos, Feliciano, & Pires, 2003; Dhanaraj, Geethalakshmi, & Jagannathan, 2004; Fragoso, de Mello Donegá, & Longo, 2003; Joly, Chen, Zhang, & Wang, 2007; Sun, Qi, Lee, & Lee, 2004). These Eu³⁺ ions-doped materials have been used as various display materials (Rambabu, Khanna, & Rao, 1998; Rambabu, Mathur, & Buddhudu, 1999; Ronda, 1997; Tanner & Wong, 2004). Thermoluminescence, electroluminescence, and cathodoluminescence properties of these materials are significantly applied in radiation detector, display material as field emission and electroluminescent display (FED, ELD) and cathode ray tube (CRT) (Oshio et al., 1999; Ravichandran, Roy, & White, 1997; Shea, McKittrick, & Lopez, 1996).

La₂O₃ is renowned as an outstanding host lattice for rare earth-doped oxide phosphors due to their good photoluminescence properties and moderately low cost. RE ions-activated metal lanthanates phosphors having general formula MLn₂O₄ (where M = Sr, Ba, and Ln = Y, La, Gd) have been proved outstanding phosphor materials (Yang, Xiao, Ding, Yang, & Wang, 2009; Zhou, Shi, & Gong, 2007). Different synthetic methods such as spray pyrolysis (Medina, Orozco, Hernandez, Hernandez, & Falcony, 2011), precipitation (Gunawidjaja, Diez-y-Riega, & Eilers, 2015), sol–gel Pechini method (Méndez et al., 2010), glycine nitrate solution combustion synthesis (Shang, Jiang, Shang, Li, & Zhao, 2011), etc. are used for the preparation of lanthanates-based phosphor materials. In our present work, we have synthesized and investigated the luminescence properties of these nanocrystalline MLa₂O₃:Eu³⁺ (M = Sr, Ca, Mg) phosphors with facile rapid combustion synthesis process. The prepared phosphors are having very fine particle size because of the released heat energy and gases of exothermic reaction of fuel and metal nitrates (Lou & Chen, 2008). The synthesized series of phosphors are characterized by PL, X-ray powder diffraction (XRD), SEM, and TEM analysis. The synthesized nanomaterials are having better optical properties than the La₂O₃:Eu³⁺ that could be effectively used in various display devices.

2. Experimental details

2.1. Syntheses of nanomaterials

High-purity chemicals [Eu(NO₃)₃.6H₂O], [M(NO₃)₂].xH₂O, (M = Sr, Ca, Mg), [La(NO₃)₃.6H₂O] and hexamethylenetetramine [C₆H₁₂N₄] were taken as raw materials. Nanophosphors having general formula MLa_(2-z)O₄:zEu³⁺ (where z = 1–5 mol%) were synthesized (Singh et al., 2015) by combusting a hydrated concentrated mixture having a considered stoichiometric amount of metal nitrates and hexamethylenetetramine as fuel. The amount of hexamethylenetetramine fuel was calculated by considering oxidizing and reducing valencies (Ekambaram & Patil, 1997). All these chemicals were mixed with deionized water and heated on hot plate till a semisolid paste was prepared. This gel mixture was then subjected to muffle furnace set already at 500°C. The material underwent rapid dehydration followed with degradation and production of numerous gases. These combustible gases ignited and burnt rapidly leaving a fluffy white colored fine powder. The solid powder obtained was reheated at 750 and 950°C for 1 h to study its effect on materials (Figure 1). The equation for the synthesis of this series of nanophosphor is shown as:$$M{\left({\text{NO}}_3\right)}_2.x{\text{H}}_2\text{O}+\left(2-z\right)\text{La}{\left({\text{NO}}_3\right)}_3.6{\text{H}}_2\text{O}+z\text{Eu}{\left({\text{NO}}_3\right)}_3.6{\text{H}}_2\text{O}+\left[{\text{C}}_6{\text{H}}_{12}{\text{N}}_4\right]\to {\text{MLa}}_{(2-z)}{\text{O}}_4:z{\text{Eu}}^{3+}+\text{gaseous products and water}$$

Figure 1. A schematic presentation for the synthesis of Eu³⁺-activated MLa₂O₄ (M = Sr, Ca, Mg) nanophosphors.

where M may be Sr/Ca/Mg, x is the number of moles of water in accordance to the metal nitrates and z is the number of moles of Europium nitrates (z varies from 1 to 5 mol%).

2.2. Instrumentation

Nanophosphors were characterized by X-ray diffraction profiles of the samples by the use of Rigaku Ultima IV X-ray diffractometer with Cu–Ka radiation for 2θ = 20–80°. Transmission electron micrographs (TEM) were taken using the Hitachi F-7500 transmission electron microscope. Scanning electron microscopy was carried out using JEOL JSM-6360LV scanning electron microscope. Emission and excitation spectra were measured with a Fluorimeter SPEX Fluorolog 1680 (USA) equipped with the SPEX 1934 D phosphorimeter having Xenon lamp as excitation source at room temperature in the UV–Visible region. All the characterizations were done at room temperature.

3. Results and discussion

3.1. Optical characterization

MLa₂O₄:Eu³⁺ (M = Mg, Ca, Sr) materials show different peaks (350–410 nm range) in their excitation spectra. These excitation lines are due to ⁷F_0,1→⁵D₄ (360–362 nm), ⁷F_0,1→ ⁵L₇ (377–380 nm), and ⁷F_0,1→ ⁵L₆ (394–396 nm) transitions (f–f) of Eu³⁺ ion. The ⁷F_0,1 to ⁵L₆ at 394–396 nm is observed as the strongest absorption for the synthesized materials. The emission spectra of MLa₂O₄:Eu³⁺ upon excitation with 395 nm show a series of emission lines in visible red region as shown in Figures 2–5. These emission lines are assigned to ⁵D₀ →⁷F_J (J→0–3) transitions of the 4f⁶ configuration of Eu³⁺ ion present in their lattices. All the emission peaks are positioned in 560–660 nm range and the most intense emission band is observed at around 613–618 nm ascribed to the ⁵D₀→⁷F₂ transition of Eu³⁺ ion for prepared series of nanophosphors (MLa₂O₄:Eu³⁺). The orange emission is observed at 595 nm which appears due to the magnetic dipole ⁵D₀→⁷F₁ transition of europium ion and this transition rarely varies with the crystal field strength (Volanti et al., 2009). The emission at 614–616 and 628–629 nm is ascribed to the electric dipole transition of ⁵D₀→⁷F₂ transition of Eu³⁺, and this transition depends on the crystal field (Liu & Wang, 2007) of the prepared lattice. As excitation and emission overlay graphs (Figure 5) are showing the maximum intensity of emission in Eu³⁺-doped SrLa₂O₄ lattice.

Figure 2. Photoluminescence spectra of SrLa₂O₄:Eu³⁺ excited at 395 nm. (a) Temperature variation (b) Concentration variation.

Figure 3. Photoluminescence spectra of CaLa₂O₄:Eu³⁺ excited at 395 nm. (a) Temperature variation (b) Concentration variation.

Figure 4. Photoluminescence spectra of MgLa₂O₄:Eu³⁺ excited at 395 nm. (a) Temperature variation (b) Concentration variation.

Figure 5. Photoluminescence spectra of La₂O₃:Eu³⁺ and MLa₂O₄:Eu³⁺ (M = Sr, Ca, Mg) nanophosphor showing variation of metal ions in host lattice.

Luminescence properties of nanophosphors frequently depend on activator concentration and crystallinity of the material. Luminescence intensity varies with Eu³⁺ ions concentration in MLa₂O₄:Eu³⁺ samples (z = 1–5 mol%), synthesized at 500°C which are presented in Figures 2(b), 3(b), 4(b). It is analyzed that emission intensity in prepared host lattice increased with the increase of europium ion concentration and reached at maximum when z is doped up to 4 mol% in the host lattice. Further increase in activator concentration results in overdoping, which with further increase of non-radiative energy transfer between adjacent Eu³⁺ ions, resulting in lowering of their luminescence intensity (Blasse & Grabmaier, 1994). Figures 2(a), 3(a), 4(a) show the emission spectra of MLa₂O₄ doped with 4 mol% of europium ions, synthesized at 500°C, sintered at 750 and 950°C temperatures, upon excitation at 395 nm. The increase in the intensity of emission spectral lines is accredited to the increase of crystallinity of nanophosphors particles with the increase of sintering temperature.

The powders sintered at high temperatures show better red emission and this intensity of red color also increased with increase in dopant concentration. The best sample is observed when concentration of dopant is taken 4 mol% and calcined at 950°C. This is well described by color coordinates given in Table 1. The ratio R of the intensities I(⁵D₀→⁷F₂)/I(⁵D₀→⁷F₁) provides the percentage purity of the red luminescent color of phosphors. As confirmed by XRD, an additional phase La₂O₃ along with the main MLa₂O₄ phase is also present. These two phases in the lattice resulted into crystal field splitting of ⁵D₀→⁷F₂ into two lines at 613–618 nm and 628–629 nm. Figures 2–5 reveal that the intensity of ⁵D₀→⁷F₂ transitions is more; as a result, R > 1 is obtained. So the red color is more prominent in MLa₂O₄:Eu³⁺ phosphors than the La₂O₃:Eu³⁺.

Table 1. Color co-ordinates of phosphors at varying concentrations and at varying calcination temperatures

Phosphor compounds	Color co-ordinates
	500°C	750°C	950°C
SrLa2O4:Eu^3+ (0.01)	x-0.5086	x-0.5178	x-0.5245
	y-0.2878	y-0.2838	y-0.2829
SrLa2O4:Eu^3+ (0.02)	x-0.5184	x-0.5250	x-0.5363
	y-0.2868	y-0.2824	y-0.2804
SrLa2O4:Eu^3+ (0.03)	x-0.5354	x-0.5463	x-0.5563
	y-0.2846	y-0.2831	y-0.2816
SrLa2O4:Eu^3+ (0.04)	x-0.5446	x-0.5856	x-0.5909
	y-0.2783	y-0.2734	y-0.2723
SrLa2O4:Eu^3+ (0.05)	x-0.5261	x-0.5310	x-0.5423
	y-0.2853	y-0.2843	y-0.2807
CaLa2O4:Eu^3+ (0.01)	x-0.5289	x-0.4335	x-0.4382
	y-0.2838	y-0.2765	y-0.2766
CaLa2O4:Eu^3+ (0.02)	x-0.5301	x-0.5376	x-0.5423
	y-0.2786	y-0.2698	y-0.2665
CaLa2O4:Eu^3+ (0.03)	x-0.5324	x-0.5423	x-0.5489
	y-0.2764	y-0.2709	y-0.2679
CaLa2O4:Eu^3+ (0.04)	x-0.5347	x-0.5434	x-0.5567
	y-0.2738	y-0.2643	y-0.2635
CaLa2O4:Eu^3+ (0.05)	x-0.5310	x-0.5406	x-0.5468
	y-0.2745	y-0.2712	y-0.2658
MgLa2O4:Eu^3+ (0.01)	x-0.5235	x-0.5307	x-0.5398
	y-0.2769	y-0.2669	y-0.2650
MgLa2O4:Eu^3+ (0.02)	x-0.5276	x-0.5328	x-0.5409
	y-0.2749	y-0.2726	y-0.2713
MgLa2O4:Eu^3+ (0.03)	x-0.5293	x-0.5346	x-0.5476
	y-0.2736	y-0.2702	y-0.2649
MgLa2O4:Eu^3+ (0.04)	x-0.5321	x-0.5387	x-0.5439
	y-0.2766	y-0.2734	y-0.2665
MgLa2O4:Eu^3+ (0.05)	x-0.5289	x-0.5327	x-0.5432
	y-0.2735	y-0.2716	y-0.2709

CIE invented $\overline{\text{x}}$(λ), $\overline{\text{y}}$(λ), and $\overline{\text{z}}$(λ) [three color corresponding functions] subsequent to three primary colors i.e. blue, green, and red. By the use of these three functions, calorimetric coordinates (x, y) are calculated from the tristimulus value as shown in Equations (1) and (2).(1) $$x=\overline{x}\mathit{\lambda }/\overline{x}(\mathit{\lambda }),\overline{y}(\mathit{\lambda }),\text{and}\overline{z}(\mathit{\lambda })$$(1) (2) $$y=\overline{y}\mathit{\lambda }/\overline{x}(\mathit{\lambda }),\overline{y}(\mathit{\lambda }),\text{and}\overline{z}(\mathit{\lambda })$$(2)

The calculated values of color co-ordinates are shown in Table 1 corresponding to red color of the visible region on the color gamut calculated from the emission spectra of MLa₂O₄:Eu³⁺ nanophosphor prepared at various annealing temperatures and at different concentrations. The color co-ordinates of MLa₂O₄:Eu³⁺ (0.04 mol) materials calcined at 950°C are shown in color triangle in Figure 6. Samples heated at 750 and 950°C show the CIE co-ordinates in the intense red color region of the color triangle.

Figure 6. Chromaticity diagram of synthesized MLa₂O₄:Eu³⁺ (0.04 mol) nanophosphors calcined at 950°C. (a) MgLa₂O₄:Eu³⁺ (b) CaLa₂O₄:Eu³⁺ (d) SrLa₂O₄:Eu³⁺.

3.2. Morphological study of the phosphor

The morphology of Eu³⁺-doped MLa₂O₄ nanoparticles was studied by scanning electron micrographs and transmission electron micrographs. The surface morphology of Eu³⁺ (4 mol%)-doped MLa₂O₄ nanoparticles prepared by present facile combustion method were studied using SEM. SEM photomicrographs of MLa₂O₄:Eu³⁺ samples are presented in Figures 7(a, b, c). The SEM micrographs showed that as-synthesized phosphors have spherical agglomerated nanocrystalline particles. As in combustion synthesis, numerous gasses evolved within very short period that generally produced nanosized materials with pores and voids. So it is analyzed from SEM images that the powders show large voids, cracks, and pores. The TEM Figures 8(a, b, c) are showing highly agglomerated nanoparticles with an average size of ~10–50 nm. TEM analysis has provided further additional microstructural details of these nanostructures. Furthermore, the crystallite size according to Scherrer’s equation is much closer to TEM observation.

Figure 7. SEM micrograph of (a) SrLa₂O₄:Eu³⁺ (b) CaLa₂O₄:Eu³⁺ (c) MgLa₂O₄:Eu³⁺.

Figure 8. TEM micrograph of (a) SrLa₂O₄:Eu³⁺ (b) CaLa₂O₄:Eu³⁺ (c) MgLa₂O₄:Eu³⁺.

3.3. X-ray diffraction study

The powder XRD patterns of the MLa₂O₄:Eu³⁺ (M = Sr, Ca, Mg) nanophosphors prepared by facile combustion synthesis at 500°C, calcined at 750 and 950°C, are shown in Figures 9–11. By using JCPDS data, multiphase components are observed at 500°C because of the occurrence of cubic La₂O₃ phase attributed as JCPDS No. 005-0602. In SrLa₂O₄:Eu³⁺, numerous supplementary peaks consequent to Sr(NO₃)₂ phase (JCPDS No. 004-0310) are also found. The XRD patterns of SrLa₂O₄:Eu³⁺ powder sintered at 750 and 950°C are assigned to SrLa₂O_x phase matched with JCPDS card No. 042-0343. XRD pattern of sintered CaLa₂O₄ nearly matched with SrLa₂O₄ but no further records are found for CaLa₂O₄ and MgLa₂O₄. The as-prepared CaLa₂O₄ and MgLa₂O₄ samples also showed many additional peaks for M(NO₃)₂ and cubic La₂O₃ phase. But with the increase of calcination temperature, these additional peaks diminished. So it appears that 500°C was not an adequate temperature for the degradation of M(NO₃)₂ to prepare pure MLa₂O₄ phase. It is observed from XRD graphs that the intensities of peaks increased with increase of sintering temperature, which specify the improvement of crystallinity and also assign the increase in size of particles. With the increase of temperature of calcination, the intensity of XRD pattern gets increased and full with half the maximum of peaks slightly decreased. All this indicated the enhancement in crystalline nature of the prepared compounds. Eu³⁺ ions (0.95 A°) replaced La³⁺ (1.061 A°) ions in the host lattices instead of Mg²⁺ (0.72 A°), Ca²⁺ (0.99 A°) and Sr²⁺ (1.12 A°) ions. Here due to both size and ionic charge issues, it is difficult for Eu³⁺ to replace M²⁺ ions, though size of Ca²⁺ is nearly same to Eu³⁺, but here also charge compensation issue predominate. Because the differences in sizes of ionic radii of La³⁺ and Eu³⁺ is small and the charges of both ions are also same. Therefore, Eu³⁺ ions easily substitute La³⁺ ions.

Figure 9. XRD patterns of synthesized SrLa₂O₄:Eu³⁺ phosphors.

Figure 10. XRD patterns of synthesized CaLa₂O₄:Eu³⁺ phosphors.

Figure 11. XRD patterns of synthesized MgLa₂O₄:Eu³⁺ phosphors.

The average crystallite sizes (D) are calculated using Scherrer’s formula from the line broadening obtained from the X-ray powders.$$D=K\mathit{\lambda }/\mathit{\beta }\text{cos}\mathit{\theta }$$

where, “K” is constant, “λ” is wavelength of X-rays, and “β” is FWHM.

Tables 2–4 provided the detailed description related to size of particles and phase of prepared phosphors.

Table 2. Detailed description of size and phase of particle of SrLa₂O₄:Eu³⁺

Nanophosphor	SrLa2O4:Eu^3+
Temperature	500°C	750°C	950°C
Phase	Major La2O3 + Minor La2SrOx	La2SrOx	La2SrOx
2θ value	18.32	30.06	30.06
FWHM (radian)	0.02067	0.01135	0.00697
Particle size (nm)	7.07	14.12	19.91

Table 3. Detailed description of size and phase of particle of CaLa₂O₄:Eu³⁺

Nanophosphor	CaLa2O4:Eu^3+
Temperature	500°C	750°C	950°C
Phase	Major La2O3 matches with JCPDS No. 005-0602	Nearly matched with XRD of SrLa2O4:Eu^3+ No further records are found up to now
2θ value	29.498	30.06	30.06
FWHM (radian)	0.016048	0.01186	0.00942
Particle size (nm)	9.927	13.0493	16.0228

Table 4. Detailed description of size and phase of particle of MgLa₂O₄:Eu³⁺

Nanophosphor	MgLa2O4:Eu^3+
Temperature	500°C	750°C	950°C
Phase	Major La2O3 matches with JCPDS No. 005-0602	No records are found up to now
2θ value	29.498	30.036	30.036
FWHM (radian)	0.019223	0.01238	0.009507
Particle size (nm)	8.296	12.938	16.848

4. Conclusions

Structurally and morphologically well-defined nanophosphor compounds were efficiently prepared by present facile rapid gel combustion process. XRD patterns showed the presence of multiphase components at 500°C, but on increasing sintering temperature, crystallinity of phosphors increased and single phase of phosphors was obtained at 750 and 950°C. Crystallite size of MLa₂O₄:Eu³⁺ phosphor particles were found below 50 nm as calculated from XRD using Scherrer equation. On increasing sintering temperature, sizes of particles were increased, which showed enhancement in the crystallinity of phosphors. Moreover, SEM and TEM characterization confirmed nanocrystalline size of prepared particles. Photoluminescence spectra obtained for all MLa₂O₄:Eu³⁺ phosphors had dominant red emission peak at 613–618 nm. The photoluminescence intensity of all the prepared nanomaterials was found to be increased with the increasing of the sintering temperature. Intensity of luminescence of SrLa₂O₄:Eu³⁺ nanophosphor was found maximum when sintered at 950°C. All prepared nanomaterials had tremendous red photoluminescence properties so they could be made functional in different optoelectronic display applications.

Additional information

Funding

The authors gratefully recognize the financial support [MRP-40-73/2011(SR)] from the University Grant Commission (UGC), New Delhi, India.

Notes on contributors

Devender Singh

Devender Singh is presently working as an assistant professor (Stage-III) of Chemistry at Maharishi Dayanand University, Rohtak, Haryana. He has completed his MSc (2001) and PhD (2005) in Inorganic Chemistry under the supervision of Prof. Ishwar Singh and Prof. Sang Do Han (KIER, S. Korea) from the Maharshi Dayanand University, Rohtak, India. His current research interest focuses on light-emitting materials and their fabrication for optoelectronic display applications. He has published more than 50 research papers in well-reputed international journals. Presently he is a principal investigator for the Major Research Project funded by the University Grant Commission, New Delhi.