High-efficiency energy transfer in the strong orange-red-emitting phosphor CeO₂:Sm³⁺, Eu³⁺

Abstract

High-efficiency energy transfer (ET) from Sm³⁺ to Eu³⁺ leads to dominant red emission in Sm³⁺, Eu³⁺ co-doped single-phase cubic CeO₂ phosphors. In this work, a series of Sm³⁺ singly and Sm³⁺/Eu³⁺ co-doped CeO₂ cubic phosphors was successfully synthesized by solution combustion followed by heat treatment at 800 °C in air. The crystal structure, morphology, chemical element composition, and luminescence properties of the obtained phosphors were investigated using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and photoluminescence analysis. Under 360 nm excitation, the Sm³⁺ singly doped CeO₂ phosphor emitted strong yellow-red light at 573 nm (⁴G_5/2–⁶H_5/2) and 615 nm (⁴G_5/2–⁶H_7/2). Meanwhile, the CeO₂:Sm³⁺, Eu³⁺ phosphors showed the emission characteristic of both Sm³⁺ and Eu³⁺, with the highest emission intensity at 631 nm. The emission intensity of Sm³⁺ decreased with increasing Eu³⁺ content, suggesting the ET from Sm³⁺ to Eu³⁺ in the CeO₂:Sm³⁺, Eu³⁺ phosphors. The decay kinetics of the ⁴G_5/2–⁶H_5/2 transition of Sm³⁺ in the CeO₂:Sm³⁺, Eu³⁺ phosphors were investigated, confirming the high-efficiency ET from Sm³⁺ to Eu³⁺ (reached 84%). The critical distance of energy transfer (R_C = 13.7 Å) and the Dexter theory analysis confirmed the ET mechanism corresponding to the quadrupole–quadrupole interaction. These results indicate that the high-efficiency ET from Sm³⁺ to Eu³⁺ in CeO₂:Sm³⁺, Eu³⁺ phosphors is an excellent strategy to improve the emission efficiency of Eu³⁺.

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1. Introduction

White-light emitting diodes (WLEDs) have been extensively used in many fields of application, such as in solid lighting, display devices, and optoelectronic devices, because of their high luminous efficiency, long lifetime, energy saving, and environment friendliness.^1–3 A popular method for manufacturing WLEDs is combining tricolor phosphor powder (blue, green, and red phosphors) with an ultraviolet (UV) InGaN chip.^4,5 However, these WLEDs present a high correlated color temperature and low color rendering index due to the lack of a red component.^6,7 To overcome these drawbacks, scholars should explore new red phosphors for WLED applications.

The europium trivalent ion (Eu³⁺) is an important rare-earth (RE) ion that has been widely used as an activator in red-emitting phosphors for WLEDs.^8–10 The red emission of Eu³⁺ is originally from electric dipole transitions. Notably, Eu³⁺-doped phosphors typically exhibit relatively narrow absorption in UV and near-UV regions because of the spin-forbidden transition of Eu³⁺, resulting in low emission efficiency.^11,12 This defect can be compensated by introducing sensitizing ions, such as Tb³⁺, Bi³⁺, Gd³⁺, and Sm³⁺,^13–15 which can absorb excitation energy efficiently and transfer it to Eu³⁺. Sm³⁺ is a popular sensitizer for improving the efficiency emission of Eu³⁺ ion due to the small energy difference between the ⁴G_5/2 level of Sm³⁺ and the ⁵D₀ level of Eu³⁺ (about 600 cm⁻¹), leading to easy phonon-assisted energy transfer (ET).¹³ Hence, the energy transfer between Sm³⁺ and Eu³⁺ ions was widely investigated in a variety of host lattices.^12,16,17 J. Wu et al.¹⁶ found that the ET efficiency from Sm³⁺ to Eu³⁺ up to 65% in YPO₄:Sm³⁺, Eu³⁺ phosphor corresponds to the electric dipole–electric dipole interaction mechanism. Y. Li et al.¹⁷ reported that ET efficiency from Sm³⁺ to Eu³⁺ was 13.7% in La₂CaB₁₀O₁₉:Sm³⁺, Eu³⁺ phosphor, further confirmed by Judd–Ofelt theory. Meanwhile, X. Zhang et al.¹² developed Ca₂GdNbO₆:Sm³⁺, Eu³⁺ phosphor with high quantum yield (82.7%), excellent thermal stability, and up to 28.6% ET efficiency. In addition, the LED device fabricated based on this phosphor emits bright white light, and CCT = 5400 K, R_a = 92.8. Clearly, the addition of two or more RE ions into a luminescent material can effectively control multi-color luminescence and achieve ET, thereby improving the luminous efficiency of the phosphors.^7,11,13,18 Besides, several parameters, including the degree of crystallinity, the dopant concentration, and the host material, influence the emission efficiency of the phosphors. In this case, the host lattice selection is important since it influences the luminescent efficiency and the application potential of the material.

Cerium oxide (CeO₂) host lattice exhibits low phonon energy, high thermal stability, low toxicity, and excellent physicochemical properties.^19–21 It is an attractive UV-excited host material due to its strong light absorption through the O²⁻–Ce⁴⁺ charge transfer band. Therefore, RE-doped CeO₂ was widely investigated for many applications, such as catalysis, sensors, optoelectronic devices, and UV-LEDs.^20,22 The small difference between the ionic radius of Ce⁴⁺ and RE³⁺ suggests that it can provide favorable sites for introducing RE ions into the CeO₂ host lattice. Furthermore, the spectral overlap of the CeO₂ host's charge transfer band and the 4f–4f transitions of the RE ions leads to highly efficient energy transfer from host to RE ions.^23,24 G. Vimal et al.²³ have observed the energy transfer from the CeO₂ host to Eu³⁺ in CeO₂:Sm³⁺, Eu³⁺ phosphor leading to improved efficiency emission of the systems. Meanwhile, an intense red emission of Eu³⁺ was achieved in CeO₂:Eu³⁺, Bi³⁺,²⁴ and CeO₂:Eu³⁺, M⁺ (M: Na, K, Li)²⁵ phosphors through change the symmetry of the host and charge compensation mechanism, respectively. However, achieving a high-efficiency energy transfer in orange-red-emitting phosphor CeO₂:Sm³⁺, Eu³⁺ synthesized by solution combustion and its energy transfer mechanism investigation have not been well documented.

In this work, a series of Sm³⁺ singly and Sm³⁺/Eu³⁺ co-doped CeO₂ phosphors was prepared using solution combustion method. The crystal structure and chemical element composition were explored using X-ray diffraction and energy-dispersive X-ray spectroscopy analyses. The luminescence properties and ET mechanisms were systematically investigated. The phosphors emitted orange to red emissions with an enhancement of Eu³⁺ doping because the efficiency of ET from Sm³⁺ to Eu³⁺ reached 84%. Furthermore, the ET mechanism between Sm³⁺ and Eu³⁺ was discussed in detail.

2. Experimental procedure

A series of CeO₂:xSm³⁺ (x = 0.02, 0.04, 0.06, 0.08, 0.10, and 0.12 mol%), CeO₂:0.04Eu, and CeO₂:0.04Sm³⁺, yEu³⁺ (y = 0, 0.02, 0.04, 0.06, 0.08, and 0.10 mol%) phosphors was prepared using solution combustion followed by heat treatment at 800 °C for 4 h in air. Ce(NO₃)₃·6H₂O (Sigma-Aldrich, 99.9%), Sm(NO₃)₃·6H₂O (Sigma-Aldrich, 99.99%), urea, and Eu₂O₃ (Sigma-Aldrich, 99.99%) were used as raw materials. These materials were exactly weighed and dissolved in deionized water and HNO₃ solution (Eu₂O₃) under magnet-stirred conditions to obtain aqueous solutions Ce³⁺ (0.5 M), Sm³⁺ (0.5 M), and Eu³⁺ (0.5 M), respectively. The stoichiometric amounts of the solutions Ce³⁺, Sm³⁺, and Eu³⁺ were added together to get 0.5 M aqueous solution and stirred for 30 min at room temperature. After that, the 20 mol% urea (compared with the Ce³⁺ content) was added to the system under magnetic stirring for 3 h at 80 °C. Next, the system was transferred to a corundum crucible (200 ml) for combustion reaction at 400 °C for 4 h. Then, the powder was heat-treated at 800 °C for 4 h in air, with a heating rate of 3 °C per minute. Finally, obtained phosphor powder was naturally cooled to room temperature and characterized. The crystal structure and chemical element composition of the phosphors were determined by X-ray diffraction (Bruker D8 Advance) and energy-dispersive X-ray spectroscopy (JEM 1010, JEOL Technique, Tokyo, Japan) analyses. The luminescence properties, lifetime, and ET mechanisms were investigated using a NANOLOG spectrophotometer (Horiba, USA). Decay curves of the investigated phosphors were also evaluated.

3. Results and discussion

3.1. XRD analysis

Fig. 1a presents the XRD patterns of CeO₂:0.04Sm³⁺ phosphors without and co-doped with Eu³⁺. All the diffraction peaks are consistent with the standard card (PDF # 01-075-8371), confirming the formation of single-phase cubic fluorite-type CeO₂ with space group Fm3m and no impurity phase is present. The crystal structure of CeO₂ includes Ce⁴⁺ sites with eight coordinates (including eight nearest-neighbor oxygen anions) and O²⁻ sites with four coordinates (surrounded by four nearest-neighbor cerium cations).²⁶ High-intensity diffraction peaks were observed corresponding to typical planes of cubic phase CeO₂ including (111), (200), (220), (311), (222), and (400). Significantly, the diffraction peaks shifted toward a lower 2θ angle (Fig. 1b) after being doped with a larger ionic radius Eu³⁺ (1.06 Å) since the ionic radius of Ce⁴⁺ is 0.97 Å, leading to a lattice expansion,¹⁸ confirming that Eu³⁺ ions have been incorporated in the Ce⁴⁺ cites of the CeO₂ host. Furthermore, the crystallite size of all investigated phosphors was determined using the Scherrer equation:²⁷where D is crystallite size (nm), 0.9 is Scherrer constant, λ is the wavelength of the X-ray sources (0.15406 nm), β (radians) and θ (radians) are full width at half maximum (FWHM) and peak position, respectively. The diffraction peak corresponding to the (111) plane of CeO₂ was chosen to calculate crystallite size, as shown in Table 1. Notably, the crystallite size slightly increases with increasing Eu³⁺ doping, suggesting a successful introduction of Eu³⁺ in the systems. These results confirm that the presence of Eu³⁺ in the systems leads to enhanced crystallinity and crystallite size, which could improve the emission efficiency of the phosphors.

Fig. 1 (a) XRD patterns of the investigated samples CeO₂:0.04Sm³⁺, yEu³⁺ (y = 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10), and (b) XRD patterns at 2θ angle of 27.5° to 29.5° of all samples.

Table 1 The crystallite size of samples with different Eu³⁺ doping content

CeO2:0.04Sm^3+, yEu^3+	FWHM	2θ°	Cos(θ)	Crystal size (nm)
y = 0.00	0.005655	28.60	0.969016	25.3
y = 0.02	0.005498	28.58	0.969059	26.1
y = 0.04	0.005236	28.54	0.969145	27.3
y = 0.06	0.004992	28.52	0.969188	28.6
y = 0.08	0.004817	28.52	0.969209	29.7
y = 0.10	0.004643	28.50	0.969231	30.8

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3.2. Morphology and chemical element composition analysis

Fig. 2a and b show the SEM images of CeO₂:0.04Sm³⁺ and CeO₂:0.04Sm³⁺, 0.04Eu³⁺, respectively. The un-doped sample (CeO₂:0.04Sm³⁺) shows the agglomeration of spherical-like nanoparticles with an average size of approximately 40.5 nm (inset of Fig. 2a). Meanwhile, the doped sample (CeO₂:0.04Sm³⁺, 0.04Eu³⁺) shows uniform grains, with boundary distribution and an average particle size of about 46.5 nm (inset of Fig. 2b). This finding suggests the high crystallinity of the obtained phosphors. However, the morphology of the samples did not significantly change, indicating that it is not a main parameter that can influence luminescent properties. Chemical element composition was analyzed using EDS to confirm the presence of ions doped into the CeO₂ host. Fig. 2c shows the presence of Ce, O, and Sm, which are attributed to the host lattice and doping ion. Meanwhile, Fig. 2d indicates the presence of Ce, O, Eu, and Sm with percent composition shown in the inset of Fig. 2d. Impurity elements were not observed, thereby confirming the high purity of the obtained phosphors.

Fig. 2 SEM image of CeO₂:0.04Sm³⁺ (a) and CeO₂:0.04Sm³⁺, 0.04Eu³⁺ (b); and EDS spectra of CeO₂:0.04Sm³⁺ (c) and CeO₂:0.04Sm³⁺, 0.04Eu³⁺ (d).

3.3. Luminescence properties

3.3.1. Luminescence properties of CeO₂:Sm and CeO₂:Sm, Eu phosphors. Fig. 3a shows the photoluminescence excitation (PLE) spectra of CeO₂:0.02Sm³⁺ with monitoring emission wavelengths of 573 and 615 nm. The absorption band from 250 nm to 300 nm is attributed to the charge transfer band from O²⁻ to Ce⁴⁺. The highest band absorption with a peak at 360 nm corresponds to the overlap between the ⁶H_5/2–⁴D_3/2 transition of Sm³⁺ and the charge transfer band O²⁻–Ce⁴⁺/Sm³⁺. The weak peak absorption at 405 nm corresponds to the ⁶H_5/2–⁴F_7/2 transition of Sm³⁺. The band absorption with 573 nm emission is higher than that with 615 nm, indicating the dominant orange emission of the phosphors. Meanwhile, Fig. 3b presents the emission spectra of all investigated samples of CeO₂:xSm³⁺ (x = 0.02, 0.04, 0.06, 0.08, 0.10, and 0.12 mol%) monitoring at excited wavelength of 360 nm. Under 360 nm excitation, the obtained phosphor showed characteristic emission of Sm³⁺, such as at 573 (⁴G_5/2–⁶H_5/2), 631 (⁴G_5/2–⁶H_6/2), and 660 nm (⁴G_5/2–⁶H_9/2). The emission intensity of the samples reached the maximum value of 0.04 mol% Sm³⁺ doping and then decreased with increasing Sm³⁺ content due to the concentration quenching effect. The sample CeO₂:0.04 Sm³⁺ showed the highest emission intensity and was selected for synthesis of CeO₂:0.04Sm³⁺, yEu³⁺ phosphor.

Fig. 3 (a) PLE spectra of CeO₂:0.04Sm³⁺ and (b) PL spectra of CeO₂:xSm³⁺ (x = 0.02, 0.04, 0.06, 0.08, 0.10, and 0.12 mol%).

Fig. 4a presents the PLE spectra of samples CeO₂:0.04Eu³⁺ and CeO₂:0.04Sm³⁺, 0.04Eu³⁺ with monitoring emission wavelength at 631 nm. In the typical two samples, the absorption bands from 250 nm to 300 are attributed to the charge transfer band from O²⁻ to Ce⁴⁺, similar to the PLE spectra of CeO₂:0.04Sm³⁺. The highest absorption band around at 360 nm corresponds to the overlap between the ⁶H_5/2–⁴D_3/2 transition of Sm³⁺²⁸ and the charge transfer band (O²⁻ to Ce⁴⁺/Sm³⁺/Eu³⁺) of the host lattice (sample CeO₂:0.04Sm³⁺, 0.04Eu³⁺). Fig. 4a shows typical absorption peaks at 405 nm corresponding to the ⁶H_5/2–⁴F_7/2 transition of Sm³⁺, while the absorption peak at 395 nm (⁷F₀–⁵L₆), 466 nm (⁷F₀–⁵D₂), 526/532 (⁷F₀–⁵D₁/⁷F₁–⁵D₁) nm and 580/587 nm (⁷F₀–⁵D₀/⁷F₁–⁵D₀) is attributed to the Eu³⁺. These results confirm the presence of dopant ions in the host lattice and the successful synthesis of the phosphors. In this case the absorption intensity from 250–400 nm of CeO₂:0.04Sm³⁺, 0.04Eu³⁺ was much higher than the CeO₂:0.04Eu³⁺. Fig. 4b shows the difference in emission spectra between three phosphors (CeO₂:0.04Sm³⁺, CeO₂:0.04Eu³⁺ and CeO₂:0.04Sm³⁺, 0.04Eu³⁺) under 360 nm excitation. Notably, the CeO₂:0.04Sm³⁺ phosphor emits a dominant peak of yellow light (573 nm), and the CeO₂:0.04Eu³⁺ phosphor emits an orange light (590 nm); however, the 0.04Sm³⁺/0.04Eu³⁺ co-doped CeO₂ exhibited a dominant peak red emission (631 nm). The photoluminescence spectra of all investigated samples are shown in Fig. 4c. Under 360 nm excitation wavelengths, the CeO₂:0.04Sm³⁺, yEu³⁺ (y = 0, 0.02, 0.04, 0.06, 0.08, and 0.10) phosphors emitted strong orange-red band at 590 nm (⁵D₀–⁷F₁) and 610–631 nm (⁵D₀–⁷F₂) of Eu³⁺. Notably, the emission intensities at 573 nm corresponding to the ⁴G_5/2–⁶H_5/2 transition of Sm³⁺ decreased with increasing Eu³⁺ doping content. By contrast, the emission intensities at 631 nm depended on Eu³⁺ content and reached the maximum value at 0.04 mol% Eu³⁺ doping (Fig. 4d). Hence, the red-emitting phosphors (CeO₂:0.04Sm³⁺, 0.04Eu³⁺) could be excited by 360 nm commercial LED chip, making it suitable for solid lighting applications. Furthermore, the red/orange (R/O) emission intensity ratios of the CeO₂:0.04Sm³⁺, yEu³⁺ (y = 0, 0.02, 0.04, 0.06, 0.08, and 0.10) phosphors can estimate the asymmetry around the Eu³⁺ sites in the host lattice,²⁹ which was calculated as shown in Fig. 4e. The R/O ratio increases with the enhancement of Eu³⁺ content, confirming the asymmetry of the crystal field increased. These results supported increasing the electric-dipole transition probability of the Eu³⁺, resulting in dominant red emissions. In addition, the Commission Internationale de I'Eclairage (CIE) chromaticity coordinates of resulted phosphors (Fig. 4f) were calculated from their emission spectra, as shown in Table 2. CeO₂:0.04Sm³⁺, 0.04Eu³⁺ shows CIE coordinates (0.622, 0.375) close to pure red.¹⁴ Furthermore, these CIE coordinates were used for calculating the color purity of the investigated phosphors, which can be described as eqn (2):³⁰

where (x, y) and (x_i = 0.3333, y_i = 0.3333) are the color coordinates of the investigated samples and the white light source, respectively; (x_d, y_d) corresponds to the color coordinates of the dominant wavelength.³¹ These values (x, y), (x_i, y_i), and (x_d, y_d) were plugged into eqn (2), and the color purity of the resulting phosphors is displayed in Table 2. The color purity of the samples increased with enhancing Eu³⁺ doping content and reached a maximum value of 89.74% (Table 2). The findings confirm the high efficiency ET from Sm³⁺ to Eu³⁺ in the systems developed. Furthermore, the correlation of phosphors with color temperature (CCT) was calculated to explore the nature of the emitted light by using McCammys' equation:³²

CCT = 5520.33 − 6823n + 3525n² − 449n³ (3)

where (x, y) is the CIE coordinates of all samples; (x_e = 0.338, y_e = 0.186) is the chromaticity epicenter; n = (x − x_e)/(y − y_e). The resulting CCT values are shown in Table 2. The values increased from 1790.8 K to 1973 K with increasing Eu³⁺ doping content.

Fig. 4 (a) PLE spectra of samples CeO₂:0.04Eu³⁺ and CeO₂:0.04Sm³⁺, 0.04Eu³⁺; (b) emission spectra of CeO₂:Sm³⁺, CeO₂:Eu³⁺, and CeO₂:Sm³⁺, Eu³⁺ phosphors; (c) PL emission spectra of samples CeO₂:0.04Sm³⁺, yEu³⁺ (y = 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10); (d) dependence of the ⁴G_5/2–⁶H_5/2 (Sm³⁺) and ⁵D₀–⁷F₁/⁷F₂ (Eu³⁺) transitions on the Eu³⁺ doping content; (e) red/orange (R/O) emission intensity ratios of investigated samples as Eu³⁺ content increased; (f) CIE coordinate chromaticity of CeO₂:0.04Sm³⁺, yEu³⁺ phosphors (y = 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10).

Table 2 CIE chromaticity coordinates, color purity, and CCT values of all investigated samples

Samples	Chromaticity coordinates (x, y)	Color purity (%)	CCT (K)
CeO2:0.04Sm	(0.565, 0.425)	77.60	1847
CeO2:0.04Sm, 0.02Eu	(0.614, 0.383)	86.93	1790.8
CeO2:0.04Sm, 0.04Eu	(0.622, 0.375)	88.95	1885.8
CeO2:0.04Sm, 0.06Eu	(0.624, 0.371)	89.36	1936.2
CeO2:0.04Sm, 0.08Eu	(0.626, 0.370)	89.74	1974.8
CeO2:0.04Sm, 0.10Eu	(0.621, 0.366)	88.10	1973.1

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3.3.2. Decay curves and ET efficiency of all obtained phosphors. The decay curve of Sm³⁺ in CeO₂:0.04Sm³⁺, yEu³⁺ phosphor (y = 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10) can be used to study the emission mechanism and confirm the efficiency of ET from Sm³⁺ to Eu³⁺. The luminescence decay curves associated with the transition ⁴G_5/2–⁶H_5/2 of Sm³⁺ of all samples were monitored at the emission wavelength of 573 nm and the excitation wavelength of 360 nm (Fig. 5a). The radiative lifetime (τ) of the samples well fitted by bi-exponential function:³³

y = y₀ + A₁e^−t/τ₁ + A₂e^−t/τ₂ (4)

where y is the emission intensity at time t; y₀ is the initial emission intensity; A₁ and A₂ are constants; τ₁ and τ₂ are decay times. τ₁ is the decay component attributed to Sm³⁺ on the surface or near defect sites. By contrast, τ₂ is the decay component associated with Sm³⁺ sites substituted for Ce⁴⁺ ions on the CeO₂ host lattice.³³ The average lifetimes (τ) can be described as eqn (5):³⁴

Fig. 5 (a) Decay curve of samples monitored at the excitation wavelength of 360 nm and the emission wavelength of 573 nm. (b) Lifetime of all sample depended on Eu³⁺ content (blue line) and ET efficiency from Sm³⁺ to Eu³⁺ (red line).

The τ values are 1.701, 0.788, 0.653, 0.512, 0.319, and 0.303 ms, corresponding to CeO₂:0.04Sm³⁺, yEu³⁺ phosphors (y = 0, 0.02, 0.04, 0.06, 0.08, and 0.10 mol%, respectively), as shown in Table 3. The average lifetime decreased with increasing Eu³⁺ doping content, thereby confirming the ET from Sm³⁺ to Eu³⁺.^12,35 Furthermore, ET efficiency was estimated using eqn (6):³⁶

where τ_S and τ_S0 are the lifetimes of Sm³⁺ with and without Eu³⁺, respectively. The calculated ET efficiency values are shown in Fig. 5b. The ET efficiency from Sm³⁺ to Eu³⁺ increased with increasing doping content of Eu³⁺ and reached the maximum value of 84% with 0.10 mol% Eu³⁺. The ET efficiency in this work is significantly higher than in other previous works (Table 4). These results indicated that the phosphor conferred it as a suitable red-emitting phosphor for WLEDs.

Table 3 Fitting decay curves for CeO₂:0.04Sm³⁺, xEu³⁺ (x = 0, 0.02, 0.04, 0.06, 0.08 and 0.10 mol%)

CeO2:0.04Sm, yEu	A1	τ1	A2	τ2	τaverage (ms)
y = 0.00	82.25	1.11	227	0.23	1.701
y = 0.02	319.7	1.12	926	0.23	0.788
y = 0.04	932	0.21	260	0.99	0.653
y = 0.06	131	0.7	743	0.17	0.512
y = 0.08	166.9	0.85	746	0.2	0.319
y = 0.10	653.6	0.15	76.76	0.62	0.309

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Table 4 Comparison of ET efficiency from Sm³⁺ to Eu³⁺ on the proposed phosphor and other existing phosphors

Phosphors	Content of Eu^3+ (mol%)	ET efficiency (%)	References
CeO2:Sm^3+, Eu^3+	0.04	84.00	This work
La2MgTiO6:Sm^3+, Eu^3+	0.20	73.50	35
YPO4:Sm^3+, Eu^3+	0.08	65.00	16
Ca2GdNbO6:Sm^3+, Eu^3+	0.30	28.60	12
Sr7Sb2O12:Sm^3+, Eu^3+	0.09	42.40	7
Sr3In2WO9:Sm^3+, Eu^3+	0.02	62.69	18
Sr3Y(BO3)3:Sm^3+, Eu^3+	0.07	55.69	33

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3.3.3. ET mechanism. The exchange interaction or multipolar interaction is responsible for the main ET mechanism in the CeO₂:Sm³⁺, Eu³⁺, which is estimated by the average critical distance (R_C) between Sm³⁺ and Eu³⁺. Notably, the multipolar interaction is attributed to the R_C value higher 5 Å and when the R_C is about 5 Å corresponding to exchange interaction.³⁷ The R_C value can be determined by eqn (7):³⁷here, the cell volume V is 430.32 ų; the critical doping content χ_C is 0.08; N is the available host cations in the cell.³⁸ Putting all these values in the eqn (7), the R_C value in CeO₂:0.04Sm³⁺, 0.04Eu³⁺ was observed to be ∼13.7 Å, thus, the ET mechanism from Sm³⁺ to Eu³⁺ occurs by multipolar interaction. In this case, the Dexter theory was explored to evaluate the multipolar interaction between Sm³⁺ and Eu³⁺.³⁹ Therefore, the rate of ET from Sm³⁺ to Eu³⁺ against the concentration of Eu³⁺ can be estimated as follows:³⁹where C is the total doping ion content (Sm³⁺ and Eu³⁺). I_S0 and I_S are the emission intensity of Sm³⁺ without and with Eu³⁺ doping. The n is an electric multipolar character equal to 6, 8, and 10, corresponding to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions.¹¹ Fig. 6 shows the relationship between C^n/3 and I_S0/I_S, including the values of the fitting factor R². Previous works^18,33,35,40 indicate that the R² highest value (close to 1) determines the ET mechanism in the system. Therefore, the R² highest value in this work is 0.986, indicating the ET mechanism in CeO₂:Sm³⁺, Eu³⁺ corresponding to quadrupole–quadrupole interaction.

Fig. 6 Dependence of the I_S0/I_S intensity ratio of Sm³⁺ on C^6/3, C^8/3, and C^10/3 in CeO₂:0.04Sm³⁺, yEu³⁺ phosphors (y = 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10).

3.3.4. Energy level diagram of the phosphors. Furthermore, the ET process from Sm³⁺ to Eu³⁺ can be explained using an energy level diagram (Fig. 7a). Under 360 nm wavelength excitation, the electron at the ⁶H_5/2 state of Sm³⁺ absorption photons move to the ⁴D_3/2 level. In addition, the population of the ⁴D_3/2 level is enhanced by absorption due to the charge transfer from O²⁻ to Ce⁴⁺ of the host lattice. The electrons in the ⁴D_3/2 level relax to the ⁴G_5/2 state through non-radiation transition. Finally, the electrons at the ⁴G_5/2 of Sm³⁺ transfer energy to the ⁵D₀ state of Eu³⁺ and then go back to the ground state ⁷F_j (j = 0, 1, 2, and 4), thereby producing intense red emission band. The ET was confirmed by the spectral overlap between the excitation of Eu³⁺ and the emission of Sm³⁺, as shown in Fig. 7b.

Fig. 7 (a) Schematic energy-level diagram of CeO₂:0.04Sm³⁺, yEu³⁺ phosphors with excitation, emission, and energy transfer; (b) the overlap between the excitation spectra of sample CeO₂:0.04Eu³⁺ and the emission spectra of sample CeO₂:0.04Sm³⁺.

4. Conclusion

CeO₂:Sm³⁺ and CeO₂:Sm³⁺, Eu³⁺ phosphors were successfully synthesized using solution combustion. The XRD analysis confirmed the formation of single-phase cubic CeO₂ with space group Fm3m. Under 360 nm excitation, CeO₂:Sm³⁺ phosphors emitted strong yellow-red band at 573 and 615 nm, corresponding to the ⁴G_5/2–⁶H_j (j = 5/2, and 6/2) transitions of Sm³⁺. Meanwhile, CeO₂:Sm³⁺, Eu³⁺ showed the emission characteristics of Sm³⁺ (573 nm) and Eu³⁺ at 590/610–631 nm. The photoluminescence spectra showed the ET from Sm³⁺ to Eu³⁺ because of the small energy difference between the ⁴G_5/2 level of Sm³⁺ and the ⁵D₀ level of Eu³⁺ ions. Significantly, the decay kinetics of the ⁴G_5/2–⁶H_5/2 transition (Sm³⁺) in CeO₂:Sm³⁺, Eu³⁺ confirmed the high-efficiency ET from Sm³⁺ to Eu³⁺ (up to 84%). The quadrupole–quadrupole interaction between Sm³⁺ and Eu³⁺ was responsible for the ET mechanism. These results indicate that the high-efficiency ET from Sm³⁺ to Eu³⁺ in CeO₂:Sm³⁺, Eu³⁺ phosphors are an excellent strategy to improve the emission efficiency of Eu³⁺.

Author contributions

Nguyen Van Hai: data curation, format analysis, resources, supervision, writing – original draft. Bui Thi Hoan: investigation, data curation. Nguyen Minh Tu: conceptualization, format analysis. Dinh Thi Hien: methodology, resources. Nguyen Thi Khanh Linh: format analysis, experiment, methodology, data curation. Duy-Hung Nguyen: methodology, data curation. Vu Tuan Anh: format analysis, experiment. Vuong-Hung Pham: format analysis, conceptualization. Hoang Nhu Van: investigation, format analysis, experiment, conceptualization, methodology, writing – original draft, writing – review & editing.

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

This work is funded by the Ministry of Education and Training of Vietnam, under the project B2023-SPH-06.

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