Energy transfer and multicolour photoluminescence in phosphors La₂O₃ doped by Tm³⁺, Ho³⁺, and Eu³⁺ for white LED and security applications

Abstract

As the world evolves, the demand for efficient lighting sources is rising rapidly, and to cater to the burgeoning demand for swift expansion, technological advancement in white light-emitting diodes (WLEDs) and lighting materials is crucial. In this work, the photoluminescence of La₂O₃ single-doped (Tm, Ho, Eu), double-doped and triple-doped phosphors prepared by combustion synthesis was studied. XRD confirms hexagonal phase (-P6c2c) with lattice contraction on doping, supported by Rietveld refinement, while SEM shows agglomerated particles (~ 12 µm) and FTIR confirms La–O bonding. Under UV excitation, Tm³⁺, Ho³⁺ , and Eu³⁺ give blue, green, and orange-red emission, respectively; in Ho³⁺ doped samples, the small ⁵F₄–⁵S₂ gap (~ 200 cm⁻¹) compared to host phonon energy (~ 400 cm⁻¹) leads to merged emission. Efficient energy transfer (Tm → Ho, Ho → Eu, Tm → Eu) is observed in co-doped and tri-doped systems, with La₂O₃:Tm/Ho/Eu phosphor showing a strong red component. These results suggest La₂O₃—based phosphors are promising candidates for WLEDs, barcode security, and multicolour LED applications.

Introduction

In the era of rapid paradigm shifts, the demand for efficient lighting sources is increasing exponentially. To accommodate the demographic imperative of growing population, technological advancements in white light-emitting diode (WLEDs) and lighting technologies are needed. LED applications demand different parameters: CCT tuning provides warm (low CCT) or cold (high CCT) white light for ambience and efficiency; color purity ensures vivid, accurate display and lighting performance; and balancing these enables LEDs to meet needs ranging from cozy indoor lighting to bright outdoor and high-precision display applications^1,2. Luminescent phosphors have widespread applications in lighting devices such as fingerprint detection³, barcode security ⁴, biomedical applications⁵, anti-counterfeiting⁶, radiation dosimetry⁷, solar energy conversion⁸, displays⁷, scintillators⁹, Leaser ¹⁰and phototherapy¹¹. Red deficiency in WLEDs phosphor in commercial phosphor can one of the problem in modern lighting devices ^12,13.

Rare earth elements are considered modern industrial vitamins because they are applicable in the development of new technologies. Rare earth elements are used in fields of catalysis, chemical industry, metallurgy, LED technology and green energy. Rare earth elements have a vacant 4f. shell that contributes to their excellent optical, electrical, ceramic, magnetic ,thermal and chemical properties^14,15. Among all rare earth elements, La₂O₃ is financial efficient and easily available rare earth element. La₂O₃ is p-type semiconductor with a band gap of 4.3 eV, and it has strongest basicity. La₂O₃ possesses a relatively low phonon energy of about 400 cm⁻¹, which facilitates the merging of two closely spaced energy levels with a gap smaller than 400 cm⁻^116–18. This results in a single emission peak with a larger full width at half maximum (FWHM), thereby reducing non-radiative losses in the host lattice. This makes it a highly exemplary prospect for rare-earth doping in the preparation of efficient phosphor materials. La₂O₃ in the hexagonal phase has a single site to replace the dopant in host lattice which make it more prominent candidate for sharp emission bands^19–22. Refractive index is an essential parameter for studying the optical properties of the prepared sample. It provides valuable insight into how light is absorbed, scattered, and confined within photoactive materials. In the present work, La₂O₃(Lanthanum oxide) acts as the host matrix, and its high refractive index contributes to enhanced light–matter interaction, improved optical transparency, and efficient energy transfer within the doped system ²³.

The emission spectra of prepared phosphor depend on the host lattice and the dopant. The choice of dopant depends on the application or the objective of synthesis. Dopants like europium (Eu) emit blue and red light in + 2 and + 3 oxidation states, respectively ^24,25. Lanthanide dopants give sharp emission peaks , while transition metals like Bi and Mn²⁺ give blue emission ²⁶. The incorporation of dopant ions into the host lattice also depends strongly on the ionic radii of both the dopant and host atoms. A closer match in their ionic radii facilitates effective substitution and minimizes lattice distortion, thereby improving the structural stability and optical performance of the La₂O₃-based material ²⁷.The emission of phosphor also depends on the lattice (host). C.X. Chen et al. studied the Bi/Tb/Sm doped CaY₂Al₄SiO₁₂ phosphor prepared by the high-temperature solid state reaction method. The prepared phosphor was used for WLEDs applications with high colour rendering index 87 and a low correlated color temperature (CCT, 4747.56 K) ²⁸. Govind B. Nair et al. prepared up conversion Ceramic phosphors Tm_2-xWO₆:_xYb³⁺ using solid state method. They studied the emission of Tm doped phosphor at excitation wavelength 980 nm ²⁹.

Combustion synthesis method is rapid , safe, energy efficient, taking less time ^12,30,suitable for large-scale production, and cost effective method as compare to other method. Gases releases during the synthesis helps make prepare material porous. Porous nature help to enhance the optical property ^31,32.

In this study , we synthesized the La₂O₃ doped with Tm, Ho, and Eu phosphor by combustion synthesis using hydrazine as fuel. We carried out structural characterization using XRD(x-ray diffraction), morphological study using SEM (scanning electron microscopy), EDX (Energy-Dispersive X-ray ) for elemental study, optical study using (photoluminescence), and bond vibration is studied using FT-IR (Fourier-Transform Infrared spectroscopy). This study mainly focuses on development on green, red, blue and WLEDs phosphors.

Synthesis

We synthesized lanthanum dioxide (La₂O₃) using a combustion method, with adding various rare earth dopants. The synthesis process involved both individual and co-doping strategies, culminating in triple-doping trials to evaluate the influence of multiple dopants on the material properties. Initially, lanthanum nitrate [La(NO₃)₂] was dissolved in distilled water and stirred using a magnetic stirrer to obtain a homogeneous precursor solution at 300 rpm. In parallel, the selected dopants (Tm, Eu, and Ho) are each dissolved in nitric acid within separate test tubes and subjected to mild heating in a hot air oven to ensure complete dissolution. Once the dopants were fully dissolved, they were gradually introduced into the lanthanum nitrate solution under constant stirring to achieve uniform mixing. Following this, hydrazine hydrate (5 mL) was added as a fuel to initiate the combustion reaction. The resulting solution was transferred to a china dish and placed in a preheated vertical furnace at 550 °C. This intermediate product was then manually ground using a mortar and pestle to ensure fine and consistent particle size. The ground material was subsequently calcined in a muffle furnace at 800 °C. The synthesized La₂O₃ samples—undoped, singly doped, co-doped, and triply doped—were then subjected to further characterization and testing to evaluate their structural, morphological, and optical properties. The schematic representation of the synthesis is shown in Fig. 1.

Fig. 1.

Schematic representation of combustion synthesis.

Results and Discussion:

XRD analysis

The XRD pattern of the prepared sample was recorded using an instrument Rigaku Miniflex D 600. XRD machine is enabled with Cu Kα radiation (λ = 0.154056 nm), operating at 40 kV and 15 mA. The XRD patterns were recorded from 10 ⁰ to 80 ⁰ at 0.04 ⁰ step size. Figure 2 shows the XRD pattern of La₂O₃, La₂O₃:Tm/Ho, La₂O₃:Tm/Ho,La₂O₃:HO/Eu, and La₂O₃:Tm/Ho/Eu . The concentration of dopant La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(0.5 mol%), La₂O₃: Ho³⁺(1.5 mol%)/Eu³⁺(2.0 mol%), La₂O₃:Tm³⁺(1.5 mol%)/Eu³⁺(1.0 mol%), and La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(1.0 mol%)/Eu³⁺(4.0 mol%) were selected so that they have maximim energy transfer eiffecncy at that concentration. All XRD spectra of prepared phosphor were well matched with the standard ICDD data file no 01–074-2430. Impurity such as Eu₂O₃ , Ho₂O₃, and Tm₂O₃ used in the synthesis does not induce any impurity peak or add other phase. This confirms that the added impurities were completely incorporated into the host lattice. The XRD peaks of all prepared samples are sharp, demonstrating the high crystalline nature of the prepared phosphors. The prepared phosphors crystallized in a hexagonal structure with the space group -P 6c 2c. The addition of dopants into the host lattice did not change the phase and space group of the co-doped phosphor.

Fig. 2.

XRD pattern of La₂O₃, La₂O₃:Tm/Ho, La₂O₃:Tm/Ho, La₂O₃:Ho/Eu, and La₂O₃:Tm/Ho/Eu with the standard data.

Figure 3 shows the shifting of the XRD pattern of La₂O₃,La₂O₃:Tm/Ho, La₂O₃:Tm/Ho, La₂O₃:HO/Eu, and La₂O₃:Tm/Ho/Eu . The shifting of the XRD patterns toward higher angles is due to the addition of impurities like Eu (0.947 Å), Ho (0.901 Å), and Tm (0.880 Å) which have smaller ionic radii compared to La (1.032 Å). The shift occurs due to the lattice contraction in host lattice due to the addition of the impurities in the host lattice having the ionic radii smaller than that of the host ^33,34. Order of ionic radii of ions are La > Eu > Ho > Tm. In the shifting graph, La₂O₃:Tm/Eu and La₂O₃:Tm/Ho has maximum shift from La₂O₃ due to the larger mismatch in ionic radii. The introduction of dopants into the lattice tends to reduce the crystallinity of prepared samples. The FWHM of the doped samples is larger than that of pure La₂O₃, which indicates an increase in lattice strain or defects in the crystal structure. The crystalline size of pure La₂O₃ is larger than that of the co-doped phosphor. La₂O₃:Ho/Eu has the maximum crystalline size while La₂O₃:Tm/Ho/Eu has the minimum crystalline size in co-doped phosphor. The detailed crystalline size is shown in table: Crystalline Size of La₂O₃ has maximum size of 80.47968115 nm and minimum at La₂O₃:Tm/Ho/Eu with 55.33576 nm. Adding dopant, in crystal structure leads to decreases in crystalline Size from 80.47968115 nm to 55.33576 nm as shown in Table 1.

Fig. 3.

XRD shift in pattern of La₂O₃, La₂O₃:Tm/Ho, La₂O₃:Tm/Ho, La₂O₃:Ho/Eu, and La₂O₃:Tm/Ho/Eu.

Table 1.

Crystalline Size of La₂O₃, La₂O₃:Tm/Ho, La₂O₃:Tm/Ho,La₂O₃:Ho/Eu, and La₂O₃:Tm/Ho/Eu.

Crystalline Size
La2O3	La2O3:Tm/Ho	La2O3:Tm/Eu	La2O3:Ho/Eu	La2O3:Tm/Ho/Eu
80.47968115 nm	61.41207426 nm	58.25336 nm	67.455 nm	55.33576 nm

The effect of doping on the crystal structure of the prepared sample was studied by using Rietveld refinement. The Rietveld refinement of La₂O₃, La₂O₃:Ho/Eu, La₂O₃:Tm/Ho, La₂O₃:Tm/Ho, and La₂O₃:Tm/Ho/Eu are shown in Fig. 4a–e respectively. The Rietveld refinement was carried out using full proof software by assuming a peak shape Pseudo-Voigt profile function and linear background. The parameters of rietveld refinement such as Rp, R_wp, R_exp , and χ² are mentioned in Table 2. Doping led to changes in lattice parameters, and the refined parameters compared with the standard data are shown in Table 3. The effect of doping on the position of atom in crystal structure is shown in Table 4. The addition of dopants is in such small quantities that does not reflect in crystal structure. The crystal structure La₂O₃, La₂O₃:Ho/Eu, La₂O₃:Tm/Ho, La₂O₃:Tm/Ho, and La₂O₃:Tm/Ho/Eu is shown in Fig. 5a–e, respectively.

Fig. 4.

Rietveld refinement of La₂O₃, La₂O₃:Tm/Ho, La₂O₃:Tm/Ho La₂O₃:Ho/Eu, and La₂O₃:Tm/Ho/Eu.

Table 2.

Rp, R_wp, R_exp , and χ² parameter of reitveld refinement.

Sr no	Name	Rp	Rwp	Rexp	χ2
1	La2O3	17.5	16.6	9.93	2.79
2	La2O3:Tm/Ho	20.1	20.7	9.72	4.54
3	La2O3:Ho/Eu	18.8	17.5	10.17	2.94
4	La2O3:Tm/Eu	18.4	17.4	10.14	2.93
5	La2O3:Tm/Ho/eu	19.0	17.7	10.31	2.94

Table 3.

Lattice parameter of La₂O₃, La₂O₃:Tm/Ho, La₂O₃:Tm/Ho ,La₂O₃:Ho/Eu, and La₂O₃:Tm/Ho/Eu with standard data.

Sr no	Name	La2O3	La2O3:Tm/Ho	La2O3:Ho/Eu	La2O3:Tm/Eu	La2O3:Tm/Ho/eu	Standard
1	a (Å)	3.937225	3.922530	3.935604	3.925273	3.916989	3.9373
2	c (Å)	6.130284	6.110597	6.131602	6.115010	6.103198	6.1299
3	α (In degree)	90.000 o	90.000 o	90.000 o	90.000 o	90.000 o	90.000 o
4	γ (In degree)	120 o	120 o	120 o	120 o	120 o	120 o
5	Volume (Å3)	82.2985	81.4229	82.2484	81.5958	82.2964	82.30

Table 4.

Atom position of La₂O₃, La₂O₃:Tm/Ho, La₂O₃:Tm/Eu, La₂O₃:Ho/Eu, and La₂O₃:Tm/Ho/Eu.

Sr no	Name	Element	X	Y	Z
1	La2O3	La1	0.33330	0.66670	0.24521
		O1	0.00000	0.00000	0.00000
		O2	0.33330	0.66670	0.66609
2	La2O3:Tm/Ho	La1	0.33330	0.66670	0.23400
		O1	0.00000	0.00000	0.00000
		O2	0.33330	0.66670	0.70283
		Tm1	0.33330	0.66670	0.23400
		Ho1	0.33330	0.66670	0.23400
3	La2O3:Tm/Eu	La1	0.33330	0.66670	0.24016
		O1	0.00000	0.00000	0.00000
		O2	0.33330	0.66670	0.65989
		Tm1	0.33330	0.66670	0.24016
		Eu1	0.33330	0.66670	0.24016
4	La2O3:Ho/Eu	La1	0.33330	0.66670	0.24557
		O1	0.00000	0.00000	0.00000
		O2	0.33330	0.66670	0.66297
		Ho1	0.33330	0.66670	0.24557
		Eu1	0.33330	0.66670	0.24557
5	La2O3:Tm/Ho/Eu	La1	0.33330	0.66670	0.24971
		O1	0.00000	0.00000	0.00000
		O2	0.33330	0.66670	0.66439
		Tm1	0.33330	0.66670	0.24971
		Ho1	0.33330	0.66670	0.24971
		Eu1	0.33330	0.66670	0.24971

Fig. 5.

Crystal structure of La₂O₃, La₂O₃:Tm/Ho, La₂O₃:Tm/Ho , La₂O₃:Ho/Eu, and La₂O₃:Tm/Ho/Eu.

The addition of dopants does not alter the hexagonal phase of the crystal structure. All the prepared samples show no alteration in α and γ because nearly the same radii ion that is replaced as a dopant. The variation in parameters a and c shows a reducing trend compared to the pure phosphor. This reduction occurs because smaller-radius dopants shrink the lattice structure. La₂O₃:Tm/Ho/Eu shows the maximum variation in lattice parameter due to maximum shrinking in crystal structure compared to other prepared phosphors. Position of atoms in crystal after Rietveld refinement is shown in Table 4. In the hexagonal phase of La₂O₃, there is only one crystallographic site for La³⁺ ions in the lattice. Therefore, dopants like Tm³⁺, Ho³⁺, and Eu³⁺ has only one cryptographically identical site present in the lattice. The position of atoms also indicates that the ions like Tm³⁺, Ho³⁺, and Eu³⁺ replace the position of La³⁺ in the lattice. Oxygen is placed in two different cryptographically identical positions and is present in the crystal i.e. O1 and O2. The site of O1 does not change by adding dopant, while the z coordinate of O2 changes as we add dopant. In the first type of O²⁻ coordination site, each oxygen ion is bonded to four chemically equivalent La³⁺ ions, forming OLa₄ tetrahedra. These tetrahedra are connected by sharing corners with six OLa₆ octahedra, sharing corners with another six OLa₄ tetrahedra, and sharing edges with three OLa₆ octahedra as well as three OLa₄ tetrahedra. In the second O²⁻ site, the oxygen is coordinated to six La³⁺ ions, resulting in OLa₆ octahedra. These octahedra connect through edge-sharing with six similar OLa₆ octahedra, sharing corners with twelve OLa₄ tetrahedra, and also sharing edges with six OLa₄ tetrahedra. Figure 4a shows the crystal structure of La₂O₃.

SEM and EDX analysis

Figures 6a–c show the SEM images of La₂O₃ at 10, 10, and 2 micrometer magnification, respectively. The particle seems to be agglomerate due to uncontrolled heating during the calcination process. The particles are porous in nature, due to the release of gases during calcination and combustion. The gases released during synthesis are O₂ , NH_3, and water vapor. The particles exhibit irregular shapes and sizes. Figure 6d shows the particle size distribution of the prepared La₂O_3.The average particle size of prepared phosphor is 12 micrometer. Larger particle size is due to agglomeration of particles. The porous nature of the particles is useful in lighting applications. Figure 6e shows the EDX spectra of La₂O₃. EDX spectra conation peaks due to the La (lanthanum) and O (Oxygen) that confirm the formation of La₂O_3.

Fig. 6.

SEM image, particle size distribution curve, and EDX spectra of La₂O_3.

PL analysis

La₂O₃:Eu

Figure 7 shows the excitation spectra of La₂O₃: Eu³⁺ at different concentration of Eu ion varying from 1 mol% to 5 mol %. The excitation spectra were recorded from 220 to 480 nm at emission wavelength fixed at 627 nm. The excitation spectra contain one major peak at 291 nm and several minor peaks at 363 nm, 383 nm, 395 nm, and 467 nm. The broad peak from 220 to 333 nm is centered at 291 nm is due to the charge transfer phenomenon between Eu³⁺ to O^2-. This excitation range is in mercury execution so it is hazardous for environment and it is not useful foe LEDs (Light Emitting Diode) application. Peak at 363 nm, 383 nm, 395 nm, and 467 nm is due to ⁷F₀ → 5D₄ , ⁷F₀ → 5G₆/5L₆ , ⁷F₀ → 5L₆, and ⁷F₀ → 5D₂ transitions. Transition respectively. Selection of peak at 395 nm is due to. maximum intensity and fall under UV-range (300 -400 nm ) is very useful for led application²⁵.

Fig. 7.

Excitation spectra of La₂O₃: Eu³⁺ at emission wavelength 627 nm.

The emission spectra of La₂O₃: Eu³⁺ is recorded at the excitation wavelength 395 nm from 525 to 650 nm as shown in Fig. 8. The emission spectra are recorded for different concentration of Eu³⁺ such as 1.0 mol %,2.0 mol%, 3.0 mol%, 4 mol%, and 5.0 mol%. The emission spectra contain one major peak at 627 nm and several minor peaks at 539 nm, 580 nm, 587 nm, 596 nm, and 614 nm. Peak at 580 nm is due to 5D₀ → 7F₀ transition. Peak at 587 nm, 596 nm is due to 5D₀ → 7F₁ transition and peak at 614 nm and 627 nm is due to 5D₀ → 7F₂ transition^25,35. Two peaks at a single transition are due to stark splitting. Intensity of emission spectrum increases from 1 mol% to 4 mol%. At 4 mol% it shows maximum intensity, then after that the intensity of spectrum decreases as shown in Fig. 9. This phenomenon is known as concentration quenching. Figure 8 shows the variation in intensity of peak at the 627 nm, 614 nm, and 587 nm with the concentration of Eu ion in the host lattice.

Fig. 8.

Emission spectra of La₂O₃: Eu³⁺ at excitation wavelength 395 nm.

Fig. 9.

Concentration quenching of La₂O₃: Eu³⁺.

Concentration quenching phenomenon depends on critical distance (R_c). Critical distance is the distance where the activator ion is close to the optimum distance at specific optimum concentration. Multiple interaction and exchange interaction phenomenon is mainly used for the transfer of energy between the activator ion and the host lattice. Critical distance (R_c) can be approximately calculated by using Blasse’s formula^36,37:

1

where V denotes the volume of the unit cell, N denotes the number of cations within the unit cell, and X_c denotes the critical concentration of the activator ion. In the prepared La₂O₃:Eu phosphor, the value of V = 82.2985 ų, N = 1, and X_c = 4.0 mol%. By using formula 1 value of R_c is found to be 3.40 which is less than 5 Å. This indicates the quenching phenomenon due to the exchange interaction.

The change in the emission intensity of the prepared phosphor when the level is impacted by the ion interaction and that strength is found by using the equation below. Relation between the intensity and concentration of activator ion can be estimated by using below equation³⁸.

2

This equation can be modified as :

3

Here k and β are constants with same matrix and excitation condition. Figure 10 show the relationship between Log(I/x) vs Log(x). Slope of Log(I/x) vs log(x) of La₂O₃: Eu³⁺ is 3.127. Slope factor of is found to be 9.381. The value of slope factor is near about 10 so quadrupole–quadrupole interaction is confirmed ^39,40.

Fig. 10.

Log(I/x) vs log(x) of La₂O₃:Eu.

La₂O₃:Ho

Figure 11 shows the excitation spectra of La₂O₃:Ho at emission wavelength of 546 nm recorded from 250 to 400 nm. The emission spectra consist of one major peak at 363 nm and 2 minor peaks at 336 nm, 345 nm. Peak at 363 nm is due to ⁵I₈ → ⁵G₂,³H₅ transition while peak at 336 nm, and 345 nm is due to ⁵I₈ → ⁵G₃ ​, and ⁵I₈ → ³H₆ transition respectively⁴¹. The emission spectra are recorded at the excitation wavelength of 363 nm as shown in Fig. 12. Emission spectra are recorded from 500 to 600 nm of different concentration of Ho³⁺ ions varies from 0.5 mol % to 2.5 mol%. Emission peak consists of one major peak at 546 nm is due to ⁵F₄​/⁵S₂​ → ⁵I₈ transition. Peak at 546 nm is combine of two sharp peaks. The two closely spaced sharp peaks arise from the transitions between two different energy levels, rather than from crystal defects, since only a single identical crystallographic site is available in the host lattice ^42,43. The two ⁵F₄​ and ⁵S₂ excited states of Ho³⁺ are very close in energy, with a separation of about 200–300 cm⁻¹, which is comparable to the phonon energies of oxide hosts (400–600 cm⁻¹). Due to this small gap, lattice vibrations can easily bridge the levels, leading to fast non-radiative thermal coupling between them. Hence, both states remain populated under excitation and take part in the emission process. The radiative transitions from ⁵F₄ and ⁵S₂ to ⁵I₈ the ground state strongly overlap, giving rise to a single broad green band near 546 nm^43,44. In some high-resolution spectra, a slight asymmetry or weak splitting can be noticed, which further supports the combined contribution of the two levels. As the concentration of Ho³⁺ ion increases from 0.5 mol% to 1.5 mol% intensity increases while after that intensity of emission is decreases gradually until the concentration up to 2.5 mol% Fig. 13. This phenomenon is due to concentration quenching. To find critical distance (R_c) of La₂O₃:Ho, the values of V = 82.2985 ų, X_c = 1.5, and N = 1. By using Eq. 1 the value of R_c is found to be 4.71. The value of which is less than 5 Å, that indicate the quenching phenomenon due to exchange interaction. By using Eq. 3 slope of the Log(I/x) vs log(x) graph (Fig. 14) is 1.74806. The slope factor is 5.24418, which is near to 6 therefore dipole–dipole interaction is confirmed.

Fig. 11.

: Excitation spectra of La₂O₃:Ho at emission wavelength 546 nm.

Fig. 12.

Emission spectra of La₂O₃:Ho at excitation wavelength 363 nm.

Fig. 13.

Variation of intensity with variation of concentration of La₂O₃:Ho.

Fig. 14.

Log(I/x) vs Log(x) of La₂O₃:Ho.

La₂O₃:Tm

Figure 15 shows the emission spectra of La₂O₃:Tm at different concentration of Tm³⁺ varies from 0.5 mol % to 2.5 mol%. Emission spectra of La₂O₃:Tm is recorded from 220 to 400 nm at emission wavelength 458 nm. Emission spectra contain one major peak at 237 nm and one minor peak at 361 nm. Broad peak centered at 237 is due to charge transfer between Tm³⁺ to O^2-. Peak at 361 nm is due to ³H₆ → ¹D₂^29,45.

Fig. 15.

Excitation spectra of La₂O₃:Tm at emission wavelength 458 nm.

The emission spectra of La₂O₃:Tm was recorded at the excitation wavelength of 361 nm at different concentration of Tm varying from 0.5 mol % to 2.5 mol % (Fig. 16).The emission spectra of La₂O₃:Tm had one major at 458 nm which is due to the ¹D₂ ​ → ³F₄​ transition^46,47. In hexagonal phase there is only one cryptographic site present in lattice, therefore Tm³⁺ ion has to occupy one single site. This results in single emission and narrow peak of emission. Emission peak intensity at 1.5 mol % is the highest among all the concentrations before and after that the intensity of emission spectra decreases. This phenomenon is due the concentration quenching (Fig. 17). Concentration quenching depends on the value of R_c. For La₂O₃:Tm, value N = 1, X_c = 1.5, V = 82.2985 ų. The value of R_c = is found to be 4.71. The value of which is less than 5 Å, that indicate the quenching phenomenon due to exchange interaction. By using Eq. 3 slope of Log(I/x) vs log(x) graph (Fig. 18) is 1.74806. The slope factor is 5.502 which is near to 6 therefore dipole–dipole interaction is confirmed.

Fig. 16.

Emission spectra of La₂O₃:Tm at excitation wavelength 361 nm.

Fig. 17.

Variation of intensity with variation of concentration.

Fig. 18.

Log(I/x) vs Log(x) of La₂O₃:Tm.

La₂O₃:Tm/Ho

Figure 19 shows the overlapping spectra of La₂O₃:Tm and La₂O₃:Ho. In Fig. 19 black line indicates the excitation spectra of La₂O₃:Ho at emission wavelength of 546 nm, while red line indicates the excitation spectra La₂O₃:Tm at emission wavelength 458 nm. In the overlapping spectra, both excitation spectra are overlapped at two points at 306 nm and 361 nm. Broad peaks from 280 to 340 nm centered at 306 nm are observed only in the co-doped phosphor excitation spectra as compared to single doped phosphor. In the Fig. 19, the excitation-excitation overlap occurred. So, energy transfer is from lower wavelength (458 nm) of Tm³⁺ ion to higher wavelength (546 nm) of Ho³⁺ ion. To study the energy transfer between them we keep the lower wavelength ion (Tm) is constant at 1.5 mol% where the concentration quenching is happens. The concentration of higher wavelength ion (Ho³⁺ ) varies from 0.5 mol% to 2.5 mol%. So, to study the energy transfer in La₂O₃:Tm/Ho we recorded the emission spectra at 306 nm and 361 nm.

Fig. 19.

Overlap graph of La₂O₃:Tm/Ho.

The emission spectra of La₂O₃:Tm/Ho is shown in the Fig. 20 at the excitation wavelength of 361 nm. The emission spectra of La₂O₃:Tm/Ho contains major 2 peaks at 458 nm and 547 nm. The peaks at 458 nm and 547 nm are due to ¹D₂ ​ → ³F₄​ and ⁵F₄​/⁵S₂​ → ⁵I₈ transitions, respectively. As the concentration of Ho³⁺ ion increases from 0.5 mol% to 2.5 mol%, the emission intensity of peak at 458 nm (Tm³⁺) ion decreases. The emission intensity of Ho³⁺ ion increases gradually. The decrease in energy is due to the transfer of the energy from Tm³⁺ ion Tm³⁺ ion to the Ho³⁺ ion in the host lattice. This variation of emission intensity in specific manner concludes that the energy transfer in La₂O₃:Tm/Ho has occurred. The variation of intensity is shown in the Fig. 21. Energy transfer efficiency can be find using following equation:

4

where I_a is intensity of the acceptor atom and I_ao is intensity of acceptor ion without donor atom. In the above case, energy transfer happens from the Tm ion to the Ho ion, so Ho is the acceptor ion while Tm is donor atom. Energy transfer is ranges from 8.18809 to 54.52997%. As increasing the concentration of Ho ion efficiency start decreases as shown in Table 5.

Fig. 20.

Emission spectra of La₂O₃:Tm/Ho at excitation wavelength 361 nm.

Fig. 21.

variation of intensity in La₂O₃:Tm/Ho at excitation wavelength 361 nm.

Table 5.

Energy transfer efficiency of La₂O₃: Tm³⁺/Ho³⁺

sr	Name	IEr	IEro	Efficiency (%)
1	La2O3: Tm3+(1.5 mol%)/Ho3+(0.5 mol%)	775.1	352.4382	54.52997
2	La2O3: Tm3+(1.5 mol%)/Ho3+(1.0 mol%)	845.526	385.25	54.43665
3	La2O3: Tm3+(1.5 mol%)/Ho3+(1.5 mol%)	1641.372	882.05	46.26142
4	La2O3: Tm3+(1.5 mol%)/Ho3+(2.0 mol%)	1229.488	938.538	23.66432
5	La2O3: Tm3+(1.5 mol%)/Ho3+(2.5 mol%)	1227.368	1126.87	8.18809

Figure 22 shows the emission spectra of La₂O₃:Tm/Ho at excitation wavelength 306 nm. Emission spectra contain one broad peak from 410 to 530 nm humped at 450 nm and 474 nm, while one sharp peak at 546 nm. Broad peak is humped at 450 and 474 nm is due to ¹D₂ → ³F₄ transition while the peak at 546 nm is due to ⁵F₄/⁵S₂ → ⁵I₈ transition of Ho³⁺ ion. Intensity of both broad and sharp peak increases as their increase in the concentration of Ho³⁺ ion in host lattice. The phenomenon of energy transfer does not manifest in that specific emission spectra. In emission spectra at excitation wavelength 306 nm, it has anomalous behavior compared to that of emission spectra at 361 nm.

Fig. 22.

Emission spectra of La₂O₃:Tm/Ho at excitation wavelength 306 nm .

Two different types of emission at different excitation in same host lattice is due to the difference in energy transfer and energy absorption mechanism. Excitation at 361 nm is directly resembles to the ³H₆ → ¹D₂ transition of Tm³⁺ ion, this populates the excitation states and resultant gives blue emission at 458 nm (¹G₄ → ³H₆), by ion-ion cross-relaxation phenomenon. The energy from Tm³⁺ ion is transfer to the Ho³⁺ ion. Transfer of energy give sharp peak at 547 nm due to transition ⁵S₂ → ⁵I₈. When we excite the La₂O₃:Tm/Ho with excitation wavelength 306 nm , absorption of high energy emerges primarily from the host lattice not dopant ions(Tm³⁺,Ho³⁺). Host lattice absorbs the energy by storing energy in broad electron–hole pair or in charge transfer state. Ho³⁺ ion act as high and efficient trap for mobile energy due to its high optimal energetic alignment with high-lying Ho³⁺ energy levels like ⁵G. This high energy absorption leads to where instance host-ion transfer overpopulate the ⁵S₂ state of Ho³⁺ ion, which result in the occurrence of intense and sharp emission at 546 nm is observed. The process of recombination of energy in Ho ion is so fast so that Tm³⁺ quenches its luminescence. Broad spectra is due to self emission of host lattice due to host lattice defect. Humped broad background from 410–530 nm represents only the small fraction of exactions that recombine radioactively at defect sites before being captured by Ho³⁺.

La₂O₃:Ho/Eu

Figure 23 shows the overlap spectra of La₂O₃:Ho/Eu. In Fig. 23 black line indicate the excitation spectra La₂O₃:Eu at emission wavelength 616 nm, while red line indicates excitation spectra of La₂O₃:Ho at emission spectra 545 nm. Both spectra overlap at 2 points in near UV region. In He, excitation-excitation overlaps happen. To study the energy transfer between these ions, we keep the lower wavelength (545 nm) constant at 1.5 mol%, where the quenching occurs. Keep varying the concentration of higher wavelength (616 nm) of Eu³⁺ ion.

Fig. 23.

: Overlap Spectra of La₂O₃:Ho/Eu.

Emission spectra of La₂O₃:Ho/Eu is recorded at excitation wavelength 363 nm is shown in Fig. 24. Emission spectra of La₂O₃:Ho/Eu contain one major peak at 545 nm and some minor peaks at 587 nm, 596 nm, 613 nm, and 626 nm. Peak at 545 nm is due to the transition of Ho ion due to ⁵F₄​ /⁵S₂​ → ⁵I₈ transition. Peak at 587 nm, and 596 nm, 613 nm, and 626 nm are due to 5D₀ → 7F₀, 5D₀ → 7F₁ transition of Eu³⁺ ion respectively. As the concentration of Eu³⁺ increases, the intensity of the Ho³⁺ emission peak at 545 nm decreases, while the intensities of the Eu³⁺ emission peaks at 587 nm, 596 nm, 613 nm, and 626 nm increase (Fig. 25). That conclude the energy transfer in La₂O₃:Ho/Eu has occurred. Energy transfer efficiency of La₂O₃:Ho/Eu is calculate using Eq. 4 and shown in the Table 6. Acceptor ion is Eu and donor atom is Ho. Maximum energy transfer happens at La₂O₃: Ho³⁺(1.5 mol%)/Eu³⁺(2.0 mol%) with efficiency of 93.75481%.

Fig. 24.

Emission spectra of La₂O₃:Ho/Eu at excitation wavelength 363 nm.

Fig. 25.

: variation of intensity in La₂O₃:Ho/Eu at excitation wavelength 361 nm.

Table 6.

Energy transfer efficiency of La₂O₃: Ho³⁺/Eu³⁺

Sr	Name	IEr	IEro	Efficiency (%)
1	La2O3: Ho3+(1.5 mol%)/Eu3+(1.0 mol%)	237.6543	35.6527	84.99808
2	La2O3: Ho3+(1.5 mol%)/Eu3+(2.0 mol%)	671.6266	41.94435	93.75481
3	La2O3: Ho3+(1.5 mol%)/Eu3+(3.0 mol%)	735.1859	63.17407	91.40706
4	La2O3: Ho3+(1.5 mol%)/Eu3+(4.0 mol%)	1353.832	95.87116	92.91853
5	La2O3: Ho3+(1.5 mol%)/Eu3+(5.0 mol%)	842.2185	119.839	85.77104

La₂O₃:Tm/Eu

In Fig. 26 shows the overlap spectra of La₂O₃:Tm/Eu. In Fig. 26 black line shows the excitation spectra of La₂O₃:Tm at emission wavelength 458 nm, while red line shows the excitation spectra LE at emission wavelength 627 nm. Both spectra are overlap at 362 nm. In excitation-excitation overlap spectra, we keep lower wavelength constant (Tm) at quenching concentration, while higher wavelength (Eu) is varying.

Fig. 26.

Overlap spectra of La₂O₃:Tm/Eu.

Figure 27 shows the emission spectra La₂O₃:Tm at excitation wavelength 362 nm. Emission spectra of La₂O₃:Tm/Eu contain one major peak at 458 nm and minor peaks at 540 nm, 588 nm, 615 nm, and 627 nm. Peak at 458 nm, 540 nm, 588 nm, 615 nm, and 627 nm is due ¹D₂ ​ → ³F₄​ 5D₀ → 7F₀, 5D₀ → 7F₁ transition. As the concentration of Eu ion increases from 1 mol% to 5 mol% intensity of peak at 458 nm due to Tm transition decreases. As increasing the concentration of Eu ion increases, the intensity of peak due to the Eu transition is increases as shown in Fig. 28. This is concluded that energy transfer has occurred. The intensity of the Tm peak is significantly higher compared to that of the Eu ions. Under 362 nm excitation, Tm ions strongly absorb and predominantly radiates intense blue light, while transferring only a small fraction of energy to Eu ions. This limited energy transfer results in the observed difference in emission intensities. Table 7 shows calculated energy transfer efficiency using Eq. 4 of La₂O₃:Tm/Eu. In above case donor atom is Tm and acceptor ion is Eu. Energy transfer efficiency varies from 90.70919 to 75.60775% with maximum at La₂O₃:Tm³⁺(1.5 mol%)/Eu³⁺(1.0 mol%).

Fig. 27.

Emission spectra of La₂O₃:Tm/Eu at excitation wavelength 362 nm.

Fig. 28.

variation of intensity in La₂O₃:Tm/Eu at excitation wavelength 361 nm.

Table 7.

Energy transfer efficiency of La₂O₃: Tm³⁺/Eu³⁺

sr	Name	IEr	IEro	Efficiency (%)
1	La2O3:Tm3+(1.5 mol%)/Eu3+(1.0 mol%)	237.6543	22.08	90.70919
2	La2O3: Tm3+(1.5 mol%)/Eu3+(2.0 mol%)	671.6266	79.626	88.1443
3	La2O3: Tm3+(1.5 mol%)/Eu3+(3.0 mol%)	735.1859	113.942	84.50161
4	La2O3: Tm3+(1.5 mol%)/Eu3+(4.0 mol%)	1353.832	180.228	86.68757
5	La2O3: Tm3+(1.5 mol%)/Eu3+(5.0 mol%)	842.2185	205.436	75.60775

La₂O₃:Tm/Ho/Eu

In the preceding study we analyzed the energy transfer between the La₂O₃:Tm/Ho/Eu, La₂O₃:Tm/Ho, and La₂O₃:Tm/Eu. To achieve the complete WLEDs phosphor with high red color component. We synthesis the triple dope phosphor in same host. Figure 29 shows the overlap spectra of the at excitation wavelength of La₂O₃:Tm/Ho/Eu. Above Fig. 29 has 3 excitation spectra at 3 different emission wavelengths. Emission wavelength at 616 nm, 545 nm, and 458 nm is associated with the Eu³⁺, Ho³⁺, and Tm³⁺ ion excitation. All excitation spectra are overlap at 363 nm. To study the triple doped phosphor, we keep the concentration of Tm and Ho ion constant at 1.5 mol% each. This concentration has balaced components of both blue and green colour which is very essential to achieve WLEDs phosphor.The concentration of Eu³⁺ ion is changes from 1.0 mol% to 5 mol%.

Fig. 29.

Overlap spectra of La₂O₃:Tm/Ho/Eu.

Figure 30 shows the excitation spectra of La₂O₃:Tm/Ho/Eu at excitation wavelength 363 nm. Emission spectra contain majorly 4 peaks. One peak in blue region at 459 nm, second in green region at 547 nm, nest at 587 nm in orange region, while last peak at 627 nm is in red region. Peaks at 459 nm, 547 nm, 587 nm, and 627 nm are associate with ¹D₂ ​ → ³F₄​ ,⁵F₄​/⁵S₂​ → ⁵I_8, 5D₀ → 7F₀, and 5D₀ → 7F₁ transition, respectively. As we increase the concentration of Eu³⁺ ion, intensity peaks at 459 nm, and 547 nm decreases continuously, while the intensity of peak at 587 nm, and 627 nm due to Eu³⁺ ion increases continuously. Detailed energy transfer is shown in Fig. 31. Table 8 shows the energy transfer efficiency of La₂O₃:Tm/Ho/Eu with maximum at La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(1.0 mol%)/Eu³⁺(2.0 mol%) about 77.2498% calculated using Eq. 4. High charge carrier mobility can be achieved by properly designing the material, adding suitable dopants, and optimizing the crystal structure. By reducing the charge recombination loss leads to efficient energy transfer in the phosphor⁴⁸ .

Fig. 30.

Emission spectra of La₂O₃:Tm/Ho/Eu at excitation wavelength 363 nm.

Fig. 31.

Energy level diagram of Tm³⁺, Ho³⁺, Eu³⁺ triple doped La₂O₃ phosphor.

Table 8.

Energy transfer efficiency in La₂O₃:Tm/Ho/Eu.

sr	Name	IEr	IEro	Efficiency (%)
1	La2O3: Tm3+(1.5 mol%)/Ho3+(1.0 mol%)/Eu3+(1.0 mol%)	237.6543	133.4	43.86804
2	La2O3: Tm3+(1.5 mol%)/Ho3+(1.0 mol%)/Eu3+(2.0 mol%)	671.6266	152.7964	77.2498
3	La2O3: Tm3+(1.5 mol%)/Ho3+(1.0 mol%)/Eu3+(3.0 mol%)	735.1859	206.1564	71.95861
4	La2O3: Tm3+(1.5 mol%)/Ho3+(1.0 mol%)/Eu3+(4.0 mol%)	1353.832	266.8	80.29298
5	La2O3: Tm3+(1.5 mol%)/Ho3+(1.0 mol%)/Eu3+(5.0 mol%)	842.2185	333.5	60.4022

CIE analysis

Parameters of emission of phosphor can be analyzed using Commission de I Eclairage (CIE) coordinates. CIE coordinates are giving the theoretical color of emission in CIE diagram. CIE co-ordinates are also used to find the color purity and CCT (Color co-related temperature) of prepared phosphor. Color purity of prepared sample is calculated by using following equation⁴⁹:

5

where, (X, Y) are CIE chromaticity co-ordinate, (Xi, Yi) are co-ordinate of white perfect light and (Xd, Yd) is co-ordinate of dominant wavelength. Value (Xi, Yi) used in calculation are (0.33, 0.33). Color co-related temperature of phosphor can be calculated by using formula ²⁴:

CCT =—437n³ + 3601n² +—6861n + 5514.31 ….(6).

La₂O₃:Eu

Figure 32 shows the CIE diagram of La₂O₃:Eu. CIE co-ordinates are in red region that confirm the emission of red color phosphor. As the concentration of Eu³⁺ ion increases from 1.0 mol% to 5 mol%, CIE coordinates shift from orange-red region to red region in CIE diagram. Table 9 shows the CIE coordinate, color purity and CCT of prepared sample. La₂O₃:Eu³⁺(1.0 mol%) and La₂O₃:Eu³⁺(2.0 mol%) shows the orange-red emission with moderate colour purity , that makes it more prominent candidate for display coating, decorative lighting. La₂O₃:Eu³⁺(3.0 mol%) and La₂O₃:Eu³⁺(4.0 mol%) has intense red emission with high colour purity (more than 80%) therefore this phosphor can be efficacious for solid-state lighting, display backlighting, phosphor-converted LEDs. Quenching in colour purity happens at La₂O₃:Eu³⁺(5.0 mol%) concentration still greater than 80%. La₂O₃:Eu³⁺(5.0 mol%) can be used for solid-state lighting, display backlighting, phosphor-converted LEDs.

Fig. 32.

CIE Diagram of La₂O₃:Eu.

Table 9.

CIE coordinate, color purity, and CCT of La₂O₃:Eu.

La2O3:Eu3+
Sr. No	Name	x	y	xd	yd	CCT (k)	Color Purity (%)
1	La2O3:Eu3+(1.0 mol%)	0.531	0.465	0.701	0.299	1155	68.08
2	La2O3:Eu3+(2.0 mol%)	0.569	0.428	0.702	0.298	1728.58	71.89
3	La2O3:Eu3+(3.0 mol%)	0.627	0.372	0.704	0.296	2267.34	81.59
4	La2O3:Eu3+(4.0 mol%)	0.641	0.359	0.704	0.296	2530.67	84.60
5	La2O3:Eu3+(5.0 mol%)	0.623	0.376	0.704	0.296	2229.14	80.77

La₂O₃:Ho

Figure 33 shows the CIE co-ordinate of La₂O₃:Ho. In CIE diagram the coordinates are in green region that confirm the emission in green region. Table 10 shows the CIE co-ordinate, color purity and CCT of prepared sample. La₂O₃:Ho³⁺ phosphor has high intense green emission with high colour purity ranging from 96% to 98.98%. La₂O₃:Ho³⁺ phosphor has narrow and single peak emission that makes it more useful in green LEDs phosphor, leaser. La₂O₃:Ho³⁺(0.5 mol%) and La₂O₃:Ho³⁺(1.0 mol%) has high CCT around 7300 with stable and intense green emission which makes it more prominent in display coatings and decorative lighting. La₂O₃:Ho³⁺(1.5 mol%) and La₂O₃:Ho³⁺(2.0 mol%) has improvement in colour purity till 98.98% with decreases in CCT so this phosphor have applications in solid-state lighting, laser applications, and optical devices. La₂O₃:Ho³⁺(2.5 mol%) has decreases in colour purity and stable CCT.

Fig. 33.

CIE Diagram of La₂O₃:Ho.

Table 10.

CIE co-ordinate , Colour purity, and CCT of La₂O₃:Ho.

La2O3:Ho3+
Sr. No	Name	x	y	xd	yd	CCT (k)	Colour Purity (%)
1	La2O3:Ho3+(0.5 mol%)	0.224	0.718	0.273	0.718	7305.11	96.84
2	La2O3:Ho3+(1.0 mol%)	0.227	0.717	0.280	0.712	7360.44	97.12
3	La2O3:Ho3+(1.5 mol%)	0.251	0.710	0.287	0.705	6762.32	98.24
4	La2O3:Ho3+(2.0 mol%)	0.224	0.728	0.280	0.712	7034.52	98.98
5	La2O3:Ho3+(2.5 mol%)	0.238	0.715	0.280	0.712	7005.78	96.10

La₂O₃:Tm

Figure 34 shows the CIE co-ordinate of La₂O₃:Tm. In CIE diagram the coordinates are in blue region that confirms the emission in green region. Table 11 shows the CIE co-ordinate, color purity and CCT of prepared sample. La₂O₃:Tm³⁺ phosphor have intense blue emission in blue region with high colour purity (> 95%). La₂O₃:Tm³⁺ has narrow emission with single emission peak which makes it more suitable for laser application. La₂O₃:Tm³⁺(0.5 mol%) and La₂O₃:Tm³⁺(1.0 mol%) has commendable colour purity among all and low CCT values which makes it more beneficial to be used in decorative lighting, display technologies, and anti-counterfeiting coatings. Increasing concentration from 1.5 mol% to 2.5 mol% , colour purity decreases by small value which again makes it preferable prospect it useful in decorative lighting, display technologies, and anti-counterfeiting coatings.

Fig. 34.

CIE diagram of La₂O₃:Tm.

Table 11.

CIE coordinate, color purity, and CCT of La₂O₃:Tm.

La2O3:Tm3+
Sr. No	Name	x	y	xd	yd	CCT (k)	Colour Purity (%)
1	La2O3:Tm3+(0.5 mol%)	0.145	0.041	0.148	0.025	2813.01	96.29
2	La2O3:Tm3+(1.0 mol%)	0.145	0.037	0.145	0.028	1767.06	97.66
3	La2O3:Tm3+(1.5 mol%)	0.143	0.045	0.147	0.027	4589.36	95.94
4	La2O3:Tm3+(2.0 mol%)	0.143	0.046	0.147	0.027	NA	95.68
5	La2O3:Tm3+(2.5 mol%)	0.143	0.048	0.147	0.027	NA	95.17

La₂O₃:Tm/Ho

Figure 35 shows the CIE diagram of La₂O₃:Tm/Ho at excitation wavelength of 306 nm. As the concentration of Ho³⁺ ion increases from 0.5 mol% to 2.5 mol%, CIE coordinate start shifting from blue region to green region because of increasing green component in emission spectra as compared to blue components. CIE coordinate, CCT and color purity of La₂O₃:Tm/Ho at excitation wavelength 306 nm shown in Table 12. La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(0.5 mol%) phosphor has emission in bluish green with comparably high purity around 60% - 65% which is suitable for anti-counterfeiting inks and display coatings. At La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(1.5 mol%) phosphor is move toward the green region with moderate purity (~55.7%) with balanced blue and green emission. La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(2.0 mol%) and La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(2.5 mol%) phosphor has CIE coordinate move toward green region with decreasing the colour purity around (34% -38%) due to concentration effects, which can be leveraged for broad-spectrum. Overall, the concentration Ho³⁺ provide 0.5–1.5 mol% provides the best trade-off between vivid color and purity, while ≥2.0 mol% favours greener, less saturated emission.

Fig. 35.

CIE diagram of La₂O₃:Tm/Ho at excitation wavelength 306 nm.

Table 12.

CIE coordinate, color purity, and CCT of La₂O₃:Tm/Ho at 306 nm.

La2O3: Tm3+/Ho3+ at 306 nm
Sr. No	Name	x	y	xd	yd	CCT (k)	Colour Purity (%)
1	La2O3: Tm3+(1.5 mol%)/Ho3+(0.5 mol%)	0.155	0.194	0.106	0.094	13,705,515.21	65.44
2	La2O3: Tm3+(1.5 mol%)/Ho3+(1.0 mol%)	0.155	0.211	0.119	0.068	13,705,515.21	59.82
3	La2O3: Tm3+(1.5 mol%)/Ho3+(1.5 mol%)	0.162	0.227	0.116	0.074	134,129.67	55.69
4	La2O3: Tm3+(1.5 mol%)/Ho3+(2.0 mol%)	0.169	0.255	0.280	0.712	47,359.82	38.73
5	La2O3: Tm3+(1.5 mol%)/Ho3+(2.5 mol%)	0.174	0.279	0.273	0.718	29,707.47	34.95

Figure 36 show the CIE coordinate of La₂O₃:Tm/Ho at excitation wavelength 363 nm. As the concentration of Ho³⁺, CIE coordinates start shifting from intense blue to intense green region. Table 13 consists of the detailed value of CCT, color purity, and CCT values for prepared sample. At La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(0.5 mol%) phosphor has bluish emission as the emission is mainly due to Tm³⁺ emission with moderate colour purity of 34%. At La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(1.0 mol%) has bluish- green emission with 31 % colour purity for cool display application. At La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(1.5 mol%) and La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(2.0 mol%) has emission in green-yellow with slightly improved purity (~32–36%), indicating balanced emission effective for tuneable lighting. Among all prepared phosphor La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(2.5 mol%) has maximum colour purity around 48%, that makes it more prominent application in displays, bio-imaging.

Fig. 36.

CIE diagram of La₂O₃:Tm/Ho at excitation wavelength 363 nm.

Table 13.

CIE coordinate, color purity, and CCT of La₂O₃:Tm/Ho at 363 nm.

La2O3: Tm3+/Ho3+ at 363 nm
Sr. No	Name	x	y	xd	yd	CCT (k)	Colour Purity (%)
1	La2O3: Tm3+(1.5 mol%)/Ho3+(0.5 mol%)	0.189	0.224	0.145	0.028	104,763.98	34.26
2	La2O3: Tm3+(1.5 mol%)/Ho3+(1.0 mol%)	0.212	0.284	0.145	0.028	33,888.44	31.08
3	La2O3: Tm3+(1.5 mol%)/Ho3+(1.5 mol%)	0.230	0.418	0.273	0.718	9275.05	32.10
4	La2O3: Tm3+(1.5 mol%)/Ho3+(2.0 mol%)	0.241	0.444	0.273	0.718	8184.02	36.01
5	La2O3: Tm3+(1.5 mol%)/Ho3+(2.5 mol%)	0.248	0.497	0.280	0.712	7525.85	48.16

La₂O₃:Tm/Ho phosphor is showing different emission color at different concentration of Ho³⁺ ion in host lattice. At low concentration one color is dominate while at high concentration different transitions activate due to cross relaxation and energy transfer, so color of emission is change. This property of multi color emission from single material can be useful in barcode application. In normal light barcode looks like normal black and white image but under 361 nm excitation different colour comes out according to Ho³⁺ concentration region. This dual mode appearance gives extra verification to barcode system. Normal barcode can be easily copy by printing but this type of multi emission barcode cannot be reproduce without exact knowledge of phosphor synthesis and doping level. That makes it highly secured system. Multi emission verification system is very useful in anti-counterfeiting. Prototype figure 37 shows barcode under room light and same barcode under 361 nm UV light, where different concentration region gives different color. By arranging phosphor with different concentration in barcode pattern we can prepare unique code which is very difficult to copy. Such type of feature is also useful in anti-counterfeiting of currency notes. Currency note looks normal in daylight but under UV source secret emission pattern will appear with multiple colors. This give two step verification for authentication. Such technology can also use in document, brand protection and other valuable goods. Advantage is one host material itself giving multiple emission colors only by changing Ho³⁺ concentration. This make system more simple but strong against duplicate. Therefore La₂O₃:Tm/Ho phosphor with Ho³⁺ concentration dependent emission is a very promising material for barcode security, anti-counterfeit and advanced authentication system.

Fig. 37.

La₂O₃:Tm/Ho phosphor applied barcode.

La₂O₃:Tm/Eu

Figure 38 shows the CIE diagram of La₂O₃:Tm/Eu. As the concentration of Eu ion increases, CIE coordinates start shifting from blue region to white region in CIE diagram. CIE coordinate, color purity, and CCT values are shown in table 14. La₂O₃:Tm³⁺(1.5 mol%)/Eu³⁺(1.0 mol%) and La₂O₃:Tm³⁺(1.5 mol%)/Eu³⁺(2.0 mol%) phosphor have moderate colour purity (~38–43%) with low CCT (~1900–2100 K), these values makes them useful for low intensity optoelectronic devices. With increasing concentration of Eu to 3 mol% and 4 mol%. Color purity slightly increases so it can be beneficial for manufacturing of warm white LEDs. La₂O₃: Tm³⁺(1.5 mol%)/Eu³⁺(5.0 mol%) has low colour purity among all prepared sample because of the incongruity between the red and blue colour. This is also useful in low-cost lighting and security ink application where low purity is needed.

Fig. 38.

CIE Diagram of La₂O₃:Tm/Eu.

Table 14.

CIE coordinate, color purity, and CCT of La₂O₃:Tm/Eu.

La2O3:Tm3+/Eu3+
Sr. No	Name	x	y	xd	yd	CCT (k)	Color Purity (%)
1	La2O3:Tm3+(1.5 mol%)/Eu3+(1.0 mol%)	0.252	0.186	0.148	0.025	2143.21	42.77
2	La2O3: Tm3+(1.5 mol%)/Eu3+(2.0 mol%)	0.259	0.202	0.147	0.027	1937.9	37.67
3	La2O3: Tm3+(1.5 mol%)/Eu3+(3.0 mol%)	0.264	0.202	0.145	0.028	2041.07	37.07
4	La2O3: Tm3+(1.5 mol%)/Eu3+(4.0 mol%)	0.253	0.181	0.147	0.027	1750.49	44.19
5	La2O3: Tm3+(1.5 mol%)/Eu3+(5.0 mol%)	0.306	0.277	0.148	0.025	1758.54	11.81

La₂O₃:Ho/Eu

CIE co-ordinate of La₂O₃: Ho/Eu is shown in the figure 39. As the concentration of Eu³⁺ ion increases from 1.0 mol% to 5 mol%, CIE coordinate starts shifting from green region to red region. Color coordinates, color purity and CCT are shown in table 15. La₂O₃: Ho³⁺(1.5 mol%)/Eu³⁺(1.0 mol%) and La₂O₃: Ho³⁺(1.5 mol%)/Eu³⁺(2.0 mol%) phosphor coordinate starts shifting from green to red with relatively high CCT (~4700–5100 K), this makes this phosphor very suitable for WLEDs phosphor and display where mixed color output is needed. At La₂O₃: Ho³⁺(1.5 mol%)/Eu³⁺(3.0 mol%) and La₂O₃: Ho³⁺(1.5 mol%)/Eu³⁺(4.0 mol%) has shifting toward stronger red and lower CCT (~2900–4300 K)that makes it useful for warm white LEDs application. At La₂O₃: Ho³⁺(1.5 mol%)/Eu³⁺(5.0 mol%), CIE coordinate approaches near (x=0.497, y=0.499) with the lowest CCT (~2504 K), which is ideal for solid-state lighting.

Fig. 39.

CIE of La₂O₃: Ho/Eu.

Table 15.

CIE coordinate, color purity, and CCT of La₂O₃: Ho/Eu.

La2O3: Ho3+/Eu3+
Sr. No	Name	x	y	xd	yd	CCT (k)	Color Purity (%)
1	La2O3: Ho3+(1.5 mol%)/Eu3+(1.0 mol%)	0.422	0.572	0.273	0.718	5093.73	69.20
2	La2O3: Ho3+(1.5 mol%)/Eu3+(2.0 mol%)	0.432	0.563	0.273	0.718	4723.46	68.22
3	La2O3: Ho3+(1.5 mol%)/Eu3+(3.0 mol%)	0.442	0.552	0.266	0.724	4313.94	65.88
4	La2O3: Ho3+(1.5 mol%)/Eu3+(4.0 mol%)	0.489	0.507	0.704	0.296	2918.75	66.32
5	La2O3: Ho3+(1.5 mol%)/Eu3+(5.0 mol%)	0.497	0.499	0.704	0.296	2504.35	66.29

La₂O₃:Tm/Ho/Eu

Figure 40 is showing CIE diagram of La₂O₃:Tm/Ho/Eu. CIE co-ordinate of La₂O₃:Tm/Ho/Eu is in white or near white region. Color coordinate, color purity and CCT are shown in table 16. As concentration of Eu³⁺ ion increases, CIE coordinates start shifting from blue to white region in CIE diagram. At La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(1.0 mol%)/Eu³⁺(1.0 mol%) and La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(1.0 mol%)/Eu³⁺(2.0 mol%) lie in bluish green region where Tm and Ho ion dominance. This phosphor has low colour purity (8-11 %) with high CCT ~6400–7700 K) which makes this phosphor suitable for cool-white display applications. At La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(1.0 mol%)/Eu³⁺(3.0 mol%) and La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(1.0 mol%)/Eu³⁺(4.0 mol%) has co-ordinate in white region with the red component. CCT of prepared phosphor is decreases (~3700–4700 K), and color purity improves (~17–19%), indicating potential for tuneable white-light sources and decorative lighting. This phosphor has proportionated blue, green and red component in emission, which is favourable for white-light generation. At La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(1.0 mol%)/Eu³⁺(5.0 mol%) have emission shifting toward the reddish region with the lowest CCT (~2497 K). Importantly, this higher Eu³⁺ doping compensates for the red deficiency often observed in conventional WLEDs, thereby enabling the production of warm white light with improved color rendering. The La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(1.0 mol%)/Eu³⁺(3.0 mol%) composition is most suitable for WLEDs. Its CCT (~4759 K) and balanced CIE coordinates provide warm-white emission with moderate color purity, ideal for natural white light.

Fig. 40.

CIE Diagram of La₂O₃:Tm/Ho/Eu.

Table 16.

CIE coordinate, color purity, and CCT of La₂O₃:Tm/Ho/Eu.

La2O3: Tm3+/Ho3+/Eu3+
Sr. No	Name	x	y	xd	yd	CCT (k)	Color Purity (%)
1	La2O3: Tm3+(1.5 mol%)/Ho3+(1.0 mol%)/Eu3+(1.0 mol%)	0.288	0.339	0.280	0.712	7778.77	8.00
2	La2O3: Tm3+(1.5 mol%)/Ho3+(1.0 mol%)/Eu3+(2.0 mol%)	0.311	0.360	0.280	0.712	6377.15	11.05
3	La2O3: Tm3+(1.5 mol%)/Ho3+(1.0 mol%)/Eu3+(3.0 mol%)	0.363	0.372	0.280	0.712	4759.22	19.38
4	La2O3: Tm3+(1.5 mol%)/Ho3+(1.0 mol%)/Eu3+(4.0 mol%)	0.376	0.326	0.704	0.296	3715.36	16.90
5	La2O3: Tm3+(1.5 mol%)/Ho3+(1.0 mol%)/Eu3+(5.0 mol%)	0.436	0.350	0.705	0.295	2497.06	32.97

FTIR analysis

Figure 41 shows the FT-IR spectra of La₂O₃. Spectrum of La₂O₃ is recorded from 400 cm^-1 to 4000 cm^-1. Spectra contain one major peak at 630 cm^-1 is due to La–O vibration present in the La₂O₃. Some minor peaks are observed at 1367 cm^-1 and 1485 cm^-1 is due to –C = O vibration. Relatively small peak is observed at 3607 cm^-1 is due to –O–H vibration in lattice. Tendency of absorbing the moisture from air ant turns into La(OH)₃ is small ^50–52.

Fig. 41.

FT-IR spectra of La₂O₃.

Conclusion

Photoluminescence property of La₂O₃ is single-doped with Tm, Ho and Eu, double-doped and triple-doped phosphor was studied. The phosphor was prepared by combustion synthesis method using hydrazine as fuel. The pure and doped La₂O₃ phosphor shows hexagonal phase with space group -P6c2c. With doping of rare earth ions, XRD peaks are shifted towards larger angle side which indicates lattice contraction. Rietveld refinement result confirms that the lattice parameter decreases after doping and also change in z–coordinate of atom is observed. SEM image shows particles are agglomerated with irregular sizes and shapes with the average particle size around 12 µm. FTIR spectrum confirms the presence of La–O bonding in prepared phosphor. Tm, Ho and Eu ion doping gives blue, green and orange-red emissions respectively under UV excitation. Single crystallographic site of host helps Tm³⁺ ion to emit sharp peak in blue region. In La₂O₃:Ho³⁺ phosphor, energy gap between ⁵F₄ and ⁵S₂ level of Ho³⁺ (~ 200 cm⁻¹) is smaller than the phonon energy gap of La₂O₃ host (~ 400 cm⁻¹), so both levels to merge and give single emission band with increasing in FWHM. The energy transfer between Tm → Ho, Ho → Eu and Tm → Eu is studied in co-doped and tri-doped phosphor. The energy transfer efficiency of co-doped and triple doped phosphor is maximum at La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(0.5 mol%), La₂O₃: Ho³⁺(1.5 mol%)/Eu³⁺(2.0 mol%), La₂O₃:Tm³⁺(1.5 mol%)/Eu³⁺(1.0 mol%), and La₂O₃: Tm³⁺(1.5 mol%)/Ho³⁺(1.0 mol%)/Eu³⁺(4.0 mol%) about 54.52, 93.75, 90.70, and 77.24%respectivelyIn La₂O₃:Tm/Ho/Eu phosphors, efficient energy absorption and energy transfer are observed. For white LEDs, the La₂O₃:Tm³⁺(1.5 mol%)/Ho³⁺(1.0 mol%)/Eu³⁺(3.0 mol%) composition is preferable, with Tm³⁺ contributing blue, Ho³⁺ green, and Eu³⁺ red emission. For blue LEDs, La₂O₃:Tm is suitable; for green LEDs, La₂O₃:Ho; for red LEDs, La₂O₃:Eu; and La₂O₃:Tm/Ho is ideal for barcode applications due to its blue–green emission. Prepared phosphor can be applied in WLEDs, barcode security, and multicolor LED application.

Author contributions

Praful P. Khode: Methodology, Investigation, Writing—original draft. Aditi Deshmukh: Methodology. Mateusz Czerwiński: Methodology, Analysis. Marta Michalska-Domańska: Analysis, Writing—review & editing. S. J. Dhoble: Supervision, Writing—review & editing.

Data availability

The datasets generated and/or analysed during the current study are not publicly available due to intellectual property protection but are available from the corresponding author on reasonable request.