Simultaneous influence of Zn²⁺/Mg²⁺ on the luminescent behaviour of La₂O₃:Tm³⁺–Yb³⁺ phosphors

Abstract

The structural characterizations of La₂O₃ phosphors doped/codoped with Tm³⁺ and Yb³⁺ ions synthesized by the urea assisted solution combustion technique were performed using X-ray diffraction analysis, Fourier transform infrared spectroscopy and scanning electron microscopy. Codoping with Zn²⁺/Mg²⁺ ions causes an increase in the particle size and aggregation for the La₂O₃:Tm³⁺–Yb³⁺ phosphor. The upconversion (UC) study under 980 nm excitation shows four UC emission bands centred around 476 nm (¹G₄ → ³H₆), 653 nm (¹G₄ → ³F₄), 702 nm (³F₂ → ³H₆) and 795 nm (¹G₄ → ³H₅). The effect of codoping with Zn²⁺ and Mg²⁺ ions in the Tm³⁺–Yb³⁺ codoped La₂O₃ phosphor was investigated. Decay curve analysis of the prepared phosphors was done to understand the mechanism of the upconversion emission intensity variation due to codoping with Zn²⁺/Mg²⁺ ions. The purity of colour emitted from the sample does not show any change with variations in pump power density.

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1. Introduction

Owing to their excellent luminescent behaviour, luminescence materials have found wide applications in the field of optical data storage, lasers, temperature sensors, detection of infrared radiation, colour displays, biomedical diagnostics, upconverters, optical amplifiers, telecommunication, plasma display panels, high definition televisions (HDTVs), light emitting diodes, cathode ray tubes (CRTs), finger print detection, scintillators, etc.^1–8 In these materials, luminescence takes place due to energy transition within the energy levels of rare earth ions.¹ Rare earth (RE) ions are characterized by a partially filled 4f shell that is shielded by completely filled outer 5s² and 5p⁶ orbitals, which causes sharp luminescence, followed by intra 4f shell transitions. The energy levels of the rare earth ions are therefore largely insensitive to the host environment in which they are placed and that leads to their radiative transitions in solid hosts which resemble those of the free ions.⁹ Lanthanide elements when incorporated into crystalline or amorphous hosts, exist in a triply ionized state, or occasionally, a doubly ionized oxidation state.

The upconversion emission from the singly thulium doped luminescent materials has been observed by many researchers^10–14 but stronger luminescence was observed when thulium was codoped with ytterbium.^12,15,16 This is because of the high absorption cross-section and favourable energy level structure of Yb³⁺ ions for being excited to the higher energy level upon 980 nm excitation.^17,18 The selection of appropriate host material is important in the preparation of micro/nanocrystalline luminescent materials with efficient optical properties. Of the various solid host materials, oxides and fluorides are mostly preferred because of their low cut-off phonon frequencies which results in lesser energy loss due to non-radiative relaxation channels.¹⁹ La₂O₃, a solid material, is expected to be suitable for the preparation of highly luminescent phosphor materials due to its low cost, good chemical durability and thermal stability, low phonon frequency (∼400 cm⁻¹) etc.^20,21 Furthermore, La₂O₃ has been considered as an attractive host material for codoping of triply ionised lanthanides owing to its comparable ionic radius. It is known that the codoping with rare-earth ions increases the pump efficiency and hence the intensity of the upconversion (UC) emission, but the effect of codoping with non-rare earth ions has rarely been studied. Researchers have studied the effects of co-doping with zinc and magnesium individually on the electrical, structural, electrochemical and photoluminescence properties of rare earth doped/codoped luminescent materials. The UC luminescence enhancement of about ∼37 times for the green band at ∼551 nm in the Nd³⁺–Yb³⁺–Zn²⁺ codoped Y₂O₃ phosphor upon excitation at 980 nm has been observed by Dey et al.²² The effect of magnesium doping on the structural as well as optical properties of Zn₂GeO₄:Mn phosphor has been reported by Anoop et al.²³ The enhancement of about ∼60 times in the green UC intensity has been observed due to codoping with Mg²⁺ ions in the Er³⁺–Yb³⁺ codoped CaAl₁₂O₁₉ phosphor.²⁴ But the effects of codoping by a combination of non-rare earth ions like zinc and magnesium on the UC luminescence have not been reported until now to the best of our knowledge.

Here, we report the synthesis and characterization of the prepared phosphors. The near infrared (NIR) to visible frequency UC emission of the Tm³⁺–Yb³⁺ codoped La₂O₃ phosphors and the effects of Zn²⁺–Mg²⁺ codoping on the UC emissions arising from the thulium ions have been studied. XRD, FTIR, pump power dependence and decay curve analysis have been studied in order to obtain information on the mechanism involved in the frequency of UC emissions. The effect of the variations of pump power density on the colour emitted from the developed phosphors has also been examined.

2. Experimental

2.1 Material synthesis

The Tm³⁺, Tm³⁺–Yb³⁺, Tm³⁺–Yb³⁺–Zn²⁺ and Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ doped/codoped La₂O₃ phosphors have been prepared by a combustion route. The compositions taken for the synthesis of these phosphor powders are as follows:

(100 − p − q)La₂O₃ + pTm₂O₃ + qYb₂O₃ (1)

where p = 0.1 mol% and q = 0.0, 1.0, 3.0, 5.0 mol% and

(100 − p − q − r)La₂O₃ + pTm₂O₃ + qYb₂O₃ + rZnO (2)

where p = 0.1 mol%, q = 3.0 mol%, r = 5.0, 10.0, 15.0 and 25.0 mol%, and

(100 − p − q − r − s)La₂O₃ + pTm₂O₃ + qYb₂O₃+ rZnO + sMgCO₃ (3)

where p = 0.1 mol% and q= 3.0 mol%, r = 15.0 mol% and s = 3.0, 5.0, 7.0 and 10.0 mol%. Stoichiometric amounts of the precursor chemicals were taken and treated with nitric acid to form their corresponding nitrates. After that, they were mixed together and an appropriate amount of urea solution was mixed with the nitrate mixtures. The solutions were stirred at 950 rpm for about 3 hours at a constant temperature of 60 °C to obtain gel-like products. The gels were then transferred into different alumina crucibles and kept inside an electric furnace at 550 °C. Within 2–5 minutes, the combustion process was complete. The whitish products obtained were ground into fine powder and further annealed at 800 °C for 3 hours. The annealed samples were used for the structural and optical analysis.

2.2 Characterization

For the structural study, X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) have been done using Bruker D8 advanced X-ray diffractometer and HITACHI (S-3400N) scanning electron microscope, respectively. To study the presence of impurities in the samples, Fourier transform infrared (FTIR) spectra of the samples were recorded in the 400–4000 cm⁻¹ spectral region on a Perkin Elmer Spectrum RX1 spectrophotometer. The UC emission spectra of the phosphor powders were recorded through a monochromator attached with a photomultiplier tube (PMT) upon excitation with 980 nm continuous wave (CW) diode laser. The lifetime analysis was performed by chopping the 980 nm laser beam by using a mechanical chopper and measurements were recorded with the help of a quick start digital oscilloscope. All the measurements were performed at room temperature.

3. Results and discussion

3.1. XRD analysis

The powder XRD patterns of the Tm³⁺–Yb³⁺; Tm³⁺–Yb³⁺–Zn²⁺ and Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphor samples were obtained between the range of 20 and 90 degrees (Fig. 1). All the XRD patterns have the same baselines and patterns, which shows that the crystal structure and phase of these prepared materials have not been altered due to codoping with rare-earth as well as non-rare-earth elements. A large number of intense diffraction peaks for different 2θ values of ∼26.71°, ∼29.6°, ∼30.55°, ∼39.94°, ∼46.66°, ∼52.72°, ∼55.53°, ∼63.04°, ∼72.88°, ∼75.94°, ∼79.42°, ∼81.37°, ∼84.37°, ∼85.72° corresponding to the diffraction from the (100), (002), (011), (012), (110), (103), (112), (202), (203), (211), (114), (122), (015), (300) planes, respectively, were observed. The observed structure of the prepared phosphors was indexed suitably with JCPDS file no. 74-1144 that shows the hexagonal phase of the La₂O₃ with lattice parameters a = b = 3.93 Å and c = 6.12 Å.

Fig. 1 XRD pattern of (A) JCPDS file no. 74-1144, (B) Tm³⁺–Yb³⁺, (C) Tm³⁺–Yb³⁺–Zn²⁺, and (D) Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphors.

The average crystallite size of synthesized phosphors was calculated using the Debye–Scherrer equation:²⁰

d = 0.89λ/β cos θ (4)

where “d” is the crystallite size, “λ” is the wavelength of X-ray (∼1.5405 Å) used, “β” is the full-width half maxima (FWHM) and “θ” is the diffraction angle. The calculated values of the average crystallite size were found to be ∼13 nm, ∼15 nm and ∼25 nm for La₂O₃:Tm³⁺–Yb³⁺, La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺ and La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ phosphors, respectively.

In addition to the Debye–Scherrer method, the crystallite size of the prepared materials was also estimated with the help of Williamson–Hall analysis which satisfies the equation²⁵

β cos θ = (0.89λ/d) + 4ε sin θ (5)

where “ε” is the strain present in the sample and other symbols have their usual meanings.

The strain present in the material and the crystallite size could be extracted from the slope and the intercept, respectively, of the 4 sin θ versus β cos θ plot. The Williamson–Hall plots for Tm³⁺–Yb³⁺, Tm³⁺–Yb³⁺–Zn²⁺ and Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphors are shown in Fig. 2.

Fig. 2 Williamson–Hall plot for (A) Tm³⁺–Yb³⁺, (B) Tm³⁺–Yb³⁺–Zn²⁺, and (C) Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphors.

The values of the crystallite size as calculated by Williamson–Hall analysis are 15.65 nm, 16.81 nm and 28.29 nm for La₂O₃:Tm³⁺–Yb³⁺, La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺ and La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ phosphors, respectively. Also, by analysing the results it can be concluded that the crystallite size calculated by both methods (Debye–Scherrer Method and Williamson–Hall method) for all three samples are comparable and a slight increase in the crystallite size is observed as we move from La₂O₃:Tm³⁺–Yb³⁺ to La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ via La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺ phosphors. The crystallite size in each case was found to be in the nanometer range, which indicates that the developed phosphors are nanocrystalline in nature.

3.2 FTIR Spectroscopy

The FTIR spectra of the prepared materials annealed at 800 °C temperature were recorded in the 400–4000 cm⁻¹ region (Fig. 3). The presence of different impurities in the prepared materials was traced by matching the vibrational frequencies corresponding to each impurity in the FTIR spectrum. From the FTIR spectrum of the prepared samples it can be concluded that functional groups like –OH, –CH and –NO are present in all the samples. The peaks observed around 3613 cm⁻¹ and 2382 cm⁻¹ are due to asymmetric stretching vibration of O–H. The peaks around 2930 cm⁻¹ and 1475 cm⁻¹ are due to C–H and N–O stretching vibrations, respectively. It is also observed from the FTIR spectrum that the presence of O–H impurity observed around 3613 cm⁻¹ was reduced in the case of La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ phosphor compared to other cases. The peak around 650 cm⁻¹ is due to the lattice vibration of La–O.

Fig. 3 FTIR spectrum of (A) Tm³⁺, (B) Tm³⁺–Yb³⁺, (C) Tm³⁺–Yb³⁺–Zn²⁺, and (D) Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphors.

3.3 SEM Image

The surface morphology and crystallinity of solid host materials are important parameters which determine the emission characteristics of phosphors. The microstructural analysis of La₂O₃:Tm³⁺–Yb³⁺ and La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ samples was performed with the help of SEM examination (Fig. 4). From Fig. 4 it is clear that the aggregation and particle sizes of the La₂O₃:Tm³⁺–Yb³⁺ phosphors increase with codoping Zn²⁺/Mg²⁺ ions. The SEM image shows that the particles are not uniformly distributed throughout the surface, which is mainly caused by the inhomogeneous distribution of temperature during the synthesis of the material by the combustion technique. Also, it is clearly visualized that the particles are in the agglomerated form with their size in the micrometer range.

Fig. 4 SEM images of (A) Tm³⁺–Yb³⁺ codoped La₂O₃ phosphor and (B) Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphor.

3.4 Upconversion emissions study of La₂O₃:Tm³⁺–Yb³⁺ phosphor

The room temperature frequency upconversion spectra of Tm³⁺/Tm³⁺–Yb³⁺ doped/codoped La₂O₃ phosphor powders upon excitation with 980 nm CW diode laser were recorded in the 450–900 nm range (Fig. 5). From Fig. 5, it is clear that no UC emission was observed under a 980 nm diode laser excitation from singly Tm³⁺ doped La₂O₃ phosphor. But in the case of La₂O₃:Tm³⁺–Yb³⁺ phosphor, four UC emission bands were observed around 476 nm, 653 nm, 702 nm and 795 nm corresponding to the ¹G₄ → ³H₆, ¹G₄ → ³F₄, ³F₂ → ³H₆ and ¹G₄ → ³H₅ transitions, respectively.

Fig. 5 The upconversion emission spectra for Tm³⁺/Tm³⁺–Yb³⁺ codoped La₂O₃ phosphors.

To get the optimum UC intensity, the concentration of Yb³⁺ ions was varied from 1.0 to 5.0 mol% with the Tm³⁺ ions concentration fixed at 0.1 mol%. The maximum UC emission intensity was observed for the 0.1 mol% Tm³⁺ + 3.0 mol% Yb³⁺ combination. The light emitted from the phosphor appears blue which could be detectable by the naked eye. The required NIR photons for the origin of these UC bands can be explained with the help of the pump power dependence study.

It is well known that for extremely small upconversion rates the number of photons involved in populating the upconversion emitting level can be calculated directly from the slope of the logarithmic plot of intensity versus pump power. But, as we increase the pump power to a higher value the upconversion luminescence caused by ‘n’ photon absorption is found to obey the relationship I_UC < Pⁿ.²⁶

We know that UC emission intensity (I_UC) is directly proportional to pump excitation power (P) in the low pump power region by the relationship,²⁷

I_UC ∝ P^k (6)

where “k” represents the number of pump photons required to excite the emitting states.

The slopes corresponding to 476 nm and 653 nm bands are calculated to be ∼1.93 and ∼2.08, which shows that both the blue and red emissions are due to two photon absorption processes.

The origin of the UC emission bands can be explained with the help of an energy level diagram as shown in Fig. 6. The UC emissions observed from Tm³⁺–Yb³⁺ codoped La₂O₃ phosphor are due to the cooperative energy transfer (CET) from Yb³⁺ to Tm³⁺ ions only, as no UC emission is observed in the singly doped Tm³⁺:La₂O₃ phosphor. The Yb³⁺ ions present in the ²F_7/2 ground state, after absorption of 980 nm NIR photon, get excited to the ²F_5/2 manifold. In the ²F_5/2 excited state, the excited donor Yb³⁺ ion transfers its excitation energy to the acceptor Yb³⁺ ions in the ²F_5/2 state and these acceptor ions are further excited to the virtual level (V). The energy of the virtual level ‘V’ of Yb³⁺ ions is in resonance with the ¹G₄ (∼21 015 cm⁻¹) level of Tm³⁺ ions. So, the Yb³⁺ ions in the virtual level transfer their excitation energy to the Tm³⁺ ions in ground state. The ground state Tm³⁺ ions then get excited to the ¹G₄ level. Finally, the Tm³⁺ ions in ¹G₄ level transit downward to the ³H₆, ³F₄ (∼5765 cm⁻¹) and ³H₅ (∼8258 cm⁻¹) levels, emitting blue, red and NIR emissions around 476 nm, 653 nm and 795 nm, respectively. The ³H₄ (∼12 556 cm⁻¹) and ³F₂ (∼15 240 cm⁻¹) levels of Tm³⁺ ion are populated by the cross-relaxation energy transfer mechanisms, CR₁ and CR₂ viz. (¹G₄, ³H₆ → ³H₅, ³H₄) and (³H₅, ³H₄ → ³F₄, ³F₂) respectively. After that, the Tm³⁺ ions in the ³F₂ level decay radiatively to the ³H₆ level giving a photon at ∼702 nm corresponding to the ³F₂ → ³H₆ transition. The ³F₃ (∼14 627 cm⁻¹) level is populated by non-radiative relaxation from the ³F₂ level followed by the emission of a phonon. The emission peak intensity ratio between the blue and red emissions was ∼30 and that between the blue to NIR emissions was ∼5. This large UC emission intensity corresponding to the ¹G₄ → ³H₆ transition is basically due to the direct cooperative energy transfer (CET) from the Yb³⁺ ions to the Tm³⁺ ions followed by the absorption of two NIR photons.

Fig. 6 Energy level diagram and energy transfer mechanism in Tm³⁺–Yb³⁺ codoped La₂O₃ phosphors.

3.5 Effect of Zn²⁺ and Mg²⁺ codoping in La₂O₃:Tm³⁺–Yb³⁺ phosphor

From Fig. 7 it is clearly observed that the UC intensity of all the bands is enhanced by the addition of Zn²⁺ and Mg²⁺ ions in the La₂O₃:Tm³⁺–Yb³⁺ phosphor. The variations in UC intensity with different doping concentrations of Zn²⁺ and Mg²⁺ ions were investigated. The enhancement of ∼2 times for the blue UC bands was observed with the codoping of 15.0 mol% Zn²⁺ ions in the Tm³⁺–Yb³⁺ codoped La₂O₃ phosphor. It was also observed that the intensities of UC bands are further increased when the Mg²⁺ ion is incorporated into the Tm³⁺–Yb³⁺–Zn²⁺ codoped La₂O₃ phosphor. The optimum concentration of Mg²⁺ ion is found to be 5.0 mol%. Enhancements of about 36 times, 34 times and 11 times in the intensity of UC peaks at ∼476 nm, ∼653 nm and ∼795 nm have been observed respectively with the codoping of 5.0 mol% Mg²⁺ ions compared to that of Tm³⁺–Yb³⁺ codoped La₂O₃ phosphor. The intensity of upconversion emission bands lying in the blue, red and NIR regions is increased with increasing concentration of Zn²⁺ and Mg²⁺ ions up to 15.0 mol% and 5.0 mol% respectively. The enhancement in the UC intensity is caused by the creation of the oxygen vacancies due to codoping with the Zn²⁺/Mg²⁺ ions in the La₂O₃ phosphor. These oxygen vacancies act as sensitizers for energy transfer due to the mixing of charge transfer states.²⁸ This enhancement in the UC emission intensity may also be explained by the broadening in the absorption bands, followed by the overlap between the defect states produced by the codopants with the rare earth ions charge transfer states,²⁹ which is beyond the scope of the present study.

Fig. 7 The upconversion emission spectra of Tm³⁺–Yb³⁺, Tm³⁺–Yb³⁺–Zn²⁺ and Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphors.

The UC emission peak positions remain unaltered with the codoping of Zn²⁺/Mg²⁺ ions. The non-occurrence of the spectral shift and the presence of a sharp UC emission peak are due to the effect of the shielding of 4f electrons by the outer 5s and 5p orbitals. The UC intensity for all of the bands is quenched beyond the optimum concentration of Zn²⁺ and Mg²⁺ ions. The reduction in the UC intensity is most probably due to imperfect atomic arrangement in the La₂O₃ phosphor. These defects present in solid host material may lead to quenching in the emission intensity.

The ionic radii of La³⁺, Tm³⁺, Yb³⁺, Zn²⁺ and Mg²⁺ are 103 pm, 88 pm, 86.8 pm, 74 pm and 72 pm, respectively. The ionic radii of Tm³⁺, Yb³⁺, Zn²⁺ and Mg²⁺ are smaller than that of La³⁺ (which has largest ionic radius compared to other rare earth ions). Also, there is a valency mismatch between Zn²⁺/Mg²⁺ and La³⁺ ion. Therefore, the excess codoping of Zn²⁺/Mg²⁺ ions can introduce distortion and hence defects into the La₂O₃ host. Thus, when the codoping concentration of Zn²⁺/Mg²⁺ ions is beyond the optimum value, the UC emission intensity is reduced.

Although many researchers have demonstrated that the enhancement in luminescence intensity is caused by Mg²⁺ incorporation, no one has reported such a large enhancement with Mg²⁺ codoping to the best of our knowledge.^24,29,30 An enhancement of about 10 times in the red UC band by codoping of Mg²⁺ ions in CaAl₁₂O₁₉:Er³⁺:Yb³⁺ phosphor has been observed by Singh et al.²⁴ A photoluminescence enhancement by a factor of 1.7 and 1.3 times for the red emission band in Eu³⁺ doped Y₂O₃ phosphor due to codoping with Mg²⁺ and Al³⁺ ions has been observed by Chong et al.²⁹ Pan et al. have reported enhancement of more than 3 times in luminescence efficiency of CaAl₁₂O₁₉:Mn⁴⁺ phosphor due to the codoping of Mg²⁺ ions.³⁰

From the FTIR spectrum (Fig. 3), it is clear that the presence of –OH impurity around 3613 cm⁻¹ which acts as a quencher is reduced for La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ phosphor compared to other samples. This also supports why the UC emission intensity is significantly enhanced in the La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ phosphor. Therefore, such a large enhancement in UC luminescence intensity, as observed in La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺, has made the present material a promising candidate as a good blue upconverter.

Decay curves analysis corresponding to the ¹G₄ → ³H₆ (476 nm) blue transition of Tm³⁺ ions in the La₂O₃:Tm³⁺–Yb³⁺, La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺ and La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ phosphors powder samples was performed (Fig. 8). The calculated values of decay time for the blue emission from the decay curves were found to be ∼1.78 ± 0.47 ms, ∼1.13 ± 0.12 ms, and ∼0.98 ± 0.06 ms for the La₂O₃:Tm³⁺–Yb³⁺, La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺ and La₂O₃:Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ phosphors, respectively. This observed variation in decay times of emitting level also support the increase in radiative transition probability of emitting state due to the incorporation of Zn²⁺ and Mg²⁺ ions into the La₂O₃:Tm³⁺–Yb³⁺ phosphor. The decrease in decay time results in an increase of the radiative transition probability of emitting state. The reason behind this increase in radiative transition probability due to Zn²⁺ and Mg²⁺ ions codoping is the sensitizing effect caused by the creation of oxygen vacancies, as discussed earlier.

Fig. 8 The fluorescence decay curves corresponding to the ¹G₄ → ³H₆ (476 nm) blue transition of thulium ion in (A) Tm³⁺–Yb³⁺, (B) Tm³⁺–Yb³⁺–Zn²⁺ and (C) Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphors.

To compare the colours emitted by the Tm³⁺–Yb³⁺, Tm³⁺–Yb³⁺–Zn²⁺ and Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped phosphors, the CIE colour coordinates corresponding to these three samples at 10.04 W cm⁻² power density were calculated and represented in Fig. 9(a), from which it is clearly visible that all these three phosphors are emitting blue colour but with the incorporation of Zn²⁺ and Mg²⁺ ions the colour is tuned towards the more bluish region. This indicates that the purity of the colour emitted by the sample is improved by codoping with Zn²⁺ and Mg²⁺ ions. In order to check the tunability of the colour emitted from the Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphor, the CIE colour coordinates of the sample were calculated at different excitation power densities as shown in Fig. 9(b), which shows that the present material is an efficient blue emitter. Based on the observed results, we can say that the present phosphor can be used as an efficient NIR to blue upconverter. It is observed from Fig. 9(b) that the blue colour emitted by the sample is independent of the pump power density, so it can be used in the fabrication of display devices. Also, the intense blue light emitted from the Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphor may be used in photodynamic therapy (PDT) for the diagnosis and localization of cancer cells.⁷

Fig. 9 (a) Comparison of CIE colour coordinates corresponding to Tm³⁺–Yb³⁺, Tm³⁺–Yb³⁺–Zn²⁺ and Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphors, (b) CIE color coordinates of Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphors at different pump power densities.

To examine the quality of white light, we calculated the Colour Correlated Temperature (CCT) values at different excitation powers densities using McCany empirical formula³¹ which is expressed as,

CCT = −449n³ + 3525n² − 6823n + 5520.33 (7)

where n = [(x − x_e)/(y − y_e)] is the inverse slope line with x_e = 0.332 and y_e = 0.186.

The calculated values of CCT corresponding to different colour coordinates are presented in Table 1. The colour correlated temperatures for different power densities fall in the region of ∼4171 K to ∼5512 K. So, from the results it can be concluded that the CCT values lie in the cold white light region.

Table 1 The calculated CCT values for different excitation power densities

Power densities (W cm^−2)	CIE (x, y) coordinates		CCT value (K)
	x coordinate	y coordinate
1.32	0.12	0.13	5848.70
1.75	0.12	0.13	5848.70
4.63	0.12	0.12	5093.34
5.91	0.12	0.12	5093.34
7.36	0.12	0.12	5093.34
10.04	0.12	0.11	4170.64
13.10	0.12	0.11	4170.64
16.73	0.12	0.11	4170.64
17.87	0.12	0.11	4170.64

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4. Conclusion

The Tm³⁺–Yb³⁺, Tm³⁺–Yb³⁺–Zn²⁺ and Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphors have been successfully prepared by the combustion method. The XRD patterns of the synthesized materials shows that these phosphors exhibit the hexagonal phase. The crystallite sizes of the materials calculated by Scherrer's formula as well as by the Williamson–Hall analysis show that the phosphor materials are nanocrystalline. The SEM images of the prepared materials reveal agglomerations within the materials with particle size in the micrometer range. The SEM analysis shows an increase in the particle size due to the introduction of Zn²⁺/Mg²⁺ ions in the codoped phosphors. The impurities present in the prepared materials are confirmed by FTIR analysis. The analysis of the upconversion spectra and the decay curve collectively reveals that the maximum enhancement of the blue UC band is observed for the Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphor upon excitation with a 980 nm NIR laser source. The CIE colour coordinates corresponding to the Tm³⁺–Yb³⁺–Zn²⁺–Mg²⁺ codoped La₂O₃ phosphor does not show any significant change with variations in the pump power density. As a result, the present phosphor can be used in making blue light upconverters, display devices and devices for the diagnosis and localization of cancer cells in photodynamic therapy.

Acknowledgements

The authors are thankful to the University Grants Commission (UGC), New Delhi, India and Indian School of Mines, Dhanbad, India for providing financial support.

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