Color tunable luminescence in ThO₂:Er³⁺,Yb³⁺ nanocrystals: a promising new platform for upconversion

Abstract

Lanthanide-doped luminescent nanoparticles are an appealing system for many applications in the area of biomedical, solar cell, thermometry, anti-counterfeiting, etc. due to their sensitivity, reliability, high photochemical stability, and high optical transparency in the visible-NIR range. A color-tunable upconversion-luminescence (UCL) in a new low phonon energy ThO₂ host based on modulating sensitizer concentration has been realized in this work and it may work as a potential candidate to replace corrosive and toxic fluoride based hosts in the future. Er³⁺–Yb³⁺ co-doped thoria nanoparticles were prepared using a gel combustion route and their structural and luminescence properties were determined as a function of the Yb³⁺ concentration. Phonon dispersion measurements have established the dynamic structural stability of the thoria nanoparticles. Density functional theory (DFT) was used to calculate the defect formation energy, highlighting the feasibility of dual ion (Er³⁺ and Yb³⁺) doping in thoria. The morphology and average size of the doped thoria was studied using high resolution transmission electron microscopy (HRTEM), and any defects evolving as a result of aliovalent doping were probed using positron annihilation lifetime spectroscopy (PALS). With 980 nm laser excitation, the nanothoria emits green and near-red light. A significant enhancement of the red-to-green intensity ratio of Er³⁺ ions in nanothoria was observed with an increase in Yb³⁺ concentration which resulted in beautiful color tunability from green to yellow light in going from lower (up to ∼5 mol%) to higher (10 and 15 mol%) Yb³⁺ concentration. The power dependence and the dynamics of the upconverted emission confirm the existence of two-photon upconversion processes for the green and red emissions.

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1. Introduction

Upconversion nanophosphors (UCNPs) are regularly studied in the scientific community owing to their application in many areas encompassing health, energy and the environment.^1–7 Their applicability has been attributed to the conversion of low energy photons to high energy photons exhibiting anti Stoke’s shifts and displaying bright emission, lower autofluorescence, deeper penetration, negligible photobleaching, higher photo stability, etc.^5,8 The three basic requirements to achieve efficient upconversion luminescent materials are dopant, sensitizer and host matrix. In most of the reported works, Er³⁺, Ho³⁺ and Tm³⁺ are used as dopant ions whereas Yb³⁺ is used as the sensitizer whenever the pumping energy is 980 nm.^9,10 The choice of sensitizer can vary depending upon the excitation wavelength in the upconversion process, whereas the dopant can also be changed based on the required energy conversion region. Among the lanthanide ions, the Er³⁺–Yb³⁺ couple is commonly used for realizing efficient upconversion. Fluoride based hosts are considered to be an excellent luminescence host for UC compared to oxides owing to low phonon energy which may lower the non-radiative channels.¹¹ However, a fluoride based matrix is corrosive in nature and handling them in a normal laboratory set-up may be an issue.^2,12 So there is a huge demand within the scientific community working in the area of upconversion phosphors, to look for inorganic host matrices which stringently fulfil the important and necessary criteria: this includes that the host matrix should be thermally/structurally/chemically stable, should possess a wide band gap, easy to synthesize and exhibit low phonon frequency. In that context ThO₂ is considered an excellent luminescence host owing to its stable crystal structure, high chemical stability, single valence state of cation and low phonon frequency.^13–15 The ability to accommodate a large amount of fluorescence active dopant ions in its open calcium fluorite structures (space group Fm3̄m) together with superior chemical and mechanical stability are additional benefits. Thorium oxide has shown tremendous potential in the area of electro and photocatalysis, in solid oxide fuel cells (SOFC), sensor materials, photoluminescence host matrix, etc.^14,16–24 Despite these advantages, there are absolutely no reports exploring thoria as an upconversion luminescence host.

Lin et al. have explored the optical properties of ThO₂ and Eu-doped ThO₂ nanotubes.²⁵ Harvey and his group on the other hand have explored the thermoluminescence (TL) of undoped and doped ThO₂ ²⁶ whereas our group have also worked on Eu, Dy and Tb doped ThO₂ nanoparticles.¹⁶ Kristianpoller et al. have investigated luminescence emitted during X-irradiated (XL) as well as the TL of ThO₂:Ca single crystals.²⁷ Pandey et al. have recently explored bismuth luminescence in thoria and proposed a dual band endowed by oxygen vacancies and Bi³⁺.²¹ Our group has also explored the emission of uranium ions in thoria.²⁸ But all this work deals with the conversion of high energy photons into low energy ones known as downconversion luminescence (DCL) which curtails their applicability in the area of bioimaging owing to lower tissue penetration and harmful effects of UV photons. Here in this work for the first time we are exploring the potential of thoria nanoparticles as an Er³⁺–Yb³⁺ upconversion host. The gel-combustion synthesized²⁹ UCNPs ThO₂:1%Er³⁺,x%Yb³⁺ (x = 1–15) were characterized using powder X-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). With Th⁴⁺ being tetravalent and the dopant and sensitizer being trivalent in nature; we have probed the defects in such materials due to such aliovalent substitution using positron annihilation lifetime spectroscopy (PALS). Density functional theory (DFT) calculations were also carried out to calculate the phonon frequency of thoria and defect formation energy of doping Er³⁺ and co-doping Er³⁺ and Yb³⁺ in ThO₂.

2. Experimental

2.1. Synthesis

Undoped and Er, Yb codoped ThO₂ nanopowders were synthesized using a gel-combustion method using Th(NO₃)₄·5H₂O as the oxidant and urea as the fuel in a 1 : 2 molar ratio using a previous procedure.¹³ For doping with Er³⁺ and Yb³⁺, stock solutions of the respective trivalent ions were prepared by dissolving Er₂O₃ and Yb₂O₃ in conc. HNO₃ and the requisite amounts of the stock solutions were then added to the precursor solution of oxidant and fuel. The schematic depicting the gel-combustion synthesis of Er³⁺, Yb³⁺ co-doped thoria (YET) upconvertible nanoparticles (UCNPs) is shown in Scheme 1.

Scheme 1 Schematic showing gel-combustion synthesis of Er³⁺, Yb³⁺ co-doped thoria UCNPs.

2.2. Instrumentation and computation details

X-ray diffraction (XRD) studies of the powder samples of undoped and doped thoria have been carried out on a benchtop X-ray diffractometer (Proto make) in the 2θ range of 15–80° using Cu Kα as the X-ray source. Rietveld refinement was carried out using FullProf software. Raman spectrum was recorded on Seki's STR300 spectrometer with a laser excitation wavelength of 532 nm. TEM, HRTEM and Selected Area Electron Diffraction (SAED) patterns were taken in a field emission transmission electron microscope of JEOL make (2100F model). Positron annihilation lifetime spectra were measured with powder thoria samples under ambient conditions with a lifetime spectrometer having a time resolution of 175 ps. The spectra were analysed using PALSFit software.³⁰ The UC emission measurements were carried out using an Edinburgh Instruments Fluorimeter of series FLS 1000 which is equipped with both a visible and NIR PMT using a monochromatic diode laser of excitation wavelength 980 nm as the NIR source.

Density of states defect formation energies are calculated using DFT. More details of the calculation methodology are given in section S1 in ESI.†

3. Results and discussion

3.1. Phase analysis, density of states calculations and phonon dispersion

Fig. 1a shows the XRD pattern of undoped and ThO₂:1%Er³⁺,x%Yb³⁺ (x = 1–15). The entire XRD pattern completely matches with the standard pattern reported for thoria with JCPDS-01-071-6407. None of the samples depicted any impurity peaks suggesting that dopants are occupying the lattice Th⁴⁺ site. The peaks were marginally shifted to a higher angle with doping. The lattice constants have been calculated via Rietveld analysis. The typical Rietveld fitted spectra are shown in Fig. 1b. All the Rietveld fitted spectra and corresponding data are shown in Fig. S1 and Table S1 of ESI.† The lattice parameters obtained are shown in the Fig. 1c and the reduction in unit cell lattice parameter with doping is in line with the ionic radii of the dopants being smaller than Th⁴⁺. Further, the crystallite sizes were calculated using the Debye Scherrer equation (see S2 of ESI,† for details) and are listed in Table 1. All the samples are nano sized and smaller than 25 nm. But interestingly crystallite size decreases monotonically from ∼24.4 to 11.8 nm with an increase in doping suggesting hindrance to the crystallite growth. This is also in sync with TEM results where the average particle size at 1% Yb³⁺ doping was 51 nm and it reduces to 28 nm when Yb³⁺ doping is raised to 15%.

Fig. 1 (a) XRD pattern of ThO₂:1%Er³⁺,x%Yb³⁺ nanophosphor (x = 1, 2, 5, 10 and 15), (b) typical Rietveld fitting of the spectrum, (c) variation of lattice parameters with doping.

Table 1 Crystallite size of nanostructure ThO₂:1%Er³⁺,x%Yb³⁺ (x = 1–15)

Dopant concentration in ThO2 sample	Crystallite size (nm)
1%Er+1%Yb	24.4
1%Er+2%Yb	14.4
1%Er+5%Yb	13.2
1%Er+10%Yb	12.2
1%Er+15%Yb	11.8

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All the DFT calculations have been carried out using the fluorite type structure of ThO₂ (space group: Fm3̄m, No. 225) (Fig. S2, ESI†). The calculated cell parameter was found to be 5.6158 Å, which is very close to the experimentally reported value.^31,32 The calculated band gap under the HSE level of calculation is found to be 6 eV, which is in excellent agreement with the experimentally observed value.³³ This justifies the choice of the present computational parameters. Fig. 2a shows the density of states (DOS) of ThO₂, which indicates that the valence band maximum (VBM) is dominated by O p states, with a small contribution from Th d and f states. On the other hand, the conduction band minimum (CBM) is contributed by Th f states along with a small contribution from the Th d and O p states. Fig. 2b shows the phonon dispersion curve for ThO₂ calculated within the GGA level along the high symmetric directions of Γ_X _W_Γ_L. It can be observed that no imaginary frequencies were obtained, indicating that the ThO₂ structure is dynamically stable.

Fig. 2 (a) Density of states of ThO₂. The vertical dashed line indicates the Fermi level. (b) Phonon dispersion curve for ThO₂ calculated within the GGA level where the frequency is in THz.

3.2. Doping Efficacy of Yb³⁺ and Er³⁺ ion in ThO₂

Now we proceed to investigate the role of doping with Yb and Er into ThO₂. To model the (Er,Yb)-doped ThO₂, we have chosen a 96 atoms cell, and replaced one of the Th atoms with Er/Yb. For (Er,Yb)-doped ThO₂, we have considered two different structures by varying the relative distance between Yb and Er. Both the structures are energetically close to each other. The defect formation energies have been calculated using the mathematical relationship:^34,35

E_formation = E_defect − E_perfect + q∑n_Xμ_X (1)

where, E_defect and E_perfect indicate the energies of the doped and perfect ThO₂, respectively, calculated with a similar cell size, μ_X is the chemical potential of the element X and n_X indicates the number of elements added (q = −1) or removed (q = +1) to generate the doped system. The variation of defect formation energies for the doping of Yb/Er into ThO₂ as a function of host chemical potential are shown in the Fig. 3. It can be observed that doping of both Yb and Er is more favorable in Th-poor condition. Interestingly, the defect formation energy is found to be reduced significantly for the cases of (Er,Yb)-doped ThO₂ under Th-poor condition. This indicates that the formation of the (Er,Yb)-doped ThO₂ is energetically more feasible (under Th-poor condition) than the doping with individual elements.

Fig. 3 Variation of defect formation energies for the doped ThO₂.

3.3. Transmission electron microscopy

TEM images of the representative ThO₂:1%Er³⁺,x%Yb³⁺ (x = 1, 5 and 15) are shown respectively in Fig. 4a, d and g. The formation of nanoparticles is clearly seen from these TEM images though the particles are slightly aggregated. This could be due to the large heat of combustion generated during the combustion reaction involving precursors and the fuel. We have further calculated the average size of these nanoparticles using ImageJ software and the values are 51, 47 and 28 nm respectively for 1, 5 and 15% Yb³⁺ doping. Reduction in crystallite size with higher Yb³⁺ doping was also evident from the XRD as discussed earlier. The HRTEM images of ThO₂:1%Er³⁺,x%Yb³⁺ (x = 1, 5 and 15) are shown respectively in Fig. 4b, e and h which clearly show the lattice fringes. The interplanar distance was obtained as 0.320 and 0.281 nm which corresponds to the (111) and (200) crystal planes in fluorite structured ThO₂.¹⁸ The selected area electron diffraction (SAED) patterns shown respectively in Fig. 4c, f and i, confirm the formation of cubic phase ThO₂, and the ring patterns depict the high crystallinity of doped thoria nanoparticles. The crystal planes corresponding to the diffraction spots are marked in the figure.¹⁵

Fig. 4 (a), (d) and (g) TEM images, (b), (e) and (h) HRTEM images and (c), (f) and (i) SAED patterns of ThO₂:1%Er³⁺,X%Yb³⁺ (X = 1, 5 and 15) UCNPs.

3.4. Raman spectroscopy

Fig. 5 shows the Raman spectra of ThO₂:1%Er³⁺,x%Yb³⁺ (x = 1–15) UCNPs. For pure thoria, a single peak in the range 460–465 cm⁻¹ corresponds to the T_2g stoichiometric fluorite structure of ThO₂, and is well reported.¹³ In the present case, this peak is at 465 cm⁻¹ in the samples with lower dopant concentration and is marginally red shifted when the dopant concentration is very high. Such a shift with dopants of different ionic radii is known in other fluorite structures like CeO₂. The other bands are mostly due to defects induced by doping with trivalent lanthanides which differ in size as well as charge, that leads to charge compensating vacancies. The Raman peaks are broader in samples with higher dopant concentration showing the increased disorder in the system. The band around 500 cm⁻¹ might be from oxygen vacancies. In general, the band from oxygen vacancies in thoria is reported in the range 550–580 cm⁻¹ and here, though speculative, it appears that the oxygen vacancies are associated with the trivalent dopants more and might form RE-Vo-RE type vacancy complexes. The Raman peak around 625 cm⁻¹ is due to REO₈ type complexes whose intensity relative to the peak at 500 cm⁻¹ is reduced, due to enhanced oxygen vacancy formation.³⁶ The other bands are similar to those reported in the defect induced Raman spectra of ThO₂ and CeO₂.^37,38

Fig. 5 Raman spectra of ThO₂:1%Er³⁺,x%Yb³⁺ nanophosphor (x = 1, 2, 5, 10 and 15).

3.5. Positron annihilation lifetime spectroscopy

The positron lifetime spectra was fitted to three lifetime components as depicted in Fig. 6A and the intensity of the third component was less than 0.5%. For the other two lifetimes, the corresponding intensities and the average lifetime are given in Fig. 6B. The first positron lifetime arises due to positron annihilation in the bulk. The first positron lifetime (τ₁) in undoped thorium oxide is 192 ps. A positron lifetime of 184 ps with 88% intensity was reported by Upadhyaya et al.³⁹ in sintered thoria. The lifetime is marginally higher in the present case and the contribution to this lifetime from annihilations in oxygen vacancies could be responsible for this. The second lifetime component is due to vacancy clusters near the surface. Upon initial doping with Er and Yb, the intensity of the second positron lifetime component drastically increased from 15% to 26% showing the formation of vacancy defects near the surfaces. The XRD patterns suggest reduced crystallinity in the doped samples and this would facilitate the positron reaching the surface of the crystallites increasing this intensity. With higher doping of trivalent lanthanides, the first positron lifetime component increased and the fraction of positrons reaching the surface states reduced due to the enhanced concentration of isolated oxygen in the bulk as can be seen from Fig. 6B and also supported by the increased intensity of the 500 cm⁻¹ peak in the Raman spectra. The magnitude of the second lifetime suggests that the size of the vacancy cluster increased, or with higher doping, the number of oxygen vacancies agglomerating near the surface increased.

Fig. 6 (A) Typical fitting of PAL spectrum showing extraction of the positron lifetimes (B) positron lifetimes and intensities in doped thoria samples, the data corresponding to undoped thoria are shown at x-coordinate of ‘0’.

3.6. Upconversion photoluminescence spectroscopy

The UC emission spectra in the visible region for a series of ThO₂:1%Er³⁺,x%Yb³⁺ (x = 1, 2, 5, 10 and 15 mol%) samples under 980 nm diode laser pumping is shown in Fig. 7a. The emission spectra depict an intense green (530–550 nm) and red (670 nm) emission bands, with the former having relatively higher intensity than the latter up to 5 mol% Yb³⁺ doping, and situation reverses when sensitizer doping is 10 and 15%. This is explained in detail later in this section. These peaks are ascribed to ²H_11/2 → ⁴I1_5/2 (∼530 nm), ⁴S_3/2 → ⁴I1_5/2 (∼550 nm) and ⁴F_9/2 → ⁴I1_5/2 (∼670 nm) transitions of the trivalent erbium ion. An enlarged version of the 5–10% Yb³⁺ co-doped samples UC spectra is shown in Fig. 7b for better visualization. Severe stark splitting of these electronic levels is evident from the UC emission spectra which could be attributed to the crystalline local field of the cubic thorium oxide with defect fluorite structure. The visible emission here on absorption of 980 nm photons, arises through energy transfer from the Yb³⁺ (sensitizer) to the Er³⁺ (dopant) promoting electrons from the ground to the various excited states. Also, the emission intensity of the UC phosphor was investigated with varying Yb³⁺ ion concentration wherein the Er³⁺ concentration was fixed at 1 mol%, and the concentration of the Yb³⁺ ion was changed from 1 to 15 mol%. But to our surprise at 2.0 mol% concentration, quenching happened. This can be seen from Fig. 7c which clearly depicts the monotonic reduction in overall upconversion intensity (for both green and red bands) as a function of Yb³⁺ concentration. This could be understood by taking into account two phenomenon which are happening beyond 1.0 mol%: (A) at higher concentration of Yb³⁺ ions, the large photon flux of 980 nm wavelength will be absorbed and more energy transferred by excited photons; and (B) at higher Yb³⁺ doping levels it can act as a trapping level and dissipate energy through non-radiative pathways. So here the concentration quenching happening beyond 1.0 mol% of Yb³⁺ ions is ascribed to process (B) which may be more dominant than process A. The other possible reason could be higher density of creation of charge, compensating the defects from aliovalent substitution of Yb³⁺ on Th⁴⁺ site in the form of oxygen vacancies which may provide additional pathways for non-radiative transitions and degrade the upconversion intensity. This proportionally increases with increasing Yb³⁺ concentration and the formation of oxygen vacancies is well supported by the positron annihilation lifetime spectroscopy results.

Fig. 7 (a) UC emission spectra, (b) magnified UC spectra of ThO₂:1%Er³⁺,x%Yb³⁺ nanophosphor with x = 5, 10 and 15 and (c) variation of red and green emission intensity with Yb³⁺ concentrations, ThO₂:1%Er³⁺,x%Yb³⁺ nanophosphor (x = 1, 2, 5, 10 and 15).

The red to green ratio (R/G) is very informative in terms of designing color tunable UC phosphors. Here in this case we have also calculated the R/G ratio at different Yb³⁺ concentrations and plotted the same in Fig. 8a. Interestingly it was seen that in the lower concentration domain of the Yb³⁺ ion of 1–5%; the green intensity was overpowering the red emission (red and black UC spectra in the lower left side inset) whereas at 10–15%; the red intensity is much higher than the green intensity (green and pink UC spectra in the top right side inset).

Fig. 8 (a) Variation of red to green emission ratio. Insets show UC spectra in the low and high concentration regions of Yb³⁺ sensitizer, and (b) color coordinate diagram depicting variation in CIE values with Yb³⁺ concentrations of ThO₂:1%Er³⁺,X%Yb³⁺ nanophosphor (x = 1, 2, 5, 10 and 15).

Based on earlier publications on phosphor with lower doping levels of sensitizer; Yb³⁺ ions find it difficult to substantially populate ⁴I_11/2 and ⁴F_7/2 energy levels,⁴⁰ therefore, the excited Er³⁺ ions occupying ⁴F_7/2 level will prefer to relax to the mixed ²H_11/2 + ⁴S_3/2 states leading to more green emission than the red ones. This can be clearly seen from Fig. 8a in the R/G plot as well as UC spectral plots. However, when Yb³⁺ concentrations exceed 5.0 mol%, the mechanism of UC emission changes. At higher sensitizer doping, the Yb³⁺ → Er³⁺ energy transfer process becomes more facile to populate the excited levels of trivalent erbium ions, providing an additional pathway for cross relaxation (CR1) to populate preferably the ⁴F_9/2 level associated with the red UC band. This CR1 pathways would be: Er³⁺ (⁴F_7/2) + Er³⁺ (⁴I_11/2) → Er³⁺ (⁴F_9/2) + Er³⁺ (⁴F_9/2)⁴¹ which would lead to enhanced red UC emission with respect to green emission bands in thoria nanoparticles.

The beautiful color tuning in ThO₂:1%Er³⁺,x%Yb³⁺ nanophosphor (x = 1, 2, 5, 10 and 15) as a function of Yb³⁺ concentration is also perceived in the value of color coordinates evaluated and mentioned in Table S2 (ESI†). It can be seen from this table that the values are quite different in the lower Yb³⁺ concentration regime (1–5%) and higher Yb³⁺ concentration (10 and 15%). The chromaticity diagram of these nanophosphors at different Yb³⁺ concentrations is shown in Fig. 8b. It can be clearly seen that color is tuned from green in the lower Yb³⁺ concentration regime (1–5%) to yellow at Yb³⁺ concentration (10 and 15%).

3.7. Pump power dependent upconversion

For studying the UC mechanism involved in the materials, the pump power dependence on two of the representative samples ThO₂:1%Er³⁺,1%Yb³⁺ and ThO₂:1%Er³⁺,15%Yb³⁺ has been carried out. The number of photons involved in the UC emission was obtained from the following relation:⁶

I_UC ∝ Pⁿ_Ex (2)

where I_UC represents the intensity of UCL, P_Ex is the excitation power and n is the number of photons involved in the UC process. Fig. 9a shows the UC emission spectra of ThO₂:1%Er³⁺,1%Yb³⁺ with different laser power starting from 310 to 2820 mW. With an increase in laser power the emission intensity is also increased up to 2200 mW. It can be seen that green emission is dominant compared to red emission in this particular composition of Yb and Er at all laser powers. Fig. 9b shows the UC emission spectra of ThO₂:1%Er³⁺,15%Yb³⁺ with different laser power starting from 310 to 2820 mW and here the red emission is dominant over the green emission. This can be explained on the basis of the cross relaxation mechanism ⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2 which increases the ⁴F_9/2 level population,⁴² as discussed earlier. With an increase in the laser power the emission intensity enhanced up to 651 mW, after which a reduction in intensity was observed. The reduction in UC emission intensity at elevated pump power is ascribed to laser induced heating which enhances the multiphonon relaxation rate and causes a reduction in the radiative transitions.⁴³

Fig. 9 UC emission spectra of (a) ThO₂:1%Er³⁺,1%Yb³⁺, (b) ThO₂:1%Er³⁺,15%Yb³⁺ and (c) dependence of pump power on green and red emission in ThO₂:1%Er³⁺,1%Yb³⁺.

The log–log plot of laser excitation power and integrated emission intensity is shown in Fig. 9c. It can be seen that the number of photons (n) involved in the UC process is obtained as 1.52 and 1.66 for green and red emission which is much higher than 1, so the upconversion in thoria nanoparticles is a biphotonic process. In this two-photon UC, the green emitting levels ²H_11/2 and ⁴S_3/2, are populated through the energy transfer upconversion mechanism, whereas the red emitting state, ⁴F_9/2, is subsequently populated via²H_11/2, ⁴S_3/2 → ⁴F_9/2 multiphonon relaxation. The decrease in the slope value at higher laser power is also explained by Nascimento et al.⁴⁴ Due to competition between linear decay and the UC process, saturation effects dominate at higher laser power which reflects in the reduction of slope value.

Fig. 10 represents the decay curve of ThO₂:1%Er³⁺,x%Yb³⁺ (x = 1, 3, 5, 10 and 15) samples with an excitation wavelength of 980 nm and emission wavelength of 676 nm. The decay follows a bi-exponential decay as in the following equation:⁴⁵

I = A₀ + A₁ exp(−t/T₁) + A₂ exp(−t/T₂) (3)

where T₁ and T₂ are the lifetime of the fast and slowly decaying components, respectively, A₁ and A₂ are the fitting parameters and A₀ is the background or zero-offset. The fitted lifetime values are shown in Table S3 of ESI.† Inset of Fig. 10 shows the variation in average fluorescence lifetimes with Yb concentration. It can be seen that with increasing concentration of Yb³⁺ from 1.0 to 2.0 mol% the lifetime value also increases. With further increase in the concentration of Yb³⁺ beyond 2.0 mol% the lifetime value reduces. This suggests that the UC luminescence lifetime of the selected red emissions exhibit a decreasing trend with the increase in sensitizer concentration beyond 2.0 mol%. This photophysical process indicated that increasing Yb³⁺ concentration may invoke an additional mechanism to quench ⁴S_3/2 and ⁴F_9/2 transition emission. This observation can be explained on the basis of back energy transfer (BET) from Er³⁺ to Yb³⁺. After excitation, the ⁴F_7/2 and ²H_11/2 of Er³⁺ can transfer energy to Yb³⁺ ions in the ground state and on increasing the concentration of Yb³⁺, the back energy transfer become stronger and leads to the reduction in the lifetime value.⁴⁰

Fig. 10 UC luminescence decay profile of ThO₂:1%Er³⁺,x%Yb³⁺ nanophosphor (x = 1, 2, 5, 10 and 15). Inset shows the variation in average fluorescence lifetimes with Yb doping.

4. Conclusion

A color-tunable upconversion-luminescence in a new low phonon energy ThO₂ host based on modulating sensitizer concentration has been realized in this work. The high dynamic stability of the thoria structure is validated using phonon dispersion calculation. ThO₂ doped with Er³⁺ and varying concentrations of Yb³⁺ ions have been synthesized using the gel-combustions route. DFT calculated defect formation energy shows that doping with both these ions is highly energetically favourable in thoria. The phase purity and the crystal structures have been characterized by powder XRD and Raman spectroscopy. TEM images show the formation of spherical nanoparticles in the size domain ∼20–50 nm. Raman spectroscopy and PALS have suggested enhanced oxygen vacancy formation at higher Yb³⁺ concentrations which lead to a reduction in UCL intensity at concentrations higher than 1% Yb³⁺. The oxygen vacancies are associated with the trivalent dopants more and might form RE-Vo-RE type vacancy complexes. With 980 nm laser excitation, color-tunable UC luminescence from green to yellow was easily achieved by tuning the doping concentration of Yb³⁺ from 1.0 to 15% in the system ThO₂:1%Er³⁺,x%Yb³⁺. Based on the UCL dependence on the laser power, both the green and red emission band could be attributed to two-phonon absorption. Our work suggests that thoria can be utilized for other upconverting rare earth ions with improved color hue and brightness and can be potential materials for UC based white lighting and optoelectronic devices.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors would like to thank Achal S. Kumar from IIT Guwahati for his help in TEM measurements and Dr Rajesh V. Pai from FCD, BARC for providing thorium nitrate stock solutions. The research was funded by the Government of India through the Department of Atomic Energy, India.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cp00199k

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