Stark sublevels in Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ nanophosphor for multifunctional applications

Abstract

The phase and crystal structure of the Na₂Y₂B₂O₇:Tm³⁺–Yb³⁺ inorganic phosphor prepared by solution combustion method have been identified by powder X-ray diffraction technique. Surface morphology and particle size have been examined using field emission scanning electron microscopy and high resolution transmission electron microscopy characterizations of the prepared materials. No absorption band around 980 nm has been observed in the Tm³⁺ doped phosphors, whereas a broad band around 980 nm in the Tm³⁺–Yb³⁺ codoped phosphors corresponding to the ²F_7/2 ← ²F_5/2 absorption transition of Yb³⁺ ion has been detected. Upconversion emission bands have been observed in the UV, visible and NIR regions upon excitation with a 980 nm laser diode. The temperature sensing behaviour and the concept of a nanoheater of the developed nanophosphor have been demonstrated by using the stark sublevels of the ¹G₄ level of the Tm³⁺ ion, which are responsible for the blue upconversion emission. A maximum sensor sensitivity of about 4.54 × 10⁻³ K⁻¹ at 300 K for the developed multifunctional nanophosphor has been determined. A temperature gain of about ∼435 K has been observed at a laser power density of 66.88 W cm⁻² and the colour coordinates do not change with the variation of the pump power density. For localizing and heating hyperthermia based cancer cells using NIR radiation, a very low pump power density of about ∼7.0 W cm⁻² has been established. The experimental observations prove that the developed material can be used as a multifunctional nanomaterial in optical devices and biological applications.

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1. Introduction

Phosphors doped with rare earth (RE) ions are very attractive for many applications, namely in the fields of display devices, temperature sensors, optical nanoheaters, for solar cell efficiency enhancement, bio-imaging, fingerprint detection.^1–6 For preparation of good phosphors (luminescent materials) doped with RE ions, the selection of the host material is a very crucial factor to be considered. Therefore, much effort has to be spent on the synthesis and characterization of the solid materials. Researchers are still looking for suitable luminescent materials for different applications based on upconversion. The photon upconversion (UC) is basically a nonlinear optical effect in which two or more low energy (NIR) photons are used to generate a high energy photon.⁷ For the growth of good quality UC based phosphor materials, researchers have tried to incorporate different combinations of RE ions in suitable hosts.^8–11 The complex inorganic phosphors are capable of producing efficient luminescence and creating a new field for spectroscopic based various optical studies.^9,12 Recently, attention has been attracted towards the new complex inorganic host Na₂Y₂B₂O₇ for the development of RE doped phosphor materials. Na₂Y₂B₂O₇ is a low phonon frequency (∼500 cm⁻¹) host having an iso-structure to the Na₂Gd₂B₂O₇ phosphor^11,13. Additionally, Na₂Y₂B₂O₇ facilitates all the triply ionized RE ions for the doping purposes and has been used as an excellent luminescence producing host.¹⁴ There is a tremendous demand for getting improved luminescence in RE doped phosphors based on the choice of activator and a suitable sensitizer. Among the sensitizers, the ytterbium ion serves as an admirable and most effective sensitizer in various oxide and halide based host materials.^15–17

In recent years, the optical temperature sensing study by using the RE doped phosphors with the help of thermally coupled levels has become a very interesting objective. The radiation emitted from the thermally coupled levels upon excitation with suitable wavelength can be used to sense the temperature accurately. According to Boltzmann distribution law the temperature can affect the population of certain level as well as the upper energy levels. The fluorescence intensity ratio (FIR) and fluorescence lifetime (FL), techniques are the most excellent promising approach for temperature sensing.¹⁸ The fluorescence intensities arising from two thermally coupled energy levels from the similar RE ions can be used in optical thermometry. Since any small change in the fluorescence intensity arising due to the transition from higher level to lower level depends upon the population of particular level and hence the rate of spontaneous emission transition. Therefore, in the case of thermometric measurement the FIR technique is applicable, as the population of individual two close thermally coupled levels is directly proportional to the total population.¹⁹

The temperature sensing behaviour by using the FIR technique in a group of RE ions viz. Ho³⁺/Yb³⁺, Tm³⁺/Yb³⁺, Er³⁺/Yb³⁺ with thermally coupled levels have been reported by several workers.^1,20,21 Not only the two closely spaced thermally coupled levels but it has been possible to apply the FIR technique for stark sublevels of a level to monitor the temperature.^20,22 When the RE ion is incorporated in a host material the energy level of the rare earth ion is generally splitted into number of sublevels due to the ligand field/crystal field. These sublevels are known as the stark sublevels. The current challenges in the optical temperature sensing are to synthesize the luminescent materials by using better and cost effective chemical approaches. The developed material should work in a wide temperature range with higher temperature sensitivity and high accuracy. In case of the thermally coupled energy levels the FIR is independent of the excitation power fluctuations, spectrum losses, and electromagnetic compatibility problem, etc.^23,24 In the FIR technique, the optical temperature sensors utilize the light signals coming at a distance from the objects or sources to measure the temperature.²³

The novel materials that could be useful to generate heat in their nano or micro scale structure, known as ‘optical nanoheater’, on the excitation of NIR laser sources have been reported by different researchers, which are promising in hyperthermia, drug delivery and cancer treatment applications.^21,25–27 The NIR excitation has some advantages over other excitation sources such as small light scattering, less damaging to cell, deep penetration capability, high signal to noise ratio for biological studies and clinical applications.^1,28 To the best of our knowledge no one has reported the frequency upconversion, temperature sensing and optical nanoheater based on the stark sublevels of the ¹G₄ level of Tm³⁺ ion responsible for blue UC emission band in the Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor till date.

The present work describes the synthesis and characterization of the Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor prepared by solution combustion method. The effect of codoping with Yb³⁺ ions on the upconversion emissions from the Na₂Y₂B₂O₇:Tm³⁺ phosphor upon excitation at 980 nm has been studied. The temperature sensing behaviour and optical heating produced on increasing the excitation power density by using the fluorescence intensity ratio technique has been reported. The effect of pump power density on the colour emitted from the codoped phosphor has also been demonstrated.

2. Materials and method

2.1. Phosphor preparation

The Tm³⁺/Tm³⁺–Yb³⁺ doped/codoped Na₂Y₂B₂O₇ phosphors have been synthesized by using the solution combustion method. The starting raw materials, namely, Y₂O₃, H₃BO₃, Na₂CO₃, Tm₂O₃ and Yb₂O₃ (all 99.9% pure) were taken. The inorganic Na₂Y₂B₂O₇ phosphors were prepared according to the following chemical reactions,

2Y₂O₃ + 8HNO₃ → 4Y(NO₃)₂ + 4H₂O + O₂↑

4H₃BO₃ + 8HNO₃ → 4B(NO₃)₂ + 10H₂O + O₂↑

Na₂CO₃ + 2HNO₃ → 2NaNO₃ + CO₂↑ + H₂O

4Y(NO₃)₂ + 4B(NO₃)₂ + 4NaNO₃ → 2Na₂Y₂B₂O₇ + 20NO₂↑ + 3O₂↑

The compositional equations used for the synthesis of the series of samples were as follows,

(100 − x)Na₂Y₂B₂O₇ + xTm₂O₃

where x = 0.1, 0.4, 0.8, 1.2 wt%

and (100 − x − y)Na₂Y₂B₂O₇ + xTm₂O₃ + yYb₂O₃

where x = 0.4 wt% and y = 0.5, 1.0, 2.0, 3.0, 4.0 wt%.

The nitrates of all the oxides were obtained by dissolving the raw materials in concentrate nitric acid (HNO₃). The formed nitrates obtained by the stoichiometric amounts of the precursor materials were mixed with urea as reducing fuel in the 1 : 3 ratio. The mixture (nitrate solution + urea solution) was stirred for 3 hours at 1000 R.P.M at 70 °C. After continuous stirring of the mixtures a transparent homogeneous gel was formed. The formed gel was then transferred into an alumina crucible and placed inside an electrical furnace preheated at 650 °C. The combustion took place and finally dry fluffy mass like as-synthesize samples were produced. The as-synthesized samples were taken out and grinded by using an agate mortar to get uniform fine power. Finally, the as-synthesized samples were annealed at high temperature at 800 °C for three hours. These annealed samples were further used for all the measurement purposes.

2.2. Measurements and characterizations

The X-ray diffraction (XRD) pattern of the optimized Tm³⁺/Tm³⁺–Yb³⁺ doped/codoped Na₂Y₂B₂O₇ phosphors have been recorded in the range of 5° to 90° (2θ degree) by using X-ray diffractrometer. Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) characterizations have been done to have the clear information about the morphology and particle size of the resulting phosphors. The absorption spectra of the prepared phosphors have been recorded in the 300–1300 nm range. The UC emission studies have been done by using 980 nm continuous wave (CW) diode laser excitation source and a monochromator having photomultiplier tube (PMT) attached with a personal computer. Temperature dependent UC emission intensity measurements have been performed with small square shaped homemade furnace and the temperature produced inside the furnace was monitored by holding a copper thermocouple. The temperature of the sample was measured at distance of ∼2 mm apart from the laser irradiated focal point. Diode laser focal spot size (area ∼1.54 mm²) and thickness (∼0.5 mm) of the samples were kept fixed in all the conditions. All the measurements were performed at ∼27 °C temperature.

3. Results and discussion

3.1. XRD study

Fig. 1 shows the XRD pattern of optimized Tm³⁺/Tm³⁺–Yb³⁺ doped/codoped Na₂Y₂B₂O₇ phosphor. The XRD pattern confirms that the prepared phosphor is of monoclinic phase with space group P121/C and cell parameters, a = 10.5969 Å, b = 6.2291 Å, c = 10.2216 Å; α = γ = 90°, β = 117.75°; V = 597.1022 ų.¹¹ From the figure, XRD pattern matches well with the ICSD Collection Code no. 245226 and no any additional peak has been found, which clearly indicates that the dopants (Tm³⁺ and Tm³⁺–Yb³⁺) are successfully introduced in the host lattice.^11,13 The average crystallite size for doped/codoped Na₂Y₂B₂O₇ phosphor has been calculated from the XRD peaks analysis by using well known Debye Scherrer's formula.⁸ The average crystallite size for both the phosphors is found around ∼18 nm and ∼20 nm respectively.

Fig. 1 XRD pattern of Tm³⁺/Tm³⁺–Yb³⁺ doped/codoped Na₂Y₂B₂O₇ phosphor.

3.2. FE-SEM and TEM analysis

The FE-SEM characterizations for the developed phosphors have been performed. Fig. 2 shows the FE-SEM micrograph of the optimized Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor. The surface morphology of the prepared phosphor tells that the large numbers of small particles are in agglomerated structure having size in nanometre range. The inhomogeneous distribution of the particles is due to the unbalanced flow of temperature and mass in combustion flame during the combustion synthesis process.¹ To get a clear view about the size of the particles, TEM and HRTEM images of the Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphors have been taken. Fig. 3(a) and (b) shows the low magnification TEM images with different resolutions. The HRTEM image of the Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor is shown in Fig. 3(c). The interplanar spacing has been calculated with the help of sensible lattice fringes observed in the HRTEM image. The interplanar spacing is found to be 0.295 nm, which corresponds to the (120) plane of monoclinic Na₂Y₂B₂O₇ nanoparticle (ICSD Collection Code no. 245226). The average size of the particles is observed to be ∼20 ± 5 nm which is reasonably in good agreement with the XRD study. Fig. 3(d) represents the electron diffraction pattern of the prepared Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor. The detectable circular rings clearly identify the poly-nanocrystalline nature of the prepared phosphor.

Fig. 2 FE-SEM micrograph of the Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor.

Fig. 3 Transmission electron microscopy images at (a) 100 nm resolution (b) 20 nm resolution (c) high resolution transmission electron microscopy image and (d) electron diffraction pattern of the Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor.

3.3. Absorption study

The electronic absorption spectra of the Tm³⁺/Tm³⁺–Yb³⁺ doped/codoped Na₂Y₂B₂O₇ phosphors have been recorded. Four bands around ∼460 nm, ∼684 nm, ∼789 nm and ∼1208 nm corresponding to the transitions from the ³H₆ (ground state) to different excited states namely ¹G₄, ³F₃, ³H₄ and ³H₅ for Tm³⁺ doped Na₂Y₂B₂O₇ phosphors have been observed in the 300–1300 nm wavelength range, whereas for the Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphors one extra band around ∼980 nm along with all the above said absorption bands have been detected (Fig. 4). In the case of Tm³⁺ singly doped phosphor no absorption at 980 nm is observed. The band around 980 nm is due to the ²F_7/2 ← ²F_5/2 absorption transition of Yb³⁺ ion.

Fig. 4 The absorption spectra for Tm³⁺/Tm³⁺–Yb³⁺ doped/codoped Na₂Y₂B₂O₇ phosphor in 300–1300 nm range.

3.4. Upconversion emission study

The UC emission spectra for Tm³⁺/Tm³⁺–Yb³⁺ doped/codoped Na₂Y₂B₂O₇ phosphors in the range of 400–900 nm range have been recorded by 980 nm CW diode laser excitation. A comparison of UC emission bands for doped/codoped Na₂Y₂B₂O₇ phosphor has been shown in Fig. 5. Three leading UC emission bands for the ¹G₄ → ³H₆ (488 nm), ¹G₄ → ³F₄ (655 nm) and ³H₄ → ³H₆ (814 nm) transitions have been monitored. From this figure, it is clearly observed that the codoping of Yb³⁺ ions enhanced the UC emission intensity significantly. In addition to these UC emission bands some others bands in the Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor are also observed around ∼264 nm, ∼298 nm, and ∼362 nm due to the ³P₂ → ³H₆, ¹I₆ → ³H₆ and ¹D₂ → ³H₆ transitions respectively (Fig. 5). No UC emission band in the UV region was detected for the singly Tm³⁺ doped Na₂Y₂B₂O₇ phosphor. The appearance of UC emission bands in the UV region emitted from the higher levels of Tm³⁺ in case of the codoped phosphor is basically due to the strong energy transfer from the Yb³⁺ to Tm³⁺ ions.

Fig. 5 Comparison of UC emission spectra for Tm³⁺/Tm³⁺–Yb³⁺ doped/codoped Na₂Y₂B₂O₇ phosphor in 400–900 nm range. The inset shows the UC emission spectra for Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor in 200–400 nm range.

For the optimization of dopant ions concentration a series of Tm³⁺/Tm³⁺–Yb³⁺ doped/codoped Na₂Y₂B₂O₇ phosphors have been prepared by varying the rare earths ions concentration. The graph of UC emission intensity versus dopants concentration has been shown in Fig. 6. The optimum concentrations for the Tm³⁺ and Tm³⁺–Yb³⁺ in the doped/codoped Na₂Y₂B₂O₇ phosphors are noted to be 0.4 wt% (Tm³⁺) and 0.4 wt% (Tm³⁺)–2 wt% (Yb³⁺) respectively (Fig. 6). On further increasing the dopants ions concentration above the optimum value, due to the effect of concentration quenching a gradual reduction in the UC emission intensity was observed.

Fig. 6 Dopants (Tm³⁺, Yb³⁺) concentration dependence UC emission intensity for ¹G₄ → ³H₆ (488 nm), ¹G₄ → ³F₄ (655 nm) and ³H₄ → ³H₆ (814 nm) transitions.

3.5. Energy level diagram, pump power dependence study and colour tunability

In singly Tm³⁺ doped Na₂Y₂B₂O₇ phosphor three UC emission bands viz. blue (∼488 nm), red (∼655 nm) and NIR (∼814 nm) have been observed corresponding to the ¹G₄ → ³H₆, ¹G₄ → ³F₄ and ³H₄ → ³H₆ transitions respectively. The number of NIR photons responsible for these UC emissions can be calculated from the pump power versus UC emission dependence study,

I ∝ pⁿ (i)

where, ‘I’ is the UC emission intensity, ‘p’ is the pump power and ‘n’ is the number of NIR pump photons linked in the UC emission process. From the pump power dependence study the calculated slope values are noted to be 2.76, 2.62 and 1.58 for the UC bands in the blue, red and NIR regions respectively (Fig. 7). In case of the Tm³⁺ doped phosphor, ³H₄ and ¹G₄ level is populated by the involvement of two and three NIR photons respectively.^29,30 No absorption band around ∼980 nm in the Tm³⁺ doped phosphor is detected (Fig. 4). The stepwise non-resonant sequential 980 nm photon absorption in the Tm³⁺ is accountable for these UC emissions.³¹ The ground state absorption (GSA) from the ³H₆ is non-resonant with the ³F₄ level of Tm³⁺ ion; therefore, GSA absorption promotes the ground state population to the ³F₄ level non-radiatively (Fig. 9). After that, the ³F₄ level population by absorbing 980 nm photon gets lifted into the ³F₂ level through first excited state absorption (ESA-1) process. The ³F₂ and ³F₃ levels are very close and therefore treated as thermally coupled levels. The population of ³F₂ (∼14 975 cm⁻¹) level relaxes non-radiatively to the ³H₄ (∼12 517 cm⁻¹) level via the emission of five number of phonons. The ³H₄ level is again partially depopulated radiatively and emits a NIR photon through the ³H₄ → ³H₆ transition. The rest part of population accumulated in the ³H₄ level helps in populating the ¹G₄ level via the second excited state absorption (ESA-2) process. The population of ¹G₄ level decays radiatively to the ³H₆ and ³F₄ levels and emits two radiative photons in the blue and red regions corresponding to the ¹G₄ → ³H₆ and ¹G₄ → ³F₄ transitions respectively. The blue/red and blue/NIR emission intensity ratio is found to be 6.93 and 0.29 respectively in the singly Tm³⁺ doped phosphor.

Fig. 7 Pump power dependence study for Tm³⁺ doped Na₂Y₂B₂O₇ phosphor.

Fig. 8 Pump power dependence study for Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor.

Fig. 9 Schematic energy level diagram for Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor.

For Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor in addition to the aforementioned emission bands three more UC emission bands have been observed under the 980 nm diode laser excitation. These additional UC emission bands are observed at ∼264 nm, ∼298 nm, ∼362 nm corresponding to the ³P₂ → ³H₆, ¹I₆ → ³H₆ and ¹D₂ → ³H₆ transition respectively. Energy level diagram with their UC mechanisms for the Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor irradiated by 980 nm CW diode laser source has been depicted in Fig. 9. In order to discuss the involved processes responsible for the observed UC emissions, pump power dependence study has been performed (Fig. 8).

From Fig. 8, the slope values for, ∼264 nm, ∼362 nm, ∼488 nm, ∼655 nm and ∼814 nm transitions are 3.80, 2.62, 2.48, 1.84 and 1.54 respectively. Since the absorption cross-section of Yb³⁺ ion is large and hence it absorbs the pump photons efficiently when excited with a less expensive 980 nm laser diode.¹⁵ As in the codoped samples a broad band around ∼980 nm corresponding to the Yb³⁺ ion is observed (Fig. 4), therefore the NIR photons are efficiently absorbed by the Yb³⁺ ions. In the Tm³⁺–Yb³⁺ codoped system the concentration of Yb³⁺ is generally high; therefore the UC mechanism is mainly due to strong energy transfer (ET) from Yb³⁺ to Tm³⁺ ions.³² The excited Yb³⁺ ions after absorbing 980 nm photon, transfer its excitation energy to the Tm³⁺ ions by the ET-1 process non-resonantly. The ground state Tm³⁺ ions after getting the energy from the Yb³⁺ ions by the ET-1 process gets excited to the ³H₅ level (∼8229 cm⁻¹) by the emission of four numbers of phonons.³³ The ³H₅ level is depopulated through nonradiative relaxations into the ³F₄ level (∼5545 cm⁻¹) by multiphonon emission process. The population of ³F₄ level is again promoted via ET-2 process into the ³F₂ level. The ³F₂ (∼14 975 cm⁻¹) level is depopulated to the ³H₄ (∼12 517 cm⁻¹) level by the emission of five number of phonons.³³ A part of the population of the ³H₄ level is transferred to the ¹G₄ level via ET-3 process and the remaining population via the radiative relaxation to the ground state emits a photon at ∼814 nm corresponding to the ³H₄ → ³H₆ transition. The population available in the ¹G₄ level relaxes to the ³H₆ and ³F₄ level through the radiative transitions and emit photons in the blue and red regions respectively. From the pump power study the slope values for UC bands at ∼488 nm and ∼362 nm are noted to be ∼2.48 and ∼2.62 respectively. The decrease in the slope values is due to the effect of energy transfer and cooperative sensitization which causes an increase in the energy transfer rate from Yb³⁺ to Tm³⁺ ions.^20,34–37 In the codoped phosphor blue UC emission intensity is large enough compared to the other emissions. The intensity enhancement due to codoping with Yb³⁺ ions about ∼1000, ∼160 and ∼98 times for blue, red and NIR bands respectively have been resulted. Moreover, the blue/red and blue/NIR emission intensity ratio in codoped phosphor is improved from 6.93 to 42.90 and 0.29 to 3.0 respectively. This giant enhancement for the UC emission intensity in the codoped phosphor makes it superior from the other reported materials.^30,38–41 Thus the energy transfer and the feasible cooperative sensitization processes are responsible for such an enormous enhancement. Moreover, the cooperative sensitization and cross-relaxation (CR-1) process (³H₅ + ¹G₄ → ³H₆ + ¹D₂) helps in populating the ¹D₂ level. The energy mismatch during the cross-relaxation process is compensated by the emission of three number of phonons. The A part of the ¹D₂ level population appears to be promoted to the ³P₂ level by the ET-4 process.³⁵ The rest of the population stored in the ¹D₂ level is depopulated by the emission of a photon peaking at ∼362 nm through the ¹D₂ → ³H₆ transition.³⁶ As the slope value for ∼264 nm UC emission band is 3.80 which imply that four number of 980 nm photons are responsible for populating the ³P₂ level (Fig. 8). The ³P₂ level may also be populated through the cross-relaxation (CR-2) process (¹G₄ + ³F₂ → ³H₆ + ³P₂). The ³P₂ level is depopulated in two steps, in the first step one part of the population is utilized in the radiative emission at ∼264 nm through the ³P₂ → ³H₆ transition and in the second step the rest of the population relaxes nonradiatively to the ¹I₆ level through the multiphonon emissions.^35,36 The radiative transition corresponding to the ¹I₆ → ³H₆ transition emits a photon at ∼298 nm.³⁶ The decay curve analysis for the blue emitting level (¹G₄) has been performed (Fig. 10). The observed decay (τ) time is found to be ∼685 ± 0.1 μs and ∼432 ± 0.7 μs for doped and codoped phosphor, respectively. The observed decay time in the case of codoped phosphor is shorter than the singly Tm³⁺ doped phosphor which also proves the effect of efficient energy transfer from Yb³⁺ to Tm³⁺ ion.^1,31 It is well known that the radiative transition probability follows an inverse law with decay time of the emitting level.^42,43 In the present case it is observed that the decay time corresponding to blue light emitting level has been reduced due to addition of Yb³⁺ ion and hence the radiative transition probability is increased.¹ This result further support the existence of energy transfer mechanism in the codoped phosphor.

Fig. 10 Decay curve analysis for (a) Tm³⁺ doped Na₂Y₂B₂O₇ (b) Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor.

For Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor the colour coordinates at different pump power densities have been calculated by using the GoCIE software. The colour coordinates do not vary with the pump power density and the all colour coordinates lye in the blue region of the chromaticity diagram. The small deviation in the colour coordinates may be due to the fluctuations in the pump power at different scans (Table 2). The average value of the colour coordinate (X = 0.09, Y = 0.17) is found to be within the blue region. Thus the prepared material emits efficient blue colour which is not tunable even at higher excitation densities from 1.36 to 66.88 W cm⁻² and hence may be useful for making the efficient blue upconverter and display devices.

Table 1 Comparative list for FIR based temperature sensitivity reported by different researchers

RE doped sensing material	Transitions	Range of temperature	Maximum sensitivity	Reference
Yb2Ti2O7 codoped Er^3+–Mo	^2H11/2, ^4S3/2 → ^4I15/2	290–610 K	7.40 × 10^−3 K^−1 at 340 K	46
Y2SiO5 codoped Er^3+–Yb^3+	^2H11/2, ^4S3/2 → ^4I15/2	300–600 K	5.60 × 10^−3 K^−1 at 375 K	52
Al2O3 codoped Er^3+–Yb^3+–Mo	^2H11/2, ^4S3/2 → ^4I15/2	294–973 K	5.10 × 10^−3 K^−1 at 443 K	51
Na2Y2B2O7 doped Tm^3+–Yb^3+	^1G4(i), ^1G4(j) → ^3H6	300–623 K	4.54 × 10^−3 K^−1 at 300 K	Present work
Gd2O3 codoped Er^3+–Yb^3+	^2H11/2, ^4S3/2 → ^4I15/2	300–900 K	3.90 × 10^−3 K^−1 at 300 K	50
Y2O3 codoped Tm^3+–Yb^3+	^1G4(a), ^1G4(b) → ^3H6	303–753 K	3.50 × 10^−3 K^−1 at 303 K	20
Y2O3 codoped Ho^3+–Yb^3+–Zn^2+	^3K8, ^5F3 → ^5I8	300–673 K	3.01 × 10^−3 K^−1 at 673 K	1
Na0.82Ca0.08Er0.16Y0.853F4 doped Er^3+	^2H11/2, ^4S3/2 → ^4I15/2	5–300 K	2.20 × 10^−3 K^−1 at 338 K	48
Strontium Barium Niobate (SBN) glass ceramic doped Nd^3+	^4F5/2, ^4F3/2 → ^4I9/2	300–700 K	1.50 × 10^−3 K^−1 at 600 K	49

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Table 2 Calculated colour coordinate at different excitation power density excited by 980 nm diode laser

Excitation power density	Colour coordinate values (X, Y)
1.36 W cm^−2	(0.09, 0.16)
7.01 W cm^−2	(0.09, 0.20)
18.50 W cm^−2	(0.09, 0.20)
23.63 W cm^−2	(0.09, 0.18)
29.41 W cm^−2	(0.09, 0.18)
40.12 W cm^−2	(0.10, 0.17)
45.06 W cm^−2	(0.10, 0.17)
50.45 W cm^−2	(0.10, 0.16)
55.84 W cm^−2	(0.10, 0.16)
66.88 W cm^−2	(0.10, 0.16)

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3.6. Optical temperature sensing study

The UC emission spectra of the codoped nanophosphor at different temperatures have been recorded. Fig. 11 shows the UC emission spectra in the wavelength range from 440–520 nm at measured temperatures of 300 K, 443 K and 623 K for Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor. The pump power density of 980 nm CW laser diode has been fixed at 34.87 W cm⁻². Two distinct blue UC emission peaks centred at 477 and 488 nm corresponding to the ¹G₄ → ³H₆ transition of Tm³⁺ ions have been observed.²⁰ These two peaks are due to the transitions from the stark sublevels of the ¹G₄ level to ³H₆ level. On varying the temperature range from 300 to 623 K, no change in the peak positions has been noted but the intensity ratio know as ‘fluorescence intensity ratio (FIR)’ of two thermally coupled sublevels are altered significantly. Due to the thermal effect the intensity of the whole band lying in the blue region is reduced very much but the variations in the intensity of these two peaks (stark components) is different. The intensity of the peak at ∼477 nm increases gradually with increasing the temperature whereas the intensity of the peak at ∼488 nm decreases. Pérez-Rodríguez et al.⁴⁴ reported, that there is a high probability of non-radiative relaxations between the two sublevels that keeps them thermally coupled (i.e. each level is optically coupled to a lower common level). Due to the small energy gap (∼473 cm⁻¹) between two thermally coupled ¹G_4(i) and ¹G_4(j) sublevels of the ¹G₄ level, these sublevels are considered as in quasi-thermal equilibrium with each other.^20,24 Therefore, the population of these sublevels follow Boltzmann-type population distribution.^24,45 As the UC emission intensity ratio corresponding to the ¹G_4(j) → ³H₆ and ¹G_4(i) → ³H₆ transitions vary with change in temperature. Therefore the intensity ratio can be described by the following expression,⁴⁶where, I₄₇₇ and I₄₈₈ is the integrated intensity of the stark components peaking at 477 nm and 488 nm respectively, R is the proportionality constant, ΔE is the energy gap between the two stark sublevels, k and T is the Boltzmann's constant and absolute temperature respectively.

Fig. 11 Blue UC emission spectra of Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor in the wavelength range 440–520 nm at different temperatures.

The integrated intensity for both I₄₇₇ and I₄₈₈ of the UC emission bands have been taken for the calculation of the intensity ratio (I₄₇₇/I₄₈₈). A logarithmic plot for I₄₇₇/I₄₈₈ versus inverse of temperature is given in Fig. 12. It has been observed that the FIR changes slowly from 0.64 to 1.84 as the temperature increases from 300 to 623 K. The eqn (ii) has been fitted well with the experimentally observed values. By using the slope calculated from the Fig. 12, the energy gap between the two stark sublevels has been determined. From the estimated slope value, the energy gap ‘ΔE’ is ∼445.80 cm⁻¹. The observed energy gap between these two sublevels is ∼473 cm⁻¹ and is in close agreement with the experimentally determined energy gap (∼445.80 cm⁻¹).

Fig. 12 The plot of monolog FIR versus inverse absolute temperature.

The sensor sensitivity is an important parameter for the development of highly sensitive optical temperature sensor. The change in FIR for thermally coupled sublevels with temperature results the sensitivity, the sensitivity of the Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor has been calculate by using the following relation,²³

In the dual-Y axis plot of Fig. 13, calculated sensitivity of Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor with the variation of temperature has been plotted. The maximum sensitivity of Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor is 4.54 × 10⁻³ K⁻¹ at 300 K and seems to increase in the low temperature region. The minimum sensitivity ∼3.03 × 10⁻³ K⁻¹ within 300–623 K temperature range is observed at 623 K. On further increasing the temperature, the fluorescence intensity arising from these thermally coupled sublevels are diminished very much therefore the measurement of FIR becomes unfeasible. Moreover, we have compared our results with the other reported similar works based on sensitivity of two coupled levels (Table 1). Table 1 contains sensor sensitivity at particular temperature of different RE ions doped sensing materials with their optical transitions. From the Table 1, it is concluded that the sensor sensitivity of Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor is significant for temperature sensing measurement in the range of 300–623 K. The above demonstration supports that the Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor is suitable for the temperature sensor and thermometry measurement within the 300–623 K temperature range with high sensitivity.

Fig. 13 Variation of FIR and sensitivity as a function of temperature for Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor.

3.7. Optical nanoheater based study

In order to monitor the internal heating produced in the codoped phosphor at a particular laser excitation power density, we have calculated the FIR at different pump power densities from 1.36 to 66.88 W cm⁻² for the blue UC emissions at 477 nm and 488 nm. A plot of FIR (i.e. I₄₇₇/I₄₈₈) versus pump power density at room temperature is shown in Fig. 14. The value of FIR appears to increase from 0.44 to 0.96 with increasing the pump power density from 1.36–66.88 W cm⁻². Variation in FIR with the pump power density permits the study of temperature rising in the developed sample and hence leads to the thought of nano-volume based optical nanoheater.^2,47 Eqn (ii) can be subsequently changed into an easy form given below.where, all the terms have their usual meanings. This equation indicates that FIR is apparently related to the temperature ‘T’ of the emitting sample. The value of constant ‘R’ and ‘ΔE’ is experimentally determined by using the temperature dependent UC emission study of the prepared sample (Fig. 12 and 13). By using eqn (iv), temperature gain has been calculated at different pump power densities and plotted in the right side of Y-axis dual plot (Fig. 14). At maximum laser power density 66.88 W cm⁻² a temperature rise of about 440 K has been observed for the Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor. In order to verify the temperature rise we have placed the values of FIR obtained at that particular pump power density in the temperature dependence FIR variations (shown in the Fig. 13). The temperature gain of about 435 K for FIR ∼0.96 corresponding to the pump power density of 66.88 W cm⁻² is marked. This value (∼435 K) is reliable and is in close agreement with the temperature gain (440 K) calculated by using the eqn (iv).

Fig. 14 Variation of FIR and temperature as a function of pump power density for Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor.

The nanocrystalline nature of the prepared Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ phosphor can efficiently generate the heat by 980 nm diode laser excitation. By the optical excitation large number of mobile charge carriers may interact through the electric field of the radiation used for the excitation inside the nano-crystalline phosphor. Therefore, the energy gaining electrons are capable to renovate the excitation energy into heat via nonradiative channels or electron-phonon coupling.^21,26 Verma et al. have reported that heat developed from the nano-crystalline phosphor turns to increase the temperature of the surrounding volume.²¹ The quantum confinement of phonons in the nano-crystalline phosphor cannot be neglected, as it promotes the electron-phonon coupling,^2,26 which is beyond the scope of present investigation. Therefore, the prepared Tm³⁺–Yb³⁺ codoped nanophosphor also plays a dominant role for converting the absorbed energy into the thermal energy around the surrounding environment due to their large surface/volume ratio.²¹

Recently, hyperthermia has been helpful as a heating therapy with a variety of cancer treatments such as radiotherapy, chemotherapy, drug delivery and other radiation treatments. For hyperthermia treatment the temperature requirement is in the range of 314–318 K, which is useful for heating the affected cells or tissues and has a direct cell-killing effect, specifically in defective parts of the tumour. The developed phosphor is competent to produce hyperthermia treatment temperature at low pump power density ∼7.0 W cm⁻² upon excitation at 980 nm radiation (Fig. 13).

4. Conclusion

The efficient blue frequency upconversion upon excitation at 980 nm in the monoclinic phase Tm³⁺–Yb³⁺ codoped Na₂Y₂B₂O₇ nanophosphor synthesized successfully by solution combustion method and characterized by XRD, FE-SEM, TEM and absorption study techniques have been reported. The maximum enhancement about ∼1000 times in the blue UC emission band on codoping with Yb³⁺ ions has been observed and explained due to the energy transfer and cooperative sensitization processes. The maximum sensor sensitivity ∼4.54 × 10⁻³ K⁻¹ has been determined at room temperature and appears to increase in the lower temperature region. The laser induced optical heating behaviour has been performed by considering the FIR variation as a function of pump power density. The maximum temperature gain of about ∼435 K calculated at 66.88 W cm⁻² laser power density has been detected. The NIR laser induced study suggests that the developed nanophosphor may be of significant interest in hyperthermia based treatment. The colour coordinates at different pump power densities have been calculated and no significant deviation has been reported. On the basis of the observed experimental data, the developed nanophosphor can be used as multifunctional material for a variety of applications viz. NIR to blue upconverter, temperature sensor, medical diagnosis and blue display devices.

Acknowledgements

Authors acknowledge the financial support from University Grant Commission (UGC), New Delhi, India. Mr Abhishek Kumar Soni is also very much thankful to Indian School of Mines, Dhanbad, India for providing the financial assistance.

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