Spectroscopic characterization of SrB₄O₇:Tm²⁺, a potential laser material and optical temperature sensor

Abstract

Polycrystalline samples of SrB₄O₇:1% Tm²⁺ were prepared in air by a solid state reaction method. High resolution spectra and decay curves for infrared luminescence related to the intraconfigurational ²F_5/2 → ²F_7/2 transition and for visible luminescence related to the interconfigurational transition between the lowest energy state of the 4f¹²5d configuration and the ground ²F_7/2 state of the 4f¹³ configuration of Tm²⁺ were recorded as a function of temperature between 5 and 350 K. Energies of crystal field levels of the ²F_5/2 and ²F_7/2 multiplets and the ²F_5/2 lifetime value were determined from low temperature measurements. With these data room temperature emission and absorption spectra were calibrated in the cross-section units. It was found that the infrared emission in SrB₄O₇:Tm²⁺ is long-lived with a lifetime value of 9.5 ms and its spectrum indicates that stimulated emission near 1.18 nm may be feasible. Examination of recorded decay curves for the visible luminescence revealed that the lifetime of the lowest energy state of the 4f¹²5d configuration depends weakly on temperature in the region 5–280 K and then shows a steep decrease when the temperature grows from about 290 K to 350 K. The evaluated values of the thermal sensitivity parameter S for SrB₄O₇:Tm²⁺ in the region 300–330 K (27–57 °C) exceed 3.9% per K with a peak at 320 K (47 °C) equal to 6.30% per K. The S value is the highest reported thus far for luminescence thermometers operating in physiological temperatures.

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1 Introduction

There is a long lasting interest in rare earth-doped crystals and glasses. In the past attention has been directed mainly toward finding new laser crystals and designing laser devices. In an enormous number of published papers, laser operation for Nd³⁺, Ho³⁺, Er³⁺, Tm³⁺ and Yb³⁺ in a variety of host crystals has been reported and solid state lasers employing some of these systems are now commercially available. Physicochemical and laser properties of these materials have been reported in several review works, e.g. in an exhaustive review by Kaminskii.¹ The search for new laser active crystals is continuing and is directed mainly toward finding better and cheaper laser hosts.^2–4 Crystals doped with divalent rare earth ions have been studied, too. Interest in these systems resulted from an expectation that owing to low energies of the 4f^N−15d excited configurations the broad and intense absorption bands related to parity allowed inter-configurational transitions in divalent rare earth ions would be beneficial for optical pumping. This expectation has not been corroborated, however. In early works the alkaline fluoride host crystals have been chosen and laser operation has been demonstrated in CaF₂:Sm²⁺,^5,6 SrF₂:Sm²⁺,^7,8 CaF₂:Dy²⁺ (ref. 9–11) and CaF₂:Tm²⁺,^12,13 but only at cryogenic temperatures. Attempts to achieve room temperature laser operation in crystals doped with Sm²⁺ or Dy²⁺ were unsuccessful because their luminescence bandwidths were found to grow adversely with increasing temperature. Room temperature laser operation in CaF₂:Tm²⁺ could not be achieved as a consequence of an adverse ground state absorption of Tm ³⁺ ions (about 80% of thulium ions in CaF₂ host was present in the trivalent state).

Recently, a considerable interest has been directed to optical temperature sensing that offers a possibility to measure the temperature remotely. Numerous papers published during last decade have been devoted to elaborate various methods of temperature sensing and potential sensors that may be employed for this purpose. A variety of thermometers for thermal sensing based on organic fluorophores, quantum dots, bio-molecules and rare earth-doped systems have been described and their sensing capabilities compared. Achievements and current knowledge in this field are presented in valuable review works published recently e.g.^14–17 Phosphor thermometers based on luminescent rare earth ions in inorganic hosts are especially attractive because they offer rich energy level structures and are able to show luminescence transitions in a vast spectral region from UV to near infrared. Phosphor thermometers investigated thus far contained trivalent luminescent rare earth ions Nd³⁺, Eu, Dy, Ho, Er, Tm.

In the present paper we deal with spectroscopic features of divalent thulium ions in SrB₄O₇ host. Currently, the interest in oxide crystals doped with divalent rare earth ions is stimulated by the search for efficient phosphors able to convert the radiation of blue light emitting diodes (LED) into white light, providing thereby new sources for lighting purposes. The discovery of intense luminescence in SrB₄O₇ host doped with Eu²⁺ (ref. 18) and with Sm²⁺ ions¹⁹ has proved that oxide host crystals are worth considering. In fact, numerous visible phosphors based on Eu²⁺-doped hosts have been recently fabricated and characterized, e.g.: Ca₂SiO₄:Eu²⁺,²⁰ CaSrSiO₄:Eu²⁺,²¹ Ba₃Si₆O₁₂N₂:Eu²⁺ (ref. 22) and many sulphides.²³

Considerable less attention has been paid to Sm²⁺-doped systems. Nevertheless recent papers dealing with the SrB₄O₇:Sm²⁺ phosphor have demonstrated its suitability for application as optical pressure sensors at high temperatures.^24–26 Peculiarities of emission spectra related to the intra-configurational transitions of Sm²⁺ in SrB₄O₇:Sm²⁺ have been reported in ref. 27–29. Effect of temperature on intensities of inter- and intra-configurational transitions and on luminescence lifetime of SrB₄O₇:Sm²⁺ have been studied in ref. 30 and 31. Just recently it has been demonstrated that SrB₄O₇:5% Sm²⁺ system is promising for application as an optical sensor characterized by a high relative decay time temperature sensitivity in a wide temperature region with a record value of 3.36% per K at 550 K.³²

The search for phosphors based on divalent thulium ions is hampered by the fact that these ions are less stable than Eu²⁺ and Sm²⁺ ions. As far as we know only two papers published in the past provide fundamental information on luminescence of Tm²⁺ ions in oxide crystals. The paper by Schipper et al.³³ has reported the first observation of the 4f¹²5d → 4f¹³ emission of Tm²⁺ in SrB₄O₇ powder samples prepared by a solid state reaction in a reducing atmosphere consisting of 25% H₂ and 75% N₂. Authors have analysed and interpreted emission and excitation spectra recorded at 4.2 K and proposed mechanisms governing the excited state relaxation dynamics in the temperature region between 4.2 K and 300 K. In the paper by Peterson et al.³⁴ the room temperature diffuse reflectance spectra of SrB₄O₇:Tm²⁺ powders fired at temperatures ranging from 650 °C to 900 °C in air or in Ar/H₂ atmosphere have been recorded and analysed. Authors have shown that the reduction from Tm³⁺ to Tm²⁺ in SrB₄O₇ occurred also for samples fired in air though a significant fraction of incorporated thulium ions still was present in the trivalent state.³⁴ The outstanding feature of the SrB₄O₇:RE²⁺ (RE = Eu, Sm, Tm) stems from the stability of oxidation state of incorporated rare earth ions combined with a large transparency region of the host crystal stretching from UV to about 3000 nm.³⁵

Intention of the present work is to determine spectroscopic properties of the SrB₄O₇:Tm²⁺ system that are relevant to its potential for practical application. First, spectral and temporal characteristics of transitions within the 4f¹³ configuration of Tm²⁺ are analysed to assess the feasibility of resonantly pumped infrared laser operation near 1.2 μm. Second, the effect of temperature on spectra and decay of visible luminescence related to the inter-configurational 4f¹²5d → 4f¹³ transition of Tm²⁺ is investigated aiming at assessment of utility of the SrB₄O₇:Tm²⁺ as an optical temperature sensor.

2 Experimental

Synthesis

SrB₄O₇ polycrystalline samples were prepared by a solid state reaction method. As starting materials SrCO₃ (4N5), B₂O₃ (4N), and Tm₂O₃ (5N) were used. Powders were dried for 10 hours and then their appropriate amounts were weighted to obtain following molar composition: Sr_0.99Tm_0.01B₄O₇. Mixed SrCO₃ and B₂O₃ were formed into pellets and heated in air for 10 hours in 300 °C and 10 hours in 800 °C. First step of heating is necessary to avoid melting of B₂O₃ before getting in reaction with strontium carbonate. When reaction begins the CO₂ is starting to volatilize and heated pellets begins to swell. Second step of heating causes complete removal of CO₂ from the material. Thoroughly grounded and formed into pellets materials were heated in air again for 24 hours in 800 °C to remove other SrO–B₂O₃ phases and obtain pure SrB₄O₇ phase. Then 1% of Tm₂O₃ was added and mixture was heated for the third time for 24 hours in 800 °C. To perform spectroscopic measurement the polycrystalline pellets 17 mm in diameter and 2 mm thick were prepared.

Spectroscopic measurements

High resolution emission spectra were recorded upon ion argon laser excitation 488 nm line with DongWoo Optron setup composed of 750 mm focal length monochromator DM711 (1 cm⁻¹ spectral resolution FWHM for 0.02 mm slits), and detection systems: (VIS) PDS-1 with photomultiplier tube R3896 (Hamamatsu) or (IR) PS/TC-1 controller with InGaAs detector model IGA-030-TE2-H Electro-Optical Systems Inc Kinetic measurements were taken employing an experimental set-up consisting of a tunable optical parametric oscillator (OPO) pumped by a third harmonic of a Nd:YAG laser, a double grating monochromator with a 1000 mm focal length, a detection unit incorporating a photomultiplier operating in the visible or a cooled InSb detector operating in near infrared and a Tektronix MDO 4054B-3 Mixed Domain Oscilloscope. The OPO source delivered a train of light pulses with duration of 7 ns and pulse energy of several mJ (depending on wavelength within the 430–600 nm region) with the repetition rate of 20 Hz. The same apparatus was employed to record excitation (PLE) spectra. To perform measurements at low temperature, the samples were installed in a continuous flow liquid helium cryostat equipped with a temperature controller.

Crystallographic structure determination

Phase analysis and structural refinement were performed with a Siemens D5000 diffractometer (Ni-filtered CuKα radiation). Data were collected in the range 20–100° with a step of 0.02° and averaging time of 10 s per step. XRD pattern (Fig. 1) shows pure SrB₄O₇ phase (JCPDS Card No. 15-0801) for undoped sample. In thulium doped SrB₄O₇ beside the main tetraborate phase there is a little amount of metaborate (JCPDS Card No. 15-0779) phase visible.

Fig. 1 Diffraction patterns of SrB₄O₇ and SrB₄O₇:1 at% Tm polycrystalline pellets.

3 Results and discussion

Luminescence of Tm²⁺ ions in crystals may be related to vibronic inter-configurational transitions between the lowest energy state of the 4f¹²5d configuration and the ground ²F_7/2 state of the 4f¹³ configuration and to an intra-configurational ²F_5/2 → ²F_7/2 transition. The spectral region of the latter transition is close to that of the ²F_5/2 → ²F_7/2 transition of isoelectronic Yb³⁺ ions and depends weakly on the host crystal. Resulting luminescence band in CaF₂:Tm²⁺ system has been observed near 1.12 μm and the low temperature laser operation at 1.116 μm has been obtained in the past.^12,13 Peculiarities of the ²F_5/2 → ²F_7/2 luminescence for the SrB₄O₇:Tm²⁺ system have not been determined in previously reported study because weak luminescence lines could not be separated from the background.³³ Also, the room temperature absorption (diffuse reflectance measurement) spectrum recorded for the ²F_7/2 → ²F_5/2 transition in SrB₄O₇:Tm²⁺ did not provide reliable information because of superposed absorption of residual Tm³⁺ ions.³⁴ It will be shown in the following paragraph that spectra and decay curves recorded in a wide temperature region are able to provide new information on the ²F_5/2 → ²F_7/2 infrared luminescence in SrB₄O₇:Tm²⁺.

3.1 Effect of temperature on the intra-configurational ²F_5/2 → ²F_7/2 infrared luminescence

Fig. 2 compares luminescence spectra of Tm²⁺ in the near infrared region, recorded at several different temperatures in the region of 5–300 K.

Fig. 2 The ²F_5/2 → ²F_7/2 luminescence spectra of Tm²⁺ ions in SrB₄O₇ host recorded at several different temperatures upon excitation at 488 nm.

The site symmetry for Tm²⁺ ions in this host is lower than cubic, therefore four crystal field components for the ground ²F_7/2 multiplet and three crystal field components for the ²F_5/2 excited multiplet are predicted. Since the population of the crystal field components is governed by the Boltzmann statistics the luminescence spectra are expected to be temperature dependent. It can be seen in Fig. 2 that at 5 K the luminescence spectrum consists of four well separated lines located at 1131.6 nm (8837 cm⁻¹), 1141.3 nm (8762 cm⁻¹), 1152.95 nm (8673 cm⁻¹) and 1180 nm (8475 cm⁻¹). We assign these lines to transitions from the lowest crystal field component of the ²F_5/2 excited multiplet to four crystal field components of the ground state. Accordingly, the line located at 1131.6 nm is related to the transition ending on the lowest crystal field component of the ground multiplet (0–0 line) and remaining lines are related to transitions ending on the higher crystal field components located at 75, 164 and 362 cm⁻¹. In the spectrum of the 50 K luminescence an additional line related to transition from the second higher energy component of the initial ²F_5/2 multiplet appears at 1120.25 nm (8927 cm⁻¹). Above 200 K a weak line peaking at 1093.3 nm (9147 cm⁻¹) contributes to the spectrum. Assuming that it is related to transitions from the highest energy component of the initial state we locate components of the ²F_5/2 excited multiplet at 8837, 8927 and 9147 cm⁻¹. At higher temperatures the transitions from all crystal field levels of the ²F_5/2 multiplet contribute to luminescence band and recorded spectra do not show significant differences.

The ²F_5/2 → ²F_7/2 infrared luminescence of Tm²⁺ in SrB₄O₇ is found to be long-lived. Decay curves of this luminescence were recorded at different temperatures in the 5–450 K region. All of them were consistent with a single exponential time dependence providing thereby a set of reliable lifetime values. A plot of measured luminescence lifetimes versus temperature is shown in Fig. 3.

Fig. 3 Lifetime of the ²F_5/2 multiplet of Tm²⁺ in SrB₄O₇ host plotted versus temperature.

Observed increase of the lifetime with growing temperature is due likely to a lengthening effect of self-absorption, commonly encountered during investigation of the ²F_5/2 → ²F_7/2 emission in crystals doped with Yb³⁺. Therefore we consider the value of 9.5 ms determined from a decay curve recorded at 5 K to be actual ²F_5/2 lifetime.

In principle the relaxation of the ²F_5/2 metastable level of Tm²⁺ in SrB₄O₇ is governed by radiative transitions and nonradiative multiphonon transitions. The rate of multiphonon relaxation is expected to be low since a simultaneous emission of six phonons with the highest energy of about 1400 cm⁻¹ that are available in the host would be needed to bridge the energy difference of about 8400 cm⁻¹ between the ²F_5/2 and ²F_7/2 levels. Having this in mind we assume that the measured luminescence lifetime of 9.5 ms is close to the ²F_5/2 radiative lifetime and we calibrate the ²F_5/2 → ²F_7/2 emission band in units of emission cross-section σ_em(λ) employing the Füchtbauer–Ladenburg relation:

where β is the luminescence branching ratio (equal to 1 for this transition), I(λ) is the emission spectrum intensity, τ_rad is the radiative lifetime of the ²F_5/2 level and n denotes the index of refraction of the crystal. Solid line in Fig. 4 represents the resulting σ_em(λ) spectrum for the experimental ²F_5/2 → ²F_7/2 infrared luminescence band recorded at 300 K. The peak value for the emission cross section of 3.5 × 10⁻²⁰ cm² is advantageously high. However, the terminal level of this transition is the ground state and an actual ability of the system for light amplification will be markedly smaller due to an adverse effect of self-absorption. To assess quantitatively this shortcoming we calculate the spectrum for the ²F_7/2 → ²F_5/2 absorption cross-section employing a so called reciprocity relation. The method is based on the relation between absorption cross-section σ_abs(λ) and emission cross-section σ_em(λ):where Z_low, Z_up are the partition functions for the lower and upper levels, respectively, defined as:where g_i and (g_j) are the degeneration of a crystal field level having the E_i and (E_j) energy, respectively. E_zl is energy difference between lowest crystal field components of the upper and lower multiplets and k_B is the Boltzmann constant. Inserting values for energies of crystal field levels determined above to eqn (2) and (3) we obtain the spectrum for the absorption cross section σ_abs represented by a dashed line in Fig. 4.

Fig. 4 Room temperature ²F_5/2 → ²F_7/2 luminescence spectrum σ_em (solid line) and derived ²F_7/2 → ²F_5/2 absorption spectrum σ_abs (dashed line) calibrated in cross-section units for the SrB₄O₇:Tm²⁺ system. Inset shows an effective emission cross-section spectrum σ^eff_em(λ) calculated according to eqn (4).

It can be seen that the curve of emission cross-section dominates significantly that of absorption cross-section in the spectral region between 1150 and 1230 nm pointing thereby at positive values of optical amplification coefficient. To predict the wavelength of a potential laser operation the effective emission cross-section function σ^eff_em(λ) was evaluated according to the relation:

σ^eff_em(λ) = Kσ_em(λ) − (1 − K)σ_abs(λ) (4)

where K is the population inversion parameter defined as the density of excited Tm²⁺ ions divided by the overall density of Tm²⁺ ions in the crystal host. Resulting σ^eff_em(λ) spectra calculated for several reasonable values of K are shown in the inset to Fig. 4. It follows from these spectra that the laser operation in SrB₄O₇:Tm²⁺ at 1180 nm can be feasible for low K values. It is worth noticing here that an excited state absorption (ESA) due to allowed transitions from the metastable level to states of the 4f^N−15d configuration (ESA), detrimental for stimulated emission in SrF₂:Sm²⁺,³⁶ cannot affect the emission around 1180 nm in SrB₄O₇:Tm²⁺ because of a large ²F_5/2–4f^N−15d energy gap.

3.2 Effect of temperature on the 4f¹²5d → 4f¹³ visible luminescence

In contrast to the ²F_5/2 level of the 4f¹³ configuration considered above, the lowest energy state of the 4f¹²5d configuration of Tm²⁺ ions depends strongly on the host. Based on careful analysis of low temperature luminescence spectrum it has been located at 17 210 cm⁻¹ SrB₄O₇:Tm²⁺ system,³³ that is markedly higher than about 14 000 cm⁻¹ found in the past for CaF₂:Tm²⁺ system.^12,13 Due to larger energy gap between the bottom of the 4f¹²5d configuration and the next lower energy ²F_5/2 state the nonradiative relaxation is suppressed in SrB₄O₇:Tm²⁺ system and an efficient visible emission related to the 4f¹²5d → 4f¹³ (²F_7/2) transition is observed.

Fig. 5 shows spectra of visible luminescence in the SrB₄O₇:Tm²⁺ system recorded at several sample temperatures in the 300–5 K temperature region. At 300 K the spectrum has a maximum at 595.4 nm and consists of a single broad and smooth band, typical for vibronic transitions. Decrease of temperature down to 110 K affects little the shape of the luminescence band but it brings about a monotonic shift of the band maximum to about 601.2 nm.

Fig. 5 Effect of temperature on spectral characteristics of visible luminescence in SrB₄O₇:Tm²⁺ system. Excitation wavelength was 488 nm.

During further decrease of temperature the position of maximum is arrested but the intensity of a structure on the short wavelength side of the band, hardly visible at 110 K, grows steadily and eventually at 5 K a group of narrow lines contributes to the spectrum. Details of the 5 K luminescence spectrum shown in Fig. 5 are consistent with those recorded at 4.2 K and reported in the work by Schipper et al.³³ proving thereby that we are dealing with the same material. According to the assignment proposed in³³ the structure that appears in the low temperature spectrum of the 4f¹²5d → 4f¹³ transition of Tm²⁺ in SrB₄O₇ consists of zero phonon lines and related progressions. Measurement of the excited state relaxation dynamics was performed when changing the sample temperature in a vast region 5–360 K. It was found that the decay curves of luminescence follow a single exponential time dependence for all temperature points considered. Fig. 6 shows a plot of evaluated time constants (luminescence lifetime values) versus sample temperature.

Fig. 6 Lifetime of the 4f¹²5d → 4f¹³ (²F_7/2) visible emission (red open circles) and calculated thermal sensitivity parameter S (full blue squares) plotted versus temperature for the SrB₄O₇:Tm²⁺ system. Lines are only guides for eyes.

It can be seen that initially the lifetime values decrease gently with increasing temperature and then at about 290 K a very steep decrease occurs. Eventually at 360 K the lifetime value of 5 us was measured. Observed change of lifetime induced by variation of the temperature points at the potential of the system under study for optical thermometry. Commonly, the quality of optical sensors based on temperature-dependent luminescence lifetime is assessed employing a thermal sensitivity parameter S defined by the relation:³²

where τ denotes the lifetime value and T denotes the temperature.

The sensitivity parameter was calculated according to eqn (5) above and plotted versus temperature in Fig. 6.

The evaluated values of thermal sensitivity parameter S for the SrB₄O₇:Tm²⁺ system in the 300–330 K (27–57 °C) region exceed 3.9% per K with a peak value of 6.30% per K at 320 K (47 °C). It can be seen that in the temperature region between 10 °C and 47 °C the luminescence lifetime values are advantageously high and the S parameter depends linearly on the temperature.

In addition, the SrB₄O₇:Tm²⁺ system shows a broad and intense absorption band stretching from 450 nm to 550 nm favouring thereby an efficient excitation. Moreover, the PLE spectra recorded at 15 °C, 25 °C and 40 °C and compared in Fig. 7 indicate that the excitation efficiency is weakly affected by the temperature in this region. It is worth mentioning that in contrast to crystals doped with trivalent rare earth ions the ion–ion energy transfer processes do not affect significantly spectra and excited state relaxation dynamics of Tm²⁺ ions in SrB₄O₇. Indeed, low temperature luminescence spectra and lifetime of inter-configurational transition reported for SrB₄O₇:0.5% Tm²⁺ in ref. 33 do not differ from those presented above for SrB₄O₇:1% Tm²⁺. All these features indicate that the SrB₄O₇:Tm²⁺ system may be a promising optical sensor operating in the physiological temperature region.

Fig. 7 Excitation spectra of emission at 600 nm recorded at 15, 25, and 40 °C.

Ability to measure temperature changes in living cells is of paramount importance for the detection of pathological states e.g. of tumour deceases. Therefore numerous papers published during last decade have been devoted to elaborate various methods of temperature sensing and potential sensors that may be employed for this purpose. In an excellent review paper published recently by Tingting Bai and Ning Gu¹⁷, presenting the state of the art in this topic, thermometers for thermal sensing and imaging of living cells and biological tissues based on organic fluorophores, quantum dots, bio-molecules and rare earth-doped systems are described and their sensing capabilities compared. Reported values of thermal sensitivity parameter S refer to various spans of temperature within the 10–60 °C region. Among non-contact luminescent thermometers the highest values of S parameters have been found for organic fluorophores, e.g.: 3.9% per K in the temperature range 32–37 °C for ns decay of “ER thermo yellow” compound³⁷ and for rare earth metal complexes, e.g.: 2.74% per K in the temperature range 25–50 °C for Eu-TTA.³⁸ Evaluated thermal sensitivity parameters for inorganic rare earth-doped thermometers are markedly lower, however^39–42 with the highest S parameter value of 1.6% per K in the temperature range 10–36 °C reported for NaYF₄:Er³⁺,Yb³⁺.⁴³ Recently, using a Cr³⁺ to Nd³⁺ emission intensity ratio, the highest 3.48% per K sensitivity has been obtained in the physiological temperature range.⁴⁴ The evaluated values of thermal sensitivity parameter S for the SrB₄O₇:Tm²⁺ system in the 300–330 K (27–57 °C) region exceed 3.9% per K with a peak value of 6.30% per K at 320 K (47 °C), and to our knowledge they are higher than those reported thus far for other luminescent thermometers operating in this temperature region.

4 Conclusions

Based on analysis of the effect of temperature on luminescence spectrum related to the ²F_5/2 → ²F_7/2 intra-configurational transition of Tm²⁺ three crystal field levels of the ²F_5/2 initial multiplet were located at 8837, 8927 and 9147 cm⁻¹ and four crystal field levels of the ground ²F_7/2 multiplet were located at 75, 164 and 362 cm⁻¹ for the SrB₄O₇:Tm²⁺ system. Observed luminescence is long-lived with a lifetime value of 9.5 ms at 5 K. Comparison of the room temperature ²F_5/2 → ²F_7/2 luminescence band calibrated in the cross-section units according to the Füchtbauer–Ladenburg relation to that of the ²F_7/2 → ²F_5/2 absorption band derived by a reciprocity method points at a positive value of optical amplification coefficient in a spectral region between 1150 and 1230 nm. Adverse effect of self-absorption was considered and its importance was assessed based on the calculated effective cross-section spectrum. It was also concluded that an excited state absorption (ESA) due to allowed transitions from the metastable level to states of the 4f^N−15d configuration (ESA), cannot affect the emission around 1.18 μm in SrB₄O₇:Tm²⁺ because of large 4f^N−15d → ²F_5/2 energy gap. In SrB₄O₇:Tm²⁺ system the lifetime (τ) of intense visible emission related to vibronic inter-configurational transitions between the lowest energy state of the 4f¹²5d configuration and the ground ²F_7/2 state of the 4f¹³ configuration of Tm²⁺ changes abruptly when the temperature (T) varies in the region 280–350 K implying utility of this system for luminescence- based thermometry. The evaluated values of thermal sensitivity parameter S for the SrB₄O₇:Tm²⁺ system in the 300–330 K (27–57 °C) region exceed 3.9% per K with a peak value of 6.30% per K at 320 K (47 °C). Therefore, such a phosphor can be used as temperature sensor in biological region of temperatures.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding sources

NCN under project DEC-2013/09/D/ST5/03878.

Abbreviations

4N	99.99%
4N5	99.995%
5N	99.999%
ESA	Excited state absorption
Eu-TTA	Europium(III) thenoyltrifluoroacetonate trihydrate
FWHM	Full width at half maximum
IR	Infrared
JCPDS	Joint Committee on Powder Diffraction Standards
LED	Light emitting diode
NCN	National Science Centre
S	Thermal sensitivity parameter
VIS	Visible
XRD	X-ray diffraction

Acknowledgements

The work was funded by the National Science Centre (NCN) on the basis of the decision number DEC-2013/09/D/ST5/03878.

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