Investigation into optical heating and applicability of the thermal sensor bifunctional properties of Yb³⁺ sensitized Tm³⁺ doped Y₂O₃, YAG and LaAlO₃ phosphors

Abstract

Yb³⁺/Tm³⁺ doped Y₂O₃, YAG and LaAlO₃ phosphors have been successfully fabricated by the sol–gel method. The phosphors show intense blue emissions under excitation at 980 nm. The possible upconversion (UC) mechanisms have been proposed based on the power-dependence of the UC luminescence (UCL) intensities. The Stark levels (¹G₄₍₂₎/¹G₄₍₁₎) of Tm³⁺ are used to determine the temperature sensing performance by analyzing the temperature dependent UCL spectra from 303 to 573 K. The intensity ratio of the ¹G₄₍₂₎ → ³H₆ and ¹G₄₍₁₎ → ³H₆ transitions has been found to increase with the increase in laser pump power, which can be used to estimate the internal temperature of materials. Furthermore, the sensor sensitivities of the samples have been calculated and the factors affecting the sensitivity are discussed in detail through chemical bond theory. All the experimental results indicate that the blue emissions from the phosphors may be used for photothermal ablation (PTA) therapy and fluorescent bio-labels.

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Introduction

In recent years, the UCL properties of rare earth-doped micro-/nano-materials upon near infrared (NIR) excitation have gained significant interest, since they can be widely used in the industrial, biological and pharmaceutical fields, in solar cells, color display devices, biosensors, temperature sensors and magnetic resonance imaging.^1–3 In particular, increased attention is being paid to the determination of intracellular temperature by using fluorescence intensity ratio (FIR) technology, in which the thermally coupled energy levels (TCLs) of rare earth ions play an important role. Generally, the energy difference ΔE of TCLs ranges from 200 cm⁻¹ to 2000 cm⁻¹ and their population is supposed to be in quasi-equilibrium.⁴ The energy gap between the ¹G₄₍₂₎ and ¹G₄₍₁₎ levels of Tm³⁺ is about 315 cm⁻¹, according to previous publications, which indicates that it has potential application in optical thermometry, using the TCLs at around room temperature.^5,6 Y₂O₃, YAG and LaAlO₃ phosphor hosts have many advantages, including preeminent stability, low toxicity and low phonon energies,^7,8 which reduce the probability for non-radiative transition of the measured optical emissions. Therefore, these hosts are considered as ideal potential candidates for optical temperature sensing.^1,9,10

Some materials can absorb energy from pump lasers and convert it into heat, resulting in the increase in internal temperature, which can be explained by electron–phonon coupling;^11,12 the induced heat generated by the pump laser can be used in PTA therapy. In particular, the near-infrared laser is selected to induce PTA, due to its ability to effectively convert thermal energy in rare earth ion-doped UCL materials. Additionally, the NIR laser radiation is absorbed less by biological tissues, resulting in the “optical transmission window” (λ = 700–1100 nm) of the biological tissues in the NIR range allowing deeper light penetration for treatment, which results in less autofluorescence and light scattering, without damaging the normal biological tissues.^13,14 Therefore, effective UCL materials could provide heat for medical treatment, and at the same time, accurately measure the temperature in situ around cancer cells for PTA therapy.¹⁵

In this work, the UCL of Tm³⁺/Yb³⁺ co-doped in Y₂O₃, YAG and LaAlO₃ phosphors excited by a 980 nm diode laser have been studied. The optical emissions from the ¹G₄₍₂₎ and ¹G₄₍₁₎ TCLs of Tm³⁺ are used to investigate the optical temperature sensing properties, and the factors affecting the sensor sensitivity have been analyzed and discussed in detail, according to both the experimental results and theoretical calculations. The UCL spectra were obtained after excitation at 980 nm with varying power, and the temperatures of the samples were calculated using FIR. This indicates that the samples have potential to be regarded as optical sensors and heaters for use in PTA.

Experimental

Synthesis

Synthesis of Yb³⁺/Tm³⁺ co-doped Y₂O₃. The Y₂O₃: 0.1% Tm³⁺, 2% Yb³⁺ (mole fraction) phosphor was prepared by the sol–gel process. All the chemicals are commercially available and were used without further purification. Rare earth nitrate stock solutions (Y(NO₃)₃ (0.08 M), Tm(NO₃)₃ (0.002 M) and Yb(NO₃)₃ (0.05 M)) and citric acid (C₆H₈O₇·H₂O) were taken as raw materials. The rare earth nitrate stock solutions were mixed together according to their chemical formulas, with stirring. Next, the C₆H₈O₇·H₂O was added to the metal ion solutions as the chelating agent. The molar ratio of citric acid to total metal ions was 2 : 1. The mixtures were stirred for a few minutes to obtain a highly transparent solution. After the transparent solution was dried at 90 °C for 24 h, dry, light-brown gels were obtained; after firing at 500 °C for 2 h, the precursors were obtained. Finally, the corresponding phosphors were formed by firing in air at 1200 °C for 4 h, after grinding in an agate mortar.

Synthesis of Tm³⁺/Yb³⁺ co-doped YAG. The YAG: 0.1% Tm³⁺, 2% Yb³⁺ (mole fraction) sample was synthesized by a method similar to that of Y₂O₃: 0.1% Tm³⁺, 2% Yb³⁺, except for the use of Al(NO₃)₃·9H₂O (0.9378 g), which was added to the nitrate solution (Y(NO₃)₃, Tm(NO₃)₃ and Yb(NO₃)₃).

Synthesis of Tm³⁺/Yb³⁺ co-doped LaAlO₃. The LaAlO₃: 0.1% Tm³⁺, 2% Yb³⁺ phosphors were synthesized by a method similar to that of YAG: 0.1% Tm³⁺, 2% Yb³⁺, except for the sintering at 900 °C for 4 h.

Characterization

The powder X-ray diffraction (XRD) was recorded on a Rigaku-Dmax 2500 diffractometer equipped with Cu Kα radiation (λ = 0.15405 nm). All the measurements were performed at room temperature. The UCL spectra were obtained by exciting the samples with a 980 nm laser diode (LD), and were acquired on an Omni-λ300 spectrograph with a CCD detector equipped with a monochromator used for signal collection in the range of 450–700 nm. The Yb³⁺/Tm³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors were fixed in an iron sample cell and the power of the 980 nm LD was increased from 0.092 W to 0.843 W.

Results and discussion

Structural investigation

Fig. 1(a) gives the XRD patterns of the Yb³⁺/Tm³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors. The results indicate that the Y₂O₃ phosphor is in the pure cubic phase; all the diffraction peaks are consistent with the standard card of Y₂O₃ (JCPDS no. 41-1105). The XRD pattern of YAG shows that the YAG sample exhibits the pure cubic phase (JCPDS no. 09-0310). All diffraction peaks of LaAlO₃ could be readily indexed to the hexagonal phase of LaAlO₃ (JCPDS no. 31-0022), and no other impurities were observed. The FE-SEM images of the samples are shown in Fig. 1(b). All phosphors were microsized, with flaky morphology.

Fig. 1 (a) X-ray diffraction patterns for Yb³⁺/Tm³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors after firing. The standard data for Y₂O₃ (JCPDS card no. 41-1105), YAG (09-0310) and LaAlO₃ (31-0022) are shown for reference; (b) the FE-SEM images of samples.

UCL and energy diagram

We chose Y₂O₃ to introduce relative properties among Yb³⁺/Tm³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors. The UC emission spectrum of the Yb³⁺/Tm³⁺ co-doped Y₂O₃ phosphor under the 980 nm excitation, is displayed in Fig. 2; it is composed of two intense blue emissions and an intense red emission in the visible region, centered at 476, 489 and 654 nm, originating from ¹G₄₍₂₎ and ¹G₄₍₁₎ excited states to ³H₆ ground state and from ¹G₄₍₁₎ to ³F₄ excited states of Tm³⁺ ions, respectively. Interestingly, it can be observed that the blue emission band is composed of two sections, due to the impact of interactions among electrons and spin–orbit coupling, which result in the existence of the Stark sublevels ¹G₄₍₂₎ and ¹G₄₍₁₎.¹⁶ In addition, the UC emission spectra of the Yb³⁺/Tm³⁺ co-doped YAG and LaAlO₃ phosphors under the excitation of the 980 nm diode laser are provided in Fig. S1 and S2 (ESI†), respectively.

Fig. 2 The UC emission spectrum of the Yb³⁺/Tm³⁺ co-doped Y₂O₃ phosphor under the excitation of the 980 nm diode laser.

Fig. 3 reveals the dependence of the UCL intensity of the Yb³⁺/Tm³⁺ co-doped Y₂O₃ phosphor on the 980 nm excitation, which suggests that the UCL intensity increases remarkably with the increase in pump power. In addition, it can be observed that the visible UCL intensity is proportional to the infrared excitation power I_up ∝ (P_pump)ⁿ, where n represents the number of laser photons used to populated the upper emission state, I_up and P_pump are the UCL intensity and power, respectively.^17–19 From Fig. 3, it can be seen that the values of n are 2.97 and 2.68 for the ¹G₄₍₂₎ → ³H₆ and ¹G₄₍₁₎ → ³H₆ emissions, respectively. The results indicate that the population of the ¹G₄₍₂₎/¹G₄₍₁₎ states comes from three-photon UC processes in the Y₂O₃: 0.1% Tm³⁺, 2% Yb³⁺ system. Furthermore, the n values of YAG (0.1% Tm³⁺, 2% Yb³⁺) and LaAlO₃ (0.1% Tm³⁺, 2% Yb³⁺) phosphors were also obtained, which are 3.46 and 3.40 for the ¹G₄₍₂₎ → ³H₆ and ¹G₄₍₁₎ → ³H₆ emissions in the YAG: 0.1% Tm³⁺, 2% Yb³⁺ (Fig. S3, ESI†); for LaAlO₃: 0.1% Tm³⁺, 2% Yb³⁺, the n values are 2.55 and 2.52 for the ¹G₄₍₂₎ → ³H₆ and ¹G₄₍₁₎ → ³H₆ emissions (Fig. S4, ESI†), respectively. The results indicate that one UC photon is produced by three IR excitation photons in the systems of Y₂O₃, YAG and LaAlO₃.^20,21

Fig. 3 The log–log plots of the UC emission intensity of Y₂O₃: 0.1% Tm³⁺, 2% Yb³⁺ as a function of the 980 nm laser pumping power.

For the Tm³⁺-doped materials, the blue emission of the ¹G₄₍₂₎/¹G₄₍₁₎ → ³H₆ transitions and red emission of the ³F_2,3 → ³H₆ transition result in two main emission bands. The UCL mechanism has been investigated for Tm³⁺-doped materials in previous reports (Fig. 4).¹⁷ The metastable energy level, ³H₅, of Tm³⁺ ions is populated by an energy transfer (ET, Yb³⁺ → Tm³⁺) step: ³H₆(Tm³⁺) + ²F_5/2(Yb³⁺) → ³H₅(Tm³⁺) + ²F_7/2(Yb³⁺), or ground state absorption (GSA) process: ³H₆(Tm³⁺) + one photon → ³H₅(Tm³⁺). Multiphonon relaxation then results in the population of the lower level, ³F₄. Another ET is then required to excite the population of the ³F₄ state to higher ³F_2,3 states: ³F₄(Tm³⁺) + ²F_5/2(Yb³⁺) → ³F_2,3(Tm³⁺) + ²F_7/2(Yb³⁺), or an excited state absorption (ESA) process: ³F₄(Tm³⁺) + one photon → ³F_2,3(Tm³⁺); then, the ³H₄ level is populated by non-radiative relaxation. After the ET process, ²F_5/2(Yb³⁺) + ³H₄(Tm³⁺) → ²F_7/2(Yb³⁺) + ¹G₄(Tm³⁺), the radiative relaxation from TCLs (¹G₄₍₂₎/¹G₄₍₁₎) of the ¹G₄ state to the GS produces two blue emissions centered at 476 and 489 nm, respectively. The population at the ¹G₄ level decays to the ³F₄ state level and radiative transitions from the ³F_2,3 levels to the GS ³H₆ give red emission at 654 nm and 695 nm, respectively. The population of the ³F_2,3 level moves to the lower ³H₄ level by the NR process, then NIR emission centered at 798 nm is obtained, due to the radiative relaxation from the ³H₄ level to GS ³H₆.²²

Fig. 4 The energy level diagram of Yb³⁺/Tm³⁺ co-doped Y₂O₃ under excitation of 980 nm, and possible UC processes.

Temperature sensing behavior

The UCL properties of the Y₂O₃: 0.1% Tm³⁺, 2% Yb³⁺ phosphor, based on TCLs, were investigated for its potential application as an optical temperature sensor. The UC emission spectra of the Y₂O₃: 0.1% Tm³⁺, 2% Yb³⁺ phosphor at 300 K, 400 K and 500 K are shown in Fig. 5, with the 980 nm pump power at 1 W. It was found that the fluorescence intensities decreased with the increase in temperature, and simultaneously, we found that the FIR (¹G₄₍₂₎/¹G₄₍₁₎) increased with increasing temperature, which is shown in Fig. 6(b). In fact, the energy gap is narrow between the ¹G₄₍₁₎ and ¹G₄₍₂₎ levels and they are thermally coupled, which was reported by Suo.²² It has been shown that the relative population of such TCLs follows a Boltzmann type population distribution.²³

Fig. 5 Temperature evolution of the Tm³⁺ blue UC emission spectra, with excitation by the 980 nm pump laser for the Y₂O₃: 0.1% Tm³⁺, 2% Yb³⁺ phosphor at 300 K, 400 K and 500 K.

Fig. 6 (a) The plot of the UCL intensity ratio, FIR, versus 1/T for the Y₂O₃ phosphor; (b) FIR (I₄₇₆/I₄₈₉) of Tm³⁺, blue UCL for the ¹G₄₍₂₎/¹G₄₍₁₎ → ³H₆ transitions as a function of temperature in the range of 303–573 K.

Thus, the ¹G₄₍₁₎ level can be effectively populated to the ¹G₄₍₂₎ level through a thermalization process, leading to population variations in the ¹G₄₍₂₎ and ¹G₄₍₁₎ levels. Furthermore, there is a change in fluorescence intensity of TCLs, due to the existence of thermal quasi-equilibrium. The small changes in temperature will then induce large variations in relative florescence intensities. In addition, the blue UC emission spectra of YAG (0.1% Tm³⁺, 2% Yb³⁺) and LaAlO₃ (0.1% Tm³⁺, 2% Yb³⁺) at 300 K, 400 K and 500 K are shown in Fig. S5 and S6 (ESI†), respectively. Since the population of each energy level is related to the emission intensity, the FIR from the closely spaced levels of Tm³⁺ ions can be defined as follows:^24–26

where

For the radiative relaxations from ¹G₄₍₂₎ and ¹G₄₍₁₎ levels to the ³H₆ level, I₄₇₆ and I₄₈₉ are the integrated intensities, N(¹G₄₍₂₎) and N(¹G₄₍₁₎) denote the populations, g_2j and g_1j represent the degeneracy, σ₂ and σ₁ are the emission cross-section, and ω_2j and ω_1j symbolize angular frequency, respectively. ΔE represents the energy separation between the two TCLs (¹G₄₍₂₎/¹G₄₍₁₎), K_B denotes the Boltzmann constant and T is the absolute temperature.²⁷

Fig. 6(a) shows the FIR (¹G₄₍₂₎/¹G₄₍₁₎ → ³H₆ transitions) as a function of inverse absolute temperature from 303 K to 573 K. The experimental results display that the data are perfectly fitted by a straight line with the slope of about 598, which represents the value of ΔE/K_B. The FIR of blue UC emissions as a function of the temperature in the range of 303–573 K, are represented in Fig. 6(b), which suggests that the FIR of two emissions from the ¹G₄₍₂₎/¹G₄₍₁₎ → ³H₆ transitions increases with the increase in temperature. The experimental data are fitted to eqn (1), the value of coefficient B is about 2.63. Fig. S7(a) and S8(a)† show the FIR (¹G₄₍₂₎/¹G₄₍₁₎ → ³H₆ transitions) of YAG (0.1% Tm³⁺, 2% Yb³⁺) and LaAlO₃ (0.1% Tm³⁺, 2% Yb³⁺) as a function of inverse absolute temperature from 303 K to 573 K. The values of ΔE/K_B are 550.8 and 207.5, respectively. Fig. S7(b) and S8(b)† exhibit the FIR in blue UC emissions of YAG (0.1% Tm³⁺, 2% Yb³⁺) and LaAlO₃ (0.1% Tm³⁺, 2% Yb³⁺) as a function of the temperature in the range of 303–573 K. The values of coefficient B are about 2.26 and 2.53, respectively.

For an optical sensor, it is important to know the change in FIR at a certain change in temperature for its application. The sensitivity of the optical temperature sensor can be written as follows:^26,28,29

The temperature-dependent variation in sensitivity for Y₂O₃: 0.1% Tm³⁺, 2% Yb³⁺ is presented in Fig. 7, where it was found that the maximum of sensitivity is about 0.00242 K⁻¹ at 303 K, in the biophysical temperature range. The results suggest that the phosphor has potential as a temperature sensor in the biology field. In addition, it can also be observed from Fig. S9 and S10† that the maxima of sensitivities for YAG (0.1% Tm³⁺, 2% Yb³⁺) and LaAlO₃ (0.1% Tm³⁺, 2% Yb³⁺) are about 0.00208 K⁻¹ and 0.00215 K⁻¹ at 363 K and 303 K, respectively. Moreover, the comparison of sensitivities of different materials is shown in Table 1. It can be observed that the sensitivities of Yb³⁺/Tm³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors are not very high; in fact, they are common.

Fig. 7 The sensor sensitivity S = dFIR/dT as a function of temperature for the Y₂O₃: 0.1% Tm³⁺, 2% Yb³⁺ phosphor.

Table 1 The comparison of sensitivities of different materials

Fluorescence centers	Hosts	Temperature range (K)	Tmax (K)	Smax (%K^−1)	Ref.
Tm^3+	LiNbO3 crystal	323–773	773	0.02	30
Tm^3+	NaNbO3 NCs	293–353	—	0.08	31
Tm^3+	NaYF4 NCs	300–500	417	1.53	32
Tm^3+	Ba5Gd8Zn4O21 phosphors	300–510	300	0.61	33
Tm^3+	PbF2 GC	300–700	700	0.03	34
Tm^3+	Y2O3 phosphors	303–573	303	0.242	This work
Tm^3+	YAG phosphors	303–573	363	0.208	This work
Tm^3+	LaAlO3 phosphors	303–573	303	0.215	This work

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According to eqn (2), the S is mainly decided by B and ΔE, and the ΔE of TCLs has little difference for rare ion-doped materials. Thus, the S is mainly decided by B, which is written as follows:¹²

We find that B is mainly decided by Ω_λ=2,4,6.¹² Ω_λ denotes the crystal parameters of the samples, and some properties can be represented by it. Ω₆ represents the rigidity of the medium; the bond properties and structural symmetry of the materials have little effect on Ω₄ and Ω₆, but Ω₂ is affected strongly by the symmetry, which is related to the constituents of the samples. The nephelauxetic effect is dependent on crystal structures, bond lengths and covalency. Compared with other J–O parameters, the environment has a larger effect on Ω₂. Thus, B is primarily decided by Ω₂. In addition, Ω₂ is closely related to covalent chemical bonding (see the ESI†).³⁵ We can conclude that the sensitivity of materials is related to covalent chemical bonding. According to chemical bond theory,³⁶ the dielectric susceptibility

χ^μ = (4π)⁻¹(ℏΩ^μ_p/E^μ_g)², (4)

defines the appropriate average energy gap for the μ-type bonds, E^μ_g in terms of the average susceptibility χ. This E_g^μ does not correspond to any particular gap in the band picture, but it is an average over all the bands. Ω_p^μ is the plasma frequency.

(E_g^μ)² can be separated into the homopolar (E_h^μ)² and heteropolar (C^μ)² parts, according to the following relationship:³⁷

(E^μ_g)² = (E^μ_h)² + (C^μ)², (5)

V_b^μ denotes the bond volume of μ-type chemical bonds,where d^μ is the bond length (in Å), and the denominator is merely the required normalization factor, and the sum over ν runs over all of the different types of bonds. N_b^ν is the number of bonds of type ν in cubic centimeters.³⁸

The Fermi energy is given as

E^μ_F = (ℏk_F)²/(2m), (7)

k_F = (3π²N_e)^1/3 (8)

The covalent f_c and ionic f_i of chemical bond are given as

We calculated the bond lengths and bond covalency of Y–O and La–O. In addition, the related parameters of Y₂O₃, YAG and LaAlO₃ phosphors are listed in Table 2. It was found that the sensor sensitivity is proportional to the bond covalency, which is consistent with our previous conclusion.³⁹

Table 2 Mean value of chemical parameters and calculated B and S in Yb³⁺/Tm³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors

Crystal	Bond	d̄^μ^a (Å)	f̄c^b	B	S (K^−1)
a Average bond length.b Average bond covalency.
Y2O3	Y–O	2.285	0.155	2.63	0.00242
LaAlO3	La–O	2.665	0.122	2.53	0.00215
YAG	Y–O	2.367	0.066	2.26	0.00208

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Analysis of thermal effect

Some materials can absorb energy from pump power and convert it into heat, resulting in the increase in the internal temperature. The pump power dependent, blue UC emission spectra of Y₂O₃: 0.1% Tm³⁺, 2% Yb³⁺ are used to investigate the laser induced thermal effect, which is shown in Fig. 8, and the inset shows that the intensity ratio of two closely spaced transitions (¹G₄₍₂₎ → ³H₆ and ¹G₄₍₁₎ → ³H₆) increases linearly with the increase in pump power. The UC emission spectra of Yb³⁺/Tm³⁺ co-doped YAG and LaAlO₃ phosphors with different pump power are shown in Fig. S11 and S12 (ESI†), respectively; the inset presents the variation of FIR with increasing power. In addition, we can find that the temperature of the sample under different pump power can be estimated by the corresponding fluorescence intensity ratio. We obtained the temperature of the material under different pump power by following eqn (11), which is rearranged from eqn (1):⁴⁰where all the terms have been previously mentioned in eqn (1). The result is shown in Fig. 9, where the lattice temperature of Y₂O₃: 0.1% Tm³⁺, 2% Yb³⁺ reaches to 651 K at 0.843 W. It can be seen that the lattice temperature of YAG (0.1% Tm³⁺, 2% Yb³⁺) and LaAlO₃ (0.1% Tm³⁺, 2% Yb³⁺) reaches to 436 K and 424 K at 0.843 W, respectively. We also found that different materials have different temperatures at the same pump power. The increased temperature rate of Y₂O₃ is higher than the rate of the YAG and LaAlO₃ phosphors. According to the above results, non-radiative relaxation (electron–phonon coupling) is prominent for induced heat in UC emission materials at higher laser pump power. At the same time, the crystalline nature of the materials also contribute to the temperature rise under pump power.^41–44 The ΔE between ¹G₄₍₂₎ and ¹G₄₍₁₎ levels is narrow (ΔE ≈ 315 cm⁻¹), which results in the temperature increasing at lower pump power. Therefore, it may be used for investigating the thermometric ability in cells without damaging them.⁴⁵ In addition, the “optical transmission window” lies in the range of 700–1100 nm; thus, the near infrared light possesses low autofluorescence and deep penetration, simultaneously reducing the photodamage effects and light scattering. In particular, the UC materials exhibit tunable emission, long lifetime, sharp emission bandwidth, low cytotoxicity and high photostability, which drive them to be used for bio-imaging applications.^13,14,46

Fig. 8 The UCL spectra of the blue band ¹G₄₍₂₎/¹G₄₍₁₎ → ³H₆ of Tm³⁺ with different pumping power for Y₂O₃: 0.1% Tm³⁺, 2% Yb³⁺. Inset: FIR is a function of laser pump power.

Fig. 9 Comparison of temperature increase under excitation at 980 nm for Yb³⁺/Tm³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors.

Conclusions

Strong UCL from Yb³⁺/Tm³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors have been observed in the blue region. The UC mechanism is proposed, in which energy transfer plays an important role. The intensity ratio (I₄₇₆/I₄₈₉) of two closely spaced blue UC emissions of Tm³⁺ is found to increase linearly with input pump power; simultaneously, the sensor sensitivities of Yb³⁺/Tm³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors are also calculated, based on the FIR technique. The results show that the maximum sensitivities of materials are obtained near the biological temperature range, suggesting that the materials are good candidates for application in the biology field. The factors affecting the sensor sensitivity are speculated and discussed in detail, according to the experimental results and theoretical calculations, where the values of B and S increase with the increase in average bond covalency. Moreover, the thermal effect induced by the 980 nm laser is also investigated and the lattice temperature of the Y₂O₃ sample reached to 651 K at 0.843 W. The sample lattice temperatures of YAG and LaAlO₃ phosphors are also calculated under different pump powers. Due to the capability of transferring the pump energy into heat, Yb³⁺/Tm³⁺ co-doped Y₂O₃, YAG and LaAlO₃ phosphors should have good potential to act as optical heaters for use in PTA therapy.

Acknowledgements

This work was supported by the Science and Technology Development Planning Project of Jilin Province (20160101294JC), partially sensored by China Postdoctoral Science Foundation and by the National Science Foundation of China (no. 21521092), supported by the National Found for Fostering Talents of Basic Science (no. J1103202) and by the Outstanding Young Teacher Cultivation Plan in Jilin University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15814e

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