Multifunctionalities of near-infrared upconversion luminescence, optical temperature sensing and long persistent luminescence in La₃Ga₅GeO₁₄:Cr³⁺,Yb³⁺,Er³⁺ and their potential coupling

Abstract

The multifunctionalities of La₃Ga₅GeO₁₄:Cr³⁺,Yb³⁺,Er³⁺ (LGG:Cr³⁺,Yb³⁺,Er³⁺) with respective functions of near-infrared (NIR) upconversion (UC) luminescence, optical temperature sensing (OTS) and long persistent luminescence (LPL) were carried out and investigated in detail, which makes the materials attractive for bioapplications. The NIR UC luminescence at ∼830 nm with deep penetration in tissues is ascribed to the ⁴T₂ → ⁴A₂ transition of Cr³⁺. The OTS function based on the intensity ratio variation of ²H_11/2 → ⁴I_15/2 to ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ ion detected that the thermal effects of the UC material caused by the laser irradiation ranges from 307 to 332 K for pumping power of 60 to 86 mW mm⁻². It also showed LPL of Cr³⁺ with a defect trap depth of ∼0.77 eV below the conduction band, benefiting excitation-free and noise-free imaging in tissues. Additionally, anomalous temperature dependant UC emission behaviours of Cr³⁺ and Er³⁺ in this material were also characterized and discussed, which can be understood by the configurational coordinate (CC) model and the complex forward and backward energy transfer processes among the dopants, respectively. The potential coupling of these functions was further discussed, such as UC induced NIR LPL, which would repetitively restrengthen the fading LPL signals and alert the operators to thermal damage of the biosystems by the NIR laser in the tissue imaging.

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I. Introduction

Coupled multifunctionalities would result in novel functional materials with potential applications in some specific fields, for instance, luminescent materials with multi-modes in biosystems. UC luminescence is a process in which long-wavelength (low energy) photons are converted into a shorter wavelength (high energy) photon.¹ Great attention has been paid to UC luminescent materials due to their potential applications in various areas such as color displays, medical, biological labels, anti-counterfeiting, solar cells and so on.^2–5 Specially, the research of NIR UC luminescent materials is essential for optical probes in biosystems because of the low background noise and deep tissue penetration compared to visible UC materials.^6–8 Tm³⁺ is the most commonly used NIR emitting ion in the Yb³⁺ codoped UC materials.^6,9,10 However, owing to the well shield 4f electrons by outer shell 5s5p electrons, the tunability of luminescence and multifunctionality for the lanthanide ions are deficient. Whereas, transition metal (TM) ions have the complementally virtues and might be adopted in NIR UC luminescent materials. It was recently reported that Mn²⁺ could show anomalous NIR UC emission in KZnF₃:Yb³⁺,Mn²⁺ and MgGa₂O₄:Yb³⁺,Mn²⁺.^11,12 Besides the Mn²⁺, Cr³⁺ is another known NIR emitting TM ion and Yb³⁺–Cr³⁺ is a potential efficient NIR UC system.^13–16

LPL is a process with luminescence last for minutes to hours at room temperature after the stoppage of the excitation.¹⁷ The NIR LPL is extremely attractive in biolabels for its excitation-free and noise-free imaging conditions.¹⁸ Again, Cr³⁺ is the most potential NIR LPL ion with outstanding performance.^19–21 It is recently reported that the gallogermanate La₃Ga₅GeO₁₄:Cr³⁺ exhibits attractive NIR LPL property.²²

Additionally, pumping the UC materials by the high power laser in order to get strong fluorescence signals in the bioapplications would inevitably cause temperature rise of the materials.^23,24 It should be necessary and possible to incorporate the OTS function in the UC system to detect the thermal temperature. Yb³⁺–Er³⁺ is the most commonly used ions for OTS owing to that the intensity ratio of ²H_11/2 → ⁴I_15/2 to ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ ion is independent of luminescence loss and fluctuations in excitation intensity.^25–27 Overall, codoping Er³⁺ in Yb³⁺–Cr³⁺ UC system might realize the multifunctionalities of NIR UC emission, OTS and LPL. In addition, the host of La₃Ga₅GeO₁₄ (LGG) offers proper sites for accommodating the Cr³⁺ ions and Yb³⁺/Er³⁺ ions for triply doping since the radius of Cr³⁺(coordination number CN = 6), Ga³⁺ (CN = 6), Yb³⁺ (CN = 8), Er³⁺ (CN = 8) and La³⁺ (CN = 8) are 0.615 Å, 0.62 Å, 0.985 Å, 1.004 Å and 1.16 Å, respectively.²⁸ In the structure of the host, Ge⁴⁺ and partial Ga³⁺ are formed by (Ga/Ge)O₄ tetrahedrons and GaO₄ tetrahedrons with coordination of four, while some of Ga³⁺ are formed by GaO₆ octahedra with coordination of six. The energy transfer processes among these dopants were detailedly investigated in our previous work.²⁹ Hence, we are convinced that this La₃Ga₅GeO₁₄:Cr³⁺,Yb³⁺,Er³⁺ material could fulfill the multifunctionalities of NIR UC emission, OTS and LPL. Further, we attempt to give an insight to the temperature dependant luminescence behaviours and couple these functions to accomplish some more novel function.

II. Experimental

The LGG:xCr³⁺,yYb³⁺,zEr³⁺ (short for La_1−y−zGa_5−xGeO₁₄:xCr³⁺,yYb³⁺,zEr³⁺) phosphors were synthesized by a solid-state reaction method as reported in our previous work.²⁹ Typically, stoichiometric amounts of starting materials La₂O₃ (99.99%), Ga₂O₃ (analytical reagent, A.R.), GeO₂ (99.99%), Cr₂O₃ (A.R.), Yb₂O₃ (99.99%), Er₂O₃ (99.99%) and NH₄Cl (A.R., 10 wt%) were weighted and mixed thoroughly in an agate mortar. NH₄Cl was added as flux. Then the mixture was sequentially fired at 1273 K and 1573 K for 4 h with intermediate grindings.

Phase purity was checked by PANalytical X' pert Pro X-ray diffractometer with Cu anode target (Kα1 = 1.54059 Å). XRD data for refinement was collected on a Bruker D8 ADVANCE X-ray diffractometer with Cu target and Ni filter. The step-scan mode was used with setup of 40 kV × 40 mA, step 0.01°, 0.2 s per step. Rietveld refinement was conducted by TOPAS-Academic program. UC emission spectra were recorded on the Jobin-Yvon TRIAX320 spectrofluorimeter equipped with a R928 photomultiplier tube as the detector and a 976 nm laser diode (LD, Coherent Corp.) as monochromatic light source. To study temperature dependant behaviour of the UC luminescence, the same spectrofluorometer was equipped with a TAP-02 high-temperature fluorescence accessory (Tian Jin Orient–KOJI instrument Co., Ltd.). Thermo-luminescence glow curves were measured with a FJ-427A TL meter (Beijing Nuclear Instrument Factory) by heating the irradiated samples from 313 to 473 K. The samples were pre-irradiated by using a simulative sunlight lamp for 5 min and then heated at a linear rate of 2 K s⁻¹ to release the energy reserved in the material. For testing the thermo-luminescence (TL) curves with laser, the sample was excited by a 976 nm laser for 5 min before testing. LPL decay curve was tested on a long afterglow Material Optical Test System (Hangzhou, Everfine corporation), and the sample was irradiated by 365 nm laser for 10 min before testing.

III. Results and discussion

3.1 XRD patterns and crystal structure

Fig. 1 is the X-ray powder diffraction patterns of La₃Ga₅GeO₁₄ and some typical samples La_2.82Ga_5−xGeO₁₄:xCr³⁺,0.12Yb³⁺,0.06Er³⁺ (x = 0.15, 0.30). It can be observed that all the samples are in agreement with ICSD #20783, except for the sample of La_0.82Ga_4.7GeO₁₄:0.30Cr³⁺,0.12Yb³⁺,0.06Er³⁺ with an obvious impurity phase of LaGaO₃ (JCPDS 54-870, marked with inverted triangle). It clearly suggests that Cr³⁺, Yb³⁺ and Er³⁺ ions have been successfully incorporated into the host lattice for those samples with dopant contents less than that of La_0.82Ga_4.7GeO₁₄:0.30Cr³⁺,0.12Yb³⁺,0.06Er³⁺. Reitveld refinement graph of LGG sample is illustrated in Fig. 2 and some of the refinement results are listed in Table 1. LGG has a space group of P321, its 2d site is disorderly occupied by 50% of Ga(2) and 50% of Ge. Layers formed by (Ga/Ge)O₄ tetrahedrons and GaO₄ tetrahedrons sharing the corner are connected by GaO₆ octahedra along c axis, with La³⁺ ions in the forming cages of the framework (as seen in Fig. 3). Some typical cation–cation distances are also listed in Table 1. It should be noted that the distance between La–Ga is shorter than that of La–La.

Fig. 1 XRD patterns of some typical samples (La_1−y−zGa_5−xGeO₁₄:xCr³⁺,yYb³⁺,zEr³⁺; LGG:CYE), the inverted triangle shows an impurity phase.

Fig. 2 The experimental (blue dotted line) and calculated (red solid line) XRD patterns and their difference (gray solid line) for LGG (blue solid discrete vertical lines; traced β-Ga₂O₃ as a minor phase with gray solid discrete vertical lines).

Table 1 Some of the refinement results of LGG sample^a

Cation–cation distance (Å)
a Space group: P321 (no. 150), Z = 1, V = 297.80(9) Å^3, a = 8.2049(6) Å, c = 5.1080(4) Å, Rwp = 6.89%.
La–Ga1	3.4387	La–Ga2	3.4391	La–Ga3	3.8209
La–Ga2/Ge	3.6556	La–Ga3	3.9288
La–La	5.9560	La–La	5.1090	La–La	4.2598

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Fig. 3 Crystal structure of LGG sample (upper, view along a axis); cation surroundings and cation–cation distances (lower, anion is omitted for clarity).

3.2 NIR UC luminescence spectra

Fig. 4 shows the UC emission spectrum of LGG:0.15Cr³⁺,0.12Yb³⁺,0.06Er³⁺ at room temperature (RT) pumped by a 976 nm laser diode (∼60 mW mm⁻²). It can be observed that all the spectra exhibit four main emission peaks centered at 524, 548, 650, and 830 nm, which can be ascribed to the ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺and ⁴T₂ → ⁴A₂ transition of Cr³⁺, respectively. Notably, the intense NIR UC emission peak at ∼830 nm of Cr³⁺ is gained in this LGG material, which is highly desired for bioimaging. According to our previous work,²⁹ detailed energy transfer processes among Yb³⁺, Er³⁺ and Cr³⁺ were discussed and there exist forward and backward energy transfer processes. Our previous work also reveals that both the UC processes of Er³⁺ and Cr³⁺ are the two-photon processes.

Fig. 4 UC luminescence spectra of LGG:015Cr³⁺,0.12Yb³⁺,0.06Er³⁺ upon 976 nm laser diode excitation at RT (∼60 mW mm⁻²).

3.3 OTS and temperature dependent NIR UC behaviour

Yb³⁺–Er³⁺ doped phosphors can be used as temperature sensors for the reasons as follow.²⁶ The energy gap between the ²H_11/2 and ⁴S_3/2 levels is about 840 cm⁻¹ according to the green UC emissions spectra. This energy separation allows the ²H_11/2 level populated from ⁴S_3/2 level by thermal agitation, leading to variation in the transitions of ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 of Er³⁺ at different temperature. With the thermalization of population at the two levels and ignoring the effects of self-absorption of the fluorescence, the fluorescence intensity ratio (FIR) of the two green UC emissions can be written as eqn (1):³⁰where I₅₂₄ and I₅₄₈ are the integrated intensity of the ²H_11/2 → ⁴I_15/2 (510–535 nm) and ⁴S_3/2 → ⁴I_15/2 (536–570 nm) emissions, respectively. N(²H_11/2) and N(⁴S_3/2) represent the population numbers of the ²H_11/2 and ⁴S_3/2 levels. N, g, σ, ω, are the number of ions, the degeneracy, the emission cross-section, the angular frequency of fluorescence transitions from the ²H_11/2, ⁴S_3/2 levels to ⁴I_15/2 level, respectively. ΔE is the energy gap between the ²H_11/2 and ⁴S_3/2 levels, k is the Boltzmann constant, T is the absolute temperature, C = g_Hσ_Hω_H/g_Sσ_Sω_S represent the pre-exponential constant.

Fig. 5(a) depicts the green UC emissions spectra for LGG:0.15Cr³⁺,0.12Yb³⁺,0.06Er³⁺ at the temperature of 323, 373 and 523 K. Obviously, the peak locations of two emissions show little change with temperature, but the FIR of the 524 and 548 nm emission peaks varied. Fig. 5(b) illustrates a monolog plot of the FIR of 524 and 548 nm UC emission peaks as a function of inverse absolute temperature in the range of 323–573 K. The slope of the fitting line is about 546.55 according to the experimental data. Then, the FIR of these two peaks relative to the temperature in the range of 323–523 K was shown in Fig. 5(c). The experimental data are fitted to exponential curve with the coefficient C value of 3.51 in eqn (1). The sensitivity of the sensor can be defined as eqn (2):³⁰

Fig. 5 Temperature sensing of the LGG:015Cr³⁺,0.12Yb³⁺,0.06Er³⁺. (a) The green UC emissions spectra at various temperature, (b) monolog plot of the FIR of the 524 and 548 nm emission peaks as a function of inverse absolute temperature. Insert shows monolog plot of the FIR of the 524 and 548 nm emission peaks as a function of power intensity ranging from 60 to 86 mW mm⁻² (c) FIR of the 524 and 548 nm emission peaks versus the temperature, (d) sensitivity as a function of the temperature.

The corresponding resultant curve is described in Fig. 5(d). At the temperature of 323 K, the sensitivity of LGG:0.15Cr³⁺,0.12Yb³⁺,0.06Er³⁺is about 0.0034 K⁻¹. And the sensitivity deceased to 0.0025 K⁻¹ as temperature rises.

The insert of Fig. 5(b) shows a plot of the FIR of the green UC emissions at 524 and 548 nm as a function of power density. As the power density increases, the Ln(R) varied from approximate −0.483 to −0.346. As we known, NIR laser could lead to thermal release.^23,24 This variation can be related to thermal temperature, as blue line shown, the corresponding temperature was ranging from 307 to 332 K for pumping power from 60 to 86 mW mm⁻².

OTS is an important feature of this material, and for most luminescence materials and devices, their application temperature are usually over 300 K.³¹ Thus, it needs to investigate the NIR UC behaviours of the phosphors above RT due to that there may be thermal quenching phenomenon and additional energy transfer process in these triply doped phosphors. Therefore, the temperature-dependent UC emission of LGG:xCr³⁺,0.12Yb³⁺,0.06Er³⁺(x = 0.02, 0.04, 0.06, 0.1, 0.15, 0.30) were investigated.

Fig. 6(a, c and e) shows the typical spectra of LGG:xCr³⁺,0.12Yb³⁺,0.06Er³⁺(x = 0.02, 0.06, and 0.15) at temperature from 323 to 573 K. The integrated intensity of different peaks (510 to 570 nm for green emission of Er³⁺, 640 to 690 nm for red emission of Er³⁺, 730 to 870 nm for NIR emission of Cr³⁺) with different concentration of Cr³⁺ at various temperature were presented in Fig. 6(b, d and f). The other samples can be seen in the ESI (Fig. S1†). Interestingly, the NIR emission intensities of all samples show a rise up firstly then decline with the increase of temperature while that of green emission of Er³⁺ behave distinctively for samples with different concentration of Cr³⁺. For the sample with low concentration (x = 0.02) of Cr³⁺ (Fig. 6(b)), it decreases as temperature rises; while for the sample with higher concentration (x = 0.04 and 0.06) of Cr³⁺ (Fig. 6(d) and S1(b)†), it show distinct increase first and then decrease as temperature rises; and for the sample with large concentration (x = 0.1, 0.15 and 0.30) of Cr³⁺ (Fig. 6(f) and S1(d and f)†), it varies little with different temperature. Additionally, the red emission intensities of all samples do not vary dramatically with temperature. Normally, thermal quenching behaviour is expected as the temperature arises because the probabilities of multi-phonon relaxation and energy transfer for quenching the emitting levels are enhanced.^32,33 However, anti-thermal-quenching behavior was observed in this system as seen in Fig. 6 and S1.† A possible reason for this phenomenon is that there may exist thermo-luminescence in this system.³⁴ With temperature increases, the photons may be released due to thermal disturbance, so the intensity of luminescence may arise firstly. This is convinced by the fact that LGG:Cr³⁺ phosphor possesses LPL characteristic.²² Thus, the interpretation of temperature-dependent of UC emission will be discussed after the characterization of LPL behavior.

Fig. 6 UC luminescence spectra and corresponding integrated intensity of three emission peaks of LGG:xCr³⁺,0.12Yb³⁺,0.06Er³⁺ (for (a and b), x = 0.02; for (c and d), x = 0.06; for (e and f), x = 0.15).

3.4 LPL behaviour

To find out whether the Yb³⁺–Er³⁺–Cr³⁺ triply doped LGG behaves LPL, thermo-luminescence of the samples were carried out and a typical result of LGG:0.15Cr³⁺,0.12Yb³⁺,0.06Er³⁺ is illustrated in Fig. 7. After heated (black pentacle dots in Fig. 7), it shows a horizontal line at the temperature ranging 313 to 473 K. When the sample was excited by 976 nm laser for 5 min again in a black box, the TL spectrum still appear with a horizontal line as shown in Fig. 7 (red prismatic dots). The TL peak shows up when the sample exposure to the stimulated sunlight for 5 min. These facts suggest that the anomalous temperature dependent UC emissions cannot be ascribed to the LPL of these phosphors since the excitation of 976 nm LD laser on the samples could not lead to any TL peaks. The recovering ability of the temperature-dependent UC spectra of the samples (see that of a typical sample in Fig. S2†) also reveal that there is no TL effect in the temperature-dependent UC process. However, the LGG:0.15Cr³⁺,0.12Yb³⁺,0.06Er³⁺ phosphor indeed possess the LPL function with afterglow time of 19.6 min (5% of luminance left) according to the TL curves in the insert of Fig. 7. The energy depth of the defects that trapped the photo-excited electrons is about 0.77 eV following the eqn (3):³⁵where, T_m is the temperature for which the glow curve reaches a maximum. Such trapping centers of defects may be the defect clusters of oxygen vacancy-Cr³⁺.²²

Fig. 7 Thermoluminescence curves of LGG:0.15Cr³⁺,0.12Yb³⁺,0.06Er³⁺ under different condition (black pentacle dots: after heated; red prismatic dots: exposed to 980 nm laser for 5 min; blue sphere dots: exposed to simulative sunlight for 5 min). The inset is the LPL decay curve after 10 min of 365 nm laser.

3.5 Luminescence models and the potential coupled multifunctionality

Upon a 976 nm laser excitation at RT, Yb³⁺ ion firstly transfer the absorbed 976 nm photon energy to Er³⁺ ions via energy transfer process and populate the ⁴I_11/2 level. Then, the Er³⁺ ion absorbed another 976 nm photon from excitation laser or from energy transfer of Yb³⁺ to populate the ⁴F_7/2 of Er³⁺ ions. The Er³⁺ first relax nonradiatively to the ²H_11/2, resulting in the ²H_11/2 → ⁴I_15/2 emission. Sequentially, the ⁴S_3/2 level is populated by partial energy relaxation from the ²H_11/2 level, producing the ⁴S_3/2 → ⁴I_15/2 emission. Alternatively, it can populate the ⁴F_9/2 through further relaxation, leading to the red emission of ⁴F_9/2 → ⁴I_15/2. As for NIR UC of Cr³⁺, an cooperation sensitized upconversion is proposed as a three-ion process, in which two excited Yb³⁺ ions might simultaneously transfer their absorbed energy to a neighboring Cr³⁺ ion.¹⁴ Since the anomalous temperature-dependent UC behaviour can not be caused by TL, it should be the essential attribute of luminescent centers itself in this material. Then we attempt to interpret the abnormal temperature-dependent NIR UC emission of Cr³⁺ by the configurational coordinate (CC) diagram, as illustrated in Fig. 8. Upon the 976 nm laser excitation, the Yb³⁺ ions transfer the absorbed two 976 nm photons' energy to Cr³⁺ ions, resulting in population of ⁴T₁ level from R₀ point according to the Frank–Condon principle. Then the electrons at ⁴T₁ level first relax to the equilibrium state of the ⁴T₁ level (point R₁) and subsequently relax nonradiatively to the ⁴T₂ level (green dash line, process I), when the electron returns to the equilibrium state (point R₂), it would result in 830 nm emission via process II (green line), which is dominative emission at RT. As the temperature rises, thermal activated electrons are pumped from ⁴T₂ level to a higher sublevel ⁴T₂′ with the assistance of phonons in LGG host and then electrons return to the ground state via process IV (pink line). It is noticed that the energy barrier ΔE (the energy difference between the crossing point S and the bottom of the excited state) becomes larger (ΔE₂ > ΔE₁). As a result, the probability of nonradiative transition across S₂ decreases at elevated temperature.^36–38 However, when the temperature is getting higher, the vibrational energy makes the excited electrons to reach to the S₂ point and then relax to the ground state nonradiatively along the route of R₂′ → S₂ → R₀, resulting in the thermal quenching of luminescence. This can interpret why the NIR emission intensity of Cr³⁺ goes up firstly and then decline in Fig. 6(b, d and f), (circle pink dots) and Fig. S1(b, d and f),† (circle pink dots).

Fig. 8 Configurational coordinate model of Cr³⁺ luminescence.

However, the luminescence intensity behaves distinctly for Er³⁺. Compared with the d–d transitions of TM ions, the f–f transitions of RE ions with sharp line shape of the emission peaks are insensitive to the local chemical environment. Hence the temperature dependent UC behaviour of Er³⁺ could not be well understood by configurational coordinate. According to our previous work,²⁹ there are several forward and backward energy transfer processes in LGG:Cr³⁺,Yb³⁺,Er³⁺, in which Cr³⁺ → Yb³⁺ energy transfer is efficient and Yb³⁺ acts as an imperative bridge that makes transfer energy from Cr³⁺ to Er³⁺ efficient. For x = 0.02, the green UC emission of Er³⁺ declines monotonously with temperature, which might be due to the normal thermal quenching. Then it behaves analogously to that of NIR UC emission of Cr³⁺ when the Cr³⁺ concentration x is 0.04 and 0.06, which is ascribed to the backward energy transfer from Cr³⁺ (⁴T₁ level) to Er³⁺ via Yb³⁺ bridge (²F_5/2 level) (as demonstrated in Fig. 9, blue solid arrow). While for Cr³⁺ concentration x higher than 0.06, the changeless of the green UC emission could be contributed by that larger possibility of forward energy transfer from Yb³⁺ to Cr³⁺ than that from Yb³⁺ to Er³⁺. This can be deduced by the fact that the distance between Yb³⁺ (La³⁺) and Cr³⁺ (Ga³⁺) is significantly shorter than that between Yb³⁺ (La³⁺) and Er³⁺ (La³⁺) and more possibility for Cr³⁺ (Ga³⁺) to show up around Yb³⁺ (La³⁺), as demonstrated in Fig. 3 and Table 1. Whereas, the invariant red UC emission of Er³⁺ for samples with x < 0.06 may be owed to the dissipation of red emission through backward transfer to Cr³⁺ (blue dashed line), based on the fact that the emission of ⁴F_9/2 → ⁴I_15/2 of Er³⁺ fully overlaps with the ⁴A₂–⁴T₂ absorption of Cr³⁺.²⁹

Fig. 9 The schematic mechanisms of UC luminescence in LGG:xCr³⁺,0.12Yb³⁺,0.06Er³⁺. Blue solid arrows and blue dashed lines denote the energy transfer processes from Cr³⁺ to Er³⁺ via Yb³⁺ bridge and resonant energy transfer from of Er³⁺ (red radiation) to Cr³⁺ in the temperature-dependent UC emission processes, respectively. Orange dashed arrows denote the potential coupling effect for multifunctionality through implied energy transfer pathway.

For LPL process (as shown in Fig. 9), a high energy photon (UV light)²² excites an electron of Cr³⁺ from ground state ⁴A₂ to conduction band (CB) of LGG, then, the electrons delocalize to be trapped by the defects. With the thermal activation, the trapped electron is activated to CB then relax to ⁴T₂ level, and finally back to the ground state of ⁴A₂ with NIR emission.

In summary, the luminescence models of LGG:Cr³⁺,Yb³⁺,Er³⁺ with multifunctionality of NIR UC, OTS and LPL is schematically illustrated in Fig. 9. It should be very interesting if coupling these function together, e.g., if the NIR LPL, NIR UC emission and OTS are caused by a single pumping source of 976 nm laser, as the orange dotted curved arrows depicted in Fig. 9. It would make such material rather attractive for bioapplications, since the fading NIR LPL signals in deep tissues could be restrengthened repetitively through NIR laser irradiation. The problem is that the 976 nm laser could not cause delocalized electrons that would be trapped by the defects in this system, which is essential for the thermal perturbation induced luminescence. One possible reason may be that the pumping power of the laser is not high enough to cause delocalized electrons, or in other words, the band gap of LGG is too large. Considering the application of such materials, the pumping power should not be as high as possible. Therefore, choosing the host with narrow band gap and Er³⁺ ions with proper energy levels below and above conduction band involving in the UC process should be preferential options. Of course the depth of the controllable defects should be another significant factor to be considered. Furthermore, the choice of Er³⁺ ion in such material with OTS function would alert to prevent the biosystem from thermal damage caused by the intensive laser irradiation.

IV. Conclusions

In conclusion, we have investigated the mutifunctionalities of LGG:Cr³⁺,Yb³⁺,Er³⁺ with respective functions of strong NIR UC, OTS and LPL. NIR UC and LPL are ascribed to Cr³⁺ emission, while OTS is based on the transition characteristic of Er³⁺. The laser induced thermal effect would be ranging from 307 to 332 K for pumping power of 60 to 86 mW mm⁻² incident on the UC material according to OTS function. The temperature-dependent UC spectra demonstrated that the UC emission intensity of Cr³⁺ was quite different from that of Er³⁺, which can be interpreted by the CC model for the former and forward/backward energy transfer processes for the latter. If the LPL could be stimulated by the 976 nm laser, all the three functions would coupling together. It would make such kind of materials with the capacities of repetitively restrengthen the fading LPL signals and alerting the operators to thermal damage of the biosystems in the deep tissue imaging.

Acknowledgements

This work is financially joint supported by the NSFC (Grant no. 51125005, 21101065), Outstanding Young Teacher Training Program of Guangdong provincial Institute of higher education (Yq2013011) and Guangdong Natural Science Funds for Distinguished Young Scholar (2014A030306009).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra09095d

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