Anomalous thermal activation of green upconversion luminescence in Yb/Er/ZnGdO self-assembled microflowers for high-sensitivity temperature detection

Abstract

Non-contact optical temperature detection has shown a great promise in biological systems and microfluidics because of its outstanding spatial resolution, superior accuracy, and non-invasive nature. However, the thermal quenching of photoluminescence significantly hinders the practical applications of optical temperature probes. Herein, we report thermally enhanced green upconversion luminescence in Yb/Er/ZnGdO microflowers by a defect-assisted thermal distribution mechanism. A 1.6-fold enhancement in green emission was demonstrated as the temperature increased from 298 K to 558 K. Experimental results and dynamic analysis demonstrated that this behavior of thermally activating green upconversion luminescence originates from the emission loss compensation, which is attributed to thermally-induced energy transfer from defect levels to the green emitting level. In addition, the Yb/Er/ZnGdO microflowers can act as self-referenced radiometric optical thermometers. The ultrahigh absolute sensitivity of 1.61% K⁻¹ and an excellent relative sensitivity of 15.5% K⁻¹ based on the ⁴F_9/2/²H_11/2(2) level pair were synchronously achieved at room temperature. These findings provide a novel strategy for surmounting the thermal quenching luminescence, thereby greatly promoting the application of non-contact sensitive radiometric thermometers.

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New concepts

The conventional thermal quenching of luminescence greatly limits the applications of upconversion materials in temperature detection and inevitably degrades device performance. This limitation can be alleviated by minimizing the thermal quenching (TQ) of luminescence in upconversion luminescent materials. Previous studies have proposed several theories that focused on minimizing TQ in small-scale nanomaterials. However, effective mechanisms to clarify the thermal activation of luminescence in micromaterials are not clear. Herein, we propose a new “defect-assisted thermal distribution” mechanism for the thermal enhancement of green upconversion luminescence in the Yb³⁺/Er³⁺/ZnGdO microflowers. To the best of our knowledge, this is the first paper to experimentally demonstrate that defect-assisted thermal distribution can induce the thermally enhanced upconversion luminescence phenomenon. Experimental results and dynamic analyses indicated energy transfer from defects to ⁴S_3/2(1)via thermal excitation and a quasi-thermal equilibrium, which contribute to the compensation of green emission losses caused by non-radiative transitions. Therefore, a 1.6-fold enhancement in green emission was demonstrated in parallel with prominent temperature sensing performance (S_a up to 15.5% K⁻¹). These findings provide a new insight into surmounting the thermal quenching of luminescence, thereby greatly promoting the application of non-contact sensitive radiometric thermometers.

1. Introduction

The technology of temperature measurement has been extensively integrated into industrial and scientific fields.^1,2 Conventional thermometers, including thermocouples, thermistors, and pyrometers, are generally utilized to detect the temperature of an object from the direct heat transfer and thermal equilibrium between the sensor and the object.^3,4 These contact temperature sensors are inappropriate for temperature measurements at scales below 10 μm,^1,5 such as temperature mapping of microcircuits and intracellular temperature fluctuations, and are easily interfered by electromagnetic waves.^6,7 These unavoidable disadvantages have encouraged the development of non-contact optical temperature sensors because of their unique advantages. The non-contact optical thermometry can realize temperature measurements under extreme environments through remote control, which is immune to electromagnetic interferences and dimensional limits.^3,8–11

Recently, lanthanide-doped (e.g., Er³⁺, Tm³⁺, and Ho³⁺) upconversion optical temperature sensing materials have attracted much attention because of their significant potential applications in non-contact optical temperature detection.^12–15 Among the various pathways to the upconversion temperature sensing technique, the fluorescence intensity ratio (FIR) technique is a great research hotspot.^16–18 Additionally, the FIR technique is independent of external interferences and spectral losses and possesses the advantages of rapid response and high spatial and temperature resolutions. Nevertheless, the emission loss of non-radiative transitions with increasing temperature, namely thermal quenching, seriously affects the luminescence efficiency and impedes the application of upconversion temperature sensors.¹⁹

Attempts have been made to minimize thermal quenching luminescence in lanthanide-doped upconversion nanoparticles (LUCNPs) by weakening phonon confinement and surface-related processes.^20–23 In particular, it has been recently demonstrated that phonon-assisted energy transfer plays a positive role in surmounting thermal quenching in nanoparticles.²⁰ Some missing low-energy phonon modes can recover via the weakening of phonon confinement with increasing temperature, promoting energy transfer between Yb³⁺ and Er³⁺. In addition, the alleviation of surface quenching by a heating mechanism has been illustrated for thermally enhanced luminescence of LUCNPs. The heating-induced luminescence enhancement occurs through eliminating the surface-absorbed water in small nanoparticles as the temperature increases.^21,22 However, these mechanisms are currently limited to small-scale nanomaterials and are not suitable for large-scale materials, such as micromaterials.

In this work, a novel “defect-assisted thermal distribution” mechanism for the thermal enhancement of green upconversion luminescence is proposed in Yb³⁺/Er³⁺/ZnGdO microflowers. Experimental results and dynamic analysis indicated that energy transfer from the defect energy level to the ⁴S_3/2(1) energy level occurred via thermal excitation and a quasi-thermal equilibrium, contributing to the compensation of green emission losses caused by non-radiative transitions. The green upconversion luminescence intensity was continuously improved as the temperature was elevated from 298 K to 558 K. The multiple temperature sensing performances were concurrently implemented in the Yb³⁺/Er³⁺/ZnGdO microflowers. Ultrahigh S_r, superb S_a, and excellent signals were simultaneously obtained based on the ⁴F_9/2/²H_11/2(2) level pair. This study may provide new understanding and perspectives for the thermal activation of luminescence in upconversion micromaterials.

2. Experimental section

2.1. Sample preparation

The Yb/Er/ZnGdO self-assembled microflowers were prepared by a coprecipitation method and a thermal treatment process. The raw materials including Zn(NO₃)₂·6H₂O, Gd(NO₃)₃·6H₂O, Yb(NO₃)₃·5H₂O and Er(NO₃)₃·5H₂O, NaOH, and citric acid were purchased from Sigma-Aldric and used without further purification. First, 0.8032 g of Gd(NO₃)₃·6H₂O (1.78 mmol), 0.0898 g of Yb(NO₃)₃·5H₂O (0.2 mmol) and 0.0088 g of Er(NO₃)₃·5H₂O (0.02 mmol) were dissolved in 10 mL of ethanol (solution A). Then, 0.595 g of Zn(NO₃)₂·6H₂O (2 mmol) and 0.3842 g of citric acid (2 mmol) were dissolved in 70 mL of aqueous solution (solution B). Next, solution A and solution B were mixed thoroughly under stirring. Then,10 mL and 30 mL of aqueous solution of NaOH (2 M) were added in turn and the reaction lasted for 30 min, respectively. Subsequently, a certain amount of citric acid (0 g, 2.3052 g, 2.8815 g, 3.4583 g and 4.0341 g, respectively) was added to the above mixed solution and stirred for 30 min. The white precursor was harvested by several rinse-centrifugation cycles with H₂O and ethanol, and dried in air at 60 °C. The acquired precursor was annealed at 600 °C for 3 h, forming the Yb/Er/ZnGdO microflowers.

2.2. Material characterizations

The morphologies and structures of the samples were examined by a Helios Nanolab 600i high-resolution field emission scanning electron microscope (SEM) and TecnaiG2F30 transmission electron microscope (TEM). The chemical composition of the samples was analyzed by X-ray photoelectron spectroscopy via a PHI 5700-ECSA system, and energy-dispersive X-ray spectroscopy (EDX) attached to the Tecnai G2F30 instrument. The crystal phases of the samples were analyzed by a Panalytical Empyrean powder X-ray diffractometer (XRD). Upconversion PL spectra were recorded using a Zolix-SBP300 grating spectrometer equipped with a CR131 photomultiplier tube. The 980 nm diode laser was used as a pump source, and a signal generator was used to convert a continuous 980 nm excitation source into a pulse excitation source. The temperature-dependent upconversion luminescence measurements were performed on a temperature controlling stage. The temperature was detected by a copper-constantan thermocouple and controlled by a proportional-integral derivative loop feedback temperature control system.

3. Results and discussion

3.1. Structure and morphology characterization

The controllable synthesis of upconversion micromaterials with desirable size/morphology and crystal structure is of practical significance and has attracted much attention in recent years. Here, Yb/Er/Gd co-doped ZnO (Yb/Er/ZnGdO) self-assembled microflowers were synthesized via a coprecipitation method and subsequent thermal treatment, as illustrated in Scheme 1. In a typical synthetic process, the first step is the preparation of precursor microflowers via the coprecipitation method, in which the precursor morphology can be easily adjusted by changing the citric acid content. The next step is thermal treatment. After heat treatment, the –OH and –CO₃ groups can be removed from the structure by thermal decomposition reaction, resulting in the formation of Yb/Er/ZnGdO microflowers. Under controllable synthesis conditions, the size/morphology and crystalline structure of the Yb/Er/ZnGdO microflowers also can be adjusted.

Scheme 1 Schematic illustration of synthesis of Yb/Er/ZnGdO microflowers.

To investigate the effects of synthesis conditions on the morphology of Yb/Er/ZnGdO microflowers, SEM and TEM studies were carried out. All precursors displayed the microflower morphology with uniform size distribution, as shown in Fig. 1(a)–(e) and Fig. S1 (ESI†). As the citric acid content increased, the morphology/size of the microflowers changed accordingly. After heat treatment, the microflower morphology of the Yb/Er/ZnGdO samples were well maintained (Fig. 1(f)–(g) and Fig. S2, ESI†). When the citric acid content increased from 0 mM to 18 mM, the average particle size of Yb/Er/ZnGdO microflowers firstly increased, then decreased, and reached the maximum of 2.08 μm at 12 mM (Fig. S3, ESI†). On further increasing the citric acid content, the microflower size slightly increased. This may be because the citric acid content affected the nucleation and growth process of the samples, resulting in different microflower morphologies/sizes under different citric acid concentrations. Thus, the controllable microflower morphology/size of the Yb/Er/ZnGdO sample was achieved by varying the content of citric acid. The microflower structures of the Yb/Er/ZnGdO samples (citric acid content 0 and 12 mM) were further elucidated by TEM, as displayed in Fig. 1(k) and (l). A high-resolution transmission electron microscope (HRTEM) image of Yb³⁺/Er³⁺ co-doped ZnGdO (citric acid 12 mM) is shown in Fig. 1(m), where clear lattice fringes with a spacing of 0.304 nm were indexed to the (110) of hexagonal ZnO. Obvious distortion of the lattice arrays was also observed from the HRTEM (Fig. 1(m)), illustrating the formation of defects induced by the substitution of Gd/Yb/Er for Zn atoms. The selected area electron diffraction (SAED) patterns proved the existence of polycrystalline ZnGdO nanocrystals (Fig. 1(n)). Scanning and transmission analytical electron microscopy (STEM-EDX) line scan results (Fig. 1(o) and (p)) demonstrated that the Zn, Gd, Yb, Er and O elements were homogeneously distributed in the microflowers, indicating the successful preparation of Yb/Er/ZnGdO particles.

Fig. 1 SEM images of precursor microflowers with different citric acid contents: (a) 0 mM, (b) 12 mM, (c) 15 mM, (d) 18 mM, and (e) 21 mM. SEM images of Yb/Er/ZnGdO microflowers with different citric acid contents: (f) 0 mM, (g) 12 mM, (h) 15 mM, (i) 18 mM, and (j) 21 mM. TEM images of Yb/Er/ZnGdO microflowers with a citric acid content of (k) 0 mM and (l) 12 mM. Yb/Er/ZnGdO microflowers with a citric acid content of 12 mM: (m) HRTEM image, (n) corresponding SAED pattern, (o) HAADF-STEM image, and (p) the corresponding EDS line scans.

To evaluate the relationship between citric acid content, morphology, and crystal structure, X-ray powder diffraction (XRD) studies were conducted. The diffraction peaks were assigned to the JCPDS data (36-1451) and correlated with the hexagonal phase of ZnO (Fig. S4, ESI† and Fig. 2(f)). Fig. S4 and Table S1 (ESI†) indicate that the citric acid content has an important effect on the crystallinity of the sample. The variation trend of crystallinity with citric acid content is consistent with that of particle size with citric acid content. This may be because the large particle size is beneficial to the degree of long-range ordering of the sample, thus resulting in better crystallinity. Therefore, it was concluded that the citric acid content can greatly affect the morphology/size and crystallinity of the products. Besides, all of peak positions shifted slightly towards lower angles (Fig. S4, ESI†), which could be due to the increase in the lattice layer resulting from the successful substitution of Gd/Yb/Er ions into the ZnO matrix.^24,25

Fig. 2 (a) XPS results of Yb/Er/ZnGdO particles with a citric acid content of 12 mM: survey spectra; high-resolution spectra of (b) Zn 2p, (c) Gd 4d, (d) Yb 4d and Er 4d, and (e) O 1s. (f) XRD patterns of Yb/Er/ZnGdO particles with a citric acid content of 12 mM. (g) EPR spectra. (h) The conversion process of the crystal structure-change from ZnO to defect-Yb/Er/Gd/ZnO.

To further determine the chemical composition and states of Yb/Er/ZnGdO microflowers, X-ray photoelectron spectroscopy (XPS) was carried out. The chemical elements Zn Gd, Yb, Er and O could be detected in the survey spectrum (Fig. 2(a)). The Yb/Er/ZnGdO sample (citric acid 12 mM) showed typical peaks of Zn 2p_3/2 and Zn 2p_1/2 at binding energies of ∼1022.2 eV and ∼1045.2 4 eV, corresponding to the results of Zn²⁺ in the ZnO configuration (Fig. 2(b))^26,27 In the Gd 4d spectra (Fig. 2(c)), two obvious peaks at 139.04 eV for Gd 4d_5/2 and 141.66 eV for Gd 4d_3/2 were demonstrated, verifying the presence of the Gd³⁺ oxidation state.²⁸ The characteristic peaks at 198.3 and 169.6 eV were observed from the Fig. 2(d), which correspond to the binding energies of Yb 4d and Er 4d, respectively.^29,30 The replacement of Zn by Gd/Yb/Er also generated oxygen vacancies in Yb/Er/ZnGdO samples to balance the electroneutrality of the crystal. The O 1s spectra could be divided into three peaks, as shown in Fig. 2(e). The peak at 530.1 eV is due to Zn–O and the peak at 531.4 eV is related to Gd–O.^26,31,32 Besides, the XPS O 1s spectrum presented an obvious peak at the binding energy of ∼532.4 eV, illustrating the formation of oxygen vacancies. The existence of oxygen vacancies was also evidenced by EPR analysis (Fig. 2(g)); the signal at g = 2.003 originated from unpaired electrons trapped on surface oxygen vacancies (Fig. 2(h)).

3.2. Upconversion luminescence properties

The upconversion luminescence spectra of various Yb/Er/ZnGdO microflowers at different citric acid contents are shown in Fig. S5 (ESI†). Under 980 nm excitation, the upconversion nanoparticles exhibited typical green and red emission peaks of Er³⁺ due to the ²H_11/2/⁴S_3/2→ ⁴I_15/2 and ⁴F_9/2→ ⁴I_15/2 transitions, respectively.³³ Interestingly, an extra emission peak at 611 nm was observed, which could be due to the defect upconversion emission induced by hetero-valence doping.^34–37 The possible upconversion mechanisms for Yb/Er/ZnGdO are presented via the simplified energy level diagram as displayed in Fig. S6 (ESI†). It should be noted that all products exhibited similar spectral shapes and emission bands, except for the difference in the emission intensity. As the content of citric acid increased from 0 to 18 mM, the upconversion emission intensity presented a trend of first increasing and then decreasing (Fig. S5b, ESI†). When the citric acid content was 12 mM, the upconversion luminescence intensity was the strongest. Further enhancing the citric acid content to 21 mM, the emission intensity slightly increased. The difference in upconversion emission intensities could be attributed to the combined roles of the crystal structure and morphology/size. Based on SEM and the corresponding particle size results of the as-prepared products at different citric acid contents, the change trend of particles size was found to be consistent with that of luminescence intensity when the citric acid content was elevated. According to the reported surface quenching mechanism, the size/morphology of the samples contributed to their surface area, directly influencing the density of surface defects and emission intensity.³⁸ With increasing microflower size, the Er³⁺ ion concentration on the crystal surface was reduced significantly, leading to lower surface defects and stronger emission. Clearly, the morphology/size affected the upconversion luminescence via surface related processes.

Apart from the morphology/size, the crystallinity of Yb/Er/ZnGdO is an important factor that affects the upconversion luminescence properties. Based on the XRD results, the changing trend of crystallinity with citric acid content is consistent with that of luminescence intensity with citric acid content. This result indicates that high crystallinity is favorable for the upconversion emission, consistent with the previous reports.^39,40 Thus, the luminescence properties may be determined by the morphology/size and crystal structure.

3.3. Zero-thermal-quenching upconversion luminescence and temperature sensing performance

Fig. 3(a) and Fig. S7 (ESI†) show the temperature-dependent upconversion emission spectra for representative Yb/Er/ZnGdO microflowers with citric acid content of 12 mM in the temperature range from 298 K to 558 K. The green emission level of ²H_11/2 split into ²H_11/2(1) and ²H_11/2(2) levels, and the ⁴S_3/2 level split into ⁴S_3/2(1) and ⁴S_3/2(2) levels. Correspondingly, the upconversion emission peaks at 520 nm, 534 nm, 541 nm, and 552 nm were attributed to transitions from the ²H_11/2(2) → ⁴I_15/2, ²H_11/2(1) → ⁴I_15/2, ⁴S_3/2(2) → ⁴I_15/2, and ⁴S_3/2(1) → ⁴I_15/2, respectively. As the temperature was varied, the positions of the emission bands at 520 nm, 534 nm, 541 nm, and 552 nm hardly changed. Intriguingly, the green emission intensity obviously varied as the temperature increased, as shown in Fig. 3(a). Fig. 3(b) shows the variation in emission intensity of different green emission peaks along with temperature. The total green emission intensity increased with the increase in temperature (Fig. 3(c)), and a 1.6-fold improvement was achieved at 558 K. Bright green upconversion luminescence under different temperatures were clearly observed and the emission intensity was well maintained (as shown in Fig. 3(e)), which is greatly beneficial to practical applications. Under various pump powers, the green upconversion emission intensity also presented the same trends of enhancement as the temperature was elevated (Fig. 3(d)). This indicates that the thermally enhanced green upconversion luminescence is independent of pump power.

Fig. 3 Temperature-dependent (a) upconversion luminescence mapping of Yb/Er/ZnGdO particles with a citric acid content of 12 mM and (b) emission intensity variation of different green emission peaks at 520 nm, 534 nm, 541 nm, and 552 nm. (c) Statistical diagram illustrating the improvements of green emissions under 29 mW. (d) Statistical diagram illustrating the improvements of green emissions under different pump powers. (e) Luminescence photographs under different temperatures.

The thermal enhancement of green upconversion offers great opportunities for the practical application of optical thermometry based on the FIR technique. A radiometric thermometer was constructed based on the thermally coupled level pair ²H_11/2(2)/⁴S_3/2(1) for Yb/Er/ZnGdO microflowers. The energy space between the ²H_11/2(2) and ⁴S_3/2(1) levels was about 1115 cm⁻¹ according to the upconversion luminescence spectra in Fig. S7 (ESI†), obeying the Boltzmann distribution law. Consequently, this energy gap makes the energy from the ⁴S_3/2(1) level populate the ²H_11/2(2) level with the increasing temperature via thermal excitation and a quasi-thermal equilibrium. Correspondingly, the FIR of 520 nm and 552 nm varied regularly as the temperature increased. The FIR between two thermally coupled levels can be expressed as equation (eqn (1)):^41–43

where FIR represents the integrated intensity ratio of emission bands; I_H and I_S are the integrated emission intensities of different green luminescence energy levels; ΔE is the energy space between two thermally coupled levels; k, T and C are the Boltzmann constant, the absolute temperature and corresponding constant, respectively.

Eqn (1) can be converted to the following equation:

As displayed in Fig. 4(a), the Yb/Er/ZnGdO sample based on the TCL pair ²H_211/2(2)/⁴S_3/2(1) showed an excellent linear relationship between the ln(FIR) and 1/T in the temperature range 298–558 K, and the slope value (−ΔE/k) of 1336.4 was obtained. The temperature sensitivity, including absolute sensitivity (S_a) and relative sensitivity (S_r), are vital parameters for evaluating the capability of a thermometer.⁹S_a represents the change rate of FIR with response to the temperature variation, while S_r is defined as the relative change in FIR per degree of temperature change. The values of S_a-max and S_r-max represent the maximum ability of temperature sensitive performance and are usually used for comparing the temperature sensing performances of different materials.⁹ To better evaluate the temperature sensing performance, S_a and S_r are calculated by eqn (3) and (4).⁴⁴

Fig. 4 (a) Monolog plot of the FIR based on the sublevel TCL pair ²H_211/2(2)/⁴S_3/2(1)vs. the inverse absolute temperature. (b) Temperature-dependent S_r based on TCL pairs ²H_211/2(2)/⁴S_3/2(1). (c) Temperature-dependent S_a based on TCL pairs ²H_11/2(2)/⁴S_3/2(1) (G₁/G₄) and ⁴S_3/2(1)/²H_11/2(2) (G₄/G₁). (d) Monolog plot of the FIR based on the level pair ⁴F_9/2/²H_11/2(2)vs. the inverse absolute temperature. (e) Temperature-dependent S_r based on the level pair ⁴F_9/2/²H_11/2(2). (f) Temperature-dependent S_a based on the level pair ⁴F_9/2/²H_11/2(2).

S _r was calculated and the corresponding resultant curve is presented in Fig. 4(b); it is obvious that S_r is negatively correlated with temperature. At 298 K, the S_r based on the TCL pair of ²H_211/2(2)/⁴S_3/2(1) of the Yb/Er/ZnGdO sample reached the maximal value of about 1.51% K⁻¹. In contrast, the S_a of the TCL pair ²H_11/2(2)/⁴S_3/2(1) of the Yb/Er/ZnGdO sample was enhanced with the continuous increase in temperature. When the temperature reached 558 K, S_a attained the maximal value of about 2.16% K⁻¹. The primary performance parameters of the most representative luminescence thermometers are summarized in Table 1. The absolute sensitivity and relative sensitivity showed opposite trends with increasing temperature. To overcome this problem, the inverted FIR technique was utilized viaeqn (5) and (6).⁴⁵

Table 1 The comparison of the maximum sensitivity in Er³⁺-doped upconversion luminescent materials

Materials	Coupled pair	S r-max (%)	S a-max (%)	Δλ (nm)	T-range (K)	Ref.
YF3:Er^3+	^4F9/2–^4S3/2	0.46	12.7	112	303–500	2
NaBiF4:Yb,Er	^2H11/2–^4S3/2	—	0.65	19	223–548	8
Ba3Y4O9:Er^3+/Yb^3+	^2H11/2–^4S3/2	1.34	0.458	23	298–573	46
NaBiF4:Er^3+/Yb^3+	^2H11/2–^4S3/2	1.24	0.57	—	303–483	47
YF3:Yb^3+,Er^3+	^2H11/2–^4S3/2	1.20	—	20	295–478	48
GdVO4:Er^3+,Eu^3+,Yb^3+	^2H11/2–^4S3/2	1.11	0.85	31	307–473	49
Bi2SiO5:Yb^3+,Er^3+	^2H11/2–^4S3/2	0.99	0.385	20	298–600	50
La2Ti2O7:Er^3+/Yb^3+	^2H11/2–^4S3/2	0.36	0.57	24	333–553	51
Cs2NaInCl6:Er^3+/Yb^3+	^2H11/2–^4S3/2	0.94	—	25	80–500	52
ZnGdO:Yb^3+,Er^3+	^4S3/2(1)–H11/2(2)	1.51	2.54	32	298–558	This work
ZnGdO:Yb^3+,Er^3+	^4F9/2–^2H11/2(2)	1.61	15.5	149	298–558	This work

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The corresponding S_a and S_r were obtained from eqn (7) and (8):⁴⁸

The plot of ln(I_S/I_H) based on the ⁴S_3/2(1)/²H_11/2(2) level pair versus 1/T is displayed in Fig. S8 (ESI†). Note that the relative sensitivity did not change after inversing the FIR because the S_r is only related to the slope value (ΔE/k). The plot of the absolute sensitivity based on the TCL pair ⁴S_3/2(1)/²H_11/2(2) against temperature in Fig. 4(c) revealed that S_a improves along with the increase in temperature, and a maximum S_a of 2.54% K⁻¹ was observed at 298 K, surmounting the contradiction between S_a and S_r of the traditional FIR technique.

The temperature sensing behavior of the non-thermal coupled level pair ⁴F_9/2/²H_11/2(2) was investigated due to its outstanding signal discriminability. As shown in Fig. S9 (ESI†), the green emission at 520 nm showed increased intensity as the temperature was enhanced, while the red emission at 669 nm dropped obviously. The FIR value of level pair ⁴F_9/2/²H_11/2(2) exhibited the same dependent relationship between temperature and the TCL pair. The ln(FIR) and 1/T plot gave a good linear fit (eqn (6)) in the temperature range from 298 K to 558 K, as presented in Fig. 4(d). The slope of the coupled pair ⁴F_9/2/²H_11/2(2) outperformed that in the TCL pair H_11/2(2)/⁴S_3/2(1). The S_a and S_r of coupled pair ⁴F_9/2/²H_11/2(2) were calculated using eqn (7) and (8). Fig. 4(e) displays the plot of the relative sensitivity of the coupled pair ⁴F_9/2/²H_11/2(2) and temperature. The S_r-max value of the level pair ⁴F_9/2/²H_11/2(2) was 1.61% K⁻¹ at 298 K, which is better than the traditional upconversion thermometric materials mentioned in Table 1. Importantly, the S_a value-based coupled pair ⁴F_9/2/²H_11/2(2) was larger than 1.0% K over the 298–518 K range, and a maximum absolute sensitivity of 15.5% K was achieved at 298 K (Fig. 4(f)). This indicates that Yb/Er/ZnGdO microflowers are greatly promising for applications in temperature detection.

3.4. Mechanistic investigation of the zero-thermal-quenching green upconversion luminescence process

From the ESR, XPS and TEM results, as well as luminescence spectra (Fig. 6(a)), it can be inferred that interior defects of O vacancies induced by Ln³⁺ (Gd/Er/Yb) doping can result in the energy level observed at 611 nm. In view of the defect's emission, the thermally enhanced green emission phenomenon can be explained by the defect-assisted thermal distribution, as illustrated in Fig. 5. At room temperature, the electrons in the 4f ground state of Er³⁺ are excited into the ⁴I_13/2 excitation states via energy transfer from Yb³⁺ to Er³⁺ and nonradiative relaxation processes, followed by energy transfer from Yb³⁺ to the defect level, and electrons are trapped by the defect energy, achieving orange emission. When the temperature increases, the energy is transferred from defect levels to the Er^{3+ 4}S_3/2(1) level, which leads to an increase in green emission to compensate for the emission loss because of nonradiative transitions at high temperatures. Since the compensated emission loss outbalances nonradiative transitions, thermally enhanced green emission can be realized.

Fig. 5 Schematic diagram of thermally activated green upconversion emission.

To clarify the defect interaction via the relationship between temperature and green emitting levels, the variations in the emission intensity ratio of the green emission and defect with temperature were studied. The luminescence intensity ratio of green emission and defect emission with temperature was consistent with exponential fitting, same as the relationship between traditional thermally coupled energy pairs of Er³⁺ ions with temperature (Fig. 6(b)). Considering that the energy space of the defect energy level and ⁴S_3/2(1) energy level is about 1749 cm⁻¹, following the Boltzmann distribution, it can be supposed that the improvement in the emission intensity with increasing temperature is most likely due to the fact that the increment in the thermal energy is beneficial for efficient energy transfer from the defect energy level to the ⁴S_3/2(1) energy level via the Boltzmann distribution.

Fig. 6 (a) Locally amplified upconversion luminescence spectra of defect levels. (b) Temperature-dependent FIR(I_green/I_defect) value. (c) The Er³⁺ lifetime at 520 nm, 552 nm, and 669 nm versus temperature. (d) Power dependence of green emission in Yb/Er/ZnGdO particles with a citric acid content of 12 mM. (e) Power dependence of red emission in Yb/Er/ZnGdO particles with citric acid content 12 mM.

To prove the thermally assisted energy transfer from the defect level to the ⁴S_3/2(1) energy level via the Boltzmann distribution, the decay behaviors of Er³⁺ emissions at wavelengths 520 nm, 552 nm and 669 nm for Yb/Er/ZnGdO microflowers with citric acid content 12 mM were measured as shown in Fig. S10–S12 (ESI†). All the decay curves could be well fitted by a double-exponential function as follows:⁵³

where I represents the emission intensity, τ₁ and τ₂ are the fast and slow components of the decay time, and A₁ and A₂ stand for the corresponding fitting parameters.

The fluorescence lifetimes are shown in Table S2 (ESI†). The fluorescence lifetime slightly shortened from 486 μs to 471 μs as the temperature increased from 298 K to 558 K in the case of the Er³⁺ emissions at 552 nm for the Yb/Er/ZnGdO microflowers with citric acid content of 12 mM. The lifetime at 520 nm gradually became longer from 428 μs to 452 μs with the increase of the temperature. In contrast, the lifetime at 669 nm significantly decreased from 467 μs to 383 μs as the temperature increased. Dynamic analysis showed that the variation tendency of the lifetime is coincident with that of the upconversion emission intensity. According to the report, temperature has an important impact on luminescence lifetimes due to the nonradiative relaxation process.^54,55 In general, as the temperature rises, the nonradiative relaxation process increases, leading to the shortening of the lifetime. Compared to the significant change in lifetime at 669 nm with temperature, the lifetime at 552 nm was only slightly shortened. This is because of the energy transfer from the defects energy level to the ⁴S_3/2(1) energy level via the Boltzmann distribution. The prolonged lifetime at 520 nm with temperature further proved that energy transfer occurred from the defect energy level to the ⁴S_3/2(1) level and then from the ⁴S_3/2(1) level to the ²H_11/2 level via Boltzmann distribution. Thus, the thermally enhanced upconversion emission is due to the energy transfer process from the defect level to the ⁴S_3/2(1) energy level.

To further support these arguments experimentally, the power-dependent upconversion process was investigated. It is worth mentioning that the photon number (n) of green emissions was reduced with the increment of temperature from 298 to 438 and 558 K, while the n value of red emission was elevated as the temperature increased (Fig. 6(d)). This may be because the defect-assisted thermal distribution increased energy transfer between the defect and the green luminescence level, and the enhanced upconversion process resulted in a decrease in the photon number of green emissions. For the red emission, the non-radiative radiation probability increased when the temperature was elevated from 298 K to 438 K and 558 K, leading to the improvement in the linear decay process and the corresponding increment in the n value of red emission.

4. Conclusions

The thermal activation of green upconversion luminescence was observed in lanthanide-doped ZnGdO microflowers. This phenomenon is ascribed to the compensation of emission losses through energy transfer from the defect energy level to the green luminescence level via thermal excitation and a quasi-thermal equilibrium. Outstanding multiple temperature sensing behavior was realized in Yb/Er/ZnGdO microflowers. S_a and S_r up to 1.61% K⁻¹ and 15.5% K⁻¹ were achieved by utilizing the coupled pair ⁴F_9/2/²H_11/2(2) in parallel with excellent signal discriminability. This thermally activated luminescence not only provides new insights but also pushes forward the application of nano-contact optical sensing materials in temperature detection and optoelectronic devices.

Author contributions

Conceptualization: B. S., W. Z. and A. H.; data curation: W. Z., A. H., H. M. and B. S.; formal analysis: W. Z., A. H., J. C., and B. J.; funding acquisition: A. H. and B. S.; investigation: W. Z. and Y. L.; project administration: A. H. and B. S. resources: A. H. and B. S. software: W. Z., Y. L. and X. Y.; supervision: A. H. and B. S.; validation: A. H., C. C. and B. S. visualization: W. Z. and H. M. writing – original draft: W. Z., A. H., H. M. and B. S.; writing – review & editing: W. Z., A. H., H. M., B. J. and B. S.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work is supported by the China Postdoctoral Science Foundation (2023M732781).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3mh01360j

‡ Equally contributed.

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