Excellent red upconversion luminescence in GdLaO₃:Er³⁺/Yb³⁺/Sc³⁺ under 980 nm laser excitation

Abstract

A series of (Gd_0.8−xEr_0.1Yb_0.1Sc_x)LaO₃ and (Gd_0.8Er_0.1Yb_0.1)(La_1−xSc_x)O₃ upconversion luminescence (UCL) phosphors were prepared by solid state sintering technology. The structure, diffuse reflection, and UCL spectra were investigated. (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ represents excellent red UCL, which is comparable to commercial red β-NaYF₄:Er³⁺/Yb³⁺. Er³⁺/Yb³⁺ are distributed in a continuous double layer structure, blocked by a single layer of Sc³⁺/La³⁺. This structure allows cross relaxation (CR) to occur only in the double-layer structure and improves the efficiency of energy transfer with CR in the double-layer, resulting in the emission of outstanding red UCL. Using the luminescence branching intensity ratio (LIR) technique, the maximum value of relative temperature sensitivity is calculated as 0.01295 K⁻¹ at 303 K. GdLaO₃:Er³⁺/Yb³⁺/Sc³⁺ has excellent pure red UCL and can be applied in luminescence display and temperature sensing.

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1. Introduction

The upconversion luminescence (UCL) process is the process of absorbing multiple low-energy photons to emit a high-energy photon.¹ Due to this unique property, it can be widely used in solar cells,^2,3 sterilization,⁴ anti-counterfeiting,^5,6 medical treatment,^7,8 temperature sensing,^9–11 and so on. The UCL phosphor is obtained by doping the luminescence center (Er³⁺, Tm³⁺, Ho³⁺) and sensitization center (Yb³⁺) into a suitable host. In general, this host structure determines the UCL intensity. Now, β-NaYF₄ is currently recognized as the optimal host.^12,13 However, the chemical stability of fluoride is poor, and there is certain toxicity, so there are certain limitations on the above applications. Therefore, the use of a non-toxic, chemically stable oxide as a host to replace fluoride is a reasonable choice. However, a large number of experimental studies have shown that the UCL intensity of oxide phosphors is usually much lower than that of fluoride. Therefore, obtaining oxide phosphors with UCL intensity comparable to β-NaYF₄ is a challenge.

There are four main factors affecting UCL intensity. (1) Low phonon energy can reduce the non-radiative transition and help to obtain high UCL intensity.^14,15 (2) The low symmetry of the luminous center helps to break the imprisonment of the 4f–4f transition and improve the UCL intensity.^16–18 (3) The appropriate distance of rare earth ions is helpful to transfer energy from the sensitized center to the luminescent center and enhance UCL intensity.¹⁹ (4) Distribution of rare earth ions in the host:²⁰ a suitable distribution of rare earth ions in the host helps to reduce and enhance the cross relaxation (CR) between rare earth ions and can regulate the visible green and red emission ratio for Yb³⁺/Er³⁺ co-doped samples.^21,22 In our previous study,^23–25 it was found that the BiTa₇O₁₉ host has low phonon energy and low luminescence center symmetry, while Er³⁺/Yb³⁺ are distributed in a single layer, obtaining excellent pure green UCL. Based on the inspiration of the previous experiments, the search for a low phonon and low luminescent center symmetry oxide host with Er³⁺/Yb³⁺ distributed in a continuous double layer structure and blocked by other atomic layers is expected to obtain excellent pure red-light emission. Based on the above analysis, a low phonon energy (∼419 cm⁻¹) GdLaO₃ host with space group C2/m (No. 12) is expected to be a good choice.^26,27 Gd³⁺/La³⁺ and Gd³⁺ sites form a continuous double layer structure, which is blocked by a single layer of La³⁺. Therefore, we regulate doping different elements so that Er³⁺/Yb³⁺ replaces Gd³⁺/La³⁺ and Gd³⁺ sites, making Er³⁺/Yb³⁺ distribute in a double layer structure, blocked by a La³⁺ single layer, and it is expected to obtain outstanding red UCL intensity. Stronger absorption of biological tissues for green UCL than red UCL is observed. Therefore, red UCL can have a deeper tissue detection depth, which is more suitable to be utilized in biomedical treatment.^28,29

On the basis of Er³⁺/Yb³⁺ co-doping, foreign ion doping to break the symmetry of the crystal field in the luminous center will significantly improve the UCL intensity. Some studies have shown that the smallest trivalent rare earth ion (Sc³⁺) doping will decrease the crystal field symmetry and cause greater local structural distortion, leading to UCL enhancement.^30–32 Additionally, due to the small ionic radius and the same valence, Sc³⁺ ions can easily enter into other rare-earth-based host lattices as dopants for modulation of the crystallographic parameters toward UCL tuning.³³ Therefore, the sites occupied by Er³⁺/Yb³⁺/Sc³⁺ can be controlled by changing the molar ratio of the raw materials to obtain excellent red UCL intensity in Er³⁺/Yb³⁺/Sc³⁺ co-doped GdLaO₃ phosphors.

In this work, (Gd_0.8−xEr_0.1Yb_0.1Sc_x)LaO₃ (x = 0, 0.05, 0.1, 0.15, 0.2) and (Gd_0.8Er_0.1Yb_0.1)(La_1−xSc_x)O₃ (x = 0, 0.05, 0.1, 0.15, 0.2) phosphors were successfully prepared by high temperature solid phase sintering. (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ has excellent red UCL intensity comparable to β-NaYF₄:Er³⁺/Yb³⁺ under 980 nm laser excitation. Employing the luminescence branching intensity ratio (LIR) technique, the maximum value of relative temperate sensitivity of 0.01295 K⁻¹ at 303 K was reached. GdLaO₃:Er³⁺/Yb³⁺/Sc³⁺ is expected to replace β-NaYF₄:Er³⁺/Yb³⁺ in the applied field of luminescence display and temperature sensing.

2. Experimental

2.1. Sample synthesis

All samples were prepared using the traditional high temperature solid state reaction method. The materials used in the production process included Gd₂O₃ (99.9%), La₂O₃ (99.9%), Er₂O₃ (99.9%), Yb₂O₃ (99.9%), and Sc₂O₃ (99.9%), weighed following the stoichiometric ratio. The weighed samples were mixed in an agate mortar, ground for 30 minutes, and then sintered in a resistance furnace at 1500 °C for 8 h. After the furnace cooling process was completed, the sintered samples were further ground into powders for the test.

2.2. Characterizations

Powder X-ray diffraction (XRD) analysis of the samples was conducted using a Rigaku X-ray diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the 10°–90° range. The Rietveld refinement of the XRD data was performed with the general structural analysis system (GASA) software.³⁴ The diffuse reflectance spectra of the samples were analyzed using a UV3600 spectrophotometer (Shimadzu, Japan). The emission spectra of the samples at variable temperatures (303 to 663 K) were measured using a Hitachi F-4600 fluorescence spectrophotometer. Temperature control was achieved with a home-made device (DMU-TC 450), ensuring an accuracy better than ± 0.5 °C.

3. Results and discussion

Powder X-ray diffraction (XRD) pattern analysis was conducted to verify the crystal structure and phase purity of the synthesized phosphors. The XRD patterns are illustrated in Fig. 1(a) and (b). The diffraction peaks of the synthesized phosphors closely match the standard card of GdLaO₃ (ICSD#190648) with space group C2/m (No. 12), and lattice parameters (a, b, c) = (14.433, 3.689, 8.997 Å) with a B-type monoclinic structure. Notably, when the Sc³⁺ concentration is below 10 mol%, no peaks related to the raw materials of Gd₂O₃, La₂O₃, Sc₂O₃, or any other impurities were detected, except for the signals originating from the GdLaO₃ host. However, when the Sc³⁺ concentration exceeds 10 mol%, Sc₂O₃ impurities are observed. The enlarged angular regions from 27° to 30° are shown in the right patterns of Fig. 1(a) and (b), respectively. Evidently, the absence of peak shifting was confirmed when the sample was synthesized using stoichiometric means with Sc³⁺ ions molar replacing Gd³⁺ ions molar. However, when the sample was synthesized by stoichiometric means with Sc³⁺ ions molar replacing La³⁺ ions molar, the diffraction peaks shifted towards the smaller angle direction. This shows that the sintered sample with Sc³⁺ ions molar replacing La³⁺ ions molar has a more obvious structural change as the composition of Sc³⁺ increases than the sample with Sc³⁺ ions molar replacing Gd³⁺ ions molar, and the volume decreases as the doping of Sc³⁺ with a small radius increases.

Fig. 1 Powder X-ray diffraction (XRD) patterns of (Gd_0.8−xEr_0.1Yb_0.1Sc_x)LaO₃ (x = 0, 0.05, 0.1, 0.15, 0.2) phosphors (a) and (Gd_0.8Er_0.1Yb_0.1)(La_1−xSc_x)O₃ (x = 0, 0.05, 0.1, 0.15, 0.2) phosphors (b). The standard pattern of GdLaO₃ (ICSD#190648) is presented as a reference. The right patterns in pictures (a) and (b) depict an enlarged angular region from 27° to 30°, respectively.

The diffuse reflection (DR) spectra of (Gd_0.8Er_0.1Yb_0.1)LaO₃, (Gd_0.8Er_0.1Yb_0.1Sc_0.1)LaO₃, and (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ phosphors are presented in Fig. 2(a). The absorption peaks in the DR spectra, ranging from 200 to 1800 nm, are mainly attributed to the transitions of Er³⁺ from ⁴I_15/2 to ⁴G_11/2, ⁴F_7/2, ²H_11/2, ⁴F_9/2, ⁴I_9/2, ⁴I_11/2, and ⁴I_13/2, respectively. Notably, these peaks at ∼976 nm are from the transitions of Yb³⁺ from ²F_7/2 to ²F_5/2 and Er³⁺ from ⁴I_15/2 to ⁴I_11/2. Fig. 2(b) illustrates the corresponding bandgap values of the three phosphors, which are 4.61, 4.70, and 4.74 eV, respectively. Comparing the bandgap values among the three samples, this investigation reveals that the bandgap of the Er³⁺/Yb³⁺ co-doped GdLaO₃ phosphor is smaller than that of the Er³⁺/Yb³⁺/Sc³⁺ triple-doped GdLaO₃ phosphors. Furthermore, the bandgap value of the phosphor synthesized by stoichiometric means with Sc³⁺ ions molar replacing Gd³⁺ ions molar is smaller than that of the phosphor synthesized by stoichiometric means with Sc³⁺ ions molar replacing La³⁺ ions molar. Additionally, Fig. S1 (ESI†) displays the DR spectra and corresponding bandgap values for all samples. Fig. S2 (ESI†) presents relationships between bandgap and doped Sc³⁺ concentration in the (Gd_0.8−xEr_0.1Yb_0.1Sc_x)LaO₃ (x = 0, 0.05, 0.1, 0.15, 0.2) and (Gd_0.8Er_0.1Yb_0.1)(La_1−xSc_x)O₃ (x = 0, 0.05, 0.1, 0.15, 0.2) phosphors. The experimental results show that Sc³⁺ doping can increase the bandgap values. It is noteworthy that when the concentration of Sc³⁺ exceeds 10 mol%, the samples prepared by replacing La³⁺ ions molar with Sc³⁺ consistently exhibit a higher bandgap compared to the samples prepared by replacing Gd³⁺ ions molar with Sc³⁺. The dosing according to different chemical formulas will lead to different lattice structures, which will change the bandgap values.

Fig. 2 (a) Diffuse reflectance spectra of (Gd_0.8Er_0.1Yb_0.1)LaO₃, (Gd_0.8Er_0.1Yb_0.1Sc_0.1)LaO₃, and (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ phosphors. (b) Corresponding calculated bandgap values.

In order to confirm the positions occupied by Er³⁺/Yb³⁺/Sc³⁺, we performed Rietveld refinement of the XRD data. For (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃, we first carried out the replacement of Rietveld refinement according to the molar ratio of raw materials, that is, supposing Er³⁺/Yb³⁺ replaces Gd³⁺ and Sc³⁺ replaces La³⁺. The result of this refinement is shown in Fig. S3(a) (ESI†). This refinement result is very unsatisfactory, and some peaks correspond very poorly, so it is unreasonable to refine according to the molar ratio of this chemical formula. Therefore, we assume that Er³⁺/Yb³⁺/Sc³⁺ can randomly replace Gd³⁺ and La³⁺ sites for refinement. The refinement result is illustrated in Fig. 3(e). This refined result is more reasonable than Fig. S3(a) (ESI†). R_p is 12.6, and R_wp is 16, and these values are relatively high. Therefore, in order to verify the rationality of the above refinement, we conducted XRD slow scanning of the sample to improve the diffraction intensity of XRD, and then carried out the same refinement operation. R_p and R_wp become 4.32 and 5.55, respectively, as shown in Fig. S3(b) (ESI†). The R_p and R_wp values are very reasonable and once again confirm the accuracy of the structural refinement. With the same operation, the XRD results of (Gd_0.8Er_0.1Yb_0.1)LaO₃ and (Gd_0.8Er_0.1Yb_0.1Sc_0.1)LaO₃ are also refined, as shown in Fig. 3(a) and (c). For (Gd_0.8Er_0.1Yb_0.1)LaO₃ and (Gd_0.8Er_0.1Yb_0.1Sc_0.1)LaO₃,Er³⁺/Yb³⁺ replace the Gd³⁺, Gd³⁺/La³⁺, and La³⁺ sites, and the distribution of Er³⁺/Yb³⁺ in the host belongs to three-dimensional (3D). For (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃, Er³⁺/Yb³⁺ replace the Gd³⁺ and Gd³⁺/La³⁺ sites, and most Er³⁺/Yb³⁺ replace the Gd³⁺ sites. Sc³⁺ replaces La³⁺ sites. Here, Er³⁺/Yb³⁺ is distributed in a continuous double layer structure, blocked by a single layer of Sc³⁺/La³⁺. The distribution of Er³⁺/Yb³⁺ in (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ is the case between 3D and monolayer (2D). This structure can prevent the energy transfer between different bilayer structures and promote cross relaxation between Er³⁺ and Er³⁺, Yb³⁺ and Er³⁺ in the double layers, and therefore is expected to obtain outstanding red UCL. In addition, the nearest distances between Yb³⁺ and Er³⁺ or Er³⁺ and Er³⁺ in the (Gd_0.8Er_0.1Yb_0.1)LaO₃, (Gd_0.8Er_0.1Yb_0.1Sc_0.1)LaO₃, and (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ phosphors are 3.42(4), 3.37(6), and 3.65(8) Å, respectively, which are labeled in Fig. 3(b), (d), and (f).

Fig. 3 Rietveld refinement of (Gd_0.8Er_0.1Yb_0.1)LaO₃ (a), (Gd_0.8Er_0.1Yb_0.1Sc_0.1)LaO₃ (c), and (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ (e) phosphors. The crystal structures (b), (d), and (f) correspond to the above refined results, respectively. All the different coordination environments in GdLaO₃ are also shown below.

Fig. 4(a) and (b) display the UCL spectra of (Gd_0.8−xEr_0.1Yb_0.1Sc_x)LaO₃ (x = 0, 0.1, 0.2) and (Gd_0.8Er_0.1Yb_0.1)(La_1−xSc_x)O₃ (x = 0, 0.1, 0.2) under 980 nm laser excitation. The samples with a concentration of 10 mol% Sc³⁺ exhibit the highest UCL intensity. The UCL spectra of other trivalent cation (Al³⁺, Y³⁺, Bi³⁺, and La³⁺) doped samples, respectively, replacing Gd³⁺ and La³⁺ in molar ratios for weighing, were also tested and shown in Fig. S4(a) and (c) (ESI†). The red UCL intensity integral areas are calculated and displayed in Fig. S4(b) and (d) (ESI†). This result shows that, compared with other trivalent elements, Sc³⁺ doping can obtain the highest red UCL intensity.

Fig. 4 UCL spectra of (Gd_0.8−xEr_0.1Yb_0.1Sc_x)LaO₃ (x = 0, 0.1, 0.2) (a) and (Gd_0.8Er_0.1Yb_0.1)(La_1−xSc_x)O₃ (x = 0, 0.1, 0.2) (b) under 980 nm laser excitation.

Fig. 5(a) depicts the red UCL integral intensity of the (Gd_0.8Er_0.1Yb_0.1)LaO₃, (Gd_0.8Er_0.1Yb_0.1Sc_0.1)LaO₃, (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃, and β-NaYF₄:Er³⁺/Yb³⁺ phosphor under 980 nm laser excitation with different excitation power densities. The red UCL intensity of all samples increases with increasing excitation power density. The UCL performance of (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ is better than that of (Gd_0.8Er_0.1Yb_0.1Sc_0.1)LaO₃, and much better than that of (Gd_0.8Er_0.1Yb_0.1)LaO₃. Under 130.96 W cm⁻², the red UCL intensity of the (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ sample reaches 100% of the strength exhibited by the commercial β-NaYF₄:Er³⁺/Yb³⁺ phosphor. The corresponding UCL spectra are shown in Fig. 5(b) under 130.96 W cm⁻² excitation.

Fig. 5 (a) Red UCL integral intensity of (Gd_0.8Er_0.1Yb_0.1)LaO₃, (Gd_0.8Er_0.1Yb_0.1Sc_0.1)LaO₃, (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃, and commercial β-NaYF₄:Er³⁺/Yb³⁺ phosphors under 980 nm laser excitation with different excitation power densities. (b) UCL spectra when the pump power density of the 980 nm laser was fixed at 130.96 W cm⁻².

The logarithmic curves of the red and green UCL intensity of (Gd_0.8Er_0.1Yb_0.1)LaO₃, (Gd_0.8Er_0.1Yb_0.1Sc_0.1)LaO₃, and (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ as a function of the pump power density are shown in Fig. 6(a), (b) and (c), respectively. The relationship between the pump power density (P) and the integrated UCL intensity (I) in a multiphoton process can be expressed as follows:

ln(I) ∝ n ln(P) (1)

where n is the number of photons. The linear fitting of the integrated red UCL yields slope n values of 1.997, 1.855, and 2.134 for (Gd_0.8Er_0.1Yb_0.1)LaO₃, (Gd_0.8Er_0.1Yb_0.1Sc_0.1)LaO₃, and (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃, respectively. Similarly, the corresponding green UCL slope n values are 2.199, 2.070, and 2.320, respectively. Fig. 6(d) illustrates the relationship between n values and the nearest distance between Yb³⁺ and Er³⁺ or Er³⁺ and Er³⁺. This result shows that the change in n value is related to the nearest distance, and for the GdLaO₃ host, the n value increases with the increase in the nearest distance. This indicates that the n value is closely related to the energy transfer efficiency from CR between Yb³⁺ and Er³⁺ or Er³⁺ and Er³⁺ under 980 nm laser excitation.

Fig. 6 Double logarithmic plots of the integrated red and green UCL intensity as a function of pump power of (a) (Gd_0.8Er_0.1Yb_0.1)LaO₃, (b) (Gd_0.8Er_0.1Yb_0.1Sc_0.1)LaO₃, and (c) (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃. (d) Relationship between the slope of the curve and nearest distance between Yb³⁺ and Er³⁺ or Er³⁺ and Er³⁺ in the host.

The possible UCL luminescence process under 980 nm pump excitation is illustrated in Fig. 7. UCL processes contain ground state absorption (GSA), excited state absorption (ESA), energy transfer (ET), multiphonon non-radiative transition (MPR), and cross relaxation (CR). The CR process is significantly related to the distance between Yb³⁺ and Er³⁺ or Er³⁺ and Er³⁺. Yb³⁺ exhibits a large absorption cross section of 980 nm, and the electron absorbs 980 nm light energy and jumps from ²F_7/2 to ²F_5/2. Firstly, Er³⁺ undergoes two energy transfers from Yb³⁺: ⁴I_15/2 (Er³⁺) + ²F_5/2 (Yb³⁺) → ⁴I_11/2 (Er³⁺) + ²F_7/2 (Yb³⁺) (ET 1) and ⁴I_11/2 (Er³⁺) + ²F_5/2 (Yb³⁺) → ⁴F_7/2 (Er³⁺) + ²F_7/2 (Yb³⁺) (ET 2), resulting in the population of ⁴F_7/2 of Er³⁺. The ²H_11/2 and ⁴S_3/2 are then populated through MPR, resulting in green light emission through radiative transitions with ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 in Er³⁺. ⁴F_9/2 in Er³⁺ can be populated through MPR with ⁴S_3/2 → ⁴F_9/2. However, due to the low phonon energy of the GdLaO₃ host, MPR with ⁴S_3/2 → ⁴F_9/2 is weak. Therefore, it is believed that this transition pathway is not the main reason for the strong red UCL emission. Additionally, the red UCL pathway also involves electrons that are populated at ⁴I_13/2 through MPR with ⁴I_11/2 → ⁴I_13/2 and then jump to ⁴F_9/2 through energy transfer: ⁴I_13/2 (Er³⁺) + ²F_5/2 (Yb³⁺) → ⁴F_9/2 (Er³⁺) + ²F_7/2 (Yb³⁺) (ET 3), resulting in red UCL emission. However, this MPR with ⁴I_11/2 → ⁴I_13/2 is also weak, so this transition pathway should not be the main cause of the intense red UCL emission. In addition, red UCL emission can be achieved by CR 1, CR 2, and CR 3. In general, for oxide UCL phosphors with low phonon energy, red UCL should come mainly from CR, which has also been discussed in detail in some other studies.^35–38 The three CR 1, CR 2 and CR 3 processes involve the following transitions: ⁴S_3/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴I_13/2 (Er³⁺) + ²F_5/2 (Yb³⁺) (CR 1), ⁴I_13/2 (Er³⁺) + ²F_5/2 (Yb³⁺) → ⁴F_9/2 (Er³⁺) + ²F_7/2 (Yb³⁺) (CR 2), and ⁴F_7/2 (Er³⁺) + ⁴I_11/2 (Er³⁺) → ⁴F_9/2 (Er³⁺) + ⁴F_9/2 (Er³⁺) (CR 3). CR 1 can reduce the populated electron number at ⁴S_3/2, which produces green UCL. CR 2 and CR 3 can enhance the electron population of ⁴F_9/2 in Er³⁺ with emitting red UCL. In summary, the strong red UCL emission mainly comes from the simultaneous occurrence of CR 1 and CR 2 between Er³⁺ and Yb³⁺, and CR 3 between Er³⁺ and Er³⁺. For (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃, Er³⁺ and Yb³⁺ are distributed in a continuous double layer structure, blocked by a single layer of Sc³⁺/La³⁺. Therefore, this structure allows CR 1, CR 2, and CR 3 to occur only in the double-layer structure and improves the efficiency of energy transfer with CR 1, CR 2, and CR 3 in the double-layer. These CR 1 and CR 2 will make the number of photons n emitted by red light become super-quadratic dependence of the red intensity on 980 nm laser power density. CR 3 will cause a sub-quadratic dependence of red intensity on 980 nm laser power density because three 980 nm photons generate two red photons.³⁸ Therefore, for (Gd_0.8Er_0.1Yb_0.1)LaO₃ and (Gd_0.8Er_0.1Yb_0.1Sc_0.1)LaO₃, CR 3 may have a major role. For (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃, CR 1 and CR 2 may play a major role. In these three samples, the difference in photon number n and the difference in electron transition path should be due to the difference in Er³⁺/Yb³⁺/Sc³⁺ substitution positions and the change in distance between Er³⁺ and Yb³⁺ or Er³⁺ and Er³⁺.

Fig. 7 Schematic illustration of possible UC emission processes under 980 nm laser excitation.

Fig. 8(a) illustrates the temperature-dependent UCL spectra of (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ within the temperature range of 303–663 K under 980 nm laser excitation. It has been demonstrated that the UCL integral intensity decreases as the temperature increases. To evaluate the temperature sensing property of (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃, the UCL integral intensity of ²H_11/2 → ⁴I_15/2 (I_H), ⁴S_3/2 → ⁴I_15/2 (I_S), and ⁴F_9/2 → ⁴I_15/2 (I_F) were calculated under different temperatures. The LIR (I_H/I_S, I_H/I_F) values and their relationship with temperature can be calculated and fitted using the following formula:

B₁ and B₂ are constants, and ΔE₁ and ΔE₂ represent the bandgaps between the energies ²H_11/2 and ⁴S_3/2, ²H_11/2 and ⁴F_9/2, respectively, K_B is the Boltzmann constant, and T is the absolute temperature. The experimental results are depicted in Fig. 8(b). Based on the data presented in Fig. 8(b), the absolute sensitivity (S_A) and relative sensitivity (S_R) can be calculated as follows:

Fig. 8 (a) Temperature-dependent UCL emission spectra of the (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ phosphor at 303–663 K under 980 nm laser excitation. The inset shows an enlarged wavelength region from 500 to 600 nm. (b) Relationship between LIR values and temperature. (c) Absolute sensitivity (S_A). (d) Relative sensitivity (S_R).

The experimental results of absolute sensitivity (S_A) and relative sensitivity (S_R) are presented in Fig. 8(c) and (d), respectively. The maximum values of absolute sensitivity (S_A) are 0.004116 K⁻¹ at 552 K and 0.000286 K⁻¹ at 595 K, respectively. Similarly, the maximum values of the relative sensitivities (S_R) are 0.01206 K⁻¹ and 0.01295 K⁻¹ at 303 K, respectively. The maximum absolute and relative sensitivity are compared with other materials, as shown in Table 1. This result indicates that (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ exhibits excellent temperature sensing performance under 980 nm laser excitation and holds promising potential for temperature sensing applications.

Table 1 Values of maximum absolute sensitivity (S_A) and relative sensitivity (S_R) of different materials

Materials	S A (K^−1)	S R (K^−1)	Ref.
SrLu2O4:Er^3+/Yb^3+	0.00126	0.00260	39
BiLaWO6:Er^3+/Tm^3+/Yb^3+	0.00120	0.00430	40
Ba3Y4O9:Er^3+/Yb^3+	0.00371	—	41
Gd2(WO4)3:Er^3+/Yb^3+	0.00130	—	42
NaGdF4:Er^3+	0.001	—	43
Gd2O3:Er^3+@Gd2O3:Yb^3+	0.0007	—	44
LaGdO3:Er^3+/Yb^3+	0.0034	0.0108	45
BaY2O4:Er^3+/Yb^3+	0.0019	—	46
(Gd0.8Er0.1Yb0.1)(La0.9Sc0.1)O3	0.0041163	0.01295	This work

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4. Conclusion

GdLaO₃:Er³⁺/Yb³⁺/Sc³⁺ phosphors with space group C2/m (No. 12) are successfully synthesized by high temperature solid state sintering and have excellent red UCL intensity. (Gd_0.8Er_0.1Yb_0.1)(La_0.9Sc_0.1)O₃ represents the highest red UCL intensity in all the samples, which is comparable to commercial red β-NaYF₄:Er³⁺/Yb³⁺. The Rietveld refinement result of the XRD data reveals that Er³⁺/Yb³⁺ are distributed in a continuous double layer structure, blocked by a single layer of Sc³⁺/La³⁺. This special structure allows cross relaxation between Er³⁺ and Er³⁺, Er³⁺ and Yb³⁺ to occur only in the double-layer structure and improves the efficiency of energy transfer with cross relaxation in the double-layer. The maximum value of relative temperature sensitivity reaches 0.01295 K⁻¹ at 303 K. GdLaO₃:Er³⁺/Yb³⁺/Sc³⁺ phosphors with excellent pure red UCL emission have good application prospects in luminescence display and temperature sensing.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This study was supported in part by NSFC (National Natural Science Foundation of China, grant no. 11774042, 52071048 and 51772159), Fundamental Research Funds for the Central Universities (grant no. 3132023196 and 3132023519), and China Postdoctoral Science Foundation (grant no. 2016M591420 and 3620080711).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc01015a

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