Upconversion and Downconversion Luminescence of CaLaLiTeO₆:Mn⁴⁺/Er³⁺ Phosphors for Dual-Mode Optical Thermometry and Anti-Counterfeiting Application

Abstract

Multifunctional phosphors that integrate optical temperature measurement and counterfeit detection capabilities have garnered considerable interest owing to their diverse application potential. In this study, novel CaLaLiTeO₆:Mn⁴⁺/Er³⁺ phosphors were prepared via the high-temperature solid-phase method. The phase structure and morphology characterization confirmed the successful synthesis of CaLaLiTeO₆ material with effective doping of Mn⁴⁺ and Er³⁺ into the host lattice. Upon excitation at 379 nm, the CaLaLiTeO₆:Mn⁴⁺/Er³⁺ material exhibits far-red emission at 716 nm (Mn⁴⁺:²E_g → ⁴A_2g) and green emission at 525/548 nm (Er³⁺:²H_11/2/⁴S_3/2 → ⁴I_15/2). The emission peak intensities of Er³⁺ and Mn⁴⁺ ions in the CaLaLiTeO₆:0.015Mn⁴⁺/0.01Er³⁺ sample displayed distinct variations with temperature under different excitation wavelengths (325 nm, 379 nm, and 980 nm). Subsequently, a dual-mode optical temperature sensing system was developed based on the fluorescence intensity ratio and the dual excitation single-band ratiometric method, which achieved a maxed relative sensitivity of 1.12% K⁻¹ at 343 K. Moreover, the excitation-dependent luminescence color changes of CaLaLiTeO₆:Mn⁴⁺/Er³⁺ make it particularly suitable for anti-counterfeiting applications. The present study underscores the dual-functional capabilities in sophisticated non-contact optical temperature measurement and anti-counterfeiting applications.

1. Introduction

With the advancement of society, there is a progressively increasing need for temperature sensing systems that demonstrate excellent resolution, exceptional sensitivity, and nondestructive characteristics [1,2,3,4]. The emergence of new thermometers that utilize the optical response of materials offers distinct advantages, including outstanding light efficiency, resilience to environmental factors, and swift reaction times, positioning them as a significant focus of contemporary research [5]. Luminescent materials possess specific optical characteristics that respond to temperature variations, which include the shape of the emission spectrum, position of the emission maximum, spectral width, fluorescence intensity ratio (FIR), polarization dependence, fluorescence strength, and fluorescence decay time [6]. Of these characteristics, the FIR temperature measurement technique is acknowledged as a promising approach because of its decreased reliance on measurement conditions and resilience to spectral degradation [7].

Most FIR thermometers primarily rely on thermally coupled energy levels (TCLs) of rare earth (RE) ions. The enhancement of relative sensitivity necessitates a broader TCL energy gap; however, the stable energy gap of TCLs (ranging from 200 cm⁻¹ to 2000 cm⁻¹) imposes limitations on the improvement of relative sensitivity. To enhance thermometric sensitivity, co-doping independent dual emitting centers has garnered significant interest due to its substantial variation in FIR with temperature, resulting in relatively high sensitivity. This approach is typically achieved by co-doping a RE ion with a transition metal (TM) ion, which serve as the reference and temperature probe signals, respectively [8,9]. The FIR based on the dual emission peak ratio under a single excitation source exhibits greater stability owing to its reduced sensitivity to measurement conditions and external interferences [8]. The photoluminescence (PL) properties of TM ions are particularly susceptible to surrounding conditions due to their unshielded 3d shell, rendering TM-based luminescence more effective for temperature sensing in comparison to RE-based counterparts. For instance, Yan et al. reported a maximum relative sensitivity of 0.7% K⁻¹ for Lu₃Al₅O₁₂:Eu³⁺/Mn⁴⁺ phosphors [10]. Fang et al. demonstrated that the CaGdAlO₄:Mn⁴⁺, Tb³⁺ phosphors achieved a maximum relative sensitivity of 2.3% K⁻¹ [11]. In addition to the FIR mode associated with dual different emission centers, an innovative measurement strategy has emerged for assessing the temperatures of objects, referred to as the dual-excitation single-band ratiometric (SBR) thermometry strategy. Using the SBR technique, Trejgis et al. developed an optical thermometer through the use of the excited state absorption mechanism [12]. Yang et al. also developed an innovative SBR-based temperature sensor that leverages the redshift of the charge transfer band edge [13]. Integrating multiple modes within a single phosphor may represent an effective strategy for enhancing thermometric performance and providing self-calibrating capabilities. In comparison to single-mode temperature sensors, multi-mode thermometers exhibit superior performance, characterized by higher accuracy and sensitivity [9]. Energy transfer mechanisms between impurity ions can extend beyond classical resonant models to include processes mediated by the host lattice, such as charge carrier trapping and thermal release or exciton migration, as demonstrated in LaPO₄:Ce,Tb nanocrystals, where the microsecond-scale rise time of Tb³⁺ emission and its strong thermal quenching at low temperatures reflect a temperature-dependent transfer efficiency contrary to resonant behavior [14]. In YAG:Nd,Er,Cr systems, Cr³⁺ acts as a sensitizer via phonon-assisted energy transfer to lanthanides, while Cr⁴⁺ serves as an energy bridge between Nd³⁺ and Er³⁺ [15], with both processes exhibiting pronounced temperature sensitivity due to their phonon-assisted nature, leading to enhanced emission intensity and thermometric performance at elevated temperatures.

In recent years, the double-perovskite structure of AA’BB’O₆ has garnered significant research attention [16]. Contrasted with the ABO₃ perovskite, the double-perovskite structure offers a greater number of lattice sites for substituting various elements, particularly high-valence ions such as Nb⁵⁺, Sb⁵⁺, Te⁶⁺, and W⁶⁺, thus facilitating the formation of a diverse lattice framework. This diversity facilitates the formation of multiple crystalline phases, including cubic, monoclinic, and other crystal systems. The abundant double-perovskite structures provide numerous [BO₆] and [B’O₆] octahedral sites capable of accommodating Mn⁴⁺ substitution. Noteworthy is the fact that the replacement of ions at the A, A’, B, and B’ sites induces distortions, contractions, and other alterations in the octahedral groups, thereby imparting new luminescent behaviors to Mn⁴⁺. One exemplary host of the double-perovskite structure is CaLaLiTeO₆ (CLLT), which facilitates PL across different bands through the activation of various ions [16,17,18,19]. In this study, TM Mn⁴⁺ ions and RE Er³⁺ ions were successfully doped into the CLLT double-perovskite structure. The synthesized products underwent characterization of phase structure and morphology through X-Ray diffraction (XRD), scanning electron microscope (SEM), and elemental mapping analyses. Moreover, the PL characteristics of the specimens with varying doping levels were thoroughly investigated at different excitation wavelengths. Leveraging the distinctive thermal behaviors of Er³⁺ and Mn⁴⁺ luminescence, a dual-mode optical thermometry was developed by combining dual emission LIR and dual excitation SBR methods. Furthermore, the co-doped CLLT:Mn⁴⁺/Er³⁺ phosphor demonstrates practical applications in the production of anti-counterfeiting tags, leveraging its excitation-dependent color-switching properties.

2. Results and Discussion

2.1. Crystalline Structure and Morphology

The crystal structure and phase purity of the synthesized phosphors were characterized by XRD patterns. The XRD patterns of samples including CLLT, CLLT:0.015Mn⁴⁺, CLLT:0.01Er³⁺, CLLT:0.015Mn⁴⁺/0.01Er³⁺, and CLLT:0.015Mn⁴⁺/0.10Er³⁺ closely matched those of the reference card (PDF # 04-018-0711) for CLLT, as depicted in Figure 1a. These phosphors exhibited a monoclinic crystal structure identical to that of the CLLT crystal, belonging to the P21/c space group. Notably, no impurity phases were detected in the XRD patterns, indicating that the codoping with Mn⁴⁺ and Er³⁺ did not induce significant structural distortions in the double-perovskite system.

In assessing ion substitution within the crystal structure, the difference in ionic radius (D_r) was determined using the following equation:

$$D_r={\displaystyle \frac{R_s(CN)-R_d(CN)}{R_s(CN)}}\times 100\%$$

(1)

where D_r signifies the percentage variation in radius, CN indicates the coordination number, and R_s and R_d refer to the radii of the central and substituted ions, respectively. Typically, Er³⁺ substitutes ions with greater radii, such as Ca²⁺ or La³⁺, within this structure. The calculated ionic radius differences are determined as D_r(Er-La) = 13.44% and D_r(Er-Ca) = 10.35%. Given their similar positions and sizes, Er³⁺ is expected to be disordered and occupy La³⁺ or Ca²⁺ sites. This behavior is consistent with observations in other rare-earth-doped phosphors with the identical host material, such as CaLaLiTeO₆:Tm³⁺/Mn⁴⁺ [17] and CaLaLiTeO₆:Yb³⁺,Tm³⁺ [20]. Conversely, Mn⁴⁺ is improbable as a substitute for La³⁺ or Ca²⁺ owing to the significant difference in ionic radii. Mn⁴⁺ typically occupies sites with octahedral coordination and emits red light. Within the CLLT matrix, two octahedral structures exist: [TeO₆]⁶⁻ and [LiO₆]¹¹⁻. Calculations based on elemental radii differences show D_r(Mn-Te) as 5.35% and D_r(Mn-Li) as 30.26%. Consequently, Mn⁴⁺ ions preferably substitute Te⁶⁺ within the CLLT matrix as opposed to Li⁺ sites, following the principle of radius similarity.

The substitution of Ca²⁺ by Er³⁺ and Te⁶⁺ by Mn⁴⁺ introduces localized charge imbalances that necessitate compensation mechanisms. Due to the low concentrations of Er and Mn, their core-level peaks could not be directly detected in the X-Ray photoelectron spectroscopy (XPS) survey spectrum of CaLaLiTeO₆:0.015Mn⁴⁺/0.01Er³⁺ (Figure S1a, Supplementary Information). Instead, the high-resolution O 1s spectrum was deconvoluted into two components, corresponding to oxygen vacancy and lattice oxygen (Figure S1b, Supplementary Information). These oxygen vacancies serve as the primary charge compensation mechanism for both the positive charge excess created by Er³⁺ substituting for Ca²⁺ and the negative charge deficit created by Mn⁴⁺ substituting for Te⁶⁺.

In order to validate the crystal structure further, Rietveld structure refinement of CLLT:0.015Mn⁴⁺/0.01Er³⁺ was performed utilizing the standard structures of PDF # 04-018-0711 as the initial data. As shown in Figure 1b, the refinement yielded Rwp = 6.25%, Rp = 4.83%, and χ² = 1.65, indicating good agreement between the experimental and standard data. The cell volume was calculated as 245.83 ų, with an average Ca-O bond distance of 2.58 Å for the CLLT:0.015Mn⁴⁺/0.01Er³⁺ samples. The refined atomic parameters for CLLT:0.015Mn⁴⁺/0.01Er³⁺ are summarized in Table S1 provided in the Supplementary Information. The crystal structures extracted from the refinement findings are depicted in Figure 1c. The skeletal structure of CLLT comprises alternating octahedra [TeO₆]⁶⁻ and [LiO₆]¹¹⁻, with Ca²⁺ and La³⁺ ions occupying the interspaces between these octahedra to maintain overall charge balance and structural stability. The Ca²⁺/La³⁺/Er³⁺ cations belong to the 8-coordinated 4e Wyckoff position, while the Li⁺ and Te⁶⁺/Mn⁴⁺ cations occupy the 6-coordinated 2d and 2a Wyckoff positions, respectively.

The morphology and crystallinity of particles significantly influence the luminescent properties of phosphors. The SEM image inset in Figure 1d illustrates CLLT:0.015Mn⁴⁺/0.01Er³⁺ phosphors with irregularly shaped grains that are agglomerated. The presence of well-grown grains with smooth surfaces enhances their performance as luminescent materials. The SEM analysis indicates an average particle size of 4.67 μm within a range of 2.07–9.70 μm. The Energy-Dispersive X-Ray Spectroscopy (EDX) and elemental mappings, as shown in Figure 1d,e, demonstrate the uniform distribution of elements (Ca, La, Te, O, Mn, and Er) on the particle surfaces. The atomic percentages obtained from the EDX quantitative analysis are summarized in Table S2 (Supplementary Information). The results confirm the successful incorporation of both Mn⁴⁺ and Er³⁺ into the host lattice without significant segregation or impurity formation.

2.2. Diffuse Reflection Spectra and Electronic Band Structure

Figure 2a presents the UV–visible diffuse reflectance spectra of the phosphors within the wavelength range of 240–800 nm. The f-f absorption transition of Er³⁺ is observed at 379 nm, corresponding to the ⁴I₁₅/₂ → ²G₁₁/₂ transitions. Moreover, absorption bands at approximately 318 nm and 476 nm are attributed to the ⁴A_2g → ⁴T_2g and ⁴A_2g → ⁴T_1g transitions of Mn⁴⁺, respectively [17]. The Kubelka–Munk formula was applied to determine the bandgap energy (E_g) of the samples derived from the diffuse reflectance spectra, as expressed in the following equation [21]:

$$F(R)={\displaystyle \frac{{(1-R)}^2}{2R}}$$

(2)

where R denotes the diffuse reflection coefficient. The relationship between the bandgap energy E_g and the Kubelka–Munk function is represented by the equation [22]:

$${(F(R)h\nu )}^n=c(h\nu -E_g)$$

(3)

Here, h denotes Planck’s constant, ν is the frequency of incident light, and c is a constant. The exponent n takes values of 2, 1/2, 2/3, and 1/3 for direct–allowed, indirect–allowed, direct–forbidden, and indirect–forbidden transitions, respectively. Detailed density functional theory (DFT) calculations of the electronic properties were conducted, and the calculated band structure of the CLLT host is illustrated in Figure 2b. The theoretical calculations indicate that CLLT exhibits an indirect bandgap of 3.36 eV, with the valence band maximum (VBM) situated at the Z point and the conduction band minimum (CBM) located at the Γ point in the Brillouin zone. Given that CLLT demonstrates indirect bandgap characteristics, the value of n is considered to be 1/2 for the calculations presented herein.

The inset of Figure 2a depicts the [F(R)hν]^1/2 versus hν plot for the CLLT host, resulting in a calculated bandgap energy of 3.55 eV. Notably, the experimental bandgap value indicated in the inset of Figure 2a is slightly elevated compared to the computed value presented in Figure 2b. The theoretical prediction is typically smaller than the experimentally determined bandgap [23]. This deviation can primarily be attributed to abrupt changes in the derivative of the total energy E_g concerning the electron count N, within the context of DFT, as well as the self-interaction error of electrons [24]. These findings unequivocally confirm the indirect bandgap semiconductor nature of this material. Additionally, its wide bandgap characteristic (>3.3 eV) is essential for enabling effective electronic transitions in RE or TM luminescent centers.

2.3. Photoluminescence Properties

The photoluminescence excitation (PLE) and PL spectra of Mn⁴⁺-single-doped CLLT are illustrated in Figure 3a. The excitation spectrum, monitored at 716 nm, exhibits two broad excitation bands centered around 312 nm and 476 nm, which correspond to the ⁴A_2g → ⁴T_1g and ⁴A_2g → ⁴T_2g spin-allowed transitions, respectively [25,26]. When excited at 312 nm, the CLLT:0.015Mn⁴⁺ phosphor emits a broad and asymmetric spectrum centered at 716 nm, with an accompanying hump at approximately 703 nm. This emission can be attributed to the ²E_g → ⁴A_2g transition of the Mn⁴⁺ ion, while the observed hump results from the splitting of the ²E_g levels due to the influence of the crystal field [26].

Figure 3b displays the PL spectra and integral intensity for CLLT:xMn⁴⁺ (where x = 0.005, 0.01, 0.015, 0.02, 0.03, and 0.04). As the concentration of Mn⁴⁺ increases, the emission intensity initially rises, reaching its peak at x = 0.015, before subsequently declining. Notably, the locations of the emission peaks stay relatively consistent across the concentration range. Consequently, the optimal concentration of Mn⁴⁺ for enhanced luminescence is determined to be x = 0.015, which is significantly higher than that observed in other concentrations.

As the concentration of Mn⁴⁺ ions increases, the enhanced number of Mn⁴⁺ luminescent centers leads to a gradual increase in the luminescence intensity of the CLLT:xMn⁴⁺ phosphors. However, for concentrations exceeding 0.015, the decreased inter-ionic distance leads to concentration quenching of the Mn⁴⁺ ions, contributing to a gradual decline in spectral intensity. The critical distance R_c between Mn⁴⁺ ions can be estimated using the following equation [27]:

$$R_c=2{({\displaystyle \frac{3V}{4\pi x_{c}N}})}^{{\displaystyle \frac{1}{3}}}$$

(4)

where V (245.83 ų) represents the unit cell volume, x_c (0.015) denotes the critical doping concentration for Mn⁴⁺, and N (2) indicates the number of available Mn⁴⁺ sites within the unit cell. Thus, the estimated R_c value is approximately 25.02 Å. Three mechanisms contribute to concentration quenching: exchange interaction, radiation re-absorption, and electric multipolar interaction. The exchange interaction is pertinent when the R_c value is less than 5 Å; however, the calculated R_c value for the prepared sample is substantially greater than this threshold. Radiation re-absorption arises when spectral overlap occurs between the sensitizer’s emission and the activator’s PLE spectra. Consequently, the predominant mechanism for concentration quenching in this study is attributed to electric multipolar interactions.

The electric multipolar interaction is further analyzed with values of θ assigned as 6, 8, and 10, corresponding, respectively, to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions. The θ value can be derived from the following equation [28]:

$$\lg (I/x)=\lg (K/\beta )-(\theta /3)\mathrm{l}\mathrm{g}(x)$$

(5)

In this equation, x represents the dopant concentration, I denotes the luminescence intensity, and θ serves as an index of the electric multipolar character. Parameters K and β are constants specific to a given host material under identical excitation conditions. Upon conducting calculations, a slope of approximately −θ/3 is obtained, yielding a θ value of 6.69, which is close to 6. Hence, we conclude that the electric multipolar interaction corresponds to the dipole–dipole interaction.

The PLE and PL spectra of CLLT:Er³⁺ are presented in Figure 3c. The emission peaks of Er³⁺ stemming from the ⁴S_3/2 → ⁴I_15/2 transition at 545 nm reveal an excitation spectrum characterized by several narrow peaks associated with intra-4f transitions of Er³⁺, specifically at 365 nm (⁴I_15/2 → ⁴G_9/2), 379 nm (⁴I_15/2 → ⁴G_11/2), 407 nm (⁴I_15/2 → ²H_9/2), 452 nm (⁴I_15/2 → ⁴F_5/2), and 489 nm (⁴I_15/2 → ⁴F_7/2), respectively [1]. Under the strongest PLE peak at 379 nm, electrons are promoted to the ⁴F_5/2 state of Er³⁺. By means of phonon assistance, these electrons transition to the ²H_11/2 and ⁴S_3/2 excited states of Er³⁺ via a non-radiative relaxation process, ultimately returning to the basal state ⁴I_15/2 level, resulting in the characteristic green emission of Er³⁺ [5,29]. A series of CLLT:yEr³⁺ samples with varying Er³⁺ doping concentrations were prepared, and their PL spectra are provided in Figure 3d. The emission intensity of Er³⁺ initially increases and then decreases with increasing doping concentration y, reaching a maximum at y = 0.01. Furthermore, the positions of the Er³⁺ emission peaks at 524 nm and 545 nm remain nearly unchanged across different doping levels.

Figure 4a presents the PL and PLE spectra for CLLT:0.015Mn⁴⁺/0.01Er³⁺. The PLE spectra of CLLT:0.015Mn⁴⁺/0.01Er³⁺ while monitoring emission peaks at 524 nm, 545 nm, and 716 nm are presented. The three plots distinctly illustrate the unique PLE profiles of Er³⁺ and Mn⁴⁺, which correspond with the PLE data of CLLT:0.015Mn⁴⁺ and CLLT:0.01Er³⁺, presented in Figure 3a,c, respectively. Notably, the broad PLE band of Mn⁴⁺ significantly overlaps with the narrow PLE band of Er³⁺, with the prominent PLE maximum for Er³⁺ occurring at 379 nm. Consequently, the 379 nm wavelength can be effectively utilized as an excitation stimulant to induce luminescence from Er³⁺ as well as Mn⁴⁺. It is notable that a slight spectral overlap exists between the emission peaks of Er³⁺ and the excitation spectrum of Mn⁴⁺, indicating the potential for energy transfer (ET) from Er³⁺ to Mn⁴⁺.

A series of CLLT:0.015Mn⁴⁺/yEr³⁺ phosphors (where y = 0.005, 0.01, 0.04, 0.07, and 0.10) were synthesized, maintaining a fixed concentration of Mn⁴⁺ while varying the concentration of Er³⁺. The PL spectra obtained under excitation at 379 nm are depicted in Figure 4b. The CLLT:0.015Mn⁴⁺/yEr³⁺ phosphors exhibit three prominent emission peaks at 524 nm, 545 nm, and 716 nm, corresponding to the transitions ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 of Er³⁺, as well as the ²E_g → ⁴A_2g transition of Mn⁴⁺. Notably, the positions of their PL peaks remain largely unaffected by the varying concentrations of Er³⁺, while significant variations in PL intensity are observed. As the concentration of Er³⁺ (y) increases, the PL intensity reaches its maximum at y = 0.01, after which it begins to decline. Considering the comparative PL intensities of both Mn⁴⁺ and Er³⁺, a concentration of y = 0.01 was chosen for additional investigations in the following portions.

To investigate the multi-photon upconversion (UC) luminescence mechanism of CLLT:0.015Mn⁴⁺/0.01Er³⁺ luminescent materials, intensity-dependent UC emission spectra were recorded, as illustrated in Figure 4c. It is clear that the locations of the UC emission peaks stay constant while the excitation power rises. Furthermore, the emission strength of UC shows a remarkable increase as the excitation intensity escalates. The three distinct luminescence peaks correspond to wavelengths of 524, 545, and 661 nm, which are attributed to the transitions ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 of Er³⁺, respectively [30]. Additionally, a weaker luminescence peak at 716 nm is related to the ²E_g → ⁴A_2g transition of Mn⁴⁺ ions. The logarithmic curves of pump power (0.35–1.02 W) versus upconversion radiation were analyzed as shown in Figure 4d. When stimulated by a 980 nm wavelength light, the inclinations of the fitting curves for green and red light were determined to be 1.98 and 1.94, suggesting their involvement in the two-photon absorption process.

The diagram of energy levels presented in Figure 5a illustrates the ET processes involving Er³⁺ and Mn⁴⁺ ions under the irradiation of 980 nm light. Specifically, Er³⁺ undergoes transitions from ⁴I_15/2 to ⁴I_11/2 via ground state absorption under laser irradiation, followed by transitions to ⁴F_7/2 stimulated by excited state absorption. Subsequently, the electrons at this energy level go through non-radiative relaxation to ²H_11/2 and ⁴S_3/2, before returning to the ground state ⁴I_15/2, thereby generating green radiation at 524 and 545 nm. For red emission, two pathways exist whereby the Er³⁺ ions at the ⁴S_3/2 level undergo non-radiative relaxation to the ⁴F_9/2 state before transitioning back to the ground ⁴I_15/2 state, emitting red light at 661 nm. In the UC process, the green luminescence arising from the transitions ²H_11/2 or ⁴S_3/2 → ⁴I_15/2 could promote the electrons from the fundamental state ⁴A_2g of Mn⁴⁺ to the excited configuration ⁴T_2g energy level. This indicates the ET from the green emission levels ²H_11/2 or ⁴S_3/2 of Er³⁺ to the ⁴T_2g level of Mn⁴⁺ ions, resulting in the faint luminescence observed at 716 nm in Figure 4c. These findings indicate the occurrence of partial energy transfer from Er³⁺ to Mn⁴⁺ in both the upconversion and downconversion luminescence of the CLLT:0.015Mn⁴⁺/0.01Er³⁺ sample. The singly doped CLLT:0.01Er³⁺ and the co-doped CLLT:0.015Mn⁴⁺/0.01Er³⁺ phosphors are shown in Figure 5b. The lifetime of Er³⁺ was determined to be 106.46 μs in the absence of Mn⁴⁺ and 93.64 μs in its presence, using the formula for energy transfer efficiency, ${\eta }_{ET}=1-\tau /{\tau }_0$, where τ0 and τ are the lifetimes without and with the acceptor (Mn⁴⁺), respectively. The calculated ET efficiency is approximately 12%. This result confirms the presence of weak energy transfer from Er³⁺ to Mn⁴⁺.

2.4. Temperature Sensing Properties

2.4.1. Dual Emission FIR Method

The temperature-sensitive PL spectra of CLLT:0.015Mn⁴⁺/0.01Er³⁺ spanning from 303 to 463 K are shown in Figure 6a. Clearly, the temperature has a substantial effect on the PL intensities of Er³⁺. As the temperature rises, the PL intensities of Er³⁺ at 545 nm exhibit a rapid decrease, while those at 524 nm demonstrate a contrasting trend. Conversely, the PL intensity of Mn⁴⁺ demonstrates minimal variation with temperature, initially decreasing gradually, then increasing, and ultimately decreasing once more. Notably, no observable peak shifts or significant changes in width occur during the temperature variation. The integral PL intensities as a function of temperature are depicted in Figure 6b.

The ²H_11/2 and ⁴I_15/2 excited states of Er³⁺ represent two thermally coupled energy levels, whereby the electron population is significantly affected by temperature changes. As the temperature increases, species located in the ⁴S_3/2 state may experience transitions to the ²H_11/2 state via thermal excitation. Consequently, the relative luminescence intensity of the ²H_11/2 → ⁴I_15/2 transition compared to the ⁴S_3/2 → ⁴I_15/2 transition depends on the complete lateral distribution of these two thermally coupled energy states [31]. The combination of electrons excited by direct means and electrons activated by thermal energy allows for emission transitions to the fundamental state ⁴I_15/2, resulting in an escalation of intensity with increasing thermal conditions.

In addition, Figure S2 in the Supplementary Information presents the temperature-dependent PLE spectra of CLLT:0.015Mn⁴⁺/0.01Er³⁺ spanning from 303 to 463 K. As the temperature increases, the wide excitation bands corresponding to the ⁴A_2g → ⁴T_g spin-allowed transition of Mn⁴⁺ exhibit a shift towards longer wavelengths, with the peak position moving from approximately 322 nm (303 K) to 328 nm (463 K). Consequently, the changes in the intensity of the band edge (379 nm) with temperature are minimal. Specifically, the intensity of the band edge (379 nm) exhibits a gradual decrease, followed by an increase and eventually a decrease, aligning with the fluctuation observed in the PL intensity of Mn⁴⁺ as depicted in Figure 6a,b.

There are two valid approaches to evaluate the FIR responsiveness of the luminescent material CLLT:0.015Mn⁴⁺/0.01Er³⁺ using the PL intensity ratio analysis method: (i) the ratio of green emissions from Er³⁺ at 524 nm and 545 nm, which arises from thermal interaction between the ²H_11/2 and ⁴S_3/2 energy levels, and (ii) the intensity ratio comparing the emissions of Mn⁴⁺ at 716 nm and Er³⁺ at 545 nm, which demonstrate distinct thermal behaviors.

The FIR (I_524nm/I_545nm) associated with the interaction of thermal energies in Er³⁺ can be represented by the following Equation (6) [29]:

$$FIR=A{\exp }^{(-{\displaystyle \frac{B}{T}})}+C$$

(6)

In this equation, A, B, and C are constants. Given that FIR (I_524nm/I_545nm) values can be derived from experimental data across a range of temperatures, Figure 6c illustrates the relationship between FIR (I_524nm/I_545nm) and temperature (T). The FIR (I_524nm/I_545nm) values exhibit a satisfactory fit, with an R-squared value of 0.998. The determined values of A, B, and C for FIR (I_524nm/I_545nm) are 259.91, 1957.84, and 0.4216, respectively. These results demonstrate that temperature significantly influences the variation of the FIR (I_524nm/I_545nm) value. This indicates that the CLLT:0.015Mn⁴⁺/0.01Er³⁺ material is suitable for temperature measurement based on the dual emission FIR method.

The ratio of intensities at 524 nm and 545 nm (FIR, I_524nm/I_545nm) exhibits a monotonic increase with temperature. To assess the potential of FIR (I_524nm/I_545nm) for temperature detection, two crucial parameters, absolute sensitivity S_a and relative sensitivity S_r, were computed, as expressed in Equations (7) and (8) [31]:

$$S_a=\left|{\displaystyle \frac{\partial FIR}{\partial T}}\right|$$

(7)

$$S_r=100\%\times |{\displaystyle \frac{1}{FIR}}\times {\displaystyle \frac{\partial FIR}{\partial T}}|$$

(8)

The corresponding values of S_r and S_a depicted in Figure 6d demonstrate a rising trend for S_a with increasing temperature, with S_a peaking at 0.0348 K⁻¹ at 463 K. However, S_r portrays an initial increase followed by a subsequent decrease as temperature ascends, reaching a maximum value of 1.12% K⁻¹ at 343 K.

Calculations of the intensity ratio at 716 nm and 545 nm (FIR, I_716nm/I_545nm) were conducted based on the integrated PL intensities of Mn⁴⁺ (716 nm) and Er³⁺ (545 nm). By deriving FIR (I_716nm/I_545nm) values from experimental results at different temperatures (T), a relationship between FIR (I_716nm/I_545nm) and temperature (T) is illustrated in Figure 6e, which can be fitted using Equation (6). The fitted parameters A, B, and C are determined to be 327.07, 2089.92, and 0.62, respectively, differing from the parameters in FIR (I_524nm/I_545nm). To quantify the thermally induced variation of FIR (I_716nm/I_545nm), the S_a and S_r values of thermometers were computed using Equations (4) and (5) and are presented in Figure 6f. Observations indicate that Sr initially increases, reaching its peak value at 373 K before declining. The maximum values of Sr and Sa were recorded at 1.01% K⁻¹ at 373 K and 0.035 K⁻¹ at 463 K, respectively.

2.4.2. Dual Excitation SBR Method

The temperature-sensitive PL of Mn⁴⁺ in CLLT:0.015Mn⁴⁺/0.01Er³⁺ under 325 nm irradiation is displayed in Figure 7a. The integrated PL intensity histogram for excitations at 325 nm (band center) and 379 nm (band edge) is illustrated in Figure S3. Under 325 nm excitation, the luminescence intensity exhibits a pronounced decrease with increasing temperature. Arrhenius fitting was performed for the thermal quenching of both the Mn⁴⁺ emission and the Er³⁺ emission. The corresponding plots for the activation energy of Er³⁺ and Mn⁴⁺ are included in Figure S5 of the Supplementary Information. From the linear fits, the activation energies for thermal quenching were determined to be 0.187 eV for Er³⁺ and 0.244 eV for Mn⁴⁺.

This thermal quenching behavior contrasts sharply with the trend observed under 379 nm excitation, highlighting its potential applicability as an SBR-based optical thermometer. The SBR (I_{716nm,λex=379nm}/I_{716nm,λex=325nm}) of Mn⁴⁺ can be accurately fitted using equation (6), as demonstrated in Figure 7c, with the parameters A, B, and C found to be 43.62, 1809.42, and 0.3881, respectively. The corresponding absolute sensitivity (S_a) and relative sensitivity (S_r) values, calculated using Equations (4) and (5), are depicted in Figure 7d. A maximum relative sensitivity of 0.62% K⁻¹ was achieved at 403 K. Meanwhile, a maximum absolute sensitivity of 0.0074 K⁻¹ was achieved at a higher temperature of 463 K.

Figure 7b shows the upconversion PL spectra of Er³⁺ ions in CLLT:0.015Mn⁴⁺/0.01Er³⁺ under 980 nm excitation at various temperatures. The temperature-dependent PL intensity trend exhibits a distinct variation compared to the downconversion emission observed under 379 nm excitation, as shown in Figure 6a, with their integrated PL intensity variations with temperature presented in Figure S4. The SBR (I_{524nm,λex=379nm}/I_{716nm,λex=980nm}) values and the corresponding fitting curve obtained using Equation (3) are illustrated in Figure 7e. The maximum absolute sensitivity S_a and relative sensitivity S_r reach values of 0.02 and 0.59% K⁻¹, respectively, as shown in Figure 7f.

Based on the temperature sensing performance of the CLLT:0.015Mn⁴⁺/0.01Er³⁺ phosphor, the dual emission FIR method demonstrates superior relative sensitivity, achieving a maximum value of 1.12% K⁻¹ at 343 K using the ratio of Er³⁺ emissions at 524 nm and 545 nm and 1.01% K⁻¹ at 373 K using the ratio of Mn⁴⁺ emission at 716 nm to Er³⁺ emission at 545 nm. In comparison, the dual excitation SBR method, which relies on the intensity ratio of the same emission under different excitation wavelengths, yields maximum relative sensitivities of 0.62% K⁻¹ at 403 K for Mn⁴⁺ emission and 0.59% K⁻¹ for Er³⁺ emission. These results indicate that the FIR strategy offers higher sensitivity, making the developed phosphor highly competitive for dual-mode optical thermometry applications.

The absolute sensitivity S_a and relative sensitivity S_r values of CLLT:0.015Mn⁴⁺/0.01Er³⁺, derived from the dual-mode thermometry, are comparable to many reported materials. A comparative summary of these values is provided in

Table 1. Comparative analysis of the S_a and S_r values obtained in the current study with those documented for other temperature-sensing materials utilizing Er³⁺ or Mn⁴⁺ ions.

Phosphor	Temperature Range (K)	Max Sa (K−1)	Max Sr (%K−1)	Reference
NaYb(MoO4)2: Er3+	300–570	0.013	0.69	[32]
SrWO4: Yb3+/Er3+	303–573	0.013	1.03	[33]
Gd2O3: Yb3+/Ho3+/Er3+	303–483	--	0.98	[34]
YbTaO4: Er3+	300–570	0.0038	1.00	[35]
α-SiAlON: Er3+	298–1273	0.0034	0.59	[36]
Gd2Zr2O7: Yb3+/Er3+	298–573	--	1.10	[37]
La2Mo2O9: Er3+/Tm3+	303–478		0.83	[38]
MgLaLiTeO6: Er3+	298–573	0.0037	1.03	[18]
NaYF4: Er3+	303–423	--	1.06	[39]
NaLaMgWO6: Mn4+/Er3+	303–523	0.0017	1.31	[29]
Lu3Al5O12: Eu3+/Mn4+	303–358	0.07	0.7	[10]
Gd2Ti2O7: Er3+/Yb3+	303–633	0.0038	1.12	[40]
NaLaMgWO6: Mn4+/Eu3+	303–523	0.0030	0.86	[41]
Sr3(PO4)2: Er3+/Yb3+	303–623	0.0070	0.88	[42]
CaLaLiTeO6: Mn4+/Er3+	303–463	0.0348	1.12	This work

. For temperature monitoring ranging from 303 K to 463 K, the maximum sensitivity is comparable to the best-reported values in Table 1 for temperature measurement materials doped with Mn⁴⁺ or Er³⁺. The results indicate that CLLT:0.015Mn⁴⁺/0.01Er³⁺ is a promising candidate for optical temperature measurement.

2.5. Application in Optical Anti-Counterfeiting

Drawing inspiration from the excitation-sensitive PL properties of CLLT:0.015Mn⁴⁺/0.01Er³⁺ phosphor, we conducted an investigation into its possible application in sophisticated optical anti-counterfeiting. The CIE color coordinates corresponding to the sample CLLT:0.015Mn⁴⁺/0.01Er³⁺ under 325 nm and 379 nm light sources are shown in Figure 8a, demonstrating emission of orange-red and bright yellow-green light. The distinct emission colors of this luminescent material allow for easy differentiation by both human observers and cameras. In Figure 8b–g, luminescence images of the letter “H” and the number “8” patterns under various excitation conditions at room temperature are displayed. Under normal lighting, the anti-counterfeiting patterns appear milky white. However, when excited by 379 nm light, the patterns emit a vibrant yellow-green luminescence. Particularly noteworthy is the shift in emission color to orange-red when excited at 325 nm, primarily attributed to Mn⁴⁺ luminescence. This outcome underscores the remarkable ability of CLLT:0.015Mn⁴⁺/0.01Er³⁺ phosphor to dynamically adjust its emitted color through adjustable excitation wavelengths, presenting significant benefits for anti-counterfeiting applications.

The overall merits of the CaLaLiTeO₆:Mn⁴⁺/Er³⁺ phosphor lie in the combination of dual-mode self-calibration, multifunctionality, cost-effectiveness, and ease of synthesis. A key advantage is its capability for dual-mode self-calibration through both FIR and SBR thermometric methods within a single material, enhancing measurement reliability. Furthermore, the material exhibits excitation-dependent luminescence, enabling dynamic color switching between orange-red and yellow-green emissions, which makes it highly suitable for advanced anti-counterfeiting applications. This dual functionality serves as both a temperature sensor and a security material. Moreover, the synthesis via a conventional solid-state reaction at a moderate temperature (1050 °C) in air offers notable cost-effectiveness, scalability, and simplicity compared to methods requiring complex processes or controlled atmospheres. Thus, the CaLaLiTeO₆:Mn⁴⁺/Er³⁺ phosphor presents a compelling and practical alternative for applications where a balance among performance, stability, ease of fabrication, and cost is essential.

3. Experimental Section

3.1. Synthetic Method

The CLLT:Mn⁴⁺/Er³⁺ materials were synthesized via a high-temperature solid-state reaction. The initial reactants, sourced from Aladdin, included Li₂CO₃ (99.9%), CaCO₃ (99.9%), La₂O₃ (99.9%), TeO₂ (99.9%), MnO₂ (99.9%), and Er₂O₃ (99.9%), which were measured based on their respective stoichiometric proportions. The doping concentrations for Mn or Er (e.g., ‘x’ in CLLT:xEr³⁺) are expressed in mol%, representing the molar percentage of Er³⁺ ions intended to substitute for the combined (Ca²⁺ + La³⁺) cations on their shared crystallographic site. Subsequently, these substances were finely milled and blended in an agate mortar for 30 min, after which they were transferred to a crucible for preliminary sintering at 600 °C for 6 h in ambient air. Following preliminary sintering, the substances were pulverized once more before undergoing sintering at 1050 °C for 12 h in an air environment. After reaching room temperature, the substances were pulverized for further analyses. For the preparation of anti-counterfeiting materials, a composite of polydimethylsiloxane (PDMS) and CLLT:Mn⁴⁺/Er³⁺ phosphors was utilized. The luminescent material and resin mixture (with a PDMS-to-curing-agent ratio of 10:1) were weighed in a mass ratio of 1:2, mixed, and stirred magnetically for 20 min to achieve a uniform consistency. Anti-counterfeiting patterns resembling the characters “H” and “8” were then painted onto a glass substrate. The substrate was placed in an oven and maintained at 80 °C for 2 h to complete the formation of the anti-counterfeiting label.

3.2. Characterization

X-Ray powder diffraction patterns were acquired utilizing a Rigaku Miniflex600 X-Ray diffractometer (Rigaku, Tokyo, Japan), which was outfitted with Cu Kα radiation (λ = 0.15406 nm). The morphological features and elemental mapping of the materials were examined using a Sigma 300 field emission SEM (Zeiss, Oberkochen, Germany). Diffuse reflectance (DR) spectra were obtained at ambient temperature using a UV–visible spectrophotometer (UV3600 PLUS, Shimadzu, Kyoto, Japan). The excitation and emission spectra were evaluated using a fluorescence spectrophotometer (model F-4600, Hitachi, Tokyo, Japan), which was furnished with a 150 W Xenon lamp as the light source for excitation.

3.3. Density Functional Theory Calculations

All DFT calculations were carried out using the Vienna Ab initio Simulation Package (VASP 5.4.4) [43]. In these calculations, the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) was employed to handle the exchange-correlation functional [44]. The electron–ion interactions were treated with the projector-augmented-wave (PAW) pseudopotentials, with the following valence electron configurations considered: Ca (3s² 3p⁶ 4s²), La (5s² 5p⁶ 5d¹ 6s²), Li (2s¹), Te (5s² 5p⁴), and O (2s² 2p⁴). The calculations employed a plane-wave cutoff energy of 520 eV and a Γ-centered Monkhorst–Pack k-point grid of 7 × 6 × 4. The convergence criteria were set to 1 × 10⁻⁶ eV for the electronic energy and 0.02 eV/Å for the Hellmann–Feynman forces during geometry relaxation.

4. Conclusions

In summary, the dual-function material CLLT:Mn⁴⁺/Er³⁺ phosphors, intended for optical temperature measurement and multicolored anti-counterfeiting applications, were effectively produced through a conventional high-temperature solid-state process. XRD and Rietveld analysis confirmed the high quality of the crystal phase, with no detectable impurities. The optimal composition was determined to be CLLT:0.015Mn⁴⁺/0.01Er³⁺, which exhibits bright emissions in the far red, yellow-green, and green regions when excited by 379 nm light. The luminescence intensity of Mn⁴⁺ and Er³⁺ ions demonstrates significantly different trends as temperature increases. For temperature monitoring ranging from 303 K to 463 K, the dual emission FIR method based on the Er³⁺ emissions (I₅₂₄ₙₘ/I₅₄₅ₙₘ) achieved a maximum relative sensitivity (Sᵣ) of 1.12% K⁻¹ at 343 K and a maximum absolute sensitivity (Sₐ) of 0.0348 K⁻¹ at 463 K. In comparison, the dual excitation SBR method yielded a maximum Sᵣ of 0.62% K⁻¹ at 403 K and a maximum Sₐ of 0.0074 K⁻¹ at 463 K. Furthermore, the variation in PL color with excitation wavelength enhances the appropriateness of CLLT:0.015Mn⁴⁺/0.01Er³⁺ for optical anti-counterfeiting applications. This study introduces an approach for developing versatile materials that combine optical detection capabilities and anti-counterfeiting attributes, thereby opening up possibilities for their potential use in biomedical research, intelligent packaging, and industrial applications.