Luminescence Properties and Energy Transfer in SrLa2Sc2O7 Co-Doped with Bi³⁺/M (M = Eu³⁺, Mn⁴⁺, or Yb³⁺)

Abstract

Series of Eu³⁺/Mn⁴⁺/Yb³⁺-doped SrLa₂Sc₂O₇:Bi³⁺ (SLSO: Bi³⁺) were synthesized by a high-temperature solid-state method, and the energy transfer of Bi³⁺→Eu³⁺/Mn⁴⁺/Yb³⁺ was observed. Under ultraviolet radiation, a 550 nm emission peak was observed, which is attributed to Bi³⁺ occupying the Sr²⁺/La³⁺ sites. Additionally, the other peaks were found to be 615, 707, and 980 nm, which are assigned to the Re³⁺ (Eu³⁺ and Yb³⁺) and Mn⁴⁺ occupying two different cationic sites. An obvious energy transfer (ET) from Bi³⁺ to Eu³⁺/Mn⁴⁺/Yb³⁺ was observed, and the tunable color, emitting from yellow to red, was obtained; the ET efficiency was about 86.2%, 78.6%, and 27.5% in SLSO, respectively. We found that the large overlap area between the emission spectrum of the sensitizer and the excitation spectrum of the activator could produce efficient energy transfer, which provided the idea for designing experiments in the future for some highly efficient energy transfer processes.

1. Introduction

Recently, luminescent materials have been widely used in light-emitting diodes (LEDs), medical devices, 3D displays, temperature sensing, and other applications [1,2]. Being an essential component of LED devices, the different color emitting phosphors have been investigated and discussed; they are considered one of the biggest challenges in the field of lighting and backlighting display [3,4]. Generally, the different emitting phosphors can be achieved by energy transfer (ET) [5,6,7,8,9,10,11,12]. As is well known, Eu²⁺, Ce³⁺, and Bi³⁺ are effective sensitizers because they can emit broad bands in various hosts, depending on their 4f⁶5d^l-4f⁷, 5d¹-4f^l, and ³P₁-¹S₀ allowed transitions [13,14,15], respectively, and can transfer the energy to activators, such as rare earth ions (Re³⁺ = Eu³⁺, Sm³⁺, Tb³⁺, Yb³⁺, Nd³⁺, etc.), and achieve the different color emitting phosphors [16,17,18]. The 6s² configuration of the Bi³⁺ ion is sensitive to its surroundings of the local crystal field but is more stable than other valence states such as Bi⁰, Bi²⁺, and Bi⁵⁺; hence, Bi³⁺ ion exhibits a variety of emissions involving ultraviolet, blue, green, yellow, and red [19,20,21]. In contrast, unique spectral features such as narrow emission bands, long luminescence lifetimes, and diverse emission states are presented by Re³⁺ ions with a 4f electronic configuration [22]. Therefore, both are considered candidates for the potential of multiple luminescent color materials. In this work, in order to improve the luminescent properties of activator Eu³⁺/Mn⁴⁺/Yb³⁺, we select SrLa₂Sc₂O₇ as the host and Bi³⁺ as the sensitizer for multi-colored phosphors. In scandium salts, the Sc³⁺ ion does not follow the contraction law of the lanthanides and possesses a smaller ionic radius. Due to the lack of 4f electrons, it has a different electron configuration than other rare earth elements, giving it special physicochemical properties that allow Sc-based matrices to exhibit excellent photoluminescence. We, therefore, chose SrLa₂Sc₂O₇ matrix crystals. In this paper, we choose the co-doping of Bi³⁺→Eu³⁺; Bi³⁺→Mn⁴⁺; Bi³⁺→Yb³⁺ to achieve visible to near-infrared light emission in the SrLa₂Sc₂O₇ matrix, with Bi³⁺ as the sensitizer and Eu³⁺/Mn⁴⁺/Yb³⁺ as the activator. It is noted that SrLa₂Sc₂O₇ had three cationic environments: SrO₁₂, Sr/LaO₉, and ScO₆ polyhedrals, respectively. The emission color can turn from yellow to red and near-infrared light by the Eu³⁺, Yb³⁺, and Bi³⁺ entering into the Sr/LaO₉ polyhedral in terms of ion valence and radius and because the Mn⁴⁺ ion has a small radius, which can occupy the ScO₆ site to produce the yellow to deep red light. Moreover, the luminescence properties and the energy transfer progress of Eu³⁺/Mn⁴⁺/Yb³⁺/Bi³⁺ in SrLa₂Sc₂O₇ are also discussed. We also explore the interaction between the sensitizer and the activator and find that the large overlap area between the emission spectrum of the sensitizer and the excitation spectrum of the activator produces efficient energy transfer, which provides the idea for designing experiments in the future for some highly efficient energy transfer processes.

2. Materials and Methods

A series of SrLa₂Sc₂O₇:0.06Bi³⁺, Re³⁺ (Re = Eu and Yb) and SrLa₂Sc₂O₇:0.06Bi³⁺, Mn⁴⁺ was synthesized by the high-temperature solid-state method. High purity SrCO₃ (99.99%), La₂O₃ (99.99%), Sc₂O₃ (99.99%), Bi₂O₃ (99.99%), Eu₂O₃ (99.99%), MnO₂ (99.99%), and Yb₂O₃ (99.99%) were used as raw materials. The ingredients are mixed together thoroughly in an agate mortar for 10 min according to the stoichiometric proportions. The raw materials were then placed in a crucible and heated at 900 °C for 6 h, and then at 1500 °C for 6 h. Finally, by cooling the synthetic samples to room temperature, it was then ground into a powder for subsequent measurement.

Samples were analyzed by a Bruker D8 X-ray diffractometer (XRD) under Cu Kα radiation at 40 kV and 40 mA, with the radiation source parameters set to λ = 1.5406 Å, 2θ = 10–80°, and step sizes = 0.05 s/step. Rietveld structure were carried out using the General Structural Analysis System (GSAS) software (version 1251). The excitation and emission spectra were measured by a Japan Hitachi F-7000 fluorescence spectrometer; its resolution can reach 1 nm. The decay curves were recorded on a HORIBA FLuorolog-3 fluorescence spectrometer; the instrument can measure lifetime values from 10 ps to 10 s.

3. Results and Discussion

3.1. Phase Information

Generally, the performance of phosphor is affected by the crystal structure [23]. Figure 1a depicts the crystal structure of the SrLa₂Sc₂O₇ unit cell and the crystal structure of the SrLa₂Sc₂O₇ orthorhombic system and the Pmmm space group. It is revealed that the Sr1, Sr2/La, and Sc sites are surrounded by oxygen atoms with the coordination numbers of 12, 9, and 6, respectively. It is worth noting that the [Sr1O₁₂], [Sr2O₉] polyhedral and the [ScO₆] octahedra are joined together by shared edges to form the basic unit. Figure 1b–d present the standard XRD pattern of SrLa₂Sc₂O₇ (SLSO) (ICSD#67625), the XRD patterns of SLSO:0.06Bi³⁺, yEu³⁺ (y = 0, 0.0125, 0.025, 0.05, 0.1, and 0.15), SLSO:0.06Bi³⁺, mMn⁴⁺ (m = 0, 0.001, 0.003, 0.005, 0.007, 0.01, and 0.015), and SLSO:006Bi³⁺, zYb³⁺ (z = 0. 01, 0.015, 0.02, 0.05, and 0.15). The Rietveld refinement of the XRD patterns obtained by slow sweeping using the GSAS program is required. Here, a series of SLSO: Bi³⁺, Eu³⁺, SLSO: Bi³⁺, Mn⁴⁺, and SLSO: Bi³⁺, Yb³⁺ was refined separately, and the diffraction peaks of all samples were basically consistent with the standard card (ICSD#67625). Figure 1e–g show the Rietveld refinement results for the representative samples of the Eu³⁺/Mn⁴⁺/Yb³⁺-doped SLSO: Bi³⁺. The black “-” indicates the intensity obtained from experimental tests, the blue dot represents the calculated intensity, the green line indicates the background, the green horizontal line indicates the error between the intensity obtained from experimental tests and the calculated intensity, and the plum-red “|” indicates the Bragg reflection position of the calculated pattern. The refinement parameters R_wp < 15%, R_p < 10%, and χ² < 5 are within a reasonable range. These indicate that the Eu³⁺/Mn⁴⁺/Yb³⁺ and Bi³⁺ incorporation into the SLSO could maintain the phase purity and no impurity phase was generated at the current doping level.

3.2. Luminescence Properties of SLSO:0.06Bi³⁺, yEu³⁺

Figure 2a shows the emission and excitation spectra of SLSO:0.06Bi³⁺, 0.05Eu³⁺. For the 615 nm emission peak, the band centered at 350 nm, along with the characteristic absorption bands of Eu³⁺ ions at 394, 465, and 537 nm, which can be attributed to the ⁷F₀-⁵L₆, ⁷F₀-⁵D₂, and ⁷F₀-⁵D₁ of Eu³⁺ [24]. Moreover, the wide band is comprised of a ¹S₀→³P₁ transition of Bi³⁺ ions and a charge transfer band (CTB) transition (Eu³⁺-O^2-), while these sharp peaks are associated with the 4f-4f characteristic transitions of Eu³⁺ ions. Under the 350 nm excitation, as shown in Figure 2b, not only a broad emission band centered at 550 nm but also several sharp peaks at 581, 590, 598, 615, 658, and 707 nm were observed from the emission spectra of SLSO:0.06Bi³⁺, yEu³⁺, which correspond to the ⁵D₀→⁷F_J (J = 0, 1, 2, 3, 4) characteristic transitions of Eu³⁺ ions. Among them, the 590 and 598 nm emission peaks, corresponding to the ⁵D₀→⁷F₁ transition, are magnetic dipole transitions and are insensitive to positional symmetry in the crystal structure, while the red emission at 615 nm is attributed to the ⁵D₀→⁷F₂ electric dipole transition [25,26,27], which arises from the lack of inversion symmetry at the Eu³⁺ site; its emission intensity is much stronger than that of the transition ⁷F₁ level. When Eu³⁺ ions are in a low-local symmetric environment, the ⁵D₀-⁷F₂ is dominant [28]. Considering the similar ionic radius and the same charge, Eu³⁺ ions tend to occupy the position of La³⁺ in SLSO. As can be observed in the spectrum, the ⁵D₀-⁷F₂ emission of Eu³⁺ at 615 nm is the strongest emission peak. In Figure 2c, with the increase in Eu³⁺ concentration from y = 0 to y = 0.15, the red emission intensity at 615 nm appears to increase significantly, while the yellow emission intensity at 550 nm is constantly decreased, which shows the energy transfer from Bi³⁺ to Eu³⁺. The critical distance (R_c) is of great significance for evaluating the energy transfer mechanism from Bi³⁺ to Eu³⁺ in SLSO: Bi³⁺, Eu³⁺. The following equation can be used (see [29,30]).

$$R\mathrm{c} \approx 2(\frac{3V}{{4\pi x}_{\mathrm{c}}N}{)}^{1/3}$$

(1)

where the x_c value is the critical concentration of dopant ions (total concentration of Bi³⁺ and Eu³⁺), referring to the luminescence intensity of Bi³⁺ in SLSO: Bi³⁺, Eu³⁺, which is half of the initial value luminescence intensity; V is the cell volume size of 682.987 ų; N = 4. Therefore, the calculated R_c is 12.68 Å, excluding the exchange interaction mechanism (R_c ≈ 5Å). Thus, for SLSO: Bi³⁺, Eu³⁺, the electric multipole interaction is used for the energy transfer of Bi³⁺-Eu³⁺. The energy transfer equation for multipolar interactions was derived according to the theory of Dexter and Reisfeld [31].

$$\frac{{\tau }_{\mathrm{S}0}}{{\tau }_{\mathrm{S}}}\propto {\mathrm{C}}^{\alpha /3}$$

(2)

where C is the total concentration of Bi³⁺ and Eu³⁺ ions. τ_S and τ_S0 represent the lifetime values of Eu³⁺-doped and non-Eu³⁺-doped, respectively. The values α = 6, 8, and 10 indicate different types of dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. As visualized in Figure 2d, the fitting factors of SLSO:0.06Bi³⁺, yEu³⁺ samples are equal to 0.899, 0.946, and 0.974 for α = 6, 8, and 10, respectively, where R² obtains the maximum at α = 10; hence, the energy transfer mechanism of Bi³⁺-Eu³⁺ is quadrupole–quadrupole interaction.

Time-resolved photoluminescence (TRPL) spectroscopy reveals the dynamic behavior of doped ions with time in the emission spectrum, providing the ability to discriminate luminescent ions. Under the 350 nm Nano-LED lamp excitation, the TRPL spectra and fluorescence decay curves of SLSO:0.06Bi³⁺, 0.05Eu³⁺ are shown in Figure 3. Since the lifetimes of Bi³⁺ and Eu³⁺ are not on the corresponding level of magnitude, it can be noticed that the TRPL spectra of Bi³⁺ and Eu³⁺ do not appear at the same time in the coordinating system, as is visualized in Figure 3a,c. Figure 3b,d show the decay curves of Bi³⁺ and Eu³⁺ in SLSO:0.06Bi³⁺, yEu³⁺, respectively, and the lifetimes are calculated as follows [32,33,34,35]:

$${{\rm I}\left(t\right)=A}_1{\exp (-t/\tau }_1{)+A}_2{\exp (-t/\tau }_2)$$

(3)

where t represents the decay time, τ₁ represents the fast decay lifetime, and τ₂ represents the slow decay lifetime. The average lifetime of the activated ion (τ_av) can be expressed by the following equation:

$${\tau }_{\mathit{av}}{=(A}_1{\tau }_1{}^2{+A}_2{\tau }_2{}^2{)/(A}_1{\tau }_1{+A}_2{\tau }_2)$$

(4)

The lifetimes of Bi³⁺ decrease from 473.2 to 65.3 ns with increasing Eu³⁺ concentration, while the lifetime of Eu³⁺ increases sustainably from 376.7 to 573.4 μs. This means there is an energy transfer from Bi³⁺ to Eu³⁺.

As shown in Figure 4a, when y is at 0.15Eu³⁺, energy transfer efficiency can reach 86.2%, and its value increases continuously with increasing Bi³⁺ concentration. This is because the emission peak of Bi³⁺ has an overlapping region with the excitation peak of Eu³⁺ to promote radiation reabsorption progress and, hence, energy transfer efficiency. Figure 4b illustrates the energy level diagram of Bi³⁺ and Eu³⁺ in SLSO. Figure 4c depicts the Commission International de L’ Eclairage (CIE) chromaticity coordinates of SLSO:0.06Bi³⁺, yEu³⁺. Obviously, SLSO: 0.06Bi³⁺, yEu³⁺ can emit yellow and red light, and the CIE values are from (0.377, 0.483) to (0.590, 0.376) with increasing concentrations of Eu³⁺.

3.3. Luminescence Properties of SLSO:0.06Bi³⁺, mMn⁴⁺

To further increase the luminescence intensity in the red region, we chose MnO₂ as the raw material to provide Mn⁴⁺ ion co-doped with Bi³⁺ ion for luminescence, which provides the basis for the preparation of white LEDs with a high color rendering index and a low correlated color temperature. Figure 5a,b show the emission and excitation spectra of SLSO: mMn⁴⁺ (m = 0.001, 0.003, 0.005, 0.007, 0.01, and 0.015). As the samples were prepared in air, it was speculated that the Mn ions were not reductive and we did not consider the red luminescence to be Mn²⁺. Secondly, most of the luminescence of Mn²⁺ is a green or red broadband spectrum, whereas the Mn luminescence is a narrow band from 650 to 800 nm in Figure 5a. Afterwards, to further figure out the Mn⁴⁺ luminescence center, we surveyed some published literature and found that the luminescence around 707 nm, with narrowband emissions, is that of Mn⁴⁺ ions. It can be seen that the excitation spectrum contains the 200–450 nm broadband, which is attributed to the charge migration band of Mn⁴⁺-O^2- and the ⁴A_2g→⁴T_1g energy level transition of Mn⁴⁺; the weaker excitation peak at 395 nm is ascribed to the ⁴A_2g→²T_2g transition of Mn⁴⁺; the other excitation peaks at 450–600 nm are considered to be the ⁴A_2g→⁴T_2g energy level transition. SLSO: mMn⁴⁺ presents the same spectral shape, and the emission intensity can reach a maximum at 0.007 Mn⁴⁺; there is a concentration quenching effect. Figure 5c shows that the emission spectra of SLSO: Bi³⁺, Mn⁴⁺ contain the emission peaks of Bi³⁺ and Mn⁴⁺, as shown in Figure 5d; with increasing Mn⁴⁺ concentrations, the emission intensities of Bi³⁺ decrease and the emission intensities of Mn⁴⁺ increase. This indicates that there may be an energy transfer from Bi³⁺ to Mn⁴⁺ in SLSO:0.06Bi³⁺, mMn⁴⁺. In our previous research, it was found that Bi³⁺ ions enter the lattice sites Sr1 and Sr2(La) in SLSO; however, in this paper, the radius of Mn⁴⁺ ions r = 0.67Å (CN = 6) is similar to Sc³⁺ ion radius r = 0.745Å (CN = 6), which is suitable for Mn⁴⁺ ions to dope into. Therefore, it is believed that Mn⁴⁺ enters the [ScO₆] octahedron.

Figure 6a depicts the energy transfer process of Bi³⁺ and Mn⁴⁺, where the electrons were pumped to the excited states ¹P₁ and ³P₁ from the ground state ¹S₀ under 250 and 350 nm excitation, according to the 6S² outer-electron [36]. Meanwhile, the electrons on the ground state ⁴A_2g of Mn⁴⁺ were excited by the UV light source transition to the excited states ⁴T_1g, ²T_2g, and ⁴T_2g [37]. Then, the electrons relaxed to the lowest excited states ³P₁ and ²E_g and released photons to emit 550 and 707 nm light through the radiation process back to the ground state. The variation in the fluorescence lifetime of the sensitizer ions doped in the matrix is direct evidence that energy transfer occurs. Figure 6b presents the decay curves of SLSO: Bi³⁺, mMn⁴⁺. All decay curves can be well fitted to a double exponential function, and the lifetime values of Bi³⁺ were 458.3, 426.4, 359.3, 250.4, 158.7, 123.2, and 97.8 ns. Moreover, the decay curves of Mn⁴⁺ in SLSO:0.06Bi³⁺, mMn⁴⁺ were also measured, and the lifetime values of Mn⁴⁺ increased from 12.4 to 57.5 μs, as displayed in Figure 6c. Moreover, the energy transfer efficiency can reach 78.60%, as shown in Figure 6d. In this process, although some energy is lost because of the non-radiative transition, it is clear that Bi³⁺-Mn⁴⁺ can still exhibit an efficient energy transfer, which is mainly attributed to the fact that Bi³⁺ and Mn⁴⁺ ions have not only the same excitation band but also the same radiative reabsorption process because the emission spectrum of Bi³⁺ and the excitation spectrum of Mn⁴⁺ have a large overlap region.

The energy transfer process may be caused by the exchange interaction or the multipolar interaction; the former should be less than 5 Å, according to the critical distance (R_c). The R_c value between Bi³⁺ and Mn⁴⁺ can be evaluated using Blasse’s theory [38], and the calculated R_c is 17.31 Å. It is proposed that the energy transfer mechanism may be the electron multipole–multipole interaction. In addition, the Dexter theory and the Reisfeld approximation can be utilized to analyze the multipole interaction type of the energy transfer mechanism. The values α = 6, 8, and 10 correspond to dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The plot of τ_S0 /τ₀ and C^α/3 was fitted linearly, and the results are shown in Figure 7. By comparing the coefficient of determination (R² value), the best fit was obtained for α = 6, and the outcome clearly illustrates that the energy transfer from Bi³⁺ to Mn⁴⁺ in SLSO is primarily defined by electric dipole–dipole interaction. The Mn⁴⁺ ion can emit strong deep red light, which can be excited in near ultraviolet for the lighting equipment of plant growth.

3.4. Luminescence Properties of SLSO:0.06Bi³⁺, zYb³⁺

In order to explore the possibility of near-infrared luminescence in SLSO, we co-doped Yb³⁺ with Bi³⁺. Figure 8a shows the emission spectra of SLSO:0.06Bi³⁺, zYb³⁺ (z = 0.01, 0.015, 0.02, 0.05, and 0.15), which can range from 400 to 1350 nm under the 350 nm excitation. Figure 8b depicts the excitation spectrum and the emission spectrum of SLSO:0.06Bi³⁺, 0.02Yb³⁺, and the 980 nm emission peak reaches the high excited state ²F_5/2 to ground state ²F_7/2 of the Yb³⁺ ions [39]; the 550 nm emission peak is due to the excited state ³P₁ to ground state ¹S₀ of the Bi³⁺ ions. As shown in Figure 8c, obviously, with an increase in Yb³⁺ concentrations, the emission intensities of 980 nm increase and those of 550 nm decrease; it means there may be an energy transfer from Bi³⁺ to Yb³⁺. According to the critical distance (R_c) [40], for SLSO: Bi³⁺, Yb³⁺, N = 4, V = 683.702 ų, and x_c = 0.11, the critical distance is R_c = 14.37 Å. The plot of τ_S0 /τ₀ and C^α/3 is fitted linearly in Figure 8d, where α = 6, 8, and 10 corresponds to dipole–dipole (d–d), dipole–quadrupole (d–q), and quadrupole–quadrupole (q–q) interactions, respectively. The fitting factors of R² are 0.957, 0.931, and 0.908, respectively, and the best fit was obtained for α = 6, showing that Bi³⁺ and Yb³⁺ belong to the d–d interaction.

Figure 9a depicts the energy transfer process of Bi³⁺-Yb³⁺ [41]. Figure 9b shows the decay curves of Bi³⁺ in SLSO: Bi³⁺, zYb³⁺, and the lifetime values were 512.2, 486.3, 481.0, 480.7, 437.6, and 371.3 ns. Obviously, the lifetime of Bi³⁺ ions gradually decayed, and Figure 9c presents the lifetime values of a Yb³⁺ increase from 374.1 to 558.1 μs, which means the energy transfer from Bi³⁺ ions to Yb³⁺ ions. The energy transfer efficiency was calculated as follows: η = 1-τ_S0 /τ₀ [42], and Figure 9d shows that the η value can reach a maximum of η = 27.5% at z = 0.15. Since the excitation spectrum of Yb³⁺ ion is concentrated in the near-UV region and the emission spectrum of Bi³⁺ is in the visible region, they have fewer overlapping areas, so the energy transfer efficiency between them is low. However, it is found that the Bi³⁺-Yb³⁺ co-doping greatly broadens the emission range; this means that SLSO: Bi³⁺, zYb³⁺ has a promising application in the visible illumination region as well as the near-infrared detection region.

4. Conclusions

In summary, a series of SLSO:0.06Bi³⁺, yEu³⁺, SLSO:0.06Bi³⁺, mMn⁴⁺, and SLSO:0.06Bi³⁺, zYb³⁺ was synthesized by the high-temperature solid-state method and could create tunable color emissions by the energy transfer from Bi³⁺ to Eu³⁺/Mn⁴⁺/Yb³⁺, which was proven by the decay curves, time-resolved spectra, and spectral properties. The experiments have not only discussed the process of energy transfer from Bi³⁺ ions to other luminescent centers but have also investigated the interactions between the sensitized and activated ions. Based on the above studies, we believe that a large overlapping area between the emission spectrum of the sensitizer and the excitation spectrum of the activator will result in high energy transfer efficiency. Therefore, in order to prepare multifunctional luminescent materials with both visible and near-infrared emissions, the intense luminescence of the Yb³⁺ ion needs to be improved so that the emission spectrum of Bi³⁺ can be modulated to blue-shift to get more overlapping regions in the future in order to obtain efficient energy transfer progress between Bi³⁺ and Yb³⁺, which can provide a novel tunable wavelength luminescent material for high-quality lighting and detection applications.