Novel BaCa₁₃Mg₂(SiO₄)₈: Ce³⁺, Mn²⁺ Phosphor for Plant Lighting with Robust Thermal Stability and High Energy Transfer Efficiency

Abstract

The blue and deep-red wavelengths emitted by plant growth lamps effectively encompass the entire spectrum of plant pigments essential for optimal growth. Nevertheless, inadequate thermal stability and poor quantum efficiency impede advancements in plant lighting technology. In this study, Ce³⁺ and Mn²⁺ ions are co-doped into the BaCa₁₃Mg₂(SiO₄)₈ matrix to develop a phosphor that encompasses the entire absorption wavelength range of plant pigments. Ce³⁺ and Mn²⁺ ions can emit blue light and red light in the range of 380–550 nm and 600–760 nm, respectively. As an excellent sensitizer, Ce³⁺ ions can significantly enhance the luminescence efficiency of Mn²⁺ ions, with a quantum efficiency of 51.03%. The analysis of crystal structure, spectroscopy, and energy transfer mechanisms clarifies the fundamental factors contributing to the observed high quantum efficiency. Theoretical calculations show that dipole–dipole interactions dominate the energy transfer process. At 140 °C, the emission intensity of the whole sample region can reach 92.83% of the integrated emission intensity. The electroluminescence spectrum of the sample overlaps substantially with the spectrum required for optimal absorption by the plant. The results show that BaCa₁₃Mg₂(SiO₄)₈: Ce³⁺, Mn²⁺ phosphors have excellent properties and inspire broad application prospects in the plant lighting field.

1. Introduction

Light plays a pivotal role not only in supporting plant growth but also serves as a fundamental component in contemporary agricultural practices. The process of photosynthesis in plant pigments converts light energy into chemical energy [1,2]. The four primary plant pigments, chlorophyll A, chlorophyll B, phytochrome PR, and phytochrome PFR, are sensitive to blue (400–500 nm), red (600–700 nm), and near-infrared light (700–780 nm) [3]. However, traditional indoor plant growth lighting sources have limitations, including poor thermal stability, low quantum efficiency, short lifespan, high energy consumption, and spectral mismatch, posing challenges for their application in the field of plant lighting [4,5,6]. The combination of different luminescent materials to cover the absorption peaks of all plant pigments will lead to reabsorption, resulting in inconsistent color drift and thermal stability. Using multi-chip light-emitting diode (LED) combinations for plant lighting can lead to a mismatch between the light spectrum and the photosynthesis spectrum (PAS) of plant chlorophyll [7]. The blue light emission and broadband deep infrared emission in a single substrate by energy transfer can overcome the shortcomings of the above strategies. Therefore, it is of great practical significance to find a single matrix luminescent material that can cover the pigment absorption area of plants with excellent thermal stability and high quantum efficiency.

Mn²⁺ ion, as an excellent activator ion, has a 3d⁵ electron configuration, with the electrons in the outer orbital [8]. The emission band wavelength of the Mn²⁺ ion is typically influenced by the crystal field of the matrix [9]. The transition of Mn²⁺ ions from the ⁴T₁-⁶A₁(⁶S) state results in distinct emission colors depending on the coordination environment. Specifically, the four-coordinated weak crystal field facilitates green light emission, whereas the six-coordinated strong crystal field leads to emissions ranging from orange to red [10,11,12]. The energy level splitting increases as the crystal field strength increases from weak to strong. As a result, the lowest energy level, designated ⁴T₁ (⁴G), shifts downward, leading to a redshift in the emitted light that may approach the near-infrared region [13]. The overlap between the deep-red emission of Mn²⁺ and the absorption spectrum of plant pigments makes it have broad application prospects in plant lighting. The excitation energy absorption exhibited by Mn²⁺ is typically weak, and its luminescence intensity is notably low, which can be attributed to the violation of the selection rule. Co-doped sensitizers are typically chosen to enhance absorption intensity, thereby ultimately increasing luminescence intensity [14,15,16]. The luminescence of Ce³⁺ ions is notably efficient owing to the parity-allowed 4f-5d transitions. This property enables them to function effectively as sensitizers, facilitating the transfer of excitation energy to the Mn²⁺ activator ions [17,18,19,20,21,22,23,24,25,26,27]. BaCa₁₃Mg₂(SiO₄)₈ is selected as the matrix material with a calcium silicate structure and belongs to the Pnn2 (34) space group of the orthorhombic system. In the crystal structure of BaCa₁₃Mg₂(SiO₄)₈, the incorporation of rare-earth ions is facilitated by the existence of multiple sites, each offering a distinct crystal-field environment [28,29]. Recently, a highly effective Ce³⁺ → Mn²⁺ energy transfer has been reported in many publications, making these materials exhibit excellent luminescence properties, such as NaMgBO₃: Ce³⁺, Mn²⁺ [30] and CaY₂Al₄SiO₁₂: Ce³⁺, Mn²⁺ [31] with excellent thermal stability. However, materials like Mg₂Gd₈(SiO₄)₆O₂: Ce³⁺, Mn²⁺ [32] and Ca₂Gd₈(SiO₄)₆O₂: Ce³⁺, Mn²⁺ [33] still exhibit low quantum efficiency. Thus, it is necessary to explore a luminescent material with excellent thermal stability and high emission quantum yield.

In this paper, a new phosphor BaCa₁₃Mg₂(SiO₄)₈: Ce³⁺, Mn²⁺ is synthesized by the high-temperature solid-phase method with Ce³⁺ as a sensitizer and Mn²⁺ as an activator. The characteristics of the samples, including their crystal structure, photoluminescence, decay profiles, and thermal stability, were thoroughly investigated. The forbidden ⁴T₁-⁶A₁ transition of Mn²⁺ ion is compensated by the energy transfer from Ce³⁺ to Mn²⁺ ions, thereby improving the emission intensity. The critical distance has been calculated, and an extensive investigation has been conducted to analyze the efficiency and mechanism of energy transfer from Ce³⁺ to Mn²⁺ ions. The results show that the BaCa₁₃Mg₂(SiO₄)₈: Ce³⁺, Mn²⁺ phosphor can be used in the field of plant growth lighting.

2. Materials and Methods

BaCa₁₃Mg₂(SiO₄)₈: Ce³⁺, Mn²⁺ phosphors were synthesized by using BaCO₃ (99%, Aladdin, Shanghai, China), CaCO₃ (99.5%, Aladdin, Shanghai, China), MgCO₃ (99%, Aladdin, Shanghai, China), SiO₂ (99%, Macklin, Tianjin, China), CeO₂ (99.99%, Macklin, Tianjin, China) and MnO (99%, Macklin, Tianjin, China) as raw materials. The raw materials were ground with ethanol (G.R., Aladdin, Shanghai, China) for 30 min. After uniform grinding, the powder reagent was placed in an alumina crucible and sintered at 1400 °C for 4 h in a tube furnace under the protection of nitrogen/hydrogen. Finally, the sample was naturally cooled to room temperature, and then reground for subsequent analysis.

XRD (DX-2700BH, Haoyuan Instrument Co., Ltd., Dandong, China) is used to identify the phase purity and characterize the crystal structure by Cu-Ka radiation at 40 kV and 30 mA. GSAS program (Version II, Argonne National Laboratory, Argonne, IL, USA) is applied to fitting the calculated data and the measured data obtained by XRD Rietveld refinements. The morphology and element distribution are measured by Hitachi S-4800 (Hitachi Ltd., Tokyo, Japan). The photoluminescence (PL) spectrum, decay curves and PL spectra depended on temperature are measured by FS5-MCS fluorescence spectrophotometer (FS5-MCS, Edinburgh Instruments, Livingston, UK) with microsecond light source and Xe lamp. The internal quantum efficiency was investigated using the FLS1000 instrument (Edinburgh Instruments, Livingston, UK).

3. Results

3.1. Purity of the Crystal Phase and Crystal Structure Analysis

Figure 1a exhibits the XRD patterns of BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, xMn²⁺ (5% ≤ x ≤ 40%) samples. The prominent peaks of the diffraction pattern of the powder matched precisely with the standard peaks from the reference card (PDF#00-027-1060), and no other impurity phases were generated. Research indicates that Mn²⁺ and Ce³⁺ ions are thoroughly incorporated into the BaCa₁₃Mg₂(SiO₄)₈ matrix, with no significant alterations observed in the crystal structure. The results of the Rietveld refinement are illustrated in Figure 1b and serve to validate the phase structure. Additionally, Tables S1 and S2 provides the cell characteristics. The results of the refinement indicate that the synthesized BaCa₁₃Mg₂(SiO₄)₈ crystallizes in the orthorhombic Pnn2 (34) space group. The low residual factors (Rwp = 12.95% and Rp = 7.28%) suggest that the simulation results are within an acceptable range. Figure 1c illustrates the crystal structure of BaCa₁₃Mg₂(SiO₄)₈. Within this structure, a singular type of Ba site is coordinated with twelve oxygen atoms, while a singular type of Mg site is coordinated with six oxygen atoms. There exist three distinct types of calcium sites in BaCa₁₃Mg₂(SiO₄)₈: nine-coordinated CaI (CaI9), ten-coordinated CaII (CaII10), and twelve-coordinated CaIII (CaIII12). The given formula is used to accurately pinpoint the exact cation sites within the host lattice that are occupied by doped ions [34]:

$$D_r={\displaystyle \frac{[R_m\left(CN\right)-R_d\left(CN\right)]}{R_m\left(CN\right)}}$$

(1)

where CN represents the coordination number, Rm (CN) stands for the radius of the host cation and Rd (CN) denotes the radius of the substituent ion. Table 1 presents the effective ionic radii of Ba²⁺, Ca²⁺, Mg²⁺, Ce³⁺, and Mn²⁺ across various coordination environments. The percentage deviation in their ionic radii must remain below 30%. Consequently, it can be inferred that Ce³⁺ ions are likely to occupy the structural sites of Ba²⁺ and Ca²⁺, while Mn²⁺ ions are expected to occupy the Mg²⁺ sites.

Figure 1. (a) XRD patterns of BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, xMn²⁺ (5% ≤ x ≤ 40%); (b) Rietveld refinement results of BaCa₁₃Mg₂(SiO₄)₈ sample; (c) Crystal structure of BaCa₁₃Mg₂(SiO₄)₈.

Table 1. Relative difference in the ionic radii between the host cations and the dopant.

Ions	Radius	CN	Dr (Ce3+)	Dr (Mn2+)
Ba2+	1.61	12	16.8%
Ca2+	1.18	9	1.4%
	1.23	10	1.6%
	1.34	12	0%
Mg2+	0.72	6	40.3%	15.3%
Ce3+	1.01	6
	1.196	9
	1.25	10
	1.34	12
Mn2+	0.83	6

Figure S1a,b illustrate the crystal morphology of BaCa₁₃Mg₂(SiO₄)₈, as measured using scanning electron microscopy (SEM) images. The micron-sized crystals, exhibiting particle sizes of approximately 8 to 9 μm, demonstrate minimal agglomeration. Figure S1c–k present the energy dispersive spectroscopy (EDS) data along with mapping images for BaCa₁₃Mg₂(SiO₄)₈ doped with 0.3% Ce³⁺ and 20% Mn²⁺. The results indicate that the spectral peaks reveal an even distribution of the elements Ba, Ca, Mg, Si, O, Mn, and Ce throughout the particles. Based on the structural data derived from the refinement results, Figure S2a shows that the band gap is 4.411 eV. The valence band maximum (VBM) and conduction band minimum (CBM) of BaCa₁₃Mg₂(SiO₄)₈ are located at different points, indicating an indirect bandgap. The total density of states (TDOS) for the BaCa₁₃Mg₂(SiO₄)₈, along with the partial density of states (PDOS) for various atomic species, is represented in Figure S2b. Notably, the O-2p orbital is a significant contributor at the apex of the valence band, whereas the bottom of the conduction band is primarily composed of cations such as Ca, Ba, and Si. These findings from the band structure and the density of states analysis indicate a favorable environment for the transitions of Ce³⁺ and Mn²⁺ within the band gap of BaCa₁₃Mg₂(SiO₄)₈.

3.2. Luminescence Properties Analysis

The excitation and emission spectra of Ce³⁺ and Mn²⁺ doped BaCa₁₃Mg₂(SiO₄)₈ samples are constructed in Figure 2a. At 380–420 nm, the PL spectra of Ce³⁺ ions and the PLE spectra of Mn²⁺ ions clearly overlap, indicating energy transfer between Ce³⁺ and Mn²⁺. By adjusting the Ce³⁺ ion doping concentration while maintaining a constant Mn²⁺ ion doping concentration, energy is transferred to the Mn²⁺ ions, thereby enhancing luminescence. Figure 2b presents the PLE spectra of BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, xMn²⁺ (5% ≤ x ≤ 40%). Upon monitoring at 690 nm, the diagram reveals distinct excitation peaks corresponding to both Ce³⁺ and Mn²⁺. It is noteworthy that due to the forbidden nature of the d-d transition in Mn²⁺, the resultant emission intensity is considerably low. However, the sensitization provided by Ce³⁺ can markedly enhance this intensity. Simultaneously, the excitation peak of the Ce³⁺ ion is distinctly observable while monitoring the emission peak of the Mn²⁺ ion at 690 nm, indicating the occurrence of energy transfer between the Ce³⁺ and Mn²⁺ ions. Figure 2c illustrates the emission spectra obtained under excitation at 365 nm, demonstrating that, with a fixed concentration of 0.3% Ce³⁺, the emission intensity of Ce³⁺ decreases progressively as the concentration of Mn²⁺ ions increases. This observation indicates that an energy transfer process is occurring within the sample. The relationship between normalized luminescence intensity and Mn²⁺ doping concentration is illustrated in Figure S3. It is observed that the emission intensity associated with the characteristic peak of the Mn²⁺ ion reaches its maximum at a doping concentration of 20%, indicating the occurrence of concentration quenching at this level. The CIE coordinates of BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, xMn²⁺ (5% ≤ x ≤ 40%) is shown in Figure S4 and Table S3 presents the exact CIE (x, y) coordinates and color purity. The energy level diagrams for Mn²⁺ and Ce³⁺ ions have been constructed to depict the luminescence mechanism and energy transfer processes, as illustrated in Figure 2d. In BaCa₁₃Mg₂(SiO₄)₈: Ce³⁺, Mn²⁺ phosphors, the free electrons are capable of absorbing ultraviolet light and transitioning from the ground state (²F_5/2) to an excited state (5d). The subsequent 5d → 4f transition facilitates the emission of blue light by the Ce³⁺ ions. Notably, the 5d energy level of Ce³⁺ is situated higher than the ⁴E, ⁴A₁ levels of Mn²⁺. Consequently, the excitation energy can be transferred from the Ce³⁺ 5d level to the high-lying excited states of Mn²⁺, followed by non-radiative relaxation to the ⁴T₁ state, and finally the ⁴T₁ → 6A₁ radiative transition yields the red emission at ~690 nm.

Figure 2. (a) Excitation and emission spectra of BaCa₁₃Mg₂(SiO₄)₈: Ce³⁺ and BaCa₁₃Mg₂(SiO₄)₈: Mn²⁺ phosphors; (b,c) PLE and PL spectra of BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, xMn²⁺ (5% ≤ x ≤ 40%) phosphors; (d) the energy level diagrams of Ce³⁺ and Mn²⁺ ions and the ET process. The corresponding wavenumbers of the energy levels are also labeled on the right side.

The energy transfer process from Ce³⁺ to Mn²⁺ can be proved by examining the decay curves. Figure 3a exhibits the decay curves of BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, xMn²⁺ (5% ≤ x ≤ 40%). The curves can be fitted using the double exponential formula [35,36]:

$$I\left(t\right)=A_1\mathit{exp}\left(-{\displaystyle \frac{t}{{\tau }_1}}\right)+A_{2}exp(-{\displaystyle \frac{t}{{\tau }_2}})$$

(2)

where τ₁ and τ₂ denote the decay times of exponential components, t stands for the time, A₁ and A₂ represent constants, and I(t) denotes the luminescence intensity. Due to the energy transfer from Ce³⁺ to Mn²⁺, the lifetime of Ce³⁺ decreases from 43.8 to 35.06 ns. The energy transfer efficiency (η_ET) can be estimated using the following formula [29,37]:

$${\eta }_{ET}=1-{\displaystyle \frac{{\tau }_s}{{\tau }_{s0}}}$$

(3)

where τ_S and τ_S0 denote the decay lifetimes of the sensitizer (Ce³⁺) ion with and without activator (Mn²⁺) ion present, respectively. The energy transfer efficiency monotonously ascends with continuous increase in Mn²⁺ concentration. The η_ET values for it were calculated as 19.95%. The critical distance (Rc) between sensitizers and activators is a pivotal parameter for assessing the energy transfer process. Rc can be determined using the following formula [38]:

$$R_c=2[{\displaystyle \frac{3V}{4\pi X_{C}N}}{]}^{1/3}$$

(4)

where V represents the unit cell volume. X_C stands the total critical concentration, and N denotes the number of which the dopant can occupy sites in the cell. For BaCa₁₃Mg₂(SiO₄)₈ host, V = 1345.215 Å3, X_C = 0.02 and N = 9. So Rc is 26.7 Å after calculation. Energy transfer primarily influenced by exchange interactions is observed only when the distance, Rc is less than 5 Å. The calculation results indicate that the electro–multipole interaction facilitates energy transfer between two ions. Utilizing Dexter’s theory and the Reisfeld approximation, the mechanism of energy transfer is elucidated, as detailed in the formula presented below [39]:

$${\displaystyle \frac{I_{S0}}{I_S}}\propto C^{n/3}$$

(5)

where I_S0 and I_S represent the luminescence intensity of Ce³⁺ ions with and without Mn²⁺ ions doping. C denotes the ion doping concentration, and n equal to 6, 8, and 10 correspond to dipole–dipole, dipole–quadrupole, quadrupole–quadrupole interactions. Figure 3c exhibits the I_S0/I_S vs. C^n/3 curve with varying n as 6, 8 and 10. As is observed, the curves of I_S0/I_S versus C^n/3 at n = 6 follow a linear relationship. It suggests that dipole–dipole interaction dominates energy transfer between Ce³⁺ and Mn²⁺ ions. Figure 3d depicts the internal quantum efficiency spectra of the samples. It can be determined using the below formula [40]:

$$\eta ={\displaystyle \frac{\int L_S}{\int E_R-\int E_{RS}}}$$

(6)

where E_R denotes the spectrum from the excitation light of BaSO₄, L_S represents the emission spectrum of the sample and E_S is the excitation spectrum of the sample. Under 365 nm excitation, the quantum efficiency of the sample is calculated to be 51.03%.

Figure 3. (a) Decay curves of BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, xMn²⁺ (5% ≤ x ≤ 40%) excited by 365 nm and monitored at 440 nm; (b) emission intensity and energy transfer efficiency of BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, xMn²⁺ samples with different doping concentrations; (c) dependence of IS0/IS of Ce³⁺ on C^6/3, C^8/3, and C^10/3, The red and black squares represent the actual (experimental) values; (d) internal quantum efficiency of BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, 20% Mn²⁺ samples.

3.3. Thermal Stability Analysis and EL Spectrum

Temperature-dependent emission spectra of BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, 20% Mn²⁺ were measured at 20–230 °C, as constructed in Figure 4a. Compared with the same Pnn2 (34) host BaCa₁₃Mg₂(SiO₄)₈: Ce³⁺,K⁺ and the garnet-type silicate Ca₂LuScAl₂Si₂O₁₂: Ce³⁺, Mn²⁺, the present BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, 20% Mn²⁺ phosphor retains 88.13% of its room-temperature emission intensity and 92.83% of its integrated emission area. These results demonstrate the excellent thermal stability of the present phosphor [29,41]. Figure 4b illustrates the variation in emission intensity and its integral as the temperature increases. At 140 °C, the emission intensity and integral area of the sample co-doped with 0.3% Ce³⁺ and 20% Mn²⁺ reach 88.13% and 92.83%, respectively, relative to room temperature. Furthermore, as the temperature rises, a blue shift in the emission peak of the Mn²⁺ ion is observed. This phenomenon can be attributed to thermally activated phonon-assisted excitation, which facilitates the transition of Mn²⁺ from a lower excited state to a higher excited state. The increase in non-radiative transition probability leads to a corresponding decrease in emission intensity. Moreover, this thermal quenching mechanism can be substantiated through the Arrhenius equation [42]:

$$I\left(T\right)=I_0[1+Aexp\left(-{\displaystyle \frac{∆E}{kT}}\right){]}^{-1}$$

(7)

where k stands for the Boltzmann constant, I₀ represents the original intensity, I denotes the intensity at a given temperature, T represents the temperature, and ΔE stands for the activation energy. The ΔE can be determined as 0.2027 eV based on the linear fitting results of ln[(I₀/I_T) − 1] vs. 1/kT. Consequently, the phosphor exhibits a high activation energy and demonstrates low thermal quenching performance, indicating significant potential for a wide range of applications. To investigate the potential applications in the domain of plant lighting, LED devices were fabricated utilizing BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, 20% Mn²⁺ phosphors. Figure 4c illustrates the electroluminescent spectrum of the assembled LED device under a forward bias current of 225 mA, which is compared to the absorption spectrum of visible light by plant pigments. As illustrated in Figure 4d, both the blue light region associated with the emission of Ce³⁺ ions and the red light region corresponding to the emission of Mn²⁺ ions significantly overlaps with the spectra required for plant absorption. Consequently, it can be inferred that the synthesized samples exhibit considerable potential for applications in plant lighting.

Figure 4. (a) BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, 20% Mn²⁺ phosphor excited by 365 nm (T = 20–230 °C), the inset indicates the linear fitted result of Ln(I₀/I_T − 1) on 1/kT for phosphor; (b) the temperature-dependent emission intensity and integrated area; (c) EL spectrum (225 mA) with a chip (365 nm) and BaCa₁₃Mg₂(SiO₄)₈: 0.3% Ce³⁺, 20% Mn²⁺, The purple curve represents the NUV chip, and the blue-to-red curve represents the emission of the phosphor (d) the absorption spectra of plant pigment and the emission spectrum of the sample.

4. Conclusions

In summary, Ce³⁺ and Mn²⁺ ions co-doped BaCa₁₃Mg₂(SiO₄)₈ blue phosphor were developed, and its luminescence properties were studied systematically. The optimal excitation wavelength for the sample is established at 365 nm. Furthermore, the reduction in lifetime corroborates the energy transfer mechanism occurring between Ce³⁺ and Mn²⁺ co-doped BaCa₁₃Mg₂(SiO₄)₈. The energy transfer efficiency from Ce³⁺ to Mn²⁺ in BaCa₁₃Mg₂(SiO₄)₈ is determined to be 70.9%. Calculations indicate that the mechanism of energy transfer is dominated by dipole–dipole interactions, with a critical distance measured at 26.7 Å. The synthesized phosphor exhibits exceptional thermal stability, maintaining approximately 92.83% of its original emission intensity at room temperature when subjected to 140 °C. Additionally, the quantum efficiency of the sample is 51.03% under 365 nm excitation. The emission spectrum and the electroluminescence spectrum are in agreement with the spectrum required for plant absorption, suggesting potential applicability in the field of plant lighting [43].