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. 2022 Feb 24;2(3):374–380. doi: 10.1021/acsmaterialsau.1c00081

Eu2+ Doping Concentration-Induced Site-Selective Occupation and Photoluminescence Tuning in KSrScSi2O7:Eu2+ Phosphor

Shunqi Lai †, Ming Zhao ‡, Yifei Zhao †,§, Maxim S Molokeev ∥,⊥,@,#, Zhiguo Xia †,*
PMCID: PMC9928192  PMID: 36855382

Abstract

graphic file with name mg1c00081_0007.jpg

Regulation of Eu2+ dopants in different cation sites of solid-state materials is of great significance for designing multicolor phosphors for light-emitting diodes (LEDs). Herein, we report the selective occupation of Eu2+ for multiple cationic sites in KSrScSi2O7, and the tunable photoluminescence from blue to cyan is realized through Eu2+ doping concentration-dependent crystal-site engineering. Eu2+ preferably occupies the K and Sr sites in KSrScSi2O7 at a low doping concentration, resulting in a 440 nm blue emission. As the Eu2+ concentration increases, a new Eu2+ substitution pathway is triggered, that is, Eu2+ enters the Sc site, leading to the red-shifted emission spectra from 440 to 485 nm. The doping mechanism and photoluminescence properties are corroborated by structural analysis, optical spectroscopy study, and density functional theory calculations. The optical properties of the as-fabricated white LEDs are studied, which demonstrates that these phosphors can be applied to full-spectrum phosphor-converted LEDs. This study provides a new design strategy to guide the development of multicolor Eu2+-doped oxide phosphors for lighting applications.

Keywords: photoluminescence, silicates, crystal-site engineering, doping, light-emitting diodes

Introduction

Currently, healthy lighting with full spectrum and high quality has been favored in our daily life.1 The fabrication of full-spectrum white light-emitting diodes (WLEDs) has become a research hot spot in the field of lighting due to their advantages not only to be close to the solar spectrum but also energy saving, high efficiency, and environmental friendliness.2,3 Generally, traditional WLEDs are mainly obtained by combining a blue InGaN chip with the commercial yellow-emitting phosphor Y3Al5O12:Ce3+ (YAG:Ce).4−7 Nevertheless, the absence of red spectral components would result in high-correlated color temperature (CCT > 5000 K) and poor color-rendering index (Ra < 80), which cannot meet the demands of healthy lighting.8 In addition, the harsh blue light generated by blue light-emitting diode (LED) chips could hurt the endocrine system, human emotion, thoughtseize, and even the pace of life.9 Hence, some researchers proposed to fabricate a lighting device using the phosphors of three primary colors combined with near-ultraviolet (n-UV) or ultraviolet (UV) chips that can diminish blue light and enable high-quality white light.10−12 However, there still exists a cavity in the region of 480–520 nm compared with sunlight, which is called the “cyan gap”.13 This gap hinders the WLED devices from obtaining a true continuous white light spectrum similar to sunlight, and it also has a negative impact on the rhythm of life.14,15 Consequently, it is very important to design a new kind of phosphor emitting in the blue-to-cyan spectral region to achieve full-spectrum lighting by cyan light compensation.16−18

As one of the most common and highly efficient activators, an Eu2+ ion possesses broadband absorption in the UV to blue regions (350–460 nm), as well as multicolor emission from 400 to 740 nm, originated from the highly efficient 4f65d1–4f7 parity-allowed electric dipole transition.19,20 Furthermore, due to the exposed 5d electrons on the surface of Eu2+ ions, its energy state is susceptible to the local surroundings around the ion. Therefore, the multicolor emission can be achieved by intentionally controlling the local site symmetry, crystal field splitting, and the covalency between the activator and ligand.21−25 In this regard, many strategies were put forward to regulate the site-selective occupation of Eu2+ and design the phosphor with the desired emission, such as modifying the synthetic route, neighboring cation/anion substitution, changing the doping concentration of Eu2+, and so on.26,27 Among them, changing the Eu2+ concentration has been served as an efficient way to regulate the local environment and site occupation of Eu2+ to harvest multicolor phosphors.28 For example, Kakihana et al. successfully modulated the Eu2+ occupation site from Ca(1n) sites to Ca(2n) sites, which is smaller, to reach the tuning of photoluminescence from orange to red in Ca2SiO4:Eu2+ by simply changing the Eu2+ concentration.29 Tang studied the multicolor luminescence in Li2Sr2Al(PO4)3:Eu2+ in the same way.30 All of these systems have one specific feature in common, containing at least two cationic sites with different coordination environments.

In 2010, Maria reported a new scandium silicate compound KSrScSi2O7, which includes three different cationic sites (K, Sr, Sc) that may be occupied by Eu2+.31 Subsequently, Chen et al. doped Eu2+ ions into this host obtaining a blue-emitting phosphor, which was attributed to the Eu2+ ions locating in Sr sites.32,33 As reported in A3BSi2O7 (A refers to alkali metal ions and B stands for inert rare-earth ions), Eu2+ ions can occupy A and B sites, obtaining a stable system and bright emission.34,35 Herein, taking the blue-emitting silicate phosphor KSrScSi2O7:Eu2+ (KSS:Eu) as the research object, we intend to realize different occupations of Eu in KSS:Eu via changing the Eu2+ concentration to achieve the tunable emissions.

Accordingly, KSS:xEu2+ phosphors dependent on different Eu2+ doping concentrations were synthesized, and the luminescence properties and mechanisms were studied in detail. At a low Eu2+ concentration, Eu2+ occupies K and Sr sites enabling blue emission; while increasing the concentration of Eu2+, Eu2+ can be chemically driven to K and Sc sites, thereby achieving cyan emission. In this way, this cyan-emitting phosphor can be potentially used for full-spectrum lighting. This work reveals novel insights into the search for multicolor phosphors suitable for the full-spectrum LED illumination by Eu2+ site-selective occupation.

Experimental Section

Materials and Preparation

The designed samples of KSS:xEu (x = 0–20%) were synthesized using the conventional high-temperature solid-state reaction method. Stoichiometric amounts of K2CO3 (A.R.), SrCO3 (A.R.), SiO2 (A.R.), Sc2O3 (A.R.), and Eu2O3 (99.99%) were mixed evenly by grounding them with ethanol for an appropriate time, and all of the chemicals were commercially purchased from Aladdin without further purification. First, the mixtures were sintered at 600 °C for 6 h under air and then sintered at 1150 °C for 4 h under a reducing atmosphere of 80%N2–20%H2. The final products were naturally cooled to room temperature (RT) and ground into fine powders for further measurement.

Characterization

X-ray diffraction (XRD) patterns were collected on an Aeris X-ray diffractometer (PANalytical Corporation, The Netherlands) operating at 40 kV and 15 mA with monochromatized Cu Kα radiation (λ = 1.5406 Å). Rietveld analyses of the powder XRD patterns were carried out by TOPAS 4.2 program.36 Photoluminescence excitation (PLE) and emission (PL) spectra were recorded by an FLS1000 fluorescence spectrometer (Edinburgh Instruments Ltd., U.K.) equipped with a continuous 450 W xenon (Xe) lamp. The internal/external quantum efficiency (IQE/EQE) was checked using an integrated sphere on the same FLS1000 instrument. Temperature-dependent PL spectra were collected from 300 to 420 K using the FLS1000 with heating equipment. Decay curves were collected by the FLS1000 equipped with a 340 nm pulse diode, and the count number of photons is 5,000. All of the decay curves were fitted by FAST software supplied with the FLS1000.

LED Fabrication

WLED devices were fabricated based on n-UV chips (λ = 365 nm) encapsulated in epoxy resin containing selected phosphors. The photoelectric performance of the LEDs, namely, the emission spectra, luminous efficacy, CCT, Ra, and Commission Internationale de L’Eclairage (CIE) chromatic coordinates, were measured on an integrating sphere spectroradiometer system (ATA-1000, Everfine).

Computational Methodology

Density functional theory (DFT) calculations were implemented adopting the Vienna Ab initio Simulation Package (VASP).37,38 The generalized gradient approximation (GGA) and Perdew–Burke–Ernzerhof (PBE) functionals were selected to represent the exchange–correlation interactions within the material systems.39−41 For all of the calculations, the plane-wave basis cutoff energy was set to 400 eV. The 3 × 3 × 3 k-meshes centered on the Γ point were employed. The following convergence criteria were selected for the geometry optimizations: (1) the Hellmann–Feynman forces should not be greater than 0.05 eV/Å and (2) the total energy difference should be under 5 × 10–5 eV/atom. The formation energies are calculated based on a 2 × 3 × 2 KScSrSi2O7 supercell using the following equation

graphic file with name mg1c00081_m001.jpg 1

where Etot (perfect) and Etot (defect) are the total energy of the cells before and after introducing the dopants; μi refers to the referenced chemical potential of atoms that are involved in the defect formation; and ni stands for the corresponding number of atoms. Each μi is derived from the corresponding single substance in the conventional phase.

Results and Discussion

As shown in Figure 1a, the XRD patterns of KSS:xEu (x = 1–20%) with different Eu2+ doping concentrations are compared with the standard pattern of KSrScSi2O7 (ICSD card no.166997).31 Almost all diffraction peaks can be well indexed by the monoclinic space system (P21/n), except for small peaks of Sc2O3 and SrSiO3 impurities. The parameters resemble those in the KSrScSi2O7 phase, which proves that the KSS:Eu phosphors have been successfully synthesized. To further understand the local structure and substitution mechanism of Eu2+ ions, we performed Rietveld refinement for KSS:xEu (x = 1–20%), as shown in Figure S1. All of the refinement results were sufficiently reliable for its low R-factors (Table S1). Moreover, Table S2 listed the isotropic displacement parameters and fractional atomic coordinates of the studied phases for comparison. Figure 1b shows the schematic illustration of a KSrScSi2O7 crystal structure. Based on the isolated Si2O7 groups, the basic framework of the KSrScSi2O7 structure is formed by connecting nine-coordinated Sr cations, nine-coordinated K cations, and six-coordinated Sc cations. Adjacent ScO6 polyhedra are linked to Si2O7 groups by sharing oxide atoms, and the Si–Si connected linearly along the c-axis, which creates the ScO6 polyhedra layers in the ab plane and forms a three-dimensional (3D) skeleton with K and Sr cations located in the voids.31

Figure 1.

Figure 1

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(a) XRD patterns of KSS:xEu (x = 1–20%) and the theoretical pattern calculated on the base of ICSD card no.166997 as a reference. (b) Crystal structure of KSrScSi2O7. (c) Dependence of cell volume with the Eu concentration, and the nonlinear trend can be associated with different Eu occupation mechanisms.

In the KSrScSi2O7 structure, theoretically, Eu2+ ions can be located in cation sites of K, Sr, and Sc. Table S3 lists the specific radius of cations with different coordination numbers. As shown in Figure 1c, while the Eu ion concentration x ranges from 0 to 10%, the cell volume decreases first as x increases, and this indicates that Eu2+ ions mainly occupy K and/or Sr sites because the ionic radii of Eu2+ are smaller than those of K+ and Sr2+ with the same coordination number while larger than that of Sc3+. However, with the continuous increase of x, the cell volume begins to increase, indicating that a new site occupation mechanism of Eu2+ emerged, which would also involve the substitution in Sc sites. Then, the DFT simulations were carried out to further investigate the preferential site occupation of Eu2+ upon different doping concentrations, and the results of each model are summarized in Figure 2a. Considering the complex scenarios involving extra defects for charge balances, we have attempted to treat the substitutions as neutral defects first. Within a low doping concentration (the single Eu dopant in K48Sr48Sc48Si96O336, giving an Eu molar ratio of 2%), Eu prefers to enter Sr and K sites, according to the relatively low formation energies. While provided with a higher Eu dosage (such as dual Eu substitution in the same pristine lattice, leading to Eu% = 4 mol %), Eu2+ ions became more likely to occupy K and Sc sites instead. Also, if the charge balances (e.g., oxygen defect) are included for the naturally charge-imbalanced models, the differences in formation energies would only be larger. Therefore, these DFT simulations could also suggest the concentration-dependent Eu2+ site preference from a theoretical perspective.

Figure 2.

Figure 2

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(a) Calculated formation energy for different variants of Eu substitution in KSrScSi2O7. (b) PLE (dotted line) and PL (solid line) spectra of KSS:Eu phosphors monitored at corresponding optimal wavelengths. (c) FWHM and the integrated intensity of KSS:Eu phosphors. The low-temperature emission spectra and Gaussian fitting results of (d) KSS:1%Eu and (e) KSS:15%Eu. (f) Decay curves of KSS:xEu (x = 1–20%) at RT excited at 340 nm and monitored at the corresponding optimal emission wavelength.

Figure 2b shows the PL and PLE spectra of KSS:xEu2+ (x = 1–20%) at RT. The f–d transition of Eu2+ ions produces a broad excitation band ranging from 300 to 420 nm, and its peak position is almost unchanged with increasing Eu2+ concentration, besides slight increasing of bandwidth. As the Eu2+ concentration increases, the PL spectra excited at 365 nm present a continuous red shift from 440 blue light to 485 nm cyan light. To analyze the changing trend of emission spectra with the increase of the Eu2+ concentration in higher accuracy, we reduced the concentration step from 5 to 2%. As shown in Figure 2c, as x increases, the PL intensity experiences increases and decreases twice, and the full width at half-maximum (FWHM) increases first and then reduces. It is obvious that the point with the weakest PL intensity corresponds to the widest FWHM. We further tested the quantum yield (IQE and EQE), as shown in Table S4, and its variation was consistent with that of the luminescence intensity. All of those are related to the different Eu2+ occupation mechanisms, which will be discussed later.

The connection of the luminescent centers and the crystallographic positions in the crystal structure was further confirmed by measuring low-temperature spectra. The PL spectra of two selected samples of KSS:1%Eu and KSS:15%Eu at 77 K are depicted in Figure 2d,e, respectively. It can be seen that two Gaussian peaks can be well fitted to the spectral profile. Using the empirical eq 2 proposed by Van Uitert, we estimated the d-band-edge position of Eu2+ (E) to analyze the origin of the PL peaks42

graphic file with name mg1c00081_m002.jpg 2

where Q is 34 000 cm–1 (the d-band-edge position of Eu2+ free ions) and V is 2 (the valence of Eu2+). Ea is the electron affinity of the anion, regarded as a constant here, n stands for the coordination number of Eu2+, and r represents the substituted ion radius in the host. That is, E (cm–1) is proportional to n and r. As n = 9 for the Sr and K sites, n = 6 for the Sc site, and r (K) > r (Sr) > r (Sc), the peak at 433 nm (435 nm) can be assigned to Eu2+ at the K site, the peak at 455 nm can be assigned to Eu2+ at the Sr site, and the peak at 481 nm can be assigned to Eu2+ at the Sc site.

Combined with the above structural analysis and DFT calculation results, it can be concluded that Eu2+ preferentially occupies Sr and K sites when the Eu2+ doping concentration is low. The optimal concentration is x = 4%, and the intensity of the emission spectrum starts to decrease when x is higher than 4%, resulting from the concentration quenching effect. As the Eu2+ concentration increases continuously, Eu2+ prefers to enter K and Sc sites, and it would lead to the continuous change of FWHM and the emission intensity, as shown in Figure 2c. The Sc site is a new site that can be occupied by Eu2+, and the luminescence intensity of Eu2+ at the Sc site increases first and then decreases with increasing Eu2+ doping concentration. Figure 3 shows the schematic diagram of the luminescence mechanism changing with the Eu2+ concentration, and some detailed description will also be given later.

Figure 3.

Figure 3

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Schematic diagram of the optical tuning mechanism of KSS:Eu.

To further evaluate the PL properties, the decay curves of KSS:xEu phosphors with different concentrations monitored at the corresponding optimal emission wavelength and excited at 340 nm were also measured (Figure 2f). The single exponential function (3) listed below is used to fit the decay curves43

graphic file with name mg1c00081_m003.jpg 3

In the function, A is a constant, τ stands for the decay time, and I corresponds to the luminescence intensity at time t. As can be seen from Table S5, the decay time increases gradually with increasing Eu2+ concentration. Normally, the lifetime would monotonously decrease with increasing Eu2+ concentration due to the increase of self-absorption rate and nonradiative energy transfer among Eu2+ ions. While the lifetime increases with the increase of Eu2+ concentration, it could be related to the change of the luminescence center because the decay behaviors would change with the local environment of Eu2+. Comparing the decay times of phosphors for a certain excitation wavelength, different emission wavelengths can be used to demonstrate the presence of multiple Eu2+ luminescence centers. The lifetime of the luminescence varies greatly due to the influence of additional nonradiative contributions on the decay process as the luminescence center changes. Hence, the decay curves of KSS:1%Eu were monitored at different emission wavelengths (Figure S2 and Table S6). The lifetime curves of different monitored emission wavelengths show different decay trends. When the monitoring wavelengths are in the range of 400–460 nm, they are double exponential decay, while when the monitoring wavelengths are in the range of 470–500 nm, they are single exponential decay. The difference is due to the lifetime of the single luminescence center corresponding to monoexponential decay, while in the area where the luminescence peaks corresponding to different luminescence centers coincide, the lifetime is affected by multiple luminescence centers and presents multiexponential decay. In Figure 4a, the lifetime values are fitted as a function of the monitoring wavelength. Some significant differences in decay times can be easily observed, suggesting that there are different luminescence centers in KSS:1%Eu, and it corresponds to the result that Eu2+ occupies both K and Sr sites at a low concentration. Furthermore, the decay curves of KSS:15%Eu2+ recorded at different wavelengths are depicted in Figures S3 and 3b, and the fitting result is listed in Table S7. The lifetimes also change greatly, verifying the two different luminescence centers (EuK and EuSc), as also given in Figure 3.

Figure 4.

Figure 4

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(a) Dependence of the KSS:1%Eu lifetime value on monitoring wavelength in the range of λem = 400–500 nm. (b) Dependence of the KSS:15%Eu lifetime value on monitoring wavelength in the range of λem = 410–500 nm. (c) PL spectra measured at different temperatures. (d) Dependence of temperature with an integrated emission intensity of KSS:4%Eu and KSS:15%Eu ranging from 300 to 420 K excited at 365 nm.

The temperature quenching performance of KSS:xEu powders were also investigated, which is one of the key parameters for evaluating a new type of WLED phosphor. Upon the excitation with the UV light of 365 nm, the temperature-dependent emission spectra of KSS:4%Eu and KSS:15%Eu at 300–420 K are shown in Figure 3e. Due to the increased probability of nonradiative transition, they both decrease slowly with increasing temperature. It can be seen from Figure 3f that at 420 K, the integrated intensity of KSS:4%Eu is 90% of that at RT, and the integrated intensity of KSS:15%Eu is 87% of that at RT, both of which are suitable for WLED device applications.

To further assess the application prospects of KSS:Eu in WLEDs, we fabricated the WLED lamp (LED 1) using the blue-emitting KSS:4%Eu, commercial green-emitting (Sr,Ba)2SiO4:Eu2+ phosphor, and commercial red-emitting Sr2Si5N8:Eu2+ phosphor on an n-UV LED chip (λ = 365 nm). LED 1 shows warm white light with a CIE chromaticity coordinate of (0.3644, 0.3762), CCT as 4458 K, and Ra as 88.2 under 20 mA drive current. As shown in Figure 5a, due to the cyan gap, the Ra value of the LED 1 device is limited, which is difficult to exceed 90. However, after adding the cyan-emitting KSS:15%Eu phosphor to the device, LED 2 with a similar chromaticity coordinate (0.3626, 0.3581) and CCT (4566 K) can be obtained (Figure 5b). Meanwhile, the Ra can be improved obviously to 94.6, suggesting the high value of full-spectrum lighting. All of those demonstrate the great application prospects of these multicolor phosphors in high-quality full-spectrum lighting.

Figure 5.

Figure 5

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PL spectra of WLEDs fabricated using commercial phosphors (Sr,Ba)2SiO4:Eu and Sr2Si5N8:Eu with (a) KSS:4%Eu or with (b) KSS:4%Eu and KSS:15%Eu on an n-UV LED chip (λ = 365 nm) under 20 mA drive current.

Conclusions

To sum up, we proposed a crystal-site engineering method to detect the Eu2+ luminescence in KSrScSi2O7 with diverse crystallographic sites. Photoluminescence tuning from blue to cyan was realized in KSS:Eu phosphors depending on Eu2+ doping concentrations. At low Eu2+ concentrations, Eu2+ prefers to occupy the K and Sr sites, whereas, with increasing Eu2+ concentration, Eu2+ can be chemically driven to the K and Sc sites to reach a more stable environment. The analysis of DFT calculation and decay curves further confirms the site-selective occupation mechanism. The fabricated WLED based on KSS:15%Eu, KSS:4%Eu, and commercial phosphors (Sr,Ba)2SiO4:Eu2+ and Sr2Si5N8:Eu2+ shows a high Ra (94.6) and a low CCT (4566 K), indicating that the cyan-emitting KSS:15%Eu phosphor can be applied to high-quality pc-WLEDs in full-spectrum lighting. The design principle on the Eu2+ doping concentration-induced site-selective occupation provides a new gateway to find multicolor phosphors suitable for full-spectrum lighting applications.

Acknowledgments

This work was supported by the International Cooperation Project of National Key Research and Development Program of China (2021YFE0105700), the National Natural Science Foundations of China (Grant Nos. 51972118 and 51961145101), the Guangzhou Science & Technology Project (202007020005), the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01X137), and the China Postdoctoral Science Foundation (2021M700298). This work was also funded by RFBR according to research project No. 19-52-80003.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsmaterialsau.1c00081.

  • Rietveld refinement results (Figure S1, Tables S1–S3); PLQY (Table S4); and lifetimes (Figures S2 and S3, Tables S5–S7) (PDF)

Author Contributions

∇ S.L. and M.Z. contributed equally.

The authors declare no competing financial interest.

Supplementary Material

mg1c00081_si_001.pdf (550.8KB, pdf)

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