Enhanced Yellow Persistent Luminescence in Sr3SiO5:Eu2+ through

Jun 14, 2019 - PDF (4 MB). Supporting Info ...... pdf. ic9b01020_si_001.pdf (792.91 kb) .... Download ...... Proceedings of the Royal Society of Londo...
3 downloads 0 Views 4MB Size
Download PDF
Article Cite This: Inorg. Chem. 2019, 58, 8694−8701

pubs.acs.org/IC

Enhanced Yellow Persistent Luminescence in Sr3SiO5:Eu2+ through Ge Incorporation Zhizhen Wang,† Zhen Song,† Lixin Ning,*,‡ Zhiguo Xia,† and Quanlin Liu*,† †

Downloaded via UNIV OF SOUTHERN INDIANA on July 21, 2019 at 10:45:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China ‡ Anhui Key Laboratory of Optoelectric Materials Science and Technology, Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Normal University, Wuhu 241000, PR China S Supporting Information *

ABSTRACT: The Sr3SiO5:Eu2+ phosphor has attracted considerable attention for applications in white LEDs owing to its highly efficient yellow emission under violet-blue excitation. We report herein an enhancement of yellow persistent luminescence in Sr3SiO5:Eu2+ through Ge incorporation. The strongest persistent luminescence intensity is observed for Sr3(Si1−xGex)O5:Eu2+ with x = 0.005 with a peak emission wavelength at ∼580 nm and a persistent time of ∼7000 s at the 0.32 mcd/m2 threshold value after UV radiation. A combination of thermoluminescence measurements and density functional theory (DFT) calculations reveals that the afterglow enhancement is due to a significant increase in the number of oxygen vacancies that act as electron trapping centers with appropriate trap depths. This investigation is anticipated to encourage more exploration of GeSi substitution to design and improve Si-containing persistent phosphors with superior functionalities.

■

applications in white light-emitting diodes (LEDs).15−18 In contrast, PerL properties of Sr3SiO5:Eu2+ have also been investigated through introduction of codopant Dy3+ which led to a prolonged afterglow as long as 6 h after UV irradiation.19,20 In this work, we demonstrate that Ge substitution for Si in Sr3SiO5:Eu2+ can also result in an enhancement of the afterglow with a duration up to 7000 s. A fundamental understanding of the afterglow mechanism is achieved from results of thermoluminescence measurements and DFT calculation. This study may open a new perspective for designing persistent oxide phosphors through cation substitution.

INTRODUCTION Persistent luminescence (PerL or afterglow) phosphors, which can emit light for minutes, hours, or even days after withdrawal of the excitation source, are widely applied in many areas such as road markers in the dark, emergency signs, and optical information storage.1,2 The past decades have witnessed a rapid growth in the development of afterglow materials since the discovery of SrAl2O4:Eu2+,Dy3+ in 1996.3 Many efficient oxide afterglow phosphors have been developed through codoping of Eu2+ with other lanthanide ions, such as Sr 4 Al 1 4 O 2 5 :Eu 2 + ,Dy 3 + , 4 , 5 CaAl 2 O 4 :Eu 2 + ,Nd 3 + , 6 , 7 and Sr2MgSi2O7:Eu2+,Dy3+,8 which emit mostly in the blue-green spectral range. Afterglow phosphors with longer-wavelength emissions are rather rare, such as CaS:Eu2+,Tm3+,9 SrS:Eu2+,10 Ca 2 SiS 4 :Eu 2+ ,Nd 3+ , 11 Ca 2 Si 5 N 8 :Eu 2+ , 12,13 and (Ca,Sr)AlSiN3:Eu2+.14 Most of them displayed relatively weak and short persistent luminescence compared to that of Eu2+activated blue or green phosphors. Besides, sulfides are chemically unstable, and nitrides require harsh synthetic conditions such as high temperature and high pressure. Thus, the development of efficient afterglow phosphors emitting in the yellow-red region is still a challenge. Eu2+-activated Sr3SiO5 phosphor displays efficient yellow emission under violet-blue excitation. The luminescence is due to 5d → 4f transition of Eu2+ and is sensitive to the local coordination environment in the host. Various strategies have been employed to optimize its luminescence properties for © 2019 American Chemical Society

■

EXPERIMENTAL SECTION

Materials and Synthesis Procedures. Sr3(Si1−xGex)O5:0.01% Eu2+ (abbreviated as Sr3(Si1−xGex)O5:Eu2+) (x = 0−1) phosphors and Sr3(Si1−xGex)O5 (x = 0.005):1%Ln3+ (Ln = Eu and Ce) were prepared by chemical coprecipitation combined with a solid-state reaction method, using stoichiometric amounts of Sr(NO3)2 (99.9%, Sinopharm), nano-SiO2 (15 nm, Aladdin), GeO2 (99.9%, Sinopharm), NH3·H2O (25−28 wt %, Aladdin), (NH4)2CO3 (99.9%, Sinopharm), Ce(NO3)3·6H2O (99.9%, Aladdin), and Eu(NO3)3· 6H2O (99.9%, Aladdin). (NH4)2CO3 was used as the precipitating agent, and NH3·H2O was used as the coating agent. Sr(NO3)2, Eu(NO3)3·6H2O, nano-SiO2, and GeO2 were put in 30 mL of Received: April 9, 2019 Published: June 14, 2019 8694

DOI: 10.1021/acs.inorgchem.9b01020 Inorg. Chem. 2019, 58, 8694−8701

Article

Inorganic Chemistry

Figure 1. (a) XRD patterns of Sr3(Si1−xGex)O5:Eu2+ (x = 0−1) samples. (b) Dependence of the lattice parameters and unit cells volume on x value. (c) Crystal structure of Sr3(Si1−xGex)O5:Eu2+ and 6-fold coordinate Sr1/Eu1, Sr2/Eu2 polyhedron.

■

deionized water (solution A), which was mixed with 10 mL of NH3· H2O (solution B) under a constant stirring at 60 °C. After they were thoroughly mixed together, 3 g of (NH4)2CO3 was added, and the solution color changed from light white to milk white rapidly. The mixtures were thoroughly washed with deionized water several times and dehydrated in an oven at 80 °C until they were completely dried. The dried mixtures were ground into fine powders in an agate mortar and calcined at 900 °C for 3 h. The obtained samples were naturally cooled down to room temperature, and ground into powders again with a mortar, then transferred into an alumina crucible and sintered at 1450 °C for 4 h under a reducing atmosphere of 5% H2/95% N2. The samples were cooled down to room temperature naturally and ground to fine powders for subsequent measurements. Characterization. The X-ray diffraction (XRD) data were collected on a Rigaku TTR III diffraction (Cu Kα radiation, λ = 1.5406 Å) in the 2θ range from 20 to 60°with a continuous scanning rate of 8°/min. The persistent time were measured by a phosphor photometer (PR-305, SENSINGM), with a simulated daylight source (1000 lx, 290−800 nm). The steady photoluminescence spectra were measured by an Edinburgh Instruments, FLS 920 fluorescence spectrophotometer, equipped with a 450 W xenon lamps as radiating source. Thermoluminescence (TL) curves were obtained by a TOSL3DS instrument (Ai-dirui-sheng Company, Guangzhou, China), equipped with an Hg lamp as UV radiating source in the temperature range of 303−623 K with a heating rate of 5 K/s. For the thermoluminescence excitation spectra (TLEs), prior to measurements, the samples were heated to 673 K in order to empty all the traps. The TLE analysis is carried out by obtaining a series of TL curves under various (monochromatic) excitation wavelengths from 254 to 500 nm with an interval of about 20 nm. The integrated TL values were displayed as a function of the excitation wavelengths, then corrected by the known spectral profile of the xenon lamp emission; thus, the so-called TLEs is obtained.1 The excitation spectra in the VUV−UV range were measured on the beamline 4B8 of the Beijing Synchrotron Radiation Facility (BSRF) under normal operating conditions by using the spectrum of sodium salicylate (oC6H4OHCOONa) as a reference.

COMPUTATIONAL METHODOLOGY

The Sr3SiO5 crystal was modeled by using a 2 × 2 × 1 supercell containing 48 Sr, 16 Si, and 80 O atoms, in which one of the Sr atoms at the Sr1 or Sr2 site was substituted by an Eu to investigate its site preference. To study the change of energetic and electronic properties of oxygen vacancies with Ge doping concentration, four Ge doping levels were considered for Sr3(Si1−xGex)O5 by substituting 0, 1, 2, and 4 Ge atoms for Si atoms in the supercell, corresponding to x = 0, 0.0625, 0.125, and 0.25, respectively. Then, one of the O atoms at the O1 or O2 site was removed to form a vacancy. The nearest distance between the defects is larger than 10 Å. For the supercells with x = 0.0625, 0.125, and 0.25, there are respectively 2, 7, and 49 crystallographically independent Ge configurations. On the basis of periodic DFT calculations with the PBE sol functional,21 the most stable configurations were selected for investigation of oxygen vacancies with the hybrid DFT in the PBE0 scheme,22 and if several configuration are comparably stable, then the average values of relevant quantities were taken. The fraction of HF exchange was increased to 29% in the PBE0 functional in order to match the calculated band gap (6.66 eV) with the experimentally estimated value (6.68 eV). Sr(4s24p65s2), Si(3s23p2), Ge(4s24p2), O(2s22p4), and Eu(5s25p64f76s2) were treated as valence electrons, and their interactions with the cores were described by the projected augmented wave (PAW) approach.23 The atomic structures of the supercells were fully optimized until the total energies and the forces on the atoms were converged to 10−6 eV and 0.01 eV Å−1. One kpoint Γ was used to sample the Brillouin zone, and the cut off energy of the plane-wave basis was set to 530 eV. The DFT calculations were performed using the VASP package.24,25 The formation energy (ΔEf) of a neutral VO was calculated from the total energies of supercells by using the standard formalism:26 ΔEf = E(doped) − E(undoped) + μO where E(undoped) and E(doped) denote the DFT total energies of the undoped and VO-incorporated supercells. μO is the chemical potential of the O atom, and was calculated by using the relationship 3μSr + (1 − x)μSi + xμGe + 5μO = μ bulk 8695

DOI: 10.1021/acs.inorgchem.9b01020 Inorg. Chem. 2019, 58, 8694−8701

Article

Inorganic Chemistry

Figure 2. (a) PL spectra of Sr3(Si1−xGex)O5:Eu2+ (x = 0−0.2) measured under 420 nm excitation at room temperature. (b) Peak fitted curves of Sr3(Si1−xGex)O5:Eu2+ (x = 0.005) using two Gaussian bands, with the integrated intensity of band I being around two times larger than that of band II. (c) VUV−UV and visible region PLE spectra monitoring 580 nm of the Sr3(Si1−xGex)O5:Eu2+ (x = 0−0.2) phosphors.

peaking at 17 345 and 16 437 cm−1, respectively, with the integrated intensity of band I about 2 times larger than that of band II. Considering the number of the Sr1 sites is 2 times larger than that of the Sr2 sites in Sr3(Si1−xGex)O5, we suppose that bands I and II originate from Eu2+ located at Sr1 and Sr2 sites, respectively, assuming both substitutions have equal probabilities. This is confirmed by DFT energetic calculations on EuSr1- and EuSr2-doped Sr3SiO5 supercells, showing that Eu2+ occupations at the two Sr sites are almost equally stable with a marginal total energy difference of 25 meV. In view of the very close ionic radii of Eu2+ (1.17 Å) and Sr2+ (1.18 Å) in 6-fold coordination, this result reflects that the size match between the two ions, rather than the local coordination structure, is an important factor determining the site distribution of Eu2+.30 Figure 2c shows the excitation spectra of Sr3(Si1−xGex)O5:Eu2+ (x = 0−0.2) in the VUV−visible region by monitoring the emission at 580 nm. The excitation band at ∼200 nm is attributed to the host excitonic absorption, from which the host band gap is estimated to be 6.68 eV after taking into account the electron−hole binding energy of the exciton.31 The excitation spectra at longer wavelengths are ascribed to 4f → 5d transitions of Eu2+, with similar spectral profiles for different Ge contents. Furthermore, we have also measured low-temperature excitation spectra by monitoring the emission at 550 and 660 nm, which are basically due to Eu2+ at the Sr1 and Sr2 sites, respectively. The results show that there is no obvious relative shift between the onsets of the two excitation spectra (Figure S1), reflecting the lowest 4f → 5d transition energy of Eu2+ at the two sites being similar. This is also consistent with the comparison of energy positions and fwhm’s between the two Gaussian bands in Figure 2b. That is, Eu2+ emission at the Sr2 site (band II) is at lower energy, displaying a larger Stokes shift and thus a wider fwhm. Persistent Luminescence. From the digital photographs of Sr3(Si1−xGex)O5:Eu2+ (x = 0−0.2), it is shown that all the

where μbulk is the total energy per formula unit of the bulk crystal. μSr, μSi, and μGe are the chemical potentials of the corresponding atoms and can be approximated by their energies in the bulk materials due to the fact that Sr3(Si1−xGex)O5:Eu2+ was synthesized in reducing atmosphere. Calculation for metallic Sr (1 atom cell), bulk Si, and Ge (8 atom cell) were performed with the same convergence criteria as above and a 12 × 12 × 12 k-point grid.

■

RESULTS AND DISCUSSION Crystal Structure and Photoluminescence. Figure 1a shows the X-ray diffraction (XRD) patterns of Sr3(Si1−xGex)O5:Eu2+ (x = 0−1). All the samples are pure phase and crystallized in the tetragonal space group P4/ncc (No.130). In addition, all the diffraction peaks shift toward smaller angles with x increasing from 0 to 1.0, indicating an expansion of the unit cell. Lebail refinements of the XRD patterns were performed using the FullProf program.27,28 The derived lattice constants and cell volumes are listed in Table S1 and are displayed in Figure 1b, showing a linear relationship with Ge content. This also implies the formation of continuous solid solution for Sr3(Si1−xGex)O5. The crystal structure of Sr3(Si1−xGex)O5:Eu2+ is shown in Figure 1c. It consists of two crystallographically different Sr sites, both coordinated by six oxygens with a corner-sharing O atom.29 The size of the Sr1 site is slightly smaller than that of the Sr2 site, with the average Sr−O bond length smaller by ∼0.04 Å. In Sr3SiO5:Eu2+, when the dopant Ge content exceeds x = 0.2, the afterglow performance is dramatically degraded, and this observation will be discussed later. Here we first discuss the samples with Ge content x ≤ 0.2. Figure 2a displays the emission spectra of Sr3(Si1−xGex)O5:Eu2+ (x = 0−0.2) measured under 420 nm excitation at room temperature. The emission spectral profiles almost overlap with each other, with a very slight blueshift of the peak from 584 to 575 nm with increasing x. In Figure 2b, the emission spectrum of the x = 0.005 sample is deconvoluted into two Gaussian bands 8696

DOI: 10.1021/acs.inorgchem.9b01020 Inorg. Chem. 2019, 58, 8694−8701

Article

Inorganic Chemistry

Figure 3. (a) Digital photographs of Sr3(Si1−xGex)O5:Eu2+ (x = 0−0.2) under excitation of 365 nm NUV lamp. (b) Digital photographs of Sr3(Si1−xGex)O5:Eu2+ (x = 0−0.2) after 365 nm irradiation for 15 s. (c) PerL decay curves of Sr3(Si1−xGex)O5:Eu2+ (x = 0−0.2) after 100 s excitation with simulated daylight (290−800 nm) lamp at room temperature. (d) TL glow curves of Sr3(Si1−xGex)O5:Eu2+ (x = 0−0.2) after UV charging from a Hg lamp. The TL peak temperatures are about 379 K (marked as 1) and 481 K (marked as 2).

Figure 3c. Similar experimental phenomena also appeared in Y3Al2Ga3O12:Ce3+,Yb3+ phosphor by codoping B3+.32 The trap depth shows a gradual decrease with increasing x value for both TL peak, and the estimated trap depth values are listed in Table 1 by using the crude relationship ET = T/500 eV,33−35

samples present an orange body color under 365 nm irradiation, and the brightness of the color reaches its maximum at x = 0.01 and then decreases with further increase of Ge content. Figure 3b shows the persistent luminescence of the samples after 365 nm irradiation for 15 s. The sample with x = 0 shows almost no afterglow. However, the brightness of the afterglow goes up with x value increasing, reaching its maximum at x = 0.005. Further increasing Ge concentration weakens the persistent luminescence, which becomes almost invisible at x = 0.2. Figure 3c shows the afterglow brightness as a function of decay duration (log−log plot) for samples with different Ge concentrations after 100 s excitation with a simulated daylight (290−800 nm) lamp at room temperature. The red dashed line fixed at 0.32 mcd/m2 represents the threshold value used in the industrial standard, defined as about 100 times the sensitivity of human eyes.7 Clearly, the brightness and time duration of the afterglow increase first with increasing x value, reaching its maximum at x = 0.005, which is consistent with the observation shown in Figure 3b. The PerL duration is obviously extended from about 180 s for Sr3SiO5:Eu2+ to 7000 s for Sr3(Si1−xGex)O5:Eu2+ (x = 0.005). To investigate the electron-trapping centers responsible for the persistent luminescence observed in Sr3(Si1−xGex)O5:Eu2+, TL measurements were performed, and the results are displayed in Figure 3d. Two main TL peaks are present for all samples, and the intensity of peak 1 is much stronger than that of peak 2. The change of Ge concentration affects not only the TL peak intensity (and thus the trap concentration) but also the peak position (and thus the trap depth) which is especially pronounced for peak 1 at lower temperature. The maximum TL intensity of peak 1 is attained at x = 0.005 consistent with the results of the afterglow intensity depicted in

Table 1. Trap Depths in Sr3(Si1−xGex)O5:Eu2+ (x = 0−0.2) Derived from Thermoluminescence Measurements peak 1

peak 2

Ge

T (K)

trap depth (eV)

T (K)

trap depth (eV)

0 0.001 0.005 0.01 0.02 0.05 0.1 0.2

379 361 366 356 357 348 342 338

0.76 0.72 0.73 0.71 0.71 0.70 0.68 0.67

481 488 487 455 464 450 440 400

0.98 0.98 0.97 0.91 0.93 0.90 0.88 0.80

where the temperature T is in units of kelvin (K). The trap depths derived from the positions of TL peak 1 are in the range of 0.67−0.76 eV, which is relevant to the afterglow observed at room temperature.36 The dominant crystal defects associated with the traps are supposed to be oxygen vacancies (VO) which were formed due to the reducing condition during the material synthesis. To clarify this point, the defect formation energies of VO and the associated thermodynamic charge transition levels at different Ge doping contents were calculated using hybrid DFT. A 2 × 2 × 1 supercell containing 16 Si, 64 O1, and 16 O2 atoms was employed to model the Sr3SiO5 crystal, where the O1 atoms 8697

DOI: 10.1021/acs.inorgchem.9b01020 Inorg. Chem. 2019, 58, 8694−8701

Article

Inorganic Chemistry are connected with Si to form SiO4 tetrahedral and the O2 atoms are isolated. Four Ge doping levels were considered for Sr3(Si1−xGex)O5 by substituting 0, 1, 2, and 4 Ge atoms for Si, corresponding to x = 0, 0.0625, 0.125, and 0.25, respectively. Table 2 lists the defect formation energies of a VO in the above four supercells. First, in the Sr3SiO5 supercell (x = 0), Table 2. Calculated Formation Energies (ΔEf in eV) for a VO in Sr3(Si1−xGex)O5 (x = 0, 0.0625, 0.125, and 0.25)a x=0 x = 0.0625 x = 0.125 x = 0.25

VO1−Si

VO2

VO1−Ge

0.870 0.747 1.261 1.197

1.199 1.058 1.586 1.493

−1.360 −1.014 −0.862

a The symbols VO1−Si, VO1−Ge, and VO2 represent the VO1 connected with Si and Ge, and the isolated VO2, respectively.

Figure 4. Calculated thermodynamic charge transition levels for the most stable oxygen vacancies in Sr3(Si1−xGex)O5 (x = 0, 0.0625, 0.125, and 0.25). The symbols VO1−Si and VO1−Ge represent the VO1 connected with Si and Ge, respectively. The calculated band gaps for the materials are also indicated.

the formation energy of a VO1 is lower than that of a VO2 by 329 meV, showing that the removal of an oxygen connected with Si is easier than an isolated oxygen. Next, in the Sr3(Si1−xGex)O5:Eu2+ (x = 0.0625) supercell, the formation energy of a VO1 connected with Si is still lower than that of a VO2 by 311 meV, similar to the above result for pure Sr3SiO5. However, the formation energy of a VO1 connected with Ge is much lower (by 2107 meV) than that of a VO1 connected with Si. This is a striking result which indicates that the substitution of Ge for Si dramatically reduces the cost in energy paid to remove a coordinating oxygen. This can lead to a significant increase of VO1 concentration, which successfully explains the experimental observation that the TL intensity increases considerably upon Ge/Si substitution. Then, in the Sr3(Si1−xGex)O5:Eu2+ (x = 0.125) supercell, the formation energy of a VO1 connected with Ge is still lower than those of a VO1 connected with Si and a VO2 but is higher by 346 meV than that of a VO1 connected with Ge in the Sr3(Si1−xGex)O5:Eu2+ (x = 0.0625) supercell. This indicates that with increasing Ge concentration the formation of a VO1 connected with Ge becomes more difficult. This trend is further illustrated by the results for Sr3(Si1−xGex)O5:Eu2+ (x = 0.25) supercell, which shows that the formation energy of a VO1 connected with Ge is lower by 152 meV than that in the x = 0.125 supercell. This may explain the experimental observation that the afterglow intensity is decreased with increasing Ge doping concentration after reaching its maximum. The calculated thermodynamic charge transition levels for the most stable oxygen vacancies are depicted in Figure 4. The charge transition level indicates the Fermi level at which the formation energies of a defect are equal in two charge states. It shows that a VO can behave as a trapping center of electrons from the conduction band. The ε(0/−) levels that are associated with trapping an electron by a neutral VO and changing its charge state to −1 are predicted to be in the range of 0.67−0.96 eV below the bottom of the conduction band in the four supercells. These values are close to the trap depths estimated for TL peak 1 as listed in Table 1. Therefore, the trapping levels responsible for afterglow are most likely due to the ε(0/−) levels of oxygen vacancies connected with Ge in Sr3(Si1−xGex)O5. Construct HRBE and VRBE Diagram. Persistent luminescence is closely related to trap levels. Electron charging and detrapping processes are determined by trap depth, 4f/5d levels of Eu2+, and positions of valence/conduction bands.

Herein, we construct the host-referred binding energy (HRBE) and vacuum-referred binding energy (VRBE) schemes to demonstrate the relative positions of various energy levels according to Dorenbos et al.37−40 To construct the HRBE diagram of Sr3(Si1−xGex)O5 (x = 0.005), spectroscopic data, including the PL and PLE spectra of Eu2+-, Eu3+-, and Ce3+doped Sr3(Si1−xGex)O5 (x = 0.005), are required and are shown in Figure S2. The band gap energy E vc for Sr3(Si1−xGex)O5 (x = 0.005) was estimated to be 6.64 eV, which agrees well with the broad peak located at 207 nm (5.99 eV) reported in Sr3SiO5:Ce3+ by Dorenbos et al.41 In the PLE of Sr3(Si1−xGex)O5:Eu3+ (x = 0.005) (Figure S2b), there is one broad band due to the charge transfer (CT) process from O2− to Eu3+ in the range of 250−400 nm, with a peak wavelength 336 nm, corresponding to 3.69 eV. The multiple f−f transitions of Eu3+ in the range of 400−500 nm were also observed. In the low-temperature PLE spectra of Sr3(Si1−xGex)O5:Ce3+ (x = 0.005) (Figure S2d), a total of 5 excitation bands were identified. With the consideration of two different crystallographic sites for Ce3+ substitution, we conclude that the bands from two different sites cannot be distinguished under 78 K due to the similar crystal coordination environment. Hence, the centroid shift (εc) for Ce3+ 5d energy level was estimated to be 1.61 eV, and U (6, Sr3(Si1−xGex)O5) was estimated to be 6.81 eV. For the lowest 5d level, the value of Efd iscalculated as 2.58 eV (Figure S2c).42 HRBE and VRBE for Sr3(Si1−xGex)O5 (x = 0.005) host are shown inFigure S3, with all the parameters listed in Table S2. Thermoluminescence Excitation (TLE) Spectra and Electron Charging and Discharging Processes. To examine the significant effect of Ge4+ on the PerL property and further explore the PerL mechanism, thermoluminescence excitation (TLE) spectra and 3D-TL curves are recorded.Figure 5a presents TL glow curves of Sr3(Si1−xGex)O5:Eu2+ (x = 0.005) phosphors excited at 254, 280, 320, 360, and 420 nm. All glow curves consist of two overlapping peaks located at around 360 and 490 K. Different excitation wavelengths only change the relative intensities of the peaks. This suggests that the traps belong to the same type, independent of the excitation wavelength. On the basis of these TL spectra, the TLE spectra of Sr3(Si1−xGex)O5:Eu2+ (x = 0.005) phosphors 8698

DOI: 10.1021/acs.inorgchem.9b01020 Inorg. Chem. 2019, 58, 8694−8701

Article

Inorganic Chemistry

Figure 5. (a) TL glow curves excited by different wavelengths of Sr3(Si1−xGex)O5:Eu2+ (x = 0.005) phosphor, with heating rate of 5 K/s. (b) Photoluminescence excitation (PLE) and thermoluminescence excitation (TLE) spectra of Sr3(Si1−xGex)O5:Eu2+ (x = 0.005) phosphor. The PLE spectrum is obtained by monitoring at 580 nm at room temperature. (c) 3D-TL curve for Sr3(Si1−xGex)O5:Eu2+ (x = 0.005) phosphor. (d) Photoluminescence (PL) spectrum excited at 420 nm and PerL spectrum of Sr3(Si1−xGex)O5:Eu2+ (x = 0.005).

are obtained as the red dotted line in Figure 5b. By comparing TLE with PLE spectra (blue curve), we notice that for excitation wavelength longer than 400 nm the intensity of TLE spectrum is close to zero, whereas the intensity of the PLE spectrum is very high. Thus, in this region the traps are unable to be efficiently charged by electrons excited to lower 5d levels. On the contrary, electrons excited directly from the 4f7 ground state of Eu2+ to the higher 5d levels closed or submerged in the conduction band mostly contribute to the persistent luminescence. The 3D-TL curve for the Sr3(Si1−xGex)O5:Eu2+ (x = 0.005) sample was then measured to gain more information on the traps, as shown in Figure 5c. From the iso-intensity contours at room temperature, the PerL spectrum could be obtained. The persistent luminescence has the same spectral shape as that of PL spectrum (Figure 5d), which confirms that electrons released from traps transfer to Eu2+ through conduction band and that the emission band still originate from Eu2+. Persistent Luminescence Mechanism. After the discovery of SrAl2O4: Eu2+,Dy3+, several conceptual models were proposed to explain the mechanism of persistent luminescence.14,43−46 Herein, based on experimental and calculated results, we construct a model based on HRBE diagram to better understand the persistence luminescence mechanism of Sr3(Si1−xGex)O5:Eu2+ phosphor. All the needed parameters are shown in Table 3. Figure 6 shows the schematic mechanism of persistent luminescence for Sr3(Si1−xGex)O5:Eu2+ (x = 0.005) phosphor. The substitution of Ge for Si can lead to a significant increase of shallow electron traps (marked as 1 in the figure). In the charging process (arrows ① in the figure), electrons are only excited from the 4f ground state to the higher 5d levels that are very close to or emerged in the conduction band by the ultraviolet irradiation with energy higher than 3.10 eV,

Table 3. Parameters Used to Construct Host Referred Binding Energy Diagram to Understand the Persistence Luminescence Mechanism of Sr3(Si1−xGex)O5:Eu2+ parameters

value (eV)

Eex ECT E4f(7,2+) E5d(7,2+) trap depth 1 trap depth 2 PerL emission energy threshold of TLES

6.05 3.69 3.69 6.27 0.73 0.98 2.14 3.10

Figure 6. Schematic mechanism of persistent luminescence.

corresponding to wavelength about 400 nm. This is based on the thermoluminescence excitation spectra as shown in Figure 5b. The mobile electrons are then captured by various depth traps, as shown by the bluish violet arrow (marked as (2) in Figure 6). Traps will release electrons into the conduction band under room or higher temperature due to 8699

DOI: 10.1021/acs.inorgchem.9b01020 Inorg. Chem. 2019, 58, 8694−8701

Article

Inorganic Chemistry the thermal stimulations (arrows marked as (3)), which finally return to excited level of Eu2+ and produce the orange persistent luminescence. The release rates depend on the depth of trap: Shallow traps need a lower temperature, and deep traps need a higher temperature.

M2Si5N8:Eu (M = Ca, Sr, Ba) persistent phosphors. J. Lumin. 2012, 132, 682−689. (2) Saito, M.; Adachi, N.; Kondo, H. Full-color illumination that needs no electric power. Opt. Express 2007, 15, 1621−1626. (3) Matsuzawa, T.; Aoki, Y.; Takeuchi, N.; Murayama, Y. A New Long Phosphorescent Phosphor with High Brightness, SrAl2O4:Eu2+, Dy3+. J. Electrochem. Soc. 1996, 143, 2670−2673. (4) Lin, Y.; Tang, Z.; Zhang, Z. Preparation of long-afterglow Sr4Al14O25-based luminescent material and its optical properties. Mater. Lett. 2001, 51, 14−18. (5) Lin, Y.; Tang, Z.; Zhang, Z.; Nan, C. W. Anomalous luminescence in Sr4Al14O25:Eu, Dy phosphors. Appl. Phys. Lett. 2002, 81, 996−998. (6) Aitasalo, T.; Hölsä, J.; Jungner, H.; Lastusaari, M.; Niittykoski, J. Thermoluminescence Study of Persistent Luminescence Materials: Eu2+- and R3+-Doped Calcium Aluminates, CaAl2O4:Eu2+, R3+. J. Phys. Chem. B 2006, 110, 4589−4598. (7) Jia, W.; Yuan, H.; Lu, L.; Liu, H.; Yen, W. M. Crystal growth and characterization of Eu2+, Dy3+:SrAl2O4 and Eu2+, Nd3+:CaAl2O4 by the LHPG method. J. Cryst. Growth 1999, 200, 179−184. (8) Lin, Y.; Tang, Z.; Zhang, Z.; Wang, X.; Zhang, J. Preparation of a new long afterglow blue-emitting Sr2MgSi2O7-based photoluminescent phosphor. J. Mater. Sci. Lett. 2001, 20, 1505−1506. (9) Jia, D. Enhancement of Long-Persistence by Ce Co-Doping in CaS:Eu2+, Tm3+ Red Phosphor. J. Electrochem. Soc. 2006, 153, H198− H201. (10) Kojima, Y.; Takahashi, A.; Umegaki, T. Synthesis of orange-redemitting Eu2+, Pr3+ codoped SrS long afterglow phosphor. J. Lumin. 2014, 146, 42−45. (11) Smet, P. F.; Avci, N.; Poelman, D. Red Persistent Luminescence in Ca2SiS4: Eu, Nd. J. Electrochem. Soc. 2009, 156, H243−H248. (12) Van den Eeckhout, K.; Smet, P. F.; Poelman, D. Luminescent Afterglow Behavior in the M2Si5N8: Eu Family (M = Ca, Sr, Ba). Materials 2011, 4, 980. (13) Wang, J.; Zhang, H.; Liu, Y.; Dong, H.; Lei, B.; Zheng, M.; Xiao, Y.; Peng, M.; Wang, J. Insights into luminescence quenching and detecting trap distribution in Ba2Si5N8:Eu2+ phosphor with comprehensive considerations of temperature-dependent luminescence behaviors. J. Mater. Chem. C 2015, 3, 9572−9579. (14) Liu, X.; Song, Z.; Wang, S.; Liu, Q. The red persistent luminescence of (Sr,Ca)AlSiN3:Eu2+ and mechanism different to SrAl2O4:Eu2+,Dy3+. J. Lumin. 2019, 208, 313−321. (15) Jang, H. S.; Jeon, D. Y. Yellow-emitting Sr3SiO5:Ce3+,Li+ phosphor for white-light-emitting diodes and yellow-light-emitting diodes. Appl. Phys. Lett. 2007, 90, 041906. (16) Park, J. K.; Kim, C. H.; Park, S. H.; Park, H. D.; Choi, S. Y. Application of strontium silicate yellow phosphor for white lightemitting diodes. Appl. Phys. Lett. 2004, 84, 1647−1649. (17) Park, J. K.; Choi, K. J.; Yeon, J. H.; Lee, S. J.; Kim, C. H. Embodiment of the warm white-light-emitting diodes by using a Ba2+ codoped Sr3SiO5:Eu phosphor. Appl. Phys. Lett. 2006, 88, 043511. (18) Ci, Z.; Que, M.; Shi, Y.; Zhu, G.; Wang, Y. Enhanced Photoluminescence and Thermal Properties of Size Mismatch in Sr2.97‑x‑yEu0.03MgxBaySiO5 for High-Power White Light-Emitting Diodes. Inorg. Chem. 2014, 53, 2195−2199. (19) Sun, X.; Zhang, J.; Zhang, X.; Luo, Y.; Wang, X. Long lasting yellow phosphorescence and photostimulated luminescence in Sr3SiO5:Eu2+ and Sr3SiO5:Eu2+, Dy3+ phosphors. J. Phys. D: Appl. Phys. 2008, 41, 195414. (20) Sun, X.; Zhang, J.; Zhang, X.; Luo, Y.; Hao, Z.; Wang, X.-J. Effect of retrapping on photostimulated luminescence in Sr3SiO5:Eu2+, Dy3+ phosphor. J. Appl. Phys. 2009, 105, 013501. (21) Perdew, J. P.; Ruzsinszky, A.; Csonka, G. I.; Vydrov, O. A.; Scuseria, G. E.; Constantin, L. A.; Zhou, X.; Burke, K. Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces. Phys. Rev. Lett. 2008, 100, 136406.

■

CONCLUSIONS In summary, a series of Sr3(Si1−xGex)O5:Eu2+ phosphors have been synthesized by chemical coprecipitation combined with the solid-state reaction method successfully. The persistent luminescence performance is obviously enhanced by Ge doping, and the sample with x = 0.005 Ge4+ substitution has the best performance. TL measurements and DFT calculations reveal that the enhanced persistent luminescence is due to Ge4+ addition leading to more oxygen vacancies that act as electron-trapping centers with suitable trap depths. Comparing TLE and PLE spectra, we conclude that electrons are directly excited from the 4f7 ground state to higher 5d levels submerged in the conduction band, rather than to the lowest 5d level. In addition, the same shape of persistent emission spectrum and photoluminescence spectrum of Sr3(Si1−xGex)O5:Eu2+ confirms that persistent luminescence still originates from Eu2+ ions. Furthermore, based on experimental data and calculated results, HRBE model was constructed to explain the persistent luminescence mechanism in Sr3(Si1−xGex)O5:Eu2+ phosphor. This work will help us to open a novel perspective to improve persistent luminescence performance and to design new persistent phosphors.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01020. Low-temperature excitation spectra monitoring the emission at 550 and 660 nm of Sr3(Si1−xGex)O5:Eu2+ (x = 0.005) phosphor; room-temperature PLE and PL spectra; HRBE and VRBE for Sr3(Si1−xGex)O5 (x = 0.005); main parameters of refinement; parameters used to construct energy level scheme (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.L.). *E-mail: [email protected] (L.N.). ORCID

Zhen Song: 0000-0002-7251-5703 Lixin Ning: 0000-0003-2311-568X Zhiguo Xia: 0000-0002-9670-3223 Quanlin Liu: 0000-0003-3533-7140 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Nos. 51672027, 51832005, and 11574003).

■

REFERENCES

(1) Smet, P. F.; Van den Eeckhout, K.; Bos, A. J. J.; van der Kolk, E.; Dorenbos, P. Temperature and wavelength dependent trap filling in 8700

DOI: 10.1021/acs.inorgchem.9b01020 Inorg. Chem. 2019, 58, 8694−8701

Article

Inorganic Chemistry

(44) Dorenbos, P. Mechanism of Persistent Luminescence in Eu2+ and Dy3+ Co doped Aluminate and Silicate Compounds. J. Electrochem. Soc. 2005, 152, H107−H110. (45) Clabau, F.; Rocquefelte, X.; Le Mercier, T.; Deniard, P.; Jobic, S.; Whangbo, M. H. Formulation of Phosphorescence Mechanisms in Inorganic Solids Based on a New Model of Defect Conglomeration. Chem. Mater. 2006, 18, 3212−3220. (46) Van den Eeckhout, K.; Smet, P. F.; Poelman, D. Persistent Luminescence in Eu2+-Doped Compounds: A Review. Materials 2010, 3, 2536.

(22) Perdew, J. P.; Ernzerhof, M.; Burke, K. Rationale for mixing exact exchange with density functional approximations. J. Chem. Phys. 1996, 105, 9982−9985. (23) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (24) Kresse, G.; Furthmüller, J. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (25) Kresse, G.; Joubert, D. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (26) Freysoldt, C.; Grabowski, B.; Hickel, T.; Neugebauer, J.; Kresse, G.; Janotti, A.; Van de Walle, C. G. First-principles calculations for point defects in solids. Rev. Mod. Phys. 2014, 86, 253−305. (27) Rodriguez-Carvajal, J. FULLPROF: a program for Rietveld refinement and pattern matching analysis. In Satellite Meeting on Powder Diffraction of the XV Congress of the IUCr; Toulouse, France, 1990. (28) Roisnel, T.; Rodríquez-Carvajal, J. WinPLOTR: a windows tool for powder diffraction pattern analysis. Mater. Sci. Forum 2001, 378381, 118−123. (29) Glasser, L. S. D.; Glasser, F. P. Silicates M3SiO5. I. Sr3SiO5. Acta Crystallogr. 1965, 18, 453−454. (30) Ning, L.; Huang, X.; Huang, Y.; Tanner, P. A. Origin of the green persistent luminescence of Eu-doped SrAl2O4 from a multiconfigurational ab initio study of 4f7 4f65d1 transitions. J. Mater. Chem. C 2018, 6, 6637−6640. (31) Dorenbos, P. The Eu3+ charge transfer energy and the relation with the band gap of compounds. J. Lumin. 2005, 111, 89−104. (32) Zhou, D.; Wang, Z.; Song, Z.; Wang, F.; Zhang, S.; Liu, Q. Enhanced Persistence Properties through Modifying the Trap Depth and Density in Y3Al2Ga3O12:Ce3+, Yb3+ Phosphor by Co-doping B3+. Inorg. Chem. 2019, 58, 1684−1689. (33) Randall, J. T.; Wilkins, M. H. F. Phosphorescence and electron traps - I. The study of trap distributions. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences 1945, 184, 365. (34) Bos, A. J. J. Theory of thermoluminescence. Radiat. Meas. 2006, 41, S45−S56. (35) Randall, J. T.; Wilkins, M. H. F. Phosphorescence and electron traps II. The interpretation of long-period phosphorescence. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences 1945, 184, 390. (36) Clabau, F.; Rocquefelte, X.; Jobic, S.; Deniard, P.; Whangbo, M.-H.; Garcia, A.; Le Mercier, T. On the phosphorescence mechanism in SrAl2O4:Eu2+ and its codoped derivatives. Solid State Sci. 2007, 9, 608−612. (37) Dorenbos, P. Energy of the first 4f7→4f65d transition of Eu2+ in inorganic compounds. J. Lumin. 2003, 104, 239−260. (38) Dorenbos, P. Systematic behaviour in trivalent lanthanide charge transfer energies. J. Phys.: Condens. Matter 2003, 15, 8417. (39) Dorenbos, P. Ce3+ 5d-centroid shift and vacuum referred 4felectron binding energies of all lanthanide impurities in 150 different compounds. J. Lumin. 2013, 135, 93−104. (40) Dorenbos, P. Lanthanide 4f-electron binding energies and the nephelauxetic effect in wide band gap compounds. J. Lumin. 2013, 136, 122−129. (41) Luo, H.; Bos, A. J. J.; Dobrowolska, A.; Dorenbos, P. Lowtemperature VUV photoluminescence and thermoluminescence of UV excited afterglow phosphor Sr3AlxSi1‑xO5:Ce3+, Ln3+ (Ln = Er, Nd, Sm, Dy and Tm). Phys. Chem. Chem. Phys. 2015, 17, 15419−15427. (42) Cui, D.; Song, Z.; Xia, Z.; Liu, Q. Luminescence Tuning, Thermal Quenching, and Electronic Structure of Narrow-Band RedEmitting Nitride Phosphors. Inorg. Chem. 2017, 56, 11837−11844. (43) Bos, A. J. J.; van Duijvenvoorde, R. M.; van der Kolk, E.; Drozdowski, W.; Dorenbos, P. Thermoluminescence excitation spectroscopy: A versatile technique to study persistent luminescence phosphors. J. Lumin. 2011, 131, 1465−1471. 8701

DOI: 10.1021/acs.inorgchem.9b01020 Inorg. Chem. 2019, 58, 8694−8701