Near-infrared emitting microspheres of LaAlO3:Mn4+: Defects engineering via Ge4+ doping for greatly enhanced luminescence and improved afterglow

Journal Pre-proof 4+ 4+ Near-infrared emitting microspheres of LaAlO3:Mn : Defects engineering via Ge doping for greatly enhanced luminescence and improved afterglow Siyuan Li, Qi Zhu, Xiaodong Li, Xudong Sun, Ji-Guang Li PII:

S0925-8388(20)30728-3

DOI:

https://doi.org/10.1016/j.jallcom.2020.154365

Reference:

JALCOM 154365

To appear in:

Journal of Alloys and Compounds

Received Date: 26 November 2019 Revised Date:

12 February 2020

Accepted Date: 13 February 2020

Please cite this article as: S. Li, Q. Zhu, X. Li, X. Sun, J.-G. Li, Near-infrared emitting microspheres of 4+ 4+ LaAlO3:Mn : Defects engineering via Ge doping for greatly enhanced luminescence and improved afterglow, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154365. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Credit Author Statement Q.Z. and J.-G.L. conceived the project; S.Y.L. carried out the experiments and data analysis; Q.Z. and S.Y.L. drafted the manuscript. All the authors were involved in the results discussion, and have read and approved the final manuscript.

Near-infrared emitting microspheres of LaAlO3:Mn4+: defects engineering via Ge4+ doping for greatly enhanced luminescence and improved afterglow

Siyuan Li a,b, Qi Zhu a,b,*, Xiaodong Li a,b, Xudong Sun a,b, Ji-Guang Li c,* a

Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education),

Northeastern University, Shenyang, Liaoning 110819, PR China b

Institute of Ceramics and Powder Metallurgy, School of Materials Science and

Engineering, Northeastern University, Shenyang, Liaoning 110819, PR China c

Research Center for Functional Materials, National Institute for Materials Science,

Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan

*Corresponding author: Dr. Qi Zhu Northeastern University Tel: +86-24-8368-1680 E-mail: [email protected]

Dr. Ji-Guang Li National Institute for Materials Science Tel: +81-29-860-4394 E-mail: [email protected] 1

Abstract Perovskite-type microspheres of LaAlO3:xMn4+,yGe4+ have been fabricated through thermal decomposition of the amorphous precursors autoclaved from the nitrates and ethylene glycol in the prescence of citric acid. LaAlO3:xMn4+ spheres emit broad and bright near infrared (NIR) emission ranging from 680 nm to 750 nm with the maximum at around 730 nm. The quenching concentration for Mn4+ in LaAlO3 is found at x = 0.1%. In the absence of light source, the spheres emit intense NIR afterglow for ten minutes at room temperature, mainly due to the formation of [MnAl] defects. Doping Ge4+ does not significantly change the morphology, crystal structure, band gap, and thermal stability of LaAlO3:Mn4+ sample, but it induces stronger asymmetry of MnO6 and more isolated Mn4+ ions, which thus contributes to greatly enhanced NIR emission intensity by 260 %. Ge4+ ions occupying Al site leads to the formation of positive [GeAl] defects, which are closely related to the trap concentration rather than the trap depth. Incorporation of Ge4+ gives rise to the improved persistent luminescence, and the most intense afterglow is found at y = 0.6%. The spheres exhibit repeatable afterglow by a proper heat processing in the absence of light source, indicating their potential application in optical data storage.

Keywords:

LaAlO3;

microsphere;

near-infrared

luminescence; defects engineering; Mn4+

2

(NIR)

emission;

persistent

1. Introduction Mn4+ ions (3d3) doped a crystalline host has attracted a lot of attention, because it shows intense deep red or near-infrared (NIR) emission depending on the crystal field strength of the host lattices. Up to date, the hosts for Mn4+ ions doping are mainly divided into two categories, including fluoride and oxide, which have been widely studied and exhibited excellent performance [1-3]. Previous reports suggested Mn4+ ions could be stable in a six-coordinated octahedron, such as [MnO6] and [MnF6] [3-6]. Mn4+ ions doped fluorides, such as K2TiF6 [7], Cs2SiF6 [8], and KTeF5 [9] are always used as a red emission component in white light emitting dioxides (w-LEDs), because of their low correlated color temperature, high color rendering index and high quantum efficiency. However, fluorides are unstable in moist environments, and the toxic HF used in the fluoride phosphors synthesis process is harmful to the human body and the environment. Thus, more and more attention has been paid to the study of Mn4+ ions doped oxide, because Mn4+ ions in oxide hosts exhibit broad and strong absorption spectrum (300-500 nm) and sharp emission spectrum in the deep red or NIR region (600-760 nm) [4-6,10]. For example, Mg3Ga2GeO8:Mn4+ has been reported as a deep red phosphor with high color purity, outstanding quantum efficiency (64.7%) and excellent thermal stability (a small emission loss of 28% at 423 K) [10]. Moreover, to acquire stronger emission intensity, further incorporation of Mg2+ and Ge4+ ions in the oxide phosphors were employed for manipulating the radiative transition probability of Mn4+ [11,12]. Co-doping of Mg2+ and Mn4+ in BaMgAl10O17, contributes to more isolated Mn4+ ions and Mn4+-Mg2+ pairs, which 3

increases the radiative transition probability of Mn4+ in the excited state [11]. While doping Ge4+ ions in Y3Al5O12:Mn4+ efficiently interrupts the probability of energy transfer among Mn4+ ions via Mn4+-Ge4+-O2- pairs replacing Mn4+-Mn4+-O2- pairs [12]. Recently, perovskite-type oxides, including LaAlO3 [4], La2MgGeO6 [5], GdAlO3 [6], AE2LaTaO6 (AE= Ca, Sr, Ba) [13] and so on, have been used as red phosphor hosts for Mn4+ doping, because of their advantages on chemical stability, luminescence efficiency, and green preparation procedure. Interestingly, LaAlO3 [4], La2MgGeO6 [5] and GdAlO3 [6] have been reported as novel hosts for fabricating NIR persistent phosphors by doping Mn4+, which emit NIR afterglow at ~720 nm. LaAlO3 has attracted great interest due to its extensively application in high frequency capacitors, huge magnetoresistance, magnetic fluid generators, ferroelectric thin films and superconducting substrate [14,15]. LaAlO3 belongs to a perovskite family with the general formula of ABO3. La3+ occupies the A site coordinated with twelve oxygen atoms, while Al3+ occupies the B site coordinated with six oxygen atoms to form a octahedron. Depending on the similar radius of the ions and the stability of perovskite structure, the activators can replace La3+ or Al3+ without altering the crystal structure significantly. Rare earth ions, including Eu2+ or Eu3+ [16], Er3+ [17], Sm3+ or Tb3+ [18], can replace La3+ easily to form rare-earth phosphors, so LaAlO3 is the suitable host for these activators. Because the rare earth sources are expensive, some non-rare-earth transition metal ions, such as Cr3+ and Mn4+, have been investigated for LaAlO3 based phosphors, because they can replace Al3+ to form new NIR illuminators [4,19,20]. Luo et al. [19] fabricated a novel Cr3+ doped LaAlO3 4

NIR emitting phosphor with the emission maximum at ~740 nm. In this system, Cr3+ can act not only as an electron-trapping center, but also as a deep hole-trapping center. Among all the methods for preparing LaAlO3-based phosphors, solid-state reaction method is the most widely used. Nevertheless, the calcination temperature is too high (>1400 oC), which results in the particles exhibiting agglomeration and broad size distribution [4,19,20]. However, for high-definition displays and practical application, phosphor particles of spherical shape and narrow size distribution are necessary for improving the overall luminescent performance. Therefore, spherical engineering of LaAlO3-based phosphors is of great importance, but remains a considerable challenge in particle science. Chen et al. [18] synthesized LaAlO3:Ln3+ (Ln=Eu, Tb, and Sm) hollow spheres with diameters of 166±26 nm through calcining the amorphous colloidal spheres synthesized via hydrothermal processing. However, the hollow structure and the tendency of the structure to collapse under heat in luminescence applications may degrade the luminescence performance. In this work, micron-sized spheres of LaAlO3:xMn4+,yGe4+ have been successfully synthesized by thermal decomposition of amorphous precursors autoclaved from the nitrates and ethylene glycol (EG) in the prescence of citric acid. The spherical particles of LaAlO3:xMn4+ exhibited bright NIR emission at around 730 nm, which can last for at least ten minutes without any light source, mainly due to the formation of [MnAl] defects. Incorporation of Ge4+ ions not only greatly enhance the NIR emission intensity, but also improve the NIR persistent luminescence. The influence of Ge4+ doping on the local structure, trap depth and concentration, and luminescence behavior has been 5

investigated via detailed characterizations by combined techniques of XRD, FT-IR, FE-SEM, TEM, SAED, TL, Raman, PLE/PL spectroscopy, and fluorescence decay analysis. We believe that the outcomes of this work may enrich the applications of the lanthanum aluminate materials and may offer wide implications to NIR persistent luminescence.

2. Experimental 2.1 Synthesis of LaAlO3:xMn4+,yGe4+ spheres The lanthanum (La) source is La2O3, 99.99 % pure product purchased from Huizhou Ruier Rare-Chem. Hi-Tech. Co. Ltd (Huizhou, China). Analytical grade nitric acid (HNO3, 63 wt%), ethylene glycol (EG, ≥99.0 % pure), citric acid (CA, ≥99.8 % pure), manganous nitrate (Mn(NO3)2, 50.0 wt%), germanium oxide (GeO2, 99.999 % pure) and aluminum nitrate nonahydrate (Al(NO3)3, ≥99.0 % pure) were purchased from China pharmaceutical group chemical reagent Co. LTD (Shanghai, China). The nitrate solution of La3+ was prepared by dissolving the corresponding oxide with a slightly excess amount of nitric acid, followed by evaporation at ~90 °C to dryness to remove the superfluous acid. In a typical hydrothermal processing, La(NO3)3, Al(NO3)3, GeO2, Mn(NO3)2 and citric acid monohydrate (CA) were dissolved in 20 mL of deionized water according to the intended chemical formula of LaAlO3:xMn4+,yGe4+. In all the cases, the molar ratio of the CA to all the metal ions was kept at 1:1. Then 40 mL of ethylene glycol (EG) was added in the mixture, which was homogenized under magnetic stirring at 25 °C for 30 min. The final transparent mixture was transferred to a Teflon-lined stainless steel autoclave with a 100 mL 6

capacity. The autoclave was tightly sealed and put into an electric oven preheated to 180 °C. After 4 h of reaction, the autoclave was left to cool to room temperature naturally and the hydrothermal products were collected via centrifugation, followed by washing with water three times and ethanol once, and then air drying at 50 °C for 24 h. The NIR emitting persistent phosphors of LaAlO3:xMn4+,yGe4+ were obtained through calcining the precursors under flowing O2 gas at a predetermined temperature for 4 h, with a heating rate of 8 °C min-1 at the ramp stage.

2.2 Characterization techniques Phase identification was performed by X-ray diffraction (XRD, Model SmartLab, Rigaku, Tokyo, Japan), operating at 40 kV/40 mA using nickel-filtered Cu Kα radiation (λ=0.15406 nm) and a scanning speed of 6.0° 2θ/min. The XRD data for Rietveld refinement were obtained in the step-scan mode, using a step interval of 0.02° and a counting time of 2.5 s per step. The product morphology was analyzed by field emission scanning electron microscopy (FE-SEM, Model JSM-7001F, JEOL, Tokyo) and transmission electron microscopy (TEM, Model JEM-2000FX, JEOL, Tokyo). Average diameter and standard size deviation of the spheres were determined from at least 100 individual particles in the FE-SEM micrograph, using the ‘‘SMile View” image processing software (Ver. 2.1, JEOL). Fourier transform infrared spectroscopy experiments (FT-IR, Nicolet iS5, Thermal Fisher Scientific, USA) were under

taken

using

the

standard

KBr

method.

Thermogravimetry

(TG,

Setsys18TG/DAT/DSC, France) was performed in flowing oxygen gas (30 mL·min-1) with a constant heating rate of 10 °C·min-1. Diffuse reflectance spectra of the samples 7

were taken on a UV-Vis-NIR spectrophotometer (UV-3600 Plus, Shimadzu, Kyoto, Japan) at room temperature. The Raman scattering measurements were carried out using the Raman microscope (Model R-XploRA Plus, Horiba, Kyoto) with a 532 nm He-Ne laser as the excitation source. The laser power was 49.1 mW at the sample. Thermoluminescence (TL) glow curves of the samples were recorded from 283 K (10 °C) to 475 K (202 °C) with a heating rate of 1 K·s

−1

on a spectrometer (Model

FJ427A TL, Beijing Nuclear Instrument Factory), after exposure to UV lightsource for 5 min at room temperature. The photoluminescence and the persistent luminescence

of

phosphors

were

analyzed

with

a

model

JY

FL3-21

spectrophotometer (Horiba, Kyoto).

3. Results and discussion 3.1. Synthesis, crystal structure, and morphology of Mn4+/Ge4+ doped LaAlO3 spheres Fig. 1a shows the morphology of the precursor for LaAlO3:0.1%Mn4+ synthesized by solvothermal processing in the presence of CA and EG, and the products feature spherical shape and narrow size distribution of 3±1 µm. After calcination at 1000 oC for 4 h, LaAlO3:0.1%Mn4+ spheres were obtained with maintaining the original morphology of their precursors (Fig. 1b,c). However, their diameters shrunk from 3±1 µm to 2.5±1 µm, because of dehydration and decomposition during the calcination process. Through careful observation in Fig. 1c, LaAlO3:0.1%Mn4+ spheres are found to be consisting of a number of nanocrystals, which are smaller than 100 nm. The selected-area electron diffraction (SAED) 8

analysis further indicates that the spheres are of polycrystalline, owing to the featured diffraction ring (Fig. 1d). The well-resolved planes of (110), (210) and (220) in SAED patterns suggest the spheres are well crystallized. Focus on analyzing the compositional nanocrystals finds that they are of single crystalline. The well-resolved lattice fringes in the high-resolution transmission electron microscopy (HR-TEM) image (Fig. 1f) suggest the excellent crystallinity with the spacings of ~0.38 nm and ~0.27 nm corresponding to the (001) and (110) planes of LaAlO3 (d(001)=~0.38nm, d(110)=~0.27nm, arising from JCPDS No. 85-0848). The spheres are homogeneous solid solutions, because elemental mapping results show that all elements are distributed among the particles, as shown in Fig. 1g. Fig. 2a,b shows the Rietveld structure refinements for LaAlO3 host and LaAlO3:0.1%Mn4+ with the crystallographic data of standard LaAlO3 (ICSD 28629) as initial structure model. Through comparing the calculated data with experimental spectra, we found that all the peaks are in good agreement, without any impurity phase in the samples. The stable results and acceptable residual factor values summarized in Table 1 and Table S1 indicate that the two samples both crystallize in a rhombohedral structure with the space group of R3mR. LaAlO3 belongs to a perovskite family, which corresponds to a rhombohedral symmetry and involves two types of units: AlO6 octahedral and LaO12 polyhedral (Fig. 2c). Mn4+ ions substituting for Al3+ ions did not change the crystal structure significantly. Due to the smaller ions radius of Mn4+ (0.530Å for Mn4+ and 0.535Å for Al3+, CN=6), the cell parameters (a=b=c=3.7894(1) Å) of LaAlO3:0.1%Mn4+ are a little smaller than the values of 9

LaAlO3 host (a=b=c=3.7911(1) Å). However, Mn4+ ions occupying the Al3+ site would give rise to the positive defects ([MnAl]) inevitably, because of the charge imbalance. In order to keep charge balance, negative interstitial defects O2- ([OI]) may appear in LaAlO3:0.1%Mn4+ system with a [OI] being assigned to a pair of [MnAl] [12,21]. Because the defects of [OI] and [MnAl] can be used as traps for capturing and releasing electrons, they would contribute to the persistent luminescence.

Table 1 Primitive cell parameters and reliability factors of the Rietveld refinements of LaAlO3 and LaAlO3:0.1%Mn4+ Formula LaAlO3 LaAlO3:0.1%Mn4+ Space group R3mR (No.160) R3mR (No.160) Symmetry rhombohedral rhombohedral a (Å) 3.7911(1) 3.7894(1) 90.02(2) 90.06(1) β (°) 3 V (Å ) 54.490(6) 54.416(4) 6.38% 5.00% Rexp Rwp 8.90% 8.49% 6.39% 5.73% Rp 2 χ 1.39% 1.70% To reveal the formation mechanism of LaAlO3:0.1%Mn4+ spheres, FT-IR spectra and DTA/TG curves were analyzed in Fig. S1 and Fig. 3a. Because the precursor is amorphous (confirmed by the XRD patterns in Fig. 3b), its composition is unknown. Fig. S1 shows FT-IR spectra of the precursor and the products calcined at different temperatures, in order to analyze the composition of the amorphous precursors. The broad absorption band at ~3300-3700 cm-1 (centered at ~3500 cm-1) and absorption peak ~1640 cm-1 are assignable to the O-H (ν1 and ν3) and H-O-H bending mode (ν2) vibrations, respectively [22-24], indicating the existence of hydration water and hydroxyl group in the precursor and the participation of EG in the reaction. The 10

strong absorption bands at 1587 and 1401 cm-1 are assigned to the vibration of COO[25,26], arising from the initial reactant of CA. The absorption peak at 1308 cm-1 corresponds to the C-H vibration [25,26], indicating the existence of CH2 or CH3 group in precursor. Therefore, the sharp exothermic peak at 420 oC in DTA curve (Fig. 3a) is due to the combustion of carbon monoxide decomposed by the organic composition. While the absorption bands at ~1434 and ~791 cm-1 detected at 500 oC and 700 oC (Fig. S1) are indicative of CO32-, suggested the combustion induced the formation of carbonate ions. Therefore, the two endothermic peaks at 822 and 868 oC are attributed the decomposition of carbonate ions. Because the decomposition completes at the temperature up to ~870 °C (Fig. 3a), the amorphous precursor did not transform into rhombohedral-structured LaAlO3:0.1%Mn4+ until the calcination temperature above 700 oC (Fig. 3b). At the temperature of ≥ 900 oC, the vibration of carbonate ion disappear and new peaks at 665 and 450 cm-1 assigning to metal-oxygen vibration are observed [26,27], further confirming the formation of rhombohedral solid solution (Fig. S1). Inferred from above results, a possible formation mechanism for amorphous spheres is proposed. Cit3- (CA) initially reacted with La3+, Al3+, and Mn2+ to form La3+- Cit3-- Al3+- Mn2+ complexes. The concentration of free cations would be restricted by the formation of such complexes, which help to control nucleation and growth. In addition, EG is not only a solvent, but also a ligand [18,28]. Acting as a structure-directing agent, Cit3- and EG could absorb to the surface of the amorphous particles, then resulting in microspheres being formed from aggregation of amorphous particles. Calcining these amorphous spheres at ≥ 900 11

o

C yielded rhombohedral-structured products. A continuous progression in peak shape

and intensity was observed at higher calcination temperatures (Fig. 3b), indicative of crystallite growth and perfect. Calculation by applying the Scherrer equation, the average crystallite size increases from ~24 nm to ~60 nm with increasing the temperature from 900 to 1500 oC. The spherical shape and excellent dispersion of the original particles were well retained for LaAlO3:0.1%Mn4+ up to 1000 °C (Fig. 4). However, elevating the sintering temperature above 1000 °C induced the serious aggregation and the damage of spherical shape (Fig. 4). Therefore, 1000 °C was selected as the optimal calcination temperature. Through the same solvothermal processing, followed by calcination at 1000 °C for 4h, rhombohedral-structured microspheres of LaAlO3:xMn4+ (x=0.05%-0.175%) (Fig. 5a and Fig. S2) and LaAlO3:0.1%Mn4+,yGe4+ (y=0-1.2%) (Fig. 5b and Fig. S3) were successfully synthesized. In this work, incorporation of Mn4+ and Ge4+ did not significantly affect the crystal structure, because no any other phases and impurities were found. Because the ionic radius of Ge4+ (0.530 Å, CN=6) and Mn4+ (0.530 Å, CN=6) are smaller than that of Al3+ (0.535 Å, CN=6), the calculated cell parameters a=b=c=~3.7890(1) Å for LaAlO3:0.1%Mn4+,0.6%Ge4+ are smaller than that a=b=c=~3.7894(1) Å for LaAlO3:0.1%Mn4+ (Fig. S4, Table S2 and Table S3). However, Mn4+ and Ge4+ ions occupying the Al3+ site would give rise to the positive defects ([MnAl] and [GeAl]) inevitably, because of the charge imbalance. In order to keep charge balance, negative interstitial defects O2- ([OI]) may appear, and its content would increase with increasing the incorporation content of Mn4+ and Ge4+. In order to investigate the 12

variation of local microstructure, Raman spectra were recorded (Fig. 6). There are two peaks at ~124 cm−1 and ~154 cm−1, which are assigned to the rotation of the oxygen octahedron along the [001]h crystal direction and the vibration of M-O (M=La and Al) in the (001)h crystal plane, respectively [20,29]. Obviously, the full width at half maximum (FWHM) value is 23 cm−1 for LaAlO3 (x=0, y=0), and the value increased with Mn4+ and Ge4+ ions substituting for Al3+ ions. The FWHM value reaches 40 cm−1 with the x and y increasing to 0.1% and 0.9%. These are mainly due to the increased content of interstitial defects O2- ([OI]), because defects in the crystal contribute to broadening of the vibrational Raman bands [20,30]. It is interesting found that the peak at ~154 cm−1 became weaker at a higher concentration of Mn4+ and Ge4+. Because the ionic radius of Ge4+ (0.530 Å, CN=6), Mn4+ (0.530 Å, CN=6) are smaller than that of Al3+ (0.535 Å, CN=6), Mn4+ and Ge4+ ions substituting for Al3+ ions leads to smaller octahedron of MnO6 and GeO6, which are close to the octahedron AlO6 and the polyhedron LaO12 with sharing the oxygen atom. Therefore, the symmetry of the octahedron AlO6 and the polyhedron LaO12 became weaker, because of the movement of the sharing oxygen atom arising from the substitution of Mn4+ and Ge4+ (Fig. 7). These contributed to the weakened vibration at ~154 cm−1. Also due to the movement of the sharing oxygen atom, the octahedron MnO6 and GeO6 became asymmetry, and the degree of asymmetry increased with more substitution of Mn4+ and Ge4+.

3.2 Luminescence of LaAlO3:xMn4+ spheres Diffuse reflection spectra of LaAlO3 and LaAlO3:0.1%Mn4+ are shown in Fig. 8a. Different from the spectra of LaAlO3, two relevant absorption bands were observed 13

for LaAlO3:0.1%Mn4+, which are the spin-allowed 4A2g-4T2g transition of Mn4+ at ~500 nm, and the overlaps of Mn4+-O2- charge transfer band, 4A2g-4T1g transition and 4

A2g-2T2g transition of Mn4+ covering the spectra from 250 nm to 440 nm [5]. Fig.

8(b,c,d) shows the PLE and PL spectra for the LaAlO3:xMn4+ spheres. Several broad excitation bands are observed ranging from 250 to 600 nm, which can be well-decomposed into five components based on a Gaussian function shown in Fig. 8c, with the maxima at 291, 350, 428 and 499 nm accounting for Mn4+-O2- charge transfer band, 4A2g-4T1g, 4A2g-2T2g and 4A2g-4T2g transitions of Mn4+ [4,5,31-33]. The relevant fitting results are consistent with diffuse reflection spectra. Under the excitation of 330 nm, the spheres exhibit seven sharp emissions peaks at 689, 697,703, 710, 717, 723, and 730 nm, assigned to the 2E-4A2 transition of Mn4+ and the vibrational side bands of zero-phonon R-line with phonon assistance [4,5]. The emission spectra almost locates at the near-infrared (NIR) range, with the 730-nm peak taking the dominate role, indicating their potential application in bio imaging. Enhanced emission intensity was observed by increasing the Mn4+ content (x value) from 0.05% to 0.1%, but it then decreases at a higher Mn4+ content, indicating the quenching concentration of 0.1% (inset of Fig. 8d). Although the excitation and emission intensity are closely related to the Mn4+ content, the positions of PLE and PL bands did not appreciably alter at various Mn4+ contents. It is widely accept that the luminescence behavior of Mn4+ is affected by the crystal field strength and site symmetry of the host, according to the coupling of Mn4+ 3d3 electrons to lattice vibration [11]. As shown in Fig. 8e, it clearly illustrates that the energies of most 14

multiples are strongly dependent on the crystal-field strength except the 2T1 and 2Eg levels. To understand the effect of the crystal field strength on NIR emission of the Mn4+ doped phosphors, the crystal field strength (Dq) and the Racah parameters (B and C) were calculated by analyzing the PLE and PL spectra of LaAlO3:0.1%Mn4+. The energy levels of 4T2g, 4T1g, and 2Eg in the LaAlO3 host are determined at 20040 cm-1, 28572 cm-1, and 13698 cm-1, which are related to the PLE fitting peaks and the maximum PL peak, respectively. Dq can be determined by the following equation [1,11]: Dq =

E (2 T2g − 4 A 2g ) (1) 10

B can be determined by the following equation:

Dq B

=

15( x − 8) (2) x 2 − 10 x

Where parameter x can be evaluated from the expression: x=

E( 4 A 2g − 4 T1g ) − E( 4 A 2g − 4 T2g ) Dq

(3)

The Racah parameter C is defined as:

E( 2 E g − 4 A 2g ) B

=

3.05C 1.8B − + 7.9 (4) B Dq

From equations (1)-(4), the final calculated values of crystal field parameters of Dq, B, and C are 2004, 871, and 2459 cm-1, respectively. The ratio of Dq/B equals ~2.30, indicating that Mn4+ ions experience a strong crystal field in the LaAlO3 host, which is close to that reported in the reference [20]. Because Mn4+ ions occupying the Al3+ site gives rise to the positive defects ([MnAl]) and negative interstitial defects O2- ([OI]), which contribute to the persistent 15

luminescence, the spherical phosphors exhibit long NIR persistent luminescence. The NIR-persistent luminescence decay curves of LaAlO3:xMn4+ spheres are shown in Fig. 9a, which are monitored at 730 nm after 5 min irradiation under UV light at room temperature. Clearly, the intensity of all samples dropped rapidly in the first stage and then slowly decreased. This can be attributed to the fast and slow release of the stored excitation energy from the shallow and deep traps, respectively [34]. It is founded that the afterglow intensity at x=0.15% is most intense, and further increasing the content of Mn4+ to 0.175% yielded a lower one. In LaAlO3:xMn4+, Mn4+ is not only the emitting center, but also the defect center. Previous literature [4] reported that [MnAl] is attributed to the shallow trap, while [OI] is ascribed to the deep trap. But the formation of defect clusters [35,36] arising from more Mn4+ incorporation, would decrease the effective traps and thus leading to a poor afterglow. Thermoluminescence (TL) curves for LaAlO3:xMn4+ are displayed in Fig. 9b, in order to investigate trap depth responsible for the NIR-persistent luminescence. The estimated values of trap depth are calculated by using following equation [37]: E = Tm/500

(5)

where Tm is the temperature of maximum peak in TL curve. The trap depths continually decrease from 0.834 to 0.772 eV with x increasing from 0.05% to 0.15%. Because the captured electrons are not easy to escape in the deep trap, the sample with shallower trap depth would exhibit a better afterglow at the initial stage and a worse one at the following stage [38]. Therefore, the x=0.15% sample possessing the shallowest trap depth exhibited the most intense persistent luminescence during the 16

test range of 0-300 s. However, the persistent luminescence of x=0.10% and x=0.125% samples exhibited higher intensity than that for x=0.15% sample at the

300-600 s, because of the exhaustion of shallow electronic traps. It is noted that further increasing x from 0.15% to 0.175% yielded almost the same trap depth of 0.772 eV. However, the sample with a higher content of Mn4+ exhibited a worse performance, mainly due to the formation of defect clusters arising from more Mn4+ incorporation [35,36]. The persistent luminescence mechanism is illustrated in Fig. 9c. Upon UV light source excitation, the electrons are promoted from valence band (VB) to conduction band (CB) due to the incident photons are absorbed by LaAlO3 host (process 1). Then electron traps capture the free electrons from CB via nonradiative relaxation (process 2). After a sufficient irradiation time, shallow traps are filled with electrons. Then the deep traps are filled mainly through nonradiative relaxation from the shallow traps. After the stoppage of irradiation, the electrons in the shallow traps escape thermally into the CB and recombine with the ionized Mn4+ ions (process 3). This process dominates the initial intense persistent luminescence and gives first step of NIR afterglow. When the depletion of the electrons in the shallow traps in a short time, most of the electrons in the deep traps directly tunnel a short distance to the nearby ionized Mn4+ ions and are captured into the matched energy levels of Mn4+ (process 4). While the long persistent luminescence is mainly due to the tunneling recombination for other electrons released from deep traps [34].

3.3 The influence of Ge4+ doping on luminescence behavior In Mn4+ doped LaAlO3, Mn4+ is not only the emitting center, but also the defect 17

center. Although incorporation of Mn4+ could manipulate the trap depth, which thus affect the persistent luminescence, the quench concentration of Mn4+ would lead to a weakened lightness. Therefore, further doping Ge4+ at the optimal concentration of Mn4+ may be an acceptable plan to improve the luminescence. Indeed, incorporation of Ge4+ in LaAlO3:0.1%Mn4+ did not affect the absorption of Mn4+ and bandgap significantly

(Fig.

S5).

However,

the

luminescence

behavior

of

LaAlO3:0.1%Mn4+,yGe4+ (y=0-1.2%) are dependent on the concentration of Ge4+ ions. Fig. 10 presents PLE and PL spectra of the LaAlO3:0.1%Mn4+,yGe4+ phosphors. There are no obvious changes in peak position with increasing the concentration of Ge4+, but enhanced emission intensities were observed. Increasing the concentration of Ge4+ from 0 to 0.6%, resulted in an increase of 260 % at 730 nm for emission intensity. Further increasing the concentration of Ge4+ above 0.6% yielded weakened emission intensity. In Mn4+ doped phosphors, the luminescence intensity is determined by the competition between the isolated Mn4+ ions and the interaction among the Mn4+ ions. Obviously, more isolated Mn4+ ions contribute to enhanced luminescence while the Mn4+-Mn4+ interaction arising from more incorporation of Mn4+ gives rise to the emission quench. In the absence of Ge4+ ions, two Mn4+ ions tend to replacing a couple of neighboring Al3+ sites through the formation of a Mn4+-Mn4+ pair. In order to maintain charge balance, one O2- trapped at an interstitial site will be introduced to form a Mn4+-Mn4+-O2- pair [12,21]. However, a Mn4+-Ge4+-O2- pair can replace the Mn4+-Mn4+-O2- pair through doping Ge4+ ions, which contributes to the emergence of more isolated Mn4+ ions and thus enhanced 18

emission intensity via reducing the energy loss from the interaction between Mn4+ ions. Nevertheless, an increase of [OI] defects arising from excessive doping of Ge4+ (y > 0.6%) would weaken the luminescence intensity, because of the energy migration from Mn4+ to [OI] quenching centers [21]. Chen et al. and Shu et al. also reported similar luminescence phenomenon for Mn4+/Ge4+ co-doped Y3Al5O12 [12] and CaAl12O19 [21] previously. The ratio of Dq/B for LaAlO3:0.1%Mn4+,yGe4+ (y=0-1.2%) samples is determined to be ~2.32, which is close to the value ~2.30 for LaAlO3:Mn4+ samples, indicating that Ge4+ doping did not appreciable alter the crystal field of Mn4+ in the LaAlO3 host. However, the increased asymmetry of MnO6 arising from more incorporation of Ge4+ may also contribute to the enhanced luminescence intensity [39]. Usually, the emission intensity of phosphors would gradually decrease with increased temperatures due to the magnified population of higher density phonons and vibration levels as well as the probability of nonradiative transfer [8]. Fig. 11a shows the relative PL intensity of LaAlO3:0.1%Mn4+,yGe4+ in the temperature range of 298-473 K. Since a similar attenuation trend was observed for all the samples, the integrated

emission

intensities

of

LaAlO3:0.1%Mn4+

and

LaAlO3:0.1%Mn4+,0.6%Ge4+ phosphors were analyzed in Fig. 11b. The almost coincident curves, indicating the incorporation of Ge4+ did not appreciably affect the thermal stability. Elevating the temperature from 298 K (25 oC) to 373 K (100 oC) yielded a 58% intensity attenuation. According to the Arrhenius formula (equation (6)), the activation energy (Ea) for thermal quenching of LaAlO3:0.1%Mn4+,0.6%Ge4+ 19

phosphor can be obtained [10].

IT =

I0 (6)  Ea  1 + cexp −   kT 

where I0 and IT are the integrated emission intensity at room temperature and temperature T, respectively, c is a constant, and k is the Boltzmann constant (8.629×10-5 eV). From the slope of the ln(I0/IT-1) vs 1/kT correlation showed in Fig. 11c,

it

can

be

determined

that

the

thermal

activation

energy

of

LaAlO3:0.1%Mn4+,0.6%Ge4+ is about ~0.41 eV, which locates in the range of reported values from 0.2 to 0.45 eV for Mn4+ doped phosphors [10-12], indicating a good thermal stability. In order to investigate the influence of Ge4+ doping on persistent luminescence behavior, the NIR-persistent luminescence decay curves of LaAlO3:0.1%Mn4+, yGe4+ spheres were analyzed in Fig. 12a. In all cases, the samples containing Ge4+ exhibit a better persistent luminescence than y = 0 sample. Increasing the Ge4+ content gives rise to the enhanced persistent luminescence intensity until y reaches to 0.6%. Then weakened afterglow intensity was observed at more Ge4+ incorporation. Fig. 12b shows TL glow curves of LaAlO3:0.1%Mn4+,yGe4+ (y=0-1.2%) and the calculated trap depth decreases from ~0.812 eV for y=0 to ~0.796 eV for y=0.3%. However, the trap depth almost keep at a constant of ~0.796 for y=0.3%-1.2%, indicating an independence on Ge4+ incorporation at this range. Similar to [MnAl] defects, doping Ge4+ ions would induce the formation of [GeAl] defects with Ge4+ occupying Al3+ site. Although Ge4+ and Mn4+ ions have the same valence state and similar radius, [GeAl] 20

defects mainly contribute to the trap concentration rather than the trap depth, which are different from [MnAl] defects. Also due to the formation of defects cluster arising from more Ge4+ incorporation [35,36], further increasing the doping content of Ge4+ over 0.6% leads to weakened afterglow intensity (Fig. 12a). According to above results, LaAlO3:0.1%Mn4+,0.6%Ge4+ spheres show the strongest emission intensity and the best NIR-persistent luminescence at room temperature. However, the afterglow became very weak after ten minutes (Fig. 12a). To estimate the persistent luminescence behavior at a high temperature, the sample was heated at 150 oC for four times. After heating the phosphors at 150 oC for 3 min in the absence of light source, more intense afterglow was observed than the original one. The strong afterglow maintained after the second-round heating (Fig. 13a). These are owing to the release of electrons from deep traps with the assistance of heating (Fig. 13d). However, the exhaustion of electrons contributed to the weakest afterglow signal after the third- and fourth- round heating [35,36]. Fig. 13b shows the afterglow curves for LaAlO3:0.1%Mn4+,yGe4+ (y=0-1.2%) samples after heating at 150 oC for 3 min, and the afterglow curves exhibit an almost the same law to that before heating (Fig. 12a). However, after the second-round heating, the afterglow intensities of y = 0.9% and y = 1.2% samples are stronger than that of y = 0.6% sample (Fig. 13c). More Ge4+ doping results in a higher trap concentration, thus more electrons captured by the deep traps contribute to the more intense persistent luminescence after heat processing. This repeatable persistent luminescence by a proper heating without light irradiation indicates a potential application of optical data storage. 21

4. Conclusion In this work, micron-sized spheres of LaAlO3:xMn4+,yGe4+ have been successfully synthesized by thermal decomposition of amorphous precursors autoclaved from the nitrates and ethylene glycol in the prescence of citric acid. The polycrystalline phosphors could maintain spherical morphology and good dispersion of their original precursors at the calcination temperature up to 1000 oC. LaAlO3:xMn4+ exhibited bright NIR emission at around 730 nm, which can last for at least ten minutes in the absence of light source, mainly due to the formation of [MnAl] defects. The quenching concentration for Mn4+ in LaAlO3 is found at x = 0.1%. Incorporation of Ge4+ ions did not change the morphology, crystal structure, band gap, and thermal stability of LaAlO3:Mn4+ significantly. However, doping Ge4+ gave rise to emergence of more isolated Mn4+ ions and stronger asymmetry of MnO6, which thus induced a greatly enhanced NIR emission intensity by 260%. Ge4+ substituting for Al3+ formed the positive defects of [GeAl], which contributed to the trap concentration rather than the trap depth. Continuously improved persistent luminescence was found at a higher Ge4+ concentration, until it was over 0.6%. Heat processing at 150 oC would induce repeated afterglow in the absence of light source, until the exhaustion of electrons in the traps.

Acknowledgments This work was supported in part by the National Natural Science Foundation of China (Grant 51672039 and 51972047) and the Fundamental Research Funds for the Central 22

Universities (Grant N172002001). Q. Zhu acknowledges the financial support from the China Scholarship Council (Grant 201806085026).

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28

Figure captions Fig. 1. TEM and FE-SEM images showing morphology of the precursor for LaAlO3:0.1%Mn4+ (a) and LaAlO3:0.1%Mn4+ spheres calcined at 1000 oC (b,c). (d-g) are SAED patterns (d,e), HR-TEM (f) and element distribution (g) of the LaAlO3:0.1%Mn4+ spheres.

Fig. 2. Rietveld refinements of LaAlO3 (a) and LaAlO3:0.1%Mn4+ (b), where the black crosses, red solid line, blue solid line and purple short vertical lines represent the observed, calculated and difference XRD profiles and the positions of Bragg reflections respectively. (c) is crystal structure of LaAlO3, showing the substitution of Mn4+, with coordination polyhedron of Al3+ and La3+.

Fig. 3. (a) DTA/TG curves for precursor of LaAlO3:0.1%Mn4+, and (b) XRD patterns of the products calcined from the precursor at various temperatures for 4 h.

Fig. 4. FE-SEM morphology of the precursor for LaAlO3:0.1%Mn4+ (a) and the products calcined from the precursor at (b) 900 oC, (c) 1000 oC, (d) 1200 oC, (e) 1300 o

C, (f) 1500 oC.

Fig.

5.

XRD

patterns

of

LaAlO3:xMn4+

(x=0.05%-0.175%)

(a)

and

LaAlO3:0.1%Mn4+,yGe4+ (y=0-1.2%) (b) calcined at 1000 oC for 4h.

Fig. 6. Raman spectra of LaAlO3:xMn4+,yGe4+ calcined at 1000 oC for 4h. Fig. 7. Crystal structure of LaAlO3, showing the microstructural distortion with substitution of Mn4+/Ge4+.

Fig. 8. (a) Diffuse reflection spectra of LaAlO3 and LaAlO3:0.1%Mn4+. (b) and (d) are PLE and PL spectra of LaAlO3:xMn4+ spheres. (c) is PLE spectra of 29

LaAlO3:0.1%Mn4+ fitted by the Gaussian function. (e) is Tanabe-Sugano energy-level diagram for Mn4+ in the octahedral site of LaAlO3 host[20].

Fig. 9. (a) NIR-persistent luminescence decay curves and (b) TL glow curves of LaAlO3:xMn4+ after 5 min of irradiation of UV light source. (c) is schematic illustration of NIR-persistent luminescence mechanism at room temperature.

Fig. 10. (a) PLE and (b) PL spectra of LaAlO3:0.1%Mn4+, yGe4+ (y=0-1.2%). Fig. 11. (a) Relative PL intensity of LaAlO3:0.1%Mn4+,yGe4+ versus temperatures (y=0-1.2%). (b) is relative PL intensity of LaAlO3:0.1%Mn4+,yGe4+(y=0, 0.6%). (c) is the ln(I0/IT-1) vs 1/kT plot for determination of the activation energy of thermal quenching for LaAlO3:0.1%Mn4+,0.6%Ge4+.

Fig. 12. (a) NIR-persistent luminescence decay curves and (b) TL glow curves of LaAlO3:0.1%Mn4+, yGe4+ (y=0-1.2%) after irradiation by UV light source for 5 min.

Fig. 13. (a) NIR-persistent luminescence decay curve of LaAlO3:0.1%Mn4+,0.6%Ge4+ after irradiation by UV lightsource for 5 min and then 3-min heating at 150 oC for four rounds. (b) and (c) are NIR-persistent luminescence decay carves of LaAlO3:0.1%Mn4+,yGe4+ (y=0-1.2%) after heating at 150 oC for two rounds: (b) first round heating, (c) second round heating. (d) is the schematic illustration of NIR-persistent luminescence mechanism after heat processing.

30

1. Perovskite-type microspheres of LaAlO3:Mn4+/Ge4+ were originally synthesized. 2. Doping Ge4+induced a greatly enhanced NIR emission intensity by 260 %. 3. Incorporation of Ge4+ gave rise to the improved persistent luminescence. 4. The spheres exhibited repeated illumination by heating without light source.

Declaration of Interest Statement There are no conflicts of interest to declare.