Efficient Luminescence Enhancement of Mg2TiO4:Mn4+ Red

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Functional Inorganic Materials and Devices

Efficient Luminescence Enhancement of Mg2TiO4:Mn4+ Red Phosphor by Incorporating Plasmonic Ag@SiO2 Nanoparticles Leonid Dolgov, Junyu Hong, Lei Zhou, Xiaohui Li, Junhao Li, Vesna Djordjevic, Miroslav D. Drami#anin, Jianxin Shi, and Mingmei Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05781 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 11, 2019

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ACS Applied Materials & Interfaces

Efficient Luminescence Enhancement of Mg2TiO4:Mn4+ Red Phosphor by Incorporating Plasmonic Ag@SiO2 Nanoparticles Leonid Dolgov,†,‡ Junyu Hong,† Lei Zhou,*,† Xiaohui Li, † Junhao Li,† Vesna Djordjevic,⁋ Miroslav Dramicanin, ⁋ Jianxin Shi, † Mingmei Wu*,† †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, P. R. China ‡

Institute of Physics, University of Tartu, W. Ostwaldi street 1, 50411, Tartu, Estonia

⁋

University of Belgrade, Vinca Institute of Nuclear Sciences, P.O. Box 522, Belgrade 11001, Serbia

KEYWORDS: Mn4+ phosphors, localized surface plasmon resonance, plasmon enhancement, magnesium titanate, core@shell nanoparticles, silver

ABSTRACT: One of prospective ways for boosting efficiency of luminescent materials is their combination with noble metal nanoparticles. Collective, so-called plasmon, oscillations of surface electrons in nanoparticle can resonantly interact with incident or fluorescent light and cause increase in light absorption cross section or radiative rate for adjacent emitter. Plasmonic inorganic phosphors require gentle host crystallization at which added noble nanoparticles will not suffer from aggregation or oxidation. Prospective plasmonic Mg2TiO4:Mn4+ phosphor containing core@shell Ag@SiO2 nanoparticles is prepared here by spare low temperature annealing of sol-gel host precursor. It is revealed that Mn4+ luminescence non-monotonously depends on the size and concentration of 40 and 70-nm silver nanoparticles. It is demonstrated that luminescence of Mg2TiO4:Mn4+ phosphor can be up to 1.5 times increased, when Mn4+ excitation is supported by localized surface plasmon resonance in Ag@SiO2 nanoparticles.

al.5,6 Such coupling revealed possibility for additional angular control of luminescence from LED device.

1. INTRODUCTION Intriguing route for increase in efficiency of luminescent materials is their combination with noble metal nanoparticles. Electric field of incident light can resonate with collective plasmon oscillations of surface electrons in nanoparticles. As a consequence, nanoparticles redistribute and concentrate light energy and electric field in own vicinity working as optical antennas.1 This effect causes resonant increase in light absorption and scattering for host material adjacent to nanoparticles, which can be useful, for example, in plasmonic solar cells.2 At the same time more effective light absorption can be a pre-condition for more efficient light emission. Therefore, plasmon nanoparticles combined with luminescent centers are also actively studied.3-6

Nanoparticles played role of constructive layer in the mentioned works. At the same time, ability of their incorporation inside luminescent host looks even more attractive both on the theoretical7 and on the experimental3 points of view. There are some papers about combination of noble metal plasmonic nanoparticles with semiconductor quantum dots,8 up-conversion phosphors9 and glasses.3 However, provision of nice host crystallinity simultaneously with control of metal nanoparticles' size and geometry still remains a challenge. Probably because of this, crystal phosphors for LEDs combined with plasmonic nanoparticles is not frequently reported. Only some works about plasmon coupled luminescence in boron carbon oxynitride10 and rare earth activated molybdate11 crystal phosphors appeared recently. In both of these works nanoparticles were formed together with host material, which means certain difficulties in control of their monodispersity.

Thus, it is reported that noble metal nanoparticles can be prospective for increasing efficiency of light emitting diodes (LEDs).4-6 Namely, Pillai et al.4 showed that silver nanoparticles deposited in the vicinity of the p-n junction caused plasmon coupled light absorption, which resulted in two times increase in photocurrent and similar two times increase in integrated electroluminescence. Coupling of emitted light with plasmons in gold nanoparticles was considered by Lozano et

Here, we firstly report about plasmonic increase of excitation for the magnesium titanate crystal phosphor activated by Mn4+ transition metal ions and combined with silver nanoparticles. Mn4+ ions12-14 as red emitters can be more beneficial

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than rare earth ions,15-17 especially for warm white light LEDs and LEDs for indoor plant growth. Notably, Mn4+ ion has extended possibility for excitation of 4A2→4T1 and 4A2→4T2 transitions both by ultraviolet and violet light correspondingly. In addition, variation of crystal host for Mn4+ ion can fine-tune the red emission from 2Eg→4A2g transition in the spectral range 620-720 nm,14 which is definitely wider than for Sm3+ and Eu3+ ions. Oxides and fluorides are most perspective hosts for the Mn4+ ions.13 Oxide hosts are more chemically stable and can be prepared without usage of harmful hydrofluoric acid usually necessary in case of fluorides. Specifically Mg2TiO4:Mn4+ (MTO) magnesium titanate attracted our attention as a host material. Experimental and theoretical data about luminescence of Mn4+ ions in different oxide phases of magnesium titanate18,19 and in MTO crystal phase particularly20,21 served as background for the present work. Sol-gel method, namely its Pechini variant, is proposed here for combination of MTO phosphor with core-shell Ag@SiO2 nanoparticles. Silica shell prevents oxidation and aggregation of silver and protects Mn4+ luminescence from quenching by silver surface. In order to keep better monodispersity of nanoparticles we propose their preliminary preparation and further usage as gelation centers for the host material. Such ability with next MTO host crystallization is realized here. Magnesium titanate host is selected since it allows low temperature crystallization, which saves incorporated silver nanoparticles from melting and agglomeration. Combination of plasmonic nanoparticles with transition metal phosphors was not considered before, up to our knowledge. Luminescent behavior of samples containing different concentrations of Ag@SiO2 nanoparticles with different sizes of silver core is analyzed. Synthesis routes, structural properties and luminescence characteristics of obtained powders are considered in the next sections.

Figure 1. (a) General scheme of MTO samples preparation. (b) XRD spectra of MTO-40Ag224 and MTO-70Ag224 samples. MTO peaks are marked in accordance with ICDD 25-1157 card. (c) Scheme of MTO crystal structure. MTO host was prepared from the 1:2:5:20 molar ratio mixture of titanium (IV) isopropoxide, magnesium (II)-nitrate, citric acid and ethylene glycol. Ethylene glycol solution of manganese (II) nitrate hydrate salt was introduced into the mixture as a source of Mn4+ ions. Different amounts of preliminary prepared27,28 Ag@SiO2 nanoparticles (Figure S1) were re-dispersed in ethylene glycol and added to the mixture also. Reaction was started under continuous stirring at 60°C and finished after several hours at 130°C. Finally samples were pre-annealed at 350°C during 30 min and annealed at 550°C during 1 hour. More details about preparation procedure are available in the Supporting information to this article. Samples were prepared with different mole concentrations (c) of Ag@SiO2 nanoparticles relatively to the overall content of titanium isopropoxide and magnesium (II)-nitrate (Table 1).

2. EXPERIMENTAL 2.1. Preparation of samples There is a certain risk of oxidation and melting of metal nanoparticles inside crystallizing host material if the composite is prepared by traditional high temperature sintering.22 As an alternative, it is possible to use sol-gel route23 and spare low temperature annealing of samples. Magnesium titanates are promising hosts for nanoparticles because they have several low temperature crystal phases.18,24,25 In view of this, perspective MTO red phosphor26 was prepared by sol-gel route, namely by its Pechini variant adapted from Ref.20 Gelation process can start around Ag@SiO2 nanoparticles preliminary prepared and introduced into sol (Figure 1a). Low annealing temperature saves shape of these nanoparticles, which become naturally incorporated into the MTO crystal host.

2.2. Measurement techniques X-ray diffraction (XRD) measurements were done by means of Rigaku D-Max 2200 X-ray diffraction system with a Cu Kα radiation at 35 kV and 35 mA. Transmission electron microscopy was performed on the FEI Tecnai G2 F300 instrument and on the Gemini SEM 500 (Zeiss). Measurements of luminescence spectra and lifetimes of emission were done on the luminescent spectrometer FLS 920 (Edinburgh instruments). Quantum yield was measured via absolute PL quantum yield C9920 spectrometer equipped with integrating sphere adapter (Hamamatsu). Light absorbance was measured via Cary 5000 spectrophotometer (Agilent Technology).

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ACS Applied Materials & Interfaces Table 1. Marking of Mg2TiO4:Mn4+ (MTO) samples with Ag@SiO2 nanoparticles. Concentration (c) of Ag@SiO2 nanoparticles in Mg2TiO4:Mn4+ spinel samples, mol% 0 Mark of sample

MTO

0.014

0.028

0.056

0.112

0.224

MTO-40Ag014

MTO-40Ag028

MTO-40Ag056

MTO-40Ag112

MTO-40Ag224

MTO-70Ag014

MTO-70Ag028

MTO-70Ag056

MTO-70Ag112

MTO-70Ag224

Note: numbers -40 and -70 correspond to 40 nm and 70 nm size of silver cores in Ag@SiO2 nanoparticles.

3. RESULTS AND DISCUSSION All samples were formed as powders consisting of crystallites. Position and intensity of XRD peaks for all samples are corresponded to ICDD 25-1157 card of Mg2TiO4 (Figure 1b). MTO relates to the minerals with inverse spinel structure, in which tetrahedral lattice sites are occupied by Mg2+ ions (Figure 1c). Mn4+ activator ions substitute the octahedral sites in the MTO host lattice. Concentration of silver nanoparticles in the samples was varied in the range from 0.014 to 0.224 mol%. Scanning transmission electron microscopy images demonstrated (Figures 2a-c) that Ag@SiO2 nanoparticles indeed have become a centers on the top of which the host material was formed. Light absorption and luminescence of Mn4+ ion are caused by transitions of 3 electrons in the unfilled 3d shell. Excitation spectrum consists of two spectral bands (Figure 3a, blue line) with maxima at 340 and 475 nm related to 4A2→4T1 and 4 A2→4T2 parity-forbidden and spin-allowed electron transitions in Mn4+ ion respectively. 14,18 They result in luminescent band in the spectral range of red visible light 625-750 nm (Figure 3a, red line).

Figure 2. Scanning TEM and element mapping images of MTO-40Ag224 sample area: (a-c) formation of Mg on the surface of 40Ag@SiO2 nanoparticles; (d-g) general view of microcrystallite and overlapped maps of constituent elements. therefore emit more light. Precise control of distance between metal nanoparticle and emitter is one of conditions for this effect. Too short distance could provoke quenching of emission by nanoparticle; too long distance will exclude emitter from the adjacent spatial zone.31 Optimal distance is longer than units and can reach several tens of nm according to literature.31,32

It is worth to note that maximum for Mn4+ emission corresponding to 2Eg→4A2g transition can serve as a marker proving the crystal phase state of surrounding host. For example, luminescence of Mn4+ in the MgTi2O5 phase has this maximum at 700 nm.19 While it is at 660 nm (Figure 3a, red line) in the spectra of Mn4+ in the MTO phase.18-21 Luminescence intensity non-monotonously depends on the concentration of Mn4+ ions in the MTO host because of concentration quenching. We revealed that concentration quenching is minimal for 0.2 mol% of Mn4+ ions in MTO material (Figure 3b). Therefore, all prepared samples have such percentage of Mn4+ ions. This concentration is close to the value 0.1 mol% pointed in the literature data.20

So silica shell with size ~10 nm should prevent direct contact between silver core and Mn4+ emitters and thus avoid quenching.8 Distance between emitting Mn4+ centers and Ag@SiO2 nanoparticles can be varied indirectly through the change in nanoparticles concentrations in the Mg2TiO4 host. One can suggest that too high concentrations of nanoparticles will damp luminescence. It is because grains of luminescent MTO host will be un-desirably displaced from the vicinity of metal by too closely packed metal nanoparticles. Another reason for less emission can be growth in harmful re-absorption of emitted light with concentration of Ag@SiO2 nanoparticles due to their electromagnetic coupling.33

Position of Mn4+ absorption bands in the ultraviolet-violet spectral region makes them attractive for coupling with plasmon resonance of silver nanoparticles situating in the same spectral range. By such a way intensity of Mn4+ luminescence can be enhanced additionally by Ag@SiO2 nanoparticles incorporated into the MTO host. Light absorption resonance cross section for Ag@SiO2 nanoparticle can be much higher than its geometrical cross section. 1,29,30 Therefore resonance light absorption can be high also for the Mn4+ sites adjacent to Ag@SiO2 nanoparticle. So they will absorb more and

Evolution of Mn4+ luminescence was checked for MTO samples with different concentrations (c) of Ag@SiO2 nanoparticles: 0.014, 0.028, 0.056, 0.112, and 0.224 mol% calculated relatively to the summed content of Mg and Ti atoms in

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Figure 3. (a) Excitation (blue) and emission (red) spectra for referent MTO sample. (b) Luminescence intensity vs. concentration of Mn4+ ions in the MTO host. (c, d) Intensity of Mn4+ luminescence vs. concentration of 40Ag@SiO2 nanoparticles. (e,f) Comparison of MTO-Ag fluorescence (λem=660 nm) at different concentrations Ag@SiO2 nanoparticles with 40-nm and 70-nm Ag cores.

the host. Such range of concentrations correlates with interparticle distance from micron down to hundreds of nanometers. It was revealed that small 0.014 mol% concentration of 40Ag@SiO2 particles increases intensity of Mn4+ luminescence, while further growth of Ag@SiO2 content makes luminescence less intensive (Figures 3c, d). Samples of MTO-70Ag series demonstrated similar behavior, however maximum of Mn4+ luminescence was shifted to higher 0.028 mol% concentration (Figures 3e, f). Such behavior can be explained as follows. Smaller Ag@SiO2 particles are locally better concentrated in the MTO host than big ones at the same amount of silver material. It means that smaller particles are less distant in the host than bigger ones. Thus small enough and optimal for enhancement distance between nanoparticles and Mn4+ centers is achieved earlier for smaller particles than for the larger ones at the increase in their concentration. Theoretically it was supposed that linear Oz-polarized electromagnetic wave propagating along Oy axis came to the Ag@SiO2 nanoparticle (Figure 4). Size of Ag core was taken as 40 nm (Figures 4b, c) or 70 nm (Figures 4d-f). Thickness of silica shell was taken as 10 nm. It was assumed that Ag@SiO2 nanoparticle is surrounded by homogeneous dielectric medium with constant refractive index 1.72, which can be associated with MTO host.34

Figure 4. (a) Scheme of light induced electrical polarization in metal nanoparticle. (b) Electric field (E/E0)2 distribution near 40Ag@SiO2 nanoparticle in MTO host (ex=450 nm). (c) Spectra of Mn4+ excitation (red) and plasmonic absorption (blue) for 40Ag@SiO2 nanoparticle in MTO host. (d, e) Quadrupole (ex=420 nm) and dipole (ex=510 nm) electric field (E/E0)2 distributions near 70Ag@SiO2 nanoparticle in MTO host. (f) Spectra of Mn4+ excitation (red) and 70Ag@SiO2 plasmonic absorption (green).

Incident light wave can resonate with oscillations of surface electron plasma in the Ag core at certain resonance frequency (wavelength). Light induced, so-called surface plasmon, electric field spreads outside Ag core and

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ACS Applied Materials & Interfaces influences neighboring Mn4+ ions increasing light absorption for them. Distribution of light induced electric field near sphere-like 40Ag@SiO2 (Figure 4b) and 70Ag@SiO2 (Figures 4d, e) nanoparticles was calculated by means of finite difference frequency domain method.35 Electric field spreads outside the 40Ag@SiO2 nanoparticle and locally concentrates in its vicinity in form of dipole pattern (Figure 4b) at the wavelength 450 nm. This local field stimulates enhanced dipole-associated light absorption both by nanoparticle itself and by its neighborhood. Electric field near 70Ag@SiO2 nanoparticle shows resonance patterns of higher multipoles: quadrupole at 420 (Figure 4d) and dipole at 510 nm (Figure 4e).

(Figures 4c, f). Such overlapping with plasmonic absorption especially at 475 nm wavelength makes good conditions for plasmon assisted enhanced light absorption and, as consequence, light emission by Mn4+ ions. At the same time one can see that 340 nm excitation is spectrally distant from the Ag@SiO2 plasmon resonance bands. Nevertheless, it gives certain enhancement in Mn4+ luminescence. The non-plasmonic reason for such effect can imply sensitization of Mn 4+ centers by silver ions or their clusters. It can be so that a part of silver ions or their clusters penetrated from the surface of silver nanoparticles inside the host and become more close to Mn4+ ions. Silver clusters have such specific light absorption peaks in the ultraviolet spectral range: Ag2 (275, 278, 389 nm), Ag3 (222, 232, 246, 422 nm), Ag4 (286 nm), Ag5 (349, 476 nm).3,37 Since these peaks are overlapped with Mn4+ absorption, it can be possibility for energy transfer from Ag clusters sensitizing Mn4+ fluorescence. Possibility of such sensitization was reported previously for case of optical glasses activated by rare earth ions.3

Field enhancement factor can be estimated from the color of side bar in Figures 4b, d, e visualizing the squared ratio of induced to incident electric fields (E/E0)2, which is proportional to the light absorption value. Field enhancement is in the range 100-200 in the Figure 4b and 50-100 in the Figures 4d, e.

Since Ag@SiO2 nanoparticles act mainly on the stage of Mn4+ excitation, they weakly influence radiative rates or lifetimes of luminescence. Lifetimes of all prepared MTO-

Fluorescence intensity can be described by general equation36 including such factors as light absorption rate , emission yield Yem and excitation transfer yield Ytrans from upper levels to the 2Eg level, from which Mn4+ emission occurs: 𝑰 = 𝒀𝒆𝒎𝒀𝒕𝒓𝒂𝒏𝒔 ,

(1).

Let's consider sequentially each factor in equation (1). Pointed increase of local field (E/E0)2 near the nanoparticle causes growth of , which, as consequence, causes higher value of I in equation (1) for samples with silver nanoparticles. At the same time such field can favour the radiative and depress non-radiative decay channels. It can be the reason for experimentally detected 1.5 increase in Yem value for MTO40Ag014 sample in comparison with the referent MTO (ex=340 nm). It is worth to note also relation36 between fluorescence enhancement coefficient RFL, local field enhancement (E/E0)2, emission yield Yem and transfer yield Ytrans: 𝑬

𝑹𝑭𝑳 = (𝑬 )𝟐 𝒀𝒆𝒎 𝒀𝒕𝒓𝒂𝒏𝒔 𝟎

(2).

Substituting into equation (2) such numbers, 1.5 for RFL, 200 for (E/E0)2 and experimentally measured 0.098 for Yem of MTO-40Ag014 sample, one can estimate value of Ytrans as 0.077 from equation (2). Namely, practical enhancement in Mn4+ luminescence in our case is only 1.5 times, because product of emission and transfer yields is in the range of two decimal places in our case.

Figure 5. (a) Diagram of MTO-40Ag014 luminescence vs. temperature. (a-ii) Mn4+ fluorescence intensity for MTO and MTO-40Ag014 samples vs. temperature (em=660 nm). (aiii) Arrhenius plots for MTO and MTO-40Ag014 samples. Activation energies defined from the slope of the lines are 0.39 eV (black) and 0.46 eV (red). (b) Luminescence spectrum of LED device with MTO-40Ag014 phosphor powder. Photo of prepared device is in the inset. (c) CIE diagram with (0.502, 0.31) point of the MTO-40Ag014 sample emission.

Decrease of Mn4+ luminescence at higher concentrations of Ag@SiO2 nanoparticles can be explained by its re-absorption by electrically interacting more closely packed nanoparticles.33 Indeed, contribution to the optical absorption in the spectral range 400-500 nm is slightly bigger for the samples with higher concentrations of Ag@SiO2 nanoparticles (Figure S2). Here we emphasize that plasmonic spectral bands are overlapped with excitation spectrum of Mg2TiO4:Mn4+

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Ag samples were fitted by mono-exponential curves (Figure S3) giving the lifetimes in the range from 580 to 615 µs which correlates with literature data.18

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Science Foundation funded project (no. 2017M622848) and the International Postdoctoral Exchange Fellowship Program (no. 20180056). We kindly acknowledge Yiqin Xu (Guangdong Institute of Semiconductor Industrial Technology, Guangzhou, P. R. China) for the assistance with preparation of LED device, Dr. Senchuan Huang (School of Chemistry, Sun Yat-Sen University, Guangzhou, P. R. China) for the assistance with electronic microscopy measurements and Dr. Alexander Vanetsev (Institute of Physics, University of Tartu, Estonia) and Dr. Sergii Mamykin (V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine, Kyiv, Ukraine) for the useful discussions related to the preparation of nanoparticles.

LED device constructed on the basis of Mg2TiO4:Mn4+ phosphor with added Ag@SiO2 nanoparticles showed red emission excited by ultraviolet semiconductor chip (Figure 5). Its further improvement, particularly in a sense of thermal stability (Figure 5a), is a subject of our forthcoming work. 4. CONCLUSIONS Red emitting Mg2TiO4:Mn4+ crystalline phosphor powder decorated by preliminary synthesized Ag@SiO2 nanoparticles is prepared. It is demonstrated that excitation of Mn4+ emitters can be increased through more effective light absorption near Ag@SiO2 nanoparticles. Namely, samples with small (onehundredth of mole percent) concentrations of nanoparticles demonstrated up to 1.5 times increase in intensity and quantum yield of Mn4+ emission. In desire to achieve luminescence enhancement it is better to avoid higher (one-tenth of mole percent) concentrations of Ag@SiO2 nanoparticles which cause re-absorption of light and decrease in Mn4+ luminescence.

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ASSOCIATED CONTENT Supporting Information Supporting data are represented as separate file and include such sections: Preparation of samples A. Crystal Mg2TiO4 host activated by Mn4+ ions B. Ag@SiO2 nanoparticles Figures S1-S3 Supplementary references

AUTHOR INFORMATION Correthsponding Author *(L．Zhou) Email: [email protected]; *(M. M. Wu) Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by grants from the Joint Funds of the National Natural Science Foundation of China (NSFC) and Yunnan Province (No. U U1702254) and Guangdong Province (No. U1801253), NSFC (No. 51672315 and No. 21771195), Science and Technology Planning Project of Guangdong Province for Applied Science and Technology Research and Development (Nos.2017B090917001, 2016B090931007, 2015B090927002), the government of Guangzhou city for international joint-project (201704030020). Lei Zhou acknowledges China Postdoctoral

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Table of Contents Graphic We have prepared red emitting Mg2TiO4:Mn4+ crystalline phosphor powder decorated by preliminary synthesized Ag@SiO2 nanoparticles. It is demonstrated that excitation of Mn 4+ emitters can be increased through more effective light absorption near Ag@SiO2 nanoparticles. The intensity and quantum yield of Mn 4+ emission can be up to 1.5 times increased with small (one-hundredth of mole percent) concentrations of nanoparticles.

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Efficient Luminescence Enhancement of Mg2TiO4:Mn4+ Red

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