The study of composition, structure and cathodoluminescent features of YAG:Eu3+ nanoceramics. Excitation capture efficiency of Eu3+ energy levels

Journal Pre-proof The study of composition, structure and cathodoluminescent features of YAG:Eu 3+ nanoceramics. Excitation capture efficiency of Eu energy levels

3+

Kseniia Orekhova, Robert Tomala, Maria Zamoryanskaya PII:

S0925-8388(20)34095-0

DOI:

https://doi.org/10.1016/j.jallcom.2020.157731

Reference:

JALCOM 157731

To appear in:

Journal of Alloys and Compounds

Received Date: 7 August 2020 Revised Date:

15 October 2020

Accepted Date: 25 October 2020

Please cite this article as: K. Orekhova, R. Tomala, M. Zamoryanskaya, The study of composition, 3+ structure and cathodoluminescent features of YAG:Eu nanoceramics. Excitation capture efficiency 3+ of Eu energy levels, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.157731. 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 Kseniia Orekhova: Conceptualization, Investigation, Writing - Original Draft. Robert Tomala: Resources, Investigation, Writing - Review & Editing.

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Maria Zamoryanskaya: Methodology, Writing - Review & Editing, Supervision.

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The study of composition, structure and cathodoluminescent features of YAG:Eu3+ nanoceramics. Excitation capture efficiency of Eu3+ energy levels.

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Kseniia Orekhovaa),*, Robert Tomalab), Maria Zamoryanskayaa)

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a)

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b) Institute of Low Temperature and Structure Research, ul. Okolna 2, 50-422 Wroclaw, Poland

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*[email protected]

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ABSTRACT

Ioffe Institute, 26 Politekhnicheskaya st., St Petersburg 194021, Russian Federation

The focus of this research paper is upon cathodoluminescent features of YAG:Eu3+ nanoceramics. The concentration series of samples was synthesized from nano powders using low temperature high pressure sintering technique. The composition of the samples has been studied using electron-probe microanalysis. The structure and the grain size of the samples have been studied by X-ray diffraction, scanning electron microscopy and electron backscattered diffraction. Cathodoluminescent study of the samples has been performed. The concentration dependence of cathodoluminescence intensity and kinetic dependencies of CL bands have been obtained. The excitation capture efficiency of the radiative energy levels was estimated based on cathodoluminescence intensity rise rate dependence on electron beam current density. Energy transfer mechanism between the energy levels will be discussed hereinafter.

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Keywords

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YAG:Eu; Cathodoluminescence; Excitation capture efficiency; Energy level diagram; Nanoceramics.

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1. Introduction

Yttrium aluminum garnet (YAG) single crystals as well as ceramics doped with trivalent rare earth ions (RE3+) are well-known materials for a number of optical applications – solid state laser media, scintillators, phosphors, LEDs, etc. [1-5]. Such materials demonstrate good luminescent properties in a wide optical range (from UV to IR) under photo-, X-ray and highenergy electron excitation [6-10]. For the last few decades, starting from the pioneer work on YAG:Nd3+ transparent ceramics lasing performance [11], polycrystalline YAG materials attract much more attention in scientific society due to their lower production costs, less physical limitation of samples, the possibility of doping profile with the desired characteristics design, mass-production, short preparation period, etc. [12-14]. All these extend the potential of the material in the optical and scintillator applications and bring about new researches on YAG:RE3+ ceramics optical, structural and luminescent characteristics.

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Eu3+ ion demonstrates strong red luminescence. For this reason, it is commonly used as a luminescent probe, and its spectra have been well examined in various structural modifications of oxide materials [15-20]. The luminescence spectrum of rare earth Eu3+ ion consists of a set of narrow bands corresponding to transitions inside the screened f-shell. Screening reduces the influence of the matrix crystal field on the f-shell electrons, because of which the Eu3+ bands only slightly change their positions in different matrices [21-23]. All this makes it possible to unambiguously interpret the luminescence spectra of Eu3+ in different materials.

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YAG:Eu3+ has been intensively investigated in the form of single crystals [24], ceramics [25] and nanocrystals [26, 27]. Due to properties of the matrix, such as large solubility of the rareearth ions, chemical and thermal stability [24], the possibility of synthesis of nonagglomerated, spherical particles with narrow size distribution [27], the YAG:Eu exhibits potential to be used as phosphor material in cathode ray tubes (CRT), field emission displays (FED) and vacuum fluorescent display (VFD). As reported, the synthesis method and morphology of YAG:Eu strongly affect its luminescent properties. Kiyokava et al. report the anomalous temperature dependence of YAG:Eu crystals and potential to use it as fluorescence thermometer for high temperature range [24]. Upasani [28] reports that the dopant of silica enhances the photoluminescence intensity of Eu3+ ions, and decreases the crystallization temperature. Kolesnikov et al [29], report that the emission intensity of YAG:Eu is strongly dependent on europium ions concentration and that the matrix allows to incorporate high concentration of Eu3+ ions. As reported by Bekhmetyev et al. [30], the surface of the YAG:Eu phosphor allows to achieve functionalization of photodynamic cancer therapy. All these works are devoted to the study of YAG:Eu3+ luminescent properties in the red light optical range (around 600 nm).

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When YAG:Eu is irradiated with high-energy excitation (electron-beam, X-Ray or VUV irradiation), the bands corresponding to the transitions from high energy levels emerge in the blue and UV region of the luminescent spectra. For example, the host-associated luminescence of YAG or the bands corresponding to the transitions from 5D3 energy level of Eu3+(located approximately at 24390 cm-1above the ground state [31]) in YAG:Eu are usually not observed in steady-state photoluminescence (PL). With the use of cathodoluminescence (CL) technique not only is it possible to excite the high-energy transitions and study their kinetic characteristics, but also study the excitation capture efficiency [32] of each radiative level.

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The physical mechanism of scintillation process can be represented by consequent stages as follows: absorption of the radiation and creation of primary electrons; relaxation of the primary electrons and formation of numerous secondary electrons, holes, photons, plasmons and other electronic excitations; thermalization of the low-energy (10-50 eV) secondary electrons resulting in the electron-hole pairs (with energy ≈ equal to the bandgap) formation; energy transfer from electron-hole pairs to the luminescence centers and their excitation; emission from the luminescence centers [33].

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The processes of the energy transfer to the luminescence centers and their excitation play the primary role in scintillation. The mechanisms of the energy levels excitation and the energy transfer between them are continuously studied as they are unique for each material and depend on the synthesis method, the quality of the matrix, the presence of defect energy levels that may not emit but contribute to the excitation energy transfer, etc. The study of the excitation capture efficiency of different energy levels (including high energy levels) of the luminescent centers enhances the understanding of high energy interaction with the material processes and can significantly contribute to the improvement of existing scintillators and development of novel scintillator materials.

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The objective of the work was to synthesize YAG:Eu3+ nanoceramics series with different Eu3+ concentrations and to study the structural and luminescent properties of the nanoceramics samples – CL spectra, the kinetics of CL bands (decay and rise times), the intensity concentration dependences of CL bands. The main aim of the work was to investigate the excitation capture efficiency of various radiative levels in the material.

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2. Samples synthesis and methods of study

The YAG doped Eu3+ nanoceramics samples were prepared using low temperature high pressure (LTHP) sintering technique [34, 35]. Powders used as the starting material for nanoceramics fabrication were prepared by a modified Pechini method [36]. Stoichiometric amounts of yttrium oxide (99.999%) and europium oxide (99.999%) were dissolved in an aqueous solution of nitric acid. Then, the citric acid (in molar ratio of 1:5 in respect of cations) and ethylene glycol (in molar ratio of 1:1 in respect of citric acid) were added to aqueous mixture of stoichiometric amount of aluminum chloride (99.99%), and nitrates of yttrium and europium. The mixture was being stirred for one hour. Then the solution was being heated at 90°C for 1 week to extract brown resin. At the final stage, dried resin was being heated at 1200°C for 16 h. Afterwards, the powder was pressed into 5 mm diameter pellet using hand press (50 MPa), and placed in the toroid-type container. The parameters of sintering were 8 GPa and 500°C being applied for 1 min (plus 1 min for raising the temperature and 1 min for cooling time).

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The composition of the samples was studied with electron probe microanalysis (EPMA) using the electron-probe microanalyzer Camebax (Cameca) equipped with four wavelength dispersive X-ray spectrometers. A carbon film was deposited upon the surface of samples using the JEE-4C JEOL vacuum universal post to provide the electric conductivity.

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The CL properties of the samples were studied with the use of the same electron-probe microanalyzer equipped with the optical spectrometer of original construction [37] that was installed onto the optical port of the microanalyzer. The spectrometer covered the optical range from 200 to 800 nm and provided the spectral resolution of 0.1 nm. In order to obtain the kinetics of the luminescent bands the beam was deflected with adjusted frequency. The duty cycle, the pulse duration and the afterglow registration duration were programable. The technique allowed measuring the decay and rise times of CL with the time resolution of 100 ns, which was sufficient to measure forbidden transitions in Eu3+ spectrum. The homogeneity of the samples was studied in an optical microscope as well as by EPMA and CL methods. It was shown that all samples were homogeneous in the composition and the luminescent properties.

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The average size of grains in YAG:Eu nanoceramics was estimated using the Rietveld refinement technique XRD data collected on Panalytical X’PERT PRO diffractometer with Cu Kα1: 1.54060 Å source.

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Scanning electron microscopy (SEM) and Electron backscattered diffraction (EBSD) techniques were used to estimate the structural parameters of the samples – their morphology and the average grain size. The studies were performed using the Scanning Electron Microscope LYRA3 (TESCAN).

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3. Experimental results

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3.1

Composition

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The measured composition coincided with the calculated one with an accuracy of up to 10 rel%. The samples of YAG:Eu3+ nanoceramics with measured Eu concentrations are listed in Table 1.

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Table 1. The concentration of Eu3+ in YAG:Eu3+ nanoceramics samples. Calculated concentration of Eu in the initial precursor, at.% by Y 0.1

Eu concentration in YAG:Eu3+ nanoceramics samples, at.% by Y (measured by EPMA) 0.1±0.05

0.3 0.5 0.7 1 2 4

0.3±0.05 0.5±0.05 0.7±0.1 1±0.1 1.9±0.1 3.6±0.2

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3.2

Structural parameters

The structural parameters of YAG:Eu nanoceramics sample doped with 1.9 at.% of Eu3+ (YAG:1.9%Eu) were obtained. It was assumed that the samples sintered in the same conditions have the same grain and coherent scattering region size as the temperature of sintering and the pressure remained the same.

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The position of diffraction peaks of the sample coincided with The Inorganic Crystal Structure Database (ICSD) card #01-072-1315 corresponded to YAG crystal, and their halfwidth was noticeably higher, which allowed to estimate the average size of the coherent scattering region in the sample. It was estimated as 45±5 nm, X-ray diffraction (XRD) data is presented in Fig. 1.

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The average grain size was estimated by SEM and EBSD techniques with the following parameters of the electron beam – the energy of 10 keV and the current of 0.1 – 2 nA. The SEM image of YAG:1.9%Eu nanoceramics sample surface is presented in Fig. 2.

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The average grain size obtained by analyzing SEM image of the sample was estimated as 50±10 nm. The microscopic morphology and the size of the YAG:Eu nanoceramics grains have been estimated as rather uniformed according to the SEM image analysis.

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The EBSD study required smooth surface of the sample, hence the samples were additionally ion-treated with an ion-etching unit SEMPrep2 (Technoorg-Linda Co Ltd.) at the following conditions: the Ar+ ions voltage of 10 kV, the etching time of 1 hour. Preparing the sample this way is necessary to smooth the surface relief as the EBSD method provides information on the crystal structure of the first several tens of atomic monolayers (about 2050 nm), and the surface relief noticeably affects the quality of the information obtained – backscattered electrons from “lower” (or “deeper” relative to the detector) surface areas do not reach the detector and are displayed as “zero solutions” – areas without a crystal structure on the EBSD images).

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The EBSD surface map of the YAG:1.9%Eu nanoceramics sample obtained using Aztec software is shown in Fig.2 (inset). The EBSD surface map represents the image of the sample grains from which it was possible to obtain the diffraction patterns (highlighted in red). The diffraction of all grains corresponded to the Y3Al5O12 phase, and no other phases have been found. It was not possible to obtain the diffraction patterns from the rest of the surface (highlighted in dark) due to the sample relief. Additional surface treatment was required in order to obtain a more informative picture.

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Analysis of the EBSD surface maps of YAG:1.9%Eu nanoceramics sample showed that the average circle equivalent diameter [38] corresponded to the grain size of 50±10 nm in the sample.

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Therefore, the average grain size determined using these methods coincided and was estimated as 50±10 nm.

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3.3

CL spectra

The CL studies were performed with the following parameters of the electron beam: the energy set to 20keV was aimed to provide the deep electron beam penetration inside the sample (about 1.5 µm) and, consequently, minimize the influence of contamination film formed under electron beam [39]; the diameter was 5 µm to obtain the CL spectra and 50 µm to obtain the CL images; the current was set to 10 nA in order to obtain the CL spectra, and would vary from 0.01 nA to 0.1 nA in kinetic measurements. The CL spectra presented in the paper have been corrected with respect to the spectrometer sensitivity.

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The CL spectra of all studied samples are presented in Fig. 3. The spectra were interpreted according to [31, 40]. Such spectra are typical of Eu3+ in YAG, where Eu3+ substitutes Y3+ at D2 site with no inversion symmetry [41, 42]. A broad band centered at about 380 nm was observed in the samples with low Eu3+ concentration (0.1 – 1 at.%). This band is referred to YAG matrix defects [43, 44] and it is shifted to the lower energies (higher wavelengths) compared to its position in YAG:RE single crystals, as it was already reported in [45, 46]. The CL bands corresponding to the high energy transitions from 5D2, 5D3 and 5L6 energy levels were observed in the spectra. No information on PL of these energy levels in YAG:Eu3+ had been found in literature. The energy level diagram of YAG:Eu3+ nanoceramics is presented in Fig. 4. Table 2 presents the radiative transitions wavelengths with the corresponding CL bands in nm.

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Table 2. The position of the CL bands of Eu3+ transitions in YAG.

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L6 — 7FJ

D3 — 7F0 D3 — 7F1

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Wavelength, nm (± 0.1 nm)

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D3 — 7F2

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D3 — 7F3

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D0 — 7F2

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D0 — 7F3 D1 — 7F5 D0 — 7F4 D0 — 7F5

The CL image of the samples is presented in Fig. 5. It illustrates how the CL color changes with the increase of Eu3+ concentration at the same electron beam current density. The change in the CL color results from a different ratio of the CL bands of different ranges in the samples – the “blue” wide defect band and the “red” 5D0 — 7FN intensive bands.

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The concentration dependencies of the most intensive bands of the transitions from each energy level and the defect band were plotted (see Fig. 6). It was shown that the CL intensity of the YAG defect band and the band corresponding to 5D0-7F1 energy transition are inversely changing with the increase of Eu3+ concentration. This may point to the energy transfer between these energy levels. The concentration maximum of the most intensive CL band corresponding to the transition from 5D0 energy level was observed in the YAG:1.9%Eu sample.

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CL kinetics

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To study the kinetics of the transitions the sample with maximum CL intensity, YAG:1.9%Eu, was chosen. The decay times of the transitions occurring from each energy level were measured at different current density values. The decay times showed independence from the current density of the electron beam, they are presented in Table 3. The decay time of the defect band was measured for sample with 0.1 at.% of Eu by Y with the highest intensity of this band.

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Table 3. The decay times of the transitions in sample 6.

Transition (wavelength)

defect band (400 nm)

Decay time

0.9±0.2 µs

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L6 — 7F1 (406 nm)

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D3 — 7F3 (437 nm)

4±0.3 µs

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D2 — 7F1 (514 nm)

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D1 — 7F2 (551 nm)

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D0 — 7F1 (591 nm)

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D0 — 7F4 (711 nm)

5±0.1 µs

15±0.3 µs 2.3±0.1 ms 2.3±0.1 ms

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The decay times of all measured transitions had single exponential approximation and were shorter than the decay times in YAG:Eu single crystal [47]. It can be explained with higher rate of nonradiative energy losses in nanoceramics due to the presence of such defects as pores, grain boundaries, etc. The decay times of two transitions from the same energy level 5 D0 — 7F1 and 5D0 — 7F4 were measured to prove that the decay time of all CL bands corresponding to the transitions from the same energy level was equal. 3.5

Excitation capture efficiency upon electron beam irradiation

The behavior of intracenter transitions in wide gap materials is well characterized by a twolevel model described in [48 - 51].

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According to this model, the luminescence intensity of intracenter transition is proportional to the content of excited luminescent centers . The rate of the excited centers quantity

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change

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spontaneous emission. The process of excited centers quantity change can be described with the following kinetic equation.

is determined by two processes: the capture of the excitation and the

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where and stand for the quantity of the unexcited and excited luminescence centers, respectively; is the current density of the primary electron beam (in case of electron beam is the decay rate, [ms-1], and is the CL excitation capture efficiency excitation, [nA µm2]; of the radiative level coefficient, which represents the number of radiative level excitation acts per 1 ms upon excitation by electron beam with the density of 1 nA/µm2 [µm2ms-1nA-1]. Provided that the total quantity of luminescence centers = + , and at the initial moment of time the quantity of excited centers is zero ( = 0), the solution of the equation and, accordingly, the (1) gives the following dependence of the number of excited centers CL intensity ~ at the moment of the beginning of the excitation: ~

=

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(2),

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=

where is the CL decay rate, and ( + )= #$%& – the CL rise rate. The rise and decay rates are inversed decay and rise times of the CL bands, which are determined by approximating the obtained kinetic curves. The excitation capture efficiency coefficient is independent from the concentration of the luminescent centers. It can be determined from the dependences of the CL rise rate #$%& on the electron beam current density .

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The excitation capture efficiency of the radiative energy levels was estimated based on the CL rise rate dependence on the electron beam current density. The rise times of all transitions showed single exponential decomposition except for the transitions occurring from 5 D0 energy level (Fig. 7). Depending on the beam current density, the CL intensity rise of these transitions had double exponential character with indices about 0.6-1.5 ms and 50200 µs respectively. This particular behavior could be explained by 5D0 excitation capture from two different sources with different time dependencies, i.e. the processes of the electron-hole recombination on the Eu3+ ion and the energy transfer from defect energy level. It has been assumed that the YAG matrix defect energy level accounts for the most probable additional excitation source, which is due to the possible energy transfer between these levels according to the concentration dependencies. For further studies, the ms rise time duration was selected as it was close to Eu3+ 5D0 energy level decay time. Additional studies are required to explain double exponential character of 5D0 Eu3+ rise time deconvolution.

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An example of the rise rate dependence on the electron beam current density is shown in Fig. 8. The dependence is linear, the slope of the dependence corresponds to the excitation capture efficiency coefficient and its intercept coincides with the decay rate of the transition with an error of 10 rel%.

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Table 4 contains summary data on the excitation capture efficiency coefficient of different Eu3+ energy levels in the sample of YAG:1.9%Eu nanoceramics with maximum CL intensity and additionally 5D0 Eu3+ transition in samples with 0.1 and 0.3 at% of Eu by Y. This was carried out to prove the thesis of excitation capture efficiency independence on the activator concentration.

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Table 4. Excitation capture efficiency coefficient nanoceramics.

Energy level, cm-1 5 D3 5 D2 5 D1 5 D0

1.9 at.% of Eu by Y 215000±30000 120000±20000 165000±25000 1300±70

of various Eu3+ energy levels in YAG:Eu

L, µm2ms-1nA-1 0.1 at.% of Eu by Y 1500±125

0.3 at.% of Eu by Y 1120±30

264 The dependence of the excitation capture efficiency coefficient on the energy of the radiative levels in YAG:1.9%Eu nanoceramics sample was plotted according to the obtained data (Fig. 9).

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As can be seen in Fig. 9, the excitation capture coefficient value increases monotonically with the increase of the energy for all levels except for 5D1 level, accounting for the measurement error. This can be explained by the fact that the closer the level is to the conduction band, the greater its probability of the excitation capture. A similar situation has been observed in the YAG:Eu3+ single crystal excitation capture efficiency study [32]. From general considerations, the excitation capture efficiency coefficient of Eu3+ energy levels should correlate with their absorption, as it reflects the ability of the level to absorb the excitation. According to the literature data on Eu3+ energy levels absorption in YAG [52, 53], the absorption intensities of 5D1 and 5D3 energy levels are comparable, and the absorption intensity of 5D2 energy level is lower.

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4. Conclusions

The study of the structural and CL properties of YAG:Eu nanoceramics with different Eu3+ concentrations sintered using LTHP technique was performed. The CL spectra of the samples were obtained, whereas the CL bands corresponding to high energy levels of Eu3+ (5L6, 5D3, 5D2) have been observed in the blue spectral region. The wide band corresponding to the defects in YAG matrix has been observed in the CL spectra of the samples with low Eu3+ concentration. The concentration dependence of the most intensive CL bands of each Eu3+ transition and the defect YAG band points to the possible energy transfer between the defect band and 5D0 Eu3+ energy level. The further study is required for a more detailed interpretation of the energy transfer mechanisms.

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A technique for determination of the excitation capture efficiency was applied to YAG:Eu nanoceramics. The excitation capture efficiency coefficient has proved to be independent from the concentration of the activator. It has been shown that the excitation capture efficiency coefficient of 5D0, 5D2 and 5D3 Eu3+ energy levels depends linearly on the energy of the level, and distracts from the linear dependency for 5D1 energy level. The obtained information on the kinetics of CL may be used for further development of luminescent schemes including energy transfer between high energy levels in novel scintillator and cathodoluminophore materials based on YAG:Eu ceramics.

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The development of YAG:Eu nanoceramics LTHP sintering technique will allow using it as a commercial scintillation material. The method of production is reproducible, fast and, in opposite to single crystal, low cost.

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Acknowledgements

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Authors thank Dr. Andrey Kudryavtsev from Tescan Ltd. Demo Laboratory (Saint Petersburg, Russia) for his assistance in the SEM and EBSD studies. Authors are also grateful to Mr. Vlad Kravets form Ioffe Institute (Saint Petersburg, Russia), who helped to interpret the high energy range of the CL spectra. References

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Figure captions

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Fig. 1.The XRD pattern of YAG:1.9%Eu nanoceramics sample

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Fig. 2. The SEM image of YAG:1.9%Eu nanoceramics sample. The EBSD surface map of

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YAG:1.9%Eu nanoceramics sample (inset).

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Fig. 3. CL spectra of samples

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Fig. 4. The energy level diagram of YAG:Eu3+ nanoceramics.

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Fig. 5. CL image of samples.

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Fig. 6. The CL intensity concentration dependences of the most intensive bands of the transitions

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from each energy level and the defect band.

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Fig. 7. The example of the CL intensity rise time dependences of various Eu3+ energy levels.

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Fig. 8. The dependence of 591 nm CL band corresponding to the transition from 5D0 Eu3+ energy level

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rise rate on the electron beam current density.

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Fig. 9. The dependence of the excitation capture efficiency coefficient level in the sample of YAG:1.9%Eu nanoceramics.

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on the energy of the radiative

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Highlights

• YAG:Eu3+ nanoceramics structural and cathodoluminescent features were investigated •

Excitation capture efficiency of various Eu3+ energy levels was estimated

• Energy transfer between YAG defect band and 5D0 Eu3+ energy levels was discussed

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• YAG:Eu3+ nanoceramics is promising to be used as a cathodoluminophore material

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: