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DOI: 10.1016/j.jmrt.2018.10.013
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Available online 12 January 2019
Deep insight into the photoluminescent monocrystalline particles: Heat-treatment, structure, mechanisms and mechanics
Gözde Alkana,1,
Corresponding author

Corresponding author.
, HakanYavasb,c,1, Berfu Göksela,1, Lidija Mancicd, Bernd Friedricha, Olivera Milosevicd
a IME – Process Metallurgy and Metal Recycling, RWTH Aachen University, Aachen, Germany
b Department of Mechanical Engineering, Johns Hopkins University, Baltimore 21218, MD, USA
c Faculty of Electrical Engineering, Czech Technical University, KarlovoNamesti 13, Prague 4, Czech Republic
d Institute of Technical Sciences of Serbian Academy of Sciences and Arts, KnezMihailova 35, 11000 Belgrade, Serbia
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Tables (1)
Table 1. Refined structural parameters of the as-prepared and heat treated Ag@Y2O3:Eu3+particles.

The red light emitting down-converting Ag@Y2O3:Eu3+ phosphor particles were synthesized by one-step ultrasonic spray pyrolysis and exposed further to the heat treatment at 1000°C (12h). A detailed investigation on structural and functional properties of the as-prepared and heat treated particles was conducted in a comparative manner. High-resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRPD) and focus ion beam milling (FIB) revealed in a great consistency the poorly crystallized and porous nature of the as-prepared particles. Well-crystallized coarser primary nanocrystals of Y2O3:Eu3+and Ag, which are hierarchically organized in dense spherical Ag@Y2O3:Eu3+ phosphor particles, were obtained through the heat treatment. Along with the change of structural properties, down conversion (red luminescence at 612nm owing to the Eu3+5D07F2 electric dipole transition) and mechanical endurance were enhanced 4-fold and 5-fold via heat treatment, respectively. This comparative study implies a good correlation between mechanical and luminescence behavior of phosphors, both strongly influenced by the particles structural properties.

Rare earth oxides
Silver nanoparticles
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The demand for phosphor materials grows rapidly due to their extensive application in mobile phones, LED‘s, traffic displays and security markers [1–4]. Black banknotes and counterfeiting credit cards are becoming more threading and necessitate high efficient security inks which usually include a fluorescent component that could be activated with a ultraviolet or visible light for straightforward checking [5]. Metal-rare earth oxide nanostructures are considered as promising candidates for security ink applications owing to their superior luminescence efficiency, good corrosion resistance, high chemical and thermal stability [6–8]. Europium-doped yttrium oxide (Y2O3:Eu3+), which is a down-converting phosphor that exhibit red-orange emission at 612nm due to the 5D07F2 electric dipole transition of Eu3+, is an indispensable candidate which possess excellent atmospheric stability, reduced degradation under applied voltages, high quantum efficiency and lack of hazardous constituents (unlike sulphide phosphors) [9,10]. The host lattice manipulation, energy transfer modulation, surface passivation, surface plasmon coupling, broadband sensitization, and photonic crystal engineering were recently reported as possible strategies that could even improve its luminescence response [11]. Among these, surface plasmon coupling has gained most attention due to the fact that noble metal nanoparticles (such as Au, Pt andAg) incorporated into oxide structure or present in close proximity of emitters enhances photoluminescence (PL) efficiency by energy transfer (ET) and local field enhancement (LCE) [12–14].

Ag incorporated Y2O3:Eu3+ synthesized by USP are chosen to be the focus of this study due to the superior PLA (when compared with Y2O3:Eu), as reported in [15]. The plasmon effect observed on the Ag nanoparticles [15] contributed to an increase of the relative luminescence intensity of Eu3+-doped Y2O3 host lattice for about 34%, and could be enhanced more with the control of the particle morphology. There are various methods capable in producing hierarchically structured nanophosphors [16–21], however majority are challenging in terms of their cost efficiency and scale-up potential. Ultrasound assistant spray pyrolysis (USP) method is promising for the production of low-cost nanophosphors. In the USP process, the aerosol droplets experience evaporation/drying, precipitation and thermolysis in a single-step process under extreme synthesis conditions (high droplet/particle heating rate and high surface reactivity) resulting in the formation of spherical, hierarchically organized nanostructured particles through the mechanisms of primary nano crystals coalescence and collision [16,21]. However, due to fast reaction kinetics subsequent heat treatment may be required to improve particles crystallinity [22,23]. Since crystal quality, uniform distribution of constituents and defect concentration have direct influence on PL efficiency [24], and hence on mechanical properties too, it is intriguing to establish a correlation which will reveal uncovered features of PL using scalable and applicable system parameter, i.e. heat-treatment with nanoidentation for designing of the next-generation of efficient phosphors. Nanoindentation enables evaluation of mechanical properties in the small volume providing in such way precise detection of materials elastic and plastic behavior [25,26], which in turn could be correlated with some other property, in this case with the photoluminescence. However, it is worth to mention that measuring the single particle mechanical property is a challenging task which requires extra attention to exclude the contribution of the artificial deformation. There are only a few computational and experimental efforts [27–29] which can be used as guidance but the technical gap still remains.

Microstructural, PL and mechanical properties of as-prepared and heat treated Ag@Y2O3:Eu3+particles synthesized by USP were elucidated in a comparative manner by HRTEM, XRPD, FIB, fluorescence measurement, and nanoindentation to establish an indirect relation between PL and mechanical properties. In brief, we “deeply” investigated structural properties of nanoparticles to reveal links at different length-scales, i.e. from atomic scale to microscale deformation for achieving glamorous PL properties.

2Experimental procedure

Yttrium nitrate (Y(NO3)3·6H2O, Aldrich, purity >99%), europium nitrate (Eu(NO3)3·5H2O, Aldrich, purity >99%) and silver nitrate (AgNO3, AppliChem, purity >98%) commercial precursors were used as yttrium oxide, europium, and silver precursors, respectively. The precursor solutions were prepared by dissolving relative amounts of nitrates, to obtain yttrium oxide doped with 5at.% of Eu and decorated with 5wt.% of Ag, in de-ionized water. Classical ultrasonic spray pyrolysis (USP) production line comprised ultrasound generator (1.7MHz, Gapusol, RBI, France), a carrier gas (air) inlet connected over a flow regulator (1.5l/min), horizontal wall heated furnace with a quartz tube and washing bottles for the collection of nanoparticles. Details of the experimental setup can be found elsewhere [30]. The precursoris atomized and carried by air to a heating zone, set at target temperature previously to experience evaporation, precipitation and thermal decomposition in a single-step. Synthesized product was exposed to post heat treatment at 1000°C for 12h in inert argon ambient to prevent the oxidation of silver.

The crystal structure of powders was elucidated by X-ray powder diffraction (XRPD) (Rigaku, 40kV/40mA). The patterns were recorded from 10° to 80° with a step scan of 0.02°/min and accounting time of 5s per step. Refined structural data were evaluated through Rietveld analysis using Topas 4.2. software and starting from ICSD data 086813 and 044387 (for Y2O3 and Ag, respectively). The average crystallite size was calculated using volume weighted mean column height broadening modeled by a Voigt function. Conventional and High-resolution transmission electron microscopy (HRTEM) Jeol JEM-2100, Tecnai 20-F were used to reveal morphologic features and details of the microstructure. The chemical composition was determined using a Jeol JED-2300 Series energy 90 dispersive spectrometer (EDS). In addition, Gatan Digital Micrograph TM software was used to perform Fast Fourier Transformation (FFT) of the high-resolution transmission electron microscopy images (HRTEM) and confirmation of the crystal structure. In addition to the HRTEM analyses, we also performed scanning transmission electron microscopy (STEM) then cut the cross-sections of particles via Ga focused ion beam (FEI Helios Dual Beam STEM, CA) to reveal microstructural changes after heat-treatment. To do that, first, we choose an isolated particle to prevent plausible interactions with the neighbor particles and applied ion beam cross-cut at 52˚ with a 30kV accelerating voltage and 0.79nA ion current. The photoluminescence spectra of the particles were recorded at room temperature by the Fluorescence Spectrophotometer FP-8600 (JASCO) with a scan rate of 200nm/min and an excitation wavelength of 230nm.

Nanoindentation load-sensitive experiments were performed with iNano (Nanomechanics Inc., TN) and Hysitron TI 950 (Hysitron Inc., MN) nanoindenters with a diamond Berkovich type. It should be noted that to eliminate possible oscillation based uncertainties, i.e. woodpecker effect, the dynamic force oscillation was disabled first and then indentations were carried out following quasi-static loading protocols. Prior to the indentations, an AFM like piezo-automated imaging mode was turned on to detect exact locations of particles with a scan rate of 1Hz and 60μm/s tip velocity. During the indentation, a constant strain rate loading was applied while keeping the depth at 500nm. For the same depth level, each indent (total of 15 indentations) is displaced by 5 microns in all directions. Thermal drift was digitally monitored for the 20s prior to each indentation and kept lower than 0.1nm/s. All tests were performed at ambient conditions.

3Results and discussion

Fig. 1a and b reveals low-resolution TEM (LRTEM) images of as prepared and heat-treated particles. Both samples exhibit typical USP spherical morphology assembled by fine primary crystals through collision/coalescence mechanism, as reported previously [22]. In as prepared particles, represented in Fig. 1a, a certain amount of porosity is detected, which is again consistent with previous studies. Since USP is a process with high heating/cooling rates (500°C/s) and short reaction times (less than 1s), reaction time does not allow completion of solid particle formation resulting in the porous microstructure. However, through the heat treatment, when extra time and heat provided for diffusion, the growth of primary particles results in the denser microstructure, as revealed in Fig. 1b.

Fig. 1.

LRTEM micrographs of (a) as prepared, (b) heat treated samples and HRTEM micrographs with FFT analyses of (c) and (d), (e) EDX analyses of the as-prepared sample, (f) X-ray powder diffraction patterns of as-synthesized and heat treated samples in which Ag related (111) plane is marked with red *.


During the formation of multicomponent structures via USP, all constituents experience evaporation, precipitation, and thermal decomposition simultaneously toward particle formation. Since europium and yttrium nitrate precursors have a low melting point (around 50°C) and similar thermal decomposition mechanisms, their highly uniform distribution were achieved in the particles. However, AgNO3 starts to experience thermal decomposition after these two constituents. Due to the differences in lattice parameters among the silver and the mentioned components and the consecutive precipitation, silver does not incorporate into the lattice of yttrium oxide, but rather precipitates on the surface of either primary or secondary Y2O3:Eu3+ particles forming a nanocomposite structure that might exhibit a plasmon effect on luminescence. Details of the formation mechanism of the multicomponent hierarchical systems obtained by USP can be found elsewhere [31]. Fig. 1 clearly supports the above statements. HRTEM images given in Fig. 1c and d highlights the details of both microstructures. Moiré like fringes and poorly organized structure were observed within the as-prepared particles, while after heat treatment well-crystalline particles free of defects were obtained. FFT analysis helps in elucidation of the crystal structure in selected regions. As one could see, both regions (insets in Fig. 1c, d) exhibited crystal planes of Y2O3 phase (JCPDS card 89-5591). However, EDX analysis (Fig. 1e) of the selected area imply the presence of silver inside the secondary as-prepared particle, presumably distributed either in between or on the surface of primary ytria particles. XRPD patterns of both samples presented in Fig. 1f additionally confirm HRTEM findings. Poorly crystallized as-prepared particles were transferred into particles with a higher crystallinity, as it is implied by sharper diffraction lines with a higher intensity in upper diffractogram. In the same diffractogram it is possible to observe one of the most intense Ag diffraction line, (111), suggesting lack of the Ag crystallization in the as-prepared samples. In order to confirm its presence in the as-prepared particles, EDX was performed. Structural parameters of both samples evaluated by Rietveld refinement are listed in Table 1.

Table 1.

Refined structural parameters of the as-prepared and heat treated Ag@Y2O3:Eu3+particles.

  As-prepared  Heat treated 
Crystal structure  Cubic, Ia-3  Cubic, Ia-3 
Unit cell parameter a (Å)  10.6248(5)  10.6193(1) 
Crystallite size (nm)  16.5(3)  44.5(6) 
OccY1 (C2)a
OccY2 (S6)a 
Strain  0.265(6)  0.090(2) 
RBragg  1.962  1.123 
GoF  1.266  1.148 
RwP  7.884  6.456 
Y1:O (Å)  2.2101(76)  2.2119(36) 
Y2:O (Å)  2.3141(77)  2.2938(37) 

Occupation of C2 and S6 positions with Eu3+could be calculated as 1-OccY1 and 1-OccY2, respectively.

Presented data implies that both samples crystallized in the BCC cubic structure with the space group la-3, JCPDS card 89-5591 [32]. Increased lattice parameters of the Y2O3:Eu3+phase detected in both samples implies that Eu3+partially replaced Y3+ ions in the crystal lattice (lattice parameter of Y2O3 is 10.60Å, ionic radii of Eu3+and Y3+ ions are 0.947 and 0.90Å respectively). Better and more homogeneous accommodation of Eu3+ ions are obtained in the heat treated particles. This is certain from the observed changes of following parameters: lattice parameter and microstrain decreased as a consequence of structural defect diminishing while equalization of the C2 and the S6 occupancy with the Y3+ ions is followed with a change of the Y-O bonds. Heat treatment provokes additional change in the crystallite size from 16.5 to 44.5nm. Better particle crystallinity bring on microstrain reducing for more than 3 times. As it will be shown further all these affect luminescence emission efficiency through diminishing effect of quenching in Ag@Y2O3:Eu3+ heat treated sample.

Fig. 2 shows the averaged load–responses of the as-prepared and heat treated samples. The inset presented in Fig. 2 demonstrates a representative piezo-automation image of indentation at the single particle. It is worth to mention that for the conventional nanoindentation, obtained hysteresis comprises three distinct regions: (i) loading, where the deformation is controlled by both elasticity and plasticity; (ii) dwell period/creep region; and (iii) elastic unloading. Since inter/intra-granular nature of the deformation might arise uncertainties during the unloading [25,26] we focused on the region i: level deformation loading in elasto-plastic region. At that stage, the resistive energy of the material will stop the motion of the nanoindenter tip at the predefined depth to underline the current mechanical state of the material. At this point, one can quantitatively extrapolate that the material with higher hardness will show superior load–response in comparison to its soft counterpart. As can be seen in Fig. 2, after particles heat treatment their load–response increased from 0.6 to 3.8mN. Approximately 6 times higher load response obtained is a consequence of the better crystallinity, strain-free lattice and denser structure of the heat treated particle, confirmed through HRTEM and XRPD analyses.

Fig. 2.

Nanoindentation load responses of the as-prepared and heat prepared particles. The inset shows the way of applying single particle indentation using the piezo-scanning method.


Fig. 3a reveals excitation spectra of as prepared and heat treated samples, in a comparative manner. Due to the relatively lower excitation intensity of the as-prepared sample, its magnified view is given as inset in Fig. 3a. Y3+–O2− excitonic band at 210nm and Eu3+–O2− charge transfer band at 250nm (related to Eu3+ at C2 position) are present in both spectra and are characteristic of the Eu3+ doped Y2O3 phase. The dramatic increase of the latter in the heat treated sample indicates an increasing rate of energy transfer between excited Eu3+ ions in C2 and S6 positions. The course of this transition is revealed through deconvolution and integration of the second peak in both spectra and obtained data implies that with heat treating Eu3+ ions migrate from S6 to C2, as it was shown through Rietveld refinement of the XRPD. It should be also noted that in the spectrum of the as-prepared sample additional band is visible at 314nm. This may be due to the excitation of Ag or due to the 4f–f transition of Eu3+[33,34]. Fig. 3b represents the emission spectra of both samples excited at 230nm. Typical red-light emission peak located at 612nm is due to 5D07F2transition of Eu3+ ions locating at the C2 site. Since this peak is hypersensitive to local environment changes, it was considered as a comparison criterion between as prepared and heat-treatedsample which shows an approximately five-fold increase in emission intensity of the latter one. This enhancement in emission is in agreement with the improvement of the structural and specific crystallographic site occupation analysis. Since one of the most important properties of a phosphor is the purity of emitted color, CIE chromaticity coordinates X and Y were calculated for both samples, Fig. 3b. The chromaticity coordinates of (0.27, 0.21) and (0.64, 0.33) were obtained for the as prepared and heat treated samples, respectively. It is notable that color coordinates change dramatically with the heat treatment approaching the ideal red chromaticity defined by (0.67, 0.33) [35]. This indicates the feasibility of the USP synthesized particles to heat treatment for the target red-light-emitting phosphor applications.

Fig. 3.

(a) Excitation spectra of samples, (b) emission spectra of samples excited by 230nm and (c) corresponding CIE diagram revealing the visible color output.


To correlate the mechanical and luminescence properties of Y2O3:Eu3+@Ag nanocomposite powders, structural data were used, as given in Fig. 4. As it was mentioned earlier during USP processing, droplet/particle residence time in the reaction zone was less than second, more precise 0.4s. A high-temperature gradient was formed in the droplet instantly preventing the release of the gaseous product from decomposed precursor creating in that way porous particle structure or even the broken ones. Extremely short reaction time provokes nucleation of many interconnected primary particles with a poor crystallinity in a secondary spherical one. All that result in low mechanical endurance and weak luminescence emission. When additional heat treatment is applied, more uniform distribution of europium ion in Y2O3 matrix and almost complete neutralization of stress led to improving maximum load (5 fold) and red light emission (4 fold). In Fig. 4, the detailed microstructural analyses of the sample before (left) and after (right) heat treatment are presented revealing information from the particles inner structure. It highlights the fact that secondary spheres are really assembled from primary nanocrystals which size is consistent with ones obtained through Rietveld refinement. Porosity present in the as-prepared particle (left) disappeared during annealing, so dense spherical particles composed form well crystallized primary ones were formed (right). Interconnected crystal islands which assemble the heat treated particle are responsible for the improved load response. Moreover, energy dispersive X-ray spectroscopy revealed a silver distribution on the surface of the primary Eu3+-doped Y2O3 particles, amplifying their plasmonic effect on the luminescence.

Fig. 4.

Correlation of emission intensity, crystallinity, and mechanical properties.


In this study, the effect of heat treatment on hierarchically organized Y2O3:Eu3+@Ag nanoparticles and their structural, mechanical and luminescence properties were reported. Local deformation characteristics and detailed structural analyses were correlated successfully with the photoluminescence efficiency of the particles. Detailed structural characterization revealed that poorly crystallized porous as-prepared particles obtained through USP were converted into highly crystallized dense ones with a uniform distribution of all constituents through additional heat treatment. Such particles emit red light which CIE coordinates (0.63, 0.33) are close to the pure red light (0.67, 0.33) and are characterized with the 5 fold enhancement in mechanical strength, so could be considered as an advanced phosphor nanocomposites that could be used in security ink applications.

Conflicts of interest

The authors declare no conflicts of interest.


Authors would like to thank CEITEC Research Infrastructure in Brno (CZ) and Johns Hopkins University (Maryland, USA) – Mechanical Engineering Department for the help with FIB and nanoindentation. We also like to thank Friedrich Rosen for his technical support in the laboratory and to Profs. Rebeka Rudolf, Satoshi Ohara, Sayaka Tamura, Koji Tomita, Zhenquan Tan, Fefei Sun and Rebeka Rudolf for their support in characterization. The support of Ministry of Education, Science and Technological Development, Republic of Serbia (Project #172035) is also kindly acknowledged.

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These authors contributed equally to this work.

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