Surface Plasmon Enhancement of Eu3+ Emission Intensity in LaPO4/Ag Nanoparticles

A promising way to improve the performance of luminescent materials is to combine them with noble metal nanoparticles. Herein, a set of silver/europium-doped lanthanum orthophosphate (Ag/La0.95Eu0.05PO4) nanostructures with different concentrations of silver nanoparticles were prepared and investigated. The presented overlap between the strongest europium (Eu3+) excitation line and the broad silver nanoparticle surface plasmon resonance makes the combination prospective for coupling. X-ray powder diffraction confirmed the monoclinic monazite structure. The transmission electron microscopy revealed particles with a rod-like shape and ~4 aspect ratio. Photoluminescence spectra show characteristic Eu3+ ion red emission. One of the requirements for an enhanced luminescence effect is the precise control of the distance between the noble metal nanoparticles and the emitter ion. The distance is indirectly varied throughout the change of Ag nanoparticle concentration in the La0.95Eu0.05PO4 host. The emission intensity increases with the increase in Ag nanoparticles up to 0.6 mol %, after which the luminescence decreases due to the nanoparticles’ close packing and aggregation leading to the displacement of La0.95Eu0.05PO4 from the vicinity of the metal particles and reabsorption of the emitted light. The emission intensity of La0.95Eu0.05PO4 increases more than three times when the Eu3+ excitation is supported by the localized surface plasmon resonance in the Ag/La0.95Eu0.05PO4 nanostructures.


Introduction
Advanced materials are crucial for social and economic development, with applications in industries aimed to meet the challenges of renewable and clean energy, climate changes, national security, as well as human health and welfare. Therefore, the development and exploitation of innovative materials is critical in achieving global competitiveness in the 21st century. To meet, among others, demands for better physical and chemical sensors, brighter phosphors, or biocompatible drug-delivery molecules, material scientists are searching for new molecular combinations and structures [1][2][3][4].
The design and preparation of phosphors, inorganic materials activated by lanthanide (Ln 3+ ) or transition metal (TM) ions, with different particle size and/or distinctive morphology is a challenging task that has attracted researchers' attention. Phosphors are used in a variety of applications, such as displays, lighting, optical sensing (physical and chemical), catalysis, and medicine, just to mention a few [5][6][7]. Phosphor's luminescent properties depend on the characteristics of the optically active ion and its local environment provided by the host lattice. In recent years, the lanthanum orthophosphate (LaPO 4 ), non-toxic, biocompatible, thermally, chemically, and photo-stable material has been recognized Synthesis of silver colloid: An appropriate amount of silver nitrate (AgNO3, Merck) was dissolved in the water previously purged with argon for 30 min. The reducing sodium borohydride (NaBH4) agent was added to the solution by vigorous stirring. A large excess of NaBH4 was required to reduce the silver ions and to stabilize the formed silver nanoparticles. The obtained colloids were left in an argon atmosphere for an additional 30 minutes. The concentrations of obtained silver colloids were: 1 × 10 −4 M; 2 × 10 −4 M; 1 × 10 −3 M; 2 × 10 −3 M and 3 × 10 −3 M, and they were used without any further purification.

Instruments and Measurements
X-ray diffraction (XRD) measurements were performed using the Rigaku SmartLab diffractometer, Rigaku Corporation, Tokyo, Japan. Diffraction data were recorded in a 2θ range from 10° to 90°, counting 0.1°/min in 0.02° steps. Transmission electron microscopy was performed on the FEI TECNAI G2 X-TWIN microscope. Diffuse spectral reflectance measurements were performed on the Thermo Evolution 600 spectrometer equipped with an intergrading sphere, using BaSO4 as a blank. The photoluminescent emission spectra were collected using a Fluorolog-3 Model FL3-221 spectrofluorometer system (Horiba-Jobin-Yvon) under continuous excitation using a 450W xenon lamp (λex = 393 nm). The photoluminescent excitation spectra were recorded while monitoring emission at λem = 611 nm.

Structural Analysis
X-ray powder diffraction patterns of the representative LPO and two Ag/La0.95Eu0.05PO4 (LPO-Ag0.6 and LPO-Ag1.8) samples confirmed the monoclinic monazite structure with space group P 121/n 1(14) (see Figure 2a, all diffraction peaks are indexed according to the COD card no. 9001647). The absence of impurity phases indicates that the dopant Eu 3+ ions are successfully incorporated into the LaPO4 matrix due to the equal valence (+3) and similar ionic radii between the Eu 3+ (a = 0.112 nm) and La 3+ ions (a = 0.122 nm) [35]. In this crystal structure, the lanthanide ions are coordinated with Synthesis of silver colloid: An appropriate amount of silver nitrate (AgNO 3, Merck) was dissolved in the water previously purged with argon for 30 min. The reducing sodium borohydride (NaBH 4 ) agent was added to the solution by vigorous stirring. A large excess of NaBH 4 was required to reduce the silver ions and to stabilize the formed silver nanoparticles. The obtained colloids were left in an argon atmosphere for an additional 30 minutes. The concentrations of obtained silver colloids were: 1 × 10 −4 M; 2 × 10 −4 M; 1 × 10 −3 M; 2 × 10 −3 M and 3 × 10 −3 M, and they were used without any further purification.

Instruments and Measurements
X-ray diffraction (XRD) measurements were performed using the Rigaku SmartLab diffractometer, Rigaku Corporation, Tokyo, Japan. Diffraction data were recorded in a 2θ range from 10 • to 90 • , counting 0.1 • /min in 0.02 • steps. Transmission electron microscopy was performed on the FEI TECNAI G2 X-TWIN microscope. Diffuse spectral reflectance measurements were performed on the Thermo Evolution 600 spectrometer equipped with an intergrading sphere, using BaSO 4 as a blank. The photoluminescent emission spectra were collected using a Fluorolog-3 Model FL3-221 spectrofluorometer system (Horiba-Jobin-Yvon) under continuous excitation using a 450W xenon lamp (λ ex = 393 nm). The photoluminescent excitation spectra were recorded while monitoring emission at λ em = 611 nm.

Structural Analysis
X-ray powder diffraction patterns of the representative LPO and two Ag/La 0.95 Eu 0.05 PO 4 (LPO-Ag0.6 and LPO-Ag1.8) samples confirmed the monoclinic monazite structure with space group P 121/n 1(14) (see Figure 2a, all diffraction peaks are indexed according to the COD card no. 9001647). The absence of impurity phases indicates that the dopant Eu 3+ ions are successfully incorporated into the LaPO 4 matrix due to the equal valence (+3) and similar ionic radii between the Eu 3+ (a = 0.112 nm) and La 3+ ions (a = 0.122 nm) [35]. In this crystal structure, the lanthanide ions Materials 2020, 13, 3071 4 of 10 are coordinated with nine oxygen atoms forming polyhedrons (LaO 9 ) that a share corner with PO 4 tetrahedra in which all four P-O bonds are equivalent (Figure 2b,c) [13,36]. The average crystallite size of~6 nm was determined using built-in software and was similar for all the samples.
Materials 2020, 13, x FOR PEER REVIEW 4 of 10 nine oxygen atoms forming polyhedrons (LaO9) that a share corner with PO4 tetrahedra in which all four P-O bonds are equivalent (Figure 2b,c) [13,36]. The average crystallite size of ~6 nm was determined using built-in software and was similar for all the samples.

Microstructural Characterization
Transmission electron microscopy images (Figure 3a-c) of the representative LPO-Ag0.6 sample show particles with a rod-like shape, expected for the given monoclinic crystal system. Furthermore, the TEM images reveal an aggregation tendency also found in higher condensed La-based phosphates [37]. An average length/diameter aspect ratio was calculated and found to be around 4 (Figure 3d-f). According to the literature [38,39], when synthesized under alkaline conditions (pH > 7) the particles are sphere-like. On the other hand, when synthesized under acidic conditions (pH < 7), the particles become rod-like with a different length/diameter ratio corresponding to the different pH values. In a strongly acidic solution, rod-like morphology becomes fiber-like having a width of 5-20 nm and a length of several micrometers. Herein, the prepared mildly acidic precursor solutions (pH ~5) yield to, as expected, rod-like particles with ~4 aspect ratio.

Microstructural Characterization
Transmission electron microscopy images (Figure 3a-c) of the representative LPO-Ag0.6 sample show particles with a rod-like shape, expected for the given monoclinic crystal system. Furthermore, the TEM images reveal an aggregation tendency also found in higher condensed La-based phosphates [37]. An average length/diameter aspect ratio was calculated and found to be around 4 (Figure 3d-f). According to the literature [38,39], when synthesized under alkaline conditions (pH > 7) the particles are sphere-like. On the other hand, when synthesized under acidic conditions (pH < 7), the particles become rod-like with a different length/diameter ratio corresponding to the different pH values. In a strongly acidic solution, rod-like morphology becomes fiber-like having a width of 5-20 nm and a length of several micrometers. Herein, the prepared mildly acidic precursor solutions (pH~5) yield to, as expected, rod-like particles with~4 aspect ratio.   Figure 4 shows the diffuse reflectance spectra of the representative LPO and LPO-Ag0.6 samples. The sample with the Ag NPs clearly shows a typical broad band centred at 430 nm originating from the Ag surface plasmon resonance [40,41]. On the other hand, the sample without Ag NPs shows only weak bands that correspond to the Eu 3+ transitions with the most distinguished 7 F0→ 5 L6 absorption placed ~ 393 nm. An enlarged inset clearly shows how Ag plasmon absorption overlaps the 7 F0→ 5 L6 absorption band of the Eu 3+ ion [42,43].   Figure 4 shows the diffuse reflectance spectra of the representative LPO and LPO-Ag0.6 samples. The sample with the Ag NPs clearly shows a typical broad band centred at 430 nm originating from the Ag surface plasmon resonance [40,41]. On the other hand, the sample without Ag NPs shows only weak bands that correspond to the Eu 3+ transitions with the most distinguished 7 F 0 → 5 L 6 absorption placed~393 nm. An enlarged inset clearly shows how Ag plasmon absorption overlaps the 7 F 0 → 5 L 6 absorption band of the Eu 3+ ion [42,43].   Figure 4 shows the diffuse reflectance spectra of the representative LPO and LPO-Ag0.6 samples. The sample with the Ag NPs clearly shows a typical broad band centred at 430 nm originating from the Ag surface plasmon resonance [40,41]. On the other hand, the sample without Ag NPs shows only weak bands that correspond to the Eu 3+ transitions with the most distinguished 7 F0→ 5 L6 absorption placed ~ 393 nm. An enlarged inset clearly shows how Ag plasmon absorption overlaps the 7 F0→ 5 L6 absorption band of the Eu 3+ ion [42,43].    Figure S1. The excitation spectrum of the LPO-Ag0.6 sample (λ em = 611nm) consists of several sharp lines that correspond to the transitions within the 4f 6 configuration of Eu 3+ ions. The strongest Eu 3+ line in the LPO-Ag0.6 excitation spectrum ( 7 F 0 → 5 L 6 at 393 nm) overlaps with the silver nanoparticles' plasmon resonance, making this combination prospective for coupling. The incident light wave resonates with oscillations of the surface electron plasma in the Ag NPs at a certain resonance frequency/wavelength. The light-induced, so-called surface plasmon, electric field spreads outside the Ag NPs and influences the neighbouring Eu 3+ ions by increasing their light absorption. Thus, the Eu 3+ ions will absorb more and therefore emit more light. In that way, by the incorporation of Ag NPs into the LPO host, the intensity of Eu 3+ emission could be enhanced [23].

Photoluminescence Measurements
Materials 2020, 13, x FOR PEER REVIEW 6 of 10 Figure 5 presents the UV-VIS absorption spectrum of the representative Ag colloid (1 × 10 −3 M) together with the theoretical extinction efficiency curve for 6 nm spherical Ag particles in water and the excitation spectrum of the representative LPO-Ag0.6 sample. The UV-VIS absorption spectra of all silver colloids used for the synthesis (1 × 10 −4 M; 2 × 10 −4 M; 1 × 10 −3 M; 2 × 10 −3 M; 3 × 10 −3 M) are given in the Supporting Material as Figure S1. The excitation spectrum of the LPO-Ag0.6 sample (λem = 611nm) consists of several sharp lines that correspond to the transitions within the 4f 6 configuration of Eu 3+ ions. The strongest Eu 3+ line in the LPO-Ag0.6 excitation spectrum ( 7 F0 ➝ 5 L6 at 393 nm) overlaps with the silver nanoparticles' plasmon resonance, making this combination prospective for coupling. The incident light wave resonates with oscillations of the surface electron plasma in the Ag NPs at a certain resonance frequency/wavelength. The light-induced, so-called surface plasmon, electric field spreads outside the Ag NPs and influences the neighbouring Eu 3+ ions by increasing their light absorption. Thus, the Eu 3+ ions will absorb more and therefore emit more light. In that way, by the incorporation of Ag NPs into the LPO host, the intensity of Eu 3+ emission could be enhanced [23].  It is well known that the 5 D0 → 7 F1 transition of Eu 3+ ion is the parity-allowed magnetic dipole transition (ΔJ = 1) and its intensity does not vary with the host. On the other hand, red 5 D0 → 7 F2 electric dipole transition (ΔJ = 2) is highly sensitive to the local environment, and its intensity decreases with an increase in the symmetry of the crystal field around the europium ion. In the given structure, the Eu 3+ ion replaces La 3+ in the non-centrosymmetric C1 crystallographic site. It is generally acknowledged that the observation of the dominant 5 D0 → 7 F2 line and completely forbidden 5 D0 → 7 F0 transition indicates that Eu 3+ ion is located in a structural site without a inversion centre (such as C1) [42,43]. This is in agreement with the observed photoluminescence spectra where the red emission arising from the 5 D0 → 7 F2 transition is more intense compared to the 5 D0 → 7 F1 emission.

Photoluminescence Measurements
One of the conditions for enhanced luminescence effect is the precise control of a distance between the plasmon particles and the emitter. Here, the distance is indirectly varied by changing the Ag nanoparticle's concentration in the La0.95Eu0.05PO4 host. Figure 6b shows the normalized integrated area of the photoluminescent emission spectra as a function of Ag NPs mol % in nanostructures. The emission intensity increases up to 0.6 mol % of Ag NPs while higher Ag concentrations cause emission intensity decrease. When the concentration of Ag NPs is ≤ 0.6 mol %, phosphor particles are located in the vicinity of plasmon particles which results in the increase in  Figure 6a shows the photoluminescence spectra of all six samples, the LPO and the five samples with different concentrations of Ag NPs. Five characteristic Eu 3+ emission bands associated with 5 D 0 → 7 F J (J = 0, 1, 2, 3, 4) spin forbidden f -f transitions, are visible around 578, 587, 611, 652, and 698 nm, respectively. It is well known that the 5 D 0 → 7 F 1 transition of Eu 3+ ion is the parity-allowed magnetic dipole transition (∆J = 1) and its intensity does not vary with the host. On the other hand, red 5 D 0 → 7 F 2 electric dipole transition (∆J = 2) is highly sensitive to the local environment, and its intensity decreases with an increase in the symmetry of the crystal field around the europium ion. In the given structure, the Eu 3+ ion replaces La 3+ in the non-centrosymmetric C1 crystallographic site. It is generally acknowledged that the observation of the dominant 5 D 0 → 7 F 2 line and completely forbidden 5 D 0 → 7 F 0 transition indicates that Eu 3+ ion is located in a structural site without a inversion centre (such as C1) [42,43]. This is in agreement with the observed photoluminescence spectra where the red emission arising from the 5 D 0 → 7 F 2 transition is more intense compared to the 5 D 0 → 7 F 1 emission.
One of the conditions for enhanced luminescence effect is the precise control of a distance between the plasmon particles and the emitter. Here, the distance is indirectly varied by changing the Ag nanoparticle's concentration in the La 0.95 Eu 0.05 PO 4 host. Figure 6b shows the normalized integrated area of the photoluminescent emission spectra as a function of Ag NPs mol % in nanostructures. The emission intensity increases up to 0.6 mol % of Ag NPs while higher Ag concentrations cause emission intensity decrease. When the concentration of Ag NPs is ≤ 0.6 mol %, phosphor particles are located in the vicinity of plasmon particles which results in the increase in emission intensity due to more effective light absorption. The optimal Ag NP concentration is 0.6 mol % and it results in the3 emission enhancement factor (distribution of the light-induced electric field near Ag nanoparticles calculated by the means of the discrete dipole approximation, given and discussed in the Supporting Material as Figure S2). On the other hand, the high Ag NP concentrations (>0.6 mol %) cause the close packing and aggregation of the plasmon particles leading to: (i) the displacement of La 0.95 Eu 0.05 PO 4 phosphor particles from the vicinity of the metal particles and consequently a luminescence efficiency decrease [20]; (ii) the reabsorption of emitted light, due to the electromagnetic coupling between the neighbouring particles in the aggregates. Quinten et al. reported that the Ag aggregate spectra clearly show single-particle resonance splitting into new resonances, most of which contribute at longer wavelengths (~500-600 nm) than the resonance wavelength of the single particle (~400 nm). Thus, the strongest Eu 3+ excitation line (~393 nm) does not overlap efficiently with the silver nanoparticles' plasmon resonance [44].
emission intensity due to more effective light absorption. The optimal Ag NP concentration is 0.6 mol % and it results in the ~ 3 emission enhancement factor (distribution of the light-induced electric field near Ag nanoparticles calculated by the means of the discrete dipole approximation, given and discussed in the Supporting Material as Figure S2). On the other hand, the high Ag NP concentrations (>0.6 mol %) cause the close packing and aggregation of the plasmon particles leading to: i) the displacement of La0.95Eu0.05PO4 phosphor particles from the vicinity of the metal particles and consequently a luminescence efficiency decrease [20]; ii) the reabsorption of emitted light, due to the electromagnetic coupling between the neighbouring particles in the aggregates. Quinten et al. reported that the Ag aggregate spectra clearly show single-particle resonance splitting into new resonances, most of which contribute at longer wavelengths (~500-600 nm) than the resonance wavelength of the single particle (~400 nm). Thus, the strongest Eu 3+ excitation line (~393 nm) does not overlap efficiently with the silver nanoparticles' plasmon resonance [44].

Conclusions
The brightness of phosphors can be increased due to stronger light absorption when phosphor particles are located in the vicinity of plasmon particles, for example noble metal clusters. Herein, we showed that the surface plasmon resonance facilitated the enhancement of the Eu 3+ -doped LaPO4 emission intensity by the addition of Ag nanoparticles. One of the conditions for the enhancement of the phosphors' emission intensity was the precise control of the distance between the plasmon particles and the phosphor. The distance is indirectly varied by the Ag NPs concentration in the structure. When the Ag NPs concentration is low, i.e., when there is a small number of Ag NPs, the distance between the Ag and phosphor particles is long. With the Ag NPs concentration increase, the distance becomes shorter, reaching the optimum at 0.6 mol % Ag and resulting in a three times larger value than the initial photoluminescent intensity. One could expect that with the further increase in concentration (and number of NPs), the distance would be even shorter, leading to further photoluminescent intensity enhancement. However, that is not the case due to the close packing and aggregation of the plasmon particles which causes both the reabsorption of emitted light and the displacement of the La0.95Eu0.05PO4 phosphor particles from the vicinity of the metal particles. The distance between the phosphor particles and the Ag NPs can also be adjusted by coating phosphor particles with a different thickness SiO2 shell, which will be a subject of our future work.

Conclusions
The brightness of phosphors can be increased due to stronger light absorption when phosphor particles are located in the vicinity of plasmon particles, for example noble metal clusters. Herein, we showed that the surface plasmon resonance facilitated the enhancement of the Eu 3+ -doped LaPO 4 emission intensity by the addition of Ag nanoparticles. One of the conditions for the enhancement of the phosphors' emission intensity was the precise control of the distance between the plasmon particles and the phosphor. The distance is indirectly varied by the Ag NPs concentration in the structure. When the Ag NPs concentration is low, i.e., when there is a small number of Ag NPs, the distance between the Ag and phosphor particles is long. With the Ag NPs concentration increase, the distance becomes shorter, reaching the optimum at 0.6 mol % Ag and resulting in a three times larger value than the initial photoluminescent intensity. One could expect that with the further increase in concentration (and number of NPs), the distance would be even shorter, leading to further photoluminescent intensity enhancement. However, that is not the case due to the close packing and aggregation of the plasmon particles which causes both the reabsorption of emitted light and the displacement of the La 0.95 Eu 0.05 PO 4 phosphor particles from the vicinity of the metal particles. The distance between the phosphor particles and the Ag NPs can also be adjusted by coating phosphor particles with a different thickness SiO 2 shell, which will be a subject of our future work.