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Plasmon-enhanced fluorescence of CaF2:Eu2+ nanocrystals by Ag nanoparticles
Journal of Alloys and Compounds 719 (2017) 159e170
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Plasmon-enhanced fluorescence of CaF2:Eu2þ nanocrystals by Ag nanoparticles Weihao Ye a, Qiying Huang a, Xianfu Jiao a, Xiaotang Liu a, b, *, Guangqi Hu a a
Guangdong Provincial Engineering Technology Research Center for Optical Agricultural, College of Materials and Energy, South China Agricultural University, China b Department of Physics, University of Texas at Arlington, Arlington, TX, 76019, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 March 2017 Received in revised form 15 May 2017 Accepted 18 May 2017 Available online 19 May 2017
We report plasmon-enhanced fluorescence in Eu2þ doped CaF2 (CaF2:Eu2þ) nanocrystals assembled with Ag nanoparticles (NPs) using a simple hydrothermal method. The structure, morphology and composition of the nanocrystals are confirmed by X-ray diffraction (XRD), transmission electron Microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and UVevis absorption spectrum. Ag nanoparticles are ideal building blocks for plasmonic materials that can exhibit a wide range of unique and potentially useful optical phenomena. Surface plasmon enhancement makes significant contribution to enhance the blue fluorescence emission of CaF2:Eu2þ. The enhancement on the Eu emission in visible regions is up to a factor of 12.3-fold. As shown in time-resolved measurements, surface Plasmon Resonance (SPR) can reduce the fluorescence lifetime and increase the quantum yield. Furthermore, with increasing Ag concentration, the fluorescence intensity increases to its maximum and then decreases. © 2017 Elsevier B.V. All rights reserved.
Keywords: Calcium fluoride Europium Ag nanoparticles Surface plasmon Photoluminescence
1. Introduction In recent years lanthanide-doped nanomaterials for photonic applications have attracted great attention and been widely investigated for many fields, such as semiconductor [1], UV lasers [2], display [3] and biological labels [4]. What makes nanocrystals special is that each size has a completely distinct set of physical and chemical properties. As nanometer crystals can be precisely prepared now, materials with designed properties can be produced not only by changing their chemical composition but by controlling their sizes [5]. However, the properties of nanometer scale crystals differ from their bulk materials [6]. The nanocrystals with welldefined morphologies and accurately tunable sizes remains a challenge. Their physical and chemical properties vary drastically by geometrical factors such as shape, dimensionality, and size. So far, the host materials for fluoride nanocrystals have been mainly focused on NaYF4 and LaF3 [7,8]. But, lanthanide doping in these host lattice are induced by a high-intensity pulse laser or have
* Corresponding author. Guangdong Provincial Engineering Technology Research Center for Optical Agricultural, College of Materials and Energy, South China Agricultural University, China. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.jallcom.2017.05.181 0925-8388/© 2017 Elsevier B.V. All rights reserved.
low quantum yield. In contrast, calcium fluoride (CaF2) is very suitable as a host for rare earth ions due to its desirable properties, such as low refractive index, low phonon energy, high laser damage threshold [9e11]. To date, RE-doped CaF2 nanomaterials have attracted more attention because of its robustness among other fluorides. Among the rare-earth ions, CaF2:Eu2þ exhibit excellent properties because of 4f7-4f65d transition and the emission varies from ultraviolet to red region [12]. Particularly, increasing technological and industrial interest is given to research on the luminescence of optical nanomaterials due to their novel optical properties about emission lifetime, luminescence quantum efficiency, and concentration quenching [13]. It has been investigated that luminous efficiency depends on the particle size [14,15]. When particles are prepared in small sizes, the following factors become critical for luminescence intensity: (1) surface defect; (2) ratio between thermalization length of charge carriers and the nanoparticle sizes [16]. The reduction of the nanocrystals size leads to significant luminescence intensity decrease, which limits their further applications. To solve this question, extensive work has been carried out, such as modulating reaction condition [17], coating surfactants or shell [17,18], and preparing composite materials [19]. However, the effect of these enhancement mechanism is not obvious. By coupling with noble metals on the surface of luminescent
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nanocrystals, luminescence can be enhanced due to Surface Plasmon Resonance (SPR) [20e22]. Surface plasmon enhanced fluorescence has been widely studied and used in various fields starting with classic reports of Drexhage [23]. This can be an effective promoter of fluorescence when optimized to provide a large and highly confined local electric field enhancement that arises from the collective oscillations of the electrons due to resonance [24e26]. In particularly, metal nanoparticles such as gold, silver, and copper nanoparticles can show intense surface plasmon resonance absorption band in UVevis and near infrared (NIR) region [27,28]. Recently, most researchers have used Ag-NPs with desirable SPR to enhance luminous efficiency. For instance, according Xin Xu's report [29], the visible emission of Sr2Si5N8:Eu2þ phosphor can be enhanced by coupling with the localized surface plasmon oscillation of Ag nano-particles. Qiang Shi [30] reported on highly tunable enhancement of red emission from Mg0.1Zn0.9O phosphors by regulation of the LSP resonance of Ag nanoparticles. Hyungduk Ko [31] demonstrated a novel anticounterfeit device based on NIR-to-Vis upconversion luminescence from b-NaYF4:Yb3þ,Er3þ enhanced by the Ag nanowires-driven plasmonic effects. Similarly, Wen Xu [32], Hongwei Song [33], and other researchers [34e36] enhanced upconversion emission of NaYF4:Yb, Er nanocrystals by using noble metal. However, among these studies, a large number of researches are about up-converting materials and noble metal with small grain size. There are few reports on down-converting materials with resonance metal. With this motivation, we report herein a facile one-pot process to synthesize down-converting CaF2:Eu2þ attaching Ag nanoparitcles without any calcination. Using this method, we are able to synthesize nanoparticles with particle size about 60 nm. Meanwhile, a maximum luminescence enhancement of 12.3-fold is observed when the fluorescent intensifier Ag nanoparticles with a relatively large size of 85 nm are used. Tuning by switching between fluorescence enhancements and quenching is shown as a function of the concentration of Ag NPs coated on the surface of CaF2, a parameter easily controlled during synthesis. In reaction process, a reduction of Eu3þ/Eu2þ happens under the catalytically active Ag nanoparticles. In addition, the influences of the reaction time, temperature and pH value are also discussed. 2. Experimental 2.1. Synthesis of Ag nanoparticles The nanoparticles were synthesized using method described by P.C.Lee and D.Meisel [37]. Briefly, 45 mg AgNO3 was dissolved in 250 ml boiling water and the solution was kept boiling for half an hour. Under stirring, 1% sodium citrate aqueous solution (5 ml) was rapidly injected into above solution and the mixture was further refluxed for another 1 h. Sodium citrate as a mild reducing agent gradually changed the growth solution from colorless to brownish. The suspension was then cooled to room temperature under stirring and stored at 4 C for later use. 2.2. Synthesis of CaF2: Eu2þ(2.5%) nanoparticles CaF2:Eu2þ nanoparticles were prepared by hydrothermal method. The EuCl3$6H2O were obtained by adding hydrochloric acid to Eu2O3. In a typical synthesis, 3.40 mmol CaCl2$2H2O and 0.43 mmol EuCl3$6H2O were dissolved in 15 ml distilled water under stirring followed by dropwise addition of 15 ml 68.30 mmol NH4F. After a white homogeneous dispersion appeared, a certain amount of Cit-Ag colloids were injected and a well-controlled amount of NaOH solution was added under magnetic stirring to reach different pH values. Next, 0.40 g NaBH4 was added and the
mother solution containing white suspension was sealed in 50 ml Teflon-lined stainless steel autoclave. The mixture was treated at different temperature and time. Temperature rate is 10 C/min. Finally, the obtained suspensions were washed with ethanol and deionized water for several times and dried in a vacuum oven at 60 C. 2.3. Characterization The XRD measurements of synthesized samples were carried out using a XD-2X/M4600 X-ray diffractometer with Cu-Ka radiation (l ¼ 0.154056 nm) at 40 kV and 30 mA in a scan range 10e80 . The morphology and EDS were observed by a JSM-7001F field emission scanning electron microscope (FESEM). A JEOL-2100F High-Resolution Transmission Electron Microscope (HRTEM) operated at 200 kV was used to observe the grain size and examine the lattice fringes. Samples for TEM were prepared by depositing a drop of sample dispersed in ethanol onto a holey carbonecoater copper grid. The photoluminescence (PL) of the powder samples was measured by a Shimadzu RF5301pc fluorescence spectrophotometer. UVevis absorption spectra were measured using a Shimadzu UV-2550 spectrophotometer with a wavelength range from 200 to 800 nm. The concentration of Ag nanoparticles was analyzed by Varian 810 ICP-MS. EPR spectra were acquired with a Bruker A300-10/12 spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed in a ESCALAB 250Xi with an Al Ka X-ray source with constant pass energy of 30 eV. 3. Results and discussion 3.1. Structure and formation of CaF2:Eu2þ nanoparticles The composition and crystal phase of the nanocrystals are characterized by XRD. Fig. 1a and b shows the XRD patterns of 2.5mol% Eu2þ doped CaF2 nanoparticles with different amount of Cit-Ag nanoparticles and heating temperatures, respectively. Based on the XRD diffraction peaks, the samples are well-crystallized. The diffraction peaks in Fig. 1 could easily be assigned to the (111), (220), (311), (400) and (331) planes of face-centered cubic (fcc, space group: Fm3m [38]). There is no impurity peaks observed. These values obtained from all the samples match very well with CaF2 standard JCPDS cards (35-0816). There is no apparent difference observed after Eu2þ doping or Cit-Ag nanoparticles addition. The lattice constant for the CaF2:Eu2þ is calculated to be 5.39 Å, which is close to 5.46 Å for pure CaF2 nanocrystals [39]. The Eu2þ ions may occupy the Ca2þ sites in the CaF2 host lattice. Based on the halfmaximum (FWHM) values of the (111) peaks, the mean size of the CaF2 nanocrystals is evaluated to be about 56 nm by DebyeeScherrer formula [40]. As shown in Fig. 1a, the strong and sharp diffraction peaks indicate that the CaF2 with different amount of CitAg nanoparticles are well crystallized. As heat treatment temperatures rise, the intensity of the diffraction peaks increase significantly (Fig. 1b), which implies a better crystallinity. The broadening of these diffraction peaks is an indication of small size samples. XPS can be used to check the composition of surfaces, which is carried out in the 0e1300 eV binding energy range to identify the elements on the surface of samples. Fig. 2a depicts the survey of the samples, which indicates that all the samples are mainly consist of Ca, F, Eu, and some impurity elements of C, O. The 2p levels of Ca show the spin-orbit doublet with the binding energies around 348.6 eV (2p3/2) and 352.2 eV (2p1/2) in Fig. 2b [41]. Moreover, the Ag 3d5/2 and Ag 3d3/2 core level binding energies for Ag NPs sample appear at 368.1 and 374.1 eV in Fig. 2c, which are higher than the reference value from J. S. Hammond [42] and Schon, G [43] due to the larger size [44]. However, a broad peak of the Ag 3d5/2 peaks as
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Fig. 1. XRD patterns of the CaF2: Eu(2.5%) nanoparticles (a) with different amount of Cit-Ag nanoparticles at 200 C (b) heated at different temperatures with 200 mL Cit-Ag nanoparticles.
the oxidation state is observed. The broad Ag 3d5/2 peak has been deconvoluted into two peaks which are related to Ag2O (365.2 eV) and AgO (364.4 eV). With exposure to damp air, the Ag 3d5/2 peaks shift continuously to lower binding energy as oxidation occured on the surface of Cit-Ag nanoparticles. Fig. 2d shows XPS spectrum of Eu ions. The peaks at 1135.2 and 1164.6 eV represent the existence of Eu3þ ions, while the peaks at 1124.8 and 1154.4 eV reveal the existence of Eu2þ ions [45,46]. Actually, the peaks of Eu ions are relatively weak as the low doping concentration of Eu. Their valence state cannot be resolved due to their weak binding energies. One of the most informative methods to study the structure of defects formed by paramagnetic ions is the EPR method. The electronic structure of the doping ions in the diamagnetic crystal matrix of the crystal is very sensitive to variations in the fine and hyperfine structure of the fields [47]. Fig. 2e shows the EPR spectra of CaF2:Eu2þ nanoparticles observed at room temperatures(293 K). The spectra exhibit the resonance signals with the effective g values at 1.975, which is consistent with R.S. Title's result [48]. The Eu2þ ion has a half-filled electronic configuration of 4f7 which generates an 8S7/2 ground state. The strong hyperfine interaction of Eu2þ splits each energy level into six sublevels. However, the lines in higher fields are wide and structureless. The reasons for this are the superposition of signals from the two uneven Eu isotopes, 151Eu and 153 Eu [49] and large anisotropy [50]. Thus, only a hyperfine structure line of the central transition <þ1/2j4<1/2j of Eu2þ EPR spectra is observed. Based on the EPR result, the reduction of Eu3þ/Eu2þ has occurred by the Cit-Ag catalyst. EDS data in Fig. 3 taken from a large area containing several crystals further shows atom percentage of F, Ca, Eu, Ag of 40.32%, 20.34%, 0.58%, 0.01%, respectively, The values are almost consistent with their molar concentration in the molecular formula CaF2: Eu2þ(2.5%). However, due to sensitivity limit of equipment, the proportion of Ag is not inaccurate. No other impurities are evident in the figure implying that the resulting CaF2: Eu2þ nanocrystals have high purity in chemical composition. It should be pointed out that the weak signals of C are originated from conducting resin used to support the SEM sample. 3.2. TEM TEM measurement is carried out to observe the size and morphology of the Ag nanoparticles prepared by the method from P.C.Lee and D.Meisel. In the syntheses of Ag nanoparticles using citrate reduction, the reactivity of silver precursor is almost not affected by the molar ratio or pH value since the coordination of Ag ions is dominated by citrate in a wide range of pH [51]. As shown in
Fig. 4a, all the particles prepared are quasi-spherical. The particles agglomerate severely due to the high speed centrifugation for preparing TEM samples. The size of Cit-Ag NPs is about 80e100 nm with a relatively large range, which is consistent with the results reported previously [52]. It is noticed that, when the Ag NPs are prepared at boiling temperature, a large average size of Ag NPs with a broad size distribution is observed since heat-treatment speeds up the Oswald ripening process. Fig. 4b is a high resolution TEM image obtained at the edge of a typical Ag nanoparticle, which shows a highly ordered lattice fringe. The lattice spacing is measured as 0.2500 nm, which is corresponding to the (0 0 4) plane of Ag nanoparitcles. The inset shows the sample's SAED diffraction pattern. The presence of ring-shaped patterns is not in conflict with their single-crystalline nature, because the SAED patterns were recorded from an area containing a large number of nanoparticles. Fig. 5 displays the HRTEM images of CaF2: Eu2þ (2.5%) nanoparticles treated with 200ul Cit-Ag nanoparticles. The average grain size is about 50e58 nm, which is in good agreement with the calculated result from DebyeeScherrer formula. CaF2 nanocrystals are nearly spherical particle with relatively even sizes, except that one is very small. It is attributed to the Ostwald ripening process. The large particles receive atoms from small dissolved particles. Thus, small nanoparticles shrink and eventually disappear while larger nanoparticles grow larger and have better crystallinity [53]. Moreover, for precipitation formation, the rate of nucleation is much higher than crystal growth when the supersaturation ratio is high. Fig. 5d is a schematic diagram of CaF2:Eu2þ crystal growth. The morphology of nanoparticles is basically cubic and large at low supersaturated condition. However, previous study has demonstrated that the nanoparticles change into hexagonal or spherical and smaller with increasing supersaturation ratio [54]. The grain size is still reducing due to a more rapid rate of nucleation, even at higher concentration of the component ions. According to highresolution TEM micro-graphs, the CaF2 nanoparticles are single crystalline. A selected-area electron diffraction (SAED) pattern in Fig. 5b clearly shows the characteristic spots of single-crystal CaF2:Eu2þ, which is ascribed to specific crystal planes of their corresponding XRD patterns. The periodicity of the lattice fringes obtained for the nanocrystals is d ¼ 0.31546 nm in Fig. 5c, corresponding to the (111) planes of CaF2. The TEM images also indicate that the surface region of CaF2:Eu2þ is highly ordered. 3.3. SEM The general morphologies of the as-prepared samples are not affected by doped Eu2þ ions or Ag addition in CaF2. The influences
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Fig. 2. XPS spectra of CaF2:Eu2þ nanoparticles; (a) is a survey while (b),(c) and (d) show the spectrum of Ca,Ag, and Eu, respectively. (e) is EPR spectrum of the central transition <þ1/2j4<1/2j of Eu2þ in CaF2:Eu2þ.
of heating temperature on CaF2 nanoparticle phase and crystallinity have been investigated by FESEM. Fig. 6 shows SEM images of CaF2:Eu nanoparticles treated with 200 ml Cit-Ag at 20 h for 150 C, 160 C, 170 C, 18 C, 190 C and 200 C. It is found that temperature had significant influence on the morphology of final products. With temperature increasing, the size of obtained samples is changed significantly and about 50e70 nm. Serious aggregation can be seen in the phosphors heated at those higher temperatures. For the samples heated at 200 C (Fig. 6a) and 190 C (Fig. 6b), hard particle aggregation with grain size of 60e75 nm have been observe and most of the microspheres are attached to each other to form irregular bulk-like aggregates of several microns via a particle growth process. As seen in Fig. 6c, the as-prepared phosphors that were heated for 180 C have displayed initial aggregation. They are about 60 nm and nonspherical. However, after reducing the
reaction temperature, the morphology of phosphors is notably improved. With the hydrothermal treatment for 170 C (Fig. 6d), a large number of homogeneous and monodisperse spherical nanoparticles are observed, with narrow size distribution of 55e60 nm. No irregular bulk-like aggregates but only nanoparticles with diameters about 55 nm are found when the reaction temperature is 160 C (Fig. 6e) and 150 C (Fig. 6f). On the basis of the aforementioned results, it is concluded that with the reaction temperature increasing, the sample size is increased and morphology becomes irregular bulk-like aggregates. Chemical composition is also of importance in the luminescent properties of phosphors [55]. Elemental mapping was conducted to investigate chemical composition inside the nanocrystals using FESEM with energy dispersive X-ray spectroscopy (EDS). Fig. 7 shows the mapping of CaF2:Eu2þ nanocrystals at 200 C for 20 h.
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Fig. 3. EDS spectrum of the CaF2: Eu(2.5%) heated at 200 C for 20 h with 200 mL Cit-Ag nanoparticles.
Fig. 4. TEM images of Cit-Ag nanoparticles at (a) low-resolution and (b) high-resolution. The inset in (b) shows the SAED pattern of Ag nanoparticles.
Fig. 5. (a) and (c) are TEM images of CaF2:Eu2þ at different resolutions. (b) SAED pattern of CaF2: Eu2þ. (d) shows a schematic diagram of CaF2:Eu2þ crystal growth.
Ca and F are main component and well distributed inside asprepared nanocrystals. The spatial distribution of Ca rich crystallites on surface correlates with cubic face-centered lattice. Eu atoms
distribute relatively well, indicating a homogeneous Eu2þ doping in the sample. Moreover, a relatively weak Ag signal is also observed on the surfaces of samples, which could have an affect on
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Fig. 6. SEM images of the CaF2 samples prepared at (a) 200 C for 20 h, (b) 190 C for 20 h, (c) 180 C for 20 h, (d) 170 C for 20 h, (e) 160 C for 20 h and (f) 150 C for 20 h.
Fig. 7. EDS mapping of CaF2:Eu phosphors.
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luminescent properties. 3.4. UVevis and ICP-MS spectroscopy analysis of Ag nanoparticles suspensions The metal colloid exhibits unique absorption bands or broad regions of absorption in the ultravioletevisible range due to the excitation of plasma resonances or interband transitions. These complex properties are due to conducting electrons which migrate freely but experience some resistance to the interband transitions present in most metals [56]. When visible light at an appropriate frequency is incident, the noble metals that possess subwavelength metallic features support surface plasmons e collective excitations of free electrons in a metal e that can locally amplify incident electromagnetic fields by orders of magnitude at the metal surface. Surface plasmon resonance is based on the coupling of electromagnetic radiation into a conducting metal. It is a very characteristic property of the metallic nature of the particles [57]. The position and the number of peaks are also an indication of the particle shapes. The UVevis absorption spectrum of prepared sample is shown in Fig. 8. The obtained Cit-Ag NPs have an absorption peak at 426 nm which display the characteristic surface plasmon resonance (SPR) band of 4d/5s from 350 to 600 nm. The broadness of the peak indicates a broad distribution of particle sizes in the solution. For spherical or near-spherical Ag particles, there is only one band centered at about 400 nm [58]. In our case, the peak shifts to the red with increased particle's diameter [59]. The total amount of elemental silver attached to CaF2 nanoparticles is determined by ICP-MS. The concentration of mother solution of Cit-Ag is 1.039 103 mol/L. The Ag nanoparticles attached to the phosphors prepared with different volume of Cit-Ag suspensions affect emission intensities due to the surface plasmon resonance. However, the amount of attached Ag nanoparticles is very low (Table 1). 3.5. Photoluminescence properties of CaF2:Eu2þ and plasmonenhanced photoluminescence by Ag NPs The Eu2þ ion enters the CaF2 lattice by substitutional Ca2þ ions and will be in cubic surroundings [60]. Each Eu2þ ion is coordinated with eight fluorine ions. Eu2þ only luminesce on a divalent or a monovalent cation site. Fig. 9 shows photoluminescence spectra of CaF2:Eu. An intense emission peaking at 425 nm is seen in the sample under 337 nm excitation and this is a characteristic Eu2þ emission band, which indicate catalytic performance of Cit-Ag nanoparticles. The wide emission from 400 to 480 nm with a
Fig. 8. UVevis absorption spectra of Cit-Ag NPs suspension.
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Table 1 The concentration of Ag nanoparticles attached on phosphors. Cit-Ag suspension volume/ml
Amount/ppm
100 200 300 400 500
23.829391144 55.389243026 85.099877632 125.180735724 204.201339872
peak at 425 nm is due to allowed electron dipole transitions from the excited state of the mixed 4f65d1 configuration of the Eu2þ ion to the 8S7/2 level of the 4f7 configuration in the ground state [61]. Fig. 9a also shows the excitation of CaF2:Eu2þ heated at 200 C for 20 h with 200 mL Cit-Ag sols. In the excitation spectrum, a weak band is located at around 245 nm and several strong bands in the form of a typical staircase structure are observed around 337, 355, 367, 377 and 397 nm. Depending on the strength of the octahedral crystal field, the excited 5d level of Eu2þ can be split into two degenerated levels and the energy difference between the two groups of degenerated 5d levels are highly affected by the state of chemical bonds between Eu2þ ions and the anions [62e64]. These two absorption bands correspond to the electron transitions in lower excited states 4f7[8S7/2]/4f65d1[eg] and higher excited states 4f7[8S7/2]/4f65d1[t2g] respectively [63,65]. The lower eg bands are seven narrow bands, referred to as a “staircase spectrum,” which retain the character of the seven (Eu2þ) 4f6 levels (7F02þ 6) levels. The “staircase spectrum” is due to the transition from Eu 8 S7/2 ground state to the lowest levels of the excited configuration of Eu2þ are of the type 4f6[7FJ ]5d1, where the 7F term is spineorbit split into the 7FJ multiplets with J ¼ 0-6. In addition, the 5d electron can also be found in higher 5d levels, and in most compounds the 4f n1 7 [ FJ]5d2 levels overlap with the first ‘staircase’, leading to broad excitation or absorption bands where all structure is washed out [66]. No absorption peaks related with Eu3þ ions are detected [67]. Fig. 9b shows the spectra of CaF2:Eu without any Cit-Ag nanoparitcles. A few minor emission peaks are observed at 570, 589, 613 and 627 nm, which are ascribed to fef transitions of Eu3þ of 5D0 to 7 F0, 7F1, 7F2, and 7F3, respectively [68]. However, the characteristic emission peak at 425 nm of Eu2þ is not observed, which indicate the conversion of Eu3þ/Eu2þ dose not occur without catalytic CitAg nanoparticles. The peaks near 469 nm are due to emission of excited states of Xe atom [69]. Fig. 10a shows the emission spectra of CaF2:Eu2þ nanoparticles calcined with different amount of Cit-Ag suspension using optimum excitation of 377 nm. The strong emission band peaked at 425 nm is generally assigned to 4f65d1/4f7(8S7/2). The spectra reveals that the fluorescence intensity of the nanocrystals is enhanced with increasing amount of Cit-Ag NPs from 100 mL to 200 mL, and then quenched as the amount is over 200 mL. The best amount is found to about 200 mL and a maximum luminescence enhancement of 12.3-fold is observed. Meanwhile, the emissions of Eu3þ are relatively weak due to the strong catalytic activity of CitAg nanoparticles and the intensity of Eu3þ emission at 570 nm, 589 nm and 613 nm is declined gradually with the increase amount of Cit-Ag nanoparticles shown in Fig. 10b. The enhancement effect from the Ag NPs may be attributed to surface plasmon resonance (SPR). The mechanism of the enhancement is investigated in terms of two prominent aspects related to the noble metals: excitation enhancement and emission enhancement. The excitation enhancement is an increase of the excitation rate or effective excitation flux due to the metal amplifying the incident field. The emission enhancement is an increase of the intrinsic radiative decay rate of phosphors by surface plasmon-coupled emission (SPCE) [70e73]. The interactions of phosphors with metallic
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Fig. 9. Excitation and emission spectra of (a) CaF2: Eu2þ calcined at 200 C for 20 h with addition of 200 mL Cit-Ag sols; (b) CaF2:Eu without Cit-Ag sols.
surfaces have been used to obtain increased phosphors luminescence intensities. In general, the quenching and enhancement are determined by the distance-dependent mechanism. The quenching of fluorescence happened when the metal nanoparticles are in close proximity to phosphors. However, our result is in stark contrast to the usual perception that the emission is strongly quenched and lifetime drop dramatically when the phosphors are close to the metal [71,74e76]. This quenching effect is attributed to losing surface waves (LSWs) [23]. The enhancement or quenching of phosphors can be explained by radiating plasmons (RPs) model [20] and classical electrodynamics. Fig. 11 shows the schematic
illustration of radiating plasmons (RPs) model. The surface plasmons are induced in the metal by the nearby excited fluorophores or incident light and radiate to the far-field to enhance the emission of phosphors. The surface plasmon is used to describe the collective oscillations of a group of electrons in a metal [77]. The interaction of fluorophores with metals can be modeled as the interaction of an oscillating dipole with a conducting metal surface. The interactions of incident light or nearby excited fluorophores with metallic surfaces requires wavevector matching at the phosphorsemetal interface. At a dielectricemetal interface the surface plasmon wavevector is given by
Fig. 10. (a) Emission spectra of CaF2: Eu2þ phosphors with addition of different amount of Cit-Ag nanoparticles. The inset reveals CaF2:Eu2þ luminescence intensity as a function of Cit-Ag amount (b) The volume of Ag NPs added affects luminescence intensity of different emission peaks in CaF2: Eu2þ phosphors.
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Fig. 11. Schematic illustration of radiating plasmons (RPs) model.
ksp ¼ k0
ε1 εm ε1 þ εm
1=2 (1)
where k0 is the wavevector in a vacuum, ε0 is the dielectric constant and ε1 is the dielectric constant of a propagating light in a medium. However, the dielectric constant km experienced by a wave incident on a metal depends on the incident light frequency
km ¼ nm c
u
(2)
pffiffiffiffiffiffi εm
(3)
nm ¼
εm ¼ 1
u2p u2
(4)
where nm is the refractive index of metal, εm is the dielectric constant of metal, up and u are the plasmon frequency of the electrons and frequency of incident light, respectively. The wavevector match at the interface and plasmon radiate out of the metal when ksp equals km . The surface plasmons on the metal surface create an
evanescent field which penetrates into the phosphors and fluorophore emission is observed as plasmon-coupled emission [78]. According to H. Raether [79], surface plasmon resonances in the metal are observed using incident light on the metal through a high refractive index prism or some higher dielectric-constant materials. The wavelength of incident light is shorter using prism, which satisfy the conditions of phosphors-metal interaction. However, in our results, surface plasmon resonances created in the metal at short distances enhance fluorescence without prism or higher dielectric-constant material. It is ascribe to Ag nanoparticles with roughened surfaces displayed by the TEM image of Cit-Ag nanoparticles in Fig. 4a, which increase the intensity of scattered plasmon emission. The light is scattered at incident wavelength, resulting in an increase in coupling efficiency away from the prism [80]. Fig. 12 shows the excitation spectra of CaF2:Eu2þ nanoparticles calcined with different amount of Cit-Ag suspension using optimum emission of 425 nm. The excitation peaks at 355 nm and 377 nm are decreasing with increasing amount of Cit-Ag nanoparticles. The reason may be that the absorption of Ag nanoparticles (shown in Fig. 7) overlaps well with the major emission peak (425 nm) and excitation peaks (355 nm and 377 nm) in the
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decrease in the lifetime of fluorophores. This agrees with our results, which show that the fluorescence intensity is higher, but lifetime is shorter. The higher quantum yield for CaF2:Eu2þ near a metal surface can be understood as a result of rapid energy transfer to the plasmons which then radiate to the far-field [82e84]. However, the quantum yield shows a dramatic decrease when the amount of Ag NPs is 500 mL. This decrease is interpreted by lossy surface waves (LSWs) which quench the fluorescence [85]. When CaF2:Eu2þ surface is covered with tremendous amounts of Cit-Ag nanoparticles, the decrease of quantum yield is due to at least two mechanisms. One is that the excited state population is reduced by LSWs. The other mechanism is that absorption of incident light in the Ag nanoparticles is dominated process and dissipated, which transfer to nonradiative decay energy.
4. Conclusions Fig. 12. Excitation spectra of CaF2:Eu2þ nanoparticles calcined with different amount of Cit-Ag suspension using optimum emission of 425 nm.
In summary, we have demonstrated a simple hydrothermal
Fig. 13. (a) PL decay curve and (b) quantum yield of CaF2:Eu2þ prepared with different amount of Cit-Ag nanoparticles.
CaF2:Eu2þ. Fig. 13a shows the PL decay curve of prepared CaF2:Eu2þ with Cit-Ag nanoparticles. The normalized curves fit well in single exponential function. A decrease in the fluorescence lifetime from 19.8 ms to 16.5 ms is observed with increasing amount of Ag NPs. Fig. 13b provides quantum yields of the samples at same amount of Cit-Ag nanoparticles. The absolute quantum yield of samples are determined to be 20.61%, 47.78% and 14.2%, corresponding to the addition of 100 mL, 200 mL and 500 mL of Ag NPs, respectively. The change of quantum yield of samples with increasing Ag NP amount proves the existence of surface plasmon resonance. Since Ag NPs also play a role as catalyst, the lifetime and quantum yield of the samples without Ag NPs are not given. Actually, the quantum yields and lifetimes are governed by the magnitudes of the radiative rate G and the sum of the nonradiative decay rates knr . The quantum yield Q and lifetime t of the fluorophore are given by Ref. [81]:
G G þ knr
(5)
t ¼ ðG þ knr Þ1
(6)
Q¼
The outcomes of higher radiative rate and lower nonradiative decay rates result in an increase in the quantum yield and a
method to synthesize CaF2:Eu2þ nanoparticles without using calcination in reducing atmosphere. EPR has confirmed that Eu ions are successfully reduced from Eu3þ to Eu2þ by the Cit-Ag catalyst. The obtained CaF2:Eu2þ phosphors exhibit intensive and broad emission centered at 423 nm due to the 4f65d1/4f7(8S7/2) transition. The presence of small amount of Cit-Ag nanoparticles not only catalyze the reduction of Eu ions, but also affect luminescence intensity of the phosphors by surface plasmon resonance. The surface plasmons are induced in the metal by the nearby excited fluorophores or incident light and radiate to the far-field to enhance the emission of phosphors. The interaction of fluorophores with metals can be modeled as the interaction of an oscillating dipole with a conducting metal surface. The interactions of incident light or nearby excited fluorophores with metallic surfaces requires wavevector matching at the phosphorsemetal interface. Although the metal nanoparticles are in close proximity to phosphors, the Ag nanoparticles with roughened surfaces increase the intensity of scattered plasmon emission and enhance luminescence intensity. The higher radiative rate and lower nonradiative decay rates result in an increase of quantum yield and a decrease of luminescence lifetime of the CaF2:Eu2þ phosphors. Moreover, radiating plasmons model can be used to explain the luminescence enhancement or quenching.
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Acknowledgments This work is supported by the National Natural Science Foundations of China (No. 21271074), Teamwork Projects funded by the Guangdong Natural Science Foundation (No. S2013030012842), Science and Technology Innovating Project funded by the Guangdong Province (No. 2014B090901045), Guangdong Provincial Science & Technology Project (No. 2015B090903074), and Guangzhou Science & Technology Project (201605030005). References [1] Y. Huang, C.M. Lieber, Integrated nanoscale electronics and optoelectronics: exploring nanoscale science and technology through semiconductor nanowires, Pure Appl. Chem. 76 (2004) 2051e2068. [2] M. Bruchez, M. Moronne, P. Gin, A.P. Alivisatos, Semiconductor nanocrystals as fluorescent biological labels, Science 281 (1998) 2013e2015. [3] Q.H. Wang, A.A. Setlur, J.M. Lauerhaas, J.Y. Dai, E.W. Seelig, R.P.H. Chang, A nanotube-based field-emission flat panel display, Appl. Phys. Lett. 72 (22) (1998) 2912e2913. [4] Heon-Jin Choi, Justin C. Johnson, Rongrui He, Sang-Kwon Lee, Franklin Kim, Peter Pauzauskie, Joshua Goldberger, Richard J. Saykally, Peidong Yang, Selforganized GaN quantum wire UV lasers, J. Phys. Chem. B 107 (34) (2003) 8721e8725. [5] A.P. Alivisatos, Nanocrystals: building blocks for modern materials design, Endeavour 21 (2) (1997) 56e60. [6] Y. Volokitin, J. Sinzig, L.J. de Jongh, G. Schmid, M.N. Vargaftik, I.I. Moiseevi, Quantum-size effects in the thermodynamic properties of metallic nanoparticles, Nature 384 (6610) (1996) 621e623. [7] F. Wang, R. Deng, X. Liu, Preparation of core-shell NaGdF4 nanoparticles doped with luminescent lanthanide ions to be used as upconversion-based probes, Nat. Protoc. 9 (2014) 1634e1644. [8] L.M. Jin, X. Chen, C.K. Siu, F. Wang, S.F. Yu, Enhancing multiphoton upconversion from NaYF4:Yb/Tm@NaYF4 core-shell nanoparticlesvia the use of laser cavity, ACS Nano 11 (2017) 843e849. [9] L. Song, J. Gao, R. Song, Synthesis and luminescent properties of oleic acid (OA)-modified CaF2: Eu nanocrystals, J. Lumin 130 (2010) 1179e1182. [10] P. Samuel, H. Ishizawa, Y. Ezura, K.I. Ueda, S.M. Babu, Spectroscopic analysis of Eu doped transparent CaF2 ceramics at different concentration, Opt. Mater 33 (2011) 735e737. [11] O. Mohammed, D. Pines, J. Dreyer, E. Pines, E. Nibbering, Sequential proton transfer through water bridges in acid-base, Science 310 (2005) 83e86. [12] K. Biswas, A.D. Sontakke, R. Sen, K. Annapurna, Luminescence properties of dual valence Eu doped nano-crystalline BaF2 embedded glass-ceramics and observation of Eu2þ / Eu3þ energy transfer, J. Fluoresc. 22 (2012) 745e752. [13] W. Zhang, W. Zhang, P. Xie, M. Yin, H. Chen, L. Jing, Y. Zhang, L. Lou, S. Xia, Optical properties of nanocrystalline Y2O3:Eu depending on its odd structure, J. Colloid Interf. Sci. 262 (2003) 588e593. [14] K. Riwotzki, H. Meyssamy, H. Schnablegger, A. Kornowski, M. Haase, Synthese von Kolloiden und redispergierbaren Pulvern stark lumineszierender LaPO4: Ce,TbNanokristalle, Angew. Chem. Int. Ed. 113 (2001) 574e578. [15] G. Wakefield, E. Holl, P.J. Dobson, J.L. Hutchison, Luminescence properties of nanocrystalline Y2O3:Eu, Adv. Mater 13 (2001) 1557e1560. [16] V.V. Vistovskyy, A.V. Zhyshkovych, N.E. Mitina, A.S. Zaichenko, A.V. Gektin, A.N. Vasil Ev, A.S. Voloshinovskii, Relaxation of electronic excitations in CaF2 nanoparticles, J. Appl. Phys. 112 (2012) 024325. [17] C. He, Y. Guan, L. Yao, W. Cai, X. Li, Z. Yao, Synthesis and photoluminescence of nano-Y2O3:Eu3þ phosphors, Mater. Res. Bull. 38 (2003) 973e979. [18] W. Ye, X. Liu, Q. Huang, Z. Zhou, G. Hu, Co-precipitation synthesis and selfreduction of CaF2:Eu2þ nanoparticles using different surfactants, Mater. Res. Bull. 83 (2016) 428e433. [19] Nanguo Liu, Bradley S. Prall, Victor I. Klimov, Hybrid gold/silica/nanocrystalquantum-dot Superstructures: synthesis and analysis of SemiconductorMetal interactions, J. Am. Chem. Soc. 128 (48) (2006) 15362e15363. [20] Joseph R. Lakowicz, Radiative decay engineering 5: metal-enhanced fluorescence and plasmon emission, Anal. Biochem. 337 (2) (2005) 171e194. [21] Wounjhang Park, Dawei Lu, Sungmo Ahn, Plasmon enhancement of luminescence upconversion, Chem. Soc. Rev. 44 (10) (2015) 2940e2962. [22] R.J. Amjad, M.R. Sahar, M.R. Dousti, S.K. Ghoshal, M.N.A. Jamaludin, Surface enhanced Raman scattering and plasmon enhanced fluorescence in zinctellurite glass, Opt. Express 21 (2013) 14282e14290. [23] K.H. Drexhage, Influence of a dielectric interface on fluorescence decay time, J. Luminescence 1e2 (1970) 693e701. [24] P. Yuan, Y.H. Lee, M.K. Gnanasammandhan, Z. Guan, Y. Zhang, Q. Xu, Plasmon enhanced upconversion luminescence of NaYF4:Yb,Er@SiO2@Ag coreeshell nanocomposites for cell imaging, Nanoscale 4 (2012) 5132e5137. [25] H. Zhang, Y. Li, I.A. Ivanov, Y. Qu, Y. Huang, X. Duan, Plasmonic modulation of the upconversion fluorescence in NaYF4:Yb/Tm hexaplate nanocrystals using gold nanoparticles or nanoshells, Angew. Chem. Int. Ed. 49 (2010) 2865e2868.
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Plasmon-enhanced fluorescence of CaF2:Eu2þ nanocrystals by Ag nanoparticles Weihao Ye a, Qiying Huang a, Xianfu Jiao a, Xiaotang Liu a, b, *, Guangqi Hu a a
Guangdong Provincial Engineering Technology Research Center for Optical Agricultural, College of Materials and Energy, South China Agricultural University, China b Department of Physics, University of Texas at Arlington, Arlington, TX, 76019, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 March 2017 Received in revised form 15 May 2017 Accepted 18 May 2017 Available online 19 May 2017
We report plasmon-enhanced fluorescence in Eu2þ doped CaF2 (CaF2:Eu2þ) nanocrystals assembled with Ag nanoparticles (NPs) using a simple hydrothermal method. The structure, morphology and composition of the nanocrystals are confirmed by X-ray diffraction (XRD), transmission electron Microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and UVevis absorption spectrum. Ag nanoparticles are ideal building blocks for plasmonic materials that can exhibit a wide range of unique and potentially useful optical phenomena. Surface plasmon enhancement makes significant contribution to enhance the blue fluorescence emission of CaF2:Eu2þ. The enhancement on the Eu emission in visible regions is up to a factor of 12.3-fold. As shown in time-resolved measurements, surface Plasmon Resonance (SPR) can reduce the fluorescence lifetime and increase the quantum yield. Furthermore, with increasing Ag concentration, the fluorescence intensity increases to its maximum and then decreases. © 2017 Elsevier B.V. All rights reserved.
Keywords: Calcium fluoride Europium Ag nanoparticles Surface plasmon Photoluminescence
1. Introduction In recent years lanthanide-doped nanomaterials for photonic applications have attracted great attention and been widely investigated for many fields, such as semiconductor [1], UV lasers [2], display [3] and biological labels [4]. What makes nanocrystals special is that each size has a completely distinct set of physical and chemical properties. As nanometer crystals can be precisely prepared now, materials with designed properties can be produced not only by changing their chemical composition but by controlling their sizes [5]. However, the properties of nanometer scale crystals differ from their bulk materials [6]. The nanocrystals with welldefined morphologies and accurately tunable sizes remains a challenge. Their physical and chemical properties vary drastically by geometrical factors such as shape, dimensionality, and size. So far, the host materials for fluoride nanocrystals have been mainly focused on NaYF4 and LaF3 [7,8]. But, lanthanide doping in these host lattice are induced by a high-intensity pulse laser or have
* Corresponding author. Guangdong Provincial Engineering Technology Research Center for Optical Agricultural, College of Materials and Energy, South China Agricultural University, China. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.jallcom.2017.05.181 0925-8388/© 2017 Elsevier B.V. All rights reserved.
low quantum yield. In contrast, calcium fluoride (CaF2) is very suitable as a host for rare earth ions due to its desirable properties, such as low refractive index, low phonon energy, high laser damage threshold [9e11]. To date, RE-doped CaF2 nanomaterials have attracted more attention because of its robustness among other fluorides. Among the rare-earth ions, CaF2:Eu2þ exhibit excellent properties because of 4f7-4f65d transition and the emission varies from ultraviolet to red region [12]. Particularly, increasing technological and industrial interest is given to research on the luminescence of optical nanomaterials due to their novel optical properties about emission lifetime, luminescence quantum efficiency, and concentration quenching [13]. It has been investigated that luminous efficiency depends on the particle size [14,15]. When particles are prepared in small sizes, the following factors become critical for luminescence intensity: (1) surface defect; (2) ratio between thermalization length of charge carriers and the nanoparticle sizes [16]. The reduction of the nanocrystals size leads to significant luminescence intensity decrease, which limits their further applications. To solve this question, extensive work has been carried out, such as modulating reaction condition [17], coating surfactants or shell [17,18], and preparing composite materials [19]. However, the effect of these enhancement mechanism is not obvious. By coupling with noble metals on the surface of luminescent
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nanocrystals, luminescence can be enhanced due to Surface Plasmon Resonance (SPR) [20e22]. Surface plasmon enhanced fluorescence has been widely studied and used in various fields starting with classic reports of Drexhage [23]. This can be an effective promoter of fluorescence when optimized to provide a large and highly confined local electric field enhancement that arises from the collective oscillations of the electrons due to resonance [24e26]. In particularly, metal nanoparticles such as gold, silver, and copper nanoparticles can show intense surface plasmon resonance absorption band in UVevis and near infrared (NIR) region [27,28]. Recently, most researchers have used Ag-NPs with desirable SPR to enhance luminous efficiency. For instance, according Xin Xu's report [29], the visible emission of Sr2Si5N8:Eu2þ phosphor can be enhanced by coupling with the localized surface plasmon oscillation of Ag nano-particles. Qiang Shi [30] reported on highly tunable enhancement of red emission from Mg0.1Zn0.9O phosphors by regulation of the LSP resonance of Ag nanoparticles. Hyungduk Ko [31] demonstrated a novel anticounterfeit device based on NIR-to-Vis upconversion luminescence from b-NaYF4:Yb3þ,Er3þ enhanced by the Ag nanowires-driven plasmonic effects. Similarly, Wen Xu [32], Hongwei Song [33], and other researchers [34e36] enhanced upconversion emission of NaYF4:Yb, Er nanocrystals by using noble metal. However, among these studies, a large number of researches are about up-converting materials and noble metal with small grain size. There are few reports on down-converting materials with resonance metal. With this motivation, we report herein a facile one-pot process to synthesize down-converting CaF2:Eu2þ attaching Ag nanoparitcles without any calcination. Using this method, we are able to synthesize nanoparticles with particle size about 60 nm. Meanwhile, a maximum luminescence enhancement of 12.3-fold is observed when the fluorescent intensifier Ag nanoparticles with a relatively large size of 85 nm are used. Tuning by switching between fluorescence enhancements and quenching is shown as a function of the concentration of Ag NPs coated on the surface of CaF2, a parameter easily controlled during synthesis. In reaction process, a reduction of Eu3þ/Eu2þ happens under the catalytically active Ag nanoparticles. In addition, the influences of the reaction time, temperature and pH value are also discussed. 2. Experimental 2.1. Synthesis of Ag nanoparticles The nanoparticles were synthesized using method described by P.C.Lee and D.Meisel [37]. Briefly, 45 mg AgNO3 was dissolved in 250 ml boiling water and the solution was kept boiling for half an hour. Under stirring, 1% sodium citrate aqueous solution (5 ml) was rapidly injected into above solution and the mixture was further refluxed for another 1 h. Sodium citrate as a mild reducing agent gradually changed the growth solution from colorless to brownish. The suspension was then cooled to room temperature under stirring and stored at 4 C for later use. 2.2. Synthesis of CaF2: Eu2þ(2.5%) nanoparticles CaF2:Eu2þ nanoparticles were prepared by hydrothermal method. The EuCl3$6H2O were obtained by adding hydrochloric acid to Eu2O3. In a typical synthesis, 3.40 mmol CaCl2$2H2O and 0.43 mmol EuCl3$6H2O were dissolved in 15 ml distilled water under stirring followed by dropwise addition of 15 ml 68.30 mmol NH4F. After a white homogeneous dispersion appeared, a certain amount of Cit-Ag colloids were injected and a well-controlled amount of NaOH solution was added under magnetic stirring to reach different pH values. Next, 0.40 g NaBH4 was added and the
mother solution containing white suspension was sealed in 50 ml Teflon-lined stainless steel autoclave. The mixture was treated at different temperature and time. Temperature rate is 10 C/min. Finally, the obtained suspensions were washed with ethanol and deionized water for several times and dried in a vacuum oven at 60 C. 2.3. Characterization The XRD measurements of synthesized samples were carried out using a XD-2X/M4600 X-ray diffractometer with Cu-Ka radiation (l ¼ 0.154056 nm) at 40 kV and 30 mA in a scan range 10e80 . The morphology and EDS were observed by a JSM-7001F field emission scanning electron microscope (FESEM). A JEOL-2100F High-Resolution Transmission Electron Microscope (HRTEM) operated at 200 kV was used to observe the grain size and examine the lattice fringes. Samples for TEM were prepared by depositing a drop of sample dispersed in ethanol onto a holey carbonecoater copper grid. The photoluminescence (PL) of the powder samples was measured by a Shimadzu RF5301pc fluorescence spectrophotometer. UVevis absorption spectra were measured using a Shimadzu UV-2550 spectrophotometer with a wavelength range from 200 to 800 nm. The concentration of Ag nanoparticles was analyzed by Varian 810 ICP-MS. EPR spectra were acquired with a Bruker A300-10/12 spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were performed in a ESCALAB 250Xi with an Al Ka X-ray source with constant pass energy of 30 eV. 3. Results and discussion 3.1. Structure and formation of CaF2:Eu2þ nanoparticles The composition and crystal phase of the nanocrystals are characterized by XRD. Fig. 1a and b shows the XRD patterns of 2.5mol% Eu2þ doped CaF2 nanoparticles with different amount of Cit-Ag nanoparticles and heating temperatures, respectively. Based on the XRD diffraction peaks, the samples are well-crystallized. The diffraction peaks in Fig. 1 could easily be assigned to the (111), (220), (311), (400) and (331) planes of face-centered cubic (fcc, space group: Fm3m [38]). There is no impurity peaks observed. These values obtained from all the samples match very well with CaF2 standard JCPDS cards (35-0816). There is no apparent difference observed after Eu2þ doping or Cit-Ag nanoparticles addition. The lattice constant for the CaF2:Eu2þ is calculated to be 5.39 Å, which is close to 5.46 Å for pure CaF2 nanocrystals [39]. The Eu2þ ions may occupy the Ca2þ sites in the CaF2 host lattice. Based on the halfmaximum (FWHM) values of the (111) peaks, the mean size of the CaF2 nanocrystals is evaluated to be about 56 nm by DebyeeScherrer formula [40]. As shown in Fig. 1a, the strong and sharp diffraction peaks indicate that the CaF2 with different amount of CitAg nanoparticles are well crystallized. As heat treatment temperatures rise, the intensity of the diffraction peaks increase significantly (Fig. 1b), which implies a better crystallinity. The broadening of these diffraction peaks is an indication of small size samples. XPS can be used to check the composition of surfaces, which is carried out in the 0e1300 eV binding energy range to identify the elements on the surface of samples. Fig. 2a depicts the survey of the samples, which indicates that all the samples are mainly consist of Ca, F, Eu, and some impurity elements of C, O. The 2p levels of Ca show the spin-orbit doublet with the binding energies around 348.6 eV (2p3/2) and 352.2 eV (2p1/2) in Fig. 2b [41]. Moreover, the Ag 3d5/2 and Ag 3d3/2 core level binding energies for Ag NPs sample appear at 368.1 and 374.1 eV in Fig. 2c, which are higher than the reference value from J. S. Hammond [42] and Schon, G [43] due to the larger size [44]. However, a broad peak of the Ag 3d5/2 peaks as
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Fig. 1. XRD patterns of the CaF2: Eu(2.5%) nanoparticles (a) with different amount of Cit-Ag nanoparticles at 200 C (b) heated at different temperatures with 200 mL Cit-Ag nanoparticles.
the oxidation state is observed. The broad Ag 3d5/2 peak has been deconvoluted into two peaks which are related to Ag2O (365.2 eV) and AgO (364.4 eV). With exposure to damp air, the Ag 3d5/2 peaks shift continuously to lower binding energy as oxidation occured on the surface of Cit-Ag nanoparticles. Fig. 2d shows XPS spectrum of Eu ions. The peaks at 1135.2 and 1164.6 eV represent the existence of Eu3þ ions, while the peaks at 1124.8 and 1154.4 eV reveal the existence of Eu2þ ions [45,46]. Actually, the peaks of Eu ions are relatively weak as the low doping concentration of Eu. Their valence state cannot be resolved due to their weak binding energies. One of the most informative methods to study the structure of defects formed by paramagnetic ions is the EPR method. The electronic structure of the doping ions in the diamagnetic crystal matrix of the crystal is very sensitive to variations in the fine and hyperfine structure of the fields [47]. Fig. 2e shows the EPR spectra of CaF2:Eu2þ nanoparticles observed at room temperatures(293 K). The spectra exhibit the resonance signals with the effective g values at 1.975, which is consistent with R.S. Title's result [48]. The Eu2þ ion has a half-filled electronic configuration of 4f7 which generates an 8S7/2 ground state. The strong hyperfine interaction of Eu2þ splits each energy level into six sublevels. However, the lines in higher fields are wide and structureless. The reasons for this are the superposition of signals from the two uneven Eu isotopes, 151Eu and 153 Eu [49] and large anisotropy [50]. Thus, only a hyperfine structure line of the central transition <þ1/2j4<1/2j of Eu2þ EPR spectra is observed. Based on the EPR result, the reduction of Eu3þ/Eu2þ has occurred by the Cit-Ag catalyst. EDS data in Fig. 3 taken from a large area containing several crystals further shows atom percentage of F, Ca, Eu, Ag of 40.32%, 20.34%, 0.58%, 0.01%, respectively, The values are almost consistent with their molar concentration in the molecular formula CaF2: Eu2þ(2.5%). However, due to sensitivity limit of equipment, the proportion of Ag is not inaccurate. No other impurities are evident in the figure implying that the resulting CaF2: Eu2þ nanocrystals have high purity in chemical composition. It should be pointed out that the weak signals of C are originated from conducting resin used to support the SEM sample. 3.2. TEM TEM measurement is carried out to observe the size and morphology of the Ag nanoparticles prepared by the method from P.C.Lee and D.Meisel. In the syntheses of Ag nanoparticles using citrate reduction, the reactivity of silver precursor is almost not affected by the molar ratio or pH value since the coordination of Ag ions is dominated by citrate in a wide range of pH [51]. As shown in
Fig. 4a, all the particles prepared are quasi-spherical. The particles agglomerate severely due to the high speed centrifugation for preparing TEM samples. The size of Cit-Ag NPs is about 80e100 nm with a relatively large range, which is consistent with the results reported previously [52]. It is noticed that, when the Ag NPs are prepared at boiling temperature, a large average size of Ag NPs with a broad size distribution is observed since heat-treatment speeds up the Oswald ripening process. Fig. 4b is a high resolution TEM image obtained at the edge of a typical Ag nanoparticle, which shows a highly ordered lattice fringe. The lattice spacing is measured as 0.2500 nm, which is corresponding to the (0 0 4) plane of Ag nanoparitcles. The inset shows the sample's SAED diffraction pattern. The presence of ring-shaped patterns is not in conflict with their single-crystalline nature, because the SAED patterns were recorded from an area containing a large number of nanoparticles. Fig. 5 displays the HRTEM images of CaF2: Eu2þ (2.5%) nanoparticles treated with 200ul Cit-Ag nanoparticles. The average grain size is about 50e58 nm, which is in good agreement with the calculated result from DebyeeScherrer formula. CaF2 nanocrystals are nearly spherical particle with relatively even sizes, except that one is very small. It is attributed to the Ostwald ripening process. The large particles receive atoms from small dissolved particles. Thus, small nanoparticles shrink and eventually disappear while larger nanoparticles grow larger and have better crystallinity [53]. Moreover, for precipitation formation, the rate of nucleation is much higher than crystal growth when the supersaturation ratio is high. Fig. 5d is a schematic diagram of CaF2:Eu2þ crystal growth. The morphology of nanoparticles is basically cubic and large at low supersaturated condition. However, previous study has demonstrated that the nanoparticles change into hexagonal or spherical and smaller with increasing supersaturation ratio [54]. The grain size is still reducing due to a more rapid rate of nucleation, even at higher concentration of the component ions. According to highresolution TEM micro-graphs, the CaF2 nanoparticles are single crystalline. A selected-area electron diffraction (SAED) pattern in Fig. 5b clearly shows the characteristic spots of single-crystal CaF2:Eu2þ, which is ascribed to specific crystal planes of their corresponding XRD patterns. The periodicity of the lattice fringes obtained for the nanocrystals is d ¼ 0.31546 nm in Fig. 5c, corresponding to the (111) planes of CaF2. The TEM images also indicate that the surface region of CaF2:Eu2þ is highly ordered. 3.3. SEM The general morphologies of the as-prepared samples are not affected by doped Eu2þ ions or Ag addition in CaF2. The influences
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Fig. 2. XPS spectra of CaF2:Eu2þ nanoparticles; (a) is a survey while (b),(c) and (d) show the spectrum of Ca,Ag, and Eu, respectively. (e) is EPR spectrum of the central transition <þ1/2j4<1/2j of Eu2þ in CaF2:Eu2þ.
of heating temperature on CaF2 nanoparticle phase and crystallinity have been investigated by FESEM. Fig. 6 shows SEM images of CaF2:Eu nanoparticles treated with 200 ml Cit-Ag at 20 h for 150 C, 160 C, 170 C, 18 C, 190 C and 200 C. It is found that temperature had significant influence on the morphology of final products. With temperature increasing, the size of obtained samples is changed significantly and about 50e70 nm. Serious aggregation can be seen in the phosphors heated at those higher temperatures. For the samples heated at 200 C (Fig. 6a) and 190 C (Fig. 6b), hard particle aggregation with grain size of 60e75 nm have been observe and most of the microspheres are attached to each other to form irregular bulk-like aggregates of several microns via a particle growth process. As seen in Fig. 6c, the as-prepared phosphors that were heated for 180 C have displayed initial aggregation. They are about 60 nm and nonspherical. However, after reducing the
reaction temperature, the morphology of phosphors is notably improved. With the hydrothermal treatment for 170 C (Fig. 6d), a large number of homogeneous and monodisperse spherical nanoparticles are observed, with narrow size distribution of 55e60 nm. No irregular bulk-like aggregates but only nanoparticles with diameters about 55 nm are found when the reaction temperature is 160 C (Fig. 6e) and 150 C (Fig. 6f). On the basis of the aforementioned results, it is concluded that with the reaction temperature increasing, the sample size is increased and morphology becomes irregular bulk-like aggregates. Chemical composition is also of importance in the luminescent properties of phosphors [55]. Elemental mapping was conducted to investigate chemical composition inside the nanocrystals using FESEM with energy dispersive X-ray spectroscopy (EDS). Fig. 7 shows the mapping of CaF2:Eu2þ nanocrystals at 200 C for 20 h.
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Fig. 3. EDS spectrum of the CaF2: Eu(2.5%) heated at 200 C for 20 h with 200 mL Cit-Ag nanoparticles.
Fig. 4. TEM images of Cit-Ag nanoparticles at (a) low-resolution and (b) high-resolution. The inset in (b) shows the SAED pattern of Ag nanoparticles.
Fig. 5. (a) and (c) are TEM images of CaF2:Eu2þ at different resolutions. (b) SAED pattern of CaF2: Eu2þ. (d) shows a schematic diagram of CaF2:Eu2þ crystal growth.
Ca and F are main component and well distributed inside asprepared nanocrystals. The spatial distribution of Ca rich crystallites on surface correlates with cubic face-centered lattice. Eu atoms
distribute relatively well, indicating a homogeneous Eu2þ doping in the sample. Moreover, a relatively weak Ag signal is also observed on the surfaces of samples, which could have an affect on
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Fig. 6. SEM images of the CaF2 samples prepared at (a) 200 C for 20 h, (b) 190 C for 20 h, (c) 180 C for 20 h, (d) 170 C for 20 h, (e) 160 C for 20 h and (f) 150 C for 20 h.
Fig. 7. EDS mapping of CaF2:Eu phosphors.
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luminescent properties. 3.4. UVevis and ICP-MS spectroscopy analysis of Ag nanoparticles suspensions The metal colloid exhibits unique absorption bands or broad regions of absorption in the ultravioletevisible range due to the excitation of plasma resonances or interband transitions. These complex properties are due to conducting electrons which migrate freely but experience some resistance to the interband transitions present in most metals [56]. When visible light at an appropriate frequency is incident, the noble metals that possess subwavelength metallic features support surface plasmons e collective excitations of free electrons in a metal e that can locally amplify incident electromagnetic fields by orders of magnitude at the metal surface. Surface plasmon resonance is based on the coupling of electromagnetic radiation into a conducting metal. It is a very characteristic property of the metallic nature of the particles [57]. The position and the number of peaks are also an indication of the particle shapes. The UVevis absorption spectrum of prepared sample is shown in Fig. 8. The obtained Cit-Ag NPs have an absorption peak at 426 nm which display the characteristic surface plasmon resonance (SPR) band of 4d/5s from 350 to 600 nm. The broadness of the peak indicates a broad distribution of particle sizes in the solution. For spherical or near-spherical Ag particles, there is only one band centered at about 400 nm [58]. In our case, the peak shifts to the red with increased particle's diameter [59]. The total amount of elemental silver attached to CaF2 nanoparticles is determined by ICP-MS. The concentration of mother solution of Cit-Ag is 1.039 103 mol/L. The Ag nanoparticles attached to the phosphors prepared with different volume of Cit-Ag suspensions affect emission intensities due to the surface plasmon resonance. However, the amount of attached Ag nanoparticles is very low (Table 1). 3.5. Photoluminescence properties of CaF2:Eu2þ and plasmonenhanced photoluminescence by Ag NPs The Eu2þ ion enters the CaF2 lattice by substitutional Ca2þ ions and will be in cubic surroundings [60]. Each Eu2þ ion is coordinated with eight fluorine ions. Eu2þ only luminesce on a divalent or a monovalent cation site. Fig. 9 shows photoluminescence spectra of CaF2:Eu. An intense emission peaking at 425 nm is seen in the sample under 337 nm excitation and this is a characteristic Eu2þ emission band, which indicate catalytic performance of Cit-Ag nanoparticles. The wide emission from 400 to 480 nm with a
Fig. 8. UVevis absorption spectra of Cit-Ag NPs suspension.
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Table 1 The concentration of Ag nanoparticles attached on phosphors. Cit-Ag suspension volume/ml
Amount/ppm
100 200 300 400 500
23.829391144 55.389243026 85.099877632 125.180735724 204.201339872
peak at 425 nm is due to allowed electron dipole transitions from the excited state of the mixed 4f65d1 configuration of the Eu2þ ion to the 8S7/2 level of the 4f7 configuration in the ground state [61]. Fig. 9a also shows the excitation of CaF2:Eu2þ heated at 200 C for 20 h with 200 mL Cit-Ag sols. In the excitation spectrum, a weak band is located at around 245 nm and several strong bands in the form of a typical staircase structure are observed around 337, 355, 367, 377 and 397 nm. Depending on the strength of the octahedral crystal field, the excited 5d level of Eu2þ can be split into two degenerated levels and the energy difference between the two groups of degenerated 5d levels are highly affected by the state of chemical bonds between Eu2þ ions and the anions [62e64]. These two absorption bands correspond to the electron transitions in lower excited states 4f7[8S7/2]/4f65d1[eg] and higher excited states 4f7[8S7/2]/4f65d1[t2g] respectively [63,65]. The lower eg bands are seven narrow bands, referred to as a “staircase spectrum,” which retain the character of the seven (Eu2þ) 4f6 levels (7F02þ 6) levels. The “staircase spectrum” is due to the transition from Eu 8 S7/2 ground state to the lowest levels of the excited configuration of Eu2þ are of the type 4f6[7FJ ]5d1, where the 7F term is spineorbit split into the 7FJ multiplets with J ¼ 0-6. In addition, the 5d electron can also be found in higher 5d levels, and in most compounds the 4f n1 7 [ FJ]5d2 levels overlap with the first ‘staircase’, leading to broad excitation or absorption bands where all structure is washed out [66]. No absorption peaks related with Eu3þ ions are detected [67]. Fig. 9b shows the spectra of CaF2:Eu without any Cit-Ag nanoparitcles. A few minor emission peaks are observed at 570, 589, 613 and 627 nm, which are ascribed to fef transitions of Eu3þ of 5D0 to 7 F0, 7F1, 7F2, and 7F3, respectively [68]. However, the characteristic emission peak at 425 nm of Eu2þ is not observed, which indicate the conversion of Eu3þ/Eu2þ dose not occur without catalytic CitAg nanoparticles. The peaks near 469 nm are due to emission of excited states of Xe atom [69]. Fig. 10a shows the emission spectra of CaF2:Eu2þ nanoparticles calcined with different amount of Cit-Ag suspension using optimum excitation of 377 nm. The strong emission band peaked at 425 nm is generally assigned to 4f65d1/4f7(8S7/2). The spectra reveals that the fluorescence intensity of the nanocrystals is enhanced with increasing amount of Cit-Ag NPs from 100 mL to 200 mL, and then quenched as the amount is over 200 mL. The best amount is found to about 200 mL and a maximum luminescence enhancement of 12.3-fold is observed. Meanwhile, the emissions of Eu3þ are relatively weak due to the strong catalytic activity of CitAg nanoparticles and the intensity of Eu3þ emission at 570 nm, 589 nm and 613 nm is declined gradually with the increase amount of Cit-Ag nanoparticles shown in Fig. 10b. The enhancement effect from the Ag NPs may be attributed to surface plasmon resonance (SPR). The mechanism of the enhancement is investigated in terms of two prominent aspects related to the noble metals: excitation enhancement and emission enhancement. The excitation enhancement is an increase of the excitation rate or effective excitation flux due to the metal amplifying the incident field. The emission enhancement is an increase of the intrinsic radiative decay rate of phosphors by surface plasmon-coupled emission (SPCE) [70e73]. The interactions of phosphors with metallic
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Fig. 9. Excitation and emission spectra of (a) CaF2: Eu2þ calcined at 200 C for 20 h with addition of 200 mL Cit-Ag sols; (b) CaF2:Eu without Cit-Ag sols.
surfaces have been used to obtain increased phosphors luminescence intensities. In general, the quenching and enhancement are determined by the distance-dependent mechanism. The quenching of fluorescence happened when the metal nanoparticles are in close proximity to phosphors. However, our result is in stark contrast to the usual perception that the emission is strongly quenched and lifetime drop dramatically when the phosphors are close to the metal [71,74e76]. This quenching effect is attributed to losing surface waves (LSWs) [23]. The enhancement or quenching of phosphors can be explained by radiating plasmons (RPs) model [20] and classical electrodynamics. Fig. 11 shows the schematic
illustration of radiating plasmons (RPs) model. The surface plasmons are induced in the metal by the nearby excited fluorophores or incident light and radiate to the far-field to enhance the emission of phosphors. The surface plasmon is used to describe the collective oscillations of a group of electrons in a metal [77]. The interaction of fluorophores with metals can be modeled as the interaction of an oscillating dipole with a conducting metal surface. The interactions of incident light or nearby excited fluorophores with metallic surfaces requires wavevector matching at the phosphorsemetal interface. At a dielectricemetal interface the surface plasmon wavevector is given by
Fig. 10. (a) Emission spectra of CaF2: Eu2þ phosphors with addition of different amount of Cit-Ag nanoparticles. The inset reveals CaF2:Eu2þ luminescence intensity as a function of Cit-Ag amount (b) The volume of Ag NPs added affects luminescence intensity of different emission peaks in CaF2: Eu2þ phosphors.
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Fig. 11. Schematic illustration of radiating plasmons (RPs) model.
ksp ¼ k0
ε1 εm ε1 þ εm
1=2 (1)
where k0 is the wavevector in a vacuum, ε0 is the dielectric constant and ε1 is the dielectric constant of a propagating light in a medium. However, the dielectric constant km experienced by a wave incident on a metal depends on the incident light frequency
km ¼ nm c
u
(2)
pffiffiffiffiffiffi εm
(3)
nm ¼
εm ¼ 1
u2p u2
(4)
where nm is the refractive index of metal, εm is the dielectric constant of metal, up and u are the plasmon frequency of the electrons and frequency of incident light, respectively. The wavevector match at the interface and plasmon radiate out of the metal when ksp equals km . The surface plasmons on the metal surface create an
evanescent field which penetrates into the phosphors and fluorophore emission is observed as plasmon-coupled emission [78]. According to H. Raether [79], surface plasmon resonances in the metal are observed using incident light on the metal through a high refractive index prism or some higher dielectric-constant materials. The wavelength of incident light is shorter using prism, which satisfy the conditions of phosphors-metal interaction. However, in our results, surface plasmon resonances created in the metal at short distances enhance fluorescence without prism or higher dielectric-constant material. It is ascribe to Ag nanoparticles with roughened surfaces displayed by the TEM image of Cit-Ag nanoparticles in Fig. 4a, which increase the intensity of scattered plasmon emission. The light is scattered at incident wavelength, resulting in an increase in coupling efficiency away from the prism [80]. Fig. 12 shows the excitation spectra of CaF2:Eu2þ nanoparticles calcined with different amount of Cit-Ag suspension using optimum emission of 425 nm. The excitation peaks at 355 nm and 377 nm are decreasing with increasing amount of Cit-Ag nanoparticles. The reason may be that the absorption of Ag nanoparticles (shown in Fig. 7) overlaps well with the major emission peak (425 nm) and excitation peaks (355 nm and 377 nm) in the
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decrease in the lifetime of fluorophores. This agrees with our results, which show that the fluorescence intensity is higher, but lifetime is shorter. The higher quantum yield for CaF2:Eu2þ near a metal surface can be understood as a result of rapid energy transfer to the plasmons which then radiate to the far-field [82e84]. However, the quantum yield shows a dramatic decrease when the amount of Ag NPs is 500 mL. This decrease is interpreted by lossy surface waves (LSWs) which quench the fluorescence [85]. When CaF2:Eu2þ surface is covered with tremendous amounts of Cit-Ag nanoparticles, the decrease of quantum yield is due to at least two mechanisms. One is that the excited state population is reduced by LSWs. The other mechanism is that absorption of incident light in the Ag nanoparticles is dominated process and dissipated, which transfer to nonradiative decay energy.
4. Conclusions Fig. 12. Excitation spectra of CaF2:Eu2þ nanoparticles calcined with different amount of Cit-Ag suspension using optimum emission of 425 nm.
In summary, we have demonstrated a simple hydrothermal
Fig. 13. (a) PL decay curve and (b) quantum yield of CaF2:Eu2þ prepared with different amount of Cit-Ag nanoparticles.
CaF2:Eu2þ. Fig. 13a shows the PL decay curve of prepared CaF2:Eu2þ with Cit-Ag nanoparticles. The normalized curves fit well in single exponential function. A decrease in the fluorescence lifetime from 19.8 ms to 16.5 ms is observed with increasing amount of Ag NPs. Fig. 13b provides quantum yields of the samples at same amount of Cit-Ag nanoparticles. The absolute quantum yield of samples are determined to be 20.61%, 47.78% and 14.2%, corresponding to the addition of 100 mL, 200 mL and 500 mL of Ag NPs, respectively. The change of quantum yield of samples with increasing Ag NP amount proves the existence of surface plasmon resonance. Since Ag NPs also play a role as catalyst, the lifetime and quantum yield of the samples without Ag NPs are not given. Actually, the quantum yields and lifetimes are governed by the magnitudes of the radiative rate G and the sum of the nonradiative decay rates knr . The quantum yield Q and lifetime t of the fluorophore are given by Ref. [81]:
G G þ knr
(5)
t ¼ ðG þ knr Þ1
(6)
Q¼
The outcomes of higher radiative rate and lower nonradiative decay rates result in an increase in the quantum yield and a
method to synthesize CaF2:Eu2þ nanoparticles without using calcination in reducing atmosphere. EPR has confirmed that Eu ions are successfully reduced from Eu3þ to Eu2þ by the Cit-Ag catalyst. The obtained CaF2:Eu2þ phosphors exhibit intensive and broad emission centered at 423 nm due to the 4f65d1/4f7(8S7/2) transition. The presence of small amount of Cit-Ag nanoparticles not only catalyze the reduction of Eu ions, but also affect luminescence intensity of the phosphors by surface plasmon resonance. The surface plasmons are induced in the metal by the nearby excited fluorophores or incident light and radiate to the far-field to enhance the emission of phosphors. The interaction of fluorophores with metals can be modeled as the interaction of an oscillating dipole with a conducting metal surface. The interactions of incident light or nearby excited fluorophores with metallic surfaces requires wavevector matching at the phosphorsemetal interface. Although the metal nanoparticles are in close proximity to phosphors, the Ag nanoparticles with roughened surfaces increase the intensity of scattered plasmon emission and enhance luminescence intensity. The higher radiative rate and lower nonradiative decay rates result in an increase of quantum yield and a decrease of luminescence lifetime of the CaF2:Eu2þ phosphors. Moreover, radiating plasmons model can be used to explain the luminescence enhancement or quenching.
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