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Yb3+ phosphor
Accepted Manuscript Plasmonic enhancement of upconversion emission of Ag@NaYF4:Er 3+ Yb phosphor
3+
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S.K. Maurya, S.P. Tiwari, A. Kumar, K. Kumar PII:
S1002-0721(18)30292-8
DOI:
10.1016/j.jre.2018.03.003
Reference:
JRE 159
To appear in:
Journal of Rare Earths
Received Date: 3 November 2017 Revised Date:
2 March 2018
Accepted Date: 5 March 2018
Please cite this article as: Maurya SK, Tiwari SP, Kumar A, Kumar K, Plasmonic enhancement of 3+ 3+ upconversion emission of Ag@NaYF4:Er /Yb phosphor, Journal of Rare Earths (2018), doi: 10.1016/ j.jre.2018.03.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Plasmonic enhancement of upconversion emission of Ag@NaYF4:Er3+/Yb3+phosphor S. K. Maurya, S. P. Tiwari, A. Kumar, K. Kumar*
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Optical Materials and Bio-imaging Research Laboratory, Department of Applied Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad-826004, India
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Abstract: In this article upconversion luminescence of silver nanoparticles (AgNPs) coated NaYF4:Er3+/Yb3+phosphor nano-particles are investigated. The prepared samples were characterized through various techniques. The surface plasmon band was observed for prepared AgNPs by analyzing UV-Vis. measurements and was used to enhance the upconversion emission. From the upconversion measurement the emission bands are observed at 522, 546, and 656 nm corresponding to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 levels, respectively. The upconversion emission intensity of the above bands is found to enhance for sample containing 1 mmolAgNPs. Decay time of 4S3/2 and 4F9/2 levels is found to decrease on coating of AgNPs and hence intensity enhancement is assumed due to the surface plasmon resonance (SPR) effect.
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Keywords: Upconversion phosphor, Surface plasmon resonance, Decay time analysis, CIE diagram; Rare earths
Foundation item: Project supported by the Indian Institute of Technology (Indian School of Mines), Dhanbad, India and the Council of Scientific & Industrial Research (CSIR), New Delhi, India (03(1303)13/EMR). *Corresponding author: K. Kumar (E-mail:[email protected], Tel: +91-3262235754)
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1. Introduction
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Lanthanide doped/codopedupconversion materials have attracted much attention for their applications in the field of solar cell, biological fluorescence imaging, fingerprint detection, cancer therapy, etc. [1-7]. Upconversion phosphors possess intense emission through 4f-4f or 4f-5d transitions with unique properties such as sharp emission spectra and long emission time, [8]. The utility of single rare earth ion doped phosphor such as Er3+, Tm3+, Ho3+ is limited due to the poor quantum efficiency and low absorption cross section. To improve the emission efficiency the researchers are trying with various means such as incorporating silver/gold nanoparticles in the host, choosing the host matrices or co-doping other ions to improve the emission intensity. The matrices such as bromides, sulphides, oxysulfides have low phonon energies and good chemical stability [3]. These halide hosts are found hygroscopic in nature and hence have limited use in bio-imaging and photo-dynamic therapy [3-5]. In comparison to other halides and oxides, fluorides exhibit low phonon energy and high chemical stability. NaYF4 is considered most popular host among phosphor materials as it possesses the highest upconversion luminescence quantum efficiency[3, 8]. From the last decade, researchers are trying to improve the upconversion emission intensity by utilizing surface plasmon resonance (SPR) of metal nanoparticles[9-11]. In order to obtain plasmonenhanced upconversion emission, many research articles have been published on designing the core shell structures[10-17] and constituting of photonic crystals[11, 13]. The plasmonic enchancement is reported to work by two mechanisms: (a) SPR induced enhancement in absorption of emitting centers due to the local field enhancement effect; (b) increase of radiative decay rate of emitting centers due to the resonance of emission of surface plasmon of metallic particles [14, 15]. The plasmon absorption band is dependent on size and shape of metallic particles, hence by tuning the size and shape the maximum upconversion enhancement could be achieved. Collective oscillation of electrons of metal nanoparticle generates SPR. The electron oscillation in metallic particles affects the local field around the emitting ion and this interaction is near field[14, 18, 19]. The Au and Ag are well known plasmonic particles whose SPR absorption lies in the visible region and hence incorporation of these metals particles in phosphor materials has been studied for upconversion enhancement of organic dyes[20-22] and quantum dots[23]. Yin et al.[24] have reported the enhancement in upconversion emission intensity due to the local field modulation induced combining SPR effect and photonic crystal effect in NaYF4/AuNRs hybrids film. Schietinger et al.[25] have studied the upconversion emission enhancement from NaYF4:Er3+/Yb3+ nanoparticles coated with Au spheres and found several foldenhanced emission intensity. Tiwari et al.[26] have studied the upconversion enhancement in La2O3:Er3+/Yb3+ phosphor by introducing Ag nanoparticlesat different concentrations. Yuan et al.[27]have reported NaYF4:Er3+/Yb3+@SiO2@Ag upconversion nanoparticles and used silica as spacer between the upconverting core and Ag shell. Severalother researchers have also tried to improve the upconversion emission through direct coupled spherical Ag and Au particles[28-31].
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ACCEPTED MANUSCRIPT In this report, authors have investigated plasmon enhanced upconversion in Ag@NaYF4:Er3+/Yb3+ phosphornanoparticles by illuminating 976 nm diode laser. Remarkable enhancement in upconversion emission intensity of NaYF4:Er3+/Yb3+ nanoparticles coated with AgNPs are recorded up to 12 fold compared to bare NaYF4:Er3+/Yb3+ sample. The mechanism emission enhancement due to the SPR effect is discussed.
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2. Experimental 2.1.Chemicals
2.1.1. Synthesis of silver nanoparticles
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All the preparatory materials (chemicals) used for synthesis of NaYF4:Er3+/Yb3+ and AgNPs were yttrium oxide (Y2O3, 99.99% Otto, India), erbium oxide (Er2O3, 99.99% Otto, India) and ytterbium oxide (Yb2O3, 99.99% Otto, India), sodium nitrate (NaNO3, 99% Merck, India), ammonium fluoride (NH4F, 99.9% LobaChemie, India), silver nitrate (AgNO3, 99.99% Merck, India), nitric acid (HNO3, 69% dilution, Merck, India) and ethylenediaminetetraacetic acid (C10H16N2O8, 99% LobaChemie, India).
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AgNPs were synthesized by reducing silver nitrate (AgNO3) with tri-sodium citrate[32]. In a typical preparation procedure, 0.11 mmol of AgNO3 was dissolved in 100 ml deionized water and heated to 100 ºC. At this temperature 2 mL of 1 % sodium citrate solution was dropwise mixed under vigorous stirring. After 30 min the solution was cooled down to room temperature. Centrifugation of sample was done and collected precipitate was dried whole night at 70 ºC to get the dispersible powder.
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2.1.2. Synthesis of NaYF4:Er3+/Yb3+ and Ag@NaYF4:Er3+/Yb3+
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Sodium yttrium fluoride (NaYF4) nano-crystals doped with Er3+(3%)/Yb3+(17%)[33] ions were prepared by co-precipitation method[34, 35]. Rare earth nitrates were first prepared by refluxing stochiometric amounts of rare earth oxides (1.5 mmol of Y2O3, 0.32 mmol of Yb2O3 and 0.06 mmol of Er2O3) with nitric acid at 80 ºC until transparentgel was obtained and again heated upto 90 ºC for 15 min to remove the excess amount of un-reacted nitric acid (solution A). After that 0.3239 gm NaNO3was dissolved in 10 mL deionized water to form solution B and 0.5647 gm NH4F was also dissolved in 10 mL deionized water to form solution C. 5 mL of EDTA was mixed to solution as chelating agent to stabilize the particle’s growth. Now solutions B and C were mixed into solution A dropwise under vigorous stirring at 85 ºC. After half an hour white precipitate was obtained after centrifuging and was washed three times with ethanol and then dried at 70 ºC overnight. Further, powder form of AgNPs was dispersed in deionized water using ultrasonic bath. TheNaYF4:Er3+/Yb3+sample solution was mixed with three different molar concentrations (1, 2, 3 mmol) of colloidal AgNPs. The mixture was left 15 min for coagulation and then dried at 70 ºC. Collected samples having 0, 1, 2 and 3 mmol AgNPs@NaYF4:Er3+/Yb3+ were used for further study.
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Crystal phase of all prepared samples were investigated using X-ray diffractometer (Bruker-Nonius FR-590). Impurities present in samples were investigated by Fourier transform infrared (FTIR) analysis on Spectrum RX-1 (PerkinElmer, USA). Particles size and surface morphology were studied on a field emission scanning electron microscope (FESEM) images using Supra-55 (Carl Zeiss, Germany). The transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were recorded on Tecnai 20 G2, USA for the confirmation of AgNPs. The UV-Vis absorption spectra of samples were monitored in an absorbance modeon Lambda 35 (PerkinElmer, USA) spectrophotometer. For absorption measurement the powder phosphor sample was dispersed in ethanol (1:10) and under sonication for 10 min. Immediately after sonication UV-Vis absorption measurement was done. The upconversion emission spectra were recorded on a CCD spectrometer (ULS2048X64, Avantes, USA) using 976 nm diode laser as excitation sourcehaving spot size of 1.2 mm at the sample surface. A 750 nm (model: FES0750, Thorlabs, USA) short pass filter was used to block the unwanted laser excitation. The decay curves were measured using 976 nm diode laser and InGaAs detector coupled with a digital oscilloscope. All measurements were analyzed at room temperature. 3. Results and discussion 3.1. Structural analysis 3.1.1. X-ray diffraction analysis
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The crystal structures of 0, 1, 2 and 3 mmol of AgNPs mixed NaYF4:Er3+/Yb3+ samples were studied by XRD analysis and spectra are shown in Fig. 1. The XRD peaks could be assigned to mixed phase of cubic (JCPDS No. 06-0342) and hexagonal (JCPDS No. 16-0334) NaYF4 hosts. Observed dual crystal phase in XRD pattern is the result of used preparation conditions during synthesis. Debye-Scherrer formula was used to calculate the crystallite size of samples[36].
where,
Cu Kα,
is the average crystallite size,
(1) the X-ray diffracted wavelength (0.15406 nm) for
the full width at half maximum (FWHM) in radian of the X-ray diffraction peak
and the Braggs' angle (degree). The average crystallite size was calculated around 42, 38, 35 and 30 nm for 0, 1, 2 and 3 mmol Ag coated samples, respectively.
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Fig. 1. X-ray diffraction patterns of NaYF4 with 0 mmol AgNPs (c), 1 mmol AgNPs (d), 2 mmol AgNPs (e), and 3 mmol AgNPs (f). JCPDS data of cubic phaseand hexagonal phase are shown in (a) and (b), respectively. The star (*) sign showspresence of Ag metal.
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3.1.2.Fourier trans form infrared absorption study
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In order to investigate the presence of organic impurities in the synthesized samples the FTIR spectra were recorded for the range of 4000 – 400 cm–1 and spectra are shown in Fig. 2. From this figure, it isobserved that O-H and NO3-vibrational bands are present in all samples, however band intensities of these vibrations are found lowest in case of sample having 3 mmolAgNPs. The broad peak found at 3200–3600 cm–1 is assigned to O–H stretching of water molecules. This band shows that synthesized sample contains small amount of water. The band in the range of 1680–1300 cm–1 occurs due to the presence of NO3− in the sample while band found at 1500–1250 cm–1 is observed due to C-O bond (maybe due to EDTA). The band monitored at 648 cm–1 is due to the presence of stretching vibration of Ag–O which is absent in AgNPs free sample[37, 38].This Ag–O is expected to form due to partial oxidation of AgNPs.
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Fig. 2. Fourier transform infra-red spectra of NaYF4:Er3+/Yb3+ upconversion nanoparticles mixed with 0 mmol (1), 1 mmol (2), 2 mmol (3) and 3 mmol (4) AgNPs.
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3.1.3.Field emission scanning electron microscopy analysis
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FESEM images of samples ave shown in Fig. 3. Sample without AgNPs shows uniform distribution of particles with average size of ~65 nm (Fig. 3 (a)). The size distribution of particles was calculated by using Image J software. After coating with AgNPs (~20 nm) the particle size is seen to increase, as shown in Fig. 3(b, e, f). Statistical distribution of particle size has shown in inset of each SEM image and comparison indicates that overall particle size increases with increase in concentration of AgNPs.The EDX spectra were taken to confirm the presence of Ag. In EDX spectrum of sample coated with 1 mmolAgNPs (Fig. 3(d)) shows the presence of Ag. This Ag peak is absent incase of sample without AgNPs as shown in Fig. 3(c).This result indicates that AgNPs have been coated on the surface of NaYF4:Er3+/Yb3+particles. After coating with AgNPs the sample morphology is seen to change and particle surface looks like being covered with small particles.
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Fig. 3. Field emission scanning electron microscopy images for the samples 0 mmol (a), 1 mmol (b), EDX spectra for (a) (c), EDX spectra for (b) (d), 2 mmol (e), 3 mmol (f) AgNPs. Inset figure shows the statistical distribution of the particles of Ag@NaYF4:Yb3+/Er3+. Average particle size of samples containing 0, 1, 2 and 3 mmol AgNPs has found to 65.7, 72.9, 75.1 and 74.5 nm, respectively.
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3.1.4. Transmission electron microscopy analysis To investigate the presence of AgNPs, the TEM of samples 0 and 1 mmol of AgNPs were recorded. The images are shown in Fig. 4 with selected area electron diffraction (SAED) patterns. Both images show the nearly spherical shape but second image looks random in shape. In Fig. 4(b) the image of 1 mmol AgNPs sample shows the presence of small spherical black spots due to the presence of AgNPs in the sample. The SAED pattern of 0 mmol sample (inset of Fig. 4(a)) shows distinct bright spots in hexagonal arrangement and there is no ring pattern. However, SAED image of 1 mmol sample (inset of Fig. 4(b)) shows the presence of ring pattern. The ring diameter was used to calculate the lattice constant and was found to be 0.233 nm, 0.124 nm and 0.083 nm. These values correspond to the Ag crystalline plane (d111 = 0.233, d311 = 0.124 and d422 = 0.083 nm, JCPDS No. 04-0783). Hence SAED again confirms the presence of AgNPs in NaYF4:Er3+/Yb3+ particles.
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Fig.4.Transmission electron microscope (TEM) images of 0 mmol AgNPs in NaYF4:Er3+/Yb3+ (a), and 1 mmol AgNPs in NaYF4:Er3+/Yb3+ (b). Corresponding SAED patterns are shown in inset of each image. Ring pattern in sample containing 1 mmol AgNPs sample indicates the presence of AgNPs.
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4. Optical studies 4.1. UV-Vis absorption study
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Fig. 5 shows the absorption spectra of samples having 0, 1, 2 and 3 mmol AgNPs. The absorption bands associated with Er3+ and Ag metal are visible in the graph. Pure AgNPs sample shows broad absorption in 325–560 nm due to the surface plasmons. NaYF4:Er3+/Yb3+ containing AgNPs samples also show the absorption band of AgNPs but with weak absorption and little shift towards higher wavelength due to the clustering effect. No absorption in this region is found in case of pure NaYF4:Er3+/Yb3+ sample. Pure NaYF4:Er3+/Yb3+ sample however shows sharp absorption bands due to the Er3+ion at 380, 490, 522, 546 and 656 nm wavelengths and are assigned to 4G11/2 ← 4I15/2, 4F5/2 ← 4I15/2, 4 H11/2 ← 4I15/2, 4S3/2 ← 4I15/2,and4F9/2 ← 4I15/2 transitions, respectively. Er3+ absorption bands are not observed for samples containing AgNPs and it may be due to the hindering of NaYF4:Er3+/Yb3+ particles from incident photons. The incident light is mostly scattered by AgNPs covering the surface of NaYF4:Er3+/Yb3+ phosphor nanoparticles.
Fig. 5.Acomparison of absorption spectra of the 0, 1, 2 and 3 mmol AgNPs at room temperature. The inset shown the plasmonic bands of AgNPs at 455 nm. 8
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Upconversion emission measurements of samples were carried out by illuminating the samples at 976 nm wavelength from a diodelaser. The spectra recorded at 32 W/cm2 laser power density are compared in Fig. 6. The emission bands are observed at 522, 546 (green) and 656 nm (red) are due to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ ions, respectively. The assignments to the emission bands are given on the basis of other reports[36, 39].
Fig. 6. Comparative upconversion emission spectra for different concentrations of AgNPs excited with 976 nm diode laser source. The snap shot of sample color (red) after illumination is shown in inset.
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On the basis of upconversion measurements it is noticed that sample containing 1 mmolAgNPs has optimum emission intensity. Above this concentration, emission quenching is seen. Similar observation has also been observed by Wu et al.[40] in the glass medium. The maximum enhancement in emission for green and red bands is found to be 12 and 9 folds, respectively, when compared with 0 mmolAgNPs. This remarkable enhancement is due to the SPR phenomenon of AgNPs[40-42]. The excitation wavelength of 976 nm excites the higher lying 4F7/2 level of Er3+ ions and AgNPs partially absorb emission coming from Er3+ ions and thus plasmongets excited. There are two enhancement mechanisms as given in the literature[14, 18] but it is hard to predict which one is working in the present case. The change in local field around the Er3+ ions could not be ignored and there is some contribution of it in enhancing the upconversion emission. The UV-Vis absorption measurement could not help to monitor enhancement in absorption of excitation radiation. The change in radiative decay rate is also possible and decay time was measured for both the cases. The huge enhancement in emission intensity is highly probable for green band because atthis wavelength SPR band of AgNPs matched well.The decrease in upconversion emission intensity of all the emission bands beyond 1 mmol Ag concentration is observed. Above a certain concentration of Ag particles the excitation energy of Er3+ migrates to nearby atoms/ions through the Ag particles. The emission quenching by plasmonic nanoparticles in luminescent phosphors is a general phenomenon and is widely reported[40, 43].This decrease in intensity is due to the loss of localization of the surface plasmon of individual Ag particle. As higher concentration Ag particles form aggregates which act as emission quencher rather 9
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than enhancer. A schematic diagram of enhanced upconversion emission intensity through SPR is shown in Fig. 7. In the excitation process of Er3+ ion both energy transfer (ET) and excited state absorption (ESA) followed by ground state absorption (GSA) mechanism are found to be involved. The most probable excitation pathways can be written as[44, 45]: GSA (I): 4I15/2 (Er3+) + hν → 4I11/2 (Er3+), ESA (I): 4I11/2 (Er3+) + hν → 4F7/2 (Er3+), ESA (II): 4I13/2 (Er3+) + hν → 4F9/2 (Er3+), ESA (III): 4F9/2 (Er3+) + hν → 2H9/2 (Er3+), 2 ET (I): F5/2 (Yb3+) + 4I15/2 (Er3+) →2F7/2 (Yb3+) + 4I11/2 (Er3+), ET (II): 2F5/2 (Yb3+) + 4I13/2 (Er3+) → 2F7/2 (Yb3+) + 4F9/2 (Er3+), ET (III): 2F5/2 (Yb3+) + 4I11/2 (Er3+) → 2F7/2 (Yb3+) + 4F7/2 (Er3+), ET (IV): 2F5/2 (Yb3+) + 4F9/2 (Er3+) → 2F7/2 (Yb3+) + 2H9/2 (Er3+). The excitation and emission pathways ave shown in Fig. 8, while power dependent emission of 1 mmolAgNPs optimized sample is shown in Fig. 9. The luminescence intensity increases with increasing the pump power density due to the increased population inversion at higher emitting states. As the excitation power has increased, the population inversion of atoms in 2 H11/2 and 4S3/2 levels increases. Consequently, the emission intensity of corresponding band increases (Fig. 9). Here, it is worth to note that as the excitation power has increased above 34.30 W/cm2, the green band observed at 522 and 546 nm decreases while the red band at 660 nm increases. This mechanism may be explained by assuming the energy level diagram. When the power of incident laser beam has increased above a certain power (34.30 W/cm2) then population inversion of atoms also increases in 2H11/2 and 4S3/2 levels; this results in the internal heating effect due to the large amount of atoms in same level. Because of the internal heating effect, non-radiative decay channel starts and transfers population from 2H11/2 and 4 S3/2 level to 4F9/2 level. This may be a reason for decrease in green emission while increase in red emission above a certain excitation power. The CIE (Commission Internationale de l’Elcairage) color coordinates of all prepared samples at fixed laser power density (17.80 W/cm2) were calculated. From the color coordinate diagram it is clear that the purity of color increases upto 1 mmolAgNPs sample and further starts to decrease (inset of Fig. 9) for other increasing Ag concentration.
Fig. 7. Model representing the description of Ag@NaYF4:Er3+/Yb3+phosphornanoparticles with generation of SPR.
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Fig. 8.A simplified energy level diagram of Er3+/Yb3+system embedded with AgNPs and possible upconversion pathways.
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To investigate the upconversion emission enhancement, the decay time of 4S3/2 and 4F9/2 emitting levels was measured for 0 mmol and 1mmolAgNPsprepared samples. The decay curves are shown in Fig. 10 and best fitted to the double exponential function: (2)
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where, and are the fitting parameters and and are the decay times. With fitting function given in Eq. (2), decay time was calculated. The calculated decay time and rise time of different bands are given in Table 1. Rise time calculation is done using procedure given elsewhere[46, 45, 25].
Fig. 9. Comparison of upconversion spectra of 1 mmolAgNPs sample at different power densities excited by 976 nm diode laser source. Inset in CIE color coordinate diagram of the samples (0, 1, 2 and 3 mmolAgNPs).
Decrease in decay time for sample containing 1 mmolAgNPsis observed. This decreased decay time indicates increase in radiative transition rate of emitting levels. The rise time of the samples with respect to 4S3/2 → 4I15/2 and4F9/2→ 4I15/2 emitting levels was also measured 11
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and found to be almost invariant[47, 48]. It is well reported that plasmonic nanoparticles reduce the decay time of rare earth levels. Maximum reduction occurs for emission band whose wavelength better matches for SPR of plasmonic particles. Reduced decay time of green and red emission bands are found to be 83.9% and 78.2%, respectively as calculated by Table 1. This quantitative explanation of reduced decay time supports the increased upconversion emission of green (12-fold) and red (9-fold) with SPR of the AgNPs better matches for green emission band.The reduced lifetime has also been reported by other researchers by correlating the emission mechanism using SPR waves[49-51]. From the decrease in decay time it could be predicted that in present case increase in radiative decay rate due to SPR is dominant over the enhancement due to the change in local filed metal particles.
τr (μs) 264 267
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τd (μs) 155 130
τd (μs) 330 258
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0 mmol 1 mmol
τr (μs) 242 247
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Table 1 Decay time and rise time details of 0 mmol and 1 mmolAgNPs prepared samples associated with upconversion emission resulting in green (4S3/2 → 4I15/2) and red (4F9/2→ 4I15/2)emissions. 4 4 Sample S3/2 → 4I15/2 F9/2 → 4I15/2
Fig. 10. Decay curves of 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions for the samples containing 0 mmol and 1 mmolAgNPs.
5. Conclusions
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NaYF4:Er3+/Yb3+upconversion nanoparticles coated with AgNPs were successfully synthesized by co-prepetition route. Mixed phase (cubical and hexagonal phase)of NaYF4:Er3+/Yb3+ is confirmedfrom the XRD pattern of upconversion nanoparticles. EDX, TEM and UV-Vis analysis indicate that AgNPs are coated on the NaYF4:Er3+/Yb3+ upconversion nanoparticles. The maximum enhancement of 12 fold for green emission is observed for sample containing 1 mmolAgNPs. Above this concentration of the AgNPs the emission quenching is seen. The decay time was measured for the green and red emissions and it is found that AgNPs reduces the decay time for both the emissions. This reduction in emission lifetime indicates that surface plasmon resonance (SPR) is the dominant mechanism over the local field effect. Acknowledgments
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One of authors S.K. Mauryaacknowledges Indian Institute of Technology (Indian School of Mines), Dhanbadfor providing fellowship to carry out doctoral research programme and Dr. N.P. Lalla UGC-DAE Consortium for scientific Research, Indore (India) for TEM measurements.The author thanks Prof. C.K. Jaya Sankar, Department of Physics, Sri Venkateswara University, Tirupati, India for lifetime measurement.Dr. K. Kumar thankfully acknowledges CSIR, New Delhi (India) for the financial support to carry out theresearch project [03(1303)13/EMR]. References
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ACCEPTED MANUSCRIPT [22] Li XD, Zhang DW, Chen S, Wang ZA, Sun Z, Yin XJ, Huang SM. Enhancing efficiency of dye-sensitized solar cells by combining use of TiO2 nanotubes and nanoparticles. Mater Chem Phys. 2010; 124: 179. [23] Zhang DW, Li XD, Chen S, Sun Z, Yin XJ, Huang SM. Performance of dyesensitized solar cells with various carbon nanotube counter electrodes. Microchim Acta. 2011; 174: 73.
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[26] Tiwari SP, Kumar K, Rai VK. Plasmonic enhancement in upconversion emission of La2O3: Er3+/Yb3+ phosphor via introducing silver metal nanoparticles. Appl Phys B. 2015; 121: 221. [27] Yuan P, Lee YH, Gnanasammandhan MK, Guan Z, Zhang Y, Xu QH. Plasmon enhanced upconversion luminescence of NaYF4: Yb, Er@ SiO2@ Ag core–shell nanocomposites for cell imaging. Nanoscale. 2012; 4: 5132.
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[31] Priyam A, Idris NM, Zhang Y. Gold nanoshell coated NaYF4 nanoparticles for simultaneously enhanced upconversion fluorescence and darkfield imaging. J Mater Chem. 2012; 22: 960. [32] Ding YL, Zhang XD, Gao HB, Xu SZ, Wei CC, Zhao Y. Plasmonic enhanced upconversion luminescence of β-NaYF4:Yb3+/Er3+ with Ag@SiO2 core-shell nanoparticles. J Lumin. 2014; 147: 72. [33] Lin C, Berry MT, Anderson R, Smith S, May PS. Highly luminescent NIR-to-visible upconversion thin films and monoliths requiring no high-temperature treatment. Chem Mater. 2009; 21: 3406. [34] Yi GS, Lu HC, Zhao SY, Ge Y, Yang WJ, Chen DP, et al. Synthesis, characterization, and biological application of size-controlled nanocrystalline NaYF4:Yb,Er infraredto-visible up-conversion phosphors. Nano Lett. 2004; 4: 2191.
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ACCEPTED MANUSCRIPT [35] Li ZQ, Zhang Y. Monodisperse silica‐coated polyvinylpyrrolidone/NaYF4 nanocrystals with multicolor upconversion fluorescence emission. Angew Chem Int Ed. 2006; 45: 7732. [36] Tiwari SP, Mahata MK, Kumar K, Rai VK. Enhanced temperature sensing response of upconversion luminescence in ZnO-CaTiO3:Er3+/Yb3+ nano-composite phosphor. Spectrochim Acta Part A Mol Bio Spect. 2015; 150: 623.
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[45] Kumar A, Tiwari SP, Kumar K, Rai VK. Structural and optical properties of thermal decomposition assisted Gd2O3: Ho3+/Yb3+ upconversion phosphor annealed at different temperatures. Spectrochim Acta Part A Mol Bio Spect. 2016; 167: 134. [46] Chen Z, Kang SL, Zhang H, Wang T, Lv SC, Chen QQ, et al. Controllable optical modulation of blue/green up-conversion fluorescence from Tm3+ (Er3+) single-doped glass ceramics upon two-step excitation of two-wavelengths. Sci Rep. 2017; 7: 45650. [47] Tian LJ, Xu Z, Zhao SL, Cui Y, Liang ZQ, Zhang JJ, et al. The upconversion luminescence of Er3+/Yb3+/Nd3+ triply-doped β-NaYF4 nanocrystals under 808-nm excitation. Materials. 2014; 7: 7289. [48] Biswas A, Maciel GS, Friend CS, Prasad PN. Upconversion properties of a transparent Er3+–Yb3+ co-doped LaF3-SiO2 glass-ceramics prepared by sol–gel method. J Non-Cryst Solids. 2003; 316: 393. 16
ACCEPTED MANUSCRIPT [49] Saboktakin M, Ye X, Oh SJ, Hong SH, Fafarman AT, Chettiar UK, Engheta N, Murray CB, Kagan CR. Metal-enhanced upconversion luminescence tunable through metal nanoparticle–nanophosphor separation. ACS nano. 2012; 6: 8758. [50] Feng AL, Lin M, Tian L, Zhu HY, Guo H, Singamaneni S, Duan Z, Lu TJ, Xu F. Selective enhancement of red emission from upconversion nanoparticles via surface plasmon-coupled emission. RSC Adv. 2015; 5: 76825.
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Graphical Abstract:
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Upconversion emission enhancement through different coating concentrations of silver nanoparticles (AgNPs) on NaYF4:Er3+/Yb3+ phosphor nanoparticles is shown. Surface plasmonic resonance effect enhances the upconversion emission at 522, 546 and 656 nm corresponding to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions, respectively.
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S.K. Maurya, S.P. Tiwari, A. Kumar, K. Kumar PII:
S1002-0721(18)30292-8
DOI:
10.1016/j.jre.2018.03.003
Reference:
JRE 159
To appear in:
Journal of Rare Earths
Received Date: 3 November 2017 Revised Date:
2 March 2018
Accepted Date: 5 March 2018
Please cite this article as: Maurya SK, Tiwari SP, Kumar A, Kumar K, Plasmonic enhancement of 3+ 3+ upconversion emission of Ag@NaYF4:Er /Yb phosphor, Journal of Rare Earths (2018), doi: 10.1016/ j.jre.2018.03.003. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Plasmonic enhancement of upconversion emission of Ag@NaYF4:Er3+/Yb3+phosphor S. K. Maurya, S. P. Tiwari, A. Kumar, K. Kumar*
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Optical Materials and Bio-imaging Research Laboratory, Department of Applied Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad-826004, India
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Abstract: In this article upconversion luminescence of silver nanoparticles (AgNPs) coated NaYF4:Er3+/Yb3+phosphor nano-particles are investigated. The prepared samples were characterized through various techniques. The surface plasmon band was observed for prepared AgNPs by analyzing UV-Vis. measurements and was used to enhance the upconversion emission. From the upconversion measurement the emission bands are observed at 522, 546, and 656 nm corresponding to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 levels, respectively. The upconversion emission intensity of the above bands is found to enhance for sample containing 1 mmolAgNPs. Decay time of 4S3/2 and 4F9/2 levels is found to decrease on coating of AgNPs and hence intensity enhancement is assumed due to the surface plasmon resonance (SPR) effect.
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Keywords: Upconversion phosphor, Surface plasmon resonance, Decay time analysis, CIE diagram; Rare earths
Foundation item: Project supported by the Indian Institute of Technology (Indian School of Mines), Dhanbad, India and the Council of Scientific & Industrial Research (CSIR), New Delhi, India (03(1303)13/EMR). *Corresponding author: K. Kumar (E-mail:[email protected], Tel: +91-3262235754)
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1. Introduction
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Lanthanide doped/codopedupconversion materials have attracted much attention for their applications in the field of solar cell, biological fluorescence imaging, fingerprint detection, cancer therapy, etc. [1-7]. Upconversion phosphors possess intense emission through 4f-4f or 4f-5d transitions with unique properties such as sharp emission spectra and long emission time, [8]. The utility of single rare earth ion doped phosphor such as Er3+, Tm3+, Ho3+ is limited due to the poor quantum efficiency and low absorption cross section. To improve the emission efficiency the researchers are trying with various means such as incorporating silver/gold nanoparticles in the host, choosing the host matrices or co-doping other ions to improve the emission intensity. The matrices such as bromides, sulphides, oxysulfides have low phonon energies and good chemical stability [3]. These halide hosts are found hygroscopic in nature and hence have limited use in bio-imaging and photo-dynamic therapy [3-5]. In comparison to other halides and oxides, fluorides exhibit low phonon energy and high chemical stability. NaYF4 is considered most popular host among phosphor materials as it possesses the highest upconversion luminescence quantum efficiency[3, 8]. From the last decade, researchers are trying to improve the upconversion emission intensity by utilizing surface plasmon resonance (SPR) of metal nanoparticles[9-11]. In order to obtain plasmonenhanced upconversion emission, many research articles have been published on designing the core shell structures[10-17] and constituting of photonic crystals[11, 13]. The plasmonic enchancement is reported to work by two mechanisms: (a) SPR induced enhancement in absorption of emitting centers due to the local field enhancement effect; (b) increase of radiative decay rate of emitting centers due to the resonance of emission of surface plasmon of metallic particles [14, 15]. The plasmon absorption band is dependent on size and shape of metallic particles, hence by tuning the size and shape the maximum upconversion enhancement could be achieved. Collective oscillation of electrons of metal nanoparticle generates SPR. The electron oscillation in metallic particles affects the local field around the emitting ion and this interaction is near field[14, 18, 19]. The Au and Ag are well known plasmonic particles whose SPR absorption lies in the visible region and hence incorporation of these metals particles in phosphor materials has been studied for upconversion enhancement of organic dyes[20-22] and quantum dots[23]. Yin et al.[24] have reported the enhancement in upconversion emission intensity due to the local field modulation induced combining SPR effect and photonic crystal effect in NaYF4/AuNRs hybrids film. Schietinger et al.[25] have studied the upconversion emission enhancement from NaYF4:Er3+/Yb3+ nanoparticles coated with Au spheres and found several foldenhanced emission intensity. Tiwari et al.[26] have studied the upconversion enhancement in La2O3:Er3+/Yb3+ phosphor by introducing Ag nanoparticlesat different concentrations. Yuan et al.[27]have reported NaYF4:Er3+/Yb3+@SiO2@Ag upconversion nanoparticles and used silica as spacer between the upconverting core and Ag shell. Severalother researchers have also tried to improve the upconversion emission through direct coupled spherical Ag and Au particles[28-31].
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ACCEPTED MANUSCRIPT In this report, authors have investigated plasmon enhanced upconversion in Ag@NaYF4:Er3+/Yb3+ phosphornanoparticles by illuminating 976 nm diode laser. Remarkable enhancement in upconversion emission intensity of NaYF4:Er3+/Yb3+ nanoparticles coated with AgNPs are recorded up to 12 fold compared to bare NaYF4:Er3+/Yb3+ sample. The mechanism emission enhancement due to the SPR effect is discussed.
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2. Experimental 2.1.Chemicals
2.1.1. Synthesis of silver nanoparticles
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All the preparatory materials (chemicals) used for synthesis of NaYF4:Er3+/Yb3+ and AgNPs were yttrium oxide (Y2O3, 99.99% Otto, India), erbium oxide (Er2O3, 99.99% Otto, India) and ytterbium oxide (Yb2O3, 99.99% Otto, India), sodium nitrate (NaNO3, 99% Merck, India), ammonium fluoride (NH4F, 99.9% LobaChemie, India), silver nitrate (AgNO3, 99.99% Merck, India), nitric acid (HNO3, 69% dilution, Merck, India) and ethylenediaminetetraacetic acid (C10H16N2O8, 99% LobaChemie, India).
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AgNPs were synthesized by reducing silver nitrate (AgNO3) with tri-sodium citrate[32]. In a typical preparation procedure, 0.11 mmol of AgNO3 was dissolved in 100 ml deionized water and heated to 100 ºC. At this temperature 2 mL of 1 % sodium citrate solution was dropwise mixed under vigorous stirring. After 30 min the solution was cooled down to room temperature. Centrifugation of sample was done and collected precipitate was dried whole night at 70 ºC to get the dispersible powder.
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2.1.2. Synthesis of NaYF4:Er3+/Yb3+ and Ag@NaYF4:Er3+/Yb3+
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Sodium yttrium fluoride (NaYF4) nano-crystals doped with Er3+(3%)/Yb3+(17%)[33] ions were prepared by co-precipitation method[34, 35]. Rare earth nitrates were first prepared by refluxing stochiometric amounts of rare earth oxides (1.5 mmol of Y2O3, 0.32 mmol of Yb2O3 and 0.06 mmol of Er2O3) with nitric acid at 80 ºC until transparentgel was obtained and again heated upto 90 ºC for 15 min to remove the excess amount of un-reacted nitric acid (solution A). After that 0.3239 gm NaNO3was dissolved in 10 mL deionized water to form solution B and 0.5647 gm NH4F was also dissolved in 10 mL deionized water to form solution C. 5 mL of EDTA was mixed to solution as chelating agent to stabilize the particle’s growth. Now solutions B and C were mixed into solution A dropwise under vigorous stirring at 85 ºC. After half an hour white precipitate was obtained after centrifuging and was washed three times with ethanol and then dried at 70 ºC overnight. Further, powder form of AgNPs was dispersed in deionized water using ultrasonic bath. TheNaYF4:Er3+/Yb3+sample solution was mixed with three different molar concentrations (1, 2, 3 mmol) of colloidal AgNPs. The mixture was left 15 min for coagulation and then dried at 70 ºC. Collected samples having 0, 1, 2 and 3 mmol AgNPs@NaYF4:Er3+/Yb3+ were used for further study.
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Crystal phase of all prepared samples were investigated using X-ray diffractometer (Bruker-Nonius FR-590). Impurities present in samples were investigated by Fourier transform infrared (FTIR) analysis on Spectrum RX-1 (PerkinElmer, USA). Particles size and surface morphology were studied on a field emission scanning electron microscope (FESEM) images using Supra-55 (Carl Zeiss, Germany). The transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were recorded on Tecnai 20 G2, USA for the confirmation of AgNPs. The UV-Vis absorption spectra of samples were monitored in an absorbance modeon Lambda 35 (PerkinElmer, USA) spectrophotometer. For absorption measurement the powder phosphor sample was dispersed in ethanol (1:10) and under sonication for 10 min. Immediately after sonication UV-Vis absorption measurement was done. The upconversion emission spectra were recorded on a CCD spectrometer (ULS2048X64, Avantes, USA) using 976 nm diode laser as excitation sourcehaving spot size of 1.2 mm at the sample surface. A 750 nm (model: FES0750, Thorlabs, USA) short pass filter was used to block the unwanted laser excitation. The decay curves were measured using 976 nm diode laser and InGaAs detector coupled with a digital oscilloscope. All measurements were analyzed at room temperature. 3. Results and discussion 3.1. Structural analysis 3.1.1. X-ray diffraction analysis
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The crystal structures of 0, 1, 2 and 3 mmol of AgNPs mixed NaYF4:Er3+/Yb3+ samples were studied by XRD analysis and spectra are shown in Fig. 1. The XRD peaks could be assigned to mixed phase of cubic (JCPDS No. 06-0342) and hexagonal (JCPDS No. 16-0334) NaYF4 hosts. Observed dual crystal phase in XRD pattern is the result of used preparation conditions during synthesis. Debye-Scherrer formula was used to calculate the crystallite size of samples[36].
where,
Cu Kα,
is the average crystallite size,
(1) the X-ray diffracted wavelength (0.15406 nm) for
the full width at half maximum (FWHM) in radian of the X-ray diffraction peak
and the Braggs' angle (degree). The average crystallite size was calculated around 42, 38, 35 and 30 nm for 0, 1, 2 and 3 mmol Ag coated samples, respectively.
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Fig. 1. X-ray diffraction patterns of NaYF4 with 0 mmol AgNPs (c), 1 mmol AgNPs (d), 2 mmol AgNPs (e), and 3 mmol AgNPs (f). JCPDS data of cubic phaseand hexagonal phase are shown in (a) and (b), respectively. The star (*) sign showspresence of Ag metal.
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3.1.2.Fourier trans form infrared absorption study
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In order to investigate the presence of organic impurities in the synthesized samples the FTIR spectra were recorded for the range of 4000 – 400 cm–1 and spectra are shown in Fig. 2. From this figure, it isobserved that O-H and NO3-vibrational bands are present in all samples, however band intensities of these vibrations are found lowest in case of sample having 3 mmolAgNPs. The broad peak found at 3200–3600 cm–1 is assigned to O–H stretching of water molecules. This band shows that synthesized sample contains small amount of water. The band in the range of 1680–1300 cm–1 occurs due to the presence of NO3− in the sample while band found at 1500–1250 cm–1 is observed due to C-O bond (maybe due to EDTA). The band monitored at 648 cm–1 is due to the presence of stretching vibration of Ag–O which is absent in AgNPs free sample[37, 38].This Ag–O is expected to form due to partial oxidation of AgNPs.
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Fig. 2. Fourier transform infra-red spectra of NaYF4:Er3+/Yb3+ upconversion nanoparticles mixed with 0 mmol (1), 1 mmol (2), 2 mmol (3) and 3 mmol (4) AgNPs.
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3.1.3.Field emission scanning electron microscopy analysis
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FESEM images of samples ave shown in Fig. 3. Sample without AgNPs shows uniform distribution of particles with average size of ~65 nm (Fig. 3 (a)). The size distribution of particles was calculated by using Image J software. After coating with AgNPs (~20 nm) the particle size is seen to increase, as shown in Fig. 3(b, e, f). Statistical distribution of particle size has shown in inset of each SEM image and comparison indicates that overall particle size increases with increase in concentration of AgNPs.The EDX spectra were taken to confirm the presence of Ag. In EDX spectrum of sample coated with 1 mmolAgNPs (Fig. 3(d)) shows the presence of Ag. This Ag peak is absent incase of sample without AgNPs as shown in Fig. 3(c).This result indicates that AgNPs have been coated on the surface of NaYF4:Er3+/Yb3+particles. After coating with AgNPs the sample morphology is seen to change and particle surface looks like being covered with small particles.
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Fig. 3. Field emission scanning electron microscopy images for the samples 0 mmol (a), 1 mmol (b), EDX spectra for (a) (c), EDX spectra for (b) (d), 2 mmol (e), 3 mmol (f) AgNPs. Inset figure shows the statistical distribution of the particles of Ag@NaYF4:Yb3+/Er3+. Average particle size of samples containing 0, 1, 2 and 3 mmol AgNPs has found to 65.7, 72.9, 75.1 and 74.5 nm, respectively.
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3.1.4. Transmission electron microscopy analysis To investigate the presence of AgNPs, the TEM of samples 0 and 1 mmol of AgNPs were recorded. The images are shown in Fig. 4 with selected area electron diffraction (SAED) patterns. Both images show the nearly spherical shape but second image looks random in shape. In Fig. 4(b) the image of 1 mmol AgNPs sample shows the presence of small spherical black spots due to the presence of AgNPs in the sample. The SAED pattern of 0 mmol sample (inset of Fig. 4(a)) shows distinct bright spots in hexagonal arrangement and there is no ring pattern. However, SAED image of 1 mmol sample (inset of Fig. 4(b)) shows the presence of ring pattern. The ring diameter was used to calculate the lattice constant and was found to be 0.233 nm, 0.124 nm and 0.083 nm. These values correspond to the Ag crystalline plane (d111 = 0.233, d311 = 0.124 and d422 = 0.083 nm, JCPDS No. 04-0783). Hence SAED again confirms the presence of AgNPs in NaYF4:Er3+/Yb3+ particles.
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Fig.4.Transmission electron microscope (TEM) images of 0 mmol AgNPs in NaYF4:Er3+/Yb3+ (a), and 1 mmol AgNPs in NaYF4:Er3+/Yb3+ (b). Corresponding SAED patterns are shown in inset of each image. Ring pattern in sample containing 1 mmol AgNPs sample indicates the presence of AgNPs.
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4. Optical studies 4.1. UV-Vis absorption study
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Fig. 5 shows the absorption spectra of samples having 0, 1, 2 and 3 mmol AgNPs. The absorption bands associated with Er3+ and Ag metal are visible in the graph. Pure AgNPs sample shows broad absorption in 325–560 nm due to the surface plasmons. NaYF4:Er3+/Yb3+ containing AgNPs samples also show the absorption band of AgNPs but with weak absorption and little shift towards higher wavelength due to the clustering effect. No absorption in this region is found in case of pure NaYF4:Er3+/Yb3+ sample. Pure NaYF4:Er3+/Yb3+ sample however shows sharp absorption bands due to the Er3+ion at 380, 490, 522, 546 and 656 nm wavelengths and are assigned to 4G11/2 ← 4I15/2, 4F5/2 ← 4I15/2, 4 H11/2 ← 4I15/2, 4S3/2 ← 4I15/2,and4F9/2 ← 4I15/2 transitions, respectively. Er3+ absorption bands are not observed for samples containing AgNPs and it may be due to the hindering of NaYF4:Er3+/Yb3+ particles from incident photons. The incident light is mostly scattered by AgNPs covering the surface of NaYF4:Er3+/Yb3+ phosphor nanoparticles.
Fig. 5.Acomparison of absorption spectra of the 0, 1, 2 and 3 mmol AgNPs at room temperature. The inset shown the plasmonic bands of AgNPs at 455 nm. 8
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Upconversion emission measurements of samples were carried out by illuminating the samples at 976 nm wavelength from a diodelaser. The spectra recorded at 32 W/cm2 laser power density are compared in Fig. 6. The emission bands are observed at 522, 546 (green) and 656 nm (red) are due to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ ions, respectively. The assignments to the emission bands are given on the basis of other reports[36, 39].
Fig. 6. Comparative upconversion emission spectra for different concentrations of AgNPs excited with 976 nm diode laser source. The snap shot of sample color (red) after illumination is shown in inset.
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On the basis of upconversion measurements it is noticed that sample containing 1 mmolAgNPs has optimum emission intensity. Above this concentration, emission quenching is seen. Similar observation has also been observed by Wu et al.[40] in the glass medium. The maximum enhancement in emission for green and red bands is found to be 12 and 9 folds, respectively, when compared with 0 mmolAgNPs. This remarkable enhancement is due to the SPR phenomenon of AgNPs[40-42]. The excitation wavelength of 976 nm excites the higher lying 4F7/2 level of Er3+ ions and AgNPs partially absorb emission coming from Er3+ ions and thus plasmongets excited. There are two enhancement mechanisms as given in the literature[14, 18] but it is hard to predict which one is working in the present case. The change in local field around the Er3+ ions could not be ignored and there is some contribution of it in enhancing the upconversion emission. The UV-Vis absorption measurement could not help to monitor enhancement in absorption of excitation radiation. The change in radiative decay rate is also possible and decay time was measured for both the cases. The huge enhancement in emission intensity is highly probable for green band because atthis wavelength SPR band of AgNPs matched well.The decrease in upconversion emission intensity of all the emission bands beyond 1 mmol Ag concentration is observed. Above a certain concentration of Ag particles the excitation energy of Er3+ migrates to nearby atoms/ions through the Ag particles. The emission quenching by plasmonic nanoparticles in luminescent phosphors is a general phenomenon and is widely reported[40, 43].This decrease in intensity is due to the loss of localization of the surface plasmon of individual Ag particle. As higher concentration Ag particles form aggregates which act as emission quencher rather 9
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than enhancer. A schematic diagram of enhanced upconversion emission intensity through SPR is shown in Fig. 7. In the excitation process of Er3+ ion both energy transfer (ET) and excited state absorption (ESA) followed by ground state absorption (GSA) mechanism are found to be involved. The most probable excitation pathways can be written as[44, 45]: GSA (I): 4I15/2 (Er3+) + hν → 4I11/2 (Er3+), ESA (I): 4I11/2 (Er3+) + hν → 4F7/2 (Er3+), ESA (II): 4I13/2 (Er3+) + hν → 4F9/2 (Er3+), ESA (III): 4F9/2 (Er3+) + hν → 2H9/2 (Er3+), 2 ET (I): F5/2 (Yb3+) + 4I15/2 (Er3+) →2F7/2 (Yb3+) + 4I11/2 (Er3+), ET (II): 2F5/2 (Yb3+) + 4I13/2 (Er3+) → 2F7/2 (Yb3+) + 4F9/2 (Er3+), ET (III): 2F5/2 (Yb3+) + 4I11/2 (Er3+) → 2F7/2 (Yb3+) + 4F7/2 (Er3+), ET (IV): 2F5/2 (Yb3+) + 4F9/2 (Er3+) → 2F7/2 (Yb3+) + 2H9/2 (Er3+). The excitation and emission pathways ave shown in Fig. 8, while power dependent emission of 1 mmolAgNPs optimized sample is shown in Fig. 9. The luminescence intensity increases with increasing the pump power density due to the increased population inversion at higher emitting states. As the excitation power has increased, the population inversion of atoms in 2 H11/2 and 4S3/2 levels increases. Consequently, the emission intensity of corresponding band increases (Fig. 9). Here, it is worth to note that as the excitation power has increased above 34.30 W/cm2, the green band observed at 522 and 546 nm decreases while the red band at 660 nm increases. This mechanism may be explained by assuming the energy level diagram. When the power of incident laser beam has increased above a certain power (34.30 W/cm2) then population inversion of atoms also increases in 2H11/2 and 4S3/2 levels; this results in the internal heating effect due to the large amount of atoms in same level. Because of the internal heating effect, non-radiative decay channel starts and transfers population from 2H11/2 and 4 S3/2 level to 4F9/2 level. This may be a reason for decrease in green emission while increase in red emission above a certain excitation power. The CIE (Commission Internationale de l’Elcairage) color coordinates of all prepared samples at fixed laser power density (17.80 W/cm2) were calculated. From the color coordinate diagram it is clear that the purity of color increases upto 1 mmolAgNPs sample and further starts to decrease (inset of Fig. 9) for other increasing Ag concentration.
Fig. 7. Model representing the description of Ag@NaYF4:Er3+/Yb3+phosphornanoparticles with generation of SPR.
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Fig. 8.A simplified energy level diagram of Er3+/Yb3+system embedded with AgNPs and possible upconversion pathways.
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To investigate the upconversion emission enhancement, the decay time of 4S3/2 and 4F9/2 emitting levels was measured for 0 mmol and 1mmolAgNPsprepared samples. The decay curves are shown in Fig. 10 and best fitted to the double exponential function: (2)
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where, and are the fitting parameters and and are the decay times. With fitting function given in Eq. (2), decay time was calculated. The calculated decay time and rise time of different bands are given in Table 1. Rise time calculation is done using procedure given elsewhere[46, 45, 25].
Fig. 9. Comparison of upconversion spectra of 1 mmolAgNPs sample at different power densities excited by 976 nm diode laser source. Inset in CIE color coordinate diagram of the samples (0, 1, 2 and 3 mmolAgNPs).
Decrease in decay time for sample containing 1 mmolAgNPsis observed. This decreased decay time indicates increase in radiative transition rate of emitting levels. The rise time of the samples with respect to 4S3/2 → 4I15/2 and4F9/2→ 4I15/2 emitting levels was also measured 11
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and found to be almost invariant[47, 48]. It is well reported that plasmonic nanoparticles reduce the decay time of rare earth levels. Maximum reduction occurs for emission band whose wavelength better matches for SPR of plasmonic particles. Reduced decay time of green and red emission bands are found to be 83.9% and 78.2%, respectively as calculated by Table 1. This quantitative explanation of reduced decay time supports the increased upconversion emission of green (12-fold) and red (9-fold) with SPR of the AgNPs better matches for green emission band.The reduced lifetime has also been reported by other researchers by correlating the emission mechanism using SPR waves[49-51]. From the decrease in decay time it could be predicted that in present case increase in radiative decay rate due to SPR is dominant over the enhancement due to the change in local filed metal particles.
τr (μs) 264 267
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τd (μs) 155 130
τd (μs) 330 258
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0 mmol 1 mmol
τr (μs) 242 247
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Table 1 Decay time and rise time details of 0 mmol and 1 mmolAgNPs prepared samples associated with upconversion emission resulting in green (4S3/2 → 4I15/2) and red (4F9/2→ 4I15/2)emissions. 4 4 Sample S3/2 → 4I15/2 F9/2 → 4I15/2
Fig. 10. Decay curves of 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions for the samples containing 0 mmol and 1 mmolAgNPs.
5. Conclusions
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NaYF4:Er3+/Yb3+upconversion nanoparticles coated with AgNPs were successfully synthesized by co-prepetition route. Mixed phase (cubical and hexagonal phase)of NaYF4:Er3+/Yb3+ is confirmedfrom the XRD pattern of upconversion nanoparticles. EDX, TEM and UV-Vis analysis indicate that AgNPs are coated on the NaYF4:Er3+/Yb3+ upconversion nanoparticles. The maximum enhancement of 12 fold for green emission is observed for sample containing 1 mmolAgNPs. Above this concentration of the AgNPs the emission quenching is seen. The decay time was measured for the green and red emissions and it is found that AgNPs reduces the decay time for both the emissions. This reduction in emission lifetime indicates that surface plasmon resonance (SPR) is the dominant mechanism over the local field effect. Acknowledgments
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One of authors S.K. Mauryaacknowledges Indian Institute of Technology (Indian School of Mines), Dhanbadfor providing fellowship to carry out doctoral research programme and Dr. N.P. Lalla UGC-DAE Consortium for scientific Research, Indore (India) for TEM measurements.The author thanks Prof. C.K. Jaya Sankar, Department of Physics, Sri Venkateswara University, Tirupati, India for lifetime measurement.Dr. K. Kumar thankfully acknowledges CSIR, New Delhi (India) for the financial support to carry out theresearch project [03(1303)13/EMR]. References
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Graphical Abstract:
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Upconversion emission enhancement through different coating concentrations of silver nanoparticles (AgNPs) on NaYF4:Er3+/Yb3+ phosphor nanoparticles is shown. Surface plasmonic resonance effect enhances the upconversion emission at 522, 546 and 656 nm corresponding to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions, respectively.
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