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Intensification of luminescence of Europium-EDTA complex in polyvinyl pyrrolidone films by copper nanoparticles
Optical Materials 59 (2016) 3 3 3 e7
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Intensification of luminescence of Europium-EDTA complex in polyvinyl pyrrolidone films by copper nanoparticles* Renata Reisfeld a, b, *, Viktoria Levchenko a, b, Agata Lazarowska c, Sebastian Mahlik c, Marek Grinberg c a
Institute of Chemistry, The Hebrew University of Jerusalem, E. Safra Campus, Givat Ram, 91904 Jerusalem, Israel The Harvey M. Krueger Family Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Edmond J. Safra Campus, 91904 Jerusalem, Israel c sk University, Wita Stwosza 57, 80-952 Gdan sk, Poland Institute of Experimental Physics, Gdan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 November 2015 Received in revised form 15 March 2016 Accepted 25 March 2016 Available online 12 May 2016
Stable copper nanoparticles (CuNPs) were prepared and incorporated into polyvinylpyrrolidone (PVP) films together with pre-prepared complex of europium-ethylenediaminetetraacetic acid (EuEDTA). From the comparison of the excitation spectrum of the complex alone and of the complex in conjunction with CuNPs an increased fluorescence intensity of the complex is observed as the result of interaction of the complex with surface plasmons of copper. This effect is maximal when the extinction band of CuNPs coincides with the absorption maximum of the complex, as much more light reaches the excited state of europium in the complex during the excitation events as the result of light scattered by copper plasmons. An additional band was observed in the excitation spectrum of the complex in co-doped by the CuNPs around 320÷390 nm which we attribute to electron transfer from CuNPs to excited state of europium. © 2016 Elsevier B.V. All rights reserved.
Keywords: Stable copper nanoparticles Surface plasmons of copper Complex of europiumethylenediaminetetraacetic acid (EuEDTA) Scattered light Enhancement of fluorescence Rare earths
1. Introduction The lanthanides are a fascinating group of elements the optical properties of which arise from the inner f-electrons which are starting with one in Cerium and terminate with thirteen in Ytterbium. The transition probabilities within the 4 f-orbital are forbidden by Laporte rule and become partially allowed either by mixing with 5 d-orbital of Ln ion or with a charge transfer states of the neighboring ligands [1,2]. The basic theory of the electronic spectra can be found in Refs. [3e5]. Lanthanide luminescence is currently applied in the following fields. Luminescent solar concentrators [6,7], computer displays; light emitting diodes [8], semiconductor quantum dots [9], luminescent biosensors [10], bioassays [11], active optical waveguides [12], materials based on upconversion from IR to visible light [13],
*
Enrique Berman Professor of Solar Energy. * Corresponding author. Institute of Chemistry, The Hebrew University of Jerusalem, E. Safra Campus, Givat Ram, 91904 Jerusalem, Israel. E-mail address: [email protected] (R. Reisfeld). http://dx.doi.org/10.1016/j.optmat.2016.03.046 0925-3467/© 2016 Elsevier B.V. All rights reserved.
materials for optoelectronics [14]. The importance of lanthanides in solar energy conversion can be found in excellent book [15]. The luminescent spectra of the europium are intensified greatly by complexation of lanthanide ions with organic ligands. These lanthanide ions form stable crystalline complexes with heterocyclic ligands, such as bipyridyl (bpy) and phenanthroline (phen), which exhibit efficient energy transfer to the chelated lanthanide ions [16,17] and cryptates [18]. However, lanthanides complexes with organic ligands for practical uses are limited because of poor thermal stability and mechanical properties, although they also have good phosphor characteristics. This drawback can be overcome by protecting them by a glassy or stable polymer surrounding [19]. Recently nanoparticles (NPs) of lanthanides having high luminescence have shown numerous applications in medicine and other fields. In order to take the advantage of the properties of the lanthanides additional ways of increasing their fluorescence intensities are studied. This is done by incorporating into the host matrices NPs of silver, gold and copper [20e22] in conjunction with the luminescent species. In the present paper we describe the intensification of EuEDTA
R. Reisfeld et al. / Optical Materials 59 (2016) 33e7
4
Fig. 1. The 3D-structure of PVP, red atom is oxygen, blue e nitrogen, dark grey e carbon, light grey e hydrogen.
complex incorporated in PVP and study the steady state and dynamic spectroscopy. The size and distribution of the CuNPs are obtained by scanning electron microscope (SEM) and transmission electron microscope (TEM).
2.3. Preparation of the samples
2. Experimental
1) EuEDTA e PVP film doped by EuEDTA complex 2) EuEDTACuNPs e PVP film doped by EuEDTA complex and nanoparticles of copper
2.1. Preparation of EuEDTA complex in PVP
Films were formed on microscope slides using drop-casting process, dried at 28 C for 24 h and heated at 100 C for 1 h. Two types of films were obtained by drop casting method:
The EuEDTA complex in PVP matrix, see the structure of PVP in the Fig. 1, was formed as follows: appropriate stock solutions of desired metal ion europium chloride and ligands - ethylenediamine tetraacetic acid (see the structure of EDTA in Fig. 2) in 1:1 ratio were mixed in solution of sodium hydroxide/water/ethanol. Then 5 ml of this solution were incorporated in 5 ml of 15% PVP solution in ethanol followed by stirring for 3 h at 70 C. Fig. 3 demonstrates the chemical structure of EuEDTA chelated complex. Concentration of Euþ3 ions in PVP solution was 0.013 mM/mL. 2.2. Preparation of nanoparticles of copper The synthesis of CuNPs is based on a previous paper [22] modified as follows. The starting materials consist of Cu(NO3)2∙5H2O e 0.3 M, sodium citrate - 0.3 M, 100 ml of distillated water, surfactant (C16H33)N(CH3)3Br (CTAB) - 0.05 M, aqueous NaOH edrop by drop up to pH 7, 0.01 ml of polyethylene glycol and 0.03 ml of glycerin. The solution is stirred with magnetic stirrer at temperature 80 C. During the process, the blue solution turns reddish the extinction spectrum of which (vide infra) is the evidence of presence of CuNPs. The extinction spectrum of the CuNPs in solution is presented in Fig. 4.
Fig. 3. The chemical structure of EuEDTA chelated complex.
Fig. 2. The chemical structures of EDTA at classic (a), high pH alkaline (b) conditions.
R. Reisfeld et al. / Optical Materials 59 (2016) 3 3e7
5
Fig. 4. The extinction spectrum of the CuNPs in solution.
3. Measurements
3.2. SEM and TEM measurements
3.1. Spectroscopy
High resolution transmission electron microscopy (HRTEM) observations were carried out using a TECNAI F20 G2 electron microscope (FEI Company) operated at 200 kV and equipped with an EDAX EDS detector. The film samples were deposited on 300mesh cooper grids. High resolution scanning electron microscopy (HRSEM) (Sirion FEI Company) uses Shottky type Field Emission Source and allows wide range of accelerating voltages from 200 V to 30 kV. It is able to achieve resolutions of 1.5 nm at 10 kV. The Fig. 5 shows the HRSEM image of CuNPs in PVP film and histogram of sizes distribution.
The photoluminescence excitation and emission spectra were acquired using a FluoroMax-4P TCSPC spectrofluorometer from Horiba. The excitation source in this system was a 150-W ozone-free Xenon lamp. The luminescence was collected using CzernyeTurner monochromators and photomultiplier R928 Side-on. The experimental setup for luminescence kinetics [23] consists of YAG:Nd laser of PL 2143 A/SS type and the parametric optical generator PG 401/SH. The laser generated pulses of the 355 nm wavelengths with frequency of 10 Hz and pump PG generator could produce 30 ps light pulses with wavelength ranging from 220 nm to 2200 nm. Emission signal was analyzed by the spectrometer 2501S (Bruker Optics) and a Hamamatsu Streak Camera model C4334-01 with a final spectral resolution of 0.5 nm.
4. Results EDTA is a polyprotic acid for complexing metal ions. It is contains four carboxylic acid groups connected to europium ion. Fluorescence excitation spectra monitored at 616.6 nm of the
Fig. 5. HRSEM image of CuNPs in PVP film and histogram of sizes distribution.
6
R. Reisfeld et al. / Optical Materials 59 (2016) 3 e7
Fig. 6. The excitation spectra of the EuEDTA luminescence in PVP film (dashed line) and EuEDTA doped with copper NPs in PVP film (solid line). The excitation spectra were measured observing the maximum of Eu3þ luminescence at 616.6 nm.
Fig. 8. Experimental decay and fitted decay (red line) of Eu3þ in EuEDTA with and without Cu NPs observing emission at 616.6 nm.
EuEDTA and EuDTACuNPs are presented in Fig. 6. The excitation spectrum of EuEDTA (dotted curve) consists only of sharp lines related to internal transition in the 4f6 electronic configuration of Eu3þ. Solid line presents the excitation of co-doped sample and it shows an additional maximum at 345 nm. This band can be attributed to electron transfer from the host to Europium [24].
Steady state luminescence spectra un This band can be attributed to electron transfer from the host to Europium der excitation at 394 nm (corresponding to 7F0/5L6 transition in Eu3þ) is presented in Fig. 7a whereas luminescence spectra obtained under excitation with 320 nm (that corresponds to excitation through CuNPs are presented in Fig. 7b). One notices that for both excitation and emission intensity of the luminescence is much broader for EuDTACuNPs than for EuEDTA. The amplification is more than 100%. When the system is excited at 320 nm apart of the Eu3þ sharp lines luminescence the broad band emission with maximum at 410 nm is observed. The intensity of this band (attributed to the emission of the host) also increases for EuDTACuNPs, intensified also by CuNPs. Photoluminescence decay of Eu3þ luminescence monitored at 616.6 nm, which corresponds to 5D0/7F2 transition are presented in Fig. 8. It is seen that the decays are almost single exponential. When the system is co-doped with CuNPs the luminescence decay becomes faster (the lifetime diminishes) indicating the some nonradiative energy transfer occurs to CuNPs situating at very close distance to the complex. At higher distances between the complex and CuNPs intensification by CuNPs is clearly seen, see Fig. 7a, b.
Fig. 7. Luminescent spectra of Eu3þ EDTA complex (dashed line) in PVP film and codoped by copper NPs (solid line). Excitation wavelength is 394 nm (a) and 320 nm (b).
Fig. 9. Experimental decay curve of EuEDTA with and without Cu NPs observing emission at 420 nm.
R. Reisfeld et al. / Optical Materials 59 (2016) 3 e7
In Fig. 9 the decay of the luminescence monitored at 420 nm (host emission) is presented. The decay is strongly multiexponential. It is seen that in contrast to the Eu3þ luminescence related to the 5D0/7F2 transition, the host emission of the EuDTACuNPs decays slower than the host emission of EuEDTA which may be the result of back energy transfer from the complex to the CuNPs. The increase of luminescence of europium complex in presence of CuNPs may be the result of the scattered light by the nanoparticles which during the relatively long lifetime of the excited state of the europium can affect many excitations in the excited state. The second effect that can cause the decreasing of the luminescence lifetime is increasing of the 5D0/7F2 radiative transition probability. Even weak axial electric field created by CuNP in the vicinity of Eu3þ can enhanced of quantity transition moment. 5. Conclusion Co-doping with CuNPs magnifies the Eu3þ luminescence and host luminescence. It seems that multiexponential host luminescence decay is related to nonradiative energy transfer form host to Eu3þ. We observe the energy transfer from CuNPs to the EDTA host. It seems that there is direct energy transfer from CuNPs to Eu3þ that does not involve the host excitation (the elongation of host emission of EuEDTACuNPs in comparison to EuEDTA). The increased luminescence of the complex and the matrix results from: 1) The scattered light which allows many excitation events because of the long excited state lifetime. 2) Electron transfer from CuNPs to excited state of Eu in the complex results in shifting the excitation band to longer wavelengths (this is very important to solar energy utilization). From the increase of the host luminescence presented in Figs. 6 and 7a, b we come to the conclusion that the scattered light by CuNPs allows many absorption events during excitation of luminescence when the excited state has a relatively long lifetime. References [1] C.K. Jorgensen, Modern Aspects of Ligand Field Theory, North-Holland Pub. Co,
7
1971. [2] R. Reisfeld, C.K. Jorgensen, Lasers and Excited States of Lanthanides, SpringerVerlag, Berlin Heidelberg, New-York, 1977. [3] J.C. Slater, The theory of complex spectra, Phys. Rev. 34 (1929) 1293e1322. [4] G. Racah, Theory of complex spectra, Phys. Rev. 76 (1949) 186e197. [5] Invited review, in memory of Christian K. Jørgensen R. Reisfeld, Optical properties of lanthanides in condensed phase, Theory Appl. Mater. Sci. 2 (2015) 37e60. [6] R. Reisfeld, Luminescent solar concentrators and the ways to increase their efficiencies, in: D. Levy, M. Zayat (Eds.), Handbook Solegel, 3, Wiley-VCH, GmbH, 2015, pp. 1281e1308. [7] R. Reisfeld, New developments in luminescence for solar energy utilization, Opt. Mater. 32 (2010) 850e856. [8] J.C. Bunzli, C. Piguet, Taking advantage of luminescent lanthanide ions, Chem. Soc. Rev. 34 (2005) 1048e1077. [9] R. Reisfeld, M. Gaft, T. Saraidarov, G. Panczer, M. Zelner, Nanoparticles of cadmium sulfide with europium and terbium in zirconia films having intersified luminescence, Mater. Lett. 45 (2000) 154e156. [10] J. Zhang, Y. Fu, K. Ray, Y. Wang, J.R. Lakowicz, Luminescent properties of Eu(III) chelates on metal nanorods, J. Phys. Chem. C 117 (2013) 9372e9380. [11] S. Zhou, W. Zheng, Z. Chen, D. Tu, Y. Liu, E. Ma, R. Li, H. Zhu, M. Huang, X. Chen, Dissolution-enhanced luminescent bioassay based on inorganic lanthanide nanoparticles, Angew. Chem. 53 (2014) 12498e12502. [12] R. Reisfeld, Fluorescence-active waveguides by the sol-gel method, Theory Appl. Z. fur Naturfor. B 69 (2014) 131e140. [13] F. Auzel, Upconversion and anti-stokes processes with f and d ions in solids, Chem. Rev. 104 (2004) 139e174. [14] A.J. Kenyon, Recent developments in rare-earth doped materials for optoelectronics, Prog. Quant. Electron 26 (2002) 225e284. [15] J.-C.G. Bünzli, A.-S. Chauvin, Lanthanides in solar energy conversion, in: J.C.G. Bünzli, V.K. Pecharsky (Eds.), Handbook on the Physics and Chemistry of Lanthanides, Elsevier B.V., North-Holland, 2014. [16] C.K. Jorgensen, R. Reisfeld, Top. Curr. Chem. 100 (1982) 126e166. [17] R. Reisfeld, C.K. Jorgensen, Excited state phenomena in vitreous materials, in: Handbook on the Physics and Chemistry of Lanthanides, 9, 1987, pp. 1e90. [18] J.-M. Lehn, Supramolecular chemistry e scope and perspectives molecules, supermolecules, and molecular devices (Nobel Lecture), Angew. Chem. Int. Ed. Engl. 27 (1988) 89e112. [19] R. Reisfeld, C.K. Jorgensen, Optical properties of colorants or luminescent species in sol-gel glasses, Struct. Bond. 77 (1992) 207e256. [20] R. Reisfeld, T. Saraidarov, G. Panzer, V. Levchenko, New optical material Europium EDTA complex in polyvinyl pyrrolidone films with fluorescence enhanced by silver plasmons, Opt. Mater 34 (2011) 351e355. [21] V. Levchenko, M. Grouchko, S. Magdassi, T. Saraidarov, R. Reisfeld, Enhancement of luminescence of Rhodamine B by gold nanoparticles in thin films on glass for active optical materials applications, Opt. Mater. 34 (2011) 360e365. [22] R. Reisfeld, V. Levchenko, F. Piccinelli, M. Bettinelli, Ampliphication of light emission of chiral pyridine Eu(III) complex by copper nanoparticles, J. Lumin 170 (2016) 820e824. ski, Time-resolved [23] A. Kubicki, P. Bojarski, M. Grinberg, M. Sadownik, B. Kuklin streak camera system with solid state laser and optical parametric generator in different spectroscopic applications, Opt. Commun. 269 (2006) 275e280. [24] Klixbüll Jørgensen, Electron transfer spectra, in progress, in: S.J. Lippard (Ed.), Inorganic Chemistry, 12, John Wiley & Sons, Inc., Hoboken, NJ, USA, 1970.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Intensification of luminescence of Europium-EDTA complex in polyvinyl pyrrolidone films by copper nanoparticles* Renata Reisfeld a, b, *, Viktoria Levchenko a, b, Agata Lazarowska c, Sebastian Mahlik c, Marek Grinberg c a
Institute of Chemistry, The Hebrew University of Jerusalem, E. Safra Campus, Givat Ram, 91904 Jerusalem, Israel The Harvey M. Krueger Family Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Edmond J. Safra Campus, 91904 Jerusalem, Israel c sk University, Wita Stwosza 57, 80-952 Gdan sk, Poland Institute of Experimental Physics, Gdan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 November 2015 Received in revised form 15 March 2016 Accepted 25 March 2016 Available online 12 May 2016
Stable copper nanoparticles (CuNPs) were prepared and incorporated into polyvinylpyrrolidone (PVP) films together with pre-prepared complex of europium-ethylenediaminetetraacetic acid (EuEDTA). From the comparison of the excitation spectrum of the complex alone and of the complex in conjunction with CuNPs an increased fluorescence intensity of the complex is observed as the result of interaction of the complex with surface plasmons of copper. This effect is maximal when the extinction band of CuNPs coincides with the absorption maximum of the complex, as much more light reaches the excited state of europium in the complex during the excitation events as the result of light scattered by copper plasmons. An additional band was observed in the excitation spectrum of the complex in co-doped by the CuNPs around 320÷390 nm which we attribute to electron transfer from CuNPs to excited state of europium. © 2016 Elsevier B.V. All rights reserved.
Keywords: Stable copper nanoparticles Surface plasmons of copper Complex of europiumethylenediaminetetraacetic acid (EuEDTA) Scattered light Enhancement of fluorescence Rare earths
1. Introduction The lanthanides are a fascinating group of elements the optical properties of which arise from the inner f-electrons which are starting with one in Cerium and terminate with thirteen in Ytterbium. The transition probabilities within the 4 f-orbital are forbidden by Laporte rule and become partially allowed either by mixing with 5 d-orbital of Ln ion or with a charge transfer states of the neighboring ligands [1,2]. The basic theory of the electronic spectra can be found in Refs. [3e5]. Lanthanide luminescence is currently applied in the following fields. Luminescent solar concentrators [6,7], computer displays; light emitting diodes [8], semiconductor quantum dots [9], luminescent biosensors [10], bioassays [11], active optical waveguides [12], materials based on upconversion from IR to visible light [13],
*
Enrique Berman Professor of Solar Energy. * Corresponding author. Institute of Chemistry, The Hebrew University of Jerusalem, E. Safra Campus, Givat Ram, 91904 Jerusalem, Israel. E-mail address: [email protected] (R. Reisfeld). http://dx.doi.org/10.1016/j.optmat.2016.03.046 0925-3467/© 2016 Elsevier B.V. All rights reserved.
materials for optoelectronics [14]. The importance of lanthanides in solar energy conversion can be found in excellent book [15]. The luminescent spectra of the europium are intensified greatly by complexation of lanthanide ions with organic ligands. These lanthanide ions form stable crystalline complexes with heterocyclic ligands, such as bipyridyl (bpy) and phenanthroline (phen), which exhibit efficient energy transfer to the chelated lanthanide ions [16,17] and cryptates [18]. However, lanthanides complexes with organic ligands for practical uses are limited because of poor thermal stability and mechanical properties, although they also have good phosphor characteristics. This drawback can be overcome by protecting them by a glassy or stable polymer surrounding [19]. Recently nanoparticles (NPs) of lanthanides having high luminescence have shown numerous applications in medicine and other fields. In order to take the advantage of the properties of the lanthanides additional ways of increasing their fluorescence intensities are studied. This is done by incorporating into the host matrices NPs of silver, gold and copper [20e22] in conjunction with the luminescent species. In the present paper we describe the intensification of EuEDTA
R. Reisfeld et al. / Optical Materials 59 (2016) 33e7
4
Fig. 1. The 3D-structure of PVP, red atom is oxygen, blue e nitrogen, dark grey e carbon, light grey e hydrogen.
complex incorporated in PVP and study the steady state and dynamic spectroscopy. The size and distribution of the CuNPs are obtained by scanning electron microscope (SEM) and transmission electron microscope (TEM).
2.3. Preparation of the samples
2. Experimental
1) EuEDTA e PVP film doped by EuEDTA complex 2) EuEDTACuNPs e PVP film doped by EuEDTA complex and nanoparticles of copper
2.1. Preparation of EuEDTA complex in PVP
Films were formed on microscope slides using drop-casting process, dried at 28 C for 24 h and heated at 100 C for 1 h. Two types of films were obtained by drop casting method:
The EuEDTA complex in PVP matrix, see the structure of PVP in the Fig. 1, was formed as follows: appropriate stock solutions of desired metal ion europium chloride and ligands - ethylenediamine tetraacetic acid (see the structure of EDTA in Fig. 2) in 1:1 ratio were mixed in solution of sodium hydroxide/water/ethanol. Then 5 ml of this solution were incorporated in 5 ml of 15% PVP solution in ethanol followed by stirring for 3 h at 70 C. Fig. 3 demonstrates the chemical structure of EuEDTA chelated complex. Concentration of Euþ3 ions in PVP solution was 0.013 mM/mL. 2.2. Preparation of nanoparticles of copper The synthesis of CuNPs is based on a previous paper [22] modified as follows. The starting materials consist of Cu(NO3)2∙5H2O e 0.3 M, sodium citrate - 0.3 M, 100 ml of distillated water, surfactant (C16H33)N(CH3)3Br (CTAB) - 0.05 M, aqueous NaOH edrop by drop up to pH 7, 0.01 ml of polyethylene glycol and 0.03 ml of glycerin. The solution is stirred with magnetic stirrer at temperature 80 C. During the process, the blue solution turns reddish the extinction spectrum of which (vide infra) is the evidence of presence of CuNPs. The extinction spectrum of the CuNPs in solution is presented in Fig. 4.
Fig. 3. The chemical structure of EuEDTA chelated complex.
Fig. 2. The chemical structures of EDTA at classic (a), high pH alkaline (b) conditions.
R. Reisfeld et al. / Optical Materials 59 (2016) 3 3e7
5
Fig. 4. The extinction spectrum of the CuNPs in solution.
3. Measurements
3.2. SEM and TEM measurements
3.1. Spectroscopy
High resolution transmission electron microscopy (HRTEM) observations were carried out using a TECNAI F20 G2 electron microscope (FEI Company) operated at 200 kV and equipped with an EDAX EDS detector. The film samples were deposited on 300mesh cooper grids. High resolution scanning electron microscopy (HRSEM) (Sirion FEI Company) uses Shottky type Field Emission Source and allows wide range of accelerating voltages from 200 V to 30 kV. It is able to achieve resolutions of 1.5 nm at 10 kV. The Fig. 5 shows the HRSEM image of CuNPs in PVP film and histogram of sizes distribution.
The photoluminescence excitation and emission spectra were acquired using a FluoroMax-4P TCSPC spectrofluorometer from Horiba. The excitation source in this system was a 150-W ozone-free Xenon lamp. The luminescence was collected using CzernyeTurner monochromators and photomultiplier R928 Side-on. The experimental setup for luminescence kinetics [23] consists of YAG:Nd laser of PL 2143 A/SS type and the parametric optical generator PG 401/SH. The laser generated pulses of the 355 nm wavelengths with frequency of 10 Hz and pump PG generator could produce 30 ps light pulses with wavelength ranging from 220 nm to 2200 nm. Emission signal was analyzed by the spectrometer 2501S (Bruker Optics) and a Hamamatsu Streak Camera model C4334-01 with a final spectral resolution of 0.5 nm.
4. Results EDTA is a polyprotic acid for complexing metal ions. It is contains four carboxylic acid groups connected to europium ion. Fluorescence excitation spectra monitored at 616.6 nm of the
Fig. 5. HRSEM image of CuNPs in PVP film and histogram of sizes distribution.
6
R. Reisfeld et al. / Optical Materials 59 (2016) 3 e7
Fig. 6. The excitation spectra of the EuEDTA luminescence in PVP film (dashed line) and EuEDTA doped with copper NPs in PVP film (solid line). The excitation spectra were measured observing the maximum of Eu3þ luminescence at 616.6 nm.
Fig. 8. Experimental decay and fitted decay (red line) of Eu3þ in EuEDTA with and without Cu NPs observing emission at 616.6 nm.
EuEDTA and EuDTACuNPs are presented in Fig. 6. The excitation spectrum of EuEDTA (dotted curve) consists only of sharp lines related to internal transition in the 4f6 electronic configuration of Eu3þ. Solid line presents the excitation of co-doped sample and it shows an additional maximum at 345 nm. This band can be attributed to electron transfer from the host to Europium [24].
Steady state luminescence spectra un This band can be attributed to electron transfer from the host to Europium der excitation at 394 nm (corresponding to 7F0/5L6 transition in Eu3þ) is presented in Fig. 7a whereas luminescence spectra obtained under excitation with 320 nm (that corresponds to excitation through CuNPs are presented in Fig. 7b). One notices that for both excitation and emission intensity of the luminescence is much broader for EuDTACuNPs than for EuEDTA. The amplification is more than 100%. When the system is excited at 320 nm apart of the Eu3þ sharp lines luminescence the broad band emission with maximum at 410 nm is observed. The intensity of this band (attributed to the emission of the host) also increases for EuDTACuNPs, intensified also by CuNPs. Photoluminescence decay of Eu3þ luminescence monitored at 616.6 nm, which corresponds to 5D0/7F2 transition are presented in Fig. 8. It is seen that the decays are almost single exponential. When the system is co-doped with CuNPs the luminescence decay becomes faster (the lifetime diminishes) indicating the some nonradiative energy transfer occurs to CuNPs situating at very close distance to the complex. At higher distances between the complex and CuNPs intensification by CuNPs is clearly seen, see Fig. 7a, b.
Fig. 7. Luminescent spectra of Eu3þ EDTA complex (dashed line) in PVP film and codoped by copper NPs (solid line). Excitation wavelength is 394 nm (a) and 320 nm (b).
Fig. 9. Experimental decay curve of EuEDTA with and without Cu NPs observing emission at 420 nm.
R. Reisfeld et al. / Optical Materials 59 (2016) 3 e7
In Fig. 9 the decay of the luminescence monitored at 420 nm (host emission) is presented. The decay is strongly multiexponential. It is seen that in contrast to the Eu3þ luminescence related to the 5D0/7F2 transition, the host emission of the EuDTACuNPs decays slower than the host emission of EuEDTA which may be the result of back energy transfer from the complex to the CuNPs. The increase of luminescence of europium complex in presence of CuNPs may be the result of the scattered light by the nanoparticles which during the relatively long lifetime of the excited state of the europium can affect many excitations in the excited state. The second effect that can cause the decreasing of the luminescence lifetime is increasing of the 5D0/7F2 radiative transition probability. Even weak axial electric field created by CuNP in the vicinity of Eu3þ can enhanced of quantity transition moment. 5. Conclusion Co-doping with CuNPs magnifies the Eu3þ luminescence and host luminescence. It seems that multiexponential host luminescence decay is related to nonradiative energy transfer form host to Eu3þ. We observe the energy transfer from CuNPs to the EDTA host. It seems that there is direct energy transfer from CuNPs to Eu3þ that does not involve the host excitation (the elongation of host emission of EuEDTACuNPs in comparison to EuEDTA). The increased luminescence of the complex and the matrix results from: 1) The scattered light which allows many excitation events because of the long excited state lifetime. 2) Electron transfer from CuNPs to excited state of Eu in the complex results in shifting the excitation band to longer wavelengths (this is very important to solar energy utilization). From the increase of the host luminescence presented in Figs. 6 and 7a, b we come to the conclusion that the scattered light by CuNPs allows many absorption events during excitation of luminescence when the excited state has a relatively long lifetime. References [1] C.K. Jorgensen, Modern Aspects of Ligand Field Theory, North-Holland Pub. Co,
7
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