- Email: [email protected]
PVP blended polymer composites
Journal Pre-proof Energy-transfer-based NIR photoluminescence of Pr3+ /Yb3+ co-doped PEO/PVP blended polymer composites K. Naveen Kumar, Jungwook Choi
PII:
S0030-4026(19)31998-9
DOI:
https://doi.org/10.1016/j.ijleo.2019.164099
Reference:
IJLEO 164099
To appear in:
Optik
Received Date:
18 November 2019
Accepted Date:
18 December 2019
Please cite this article as: Naveen Kumar K, Choi J, Energy-transfer-based NIR photoluminescence of Pr3+ /Yb3+ co-doped PEO/PVP blended polymer composites, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.164099
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Energy-transfer-based NIR photoluminescence of Pr3+/Yb3+ co-doped PEO/PVP blended polymer composites K. Naveen Kumar* and Jungwook Choi* a
School of Mechanical Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan, Gyeongbuk 38541, Republic of Korea. *
Email: [email protected], [email protected]
Abstract
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This study investigates the near infrared (NIR) photoluminescent properties of polyethylene oxide (PEO) and polyvinyl pyrrolidone (PVP) blended polymer composites doped with Pr3+ and Yb3+ ions. The doped composites were synthesized using the conventional solution
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casting method. X-ray diffraction confirmed the amorphous nature of the composites, while
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Fourier-transform infrared spectroscopy revealed the intermolecular and ion-polymeric interactions. The Yb3+-doped polymer composites possessed a broad emission centered at
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1.01 µm (2F5/2→2F7/2). Significant NIR emission was observed at 1.47 µm (1D2→1G4) in the Pr3+-doped polymer composites. This peak was enhanced by increasing the Pr3+ concentration
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in the matrix. Prevalent luminescence (PL) was observed at the optimal concentration of 0.2 wt% of Pr3+. The PL intensity diminished at concentrations higher than the optimal
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concentration because of the quenching effect. The NIR emission at 1.47 µm (1D2→1G4) was intensified after co-doping Pr3+ with Yb3+, owing to energy migration from Yb3+ to Pr3+. The
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optimal Yb3+ concentration, or optimal sensitizer concentration, in the co-doped matrix was 0.15 wt%. The energy transmission pathway from Yb3+ to Pr3+ was illustrated using an energy level scheme diagram. The prepared (0.2 wt%) Pr3+/(0.15 wt%) Yb3+:PEO/PVP blended polymer composite has potential in various NIR photonic and optoelectronic applications.
Keywords: Pr3+/Yb3+; Photoluminescence; Polymers; NIR emission; Energy migration 1
1. Introduction Rare earth (RE) ion-doped materials are materials commonly used in the field of photonics because of their fascinating optical properties. These materials have several potential applications such as in displays, cathode ray oscilloscope tubes, televisions, medical diagnostics and other biological applications [1]. Recently, they have been studied for their utility in several devices, because these materials are convenient and flexible compared to other classes of materials and are more suitable in terms of weight, eco-friendliness,
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transparency, and high impact strength [2]. These properties would enable the development multichannel systems with large networks. Furthermore, these materials are thermally stable and suitable for applications in light-emitting diodes, lasers, and display devices. They are
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also vitally used emissive layers in electroluminescent devices and are considered as promising materials for optical telecommunication applications [3]. Recent advancements
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improved their usage in the field of photonics and are also considered as light converting
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molecular devices. These materials unveiled a new class of materials which can be used commercially for luminescent devices and photonics. In a previous work, we have been identified a new class of blended polymer matrix, with more efficacy in all respects compared
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to conventional materials, which consists of polyethylene oxide (PEO) and polyvinyl pyrrolidone (PVP) [4]. PEO is a polymer commonly used as polymer electrolyte in lithium
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ion polymer batteries due to its good thermal stability and high reactivity. PEO can solvate a
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wide variety of salts and form new composites due to its polar nature, chemical stability, low glass transition temperature, and etheric oxygen cations on its side chain [5, 6]. On the other hand, PVP is an inexpensive polymer which has a high solubility in polar solvents. It is a crosslinked polymer with high thermal stability and mechanical strength. The amorphous nature of PVP permits faster ionic mobility compared to other polymers. It can form a wide variety of complexes with inorganic salts due to the carbonyl group in its side chain [7].
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Some advantages of PVP are its environmental stability, ease of processing, and appreciable electrical properties. In all respects, the PEO and PVP blended polymers have more appreciable characteristics, such as thermal stability and mechanical strength, when compared to individual PEO and PVP. Here, we have chosen an optimized blended complex of PEO/PVP as the host material for RE ion-doping. RE ions are considered as suitable for luminescence due to their exclusive electronic f-f transitions. The fluorescence of the RE ions emanates from partly filled 4f-4f electrons, which are shielded by fully filled 5s and 5p shells
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[8]. Because of this principal character of the RE ions, they display unique properties such as sharp emission, large stokes shift, good color purity, and long lifetime. RE ions consisted compounds play a pivotal role as laser sources and optical amplifiers in modern broadband
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telecommunication technology. Because of restricted gain bandwidth character of RE ions, they can cover C, L, S, E and O band emissions/amplifications [9]. Praseodymium (Pr3+) is
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an attractive and promising activator ion among other RE ions, because of its significant near
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infrared (NIR) emission between 1300 and 1600 nm. NIR and mid-infrared (MIR) emissive materials are possible materials for various applications such as in optical communications, eye-safe laser radars, medical surgeries, atmospheric pollution monitoring, and remote
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sensing. The wavelength range from 1.3 to 3.0 µm is the most significant region for broadband optical amplifiers, optoelectronics, photonics, and telecommunication systems
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[10]. Praseodymium trivalent ions possess unique optical characteristics such as multi-
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metastable states of 3P0, 1D2 & 1G4, and it has two different emission energy levels of 3P0 and 1
D2 due to its inter-configurational (f-d) and intra-configurational (f-f) electronic transitions.
Materials induced with Pr3+ ions are important for their NIR and MIR emission region, which is essential in the field of fiber optic amplifiers, optoelectronics, medical, and optical communication systems. Many researchers have investigated the prominent emission band at 1.47 µm (1D2→1G4) pertaining to Pr3+ ions in several glassy materials and other materials
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[11]. However, to the best of our knowledge, no investigation has been reported on PEO/PVP polymer composites with Pr3+ focusing on its NIR emission. In order to further enhance the Pr3+ ion emission bandwidth and the luminescence efficacy, we have investigated co-doping strategy with Yb3+ ions in PEO/PVP polymer matrix. However, Yb3+ ions are widely utilized as energy promoter to enhance the activator efficacy of luminescence [12]. Hence, we have undertaken the Yb3+ ions for sensitizing the Pr3+ ions emission. Here, Yb3+ ions are acting as sensitizer and Pr3+ ions are working as activator ions. We can obtain improved NIR
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luminescence characteristics of Pr3+ ions by co-doping with Yb3+ ions in PEO/PVP polymer environment. It might be a possible scenario for obtaining the significant NIR emission results by increasing the co-doping concentration of Yb3+ ion due to energy promotion from
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Yb3+ ions to Pr3+ ions. At particular concentration of sensitizer Yb3+ ions, we can get prominent NIR emission results of Pr3+ ions, which can noticed as optimized concentration of
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Yb3+ ions. We can observe diminishing behavior of emission performance pertaining to Pr3+
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ions by co-doping with Yb3+ ions at above optimized concentration, which could be occurred due to concentration quenching effect [13]. At higher concentration of rare earth ions, the possibility of proximity could be existed between two adjacent rare earth ions. The
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luminescence quenching can be originated from increasing effect of non-radiative relaxation rate due to energy transfer by cross-relaxation mechanism. In our present work, we reported
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the NIR emission of 1.47 µm (1D2→1G4) pertaining to Pr3+ ions in PEO/PVP blended
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polymer composites. Moreover, we have investigated the NIR photoluminescence of the Pr3+ ions in the PEO/PVP blended polymer composites by co-doping with Yb3+ ions.
2. Experimental Studies Pr3+- and Yb3+-doped and Pr3+/Yb3+ co-doped PEO/PVP blended polymer composite materials were synthesized by employing the solution-casting method. Fig. 1 demonstrates
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the process of solution casting. Initially, we have purchased PEO (MW = 6×105), PVP (MW = 13×105), praseodymium nitrate pentahydrate Pr(NO3)3·5H2O and ytterbium nitrate pentahydrate Yb(NO3)3·5H2O from Sigma–Aldrich. Precursor polymer materials of PEO and PVP were taken in an appropriate weight percentage and separately dissolved in adequate amount of double-distilled (DD) water in two beakers as shown in Fig. 1. The appropriate amount (in weight percentage) of RE Pr3+ and Yb3+ ions was dissolved in an adequate amount of DD water. The RE ions solution was poured into the PEO/PVP solution and was stirred for
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10 h at room temperature. The final solution was poured into a polypropylene dish and placed in an evaporation chamber (dust free environment) for 72 h at room temperature. The obtained RE ion-doped polymer composites were peeled up from the surface of Petri dish and
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stored in a desiccator for further investigation of characterization. The prepared PEO/PVP:Pr3+, PEO/PVP:Yb3+, and PEO/PVP:Yb3+/Pr3+ polymer composite samples had a
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thickness of ~100 μm.
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Structural investigation was carried out for all the prepared composites via X-ray diffraction (XRD, SEIFERT 303 TT X-ray diffractometer with CuKα radiation). Intermolecular interactions and functional group assignments were elucidated by Fourier-
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transform infrared (FTIR) analysis. The FTIR spectra were obtained using an EOSXB IR spectrometer in the range from 4,000 to 500 cm-1. An Edinburgh FLS980 fluorescence
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spectrometer with an excitation wavelength of 980 nm was used to acquire the NIR
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photoluminescence spectra for the RE-ion-doped PEO/PVP blended polymer composites.
3. Results and discussion 3.1 Structural analysis XRD was performed to analyze the structure of the PEO/PVP blended undoped matrix and the Pr3+-, Yb3+-, and Pr3+/Yb3+-doped PEO/PVP blended polymer composites
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(Fig. 2). We recorded the XRD profile from 10° to 90° for all the prepared samples. The blended polymer films had a semicrystalline structure. The host matrix exhibited two peaks pertaining to PEO at 19.2° and 23.6° which are assigned to (1 2 0) and (1 1 2), respectively [14]. A small peak was observed near 13.4°, which could pertain to PVP and is an indication of the non-crystalline behavior of the film. We have observed the diminishing of the peak intensity after doping and co-doping with Pr3+, Yb3+. The decrease of crystallinity after doping and co-doping of the polymer matrix could be due to the coordination interactions
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between the RE ions and etheric oxygen cations. This indicates that the Pr3+/Yb3+:PEO/PVP blended polymer matrix possessed a semi-crystalline structure, which indicates that it exhibits both amorphous and crystalline properties at room temperature. No other crystalline peaks
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have been observed in polymer composites, which indicates that there is no other impurities
3.2 Thermogravimetric analysis
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in the sample.
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The thermogravimetric analysis has been carried out for host PEO/PVP and Pr3+ (0.2 wt%)/Yb3+(0.15 wt%): PEO/PVP polymer composites and the TG profiles are shown in Fig. 3. It can be seen that the initial decomposition of the polymer complex has been noticed at
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44°C with a small amount of weight loss in host PEO/PVP and Pr3+ (0.2 wt%)/Yb3+(0.15 wt%): PEO/PVP polymer composites. This weight loss aroused due to presence of air
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humidity and moisture while sample loading. Next decomposition is observed at 330 °C and
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325 °C for both host and codoped polymer composites respectively. From these thermal analysis, the withstanding temperatures for the host and co-doped polymer composites are found to be 330 °C and 325 °C respectively. We have been observed decay behavior in TG curve after 330 °C in both the samples which could be attributed to the elimination of organic functional groups of polyethelene and pyrrolidone pertaining to PEO and PVP from the parent structure [15].
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3.3 FTIR analysis The intermolecular interactions, the complex formation details, and miscibility was investigated through FTIR spectroscopic analysis. We have recorded the FTIR spectra from 400 to 4,000 cm-1 for all the optimized samples as shown in Fig. 4. The broad bands centered at 3228 cm-1 and 3417 cm-1 could be attributed to the O-H group [16]. The peak at 2880 cm-1 indicate the asymmetric vibration of CH pertaining to the CH2 in PEO. We have observed
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multiple bands centered at 1442, 1456, 1364 and 1284 cm-1 which is attributed to CH2 scissoring, bending, wagging doublet, and twisting from the PEO, respectively [17]. Bands at 1265 cm-1 and 1095 cm-1 are ascribed to symmetric twisting of CH2 in PEO and vibrational
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mode of C–O–C in PEO, respectively [18]. These modes are sensitive to macromolecular interactions which confirms the semi-crystalline nature of the PEO. CH2 rocking vibrations
peak in the polymer complex [19]. We have also observed a PVP-related broadband in the
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1
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were observed at 945 cm-1 and methylene groups in PEO were identified through the 844 cm-
region from 1286 cm-1 to 1350 cm-1, which could be ascribed to –CH wagging, and aliphatic C-H stretching vibration was observed at 2896 cm-1 [20]. We noticed a broad band comprised
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of several overlapped bands between 1710 cm-1 and 1600 cm-1 which are assigned to symmetric and asymmetric stretching vibrations of C=O in PVP. CH2 wagging in PVP was
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observed at 1443 cm-1 [21]. We affirmed the presence of PEO and PVP in the host matrix and
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their miscibility for the blended form. We also observed a less intense and broader vibrational band in 1092 cm-1 after doping/co-doping of Pr3+, Yb3+ ions, and it was found to be slightly shifted. This is indicates the formation of coordination bonds between the RE ions and etheric oxygen cations. The PEO and PVP related band at 1456 cm-1 also slightly shifted to a smaller wavenumber which suggests that the RE ions were successfully doped in the blended polymer matrix. A decrease in intensity and shift towards a higher wavenumber was observed
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for the band in 2880 cm-1 which is ascribed to CH2-stretching vibrations. This could be attributed to interactions between the polymer matrix and the RE ions [22]. 3.4 NIR photoluminescence and mechanism of energy-transfer The photoluminescence emission spectrum was recorded in the NIR region at room temperature from 1,000 to 1,100 nm under the excitation of 980 nm for the (0.15 wt%) Yb3+doped PEO/PVP polymer blend composite. We observed a broad emission band centered at 1.01 µm. This emission is attributed to electronic transition between the higher energy state
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of 2F5/2 and the lower energy state of 2F7/2 in the Yb3+ ions in the matrix [23] as shown in Fig. 5. The NIR spectrum was also recorded for the Pr3+-doped PEO/PVP blended polymer composites in the region of 1,400 to 1,565 nm under the excitation of 980 nm as shown in
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Fig. 6. We observed an intense NIR emission band at 1.47 µm in all Pr3+:PEO/PVP polymer blended composites under the laser excitation of 980 nm. This emission could be attributed to
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the existence of an electronic transition of 1D2→1G4 in the Pr3+ ions in polymer matrix [24].
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Upon excitation with the 980 nm wavelength photons, the ground states of Pr3+ ions absorb the energy of the incident photons, enabling the electrons to reach excited states which leads to transition from the 1D2 excited state to 1G4 state. This transition is responsible for the
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emission at 1.47 µm for the broadband wavelength region. The emission intensity was significantly enhanced by increasing the concentration of Pr3+ ions, and the optimal
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concentration for NIR emission of Pr3+:PEO/PVP polymer composites was found to be 0.2
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wt%. The energy migration could also be due to the cross relaxation mechanism from donor Pr3+ ions to acceptor Pr3+ ions where 1D2 energy levels could depopulate and populate the 1G4 energy state. This leads to the electronic transition, which results to an emission at 1.47 µm is vital for the application of this material in telecommunication systems [25]. Nevertheless, emission intensity decreased as the concentration Pr3+ ions exceeded the optimal concentration of 0.2 wt%. Such reduction could be ascribed to concentration quenching effect
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[26]. Concentration quenching could have occurred due to the increase in the density of activator ions at levels where the dopant concentration is above the optimal level. Hence, the emission intensity reduction arose from the cross relaxation, non-radiative multiphonon relaxations through different channels such as 1D2 + 3H4 →1G4 +3F4 and 1D2 + 3H4 →3F4 +1G4 [27, 28] and energy migration between Pr3+ ions and Pr3+ ions. The non-radiative relaxation process lead to the depopulation of 1D2 energy state that caused the concentration quenching at higher concentration. Among various cross relaxation routes and channels pertaining to
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Pr3+ ions such as 3P0, 3P1 and 1D2, which can generate from 1D2 state. Moreover, the 1G4 energy level could be populated not only by 1D2 energy state but also by adjacent Pr3+ ions 3
H4 energy states. Here, relaxation channels were more preferably occurred from 1D2 energy
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state and it is the responsible for exhibiting NIR emission. The effect of the cross relaxation mechanism in the NIR emission quenching can be explained by Dexter’s theory. Here, the
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ground states of Pr3+ ions can absorb the emitted photons of adjacent Pr3+ ions that reemit the
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absorbed photons, which can generate the broad emission spectrum. The energy transmission could occur between donor Pr3+ ions and acceptor Pr3+ ions through cross relaxation, which can depopulate the 1D2 energy state and populate the 1G4 state. This depopulating transition
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of 1D2, which can gives the emission at 1.47 µm (1D2→1G4) and it is useful for telecommunication engineering. Moreover, the concentration quenching is generally a result
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of ion-ion interactions and multipole-multipole interactions [29]. The fluorescence of Pr3+
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ions is governed by electric dipole transitions. Thus, the exchange interactions initiated energy migration between Pr3+ ions and Pr3+ ions could be ignored. Hence, the multipolemultipole interactions between Pr3+ ions and Pr3+ ions are more responsible for concentration quenching effect. One more possibility to exist the luminescence quenching is the interaction between hydroxyl groups and Pr3+ ions in the rare earth ions doped polymer matrix, because the proximity between the vibration energies of hydroxyl group (ranging from 2000 cm-1 to
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3500 cm-1) and energy gap between energy levels pertaining to rare earth ions. The luminescence quenching can occurred by hydroxyl groups presence in huge amount due to their strong absorption bands and overtones in the NIR region, which can generate the energy migration among hydroxyl bonds assisted Pr3+ ions [30]. Nevertheless, we have obtained low intense hydroxyl absorption bands in higher wavenumber region in the FTIR spectra as shown in Fig.4, which discussed in earlier section. However, the broad and weak hydroxyl groups can affect the NIR emission in our present Pr3+:PEO/PVP polymer composites.
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Therefore, the luminescence quenching could be occurred neither hydroxyl group presence nor exchange interactions initiated energy migration between Pr3+ ions and Pr3+ ions.
To enhance the Pr3+ NIR emission performance, we co-doped the (0.2 wt%)
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Pr3+:PEO/PVP polymer composite with Yb3+ ions at various concentrations from 0.05 wt% to 0.25 wt%. The emission performance was generally enhanced after co-doping as shown in
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Fig. 7. We observed a prominent NIR emission at the co-doped concentration of 0.15 wt% of
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Yb3+ ions, which is found to be the optimal concentration. The enhancement of the NIR emission of Pr3+ ions in the PEO/PVP polymer matrix after co-doping with Yb3+ ions suggests the occurrence of energy migration from Yb3+ ions to Pr3+ ions in the polymer
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matrix [31]. Here, Pr3+ ions perform as the acceptor and Yb3+ ions act as the donor. At concentrations greater than 0.15 wt% Yb3+, the NIR emission performance reduced, due to
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concentration quenching effect. Due to the segregation of ions in the polymer matrix, the
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average distance between activator ions is reduced, which could result to luminescence quenching [32]. After the Yb3+ ions absorb energy from the incident photons during excitation, electronic transition takes place from 2F7/2 to 2F5/2. Thereafter, energy transfer takes place from the 2F5/2 of Yb3+ ions to the 1G4, 3P0 levels of Pr3+ ions. The excited electrons of 3P0 of Pr3+ relax to 1D2 through non-radiative energy transfer. Then, the populated 1D2
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electrons relax to 1G4 via radiative emission at 1,470 nm (1D2→1G4). The mechanism of energy migration is illustrated by an energy level scheme diagram as shown in Fig. 8.
4. Conclusion In
summary,
we
have
prepared
Pr3+-
and
Yb3+-doped
and
co-doped
Pr3+/Yb3+:PEO/PVP transparent polymer composites through the conventional solution casting method. The semi-crystalline nature of the composite was confirmed for all prepared
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optimized polymer composite samples via XRD. The thermal stability of the pure and optimized co-doped polymer matrices has been evaluated by thermogravimetric analysis. The intermolecular interactions and the ion-polymer interactions were investigated through FTIR
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spectroscopy analysis. We have obtained an NIR emission at 1.01 µm (2F5/2→2F7/2) from the (0.15 wt%) Yb3+:PEO/PVP blended polymer composite under the excitation wavelength of
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980 nm. An intense NIR emission band was obtained at 1.47 µm (1D2→1G4) from the
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Pr3+:PEO/PVP blended polymer composites. The intensity of this emission band increased by increasing the concentration of Pr3+ ions in the Pr3+:PEO/PVP blended polymer composites. Nevertheless, the predominant emission intensity at 1.47 µm was obtained at 0.2 wt%
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concentration of Pr3+ ions. Hence, the optimal Pr3+ ion concentration was found to be 0.2 wt%. Above the optimal concertation of Pr3+ ions, the emission intensity was reduced due to
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the concentration quenching effect. We successfully increased the intensity of the Pr3+ ion
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NIR emission by co-doping with Yb3+ ions in the PEO/PVP blended polymer complex, which is due to the energy transfer taking place from Yb3+ to Pr3+ ions. Moreover, we have observed a consistent enhancement in this NIR emission while co-doping with Yb3+ ions up to 0.15 wt%, the optimal concentration for the sensitization from Yb3+ ions. However, the emission intensity of 1.47 µm (1D2→1G4) of the Pr3+ ions reduced at concentration greater than 0.15 wt% of Yb3+ ions due to concentration quenching in the polymer matrix. The mechanism of
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energy migration from the Yb3+ to the Pr3+ ions was illustrated in an energy level scheme diagram. The optimized (0.2 wt%) Pr3+ / (0.15 wt%) Yb3+:PEO/PVP blended polymer composite material is a promising material for several NIR photonic device applications.
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
ACKNOWLEDGEMENT
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This work was supported by the 2018 Yeungnam University Research Grant.
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[24] Jiaxin Yang, Lifan Shen, Edwin Yue Bun Pun, Hai Lin, Superiority of shortwave transparent glasses with moderate phonon energy in achieving effective radiations from 1
D2 level of Pr3+, J. Lumin. 213 (2019) 51–60.
[25] V. Naresh, Byoung S. Ham, Influence of multiphonon and cross relaxations on 3P0 and 1
D2 emission levels of Pr3+ doped borosilicate glasses for broad band signal amplification,
J. Alloys Compd. 664 (2016) 321-330. [26] Fengjun Chun, Binbin Zhang, Honggang Liu, Wen Deng, Wen Li, Meilin Xie, Chao
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Luo, Weiqing Yang, Na+ and Pr3+ co-doped orange-emitting CaYAl3O7 phosphors: synthesis, luminescence properties and theoretical calculations, Dalton Trans. 47 (2018)17515-17524.
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[27] Dagupati Rajesh, Pr3+ doped new TZYN glasses and glass-ceramics containing NaYF4 nanocrystals: Luminescence analysis for visible and NIR applications, Opt. Mater. 86
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[28] Houtong Chen, Rui Lian, Min Yin, Liren Lou, Weiping Zhang, Shangda Xia and JeanClaude Krupa, Luminescence concentration quenching of 1D2 state in YPO4:Pr3+, J. Phys: Condens. Matter. 13 (2001) 1151–1158.
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[29] K. Lemański, P.J. Dereń, Luminescent properties of LaAlO3 nanocrystals, doped with Pr3+ and Yb3+ ions, J. Lumin. 146 (2014) 239–242.
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[30] D.L. Dexter, A Theory of Sensitized Luminescence in Solids, J. Chem. Phys. 21
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(1953) 836e-850.
[31] L.H. Slooff, A. van Blaaderen, A. Polman, G.A. Hebbink, S.I. Klink, F.C.J.M. Van Veggel, D.N. Reinhoudt, J.W. Hofstraat, J. Appl. Phys. 91 (7) (2002) 3955–3980.
[32] Marcos P. Belançon, Jorge D. Marconi, Mariana F. Ando, Luiz C. Barbosa, Near-IR emission in Pr3+ single doped and tunable near-IR emission in Pr3+/Yb3+ codoped tellurite tungstate glasses for broadband optical amplifiers, Opt. Mater. 36 (2014) 1020–1026.
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Fig.1
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Fig. 2
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Fig. 1 Schematic diagram of the synthesis of the PEO/PVP polymer.
Fig. 2 (a) XRD profiles of pure (a) PEO/PVP, (b) Pr3+:PEO/PVP, (c) Yb3+:PEO/PVP, and (d) (Pr3+ + Yb3+):PEO/PVP polymer composites.
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Fig.3
Fig.3 Thermogravimetry profile of host PEO/PVP and (0.2 wt%) Pr3+ / (0.15 wt%)
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Yb3+:PEO/PVP blended polymer composites.
Fig. 4
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Fig. 4 FTIR spectra of (a) PEO/PVP, (b) Pr3+:PEO/PVP, (c) Yb3+:PEO/PVP, and
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(d) (Pr3+ / Yb3+):PEO/PVP polymer composites.
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Fig. 5
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Fig. 5 NIR emission spectra of (0.15 wt%) Yb3+:PEO/PVP polymer composite film under
Fig. 6
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980 nm excitation.
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Fig. 6 NIR emission spectra of Pr3+ (0.05, 0.1, 0.15, 0.2, and 0.25): PEO/PVP polymer
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composite films at 980 nm excitation wavelength.
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Fig. 7
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Fig. 7 NIR emission spectra of co-doped Pr3+(0.2) / Yb3+ (0.05, 0.1, 0.15, 0.2, and
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0.25):PEO/PVP polymer composite films at 980 nm excitation wavelength.
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Fig. 8
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composites.
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Fig. 8 Energy-level diagram for NIR emission in (Pr3+ + Yb3+):PEO/PVP polymer
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PII:
S0030-4026(19)31998-9
DOI:
https://doi.org/10.1016/j.ijleo.2019.164099
Reference:
IJLEO 164099
To appear in:
Optik
Received Date:
18 November 2019
Accepted Date:
18 December 2019
Please cite this article as: Naveen Kumar K, Choi J, Energy-transfer-based NIR photoluminescence of Pr3+ /Yb3+ co-doped PEO/PVP blended polymer composites, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.164099
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier.
Energy-transfer-based NIR photoluminescence of Pr3+/Yb3+ co-doped PEO/PVP blended polymer composites K. Naveen Kumar* and Jungwook Choi* a
School of Mechanical Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan, Gyeongbuk 38541, Republic of Korea. *
Email: [email protected], [email protected]
Abstract
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This study investigates the near infrared (NIR) photoluminescent properties of polyethylene oxide (PEO) and polyvinyl pyrrolidone (PVP) blended polymer composites doped with Pr3+ and Yb3+ ions. The doped composites were synthesized using the conventional solution
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casting method. X-ray diffraction confirmed the amorphous nature of the composites, while
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Fourier-transform infrared spectroscopy revealed the intermolecular and ion-polymeric interactions. The Yb3+-doped polymer composites possessed a broad emission centered at
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1.01 µm (2F5/2→2F7/2). Significant NIR emission was observed at 1.47 µm (1D2→1G4) in the Pr3+-doped polymer composites. This peak was enhanced by increasing the Pr3+ concentration
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in the matrix. Prevalent luminescence (PL) was observed at the optimal concentration of 0.2 wt% of Pr3+. The PL intensity diminished at concentrations higher than the optimal
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concentration because of the quenching effect. The NIR emission at 1.47 µm (1D2→1G4) was intensified after co-doping Pr3+ with Yb3+, owing to energy migration from Yb3+ to Pr3+. The
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optimal Yb3+ concentration, or optimal sensitizer concentration, in the co-doped matrix was 0.15 wt%. The energy transmission pathway from Yb3+ to Pr3+ was illustrated using an energy level scheme diagram. The prepared (0.2 wt%) Pr3+/(0.15 wt%) Yb3+:PEO/PVP blended polymer composite has potential in various NIR photonic and optoelectronic applications.
Keywords: Pr3+/Yb3+; Photoluminescence; Polymers; NIR emission; Energy migration 1
1. Introduction Rare earth (RE) ion-doped materials are materials commonly used in the field of photonics because of their fascinating optical properties. These materials have several potential applications such as in displays, cathode ray oscilloscope tubes, televisions, medical diagnostics and other biological applications [1]. Recently, they have been studied for their utility in several devices, because these materials are convenient and flexible compared to other classes of materials and are more suitable in terms of weight, eco-friendliness,
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transparency, and high impact strength [2]. These properties would enable the development multichannel systems with large networks. Furthermore, these materials are thermally stable and suitable for applications in light-emitting diodes, lasers, and display devices. They are
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also vitally used emissive layers in electroluminescent devices and are considered as promising materials for optical telecommunication applications [3]. Recent advancements
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improved their usage in the field of photonics and are also considered as light converting
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molecular devices. These materials unveiled a new class of materials which can be used commercially for luminescent devices and photonics. In a previous work, we have been identified a new class of blended polymer matrix, with more efficacy in all respects compared
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to conventional materials, which consists of polyethylene oxide (PEO) and polyvinyl pyrrolidone (PVP) [4]. PEO is a polymer commonly used as polymer electrolyte in lithium
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ion polymer batteries due to its good thermal stability and high reactivity. PEO can solvate a
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wide variety of salts and form new composites due to its polar nature, chemical stability, low glass transition temperature, and etheric oxygen cations on its side chain [5, 6]. On the other hand, PVP is an inexpensive polymer which has a high solubility in polar solvents. It is a crosslinked polymer with high thermal stability and mechanical strength. The amorphous nature of PVP permits faster ionic mobility compared to other polymers. It can form a wide variety of complexes with inorganic salts due to the carbonyl group in its side chain [7].
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Some advantages of PVP are its environmental stability, ease of processing, and appreciable electrical properties. In all respects, the PEO and PVP blended polymers have more appreciable characteristics, such as thermal stability and mechanical strength, when compared to individual PEO and PVP. Here, we have chosen an optimized blended complex of PEO/PVP as the host material for RE ion-doping. RE ions are considered as suitable for luminescence due to their exclusive electronic f-f transitions. The fluorescence of the RE ions emanates from partly filled 4f-4f electrons, which are shielded by fully filled 5s and 5p shells
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[8]. Because of this principal character of the RE ions, they display unique properties such as sharp emission, large stokes shift, good color purity, and long lifetime. RE ions consisted compounds play a pivotal role as laser sources and optical amplifiers in modern broadband
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telecommunication technology. Because of restricted gain bandwidth character of RE ions, they can cover C, L, S, E and O band emissions/amplifications [9]. Praseodymium (Pr3+) is
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an attractive and promising activator ion among other RE ions, because of its significant near
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infrared (NIR) emission between 1300 and 1600 nm. NIR and mid-infrared (MIR) emissive materials are possible materials for various applications such as in optical communications, eye-safe laser radars, medical surgeries, atmospheric pollution monitoring, and remote
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sensing. The wavelength range from 1.3 to 3.0 µm is the most significant region for broadband optical amplifiers, optoelectronics, photonics, and telecommunication systems
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[10]. Praseodymium trivalent ions possess unique optical characteristics such as multi-
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metastable states of 3P0, 1D2 & 1G4, and it has two different emission energy levels of 3P0 and 1
D2 due to its inter-configurational (f-d) and intra-configurational (f-f) electronic transitions.
Materials induced with Pr3+ ions are important for their NIR and MIR emission region, which is essential in the field of fiber optic amplifiers, optoelectronics, medical, and optical communication systems. Many researchers have investigated the prominent emission band at 1.47 µm (1D2→1G4) pertaining to Pr3+ ions in several glassy materials and other materials
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[11]. However, to the best of our knowledge, no investigation has been reported on PEO/PVP polymer composites with Pr3+ focusing on its NIR emission. In order to further enhance the Pr3+ ion emission bandwidth and the luminescence efficacy, we have investigated co-doping strategy with Yb3+ ions in PEO/PVP polymer matrix. However, Yb3+ ions are widely utilized as energy promoter to enhance the activator efficacy of luminescence [12]. Hence, we have undertaken the Yb3+ ions for sensitizing the Pr3+ ions emission. Here, Yb3+ ions are acting as sensitizer and Pr3+ ions are working as activator ions. We can obtain improved NIR
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luminescence characteristics of Pr3+ ions by co-doping with Yb3+ ions in PEO/PVP polymer environment. It might be a possible scenario for obtaining the significant NIR emission results by increasing the co-doping concentration of Yb3+ ion due to energy promotion from
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Yb3+ ions to Pr3+ ions. At particular concentration of sensitizer Yb3+ ions, we can get prominent NIR emission results of Pr3+ ions, which can noticed as optimized concentration of
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Yb3+ ions. We can observe diminishing behavior of emission performance pertaining to Pr3+
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ions by co-doping with Yb3+ ions at above optimized concentration, which could be occurred due to concentration quenching effect [13]. At higher concentration of rare earth ions, the possibility of proximity could be existed between two adjacent rare earth ions. The
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luminescence quenching can be originated from increasing effect of non-radiative relaxation rate due to energy transfer by cross-relaxation mechanism. In our present work, we reported
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the NIR emission of 1.47 µm (1D2→1G4) pertaining to Pr3+ ions in PEO/PVP blended
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polymer composites. Moreover, we have investigated the NIR photoluminescence of the Pr3+ ions in the PEO/PVP blended polymer composites by co-doping with Yb3+ ions.
2. Experimental Studies Pr3+- and Yb3+-doped and Pr3+/Yb3+ co-doped PEO/PVP blended polymer composite materials were synthesized by employing the solution-casting method. Fig. 1 demonstrates
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the process of solution casting. Initially, we have purchased PEO (MW = 6×105), PVP (MW = 13×105), praseodymium nitrate pentahydrate Pr(NO3)3·5H2O and ytterbium nitrate pentahydrate Yb(NO3)3·5H2O from Sigma–Aldrich. Precursor polymer materials of PEO and PVP were taken in an appropriate weight percentage and separately dissolved in adequate amount of double-distilled (DD) water in two beakers as shown in Fig. 1. The appropriate amount (in weight percentage) of RE Pr3+ and Yb3+ ions was dissolved in an adequate amount of DD water. The RE ions solution was poured into the PEO/PVP solution and was stirred for
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10 h at room temperature. The final solution was poured into a polypropylene dish and placed in an evaporation chamber (dust free environment) for 72 h at room temperature. The obtained RE ion-doped polymer composites were peeled up from the surface of Petri dish and
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stored in a desiccator for further investigation of characterization. The prepared PEO/PVP:Pr3+, PEO/PVP:Yb3+, and PEO/PVP:Yb3+/Pr3+ polymer composite samples had a
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thickness of ~100 μm.
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Structural investigation was carried out for all the prepared composites via X-ray diffraction (XRD, SEIFERT 303 TT X-ray diffractometer with CuKα radiation). Intermolecular interactions and functional group assignments were elucidated by Fourier-
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transform infrared (FTIR) analysis. The FTIR spectra were obtained using an EOSXB IR spectrometer in the range from 4,000 to 500 cm-1. An Edinburgh FLS980 fluorescence
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spectrometer with an excitation wavelength of 980 nm was used to acquire the NIR
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photoluminescence spectra for the RE-ion-doped PEO/PVP blended polymer composites.
3. Results and discussion 3.1 Structural analysis XRD was performed to analyze the structure of the PEO/PVP blended undoped matrix and the Pr3+-, Yb3+-, and Pr3+/Yb3+-doped PEO/PVP blended polymer composites
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(Fig. 2). We recorded the XRD profile from 10° to 90° for all the prepared samples. The blended polymer films had a semicrystalline structure. The host matrix exhibited two peaks pertaining to PEO at 19.2° and 23.6° which are assigned to (1 2 0) and (1 1 2), respectively [14]. A small peak was observed near 13.4°, which could pertain to PVP and is an indication of the non-crystalline behavior of the film. We have observed the diminishing of the peak intensity after doping and co-doping with Pr3+, Yb3+. The decrease of crystallinity after doping and co-doping of the polymer matrix could be due to the coordination interactions
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between the RE ions and etheric oxygen cations. This indicates that the Pr3+/Yb3+:PEO/PVP blended polymer matrix possessed a semi-crystalline structure, which indicates that it exhibits both amorphous and crystalline properties at room temperature. No other crystalline peaks
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have been observed in polymer composites, which indicates that there is no other impurities
3.2 Thermogravimetric analysis
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in the sample.
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The thermogravimetric analysis has been carried out for host PEO/PVP and Pr3+ (0.2 wt%)/Yb3+(0.15 wt%): PEO/PVP polymer composites and the TG profiles are shown in Fig. 3. It can be seen that the initial decomposition of the polymer complex has been noticed at
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44°C with a small amount of weight loss in host PEO/PVP and Pr3+ (0.2 wt%)/Yb3+(0.15 wt%): PEO/PVP polymer composites. This weight loss aroused due to presence of air
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humidity and moisture while sample loading. Next decomposition is observed at 330 °C and
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325 °C for both host and codoped polymer composites respectively. From these thermal analysis, the withstanding temperatures for the host and co-doped polymer composites are found to be 330 °C and 325 °C respectively. We have been observed decay behavior in TG curve after 330 °C in both the samples which could be attributed to the elimination of organic functional groups of polyethelene and pyrrolidone pertaining to PEO and PVP from the parent structure [15].
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3.3 FTIR analysis The intermolecular interactions, the complex formation details, and miscibility was investigated through FTIR spectroscopic analysis. We have recorded the FTIR spectra from 400 to 4,000 cm-1 for all the optimized samples as shown in Fig. 4. The broad bands centered at 3228 cm-1 and 3417 cm-1 could be attributed to the O-H group [16]. The peak at 2880 cm-1 indicate the asymmetric vibration of CH pertaining to the CH2 in PEO. We have observed
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multiple bands centered at 1442, 1456, 1364 and 1284 cm-1 which is attributed to CH2 scissoring, bending, wagging doublet, and twisting from the PEO, respectively [17]. Bands at 1265 cm-1 and 1095 cm-1 are ascribed to symmetric twisting of CH2 in PEO and vibrational
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mode of C–O–C in PEO, respectively [18]. These modes are sensitive to macromolecular interactions which confirms the semi-crystalline nature of the PEO. CH2 rocking vibrations
peak in the polymer complex [19]. We have also observed a PVP-related broadband in the
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were observed at 945 cm-1 and methylene groups in PEO were identified through the 844 cm-
region from 1286 cm-1 to 1350 cm-1, which could be ascribed to –CH wagging, and aliphatic C-H stretching vibration was observed at 2896 cm-1 [20]. We noticed a broad band comprised
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of several overlapped bands between 1710 cm-1 and 1600 cm-1 which are assigned to symmetric and asymmetric stretching vibrations of C=O in PVP. CH2 wagging in PVP was
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observed at 1443 cm-1 [21]. We affirmed the presence of PEO and PVP in the host matrix and
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their miscibility for the blended form. We also observed a less intense and broader vibrational band in 1092 cm-1 after doping/co-doping of Pr3+, Yb3+ ions, and it was found to be slightly shifted. This is indicates the formation of coordination bonds between the RE ions and etheric oxygen cations. The PEO and PVP related band at 1456 cm-1 also slightly shifted to a smaller wavenumber which suggests that the RE ions were successfully doped in the blended polymer matrix. A decrease in intensity and shift towards a higher wavenumber was observed
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for the band in 2880 cm-1 which is ascribed to CH2-stretching vibrations. This could be attributed to interactions between the polymer matrix and the RE ions [22]. 3.4 NIR photoluminescence and mechanism of energy-transfer The photoluminescence emission spectrum was recorded in the NIR region at room temperature from 1,000 to 1,100 nm under the excitation of 980 nm for the (0.15 wt%) Yb3+doped PEO/PVP polymer blend composite. We observed a broad emission band centered at 1.01 µm. This emission is attributed to electronic transition between the higher energy state
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of 2F5/2 and the lower energy state of 2F7/2 in the Yb3+ ions in the matrix [23] as shown in Fig. 5. The NIR spectrum was also recorded for the Pr3+-doped PEO/PVP blended polymer composites in the region of 1,400 to 1,565 nm under the excitation of 980 nm as shown in
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Fig. 6. We observed an intense NIR emission band at 1.47 µm in all Pr3+:PEO/PVP polymer blended composites under the laser excitation of 980 nm. This emission could be attributed to
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the existence of an electronic transition of 1D2→1G4 in the Pr3+ ions in polymer matrix [24].
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Upon excitation with the 980 nm wavelength photons, the ground states of Pr3+ ions absorb the energy of the incident photons, enabling the electrons to reach excited states which leads to transition from the 1D2 excited state to 1G4 state. This transition is responsible for the
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emission at 1.47 µm for the broadband wavelength region. The emission intensity was significantly enhanced by increasing the concentration of Pr3+ ions, and the optimal
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concentration for NIR emission of Pr3+:PEO/PVP polymer composites was found to be 0.2
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wt%. The energy migration could also be due to the cross relaxation mechanism from donor Pr3+ ions to acceptor Pr3+ ions where 1D2 energy levels could depopulate and populate the 1G4 energy state. This leads to the electronic transition, which results to an emission at 1.47 µm is vital for the application of this material in telecommunication systems [25]. Nevertheless, emission intensity decreased as the concentration Pr3+ ions exceeded the optimal concentration of 0.2 wt%. Such reduction could be ascribed to concentration quenching effect
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[26]. Concentration quenching could have occurred due to the increase in the density of activator ions at levels where the dopant concentration is above the optimal level. Hence, the emission intensity reduction arose from the cross relaxation, non-radiative multiphonon relaxations through different channels such as 1D2 + 3H4 →1G4 +3F4 and 1D2 + 3H4 →3F4 +1G4 [27, 28] and energy migration between Pr3+ ions and Pr3+ ions. The non-radiative relaxation process lead to the depopulation of 1D2 energy state that caused the concentration quenching at higher concentration. Among various cross relaxation routes and channels pertaining to
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Pr3+ ions such as 3P0, 3P1 and 1D2, which can generate from 1D2 state. Moreover, the 1G4 energy level could be populated not only by 1D2 energy state but also by adjacent Pr3+ ions 3
H4 energy states. Here, relaxation channels were more preferably occurred from 1D2 energy
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state and it is the responsible for exhibiting NIR emission. The effect of the cross relaxation mechanism in the NIR emission quenching can be explained by Dexter’s theory. Here, the
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ground states of Pr3+ ions can absorb the emitted photons of adjacent Pr3+ ions that reemit the
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absorbed photons, which can generate the broad emission spectrum. The energy transmission could occur between donor Pr3+ ions and acceptor Pr3+ ions through cross relaxation, which can depopulate the 1D2 energy state and populate the 1G4 state. This depopulating transition
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of 1D2, which can gives the emission at 1.47 µm (1D2→1G4) and it is useful for telecommunication engineering. Moreover, the concentration quenching is generally a result
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of ion-ion interactions and multipole-multipole interactions [29]. The fluorescence of Pr3+
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ions is governed by electric dipole transitions. Thus, the exchange interactions initiated energy migration between Pr3+ ions and Pr3+ ions could be ignored. Hence, the multipolemultipole interactions between Pr3+ ions and Pr3+ ions are more responsible for concentration quenching effect. One more possibility to exist the luminescence quenching is the interaction between hydroxyl groups and Pr3+ ions in the rare earth ions doped polymer matrix, because the proximity between the vibration energies of hydroxyl group (ranging from 2000 cm-1 to
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3500 cm-1) and energy gap between energy levels pertaining to rare earth ions. The luminescence quenching can occurred by hydroxyl groups presence in huge amount due to their strong absorption bands and overtones in the NIR region, which can generate the energy migration among hydroxyl bonds assisted Pr3+ ions [30]. Nevertheless, we have obtained low intense hydroxyl absorption bands in higher wavenumber region in the FTIR spectra as shown in Fig.4, which discussed in earlier section. However, the broad and weak hydroxyl groups can affect the NIR emission in our present Pr3+:PEO/PVP polymer composites.
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Therefore, the luminescence quenching could be occurred neither hydroxyl group presence nor exchange interactions initiated energy migration between Pr3+ ions and Pr3+ ions.
To enhance the Pr3+ NIR emission performance, we co-doped the (0.2 wt%)
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Pr3+:PEO/PVP polymer composite with Yb3+ ions at various concentrations from 0.05 wt% to 0.25 wt%. The emission performance was generally enhanced after co-doping as shown in
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Fig. 7. We observed a prominent NIR emission at the co-doped concentration of 0.15 wt% of
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Yb3+ ions, which is found to be the optimal concentration. The enhancement of the NIR emission of Pr3+ ions in the PEO/PVP polymer matrix after co-doping with Yb3+ ions suggests the occurrence of energy migration from Yb3+ ions to Pr3+ ions in the polymer
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matrix [31]. Here, Pr3+ ions perform as the acceptor and Yb3+ ions act as the donor. At concentrations greater than 0.15 wt% Yb3+, the NIR emission performance reduced, due to
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concentration quenching effect. Due to the segregation of ions in the polymer matrix, the
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average distance between activator ions is reduced, which could result to luminescence quenching [32]. After the Yb3+ ions absorb energy from the incident photons during excitation, electronic transition takes place from 2F7/2 to 2F5/2. Thereafter, energy transfer takes place from the 2F5/2 of Yb3+ ions to the 1G4, 3P0 levels of Pr3+ ions. The excited electrons of 3P0 of Pr3+ relax to 1D2 through non-radiative energy transfer. Then, the populated 1D2
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electrons relax to 1G4 via radiative emission at 1,470 nm (1D2→1G4). The mechanism of energy migration is illustrated by an energy level scheme diagram as shown in Fig. 8.
4. Conclusion In
summary,
we
have
prepared
Pr3+-
and
Yb3+-doped
and
co-doped
Pr3+/Yb3+:PEO/PVP transparent polymer composites through the conventional solution casting method. The semi-crystalline nature of the composite was confirmed for all prepared
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optimized polymer composite samples via XRD. The thermal stability of the pure and optimized co-doped polymer matrices has been evaluated by thermogravimetric analysis. The intermolecular interactions and the ion-polymer interactions were investigated through FTIR
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spectroscopy analysis. We have obtained an NIR emission at 1.01 µm (2F5/2→2F7/2) from the (0.15 wt%) Yb3+:PEO/PVP blended polymer composite under the excitation wavelength of
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980 nm. An intense NIR emission band was obtained at 1.47 µm (1D2→1G4) from the
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Pr3+:PEO/PVP blended polymer composites. The intensity of this emission band increased by increasing the concentration of Pr3+ ions in the Pr3+:PEO/PVP blended polymer composites. Nevertheless, the predominant emission intensity at 1.47 µm was obtained at 0.2 wt%
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concentration of Pr3+ ions. Hence, the optimal Pr3+ ion concentration was found to be 0.2 wt%. Above the optimal concertation of Pr3+ ions, the emission intensity was reduced due to
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the concentration quenching effect. We successfully increased the intensity of the Pr3+ ion
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NIR emission by co-doping with Yb3+ ions in the PEO/PVP blended polymer complex, which is due to the energy transfer taking place from Yb3+ to Pr3+ ions. Moreover, we have observed a consistent enhancement in this NIR emission while co-doping with Yb3+ ions up to 0.15 wt%, the optimal concentration for the sensitization from Yb3+ ions. However, the emission intensity of 1.47 µm (1D2→1G4) of the Pr3+ ions reduced at concentration greater than 0.15 wt% of Yb3+ ions due to concentration quenching in the polymer matrix. The mechanism of
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energy migration from the Yb3+ to the Pr3+ ions was illustrated in an energy level scheme diagram. The optimized (0.2 wt%) Pr3+ / (0.15 wt%) Yb3+:PEO/PVP blended polymer composite material is a promising material for several NIR photonic device applications.
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
ACKNOWLEDGEMENT
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This work was supported by the 2018 Yeungnam University Research Grant.
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Fig.1
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Fig. 1 Schematic diagram of the synthesis of the PEO/PVP polymer.
Fig. 2 (a) XRD profiles of pure (a) PEO/PVP, (b) Pr3+:PEO/PVP, (c) Yb3+:PEO/PVP, and (d) (Pr3+ + Yb3+):PEO/PVP polymer composites.
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Fig.3
Fig.3 Thermogravimetry profile of host PEO/PVP and (0.2 wt%) Pr3+ / (0.15 wt%)
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Yb3+:PEO/PVP blended polymer composites.
Fig. 4
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Fig. 4 FTIR spectra of (a) PEO/PVP, (b) Pr3+:PEO/PVP, (c) Yb3+:PEO/PVP, and
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(d) (Pr3+ / Yb3+):PEO/PVP polymer composites.
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Fig. 5
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Fig. 5 NIR emission spectra of (0.15 wt%) Yb3+:PEO/PVP polymer composite film under
Fig. 6
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980 nm excitation.
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Fig. 6 NIR emission spectra of Pr3+ (0.05, 0.1, 0.15, 0.2, and 0.25): PEO/PVP polymer
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composite films at 980 nm excitation wavelength.
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Fig. 7
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Fig. 7 NIR emission spectra of co-doped Pr3+(0.2) / Yb3+ (0.05, 0.1, 0.15, 0.2, and
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0.25):PEO/PVP polymer composite films at 980 nm excitation wavelength.
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Fig. 8
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Fig. 8 Energy-level diagram for NIR emission in (Pr3+ + Yb3+):PEO/PVP polymer
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