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Ionic liquids for active photonics components fabrication
Optical Materials 89 (2019) 106–111
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Optical Materials journal homepage: www.elsevier.com/locate/optmat
Ionic liquids for active photonics components fabrication a
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Krzysztof Rola , Adrian Zając , Maciej Czajkowski , Andrea Szpecht , Maria Zdończyk Marcin Śmiglakb, Joanna Cybińskaa,d, Katarzyna Komorowskaa,e,∗
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PORT Sp. z o.o., Stablowicka 147 Str., 54-066 Wroclaw, Poland Material Synthesis Group, Poznan Science and Technology Park, ul. Rubiez 46, 61-612 Poznan, Poland Department of Chemistry, Adam Mickiewicz University, Poznan, Poland d Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie Str., 50-383, Wroclaw, Poland e Department of Optics and Photonics, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, 27 Wybrzeze Wyspianskiego Str. 50-370 Wroclaw, Poland b c
ARTICLE INFO
ABSTRACT
Keywords: Photonics Ionic liquids Electron beam Fluorescein Europium Polymerization
We report the new fluorescent polymerizable imidazolium-based ionic liquids for photonics application. We investigate two approaches: one ionic liquid (IL) contains the active fluorescein dianion as an intrinsic part of the polymerizable ionic liquid structure. The second liquid is based on the mixture of polymerizable ionic liquid with optically active europium(III) nitrate hydrate. We demonstrate the polymerization abilities of both liquids by means of electron beam in vacuum conditions. Furthermore, we measure the light emission properties of liquid and polymerized structures. The results reveal that IL with europium(III) nitrate hydrate yields emission spectrum with Eu3+ bands not only as a liquid but also as a polymerized solid microstructure, which is important for its application in active photonics components.
1. Introduction Design and synthesis of new materials is of great importance for developing new advanced material platforms for optics and photonics which meet special requirements of these research fields. For this specific reason, ionic liquids (ILs) are promising materials for photonics since their properties can be tuned in many different ways by introducing different functionalities into chemical structure of ions [1–5]. Moreover, this class of compounds has a unique combination of physicochemical properties, such as negligible volatility, relatively high thermal and chemical stability, low toxicity and high ionic conductivity. This has made ILs an attractive solution in many research and industrial areas including not only synthesis in chemical industry [6,7], where they can replace conventional solvents, but also energy conversion using fuel cells [8,9], energy storage [9,10] and CO2 capture [11,12] and many more. Among different functionalities that can be incorporated into ionic liquids, two of them are particularly interesting. Ionic liquids with vinyl or allyl group can be polymerized, either by adding polymerization initiators [13–17] or by exposure to high-energy radiation which can break chemical bonds [18–20]. The latter method enables one to fabricate planar microstructures with high resolution by means of lithographic/patterning techniques using ion/electron beam sources, which ∗
can be employed for nanotechnology and photonics due to good optical properties [20]. This method is very attractive because, in contrast to typical solvent-containing lithographic resists, the ionic liquids can be used in a pure form, i.e. the ionic liquid thin films can be formed by spin-coating without using any solvent, which makes the method greener and safer [20]. Apart from polymerizable ILs, fluorescent ionic liquids have also attracted the attention of researchers in the last years since they are potentially useful for optical sensing and detection of various substances, such as, for example, dihydroxybenzenes or nitroexplosives. Such task-specific ionic liquids can be realized using fluorescent organic dyes [21–30], of which fluorescein is often used as monoanion or dianion of IL [24–27]. Another approach to obtain luminescent ILs is to form rare earth elements-based ionic liquids [31–36], which could be applied in photochemistry and spectroscopy. Both aforementioned functionalities, that is polymerizability and luminesce, can be combined in order to achieve poly(ionic liquid)s containing fluorescent moieties [37] or rare earth metal ions [38,39]. In the latter case, poly(ionic liquid)s facilitate immobilization of ions and, in addition, can even slightly enhance the luminescence performance of europium(III) ions in comparison with monomeric ILs [38]. Therefore, in this work, we also address this issue but we develop a different method, i.e. a polymerizable ionic liquid is mixed with either
Corresponding author. PORT Sp. z o.o., Stablowicka 147 Str., 54-066 Wroclaw, Poland. E-mail addresses: [email protected], [email protected] (K. Komorowska).
https://doi.org/10.1016/j.optmat.2019.01.003 Received 20 November 2018; Received in revised form 4 January 2019; Accepted 7 January 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Synthesis route of fluorescein-based ionic liquid.
Fig. 2. Scheme of fabrication procedure of polymerized microstructures out of ionic liquids.
fluorescein-based ionic liquid or europium(III) nitrate and, subsequently, the prepared mixtures in the form of thin films on silicon substrates are in situ polymerized and patterned by high-energy electron beam in vacuum. The proposed technique is aimed at fabrication of functionalized microstructures that could be integrated within miniaturized devices, for example, on silicon microchip. It should be mentioned that, to the best of our knowledge, the polymerization and patterning of luminescent ionic liquids have not been reported in the literature so far. 2. Experimental The polymerizable ionic liquid used in the experiments was 1-allyl3-methylimidazolium chloride (abbreviated as [Allmim][Cl]). It was synthesized according to the procedure described in our previous work [20]. The ionic liquid with [Allmim]+ cation and fluorescein-based dianion was synthesized as follows (Fig. 1). In a round-bottomed one-neck flask (100 mL) 1-allyl-3-methylimidazolium chloride (1.00 g, 6.31 mmol) was places and dissolved in dichloromethane (10 mL). To stirred solution, a solution of fluorescein disodium salt (1.19 g, 3.16 mmol) in water (20 mL) was added in one charge at room temperature. The resulted two phase system was then stirred vigorously for 6 days at room temperature. Next, the water layer was separated, evaporated and dried under high vacuum. The resulted solid was then washed with methanol (100 mL), and obtained solution was evaporated and dried under high vacuum to give pure product (1.70 g, 93%) as a dark red amorphous solid. The mixture of [Allmim][Cl] and europium(III) nitrate hydrate (molar ratio 1:1) was prepared as follows: the proper amount of
Fig. 3. SEM images of polymerized microstructures obtained by electron beam patterning of [Allmim][Cl] containing fluorescein as an dianion (500:1): (a) influence of electron dose, (b) polymerized micropillar, (c) array of polymerized micropillars.
europium salt was dissolved in 5 mL of ethanol and, separately, the IL was also dissolved in ethanol at RT. Both solutions were put together and stirred for 10 min, then the mixture was placed into the vacuum oven for 5–6 h. The drying temperature did not exceed 40 °C. Ionic liquids or their mixtures were spin-coated on monocrystalline (100) silicon substrates in order to form thin films of about 1 μm 107
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Fig. 4. SEM image of polymerized microstructures obtained by electron beam patterning (with different dose) of [Allmim][Cl] containing Eu(III) ions.
thickness. For electron beam activated polymerization processes, the resulting samples were inserted into vacuum chamber of scanning electron microscope (SEM) FEI Helios 660 equipped with Raith Elphy Multibeam patterning system and EDAX energy dispersive X-ray spectrometer (EDS). Then, the samples were irradiated with electron beam (30 keV) according to designed patterns. The dose of electrons delivered to sample surface was varied and determined from the following formula:
D=
to an area of the ionic liquid film. As seen in Fig. 3a, the minimal amount of high-energy electrons must reach the surface of IL to cause solidification on Si substrate. In addition, this effect depends on size of an area irradiated with electrons. The usage of large circular patterns yields traces of polymerized ionic liquid even for doses as low as 7.5 mC/cm2. However, the dose must be doubled to achieve well-defined circular structures and 2-μm-wide trenches. If micropillars of 1 μm radius and trenches of 1 μm width are desired, the dose should be about 30 mC/cm2. Further increase of the dose is not beneficial due to the lateral widening of microstructures. The phenomenon of lateral widening of IL polymerized by electron beam has been discussed for pure [Allmim][Cl] in our previous paper [20]. The high precision of projection of designed patterns onto IL film during polymerization is demonstrated in Fig. 3b. The micropillar has practically ideal circular shape with smooth edges and sidewalls. Moreover, the features of produced microstructures are highly repeatable which can be seen for an array of micropillars in Fig. 3c. The results of electron beam irradiation of the ionic liquid containing Eu3+ cations are significantly different, as shown in Fig. 4. Generally, the quality of obtained microstructures is worse than for the IL/fluorescein system. Although the minimal dose needed to solidify polymerizable ionic liquid on silicon surface is decreased, the resolution is deteriorated. Moreover, the microstructures are surrounded by a material that could not be dissolved by ethanol. During experiments it was noticed that the structures were ruptured in many places, which can be attributed to the presence of europium in a film. The mixture of europium salt and ionic liquid is expected to have different physicochemical properties from the pure ionic liquid. For example, density, thermal conductivity and stiffness can be changed after mixing ionic liquid with europium nitrate. In such a case, the rupture of microstructures could be a result of the mechanical stress in polymerized films. This hypothesis is supported by the fact that the effect was considerably reduced by decreasing the thickness of IL layer on Si substrate (see Fig. S1 in supporting info). It should be also noted that ruptured pieces had sizes of about 10–100 μm. That is why the effect is hardly visible for small microstructures shown in Fig. 4. The detailed microanalysis of IL/Eu3+ thin film by x-ray spectroscopy reveals that distribution of europium in ionic liquid is highly
IB × tdwell xpitch × ypitch
where: D is electron dose (mC/cm2), IB is a beam current, tdwell is an exposure time of a one point (called dwell time), xpitch, ypitch are distances between exposure points in horizontal and vertical directions, respectively. The dose was modified by changing the dwell time (e.g. D = 1 mC/cm2 when tdwell = 1 μs) whereas other parameters were fixed: IB = 26 nA, xpitch = 50 nm, ypitch = 50 nm. After patterning process was finished, each sample was washed in ethanol to remove ionic liquid so that only polymerized microstructures remained on silicon surface. The samples were then imaged using SEM microscope and used for optical measurements. The fabrication procedure briefly illustrated in Fig. 2. The light transmission spectra were measured using Evolution 300 UV–Vis spectrophotometer and Nicolet iS50 FT-IR spectrometer by Thermo Scientific. The luminescence spectra and decay kinetics (DEC) were measured with FSL980-sm Fluorescence Spectrometer from Edinburgh Instruments Ltd. A 450 W Xenon arc lamp (PL and PLE) and 60 W Xenon flash lamp (DEC) as an excitation sources were used. Emission spectra were corrected for the recording system efficiency and excitation spectra were corrected for the incident light intensity. 3. Results 3.1. Microscopic characterization Irradiation of the [Allmim][Cl] ionic liquid containing fluorescein dianions in the proportion of 500 to 1 results in solidification of the liquid, though this process is influenced by the electron dose delivered 108
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homogenous distribution of Eu is maintained within the large polymerized microstructure. 3.2. Spectroscopic characterization The modern studies of ionic liquids, which are in agreement with the principles of the green chemistry, are focused on giving additional functionality to ionic liquids. As it is very well known, the properties of the ionic liquids can be designed and tuned by proper combination of anionic and cationic parts, which, in consequence, can lead to multitask compounds. Thus, the attempt of preparation of luminescent and polymerizable ionic liquids was made. The ionic liquid with the fluorescein as anionic part of compound was obtained according to the procedure described in the experimental part. Resulting dark red compound did not show emission, which is in agreement with previous reported study for fluorescein compounds. To avoid the emission quenching, a mixture of fluorescent polymerizable ionic liquid and additional polymerizable ionic liquid without fluorescent group can be used, since it leads to “diluted” fluorescence dyes and thus prevents quenching. Therefore, the [Allmim][Cl] ionic liquid containing fluorescein dianions in the proportion of 500 to 1 was prepared. For that prepared mixture the strong emission with the maximum around 540 nm was recorded (Fig. 6a). The shape and the location of the band is in agreement with previous reported data [25,26,40]. However, after electron beam exposure and polymerization process, no emission from the micro-size structure was found. Such quenching is a general feature of compounds with π -planar structures which are self-quenched in the aggregated state (aggregation-caused quenching- ACQ) [41]. The other reason for the lack of the luminescence can be directly connected with the high-energy electrons interacting with the material during the polymerization process, which can lead to destroying (by means of electrons themselves or heating caused by electrons), at least partially, of the luminophores groups in the structure. To preserve the luminescent properties after the electron beam exposure another strategy was used. The polymerizable ionic liquid was mixed with Eu3+ salt. Among the lanthanide ions the Eu3+ is well known as showing intense red emission under UV radiation. Moreover, the luminescence spectra of Eu3+ can be used to probe the local environment of this ions [42]. The room temperature emission spectra recorded for Eu3+ containing [Allmim][Cl] ionic liquid before and after e-beam irradiation is presented in Fig. 6b. Both spectra exhibit similar features and consist of typical Eu3+ bands assigned to the 5D0→7FJ transitions. For materials before and after polymerization, 5D0→7F2 band dominate in the spectra, and no significant differences were observed. However, the decay times traces measured for both systems are different (Fig. 7). Before the e-beam exposure the decay curve is clearly monoexponential with decay time constant (τdec) equal 323 μs. After e-beam exposure, the decay time has unexpectedly decreased about 2–3 times and the curve became more complex, not fittable with two-exponential decay function. It clearly indicates that the environment of the europium(III) cations has changed after the e-beam exposure and the resulting polymerized structure may contain the mixture of europium ions with various coordination environment. The exact composition cannot be determined without additional experiments, but here, we can recall some lifetimes of europium(III) species found in literature. The decay times, as 100–140 μs was observed for aqueous solutions of europium nitrate hydrate [43–45] and is similar to europium chloride hydrates, as described in a ref. [43]. In solid state, the monocrystal of europium nitrate hexahydrate has higher decay time - equal 180 μs [46]. The higher decay time of the europium nitrate hydrate after mixing with ionic liquid (the sample before e-beam exposure) suggests that the coordination environment of this molecule has changed. It is more close to the europium(III)-IL mixtures found in recent literature [33,47], where 1 ms and submillisecond times are reported at room temperature. The rise of decay time may be related to
Fig. 5. Results of EDS microanalysis for [Allmim][Cl] containing Eu(III) ions: (a), (b) thin liquid film on Si, (c) polymerized microstructure on Si after washing with ethanol.
uniform and all elements occurring for [Allmim][Cl] and europium(III) nitrate hydrate can be assigned to peaks in the spectrum (Fig. 5a and b). When comparing amounts of Eu and Cl in the sample (Table 1), it can be seen that they are almost equal. This confirms good mixing of constituents of IL and europium(III) nitrate. As shown in Fig. 5c, the 109
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Table 1 EDS quantitative results for [Allmim][Cl] thin film containing Eu(III) ions. Element
Weight (%)
Atomic (%)
Error (%)
C N O Cl Eu
18.8 17.1 27.9 7.6 28.6
31.7 24.7 35.4 4.4 3.8
9.1 10.2 9.7 4.1 5.2
Fig. 7. The photoluminescence decay curves, registered in thin film of [Allmim] [Cl] doped with europium(III) nitrate hydrate, measured at λem = 616 nm: (a) before electron beam treatment - excitation at λex = 275 nm and; (b) after electron beam treatment – excitation at λex = 320 nm. The filter cutting light below 475 nm was used in the measurements in the emission arm. The IRF signal was cut from the graph.
polymerizable cation, which is a new synthesized ionic liquid and opens up prospects for other organic dyes. The pure fluorescein liquid did not show any fluorescence, however the light emission typical for fluorescein was observed for fluorescein dianions diluted in imidazoliumbased chloride. The second approach, which is the use of rare earth metal ions, yielded a homogenous mixture of polymerizable imidazolium-based IL and europium(III) nitrate hydrate with molar ratio about 1:1, as confirmed by quantitative energy dispersive X-ray microanalysis. The light emission spectrum of the above-mentioned liquid included typical Eu3+ bands. Interestingly, the Eu3+ bands were observed also in the case of polymerized microstructures obtained by electron beam. In other words, exposure of polymerizable IL containing Eu3+ ions to high-energy (30 keV) electrons did not prevent the material from having luminescence properties. Nevertheless, this was not the case for polymerized IL with fluorescein where the emission typical for fluorescein was not detected. This can be attributed to solidification of IL as well as to destroying of fluorescein species by electron beam during exposure. As for electron beam activated polymerization, both considered kinds of luminescent ionic liquids could be deposited on silicon substrates as thin films and patterned to form polymerized microstructures of well-defined shapes, potentially attractive as planar photonic microcomponents. Moreover, the distribution of europium in an IL thin film was uniform even after polymerization. The polymerization process of luminescent ionic liquids can be optimized in the future research in terms of spatial quality of polymerized structures and emission properties. The study should take into consideration the search for optimal concentration of rare earth ions in IL and the selection of organic dye that gives high emission in solid polymerized structure.
Fig. 6. Emission spectra for: (a) [Allmim][Cl] with fluorescein excited at 475 nm, (b) [Allmim][Cl] with europium(III) nitrate hydrate before and after ebeam irradiation excited at 318 nm.
the lower number of H2O ligands in coordination sphere of lanthanides, as it is commonly known that the number of H2O strongly decreases the decay time [45], which is also observed in ionic liquids – e.g. for ionic liquids being trihydrate complexes the decay time is about two times lower than for similar dihydrate complexes [47].
Acknowledgements
4. Conclusions
The work was financed by the National Science Center within the Grant Opus UMO-2015/19/B/ST8/02761.
In summary, we successfully obtained two kinds of luminescent polymerizable ionic liquids, the first one – containing fluorescent organic dye, and the second one – mixed with rare earth ions. The first solution consisted of fluorescein dianions diluted in polymerizable imidazolium-based chloride. Admittedly, the fluorescein dianion was first obtained as an only anionic constituent of ionic liquid comprising
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optmat.2019.01.003. 110
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Ionic liquids for active photonics components fabrication a
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Krzysztof Rola , Adrian Zając , Maciej Czajkowski , Andrea Szpecht , Maria Zdończyk Marcin Śmiglakb, Joanna Cybińskaa,d, Katarzyna Komorowskaa,e,∗
,
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PORT Sp. z o.o., Stablowicka 147 Str., 54-066 Wroclaw, Poland Material Synthesis Group, Poznan Science and Technology Park, ul. Rubiez 46, 61-612 Poznan, Poland Department of Chemistry, Adam Mickiewicz University, Poznan, Poland d Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie Str., 50-383, Wroclaw, Poland e Department of Optics and Photonics, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, 27 Wybrzeze Wyspianskiego Str. 50-370 Wroclaw, Poland b c
ARTICLE INFO
ABSTRACT
Keywords: Photonics Ionic liquids Electron beam Fluorescein Europium Polymerization
We report the new fluorescent polymerizable imidazolium-based ionic liquids for photonics application. We investigate two approaches: one ionic liquid (IL) contains the active fluorescein dianion as an intrinsic part of the polymerizable ionic liquid structure. The second liquid is based on the mixture of polymerizable ionic liquid with optically active europium(III) nitrate hydrate. We demonstrate the polymerization abilities of both liquids by means of electron beam in vacuum conditions. Furthermore, we measure the light emission properties of liquid and polymerized structures. The results reveal that IL with europium(III) nitrate hydrate yields emission spectrum with Eu3+ bands not only as a liquid but also as a polymerized solid microstructure, which is important for its application in active photonics components.
1. Introduction Design and synthesis of new materials is of great importance for developing new advanced material platforms for optics and photonics which meet special requirements of these research fields. For this specific reason, ionic liquids (ILs) are promising materials for photonics since their properties can be tuned in many different ways by introducing different functionalities into chemical structure of ions [1–5]. Moreover, this class of compounds has a unique combination of physicochemical properties, such as negligible volatility, relatively high thermal and chemical stability, low toxicity and high ionic conductivity. This has made ILs an attractive solution in many research and industrial areas including not only synthesis in chemical industry [6,7], where they can replace conventional solvents, but also energy conversion using fuel cells [8,9], energy storage [9,10] and CO2 capture [11,12] and many more. Among different functionalities that can be incorporated into ionic liquids, two of them are particularly interesting. Ionic liquids with vinyl or allyl group can be polymerized, either by adding polymerization initiators [13–17] or by exposure to high-energy radiation which can break chemical bonds [18–20]. The latter method enables one to fabricate planar microstructures with high resolution by means of lithographic/patterning techniques using ion/electron beam sources, which ∗
can be employed for nanotechnology and photonics due to good optical properties [20]. This method is very attractive because, in contrast to typical solvent-containing lithographic resists, the ionic liquids can be used in a pure form, i.e. the ionic liquid thin films can be formed by spin-coating without using any solvent, which makes the method greener and safer [20]. Apart from polymerizable ILs, fluorescent ionic liquids have also attracted the attention of researchers in the last years since they are potentially useful for optical sensing and detection of various substances, such as, for example, dihydroxybenzenes or nitroexplosives. Such task-specific ionic liquids can be realized using fluorescent organic dyes [21–30], of which fluorescein is often used as monoanion or dianion of IL [24–27]. Another approach to obtain luminescent ILs is to form rare earth elements-based ionic liquids [31–36], which could be applied in photochemistry and spectroscopy. Both aforementioned functionalities, that is polymerizability and luminesce, can be combined in order to achieve poly(ionic liquid)s containing fluorescent moieties [37] or rare earth metal ions [38,39]. In the latter case, poly(ionic liquid)s facilitate immobilization of ions and, in addition, can even slightly enhance the luminescence performance of europium(III) ions in comparison with monomeric ILs [38]. Therefore, in this work, we also address this issue but we develop a different method, i.e. a polymerizable ionic liquid is mixed with either
Corresponding author. PORT Sp. z o.o., Stablowicka 147 Str., 54-066 Wroclaw, Poland. E-mail addresses: [email protected], [email protected] (K. Komorowska).
https://doi.org/10.1016/j.optmat.2019.01.003 Received 20 November 2018; Received in revised form 4 January 2019; Accepted 7 January 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Synthesis route of fluorescein-based ionic liquid.
Fig. 2. Scheme of fabrication procedure of polymerized microstructures out of ionic liquids.
fluorescein-based ionic liquid or europium(III) nitrate and, subsequently, the prepared mixtures in the form of thin films on silicon substrates are in situ polymerized and patterned by high-energy electron beam in vacuum. The proposed technique is aimed at fabrication of functionalized microstructures that could be integrated within miniaturized devices, for example, on silicon microchip. It should be mentioned that, to the best of our knowledge, the polymerization and patterning of luminescent ionic liquids have not been reported in the literature so far. 2. Experimental The polymerizable ionic liquid used in the experiments was 1-allyl3-methylimidazolium chloride (abbreviated as [Allmim][Cl]). It was synthesized according to the procedure described in our previous work [20]. The ionic liquid with [Allmim]+ cation and fluorescein-based dianion was synthesized as follows (Fig. 1). In a round-bottomed one-neck flask (100 mL) 1-allyl-3-methylimidazolium chloride (1.00 g, 6.31 mmol) was places and dissolved in dichloromethane (10 mL). To stirred solution, a solution of fluorescein disodium salt (1.19 g, 3.16 mmol) in water (20 mL) was added in one charge at room temperature. The resulted two phase system was then stirred vigorously for 6 days at room temperature. Next, the water layer was separated, evaporated and dried under high vacuum. The resulted solid was then washed with methanol (100 mL), and obtained solution was evaporated and dried under high vacuum to give pure product (1.70 g, 93%) as a dark red amorphous solid. The mixture of [Allmim][Cl] and europium(III) nitrate hydrate (molar ratio 1:1) was prepared as follows: the proper amount of
Fig. 3. SEM images of polymerized microstructures obtained by electron beam patterning of [Allmim][Cl] containing fluorescein as an dianion (500:1): (a) influence of electron dose, (b) polymerized micropillar, (c) array of polymerized micropillars.
europium salt was dissolved in 5 mL of ethanol and, separately, the IL was also dissolved in ethanol at RT. Both solutions were put together and stirred for 10 min, then the mixture was placed into the vacuum oven for 5–6 h. The drying temperature did not exceed 40 °C. Ionic liquids or their mixtures were spin-coated on monocrystalline (100) silicon substrates in order to form thin films of about 1 μm 107
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Fig. 4. SEM image of polymerized microstructures obtained by electron beam patterning (with different dose) of [Allmim][Cl] containing Eu(III) ions.
thickness. For electron beam activated polymerization processes, the resulting samples were inserted into vacuum chamber of scanning electron microscope (SEM) FEI Helios 660 equipped with Raith Elphy Multibeam patterning system and EDAX energy dispersive X-ray spectrometer (EDS). Then, the samples were irradiated with electron beam (30 keV) according to designed patterns. The dose of electrons delivered to sample surface was varied and determined from the following formula:
D=
to an area of the ionic liquid film. As seen in Fig. 3a, the minimal amount of high-energy electrons must reach the surface of IL to cause solidification on Si substrate. In addition, this effect depends on size of an area irradiated with electrons. The usage of large circular patterns yields traces of polymerized ionic liquid even for doses as low as 7.5 mC/cm2. However, the dose must be doubled to achieve well-defined circular structures and 2-μm-wide trenches. If micropillars of 1 μm radius and trenches of 1 μm width are desired, the dose should be about 30 mC/cm2. Further increase of the dose is not beneficial due to the lateral widening of microstructures. The phenomenon of lateral widening of IL polymerized by electron beam has been discussed for pure [Allmim][Cl] in our previous paper [20]. The high precision of projection of designed patterns onto IL film during polymerization is demonstrated in Fig. 3b. The micropillar has practically ideal circular shape with smooth edges and sidewalls. Moreover, the features of produced microstructures are highly repeatable which can be seen for an array of micropillars in Fig. 3c. The results of electron beam irradiation of the ionic liquid containing Eu3+ cations are significantly different, as shown in Fig. 4. Generally, the quality of obtained microstructures is worse than for the IL/fluorescein system. Although the minimal dose needed to solidify polymerizable ionic liquid on silicon surface is decreased, the resolution is deteriorated. Moreover, the microstructures are surrounded by a material that could not be dissolved by ethanol. During experiments it was noticed that the structures were ruptured in many places, which can be attributed to the presence of europium in a film. The mixture of europium salt and ionic liquid is expected to have different physicochemical properties from the pure ionic liquid. For example, density, thermal conductivity and stiffness can be changed after mixing ionic liquid with europium nitrate. In such a case, the rupture of microstructures could be a result of the mechanical stress in polymerized films. This hypothesis is supported by the fact that the effect was considerably reduced by decreasing the thickness of IL layer on Si substrate (see Fig. S1 in supporting info). It should be also noted that ruptured pieces had sizes of about 10–100 μm. That is why the effect is hardly visible for small microstructures shown in Fig. 4. The detailed microanalysis of IL/Eu3+ thin film by x-ray spectroscopy reveals that distribution of europium in ionic liquid is highly
IB × tdwell xpitch × ypitch
where: D is electron dose (mC/cm2), IB is a beam current, tdwell is an exposure time of a one point (called dwell time), xpitch, ypitch are distances between exposure points in horizontal and vertical directions, respectively. The dose was modified by changing the dwell time (e.g. D = 1 mC/cm2 when tdwell = 1 μs) whereas other parameters were fixed: IB = 26 nA, xpitch = 50 nm, ypitch = 50 nm. After patterning process was finished, each sample was washed in ethanol to remove ionic liquid so that only polymerized microstructures remained on silicon surface. The samples were then imaged using SEM microscope and used for optical measurements. The fabrication procedure briefly illustrated in Fig. 2. The light transmission spectra were measured using Evolution 300 UV–Vis spectrophotometer and Nicolet iS50 FT-IR spectrometer by Thermo Scientific. The luminescence spectra and decay kinetics (DEC) were measured with FSL980-sm Fluorescence Spectrometer from Edinburgh Instruments Ltd. A 450 W Xenon arc lamp (PL and PLE) and 60 W Xenon flash lamp (DEC) as an excitation sources were used. Emission spectra were corrected for the recording system efficiency and excitation spectra were corrected for the incident light intensity. 3. Results 3.1. Microscopic characterization Irradiation of the [Allmim][Cl] ionic liquid containing fluorescein dianions in the proportion of 500 to 1 results in solidification of the liquid, though this process is influenced by the electron dose delivered 108
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homogenous distribution of Eu is maintained within the large polymerized microstructure. 3.2. Spectroscopic characterization The modern studies of ionic liquids, which are in agreement with the principles of the green chemistry, are focused on giving additional functionality to ionic liquids. As it is very well known, the properties of the ionic liquids can be designed and tuned by proper combination of anionic and cationic parts, which, in consequence, can lead to multitask compounds. Thus, the attempt of preparation of luminescent and polymerizable ionic liquids was made. The ionic liquid with the fluorescein as anionic part of compound was obtained according to the procedure described in the experimental part. Resulting dark red compound did not show emission, which is in agreement with previous reported study for fluorescein compounds. To avoid the emission quenching, a mixture of fluorescent polymerizable ionic liquid and additional polymerizable ionic liquid without fluorescent group can be used, since it leads to “diluted” fluorescence dyes and thus prevents quenching. Therefore, the [Allmim][Cl] ionic liquid containing fluorescein dianions in the proportion of 500 to 1 was prepared. For that prepared mixture the strong emission with the maximum around 540 nm was recorded (Fig. 6a). The shape and the location of the band is in agreement with previous reported data [25,26,40]. However, after electron beam exposure and polymerization process, no emission from the micro-size structure was found. Such quenching is a general feature of compounds with π -planar structures which are self-quenched in the aggregated state (aggregation-caused quenching- ACQ) [41]. The other reason for the lack of the luminescence can be directly connected with the high-energy electrons interacting with the material during the polymerization process, which can lead to destroying (by means of electrons themselves or heating caused by electrons), at least partially, of the luminophores groups in the structure. To preserve the luminescent properties after the electron beam exposure another strategy was used. The polymerizable ionic liquid was mixed with Eu3+ salt. Among the lanthanide ions the Eu3+ is well known as showing intense red emission under UV radiation. Moreover, the luminescence spectra of Eu3+ can be used to probe the local environment of this ions [42]. The room temperature emission spectra recorded for Eu3+ containing [Allmim][Cl] ionic liquid before and after e-beam irradiation is presented in Fig. 6b. Both spectra exhibit similar features and consist of typical Eu3+ bands assigned to the 5D0→7FJ transitions. For materials before and after polymerization, 5D0→7F2 band dominate in the spectra, and no significant differences were observed. However, the decay times traces measured for both systems are different (Fig. 7). Before the e-beam exposure the decay curve is clearly monoexponential with decay time constant (τdec) equal 323 μs. After e-beam exposure, the decay time has unexpectedly decreased about 2–3 times and the curve became more complex, not fittable with two-exponential decay function. It clearly indicates that the environment of the europium(III) cations has changed after the e-beam exposure and the resulting polymerized structure may contain the mixture of europium ions with various coordination environment. The exact composition cannot be determined without additional experiments, but here, we can recall some lifetimes of europium(III) species found in literature. The decay times, as 100–140 μs was observed for aqueous solutions of europium nitrate hydrate [43–45] and is similar to europium chloride hydrates, as described in a ref. [43]. In solid state, the monocrystal of europium nitrate hexahydrate has higher decay time - equal 180 μs [46]. The higher decay time of the europium nitrate hydrate after mixing with ionic liquid (the sample before e-beam exposure) suggests that the coordination environment of this molecule has changed. It is more close to the europium(III)-IL mixtures found in recent literature [33,47], where 1 ms and submillisecond times are reported at room temperature. The rise of decay time may be related to
Fig. 5. Results of EDS microanalysis for [Allmim][Cl] containing Eu(III) ions: (a), (b) thin liquid film on Si, (c) polymerized microstructure on Si after washing with ethanol.
uniform and all elements occurring for [Allmim][Cl] and europium(III) nitrate hydrate can be assigned to peaks in the spectrum (Fig. 5a and b). When comparing amounts of Eu and Cl in the sample (Table 1), it can be seen that they are almost equal. This confirms good mixing of constituents of IL and europium(III) nitrate. As shown in Fig. 5c, the 109
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Table 1 EDS quantitative results for [Allmim][Cl] thin film containing Eu(III) ions. Element
Weight (%)
Atomic (%)
Error (%)
C N O Cl Eu
18.8 17.1 27.9 7.6 28.6
31.7 24.7 35.4 4.4 3.8
9.1 10.2 9.7 4.1 5.2
Fig. 7. The photoluminescence decay curves, registered in thin film of [Allmim] [Cl] doped with europium(III) nitrate hydrate, measured at λem = 616 nm: (a) before electron beam treatment - excitation at λex = 275 nm and; (b) after electron beam treatment – excitation at λex = 320 nm. The filter cutting light below 475 nm was used in the measurements in the emission arm. The IRF signal was cut from the graph.
polymerizable cation, which is a new synthesized ionic liquid and opens up prospects for other organic dyes. The pure fluorescein liquid did not show any fluorescence, however the light emission typical for fluorescein was observed for fluorescein dianions diluted in imidazoliumbased chloride. The second approach, which is the use of rare earth metal ions, yielded a homogenous mixture of polymerizable imidazolium-based IL and europium(III) nitrate hydrate with molar ratio about 1:1, as confirmed by quantitative energy dispersive X-ray microanalysis. The light emission spectrum of the above-mentioned liquid included typical Eu3+ bands. Interestingly, the Eu3+ bands were observed also in the case of polymerized microstructures obtained by electron beam. In other words, exposure of polymerizable IL containing Eu3+ ions to high-energy (30 keV) electrons did not prevent the material from having luminescence properties. Nevertheless, this was not the case for polymerized IL with fluorescein where the emission typical for fluorescein was not detected. This can be attributed to solidification of IL as well as to destroying of fluorescein species by electron beam during exposure. As for electron beam activated polymerization, both considered kinds of luminescent ionic liquids could be deposited on silicon substrates as thin films and patterned to form polymerized microstructures of well-defined shapes, potentially attractive as planar photonic microcomponents. Moreover, the distribution of europium in an IL thin film was uniform even after polymerization. The polymerization process of luminescent ionic liquids can be optimized in the future research in terms of spatial quality of polymerized structures and emission properties. The study should take into consideration the search for optimal concentration of rare earth ions in IL and the selection of organic dye that gives high emission in solid polymerized structure.
Fig. 6. Emission spectra for: (a) [Allmim][Cl] with fluorescein excited at 475 nm, (b) [Allmim][Cl] with europium(III) nitrate hydrate before and after ebeam irradiation excited at 318 nm.
the lower number of H2O ligands in coordination sphere of lanthanides, as it is commonly known that the number of H2O strongly decreases the decay time [45], which is also observed in ionic liquids – e.g. for ionic liquids being trihydrate complexes the decay time is about two times lower than for similar dihydrate complexes [47].
Acknowledgements
4. Conclusions
The work was financed by the National Science Center within the Grant Opus UMO-2015/19/B/ST8/02761.
In summary, we successfully obtained two kinds of luminescent polymerizable ionic liquids, the first one – containing fluorescent organic dye, and the second one – mixed with rare earth ions. The first solution consisted of fluorescein dianions diluted in polymerizable imidazolium-based chloride. Admittedly, the fluorescein dianion was first obtained as an only anionic constituent of ionic liquid comprising
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optmat.2019.01.003. 110
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