Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Photopatternable High‑k Fluoropolymer Dielectrics Bearing Pendent Azido Groups Konstantinos Kallitsis,† Damien Thuau,†,# Thibaut Soulestin,‡ Cyril Brochon,† Eric Cloutet,† Fabrice Domingues Dos Santos,‡ and Georges Hadziioannou*,† †
Univ. Bordeaux, CNRS, Bordeaux INP, LCPO, UMR 5629, F-33615, Pessac, France ARKEMA-Piezotech, Rue Henri-Moissan, Pierre-Benite Cedex 69493, France
‡
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S Supporting Information *
ABSTRACT: Photopatternable fluoropolymers with high dielectric constant were prepared by direct azidation of commercially available poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene). The produced fluoropolymers exhibit very high dielectric constant, while being photopatternable without the use of any additives. These cross-linked polymers appear to have reduced crystallinity compared to the pristine ones, as cross-linking is known to introduce defects in the polymer conformation, leading to a reduction in crystallinity. Cross-linking can occur both thermally and photochemically because of the presence of the pendent azido groups, which upon heating or irradiation with UV light release nitrogen and act as cross-linking sites. The cross-linked films exhibit excellent dielectric properties which greatly depend on the reaction conditions. Appropriate photolithographic protocols have been developed to obtain excellent quality patterned films using the azide-containing fluoropolymer as negative photoresist.
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with those techniques, either directly 15 or by lift-off approaches.16,17 Nevertheless, nanoimprint lithography and hot embossing have been reported as alternative approaches for micro-/ nanostructuration of FEPs.6,7 Microstructured poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] in pyramid shape has been shown to exhibit 5 times larger output voltage in energy-harvesting devices.18 Also, P(VDF-TrFE) was patterned in nanometer-sized pillars using nanoimprint lithography and subsequently used for nonvolatile low-voltage memory applications.19 The same patterning approach was applied for the fabrication of multiferroic hybrid films, exhibiting for the first time room-temperature switching of the electric polarization.20 The above findings highlight the benefit of micro-/nanostructuration of FEPs. However, despite devices fabricated using nanoembossing being extremely promising, their noncompatibility with existing production lines, which are based on
INTRODUCTION
Organic electronics are a promising alternative to conventional silicon-based electronics where electronic components such as organic light-emitting diodes (OLEDs),1,2 transistors,3 and sensors4 can be fabricated at low cost and large scale. Conversely to silicon-based electronics, those devices can be fabricated on flexible or curved substrates by printing techniques.5 Among the organic materials that allow the development of flexible electronics, fluorinated electroactive polymers (FEPs) are attracting increasingly high interest because of their unique ferroelectric and dielectric properties6,7 and their applications, among others, as gate dielectrics in organic field-effect transistors (OFETs) as well as for ferroelectric nonvolatile memories.8−11 Before FEP-based devices find wide commercial application, patterning issues need to be solved. FEPs are solution-processable materials and thus can be patterned with a wide range of printing techniques.12−14 However, their limited compatibility with photolithography, which is the method of choice for high-throughput electronics production, limits their potential, while conducting polymers are already compatible © XXXX American Chemical Society
Received: March 13, 2019 Revised: July 3, 2019
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DOI: 10.1021/acs.macromol.9b00508 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules photolithography, limits their potential. Consequently, the need arises for a photolithographic-based alternative to answer the emerging need for structuration in FEPs. To date, only few pioneering works on photolithographic patterning of FEPs and more specifically of P(VDF-TrFE)21 and poly(vinylidene fluoride-chlorotrifluoroethylene) [P(VDFCTFE)]22 have been reported in the literature. In both cases azide photochemistry was applied. To make the FEPs photocross-linkable and thus photopatternable, an expensive bis-azide photoinitiator was used as an additive. Except for the high cost, the presented bis-azide initiators raise safety issues for large-scale production because of the low carbon-to-azide ratio for the small organic azides. Another report suggested that poly(vinylidene fluoride-bromotrifluoroethylene [P(VDF-BTFE)] with a dielectric constant (εr) of 8.4 can be patterned with a photolithographic process, with the addition of radical photoinitiators in the spin-cast blend, and exposure of the film under UV light for several hours.23 The patterned fluoropolymer film was then integrated in OFETs as gate dielectric and charge mobilities exceeding 10 cm2 V−1 s−1 were measured, which are the highest charge mobilities reported to date for transistors using high-k polymer dielectrics.23−25 In this paper, we present an innovative approach which consists of the development of fluorinated electroactive polymers bearing photosensitive groups on the polymer backbone and thus being compatible with photolithographic patterning techniques without the use of any additives. To do so, we focused on P(VDF-TrFE-CTFE), one of the most interesting FEPs because of its high dielectric constant which makes it a promising candidate as a gate dielectric for low operating voltage OFETs.
Scheme 1. Grafting Reaction of Azido Groups on P(VDFTrFE-CTFE) Terpolymer, Showing the Competition between the Substitution and the Elimination Reaction
Scheme 2. Reaction Pathways That Lead to the Cross-linking of the Azide-Modified Terpolymer upon UV Irradiation
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DISCUSSION To enable the photo-cross-linking of the P(VDF-TrFE-CTFE) terpolymer, azidation of the CTFE groups was attempted, as were tested by varying the sodium azide feeding ratio with respect to the total number of repeating units as well as the reaction temperature and solvents. The products were characterized by infrared spectroscopy, following the characteristic azide band at 2100 cm−1. However, except for the expected substitution reaction which leads to azide functionalization of the polymer an additional band at 1680 cm−1 due to doublebond formation on the polymer backbone was also observed (Figure 1). The appearance of those two bands indicates the competition between the desired substitution and the elimination side reaction (Scheme 1), with the reaction conditions strongly determining which would be the dominant pathway. To verify that the substitution during the azidation occurs only in the CTFE groups, two blank tests were performed. The former test was the reaction of P(VDF-TrFE) copolymer with sodium azide in which no reaction took place, as no band correlated to the azido group appeared in the IR spectrum after modification (Figure 2a). The latter test was the reaction of PCTFE homopolymer with sodium azide in which the reaction was successful, as a very intense band at 2100 cm−1 appeared, indicating the strong presence of azido groups (Figure 2b). When the azide-containing polymer is irradiated with deep UV light at ambient temperature, nitrenes are formed as reactive intermediates29 which then rapidly undergo a wide range of reactions as shown in Scheme 2, making the polymer photocross-linkable in a single step. Some of those reactions lead to
Figure 1. ATR-IR spectra of P(VDF-TrFE-CTFE) (61.8/30.4/7.8) mol before reaction with sodium azide (black) and after reaction with different amounts of sodium azide (colored).
polymers bearing azide side groups are known to act as negative photoresists.26−28 Initially, the polymer was modified with sodium azide, substituting the chlorine groups on the polymer backbone with azido groups. Different modification conditions B
DOI: 10.1021/acs.macromol.9b00508 Macromolecules XXXX, XXX, XXX−XXX
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Figure 2. (a) ATR-IR spectra of P(VDF-TrFE) (75/25 mol) before reaction with sodium azide (black) and after reaction with sodium azide (red). (b) ATR-IR spectra of PCTFE before reaction with sodium azide (black) and after reaction with 0.25 equiv of sodium azide (green) and 0.75 equiv of sodium azide (red).
Figure 3. (a) UV−vis absorption spectra of P(VDF-TrFE-CTFE) terpolymer film functionalized with azido groups. (b) Azide-modified P(VDF-TrFECTFE) on silicon substrate patterned through a single photolithographic step. (c) Measurement of the film roughness using mechanical profilometer.
Figure 4. Differential scanning calorimetry thermograms comparing the pristine P(VDF-TrFE-CTFE) terpolymer (black) with terpolymers modified with different amounts of sodium azide (colored) after thermal cross-linking at 80 °C for 30 min: (a) second heating; (b) first cooling.
photoresist in a photolithographic process (Figure 3b), with
cross-linking of the polymer chains, making the polymer insoluble in common organic solvents. The azide-modified polymers show a deep-UV absorption peak (Figure 3a) and can be directly patterned as negative
very low surface roughness for the patterned films (Figure 3c). Another advantage of this approach is that polymer multilayers C
DOI: 10.1021/acs.macromol.9b00508 Macromolecules XXXX, XXX, XXX−XXX
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As reported in the literature, cross-linking reduces the polymer’s crystallinity by introducing defects on the polymer backbone,30−32 which in turn affect the dielectric properties of FEPs.33,34 Although the effect in terms of dielectric constant is usually negative, recent reports have shown that transistors with the highest charge mobilities to date are obtained with the crosslinked P(VDF-BTFE),23 despite the reduced dielectric constant after cross-linking. Differential scanning calorimetry (DSC) was employed to evaluate the effect that the modification and the subsequent cross-linking have on the polymer crystallinity (Figure 4). As expected, the cross-linking broadens the melting and crystallization peaks, shifting them at lower temperatures, and at the same time decreases their intensity. These results indicate that cross-linking leads to the lowering of the total crystallinity of the modified polymers, while broadening the crystal size distribution. The reaction conditions had to be fine-tuned to induce photopatternability through the azido groups, all in maintaining the relaxor ferroelectric and dielectric properties of the terpolymer (Table 1). Films of the azide-modified terpolymer were compared to films of the pristine terpolymer by atomic force microscopy (AFM) in tapping mode. The large crystal grains of the pristine P(VDF-TrFE-CTFE) (Figure 5a,b) disappear for the azidemodified terpolymer (Figure 5c,d), making the surface of the latter considerably smoother. The root mean square roughness (RMS) of the two polymers was reduced from 5.6 to 0.7 nm, as calculated from 10 μm AFM scans which are shown in Figure S4. Such feature makes the azide-modified polymers better candidates for gate dielectrics in OFETs, as smoother interface between the gate dielectric and the organic semiconductor leads to better performing OFETs.35,36 The ferroelectric properties of the azide-modified polymer with respect to the pristine terpolymer were investigated at ambient temperature. Both terpolymers exhibit relaxor-ferroelectric behavior. However, upon modification and crosslinking, the ferroelectric behavior of the polymer changes. The pristine polymer exhibits double hysteresis loop (DHL) behavior, as shown in Figure 6, characteristic of the physical pinning caused by the bulky CTFE group, which is incorporated on the polymer crystal. The presence of the CTFE group causes
Table 1. Melting Temperature and the Corresponding Enthalpies Calculated from the DSC Thermograms (Figure 4) for Thermally Cross-linked Terpolymers Modified with Different Amounts of Sodium Azide pristine terpolymer 0.008 equiv of NaN3 0.04 equiv of NaN3 0.8 equiv of NaN3
melting temperature (°C)
melting enthalpy (J/g)
127.3 109.0 107.1
11.7 7.61 3.8
Figure 5. AFM images of the height (a,c) and phase (b,d) comparing the pristine terpolymer (left) to the azide-grafted terpolymer with 0.06 equiv of NaN3 (right).
can be easily fabricated by sequential deposition of patterned layers.
Figure 6. Ferroelectric testing at 20 °C of the pristine terpolymer P(VDF-TrFE-CTFE) compared to that of terpolymer modified with sodium azide (0.06 equiv), measuring the (a) displacement and (b) the current as a function of applied electric field. D
DOI: 10.1021/acs.macromol.9b00508 Macromolecules XXXX, XXX, XXX−XXX
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Figure 7. (a, b) Broad band dielectric spectroscopy at 20 °C of the pristine terpolymer P(VDF-TrFE-CTFE) compared to that of the terpolymers modified with sodium azide measuring (a) the real part of the relative permittivity and (b) the dielectric loss as a function of frequency. (c−f) Broad band dielectric spectroscopy as a function of temperature. (c, d) Showing the real part of the relative permittivity and the dielectric loss of pristine P(VDF-TrFE-CTFE) from 20 to 80 °C for different frequencies, respectively. (e, f) Showing the real part of the relative permittivity and the dielectric loss of azide-modified P(VDF-TrFE-CTFE) from 20 to 80 °C for different frequencies, respectively.
the other hand, which is cross-linked, exhibits a narrow single hysteresis loop (SHL), as shown in Figure 6. That behavior is characteristic of permanent chemical pinning caused by the cross-linking sites, where no field-induced hysteresis can be observed.37 Similar behavior has been reported in the literature for cross-linked FEPs and was first reported on electron-beamirradiated P(VDF-TrFE).38
the interchain distance to increase, leading to relaxor-ferroelectric behavior at low electric fields. However, when the electric field becomes high enough, since the CTFE group is polar itself, it will rotate and get polarized. That way the CTFE group stops acting as a pinning point at high fields and the polymer becomes ferroelectric. That phenomenon gives rise to field-induced hysteresis during the relaxor-ferroelectric (RFE) to ferroelectric (FE) transition. The azide-modified polymer, on E
DOI: 10.1021/acs.macromol.9b00508 Macromolecules XXXX, XXX, XXX−XXX
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Figure 8. Image and subsequent calculation of the contact angle of water with (a) the pristine P(VDF-TrFE-CTFE) terpolymer and (b) the crosslinked P(VDF-TrFE-CTFE) terpolymer modified with 0.04 equiv of NaN3.
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CONCLUSIONS Photopatternable fluoropolymers with very high dielectric constant were synthesized by a single-step grafting reaction of azido groups on commercially available P(VDF-TrFE-CTFE). The dielectric constant that was obtained for the cross-linked fluoropolymers was 25 (1 kHz, 20 °C) which is the highest value reported to date for a photopatternable polymer, although slightly reduced compared to that of the starting fluoropolymer which had a dielectric constant of 35 (1 kHz, 20 °C). In addition, the cross-linked polymers yield films with significantly lower roughness than the pristine ones, making them excellent candidates for gate dielectrics in OFETs.
Broad band dielectric spectroscopy was used to investigate the effect of azide modification on the dielectric properties of the terpolymers. The azide-modified terpolymers exhibit high-k and low loss across the studied temperature and frequency range, for example, εr′ = 25 at 20 °C and 1 kHz (Figure 7a). Those values are nevertheless lower compared to those of the pristine terpolymer (εr′ = 34 at 20 °C and 1 kHz) (Figure 7a). However, they are to the best of the author’s knowledge the highest dielectric constant values reported to date for photopatternable polymers. It has to be noted that those values can be further decreased with increasing the sodium azide feeding ratio, as shown in Figure S5. The azide-modified polymers have their dielectric constant peak at the same temperature as the pristine terpolymer (Figure 7c,e) but the εr′ values are reduced across the temperature range. The degree to which the dielectric properties get influenced from the azide modification and the subsequent cross-linking step greatly depend on the reaction conditions. The surface tensions of the cross-linked and pristine polymers, as well as their dispersive and polar parts, were calculated by the drop shape method using three liquids (water, diiodomethane, and ethylene glycol) with known dispersive and polar surface tensions. Then the Owens-Wendt model39 was used to calculate the dispersive and polar parts for each polymer. When the cross-linked and pristine polymers are compared, a decrease of the dispersive/polar surface tension ratio is observed, from 16.9 at the pristine polymer down to 2.5 at the cross-linked polymer modified with 0.04 equiv of NaN3. That indicates that the adhesion of the cross-linked polymer should be better than the one of the pristine polymer on common substrates such as silicon and glass, as well as aluminum which is used as the electrode. Additionally, the interfacial tension between the polymer and water reduces upon modification and cross-linking, as presented in Figure 8 and expressed by a decrease in contact angle, making possible the deposition of water-based deposited electrodes (such as PEDOT:PSS) without the UV-ozone treatment that is normally used in such cases. The contact angle measurement data are presented in Table S1.
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EXPERIMENTAL SECTION
Materials. Unless otherwise stated, all chemicals were purchased from Acros Organics or Merck Chemicals and were used without any further purification. The statistical copolymers P(VDF-TrFE) and terpolymers P(VDF-TrFE-CTFE) (with various VDF/TrFE/CTFE molar ratios) were provided by ARKEMA-Piezotech (France). Argon was supplied by Air Liquid with at least 99.99% purity. Synthesis. A typical modification reaction was done using standard Schlenk techniques. P(VDF-TrFE-CTFE) (61.9, 29.9, 8.2) (4 g, 4.5 mmol of CTFE) and NaN3 (0.21 g, 3.2 mmol) were mixed and degassed in DMF (170 mL). The glass Schlenk was sealed under argon and heated to 55 °C in the dark for 16 h. The solvent was then evaporated under reduced pressure and the polymer was washed with deionized water and ethanol, redissolved in acetone, and dialyzed (13 kDa membrane cutoff) to remove remaining traces of salt. Then it was dried in a vacuum oven at 40 °C for 12 h. Instruments and Characterization. 1H NMR and 19F NMR spectra were recorded on a Bruker Advance DPX 400 MHz spectrometer. The samples were prepared in deuterated acetone. FTIR spectra were recorded on Vertex 70 from Bruker using diamond ATR spectroscopy. The differential scanning calorimetry (DSC) thermograms were recorded by a DSC Q100 RCS from TA Instrument. The DSC analysis was performed from 0 to 200 °C at a heating or cooling rate of 10 °C/min. The first heating was used to erase thermal history and traces of solvents, while the first cooling and second heating ramps were recorded. Displacement hysteresis loops of the metal− dielectric−metal devices were recorded at room temperature using the TF Analyzer 2000E from aixACCT Systems. A continuous triangular wave with a 10 Hz frequency was used. Broad band dielectric F
DOI: 10.1021/acs.macromol.9b00508 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Author Contributions
spectroscopy of the metal−dielectric−metal devices was performed on a Solartron 1260 A impedance analyzer. UV−visible absorption spectra were recorded on self-standing films using a Shimadzu spectrophotometer UV-3600. Surface tensions were assigned by the drop shape analysis technique on a pendent drop tensiometer (Krüss DSA-100) while the surface energies were determined by measuring contact angle between three solvents (water, diiodomethane, and ethylene glycol) and the film. The surface energies were calculated using Owens-Wendt model.39 Atomic force microscopy (AFM) was performed (Bruker Dimension Fast Scan) in tapping mode. Photolitography was performed on an EVG6200 mask aligner. Film thicknesses were measured on a Bruker Dektak XT-A stylus profilometer. Preparation of Patterned Thin fFlms. A 4 wt % solution of the modified polymer in cyclopentanone was spin-coated on a silicon wafer at 500 rpm for 5 s and then at 1000 rpm for 1 min, yielding a 250 nm thick film. The wafer was soft-baked before exposure at 60 °C for 5 min and subsequently cross-linked by UV light under nitrogen, using a photolithographic mask. Different UV doses were applied, varying from 0.5 to 2 J/cm2. Then the film was post-exposure-baked at 60 °C for 5 min. Finally, the patterned film was developed in a blend of one-third cyclopentanone and two-thirds isopropanol for 1 min. Subsequently, the wafer was rinsed with isopropanol and dried with compressed air. Device Fabrication. The metal−dielectric−metal devices were fabricated as follows. Glass substrates were cleaned and sonicated in acetone and isopropanol for 15 min. Then a layer of aluminum (80 nm) was evaporated as the bottom electrode using a thermal evaporator at a pressure of 1.10−6 mbar. Subsequently a 10% (wt) solution polymer dielectric layer was spin-coated at 500 rpm for 5 s and then 1000 rpm for 1 min. An aluminum top electrode (80 nm) was evaporated, creating a metal−insulator−metal device area of 2 mm2. The devices were annealed at 110 °C for 1 h and slowly cooled down to room temperature.
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All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ABBREVIATIONS USED k, dielectric constant; OLED, organic light-emitting diode; FEP, fluorinated electroactive polymer; OFET, organic field effect transistor; εr′, real part of the relative permittivity; Tan δ, dielectric loss
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00508.
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ACKNOWLEDGMENTS
The authors acknowledge the financial support from Arkema and Région Aquitaine as well as from the Industrial Chair (Arkema/ANR) within the Grant Agreement No. AC-2013365. K.K. acknowledges the Région Aquitaine for the financial support (Ph.D. Grant No. 2015-1R10207-00004862). This work was performed within the framework of the Equipex ELORPrintTec ANR-10-EQPX-28-01 with the help of the French state’s Initiative d’Excellence IdEx ANR-10-IDEX-00302.
ATR-IR spectra of P(VDF-TrFE-CTFE) (61.9/29.9/8.2) modified with 0.06 and 0.12 equiv of NaN3; 1H NMR spectra of P(VDF-TrFE-CTFE) (61.9/29.9/8.2) modified with 0.06 and 0.12 equiv of NaN3; 19F NMR spectra of P(VDF-TrFE-CTFE) (61.9/29.9/8.2) modified with 0.12 equiv of NaN3; calculated surface tensions of P(VDF-TrFE-CTFE) grafted with azido groups, before and after cross-linking; 50 μm AFM phase images, comparing the pristine terpolymer with the azidemodified terpolymer (0.06 equiv); broad band dielectric spectroscopy at 20 °C showing the real part of the relative permittivity of pristine P(VDF-TrFE-CTFE) (61.9/29.9/ 8.2) modified with 0.06 and 0.12 equiv of NaN3 (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Thibaut Soulestin: 0000-0002-1534-0230 Eric Cloutet: 0000-0002-5616-2979 Georges Hadziioannou: 0000-0002-7377-6040 Present Address #
Bordeaux National Polytechnic Institute (Bordeaux INP/ ENSCBP), IMS CNRS UMR 5218, University of Bordeaux, 16 Avenue Pey Berland, Pessac Cedex F-33607, France. G
DOI: 10.1021/acs.macromol.9b00508 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.9b00508 Macromolecules XXXX, XXX, XXX−XXX
