Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22575−22582
www.acsami.org
Low-Voltage Organic Transistor Memory Fiber with a Nanograined Organic Ferroelectric Film Minji Kang,†,⊥ Sang-A Lee,†,‡,⊥ Sukjae Jang,† Sunbin Hwang,† Seoung-Ki Lee,† Sukang Bae,† Jae-Min Hong,† Sang Hyun Lee,§ Kwang-Un Jeong,‡ Jung Ah Lim,∥ and Tae-Wook Kim*,†
Downloaded via BUFFALO STATE on July 21, 2019 at 13:40:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
Functional Composite Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Jeollabuk-do 55324, Republic of Korea ‡ Department of Polymer-Nano Science and Technology, Chonbuk National University, Jeonju 561-756, Republic of Korea § School of Chemical Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 61186, Republic of Korea ∥ Center for Optoelectronic Materials and Devices, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea S Supporting Information *
ABSTRACT: Wearable technology offers new ways to be more proactive about our health and surroundings in real time. For next-generation wearable systems, robust storage and recording media are required to monitor and process the essential electrical signals generated under various unpredictable strain conditions. Here, we report the first fibriform organic transistor memory integrated on a thin and flexible metal wire. A capillary tube coating system allows the formation of a thin and nanograined organic ferroelectric film on the wire. The uniform morphology imparts excellent switching stability (∼100 cycles), quasi-permanent retention (over 5 × 104 s), and low-voltage operation (below 5 V) to the fibershaped memory devices. When sewn in a stretchable textile fabric, the memory fiber achieves long retention time of more than 104 s with negligible degradation of memory window even under a constant diagonal strain of 100% that exhibits reliable data storage under tough environments. These results illustrate the possibility of the practical, wearable fiber memory for recording electronic signals in smart garment applications. KEYWORDS: organic memory, fiber electronics, organic field-effect transistor, wearable electronics, organic ferroelectrics
■
the body.11 Such fiber devices can be well incorporated into fabric and guarantee freedom of “garment design”, such as material choices or geometric patterning, to meet the demands of the smart garment. In particular, less visible, smaller, and hardly perceptible technology needs to be developed for wearable applications. Thus, the form factor of the fiber device, which can be naturally incorporated into smart garments, is more beneficial for the fabrication of wearables than conventional planar devices.11,12 As one of the key components of truly wearable electronic devices, wearable data storage media are indispensable for recording or storing electrical signals generated from other deformable devices, such as wearable processors, sensors, and displays.13 They require robust recording data that can be read and translated into valuable information under mechanical deformations such as bending, folding, and crumpling.3,13 Therefore, there have been gradual efforts to develop wearable memories such as fiber-shaped memories that can adhere well to garments or the human body while feeling comfortable.1,14,15 To demonstrate such fiber memory in electronic textiles (e-textile),
INTRODUCTION Wearable electronics have attracted attention as an emerging technology to realize practical e-textile and smart garments.1−3 The future of wearable devices is expected to include a wearable automatic system that perceives the requisite information from human skin, clothing, environment, and some objects.3 For the development of wearable devices, the integration of electronic devices on textiles is considered to be the most compatible form of electronics applied in continuous health/environmental monitoring and computing.4−7 Among various technical approaches, fiber-shaped devices, as representative candidates, have been developed to enable network formation in textiles for practical wearable applications.4,8,9 Such fiber devices can be integrated into stretchable/ flexible devices with diverse functionalities and configured into threadlike fiber devices that can be woven together.10 Because they have a one-dimensional shape and are highly compatible with textiles, fiber devices can be easily woven into textiles. They smoothly rearrange along with the clothes with body movement, resulting in less stress under mechanical deformation. In addition, fiber-based devices help to keep human skin fresh and comfortable owing to their breathable porous network geometry in the textile, whereas conventional planar-type flexible devices prevent sweat evaporation from the surface of © 2019 American Chemical Society
Received: February 26, 2019 Accepted: May 31, 2019 Published: May 31, 2019 22575
DOI: 10.1021/acsami.9b03564 ACS Appl. Mater. Interfaces 2019, 11, 22575−22582
Research Article
ACS Applied Materials & Interfaces
Figure 1. FOM device structure. (a) Schematic of the device architecture of the FOMs (left); inset: cross-sectional FOM structure indicating the major materials and optical photograph of a flexible FOM device array (right). Thicknesses of each layer: P(VDF-TrFE), 260 nm; pentacene, 50 nm; Au electrodes, 50 nm. (b) Schematic representation of a continuous reel-to-reel coating process for fabrication of the FOMs, along with an illustration and photograph of a P(VDF-TrFE)-wrapped metal wire formed by the CTAC process (right box). Schematic front and side views of the CTAC process at the transition from a solution meniscus to the solid film (bottom box).
transistors (OFETs) that can be integrated onto flexible/ stretchable substrates by simple solution processes.21−25 The organic memory devices have mainly used a ferroelectric copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)), which can be reversibly switched between two stable polarization states by applying electric field.22,26 The P(VDF-TrFE) is more favorable for the fabrication of fibershaped memory devices by virtue of environmental stability, light-weight, low cost, high scalability, and flexibility.27 However, the P(VDF-TrFE)-based memory devices suffer from high operation voltage and short retention time associated with poor film quality and uniformity.27,28 It is therefore essential to find optimized coating methods for the fabrication of a high-quality ferroelectric film. Here, we report for the fiber organic memory (FOM) based on the P(VDF-TrFE) and OFETs are developed via a facile solution process onto thin Ag wires. To achieve enhanced ferroelectric properties and reliable device performance during low-voltage operation, we introduce a new fiber coating technique using a capillary tube and controlling the solution viscosity, resulting in a highly uniform thin film on the flexible wire. The FOMs show low-voltage operation, stable switching,
the choice of appropriate materials and optimization of the specific process technology are required. Organic materials have been extensively investigated due to their low cost, simple solution process at low temperatures, softness, flexibility, and stretchability compared with inorganic materials.16−18 Owing to their intrinsic features, organic-based devices can be fabricated by direct printing onto fiber-shaped substrates and they are suitable for cost-effective reel-to-reel manufacturing.5 Recently, a variety of solution process techniques have been developed for wearable device elements.11,19 However, the wearable organic devices have rarely been explored in the field of fiber memory, and only capacitor-type memory elements have been demonstrated thus far.15,20 Despite high adaptability in textile integration for logic circuits and nondestructive read-out in a single device, a transistor-type memory with a fiber form has not yet been reported. Furthermore, the wearable fiber memories in previously reported results have not shown low-voltage operation, long-term retention and high device reliability under the mechanical stresses. As one of the promising nonvolatile memory devices, organic ferroelectric transistor memories have device structures and operating principle similar to those of organic field-effect 22576
DOI: 10.1021/acsami.9b03564 ACS Appl. Mater. Interfaces 2019, 11, 22575−22582
Research Article
ACS Applied Materials & Interfaces
Figure 2. CTAC-processed P(VDF-TrFE) film on a thin Ag wire. (a) Cross-sectional SEM images of the FOM and a top-surface SEM view (left bottom) of the source−drain electrode region. (b) Atomic force microscopy (AFM) measurements on the P(VDF-TrFE) layer on the Ag wire in contact mode. The area size is 1 μm × 1 μm, where the top image shows the topography and the bottom image shows the phase response. (c) X-ray diffraction (XRD) patterns of P(VDF-TrFE) thin films formed by the CTAC process (red line) and spin-coating (black line).
Notably, an optimization process was performed to prevent dewetting of the P(VDF-TrFE) solution from the surface of the wire substrates and acquire uniform polymer films. The surface coverage and interfacial adhesion on the wire substrate were improved by modifying the solution viscosity, depending on the concentration of the solution and the ageing time without additional additives or adhesion layers. P(VDF-TrFE) was dissolved in N,N-dimethylformamide (DMF) with concentrations of 11, 21, and 32 wt %, and the ageing time of the solution was adjusted to achieve the desired viscosity (Table S1). The solvent DMF, which has a high boiling point of 152 °C, was chosen to impart favorable solution flow by avoiding uncontrollable fast solvent drying. The gelated 21 wt % P(VDFTrFE) solution was prepared without stirring at 50 °C over 3 days and left to cool to room temperature for 1 h before printing. To facilitate meniscus-assisted coating for the deposition of uniform films, a conventional capillary tube was employed as a container for the printing ink. We speculated that the cylindrical shape and narrow diameter of the capillary tube reduced vibrations of the solution surface and potential deviations in the meniscus height during coating.17,32 Figure S2 and Table S2 compare the device yields using CTAC and general dip-coating methods. The CTAC process results in a more uniform thickness of the P(VDF-TrFE) film after the coating process, which improves device yield over the conventional dip-coating method. To gain a further understanding of the CTAC process, we illustrate the specific coating schematics and the role of the meniscus in the initial coating process of the polymer ink onto the Ag wire in Figure 1b. The top-right inset of Figure 1b shows the fluid flow of the P(VDF-TrFE) solution, starting from the initial meniscus where the gelated P(VDF-TrFE) thin film is formed. In this CTAC process, the meniscus of the solution connects the capillary tube to the wire and guides polymer film deposition on the wire.33 When the meniscus passes over the radial contact lines drawn from the outer surface of the wire to the round edge of the capillary tube, homogeneous solvent evaporation occurs along the drawing direction, caused by the equal radial gap between the capillary tube and the wire. The thin film still contains residual solvent, which is then completely dried and annealed at 140 °C through isothermal evaporation in an air convection oven. The controlled solvent evaporation leads
and quasi-permanent retention time with excellent mechanical endurance. By a conventional needle, we directly sew the resulting memories into commercially available and stretchable polypropylene (PP) fabrics. The fibriform memory achieves stable device performance within a 5 V operating voltage under a tough environment, such as uniaxial (and diagonal) strain from 0 to 73.3% (100%) and random crumpling. The features of FOMs suggest their potential as data storage media for low-power smart wearable devices.
■
RESULTS AND DISCUSSION Capillary Tube-Assisted Coating (CTAC) Process for Fibriform Device Integration. A FOM with a bottom-gate/ top-contact (BG/TC) device structure, which is a type of fieldeffect transistor (FET) architecture,5,9,14 was demonstrated on a one-dimensional metal wire by the sequential deposition of an organic semiconductor and a patterned Au electrode layer onto an organic ferroelectric layer, as shown in Figure 1a. Pentacene was used as the organic semiconductor because it shows stable memory performance in P(VDF-TrFE)-based transistor memory devices.29−31 To integrate the FOMs, we introduced a modified capillary tube-assisted coating (CTAC) process. Unlike conventional two-dimensional substrates, such as silicon wafers or flexible planar films, Figure 1b presents the schematics of the CTAC process for the precise wrapping of P(VDF-TrFE) on a microfiber, which is potentially a reel-to-reel compatible coating process for the continuous production of fiber-type electronic devices. The CTAC process was inspired by the dip-coating process, which is known to be one of the most efficient ways to minimize material waste and allows fine control of film thickness and uniformity by adjusting the coating speed or the concentration of the target solution (Figure S1).32 During the CTAC process for the formation of the organic thin film, a thin and flexible silver (Ag) wire (diameter 100 μm), as both the substrate and the gate electrode, was connected with a motorized speed controller and slowly passed through the horizontally fixed microcapillary tube. The inside of the cylindrical capillary tube with a diameter of 1.1 mm was filled with a viscous P(VDF-TrFE) solution, and finally, the wire was wrapped in the P(VDF-TrFE) thin film as it passed through the center of the tube at a constant horizontal velocity of 0.38 mm s−1. 22577
DOI: 10.1021/acsami.9b03564 ACS Appl. Mater. Interfaces 2019, 11, 22575−22582
Research Article
ACS Applied Materials & Interfaces
Figure 3. Memory characteristics of the FOMs. (a) ID−VG curves during VG sweeps ranging from −5 to +5 V at VD = −1 V. Inset: photograph of the FOM during measurement. The channel length (L) and width (W) are 15 and 157 μm, respectively. (b) Write−read−erase−read memory cycling test. (c) Retention test of the FOM, measured at a time interval of 1 min after application of writing and erasing biases for 1 s. Bending tests on the FOMs. (d) Illustration and photograph of the FOM coiled on a capillary tube with a diameter of 1.1 mm during measurement [L/W = 50/157 μm]. (e) Threshold voltage shifts and (f) on/off current states measured at VD = −1 V as a function of the diameter of cylindrical rods. The FOM is bent by coiling around rods of different diameters. (g) Photographs of the FOM at the bent and flattened states during the bending cycling test [L/W = 15/157 μm]. (h) Threshold voltage shifts and (i) on/off current states over 2000 bending cycles with a bending diameter of 2.5 mm.
to the homogeneous distribution of the crystalline domain in the thin film.17,34 CTAC-Processed Organic Thin Films for Low-Voltage Operation. Cross-sectional scanning electron microscope (SEM) images reveal that the thin P(VDF-TrFE) film uniformly covered the entire outer surface of the Ag wire, as shown in Figure 2a. The thickness of the P(VDF-TrFE) layer was measured to be approximately 260 nm. Additionally, through the CTAC process, the P(VDF-TrFE) films were well coated on several types of metal wires, such as Al, Au, Ag, and Ag deposited on a plastic fishing line, as shown in the SEM images of Figure S3. These wires demonstrated ferroelectric memory characteristics corresponding to each film (Figure S3, bottom). Thus, the CTAC process is applicable for coating organic materials on fiber-shaped one-dimensional substrates.
Afterward, atomic force microscopy (AFM) measurements were conducted to observe the morphology and microstructure of P(VDF-TrFE) thin films on both Ag wires and Si substrates. The morphology of P(VDF-TrFE) is a critical factor that considerably affects not only the ferroelectric characteristics but also the device performance.27,35 From the topography of the P(VDF-TrFE) thin film on the Ag wire, as shown in Figure 2b, we clearly observed the formation of nanograins with an average width of ∼38 nm. As presented in the 3 μm × 3 μm scale images in Figure S4a, the surface roughness was measured to be approximately 3.6 nm. Compared to the conventional spincasting process, the CTAC process allows the growth of smaller grains of P(VDF-TrFE) on the wire, resulting in a relatively smoother surface than that on the Si wafer (Figure S4b). The P(VDF-TrFE) film on the Si wafer is composed of relatively 22578
DOI: 10.1021/acsami.9b03564 ACS Appl. Mater. Interfaces 2019, 11, 22575−22582
Research Article
ACS Applied Materials & Interfaces
Figure 4. Performance of wearable FOMs under fabric deformation. (a) Photographs of a FOM array sewn into fabric with a needle and a FOM device fabricated on a Ag wire substrate in fabric both under strain and relaxed. (b) Change in threshold voltage corresponding to the memory window of a FOM sewn in fabric with the increasing mechanical stress. Photographs (left) of the FOM-sewn fabric under pristine (top), uniaxial (middle), and diagonal (bottom) strains. Graph inset: optical microscope images of the FOM embedded in the textile during stretching the fabric (at 73.3% uniaxial strain and 100% diagonal strain) and relaxing (at 0% strain). (c) Retention characteristics during diagonal stretching on the fabric weaving the device (inset), measured at a time interval of 1 min after application of writing and erasing biases for 1 s each. (d) Photographs of the FOM-sewn fabric being crumpled and the transfer characteristics of the FOM before and after crumpling.
ferroelectric phase, which is consistent with the morphology characteristics of the ferroelectric P(VDF-TrFE) film obtained from AFM analysis of Figure 2b. From the above results, we conclude that the CTAC process is a promising coating method to form high-quality organic thin films on a one-dimensional fiber-shaped substrate. Device Performances of FOMs. To evaluate the feasibility of the FOMs as wearable data storage media, we measured the current−voltage characteristics of the FOMs in pristine and various bending situations, such as different curvatures and cycles, as shown in Figure 3. The FOMs exhibited a clockwise hysteresis window and considerable threshold voltage shifts (ΔVTh) with dual sweeps of gate voltages (VG) ranging from ±1 to ±5 V (VD = −1 V), indicating low-voltage ferroelectric transistor memory characteristics (Figure S5a). The memory windows increased depending on the applied VG and finally reached a ΔVTh of 5.6 V at the VG sweep range from 5 to −5 V, showing an on/off ratio of 103, as shown in Figure 3a. The hole mobility calculated from the transfer properties in the linear regime (VD = −1 V) approached 0.009 cm2 V−1 s−1. The lowvoltage operation of the FOMs can be attributed to the low trap density and fast charge accumulation in the small channel region due to the morphological features of the CTAC-processed P(VDF-TrFE) layer, such as the high-quality thin-film and the
large, rough, and needlelike gains. Structural defects and internal disorder, such as rough surfaces, significantly hinder the switching dynamics and dipole orientation in the P(VDFTrFE) film, resulting in slow charge accumulation in the channel.36 On the other hand, the phase image in Figure 2b clearly presents circular featureless and compact grains, which are typical surface features of β-phase P(VDF-TrFE).36 For this reason, the morphological features of the CTAC-processed polymer film lead to the formation of a uniform electrical field distribution between the gate electrode and the semiconductor layer due to the lower probability of carrier scattering and leakage current paths.27,37 Although further studies will be required to understand fully this interesting structural feature, the nanogranular film tends to show better ferroelectric characteristics at lower driving voltage than the needlelike domains.31,35,38,39 To verify the ferroelectric phase of the CTAC-processed P(VDF-TrFE) films on a wire, X-ray diffraction (XRD) measurements were performed. Figure 2c shows the XRD patterns of P(VDF-TrFE) films on an Ag wire and a silicon wafer. For both samples, we observed a diffraction pattern corresponding to overlapping of the (110) and (200) peaks of the ferroelectric β-phase at 2θ of 19.6° (d = 4.52 Å). This crystalline feature is strong evidence of the formation of the 22579
DOI: 10.1021/acsami.9b03564 ACS Appl. Mater. Interfaces 2019, 11, 22575−22582
Research Article
ACS Applied Materials & Interfaces formation of homogeneous and low-defective β-phase nanograins, in accordance with the SEM and AFM results. Although organic transistor memories using P(VDF-TrFE) films thinner than 200 nm have been reported, they did not successfully perform low-voltage operation compared to that of the FOMs.29,30 Hence, morphology control, as well as trying to reducing film thickness, is significant for performance enhancement of P(VDF-TrFE)-based devices. A switching endurance test was also performed through cycles of write−read−erase− read operations. As shown in Figure 3b, repeated switching cycles were obtained by measuring drain−source currents at VD = −1 V upon applying sequential gate voltages for writing (VG = −5 V), reading (VG = 0 V), and erasing (VG = 5 V). The electric fields applied at the gate lead to the reversible polarity of the dipoles in P(VDF-TrFE), resulting in current switching of the FOMs. After the writing operation, the ID associated with the on current state is enhanced at VG = 0 V. In turn, after erasing, the memory returns to the off current state. From these results, the power consumption was calculated by P = IV, where I = 6 × 10−9 A is the on-state drain current and V = 5 V is the gate voltage. The lowest operating P for the reliable ferroelectric memory was 3 × 104 pW. Importantly, to the best of our knowledge, our resulting device indicates the lowest power consumption for memory operation compared to that of previous reports on solution-processed OFET-type ferroelectric memories.29−31,40 Figure 3c shows the retention characteristics of the FOMs, which are written and erased by applying gate voltages of −5 and 5 V for 1 s each. Although the memory device exhibited slight initial decay followed by steady slow relaxation, it maintained its current state for more than 5 × 104 s and the fitted linear plots from extrapolation with extended timescale showed long-term retention time of more than 107 s, implying promising retention characteristics (Figure S6). To investigate the stability and robustness of the FOMs against mechanical stress, various bending tests were performed by varying the bending diameters and cycles. In particular, we monitored changes in the electrical performances of the FOMs (Figures 3e,f and S5b) before and after bending by coiling them around glass cylinders with different diameters, as illustrated in Figure 3d. During bending, the FOMs on the outer shell of the Ag wire are expected to be under tensile strain, resulting in meaningful geometric changes.21 However, the ΔVTh of the FOMs remains almost unchanged and the on/off current ratio is reduced. The primary degradation behavior of the devices is inferred to be attributed to damage in the semiconductor rather than the ferroelectric layer. The reliability of the memory characteristics of our device was observed over 2000 bending cycles from a bent diameter of 2.5 mm to a flattened state (Figure 3g). When the devices were repeatedly bent by increasing the number of bending cycles, there was a negligible change in the memory window and on/off current ratio, as shown in Figures 3h,i and S5c. Additionally, a FOM with a thicker P(VDF-TrFE) film operates at high writing/easing voltages but exhibits wide memory window, high on/off ratio, good electrical endurance, and good mechanical stability (Figure S7). The results of the tests concerning the bending curvature and cycles indicate that mechanical stress slightly reduces FET performance, whereas the memory performance is significantly maintained during bending from a bent diameter larger than 2.5 mm. Therefore, we expect that the resulting FOM will stably perform in mechanically deformed fabrics. Fabric Applicability of FOMs. To prove their applicability as a data storage medium in wearables, the FOMs were directly
sewed in a piece of common compression bandage, as a highly stretchable fabric, without any additional protective coating. The target fabric consists of polypropylene (PP), which is often used in sportswear because it is nonabsorbent and keeps the body feeling dry while sweating. The PP textile is thought to be one of the best test fabrics for our FOMs in real wearable electronic applications. The fiber devices were threaded with a needle, passed inside a textile yarn, and woven with the textile without detectable damage after the sewing process, as shown in Figure 4a and Video S1. After sewing, the memory characteristics of the FOM fabrics were evaluated, as shown in Figure 4b−d. Corresponding electrical data was successfully stored under uniaxial and diagonal stretching and crumpling. Figure 4b shows the changes in memory performance due to uniaxial strain in the fabric. Upon stretching, threshold voltages corresponding to both writing and erasing current states shift toward a wider memory window (Video S2). Upon release, the shifts return to the initial state of the pristine device. The changes in the memory window can be explained by the piezoelectric property of P(VDFTrFE).21,41 When stretching the fabric, the weft, inserted over and under the warp in the woven fabric, presses down the FOM, which experiences pressure perpendicular to the direction of stretching (Figure S8). Note that the applied strain on FOM is not exactly the same as that on the fabric. The mechanical force of the weft is similar to the tensile strain parallel to the channel. When tensile strain is applied to the P(VDF-TrFE) film, the polarization increases and more charge carriers in the P(VDFTrFE)-based FOM accumulate in the channel, resulting in an increased ID and memory window. Last, electrical tests were performed at the maximum strain point at which a thread in the fabric started to unravel. When further extended to more than 72% diagonal strain, the stretched fabrics start to break. After applying a constant diagonal stretching (∼100% strain) and releasing the strain to 0%, the FOMs showed a negligible decrease in ΔVTh. In particular, Figure 4c indicates the excellent retention characteristics of the FOM after writing and erasing operations. When a constant diagonal strain of 100% was applied on the fabric, the on and off current states are retained for 104 s each. This implies that the FOMs in the fabric are significantly robust even under strong applied strains. Furthermore, the device performance was measured under irreversible crumpling of the fabric, as shown in Figure 4d. The mechanical robustness of the FOM subjected to substantial damage, such as scratches, dents, and compression, meets requirements for future data storage media in smart garments for human monitoring in real time.
■
CONCLUSIONS In summary, we demonstrated fiber-based organic transistor memories by a simple solution process that can be applied to a variety of fibers for wearable e-textiles. The capillary tubeassisted coating system allowed the formation of smooth and compact nanogranular P(VDF-TrFE) films on flexible and thin metal wires. As a result, the ferroelectric properties of the fibershaped polymer film were enhanced, allowing the memory device to operate at low voltages below 5 V. The fiber organic memory exhibits excellent memory performance (low power consumption, reliable endurance cycles (∼100 times), and longterm retention (∼5 × 104 s)) with high flexibility. Moreover, our memory device sewed into a stretchable fabric is stable under mechanically harsh conditions, including uniaxial/diagonal strain and crumpling. Our approach for coating organic thin 22580
DOI: 10.1021/acsami.9b03564 ACS Appl. Mater. Interfaces 2019, 11, 22575−22582
Research Article
ACS Applied Materials & Interfaces films can provide new functionalities and further improvement in the electrical and mechanical performances of wearable textile devices.
■
Tae-Wook Kim: 0000-0003-2157-732X Author Contributions ⊥
M.K. and S.L. contributed equally to this work. T.K. planned and supervised the project. M.K., S.L., and T.K. designed the experiments. M.K. and S.L. carried out experimental work, data collection, and data analysis. S.J. assisted in the fabrication and measurements. All authors discussed the results and contributed to the manuscript preparation. M.K. and T.K. wrote the manuscript.
EXPERIMENTAL SECTION
Materials and Device Fabrication. Silver wires (φ = 0.1 mm, 99.99%, Nilaco corp.), as the gate electrode, were used to fabricate FOMs with a bottom-gate/top-contact (BG/TC) architecture. The wires were washed with isopropyl alcohol and dried under air flow. P(VDF-TrFE) (70:30 mol % copolymer) purchased from Solvay was dissolved in N,N-dimethylformamide (DMF, 99.8% purity, SigmaAldrich) (200 mg mL−1) and deposited on the metal wires by dipcoating (0.38 mm s−1) using a capillary tube and a printing speed controller, as shown in Figure 1a. Considering the Curie temperature (118 °C) and melting temperature (150 °C),42 the P(VDF-TrFE) film was annealed at 140 °C for 2 h in an oven. Then, a 50 nm-thick layer of pentacene (99.9% purity, Sigma-Aldrich) was evaporated at a pressure of approximately 5 × 10−7 Torr. Finally, Au source/drain electrodes (50 nm thick) were deposited on the pentacene through a shadow mask using thermal evaporation. The channel length (L) and width (W) of the FOMs were 15 and 157 μm, respectively. For the curvaturedependent bending test, L and W were 50 and 157 μm, respectively. We also fabricated fiber capacitors on a Ag wire with Au/P(VDF-TrFE)/Ag layers and evaluated the capacitance at different positions along the fiber (Figure S2). The average capacitance per area was 30.1 nF cm−2 at 10 kHz. Thin Film and Device Characterization. Scanning electron microscope (SEM) measurements were performed using an FEI Nova Nano SEM 200. The P(VDF-TrFE) film on a Ag wire was imaged by atomic force microscopy (AFM) (NanoScope V multimode, Bruker). Phase analysis of the thin film was carried out by X-ray diffraction (XRD) (PANalytical Empyrean X’Pert PRO MRD) with Cu Kα radiation at a wavelength of 1.5425 Å. The electrical characteristics of the FOMs were measured using a Keithley 4200-SCS and a 4145B (HP) semiconductor characterization system in a nitrogen glovebox, as shown in Figure S9. The strain of the fabric was presented as the ratio of the total deformation to that at the initial state under applied mechanical input. It was calculated through ε = ΔL/L = l − L/L, where ε is the engineering strain, L is the initial length of the fabric, and l is the final length of the fabric.
■
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the Korea Institute of Science and Technology (KIST) Young Fellow Program, and also partially supported by the National Research Foundation of Korea (NRF-2016R1C1B2007330 and NRF2019R1A2B5B03003955), and the Ministry of Trade, Industry, and Energy (MOTIE, Korea) under the Industrial Technology Innovation Program “Development of 3D-Deformable Multilayered FPCB Devices” (Grant no. 10051162).
■
(1) Yetisen, A. K.; Martinez-Hurtado, J. L.; Ü nal, B.; Khademhosseini, A.; Butt, H. Wearables in Medicine. Adv. Mater. 2018, 30, No. 1706910. (2) Yang, T.; Xie, D.; Li, Z.; Zhu, H. Recent Advances in Wearable Tactile Sensors: Materials, Sensing Mechanisms, and Device Performance. Mater. Sci. Eng., R 2017, 115, 1−37. (3) Pailes-Friedman, R. Electronics and Fabrics: The Development of Garment-Based Wearables. Adv. Mater. Technol. 2018, 3, No. 1700307. (4) Jin, H.; Matsuhisa, N.; Lee, S.; Abbas, M.; Yokota, T.; Someya, T. Enhancing the Performance of Stretchable Conductors for E-Textiles by Controlled Ink Permeation. Adv. Mater. 2017, 29, No. 1605848. (5) Yang, A.; Li, Y.; Yang, C.; Fu, Y.; Wang, N.; Li, L.; Yan, F. Fabric Organic Electrochemical Transistors for Biosensors. Adv. Mater. 2018, 30, No. e1800051. (6) Heo, J. S.; Eom, J.; Kim, Y. H.; Park, S. K. Recent Progress of Textile-Based Wearable Electronics: A Comprehensive Review of Materials, Devices, and Applications. Small 2018, 14, No. 1703034. (7) Lund, A.; van der Velden, N. M.; Persson, N. K.; Hamedi, M. M.; Müller, C. Electrically Conducting Fibres for E-Textiles: An Open Playground for Conjugated Polymers and Carbon Nanomaterials. Mater. Sci. Eng., R 2018, 126, 1−29. (8) Heo, J. S.; Kim, T.; Ban, S. G.; Kim, D.; Lee, J. H.; Jur, J. S.; Kim, M. G.; Kim, Y. H.; Hong, Y.; Park, S. K. Thread-Like CMOS Logic Circuits Enabled by Reel-Processed Single-Walled Carbon Nanotube Transistors via Selective Doping. Adv. Mater. 2017, 29, No. 1701822. (9) de Mulatier, S.; Nasreldin, M.; Delattre, R.; Ramuz, M.; Djenizian, T. Electronic Circuits Integration in Textiles for Data Processing in Wearable Technologies. Adv. Mater. Technol. 2018, 3, No. 1700320. (10) Ma, S.; Ye, T.; Zhang, T.; Wang, Z.; Li, K.; Chen, M.; Zhang, J.; Wang, Z.; Ramakrishna, S.; Wei, L. Highly Oriented Electrospun P(VDF-TrFE) Fibers via Mechanical Stretching for Wearable Motion Sensing. Adv. Mater. Technol. 2018, 3, No. 1800033. (11) Kwon, S.; Kim, H.; Choi, S.; Jeong, E. G.; Kim, D.; Lee, S.; Lee, H. S.; Seo, Y. C.; Choi, K. C. Weavable and Highly Efficient Organic Light-Emitting Fibers for Wearable Electronics: A Scalable, LowTemperature Process. Nano Lett. 2018, 18, 347−356. (12) Yang, A.; Li, Y.; Yang, C.; Fu, Y.; Wang, N.; Li, L.; Yan, F. Fabric Organic Electrochemical Transistors for Biosensors. Adv. Mater. 2018, 30, No. 1800051. (13) Rajan, K.; Garofalo, E.; Chiolerio, A. Wearable Intrinsically Soft, Stretchable, Flexible Devices for Memories and Computing. Sensors 2018, 18, No. 367.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b03564. P(VDF-TrFE) film thickness as a function of coating rate, dielectric capacitance of the P(VDF-TrFE) layer on a Ag wire, viscosity of solution, top-view SEM images of P(VDF-TrFE)-wrapped wires, memory characteristics of fiber memories, AFM measurements on P(VDF-TrFE) layers deposited on the Ag wire and the Si wafer, retention characteristics of the FOM with extended timescale, and photographs of the FOMs embedded in PP fabrics (PDF) Sewing process of fiber devices (AVI) Measurement of the FOMs on electrical characteristics under no strain and uniaxial strain in fabric (AVI)
■
REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jae-Min Hong: 0000-0001-9238-5451 Sang Hyun Lee: 0000-0002-7784-5939 Kwang-Un Jeong: 0000-0001-5455-7224 Jung Ah Lim: 0000-0002-3007-3855 22581
DOI: 10.1021/acsami.9b03564 ACS Appl. Mater. Interfaces 2019, 11, 22575−22582
Research Article
ACS Applied Materials & Interfaces (14) Ji, Y.; Cho, B.; Song, S.; Kim, T. W.; Choe, M.; Kahng, Y. H.; Lee, T. Stable Switching Characteristics of Organic Nonvolatile Memory on a Bent Flexible Substrate. Adv. Mater. 2010, 22, 3071−3075. (15) Jo, A.; Seo, Y.; Ko, M.; Kim, C.; Kim, H.; Nam, S.; Choi, H.; Hwang, C. S.; Lee, M. J. Textile Resistance Switching Memory for Fabric Electronics. Adv. Funct. Mater. 2017, 27, No. 1605593. (16) Yokota, T.; Zalar, P.; Kaltenbrunner, M.; Jinno, H.; Matsuhisa, N.; Kitanosako, H.; Tachibana, Y.; Yukita, W.; Koizumi, M.; Someya, T. Ultraflexible Organic Photonic Skin. Sci. Adv. 2016, 2, No. e1501856. (17) Kim, H. M.; Kang, H. W.; Hwang, D. K.; Lim, H. S.; Ju, B. K.; Lim, J. A. Metal-Insulator-Semiconductor Coaxial Microfibers Based on Self-Organization of Organic Semiconductor:Polymer Blend for Weavable, Fibriform Organic Field-Effect Transistors. Adv. Funct. Mater. 2016, 26, 2706−2714. (18) Chen, D.; Pei, Q. Electronic Muscles and Skins: A Review of Soft Sensors and Actuators. Chem. Rev. 2017, 117, 11239−11268. (19) Rajan, K.; Bejtka, K.; Bocchini, S.; Perrone, D.; Chiappone, A.; Roppolo, I.; Pirri, C. F.; Ricciardi, C.; Chiolerio, A. Highly Performing Ionic Liquid Enriched Hybrid RSDs. J. Mater. Chem. C 2017, 5, 6144− 6155. (20) Kang, T. K. Highly Stretchable Non-Volatile Nylon Thread Memory. Sci. Rep. 2016, 6, No. 24406. (21) Thuau, D.; Abbas, M.; Wantz, G.; Hirsch, L.; Dufour, I.; Ayela, C. Mechanical Strain Induced Changes in Electrical Characteristics of Flexible, Non-Volatile Ferroelectric OFET Based Memory. Org. Electron. 2017, 40, 30−35. (22) Jang, S.; Jang, S.; Lee, E. H.; Kang, M.; Wang, G.; Kim, T. W. Ultrathin Conformable Organic Artificial Synapse for Wearable Intelligent Device Applications. ACS Appl. Mater. Interfaces 2019, 11, 1071−1080. (23) Wang, H.; Zhao, Q.; Ni, Z.; Li, Q.; Liu, H.; Yang, Y.; Wang, L.; Ran, Y.; Guo, Y.; Hu, W.; Liu, Y. A Ferroelectric/Electrochemical Modulated Organic Synapse for Ultraflexible, Artificial VisualPerception System. Adv. Mater. 2018, 30, No. 1803961. (24) Katsouras, I.; Zhao, D.; Spijkman, M. J.; Li, M.; Blom, P. W.; De Leeuw, D. M.; Asadi, K. Controlling the On/Off Current Ratio of Ferroelectric Field-Effect Transistors. Sci. Rep. 2015, 5, No. 12094. (25) Kim, K. L.; Lee, W.; Hwang, S. K.; Joo, S. H.; Cho, S. M.; Song, G.; Cho, S. H.; Jeong, B.; Hwang, I.; Ahn, J. H.; et al. Epitaxial Growth of Thin Ferroelectric Polymer Films on Graphene Layer for Fully Transparent and Flexible Nonvolatile Memory. Nano Lett. 2016, 16, 334−340. (26) Zhang, Y.; Wang, H.; Zhang, L.; Chen, X.; Guo, Y.; Sun, H.; Li, Y. Field-Effect Transistor Memories Based on Ferroelectric Polymers. J. Semicond. 2017, 38, 111001−111012. (27) Park, B. E.; Ishiwara, H.; Okuyama, M.; Sakai, S.; Yoon, S. M., Ferroelectric-Gate Field Effect Transistor Memories. Springer: Netherlands, 2016. (28) Song, L.; Wang, Y.; Gao, Q.; Guo, Y.; Wang, Q.; Qian, J.; Jiang, S.; Wu, B.; Wang, X.; Shi, Y.; et al. Speed up Ferroelectric Organic Transistor Memories by Using Two-Dimensional Molecular Crystalline Semiconductors. ACS Appl. Mater. Interfaces 2017, 9, 18127− 18133. (29) Khan, M. A.; Bhansali, U. S.; Alshareef, H. N. High-Performance Non-Volatile Organic Ferroelectric Memory on Banknotes. Adv. Mater. 2012, 24, 2165−2170. (30) Lee, K. H.; Lee, G.; Lee, K.; Oh, M. S.; Im, S.; Yoon, S. M. HighMobility Nonvolatile memory Thin-Film Transistors with a Ferroelectric Polymer Interfacing ZnO and Pentacene Channels. Adv. Mater. 2009, 21, 4287−4291. (31) Park, J. H.; Nayak, P. K.; Alshareef, H. N. Novel Ferroelectric Polymer Memory Coupling Two Identical Thin-Film Transistors. Adv. Electron. Mater. 2016, 2, No. 1500206. (32) Faustini, M.; Louis, B.; Albouy, P. A.; Kuemmel, M.; Grosso, D. Preparation of Sol−Gel Films by Dip-Coating in Extreme Conditions. J. Phys. Chem. C 2010, 114, 7637−7645. (33) Gu, X.; Shaw, L.; Gu, K.; Toney, M. F.; Bao, Z. The MeniscusGuided Deposition of Semiconducting Polymers. Nat. Commun. 2018, 9, No. 534.
(34) Diao, Y.; Shaw, L.; Bao, Z.; Mannsfeld, S. C. B. Morphology Control Strategies for Solution-Processed Organic Semiconductor Thin Films. Energy Environ. Sci. 2014, 7, 2145−2159. (35) Guo, D.; Stolichnov, I.; Setter, N. Thermally Induced Cooperative Molecular Reorientation and Nanoscale Polarization Switching Behaviors of Ultrathin Poly (Vinylidene Fluoride-Trifluoroethylene) Films. J. Phys. Chem. B 2011, 115, 13455−13466. (36) Asadi, K.; Wondergem, H. J.; Moghaddam, R. S.; McNeill, C. R.; Stingelin, N.; Noheda, B.; Blom, P. W. M.; De Leeuw, D. M. Spinodal Decomposition of Blends of Semiconducting and Ferroelectric Polymers. Adv. Funct. Mater. 2011, 21, 1887−1894. (37) Caraveo-Frescas, J. A.; Khan, M.; Alshareef, H. N. Polymer Ferroelectric Field-Effect Memory Device with SnO Channel Layer Exhibits Record Hole Mobility. Sci. Rep. 2014, 4, No. 5243. (38) Fabiano, S.; Usta, H.; Forchheimer, R.; Crispin, X.; Facchetti, A.; Berggren, M. Selective Remanent Ambipolar Charge Transport in Polymeric Field-Effect Transistors For High-Performance Logic Circuits Fabricated in Ambient. Adv. Mater. 2014, 26, 7438−7443. (39) Xu, T.; Xiang, L.; Xu, M.; Xie, W.; Wang, W. Excellent LowVoltage Operating Flexible Ferroelectric Organic Transistor Nonvolatile Memory with a Sandwiching Ultrathin Ferroelectric Film. Sci. Rep. 2017, 7, No. 8890. (40) Naber, R. C. G.; De Boer, B.; Blom, P. W. M.; De Leeuw, D. M. Low-Voltage Polymer Field-Effect Transistors for Nonvolatile Memories. Appl. Phys. Lett. 2005, 87, No. 203509. (41) Lee, J. H.; Lee, K. Y.; Kumar, B.; Tien, N. T.; Lee, N. E.; Kim, S. W. Highly Sensitive Stretchable Transparent Piezoelectric Nanogenerators. Energy Environ. Sci. 2013, 6, 169−175. (42) Hu, Z.; Tian, M.; Nysten, B.; Jonas, A. M. Regular Arrays of Highly Ordered Ferroelectric Polymer Nanostructures for NonVolatile Low-Voltage Memories. Nat. Mater. 2009, 8, 62−67.
22582
DOI: 10.1021/acsami.9b03564 ACS Appl. Mater. Interfaces 2019, 11, 22575−22582
