An ultrasensitive, low-power oxide transistor-based ... - ACS Publications

Subscriber access provided by UNIV OF DURHAM

Functional Inorganic Materials and Devices

An ultrasensitive, low-power oxide transistor-based mechanotransducer with microstructured, deformable ionic dielectrics Sukjin Jang, Eunsong Jee, Daehwan Choi, Wook Kim, Joo Sung Kim, Vipin Amoli, Taehoon Sung, Dukhyun Choi, Do Hwan Kim, and Jang-Yeon Kwon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09840 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

An ultrasensitive, low-power oxide transistor-based mechanotransducer with microstructured, deformable ionic dielectrics Sukjin Jang,†,‡,⊥Eunsong Jee,§,⊥ Daehwan Choi,†,‡ Wook Kim,∥ Joo Sung Kim, § Vipin Amoli, § Taehoon Sung, †,‡ Dukhyun Choi∥, Do Hwan Kim*,§ and Jang-Yeon Kwon*,†,‡

†

School of Integrated Technology, Yonsei University, ‡ Yonsei Institute of Convergence

Technology, Incheon 21983, Korea; § Department of Chemical Engineering, Hanyang University, Seoul 04763, Korea; ∥ Department of Mechanical Engineering, School of Engineering, Kyung Hee University, Yongin, Gyeonggi 17104, Korea

KEYWORDS: ultrasensitive, low-power oxide transistor, electronic skin, mechanotransducer, microstructured and deformable ionic dielectrics

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 25

ABSTRACT

The development of highly sensitive artificial mechanotransducer that mimics the tactile sensing features of human skin has been a big challenge in electronic skin research. Here, we demonstrate an ultrasensitive, low-power oxide transistor-based mechanotransducer modulated by microstructured, deformable ionic dielectrics, which is consistently sensitive to a wide range of pressures from 1 kPa to 50 kPa. To this end, we designed a visco-poroelastic and ionic thermoplastic polyurethane (i-TPU) with micro-pyramidal feature as a pressure sensitive gate dielectric for the indium-gallium-zinc-oxide (IGZO) transistor-based mechanotransducer, which leads to an unprecedented sensitivity of 43.6 kPa-1, which is 23 times higher than that of a capacitive mechanotransducer. This is because the pressure induced ion accumulation at the interface of the i-TPU dielectric and IGZO semiconductor effectively modulates the conducting channel, which attributed to the enhanced current level under pressure. We believe that ionic transistor-type mechanotransducer suggested by us will be an effective way to perceive external tactile stimuli over a wide pressure range even under low-power (< 4V), which might be one of candidates to directly emulate the tactile sensing capability of human skin.


2

Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION Recently, the five senses of humans (taste, sight, smell, sound, and touch) have been imitated by fabricating soft nanomaterials and optoelectronic devices.1–5 In particular, considerable efforts have been made to mimic the tactile sensing features of human skin using electronic skin (e-skin).6,7 Interestingly, the recently developed multimodal e-skins can detect temperature,8–11 sound,12–14 and even specific molecules through chemical reactions.15,16 Moreover, some reports have demonstrated a self-healing property17,18 as well as high performance of e-skins. However, most pressure sensor based e-skins suffer from maintaining high pressure sensitivity over a wide pressure range, which is a highly desirable feature in tactile electronics.19–21 Tactile sensors need to mimic both the high sensitivity and wide pressure sensing range of human skin for the full-range detection of human activities in robotics.22 Among the various tactile and pressure sensors based on different transduction mechanisms, transistor-type sensors have attracted considerable attention due to their integrated functionality of signal transduction and amplification.23–27 Especially, organic thin film transistors (OTFTs) that utilize pressure-sensitive rubber as gate dielectrics have been demonstrated as ultrasensitive pressure-sensing devices in low pressure regimes (< 1 kPa). However, issues related to narrow pressure sensing range (0-10 kPa) remain unresolved. Furthermore, low-power consumption is another highly desirable technological feature for specific application such as the use of electronic skins in mobile platforms. Bao et al.28,29 reported a highly sensitive organic transistor pressure sensor using microstructured pyramidal poly(dimethylsiloxane) (PDMS) as a gate insulator. While they achieved 15 times higher sensitivity than capacitive devices, the low sensitivity in mid-pressure regime (> 10 kPa) and high operating voltage (200 V) prevented the use of PDMS in low-power pressure sensors. Sensitivity and the effective pressure sensing range


3


Page 4 of 25

of transistor pressure sensors are critically affected by the geometry of the pressure sensitive gate dielectric material. In a previous report,30 the poor sensitivity caused by the small deformability of most rubber dielectrics inhibits the implementation of the e-skin with high sensitivity and wide pressure sensing range, mimicking the tactile capability of human skin. In general, ionic gels have emerged as a potential gate dielectric for low-voltage operating thin-film transistors (TFTs) and related various sensor devices.31–36 Due to the lower operation voltage and high sensitivity, the ionic gels have been considered as a candidate material for various sensor systems. Despite of the introduction of ionic gel, however, the limitation of the narrow sensing range still remained in this area. We recently reported a bio-inspired, low-voltage, piezocapacitive e-skin with high sensitivity and pressure sensing range similar to human skin, which was attributed to highly deformable (up to a strain of 800 %), visco-poroelastic, ionic elastomer electrolytes.12 Herein, we describe a first proof of concept using a microstructured37–42, deformable ionic dielectric with the aim of achieving pressure induced channel modulation in oxide TFTs-based mechanotransducer and explore its potential application for developing a low power (4 V) e-skin with ultra-sensitivity within the real human sensing range (< 50 kPa). The deformable, ionic thermoplastic polyurethane (i-TPU) that was used as a pressure sensitive gate dielectric generated a significant synergy among the microstructured geometries to induce a higher maximum stress than that of a flat dielectric under the same input pressure and visco-poroelastic deformation of the i-TPU dielectric layer.12 The indium-gallium-zinc-oxide (IGZO) transistorbased mechanotransducer with microstructured and deformable i-TPU dielectrics exhibited an unprecedented sensitivity of 43.6 kPa-1, which is 23 times higher than that of a capacitive e-skin. In addition, the experimental results clearly demonstrate that the pressure induced ion


4

Page 5 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


accumulation at the interface of the i-TPU dielectric and IGZO semiconductor effectively modulated the conducting channel, which attributed to the enhanced current level under pressure.

2. Experimental Section: Fabrication of ionic thermoplastic polyurethane (i-TPU) film. Thermoplastic polyurethane (TPU) beads were purchased from KOLON INDUSTRIES, INC. These beads are composed of a hard segment and a soft segment in the ratio of 1:4. The N, N-Dimethylformamide (DMF) used for the TPU beads solvent were purchased from Sigma-Aldrich. The 1-Ethyl-3-methy limidazolium:bis(trifluoromethylsulfonyl) imide [EMIM]+[TFSI]- was provided by C-TRI INDUSTRIE, INC. First, the TPU beads were dissolved in DMF at the weight ratio of 1:5, and the solution was stirred at 130°C for 2 hours. After all of the TPU beads have dissolved into the DMF, 20wt% ionic liquid, [EMIM]+[TFSI]-, depending on the weight of the TPU beads, was added to the solution by following additional stirring at 130°C for 1 hour. Fabrication of microstructured i-TPU film. After blending i-TPU solution with ionic liquid in DMF, the well-mixed i-TPU solution was poured on the silicon mold of the micro-pyramidal pattern that was fabricated using an anisotropic wet etching process. The solution was then cured in a box furnace at 150°C for 12 hours. During this curing process, the residual DMF solvent was fully removed from the i-TPU film. After curing, the solid-state i-TPU film was peeled off the silicon mold. Finally, a micro-pyramidal i-TPU dielectric was fabricated in order to operate IGZO transistor sensor upon applying pressure. Silicon wet etching. A thermally grown silicon substrate with SiO2 of 100 nm was used. By using conventional photolithography process, the micro-pyramidal patterns was developed. Squares of 10µm x 10µm are arranged at intervals of 10µm. All areas except for these squares


5


Page 6 of 25

are covered with photoresist. After patterning, buffered oxide etch (BOE) was used to wet-etch the SiO2 layer (at 40 oC, for 150 sec). A PR remover is used to remove the remaining PR residue. A tetramethylammonium hydroxide (TMAH) etchant is used to etch the silicon (at 60 oC, etching rate of 1.05 um/min). After the silicon etching, the remaining SiO2 is removed using BOE. Fabrication of IGZO transistor. The glass substrate (eagle 2000, Coring) was prepared by wet cleaning. For the 3 mask patterning process, we used the AZ 5214E photoresist (PR) and a maskless lithography tool (Nano System Solutions, Inc.). The PR was spin-coated at 4,000 rpm for 30 seconds and baked at 110oC for 50 seconds. The channel pattern is exposed by 30 mJ with a h-line (405nm) light source, and then the sample is baked at 120 oC for 2 minutes, followed by UV irradiation under 200 mJ. After development, the PR is removed in the unexposed area. The 50 nm-thick IGZO film is deposited by radio frequency (rf) sputter. The residue of PR on the substrate is dissolved by ultrasonication in acetone. The electrode (source/drain) is also fabricated by identical procedure that was used for the IGZO pattern. A molybdenum (Mo) thin film is deposited by direct current (dc) sputter (100 nm). Finally, SiO2 is deposited and patterned by a lift-off process (RF sputter, 150 nm) to prohibit a direct contact of i-TPU and IGZO. After the patterning process, the sample was annealed into tube furnace at 200 oC. Measurement of capacitance and drain current under pressure. The transistor-based mechanotransducer consisting of IGZO as a semiconductor and i-TPU as a gate dielectric was measured using KITHELY 4200. A custom-built sensor probe station with a programmable xyand z-axis stage (0.1 µm resolution) enabled the ionic transistor-type mechanotransducer to be used to obtain an exact pressure, and a force gauge (Mark-10) measured the load. the sensitivity ∆

of the device was identified with the slope of current change versus pressure variation (( )/∆P).


6

Page 7 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Deformation simulation of the microstructured i-TPU film. In order to verify the change of contact area according to the pressure increase, we performed a finite element method (FEM) based simulation using Midas NFX (Midas IT co.). The dimensions of the structure for modeling were determined from the FE-SEM image. The simulated area and number of pyramid array were limited to 275 µm × 275 µm and a 14 × 14 array as shown in Figure S1. The height of the pyramid structure is 7.06 µm and the bottom surface area is 100 µm2. The non-linear static FEM was utilized to reflect the mechanical deformation of i-TPU. The contact pressure was controlled from 0 to 35 kPa with 5 kPa intervals and induced on the top of the i-TPU, as shown in Figure S2. The detailed model information and boundary conditions are described in the supporting information.

3. RESULTS and DISCUSSION Figure 1a shows a schematic of an ultrasensitive, low-power transistor-based mechanotransducer consisting of IGZO as a channel and i-TPU as a gate dielectric (Figures S3, S4, and Experimental Section). Figure 1a also illustrates the molecular components of the i-TPU dielectric, which is prepared from the non-covalent association of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide ([EMIM]+[TFSI]- cation-anion pairs) loaded in the TPU. In particular, the IGZO film is utilized as a semiconductor because it has a high mobility and stable off-current. Moreover, the low processing temperature from the sputtering step, outstanding repeatability, and unrestricted patterning process renders the IGZO a suitable candidate for transistor e-skins.43 As shown in Figure 1a, the SiO2 layer acts as a protecting layer to prevent direct contact between the i-TPU and source/drain electrodes. This direct contact might generate a significant parasitic capacitance, which leads to a decrease in the source/drain current, even


7


Page 8 of 25

under high gate-bias (Figure S5). Figure 1b shows the field-emission scanning electron microscope (FE-SEM) images of the flat i-TPU film (left image) and well-defined micropyramidal features formed on the surface of the i-TPU film (right image). The flat i-TPU film has a random surface morphology over the entire area. In contrast, the micro-pyramidal pattern of the i-TPU shows a height of 6.6 µm, a size of 11.2 µm, and an inter-spacing of 8.8 µm. This pattern was prepared using a silicon master molding process (Figure S1). Figure 1c presents the optical micrograph of the IGZO transistor-based mechanotransducer with the microstructured, deformable i-TPU gate dielectric. The i-TPU dielectric is laminated onto the IGZO channel, and the channel position is directly prodded using a flat circle rod with a diameter of 3 mm in order to investigate the pressure-induced current modulation (Figure S6). Figure 2a illustrates the capacitance vs frequency characteristics of the micro-pyramidal iTPU over a wide pressure range (0 – 47 kPa). In order to measure the capacitance, we deposited an Au electrode on the top of the i-TPU film by DC sputtering. The capacitance between Au (gate in transistor) and Mo (source or drain in transistor) was measured with the micro-pyramidal i-TPU as the dielectric (see inset in Figure 2a). Further, Figure 2a clearly shows the frequency dependence of the device capacitance, which indicates the existence of the electric double layer (EDL) phenomenon. In order to understand the advantages of micro-pyramidal structure in pressure sensor geometry, we directly compared a micro-pyramidal structure with a flat i-TPU. Figure 2b reflects a pressure sensing capability that show a structural advantage of the micropyramidal i-TPU compared with the flat i-TPU under pressure. The capacitance of the flat i-TPU increased by visco-poroelastic behavior as well as surface roughness at the nano- or micro-scale (see Figure 1b), as demonstrated in our previous study.12 The measured capacitance of the micropyramidal i-TPU proportionally increases in response to the applied pressure, with a sensitivity


8

Page 9 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of 1.84 kPa-1. This high sensitivity that is at least 1.5 times higher than that of the flat i-TPU (1.13 kPa-1) is due to the gradually increasing contact area in the micro-pyramidal geometry of iTPU. In particular, we have selected a specific operating frequency of 100 Hz in order to improve the absolute value in sensitivity for both of capacitive devices, irrespective of structural effects. (Figure S7 and Table S1) Furthermore, we performed a computational study in order to address the physical change of the micro-pyramidal i-TPU geometry from a material perspective under pressure. Finite element method (FEM) simulation study (Figure 2c) illustrates the effective change in the contact area and the resulting effective stress of the micro-pyramidal i-TPU geometry under subsequent pressure change. As pressure increased, the contact area as well as effective stress increased due to the local deformation of the micro-pyramidal structure. Based on this FEM simulation, a comparison between the contact area from the computational data and the capacitance from the experimental data was carried out according to applied pressure in order to verify the effect of the micro-pyramidal i-TPU on EDL construction (Figure 2d). It is worth noting that the capacitance linearly increases against pressure, resulting from an increase of the contact area of the micro-pyramidal i-TPU, even over a wide range of pressure (up to 40 kPa). In other words, the sensitivity of the i-TPU is directly affected by the contact area of i-TPU. This sensitivity (showing 1.84 kPa-1) is significantly high, compared to the non-ionic polymer with an identical structure. As shown in Figure 2e, the capacitance changes of neat TPU with micropyramidal structures increases in the low pressure regime, but is saturated at 23 kPa. In addition, low sensitivity of 0.023 kPa-1 (low pressure regime) and 0.003 kPa-1 (medium pressure regime) was observed in the device with a micro-pyramidal, non-ionic TPU dielectric, even though the contact area linearly increases under the applied pressure similar to the micro-pyramidal i-TPU.


9


Page 10 of 25

Thus, we yielded the maximum sensitivity of the micro-pyramidal i-TPU by constructing these physical properties and structural advantages. In order to interpret the difference between the neat TPU (showing non-ionic characteristics) and the i-TPU, the schematic diagram and equivalent circuit are depicted in Figure 2f. The neat TPU consists only of segments, and thus has high resistivity. However, the i-TPU has [EMIM]+[TFSI]- cation-anion pairs that can form an EDL even under low-voltage bias, which can induce high capacitance. The equivalent circuit is shown on the right and the more detailed formulas are discussed in the supporting information. Figure 3a shows the transfer characteristics of the IGZO transistor-based mechanotransducer modulated by the micro-pyramidal i-TPU gate dielectric with a pressure range of up to 50 kPa. (Inset shows the schematic of the device). All the measurements were performed by sweeping the gate voltage from -2 V to 4 V at a drain voltage of 2 V, which is much lower than that of other transistor pressure sensors that were previously reported. The threshold voltage shifts negatively and the on-current increases with the pressure (This will be discussed in more detail later). Figure 3b shows a comparison of pressure response of the transistor- and capacitor-based mechanotransducer based on the micro-pyramidal i-TPU gate dielectric designed in this work. Note that the drain current (∆I/I0) change in the mechanotransducer significantly exceeds the capacitance change (∆C/C0) in the capacitive mechanotransducer over a wide range of pressure (almost 23 times). For the capacitive type, the pressure sensitivity (slope of ∆C/C0 vs Pressure) of 1.84 kPa-1 is measured for the entire pressure spectrum, while in the transistor type the pressure sensitivity (slope of ∆I/I0 vs Pressure) is 43.62 kPa-1 in the entire range. Also, the sensitivity can be divided into three regimes [3.4 kPa-1 (0~8 kPa), 68 kPa-1 (8~20 kPa), and 40.8 kPa-1 (20~50 kPa)], which were measured in different pressure regimes. The sensitivity of IGZO


10

Page 11 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


transistor-based mechanotransducer is 20-35 times higher than that of the capacitive mechanotransducer in the real tactile regime (8~50 kPa). Basically, in the saturation region, the drain current in transistors is expressed as =

( − ),

where µ is the field effect mobility, C is the capacitance, W/L is the channel width/length, Vg is the gate voltage, and Vth is the threshold voltage. Therefore, high sensitivity in the IGZO transistor-based mechanotransducer with the microstructured, deformable i-TPU gate dielectric can be explained by the interplay of the following two factors. First, high sensitivity can come from the change in the capacitance of the micro-pyramidal i-TPU gate dielectric upon applying pressure as mentioned in the previous work12 which can lead to the amplification of the drain current flow in the IGZO transistors. Second, high sensitivity is attributed to the channel modulation in the IGZO semiconductor that is caused by the pressure dependent contact area variation at the interface of the IGZO and the micro-pyramidal i-TPU dielectric. As shown in Figure 2f, the i-TPU dielectric includes [EMIM]+[TFSI]- cation-anion pairs at the micropyramidal tip as well as in the i-TPU bulk, which indicates that an increase of the contact area induced by the deformation of the micro-pyramidal tips can generate the enhancement of electric double layer. Finally, this allows the accumulation of a large number of [EMIM]+ or [TFSI]- free ions at the micro-pyramidal tip under pressure. In particular, under positive gate bias, the [EMIM]+ ions can relatively move to the micro-pyramidal tip, which will attempt to pull the electrons that are present in IGZO semiconducting layer to the i-TPU/IGZO interface. Furthermore, in order to elucidate the exact mechanism in the current modulation according to pressure for IGZO transistor-based mechanotransducer with the micro-pyramidal i-TPU dielectric, we briefly modeled the electron distribution in the IGZO channel as shown in Figure


11


Page 12 of 25

3c. At regimes I and II, the pressure is not enough to induce effective EDL layer, which results in disconnection of electron channel, although the partially contacted portion with the micropyramidal i-TPU has a high electron concentration. However, as pressure increases (see regime III), an inter-bridge in the channel area is well developed, and thus the channel is formed at the interface between IGZO semiconductor and micro-pyramidal i-TPU gate dielectric. Further, once the channel is formed efficiently, the drain current gradually increases upon applying pressure (see Figure 3a, 3d and Figure S8, S9), which results from an increase of the interfacial contact area as well as visco-poroelasticity that can be crucial in the i-TPU film.12 In particular, it can be noted that Figure 3d exhibits pressure sensing capability depending on inter-spacing between the micro-pyramidal patterns, which implies that an inter-bridge into the channel can be allowed even under lower pressure because the micro-pyramidal i-TPU tips that form the interfacial contacts with IGZO semiconducting layer are highly dense in the case of inter-spacing of 25 µm, as shown in Figure 3d. Unlike an inter-spacing of 25 µm and 50 µm, however, the transfer characteristics for the inter-spacing of 100 µm cannot be observed at low pressure. At pressures above 20 kPa, the i-TPU gate insulator and IGZO have sufficient contact area to induce fully connected channel. Figure S9 depicts pressure sensing capability as a function of micropyramidal inter-spacing of i-TPU gate dielectric in IGZO transistor-based mechanotransducer, which is strongly affected by the inter-spacing of the micro-pyramidal tips. More importantly, irrespective of inter-spacing, the current modulation is proportional to applied pressure over a wide range of pressure up to 50 kPa that is in the range of real tactile regime. Interestingly, as shown in Figure 4a, for IGZO transistor-based mechanotransducer with the micro-pyramidal i-TPU dielectric, a high stability for the change in drain current was obtained in response to the dynamic pressure (9.9 kPa, 16.3 kPa, 26.7 kPa, and 38 kPa). This reflects that


12

Page 13 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


IGZO transistor-based mechanotransducer can efficiently perceive the wide pressure range corresponding to real tactile stimuli. Further, it can be noted that the response against a gentle pressure (50 kPa @ 0.1 Hz) is highly stable and repeatable, which indicates that our system fully releases the dynamic force (Figure 4b). The measured on/off ratio value of the drain current (represented by blue line) against pressure was maintained up to almost 1,000 cycles during the repeated loading/unloading. Basically, the drain current in oxide transistor is determined by mobility, free carrier concentration (like electrons and oxygen vacancies), channel size, and capacitance. In particular, the free carrier concentration is influenced by bias stress that is applied during device manipulation. In the real time sensing experiment, the gate/drain voltage was fixed at 4V and 2V, respectively. This condition can cause on-bias effect, which results in a decrease of free electrons in the channel region. Therefore, in the IGZO transistor-based mechanotransducer, the drain current decreases during a few cycles, and then is saturated because of fully trapped electrons into the channel. Furthermore, for this dynamic test, the gate current (represented by red line) exhibits the identical feature upon applying pressure, which implies that the gate current through the i-TPU with ionic conductivity is affected by the contact area change, similar to the capacitance change. Figure 4c and 4d displays the comparison of pressure sensitivity in terms of sensing range and operating voltage reported in various transistor type pressure sensors in order to describe the distinction of our sensor directly.13,29,30,44,45 This implies that IGZO transistor-based mechanotransducer with the micro-pyramidal i-TPU dielectric can efficiently perceive external stimuli over a wide pressure range even under low power.

4. CONCLUSIONS


13


Page 14 of 25

In conclusion, we developed an ultrasensitive, low-power indium-gallium-zinc-oxide (IGZO) transistor-based mechanotransducer modulated by visco-poroelastic and ionic thermoplastic polyurethane (i-TPU) with micro-pyramidal structure, while consistently maintaining a high sensitivity of 43.6 kPa-1over a wide range of pressures from 5 kPa to 50 kPa. This behaviour is because the pressure induced ion accumulation at the interface of the i-TPU dielectric and IGZO semiconductor effectively modulates the conducting channel, which attributed to the enhanced current level under pressure. We expect that ionic transistor-type mechanotransducer will be an effective for the stable monitoring of external tactile stimuli over a wide pressure range even under low-power (< 4V), which might be one of e-skins to directly emulate the tactile sensing capability of human skin.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (D.H.K.). *E-mail: [email protected] (J.-Y.K.). Author Contributions ⊥

S.J. and E.J. contributed equally to this work

ACKNOWLEDGMENT This work was financially supported by the Center for Advanced Soft-Electronics under the Global Frontier Project (CASE-2014M3A6A5060932) and the Basic Science Research Program (2017R1A2B4012819 and 2017R1A5A1015596) of the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT. This research was also supported by the MIST


14

Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Ministry of Science and ICT), Korea, under the “ICT Consilience Creative Program” (IITP2018-2017-0-01015) supervised by the IITP (Institute for Information & communications Technology Promotion)

REFERENCES (1) Hauptmann, P.; Borngraeber, R.; Schroeder, J.; Auge, J. Artificial Electronic Tongue in Comparison to the Electronic Nose. State of the Art and Trends. In Frequency Control Symposium and Exhibition, 2000. Proceedings of the 2000 IEEE/EIA International; IEEE, 2000; pp 22–29. (2) Ko, H. C.; Stoykovich, M. P.; Song, J.; Malyarchuk, V.; Choi, W. M.; Yu, C.-J.; Geddes Iii, J. B.; Xiao, J.; Wang, S.; Huang, Y. A Hemispherical Electronic Eye Camera Based on Compressible Silicon Optoelectronics. Nature 2008, 454 (7205), 748. (3) Wilson, A. D.; Baietto, M. Applications and Advances in Electronic-Nose Technologies. Sensors 2009, 9 (7), 5099–5148. (4) Mannoor, M. S.; Jiang, Z.; James, T.; Kong, Y. L.; Malatesta, K. A.; Soboyejo, W. O.; Verma, N.; Gracias, D. H.; McAlpine, M. C. 3D Printed Bionic Ears. Nano letters 2013, 13 (6), 2634–2639. (5) Lee, M. H.; Nicholls, H. R. Review Article Tactile Sensing for Mechatronics—a State of the Art Survey. Mechatronics 1999, 9 (1), 1–31. (6) Hammock, M. L.; Chortos, A.; Tee, B. C.-K.; Tok, J. B.-H.; Bao, Z. 25th Anniversary Article: The Evolution of Electronic Skin (E‐skin): A Brief History, Design Considerations, and Recent Progress. Advanced materials 2013, 25 (42), 5997–6038. (7) Chortos, A.; Bao, Z. Skin-Inspired Electronic Devices. Materials Today 2014, 17 (7), 321–331. (8) Wang, C.; Xia, K.; Zhang, M.; Jian, M.; Zhang, Y. An All-Silk-Derived Dual-Mode ESkin for Simultaneous Temperature–Pressure Detection. ACS applied materials & interfaces 2017, 9 (45), 39484–39492. (9) Chen, Y.; Lu, B.; Chen, Y.; Feng, X. Breathable and Stretchable Temperature Sensors Inspired by Skin. Scientific reports 2015, 5, 11505. (10) Zhao, S.; Zhu, R. Electronic Skin with Multifunction Sensors Based on Thermosensation. Advanced Materials 2017, 29 (15), 1606151.


15


Page 16 of 25

(11) Zhang, F.; Zang, Y.; Huang, D.; Di, C.; Zhu, D. Flexible and Self-Powered Temperature– Pressure Dual-Parameter Sensors Using Microstructure-Frame-Supported Organic Thermoelectric Materials. Nature communications 2015, 6, 8356. (12) Jin, M. L.; Park, S.; Lee, Y.; Lee, J. H.; Chung, J.; Kim, J. S.; Kim, J.-S.; Kim, S. Y.; Jee, E.; Kim, D. W. An Ultrasensitive, Visco‐Poroelastic Artificial Mechanotransducer Skin Inspired by Piezo2 Protein in Mammalian Merkel Cells. Advanced Materials 2017, 29 (13), 1605973. (13) Zang, Y.; Zhang, F.; Huang, D.; Gao, X.; Di, C.; Zhu, D. Flexible Suspended Gate Organic Thin-Film Transistors for Ultra-Sensitive Pressure Detection. Nature communications 2015, 6, 6269. (14) Gao, N.; Zhang, X.; Liao, S.; Jia, H.; Wang, Y. Polymer Swelling Induced Conductive Wrinkles for an Ultrasensitive Pressure Sensor. ACS Macro Letters 2016, 5 (7), 823–827. (15) Wang, X.; Xiong, Z.; Liu, Z.; Zhang, T. Exfoliation at the Liquid/Air Interface to Assemble Reduced Graphene Oxide Ultrathin Films for a Flexible Noncontact Sensing Device. Advanced Materials 2015, 27 (8), 1370–1375. (16) Kim, S. Y.; Park, S.; Park, H. W.; Park, D. H.; Jeong, Y.; Kim, D. H. Highly Sensitive and Multimodal All‐carbon Skin Sensors Capable of Simultaneously Detecting Tactile and Biological Stimuli. Advanced Materials 2015, 27 (28), 4178–4185. (17) Tee, B. C.; Wang, C.; Allen, R.; Bao, Z. An Electrically and Mechanically Self-Healing Composite with Pressure-and Flexion-Sensitive Properties for Electronic Skin Applications. Nature nanotechnology 2012, 7 (12), 825–832. (18) Benight, S. J.; Wang, C.; Tok, J. B.; Bao, Z. Stretchable and Self-Healing Polymers and Devices for Electronic Skin. Progress in Polymer Science 2013, 38 (12), 1961–1977. (19) Hua, Q.; Sun, J.; Liu, H.; Bao, R.; Yu, R.; Zhai, J.; Pan, C.; Wang, Z. L. Skin-Inspired Highly Stretchable and Conformable Matrix Networks for Multifunctional Sensing. Nature communications 2018, 9 (1), 244. (20) Segev-Bar, M.; Landman, A.; Nir-Shapira, M.; Shuster, G.; Haick, H. Tunable Touch Sensor and Combined Sensing Platform: Toward Nanoparticle-Based Electronic Skin. ACS applied materials & interfaces 2013, 5 (12), 5531–5541. (21) Tian, H.; Shu, Y.; Wang, X.-F.; Mohammad, M. A.; Bie, Z.; Xie, Q.-Y.; Li, C.; Mi, W.T.; Yang, Y.; Ren, T.-L. A Graphene-Based Resistive Pressure Sensor with Record-High Sensitivity in a Wide Pressure Range. Scientific reports 2015, 5, 8603. (22) Saudabayev, A.; Varol, H. A. Sensors for Robotic Hands: A Survey of State of the Art. IEEE Access 2015, 3, 1765–1782. (23) Sekitani, T.; Zschieschang, U.; Klauk, H.; Someya, T. Flexible Organic Transistors and Circuits with Extreme Bending Stability. Nature materials 2010, 9 (12), 1015.


16

Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Sekitani, T.; Yokota, T.; Zschieschang, U.; Klauk, H.; Bauer, S.; Takeuchi, K.; Takamiya, M.; Sakurai, T.; Someya, T. Organic Nonvolatile Memory Transistors for Flexible Sensor Arrays. Science 2009, 326 (5959), 1516–1519. (25) Someya, T.; Sekitani, T.; Iba, S.; Kato, Y.; Kawaguchi, H.; Sakurai, T. A Large-Area, Flexible Pressure Sensor Matrix with Organic Field-Effect Transistors for Artificial Skin Applications. Proceedings of the National Academy of Sciences of the United States of America 2004, 101 (27), 9966–9970. (26) Noguchi, Y.; Sekitani, T.; Someya, T. Organic-Transistor-Based Flexible Pressure Sensors Using Ink-Jet-Printed Electrodes and Gate Dielectric Layers. Applied physics letters 2006, 89 (25), 253507. (27) Takei, K.; Takahashi, T.; Ho, J. C.; Ko, H.; Gillies, A. G.; Leu, P. W.; Fearing, R. S.; Javey, A. Nanowire Active-Matrix Circuitry for Low-Voltage Macroscale Artificial Skin. Nature materials 2010, 9 (10), 821–826. (28) Mannsfeld, S. C.; Tee, B. C.; Stoltenberg, R. M.; Chen, C. V. H.; Barman, S.; Muir, B. V.; Sokolov, A. N.; Reese, C.; Bao, Z. Highly Sensitive Flexible Pressure Sensors with Microstructured Rubber Dielectric Layers. Nature materials 2010, 9 (10), 859–864. (29) Schwartz, G.; Tee, B. C.-K.; Mei, J.; Appleton, A. L.; Kim, D. H.; Wang, H.; Bao, Z. Flexible Polymer Transistors with High Pressure Sensitivity for Application in Electronic Skin and Health Monitoring. Nature communications 2013, 4, 1859. (30) Sun, Q.; Kim, D. H.; Park, S. S.; Lee, N. Y.; Zhang, Y.; Lee, J. H.; Cho, K.; Cho, J. H. Transparent, Low‐Power Pressure Sensor Matrix Based on Coplanar‐Gate Graphene Transistors. Advanced Materials 2014, 26 (27), 4735–4740. (31) Inaba, A.; Yoo, G.; Takei, Y.; Matsumoto, K.; Shimoyama, I. A Graphene FET Gas Sensor Gated by Ionic Liquid. In Micro Electro Mechanical Systems (MEMS), 2013 IEEE 26th International Conference on; IEEE, 2013; pp 969–972. (32) Cho, S. H.; Lee, S. W.; Yu, S.; Kim, H.; Chang, S.; Kang, D.; Hwang, I.; Kang, H. S.; Jeong, B.; Kim, E. H. Micropatterned Pyramidal Ionic Gels for Sensing Broad-Range Pressures with High Sensitivity. ACS applied materials & interfaces 2017, 9 (11), 10128–10135. (33) Zhang, S.; Wang, F.; Peng, H.; Yan, J.; Pan, G. Flexible Highly Sensitive Pressure Sensor Based on Ionic Liquid Gel Film. ACS Omega 2018, 3 (3), 3014–3021. (34) Yamada, S.; Sato, T.; Toshiyoshi, H. Pressure Sensitive Ionic Gel-Fets of Extremely High Sensitivity over 2,200 KPa− 1 Operated under 2 V. In Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS), 2017 19th International Conference on; IEEE, 2017; pp 766– 769. (35) Jin, M. L.; Park, S.; Kim, J.-S.; Kwon, S. H.; Zhang, S.; Yoo, M. S.; Jang, S.; Koh, H.-J.; Cho, S.-Y.; Kim, S. Y. An Ultrastable Ionic Chemiresistor Skin with an Intrinsically Stretchable Polymer Electrolyte. Advanced Materials 2018, 30 (20), 1706851.


17


Page 18 of 25

(36) Ghittorelli, M.; Lingstedt, L.; Romele, P.; Crăciun, N. I.; Kovács-Vajna, Z. M.; Blom, P. W. M.; Torricelli, F. High-Sensitivity Ion Detection at Low Voltages with Current-Driven Organic Electrochemical Transistors. Nature Communications 2018, 9 (1), 1441. (37) Pan, L.; Chortos, A.; Yu, G.; Wang, Y.; Isaacson, S.; Allen, R.; Shi, Y.; Dauskardt, R.; Bao, Z. An Ultra-Sensitive Resistive Pressure Sensor Based on Hollow-Sphere Microstructure Induced Elasticity in Conducting Polymer Film. Nature communications 2014, 5, 3002. (38) Choong, C.-L.; Shim, M.-B.; Lee, B.-S.; Jeon, S.; Ko, D.-S.; Kang, T.-H.; Bae, J.; Lee, S. H.; Byun, K.-E.; Im, J. Highly Stretchable Resistive Pressure Sensors Using a Conductive Elastomeric Composite on a Micropyramid Array. Advanced materials 2014, 26 (21), 3451– 3458. (39) Chou, H.-H.; Nguyen, A.; Chortos, A.; To, J. W.; Lu, C.; Mei, J.; Kurosawa, T.; Bae, W.G.; Tok, J. B.-H.; Bao, Z. A Chameleon-Inspired Stretchable Electronic Skin with Interactive Colour Changing Controlled by Tactile Sensing. Nature communications 2015, 6, 8011. (40) Zhang, Y.; Fang, Y.; Li, J.; Zhou, Q.; Xiao, Y.; Zhang, K.; Luo, B.; Zhou, J.; Hu, B. Dual-Mode Electronic Skin with Integrated Tactile Sensing and Visualized Injury Warning. ACS Appl. Mater. Interfaces 2017, 9 (42), 37493–37500. (41) Cheng, W.; Yu, L.; Kong, D.; Yu, Z.; Wang, H.; Ma, Z.; Wang, Y.; Wang, J.; Pan, L.; Shi, Y. Fast-Response and Low-Hysteresis Flexible Pressure Sensor Based on Silicon Nanowires. IEEE Electron Device Letters 2018, 39 (7), 1069–1072. (42) Cheng, W.; Wang, J.; Ma, Z.; Yan, K.; Wang, Y.; Wang, H.; Li, S.; Li, Y.; Pan, L.; Shi, Y. Flexible Pressure Sensor With High Sensitivity and Low Hysteresis Based on a Hierarchically Microstructured Electrode. IEEE Electron Device Letters 2018, 39 (2), 288–291. (43) Park, I.-J.; Jeong, C.-Y.; Cho, I.-T.; Lee, J.-H.; Cho, E.-S.; Kwon, S. J.; Kim, B.; Cheong, W.-S.; Song, S.-H.; Kwon, H.-I. Fabrication of Amorphous InGaZnO Thin-Film TransistorDriven Flexible Thermal and Pressure Sensors. Semiconductor Science and Technology 2012, 27 (10), 105019. (44) Yeom, C.; Chen, K.; Kiriya, D.; Yu, Z.; Cho, G.; Javey, A. Large‐Area Compliant Tactile Sensors Using Printed Carbon Nanotube Active‐Matrix Backplanes. Advanced Materials 2015, 27 (9), 1561–1566. (45) Joo, Y.; Yoon, J.; Ha, J.; Kim, T.; Lee, S.; Lee, B.; Pang, C.; Hong, Y. Highly Sensitive and Bendable Capacitive Pressure Sensor and Its Application to 1 V Operation Pressure‐ Sensitive Transistor. Advanced Electronic Materials 2017, 3 (4), 1600455.


18

Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (a) Schematic of an ultrasensitive, low-power, transistor-based mechanotransducer consisting of IGZO as a semiconductor and micro-pyramidal i-TPU as a gate dielectric. The right box shows the molecular components of i-TPU. (b) FE-SEM images of the flat i-TPU film (left image) and the micro-pyramidal i-TPU film (right image). All scale bars are 10 µm. The height, size, and inter-spacing of the micro-pyramidal patterns is 6.6 µm, 11.2 µm, and 8.8 µm, respectively. (c) Optical micrograph of the IGZO transistor pressure sensor with the microstructured, deformable i-TPU gate dielectric. The scale bar is 1 mm. The IGZO channel shows a channel length of 100 µm and channel width of 1 mm.


19


Page 20 of 25


20

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a) Capacitance vs frequency characteristics of the micro-pyramidal i-TPU over a wide pressure range (0 – 47 kPa). Inset shows the experimental scheme of the devices. (b) Pressure sensing capability that shows a structural advantage of the micro-pyramidal i-TPU compared with the flat i-TPU (The devices are measured at 100Hz). (c) FEM simulation results of the contact area and the resulting effective stress of the micro-pyramidal i-TPU geometry under subsequent pressure change. (d) Comparison between the contact area from the computational data and the capacitance from the experimental data according to applied pressure. (e) Pressure sensing capability of neat TPU with micro-pyramidal structures. (f) Schematic diagram of equivalent circuit of capacitors based on neat TPU and i-TPU films with micro-pyramidal features under voltage bias.


21


Page 22 of 25

Figure 3. (a) Transfer characteristics of the IGZO transistor-based mechanotransducer with the micro-pyramidal i-TPU gate dielectric with a pressure range of up to 50 kPa. (Drain voltage: 2V) Inset shows the schematic of the device. (b) Comparison of pressure response of the transistorand capacitor-based mechanotransducer based on the micro-pyramidal i-TPU dielectric. (c) Schematic of pressure-induced charge accumulation at the contact regime between IGZO


22

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


semiconductor and micro-pyramidal i-TPU gate dielectric. (d) Pressure sensing capability as a function of micro-pyramidal inter-spacing of i-TPU gate dielectric in IGZO transistor-based mechanotransducer. The micro-pyramidal size of i-TPU shows 25 µm for all cases.

Figure 4. (a) Pressure sensing response of IGZO transistor-based mechanotransducer with micro-pyramidal i-TPU dielectric under stepwise pressure conditions (0.8 kPa, 9.9 kPa, 16.3 kPa, 26.7 kPa, and 38 kPa). (b) In-situ repetitive pressure response for IGZO transistor with micropyramidal i-TPU dielectric by reliability test. Under pressure of 50 kPa, 30 cycles were recorded.


23


Page 24 of 25

Comparison of (c) pressure sensitivity and (d) operating voltage reported in various transistor pressure sensors.


24

Page 25 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents Graphic

An ultrasensitive, low-power oxide transistor-based mechanotransducer with microstructured, deformable ionic dielectrics

Sukjin Jang,†,‡,⊥Eunsong Jee,§,⊥ Daehwan Choi,†,‡ Wook Kim,∥ Joo Sung Kim, § Vipin Amoli, § Taehoon Sung, †,‡ Dukhyun Choi∥, Do Hwan Kim*,§ and Jang-Yeon Kwon*,†,‡

We

demonstrate

an

ultrasensitive,

low-power

indium-gallium-zinc-oxide

(IGZO)

transistor-based mechanotransducer modulated by visco-poroelastic and ionic thermoplastic polyurethane (i-TPU) with micro-pyramidal structure, while consistently maintaining a high sensitivity of 43.6 kPa-1over a wide range of pressures from 5 kPa to 50 kPa. We believe that ionic transistor-type mechanotransducer will be an effective way to perceive external tactile stimuli over a wide pressure range even under low-power (< 4V), which might be one of e-skins to directly emulate the tactile sensing capability of human skin.


25

An ultrasensitive, low-power oxide transistor-based ... - ACS Publications

Recommend Documents