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Chemically Designed Metallic/Insulating Hybrid Nanostructures with Silver Nanocrystals for Highly Sensitive Wearable Pressure Sensors Haneun Kim, Seung-Wook Lee, Hyungmok Joh, Mingi Seong, Woo Seok Lee, Min Su Kang, Jun Beom Pyo, and Soong Ju Oh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15566 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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ACS Applied Materials & Interfaces

Chemically designed metallic/insulating hybrid nanostructures with silver nanocrystals for highly sensitive wearable pressure sensors

Haneun Kim†, Seung-Wook Lee‡, Hyungmok Joh†, Mingi Seong†, Woo Seok Lee†, Min Su Kang†, Jun Beom Pyo§ and Soong Ju Oh*† Department of Materials Science and Engineering† & Department of Semiconductor Systems Engineering‡, Korea University, Seoul 02841, Republic of Korea Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea§

KEYWORDS: wearable pressure sensors, nanocrystals, hybrid nanostructures, solution processes, interface engineering

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ABSTRACT: With the increase of interest in wearable tactile pressure sensors for e-skin, researches to make nanostructures to achieve high sensitivity have been actively conducted. However, limitations such as complex fabrication processes using expensive equipment still exist. Herein, simple lithography-free techniques to develop pyramid-like metal/insulator hybrid nanostructures utilizing nanocrystals (NCs) are demonstrated. Ligand-exchanged and unexchanged silver NC thin films are used as metallic and insulating components, respectively. The interfaces of each NC layer are chemically engineered to create discontinuous insulating layers, i.e. spacers for improved sensitivity, and eventually to realize fully solution-processed pressure sensors. Device performance analysis with structural, chemical, and electronic characterization and conductive atomic force microscopy study reveal that hybrid nanostructure based pressure sensor shows an enhanced sensitivity of higher than 500 kPa-1, reliability and low power consumption with a wide range of pressure sensing. Nano/micro hierarchical structures are also designed by combining hybrid nanostructures with conventional microstructures, exhibiting further enhanced sensing range and achieving a record sensitivity of 2.72 × 104 kPa-1. Finally, all-solution-processed pressure sensor arrays with high pixel density, capable of detecting delicate signals with high spatial selectivity much better than the human tactile threshold are introduced.


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1. INTRODUCTION Pressure sensors detect the presence, intensity as well as the position of a touch or touches. Recently, interest in pressure sensors has been continuously increasing as they serve as an essential component in electronic textiles, flexible touch screens, and soft robotics.1–5 They are particularly important in fields such as healthcare and bioelectronic applications, as they can readily collect physical signals around us all day without any effort and this information can be used to diagnose or prevent disease. To realize these applications, cheap and simple pressure sensors with high performance are needed.6 As an effort to improve the performance, pressure sensors with a variety of mechanism such as capacitance,7–10 piezo-electricity,11–13 or piezoresistivity14–17 have been investigated. Among them, piezoresistive pressure sensors have been shown to be promising for practical sensing applications because of their ease of fabrication, simple read-out mechanism, high sensitivity, and high pixel density.18,19 Piezoresistive sensors work by transducing pressure changes into a change in resistance. When pressure is applied to materials, the resistivity of the materials can change due to changes in band structures, or the resistance of the materials can vary due to changes in sensor structure or geometry.6,20–22 While there is an intrinsic limit to changing the amount of resistivity by applying pressure, it is possible to increase the amount of resistance change in response to pressure by sensor geometry design. One of the most common methods to devise a piezoresistive sensor with a large variation of resistance is to utilize nano/microscale uneven or bumpy structures such as pyramids. In these pyramid structures, an airgap is created between two conductive layers, reducing the contact area between them. As pressure is applied, the contact area rapidly increases and the contact resistance decreases. Therefore, these nano/microscale structures improve the



sensitivity, which is defined as the ratio of current (inversely proportional to resistance) change to pressure.23 Many researchers have introduced various ways to create these structures using nano/micro scale molds. In general, a silicon mold is made into a desired nano/microstructure through complicate processes such as e-beam lithography and photolithography with wet and dry etching steps.23–26 Afterwards, printing or nanoimprinting techniques are used to transfer the shape of the mold to the active layers, resulting in nano and micro-sized pillar, hole, dome, pyramids structures.27,28 While these previous examples successfully enhance the sensitivity by creating nanostructures, they rely on complicated, time-consuming, expensive, and/or toxic techniques to fabricate the structures. Furthermore, active layers and electrodes are usually formed by using processes that require high-vacuum conditions such as chemical vapor deposition, evaporation and sputtering. These are great barriers to realize low-cost flexible sensors.29–31 Alternatives under study are solution processes such as spin coating, printing, roll-to-roll processes, etc. Materials mainly used in the solution processes include 0-D nanocrystals (NCs),32–34 1-D nanowires or nanotubes,35,36 2-D nanosheets or graphene,19,37,38 and organic materials.23,39

Among theese materials, 0-D NCs are one of the most promising candidates as i)

their properties can be precisely controlled by size, shape, composition engineering, or by surface modifications;40 ii) their electronic properties and stability are outstanding compared to organic materials;29,41 and iii) wet-chemical based large scale synthesis and large area device fabrication are possible.42,43 Unfortunately, there are only few examples of fabricating pressure sensors through all-solution processes due to the following reasons. First and most important, techniques to fabricate uneven


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and bumpy structures such as nanopyramids using only solution processing have never been investigated. Second, there are some limitations when implementing orthogonal processes to complete one sensor device. These include poor adhesion between solution-processed layers and chemical instability in each layer, resulting in delamination of layers when additional top layers are formed through solution processes, especially on flexible and stretchable substrates of polydimethylsiloxane (PDMS). Last, it is even more difficult to fabricate multi-array sensors, which is a limitation when attempting to mimic real human skin. Electronic skin require multiarray tactile pressure sensors with delicate patterns having a spatial resolution less than 1-2 mm to match the sensitivity capability of human skin.6 However, techniques for multi-array sensors with elaborate patterning using only-solution processes are still under study. In this study, we developed lithography-free and all-solution-based techniques to design pseudo pyramid, metal/insulator hybrid nanostructures as pressure sensing layers, in order to address the problems mentioned earlier. Ligand exchanged, conductive silver (Ag) NCs were used as metallic components while as-synthesized insulating Ag NCs with long carbon chains were used as insulating components. We introduce a simple surface and interface engineering strategy to build partially covered and discontinuous insulating Ag NC thin films on top of metallic Ag NC thin films. The hybrid nanostructure resembles nano-pyramid strucutures where conductance and thickness vary as a function of position. Hybrid ligand exchange processes as well as chemical treatment enabled form mechanically stable, sub-100 nm scale metal/insulator hybrid nanostructures over a larger area on top of PDMS. Conductive atomic force microscopy (C-AFM) as well as chemical, optical, and electrical measurements were conducted to investigate the properties of our hybrid nanostructures. We demonstrated that chemically



designed metal/insulator hybrid nanostructures greatly enhanced the sensitivity of pressure sensors and these sensor devices showed great reliability and stability at low power consumption. We were able to combine our hybrid nanostructures with conventional microstructures to create nano/micro hierarchical structures that further enhance the sensing range and sensitivity of the pressure sensors to 2.72 × 104 kPa-1, which to the best of our knowledge is the highest among current wearable pressure sensors. Finally, we fabricated a wearable multi-array tactile pressure sensor using an all-solution process which showed higher pressure and spatial sensitivity compared to human sensing capabilities. Our unconventional fabrication methods provide a pathway to fabricate low cost, high performance, and multifunctional sensors to be used in electronic skin.


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2. EXPREIMENTAL SECTION 2.1. Materials. Silver nitrate (AgNO3, > 99.9%) was obtained from Alfa Aeser. Oleic acid (90%) and oleylamine (70%), ammonium chloride (NH4Cl, 99.998%), 1,2-ethanedithiol (EDT), (3-aminopropyl) triethoxysilane (APTES, 99%), and methanol (99.8%) were purchased from Sigma-Aldrich Co. All reagents were used without further purification. PDMS (Sylgard 184 silicone elastomer kit) was purchased from the Dow Corning Corporation. polyethylene terephthalate (PET) films with thicknesses of 100 (SKC films) and 6 μm (Mylar® thin-film) were used as flexible substrates. 2.2. Synthesis of Ag NCs. Ag NCs were synthesized using previously reported methods with a slight modification.30,42,44 AgNO3 (1.7 g), oleic acid (45 mL), and oleylamine (5 mL) were added to a three-neck flask and mixed vigorously by magnetic stirring. Afterwards, the solution was degassed at 70 ℃ for 1.5 h to remove moisture and oxygen. After degassing, the solution temperature was increased to 180 ℃ at 1 ℃ min-1. After air-cooling to room temperature, the Ag NCs were washed by centrifugation at 5000 rpm for 5 min with toluene and ethanol two or three times. The precipitated Ag NCs were dispersed in octane at various concentrations (50–200 mg mL-1). 2.3. Substrate preparation. Glass substrates were used for ultraviolet-visible (UV-vis) and Fourier transform-infrared (FT-IR) spectroscopy and electrical characterization, and Si wafers were used for scanning electron microscopy (SEM) and C-AFM. They were sequentially sonicated in acetone, isopropanol, and deionized water for 5 min to clean their surfaces. Next, UV-ozone treatment was performed to form –OH groups on the surface. Finally, they were immersed in 5 vol% 3-mercaptopropyltrimethoxysilane solution in toluene for the self-assembled



monolayer (SAM).30,44 PDMS substrates used in fabricating pressure sensors were prepared by mixing Sylgard® 184 Silicone Elastomer base and curing agent with the ratio of 10:1. After degassing for 1.5h, the mixture was spin-coated at 100 rpm and heat treated at 70 ℃ for 1h. When peeled off after heat treatment, a PDMS layer with a thickness of 500 μm was obtained. 2.4. Ligand exchange. NH4Cl ligand-exchange solutions were prepared at a concentration of 30 mM in methanol and EDT ligand-exchange solutions were prepared at a concentration of 0.01 mM in acetonitrile. Ag NC thin films were immersed in this solution for 60 s and then cleaned thoroughly with the mother solvent. 2.5. Fabrication of the pressure sensors. PDMS substrates were prepared through the method mentioned earlier. To enhance the adhesion between the substrates and the active layer, we performed pretreatment as the following. First, PDMS substrates were exposed to UV-ozone for 5 min and immersed in 5 vol% APTES solution for a few seconds. Next, as-synthesized Ag NCs at 100 mg mL-1 were spin-coated at a speed of 3000 rpm and treated with a 0.01 M EDT ligands solution for 1 min. After sufficient washing of the surfaces with methanol, the active layer was fabricated by spin-coatingas-synthesized Ag NCs with a concentration of 200 mg mL-1 at 1000 rpm and immersed in a NH4Cl solution. The same procedure was repeated once more to obtain sufficient conductivity. Finally, the nanoscale spacers were fabricated by spin coating assynthesized Ag NCs with the concentration of 10, 50, 100, 200 mg mL-1 at 1000 rpm. 2.6. Characterization Electrical characterization. A probe station (Model MST-4000A, MSTECH) was used to measure the resistance of the Ag NC thin films, and a conductive atomic force microscope (Model XE100, PSIA) was used to map the conductivity of Ag NC thin films.


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Optical characterization. An UV-vis spectroscope (Model Cary 5000, Agilent Technologies) and an FT-IR spectroscope (Model LabRam ARAMIS IR2, Horiba Jobin Yvon) were used to characterize the optical properties of the Ag NC thin film before and after exchanging the ligands. Structural characterization. A field emission transmission electron microscope (FETEM, Model Tecnai G2 F30ST, Korea Basic Science Institute) was used for the structural characterization of the Ag NCs. Scanning electronic microscopy energy dispersive X-ray (SEMEDX, Model Hitachi S-4300, Hitachi High technologies America, inc.) was used for analysis of Ag NC thin films. Pressure sensor device performance characterization. The electrical properties in accordance with pressure were measured with a multimeter (Model Fluke 289, Fluke Corporation) and a probe station (Model MST-4000A, MSTECH). Various pressures were applied by placing different weights or using homemade pressure applying devices on the pressure sensor. After integrating the top and bottom electrodes, a protective layer was placed on top to prevent the PDMS from sticking to the weight.



3. RESULTS AND DISCUSSION To design solution processable piezoresistive sensor devices, we chose Ag NC thin films as an active material for our sensor devices due to the following reasons: i) Ag is relatively cheap and is one of the most conductive materials; ii) Ag NCs can be easily synthesized by chemical methods on a large scale; iii) Ag NC solution inks enable large area fabrication of Ag NC thin films through all-solution processes, which is compatible with flexible substrates; and iv) the properties of Ag NCs can be precisely controlled in a wide range by engineering the surface ligands and chemistry.40,45–47


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Figure 1. (A) TEM image of as-synthesized Ag NCs (scale bar in the image is 5 nm), (B) FT-IR spectra, (C) UV-vis absorption spectra, and (D) current-voltage (I-V) curves of Ag NC thin film (black) before and after (blue) NH4Cl and (red) EDT treatment.

As-synthesized Ag NCs were sphere-shaped particles with 3.73 ± 0.45 nm diameter, as seen in the TEM image of Figure 1A. Ag NC thin films are formed by spincoating the NC solutions on glass or silicon wafer substrates for characterization. As-synthesized Ag NCs are encapsulated



by long carbon chain of oleate ligands, resulting in long interparticle distances. Consequently, asdeposited Ag NC thin films are highly insulating, indicating that these are promising candidates for insulating layers.44 On the other hand, to create metallic layers of electrically conductive Ag NC thin films, we conducted a ligand exchange process that replaced the long original ligands with shorter ones by dipping them in a solution containing either NH4Cl or EDT. After treatment with NH4Cl, the interparticle distances were greatly reduced and the Ag NCs were sintered and grown into larger ones, as seen in Figure S1A. EDT treatment similarly reduced the interparticle distance between the NCs but the sintering phenomenon was not observed (Figure S1B).30,44 To investigate the surface chemistry and optical properties of Ag NC thin films after ligand exchange, FT-IR and UV-vis spectroscopy were performed (Figure 1B and 1C). As seen in the spectrum of Figure 1B, as-synthesized Ag NC thin films showed a peak around 2900 cm-1 corresponding to the CH stretch of oleate ligands.48 After NH4Cl treatment, the peak intensity greatly decreased. EDX analysis shows a peak corresponding to Cl (Figure S1C). These measurements indicate the successful replacement of the original ligands with Cl-.49 Similarly, after EDT ligand exchange, the peak intensity greatly decreased.29 Free S-H peaks around 2500 ~ 2600 cm-1 were not observed on EDT-treated Ag NC thin films, which means that Ag-S bonds are directly formed.50–52 In the UV-vis spectrum of as-synthesized Ag NC thin films, a peak was shown at around 440 nm corresponding to the localized surface plasmon resonance of the Ag NCs, as seen in Figure 1C.40 After treatment with NH4Cl or EDT, the peak was dramatically quenched and shifted to 380 and 550 nm, respectively, indicating strong electronic coupling with reduced interparticle distances and coinciding with TEM analysis (Figure S1A and S1B).


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The electrical properties of the Ag NC thin films with different ligand treatments were examined with two terminal I-V curve measurements (Figure 1D). The as-synthesized Ag NC thin films showed electrically insulating properties due to the long organic ligands (black line in Figure 1D), while NH4Cl-treated Ag NC films showed a very high conductivity of (1.22 ± 0.36) × 104 S cm-1, in good agreement with our previous reports.30,44 EDT-treated Ag NC thin films also exhibited increased conductivity that reached a level of 2.83 ± 0.05 S cm-1, which is lower than NH4Cl-treated Ag NC thin films. This is in accordance with the fact that EDT is longer than Cl, and our observation that NH4Cl-treated Ag NC thin films exhibit a fusion of Ag NCs. The high conductivity of the NH4Cl-treated Ag NC thin films suggests that it is a great candidate for the active layers in piezoresistive sensor devices.



Figure 2. (A) Schematic, (B) working principles and (C) fabrication process of the Ag NC-based pressure sensors.

Ag NC thin films, showing varying properties with different ligand treatment, are matched with the function of each part of our metal/insulator hybrid nanostructure pressure sensor so that it can play an optimal role. To obtain a sufficient current to sense the signal, NH4Cl-treated Ag NCs with high conductivity were used to build the active conducting metallic layers, and assynthesized Ag NCs with high resistivity were used to create insulating spacers between the two electrodes (Figure 2A). The insulating spacers are designed to have discontinuous and partially covered structures, forming effective metal/insulator hybrid nanostructures. As seen in Figure 2B, these spacers form air gaps between top and bottom electrode reducing the contact area. For this


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reason, when pressure was applied, the contact area increased more rapidly when including a spacer component, improving the sensitivity. The overall fabrication process is shown in Figure 2C. First, 500 μm thick PDMS films were prepared by curing a spincoated mixture of silicon elastomer and curing agent. Next, conductive Ag NC thin films were deposited on PDMS substrates. However, forming conductive layers on top of PDMS was difficult as the surface of PDMS is known to be very stable.53 Although the Ag NC layers were well formed on top of PDMS by spin-coating, NH4Cl treatment caused delamination of the Ag NC thin films. As this phenomenon does not occur on silicon or glass wafers, we attributed the delamination to weak adhesion between the Ag NC thin film and the stable PDMS surface. To avoid delamination, we developed a novel chemical treatment strategy by investigating and engineering the surface and interface of each layer. We first conducted SAM treatments on PDMS substrates. The PDMS substrates were treated with UV ozone to substitute the methyl group at the PDMS surface with a hydroxyl group, and then APTES solution to form the SAM.54 This significantly suppressed the delamination phenomenon during NH4Cl treatment although slight delamination was still observed. Additionally, we conducted an EDT treatment on the Ag NC film and found that it did not cause any delamination. We believe that this is because EDT treatment is a milder process than the NH4Cl treatment since the former did not change the morphology of the Ag NCs while the latter significantly and abruptly modified it, as shown in TEM images (Figure S1A and S1B). Furthermore, we believe that the interaction between the amine group of APTES and the thiol group of EDT helps avoid delamination and that small amounts of remaining oleic acid can help the adhesion.55 Hence, we used EDT-treated Ag NC



thin films as an adhesive layer between the NH4Cl-treated Ag NC thin films and the APTEStreated PDMS, and found that the NH4Cl-treated Ag NC thin films deposited well on the predeposited EDT-treated Ag NC thin films on the PDMS, thereby completing the Ag NC thin film top electrode fabrication on PDMS. Adhesion tests confirmed the stability and durability of the Ag NC thin film electrodes combined with an adhesion layer (Figure S2). The metal/insulator hybrid nanostructure was completed by creating partially covered as-synthesized Ag NC thin films on the top of NH4Cl-treated Ag NC thin films, by spincoating as-synthesized Ag NC solutions at optimized conditions (see below). The structure made using the aforementioned method was used as the top electrode of the pressure sensor. Bottom electrodes were prepared by patterning beforehand in the form of two conductive electrodes with a 1 mm separation gap using various conductive materials such as silver paste, indium tin oxide (ITO) electrodes, gold (Au) thin films fabricated via an e-beam evaporator with a shadow mask, and Ag NC thin films created by solution-based photolithography. These bottom layers showed similar performance (see discussion in the Supporting Information and Table S1). ITO electrodes were used for general characterization as they are easily available. However, we emphasize the fact that the Ag NC thin film electrodes were made by solution processes, allowing the fabrication of all solution processed sensors. In addition, the highest sensitivity was achieved when Ag NC thin film bottom electrodes were used, as Ag NC thin films have a rougher surface compared to other electrodes. It reduced the contact area between top and bottom electrode, resulting in the highest sensitivity. The integration of the top and bottom electrodes completed the pressure sensor fabrication.


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Figure 3. (A) I-V curves of the Ag NC-based pressure sensor (top) before and (bottom) after forming nanoscale spacers (black) before and (red) after applying a pressure of 10 kPa. (B) The change in off current and the on-off ratio of the metal/insulator hybrid nanostructured pressure sensor as a function of insulating Ag NC concentration. (C) C-AFM image of the hybrid Ag NC thin film electrodes prepared with different insulating Ag NC concentration (from left to right: 0, 50, 100, and 200 mg mL-1, scale bar: 2 μm).



To investigate the effect of the nanoscale spacer with as-synthesized Ag NCs, we first compared the I-V curve before and after forming a nanoscale spacer with as-synthesized Ag NCs with the concentration of 200 mg mL-1 (Figure 3A). We monitored the current change upon applying a pressure of 10 kPa using a homemade pressure applying/measuring device and defined the current with and without pressure as the on-current (Ion) and off-current (Ioff), respectively. Before forming the spacer, large contact areas between the top and bottom electrodes were formed, creating an electrical pathway from the top to the bottom electrodes, as seen in Figure 3A (top). The Ioff was 0.12 mA under 0.1 V and increased up to 2.11 mA after applying pressure, resulting in a Ion/ Ioff of 1.71 × 101 . After coating with as-synthesized Ag NCs, spacers and gaps between the two conducting layers made the device less conductive. Ioff was decreased to 0.491 μA and Ion was 1.03 mA under 0.1 V, resulting in an increase of Ion/ Ioff up to 2.1x103 (Figure 3A (bottom)). For a more detailed investigation of the effect of the nanoscale spacer, the coverage of the insulating layer was tuned by changing the concentration of Ag NCs. As-synthesized Ag NCs dispersed in octane at a concentration of 50, 100, or 200 mg mL-1 were used. We measured the Ion, Ioff and calculated the Ion/Ioff ratio, as can be seen in Figure 3B and S3A. As the concentration increased, Ioff decreased significantly whereas Ion decreased much more slowly, resulting in an abrupt increase in the Ion/Ioff ratio. The spacer made with 200mg mL-1 of Ag NC solution showed the highest Ion/Ioff ratio of (2.06 ± 0.48) × 103 . Therefore, this optimized condition was used to fabricate pressure sensor in this work. Note that when Ag NC solutions were used at a concentration higher than 300 mg mL-1 or when a low spin-coating rpm speed was used, relatively thick insulating Ag NC layers were formed, leading to poor and unstable performance. 18 ACS Paragon Plus Environment

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To directly examine the morphology and structures of nanoscale spacers, we conducted CAFM measurements and mapped the conductance distribution of the hybrid NC thin films (Figure 3C). In the case of electrodes without a spacer, the device showed a quite uniform and high current level, but after forming the spacer, the current level fluctuated between 0 and 800 nA. As the concentration of insulating Ag NCs increased, the portion of the area exhibiting zero current and the standard deviation of current increased. 0, 50, 100, 200 mg mL-1

Ag NC thin

film coated active layers showed currents of (5.52 ± 1.05) × 102 , (4.77 ± 1.43) × 102 , (2.42 ± 1.70) × 102 , (1.63 ± 1.81) × 102 nA, respectively. The C-AFM image clearly shows discontinuous, partially covered insulating layers at the nanometer scale. FT-IR spectra of the thin films with different concentrations of Ag NC forming spacer also offered direct evidence of this phenomenon (Figure S3B). Hence, we successfully demonstrated that hybrid metalinsulating layers were formed at the nanometer scale over a large area by using only simple spincoating methods. We believe that partially covered insulating layers were effectively formed, as the underlying bi-layers of NH4Cl and EDT treated Ag NC thin films do not have uniform thickness, but showed large thickness deviation, as seen in AFM image (Figure S4). When using the spin-coating method, the thickness deviation increases as the number of layer increases. Therefore, it is difficult to obtain a fully covered insulating layer on top of a layer having such a large thickness variation. However, we took advantage of this large thickness deviation and eventually achieved discontinuous and partially covered insulating layers as a novel nanoscale structure that acts as a spacer.



Figure 4. (A) Relative current change as a function of applied pressure on pressure sensors fabricated (red) with and (black) without nanoscale spacers. (B) Response/relaxation time of the pressure sensor (inset shows the pressure sensing behavior at the microsecond scale). (C) Cycle testing of the pressure sensor with an applied pressure of 0.15 kPa.

To evaluate the performance of the pressure sensor, the sensitivity of the hybrid nanostructure device was measured. We plotted the ratio of the current change to the initial current (current change ratio, ΔI/Ioff = (Ion-Ioff)/Ioff) as a function of pressure before (black line in Figure 4A) and 20 ACS Paragon Plus Environment

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after (red line in Figure 4A) forming the nanoscale spacer. The log scale plot is shown in Figure S5A. The sensitivity ((ΔI/Ioff)/ΔP) is then calculated from the slope of the graph (Figure 4A).56 The minimum pressure that could be detected by the flat pressure sensor without a nanoscale spacer was 0.01 kPa with a ΔI/Ioff of 9.95 × 10−1 . The ΔI/Ioff increased as the pressure increased, and became almost saturated at around 5 kPa, indicating that the maximum pressure sensing level was 5 kPa. The sensitivity was 5.59 kPa-1 in the range of 0.01 to 5 kPa with an average sensitivity of 4.16 ± 2.72. However, the sensitivity greatly decreased to 0.1 kPa-1 over 5 kPa, exhibiting nearly saturated behavior. By introducing nanoscale spacers, variation in the current greatly increased, as shown in the red line in Figure 4A. The minimum pressure sensing capability was 0.01 kPa, and the ΔI/Ioff was consequentially 8.29 × 101 . The maximum pressure sensing capability with the hybrid nanostructure was 15 kPa, which is higher than the flat sensor. The sensitivity of the hybrid nanostructure pressure sensor was 530.93 kPa-1 (321.4±91.3 on average) in the range of 0.01~5 kPa, which is 100 times higher than that of the flat pressure sensor, and 35.14 kPa-1 in the range of 5~20 kPa, which is 300 times higher than that of the flat sensor (Table S1). Above 20 kPa, the top and bottom electrodes were completely in contact with each other, leading to the saturation of Ion. We believe that by engineering the shape and coverage of the spacer, we can further improve the pressure sensing range. We also investigated the effect of low voltage on the performance of the pressure sensor to realize low power pressure sensors. It can be seen that 0.05, 0.1, and 0.15 kPa are distinguishable at 1, 0.1, 0.01, and 0.001 V (Figure S5B). Since the current is measured as tens of microamperes under 0.001 V, it operates at around several nW of electric power. By implementing sensors that 21 ACS Paragon Plus Environment


can operate at very low voltages, low-power device development is possible, which would be advantageously used in wireless electronic sensor systems. Next, we examined the reliability and durability as well as the response/relaxation time of our sensors. The pressure sensor responded within a few milliseconds (100 ms for response time, 60 ms for relaxation time) when applying or releasing a pressure of 0.1 kPa (Figure 4B). Furthermore, deformation and recovery were shown to be possible many times for various pressures as seen in the supporting information, Figure S5C. Plots exhibit constant Ion and Ioff under various applied pressures (0.01 to 10 kPa). While a pressure of 0.15 kPa was applied over 200 times on the pressure sensor, both Ion and Ioff were maintained (Figure 4C). Our sensor demonstrated not only high sensitivity, but also quick response time, excellent reliability, and stability.


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Figure 5. (A) Fabrication process of the nano/micro hierarchical structure based pressure sensors. (B) I-V curve when applying a pressure of (black) 0, (blue) 0.01, (green) 0.1, (yellow) 1, and (red) 10 kPa. (C) Current change ratio corresponding to the various pressures. (D) Response/relaxation time when applying a pressure of 10 kPa onto the sensor.

To further improve the performance of our sensor, particularly to further enhance the sensitivity and achieve a wider range of measurement pressure, we devised a nano/micro hierarchical structure based pressure sensor. It consists of both nanoscale and microscale metal/insulator structures by exploiting our hybrid nanospacer and conventional microscale



periodic jagged structures. Microstructures were formed using PDMS with a conventional notchshaped mold with a width of 1 mm and a depth of 0.5 mm (Figure 5A). We expected this structure to further improve the sensitivity by changing the contact area between the two electrodes, especially when high pressure was applied. An active layer was formed in the same manner as described previously on the prepared substrate. Bottom electrodes consisted of Ag NC thin films to realize all-solution processed pressure sensors To characterize the nano/micro hierarchical structure based pressure sensor, we mapped I-V curves at various pressures (Figure 5B). When pressure was not applied, Ioff at 0.1 V was around 20 nA, which is an order of magnitude lower than that of the hybrid nanostructure at 491 nA. This is attributed to the periodic jagged microstructure reducing the contact area between the top and bottom electrodes. As applied pressure increased, the contact area increased and Ion gradually increased. At a pressure of 10 kPa, the current continuously increased to 0.226 mA, leading to Ion/Ioff of 8.8× 103 . This is about 4 times higher than the hybrid nanostructured pressure sensor without microstuctures. By introducing the microstructure, the pressure sensors showed a sharper current change over a wider range. The tendency of current change ratio as a function of pressure was also analyzed (Figure 5C). ΔI/Ioff had the value of 2.81 × 101 at the minimum detectable pressure of 0.01 kPa. As the pressure increased, it increased to 2.01× 105 under 100 kPa with little tendency of saturation. Due to the large increase in ΔI/Ioff, the sensitivity was greatly enhanced. The sensitivity under 5 kPa was 2.72 × 104 kPa-1, which to the best of our knowledge is the highest sensitivity among recently developed wearable pressure sensors. High sensitivity of 4.70× 103 kPa-1, 1.09× 102 kPa-1 are achieved at the range of 5~20 kPa, and over 20 kPa, respectively. On average, nano/micro hierarchical structure based pressure 24 ACS Paragon Plus Environment

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sensor showed a sensitivity of (1.79 ± 0.74) × 104 in the range of 5 kPa or less, (1.60 ± 0.99) × 103 in the range of 5 to 20 kPa and (2.02 ± 0.99) × 102 above 20 kPa (Table S1). We successfully show that by introducing the hierarchical nano/microstructures, not only the pressure detection range widened but also the sensitivity was improved at a pressure range of 0.01~100 kPa. The improved performance is attributed to the effect of both the nanoscale spacers and the microstructure of the PDMS. For example, in the range of pressure under 5 kPa, compared with the nano-only structure with a sensitivity of (4.91 ± 2.70) × 102 kPa-1 and micro-only structure with the sensitivity of (3.44 ± 1.46) × 101 kPa-1, the nano/micro hierarchical structure based pressure sensor showed improved sensitivity of (1.79 ± 0.74) × 104 kPa-1 than both (Table S1 and Figure S6). This indicates that both nanoscale spacers and microstructure contributed to the improvement of the sensitivity. At pressures of 20 kPa or higher, the PDMS substrate was deformed and the new surface came into contact with the lower electrode. This might increase not only the contact area between the two conducting layers, but also the number of nanoscale spacers, resulting in a high sensitivity over a wider pressure range. To evaluate the performance of the nano/micro hierarchical structure based pressure sensor, the response/relaxation time and repeatability were estimated in the same manner as the previous experiment. When a pressure of 10 kPa was applied, the response time was 200 ms and the relaxation time was 50 ms (Figure 5D). The increase in response time compared to previous nano-only pressure sensors is due to the PDMS substrate deformation time. Still, a quite short relaxation time was observed. When the applied pressure is removed, the current returns to the original Ioff value in all of the cases of 0.1, 1, and 10 kPa (Figure S7A and S7B). The experimental data clearly suggests the high sensitivity, robustness and reliability of our sensors. 25 ACS Paragon Plus Environment



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Figure 6. Application of the hybrid pressure sensor. (A) Real-time monitoring of pulse waves. The insets are a photograph of the sensor on the wrist and a magnified plot of the signal (scale bar: 2 cm). (B) An image of a 6 x 6 array pressure sensor and mapping image with distinguishable space and weight (scale bars: 2 cm). (C) The fabrication process of the multiarray tactile pressure sensor. (D) Images of (left) top and (upper middle) bottom electrodes, (lower middle) integrated devices and (right) conformally contacted 4 x 4 multi-array tactile



pressure sensor with 1 mm resolution on human skin (scale bars: 2 mm). (E) A mapping image of current change when applying pressure. By taking an advantage of simple solution processes and high sensitivity, real applications are demonstrated for the pressure sensors (Figure 6). The pressure sensor was placed at the point where the pulse of the wrist can be sensed and the current change ratio was observed at a voltage of 0.1 V. As seen in the inset of Figure 6A, it was capable of detecting a pulse for a long time repeatedly while distinguishing between percussion (P), tidal (T), and diastolic (D) waves.57 As our solution-based method is applicable for large-area fabrication, we fabricated a 6 x 6 array chessboard model (Figure 6B). Figure 6B shows an image and mapping image when a weight of 10 kPa was placed on the cross point of the two electrodes (left) and removed (middle), and when finally weights of 10 kPa and 20kPa were placed apart (right), for which the sensor clearly showed both the position and amount of applied pressure. According to Figure 6A and 6B, the pressure sensor showed high sensitivity in both low-pressure regimes (

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