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Controlling the Resistive Switching Behavior in Starch-Based Flexible Biomemristors Niloufar Raeis-Hosseini and Jang-Sik Lee* Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea S Supporting Information *

ABSTRACT: Implementation of biocompatible materials in resistive switching memory (ReRAM) devices provides opportunities to use them in biomedical applications. We demonstrate a robust, nonvolatile, flexible, and transparent ReRAM based on potato starch. We also introduce a biomolecular memory device that has a starch−chitosan composite layer. The ReRAM behavior can be controlled by mixing starch with chitosan in the resistive switching layer. Whereas starch-based biomemory devices which show abrupt changes in current level; the memory device with mixed biopolymers undergoes gradual changes. Both devices exhibit uniform and robust programmable memory properties for nonvolatile memory applications. The explicated source of the bipolar resistive switching behavior is assigned to formation and rupture of carbon-rich filaments. The gradual set/reset behavior in the memory device based on a starch−chitosan mixture makes it suitable for use in neuromorphic devices. KEYWORDS: starch, chitosan, resistive switching memory, conduction mechanism, biopolymer explore the possibility of using starch as biopolymer flexible ReRAM.20,21 Starch is an inexpensive, abundant, biodegradable, and biocompatible polymer which has been studied as solid polymer electrolyte (SPE) in electrochemical systems.24 Starch with a granular structure, contains a composition of amylose as a linear polymer and amylopectin as a branched polysaccharide composed of chains similar to amylose (Supporting Information, SI, Scheme S1a,b).25 Potato starch in particular contains both tightly- and loosely bound water molecules; it has reasonable ionic conductivity due to the presence of loosely bound water molecules in its crystallite network.26 Chitosan (Scheme S1c) is another ubiquitous biopolymer; it is an abundant waste product of crustacean shells.27 Our previous work has examined Ag-doped chitosan as the active layer in ReRAM devices.20,21 A blend of starch and chitosan can provide a biodegradable composite film because of their excellent miscibility. Incorporation of chitosan into the starch-based film reduces water affinity of the film while improving its mechanical properties by encouraging the formation of intermolecular hydrogen bonds.22,28 The proposed material is harmless and even edible. It is environmentally benign material with disposability and biodegradability at prescribed time. Thus, it is skin-attachable and can get applied in wearable and human body-desorbable devices.

1. INTRODUCTION Flexible nonvolatile memory devices based on organic materials constitute an emerging technology for nanoelectronics.1 Organic materials including polymers and molecules have been evaluated for use in diverse flexible electronic applications such as flexible displays, thin-film transistors, organic photovoltaic cells, electronic skins, and data storage devices.2,3 Utilization of organic electronics is mainly motivated by amenability to large-area production, pliability, room-temperature processability, reasonably low cost, and compatibility with available substrates like plastic, glass, metal foils, and biocompatible substrates.4 Resistance switching random access memory (ReRAM) has advantages of scalability, reliability, low power consumption, and fast switching.5−9 ReRAMs that use organic materials as their active layer have the merits of flexibility,1,10 transparency,11,12 and compatibility with various substrates.2 Polymer-based ReRAM is a unique technology for flexible and large area nonvolatile memory applications. Compared to conventional devices, polymeric memory is inexpensive and easy to fabricate.13−16 Natural organic materials are ubiquitous, inexpensive, and biodegradable, and are candidates to supersede rigid silicon-based electronics.17,18 Development of environmentally benign biodegradable nanoelectronic devices requires use of biopolymers that are electrically and mechanically stable. Polysaccharides like chitin, chitosan,19−22 and cellulose23 have been considered in both transistors, ReRAMs, and synaptic devices. On the basis of successful implementation of polysaccharides in nanoelectronic memory devices, we © XXXX American Chemical Society

Received: February 5, 2016 Accepted: February 26, 2016

A

DOI: 10.1021/acsami.6b01559 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces The objective of this study is to fabricate and characterize flexible and natural nanoscale memory devices based on starch as a candidate for use in bioinspired electro-active polymer materials. We also compare the resistive switching properties of ReRAM devices based on starch and starch−chitosan composite. The progressive set and reset characteristic of starch−chitosan-based biomemristors makes it a potential candidate for analog-based synaptic devices.29,30

2. EXPERIMENTAL SECTION 2.1. Materials and Device Fabrication. ReRAM devices with MIM structure of Au/starch/ITO and Au/starch−chitosan/ITO were fabricated on a PET flexible substrate. The PET substrates were cleaned ultrasonically using acetone, 2-propanol, and distilled water for 15 min, then dried using N2 gas. The cleaned substrates then treated using UV ozone cleaner (JSE, Korea) to achieve high coverage of biopolymers on ITO surface with uniformity. Starch solution was synthesized using 4 wt % potato starch powder (Sigma-Aldrich) in DI water with 86−89% glycerol solution as plasticizer and filler31 at ambient temperature and constant stirring for 1 h. Chitosan with a medium molecular weight (deacetylation degree 75−85%, SigmaAldrich) derived from crab shells was dissolved (1 wt/v %) in 1% (v:v) acetic acid solution in distilled water, and then mixed overnight under ambient temperature and constant stirring at 120 rpm. Both of the solutions were filtered through 0.4-μm PVDF syringe filters. Starch solution was mixed with the chitosan solution at 1:1 (v:v), then the resulting solution was filtered through a 0.2-μm PVDF filters. The starch-based solution or the mixed solution of starch and chitosan were spin-coated on the transparent and flexible substrate at 500 rpm for 5 s then at 1200 rpm for 40 s. The films were dried at ambient temperature overnight and then thermally annealed at 60 °C in a vacuum oven to form starch and starch−chitosan resistive switching layers with a thickness of 100 and 259 nm (Figure S1). Soon after the thin films had been annealed, Au electrodes were patterned using thermal evaporation to make ReRAM devices with a 100-μm diameter. 2.2. Characterization. 2.2.1. Electrical Characterizations. Electrical characteristics of the fabricated devices were analyzed at atmospheric pressure and ambient temperature using a semiconductor parameter analyzer (Keithley 4200SCS, U.S.A.) to apply voltage and to measure current. In a typical test configuration, the samples were placed in a probe station, and bias voltages were applied to Au electrode while the ITO electrode was grounded. The I−V measurements were performed with forward and backward voltage sweeps from +3 V to −4 at 0.05 V/step. 2.2.2. Electrochemical Characterizations. The redox properties of the SPE were measured using a potentiostat (Gamry instruments, Reference 3000, U.S.A.). Cyclic voltammetry (CV) curves were obtained with a Pt sheet as a counter electrode and Ag/AgCl reference electrode, with the desired thin film on an ITO-coated PET as the working electrode. The supporting electrolyte was 0.01 M phosphate buffer solution; the thin film was protected using nafion to keep the SPE thin film from dissolving during the CV measurement 2.2.3. Optical Characterizations. A Cary 100 UV−vis spectrophotometer (Agilent Technologies, CA, U.S.A.) equipped with a transmittance accessory was used to record the optical spectrum of the samples over the wavelength range of 200−800 nm. The transmittance spectra were collected from starch-based and starch− chitosan mixed thin films on the transparent ITO coated substrate.

Figure 1. Schematic illustration of fabricated flexible resistive switching devices. (a) Flexible ReRAM with an Au/starch/ITO/PET and Au/ starch−chitosan/ITO/PET configuration; (b) a magnified optical image of the memory arrays on a flexible substrate; the device is wrapped around a glass rod with a 2.5 mm diameter; chemical formula of starch combined with amylose (c), and of amylopectin (d) as the resistive switching layer in ReRAM memory device.

around a glass rod with 2.5 mm diameter (Figure 1b); they are highly transparent (Figure 1b: inset) due to transparency of chitosan and starch solutions. Potato-derived starch with a combined structure of amylose and amylopectin (Figure 1c,d) was used as the resistive switching layer in the devices. Electrical characterizations of starch-based and starch− chitosan-based ReRAMs were demonstrated under ambient conditions; in both devices, the measured current−voltage (I− V) curves indicate typical nonvolatile resistive switching (RS) behavior (Figure 2a,b). During measurement of the I−V responses of the Au/starch/ITO/PET and Au/starch− chitosan/ITO/PET devices, dc bias voltages were swept on Au top electrodes in the sequence 0 V → 3 V → 0 V → − 4 V → 0 V at 0.05 V/step while the ITO bottom electrode was grounded. In the starch-based device, during the first voltage sweep a positive bias from 0 V to set voltage Vset ≈ 0.9 V drove the resistance state of the memory device from its pristine highresistance state (HRS) to a low-resistance state (LRS); this is the “set” transition. Subsequent application of a voltage of opposite polarity changed the resistance state from LRS to HRS (“reset”) (Figure 2a). The device can be toggled repeatedly between HRS and LRS by changing the applied bias polarity. The electrical characteristics of the starch−chitosan based device were examined using the same voltage sweep and measurement method. In contrast to the starch-based device without chitosan, both the set and reset processes were gradual (Figure 2b). Upon sweeping an external voltage from 0 to 3 V, the resistance changed gradually from HRS to LRS, and the LRS was retained until an opposite voltage was applied (Figure 2b). The polarity of the bipolar switching cycle is influenced by several parameters, such as electrode materials and work functions. By using different metals for the two electrodes of a MIM structure, the polarity of the switching process can be changed.32 For bipolar memory devices, the device remains at LRS until a voltage with opposite polarity is applied which changes the device state from LRS to HRS (Figure 2a,b).

3. RESULTS AND DISCUSSION Memory devices that use starch or starch−chitosan mixture as resistive switchable materials were fabricated on indium−tin oxide (ITO)-coated polyethylene terephthalate (PET) flexible substrates. Au/starch thin film/ITO/PET and Au/starch− chitosan composite thin film/ITO/PET structures were used to demonstrate the memory devices with a metal/insulator/metal (MIM) structure (Figure 1a). The devices can be wrapped B

DOI: 10.1021/acsami.6b01559 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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are sandwiched between Au and ITO electrodes. The chemical structure of polymer thin film can determine the RS properties of the device14 so this difference in behaviors may be the result of a change in the chemical structure of the insulator layer upon mixing two polysaccharides. Starch is made out of glucose units with branched chains of amylopectine linked with linear amyloses; while the mixed polymer layer contains starch chains in contact with chitosan. In contrast to the starch-based device which only contains water-soluble starch, the starch in the mixed layer is partly acetylated due to presence of acetic acid in chitosan solution. (Figure 2c,d). In the mixed films of chitosan and starch, most of the hydroxyl groups on the starch interact with amino groups on the chitosan; this interaction changes the physicochemical properties of both molecules.27 The OH− groups of starch could readily form hydrogen bonds with NH3+ moieties in the chitosan (Scheme S2), and the high density of amino and hydroxyl groups in starch−chitosan blends yields a well-formed film. A data retention test was performed to appraise the stability of the fabricated memory devices with a reading bias of 0.25 V under ambient conditions. Starch-based ReRAM maintained an ON/OFF current ratio ∼103 without any noticeable degradation for 104 s (Figure 2e). The starch−chitosan devices also showed very stable data retention property under the same reading bias (Figure 2f), but with a small ON/OFF current ratio ∼100. The cumulative distribution of set and reset voltages was determined for the starch-based biomemristor; the average voltages required to accomplish set and reset operations were stable (Figure S3). To examine the mechanical flexibility of the fabricated devices, they were bent under tensile and compressive stresses. We also performed the bending cycle measurement up to 1000 cycles with a curvature radios of 5 mm (Figure S4a,b). Because the I−V characteristics under bending condition were similar to the I−V characteristics of flat devices, we conclude that the nonvolatile memory properties of Au/starch−chitosan/ITO/ PET and Au/starch/ITO/PET devices were reliable under tensile (Figure 3a,b) and compressive (Figure 3c,d) stresses with 5 mm radius of curvature. We used cyclic voltammetry (CV) to examine the electrochemical properties of biopolymer thin films. The current responses were recorded while the voltage was swept as 0 V → 1.5 V → 0 V → −1.5 V → 0 V for both the starch−chitosan and starch SPE thin films (Figure S5a−c). The oxidation peaks of starch and starch−chitosan films occurred at −0.42 V and −0.32 V, respectively (Figure S5a). The onset potential of reduction in the starch-based layer started at a voltage of −0.88 V, and included two wide reduction peaks between −0.88 and −1.5 V. The onset potential of reduction for starch−chitosan mixed thin film occurred at −0.86 V and showed no reduction peak, so we infer that reduction occurs at lower voltage in this film than in the pure starch film. We also performed CV and examined the maximum peak current in starch films and starch−chitosan mixed films (Figure S5b,c) while sequentially increasing scan rates from 50 to 300 mV·s−1. The redox peak currents increased linearly with scan rate; this trend indicates a small shift in potentiation and depression peak potentials. This phenomenon has been attributed to surface-controlled redox processes in the polymer chains.34 The current−voltage relationships of the starch-based biomemristors differ from those of the starch−chitosan-based devices. The starch-based device shows abrupt change in the semilogarithmic I−V curve during set and reset processes; the

Figure 2. Resistive switching (RS) characterization of starch-based memristors. I − V curves of the RS behaviors of (a) Au/starch/ITO/ PET device and (b) Au/starch−chitosan/ITO/PET devices. Current compliance of 10−4 A was applied to prevent permanent breakdown. Schematic device structures of (c) starch-based- and (d) starch− chitosan-based bio-ReRAM with chemical structures. Data retention characteristics of LRS and HRS states under continuous read-out voltages for (e) starch-based device and (f) starch−chitosan-based device.

Moreover, for both of the devices, the minimum possible compliance current was 10−4 A and lower than this amount the devices did not show stable RS behavior due to short and unstable conductive paths. We started from 10−4 A then increased the compliance current to 5 × 10−4 A and 10−3 A for both of the devices. The device showed RS behavior under different compliance currents (Figure S2a−f). There is a tradeoff between power consumption and stability of the device which is related to compliance current. Upon increasing the compliance current, more current can pass through the active layer of the device which leads to stronger filament formation and more stable device. Because high compliance current causes more power consumption, the optimized compliance current of 10−4 A was used to keep the device from breaking down permanently. The phenomenon that causes different current levels in strach-based bio-ReRAM is voltage-induced change in conductivity of SPE, which is a result of chemical modification.33 Electronic conductivity of the starch- and starch−chitosanbased thin film is determined by presence of the functional groups including hydroxyl and amino groups, respectively.33 Figure 2c,d illustrates the SPE layer of starch-based- and starch−chitosan-based biomemristors under applied bias which C

DOI: 10.1021/acsami.6b01559 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. Mechanical stability of flexible biomemristors under bent conditions. I−V characterization of the (a) Au/starch/ITO/PET and (b) Au/starch−chitosan/ITO/PET under tensile bending; inset (right): photograph of the bent device measurement, (left) schematic diagram of the device under bending condition. I−V curve of memory device with (c) starch and (d) starch−chitosan composition as SPE layer under compressive bending. The curvature radius in both the tensile and compressive stresses is 5 mm; inset (right): photograph of the bent device measurement, (left) schematic diagram of the device under bending condition.

Figure 4. Experimental and fitted I−V curves of the Au/biopolymer/ ITO/PET resistive switching memory device. Semilogarithmic I−V curve with fitted models for (a) starch- and (b) starch−chitosan-based memristors. Double-logarithmic plot for Au/starch/ITO/PET in (c) positive sweep and (d) negative sweep with the space-charge-limited model. Double-logarithmic plot for Au/starch−chitosan/ITO/PET in (e) positive sweep and (f) negative sweep with Ohmic model. The slopes are listed in the plots.

starch−chitosan-based device shows gradual changes during these processes (Figure 4a, b). The semilogarithmic I−V curve (Figure 4a) indicates that the conduction mechanism in starchbased memories changes from trap-induced space-chargelimited conduction (SCLC), to formation and rupture of conductive filaments. The conduction mechanism was determined for Au/starch/ITO/PET devices by considering double-logarithmic plots of the I−V curves during positive and negative voltage sweeping. Equations of the fitted curves indicate that current conduction of starch thin film is SCLC during HRS (I ∝ V2) and filamentary type during LRS (I ∝ V) (Figure 4c,d). We attribute the SCLC traps to defects in starch material at the connection between the branched amylopectin matrix and the linear amylose. These defects form trap sites below the conduction band that trap charge carriers. When positive bias is applied, the curve fit revealed two different responses: at low voltage the I−V curves were linear (Ohmic conduction), whereas at higher voltage it was quadratic until the set voltage was reached. Ohmic conduction occurs at low voltage due to the insufficient electric field across the device; as a result the number of injected charge carriers is lower than the number of thermally generated free charge carriers.35,36 When the voltage is increased, the trap centers are occupied by sufficient charge carriers, and the conduction mechanism follows the square-law dependency on voltage; this mechanism is controlled by traps and the ratio of free carriers to trapped ones in coherence with SCLC.37−40 The linear I−V characteristic from +3 V → 0 V implies that filamentary conduction paths form when positive bias is applied. When negative bias was applied, the occurrence of the LRS means that conduction was Ohmic, and the switch to HRS entailed a switch from SCLC-controlled conduction to Ohmic conduction (Figure

4c,d). The devices with starch−chitosan mixed film switched gradually and monotonically from LRS to HRS (Figure 4b). The double-logarithmic I−V curve was linear in both states; this relationship implies that the conduction mechanism is related to formation and breakdown of filaments (Figure 4e,f). Controlling the chemical composition of polymers leads to tuning switching dynamics and electronic properties.33 The switching mechanism in biopolymer-based ReRAM is complicated, and more than one mechanism might operate concurrently. The switching is controlled by the inherent characteristics of SPE layer. Although the active layer of biomemories is a polysaccharide and H+ and OH− transport has been achieved in polysaccharide-based proton wires and devices, it seems that there is almost no chance of proton conduction due to absence of protode and humid condition.19 Among two possible filaments in the active layer of polymerbased ReRAM devices,14 in the examined bio-ReRAMs it is thought that carbon filament is predominant to a metal filament.41,42 Because the top electrode in Au/SPE/ITO/PET memory device is an inert metal (Au), there is almost no chance of anodic dissolution of top electrode upon applying positive bias, then a conductive metallic filament is not likely to happen. A postulated governing mechanism is thought to be formation and ruptur e o f c arbon-rich filaments (CRF)3,14,41−44 due to local change in chitosan and starch D

DOI: 10.1021/acsami.6b01559 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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the weakest point which turns the device sharply to HRS state (Figure 5b). When chitosan is mixed with starch, the density of carbon atoms becomes much higher than in the previous system. When the applied electric field is sufficiently high, Joule heating exceeds heat dissipation. The weakest point of the thicker film becomes thermally damaged, so the current and temperature increase as a result of thermally activated structural change in the SPE layer. The localized high temperature regions pyrolyze the mixed layer; as a result a wide conductive carbon-rich pathway forms around the breakdown region (Figure 5c). The broader conductive path causes a gradual increase of conductivity. The reverse switching is induced by applying negative voltage which smoothly evaporates carbon localization and dissolves the CRFs. However, they are not totally ruptured; the surviving CRFs also contribute to the progressive reset (Figure 5d).41,46 Although nanostructural carbon is a stable material, but there are several studies for its failure mechanism, and its rupture is specifically caused by Joule heating. For instance, the breakdown of carbon nanotubes by Joule heating have been studied by transmission electron microscopy. It has been claimed that a progressive destruction of the carbon nanotube structure under air atmosphere is made by Joule heating. The Joule heating temperature rises at the center of the nanotube bridge and the nanotube becomes thinner in the central part of the bridge with a vast damage.47 Researchers also have modeled the carbon rupture considering the fact that rupture is happened by the combined effects of carbon nanotube length, diameter, volume fraction, and alignment with experimental verification.48 Furthermore, it has been confirmed that a high temperature caused by Joule heating combined with a tensile stress applied by the electric field causes a breakdown in carbon nanotube structure.49 In contrast to the starch based device, it is more likely that multiple conducting filaments are formed in the starch− chitosan based device. Therefore, the gradual I−V behavior is also postulated to existence of multiple CRF filaments42 (Figure 5c,d). As the bias voltage is increased, the current moderately increases to form several filaments leading to LRS state (Figure 5c). Considering that all filaments are partially ruptured, the device turns to the HRS state progressively (Figure 5d). The starch−chitosan-based ReRAM device shows a gradual change in conductivity, whereas the starch-based ReRAM without chitosan indicates an abrupt change. The abrupt and progressive changes in current level are thought to happen because of broken and unbroken46 filaments of carbon, respectively. In addition, to investigate the thickness dependence RS characteristics, thin films thicknesses were controlled by variable solution concentrations, different RPMs of spin coating, and changing the drying condition and annealing time. Remarkably, both of the memory devices were almost stable by different resistive layer thicknesses (Figure S7a,b). The optical absorbance of the prepared solutions of starch and starch−chitosan was low, but was slightly higher in the blended solution than that in the starch solution (Figure S8a). The transparency of the thin films was confirmed by UV−vis transmittance spectra. The transmittance was about 86.8% for both of these thin films at a wavelength of 452 nm (Figure S8b). These characteristics make the films appropriate for use in transparent flexible devices.

structure. A stable memory operation without dependency on the top electrode material is another conformation to CRF mechanism.41 We examined starch-based bio-ReRAM with Al and Ag top electrodes and the same RS properties were obtained (Figure S6a,b). Because the memory device can be operated with different top electrodes this can be attributed to CRF-based ReRAM due to Joule heating. The forming-free switching behavior verifies absence of metallic filament and construction of a CRF, because the need for an electroforming process is representative for metallic filament formation based on cation/anion migrations.45 Although Joule heating generally happens in unipolar resistive switching devices with thermochemical effects during reset process, in our device explained by CRF the role of Joule heating is significant in both of set and reset processes. Because the active layer in the bio-ReRAM is a polymer (which is a soft material) the applied bias leads to pyrolysis of the polymer by Joule heating during set process. This phenomenon leads to CRF formation which has been reported in bipolar switching mechanisms.41,42,44 In addition, the reset process is also affected by Joule heating to make a rupture in the formed CRFs through the polymer layer. By applying positive bias to the initial biomemristor, Joule heating initiates a local breakdown at the weakest point of the SPE which leads to formation of voids. An increase of the applied bias speeds up the local breakdown due to pyrolysis of SPE, consequently builds the localized carbon-rich region which is surrounded by voids and converts to be more conductive. A high current passes through the produced CRF because of its low resistivity and subsequently the temperature of CRF is elevated by Joule heating. Because of a big difference between thermal conductivity of carbon and SPE the increased temperature is restrained in CRF and causes a rupture, thereby leading to HRS (Figure 5a, b).41,46 In starch-based devices,

Figure 5. Proposed resistive switching mechanism. Illustration of carbon-rich filament (CRF) for (a, b) Au/starch/ITO and (c, d) Au/ starch−chitosan/ITO devices.

pyrolysis of the polymer due to Joule heating causes changes in the conductivity of the initial biopolymer. When the bias reaches Vset, a narrow CRF with high conductivity is built between top and bottom electrode, so the device switches to LRS state and current increases abruptly (Figure 5a). Additional thermal energy causes a rupture inside the CRF at E

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filament; CV, cyclic voltammetry; ITO, indium−tin oxide; PET, polyethylene terephthalate

4. CONCLUSIONS We evaluated transparent and flexible ReRAM devices that are based on a thin film of starch, or on starch−chitosan blend. Starch is stable, inexpensive, and ecologically benign under ambient conditions; these traits make it useful as an SPE in ReRAMs. The demonstrated nonvolatile and biodegradable memories were fabricated using an inexpensive and simple solution processes using starch and chitosan biopolymers. The switching uniformity was highly improved in ReRAM that had the composite starch−chitosan active layer. Both devices had low operation voltages with good mechanical reliability and retention capability. They also exhibited uniform and robust ReRAM properties for nonvolatile memory applications. In particular, the memory device based on the starch−chitosan mixture showed gradual set/reset behaviors which can be useful in neuromorphic devices. We successfully tuned the resistive switching property of starch by controlling its chemical composition.

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(1) Zhou, Y.; Han, S.-T.; Yan, Y.; Zhou, L.; Huang, L.-B.; Zhuang, J.; Sonar, P.; Roy, V. Ultra-flexible Nonvolatile Memory based on Donoracceptor Diketopyrrolopyrrole Polymer Blends. Sci. Rep. 2015, 5, 10683−10690. (2) Forrest, S. R. The Path to Ubiquitous and Low-cost Organic Electronic Appliances on Plastic. Nature 2004, 428, 911−918. (3) Ling, Q.-D.; Liaw, D.-J.; Zhu, C.; Chan, D. S.-H.; Kang, E.-T.; Neoh, K.-G. Polymer Electronic Memories: Materials, Devices and Mechanisms. Prog. Polym. Sci. 2008, 33, 917−978. (4) Berggren, M.; Nilsson, D.; Robinson, N. D. Organic Materials for Printed Electronics. Nat. Mater. 2007, 6, 3−5. (5) Lu, W.; Jeong, D. S.; Kozicki, M.; Waser, R. Electrochemical Metallization CellsBlending Nanoionics Into Nanoelectronics? MRS Bull. 2012, 37, 124−130. (6) Tappertzhofen, S.; Valov, I.; Tsuruoka, T.; Hasegawa, T.; Waser, R.; Aono, M. Generic Relevance of Counter Charges for Cation-based Nanoscale Resistive Switching Memories. ACS Nano 2013, 7, 6396− 6402. (7) Valov, I.; Linn, E.; Tappertzhofen, S.; Schmelzer, S.; Van den Hurk, J.; Lentz, F.; Waser, R. Nanobatteries in Redox-based Resistive Switches Require Extension of Memristor Theory. Nat. Commun. 2013, 4, 1771−1779. (8) Yang, J. J.; Strukov, D. B.; Stewart, D. R. Memristive Devices for Computing. Nat. Nanotechnol. 2013, 8, 13−24. (9) Linn, E.; Rosezin, R.; Kügeler, C.; Waser, R. Complementary Resistive Switches for Passive Nanocrossbar Memories. Nat. Mater. 2010, 9, 403−406. (10) Tian, H.; Chen, H.-Y.; Ren, T.-L.; Li, C.; Xue, Q.-T.; Mohammad, M. A.; Wu, C.; Yang, Y.; Wong, H.-S. P. Cost-Effective, Transfer-Free, Flexible Resistive Random Access Memory Using Laser-Scribed Reduced Graphene Oxide Patterning Technology. Nano Lett. 2014, 14, 3214−3219. (11) Kim, H.-D.; Yun, M. J.; Lee, J. H.; Kim, K. H.; Kim, T. G. Transparent Multi-level Resistive Switching Phenomena Observed in ITO/RGO/ITO Memory Cells by the Sol-Gel Dip-Coating Method. Sci. Rep. 2014, 4, 4614−4619. (12) Lee, T.; Chen, Y. Organic Resistive Nonvolatile Memory Materials. MRS Bull. 2012, 37, 144−149. (13) Bhansali, U. S.; Khan, M. A.; Cha, D.; AlMadhoun, M. N.; Li, R.; Chen, L.; Amassian, A.; Odeh, I. N.; Alshareef, H. N. Metal-Free, Single-Polymer Device Exhibits Resistive Memory Effect. ACS Nano 2013, 7, 10518−10524. (14) Lin, W. P.; Liu, S. J.; Gong, T.; Zhao, Q.; Huang, W. PolymerBased Resistive Memory Materials and Devices. Adv. Mater. 2014, 26, 570−606. (15) Song, S.; Cho, B.; Kim, T. W.; Ji, Y.; Jo, M.; Wang, G.; Choe, M.; Kahng, Y. H.; Hwang, H.; Lee, T. Three-Dimensional Integration of Organic Resistive Memory Devices. Adv. Mater. 2010, 22, 5048− 5052. (16) Wu, S.; Tsuruoka, T.; Terabe, K.; Hasegawa, T.; Hill, J. P.; Ariga, K.; Aono, M. A Polymer-Electrolyte-Based Atomic Switch. Adv. Funct. Mater. 2011, 21, 93−99. (17) Irimia-Vladu, M.; Głowacki, E. D.; Voss, G.; Bauer, S.; Sariciftci, N. S. Green and Biodegradable Electronics. Mater. Today 2012, 15, 340−346. (18) Hung, Y.-C.; Hsu, W.-T.; Lin, T.-Y.; Fruk, L. Photoinduced Write-once Read-many-times Memory Device based on DNA Biopolymer Nanocomposite. Appl. Phys. Lett. 2011, 99, 253301− 253304. (19) Deng, Y.; Josberger, E.; Jin, J.; Rousdari, A. F.; Helms, B. A.; Zhong, C.; Anantram, M.; Rolandi, M. H+-type and OH−-type Biological Protonic Semiconductors and Complementary Devices. Sci. Rep. 2013, 3, 2481−2487.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01559. Schematic formula of amylose, amylopectin, and chitosan, postulated interaction of starch and chitosan, cross-sectional SEM images, I−V characteristics with different compliance currents, statistical cumulative distribution of the SET and RESET voltages for starchbased device, mechanical flexibility with continuous bending cycles, electrochemical CV of SPE layer including starch and starch−chitosan biopolymers, effect of different electrodes on RS properties, effect of thickness on RS properties for both of the bio-ReRAMs, and optical characterization in Schemes S1 and S2 and Figures S1−S8 (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.-S.L). Author Contributions

J.S.L. conceived, supervised, and directed the research. N.R.H. designed and performed the experiment. N.R.H. and J.S.L. wrote the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Research Foundation of Korea (NRF-2015R1A2A1A15055918). This work was also supported by Future Semiconductor Device Technology Development Program (10045226) funded by the Ministry of Trade, Industry & Energy (MOTIE)/Korea Semiconductor Research Consortium (KSRC). In addition, this work was partially supported by Brain Korea 21 PLUS project (Center for Creative Industrial Materials).

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ABBREVIATIONS SPE, solid-polymer electrolyte; ReRAM, resistive switching random access memory; RS, resistive switching; LRS, low resistance state; HRS, high-resistance state; CRF, carbon-rich F

DOI: 10.1021/acsami.6b01559 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.6b01559 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX