Memristor-Integrated Voltage ... · memristor with PCBM-based supercapacitor. PCBM-based ﬂ ex-ible supercapacitor was ﬁ rst fabricated by using the NIL tech-nique, which delivered

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Memristor-Integrated Voltage-Stabilizing Supercapacitor System

Bin Liu , Boyang Liu , Xianfu Wang , Xinghui Wu , Wenning Zhao , Zhimou Xu , * Di Chen , * and Guozhen Shen *

In this work, for the fi rst time, we fulfi lled a new voltage-sta-bilizing supercapacitor system by the integration of SnO 2 -based memristor with PCBM-based supercapacitor. PCBM-based fl ex-ible supercapacitor was fi rst fabricated by using the NIL tech-nique, which delivered excellent cycling life, enhanced areal capacitance, and stable electrical stability under bending. Then, a memristor-integrated voltage-stabilizing supercapacitor was proposed, which facilitates a potential feasibility of functional relationship between the fundamental circuit-components (Figure S1, Supporting Information). Our work here opens up a new approach to fabricate new voltage-stabilizing systems and will bring a new boost for future electronics.

To fabricate the fl exible supercapacitor with patterned elec-trodes, nanoimprint lithography (NIL) technique was applied. NIL stands out as a promising technology for high throughput and resolution nanoscale patterning, which has been intro-duced into various applications. [ 15–20 ] Beyond its fundamental nano-patterning interest, it is highly desired to develop this effi -cient and facile way to fabricate lightweight, fl exible, and trans-parent electrodes for advanced energy-storage devices, espe-cially supercapacitors. [ 21–31 ] The typical fabrication processes for the supercapacitors were illustrated in Figure 1 a, which consists of fi ve steps: (1) nanoimprint lithography (NIL), (2) sputtering, (3) spraying, (4) assembling, and (5) molding. In the NIL pro-cess, intermediate polymer stamp (IPS) fi lms were pressed with Si mold to form the nanorod arrays after demolding. Interestingly, the as-prepared IPS fi lms have outstanding trans-parent and fl exible features (Figure 1 b and Figure S2, S3, Sup-porting Information), indicating their great potentials in future transparent/stretchable electronics. The remarkably uniform nanorod arrays were verifi ed by atomic force microscopy (AFM) measurement (Figure 1 c). The 3D AFM image further shows that the height of the nanorods is about 200 nm (Figure 1 d). Their corresponding nanopatterning is observed by scanning electron microscope (SEM) after sputtering Pt layer. The SEM images show the formation of orderly IPS nanorod arrays on a plane (Figure 1 e and Figure S4) and their diameters is approxi-mately 200 nm (Figure 1 f).

Previously, PCBM ([6,6]-phenyl-C61-butyric acid methyl ester), a fullerene derivative, was widely applied in solar cells due to its high electron mobility, good solubility, and excellent phase separation. It is expected that breakthrough advance-ments could be gained in novel energy storage units by com-bining PCBM active materials with the NIL technique for future electronics. Here, our nanopatterned PCBM/Pt/IPS electrodes were fabricated by coat-spraying method. The optical image of the PCBM solution is shown in Figure 1 g (The detailed descrip-tion of its fabrication process is stated in Experimental Sec-tion). It was observed that all the IPS nanorods are uniformly

Recently, a great demand for high-performance multifunctional integrated systems has arisen for use in integrated electronics because of the maximum functionality of these integrated sys-tems at minimized size. Considerable research efforts have been made to integrate different functional units into single multifunctional micro/nanosystems, such as solar cell-inte-grated photo-supercapacitor, [ 1–4 ] solar cell-electrochromic smart windows, [ 5 ] hydrogen generation-supercapacitor, [ 6 ] and photo-detector-supercapacitor nanosystems, [ 7 ] etc. These indicated designed integrated systems can not only avoid some space and energy consumption form the external connection circuit but also achieve some new functionalities by incorporating different devices into single system.

As one of the most potential energy storage devices, super-capacitors have attracted increasing interest in the past few years owing to their high power density, long cycling life, and high energy effi ciency. [ 8–12 ] However, one drawback associated with supercapacitors is that their voltage often signifi cantly decreases once they begin to discharge as power sources, which greatly affects the smooth operation of the electronic/opto-electronics devices. To avoid this drawback, one should con-sider introducing voltage-stabilizing unit into supercapacitors. However, the up-to-date existing voltage-stabilizing units still possess several unsatisfying features, including redundant cir-cuits, complicated and high cost procedures, etc, which makes it hard to integrate with supercapacitor to fulfi ll multifunctional demands at minimized scales. As the fourth circuit element, memristors have observable effects in resistance switching memory. [ 13,14 ] Considering the unique features of memristors, voltage-stabilizing characteristics may be fulfi lled if one could integrate a memristor with a supercapacitor.

DOI: 10.1002/adma.201401017

B. Liu,[+] Prof. G. Z. Shen State Key Laboratory for Superlattices and Microstructures Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 , PR China E-mail: [email protected] B. Liu, B. Y. Liu,[+] X. F. Wang, X. H. Wu, W. N. Zhao, Prof. Z. M. Xu, Prof. D. Chen Wuhan National Laboratory for Optoelectronics (WNLO) and College of Optical and Electronic Information Huazhong University of Science and Technology (HUST) Wuhan 430074 , PR China E-mail: [email protected] ; [email protected]

[+]B. Liu and B. Y. Liu contributed equally to this work.

Adv. Mater. 2014, DOI: 10.1002/adma.201401017

http://doi.wiley.com/10.1002/adma.201401017

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covered with PCBM, as shown in Figure 1 h. The diameter of the fi nal regular rods with PCBM coating increased to about 400 nm, and the gap between these adjacent nanorods are fi lled with PCBM materials, which leads to a PCBM/IPS core/shell architecture as an electrode, as shown in Figure 1 i. After the fabrication of the PCBM/IPS core/shell electrodes, two of the as-fabricated electrodes were assembled into an all-solid-state supercapacitor, as depicted in Figure 1 j, which still keeps the features of excellent fl exibility.

The capacitive features of the as-assembled fl exible super-capacitor, which includes two PCBM/Pt/IPS composite electrodes, PVA/H 3 PO 4 electrolyte, and carbon electrodes ( Figure 2 a), were demonstrated in Figure 2 . The cyclic vol-tammograms (CV) of the fl exible supercapacitor show a typical symmetrical rectangular shape at various scan rates of 20, 30, 50, 80, and 100 mV s −1 , which indicates their elec-trical double-layer capacitor features (Figure 2 b) according to previous reports. [ 32–36 ] The capacitance of the as-fabricated supercapacitors is associated with the accumulation of electro-static charge in an electrochemical double layer formed at the PCBM/electrolyte interfaces. To better verify the advantages of the nanopatterning-based electrodes, the CV curves of a com-parative supercapacitor with non-patterning electrodes was also measured (Figure 2 c). It is observed that these electrodes have an extremely small area surrounded by CV curves at the corresponding scan rates, suggesting that their capacitive fea-ture is worse than that of the PCBM/Pt/IPS nanopatterned

electrodes. We can easily attribute it to the more enhanced surface area of the patterned PCBM/Pt/IPS electrodes in con-trast to non-patterned electrodes. Figure 2 d shows the charge/discharge curves of the patterned PCBM/Pt/IPS electrodes, and the corresponding calculated capacitances are 33, 28, 26, and 18 mF cm −2 at 0.2, 0.5, 1.0, and 2.0 mA cm −2 , respectively, as summarized in Figure 2 e.

The cyclic stability test of the devices is shown in Figure 2 f. From the plot, after as long as 1000 cycles at 0.2 mA cm −2 , there is an only ≈6.4% drop in the capacitance of the device. Figure 2 g demonstrates the fi rst 4 charge/discharge curves, revealing the excellent cycling reproducibility of the patterned electrodes. Using our patterned PCBM/Pt/IPS fi lms, the as-assembled highly fl exible supercapacitor devices may fi nd many potential applications in next generation fl exible electronics. Figure 2 h shows the CV curves of the as-fabricated fl exible supercapacitor device under different bending states with different bending curvatures (30, 25, and 20 mm). The three curves are nearly unchanged at different bending states, revealing that the elec-trical stability of the fl exible device is hardly affected by external bending stress. Figure 2 i shows the CV curves of the fl exible devices after bending for different cycles (50, 100, and 200 bending cycles) to further evaluate their corresponding folding endurance, which further verifi ed the excellent mechanical fl ex-ibility of the as-fabricated fl exible supercapacitor.

Figure 3 shows the characterizations of the memris-tors based on SnO 2 fi lm, which was prepared from a sol-gel


Figure 1. Schematic diagram of the construction procedures and structure characterizations of the NIL-fabricated supercapacitor. a) Fabricated process fl ow of the fl exible supercapacitors via a NIL technique. b) The photo of the patterned fi lm. c,d) The AFM images for the surface of the samples. e,f) The SEM images of the Pt/IPS composites fabricated by sputtering approach. g) The photo of the as-prepared PCBM solution. h,i) The SEM images of the PCBM/Pt/IPS samples after spraying. j) The photograph of the resulting fl exible device.

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method. The structure of the as-fabricated memristor was shown in Figure 3 a, containing the active SnO 2 fi lm on FTO glass, and the Ni electrode, which is measured on a probing station (Figure 3 b). Th crystallographic structure of the sol-gel synthesized product is analyzed by X-ray diffraction (XRD). All the diffraction peaks in Figure 3 c can be readily indexed as SnO 2 , which are well consistent with the values in the standard card (JCPDS Card No. 46–1088). The AFM images of the SnO 2 thin fi lm in Figure 3 d and Figure S5 reveals the extremely high smoothness of the SnO 2 fi lm with the surface roughness (Rq) of 3.11 nm. Figure 4 a is the corresponding I–V curve of the memristor device in the voltage window of −5.4–5.4 V. From the curve, it can be observed that the I–V curve represents an inverse proportion between device voltage and resistance. To further confi rm the unique properties of the memristors, many I−V relationships in several models, icluding I ∝ V 2 for the space charge limited current (SCLC), ln( I ) ∝ Sqrt( V ) for the Schottky emission, ln( I / V ) ∝ Sqrt( V ) for the Poole–Frenkel (PF) emis-sion, and ln( I / V 2 ) ∝ 1/ V for the Fowler-Nordeim tunneling,

were used to fi t the typical I–V results by Matlab software. [ 37 ] Compared to other models, the fi tted value of PF model is best consistent with the measured value from the I–V curves, as shown in Figure 4 b. The voltage region is dominated by the Poole-Frenkel (PF) emission at the interface between the elec-trodes and SnO 2 fi lm. According to previous reports, the PF emission is a property that memoristors have in high resistance state. With similar behavior, as a conclusion, our SnO 2 based device could be seen as a memoristor. [ 37 ]

As the fourth circuit element, memristors gained great research attention and till now, almost all of the researches are focused on the essential characteristics of memristors, while few work on the investigation of the functional relationship between memristor and the other passive elements (Resistor, Capacitor, and Inductor). To demonstrate the application of memristor as voltage-stabilizing unit, we designed a new mem-ristor integrated supercapacitor system by the integration of the SnO 2 -based memrister with the patterned PCBM-based super-capacitor. A photograph of the memristor-supercapacitor (M-S)


Figure 2. Performance characterizations of the as-assembled supercapacitor. a) A schematic of several components of supercapacitor. b,c) Typical CV curves of the nanopatterning and non-nanopatterning devices at various scan rates between 0–1.5 V. respectively. d) The charge-discharge curves of these electrodes at 0.2, 0.5, 1.0, and 2.0 mA cm −2 . e) The corresponding rate capability. f) Variation of capacitance stability of the devices with cycle number. g) The galvanostatic charge/discharge curves of the devices at 0.2 mA cm −2 . h,j) CV curves collected at the scan rate of 40 mV s −1 for the fl exible device bent at various states, respectively.

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integrated device was shown in Figure 5 a. For the M-S inte-grated device, the supercapacitor (S) as a power source begun to work once it was charged to 1 V, as shown in Figure 5 b. A stable discharging curve is obtained for the integrated device at about 0.8 V for as long as 200 s. and only about 0.11 V voltage drop was observed as shown in Figure 5 c. While for the single supercapacitor (also shown in Figure 5 b), once discharging, the voltage drops dramatically from 0.86 V to 0.3 V within the measured discharging time (200 s). Similar phenomena were also observed once both the devices, the integrated device and the single supercapacitor device, were charged to 1.5 V (Figure 5 d, 5 e). Importantly, the integrated device delivers an extremely low voltage-drop of only ∼0.06 V in 200 s, revealing the possible application of the integrated device as stable energy unit for electronic and optoelectronic devices.

To explain the reason why the integrated M-S device delivered stable voltage during the discharging process, we draw the cor-responding circuit schematics of both the single supercapacitor (Figure 5 f) and the integrated M-S device (Figure 5 g). In this fi gure, I 1 , R 1 , U 1 , I 2 , R 2 , U 2 , and I 3 , R 3 , U 3 refer to electric cur-rent, resistance, and voltage of supercapacitor, memristor, and measured output-circuit, respectively. For the single supercapac-itor system, the device as a power source begun to control the total circuit after it was fully charged. Since the electrical double-layer supercapacitors deliver quite rapid ion absorption and des-orption in double layers at the interface between materials and electrolyte, leading to the inevitable on-going voltage decrease ( U 3 ) of single supercapacitors without voltage-stabilizing com-ponents (Figure 5 f). While considering an inverse relationship between U 2 and R 2 (Figure 4 a), U 2 should gradually reduce for the discharging process of devices, which leads to rapid decrease of I 2 in the integrated M-S system (Figure 5 g). According to the circuit principle: I 1 = I 2 + I 3 . Here, lower electric current ( I 2 )

derived from lower voltage ( U 2 ) may cause more compensation for external output circuit ( I 3 ). Finally, the synergistic effects of integrated devices can enable U 3 to smoothly keep almost con-stant. Therefore, memristor presented here plays a key role in circuit equilibration and regulation, which facilitates the reali-zation of voltage-stabilizing characteristic. All these results indicate that it is a stirring breakthrough for future electronics: such a simplifi ed voltage-stabilizing circuit formed with two fundamental passive elements (memristor and capacitor).

To demonstrate the real application of the integrated M-S system as stabilized energy units, we attempted to control com-mercial red LED by using the as-designed M–S system. From Figure 6 a–c, for single a supercapacitor, it can be observed that the brightness of a lighten LED decreased very fast and then even fully disappeared within 100 s during a continuous discharging process. While for the M–S integrated system with voltage-stabilizing ability, the LED was found to be still light-ened after 100 s, as shown in Figure 6 d–f. The results confi rm the voltage stabilizing features of our designed memristor-supercapacitor system.

In summary, we designed a new memristor-integrated supercapacitor system with voltage-stabilizing features. The supercapacitor used in the system was fabricated by the nano-imprint lithography process with the features of excellent cycle


Figure 3. Structure characterizations of the SnO 2 -based memristor. a) The schematic drawing of the SnO 2 -based memristors. b) An optical image of device measurement confi guration. c) The XRD pattern of the SnO 2 samples. d) The AFM image of the surface of the SnO 2 -based memristors.

Figure 4. a) Typical current-voltage ( I–V ) characteristic. b) The fi tting I–V relationships in the model of the Poole–Frenkel (PF) emission.

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life, enhanced capacitance, and outstanding electrical stability under bending. Integrating the supercapacitor with a SnO 2 -based memristor, the fi nal integrated system delivered obvious voltage-stabilizing effects as confi rmed from both the circuit analyses and the display demonstration. Our work demon-strates the successful integration between the two basic circuit elements (capacitor and memristor) to fulfi ll new functions. By design of fl exible memristor and reducing the size of both the memristor and the supercapacitor, the fully fl exible memristor-supercapacitor may fi nd real applications in current micro/nanoelectronics fi elds.

Experimental Section Fabrication of NIL-Based PCBM/Pt/IPS Electrodes for Supercapacitors :

The cleaned intermediate polymer stamp (IPS) fi lm was heated at 155 °C and then pressed by a patterned Si mold by nanoimprint lithography (NIL) technique (Eitre3 Nano Imprinter) to obtain resulting nanorod arrays fi lm. A Pt layer was sputtered onto this IPS thin fi lm by using a precision etching coating system (PECS), Gatan, Model 682. In addition, 5 mg PCBM, [6, 6]-phenyl-C61-butyric acid methyl ester, was dissolved in 1 mL o-dichlorobenzene solution under continuous stirring for 12 h, and then the mixture was spin-coated onto IPS/Pt thin fi lm at 2000 rpm for 60 s (KW-4A spin coater). After drying at 50 °C in vacuum oven, PCBM/Pt/IPS thin fi lm electrodes were fi nally fabricated.

Figure 5. The voltage-stabilizing properties of the supercapacitor-memristor integrated devices. a) The optical image of the integrated circuit in parallel. The discharging curves of the integrated devices from b,c) 1 V and d,e) 1.5 V. respectively. f,g) In addition, the working analysis of supercapacitor and supercapacitor-memristor circuits, respectively, is shown.

Figure 6. Display comparison of the single and integrated systems. A red LED driven by a–c) supercapacitor, and d–f) supercapacitor-memristor systems, respectively.

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Preparation of the Ni/SnO 2 /FTO Memristor Devices : SnO 2 thin fi lm was fabricated by sol-gel method. Briefl y, SnCl 4 ·5H 2 O was dissolved in absolute ethanol. The solution was transferred into a three fl asks and then heated at 30 °C for 12 h under stirring. The aqueous SnO 2 solution was spin-coated on the FTO glass, and then the fi lm was baked on the hot plate at 130 °C for 10 min. Finally, the samples were annealing in the furnace at 560 °C for 3 h. For electrical measurement, Ni top electrodes were deposited on SnO 2 fi lms by DC magnetron sputtering using a metal shadow mask.

Material Characterization : The morphology was investigated by fi eld-emission scanning electron microscope (FESEM, FEI Sirion 200). The phase of the products was identifi ed using a X-ray diffraction (XRD) diffractometer (X’Pert PRO, PANalytical B.V., Holland) with Cu K α1 irradiation (λ = 1.5406 Å).

Device Assembly : The fl exible supercapacitors were both composed of a sandwiched structure with two patterned PCBM-coated IPS fi lms and a layer of PVA/H 3 PO 4 solid electrolyte. Then, the obtained PCBM-based supercapacitor was integrated with an above Ni/SnO 2 /FTO memristor to form a voltage-stabilizing circuit.

Performance Measurement : The I–V curve of memristors was measured by semiconductor characteristic measurement system (Keithley 4200 SCS) at room temperature in ambient condition. The fi tted I−V characteristics were obtained using Matlab software. The self-discharging curves of the integrated devices were performed on the electrochemical station (CHI 760D).

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by the National Natural Science Foundation (91123008, 61377033), the 973 Program of China (2011CB933300), the Program for New Century Excellent Talents of the University in China (grant no. NCET-11–0179), the Fundamental Research Funds for the Central Universities (HUST: 2013NY013) and the Wuhan Science and Technology Bureau (20122497).

Received: March 5, 2014 Revised: March 27, 2014

Published online:

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Documents

Memristor-Integrated Voltage ... · memristor with PCBM-based supercapacitor. PCBM-based ﬂ ex-ible supercapacitor was ﬁ rst fabricated by using the NIL tech-nique, which delivered