A Universal Strategy for Stretchable Polymer Nonvolatile Memory via Tailoring Nanostructured Surfaces

Abstract

Building stretchable memory is an effective strategy for develo** next-generation memory technologies toward stretchable and wearable electronics. Here we demonstrate a universal strategy for the fabrication of high performance stretchable polymer memory via tailoring surface morphology, in which common conjugated polymers and sharp reduced graphene oxide (r-rGO) films are used as active memristive layers and conductive electrodes, respectively. The fabricated devices feature write-once-read-many-times (WORM) memory, with a low switching voltage of 1.1 V, high ON/OFF current ratio of 10⁴, and an ideal long retention time over 12000 s. Sharp surface-induced resistive switching behavior has been proposed to explore the electrical transition. Moreover, the polymer memory show reliable electrical bistable properties with a stretchability up to 30%, demonstrating their great potential candidates as high performance stretchable memory in soft electronics.

Introduction

Polymer memory, a promising emerging memory technology, have captured great attention owing to their fast operating speed, low power consumption, simple structure and high integrated density¹. Specially, the intrinsically flexible properties of polymers endow polymer memory with high flexibility, enabling them as basic building blocks in flexible electronics and skin electronics for data storage^2,3,4, logic, neuromorphic computing⁵ and artificial synapses^6,7. In the past decade, flexible memory materials mainly include polymer composites and pure polymers comprising complex structures, such as donor-acceptor and conformational-change systems⁷. Flexible electrodes normally consist of graphene or reduced graphene oxide (rGO)^7,8, conductive polymer⁹, silver nanowire and several metal conductors^7,10. Despite extensive effort devoted to flexible polymer memory, the stretchability of memory devices has not been well solved, limiting their uses for growing demands in stretchable electronics, wearable electronics and smart electronics.

In the pursuit of memory devices with high stretchability, several strategies have been exploited to retain the electrical hysteresis behaviors of memory device under tensile deformation. The prevailing approach is to design device array with stretchable interconnectors between the individual rigid devices^{11,12,13,14,15,16,17,18,19,20}. For example, Kim et al. reported a stretchable carbon nanotube charge-trap floating-gate memory arrays by integrating rigid transistor memory device and logic gates with serpentine interconnections for enhanced deformability of the integrated system²⁰. Another strategy is to shape rigid materials with mechanically deformable structures on elastic substrates to achieve stretchable floating-gate memories, such as buckled-, spring-, and meshed-structures^21,22,23,24. Recently, Chen et al. demonstrated a stretchable organic nonvolatile memory diode with buckled structure by transferring flexible compounds on pre-strained elastomer, with mixed semiconducting/insulating polymers as active layer²⁵. Such research facilitates the development of data storage devices toward stretchable electronic applications. However, these strategies face the challenges, for example, complex fabrication process owing to their complicated structures or multicomponent of memristive materials, impeding their universal and low cost manufacturing as well as high density integration. Therefore, develo** a general strategy to fabricate stretchable polymer memory devices with diverse common materials and simple diode structure is highly desirable.

Here we report a facile and universal strategy for high-throughput fabrication of high performance stretchable polymer memory device by direct depositing polymer diode onto a pre-strained poly(dimethylsiloxane) (PDMS) elastic substrate. By utilizing pure conjugated polymer Poly(9-vinylcarbazole (PVK) active layer sandwiched between alumina (Al) top electrode and rough reduced graphene oxide (r-rGO) bottom electrode, the as-fabricated polymer device exhibits typical electrical bistable behavior and nonvolatile memory effect, with the merits of high ON/OFF ratio, low switching voltage, outstanding retention ability and excellent reproducibility. Moreover, the memory device possesses reliable ability to operate under uniaxial tensile strain. Meanwhile, we have also discussed the carrier transport process and memory mechanism. Importantly, this method is general and can be used to prepare other stretchable polymer memory devices comprising pure conjugated polymers, including poly[2-methoxy-5-(2-ethylhexyloxy) phenylenevinylene-1,4-diyl] (MEH:PPV) and poly(9, 9-di-n-octylfluorenyl-2,7-diyl) (PFO).

Results

The graphene electrode used in this work was made of rGO film prepared by our previous reported method. The evolution of surface morphology in rGO film made from GO precursor under five diverse sonication time is clearly shown in Fig. 1. The atomic force microscope (AFM) images show the top surface of 3 h rGO is covered with dense sharps, resulting in a relative higher surface roughness, while its counterparts (18 h rGO) demonstrates a smoother surface without obvious sharp shape, indicating that the surface roughness of rGO film decreases with the sonication time increase, with a value of 4.54 nm at 3 h, 3.78 nm at 6 h, 3.73 nm at 9 h, 3.51 nm at 12 h, and 2.76 nm at 18 h. The rGO films with the highest and lowest roughness are referred as rough rGO (r-rGO) and smooth rGO (s-rGO), respectively. Therefore, the surface morphology of rGO film could be effectively controlled by adjusting sonication time of GO solution.

Figure 1

Morphology evolution of rGO films made from different graphene oxide solution under various sonication times. AFM images (a–e) and corresponding 3D-mode AFM images (f–j) of rGO films.

The fabrication procedure of stretchable polymer memory devices comprising pure conjugated polymer active layer and r-rGO electrodes is posed in Fig. 2a (see the Experimental Section for details). Briefly, r-rGO film with a sheet resistance of 1000 Ω/sq. was detached and transferred onto target pre-strained PDMS substrate. Then, semiconductive polymer PVK was spin-coated onto r-rGO electrodes, followed by drying in oven. Finally, 200 nm top Al electrodes were thermally evaporated onto polymer layer and stretchable polymer devices were obtained after carefully relaxing the pre-strained system. Similarly, s-rGO was used to replace r-rGO and underwent the identical procedure to fabricate a reference device in order to study the effect of surface morphology of rGO electrode on device performance.

Figure 2

Experimental analysis of the fabricated memory. (a) The fabrication process of stretchable polymer memory devices. (b) The I–V characteristic of r-rGO/PVK/Al and s-rGO/PVK/Al. (c) The sonication time dependence of switching voltage. (d) ON/OFF ratio as a function of forward bias. (e) Statistics histograms of the switching voltages of r-rGO/PVK/Al memory devices from 30 memory cells. (f) The retention ability of r-rGO/PVK/Al memory device at reading voltage of +0.5 V.

To investigate the memory effects of fabricated polymer devices, the current-voltage (I–V) characteristic of the r-rGO/PVK/Al device was measured and shown in the red dots curve of Fig. 2b. The I–V curves showed a typical electrical bistable phenomenon. The applied voltage was swept with a step of 0.02 V in a cycle from 0 to 5 V and then from 5 to 0 V to realize the electrical transition of device from initial low conductivity state to high conductivity state. Initially, in the range of 0–1.1 V, our device exhibited a high resistance state (HRS, labelled as OFF state). The current increased abruptly at about 1.1 V, indicating a transition from HRS to the low resistance state (LRS, labelled as ON state) of our device. Obviously, once the ON state was achieved, the ON state was maintained even in the absence of power, indicating a non-volatile nature of this memory device. Importantly, the device cannot regain the OFF state during a large negative voltage sweep, indicative of a nonvolatile write-once-read-many times (WORM) characteristic. Moreover, the measured high ON/OFF current ratio of ~10⁴ (Fig. 2d) within individual memory cell, promising low misreading probability in read operation. In contrast, as shown in black dots curve of Fig. 2b, no resistive switching behavior was observed in the reference device with a configuration of s-rGO/PVK/Al. By studying the effect of surface roughness (induced by sonication time) on the memory performance, results showed that the devices performed switching behavior from high resistance state to low resistance state with the ultrasonication time of GO precursor ranging from 3~9 h. Figure 2c demonstrated that the sonication time dependence of switching voltage, indicating that the switching voltage increase with the sonication time increase. However, this WORM switching behavior disappeared as the ultrasonication time applied to the GO precursor overwhelmed 12 h, suggesting that surface sharp morphologies of rGO electrodes play a crucial role in resistive switching behaviors of polymer memory devices, which was experimentally confirmed by works we demonstrated previously.

To evaluate the reproducibility and stability of the polymer memory device, other memory performances, including operation voltage distribution and retention abilities, were measured and investigated systematically. Figure 2e shows that the statistical distribution of switching voltage was calculated from 30 randomly selected memory cells, with an average value of 1.1 V, which is lower than that in most of previous reported PVK memory devices

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Acknowledgements

We thank primary financial supports by the National Key R&D Program of China (2017YFB1002900), the National Natural Science Foundation of China (61622402 and 61376088), the Research Fund for Postgraduate Innovation Project of Jiangsu Province (KYCX18_1123) and the Jiangsu Specially-Appointed Professor programme, the Six Talent Plan (2015XCL015).

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Authors and Affiliations

1. Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Nan**g Tech University (Nan**gTech), 30 South Puzhu Road, Nan**g, 211816, China

   Chaoyi Ban, **ang**g Wang, Zhe Zhou, Huiwu Mao, Shuai Cheng, Zepu Zhang, Zhengdong Liu, Hai Li, Juqing Liu & Wei Huang

2. Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, **’an, 710072, China

   Wei Huang

3. Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials (IAM), SICAM, Nan**g University of Posts & Telecommunications, 9 Wenyuan Road, Nan**g, 210023, China

   Wei Huang

Contributions

J.L. and W.H. conceived and supervised the experiments. C.B., J.L. H.M. and X.W. performed the device fabrication and electrical measurements. H.L. carried out the AFM characterization. C.B., Z.Z. and X.W. performed the theoretical calculation. C.B., Z.Z., S.C., Z.L., J.L. and W.H. performed the GO synthesis, characterization, collected data and wrote the manuscript. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Juqing Liu or Wei Huang.

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A Universal Strategy for Stretchable Polymer Nonvolatile Memory via Tailoring Nanostructured Surfaces

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Ban, C., Wang, X., Zhou, Z. et al. A Universal Strategy for Stretchable Polymer Nonvolatile Memory via Tailoring Nanostructured Surfaces. Sci Rep 9, 10337 (2019). https://doi.org/10.1038/s41598-019-46884-4

• Received: 19 November 2018

• Accepted: 24 June 2019

• Published: 17 July 2019

• DOI: https://doi.org/10.1038/s41598-019-46884-4

• Springer Nature Limited