From dead leaves to sustainable organic resistive switching memory

Accepted Manuscript From dead leaves to sustainable organic resistive switching memory Bai Sun, Shouhui Zhu, Shuangsuo Mao, Pingping Zheng, Yudong Xia, Feng Yang, Ming Lei, Yong Zhao PII: DOI: Reference:

S0021-9797(17)31397-8 YJCIS 23079

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Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

7 October 2017 1 December 2017 3 December 2017

Please cite this article as: B. Sun, S. Zhu, S. Mao, P. Zheng, Y. Xia, F. Yang, M. Lei, Y. Zhao, From dead leaves to sustainable organic resistive switching memory, Journal of Colloid and Interface Science (2017), doi: https://

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From dead leaves to sustainable organic resistive switching memory Bai Sun a,b,*, Shouhui Zhu a,b, Shuangsuo Mao a,b, Pingping Zheng a,b, Yudong Xia a,b,c , Feng Yang b, Ming Lei b, and Yong Zhao a,b,* a

School of Physical Science and Technology, Southwest Jiaotong University, Chengdu, Sichuan 610031, China b Key Laboratory of Magnetic Levitation Technologies and Maglev Trains, Ministry of Education of China, and Superconductivity and New Energy R&D Center, Southwest Jiaotong University, Chengdu, Sichuan 610031, China c State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 610054, China

ABSTRACT An environmental-friendly, sustainable, pollution-free, biodegradable, flexible and wearable electronic device hold advanced potential applications. Here, an organic resistive switching memory device with Ag/Leaves/Ti/PET structure on a flexible polyethylene terephthalate (PET) substrate was fabricated for the first time. We observed an obvious resistive switching memory characteristic with large switching resistance ratio and stable cycle performance at room temperature. This work demonstrates that leaves, a useless waste, can be properly treated to make useful devices. Furthermore, the as-fabricated devices can be degraded naturally without damage to the environment. Keywords: Dead leaves; Organic; Flexible; Resistive switching; Memory device.

* Corresponding author

Tel: +86-28-87600787

E-mail: [email protected];

[email protected]


1. Introduction Recently, with the progress of technology and the deterioration of environment, an environmental-friendly, sustainable, pollution-free, biodegradable, biodegradable electronic device will be favored by researchers [1-3]. Of course, with the quickening pace of life and improvement of work efficiency, an easily to carry and even wearable electronic product become the potential practical application value in the future [4]. Therefore, the preparation of an electronic device which meets the above requirements has attracted the attention of researchers. It would be the development trend of the next generation new concept electronic devices. Therefore, biomaterials-based resistive switching devices, specially using plant materials, have recently attracted increasing attention [5]. Of course, a leaves-based memory device is one of the most promising options because leaves are the most abundant renewable material on the earth. In the past few years, the preparation technique of electronic devices has been undergone revolutionary changed with the emergence of nanotechnology [6]. In terms of storage technology, the pursuit of higher storage density, faster read-write speed and higher integration density seems to be a significant trend of development [7]. However, after continuous screening, resistance random memory (RRAM) has almost become the most promising one [8]. The storage mechanism of RRAM cell is that the resistance of an active materials can be reversible switched between a high resistance state (HRS or “OFF”) and a low resistance state (LRS or “ON”) under the applied voltage pulse or current pulse [9, 10]. If the HRS or “OFF” is defined as logic "0", the LRS or “ON” is defined as logic "1", thus the switch memory behaviour can be applied for the storage of information. Herein, we demonstrate an environment-friendly, sustainable and flexible nonvolatile memory device composed mainly of leaves. Our memory device consists of leaves-based


active layer and a flexible substrate. The device exhibited electrochemical redox-based nonvolatile resistive switching behavior with a large switching resistance ratio and stable cycle performance at room temperature.

2. Materials and methods 2.1 Materials preparation Banyan leaves were collected from campus. Then we cleaned these leaves, cut and dried at 60 oC in an oven until it attains constant weight. Finally, these dried leaves were crushed, filtrated and dried to get fine leaves powder. 2.2 Preparation of device Ti bottom electrode was fabricated on PET substrates by vacuum deposition at room temperature. Then leaves film was spin-coated on Ti electrode. After leaves film deposition, the sample was dried at 60 oC in vacuum for 24 hours to remove toluene. Finally, the Ag top electrodes with the area of ~1 mm2 were deposited on the same side, thus we get a device with Ag/Leaves/Ti/PET layered structure, in which the as-prepared device is composed of ~95 vol.% leaves but the Ag and Ti is only about 5 vol.%. Therefore, the leaves are major component in our device. In addition, Ti and Ag are non-toxic and non-polluting for the environment, and can be recycled and reused. 2.3 Characterization Microstructure of leaves powder was characterized by X-ray diffraction (XRD, Shimadzu XRD-7000 X-ray diffractometer) with Cu Ka radiation. The chemical state of leaves powder was analyzed by X-ray photoelectron spectroscopy (XPSESCALAB250). In the test of resistive switching characteristics, Ag acts the top and the Ti as bottom electrodes. All the electric measurements were measured by the electrochemical workstation using cyclic scanning model at room temperature.


3. Result and discussion Fig. 1a shows the schematic image of a leaves-based memory device. The thickness of the PET substrate was ~50 μm. Then, the approximately 300 nm Ti thin film, which was used as bottom electrode, was deposited onto the PET substrate by vacuum magnetron sputtering. The resistive-switching active layer was created by spin-coating leaves powder. The thickness of the resistive-switching layer was ~400 nm. Finally, an Ag electrode was deposited onto the surface of leaves layer by a metal mask. A photograph of the as-prepared memory device is shown in Fig. 1a. In particularly, the as-prepared devices represent excellent flexibility from Fig. 1a. Fig. 1b shows the XRD pattern of the as-extracted leaves powder. The two broad peaks around 2 = 23o and 38o would be located in carbon [11]. The more detailed information about element constitutes of as-prepared leaves powder was further detected by X-ray photoelectron spectroscopy (XPS) measurements and the analysis results are shown in Fig.1c. Actually, it is known that the K, N and Na elements always existed in the leaves besides C, O, Si and Ca elements. However, the XPS used for element analysis has a limit amount of detection, we can only observe the four peaks at 284.8, 531.0 eV, 102.0 eV and 347.0 eV, which can be attributed to C 1s, O 1s, Si 2p and Ca 2p respectively. The extra K element is displayed by energy-dispersive X-ray (EDX) spectra in the inset of Fig. 1c.


Fig. 1 (a) The preparation process of leaves based resistive switching memory device. (b) XRD spectra of as-prepared leaves powder. (c) XPS survey spectra, the inset shows the EDX spectra of leaves powder.

Recently, bio-materials based memristor devices has been reported in many previous literatures and has received great attention [12-15]. To probe the resistive switching memory performance of leaves powder, a cycling scanning current-voltage (I–V) curve of Ag/Leaves/Ti/PET structure was investigated under a direct current sweeping mode by applying voltage to the top Ag electrode while keeping the Ti bottom electrode grounded. To avoid electrical permanent breakdown for the dielectric layer, the compliance current (CC) of 10 mA was used during the test process. We observe that the circular I-V characteristics curves of Ag/Leaves/Ti/PET device exhibit an asymmetric hysteresis behaviour (Fig. 2a) where these


arrows in our figure denote the sweeping directions of the applied voltage. In other words, the direct-current (DC) voltage was swept from 0 → 1.5 V → 0 → -1.5 V → 0. It is highly obvious that the as-prepared device display an obvious resistive switching memory behaviour with excellent conversion and switching reproducibility. Fig. 2b shows prominent I-V curve with resistive switching effects on a logarithmic scale. Simply speaking, our device is comparable to a bio-materials based resistive switching device reported in previous works [16].

Fig. 2 (a) The typical I-V characteristics curve of Ag/Leaves/Ti/PET structure. (b) The first cycle I–V curves on a logarithmic scale. (c) I-V curve of continuously switching 100 times. (d) Retention test of the Ag/Leaves/Ti/PET memory device under a read voltage of 0.3 V.

It is very obvious that the Ag/Leaves/Ti/PET device present a resistive switching memory effect with two stable resistance states when applying an electric field (Fig. 2b), sweeping the voltage from zero to positive up to the set voltage (VSet = 1.0 V) resulted in the device 6

switching from a HRS or “OFF” state to a LRS or “ON” state, which is denoted as the set process. When applied voltage subsequent sweep from zero to a negative voltage of about -1.5 V (VReset), our device reversibly exhibit a reset process from a LRS or “ON” state to HRS or “OFF” state. The two well-resolved states provide two memory logic states of the device. In other word, the two different logic states can write and read the data bits a RRAM element [17, 18]. Fig. 2c shows the I-V curve of continuously switching 100 times, we can see that the switching stability of the device is relatively stable, and there is a litter attenuation about 1 mA after 100 cycles. The resistance retention of both the HRS and LRS for the Ag/Leaves/Ti/PET device was tested and is shown in Fig. 2d. It is usually set at a low bias voltage value for fatigue testing to avoid the possible influence of read voltage [19, 20]. We can see that the resistance value is approximately 3.2 MΩ at HRS but it is only ~0.11 MΩ at LRS, indicating that high resistance OFF/ON ratios as large as 50 can be achieved in leaves-based RRAM devices, which generally implies a lower misreading rate during read operation for data storage [21]. More importantly, the resistance values in both states are stable and exhibit minimal degradation over 10 3 sec, illustrating that the resistive switching effects are stable, confirming the nonvolatile and the nondestructive readout properties of the device [22-24]. According to the above results, the resistive switching memory behavior in the Ag/Leaves/Ti/PET device strong evidence that our leaves-based resistive switching memory device is suitable and reliable for potential RRAM memory applications. Next, to comprehend the conduction and switching mechanisms of the device, the I –V curve of the first sweeping cycle in the positive and negative voltage regions for the Ag/Leaves/Ti/PET device were plotted and fitted on a log–log scale in Fig. 3a and b. Indeed, for the LRS (Fig. 3a, b), an Ohmic behavior (I ∝ V) is observed. However, for the HRS, an


Ohmic behavior (I ∝ V) is followed by a space-charge-limited (SCL) behavior (I ∝ V2) at higher voltage region, suggesting a change from the trap-unfilled SCL conduction to the trapfilled SCL conduction (Fig. 3b). While a larger slope (~3) emerges in the positive region. The increase of the slope could be due to the density and energy distribution of the traps (Fig . 3a) [25, 26].

Fig. 3 (a-b) The current-voltage (I–V) curves in log–log scale for Ag/Leaves/Ti/PET structure device, the scatters are experimental data and the straight lines are the fitting curves from the theoretical models. (c) A schematic diagram of the proposed model demonstrating the conduction process.


In many previous reports, the mechanisms for resistive switching in metal/organic/oxide structure have been extensively investigated [27-29]. The electrolytic migration of cations and oxygen vacancy in an insulator layer is one of the most common phenomena in electronics, which leads to the formation of metallic filaments in resistive switching devices [30, 31]. The transition between HRS and LRS states can be explained by fracturing and reformation of the filaments embedded in the insulating layer. In this work, the Ag ions, K ions and oxygen vacancy











Ag/Leaves/Ti/PET device and it can be described with the conducting filament model. A schematic diagram is shown in Fig. 3c-h. It is well known that the Ag atoms can be ionized into Ag ions under the electric field [32], which could be described as Ag → Ag+ + e-. At the same time, K atoms can be ionized into K ions under the electric field, which could be described as K → K+ + e-, and the DC electric field between anode and cathode will cause the Ag+, K+ and oxygen vacancy to migrate across the media layer (Fig. 3c). The mobility of the Ag+, K+ and oxygen vacancy could be enhanced by the local Joule heating under high current, and a high electrical field is effective to accelerate the electro-migration process, and the molecular chain of leaves can act a mobile channel for Ag+, K+ and oxygen vacancy, as shown in Fig. 3d. When the Ag+, K+ and oxygen vacancy are constantly reduced and accumulated to a certain extent, the conductivity of material would be increased greatly because the reduced Ag+, K+ and oxygen vacancy would play the role of conductive filaments, which can suddenly enhance the conductivity of the leaves layer to complete the “Set” process [33-35], as shown in Fig. 3d. In other words, the positive voltage (> VSet) on the Ag electrodes can generate a high electric field that drives Ag+, K+ and oxygen vacancy into the leaves layer to form conducting filaments inside the leaves layer for the device achieve the LRS or ‘ON’ state (Fig. 3e, f). After the ‘Set’ process, the device retains the ‘ON’ state unless a sufficient large voltage of opposite


polarity (< VReset) is applied, due to the Coulomb repulsion effect, the Ag+, K+ and oxygen vacancy will be pushed back to top electrode, and as a result, a large tunneling gap is formed after such a process. The number of electron hopping paths is significantly reduced after such a reset process, which results in the OFF state (HRS) (Fig. 3g), followed by the electric-field driven Ag+, K+ and oxygen vacancy drift back to the top electrode (Fig. 3h). Therefore, our devices can be reversibly switched by connect and disconnect of conducting filaments.

4. Conclusions In conclusion, the leaves powder was prepared from dry waste in the form of dead plant leaves without any external activation and studied for organic resistive switching memory application. The Ag/Leaves/Ti/PET device represent a high resistance OFF/ON ratio of about 30 and a long retention time of more than 10 3 s have been obtained for the resistance random access memory (RRAM). Our data suggest that the outstanding resistive switching memory in the Ag/Leaves/Ti/PET system results from formation and annihilation of conductive filaments. Our findings have provided important guidelines for not only the optimization of memory performance, but also the applications study of the resistive switching effect based on biomaterials. The excellent performance of such leaves-based resistive switching devices suggests that such organic memories complement the conventional inorganic counterparts, and these environmentally friendly biomaterials possess the potential for green and sustainable electronics and data storage.

Acknowledgements This work was supported by the National Magnetic Confinement Fusion Science Program (No. 2013GB114003-1, 2013GB110001), the National Natural Science Foundation of China (No.


51377138, 11504303), the 863 Program (No. 2014AA032701), the Sichuan Province Science Foundation (No. 2017JY0057), the application Infrastructure Projects in Sichuan Province (No. 2017JY0057).

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