- Email: [email protected]
Bistable electrical switching and nonvolatile memory effect in poly (9,9-dioctylfluorene-2,7-diyl) and multiple-walled carbon nanotubes
Organic Electronics 74 (2019) 110–117
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Bistable electrical switching and nonvolatile memory effect in poly (9,9dioctylfluorene-2,7-diyl) and multiple-walled carbon nanotubes
T
Ying Xina, Xiaofeng Zhaob, Hongyan Zhanga, Shuhong Wanga,*, Cheng Wanga,c,**, Dongge Mad, Pengfei Yanc,*** a
School of Chemical Engineering and Materials, Heilongjiang University, Harbin, 150080, PR China School of Electronic Engineering, Heilongjiang University, Harbin, 150080, PR China c Key Laboratory of Functional Inorganic Material Chemistry, (Heilongjiang University), Ministry of Education, Harbin, 150080, PR China d School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510640, PR China b
ARTICLE INFO
ABSTRACT
Keywords: PFO MWCNTs Flash memory device Nonvolatile behavior
The Suzuki coupling reaction was used to synthesize the poly (9,9-dioctylfluorene-2,7-diyl) (PFO), MWCNTs were doped in PFO by physical blending. Memristors were prepared with PFO and PFO:MWCNTs as active layer, and the effect of MWCNTs content on the electrical characteristics of the device was proved. The results reveal that the devices with doped and undoped MWCNTs had bistable nonvolatile memory behavior. After doping MWCNTs, the ON/OFF state current ratio is obviously improved and the device with 1.6% MWCNTs content shows the maximum ON/OFF curent ratio of 2.6 × 103 and lower threshold voltage approximately −0.7 V. Futhermore, the device remains stable for 3 h and the curent has no obvious change after 9000 read cycles, whether it is ON state or OFF state.
1. Introduction
a great application prospect in electronic devices, polymer reinforcement materials and other fields [12–15]. According to reports, CNTs were studied more and more in memristor based on organic materials, including polystyrene (PS) [16], poly (3,4-ethylenedioxythio-phene):poly (styrenesulfonate) (PEDOT: PSS) [17], poly (vinyl alcohol) (PVA) [18], polyurethane (PU) [19], poly (mehyl methacrylate) (PMMA) [20] and so forth. The researches show that the device contains 1 wt% MWCNTs [21], it exhibits WORM memory device and electrical performance is improved, the MWCNTs content is increased to 2 wt% [22], and the difference is that the device is a rewritable memory behavior. Based on the above analysis, we synthesized poly (9,9-dioctylfluorene-2,7-diyl) (PFO) through the Suzuki coupling reaction method in the first place. Secondly, we prepared PFO solution containing different concentrations of MWCNTs, and made the memristors based on four different solutions by spin coating. In order to clarify the impact of MWCNTs on devices, we made current-voltage characteristic curve and stability of the devices. The results indicate that although the content of MWCNTs is beneficial to the improvement of switching ratio, it has not always increased. When the content of MWCNTs is 1.6%, the ON/OFF current ratio reaches the maximum value of 2.6 × 103. It is worth noting that the devices have a good stability no matter how much the
In today's society, the amount of information is increasing rapidly, so the demand for information storage is becoming more and more important. In recent years, Organic resistive random access memory (ORRAM) devices have attracted the attention of many researchers, the memory device is a nonlinear resistance that can memorize the current, and by virtue of its superior characteristics, it will become a promising storage element in the future [1–4]. By controlling current of the device, the resistance can be changed, if the low conduction state is defined as “1” and the high conduction state is defined as “0”, like this resistance, storage information can be achieved [5,6]. At present, many researchers have put their eyes in the direction of the memristors based on the doping of organic/inorganic materials, especially the doping of organic/carbon-based composite organic materials. For example, GO is doped in the following organic materials: poly (3-hexylthiophene) (P3HT) [7], poly (N-vinylcarbazole) (PVK) [8], poly (vinylidene fluoride) (PVDF) [9], poly (4-vinyl phenol) [10], P3HT:PCBM [11] and so on. In addition, due to CNTs have a unique small size effect, large specific surface area, high strength, high modulus, high chemical and thermal stability, and good conductivity. It has *
Corresponding author. Corresponding author. School of Chemical Engineering and Materials, Heilongjiang University, Harbin, 150080, PR China. *** Corresponding author. E-mail addresses: [email protected] (S. Wang), [email protected] (C. Wang). **
https://doi.org/10.1016/j.orgel.2019.07.003 Received 20 May 2019; Received in revised form 26 June 2019; Accepted 1 July 2019 Available online 02 July 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
Organic Electronics 74 (2019) 110–117
Y. Xin, et al.
were dissolved in 10 mL isopropanol under ultrasonic for 50 min to form a homogeneous suspension, furthermore, the MWCNTs suspension was mixed with the previously filtered PFO solution and stirred well at least 4 h. Samples with different concentrations of MWCNTs were prepared, as shown in Table 1. Finally, The ITO/PFO:MWCNTs/Al devices were prepared by using the following procedure: the prepared solution was spin-coated on the ITO glass at a speed rate of 900 rpm for 15 s and then 4000 rpm for 45 s, and then vacuum drying at 80 °C for 8 h. Next, the Al top electrode layer about 300 nm was vacuum thermally evaporated under high vacuum of 10−7 Torr through a shadow mask. The structure of ITO/PFO:MWCNTs/Al device is shown in Fig. 2(b). 2.3. Measurement
Fig. 1. The transmission electron microscope images of MWCNTs, (a) low magnification and (b) high magnification.
The I–V characteristic curve was carried out by a Keithley 4200-SCS semiconductor parameter analyzer with a probe station at room temperature. The morphologies of PFO and PFO:MWCNTs composite films were performed by field-emission scanning electron microscopy (FESEM) (Hitachi, S-4800). The molecular orbitals were tested by the density function theory (DFT) using Parr correlation functional method (B3LYP) and the 6-31G basis set. 3. Results and discussion 3.1. SEM of PFO and PFO:MWCNTs composite film
Fig. 2. (a) The chemical structure of PFO, (b) Schematic of ITO/PFO: MWCNTs/Al device.
2. Experimental details
Fig. 3 reveals the SEM images of ITO/PFO/Al and ITO/ PFO:MWCNTs/Al. From the top to bottom indicates PFO composite film, ITO film and glass are shown in Fig. 3(a), and The PFO:MWCNTs composite film, the ITO film and glass are respectively expressed from top to bottom in Fig. 3(b). The thickness of active film is 130 nm and 148 nm, respectively.
2.1. Materials
3.2. Characteristics of the memory device
All the organic solvents and anhydrous sodium carbonate in the experiment were purchased from Sinopharm Co. Ltd. 9,9-Dioctyl-2,7dibromofluorene and 9,9-Dioctyfluorene-2,7-diboronic acid bis (1,3propanediol) ester were purchased from Sigma-Aldrich without further purification. MWCNTs have been treated by acidification for 3 h. The transmission electron microscope images of the MWCNTs as shown in Fig. 1. PFO was synthesized by the Suzuki method [23]. Fig. 2(a) shows chemical structures of PFO.
3.2.1. Current-voltage curve of the memory device For the memory device, the I–V characteristic curve is a very important parameter, and the large current range is beneficial for storing data. Therefore, we have conducted a corresponding I–V test for the device with doped and undoped MWCNTs, as shown in Fig. 4(a). From the curves, we can find that the five samples all exhibit similar memory behavior, which is a bistable nonvolatile flash type. For the ITO/PFO/Al device (sample A), the first scan voltage is from 0 to −6 V, as the voltage increases, the current is also slowly rising, but when the voltage increases to −1.2 V, the current rapidly rises from 1.07 × 10−3 to 2.16 × 10−2, which indicates that the device changes from low conduction state (OFF) to high conduction state (ON). This process is “writing”. In the subsequent voltage from −6 to 0 V, the device has been in the ON state, and the process is consistent with “read”. When the reverse threshold voltage is about 4.3 V, it can be clearly seen from the figure that the current drops suddenly, the process is defined as “erasing”. After applying a voltage of 6 to 0 V, the device returned to its
MWCNTs content is in the device.
2.2. Fabrication of the memory device In order to fabricate the devices, first of all, ITO conductive glass of dimensions 2 cm × 1 cm (sheet resistance R□ = 6–9 Ω/□) were precleaned sequentially with the detergent, deionized water, acetone, isopropanol and methanol in an ultrasonic bath for 20 min, respectively, and dried in a vacuum oven at 60 °C for 24 h, secondly, we prepared the PFO toluene solution of 3 mg/mL, and then MWCNTS Table 1 Different concentration of the composite films used in each sample. Sample
Content of MWCNTs isopropanol solution (mg/mL)
Volume of MWCNTs solution (mL)
Volume of PFO toluene solution (3 mg/ mL) (mL)
MWCNTs Content in the composite film (%)
A B C D E
0 0.05 0.1 0.2 0.3
1 1 1 1 1
1 1 1 1 1
0 1.6 3.2 6.3 9.1
111
Organic Electronics 74 (2019) 110–117
Y. Xin, et al.
Fig. 3. Cross-section scanning electron microscopic images of the device based on (a) ITO/PFO/Al and (b) ITO/PFO:MWCNTs/Al.
Fig. 4. (a) I–V curves and (b) Relationship between the ON/OFF ratio of current and the applied voltage for the device based on Sample A, Sample B, Sample C, Sample D and Sample E.
original state (OFF). The above process makes a cycle. The rest of four samples with doping MWCNTs exhibit the same trend. For the device with 1.6% MWCNTs (sample B), the current rises with the increase of voltage, the turn-on voltage is −0.7 V, the current is rapidly from 4.23 × 10−6 to 1.08 × 10−2. When the reverse positive bias is applied to 3.9 V, the device completes the “erasing” process and recurs the low conduction state. Similarly, the trend of the four curves for the sample C is the same as that of the previous ones. That is, the current rapidly increases from 4.06 × 10−5 to 3.75 × 10−2 when the threshold voltage is reached (−1.8 V). During subsequent voltage application, the device is always in a low resistance state, but when the reverse voltage is applied to 3.8 V, the current begins to decrease obviously, which indicates the device is converted to the closed state. The threshold voltage of the device (sample D) is −1.1 V and 4.2 V. In the process of writing, the current increases from 4.98 × 10−5 to 1.90 × 10−2. Thereafter, the device maintains low resistance state until the positive voltage applied to 4.2 V, and then it begins to the high resistance state. The relationship between the voltage and current of the device (sample E) is consistent with the memory behavior. Fig. 4(a) reveals that, at the beginning, the current gradually increases, and then when the turn-on voltage is near −1.3 V, the current suddenly increases to1.11 × 10−2. The device has been kept ON state until the turn-off voltage is 5.7 V. In order to more intuitively observe the influence of different MWCNTs content on the memristors, we construct the relationship between voltage and current, such as Fig. 4(b). It is easily perceived that the ON/OFF state current ratio of the device with doped MWCNTs is higher than that of the undoped, furthermore, the switch ratio of the device contains 1.6% MWCNTs reaches the maximum of 2.6 × 103.
3.2.2. Stability of the memory device In addition to the I–V curve, the stability of the device is also an important basis for measuring the memristor. Therefore, we have measured the retention time and the cycle number of the device, such as Fig. 5 and Fig. 6. Fig. 5 reveals the result of the retention time for all samples. In order to ensure the reliability of the result, our test conditions are the same, that is, the applied voltage is 2 V and the test time is 3 h at room temperature. For all devices, whether it is in ON state or OFF state, the current is basically unchanged, and the switching current ratio is about 20, 2600, 920, 380, 140, respectively, it is consistent with the original data. As shown in Fig. 6, the cycle numbers of the four samples were measured at 2 V pulse (pulse period = 2 ms, pulse width = 1 ms) and the result shows that the device is stable. Based on all of the above analysis, we can conclude that the memristor is not only very stable but also has application value in storing information. By analyzing the I–V curve and stability of the device, we can realize that doping MWCNTs has a certain effect on the electrical performance of the device. In order to understand the impact of MWCNTs on the device, the curve of MWCNTs content, threshold voltage and ON/OFF state current ratio is shown in Fig. 7. Fig. 7 indicates that with the increase of MWCNTs content, the threshold voltage decreases, and the ON/OFF state current ratio increases to the maximum value and follows by a downward trend. Although the switch ratio is reduced, it is higher than that of the device without doped MWCNTs. Because, with the increase of MWCNTs content, the effective distance between isolated MWCNTs is shortened, which is propitious to form a conductive pathway in the device. As a result, a large number of carriers pass through and continuous jump between two electrodes, which leads to increase switch ratio [24–26]. Subsequently, due to the distance
112
Organic Electronics 74 (2019) 110–117
Y. Xin, et al.
Fig. 5. Retention performance on the ON and OFF states of the device based on (a) Sample A (b) Sample B (c) Sample C (d) Sample D and (e) Sample E.
between the individual MWCNTs is getting smaller and smaller, which count against the carriers hopping, so the ON/OFF state current ratio decreases, and then the device is closer to the conductor behavior [27].
1.12, 1.08, 1.11, respectively, which is consistent with Ohm's law; (b) at high voltage region, the slope of the devices are 1.93, 1.80, 1.81, 1.81, 1.81 (nearly 2), which indicates that the voltage and current are nonlinear, but in accordance with the Child's law (I ∝ V2) [28–30]. In this process, with the increase of voltage, the current increases slowly, mainly because carbon nanotubes act as trap to capture carriers, and when the voltage increases to the threshold voltage, the traps are all filled, forming conductive filaments, so the current increases rapidly. Continuing to increase the voltage, the device maintains a high conductivity because the carriers maintain a stable output after the traps are all filled.
3.3. Switching mechanism and stored procedure of the memory device 3.3.1. Mechanism of the memory device In order to clarify the switching mechanism of the memory device, we have a linear fitting of the I–V curve. Log-log plotting is performed in the ON and OFF states during the RESET process. Fig. 8 indicates the linear fitting of the device contains a different concentration of MWCNTs. It is noteworthy that they are composed of Ohmic conduction and SCLC conduction. In the beginning, the devices based on samples A, B, C, D and E are in a high conductive state (ON state), the slopes are 0.94, 0.99, 1.05, 1.00, 1.02 (nearly 1), indicating a direct proportion between voltage and current (I ∝ V), which is consistent with Ohm's law. The cause of this phenomenon is the formation of a filament path in the device. On the contrary, in the high resistance state (OFF state), the fitting result is accord with the space charge limited current (SCLC) conduction model. The SCLC mechanism includes two parts: (a) the slope of the devices are close to 1 at low voltage, which is 1.08, 1.10,
3.3.2. Stored procedure of the memory device Although we have fully discussed the switching mechanism of the memristor, the specific stored procedure has not been carefully discussed. So, we mainly use HOMO-LUMO and energy level diagram of the device to analyze their stored procedures. We use DFT (B3LYP/631G) to calculate the HOMO, LUMO and band gap of PFO. HOMO is the highest occupied molecular orbital, which is related to the ability to supply electrons, nevertheless, LUMO (lowest unoccupied molecular orbital) represents the accept electronics capability [31]. The band gap is the difference between HOMO and LUMO. The corresponding energy
113
Organic Electronics 74 (2019) 110–117
Y. Xin, et al.
Fig. 6. Stimulus effect of read pulses on the ON and OFF states of the device based on (a) Sample B (b) Sample C (c) Sample D and (d) Sample E.
understand the storage process of the memristors. For the ITO/PFO/Al device (Fig. 9(a)), Al is the top electrode (−4.30 eV), ITO is the bottom electrode (−4.80 eV) [32]. The energy barrier between the ITO electrode and the HOMO of the PFO is 0.96 eV, while the energy barrier between the Al electrode and the LUMO of the PFO is 3.26 eV, thus the holes injection are easier than the electron injection. At low bias, the holes are gradually injected from ITO to HOMO of the polymer and jump along the polymer chain, at this point the device is in a high resistance state (OFF). When the scanning voltage is increased to the threshold voltage, the electrons overcome the energy barrier between the Al electrode and LUMO of the PFO and then a large number of electrons are injected into the device. Soon afterwards, most of the charges trapping centers are filled, and the carriers increase rapidly, so the device converts to a low resistance state. With the increase of voltage, the carriers can move freely between the two electrodes, the device forms a conductive filament in the meantime. When the reverse voltage is applied, the device keeps ON state and produces a lot of heat. But when the Vreset is 4.3 V, the charges exceed the capacity of the filament, and the superfluous heats destroy the filament, therefore the device returns to original state [33,34]. Fig. 9(b) represents stored process of the device with doped MWCNTs. The function of MWCNTs is to capture and transport electronics. The work function of MWCNTs is lower than the LUMO of the polymer [35], so the electrons enter into MWCNTs from the Al
Fig. 7. The switch-on voltage and the ON/OFF state current ratio of the device with different MWCNTs content.
levels are −5.76 eV, −0.74 eV and 5.02 eV, respectively, as shown in Table 2. As shown in Fig. 9, we have done the energy level diagram of the device with undoped MWCNTs and the doped MWCNTs to better
114
Organic Electronics 74 (2019) 110–117
Y. Xin, et al.
Fig. 8. Linear fitting and corresponding slopes for the I–V curves of the device based on (a) Sample A (b) Sample B (c) Sample C (d) Sample D and Sample E.
the device, and then the electrons and holes move between the two electrodes and the polymer. When the turn-on voltage is reached, the carriers overcome the energy barrier and enter into the LUMO of the polymer. At this time the phenomenon is that the current suddenly increases and then the devices turn from the OFF state to the ON state. After the application of the reverse voltage, the heat generated by the device destroys the conduction channel, so it returns the OFF state. The above analyses fully show that the device is a bistable nonvolatile behavior [36–38].
Table 2 Molecular simulation result for the PFO. Frontier molecular orbital
Energy level (eV)
HOMO
−5.76
LUMO
−0.74
4. Conclusions In brief, we have synthesized PFO by Suzuki coupling reaction, and doped a different content of MWCNTs to prepare memristors, subsequently, a series of tests were carried out. The results reveal that all devices are bistable nonvolatile memory device, and with the increase of MWCNTs content, the ON/OFF state current ratio increases to the maximum of 2.6 × 103 and then reduces. The threshold voltage is also reduced from −1.2 V to −0.7 V. In addition, the retention time of the devices was under the condition of 2 V and the test lasted for 3 h. The results exhibit that the current has no obvious mutation, which is almost consistent with the original value. By testing the cycle numbers of the memristors at 2 V read pulse, the current is basically stable after 9000 cycles. Based on the above tests, it indicates that the device has a good stability, which can reduce the misreading rate of the device, which is beneficial to the application of memristor in data storage.
electrode are easier than the holes inject from ITO to the HOMO of the PFO. At the beginning, the voltage is low, and the electrons get into the MWCNTs and move along the MWCNTs tube, the devices show a low conduction state. As the voltage increases, the holes are injected into 115
Organic Electronics 74 (2019) 110–117
Y. Xin, et al.
Fig. 9. Energy level diagrams for the device (a) pure PFO (b) PFO:MWCNTs.
Acknowledgements
[17] J.J.L. Hmar, Flexible resistive switching bistable memory devices using ZnO nanoparticles embedded in polyvinyl alcohol (PVA) matrix and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), RSC Adv. 8 (2018) 20423–20433. [18] S. Chandrakishore, A. Pandurangan, Facile synthesis of carbon nanotubes and their use in the fabrication of resistive switching memory devices, RSC Adv. 4 (2014) 9905–9911. [19] Y.M. Sun, F.J. Miao, R. Li, Bistable electrical switching and nonvolatile memory effect based on the thin films of polyurethane-carbon nanotubes blends, Sensor Actuat A-Phys. 234 (2015) 282–289. [20] L. Li, D.Z. Wen, Memory behavior of multi-bit resistive switching based on multiwalled carbon nanotubes, Org. Electron. 34 (2016) 12–17. [21] G. Liu, Q.D. Ling, E.T. Kang, K.G. Neoh, Bistable electrical switching and write-once read-many-times memory effect in a donor-acceptor containing polyfluorene derivative and its carbon nanotube composites, J. Appl. Phys. 102 (2007) 0245021–0245028. [22] G. Liu, Q.D. Ling, E.Y.H. Teo, C.X. Zhu, D.S.H. Chan, K.G. Neoh, E.T. Kang, Electrical conductance tuning and bistable switching in poly(Nvinylcarbazole)?Carbon nanotube composite films, ACS Nano 3 (2009) 1929–1937. [23] Y. Xin, X.F. Zhao, X.K. Jiang, Q. Yang, J.H. Huang, S.H. Wang, R.R. Zheng, C. Wang, Y.J. Hou, Bistable electrical switching and nonvolatile memory effect by doping different content of GO in Poly (9,9-dioctylfluorene-2,7-diyl), RSC Adv. 8 (2018) 6878–6886. [24] Y.M. Sun, J.G. Lu, C.P. Ai, D.Z. Wen, Nonvolatile memory devices based on poly (vinylalcohol) + graphene oxide hybrid composites, Phys. Chem. Chem. Phys. 18 (2016) 11341–11347. [25] W.B. Zhang, C. Wang, G. Liu, X.J. Zhu, X.X. Chen, L. Pan, H.W. Tan, W.H. Xue, Z.H. Ji, J. Wang, Y. Chen, R.W. Li, Thermally-stable resistive switching with a large ON/OFF ratio achieved in poly(triphenylamine), Chem. Commun. 50 (2014) 11856–11858. [26] Y.Q. Li, H.H. Xu, X. Tao, K.J. Qian, S. Fu, Y.Z. Shen, S.J. Ding, Synthesis and memory characteristics of highly organo-soluble polyimides bearing a noncoplanar twisted biphenyl unit containing aromatic side-chain groups, J. Mater. Chem. 21 (2011) 1810–1821. [27] C.J. Chen, J.H. Wu, G.S. Liou, Thermally stable and high ON/OFF ratio non-volatile memory devices based on poly(triphenylamine) with pendent PCBM, Chem. Commun. 50 (2014) 1335–1337. [28] T.W. Kim, S.H. Oh, H.J. Choi, G. Wang, H.H. Wang, D.Y. Kim, T. Lee, Reversible switching characteristics of polyfluorene-derivative single layer film for nonvolatile memory devices, Appl. Phys. Lett. 92 (2008) 2533081–2533083. [29] J.Q. Liu, Z.Y. Zeng, X.H. Cao, G. Lu, L.H. Wang, Q.L. Fan, W. Huang, H. Zhang, Preparation of MoS2 –polyvinylpyrrolidone nanocomposites for flexible nonvolatile rewritable memory devices with reduced graphene oxide electrodes, Small 8 (2012) 3517–3522. [30] B.J. Ree, W. Kwon, K. Kim, Y.G. Ko, Y.Y.K. IM, h. Lee, M. Ree, Clues to the electrical switching mechanism of carbazole-containing polyimide thin films, ACS Appl. Mater. Interfaces 6 (2014) 21692–21701. [31] P.M. Niamien, F.K. Essy, A. Trokourey, A. Yapi, H.K. Aka, D. Diabate, Correlation between the molecular structure and the inhibiting effect of some benzimidazole derivatives, Mater. Chem. Phys. 136 (2012) 59–65. [32] D. Wang, H. Li, N.J. Li, Y. Zhao, Q.H. Zhou, Q.F. Xu, J.M. Lu, L.H. Wang, A new DRAM-type memory devices based on polymethacrylate containing pendant 2methylbenzothiazole, Mater. Chem. Phys. 134 (2012) 273–278. [33] Y.M. Sun, L. Li, D.Z. Wen, X.D. Bai, G. Li, Bistable electrical switching and nonvolatile memory effect in carbon nanotube – poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) composite films, Phys. Chem. Chem. Phys. 17 (2015) 17150–17158. [34] Y.C. Chang, Y.H. Wang, ACS appl. Resistive switching behavior in gelatin thin films for nonvolatile memory application, Mater. Interfaces 6 (2014) 5413–5421. [35] S.P. Somani, P.R. Somani, M. Umeno, Carbon nanotube incorporation: a new route
The authors are grateful to the support of the National Natural Science Foundation of China (grant numbers 51527804), Heilongjiang university JMRH project (HDJMRH201913), and Natural Science Foundation of Heilongjiang Province of China (grant number B2017010). References [1] W.T. Kim, J.H. Jung, T.W. Kim, Carrier transport mechanisms in nonvolatile memory devices fabricated utilizing multiwalled carbon nanotubes embedded in a poly-4-vinyl-phenol layer, Appl. Phys. Lett. 95 (2009) 1–3. [2] W.P. Lin, S.J. Liu, T. Gong, Q. Zhao, W. Huang, Polymer-based resistive memory materials and devices, Adv. Mater. 26 (2014) 570–606. [3] G.L. Ding, Y.X. Wang, G.X. Zhang, K. Zhou, K.L. Zeng, Z.X. Li, Y. Zhou, C. Zhang, X.L. Chen, S.T. Han, 2D metal–organic framework nanosheets with time-dependent and multilevel memristive switching, Adv. Funct. Mater. 29 (2019) 1–11. [4] Y.Y. Ren, H.L. Ma, W. Wang, Z.Q. Wang, H.Y. Xu, X.N. Zhao, W.Z. Liu, J.G. Ma, Y.C. Liu, Cycling-induced degradation of organic–inorganic perovskite-based resistive switching memory, Adv. Mater. Technol. 4 (2019) 1–6. [5] Z.J. Wang, K. Sierros, M.S. Seehra, D. Korakakis, Development of indigo-based nonvolatile write-once-read-many-times memory device, Mater. Lett. 206 (2017) 128–131. [6] Y.Y. Shan, Z.S. Lyu, X.W. Guan, A. Younis, G.L. Yuan, J.L. Wang, S. Li, Tom Wu, Solution-processed resistive switching memories based on hybrid organic-inorganic materials and composites, Phys. Chem. Chem. Phys. 20 (2018) 23837–23846. [7] D. Chaudhary, S. Munjal, N. Khare, V.D. Vankar, Bipolar resistive switching and nonvolatile memory effect in poly(3-hexylthiophene) ecarbon nanotube composite films, Carbon 130 (2018) 553–558. [8] F.Y. Sun, D.Z. Wen, Y.M. Sun, Influence of blending ratio on resistive switching effect in donor-acceptor type composite of PCBM and PVK-based memory devices, Org. Electron. 65 (2019) 141–149. [9] A. Thakre, H. Borkar, B.P. Singh, A. Kumar, Electroforming free high resistance resistive switching of graphene oxide modified polar-PVDF, RSC Adv. 5 (2015) 57406–57413. [10] L.Y. Xiang, J. Ying, J.H. Han, L.T. Zhang, W. Wang, High reliable and stable organic field-effect transistor nonvolatile memory with a poly(4-vinyl phenol) charge trapping layer based on a pn-heterojunction active layer, Appl. Phys. Lett. 108 (2016) 1–4. [11] P. Aruna, K. Suresh, C.M. Joseph, Effect of fullerene doping on the electrical propertie of P3HT/PCBM layers, Mater. Sci. Semicond. Process. 36 (2015) 7–12. [12] G.Z. Sun, J.Q. Liu, L.X. Zheng, W. Huang, H. Zhang, Preparation of weavable, allcarbon fibers for non-volatile memory devices, Angew. Chem. 125 (2013) 13593–13597. [13] J. Aimi, C.T. Lo, H.C. Wu, C.F. Huang, T. Nakanishi, M. Takeuchi, W.C. Chen, Phthalocyanine-cored star-shaped polystyrene for nano floating gate in nonvolatile organic transistor memory device, Adv. Electron. Mater. 2 (2016) 1–9. [14] G. Koplovitz, D. Primc, O.B. Dor, S. Yochelis, D. Rotem, D. Porath, Y. Paltiel, Magnetic nanoplatelet-based spin memory device operating at ambient temperatures, Adv. Mater. 29 (2017) 1–6. [15] S.P. Ge, Y.X. Huang, X.J. Chen, X.R. Zhang, Z.C. Xiang, R.X. Zhang, W.P. Li, Y.M. Cui, Silver iodide induced resistive switching in CsPbI3 perovskite-based memory device, Adv. Mater. Interfaces 6 (2019) 1–9. [16] J. Wang, X. Wang, J.J. Xu, L.H. Xie, M.D. Yi, W. Huang, The trapping, detrapping, and transport of the ambipolar charges in the electret of Polystyrene/C60 blend films, Org. Electron. 44 (2017) 247–252.
116
Organic Electronics 74 (2019) 110–117
Y. Xin, et al. to improve the performance of organic–inorganic heterojunction solar cells, Diam. Relat. Mater. 17 (2008) 585–588. [36] B.L. Hu, C.Y. Wang, J. Zhang, K. Qian, P.S. Lee, Q.C. Zhang, Organic memory effect from donor–acceptor polymers based on 7-perfluorophenyl-6H-[1,2,5]-thiadiazole [3,4-g]benzoimidazole, RSC Adv. 5 (2015) 77122–77129. [37] C.C. Hung, Y.C. Chiu, H.C. Wu, C. Lu, C. Bouilhac, L. Otsuka, S. Halila, R. Borsali,
S.H. Tung, W.C. Chen, Conception of stretchable resistive memory devices based on nanostructure-controlled carbohydrate-block-polyisoprene block copolymers, Adv. Funct. Mater. 27 (2017) 1–10. [38] A. Rani, D.B. Velusamy, F.M. Mota, Y.H. Jang, R.H. Kim, C. Park, D.H. Kim, Onestep all-solution-based Au–GO core–shell nanosphere active layers in nonvolatile ReRAM devices, Adv. Funct. Mater. 27 (2017) 16046041–16046048.
117
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Bistable electrical switching and nonvolatile memory effect in poly (9,9dioctylfluorene-2,7-diyl) and multiple-walled carbon nanotubes
T
Ying Xina, Xiaofeng Zhaob, Hongyan Zhanga, Shuhong Wanga,*, Cheng Wanga,c,**, Dongge Mad, Pengfei Yanc,*** a
School of Chemical Engineering and Materials, Heilongjiang University, Harbin, 150080, PR China School of Electronic Engineering, Heilongjiang University, Harbin, 150080, PR China c Key Laboratory of Functional Inorganic Material Chemistry, (Heilongjiang University), Ministry of Education, Harbin, 150080, PR China d School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510640, PR China b
ARTICLE INFO
ABSTRACT
Keywords: PFO MWCNTs Flash memory device Nonvolatile behavior
The Suzuki coupling reaction was used to synthesize the poly (9,9-dioctylfluorene-2,7-diyl) (PFO), MWCNTs were doped in PFO by physical blending. Memristors were prepared with PFO and PFO:MWCNTs as active layer, and the effect of MWCNTs content on the electrical characteristics of the device was proved. The results reveal that the devices with doped and undoped MWCNTs had bistable nonvolatile memory behavior. After doping MWCNTs, the ON/OFF state current ratio is obviously improved and the device with 1.6% MWCNTs content shows the maximum ON/OFF curent ratio of 2.6 × 103 and lower threshold voltage approximately −0.7 V. Futhermore, the device remains stable for 3 h and the curent has no obvious change after 9000 read cycles, whether it is ON state or OFF state.
1. Introduction
a great application prospect in electronic devices, polymer reinforcement materials and other fields [12–15]. According to reports, CNTs were studied more and more in memristor based on organic materials, including polystyrene (PS) [16], poly (3,4-ethylenedioxythio-phene):poly (styrenesulfonate) (PEDOT: PSS) [17], poly (vinyl alcohol) (PVA) [18], polyurethane (PU) [19], poly (mehyl methacrylate) (PMMA) [20] and so forth. The researches show that the device contains 1 wt% MWCNTs [21], it exhibits WORM memory device and electrical performance is improved, the MWCNTs content is increased to 2 wt% [22], and the difference is that the device is a rewritable memory behavior. Based on the above analysis, we synthesized poly (9,9-dioctylfluorene-2,7-diyl) (PFO) through the Suzuki coupling reaction method in the first place. Secondly, we prepared PFO solution containing different concentrations of MWCNTs, and made the memristors based on four different solutions by spin coating. In order to clarify the impact of MWCNTs on devices, we made current-voltage characteristic curve and stability of the devices. The results indicate that although the content of MWCNTs is beneficial to the improvement of switching ratio, it has not always increased. When the content of MWCNTs is 1.6%, the ON/OFF current ratio reaches the maximum value of 2.6 × 103. It is worth noting that the devices have a good stability no matter how much the
In today's society, the amount of information is increasing rapidly, so the demand for information storage is becoming more and more important. In recent years, Organic resistive random access memory (ORRAM) devices have attracted the attention of many researchers, the memory device is a nonlinear resistance that can memorize the current, and by virtue of its superior characteristics, it will become a promising storage element in the future [1–4]. By controlling current of the device, the resistance can be changed, if the low conduction state is defined as “1” and the high conduction state is defined as “0”, like this resistance, storage information can be achieved [5,6]. At present, many researchers have put their eyes in the direction of the memristors based on the doping of organic/inorganic materials, especially the doping of organic/carbon-based composite organic materials. For example, GO is doped in the following organic materials: poly (3-hexylthiophene) (P3HT) [7], poly (N-vinylcarbazole) (PVK) [8], poly (vinylidene fluoride) (PVDF) [9], poly (4-vinyl phenol) [10], P3HT:PCBM [11] and so on. In addition, due to CNTs have a unique small size effect, large specific surface area, high strength, high modulus, high chemical and thermal stability, and good conductivity. It has *
Corresponding author. Corresponding author. School of Chemical Engineering and Materials, Heilongjiang University, Harbin, 150080, PR China. *** Corresponding author. E-mail addresses: [email protected] (S. Wang), [email protected] (C. Wang). **
https://doi.org/10.1016/j.orgel.2019.07.003 Received 20 May 2019; Received in revised form 26 June 2019; Accepted 1 July 2019 Available online 02 July 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
Organic Electronics 74 (2019) 110–117
Y. Xin, et al.
were dissolved in 10 mL isopropanol under ultrasonic for 50 min to form a homogeneous suspension, furthermore, the MWCNTs suspension was mixed with the previously filtered PFO solution and stirred well at least 4 h. Samples with different concentrations of MWCNTs were prepared, as shown in Table 1. Finally, The ITO/PFO:MWCNTs/Al devices were prepared by using the following procedure: the prepared solution was spin-coated on the ITO glass at a speed rate of 900 rpm for 15 s and then 4000 rpm for 45 s, and then vacuum drying at 80 °C for 8 h. Next, the Al top electrode layer about 300 nm was vacuum thermally evaporated under high vacuum of 10−7 Torr through a shadow mask. The structure of ITO/PFO:MWCNTs/Al device is shown in Fig. 2(b). 2.3. Measurement
Fig. 1. The transmission electron microscope images of MWCNTs, (a) low magnification and (b) high magnification.
The I–V characteristic curve was carried out by a Keithley 4200-SCS semiconductor parameter analyzer with a probe station at room temperature. The morphologies of PFO and PFO:MWCNTs composite films were performed by field-emission scanning electron microscopy (FESEM) (Hitachi, S-4800). The molecular orbitals were tested by the density function theory (DFT) using Parr correlation functional method (B3LYP) and the 6-31G basis set. 3. Results and discussion 3.1. SEM of PFO and PFO:MWCNTs composite film
Fig. 2. (a) The chemical structure of PFO, (b) Schematic of ITO/PFO: MWCNTs/Al device.
2. Experimental details
Fig. 3 reveals the SEM images of ITO/PFO/Al and ITO/ PFO:MWCNTs/Al. From the top to bottom indicates PFO composite film, ITO film and glass are shown in Fig. 3(a), and The PFO:MWCNTs composite film, the ITO film and glass are respectively expressed from top to bottom in Fig. 3(b). The thickness of active film is 130 nm and 148 nm, respectively.
2.1. Materials
3.2. Characteristics of the memory device
All the organic solvents and anhydrous sodium carbonate in the experiment were purchased from Sinopharm Co. Ltd. 9,9-Dioctyl-2,7dibromofluorene and 9,9-Dioctyfluorene-2,7-diboronic acid bis (1,3propanediol) ester were purchased from Sigma-Aldrich without further purification. MWCNTs have been treated by acidification for 3 h. The transmission electron microscope images of the MWCNTs as shown in Fig. 1. PFO was synthesized by the Suzuki method [23]. Fig. 2(a) shows chemical structures of PFO.
3.2.1. Current-voltage curve of the memory device For the memory device, the I–V characteristic curve is a very important parameter, and the large current range is beneficial for storing data. Therefore, we have conducted a corresponding I–V test for the device with doped and undoped MWCNTs, as shown in Fig. 4(a). From the curves, we can find that the five samples all exhibit similar memory behavior, which is a bistable nonvolatile flash type. For the ITO/PFO/Al device (sample A), the first scan voltage is from 0 to −6 V, as the voltage increases, the current is also slowly rising, but when the voltage increases to −1.2 V, the current rapidly rises from 1.07 × 10−3 to 2.16 × 10−2, which indicates that the device changes from low conduction state (OFF) to high conduction state (ON). This process is “writing”. In the subsequent voltage from −6 to 0 V, the device has been in the ON state, and the process is consistent with “read”. When the reverse threshold voltage is about 4.3 V, it can be clearly seen from the figure that the current drops suddenly, the process is defined as “erasing”. After applying a voltage of 6 to 0 V, the device returned to its
MWCNTs content is in the device.
2.2. Fabrication of the memory device In order to fabricate the devices, first of all, ITO conductive glass of dimensions 2 cm × 1 cm (sheet resistance R□ = 6–9 Ω/□) were precleaned sequentially with the detergent, deionized water, acetone, isopropanol and methanol in an ultrasonic bath for 20 min, respectively, and dried in a vacuum oven at 60 °C for 24 h, secondly, we prepared the PFO toluene solution of 3 mg/mL, and then MWCNTS Table 1 Different concentration of the composite films used in each sample. Sample
Content of MWCNTs isopropanol solution (mg/mL)
Volume of MWCNTs solution (mL)
Volume of PFO toluene solution (3 mg/ mL) (mL)
MWCNTs Content in the composite film (%)
A B C D E
0 0.05 0.1 0.2 0.3
1 1 1 1 1
1 1 1 1 1
0 1.6 3.2 6.3 9.1
111
Organic Electronics 74 (2019) 110–117
Y. Xin, et al.
Fig. 3. Cross-section scanning electron microscopic images of the device based on (a) ITO/PFO/Al and (b) ITO/PFO:MWCNTs/Al.
Fig. 4. (a) I–V curves and (b) Relationship between the ON/OFF ratio of current and the applied voltage for the device based on Sample A, Sample B, Sample C, Sample D and Sample E.
original state (OFF). The above process makes a cycle. The rest of four samples with doping MWCNTs exhibit the same trend. For the device with 1.6% MWCNTs (sample B), the current rises with the increase of voltage, the turn-on voltage is −0.7 V, the current is rapidly from 4.23 × 10−6 to 1.08 × 10−2. When the reverse positive bias is applied to 3.9 V, the device completes the “erasing” process and recurs the low conduction state. Similarly, the trend of the four curves for the sample C is the same as that of the previous ones. That is, the current rapidly increases from 4.06 × 10−5 to 3.75 × 10−2 when the threshold voltage is reached (−1.8 V). During subsequent voltage application, the device is always in a low resistance state, but when the reverse voltage is applied to 3.8 V, the current begins to decrease obviously, which indicates the device is converted to the closed state. The threshold voltage of the device (sample D) is −1.1 V and 4.2 V. In the process of writing, the current increases from 4.98 × 10−5 to 1.90 × 10−2. Thereafter, the device maintains low resistance state until the positive voltage applied to 4.2 V, and then it begins to the high resistance state. The relationship between the voltage and current of the device (sample E) is consistent with the memory behavior. Fig. 4(a) reveals that, at the beginning, the current gradually increases, and then when the turn-on voltage is near −1.3 V, the current suddenly increases to1.11 × 10−2. The device has been kept ON state until the turn-off voltage is 5.7 V. In order to more intuitively observe the influence of different MWCNTs content on the memristors, we construct the relationship between voltage and current, such as Fig. 4(b). It is easily perceived that the ON/OFF state current ratio of the device with doped MWCNTs is higher than that of the undoped, furthermore, the switch ratio of the device contains 1.6% MWCNTs reaches the maximum of 2.6 × 103.
3.2.2. Stability of the memory device In addition to the I–V curve, the stability of the device is also an important basis for measuring the memristor. Therefore, we have measured the retention time and the cycle number of the device, such as Fig. 5 and Fig. 6. Fig. 5 reveals the result of the retention time for all samples. In order to ensure the reliability of the result, our test conditions are the same, that is, the applied voltage is 2 V and the test time is 3 h at room temperature. For all devices, whether it is in ON state or OFF state, the current is basically unchanged, and the switching current ratio is about 20, 2600, 920, 380, 140, respectively, it is consistent with the original data. As shown in Fig. 6, the cycle numbers of the four samples were measured at 2 V pulse (pulse period = 2 ms, pulse width = 1 ms) and the result shows that the device is stable. Based on all of the above analysis, we can conclude that the memristor is not only very stable but also has application value in storing information. By analyzing the I–V curve and stability of the device, we can realize that doping MWCNTs has a certain effect on the electrical performance of the device. In order to understand the impact of MWCNTs on the device, the curve of MWCNTs content, threshold voltage and ON/OFF state current ratio is shown in Fig. 7. Fig. 7 indicates that with the increase of MWCNTs content, the threshold voltage decreases, and the ON/OFF state current ratio increases to the maximum value and follows by a downward trend. Although the switch ratio is reduced, it is higher than that of the device without doped MWCNTs. Because, with the increase of MWCNTs content, the effective distance between isolated MWCNTs is shortened, which is propitious to form a conductive pathway in the device. As a result, a large number of carriers pass through and continuous jump between two electrodes, which leads to increase switch ratio [24–26]. Subsequently, due to the distance
112
Organic Electronics 74 (2019) 110–117
Y. Xin, et al.
Fig. 5. Retention performance on the ON and OFF states of the device based on (a) Sample A (b) Sample B (c) Sample C (d) Sample D and (e) Sample E.
between the individual MWCNTs is getting smaller and smaller, which count against the carriers hopping, so the ON/OFF state current ratio decreases, and then the device is closer to the conductor behavior [27].
1.12, 1.08, 1.11, respectively, which is consistent with Ohm's law; (b) at high voltage region, the slope of the devices are 1.93, 1.80, 1.81, 1.81, 1.81 (nearly 2), which indicates that the voltage and current are nonlinear, but in accordance with the Child's law (I ∝ V2) [28–30]. In this process, with the increase of voltage, the current increases slowly, mainly because carbon nanotubes act as trap to capture carriers, and when the voltage increases to the threshold voltage, the traps are all filled, forming conductive filaments, so the current increases rapidly. Continuing to increase the voltage, the device maintains a high conductivity because the carriers maintain a stable output after the traps are all filled.
3.3. Switching mechanism and stored procedure of the memory device 3.3.1. Mechanism of the memory device In order to clarify the switching mechanism of the memory device, we have a linear fitting of the I–V curve. Log-log plotting is performed in the ON and OFF states during the RESET process. Fig. 8 indicates the linear fitting of the device contains a different concentration of MWCNTs. It is noteworthy that they are composed of Ohmic conduction and SCLC conduction. In the beginning, the devices based on samples A, B, C, D and E are in a high conductive state (ON state), the slopes are 0.94, 0.99, 1.05, 1.00, 1.02 (nearly 1), indicating a direct proportion between voltage and current (I ∝ V), which is consistent with Ohm's law. The cause of this phenomenon is the formation of a filament path in the device. On the contrary, in the high resistance state (OFF state), the fitting result is accord with the space charge limited current (SCLC) conduction model. The SCLC mechanism includes two parts: (a) the slope of the devices are close to 1 at low voltage, which is 1.08, 1.10,
3.3.2. Stored procedure of the memory device Although we have fully discussed the switching mechanism of the memristor, the specific stored procedure has not been carefully discussed. So, we mainly use HOMO-LUMO and energy level diagram of the device to analyze their stored procedures. We use DFT (B3LYP/631G) to calculate the HOMO, LUMO and band gap of PFO. HOMO is the highest occupied molecular orbital, which is related to the ability to supply electrons, nevertheless, LUMO (lowest unoccupied molecular orbital) represents the accept electronics capability [31]. The band gap is the difference between HOMO and LUMO. The corresponding energy
113
Organic Electronics 74 (2019) 110–117
Y. Xin, et al.
Fig. 6. Stimulus effect of read pulses on the ON and OFF states of the device based on (a) Sample B (b) Sample C (c) Sample D and (d) Sample E.
understand the storage process of the memristors. For the ITO/PFO/Al device (Fig. 9(a)), Al is the top electrode (−4.30 eV), ITO is the bottom electrode (−4.80 eV) [32]. The energy barrier between the ITO electrode and the HOMO of the PFO is 0.96 eV, while the energy barrier between the Al electrode and the LUMO of the PFO is 3.26 eV, thus the holes injection are easier than the electron injection. At low bias, the holes are gradually injected from ITO to HOMO of the polymer and jump along the polymer chain, at this point the device is in a high resistance state (OFF). When the scanning voltage is increased to the threshold voltage, the electrons overcome the energy barrier between the Al electrode and LUMO of the PFO and then a large number of electrons are injected into the device. Soon afterwards, most of the charges trapping centers are filled, and the carriers increase rapidly, so the device converts to a low resistance state. With the increase of voltage, the carriers can move freely between the two electrodes, the device forms a conductive filament in the meantime. When the reverse voltage is applied, the device keeps ON state and produces a lot of heat. But when the Vreset is 4.3 V, the charges exceed the capacity of the filament, and the superfluous heats destroy the filament, therefore the device returns to original state [33,34]. Fig. 9(b) represents stored process of the device with doped MWCNTs. The function of MWCNTs is to capture and transport electronics. The work function of MWCNTs is lower than the LUMO of the polymer [35], so the electrons enter into MWCNTs from the Al
Fig. 7. The switch-on voltage and the ON/OFF state current ratio of the device with different MWCNTs content.
levels are −5.76 eV, −0.74 eV and 5.02 eV, respectively, as shown in Table 2. As shown in Fig. 9, we have done the energy level diagram of the device with undoped MWCNTs and the doped MWCNTs to better
114
Organic Electronics 74 (2019) 110–117
Y. Xin, et al.
Fig. 8. Linear fitting and corresponding slopes for the I–V curves of the device based on (a) Sample A (b) Sample B (c) Sample C (d) Sample D and Sample E.
the device, and then the electrons and holes move between the two electrodes and the polymer. When the turn-on voltage is reached, the carriers overcome the energy barrier and enter into the LUMO of the polymer. At this time the phenomenon is that the current suddenly increases and then the devices turn from the OFF state to the ON state. After the application of the reverse voltage, the heat generated by the device destroys the conduction channel, so it returns the OFF state. The above analyses fully show that the device is a bistable nonvolatile behavior [36–38].
Table 2 Molecular simulation result for the PFO. Frontier molecular orbital
Energy level (eV)
HOMO
−5.76
LUMO
−0.74
4. Conclusions In brief, we have synthesized PFO by Suzuki coupling reaction, and doped a different content of MWCNTs to prepare memristors, subsequently, a series of tests were carried out. The results reveal that all devices are bistable nonvolatile memory device, and with the increase of MWCNTs content, the ON/OFF state current ratio increases to the maximum of 2.6 × 103 and then reduces. The threshold voltage is also reduced from −1.2 V to −0.7 V. In addition, the retention time of the devices was under the condition of 2 V and the test lasted for 3 h. The results exhibit that the current has no obvious mutation, which is almost consistent with the original value. By testing the cycle numbers of the memristors at 2 V read pulse, the current is basically stable after 9000 cycles. Based on the above tests, it indicates that the device has a good stability, which can reduce the misreading rate of the device, which is beneficial to the application of memristor in data storage.
electrode are easier than the holes inject from ITO to the HOMO of the PFO. At the beginning, the voltage is low, and the electrons get into the MWCNTs and move along the MWCNTs tube, the devices show a low conduction state. As the voltage increases, the holes are injected into 115
Organic Electronics 74 (2019) 110–117
Y. Xin, et al.
Fig. 9. Energy level diagrams for the device (a) pure PFO (b) PFO:MWCNTs.
Acknowledgements
[17] J.J.L. Hmar, Flexible resistive switching bistable memory devices using ZnO nanoparticles embedded in polyvinyl alcohol (PVA) matrix and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), RSC Adv. 8 (2018) 20423–20433. [18] S. Chandrakishore, A. Pandurangan, Facile synthesis of carbon nanotubes and their use in the fabrication of resistive switching memory devices, RSC Adv. 4 (2014) 9905–9911. [19] Y.M. Sun, F.J. Miao, R. Li, Bistable electrical switching and nonvolatile memory effect based on the thin films of polyurethane-carbon nanotubes blends, Sensor Actuat A-Phys. 234 (2015) 282–289. [20] L. Li, D.Z. Wen, Memory behavior of multi-bit resistive switching based on multiwalled carbon nanotubes, Org. Electron. 34 (2016) 12–17. [21] G. Liu, Q.D. Ling, E.T. Kang, K.G. Neoh, Bistable electrical switching and write-once read-many-times memory effect in a donor-acceptor containing polyfluorene derivative and its carbon nanotube composites, J. Appl. Phys. 102 (2007) 0245021–0245028. [22] G. Liu, Q.D. Ling, E.Y.H. Teo, C.X. Zhu, D.S.H. Chan, K.G. Neoh, E.T. Kang, Electrical conductance tuning and bistable switching in poly(Nvinylcarbazole)?Carbon nanotube composite films, ACS Nano 3 (2009) 1929–1937. [23] Y. Xin, X.F. Zhao, X.K. Jiang, Q. Yang, J.H. Huang, S.H. Wang, R.R. Zheng, C. Wang, Y.J. Hou, Bistable electrical switching and nonvolatile memory effect by doping different content of GO in Poly (9,9-dioctylfluorene-2,7-diyl), RSC Adv. 8 (2018) 6878–6886. [24] Y.M. Sun, J.G. Lu, C.P. Ai, D.Z. Wen, Nonvolatile memory devices based on poly (vinylalcohol) + graphene oxide hybrid composites, Phys. Chem. Chem. Phys. 18 (2016) 11341–11347. [25] W.B. Zhang, C. Wang, G. Liu, X.J. Zhu, X.X. Chen, L. Pan, H.W. Tan, W.H. Xue, Z.H. Ji, J. Wang, Y. Chen, R.W. Li, Thermally-stable resistive switching with a large ON/OFF ratio achieved in poly(triphenylamine), Chem. Commun. 50 (2014) 11856–11858. [26] Y.Q. Li, H.H. Xu, X. Tao, K.J. Qian, S. Fu, Y.Z. Shen, S.J. Ding, Synthesis and memory characteristics of highly organo-soluble polyimides bearing a noncoplanar twisted biphenyl unit containing aromatic side-chain groups, J. Mater. Chem. 21 (2011) 1810–1821. [27] C.J. Chen, J.H. Wu, G.S. Liou, Thermally stable and high ON/OFF ratio non-volatile memory devices based on poly(triphenylamine) with pendent PCBM, Chem. Commun. 50 (2014) 1335–1337. [28] T.W. Kim, S.H. Oh, H.J. Choi, G. Wang, H.H. Wang, D.Y. Kim, T. Lee, Reversible switching characteristics of polyfluorene-derivative single layer film for nonvolatile memory devices, Appl. Phys. Lett. 92 (2008) 2533081–2533083. [29] J.Q. Liu, Z.Y. Zeng, X.H. Cao, G. Lu, L.H. Wang, Q.L. Fan, W. Huang, H. Zhang, Preparation of MoS2 –polyvinylpyrrolidone nanocomposites for flexible nonvolatile rewritable memory devices with reduced graphene oxide electrodes, Small 8 (2012) 3517–3522. [30] B.J. Ree, W. Kwon, K. Kim, Y.G. Ko, Y.Y.K. IM, h. Lee, M. Ree, Clues to the electrical switching mechanism of carbazole-containing polyimide thin films, ACS Appl. Mater. Interfaces 6 (2014) 21692–21701. [31] P.M. Niamien, F.K. Essy, A. Trokourey, A. Yapi, H.K. Aka, D. Diabate, Correlation between the molecular structure and the inhibiting effect of some benzimidazole derivatives, Mater. Chem. Phys. 136 (2012) 59–65. [32] D. Wang, H. Li, N.J. Li, Y. Zhao, Q.H. Zhou, Q.F. Xu, J.M. Lu, L.H. Wang, A new DRAM-type memory devices based on polymethacrylate containing pendant 2methylbenzothiazole, Mater. Chem. Phys. 134 (2012) 273–278. [33] Y.M. Sun, L. Li, D.Z. Wen, X.D. Bai, G. Li, Bistable electrical switching and nonvolatile memory effect in carbon nanotube – poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) composite films, Phys. Chem. Chem. Phys. 17 (2015) 17150–17158. [34] Y.C. Chang, Y.H. Wang, ACS appl. Resistive switching behavior in gelatin thin films for nonvolatile memory application, Mater. Interfaces 6 (2014) 5413–5421. [35] S.P. Somani, P.R. Somani, M. Umeno, Carbon nanotube incorporation: a new route
The authors are grateful to the support of the National Natural Science Foundation of China (grant numbers 51527804), Heilongjiang university JMRH project (HDJMRH201913), and Natural Science Foundation of Heilongjiang Province of China (grant number B2017010). References [1] W.T. Kim, J.H. Jung, T.W. Kim, Carrier transport mechanisms in nonvolatile memory devices fabricated utilizing multiwalled carbon nanotubes embedded in a poly-4-vinyl-phenol layer, Appl. Phys. Lett. 95 (2009) 1–3. [2] W.P. Lin, S.J. Liu, T. Gong, Q. Zhao, W. Huang, Polymer-based resistive memory materials and devices, Adv. Mater. 26 (2014) 570–606. [3] G.L. Ding, Y.X. Wang, G.X. Zhang, K. Zhou, K.L. Zeng, Z.X. Li, Y. Zhou, C. Zhang, X.L. Chen, S.T. Han, 2D metal–organic framework nanosheets with time-dependent and multilevel memristive switching, Adv. Funct. Mater. 29 (2019) 1–11. [4] Y.Y. Ren, H.L. Ma, W. Wang, Z.Q. Wang, H.Y. Xu, X.N. Zhao, W.Z. Liu, J.G. Ma, Y.C. Liu, Cycling-induced degradation of organic–inorganic perovskite-based resistive switching memory, Adv. Mater. Technol. 4 (2019) 1–6. [5] Z.J. Wang, K. Sierros, M.S. Seehra, D. Korakakis, Development of indigo-based nonvolatile write-once-read-many-times memory device, Mater. Lett. 206 (2017) 128–131. [6] Y.Y. Shan, Z.S. Lyu, X.W. Guan, A. Younis, G.L. Yuan, J.L. Wang, S. Li, Tom Wu, Solution-processed resistive switching memories based on hybrid organic-inorganic materials and composites, Phys. Chem. Chem. Phys. 20 (2018) 23837–23846. [7] D. Chaudhary, S. Munjal, N. Khare, V.D. Vankar, Bipolar resistive switching and nonvolatile memory effect in poly(3-hexylthiophene) ecarbon nanotube composite films, Carbon 130 (2018) 553–558. [8] F.Y. Sun, D.Z. Wen, Y.M. Sun, Influence of blending ratio on resistive switching effect in donor-acceptor type composite of PCBM and PVK-based memory devices, Org. Electron. 65 (2019) 141–149. [9] A. Thakre, H. Borkar, B.P. Singh, A. Kumar, Electroforming free high resistance resistive switching of graphene oxide modified polar-PVDF, RSC Adv. 5 (2015) 57406–57413. [10] L.Y. Xiang, J. Ying, J.H. Han, L.T. Zhang, W. Wang, High reliable and stable organic field-effect transistor nonvolatile memory with a poly(4-vinyl phenol) charge trapping layer based on a pn-heterojunction active layer, Appl. Phys. Lett. 108 (2016) 1–4. [11] P. Aruna, K. Suresh, C.M. Joseph, Effect of fullerene doping on the electrical propertie of P3HT/PCBM layers, Mater. Sci. Semicond. Process. 36 (2015) 7–12. [12] G.Z. Sun, J.Q. Liu, L.X. Zheng, W. Huang, H. Zhang, Preparation of weavable, allcarbon fibers for non-volatile memory devices, Angew. Chem. 125 (2013) 13593–13597. [13] J. Aimi, C.T. Lo, H.C. Wu, C.F. Huang, T. Nakanishi, M. Takeuchi, W.C. Chen, Phthalocyanine-cored star-shaped polystyrene for nano floating gate in nonvolatile organic transistor memory device, Adv. Electron. Mater. 2 (2016) 1–9. [14] G. Koplovitz, D. Primc, O.B. Dor, S. Yochelis, D. Rotem, D. Porath, Y. Paltiel, Magnetic nanoplatelet-based spin memory device operating at ambient temperatures, Adv. Mater. 29 (2017) 1–6. [15] S.P. Ge, Y.X. Huang, X.J. Chen, X.R. Zhang, Z.C. Xiang, R.X. Zhang, W.P. Li, Y.M. Cui, Silver iodide induced resistive switching in CsPbI3 perovskite-based memory device, Adv. Mater. Interfaces 6 (2019) 1–9. [16] J. Wang, X. Wang, J.J. Xu, L.H. Xie, M.D. Yi, W. Huang, The trapping, detrapping, and transport of the ambipolar charges in the electret of Polystyrene/C60 blend films, Org. Electron. 44 (2017) 247–252.
116
Organic Electronics 74 (2019) 110–117
Y. Xin, et al. to improve the performance of organic–inorganic heterojunction solar cells, Diam. Relat. Mater. 17 (2008) 585–588. [36] B.L. Hu, C.Y. Wang, J. Zhang, K. Qian, P.S. Lee, Q.C. Zhang, Organic memory effect from donor–acceptor polymers based on 7-perfluorophenyl-6H-[1,2,5]-thiadiazole [3,4-g]benzoimidazole, RSC Adv. 5 (2015) 77122–77129. [37] C.C. Hung, Y.C. Chiu, H.C. Wu, C. Lu, C. Bouilhac, L. Otsuka, S. Halila, R. Borsali,
S.H. Tung, W.C. Chen, Conception of stretchable resistive memory devices based on nanostructure-controlled carbohydrate-block-polyisoprene block copolymers, Adv. Funct. Mater. 27 (2017) 1–10. [38] A. Rani, D.B. Velusamy, F.M. Mota, Y.H. Jang, R.H. Kim, C. Park, D.H. Kim, Onestep all-solution-based Au–GO core–shell nanosphere active layers in nonvolatile ReRAM devices, Adv. Funct. Mater. 27 (2017) 16046041–16046048.
117