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Self-rectifying resistive switching behavior observed in sodium copper chlorophyllin-polyvinyl alcohol composites on a flexible and transparent substrate
Physica B 577 (2020) 411787
Contents lists available at ScienceDirect
Physica B: Physics of Condensed Matter journal homepage: http://www.elsevier.com/locate/physb
Self-rectifying resistive switching behavior observed in sodium copper chlorophyllin-polyvinyl alcohol composites on a flexible and transparent substrate Limin Chen a, Xinyu Chen b, Qinghong Lin a, Gong Chen a, Kaibin Ruan a, * a b
College of Mechanical and Electronic Engineering, Fujian Agriculture and Forestry University, Fuzhou, 350002, China Jinshan College, Fujian Agriculture and Forestry University, Fuzhou, 350002, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Sodium copper chlorophyllin Self-rectifying Resistive switching Thin films Flexible Transparent
Sodium copper chlorophyllin (SCC)- polyvinyl alcohol (PVA) composites thin films have been prepared on flexible and transparent ITO/PET substrates using Ag as the top electrode. FE-SEM shows that the prepared SCCPVA thin films are smooth and dense, SCC particles have embedded in PVA and formed nano-islands, which are distributed irregularly in the thin film. The optical transmittance spectrum demonstrates that Ag/SCC-PVA/ITO/ PET can be used as a transparent electronic device. I–V characteristics of the Ag/SCC-PVA/ITO/PET show that the device exhibits self-rectifying resistive switching performance, which could be related to the forming/rapture of Ag conductive filaments induced by the migration of Ag ions. This study demonstrates the potential uses of SCC-PVA in low-cost, flexible and transparent resistive switching memory.
1. Introduction Flexible electronic devices have been more and more popular in recent years because of their human-friendly interfaces compared to those of conventional devices. They are widely employed in a variety of wearable electronic devices [1–3]. Recently, transparent electronic de vices are also attracted much attention in devices such as transparent transistors [4] and fully transparent displays [5]. The combination of flexible and transparent properties in electronic devices, therefore, would greatly contribute to the development of future electronics. A flexible and transparent memory is expected to be an important part of electronic devices for data processing, storage, and communication. Therefore, it is reasonable to focus more attention on flexible and transparent memories studies. Resistive random access memory (RRAM) is one of the most prom ising candidates for the next generation memories owing to its simple structure and good storage properties [6–8]. Various studies have been conducted to investigate resistive switching phenomena in different materials, including inorganic and organic materials. The preparation of inorganic resistive switching materials usually needs high temperature, which is not compatible with the commonly used flexible and trans parent polymer substrates. Hence, organic materials with low temper ature and low-cost fabrication have been attracted much attention in
flexible and transparent RRAM. Sodium copper chlorophyllin (SCC) has been widely used as color additive in foods, drugs, cosmetics [9] as well as photoelectrical devices [10]. SCC could be synthesized using raw material of natural chlorophyll which can be easily distilled from green plants. It is anticipated as a candidate material for green, renewable and low-cost electronic devices [11]. However, there is no literature found concerning about SCC used in RRAM as far as we know. In this work, we propose a flexible and transparent Ag/SCC-PVA/ITO/PET device, and study its resistive switching behaviors. The results show that Ag/SCC-PVA/ITO/PET ex hibits self-rectifying resistive switching effect. This self-rectifying char acteristic feature of the device is quite promising in RRAM devices. As is widely known, the sneak-path issue is a key challenge for the integration of RRAM into crossbar arrays, and the memory device which has a self-rectifying effect in the low resistance state is a desirable solution [12–14]. So, the flexible and transparent Ag/SCC-PVA/ITO/PET device fabricated in this work would be a new candidate for self-rectifying RRAM. 2. Experimental The proposed Ag/SCC-PVA/ITO/PET devices were fabricated on commercial flexible and transparent ITO/PET substrates. The schematic
* Corresponding author. E-mail address: [email protected] (K. Ruan). https://doi.org/10.1016/j.physb.2019.411787 Received 4 August 2019; Received in revised form 11 October 2019; Accepted 12 October 2019 Available online 16 October 2019 0921-4526/© 2019 Elsevier B.V. All rights reserved.
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Physica B: Physics of Condensed Matter 577 (2020) 411787
diagrams of the fabrication process and the bending image of the device is shown in Fig. 1. 2.1. Device fabrication PVA124 was dissolved in distilled water with a concentration of 0.05 g/mL, and then stirred at 95 � C for 20 min until the solution was clear. After the above solution was cooled, SCC power with 10% weight of PVA124 was added to the solution, and kept on stirring for 2 h. The obtained solution was spin coated on ITO/PET substrates at 2000 rpm for 30 s, followed by the wet films baked under IR lamp until the thin films were dry. This spin-coating and baking processes repeated several times until the designed film thickness was achieved. Subsequently, thin films were dried at 80 � C in oven for 1 h. Then, Ag top electrodes with diameter of 0.5 mm were deposited on the above PVA-SCC thin films by thermal evaporation deposition. As a comparison, Pt and Cu top elec trodes were also deposited on PVA-SCC thin films by RF sputtering. The RF power applied on Pt target was 25 W, while the power on Cu target was 45 W. The base pressure of chambers for all deposition processes were about 10 4 Pa.
Fig. 2. FESEM morphologies of (a) PVA and (b) PVA/SCC thin films.
Agriculture and Forestry University” can be clearly seen under the ob tained Ag/SCC-PVA/ITO/PET device. While the wavelength is below about 370 nm, the optical transmittance exhibits abrupt decrease to nearly opaque. This is the optical absorption edge of SCC. As is known, the transition between valence and conduction bands is direct for SCC [15], and the absorption coefficient α as a function of photon energy could be expressed as following [16]: � (1) ðαhνÞ2 ¼ C hν Eg
2.2. Characterization techniques
Where hv is the incident photon energy, C is a constant, and Eg is the band gap energy. Fig. 3(b) shows the curve of (αhv)2 vs hv for SCC-PVA based device. The optical band gap of the thin film could be obtained by extrapolating the linear portion of the plots to hv axis. Thereby, the band gap of thin film is identified to be about 2.87 V. This is similar to that of the reported literature [15]. The optical property results in Fig. 3 demonstrate that Ag/SCC-PVA/ITO/PET can be used as a transparent resistive switching device.
The surface morphologies of thin films were examined by fieldemission scanning electron microscopy (JEOL LTD, JSM-6700 F) and images were taken at 10 kV. Optic transmittance spectra measurement in the wavelength of 250–800 nm was carried out using a Shimadzu UV2600 spectrophotometer. Current–voltage (I–V) measurements were measured using a Keithley 2420 digital source meter. 3. Results and discussion
3.3. Electrical characterization
3.1. Surface morphology
The typical I–V characteristics of Ag/SCC-PVA/ITO/PET device under various sweeping voltages and scan rates are shown in Fig. 4(a) and (b), respectively. For a comparison, I–V curve of pure Ag/PVA/ITO/ PET device is shown in the inset of Fig. 4(a). For all measurements, the current flow from top metal Ag to bottom ITO electrode is defined as the positive direction, and the schematic view could be seen in the inset of Fig. 4(b). The direction of the voltage sweep 0→Vmax→0→ Vmax→0 V is denoted by the numbered arrows in the figure. From Fig. 4(a), Ag/SCCPVA/ITO/PET device shows typical self-rectifying resistive switching performance at low operating voltages, but the pure Ag/PVA/ITO/PET device exhibits negligible hysteresis. By steadily increasing the positive voltages applied on the Ag/SCC-PVA/ITO/PET device from 0 to Vmax, the resistive transition from HRS to LRS can be observed, and the “SET” voltages are between 0.8 V and 1 V in different sweep conditions. But the resistive values are much higher and change little when applying negative bias. As is known, PVA is an isolating dielectric material itself [17]. The FE-SEM result shows that the thin film is very dense, therefore it is difficult to form conductive filaments in PVA, and the device keeps
Fig. 2 displays the surface morphologies of PVA and PVA-SCC blend thin films observed by FESEM. From Fig. 2, the surface of PVA is smooth and uniformity, no obvious cracks are observed. PVA-SCC thin film also has a smooth and dense surface morphology, but some particles with sizes of about 200–500 nm distributed irregularly in the sample can be easily seen. Obviously they are SCC particles embedded in the PVA and form PVA-SCC composites. 3.2. Optical transmittance spectra The optical transmittance spectra of Ag/SCC-PVA/ITO/PET device and ITO/PET substrate itself are exhibited in Fig. 3(a). It is shown that the transmittance of the device decreases about 10–20% in the visible region compared to that of the substrate. In the wavelength less than about 500 nm, the transmittance of light abruptly decreases, but the device still shows good transparency in the visible region. This can be confirmed by the inset of Fig. 3(a), in which the logo of “Fujian
Fig. 1. Schematic diagrams of the fabrication process and bending image for Ag/SCC-PVA/ITO/PET device. 2
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Fig. 3. (a) The optical transmittance spectra of Ag/SCC-PVA/ITO/PET device and ITO/PET substrate. The inset is the logo of “Fujian Agriculture and Forestry University” under Ag/SCC-PVA/ITO/PET device. (b) The curve of (αhv)2 vs hv for Ag/SCC-PVA/ITO/PET device.
Fig. 4. (a) I–V characteristics of Ag/SCC-PVA/ITO/PET device swept with various voltage amplitudes. The inset is the I–V curve of pure Ag/PVA/ITO/ PET. (b) I–V characteristics of Ag/SCC-PVA/ITO/PET device with different scan rates. The inset is the schematic view of the device.
high resistance and appears negligible resistive switching behavior. But SCC/PVA composite is different. PVA is used as a supportive layer in the composites thin film, and SCC powder is embedded into PVA and form nano-islands, which can be seen in Fig. 2(b). Due to these nano-islands, it is much easier to form conductive paths in SCC/PVA composites thin film. That is, when applying positive voltage to the sample, Ag metal ions would migrate from the top electrode to the grounded side through the nano-islands and form conductive filaments. Thus, the device shows low resistance [18]. For the applied voltage is low, the conductive fil aments would be thin due to the small amount accumulations of Ag ions, and the conductive filaments are not so stable, which are easy to rapture again. When applying negative voltage to the sample, the opposite electric-field would drive Ag ions drift back to the top electrode, and the conductive filaments would rupture easily from the thinnest place. Therefore, the device shows high resistance when negative voltage is applied, and I–V curves exhibit rectifying effect for the device. The rectifying ratios read at �0.5 V in the low resistance state from Fig. 4(a) are about 13, 53 and 5, for the applied Vmax are 1, 1.5 and 2 V respec tively. The decrease of rectifying ratio could be explained as follows: with increasing the Vmax, the sizes of conductive filaments would in crease, too. It will be not so easy to rupture the conductive filaments anymore while sweeping to negative voltage. So, the resistance read at negative voltage would increase, and of course the ratio would decrease accordingly. The rectifying ratio is also related to the scan rate. From
Fig. 4(b) the rectifying ratio decreases with increasing the scan rates. This could be due to the less accumulations of Ag ions under higher scan speed. To further confirm the self-rectifying resistive switching behavior of the device is related to the migration of metal ions, Pt and Cu top electrodes were deposited on SCC-PVA thin films by RF sputtering, and their I–V curves are shown in Fig. 5(a) and (b), respectively. From Fig. 5, both devices show self-rectifying resistive switching behaviors. Compared to the I–V curve of Ag/SCC-PVA/ITO/PET, devices with Pt and Cu top electrodes needs much more higher operating voltages. That is, the device with Ag top electrode needs the lowest operating voltages, while the device with Pt top electrode needs the highest operating voltages. This could be due to the difference of metal ions migration. As is known, Ag has much higher mobility than that of Cu ions, so it is much easier for Ag ions to form conductive filaments, and hence the device needs lower operating voltage. By contrary, the mobility of Pt ions is much lower than those of Ag and Cu metal ions, so it would need much higher voltages to form conductive filaments. I–V curves of SCC-PVA based thin films with different metal top electrodes indicate that the self-rectifying resistive switching behavior of SCC-PVA based devices is strongly affected by the migration of metal ions.
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voltage amplitudes and voltage scan rates, which could be due to the forming/rapture of Ag conductive filaments. The self-rectifying perfor mances of devices with Pt and Cu top electrodes further confirm that the resistive switching behaviors of SCC-PVA based devices strongly related to the metal conductive filaments. These study results implying that the flexible and transparent Ag/SCC-PVA/ITO/PET resistive switching de vice with self-rectifying property should be a new type of potential candidate for green and low-cost RRAM.
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Declaration of competing interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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The authors gratefully acknowledge financial support from Natural Science Foundation of Fujian Province of China (No. 2017J01735) and Special Fund for Scientific and Technological Innovation of Fujian Agriculture and Forestry University (No. KFA17252A).
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References
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[1] J.-L. Wang, M. Hassan, J.-W. Liu, S.-H. Yu, Adv. Mater. 30 (2018) 1803430. [2] S.-W. Yeom, S.-C. Shin, T.-Y. Kim, H.J. Ha, Y.-H. Lee, J.W. Shim, B.-K. Ju, Nanotechnology 27 (2016), 07LT01. [3] J. Ali, M.M. Rehman, G.U. Siddiqui, S. Aziz, K.H. Choi, Physica B 531 (2018) 223–229. [4] A. Liu, H. Zhu, W.-T. Park, S.-J. Kang, Y. Xu, M.G. Kim, Y.-Y. Noh, Adv. Mater. 23 (2018), 1802379. [5] H.J. Lee, J.H. Hwang, K.B. Choi, S.-G. Jung, K.N. Kim, Y.S. Shim, C.H. Park, Y. W. Park, B.-K. Ju, ACS Appl. Mater. Interfaces 5 (2013) 10397–10403. [6] F. Pan, S. Gao, C. Chen, C. Song, F. Zeng, Mat. Sci. Eng. R 83 (2014) 1–59. [7] H.J. Ji, H.-Y. Joo, Y.-M. Kim, D.H. Lee, J.-S. Kim, Y.S. Kim, T. Choi, B.H. Park, Sci. Rep. 6 (2016) 23299. [8] M. Querr� e, J. Tranchant, B. Corraze, S. Cordier, V. Bouquet, S. D�eputier, M. Guilloux-Viry, M.-P. Besl, E. Janod, L. Cario, Physica B 536 (2018) 327–330. [9] N. Ivana, K.-L. Sebojka, Electroanal 24 (2012) 1–9. [10] A.A.M. Farag, A.M. Mansour, A.H. Ammar, M. Abdel Rafea, A.M. Farid, J.Alloy. Compd. 513 (2012) 404–413. [11] K.B. Ruan, Q.C. Hu, Y.Z. Wang, B. Long, L.M. Chen, Y.B. Wu, Mater. Lett. 236 (2019) 383–386. [12] J.Y. Kwon, J.H. Park, T.G. Kim, Appl. Phys. Lett. 106 (2015) 223506. [13] S. Gao, F. Zeng, F. Li, M.j. Wang, H.j. Mao, G.y. Wang, C. Song, F. Pan, Nanoscale 7 (2015), 6031-3038. [14] H.P. Lu, X.C. Yuan, B.L. Chen, C.H. Gong, H.Z. Zeng, X.H. Wei, J. Sol. Gel Sci. Technol. 82 (2017) 627–634. [15] M.E. Aydin, A.A.M. Farag, M. Abdel-Rafea, A.H. Ammar, F. Yakuphanoglu, Synth. Met. 161 (2012) 2700–2707. [16] D.-A. Chang, P. Lin, T.-Y. Tseng, J. Appl. Phys. 77 (1995) 4445. [17] J.J.L. Hmar, RSC Adv. 8 (2018) 20423. [18] B. Sun, X. Zhang, G.D. Zhou, C.M. Zhang, P.Y. Li, Y.D. Xia, Y. Zhao, J.Alloy. Compd. 694 (2017) 464–470.
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4. Conclusions In conclusion, we have fabricated flexible and transparent Ag/SCCPVA/ITO/PET resistive switching devices. FE-SEM shows that SCC particles embedded in PVA and the forming SCC nano-islands distrib uted irregularly in the sample. The optical transmittance spectrum demonstrates that Ag/SCC-PVA/ITO/PET can be used as a transparent resistive switching device. I–V characteristics of the Ag/SCC-PVA/ITO/ PET show that the resistive switching of device exhibit self-rectifying performance. The values of rectifying ratio are related to the applied
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Contents lists available at ScienceDirect
Physica B: Physics of Condensed Matter journal homepage: http://www.elsevier.com/locate/physb
Self-rectifying resistive switching behavior observed in sodium copper chlorophyllin-polyvinyl alcohol composites on a flexible and transparent substrate Limin Chen a, Xinyu Chen b, Qinghong Lin a, Gong Chen a, Kaibin Ruan a, * a b
College of Mechanical and Electronic Engineering, Fujian Agriculture and Forestry University, Fuzhou, 350002, China Jinshan College, Fujian Agriculture and Forestry University, Fuzhou, 350002, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Sodium copper chlorophyllin Self-rectifying Resistive switching Thin films Flexible Transparent
Sodium copper chlorophyllin (SCC)- polyvinyl alcohol (PVA) composites thin films have been prepared on flexible and transparent ITO/PET substrates using Ag as the top electrode. FE-SEM shows that the prepared SCCPVA thin films are smooth and dense, SCC particles have embedded in PVA and formed nano-islands, which are distributed irregularly in the thin film. The optical transmittance spectrum demonstrates that Ag/SCC-PVA/ITO/ PET can be used as a transparent electronic device. I–V characteristics of the Ag/SCC-PVA/ITO/PET show that the device exhibits self-rectifying resistive switching performance, which could be related to the forming/rapture of Ag conductive filaments induced by the migration of Ag ions. This study demonstrates the potential uses of SCC-PVA in low-cost, flexible and transparent resistive switching memory.
1. Introduction Flexible electronic devices have been more and more popular in recent years because of their human-friendly interfaces compared to those of conventional devices. They are widely employed in a variety of wearable electronic devices [1–3]. Recently, transparent electronic de vices are also attracted much attention in devices such as transparent transistors [4] and fully transparent displays [5]. The combination of flexible and transparent properties in electronic devices, therefore, would greatly contribute to the development of future electronics. A flexible and transparent memory is expected to be an important part of electronic devices for data processing, storage, and communication. Therefore, it is reasonable to focus more attention on flexible and transparent memories studies. Resistive random access memory (RRAM) is one of the most prom ising candidates for the next generation memories owing to its simple structure and good storage properties [6–8]. Various studies have been conducted to investigate resistive switching phenomena in different materials, including inorganic and organic materials. The preparation of inorganic resistive switching materials usually needs high temperature, which is not compatible with the commonly used flexible and trans parent polymer substrates. Hence, organic materials with low temper ature and low-cost fabrication have been attracted much attention in
flexible and transparent RRAM. Sodium copper chlorophyllin (SCC) has been widely used as color additive in foods, drugs, cosmetics [9] as well as photoelectrical devices [10]. SCC could be synthesized using raw material of natural chlorophyll which can be easily distilled from green plants. It is anticipated as a candidate material for green, renewable and low-cost electronic devices [11]. However, there is no literature found concerning about SCC used in RRAM as far as we know. In this work, we propose a flexible and transparent Ag/SCC-PVA/ITO/PET device, and study its resistive switching behaviors. The results show that Ag/SCC-PVA/ITO/PET ex hibits self-rectifying resistive switching effect. This self-rectifying char acteristic feature of the device is quite promising in RRAM devices. As is widely known, the sneak-path issue is a key challenge for the integration of RRAM into crossbar arrays, and the memory device which has a self-rectifying effect in the low resistance state is a desirable solution [12–14]. So, the flexible and transparent Ag/SCC-PVA/ITO/PET device fabricated in this work would be a new candidate for self-rectifying RRAM. 2. Experimental The proposed Ag/SCC-PVA/ITO/PET devices were fabricated on commercial flexible and transparent ITO/PET substrates. The schematic
* Corresponding author. E-mail address: [email protected] (K. Ruan). https://doi.org/10.1016/j.physb.2019.411787 Received 4 August 2019; Received in revised form 11 October 2019; Accepted 12 October 2019 Available online 16 October 2019 0921-4526/© 2019 Elsevier B.V. All rights reserved.
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Physica B: Physics of Condensed Matter 577 (2020) 411787
diagrams of the fabrication process and the bending image of the device is shown in Fig. 1. 2.1. Device fabrication PVA124 was dissolved in distilled water with a concentration of 0.05 g/mL, and then stirred at 95 � C for 20 min until the solution was clear. After the above solution was cooled, SCC power with 10% weight of PVA124 was added to the solution, and kept on stirring for 2 h. The obtained solution was spin coated on ITO/PET substrates at 2000 rpm for 30 s, followed by the wet films baked under IR lamp until the thin films were dry. This spin-coating and baking processes repeated several times until the designed film thickness was achieved. Subsequently, thin films were dried at 80 � C in oven for 1 h. Then, Ag top electrodes with diameter of 0.5 mm were deposited on the above PVA-SCC thin films by thermal evaporation deposition. As a comparison, Pt and Cu top elec trodes were also deposited on PVA-SCC thin films by RF sputtering. The RF power applied on Pt target was 25 W, while the power on Cu target was 45 W. The base pressure of chambers for all deposition processes were about 10 4 Pa.
Fig. 2. FESEM morphologies of (a) PVA and (b) PVA/SCC thin films.
Agriculture and Forestry University” can be clearly seen under the ob tained Ag/SCC-PVA/ITO/PET device. While the wavelength is below about 370 nm, the optical transmittance exhibits abrupt decrease to nearly opaque. This is the optical absorption edge of SCC. As is known, the transition between valence and conduction bands is direct for SCC [15], and the absorption coefficient α as a function of photon energy could be expressed as following [16]: � (1) ðαhνÞ2 ¼ C hν Eg
2.2. Characterization techniques
Where hv is the incident photon energy, C is a constant, and Eg is the band gap energy. Fig. 3(b) shows the curve of (αhv)2 vs hv for SCC-PVA based device. The optical band gap of the thin film could be obtained by extrapolating the linear portion of the plots to hv axis. Thereby, the band gap of thin film is identified to be about 2.87 V. This is similar to that of the reported literature [15]. The optical property results in Fig. 3 demonstrate that Ag/SCC-PVA/ITO/PET can be used as a transparent resistive switching device.
The surface morphologies of thin films were examined by fieldemission scanning electron microscopy (JEOL LTD, JSM-6700 F) and images were taken at 10 kV. Optic transmittance spectra measurement in the wavelength of 250–800 nm was carried out using a Shimadzu UV2600 spectrophotometer. Current–voltage (I–V) measurements were measured using a Keithley 2420 digital source meter. 3. Results and discussion
3.3. Electrical characterization
3.1. Surface morphology
The typical I–V characteristics of Ag/SCC-PVA/ITO/PET device under various sweeping voltages and scan rates are shown in Fig. 4(a) and (b), respectively. For a comparison, I–V curve of pure Ag/PVA/ITO/ PET device is shown in the inset of Fig. 4(a). For all measurements, the current flow from top metal Ag to bottom ITO electrode is defined as the positive direction, and the schematic view could be seen in the inset of Fig. 4(b). The direction of the voltage sweep 0→Vmax→0→ Vmax→0 V is denoted by the numbered arrows in the figure. From Fig. 4(a), Ag/SCCPVA/ITO/PET device shows typical self-rectifying resistive switching performance at low operating voltages, but the pure Ag/PVA/ITO/PET device exhibits negligible hysteresis. By steadily increasing the positive voltages applied on the Ag/SCC-PVA/ITO/PET device from 0 to Vmax, the resistive transition from HRS to LRS can be observed, and the “SET” voltages are between 0.8 V and 1 V in different sweep conditions. But the resistive values are much higher and change little when applying negative bias. As is known, PVA is an isolating dielectric material itself [17]. The FE-SEM result shows that the thin film is very dense, therefore it is difficult to form conductive filaments in PVA, and the device keeps
Fig. 2 displays the surface morphologies of PVA and PVA-SCC blend thin films observed by FESEM. From Fig. 2, the surface of PVA is smooth and uniformity, no obvious cracks are observed. PVA-SCC thin film also has a smooth and dense surface morphology, but some particles with sizes of about 200–500 nm distributed irregularly in the sample can be easily seen. Obviously they are SCC particles embedded in the PVA and form PVA-SCC composites. 3.2. Optical transmittance spectra The optical transmittance spectra of Ag/SCC-PVA/ITO/PET device and ITO/PET substrate itself are exhibited in Fig. 3(a). It is shown that the transmittance of the device decreases about 10–20% in the visible region compared to that of the substrate. In the wavelength less than about 500 nm, the transmittance of light abruptly decreases, but the device still shows good transparency in the visible region. This can be confirmed by the inset of Fig. 3(a), in which the logo of “Fujian
Fig. 1. Schematic diagrams of the fabrication process and bending image for Ag/SCC-PVA/ITO/PET device. 2
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Fig. 3. (a) The optical transmittance spectra of Ag/SCC-PVA/ITO/PET device and ITO/PET substrate. The inset is the logo of “Fujian Agriculture and Forestry University” under Ag/SCC-PVA/ITO/PET device. (b) The curve of (αhv)2 vs hv for Ag/SCC-PVA/ITO/PET device.
Fig. 4. (a) I–V characteristics of Ag/SCC-PVA/ITO/PET device swept with various voltage amplitudes. The inset is the I–V curve of pure Ag/PVA/ITO/ PET. (b) I–V characteristics of Ag/SCC-PVA/ITO/PET device with different scan rates. The inset is the schematic view of the device.
high resistance and appears negligible resistive switching behavior. But SCC/PVA composite is different. PVA is used as a supportive layer in the composites thin film, and SCC powder is embedded into PVA and form nano-islands, which can be seen in Fig. 2(b). Due to these nano-islands, it is much easier to form conductive paths in SCC/PVA composites thin film. That is, when applying positive voltage to the sample, Ag metal ions would migrate from the top electrode to the grounded side through the nano-islands and form conductive filaments. Thus, the device shows low resistance [18]. For the applied voltage is low, the conductive fil aments would be thin due to the small amount accumulations of Ag ions, and the conductive filaments are not so stable, which are easy to rapture again. When applying negative voltage to the sample, the opposite electric-field would drive Ag ions drift back to the top electrode, and the conductive filaments would rupture easily from the thinnest place. Therefore, the device shows high resistance when negative voltage is applied, and I–V curves exhibit rectifying effect for the device. The rectifying ratios read at �0.5 V in the low resistance state from Fig. 4(a) are about 13, 53 and 5, for the applied Vmax are 1, 1.5 and 2 V respec tively. The decrease of rectifying ratio could be explained as follows: with increasing the Vmax, the sizes of conductive filaments would in crease, too. It will be not so easy to rupture the conductive filaments anymore while sweeping to negative voltage. So, the resistance read at negative voltage would increase, and of course the ratio would decrease accordingly. The rectifying ratio is also related to the scan rate. From
Fig. 4(b) the rectifying ratio decreases with increasing the scan rates. This could be due to the less accumulations of Ag ions under higher scan speed. To further confirm the self-rectifying resistive switching behavior of the device is related to the migration of metal ions, Pt and Cu top electrodes were deposited on SCC-PVA thin films by RF sputtering, and their I–V curves are shown in Fig. 5(a) and (b), respectively. From Fig. 5, both devices show self-rectifying resistive switching behaviors. Compared to the I–V curve of Ag/SCC-PVA/ITO/PET, devices with Pt and Cu top electrodes needs much more higher operating voltages. That is, the device with Ag top electrode needs the lowest operating voltages, while the device with Pt top electrode needs the highest operating voltages. This could be due to the difference of metal ions migration. As is known, Ag has much higher mobility than that of Cu ions, so it is much easier for Ag ions to form conductive filaments, and hence the device needs lower operating voltage. By contrary, the mobility of Pt ions is much lower than those of Ag and Cu metal ions, so it would need much higher voltages to form conductive filaments. I–V curves of SCC-PVA based thin films with different metal top electrodes indicate that the self-rectifying resistive switching behavior of SCC-PVA based devices is strongly affected by the migration of metal ions.
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Physica B: Physics of Condensed Matter 577 (2020) 411787
voltage amplitudes and voltage scan rates, which could be due to the forming/rapture of Ag conductive filaments. The self-rectifying perfor mances of devices with Pt and Cu top electrodes further confirm that the resistive switching behaviors of SCC-PVA based devices strongly related to the metal conductive filaments. These study results implying that the flexible and transparent Ag/SCC-PVA/ITO/PET resistive switching de vice with self-rectifying property should be a new type of potential candidate for green and low-cost RRAM.
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Declaration of competing interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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The authors gratefully acknowledge financial support from Natural Science Foundation of Fujian Province of China (No. 2017J01735) and Special Fund for Scientific and Technological Innovation of Fujian Agriculture and Forestry University (No. KFA17252A).
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References
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Voltage (V) Fig. 5. I–V curves of SCC-PVA based devices with (a) Pt and (b) Cu top electrodes.
4. Conclusions In conclusion, we have fabricated flexible and transparent Ag/SCCPVA/ITO/PET resistive switching devices. FE-SEM shows that SCC particles embedded in PVA and the forming SCC nano-islands distrib uted irregularly in the sample. The optical transmittance spectrum demonstrates that Ag/SCC-PVA/ITO/PET can be used as a transparent resistive switching device. I–V characteristics of the Ag/SCC-PVA/ITO/ PET show that the resistive switching of device exhibit self-rectifying performance. The values of rectifying ratio are related to the applied
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