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Resistive Switching Memory Devices Based on Proteins Hong Wang, Fanben Meng, Bowen Zhu, Wan Ru Leow, Yaqing Liu, and Xiaodong Chen* electronic systems.[31,32] Among several types of memory, resistive switching memory, which exhibits resistance switching between a high resistance state (HRS) and a low resistance state (LRS), is the most promising candidate for the new generation of memory devices. This is due to its simple structure, easy fabrication, low power consumption, and excellent scalability.[33–61] Recently, protein molecules have been demonstrated to exhibit resistance switching memory characteristics. Moreover, during the past two years, significant progress has been made in the use of proteins for memory devices.[62–69] In this Research News, we summarize the main advancements of the field, ranging from materials selection to device engineering, as well as the mechanism of protein-based resistive switching memory, and the use of proteins in future biocompatible memory devices. Furthermore, we provide insights into the future development of protein-based resistive switching memory to fulfill the future demand for biologically safe and environmentally friendly information storage, as well as biointegrated and bio-inspired electronic systems.
Resistive switching memory constitutes a prospective candidate for nextgeneration data storage devices. Meanwhile, naturally occurring biomaterials are promising building blocks for a new generation of environmentally friendly, biocompatible, and biodegradable electronic devices. Recent progress in using proteins to construct resistive switching memory devices is highlighted. The protein materials selection, device engineering, and mechanism of such protein-based resistive switching memory are discussed in detail. Finally, the critical challenges associated with protein-based resistive switching memory devices are presented, as well as insights into the future development of resistive switching memory based on natural biomaterials.
1. Introduction Advances in materials and fabrication technologies have provided synthetic routes for the construction of electronic devices that are capable of intimate, conformal integration with biological tissues for applications in surgical devices, health monitoring systems, and human–machine interfaces.[1–4] Recently, interest has been generated in the use of organic materials with excellent flexibility for the development of electronic devices that can integrate with soft and curvilinear surfaces for potential biological-related applications, such as biocompatible and wearable communications devices, as well as biotic/abiotic interfaces.[5–11] Meanwhile, natural biomaterials extracted from organisms provide a promising basis for the design of bio-integrated electronics and bio-inspired devices.[12–23] Biomaterials possess the advantages of being renewable, environmentally friendly, biocompatible, and biodegradable.[12–14] In addition, they are inexpensive, light-weight, and compatible with large scale fabrication on flexible substrates.[12] Proteins are an essential component of all organisms and thereby constitute readily available biomaterial.[24] Electron transfer through proteins has been studied thoroughly over the last several decades and has opened up possibilities for using proteins to construct solid-state electronic devices.[25–30] Functional devices based on proteins, such as transistors, diodes, and optical elements, have been proposed.[14,29] The use of proteins in the fabrication of memory devices—which are key components in data storage systems and fundamental units in integrated circuits—is highly desirable for future bio-integrated Dr. H. Wang, Dr. F. Meng, B. Zhu, W. R. Leow, Dr. Y. Liu, Prof. X. Chen School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore 639798 E-mail: [email protected]
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2. Resistive Switching Resistive switching devices generally comprise a two-terminal geometry, in which the resistive switching material is sandwiched between two conductive electrodes. By applying an electrical field across the electrodes, the resistance can be switched between the HRS and the LRS. The resistive switching behaviors of the devices can be categorized into two main types: volatile, and non-volatile switching.[70] In volatile switching, the information cannot be retained without an applied voltage, and only the OFF state (HRS) is stable, whereas the volatile ON state (LRS) can be maintained by continuous voltage pulses. The volatile resistive switching devices have great potential for dynamic random access memory (DRAM) and static random access memory (SRAM) applications.[43] In addition, the volatile resistive switching effect can be employed to build a selector to solve the sneak path problem in cross-bar arrays for high density memory applications.[71] On the other hand, resistive switching devices with non-volatile switching can retain information for a long time. Here, both the HRS and LRS are stable after the applied voltage is removed. The non-volatile switching effect can be classified as write-once-read-many-times (WORM) memory or rewritable memory, depending on whether an applied voltage can switch the ON state to the OFF state.[44,72] In WORM memory, if the device is switched from HRS to LRS, the LRS cannot be switched back to HRS. Such devices can be used as
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read-only memory components in radio frequency identification tags.[41] The rewritable switching devices can be used as random access memory in a variety of electronic systems. Depending on voltage polarity, the rewritable devices can be classified as bipolar or unipolar. For bipolar switching, opposite voltage polarities are required for switching ON and OFF, respectively, whereas for unipolar switching, the switching is induced by a voltage of the same polarity.[40] Interestingly, research on protein-based resistive switching devices is mainly focused on rewritable, nonvolatile memory. The mechanisms of resistive switching are still controversial and not clearly understood. The established switching mechanisms of inorganic and organic materials mainly involve formation and rupture of conductive filaments, trap charging and discharging, the redox reaction of the switching materials, and so on.[43,44] The switching mechanisms of protein-based memory devices are referred to as those of inorganic and organic based devices, and are described in next section.
3. Proteins for Memory Devices 3.1. Proteins Containing Metal Ions A large fraction of proteins contain metal ions, which endow them with various functionalities, such as storage, charge transfer, and signal transduction.[73] It is therefore believed that proteins which contain metal ions have the potential to be exploited as electronic memory. The first protein-based solidstate resistive switching memory was proposed by our group,
in which ferritin was used as a resistive switching material.[62] Ferritin—which is a primary intracellular iron-storage protein with a nearly spherical shell (diameter: 12 nm), and an active mineral core of hydrous ferric oxide (Fe(III)O·OH)—has been used in field effect transistors, floating gate memory devices, and as a doping element in resistive switching memory devices.[74–76] Two-terminal resistive switching devices were prepared by assembling ferritin molecules inside a 12 nm gap generated by on-wire lithography (OWL) (Figure 1a). The typical I–V characteristics of the ferritin based devices showed an obvious resistive switching effect. By sweeping the applied voltage from zero to a positive value, the current abruptly increased, and the device switched from a HRS to a LRS at a setting voltage Vset of 0.7 V, as shown in Figure 1b. The device maintained the LRS until the applied voltage was lower than –0.8 V (Vreset). This work opened the doors for the application of proteins in resistive switching memory devices. Interestingly, the structures of proteins can be tailored to fit the complex requirements for future applications.[77] It is important to know the detailed mechanism of the resistive switching effect of ferritin based devices, in order to achieve modulation of the molecule structure for further improvement in memory performance. It was demonstrated that an electrochemical process occurring in the active center of ferritin is responsible for the resistive switching effect. When apoferritin—which has a structure similar to that of ferritin, but lacks an active center—was assembled within the nanogap, no resistive switching effect was observed. However, reconstitution of the iron-core active center in apoferritin led to a clear observation of resistive switching behavior.
Figure 1. a) Schematic diagram of the first protein-based resistive switching memory, in which the ferritin molecules are embedded into the nanogaps. Reproduced with permission.[62] b) Typical I–V characteristics of the nanogap device before and after immobilization of ferritin. Reproduced with permission.[62] c) On/Off ratio of the Archaeoglobus fulgidus ferritin-based resistive switching device with different Fe loadings. Reproduced with permission.[63] d) Schematic diagram of the mechanism of ferritin based resistive switching device. Reproduced with permission.[63]
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3.2. Proteins Without Metal Ions
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Additionally, our group discovered that the memory performance of the ferritin based resistive switching devices can be modulated via controlling the amount of iron loaded in the complex core.[63] Ferritin, with varying amounts of iron loaded in its core, was obtained by treating purified Archaeoglobus fulgidus ferritin (AfFtn) with different amounts of Fe(II) ions. The ferritin molecules with different numbers of loaded Fe atoms (1200, 2400, 4800, and 7200 Fe per AfFtn complex core) were assembled inside the 12 nm gap to study the relationship between memory performance and the amount of loaded Fe atoms. As shown in Figure 1c, the memory window (current ON/OFF ratio) increased with the increase of loaded Fe ions. Most remarkably, the 7200 Fe ferritin based resistive switching devices showed a significant memory window. The ferric-oxyhydroxyphosphate complex core contains several thousand Fe(III) atoms, some of which could be reduced to Fe(II) by applying a voltage to the device.[78] The Fe(II) drifted more easily than Fe(III) under an electric field.[79] Conversely, when an opposite polarity voltage was applied, the mineral core was converted to its original state. Therefore, the reduction and oxidation of the metal oxide core is responsible for the resistive switching of ferritin based devices, as illustrated in Figure 1d. Thus, metal ion-containing proteins are promising candidates for resistive switching memory materials. In addition, the memory performance can be modulated by controlling the type and amount of ions in proteins. The bioengineered, tunable performances of devices based on such materials offer opportunities for higherorder target applications such as the integration with synapses in neuromorphic systems.
It is estimated that more than half of proteins do not contain any metal ions, of which silk proteins are the most representative materials for applications in electronic related systems. Silk, derived from Bombyx mori cocoons, consists mainly of fibroin and sericin. What particularly distinguishes silk from other proteins for electronic applications is the fact that silk can not only be processed in an all water-based, room temperature, neutral pH environment, but also is environmentally friendly, biocompatible and implantable in the human body. Recently, silk proteins have been widely studied as sustainable materials for optics and photonics, electronics, and optoelectronics applications.[14] Interestingly, silk proteins also demonstrate promising applications in future biocompatible memory devices. We discovered a remarkable resistive switching effect in an Ag/sericin/Au configuration (Figure 2a). The fabricated Ag/ sericin/Au devices exhibited random access memory behavior with a high ON/OFF ratio of 106 and long retention times of >103 s.[67] More importantly, under different applied compliance currents (to avoid a hard breakdown of the device) of 10 mA, 1 mA and100 µA in the set process, three LRS levels were obtained (Figure 2b). The device showed four distinct resistive states ((00), (01), (10), and (11)), and could be used as multilevel memory. Additionally, flexible resistive switching memory based on sericin was successfully achieved, as shown in Figure 2c and 2d. Such natural biomaterial-based flexible memory has great potential for wearable electronics, smart skin, and biomedical device applications. Interestingly, sericin
Figure 2. a) Schematic diagram of the Ag/sericin/Au device. b) Typical I-V curves of the Ag/sericin/Au device with multilevel memory. Reproduced with permission.[67] c) Optical image of flexible sericin based memory devices. Reproduced with permission.[67] d) Typical I–V curves of the Ag/sericin/Au device on flexible substrate. Reproduced with permission.[67] e) Schematic of the potential distribution of Ag/sericin/Au device. Much higher electrical fields are formed at the electrode/sericin interfaces, with a positive voltage applied to the top electrode (TE), and the bottom electrode (BE) grounded. f) Schematic of the migration of Ag nanoparticles from the TE to the BE. Driven by the electric field, the cations move towards the cathode. The neutralization of Ag cations takes place due to the incoming electrons. g,h) LRS (low resistance state) and HRS (high resistance state) formed after the removal of the electrical field.
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is a usually-discarded by-product of silk processing, thus the successful development of resistive switching memory based on sericin provides a good example for the exploitation of waste materials in developing environmentally friendly devices. As opposed to the redox processes of ferritin-based devices, the resistive switching effect in sericin films is due to the formation and rupture of hopping paths.[67] Defects induced by the structure disorder and injected Ag nanoparticles in sericin films contribute to the hopping conduction. As shown in Figure 2e,f, when a positive voltage is applied to the Ag electrode positive and negative charges are created at the top and bottom interface, respectively. A large electrical field can be created, causing the Ag nanoparticles to migrate into the sericin film. Electrons hopping through defects induced by these Ag nanoparticles lead to the observed high current at the LRS state (Figure 2g). When a negative voltage is applied to the Ag electrode, a large tunneling gap can be formed (Figure 2h), as the number of hopping paths is significantly reduced, causing the device to switch to HRS. For multi-level memory, it can be explained that more conductive paths are formed at a high set compliance current, while fewer hopping paths are formed with a lower set compliance current. Hota et al. similarly demonstrated that fibroin films exhibit nonvolatile resistive switching memory behavior in a ITO/fibroin/Al sandwich device (ITO refers to indium tin oxide),[65] which showed an ON/OFF ratio of 10, and retention time of 103 s. It was also proposed that carrier trapping and/or detrapping due to oxidation and reduction procedures in fibroin films were responsible for the resistive switching memory effect. The development of memory devices based on silk would pave the way for future improvement of proteinbased memory technology. Especially, since silk proteins have been widely used in electronics, memory devices based on silk are interesting for future silk based electronic systems due to the great potential for simplifying the integrated technology.
3.3. Protein Composites Besides using single proteins as functional materials, the resistive switching memory effect in doped or mixed protein systems has attracted significant research interest, as it enables tunable memory performance via controlling the doping level in the composite system.[68,69] So far, protein composites, such as protein/organic multilayers and protein/metal nanoparticles composites, have shown excellent resistive switching memory performance. Ko et al. demonstrated that the memory performance of ferritin-based resistive switching devices could be significantly improved by layer-by-layer (LbL) assembly of poly(allylamine hydrochloride) (PAH)/ferritin multilayers,[68] which were fabricated as a Ag/(PAH/ferritin)n/Pt structure (Figure 3a). The film thickness of (PAH/ferritin)n multilayers was increased from 0 to 107 nm by increasing the bilayer number from 0 to 15, as shown in Figure 3b. Typical resistive switching behaviors were observed in the devices with a bilayer number of 5, 10, and 15 (Figure 3c). Notably, the resistance of the HRS depended on the PAH/ferritin bilayer number, while the resistance of LRS remained nearly constant. The OFF current decreased dramatically with increased film thickness, while an ON/OFF current ratio as large as 103 was achieved for
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devices with a PAH/ferritin bilayer number of 15. Additionally, the 15 layered PAH/ferritin film based devices were found to have excellent stability, with retention time longer than 104 seconds, and cycling tests of approximately 300 cycles. The log-log scale of the I–V curve (Figure 3d) at the HRS state shows space charge limited conduction (SCLC), consisting of an Ohmic current region (slope of 1.02) at low bias and a Mott–Gurney region (slope of 2) at high bias. Moreover, the LRS of the device demonstrates Ohmic conduction behavior. The resistive switching of PAH/ferritin based devices can be explained by the charge trapping/releasing of the Fe(III)/Fe(II) redox couples within ferritin, in which the ferrihydrite cores of ferritin act as trap states.[68] To further demonstrate that the resistive switching was due to the trapping and release of charges, kelvin force microscopy (KFM) was utilized to investigate the nonvolatile memory effect, as shown in Figure 3e. The charge-trapping and charge-releasing state can be clearly seen in the KFM image, supporting the hypothesis that resistive switching originates from charges trapping/detrapping via ferrihydrite of ferritin. The memory performance of conventional inorganic and organic-based resistive switching devices could also be significantly improved by doping with nanoparticles.[80,81] Gogurla et al. demonstrated that the memory performance of resistive switching devices based on silk protein can be modulated by controlling the concentration of gold nanoparticles (Au NPs) in the silk films.[69] The tunable memory performance of the Al/silk fibroin/ITO device was studied by doping Au NPs in the fibroin composite thin films.[69] The composite thin films were prepared by spin-coating the silk-Au NPs in aqueous solution, in which the ratio of silk and Au NP concentrations can be adjusted. With a concentration ratio of 10:1 of silk to Au NPs, a reduced set voltage of 1.4 V (10 V for pure silk-based devices) and ON/OFF current ratio more than 106 (less than 10 for pure silk-based devices) were observed in these resistive switching devices. The Au NPs in the composite films acted as trap centers, and the current conduction mechanism was in good agreement with the space charge limited conduction (SCLC) controlled by trap states. As described for the PAH/ ferritin-based devices, the switching mechanism of the silk/ Au NPs based devices also originated from charge trapping/ releasing; the difference between the two systems is that different components acted as the trap states. Therefore, we propose that controlling the structure and density of trap states in protein-based resistive switching memory can effectively control memory performance. This would render the composite system a promising candidate for future protein-based memory devices. In particular, high performance memory devices that are achieved by carefully adjusting the components of protein composite systems have great potential for green information storage with remarkable performance.
4. Conclusion and Perspective Resistive switching memory based on natural biomaterials, especially proteins, has attracted a lot of attention due to the great potential for new-generation biocompatible information storage technologies and bio-integrated electronics systems applications. The resistive switching effect of a variety of
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RESEARCH NEWS Figure 3. a) Schematic diagram of the fabrication of layer-by-layer assembled PAH/ferritin multilayer-based resistive switching memory. b) Film thickness of PAH/ferritin multilayers as a function of the bilayer number. The insets are cross-sectional SEM images of 5 and 15 bilayered films. c) Typical I–V characteristics of the PAH/ferritin multilayers device with bilayer number from 5 to 15. d) I–V curve of a 15-bilayered device plotted on a log–log scale. e) Kelvin force microscopy image of PAH/ferritin multilayers for the charge-trap/release operations. Reproduced with permission.[68] Copyright 2011, American Chemical Society.
proteins has been demonstrated. In this research news, a brief summary of the materials, device structures, related resistive switching mechanisms, and potential systems for applications
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associated with protein-based resistive switching memory devices has been provided. In particular, we highlighted the essential strategies, including the control of protein structures,
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the use of doped or mixed protein systems, the selection of operation modes to improve memory performance, as well as the study of resistive switching mechanisms in the resultant devices. Nevertheless, there remain some challenging tasks to be resolved. The resistive switching mechanisms in relation to the different categories of materials need to be studied in order to achieve reliable high performance devices, as well as provide guidance for the development of configurable resistive switching devices for various applications. Additionally, it would be interesting to develop protein-based memory devices that can maintain good memory performance on soft, and curvilinear, as well as time-dynamic surfaces. These devices may—enables large-area production technologies such as inkjet printing—create opportunities for human-machine interface and other bio-integrated systems applications. Furthermore, active matrix systems such as one-diode-one resistor (1D1R) and one-transistor-one-resistor (1T1R) architectures are required to solve the cross-talk problem in cross-bar arrays for high density memory applications. Finally, the development of technology to integrate protein-based memory devices with other bio-compatible and bio-inspired electronic devices to achieve unprecedented functionality for future bio-integrated system applications is also highly desirable. By solving the scientific and technical issues described above, we believe that protein-based resistive switching memory will facilitate the new generation of bio-compatible information storage technologies and bio-integrated electronics systems.
Acknowledgements This work was supported by the Singapore National Research Foundation (NRF-RF2009–04). Received: December 15, 2014 Revised: January 23, 2015 Published online:
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[48] G. Wang, A. Lauchner, J. Lin, D. Natelson, K. Palem, J. Tour, Adv. Mater. 2013, 25, 4789. [49] C. He, J. Li, X. Wu, P. Chen, J. Zhao, K. Yin, M. Cheng, W. Yang, G. Xie, D. Wang, D. Liu, R. Yang, D. Shi, Z. Li, L. Sun, G. Zhang, Adv. Mater. 2013, 25, 5593. [50] Z. Lin, J. Liang, P. Sun, F. Liu, Y. Tay, M. Yi, K. Peng, X. Xia, L. Xie, X. Zhou, J. Zhao, W. Huang, Adv. Mater. 2013, 25, 3664. [51] K. Szot, W. Speier, G. Bihlmayer, R. Waser, Nat. Mater. 2006, 5, 312. [52] J. Yoon, J. Han, J. Jung, W. Jeon, G. Kim, S. Song, J. Seok, K. Yoon, M. Lee, C. Hwang, Adv. Mater. 2013, 25, 1987. [53] R. Shallcross, P. Zacharias, A. Köhnen, P. Körner, E. Maibach, K. Meerholz, Adv. Mater. 2013, 25, 469. [54] S. Balatti, S. Larentis, D. Gilmer, D. Ielmini, Adv. Mater. 2013, 25, 1474. [55] S. Kim, J. Son, S. Lee, B. You, K. Park, H. Lee, M. Byun, K. Lee, Adv. Mater. 2014, 26, 7480. [56] D. Lee, J. Park, J. Park, J. Woo, E. Cha, S. Lee, K. Moon, J. Song, Y. Koo, H. Hwang, Adv. Mater. 2015, 27, 59. [57] S. Ambrogio, S. Balatti, S. Choi, D. Ielmini, Adv. Mater. 2014, 26, 3885. [58] Y. Yang, J. Lee, S. Lee, C. Liu, Z. Zhong, W. Lu, Adv. Mater. 2014, 26, 3693. [59] X. Tian, S. Yang, M. Zeng, L. Wang, J. Wei, Z. Xu, W. Wang, X. Bai, Adv. Mater. 2014, 26, 3649. [60] A. Herpers, C. Lenser, C. Park, F. Offi, F. Borgatti, G. Panaccione, S. Menzel, R. Waser, R. Dittmann, Adv. Mater. 2014, 26, 2730. [61] M. Qian, Y. Pan, F. Liu, M. Wang, H. Shen, D. He, B. Wang, Y. Shi, F. Miao, X. Wang, Adv. Mater. 2014, 26, 3275. [62] F. Meng, L. Jiang, K. Zheng, C. Goh, S. Lim, H. Hng, J. Ma, F. Boey, X. Chen, Small 2011, 7, 3016. [63] F. Meng, B. Sana, Y. Li, Y. Liu, S. Lim, X. Chen, Small 2014, 10, 277.
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Resistive Switching Memory Devices Based on Proteins Hong Wang, Fanben Meng, Bowen Zhu, Wan Ru Leow, Yaqing Liu, and Xiaodong Chen* electronic systems.[31,32] Among several types of memory, resistive switching memory, which exhibits resistance switching between a high resistance state (HRS) and a low resistance state (LRS), is the most promising candidate for the new generation of memory devices. This is due to its simple structure, easy fabrication, low power consumption, and excellent scalability.[33–61] Recently, protein molecules have been demonstrated to exhibit resistance switching memory characteristics. Moreover, during the past two years, significant progress has been made in the use of proteins for memory devices.[62–69] In this Research News, we summarize the main advancements of the field, ranging from materials selection to device engineering, as well as the mechanism of protein-based resistive switching memory, and the use of proteins in future biocompatible memory devices. Furthermore, we provide insights into the future development of protein-based resistive switching memory to fulfill the future demand for biologically safe and environmentally friendly information storage, as well as biointegrated and bio-inspired electronic systems.
Resistive switching memory constitutes a prospective candidate for nextgeneration data storage devices. Meanwhile, naturally occurring biomaterials are promising building blocks for a new generation of environmentally friendly, biocompatible, and biodegradable electronic devices. Recent progress in using proteins to construct resistive switching memory devices is highlighted. The protein materials selection, device engineering, and mechanism of such protein-based resistive switching memory are discussed in detail. Finally, the critical challenges associated with protein-based resistive switching memory devices are presented, as well as insights into the future development of resistive switching memory based on natural biomaterials.
1. Introduction Advances in materials and fabrication technologies have provided synthetic routes for the construction of electronic devices that are capable of intimate, conformal integration with biological tissues for applications in surgical devices, health monitoring systems, and human–machine interfaces.[1–4] Recently, interest has been generated in the use of organic materials with excellent flexibility for the development of electronic devices that can integrate with soft and curvilinear surfaces for potential biological-related applications, such as biocompatible and wearable communications devices, as well as biotic/abiotic interfaces.[5–11] Meanwhile, natural biomaterials extracted from organisms provide a promising basis for the design of bio-integrated electronics and bio-inspired devices.[12–23] Biomaterials possess the advantages of being renewable, environmentally friendly, biocompatible, and biodegradable.[12–14] In addition, they are inexpensive, light-weight, and compatible with large scale fabrication on flexible substrates.[12] Proteins are an essential component of all organisms and thereby constitute readily available biomaterial.[24] Electron transfer through proteins has been studied thoroughly over the last several decades and has opened up possibilities for using proteins to construct solid-state electronic devices.[25–30] Functional devices based on proteins, such as transistors, diodes, and optical elements, have been proposed.[14,29] The use of proteins in the fabrication of memory devices—which are key components in data storage systems and fundamental units in integrated circuits—is highly desirable for future bio-integrated Dr. H. Wang, Dr. F. Meng, B. Zhu, W. R. Leow, Dr. Y. Liu, Prof. X. Chen School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore 639798 E-mail: [email protected]
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2. Resistive Switching Resistive switching devices generally comprise a two-terminal geometry, in which the resistive switching material is sandwiched between two conductive electrodes. By applying an electrical field across the electrodes, the resistance can be switched between the HRS and the LRS. The resistive switching behaviors of the devices can be categorized into two main types: volatile, and non-volatile switching.[70] In volatile switching, the information cannot be retained without an applied voltage, and only the OFF state (HRS) is stable, whereas the volatile ON state (LRS) can be maintained by continuous voltage pulses. The volatile resistive switching devices have great potential for dynamic random access memory (DRAM) and static random access memory (SRAM) applications.[43] In addition, the volatile resistive switching effect can be employed to build a selector to solve the sneak path problem in cross-bar arrays for high density memory applications.[71] On the other hand, resistive switching devices with non-volatile switching can retain information for a long time. Here, both the HRS and LRS are stable after the applied voltage is removed. The non-volatile switching effect can be classified as write-once-read-many-times (WORM) memory or rewritable memory, depending on whether an applied voltage can switch the ON state to the OFF state.[44,72] In WORM memory, if the device is switched from HRS to LRS, the LRS cannot be switched back to HRS. Such devices can be used as
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read-only memory components in radio frequency identification tags.[41] The rewritable switching devices can be used as random access memory in a variety of electronic systems. Depending on voltage polarity, the rewritable devices can be classified as bipolar or unipolar. For bipolar switching, opposite voltage polarities are required for switching ON and OFF, respectively, whereas for unipolar switching, the switching is induced by a voltage of the same polarity.[40] Interestingly, research on protein-based resistive switching devices is mainly focused on rewritable, nonvolatile memory. The mechanisms of resistive switching are still controversial and not clearly understood. The established switching mechanisms of inorganic and organic materials mainly involve formation and rupture of conductive filaments, trap charging and discharging, the redox reaction of the switching materials, and so on.[43,44] The switching mechanisms of protein-based memory devices are referred to as those of inorganic and organic based devices, and are described in next section.
3. Proteins for Memory Devices 3.1. Proteins Containing Metal Ions A large fraction of proteins contain metal ions, which endow them with various functionalities, such as storage, charge transfer, and signal transduction.[73] It is therefore believed that proteins which contain metal ions have the potential to be exploited as electronic memory. The first protein-based solidstate resistive switching memory was proposed by our group,
in which ferritin was used as a resistive switching material.[62] Ferritin—which is a primary intracellular iron-storage protein with a nearly spherical shell (diameter: 12 nm), and an active mineral core of hydrous ferric oxide (Fe(III)O·OH)—has been used in field effect transistors, floating gate memory devices, and as a doping element in resistive switching memory devices.[74–76] Two-terminal resistive switching devices were prepared by assembling ferritin molecules inside a 12 nm gap generated by on-wire lithography (OWL) (Figure 1a). The typical I–V characteristics of the ferritin based devices showed an obvious resistive switching effect. By sweeping the applied voltage from zero to a positive value, the current abruptly increased, and the device switched from a HRS to a LRS at a setting voltage Vset of 0.7 V, as shown in Figure 1b. The device maintained the LRS until the applied voltage was lower than –0.8 V (Vreset). This work opened the doors for the application of proteins in resistive switching memory devices. Interestingly, the structures of proteins can be tailored to fit the complex requirements for future applications.[77] It is important to know the detailed mechanism of the resistive switching effect of ferritin based devices, in order to achieve modulation of the molecule structure for further improvement in memory performance. It was demonstrated that an electrochemical process occurring in the active center of ferritin is responsible for the resistive switching effect. When apoferritin—which has a structure similar to that of ferritin, but lacks an active center—was assembled within the nanogap, no resistive switching effect was observed. However, reconstitution of the iron-core active center in apoferritin led to a clear observation of resistive switching behavior.
Figure 1. a) Schematic diagram of the first protein-based resistive switching memory, in which the ferritin molecules are embedded into the nanogaps. Reproduced with permission.[62] b) Typical I–V characteristics of the nanogap device before and after immobilization of ferritin. Reproduced with permission.[62] c) On/Off ratio of the Archaeoglobus fulgidus ferritin-based resistive switching device with different Fe loadings. Reproduced with permission.[63] d) Schematic diagram of the mechanism of ferritin based resistive switching device. Reproduced with permission.[63]
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3.2. Proteins Without Metal Ions
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Additionally, our group discovered that the memory performance of the ferritin based resistive switching devices can be modulated via controlling the amount of iron loaded in the complex core.[63] Ferritin, with varying amounts of iron loaded in its core, was obtained by treating purified Archaeoglobus fulgidus ferritin (AfFtn) with different amounts of Fe(II) ions. The ferritin molecules with different numbers of loaded Fe atoms (1200, 2400, 4800, and 7200 Fe per AfFtn complex core) were assembled inside the 12 nm gap to study the relationship between memory performance and the amount of loaded Fe atoms. As shown in Figure 1c, the memory window (current ON/OFF ratio) increased with the increase of loaded Fe ions. Most remarkably, the 7200 Fe ferritin based resistive switching devices showed a significant memory window. The ferric-oxyhydroxyphosphate complex core contains several thousand Fe(III) atoms, some of which could be reduced to Fe(II) by applying a voltage to the device.[78] The Fe(II) drifted more easily than Fe(III) under an electric field.[79] Conversely, when an opposite polarity voltage was applied, the mineral core was converted to its original state. Therefore, the reduction and oxidation of the metal oxide core is responsible for the resistive switching of ferritin based devices, as illustrated in Figure 1d. Thus, metal ion-containing proteins are promising candidates for resistive switching memory materials. In addition, the memory performance can be modulated by controlling the type and amount of ions in proteins. The bioengineered, tunable performances of devices based on such materials offer opportunities for higherorder target applications such as the integration with synapses in neuromorphic systems.
It is estimated that more than half of proteins do not contain any metal ions, of which silk proteins are the most representative materials for applications in electronic related systems. Silk, derived from Bombyx mori cocoons, consists mainly of fibroin and sericin. What particularly distinguishes silk from other proteins for electronic applications is the fact that silk can not only be processed in an all water-based, room temperature, neutral pH environment, but also is environmentally friendly, biocompatible and implantable in the human body. Recently, silk proteins have been widely studied as sustainable materials for optics and photonics, electronics, and optoelectronics applications.[14] Interestingly, silk proteins also demonstrate promising applications in future biocompatible memory devices. We discovered a remarkable resistive switching effect in an Ag/sericin/Au configuration (Figure 2a). The fabricated Ag/ sericin/Au devices exhibited random access memory behavior with a high ON/OFF ratio of 106 and long retention times of >103 s.[67] More importantly, under different applied compliance currents (to avoid a hard breakdown of the device) of 10 mA, 1 mA and100 µA in the set process, three LRS levels were obtained (Figure 2b). The device showed four distinct resistive states ((00), (01), (10), and (11)), and could be used as multilevel memory. Additionally, flexible resistive switching memory based on sericin was successfully achieved, as shown in Figure 2c and 2d. Such natural biomaterial-based flexible memory has great potential for wearable electronics, smart skin, and biomedical device applications. Interestingly, sericin
Figure 2. a) Schematic diagram of the Ag/sericin/Au device. b) Typical I-V curves of the Ag/sericin/Au device with multilevel memory. Reproduced with permission.[67] c) Optical image of flexible sericin based memory devices. Reproduced with permission.[67] d) Typical I–V curves of the Ag/sericin/Au device on flexible substrate. Reproduced with permission.[67] e) Schematic of the potential distribution of Ag/sericin/Au device. Much higher electrical fields are formed at the electrode/sericin interfaces, with a positive voltage applied to the top electrode (TE), and the bottom electrode (BE) grounded. f) Schematic of the migration of Ag nanoparticles from the TE to the BE. Driven by the electric field, the cations move towards the cathode. The neutralization of Ag cations takes place due to the incoming electrons. g,h) LRS (low resistance state) and HRS (high resistance state) formed after the removal of the electrical field.
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is a usually-discarded by-product of silk processing, thus the successful development of resistive switching memory based on sericin provides a good example for the exploitation of waste materials in developing environmentally friendly devices. As opposed to the redox processes of ferritin-based devices, the resistive switching effect in sericin films is due to the formation and rupture of hopping paths.[67] Defects induced by the structure disorder and injected Ag nanoparticles in sericin films contribute to the hopping conduction. As shown in Figure 2e,f, when a positive voltage is applied to the Ag electrode positive and negative charges are created at the top and bottom interface, respectively. A large electrical field can be created, causing the Ag nanoparticles to migrate into the sericin film. Electrons hopping through defects induced by these Ag nanoparticles lead to the observed high current at the LRS state (Figure 2g). When a negative voltage is applied to the Ag electrode, a large tunneling gap can be formed (Figure 2h), as the number of hopping paths is significantly reduced, causing the device to switch to HRS. For multi-level memory, it can be explained that more conductive paths are formed at a high set compliance current, while fewer hopping paths are formed with a lower set compliance current. Hota et al. similarly demonstrated that fibroin films exhibit nonvolatile resistive switching memory behavior in a ITO/fibroin/Al sandwich device (ITO refers to indium tin oxide),[65] which showed an ON/OFF ratio of 10, and retention time of 103 s. It was also proposed that carrier trapping and/or detrapping due to oxidation and reduction procedures in fibroin films were responsible for the resistive switching memory effect. The development of memory devices based on silk would pave the way for future improvement of proteinbased memory technology. Especially, since silk proteins have been widely used in electronics, memory devices based on silk are interesting for future silk based electronic systems due to the great potential for simplifying the integrated technology.
3.3. Protein Composites Besides using single proteins as functional materials, the resistive switching memory effect in doped or mixed protein systems has attracted significant research interest, as it enables tunable memory performance via controlling the doping level in the composite system.[68,69] So far, protein composites, such as protein/organic multilayers and protein/metal nanoparticles composites, have shown excellent resistive switching memory performance. Ko et al. demonstrated that the memory performance of ferritin-based resistive switching devices could be significantly improved by layer-by-layer (LbL) assembly of poly(allylamine hydrochloride) (PAH)/ferritin multilayers,[68] which were fabricated as a Ag/(PAH/ferritin)n/Pt structure (Figure 3a). The film thickness of (PAH/ferritin)n multilayers was increased from 0 to 107 nm by increasing the bilayer number from 0 to 15, as shown in Figure 3b. Typical resistive switching behaviors were observed in the devices with a bilayer number of 5, 10, and 15 (Figure 3c). Notably, the resistance of the HRS depended on the PAH/ferritin bilayer number, while the resistance of LRS remained nearly constant. The OFF current decreased dramatically with increased film thickness, while an ON/OFF current ratio as large as 103 was achieved for
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devices with a PAH/ferritin bilayer number of 15. Additionally, the 15 layered PAH/ferritin film based devices were found to have excellent stability, with retention time longer than 104 seconds, and cycling tests of approximately 300 cycles. The log-log scale of the I–V curve (Figure 3d) at the HRS state shows space charge limited conduction (SCLC), consisting of an Ohmic current region (slope of 1.02) at low bias and a Mott–Gurney region (slope of 2) at high bias. Moreover, the LRS of the device demonstrates Ohmic conduction behavior. The resistive switching of PAH/ferritin based devices can be explained by the charge trapping/releasing of the Fe(III)/Fe(II) redox couples within ferritin, in which the ferrihydrite cores of ferritin act as trap states.[68] To further demonstrate that the resistive switching was due to the trapping and release of charges, kelvin force microscopy (KFM) was utilized to investigate the nonvolatile memory effect, as shown in Figure 3e. The charge-trapping and charge-releasing state can be clearly seen in the KFM image, supporting the hypothesis that resistive switching originates from charges trapping/detrapping via ferrihydrite of ferritin. The memory performance of conventional inorganic and organic-based resistive switching devices could also be significantly improved by doping with nanoparticles.[80,81] Gogurla et al. demonstrated that the memory performance of resistive switching devices based on silk protein can be modulated by controlling the concentration of gold nanoparticles (Au NPs) in the silk films.[69] The tunable memory performance of the Al/silk fibroin/ITO device was studied by doping Au NPs in the fibroin composite thin films.[69] The composite thin films were prepared by spin-coating the silk-Au NPs in aqueous solution, in which the ratio of silk and Au NP concentrations can be adjusted. With a concentration ratio of 10:1 of silk to Au NPs, a reduced set voltage of 1.4 V (10 V for pure silk-based devices) and ON/OFF current ratio more than 106 (less than 10 for pure silk-based devices) were observed in these resistive switching devices. The Au NPs in the composite films acted as trap centers, and the current conduction mechanism was in good agreement with the space charge limited conduction (SCLC) controlled by trap states. As described for the PAH/ ferritin-based devices, the switching mechanism of the silk/ Au NPs based devices also originated from charge trapping/ releasing; the difference between the two systems is that different components acted as the trap states. Therefore, we propose that controlling the structure and density of trap states in protein-based resistive switching memory can effectively control memory performance. This would render the composite system a promising candidate for future protein-based memory devices. In particular, high performance memory devices that are achieved by carefully adjusting the components of protein composite systems have great potential for green information storage with remarkable performance.
4. Conclusion and Perspective Resistive switching memory based on natural biomaterials, especially proteins, has attracted a lot of attention due to the great potential for new-generation biocompatible information storage technologies and bio-integrated electronics systems applications. The resistive switching effect of a variety of
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RESEARCH NEWS Figure 3. a) Schematic diagram of the fabrication of layer-by-layer assembled PAH/ferritin multilayer-based resistive switching memory. b) Film thickness of PAH/ferritin multilayers as a function of the bilayer number. The insets are cross-sectional SEM images of 5 and 15 bilayered films. c) Typical I–V characteristics of the PAH/ferritin multilayers device with bilayer number from 5 to 15. d) I–V curve of a 15-bilayered device plotted on a log–log scale. e) Kelvin force microscopy image of PAH/ferritin multilayers for the charge-trap/release operations. Reproduced with permission.[68] Copyright 2011, American Chemical Society.
proteins has been demonstrated. In this research news, a brief summary of the materials, device structures, related resistive switching mechanisms, and potential systems for applications
Adv. Mater. 2015, DOI: 10.1002/adma.201405728
associated with protein-based resistive switching memory devices has been provided. In particular, we highlighted the essential strategies, including the control of protein structures,
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the use of doped or mixed protein systems, the selection of operation modes to improve memory performance, as well as the study of resistive switching mechanisms in the resultant devices. Nevertheless, there remain some challenging tasks to be resolved. The resistive switching mechanisms in relation to the different categories of materials need to be studied in order to achieve reliable high performance devices, as well as provide guidance for the development of configurable resistive switching devices for various applications. Additionally, it would be interesting to develop protein-based memory devices that can maintain good memory performance on soft, and curvilinear, as well as time-dynamic surfaces. These devices may—enables large-area production technologies such as inkjet printing—create opportunities for human-machine interface and other bio-integrated systems applications. Furthermore, active matrix systems such as one-diode-one resistor (1D1R) and one-transistor-one-resistor (1T1R) architectures are required to solve the cross-talk problem in cross-bar arrays for high density memory applications. Finally, the development of technology to integrate protein-based memory devices with other bio-compatible and bio-inspired electronic devices to achieve unprecedented functionality for future bio-integrated system applications is also highly desirable. By solving the scientific and technical issues described above, we believe that protein-based resistive switching memory will facilitate the new generation of bio-compatible information storage technologies and bio-integrated electronics systems.
Acknowledgements This work was supported by the Singapore National Research Foundation (NRF-RF2009–04). Received: December 15, 2014 Revised: January 23, 2015 Published online:
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