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Published on 22 September 2014. Downloaded by University of Utah on 16/10/2014 13:01:13.
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Cite this: DOI: 10.1039/c4nr03866e
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Ordered arrays of a defect-modified ferroelectric polymer for non-volatile memory with minimized energy consumption† Xiang-Zhong Chen,‡a Xin Chen,‡a Xu Guo,b Yu-Shuang Cui,b Qun-Dong Shen*a and Hai-Xiong Ge*b Ferroelectric polymers are among the most promising materials for flexible electronic devices. Highly ordered arrays of the defect-modified ferroelectric polymer P(VDF–TrFE–CFE) ( poly(vinylidene fluoride– trifluoroethylene–chlorofluoroethylene)) are fabricated by nanoimprint lithography for nonvolatile memory application. The defective CFE units reduce the coercive field to one-fifth of that of the un-
Received 10th July 2014, Accepted 21st September 2014
modified P(VDF–TrFE), which can help minimize the energy consumption and extend the lifespan of the device. The nanoimprint process leads to preferable orientation of polymer chains and delicately con-
DOI: 10.1039/c4nr03866e
trolled distribution of the defects, and thus a bi-stable polarization that makes the memory nonvolatile, as
www.rsc.org/nanoscale
revealed by the pulsed polarization experiment.
Introduction Flexible electronics are increasingly being used in portable devices because they are light-weight, foldable and/or stretchable in nature. Their easy processability, cost-effectiveness, and recyclability facilitate the boom in flexible electronic industry. While the flexible display technique is on its way to perfection, the memory units are far less than well-developed, which becomes a bottleneck for further development of active all-organic electronic devices.1 Flash memories,2 resistance random access memories (RRAM),3 and ferroelectric random access memories (FeRAM)4 have been developed for data storage in flexible electronics. Among them, ferroelectric memories attract wide attention due to their fast switching speed, low power consumption, and long data retention time (nonvolatile storage) over competing volatile memories.5–9 Conventional ferroelectric memories using inorganic ferroelectrics are not well compatible with the existing lithographic nanofabrication techniques. Therefore, the device capacities are relatively small at the megabit (Mb) level in comparison with the gigabit
a Department of Polymer Science & Engineering and Key Laboratory of High Performance Polymer Materials & Technology of MOE, School of Chemistry & Chemical Engineering, Nanjing University, Nanjing, 210093, China. E-mail: [email protected] b Department of Materials Science & Engineering and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4nr03866e ‡ These authors have contributed equally to this work.
This journal is © The Royal Society of Chemistry 2014
(Gb) capacities of flash memories and DRAM. The brittle nature of inorganic ferroelectrics makes them not ready for integration into flexible electronics. Besides, the stress gradient generated by mechanical bending of flexible substrates can drastically change the polarization states of the inorganic ferroelectrics,10 which makes memory devices less reliable. In contrast, polymer ferroelectrics have shown great potential for being readily integrated into the flexible electronics.11 A typical ferroelectric polymer is P(VDF–TrFE), i.e. a copolymer of vinylidene fluoride and trifluoroethylene. Such a copolymer exhibits bi-stable polarization states that can be switched from one to the other by an external electric field and then can be retained after removal of the field.12 The applications of P(VDF–TrFE)s in flexible field effect transistors (FETs)4–6 and ultrahighdensity (Gb inch−2) data storage arrays13–15 have been widely investigated recently. However, one notable issue of P(VDF–TrFE) is the longrange ordered and strong correlated ferroelectric domains, which make the minimum electric field (the coercive field, denoted as Ec) for switching polarization states of the copolymer extremely high (>50 MV m−1). Thus, the data writing– erasing processes, which include nucleation of the anti-parallel domains and their growth by domain wall motion, require relatively high activation energy. However, low-power consumption memories are required in many electronic devices including medically implantable devices, wireless sensors, and neural memory systems. Therefore, the high electric energy consumption becomes a significant drawback of the copolymers for portable electronics requiring low operation voltage (
Published on 22 September 2014. Downloaded by University of Utah on 16/10/2014 13:01:13.
PAPER
Cite this: DOI: 10.1039/c4nr03866e
View Journal
Ordered arrays of a defect-modified ferroelectric polymer for non-volatile memory with minimized energy consumption† Xiang-Zhong Chen,‡a Xin Chen,‡a Xu Guo,b Yu-Shuang Cui,b Qun-Dong Shen*a and Hai-Xiong Ge*b Ferroelectric polymers are among the most promising materials for flexible electronic devices. Highly ordered arrays of the defect-modified ferroelectric polymer P(VDF–TrFE–CFE) ( poly(vinylidene fluoride– trifluoroethylene–chlorofluoroethylene)) are fabricated by nanoimprint lithography for nonvolatile memory application. The defective CFE units reduce the coercive field to one-fifth of that of the un-
Received 10th July 2014, Accepted 21st September 2014
modified P(VDF–TrFE), which can help minimize the energy consumption and extend the lifespan of the device. The nanoimprint process leads to preferable orientation of polymer chains and delicately con-
DOI: 10.1039/c4nr03866e
trolled distribution of the defects, and thus a bi-stable polarization that makes the memory nonvolatile, as
www.rsc.org/nanoscale
revealed by the pulsed polarization experiment.
Introduction Flexible electronics are increasingly being used in portable devices because they are light-weight, foldable and/or stretchable in nature. Their easy processability, cost-effectiveness, and recyclability facilitate the boom in flexible electronic industry. While the flexible display technique is on its way to perfection, the memory units are far less than well-developed, which becomes a bottleneck for further development of active all-organic electronic devices.1 Flash memories,2 resistance random access memories (RRAM),3 and ferroelectric random access memories (FeRAM)4 have been developed for data storage in flexible electronics. Among them, ferroelectric memories attract wide attention due to their fast switching speed, low power consumption, and long data retention time (nonvolatile storage) over competing volatile memories.5–9 Conventional ferroelectric memories using inorganic ferroelectrics are not well compatible with the existing lithographic nanofabrication techniques. Therefore, the device capacities are relatively small at the megabit (Mb) level in comparison with the gigabit
a Department of Polymer Science & Engineering and Key Laboratory of High Performance Polymer Materials & Technology of MOE, School of Chemistry & Chemical Engineering, Nanjing University, Nanjing, 210093, China. E-mail: [email protected] b Department of Materials Science & Engineering and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4nr03866e ‡ These authors have contributed equally to this work.
This journal is © The Royal Society of Chemistry 2014
(Gb) capacities of flash memories and DRAM. The brittle nature of inorganic ferroelectrics makes them not ready for integration into flexible electronics. Besides, the stress gradient generated by mechanical bending of flexible substrates can drastically change the polarization states of the inorganic ferroelectrics,10 which makes memory devices less reliable. In contrast, polymer ferroelectrics have shown great potential for being readily integrated into the flexible electronics.11 A typical ferroelectric polymer is P(VDF–TrFE), i.e. a copolymer of vinylidene fluoride and trifluoroethylene. Such a copolymer exhibits bi-stable polarization states that can be switched from one to the other by an external electric field and then can be retained after removal of the field.12 The applications of P(VDF–TrFE)s in flexible field effect transistors (FETs)4–6 and ultrahighdensity (Gb inch−2) data storage arrays13–15 have been widely investigated recently. However, one notable issue of P(VDF–TrFE) is the longrange ordered and strong correlated ferroelectric domains, which make the minimum electric field (the coercive field, denoted as Ec) for switching polarization states of the copolymer extremely high (>50 MV m−1). Thus, the data writing– erasing processes, which include nucleation of the anti-parallel domains and their growth by domain wall motion, require relatively high activation energy. However, low-power consumption memories are required in many electronic devices including medically implantable devices, wireless sensors, and neural memory systems. Therefore, the high electric energy consumption becomes a significant drawback of the copolymers for portable electronics requiring low operation voltage (
