Polymer electronic memories materials, devices mechanisms

POLYMER ELECTRONIC MEMORIES: MATERIALS, DEVICES AND MECHANISMS LIM SIEW LAY (B. Eng. (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY NUS GRADUATE SCHOOL OF INTEGRATIVE SCIENCES AND ENGINEERING (NGS) NATIONAL UNIVERSITY OF SINGAPORE 2008 Acknowledgements Firstly, I would like to express my cordial gratitude to my supervisor Prof Kang En-Tang for his heartfelt guidance, invaluable suggestions and profound discussions during my course of study, for sharing with me his enthusiasm and active research interests, which formed a constant source for inspiration for me throughout this project. The knowledge I have gained from his supervision will accompany me through the path of my life-long study. I would like to thank Prof Zhu Chunxiang, our collaborator from the Department of Electrical and Computer Engineering, for providing precious input from a different perspective. I would also like to thank Prof Neoh Koon Gee and Dr He Chaobin for being on my thesis advisory committee. I am also grateful to all my colleagues for their kind help and encouragement, especially to Dr Ling Qidan, who was my mentor when I first joined the research group and who had shared his invaluable knowledge and experience in the research field with me. Thanks to Dr. Xu Fujian, Zhang Fan, Liu Gang, Chen Fei and all members of the research group, as well as all members of the Polymer Electronics Group. It has been a great pleasure working with all of them, and their kindness and friendship shall always remain in my memory. In addition, I also appreciate the assistance and cooperation from lab technologists, especially Xu Yanfang, instructors, professional officers and other staff members of Department of Chemical and Biomolecular Engineering. Thanks to Agency for Science, Technology and Research (A*Star), for their financial support through the A*Star Graduate Scholarship, and to NUS Graduate School of Integrative Sciences and Engineering (NGS), for their assistance on coursework and administrative matters. Finally, I would give my special thanks to my parents, family and loved ones for their continuous love, support and encouragement, without which I would not have been able to achieve yet another milestone in my life journey. Table of Contents Acknowledgements Summary List of Tables List of Figures List of Schemes 15 Nomenclature .16 Chapter Introduction .17 Chapter Literature Review 23 2.1 Polymer Electronic Memories 23 2.1.1 Types of Electronic Memories .25 2.1.2 Device Structure .27 2.1.3 Operating Mechanisms 30 2.1.4 Current Conduction Models .36 2.1.5 Properties-Tuning of Memory Devices 37 2.1.6 Hybrid Systems 40 2.1.7 Summary and Future Outlook 44 2.2 Modification of Indium-Tin Oxide (ITO) Surface 46 2.2.1 Surface Treatment 46 2.2.2 Self-Assembled Monolayers (SAMs) 48 2.2.3 Surface-Initiated Polymerization .50 Chapter Polymer Memory Based on Conformation Change 58 3.1 Introduction .58 3.2 Experimental Section 60 3.2.1 Materials 60 3.2.2 Syntheses of Polymers .61 3.2.3 Materials Characterization .63 3.2.4 Device Fabrication and Characterization .65 3.2.5 Molecular Simulation .66 3.2.6 Other Experiments .66 3.3 Results and Discussions 67 3.4 Conclusions .86 Chapter Polymer Memory Based on Charge-Transfer .88 4.1 Introduction .88 4.2 Experimental Section 89 4.3 Results and Discussions 92 4.4 Conclusions .98 Chapter Polymer Memory Based on Charge-Trapping and -Detrapping .100 5.1 Introduction .100 5.2 Experimental Section 102 5.2.1 Materials 102 5.2.2 Synthesis of AzoNEtNO2, AzoNEtOCH3 and AzoNEtBr 103 5.2.3 Synthesis of AzoONO2 and AzoOOCH3 105 5.2.4 Materials Characterization .106 5.2.5 Device Fabrication and Characterization .106 5.3 Results and Discussions 107 5.3.1 Material Properties .107 5.3.2 Electrical Characterization and Operating Mechanism .112 5.4 Conclusions .126 5.5 Acknowledgement 127 Chapter Polymer Memory Based on Heterogeneous Polyaniline-Carbon Nanotube Composites 128 6.1 Introduction .128 6.2 Experimental Section 130 6.3 6.2.1 Materials 130 6.2.2 Preparation of Polyaniline-Carbon Nanotube Composites 130 6.2.3 Fabrication and Characterization of Memory Device 131 6.2.4 Materials Characterization .131 Results and Discussions 132 6.3.1 Characterization of the Polyaniline Composites and Films .132 6.3.2 J-V Characteristics of Devices Based on the LM and EB States of Polyaniline, and of their Corresponding Composites with CNT .136 6.4 Conclusions .150 Chapter Application of Polymer Brushes in Polymer Memory Devices 151 7.1 Introduction .151 7.2 Experimental Section 153 7.2.1 Materials 153 7.2.2 Immobilization of Initiator for ATRP 154 7.2.3 Surface-Initiated ATRP 154 7.2.4 Methods of Characterization 155 7.2.5 Device Fabrication and Characterization .156 7.3 Results and Discussions 156 7.3.1 Immobilization of ATRP Initiator on ITO 156 7.3.2 Surface-Initiated ATRP from the ITO-Cl Surface and Subsequent Block Copolymerization 159 7.3.3 Device Performance and Comparison with Spin-coated Device .162 7.3.4 Topographical Study 166 7.4 Chapter Conclusions .169 Conclusions .170 Bibliography 173 List of Publications 196 Summary In this project, electroactive polymers with resistive switching properties were applied for electronic memories as an alternative or supplementary technology to the conventional semiconductor electronic memory. The mechanisms responsible for the resistive switching were determined. Different approaches for device fabrication were explored. The ability of the pendant carbazole group to undergo electric field-induced conformation change, via rotation about the C-N bond, produced resistive switching effects in single-layer devices based on two non-conjugated polymers. Non-volatile write-once-read-many-times (WORM) and volatile memory effects were observed, depending on the spacer units in the polymer structures. The transformation from regio-random to regio-regular arrangement was captured by in situ fluorescence spectroscopy and transmission electron microscopy. Besides the ability to undergo conformation change, the carbazole group, a well-known electron-donor, can also participate in charge-transfer processes when coupled with an electron-acceptor. A rewritable (flash) electronic memory device was obtained based on a copolymer containing carbazole and 1,3,4-oxadiazole (electron-acceptor) pendant groups. Plausible electronic processes during the electrical transition were deduced based on UV-visible absorption spectroscopy and molecular simulation results. In addition to inter-molecular charge-transfer processes between pendant groups in a polymer, intra-molecular charge-transfer processes can also occur in pendant groups with donor-acceptor electronic structures. In polymer electronic memory devices based on charge-trapping and –detrapping in the pendant azobenzene chromophores, WORM memory effects were observed when the presence of electron-accepting terminal moieties, such as –Br or –NO2, facilitated intra-molecular charge-transfer to stabilize the high-conductivity state, while flash memory effects were observed with electron-donating terminal moieties, such as –OCH3, as intra-molecular charge-transfers were not feasible. The proposed trapping mechanism was supported by UV-visible absorption spectroscopy results. Charge-transfer interactions can also occur between the imine nitrogen of polyaniline and carbon nanotubes (CNT) in polyaniline-CNT nanocomposites. A different type of electrical behavior was observed with each intrinsic redox state of polyaniline in memory devices based on polyaniline-CNT nanocomposites. As the polyaniline-CNT nanocomposites were oxidized from the fully reduced leucoemeraldine base to emeraldine base, negative differential resistance (NDR), rewritable memory and non-rewritable memory effects were observed at different degrees of oxidation in the devices. While the characteristics of NDR were attributed to the charge-trapping effects of the CNT, the electrical bistability subsequently displayed was due to electric field-induced charge-transfer interactions. This was confirmed by X-ray photoelectron spectroscopy and supported by cyclic voltammetry results. The wide range of electrical behaviors observed could be applied for multilevel, rewritable and WORM memories. Other than synthesizing materials with superior electrical characteristics, alternative approaches to device fabrication can also enhance the performance of polymer memory devices. Surface-initiated atom-transfer radical polymerization from the indium-tin oxide (ITO) substrate was successfully applied for the preparation of uniform and conformal films of electroactive polymer brushes tethered on the ITO surface. Compared to spin-coated polymer films, this architectural modification of the active polymer layers reduced the switching voltages of memory devices based on conformation change, while still maintaining significant ON/OFF current ratios. List of Tables Table 2-1 Basic conduction processes in insulators. [extracted from (Sze, 1981)] .37 Table 5-1 Number-average molecular weights (Mn), polydispersities (Mw/Mn), and glass- transition temperatures (Tg) of the azobenzene polymers. 107 Table 5-2 Highest-occupied-molecular-orbital (HOMO) and lowest-unoccupiedmolecular-orbital (LUMO) energy levels obtained from cyclic voltammetry and UV-visible spectroscopy. . 111 Table 5-3 Highest-occupied-molecular-orbital (HOMO) and lowest-unoccupiedmolecular-orbital (LUMO) surfaces of the monomer units of the azobenzene polymers obtained by molecular simulation. .118 Table 5-4 Molecular electrostatic potential (ESP) surfaces and quadrupole moments (field-independent basis, Debye-Ang) of the monomer units obtained by molecular simulation 119 Table 6-1 Carbon nanotube (CNT) content in the emeraldine base (EB)-CNT composite obtained with different CNT content in the polymerization mixture. (awith respect to the weight of aniline monomer; bwith respect to the weight of the composite) 133 List of Figures Figure 2-1 Classification of electronic memories (the most widely reported polymer memories are shaded gray). (ROM: read-only-memory; EPROM: erasable-programmable-ROM; WORM: write-once-read-many-timesmemory; EEPROM: electrically-erasable- programmable-ROM; RAM: random-access-memory; SRAM: static-RAM; DRAM: dynamic-RAM) .26 Figure 2-2 Schematic diagrams of (a) a test memory device, (b) a (word line) × (bit line) cross-point memory array and a (stacked layer) × (word line) × (bit line) stacked memory device, and (c) a memory cell with an integrated Si n-i-p or Si p-i-n rectifying diode between the bottom electrode and the polymer film to avoid parasitic currents .29 Figure 2-3 (a) Chemical structure of PFOxPy. (b) Typical J-V characteristics of the ITO/PFOxPy/Al device in the OFF and ON states whilst maintaining the ON state by refreshing at -1 V every 10 s. DFT molecular simulation results: (c) LUMO and HOMO energy levels for the basic unit (BU) and functional segments of PFOxPy along with the work functions of the electrodes. The energy levels of the copolymer measured by cyclic voltammetry are indicated with an asterisk. (d) LUMO, HOMO, and molecular electrostatic potential (ESP) surfaces of the BU of PFOxPy. [extracted from (Ling, Song, Lim et al., 2006)] 32 Figure 2-4 I-V curves of the device with the structure Al/PS:PCBM:TTF/Al, showing (a) the first, (b) the second, and (c) the third bias scans, respectively. [extracted from (Chu et al., 2005)] 34 Figure 2-5 (a) Current-voltage characteristics of the polyaniline nanofiber-gold nanoparticle device. The potential is scanned from (A) to +4 V, (B) +4 to V, and (C) to +4 V. (b) A schematic representation of the electric field-induced charge transfer from polyaniline to gold nanoparticle. 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P., Zhu, C. X., Chan, D. S. H., Kwong, D. L., Kang, E. T. and Neoh, K. G. (2006) "A dynamic random access memory based on a conjugated copolymer containing electron-donor and -acceptor moieties" Angew. Chem. Int. Ed., 45, 2947-2951. Ling, Q. D., Song, Y., Teo, E. Y. H., Lim, S. L., Zhu, C. X., Chan, D. S. H., Kwong, D. L., Kang, E. T. and Neoh, K. G. (2006) "WORM-type memory device based on a conjugated polymer containing europium complex in the main chain" 196 List of Publications Electrochem. Solid St., 9, G268-G271. 197 [...]... architectural modification of the active polymer layers reduced the switching voltages of memory devices based on conformation change, while still maintaining significant ON/OFF current ratios 22 Chapter 2 Chapter 2 Literature Review 2.1 Polymer Electronic Memories Polymer electronic memories can be classified into three categories by drawing the mechanistic analogy between the polymer memory element and one... (ITRS) has identified polymer memory as an emerging memory technology since year 2005 (ITRS, 2005) Polymer electronic memory is likely to be an alternative or supplementary technology to the conventional semiconductor electronic memory In this PhD project, polymeric materials which exhibit bistable resistive switching are examined for their potential applications as polymer electronic memories The materials... architecture of the active polymer layer The ability of the pendant carbazole group to undergo electric field-induced conformation change, via rotation about the C-N bond, produced resistive switching effects in single-layer devices based on two non-conjugated polymers While 19 Chapter 1 non-rewritable memory effects were observed for both polymer devices, the volatilities of the polymer memories were dependent... functional and poccessible polymers for flash, WORM, 24 Chapter 2 and DRAM applications is illustrated Resistive switching properties can be incorporated or enhanced in some polymers by the addition of suitable dopants The operating mechanisms, including trapping-detrapping, charge transfer and nanocomposite redox effects are also described 2.1.1 Types of Electronic Memories Electronic memories can be primarily... of electroactive polymers, (2) memory device fabrication and characterization of the memory performance, (3) determination of the operating mechanisms of the memory devices Polymer memories based on conformation change, charge-trapping and –detrapping, and charge-transfer are studied A heterogeneous system based on composites of carbon nanotubes is also studied Besides the design of materials, an attempt... mentioned in Chapter 1, organic, in particular, polymer electronic memories based on resistive switching have emerged as an attractive alternative to conventional memory technology due to the unique properties and processability of polymers Instead of information storage and retrieval by encoding “0” and “1” as the amount of charges stored in a cell, polymer memory devices store data based on the high and... rewritable and WORM memories Alternative approaches to device fabrication can enhance the performance of polymer memory devices Surface-initiated atom-transfer radical polymerization from the indium-tin oxide (ITO) substrate was successfully applied for the preparation of uniform and conformal films of electroactive polymer brushes tethered on the ITO 21 Chapter 1 surface Compared to spin-coated polymer films,... memories (Moller et al., 2003, Ouyang et al., 2004, Ling, Song, Lim et al., 2006) 25 Chapter 2 Types of memories Non-volatile memories ROM Volatile memories Hybrid RAM EPROM Flash SRAM WORM EEPROM DRAM Figure 2-1 Classification of electronic memories (the most widely reported polymer memories are shaded gray) (ROM: read-only-memory; EPROM: erasable-programmable-ROM; WORM: write-once-read-many-times-memory;... DRAMs, with the ability to write, read, erase and refresh the electrical states, are observed in some polymer memories 2.1.2 Device Structure Resistive switching polymer memories are usually two terminal devices having a metal/insulator/metal (MIM) sandwich device structure, with the electroactive polymer thin film sandwiched between the two electrodes Materials such as aluminum, gold, copper, p- or... Plausible electronic processes during the electrical transitions were deduced based on UV-visible absorption spectroscopy and molecular simulation results In addition to inter-molecular charge-transfer processes between pendant groups in a polymer, intra-molecular charge-transfer processes can also occur in the pendant group when a donor-acceptor electronic structure exists In electronic memory devices . POLYMER ELECTRONIC MEMORIES: MATERIALS, DEVICES AND MECHANISMS LIM SIEW LAY (B. Eng. (Hons.), NUS) A THESIS. 17 Chapter 2 Literature Review 23 2.1 Polymer Electronic Memories 23 2.1.1 Types of Electronic Memories 25 2.1.2 Device Structure 27 2.1.3 Operating Mechanisms 30 2.1.4 Current Conduction. electroactive polymers with resistive switching properties were applied for electronic memories as an alternative or supplementary technology to the conventional semiconductor electronic memory. The mechanisms