ELECTRICAL CONDUCTIVITY SWITCHING BEHAVIOR AND MEMORY EFFECTS IN ELECTROACTIVE POLYMERS AND NANOCOMPOSITES LIU GANG (M. Sci., Singapore-MIT Alliance, NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE SINGAPORE, JULY 2010 i ACKNOWLEDGEMENT First and foremost, I would like to express my sincerest and deepest appreciation to my supervisors, Professor Kang En-Tang and Professor Neoh Koon-Gee, at National University of Singapore, for their invaluable guidance, suggestion, and discussion throughout this work. I was especially fortunate to be able to work under the supervision of Professor Kang En-Tang, who leads me into the great world of polymer electronics. Professor Kang and Professor Neoh’s abundant knowledge in polymer related areas is always a source of inspiration to me in carrying out this project. Whenever I came across a problem, I knew for sure that I could turn myself to my supervisors, Professor Kang and Professor Neoh, for their sincere and unreserved suggestion and help. Their enthusiasm, diligence, patience, and preciseness enlighten me on the road of scientific research, and even my future road of life, no matter in academic or industrial area, regardless of the country in which I stay. I have benefited greatly from the collaboration with Professor Liaw Der-Jang and Dr. Chang Feng-Chyuan, from National Taiwan University of Science and Technology, and Dr. Chen Yu and Mr. Zhuang Xiaodong, from East China University of Science and Technology. I acknowledge with gratitude for the materials used in this project, provided by Professor Liaw, Dr. Chen, and their research staffs and students. I am also indebted to Dr. Zhu Chuxiang, Dr. Ling Qidan, Dr. Tong Shy Wun, Dr. Lim Siew Lay, Dr. Zhang Zhiguo, Mr. Eric Teo Yeow Hwee, Mr. Liu Yiliang, Mr. Zhang Chunfu, and Ms. Wan Dong, for their fruitful discussion and comments during this ii work. I would like to express my particular gratitude to Dr. Ling Qidan, from whose generous consultation and invaluable experience I learnt heavily for my own work. In addition, I wish to thank the laboratory technologists, Ms. Novel Chew Su Mei, Ms. Alyssa Tay Kai Si, Ms. Xu Yanfang, and Mr. Ng Kim Poi, for their time and assistance during this work. Fully as important as the scientific guidance of my supervisors and the help of my colleagues have been the love and encouragement of my devoted parents, Mr. Liu Zhiran and Ms. Zhang Guiying, and my devoted wife, Ms. Zhou Jinjia. The unconditional love and sacrifice of my family during this four-year project made me fully concentrate on my research work without concerning too much about the daily issues. Their consistent care and support enable me healthy enough, both mentally and physically, to finish this work. Nevertheless, my uncles, Professor Wang Erlin and Professor Gu Yaoxin, helped me to clarify my aspiration when I was hesitating and wandering in doubt. The continuous love, support, and encouragement of my family are always the power of advancing in my profession. Last but not least, I appreciate the financial support provided by the National University of Singapore in the form of research scholarship to carry out this work. iii TABLE OF CONTENTS ACKNOWLEDGEMENT ii TABLE OF CONTENTS . iv SUMMARY vii NOMENCLATURE . xi LIST OF FIGURES . xiv LIST OF TABLES .xx CHAPTER INTRODUCTION .1 CHAPTER LITERATURE SURVEY 2.1 Current Status of Electronic Memories . 2.2 Basic Concepts of Electronic Memories . 10 2.3 Brief History of Polymer Electronic Memories 14 2.4 Classification of Polymer Electronic Memories . 19 2.4.1 Transistor-type polymer memories 19 2.4.2 Capacitor-type polymer memories 23 2.4.3 Resistor-type polymer memories . 25 2.5 Conduction Mechanisms of Polymer Memories . 28 2.5.1 Filament conduction 28 2.5.2 Space charges and traps . 31 2.5.3 Charge transfer effects 32 2.5.4 Tunneling effects . 33 2.5.5 Conformational change effects 35 CHAPTER FLUORENE POLYMERS 36 3.1 Introduction . 37 3.2 Experimental Section 39 3.3 Results and Discussion 48 3.3.1 Bistable conductivity switching and WORM memory effects of PFPTPA . 48 3.3.2 Tristable conductivity switching and WORM memory effects of PFPCz . 55 3.3.3 Electrical properties of PFPPy 67 iv 3.4 Conclusion 71 CHAPTER IMIDE POLYMERS 72 4.1 Introduction . 73 4.2 Experimental Section 75 4.3 Results and Discussion 80 4.3.1 Bistable conductivity switching and WORM memory effects of PCz6FDA 80 4.3.2 Electrical properties of PNa6FDA 91 4.4 Conclusion 93 CHAPTER (POLY(VINYLCARBZOLE-AZOBENZENE-ACCEPTOR) COMPL EX 94 5.1 Introduction . 95 5.2 Experimental Section 98 5.3 Results and Discussion 100 5.3.1 Bistable conductivity switching and WORM memory effects of PVK-AZO-NO2 100 5.3.2 Bistable conductivity switching and WORM memory effects of PVK-AZO-2CN 109 5.4 Conclusion 111 CHAPTER POLYMER-CARBON NANOTUBE COMPOSITES 112 6.1 Introduction . 113 6.2 Experimental Section 115 6.3 Results and Discussion 118 6.3.1 Enhancement of the PFPTPA memory device performance via CNT doping 118 6.3.2 Tuning of the electrical properties and controlling of the conductivity switching behavior and memory effects of PVK-CNT composites 124 6.4 Conclusion 137 CHAPTER POLY(N-VINYLCARBAZOLE)-GRAPHENE OXIDE COMPLEX 138 7.1 Introduction . 139 7.2 Experimental Section 141 7.3 Results and Discussion 143 7.3.1 Material characterization . 143 v 7.3.2 Bistable conductivity switching and rewritable memory effects of GO-PVK 150 7.4 Conclusion 156 CHAPTER CONCLUSION 157 CHAPTER RECOMMENDATIONS FOR FUTURE WORK .162 REFERENCES 166 LIST OF PUBLICATIONS 185 vi SUMMARY Organic and polymeric materials can exhibit electric-field-induced electrical conductivity switching behavior and resistor-type electronic memory effects. The field-induced electrical bistability, together with the low-cost potential, light weight, mechanical flexibility, and the most important of all, tunable electronic properties via molecular design, make organic and polymer materials promising alternatives or supplements to inorganic semiconductors in data storage technologies. In this work, a series of polymers and polymer composite materials were explored for electronic memory applications. The focus of this work is concentrated on studying the electrical properties and the underlying switching and conduction mechanisms of the electroactive polymers and nanocomposites. Conjugated fluorene copolymers of poly(2,6-diphenyl-4-((9-ethyl)-9H-carbazole)-pyridinyl-alt-2,7-(9,9-didodecyl)-9H-fluorenyl) (PFPCz), poly(2,6-diphenyl-4-triphenylamine-pyridinyl-alt-2,7-(9,9-didodecyl)-9H-fluorenyl) (PFPTPA), and poly(2,6-diphenyl-4-pyrene-pyridinyl-alt-2,7-(9,9-didodecyl)-9H-fluorenyl) (PFPPy) (structures shown in Figure 3.3, 3.4 and 3.5, respectively) were first synthesized via Suzuki coupling polymerization reaction. The electrical behavior of these polymers was found to be dependant on the molecular structure of the macromolecules. Both write-once read-many-times (WORM) memory effects (PFPCz and PFPTPA) and insulator (PFPPy) behavior are demonstrated in the current density-voltage (J-V) characteristics of the devices with aluminium (Al)/polymer/indium-tin oxide (ITO) vii sandwich structure. The electrical conductivity switching behavior of these fluorene polymers is ascribed to electric field-induced conformational ordering and/or charge transfer (CT) interaction of the polymer film in the devices. To achieve a higher ON/OFF state current ratio and thus a lower misreading rate of the polymer electronic memories, two non-conjugated imide polymers, poly(2,6-diphenyl-4-((9-ethyl)-9H-carbazole)-pyridinyl-alt-hexafluoroisopropylidene diphthal-imide) or PCz6FDA, and poly(2,6-diphenyl-4-napathalene)-pyridinyl-alt-hexafluoroisopropylidenediphthal-imide) or PNa6FDA (structures shown in Figure 4.2 and 4.3, respectively), were synthesized via a two-step polymerization reaction, involving a ring-opening poly-addition reaction and the subsequent chemical imidization reaction. The incorporation of a stronger electron withdrawing group (as compared to the pyridine electron acceptor), hexafluoroisopropylidenediphthalimide (or 6FDA), can significantly enhance the field-induced CT characteristics of the imide polymers. The Al/PCz6FDA/ITO device exhibits electrical bistability and WORM memory effects with an ON/OFF state current ratio of 105, while the PNa6FDA device behaves as an electrical insulator. The electrical bistability of PCz6FDA device is well maintained at elevated temperatures and thermally stable electronic memory device is thus demonstrated. The bistable switching behavior of PCz6FDA is attributed to intra-molecular CT interaction under an electric field, while the lack of electrical bistability in PFPPy and PNa6FDA are ascribed to the absence of effective electron donating species in the pendant groups. viii Capitalizing on the charge trapping ability of azobenzene chromophores, electrical bistability with enhanced ON/OFF state current ratios in excess of ~ 105 were demonstrated in two donor-trap-acceptor (D-T-A) structure carbazole-azobenzene polymers. Synthesized via a post azo-coupling reaction, poly(3-(4-nitrophenyl)-diazenyl-9-vinylcarbazole-alt-9-vinylcarbazole) (PVK-AZO-NO2), and poly(3-(3,4-dicyanophenyl)-diazenyl-9-vinylcarbazole-alt-9-vinylcarbazole) (structures shown in Figure 5.3) possess pendant (PVK-AZO-2CN) electron D-A pair (carbazole-nitro/cyano) and charge trapping center (azobenzene) at the same time, allowing effective stabilizing of the intra-molecular CT state, and leading to WORM memory effects with low switching voltages and high ON/OFF state current ratios. In addition to molecular design and organic chemistry, the electronic properties of polymers can also be controlled by forming composites with other electroactive materials. The bistable switching behavior and memory effect of the fluorene polymer PFPTPA can be enhanced upon mixing the polymer with carbon nanotubes (CNTs) Furthermore, by varying the carbon nanotube content in poly(N-vinylcarbazole) (PVK) composite films, the electrical conductivity of the PVK-CNT doping system can be tuned deliberately. The Al/PVK-CNT/ITO sandwich structure exhibits insulator, bistable electrical conductivity switching (WORM memory and flash memory effects), and conductor behaviors, when the CNT content in the composite film is increased from to 3%. The conductivity switching effects of the PVK-CNT composite films are ascribed to electron trapping in the CNTs of the hole-transporting PVK matrix. ix Similar to its consanguinity of C60 and carbon nanotube, the large numbers of hexagonal aryl make graphene material a good electron acceptor. The atomic nanosheets of graphene enhance its potential application in ultrathin electronic devices. 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Chen, Y., Zhuang X. D., Liu, G., Li, P. P., Zhu, C. X., Kang, E. T. Neoh, K. G., Zhang, B. and Zhu, J. H. Conjugated Polymer Functionalized Graphene Oxide: Synthesis and Non-volatile Rewritable Memory Effect. Adv. Mater.,22, pp.1731-1735. 2010. 9. Zhang, Z. G., Zhang, K. L., Liu, G., Zhu, C. X., Neoh, K. G. and Kang, E. T. Triphenylamine-Fluorene Alternating Conjugated Copolymers with Pendant Acceptor Groups: Synthesis, Structure-Property Relationship, and Photovoltaic Application. Macromolecules, 42, pp.3104-3111. 2009. 10. Li, G. L., Liu, G., Kang, E. T., Neoh, K. G. and Yang, X. L. pH-Responsive Hollow Polymeric Microspheres and Concentric Hollow Silica Microspheres from Silica-Polymer Core-Shell Microspheres. Langmuir, 24, pp.9050-9055. 2008. 11. Tong, S. W., Zhang, C. F., Jiang, C. Y., Liu G., Ling, Q. D., Kang, E. T., Chan, D. 185 S. H. and Zhu, C. X. Improvement in the Hole Collection of Polymer Solar Cells by Utilizing Gold Nanoparticle Buffer Layer. Chem. Phys. Lett., 453, pp.73-76. 2008. 186 [...]... reported in 1974 (Henisch and Smith, 1974) Memory switching effects in polystyrene, polyethylmethacrylate and polybutylmethacrylate films were ascribed to a field-controlled polymer chain ordering and disordering Memory switching in poly(N-vinylcarbazole) (PVK) thin films was also reported, attributed to the trapping-detrapping processes associated with absorbed O2 impurities-hole traps (Sadaoka and Sakai,... been invented since 1940s So far, no practical universal storage medium exists, and all forms of storage have some drawbacks Therefore a computer system usually contains several kinds of storage, each with an individual purpose The main emphasis in this field has been placed on studying the electrical switching memory effects of inorganic materials (Lew, 1967; Ovshinsky, 1968; Lem and Spruth, 1969), and. .. mechanical characteristics, was carried out The bistable and even tristable electrical conductivity switching behavior and resistor-type electronic memory effects of these polymers and nanocomposites were explored It is the objective of this work to study the electrical switching properties and the underlying mechanisms of these polymer-based electroactive materials Through this work, the design-cum-synthesis... chloride) and polystyrene) exhibited bistable switching behavior Reproducible bistable switching was soon demonstrated in polymer thin films prepared by glow-discharge polymerization (Carchno et al., 1971) Inspired by the pioneering works, a wide variety of organic and polymer materials have been reported to show threshold or memory switching effects (Antonowicz et al, 1973; Gazso, 1974; Pender and Fleming,... transistors and capacitors The ever increasing demand of personal mobile electronic devices provides a significant incentive for renovation in memory technology with higher capacity and system performance, smaller form factor, and lower power consumption and system cost (Marsh, 2003; Pinnow and Mikolajich, 2004) However, the continuous shrinking of current Si, Ge, and GaAs semiconductors based memory devices... convert information between optical/magnetic and electrical signals (Prince, 1991), an electronic memory is a form of semiconductor storage compact in size, fast in response, and may be temporary in nature When coupled with a central processing unit (CPU, a processor), an electronic memory implements the basic Von Neumann computer model used since the 1940s, that can store the operating instructions and. .. and other digital products, as well as used in memory cards and USB flash drives for transfer of data between computers and portable electronics 12 Chapter 2 Literature Survey Dynamic-random-access memory (DRAM) is a volatile random access memory that stores each bit of data in a separate capacitor within an integrated circuit Since real-world capacitors have charge-leaking tendencies, the stored-information... memories in (a) linear scale (Ling et al., 2005) and (b) log scale (Ling et al., 2007) (c) Stability under continuous voltage stress and ON/OFF ratio (inset, Ling et al., 2007) (d) Effects of number of read cycles (Ling et al., 2007) (e) Write-read-erase-read (WRER) cycles (Tseng et al., 2005) (f) Switching time measurement (Ling et al., 2005) Figure 2.8 Schematic diagram of (a) a 3 × 3 polymer memory. .. employing these innovative technologies, the amazing fabrication cost arising from the preparation and processing of high purity silicon wafer is still inevitable, especially true when utilizing ultrahigh vacuum techniques Thus, exploiting alternative concepts and materials that are running on entirely different operation mechanisms is becoming imperative for IT industry in this concern Regarding the... stacking, and in particular, low cost solution-based techniques of spin-coating, spray-coating, dip-coating, roller-coating and ink-jet printing (Scott, 2004; Guizzo, 2004; Li et al., 2004; Fu et al., 2004; Yang et al., 2006) Other than the more elaborated vacuum evaporation and deposition techniques, organic and polymer materials can be deposited on a variety of substrates such as glass, plastic and . i ELECTRICAL CONDUCTIVITY SWITCHING BEHAVIOR AND MEMORY EFFECTS IN ELECTROACTIVE POLYMERS AND NANOCOMPOSITES LIU GANG (M. Sci., Singapore-MIT Alliance, NUS). Results and Discussion 48 3.3.1 Bistable conductivity switching and WORM memory effects of PFPTPA 48 3.3.2 Tristable conductivity switching and WORM memory effects of PFPCz 55 3.3.3 Electrical. structure exhibits insulator, bistable electrical conductivity switching (WORM memory and flash memory effects) , and conductor behaviors, when the CNT content in the composite film is increased from