Polymer electronic memories materials, devices mechanisms

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-acce

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

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

Acknowledgements 1

Summary 5

List of Tables 8

List of Figures 9

List of Schemes 15

Nomenclature 16

Chapter 1 Introduction 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 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 3 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

3.4 Conclusions 86

Chapter 4 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 5 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 6 Polymer Memory Based on Heterogeneous Polyaniline-Carbon Nanotube Composites 128

6.1 Introduction 128

6.2 Experimental Section 130

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

6.3 Results and Discussions 132

Polyaniline, and of their Corresponding Composites with CNT 136

6.4 Conclusions 150

Chapter 7 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 Conclusions 169

Chapter 8 Conclusions 170

Bibliography 173

List of Publications 196

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

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

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

Table 5-1 Number-average molecular weights (M n ), polydispersities (M w /M n), and

glass- transition temperatures (T g) of the azobenzene polymers .107

Table 5-2 Highest-occupied-molecular-orbital (HOMO) and lowest-unoccupied-

molecular-orbital (LUMO) energy levels obtained from cyclic voltammetry and UV-visible spectroscopy 111

Table 5-3 Highest-occupied-molecular-orbital (HOMO) and lowest-unoccupied-

molecular-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

memories are shaded gray) (ROM: read-only-memory; EPROM: erasable-programmable-ROM; WORM: write-once-read-many-times- memory; 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 4 (word line) × 4

(bit line) cross-point memory array and a 2 (stacked layer) × 4 (word line)

× 4 (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) 0 to +4 V, (B) +4

to 0 V, and (C) 0 to +4 V (b) A schematic representation of the electric field-induced charge transfer from polyaniline to gold nanoparticle (NDR: negative differential resistance) [extracted from (Tseng et al., 2005)] 35

Figure 2-6 (a) UV-visible absorption spectra of GNPs, PVK, and PVK:GNPs (i) in

tetrahydrofuran solutions Inset: Molecular structure of PVK and schematic structure of GNPs, (ii) films on quartz Inset: Schematic

diagram of the TaN/PVK:GNPs/Al memory device (b) Typical J-V

characteristics of the TaN/PVK:GNPs/Al device Inset: (i) The ON/OFF

current ratio as a function of the applied voltage and (ii) J-V

characteristics of the TaN/PVK/Al device [extracted from (Song et al., 2007)] 43

the ITO surface .49

Figure 2-8 Approaches for depositing organic polymers on inorganic substrates

[extracted from (Dyer, 2003)] 50

Figure 2-9 Examples of polymer brushes synthesized by ATRP using

“grafting-from” approach from various functional substrates, such as flat wafer, particles, colloids and polymers (X: halogen) [extracted from (Pyun et al., 2003)] 54

Figure 2-10 A schematic representation of the immobilization of initiator moieties on

the ITO surface for subsequent ATRP using a silane coupling agent 55

Figure 2-11 A schematic representation of an organic field effect transistor, with a

polymer layer of similar thickness at the edges and on the flat surface prepared via surface-initiated polymerization [extracted from (Rutenberg et al., 2004)] 55

Figure 2-12 A schematic representation of a polymer brush film and a spin-coated

film [extracted from (Snaith et al., 2005)] 57

Figure 3-1 (a) Molecular structures of poly(N-vinylcarbazole) (PVK),

poly(2-(N-carbazolyl)ethyl methacrylate (PMCz) and poly(9-(2-((4-vinylbenzyl)oxy)ethyl)- 9H-carbazole (PVBCz) (b)

Schematic diagram of the memory device consisting of a thin film (~50 nm) of the polymer sandwiched between an indium-tin-oxide (ITO) substrate and an aluminum top electrode 68

Figure 3-2 (a) J-V characteristics of an ITO/PVK/Al device showing a single

conductivity state; (b) and (c) J-V characteristics of an ITO/PMCz/Al

device and ITO/PVBCz/Al device, respectively, in the OFF- and ON-state, with the corresponding OFF-to-ON transitions at -1.8 V and -2.0 V (part (c) also shows the ON-state being maintained by refreshing

at -1 V with a pulse width of 10 ms every 5 s); (d) stability of the Al/PVBCz/ITO device in either OFF or ON state under a constant stress

of -1 V; (e) effect of read cycles on the OFF and ON states; (e) pulses used for (i) refreshing the ON-state and (ii) read cycles testing .69

Figure 3-3 Simulated 3D models by molecular mechanics showing the optimized

geometry corresponding to the minimum energy states in (a) PVK, (b) PMCz and (c) PVBCz 71

Figure 3-5 Fluorescence emission spectra showing changes in intensity at ~380 nm

and ~420 nm of the (a) ITO/PVBCz/Al and (b) ITO/PMCz/Al devices at

0 V, after applying a voltage bias, and after power-off .77

Figure 3-6 TEM images of (a) a PMCz film without any bias applied (OFF state); (b)

a PMCz film showing ordered microdomains after a voltage sweep from

0 V to -4 V (ON state); (c) a PMCz film in the ON state with higher magnification; (d) a PVK film; (e) a magnified region in the PVK film; (f) a PVBCz film, after a voltage sweep from 0 V to -4 V and conformation relaxation 80

Figure 3-7 Experimental and fitted J–V curves of ITO/PMCz/Al in the (a) OFF-state

and (b) ON-state and of ITO/PVBCz/Al in the (c) OFF-state and (d) ON-state Inset figures show the temperature dependence of the device currents in accordance to the fitted conduction models [SCLC-F denotes space-charge-limited current with field dependent mobility] 84

Figure 4-1 (a) Molecular structure of poly(2-(9H-carbazol-9-yl)ethyl

methacrylate-co-4-(5-(4-tert-butylphenyl-1,3,4-oxadiazol-2-yl)phenyl methacrylate) (PCzOx) The values of x and y are obtained by elemental analysis to be 0.676 and 0.324, respectively (b) J-V characteristics of an

ITO/PCzOx/Al device in the OFF- and ON-state, with the OFF-to-ON transition at -1.8 V , followed by a subsequent ON-to-OFF transition at +3.6 V .93

Figure 4-2 Experimental and fitted J–V curves of ITO/PCzOx/Al in the (a)

OFF-state and (b) ON-state [SCLC-F denotes space-charge-limited current with field-dependent mobility] 94

Figure 4-3 Energy level diagram for the ITO/PCzOx/Al device, along with the

HOMO and LUMO levels of the Cz and Ox components Energy levels

of the molecular orbitals were obtained from DFT simulation 95

Figure 4-4 Molecular orbitals (left) of the basic unit of PCzOx and the transitions

(right) from the ground state to the CT state induced by the electric field 97Figure 4-5 UV-visible absorption spectrum of PCzOx in tetrahydrofuran solution 97Figure 5-1 (a) Synthesis scheme and molecular structures of the amino

group-containing azobenzene polymers, AzoNEtNO2, AzoNEtOCH3 and

Figure 5-2 UV-visible absorption spectra of (a) the five azobenzene polymers in

dilute DMAc solution and of (b) (i) AzoNEtNO2 and (ii) AzoNEtOCH3

in dilute DMAc and THF solutions, respectively .109

Figure 5-3 J-V characteristics and effect of operation time (at -1 V) on the device

current density in the OFF and ON states of the ITO/AzoNEtNO2/Al device [(a) and (b)] and the ITO/AzoONO2/Al device [(c) and (d)] .113

Figure 5-4 J-V characteristics and the effect of operation time (at -1 V) on the

current density in the OFF and ON states of the ITO/AzoOOCH3/Al rewritable device [(a) and (b)], the ITO/AzoNEtOCH3/Al rewritable device [(c) and (d)], and the ITO/AzoNEtBr/Al WORM device [(e) and (f)] .115

Figure 5-5 Comparison of the UV-visible absorption spectra of the (a) AzoNEtNO2,

(b) AzoNEtBr and (c) AzoNEtOCH3 polymer thin films in the low-conductivity (OFF) and high-conductivity (ON) states The ON state was induced using a removable liquid mercury droplet as the top electrode 123

Figure 5-6 (a) Energy level diagram for the ITO/AzoNEtNO2/metal device with

aluminum (Al), mercury (Hg), or gold (Au) as the top electrode (b)

J-V characteristics of the ITO/AzoNEtNO2/Au memory device .125

Figure 6-1 TEM images of (a) surface-functionalized carbon nanotubes, (b) pure

emeraldine base, (c) composite of emeraldine base and carbon nanotubes (EB-CNT11.3), and (d) magnified image showing tubular core-shell structure of EB-CNT11.3 .134

Figure 6-2 AFM images of (a) the emeraldine base (EB) polymer film, (b) the

EB-CNT4.02 and (c) the EB-CNT11.3 composite films (Images are 5 μm

× 5 μm Z = 20 nm/div) 135

Figure 6-3 (a) J-V characteristics of an ITO/EB/Al device showing a single

conductivity state; (b) J-V characteristics of an ITO/ EB-CNT1.71/Al device in the OFF- and ON-state with the OFF-to-ON transitions at 2.2

V and the stability of the device in the ON-state under a constant voltage stress of 1 V; (c) effect of read cycles on the OFF and ON states of ITO/ EB-CNT1.71/Al device (inset: pulses used for read cycles testing); (d) J-V

characteristics of an ITO/ EB-CNT11.3/Al device in the OFF- and ON-state with the OFF-to-ON transitions at 2.35 V and stability of the

Figure 6-4 (a) J-V characteristics of an ITO/LM/Al device showing a single

conductivity state; (b) J-V characteristics of the NDR device of ITO/pLM-CNT/Al showing the reversibility in the conductivity states

and reading of the ON-state; (c) multilevel memory performance of

ITO/pLM-CNT/Al during write-read-erase-read cycles for five-level data

storage application The four different high-conductivity states are assigned by voltage sweeps from 0 V to -2, -3, -4 and -6 V, respectively, while the low-conductivity states are recovered by a positive bias sweep

of the same magnitude; (d) J-V characteristics of the rewritable device of ITO/oLM-CNT/Al showing an OFF-to-ON transition at about 3 V,

followed by an ON-to-OFF transition at about -3 V; (e) The current

response of the ITO/ oLM-CNT/Al device to continuous

write-read-erase-read sequences showing consecutive low-conductivity states (“0”) and high-conductivity states (“1”) when read at -/+ 1 V The

“1” state was obtained by a forward bias sweep from 0 to 4 V and the

“0” state by a reverse bias sweep from 0 to -4 V; (f) J-V characteristics of the non-rewritable device of ITO/oLM-CNT/Al-WORM device with an

ON/OFF current ratio of about 102 .139

Figure 6-5 UV-visible absorption spectra of leucoemeraldine/carbon nanotube

composite films indicating the relative degrees of oxidation when electrical characteristics of negative differential resistance (NDR), rewritable memory and write-once-read-many (WORM) memory are observed in the ITO/LM-CNT/Al devices 145

Figure 6-6 XPS N 1s core-level spectra of the emeraldine base/carbon nanotube

(EB-CNT11.3) composite film before [(a)] and after [(b)] being “switched ON” by applying a voltage sweep through a removable mercury droplet electrode 146

Figure 6-7 Cyclic voltammograms of pure emeraldine base film (EB) and composite

films of EB-CNT1.71 and EB-CNT11.3 Each film was scanned anodically, then cathodically .147

Figure 6-8 (a) A comparison of the switching voltages of the ITO/EB-CNT/Al

memory devices and the changes in the corresponding highest-occupied-molecular orbital (HOMO) level of the EB-CNT composites, and (b) ON/OFF current ratios of the ITO/EB-CNT/Al devices for increasing CNT content 149Figure 7-1 (a) Wide scan spectra of a clean indium-tin oxide (ITO) surface; (b) wide

(d) C 1s core level spectrum of the ITO substrate grafted with

2-(N-carbazolyl)ethyl methacrylate polymer brushes (ITO-g-PMCz) (inset of (d) is the N 1s core-level spectrum of the ITO-g-PMCz surface); (e) C 1s core level spectrum of the ITO substrate grafted with

9-(2-(4-vinyl(benzyloxy)ethyl)-9H-carbazole) polymer brushes (ITO-g-PVBCz) (inset of (e) are the N 1s and Cl 2p core-level spectra of the ITO-g-PVBCz surface); (f) C 1s core-level spectrum of the

ITO-g-PVBCz-b-P(HEMA) surface from 5 h of surface-initiated ATRP

of (2-hydroxyethyl methacrylate) (HEMA) from the ITO-g-PVBCz

surface 158

Figure 7-2 J-V characteristics of (a) the ITO/PMCz/Al and (b) the ITO/PVBCz/Al

devices; (c) J-V characteristics and (d) stability of the ON-state under a constant voltage stress of -0.5 V of the ITO-g-PMCz/Al device; (e) J-V

characteristics and (f) stability of the OFF- and ON-state under a

constant voltage stress of -0.5 V of the ITO-g-PVBCz/Al device [(e) also

shows also shows the ON-state being maintained by refreshing at -0.5 V with a pulse width of 10 ms every 10 s] .164

Figure 7-3 AFM images of the (a) bare ITO surface before silanization, (b) the

ITO-Cl surface obtained from reaction with the silane coupling agent

TCMPS, (c) the ITO-g-PMCz surface obtained by surface-initiated

ATRP on the ITO-Cl substrate (ATRP time = 12 h), (d) the

ITO-g-PVBCz surface obtained by surface-initiated ATRP on the ITO-Cl substrate (ATRP time = 12 h), and (e) the ITO-g-PVBCz-b-P(HEMA) surface obtained by surface-initiated ATRP on the ITO-g-PVBCz

substrate (ATRP time = 5 h) 167

Figure 7-4 Two- and three-dimensional AFM images of (a) the ITO-g-PMCz

surface and (b) the spin-coated PMCz film surface 168

dormant polymer chains bearing the active halide (X) M t n /L and M t n+1 /L

represent the concentrations of the transition metal-ligand complex in two different oxidation states, respectively [extracted from (Coessens et al., 2001)] 53

Scheme 3-1 Synthetic route for the methacrylate (MCz) and the styrene (VBCz)

monomers with carbazole pendant groups 61

Scheme 4-1 Synthetic route for the monomer (Ox) containing the 1,3,4-oxadiazole

group .90

Scheme 7-1 (1) Silanization of the indium-tin oxide (ITO) surface with

trichloro[4-(chloromethyl)phenyl]silane (TCMPS) in dry toluene; (2) surface-initiated atom transfer radical polymerizations (ATRP) of (a)

2-(N-carbazolyl)ethyl methacrylate (MCz) and (b) 9-(2-(4-vinyl(benzyloxy)ethyl)-9H-carbazole) (VBCz) monomers to

obtain ITO surfaces with grafted MCz and VBCz polymers, or the

ITO-g-PMCz and ITO-g-PVBCz surfaces, respectively; (3)

surface-initiated ATRP of 2-hydroxyethyl methacrylate (HEMA) from

the ITO-g-PVBCz surface to produce the ITO-g-PVBCz-b-P(HEMA)

surface 157

ATRP atom-transfer radical polymerization

Cz carbazole

DFT density functional theory

NDR negative differential resistance

OLED organic light-emitting diode

Ox 1,3,4-oxadiazole

P3HT poly(3-hexylthiophene)

PCBM phenyl C61-butyric acid methyl ester

PCzOx poly(2-(9H-carbazol-9-yl)ethyl methacrylate-co-4-(5-(4-tert-

butylphenyl-1,3,4-oxadiazol-2-yl)phenyl methacrylate) PEDOT:PSS poly(ethylenedioxythiophene) and poly(styrenesulfonic acid)

Chapter 1 Introduction

The microelectronics industry follows Moore’s law by shrinking transistor dimensions

As a result, the microelectronics industry has driven transistor feature size scaling from 10 μm to ~30 nm during the past 40 years (Thompson and Parthasarathy, 2006) However, transistor scaling is reaching its limit, both technologically and fundamentally, as the transistor size approaches tens of nanometers (Zhirnov et al., 2003) Although new lithography techniques such as nano-imprint lithography and extreme ultraviolet (EUV) lithography (Ito and Okazaki, 2000) are able to produce feature sizes down to a few nanometers, the physical size of the transistor can only be scaled down so much before electron tunneling through the gate occurs, making the transistor unreliable as a source of basic data (Zhirnov et al., 2003) How the microelectronics industry evolves after this limit is reached is unclear

Looking beyond data storage and memories as packets of charge in semiconductor devices, efforts have also been devoted to developing alternative technologies that exploit new materials and concepts to allow better scaling, and to enhance the memory performance These technologies include ferroelectric random-access memory (FeRAM) (Setter et al., 2006), magnetoresistive random access memory (MRAM) (De et al., 2002), phase-change memory (PCM) (Hudgens and Johnson, 2004), and organic/polymer memories (ITRS, 2005) Unlike the current memory technologies, where the memory effects are associated with a special cell structure,

the new technologies are based on electrical bistability of materials arising from changes in certain intrinsic properties, such as magnetism, polarity, phase, conformation and conductivity, in response to the applied electric field Organic and polymeric materials exhibiting bistable resistive switching (Stikeman, 2002) are promising candidates for such electronic devices in new information technologies (Kwok and Ellenbogen, 2002) Data can be written to the memory cell by applying an external voltage and the stored data read by measuring the low- or high-conductivity response, equivalent to “0” or “1” in silicon memory cells, to a small probe voltage The advantages of a simple device structure, good scalability, low-cost potential, low-power operation, multiple-state accessibility, three-dimensional (3D) stacking capability and large capacity for data storage add to the attractiveness of organic and polymer memories

Organic materials possess attractive features of miniaturized dimensions and property-tuning by molecular design and molecular synthesis A wide range of organic materials, including conjugated oligomers, organic dyes, charge transfer (CT) complexes, redox metal complexes, as well as other molecules and polymers, have been explored for memory applications Out of these organic materials, polymer materials offer good mechanical strength, flexibility and ease of processing These unique properties allow the polymers to be deposited by simple solution processes, including spin-coating, spray-coating, dip-coating, roller-coating and ink-jet printing,

on a variety of substrates, such as plastics, wafers, glass and metal foils, without the

elaborate processes of vacuum evaporation and deposition required for inorganic and organic molecular materials In fact, the International Technology Roadmap for Semiconductors (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 are molecularly designed or chosen based on their unique electroactive properties The objectives of the project include (1) the design and synthesis 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 is made to improve the device performance by modifying the 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

non-rewritable memory effects were observed for both polymer devices, the volatilities of the polymer memories were dependent 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 The device based on a copolymer containing carbazole and 1,3,4-oxadiazole (electron-acceptor) pendant groups exhibited electrical bistability with associated flash (rewritable) memory effects 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 based on charge-trapping and -detrapping processes in azobenzene polymers, the high-conductivity states were stabilized by intra-molecular charge-transfer in the pendant azobenzene chromophores WORM memory effects were observed when the presence of electron-accepting terminal moieties, such as –Br or –NO2, in the azobenzene chromophore facilitated intra-molecular charge-transfer interactions,

while flash memory effects were observed with electron-donating terminal moieties, such as –OCH3, as such processes 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 (electron donor) and carbon nanotubes (CNT) (electron acceptor) 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

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

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

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 of the three primary circuit elements, viz., capacitor, transistor and resistor (Scott and Bozano, 2007) As 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 low conductivity response to an external voltage Two conducting states can thus be observed at the same applied voltage In comparison to inorganic materials, organic materials offer the advantages of low cost, processability and, in particular, properties-tuning by appropriate syntheses The organic materials can also be processed in three-dimensional arrays for data storage with higer density (Ouyang et al., 2004) compared to the traditional inorganic semiconductor memory, which is restricted to two dimensions

Polymer memories have been demonstrated with materials such as the composite of poly(ethylenedioxythiophene) (PEDOT) and poly(styrenesulfonic acid) (PSS), or PEDOT:PSS (Moller et al., 2003), supramolecular structures doped with dyes

(Bandyopadhyay and Pal, 2003c), and other polymers containing organic electron donor and acceptor molecules, such as 8-hydroxyquinoline (8HQ), tetrathiafulvalene (TTF), fullerene (C60) and phenyl C61-butyric acid methyl ester (PCBM) (Chu et al.,

2005, Majumdar et al., 2005) A variety of heterogeneous hybrid materials containing nanoparticles (NPs) of different materials, including gold (Ouyang et al., 2004), zinc oxide (Verbakel et al., 2007), and CdSe/ZnS (Li, Son et al., 2007), as well as carbon

nanotubes (Pradhan et al., 2006) in matrices of polystyrene, poly(N-vinylcarbazole)

(PVK), poly(3-hexylthiophene) (P3HT) and other polymers have also found applications in polymer memories The electrical bistability observed in polymer materials is attributed to changes in their intrinsic properties in response to the applied electric field via processes such as charge transfer, phase change, conformation change and reduction-oxidation (redox) The different kinds of memory effects observed in organic memory devices include write-one-read-many-times (WORM) memory, dynamic random-access memory (DRAM) and flash memory In particular,

polymer WORM devices are attractive as each device occupies a cell area of only 4F2,

where F is the minimum feature size, compared to the cell area of 9F2 occupied by traditional three terminal memory cells (Ling, Liaw et al., 2007) Devices exhibiting negative differential resistance (NDR) have also been reported (Yang et al., 2006)

This literature review will focus on resistive switching memories in which the memory states are both induced and read electrically The use of molecular design in the synthesis of a series of functional and poccessible polymers for flash, WORM,

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 classified according to the volatility of the device A volatile memory requires a constant electrical power supply to retain stored data The stored data is lost immediately when power supply to the system is turned off On the other hand, a non-volatile memory can retain the stored data even when the electrical power supply has been turned off The electronic memories can be further classified, as shown in Figure 2-1 Of these categories, the write-once-read-many-times (WORM) memory, the hybrid non-volatile and rewritable (flash) memory, and the dynamic random access memory (DRAM) are among the most widely reported polymer memories (Moller et al., 2003, Ouyang et al.,

2004, Ling, Song, Lim et al., 2006)

A WORM memory, being non-volatile, can store data permanently and allows the stored data to be read repeatedly Data can only be written once and the stored data cannot be modified WORM memories can be used to store archival standards, databases and in other applications where large amounts of data has to be reliably kept and made available over a long period of time Conventional CD-Rs, DVD±Rs or programmable-read-only-memories (PROMs) are examples of WORM memories WORM memories can also be used in niche applications, such as disposable electronic labels and RFID tags

A flash memory, another form of non-volatile memory, can be electrically reprogrammed (rewritable) and has the ability to write, read, erase and retain the

stored data Flash memories are extensively used in portable electronic systems, such

as PDAs, mobile PCs, video/audio players and digital cameras The current flash memory technology is based on the metal-oxide-semiconductor field-effect transistor (MOSFET) with a floating gate An electric field transfers charge to and from the floating gate, thus modifying the threshold voltage (encoding “0” and “1” signals) of the underlying transistor

A DRAM is a random access memory in which each bit of data is stored in a separate capacitor As real-world capacitors tend to leak electrons, the capacitor charge has to

be refreshed periodically to prevent loss of the stored information This refresh requirement makes it a dynamic memory The inability to store data in the absence of

a constant power supply makes it a form of volatile memory DRAMs are used as the main memory of most computers today The volatile memory effect of 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 n-doped silicon, and indium-tin-oxide (ITO) are

commonly used as electrodes in symmetric or asymmetric device configurations A

flexible polymer memory utilizing a conductive polypyrrole film as the bottom substrate and electrode has also been reported (Li, Ling et al., 2007) Test structures usually consist of a spin-coated polymer thin film on the bottom electrode, for instance, ITO, with the top electrodes deposited through a shadow mask via thermal evaporation in a vacuum chamber The area covered by the top electrode forms the active device area The basic configuration of a test memory device is shown in

Figure 2-2(a) An x-y cross point memory array can be used for the integration and

easy addressing of memory cells The arrays can be stacked to form 3-D data storage devices with high data storage density (Ling, Liaw et al., 2007) A schematic diagram

of a 4 (word line) × 4 (bit line) cross point memory array and of a 2 (stacked layer) ×

4 (word line) × 4 (bit line) stacked memory device is shown in Figure 2-2(b)

In cross-point arrays and three-dimensional stacked memory devices, parasitic paths may exist in parallel to the selected node, through all the neighbouring nodes, if the polymers lack the current rectification properties These paths can affect the reading process, introducing parasitic leakage currents, and hinder programming To avoid the

misreading phenomenon, one rectifying diode, such as a Si n-i-p diode (when the bottom electrode acts as an anode) or a Si p-i-n diode (when the bottom electrode acts

as a cathode), can be integrated in series with the cell to prevent parasitic leakage current (Figure 2-2(c)) Some resistive switching memories with rectifying properties have been reported (Moller et al., 2003, Verbakel et al., 2006, Tan et al., 2008)

Figure 2-2 Schematic diagrams of (a) a test memory device, (b) a 4 (word line) × 4 (bit line) cross-point memory array and a 2 (stacked layer) × 4 (word line) × 4 (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

The structure of the active memory layer can range from that of a single layer, consisting of one type of material or of composite materials, to that of a multilayer

made up of different materials Devices made up of active memory layers with multilayer structures include those with an organic/metal-nanocluster/organic trilayer (Ma et al., 2002, Bozano et al., 2004), several bilayer films (Majumdar et al., 2003) and polymer/semiconductor hybrids (Moller et al., 2003), while single-layer devices make use of materials such as non-conjugated polymers (Carachano et al., 1971), conjugated organic molecules and polymers (Bandyopadhyay and Pal, 2003c), and polymers containing electron-donor and -acceptor groups (Bandyopadhyay and Pal, 2003c, Chu et al., 2005), metal dopants (Yang et al., 2006, Tseng et al., 2005), and organic or rare earth metal complexes (Fang et al., 2006, Gao et al., 2000).

2.1.3 Operating Mechanisms

Various mechanisms have been proposed to explain the change in conductance due to

an applied voltage, or voltage-induced resistive switching, in organic/polymer memory devices The switching mechanism depends on the characteristics of the materials used for the fabrication of the memory device For example, it is known that conjugated organic molecules and polymers allow conduction of current The main mechanisms proposed for resistive switching include trapping-detrapping, charge transfer and nanocomposite redox effects

Resistive switching and associated memory effects based on charge-trapping and -detrapping processes have been observed in a polymer denoted by PFOxPy (Ling, Song, Lim et al., 2006) The conjugated polymer (Figure 2-3(a)), containing the

fluorene group as the electron donor, and the oxadiazole and bipyridine groups as the electron acceptors, possesses volatile, rewritable properties and can function as a

DRAM The J-V characteristics of the ITO/PFOxPy/Al device are shown in (Figure

2-3(b)) The device was initially at a low-conductivity (OFF) state when a negative voltage sweep was applied At the switching threshold voltage of about -2.8 V, the current density of the device increased abruptly from 10-7 Acm-2 to 10-2 Acm-2 This

increase in J indicated the transition of the device from the low-conductivity (OFF)

state to the high-conductivity (ON) state, which serves as the “write” process for the device Subsequent voltage sweeps of both polarities showed that the ON state of the device was retained Thus, the device exhibits two bistable electrical states The device can be reset to the initial OFF state (the “erase” process) by a reverse voltage pulse of 3.5 V The processes of “write” and “erase” of the device can be repeated, indicating that the memory device is rewritable However, the ON-state could only be retained for less than 2 minutes before the device relaxes to the OFF state, indicating the volatile nature of the memory The ON state could be sustained by applying a refreshing pulse of 1 V every 10 s The ability to write, read, erase, and refresh the electronic states of the device fulfils the functionality of a DRAM

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)]

The electronic properties of PFOxPy were studied by density functional theory (DFT) Calculations of the electrostatic potentials (ESP), and molecular orbitals of the basic units, namely the fluorene oligomer (F), the 2,2’-bipyridine moiety (Py), and the 1,3,4-oxadiazole moiety (Ox), were carried out (Figure 2-3 (c) and (d)) An analysis

of the work functions of the electrodes and the high-occupied-molecular-orbital (HOMO) and lowest-unoccupied-molecular-orbital (LUMO) energy levels of the

different basic units suggests that hole injection from ITO into the HOMO of PFOxPy

is the favored process (lower energy barrier of 0.4 eV) The OFF-to-ON transition thus occurs under negative bias The molecular surface with continuous positive ESP along the conjugated backbone indicates an open channel for charge migration On the other hand, negative ESP regions (of darker colour), arising from electron-acceptor groups, serve as “traps” to block the mobility of charge carriers At the turn-on voltage, the charge “traps” are completely filled by charge carriers generated under the electric field, resulting in a transition to the high-conductivity state The depth of charge “traps” arising from the Ox moieties is about 0.6 eV, while that from the Py moieties is about 0.3 eV The incorporation of pyridine moieties reduces the depth of the electron traps and increases the ease of electron detrapping, leading to the volatility of the memory device based on PFOxPy

Electrical bistability based on electric field-induced charge transfer effects was observed in polymeric system containing phenyl C61-butyric acid methyl ester (PCBM), an organic electron acceptor, and tetrathiafulvalene (TTF), an organic electron donor, in a polystyrene matrix (Chu et al., 2005) (Figure 2-4) The device undergoes the transition from the OFF to the ON state at a critical voltage of 2.6 V, with an increase of current by about three orders of magnitude The OFF state can be recovered by applying a larger voltage bias of opposite polarity As shown in Figure 2-4, the device undergoes an ON-to-OFF transition at about -6.5 V The ability to program, and reprogram, the conductivity-state of the device, as well as the stability

of the conductivity-state under electrical stress and power-off conditions, fulfills the functionality of a flash device

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)]

Under high electric field, electron transfer from the HOMO of TTF to the LUMO of PCBM may occur The HOMO of TTF and the LUMO of PCBM become partially filled, resulting in TTF and PCBM being positively and negatively charged, respectively Charge carriers are thus generated and a sharp increase in conductivity is observed as a result of the charge transfer Alternating-current impedance experiments reveal a change in the apparent dielectric constant of the film, associated with the electric field-induced dipole formation between the donor and the acceptor

Non-volatile memory behavior based on nanocomposite redox effects has been

observed with nanocomposites of polyaniline nanofibers and gold nanoparticles as the active layer material (Tseng et al., 2005) The device can be switched from the OFF state to the ON state at a threshold voltage of 3 V, with a difference of about three orders of magnitude in the conductivity of the two states (Figure 2-5) The ON state can be reversed by a voltage bias of -5 V Negative differential resistance (NDR), commonly observed in systems containing nanoparticles, is observed when the voltage is increased beyond 3 V The device exhibits high stability, with no significant change in conductivity from being under constant voltage stress over a 14 hour period, and fast switching time of less than 25 ns

Figure 2-5 (a) Current-voltage characteristics of the polyaniline nanofiber-gold nanoparticle device The potential is scanned from (A) 0 to +4 V, (B) +4 to 0 V, and (C) 0 to +4 V (b) A schematic representation of the electric field-induced charge transfer from polyaniline to gold nanoparticle (NDR: negative differential resistance) [extracted from (Tseng et al., 2005)]

Under a high electric field, electrons residing on the imine nitrogen of polyaniline may gain sufficient energy to overcome the polyaniline-gold interface and move onto the gold nanoparticles The gold nanoparticles become negatively charged, while the

polyaniline nanofibers become positively charged, thus increasing the conductivity of the nanocomposite dramatically X-ray photoelectron spectroscopy and Raman spectroscopy studies revealed evidence of the nanocomposite redox effects As a result of the redox interactions, a positive shift of 0.5 eV in the binding energy of the

N 1s core electrons of polyaniline is observed, together with a negative shift of 0.2 eV

in the binding energy of the gold electrons (4f5/2) These energy shifts are indicative of the partial charges lying on polyaniline and gold nanoparticles, respectively The Raman spectrum of the polyaniline-gold nanoparticle composite revealed peaks attributed to protonated C=N+ and C-N+ species, confirming that electron transfer from polyaniline has occurred (Tseng et al., 2007)

2.1.4 Current Conduction Models

To further understand the device transition from the OFF-state to the ON-state, the current density-voltage data in both states can be fitted to theoretical models Various mechanisms have been proposed to explain electronic transport in semiconducting organic and polymer devices The space-charge-limited current model with field-dependent mobility (Murgatroyd, 1970) has been widely used for charge transport in organics Other basic conduction models used for such data fitting include the Schottky emission model, the Frenkel-Poole emission model, space-charge-limited current (Sze, 1981)

Table 2-1 Basic conduction processes in insulators [extracted from (Sze, 1981)]

A summary of mechanisms based on thermal processes such as thermionic and field emission, a variety of tunneling paths, hopping and several field-dependent mobility models is also available (Braun, 2003) Temperature-dependence studies can be used

to verify if the correct conduction model has been used

2.1.5 Properties-Tuning of Memory Devices

In the field of organic memory devices, molecular design and doping have been used

in the fabrication of memory devices to achieve different memory properties The methods used are not unlike those for color-tuning in organic light-emitting diodes (Akcelrud, 2003) and adaptation of material bandgap in organic photovoltaic cells (Hoppe and Sariciftci, 2004) While molecular design and selective structure

modifications are used to prepare materials with desirable characteristics, multilayer structures and the blending of conjugated polymers with electron accepting materials, such as C60 derivatives, cadmium selenide, and titanium dioxide, are also used to improve the performances of organic light-emitting diodes (Moliton and Hiorns, 2004) and photovoltaic cells (Coakley and McGehee, 2004), respectively

In earlier works, polymers, such as poly(styrene sulfonic acid), polyaniline and polythiophene, were used as the polyelectrolyte, matrix of a dye, or a component of a charge-transfer complex in a doped or mixed system The doping of polystyrene with gold nanoparticles and 8-hydroxyquinoline (8HQ) imparted non-volatile memory properties to the originally insulating material (Ouyang et al., 2004) The electrical bistability observed was attributed to charge transfer between the gold nanoparticles and 8HQ under a high electric field The morphology associated with doping and mixing, and its effect on device performance, had also been studied (Zhong et al., 2002) As doping and mixing can result in phase separation and ion aggregation due

to non-uniform dispersion and incompatible components, a processible polymeric material which possesses the required electronic properties with a single macromolecule is thus desirable for memory-device applications

Besides the imparting of memory properties to materials which are originally insulators, physical techniques and modifications are also used to tune the properties

of organic memory devices Conductance switching in organic films has been

observed to change from memory switching to threshold switching with the decrease

in film thickness down to the molecular limit, making them suitable for different applications in molecular memory and logic elements (Majee et al., 2004) The phenomenon was attributed to a change in the switching mechanism from electro-reduction to direct tunneling of electrons as the number of molecular layers was reduced Devices with three distinctly different properties, viz., a true insulator, a bistable device with an ON/OFF current ratio larger than 104 and a write-once-read-many-times (WORM) device, were obtained with increasing fullerene concentration in polystyrene (Majumdar et al., 2005) Tunneling was identified as the basic device mechanism for the three devices and the different behaviors were attributed to the varying degrees of charge tunneling and storage with changing fullerene concentration

Besides altering the type of memory property possessed by the device, its ON/OFF current ratio can also be tuned Material selection, molecular design and selective structural modifications of the material are more applicable than physical techniques for this purpose The ON/OFF current ratios of memory devices can also be increased

by selecting molecules with suitable functional groups and dipole moments (Li et al., 2003) Such ratios of binary memory devices based on the same backbone structure can be tuned to a value exceeding 105 via functional group modifications (Bandyopadhyay and Pal, 2003b, Bandyopadhyay and Pal, 2003a) Utilizing the technique of layer-by-layer electrostatic self-assembly, the ON/OFF current ratios of