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Rather than encoding “0” and “1” from the amount of charges stored in a cell, a polymer memory stores data, for instance, based on the high an...
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J. Phys. Chem. B 2006, 110, 23995-24001

23995

Bistable Electrical Switching and Memory Effects in a Thin Film of Copolymer Containing Electron Donor-Acceptor Moieties and Europium Complexes Qi-Dan Ling,† Wen Wang,† Yan Song,‡ Chun-Xiang Zhu,‡ Daniel Siu-Hung Chan,‡ En-Tang Kang,*,† and Koon-Gee Neoh† Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, Kent Ridge, Singapore 119260, Singapore, and SNDL, Department of Electrical and Computer Engineering, National UniVersity of Singapore, Singapore 119260, Singapore ReceiVed: July 27, 2006; In Final Form: September 26, 2006

A nonconjugated methacrylate copolymer (PCzOxEu) containing carbazole moieties (electron donors), 1,3,4oxadiazole moieties (electron acceptors), and europium complexes in the pendant groups was synthesized via free radical copolymerization of methacrylate monomers containing the respective functional groups. The molecular structure and composition of PCzOxEu was characterized by elemental analysis, FT-IR, 1H NMR, 13C NMR, UV-vis absorption and fluorescence spectroscopies, gel permeation chromatography (GPC), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CyV). The resulting copolymer exhibited a relatively high glass transition temperature (Tg ≈ 125 °C) and good solubility in common organic solvents. It could be cast into transparent films from solutions. For a thin film of PCzOxEu sandwiched between an indium-tin oxide (ITO) electrode and an Al electrode (ITO/PCzOxEu/Al), the structure behaved as a nonvolatile flash (rewritable) memory with accessible electronic states that could be written, read, and erased. The polymer memory exhibited an ON/OFF current ratio up to 105, switching response time of ∼1.5 µs, more than 106 read cycles, retention time of more than 8 h, and write/erase voltages of about 4.4 V/-2.8 V under ambient conditions. The roles of oxadiazole moieties in improving the response time and retention time of the memory device were elucidated from the molecular simulation results.

1. Introduction Electroactive organic and polymeric materials are alternatives to traditional Si, Ge, and GaAs semiconductors that have to face the problem of scaling down in feature size.1,2 Several types of organic electronics and devices, including light-emitting diodes,3,4 transistors,5 lasers,6 sensors,7 photovoltaic cells,8,9 and switches,10 have been realized. Recently, flash (rewritable) and WORM (write-once read-many-times) memories based on polymeric materials have been demonstrated.11,12 They exhibit simplicity in structure, drive-free read and write, good scalability, 3-D stacking ability, low-cost potential, and a large capacity for data storage. A polymer memory stores information in a manner entirely different from that of the silicon devices. Rather than encoding “0” and “1” from the amount of charges stored in a cell, a polymer memory stores data, for instance, based on the high and low conductivity response to an applied voltage.11 In other pioneering works on polymer memory effects,13 the polymers were used as polyelectrolytes,14,15 matrices of dyes,16 or components of charge transfer (CT) complexes17 in a doped or mixed system. The design and synthesis of a processable polymer that can provide the required electronic properties within a single macromolecule and yet still possesses good chemical, mechanical, and morphological characteristics is desirable for memory device applications. The molecular designcum-synthesis approach has allowed several types of polymer memories, including flash memory, dynamic random access memory (DRAM) ,and WORM memory, to be realized.18 In this work, a methacrylate copolymer containing electron donor-acceptor (D-A) moieties and europium complexes * To whom all correspondence should be addressed. Phone: +65-65162189. Fax: +65-6779-1936. E-mail address: [email protected]. † Department of Chemical and Biomolecular Engineering. ‡ SNDL, Department of Electrical and Computer Engineering

(PCzOxEu) in the pendant groups has been synthesized and characterized (Scheme 1). In this copolymer, the pendant carbazole group (Cz) in the ethyl acrylate spacer units serves as the electron-donor and hole-transporting moiety,19 the oxadiazole derivative (Ox) serves as the electron-acceptor and electron-transporting moiety,20 and the Eu complex (Eu), which is known to exhibit memory effects,21 is used to hold the charges. Nonvolatile memory effects were demonstrated in a metal/PCzOxEu/metal sandwich structure. A flash memory based on this nonconjugated donor-acceptor copolymer containing also the Eu complexes exhibited a significant improvement in performance, with a higher ON/OFF current ratio, longer retention time of both ON and OFF states, and much faster response over that of the memory device based on a nonconjugated vinyl copolymer containing only N-vinylcarbazole (donor) groups and similar pendant Eu complexes.18a Molecular simulation of the components of PCzOxEu was carried out to better understand the carrier transport process and memory mechanism in this electroactive copolymer. 2. Experimental Section 2.1. Instrumentation. 1H NMR spectra were measured on a Bruker ACF 300 spectrometer with d-chloroform or d-acetone as the solvent and tetramethylsilane as the internal standard. The 13C NMR spectrum was recorded on a Bruker AMX 500 spectrometer with d6-dimethyl sulfoxide (DMSO) as the solvent and tetramethylsilane as the internal standard. FT-IR spectra were recorded on a Bio-Rad FTS 165 spectrometer by dispersing the samples in KBr pellets. UV-visible absorption and fluorescence spectra were measured on a Shimadzu UV-NIR 1601 spectrophotometer and a Shimadzu RF 5301PC luminescence spectrophotometer, respectively. Thermogravimetric analysis (TGA) was conducted on a TGA 2050 thermogravimetric

10.1021/jp0647939 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/31/2006

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SCHEME 1: Synthetic Routes for the Functional Monomers and Copolymer Complex (PCzOxEu)

analyzer of the TA Instruments at a heating rate of 20 deg/min and under an air flow rate of 75 mL/min. Differential scanning calorimetry (DSC) measurement was carried out on the Mettler Toledo DSC 822e system under N2 and at a heating rate of 10 deg/min. Gel permeation chromatography (GPC) measurements were conducted on a HP 1100 HPLC system equipped with the HP 1047A RI detector and the Agilent 7991 columns. Polystyrene standards were used as the molecular weight references and THF was used as the eluent. Elemental microanalysis (for C, H, and N) was performed on a Perkin-Elmer 2400 elemental analyzer. The Eu content was determined by ethylenediamine tetraacetic acid (EDTA) titration after the copolymer was digested in concentrated HNO3/HClO4 (1:1, v/v). Cyclic voltammetry (CyV) measurement was performed on an Autolab potentiostat/galvanostat system, using a three-electrode cell under an argon atmosphere. The polymer film on a Pt disk electrode (working electrode) was scanned (scan rate: 15 mV/ s) anodically and cathodically in a solution of tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) in acetonitrile (0.1 M) with Ag/AgCl and a platinum wire as the reference and counter electrode, respectively. XPS measurements were carried on a Kratos AXIS HSi spectrometer with a monochromatized Al KR X-ray source (1486.6 eV photons) at a constant dwell time of 100 ms and pass energy of 40 eV. The anode voltage and current were set at 15 kV and 10 mA, respectively. The pressure in the analysis chamber was maintained at 5 × 10-8 Torr or lower during each measurement. 2.2. Reagents for Synthesis. Tetrahydrofuran (THF) was refluxed over sodium in the presence of benzophenone until a persistent blue color appeared and then distilled prior to use. Other GR or HPLC grade solvents were purchased from Merck Schuchardt Chemical Co., and were used without further purification (unless stated otherwise). Europium oxide of high purity (99.99%) was supplied by Shanghai Yuelong Nonferrous Metal Co. Ltd., Shanghai, China. 4-tert-Butylbenzoic hydrazide (97%) and the other chemicals were purchased from Aldrich Chemical Co. and were used as received.

2.3. Synthesis of Monomers and the Copolymer. 2-(9HCarbazol-9-yl)ethyl Methacrylate (M1). A solution of methacryloyl chloride (1.6 g, 15 mmol) in dry THF (5 mL) was added slowly to a solution of triethylamine (1.2 g, 12 mmol) and 2-(9H-carbazol-9-yl)ethanol22 (2.1 g, 10 mmol) in dry THF. The reaction mixture was stirred for 12 h at room temperature. The ammonium salt was removed by filtration and the solvent by evaporation. The residue was extracted with chloroform and further filtered through a short silica gel column with chloroform as the eluent. Yield: 1.5 g (53.7%). Mp 58-62 °C. Anal. Calcd for C18H17NO2 (wt %): C, 77.4; H, 6.13; N, 5.01. Found: C, 77.00; H, 5.92; N, 4.61. EI-MS (m/e): 279.0 (95%), 211.0, 193.0, 180.1, 167.0, 83.6, 70.5, 237.9, 222.0, 105.0, 93.0, 77.0. 1H NMR (CDCl , 300 MHz): δ (ppm) 2.27 (3H, s, -CH ), 3 3 4.52 (2H, t, -NCH2-), 4.64 (2H, t, -OCH2-), 5.48 (1H, s, -CdCH2), 5.93 (1H, s, -CdCH2), 7.24 (2H, t, Ar-H), 7.44 (2H, d, Ar-H), 7.47 (2H, d, Ar-H), 8.09 (2H, t, Ar-H). FTIR (KBr pellet, cm-1): 3051, 2931, 1718 (υCdO(CO2C)), 1629, 1598, 1486, 1458, 750, 724 (carbazole ring). 4-(5-(4-tert-Butylphenyl-1,3,4-oxadiazol-2-yl)phenol. 4-(5-(4tert-Butylphenyl-1,3,4-oxadiazol-2-yl)phenol was prepared by reaction of 4-tert-butylbenzoic hydrazide with 4-acetoxybenzoly chloride in the presence of dichloromethane and pyridine to form the corresponding hydrazide compound, which was subsequently treated with excess phosphorus oxychloride, and hydrolyzed by sodium hydroxide in ethanol.23,24 Yield: 88.2%. Mp 234-236 °C. Anal. Calcd for C18H18N2O2 (wt %): C, 73.45; H, 6.16; N, 9.52. Found: C, 73.05; H, 5.95; N, 9.32. EI-MS (m/e): 294.1 (98%), 279.2, 251.1, 236.1, 161.1, 121.0, 93.1. 1H NMR (CDCl3, 300 MHz): δ (ppm) 1.37 (9H, s, CH3), 7.01 (2H, d, Ar-H), 7.29 (2H, d, Ar-H), 7.53 (2H, d, Ar-H), 8.04 (2H, d, Ar-H), 9.10 (1H, s, -OH). FT-IR (KBr pellet, cm-1): 3200-3100, 1613, 1558, 1495, 1441, 1271, 1173, 843, 751, 725. 4-(5-(4-tert-Butylphenyl-1,3,4-oxadiazol-2-yl)phenyl Methacrylate (M2). A solution of methacryloyl chloride (1.6 g, 15 mmol) in dry THF (5 mL) was added slowly to a solution of triethylamine (1.2 g, 12 mmol) and 4-(5-(4-tert-butylphenyl-

Electrical Switching and Memory Effects in Copolymers 1,3,4-oxadiazol-2-yl)phenol (2.5 g, 8.5 mmol) in dry THF. The reaction mixture was stirred for 12 h at room temperature. The ammonium salt was removed by filtration and the solvent by evaporation. The residue was extracted with chloroform and further filtered through a silica gel column with hexane/ethyl acetate (4:1, v/v) as the eluent. Yield: 2.1 g (68.2%). Mp 70 °C. Anal. Calcd for C22H21N2O4 (wt %): C, 72.91; H, 6.12; N, 7.73. Found: C, 72.59; H, 6.38; N, 7.38. EI-MS (m/e): 362.0 (97%), 347.3, 294.2, 279.2, 236.2, 161.1, 121.1, 68.0. 1H NMR (CDCl3, 300 MHz): δ (ppm) 1.37 (9H, s, CH3), 2.08 (3H, m, -COCH3), 5.82 (1H, s, -CdCH2), 6.39 (1H, s, -CdCH2), 7.11 (2H, d, Ar-H), 7.31 (2H, d, Ar-H), 7.56 (2H, d, Ar-H), 8.17 (2H, d, Ar-H). FT-IR (KBr pellet, cm-1): 1736 (υCdO), 1613, 1555, 1493, 1416, 1316, 1208, 1165, 841, 748, 730. Europium-Methacrylate-Thenoyltrifluoroacetone-Phenanthroline (M3). Europium triisopropoxide (3.4 mmol) was dissolved in 20 mL of anhydrous 2-propanol and benzene (1:1, v/v) by heating in a flask under an argon atmosphere. A solution of thenoyltrifluoroacetone (TTA, 1.5 g, 6.8 mmol) in 10 mL of benzene was added dropwise into the flask. After refluxing for 2.5 h, a solution of methacrylic acid (0.3 g, 3.4 mmol) in 10 mL of benzene was added. After further refluxing for 1 h, a solution of 1,10-phenanathroline (phen, 0.6 g, 3.4 mmol) in 10 mL of benzene was added. The reaction mixture was refluxed for another 2 h and cooled. The excess solvent was evaporated and a yellow residue was obtained. The residue was recrystallized from acetone/ethanol (2:1, v/v). Yield: 2.1 g, 72.4%. Eu content for C32H21F6N2O6S2Eu (wt %): 17.68; Found: 18.16. FT-IR (KBr pallet, cm-1): 3110, 2921, 1629, 1597, 1574, 1539, 1501, 1469, 1454, 1428, 1411, 1357, 1305, 1284, 1247, 1186, 1139, 1229, 1103.0, 1035, 1014, 933, 862, 842, 787, 719, 752, 680, 640, 605, 580, 493 (w, υas of Eu-O) and 460 (w, υs of Eu-O). UV-visible (CHCl3, λmax, nm): 236.0 (E band of the chelate), 268.0 (TTA+phen) and 340.0 (K-band of CdC-CO). XPS (binding energy, eV): C 1s, 284.6 (C-H/C-C), 285.4 (C-S), 285.8 (CdN-C), 287.3 (C-CdO), 288.7 (O-CdO), 289.1 (F3C-CdO), 292.2 (F3C-C); N 1s, 399.0 (CdN-C). Preparation of the Copolymer (PCzOxEu). A mixture of M1, M2, and M3 in a molar ratio of 1:1:0.1 and the azo-bis(isobutyronitrile) (AIBN) initiator (1.0 wt % of the total monomer concentration) were dissolved in dry THF in a glass tube. The homogeneous solution was purged with argon for 5 min and sealed under an argon atmosphere. Copolymerization was carried out with continuous stirring at 65 °C for 48 h. The viscous copolymer solution was diluted with THF and precipitated into an excess amount of methanol under vigorous stirring. Precipitate was collected by filtration and dried under reduced pressure. Number average molecular weight (Mn) ) 2.31 × 104, polydispersity index ) 1.70. For PCzOxEu, x ) 1.00, y ) 0.08, z ) 1.10 (determined from the elemental analysis and XPS data). Anal. Found (wt %): Eu, 1.63; C, 71.9; H, 5.79; O, 12.04; N, 6.30; F, 1.10. FT-IR (KBr pellet, cm-1): 2962, 1736 (υCd O(CO2C)), 1612, 1492, 1459, 1269, 1206, 1166, 1123, 843, 751, 725(Cz), 647, 575 (Eu-O). 1H NMR (CDCl3, 300 MHz): δ (ppm) 1.70 (C(CH3)3), 2.10 (C(CH3)CH2), 5.4 (NCH2CH2O), 5.9 (OCH2CH2N), 6.3-6.5 (TTA, -C(dO)-CH-C(-O)), 6.9-7.4 (Ar, Th-H), 7.4-7.8 (Ar, Th-H), 7.9-8.2 (Ar, ThH). 13C NMR (d6-DMSO, 126 MHz): δ (ppm) 166.3 (O-Cd O), 164.1/163.1 (CdO/C-O, TTA), 155.0 (C-O-C, Ox), 152.6 (C(Ph)-O-CdO, Ox), 140.1 (CdN, phen), 136.1 (ArC-, Ox), 135.3 (Ar-C-N, Cz), 128.1 (Th-C, TTA), 126.5 (Ph-C, Ox), 126.2 (Ph-C, Ox), 125.6 (Ph-C, Ox), 123.0 (ArC, phen), 122.1 (CR and Cβ in Cz), 121.0 (C2 and C7 in Cz), 120.2 (C4 and C5 in Cz), 118.9 (C3 and C6 in Cz), 116.2

J. Phys. Chem. B, Vol. 110, No. 47, 2006 23997 (-CF3, TTA), 109.4 (C1 and C8 in Cz), 62.8 (-NCH2CH2O-, Cz), 52.7 (-NCH2CH2O-, Cz), 41.1 (-C(CH3)3), 34.8 (-CH3 in tert-butyl), 30.8 (-C(CH3)-, backbone), 17.8 (-CH3). Tg ) 125 °C, Td(onset) ) 251 °C. CyV: ERed ) -1.8 V vs Ag/ Ag+, EOx ) +1.3 V vs Ag/Ag+, ELUMO ) -2.95 eV, EHOMO ) -5.46 eV. XPS (binding energy, eV): C 1s, 284.6 (C-H/CC), 285.9 (C-N), 286.9 (CdN-C), 288.9 (O-CdO); N 1s, 399.0 (CdN-C), 399.4 (CdN-N), 399.9 (C-N-C(-C)). 2.4. Fabrication and Characterization of the Memory Device. The indium-tin oxide (ITO) glass substrate was precleaned with water, acetone, and 2-propanol, in that order, in an ultrasonic bath for 15 min. A toluene solution of PCzOxEu (15 mg/mL) was spin-coated onto the ITO substrate, followed by solvent removal in a vacuum chamber at 10-5 Torr at room temperature. The thickness of the polymer layer was about 50 nm. Finally, a 100-nm-thick Al top electrode was thermally evaporated at a pressure around 10-7 Torr. The measurements were carried out on devices of 0.4 × 0.4, 0.2 × 0.2 and 0.15 × 0.15 mm2 in size. The I-V data reported were based on devices of 0.4 × 0.4 mm2 in size. Electrical and switching time measurements were carried out respectively on a HP 4156A semiconductor parameter analyzer and an Agilent Infiniium oscilloscope under ambient conditions. 2.5. Molecular Simulation. Molecular simulations of the components of PCzOxEu were carried out with the Gaussian 03 (Revision D 01) program package on a HP XW6200 Workstation with 2 CPUs and 3 GB memory. The molecular orbitals and electronic properties were calculated by the density function theory (DFT), using the Becke’s three-parameter functional with the Lee, Yang, and Parr correlation functional method (B3LYP) and the basis set 6-31G with d function added to heavy atoms (in short, DFT B3LYP/6-31G(d)).25 3. Results and Discussion 3.1. Characterization of the Copolymer. The synthesized copolymer, PCzOxEu (Scheme 1), was characterized by GPC, elemental analysis, and FT-IR, 1H NMR, 13C NMR, UVvisible, fluorescence, and X-ray photoelectron spectroscopies. GPC measurement indicated that the number-average molecular weight of the resulting copolymer was around 23 100, with a polydispersity index of about 1.7. As revealed by cyclic voltammetry (CyV), PCzOxEu shows irreversible oxidation (EOx(onset) ) +1.3 V vs Ag/Ag+) and reduction (ERed(onset) ) -1.8 V vs Ag/Ag+) behavior, with estimated HOMO and LUMO energy levels of -5.46 and -2.95 eV, respectively.26,27 PCzOxEu is soluble in common organic solvents, such as tetrahydrofuran (THF), toluene, chloroform, N,N-dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP). It can be cast into transparent and uniform thin films from solution by spin-coating. The FT-IR spectrum of PCzOxEu shows characteristic absorption bands at 3000-2950 cm-1 for C-H stretching vibrations, 1736 cm-1 for CdO stretching, 1612 cm-1 for the 1,3,4-oxadiazole ring,28 1269 cm-1 for the -CF3 group, 1206-1068 cm-1 for C-O-C stretching vibrations, and 751 + 725 cm-1 (doublet) for the carbazole group.29 Two absorption bands at 650-640 and 580-570 cm-1, associated with the Eu-O stretching vibrations,30 are also discernible. Due to the low content of europium complex (about 0.08 units per unit of acceptor or donor) in PCzOxEu, the absorption bands associated with the ligands were buried under those of other aromatic species. However, chemical shifts of the protons in the TTA ligand were observed in the 1H NMR spectrum of PCzOxEu. The chemical shifts in the ranges of 6.9-8.2, 5.4-5.9, and 1.72.1 ppm are attributed to the aromatic, -NCH2CH2O-, and

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Figure 1. UV-visible absorption spectrum (dash curve) and fluorescence spectra from excitation at 295 (dotted curve) and 348 nm (solid curve), respectively, of PCzOxEu in diluted THF solution (1.0 × 10-5 mol/L).

other alkyl protons, respectively. In the 13C NMR spectrum of PCzOxEu, the chemical shifts in the range of 166.3-163.1 ppm are attributed to the CdO in methylacrylate and TTA moieties. The chemical shifts in the ranges of 155.0-152.6, 126.5-125.6, and 122.1-228.9 ppm are attributed to C-O-C in the Ox ring, the phenyl carbon bonded to Ox, and the aromatic carbon in the Cz ring, respectively. The weak chemical shifts at 140.1/ 123.0, 128.1, 116.2, 62.8/52.7, and 41.1/34.8 ppm are assigned to the aromatic carbons of Phen, thenoyl carbons, -CF3 of TTA, the linker carbons (-NCH2CH2O-) in Cz moiety, and the tertbutyl carbons in Ox moiety, respectively. The strong chemical shifts at 30.8 and 17.8 ppm are attributable to the aliphatic carbon atoms in the copolymer backbone. Figure 1 shows the UV-visible absorption and fluorescence spectra of PCzOxEu in dilute THF solution. The UV-visible spectrum (dash curve) consists of three main absorption bands and two shoulders in the long wavelength region. When the excitation wavelength is in the range of main absorption bands (e.g., λex ) 294 nm), only two fluorescence peaks at 350 and 365 nm (dotted curve) are detected. These emissions can be attributed to the π* f p transitions of the carbazole and oxadiazole moieties,31 respectively. However, when excited at a wavelength of 348 nm (the shoulder in the UV-visible absorption spectrum), another group of fluorescence peaks at 577, 592, and 611 nm is detected. These emission peaks are attributable to the f-f transitions of the europium ions.30 Because the 4f levels of europium ions are protected from environmental perturbations by the filled 5s2 and 5p6 orbitals (Eu3+: [Xe]4f 65s25p6), the fluorescence peaks of europium ions are expected to be sharp and narrow. Thus, the presence of europium complexes in PCzOxEu is further evidenced by the characteristic fluorescence from the europium ions. The chemical composition of PCzOxEu was determined by elemental analysis and X-ray photoelectron spectroscopy (XPS). Figure 2 shows the respective C 1s and N 1s core-level spectra of the Eu-complexed monomer (M3) and PCzOxEu. The C 1s core-level spectrum of M3 can be curve fitted with peak components (fwhm or full-width at half-maximum of 1.4 eV), having binding energies (BE’s) at 284.6 eV for the C-H/C-C species of the aliphatic carbon atoms, at 285.4 eV for the C-S species of the thiophene ring,32 at 285.8 eV for the C-N species of phenanthroline (phen), at 286.6 eV for the dC-O species of thenoyltrifluoroacetone (TTA) (chelated to the Eu ion), at 288.0 eV for the O-CdO species of the methacrylic ligand (chelated to the Eu ion),33,34 at 289.0 eV for the F3C-CdO species of TTA (chelated to the Eu ion), and at 292.2 eV for the F3C-C species of TTA (chelated to the Eu ion) and the π

Figure 2. XPS C 1s and N 1s core-level spectra of the Eu-complexed monomer (M3) and the copolymer PCzOxEu.

f p* shake-up satellites.35 The C 1s core-level spectrum of PCzOxEu can be curved-fitted with four peak components. The main peak component with BE at about 284.6 eV can be attributed to the C-H/C-C species of the aliphatic carbon atoms. The other minor peak components at the BE’s of 285.9, 286.9, and 288.9 eV can be attributed respectively to the -C-N species of the carbazole group (Cz), the -CdN species of the oxadiazole group (Ox), and the O-CdO species contributed predominantly by the methacrylate moieties.33,34 Because PCzOxEu contains only a small fraction of M3, the peak components associated with the C 1s species of M3 are obscured by other peak components. The N 1s core-level spectra provide additional information on the chemical structure of PCzOxEu. The N 1s core-level spectrum of M3 shows a single peak component at the BE of 399.0 eV. It is associated with the dN- species of the phen ligand chelated to the europium ion. The N 1s core-level spectrum of PCzOxEu consists of three peak components with BE’s at 399.0, 399.4, and 399.9 eV. The minor peak component at 399.0 eV is attributable to the dN- species of the phen ligand in the complex moiety. In comparison with the BE’s of the corresponding species in pure 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) and poly(N-vinylcarbazole) (PVK),34 the other two peak components are assigned to the -Nd species of the 1,3,4-oxadiazole group (Ox) and the -Nspecies of the carbazole group (Cz). The composition (x:y:z molar ratio) of the PCzOxEu copolymer, estimated from the area ratio of the three N 1s peak components associated with the respective carbazole, europium complex, and 1,3,4-oxadiazole moieties in the copolymer and after taking into account the nitrogen stoichiometry in each monomer, is about 1:0.07: 1.08. This composition is in agreement with that obtained from the elemental analysis results of PCzOxEu (corresponding to a x:z:y ratio of 1:0.08:1.1). However, the copolymer composition differs somewhat from the M1:M3:M2 molar feed ratio of 1:0.1: 1, indicating the presence of a small difference in reactivity among the monomers. 3.2. Memory Effects. The memory effects of PCzOxEu are demonstrated by the current-voltage (I-V) characteristics of an ITO/PCzOxEu/Al sandwich device (Figure 3). I increases progressively with the applied bias (Stage I). A sharp decrease in injection current occurs at about 4.4 V (Stage II), indicating the transition of the device from a high conductivity state (ON state) to a low conductivity state (OFF state). The transition

Electrical Switching and Memory Effects in Copolymers

Figure 3. Current-voltage characteristics of an ITO/PCzOxEu/Al device based on a spin-cast film of PCzOxEu (∼50 nm) for two sweep directions. Arrows indicate the sweep direction of the applied voltage.

Figure 4. The ON- to OFF-state current ratio as a function of applied voltage for the same sweep.

from the ON state to the OFF state is equivalent to the “writing” process in a digital memory cell. After this transition, the device remains in this state when swept between -2.7 and +4.8 V, as well as after the power has been turned off (Stage III). The I-V characteristics define the electrical bistability of PCzOxEu and also reveal the nonvolatile nature of the memory effect. One of the most important features of the present copolymer is that the OFF state can be recovered by the simple application of a reverse voltage pulse (at about -2.8 V, Stage IV). This is equivalent to the “erasing” process of a digital memory cell. The difference in transition voltages between the ON and OFF states is likely to be due to the difference in barrier heights of ITO/PCzOxEu and Al/PCzOxEu. Stage V shows the I-V characteristics of the device after application of a -2.8 V bias. The behavior is nearly identical with that of Stage I. This feature allows the application of PCzOxEu in a rewritable memory device. 3.3. Memory Performance. In addition to the rewritable capability, other parameters of importance to the performance of a memory device include read cycles, ON/OFF current ratios, retention ability, and switching time. These parameters were evaluated under ambient conditions. Figure 4 shows the ON/ OFF current ratio as a function of the applied voltage for the same sweep. An ON/OFF current ratio as high as 105 has been achieved in the memory device based on PCzOxEu. The high ON/OFF current ratio in the present device promises a low misreading rate through the precise control of the ON and OFF states. As shown in Figure 5, no resistance degradation is observed for both the ON and OFF states after more than one million (106) read cycles at 1.0 V. The ability of PCzOxEu to retain the two states was tested under ambient conditions. As also shown in Figure 6, there is no significant degradation of the device in both the ON and

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Figure 5. Effect of 1-V read pulses on the device resistance in the OFF state and ON state. The inset shows the pulses used for the measurements.

Figure 6. Effect of operation time (at 1 V) on the device resistance in the OFF state and ON state tested under ambient conditions.

Figure 7. Transient response of current vs. time, showing a short switching time from ON to OFF state. The inset shows the corresponding circuit used in the measurement.

OFF states after 8 h of the continuous stress test, indicative of the stability of both the material and the electrode/polymer interfaces. It is expected that the retention time of the device can be significantly improved when the device is properly encapsulated. Another crucial performance indicator for a memory device is the switching time. Figure 7 shows the transient response of current vs. time. The inset in Figure 7 shows the circuit layout used in the measurement. The use of double-wire cable connection to the device ensured that the voltage source and the oscilloscope shared the same ground. The ITO/PCzOxEu/ Al device has a switching time of about 1.5 µs from the ON state to the OFF state. In this yet to be optimized single-layer architecture, the switching time is almost comparable to that (∼1 µs) of a NAND (NOT/AND) flash memory based on traditional semiconductors. Memory devices based on polymeric ferroelectric materials, with a switching time on the order of

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Figure 8. HOMO and LUMO energy levels for the components in PCzOxEu, with the Cz and Ox data obtained from DFT B3LYP/631G(d) molecular simulation, Eu data from those of a similar complex in the literature, and PCzOxEu data from cyclic voltammetry results. The inset shows the plausible electronic processes induced by an electric field.

50 µs, have been suggested for use in data storage.36 Thus, the switching response of a polymer memory based on PCzOxEu is probably fast enough for data storage (and even code storage) in many hand-held device applications.36 3.4. Memory Mechanism. The good reproducibility of the switching and rewritable memory phenomena in the present device and the dependence of current magnitude on the active area (0.4× 0.4, 0.2 × 0.2, and 0.15 × 0.15 mm2) of the device ruled out the filament effect and dielectric breakdown. A combination of high field and elevated sample temperature (due to Joule heating) probably have caused the formation of low conductivity charge transfer (CT) complexes, through donoracceptor interactions,37 across the sample between the electrodes. Such CT state, once formed, persists even after the removal of applied field. Thus, the memory state is retained. A reversal of the electric field breaks the stacks and decouples the CT complexes, causing the sample to switch back to its initial state.18a In comparison with that of the earlier flash-type memory based on a vinyl copolymer containing only the N-vinylcarbazole donor groups and the pendant Eu complex moieties (PKEu),18a the performance of the present memory device based on PCzOxEu, containing both the donor and acceptor groups, an ethyl acrylate spacer between the carbazole donor and the main chain, and the pendant Eu complex moieties, has improved significantly. A higher (by 1 order of magnitude) ON/OFF current ratio, longer (by a factor of 3) retention time of both ON and OFF states, and shorter (by 1 order of magnitude) response time have been achieved. To understand the role of the additional Ox electron acceptor in the copolymer of the present memory device, molecular simulation for the Cz and Ox components of PCzOxEu was carried out at the DFT B3LYP/6-31G(d) level with the Gaussian 03 program package.25 Figure 8 shows the corresponding energy levels for the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO)38 of Cz and Ox. Simulation for the Eu component was not carried out

Ling et al. because of the extreme complexity of the electronic structure of lanthanide complexes.39 The energy levels for the Eu component are taken from those of a similar europium complex, Eu(TTA)3phen, in the literature.40 The reliability of the simulation results is verified by the HOMO and LUMO energy levels of PCzOxEu, obtained from cyclic voltammetry (Figure 8). The energy band diagram in Figure 8 shows that the presence of the Ox component does not change the energy levels of PCzOxEu significantly. It acts as a mediator to facilitate carrier transport from the respective HOMOs and LUMOs of the Cz and Eu components. The mediation effect reduces internal energy barriers and accelerates hole transport from the HOMO of Cz and electron transport from the LUMO of Eu when a threshold voltage is applied. The inset in Figure 8 shows the plausible electronic processes induced by an electric field. This mediation effect thus accounts for the faster switching time (∼1.5 µs) in the present memory based on PCzOxEu. The presence of the ethyl acrylate spacer between the carbazole group and the polymer backbone leads to a less regioregular conformation of the carbazole groups. As a result, higher activation energy is required for the CT complex formation. This conformational effect explains the increase in write/erase voltages and ON/OFF current ratio for the present device, in comparison to those of the previous device based on PKEu.18a Also from the molecular simulation results, the dipole moments of Cz and Ox are 2.46 and 4.99 D, respectively. The large dipole moment of Ox in PCzOxEu favors the holding of the separated charges and accounts for the longer retention time and improved stability of the present memory device. Parallel studies on the memory effects of polymers lacking the Eu complex (PCzOx), the oxadiazole acceptor group (PKEu), or both the Ox acceptor group and the Eu complex (PCz) from the present PCzOxEu have been carried out and can serve as control experiments. While the memory effects of PKEu and PCz have already been reported in the literature,18a,41 the device fabricated from PCzOx (containing both the donor and acceptor groups) exhibits rewritable memory behavior. To further understand the transition from the high conductivity state to the low conductivity state in the present PCzOxEu memory device, the I-V curves in both states were analyzed in terms of theoretical models. For the high conductivity state, the I-V curve is linear and can be fitted with the Ohmic model (Figure 9a). When the threshold voltage was applied, low conductivity CT complexes formed around the Eu moieties, leading to an accumulation of space charges and a redistribution of the electric field. The Ohmic current changes to the space charge limited current (SCLC) of the form J ∝ 9iµV2/8d3,42 wherein µ is the carriers mobility, c is a positive constant independent of V or T, i is the dynamic permittivity of the insulator, and d is the film thickness. Figure 9b shows that the I-V characteristics of the low conductivity state can be fitted rather well by this model. 4. Conclusions A nonconjugated methacrylate copolymer (PCzOxEu), containing the carbazole moieties (electron donors), the 1,3,4oxadiazole moieties (electron acceptors), and the europium complexes in the pendant groups, was synthesized via free radical copolymerization of the methacrylate monomers containing the respective functional groups. A nonvolatile flash (rewritable) polymer memory, based on PCzOxEu in a simple metal/polymer/metal structure, was demonstrated. The simple structure exhibited two distinct and accessible conductivity states. It remained in either state even after the power had been

Electrical Switching and Memory Effects in Copolymers

Figure 9. Experimental and fitted I-V curves of the ITO/PCzOxEu/ Al device: (a) high conductivity state fitted with the Ohmic model and (b) low conductivity state fitted with the space-charge-limitedcurrent (SCLC) model.

turned off. It also exhibited a high ON/OFF current ratio, short response time, and respectable retention ability under ambient conditions. More than a million read cycles were performed on the device under ambient conditions and in the absence of device encapsulation. The roles of donor and acceptor components in the memory mechanism of the copolymer were elucidated through molecular simulation. The copolymer memory device distinguishes itself by its solution processability and the ability to form homogeneous thin films with good mechanical properties. Furthermore, polymer memory cells may be stacked to produce a 3-D architecture, which can translate into memory devices with several times the storage capacity of flash memories based on conventional semiconductors. Acknowledgment. The authors would like to thank the Agency for Science, Technology and Research (A*STAR) of Singapore for financial support under project grant R-052-1170027. References and Notes (1) Reichmanis, E.; Katz, H.; Kloc, C.; Maliakal A. Bell Labs Tech. J. 2005, 10, 87. (2) Forrest, S. R. Nature (London) 2004, 428, 911. (3) Hagen, J. A.; Li, W.; Steckl, J.; Grote, J. G. Appl. Phys. Lett. 2006, 88, 171109. (4) Lowry, M. S.; Goldsmith, J. I.; Slinker, J. D.; Rohl, R.; Pascal, R. A., Jr.; Malliaras, G. G.; Bernhard, S. Chem. Mater. 2005, 17, 5712. (5) Drolet, N.; Morin, J. F.; Leclerc, N.; Wakim, S.; Tao, Y.; Leclerc, M. AdV. Funct. Mater. 2005, 15 (10), 1671. (6) Polson, R. C.; Vardeny, Z. V. Appl. Phys. Lett. 2004, 85 (11), 1892. (7) Juan, Z.; Swager, T. M. AdV. Polym. Sci. 2005, 177, 151. (8) Hasobe, T.; Fukuzumi, S.; Kamat, P. V. Angew. Chem., Int. Ed. 2006, 45, 755. (9) Roberson, L. B.; Poggi, M. A.; Kowalik, J.; Smestad, G. P.; Bottomley, L. A.; Tolbert, L. M. Coord. Chem. ReV. 2004, 248, 1491. (10) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature (London) 2002, 420, 800. (11) Stikeman, A. Technol. ReV. 2002, 105, 31. (12) Moller, S.; Perlov, C.; Jackson, W.; Taussig, C.; Forrest, S. R. Nature 2003, 426, 166. (13) Taylor, D. M.; Mills, C. A. J. Appl. Phys. 2001, 90, 306. (14) Bandyopadhyay, A.; Pal, A. J. AdV. Mater. 2003, 15, 1949. (15) Moller, S.; Forrest, S. R.; Perlov, C.; Jackson, W.; Taussig, C. J. Appl. Phys. 2003, 94, 7811.

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