Nonvolatile Polymer Memory Device Based on Bistable Electrical

Aug 4, 2006 - Nonvolatile Polymer Memory Device Based on Bistable Electrical Switching in a Thin Film of Poly(N-vinylcarbazole) with Covalently Bonded...
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Langmuir 2007, 23, 312-319

Nonvolatile Polymer Memory Device Based on Bistable Electrical Switching in a Thin Film of Poly(N-vinylcarbazole) with Covalently Bonded C60† Qi-Dan Ling,‡ Siew-Lay Lim,‡ Yan Song,§ Chun-Xiang Zhu,§ Daniel Siu-Hhung Chan,§ En-Tang Kang,*,‡ and Koon-Gee Neoh‡ Department of Chemical & Biomolecular Engineering, and SNDL, Department of Electrical & Computer Engineering, National UniVersity of Singapore, Kent Ridge, Singapore 119260 ReceiVed May 27, 2006. In Final Form: July 7, 2006 A functional polymer (PVK-C60), containing carbazole moieties (electron donors) and fullerene moieties (electronacceptors) in a molar ratio of about 100:1, was synthesized via covalent tethering of C60 to poly(N-vinylcarbazole) (PVK). The molecular structure and composition of PVK-C60 were characterized by FTIR, Raman, and UV-vis absorption spectroscopy, gel permeation chromatography (GPC), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CyV). The C60-modified PVK exhibited an enhanced glass-transition temperature (Tg ≈ 226 °C) and good solubility in organic solvents such as toluene, tetrahydrofuran, chloroform, and N,N-dimethylformamide (DMF). It could be cast into transparent films from solutions. For a thin film of PVK-C60 sandwiched between an indium tin oxide (ITO) electrode and an Al electrode (ITO/PVK-C60/Al), the device behaved as nonvolatile flash (rewritable) memory with accessible electronic states that could be written, read, and erased. The polymer memory exhibited an ON/OFF current ratio of more than 105 and write/erase voltages around -2.8 V/+3.0 V. Both the ON and OFF states were stable under a constant voltage stress of -1 V for 12 h and survived up to 108 read cycles at -1 V under ambient conditions.

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 transistors,4 lasers,5 sensors,6 solar cells,7 and switches,8 have been realized. Recently, the properties of flash (rewritable) and WORM (write once, read many times) memories based on polymeric materials have been demonstrated.9,10 They exhibit simplicity in structure, drive-free read and write capability, 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 silicon devices. Rather than encoding “0” and “1” from the number of charges stored in a cell, a polymer memory stores data on the basis of the high and low conductivity responses to an applied voltage.9 In other pioneering work on polymer memory effects,11 polymers were †

Part of the Stimuli-Responsive Materials: Polymers, Colloids, and Multicomponent Systems special issue. * To whom all correspondence should be addressed. E-mail: cheket@ nus.edu.sg. Tel: +65-6516-2189. Fax: +65-779-1936. ‡ Department of Chemical & Biomolecular Engineering. § SNDL, Department of Electrical & Computer Engineering. (1) Reichmanis, E.; Katz, H.; Kloc, C.; Maliakal A. AT&T Bell Lab. Techn. J. 2005, 10, 87. (2) Forrest, S. R. Nature 2004, 428, 911. (3) Hagen, J. A.; Li, W.; Steckl, J.; Grote, J. G. Appl. Phys. Lett. 2006, 88, 171109. (4) Drolet, N.; Morin, J. F.; Leclerc, N.; Wakim, S.; Tao, Y.; Leclerc, M. AdV. Funct. Mater. 2005, 15, 1671. (5) Polson, R. C.; Vardeny, Z. V. Appl. Phys. Lett. 2004, 85, 1892. (6) Juan, Z.; Swager, T. M. AdV. Polym. Sci. 2005, 177, 151. (7) Roberson, L. B.; Poggi, M. A.; Kowalik, J.; Smestad, G. P.; Bottomley, L. A.; Tolbert, L. M. Coord. Chem. ReV. 2004, 248, 1491. (8) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800. (9) Stikeman, A. Technol. ReV. 2002, 105, 31. (10) Moller, S.; Perlov, C.; Jackson, W.; Taussig, C.; Forrest, S. R. Nature 2003, 426, 166. (11) Taylor, D. M.; Mills, C. A. J. Appl. Phys. 2001, 90, 306.

used as polyelectrolytes12,13 or as matrixes for dyes,14 nanoparticles,15 and fullerenes16,17 in a doped 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 are desirable for memory device applications. The molecular design-cum-synthesis approach has allowed the development of several types of polymer memory, including flash memory,18 WORM memory,19 and dynamic random access memory (DRAM),20 to be realized. In this work, poly(N-vinylcarbazole) with covalently attached fullerene (C60) (PVK-C60) has been synthesized and characterized (Scheme 1). In this modified polymer, the carbazole group serves as the electron donor and hole-transporting moiety,21 and C60 serves as the electron acceptor.22 Nonvolatile flash memory effects were demonstrated in a metal/PVK-C60/metal sandwich structure. (12) Bandyopadhyay, A.; Pal, A. J. AdV. Mater. 2003, 15, 1949. (13) Moller, S.; Forrest, S. R.; Perlov, C.; Jackson, W.; Taussig, C. J. Appl. Phys. 2003, 94, 7811. (14) Vorotyntsev, M. A.; Skompska, M.; Pousson, E.; Goux, J.; Moise, C. J. Electroanal. Chem. 2003, 552, 307. (15) Ouyang, J.; Chu, C. W.; Szmanda, C. R.; Ma, L. P.; Yang, Y. Nat. Mater. 2004, 3, 918. (16) Majumdar, H. S.; Baral, J. K.; Osterbacka, R.; Ikkala, O.; Stubb, H. Org. Electron. 2005, 6, 188. (17) Paul, S.; Kanwal, A.; Chhowalla, M. Nanotechnology 2005, 17, 145. (18) Ling, Q. D.; Song, Y.; Ding, S. J.; Zhu, C. X.; Chan, D. S. H.; Kwong, D. L.; Kang, E. T.; Neoh, K. G. AdV. Mater. 2005, 17, 455. (19) (a) Ling, Q. D.; Song, Y.; Teo, E. Y. H.; Lim, S. L.; Zhu, C. X.; Chan, D. S. H.; Kwong, D. L.; Kang, E. T.; Neoh, K. G. Electrochem. Solid-State Lett. 2006, 9(8), G268. (b) Song, Y.; Ling, Q. D.; Zhu, C. X.; Kang, E. T.; Chan, D. S. H.; Wang, Y. H.; Kwong, D. L. IEEE Electron DeVice Lett. 2006, 27 (3), 154. (c) Teo, E. Y. H.; Ling, Q. D.; Song, Y.; Tan, Y. P.; Wang, W.; Kang, E. T.; Chan, D. S. H.; Zhu, C. X. Org. Electron. 2006, 7, 173. (20) Ling, Q. D.; Song, Y.; Lim, S. L.; Teo, E. Y. H.; Tan, Y. P.; Zhu, C. X.; Chan, D. S. H.; Kwong, D. L.; Kang, E. T.; Neoh, K. G. Angew. Chem., Int. Ed. 2006, 45, 2947. (21) Walsh, C. A.; Burland, D. M. Chem. Phys. Lett. 1992, 195, 309. (22) Martin, N.; Sanchez, L.; Illescas, B.; Gonzalez, S.; Herranz, M. A.; Guldi, D. M. Carbon 2000, 38, 1577.

10.1021/la061504z CCC: $37.00 © 2007 American Chemical Society Published on Web 08/04/2006

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Langmuir, Vol. 23, No. 1, 2007 313 Scheme 1

a Synthetic route and molecular structure of PVK-C containing about 4 wt % C or x/y ≈ 0.99:0.01. bSchematic diagram of the polymer 60 60 memory device consisting of a thin film of PVK-C60 sandwiched between an ITO bottom electrode and an aluminum top electrode.

2. Experimental Section 2.1. Instrumentation. Fourier transform infrared (FTIR) spectra were recorded on a Bio-Rad FTS 165 spectrophotometer by dispersing the samples in KBr pellets. Raman spectra were measured on a Bruker Equinox 55 spectrometer. The power of the laser (1064 nm) was set to 300 mW for all measurements. UV-vis absorption spectra were measured on a Shimadzu UV 3101PC spectrophotometer, using an integrating sphere, ISR-260, coated on the inside with a diffusely reflecting material, BaSO4. Elemental microanalysis (for C, H, and N) was performed on a Perkin-Elmer 2400 elemental analyzer. Thermogravimetric analysis (TGA) was conducted on a TA Instruments TGA 2050 thermogravimetric analyzer at a heating rate of 20 °C/min and under an air flow rate of 75 mL/min. Differential scanning calorimetry (DSC) measurements were carried out on the Mettler Toledo DSC 822e system under N2 and at a heating rate of 10 °C/min. Gel permeation chromatography (GPC) measurements were conducted on an HP 1100 HPLC system equipped with an HP 1047A RI detector and Agilent 7991 columns. Polystyrene standards were used as the molecular weight references, and THF was used as the eluent. Cyclic voltammetry (CyV) measurement was performed on an Autolab potentiostat/galvanostat system using a threeelectrode 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 electrodes, respectively. XPS measurements were carried out 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 a 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. N-Vinylcarbazole (NVK), sodium hydride, tetrabutylammonium hexafluorophosphate (n-Bu4NPF6), and C60 of high purity (>99.9%) were purchased from Aldrich Chemical Co. and were used as received. Azobisisobutyronitrile (AIBN) was purchased from Kanto Chemical Co. It was recrystallized from methanol at 40 °C, dried in a vacuum oven, and stored under an Ar atmosphere. Acetonitrile was dried over molecular sieve (4 Å). Toluene and tetrahydrofuran (THF) were purified by refluxing over sodium in the presence of benzophenone until a persistent blue color appeared and were then distilled prior to use. Other sol-

vents were of GR or HPLC grade and were purchased from Merck Schuchardt Chemical Co. They were used without further purification. 2.3. Synthesis of PVK-C60. All of the glassware was prebaked at 120 °C, cooled under vacuum, and purged with argon. The starting polymer, poly(N-vinylcarbazole), or PVK, was prepared via conventional free-radical polymerization of N-vinylcarbazole (1.93 g, 10 mmol) with AIBN (8 mg, 0.05 mmol) as the initiator in dried THF (10 mL) and purified by Soxhlet extraction with boiling acetone. PVK-C60 was prepared (Scheme 1) according to a similar method in the literature.23 A mixture of PVK (1.0 g) and sodium hydride (0.2 g, 8.3 mmol) in 30 mL of dried THF was stirred at room temperature for 24 h under an argon atmosphere. Forty milligrams (0.06 mmol) of C60 was dissolved in 60 mL of dried toluene to form a clear, purple solution. The solution was added dropwise to the reactor over a period of 2 h, followed by vigorous stirring at room temperature for 72 h. The resulting solution was precipitated into an excess amount of methanol under vigorous stirring. The precipitate was collected by filtration and washed with a mixed solvent of methanol and benzene (100:1 v/v). The polymer was redissolved in THF and filtered to remove any unreacted C60 because the solubility of C60 in THF was reported to be negligible.24 The PVK-C60 was reprecipitated from methanol to afford a gray solid, which was dried at 60 °C under reduced pressure overnight. Yield: 0.37 g (35.6%). 2.4. Fabrication of Memory Devices. The indium tin oxide (ITO) glass substrate was precleaned sequentially with water, acetone, and 2-propanolin an ultrasonic bath for 15 min. A toluene solution of PVK-C60 (10 mg/mL) was spin-coated onto the ITO substrate, followed by solvent removal in a vacuum chamber at 10-5 Torr under room temperature. The thickness of the polymer layer was about 50 nm. Finally, a 400-nm-thick Al top electrode was thermally evaporated at a pressure of around 10-7 Torr through a shadow mask. The measurements were carried out on devices of 0.4 × 0.4, 0.2 × 0.2, and 0.15 × 0.15 mm2 in size (Scheme 1). The current density-voltage (J-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 an HP 4156A semiconductor parameter analyzer and an Agilent Infiniium oscilloscope under ambient conditions. (23) Chen, Y.; Chen, S. M.; Xiao, L. X.; Cai, R. F.; Huang, Z. E.; Yan, X. M.; Pan, C. C.; Jin, W.; Wang, S. T. J. Mater. Sci. 1998, 33, 2061. (24) Patil, A. O.; Schriver, G. W.; Lundberg, R. D. Polym. Prepr. 1993, 34, 592.

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2.5. Molecular Simulation. Molecular simulations of the basic unit of PVK-C60 were carried out using the Gaussian 03 (revision D.01) program package25 on an HP XW6200 workstation with two CPUs and 3 GB of 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 6-31G basis set with the d function added to heavy atoms (in short, DFT B3LYP/6-31G(d)).

3. Results and Discussion 3.1. Characterization of PVK-C60. PVK-C60 was characterized by gel permeation chromatograph (GPC) and FTIR, Raman, UV-vis, and X-ray photoelectron spectroscopy. PVKC60 is soluble in common organic solvents, such as THF, toluene, chloroform, carbon disulfide, N,N-dimethylacetamide (DMAc), and N,N-dimethylformamide (DMF). It can be cast into transparent, uniform thin films from solutions by spin-coating. GPC measurement indicated that the weight-average molecular weight (Mw) of the starting polymer, PVK, was 55 910, with a polydispersity index (PDI) of about 1.98. After the covalent attachment of C60, the Mw of the resulting polymer, PVK-C60, increased to 58 259, with a comparable PDI of about 1.95. The weight percentage of C60 in PVK-C60 is about 4 wt %, which is comparable to the weight percentage of C60 in the reactant feed. Thus, there is approximately 1 fullerene per 100 vinylcarbazole repeat units. The composition was further verified by elemental analysis (Anal. calcd for PVK-C60 (x ) 0.99, y ) 0.01, wt %): C, 87.47, H, 5.49, N, 6.99. Found: C, 87.58, H, 5.42, N, 7.00.) As revealed by experimental results (see below), this composition of PVK-C60 can provide the required electronic properties for nonvolatile flash memory. A lower ratio of C60 will result in an insufficient number of acceptors to hold the charges, whereas a higher ratio will cause charge tunneling through the C60 moieties.16 The FTIR spectrum of PVK-C60 (Figure 1a) shows the characteristic absorption bands at 751 and 725 cm-1 (doublet) for the carbazole group.26 In addition to vibration absorptions of alkyl and aryl groups in PVK, two absorption peaks at 570 and 525 cm-1 associated with the C60 stretching vibrations27 are also discernible. The other two characteristic absorption bands of C60 at 1411 and 1170 cm-1 are buried under those of the carbazole species. Because of the extremely high symmetry of the molecule, C60 has only four FTIR absorption bands.28 However, it is Raman-active with characteristic and intensive Raman peaks at 270, 496, 772, 1468, and 1574 cm-1.28 These Raman shifts are detected in PVK-C60 but not in PVK (Figure 1b). The UV-vis absorption spectra of C60, PVK, and PVK-C60 are shown in Figure 2a. The spectrum of C60 consists of two (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.01; Gaussian, Inc.: Wallingford, CT, 2004. (26) Wainwright, M.; Griffths, J. J.; Guthrie, T.; Gates, A. P.; Murry, D. E. J. Appl. Polym. Sci. 1992, 44, 1179. (27) Frum, C. I.; Engleman, R., Jr.; Hedderich, H. G.; Bernath, P. F.; Lamb, L. D.; Huffman, D. R. Chem. Phys. Lett. 1991, 176, 504. (28) Nakamoto, K.; McKinney, M. A. J. Chem. Educ. 2000, 77, 775.

Figure 1. (a) Comparison of the FTIR spectra of PVK-C60 and C60. (b) Comparison of the Raman spectra of PVK-C60, pure C60, and PVK.

main absorption peaks at 281 and 352 nm, with the absorption edge extending through the entire visible spectrum.29,30 The unmodified PVK film exhibits three strong absorption bands at 297, 330, and 345 nm (corresponding to the 1A f 1La and 1A f 1L transitions), with the absorption edge at about 380 nm.31 b PVK-C60 shows an additional absorption peak at 261 nm that is attributable to the 1A f 1Ba transition. The absorption edge of PVK-C60 has also been extended into the long-wavelength range. Figure 2b shows the respective XPS C 1s and N 1s corelevel spectra of PVK-C60. The C 1s core-level spectrum can be curve fitted with peak components (fwhm of 1.1 eV) having binding energies (BEs) at 284.6 eV for the C-H/C-C species of the aliphatic and C60 carbon atoms32 and at 285.9 eV for the -C-N species of the carbazole group.33 The N 1s core-level spectrum of PVK-C60 shows a single peak component at a BE of 400.0 eV, with a fwhm of 1.0. It is associated with the -Nspecies of the carbazole group.33 The area ratio of the C 1s and N 1s spectra in PVK-C60 is greater than the theoretical ratio of 14:1 for pure PVK, consistent with the successful incorporation of C60. 3.2. Thermal and Electrochemical Properties. The thermal properties of PVK before and after chemical modification with C60 are compared in Figure 3a. The glass-transition temperature (Tg) of pristine PVK is about 207 °C. After the covalent attachment of C60, the Tg of PVK-C60 has increased to about 226 °C. Because C60 is quite bulky, the mobility of the polymer chain is restricted because of steric hindrance, resulting in an increase in Tg. The (29) Wang, Y. Nature 1992, 356, 585. (30) Chen, Y.; Huang, Z. E.; Cai, R. F.; Kong, S. Q. J. Instrum. Anal. 1995, 14, 41. (31) Klo¨pffer, W. Introduction to Polymer Spectroscopy; Springer-Verlag: Berlin, 1984; Chapter 3. (32) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. The Handbook of X-ray Photoelectron Spectroscopy, 2nd ed.; Perkin-Elmer Corporation (Physical Electronics): Wellesley, MA, 1992; p 216. (33) Beamson, G.; Briggs, D. High-Resolution XPS of Organic PolymersThe Scienta ESCA300 Database; Wiley: Chichester, England, 1992; p 224.

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Figure 2. (a) Comparison of the UV-vis absorption spectra of C60, PVK, and PVK-C60 measured in the solid state using an integrating sphere. (b) XPS C 1s and N 1s core-level spectra of PVK-C60.

Tg value observed for PVK-C60 is also much higher than for many of the soluble conducting polymers (Tg ≈ 50-100 °C).34 The relatively high Tg of the present polymer-C60 hybrid will be advantageous to its application as the active material in an electronic device. The thermal stability of PVK-C60 was studied by thermogravimetric analysis (TGA) in air. The temperatures for the onset of decomposition (Td) of PVK and PVK-C60 are about 435 and 457 °C, respectively. These result indicate that the thermal stability of PVK has been enhanced by chemical modification with C60. Pure C60 underwent only minor weight loss up to 800 °C. Organic memory devices operating under a high bias typically have limited lifetimes, arising from thermal and electric field-induced degradation. PVK-C60 exhibits excellent thermal stability, which will have a bearing on the device stability. Above 600 °C, there was no residue remaining for unmodified PVK. However, some residues remained even when PVK-C60 was heated to 800 °C in air. The percentage of the residues, in the form of charred materials, was about 3.4 wt %. This value is comparable to the weight percentage of C60 (4 wt %) in PVK-C60 because C60 undergoes only minor weight loss up to 800 °C.35 Matching the valence band (or the highest occupied molecular orbital (HOMO)) and conduction band (or the lowest unoccupied molecular orbital (LUMO)) energy levels of the active material to the work functions of the corresponding cathode and anode is important to the performance of a memory device. Cyclic voltammetry (CyV) is an effective method for exploring the relative ionization and reduction potentials. Figure 3b shows the CyV sweep of both the p-doping and n-doping processes of a

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Figure 3. (a) TGA and DSC (inset) traces of PVK-C60 and PVK (TGA was measured in air at a heating rate of 20 °C/min and DSC was measured in nitrogen at a heating rate of 10 °C/min). (b) Cyclic voltammetry sweep of PVK-C60 in 0.1 M n-Bu4NPF6/acetonitrile (scan rate: 0.1 V/s).

PVK-C60 film on platinum. During the anodical scan, PVKC60 exhibits quasi-reversible p-doping behavior. The strong oxidation peak appears at about 1.4 V vs Ag/AgCl, with a weak reduction peak at about 0.1 V when scanned backward. The results suggest that the carbazole group has a high tendency to donate electrons and, as expected, is an efficient hole-transport site. When scanned cathodically, the PVK-C60 film shows irreversible n-doping behavior with a reduction peak at around -1.2 V vs Ag/AgCl. This cathodic peak is not detected in the unmodified PVK film. It is thus attributable to the electronwithdrawing effect of the C60 moieties. The HOMO and LUMO energy levels of PVK-C60 can be calculated from the onset oxidation potential (EOx(onset)) and the onset reduction potential (ERed(onset)) based on the reference energy level of ferrocene (4.8 eV below the vacuum level, which is defined as zero)36,37

HOMO) -[(EOx(onset) - EFoc) + 4.8] (eV) LUMO) -[(ERed(onset) - EFoc) + 4.8] (eV) wherein EFoc is the potential of the external standard, the ferrocene/ (34) Wudl, F.; Hoger, S. U.S. Patent 5,679,757, 1997. (35) Hawker, C. J. Macromolecules 1994, 21, 4836. (36) Bredas, J. L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J. Am. Chem. Soc. 1983, 105, 6555.

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Figure 4. Comparison of the HOMO energy level and band gap of a model compound, 9,9′-(pentane-2,4-diyl)bis(9H-carbazole), which is similar to two repeat units of PVK, from various simulation methods with those obtained from the respective cyclic voltammetry (CyV) and UV-vis absorption spectrum of PVK.

ferricenium ion (Foc/Foc+) couple. The value of EFoc, determined under the same conditions as for PVK-C60, is about 0.38 V vs Ag/AgCl. EOx(onset) and ERed(onset) for PVK-C60 are +1.15 and -0.86 V vs Ag/AgCl, respectively. Thus, the HOMO and LUMO energy levels of PVK-C60 relative to the vacuum level are estimated to be -5.57 and -3.56 eV, respectively. These results indicate that the HOMO and LUMO energy levels of PVK-C60 and the respective work functions of ITO (-4.80 eV) and Al (-4.28 eV) with a similar energy barrier of about 0.7 eV match well. 3.3. Molecular Simulation. Some physical properties, such as molecular orbitals, energy levels, charge densities, dipole moments, and electrostatic potentials, of a molecule can be conveniently obtained by computation. To select a suitable method of simulating the current macromolecular system containing C60, several calculation methods for a model compound, 9,9′-(pentane2,4-diyl)bis(9H-carbazole), which is similar to two repeat units of PVK, were evaluated. The basis set 6-31G with the d function is used, whenever possible, in the evaluation. The calculated HOMO levels and band gaps are compared to the experimental results from cyclic voltammetry (CyV), and the UV-vis absorption spectrum of PVK in Figure 4. The molecular mechanics (MM2 force field) gave the worst result because it does not use a wave function or the total electron density. Semiempirical methods (ETH, MNDO, AM1, and PM3) gave better results, albeit they still deviated considerably from the experiment results because of the lack of diffuse basis functions.38 In general, ab initio calculation can give reasonable qualitative results at the expense of an enormous amount of computational resources. When approximations were introduced, such as the HF method using the central field approximation, the energies from the HF calculation are always greater than the exact energy. Density functional theory (DFT) calculations are more accurate than other methods. The DFT B3LYP method with the 6-31G(d) basis set is the method of choice for the simulation of the PVKC60 components because it can give acceptable results with reasonable computational vigor. Simulation results of the PVK-C60 components, including one unit of PVK (VK), one C60, and a representative unit of PVK-C60 (VK-C60), are summarized in Table 1. VK has a lower HOMO energy level (value) than C60, indicating that the (37) Lee, Y. Z.; Chen, X. W.; Chen, S. A.; Wei, P. K.; Fann, W. S. J. Am. Chem. Soc. 2001, 123, 2296. (38) Grant, G. H.; Richards, W. G. Computational Chemistry; Oxford University Press: New York, 1995; p 5.

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VK group has a stronger tendency to donate an electron. The electron-withdrawing ability of C60 is indicated by a high LUMO energy level. It was reported that a C60 molecule could become negatively charged and could capture up to six electrons.39 When these two components are covalently connected, the HOMO of VK-C60 is located on the VK side with an increased HOMO energy level, whereas the LUMO of VK-C60 is located on the C60 side with a decreased LUMO energy level. These features promise easier charge transfer from VK (HOMO) to C60 (LUMO). The energy band gap of C60 is much lower than that of VK, indicating that C60 can become highly conductive once it is charged under an electric field. The band gap is further reduced in VK-C60. C60 is a nonpolar symmetric molecule, and VK is polar with a total dipole moment of 1.93 D. The total dipole moment of VK-C60 is 3.32 D, which is much larger than that of VK and almost comparable to that of a highly polar molecule, such as HCl (3.57 D). This phenomenon indicates that there is a high tendency for charge separation in VK-C60 and for VK-C60 to retain charges once induced by an electric field. Electrostatic potential surfaces further support the above conclusion. 3.4. Memory Effects. The memory effects of PVK-C60 are demonstrated by the current density-voltage (J-V) characteristics of an ITO/PVK-C60/Al sandwich device (Figure 5). Initially, the device was swept positively (with ITO as the cathode and Al as the anode) from 0 to 4 V. The device remained in the low-conductivity state without any abrupt increase in current density. However, when it was swept negatively (with ITO as the anode and Al as the cathode) from 0 V to -4 V, J increased progressively with the applied bias, and a sharp increase in the injection current occurred at about -2.8 V (the second sweep in Figure 5), indicating the device transition from a lowconductivity state (OFF state) to a high-conductivity state (ON state). The transition from the OFF state to the ON state is equivalent to the “writing” process in a digital memory cell. The device exhibited good stability in this high-conductivity state during the subsequent negative sweep (the third sweep). It remained in the ON state even after turning off the power (the fourth sweep). The J-V characteristics defined the electrical bistability of PVK-C60 and also revealed the nonvolatile nature of the memory effect. One of the most important features of the present polymer memory is that the ON state can be recovered by the simple application of a reverse voltage at about 3.0 V (the fifth sweep). This is equivalent to the “erasing” process of a digital memory cell. The sixth and seventh sweeps show the J-V characteristics of the device right after the application of an erase sweep and turning off the power, respectively. The J-V characteristics were nearly identical to those of the first sweep, indicating that the device remained in the stable OFF state. This feature allows the application of PVK-C60 in a rewritable memory device. The above J-V characteristics of the device were repeatable with high accuracy. Sweeps 8-10 are the J-V characteristics of the device after 20 write-read (ON)-eraseread (OFF) switching cycles. With the decrease in active area (0.16 f 0.04 f 0.0225 mm2) of the device, the current magnitude decreased accordingly. However, the current density remained almost constant, indicating the absence of sample degradation and dielectric breakdown. 3.5. Proposed Mechanism. For PVK-C60, the HOMO and LUMO energy levels are -5.57 and -3.56 eV, which are associated with the carbazole moiety and the fullerene, respectively, as determined from the onset redox potentials in cyclic (39) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, CA, 1996.

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Table 1. Summary of DFT B3LYP/6-31G(d) Molecular Simulation Results for the Components in PVK-C60

a Field-independent basis dipole moment. b Molecular electrostatic potential surface (mapped with the total charge density). c 1 Hartree ) 4.36 × 10-18 J.

voltammetry. For the first positive sweep, ITO (Φ (work function) ) 4.8 eV) was the cathode, and Al (Φ ) 4.28 eV) was the anode. The energy barriers of Al/PVK-C60 (LUMO) and PVK-C60 (HOMO)/ITO interfaces are 1.29 and 1.24 eV, respectively. However, when ITO was used as the anode and Al was used as the cathode (negative sweep), the energy barriers of ITO/PVKC60 (HOMO) and PVK-C60 (LUMO)/Al interfaces were reduced to 0.77 and 0.72 eV, respectively (Figure 6a). The relatively high energy barriers prevented switching of the device to the ON state during the positive sweep from 0 to 4 V. Charge injections are more favorable during the negative sweep. The high HOMO energy level of the carbazole moiety indicates that PVK-C60 is a p-type material with the holes as the dominant charge carriers because carbazole is the dominant moiety in PVK-C60. However, under a low bias voltage (0 to -2.8 V), hole mobility in PVKC60 is blocked by the C60 moieties with a low HOMO energy level (-6.01 eV, Table 1). The device is at the low conductivity state (OFF state). The small increase in current density probably

arises from the interchain hopping of charge carriers. When the electric field exceeds the barrier (∼0.7 eV), holes are injected into the HOMO of the carbazole moieties, and electrons are injected into the LUMO of C60. The charged HOMO of the carbazole moiety and the charged LUMO of C60 form a channel for charge carriers through charge-transfer interactions (illustrated in Figure 6b). The polymer becomes p-doped under the induction of the electric field and switches to the high-conductivity state (ON state). Because of the strong electron-withdrawing ability of C60 with a high LUMO and the strong dipole moment of VK-C60 (Table 1), electrons trapped in C60 can be retained and coexist with the surrounding positively charged carbazole moiety (Figure 6b). As a result of the dipole formation, an internal electric filed was created, maintaining the high-conductivity state. Under a reversed bias of about +3 V, C60 loses the charged state to neutralize the positively charged carbazole moiety. The internal electric field disappears immediately, and the device returns to its initial low-conductivity state (OFF state).

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Figure 5. Current density-voltage (J-V) characteristics of a memory device: ITO/PVK-C60/Al (0.4 × 0.4 mm2). The sweep sequence and direction are indicated by the number and arrow, respectively. The fourth and seventh sweeps were conducted after the power had been turned off. Sweeps 8-10 were obtained after 20 write-read(ON)-erase-read(OFF) switching cycles.

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Figure 7. Effect of operation time (at -1 V) on the device current density in the OFF and ON states tested under ambient conditions. The inset shows the ON- to OFF-state current ratio as a function of applied voltage for the same sweep.

Figure 8. Effect of -1 V read pulses on the device current density in the OFF and ON states. The inset shows the pulses used for the measurements.

Figure 6. (a) Energy band diagram of PVK-C60 (from CyV results) and its molecular components (from calculation results). (b) Plausible electronic processes in a molecule of VK-C60 producing the memory effects.

3.6. Performance of the Memory Device. In addition to the rewritable capability, other parameters of importance to the performance of a memory device include read cycles, ON/OFF current ratios, and retention ability. These parameters were evaluated under ambient conditions. The inset in Figure 7 shows the ON/OFF current ratio as a function of the applied voltage for the same sweep. An ON/OFF current ratio of more than 105

has been achieved in the memory device based on PVK-C60. 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. In addition to a low misreading rate, the stability is always an important issue in memory device performance. The ability of PVK-C60 to retain the two states was tested under a constant stress of -1 V. There was no significant degradation of the device in both the ON and OFF states after 12 h of the continuous stress test (Figure 7), 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. The effect of continuous read pulses of -1 V on the ON and OFF states was investigated. As shown in Figure 8, no degradation in current density was observed for the ON and OFF states after more than 100 million (108) read cycles, indicating that both states are insensitive to read cycles. The obvious advantage of the present system over the other physically doped systems lies in the fact the dopants in PVK-C60 are covalently bonded to the polymer chain. Thus, the phenomena of phase separation and aggregation can be avoided when the memory device is under long-term operation.

4. Conclusions A nonvolatile flash polymer memory device, based on poly(N-vinylcarbazole) with covalently tethered C60 (PVK-C60) in a simple metal/polymer/metal structure, was fabricated. The device exhibited two distinct and accessible conductivity states with write/erase voltages around -2.8 V/+3.0 V. It also exhibited

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a high ON/OFF current ratio and respectable retention ability under ambient conditions. More than 100 million read cycles were performed on the device under ambient conditions and in the absence of device encapsulation. The present polymer memory device distinguishes itself by its solution processability and the ability of the acceptor-modified polymer to form homogeneous thin films with good mechanical properties. Furthermore, polymer memory cells may be stacked to produce a 3-D architecture,

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which can translate into memory devices with several times the storage capacity of flash memory based on conventional semiconductors. Acknowledgment. We thank the Agency for Science, Technology and Research (A*STAR) of Singapore for financial support under project grant 052-117-0027. LA061504Z