Angewandte. A Dynamic Random Access Memory Based on a Conjugated Copolymer Containing Electron- Donor and -Acceptor Moieties

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1 Polymer-Based Dynamic Memory DOI: /anie A Dynamic Random Access Memory Based on a Conjugated Copolymer Containing Electron- Donor and -Acceptor Moieties Qi-Dan Ling, Yan Song, Siew-Lay Lim, Eric Yeow-Hwee Teo, Yoke-Ping Tan, Chunxiang Zhu, Daniel Siu Hhung Chan, Dim-Lee Kwong, En-Tang Kang,* andkoon-gee Neoh Electroactive organic and polymeric materials are alternatives to traditional Si, Ge, and GaAs semiconductors that have to face the problem of scaling-down in cell size. [1,2] Several types of organic electronics and devices, [3] including light-emitting diodes, [4] transistors, [5] lasers, [6] photovoltaic cells, [7] switches, [8] and memories, [9] have been realized. Recently, flash (rewritable) memory [10] and write-once readmany-times (WORM) memory [11] based on polymeric materials has been demonstrated. Polymer memories exhibit simple structures, good scalability, low-cost potential, 3D stacking capability, and a large capacity for data storage. [12] Rather than encoding 0 and 1 as the amount of charge stored in a cell, as in silicon-based devices, polymer memory stores data, for instance, based on the high and low conductivity response to an applied voltage. [13,14] In the pioneering work, the polymers were used as polyelectrolytes [15] and as matrices for dyes, [16] gold nano-particles, [17] or organic donor acceptor (DA) systems, [18] and we have demonstrated a flash memory and a WORM memory based on a nonconjugated and a conjugated copolymer, respectively, containing lanthanide complexes. [19,20] In this work, we report a novel memory device based on a conjugated copolymer (PFOxPy, structure shown in Figure 1a) that contains both electron-donor and -acceptor groups but no metal complex. In contrast to the nonvolatile flash and WORM memory devices based on polymers containing metal complexes, the device based on PFOxPy is Angew. Chem. Int. Ed. 2006, 45, [*] Dr. Q.-D. Ling, S.-L. Lim, Prof. E.-T. Kang, Prof. K.-G. Neoh Department of Chemical and Biomolecular Engineering National University of Singapore 10 Kent Ridge Singapore (Singapore) Fax: (+ 65) cheket@nus.edu.sg Y. Song, E. Y.-H. Teo, Y.-P. Tan, Prof. C. Zhu, Prof. D. S. H. Chan SNDL Department of Electrical and Computer Engineering National University of Singapore Singapore (Singapore) Prof. D.-L. Kwong Institute of Microelectronics 11 Science Park Road, Singapore Science Park II Singapore (Singapore) Supporting Information for this article is available on the WWW under or from the author Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2947

2 Communications Figure 1. a) Molecular structure of the conjugated copolymer PFOxPy (x/y = 0.81:0.19). The inset shows a schematic of the memory device. b) Typical J V characteristics of the ITO/PFOxPy/Al device in the ON and OFF states whilst maintaining the ON state by refreshing at 1V every 10 s. a volatile memory. However, the ON state can be electrically sustained by a refreshing pulse and reset to the initial OFF state by a reverse pulse. Thus, the device behaves as a dynamic random access memory (DRAM). This polymerbased DRAM device exhibits fairly good reliability and performance: 1) a high ON/OFF current ratio of up to 10 6 promises a low misreading rate, 2) both the ON and OFF states are stable up to 10 8 read cycles at a read voltage of 1.0 V, and 3) both states are stable at a constant voltage stress of 1.0 V. PFOxPy was synthesized by the Suzuki coupling copolymerization of the monomers 9,9-dihexyl-9H-fluorene-2,7- bis(trimethylene boronate) (M1), 2,7-dibromo-9,9-dihexyl- 9H-fluorene (M2), and 5,5 -bis(5-(4-bromo-3,5-bis(2-ethylhexyloxy)phenyl)-1,3,4-oxadiazol-2-yl)-2,2 -bipyridine (M3). The number-average molecular weight of PFOxPy is , with a polydispersity index of Because of its long alkyl and alkoxy substituents, PFOxPy is soluble in common organic solvents, such as THF, toluene and chloroform, and it can be spin-cast into uniform thin films from a toluene solution (10 mgml 1 ). The memory device has the simple structure of a PFOxPy film (approx. 50 nm thick) sandwiched between an indiumtin-oxide (ITO) bottom electrode and an aluminium top electrode (about 0.5 mm thick; see inset of Figure 1 a). The device has an active area of mm 2, as dictated by the shadow-mask used for electrode deposition. The memory effect of PFOxPy is shown by the current density voltage (J V) characteristics in Figure 1b. The device was swept negatively, with the sweep direction indicated by the arrows. Initially, the device was at its low-conductivity state (OFF state). The current density in this state is quite low (on the order of Acm 2 ). When a switching threshold voltage of about 2.8 V (onset) was applied, an abrupt increase in J from 10 7 to 10 2 Acm 2 was observed, indicating the transition from a low-conductivity state to a highconductivity state (ON state). This electrical transition serves as the writing process for the memory device. Subsequent forward and backward sweeps showed that the device remains in its high-conductivity state. The distinct bi-electrical states in the voltage range of about 0 to 2V allow a voltage (e.g., 1.0 V) to read the 0 or OFF signal (before writing) and 1 or ON (after writing) signal of the memory. As indicated in the second sweep, a reverse voltage pulse of about 3.5 V resets the memory device to the initial lowconductivity state ( 0 or OFF state), which corresponds to the erasing process for the memory device. The erased state ( 0 ) can be further written to the stored state ( 1 ) when the switching threshold voltage is applied, thereby indicating that the memory device is rewritable. The slightly higher turn-on voltage for the first sweep might be caused by an initial delay in conformational change of the polymer chain under the electric field. The third sweep was carried out after turning off the power for about two minutes. It was found that the ON state had relaxed to the steady OFF state. However, the device could be reprogrammed to the ON state. The short retention time of the ON state indicates that the memory device is volatile. However, the ON state can be electrically sustained by a refreshing voltage pulse of 1 V every 10 s (Figure 1b). The above characteristics are repeatable with high accuracy, and device degradation was not observed. This ability to write, read, erase, and refresh the electronic states of the device fulfils the functionality of a DRAM. In order to understand the memory behavior of the present PFOxPy device, the electronic properties of PFOxPy were studied by density functional theory (DFT). Calculations of the optimized geometry, total charge density, electrostatic potentials (ESP), and molecular orbitals of the basic unit (BU), which includes all the functional moieties and segments, namely the fluorene oligomer (four repeat units; F), the 2,2 -bipyridine moiety (Py), and the 1,3,4-oxadiazole moiety (Ox) of PFOxPy, were carried out at the B3LYP/6-31G(d) level with the Gaussian 03 program package. [21] The alkyl groups of PFOxPy were not included in the calculation since they do not significantly affect the equilibrium geometry and thus the electronic properties. The energy levels of the HOMO and LUMO for the BU and functional fragments of PFOxPy are summarized in Figure 2a, along with the work functions (F) of the ITO and Al electrodes. In comparison with the energy levels of the F fragment, the introduction of electron-withdrawing 2,2 -bipyridine and 1,3,4-oxadiazole units lowers slightly the HOMO and LUMO energy levels of Py and Ox, respectively. The lowest energy barrier is between the F of ITO and the HOMO (0.40 ev) of the F fragment, which suggests that hole injection from ITO into the HOMO of PFOxPy (corresponding to a negative bias or with ITO as the anode) is the favored process; electron Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2006, 45,

3 Figure 2. DFT molecular simulation results (B3LYP/6-31G(d) level): a) LUMO and HOMO energy levels for the basic unit (BU) and functional segments of PFOxPy along with the work function of the electrodes. The energy levels of the copolymer measured by cyclic voltammetry are indicated with an asterisk. b) LUMO, HOMO, and molecular electrostatic potential (ESP) surfaces of the BU of PFOxPy. injection from Al into the LUMO of PFOxPy will be much more difficult due the high energy barrier (1.97 ev). Thus, PFOxPy is a p-type material and holes dominate the conduction process. The LUMO and HOMO of BU show the delocalization in PFOxPy (Figure 2b). The molecular surface with continuous positive ESP (in grey) along the conjugated backbone indicates that charge carriers can migrate through this open channel. However, there are some negative ESP regions (dark) which arise from the electron-acceptor groups. These negative regions can serve as traps to block the mobility of charge carriers. The depth of these traps, which arise from the 1,3,4-oxadiazole electronacceptor moieties, is about 0.6 ev (Figure 2a). However, in the presence of incorporated 2,2 -bipyridine groups the depth of the traps is reduced to 0.3 ev. The traps are shallow, and filled traps can easily be detrapped. These shallow traps lead to low write/erase voltages and volatility of memory devices based on PFOxPy. The operational mechanism of the memory is depicted in Figure 3a. Under a low negative bias voltage (0 to 1.3 V, with ITO as the anode), the J V curve is linear and can be fitted with the Ohmic model, corresponding to a room-temperature resistance on the order of 10 9 W (Figure 3 b). At this stage the material is insulating and the device is in its OFF state. A Schottky barrier (0.4 ev) forms under bias. Since the material is p-type in the OFF state, this barrier will be located near the Figure 3. a) Operational mechanism of the memory. Experimental and fitted J V curves of the ITO/PFOxPy/Al device: b) OFF state with the Ohmic current (< 1.3 V) and SCLC ( V) models, and c) ON state with the Ohmic current model (> 2.8 V). anode. When the field exceeds the Schottky barrier, carrier generation (hole injection) occurs near the anode, leading to an accumulation of space charges and a redistribution of the electric field. The ohmic current changes to a space-chargelimited current (SCLC; bias V) of the form of Equation (1). [22] J / 9 e imv 2 8 d 3 ð1þ Herein, m is the carriers mobility, c is a positive constant independent of V or T, e i is the dynamic permittivity of the insulator, and d is the film thickness. Figure 3b shows that the J V curve can be fitted rather well by this model. Close to the turn-on voltage the generated carriers fill some of the charge traps and the Al cathode also becomes an electron-injecting contact (to the charged HOMOs or radical cations), which leads to double injection and thus enhanced carrier concentration and mobility; the current increases rapidly to switch the device to the high-conductivity state (ON state). In the ON state (> 2.8 V), the J V curve again follows the Ohmic model, with a room-temperature resistance on the order of 10 3 W (Figure 3 c). The conductivity in the ON state is about 10 6 times that in the OFF state and can be retained as long as the current flow is maintained. As indicated by the theoretical results, the traps in PFOxPy are shallow. This feature defines the volatile characteristic of the present memory device. A reversed voltage pulse causes detrapping of the filled traps; Angew. Chem. Int. Ed. 2006, 45, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

4 Communications turning off the power for a few minutes will have a similar effect. However, a refreshing pulse of 1 V every 10 s can replenish the traps and sustain the ON state. The performance of the DRAM device based on PFOxPy was evaluated under ambient conditions. Figure 4a shows the In summary, a polymer memory device based on the conjugated copolymer PFOxPy, which contains both electrondonor and -acceptor groups and with the sandwich structure ITO/PFOxPy/Al, allows us to write, read, erase, and refresh its electronic states. It fulfils the functionality of a dynamic random access memory (DRAM). After the device has been transformed to the ON state, it can be reset to the OFF state simply by turning off the power or by applying a reverse voltage pulse. The device also exhibits low reading, writing, and erasing voltages and a high ON/OFF current ratio of more than Both the ON and OFF states are stable under a constant voltage stress of 1.0 V and survive up to 10 8 read cycles at a read voltage of 1.0 V. In this as-yet unoptimized single-layer architecture, the switching speed is about an order of magnitude slower than that (approx. 1 ms) of a NAND (NOT/AND) memory based on traditional semiconductors. However, the polymer DRAM distinguishes itself from the conventional silicon-based memory technologies by its solution processability and potential for high-density data storage through 3D stacking. Figure 4. a) Stability of the ITO/PFOxPy/Al device in either ON or OFF state under a constant stress ( 1 V) and the ON/OFF current ratio for the same sweep. b) Effect of read cycles on the ON and OFF states. The inset shows the pulses used for this measurement. ratio of the ON- to OFF-state current as a function of the applied voltage for the same sweep. An ON/OFF current ratio of more than 10 6 is achieved for this memory device. This feature promises a low misreading rate due to the precise control of the ON and OFF states. In addition to a low misreading rate, stability is always an important issue in memory device performance. The stability of the device under a constant stress of 1.0 V is displayed by Figure 4a. No degradation in current density for the ON and OFF states was observed during the test. The effect of continuous read pulses (with a read voltage of 1.0 V) on the ON and OFF states was also investigated (Figure 4b). The inset in Figure 4b shows the pulses used for the measurements. No resistance degradation was observed for the ON and OFF states after more than one hundred million (10 8 ) read cycles. Neither the voltage stress nor the read pulses cause the state transition since the applied voltage ( 1 V) is lower than the switching threshold voltage (about 2.8 V) and can maintain the current flow in the ON state. Thus, both states are stable under the voltage stress and are insensitive to read pulses. The performance of the present polymer memory device compares favorably with that of current single-layer molecular (non-polymeric) memory devices. [23] Experimental Section M1 and M2 were prepared as described in the literature. [24] M3 was synthesized by reaction of methyl 4-bromo-3,5-bis(2-ethylhexyloxy)- benzoate [25] with hydrazine in the presence of methanol to form the corresponding hydrazide compound, [26] which was subsequently treated with 5,5 -bis(chlorocarbonyl)-2,2 -bipyridine. [27] The resulting compound was refluxed in excess phosphorus oxychloride to give M3 as a light-yellow powder. Yield: 67.2%. M.p C. C,H,N analysis (%) for C 58 H 78 Br 2 N 6 O 6 : calcd. C 62.47, H 7.05, N 7.54; found C 62.85, H 6.87, N MS (EI): m/z: 1115 [M] +, 1003, 890, 778, 666 (100%), 586, 437, 357, 112, 70, H NMR (CDCl 3, 300 MHz): d = 9.48 (s, 2H, Py-H), 8.74 (d, J = 8.2Hz, 2H, Py-H), 8.61 (d, J = 2.1 Hz, 2H, Py-H), 8.58 (s, 4H, Ar-H), 4.06 (d, J = 5.6 Hz, 8H, OCH 2 ), (m, 4H, CH), (m, 12H, CH 2 ), (m, 12H, CH 2 ), ppm (m, 24 H, CH 3 ). FT-IR (KBr pellet): ñ = 2960, 2931, 2872, 2855, 1602, 1585, 1539, 1460, 1428, 1383, 1353, 1317, 1238, 1113, 1034, 1014, 846, 741 cm 1. PFOxPy: Monomers M2 (0.39 g, 0.8 mmol) and M3 (0.45 g, 0.4 mmol) were dissolved in degassed toluene (3 ml) in a small Schlenk flask under argon. M1 (0.60 g, 1.2mmol) was mixed with mol% of [Pd(PPh 3 ) 4 ] and added to the flask. A degassed aqueous solution of potassium carbonate (3 ml, 2.0m) and toluene (2mL) were added to the reactor. The mixture was stirred vigorously at C for 72h under argon. The resulting solution was added dropwise, with stirring, to excess methanol to precipitate the polymer. The fibrous solid was collected by filtration and washed with methanol and water. The material was washed by Soxhlet extraction with acetone for 2d to remove oligomers and catalyst residues. The product was dried overnight at room temperature under reduced pressure. Yield: 0.87 g (83.4%). M n = , M w = , PDI (polydispersity index) = x = 0.81, y = 0.19 (calculated from the XPS composition data). 1 H NMR (CDCl 3, 300 MHz): d = 9.44 (s, Py- H), 8.72(d, J = 8.4 Hz, Py-H), 8.61 (d, J = 2.1 Hz, Py-H), 8.59 (d, J = 2.3 Hz, Ar-H), 8.55 (d, J = 2.1 Hz, Ar-H), 7.20 (s, Ar-H), 4.04 (d, J = 5.6 Hz, OCH 2 ), (m, CH), (m, CH 2 ), (m, CH 2 ), ppm (m, CH 3 ). FT-IR (KBr pellet): ñ = 2960, 2928, 2876, 2859, 1600, 1584, 1538, 1457, 1426, 1383, 1352, 1313, 1236, 1109, 1092, 1013, 973, 870, 840, 739, 645, 594 cm 1. XPS (binding energy, ev): C 1s: (C-H/C-C), (C=N-C in pyridyl), (C-O, alkoxy), (O-C=N in oxadiazole); N1s: (C=N-C in pyridyl), (C=N-N in oxadiazole). Cyclic voltammetry (0.1m nbu 4 NPF 6 in MeCN, Pt counter electrode, Ag/AgCl): Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2006, 45,

5 E Red (onset) = 1.76 V, E Ox (onset) =+1.27 V; E LUMO = 2.54 ev, E HOMO = 5.57 ev. Calculations of the molecular orbitals and electronic properties for the basic unit and functional fragments of PFOxPy were carried out using the Gaussian 03 program package on a Compaq GS320 alpha sever with 8 CPUs and up to 4 GB memory. The molecular orbitals and static electronic properties were calculated using density functional theory (DFT) at the B3LYP/6-31G(d) level. [21] Fabrication and characterization of the memory device: The indium-tin-oxide (ITO)/glass substrate was cleaned with water, acetone, and then 2-propanol in an ultrasonic bath (15 min each). A toluene solution of PFOxPy (10 mgml 1 ) was spin-coated onto ITO, followed by solvent removal in a vacuum chamber at 10 5 Torr and 508C for 12h. The thickness of the polymer layer was about 50 nm. Aluminum top electrodes (for needle contacts) with an area of mm 2 and a thickness of 0.5 mm were thermally evaporated onto the polymer surface at about 10 7 Torr through a shadow mask. The devices were characterized, under ambient conditions, using a Hewlett Packard 4156A semiconductor parameter analyzer equipped with an Agilent 16440A SMU/pulse generator. [23] A. Bandyopadhyay, A. J. Pal, Appl. Phys. Lett. 2003, 82, 1215, and references therein. [24] Q. D. Ling, E. T. Kang, K. G. Neoh, W. Huang, Macromolecules 2003, 36, [25] A. Fürstner, F. Stelzer, A. Rumbo, H. Krause, Chem. Eur. J. 2002, 8, [26] X. W. Zhan, Y. Q. Liu, X. Wu, S. Wang, D. B. Zhu, Macromolecules 2002, 35, [27] L. S. Tan, J. L. Burkett, S. R. Simko, M. D. Alexander, Jr., Macromol. RapidCommun. 1999, 20, 16. Received: December 8, Published online: March 27, 2006 Keywords: conjugation dynamic memory materials science polymers [1] F. M. Raymo, Adv. Mater. 2002, 14, 401. [2] C. Li, D. H. Zhang, X. L. Liu, S. Han, T. Tang, C. W. Zhou, W. Fan, J. Koehne, J. Han, M. Meyyappan, A. M. Rawlett, D. W. Price, J. M. Tour, Appl. Phys. Lett. 2003, 82, 645. [3] S. R. Forrest, Nature 2004, 428, 911. [4] J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, A. B. Holmes, Nature 1990, 347, 539. [5] F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava, Science 1994, 265, [6] N. Tessler, G. J. Denton, R. H. Friend, Nature 1996, 382, 695. [7] M. Granstrom, K. Petritsch, A. C. Arias, A. Lux, M. R. Andersson, R. H. Friend, Nature 1998, 395, 257. [8] H. Goto, E. Yashima, J. Am. Chem. Soc. 2002, 124, [9] Q. Li, G. Mathur, S. Gowda, S. Surthi, Q. Zhao, L. Yu, J. S. Lindsey, D. F. Bocian, V. Misra, Adv. Mater. 2004, 16, 133. [10] J. Ouyang, C. W. Chu, C. R. Szmanda, L. P. Ma, Y. Yang, Nat. Mater. 2004, 3, 918. [11] S. Moller, C. Perlov, W. Jackson, C. Taussig, S. R. Forrest, Nature 2003, 426, 166. [12] A. Stikeman, Technol. Rev. 2002, 105, 31. [13] Z. M. Liu, A. A. Yasseri, J. S. Lindsey, D. F. Bocian, Science 2003, 302, [14] D. M. Taylor, C. A. Mills, J. Appl. Phys. 2001, 90, 306. [15] S. Moller, S. R. Forrest, C. Perlov, W. Jackson, C. Taussig, J. Appl. Phys. 2003, 94, [16] A. Bandyopadhyay, A. J. Pal, Adv. Mater. 2003, 15, [17] R. J. Tseng, J. X. Huang, J. Ouyang, R. B. Kaner, Y. Yang, Nano Lett. 2005, 5, [18] C. W. Chu, J. Ouyang, J. H. Tseng, Y. Yang, Adv. Mater. 2005, 17, [19] Q. D. Ling, Y. Song, S. J. Ding, C. X. Zhu, D. S. H. Chan, D. L. Kwong, E. T. Kang, K. G. Neoh, Adv. Mater. 2005, 17, 455. [20] Y. Song, Q. D. Ling, C. X. Zhu, E. T. Kang, D. S. H. Chan, Y. H. Wang, D. L. Kwong, IEEE Electron Device Lett., 2006, 27, [21] Gaussian 03 (Revision C.02): M. J. Frisch et al., see Supporting Information. [22] C. A. Mills, D. M. Taylor, A. Riul, A. P. Lee, J. Appl. Phys. 2002, 91, Angew. Chem. Int. Ed. 2006, 45, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim