J. Phys. Chem. B 2009, 113, 9143–9150
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Nonvolatile Unipolar and Bipolar Bistable Memory Characteristics of a High Temperature Polyimide Bearing Diphenylaminobenzylidenylimine Moieties Kyungtae Kim,† Samdae Park,† Suk Gyu Hahm, Taek Joon Lee, Dong Min Kim, Jin Chul Kim, Wonsang Kwon, Yong-Gi Ko, and Moonhor Ree* Department of Chemistry, National Research Laboratory for Polymer Synthesis and Physics, Center for Electro-Photo BehaViors in AdVanced Molecular Systems, BK School of Molecular Science, DiVision of AdVanced Materials Science, and Polymer Research Institute, Pohang UniVersity of Science & Technology, Pohang 790-784, Republic of Korea ReceiVed: March 24, 2009; ReVised Manuscript ReceiVed: May 13, 2009
This study reports the synthesis and properties (in particular, the electrical switching characteristics) of a new high-performance polyimide (PI), poly(3,3′-di(4-(diphenylamino)benzylidenyliminoethoxy)-4,4′-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB-TPAIE PI). This PI polymer bears diphenylaminobenzylidenylimine moieties as side groups and is dimensionally stable up to 280 °C and thermally stable up to 440 °C. In devices fabricated with the PI polymer as an active memory layer, the active PI polymer was found to operate at less than (2 V in electrically bistable unipolar and bipolar switching modes by controlling the compliance current. The PI polymer layer exhibits repeatable writing-reading-erasing capability with high reliability in ambient air conditions as well as at high temperatures up to 130 °C. This PI polymer also exhibits a high ON/OFF current ratio up to 109. The observed nonvolatile memory behaviors are due to Schottky emission and local filament formation. This study has demonstrated that this thermally, dimensionally stable PI polymer is a promising material for mass production at low cost for high-performance, programmable, nonvolatile memory devices that can be operated with low power consumption in unipolar and bipolar switching modes. Introduction In general, polymeric materials of any dimensions, including miniaturized and multilayer structures, can easily be processed at low cost and have high flexibility, high mechanical strength, and good scalability. Furthermore, their properties can easily be tailored through chemical synthesis. Because of these advantages, there has been significant research effort invested in recent years in the development of electrically switching polymer materials that meet the requirements of nonvolatile memory devices.1-31 Many studies have reported such polymer materials: polyaniline,1 poly(o-anthranilic acid) and its copolymers,2-6 poly(diethyl dipropargylmalonate),7 poly(3-hexylthiophene) and its derivatives,8-10 poly(N-vinylcarbazole) and its derivatives,11-15 carbazole-containing cellulose,16 carbazolecontaining polyimide,17 poly(p-phenylenevinylene),18 polyfluorene derivatives,15,19-22 anthracene-containing poly(methyl methacrylate),23 azobenzene-containing polymers,24 a hyperbranched copper phthalocyanine polymer,25 polyethylenedioxythiophene/ poly(styrene sulfonic acid),26 poly(2,2,6,6-tetramethylpiperidine1-oxylmethacrylate)/poly(4-(2,6-di-tert-butyl-(R)-(3,5-di-tert-butyl4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy)styrene),27 poly(3,3′-bis(diphenylcarbamyloxy)-4,4′-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB-DPC PI),28 poly(4,4′-aminotriphenylene hexafluoroisopropylidenediphthalimide) (6F-TPA PI),29 poly(4-triphenylamino-2,6-bis(4-phenyl)pyridinyl-2,7-(9,9didodecyl)fluorene) (TPA-PPF),30 and poly(N-(N′,N′-diphenylN′-1,4-phenyl)-N,N-4,4′-diphenylene hexafluoroisopropylidenediphthalimide) (6F-2TPA PI).31
The 6F-TPA PI contains triphenylamino (TPA) moieties and has been reported to exhibit volatile memory (i.e., dynamic random access memory (DRAM)) behavior with bipolar ONand OFF-switching characteristics.29 In contrast, the TPA-PPF polymer (which also contains TPA moieties) exhibits writeonce-read-many-times memory (WORM) behavior in negative voltage sweeps, even though the polymer contains the same triphenylamino moieties.30 On the other hand, the 6F-2TPA PI, which contains connected two TPA moieties, reveals unipolar WORM characteristic as well as DRAM behavior with polarity, depending on the film thickness.31 Thus, questions remain about the role of the triphenylamino moiety in the electrically bistable characteristics of these polymers. In addition, the development of dimensionally and thermally stable high-performance polymers for nonvolatile memory devices remains in its early stages. In this paper, we report a new polyimide bearing diphenylaminobenzylidenylimine moieties, poly(3,3′-di(4-(diphenylamino)benzylidenyliminoethoxy)-4,4′-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB-TPAIE PI) (Figure 1a), and its excellent nonvolatile memory characteristics. This PI can easily be fabricated by means of conventional solution coating and subsequent drying. The PI devices with metal electrodes demonstrate excellent nonvolatile bipolar and unipolar switching behaviors with a high ON/OFF ratio and a long retention time in ambient air conditions and even at high temperatures up to 130 °C. In addition, the switching mechanism of the nonvolatile memory devices was investigated. Experimental Section
* To whom correspondence should be addressed. Phone: +82-54-2792120. Fax: +82-54-279-3399. E-mail:
[email protected]. † These authors contributed equally to this work.
2,2′-Bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6F) and 3,3′-dihydroxy-4,4′-diphenylene diamine (HAB) were
10.1021/jp902660r CCC: $40.75 2009 American Chemical Society Published on Web 06/11/2009
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Figure 1. (a) Chemical structure of the aromatic polyimide, 6FHAB-TPAIE PI; (b) A schematic diagram of the memory device fabricated with the PI and aluminum (Al) top and bottom electrodes.
purchased from Fluorochem Company and Chriskev Company, respectively. 6F was purified by recrystallization from acetic anhydride, and HAB was dried at 100 °C under vacuum for 1 day. 4-(Diphenylamino)benzaldehyde was purchased from TCI Company. All other materials were purchased from Aldrich and used as received. 4-(Diphenylamino)benzylidenyliminoethylenyl alcohol (TPAIE) was synthesized from 4-(diphenylamino)benzaldehyde by using magnesium sulfate according to a method previously reported in the literature.32 A mixture of magnesium sulfate (1.0 g) and 4-(diphenylamino)benzaldehyde (5.00 g, 18.29 mmol) in 20 mL of tetrahydrofuran (THF) was stirred at room temperature. To the mixture, 2-ethanolamine (2.23 g, 36.58 mmol) was added, then the mixture was stirred at room temperature for 24 h. The mixture was filtered off to remove the salt. After adding 100 mL of dichloromethane, the filtered reaction mixture was washed with distilled, deionized water several times; dried over anhydrous magnesium sulfate; and filtered. The filtrate was concentrated on a rotary evaporator and further dried in vacuum, giving the target product with a yield of 82%. The obtained product was dissolved in deuterated chloroform (CDCl3) and characterized at room temperature by using a proton nuclear magnetic resonance (1H NMR) spectrometer (Bruker, model Aspect 300 MHz). 1H NMR (δ in ppm, CDCl3): 8.23 (s, 1H, CHdN), 7.57 (d, 2H, Ar-H), 7.56 (t, 4H, Ar-H), 7.13-7.02 (m, 8H, Ar-H), 3.89 (t, 2H, CH2N), 3.73 (d, 2H, CH2OH). On the other hand, a soluble polyimide (PI), poly(3,3′dihydroxy-4,4′-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB PI) was synthesized in N-methyl-2-pyrrolidone (NMP) as follows: 6F (5.00 g, 11.26 mmol) and HAB (2.43 g, 11.26 mmol) were dissolved together in dry NMP with isoquinoline (2.53 mL) as a catalyst. The reaction mixture was gently heated to 70 °C under stirring for 2 h, followed by refluxing for 5 h. The reaction solution was then poured into a mixture of methanol and water under vigorous stirring. The precipitate was filtered, then washed with methanol and dried under vacuum, giving the target PI product. 1H NMR (δ in ppm, dimethyl-d6 sulfoxide (DMSO-d6)): 10.10 (s, 1H, Ar-OH), 8.24 (d, 1H, Ar-H), 8.03 (d, 1H, Ar-H), 7.82 (d, 1H, Ar-H), 7.43 (d, 1H, Ar-H), 7.23 (d, 2H, Ar-H).
Kim et al. The obtained 6F-HAB PI was further reacted with the TPAIE synthesized above as follows. 6F-HAB PI (0.624 g, 1.0 mmol), TPAIE (0.984 g, 3 mmol) and triphenyl phosphine (0.786 g, 3 mmol) were dissolved in dry THF under nitrogen atmosphere, then to the reaction mixture, diisopropyl diazocarboxylate (0.606 g, 3 mmol) was added dropwise. The reaction mixture was stirred at room temperature for 24 h. The reaction solution was poured into methanol under vigorous stirring, giving the target polymer product as precipitate. The polymer precipitate was filtered, washed with methanol, and dried under vacuum. The obtained polymer was identified as poly(3,3′-di(4-(diphenylamino)benzylidenyliminoethoxy)-4,4′-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB-TPAIE PI) by 1H NMR spectroscopy. 1H NMR (δ in ppm, DMSO-d6): 7.89-6.72 (m, 40H, Ar-H), 4.37 (br, 4H, CH2N), 3.71 (d, 4H, CH2O). The molecular weight of the synthesized 6F-HAB PI was measured by using a gel permeation chromatography (GPC) system (model PL-GPC 210, Polymer Laboratories, England) calibrated with polystyrene standards. In these measurements, a flow rate of 1.0 mL/min was employed, and THF was used as the eluent. The glass transition temperature, Tg, of the 6FHAB-TPAIE PI was measured in the range 25-350 °C using a differential scanning calorimeter (model DSC 220CU, Seiko, Japan). In the measurements, dry nitrogen gas was purged at a flow rate of 80 cc/min, and a ramping rate of 10.0 °C/min was employed. In each run, a sample of about 5 mg was used. The Tg was taken as the onset temperature of the glass transition in the thermogram. The degradation temperature, Td, of the polymer was measured in the range 50-800 °C using a Seiko thermogravimeter (model TG/DTA-6300); dry nitrogen gas was purged at a flow rate of 100 cc/min, and a ramping rate of 10.0 °C/min was employed. Optical properties were measured using an ultraviolet-visible (UV-vis) spectrometer (Scinco model S-3100). Cyclic voltammetry (CV) was carried out in 0.1 M tetrabutylammonium tetrafluoroborate in acetronitrile by using an electrochemical workstation (IM6ex impedance analyzer) with a platinum gauze counter electrode and a Ag/AgCl (3.8 M KCl) reference electrode, and the polymer was coated on the gold (Au) electrode deposited on a silicon wafer. A scan rate of 100 mV/s was used. Thin films of the 6F-HAB-TPAIE PI polymer (Figure 1a) and its devices (Figure 1b) were prepared as follows: All nanoscale thin films of the polymer were prepared by spincoating of its solution (1.0 wt % polymer) in cyclopentanone on precleaned silicon substrates with and without metal electrode depositions at 2500 rpm for 60 s and subsequent drying in vacuum at 80 °C for 8 h. The thicknesses of the obtained PI films were determined to be about 35 nm by using a spectroscopic ellipsometer (model M2000, Woollam). Here, the polymer solutions were filtered through polytetrafluoroethylene (PTFE) membrane microfilters with a pore size of 1.0 µm. For devices, aluminum (Al) and gold (Au) bottom electrodes with a thickness of 300 nm were prepared by E-beam sputtering on precleaned silicon substrates with a thermal oxide layer of 500 nm thickness, whereas Al top electrodes with a thickness of 300 nm were prepared by thermal evaporation through shadow masks under a pressure of 10-6 Torr. Each Al top electrode had a size of 0.04 mm2. The current-voltage (I-V) measurements were carried out using a Keithley 4200 semiconductor analyzer with a maximum current compliance of 0.105 A. All the experiments were performed at room temperature under ambient conditions and also carried out with varying temperatures in a nitrogen atmosphere. In addition, atomic force microscopy (AFM) surface images were obtained using a
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tapping mode atomic force microscope (Digital Instruments, model Multimode AFM Nanoscope IIIa). A cantilever (with a 26 N/m spring constant and 268 KHz resonance frequency) was used. In addition, the polymer films’ thermal expansion behavior was measured in a nitrogen atmosphere over the temperature range 25-180 °C by spectroscopic ellipsometry. Results and Discussion In this study, a new functional compound, TPAIE was successfully synthesized with a high yield (82%), which contains both triphenylamino and imino groups (see the NMR data in the Experimental Section). Soluble 6F-HAB PI was synthesized directly from the polycondensation of the respective monomers using isoquinoline as a catalyst. Its 1H NMR spectrum showed the proton peak of the hydroxyl side groups at 10.10 ppm and the characteristic peaks of the aromatic rings in the range 7.10-8.30 ppm, but did not reveal any amino protons originating from possible amic acid residues of partially imidized polymer chains (data given in the Experimental Section), confirming that 6F-HAB PI was successfully synthesized. The 6F-HAB PI was determined to have a weight-averaged molecj w) of 36 300 and a polydispersity index of 1.9 ular weight (M by GPC analysis. In addition, the TPAIE was incorporated into the 6F-HAB PI polymer, producing 6F-HAB-TPAIE, a new polyimide containing two TPAIE groups per repeat unit. The 6F-HAB-TPAIE polymer was found to reveal the proton peak of the ethylenyl linkers over the range 3.60-4.42 ppm and the characteristic peaks of the aromatic rings in the range 6.60-8.10 ppm but did not reveal any hydroxyl protons (data given in the Experimental Section), confirming that the TPAIE-containing PI was successfully synthesized. The new functional PI was soluble in some common solvents, providing good quality thin films by means of a conventional solution spin-casting and subsequent drying process. Thin films of this PI were determined to have smooth surfaces (
