ARTICLE pubs.acs.org/JPCC
New Dibenzothiophene-Containing Donor-Acceptor Polyimides for High-Performance Memory Device Applications Cheng-Liang Liu,†,‡ Tadanori Kurosawa,†,§ An-Dih Yu,^ Tomoya Higashihara,§ Mitsuru Ueda,*,§ and Wen-Chang Chen*,^,|| ‡
Department of Organic Device Engineering, Yamagata University, Yonezawa, Yamagata 992-8510, Japan Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan ^ Department of Chemical Engineering and Institute of Polymer Science and Engineering, National Taiwan University, Taipei, Taiwan 106 )
§
bS Supporting Information ABSTRACT: We report the synthesis and characterization of two novel polyimides (PIs), PI(2,8-APDBT-6FDA) and PI(3,7APDBT-6FDA), consisting of alternating electron-donating 2,8- or 3,7-phenylenesulfanyl-substituted dibenzothiophene and electron-accepting phthalimide moieties for high-performance memory device applications. The optical band gaps of PI(2,8-APDBT-6FDA) and PI(3,7-APDBT-6FDA) films were 3.40 and 3.31 eV, respectively, indicating the significance of the linkage position. The device with the ITO/PIs/Al configuration showed the multimemory characteristics changing from high-conductance ohmic current flow to nonvolatile negative differential resistance (NDR), dynamic random access memory (DRAM), and insulator, with the corresponding film thickness of 12, 20, 25, and 45 nm, respectively. Both PIs exhibited similar memory characteristics, but PI(3,7-APDBT-6FDA) with a lower threshold voltage due to its high-lying HOMO energy level. The 20 nm PIs device exhibited the nonvolatile behavior with the NDR region and trilevel programming for the electrical stability of at least 104 s, resulting from the diffusion of Al atoms into the PIs layer. The 25 nm PIs device showed the reproducible DRAM characteristic with the high ON/OFF current ratio more than 108. The weak instantaneous dipole moment led to the unstable induced charge-transfer complex, which was confirmed by density functional theory. The experimental results suggested that the tunable switching behavior could be achieved through the appropriate design of the donoracceptor PIs structure and controllable thickness of the active memory layer.
’ INTRODUCTION Polymer- or organic-based resistive-type memory devices have been widely investigated recently due to the advantages of rich structure flexibility, low cost, solution processability, and threedimensional stacking capability.1-5 It was found that the digital data could be stored by applying or discharging electrical charges through a simple device configuration consisting of two electrode layers sandwiching the storage element. The reported polymerbased systems for memory device applications included conjugated polymers,6-9 functional polyimides (PIs),10-19 polymers with specific pendent chromophores,20-25 and polymer nanocomposites (metal nanoparticle,26-28 fullerene,29-31 carbon nanotube,32,33 or graphene oxide34 embedded). The classified primary circuit elements and the relationships among material structures, switching performance, and operation mechanism of polymer electronic memories were reviewed by Kang and co-workers recently.3,4 In particular, the charge-transfer (CT) interaction between the electron donor (D) and acceptor (A) have been demonstrated to exhibit electric bistable switching for different kinds of memory devices such as the volatile6,9,16-19 (such as static random access r 2011 American Chemical Society
memory (SRAM) or dynamic random access memory (DRAM)) and nonvolatile7-15,20-34 (such as write-once read-many times (WORM), flash, or negative differential resistance (NDR) behavior) memory. Yang et al. reported polymer memories based on electricfield-induced CT effect from doping a polymer D with a metal nanoparticle A in an organic matrix.26-29 Besides, the copolymers (random,6-8,20,23 alternating,10-19 and block25) or small molecules35-38 with covalently bonded D and A also exhibited electrical switching for data storage applications. D-A PIs have been widely studied for memory device applications due to their structural flexibility, tunable memory characteristics, and excellent thermalmechanical properties.10-19 For example, triphenylamine (TPA) moiety can act as a strong donor with a strong charge-transfer effect with acceptor in different PIs system. Kang and co-workers first reported the DRAM memory characteristics of TPA-functionalized PIs.17,18 Recently, our group also reported TPA-based PIs containing Received: September 14, 2010 Revised: February 16, 2011 Published: March 07, 2011 5930
dx.doi.org/10.1021/jp108737e | J. Phys. Chem. C 2011, 115, 5930–5939
The Journal of Physical Chemistry C
ARTICLE
Scheme 1. Synthetic Route for PI(2,8-APDBT-6FDA) and PI(3,7-APDBT-6FDA)
mono- or dual-mediated phenoxy linkages that exhibited the DRAM and SRAM behaviors, respectively.16 On the other hand, Ree and coworkers showed that di-TPA-based PIs memory devices had stable digital nonvolatile WORM and volatile DRAM characteristics depending on the film thickness.10 Besides, the electron rich sulfur-containing PIs with high polarity revealed the nonvolatile type flash memory characteristics.14 Thus, the influence of the D-A CT stability and thickness of polymer memory layer could dominate the electronic switching properties. Although various donors have been investigated for PIs-based memory devices, the conjugated thiophene donor-based PIs have not been explored for memory devices although it was widely studied in conjugated polymer systems. In this work, we reported new polyimides, consisting of electrondonating 2,8- or 3,7-substituted dibenzothiophene with phenylenesulfanyl moieties and electron-accepting 4,40 -(hexafluoroisopropylidene)diphthalic anhydride (6FDA), as shown in Scheme 1. The CT effect between 2,8-APDBT or 3,7-APDBT (D) and phthalimide moieties (A) was expected to reveal the electronic switching of the memory device. The chemical structures and their physical properties (thermal, optical, and electrochemical properties) of the new PIs were also characterized. The memory behavior was conducted by a simple sandwich device configuration consisting of spin-coated PIs films between ITO bottom electrode and Al top electrode. Various thicknesses of nanoscale PIs thin films were also fabricated for evaluating the memory characteristics. In order to clarify the switching mechanism of the memory devices, theoretical calculations under the density functional theory (DFT) method at the B3LYP level with the 6-31G(d) basic set were applied to analyze the geometry and electronic transitions of the studied polymers. This study suggested that the electrical switching performance could be tuned through the careful design of the functional D-A type PIs and their film thickness.
’ EXPERIMENTAL SECTION Materials. 4,40 -(Hexafluoroisopropylidene)diphthalic anhydride (6FDA) was purchased from TCI and purified by sublimation. 1,
3-Dimethyl-2-imidazolidinone (DMI) was also purchased from TCI and used as received. p-Aminothiophenol, anhydrous potassium carbonate, dehydrated N,N-dimethylacetamide (DMAc), dehydrated pyridine, acetic anhydride, and cyclohexane purchased from Wako (Japan) were used as received. 2,8-Bis(p-aminophenylenesulfanyl)dibenzothiophene (2,8-APDBT)39 and 3,7-dibromodibenzothiophene (3,7-DBT)40 were synthesized according to previous reports.39,40 The synthesis of new diamine monomer (3,7-APDBT) is described in the Supporting Information. Synthesis of Poly[2,8-bis(p-aminophenylenesulfanyl)dibenzothiophene-hexafluoroisopropylidenediphthalimide] (PI(2,8APDBT-6FDA)). 2,8-APDBT (0.430 g, 1.00 mmol) and dehydrated DMAc (5.27 mL) were charged into a 20 mL flask equipped with a magnetic stirrer under a nitrogen atmosphere (Scheme 1). After 2,8-APDBT was completely dissolved, 6FDA (0.444 g, 1.00 mmol) was added, and the solution was stirred at room temperature for 24 h to afford a viscous poly(amic acid) (PAA) solution. A mixture of acetic anhydride (0.148 g, 1.49 mmol) and dehydrated pyridine (0.115 g, 1.49 mmol) was added to the above PAA solution. The reaction mixture was stirred at room temperature under nitrogen atmosphere for 24 h. The resulting solution was poured into methanol, and the precipitate was collected by filtration and washed with methanol. The final product was dried at 200 C for 10 h under vacuum, and the polymer yield was 95%. The number average molecular weight (Mn) and weight average molecular weight (Mw) values estimated from size exclusion chromatography (SEC) were 1.40 104 and 2.60 104, respectively, with the polydispersity index (PDI = Mw/Mn) of 1.89. IR (KBr), ν (cm-1): 1786, 1720 (CdO stretching), 1369 (C-N stretching). 1H NMR (DMSO-d6, δ, ppm): 8.67 (s, ArH, 2H), 8.20-8.04 (m, ArH, 4H), 7.99-7.88 (m, ArH, 2H), 7.76 (s, ArH, 2H), 7.66-7.34 (m, ArH, 10H). Synthesis of Poly[3,7-bis(p-aminophenylenesulfanyl)dibenzothiophene-hexafluoroisopropylidenediphthalimide] (PI(3,7APDBT-6FDA)). PI(3,7-APDBT-6FDA) was synthesized by a similar procedure as for PI(2,8-APDBT-6FDA) using 3,7-APDBT as a diamine monomer. The yield of PI(3,7-APDBT-6FDA) was 5931
dx.doi.org/10.1021/jp108737e |J. Phys. Chem. C 2011, 115, 5930–5939
The Journal of Physical Chemistry C 90%. Mn, Mw, and PDI values of PI(APDBT-6FDA) were 1.47 104, 3.62 104, and 2.46, respectively. IR (KBr), ν (cm-1): 1786, 1724 (CdO stretching), 1369 (C-N stretching). Anal. Calcd. for C43H20N2O4F6S3: C, 61.6; H, 2.40; N, 3.34. Found: C, 61.1; H, 2.77; N, 3.14. Characterization. NMR spectra were recorded on a BRUKER DPX-300S spectrometer at the resonant frequencies at 300 MHz for 1H and 75 MHz for 13C nuclei using DMSO-d6 as the solvent and tetramethylsilane as the reference. FT-IR spectra were measured by a Horiba FT-120 Fourier transform spectrophotometer. Mn and Mw values were evaluated by SEC on a JASCO PU-2080 Plus with two polystyrene gel columns (TSK GELS GMHHR-M). N,N-Dimethylformamide (DMF) containing 0.01 M LiBr was used as an eluent at a flow rate of 1.0 mL 3 min-1 calibrated by polystyrene standard samples. The UV-visible optical absorption spectra were recorded on a Hitachi U-3210 spectrophotometer at room temperature. The absorbance of polymer solutions was evaluated in the wavelength range of 280-800 nm. Elemental analyses were performed on a Yanaco MT-6 CHN recorder elemental analysis instrument. Thermal properties were estimated from a Seiko TG/DTA 6300 thermal analysis system (TGA) and a TA Instruments DSC-Q100 differential scanning calorimeter (DSC) under a nitrogen atmosphere at a heating rate of 10 and 6 C/min, respectively. Cyclic voltammetry was performed at room temperature using a working electrode (ITO, polymer films area about 10 30 mm2), a homemade reference electrode Ag/AgCl, and a counter electrode (Pt wire) at a sweep rate of 0.1 V/s (CHI 611B electrochemical analyzer). A 0.1 M solution of tetrabutylammonium perchlorate (TBAP) in anhydrous acetonitrile was used as an electrolyte. The thickness of the polymer film was measured with a Microfigure Measuring Instrument (Surfcorder ET3000, Kosaka Laboratory Ltd.). The thin film surface image and roughness were examined by atomic force microscopy (AFM NanoScope IIIa, Digital Instruments) at room temperature in a tapping mode. Fabrication and Measurement of the Memory Device. The memory device was fabricated on the indium tin oxide (ITO)coated glass, with the configuration of ITO/polyimide/Al. Before the fabrication of the polymer layer, the ITO glass was precleaned by ultrasonication with water, acetone, and isopropanol each for 15 min. Both PI(2,8-APDBT-6FDA) and PI(3,7APDBT-6FDA) were homogeneously well-dissolved in DMAc with a concentration of 10-30 mg/mL, and then the polymer solutions were filtered through 0.45 μm pore size PTFE membrane syringe filter, spin-coated onto the ITO glass at a control speed rate of 1000 rpm for 60 s, and baked on a hot plate at 150 C for 10 min under nitrogen. The thicknesses of the thin films were determined by concentration of the prepared polyimide solution. Finally, a 300 nm thick Al or 100 nm thick Au top electrode was thermally evaporated through a shadow mask at a pressure of 10-6 torr with a uniform deposition rate of 3-5 Å/s. The electrical characterization of the memory device was performed by a Keithley 4200-SCS semiconductor parameter analyzer equipped with a Keithley 4205-PG2 arbitrary waveform pulse generator. ITO was used as the cathode (maintained as common), and the top electrode was set as the anode during the voltage sweep. The top probe tip used a 10 μm diameter tungsten wire attached to a tinned copper shaft with a point radius
