Morphology-Dependent Electrical Memory Characteristics of a Well

ARTICLE pubs.acs.org/JPCC

Morphology-Dependent Electrical Memory Characteristics of a Well-Defined Brush Polymer Bearing Oxadiazole-Based Mesogens Wonsang Kwon,†,‡ Byungcheol Ahn,†,‡ Dong Min Kim,†,‡ Yong-Gi Ko,‡ Suk Gyu Hahm,‡ Youngkyoo Kim,*,§ Hwajeong Kim,§ and Moonhor Ree*,‡ ‡

Department of Chemistry, Center for Electro-Photo Behaviors in Advanced Molecular Systems, Division of Advanced Materials Science, Pohang Accelerator Laboratory, BK School of Molecular Science, and Polymer Research Institute, Pohang University of Science & Technology, Pohang 790-784, Republic of Korea § Organic Nanoelectronics Laboratory, Department of Chemical Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea

bS Supporting Information ABSTRACT: A new oxadiazole-containing brush polymer, poly(5-phenyl-1,3,4-oxadiazol-2-yl-[1,1 0 -biphenyl]carboxyloxyn-nonyl acrylate) (PPOXBPA), was synthesized. The polymer was thermally stable up to 350 °C. Below the degradation temperature, it showed a glass transition, crystal melting transition, and a liquid crystal to isotropic melt transition. Its optical and electrochemical properties were also investigated. This brush polymer was found to always self-assemble into a multibilayer structure, with partial interdigitation between bristles from different layers occurring via the ππ stacking of the oxadiazole mesogen units. Interestingly, when the polymer film was applied in devices with a bottom and top electrode, it showed either volatile or nonvolatile memory behavior, depending on the ordering and orientation of the multibilayer structure (particularly, the ππ stacked oxadiazole mesogen units), which could be controlled via thermal annealing. The switching mechanisms of these electrical memory behaviors were investigated. Collectively, these results demonstrate that this chemically well-defined brush polymer is suitable for use as an active material for the low-cost, mass production of high-performance, programmable volatile and nonvolatile memory devices via control of the morphological structure.

’ INTRODUCTION Recently, functional polymers have received much attention as alternatives to traditional inorganic semiconductor materials for memory devices because they can be easily miniaturized in device fabrication and, furthermore, their properties can easily be tailored through chemical synthesis.13 In particular, π-conjugated polymers have shown electrically nonvolatile memory behavior.46 Polymers containing functional moieties such as carbazoles, triphenyl amines, anthracenes, fluorenes, and their derivatives also exhibited electrically volatile or nonvolatile memory characteristics.711 In fact, these functional groups are well-known to possess electron donor abilities.712 Thus, polymers bearing such functional groups have been investigated as hole-transporting materials for light-emitting devices.12 In contrast, oxadiazole has a high affinity for electrons rather than holes.13 Therefore, oxadiazole-containing polymers have been considered as candidate electron-transporting materials for light-emitting devices.13 An oxadiazole-containing polyimide, poly(4,40 -(1,3,4-oxadiazolyl)diphenoxyphenylene hexafluoroisopropylidenediphthalimide) (6F-OXDODA), was reported as a candidate material for electrical memory devices.14 This polymer r 2011 American Chemical Society

showed volatile random access memory behavior rather than nonvolatile memory characteristics. Moreover, in the polymer the oxadiazole moiety was found to act as an electron donor rather than an electron acceptor, even though it has a high electron-accepting ability.14 Thus, the development of oxadiazolebased, high-performance polymers for nonvolatile memory devices remains in its early stages. In this study, a new oxadiazole-containing brush polymer, poly(5-phenyl-1,3,4-oxadiazol-2-yl-[1,10 -biphenyl]carboxyloxyn-nonyl acrylate) (PPOXBPA) (Scheme 1), was synthesized, and its structure and properties (including electrical memory characteristics) were investigated. The polymer was easily fabricated by means of conventional solution coating (spin-, roll-, or dip-coating) and subsequent drying, producing high-quality thin films. Interestingly, analysis of grazing incidence X-ray scattering data disclosed that the brush polymer molecules in the thin films showed self-assembly abilities due to the well-defined oxadiazole Received: June 18, 2011 Revised: August 26, 2011 Published: August 30, 2011 19355

dx.doi.org/10.1021/jp205719k | J. Phys. Chem. C 2011, 115, 19355–19363

The Journal of Physical Chemistry C

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Scheme 1. Synthetic Route and Schematic Representation of PPOXBPA Polymer

mesogen in the bristle and formed a molecular multibilayer structure. Furthermore, the ordering and orientation of such multibilayer structures were improved by a thermal annealing process. These changes were found to influence the electrical memory characteristics of the brush polymer film; specifically, the as-cast films revealed excellent volatile memory behavior, whereas the thermally annealed films exhibited excellent nonvolatile memory characteristics. In addition, the switching mechanism and reliability of the observed memory behavior were investigated in terms of the chemical and morphological structures.

’ EXPERIMENTAL SECTION Materials. 4-(Methoxycarbonyl)phenylboronic acid, nonane1,9-diol, and acryloyl chloride were purchased from Tokyo Chemical Industry Company (Japan). Other chemicals were obtained from Sigma-Aldrich Company (U.S.). All reagents were used as received. Synthesis. An oxadiazole-containing monomer was synthesized in a five-step reaction as shown in Scheme 1. In the first step, N0 -benzoyl-4-bromobenzohydrazide (1) was synthesized in tetrahydrofuran (THF) from 4-bromobenzoyl chloride and

benzoic hydrazide. In the second step, the obtained compound 1 was further converted to 2-(4-bromophenyl)-5-phenyl-1,3,4oxadiazole (2). In the third step, the compound 2 was reacted with (4-(methoxycarbonyl)phenyl)boronic acid under the aid of tetrakis(triphenylphosphine)palladium(0) (Pd(pph3)4) as a catalyst, giving the product 40 -(5-phenyl-1,3,4-oxadiazol-2-yl)-[1,10 biphenyl]-4-carboxylate (3) in white powder. In the fourth step, the compound 3 was treated with aqueous HCl solution and then converted to 40 -(5-phenyl-1,3,4-oxadiazol-2-yl)-[1,10 -biphenyl]4-carboxylic acid (4) in pinkish solid. On the other hand, 9-hydroxynonyl acrylate (5) was synthesized from nonane-1,9diol and acryloyl chloride. In the final step, the compounds 4 and 5 were reacted in anhydrous dimethylformamide (DMF), producing the target monomer, 9-(acryloyloxy)nonyl 40 -(5-phenyl1,3,4-oxadiazol-2-yl)-[1,10 -biphenyl]-4-carboxylate (6). Details of the monomer synthesis are available in the Supporting Information. The brush polymer PPOXBPA was synthesized from the monomer 6 by nitroxide-mediated radical polymerization (NMP). For this polymerization, an initiator, 2,2,5-trimethyl3-(1-phenylethoxy)-4-phenyl-3-azahexane (7) and a free nitroxide, N-tert-butyl-α-isopropyl-α-phenylnitroxide, were synthesized 19356

dx.doi.org/10.1021/jp205719k |J. Phys. Chem. C 2011, 115, 19355–19363

The Journal of Physical Chemistry C according to the methodologies in the literature.15 The monomer 6 (0.82 g, 1.6 mmol) with the initiator 7 (26.15 mg, 80.3 μmol) and the free nitroxide (1.24 mg, 5.63 μmol) was degassed by three freezepumpthaw cycles in a Schlenk tube. Then, the reaction mixture was heated to 125 °C and kept at the temperature for 20 h, followed by cooling to room temperature. The polymer was purified by repeated precipitation from dichloromethane solution into a mixture of methanol and acetone (2:1 in volume). The precipitate was then collected by vacuum filtration and dried, giving the brush polymer PPOXBPA (8) in white solid. Yield: 0.51 g (62%). The obtained brush polymer was determined to have a number-average molecular weight Mn of 6440 and a polydispersity index (PDI) of 1.52. Film Preparation and Device Fabrication. All nanometerscale thin films of the obtained brush polymer were prepared by spin-coating of its solution (1.0 wt % polymer) in chloroform on precleaned silicon substrates or ITO-coated glass at 2000 rpm for 60 s and subsequent drying in vacuum at 40 °C for 12 h. Some of the thin film samples were further treated with thermal annealing at 115 °C for 1 day under vacuum. The thicknesses of the obtained polymer films were determined to be 3065 nm. Here, the polymer solutions were filtered through poly(tetrafluoroethylene) (PTFE) membrane microfilters with a pore size of 1.0 μm. For devices, indium tin oxide (ITO) bottom electrodes with a thickness of 20 nm were prepared by sputtering on precleaned glass substrates, whereas aluminum (Al) top electrodes with a thickness of 30 nm were prepared by thermal evaporation through shadow masks under a pressure of 106 torr. Characterization. Proton and carbon nuclear magnetic resonance (1H and 13C NMR) spectra were recorded on a Bruker AV300 FT-NMR spectrometer at 300 and 75 MHz, respectively. Deuterated chloroform (CDCl3) and dimethyl sulfoxide (DMSO-d6) were used as solvents with tetramethylsilane as an internal standard. Molecular weights were measured at 40 °C using a gel permeation chromatography (GPC) system (model PL-GPC 210, Polymer Laboratories, England) calibrated with polystyrene standards. In the GPC measurements, a flow rate of 1.0 mL/min was employed and THF was used as the eluent. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were carried out under a nitrogen atmosphere using a thermogravimeter (model TG/ DTA 6200, Seiko Instruments, Japan) and a calorimeter (model DSC 6200, Seiko Instruments, Japan). A rate of 10.0 °C/min was employed for heating and cooling runs. Ultravioletvisible (UVvis) spectra were recorded using a Sinco spectrometer (model S-3100, Korea). Fluorescence spectra were recorded on a fluorescence spectrometer (model MP-1, Photon Technical International, U.S.). 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 an Ag/ AgCl (3.8 M KCl) reference electrode, and the polymer was coated on the gold (Au) electrode deposited on silicon wafer. A scan rate of 100 mV/s was used. Grazing incidence X-ray scattering (GIXS) measurements were carried out at the 4C2 beamline16 of the Pohang Light Source. Thin film samples were measured at a sample-to-detector distance (SDD) of 123 mm. Scattering data were typically collected for 30 s using an X-ray radiation source of λ = 0.138 nm (wavelength) with a twodimensional (2D) charge-coupled detector (CCD) (Roper Scientific, Trenton, NJ, U.S.). The incidence angle αi of the X-ray beam was set at 0.150°, which is between the critical angles

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Figure 1. 1H NMR spectra of (a) 9-(acryloyloxy)nonyl 40 -(5-phenyl1,3,4-oxadiazol-2-yl)-[1,10 -biphenyl]-4-carboxylate and (b) PPOXBPA polymer in CDCl3.

of the polymer film and the silicon substrate (αc,f and αc,s). Scattering angles were corrected according to the positions of the X-ray beams reflected from the silicon substrate with respect to a precalibrated silver behenate powder (TCI, Japan). Aluminum foil pieces were applied as a semitransparent beam stop because the intensity of the specular reflection from the substrate was much stronger than the scattering intensity of the polymer films near the critical angle. The thicknesses of the polymer films were measured by using a spectroscopic ellipsometer (model M2000, Woollam, U.S.). The polymer films’ thermal expansion behavior was also measured in a nitrogen atmosphere over the temperature range 20160 °C by ellipsometry. The currentvoltage (IV) measurements of memory devices were carried out using a Keithley 4200 semiconductor analyzer (U.S.) 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 temperature (20160 °C) in nitrogen atmosphere. 19357



Figure 2. (a) TGA and (b) DSC thermograms of PPOXBPA polymer, which were measured at a rate of 10.0 °C/min under nitrogen atmosphere.

’ RESULTS AND DISCUSSION Synthesis. The new, acrylate monomer-containing oxadiazole mesogen (6) was synthesized in a five-step process, as shown in Scheme 1. In the first and second steps, an oxadiazole derivative, 2-(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole (2), was prepared according to a synthetic method described in the litereature.17 p-Bromobenzoyl chloride was coupled with benzoic hydrazide under basic conditions, and the resulting amide was treated with POCl3 to form oxadiazole rings via cyclization of the hydrazide. In the third step, compound 2 was converted to 3 (with a biphenyl mesogen unit) by a palladium-catalyzed Suzuki coupling reaction of 2 with 4-(methoxycarbonyl)phenylboronic acid. In the fourth step, 3 was further treated with an aqueous NaOH solution to deprotect the methyl group, producing a benzoic acid product 4. In the final step, the oxadiazole derivative 4 was incorporated into an acrylate compound, 9-hydroxynonyl acrylate (5) (which was prepared from the reaction of nonane1,9-diol and acryloyl chloride with the aid of hydroquinone and triethylamine), via conventional esterification. The esterification was performed in DMF with the aid of EDC and DMAP. The obtained acrylate monomer 6 (which bore an oxadiazole-based mesogen and a flexible alkyl spacer in the side group) was characterized by 1H and 13C NMR spectroscopy (see Figure 1a and data in the Supporting Information). From the monomer 6, a chemically well-defined brush polymer, PPOXBPA (8), was synthesized by nitroxide-mediated radical polymerization (NMP) with the aid of an initiator, 2,2,5-trimethyl-3-(1-phenylethoxy)-4-phenyl-3-azahexane, and a free nitroxide, N-tert-butyl-α-isopropyl-α-phenylnitroxide. NMP is a controlled radical polymerization technique that is metal-free and can tolerate a wide range of functional groups.18 In the 1H NMR spectrum (Figure 1a), the monomer 6 showed the characteristic resonances of the vinyl group at 5.8, 6.1, and 6.4 ppm. These NMR signals disappeared completely after the polymerization (Figure 1b), confirming that the monomer

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Figure 3. Representative 2D GIXS pattern measured at 25 °C with αi = 0.150° for 65 nm thick PPOXBPA films deposited on silicon substrate: (a) as-cast film; (b) thermally annealed film. Here the as-cast film was prepared by spin-coating of a PPOXBPA solution in chloroform and subsequent drying in vacuum at 40 °C for 12 h. The thermally annealed film was prepared by further annealing at 115 °C for 1 day under vacuum after spin-coating and subsequent drying in vacuum at 40 °C for 12 h.

molecules underwent polymerization, producing the brush polymer PPOXBPA. The obtained brush polymer showed good solubility in common organic solvents such as chloroform, THF, and chlorobenzene. The polymer was determined to have a relatively low molecular weight (Mn = 6440 and PDI = 1.52). Even with such a low molecular weight, the polymer was found to produce good quality films via conventional coating processes such as spin-, dip-, and bar-coating. Thermal Properties. The thermal properties of the PPOXBPA polymer were investigated using TGA and DSC. The TGA analysis showed that the polymer was thermally stable up to around 350 °C (Figure 2a). On the heating run, the thermogram of the polymer revealed three endothermic transitions, at 87, 105, and 215 °C (Figure 2b). These phase transitions were also observed as exothermic signals on the cooling run from the melt. The phase transition at 87 °C is typical of a glass transition; thus, the polymer was determined to have a glass transition temperature (Tg) of 87 °C. The endothermic peak at 105 °C had a much higher intensity than the other two endothermic peaks. Considering the chemical structure of the polymer, the polyacrylate backbone has an amorphous nature, while the alkyl spacer and oxadiazole-based mesogen in the bristles have a strong tendency to crystallize. Taking these factors into account, it is likely that such a strong endothermic peak originated from the melting of the ordered bristles; the melting temperature (Tm) of the crystals was therefore determined to be 105 °C. The third endothermic peak appeared as a very weak feature at 215 °C, far above the crystal melting point. Considering the mesogenic nature of the oxadiazole unit linked with phenyl and biphenyl moieties, this result indicates that the bristles of the polymer formed a liquid crystalline phase and underwent a liquid crystal to isotropic phase transition at 215 °C (=Ti). Thin Film Morphology. The PPOXBPA polymer in thin film form was investigated using GIXS analysis, before and after 19358



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Figure 4. Schematic representation of the multibilayer structure of PPOXBPA molecules: (a) as-cast film; (b) thermally annealed film. The atoms are colored: C, gray; O, red; N, blue; hydrogen atoms are omitted for clarity. In the multibilayer structure the bristles are partially interdigitated via ππ stacking of the mesogenic part containing oxadiazole unit.

thermal annealing at 115 °C. The measured scattering patterns are shown in Figure 3a and b. Figure 3a shows a representative 2D GIXS pattern for the ascast brush polymer films. As can be seen in the figure, the as-cast film exhibits three scattering rings below 10°, at 2.22°, 4.45°, and 6.68°, whose relative scattering vector lengths from the specular reflection positions are 1, 2, and 3. The first-order scattering ring at 2.22° is slightly anisotropic in intensity; namely, the intensity is slightly stronger in the in-plane direction than in the out-of-plane direction. However, the second- and third-order scattering rings are apparently isotropic rather than anisotropic. The observation of these scattering rings indicates that, in the as-cast film, the brush polymer molecules formed lamellar structures, but their overall orientation (i.e., the stacking direction of lamellae) slightly preferred the in-plane direction rather than being completely random. Considering the brush polymer’s chemical structure and the DSC results, it is likely that the individual layers in the lamellar structure consisted of the polymer bristles, arranged in an ordered fashion. The long period (i.e., layer thickness) was estimated to be 3.56 nm from the first-, second-, and third-order scattering rings. This long period value is much shorter than twice the length (5.55 nm) of the fully extended bristle, where the length of the fully extended bristle was estimated from the geometrical optimization of the bristle with the Cerius2 software package (Accelrys, San Diego, CA). These findings indicate that the brush polymer molecules (i.e., the bristles) in the lamellar structure were interdigitated to a certain degree. The 40 -(5-phenyl-1,3,4-oxadiazol-2-yl)-[1,10 -biphenyl] part in the bristle is mesogenic. Thus, the partial interdigitation of the bristles in the lamellar structure can be attributed to the possible ππ interaction of the mesogenic parts. The as-cast film showed two additional scattering rings over the range of >10° (Figure 3a). The scattering ring at around 13° was slightly anisotropic, with a higher intensity in the in-plane direction than in the out-of-plane direction. Another scattering ring at around 21° is also slightly anisotropic, but its intensity was higher in the out-of-plane direction than in the in-plane direction. The corresponding d-spacing values were determined to be 0.62 and 0.39 nm, respectively. Taking into consideration the polymer’s chemical structure as well as the lamellar structure in the film, the scattering results indicate the following. The weakly anisotropic

scattering ring with a d-spacing of 0.62 nm originates from the interdistance of the brush polymer molecules whose backbone chains are oriented slightly more in the out-of-plane of the film rather than randomly; namely, the number of vertically oriented polymer chains is slightly higher than the number of chains oriented in other directions. The weakly anisotropic scattering ring with a d-spacing of 0.39 nm is attributed to the interdistance of the ππ stacked mesogenic bristle parts in the lamellar structure. The partially interdigitated bristles are slightly more oriented in the film plane rather than in other directions. Figure 3b shows a representative 2D GIXS pattern for the thermally annealed brush polymer films. The scattering pattern shows periodic spots along the αf direction whose relative positions from the specular reflection position are 1, 2, and 3; their scattering angles are 2.22, 4.45, and 6.68°, respectively. This scattering feature is indicative of lamellar stacks in the film, with the lamellae stacked along a direction normal to the film plane. The individual layers in the lamellar structure consisted of the polymer bristles, arranged in an ordered fashion. The layer thickness was determined to be 3.56 nm (from the scattering spots), which is identical to that of the lamellar structure in the as-cast film. The thermally annealed film additionally showed highly anisotropic scattering strips around 13° and 21° (= 2θf) (Figure 3b), which are different from the weakly anisotropic rings observed for the as-cast film. Their d-spacing values were determined to be 0.62 and 0.39 nm, respectively, which are also identical to those obtained from the as-cast film. Overall, these scattering results indicate that the brush polymer backbones with a lateral interdistance of 0.62 nm are highly oriented in the film plane while the interdigitated mesogenic bristle parts with an interdistance of 0.39 nm are highly oriented in the out-of-plane of the film. The GIXS analysis results collectively inform that the brush polymer molecules in the as-cast film form lamellar structures similar to those observed in the thermally annealed film. However, the lamellar structures in the as-cast film have a lower degree of ordering and poor orientation, compared with those of the thermally annealed film. From the above structural information, molecular packing models of the brush polymer molecules in the as-cast and thermally annealed films were proposed, as shown in Figure 4. 19359



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Figure 5. 2D GIXS patterns of a thermally annealed PPOXBPA film (65 nm thick) measured with αi = 0.150° during heating at a rate of 2.0 °C/min in vacuum.

Further GIXS measurements were carried out during a heating run of the thermally annealed films (at a rate of 2.0 °C/min, in vacuum). Patterns representative of the measured GIXS data are shown in Figure 5. The scattering pattern measured at 90 °C was similar to that at room temperature (Figures 5a and 3b). In contrast, some changes were observed in the scattering patterns measured above Tm (=105 °C). The two scattering strips at around 2θf = 13° and 21° were weakened and broadened and consequently merged together; by contrast, the scattering spots along the αf direction below 10 °C were well retained (Figure 5b). Similar scattering patterns were observed up to 200 °C (=Ti) (Figure 5c and d). However, above Ti the scattering patterns revealed only one weak, broad isotropic ring at around 15° (see representative pattern in Figure 5e). Taking into account the DSC results in the earlier section, these scattering results collectively suggest the following structural characteristics. First, the crystalline-like multibilayer structure formed in the thermally annealed film was well retained up to Tm. Second, above Tm the crystalline structure was transformed to a liquid crystalline-like structure, which maintained the multibilayers and their orientation well. Finally, above Ti, the liquid crystalline-like structure was further transformed to a fully melted state without any structural features. Optical and Electrochemical Properties. Figure 6a illustrates the UVvis absorption spectra of the as-cast and thermally annealed brush polymer films. As can be seen in the figure, an absorption maximum appeared at 293 nm (=λmax) for the as-cast film. This absorption maximum can be attributed to the ππ* transition of the mesogenic 40 -(5-phenyl-1,3,4-oxadiazol-2yl)-[1,10 -biphenyl] part in the bristle. The absorption maximum

Figure 6. (a) UVvis and (b) fluorescence spectra of the as-cast (square) and thermal annealed (circle) PPOXBPA films coated on quartz substrate. (c) CV response of a PPOXBPA film fabricated with an Au electrode supported by a silicon substrate in acetonitrile containing 0.1 M tetrabutylammonium tetrafluoroborate.

was slightly blue-shifted for the thermally annealed film, to λmax = 290 nm. Moreover, the absorbance was reduced by 30% after the thermal annealing. Such a blue-shift and reduced absorbance may have resulted, in the main, from higher ordering (i.e., a higher degree of aggregation) of the mesogenic bristles via the ππ stacked interdigitation, and to a lesser extent from their preferential orientation in the out-of-plane of the film, as discussed above. A blue-shift and reduction in absorbance were previously reported for oxadiazole derivatives.19,20 Nevertherless, the as-cast and the annealed films showed the same absorption edge at 364 nm, giving a band gap (i.e., the difference between the highest occupied molecular orbital (HOMO) level and the lowest unoccupied molecular orbital (LUMO) level) of 3.40 eV. Figure 6b shows representative photoluminescence (PL) spectra for the as-cast and thermally annealed brush polymer films. Both the films exhibited an emission peak maximum at 406 nm on excitation with 293 nm wavelength light. However, the PL intensity of the annealed film was much lower than that of the ascast film. This drastically reduced PL intensity might be due to the crystalline mesogenic bristles based on the ππ stacked interdigitation. The brush polymer was further investigated by CV; the results are shown in Figure 6c. From the CV data, the oxidation onset 19360



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Figure 7. IV curves of the ITO/PPOXBPA (30 nm thick)/Al devices, which were measured with a compliance current level 0.01 A: (a) as-cast film; (A) thermal annealed film. The size of top Al electrodes was 100 100 μm2. The IV data of the OFF- and ON-state in (b) and (B) were analyzed: (c) and (C), the OFF-state fitted with a combination of Ohmic model and trap-limited space charge limited current (SCLC) model; (d) and (D), the ON-state fitted with ohmic model. The symbols are the measured data, and the solid lines are the fits obtained with the models.

potential (Eox) of the brush polymer was determined to be 0.24 V vs Ag/AgCl electrode. The external ferrocene/ferrocenium (Fc/ Fc+) redox standard potential (E1/2) was measured (in acetonitrile) to be 0.45 V vs Ag/AgCl electrode. Assuming that the HOMO level for the Fc/Fc+ standard was 4.80 eV with respect to the zero vacuum level, the HOMO level of the brush polymer was determined to be 4.58 eV. Therefore, the LUMO level of the polymer was estimated to be 1.18 eV. Electrical Memory Properties. Devices were fabricated using the as-cast or thermally annealed brush polymer films (30 nm thick) as the active layer, with an ITO bottom electrode and an Al top electrode (Figure 7a and A). As can be seen in Figure 7b, the as-cast polymer film initially exhibited a high-resistance state (OFF state) of around 1015 A. When a positive voltage was continuously applied with a current compliance of 0.01 A in ambient conditions, there

was an abrupt increase in the current at +2.7 V, indicating that the as-cast film underwent a sharp electrical transition into a high conductivity state (ON-state) of around 103 A from the OFFstate. This OFF-to-ON transition can function as a “writing” process in a memory device. Once the as-cast film has reached the ON-state, it remained in that state during reverse and forward voltage sweeps. When the power was turned off, the film returned to the OFF-state after approximately 3 min. From the currentvoltage (IV) data shown in the figure, the critical voltage to switch on the device (Vc,ON) was determined to be 2.7 V, and the ON/OFF current ratio was estimated to be 104108 over the voltage range 02.7 V. Similar memory behaviors were observed for the film in negative voltage sweeps (Figure S1 in the Supporting Information). These results collectively indicate that the as-cast film shows static dynamic random access memory (SRAM) behavior without polarity. 19361


The Journal of Physical Chemistry C The thermally annealed film also initially exhibited an OFFstate with a current level of around 1014 A (Figure 7B). In a positive voltage sweep, the film was switched to an ON-state of 103 A at +2.8 V. Once the film reached the ON-state, it remained there even after the power was turned off and during reverse and forward voltage sweeps. From the IV data, the Vc,ON was determined to be 2.8 V, and the ON/OFF current ratio was estimated to be 104107 over the voltage range 02.8 V. Similar memory behavior was observed for the film in negative voltage sweeps (Figure S1 in the Supporting Information). These results showed that the thermally annealed polymer film possessed unipolar write-once-read-many-times (WORM) memory behavior, which is quite different from the SRAM behavior of the as-cast polymer film. The measured IV data were further analyzed in detail using various conduction models,21,22 to understand the electrical switching characteristics and the current conduction mechanism of the brush polymer films in the devices. As shown in Figure 7c, the current in the as-cast film in the OFF-state increased linearly, with an initially low slope of 1.4 in the logarithmic plot (with voltages in the range 00.67 V). On increasing the voltage this behavior changed, with a steep slope of 5.5 for voltages >0.67 V. Similar IV variations were observed for the thermally annealed film in the OFF-state (Figure 7C). These results indicate that the shallow trap-limited space-charge limited conduction (SCLC) model satisfactorily fitted the IV data of both the as-cast and thermally annealed films in the OFF-state. In contrast, for both the as-cast and thermally annealed films in the ON-state, the current increased (linearly with voltage) with a slope of only 1.0, as shown in the logarithmic plots in Figures 7d and D. These results indicated that ohmic current conduction was dominant for the brush polymer films in the ON-state. The above results collectively suggest that both the SRAM behavior in the as-cast film and the WORM memory characteristics in the thermally annealed film are governed by shallow trap-limited space charge limited conduction and local filament formation. Poly(alkyl acrylate) is known to be an insulator and does not exhibit any electrical memory characteristics. Thus, the observed memory behavior of the PPOXBPA brush polymer in the devices must originate from the oxadiazole-containing bristles, which act as charge-trap sites. Furthermore, the difference in memory behavior between the as-cast and thermally annealed brush polymer films can be attributed to morphological changes in the oxadiazole-containing bristles in the polymer film during thermal annealing. In the as-cast film, the brush polymer molecules formed multibilayer structures with partially interdigitated bristles. However, the multibilayer structures had a relatively low degree of ordering, and their overall orientation showed a preference for the out-of-plane direction of the film, as discussed in an earlier section. Thus, the oxadiazole parts stacked preferentially along the out-of-plane direction of the film. The stacked oxadiazole parts may have acted as charge trap sites. They had a very short interdistance of 0.39 nm, which meant that the trapped charges could easily be transported through the stacked oxadiazole parts in the lateral direction, where the stacks were preferentially aligned in the out-of-plane direction of the film. Therefore, the SRAM behavior observed in the as-cast film can be attributed to the interdigitated oxadiazole parts whose ππ stacking direction is preferentially in the out-of-plane of the film. The thermally annealed film was also composed of multibilayer structured brush polymers, but their ordering was much

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higher than in the as-cast film. Moreover, due to the highly preferential orientation of the multibilayer structures, all the oxadiazole mesogen parts were laterally stacked along a direction parallel to the film plane. Due to this orientation of the ππ stacked oxadiazole parts and the relatively larger distance between the layers [i.e., layer thickness (3.56 nm)], the trapped charges could not be transported as easily through the molecular layers, compared with those in the as-cast film. Thus, the WORM memory behavior observed in the thermally annealed film may be due to the interdigitated oxadiazole parts whose ππ stacking direction is in the film plane.

’ CONCLUSIONS We have synthesized a chemically well-defined brush polymer, PPOXBPA, which bears oxadiazole mesogen units in its bristles. The obtained polymer was easily fabricated into films using solution-coating processes such as spin-, dip-, and roll-coating. Its optical and electrochemical properties were investigated in terms of the band gap (3.40 eV), HOMO (4.58 eV), and LUMO (1.18 eV) levels and UVvis absorption and fluorescence characteristics. The brush polymer was thermally stable up to 350 °C. It showed three temperature-dependent phase transitions: a glass transition at 87 °C, crystal melting at 105 °C, and a liquid crystal to isotropic melt transition at 215 °C. X-ray scattering analysis showed that the brush polymer molecules always self-assembled, forming a multibilayer structure with partial interdigitation of the bristles between the layers. The degree of ordering and orientation of these multibilayer structures were highly dependent on the sample preparation process. In the case of nanoscale thin films (which are highly desired in the fabrication of electrical memory devices), a much higher degree of ordering and an almost perfect in-plane orientation were achieved by thermally annealing the as-cast film, which had a multibilayer structure with a preferentially out-of-plane orientation. Temperature-dependent X-ray scattering measurements confirmed that the presence of crystals detected in the DSC analysis was due to the ordering of the interdigitated oxadiazole mesogenic parts in the bristles of the multibilayer structure, and that the liquid crystalline state originated from a multibilayer structure with partially interdigitated bristles with a certain degree of mobility. The nanoscale polymer films in devices with a bottom and top electrode were found to reveal either SRAM or WORM memory behavior, depending on the ordering and orientation of the multibilayer structure formed in the film. The as-cast films, which revealed a less ordered multibilayer structure with preferentially out-of-plane orientation (where the direction of the interdigitated (i.e., ππ stacked) oxadiazole mesogen units was preferentially in the in-plane of the film), exhibited unipolar SRAM behavior with a relatively low switching voltage (2.7 V) and a high ON/OFF current ratio of 104108. The thermally annealed films, which formed a highly ordered multibilayer structure with almost perfect in-plane orientation (where the direction of the interdigitated (i.e., ππ stacked) oxadiazole mesogen units was completely in the out-of-plane of the film), showed unipolar WORM memory behavior with a relatively low switching voltage (2.8 V) and a high ON/OFF current ratio of 104107. However, both memory behaviors were found to be governed by shallow trap-limited space charge limited conduction and local filament formation. The present findings demonstrate that the PPOXBPA polymer is a very suitable active material for the low-cost, mass 19362


The Journal of Physical Chemistry C production of high-performance, programmable SRAM and WORM memory devices via control of the morphological structure. These devices can be operated with very low power consumption, in excellent unipolar switching modes, and with a high ON/OFF current ratio.

’ ASSOCIATED CONTENT

bS

Supporting Information. Details of monomer synthesis, including 1H and 13C NMR spectroscopy data; IV curves of the ITO/PPOXBPA/Al devices in negative sweeps. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected] (M.R.), [email protected] (Y.K.); tel. +82-54-279-2120; fax +82-54-279-3399. Author Contributions †

These authors contributed equally to this work.

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Morphology-Dependent Electrical Memory Characteristics of a Well

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