New Fullerene-Based Polymers and Their Electrical Memory

Nov 18, 2014 - All GIXS measurements with a collection time of 60 s were carried out at .... might come from the average interdistances of the fullere...
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New Fullerene-Based Polymers and Their Electrical Memory Characteristics Yong-Gi Ko,† Suk Gyu Hahm,† Kimie Murata,‡ Young Yong Kim,† Brian J. Ree,† Sungjin Song,† Tsuyoshi Michinobu,*,‡ and Moonhor Ree*,† †

Department of Chemistry, Division of Advanced Materials Science, Center for Electro-Photo Behaviors in Advanced Molecular Systems, Pohang Accelerator Laboratory, Polymer Research Institute, and BK School of Molecular Science, Pohang University of Science & Technology, Pohang 790-784, Republic of Korea ‡ Department of Organic and Polymeric Materials and Global Edge Institute, Tokyo Institute of Technology, 2-12-1-S8-24 Ookayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: Covalent incorporations into polymers of fullerene were achieved via the Cu(I)-catalyzed azide−alkyne click polymerizations of a fullerene derivative monomer functionalized with 5-(trimethylsilyl)pent-4-yn-1-yl groups and a comonomer functionalized with azidomethyl groups, producing the novel fullerene polymers P1-C60 and P2-C60. Despite their extremely high fullerene loading levels, the polymers were soluble in common organic solvents and exhibited no aggregation of fullerene units in films. Moreover, devices containing these fullerene polymers were easily fabricated with common coating processes that exhibit excellent unipolar and bipolar flash memory characteristics as well as unipolar permanent memory characteristics, with high ON/OFF current ratios, long retention times, and low power consumption. These electrical switching behaviors were favorably operated by electron injection. Overall, these devices are the first n-type bipolar and unipolar digital polymer memory devices which can be operated in flash and write-once-read-many-times modes.

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INTRODUCTION Since the discovery of fullerene1 in 1985 and synthetic scheme producing macroscopic quantity2 in 1990, there has been a huge effort in understanding the properties and structure of the interesting molecule.3,4 In particular, the versatile electrical and optical properties of fullerene3,4 led to the development of high performance materials including fullerene as a component or solely consisting of fullerene.4−11 One possible approach developing high performance materials is the use of fullerene as an ingredient in the production of polymer composites via physical mixing. A number of fullerene/polymer composite systems have been reported.4−7 In particular, some of the composite systems were prepared and investigated for applications in electrical memory devices.6,7 Severe aggregations of fullerene molecules in the composite systems, however, caused unusually low stability in the associated memory devices.6,7 Fullerene aggregation, attributed to fullerene’s insolubility in most organic solvents and immiscibility in most polymers, is a common phenomenon observed among fullerene/polymer composites, even in composites with low loading levels of fullerene. Thus, to overcome these drawbacks, improvements in solubility and miscibility of fullerene have currently been attempted via chemical modifications.3,4,8 © 2014 American Chemical Society

Another possible approach in developing high performance materials is to covalently incorporate fullerene molecules into polymers as a side group component and/or a backbone component and/or an end group component, producing fullerene polymers. 4,9−11 Some fullerene-based polymer systems have been reported, although a certain level of difficulties was associated in the syntheses.4,9−11 Most of fullerene-based polymers, however, were also found to have solubility problem in solvents; higher fullerene content aggravated the solubility issue.4 Among the reported fullerene polymers, only two systems were introduced for applications in electrical memory devices.10,11 One system is poly(N-vinylcarbazole) bearing fullerene covalently (PVK-C60).10 The PVKC60 polymer was prepared by the direct coupling reaction of poly(N-vinylcarbazole) and fullerene with the aid of sodium hydride, and indeed, the incorporation of fullerene was limited to only 1 mol % with respect to the total number of repeat units in the polymer backbone. The films (ca. 50 nm thick) of PVKC60 in devices with aluminum (Al) and indium−tin-oxide (ITO) electrodes were found to reveal bipolar flash memory Received: October 20, 2014 Revised: November 10, 2014 Published: November 18, 2014 8154

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Figure 1. Synthetic schemes of (a) P1-C60 and (b) P2-C60. Memory devices with top and bottom electrodes fabricated with P1-C60 and P2-C60. (c) Schematic diagrams of the memory device cells. Optical images of memory device cells with various sizes (0.5 × 0.5, 1.0 × 1.0, and 1.5 × 1.5 mm2) fabricated on a high quality glass substrate (d).

behavior, which was similar to one of the memory characteristics (bipolar flash and write-once-read-many-times (WORM) memories) observed for films of poly(N-vinylcarbazole) itself (i.e., the mother polymer).12 Overall, its electrical memory behavior was mainly attributed to the mother polymer rather than the fullerene; perhaps the incorporated fullerene could assist the observed memory behavior in a certain level, but its role could not be understood well. The other polymer system is poly(2-(N-carbazolyl)ethyl methacrylate) whose one end is capped with fullerene (PCzMA-C60), which was synthesized via living anionic polymerization in a well-controlled manner.11 The PCzMA-C60 films (60 nm thick) in devices exhibited bipolar and unipolar flash memory behaviors as well as WORM memory behavior. In comparison, the mother polymer films revealed unipolar flash and WORM memory behaviors. These results collectively suggest that the fullerene incorporated as a minor component plays a role in tuning electrical memory characteristics of the mother polymer in positive ways. However, the observed nonvolatile memory behaviors of PCzMA-C60 films seem to be mainly driven by the mother polymer rather than the fullerene. Hence, the development of fully fullerene-based polymers with high-performance electrical memory characteristics remains at early stages. In this study, two novel fullerene-based polymers were elegantly designed and successfully synthesized by using click chemistry: poly(1,2,3-triazol-1,4-diylmethylene-1,3-(5-methoxycarbonyl-2,4,6-tris(dodecyloxy)phenylene)methylene-1,2,3-triazol-1,4-diyl(dipropylene 1,2-methano[60]fullerene-61,61-di-

carboxylate)) (P1-C60) and poly(1,2,3-triazol-1,4-diylmethylene-1,3-(5-methoxycarbonyl-2,4,6-tris(dodecyloxy)phenylene)methylene-1,2,3-triazol-1,4-diyl(diethyldipropylene bismethano[60]fullerene-61,61,62,62-tetracarboxylate)) (P2C60) (Figure 1a,b). Despite these extremely high fullerene loading levels, the polymers were found to reveal good solubility in common organic solvents as well as no fullerene aggregation. Moreover, the associated memory devices were easily fabricated via spin-coating, and exhibited excellent flashtype memory characteristics as well as permanent memory characteristics, with high ON/OFF current ratios, long retention times, and low power consumption. Remarkably, these devices exhibited both bipolar and unipolar switching behaviors. Excitingly, these are the first n-type bipolar and unipolar digital polymer memory devices; their nonvolatile memory characteristics might be driven by electron injection rather than hole injection.

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EXPERIMENTAL SECTION

Synthesis. A fullerene-containing monomer, bis(5-(trimethylsilyl)pent-4-yn-1-yl) 1,2-methano[60]fullerene-61,61-dicarboxylate (1), was prepared according to the method reported previously.13 To a 1000 mL flask, fullerene (C60: 595 mg, 0.826 mmol), bis(5-(trimethylsilyl)pent-4-yn-1-yl) malonate (314 mg, 0.825 mmol), iodine (I2: 320 mg, 1.26 mmol), and dry toluene (600 mL) were placed at 20 °C under argon. To this solution, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU: 308 μL, 2.06 mmol) was added, and then the reaction solution was stirred at 20 °C for 22 h. After filtration through a short plug [silicate (SiO2), methylene chloride (CH2Cl2)], the solvent was evaporated. 8155

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filtration, the filtrate was evaporated. Column chromatography (SiO2, hexane:chloroform (CHCl3) (= 1:1, v:v)) yielded methyl 2,4,6tris(dodecyloxy)benzoate as a colorless oil (2.05 g, 99%). To a 20 mL flask, methyl 2,4,6-tris(dodecyloxy)benzoate (345 mg, 0.500 mmol) and zinc chloride (ZnCl2: 136 mg, 1.00 mmol) were placed in nitrogen. Chloromethyl methyl ether (0.52 mL, 6.95 mmol) was added, and the mixture was stirred at 20 °C for 23 h. After the addition of ice−water, the mixture was stirred for 30 min. The organic layer was extracted with ether, washed with water, and dried over Na2SO4. After filtration, the filtrate was evaporated. Column chromatography (SiO2, hexane:CHCl3 (= 1:1, v:v) followed by purification using using a recycling preparative HPLC (Japan Analytical Industry, LC-9204, CHCl3) yielded methyl 3,5-bis(chloromethyl)-2,4,6-tris(dodecyloxy)benzoate as a colorless oil (188 mg, 48% yield). The obtained product was characterized by NMR and IR spectroscopies. 1H NMR (δ (ppm), 300 MHz, CDCl3): 0.88 (t, J = 6 Hz, 9 H), 1.27−1.57 (m, 54 H), 1.74−1.92 (m, 6 H), 3.91 (s, 3 H), 4.05 (t, J = 4.5 Hz, 4 H), 4.18 (t, J = 4.5 Hz, 2 H), 4.66 (s, 4 H). IR (neat, cm−1): 2921.6, 2852.2, 1735.6, 1583.3, 1464.7, 1437.7, 1379.8, 1319.1, 1262.2, 1202.4, 1186.0, 1163.8, 1116.6, 1089.6, 998.9, 908.3, 721.2, 666.3. The obtained methyl 3,5-bis(chloromethyl)-2,4,6-tris(dodecyloxy)benzoate (574 mg, 0.730 mmol) and sodium azide (189.8 mg, 2.92 mmol) were added to dehydrated DMF (3.0 mL) in a 50 mL flask at 20 °C and followed by stirring for 31.5 h. After the reaction was completed, water was added to the solution, and then the organic layer was extracted with ethyl acetate. The organic layer was again washed with water and dried over Na2SO4. After filtration, the filtrate was evaporated. Column chromatography (SiO2, hexane → hexane:ethyl acetate (= 100:1, v:v)) afforded the desired compound (561.4 mg, 0.7025 mmol) in 96.2% yield. The product was characterized by NMR and IR spectroscopies as well as MALDI-TOF MS spectrometry. 1H NMR (δ (ppm), 300 MHz, CDCl3): 0.88 (t, J = 6 Hz, 9 H), 1.27−1.51 (m, 54 H), 1,70−1,89 (m, 6 H), 3.89−3.98 (m, 9 H), 4.38 (s, 4 H). 13 C NMR (δ (ppm), 75 MHz, CDCl3): 14.03, 22.64, 25.85, 25.90, 29.31, 29.35, 29.38, 29.52, 29.54, 29.59, 29.61, 29.66, 30.14, 30.23, 31.88, 44.14, 52.44, 76.09, 118.74, 119.02, 157.48, 160.20, 166.49. IR (neat, cm−1): 2922.6, 2853.2, 2089.5, 1735.6, 1586.2, 1464.7, 1431.9, 1378.9, 1339.3, 1296.9, 1265.1, 1201.4, 1178.3, 1116.6, 1101.2, 1003.8, 907.3, 722.2. MALDI-TOF MS (dithranol, m/z): calcd for C46H82N6O5: 798.63 g/mol; found: 821.87 g/mol [M + Na]+. As shown in Figure 1a, P1-C60 was synthesized from the polymerization of the monomers 1 and 3. To a 100 mL flask, the monomer 1 (1.1 g, 1.0 mmol) and THF (25 mL) were placed, and the solution was stirred at 0 °C. After (nC4H9)4NF solution in THF (1 M, 2.5 mL) was added, the solution was stirred at 0 °C for 3 h. Water was added, and the organic layer was extracted with CH2Cl2. The organic layer was washed with water again and dried over Na2SO4. After filtration, the filtrate was concentrated to ca. 25 mL by evaporation. To this solution, the comonomer 3 (799.2 mg, 1.0 mmol), water (25 mL), CuSO4·5H2O (25.0 mg, 1.0 mmol), and sodium ascorbate (59.4 mg, 1.0 mmol) were added, and the mixture was stirred at 20 °C for 24 h under nitrogen. The solution was concentrated by evaporation and poured into methanol (CH3OH). The precipitate was collected by filtration and dried in vacuo. This solid was washed with hexane for 7 h by using a Soxhlet extractor and then extracted with CHCl3. The solution was concentrated by evaporation and poured into a mixture of hexane:CHCl3 (= 1:1, v:v), yielding the target polymer as a brown solid (420.7 mg). The product was characterized by NMR and IR spectroscopies. 1H NMR (δ (ppm), 300 MHz, CDCl3): 0.88 (br s, 9n H), 1.25 (br s, 54n H), 2.04−2.46 (m, 14n H), 3.92 (br s, 6n H), 4.11 (s, 3n H), 4.47 (br s, 4n H), 4.63 (t, J = 6.3 Hz, 4n H). IR (neat, cm−1): 3296.7, 2920.7, 2849.3, 1742.4, 1578.5, 1458.9, 1427.1, 1376.9, 1292.1, 1227.5, 1112.7, 1052.0, 1020.2, 693.3, 638.3. From the monomers 2 and 3, (P2-C60) was prepared (Figure 1b). To a 100 mL flask, the monomer 2 (1.18 g, 0.938 mmol) and THF (23 mL) were placed, and the solution was stirred at 0 °C. After (nC4H9)4NF solution in THF (1 M, 2.3 mL) was added, the solution was stirred at 0 °C for 3 h. Water was added, and the organic layer was extracted with CH2Cl2. The organic layer was washed with water again and dried over Na2SO4. After filtration, the filtrate was concentrated to

Column chromatography (SiO2, hexane → hexane:CH2Cl2 (= 1:9, v:v) yielded the desired product as a brown solid (508 mg, 56%). The product was characterized by means of nuclear magnetic resonance (NMR) and Fourier-transform infrared (FTIR) spectroscopies and matrix-assisted laser desorption/ionization time-of-flight mass (MALDI-TOF MS) spectrometry. NMR spectra were recorded on a JEOL AL300 spectrometer with proton (1H) and/or carbon (13C) probe; tetramethylsilane (TMS) with the solvent’s residual signal was used as an internal reference. IR spectra were measured on a spectrometer (model FT/IR-4100, JASCO). MALDI-TOF MS spectroscopy measurements were conducted on a mass spectrometer (model AXIMA-CFR, Shimadzu) using dithranol as a matrix. A solution of sample (1 g/L), silver trifluoroacetate (1 g/L), and dithranol (20 g/L) in tetrahydrofuran (THF) was placed on a sample plate, and the solvent was evaporated. 1H NMR (δ (ppm), 300 MHz, deuterated chloroform (CDCl3)): 0.17 (s, 18 H), 2.07 (q, J = 7 Hz, 4 H), 2.44 (t, J = 7 Hz, 4 H), 4.60 (t, J = 6 Hz, 4 H), where s, d, t, q, and m denote singlet, doublet, triplet, quartet, and multiplet, respectively. IR (neat, cm−1): 2955.4, 2173.4, 1744.3, 1460.8, 1427.1, 1390.4, 1317.1, 1265.1, 1229.4, 1202.4, 1185.0, 1111.8, 1052.0, 1025.9, 901.6, 837.0, 756.9, 728.0, 701.0, 638.3, 575.6. MALDI-TOF MS (dithranol, m/z): calcd for C79H30O4Si2: 1098.17 g/mol; found: 1098.69 g/mol [M+]. Another fullerene-containing monomer, b is(ethyl(5(trimethylsilyl)pent-4-yn-1-yl) bismethano[60]fullerene-61,61,62,62tetracarboxylate (2), was synthesized as follows. To a 500 mL flask, 5-(trimethylsilyl)pent-4-yn-1-ol (1.83 mL, 10 mmol), dehydrated CH2Cl2 (133 mL), and dehydrated pyridine (0.81 mL) were placed, and the solution was cooled to 0 °C under a nitrogen atmosphere. After ethyl 3-chloro-3-oxopropanoate (1.27 mL, 10 mmol) was added, the solution was stirred at 0 °C for 1 h followed by at 20 °C for 20 h. The precipitate was filtered off, and the filtrate was evaporated. Column chromatography (SiO2, CH2Cl2) afforded the desired compound (2.41 g, 8.91 mmol) in 89.1% yield. The obtained ethyl 5-(trimethylsilyl)pent-4-yn-1-yl propanedioate product was identified. 1 H NMR (δ (ppm), 300 MHz, CDCl3): 0.11 (s, 9 H), 1.25 (t, J = 6 Hz, 3 H), 1.79−1.87 (m, 2 H), 2.29 (t, J = 6 Hz, 2 H), 3.33 (s, 2 H), 4.13−4.23 (m, 4 H). 13C NMR (δ (ppm), 75 MHz, CDCl3): 0.12, 14.11, 16.51, 27.60, 41.65, 61.59, 64.10, 85.48, 105.54, 166.56, 166.58. IR (neat, cm−1): 2960.2, 2902.3, 2175.3, 1735.6, 1411.6, 1369.2, 1328.7, 1249.7, 1184.1, 1147.4, 1070.3, 1033.7, 941.1, 840.8, 759.8, 698.1, 640.3. The obtained ethyl 5-(trimethylsilyl)pent-4-yn-1-yl propanedioate (450 mg, 2.50 mmol) was added together with C60 (600 mg, 0.833 mmol) and I2 (646 mg, 2.55 mmol) to toluene (600 mL) in a 1000 mL flask, and the solution was stirred at 20 °C under argon. After DBU (0.72 mL, 4.17 mmol) was added, the solution was stirred at 20 °C for 22 h. The solution was passed through a plug (SiO2, CH2Cl2), and the solvents were evaporated. Repeated column chromatography (SiO2, hexane → CH2Cl2 and hexane → hexane:CH2Cl2 (= 1:1, v:v)) afforded the desired compound in the form of regioisomeric mixtures (456 mg, 0.363 mmol) in 43.6% yield. The target product was characterized by NMR and IR spectroscopies as well as MALDI-TOF MS spectrometry. 1H NMR (δ (ppm), 300 MHz, CDCl3): 0.15−0.20 (m, 18 H), 1.39−1.63 (m, 6 H), 1.98−2.08 (m, 4 H), 2.38−2.55 (m, 4 H), 4.43−4.74 (m, 8 H). IR (neat, cm−1): 2956.3, 2173.4, 1741.4, 1430.9, 1388.5, 1230.4, 1099.2, 1058.7, 1020.2, 837.0, 756.0, 730.9, 702.0. MALDI-TOF MS (dithranol, m/z): calcd for C86H40O8Si2: 1256.23 g/mol; found: 1279.71 g/mol [M + Na]+. A comonomer, methyl 3,5-bis(azidomethyl)-2,4,6-tris(dodecyloxy)benzoate (3), was synthesized as follows. Methyl 3,5-bis(chloromethyl)-2,4,6-tris(dodecyloxy)benzoate was prepared as described in the literature.8,14 To a 50 mL flask, methyl 2,4,6trihydroxybenzoate (553 mg, 3.00 mmol), potassium carbonate (K2CO3: 2.65 g 19.2 mmol), and dry dimethylformamide (DMF: 3.0 mL) were placed, and the mixture was stirred under nitrogen. To this suspension, 1-bromododecane (2.3 mL, 9.2 mmol) was added, and the mixture was heated to 70 °C for 18 h. After cooling to 20 °C, water was added. The organic layer was extracted with ether, washed with water and brine, and dried over sodium sulfate (Na2SO4). After 8156

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Device Fabrication. Devices of the polymers with aluminum (Al) top electrode and ITO or Al or gold (Au) bottom electrode were fabricated. ITO glasses were used as a substrate with ITO bottom electrode. Al or Au was deposited with 300 nm thickness onto silicon substrates with thermally grown oxide layer by electron beam sputtering; the deposited Al or Au layer was used as a bottom electrode. The polymer solutions were spin-cast onto ITO glass substrates or metal-deposited Si substrates and followed by drying in vacuo at 100 °C for 30 min. Then, the Al top electrodes (300 nm thick) were deposited onto the polymer film layers through a shadow mask by thermal evaporation in vacuum, with a size of 0.5 × 0.5, 1.0 × 1.0, and 1.5 × 1.5 mm2. Current−voltage (I−V) analysis was conducted under ambient air at room temperature, using a semiconductor analyzer (model 4200, Keithley).

ca. 23 mL by evaporation. To this solution, the comonomer 3 (749.6 mg, 0.938 mmol), water (23 mL), CuSO4·5H2O (23.4 mg, 0.938 mmol), and sodium ascorbate (55.8 mg, 0.938 mmol) were added and then stirred at 20 °C for 24 h under a nitrogen atmosphere. The solution was concentrated by evaporation and poured into CH3OH. The precipitate was collected by filtration and dried in vacuo. This solid was washed with hexane by using a Soxhlet extractor and then extracted with CHCl3. The solution was concentrated and poured into a mixture of CH3OH:CHCl3 (= 2:1, v:v), yielding the target polymer product as a dark brown solid (1.03 g). The product was characterized by NMR and IR spectroscopies. 1H NMR (δ (ppm), 300 MHz, CDCl3): 0.88 (br s, 9n H), 1.27 (br s, 54n H), 1.41−1.75 (m, 6n H), 2.00−2.36 (m, 14n H), 3.91−3.96 (m, 9n H), 4.38−4.53 (m, 12n H); IR (neat, cm−1): 2919.7, 2850.3, 2366.2, 2337.3, 1737.6, 1577.5, 1457.9, 1367.3, 1290.1, 1222.7, 1093.4, 1014.4, 705.8, 669.2, 630.6. Measurements. Molecular weight analysis was performed with gel permeation chromatography (GPC) combined with multiangle light scattering (MALS); a Shodex GPC system equipped with polystyrene gel columns and a detector miniDAWN Tristar were employed. THF as an eluent was used at a rate of 1.0 mL/min and 40 °C. Thermogravimetric analysis (TGA) was conducted with a ramping rate of 10.0 °C/min using a thermogravimeter (model SII TG/DTA 6200, Seiko). Differential scanning calorimetry (DSC) was carried out with a ramping rate of 10.0 °C/min using a calorimeter (model SII DSC 6220, Seiko). During these measurements, dry nitrogen gas was purged with a flow rate of 100 cm3/min. Ultraviolet−visible (UV−vis) spectroscopy measurements were conducted using a Scinco spectrometer (model S-3100). Cyclic voltammetry (CV) measurements were conducted with a scan rate of 100 mV/s in THF containing 0.1 M (n-C4H9)4NClO4 at 20 °C under an argon atmosphere using a cell composed of three-electrodes (a platinum (Pt) working electrode with a diameter of 1.6 mm, a reference electrode (Ag/Ag+/CH3CN/(n-C4H9)4NClO4), and a Pt auxiliary electrode); in the measurements, a ferrocene/ferricinium (Fc/Fc+) couple was used as internal standard. Thin Film Preparation. Each polymer was dissolved in CHCl3 with a concentration of 20 mg/mL and then filtered through polytetrafluoroethylene (PTFE) membrane microfilters with a pore size of 0.45 μm at room temperature. The polymer solutions were spin-cast on substrates such as silicon (Si) wafers and indium−tin oxide (ITO) glass substrates, followed by drying in vacuo at 100 °C for 30 min. The film thicknesses were measured by spectroscopic ellipsometry (model M2000, Woollam). The film surface morphology was measured in tapping mode by means of atomic force microscopy (AFM: model Multimode AFM Nanoscope IIIa, Digital Instruments); silicon cantilevers with a 42 N/m spring constant and 330 kHz resonance frequency were used. Synchrotron Grazing Incidence X-ray Scattering (GIXS). Grazing incidence wide-angle and small-angle X-ray scattering (GIWAXS and GISAXS) analyses were performed with a twodimensional (2D) charge-coupled detector (CCD: MAR USA) at the 3C and 9A beamlines of Pohang Accelerator Laboratory (PAL), according to the method reported in the literature.15 Measurements were conducted with an incident angle αi of 0.160°, using an X-ray radiation source (λ = 0.1380 nm wavelength). The sample-to-detector distance was 121.1 mm for GIWAXS and 2201.5 mm for GISAXS. All GIXS measurements with a collection time of 60 s were carried out at room temperature. The scattering angle correction of the measured GIXS data was made using the reflected X-ray beam position as well as polystyrene-b-polyethylene-b-polybutadiene-b-polystyrene block copolymer and silver behenate powder standards.15 Synchrotron X-ray Reflectivity (XR). XR analysis was conducted in θ−2θ scanning mode at the 3D and 8D POSCO beamlines16 of PAL. An X-ray radiation source (λ = 0.1541 nm) was used, and the beam size at the sample position was 0.1 × 2.0 mm2. The reflectivity R was measured down to just above 10−8; here the reflected beam intensity was normalized to the incident beam intensity. The measured data had undergone the geometrical correction and background subtraction procedure described in the literature.16

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RESULTS AND DISCUSSION P1-C60 was successfully prepared in THF by the polymerization of in situ deprotected 1 with 3 via copper(I)-catalyzed azide− alkyne click polymerization (Figure 1a). In a similar manner, P2-C60 was synthesized from 2 and 3 (Figure 1b). The obtained polymer products were characterized with 1H and 13C NMR and IR spectroscopies as well as with GPC-MALS. P1-C60 was found to have a weight-average molecular weight ( M w ) of 79 800 and a polydispersity index (PDI) of 1.20. P2-C60 was found to have M w = 76 500 and PDI = 1.53. Both P1-C60 and P2-C60 exhibited good solubilities in THF, CH2Cl2, CHCl3, and toluene, despite their extremely high fullerene content. These good solubilities mean that the polymers exhibit excellent film formability, affording highquality nanoscale thin films with conventional solution processes including simple spin-coating. Soluble polymers with high fullerene content are generally known to be very elusive because the strong aggregation tendencies of fullerene moieties significantly suppress polymer solubility,4,17 so these good solubility results are a significant achievement. The solubilities of these polymers are attributed to the positive contributions of the three n-dodecyloxy groups and the three ester linkers per polymer repeat unit. In particular, the three ndodecyloxy groups significantly improve the polymers’ solubilities because of their high solubility in organic solvents; they are also good steric stabilizers of the fullerene moieties. The arrangement of the three n-dodecyloxy groups at the 1,3,5benzene unit also effectively prevents the aggregation of the fullerene moieties. P1-C60 and P2-C60 were found to start degradation at 210 °C (= Td) in a nitrogen atmosphere (Figure S1a in Supporting Information). However, their glass transitions could not be discernible below 200 °C in DSC analysis (Figure S1b). The UV−vis spectroscopy analysis found that P1-C60 and P2-C60 reveal a band gap (i.e., the difference between the highest occupied molecular orbital (HOMO) level and the lowest unoccupied molecular orbital (LUMO) level) of 3.20 and 3.10 eV, respectively (Figures S2a and S3a). The reduction halfwave potential ERed of P1-C60 was determined to be as −0.89 V vs Fc/ Fc+; the ERed of P2-C60 was −1.03 V vs Fc/Fc+ (Figures S2b and S3b). The LUMO level for the Fc/Fc+ standard is assumed to be −4.80 eV with respect to the zero vacuum level. From the band gap and CV data, the HOMO and LUMO levels were estimated to be −7.11 and −3.91 eV for P1-C60 and −6.87 and −3.77 eV for P2-C60, respectively. Figure 2 shows representative GISAXS and GIWAXS patterns of P1-C60 and P2-C60 in thin films (50−60 nm thick). The GISAXS patterns of both the P1-C60 and P2-C60 films are featureless, indicating that neither of the films of the 8157

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Figure 3. Representative XR profiles of P1-C60 and P2-C60 in thin films (50−60 nm thick), which were prepared on silicon substrates with native oxide layer by solution coating and subsequent drying. The symbols are the measured data, and the solid line represents the fit curve assuming a homogeneous electron density distribution within the film. The inset shows a magnification of the region around the two critical angles: αc,f and αc,s are the critical angles of the film and the silicon substrate, respectively.

easily produce high quality films via common solution coating process. Memory devices were fabricated on ITO-glass and silicon substrates, as shown in Figure 1c,d. Surprisingly, the P1-C60 devices with Al top and ITO bottom electrodes exhibit excellent bipolar switching (i.e., flash type) memory characteristics, as shown in Figure 4a. The P1-C60 film layer in the device

Figure 2. Synchrotron GIXS data of polymer films (50−60 nm thick): (a) GISAXS and (b) GIWAXS patterns of P1-C60; (c) GISAXS and (d) GIWAXS patterns of P2-C60; (e) out-of-plane scattering profiles extracted along the αf direction at 2θf = 0.000° from the GIWAXS patterns in (b) and (d); (f) in-of-plane scattering profiles extracted along the 2θf direction at αf = 0.300° from the GIWAXS patterns in (b) and (d). The wavelength λ of the X-ray beam was 0.1380 nm; the incident angle αi of the X-ray beam was set at 0.160°.

polymers contains nanostructures. These scattering patterns further confirm that aggregation of the fullerene units incorporated into the polymer chains has been prevented during the film formation process. The films further show featureless GIWAXS patterns, again confirming the absence of ordered phases; only three amorphous scattering rings are discernible. Their d-spacings are 1.72 nm (3.70°), 1.01 nm (6.30°), and 0.47 nm (13.60°) for the P1-C60 film and 1.93 nm (3.30°), 1.10 nm (5.80°), and 0.46 nm (13.90°) for the P2-C60 film. Considering their chemical structures, the relatively strong ring scatterings at 3.70° (1.72 nm d-spacing) and 3.30° (1.93 nm) for P1-C60 and P2-C60, respectively, might come from the average interdistances of the fullerene units. The weak ring scatterings at 6.30° (1.01 nm d-spacing) and 5.80° (1.10 nm) for P1-C60 and P2-C60, respectively, might result from the average interdistances of the polymer chains without fullerene units, whereas the ring scatterings at 13.60° (0.47 nm dspacing) and 13.90° (0.46 nm) might originate from the mean interdistances of the n-dodecyloxy side groups. Figure 3 depicts representative specular XR profiles of P1-C60 and P2-C60 in thin films (50−60 nm thick). The XR profiles could be well analyzed using the algorithm proposed by Parratt.16,18 The analysis results are summarized in Table S1. The synchrotron XR analysis found that for the P1-C60 film the electron density ρe and surface roughness σ are 471.6 nm−3 and 0.1 nm, respectively, whereas for the P2-C60 film ρe = 505.6 nm−3 and σ = 0.2 nm. The P1-C60 film exhibits relatively lower ρe value than that of P2-C60. The σ values are slightly lower than the root-mean-square (rms) roughnesses (0.2 nm for the P1-C60 film and 0.3 nm for the P2-C60 film) measured by atomic force microscopy (AFM; Figure S4). Collectively, the low σ values again confirmed that both P1-C60 and P2-C60 can

Figure 4. Bipolar I−V curves of Al/polymer(60 nm thick)/ITO devices. I−V measurements were carried out during negative voltage sweeps and reverse voltage sweeps: (a) P1-C60; (b) P2-C60. The electrode contact area was 1.0 × 1.0 mm2.

is initially in an OFF-state (i.e., a low conductivity state). The polymer device switches on to an ON-state (i.e., a high conductivity state) at the critical voltage, Vc,ON, of −1.10 V, during the first voltage sweep ranging from 0 to −3.5 V with a compliance current of 0.01 A (1st sweep). The ON-state persists even after the removal of the power supply as well as during the second voltage sweep from 0 to −3.5 V with the same compliance current. Additionally, the ON-state could not be switched off by positive voltage sweeps with a compliance current of 0.10 A, which is 10 times higher than that of the first sweep (Figure S5a). The ON-state, however, switches off at 8158

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+0.90 V (= Vc,OFF, the critical switch-OFF voltage) during the reverse voltage sweep from −3.5 V to a positive voltage (second sweep). The OFF-state persists during the positive voltage sweep; moreover, the OFF-state could not be transformed to an ON-state by any further positive voltage sweeps. This whole bipolar switching process is repeatable, as shown in the third and fourth sweeps. Overall, the P1-C60 device exhibits high ON/OFF current ratio (104−105). P2-C60 devices with Al top and ITO bottom electrodes exhibited similar bipolar switching behaviors: Vc,ON = −1.44 V, Vc,OFF = +0.95 V, and the ON/OFF current ratios ranged from 104 to 107 (Figure 4b and Figure S5b). Interestingly, the P1-C60 devices with Al electrodes exhibited quite different memory behaviors from these bipolar flash memory characteristics. As shown in Figure 5a, the device in an

compliance current were observed for the devices whose operation was initiated with a positive voltage sweep (Figure 5b): Vc,ON = +1.28 V, Vc,OFF = +0.63 V, and the ON/OFF current ratio ranges from 105 to 109. Similar unipolar WORM memory and ON−OFF switching behaviors were also demonstrated for the P2-C60 devices fabricated with Al electrodes (Figure 5c,d and Figure S6b): Vc,ON = −1.67 or +1.40 V and Vc,OFF = −0.51 or +0.67 V; the ON/OFF current ratios are in the range 107−1010. Similar unipolar WORM memory behavior and ON−OFF switching behavior were demonstrated for the P1-C60 and P2C60 devices with Al top and Au bottom electrodes (Figure S7); P1-C60 devices: Vc,ON = −1.49 or +2.10 V and Vc,OFF = −0.50 or +0.81 V; P2-C60 devices: Vc,ON = −2.05 or +1.80 V and Vc,OFF = −0.50 or +0.79 V. Their ON/OFF current ratios are in the range 102−106. To evaluate P1-C60 and P2-C60 devices as bipolar nonvolatile memory devices, the write-read-erase cycles and the memory retention times were measured. Figure 6 displays the

Figure 5. Unipolar I−V curves for Al/polymer(60 nm thick)/Al devices: (a, b) P1-C60 devices; (c, d) P2-C60 devices. The first negative voltage sweep was performed with a current compliance of 0.01 A from 0 to −3.0 V to switch the device on, the second forward sweep was performed with the same current compliance (0.01 A) to test the ON-state of the device, and the third sweep was carried out with a current compliance of 0.10 A to switch the device off. The first positive voltage sweep was performed with a current compliance of 0.01 A from 0 to +3.0 V to switch the device on, the second forward sweep was performed with the same current compliance (0.01 A) to test the ON-state of the device, and the third sweep was carried out with a current compliance of 0.10 A to switch the device off. The electrode contact area was 1.0 × 1.0 mm2.

OFF-state switches on to an ON-state at −2.10 V (= Vc,ON) during a negative voltage sweep with a current compliance of 0.01 A (first sweep). The ON-state remains even after the elimination of power supply or during further forward voltage sweep (second sweep). In addition, the ON-state could not be returned to the OFF-state by any reverse voltage sweeps (Figure S6a). These results collectively indicate that the P1-C60 devices fabricated with Al electrodes exhibit WORM memory behavior when operated under a fixed compliance current. Surprisingly the switched-ON P1-C60 devices can, however, be undergone to the OFF-state by a voltage sweep with a current compliance (0.10 A) higher than that set in the first switchingON process; Vc,OFF = −0.60 V in a negative voltage sweep (see the third sweep). The ON/OFF current ratios range 105−109. Similar WORM memory behavior with a fixed compliance current and ON−OFF switching behavior with variation in the

Figure 6. Bipolar write-read-erase cycles of Al/polymer(60 nm thick)/ ITO devices: (a) voltages applied in the cycles (each single cycle: −2.5 V (write) → −1.0 V (read) → +2.0 V (erase) → −1.0 V (read)); (b) current responses of the P1-C60 device during the cycles; (c) current responses of the P2-C60 device during the cycles. The voltages were applied during the cycles with a current compliance of 0.01 A. The electrode contact area was 1.0 × 1.0 mm2.

characteristics of write-read-erase cycles, with writing voltage of −3.0 V and erasing voltage of +2.0 V. The ON-state (memory writing) and OFF-state (memory erasing) were monitored through small probe voltage pulses of −1.0 V. The current running through the device shows sensitive response to continuous write-read-erase cycles for several hours. The write8159

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stabilities (Figure 8a,c); the reading voltage of −1.5 V was used. Both the bipolar and the unipolar P2-C60 devices also revealed highly stable ON- and OFF-state over a test period of 1.0 × 104 to 4.0 × 104 s as observed for the P1-C60 devices (Figure 8b,d). Moreover, it was confirmed that the P1-C60 and P2-C60 memory devices in both the bipolar and the unipolar modes functioned without error after being kept in ambient air for 1.5 years (Figure 9), demonstrating high quality unipolar and bipolar flash memory performances and outstanding reliabilities.

read-erase cycles in unipolar switching cases were confirmed to be stable as well (Figure 7).

Figure 7. Unipolar write-read-erase cycles of Al/polymer(60 nm thick)/Al devices. P1-C60 devices: (a) unipolar write-read-erase cycles; (b) switch-ON and OFF voltages (Vc,ON and Vc,OFF) and switch-OFF (i.e., turn-OFF) currents versus voltage sweep cycles. P2-C60 devices: (c) unipolar write-read-erase cycles; (d) switch-ON and OFF voltages (Vc,ON and Vc,OFF) and switch-OFF (i.e., turn-OFF) currents versus voltage sweep cycles. In every single cycle, the first sweep was performed with a current compliance of 0.01 A from 0 to −3.0 V to switch the device on, the second sweep was performed with the same current compliance from 0 to −3.0 V to read or test the ON-state of the device, and the third sweep was carried out with a current compliance of 0.1 A to switch the device off.

The stabilities of the fullerene-based polymer devices were further evaluated under ambient air at room temperature. As shown in Figure 8a,c, the ON-states of both the bipolar and the unipolar P1-C60 devices have held stable over a test period of 1.0 × 104 to 4.0 × 104 s without loss of integrity; the reading voltage of −3.0 V was employed. In case of the OFF-states, both the bipolar and the unipolar P1-C60 devices showed high

Figure 9. Reliability of the polymer devices: (a) bipolar Al/P1-C60(60 nm thick)/ITO and (b) Al/P2-C60(60 nm thick)/ITO devices; (c, d) unipolar Al/P1-C60(60 nm thick)/Al and (e, f) Al/P2-C60(60 nm thick)/Al devices. The devices used in the reliability test were stored in air ambient for 1.5 years. The electrode contact area was 1.0 × 1.0 mm2.

For the P1-C60 and P2-C60 devices fabricated with Al top and ITO bottom electrodes, which exhibit switching-ON behavior in the first negative voltage sweeps, the energy barriers (0.37 and 0.51 eV) for electron injection into the LUMO levels from the Al electrode (Φ(work function) = −4.28 eV) are lower than those (2.31 and 2.07 eV) for hole injection into the HOMO levels from the ITO electrode (Φ = −4.80 eV). In the first negative voltage sweeps the conduction processes are thus governed by electron injection. On the other hand, in the first positive voltage sweeps, the energy barriers (2.83 and 2.59 eV) for hole injection into the HOMO levels from the Al electrode are higher than those (0.89 and 1.03 eV) for electron injection into the LUMO levels from the ITO electrode. These facts indicate that the conduction processes under the first positive voltage sweeps are also driven by electron injection. However, the energy barriers to electron injection are higher in the positive voltage sweeps than in the negative voltage sweeps. Thus, charge injection is more favorable during the negative voltage sweeps, which results in the switching of the devices to the ON-state; in contrast, the higher energy barriers in the

Figure 8. Long-time responses (i.e., retention times) of the ON- and OFF-states of the polymer devices: (a) Al/P1-C60(60 nm thick)/ITO and (b) Al/P2-C60(60 nm thick)/ITO devices in bipolar switching mode; (c) Al/P1-C60(60 nm thick)/Al and (d) Al/P2-C60(60 nm thick)/Al devices in unipolar switching mode. 8160

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Figure 10. Switching mechanism proposed for the polymer memory devices. (a) Al/P1-C60(60 nm thick)/ITO or Al/P2-C60(60 nm thick)/ITO device; (b, c) energy level diagrams of the polymer device before applying electric field; (d, e) energy level diagrams of the polymer device under applying electric field. The purple dots containing yellow C60 in (a), (c), and (e) represent the fullerene moieties present in the polymer film layer of the device. The schematic diagram in (c) additionally illustrates some fullerene moieties positioned locally across the film thickness in the highlighted local area of the polymer film layer in (a), which can act as the charge-trapping sites and also serve as the stepping stones for the flow of the charge carriers via hopping process; the schematic diagram in (e) shows the fullerene moieties being acted under applying electric field as the electron trapping sites and being served as the stepping stones (that play together as an filament) for the flow of the charge carriers via hopping process. dtr in (c) and (e) is the trap depth of the charge-trapping sites, which is a function of the induction and stabilization powers of the charge-trapping site.

Taking into account the results described above, the switching-ON processes of the P1-C60 and P2-C60 layers in the devices can be understood by considering their chargetrapping and morphological characteristics as follows. The charge-trapping sites could originate from the chemical compositions of the P1-C60 or P2-C60 chains, whose chemical repeat units are composed of a fullerene moiety, a phenyl unit, two triazole units, three or five ester linkers, and three ether linkers, as shown in Figure 1a,b. The fullerene, phenyl, and ester carbon units can play as electrophilic sites because of their electron-deficient nature. In contrast, the triazole, ester oxygen, and ether oxygen groups can function as nucleophilic sites because of their electron-rich nature. For example, consider a case where a voltage of current compliance of 0.01 A is applied to the active polymer film layer. The fullerene moieties and the other electrophilic units get enriched with electrons, thereby becoming electron-trapping sites. At that instance, the nucleophilic groups become enriched with holes, acting as charge-trapping sites. Taking into consideration induction and resonance effects, in the polymers the fullerene moiety may have the highest charge trap and stabilization abilities than any other groups. Hence, the fullerene moieties can act as major charge-trapping sites under applied bias; they enable to flow charge carriers at Vc,ON or higher. The resulting current, however, is not a simple flow of charge carriers through the entire polymer layer, but it rather occurs through hopping process via the fullerene sites (which were dispersed with the average interdistance of 1.04−1.93 nm in the film) as stepping stones. Based on these considerations and results, a switching

positive voltage sweeps prevent the devices switching on. In case of the P1-C60 and P2-C60 devices with Al top and bottom electrodes exhibiting switching-ON behavior in negative and positive voltage sweeps, the energy barriers (0.37 and 0.51 eV) for electron injection into the LUMO levels are lower than those (2.83 and 2.59 eV) for hole injection into the HOMO levels. For the P1-C60 and P2-C60 devices with Al top and Au bottom electrodes revealing unipolar switching behaviors, electron injection into the LUMO levels is also more favorable than hole injection into the HOMO levels. Thus, the conduction processes in the devices with Al top and Al or Au bottom electrodes are also dominated by electron injection. These considerations collectively show that the P1-C60 and P2C60 devices with an Al top electrode and ITO, Al, or Au bottom electrode can be favorably operated by electron injection and are thus the first n-type nonvolatile polymer memory devices. Additional analysis using various conduction models19 was carried out to characterize the conduction mechanisms and electrical switching behaviors of the polymer films. The OFFstate I−V data were found to follow trap-limited space charge limited conduction (SCLC) model (Figure S8). In comparison, the ON-state data were characterized to follow the ohmic conduction model (Figure S8). Furthermore, the switched ON devices showed current levels which were independent of the size of the device cell (Figures S9 and S10). These results suggest that heterogeneous localized filaments (i.e., hopping routes) might be formed under applied electric field. As a result, trap-limited SCLC and hopping process govern the outstanding switching behaviors of the P1-C60 and P2-C60 devices. 8161

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Macromolecules mechanism of the P1-C60 and P2-C60 devices is proposed, as shown in Figure 10. The switching-OFF processes of the P1-C60 and P2-C60 devices in their bipolar and unipolar modes involve two different mechanisms. In the bipolar switching case, applying a negative voltage with the same current compliance as the switching-ON process destabilizes the fullerene groups’ ability to maintain the charged state. Charge destabilization renders fullerene moieties neutral, thereby decommissioning the conducting pathways and switching off the device. On the contrary, the excess current from a positive voltage bias with a higher current compliance than the switching-ON process generate heat generation,20 causing repulsive Coulomb interactions among the trapped charges at the fullerene units and other groups. Heat generation and repulsive Coulomb interactions may rupture of the hopping routes made of chargetrapped fullerene moieties inside the film and switch off the device.

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CONCLUSIONS P1-C60 and P2-C60 were newly designed and synthesized, which covalently contained fullerene in extremely high levels (61.2− 69.8 wt %). Despite the high fullerene contents, the polymers demonstrated good solubility in common solvents and easy film formability, which are generally required in the fabrication of advance devices. Moreover, they exhibited no aggregation of fullerene units even in the film state. The nanoscale thin films of both P1-C60 and P2-C60 in devices revealed superior digital memory behaviors in unipolar and bipolar modes under ambient air. The polymer films additionally demonstrated highly stable permanent memory behavior. These memory devices were able to operate with high stability and reliability as well as with high ON/OFF current ratios in low power consumption. These electrical switching behaviors were favorably operated by electron injection. Overall, we succeeded to develop two novel fullerene-based polymers, P1-C60 and P2-C60, over limitations in synthesis. Their physicochemical properties were quite significant over the poor solubilities in solvents and the severe aggregations in the fullerene units of previously reported fullerene-containing materials. Moreover, the P1-C60 and P2-C60 devices exhibited the first reported n-type unipolar and bipolar digital memory characteristics with high performance and low power consumption in flash and WORM modes. ASSOCIATED CONTENT

S Supporting Information *

TGA and DSC data, UV−vis spectra, CV data, AFM images, and I−V data and analysis results. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This study was supported by the National Research Foundation (NRF) of Korea (Doyak Program 2011-0028678 and Center for Electro-Photo Behaviors in Advanced Molecular Systems (2010-0001784)) and the Ministry of Science, ICT & Future Planning (MSIP) and the Ministry of Education (BK21 Plus Program and Global Excel Program). This work was also supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (Challenging Exploratory Research, No. 22655030), and by Tokyo Institute of Technology (SATR program−Supports for Tokyo Tech Advanced Researchers). The synchrotron X-ray scattering measurements at the Pohang Accelerator Laboratory were supported by MSIP, POSTECH Foundation, and POSCO Company.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected], Tel +82-54-279-2120, Fax +82-54279-3399 (M.R.). *E-mail [email protected], Tel +81-3-5734-3774, Fax +81-3-5734-3944 (T.M.). Author Contributions

Y.-G.K. and S.G.H. equally contributed to this work. Notes

The authors declare no competing financial interest. 8162

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