Electrochemical Fabrication of a Memory Device Based on

Memory devices with sandwiched structures were fabricated by electrochemical deposition of the active layers onto indium tin oxide substrates. The act...
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18392

J. Phys. Chem. C 2007, 111, 18392-18396

Electrochemical Fabrication of a Memory Device Based on Conducting Polymer Nanocomposites Qi Chen, Lu Zhao, Chun Li, and Gaoquan Shi* Department of Chemistry and Key Laboratory of Bio-organic Phosphorus Chemistry and Chemical Biology, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed: July 28, 2007; In Final Form: October 3, 2007

Memory devices with sandwiched structures were fabricated by electrochemical deposition of the active layers onto indium tin oxide substrates. The active layers with thicknesses of 240-280 nm were composed of poly(3-hexylthiohene) and gold nanoparticles (GNPs), in which the GNP content was around 9 wt %. The devices exhibited an on/off conductivity ratio of 103 and can be read for over 104 times without a distinct performance decrease even after being stored in air without packing for over 1 month.

1. Introduction A great deal of effort has been devoted to developing fast, nonvolatile, and inexpensive techniques for data storage.1,2 Among these techniques, organic bistable devices (OBDs) have been intensively investigated,3-6 mainly due to that they are promising alternatives to conventional silicon-based memory devices. OBDs usually consist of a three-layered configuration with a sandwiched active composite layer, which can be easily addressed by X-Y cross-wire electrodes.7 To date, various methods including chemical vapor deposition,8,9 LangmuirBlodgett technique,10 layer-by-layer self-assembly,11-13 and spin-coating14,15 have been applied for fabricating the active layers of OBDs. On the other hand, OBDs based on conducting polymer composites have the advantages of a simple fabrication process and good controllability of materials.16-18 The active layers were usually prepared by spin-coating the composites of polyalkylthiophene or polyaniline and gold nanoparticles (GNPs). In this paper, we report the fabrication and properties of OBDs with an electrochemically deposited composite film of poly(3hexylthiophene) (P3HT) and GNPs as the memory element. A uniform distribution of nanometer-scaled GNPs was created by copolymerization of 3-hexylthiophene (3HT) and GNPs capped with 3-(11-mercapto-1-undecanoxyl)thiophene (MT). The film thickness can be easily modulated by controlling the total charge passed through the electrolysis cell or the numbers of cyclic voltammetry scanning.19 The OBD devices reported here are highly reproducible and show a high performances even after being stored in air for over a month. 2. Experimental Procedures 2.1. Materials. 3-Bromothiophene and 3-hexylthiophene are products of Pacific ChemSource Inc. 11-Bromo-1-undecanol was purchased from Alfa Aesar. Thiourea and hydrogen tetrachloroaurate (HAuCl4) were obtained from Beijing Chemical Reagent Co. 1-Dodecanethiol (DT) was purchased from Aldrich. All reagents were used without further purification. 2.2. Synthesis of 3-Methoxylthiophene. 3-Methoxylthiophene was synthesized according to a modified method reported previously.20 Briefly, sodium (2 g, 87 mmol) and * Corresponding author. Tel.: +86-10-6277-3743; fax: +86-10-62771149; e-mail: [email protected].

methanol (12 mL) were mixed with 20 mL of NMP. After the reaction, the temperature of the reaction system was elevated to 110 °C to remove methanol. Subsequently, 3-bromothiophene (2.9 g, 17.7 mmol) and cuprous bromide (0.29 g, 2 mmol) were added with stirring under N2 atmosphere. After reacting for 2 h, the reaction mixture was cooled to room temperature and poured into 30 mL of a saturated aqueous solution of sodium chloride. The reaction mixture was extracted 3 times with 30 mL of diethyl ether each time. The combined organic layers were dried over magnesium sulfate, and the solvent was evaporated. The final product was purified by column chromatography (CH2Cl2/petroleum ether ) 1:2, by volume) to give a colorless oil (1.7 g, 84%). 1H NMR (CDCl3, TMS): δ 7.20 (1H, dd), 6.74 (1H, dd), 6.23 (1H, dd), δ3.82 (3H, s). 2.3. Synthesis of 3-(11-Bromo-1-undecanoxyl)thiophene. 3-Methoxylthiophene (570 mg, 5 mmol) and 11-bromo-1undecanol (1.5 g, 5.98 mmol) were dissolved in 20 mL of toluene and heated to 100 °C with stirring under N2 atmosphere. Then, NaHSO4 (120 mg, 0.89 mmol) was added to the solution. The resulting methanol was extensively distilled during the reaction process. After cooling to room temperature, the reaction mixture was poured into 60 mL of water and extracted 3 times with 40 mL of diethyl ether each time. The collected organic phase was dried with magnesium sulfate and evaporated. The crude product was purified by column chromatography (CH2Cl2/petroleum ether ) 1:2, by volume) to give a pale yellow powder (1.1 g, 66%). 1H NMR (CDCl3, TMS): δ 7.16 (1H, dd), 6.74 (1H, dd), 6.22 (1H, dd), 3.95 (2H, t), 3.43 (2H, t), 1.88-1.73 (4H, m), 1.44-1.29 (14H, m). 2.4. Synthesis of MT. MT was synthesized following a published procedure.21 Thiourea (129 mg, 1.7 mmol) was added to 30 mL of ethanol (95%) and refluxed until it was extensively dissolved. Then, an ethanol solution (10 mL) of 3-(11-bromo1-undecanoxyl)thiophene (560 mg, 1.7 mmol) was dropped into the solution described above. The reaction lasted for 4 h, and then 10 mL of aqueous sodium hydroxide solution (1 wt %) was added. After reaction for 3 h, the solution was neutralized by diluted sulfuric acid. After cooling to room temperature, the mixture was extracted 3 times with 30 mL of diethyl ether each time, and the combined organic phase was washed twice with 30 mL of water each time. The product was dried with magnesium sulfate and purified by column chroma-

10.1021/jp075988z CCC: $37.00 © 2007 American Chemical Society Published on Web 11/20/2007

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Figure 2. Cyclic voltammograms of 0.1 M 3HT and 0.67 mg mL-1 MTGNPs in 50 mM CH2Cl2 solution of TBABF4 at a potential scan rate of 100 mV s-1.

Figure 1. (A) Configuration of the sandwich-structured memory device. (B) Image of the active layer and its chemical structure.

tography (CH2Cl2/petroleum ether ) 1:1 by volume) to give a white powder (0.1 g, 20%). 1H NMR (CDCl3, TMS): δ 7.16 (1H, dd), 6.75 (1H, dd), 6.22 (1H, dd), 3.95 (2H, t), 2.49 (2H, t), 1.78-1.73 (2H, m), 1.57-1.28 (16H, m). 2.5. Synthesis of MT Capped Gold Nanoparticles (MTGNPs). GNPs were prepared via a modified Brust et al.’s method.22 AuCl4- was first transferred from an aqueous HAuCl4 solution (25 mmol L-1, 20 mL) to the dichloromethane phase by tetrabutylammoniumtetrafluoroborate (TBABF4) (50 mmol L-1, 40 mL). A total of 100 mg of MT was added to the separated organic phase followed by adding a freshly prepared aqueous solution of sodium borohydride (0.4 mol L-1, 20 mL) under vigorous stirring. After stirring for 3 h, the organic phase was separated and evaporated to 5 mL. Then, 200 mL of ethanol was added, and the mixture was kept at room temperature overnight. A dark brown precipitate was obtained by centrifugation. After drying at room temperature under vacuum, the MTGNPs were obtained (200 mg). 2.6. Electrochemical Synthesis of P3HT/MTGNP Composite Active Layer. Electrosyntheses were performed in a conventional three-electrode cell with a CHI440 potentiostatgalvanostat (CH Instruments Inc.) under computer control. A patterned ITO glass sheet was used as the working electrode. A platinum sheet with an area of 30 mm × 10 mm was used as the counter electrode, and the reference electrode was a Ag/ AgCl wire. The electrolyte was a CH2Cl2 solution containing 0.1 M 3HT, 50 mM TBABF4, and 0.33 or 0.67 mg mL-1 MTGNPs. The composite films were synthesized by voltammetry scanning in the potential range of 0-2.6 V (vs Ag/AgCl) at a scan rate of 100 mV s-1 for 2 or 3 cycles, and finally, the active layers with different thicknesses (240 and 280 nm, respectively) were generated. For control experiments, MTGNPs were displaced by DT capped GNPs (DTGNPs) or no GNPs existed in the electrolytes, and the other experimental parameters were kept constant. 2.7. Fabrication of the Memory Devices. The configuration of the memory devices is presented in Figure 1, and its fabrication procedures are shown as follows. First, indium tin oxide (ITO) glasses were etched in hydrochloric acid (6 mol L-1) for 3 h and washed successively with 10% NaOH solution, ethanol, acetone, and deionized water for half an hour each.

The patterned ITO glass sheet was dried and then employed as the working electrode (with a line width of 2 mm) for depositing the active layer. Successively, an up electrode of Al (150 nm) was deposited by thermal evaporation under a vacuum of 10-5 Pa. The X-Y cross-wired, two terminated structure can be easily addressed, and the thickness of the active layer was controlled to be 120-280 nm by the numbers of cyclic voltammetry scanning as described previously. The two terminated active layers for each cell had an active area of 2 mm × 2 mm, and the sandwich-structured device was nominated as ITO/P3HT: MTGNPs/Al. 2.8. Instruments. UV-vis spectra were recorded on a Hitachi 3010 UV-vis spectrometer. Atomic force microscopy (AFM) images were taken out by using a tapping mode AFM SPA 400 instrument (Seiko Instruments Inc.). Transmission electron microscopy (TEM) images were carried out on a JEM2101 transmission electron microscope (JEOL). Samples for TEM measurement were prepared by the following procedures: a thin composite film covered on an ITO electrode (prepared by 1 cyclic voltammetric scanning in the system described in Figure 2) was cut into small pieces with a knife, and then the film was striped from the electrode surface by ultrasonication in ethanol. Finally, the ethanol dispersion was dropped onto a copper mesh for TEM characterization. Thermogravimetric analysis (TGA) was carried out by using a TGA2050 thermal degradation analyzer (TA Instruments Corp.) in air at a heating rate of 10 °C min-1. The thicknesses of the polymer nanocomposite films were measured by an ellipsometer Model GES5 (Sopra Corporation). I-V measurement was performed in a nitrogen atmosphere by a Keithley 2400 source meter, and the ITO terminal was positively biased in the forward scan. 3. Results and Discussion Figure 2 shows the successive cyclic voltammograms of 0.1 M 3HT and 0.67 mg mL-1 MTGNPs in CH2Cl2 containing 50 mM TBABF4 at a potential scan rate of 100 mV s-1. As the CV scan continued, a polymer composite film was formed on the ITO working electrode surface. The film was reduced and oxidized in the potential range of 0-1.8 V versus Ag/AgCl. The enhancement in the redox wave currents implied that the amount of the polymer composite was increased with increasing the scanning cycles. The voltammetry scanning was stopped at 0 V, and the polymer film was in the neutral state. A TEM image of the composite film with a thickness of 120 nm is illustrated in Figure 3A. It is clear from this figure that GNPs were successfully incorporated and uniformly dispersed

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Figure 3. (A) TEM image of a P3HT/MTGNPs nanocomposite film with a thickness of 120 nm. Inset: energy dispersive X-ray pattern of the film. (B) Top: 2 µm × 2 µm topographic AFM image of the P3HT/MTGNPs composite film; bottom: height image along the line section of A to B.

in the active layer without phase separation and aggregation. The diameter of the GNPs is in the range of 3-5 nm. The energy dispersive X-ray (EDX) pattern of the composite film (inset of Figure 3A) shows strong lines of Au and S, also indicating that the film contains GNPs. The AFM image of the surface of the film (Figure 3B) shows a granular morphology, and the size of the grains is in the range of 30-50 nm, about 10 times that of MTGNPs. This is a typical morphology of an electrosynthesized polythiophene nanocomposite film.23 To confirm that MTGNPs were really incorporated into the polymer films through copolymerization rather than physical inclusion, UV-vis spectra of the CH2Cl2 soaking solutions of the composite films deposited from the electrolytes containing 0.1 mol L-1 3HT and 2 mg mL-1 MTGNPs or DTGNPs were recorded (Figure S1). It was found that the composite film prepared from the electrolyte containing MTGNPs was insoluble, whereas the composite film deposited from the electrolyte containing DTGNPs was partly soluble. On the basis of the UV-vis spectral results described above and in combination with the facts that the surface density of the thiophene moiety on the MTGNPs is relatively high and that alkoxy-substituted thiophene has a lower oxidation potential with respect to alkylthiophene, it is reasonable to conclude that copolymerization occurred during electrochemical deposition. MTGNPs acted as a cross-linking agent in the composite film, and thus, GNPs were pinned in the polymer matrix. TGA of the composite films fabricated with different GNPs can give further evidence for the occurrence of copolymerization between 3HT and MTGNPs (Figure 4). Films 1 and 2 were deposited from the electrolytes containing 0.33 and 0.67 mg mL-1 MTGNPs, respectively. Films 3 and 4 synthesized from the electrolytes containing 0.33 and 0.67 mg mL-1 DTGNPs were used as a reference. It can be seen from Figure 4 that the weight residuals of films 1 and 2 were 3.44 and 8.85%, respectively, being about twice those of films 3 and 4. These results demonstrated that MTGNPs are more easily incorporated into the composite films, which is due to the presence of thiophene moieties on MTGNPs, thus resulting in the occurrence

Figure 4. TGA curves of films 1-4 recorded in air at a temperature increasing rate of 10 °C min-1 (dashed line, film 1; solid line, film 2; dash-dot-dotted line, film 3; and dotted line, film 4). Inset: magnification of TGA curves in the range of 520-600 °C.

of copolymerization with 3HT and integrating more GNPs into the composite films.24 Figure 5 shows the typical current-voltage (I-V) curves for the memory devices with different active layers, namely, ITO/ P3HT:MTGNPs/Al, ITO/P3HT:DTGNPs/Al, and ITO/P3HT/ Al. For the first device, in the first scan, a slight current jump at a threshold voltage of around 2-3 V was observed. The device transited to the high conductivity state with a higher current response at 1 V than that of the low conductivity state by over 2 orders of magnitude. Accompanied by conductivity hysteresis, negative differential resistance (NDR) was observed by subjection to a reverse voltage scan. A similar I-V curve was obtained after the device was subsequently negatively biased (-10 V). The second device showed a smaller on/off ratio than the first one, which may originate form the difference of gold content in the active layers. In contrast, the third device exhibited neither switching nor NDR behavior in the voltage scale of 0-5.5 V. These results are consistent with those of devices based on CP/GNP hybrids through spin-coating,16 except for a

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Figure 5. Current-voltage (I-V) curves for ITO/P3HT:MTGNPs/Al (9/0), ITO/P3HT:DTGNPs/Al (2/0), and ITO/P3HT/Al devices (b/ O) with the same thickness of active layer (240 nm). Arrows indicate scan directions, and applied voltage pulse is 0.5 s.

Figure 7. (A) Retention time test for device described in Figure 5 by applying a 100 Hz read pulse of 1 V at either on or off states. (B) Device current responses to 100 Hz 1 V read pulse in the stress test by applying the cycled write-read-erase pulse. Figure 6. Current responses (9 for write-erase pulses and O for read pulses) of the device shown in Figure 5 at different applied voltage pulses (solid lines) during write-read-erase pulse cycles. Write voltage was 5 V, read voltage was 1 V, and erase voltage was -10 V. Applied pulse width was 0.1 s, and time used for measuring each current was 1/60 s.

slightly higher current response in the low conductivity state for our device. This is possibly due to that the electrochemically fabricated film has a compact morphology and that GNPs were spatially separated by a polythiophene component. Voltammetric scanning cycles from 1 to 4 were used for depositing the active layer to modulate the thickness of the composite film from 120 to 240, 280, and 340 nm, respectively. The device with a 120 nm active layer was shortened during the testing process, and the device with a 340 nm active layer did not exhibit electric bistability. In contrast, the other two devices exhibited similar performances as shown in Figure 5. Furthermore, we changed the contents of MTGNPs in the electrolytes from 0 to 0.33, 0.67, or 1.33 mg mL-1 to change the GNP content of the active layer. As shown in Figure 5, the device without MTGNPs did not show the behavior of electric bistability. The devices with active layers grown from the electrolytes containing 0.67 mg mL-1 MTGNPs exhibited the highest on/off ratio. Therefore, the memory devices with the highest performances had active layers with optimized thickness of 240-280 nm and a GNP content of around 9 wt %. Write-read-erase cycling tests on the device shown in Figure 5 were performed in a nitrogen environment, and the results are demonstrated in Figure 6. The cycling pulse was designed as follows: 5 V to write, then 1 V to read, subsequently

-10 V to erase, and another 1 V to read. The current at the end of each 0.1 s voltage pulse was measured within 1/60 s. The current responses to each read pulses (represented as cycles) oscillated from 10-4 to 10-7 A, which can be repeated many times. The electrical conductivity switching possesses an on/ off ratio around 103. Stress tests were performed in nitrogen atmosphere on the device shown in Figure 5 after being stored for 40 days without packing. Figure 7A illustrates the device current responses to a repeated 1 V bias with a pulse width of 0.01 s before which a voltage of 5 or -10 V was given to change it to an on or off state. After experiencing 104 read pulses, the device maintained its original conductivity without significant change, which demonstrates its excellent retention of both states. Moreover, the devices were subjected to be cycled by write-read-erase pulses as described in Figure 6, and the current responses to read voltage are represented in Figure 7B. The on/off ratio slightly increased in the first 200 cycles and then decreased gradually to less than 10 after 1500 cycles. Accordingly, the electrochemically fabricated memory devices showed a high stability even after being stored in air for over 1 month. This is mainly due to that MTGNPs were connected to P3HT chains with covalent bonds and pinned in the matrix of the active layer without further phase separation and aggregation. In this case, the on/off ratio of the device was less than 100 because of a short potential pulse of 0.01 s. This phenomenon has also been observed by another group.16 With respect to the electric switching mechanism of this conducting polymer/GNPs composite system, the electric fieldinduced charge transfer16 in a 2-D single-electron tunneling

18396 J. Phys. Chem. C, Vol. 111, No. 49, 2007 model25 is an appropriate explanation. It was reported that the hole injection barrier, φh, between P3HT and GNPs was around 0.59 eV. When the external electric field is sufficiently high, electrons in P3HT break through the Schottky barrier and tunnel to GNPs. On one hand, the partially negatively charged GNP17 arrays form a charge transfer pathway. On the other hand, the charge carrier number in the oxidized P3HT film increases and results in the enhancement of its conductivity. Thus, the device switches to the on state. When the external electric field increases further, a coulomb blockade occurs in the GNPs array and results in the device having NDR property. However, when a sufficiently high reverse bias was applied, GNPs can be discharged, and the P3HT is consequently reduced. As a result, the device was switched back to the off state. It was proposed that the 1-dodecanethiol insulating shells of MTGNPs can stabilize their negative charges.16 For comparison, we also fabricated the memory devices based on the composite of poly(3HT) and 3-(6-mercapto-1-hexyl)thiophene thiolated GNPs; however, the device showed no significant difference in switching behavior with respect to that of the device shown in Figure 5. These results implied that the insulating shells of GNPs did not play a predominate role in electric bistability. 4. Conclusion Memory devices with active layers composed of the composites of P3HT and MTGNPs were successfully fabricated through electrochemical deposition. The devices exhibited a strong electric bistability and high environmental stability. This is mainly due to that the GNPs are pinned in the P3HT matrix via covalent bonds and uniformly dispersed in the composite film without phase separation and aggregation. Electrochemical deposition provides a direct and useful technique for fabricating the active layers of OBDs devices based on the composites of different metal nanoparticles and conducting polymers under precise control. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50533030), 973 Project (2003CB615700), and SEFDP (20060003081).

Chen et al. Supporting Information Available: Absorption spectra of CH2Cl2 soaking solutions of composite films. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Scott, J. C. Science (Washington, DC, U.S.) 2004, 304, 62. (2) Bez, R.; Camerlenghi, E.; Modelli, A.; Visconti, A. Proc. IEEE 2003, 91, 489. (3) Ma, L. P.; Liu, J.; Pyo, S.; Xu, Q. F.; Yang, Y. Mol. Cryst. Liq. Cryst. 2002, 378, 185. (4) Bandyopadhyay, A.; Pal, A. J. Appl. Phys. Lett. 2003, 82, 1215. (5) Yang, Y.; Ouyang, J.; Ma, L. P.; Tseng, R. J. H.; Chu, C. W. AdV. Funct. Mater. 2006, 16, 1001. (6) Scott, J. C.; Bozano, L. D. AdV. Mater. 2007, 19, 1452. (7) Ouyang, J. Y.; Chu, C. W.; Szmanda, C. R.; Ma, L. P.; Yang, Y. Nat. Mater. 2004, 3, 918. (8) Ma, L. P.; Liu, J.; Yang, Y. Appl. Phys. Lett. 2002, 80, 2997. (9) Ma, L. P.; Pyo, S.; Ouyang, J.; Xu, Q. F.; Yang, Y. Appl. Phys. Lett. 2003, 82, 1419. (10) Paul, S.; Pearson, C.; Molloy, A.; Cousins, M. A.; Green, M.; Kolliopoulou, S.; Dimitrakis, P.; Normand, P.; Tsoukalas, D.; Petty, M. C. Nano Lett. 2003, 3, 533. (11) Mukherjee, B.; Pal, A. J. Chem. Mater. 2007, 19, 1382. (12) Mohanta, K.; Majee, S. K.; Batabyal, S. K.; Pal, A. J. J. Phys. Chem. B 2006, 110, 18231. (13) Kolliopoulou, S.; Dimitrakis, P.; Normand, P.; Zhang, H. L.; Cant, N.; Evans, S. D.; Paul, S.; Pearson, C.; Molloy, A.; Petty, M. C.; Tsoukalas, D. Microelectron. Eng. 2004, 73-74, 725. (14) Ouyang, J. Y.; Chu, C. W.; Tseng, R. J. H.; Prakash, A.; Yang, Y. Proc. IEEE 2005, 93, 1287. (15) Ouyang, J. Y.; Chu, C. W.; Sieves, D.; Yang, Y. Appl. Phys. Lett. 2005, 86. (16) Prakash, A.; Ouyang, J. Y.; Lin, J. L.; Yang, Y. J. Appl. Phys. 2006, 100, 54309. (17) Tseng, R. J.; Huang, J. X.; Ouyang, J.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 5, 1077. (18) Mo¨ller, S.; Perlov, C.; Jackson, W.; Taussig, C.; Forrest, S. R. Nature (London, U.K.) 2003, 426, 166. (19) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 2064. (20) Keegstra, M. A.; Peters, T. H. A.; Brandsma, L. Synth. Commun. 1990, 20, 213. (21) Robert, L.; Frank, P. V. S. J. Am. Chem. Soc. 1946, 68, 2103. (22) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (23) Fan, B.; Wang, P.; Wang, L. D.; Shi, G. Q. Sol. Energy Mater. Sol. Cells 2006, 90, 3547. (24) Yan, H. L.; Shi, G. Q. Nanotechnology 2006, 17, 13. (25) Tang, W.; Shi, H. Z.; Xu, G.; Ong, B. S.; Popovic, Z. D.; Deng, J. C.; Zhao, J.; Rao, G. H. AdV. Mater. 2005, 17, 2307.