Flexible Organic Bistable Devices Based on Graphene Embedded in

May 26, 2010 - Dong Ick Son,†,| Tae Whan Kim,*,†,| Jae Ho Shim,† Jae Hun Jung,† Dea Uk Lee,†. Jung Min Lee,‡ Won Il Park,‡ and Won Kook ...
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Flexible Organic Bistable Devices Based on Graphene Embedded in an Insulating Poly(methyl methacrylate) Polymer Layer Dong Ick Son,†,| Tae Whan Kim,*,†,| Jae Ho Shim,† Jae Hun Jung,† Dea Uk Lee,† Jung Min Lee,‡ Won Il Park,‡ and Won Kook Choi§ †

Department of Information Display Engineering and Department of Electronics and Computer Engineering and Division of Materials Science and Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea and § Optoelectronic Materials Center, Korea Institute of Science and Technology, Cheongryang, P.O. Box 131, Seoul 130-650, Korea ‡

ABSTRACT The electrical properties of flexible nonvolatile organic bistable devices (OBDs) fabricated with graphene sandwiched between two insulating poly(methyl methacrylate) (PMMA) polymer layers were investigated. Current-voltage (I-V) measurements on the Al/PMMA/graphene/PMMA/indium-tin-oxide/poly(ethylene terephthalate) devices at 300 K showed a current bistability due to the existence of the graphene, indicative of charge storage in the graphene. The maximum ON/OFF ratio of the current bistability for the fabricated OBDs was as large as 1 × 107, and the endurance number of ON/OFF switchings was 1.5 × 105 cycles, and an ON/OFF ratio of 4.4 × 106 was maintained for retention times larger than 1 × 105 s. No interference effect was observed for the scaled-down OBDs containing a graphene layer. The memory characteristics of the OBDs maintained similar device efficiencies after bending and were stable during repetitive bendings of the OBDs. The mechanisms for these characteristics of the fabricated OBDs are described on the basis of the I-V results. KEYWORDS Flexible, organic bistable device, graphene, polymer, memory characteristics

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anocomposite structures have been particularly attractive because of interest in both investigations of fundamental physical properties1,2 and potential applications in next-generation electronic and optoelectronic devices operating at lower powers.3–7 Hybrid inorganic/ organic composites containing inorganic nanoparticles have currently emerged as excellent candidates for potential applications in next-generation nonvolatile memory devices.8–10 Recently, graphene materials have attracted much attention due to their fascinating physical properties, such as quantum electronic transport,11,12 a tunable band gap,13 extremely high mobility,14 high elasticity,15,16 and electromechanical modulation.17 Because of the discovery of the first graphene isolated by mechanical exfoliation of graphite crystals,18 many chemical approaches to synthesize largescale graphene have been developed, and the self-assembly of soluble graphene sheets demonstrates the possibility of low-cost synthesis and fabrication of large-scale transparent films.19,20 Fabricated graphene sheets of continuous film, with graphene from a single layer to a few layers can be grown on polycrystalline Ni by using ambient pressure chemical vapor deposition and can be transferred to a large variety of substrates.20 The graphene sheets, acting as charging and discharging layers, have been particularly

interesting due to their being promising candidates for flexible nonvolatile bistable memory devices. Flexible memory devices have emerged as excellent candidates for potential applications in next-generation flexible electronics and optoelectronic devices. Even though some works concerning the memory effects in organic bistable devices (OBDs) fabricated utilizing inorganic nanoparticles embedded in an organic layer fabricated by using a precisely controlled method under vacuum conditions have been performed,21–24 studies on the electrical bistabilities, the memory stabilities, scale-down effects, and the memory mechanisms of flexible OBDs based on graphene embedded in insulating poly(methyl methacrylate) (PMMA) polymer layers have not yet been reported. Furthermore, studies on the OBDs made of the grapheme embedded into the PMMA layers in comparison with that containing the nanoparticles embedded into the polymer layers are very important for improving their flexibility and reproducibility due to the uniform distribution of the graphene. This Letter reports data, which were obtained before bending and after bending, for the electrical bistabilities, the memory stabilities, and the memory mechanisms of threelayer structured flexible OBDs fabricated utilizing graphene sheets sandwiched between PMMA polymer layers formed by transferring graphene sheets and using a simple spincoating technique. Transmission electron microscopy (TEM) measurements were performed to investigate the microstructural properties of the PMMA/graphene/PMMA films.

* To whom correspondence should be addressed, [email protected]. |

These authors contributed equally to this work. Received for review: 02/19/2010 Published on Web: 05/26/2010 © 2010 American Chemical Society

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FIGURE 1. Schematic diagrams of the fabricating processes for the OBDs: (a) SiO2/Si substrate; (b) Ni formation on the SiO2/Si substrate by using an e-beam evaporation process; (c) graphene layer formation on the Ni/SiO2/Si substrate by using the chemical vapor deposition process; (d) PMMA polymer growth on the Ni/SiO2/Si substrate by using a spin-coating technique; (e) separation of the PMMA/graphene/Ni and the SiO2/Si substrate in a HF etching solution; (f) separation of the PMMA/graphene and the Ni in a TGF etching solution; (g) transfer from the PMMA/graphene to the ITO/PET substrate; (h) PMMA polymer growth on graphene/PMMA/ITO/PET sheets by using a spin-coating technique; (i) Al electrode deposition on PMMA/graphene/PMMA/ITO/PET sheets by using a thermal evaporation process; (j) photographs of the device after bending.

The carbon atoms segregated from the Ni films formed largearea, continuous graphene sheets during cooling.26 The transfer method for the graphene sheets deposited on Ni/SiO2/Si substrates is similar to that previously reported.20 After a PMMA (MicroChem, 950000 MW, 9-6 wt % in anisole) layer had been spun coated onto a graphene/ Ni/SiO2/Si substrate, the PMMA/graphene/Ni/SiO2/Si substrates were separated from the SiO2 substrate in a HF etching solution; subsequently, the Ni layer was removed in a TFG etching solution. Poly(ethylene terephthalate) (PET) flexible substrates coated with indium-tin oxide (ITO) films with a thickness of 100 nm and a sheet resistance of 45 Ω/square were purchased from the Sigma Aldrich Co. The PMMA/graphene sheets with an opposite direction were transferred onto the ITO/PET sheets. After the PMMA solution had been deposited onto the graphene/PMMA/ITO/PET sheets by using a spin-coating technique, an Al layer with a thickness of about 150 nm was deposited. A schematic diagram of the formation processes for the OBDs fabricated in this work is shown in Figure 1. I-V measurements were performed by using an HP 4140B unit. The switching characteristics were measured at 300 K by using an Agilent 4155C semiconductor parameter analyzer with an Agilent 33250A 80 MHz function/arbitrary

Current-voltage (I-V) measurements were carried out to investigate the electrical properties of the OBDs containing the graphene sheets embedded in the PMMA polymer. Current-time (I-t) and current-cycle measurements under flat and bent conditions were performed to investigate the memory stabilities of the OBDs. The interference effect of the scaled-down OBDs was investigated, and the memory mechanisms for the OBDs are described on the basis of the I-V results. Graphene sheets were formed to fabricate three-layer nonvolatile polymer-graphene bistable memory devices as follows:25 the SiO2-coated Si substrate was cleaned by using a routine chemical cleaning procedure of the sonification in a acetone and methanol solution and a rinse in a deionized water. Then the chemically cleaned SiO2-coated Si substrates were dried by using N2 gas, followed by the deposition of a 200 nm thick Ni layer on the SiO2-coated Si substrate by using thermal evaporation. The substrates were mounted onto a susceptor in a custom-built chemical vapor deposition chamber, where the substrates were heated to 850-1000 °C and then sustained for 30-60 sec under a H2 and Ar atmosphere. As soon as the thermal annealing process was finished, 50 sccm of methane (CH4) was introduced into the processing chamber for 10 min. The samples were then cooled to room temperature at a cooling rate of ∼10 °C/s. © 2010 American Chemical Society

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-5 V in all the cases shown in Figure 3a. The I-V curves for the devices clearly show electrical hysteresis behaviors, which are essential features for memory devices. State “1” and state “0” correspond to a relatively high-current state (ON state) and a relatively low-current state (OFF state), respectively. The state transition from the “OFF” to the “ON” states is equivalent to the “writing” process in a digital memory cell. After the transition is achieved, the “ON” state is maintained in the device, even after the power is turned off, which is shown in the high current denoted by the upper empty circles in Figure 3a. The maximum current ratio between the “ON” and the “OFF” states for the Al/PMMA/ graphene/PMMA/ITO/PET device before bending at 2 V is as large as about 1 × 107. The “OFF” state can be achieved by applying a reverse bias voltage about -5 V, which is defined as the “erase voltage” for the device. When the device current is decreased by 7 orders of magnitude, the state of the device is returned to its initial OFF state. This is equivalent to the “erasing” process of a digital memory cell. The switching process of the devices offers opportunities for promising applications in memristors.27 The current ratio between “ON” state and “OFF” state for the device without a graphene layer is negligible in comparison with that for the device with a graphene layer, which indicates that the electrical bistability of the device can be attributed to the existence of the graphene layer. Write-read-erase-read sequence test measurements were performed in air in order to investigate the switching characteristics of the nonvolatile memories. The write, the read, and the erase voltage pulses for the I-t characteristics were set as +5, +2, and -5 V, respectively, as shown in Figure 3b. These write and erase voltages are the lowest values reported for OBDs. The read voltage pulses distinguish the ON state with a current of 10-5 A from the OFF state with a current of 10-11 A. While the erase voltage pulse shifts from the ON state to the OFF state with a current below 10-6 A, the write voltage pulse changes the OFF state to the ON state with a current of 10-4 A. The difference between the current levels shown in parts a and b of Figure 3 is mainly attributed to the difference between the pulse mode and the dc mode of operation. A sequence of pulses lasting less than 1 ms is employed to confirm the memory endurance. The currents as functions of the number of cycles of the ON and the OFF states for the OBDs are shown in Figure 3c. An endurance ability of ON/OFF switching before bending of over 1.5 × 105 times, together with a rewriting capability, is clearly observed in Figure 3c. The retention ability of the OBDs was tested by keeping the device in the OFF state at -2 V and in the ON state at +2 V under ambient conditions. Figure 3d shows the currents before bending as functions of the retention time for 105 s. The ON/OFF ratio for the device before bending shows a very high value of about 4.4 × 106, and the extrapolation of the value to 10 years converges to 4.9 × 106, indicative of the very long time stability of OBDs.

FIGURE 2. (a) Cross-sectional TEM image of the PMMA/graphene/ PMMA/ITO layers and (b) cross-sectional HRTEM image of the PMMA/ graphene/PMMA layers. The inset represents an enlarged view of the graphene taken by using the HRTEM image.

waveform generator. TEM observations were performed in a JEM 2100F transmission electron microscope operating at 200 kV. A cross-sectional TEM image with graphene sheets sandwiched between two insulating PMMA polymer layers showed that the graphene sheets were clearly embedded in the PMMA layer, as shown in Figure 2a. A high-resolution TEM (HRTEM) image showed a PMMA/graphene/PMMA composite with 2 nm (about five layers) graphene sheets, as shown in Figure 2b. The inset represents an enlarged view of the graphene layer taken from the HRTEM image. Figure 3a shows the I-V curves for Al/PMMA/graphene/ PMMA/ITO/PET devices before bending. The voltage applied to the device was varied in a cycle from -5 to 0 to 5 to 0 to © 2010 American Chemical Society

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FIGURE 3. (1) Memory characterizations of the OBDs before bending: (a) current-voltage curves for the Al/PMMA/graphene/PMMA/ITO/PET device, (b) operation of a write-read-erase-read (5/2/-5/2 V) sequence, (c) cycling stress test of the Al/PMMA/graphene/PMMA/ITO/PET device under a stress voltage of 2 V per 4 ms, and (d) retention test of OBDs in an ambient environment. (2) Memory characterization after bending (bending radius, 10 mm): (e) current-voltage curves for the Al/PMMA/graphene/PMMA/ITO/PET devices (inset: A photograph of a flexible OBD bent at R ) 10 mm), (f) operation of a write-read-erase-read (5/1.8/-5/1.8 V) sequence, (g) cycling stress test of the Al/PMMA/graphene/ PMMA/ITO/PET device under a stress voltage of 1.8 V per 4 ms, and (h) retention test of OBDs in an ambient environment.

Figure 3e shows I-V curves for the Al/PMMA/graphene/ PMMA/ITO/PET device after bending. The OBDs were bent into a curve with a surface curvature radius (R) of 10 mm, as shown in the inset of Figure 3e. The voltage applied to the device was varied in a cycle from -5 to 0 to 5 to 0 to -5 V in all the cases shown in Figure 3e. The I-V curves clearly show electrical hysteresis behaviors. The maximum current ratio between the “ON” and the “OFF” states for the Al/ PMMA/graphene/PMMA/ITO/PET device after bending at 1.8 V is as large as about 1 × 107. The device maintained good © 2010 American Chemical Society

characteristics, and its ON/OFF current ratio after bending was approximately 107, as shown in parts e-h of Figure 3. Even though the current level of the “ON” state voltage is decreased after bending, the device performance is not affected by the bending. The device performances are stable and reproducible, regardless of repetitive bending. A cycling endurance of more than 1.5 × 105 ON/OFF switchings, together with a rewriting capability, for the device after bending, is clearly observed in Figure 3g. The retention ability of the OBDs was tested by maintaining 2444

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the device in an OFF state at -1.8 V and in an ON state at +1.8 V under ambient conditions. Figure 3h plots I-t, measured every 60 s after bending for 105 s, and the ON/ OFF ratio is as high as 4.4 × 106. The extrapolation of the I-t curve to 10 years converges to 9.1 × 106, indicative of long-life stability of the OBDs. The interference behavior of the scaled-down flexible OBDs was investigated by decreasing the diameters of the top electrodes from 26.71 to 17.74 µm. The distance between the two “R” and “L” electrodes of two neighboring OBDs was decreased to 17 µm, as shown in Figure 4a. When voltages from 0 to 2 V were independently applied to the “L” or the “R” electrodes, the two OBDs maintained the “OFF” state, as shown in Figure 4b. When voltages from 0 and 5 V were applied to the “R” electrodes of the two OBDs, the OBDs switched from the “OFF” to the “ON” state, and when voltages were applied to the “L” electrodes, the OBDs maintained the “OFF” state, as shown in Figure 4c. Therefore, no interference effect was observed for the scaleddown OBDs containing a graphene layer when the distance between the top electrodes of the OBDs was greater than or equal to 17 µm. Even though scaled-down experiments of the memory devices with nanoscale electrodes are very important for enhancing high storage density,28–30 the OBDs with nanoscale electrodes based on organic/inorganic nanocomposites cannot be fabricated due to the organic lithography limitation of the device fabrication. The data were fitted by using various conduction models based on the I-V curves to clarify the carrier transport and memory mechanisms of the fabricated OBDs.31–33 Figure 5 shows a log-log plot for the I-V results of Figure 3a. The low current of the OFF state from 0 to 1 V might be attributed to thermally generated electrons. However, two distinct regions with different slopes in the OFF state are clearly observed at voltages above 1 V, as shown in Figure 5. The slope of the fitting line in the applied voltage range between 1 and 2.4 V is 1, indicating that the device current is dominated by an Ohmic current in this range. This low current below 2.4 V is due to the thermally generated carriers at the interface and the PMMA layer.32 However, the slope of the fitting line in the applied voltage range between 2.4 and 3.4 V is 11.2. The number of carriers injected into the polymer layer is significantly higher in this voltage range, and the I-V data consequently follow a typical space charge limited current (SCLC) with a trap model, which is known as the trapped charge-limit current mechanism.32 The large slope value of the fitting line indicates that the traps in the PMMA layer are exponentially distributed over energy in the PMMA band gap.34 After the state transition to an ON state at 3.4 V, the I-V curve can be fitted well by using the Ohmic current model, as shown in Figure 5, as is the I-V curve in the range of negative voltages. The carrier transport due to the Ohmic current in the ON state is not affected by the direction of the current, the interface characteristics, or the space charges due to the existence of the traps. Therefore, © 2010 American Chemical Society

FIGURE 4. (a) Top view of the electrodes for the original and the scaled-down OBDs. (b) and (c) Currents as functions of the voltages for flexible OBDs with different voltages applied to the “L” and the “R” electrodes. The data for the OBDs with voltages applied to the “L” and the “R” electrodes are represented by rectangles and circles, respectively.

the ON state current might flow through a local conducting path, such as a metallic filament acting as a resistor with a low resistivity.35–37 The bistable behavior for the Al/PMMA/graphene/PMMA/ ITO/PET devices might be attributed to conducting filaments formed in the PMMA layer at the state transition. The graphene layer and the traps existing in the PMMA act as trapping sites, which capture electrons injected from the electrode. The electrons captured in the graphene and in the traps generate a local internal field in the PMMA layer, and 2445

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bending. Because the conducing filaments are actually formed in the PMMA layer, external factors, such as pressure or heat, can collapse or decrease the conducting filament for the device. In summary, flexible OBDs based on PMMA/graphene/ PMMA nanocomposites were fabricated. HRTEM images showed that graphene layers were embedded between the PMMA polymer layers. The I-V curves at 300 K for the Al/ PMMA/graphene/PMMA/ITO/PET devices exhibited electrical bistable behaviors before and after bending. I-t and current-cycle measurements in flat and bent conditions demonstrated the memory stabilities of the OBDs. The memory characteristics of the OBDs after bending maintained almost the same values as those of the devices before bending and were stable during repetitive bendings of the OBDs. No interference effect was observed for the scaleddown OBDs containing a graphene layer when the distance between the top electrodes of the OBDs was greater than or equal to 17 µm. These results indicate that flexible OBDs based on graphene and PMMA polymer composites hold promise for potential applications in next-generation transparent flexible nonvolatile memory devices.

FIGURE 5. A log-log plot of the current as a function of the applied voltage for Al/PMMA/graphene/PMMA/ITO/PET devices. The curves are fitted for the SCLC mechanism.

a conducting filament can be formed in the PMMA layer under a high internal field.38 However, the electrons captured in the traps are easily emitted to the lowest unoccupied molecular orbital (LUMO) level because the traps in the PMMA layer are exponentially distributed over energy. Because the trap density near the LUMO level is highest, many captured electrons can be emitted by a small amount of a thermal energy. The traps in the PMMA are partly occupied by the electrons. Therefore, only the electrons captured in the traps cannot generate sufficient internal field to form conducting filaments, resulting in the disappearance of the memory effect in the device without the graphene layer. However, the electrons captured in the graphene layer for the device containing the graphene layer are sufficient to generate a higher enough internal field to form the conducting filament in the PMMA layer. No metal component, except the Al electrode, is used in the device. Therefore, the conducting filament these formed might be caused by diffusion from the Al electrode during the metal deposition.39 The device current in the OFF state is attributed to an Ohmic current caused by thermally generated electrons and by the SCLC with exponential traps in the PMMA layer. When the applied voltage increases to 3.4 V, a conducting filament is formed in the PMMA layer, resulting in an abrupt current increase up to 10-4-10-3 A and a state transition to the ON state. The characteristics of the ON state currents at positive and negative voltages completely show a Ohmic current, indicating that the bistable behavior can evidently be attributed to a conducting filament with a low resistivity. The filament formed in the OBDs is constantly maintained even in the presences of an external applied cutoff voltage. The filament at an applied voltage of -5 V collapses due by Joule heating from the high current.38 After the filament collapses, the device state returns to the OFF state. The filament collapse for the bent device occurs at approximately -4 V, which is smaller than the value for the flat device. This filament collapse at low voltages might be attributed to a decrease in the conducting filament due to the device © 2010 American Chemical Society

Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. R0A-2007-000-200440). REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17)

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