NANO LETTERS
Polyaniline Nanofiber/Gold Nanoparticle Nonvolatile Memory
2005 Vol. 5, No. 6 1077-1080
Ricky J. Tseng,† Jiaxing Huang,‡ Jianyong Ouyang,† Richard B. Kaner,†,‡ and Yang Yang*,† Department of Materials Science and Engineering and Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, California 90095 Received March 25, 2005; Revised Manuscript Received April 24, 2005
ABSTRACT A nonvolatile plastic digital memory device based on nanofibers of the conjugated polymer polyaniline decorated with gold nanoparticles is reported. The device has a simple structure consisting of the plastic composite film sandwiched between two electrodes. An external bias is used to program the ON and OFF states of the device that are separated by a 3-orders-of-magnitude difference in conductivity. ON−OFF switching times of less than 25 ns are observed by electrical pulse measurements. The devices possess prolonged retention times of several days after they have been programmed. Write−read−erase cycles are also demonstrated. The switching mechanism is attributed to an electricfield-induced charge transfer from the polyaniline nanofibers to the gold nanoparticles. The active polymer layer is created by growing nanometer size gold particles within 30-nm-diameter polyaniline nanofibers using a redox reaction with chloroauric acid. This device combines two exciting research areassnanoparticles and conducting polymerssto form a novel materials system with unique functionality.
Conjugated polymers and other organic materials are uniquely suited for thin film, large area, mechanically flexible, low cost electronic devices.1,2 Their tremendous commercial potential has touched off a flurry of research, particularly on organic light-emitting diodes,3,4 transistors,5,6 solar cells7,8 and memory devices.9-14 By using nanoscale materials, highdensity electronic devices are possible with superior performance and manufacturability.15,16 Therefore, a conducting polymer decorated with metallic or semiconducting nanoparticles provides an exciting system to investigate with the possibility of designing device functionality. Recently, we have demonstrated a facile bulk-synthetic method to prepare high-quality polyaniline nanofibers with diameters tunable from 30 to 120 nm.17-19 Metal nanoparticles (Au, Ag) can be grown inside of the dedoped polyaniline nanofibers by a redox reaction with the metal ions (Au3+, Ag+).20-22 The combination of conducting polymers with nanoparticles offers a new direction for organic electronic devices. Nonvolatile organic/polymer memory represents an ideal application to take advantage of this novel materials system. Our earlier work has shown that electrical properties of an organic thin film can be dramatically modified when metallic nanoparticles are embedded within an organic film.11,12 This phenomenon has been attributed to charge storage inside the nanoparticles.12,13,23 In each case, the formation of nanoparticles is carried out by carefully controlling the deposition process. Electrical bistability and memory has also been * Corresponding author. E-mail:
[email protected]. † Department of Materials Science and Engineering. ‡ Department of Chemistry and Biochemistry. 10.1021/nl050587l CCC: $30.25 Published on Web 05/12/2005
© 2005 American Chemical Society
demonstrated by blending synthesized gold nanoparticles with electron donor molecules in an inert polystyrene polymer matrix.14 Here we take a giant leap forward by using polyaniline nanofibers decorated with gold nanoparticles as the polymer memory element. This is a one-component system that significantly simplifies the device structure and fabrication process. A relatively uniform distribution of nanometer-sized gold nanoparticles is created (Figure 1A) by controlling the time and temperature of a reaction between 30-nm-diameter polyaniline nanofibers and chloroauric acid. The device is fabricated through the following process: A bottom (column) aluminum electrode with a thickness of 80 nm is deposited by thermal evaporation in a chamber under a pressure of 1 × 10-5 Torr. The thickness is determined using a profilometer (Dektak). A 70-nm-thick active layer is formed by spin coating an aqueous solution of ∼0.1 wt % polyaniline nanofiber/gold nanoparticle composite in 1.5 wt % poly(vinyl alcohol). The poly(vinyl alcohol) serves as an electrically insulating matrix for the composite. Both the top and bottom aluminum electrodes have a width of 0.2 mm, and the device covers an area of 0.2 × 0.2 mm2 in Figure 1B. All electrical experiments are conducted under a vacuum of 1 × 10-4 Torr. Current-voltage (I-V) and device retention time characteristics are measured using a semiconductor parameter analyzer (HP 4155B). The current response of write-read-erase cycle tests is measured with a programmable power supply (HP 3245A) and recorded with a fourchannel oscilloscope (Tektronix TDS 460A). The device response time is measured by applying a pulse from an HP
Figure 2. Currrent-voltage characteristics of the polyaniline nanofiber/gold nanoparticle device. The potential is scanned from (A) 0 to +4 V, (B) +4 to 0 V, and (C) 0 to +4 V. Between +3 and +4 V, a region of negative differential resistance (NDR) is observed. The inset shows the retention time test of the ON-state (top) and OFF-state (bottom) currents when biased at +1 V every 5 s.
Figure 3. Retention time test of the ON-state and OFF-state currents when biased at a constant 1 V with a width of 0.167 s, recorded every 5 s. Figure 1. TEM image and device structure. (A) Transmission electron microscopy image of the polyaniline nanofiber/gold nanoparticle composite. The black dots are ∼1-nm gold nanoparticles contained within ∼30-nm-diameter polyaniline nanofibers. (B) The structure of the polyaniline nanofiber/gold nanoparticle bistable memory device.
214B pulse generator to the device followed by an I-V scan with an HP 4155B to determine if the device is in its ON or OFF state. The atomic force microscope image and nanoscale I-V curve are measured with a Dimension 3100 TUNA/ CAFM (platinum-iridium-coated Si tip with a radius of 15 nm) from Veeco Instruments. The polyaniline nanofiber/gold nanoparticle device exhibits very interesting bistable electrical behavior (Figure 2). As the potential is increased to +3 V, an abrupt increase in current is observed. This changes the device from a lowconductivity (10-7 amps) OFF state to a high-conductivity (10-4 amps) ON state (Figure 2, curve A). The device is stable in the ON state when the potential is lowered back to 0 V (Figure 2, curve B). The high conductivity of the ON state can be changed back to the OFF state by applying a reverse bias of -5 V. The device is then stable in the OFF state until +3 V is applied, at which point it returns to the ON state (Figure 2, curve C). If the potential is raised above +3 V, then a region of negative differential resistance (NDR) is observed. Negative differential resistance has been reported 1078
elsewhere in other memory devices13 but appears to have no effect on the performance of our device within the +3 to +4 V region. Note that if the gold nanoparticles are grown with diameters greater than 20 nm the devices can be switched on only once and they exhibit ohmic behavior in the ON state, indicating that the more metallic nature of the larger gold particles then dominates the switching. Additionally, devices made with just polyaniline nanofibers and no gold nanoparticles show no electronic switching. The electrical bistability suggests that the polyaniline nanofiber/gold nanoparticle composite can be used for nonvolatile memory. Other important characteristics of memory devices include the retention time and the ability to read, write, and erase data. The retention time of the polyaniline nanofiber/ gold nanoparticle device in the ON state was tested every couple of hours over a 3-day period with no appreciable change in conductivity observed. A stress test was carried out by applying a bias of +1 V with a duration of 0.0167 s, and the current was measured every 5 s until the 10 000 data point limit of the parameter analyzer was reached. No significant change in conductivity was noted during the 14-h stress test (Figure 3), although after several days a slight decrease in conductivity in the ON state was observed. Write-read-erase cycle tests carried out on a device are as shown in Figure 4. The upper part shows the continuous Nano Lett., Vol. 5, No. 6, 2005
Scheme 1. Schematic Structure of a Polyaniline Nanofiber/ Gold Nanoparticle after the Application of +3 Va
Figure 4. Current response (left axis) of the polyaniline nanofiber/ gold nanoparticle device to applied potentials (right axis) during write-read-erase testing cycles. A potential of +4.8 V is used to write, -6 V is applied to erase, and +1.2 V is used to read. W ) write, R ) read, and E ) erase. The duration of each cycle pulse is 0.1 s, during which time the current response is recorded using an oscilloscope.
Figure 5. I-V characteristics of the OFF and the ON states of the polyaniline nanofiber/gold nanoparticle device before and after the application of a voltage pulse of 4 V with a width of 25 ns, as shown in the inset.
voltage biases applied to the device. Various bias strengths were used to write the device to the ON state (+4.8 V), to read the ON state current (+1.2 V), to erase the device to the OFF state (-6 V), and to read the OFF state current (+1.2 V). The corresponding currents recorded by an oscilloscope are 2 × 10-5, 1 × 10-5, -5 × 10-5, and 10-6-10-7 amps, respectively, as shown in the lower part of Figure 4. (Please note that we have plotted the absolute values of current because of the log scale used along the Y axis.) The device can be cycled many times as is apparent from Figure 4. A readily distinguishable ON/OFF ratio around 20 is maintained. The polyaniline nanofiber/gold nanoparticle device exhibits a fast response to applied voltage pulses as shown in Figure 5. The device is initially in the OFF state, confirmed by the I-V curve in the range of 0 to 2.5 V. Because the turn-on bias is around 3 V, the voltage scan will not trigger the switching process. Subsequently, a pulsed voltage of 4 V with a duration of 25 ns (inset of Figure 5) generated by an HP 214B pulse generator is applied to the device. This transition and the ON state are confirmed by the second I-V scan, shown as the top I-V curve. Similarly, the device in the ON state can be turned to the OFF state by applying a Nano Lett., Vol. 5, No. 6, 2005
a An increase in charge transfer from polyaniline to the gold nanoparticles is believed to occur.
pulse of -5 V with a 25-ns duration. This pulse width is the limit of our instrument (HP 214B pulse generator), suggesting that the actual switching time may be faster. These response times are much shorter than the transition reported for organic bistable molecules,24,25 which are in the microsecond or slower regime. The nanosecond transition time suggests that the switching mechanism is due to electronic processes rather than chemical reactions, conformational changes,24 or isomerizations,25 as reported for other devices. Because the nanosecond switching of the polyaniline nanofiber/gold nanoparticle device must involve electronic processes, the following mechanism is proposed. The transition from the OFF to the ON state is attributed to an electricfield-induced charge transfer between the polyaniline nanofibers and the gold nanoparticles. Under a sufficient electric field, electrons that reside on the imine nitrogen of the polyaniline may gain enough energy to surmount the interface between the nanofibers and the gold nanoparticles and move onto the gold nanoparticles (Scheme 1). Consequently, the gold nanoparticles become more negatively charged, whereas the polyaniline nanofibers become more positively charged. The conductivity of the polyaniline nanofiber/gold nanoparticle composite will increase dramatically after the electric-field-induced charge transfer, in accordance with the transition from the OFF to the ON state. This proposed mechanism is supported by the following evidence. First, X-ray photoelectron spectra taken of the composite shows a shift from 399.2 to 399.7 eV for the N1S core electrons compared to the spectra of the undoped, emeraldine base polyaniline, indicating that the nitrogen in the polyaniline nanofiber/gold nanoparticle composite is partially positively charged. At the same time, the binding energy of the gold electrons (4f5/2) decreases from 87.7 to 87.5 eV, indicating that a partial negative charge resides on the gold nanoparticles. Second, our assumption of an interface between the polyaniline nanofibers and gold nanoparticles seems reasonable because without such an interface the instability of the device through rapid charge recombination would be expected. Additionally, because our device exhibits negative differential resistance, a mechanism involving filament formation is unlikely as discussed by Scott et al.13 To create truly nanoscale device structures with ultrahigh densities, it is important to demonstrate that the electrical bistability and memory effect are not only bulk phenomena 1079
materialssnanoparticles and conducting polymerssto form a novel materials system. We believe that this new polymer memory device could have an important impact on the future of information technology by providing the high-speed, highdensity memory needed for future advanced computers and digital electronics. Acknowledgment. We are indebted to Dr. Jun He for help with the XPS experiments and Dr. Mark Hilton from Veeco Instrument for AFM measurements. Funding for this research has been provided by the Microelectronics Advanced Research Corp. (MARCO) Focus Center on Functional Engineered Nano Architectonics (FENA) and the Air Force Office of Scientific Research. References
Figure 6. Conductive atomic force microscopy of the polyaniline nanofiber/gold nanoparticle composite. A conductive atomic force microscope tip is first used to perform the morphology scan on the polyaniline nanofiber/gold nanoparticle composite film and then to carry out the electrical characterization. The AFM tip is parked on the top of the polymer bump, and a voltage scan is taken from 0 to -5 V,, while the current is measured. The electrical bistability of the polymer composite film using the nanoscale tip is evident.
but can also be observed on the nanoscale. Therefore, we have carried out a nanoscale writing/reading process by placing a conductive atomic force microscope tip in direct contact with the polyaniline nanofiber/gold nanoparticle thin film (without poly(vinyl alcohol)) by removing the top electrode. The bottom electrode is kept to maintain the electric field. The stylus of the conductive atomic force microscope tip behaves as the top electrode in the nanoscale dimension. We first scanned the surface morphology of the composite (Figure 6, lower left) and then choose a “bump” containing nanofibers on which to perform an electrical measurement (upper right, Figure 6). The same tip was then parked on the top of the bump, and a voltage scan was applied from 0 to -5 V while measuring the current. The electrical bistability of the polymer composite film is observed. This provides evidence that the nonvolatile memory effect is valid down to nanoscale dimensions and paves the way for future nanoscale memory devices. In conclusion, a novel electrically bistable device is reported with electrical behavior that is promising for digital nonvolatile memory. The device can be switched electrically between two states with a conductivity difference of about 3 orders of magnitude, and these switches are nonvolatile. The mechanism likely involves an electric-field-induced charge transfer between the polyaniline nanofibers and the gold nanoparticles. The unique behavior of these devices provides an interesting approach that combines two useful
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