Polyaniline Nanofiber/Gold NanoparticleNonvolatile MemoryRicky J. Tseng, † Jiaxing Huang, ‡ Jianyong Ouyang, † Richard B. Kaner, †,‡ andYang Yang* ,†

Department of Materials Science and Engineering and Department of Chemistry andBiochemistry, 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 isreported. The device has a simple structure consisting of the plastic composite film sandwiched between two electrodes. An external bias isused to program the ON and OFF states of the device that are separated by a 3-orders-of-magnitude difference in conductivity. ON −OFFswitching times of less than 25 ns are observed by electrical pulse measurements. The devices possess prolonged retention times of severaldays after they have been programmed. Write −read−erase cycles are also demonstrated. The switching mechanism is attributed to an electric-field-induced charge transfer from the polyaniline nanofibers to the gold nanoparticles. The active polymer layer is created by growing nanometersize gold particles within 30-nm-diameter polyaniline nanofibers using a redox reaction with chloroauric acid. This device combines twoexciting research areas snanoparticles and conducting polymers sto form a novel materials system with unique functionality.

Conjugated polymers and other organic materials are uniquelysuited for thin film, large area, mechanically flexible, lowcost electronic devices.1,2 Their tremendous commercialpotential has touched off a flurry of research, particularlyon organic light-emitting diodes,3,4 transistors,5,6 solar cells7,8

and memory devices.9-14 By using nanoscale materials, high-density electronic devices are possible with superior perfor-mance and manufacturability.15,16 Therefore, a conductingpolymer decorated with metallic or semiconducting nano-particles provides an exciting system to investigate with thepossibility of designing device functionality. Recently, wehave demonstrated a facile bulk-synthetic method to preparehigh-quality polyaniline nanofibers with diameters tunablefrom 30 to 120 nm.17-19 Metal nanoparticles (Au, Ag) canbe grown inside of the dedoped polyaniline nanofibers by aredox reaction with the metal ions (Au3+, Ag+).20-22 Thecombination of conducting polymers with nanoparticlesoffers a new direction for organic electronic devices.

Nonvolatile organic/polymer memory represents an idealapplication to take advantage of this novel materials system.Our earlier work has shown that electrical properties of anorganic thin film can be dramatically modified when metallicnanoparticles are embedded within an organic film.11,12Thisphenomenon has been attributed to charge storage inside thenanoparticles.12,13,23In each case, the formation of nanopar-ticles is carried out by carefully controlling the depositionprocess. Electrical bistability and memory has also been

demonstrated by blending synthesized gold nanoparticleswith electron donor molecules in an inert polystyrene poly-mer matrix.14

Here we take a giant leap forward by using polyanilinenanofibers decorated with gold nanoparticles as the polymermemory element. This is a one-component system thatsignificantly simplifies the device structure and fabricationprocess. A relatively uniform distribution of nanometer-sizedgold nanoparticles is created (Figure 1A) by controlling thetime and temperature of a reaction between 30-nm-diameterpolyaniline nanofibers and chloroauric acid. The device isfabricated through the following process: A bottom (column)aluminum electrode with a thickness of 80 nm is depositedby thermal evaporation in a chamber under a pressure of 1× 10-5 Torr. The thickness is determined using a profilo-meter (Dektak). A 70-nm-thick active layer is formed byspin coating an aqueous solution of∼0.1 wt % polyanilinenanofiber/gold nanoparticle composite in 1.5 wt % poly(vinylalcohol). The poly(vinyl alcohol) serves as an electricallyinsulating matrix for the composite. Both the top and bottomaluminum electrodes have a width of 0.2 mm, and the devicecovers an area of 0.2× 0.2 mm2 in Figure 1B.

All electrical experiments are conducted under a vacuumof 1 × 10-4 Torr. Current-voltage (I-V) and deviceretention time characteristics are measured using a semicon-ductor parameter analyzer (HP 4155B). The current responseof write-read-erase cycle tests is measured with a program-mable power supply (HP 3245A) and recorded with a four-channel oscilloscope (Tektronix TDS 460A). The deviceresponse time is measured by applying a pulse from an HP

* Corresponding author. E-mail: yangy@ucla.edu.† Department of Materials Science and Engineering.‡ Department of Chemistry and Biochemistry.

NANOLETTERS

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214B pulse generator to the device followed by anI-V scanwith an HP 4155B to determine if the device is in its ON orOFF state. The atomic force microscope image and nanoscaleI-V curve are measured with a Dimension 3100 TUNA/CAFM (platinum-iridium-coated Si tip with a radius of 15nm) from Veeco Instruments.

The polyaniline nanofiber/gold nanoparticle device exhibitsvery interesting bistable electrical behavior (Figure 2). Asthe potential is increased to+3 V, an abrupt increase incurrent is observed. This changes the device from a low-conductivity (10-7 amps) OFF state to a high-conductivity(10-4 amps) ON state (Figure 2, curve A). The device isstable in the ON state when the potential is lowered back to0 V (Figure 2, curve B). The high conductivity of the ONstate can be changed back to the OFF state by applying areverse bias of-5 V. The device is then stable in the OFFstate until+3 V is applied, at which point it returns to theON 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

elsewhere in other memory devices13 but appears to haveno effect on the performance of our device within the+3 to+4 V region. Note that if the gold nanoparticles are grownwith diameters greater than 20 nm the devices can beswitched on only once and they exhibit ohmic behavior inthe ON state, indicating that the more metallic nature of thelarger gold particles then dominates the switching. Addition-ally, devices made with just polyaniline nanofibers and nogold nanoparticles show no electronic switching.

The electrical bistability suggests that the polyaniline nano-fiber/gold nanoparticle composite can be used for nonvolatilememory. Other important characteristics of memory devicesinclude the retention time and the ability to read, write, anderase data. The retention time of the polyaniline nanofiber/gold nanoparticle device in the ON state was tested everycouple of hours over a 3-day period with no appreciablechange in conductivity observed. A stress test was carriedout 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 datapoint limit of the parameter analyzer was reached. Nosignificant change in conductivity was noted during the 14-hstress test (Figure 3), although after several days a slightdecrease in conductivity in the ON state was observed.

Write-read-erase cycle tests carried out on a device areas shown in Figure 4. The upper part shows the continuous

Figure 1. TEM image and device structure. (A) Transmissionelectron microscopy image of the polyaniline nanofiber/goldnanoparticle composite. The black dots are∼1-nm gold nanopar-ticles contained within∼30-nm-diameter polyaniline nanofibers.(B) The structure of the polyaniline nanofiber/gold nanoparticlebistable memory device.

Figure 2. Currrent-voltage characteristics of the polyanilinenanofiber/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+3and +4 V, a region of negative differential resistance (NDR) isobserved. The inset shows the retention time test of the ON-state(top) and OFF-state (bottom) currents when biased at+1 V every5 s.

Figure 3. Retention time test of the ON-state and OFF-statecurrents when biased at a constant 1 V with a width of 0.167 s,recorded every 5 s.

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voltage biases applied to the device. Various bias strengthswere used to write the device to the ON state (+4.8 V), toread the ON state current (+1.2 V), to erase the device tothe OFF state (-6 V), and to read the OFF state current(+1.2 V). The corresponding currents recorded by anoscilloscope are 2× 10-5, 1 × 10-5, -5 × 10-5, and10-6-10-7 amps, respectively, as shown in the lower partof Figure 4. (Please note that we have plotted the absolutevalues of current because of the log scale used along theYaxis.) The device can be cycled many times as is apparentfrom Figure 4. A readily distinguishable ON/OFF ratioaround 20 is maintained.

The polyaniline nanofiber/gold nanoparticle device exhibitsa fast response to applied voltage pulses as shown in Figure5. The device is initially in the OFF state, confirmed by theI-V curve in the range of 0 to 2.5 V. Because the turn-onbias is around 3 V, the voltage scan will not trigger theswitching process. Subsequently, a pulsed voltage of 4 Vwith a duration of 25 ns (inset of Figure 5) generated by anHP 214B pulse generator is applied to the device. Thistransition and the ON state are confirmed by the secondI-Vscan, shown as the topI-V curve. Similarly, the device inthe ON state can be turned to the OFF state by applying a

pulse of-5 V with a 25-ns duration. This pulse width isthe limit of our instrument (HP 214B pulse generator),suggesting that the actual switching time may be faster. Theseresponse times are much shorter than the transition reportedfor organic bistable molecules,24,25 which are in the micro-second or slower regime. The nanosecond transition timesuggests that the switching mechanism is due to electronicprocesses rather than chemical reactions, conformationalchanges,24 or isomerizations,25 as reported for other devices.

Because the nanosecond switching of the polyanilinenanofiber/gold nanoparticle device must involve electronicprocesses, the following mechanism is proposed. The transi-tion from the OFF to the ON state is attributed to an electric-field-induced charge transfer between the polyaniline nano-fibers and the gold nanoparticles. Under a sufficient electricfield, electrons that reside on the imine nitrogen of thepolyaniline may gain enough energy to surmount the inter-face between the nanofibers and the gold nanoparticles andmove onto the gold nanoparticles (Scheme 1). Consequently,the gold nanoparticles become more negatively charged,whereas the polyaniline nanofibers become more positivelycharged. The conductivity of the polyaniline nanofiber/goldnanoparticle composite will increase dramatically after theelectric-field-induced charge transfer, in accordance with thetransition from the OFF to the ON state. This proposedmechanism is supported by the following evidence. First,X-ray photoelectron spectra taken of the composite shows ashift from 399.2 to 399.7 eV for the N1S core electronscompared to the spectra of the undoped, emeraldine basepolyaniline, indicating that the nitrogen in the polyanilinenanofiber/gold nanoparticle composite is partially positivelycharged. At the same time, the binding energy of the goldelectrons (4f5/2) decreases from 87.7 to 87.5 eV, indicatingthat a partial negative charge resides on the gold nanopar-ticles. Second, our assumption of an interface between thepolyaniline nanofibers and gold nanoparticles seems reason-able because without such an interface the instability of thedevice through rapid charge recombination would be ex-pected. Additionally, because our device exhibits negativedifferential resistance, a mechanism involving filamentformation is unlikely as discussed by Scott et al.13

To create truly nanoscale device structures with ultrahighdensities, it is important to demonstrate that the electricalbistability and memory effect are not only bulk phenomena

Figure 4. Current response (left axis) of the polyaniline nanofiber/gold nanoparticle device to applied potentials (right axis) duringwrite-read-erase testing cycles. A potential of+4.8 V is used towrite, -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 pulseis 0.1 s, during which time the current response is recorded usingan oscilloscope.

Figure 5. I-V characteristics of the OFF and the ON states of thepolyaniline nanofiber/gold nanoparticle device before and after theapplication of a voltage pulse of 4 V with a width of 25 ns, asshown in the inset.

Scheme 1. Schematic Structure of a Polyaniline Nanofiber/Gold Nanoparticle after the Application of+3 Va

a An increase in charge transfer from polyaniline to the goldnanoparticles is believed to occur.

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but can also be observed on the nanoscale. Therefore, wehave carried out a nanoscale writing/reading process byplacing a conductive atomic force microscope tip in directcontact with the polyaniline nanofiber/gold nanoparticle thinfilm (without poly(vinyl alcohol)) by removing the topelectrode. The bottom electrode is kept to maintain theelectric field. The stylus of the conductive atomic forcemicroscope tip behaves as the top electrode in the nanoscaledimension. We first scanned the surface morphology of thecomposite (Figure 6, lower left) and then choose a “bump”containing nanofibers on which to perform an electricalmeasurement (upper right, Figure 6). The same tip was thenparked on the top of the bump, and a voltage scan wasapplied from 0 to-5 V while measuring the current. Theelectrical bistability of the polymer composite film isobserved. This provides evidence that the nonvolatile memoryeffect is valid down to nanoscale dimensions and paves theway for future nanoscale memory devices.

In conclusion, a novel electrically bistable device isreported with electrical behavior that is promising for digitalnonvolatile memory. The device can be switched electricallybetween two states with a conductivity difference of about3 orders of magnitude, and these switches are nonvolatile.The mechanism likely involves an electric-field-inducedcharge transfer between the polyaniline nanofibers and thegold nanoparticles. The unique behavior of these devicesprovides an interesting approach that combines two useful

materialssnanoparticles and conducting polymerssto forma novel materials system. We believe that this new polymermemory device could have an important impact on the futureof information technology by providing the high-speed, high-density memory needed for future advanced computers anddigital electronics.

Acknowledgment. We are indebted to Dr. Jun He forhelp with the XPS experiments and Dr. Mark Hilton fromVeeco Instrument for AFM measurements. Funding for thisresearch has been provided by the Microelectronics Ad-vanced Research Corp. (MARCO) Focus Center on Func-tional Engineered Nano Architectonics (FENA) and the AirForce Office of Scientific Research.

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NL050587L

Figure 6. Conductive atomic force microscopy of the polyanilinenanofiber/gold nanoparticle composite. A conductive atomic forcemicroscope tip is first used to perform the morphology scan on thepolyaniline nanofiber/gold nanoparticle composite film and thento carry out the electrical characterization. The AFM tip is parkedon the top of the polymer bump, and a voltage scan is taken from0 to -5 V,, while the current is measured. The electrical bistabilityof the polymer composite film using the nanoscale tip is evident.

1080 Nano Lett., Vol. 5, No. 6, 2005