NANO LETTERS 2005 Polyaniline Nanofiber/Gold Nanoparticle Vol. 5, No. 6 Nonvolatile Memory 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 electric- field-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 demonstrated by blending synthesized gold nanoparticles suited for thin film, large area, mechanically flexible, low with electron donor molecules in an inert polystyrene poly- cost electronic devices.1,2 Their tremendous commercial mer matrix.14 potential has touched off a flurry of research, particularly Here we take a giant leap forward by using polyaniline on organic light-emitting diodes,3,4 transistors,5,6 solar cells7,8 nanofibers decorated with gold nanoparticles as the polymer and memory devices.9-14 By using nanoscale materials, high- memory element. This is a one-component system that density electronic devices are possible with superior perfor- significantly simplifies the device structure and fabrication mance and manufacturability.15,16 Therefore, a conducting process. A relatively uniform distribution of nanometer-sized polymer decorated with metallic or semiconducting nano- gold nanoparticles is created (Figure 1A) by controlling the particles provides an exciting system to investigate with the time and temperature of a reaction between 30-nm-diameter possibility of designing device functionality. Recently, we polyaniline nanofibers and chloroauric acid. The device is have demonstrated a facile bulk-synthetic method to prepare fabricated through the following process: A bottom (column) high-quality polyaniline nanofibers with diameters tunable aluminum electrode with a thickness of 80 nm is deposited from 30 to 120 nm.17-19 Metal nanoparticles (Au, Ag) can by thermal evaporation in a chamber under a pressure of 1 be grown inside of the dedoped polyaniline nanofibers by a × 10-5 Torr. The thickness is determined using a profilo- redox reaction with the metal ions (Au3+,Ag+).20-22 The meter (Dektak). A 70-nm-thick active layer is formed by combination of conducting polymers with nanoparticles spin coating an aqueous solution of ∼0.1 wt % polyaniline offers a new direction for organic electronic devices. nanofiber/gold nanoparticle composite in 1.5 wt % poly(vinyl Nonvolatile organic/polymer memory represents an ideal alcohol). The poly(vinyl alcohol) serves as an electrically application to take advantage of this novel materials system. insulating matrix for the composite. Both the top and bottom Our earlier work has shown that electrical properties of an aluminum electrodes have a width of 0.2 mm, and the device organic thin film can be dramatically modified when metallic covers an area of 0.2 × 0.2 mm2 in Figure 1B. nanoparticles are embedded within an organic film.11,12 This All electrical experiments are conducted under a vacuum phenomenon has been attributed to charge storage inside the of 1 × 10-4 Torr. Current-voltage (I-V) and device nanoparticles.12,13,23 In each case, the formation of nanopar- retention time characteristics are measured using a semicon- ticles is carried out by carefully controlling the deposition ductor parameter analyzer (HP 4155B). The current response process. Electrical bistability and memory has also been of write-read-erase cycle tests is measured with a program- mable power supply (HP 3245A) and recorded with a four- * Corresponding author. E-mail: [email protected]. † Department of Materials Science and Engineering. channel oscilloscope (Tektronix TDS 460A). The device ‡ Department of Chemistry and Biochemistry. response time is measured by applying a pulse from an HP 10.1021/nl050587l CCC: $30.25 © 2005 American Chemical Society Published on Web 05/12/2005 Figure 2. Currrent-voltage characteristics of the polyaniline nanofiber/gold nanoparticle device. The potential is scanned from (A)0to+4V,(B)+4to0V,and(C)0to+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 5s. 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 elsewhere in other memory devices13 but appears to have nanoparticle composite. The black dots are ∼1-nm gold nanopar- no effect on the performance of our device within the +3to ∼ ticles contained within 30-nm-diameter polyaniline nanofibers. +4 V region. Note that if the gold nanoparticles are grown (B) The structure of the polyaniline nanofiber/gold nanoparticle bistable memory device. with diameters greater than 20 nm the devices can be switched on only once and they exhibit ohmic behavior in 214B pulse generator to the device followed by an I-V scan the ON state, indicating that the more metallic nature of the with an HP 4155B to determine if the device is in its ON or larger gold particles then dominates the switching. Addition- OFF state. The atomic force microscope image and nanoscale ally, devices made with just polyaniline nanofibers and no I-V curve are measured with a Dimension 3100 TUNA/ gold nanoparticles show no electronic switching. CAFM (platinum-iridium-coated Si tip with a radius of 15 The electrical bistability suggests that the polyaniline nano- nm) from Veeco Instruments. fiber/gold nanoparticle composite can be used for nonvolatile The polyaniline nanofiber/gold nanoparticle device exhibits memory. Other important characteristics of memory devices very interesting bistable electrical behavior (Figure 2). As include the retention time and the ability to read, write, and the potential is increased to +3 V, an abrupt increase in erase data. The retention time of the polyaniline nanofiber/ current is observed. This changes the device from a low- gold nanoparticle device in the ON state was tested every conductivity (10-7 amps) OFF state to a high-conductivity couple of hours over a 3-day period with no appreciable (10-4 amps) ON state (Figure 2, curve A). The device is change in conductivity observed. A stress test was carried stable in the ON state when the potential is lowered back to out by applying a bias of +1 V with a duration of 0.0167 s, 0 V (Figure 2, curve B). The high conductivity of the ON and the current was measured every 5 s until the 10 000 data state can be changed back to the OFF state by applying a point limit of the parameter analyzer was reached. No reverse bias of -5 V. The device is then stable in the OFF significant change in conductivity was noted during the 14-h state until +3 V is applied, at which point it returns to the stress test (Figure 3), although after several days a slight ON state (Figure 2, curve C). If the potential is raised above decrease in conductivity in the ON state was observed. +3 V, then a region of negative differential resistance (NDR) Write-read-erase cycle tests carried out on a device are is observed. Negative differential resistance has been reported as shown in Figure 4. The upper part shows the continuous 1078 Nano Lett., Vol. 5, No. 6, 2005 Scheme 1. Schematic Structure of a Polyaniline Nanofiber/ Gold Nanoparticle after the Application of +3Va a An increase in charge transfer from polyaniline to the gold nanoparticles is believed to occur. 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 pulse of -5 V with a 25-ns duration. This pulse width is write, -6 V is applied to erase, and +1.2 V is used to read. W ) the limit of our instrument (HP 214B pulse generator), 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 suggesting that the actual switching time may be faster. These an oscilloscope. response times are much shorter than the transition reported for organic bistable molecules,24,25 which are in the micro- second 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 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.