Electrospun Hybrid Organic/Inorganic Semiconductor Schottky Nanodiode

We report on a simple method to fabricate, under ambient conditions and within seconds, Schottky nanodiodes using electrospun polyaniline nanofibers and an inorganic n-doped semiconductor. In addition to being a rectifier, the advantage of our design is the complete exposure of the rectifying nanojunction to the surrounding environment, making them attractive candidates in the potential fabrication of low power, supersensitive, and rapid response sensors as well. The diode parameters were calculated assuming the standard thermionic emission model of a Schottky junction, and the use of this junction as a gas sensor was examined. Disciplines Physical Sciences and Mathematics | Physics Comments Suggested Citation: Pinto, N.J., González, R., Johnson, A.T. and MacDiarmid, A.G. (2006). Electrospun hybrid organic/inorganic semiconductor Schottky nanodiode. Applied Physics Letters 89, 033505. © 2006 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters and may be found at http://dx.doi.org/10.1063/1.2227758 This journal article is available at ScholarlyCommons: http://repository.upenn.edu/physics_papers/205 Electrospun hybrid organic/inorganic semiconductor Schottky nanodiode Nicholas J. Pinto and Rosana González Department of Physics and Electronics, University of Puerto Rico, Humacao, Puerto Rico 00791 Alan T. Johnson, Jr. Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Alan G. MacDiarmid Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Received 21 March 2006; accepted 15 June 2006; published online 20 July 2006 We report on a simple method to fabricate, under ambient conditions and within seconds, Schottky nanodiodes using electrospun polyaniline nanofibers and an inorganic n-doped semiconductor. In addition to being a rectifier, the advantage of our design is the complete exposure of the rectifying nanojunction to the surrounding environment, making them attractive candidates in the potential fabrication of low power, supersensitive, and rapid response sensors as well. The diode parameters were calculated assuming the standard thermionic emission model of a Schottky junction, and the use of this junction as a gas sensor was examined. © 2006 American Institute of Physics. DOI: 10.1063/1.2227758 Devices and sensors based on the use of -conjugated conducting polymers are considered by many in the field to shape the next generation of cheap and disposable electronic inventions. The simplest and easiest polymer based device to fabricate is a hybrid organic/inorganic Schottky diode in which a junction of a p-doped polymer with an n-doped inorganic semiconductor is formed. This construction has been achieved via electrochemical polymerization or spin coating of the polymer onto the n-doped semiconducting substrate. Such a “wet” process, however, could result in undesirable chemical reactions with the substrate, leading to partial degradation of the semiconductor at the interface with the polymer causing detrimental effects on device operation. Furthermore, a large fraction of the resulting polymer/ inorganic active semiconductor junction, which is typically two dimensional 2D , is unexposed, being sandwiched between layers in the final device layout and thus not optimal for use as a sensor. In such relatively large 2D structures the effective area of the junction is also uncertain. We report on a simple method to fabricate in air, and within seconds, Schottky nanodiodes using polyaniline and an inorganic n-doped semiconductor. The advantage of our design is the complete exposure of the rectifying nanojunction to the surrounding environment, making them attractive candidates in the potential fabrication of low power, supersensitive, and rapid response sensors and rectifiers. The Schottky diode is prepared by using an n-doped Si wafer 111 , 0.1–1.0 cm with a 200 nm thermally grown oxide layer and polyaniline PANI . After prepatterning gold electrodes over the oxide via standard lithography and lift-off techniques the substrate is cleaved through the electrodes. The exposed cleaved surface has the edge of the gold electrode separated from the doped Si by the insulating oxide layer. For the polymer, 100 mg of emeraldine base PANI was doped with 129 mg of camphorsulfonic acid HCSA and dissolved in 10 ml CHCl3 for a period of 4 h. The resulting deep green solution was filtered, and 10 mg of polyethylene oxide PEO having molecular weight of 900 000 was added to the solution and stirred for an additional of 2 h. PEO was added to assist in fiber formation, and the solution was then filtered using a 0.45 m polytetra fluoroethylene PTFE syringe filter. Using an electrospinning technique reported earlier, individual, charged, dry, and flexible PANI nanofibers were deposited over the wafer edge, making contacts to the gold and the doped Si. These are stable with no apparent degradation or oxidation. The resulting Schottky diode is formed along the vertical edge of the substrate at the nanofiber-doped Si interface. Such a vertical orientation may offer higher levels of integration in circuitry than that provided by in-plane horizontal structures. External electrical contacts were then made, and the device current-voltage I-V characteristics were measured via a Keithley 6517A electrometer in a vacuum of 2 10−2 Torr. Figure 1 shows a schematic of the device and the external electrical circuit. Figure 2 a shows a top view of the cleaved substrate with the electrospun polymer nanofiber using a scanning electron microscope SEM . The nanofiber was seen to firmly adhere to the substrate and to reach toward and over the edge. In order to measure the diameter, an atomic force microscope AFM was used in tapping mode to scan a portion of the fiber, as indicated in the boxed section of Fig. 2 a . The result of this measurement is shown in Figs. 2 b and 2 c . The average fiber diameter was 70 nm and assumed to be that of the fiber at the Schottky nanojunction. Figure 3 shows the I-V characteristic curve at 300 K of the a Electronic mail: nj_ [email protected] b Also at: Departments of Chemistry and Physics, University of Texas at Dallas, Richardson, TX 75083. FIG. 1. Schematic of the prepatterned and then cleaved n-doped Si/SiO2 substrate and the electrospun polymer nanofiber making contacts to the gold electrode above and to the doped Si below the oxide layer. The real fiber is flexible and does not fracture as it bends over the substrate edge. The external electrical connections are also shown. APPLIED PHYSICS LETTERS 89, 033505 2006 0003-6951/2006/89 3 /033505/3/$23.00 © 2006 American Institute of Physics 89, 033505-1 Downloaded 23 Jun 2011 to 130.91.117.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions device shown in Fig. 2. Depending on whether the positive terminal of Vbias was connected to the gold or to the doped Si, the forward biased diode response lies either in the first or the third quadrant, respectively, thereby confirming the formation of Schottky barriers at the polymer/n-doped semiconductor junction. Several diodes were tested, and in general the characteristic I-V curves were asymmetrical with a turn on voltage in the range of 0.4–0.6 V and a much reduced reverse bias current that did not tend to saturate. These devices exhibited a rectifying behavior, and the ratio of the forward to reverse current at a bias voltage of ±1 V for this device was calculated to be 160 which was limited in part due to the low fiber conductivity and the series resistance of the semiconductor. Pretreating the substrate with dilute HF prior to fiber deposition is expected to improve the device rectification ratio. In order to verify the type of contact between the polymer and the gold electrode, another polymer nanofiber 150 nm diameter was electrospun so as to contact two gold electrodes on the same substrate as that of the previous diode. Figure 4 shows the corresponding I-V curve for this fiber together with a SEM image in the insert to Fig. 4. A linear response confirms that the polymer contact with the gold electrode is indeed Ohmic, implying a work function close to that of gold 5.1 eV , and the rectifying behavior seen in Fig. 3 arises primarily from the polymer-doped Si interface. An analysis of the fiber resistance obtained from Fig. 4 combined with the fiber dimensions yielded a conductivity of 5 10−2 S/cm, consistent with earlier reports. In order to quantitatively analyze the diode characteristics, we assume the standard thermionic emission model of a Schottky junction. In this model the forward biased current should obey the well known relationship