Electrospinning is a simple, versatile, and cost effective method for generating nanoscale fibers, wires, and tubes. Nanowires and nanotubes could be important building blocks for nanoscale electronics, optoelectronics, and sensors as they can function as miniaturized devices as well as electrical interconnects. We report on a simple method to fabricate free standing ceramic nanofiber heterostructures, which exhibit rectifying behavior of a p-n junction

Bender, E. T.

Bhargava, S.

Chase, G. G.

Evans, E. A.

Lotus, A. F.

Ramsier, R. D.

Reneker, Darrell Hyson

English

The University of Akron

The University of AkronIdeaExchange@UAkronCollege of Polymer Science and Polymer Engineering7-2009Electrospinning Route For the Fabrication of P-NJunction Using Nanofiber YarnsA. F. LotusS. BhargavaE. T. BenderE. A. EvansR. D. RamsierUniversity of Akron Main CampusSee next page for additional authorsPlease take a moment to share how this work helps you through this survey. Your feedback will beimportant as we plan further development of our repository.Follow this and additional works at: http://ideaexchange.uakron.edu/polymer_ideasPart of the Polymer Science CommonsThis Article is brought to you for free and open access by IdeaExchange@UAkron, the institutional repository ofThe University of Akron in Akron, Ohio, USA. It has been accepted for inclusion in College of Polymer Science andPolymer Engineering by an authorized administrator of IdeaExchange@UAkron. For more information, pleasecontact mjon@uakron.edu, uapress@uakron.edu.Recommended CitationLotus, A. F.; Bhargava, S.; Bender, E. T.; Evans, E. A.; Ramsier, R. D.; Reneker, Darrell Hyson; and Chase, G. G.,"Electrospinning Route For the Fabrication of P-N Junction Using Nanofiber Yarns" (2009). College of PolymerScience and Polymer Engineering. 83.http://ideaexchange.uakron.edu/polymer_ideas/83AuthorsA. F. Lotus, S. Bhargava, E. T. Bender, E. A. Evans, R. D. Ramsier, Darrell Hyson Reneker, and G. G. ChaseThis article is available at IdeaExchange@UAkron: http://ideaexchange.uakron.edu/polymer_ideas/83Electrospinning route for the fabrication of p-n junction using nanofiberyarnsA. F. Lotus,1 S. Bhargava,1 E. T. Bender,2 E. A. Evans,1 R. D. Ramsier,3 D. H. Reneker,4and G. G. Chase1,a1Department of Chemical and Biomolecular Engineering, The University of Akron, Akron,Ohio 44325-3906, USA2Department of Medical Physics, The University of Wisconsin–Madison, Madison, Wisconsin 53792, USA3Departments of Physics and Chemistry, The University of Akron, Akron, Ohio 44325, USA4Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, USAReceived 29 April 2009; accepted 26 May 2009; published online 2 July 2009Electrospinning is a simple, versatile, and cost effective method for generating nanoscale fibers,wires, and tubes. Nanowires and nanotubes could be important building blocks for nanoscaleelectronics, optoelectronics, and sensors as they can function as miniaturized devices as well aselectrical interconnects. We report on a simple method to fabricate free standing ceramic nanofiberheterostructures, which exhibit rectifying behavior of a p-n junction. © 2009 American Institute ofPhysics. DOI: 10.1063/1.3157206I. INTRODUCTIONElectrospinning has been recognized as an efficient tech-nique for the fabrication of submicron sized fibers from awide variety of materials including polymer, molten poly-mer, and polymeric precursors of ceramics.1–4 When the di-ameters of fibrous materials are reduced from micrometers tonanometers the surface area to volume ratio becomes unusu-ally large, so more chemically functional bonds can form andunusual mechanical properties are achieved. These outstand-ing properties make the nanofibers optimal candidates formany important applications. Among the electrospun nanofi-bers, ceramic materials are very promising for a number ofapplications, such as high temperature catalysis, filtration,hydrogen storage, energy conversion, electronic, optoelec-tronic, photonic, and sensor devices.5It is well recognized that there are considerable difficul-ties in handling a single nanofiber or making well organizedstructures from single nanofibers. By making yarns frombundles of oriented electrospun nanofibers it is possible toobtain ordered structures large enough to handle. Since thefiber bundle or the yarn is more robust than a single nanofi-ber it is more convenient to transfer it onto a substrate. Theelectrical, physical property characterization of the nanofi-bers may not require special equipment.The yarn manufacturing method6–8 involves collectingthe electrically charged fibers between two parallel and elec-trically grounded collector surfaces separated by a distancecommensurate with the diameter of the loops formed by theelectrically driven bending instability.9 One collector shapedas a hemisphere with diameter of 8 cm is rotated around itsaxis at an appropriate rate to twist the nanofibers into a mi-croyarn. The yarn is extended by translating the nonrotatingrod-shaped collector in an axial direction away from the ro-tating collector. Properties such as yarn length, diameter, fi-ber count, and twist per unit length are controlled by therotation rate of the hemispherical collector and the transla-tion rate of the rod shaped collector. Yarns of nanofibers canbe produced from any polymer or polymeric precursor ofceramic solutions that can be electrospun into fibers.We synthesized and characterized several semiconduct-ing ceramic ZnO, NiO nanofiber yarns using a modifiedversion of an electrospinning setup, which is reportedelsewhere.8 ZnO, a wide bandgap 3.4 eV II-VI compoundsemiconductor having n-type properties, has been intensivelystudied due to its unique properties and versatile applicationsin transparent electronics, ultraviolet UV light emitters,piezoelectric devices, chemical sensors, and spinelectronics.10–15 ZnO is a strong candidate for semiconductordevices operating in harsh environments, such as extraterres-trial space and nuclear reactors because ZnO is more radia-tion resistant than either Si, SiC, GaAs, or GaN. ZnO is alsovery suitable for high power and high-frequency semicon-ductor device applications, based on its large breakdownvoltage and high saturated electron drift velocity.16NiO on the other hand is one of the relatively few metaloxides that tend to have p-type properties. It has been attract-ing considerable attention because of its stable and widebandgap 3.7 eV. Nickel oxide is a very important materialextensively used in catalysis, battery cathodes, gas sensors,electrochromic films, and magnetic materials.17–21ZnO/NiO composites have been studied extensively asimportant soft ferrite materials, gas sensitive materials, andas semiconductors.22–27 Although the most interesting prop-erty of semiconductors is the p-n interface, attempts to formp-n interfaces with oxide semiconductor nanofibers are rare.We demonstrate in this paper that the p-n interface usingoxide semiconductor nanofibers can be fabricated using elec-trospun nanofibers, which do not require adhesion to a solidsubstrate. The electrospun ZnO/NiO nanofiber heterostruc-tures exhibit rectifying current-voltage I-V characteristics.aAuthor to whom correspondence should be addressed. Tel.: 330-972-7943.FAX: 330-972-5856. Electronic mail: gchase@uakron.edu.JOURNAL OF APPLIED PHYSICS 106, 014303 20090021-8979/2009/1061/014303/4/$25.00 © 2009 American Institute of Physics106, 014303-1II. EXPERIMENTALZinc oxide/nickel oxide heterojunction structures weremade from zinc oxide and nickel oxide nanofiber yarns intwo different ways. In the first approach, as spunZnCH3COO2 /PVP nanofiber yarn was made and thensome portion of this yarn was masked andNiCH3COO2 /PVP nanofibers were electrostatically spunaround the unmasked portion of the ZnCH3COO2 /PVPnanofiber yarn. The twisted yarns of ZnCH3COO2 /PVPand NiCH3COO2 /PVP nanofibers were heat treated at 873K at the rate of 1 K/min in ambient conditions to obtain thetwisted two-oxide ceramic nanofiber yarns. Here we referthis heterostructured yarn as tapered nanofiber p-n junctionyarn. In the second approach, nanofiber yarns ofZnCH3COO2 /PVP and NiCH3COO2 /PVP were made in-dividually and then were twisted around each other. Thesetwisted yarns were heat treated in the same manner to obtainthe ceramic heterojunction yarns. For convenience, we referthis heterostructured yarn as twisted nanofiber p-n junctionyarn. Figure 1 schematically illustrates the formation of p-nheterojunction nanofiber yarns.III. CHARACTERIZATIONA. Scanning electron microscopyFor scanning electron microscopy SEM HitachiS-2150 analysis, the nanofibers were placed on an alumi-num stub with a strip of carbon tape applied to the surface topromote fiber adhesion while minimizing charging effects.The samples were silver coated S150B Sputter Coater, Ed-wards and imaged using an accelerating voltage of 20 kV.Figure 2 shows optical images of the p-n heterojunctionnanofiber yarns and representative SEM images of individualyarn portion. The average diameters of the yarns were30–40 m. Yarn diameters were dependent on the durationof electrospinning. Zinc oxide nanofibers in the yarn had anaverage diameter of 60.3 nm with the standard deviation of7.4 nm. Nickel oxide nanofibers had an average diameter of95.2 nm and standard deviation of 4.8 nm.B. X-ray diffractionX-ray powder diffraction patterns were obtained with aPhilips diffractometer employing Cu K radiation, with 2in the range of 10°–60°, to examine the crystal structures ofthe zinc oxide/nickel oxide heterojunction nanofiber yarns.The x-ray source was operated at 40 kV and 35 mA. Thediffraction profiles of zinc oxide/nickel oxide p-n junctionnanofiber yarns annealed at 873 K are presented in Fig. 3.FIG. 1. Schematic diagram for the formation of a tapered nanofiber p-njunction yarn; b twisted nanofiber p-n junction yarn. For both junctions thewhite yarn is an n-type ceramic material ZnO while the gray yarn is ap-type ceramic material NiO. Both types of yarns are made up of manynanofibers not shown.FIG. 2. a and b Optical microscope image of twisted p-n junctionNiO/ZnO nanofiber yarns annealed at 873 K. c Optical microscope imageof tapered p-n junction NiO/ZnO nanofiber yarns annealed at 873 K. dSEM image of NiO nanofiber yarn annealed at 873 K. e SEM image ofZnO nanofiber yarn annealed at 873 K.FIG. 3. XRD patterns of ZnO/NiO nanofibers annealed at 873 K.014303-2 Lotus et al. J. Appl. Phys. 106, 014303 2009The x-ray diffraction XRD pattern indicates that the nanofi-ber heterostructure is well crystallized after annealing at 873K.In the XRD patterns of the heterostructures, only peaksthat could be assigned to ZnO and NiO were observed andthere was no evidence for any different crystalline phases.The peaks can be indexed separately to the cubic nickel ox-ide phase at 2=37.2° 111, 43.2° 200, and to the hexago-nal zinc oxide phase at 2=31.79° 100, 34.45° 200,36.28° 101, 47.54° 102, and 56.61° 110. These reflec-tion peak positions are also in good agreement with reportedvalues in the literature.28,29C. Auger electron spectroscopy imagingAuger electron spectroscopy AES measurements wereperformed under high vacuum conditions in a VG ES-CALAB Mk II system that had been outfitted with a customdata acquisition system enabling the digital acquisition ofimages where each pixel has an associated Auger spectrum.The major elements detected were Zn, Ni, and O, with aminor C feature, most likely due to adventitious carbon andorganic residues, as shown in Fig. 4. The two spectra shownwere constructed by averaging spectra in all pixels whereeither the nickel or zinc signal was found to be dominant.The spectra are therefore representative of the entire sampleas shown, rather than of two single points in the image. Noother impurities were detectable by AES, thereby verifyingthe degradation and removal of the polymer precursors byannealing at 873 K. Low and controlled impurity concentra-tions are important if electrospun nanofibers are to be usedfor optoelectronic, photonic, or sensor-related device appli-cations.D. Electrical properties of ZnO/NiO heterojunctionnanofiber yarnsThe electrical properties of the individual yarns as wellas heterojunction nanofiber yarns were measured with a Kei-thley 2410 sourcemeter. The yarns were placed on an insu-lating glass slide and Ohmic contacts were made by depos-iting silver paste onto the two ends of the yarn. Silver wireswere then connected to the electrodes of the sourcemeter formeasurements. The temperature dependence of the conduc-tivities of these individual ceramic nanofiber yarns ZnO,NiO is consistent with semiconducting behavior.8 Seebeckcoefficients of ZnO and NiO nanofibers were found to be45.5 and +25.5 V /K by hot probe measurements.The I-V curves for the heterojunctions without illumina-tion of light showed clear rectifying behavior while that ofindividual yarn of ZnO and NiO were linear indicatingOhmic contacts. Figure 5 shows the rectifying characteristiccurves of the ZnO/NiO heterojunction nanofiber yarns with-out light illumination. Some leakage current was observed inthe reverse bias region and the turn-on voltage was 3.0 V.The photo I-V characteristics of the twisted p-n junctionnanofiber yarns were measured at room temperature using ablack ray UV lamp model B 100 AP, wavelength range 315–390 nm. A Mannix UV light meter wavelength range 290–390 nm was used to measure the intensity of light. Figure 6shows the characteristic I-V curve under illumination of theUV light source with a power density of 18 mA /cm2. UnderUV illumination, the turn-on voltage is lowered and the cur-rent in both directions reverse bias as well as forward biasis found to increase. The observed increase in current flow isdue to the generation of electron-hole pairs, created withinthe ZnO material from exposure to the UV photons. It shouldbe noted that possible heating effects due to illumination byUV light was not accounted for in these measurements.The diode matrix was fabricated using different p- andn-type semiconducting oxides with a large lattice mismatchand with polycrystalline structures. Structural imperfectionsFIG. 4. a Secondary electron image of a twisted p-n junction NiO/ZnOnanofiber yarn; b corresponding AES summed over each region of thetwisted NiO/ZnO nanofiber yarn.FIG. 5. I-V property for p-NiO /n-ZnO heterojunction nanofiber yarns.FIG. 6. Photoresponse of twisted p-n junction NiO/ZnO nanofiber yarns toUV excitation wavelength range 315–390 nm with a power density of18 mA /cm2.014303-3 Lotus et al. J. Appl. Phys. 106, 014303 2009exist at grain boundaries and the interfaces; therefore, thesedefects contribute to the formation of diodes with less effec-tive rectification characteristics than in other materials sys-tems. However, the rectifying behavior is apparent.IV. CONCLUSIONTwisted yarns of several ceramic nanofibers were madeusing an electrospinning setup. We show that p-n junctionscan be formed using electrospun semiconducting nanofiberswhere they were not deposited on any substrate. The p-njunction fabricated from n-ZnO and p-NiO nanofiber yarnsshowed typical rectifying current-voltage characteristics,dominated in forward bias by the flow of space-charge-limited current in the n-ZnO region. The p-n junction nanofi-ber structure appears to be highly responsive to ultravioletirradiation but further measurements are required to accountfor any heating effects.ACKNOWLEDGMENTSThis work was supported by the Coalescence FiltrationNanomaterials Consortium: Ahlstrom Paper Group, Donald-son Co., Cummins Filtration, MemPro Ceramics Corpora-tion, and Parker Filtration. This work is also supported by theNational Science Foundation Grant Nos. CTS-0310429 andDMI-0403835. We further acknowledge the help from Dr.Sasa Dordevic of the Department of Physics for the UV-visSpectrophotometry.1J. Doshi and D. H. Reneker, J. Electrost. 35, 151 1995.2D. H. Reneker, A. L. Yarin, H. Fong, and S. Koombhongse, J. Appl. Phys.87, 4531 2000.3A. Theron, E. Zussman, and A. L. Yarin, Nanotechnology 12, 384 2001.4K. H. Lee, Y. H. Kim, M. S. Khil, Y. M. La, and D. R. Lee, Polymer 44,1287 2003.5Z. M. Huang, Y. Z. Zhang, M. Kotaki, and M. Ramakrishna, Compos. Sci.Technol. 63, 2237 2003.6S. F. Fennessey and R. 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Electrospinning Route For the Fabrication of P-N Junction Using Nanofiber Yarns

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Electrospinning Route For the Fabrication of P-N Junction Using Nanofiber Yarns

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