NANO LETTERS
Polarization Controlled Transport in PANI−BaTiO3 Nanofibers
2006 Vol. 6, No. 5 896-900
Maxim Nikiforov,† Haiqing Liu,‡ Harold Craighead,‡ and Dawn Bonnell*,† Department of Materials Science and Engineering, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104, and School of Applied & Engineering Physics and the Nanobiotechnology Center, Cornell UniVersity, Ithaca, New York 14853 Received October 10, 2005; Revised Manuscript Received February 22, 2006
ABSTRACT The orientation of ferroelectric domains in particles embedded in nanofibers can be manipulated externally and used to control the electronic properties of the nanowires. Specifically the resistance of a conductive polymer fiber increases with the magnitude of voltage used to orient domains perpendicular to the current flow direction. The behavior can be understood in terms of the effect of the local electrostatic field due to the ferroelectric particles on the electron scattering volume and/or the potential barriers to variable range electron hopping in the polymer.
Conductive polymers have been intensively studied over the past decade because of potential applications in an increasing number of electronic devices, such as chemical sensors1 and electrochromic displays.2 Within this class of materials polyaniline (PANI) is the most studied conductive polymer due to the ease of synthesis and the relative environmental stability.3 Recently, inorganic ferroelectric/polyanaline bulk composites have been produced as a route to increase the functionality of the polymer yielding, for example, positive temperature coefficient of resistance effects.4-6 We show here that on incorporating a ferroelectric compound into a lowdimensional polymer structure, e.g., a nanofiber, the polarization can be exploited to control properties. Specifically, electrospinning is used to produce nanofibers of conductive PANI containing BaTiO3 particles, to examine the effect of ferroelectric polarization on transport mechanisms in small systems. The dependences of electronic transport on composition and polarization orientation are described in terms of classic electrostatic and quantum mechanical models. Control of local polarization is a basis for numerous devices such as field-based sensor, biochemical sensor, memory, localized switch, etc. Furthermore, the electrospinning fabrication process is amenable to manufacturing in large quantities. To quantify the electronic properties of the nanofibers, templates of gold electrodes were patterned on oxidized Si(100) such that subsequent deposition of nanofibers resulted in microcircuits, Figure 1. Test devices were fabricated with a recently demonstrated scanned-tip electrospinning deposition method, employing a microfabricated source to deposit oriented polymeric nanowires.7 In this † ‡
University of Pennsylvania. Cornell University.
10.1021/nl052017r CCC: $33.50 Published on Web 04/04/2006
© 2006 American Chemical Society
Figure 1. Schematic diagram of the circuit used for measuring transport properties. I/V curves are measured between electrodes A and B; the average distance between the electrodes is about 10 µm.
method, a droplet of solution was placed on an arrow-shaped tip that acted as a scanned electrospinning source. The polymer-blend solution was 2 wt % PANI (Mw ) 100 000) doped with 10-camphorsulfonic acid (Sigma, Mw ) 232.2) in chloroform (0.6:1 mole ratio of 10-camphorsulfonic acid and polyaniline), and 0.25 wt % poly(ethylene oxide) (PEO) (Sigma, Mw ) 900 000). BaTiO3 nanoparticles (Aldrich, 3050 nm average particle size, 99+%) were incorporated into the polymer blend at 0.1, 1, 3, or 5 wt %. Nanoparticles were added after the dissolution of the polymer blend in
Table 1. Composition and Transport Properties of PANI-BaTiO3 Fibers current at + 1 V after poling, nA
I-V curve fitting
vol % of BaTiO3 particles in the fiber
BaTiO3 content, a.u.
0V
50 V
100 V
200 V
I0, nA
A, a.u. ×1000
0.1 0.8 7.4 7.4 7.4 7.4 7.4 19.4 19.4
na 0.014 0.043 0.036 0.035 0.024 0.031 0.151 0.066
600 102 287 131 351 294 76 529 192
583 83 165 77 301 264 56 361 124
565 69 158 80 219 199 51 311 108
562 60 148 71 196 174 43 237 94
595 100 255 125 340 285 72 510 180
0.4 2.8 3.4 3.0 3.0 2.5 3.0 4.2 4.0
chloroform and sonicated for 10-15 min in order to disperse the oxide particles. The polymer jet, electrostatically extracted from the tip, dried in transit to a substrate on a rotating counter electrode. The scanned tip was fixed 1.5 cm from the substrate and held at a potential of 8.5 kV with respect to the grounded counter electrode, and the substrate was rotated at a relative velocity of about 200 cm/s. This process produced oriented PANI/PEO nanowires with diameters in the range of 200-700 nm. Two-probe electrical conductivity measurements were performed on 10 nanofiber circuits. I-V curves were collected from -1 to +1 V (HP 4145B semiconductor analyzer equipped with HP 16058A test fixture). Ferroelectric polarization orientation of the nanoparticles within the fibers was rotated with a home-built plane capacitor with electrode separation of about 2 mm and applied voltages ranging from 0 to 200 V for 15 min. The polarization field in the capacitor was perpendicular to the sample surface (Figure 1). The coercive field of nanocrystalline BaTiO3 is about 300 V/cm.8 Scanning impedance microscopy (SIM) implemented on a commercial atomic force microscope (AFM) (Dimension 3000) was used to image the electronic properties of the reference nanofiber in situ.9 In SIM a 0.4 V ac signal was applied across the sample at the tip resonant frequency (∼ 45 kHz), and the amplitude signal at a constant sample-tip separation was proportional to the local electrical potential. The conductance of the poled fibers decreases about 20% over a 10-h period, perhaps due to thermal degradation or oxidation in air. During a typical 1.5-h experiment, this results in a 3% conductance decrease while the decrease due to poling is in the range 10-50% (Table 1). The nanofibers of PANI/PEO-BaTiO3 within the circuits were on the order of 10 µm in length and 500 nm in width. The variations of fiber size and composition were determined by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX) (JEOL 6300FV equipped with PGT Prism 60 EDX spectrometer). The differences in BaTiO3 content were estimated by comparing the normalized EDX peak intensities. Figure 2 shows the peaks for Ba and Ti at around 4.5 keV (Ba LR1,2 ) 4.4663 keV; Ti KR1,2 ) 4.5089 keV). Integrated peak intensities after a linear background subtraction between 4 and 5 keV are compared. Since the Nano Lett., Vol. 6, No. 5, 2006
Figure 2. Representative EDX spectrum of a PANI-BaTiO3 fiber. Integral intensity of the Ba LR1,2 (Ti KR1,2) was normalized with respect to the Au MR1,2 peak to obtain relative composition variations.
fiber area differs from sample to sample, intensities were normalized to the Au MR1,2 peak (Au MR1,2 ) 2.1205 keV). The procedure yields the relative oxide content rather than the absolute value, and the error is on the order of 10-20%, as shown in Table 1. Scanning electron micrographs of the starting BaTiO3 powder showed that the particle diameters ranged from 20 to 70 nm and in the dry state agglomerated into particles on the order of 100 nm. The local BaTiO3 content for each nominal composition of PANI-BaTiO3 fiber varied by as much as a factor of 2; see Table 1. At low nominal contents (0.1% and 1%) the fibers were smooth and the particles were evenly dispersed throughout the fibers. At intermediate contents (3%) electrospinning produced particles at the fiber surfaces but the particles appeared to be evenly dispersed. At high contents (5%) electrospinning became difficult and many fibers were fractured with large agglomerates decorating the surface. Inhomogeneous high BaTiO3 content fibers were not used in transport studies. The topographic AFM images on PANI/PEO and PANI/ PEO-BaTiO3 fibers confirm that the width of the fibers ranges from 300 to 500 nm (note that a tip convolution effect increases the measured width by 100 nm but does not affect the height measurement). At this scale no defects were observed on the nanofibers. Scanning impedance microscopy was performed in order to investigate the potential profile of the PANI/PEO fibers without particles. These samples served as a reference for the PANI/PEO-BaTiO3 fibers. SIM imaging during current flow with a lateral dc bias of 2 V, Figure 3b, exhibits a monotonic and nearly linear voltage drop along the fiber. The potential profile (Figure 3c) indicates non-Ohmic behavior near the electrodes, indicative of the expected contact potential. The composition and polarization dependence of electronic transport in PANI/PEO-BaTiO3 is shown in Figure 4 and compared in Table 1. Complete I-V curves were acquired at all points; however, since the behavior in this voltage range is ohmic and does not add additional insight, the current at +1 V is compared here. The magnitude of the initial fiber 897
Figure 3. Scanning impedance microscopy (SIM) of the PANI-BaTiO3 fiber between gold electrodes: (A) circuit topography; (B) SIM amplitude image in which contrast is proportional to potential; (C) line profile of the potential along the fiber extracted from (B); (D) line profile of the topographic structure of the fiber from (A); (E) three-dimensional topographic structure of PANI-BaTiO3 fiber near the biased electrode.
Figure 4. Dependence of the current through the fiber at +1 V on the magnitude of voltage used to reorient ferroelectric domains in the nanoparticles for samples with different BaTiO3 loadings.
resistance varies from 1.7 to 13 MΩ and does not appear to have a rational dependence on oxide particle content. This is due to the fact that the absolute value of the current depends on local polymer dopant level and geometric dimensions. In all cases the current decreases as the voltage used to orient the dipoles increases. Furthermore, the effect is a strong function of BaTiO3 nanoparticle content; the relative drop of the conductance with respect to initial conductance decreases monotonically with oxide particle content. When normalized to eliminate the variations in initial conductance, and presented in terms of resistance, the magnitude of the polarization effect increases with oxide particle content, Figure 5. 898
Figure 5. The dependence of the normalized resistance after +200 V polarization (corresponding to the electric field 1000 V/cm) on BaTiO3 particle content. The relative particle content is determined from EDX spectra. Normalized resistance increases with BaTiO3 particle content.
The effect of the polarization voltage on the electronic properties of the nanofibers can be considered from both classical and quantum mechanical perspectives with the same qualitative outcome. In the classical electrostatic model the structure is described as ideally insulating particles embedded in an ideally conducting media (Figure 6B). The small size of the ferroelectric particles ensures that each is a single domain that can be represented as a dipole terminating in surface charge. Free carriers in the polymer will screen the charge by concentrating near one pole and depleting near the opposite pole. The screened region will increase the “effective area” of the nonconductive region in an asymNano Lett., Vol. 6, No. 5, 2006
Figure 6. Schematic diagram illustrating the mechanism of the resistance dependence on ferroelectric domain orientation: (A) the shape of the screened volume around a polarized BaTiO3 particle; (B) the change in the area projected on the plane normal to the current flow from the screened volume ellipsoid as a function of dipole orientation (the minimum is when angle is 0°; the maximum when angle is 90°).
metric manner by an amount associated with the Debye length of the PANI (see Figure 6). Initially the dipoles are randomly oriented, but the application of an external voltage aligns the particles perpendicular to the fiber axis such that the projected area of the scattering center increases. For a fiber with resistance Rmedia filled with n insulating spheres with resistance Rinclusion the resistance is Rtot )
2Rinclusion + Rmedia + p(Rinclusion - Rmedia) 2Rinclusion + Rmedia - p(Rinclusion - Rmedia)
Rmedia
where p is the volume fraction of the inclusions in the polymer fiber.10 Assuming that Rinclusion . Rmedia, this expression is simplified to Rtot ) (2 + p)/(2 - p)Rmedia. On the basis of the ohmic behavior of the samples and the proportionality of p to polarization voltage, the equation for Rtot might be substituted to Itot (the current in the circuit at +1 V): Itot ) (2 - AV)/(2 + AV)I0, where I0 and A are fitting parameters and V is polarization voltage. I0 corresponds to the resistance of fibers with unoriented BaTiO3 particles, while A corresponds to the apparent volume fraction of scattering particles after polarization. Figure 7 shows the dependence of A and I0 on BaTiO3 particle content. At vanishing small particle contents, A approaches zero and Itot ) I0, which is 600 nA in Figures 4 and 7. As the particle content increases, dipole orientation has a larger effect. The projected scattering area increases dramatically as polarization rotates under the polarization voltage perpendicular to the fiber axis and resistance increases accordingly. Note that, as expected, the effect on I0 is not correlated to BaTiO3 content as it is a function of fiber geometry and PANI carrier concentration. At sufficiently large concentrations of particles the apparent volumes will overlap and no further effect is incurred. At this point A becomes independent of particle concentration. Table 2 compares the composition with particle spacing estimated from the composition. Saturation occurs at concentration associated with 80 nm center-toNano Lett., Vol. 6, No. 5, 2006
Figure 7. The initial current of the unpolarized fibers, I0, and apparent volume, A, determined from a fit to the data in Figure 4. Extracted from these transport properties, the random values of I0 result from compositional variations between fibers; the systematic increase in polarized volume increased with particle content, as expected.
center separation, suggesting that the screening length is on the order of 10 nm. This is consistent with the properties of PANI (scattering length ∼20 nm).11 Additionally, we have done electrodynamic finite element calculations based on this description which yield similar results. The results in Figure 5 can also be considered from the perspective of the transport mechanism in PANI. Several models have been developed to describe the temperature dependence of conductivity in PANI. Tunneling between metallic islands,12 variable range hopping,13 and combinations of these with metallic conduction12 differ only by the power in the temperature dependence. Regardless of which detailed mechanism applies, tunneling between localized states is involved. Transmission through a tunnel barrier has the form
( (
sinh
Tthrough ) 1 +
Tover ) 1 +
2
x ( )
E 1a 2 V p 0 E E 4 1V0 V0
(
sin
2
)
-1
2mV0a2
)
x ( )
)
2mV0a2 E -1 a p 2 V0 E E 4 1V0 V0
(
)
-1
where E is the electron energy, V0 is the potential barrier height, a is the tunneling length, m is the electron mass, p is Planck’s constant. The local electric field due to the ferroelectric dipole will alter the barrier height in the PANI adjacent to the particle, increasing it near the positively terminated domain and decreasing it near the negatively terminated domain. The quantum mechanical description is 899
Table 2. Effect of PANI-BaTiO3 Fiber Composition on Interparticle Distance wt % of BaTiO3 particles in the solution
wt % of BaTiO3 particles in the fiber
vol % of BaTiO3 particles in the fiber
0.01 0.1 1 3 5
0.4 4.3 30.8 57.0 69.0
0.1 0.8 7.4 19.4 28.7
based on the fact that the transmission coefficients through energy barriers with heights E + ∆E and E - ∆E differ. It should be noted that the Fermi energy in PANI lies below the conduction band; therefore, tunneling through the barrier should be considered (Tthrough) for tunneling without an additional field and tunneling over the barrier (Tover) should be considered when an external field decreases the barrier height. Comparing transmission coefficients when the barrier height is (∆E with respect to that at E, the decrease of T for energy E + ∆E is larger than the increase of T at energy E - ∆E. Hence the overall effect will be an increasing resistance when the domains are oriented perpendicular to the current flow direction. Using reasonable quantitative data for PANI properties, such as electron energy 35 meV, barrier height E ) 50 mV, ∆E ) 40 mV, and tunneling distance a ) 10 nm, the estimated conductivity decreases more that 3 times for the region with high barrier height and increases less than 2.5 times for the region with low barrier height. The overall conductivity around the nanoparticle decreases, i.e., resistance increases. In summary, the orientation of ferroelectric domains embedded in nanofibers can be manipulated externally and used to control the electronic properties of nanowires. The results demonstrate this phenomenon in fibers on the order of 300-500 nm in diameter, but the small size of the ferroelectric nanoparticles would allow reduction of fiber sizes to tens of nanometers. Furthermore, this functionality has been implemented with a fabrication process that can precisely position fibers into device configurations and is a scalable process. This scalability together with the external control of local polarization in the fibers makes PANIBaTiO3 an attractive approach for field-based sensors, biochemical sensors, memories, localized switches, etc. Acknowledgment. The authors gratefully acknowledge financial support from the Nano/Bio Interface Center at the University of Pennsylvania (DMR04-25780) and the Nanobiotechnology Center (NBTC) at Cornell University (DMR05-
900
PANI
PEO
distance between the particles in the fiber, nm
88.8 88.2 82.3 71.6 63.4
11.1 11 10.3 9 7.9
553 250 115 80 70
vol % of polymer in mixture
20020) funded by the National Science Foundation. We also acknowledge the use of facilities funded under DMR0425780 Grant. M.P.N. would like to acknowledge Jim Ferris’ help (University Penn Regional Nanotech Facility) in performing EDX measurements and Scott Slavin (UPenn Microfab Lab) for help with resistance measurements. References (1) Liu, H. Q.; Kameoka, J.; Czaplewski, D. A.; Craighead, H. G. Polymeric nanowire chemical sensor. Nano Lett. 2004, 4, 671-675. (2) Ohno, H.; Yamazaki, H. Preparation and characteristics of all solidstate electrochromic display with cation-conductive polymer electrolytes. Solid State Ionics 1993, 59, 217-222. (3) Martins, C. R.; de Freitas, P. S.; De Paoli, M. A. Physical and conductive properties of the blend of polyaniline/dodecylbenzenesulphonic acid with PSS. Polym. Bull. 2003, 49, 379-386. (4) Somani, P.; Kale, B. B.; Amalnerkar, D. P. Charge transport mechanism and the effect of poling on the current-voltage characteristics of conducting polyaniline-BaTiO3 composites. Synth. Met. 1999, 106, 53-58. (5) Patil, R. C.; Radhakrishnan, S.; Pethkar, S.; Vijaymohanan, K. Piezoresistivity of conducting polyaniline/BaTiO3 composites. J. Mater. Res. 2001, 16, 1982-1988. (6) Mazur, K. Piezoelectricity of PVDF/PUE, PVDF/PMMA and PVDF/ PMMA+BATIO3 laminates. IEEE Trans. Electr. Insul. 1992, 27, 782-786. (7) Kameoka, J.; Orth, R.; Yang, Y. N.; Czaplewski, D.; Mathers, R.; Coates, G. W.; Craighead, H. G. A scanning tip electrospinning source for deposition of oriented nanofibres. Nanotechnology 2003, 14, 1124-1129. (8) Jo, J. Y.; Kim, Y. S.; Kim, D. H.; Kim, J. D.; Chang, Y. J.; Kong, J. H.; Park, Y. D.; Song, T. K.; Yoon, J. G.; Jung, J. S.; Noh, T. W. Thickness-dependent ferroelectric properties in fully strained SrRuO3/ BaTiO3/SrRuO3 ultrathin capacitors. Thin Solid Films 2005, 486, 149-152. (9) Kalinin, S. V.; Bonnell, D. A. Scanning impedance microscopy of an active Schottky barrier diode. J. Appl. Phys. 2002, 91, 832-839. (10) Maxwell, J. C. A treatise on electricity and magnetism, 3rd ed: Oxford University Press: London, 1904; Vol. I. (11) Mukherjee, A. K.; Menon, R. Magnetotransport in doped polyaniline. J. Phys.: Condens. Matter 2005, 17, 1947-1960. (12) Pelster, R.; Nimtz, G.; Wessling, B. Fully Protonated Polyaniline Hopping Transport on a Mesoscopic Scale. Phys. ReV. B 1994, 49, 12718-12723. (13) Campos, M.; Bello, B. Mechanism of conduction in doped polyaniline. J. Phys. D: Appl. Phys. 1997, 30, 1531-1535.
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Nano Lett., Vol. 6, No. 5, 2006
