AuDoped PolyacrylonitrilePolyaniline CoreShell ... - Wiley Online Library

Field-Effect Transistors

Au-Doped Polyacrylonitrile–Polyaniline Core–Shell Electrospun Nanoﬁbers Having High Field-Effect Mobilities Wei Wang, Zhenyu Li,* Xiuru Xu, Bo Dong, Hongnan Zhang, Zhaojie Wang, Ce Wang,* Ray H. Baughman, and Shaoli Fang Increasing demand for low-cost, large-area, flexible electronic devices motivates increasing interest in organic field effect transistors (OFETs).[1] Polymeric semiconductors are promising for this application due to their light weight, tunable transport properties, mechanical flexibility, low cost, ease of processing, and scalable production.[2] However, use of polymer OFETs has been constrained by very low chargecarrier mobilities (μ), often below 0.01 cm2 V−1 s−1. In order to increase the field-effect mobility in polymers, major effort has been devoted to the control of molecular self-organization and to reducing structural defects that provide trapping and/ or scattering sites.[3] Despite some success, it is still a challenge for semiconducting polymers to surpass the field-effect mobilities of amorphous silicon (0.5–1.0 cm2 V−1 s−1).[4] Additionally, previously reported processing methods for high mobilities usually require both surface treatment and prolonged thermal annealing to optimally arrange molecules on molecular and domain length scales,[5] which compromises the requirement of ease of fabrication and can increase production costs. 1D nanostructures have attracted considerable attention in electronic device assembly. Electrospinning, a technique for the preparation of nanometer-sized fibers, has been used to fabricate pure semiconducting polymer fibers or blends. However the electrospinning of pure semiconducting polymers is usually hindered by low solubility and intrinsic brittleness, and the field-effect mobilities are smaller than 0.05 cm2 V−1 s−1.[6,7] In the case of blend fibers, the carrier polymers (e.g., poly(ethylene oxide), poly(ε-caprolactone), and so on) could block the charge transport, resulting in much smaller field-effect mobilities of below 1 × 10−3 cm2 V−1 s−1.[6,8]

Dr. W. Wang, Z. Y. Li, X. R. Xu, B. Dong, H. N. Zhang, Z. J. Wang, Prof. C. Wang Alan G. MacDiarmid Institute College of Chemistry Jilin University Changchun, 130012, China E-mail: [email protected]; [email protected] Prof. R. H. Baughman, S. Fang Alan G. MacDiarmid NanoTech Institute University of Texas at Dallas Richardson TX 75083–0688, USA DOI: 10.1002/smll.201001716

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In the present work, a simple, low-cost route is demonstrated for the fabrication of Au-doped polyacrylonitrile (PAN)–polyaniline (PANi) core–shell nanofibers, which involves electrospinning[9] of the core and subsequent gasphase polymerization of the shell. The measured field effect mobilities of our materials are up to ≈11.6 cm2 V−1 s−1, which is significantly higher than for PANi-based OFETs reported before and among the highest values reported for organic systems.[10] The super-high field-effect mobility in this study strongly depends on the doping of Au nanoparticles, which has not been previously reported. The present work may lead to a new method for fabricating high-performance polymer OFETs. PAN was chosen for the nanofiber core because of its excellent mechanical properties and environmental stability.[11] Figure 1 schematically illustrates the fabrication of a polymer FET using Au-doped PAN–PANi core– shell nanofibers. In the first step, core nanofibers comprising PAN and HAuCl4 were electrospun from a dimethylformamide (DMF) solution. Two parallel aluminum plates were used for collecting single and aligned nanofibers during the electrospinning process.[9b] The collected nanofibers were then transferred onto SiO2 wafers. Gas-phase polymerization of a PANi shell onto these nanofibers was subsequently conducted at 60 oC in saturated aniline vapor to obtain Audoped PAN–PANi core–shell nanofibers, using HAuCl4 in the as-spun nanofibers as the oxidant. Hundred-nanometerthick gold source and drain electrodes with 60 μm channel spacing were evaporated onto the core–shell nanofibers. A highly n++-doped Si wafer coated with a 1.6 μm-thick SiO2 dielectric was used as the top-gate electrode. Gaps between this mechanically contacted top-gate electrode automatically filled with dry air (5–20% relative humidity (RH)), which

Figure 1. Illustration of the fabrication of a nanofiber FET.

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Figure 2. TEM images of a) as-spun PAN/HAuCl4 nanofibers and b) Au-doped PAN–PANi core–shell nanofibers, and a higher magnification of PAN–PANi nanofibers (inset). The scale bar in the inset is 100 nm.

the PAN crystalline phase (at 2θ = 17o) corresponding to the orthorhombic PAN (110) reflection,[11] four sharp peaks at 2θ = 38.26o, 44.44o, 64.84o, and 77.60o were observed, which correspond to the face-centered cubic (fcc) Au phase (111, 220, 220, 311) according to the standard literature values reported for Au (JCPDS, No. 000–04–0784, a = 0.40788 nm). No peaks for the crystalline PANi phase were detected, confirming that the PANi shell obtained using our process is amorphous.[13] In the Fourier-transform infrared (FTIR) spectra of the Audoped PAN–PANi core–shell nanofibers (shown in Figure 3b), the characteristic peaks at 1588 and 1494 cm−1 correspond to the C=C stretching of the quinoid and benzenoid rings, the peak at 1390 cm−1 is related to the C–N+ polaron, the peak at 1140 cm−1 is assigned to the in-plane bending of C–H in the quinoid rings, and the peak at 830 cm−1 is attributed to the out-of-plane bending of C–H.[14] The FTIR observations confirm the formation of PANi. Polymer FETs were fabricated using the as-prepared Audoped PAN–PANi core–shell nanofibers. The source–drain current versus source–drain voltage (ISD–VSD) curves of the device are shown in Figure 4a. They display the typical behavior of a p-channel OFET, being initially linear with VSD and then saturating at higher VSD. A typical scanning electron microscopy (SEM) image of a single-nanofiber FET device is shown in the inset of Figure 4. The transfer characteristics of the same nanofiber transistor is depicted as the ISD values at VSD = 50 V plotted against the various gate voltages VG (Figure 4b). The threshold voltage VTH of the device is determined to be +24 V from the linear relationship between ISD1/2 versus VG, by extrapolating the measured data to ISD = 0. In the saturation regime, the hole mobility is calculated to be about 11.6 cm2 V−1 s−1 (the calculation of the mobility is provided in the Supporting Information (SI)), which is not only significantly higher than for PANi-based FETs,[15,8] but is among the highest values reported for organic systems.[10] For comparison, electrospun polyaniline– polyethylene oxide nanofiber field-effect transistor provides a very low mobility of 1.4 × 10−4 cm2 V−1 s−1,[8] and semiconducting liquid-crystalline thieno[3,2-b]thiophene polymers possess field-effect mobilities of 0.2−0.6 cm2 V−1 s−1, due to the highly organized morphology.[3c] The largest field-effect mobility of the high-molecular-weight

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served together with the SiO2 as the gate dielectric (the function of dry air as an excellent dielectric has been previously reported by Tang et al.).[12] Figure 2a shows transmission electron microscopy (TEM) images of the as-spun PAN/HAuCl4 nanofibers, which have smooth surfaces and uniform diameters ranging from 100 to 150 nm. After gasphase polymerization of PANi in satu* a rated aniline vapor for over 30 h, a smooth b * Au core–shell PAN–PANi nanofiber structure was formed, having a shell thickness of ca. 40–50 nm (Figure 2b). Au nanoparticles with diameters of about 3–10 nm are uniformly * dispersed along the nanofibers, as observed * at higher magnification (inset of Figure 2b), * indicating the reduction of HAuCl4 to Au nanoparticles during the gas-phase polymerization of PANi, which is described below. 4000 3000 2000 1000 15 30 45 60 75 The X-ray diffraction (XRD) pat-1 Wavenumber/cm 2 θ/degree tern obtained from Au-doped PAN– PANi core–shell nanofibers is shown in Figure 3. a) The XRD pattern, obtained using Cu Ka radiation, and b) the FTIR spectrum of Figure 3a. Besides the diffraction peak of Au-doped PAN–PANi core–shell nanofibers. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Au-Doped Core–Shell Nanofibers Having High Field-Effect Mobilities

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Figure 4. a) Output and b) transfer characteristics of the Au-doped PAN–PANi core–shell nanofibers FET. The inset shows a view of a core–shell nanofiber bridging the source–drain gold electrodes. The scale bar is 20 μm.

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gas-phase polymerization. The as-prepared Au-doped PAN– PANi core–shell nanofibers provide a very high field-effect mobility of up to 11.6 cm2 V−1 s−1, without crystallizing the molecular structures of polymers. This high mobility could be due to the nanofiber structure, which promotes charge transfer and reduces the grain-boundary effect, and the doping of Au nanoparticles, which serve as “conducting bridges” between the PANi semiconducting domains. This approach is also suitable for other conducting polymers. Thus the present work may eventually lead to a new and simple method for fabricating high performance polymer OFETs.

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cyclopentadithiophene-benzothiadiazole copolymer is 1.4 cm2 V−1 s−1 through control of the organization of the organic compound.[3a] The much higher field-effect mobilities of the Au-doped PAN–PANi core–shell nanofibers can be ascribed to two possible effects. One is that the quasi-1D nanostructure can promote charge transfer and reduce grain boundary effects, thereby increasing the field-effect mobility, as reported by Briseno et al.[16] The other effect may originate from the doping of Au nanoparticles. Researchers have reported that electrons residing on PANi can surmount the interface between PANi and gold nanoparticles, and move onto the gold nanoparticles.[17] During the gas-phase polymerization of PANi shell, Au nanoparticles synchronously form at the center of the PANi coating. This enables close contact between PANi and Au nanoparticles, which can promote charge transfer between the two parts, and make PANi more positively charged. On the other hand, it has been frequently proposed that the doped conducting polymer consists of metallic “islands” which are surrounded by semiconducting “beaches”.[18] In the as-prepared Au-doped PAN–PANi core–shell nanofibers, the “metallic islands” of Au nanoparticles will serve as “conducting bridges” between the PANi semiconducting domains,[19] which can also be considered as many nanoelectrodes between the gold source and drain electrodes. They increase electrical percolation and promote charge transport, consequently facilitating high chargecarrier mobility, even though PANi is amorphous. An auxiliary experiment is presented in the SI to provide more evidence of the role of Au nanoparticles (SI, Figure S3, S4). Preliminary studies show that this approach can also be used to prepare high-mobility PAN–polypyrrole (PPy) OFETs. Figure 5 presents the ISD–VSD output characteristic curves and transfer characteristic curves (VSD = 50 V) of the FET based on Au-doped PPy composite nanofibers prepared using electrospinning and gas-phase polymerization. There are two nanofibers aligned perpendicular to the source and drain electrodes, as shown in the inset. The threshold voltage is about –8.5 V, the hole mobility is determined to be about 1.2 cm2 V−1 s−1, and the maximum current on/off ratio is higher than 4 × 103. In summary, Au-doped PAN–PANi core–shell nanofibers have been fabricated via electrospinning and subsequent

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Figure 5. a) Output characteristics and b) transfer characteristics (VSD = 50 V) of the Au-doped PAN–PPy nanofiber FET. The inset shows a view of nanofibers bridging the source–drain gold electrodes. The scale bar is 20 μm.



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Experimental Section In a typical procedure, 8 wt% of polymer and 2.7 wt% of HAuCl4.4H2O were dissolved in DMF, under vigorous stirring at 60 °C for 5 h. After cooling to room temperature, the mixture was loaded into a glass syringe and connected to high-voltage power supply. 12 kV was provided between the tip of the spinning nozzle and the collector at a distance of 15 cm. To obtain single nanofibers or aligned nanofibers, a modified fiber collector was used during the electrospinning process, as previously reported.[9b] Two aluminium foils placed in a parallel side-by-side arrangement were used as collectors. After the nanofibers were transferred onto SiO2 wafers, the asspun nanofibers were exposed to saturated aniline vapor under ambient conditions (8.0 Torr at 60 °C, where 1 Torr ≈ 133 Pa). During this process, not only was a shell of PANi obtained on the surface of the as-spun polymer nanofibers, but evenly dispersed Au nanoparticles were also formed in the nanofibers. To assemble the top-contact devices, 100 nm-thick gold source and drain electrodes with 60 μm channel spacing were firstly evaporated onto the core–shell polymer nanofibers. Then a little of the polyvinylpyrrolidone aqueous solution was dipped onto the sides of SiO2 substrates as a binder for attaching the top-gate electrode. Finally, a highly n++-doped Si wafer coated with a 1.6 μm thick SiO2 dielectric was used as the top-gate electrode. Gaps between this mechanically contacted top-gate electrode automatically filled with dry air (5–20% RH), which served together with the SiO2 as the gate dielectric. The current-voltage characteristics of the OFETs were measured in a clean and metallically shielded box at room temperature in air (5–20% RH), and recorded using a Keithley 4200 SCS.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements The work has been supported by National 973 Project (No. 2007CB936203 and S2009061009), NSF China (No. 50973038 and 51003036), National 863 Project (No. 2007AA03Z324), and Robert A. Welch Foundation Grant AT-0029.

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Received: September 28, 2010 Published online: January 31, 2011

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