Conductive PVDF-HFP Nanofibers with Embedded TTF-TCNQ Charge Transfer Complex Reshef Gal-Oz, † Nilesh Patil, ‡ Rafail Khalfin, § Yachin Cohen, § and Eyal Zussman* ,‡ † Interdepartmental Program in Polymer Engineering, ‡ Department of Mechanical Engineering, and § Department of Chemical Engineering, Technion−Israel Institute of Technology, Haifa 32000, Israel ABSTRACT: Tetrathiafulvalene-tetracyanoquinodimethane charge- transfer complex (TTF-TCNQ CTC) represents a promising organic conductive system. However, application of this donor−acceptor pair is highly limited, because of its ultrafast crystallization kinetics and very low solubility. In this work, conductive organic nanofibers were generated via a coelectrospinning process of poly(vinylidene fluoride-co-hexafluor- opropylene) (PVDF-HFP) with embedded TTF and TCNQ in the shell and core solutions, respectively. Upon supply of the polymer solutions, a core−shell droplet was formed at the exit of the spinneret. The electron donor TTF and the electron acceptor TCNQ migrated toward each other, within the compound droplet, to produce conductive CTC crystals. In the presence of a sufficiently strong electric field, jetting set in at the droplet tip, which yielded solidified PVDF- HFP nanofibers embedded with aligned CTC. Fiber diameters ranged between 100 and 500 nm. X-ray analysis showed strong equatorial reflections (110,200) of oriented copolymer PVDF-HFP crystals (β-phase) with copolymer chains oriented along the fiber axis, and of CTC (001), indicating that the CTC molecular planes were aligned parallel to the nanofiber axis. In addition, reflections of unreacted TCNQ (120,220) and TTF (110) crystals were observed. The electrospun nanofibers were collected to form a fiber mat, which was evaluated as a working electrode in a three-electrode cell system, exhibiting differential conductance of 5.23 μmho. KEYWORDS: conductivity, electrospinning, polymer, TTF-TCNQ, nanofibers, X-ray ■ INTRODUCTION The electronic conductivity of organic molecules is dictated by their ability to overcome the band gap and to transfer an electron from highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO to LUMO), and along the conductive chain. One of the most studied systems is the tetrathiafulvalene-tetracyanoquinodimethane charge-transfer complex (TTF-TCNQ CTC) (Figure 1). 1−3 The electrical conductivity of the CTC, at room-temperature, is in the range of 1 × 10 3 to 1 × 10 4 S cm −1 , whereas the TTF and TCNQ, as single components, are nonconductive. 4 In addition, the two components are soluble in a variety of organic solvents, whereas the CTC, which is formed rapidly at room temperature, has relatively low solubility in both polar and nonpolar solvents. 5 The high conductivity of TTF-TCNQ CTC is attributed to a “herring bone”-type crystal structure formed by the flat TTF and TCNQ, in which orbitals on adjacent molecules overlap to form continuous one-dimensional bands. The electrical conductivity of the TTF-TCNQ couple depends on the spontaneous formation of appropriate segregated stacks of donors and acceptors, separated by less than 2 nm, 6 and on a certain degree of charge transfer between the stacks. The complex is electrically conductive over a wide range of temperatures, from 350 K down to 59 K, with a sharp metal- to-insulator transition observed at 59 K. 3 Despite these qualities, application of TTF-TCNQ in “plastic electronics” is highly limited, because of its poor processability as a result of its ultrafast crystallization kinetics and very low solubility. Odom et al. 4 explored the formation of TTF-TCNQ CTC via mechanical rupture of microencapsulated solutions of its individual components in poly(urea formaldehyde) and found that the resulting complex has the ability to partially restore the conductivity of severed gold electrodes. Liu et al. 7 prepared TTF-TCNQ CTC nanowires and dendrites of various morphologies, using a two-phase method, in which the individual components were individually dissolved in copper and silver solutions and then recrystallized together at various temperatures. Electrical measurements of individual TTF- TCNQ nanowires indicated that the helical nanowire conducts along its b-axis, with a conductivity of 295 S cm −1 . Braun et al. 8 studied, by means of ultraviolet photoelectron spectroscopy (UPS), the organic heterojunctions in multilayered thin film stacks comprised of alternating layers of TTF and TCNQ. They showed that energy level alignment at the organic− organic interfaces in the stacks depended solely upon the relative energy structure of the donor and acceptor molecules. Recently, Mukherjee et al. 9 presented fabrication of high- performance organic thin film transistors (OTFTs) with a solution-processed TTF-TCNQ CTC film serving as bottom contact source and drain electrodes. The organic charge Received: March 6, 2013 Accepted: June 10, 2013 Research Article www.acsami.org © XXXX American Chemical Society A dx.doi.org/10.1021/am400834b | ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX