CAFM on conjugated polymer nanofibers: Capable of assessing one fiber mobility Jean-Christophe Bolsée a,⇑ , Wibren D. Oosterbaan a , Laurence Lutsen b , Dirk Vanderzande a,b , Jean Manca a,b a Hasselt University, Campus Diepenbeek, Institute for Materials Research, Agoralaan Building D, 3590 Diepenbeek, Belgium b IMEC-IMOMEC, Wetenschapspark 1, 3590 Diepenbeek, Belgium article info Article history: Received 15 July 2011 Received in revised form 16 August 2011 Accepted 20 August 2011 Available online 9 September 2011 Keywords: Single conjugated polymer nanofiber Longitudinal mobility Transversal mobility CAFM abstract Poly(3-alkylthiophene) (P3AT) nanofibers (NFs) are interesting as building blocks for future organic electronic devices but also as a model system to examine transport mechanisms in semiconducting polymers. However, due to the technical problem of making good contacts at the nanometer scale, electrical characterizations on one isolated P3AT NF are rare. Here, using Conductive Atomic Force Microscopy (CAFM), charge transport investigations along the length (p–p stacking direction) and the height (alkyl chain direction) of one single NF are reported. Firstly, by developing a transistor structure with one electrode being the CAFM tip, we measured the longitudinal mobility: l L = 0.07 ± 0.03 cm 2 V 1 s 1 . Secondly, charge transport along the NF height was done in a sandwich structure and led to a trans- versal mobility of l T  10 5 –10 6 cm 2 V 1 s 1 where l T was found to decrease with increasing alkyl side chain length. With CAFM charge transport anisotropy in P3AT NF is thus evidenced at the nanometer scale, since we find at least four orders of magnitude dif- ference between l L and l T . Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Due to their high aspect ratio, mechanical and chemical stabilities, conducting polymer nanofibers (NFs) are inter- esting for future applications such as interconnecting wires, photodetectors [1], transistors [2] and solar cells [3,4]. There have been several reports on the class of NFs which form upon crystallization of regioregular poly(3- alkylthiophene) (P3AT) from solutions in marginal solvents [5–8]. Fig. 1 shows a schematic of the P3AT NF crystal structure: the polymer chains pack in lamellar sheets per- pendicularly to the NFs length and stack by p–p interac- tion along the NFs length. These well-defined nanostructures are not only inter- esting as nanosized components for future devices but also provide an ideal model system to investigate charge transport phenomena in organic semi-conductors. This is because they are grain boundary free and highly ordered. Therefore, field-effect transistor (FET) measurements were performed by several groups [2,9–11]. However, due to the technical problem of making good electrical contacts on the nanoscale, the majority of the studies has focused on webs of NFs and not on one single NF. Therefore, relatively little is known about charge transport on isolated NFs and more studies are needed before they can be used exten- sively in organic devices. Here, using Conductive Atomic Force Microscopy (CAFM), we report on the electrical characterization of one single P3AT NF. We have built a particular transistor structure- described in more detail in the next section - with one electrode being the CAFM tip and allowing us to measure the charge carrier mobility along the fiber length, l L for longitudinal mobility. Furthermore, by sand- wiching one NF between the CAFM tip and a conducting substrate, we have estimated the charge carrier mobility 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2011.08.022 ⇑ Corresponding author. E-mail address: jeanchristophe.bolsee@uhasselt.be (J.-C. Bolsée). Organic Electronics 12 (2011) 2084–2089 Contents lists available at SciVerse ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel