Step-by-step growth of epitaxially aligned polythiophene by surface-confined reaction

Abstract

One of the great challenges in surface chemistry is to assemble aromatic building blocks into ordered structures that are mechanically robust and electronically interlinked-i.e., are held together by covalent bonds. We demonstrate the surface-confined growth of ordered arrays of poly(3,4-ethylenedioxythiophene) (PEDOT) chains, by using the substrate (the 110 facet of copper) simultaneously as template and catalyst for polymerization. Copper acts as promoter for the Ullmann coupling reaction, whereas the inherent anisotropy of the fcc 110 facet confines growth to a single dimension. High resolution scanning tunneling microscopy performed under ultrahigh vacuum conditions allows us to simultaneously image PEDOT oligomers and the copper lattice with atomic resolution. Density functional theory calculations confirm an unexpected adsorption geometry of the PEDOToligomers, which stand on the sulfur atom of the thiophene ring rather than lying flat. This polymerization approach can be extended to many other halogen-terminated molecules to produce epitaxially aligned conjugated polymers. Such systems might be of central importance to develop future electronic and optoelectronic devices with high quality active materials, besides representing model systems for basic science investigations.metal-catalyzed coupling reaction | molecular wires | cis-polythiophene | scanning probe microscopy | polymerization mechanism T he tunability and diversity of the structural and electronic properties of π-conjugated organic polymers have spurred a wide interest in these materials as wires and semiconductors for future electronic devices (1, 2), although significant optimization is required for real breakthrough performance. Conventional organic synthesis can create conjugated polymers of practically any length and structure (3), yet their controlled positioning and ordering on a surface remains nontrivial. This supramolecular ordering ultimately controls properties such as the charge mobility and recombination, which are critical for any application, including nanoelectronics and light harvesting. The approach presented here proposes a way to precisely control the long-range order in conducting polymers by performing synthesis of these materials, in an epitaxial way, directly on crystalline substrates. Although electrochemical polymerization on conducting surfaces has long been used to prepare films of conjugated polymers such as polythiophene (4), the techniques for preparation and characterization of aligned arrays of polymers have only recently been developed. Promising results were obtained by using UV irradiation (5) or scanning probe microscope tip pulsing (6) to form polydiacetylenes and surface-catalyzed coupling to form polyphenylene (7). However, these polymers are not good conductors due to a very large bond length alternation in their conjugated acetylene structures. (8). Conductive ordered polythiophene wires have been prepared by pulsed electrooxidative polymerization of substituted thiophenes (9, 10). The symmetry of the...

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