In situ STM tip-induced electropolymerization of poly(3-octylthiophene)

Abstract

Poly(3-octylthiophene) has been grown on an indium-tin-oxide (ITO)-coated glass substrate by an STM tipinduced polymerization technique in which a high anodic bias is applied on the substrate. Molecular images of resultant polymer structures are obtained by a subsequent in situ STM study. We find that a three-dimensional nucleation and growth mechanism operates to deposit both nodular and helical polymer structures. Consideration of the helix diameters (range 25-70 Å) obtained points to the existence of both simple helices and superhelices. Initial nucleation occurs preferentially at the ITO grain boundaries, probably because of an altered electronic state from the segregation of dopants there.Since the discovery in 1977 [1] of the first conducting polymer, the doped polyacetylene (PA), many families of conducting polymers have been found. Among them, heterocyclic polymers based on pyrrole and thiophene have been intensively studied because of their relatively simple preparation, good chemical and thermal stability, high conductivity and potential applications in various fields. Even in the homopolymers, the macroscopic properties are governed by complex structure-property relationships relating to the linkage mode, stereoregularity and chain conformation in the polymer backbone.Recent studies have shown that scanning tunneling microscopy (STM) and related methods are powerful tools to elucidate the structures and conformations of conducting polymers [2-9]. The electropolymerization growth process has also been investigated by STM [10][11][12][13]. Early work by Yang et al. [2,3] showed the presence in doped polythiophene (PT) of helical structures with a diameter of 15 ± 3 Å and a pitch of 8 ± 1.5 Å; on polypyrrole, they found helical structures of diameter 18 Å and pitch 5 Å. Recently, another group reported the helix diameter of polythiophene to be in the range 9 − 15 ± 2 Å using a noncontact scanning probe microscopy technique [9]. These values are consistent with helices having repeat units per turn in the polymer backbone.For these unsubstituted polythiophene and polypyrrole, large helical structures of diameter 50-60 Å have also been * To whom all correspondences should be addressed observed [3]. It is thought that these structures are superhelices, in which helical polymer chains further coil on themselves to give a ternary structure. To the best of our knowledge, the largest 3-alkyl derivative reported is poly(3-methylthiophene) whose helices have been measured by STM with a diameter about 22 Å and height about 20 ± 3 Å [5].Atomistic simulations of the conformations of polythiophene, polypyrrole and their derivatives have indicated the formation of both anti-like and syn-like chain conformations and the existence of stable helical structures [14][15][16]. The calculated helix diameter agrees well with the values measured by STM and related methods.In this study, we have used the STM tip-induced eletropolymerization technique to polymerize 3-octylthiophene on ITO glass and subsequently study the polymeric...

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