The electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT) on an iodine-modified Au(1 1 1) single crystal electrode in aqueous 0.10 M HClO4 was investigated by cyclic voltammetry (CV) and electrochemical scanning tunneling microscopy (EC-STM). The cyclic voltammetric and EC-STM data revealed the stability of the iodine adlayer provided a suitable potential range for EDOT electropolymerization was controlled at 1.20V (vs. the reversible hydrogen electrode). EC-STM was used to examine the formation of the PEDOT adlayer on the iodine-modified Au(1 1 1) electrode. In situ electropolymerization of EDOT was carried out by slowly increasing the electrode potential to 1.20V. This process resulted in the formation of single molecular chains of PEDOT with a diameter of 0.90 nm and lengths of 5-7 nm. Higher resolution STM images further revealed PEDOT nanostructures with bent polymer backbones at angles of 105 degrees, 144 degrees and 180 degrees. The growth of PEDOT multi-layers was observed when the potential was held for a longer time. (C) 2011 Elsevier Ltd. All rights reserved.