2D compound formation during copper dissolution: An electrochemical STM study P. Broekmann * , N.T.M. Hai, K. Wandelt Institut fu ¨ r Physikalische und Theoretische Chemie, Universita ¨ t Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany Available online 6 May 2006 Abstract The reversible formation of a 2D-CuI film on Cu(1 0 0) is studied by means of cyclic voltammetry in combination with electrochemical scanning tunneling microscopy. Exposing the Cu(1 0 0) electrode surface to an acidic and iodide containing electrolyte (5 mM H 2 SO 4 /1 mM KI) leads to the formation of a well ordered c(p · 2)-I adsorbate layer at potentials close to the onset of the anodic copper dissolution reaction. Copper dissolution starts at slightly more positive potentials preferentially at step edges in the presence of the iodide adlayer via the removal of copper mate- rial from kink sites at step edges. This increase of mobile Cu + ions causes the local exceeding of the CuI solubility product (pKL = 11.3), thereby giving rise to the nucleation and growth of a laterally well ordered 2D-CuI film. Key structural motifs of the growing CuI film are closely related to the (1 1 1) plane of bulk CuI. Quite intriguing, the 2D-CuI film does not act as a passive layer. Copper dissolution pro- ceeds even in the presence of this binary compound via an inverse step flow mechanism. Ó 2006 Elsevier B.V. All rights reserved. Keywords: STM; Electrified interfaces; Corrosion; Anion effects 1. Introduction Copper as one of the key materials of the twenty-first century has attracted tremendous attention over the last two decades due to its nowadays realized application as wiring material in the state-of-the-art chip design [1], here replacing the former aluminium technology. However, the on-going trend of miniaturization towards the nanome- ter scale requires a more sophisticated understanding of the relevant interface properties and processes of those devices containing reactive materials like as copper. An atomic scale understanding of corrosion phenomena, corrosion inhibition by organics, oxidation, anodic dissolution and the formation of passivating films on copper is thus of vital interest and has consequently been in the focus of numer- ous fundamental studies. Since modern processing lines of chip fabrication also involves more and more ‘‘wet’’ chemical deposition processes the mastering of copper-elec- trolyte interfaces with or without additional potential con- trol can be regarded as a particular challenge. The anodic Copper Dissolution Reaction (CDR) in acidic electrolytes has been intensively studied in particu- lar by Bard et al. [2,3] and Magnussen/Behm et al. [4–8] using in situ STM. Active sites for the dissolution or deposition are exclusively kinks at step edges as stated by Stranski in 1920 [9]. Magnussen and co-workers elab- orated that the local structure of kink sites can be signif- icantly altered by the presence of specifically adsorbed anions thus leading to changes in the overall surface reac- tivity [4]. State-of-the-art video STM techniques even allow the direct correlation of ‘‘global’’ parameters such as the exchange current density on the one hand and the ‘‘local’’ and microscopic dissolution rates at kink sites on the other hand [6,7]. Substantial contributions to the microscopic under- standing of the anodic CDR in alkaline electrolytes came from Marcus/Strehblow et al. [10–13]. Quite contrary to the behavior in acidic media the CDR under alkaline 0039-6028/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.01.109 * Corresponding author. Tel.: +49 228 73 3292; fax: +49 228 73 2551. E-mail address: broekman@thch.uni-bonn.de (P. Broekmann). www.elsevier.com/locate/susc Surface Science 600 (2006) 3971–3977