Cu-Nanoclusters Produced on AuCu-Alloys with an Electrochemical Scanning Tunneling Microscope
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Cu-Nanoclusters Produced on AuCu-Alloys with an Electrochemical Scanning Tunneling Microscope S. Maupai1, A.S. Dakkouri1, M. Stratmann2, P. Schmuki1 University of Erlangen-Nuremberg, Department of Materials Science, Chair for Surface Science and Corrosion (LKO), Martensstraße 7, D-91058 Erlangen, Germany 2 Max Planck Institute for Iron Research, Department of Interface chemistry and Surface Engineering, Max Planck Str. 1, D-40237 Düsseldorf, Germany 1
ABSTRACT Cu-nanoclusters can be produced in an electrochemical environment by tip-induced metal deposition using an electrochemical scanning tunneling microscope (EC-STM). These clusters, consisting of 100-1000 atoms only, show a surprising stability against anodic oxidation. The clusters, which are 2-3 atomic layers in height dissolve slowly when the applied potential is increased step by step to 200 mV positive of the reversible Nernst potential for “normal” copper dissolution. The presented work gives evidence that the unusual stability of the clusters could be a consequence of interfacial alloying between the cluster and the underlying substrate. In order to study these effects Cu-nanoclusters have been produced on pure gold substrates and on carefully prepared Au3Cu(111)-substrates. This work compares the results obtained on both substrates.
INTRODUCTION The electrochemical scanning tunneling microscope (EC-STM) is used frequently for imaging under electrochemical conditions but also used as a tool to modify electrode surfaces on a nanoscopic scale [1-8]. Kolb and coworkers developed a method called the “tip-induced metal deposition”, that they used to deposit small metal clusters under electrochemical conditions on a metallic substrate [9]. It was shown that various metals can be deposited (Cu, Pb, Ag, Pd) on a variety of substrates (Au, Ag) [10-14]. The tip-induced clusters are surprisingly stable against anodic oxidation in the electrolyte. They are usually generated in a metal containing electrolyte at potentials slightly positive of the reversible Nernst potential for metal dissolution and persist on the substrate when the applied potential is increased up to 100-200 mV anodic to the Nernst potential. An increase of the potential leads to a comparably slow but continuous decrease of the height of the clusters. The kinetics of this dissolution was described previously [14]. However the remarkable stability of the clusters could not yet be explained satisfactorily. Early explanations suggested a shielding effect of the probing STM tip as the main stabilizing factor [9]. A perfect crystallinity was also suspected to be responsible for the stability, but could not yet be experimentally proven [10]. Newer theories try to explain the stability by a quantum confinement effect due to the size of the clusters [14]. Lorenz and coworkers explained this stability by describing the nanoscale clusters as zero-dimensional phases [8]. Recently it could be
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demonstrated, that the tip-induced nanoclusters exhibit a respectable stability even under ex-situ conditions
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