Tip induced nanostructuring of AuxCuy-alloys with an electrochemical scanning tunneling microscope

S. Maupai, A.S. Dakkouri

By tip-induced metal deposition using an EC-STM it is possible to generate small metal clusters on a metal surface in an electrochemical environment. The principle of this method is schematically shown in Figure 1. In a first step metal is electrodeposited on the apex of the tip of an electrochemical STM (a). This metal loaded tip is approached to the substrate surface (b). At a certain distance between tip and substrate, metal atoms of the tip jump to the surface and build a connective neck (c). When retracting the tip, a small amount of metal from the tip remains on the surface and forms a cluster (d). The apex of the tip is regenerated by a redeposition of metal and is immediately ready for further cluster deposi-tion (e).

Figure 1: Principle of the tip-induced metal deposition

This method was introduced by D.M. Kolb, University of Ulm, in 1992 and applied to generate metal clusters (Cu, Pb, Ag, Pd) on different substrates (Au, Ag) [1]. Clusters con-sist typically of 100-500 atoms and show an unexpected electrochemical stability positive of the reversible Nernst potential for metal dissolution in the specific system [2]. As the existing theories explain this stability not satisfactorily yet, our aim was to gain additional information by generating Cu-clusters on binary alloy surfaces instead of pure metal sur-faces. Using AuxCuy single-crystals of different composition opened the possibility to vary the tip-substrate interaction and the influence of adsorbates like monolayers of Cu that may be present on the surface due to underpotential deposition.

Figure 2: Nanoclaus of copper on Au(111) drawn with 1109 Cu-Clusters; each Cluster is 2-3 atomic layers high and consists of 100-500 Cu-atoms.

The electrochemical STM was modified to access and control the STM-scanner directly. With this modified setup it is possible to move the metal loaded STM-tip in a controlled way by applying a defined voltage pulse onto the driving voltage of the scanner. This al-lows us to generate almost any arrangement of Cu-Clusters on a substrate during the scanning process. As an example for such an arbitrary pattern a “Nanoclaus” is shown in Figure 2, consisting of more than 1000 Cu-Clusters, each 2-3 monolayers high and consisting of 100-500 atoms.

To properly investigate AuxCuy-surfaces a vigorous UHV-preparation and characterization of the alloy-single crystals is necessary [3]. Additional characterization was carried out with electrochemical current-densitiy/potential-measurements and investigations with the EC-STM. The results give evidence that the stability of the metal clusters is not intrinsic but more an effect of interfacial alloying and inhibition of surface diffusion in the system due to the adosrption of a monolayer of metal on the substrate, (under potential deposi-tion). On copper rich alloy surfaces this UPD is only partially present and thus smaller and less stable Cu-clusters are observed.

Figure 3: (300 nm x 300 nm) Cu-Clusters on Au3Cu(111) at 0 mV vs. Cu/Cu2+ (a) and 300 mV vs. Cu/Cu2+ (b)

Signs for interfacial alloying on this nanoscopic scale is the beginning of a localized corro-sion process at anodic potentials exactly at positions where the clusters were located prior to the anodic treatment (see Figure 3). This is especially obvious on the alloy surfaces where a corrosion process starts parallel to cluster dissolution and preferentially at copper rich positions of the surface.

References:

[1] D.M. Kolb, R. Ullmann and T. Will, Nanofabrication of Small Copper Clusters on Au(111) Electrodes by a Scanning Tunneling Microscope, Science, 275 (1997) 1097-1099

[2] D.M. Kolb, G.E. Engelmann und J.C. Ziegler, On the Unusual Electrochemical Stability of Nanofabri-cated Copper Clusters, Angew. Chem. Int. Ed., 39 (2000) No. 6 1123-1125

[3] G.A. Eckstein, S. Maupai, A.S. Dakkouri, M. Stratmann, M. Nielsen, M.M. Nielsen, R. Feidenhans´l, J.H. Zeysing, O. Bunk und R.L. Johnson, Surface structure of Au3Cu(001), Phys. Rev B, 60 (1999) 8321-8325