Local Formation of an Alloy by Atomic Contact between the STM Tip and the Substrate Surface

Abstract

KEYWORDS:alloys ¥ gold electrode ¥ quantum point contact ¥ scanning probe microscopy ¥ tunneling resistance A new way of nanostructuring a gold electrode surface with the STM tip will be described. Due to a force interaction between the STM tip and the substrate surface, covered by a UPD monolayer (UPD underpotential deposition) of Cu or Pd atoms, a local alloy is formed. By measuring the conductance G, which characterizes the minimal distance of the STM tip to the surface, necessary for generating the nanostructures, we get an insight into the possible mechanism. The conductance is in the range as expected from quantummechanics for a point contact. Due to this result we assume that a point contact between the STM tip and the UPD-covered surface is formed.The preparation of structures with lateral dimensions on the nanometer-scale attracts an increasing interest, since it has been shown that the physical and chemical properties of such nanostructures can strongly deviate from the properties of the corresponding bulk materials. In recent years, the electrochemical scanning tunnelling microscope (EC-STM) has often been used as a tool to modify electrode surfaces on a nanoscopic scale. Kolb et al. have demonstrated that very small metal clusters can be generated with the EC-STM by a special technique of a tip induced metal deposition, the so called ™jump-to-contact∫ method.[1] This technique involves electrolytic deposition of the metal (for example a Cu metal) onto the STM tip, followed by a controlled approach during which metal is transferred from the tip to the surface (for example a Au metal). It was shown that the metal to be deposited (Cu, Pb, Ag, Pd) as well as the substrate metal (Au, Ag) can be varied.

Nanotribology under electrochemical conditions: influence of a copper (sub)monolayer deposited on single crystal electrodes on friction forces studied with atomic force microscopy

“…The latter case, a penetration of the first metal layer is unprobable because on the pure Au surface neither such sudden increase of the friction force was observed in the range from 30 to 250 nN nor wear nor any grooves. In other experiments (not yet published) we observed the formation of grooves only at loads exceeding 1 mN (and also, on the Cu covered surface, the local formation of a surface alloy similar to that obtained by STM 36,37 ). On pure gold such a transition due to penetration of the sulfate layer is not expected because the experiments on gold were performed in sulfuric acid at an electrode potential of E (Au) = 30 mV vs. Cu/Cu 2+ where no sulfate is adsorbed.…”

Nanoscale Structure, Dynamics, and Aging Behavior of Metallic Glass Thin Films

“…Local crystallization in metallic glasses has been seen in TEM measurements 41 and nano-indentation 42 . On other surfaces, cases of cluster formation 43 and alloying 44 operating by this type of mechanism have been seen in previous STM experiments. This thermally activated change would depend exponentially on the energy transferred in the tunnel current spike, which in turn, depends linearly on the tunnel current set point, leading to a net expected exponential dependence of the merger rate on tunnel current.…”

On the surface properties of lead structures on Au(111), an atom dynamic approach

“…However, it now seems that the stability of the clusters is based an the formation of a stable alloy between the deposit and the substrate. This explanation is not only supported by computer simulations [2,3], but also by recent experiments [4,5].…”