Biomedical Engineering Reference
In-Depth Information
electrode surfaces was reported [94,96]. This so-called tip-induced
metal deposition involves conventional electrolytic deposition of a
metal onto the tip of an STM, followed by a controlled tip approach,
during which metal is transferred from the tip to the surface [97].
Small copper clusters, typically two to four atomic layers in height,
were precisely positioned on a Au(111) electrode by a process in
which copper was first deposited onto the tip of the STM, which
then acted as a reservoir from which copper could be transferred
to the surface during an appropriate approach of the tip to the
surface [98]. Tip approach and position were controlled externally
by a microprocessor unit, allowing the fabrication of various
patterns, cluster arrays, and “'conducting wires” in a very flexible
and convenient manner [94]. The formation of such clusters with the
tip of a STM is simulated by atom dynamics, and subsequently the
stability of these clusters is investigated by Monte Carlo simulation
in a grand canonical ensemble. It leads to the conclusion that optimal
systems for nanostructuring are those where the participating
metals have similar cohesive energies and negative heats of alloy
formation. In this respect, the system Cu-Pd(111) is predicted as a
good candidate for the formation of stable clusters [99]. In addition
to producing the metal nano-clusters, STMs can also be applied to
form nanoscale pits in thin conducting films of thallium (III) oxide
[100] as well as to write stable features on an atomically flat Au (111)
surface [101]. Pit formation was only observed when the process was
performed in humid ambient conditions. The mechanism involved
in pit formation was attributed to localized electrochemical etching
reactions beneath the STM tip. By applying voltage pulses (close to
3 V) across the tunneling junction in controlled atmosphere with the
presence of water or ethanol vapor, a nano-hole can be produced.
The smallest hole formed is 3 nm in diameter and 0.24 nm in depth.
This nano-hole represents the loss of about 100 Au atoms in the
top atomic layer of gold surface, and no atomic perturbation is seen
inside and outside the nano-hole. Different nanostructures (lattice
of dots, legends, map, etc.) can also be fabricated. The threshold
voltage for the formation of a nano-hole depends on the relative
humidity; however, the relationship between threshold voltage and
relative humidity is basically independent of the tip material.
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