456
12 Localized or Spatially Selective Electrodeposition Methods
or force-sensing distance for STM and AFM, respectively. The in situ imaging with
the same tip after the nanostructure fabrication is also customary.
The simplest application of ultrasmall conducting tips for electrochemical
nanopatterning is essentially analogous to the system described in Sect. 12.4.1 for
nanopillar fabrication, with the non-negligible difference that the electrode size is 3
to 4 orders of magnitude smaller (see Fig. 12.8a). In both cases, the metal ions to be
deposited are the solutes, and neither deposition nor dissolution takes place on the
tip itself which serves as anode. This method is often termed as field-induced deposition, referring to that the current can flow in the close proximity of the point-like
tip electrode only. In an early studies of this technique [119] 60 µs pulses of 4 V
were applied to plate Ag clusters from AgF solution onto HOPG surface, resulting
in nanodots of about 7 nm height and 25 nm width. The latter aspect ratio is typical
for nanotip-assisted deposition processes. Later, a nanoscopic electrochemical cell
was implemented by subsequent field-induced electrochemical deposition of Ag and
Cu nanodots serving as cathode and anode, respectively [120]. With the opposite
polarization of the tip and the substrate, the oxidation of the substrate surface can
be achieved locally under the tip (Fig. 12.8b). A demonstration of this technique
by using an AFM tip for the oxidation of thin Nb films showed [121] that not only
Nb 2 O 5 nanodots but surface patterns of essentially arbitrary shapes can be produced,
also with a complete in-depth oxidation of the Nb film.
There is also an opportunity for metal cluster formation on a substrate when the
field is yet small below the tip and the tip itself is not used for the generation of a
flush of the reactant. This intermediate case between the field-induced and collectorgenerator role of the tip is exemplified by the deposition of Cu clusters on Au when
the electrode potential of both the tip and the substrate is just slightly negative as
compared to the Cu
2+ /Cu equilibrium potential [122].
Nanotips can be used as a source of the material to be deposited onto a substrate
[123–126]. In this method, the tip, which is made of an inert material, is under
potential control. First, the negative polarization of the tip makes it possible to collect
a deposit from the solution. Then, the tip polarization is changed to a sufficiently
positive potential at which the dissolution of the previously collected metal takes
place. This dissolution step is often called “burst” in the literature. During the entire
collection–emission sequence on the tip, the substrate potential is invariant and just
slightly positive relative to the deposition potential of the same metal from the bulk
solution. However, at the large ion concentration generated locally by the burst, the
metal deposition becomes thermodynamically possible, which makes the deposition
process locally selective to the close vicinity of the tip. The process can be repeated
sequentially for patterning the surface with the dots of the deposit, and the constant
potential of the substrate stabilizes the previously deposited islands. One cycle of the
electrochemical process takes a few tens of milliseconds, the majority of which is the
collection time for the metal deposition onto the STM tip, and the typical length of
a burst is about 5 ms. Calculations show [124] that the metal ion concentration as a
result of the dissolution achieves a steady-state value within less than 200 µs, which
means that the metal ion release takes place under diffusion-controlled conditions.
Due to the shape of the diffusion field around the tip, the metal ions released on the side
12 Localized or Spatially Selective Electrodeposition Methods
or force-sensing distance for STM and AFM, respectively. The in situ imaging with
the same tip after the nanostructure fabrication is also customary.
The simplest application of ultrasmall conducting tips for electrochemical
nanopatterning is essentially analogous to the system described in Sect. 12.4.1 for
nanopillar fabrication, with the non-negligible difference that the electrode size is 3
to 4 orders of magnitude smaller (see Fig. 12.8a). In both cases, the metal ions to be
deposited are the solutes, and neither deposition nor dissolution takes place on the
tip itself which serves as anode. This method is often termed as field-induced deposition, referring to that the current can flow in the close proximity of the point-like
tip electrode only. In an early studies of this technique [119] 60 µs pulses of 4 V
were applied to plate Ag clusters from AgF solution onto HOPG surface, resulting
in nanodots of about 7 nm height and 25 nm width. The latter aspect ratio is typical
for nanotip-assisted deposition processes. Later, a nanoscopic electrochemical cell
was implemented by subsequent field-induced electrochemical deposition of Ag and
Cu nanodots serving as cathode and anode, respectively [120]. With the opposite
polarization of the tip and the substrate, the oxidation of the substrate surface can
be achieved locally under the tip (Fig. 12.8b). A demonstration of this technique
by using an AFM tip for the oxidation of thin Nb films showed [121] that not only
Nb 2 O 5 nanodots but surface patterns of essentially arbitrary shapes can be produced,
also with a complete in-depth oxidation of the Nb film.
There is also an opportunity for metal cluster formation on a substrate when the
field is yet small below the tip and the tip itself is not used for the generation of a
flush of the reactant. This intermediate case between the field-induced and collectorgenerator role of the tip is exemplified by the deposition of Cu clusters on Au when
the electrode potential of both the tip and the substrate is just slightly negative as
compared to the Cu
2+ /Cu equilibrium potential [122].
Nanotips can be used as a source of the material to be deposited onto a substrate
[123–126]. In this method, the tip, which is made of an inert material, is under
potential control. First, the negative polarization of the tip makes it possible to collect
a deposit from the solution. Then, the tip polarization is changed to a sufficiently
positive potential at which the dissolution of the previously collected metal takes
place. This dissolution step is often called “burst” in the literature. During the entire
collection–emission sequence on the tip, the substrate potential is invariant and just
slightly positive relative to the deposition potential of the same metal from the bulk
solution. However, at the large ion concentration generated locally by the burst, the
metal deposition becomes thermodynamically possible, which makes the deposition
process locally selective to the close vicinity of the tip. The process can be repeated
sequentially for patterning the surface with the dots of the deposit, and the constant
potential of the substrate stabilizes the previously deposited islands. One cycle of the
electrochemical process takes a few tens of milliseconds, the majority of which is the
collection time for the metal deposition onto the STM tip, and the typical length of
a burst is about 5 ms. Calculations show [124] that the metal ion concentration as a
result of the dissolution achieves a steady-state value within less than 200 µs, which
means that the metal ion release takes place under diffusion-controlled conditions.
Due to the shape of the diffusion field around the tip, the metal ions released on the side