460
12 Localized or Spatially Selective Electrodeposition Methods
Interestingly, the Fe cluster height was about 2-3 monolayers, similarly to other
metals, but the cluster diameter was rather high, around 120 nm.
When the STM tip near the substrate surface is used to etch the surface, a large
variety of nanoscale surface damage or nanocavity can be initiated, as shown in the
scheme of Fig. 12.8c. Relevant examples are the cavity formation on gold [137–
140], copper [141] or H-terminated Si [142]. While a relatively large voltage is
necessary to induce the nanocavity formation on gold, in the rest of the examples the
substrate voltage is in the range where no dissolution is expected on a flat surface
without another electrode in the close vicinity. Therefore, it was assumed that the
overlap of either the electrical double layers or the other Helmholtz planes plays an
important role in the dissolution/oxidation of the substrate [141]. The direct electron
tunnelling between the substrate and the tip was also assumed, hence leading to
localized substrate dissolution. It was also a common experience that the tip-induced
nanocavity was a preferred site for the nucleation of nanodots formed from a foreign
metal as compared to the rest of the substrate surface [138, 140], which was explained
with both structural and electronic effects.
Another interesting application of the scanning probe tips is the so-called electrochemical nanolithography. In this method, the tip is scanning the surface that is
covered with a layer, made from either an organic monomer [143] or a metal [144].
The tip potential is negative for both the oxidative polymerization of the monomer
coating and during the anodic etching of the substrate. In both cases, the substrate
potential is in the range where the oxidation process would also take place on a
macroscopic electrode, too, and the localization of the process at a particular part of
the substrate is highly due to the vicinity of the point-like anode tip.
Finally, it has to be noted that tips are suitable to initiate electrochemical polymerization of various monomer coatings. Such a reaction may work with an STM tip
[145] or with an AFM tip with conductive coatings [146]. In neither of these cases
was an electrolyte solution used, although the monomer film contacting the tip might
form a liquid meniscus between the tip and the substrate surface. The cell voltage
of a few volts with polarization direction corresponding to the monomer properties
was applied to polymerize the film just under the tip, hence forming a conducting
covalent network attached to the surface. The feature size in this method depends
primarily on the tip size. Manipulation of a self-assembled monolayer (SAM) is also
possible with an STP tip under dry conditions [147]. In the latter study, an Ag-coated
STM tip was used. The first step of the process was the removal of the SAM by
the anodic and cathodic polarization of the substrate and the tip, respectively. Then,
by reversing the polarization conditions, Ag was directly transferred from the tip to
the bare part of the substrate where SAM was previously removed. These examples
clearly show that electrochemical reactions by using tips can be carried out under
circumstances that are not convenient in electrochemistry, although the processes
taking place are electrochemical in nature.
12 Localized or Spatially Selective Electrodeposition Methods
Interestingly, the Fe cluster height was about 2-3 monolayers, similarly to other
metals, but the cluster diameter was rather high, around 120 nm.
When the STM tip near the substrate surface is used to etch the surface, a large
variety of nanoscale surface damage or nanocavity can be initiated, as shown in the
scheme of Fig. 12.8c. Relevant examples are the cavity formation on gold [137–
140], copper [141] or H-terminated Si [142]. While a relatively large voltage is
necessary to induce the nanocavity formation on gold, in the rest of the examples the
substrate voltage is in the range where no dissolution is expected on a flat surface
without another electrode in the close vicinity. Therefore, it was assumed that the
overlap of either the electrical double layers or the other Helmholtz planes plays an
important role in the dissolution/oxidation of the substrate [141]. The direct electron
tunnelling between the substrate and the tip was also assumed, hence leading to
localized substrate dissolution. It was also a common experience that the tip-induced
nanocavity was a preferred site for the nucleation of nanodots formed from a foreign
metal as compared to the rest of the substrate surface [138, 140], which was explained
with both structural and electronic effects.
Another interesting application of the scanning probe tips is the so-called electrochemical nanolithography. In this method, the tip is scanning the surface that is
covered with a layer, made from either an organic monomer [143] or a metal [144].
The tip potential is negative for both the oxidative polymerization of the monomer
coating and during the anodic etching of the substrate. In both cases, the substrate
potential is in the range where the oxidation process would also take place on a
macroscopic electrode, too, and the localization of the process at a particular part of
the substrate is highly due to the vicinity of the point-like anode tip.
Finally, it has to be noted that tips are suitable to initiate electrochemical polymerization of various monomer coatings. Such a reaction may work with an STM tip
[145] or with an AFM tip with conductive coatings [146]. In neither of these cases
was an electrolyte solution used, although the monomer film contacting the tip might
form a liquid meniscus between the tip and the substrate surface. The cell voltage
of a few volts with polarization direction corresponding to the monomer properties
was applied to polymerize the film just under the tip, hence forming a conducting
covalent network attached to the surface. The feature size in this method depends
primarily on the tip size. Manipulation of a self-assembled monolayer (SAM) is also
possible with an STP tip under dry conditions [147]. In the latter study, an Ag-coated
STM tip was used. The first step of the process was the removal of the SAM by
the anodic and cathodic polarization of the substrate and the tip, respectively. Then,
by reversing the polarization conditions, Ag was directly transferred from the tip to
the bare part of the substrate where SAM was previously removed. These examples
clearly show that electrochemical reactions by using tips can be carried out under
circumstances that are not convenient in electrochemistry, although the processes
taking place are electrochemical in nature.