458
12 Localized or Spatially Selective Electrodeposition Methods
nanodot deposition was a few tens of µs only. A reproducible defect initiation was
well detected in subsequent cycles; nevertheless, the amount of Ag deposit produced
was always the largest in the first cycle and then it gradually decreased with cycle
number because of an unknown reason.
Since the fully electrochemical processes did not prove to be reproducible at the
desired level, a half-electrochemical process was proposed by Kolb et al. [128] in
which the STM tip serves as cathode for the collection of metal but its relocation to
the substrate does not involve any charge transfer (see Fig. 12.8f). The Au substrate
was held at a potential where bulk Cu deposition is not possible, but the surface is
covered with a Cu UPD layer. This corresponds to about +10… + 30 mV potential as
compared to the reversible Cu
2+ /Cu potential in the same solution. After the collection
of the sufficient amount of Cu on the tip held in the bulk Cu deposition potential
regime, the electronics drives the tip towards the surface until reaching a “jumpto-contact” level in the tunnelling current, when the tip is withdrawn. However, the
contact is not broken where it is formed. Since the mobility of Cu is sufficient for a
rearrangement, the system tends to decrease the shape and curvature asymmetry, and
a neck is formed from the Cu atoms that originally belonged to the tip. Therefore, a
well-defined amount of Cu is “dropped” onto the substrate surface when the neck is
torn apart upon the withdrawal of the tip. Meanwhile, the tip potential did not need
any adjustment as a function of the distance from the surface.
Although the cluster left behind on the surface by the light contact with the Cucovered tip is by far thicker than the UPD layer already being present therein, the
clusters proved to be stable at the potential of the deposition, although the opportunity
for a long-term ex situ investigation was quite limited [129]. It required a few hundred
mV more positive potential to strip off the mechanically deposited clusters together
with the underlying UPD layer. This made it possible to manufacture very large
arrays of copper nanodots on Au (111) surface. The largest reported Cu nanodot
array covered a 1.1 × 1.1 µm
2 surface area [130]. Later, it was argued [131–133]
that the stability of the clusters may originate from alloying effects, especially when
various Au–Cu alloys were used as substrate. However, the alloying theory is in
contradiction to the observation that dissolution of the Cu clusters leave behind an
apparently perfect Au surface without any trace of reconstruction [122, 134]. The
controversy of the stability issue is yet to be resolved.
A detailed scrutiny of the tip-assisted transfer process revealed that the Cu cluster
height strongly depends on the distance of maximum approach to the substrate surface
[128, 135]. Surprisingly, a closer approach within a 1-nm distance led to a decrease
of the cluster height, whose maximum was about 0.8–0.9 nm at a 1-nm closest
tip approach. Concerning the substrate dependence of the process, it was revealed
that Ag(111) does not serve as a good receptive substrate for cluster deposition
but a monoatomic Cu layer is formed, although no UPD of Cu is possible on this
surface. Construction of Cu clusters on Ag(111) surface could be carried out with
sequential deposition cycles by holding the tip in identical lateral position [135].
The comparison of a few structures produced with the jump-to-contact method is
presented in Fig. 12.10.
12 Localized or Spatially Selective Electrodeposition Methods
nanodot deposition was a few tens of µs only. A reproducible defect initiation was
well detected in subsequent cycles; nevertheless, the amount of Ag deposit produced
was always the largest in the first cycle and then it gradually decreased with cycle
number because of an unknown reason.
Since the fully electrochemical processes did not prove to be reproducible at the
desired level, a half-electrochemical process was proposed by Kolb et al. [128] in
which the STM tip serves as cathode for the collection of metal but its relocation to
the substrate does not involve any charge transfer (see Fig. 12.8f). The Au substrate
was held at a potential where bulk Cu deposition is not possible, but the surface is
covered with a Cu UPD layer. This corresponds to about +10… + 30 mV potential as
compared to the reversible Cu
2+ /Cu potential in the same solution. After the collection
of the sufficient amount of Cu on the tip held in the bulk Cu deposition potential
regime, the electronics drives the tip towards the surface until reaching a “jumpto-contact” level in the tunnelling current, when the tip is withdrawn. However, the
contact is not broken where it is formed. Since the mobility of Cu is sufficient for a
rearrangement, the system tends to decrease the shape and curvature asymmetry, and
a neck is formed from the Cu atoms that originally belonged to the tip. Therefore, a
well-defined amount of Cu is “dropped” onto the substrate surface when the neck is
torn apart upon the withdrawal of the tip. Meanwhile, the tip potential did not need
any adjustment as a function of the distance from the surface.
Although the cluster left behind on the surface by the light contact with the Cucovered tip is by far thicker than the UPD layer already being present therein, the
clusters proved to be stable at the potential of the deposition, although the opportunity
for a long-term ex situ investigation was quite limited [129]. It required a few hundred
mV more positive potential to strip off the mechanically deposited clusters together
with the underlying UPD layer. This made it possible to manufacture very large
arrays of copper nanodots on Au (111) surface. The largest reported Cu nanodot
array covered a 1.1 × 1.1 µm
2 surface area [130]. Later, it was argued [131–133]
that the stability of the clusters may originate from alloying effects, especially when
various Au–Cu alloys were used as substrate. However, the alloying theory is in
contradiction to the observation that dissolution of the Cu clusters leave behind an
apparently perfect Au surface without any trace of reconstruction [122, 134]. The
controversy of the stability issue is yet to be resolved.
A detailed scrutiny of the tip-assisted transfer process revealed that the Cu cluster
height strongly depends on the distance of maximum approach to the substrate surface
[128, 135]. Surprisingly, a closer approach within a 1-nm distance led to a decrease
of the cluster height, whose maximum was about 0.8–0.9 nm at a 1-nm closest
tip approach. Concerning the substrate dependence of the process, it was revealed
that Ag(111) does not serve as a good receptive substrate for cluster deposition
but a monoatomic Cu layer is formed, although no UPD of Cu is possible on this
surface. Construction of Cu clusters on Ag(111) surface could be carried out with
sequential deposition cycles by holding the tip in identical lateral position [135].
The comparison of a few structures produced with the jump-to-contact method is
presented in Fig. 12.10.