428
12 Localized or Spatially Selective Electrodeposition Methods
evolution rate could not be measured in the thin-layer cell, the pH variation related
to a side process could not be evidenced.
Another cell configuration with a cylindrical arrangement and with a tip cathode
connected to the centre of the upper closing plate and with an anode ring was also
reported [10, 11]. In this setup, the cover plate was prepared from glass which made
possible the visual observation of the processes, while other parts of the cell were
essentially unchanged. By using a glass top cover, the adherence of the deposit to
the solid surface was not of interest. A non-negligible difference as compared to
the rectangular version is that the envelope around the end of the growing spikes is
expanding during the deposit formation, which naturally leads to the branching of the
filaments and a fractal-like overall appearance. It is the branching that provides the
nearly equal lateral density of the filaments along the radius. In many other aspects
of the growth, however, the observation was similar to the rectangular cells at the
microscopic scale (oscillation during the growth, formation of periodic corrugated
structures, Cu 2 O formation during the copper deposition).
After the early works describing the basic characteristics of the chilled thin-layer
cells, the technique was significantly improved, allowing a thoughtful design of
nanostructures. All further works discussed were prepared with rectangular cells.
Parallel nanowire arrays with homogeneous component distribution and adjustable
areal density but without either ramification or corrugation were reported [13]. The
technical background of the latter work is highly common with that of the studies
mentioned above. The only apparent difference is that the cathode surface was a
silicon wafer with a native oxide at the surface covered with a hydrophobic octadecyltrichlorosilane monolayer. Although a surprisingly good uniformity ranging to
millimetre length scale was reported, the reason leading to this improvement was
not clearly revealed. Another interesting improvement of the thin-layer technique
came from the discovery that the metal deposition is preferred at an artificial threephase boundary line [14]. The silicon substrate commonly used in the thin-layer
experiments was covered with poly(methyl-metacrylate) (PMMA) strips that were
perpendicular to the electrodes. The deposition in this case led to a copper wire formation at the concave corner of the solution–silicon–PMMA boundary line. The growth
preference at the three-phase boundary (triple junction) was so strong that it could
easily overwrite the impact of the electric field also in the cases when the PMMA
pattern was not perpendicular to the electrodes due the occurrence of micrometrescale hemispherical defects. The width of the longitudinally growing Cu wire was
as small as 50 nm.
The freezing technique used for the deposition of various Cu structures was
adapted for the deposition of zinc and zinc oxide [18]. The solution used contained
solely ZnSO 4 without any supporting electrolyte. Galvanostatic deposition led to
potential oscillation whose frequency increased with both the current density and the
pH. The deposit consisted of well-adherent needle structures on the cover glass plate
of the cell. The diameter of the needles was less than 2 µm. The growing Zn tips
exhibited a ramification-rich tree-like structure with regular 60
o corners between
the branches; therefore, the deposits were much less regular than that of the Cucontaining nanowires. The structural study indicated the presence of Zn and ZnO.
12 Localized or Spatially Selective Electrodeposition Methods
evolution rate could not be measured in the thin-layer cell, the pH variation related
to a side process could not be evidenced.
Another cell configuration with a cylindrical arrangement and with a tip cathode
connected to the centre of the upper closing plate and with an anode ring was also
reported [10, 11]. In this setup, the cover plate was prepared from glass which made
possible the visual observation of the processes, while other parts of the cell were
essentially unchanged. By using a glass top cover, the adherence of the deposit to
the solid surface was not of interest. A non-negligible difference as compared to
the rectangular version is that the envelope around the end of the growing spikes is
expanding during the deposit formation, which naturally leads to the branching of the
filaments and a fractal-like overall appearance. It is the branching that provides the
nearly equal lateral density of the filaments along the radius. In many other aspects
of the growth, however, the observation was similar to the rectangular cells at the
microscopic scale (oscillation during the growth, formation of periodic corrugated
structures, Cu 2 O formation during the copper deposition).
After the early works describing the basic characteristics of the chilled thin-layer
cells, the technique was significantly improved, allowing a thoughtful design of
nanostructures. All further works discussed were prepared with rectangular cells.
Parallel nanowire arrays with homogeneous component distribution and adjustable
areal density but without either ramification or corrugation were reported [13]. The
technical background of the latter work is highly common with that of the studies
mentioned above. The only apparent difference is that the cathode surface was a
silicon wafer with a native oxide at the surface covered with a hydrophobic octadecyltrichlorosilane monolayer. Although a surprisingly good uniformity ranging to
millimetre length scale was reported, the reason leading to this improvement was
not clearly revealed. Another interesting improvement of the thin-layer technique
came from the discovery that the metal deposition is preferred at an artificial threephase boundary line [14]. The silicon substrate commonly used in the thin-layer
experiments was covered with poly(methyl-metacrylate) (PMMA) strips that were
perpendicular to the electrodes. The deposition in this case led to a copper wire formation at the concave corner of the solution–silicon–PMMA boundary line. The growth
preference at the three-phase boundary (triple junction) was so strong that it could
easily overwrite the impact of the electric field also in the cases when the PMMA
pattern was not perpendicular to the electrodes due the occurrence of micrometrescale hemispherical defects. The width of the longitudinally growing Cu wire was
as small as 50 nm.
The freezing technique used for the deposition of various Cu structures was
adapted for the deposition of zinc and zinc oxide [18]. The solution used contained
solely ZnSO 4 without any supporting electrolyte. Galvanostatic deposition led to
potential oscillation whose frequency increased with both the current density and the
pH. The deposit consisted of well-adherent needle structures on the cover glass plate
of the cell. The diameter of the needles was less than 2 µm. The growing Zn tips
exhibited a ramification-rich tree-like structure with regular 60
o corners between
the branches; therefore, the deposits were much less regular than that of the Cucontaining nanowires. The structural study indicated the presence of Zn and ZnO.