4.3 Non-UPD Deposition of Ultrathin Metallic Layers
113
continuous layers can be found in the works of Guo and Searson [166–168]. Here,
we will deal with the systems only whose behaviour was described the best at the
atomic scale. Detailed overviews of the topic were offered by Allongue and Maroun
[169, 170].
In many aspects, the appropriate measurement conditions for the in situ observation of the early stage of electrodeposition are similar to UPD-based processes (e.g.,
small cell volume, concentration of the electroactive species in the mM concentration range, ultrapure conditions with the removal of oxygen with an inert gas
purging, application of a precisely-cut single crystal with well-defined surface orientation as substrate). The small concentration is necessary so that a sufficiently small
nucleation density and a slow enough deposition process are provided, hence the
changes caused by the deposition process can be easily followed with the available
time resolution of the measurement. The current density is often of the μA cm
−2
level and the corresponding growth rate is in the monolayer/minute range or even
less. The amount of deposit depends on the time of potentiostatic polarization in a
nonlinear manner, typically showing a decrease with deposition time. Although the
data obtained for dilute solutions are not transferable for fairly concentrated solutions used in conventional plating systems, the observations yield a valid guidance
for the phenomena to be considered under different circumstances. The need for the
potentiostatic control of the experiment is evident. The above mentioned conditions
are valid for the in situ magnetization studies and will not be mentioned again. Many
studies apply in situ observations of the atomic structure of the deposits and magnetic
properties in parallel. The general aspects of the atomic structure will be dealt with
in this chapter; however, where the observation of the structure at the atomic scale is
indispensable for the elucidation of the magnetization behaviour, the structural data
will be mentioned therein.
Concerning the observation method of the atomic arrangement, one possibility
is the application of the scanning tunnelling microscopy (STM). In this case, the
electrode potential of the tip and of the substrate must be controlled independently,
hence the bipotentiostatic control of the experimental setup is needed. The side of
the STM tip is insulated for reducing the parasitic current as much as possible.
Most of the observations of the atomic structure of the non-UPD type deposits
refer to Au(111) or Cu(111) as substrate and Ni, Co and Fe as host metal. The general
experience is that the growth of the host metal starts at the kink positions or at the
step edges of the substrate. This nucleation is followed by the growth of an epitaxial
atomic layer. The thickness where a full coverage of the substrate can be achieved is
the higher, the larger the overpotential of the deposition. Representative examples for
the growth of subsequent atomic layers as observed by STM are shown in Fig. 4.13.
For the most commonly used Au(111) substrate and the growth of nickel, the small
overpotential of the deposition leads to a place exchange of the host and substrate
metal atoms, while an exchange-free nucleation at the step edges takes place at higher
overpotentials [171]. This behaviour was not observed for other metals deposited;
however, Ni deposited on Ag(111) showed a very similar character [173]. It was
shown also for the Au(111)/Ni system that a monoatomic coverage can be achieved
without the nucleation of the second atomic layer. In the latter case, if the growth rate
113
continuous layers can be found in the works of Guo and Searson [166–168]. Here,
we will deal with the systems only whose behaviour was described the best at the
atomic scale. Detailed overviews of the topic were offered by Allongue and Maroun
[169, 170].
In many aspects, the appropriate measurement conditions for the in situ observation of the early stage of electrodeposition are similar to UPD-based processes (e.g.,
small cell volume, concentration of the electroactive species in the mM concentration range, ultrapure conditions with the removal of oxygen with an inert gas
purging, application of a precisely-cut single crystal with well-defined surface orientation as substrate). The small concentration is necessary so that a sufficiently small
nucleation density and a slow enough deposition process are provided, hence the
changes caused by the deposition process can be easily followed with the available
time resolution of the measurement. The current density is often of the μA cm
−2
level and the corresponding growth rate is in the monolayer/minute range or even
less. The amount of deposit depends on the time of potentiostatic polarization in a
nonlinear manner, typically showing a decrease with deposition time. Although the
data obtained for dilute solutions are not transferable for fairly concentrated solutions used in conventional plating systems, the observations yield a valid guidance
for the phenomena to be considered under different circumstances. The need for the
potentiostatic control of the experiment is evident. The above mentioned conditions
are valid for the in situ magnetization studies and will not be mentioned again. Many
studies apply in situ observations of the atomic structure of the deposits and magnetic
properties in parallel. The general aspects of the atomic structure will be dealt with
in this chapter; however, where the observation of the structure at the atomic scale is
indispensable for the elucidation of the magnetization behaviour, the structural data
will be mentioned therein.
Concerning the observation method of the atomic arrangement, one possibility
is the application of the scanning tunnelling microscopy (STM). In this case, the
electrode potential of the tip and of the substrate must be controlled independently,
hence the bipotentiostatic control of the experimental setup is needed. The side of
the STM tip is insulated for reducing the parasitic current as much as possible.
Most of the observations of the atomic structure of the non-UPD type deposits
refer to Au(111) or Cu(111) as substrate and Ni, Co and Fe as host metal. The general
experience is that the growth of the host metal starts at the kink positions or at the
step edges of the substrate. This nucleation is followed by the growth of an epitaxial
atomic layer. The thickness where a full coverage of the substrate can be achieved is
the higher, the larger the overpotential of the deposition. Representative examples for
the growth of subsequent atomic layers as observed by STM are shown in Fig. 4.13.
For the most commonly used Au(111) substrate and the growth of nickel, the small
overpotential of the deposition leads to a place exchange of the host and substrate
metal atoms, while an exchange-free nucleation at the step edges takes place at higher
overpotentials [171]. This behaviour was not observed for other metals deposited;
however, Ni deposited on Ag(111) showed a very similar character [173]. It was
shown also for the Au(111)/Ni system that a monoatomic coverage can be achieved
without the nucleation of the second atomic layer. In the latter case, if the growth rate