144
5 Compositionally Modulated and Multilayered Deposits
(see also Chap. 2.7). Since the current density for the deposition of an LN metal-rich
layer is typically high, the resulting current is sensitive to the reference electrode
position that determines the ratio of the interfacial potential drop at the cathode and
the Ohmic drop occurring in the solution. The current density is determined merely
by the interfacial potential drop. However, the accurate correction of the Ohmic drop
is seldom carried out in works related to preparative electrochemistry, which may
lead to a weak reproducibility if the conditions are not properly reported.
Concerning the deposition of the MN metal layer, the viewpoints for the choice
of the regulated parameter and its setup value are different. When the anodic partial
polarization curve of the LN metal is like the line 2b in Fig. 5.3 (i.e., the LN metal is
relatively inert), the deposition conditions for the MN metal can be selected in a wide
range of parameters, either in the G or the P mode. In this case, the dissolution of the
LN metal during the deposition of the MN one will not occur. The current density
can be set to any value lower than the diffusion-limited one; or, alternatively, the
electrode potential can chosen in the interval of the onset of the deposition potentials
of the two metals.
However, when the LN metal is not inert in the potential interval of the deposition
of the MN metal, the deposition conditions of the MN metal have to be chosen very
accurately so that the thickness of the LN metal can be designed with the help of
the charge passed in the corresponding deposition pulse. The current control in this
case is not an option, especially when the MN metal layer is to be obtained in a pure
form. If the current density is too small, the LN metal dissolves; when it is too large,
the LN metal is codeposited with the MN one. If potential control is applied, the
“neutral point” of the LN metal can be found where the partial polarization curve
of the LN metal crosses the abscissa. Here, the MN metal deposition is the only
reaction, regardless of the relative position of the polarization curves.
The first method to find the potential of no reaction of the LN metal was the
measurement of the equilibrium potential of the LN metal in the solution used for
multilayer deposition but without the ions of the MN metal [37]. While this method
proved to be satisfactory for a single-component LN metal, it cannot guarantee that
the equilibrium potential of the LN metal is the same as the rest potential of the
same metal in a partially depleted solution. The depletion always occur when the
deposition of the LN metal is just finished, and the rest potential may vary as the
mass transport leads to the alleviation of the metal ion concentrations. Therefore, a
more complex optimization method was suggested on the basis of the observation
of the potentiostatic current transient during the pulse for the MN metal deposition
[35]. The chronoamperometric records are compared for various potentials applied
in the pulse for the deposition of the MN metal (and meanwhile, the pulse parameters
for the deposition of the LN metal were constant). A demonstrative collection of the
chronoamperometric records is shown in Fig. 5.4. The potential optimum for the
deposition of the MN metal from the viewpoints of both the conservation of the
previously deposited LN metal and sharpness of the resulting interface is when the
anodic transient is as short as possible, and the diffusion-limited current density sets in
without observing higher cathodic current density values. The method of the analysis
of the chronoamperometric transients was originally applied for the Co/Cu [35] and
5 Compositionally Modulated and Multilayered Deposits
(see also Chap. 2.7). Since the current density for the deposition of an LN metal-rich
layer is typically high, the resulting current is sensitive to the reference electrode
position that determines the ratio of the interfacial potential drop at the cathode and
the Ohmic drop occurring in the solution. The current density is determined merely
by the interfacial potential drop. However, the accurate correction of the Ohmic drop
is seldom carried out in works related to preparative electrochemistry, which may
lead to a weak reproducibility if the conditions are not properly reported.
Concerning the deposition of the MN metal layer, the viewpoints for the choice
of the regulated parameter and its setup value are different. When the anodic partial
polarization curve of the LN metal is like the line 2b in Fig. 5.3 (i.e., the LN metal is
relatively inert), the deposition conditions for the MN metal can be selected in a wide
range of parameters, either in the G or the P mode. In this case, the dissolution of the
LN metal during the deposition of the MN one will not occur. The current density
can be set to any value lower than the diffusion-limited one; or, alternatively, the
electrode potential can chosen in the interval of the onset of the deposition potentials
of the two metals.
However, when the LN metal is not inert in the potential interval of the deposition
of the MN metal, the deposition conditions of the MN metal have to be chosen very
accurately so that the thickness of the LN metal can be designed with the help of
the charge passed in the corresponding deposition pulse. The current control in this
case is not an option, especially when the MN metal layer is to be obtained in a pure
form. If the current density is too small, the LN metal dissolves; when it is too large,
the LN metal is codeposited with the MN one. If potential control is applied, the
“neutral point” of the LN metal can be found where the partial polarization curve
of the LN metal crosses the abscissa. Here, the MN metal deposition is the only
reaction, regardless of the relative position of the polarization curves.
The first method to find the potential of no reaction of the LN metal was the
measurement of the equilibrium potential of the LN metal in the solution used for
multilayer deposition but without the ions of the MN metal [37]. While this method
proved to be satisfactory for a single-component LN metal, it cannot guarantee that
the equilibrium potential of the LN metal is the same as the rest potential of the
same metal in a partially depleted solution. The depletion always occur when the
deposition of the LN metal is just finished, and the rest potential may vary as the
mass transport leads to the alleviation of the metal ion concentrations. Therefore, a
more complex optimization method was suggested on the basis of the observation
of the potentiostatic current transient during the pulse for the MN metal deposition
[35]. The chronoamperometric records are compared for various potentials applied
in the pulse for the deposition of the MN metal (and meanwhile, the pulse parameters
for the deposition of the LN metal were constant). A demonstrative collection of the
chronoamperometric records is shown in Fig. 5.4. The potential optimum for the
deposition of the MN metal from the viewpoints of both the conservation of the
previously deposited LN metal and sharpness of the resulting interface is when the
anodic transient is as short as possible, and the diffusion-limited current density sets in
without observing higher cathodic current density values. The method of the analysis
of the chronoamperometric transients was originally applied for the Co/Cu [35] and