98
4 Ultrathin Layers
The above deposition mode can be called accumulative underpotential deposition. It was shown to be feasible for the deposition of, e.g., stoichiometric PbS from
one single solution, leading to crystals of micrometre-range lateral dimensions [99].
Another attempt to deposit PbS in a single step was based on a hot saturated solution
of in situ formed PbS particles [100]. In the latter study, the deposition took place in
a UPD manner from the small-concentration solution from which the whole particles
themselves did not incorporate into the deposit but served as the source of the precursors by saturating the solution via their dissolution. The accumulative underpotential
deposition from a single solution is also feasible if the solution concentrations, like
in conventional baths for alloy deposition, are not limited by the low solubility of
the precursor(s). This method was employed for a variety of binary (CdTe [101],
CuTe [102] and PbTe [103]) and ternary deposits (Bi 2 Te 3−y Se y [26], Bi 1−x Sb x and
Bi 2−x Sb x Te 3 [79]).
4.2 Combination of UPD with Other Surface-Area-Limited
Processes
4.2.1 Surface-Limited Redox Replacement Processes
It is commonly known that once a relatively non-noble metal (like iron) is immersed
into the solution of the ions of a more noble metal, an exchange (or displacement)
reaction takes place between these metals, such as Fe + Cu
2+
Fe
2+
+ Cu in the
above example. Such processes are also called cementation in the technical literature.
The cementation process takes place spontaneously in the presence of the suitable
reactants and it does not require an external current source. If the metal deposited
cannot cover conformally the one that is displaced, the process does not stop until
one of the reactants is fully consumed and is accompanied with the occurrence of
porosity in both the displaced and deposited metals.
Surface-limited redox replacement (SLRR) is a similar process to cementation
concerning the driving force of the process. The limitation comes from the fact that
the component to be displaced (i.e., the sacrificial component) covers the substrate
surface in a thin layer which is often restricted to a UPD layer. Hence, the displacement reaction has to consume an at most monoatomic layer. Therefore, the accomplishment of the displacement process results in a similarly thin layer of the displacing
metal on surface average; i.e., the atomic distribution of the displacing metal is not
certainly as surface conformal as that of the displaced layer. The diversified aspects of
SLRR processes and the deposits thus produced were recently reviewed by Dimitrov
[104].
The deposition of a single atomic layer by SLRR can be carried out with subsequent immersion into different solutions coupled with a suitable electrode polarization, as it was described in the papers on the elaboration of the SLRR technique [105,
106]. This approach requires a thorough control of the experiments by maintaining
4 Ultrathin Layers
The above deposition mode can be called accumulative underpotential deposition. It was shown to be feasible for the deposition of, e.g., stoichiometric PbS from
one single solution, leading to crystals of micrometre-range lateral dimensions [99].
Another attempt to deposit PbS in a single step was based on a hot saturated solution
of in situ formed PbS particles [100]. In the latter study, the deposition took place in
a UPD manner from the small-concentration solution from which the whole particles
themselves did not incorporate into the deposit but served as the source of the precursors by saturating the solution via their dissolution. The accumulative underpotential
deposition from a single solution is also feasible if the solution concentrations, like
in conventional baths for alloy deposition, are not limited by the low solubility of
the precursor(s). This method was employed for a variety of binary (CdTe [101],
CuTe [102] and PbTe [103]) and ternary deposits (Bi 2 Te 3−y Se y [26], Bi 1−x Sb x and
Bi 2−x Sb x Te 3 [79]).
4.2 Combination of UPD with Other Surface-Area-Limited
Processes
4.2.1 Surface-Limited Redox Replacement Processes
It is commonly known that once a relatively non-noble metal (like iron) is immersed
into the solution of the ions of a more noble metal, an exchange (or displacement)
reaction takes place between these metals, such as Fe + Cu
2+
Fe
2+
+ Cu in the
above example. Such processes are also called cementation in the technical literature.
The cementation process takes place spontaneously in the presence of the suitable
reactants and it does not require an external current source. If the metal deposited
cannot cover conformally the one that is displaced, the process does not stop until
one of the reactants is fully consumed and is accompanied with the occurrence of
porosity in both the displaced and deposited metals.
Surface-limited redox replacement (SLRR) is a similar process to cementation
concerning the driving force of the process. The limitation comes from the fact that
the component to be displaced (i.e., the sacrificial component) covers the substrate
surface in a thin layer which is often restricted to a UPD layer. Hence, the displacement reaction has to consume an at most monoatomic layer. Therefore, the accomplishment of the displacement process results in a similarly thin layer of the displacing
metal on surface average; i.e., the atomic distribution of the displacing metal is not
certainly as surface conformal as that of the displaced layer. The diversified aspects of
SLRR processes and the deposits thus produced were recently reviewed by Dimitrov
[104].
The deposition of a single atomic layer by SLRR can be carried out with subsequent immersion into different solutions coupled with a suitable electrode polarization, as it was described in the papers on the elaboration of the SLRR technique [105,
106]. This approach requires a thorough control of the experiments by maintaining