4.1 Layer Preparation Methods Based Solely on UPD Processes
97
depositing metal clusters on a surface where UPD of these metals does not take place
and even at potentials slightly negative of their reversible potential value, no effective
control of the layer thickness and/or morphology is possible due to the occurrence
of the bulk deposition. This is the case, for instance, for the iron-group metals on
silver surface.
In the first step of the SEBALD process, a non-metallic monolayer is produced at
the surface, just like in any other EC-ALD procedure. In the studies establishing this
method, it was a sulphur monolayer on Ag(111), that can later be modified with metals
that are otherwise not possible to deposit onto silver surface with an atomic-level
control. The EC-ALD cycles then lead to the growth of a MeS deposit with controlled
thickness (Me: Cd, Fe, Co, Ni or an alloy of the iron-group metals). As a final step,
sulphur is stripped off with a fairly negative cathodic pulse, when the remaining
metal atoms are rearranged at the surface and form clusters in a size range that is
not accessible with a direct deposition. The size and the areal density of the metal
clusters left behind by the electrodesorption of sulphur are tunable by the thickness
of the EC-ALD layer of the MeS compound. This surface modification method may
gain important role in the elaboration of mixed catalysts [97] where the synergetic
effect of nanoscale metal clusters has an importance for various elementary steps of
reactions with a complicated mechanism.
As opposed to the SEBALD-type process where the deposition cycles were
followed by a single dealloying step at the end, the EC-ALD process can also be
completed with a cathodic stripping peak in each cycle. Here, the EC-ALD deposit
has the metal/chalcogenide/metal atomic layer sequence near the surface when a
cathodic stripping pulse is applied to remove the chalcogenide atomic layer partly
buried down beneath the topmost metal atom layer. Thereafter, the chalcogenide
deposition—metal deposition—chalcogenide removal steps can be repeated in the
further cycles. The method named by the authors as a “bait and switch” surfacearea-limited reaction [98] was suitable to produce a continuous film from germanium, a metal whose deposition is usually self-limited (see Sect. 4.3.1) by using the
Te content of EC-ALD GeTe deposits as a sacrificial component.
A great advantage of the EC-ALD process is that the deposition conditions of
the constituents can be tuned absolutely independently of each other, hence gaining
a large degree of freedom in the pursuit of the successive formation of complete
monolayers of alternating components on each other. However, the separation of the
deposition steps in time is not necessary if the following conditions are fulfilled: (i)
The goal is not to form a layer of as even thickness as possible, but some thickness
fluctuation is allowed (which, in turn, occurs also for EC-ALD layers, as it was shown
above, and does not necessarily impact the functionality negatively). (ii) The UPD
of either of the components onto the monolayer of the other components is possible
from a properly chosen solution, hence being able to apply one single bath. Such
processes have to be operated under potentiostatic control in order to provide the
right stoichiometry of the deposit. Although the process rate is rather limited due to
the low concentration of the solution, it may be acceptable for the deposition of a
nanoscale layer.
97
depositing metal clusters on a surface where UPD of these metals does not take place
and even at potentials slightly negative of their reversible potential value, no effective
control of the layer thickness and/or morphology is possible due to the occurrence
of the bulk deposition. This is the case, for instance, for the iron-group metals on
silver surface.
In the first step of the SEBALD process, a non-metallic monolayer is produced at
the surface, just like in any other EC-ALD procedure. In the studies establishing this
method, it was a sulphur monolayer on Ag(111), that can later be modified with metals
that are otherwise not possible to deposit onto silver surface with an atomic-level
control. The EC-ALD cycles then lead to the growth of a MeS deposit with controlled
thickness (Me: Cd, Fe, Co, Ni or an alloy of the iron-group metals). As a final step,
sulphur is stripped off with a fairly negative cathodic pulse, when the remaining
metal atoms are rearranged at the surface and form clusters in a size range that is
not accessible with a direct deposition. The size and the areal density of the metal
clusters left behind by the electrodesorption of sulphur are tunable by the thickness
of the EC-ALD layer of the MeS compound. This surface modification method may
gain important role in the elaboration of mixed catalysts [97] where the synergetic
effect of nanoscale metal clusters has an importance for various elementary steps of
reactions with a complicated mechanism.
As opposed to the SEBALD-type process where the deposition cycles were
followed by a single dealloying step at the end, the EC-ALD process can also be
completed with a cathodic stripping peak in each cycle. Here, the EC-ALD deposit
has the metal/chalcogenide/metal atomic layer sequence near the surface when a
cathodic stripping pulse is applied to remove the chalcogenide atomic layer partly
buried down beneath the topmost metal atom layer. Thereafter, the chalcogenide
deposition—metal deposition—chalcogenide removal steps can be repeated in the
further cycles. The method named by the authors as a “bait and switch” surfacearea-limited reaction [98] was suitable to produce a continuous film from germanium, a metal whose deposition is usually self-limited (see Sect. 4.3.1) by using the
Te content of EC-ALD GeTe deposits as a sacrificial component.
A great advantage of the EC-ALD process is that the deposition conditions of
the constituents can be tuned absolutely independently of each other, hence gaining
a large degree of freedom in the pursuit of the successive formation of complete
monolayers of alternating components on each other. However, the separation of the
deposition steps in time is not necessary if the following conditions are fulfilled: (i)
The goal is not to form a layer of as even thickness as possible, but some thickness
fluctuation is allowed (which, in turn, occurs also for EC-ALD layers, as it was shown
above, and does not necessarily impact the functionality negatively). (ii) The UPD
of either of the components onto the monolayer of the other components is possible
from a properly chosen solution, hence being able to apply one single bath. Such
processes have to be operated under potentiostatic control in order to provide the
right stoichiometry of the deposit. Although the process rate is rather limited due to
the low concentration of the solution, it may be acceptable for the deposition of a
nanoscale layer.