12.2 Two-Plate Thin-Layer Cells for Electron Microscopic …
439
are seldom considered to be the core part of electrochemistry but are rather classified
as radiolysis, it seems to be important to deal also with such an emerging field among
other thin-layer cell techniques.
Concerning the chemical impact of the electron beam, one has to consider the
absorption characteristics of the high-energy electrons. As it is well known from
the EDS measurements, absorption characteristics of high-energy particles cannot
be simply described with formulas similar to the Lambert–Beer law; instead, characteristic collision cascades have to be considered and the mean depth of energy
absorbance has to be taken into account. As it has been recently shown [53], for
a liquid cell filled with water, the presence of interfaces between water and the
surrounding surfaces causes a spatial variation in the energy absorbed by the water
near the walls. The mechanism for this effect is that the walls act as a source of
secondary and backscattered electrons which diffuse and deposit energy in the water
nearby. This increased dose rate then changes the local concentrations of reactive
species. The dose rate can increase by several times within tens of nanometres of
a water/metal interface, locally increasing the concentrations of species such as
the hydrated electron. In accord with the electrode arrangement, such interfaceinfluenced species density can cause lateral and in-depth variations in the composition
of the liquid medium, which may modify the conditions of the reactions studied.
The discrimination of irradiation-induced deposit from electrodeposited crystals
proved to be rather easy for gold [54]. A clear bimodality was observed for the particle
size distribution. Larger particles were obtained on the carbon electrode used for
electrodeposition, while smaller particles appeared on all illuminated surfaces. Not
surprisingly, the morphology of the particles also differed. While electrodeposited
particles were nearly hemispherical (indicating a mixed diffusion-activation control
during the agglomeration process), the illumination-induced deposit showed an acicular morphology, which is characteristic of diffusion-limited aggregation. Since the
growth rate of the illumination-induced particles was smaller than that of the electrodeposit, it is clear that the diffusion limitation must come from the transport of a
dilute reactant that is produced by the electron beam.
Electron beams can also be used to assist the deposition which is purely electrochemical in nature. Due to the focusing of the electron beam, this offers a tool of
performing laterally selective electrodeposition; in other words, writing with the electron beam. This was shown for Cu deposition on Au [55]. When the electrochemical
behaviour of the nucleation zone was studied, it turned out that there is a pronounced
difference in the electrochemical nucleation rate in the activation-controlled potential regime in the absence and presence of chloride ions. In the potential interval
where chloride ions present in a few tens of ppm concentration, nucleation of Cu
on Au was hindered, which could be overcome by applying the illumination with
an electron beam. Hence, the electron illumination area could be covered selectively
without patterning the substrate.
When the only source of the reducing agent is the electron beam, the preferential deposition site is again the wall of the cell, whose quality has an impact on
the deposition process. This was exemplified for Pt deposition in an in situ liquid
SEM system [56]. Here, the focus of the electron beam is adjusted to the solution
439
are seldom considered to be the core part of electrochemistry but are rather classified
as radiolysis, it seems to be important to deal also with such an emerging field among
other thin-layer cell techniques.
Concerning the chemical impact of the electron beam, one has to consider the
absorption characteristics of the high-energy electrons. As it is well known from
the EDS measurements, absorption characteristics of high-energy particles cannot
be simply described with formulas similar to the Lambert–Beer law; instead, characteristic collision cascades have to be considered and the mean depth of energy
absorbance has to be taken into account. As it has been recently shown [53], for
a liquid cell filled with water, the presence of interfaces between water and the
surrounding surfaces causes a spatial variation in the energy absorbed by the water
near the walls. The mechanism for this effect is that the walls act as a source of
secondary and backscattered electrons which diffuse and deposit energy in the water
nearby. This increased dose rate then changes the local concentrations of reactive
species. The dose rate can increase by several times within tens of nanometres of
a water/metal interface, locally increasing the concentrations of species such as
the hydrated electron. In accord with the electrode arrangement, such interfaceinfluenced species density can cause lateral and in-depth variations in the composition
of the liquid medium, which may modify the conditions of the reactions studied.
The discrimination of irradiation-induced deposit from electrodeposited crystals
proved to be rather easy for gold [54]. A clear bimodality was observed for the particle
size distribution. Larger particles were obtained on the carbon electrode used for
electrodeposition, while smaller particles appeared on all illuminated surfaces. Not
surprisingly, the morphology of the particles also differed. While electrodeposited
particles were nearly hemispherical (indicating a mixed diffusion-activation control
during the agglomeration process), the illumination-induced deposit showed an acicular morphology, which is characteristic of diffusion-limited aggregation. Since the
growth rate of the illumination-induced particles was smaller than that of the electrodeposit, it is clear that the diffusion limitation must come from the transport of a
dilute reactant that is produced by the electron beam.
Electron beams can also be used to assist the deposition which is purely electrochemical in nature. Due to the focusing of the electron beam, this offers a tool of
performing laterally selective electrodeposition; in other words, writing with the electron beam. This was shown for Cu deposition on Au [55]. When the electrochemical
behaviour of the nucleation zone was studied, it turned out that there is a pronounced
difference in the electrochemical nucleation rate in the activation-controlled potential regime in the absence and presence of chloride ions. In the potential interval
where chloride ions present in a few tens of ppm concentration, nucleation of Cu
on Au was hindered, which could be overcome by applying the illumination with
an electron beam. Hence, the electron illumination area could be covered selectively
without patterning the substrate.
When the only source of the reducing agent is the electron beam, the preferential deposition site is again the wall of the cell, whose quality has an impact on
the deposition process. This was exemplified for Pt deposition in an in situ liquid
SEM system [56]. Here, the focus of the electron beam is adjusted to the solution