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12 Localized or Spatially Selective Electrodeposition Methods
12.2 Two-Plate Thin-Layer Cells for Electron Microscopic
Studies of Electrodeposition Processes
12.2.1 Electrochemical Cells in High-Vacuum Environment
Although the TEM observations in thin-layer cells do not make a part of the preparative works related to nanostructure formation, it was found that this emerging field
cannot be omitted when thin-layer techniques are discussed. Many details of this
technique are already available in various reviews and textbooks [24–27], and the
electrical engineering aspects of accurate electrochemical measurement under TEM
conditions have also been addressed [28]. Therefore, a few important aspects related
to electrodeposition and electrochemical nanofabrication will only be summarized
below.
One phase in electrochemical experiments is usually a liquid, which is not compatible to the conventional TEM environment where the high-vacuum condition is a
prerequisite for imaging with the electron beam. In situ electrochemical TEM studies
can be, however, performed with droplets of ionic liquids whose vapour pressure is
small enough not to disturb the imaging with the electron beam. The application of
ionic liquids is the only case when electrochemical TEM studies can be performed
without a well-sealed cell. Even with the application of ionic liquids, the sufficient
transparency for the electron beam has to be provided. This means from the practical
point of view that the total thickness of the layer that the electron beam has to pass
through should not exceed 2 micrometres, while a few hundred nanometre thickness
is rather ideal.
If an electrochemical cell is necessary, the key part of the cell is the window that
is thin enough for being transparent for the electron beam as well as strong enough
to endure the pressure difference between the internal and external parts of the cell.
This pressure difference can be the atmospheric pressure or higher when the cell
is not completely sealed, and this is the case when flow-cell conditions are applied
(the pumping system is always outside the TEM instrument). The window is made
exclusively from silicon nitride (Si 3 N 4 ). The lateral dimension of the Si 3 N 4 window
is below the millimetre scale. The in situ electrochemical cell is usually made from a
bottom and a top part, each having a Si 3 N 4 window facing with each other. The nitride
layer is manufactured on a silicon wafer with usual microelectronic techniques, and
then the rest of the silicon is etched away from the window area. The two parts of the
cell are pinned at the desired distance with the help of strips also prepared directly at
the silicon surface. Since the thickness of the cell is very small, manual adjustment
of a thin sealing layer is highly undesired.
From the electrochemical viewpoint, a problem of high importance is the arrangement of the electrodes. Due to the miniaturized nature of the cell, electrodes (metals,
mostly Au, Pt and Ti; or carbon) are pre-deposited onto the desired surface of the
cell wall (mostly bottom plate) during the manufacturing process. For creating the
external contacts, the metal deposition can reach to the outside of the solution-filled
volume of the cell. An elegant contact mode is to apply a polycrystalline silicon layer
12 Localized or Spatially Selective Electrodeposition Methods
12.2 Two-Plate Thin-Layer Cells for Electron Microscopic
Studies of Electrodeposition Processes
12.2.1 Electrochemical Cells in High-Vacuum Environment
Although the TEM observations in thin-layer cells do not make a part of the preparative works related to nanostructure formation, it was found that this emerging field
cannot be omitted when thin-layer techniques are discussed. Many details of this
technique are already available in various reviews and textbooks [24–27], and the
electrical engineering aspects of accurate electrochemical measurement under TEM
conditions have also been addressed [28]. Therefore, a few important aspects related
to electrodeposition and electrochemical nanofabrication will only be summarized
below.
One phase in electrochemical experiments is usually a liquid, which is not compatible to the conventional TEM environment where the high-vacuum condition is a
prerequisite for imaging with the electron beam. In situ electrochemical TEM studies
can be, however, performed with droplets of ionic liquids whose vapour pressure is
small enough not to disturb the imaging with the electron beam. The application of
ionic liquids is the only case when electrochemical TEM studies can be performed
without a well-sealed cell. Even with the application of ionic liquids, the sufficient
transparency for the electron beam has to be provided. This means from the practical
point of view that the total thickness of the layer that the electron beam has to pass
through should not exceed 2 micrometres, while a few hundred nanometre thickness
is rather ideal.
If an electrochemical cell is necessary, the key part of the cell is the window that
is thin enough for being transparent for the electron beam as well as strong enough
to endure the pressure difference between the internal and external parts of the cell.
This pressure difference can be the atmospheric pressure or higher when the cell
is not completely sealed, and this is the case when flow-cell conditions are applied
(the pumping system is always outside the TEM instrument). The window is made
exclusively from silicon nitride (Si 3 N 4 ). The lateral dimension of the Si 3 N 4 window
is below the millimetre scale. The in situ electrochemical cell is usually made from a
bottom and a top part, each having a Si 3 N 4 window facing with each other. The nitride
layer is manufactured on a silicon wafer with usual microelectronic techniques, and
then the rest of the silicon is etched away from the window area. The two parts of the
cell are pinned at the desired distance with the help of strips also prepared directly at
the silicon surface. Since the thickness of the cell is very small, manual adjustment
of a thin sealing layer is highly undesired.
From the electrochemical viewpoint, a problem of high importance is the arrangement of the electrodes. Due to the miniaturized nature of the cell, electrodes (metals,
mostly Au, Pt and Ti; or carbon) are pre-deposited onto the desired surface of the
cell wall (mostly bottom plate) during the manufacturing process. For creating the
external contacts, the metal deposition can reach to the outside of the solution-filled
volume of the cell. An elegant contact mode is to apply a polycrystalline silicon layer