Chapter 4
Ultrathin Layers
4.1 Layer Preparation Methods Based Solely on UPD
Processes
4.1.1 Atomic Layer Deposition Processes
Atomic layer deposition (ALD) is a method originally developed for coating surfaces
with layers that are produced in a reaction of surface-bound species with some
gaseous molecules. The principle of the method is that functional groups that can
react with each other belong to different phases; i.e., to either the surface layer or to
the molecules in the gas phase. Therefore, the reaction between them cannot proceed
until the full consumption of the gaseous reactant but is limited to the number of
active surface sites.
An example for atomic layer deposition processes is the formation of SiO 2 layers
on an activated surface. Assume that the surface of a solid sample is already terminated with silicon-containing molecular fragments that are hydrated; therefore, the
surface is dominantly covered with ≡Si–OH groups (the ≡ sign indicates that the
other three covalent bonds of the Si atom belong to atoms below the topmost atomic
layer). If SiCl 4 molecules arrive at the surface, they react with the surface-bound
entities. The reaction leads to the elimination of HCl, and another Si-containing
atomic layer is attached to the surface. Depending on the new bonds formed, the
freshly chemisorbed species will be the terminal part of a (≡Si–O–) x SiCl 4−x surface
site, where 1 ≤ x ≤ 3. If then the surface site can react with water molecules, the
chlorine atoms are exchanged to –OH groups again. The latter process is accompanied by HCl elimination, similarly to the previous reaction. With the formation
of the –OH groups at the surface, the starting state is reproduced. With the repetition of the above described cycle, the number of atomic layers can be controlled
by the number of the cycle repetition. Similarly to the formation of the ALD SiO 2
layers, various covalent and ionic metal oxide layers can be produced with the reaction of metal halogenides and water vapour, among which Al 2 O 3 and TiO 2 have the
© Springer Nature Switzerland AG 2021
L. Péter, Electrochemical Methods of Nanostructure Preparation,
Monographs in Electrochemistry, https://doi.org/10.1007/978-3-030-69117-2_4
79
Ultrathin Layers
4.1 Layer Preparation Methods Based Solely on UPD
Processes
4.1.1 Atomic Layer Deposition Processes
Atomic layer deposition (ALD) is a method originally developed for coating surfaces
with layers that are produced in a reaction of surface-bound species with some
gaseous molecules. The principle of the method is that functional groups that can
react with each other belong to different phases; i.e., to either the surface layer or to
the molecules in the gas phase. Therefore, the reaction between them cannot proceed
until the full consumption of the gaseous reactant but is limited to the number of
active surface sites.
An example for atomic layer deposition processes is the formation of SiO 2 layers
on an activated surface. Assume that the surface of a solid sample is already terminated with silicon-containing molecular fragments that are hydrated; therefore, the
surface is dominantly covered with ≡Si–OH groups (the ≡ sign indicates that the
other three covalent bonds of the Si atom belong to atoms below the topmost atomic
layer). If SiCl 4 molecules arrive at the surface, they react with the surface-bound
entities. The reaction leads to the elimination of HCl, and another Si-containing
atomic layer is attached to the surface. Depending on the new bonds formed, the
freshly chemisorbed species will be the terminal part of a (≡Si–O–) x SiCl 4−x surface
site, where 1 ≤ x ≤ 3. If then the surface site can react with water molecules, the
chlorine atoms are exchanged to –OH groups again. The latter process is accompanied by HCl elimination, similarly to the previous reaction. With the formation
of the –OH groups at the surface, the starting state is reproduced. With the repetition of the above described cycle, the number of atomic layers can be controlled
by the number of the cycle repetition. Similarly to the formation of the ALD SiO 2
layers, various covalent and ionic metal oxide layers can be produced with the reaction of metal halogenides and water vapour, among which Al 2 O 3 and TiO 2 have the
© Springer Nature Switzerland AG 2021
L. Péter, Electrochemical Methods of Nanostructure Preparation,
Monographs in Electrochemistry, https://doi.org/10.1007/978-3-030-69117-2_4
79