110
4 Ultrathin Layers
A large family of nanocoatings where the small layer thickness occurs naturally is the group of so-called self-limited electrodeposit. The self-limiting growth
is common for electropolymerization if the resulting coating is insulating. A wellknown example is the oxidative electropolymerization of phenol in aqueous solution
that leads to a coating of about 20 nm limiting thickness. This reaction is often used
for sealing the side surface of STM tips to be used under an electrolyte solution in
order to avoid parasitic reactions of the solution component at the relatively large
surface of the side wall of the STM tip, leaving the end of the tip uncoated. Like any
other surface-area-limited process, self-limiting electropolymerization of phenol is
suitable to achieve conformal coatings on porous materials [156]. Self-limiting electropolymerization of other organic monomers can yield a similar limiting thickness
to that of poly(phenylene oxide) [157]. However, if the monomer to be polymerized is a metal complex, the limiting thickness achievable by electropolymerization can be tuned in a wider interval. This was exemplified in a study dealing with
electropolymerization of tris(5-amino-1,10-phenanthroline) metal complexes with
various central metal atoms, Fe and Ru [158], leading to limiting thicknesses of cca.
50 and 2 nm, respectively. A self-limiting growth process takes place during the electrochemical oxidation of thiourea in a two-step process [159], where the electrically
insulating nature of the sulphur layer is responsible for the stop of the layer growth,
similarly to the insulating polymers.
It has a great technological importance that platinum exhibits a self-limited deposition process under suitable conditions. The major goal here, similarly to SLRR
processes, is the efficient use of the expensive catalyst material, partly in combination
with the enhancement of the catalytic effect due to the deposit–substrate interaction
at the atomic level. The principle of the platinum self-limited electrodeposition is
as follows [160–162]: Platinum is deposited in a dilute solution of a platinum salt.
With an appropriate choice of the electrode potential, the discharge of the Pt(II) or
Pt(IV) species at the cathode surface is followed by the adsorption of hydrogen on
the Pt atoms. Hence, although the deposition of platinum itself is not a UPD process,
the follow-up UPD of the hydrogen atoms on the newly-formed Pt surface prevents
the further reduction of the platinum salts. If the potential is then switched to a sufficiently positive value where the surface of the working electrode is cleaned and the
adsorbed hydrogen atoms are oxidized, the surface becomes receptive again to the
deposition of a new Pt atomic layer. The voltammetric behaviour of the system and
the current response to a suitable potential pulse programme is presented in Fig. 4.12.
The analysis offered by the authors of the original study on the self-limitation of the
Pt deposition stressed that the origin of the self-limiting nature of the process is due to
the change in the double layer structure caused by the H adsorption on Pt atoms. The
H-terminated Pt surface at potentials more negative of the potential of zero charge
makes the proton reduction possible (protons access the negatively charged surface
easily), while anionic Pt complexes such as [PtCl 4 ]
2− are strongly repelled by the
Coulombic interaction, hence completely quenching the metal deposition process.
Although the self-limiting Pt deposition proved to be a linear function of the
cycle number in the two-pulse plating process, it turned out that the surface coverage
is also a function of the substrate. Au(111) surface allows one monolayer (ML)
4 Ultrathin Layers
A large family of nanocoatings where the small layer thickness occurs naturally is the group of so-called self-limited electrodeposit. The self-limiting growth
is common for electropolymerization if the resulting coating is insulating. A wellknown example is the oxidative electropolymerization of phenol in aqueous solution
that leads to a coating of about 20 nm limiting thickness. This reaction is often used
for sealing the side surface of STM tips to be used under an electrolyte solution in
order to avoid parasitic reactions of the solution component at the relatively large
surface of the side wall of the STM tip, leaving the end of the tip uncoated. Like any
other surface-area-limited process, self-limiting electropolymerization of phenol is
suitable to achieve conformal coatings on porous materials [156]. Self-limiting electropolymerization of other organic monomers can yield a similar limiting thickness
to that of poly(phenylene oxide) [157]. However, if the monomer to be polymerized is a metal complex, the limiting thickness achievable by electropolymerization can be tuned in a wider interval. This was exemplified in a study dealing with
electropolymerization of tris(5-amino-1,10-phenanthroline) metal complexes with
various central metal atoms, Fe and Ru [158], leading to limiting thicknesses of cca.
50 and 2 nm, respectively. A self-limiting growth process takes place during the electrochemical oxidation of thiourea in a two-step process [159], where the electrically
insulating nature of the sulphur layer is responsible for the stop of the layer growth,
similarly to the insulating polymers.
It has a great technological importance that platinum exhibits a self-limited deposition process under suitable conditions. The major goal here, similarly to SLRR
processes, is the efficient use of the expensive catalyst material, partly in combination
with the enhancement of the catalytic effect due to the deposit–substrate interaction
at the atomic level. The principle of the platinum self-limited electrodeposition is
as follows [160–162]: Platinum is deposited in a dilute solution of a platinum salt.
With an appropriate choice of the electrode potential, the discharge of the Pt(II) or
Pt(IV) species at the cathode surface is followed by the adsorption of hydrogen on
the Pt atoms. Hence, although the deposition of platinum itself is not a UPD process,
the follow-up UPD of the hydrogen atoms on the newly-formed Pt surface prevents
the further reduction of the platinum salts. If the potential is then switched to a sufficiently positive value where the surface of the working electrode is cleaned and the
adsorbed hydrogen atoms are oxidized, the surface becomes receptive again to the
deposition of a new Pt atomic layer. The voltammetric behaviour of the system and
the current response to a suitable potential pulse programme is presented in Fig. 4.12.
The analysis offered by the authors of the original study on the self-limitation of the
Pt deposition stressed that the origin of the self-limiting nature of the process is due to
the change in the double layer structure caused by the H adsorption on Pt atoms. The
H-terminated Pt surface at potentials more negative of the potential of zero charge
makes the proton reduction possible (protons access the negatively charged surface
easily), while anionic Pt complexes such as [PtCl 4 ]
2− are strongly repelled by the
Coulombic interaction, hence completely quenching the metal deposition process.
Although the self-limiting Pt deposition proved to be a linear function of the
cycle number in the two-pulse plating process, it turned out that the surface coverage
is also a function of the substrate. Au(111) surface allows one monolayer (ML)