4.3 Non-UPD Deposition of Ultrathin Metallic Layers
119
growth with larger thickness shows an example for the Stransky–Krastanov type
growth above the thickness of two monolayers.
The sequential build-up of structures comprising various atomic layers yield an
opportunity to tune the deposit structure at the atomic level. For instance, a submonolayer of Pd on Au(111) was found to impede the surface reconstruction, hence
providing an immense change in the deposition conditions [181]. The difference in
Ni deposition on reconstructed and unreconstructed Au(111) surface was attributed
to a larger extent to the difference in the atomic structure of the surface than to the
modified chemical environment caused by the Pd adatoms.
Although STM observations yield very spectacular images on both atomic
arrangement and distances of the deposit surface, STM is by far not the sole
method that can reveal the deposit structure at very small thickness. X-ray diffraction as applied in the grazing incidence mode (GIXRD) can also reveal the in-plane
atomic distances, although such investigations require a high-intensity source (like
synchrotron beamline) due to the small number of atoms detected. This method has
been applied from the early phase on the investigation of ultrathin electrodeposited
layers. GIXRD was successfully used to indicate the structural transformation of
Co deposited onto Cu as a function of the layer thickness [186]. As often seen for
substrate/deposit systems with a misfit, the pseudomorphic (epitaxial) near-substrate
layers are not reproduced after several layers, and a relaxed structure is obtained as
the deposit grows.
Ultrahigh vacuum methods like low-energy electron diffraction (LEED) can be
used ex situ. For example, an LEED study of electrodeposited Pd on Au(100) revealed
an expanded Pd deposit (as compared to the relaxed bulk Pd atomic distances) up
to at least 6 ML coverage, and the bulk lattice distances were achieved near the
coverage of 20 ML. Interestingly, the three-dimensional island growth started only
at 30 ML. The relatively large threshold thickness for the three-dimensional island
growth was attributed to the small lattice misfit (4.5%). The difference between the
atomic distances between the bulk Pd and thin Pd layers is to be taken into account
for the explanation of the modified catalytic activity (see examples of Sect. 4.2, too).
It is worthwhile of mentioning that the study of the dissolution of ultrathin layers
clearly revealed that the dissolution is by far not the reversal of the deposition in
the sense of the order of immobilization and removal of the atoms. Even in the case
when a layer-by-layer growth could be observed for several monolayers (like Ni on
Au), the dissolution induced etched pits that also served as dissolution sites, similar
to the already existing step edges [171]. The formation of pits was also found for the
dissolution process of Co [179] where the fragmentation of the initial biatomic layer
was even more pronounced.
4.3.3 Magnetization of Ultrathin Electrodeposited Layers
Bulk magnetism originates from the parallel spin orientation in an assembly of species
that are paramagnetic as stand-alone entities. Ferromagnetism in metallic elements
119
growth with larger thickness shows an example for the Stransky–Krastanov type
growth above the thickness of two monolayers.
The sequential build-up of structures comprising various atomic layers yield an
opportunity to tune the deposit structure at the atomic level. For instance, a submonolayer of Pd on Au(111) was found to impede the surface reconstruction, hence
providing an immense change in the deposition conditions [181]. The difference in
Ni deposition on reconstructed and unreconstructed Au(111) surface was attributed
to a larger extent to the difference in the atomic structure of the surface than to the
modified chemical environment caused by the Pd adatoms.
Although STM observations yield very spectacular images on both atomic
arrangement and distances of the deposit surface, STM is by far not the sole
method that can reveal the deposit structure at very small thickness. X-ray diffraction as applied in the grazing incidence mode (GIXRD) can also reveal the in-plane
atomic distances, although such investigations require a high-intensity source (like
synchrotron beamline) due to the small number of atoms detected. This method has
been applied from the early phase on the investigation of ultrathin electrodeposited
layers. GIXRD was successfully used to indicate the structural transformation of
Co deposited onto Cu as a function of the layer thickness [186]. As often seen for
substrate/deposit systems with a misfit, the pseudomorphic (epitaxial) near-substrate
layers are not reproduced after several layers, and a relaxed structure is obtained as
the deposit grows.
Ultrahigh vacuum methods like low-energy electron diffraction (LEED) can be
used ex situ. For example, an LEED study of electrodeposited Pd on Au(100) revealed
an expanded Pd deposit (as compared to the relaxed bulk Pd atomic distances) up
to at least 6 ML coverage, and the bulk lattice distances were achieved near the
coverage of 20 ML. Interestingly, the three-dimensional island growth started only
at 30 ML. The relatively large threshold thickness for the three-dimensional island
growth was attributed to the small lattice misfit (4.5%). The difference between the
atomic distances between the bulk Pd and thin Pd layers is to be taken into account
for the explanation of the modified catalytic activity (see examples of Sect. 4.2, too).
It is worthwhile of mentioning that the study of the dissolution of ultrathin layers
clearly revealed that the dissolution is by far not the reversal of the deposition in
the sense of the order of immobilization and removal of the atoms. Even in the case
when a layer-by-layer growth could be observed for several monolayers (like Ni on
Au), the dissolution induced etched pits that also served as dissolution sites, similar
to the already existing step edges [171]. The formation of pits was also found for the
dissolution process of Co [179] where the fragmentation of the initial biatomic layer
was even more pronounced.
4.3.3 Magnetization of Ultrathin Electrodeposited Layers
Bulk magnetism originates from the parallel spin orientation in an assembly of species
that are paramagnetic as stand-alone entities. Ferromagnetism in metallic elements