Abstract
In this work the growth behavior of ionic insulators
on stepped and kinked metal surfaces was investigated. The substrate materials
are the metals copper and silver while the deposits are the alkali halides
sodium chloride and potassium chloride. As an inherent feature, stepped
and and kinked metal surfaces are characterized by a charge modulation due
to the Smoluchowski smoothing effect. A geometrical matching between the
ionic charges of the adlayer and the charge modulation of the template leads
to an enhanced interfacial stability. This energetic preference was exploited
(i) to grow smooth films on the original substrate, (ii) to create a one-dimensional
"hill-and-valley" facet structure, and (iii) to fabricate a two-dimensional
pyramidal facet structure. On stepped surfaces, alkali halide layers adopt
the preferred (100)-termination with the polar <110> in-plane directions
oriented parallel and perpendicular to the intrinsic steps of the underlying
template. Consequently, the growth mode is determined by the ratio between
the spacing of the intrinsic steps of the metal template and the lateral
distance of equivalent ions in the adlayer.
If this ratio is close to one (or two), smooth layer growth occurs as described
for the systems NaCl/Cu(311), KCl/Cu(311), and NaCl/Cu(221)
. These systems show that for low coverages (~1 ML), the electrostatic interactions
between adlayer and substrate are strong enough to overcome epitaxial strain
up to 6% in the polar in-plane direction perpendicular to the steps. For
higher coverages (>3 ML) the adlayer induces the formation of defect steps
in the metal template to compensate the strain.
If the substrate surface geometry does not favor smooth layer growth while
there is a preferred substrate geometry available close to the macroscopic
surface orientation, a one-dimensional faceting process can occur. This
was shown for the system KCl/Ag(211). In this case, the (311) facet
orientation which fulfills the criterion for layer growth, is tilted by
only 10° relative to the macroscopic surface. For deposition temperatures
of room temperature or higher sufficient substrate adatom mobility allows
for the required mass transport to achieve facets with a KCl overlayer in
the desired (311) orientation and bare Ag(111) facets to preserve the overall
macroscopic surface orientation.
The quasi one-dimensional faceting process found for stepped surfaces was
extended to kinked surfaces to realize a two-dimensional facet structure.
In detail, NaCl growth on the kinked surface Cu(532) was
studied. For this system, a pyramidal facet structure is obtained which
consists of three facet types: bare Cu(111), NaCl-covered (311), and NaCl-covered
(531) facets. To match the charge modulation of the (531) facet with the
ionic charges of the NaCl adlayer, regular defect steps are incorporated
into this facet. The chemical selectivity of this facet structure was verified
by adsorbing CO molecules on the Cu(111) facets only. No CO adsorption was
observed on the chemically inert NaCl-covered facets. Furthermore, by the
deposition of a metal (Ag) onto the initial facet structure a new surfactant
growth mode was found, where Ag diffuses into the interface between the NaCl
adlayer and the Cu template. Hence, the NaCl layer stabilizes the growth
of ultrathin Ag layers exhibiting an open structure.
|
