Chemistry Reference
In-Depth Information
Fig. 4.27
Lower panel
: plots of the Mg 2p oxide component in X-ray photoemission spectroscopy
(XPS) as a function of film thickness obtained in several experimental runs. Data with the same
symbol correspond to the same O
2
exposure.
Upper panel
: photoemission intensity at the Fermi
level. The observed tuning of the oxidation rate is directly correlated with the oscillations in the
density of states at the Fermi level, due to the QSE. The oscillatory reactivity has been attributed
to oscillatory dissociation barriers [
10
]
[
30
]. Fig.
4.28
a shows the STM image of a flat-top Pb mesa grown on Si(111) 7
7
surface which accomodates Pb thicknesses between 10 and 16ML due to stepped
substrate structure (600
×
600 nm
2
image). Panel (b) shows the cros section of this
mesa. Panel (d) shows the STM image of the thickness-dependent contrast devel-
oped after 60 L oxygen exposure at 100 K substrate temperature. From 9 to 12ML
thick regions are labeled on the image. Bright spots are the chemisorbed oxygen
clusters with a typical size of 1.2 nm. Although the crystal orientation is the same
Pb(111) for all the thicknesses, oxygen adsorption on even layers is clearly larger
than that on the odd layers. This behavior is consistent with the aforementioned
quantum electronic struture and local work function of the mesa [
51
]. Oxidation
occurs at room temperature with additional oxygen exposure of 120 L. This process
again turns out to be thickness dependent and shown in panel (e). Panel (f) is a close
up to the PbO nanocrystals.
So, both adsorption and oxidation are strongly enhanced when the Pb thickness
is an even number of atomic layers. Quantitative analyses of the adsoption coverage
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