Chemistry Reference
In-Depth Information
700 nm
2
STM images of Nano-patterned 9ML Pb(111) films and the correspond-
ing hysteresis loops.
(a)
Film with 2ML tall mesas,
(b)
film with 2ML deep voids,
(c)
hysteresis
loop of film with mesas, and
(d)
hysteresis loop of film with voids. For both samples, the lateral
area is 3
Fig. 4.26
700
×
3mm
2
and the magnetic data are taken at 2 K. Reproduced from [
15
]
×
voids, are due to strong flux pinning inside these blind holes and directly related to
roboust superconducting current density,
J
C
, which reaches as high as 10% of the
theoretical depairing density. Also, this reference gives a simple model to calculate
J
C
based on pinning properties of the voids (Fig.
4.26
).
4.6.6 Surface Reactivity
QSE also plays an important role in the surface reactivity. Au is the most nobel
element, still it becomes reactive at the nanometer scale [
69
]. In recent years,
many experimental results have been published showing strong QSE modulations
on ultrathin metal films [
10
,
11
,
30
,
70
-
73
]. For example, Bartynski et al. reported
thickness-dependent oscillations in the CO desorption from Cu films [
11
]. These
oscillations were correlated with 2D subbands crossing the Fermi level periodically.
Similarly, oxidation rate of ultrathin Mg(0001) exhibits oscillations as the film thick-
ness is varied (Fig.
4.27
)[
10
]. Maximum oxidation rate is achieved when a new
quantum well state crosses the Fermi energy with increasing film thickness. These
strong oscillations are related to the oscillatory decay lengths of the quantum well
states, which affects the work function and electron transfer between the metal sub-
strate and the anti-bonding orbitals of the incoming O
2
. Hence, the dissociation bar-
rier and initial oxidation rate are ultimately controlled by QSE. Also, first-principles
calculations were supplied to verify the validity of this basic mechanism [
10
].
Recently, STM observations of an oxidized Pb mesa in the quantum regime
brought the most direct proof of the thickness modulated adsorption and reactivity
