Chemistry Reference
In-Depth Information
The ability to exploit quantum mechanical principles while tuning and assembling
nanostructures offers a whole new range of opportunities for exploring the physi-
cal and chemical properties of functional nanomaterials with precisely controlled
dimensions [
10
-
15
].
Atomically flat metal thin films on a proper substrate, such as Ag/GaAs(110),
Pb/Si(111), Pb/Ge(111), Al(110)/Si(111), In/Si(111), and Mg(0001)/Si(111) films,
have been the model systems for exploring electronically driven self-assembly, a
process dubbed “electronic growth” or “quantum growth” [
4
]. In these systems,
the interplay between the quantum size effect and the morphological evolution of
the films received considerable attention following the discovery of “magic thick-
nesses” or “critical thicknesses” and perferred island heights in ultrathin metal films
grown at moderately low tempertures. In particular, Pb is a soft metal and hence
electronic effects in the formation stages of the film could easily overwhelm lattice
strain contributions to the total free energy of the film [
16
]. An added consideration
is that the Fermi wavelength
λ
F
is nearly commensurate with the interatomic layer
spacing
d
along the
direction, which amplifies the role of quantum size effects
and Friedel oscillations on the growth mode of the Pb films.
Studies of the quantum size effect (QSE) in thin films date back to the 1970s.
Jaklevic and Lambe first observed the QSE by means of tunneling experiments in
metal-oxide-metal junctions [
17
]. Schulte carried out self-consistent studies of thin
films and showed that the work function oscillates with the thickness of the films
[
18
]. Such oscillations were also confirmed beyond the jellium model by Feibelman
[
19
], who also found oscillations in the surface energy.
In the 1990s, researchers found that atomically smooth metal films on semicon-
ductors are sometimes formed at low temperatures, under growth conditions that
seem to be far away from thermodynamic equilibrium [
3
]. However, because the
adatom diffusion still appears to be significant in this growth regime, thermody-
namic considerations can still be applied at least locally. This novel growth mode is
qualitatively different from the three well-established classical growth modes [
20
]
and appears to be related to quantum size effects that provide a non-trivial thickness-
dependent contribution to the free energy of the film, one that easily overwhelms the
strain contributions due to lattice mismatch. This has led to the surprising observa-
tion that smooth film growth in the quantum regime is possible only
above
a cer-
tain critical thickness [
8
,
21
,
22
]. Qualitatively, this newly defined critical thickness
is just the inverse of the transition at the critical thickness defined in the classic
Stranski-Krastanov (SK) growth mode [
23
].
The first striking example of the quantum growth mode of Ag on GaAs was
revealed in a two-step process [
3
], deposition at low temperature and subsequent
annealing to room temperature. At very low temperature, since the mobility is very
low, only tiny grains or “nanoclusters” are formed as opposed to smooth films or
large 3D clusters. Upon heating the sample to room temperature, the atoms acquired
sufficient mobility to self-organize into an atomically smooth film with a thickness
of six monolayers (ML). If the nominal deposition amount is slightly less, then the
film contains large holes exposing the bare GaAs substrate. Evidently, the “under-
dosed” Ag film phase separates into a 6ML thick Ag film and holes exposing the
111
