Nanoscale Quantum Physics Group
Research - Past Findings
Growth of C60 on Ge and Si surfaces
Growth of Ge and Si clusters on C60
The overall goal of this project is to develop C60 overlayers as a substrate on
which to fabricate nano structures of semiconductors or metals. The C60 layer has a band gap
close to 2eV, which will prevent the carrier in the nano structure from leaking out to the substrate. The C
60 molecule is also stable and relatively inert. Some recent findings resulting from our investigations of C
60 thin film formation are summarized below.
A new mechanism by which the ordering of molecules on anisotropic surfaces takes place
has been determined. Because the interaction potential between the C60 molecule and the surface
is highly anisotropic, it only defines the ordering of the adsorbate in one principal axis while the balance between
the repulsive and attractive forces among the molecules determines ordering along the other axis.
We have shown that from STM images it is possible to infer some of the events that result in
energy dissipation of the thermal molecules prior to being bound onto the surface. This realization came from the
observation that C60 molecules prefer to form a pair occupying certain sites on the surface, which
could only be explained by an inelastic collision and sticking process, not diffusive processes.
The microscopic origin that leads to the rotational epitaxy for multi-layer C60 on
Ge and Si surfaces has been determined. Such rotation occurs gradually throughout a number of layers, probably
to diffuse strain. This coherent rotational epitaxy has not be reported before.
A novel bi-step flow growth mode for epitaxial growth of large molecules on atomic substrates
has been observed in the C60/Ge and C60/GeAs systems. This occurs when the size
of adsorbate molecules is much larger than the monoatomic step height. This has broad implications for epitaxial
growth of organic molecules or patterning passivated quantum dots on atomic substrates, an emerging technique
for fabrication of hybrid devices.
Isolated Si and Ge clusters have been grown on ordered C60 overlayers on Si
and Ge substrates. Si and Ge clusters nucleate in the interstices of the C60 layer, and reach a
typical lateral size of 10 Å before coalescing. Nontetrahedral coordination of Si atoms in the clusters was
detected from Auger spectra results. Room temperature tunneling spectroscopy of isolated clusters reveals an
enlarged band gap consistent with the effect of quantum confinement, and a quantized conductance attributable
to the Coulomb blockade phenonmena in a tip/cluster/substrate double junction barrier. Our preliminary
experiments show that metallic dots can also be fabricated using this technique.
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