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For our first application, we apply our t matrix
formalism to study the conductance of 1,4 benzene-dithiolate, a
MW of recent experimental interest[3]. The molecule
consists of a benzene molecule with the hydrogen atoms at the 1
and 4 positions replaced with sulphur atoms. The sulphur atoms
act like alligator clips when they bond to the gold leads. We
calculate the conductance of the MW geometries shown in
Fig. 2. The gold leads are oriented in the
(111) direction. Attached to the molecule are gold clusters
which form the tips for the lead. Experimentally it has been
found that sulphur atoms preferentially bind over the hollow
sites formed on gold surfaces and so for our simulation the BDT
molecule is bonded over the hollow site on each tip.
Figure 2:
Atomistic diagram of gold (111) leads bonded to a BDT
molecule. (a) Case of strong bonding to both leads. (b) Case
of weak bonding to one of the leads.
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The strength of the bond between the molecule and the gold
surface plays an important role in determining the transmission
characteristics of the BDT MW. Isolated BDT has a discrete set
of MO's with the highest occupied molecular orbital (HOMO)
calculated to be around -10.5 eV and the lowest unoccupied
molecular orbital (LUMO) found to be around -8.2 eV. These
levels when bonded with the leads become part of the continuum
of energy states that exist within the metallic reservoirs.
For strong bonding they can become significantly altered as
their chemical nature becomes mixed with the surface states of
the gold tips. For weaker bonding the MO's retain the
character of the isolated molecule.
We consider strong binding to the (111) leads first. The
transmission diagram is shown in Fig. 3a. There is
strong transmission in the energy regions where the gold tip
states have mixed with the molecular states. This occurs most
prominently around -11.5 eV, where there exist resonances that
can be connected with the HOMO states of the molecule. The HOMO
and states around it in the isolated molecule have been mixed
and lowered in energy due to the bonding to the lead. The
other region of significant transmission is at around -8 eV,
which is due to states connected with the LUMO of the BDT. The
region in between has resonances that arise from those states
that are complex admixtures of gold tip and molecule levels.
The differential conductance was calculated with a Fermi level
chosen at -10 eV which lies in the HOMO-LUMO gap. The molecule
seems to be very conductive when attached strongly to the (111)
oriented wide leads.
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