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Conductance of BDT

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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,width=0.5\textwidth,clip]{emberly.fig2.ps}

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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