1. By: Adam Krause 4/17/07 Physics 672
Molecular Conductors
By: Adam Krause
4/17/07
Physics 672

2. Molecular Conductor Quick Intro.
Two types of molecules: carbon nanotubes and polyphenylene-based molecules
Multiple approaches to conductance experiments
Molecular conductors as applied to molecular electronics

3. Polyphenylene-based molecules
Figure adapted from: Ellenbogen, J. C. and J. C. Love, Proceedings of the IEEE, Vol. 88 No. 3, (2000) 386

4. Mechanically Controlled Break-Junction Method
Benzene-1,4-dithiolate
Piezo-element
Stretched until broken in a solution of Benzene-1,4-dithiolate
Figures from: Reed, M. A., et al., Science 278, (1997) 252

5. Mechanically Controlled Break-Junction Method
Benzene-1,4-dithiolate
I(V) and dI/dV curves
Three dI/dV curves showing the reproducibility of the conductance curve
One particular experiment that exhibited half of the average maximum resistance, suggesting there were two molecules bridging the gap
Figures from: Reed, M. A., et al., Science 278, (1997) 252

6. Crossed-Wire Method Oligo(phenylene ethynylene)
Purpose is to explore the gold-molecule junction.
One wire coated with SAM.
Wires pulled together by Lorentz Force while resistance is measured.
At high force the molecules distort and the resistance decreases, want to work in constant resistance region.
Figure from: Kushmerick, J. G., et al., Phys. Rev. Lett 89, (2002)

7. Crossed-Wire Method Oligo(phenylene ethynylene)
1.The open symbols are for positive voltage. Closed symbols for negative voltage.
2. The symmetrical molecule conducts the same in both positive and negative bias conditions.
3. The asymmetrical molecule conducts much more for a positive bias voltage than for a negative.
4. The conductive direction flows electrons more readily from the side missing the thioacetyl group toward the side with the thioacetyl group.
Figure from: Kushmerick, J. G., et al., Phys. Rev. Lett 89, (2002)

8. STM Break-Junction Method
Many Thiolated Molecules Used
Gold coated STM head held at a constant bias voltage.
The STM head strikes a gold surface covered in a solution of the molecule being studied.
The molecule can bond using the thiol group and bridge the gap.
As the tip is drawn backward the resistance data is measured.
Figure from: Xiao, X., et al., Nano. Lett. 4, (2004) 267

9. STM Break-Junction Method
Many Thiolated Molecules Used
Due to the type of setup in this experiment, it’s easy to continuously repeat it.
1000 runs at various bias voltages (13mV, 50mV, 120mV are shown in the histograms).
The current peaks in the histogram are plotted as a function of bias voltage. First three peaks shown.
When the second peak current is divided by 2 and the third peak current is divided by three, the data falls on top of the first peak data.
Figure from: Xu, B. Q.and N. J. Tao, Science 307, (2003) 1221

10. Conductance Summary
The various polyphenylene-based molecules shown here exhibit conductive properties.
These properties can change based on the electrode-molecule bond.
There are several approaches to measuring the conductance and current of a molecule
Now for a few applications.

11. Applications
Diode
1.) Polyphenylene molecule with an “electron donor” group (X), and “electron acceptor” group (Y), and a semi-insulating molecule (R) joining the two sides.
2.) Without bias voltage, the Lowest Unoccupied Molecular Orbital (LUMO) of the Donor half is higher than the LUMO of the Acceptor half.
3.) With bias voltage the Fermi energy is below the LUMO.
4.) With forward bias voltage (higher voltage on the left), the LUMO’s of both sides move closer to each other. Also the electrons in the right contact increase in energy past the level of the Acceptor LUMO.
5.) Reverse Bias
Figure from: Ellenbogen, J. C. and J. C. Love, Proceedings of the IEEE, Vol. 88 No. 3, (2000) 386

12. Applications Logic Gates
Figure from: Ellenbogen, J. C. and J. C. Love, Proceedings of the IEEE, Vol. 88 No. 3, (2000) 386

13. Applications
Memory
Figures from: Chen, J., et al., Ann. N.Y. Acad. Sci. 960, (2002) 69

14. Summary
The conductance of functionalized polyphenylene-based molecules can be tailored to behave in a desired manner.
In theory, diode molecules are possible.
These diodes can be used to build more complex molecular electronic devices.
The persistent conductance states of some molecules can be utilized for molecular memory applications.

15. References
Ellenbogen, J. C. and J. C. Love, Proceedings of the IEEE, Vol. 88 No. 3, (2000) 386
Reed, M. A., et al., Science 278, (1997) 252
Kushmerick, J. G., et al., Phys. Rev. Lett 89, (2002)
Xiao, X., et al., Nano. Lett. 4, (2004) 267
Xu, B. Q.and N. J. Tao, Science 307, (2003) 1221
Chen, J., et al., Ann. N.Y. Acad. Sci. 960, (2002) 69