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

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

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

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

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

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) 086802

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) 086802

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

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

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.

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

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

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

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.

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) 086802 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