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Conjugated molecular systems containing a high degree of π-conjugation have attracted a great deal of interest because of their novel and significantly improved physical, chemical, and optical properties and phenomena that are unique to this size scale and may not be as predictable as those observed at larger scales.  While there are continuous efforts toward developing and characterizing new nanoscale electrical components, the methods to connect such components are poorly developed.  One of the main goals of this work is to develop the basic principles for mechanically stable, flexible and reliable nanoscale electrical interconnects based on surface structures of single or a small number of conducting polymer chains.  Another research direction within this program is directed towards exploring alternative types of metal-molecule contacts other than typical single thiol-metal linker with the aim of improving the electrical efficiency and reliability of molecular junctions.  Developing the basic principles for such small electrical contacts will enable the development of nearly all nanoscale electrical components as they will all require contacts of comparable dimensions, for example between metal or metal/semiconductor structures.

We have successfully studied the electrical conduction properties of metal-molecule-metal junctions formed between Au supported self-assembled monolayers (SAMs) of symmetric tetrathiafulvalene (TTF) derivatives and a Pt-coated atomic force microscope (AFM) tip.  The symmetric TTF molecule offers a new type of indirect linkage between TTF and gold by two thiol groups on both sides of the molecule.  In this study, SAMs of symmetric TTF and 1-decanethiol (C₁₀) were prepared and conducting probe (CP) AFM was used to probe the molecular conduction properties of the two films at the molecular nano-junction.  Decanethiol serves as a control as it is a well studied molecule of a comparable to the TTF molecular length.  Figure 1 shows current-voltage (I(V)) curves for TTF (x) and C₁₀ (□) SAMs in bicyclohexyl solvent under a fixed loading force of 6 nN.  The measured I(V) curves clearly indicate the TTF SAM is significantly more conductive than the 1-decanethiol SAM, which is expected for a conjugated molecule.  To quantify these results, the I-V curves are compared to the Simmons model for nonresonant tunneling through metal-insulator-metal junction.  The best-fit curves are shown by solid lines in Figure 1.  From the fit, the potential barrier for the TTF system was found to be 0.72 eV, significantly lower than 1.51 eV determined for C_10.

All I(V) curves for both molecular systems display linear current-voltage region within ±0.2 V surface bias, consistent with the Ohm's law.  The linear Ohmic region was used to estimate single molecule resistance for both molecules.  The single molecule resistance obtained using this approach is 950 ± 10 GW for the C₁₀ molecule and 14.7 ± 0.7 GW for the TTF molecule.  The value of the TTF single molecule resistance is used to estimate the molecular resistivity of 390 Ω•cm, which is comparable with the intrinsic resistivity of bulk germanium (~65 W•cm).  This result indicates a remarkable conductivity for an organic molecule suggesting that the dithiol contact geometry such as that for the TTF unit provides a better overlap of the electronic wave functions of the molecule and substrate.  Current study provide quantitative example of how the electrical conduction can be significantly improved by using contact geometries other than single thiol-gold chemical bond.  Such alternative contact geometry indicates that the molecular systems based on the TTF unit can be used as efficient metal-molecule interconnects with improved charge transfer properties.

To determine the mechanical stability of the TTF-Au contact and how it effects the electrical properties, the electrical conduction measurements were performed using CP-AFM under different applied forces (hence pressure).  Figure 2 shows the averaged junction resistance versus loading force for the TTF SAM.  Crosses are averaged data and the error bars represent the standard deviation of the mean for a series of ~50 repeated measurements at different sample regions under a particular loading force.  A clear increase in the TTF junction resistance is observed for the loading forces between 6 and 33.3 nN with the decrease in the resistance at 40 nN.  The effect is highly reproducible and its origin lies in the change of intermolecular interactions between neighboring TTF molecules.  Such nonlinear response of the TTF SAM resistance to the applied loading force may have practical applications as a possible nano-switch device where the current flow through the SAM can be efficiently controlled by the applied pressure.

The research work strongly impacts the education and training of undergraduate and graduate students.  The PI is involved on a daily basis at the educational aspect of the research.  Students get invaluable experience with cutting edge microscopy instruments; improve their analytical, physical, synthetic, writing and presentation skills, all tremendously important towards developing new generation of experienced scientists in the fast growing research fields of Nanoscience and Nanotechnology.  At this stage, the work on the TTF molecular linker is finished and results have been submitted for publication.  Currently, research is focused on other types of molecular systems, both short conjugated oligomers and conducting polymers with different molecular length, structure, and molecule-metal electrode contact geometry. 

Figure 1. Averaged current-voltage curves for TTF (x) and C₁₀ (□) SAMs in bicyclohexyl solvent under a fixed loading force of 6 nN.  Each data symbol represents the mean value of current at a particular surface bias and the solid lines are fits to the Simmons model.

Figure 2. Plots of averaged junction resistance versus loading force for TTF SAM.