| AVS 56th International Symposium & Exhibition | |
| Electronic Materials and Processing | Wednesday Sessions |
| Session EM-WeM |
| Session: | Organic & Molecular Electronics |
| Presenter: | D. Kockmann, University of Twente, Netherlands |
| Authors: | D. Kockmann, University of Twente, Netherlands H.J.W. Zandvliet, University of Twente, Netherlands B. Poelsema, University of Twente, Netherlands |
| Correspondent: | Click to Email |
We have studied (single) octanethiol molecules adsorbed on a Pt-modified Ge(001) surface, using scanning tunneling microscopy/spectroscopy.
On a clean Ge(001) surface we deposited a submonolayer amount of platinum by evaporation. Patches of dimerized atomic chains form via self-organization on the surface during the subsequent annealing-step. We have decorated the Pt-modified Ge(001) surface with octanethiol molecules. STM at 77 K revealed that at low coverage the molecules selectively adsorb on the Pt chains and not on the underlying terrace. The molecules lay down on the Pt chains, in contrast to SAM's, where they stand upwards.
In order to distinguish between the octane-tail and the thiol-head of the molecule, we have performed STS above the different regions of the molecule, again at 77 K. We observed that the I(V) spectra recorded above the thiol-head were conspicuously different from the spectra recorded above the octane-tail. From that we could determine the molecular orientation of the adsorbent.
In addition, we have measured current-time traces on the adsorbed octanethiol molecules. Throughout these experiments we turned off the feedback-loop of the STM. Then each measurable rearrangement or conformational change of the molecule is reflected in the I(t) traces. During these measurements we occasionally found a sudden dramatic increase in current from 1 nA (set point current) to values between 10-15 nA. The residence times at this high current varied between 10-40 seconds. In most cases the current jumps back to its original set point value of 1 nA within the open-loop measurements, which typically last for 50-100 seconds. STM images recorded after the open-loop experiments revealed that the octanethiol molecules remained at their original position.
From these observations we concluded that during the I(t) measurements the molecule wagged its tail upward, thereby making contact with the tip of the STM. Hence, we measured electron transport through the molecule instead of electron tunneling from tip to molecule. The derived single molecule resistance, 100-150 MΩ, is in accordance with literature.