The adsorption and tip-induced switching of single 1,5 cyclooctadiene molecules (COD) on the Si(100) surface was studied by low-temperature STM at 7 K. COD (C_8H_12) is a cyclic alkene with two opposite C=C double bonds and a twisted boat configuration. There is still controversy concerning the adsorption geometry of this molecule on Si(100): Previous experimental studies at monolayer coverage reported that COD adsorbs on top of a single Si dimer in an upright structure implying that only one of the two C=C bonds is involved in the reaction with the dimerized Si(100) surface [1]. Density functional calculations, on the other hand, predicted that the bridge structure with both C=C bonds reacting with two adjacent Si dimers is more stable than the upright structure [2].
In our experiment, 0.1 L COD was dosed at room temperature followed by Si(100) sample cooling to 7 K. In constant-current imaging the molecule appears as a uniform protrusion on top of the dimer rows except for sample biases close to the band gap edge (-1 V) where it appears as a depression. Using Si dimer vacancies and the inherent dimer buckling as markers for the position of the Si atoms within dimer rows we find that the molecule is centered between two adjacent dimers verifying that COD adsorbs in the bridge structure. We also performed time-dependent measurements of the tunneling current with the tip positioned over the molecule for different biases and initial tunneling currents. The current response appears as telegraph noise, i.e. fluctuations between two levels of current signal with the same population probability and is sensitive to the sample bias and the tip-to-sample distance. Higher bias and/or smaller distance (and therefore higher current) result in an increased switching rate (about 10 to 100 Hz for the present experimental conditions) suggesting that the fluctuations are either due to the electric field within the tunnel junction or due to inelastic electron tunneling. We interpret this fluctuation as a dynamic conformational change between two degenerate bridge structures of the adsorbed molecule which can be triggered and controlled by vertical charge transport.
[1] J. S. Hovis et al., J. Phys. Chem. B 101, 9581 (1997)
[2] J.-H. Cho et al., Phys. Rev. B 64, 241306 (2001)
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