Single-molecule switching on semiconductor surfaces
Apart from its relevance to fundamental surface chemistry, the interaction of organic molecules with Si surfaces is in the focus of new device concepts combining conventional semiconductors with functionalities based on molecular structures. A prototype functionality of interest is, e.g., a bistable molecule that can be switched by an external stimulus. In this context, we study single 1,5 cyclooctadiene (COD) molecules adsorbed on Si(001) and explore the conformational switching of this cyclic alkene upon local excitation. The excitation is carried out by vertical charge transport through the molecule in the tunnel junction of a LTSTM. We observe a switching between two levels of tunnel current at a rate tunable from 0.1 to 100 s-1 when applying currents from 10 pA to 10 nA. The involved excitation process is due to inelastic electron tunneling (IET) in the case of an adsorbed bistable molecule characterized by a double-minimum potential energy surface. Quantum chemical calculations rationalize the IET-induced switching and the effect of temperature, in full agreement with experiment. COD/Si(001) molecules represent an instructive model case of single-molecule switching based on intramolecular conformational conversion without any bond breaking and rebonding involved.
Figure:
STM topograph (65 Å x 65 Å) of five COD molecules covalently linked to Si(001) (A); the color coded image shows COD as red protrusions on top of the Si dimer rows (dark blue) whereas single-dimer vacancies appear as green depressions. (B) adds a structure model of the double dimer-bound COD adsorption geometry and (C) shows a tunnel current time spectrum measured with the tip positioned over the molecule indicating the binary switching.
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