Remote Single-Molecule Switching: Identification and Nanoengineering of Hot Electron-Induced Tautomerization

Molecular electronics where single molecules perform basic functionalities of digital circuits is a fascinating concept that one day may augment or even replace nowadays semiconductor technologies. The tautomerization of molecules, that is, the bistable functional position of hydrogen protons within an organic frame, has recently been intensively discussed as a potential avenue toward nanoscale switches. It has been shown that tautomerization can be triggered locally or nonlocally, that is, by a scanning tunneling microscope (STM) tip positioned directly above or in close vicinity to the molecule. Whereas consensus exists that local switching is caused by inelastic electrons that excite vibrational molecular modes, the detailed processes responsible for nonlocal tautomerization switching and, even more important in the context of this work, methods to control, engineer, and potentially utilize this process are largely unknown. Here, we demonstrate for dehydrogenated H₂Pc molecules on Ag(111) how to controllably decrease or increase the probability of nonlocal, hot electron-induced tautomerization by atom-by-atom designed Ag nanostructures. We show that Ag atom walls act as potential barriers that exponentially damp the hot electron current between the injection point and the molecule, reducing the switching probability by up to 83% for a four-atom wide wall. By placing the molecule in one and the STM tip in the other focal point of an elliptical nanostructure, we could coherently focus hot electrons onto the molecule that led to an almost tripled switching probability. Furthermore, single and double slit experiment based on silver atom structures were used to characterize the spatial extension of hot electron packets. The absence of any detectable interference pattern suggests that the coherence length of the hot electrons that trigger tautomerization processes is rather short. Our results demonstrate that the tautomerization switching of single molecules can remotely be controlled by utilizing suitable nanostructures and may pave the way for designing new tautomerization-based switches.