Novel light-illumination scanning tunneling microscopy equipped with optical fiber probe

Single molecular photocurrent generation was investigated using a novel light-illumination scanning tunneling microscope (LI-STM) equipped with an optical fiber probe. For this study, self-assembled monolayers (SAMs) of chimera proteins composed of cytochrome b₅₆₂ mutant proteins fused with an enhanced green fluorescent protein (EGFP) were prepared. With light illumination, the EGFP was excited, resulting in photocurrent generation due to the energy transfer to the cytochrome part instead of a typical fluorescence. Two different types of measurements were carried out: one employed a conventional LI-STM with a metallic probe and far-field optics, while the metallic probe was replaced with a laboratory-built doubly metal-coated optical fiber probe in another setup, where a local near-field illumination was possible. The comparison of these techniques indicated the superior sensitivity of the new technique to the conventional one, showing the evidence of the intramolecular energy transfer at a single molecular level.