Nano Lett. 18, 2165−2171 (2018)

Scanning tunnelling spectroscopy has developed into a versatile tool to investigate standing waves of hot carriers in a variety of materials, such as noble metals, high-temperature superconductors, Weyl semimetals, or topological insulators. Yet, the position of the probe tip determines both the point of charge injection and the detection point. Hence, the mapping of hot carriers away from the injection spot becomes impossible. Leisegang and colleagues now use a single molecule as a sensor for hot carriers. By doing so, they can separate the injection from the detection and track the evolution of the carrier concentration away from the tip position.

The researchers deprotonate a free-base phthalocyanine molecule on a Ag(111) surface, creating a tautomerization switch sensitive to hot electrons. With the tip positioned at a spot away from the molecule, voltage pulses inject hot carriers, which eventually switch the position of the hydrogen atom in the molecule. The switching probability depends on the carrier concentration at the molecule site. Leisegang and co-workers then move the tip over the molecule and read out the state of the switch by non-invasive topographic imaging. To demonstrate the wave nature of hot electrons, they built an atomic-scale interferometer, where two pairs of Ag adatoms act as mirrors. Constructive or destructive interference occur between direct and scattered electron waves depending on the distance between the atomic mirrors. If a second molecule and a second interferometer are added, carriers are more likely to travel to one of the two molecules when their wavelength matches the interference conditions of the respective interferometer.