Incremental charging of a molecule at room temperature using the scanning tunnelling microscope

Abstract

ADDING electrons to, or extracting them from, a sample of microscopic dimensions is opposed by a charging energy that depends on the object's capacitance. The stepwise charging ('Coulomb staircase') that is expected as a result has been observed for metal films and droplets, using the scanning tunnelling microscope (STM) to inject the charge^1–3. But because of the very small charging energy, the effect was observable only at very low temperatures (∼4 K). Here I present evidence for incremental charging of a single molecule (a liquid crystal) from STM studies conducted at room temperature. I observe quasiperiodic variations in the current–voltage curve which I attribute to discrete changes in the molecule's charge. The width of the steps and the asymmetry between positive and negative bias, however, remain to be explained.