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 charge1–3. 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.