  Introducing, observing, and manipulating individual impurities coupled to a
host material offers the opportunity to create new device concepts based on
single spin and charge states. Because of potential applications in spintronics
and magneto-electrics, such an approach would be particularly useful for
topological insulators (TI), a recently discovered material class hosting
spin-momentum- locked surface states. To make them useful for new technologies,
a robust control of their interaction with external perturbations is required.
However, traditional approaches such as metal electrodes or doping proved to be
problematic and resulted in strong mesoscopic fluctuations making the
spin-momentum locking ill-defined. Here, we demonstrate the effective gating of
TIs by coupling molecules to their surface which, by using electric fields,
allow to dynamically control the interface charge state by adding or removing
single electrons. This process creates a robust transconductance bistability
resembling a single-electron transistor. Our findings make hybrid molecule/TI
interfaces functional elements while at the same time pushing miniaturization
at its ultimate limit. This opens a new avenue for all electric-controlled
spintronic devices based on these fascinating materials.
