  We report that the ionic-liquid gating of bulk single crystals of a
topological insulator can control the type of the surface carriers and even
results in ambipolar transport. This was made possible by the use of a highly
bulk-insulating BiSbTeSe2 system where the chemical potential is located close
to both the surface Dirac point and the middle of the bulk band gap. Thanks to
the use of ionic liquid, the control of the surface chemical potential by
gating was possible on the whole surface of a bulk three-dimensional sample,
opening new experimental opportunities for topological insulators. In addition,
our data suggest the existence of a nearly reversible electrochemical reaction
that causes bulk carrier doping into the crystal during the ionic-liquid gating
process.
