  The surface band bending tunes considerably the surface band structures and
transport properties in topological insulators. We present a direct measurement
of the band bending on the Bi2Se3 by using the bulk sensitive angular-resolved
hard x-ray photospectroscopy (HAXPES). We tracked the depth dependence of the
energy shift of Bi and Se core states. We estimate that the band bending
extends up to about 20 nm into the bulk with an amplitude of 0.23--0.26 eV,
consistent with profiles previously deduced from the binding energies of
surface states in this material.
