  A topological p-n junction (TPNJ) is an important concept to control spin and
charge transport on a surface of three dimensional topological insulators
(3D-TIs). Here we report successful fabrication of such TPNJ on a surface of
3D-TI Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$ thin films and experimental observation
of the electrical transport. By tuning the chemical potential of n-type
topological Dirac surface of BSTS on its top half by employing
tetrafluoro-7,7,8,8-tetracyanoquinodimethane as an organic acceptor molecule, a
half surface can be converted to p-type with leaving the other half side as the
opposite n-type, and consequently TPNJ can be created. By sweeping the
back-gate voltage in the field effect transistor structure, the TPNJ was
controlled both on the bottom and the top surfaces. A dramatic change in
electrical transport observed at the TPNJ on 3D-TI thin films promises novel
spin and charge transport of 3D-TIs for future spintronics.
