  Three dimensional (3D) topological insulators are quantum materials with a
spin-orbit induced bulk insulating gap that exhibit quantum-Hall-like phenomena
in the absence of applied magnetic fields. The proposed applications of
topological insulators in device geometries rely on the ability to tune the
chemical potential on their surfaces in the vicinity of the Dirac node. Here,
we demonstrate a suite of surface control methods based on a combination of
photo-doping and molecular-doping to systematically tune the Dirac fermion
density on the topological (111) surface of Bi2Te3. Their efficacy is
demonstrated via direct electronic structure topology measurements using high
resolution angle-resolved photoemission spectroscopy (ARPES). These results
open up new opportunities for probing topological behavior of Dirac electrons
on the Bi2Te3 surface. At least one of the methods demonstrated here can be
successfully applied to other topological insulators such as the Bi{1-x}Sb{x},
Sb2Te3 and Bi2Se3 which will be shown elsewhere. More importantly, our methods
of topological surface state manipulation demonstrated here are highly suitable
for future spectroscopic studies of topological phenomena which will complement
the transport results gained from the traditional electrical gating techniques.
