  We report a systematic angle-resolved photoemission spectroscopy on
topological insulator (TI) TlBi1-xSbxTe2 which is bulk insulating at 0.5 < x <
0.9 and undergoes a metal-insulator-metal transition with the Sb content x. We
found that this transition is characterized by a systematic hole doping with
increasing x, which results in the Fermi-level crossings of the bulk conduction
and valence bands at x~ 0 and x~1, respectively. The Dirac point of the
topological surface state is gradually isolated from the valence-band edge,
accompanied by a sign reversal of Dirac carriers. We also found that the Dirac
velocity is the largest among known solid-solution TI systems. The
TlBi1-xSbxTe2 system thus provides an excellent platform for Dirac-cone
engineering and device applications of TIs.
