In this talk, I will present my angle resolved photoelectron spectroscopy
(ARPES) and density functional theory results on quaternary
topological insulator (TI) BiSbTe1.25Se1.75(BSTS). This study confirm the
non-trivial topology of the surface state bands (SSBs) in BSTS.
The SSBs are sensitive to the atomic composition of the terminating
surface and have a partial 3D character. Further study shows that the band
bending (BB) effects  in BSTS and the Dirac point (DP) shifts by more
than two times compared to that in Bi2Se3 to reach the saturation. The
stronger BB in BSTS could be due to the difference in screening of the
surface charges. From momentum density curves (MDCs) of the ARPES data we
obtained an energy dispersion relation showing the warping strength of the
Fermi surface in BSTS to be intermediate between those found in
Bi2Se3 and Bi2Te3 and also to be tunable by controlling the ratio of
chalcogen/pnictogen atoms. In addition, experiments also reveal
that the nature of the BB effects are highly sensitive to the exposure of
the fresh surface to various gas species. These findings
have important implications in the tuning of DP in TIs for technological
applications