  The helical Dirac fermions on the surface of topological insulators host
novel relativistic quantum phenomena in solids. Manipulating spins of
topological surface state (TSS) represents an essential step towards exploring
the theoretically predicted exotic states related to time reversal symmetry
(TRS) breaking via magnetism or magnetic field. Understanding Zeeman effect of
TSS and determining its g-factor are pivotal for such manipulations in the
latter form of TRS breaking. Here, we report those direct experimental
observations in Bi2Se3 and Sb2Te2Se by spectroscopic imaging scanning tunneling
microscopy. The Zeeman shifting of zero mode Landau level is identified
unambiguously by judiciously excluding the extrinsic influences associated with
the non-linearity in the TSS band dispersion and the spatially varying
potential. The g-factors of TSS in Bi2Se3 and Sb2Te2Se are determined to be 18
and -6, respectively. This remarkable material dependence opens a new route to
control the spins in the TSS.
