  Chemical functionalization is an effective means to tune electronic and
crystal structure of two-dimensional material, which may be crucial for moder
microelectronics industry. Based on the first-principle calculation and an
iterative solution of Boltzmann transport equation, we find that antimony films
are potential excellent thermoelectrical materials with rather low thermal
conductivities $k$ ($<$ 2.5 W/mK). The chemical functionalization can induce
the reduction in $k$ to some extent, which is mainly due to the reduction of
phonon lifetimes limited by the anharmonic scattering. More interesting, the
origin of the reduction in $k$ is not the anharmonic interaction but the
harmonic interaction from the depressed phonon spectrum mechanism, and for some
chemical functional atom in halogen, the flat modes appearing in the low
frequency range play also a key factor in the reduction of $k$ by significantly
increasing the three-phonon scattering channels. Our work analyzes the
reduction mechanism in $k$ from the chemical functionalization for antimonene,
and provides a new view to adjust the thermal conductivity which can benefit
thermoelectric material design.
