  Tin-chalcogenides SnX (X = Te, Se and S) have been arousing research interest
due to their thermoelectric physical properties. The two-dimensional (2D)
counterparts, which are expected to enhance the property, nevertheless, have
not been fully explored because of many possible structures. Generating
variable composition of 2D Sn$_{1-x}$X$_{x}$ systems (X = Te, Se and S) has
been performed using global searching method based on evolutionary algorithm
combining with density functional calculations. A new hexagonal phase named by
$\beta'$-SnX is found by Universal Structure Predictor Evolutionary
Xtallography (USPEX), and the structural stability has been further checked by
phonon dispersion calculation and the elasticity criteria. The $\beta'$-SnTe is
the most stable among all possible 2D phases of SnTe including those
experimentally available phases. Further, $\beta'$ phases of SnSe and SnS are
also found energetically close to the most stable phases. High thermoelectronic
(TE) performance has been achieved in the $\beta'$-SnX phases, which have
dimensionless figure of merit (ZT) as high as $\sim$0.96 to 3.81 for SnTe,
$\sim$0.93 to 2.51 for SnSe and $\sim$1.19 to 3.18 for SnS at temperature
ranging from 300 K to 900 K with practically attainable carrier concentration
of 5$\times$10$^{12}$ cm$^{-2}$. The high TE performance is resulted from a
high power factor which is attributed to the quantum confinement of 2D
materials and the band convergence near Fermi level, as well as low thermal
conductivity mainly from both low elastic constants due to weak inter-Sn
bonding strength and strong lattice anharmonicity.

Materials Science