  By means of extensive ab initio calculations, a new two-dimensional (2D)
atomic material tin selenide monolayer (coined as tinselenidene) is predicted
to be a semiconductor with an indirect gap (1.45 eV) and a high hole mobility
(of order 10000 cm2V-1S-1), and will bear an indirect-direct gap transition
under a rather low strain (<0.5 GPa). Tinselenidene has a very small Young's
modulus (20-40 GPa) and an ultralow lattice thermal conductivity (<3 Wm-1K-1 at
300 K), making it probably the most flexible and most heat-insulating material
in known 2D atomic materials. In addition, tinseleniden has a large negative
Poisson's ratio of -0.17, thus could act as a 2D auxetic material. With these
intriguing properties, tinselenidene could have wide potential applications in
thermoelectrics, nanomechanics and optoelectronics.
