Polymorphic Layered MoTe2 from Semiconductor, Topological Insulator, to Weyl Semimetal

Large size (∼2 cm) single crystals of layered MoTe2 in both 2H- and 1T′-types were synthsized using TeBr4 as the source of Br2 transport agent in chemical vapor transport growth. The crystal structures of the as-grown single crystals were fully characterized by X-ray diffraction, Raman spectroscopy, scanning transmission electron microscopy, scanning tunneling microscopy (STM), and electrical resistivity (ρ) measurements. The resistivity ρ(T), magnetic susceptibility χ(T), and heat capacity Cp(T) measurement results reveal a first order structural phase transition near ∼240 K for 1T′-MoTe2, which has been identified to be the orthorhombic Td-phase of MoTe2 as a candidate of Weyl semimetal. The STM study revealed different local defect geometries found on the surface of 2H- and Td-types of MoTe6 units in trigonal prismatic and distorted octahedral coordination, respectively.