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

Large size (∼2 cm) single crystals of layered MoTe 2 in both 2 H- and 1 T ′-types were synthsized using TeBr 4 as the source of Br 2 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 C p ( T ) measurement results reveal a first order structural phase transition near ∼240 K for 1 T ′-MoTe 2, which has been identified to be the orthorhombic T d-phase of MoTe 2 as a candidate of Weyl semimetal. The STM study revealed different local defect geometries found on the surface of 2H - and T d-types of MoTe 6 units in trigonal prismatic and distorted octahedral coordination, respectively.