Synthesis and properties of turbostratically disordered, ultrathin WSe ₂ films

Turbostratically disordered tungsten diselenide (WSe₂) thin films with as few as two c-axis-oriented (basal plane) structural units were synthesized from modulated elemental reactants. By varying the number of elemental W-Se bilayers deposited, the thickness could be controllably varied from two to eighty such structural units. The sample roughness decreases with increasing annealing time and temperature as the crystalline WSe₂ basal plane units self-assemble from the amorphous precursors. Low-angle X-ray diffraction data show that the thickness of the WSe₂ films is highly uniform after annealing, with estimated roughness of less than 0.2 nm, and highly oriented, with the c axis of the structural units oriented within 0.1̊ of the substrate normal as determined from rocking curves of the specular 00L-type diffraction peaks. Pole figures of hk0-type reflections indicate that c-axis-oriented basal plane structural units are randomly oriented within the a-b plane. The widths of diffraction peaks of type hk0, 00L, and hkl (h, k ≠0; l ≠ 0) indicate coherence lengths of about 6-7 nm in the a-b plane, the full thickness of the film along the c axis, and 1-2 nm in mixed-index directions. Scanning transmission electron microscopy imaging corroborated the X-ray scattering results, providing direct evidence of strong c-axis texture, rotational disorder between adjacent basal plane structural units, and an intraplanar grain size of several nanometers. The combination of intraplanar crystallinity and interplanar rotational disorder explains the significant anisotropy of the thermal conductivity, which is 20-30 times higher in the a-b plane than along the c axis. Electrical measurements within the a-b plane indicate that the films exhibit n-type semiconducting behavior.