TEM AND STM STUDIES OF SOME TRANSITION-METAL CHALCOGENIDES WITH REDUCED DIMENSIONALITY

摘要

Because of their potential application possibilities a great interest was recently paid to a variety of low-dimensional structures. Among those thoroughly studied were also compounds, which naturally grow in a very non-isotropic way. Most transition-metal di-, tri- and tetrachalcogenides crystallize as quasi two- or quasi one-dimensional structures and show peculiar physical properties, whose origin is in their restricted dimensionality. Structural distortions of the basic structures are common in these compounds. Many, though not all, are driven by charge density waves (CDW), often incommensurate (IC) with the underlying lattice of the undistorted structure. In this paper the structural features of a few selected compounds belonging to this family are described. These include among the layered compounds the surface and domain structure of NbTe_2 and the superstructures formed by intercalation in A_xNbX_2 (0≤x≤l/3; A = Cu, Fe, Co; X = S, Se) and among the one-dimensional compounds two composition and temperature dependent 1C phases detected at the Nb-rich side of Nb_(1-x)Ta_xTe_4, charge instability and compositional modulation in ZrX_3 (X=Se,Te), sliding CDWs in NbSe_3, and the CDW driven IC superstructures in A_xNb_3X_4 (0≤x≤l/2; A = In, Tl; X = S, Se, Te). All crystals were prepared by the chemical transport method with iodine as the transport agent and investigated by transmission electron microscopy (TEM) and diffraction (TED), scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). The last two surface sensitive methods were carried out under ultra-high vacuum conditions.