Structure, electronics and transport properties in nanostructured materials

Transition metal chalcohalides MX2 can form a wealth of diverse nanostructures, largely as a function of the synthesis conditions. The obtained structures range from large octahedral and fullerene-like hollow clusters and cylindrical nanotubes close to the nominal composition M:X = 1:2 to smaller, two-dimensional platelet-shaped clusters under sulfur excess and to one-dimensionally elongated nanowires under sulfur-deficient conditions. All of those structures exhibit specific electronic properties that differ from the ones of the pure bulk and open up a large spectrum of nanostructure applications, that still includes the lubricant aspect, but reaches further to catalysis and electronic transport. One-dimensionally delocalized electronic states provide the basis for the higher activity, reactivity and conductivity in nanostructured MX2. Larger triangular and hexagonal platelets exhibit a so-called 'brim'-state along the circumference of the cluster, which assists the activation of sulfur atoms contained in raw oil; hence platelet-based catalysts are employed for desulfurization. One-dimensional wires with compositions MX an M2X3 are composed of a central metallic wire coated by an insulating sulfur and/or halide shell. They exhibit a very high structural regularity, hence, ballistic conductivity may be obtained in such structures. The stability, structure and metallicity of such wires have been calculated in excellent accordance with recent scanning-probe experiments. In addition, the calculations showed, that wires can act as electromechanical switches, because they undergo a symmetry-dependent metal-insulator transition upon twisting. Thus, metal chalcohalide nanostructures are versatile building blocks for nanoscale catalytic and electronic devices.