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Size-effects in 2D Transition Metal Chalcogenides

Gemming, S.; Seifert, G.


Nanoscale Mo_mS_n compounds exist in a large variety of compositions with many different characteristic structural elements and related electronic properties. The structural wealth of Mo_mS_n compounds comprises finite structures such as small clusters and hollow inorganic fullerenes, one-dimensionally extended wires and stripes, or two-dimensional platelets. The structural elements are based either on a three-dimensional Mo_m framework or on platelet-shaped elements derived from the layered MoS_2 bulk structure. The electronic features range from large energy gaps in closed-shell systems over the semiconducting two-dimensional MoS_2 sheet to the metallic conductivity of the brim state in platelets or along the Mo core of wires.
With the help of density-functional calculations the preferred structural elements and the relative stabilities could be correlated with the chemical potentials of sulfur and molybdenum in the system. As structural elements and electronic features are related, this result is the basis for a further tailoring of electronic properties via the preparation conditions. For very small systems Mo_mS_n, with m <= 4, n <= 14 the cluster-platelet transition depends on a sulfur excess of the system of at least one additional S atom per MoS_2 formula unit, i.e. to MoS_3. In the small size regime the most important species are the extremely stable large-gap Mo_4S_6 cluster, Mo_3S_6 as smallest member of (MoS_2)_n platelet family and Mo_3S_3 and Mo_3S_5 as stable building blocks of wires. For one-dimensionally extended conducting structures the termination exhibits a length-dependent crossover of the termination group from (Mo_3S_3)_nS_2 to (Mo_3S_3)_n-1(MoS_4)_2 provided that the chemical potential of sulfur is low and constant. Sulfur-terminated two-dimensional (MoS_2)_nS_m platelets are stable only at the enhanced sulfur chemical potential required to saturate the platelet edges with sulfur up to a stoichiometry of Mo:S = 1:3 in the smallest clusters. In even more sulfur-rich conditions all systems tend to form chemically active, but electronically inert S_2^2- ions.

Keywords: density-functional; molybdenum sulfide; 2D electronics; molecular electronics; inorganic nanotubes; nanotubes; platelet

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