Abstract
Misfit layered compounds (MLCs) MX-TX2, where M, T = metal atoms and X = S, Se, or Te, and their nanotubes are of significant interest due to their rich chemistry and unique quasi-1D structure. In particular, LnX-TX2 (Ln = rare-earth atom) constitute a relatively large family of MLCs, from which nanotubes have been synthesized. The properties of MLCs can be tuned by the chemical and structural interplay between LnX and TX2 sublayers and alloying of each of the Ln, T, and X elements. In order to engineer them to gain desirable performance, a detailed understanding of their complex structure is indispensable. MLC nanotubes are a relative newcomer and offer new opportunities. In particular, like WS2 nanotubes before, the confinement of the free carriers in these quasi-1D nanostructures and their chiral nature offer intriguing physical behavior. High-resolution transmission electron microscopy in conjunction with a focused ion beam are engaged to study SmS-TaS2 nanotubes and their cross-sections at the atomic scale. The atomic resolution images distinctly reveal that Ta is in trigonal prismatic coordination with S atoms in a hexagonal structure. Furthermore, the position of the sulfur atoms in both the SmS and the TaS2 sublattices is revealed. X-ray photoelectron spectroscopy, electron energy loss spectroscopy, and X-ray absorption spectroscopy are carried out. These analyses conclude that charge transfer from the Sm to the Ta atoms leads to filling of the Ta 5dz2 level, which is confirmed by density functional theory (DFT) calculations. Transport measurements show that the nanotubes are semimetallic with resistivities in the range of 10–4 Ω·cm at room temperature, and magnetic susceptibility measurements show a superconducting transition at 4 K.
Highlights
Misfit layered compounds (MLCs) are a class of twodimensional (2D) materials receiving considerable attention due to their unique structure, crystallographic diversity, and chemically tailorable characteristics.[1−8] Among the MLCs, the chalcogenide-based MLCs are of special interest due to their metallic and semiconducting properties
The MLC nanotubes of (SmS)1.19TaS2 were produced by a well-established chemical vapor transport protocol with slight modification in growth temperature to improve the yield
The structure and some physical properties of nanotubes and flakes formed from the misfit layered compound (SmS)1.2TaS2 were studied
Summary
Misfit layered compounds (MLCs) are a class of twodimensional (2D) materials receiving considerable attention due to their unique structure, crystallographic diversity, and chemically tailorable characteristics (vide inf ra).[1−8] Among the MLCs, the chalcogenide-based MLCs are of special interest due to their metallic and semiconducting properties. The charge transfer from LnS to TaS2 stabilizes MLC structures and alters the electronic properties of the two subunits, appreciably.[39] The amount of charge transfer can be tuned by alloying the rocksalt (LnS) unit with other rare earth or heteroatoms.[40,41] The role of the charge transfer from the Ln atoms to the Ta atoms has been elucidated.[25,41] In particular, since the work function of the LnS subunit is smaller than that of the hexagonal TaS2, charge transfer occurs from the rare-earth atom to the partially occupied 5dz[2] level of the tantalum atom This charge transfer modifies the effective valence state of the rare-earth atom (2+) closer to the more stable 3+ state. Nanotubes (and flakes) of the misfit compound (SmS)1.19TaS2 were studied in detail to address the structural aspects atom by atom and correlate it with its properties. As for the length of the channel used in the calculations, the distance in between the inner contacts was used, which again resulted in a slight overestimation of the resistivity
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