Dr. Reshef Tenne

Abstract:

Among the 2D-materials, misfit layered compounds make a special class with incommensurate and non-stoichiometric lattice made of an alternating layer with rocksalt structure, like LaS (O) and a layer with hexagonal (octahedral) structure, like TaS₂ (T). The lack of lattice commensuration between the two slabs leads to a built-in strain, which can be relaxed via bending. Consequently, nanotubes have been produced from numerous MLC compounds over the last decade and their structure was elucidated.

Owing to their large surface area, nanostructures are generally metastable and tend to recrystallize into microscopic (macroscopic) crystallites via different mechanisms, like Ostwald ripening, or chemically decompose and then recrystallize. The stability of nanostructures at elevated temperatures has been investigated quite scarcely, so far. As for the chemical selectivity, entropic effects are expected to dictate random distribution of the chalcogen atoms on the anion sites of the MLC nanotubes at elevated temperatures. Surprisingly, the sulfur atoms were found to bind exclusively to the rare-earth atom (Ln= La, Sm) of the rocksalt slab, and the selenium to the tantalum of the hexagonal TX₂ slab [1].

In other series of experiments, the lack of utter symmetry in the multiwall nanotubes leads to exclusions of certain X-ray (0kl) reflections, which was used to distinguish them from the bulk crystallites.  The transformation of Ln-based MLC nanotubes into microscopic flakes was followed as a function of the synthesis temperature (800-1200 °C) and synthesis time (1-96 h) [2, 3]. Furthermore, sequential high-temperature transformations of the (O-T) lattice into (O-T-T) and finally (O-T-T-T) phases via deintercalation of the LnS slab was observed. This autocatalytic process is reminiscent of the deintercalation of alkali atoms from different layered structure materials. Annealing at higher temperatures and for longer periods of time leads eventually to the decomposition of the ternary MLC into binary metal-sulfide phases as well as partial oxidation of the product. This study sheds light on the complex mechanism of high-temperature chemical stability of nanostructures.