Asymmetric 2D structures (often named Janus), like SeMoS, and their nanotubes have tremendous scope in material chemistry, nanophotonics and nanoelectronics due to a lack of inversion symmetry and time-reversal symmetry. The synthesis of these structures is fundamentally difficult owing to the entropy-driven randomized distribution of chalcogens. Indeed no Janus nanotubes were experimentally prepared, so far. Serendipitously, a family of asymmetric misfit layer superstructures (tubes and flakes), including LaX-TaX2 (where X= S/Se) were synthesized by high-temperature chemical vapor transport reaction where the Se binds exclusively to the Ta atoms and La binds to S atoms, rather than the anticipated random distribution. With increasing Se concentration, the LaS-TaX2 misfit structure gradually transformed into a new LaS-TaSe2-TaSe2 superstructure. No misfit structures were found for xSe=1. These counterintuitive results shed new light on the chemical selectivity and stability of misfit compounds and 2D-alloys, in general. The lack of inversion symmetry in these asymmetric compounds induces very large local electrical dipoles. The loss of inversion and time-reversal symmetries in the chiral nanotubes offers intriguing physical observations and applications.
Sreedhara et al., Asymmetric misfit nanotubes: Chemical affinity outwits the entropy at high-temperature solid-state reactions. PNAS 2021, 118, (35), e2109945118