The phenomenon of a partial 2H 1T phase transition within multiwalled WS 2 nanotubes under 5 substitutional Rhenium doping is discovered by means of high-resolution transmission electron microscopy. Using density-functional calculations for the related MoS 2 compound we consider a possible origin of this phase transition, which was known formerly only for WS 2 and MoS 2 intercalated by alkali metals. An interplay between the stability of layered or nanotubular forms of 2H and 1T allotropes is found to be intimately related with their electronic structures and electro-donating ability of an impurity. 10 65 Notes

Transition metal dichalcogenide materials have recently been shown to exhibit a variety of intriguing optical and electronic phenomena. Focusing on the optical properties of semiconducting WS2 nanotubes, we show here that these nanostructures exhibit strong light-matter interaction and form exciton-polaritons. Namely, these nanotubes act as quasi 1-D polaritonic nano-systems and sustain both excitonic features and cavity modes in the visible-near infrared range. This ability to confine light to subwavelength dimensions under ambient conditions is induced by the high refractive index of tungsten disulfide. Using "finite-difference time-domain" (FDTD) simulations we investigate the interactions between the excitons and the cavity mode and their effect on the extinction spectrum of these nanostructures. The results of FDTD simulations agree well with the experimental findings as well as with a phenomenological coupled oscillator model which suggests a high Rabi splitting of ∼280 meV. These findings open up possibilities for developing new concepts in nanotube-based photonic devices.

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We image the ultraslow exciton-polariton propagation in a WSe2 nanoslab with extreme spatiotemporal resolution (50 nm, <45 fs).