Superconductivity in the metastable 1T′ and 1T′″ phases of MoS2 crystals

Transition-metal dichalcogenides open novel opportunities for the exploration of exciting new physics and devices. As a representative system, $2H\text{-Mo}{\mathrm{S}}_{2}$ has been extensively investigated owing to its unique band structure with a large band gap, degenerate valleys, and nonzero Berry curvature. However, experimental studies of metastable $1T$ polytypes have been a challenge for a long time, and electronic properties are obscure due to the inaccessibility of single phase without the coexistence of $1{T}^{\ensuremath{'}}$, $1{T}^{\ensuremath{''}}$, and $1{T}^{\ensuremath{'}\ensuremath{''}}$ lattice structures, which hinder the broad applications of $\mathrm{Mo}{\mathrm{S}}_{2}$ in future nanodevices and optoelectronic devices. Using ${\mathrm{K}}_{x}{({\mathrm{H}}_{2}\mathrm{O})}_{y}\mathrm{Mo}{\mathrm{S}}_{2}$ as the precursor, we have successfully obtained high-quality layered crystals of the metastable $1{T}^{\ensuremath{'}\ensuremath{''}}\text{-Mo}{\mathrm{S}}_{2}$ with $\sqrt{3}a\ifmmode\times\else\texttimes\fi{}\sqrt{3}a$ superstructure and metastable $1{T}^{\ensuremath{'}}\text{-Mo}{\mathrm{S}}_{2}$ with $a\ifmmode\times\else\texttimes\fi{}2a$ superstructure, as evidenced by structural characterizations through scanning tunneling microscopy, Raman spectroscopy, and x-ray diffraction. It is found that the metastable $1{T}^{\ensuremath{'}}\text{-Mo}{\mathrm{S}}_{2}$ is a superconductor with onset transition temperature $({T}_{\mathrm{c}})$ of 4.2 K, while the metastable $1{T}^{\ensuremath{'}\ensuremath{''}}\text{-Mo}{\mathrm{S}}_{2}$ shows either superconductivity with ${T}_{\mathrm{c}}$ of 5.3 K or insulating behavior, which strongly depends on the synthesis procedure. Both of the metastable polytypes of $\mathrm{Mo}{\mathrm{S}}_{2}$ crystals can be transformed to the stable $2H$ phase with mild annealing at about 70 \ifmmode^\circ\else\textdegree\fi{}C in He atmosphere. These findings provide pivotal information on the atomic configurations and physical properties of $1T$ polytypes of $\mathrm{Mo}{\mathrm{S}}_{2}$.