Superconductivity in the metastable 1T' and 1T''' phases of MoS$_2$ crystals

TL;DR

This study successfully synthesized metastable 1T' and 1T''' phases of MoS₂ crystals, revealing their superconducting and insulating properties, and demonstrated their reversible transformation to the stable 2H phase, advancing understanding of their atomic structures and potential applications.

Contribution

First experimental realization of high-quality metastable 1T' and 1T''' MoS₂ crystals with detailed structural and electronic characterization.

Findings

1T'-MoS₂ is a superconductor with Tc of 4.2 K.

1T'''-MoS₂ exhibits superconductivity at 5.3 K or insulating behavior depending on synthesis.

Both metastable phases can revert to the stable 2H phase with mild annealing.

Abstract

Transition-metal dichalcogenides open novel opportunities for the exploration of exciting new physics and devices. As a representative system, 2H-MoS$_2$ has been extensively investigated owing to its unique band structure with a large band gap, degenerate valleys and non-zero 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 1T', 1T'' and 1T''' lattice structures, which hinder the broad applications of MoS$_2$ in future nanodevices and optoelectronic devices. Using ${K_x(H_2O)_yMoS_2}$ as the precursor, we have successfully obtained high-quality layered crystals of the metastable 1T'''-MoS$_2$ with $\sqrt{3}a\times\sqrt{3}a$ superstructure and metastable 1T'-MoS$_2$ with a$\times$2a superstructure, as evidenced by structural characterizations through scanning tunneling microscopy, Raman spectroscopy and X-ray diffraction. It is found that the metastable 1T'-MoS$_2$ is a superconductor with onset transition temperature (${T_c}$) of 4.2 K, while the metastable 1 T'''-MoS$_2$ shows either superconductivity with Tc of 5.3 K or insulating behavior, which strongly depends on the synthesis procedure. Both of the metastable polytypes of MoS$_2$ crystals can be transformed to the stable 2H phase with mild annealing at about 70 $^{\circ}$C in He atmosphere. These findings provide pivotal information on the atomic configurations and physical properties of 1T polytypes of MoS$_2$.