Low-energy nine-layer rhombohedral stacking of transition metal dichalcogenides

TL;DR

This paper predicts a new stable rhombohedral 9R phase in transition-metal dichalcogenides, with unique electronic and optical properties, potentially useful for piezotronics and valleytronics applications.

Contribution

It introduces a hypothetical 9R phase in TMDs, demonstrating its stability and distinctive properties compared to known phases.

Findings

9R phase is energetically comparable to 3R and can be stabilized by strain.

9R exhibits distinctive Raman peaks and enhanced piezoelectricity.

Unique band splitting in 9R suggests potential for valleytronic applications.

Abstract

Transition-metal dichalcogenides (TMDs) show unique physical, optical, and electronic properties. The known phases of TMDs are 2H and 3R in bulk form, 1T and associated reconstructions, and 1H in monolayer form. This paper reports a hypothetical phase, 9R, that may exist in TMDs (Mo, W)(S, Se, Te)$_2$, meeting both dynamical stability and elastic stability criteria. 9R phase has the same space group as 3R, $i.e.$ rhombohedral $R3m$ without inversion symmetry, and has 9 layers in a conventional unit cell. We find that 9R has an energy within 1 meV per formula unit of 3R and can be energetically favored by a particular strain condition. We further calculate the electronic, elastic, piezoelectric, Raman, and second-harmonic generation signatures of 9R TMDs and compare them with the corresponding 2H and 3R phases. 9R has similar properties to 3R but shows distinctive Raman peaks in the low-frequency regime, improved piezoelectric properties, and unique band splitting arising from layer coupling at the conduction band minimum. These distinct properties make 9R an attractive candidate for applications in piezotronics and valleytronics.