Tungsten doping-induced phase transition in CVD-grown MoS2 bilayers

TL;DR

This study demonstrates that tungsten doping can induce a phase transition in bilayer MoS2, changing its stacking order and optical properties, which could enable new device functionalities.

Contribution

It reveals a novel doping-induced phase transition in bilayer MoS2, combining spectroscopic and microscopy techniques to understand structural and optical changes.

Findings

W doping causes a transition from AA to AB' stacking in 2L MoS2.

Doped samples show vanishing SHG signal and stiffened LB mode.

W distribution correlates with structural phase change.

Abstract

Controlling the crystal phase of two-dimensional (2D) transition metal dichalcogenides (TMDs) is essential for tailoring their optical and electronic properties. While phase transitions in monolayer TMDs and semiconductor-to-metal conversions have been widely studied, structural transitions between semiconducting polytypes - particularly in bilayer (2L) systems - remain underexplored. Here, we demonstrate a W doping-induced phase transition from non-centrosymmetric AA stacking to centrosymmetric AB' in 2L MoS2 synthesized by chemical vapor deposition (CVD). Using polarization-resolved second harmonic generation (SHG) and low-frequency Raman spectroscopy, we identify a phase transition correlated with increasing tungsten (W) concentration. The dilute W-doped 2L system exhibits a vanishing SHG signal and a stiffening of the layer-breathing (LB) vibrational mode, in contrast to undoped samples with strong SHG and a softer LB mode. Aberration corrected scanning transmission electron microscopy (AC-STEM) demonstrates the spatial distribution of W concentration and associated structural changes. These findings highlight W-doping as an effective strategy for inducing phase transitions in 2L TMDs, opening new possibilities for engineered heterostructures, phase-controlled device applications or as a source of single photon emitters.