Identification of Polytypism and Their Dislocations in Bilayer MoS2 Using Correlative Transmission Electron Microscopy and Raman Spectroscopy

TL;DR

This study combines transmission electron microscopy and Raman spectroscopy to accurately identify polytypism and dislocations in bilayer MoS2, revealing phase transitions and enabling defect engineering in 2D materials.

Contribution

It introduces a correlative TEM-Raman methodology for distinguishing stacking sequences and dislocations in 2D materials, demonstrating its effectiveness on bilayer MoS2.

Findings

Identified perfect and partial dislocations linked to stacking sequences.

Validated polytypes using 3D electron diffraction and Raman spectroscopy.

Discovered phase transition during mechanical exfoliation of bulk crystal.

Abstract

Stacking orders and topological defects substantially influence the physical properties of 2D van der Waals (vdW) materials. However, the inherent features of 2D materials challenge the effectiveness of single characterization techniques in identifying stacking sequences, necessitating correlative approaches. Using bilayer MoS2 as a benchmark, we differentiate its polytypism and specific dislocations through transmission electron microscopy (TEM) and Raman spectroscopy. Perfect and partial dislocations were revealed in TEM, which are closely linked to the stacking sequences, thus indirectly indicating the 2H and 3R polytypes. 3D electron diffraction reconstruction on relrods and low-frequency Raman spectroscopy further validated these polytypes owing to their reliance on crystal symmetry. Surprisingly, we unexpectedly resolved both polytypes despite starting with 2H bulk crystal, pointing to a possible phase transition during mechanical exfoliation. The correlative TEM-Raman approach can be extended to other 2D materials, paving the way for property alteration via stacking and defect engineering.