Mapping 1D Confined Electromagnetic Edge States in 2D Monolayer Semiconducting MoS₂ Using 4D-STEM

Area of Science:

• Materials Science
• Condensed Matter Physics
• Surface Science

Background:

• Two-dimensional (2D) materials like molybdenum disulfide (MoS2) possess unique electronic properties influenced by their edges.
• Understanding edge atomic structure and electric fields is crucial for optimizing 2D material applications.
• Conventional imaging techniques struggle to resolve atomic-scale features and electric fields at material edges.

Purpose of the Study:

• To investigate the atomic-scale electric fields at the edges of 2D semiconducting monolayer MoS2.
• To characterize the nature and spatial variations of these edge electric fields.
• To correlate experimental observations with theoretical calculations.

Main Methods:

• Utilizing four-dimensional (4D) scanning transmission electron microscopy (STEM) for high-resolution electric field mapping.
• Employing integrated differential phase contrast (iDPC) reconstructions to identify low Z number atoms.
• Performing density functional theory (DFT) calculations to model edge states and electric fields.

Main Results:

• Observed sub-nanometer 1D features in electric field maps at MoS2 monolayer edges (zigzag and MoS-terminated).
• Detected atomic-scale oscillations in the 1D electromagnetic edge state magnitude, dependent on edge reconstructions.
• Identified low Z number atoms at edges, often missed by conventional annular dark field STEM.
• DFT calculations confirmed periodic 1D edge states and electric field enhancement at certain terminations.

Conclusions:

• 4D-STEM is a powerful tool for mapping atomic-scale electric fields and structural fluctuations at 2D material edges.
• The observed electric fields arise from unique atomic configurations at the edges, differing from bulk bonding.
• Results provide insights into edge termination effects on electronic properties of 2D materials.