Visualizing Grain Statistics in MOCVD WSe₂ through Four-Dimensional Scanning Transmission Electron Microscopy

Area of Science:

• Materials Science
• Solid State Physics
• Nanotechnology

Background:

• Tungsten diselenide (WSe2) is a crucial 2D material with applications in electronics and optoelectronics.
• Controlling grain structure in WSe2 films is essential for device performance.
• Metal organic chemical vapor deposition (MOCVD) is a common technique for growing 2D materials like WSe2.

Purpose of the Study:

• To develop and apply a method for visualizing grain boundaries in MOCVD-grown WSe2.
• To investigate the relationship between grain structure and material properties.
• To understand the nucleation mechanism of new WSe2 layers.

Main Methods:

• Four-dimensional scanning transmission electron microscopy (4D-STEM) was employed for high-resolution imaging.
• Ion beam analysis was used to quantify elemental composition and identify deficiencies.
• Image reconstruction techniques were applied to create orientation maps.

Main Results:

• Direct visualization of grains and grain boundaries in WSe2 grown on silicon dioxide.
• Observation of a high density of small grains, correlating with measured selenium deficiency.
• Orientation maps revealed triangular domains with consistent orientation, inducing tensile strain and affecting lattice parameters.

Conclusions:

• Selenium deficiency in MOCVD-grown WSe2 contributes to a high density of small grains.
• The nucleation of new layers is influenced by the orientation of underlying layers, leading to strain.
• 4D-STEM provides critical insights into the microstructural properties of 2D materials.