Electronic Structure, Surface Doping, and Optical Response in Epitaxial WSe2 Thin Films

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Transition metal dichalcogenides (TMDCs) like tungsten diselenide (WSe2) are promising 2D materials with unique electronic and optical properties.
• Bilayer graphene (BLG) offers a versatile platform for constructing heterostructures with tailored functionalities.

Purpose of the Study:

• To grow high-quality WSe2 films on BLG with precise thickness control.
• To investigate the atomic, electronic, and optical properties of WSe2/BLG heterostructures.
• To explore methods for tuning the electronic properties of WSe2.

Main Methods:

• Molecular beam epitaxy (MBE) for layer-by-layer WSe2 film growth on BLG.
• Angle-resolved photoemission spectroscopy (ARPES) to probe electronic band structure.
• Scanning tunneling microscopy/spectroscopy (STM/STS) for atomic and electronic imaging.
• Optical absorption spectroscopy to analyze optical response.

Main Results:

• WSe2 films grown on BLG exhibit layer-dependent electronic structures.
• Bilayer WSe2 in a heterostructure acts as a direct bandgap semiconductor, shifting the crossover to trilayer WSe2.
• Monolayer WSe2 shows a record spin-splitting of 475 meV at the K point.
• Exciton binding energy in monolayer WSe2/BLG is 0.21 eV, higher than 3D semiconductors but lower than some other 2D TMDCs.
• Alkali metal surface doping effectively modifies the electronic structure of WSe2/BLG.

Conclusions:

• Precise control over WSe2 film thickness on BLG enables tuning of its electronic properties.
• WSe2/BLG heterostructures present unique semiconductor characteristics and significant spin-valley coupling.
• Alkali metal doping provides a viable pathway for further engineering the electronic behavior of these 2D materials.