Terahertz Surface Emission from MoSe₂ at the Monolayer Limit.

Area of Science:

• Materials Science
• Condensed Matter Physics
• Surface Science

Background:

• Two-dimensional materials like Molybdenum Diselenide (MoSe2) exhibit unique surface properties crucial for electronic applications.
• Understanding surface charge dynamics is essential for characterizing and optimizing these materials.
• Terahertz (THz) spectroscopy offers a non-destructive method for probing surface phenomena.

Purpose of the Study:

• To directly analyze the surface-charge region of bulk and monolayer MoSe2.
• To elucidate the contributions of different mechanisms to THz surface emission.
• To establish THz spectroscopy as a tool for surface characterization of 2D materials.

Main Methods:

• Utilizing terahertz (THz) surface emission spectroscopy.
• Analyzing both bulk and monolayer MoSe2 samples.
• Differentiating contributions from surface nonlinear optical polarization and surface field-induced photocurrent.

Main Results:

• Both surface optical rectification and photogenerated carrier acceleration by surface depletion fields contribute to THz emission.
• Surface optical rectification contribution is similar for bulk and monolayer MoSe2 due to surface symmetry.
• Surface field-induced photocurrent contribution is significantly higher in bulk MoSe2 (94.2%) compared to monolayer MoSe2 (74.5%).
• Bulk MoSe2 exhibits a larger surface depletion field (2.54 × 10^7 V/m) than monolayer MoSe2 (5.42 × 10^5 V/m).
• Total THz emission from bulk MoSe2 is approximately four times greater than from monolayer MoSe2.

Conclusions:

• The THz radiation mechanism in MoSe2 crystals is clearly identified.
• Terahertz spectroscopy provides a valuable technique for the non-destructive surface characterization of two-dimensional materials.
• Differences in surface depletion fields significantly impact THz emission intensity between bulk and monolayer MoSe2.