Exciton mapping at subwavelength scales in two-dimensional materials

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Understanding interfaces in layered materials like MoS2 and MoSe2 is crucial for their electronic and optical properties.
• Traditional optical methods lack the spatial resolution to probe nanoscale phenomena at these interfaces.
• Electron-energy-loss spectroscopy (EELS) offers a pathway to investigate electronic excitations with high spatial resolution.

Purpose of the Study:

• To investigate excitonic properties at diffuse interfaces between MoS2 and MoSe2 single layers with nanometer-scale resolution.
• To correlate excitonic signatures with chemical composition and interface characteristics.
• To demonstrate the capability of spatially resolved EELS for mapping optical band gaps at the nanoscale.

Main Methods:

• Spatially resolved electron-energy-loss spectroscopy (EELS) using a monochromated electron source (20 meV).
• Acquisition of low-loss spectra to probe excitons and core-loss spectra to determine chemical composition.
• Mapping of excitonic signatures and optical band gap across the MoS2-MoSe2 interface with nanometer precision.

Main Results:

• Exciton maps reveal significant variations over separations as small as 10 nm.
• Optical band gap measurements achieved with nanometer-scale resolution, significantly smaller than photon wavelengths.
• Variations in excitonic signatures directly correlate with changes in chemical composition.
• Broader exciton peaks observed at interfaces and heterogeneous regions, attributed to roughness and alloying.
• Absence of significant exciton peak shifts across the interface, suggesting interface width is comparable to the exciton Bohr radius.

Conclusions:

• Spatially resolved EELS is a powerful technique for probing nanoscale excitonic behavior at material interfaces.
• The study demonstrates the ability to map optical band gaps with unprecedented spatial resolution.
• Interface properties like roughness and alloying significantly influence excitonic behavior in layered heterostructures.