Synthesis-Related Nanoscale Defects in Mo-Based Janus Monolayers Revealed by Cross-Correlated AFM and TERS Imaging

Area of Science:

• Materials Science
• Nanotechnology
• Surface Science

Background:

• Two-dimensional (2D) Janus transition metal dichalcogenides (TMDs) are essential for advanced applications like non-linear optics, energy harvesting, and catalysis.
• Precise nanoscale characterization of Janus TMDs is vital for optimizing synthesis and ensuring high-performance device applications.
• Current synthesis methods often result in defects, necessitating detailed morphological and compositional analysis.

Purpose of the Study:

• To investigate the influence of growth substrate and precursor material on the nanoscale morphology and composition of 2D Janus TMDs.
• To identify and characterize nanoscale defects and heterostructures within synthesized Janus TMD monolayers.
• To demonstrate the efficacy of combined atomic force microscopy (AFM) and tip-enhanced Raman spectroscopy (TERS) for nanoscale analysis of Janus TMDs.

Main Methods:

• Synthesis of 2D Janus TMD monolayers via hydrogen plasma-assisted chemical conversion of MoSe₂ and MoS₂.
• Cross-correlated atomic force microscopy (AFM) for high-resolution morphological and topographical imaging.
• Tip-enhanced Raman spectroscopy (TERS) for nanoscale chemical composition and structural analysis.

Main Results:

• The choice of growth substrate and starting TMD material significantly influences residual strain, dictating the nanoscale morphology of Janus TMDs.
• TERS imaging revealed the presence of ≈20 nm nanoscale islands of MoSe₂-${\mathrm{Mo}}_{\mathrm{Se}}^S$ and MoS₂-${\mathrm{Mo}}_S^{\mathrm{Se}}$ vertical heterostructures.
• These heterostructures originate from bilayer nanoislands present in the precursor monolayer crystals.

Conclusions:

• Understanding the origins of nanoscale defects in Janus TMDs is key to optimizing synthesis for uniform, defect-free materials.
• Cross-correlated AFM and TERS imaging provides a powerful and accessible method for detailed nanoscale characterization of Janus TMD monolayers.
• This approach facilitates the development of high-quality Janus TMDs for next-generation electronic and photonic devices.