Designable excitonic effects in van der Waals artificial crystals with exponentially growing thickness

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Two-dimensional (2D) transition metal dichalcogenides (TMDCs) display unique optical properties due to excitonic effects when exfoliated into monolayers.
• Reassembling these 2D TMDCs into bulk excitonic crystals can enhance optical performance and enable new optoelectronic and valleytronic applications.
• Controlling 2D excitonic properties within bulk structures or superlattices remains a significant challenge.

Purpose of the Study:

• To develop a precise method for fabricating artificial excitonic crystals with tunable layer numbers.
• To investigate the retention and manipulation of monolayer-like exciton properties in multi-layered structures.
• To explore enhanced optical absorption, photoluminescence, and interlayer exciton emission in engineered vdW structures.

Main Methods:

• Introduced the "2^N method" for constructing m∙2^N-layer artificial excitonic crystals using a minimal number of stacking operations.
• Fabricated a millimeter-scale, 16-layer MoS₂ single crystal with zero interlayer twist angle.
• Constructed a WSe₂/(MoS₂/WSe₂)₃/MoS₂ superlattice from monolayer WSe₂ and MoS₂.

Main Results:

• The 16-layer MoS₂ crystal retained monolayer-like exciton properties, showing up to 643% and 646% enhancement in absorption and photoluminescence (PL), respectively.
• The fabricated superlattice exhibited up to a 400% intensity increase in quadrupolar interlayer exciton (IX) emission compared to its bilayer counterpart.
• Demonstrated the successful bottom-up fabrication of complex vdW structures with significantly boosted optical functionalities.

Conclusions:

• The "2^N method" offers a promising approach for designing and fabricating advanced excitonic crystals.
• This technique facilitates the exploration of excitonic physics in complex van der Waals (vdW) heterostructures.
• The engineered crystals show potential for next-generation optoelectronic and valleytronic devices.