Meshal Alawein

EECS Department, University of California, Berkeley

Technical Report No. UCB/EECS-2025-197

December 16, 2025

http://www2.eecs.berkeley.edu/Pubs/TechRpts/2025/EECS-2025-197.pdf

What happens when you squeeze a 2D semiconductor beyond its comfort zone? In this thesis, I demonstrate that mechanical rippling induced by controlled compression can carve out flat valence bands in 2D group-VI TMD monolayers (MoS2, WS2, MoSe2, and WSe₂), transforming them into self-assembled lateral heterostructures with nothing more than a simple squeeze. With first-principles calculations in hand, I follow the cascade that links mechanical rippling to electronic flat bands. Under compression, the monolayer's buckling instabilities spawn ripples that reconfigure the band structure, shifting $E_K$ downward and $E_F$ upward (the latter sitting between a nearly flat segment along $\Gamma X$ and the precisely flat line $\Gamma Y$). As the ripple amplitude grows further, a critical point is reached where $E_F$ overtakes $E_K$ and the valence band maximum relocates to $\Gamma$, making the gap indirect and signaling the birth of massive holes and a new electronic identity. This mechanical squeezing accomplishes what typically requires elaborate stacking procedures or precise twist-angle engineering. The periodic ripples effectively carve a built-in lateral heterostructure within a single, chemically homogeneous sheet. The curvature-driven transformation creates a striking electronic asymmetry: holes become pinned in high-curvature regions with dramatically enhanced effective masses, while electrons remain free to roam a quantum-mechanical sorting that emerges purely from geometry. This ripple-induced carrier segregation opens new possibilities for exciton trapping, hole-based electronics, and the study of correlated electronic phases in 2D systems.

Advisors: Ali Javey

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BibTeX citation:

@phdthesis{Alawein:EECS-2025-197,
    Author= {Alawein, Meshal},
    Title= {Strain-Induced Flat Bands and Lateral Heterostructures: Hole Localization in Rippled Monolayer TMDs},
    School= {EECS Department, University of California, Berkeley},
    Year= {2025},
    Month= {Dec},
    Url= {http://www2.eecs.berkeley.edu/Pubs/TechRpts/2025/EECS-2025-197.html},
    Number= {UCB/EECS-2025-197},
    Abstract= {What happens when you squeeze a 2D semiconductor beyond its comfort zone? In this thesis, I demonstrate that mechanical rippling induced by controlled compression can carve out flat valence bands in 2D group-VI TMD monolayers (MoS2, WS2, MoSe2, and WSe₂), transforming them into self-assembled lateral heterostructures with nothing more than a simple squeeze. With first-principles calculations in hand, I follow the cascade that links mechanical rippling to electronic flat bands. Under compression, the monolayer's buckling instabilities spawn ripples that reconfigure the band structure, shifting $E_K$ downward and $E_F$ upward (the latter sitting between a nearly flat segment along $\Gamma X$ and the precisely flat line $\Gamma Y$). As the ripple amplitude grows further, a critical point is reached where $E_F$ overtakes $E_K$ and the valence band maximum relocates to $\Gamma$, making the gap indirect and signaling the birth of massive holes and a new electronic identity. This mechanical squeezing accomplishes what typically requires elaborate stacking procedures or precise twist-angle engineering. The periodic ripples effectively carve a built-in lateral heterostructure within a single, chemically homogeneous sheet. The curvature-driven transformation creates a striking electronic asymmetry: holes become pinned in high-curvature regions with dramatically enhanced effective masses, while electrons remain free to roam a quantum-mechanical sorting that emerges purely from geometry. This ripple-induced carrier segregation opens new possibilities for exciton trapping, hole-based electronics, and the study of correlated electronic phases in 2D systems.},
}

EndNote citation:

%0 Thesis
%A Alawein, Meshal 
%T Strain-Induced Flat Bands and Lateral Heterostructures: Hole Localization in Rippled Monolayer TMDs
%I EECS Department, University of California, Berkeley
%D 2025
%8 December 16
%@ UCB/EECS-2025-197
%U http://www2.eecs.berkeley.edu/Pubs/TechRpts/2025/EECS-2025-197.html
%F Alawein:EECS-2025-197