[Paper Review] Variation of the fundamental band gap nature in curved two-dimensional WS2

Elena Blundo, Marco Felici+8 more|arXiv (Cornell University)|Oct 25, 2019

2D Materials and Applications51 references33 citations

TL;DR

The paper demonstrates a strain-induced direct-to-indirect band gap transition in curved WS2 monolayers created by proton irradiation and dome formation, mapped with micro-photoluminescence to link band-gap nature to mechanical stress.

ABSTRACT

We report a strain-induced direct-to-indirect band gap transition in mechanically deformed WS2 monolayers (MLs). The necessary amount of strain is attained by proton irradiation of bulk WS2 and the ensuing formation of one-ML-thick, H2-filled domes. The electronic properties of the curved MLs are mapped by spatially- and time-resolved micro-photoluminescence revealing the mechanical stress conditions that trigger the variation of the band gap character. This general phenomenon, also observed in MoS2 and WSe2, further increases our understanding of the electronic structure of transition metal dichalcogenide MLs and holds a great relevance for their optoelectronic applications.

Motivation & Objective

• Understand how mechanical deformation of WS2 monolayers affects the fundamental band gap nature.
• Link local strain conditions to changes from direct to indirect band gaps in two-dimensional WS2.
• Extend insight to related transition metal dichalcogenides and their optoelectronic implications.

Proposed method

• Create curved WS2 monolayers via proton irradiation of bulk WS2 followed by formation of H2-filled domes.
• Map electronic properties of curved monolayers using spatially and time-resolved micro-photoluminescence.
• Correlate observed band-gap changes with mechanically induced stress conditions.
• Compare behavior with similar phenomena in MoS2 and WSe2 to generalize the effect in TMD monolayers.

Experimental results

Research questions

• RQ1 Can mechanical curvature and local strain induce a direct-to-indirect band gap transition in WS2 monolayers?
• RQ2 What are the stress conditions that trigger the band-gap nature variation in curved WS2?
• RQ3 Is the strain-induced behavior observed in WS2 also present in related TMD monolayers like MoS2 and WSe2?
• RQ4 How can spatially resolved photoluminescence map the relationship between curvature, strain, and optoelectronic properties?

Key findings

• Strain from mechanical deformation can trigger a direct-to-indirect band gap transition in WS2 monolayers.
• Spatially and time-resolved micro-photoluminescence maps reveal the correlation between mechanical stress and band-gap character changes.
• The phenomenon is reported to be general to similar TMD monolayers (MoS2 and WSe2) and relevant for optoelectronic applications.
• Curved 2D WS2 monolayers exhibit altered electronic properties tied to their curved geometry and internal stress.