Heterostrain‐Driven Bandgap Increase in Twisted WS₂: A Nanoscale Study

Abstract

Twisted 2D materials present an enticing platform for exploring diverse electronic properties owning to the tunability of their bandgap energy. However, the intricate relationship between local heterostrain fields, thickness, and bandgap energy remains insufficiently understood, particularly at the nanoscale. Here, it presents a comprehensive nanoscale study elucidating the remarkable sensitivity of the bandgap energy to both thickness and heterostrain fields within twisted WS2 nanostructures. This approach integrates electron energy‐loss spectroscopy (EELS) enhanced by machine learning with 4D scanning transmission electron microscopy (STEM). Through this synergistic methodology, enhancements up to 20% in the bandgap energy is unveiled depending on the specimen thickness. This phenomenon is traced back to sizable deformation angles present within individual layers, which can be directly linked to distinct variations in local heterostrain fields. The findings represent a significant advancement in comprehending the electronic behavior of twisted 2D materials and introduce a novel methodological framework with far‐reaching implications for twistronics and the investigation of other materials within the nanoscience domain.

4D‐STEM Nanoscale Strain Analysis in van der Waals Materials: Advancing beyond Planar Configurations

“…Especially when coupled with spatially-resolved measurements like the band gap and the dielectric function procured from electron energy loss spectroscopy (EELS) on the same specimen, as demonstrated for the case of internally twisted WS 2 nanostructures. [43] The corresponding software framework, dubbed StrainMAPPER, is now available to the global community via its GitLab repository, complemented with comprehensive documentation and illustrative examples. We anticipate that this method will become a valuable resource for scientists aiming to understand the implications of nanoscale strain fields on the properties of the materials under study, a key step in bridging the gap between fundamental vdW material science and their implementation in technological applications.…”

“…This NBED pattern is taken from a internally twisted WS 2 bulk crystal. [43] The intensity distribution in such an NBED pattern directly relates to the crystal lattice parameters. However, this pattern is also susceptible to variations in the specimen thickness and to the tilt or orientation of the specimen, as can be seen in Figure 1c.…”