Atomic-scale studies of bending in two-dimensional materials using transmission electron microscopy

As advancements in technology push electronics to become smaller and thinner, the realization of flexible and deformable electronics—including wearable devices and sensors—have become highly sought after. Two-dimensional (2D) materials are uniquely suited for these applications, as they offer the flexibility of soft matter with the mechanical robustness and electronic performance of hard matter. In these systems, the bending stiffness—or the material’s resistance to deform out-of-plane or bend—is critical in governing the nanoscale deformations of 2D materials. However, conventional techniques to measure their bending stiffness have produced a broad range in values—spanning orders of magnitude—which appear to be in conflict. Until now, the bending mechanics of 2D materials spanning from low to high bending regimes has not been well understood. In this thesis, I will discuss three projects that investigate the underlying mechanics of nanoscale bending in 2D materials using scanning transmission electron microscopy. In our first work, we develop a scalable and widely applicable electron microscopy technique to measure the bending stiffness of 2D materials, multilayers, and heterostructures. In studying few-layer graphene, we discover a spread in bending stiffness that is inversely proportional to bending angle. We determine a new bending mechanism in 2D materials—governed by shear and slip between the layers—and present unifying model that may explain previous divergent measurements. Our findings demonstrate that 2D materials—even as multilayers—are among the most deformable electronic materials known. Realizing this shear-slip bending mechanism has profound implications on 2D heterostructures, in which we can freely stack individual monolayers—held together by weak van der Waals forces—to manipulate the interfacial interactions. We apply our electron microscopy technique on 2D bilayers and heterostructures comprised of graphene and MoS2, and we find that the bending angle dependence of bending stiffness can be suppressed by introducing misalignment between adjacent layers of a 2D stack, resulting in tuning of their bending stiffness by several hundred percent. From our computational studies, we determine that misaligned twisted and heterointerfaces are nearly frictionless, with interfacial sliding energy barriers that are two to three orders of magnitude lower than that of aligned homointerfaces. Finally, we develop an analytical model that predict and design the deformability of 2D heterostructures and how it evolves with the composition, interfacial arrangements, and geometry of the structure. These findings demonstrate design rules for tuning the bending stiffness of 2D materials through careful manipulation of stacking order and interfacial interactions. Finally, a critical challenge in 2D materials is to assess how does bending impact their performance in electronics. As a prime example, we fabricate buckles in multilayer α-In2Se3 to investigate how its out-of-plane polarization interplays with out-of-plane deformations. With combined electron microscopy and computational studies, we find that kink formation induces a structural transformation that results in polarization switching, and we determine a critical bending angle above which kinks are energetically favorable to occur. Lastly, we demonstrate control by transferring α-In2Se3 onto trenched substrates, designed to induce kink formation—and subsequent polarization switching—at specific locations. Our results simultaneously establish limitations in ferroelectric domain stability to bending in α-In2Se3, as well opportunities for new avenues of controlling the electrical polarization. Overall, the investigations in this thesis establish our understanding of mechanical and electronic properties under bending and lays the foundation for the development of 2D material-based flexible and deformable devices.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Materials Science & Engr

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Han, Edmund

Contributors dc:contributor

• Huang, Pinshane Y
• Cao, Qing
• van der Zande, Arend M
• Zuo, Jian-Min

• Scanning Transmission Electron Microscopy
• Two-dimensional Materials
• Graphene
• Mos2
• In2se3
• Bending Stiffness
• Ferroelectricity
• Nanomaterials

Statement dc:rights

• Copyright 2022 Edmund Han

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/117636

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F117636