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Towards designer strain distributions in two-dimensional materials
Date
2025
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
The first demonstration of graphene's superior electronic properties in 2004 sparked an extensive amount of research into the behavior of two-dimensional materials (2D) and their applications. As of the year 2022, over 6,000 types of 2D materials have been discovered and led to over 50,000 publications. Within this family can be found a wide range of materials, from insulators and semiconductors to ferromagnets and topological insulators, with applications across electronics, photonics, and quantum information science. ☐ Recent efforts have been devoted to expanding this diverse set of 2D material properties and applications even further. Among the developed methods, strain engineering has quickly become one of the most promising. The reduced dimensionality and superior mechanical properties of 2D materials enable significant modulation of the bandstructure via mechanical deformation, giving rise to a range of material-dependent effects, from increased carrier mobility to the generation of pseudomagnetic fields. While significant progress has been made in our capabilities to control strain in 2D materials, current methods do not exhibit the deterministic strain control required for many proposed devices. ☐ In this dissertation, we contribute to the ongoing efforts towards attaining deterministic control of strain in 2D materials by leveraging the in-plane strain that arises through out-of-plane deformation. We first introduce a fabrication method to produce silicon oxide nanostructures (probes) with angled sidewalls and deterministic placement to be used as tools in the local strain engineering of 2D materials. The probes are then implemented in the static and non-uniform strain engineering of 2D materials using patterned substrates. We focus on strain engineering of the van der Waals semiconductors gallium selenide (GaSe) and tungsten \ch{WS2} and find that careful design of the nanostructures enables precise control of the in-plane strain in locally-suspended regions transferred flakes. We further show that complex strain distributions can be predicted using finite element analysis and verified experimentally through micro-photoluminescence (PL) and Raman scattering mapping. ☐ We then present a novel platform for dynamically engineering local strain in suspended 2D materials via nano-indentation. Central to our approach is the design and fabrication of a silicon-on-insulator (SOI) based micro-spring (MS) with patterned nanoscale probes at its apex. While AFM probe-based indentation induces in-plane biaxial strain at the point of contact, control over the probe geometry introduces an additional degree of freedom to tune the local strain distribution. We demonstrate this concept using a ring-shaped probe to induce strain in suspended trilayer \ch{WS2} and use finite element analysis to understand the arising strain distributions. Simulations reveal that the ring probe induces nearly uniform biaxial strain across the ring diameter. Experimentally, the arising strain manifests as a measurable shift in the photoluminescence spectra and is found to be reversible. We further show that various strain distributions can be designed within the finite element framework and provide examples of probes that can be used to induce point-like, uniaxial, biaxial, and triaxial strain distributions. ☐ While the presented methodologies greatly advance our capabilities to engineer strain distributions in two-dimensional materials, we have only begun to understand the extent to which strain can be controlled deterministically using these platforms.
Description
"At the request of the author or degree granting institution, this graduate work is not available to view or purchase until April 27 2026"--ProQuest abstract/details page.
Keywords
Micro-spring, Nanocones, Strain engineering, Two-dimensional materials, Electronic properties