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Title:Strain and interfacial engineering of 2D materials
Author(s):Cho, Chullhee
Director of Research:Nam, SungWoo
Doctoral Committee Chair(s):Nam, SungWoo
Doctoral Committee Member(s):Aluru, Narayana R; Cai, Lili; Park, Cheol
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):Strain engineering
Interfacial engineering
2D materials
Flexible/wearable electronics
optoelectronics
Abstract:Deformable device technology with advanced functionalities spanning from wearable/personal electronics to flexible bio-implantable sensors, which require both static performance stability and dynamic sensitivity, has been ever-evolving with the substantial development in the field of two-dimensional (2D) materials. 2D materials are atomically thin, layered crystalline materials that possess strong intralayer (in-plane) covalent bond and weak interlayer (out-of-plane) van der Waals interaction resulting in extraordinary properties including high mechanical strength (in-plane) and extremely low bending stiffness (out-of-plane). Distinct from 3D bulk materials, the reduced dimensionality and the spatial confinement of 2D materials result in unique characteristics such as excellent electrical transport behavior, strong light-matter interaction, and vanishing flexural rigidity at the ultimate thickness limit, which will permit the achievement of next-generation flexible electronics beyond the capability of conventional semiconductor technology. Therefore, 2D materials have demanded substantial attention from the scientific community for both fundamental quantum physics/mechanics and device-level applications. Because of the exceptional mechanical strength and intrinsic flexibility, however, 2D materials can be strongly affected by external influences. Mechanical deformations, especially out-of-plane deformation, are commonly observed with 2D materials-based systems in the forms of intrinsic corrugation, wrinkles, delaminated buckles or crumples, and other non-linear complex deformations in an uncontrolled manner, which have been considered as inevitable topological defects. Furthermore, due to weak interlayer interactions of 2D materials via van der Waals interactions with surrounding environments, structural instability of delamination at the interfaces between 2D materials and substrate surfaces can occur. These mechanical instabilities are often considered as parameters deteriorating the mechanical integrity and functional performances of 2D materials-based systems. Contrary to such conventional wisdom, I have sought ways to leverage mechanical instabilities of 2D materials to advance functionalities of 2D materials-based systems including electrical sustainability, excitonic characteristics, energy generation, and novel optoelectronic phenomena by manipulating interfacial characteristics (interfacial engineering) and mechanical deformation (strain-engineering) of the constituent 2D material layers. First, I discussed a readily adaptable approach of inserting 2D materials to thin-film metal based flexible electrodes (2D-interfacial engineering) where we achieved several orders-of-magnitude enhanced strain resilient electrical functionality (which we termed ‘electrical ductility’) by manipulating mechanical fracture behaviors of thin-film metals from rapid straight cracking to progressive tortuous cracking via a buckle-guided fracture mechanism induced by 2D materials at the interface. I demonstrated that our 2D-interfacial engineering is not limited to a certain combination of metals and 2D materials, which can be incorporated into the existing flexible/wearable electronics applications. Next, I demonstrated how the wrinkling deformation of artificially stacked 2D materials via strain engineering affects the optical characteristics of interlayer excitons in heterobilayer system where the effect of mechanical strain remains relatively uncharacterized. We observed highly strain-tunable interlayer excitons with non-monotonic photoluminescence characteristics in MoS2/WSe2 heterobilayer. I further provided an insight on the competition between in-plane strain and out-of-plane interlayer coupling effects on the photoluminescence characteristics, which can be an additional tuning knob to manipulate excitonic behaviors in 2D-multilayered systems. As an extension of the strain effect on the heterobilayer system, I explored how crumpling deformation of 2D materials affects both mechanical (the effective stiffness) and electromechanical (piezoelectricity) properties. Our results suggest that the effective elastic modulus can be reduced for the controlled crumpled heterostructure where the effective modulus decreased as the aspect ratio of formed crumples decreased. I further demonstrated that the crumpled MoS2/graphene heterostructures can be utilized as an effective active layer for mechanical-to-electrical energy conversion under both instantaneous and continuous strain-driven modes of stretching, bending, acoustic and mechanical vibrations. Finally, I introduced a strain-control platform to create freestanding wrinkled structures of monolayer 2D materials for the first time, offering exciting opportunities to further investigate the fundamental strain-tunability of various materials properties in 2D materials. In particular, I showed the spatial modulation of photo-induced force (near-field dipole-dipole interactions) in freestanding wrinkled 2D materials, which we attributed to a combination of the in-plane strain-induced piezoelectric effect and the out-of-plane strain gradient-induced flexoelectric effect. In conclusion, I demonstrated that controlling the mechanical deformation of 2D materials permits emergent functionalities ranging from the nanoscale to macroscale including electrical ductility, controllable excitonic behaviors, and even energy generation. Furthermore, since the deformation of 2D materials can directly manipulate bond length and angle in the atomic lattice, deformation can tune the electronic structure and interfacial characteristics. Thus, strain and interfacial engineering can serve as effective strategies to explore advanced functionalities of 2D materials. I believe our approaches to manipulate interfacial characteristics and deformation of 2D materials-based systems, as well as in a freestanding form, contribute to the larger research community by opening up exciting opportunities to study fundamental strain physics and to advance multifunctional deformable device technologies including robust flexible electronics, novel excitonic sensors, and self-powering wearable/implantable devices for health and structural monitoring.
Issue Date:2021-12-01
Type:Thesis
URI:http://hdl.handle.net/2142/114083
Rights Information:©2021 by Chullhee Cho. All rights reserved
Date Available in IDEALS:2022-04-29
Date Deposited:2021-12


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