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Title:Mechanical self-assembly of deformed 2D materials for advanced functionalities
Author(s):Leem, Juyoung
Director of Research:Nam, SungWoo
Doctoral Committee Chair(s):Nam, SungWoo
Doctoral Committee Member(s):Murphy, Catherine J; Mason, Nadya; Cai, Lili
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):2D Materials
Mechanical self-assembly
Abstract:Introducing three-dimensionality to two-dimensional (2D) materials opened new possibilities and novel applications in material system design. Extraordinary intrinsic properties of 2D materials have attracted scientific research communities, and various 2D material – based structures and devices have been proposed and demonstrated. Given the fascinating electrical, optical, and thermal properties of 2D materials, constructing three-dimensional (3D) structures out of 2D materials would make 2D systems even more interesting by providing means of extrinsic modulation of material properties of 2D materials. 2D materials frequently undergo out-of-plane deformation because the atomic thinness of 2D materials leads to rippling, wrinkling, and folding at the surface during synthesis and transfer processes. Those out-of-plane deformations are neither organized nor controllable structures, and therefore, they have often been considered inevitable defects. However, recent studies have reported tunable electrical, chemical, optical, and mechanical properties when a controlled in-plane strain gradient is applied to 2D lattice structures. This implies that such out-of-plane deformations with structural control can be useful tools for material engineering. Therefore, we seek ways to manipulate the deformation of 2D materials to modulate material properties for advanced functionalities. One of our strategies to architecture 2D materials is inspired by fine wrinkle formation that occurs when there is a strain mismatch between a thin film and an underlying substrate. We use extreme-case strain mismatch, often up to 300 percent or more. In addition, we control the direction of prestrains to create uniaxially or biaxially crumpled 2D materials with a uniaxial prestrain or biaxial prestrains, respectively. The resulting structure is a mechanically self-assembled, buckle-delaminated structure with delocalized crumples and inhomogeneous strain in the crumpled 2D lattice. Furthermore, we create mixed-dimensional structures by combining our architecturing strategy with unconventional crack lithography. Through the crack lithography – inspired strategy, heterogeneous 2D-3D mixed-dimensional structures are formed with localized crumples. These strategies are universally applicable to 2D materials and 2D material – based hybrid structures, such as graphene, molybdenum disulfide (MoS2), and graphene – gold nanoparticles (Au NPs) hybrid structures. We further explore advanced functionalities of buckle-delaminated crumpled structures of 2D materials and 2D materials – based hybrid structures. The crumpled graphene – Au NPs hybrid structure demonstrates advanced functionality based on the topography of deformed 2D materials. In the hybrid structure, Au NPs are formed on a graphene surface, through thermal dewetting of gold thin film. The graphene is then deformed into a 3D crumpled structure. The crumpled structure effectively enhances localized electromagnetic fields between adjacent Au NPs by reducing the gap between Au NPs and helps utilize the 3D focal volume of the incident laser. Therefore, the crumpled hybrid structure – based surface-enhanced Raman spectroscopy (SERS) sensor exhibits an order of magnitude higher sensitivity, compared to a flat hybrid structure – based SERS sensor. Additionally, the crumpled hybrid structure – based SERS sensor can be easily applied on an arbitrary curvilinear surface demonstrating its potential for in situ SERS assays. We further applied the crumpled hybrid structure of graphene – Au NPs to create a photodetector with plasmonically enhanced photoresponsivity and high stretchability. Gold nanoparticles in the hybrid structure effectively enhance photoresponsivity, and further enhancement is achieved by material densification by crumpling the hybrid structure. As a result, we demonstrate 1200% enhanced photoresponsivity over a flat graphene device by combining the plasmonic effect and material densification. The crumpled structure also provides an exceptional 200% stretchability. The fabricated structure is mechanically robust, with no failure observed after 1000 cycles of stretching and releasing. The deformed 2D material system also provides a material platform for strain engineering of 2D materials. We created a 3D crumpled structure with a semiconducting 2D material, MoS2, to create inhomogeneous strain in the MoS2 lattice. This continuous strain change across the deformed structure induces a bandgap energy gradient, and therefore, provides efficient transport paths for photoexcited excitons in the deformed 2D lattice. We further demonstrated dynamic photoresponsivity modulation by structural modulation of a deformed 2D material, implying exciton drift modulation in a crumpled and flattened lattice structure. In conclusion, we have demonstrated effective strategies to create deformed 2D material systems with various levels of structural complexity, and related applications utilizing the unique topography and the strain gradient simultaneously created in the deformed lattice. We believe our approach to deforming 2D materials and achieving advanced functionalities contribute to the related research communities by demonstrating a way to extrinsically manipulate 2D materials' topography, and therefore modulate intrinsic properties.
Issue Date:2020-02-11
Rights Information:Copyright 2020 Juyoung Leem
Date Available in IDEALS:2020-08-27
Date Deposited:2020-05

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