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Title:Impact of the van der Waals interface on the mechanics of 2D nanoelectromechanical systems
Author(s):Kim, Sunphil
Director of Research:van der Zande, Arend M
Doctoral Committee Chair(s):van der Zande, Arend M
Doctoral Committee Member(s):Huang, Pinshane Y; Ertekin, Elif; Aluru, Narayana R
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):NEMS, MEMS, 2D materials, van der Waals interface, nonlinearity
Abstract:Two-dimensional materials, such as graphene and MoS2, represent the ultimate limit of both electronic materials and mechanical atomic membranes due to their intrinsic molecular scale thickness. While 2D materials exhibit many useful properties, the most exciting phenomena and applications arise from the van der Waals interface. Electrically, the van der Waals interface enables the construction of heterostructures and molecular scale electronics. Mechanically, the van der Waals interface displays superlubricity or solitons depending on whether the interface is aligned. An important question is how the van der Waals interface affects the mechanical properties of 2D membranes. Answering this question is important to incorporating 2D heterostructure electronics into building mechanically active devices such as highly tunable nanoelectromechanical systems (NEMS) from suspended 2D membranes, stretchable electronics from crumpled 2D materials, and origami/kirigami nano-machines. In this thesis, we examine the mechanics of prototypical interfaces in 2D materials including commensurate (Bernal-stacked) bilayer graphene, incommensurate (twisted) bilayer graphene, and a graphene- MoS2 heterostructure (2D bimorph). The approach is to fabricate drumhead resonators from each material. The low mass of these atomically-thin drumhead resonators makes them exquisitely sensitive to changes in stiffness and dissipation, enabling us to directly probe the influence of the interface on the mechanics. In the first part of the thesis, we demonstrate NEMS based on 2D heterostructures made from 2D bimorph. From the resonance frequencies and eigenmodes measurements, we find that the 2D bimorphs show similar properties to monolayer or few layer materials, behaving as membranes with asymmetric in-plane tension. However, when measuring the electrostatic tuning of the resonators we observe distinct kinks in the tuning curve leading to strong softening of the frequency as well as an asymmetric tuning of the quality factor which are not observed in monolayer or few layer materials. We relate this new phenamena to interlayer slippage. In the second part, we use as-grown bilayer graphene patches to further study mechanics of van der Waals interface. The use of as-grown bilayer graphene ensures the interface is atomically clean. Raman spectroscopy reveals that some of the patches are Bernal-stacked while others are twisted. For Bernal-stacked bilayers, we are able to probe the dynamics of individual solitons and demonstrate that the sensitivity of 2D resonators are sufficient to probe the nonlinear mechanics of single dislocations in an atomic membrane. We observe the creation and destruction of individual solitons manifesting as stochastic jumps in the mechanical resonance frequency tuning. We develop a simple model relating the magnitude of the stress induced by soliton dynamics across length scales. For twisted bilayers, we present superlubric NEMS and model their dissipation for the first time. We propose interlayer frictional slip causes the additional dissipation in twisted bilayer graphene and present a model of structural damping to relate the additional dissipation to interlayer friction stress. Additionally, we measure the energy dissipation from 4 K to 300 K and find that the superlubric friction stress decreases with a decrease in temperature, giving the first measurements of the temperature dependence of superlubricity. In the last part of the thesis, we use the techniques that we developed for studying 2D mechanical resonators and apply them to linear phononic mechanical lattices as a phononic waveguide to explore how phonon propagates under buckling. The buckled phononic mechanical lattice displays an additional frequency band, that has not been observed in previously studied flat phononic lattices. By investigating temperature dependent frequency response, we show that the new band originates from buckling. In conclusion, our work reveals that the van der Waals interfaces strongly affect stress and dissipation of many multilayer 2D atomic membranes and provide a foundation to understand and directly probe the mechanics of other van der Waals interfaces in 2D heterostructures. The unique properties like interfacial slip or the ultra-low bending modulus in 2D heterostructure can bring new capabilities to NEMS such as slippable NEMS using van der Waals interface or integration of 2D membranes to current M/NEMS to obtain new functionalities. For studying mechanics of 1D phononic lattices, we find that small perturbations such as buckling significantly influence wave propagations: an opportunity for tailoring wave propagation in nano and micro sized phononic crystals and metamaterials.
Issue Date:2020-02-26
Rights Information:Copyright 2020 SUNPHIL KIM
Date Available in IDEALS:2020-08-26
Date Deposited:2020-05

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