Deformable two-dimensional materials: From strain engineerable properties to strain resilient electronics

Hossain, Mohammad Abir

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https://hdl.handle.net/2142/120490 Copy

Description

Title

Deformable two-dimensional materials: From strain engineerable properties to strain resilient electronics

Author(s)

Hossain, Mohammad Abir

Issue Date

2023-02-03

Director of Research (if dissertation) or Advisor (if thesis)

van der Zande, Arend M

Doctoral Committee Chair(s)

van der Zande, Arend M

Committee Member(s)

• Miljkovic, Nenad
• Ertekin, Elif
• Huang, Pinshane Y

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• 2D materials
• Strain engineering
• Mechanical deformation, van der Waals interfaces, Optoelectronic property

Abstract

Our current electronics technology is primarily based on silicon that is suitable for rigid electronic devices and offers incredible computational capabilities. However, further progress critically depends on the continuous scaling and performance improvement of Si transistors, which is becoming increasingly challenging due to reduced channel control. Layered two-dimensional (2D) van der Waals materials are very promising to overcome the challenges of Si based logic devices because of their excellent and unique physical properties even with thickness at the atomic scale (<1 nm). Moreover, new technologies like wearable electronics demand for materials capable of deforming or being shaped into complex 3D form factors, while still maintaining high electronic mobility. Due to their low bending stiffness and high fracture strength, 2D materials are highly deformable, and high strain sensitivity makes them ideal candidates for strain engineering to manipulate their electronic structure and material properties. On the other hand, the ability to curve out-of-plane via crumpling or patterned origami folds enables 2D materials to minimize or control strain transfer even under very large substrate deformations. Nevertheless, these 3D deformation strategies on soft substrates cannot be integrated into traditional CMOS devices. So, there is also opportunity to introduce CMOS compatible approaches for strain engineering in 2D monolayers and heterostructures. This thesis is focused on understanding the coupling between mechanical deformation, strain and optoelectronic properties of 2D materials and heterostructures, and how can we leverage this deformability to engineer desired material properties or new device applications. The first part of the thesis is focused on understanding the interfacial interaction between 2D materials/ heterostructures (i.e., graphene, transitional metal dichalcogenides or TMDCs) and elastomer substrates under compression, resulting in either crumpling through buckle delamination, or conformal wrinkling. By bending out of plane, 2D membranes minimize the compressive strain from the substrate. However, we learned that interfacial mechanics between the 2D material and soft elastomer substrate plays a key role in determining the resulting morphology and residual strain transferred to the membrane. A low adhesion interface leads to delaminated buckling of 2D membrane that creates crumples and folds. On the other hand, high adhesion generates conformally wrinkled 2D membrane. In order to study how strain is transferred to the 2D membrane under 3D deformation, we transferred graphene-TMDC (MoS2, WSe2) heterostructure and WSe2-Al2O3 structure on prestretched elastomer substrates. Upon release, the substrate applies compressive strain and these 2D membranes undergo out of plane mechanical deformations. Topography analysis using atomic force microscopy shows that the graphene-TMDC heterostructure creates crumpling via buckle delamination whereas WSe2-Al2O3 structure creates conformal wrinkling. Using photoluminescence (PL) spectroscopy, we characterized the strain transfer from substrate into these deformed membranes. In crumpled structure, PL shifts by <2 meV between flat and 15% biaxial crumpling, corresponding to a change in strain <0.05%. On the other hand, wrinkling introduces a periodic modulation of the bandgap by 47 meV, corresponding with strain modulation from 0.67% tensile strain at the wrinkle crest to 0.31% compressive strain at the trough. Moreover, cycling the substrate strain mechanically reconfigures the magnitude and direction of wrinkling and resulting band tuning. In order to demonstrate strain resilient application, we implemented the crumpled graphene-TMDC heterostructure as a phototransistor where graphene works as contact and monolayer TMDC acts as channel, and measured the photocurrent. The photoresponsivity scales as P–0.38 and reaches 20 A/W under an illumination power density of 4 μW/cm2 at 20 V bias, a performance comparable to flat photosensors. Using scanning photocurrent microscopy, we observed that the photoresponsivity increases by only 20% after crumpling. Both the PL and photoresponse confirm that crumpling and delamination prevent the buildup of compressive strain leading to highly deformed materials and devices with similar performance to their flat analogs. Our results from out of plane deformation show that engineering the interfacial mechanics between 2D material and soft substrate leads to controlled strain transfer with applications in strain resilient electronics or heterogeneous strain engineering. These results pave the way towards stretchable multifunctional devices based on deformed 2D materials. While crumpling and wrinkling techniques are great for stretchable electronics/strain tuning the material properties, they are difficult to implement in actual logic devices due to dimensional limitations and device integration issues. So, we need a method to induce mechanical deformation in 2D monolayers and heterostructures that can enable strain engineering of material properties, while maintaining compatibility with traditional silicon-based devices and circuits. In this part of the thesis, we show a strain engineering technique using in-plane mechanical deformation in 2D monolayers and van der Waals heterostructures that is compatible with Si based logic devices. To induce the in-plane deformation, we implement techniques developed for strain engineering in the semiconductor industry by depositing patterned thin film stress capping layers i.e., MgO on monolayer MoS2 and MoS2-WSe2 heterostructure. This approach applies the strain from the top down, suggesting the enticing possibility for controlled strain in 2D monolayers and heterostrain in a van der Waals heterostructure. Using hyperspectral Raman and PL spectroscopy, we characterized the local strain and band gap modulation in the 2D layers. In both monolayer and heterostructure, the maximum biaxial tensile strain reaches up to ∼1%. However, there is a strong strain gradient from center towards the edge of the stressor in monolayer. PL map shows that photoluminescence emission energy gets modulated at a rate of 92 meV/% strain along this gradient. In contrast, strain profile is almost homogeneous in the top layer of a heterostructure and strain transmission is minimum into the bottom layer. This indicates that low friction at the van der Waals interface causes this homogeneous strain profile and prevents strain from transmitting into the bottom layer. This heterostrain approach opens more flexible ways for tuning the moiré superlattice in nearly commensurate 2D heterostructures by modulating the relative lattice constants between the layers. Moiré systems show diverse physics, which are extremely sensitive to the structure and size of the moiré superlattice. Developing new strategies to induce and tailor heterostrain will create new dimensions of materials design in moiré superlattices. Our results on patterned thin film stressor on 2D monolayers and heterostructures shows great promise because it is a viable approach for tuning the optoelectronic/ quantum properties in TMDC devices to make them complement existing Si based technologies.

Graduation Semester

2023-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Mohammad Hossain

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