Indentation and nanomechanics of graphene-substrate interfaces

Yaacoub, Jad Jean

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https://hdl.handle.net/2142/120356 Copy

Description

Title

Indentation and nanomechanics of graphene-substrate interfaces

Author(s)

Yaacoub, Jad Jean

Issue Date

2023-04-17

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

Tawfick, Sameh H

Doctoral Committee Chair(s)

• Tawfick, Sameh H
• Johnson, Harley T

Committee Member(s)

• Said, Taher A
• Zhang, Yingjie

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
• Graphene
• Indentation

Abstract

Graphene, a 2D material made of carbon atoms bonded in a hexagonal arrangement, is gaining considerable attention owing to the combination of its single atomic thinness, chemical and mechanical stability, and its ability to improve the desirable mechanical, electrical, or optical properties of devices and composites. In particular, graphene is proposed for use as a nanoscale filler to enhance the mechanical properties of nanocomposites, or as single atomic coating layer to enhance the surface mechanical behavior of substrates. Instrumented nanoindentation has been widely adopted for the characterization of the mechanical properties of materials at the micro and nanoscale, where a measured load-displacement response is fit to a mathematical model for mechanical property extraction. While tensile testing remains the standard method for assessing the bulk properties of materials, nanoindentation is the most widely used technique for surface mechanics such as surface modulus and hardness. When combined with modeling, nanoindentation offers unique capabilities to measure multi-layered thin films with vastly different thicknesses and elastic properties. In this work, the nanomechanics of 3 different graphene-substrate interfaces are investigated using the nanoindentation method. By combining tests on different types of substrates and graphene morphologies, this thesis reveals the important effects of the method of deposition and the graphene-substrate interaction. The first substrate considered is copper (Cu), which is a catalyst for graphene synthesis using Chemical Vapour Deposition (CVD). Selecting a catalyst material to probe the surface mechanical behavior ensures that graphene, after synthesis, is layered uniformly on the underlying substrate and, in some cases, epitaxially conforming to the substrate. Graphene growth on Cu is predominantly a surface-mediated mechanism, where dissociated carbon atoms from the highly diluted precursor gas adsorb to the surface of the catalyst, diffuse along the surface and coalesce to nucleate graphene domains. This growth mechanism leads to an almost ideal nanoindentation model system where the graphene is conformal to the outermost surface of the substrate without significant carbon precipitates or other carbon impurities beneath the copper surface. The second substrate considered is platinum (Pt), which is also a catalyst for graphene synthesis using CVD. Graphene growth on Pt is dictated by both surface- (similar to Cu) and bulk-diffusion and segregation mechanisms, where carbon atoms diffuse through the bulk and segregate to the surface during cooling for graphene nucleation and growth. The combination of these two mechanisms of graphene synthesis lead to interesting surface and sub-surface morphologies of the graphene on the Pt. The third substrate considered is a stiff crosslinked polymer widely used in microfabrication, SU-8. The graphene for this case is not directly grown on the substrate but rather transferred post-synthesis on the receiving substrate of interest. In this study, periodic graphene wrinkles form on the surface during the sample preparation. Hence, the effect of this wrinkling morphology on the nanoindentation response is also investigated. In all three cases, the experimental nanoindenation study measured statistically significant elasticity change in the graphene (Gr) coated substrate. Surface stiffening effects are observed when indenting the Gr-Cu (between 8 and 12% modulus increase) and Gr-SU8 (20-400%) systems. The stiffening observed in Gr-Cu and Gr-SU8 systems is attributed to graphene's high in-plane stiffness resulting in significant stretching forces under the action of the indenter. A finite element analysis model is used to explain the large stiffening observed during the nanoindentation of graphene-Cu. Moreover, in the limiting case of the absence of tangential interaction between the graphene an copper in the neighborhood of the indented region during unloading, an equal-displacement analytical model is proposed. This model is based on total elastic unloading energy, treating the graphene as an ultra-stiff stretchable membrane providing a restoring force when stretched, which acts alongside the force provided by the indented substrate. Conversely, an elastic softening effect is observed when indenting Gr-Pt systems. The softening phenomenon observed in Gr-Pt systems is attributed to microstructural changes near the surface of the substrate as a direct consequence of atomic diffusion and precipitation near the surface during graphene growth on Pt. Transmission Electron Microscopy (TEM) cross-sections were taken to directly and accurately observe and characterize changes in near-surface platinum microstructure and micrographs are presented. We find the presence of both sub-surface and surface impurity layers, depending on growth conditions. In-situ energy-dispersive X-ray spectroscopy (EDS) was used to chemically identify the dominant elements in the impurity layers seen in the TEM cross-section micrographs. Carbon was found to be dominant. The impurity layers in each case was found to be responsible for the observed elastic softening. Subsurface impurity layers support a "growth-by-segregation" while surface impurity layers support a "growth-by-adsorption" model. Overall, the work presented in this thesis provides insight into the mechanics of graphene-substrate interfaces. We provide a physics-informed, mechanical model for indentation problems of 2D-materials-on-substrate. This work demonstrates why 2D-materials, when layered on substrates, are best treated as stretchable membranes instead of thin-films. This work also demonstrates the intimate relationship between process, structure and property of composites. Further studies on low-depth indentation of graphene-covered substrates using atomic force microscopy can further elucidate the properties of graphene-substrate interface.

Graduation Semester

2023-05

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/120356

Copyright and License Information

Copyright 2023 Jad Yaacoub

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