Atomistic modeling of graphene-catalyst interface and graphene edge effects on near melting temperature substrates

Ananthakrishnan, Ganesh

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https://hdl.handle.net/2142/124621 Copy

Description

Title

Atomistic modeling of graphene-catalyst interface and graphene edge effects on near melting temperature substrates

Author(s)

Ananthakrishnan, Ganesh

Issue Date

2024-02-09

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

Johnson, Harley

Doctoral Committee Chair(s)

Johnson, Harley

Committee Member(s)

• Tawfick, Sameh
• Bellon, Pascal
• Schleife, André
• Trinkle, Dallas
• Pochet, Pascal

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• graphene
• interface
• Chemical Vapor Deposition
• Atomistics
• Molecular Dynamics
• Green's functions

Abstract

The integration of graphene into various devices is hindered by the challenge of producing high-quality, ultra-flat, and defect-free graphene, along with optimizing graphene-metal contacts for device applications. Chemical vapor deposition (CVD) has emerged as a cost-effective technique for producing high-quality, large-area graphene. However, CVD-grown graphene often exhibits faceting at the graphene-catalyst interface, which can adversely affect graphene's properties. This faceting phenomenon is common on various metallic catalyst substrates. In contrast, CVD on liquid substrates offers advantages in terms of producing ultra-flat and large-area graphene but poses distinct challenges. A modeling perspective is essential for engineering graphene-metal interfaces and, consequently, improving the quality of graphene. However, first principles calculations are limited by their inability to model the large length scales necessary to capture the critical features of the graphene-metal interface, such as faceting and the ordering of graphene flakes on liquid copper. On the other hand, Continuum-scale methods lack the atomic-scale resolution required to understand these phenomena. Molecular dynamics and statics, utilizing empirical and semi-empirical interatomic potentials, are invaluable for modeling length scales involving millions of atoms, allowing for the discovery of atomic-scale phenomena. We discuss the need to engineer the graphene-catalyst interfaces and the different features observed at the interface in chapter 1. We further delve into the background of the problems addressed in this thesis and the need to use molecular dynamics and statics simulations to understand them. Chapter 2 of this thesis focuses on the kinetics of a graphene-covered copper surface, revealing the contrast between bare and graphene-covered surfaces. While high diffusivity and surface pre-melting at elevated temperatures result in a flattened metal surface, graphene-covered surfaces exhibit significant roughness in the form of faceted structures. Molecular dynamics simulations demonstrate the stabilizing effect of graphene on the graphene-covered metal surface, preserving the faceted surface morphology observed in metal catalysts following CVD growth of graphene. We show that graphene suppresses surface melting, maintaining a crystalline metal surface even at temperatures slightly above the bulk copper melting point. Our analysis of mean squared displacements of atoms on copper surfaces with different facet orientations and graphene coverage reveals an anisotropic and surface specific surface diffusivity suppression effect of graphene. These findings align with experimental observations and underscore the thermomechanical surface-stabilizing role of graphene. In Chapter 3, we investigate the growth of graphene on liquid copper, uncovering a self-assembly and ordering process driven by long-range attractive capillary forces and short-range repulsive forces. Marangoni flows, resulting from surface tension gradients at the edges of graphene flakes in liquid copper, are identified as crucial factors that influence self-alignment behavior. This research deepens our understanding of the fundamental physics governing graphene growth on liquid copper, offering insights into controlled growth of 2D materials on diverse substrates. Chapter 4 delves into the thermodynamics of the graphene-metal interface, revealing the intricate interplay of thermal mismatch strain, interfacial energy, bending strain within graphene, and substrate-induced strains. Our analysis incorporates a newly developed technique for characterizing surface facets, which highlights specific orientations observed throughout the faceted interface. We analyze surface facet orientations in relation to interfacial energy and shear stresses, providing insights into the factors governing the observed orientations in experiments, especially for vicinal copper surfaces. Chapter 5 explores micromechanics models to understand stress distributions around steps and their influence on faceting at the interface. While these models have been useful for understanding faceting of stressed metallic surfaces, our analysis shows that graphene-covered surfaces do not induce similar stress fields. Therefore, we cannot predict the wavelengths of faceting using this approach. Alternative mechanisms defining faceting wavelengths in the graphene-metal interface are discussed, extending our understanding of 2D material-substrate interfaces and their distinct behavior compared to stressed thin films. The thesis concludes in Chapter 6, highlighting future research directions and potential outlooks.

Graduation Semester

2024-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2024 Ganesh Ananthakrishnan

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