From Micro- to Nano-porous Cellular Materials with Layered 2D Microstructure
Creators: name: Nakanishi, Kenichi
Publisher: name: Apollo - University of Cambridge Repository
Year: 2020
Subjects: Graphene
CVD
Foam
Mechanics
TMDC
MOCVD
Nanoindentation

description: A large body of work has been committed to studying the unique properties of 2D materials such as graphene, with advancements in both the material quality and scale of mechanical exfoliation and chemical vapour deposition (CVD) methods. These emergent 2D materials have recently been engineered as the cell walls in three-dimensional structures, but their superb material properties are yet to be fully realized in this new form. This thesis investigates the CVD processing of a range of catalytic templates to open new routes towards the controlled fabrication of graphitic foams and lattices. As part of a full feedback loop, mechanical characterization of these unique cellular materials was undertaken in order to examine their deformation and failure mechanisms, including capturing their behaviour in a new hierarchical model framework. These novel structures have the potential to combine the properties of structured porous materials, i.e. low density, high geometric surface area, permeability and mechanical stability, with the intrinsic properties of 2D materials such as enhanced electrical and thermal conductivity, high mechanical strength and stiffness as well as resistance to damage from extreme temperatures and chemical attack. Such high quality 2D-material based cellular structures have manifold potential applications in electrochemistry, catalysis and filtration. Herein, freestanding graphitic foams are fabricated across a range of relative densities, and their uniaxial compressive responses are measured to investigate the operative deformation and failure mechanisms that govern the mechanical response of such foams. For this purpose, a hierarchical micromechanical model is developed, which traces the deformation of the hollow cell struts to the axial stretching of the cell walls. The waviness of the multilayered graphitic wall increases the axial compliance of each cell wall, and it is established that axial straining within the cell wall occurs by interlayer shearing. Crucially, this mechanism demonstrates that the continuum properties of such foams are dictated by the weak out-of-plane shear properties of the layered cell wall material, leading to a large knockdown in the macroscopic mechanical properties of the foam. Ordered graphene gyroid lattices possessing nanoscale unit cell sizes are then fabricated and characterized through a combination of nanoindentation and a multi-scale finite element analysis (FEA) study. These structured nanolattices were found to be highly conductive and possessed a high degree of elastic recovery and strength owing to the structural efficiency afforded by the stretching-dominated cellular architecture. However, the nanoscale interlayer shearing deformation mechanism was again found to be active in the cell walls of these structures, attenuating the continuum response of the lattice. The hierarchical micromechanical model developed herein rationalizes why CVD-grown multilayer graphitic foams and lattices possess diminished continuum elastic moduli and yield strengths in comparison to the exemplary in-plane mechanical properties of 2D materials, presenting a first step towards the understanding of porous materials whose cell walls are comprised of emergent 2D materials. In addition, the direct shrinkage of commercial polymer foams and 3D printed templates is used herein to offer a very simple and low-cost method for reaching identically-shaped structures with sub-200 μm unit cell sizes. The conformal addition of different thicknesses of alumina is shown to control the level of isotropic shrinkage, reducing the shrinkage ratio from 125x to 4x after addition of 25 nm of alumina, while inducing a surface stress mismatch that drastically increases the surface roughness of the material. Furthermore, efficient graphitization was demonstrated through the use of an electrolessly deposited Nickel film, resulting in the formation of a conductive multilayer graphenic coating at temperatures below 1100°C. These processes present the flexible production of multifunctional cellular materials with sub-mm unit cells, tuneable size, roughness and conductivity. A final study investigates the preparation of a nascent 2D material, WS$_2$, through the use of a deconstructed metal organic chemical vapour deposition (MOCVD) process which allowed insight into the role of each process step. The catalytic effect of an Au substrate is unambiguously demonstrated, which allowed for a reduction in the precursor partial pressures required to nucleate and grow WS$_2$ by over an order of magnitude in comparison to competing methods. This enabled the efficient low-pressure growth of WS$_2$ films with low levels of carbon contamination. Furthermore, the reaction process developed herein exhibited a self-limiting monolayer growth behaviour with exposure cycles lasting just 10 minutes, a significant improvement over prior MOCVD processes requiring growth times in excess of 1 hour. These insights foster our understanding of the key underlying mechanisms of WS$_2$ growth for future integrated manufacturing of transition metal dichalcogenides (TMDCs) and other 2D materials.
description: Funded by the EPSRC (EP/G037221/1) - Cambridge NanoScience through Engineering to Application Doctoral Training Centre: Assembly of Functional NanoMaterials and NanoDevices, EPSRC (EP/K016636/1) - CVD enabled Graphene Technology and Devices (GRAPHTED), ERC (279342) - In-situ metrology for the controlled growth and interfacing of nanomaterials and ERC (206409) - Multi-phase lattice materials.