Scalable manufacturing of the few-layer van der Waals materials from bulk

Advances in nanoscale materials and nanomanufacturing offer a way to improve the performance of integrated devices. Traditional silicon manufacturing is hindered by the enormous expense of shrinking geometries and the difficulty of managing waste heat. Nanoscale materials, specifically van der Waals (vdW) materials, offer a possible solution. These are a class of materials composed of atom-thick layers, with appealing mechanical, electrical and optical properties. While graphene is renowned for its ability to transport electrons “ballistically”, other layered materials can also exist stably in air as a single atomic layer. In particular, transition metal dichalcogenides (TMDCs) such as molybdenum disulfide (MoS2) offer desirable properties as few- or monolayer films. Monolayer MoS2 exhibits a direct bandgap, which is ideal for applications in optoelectronics, such as photovoltaics and energy storage. These materials are expected to see widespread adoption, including in optical, electronic, sensing and biomedical devices. Accessing monolayer graphene and TMDCs repeatably, as part of a predictable high-yield manufacturing process, will be critical to realizing these applications. This project targets mechanical exfoliation, which receives less attention than competing techniques such as chemical vapor deposition (CVD) and atomic layer deposition (ALD), but yields monolayers with unrivaled electronic quality. Our project seeks to overcome the mechanical limitations of manual, low-yield exfoliation processes, by developing an automated, repeatable process with controlled yield and throughput. The far superior electronic performance of these materials compared to those made by competing processes demands investment to turn exfoliation into a viable manufacturing process. We will achieve our goal by studying—computationally and experimentally—the interlayer energy release behavior leading to fracture of the layers. We will then modify the mechanical parameters of our multiplexed exfoliation system to control interlayer fracture. We expect variations in the applied load and the mechanical deformation of the exfoliating material to influence the fracture behavior. The manufacturing process can be characterised by the yield, thickness and electronic quality of the material produced. An important aim of this work is to overcome problems associated with natural (mined) samples: poor flatness, debris generation and varying layer thickness. Patterning the samples in advance of exfoliation makes it possible for an adhesive layer to make conformal contact to regions with different initial height. Thus we will use laser ablation to pattern individual islands within a layer, allowing us to make conformal contact to TMDC layers and control crack initiation and propagation during the exfoliation process. This laser patterning will take place at Imperial College, while system design, exfoliation trials, and characterisation will be carried out at UC Berkeley.

Eric Yeatman
Eric Yeatman

Prof. Eric Yeatman FREng, FIEEE is Head of Imperial College’s Department of Electrical and Electronic Engineering. His research track record includes development of the first surface plasmon microscope, co-invention of the surface tension self-assembly technique for 3D MEMS, and integrated optical amplifiers by meta-stability in rare-earth doped glasses. He now leads one of the world’s most cited groups in micro-scale energy harvesting. He is a Silver Medalist of the Royal Academy of Engineering, and co-founder and director of Microsaic Systems plc, which develops and markets miniature mass spectrometers based on MEMS components.

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