The project
My Ph.D. project investigates defect generation in single-layer graphene upon magnetron sputter deposition of thin metal layers (Au, Ag, Cu, and Al). While magnetron sputtering offers a scalable route for fabricating metal contacts on 2D materials, energetic plasma species can damage 2D crystal lattices. By combining experimental methods and computational modelling, this project aims to elucidate the atomic-scale mechanisms controlling defect formation in graphene. Metal/graphene heterostructures are synthesized under ultra-high vacuum and characterized using Raman spectroscopy, SEM, AFM, TEM, and synchrotron-based XPS/NEXAFS. Target (i.e., cathode) sputtering and gas phase transport are modeled using Molecular Dynamics and Test-Particle Monte Carlo simulations. The project is expected to provide the scientific foundation for engineering defect-free metal-graphene interfaces for key enabling (e.g., nano-electronics) devices.
Why does it matter?
Two-dimensional (2D) materials, including graphene, hold the promise of revolutionizing technology in the form of ultralight nano-electronic components, catalytic platforms, and sensors. To harness the unique physical properties of 2D materials in key enabling devices, multifunctional metal contact layers often need to be deposited on the 2D crystal surface and form heterostructures with well controlled interface properties. Metal nanostructures and thin films are routinely synthesized on 2D materials via vapor condensation using established methods, such as thermal and electron beam evaporation. Besides vapor-based methods, wet-chemistry approaches and electroless depositions have been employed for decorating graphene-based surfaces with metal nanoparticles. All the above mentioned deposition strategies suffer from lack of lateral thickness, morphology and property uniformity, while they are not easily scalable.
Magnetron sputtering is a vapor-based deposition technique that meets the requirements of large scale film thickness and property uniformity. However, the magnetron sputtering process, which relies on an Ar-based plasma discharge, generates ample amounts of energetic neutral and ionized species. These species may create defects, which can compromise the pristine electronic and transport properties of graphene and other 2D materials when depositing the metal layer. Hence, magnetron sputtering has been thus far deemed incompatible with metal-layer fabrication on 2D materials. A 2021 study by Pliatsikas et al. focused on the study of defect generation on graphene during magnetron-sputter deposition of Au layers. The data presented therein provided evidence that defects primarily emanate from energetic species generated during neutralization and backscattering of Ar+ ions at the sputtering cathode (target). Using this prior work as a vantage point, I have been expanding research to different target materials (Au, Ag, Cu, and Al) and different sputtering gases (Ar, Kr, Xe, and Ne). This is motivated by the fact that different target-gas combinations exhibit distinct mass-dependent backscattering propensities, generate sputtered and backscattered atom distributions and vary over a wide range, and exhibit different energy/momentum transfer profiles to graphene that may modify defect generation. I seek to leverage these distinct behaviors for understanding the fundamental processes that drive defect formation during metal-layer magnetron-sputter-deposition on graphene and create the scientific foundation that will enable fabrication of metal-graphene heterostructures with tailored defect density and types, including defect-free heterostructures.