The mechanical response of materials at the nanoscale via simulations

Abstract

We use both molecular dynamics (MD) simulations and density functional theory (DFT) calculations to investigate various nanomechanical properties of nanoparticles under compression, exfoliation of perovskite heterogeneous systems and epitaxial methods. The compression of nanoparticles is of fundamental importance both scientifically and to various other applications such as tribology, targeted drug delivery, and biosensors. We embark on an extensive investigation of the response of a nanoparticle to external compression as this field is still short on answering some lingering doubts and details. We find a size-dependent brittle to ductile transition. We formulate a Griffith based fracture model that calculates r_critical to the same order of magnitude. We report a tradeoff: On one hand compression strengthens the surface but on the other it accumulates more and earlier shear. That is critical as the advantages of material strength can then be weighed in design against change in mechanical properties and based on what a specific application is aimed for. We also apply this to more experimentally relevant systems as in exfoliation. We design build and run DFT calculations on heterogenous perovskite systems to investigate exfoliation. It is an invaluable means of detaching epitaxial layers from substrates to produce membranes that are essential in various applications such as optoelectronics and high-speed computing. We devise an exfoliation regime that matches experimental results in every case. We back that up further using MD simulations. This can open the door to experiment with and confirm many other systems that can now be more easily tested. Which makes the process more general, efficient, and cost effective. We demonstrate that the presence of a stressor is a necessary but not sufficient condition for exfoliation. As successful peeling is contingent on defect free film-interface-substrate, we use MD to simulate and confirm that graphene nanopatterning allows for great reduction of defects in freestanding single-crystalline membranes using deposition simulations of Germanium on Silicon. We show that as graphene coverage increases the dislocation density is greatly reduced. We successfully generalize epitaxy to include multiple layers. We simulate and show the effectiveness of growing and harvesting multilayered epitaxial systems through multiple graphene layers. This results in layer-by layer peeling culminating in multiple free-standing membranes. This offers a high throughput and low-cost production of single crystal membranes needed in many applications such as high-power electronics.