Seminars

Abstract: The mechanical response of materials is best understood by studying how atoms move around under strain and stress. Transmission electron microscopy provides the most accurate images of atomic structure and provides a window into the unique deformations that occur at the nanoscale during elastic and plastic deformations. In this talk I will present the work we have done on providing the fundamental quantification of several key atomic features in 2D crystals, dislocations, strain maps, and crack tips and their propagation. The highly flexible nature of 2D crystals enables large distortions to take place, but tracking these in real time and at the atomic level can be challenging. I will cover results related to graphene and transition metal dichalcogenides (TMDs), and also some tiny sub-nm 1D wires of MoS as they bend, twist, shear, crack, pop, fold, expand and contract. In graphene, the point dislocation will be presented, and its glide and climb captured at the atomic scale, along with partial dislocation formation coupled to in-plane tension. I will show the first experimental evidence taken to prove atomically sharp crack tips exist, which a crack tip sharpens to a single bond in TMDs. The real time dynamics of crack propagation in TMDs is captured, showing dislocation formation at the crack tips, filamentation around crack tips, tip blunting during propagation through highly defective areas of MoS2, and transitions from brittle response to ductile. I will show how faceted voids in TMDs cause stress concentration at their corners that lead to early failure onset compared to circular voids in 2D crystals. Finally, I will present work on bilayer 2D TMD crystals where the interlayer S-S coupling causes friction that impacts the crack propagation and leads to unexpected crack branching and crack tip separation in each layer. The experimental atomic scale images are compared to theoretical models using molecular dynamics to help understand the mechanisms behind the dynamics and the predicted failure pathways. These results help provide an atomic level framework for the basic defect structures in 2D crystals that can be used to transition into the continuum models for large scale material predictions.

Bio: Dr. Jamie H. Warner joined the Department of Mechanical Engineering at University of Texas at Austin in January 2020 to lead the new Electron Microscopy Facility located in the Engineering Education and Research Center, Texas Materials Institute and the Cockrell School of Engineering. Prior to this he spent 13 years in the Department of Materials at the University of Oxford, where he held the position of Professor of Materials and led the Nanostructured Materials Group. He completed a PhD in Physics at the University of Queensland in 2004. In 2008, he started the University of Oxford's Glasstone Fellowship in Science. In 2010, he started a Royal Society University Research Fellowship at Oxford. In 2014, he was promoted to Full Professor in the Department of Materials in Oxford. He has been visiting Professor at MIT twice, and at Sungkyunkwan University four times. In 2019 he became a Fellow of the Royal Society of Chemistry. In 2018, he was listed as one of the top 10 ‘highly prolific’ authors for ACS Nano since its inception. In 2019 he was the ACS Nano Lectureship winner. He is the Editor-in-Chief of the new journal Materials Today Advances, and is on the Editorial Boards of Materials Today, Materials Today Chemistry, Applied Materials Today, Materials Today Nano, and Materials Today Energy. In 2020 he joined the Editorial Advisory Board of ACS Nano. He has >300 peer reviewed publications, with >160 as the corresponding author, including Science, Nature Materials, Nature Nanotechnology, Nature Communications (x3), Nano Letters (x16), Advanced Materials (x4), ACS Nano (x60), on the topics of nanomaterials for opto-electronics, energy, bio-applications, and quantum materials.