Fall 2020 Seminar Series: Matthew Rosenberger
US Naval Research Laboratory
Host: Mingwei Chen
Strain, Defects, and Interfaces in Two-Dimensional Materials
Two-dimensional materials (2DM) are atomically thin materials with extraordinary mechanical, electrical, and chemical properties that make them promising for next generation technologies in sensing (e.g. internet of things), flexible and transparent electronics and optoelectronics (e.g. biological interfaces), energy conversion (e.g. selective catalysis), and membrane technology (e.g. DNA sequencing). The realization of new technologies based on 2DM requires both fundamental research on the materials science of 2DM, as well as research that aims to bridge the gap between materials science and the engineering of real devices and systems. In this talk, I will describe my recent work on understanding the fundamental physics of strain, defects, and interfaces in 2DM and leveraging that understanding to control material behavior. First, I will discuss my work on controlling the mechanical state of 2DM at the nanometer-scale using atomic force microscope (AFM)-based techniques that I developed.[1-2] The extreme mechanical flexibility of 2DM is one of their most exciting attributes, but this flexibility can lead to the unwanted formation of bubbles or wrinkles (similar to a film of plastic wrap) which obscure observations of 2DM intrinsic properties. I addressed this ubiquitous problem by using an AFM to controllably manipulate 2DM layers in order to create flat and homogeneous 2DM interfaces, which enables precise characterization of 2DM intrinsic properties. In addition to removing unwanted mechanical perturbations, I invented a novel and general approach for encoding strain into 2DM with nanometer-scale precision. Using this technique, I was able to write strain gradients into a 2DM semiconductor, resulting in deterministic placement of quantum emitters. Quantum emitters are a promising technology for realizing secure quantum communications and 2DM provide potential advantages over alternative materials. Next, I will discuss my work on directly correlating nanometer-scale material defects with properties that govern optoelectronic and electronic device behavior, such as light emission and electrical conductivity. Using techniques that I developed, we were able to demonstrate a pronounced inverse relationship between photoluminescence intensity and defect density. I will also present a model that agrees well with the data and provides a guideline for further optimization of material and device behavior. Finally, I will discuss our recent investigations of the interfaces between 2DM layers, with a particular focus on the relative twist angle between layers. I will present experimental evidence of atomic reconstruction at the interface between semiconducting 2DM layers, which has significant implications for the behavior of 2DM heterostructure devices.