Tutorial QN02—Defects and Magnetic Properties of Two-Dimensional Materials

Defects in crystalline solids are ubiquitous. It is the second law of thermodynamics that gives rise to the appearance of a certain amount of disorder in crystalline materials at finite temperatures. Moreover, defects can be present in synthetic materials well above the equilibrium concentration due to the imperfections in material production processes or due to the exposure of the system to irradiation with energetic particles. Such lattice imperfections have a strong influence on the electronic, magnetic, optical, thermal and mechanical properties of the solids, normally deteriorating their characteristics. However, defects do not always have detrimental effects on material properties, with the most prominent example being the doping of semiconductors by controllable introduction of impurities using ion implantation.

All of the above is relevant to two-dimensional (2D) materials, such as graphene and hexagonal boron nitride, or transition-metal dichalcogenides (TMDs). It is intuitively clear that due to the reduced dimensionality the defects should have a much stronger influence on the properties of 2D materials, as compared to their bulk counterparts. Moreover, due to the morphology of 2D systems, it is much easier to introduce defects into them in a controllable manner and add new functionalities. Furthermore, the experimental realization of ferromagnetism at the monolayer level in 2D van der Waals materials beyond graphene has drawn a great deal of research interest in the recent past. In addition, these materials have exciting prospects for next-generation low-power ultra-compact spintronic applications. This tutorial will review the recent developments in the rapidly growing field of defects and magnetic properties of a broad spectrum of 2D materials through a combination of theoretical and sensitive experimental approaches, and will immensely benefits scientists at all levels.

1:30 pm—Defects in 2D Materials—Theory
Arkady V. Krasheninnikov, Helmholtz-Zentrum Dresden-Rossendorf

In this tutorial, Krasheninnikov will present the "state of the art" in the physics of defects in two-dimensional (2D) inorganic materials with the main focus on the theoretical developments. The computational and analytical methods used in theoretical physics to get insights into defect behavior will be briefly summarized, and then the effects of impurities and point/line defects on various properties of 2D inorganic materials will be addressed. He will further discuss defect- and impurity-mediated engineering of the electronic structure of inorganic 2D materials. Krasheninnikov will also present the results of the theoretical studies of electron-beam-induced phase transformations in 2D transition-metal dichalcogenides (TMDs) when electric charge, mechanical strain and vacancies are present.

2:30 pm—BREAK

3:00 pm—Defects in 2D Materials—Electron Paramagnetic Resonance Spectroscopy
Andre Stesmans, KU Leuven

In this tutorial, Stesmans will deal with some basic principles and methodology of electron paramagnetic resonance (EPR) spectroscopy, outlining it as a reliable "magnetic" technique based on non-destructively sensing of unpaired electrons, which is successfully applied in a broad range of scientific fields. Next, attention will be directed to its application in tracing the nature of point defects, both intrinsic as well as of an impurity-related nature, in 2D semiconducting materials.  In an exploring attitude, the latter include bulk TMDs, both of geological origin as well as synthetically composed, where the research is focused on robust p-type doping by covalently bonded impurities. It will be outlined how EPR can arrive at in-depth reliable characterization of these dopants, including solid atomic identification, accurate quantification, spatial distribution, and inference of electrically key properties such as their thermal activation energy, and a fortiori, defect level(s) in the semiconductor bandgap. In combination with its outstanding selectivity, EPR takes a unique position when it comes to selectively dopant characterization on a true atomic level. A separate part will deal with intrinsic defects in synthetic large-area 2D TMD layers deposited on dielectrics, where intrinsic point defects are revealed as an inherent aspect, and hence performance degrading, of current state-of-the-art fabrication methods. The main attention here will go to identification and quantification of defects, and monitoring of their behavior and stability under thermal load.

4:00 pm—Magnetic Properties in 2D Materials
Roland Kawakami, The Ohio State University

Kawakami will cover some of the advances for both the intrinsic 2D magnets and extrinsic magnetism in 2D materials with dilute magnetic doping. With the goal of getting participants up to speed on this fast moving topic, the tutorial will blend a number of experimental and theoretical concepts in the topics of sample fabrication and characterization, exchange coupling in intrinsic and extrinsic magnetic systems, considerations for stability of magnetic order in 2D systems, electric-field control of magnetism, spin transport in magnetotunnel junctions, and prospects for future science and applications.