CAREER: Designing Functionality Into Two-Dimensional Materials Through Defects, Topology, and Disorder

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NONTECHNICAL SUMMARYThis CAREER award supports integrated research and education activities centered on computational and theoretical design of functional two-dimensional materials through controlling defect-structure relationships. Defects are deviations from the ideal expected structure of a material and include: missing atoms, the appearance of impurity atoms, and rearrangements of atoms. Most conventional three-dimensional semiconductor materials, such as those that make up the transistors in computers, rely on the intentional incorporation of defects to modify their properties to enable them to function as required for the application. For instance, photovoltaic solar cells rely on incorporation of defects to enable sunlight to electricity conversion as a renewable energy source. Thermoelectric materials rely on defect incorporation into semiconductors to convert heat to electricity, which is used to power satellites in space and may be useful for waste heat harvesting on earth. The properties of defects in semiconductors are closely linked to the behavior of electrons in the material, which is governed by the laws of quantum mechanics. The goal of this research is to develop and apply predictive computational tools to understand how defects control the properties of emerging two-dimensional materials that are promising for applications. Single atomic-layer thick graphene with a honeycomb arrangement of carbon atoms that resembles atomic-scale chicken wire provides one example. In comparison to three-dimensional materials, the defect-property relationships in two-dimensional are not well understood. A unique coupling between material structure and electronic properties arises in 2D materials and leads to a distinct and rich defect physics that is different from 3D materials. The outcomes of this research will be the development of computational tools to establish these relationships, and an enhanced understanding of defect engineering in two-dimensional systems to help realize better performing devices in electronics, photonics, energy harvesting and storage, and other areas. The research effort will be integrated with educational and outreach activities to increase the diversity of the next generation technical workforce. Through the introduction of a new course entitled "Community Outreach for Science and Engineering Researchers," the effort targets several key junctures of the student development pipeline from middle school to graduate students. Graduate and undergraduate students enrolled in the course will be directly engaged in outreach activities to local schools. They will design, test, revise, deploy, and disseminate active learning modules both to local middle schools in the Champaign-Urbana region and at weeklong residential summer camps for high school students held on the Illinois campus. The learning activities will be disseminated to the community through annual teacher training workshops and an online dedicated website. Software developed in the course of the research will be disseminated as open source to the broader community through GitHub.TECHNICAL SUMMARYThis CAREER award supports integrated research and education activities centered on computational and theoretical design of functional two-dimensional materials through tailored defect and topological structure. Recent advances in 2D materials have raised intriguing possibilities with applications in electronics, photonics, energy harvesting and storage, and other areas. A critical barrier is that defects in 2D materials exhibit behavior from those in 3D materials, and currently they are not well understood. A unique coupling between topological structure and electronic structure arises in 2D materials and gives rise to a distinct and rich defect physics. The goal of this work is to exploit this unique coupling to enable unprecedented functionality through the purposeful addition of defects in a controlled way.To merge aspects of topological structure and electronic structure in 2D materials, the PI will invoke approaches that span length scales from the atomic scale where a quantum mechanical description based on the interacting Schrodinger equation is required to micrometer-scale topological defect structures. Statistical mechanics frameworks that predict the defect topology of a 2D material that deforms in 3D will be directly linked with first-principles electronic structure methods to demonstrate that topology and chemical doping can be integrated in ways that enable new functionality. First-principles approaches will utilize both hybrid density functional theory and emerging, high-accuracy quantum Monte Carlo methods. Since quantitative first-principles descriptions of defects require extraordinarily high accuracy, potential limitations to the accuracy attainable will be assessed. The integrated approach will be used to answer several outstanding questions on defect physics in 2D systems. These include: (a) How can topological structure and functionalization be integrated to design high-performance 2D thermoelectrics? (b) How can topological structure and chemical doping be integrated to design high-performance 2D photovoltaics? (c) Do 2D materials undergo defect-mediated ductile/brittle transitions as their 3D counterparts? (d) What is the nature of the defect-induced insulator to metal transitions in 2D materials? The research effort will be integrated with educational and outreach activities to increase the diversity of the next generation technical workforce. Through the introduction of a new course entitled "Community Outreach for Science and Engineering Researchers," the effort targets several key junctures of the student development pipeline from middle school to graduate students. Graduate and undergraduate students enrolled in the course will be directly engaged in outreach activities to local schools. They will design, test, revise, deploy, and disseminate active learning modules both to local middle schools in the Champaign-Urbana region and at weeklong residential summer camps for high school students held on the Illinois campus. The learning activities will be disseminated to the community through annual teacher training workshops and an online dedicated website. Software developed in the course of the research will be disseminated as open source to the broader community through GitHub.