NSF/DMR-BSF: Stochastic Electronic Structure Approaches Applied to Study Low-Dimensional Black-Phosphorene Systems

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NONTECHNICAL SUMMARYThe National Science Foundation and the United States -- Israel Binational Science Foundation (BSF) jointly support this collaboration between a US-based researcher and an Israel-based researcher. The NSF Divisions of Materials Research and Chemistry fund this award jointly. The award supports computational research and education on the mechanical and electronic properties of black phosphorene. Black phosphorene is a material composed of elemental phosphorous that has been discovered and synthesized only recently. It is very similar to graphene, it forms a two-dimensional sheet, but it offers a significant advantage in that it behaves as a semiconductor: it exhibits a sizeable energy gap while supporting at the same time high-mobility charge carriers. In addition, it turns out that the electronic properties of black phosphorene can be readily tuned by applying mechanical stress and other methods. Therefore, just as silicon emerged as an ideal material for three-dimensional electronics, black phosphorene seems to emerge as the ideal infrastructure material for two-dimensional electronics technology. The principal goal of this project is to study, understand, and eventually predict the properties of black phosphorene and its derivatives using theoretical and computational methods. Because of the way two-dimensional black phosphorene responds to mechanical stresses and to chemical perturbations, studying its properties using conventional computational methods is a highly challenging task, requiring the simulation of exceedingly large systems. Therefore, the first challenge the project will need to overcome is to build a computational tool that tackles large electronic systems containing thousands of atoms, and apply it to the study of black phosphorene. This challenge can be met by using methodologies developed by the PIs that can generally be described as stochastic computational techniques. These approaches are akin to statistical methods used in polling, and make it possible to accurately simulate material properties for much larger systems than is possible with traditional, non-stochastic approaches. Once the tools are developed, the PIs will use them to study realistic two-dimensional black-phosphorene sheets, providing a theoretical framework for fundamental understanding and for guiding future experiments on this material.The theoretical tools developed and used in this project will be made available to the community, with an aim to increase access to the developed methodology. The educational component of the project emphasizes the use of computational tools relevant for the research for the training and education of graduate students and postdoctoral fellows. Undergraduate and high school students will be involved in the research, especially in the running of large-scale simulations. All will benefit from close interactions with the Israeli collaborators.TECHNICAL SUMMARYThe National Science Foundation and the United States -- Israel Binational Science Foundation (BSF) jointly support this collaboration between a US-based researcher and an Israel-based researcher. The NSF Divisions of Materials Research and Chemistry fund this award jointly. The award supports computational research and education on the mechanical and electronic properties of black phosphorene. Traditional large-scale simulations of these properties are prohibitively expensive either for accurate density functional theory with exact exchange (DFT), or for highly accurate electronic structure methods based on Green's function (GW). This project will explore hitherto inaccessible regimes using the stochastic formulations of traditional DFT and GW. The stochastic formulations replace some, or all of the multitude of summations over orbitals inherent in electronics structure methods with a stochastic sampling of combinations of these orbitals. The stochastic approaches are designed to calculate from first principles the atomic and electronic structure of systems containing 10,000 atoms or more. Sizes of that magnitude are necessary for establishing realistic environments, which enable the study of anisotropic properties. Such capability allows the construction of a theoretical framework for fundamental understanding of black phosphorene, which in turn will help in guiding experiments and synthetic efforts by pointing out possible directions to achieve desired properties. The unique ability to accurately simulate large black-phosphorene systems allows the exploration of electronic structure under a variety of experimental conditions, which include temperature, strain, and optical excitation. Using a combination of stochastic DFT and Langevin dynamics, the structure and phonon dispersion curves of large, layered black-phosphorene nanoribbons and nanotubes will be investigated, with and without mechanical stress. The structure of defects and adsorbates on the surface and perimeters will be studied as well.The theoretical tools developed and used in this project will be made available to the community, with an aim to increase access to the developed methodology. The educational component of the project emphasizes the use of computational tools relevant for the research for the training and education of graduate students and postdoctoral fellows. Undergraduate and high school students will be involved in the research, especially in the running of large-scale simulations. All will benefit from close interactions with the Israeli collaborators.