Collaborative Research: Novel silicon-based optoelectronic materials

Nontechnical description: Silicon is an essential semiconductor for the majority of modern electronic and solar-energy devices. Nevertheless, the normal crystalline structure of silicon that is currently used has physical properties that limit light absorption/emission processes and other advanced technological applications. In contrast, different crystalline forms of silicon with alternative physical properties can overcome these challenges and impact a range of technologies including solid-state detectors, optical communication, and energy conversion devices, while simultaneously maintaining the intrinsic advantages of silicon, such as natural abundance and low toxicity. This collaborative research project aims to develop and discover completely new crystalline structures of silicon and silicon-based compounds with enhanced and/or complementary optical and electronic properties using a joint theoretical and experimental strategy. Research is focused on developing recently discovered crystalline forms of silicon, and on revealing novel synthetic approaches to achieve additional silicon-based materials with computational guidance. In contrast to conventional synthetic approaches that take place at high temperatures and low pressures, access to new silicon structures in this project is enabled by the utilization of very high pressures (up to one hundred thousand times atmospheric pressure) and moderate temperatures. These unique processing conditions provide access to new silicon structures possessing a range of physical properties that extend beyond those of the normal form of silicon that is currently used. This research project is executed within an educational environment that promotes the academic development of students and postdoctoral scholars and emphasizes science, technology, engineering and math (STEM) career trajectories. The methodologies developed for this project are expected to be generalizable to other classes of materials beyond silicon. Technical description: Modern computational methods predict the existence of new materials and their properties with remarkable accuracy. Nevertheless, practical synthetic strategies are needed to access a plethora of hypothetical materials with superlative properties. This collaborative research project explores the depth of realizable materials for silicon and probes the relationships between metastable allotropes/compounds and optoelectronic properties in order to achieve new structures of silicon with properties that exceed or complement the normal diamond-cubic form. Accompanying the development of two novel silicon allotropes (Si24 and 4H-Si) via crystal growth, doping, strain engineering and properties optimization, the discovery of additional silicon allotropes and compounds is enabled using unique high-pressure synthetic methods guided by ab initio transition pathway and structure searching predictions. The comprehensive exploration of complex potential energy surfaces is facilitated through the development of computationally efficient machine learning methodologies. The research expands the library of synthetic routes to kinetically controlled silicon-based materials using novel precursors, and the intrinsic optical and electronic transport properties of new silicon allotropes and compounds are determined experimentally. The overall goal of the project is to produce and characterize new silicon phases with enhanced optoelectronic function and the potential to inform next-generation technology.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.