Tunable Ferroelectricity in Group IV Monochalcogenide Van der Waals Semiconductors

About This Grant

Nontechnical Description Ferroelectric materials with spontaneous electric polarization have applications in information storage, energy conversion, and novel computing concepts. Among inorganic ferroelectrics, electrically insulating three-dimensional (3D) crystalline oxides are most advanced in terms of understanding the underlying mechanisms and the development of crystal growth techniques. Two-dimensional (2D) materials can yield atomically thin ferroelectrics, but truly 2D crystals carrying a spontaneous polarization have remained rare. This project focuses on a third class of ferroelectric materials consisting of several 2D layers held together by weak van der Waals forces. Such layered crystals can have advantages over both 3D and 2D ferroelectrics. They are often semiconductors capable of conducting electrical currents and interacting with visible or near-infrared light, and their inert surfaces allow combining them with many other materials. Due to their unique structure, van der Waals ferroelectrics present numerous possible symmetry-breaking mechanisms, complicating the identification of the polar structures and creating the need to develop synthesis processes that favor the ferroelectric phase among other competing crystal structures. The goal of the project is to address these challenges for a representative class of layered semiconductors, thereby contributing to the science basis for turning van der Waals ferroelectrics into emerging technologies benefiting society. Through training of the participating students, the project will help build tomorrow’s technology workforce. An integrated outreach program aims at enhancing pre-college science education by developing hands-on learning activities. Through school visits, the project team will build lasting relationships to create excitement for science and engineering among students of different backgrounds. Technical Description Two-dimensional and layered van der Waals ferroelectrics have properties that make them interesting for device applications, e.g., narrow bandgaps, anisotropic current conduction, and facile integration with other materials via high-quality interfaces. Whereas the polarization in 2D ferroelectrics is clearly linked to the structure of the single layer, van der Waals ferroelectrics can have a multitude of underlying mechanisms, making their identification nontrivial and often controversial. This project seeks to find solutions to this challenge by studying the growth, structure, and properties of the polar phase of group IV monochalcogenide van der Waals semiconductors. The research leverages a breakthrough in growing macroscopic layered tin chalcogenide crystals with ubiquitous ferroelectric domains which, combined with transfers to arbitrary supports, opens up unprecedented opportunities for investigating the origin of symmetry breaking and ferroelectricity in layered crystals; identifying key properties such as domain patterns, phase transitions, and Curie temperatures; manipulating domains and probing domain wall functionality; developing rational growth processes informed by fundamental characteristics of the polar phase; and exploring the interplay between interfaces and ferroelectric domains in heterostructures. Powerful experimental methods such as in-situ electron microscopy, cathodoluminescence, and measurements of local and global charge transport, applied to large polar crystals, will enable investigating ferroelectricity and domain structures unaffected by finite-size effects. The successful pursuit of the project goals will establish novel avenues for analyzing ferroelectricity in layered semiconductors and support the utilization of van der Waals ferroics in emerging technologies. 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.