Theory and Simulations of Open Semiconductors and Insulators

Arizona State University, Scottsdale AZ

Investigators

Abstract

9986706 Sankey This grant supports theoretical research of a novel materials class that is gaining increased attention from engineering and synthesis communities - that is of open framework semiconductors and insulators. These include (i) Si, Ge or Sn clathrate materials in which 4-coordinated atoms form cages tightly linked into crystals with guest atom residents in the cages, and (ii) semiconductors linked via chalcogens into supertetrahedron or linked open structures via small molecules. Density functional calculations and simulations will be done to determine their properties, particularly electronic. An important application is thermo-electric properties of open semiconductors. Besides desirable electronic conductivity and Seebeck qualities of clathrates, trapped guest atoms inside cages produce localized Einstein-like "rattling" oscillators which hybridize with and scatter heat-carrying phonons of the framework. This engineers the thermal conductivity in an advantageous manner. Rattlers will be modeled by first applying electronic structure methods to determine the restoring forces, and then investigating the rattler's effect on the thermal conductivity by a Kubo linear response analysis. These focused studies of the thermo-electric properties will be performed in tandem with general studies. The novel structure of semiconductor clathrates offers a unique opportunity to investigate general properties of semiconductors from an unusual perspective. Non-conventional semiconductor alloys of Group IV elements with Group III (or V) elements compensated by cation guests will be investigated (Zintl-alloys). The unusual topology of clathrates motivates a search for new polymorphic states. In addition, guest mediated softening of silicon's covalent bonds will be investigated - an effect not possible in a dense phase. High risk projects involving new open framework materials more similar to zeolites will also be undertaken. For example, solids with building blocks not of a single tetrahedron, but rather of a tetrahedron of tetrahedra will be studied by electronic structure methods for the first time. %%% This grant supports theoretical research of a novel materials class that is gaining increased attention from engineering and synthesis communities - that is of open framework semiconductors and insulators. An important application is thermo-electric properties of these open semiconductors. These focused studies of the thermo-electric properties will be performed in tandem with more general studies. The novel structure of semiconductor clathrates offers a unique opportunity to investigate general properties of semiconductors from an unusual perspective. These materials are of fundamental interest and may have many technological applications. ***