Diffusion of Guests, Dopants, and Impurity Atoms Through Open Cage Allotropes of Si and Related Structures

Colorado School Of Mines, Golden CO

Investigators

Abstract

Non-technical Description: There is a great need for development of next generation materials, derived from earth abundant elements, that would revolutionize the electronics industry for future generations of devices including computer chips, lasers, and detectors. Silicon is the second most abundant element on the Earth's crust and the most technologically significant. Many exotic forms of silicon, which exhibit caged or tunnel crystalline structures, hold the promise of high efficiency and low cost disruptive electronic and photonic properties that would push silicon to a new level. These exotic silicon structures, however, tend to form around ionic guest atoms. Their potential can be only realized with complete ionic guest removal. This research project addresses this vital requirement by elucidating the fundamental diffusion and mobility pathways for guest ions in silicon caged/tunnel structures and unraveling the complexities of the mechanistic pathways to guest diffusion. The research will create a fundamental understanding of ionic motion in open silicon structures that has not been achieved to-date. Closely integrated with the research are strong education and outreach programs to provide significant training and mentoring opportunities for undergraduate, graduate and underrepresented groups in materials science, including summer workshops and internship programs for undergraduate and high school students, and community outreach. The team also collaborates with the Rocky Mountain Camp for Dyslexic Kids to provide summer camps developed for students with dyslexia who have interest in and aptitude for science, technology, engineering, and mathematics (STEM). Technical Description: Many exotic forms of earth abundant silicon are theoretically predicted to have properties, such as a direct bandgap, that would revolutionize electronics and photonics industries. This potential has not been realized and the key limitation is the critical requirement of complete removal of the guest atoms around which these open crystal structures form. The ultimate goal of this project is to elucidate the ionic and atomic diffusional pathways and mobility of alkali and alkaline earth ions, inert guest atoms, and dopants in silicon caged and tunnel structures. Although there have been a number of exotic forms of silicon investigated via theory, simulation, and small-scale synthesis, a fundamental knowledge of the guest diffusion/mobility mechanisms is severely lacking. The major barrier that prohibits guest diffusion/mobility studies is the inability to produce sufficient quantities of phase pure material in thin-film, powder and bulk form. This barrier can be overcome by synthesis approach undertaken by the research team, allowing the research to explore: which guest diffusion pathways are preferable, i.e., via small or large cages, five-fold or six-fold rings of cages; the role of non-idealities, like vacancies, or impurities, and how partial occupancy and ion-ion interaction create unusual diffusion/solubility kinetics; the effect of charged versus neutral species and electron donation on diffusion. The research efforts include: synthesis and development of thin films and powders (on the grams-scale) of phase pure silicon allotropes with well-defined diffusive properties; investigation of macroscopic diffusion through the crystalline lattice of thin films of silicon allotropes; exploring the guest location/environment and dynamics, the defect impurities, and intrinsic defects in both films and powders. Key techniques include: time-of-flight secondary ion mass spectroscopy, electron spin resonance, and solid-state nuclear magnetic resonance spectroscopy. 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.