Speaker: Daniel Rhodes, University of Wisconsin-Madison
Abstract: Compared to industrial semiconductor materials, defects in semiconducting transition metal dichalcogenides (TMDs) are substantially high. Despite several different approaches to modifying growth parameters, monolayer TMDs grown by chemical vapor processes have remained limited to mobilities of just a few thousand cm2/Vs at cryogenic temperatures. The carrier mobilities in TMDs are limited by both charged defects (typically around 0.1% to 1% of all atomic sites) and isovalent defects (~1% to 5% of all atomic sites), compared to hBN and graphene which have point defect densities of just .0001% of all atomic sites and mobilities that are primarily limited by edge disorder. In recent years, a promising synthesis route has been identified for producing high-quality transition metal selenides and tellurides using excess chalcogen as a flux; reducing defect densities to .001% and .1% for charged and isovalent defects, respectively. While this chalcogen self-flux method has improved TMDs considerably (with measured hole mobilities of nearly 70,000 cm2/Vs), the residual defect densities remain relatively high compared to state-of-the-art semiconductors like Si or GaAs. Additionally, this method is not compatible for growth of higher vapor pressure TMDs, like MoS2 and WS2, which are target materials for the semiconducting industry. In this talk, I will discuss some of the insights we have gained over the years into the limitations of reducing point defect densities further in transition metal selenides and tellurides and our understanding of how different growth parameters integrated into the self-flux approach impact defect densities for both charged and isovalent defects. I will also discuss new growth methods that we have developed to overcome the challenges associated with growing high-quality transition metal sulfides and give a perspective on where the next opportunities are for further improving this intriguing materials family.
Biography: Dr. Daniel Rhodes has been an assistant professor in the Department of Materials Science and Engineering and an affiliate of the Department of Physics since 2020 at the University of Wisconsin-Madison. Dr. Rhodes earned his Ph.D. in physics at Florida State University in 2016, where he synthesized novel bulk single crystals and performed magnetotransport experiments. From 2016 to 2019, he was a postdoc at Columbia University, where he focused on developing techniques for ultraclean semiconducting single crystals and magnetotransport of two-dimensional heterostructures. His group now focuses on combining his love for both efforts, novel materials synthesis and magnetotransport in 2D heterostructures, to explore new magnetotransport phenomena in novel 2D materials. He is a recipient of the Grainger Professorship at UW-Madison, the NSF CAREER award, and the 2025 ARO-sponsored PECASE award.