Materials Science Research Lecture

Abstract:

Atomically thin or two-dimensional (2D) materials can be assembled into bespoke heterostructures to produce some extraordinary physical phenomena. One exciting and relatively recent example is the formation of moiré superlattices from azimuthally misoriented layers. These moiré superlattices result in flat electronic bands that lead to an array of correlated electronic phases. However, complex strain relaxation strongly influences the electronic states of the material. Precise characterization of these materials and their properties is therefore critical to the understanding of the physical and chemical behavior of these moiré materials (and 2D heterostructures in general). The talk will primarily discuss the spontaneous mechanical relaxation (atomic reconstruction) and resultant intralayer strain fields at moiré superlattices, beginning with twisted bilayer graphene, then transition metal dichalcogenides, and finally twisted trilayer graphene. The talk will describe how these materials have been quantitatively imaged using a newly developed technique—4D-STEM Bragg interferometry—and the impact of spontaneous mechanical deformations on the electronic band structure. Finally a brief introduction to magnetic intercalant superlattices will be provided. These materials are formed by chemically inserting transition metal ions between atomically thin layers as a means of synthesizing few-layer magnetic materials.

More about the Speaker:

Kwabena was born in Ghana, West Africa. He moved to the US in 2004 for his undergraduate studies in Chemistry at Calvin University, MI, graduating with honors in 2008. In 2009, he began his graduate studies in Inorganic Chemistry with Prof. Daniel Nocera at MIT (and later Harvard University). After receiving his Ph.D. in 2015 from Harvard University, Kwabena began postdoctoral work in Prof. Philip Kim's group in the Department of Physics at Harvard. In July 2018, Kwabena joined the faculty of the UC Berkeley Department of Chemistry.