I will present our studies of the unusual electric, optical, thermal, and piezoelectric properties of a new family of two-dimensional (2D) materials, black phosphorus and its corresponding group IV-VI isoelectronic materials. For black phosphorus, we predict the intrinsic, fundamental properties such as quasiparticle band gaps, excitons, and anisotropic thermal conductance, and further engineered properties such as strain-tunable anisotropic electrical conductance and realizing Dirac cones and graphene electronics. Most of these predictions have been confirmed by recent experiments. Beyond black phosphorus, we find that the group IV-VI isoelectronic materials (monolayer GeS, GeSe, SnS, and SnSe) may exhibit dramatically enhanced piezoelectric effects; their characteristic piezoelectric coefficients are about two orders of magnitude larger than those of monolayer transition metal dichalcogenides (TMDCs) and conventional bulk piezoelectric materials. More recently, we find that these monolayer group IV monochalcogenides are essentially spontaneously polarized and exhibit the ferroelectric effect above room temperature. These "giant" piezoelectricity and ferroelectricity may open the door for energy production, memory, and sensors in wearable and transparent devices.

Li Yang received his BS (1997) and MS (2000) from the Beijing Normal University (Beijing, China) and PhD from the Georgia Institute of Technology (2006). From 2006 to 2009, he had worked as a postdoctoral fellow at the University of California, Berkeley. In 2009, he joined the faculty of the physics department of the Washington University in St Louis. He received the Faculty Early Career Development Award (CAREER) from the National Science Foundation (NSF) in 2015. He has published 62 peer-reviewed articles with around 7,000 citations. Combining first-principles simulations and relevant models, his group are working on condensed matter physics and studying fundamental electronic structures, transport and excited-state properties of reduced dimensional materials. He is particularly interested in predicting and designing novel materials with enhanced excitonic, thermoelectric, piezoelectric, and ferroelectric effects.