Recent experiments on van derWaals heterostructures involving transition metal dichalcogenides (TMDs) reveal the formation of highly-ordered mirror twin boundaries (MTBs), dividing domains of single 2H phase. Using a multi-scale approach, we show that the formation of the MTB networks is a result of a delicate balance between strain accumulation in the hetero interface, which has thus far been perceived to be negligible, and single-crystal preference of the overlayer. Such a counterintuitive finding demonstrates the importance of collective effects in weakly-interacting systems, e.g., in van der Waals epitaxy. As intriguingly, photo-excitation can also induce ordering and even a phase transition in MoTe2, as revealed by experiment. While the current theory for such a phenomenon requires a collective sliding of an entire Te layer, which is thermodynamically highly unlikely, we show again by a multi-scale approach that photo-induced nonequilibrium dynamics is the key for such a 2H-to-1T’ transition. The process is initiated by an ordering of photo-generated Te vacancies to form a local 1T’ structure, which serves as a seed to gather more vacancies into ordering and a subsequent growth of the 1T’ phase. Remarkably, such a process is concomitant with photo-enhanced vacancy diffusion, whereby increasing the speed of vacancy ordering.
Dr. Zhang received his Ph. D. in Physics from the University of California at Berkeley in 1989, under the supervision of Professor Marvin L. Cohen. Dr. Zhang then joined Xerox PARC in Palo Alto, California, where he performed postdoctoral research with Dr. Jim Chadi and Dr. John Northrup. In 1991, he moved to the National Renewable Energy Laboratory (NREL) in Golden, Colorado and became group leader for Computational Materials Science in 2005. In 2008, he was appointed Senior Kodosky Constellation Chair at Rensselaer Polytechnic Institute in Troy, NY.
Dr. Zhang has a broad theoretical research background in computational materials physics, which covers a range of inorganic and organic semiconductors and solids for bulk properties, defect structures, and surface physics. His most recent work involves earth-abundant photovoltaic materials, phase change memory materials, van der Waals interaction in organic semiconductors, lithium battery materials, hydrogen storage, topological insulators, graphene, and excited state dynamics. Dr. Zhang has more than 250 peer-reviewed publications with 8,700 citations (H index = 50). He has been a Fellow of the American Physical Society since 2001.