Seminar by Haimei Zheng

Revealing Unseen Materials Dynamics in Liquids at the Atomic Level
Haimei Zheng
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California
An understanding of how materials grow and transform in their working environments is essential to the development of functional materials for various applications. My group studies a variety of physical and chemical processes of materials in liquids including gases by the development and applications of in situ transmission electron microscopy (TEM). In this talk, I will first present our recent study of shape evolution of colloidal nanocrystals in a liquid cell using TEM. An understanding of nanocrystal shape controlling mechanisms is significant since nanoparticle catalytic and other surface-enhance properties are highly dependent on their shape. However, how facets develop during growth is largely unknown due to the lack of direct observation. Using in situ liquid cell TEM, we have been able to identify unique growth mechanisms of nanocrystals and have discovered rules applied to bulk systems break down at the nanoscale. For instance, the facet development of Pt nanocubes during growth drastically differs from what is predicted by the widely accepted surface energy minimization rule, i.e., Wulff construction. We found that the growth rates of  the {100}, {110} and {111} facets are similar until the {100} facets stop growth. Subsequently, the {110} facets are eliminated when two adjacent {100} facets meet. Eventually, the growth of {111} facets fills the corners to complete a nanocube. Theoretical calculation suggests oleylamine ligand mobility on the facet is likely responsible for the arresting of {100} growing facets. In the second part of the talk, I will present the in situ study of electrochemical processes for battery applications using electrochemical liquid cells. Dissolution-deposition at the electrode-liquid electrolyte interfaces including phase transformations of the precipitates during charge cycles in a Li-S system will be discussed.
Dr. Haimei Zheng is a staff Scientist in Materials Sciences Division at Lawrence Berkeley National Laboratory and an adjunct faculty in Materials Science Department at University of California, Berkeley. Dr. Zheng did her Ph.D thesis with Prof. Ramamoorthy Ramesh from University of Maryland, College Park. She was a postdoc with Prof. Paul Alivisatos at University of California, Berkeley and jointly at the National Center for Electron Microscopy of Lawrence Berkeley National Laboratory.
Dr. Zheng’s research interests lie in physical and chemical processes of materials at the atomic level. The research topics in her group are centered on materials transformations in energy conversation and energy storage applications. By developing and applying in situ transmission electron microscopy, her group has been able to reveal many dynamic processes of materials in liquids or gases that are previously not available. She has also made significant effort on developing depth understanding of the processes through collaborative theoretical calculation. Dr. Zheng has about 70 publications with the total citations of over 5000 times. She received Lawrence Berkeley National Laboratory Director’s award for exceptional scientific achievements in 2013, DOE Office of Science Early Career Award in 2011 and MRS Graduate Student Gold Medal Award in 2003.

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