Abstract:
Wurtzite-ZnO is a wide-bandgap polar material with a ferroelectric-switching barrier that is too high to utilize, but the barrier can be reduced and switching observed in substituted materials such as Zn0.5Mg0.5O. Here, we seek to understand atomic-scale features that control concerted polarization switching in these and related systems, focusing on the planar hexagonal structures h-ZnO and h-Zn0.5Mg0.5O that may act as metastable intermediate phases along the switching pathway. h-ZnO is well known in thin films where it is stabilized by surface interactions, and once has been reported as a metastable phase of ZnO nanocrystals under ambient conditions. The reported crystal parameters were unrealistic, and we re-refine the structure using advanced phase-matching techniques to obtain a realistic outcome. A wide range of pure and dispersion-corrected density-functional theory (DFT) computational approaches, as well as ab initio Hartree-Fock (HF), Møller-Plesset perturbation-theory (MP2), and random-phase approximation (RPA) calculations, are applied to understand this result. The perceived stability of h-ZnO is found to be strongly influenced by the applied dispersion correction, with the consensus being that dispersion interactions are insufficient to stabilize h-ZnO as a metastable phase in infinite crystals. Hence the stability of the observed nanocrystals is attributed to surface interactions. In contrast, h-Zn0.5Mg0.5O is consistently predicted to be at least metastable, with some dispersion-corrected DFT approaches predicting it to be more stable than its wurtzite form; all DFT methods overestimate its stability compared to MP2 and RPA. Dispersion forces are found to be most significant for hypothetical planar hexagonal structures constrained to the lattice vectors of the wurtzite phases. In general, our results demonstrate that an accurate treatment of dispersion forces is essential when describing polarization switching and ferroelectric behaviour in wurtzite-structured materials.
Biography:
Jeff Reimers studied organic spectroscopy under Ian Ross and Gad Fischer before doing a PhD with Bob Watts on the structure, thermodynamics, and spectroscopy of water and ice. He then studied semiclassical quantum mechanics in USA under Kent Wilson and Rick Heller, before returning to Australia to be an ARC Research Fellow from 1985 to 2010 at the University of Sydney and there as a professor untill 2013. There he collaborated extensively with Noel Hush and Max Crossley on problems involving electron transfer, molecular electronics, porphyrin chemistry, self-assembly, electronic-structure theory, and photosynthesis. In 2014 he moved to a joint appointment at University of Technology Sydney and Shanghai University, focusing mostly on basic chemistry and spectroscopy, nanophotonics, molecular electronics and electronic-structure computational methods. His work spans a wide range of chemical applications, from biochemical function to electronic devices to the origins of consciousness. He has received the RACI Physical Chemistry Division Medal and the H.G. Smith Medal, the David Craig Medal of the Australian Academy of Science, and the Shanghai Magnolia Medal; he is a Fellow of the RACI, the Royal Society of NSW, and the Australian Academy of Science.
Seminar Series by the NYU-ECNU Center for Computational Chemistry at NYU Shanghai
This event is open to the NYU Shanghai, NYU, ECNU community and the computational chemistry community.