The innovation engine for new materials

Quantifying Local Structure and Chemistry of Oxides and Nitrides with Scanning Transmission Electron Microscopy

Seminar Group: 


Professor James LeBeau


Dept. of Materials Science and Engineering
North Carolina State University


Friday, June 2, 2017 - 11:00am


ESB 1001


Prof. Susanne Stemmer

Unit-cell level structural distortions, occurring due to local chemical fluctuations or at surfaces, play a major role in determining material properties. Measurements of these distortions, however, are often either indirect or are limited by a number of factors. While scanning transmission electron microscopy can provide direct imaging of atomic structure, accuracy and precision of structural measurements have been traditionally hampered by sample drift and other scan distortion. This limitation has obscured the capabilities to directly correlate local structure in bulk or at surfaces with chemistry and/or electronic structure.

In this talk, I will introduce revolving scanning transmission electron microscopy (RevSTEM). The method achieves sub-0.1% accuracy and picometer level precision. Multiple case studies will be presented to demonstrate the capabilities of         this new technique to characterize a range of oxide and nitride materials. For example, I will show how picometer level precision enables the direct observation of static atomic displacements correlated with local chemistry within complex oxide solid solutions. Specially, I will highlight our work correlating local structure and chemistry in a prototypical oxide (LSAT) and the relaxor ferroelectric PbMg1/3Nb2/3O3. In nitrides, I will provide an example study of AlGaN/AlN quantum wells, where I will show how accurate STEM lattice parameter measurements can be used to directly determine composition and reveal the origins of compositional pulling.

Turning to surfaces, I will also show how STEM can serve as a powerful tool for surface science, enabled by a new sample preparation approach we developed to minimize surface contamination. Using this method, I will highlight our work on directly imaging polar (110) SrTiO3 surface dynamics via in-situ STEM at temperatures up to 900 ºC. Combining STEM observations, atomic resolution electron energy loss spectroscopy, and density functional theory, I will discuss the combined role of lattice misfit and charge compensation to determine the structure and chemistry of the observed polar surface reconstruction. Finally, I will also present recent STEM results exploring the formation of native crystalline oxides on III-nitride surfaces.