Atomic resolution imaging of the strength and 3D direction of atomic displacements in functional oxide thin films

Atomic resolution imaging is key to understanding thin film growth and how a particular set of conditions influences properties. Whilst such imaging in the scanning transmission electron microscope (STEM) has had a transformative impact in nanoscience, it forms projection images and provides no direct information about displacements perpendicular to the image plane. In this article, we show that it is possible to make atomic resolution maps of the direction and magnitude of La displacements at <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mo>∼</a:mo><a:msup><a:mn>30</a:mn><a:mo>∘</a:mo></a:msup></a:mrow></a:math> to the imaging plane in a <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mi mathvariant="normal">L</b:mi><b:msub><b:mi mathvariant="normal">a</b:mi><b:mn>2</b:mn></b:msub><b:mi>CoMn</b:mi><b:msub><b:mi mathvariant="normal">O</b:mi><b:mn>6</b:mn></b:msub></b:mrow></b:math> thin film on (111) LSAT <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mo>(</f:mo><f:mrow><f:mi>LaAl</f:mi><f:msub><f:mi mathvariant="normal">O</f:mi><f:mn>3</f:mn></f:msub><f:mrow><f:mo>−</f:mo><f:mi>La</f:mi></f:mrow><f:mrow><f:mo>(</f:mo><f:mrow><f:mi>Sr</f:mi><f:mo>,</f:mo><f:mi>Ta</f:mi></f:mrow><f:mo>)</f:mo></f:mrow><f:msub><f:mi mathvariant="normal">O</f:mi><f:mn>3</f:mn></f:msub></f:mrow><f:mo>)</f:mo></f:mrow></f:math> using a four-dimensional STEM (4DSTEM) methodology. This reveals that the La modulation lies preferentially in the interface plane, and is strongly suppressed close to the epitaxial interface, and further reveals how the modulation varies with distance from the interface with unit cell resolution. These details would be completely invisible to all prior techniques in electron microscopy and this sheds light on why this particular substrate in this orientation best promotes double perovskite cation ordering, and the consequent optimal magnetic ordering for this thin film system. The approach used herein of fitting atomic resolution 4DSTEM data to determine crystal parameters opens the door for a new era of atomic-resolution crystallography.