Dark field electron microscopy: A new way of imaging embedded defects with high resolution
RODRIGUEZ LAMAS R. 1, BUGALLO D. 2,5, LANGENBERG E. 4, YILDIRIM C. 1, RIVADULLA F. 2,3, DETLEFS C. 1
1 European Synchrotron Radiation Facility, Grenoble, France; 2 Centro de Investigación en Química Biológica y Materiales Moleculares (CIQUS), Universidade de Santiago de Compostela, Santiago de Compostela, Spain; 3 Departamento de Química-Física, Facultad de Química, Universidad de Santiago de Compostela, Santiago de Compostela, Spain; 4 Department of Condensed Matter Physics, Universidad de Barcelona , Barcelona , Spain; 5 Department of Materials Science and Engineering, Drexel University, Philadephia, United States
The embedded defects in the lattice, their organization, and the formation of substructures within a crystal play a crucial role in modifying its physical properties. This hierarchical organization occurs on a multilevel scale, from vacancies or isolated point defects to dislocations and dislocation networks extending through the crystal, or domain walls and grain boundaries separating domains and grains. The structure should be studied from the atomic scale extending up to the millimetre. This multiscale structuring requires techniques capable of moving between scales.
The current characterization tools are either destructive or limited to surfaces. Dark field x ray microscopy (DFXM), a full-field imaging technique, offers the ability to map the phase, orientation, and degree of strain of crystalline elements embedded in a sample volume in a non-destructive manner[1]. Making use of an X-ray objective aligned in the path of the diffracted beam, DFXM allows to visualize the internal structure of the material at different scales. DFXM is an ideal tool to study texture, detect defects or observe in-situ deformation, since it allows mapping the crystal structure through up to 1mm3 of matter and offers large field of view (~500x500 µm2). With a direct space resolution of ~150 nm, an angular resolution of 0.001o and a strain field sensitivity down to 10-5. In combination with external stimuli, image collection and rapid mapping allow observing structural changes in situ.
We present a case study on dislocation arrays in LaCoO3/SrTiO3 (LCO/STO) superlattices grown on STO substrates. Previous studies focused on ferroelastic and ferromagnetic properties through transition electron microscopy[2,3], but lacked micrometric scale defect distribution. Atomic force microscopy studies found interesting features on 3x3 µm2 regions, but were limited by the need for low temperature and magnetic fields [4]. With DFXM, we conducted a systematic study on (LCO/STO)n//STO superlattices of different thickness. Thorough texture and strain mapping were performed selectively for LCO and STO contributions. We found dislocation arrays matching STO lattice spacing, with defect-free parcels, both in LCO and STO, ranging from hundreds of nanometres to several microns, visible at room temperature over hundreds of microns. The findings of this study could have implications for future research in the field of strain engineering and materials science, and may help to advance the development of new materials with improved properties.
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