Direct Visualization of Local Electromagnetic Field Structures by Scanning Transmission Electron Microscopy

Abstract

The functional properties of materials and devices are critically determined by the electromagnetic field structures formed inside them, especially at nanointerface and surface regions, because such structures are strongly associated with the dynamics of electrons, holes and ions. To understand the fundamental origin of many exotic properties in modern materials and devices, it is essential to directly characterize local electromagnetic field structures at such defect regions, even down to atomic dimensions. In recent years, rapid progress in the development of high-speed area detectors for aberration-corrected scanning transmission electron microscopy (STEM) with sub-angstrom spatial resolution has opened new possibilities to directly image such electromagnetic field structures at very high-resolution. In this Account, we give an overview of our recent development of differential phase contrast (DPC) microscopy for aberration-corrected STEM and its application to many materials problems. In recent years, we have developed segmented-type STEM detectors which divide the detector plane into 16 segments and enable simultaneous imaging of 16 STEM images which are sensitive to the positions and angles of transmitted/scattered electrons on the detector plane. These detectors also have atomic-resolution imaging capability. Using these segmented-type STEM detectors, we show DPC STEM imaging to be a very powerful tool for directly imaging local electromagnetic field structures in materials and devices in real space. For example, DPC STEM can clearly visualize the local electric field variation due to the abrupt potential change across a p-n junction in a GaAs semiconductor, which cannot be observed by normal in-focus bright-field or annular type dark-field STEM imaging modes. DPC STEM is also very effective for imaging magnetic field structures in magnetic materials, such as magnetic domains and skyrmions. Moreover, real-time imaging of electromagnetic field structures can now be realized through very fast data acquisition, processing, and reconstruction algorithms. If we use DPC STEM for atomic-resolution imaging using a sub-angstrom size electron probe, it has been shown that we can directly observe the atomic electric field inside atoms within crystals and even inside single atoms, the field between the atomic nucleus and the surrounding electron cloud, which possesses information about the atomic species, local chemical bonding and charge redistribution between bonded atoms. This possibility may open an alternative way for directly visualizing atoms and nanostructures, that is, seeing atoms as an entity of electromagnetic fields that reflect the intra- and interatomic electronic structures. In this Account, the current status of aberration-corrected DPC STEM is highlighted, along with some applications in real material and device studies.

Imaging Beam‐Sensitive Materials by Electron Microscopy

“…The scattered electrons are collected in diffraction plane by STEM detector 52. In order to complete millions of scanning points within a few seconds, scintillator/photomultiplier tubes (PMTs) are used for fast readout 257. The electrons strike the scintillator and the resulting photons are transferred into the PMT and output as an electronic pulse 258…”

“…iDPC‐STEM has also been shown to decrease the dose rate for atomic‐resolution imaging of a zeolite by a factor of five 270. Additionally, iDPC‐STEM technique is quite sensitive and can be used to map the strain fields,271 electric fields,265 and electromagnetic fields 257…”

Effect of strain on magnetic and orbital ordering of LaSrCrO3/LaSrMnO3 heterostructures

“…of the supplemental materials for the [2 LSCO/ 2 LSMO] and [2 LSCO/ 4 LSMO] SLs on LAO and STO 48. While the absolute values of the XMCD-derived magnetic moments may be lower than the moments predicted by SQUID due to the finite probe depth ( 3 nm) of the TEY mode,51 the assumption that the magnetic dipole term is negligible52 and errors due to the overlap of the L 2 and L 3 peaks, the trends for samples with similar thicknesses can be compared. For the [2 LSCO/ 2 LSMO] SLs, m s…”

Iterative Algorithm of Atomic Potential Reconstruction Based on DPC Signal from Thick Specimens

“…This imaging technique is called differential phase contrast (DPC). Electric field measurement can be carried out at atomic resolution, and electric field between nuclei and electrons has been visualized [1][2][3][4]. According to Maxwell's equation, static electric field can be converted into charge density map.…”