Single-atom dynamics in scanning transmission electron microscopy
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Introduction The properties of materials are critically influenced by the presence or absence of single atoms. For example, the electronic, magnetic, and optical properties of semiconductors depend upon the concentration of dopant atoms. Likewise, the segregation of impurity atoms to defects, such as grain boundaries, often results in catastrophic failure of structural materials.1 Impurity atoms at grain boundaries also lead to dramatic changes in the electronic2 and ionic conductivity of a material.3 Point defects in materials, such as phosphorous impurities in silicon and nitrogen-vacancy centers in diamond, are some of the leading candidates for the emerging field of quantum computing.4 Imaging the location of single impurity or dopant atoms within such materials, ideally along with information about its dynamical behavior is, therefore, an important pursuit in materials science, with the ultimate goal being the ability to control the position of atoms within the material.5,6 The correction of aberrations in the scanning transmission electron microscope (STEM) has made imaging and spectroscopy of individual atomic columns a routine task. It is also possible to identify the three-dimensional (3D) structure of a material using electron tomography incorporating STEM annular dark-field (ADF) imaging.7 The improvement in the depth-of-focus in aberration-corrected STEM, when
combined with image simulations, now allows identification of the depth location of isolated dopant atoms from the contrast in STEM ADF images.8–10 Aberration correction also improves the signal-to-noise ratio in ADF images, which allows atomic-resolution images to be obtained at faster scan rates with lower beam currents and at low-to-moderate acceleration voltages (20–300 kV).11 Together, these factors also lead to an improvement in the temporal resolution for acquiring STEM images. It is now possible to sequentially acquire multiple atomic-resolution images of materials at a fast scan rate of a few seconds, or less, per frame. Moreover, the use of acceleration voltages below the threshold for knockon damage of the bulk atoms allows various diffusion processes to be deliberately excited by the beam and then to be observed in real-time using time-sequential STEM ADF imaging. This ability has opened up a new frontier in materials characterization by enabling direct observations of the dynamics of single dopant or impurity atoms in materials. In the past few years, time-sequential STEM imaging has led to new insights about diffusion processes both on the surface and within materials, motion of vacancies, single impurity atoms and few-atom clusters in two-dimensional (2D) materials, dislocation motion in materials, behavior of impurity atoms at grain boundaries and atomic-scale insights into phase transformations.
Rohan Mishra, Department of Mechanical Engineering and Materials Science; and Institute of Materials Science and Engineering, Washington University, USA; [email protected] Ryo Ishikawa, Institute of Engineering Innovation, The University o
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