Ultrafast nanodiffraction and detection of nanoscale propagating waves

Electron microscopes are the eyes of a materials engineer and physical metallurgist; and, the interpretation of an electron microscopic image is not straight-forward, making this process of looking at structures at the atomic and meso-scale levels both exciting and challenging. This is the reason why, even though I never looked at any structure under the microscope, I still am excited about advances in microscopy — my more knowledgeable colleagues will then be able look at things and tell us what they find, which, in turn might give us some ideas and understanding of structures and hence their properties.

Here is a paper in the latest PNAS on nanodiffraction to study wave propagation at the nanoscale:

Coherent atomic motions in materials can be revealed using time-resolved X-ray and electron Bragg diffraction. Because of the size of the beam used, typically on the micron scale, the detection of nanoscale propagating waves in extended structures hitherto has not been reported. For elastic waves of complex motions, Bragg intensities contain all polarizations and they are not straightforward to disentangle. Here, we introduce Kikuchi diffraction dynamics, using convergent-beam geometry in an ultrafast electron microscope, to selectively probe propagating transverse elastic waves with nanoscale resolution. It is shown that Kikuchi band shifts, which are sensitive only to the tilting of atomic planes, reveal the resonance oscillations, unit cell angular amplitudes, and the polarization directions. For silicon, the observed wave packet temporal envelope (resonance frequency of 33 GHz), the out-of-phase temporal behavior of Kikuchi’s edges, and the magnitude of angular amplitude (0.3 mrad) and polarization Graphic elucidate the nature of the motion: one that preserves the mass density (i.e., no compression or expansion) but leads to sliding of planes in the antisymmetric shear eigenmode of the elastic waveguide. As such, the method of Kikuchi diffraction dynamics, which is unique to electron imaging, can be used to characterize the atomic motions of propagating waves and their interactions with interfaces, defects, and grain boundaries at the nanoscale.

Have fun!

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