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This allows us to add two spatial dimensions to this technique and enables four-dimensional EELS (4D-EELS) measurements in a single nanostructure at nanometer scale. With a slot aperture, phonon dispersion data can be acquired in parallel within much shorter acquisition time 17. Here, we report an efficient acquisition method for real-space mapping of phonon dispersions in a single nanostructure. To date, nanoscale position-dependent phonon dispersion measurement in a single nanostructure has not been reported to the best of our knowledge.
#Phonon dispersio dahed vertical serial
Long acquisition time (up to ~10 h for each dispersion diagram at a single spatial location) of the serial acquisition method 15 also precludes the possibility to perform a 2D scan or even a line scan. Although previous studies have demonstrated high sensitivity and large momentum transfer range of this technique, they only focused on flakes and two-dimensional (2D) sheets that are essentially homogeneous in space, and thus did not take advantage of the high spatial resolution of electron microscopes. Recently, momentum-resolved vibrational measurements of hexagonal-boron nitride (h-BN) and graphite flakes using STEM-EELS have also been reported 15, 16. Many spatially resolved measurements such as atomic-resolved phonon spectroscopy 6– 9, phonon polariton (PhP) mapping 10– 12, isotope identification 13, and temperature measurement 14 are now attainable. Recent developments of aberration correctors and monochromators within in scanning transmission electron microscopes (STEMs) have enabled kiloelectronvolt electron beams with sub-10 meV energy resolution and atomic spatial resolution, extending electron energy loss spectroscopy (EELS) measurements into lattice vibration properties in the past decade 4, 5. Other vibrational spectroscopy techniques such as inelastic X-ray/neutron scattering are capable of measuring phonon dispersions for bulk crystals, but the lack of spatial resolution (limited by their beam size and low sensitivity) results in scattering signal averaged over large crystals or ensembles of nanostructures, hence precluding phonon dispersion measurements in individual nanostructures 2, 3. Although the tip enhanced Raman spectroscopy and scanning near-field optical microscopy 1 can reach nanometer spatial resolution, their momentum transfer is much smaller than the typical Brillouin zone (BZ) size and thus inaccessible to the high momentum phonons. However, such a measurement is very challenging for crystal defects, heterointerfaces and nanostructures, where the tiny size requires high spatial resolution and high detection sensitivity. There has been significant interest in measuring phonon dispersion in the hope of gaining mechanistic understandings and optimizing materials’ properties in materials science and condensed matter physics. Phonon plays a fundamental role in mechanical, electrical, optical, and thermal properties of materials. This work not only provides insights into vibrational properties of boron nitride nanotubes, but also demonstrates potential of the developed technique in nanoscale phonon dispersion measurements. Interestingly, acoustic phonons are sensitive to defect scattering, while optical modes are insensitive to small voids. Our measurements show that the phonon dispersion of multi-walled nanotubes is locally close to hexagonal-boron nitride crystals. By scanning the electron beam in real space while monitoring both the energy loss and the momentum transfer, we are able to reveal position- and momentum-dependent lattice vibrations at nanometer scale. Here, we demonstrate a four-dimensional electron energy loss spectroscopy technique, and present position-dependent phonon dispersion measurements in individual boron nitride nanotubes. However, this requires high detection sensitivity and combined spatial, energy and momentum resolutions, thus has been elusive. Directly mapping local phonon dispersion in individual nanostructures can advance our understanding of their thermal, optical, and mechanical properties.
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