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Femtosecond electron imaging of defect-modulated phonon dynamics.

Cremons DR, Plemmons DA, Flannigan DJ - Nat Commun (2016)

Bottom Line: Here we report direct, real-space imaging of the emergence and evolution of acoustic phonons at individual defects in crystalline WSe2 and Ge.Via bright-field imaging with an ultrafast electron microscope, we are able to image the sub-picosecond nucleation and the launch of wavefronts at step edges and resolve dispersion behaviours during propagation and scattering.These observations provide unprecedented insight into the roles played by individual atomic and nanoscale features on acoustic-phonon dynamics.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, USA.

ABSTRACT
Precise manipulation and control of coherent lattice oscillations via nanostructuring and phonon-wave interference has the potential to significantly impact a broad array of technologies and research areas. Resolving the dynamics of individual phonons in defect-laden materials presents an enormous challenge, however, owing to the interdependent nanoscale and ultrafast spatiotemporal scales. Here we report direct, real-space imaging of the emergence and evolution of acoustic phonons at individual defects in crystalline WSe2 and Ge. Via bright-field imaging with an ultrafast electron microscope, we are able to image the sub-picosecond nucleation and the launch of wavefronts at step edges and resolve dispersion behaviours during propagation and scattering. We discover that the appearance of speed-of-sound (for example, 6 nm ps(-1)) wavefronts are influenced by spatially varying nanoscale strain fields, taking on the appearance of static bend contours during propagation. These observations provide unprecedented insight into the roles played by individual atomic and nanoscale features on acoustic-phonon dynamics.

No MeSH data available.


Related in: MedlinePlus

Analysis of distinct localized phonon modes in WSe2.(a) Surface plot revealing the evolution of two primary modes, as observed in the region of interest highlighted in the accompanying images (t=0 and 2,500 ps; purple boxes). The images were acquired with a 25-kHz repetition rate and an 18-s integration time per frame (see also the captions for Supplementary Video 5 for further experimental details). The coloured lines demarcate the specific subregions analysed. The thin, near-vertical streaks predominantly in the sub-500-ps range and spanning the entire step interface region (600 to 1,200 nm; magnified in the inset) are produced by a relatively high-frequency mode, whereas the large amplitude, broad, diagonal features continuing out to the time-window limit are due to a more slowly propagating low-frequency mode. (b) Time-domain Fourier transform of traces obtained from the subregions marked by the coloured lines on the surface plot in a. The peaks arising from echoing of longitudinal acoustic phonons () are labelled, while the travelling-wave modes (A0, ) are indicated by the dashed lines, highlighting propagation dispersion (see also Supplementary Fig. 4, which illustrates dispersion observed in the Ge crystal, and Supplementary Video 6, which shows the effects of edge shape on the appearance of propagating diffraction contrast). The superscript i indicates waves emanating from the thicker step-edge region (i=1) and from that bounded by the crystal-vacuum interface (i=2). The spectra are offset for clarity. (c) Schematic of the symmetry of the propagating phonon modes with magnified view of a single layer. The dilatational modes (, top) occur near 25 and 40 GHz, while the flexural modes (A0, bottom) range from 2 to 5 GHz.
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f4: Analysis of distinct localized phonon modes in WSe2.(a) Surface plot revealing the evolution of two primary modes, as observed in the region of interest highlighted in the accompanying images (t=0 and 2,500 ps; purple boxes). The images were acquired with a 25-kHz repetition rate and an 18-s integration time per frame (see also the captions for Supplementary Video 5 for further experimental details). The coloured lines demarcate the specific subregions analysed. The thin, near-vertical streaks predominantly in the sub-500-ps range and spanning the entire step interface region (600 to 1,200 nm; magnified in the inset) are produced by a relatively high-frequency mode, whereas the large amplitude, broad, diagonal features continuing out to the time-window limit are due to a more slowly propagating low-frequency mode. (b) Time-domain Fourier transform of traces obtained from the subregions marked by the coloured lines on the surface plot in a. The peaks arising from echoing of longitudinal acoustic phonons () are labelled, while the travelling-wave modes (A0, ) are indicated by the dashed lines, highlighting propagation dispersion (see also Supplementary Fig. 4, which illustrates dispersion observed in the Ge crystal, and Supplementary Video 6, which shows the effects of edge shape on the appearance of propagating diffraction contrast). The superscript i indicates waves emanating from the thicker step-edge region (i=1) and from that bounded by the crystal-vacuum interface (i=2). The spectra are offset for clarity. (c) Schematic of the symmetry of the propagating phonon modes with magnified view of a single layer. The dilatational modes (, top) occur near 25 and 40 GHz, while the flexural modes (A0, bottom) range from 2 to 5 GHz.

Mentions: The information contained in the image series can be further illustrated via a space–time surface plot (see ‘Methods' section). In Fig. 4a, an analysis of contrast dynamics observed in the region between the WSe2 crystal–vacuum interface and the step edge is summarized (see also Supplementary Video 5). Each streak corresponds to one period of an acoustic phonon, with the slope and width indicative of phase velocity and frequency, respectively. Such an analysis reveals the presence of multiple modes in this region, with a relatively high-frequency oscillation (phase velocity=5.5 nm ps−1) generated during the initial moments of excitation, and slower (0.9 nm ps−1), lower-frequency dynamics dominating after a few-hundred picoseconds. Similar spectral features identified with Brillouin light scattering from thin silicon membranes have been attributed to Lamb-wave modes44. Analogously here, the confinement of longitudinal acoustic phonons within the specimen thickness gives rise to in-plane propagating modes. These consist of travelling symmetric () and antisymmetric (A0) interlayer displacements (Fig. 4c). It has been predicted that such dilatational and flexural acoustic modes significantly influence thermal transport in layered materials45.


Femtosecond electron imaging of defect-modulated phonon dynamics.

Cremons DR, Plemmons DA, Flannigan DJ - Nat Commun (2016)

Analysis of distinct localized phonon modes in WSe2.(a) Surface plot revealing the evolution of two primary modes, as observed in the region of interest highlighted in the accompanying images (t=0 and 2,500 ps; purple boxes). The images were acquired with a 25-kHz repetition rate and an 18-s integration time per frame (see also the captions for Supplementary Video 5 for further experimental details). The coloured lines demarcate the specific subregions analysed. The thin, near-vertical streaks predominantly in the sub-500-ps range and spanning the entire step interface region (600 to 1,200 nm; magnified in the inset) are produced by a relatively high-frequency mode, whereas the large amplitude, broad, diagonal features continuing out to the time-window limit are due to a more slowly propagating low-frequency mode. (b) Time-domain Fourier transform of traces obtained from the subregions marked by the coloured lines on the surface plot in a. The peaks arising from echoing of longitudinal acoustic phonons () are labelled, while the travelling-wave modes (A0, ) are indicated by the dashed lines, highlighting propagation dispersion (see also Supplementary Fig. 4, which illustrates dispersion observed in the Ge crystal, and Supplementary Video 6, which shows the effects of edge shape on the appearance of propagating diffraction contrast). The superscript i indicates waves emanating from the thicker step-edge region (i=1) and from that bounded by the crystal-vacuum interface (i=2). The spectra are offset for clarity. (c) Schematic of the symmetry of the propagating phonon modes with magnified view of a single layer. The dilatational modes (, top) occur near 25 and 40 GHz, while the flexural modes (A0, bottom) range from 2 to 5 GHz.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4835536&req=5

f4: Analysis of distinct localized phonon modes in WSe2.(a) Surface plot revealing the evolution of two primary modes, as observed in the region of interest highlighted in the accompanying images (t=0 and 2,500 ps; purple boxes). The images were acquired with a 25-kHz repetition rate and an 18-s integration time per frame (see also the captions for Supplementary Video 5 for further experimental details). The coloured lines demarcate the specific subregions analysed. The thin, near-vertical streaks predominantly in the sub-500-ps range and spanning the entire step interface region (600 to 1,200 nm; magnified in the inset) are produced by a relatively high-frequency mode, whereas the large amplitude, broad, diagonal features continuing out to the time-window limit are due to a more slowly propagating low-frequency mode. (b) Time-domain Fourier transform of traces obtained from the subregions marked by the coloured lines on the surface plot in a. The peaks arising from echoing of longitudinal acoustic phonons () are labelled, while the travelling-wave modes (A0, ) are indicated by the dashed lines, highlighting propagation dispersion (see also Supplementary Fig. 4, which illustrates dispersion observed in the Ge crystal, and Supplementary Video 6, which shows the effects of edge shape on the appearance of propagating diffraction contrast). The superscript i indicates waves emanating from the thicker step-edge region (i=1) and from that bounded by the crystal-vacuum interface (i=2). The spectra are offset for clarity. (c) Schematic of the symmetry of the propagating phonon modes with magnified view of a single layer. The dilatational modes (, top) occur near 25 and 40 GHz, while the flexural modes (A0, bottom) range from 2 to 5 GHz.
Mentions: The information contained in the image series can be further illustrated via a space–time surface plot (see ‘Methods' section). In Fig. 4a, an analysis of contrast dynamics observed in the region between the WSe2 crystal–vacuum interface and the step edge is summarized (see also Supplementary Video 5). Each streak corresponds to one period of an acoustic phonon, with the slope and width indicative of phase velocity and frequency, respectively. Such an analysis reveals the presence of multiple modes in this region, with a relatively high-frequency oscillation (phase velocity=5.5 nm ps−1) generated during the initial moments of excitation, and slower (0.9 nm ps−1), lower-frequency dynamics dominating after a few-hundred picoseconds. Similar spectral features identified with Brillouin light scattering from thin silicon membranes have been attributed to Lamb-wave modes44. Analogously here, the confinement of longitudinal acoustic phonons within the specimen thickness gives rise to in-plane propagating modes. These consist of travelling symmetric () and antisymmetric (A0) interlayer displacements (Fig. 4c). It has been predicted that such dilatational and flexural acoustic modes significantly influence thermal transport in layered materials45.

Bottom Line: Here we report direct, real-space imaging of the emergence and evolution of acoustic phonons at individual defects in crystalline WSe2 and Ge.Via bright-field imaging with an ultrafast electron microscope, we are able to image the sub-picosecond nucleation and the launch of wavefronts at step edges and resolve dispersion behaviours during propagation and scattering.These observations provide unprecedented insight into the roles played by individual atomic and nanoscale features on acoustic-phonon dynamics.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, USA.

ABSTRACT
Precise manipulation and control of coherent lattice oscillations via nanostructuring and phonon-wave interference has the potential to significantly impact a broad array of technologies and research areas. Resolving the dynamics of individual phonons in defect-laden materials presents an enormous challenge, however, owing to the interdependent nanoscale and ultrafast spatiotemporal scales. Here we report direct, real-space imaging of the emergence and evolution of acoustic phonons at individual defects in crystalline WSe2 and Ge. Via bright-field imaging with an ultrafast electron microscope, we are able to image the sub-picosecond nucleation and the launch of wavefronts at step edges and resolve dispersion behaviours during propagation and scattering. We discover that the appearance of speed-of-sound (for example, 6 nm ps(-1)) wavefronts are influenced by spatially varying nanoscale strain fields, taking on the appearance of static bend contours during propagation. These observations provide unprecedented insight into the roles played by individual atomic and nanoscale features on acoustic-phonon dynamics.

No MeSH data available.


Related in: MedlinePlus