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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

Femtosecond-resolved phonon nucleation and launch at a crystal step-edge.(a) Bright-field image highlighting two step-edges (indicated by the partial dashed white lines) in the WSe2 flake. The coloured lines represent regions from which the mean intensity was quantified and used to generate the time traces in g. The images were acquired with a 50-kHz repetition rate and a 20-s integration time per frame (see also the captions for Supplementary Videos 3 and 4 for further experimental details). (b–f) Select processed micrographs revealing the nucleation and emergence of a localized phonon wavefront (red). The dotted white lines indicate the position of the step-edge from which the wavefront emerges. (g) Intensity measurements obtained at the coloured lines in a and plotted as a function of time delay. The slope of the line passing through the peaks of the first oscillation period reflects the wavefront velocity of 5.5 nm ps−1. Scale bars, 200 nm.
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f3: Femtosecond-resolved phonon nucleation and launch at a crystal step-edge.(a) Bright-field image highlighting two step-edges (indicated by the partial dashed white lines) in the WSe2 flake. The coloured lines represent regions from which the mean intensity was quantified and used to generate the time traces in g. The images were acquired with a 50-kHz repetition rate and a 20-s integration time per frame (see also the captions for Supplementary Videos 3 and 4 for further experimental details). (b–f) Select processed micrographs revealing the nucleation and emergence of a localized phonon wavefront (red). The dotted white lines indicate the position of the step-edge from which the wavefront emerges. (g) Intensity measurements obtained at the coloured lines in a and plotted as a function of time delay. The slope of the line passing through the peaks of the first oscillation period reflects the wavefront velocity of 5.5 nm ps−1. Scale bars, 200 nm.

Mentions: To precisely resolve the phonon dynamics at an individual step edge, image series of WSe2 were acquired with increased magnification and finer temporal sampling (500-fs steps). Within the region of interest highlighted in Fig. 3, the intensity was observed to initially increase at a step edge in the first few-hundred femtoseconds and continue to grow for approximately 10 ps before relaxation via emission of a travelling wave approximately perpendicular to the interface (see Supplementary Videos 3 and 4). The processed difference images (Fig. 3b–f; see ‘Methods' section) and corresponding time-dependent intensity traces (Fig. 3g) display the emergence mechanism of the in-plane acoustic phonons shown in Fig. 2j–o. Notably, the frequency of dynamic intensity at the step edge is in accord with the interlayer echoing of back-and-forth acoustic phonons and resulting oscillating moiré fringes414243 (see also Supplementary Fig. 3). It is therefore likely that the differential stress imparted on the interface by dephasing of the longitudinal c axis waves across regions of differing height gives rise to the formation of the in-plane travelling phonons. Systematic studies to probe this and other aspects associated with the dynamic contrast mechanisms are currently in progress and will be described elsewhere.


Femtosecond electron imaging of defect-modulated phonon dynamics.

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

Femtosecond-resolved phonon nucleation and launch at a crystal step-edge.(a) Bright-field image highlighting two step-edges (indicated by the partial dashed white lines) in the WSe2 flake. The coloured lines represent regions from which the mean intensity was quantified and used to generate the time traces in g. The images were acquired with a 50-kHz repetition rate and a 20-s integration time per frame (see also the captions for Supplementary Videos 3 and 4 for further experimental details). (b–f) Select processed micrographs revealing the nucleation and emergence of a localized phonon wavefront (red). The dotted white lines indicate the position of the step-edge from which the wavefront emerges. (g) Intensity measurements obtained at the coloured lines in a and plotted as a function of time delay. The slope of the line passing through the peaks of the first oscillation period reflects the wavefront velocity of 5.5 nm ps−1. Scale bars, 200 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f3: Femtosecond-resolved phonon nucleation and launch at a crystal step-edge.(a) Bright-field image highlighting two step-edges (indicated by the partial dashed white lines) in the WSe2 flake. The coloured lines represent regions from which the mean intensity was quantified and used to generate the time traces in g. The images were acquired with a 50-kHz repetition rate and a 20-s integration time per frame (see also the captions for Supplementary Videos 3 and 4 for further experimental details). (b–f) Select processed micrographs revealing the nucleation and emergence of a localized phonon wavefront (red). The dotted white lines indicate the position of the step-edge from which the wavefront emerges. (g) Intensity measurements obtained at the coloured lines in a and plotted as a function of time delay. The slope of the line passing through the peaks of the first oscillation period reflects the wavefront velocity of 5.5 nm ps−1. Scale bars, 200 nm.
Mentions: To precisely resolve the phonon dynamics at an individual step edge, image series of WSe2 were acquired with increased magnification and finer temporal sampling (500-fs steps). Within the region of interest highlighted in Fig. 3, the intensity was observed to initially increase at a step edge in the first few-hundred femtoseconds and continue to grow for approximately 10 ps before relaxation via emission of a travelling wave approximately perpendicular to the interface (see Supplementary Videos 3 and 4). The processed difference images (Fig. 3b–f; see ‘Methods' section) and corresponding time-dependent intensity traces (Fig. 3g) display the emergence mechanism of the in-plane acoustic phonons shown in Fig. 2j–o. Notably, the frequency of dynamic intensity at the step edge is in accord with the interlayer echoing of back-and-forth acoustic phonons and resulting oscillating moiré fringes414243 (see also Supplementary Fig. 3). It is therefore likely that the differential stress imparted on the interface by dephasing of the longitudinal c axis waves across regions of differing height gives rise to the formation of the in-plane travelling phonons. Systematic studies to probe this and other aspects associated with the dynamic contrast mechanisms are currently in progress and will be described elsewhere.

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