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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

Morphological heterogeneity and atomic-scale order of the WSe2 and Ge specimen regions of interest.(a,d) Bright-field images of (a) a WSe2 flake and (d) a thin Ge crystal. The red, dashed circle denotes the position of the selected-area aperture used to generate the diffraction patterns shown in b and e. Scale bars, 1 μm. (b,e) Corresponding selected-area diffraction patterns obtained approximately along the [001] and [110] zone axes for WSe2, and Ge, respectively, with several Bragg spots indexed. Scale bars, 5 nm−1 (b); 2 nm−1 (e). (c,f) Crystal structures of (c) WSe2 and (f) Ge, as viewed down the [001] and [110] zone axes, respectively. In c: yellow spheres=Se; blue spheres=W, as labelled.
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f1: Morphological heterogeneity and atomic-scale order of the WSe2 and Ge specimen regions of interest.(a,d) Bright-field images of (a) a WSe2 flake and (d) a thin Ge crystal. The red, dashed circle denotes the position of the selected-area aperture used to generate the diffraction patterns shown in b and e. Scale bars, 1 μm. (b,e) Corresponding selected-area diffraction patterns obtained approximately along the [001] and [110] zone axes for WSe2, and Ge, respectively, with several Bragg spots indexed. Scale bars, 5 nm−1 (b); 2 nm−1 (e). (c,f) Crystal structures of (c) WSe2 and (f) Ge, as viewed down the [001] and [110] zone axes, respectively. In c: yellow spheres=Se; blue spheres=W, as labelled.

Mentions: To locate and characterize specific defects and other nanoscale imperfections of interest, the WSe2 and Ge specimens were initially surveyed using bright-field imaging and parallel-beam electron diffraction. Static structural and morphological characterization of specific specimen regions of interest, on which subsequent femtosecond electron imaging studies were conducted, are summarized in Fig. 1. As can be seen from the diffraction patterns, the WSe2 and Ge specimens are oriented such that the electron beam travels approximately down the [001] and [110] zone axes, respectively (Fig. 1c,f)39. Combined, the bright-field images and corresponding diffraction patterns illustrate the macroscopically crystalline but microscopically disordered nature of the regions. From the images, several features of interest for the present study can be identified, which, conversely, are not readily apparent from the diffraction patterns alone. These include step edges and terraces, wrinkles, folds, vacuum-crystal interfaces and cracks. In addition, bend contours are quite prominent and widespread, as are moiré fringes in the WSe2 specimen. See the ‘Methods' section below for descriptions of how the specimens were prepared.


Femtosecond electron imaging of defect-modulated phonon dynamics.

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

Morphological heterogeneity and atomic-scale order of the WSe2 and Ge specimen regions of interest.(a,d) Bright-field images of (a) a WSe2 flake and (d) a thin Ge crystal. The red, dashed circle denotes the position of the selected-area aperture used to generate the diffraction patterns shown in b and e. Scale bars, 1 μm. (b,e) Corresponding selected-area diffraction patterns obtained approximately along the [001] and [110] zone axes for WSe2, and Ge, respectively, with several Bragg spots indexed. Scale bars, 5 nm−1 (b); 2 nm−1 (e). (c,f) Crystal structures of (c) WSe2 and (f) Ge, as viewed down the [001] and [110] zone axes, respectively. In c: yellow spheres=Se; blue spheres=W, as labelled.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Morphological heterogeneity and atomic-scale order of the WSe2 and Ge specimen regions of interest.(a,d) Bright-field images of (a) a WSe2 flake and (d) a thin Ge crystal. The red, dashed circle denotes the position of the selected-area aperture used to generate the diffraction patterns shown in b and e. Scale bars, 1 μm. (b,e) Corresponding selected-area diffraction patterns obtained approximately along the [001] and [110] zone axes for WSe2, and Ge, respectively, with several Bragg spots indexed. Scale bars, 5 nm−1 (b); 2 nm−1 (e). (c,f) Crystal structures of (c) WSe2 and (f) Ge, as viewed down the [001] and [110] zone axes, respectively. In c: yellow spheres=Se; blue spheres=W, as labelled.
Mentions: To locate and characterize specific defects and other nanoscale imperfections of interest, the WSe2 and Ge specimens were initially surveyed using bright-field imaging and parallel-beam electron diffraction. Static structural and morphological characterization of specific specimen regions of interest, on which subsequent femtosecond electron imaging studies were conducted, are summarized in Fig. 1. As can be seen from the diffraction patterns, the WSe2 and Ge specimens are oriented such that the electron beam travels approximately down the [001] and [110] zone axes, respectively (Fig. 1c,f)39. Combined, the bright-field images and corresponding diffraction patterns illustrate the macroscopically crystalline but microscopically disordered nature of the regions. From the images, several features of interest for the present study can be identified, which, conversely, are not readily apparent from the diffraction patterns alone. These include step edges and terraces, wrinkles, folds, vacuum-crystal interfaces and cracks. In addition, bend contours are quite prominent and widespread, as are moiré fringes in the WSe2 specimen. See the ‘Methods' section below for descriptions of how the specimens were prepared.

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