Origin of Axial and Radial Expansions in Carbon Nanotubes Revealed by Ultrafast Diffraction and Spectroscopy

Abstract

The coupling between electronic and nuclear degrees of freedom in low-dimensional, nanoscale systems plays a fundamental role in shaping many of their properties. Here, we report the disentanglement of axial and radial expansions of carbon nanotubes, and the direct role of electronic and vibrational excitations in determining such expansions. With subpicosecond and subpicometer resolutions, structural dynamics were explored by monitoring changes of the electron diffraction following an ultrafast optical excitation, whereas the transient behavior of the charge distribution was probed by time-resolved, electron-energy-loss spectroscopy. Our experimental results, and supporting density functional theory calculations, indicate that a population of the excited carriers in the antibonding orbitals of the nanotube walls drives a transient axial deformation in ∼1 ps; this deformation relaxes on a much longer time scale, 17 ps, by nonradiative decay. The electron-driven expansion is distinct from the phonon-driven dynamics observed along the radial direction, using the characteristic Bragg reflections; it occurs in 5 ps. These findings reveal the nonequilibrium distortion of the unit cell at early times and the role of the electron(phonon)-induced stress in the lattice dynamics of one-dimensional nanostructures.

Rippling ultrafast dynamics of suspended 2D monolayers, graphene

“…To investigate the realtime response of the atomic structure of the suspended graphene to the ultrafast optical excitation, we used ultrafast electron crystallography (UEC), which represents a unique tool for investigating energy transport processes and structural dynamics of nanomaterials (22)(23)(24). Compared with other ultrafast optical and X-ray time-resolved techniques, UEC offers not only a higher spatial resolution, down to the atomic scale, but also a high sensitivity to small material volumes, such as monoatomic layers, due to a higher electron/matter scattering cross-section.…”

Sub-phonon-period compression of electron pulses for atomic diffraction

“…In condensed matter, the half periods of optical phonons range down to 20 fs, defining the fastest paths towards structural phase transformations. For example, tens-of-fs dynamics is probably decisive for charge–density wave transitions 9 , vanadium dioxide 4 12 , graphite 13 , carbon nanotubes 54 and some molecular switches 55 . With reasonable statistics, the demonstrated FWHM pulse duration of 28±5 fs offers a temporal resolution below 20 fs, sufficiently short for capturing the entire range of possible atomic motions in condensed matter and molecules, in addition to the slower 100-fs dynamics which is also important 56 .…”

Four-dimensional electron microscopy: Ultrafast imaging, diffraction and spectroscopy in materials science and biology

“…Recent developments in ultrafast diffraction techniques have enabled the direct probing of the atomic-scale dynamics in several nanosystems using both transmission and reflection geometry. The transmission configuration is ideal for the investigation of free-standing thin layers [15,16], single particles [17], nanotubes networks [18,19], and biochemical specimens [20,21], whereas the high surface sensitivity of the reflection geometry (grazing incidence) enables the observation of crystal surfaces [22], adsorbates [23], and epitaxial nanostructures [24,25,26].…”

“…This method represents a powerful approach to investigate the electron density distribution and the electronic structure of materials [33]. When EELS is implemented in the 4D-EM setup described above, the collective dynamics of valence and core electrons can be obtained with femtosecond temporal resolution, enabling the spatiotemporal mapping of electronic structure changes [34,18,35].…”