Clocking the anisotropic lattice dynamics of multi-walled carbon nanotubes by four-dimensional ultrafast transmission electron microscopy

TL;DR

This paper demonstrates the use of four-dimensional ultrafast transmission electron microscopy to visualize atomic-level anisotropic lattice dynamics in multi-walled carbon nanotubes, revealing ultrafast relaxation and expansion behaviors.

Contribution

It introduces a versatile 4D-UTEM capable of high-resolution atomic imaging in different modes and applies it to uncover detailed anisotropic lattice dynamics in MWCNTs.

Findings

Ultrafast lattice relaxations occur within a few picoseconds.

Distinct lattice expansion along the radial direction observed.

Anisotropic behaviors linked to chemical bonding differences.

Abstract

Recent advances in the four-dimensional ultrafast transmission electron microscope (4D-UTEM) with combined spatial and temporal resolutions have made it possible to directly visualize structural dynamics of materials at the atomic level. Herein, we report on our development on a 4D-UTEM which can be operated properly on either the photo-emission or the thermionic mode. We demonstrate its ability to obtain sequences of snapshots with high spatial and temporal resolutions in the study of lattice dynamics of the multi-walled carbon nanotubes (MWCNTs). This investigation provides an atomic level description of remarkable anisotropic lattice dynamics at the picosecond timescales. Moreover, our UTEM measurements clearly reveal that distinguishable lattice relaxations appear in intra-tubular sheets on an ultrafast timescale of a few picoseconds and after then an evident lattice expansion along the radical direction. These anisotropic behaviors in the MWCNTs are considered arising from the variety of chemical bonding, i.e. the weak van der Waals bonding between the tubular planes and the strong covalent sp2-hybridized bonds in the tubular sheets.