  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.
