Dynamics of single atoms and molecules encapsulated in carbon nanotubes

Abstract text (incl. figure legends and references)

Introduction and Objectives

Molecular motion and bond dissociation are two of the most fundamental phenomena underpinning properties of molecular materials. We have entrapped single atoms such as atoms (Kr) or basic molecules (HF and H2) within the fullerene C60 cage, encapsulated within a single-walled carbon nanotube. The use of the electron beam simultaneously as a stimulus of chemical reactions in molecules and as a sub-Å resolution imaging probe allows investigations of the molecular dynamics and reactivity in real time and at the atomic scale, which are probed directly by chromatic and spherical aberration-corrected high-resolution transmission electron microscopy (CC/CS-corrected HRTEM) imaging.

Methods

Working at reduced voltages, however, decreases the obtainable resolution to about 0.2 nm at 80 kV when using standard 3rd order aberration correctors such as the hexapole-type correctors [1]. The correction of 5th order geometric aberration correction together with chromatic aberration correction is necessary to archive atomic resolution at voltages at voltages even at 20 kV. The dedicated CS/CC SALVE (sub-Ångström low voltage electron microscopy) TEM [2] operates between 20 to 80 kV and still delivers sub-Å resolution at 40 kV. EDX and monochromated EELS spectroscopy used to verify elemental composition of the tube fillings and to determine bonding information. Molecular dynamic (MD) and density functional theory (DFT) calculations are used to describe and understand the reaction processes.

Results

We present the study of single atoms and simple molecules HF and H2O embedded into C60 clusters using atomic resolved HRTEM and STEM accompanied by local electron energy loss spectroscopy (EELS) and X-Ray spectroscopy (EDX). In contrary to plain C60 clusters the embedded atoms and molecules change the behavior of dimerization and general damage behavior of the clusters. The neutral Kr atoms are trying to keep a van-der-Waals distance of 0.4 nm and this effects the dimerization behavior. For the case of H2O and HF we demonstrate the rotation of the HF@C60 resp. H2O@C60 clusters under the influence of the electron beam (Fig.1) [3]. Depending on the electron voltage and the atom species the H2O and HF molecules dissociates at different rates.

Figure captions

Figure 1. (a) Experimental time series (2 s time interval) of 30 keV Cc/Cs-corrected HRTEM images of (HF@C60)@SWNT, showing blinking of the F-atoms. The green arrows indicate examples when the F-atoms are clearly visible, these appear to be visible and invisible in the same molecules over time (see Supporting Video 1). (b) Line profile across the centre of a fullerene molecule (as shown in green and white boxes indicated in micrographs (a)) showing variation of image contrast when F-atom is visible (green plot) or invisible (black plot) within the same molecule. (c) Model and corresponding HRTEM image simulations for four different orientations of HF@C60 in SWNT (four principal projections along elements of symmetry of the C60 cage C2, S6, C1 and C5). Only some orientations are able to produce a "dot" contrast at the centre of the fullerene cage due to an overlap of F-atom with two C-atoms of C60, indicated with green arrows.

References

[1] M. Haider et al. Ultramicroscopy, 75 (1998) 53

[2] M. Linck et al. Phys. Rev. Let. 117 (2016) 076101

[3] Biskupek et al. ACS Nano 14 (2020) 11178