Approaches to modelling irradiation-induced processes in transmission electron microscopy

Abstract

The recent progress in high-resolution transmission electron microscopy (HRTEM) has given rise to the possibility of in situ observations of nanostructure transformations and chemical reactions induced by electron irradiation. In this article we briefly summarise experimental observations and discuss in detail atomistic modelling of irradiation-induced processes in HRTEM, as well as mechanisms of such processes recognised due to modelling. Accurate molecular dynamics (MD) techniques based on first principles or tight-binding models are employed in the analysis of single irradiation-induced events, and classical MD simulations are combined with a kinetic Monte Carlo algorithm to simulate continuous irradiation of nanomaterials. It has been shown that sulphur-terminated graphene nanoribbons are formed inside carbon nanotubes as a result of an irradiation-selective chemical reaction. The process of fullerene formation in HRTEM during continuous electron irradiation of a small graphene flake has been simulated, and mechanisms driving this transformation analysed.

The atomistic mechanism of carbon nanotube cutting catalyzed by nickel under an electron beam

“…The relative stability of two-coordinate carbon atoms in chains is evident by the vast abundance of these atoms compared, for example, to one-coordinate carbon atoms (Table 1). It should be noted that similar carbon chain formation reactions have been frequently observed in irradiation-induced 13,31 or thermally activated 29,54,55 graphene-fullerene transformation processes and for graphene edge etching under the e-beam in the TEM. 56,57 If a chain of two-coordinate carbon atoms is formed and then adsorbed onto the metal cluster, and does not incorporate back to nanotube structure immediately, electron impacts can lead to dissociation of the chain into atoms that are then adsorbed or dissolved into the nickel cluster (Figure 4c).…”

“…This method enables to create a gap between the ends of the cut nanotubes in a reproducible manner without chemical contamination, with the size of the gap controlled by the size of the nickel cluster of ~1 nm, which is important for the fabrication of nanodevices. Molecular dynamics simulations based on the CompuTEM algorithm 13,30,31 confirm cutting of the (5,5) nanotube by a nickel cluster adsorbed on the hole in the nanotube sidewall prior to the cutting process. Detailed analysis of the mechanism reveals that the cutting by the 80 keV electron beam takes place over a timeframe of approximately 10 4 s and involves the ejection of 100 atoms.…”

“…(6) of Ref. 13). In general, the reactions of chain formation and chain dissociation should not require a significant energy transfer from the e-beam as they can be achieved through a sequence of steps in which only one carbon-carbon bond is broken at a time.…”

“…Effective modelling of the processes induced by electron irradiation were achieved using the CompuTEM algorithm 13,30,31 in which only interactions between incident electrons and atoms which lead to changes in the atomic structure (i.e. irradiation-induced events) are taken into account.…”

“…To investigate the detailed mechanism and kinetics of the nickel-assisted cutting of carbon nanotubes we performed reactive empirical MD simulations of this process based on the recently developed CompuTEM algorithm. 13,30,31 A (5,5) carbon nanotube 43 nm in length is considered (Figure 2a) in which the initial structure of the nanotube is geometrically optimized. The two-coordinate edge carbon atoms of the nanotube (in the present paper, only the nearest-neighbour carbon atoms are included in the coordination number) are fixed to prevent displacement of the nanotube.…”