2, CIC nanoGUNE Consolider, San Sebastian, , Spain
4, University of Leeds, Leeds, , United Kingdom
3, University of Ulm, Ulm, , Germany
Transmission electron microscopy (TEM) is traditionally used as a tool to characterise materials, providing atomic resolution imaging of low dimensional nanostructures such as graphene and carbon nanotubes. In this context, damage to materials imaged by TEM (caused by collisions with the highly energetic electrons) is generally considered as something to be avoided or limited. However, with detailed understanding of the effects of the electron beam (e-beam), the energy transmitted from it can be used to drive chemical reactions that would be otherwise unfeasible.
A mechanistic understanding of beam-driven chemical reactions can be achieved with the comparison of experimental TEM images to the results of modelling. The dynamic response of nanotube-encapsulated organic species to the stimulus of the e-beam has been simulated using density functional theory (DFT) molecular dynamics. By combining these results with an accurate analytical model of the interaction of relativistic electrons and atomic nuclei, the experimentally observed behaviour of these systems under the e-beam has been quantitatively characterised.
The elemental dependence of the transfer of energy from the e-beam was shown to play a key role in determining reaction products, and is responsible for the very high susceptibility of carbon-hydrogen bonds to irradiation damage.1 Deuteration is an effective remedy for overcoming this limitation, increasing lifetimes of organic molecules under electron irradiation and therefore enhancing the accuracy of structural analysis by TEM.2 A close iterative collaboration between theory and microscopy was used to establish TEM as an effective tool for chemical reaction discovery and the characterisation of previously unknown reaction mechanisms.3 This has initially been demonstrated with two example reactions, in which organic molecule precursors are activated by the e-beam, eventually forming novel one-dimensional materials.4
As another test case, we study transformations of point defects in graphene.5 The cross-sections and threshold energies of irreversible (atom emission) and reversible (bond rotation) processes are measured. Observation of statistically significant number of events at variable experimental conditions allows us to decouple beam induced and thermal reaction pathways and obtain independent estimations of the cross-sections and activation energies for direct and back bond rotations. The back rotation is characterized by a very high value of the cross-section. Comparisons to theoretical estimations indicate that the assumed mechanism of direct knock-on damage cannot be the main cause of SW defect healing under electron beam.
1. Nanoscale, 2013, 5, 6677–6692
2. Small, 2015, 11, 622–629
3. Acc. Chem. Res., 2017, 50, 1797-1807
4. ACS Nano, 2017, 11, 2509–2520.
5. Carbon, 2016, 105, 176-182