Abstract
Computer image simulations provide a crucial aid to high-resolution transmission electron microscopy (HRTEM) in gaining fundamental understanding of the structure of materials. Interpretation of HRTEM images is, however, complicated due to continuous structure deformation caused by the imaging electron beam. A computational methodology has been implemented that takes into account the effects of the electron beam on deformation of sample structure during observation and imaging in HRTEM. The evolution of the sample structure is described as a sequence of externally initiated discrete damage events with a frequency determined by the cross section, which depends on the energy of the electron beam. A series of images showing structure evolution with time is obtained by coupling molecular dynamics simulations with the image simulation. These simulation parts are linked by two experimental parameters: the energy of the electron beam and the electron dose rate. As the energy of the electron beam also determines resolution and contrast of the obtained HRTEM image, a careful selection of its value is required to achieve a fine balance between reduction of the sample damage caused by the electron beam and the quality of the acquired image. The proposed computational approach is used to simulate the recently observed process of structural transformation of a small graphene flake into a fullerene cage. The simulated series of images showing the evolution of a graphene flake under the 80 keV electron beam closely reproduces experimental HRTEM images with regard to the structure evolution route, evolution rate, and signal-to-noise ratio. We show that under the increased electron beam energy of 200 keV a similar observation will be obscured by high damage rate or low signal-to-noise ratio.
Highlights
Recent advances in electron microscopy, in particular implementation of aberration correction of electromagnetic lenses, have stimulated unprecedented growth of interest in low-voltage high-resolution transmission electron microscopy (HRTEM) capable of spatial resolution at the atomic level when using energy of the imaging electrons near or below the ejection threshold
A subsequent evolution of the sample structure under a continuous flow of high-energy electrons is described using five basic iteration steps: (1) topological analysis of a sample structure, classification of every atom with respect to the probability of an external damage event, and an assignment of the corresponding cross sections; (2) simulation of a damage event based on the probability defined by the event cross section; (3) molecular dynamics (MD) simulation of the structure evolution at the elevated temperature for the annealing to the equilibrium configuration; (4) molecular dynamics simulations at 300 K to provide “frozen phonons” configurations at the equilibrium for subsequent image simulations; (5) image simulation step which utilizes the obtained atomic configurations and applies electron statistics followed by the time upscaling
We have developed a general computational approach for the inclusion of the dynamics of atomic rearrangements under the imaging electron beam into transmission electron microscopy image simulations
Summary
Recent advances in electron microscopy, in particular implementation of aberration correction of electromagnetic lenses, have stimulated unprecedented growth of interest in low-voltage high-resolution transmission electron microscopy (HRTEM) capable of spatial resolution at the atomic level when using energy of the imaging electrons near or below the ejection threshold. Many practically useful materials have been studied using HRTEM with great emphasis on carbon nanostructures.[1–14] The ability of HRTEM to observe the dynamics of individual atoms under the controlled influence of the electron beam (e-beam) brings another dimension to the method potentially providing a tool for direct measurements of diffusion coefficients, cross sections of the damage events, chemical constants, and other characteristics of the dynamic processes that take place at the atomic scale These advances in experimental HRTEM techniques require theoretical solutions capable of accurate treatment of the dynamic evolution of structures under the e-beam combined with subsequent image simulation at exactly the same e-beam conditions. The radiation effects of high-energy electrons on nanomaterials may include a large variety of processes (see, for example, a recent review in Ref 30) such as radiolysis triggered by ionization, local heating, direct atom ejection damage, atom rearrangements, and chemical reactions with residual gases, to mention a few
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