The most practical method for TEM image simulations is the multislice method, which is known to be an accurate numerical procedure for solving the quantum mechanical electron-specimen interaction. Although most simulation codes treat the scattering process as purely elastic and coherent, inelastic scattering cannot be neglected and it has to be included in realistic simulations. Inelastic phonon scattering is often incorporated by using the frozen phonon model [1, 2] and the electronic excitations by using the density matrix approach [3, 4]. Nowadays, new computer technologies allow us to perform large TEM calculations with inclusion of accurate approximations of the electron-specimen interaction in an acceptable amount of time.

A general overview of the MULTEM program along with a number of examples has been reported in [5]. In this work we present a new version of the open source MULTEM program, which adds key features including a graphical user interface, tapering truncation of the atomic potential, CPU multithreading, single/double precision calculations, STEM simulations using experimental detector sensitivities, ISTEM simulations, EFTEM simulations, STEM-EELS simulations along with other improvements in the algorithms. A screenshot of the user interface is shown in Fig. 1. This figure shows the main available options of the program. In Fig. 2, a simulated HRTEM image and ED pattern of an isolated gold nanoparticle of 21127 atoms are shown. In these simulations, plane-wave illumination is assumed with the following electron microscope setting: acceleration voltage (300 keV), spherical aberration (0.002 mm), defocus (19.8 Å), defocus spread (30 Å) and beam divergence angle (0.1 mrad). A numerical real space grid of 4096×4096 pixels has been used. The frozen atom simulation is performed by using the Einstein model with 200 configurations, slice thickness of 0.5Å and the three-dimensional rms displacements of all the atoms are set to 0.085Å. For the HRTEM simulation, the spatial and temporal incoherences are included by applying the partially coherent microscope transfer function to each exit wave of the frozen atom. A multislice simulation of each frozen atom configuration only took 2.6 s on a Nvidia GeForce GTX TITAN GPU card.

The MULTEM’s C++ classes, Matlab mex functions and the GUI are available in the GitHub repository https://github.com/Ivanlh20/MULTEM.

References

1. E.J. Kirkland. Springer, New York and London, (1998).

2. D. Van Dyck. Ultramicroscopy 109, 677(2009).

3. L.J. Allen and T.W. Josefsson. Physical Review B 52, 3184 (1995).

4. J. Verbeeck, P. Schattschneider, and A. Rosenauer. Ultramicroscopy 109, 350, (2009).

5. I. Lobato and D. Van Dyck.  Ultramicroscopy 156, 9 (2015).

 Acknowledgement

The authors acknowledge financial support from the Research Foundation Flanders (FWO,Belgium) through project fundings (G.0374.13N, G.0369.15N and G.0368.15N). The research leading to these results has also received funding from the European Union Seventh Framework Programme [FP7/2007- 2013] under Grant agreement no. 312483 (ESTEEM2).

Figures:

Figure 1: A screenshot of MULTEM under Windows platform demonstrating the main available options of the program

Figure 2: (a) Gold nanoparticle consisting of 21127 atoms, which has been used as an input to simulate (b) a HRTEM image and (c) an ED pattern.

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EMC Abstracts - https://emc-proceedings.com/abstract/accurate-and-fast-electron-microscopy-simulations-using-the-open-source-multem-program/