Associate Professor Andre Mkhoyan

CSENG Chemical Eng & Mat Sci
College of Science & Engineering
Twin Cities
Project Title: 
Transmission Electron Microscopy

This lab specializes in the atomic scale characterization of nanomaterials using analytical transmission electron microscopy (TEM). In order to qantitatively analyze the experimental TEM data, they simulate the propagation of the electron beam through the sample. This pairing of simulation with experiments provides new insights into the atomic and electronic structures in a variety of technologically important materials. The group's simulations are primarily performed using the multislice method based on the TEMsim code published by Kirkland. The researchers are currently working on three project areas that use MSI resources:

  • Perovskite Oxides: In perovskite oxides, various crystalline structures such as defects and interfaces are present, which often form unique atomic and electronic structures. The researchers are studying the local crystalline structures in thin-film perovskite oxides using analytical TEM. To interpret TEM images and spectroscopy, they simulate the electron beam propagation in the materials and the resultant scanning TEM (STEM) images.
    • The interface of dissimilar perovskite oxide is of particular research interest. In a recent study, the electron beam channeling through the BaSnO3/LaAlO3 bilayer was investigated using the multislice method and a 2D misfit dislocation network formed at the BaSnO3/LaAlO3 interface was visualized via experimental and computational STEM images.
    • Atomic and compositional structures of 1D defects in BaSnO3 are also investigated. ADF-STEM images at different TEM conditions are simulated using the Multislice program for proper interpretation of experimental images. The electronic structure of 1D defects is also explored via ab initio calculation.
  • 2D Materials: The group collaborates with sysnthesis groups to discover new 2D materials and study fundamental chemical and physical properties of 2D materials.
    • Black arsenic is one of the most promising 2D materials due to its high carrier mobility and anisotropy. Atomic-resolution STEM is used to demonstrate the anisotropy of black arsenic, and a combination of STEM and electron energy-loss spectroscopy (EELS) is employed to explore the atomic and electronic structure of black arsenic as a function of the number of layers. To this end, ADF-STEM images are computed using the multislice method and compared with experimental STEM data.
    • The formation of a metal arsenide by inter-diffusion of metal into the black arsenic, could be a practical route to synthesize binary 2D materials. This approach is being studied using in-situ TEM heating, where atomic resolution ADF-STEM imaging is employed to understand the mechanism of phase transformation. Simulation of the ADF-STEM images using multislice simulation is necessary to compare and validate the experimental ADF STEM data. 
  • Zeolites and Metal Organic Frameworks (MOFs): Zeolites and MOFs are porous materials that are used in applications of catalysis and membrane separation applications. These materials are very sensitive to damage by electron beam exposure, hence image simulations (CTEM, Electron Diffraction and HAADF-STEM) are needed in addition to experiments to fully analyze the crystal structure.

    • The researchers study the MFI system of zeolites for which <10 nm thick uniform nanosheets can be obtained via established synthesis methods. They use HAADF-STEM images to study the atomic structure of these systems and simulations are performed in these systems to identify and analyze defects.

    • Multislice simulations are also used to solve and visualize defects in the ZIF (Zeolitic Imidazolate Framework) systems. MOFs are extremely beam sensitive making them almost impossible to study at the atomic length-scale but having a high S/N low dose camera as an add-on to a TEM, this is possible and image simulations will be used to further analyze the structure.

Research by this group was featured on the MSI website in June 2017: A New Process to Create Zeolite Nanosheets.

Project Investigators

Supriya Ghosh
Jacob Held
Prashant Kumar
Associate Professor Andre Mkhoyan
Nhan Tran
Hwanhui Yun
 
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