Professor Andre Mkhoyan

This lab specializes in the atomic scale characterization of nanomaterials using analytical transmission electron microscopy (TEM). In order to quantitatively 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. Additionally, DFT calculations utilize MSI resources for structural relaxation of uniques defects found in perovskite systems. 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.
  • Properties of dislocations, the most common 1-D defects in perovskite oxide thin films, are explored at the atomic-scale using analytical STEM. Using STEM images and the Energy-dispersive X-ray spectroscopy, atomic configurations and non-stoichiometry at the defects are experimentally investigated, and the results are explained by the structure optimization simulation based on DFT. Additionally, core-loss spectroscopy data from dislocations are collected from dislocations and interpreted via DFT based simulations.
  • Ruddlesden-popper phase in perovskite stannates (BaSnO₃, SrSnO₃) are studied. Dielectric function of the phase is calculated as a function of multiple structural parameters using ab initio calculations and compared with experimentally obtained spectroscopic data.
• 2D Materials: The group collaborates with synthesis 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. 
  • Topological insulators (TIs) are materials that behave as insulators in their interiors but whose surfaces contain conducting states, meaning the electrons can only move along the surface of the materials. They are special because their surface states are symmetry-protected, which leads to high charge to spin conversion value. 2D TIs have special spin textures, offering an additional spin conversion system. These properties allow for applying TIs in spintronic devices and achieve efficient charge-to-spin conversion. In the group's collaboration with the synthesis group, they characterized BixSe(1–x) films under HAADF-STEM and obtained many structural information for this material. In order to compare and validate the experimental ADF STEM data, the materials interface is simulated using the multislice method.

• 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 data.

  • 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.