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Scanning/Transmission Electron Microscopy of Electronic and Magnetic Two-Dimensional and Layered Materials

Trout, Amanda Hanks
http://rave.ohiolink.edu/etdc/view?acc_num=osu1619124670667218

Abstract Details

2021, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
Two-dimensional (2D) and layered materials have been extensively studied since their discovery in 2004. The reduced third dimension results in interesting physical and electronic properties that can be tuned by various parameters, therefore, providing numerous opportunities for novel applications. 2D materials can be categorized by their synthesis method of which there are two main types; these are top down and bottom up synthesis methods. Top down methods include liquid and mechanical exfoliation, typically used for van der Waals layered materials. Bottom up synthesis methods include growth techniques such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). These techniques allow for monolayer growth of a wide variety of materials. One of the main concepts of materials science is that the properties of a material are affected by the structure, composition, and synthesis method. Therefore, characterization plays a critical role in developing structure-property relationships. Due to their structurally thin nature, characterization of 2D materials requires high spatial resolution techniques. One of the most influential and powerful characterization techniques is scanning/transmission electron microscopy (S/TEM). S/TEM combines high resolution imaging with spectroscopy techniques such as energy dispersive spectroscopy (EDS) and electron energy-loss spectroscopy (EELS) allowing for direct correlation between the structure and composition. Presented here is the S/TEM characterization of various 2D and layered materials synthesized by either top down or bottom up methods. Materials that were synthesized via top down methods include NaSn2As2, Eu1-xNaxSn2As2, NaFe3-xGeTe2, GeH, and BaSn2. Since exfoliation methods are difficult to control, S/TEM is used to analyze the size, thickness, and quality of exfoliated flakes. For NaSn2As2, Eu1-xNaxSn2As2, NaFe3-xGeTe2, high resolution TEM (HRTEM) and selected area electron diffraction (SAED) patterns are used to confirm the crystal structure and lattice parameters. Additionally, EDS maps show the distribution of each element within the exfoliated flakes. Electron diffraction patterns of GeH and NaFe3-xGeTe2 show that rotational disorder along the c-axis, known as turbostratic disorder, is observed in these layered materials. Density functional theory (DFT) calculations provide further evidence for the presence of turbostratic disorder in GeH, as well as an understanding of how this disorder affects the electronic properties of the material. The first S/TEM characterization of BaSn2, including chemical analysis via EDS and EELS, is also presented. The results show that in addition to being air-sensitive, the material is also sensitive to high vacuum exposure, leading to the disproportionation of BaSn2 particles into β-Sn + BaxSny. S/TEM characterization of materials grown by either CVD or MBE is also presented. These materials include MnSex, CoFe2O4 (CFO), and V[TCNE]x. Cross-section images and EDS analysis of MnSex films grown on GaSe base layers provides supporting evidence of monolayer 1T-MnSe2, which exhibits room temperature ferromagnetism. Moreover, the high resolution images show the presence of multiple polytypes and occasional defects in the GaSe layer. The characterization of Pt/CoFe2O4/MgO heterostructures is also presented. Induced magnetism in the polycrystalline Pt layer due to spin manipulation at the Pt/CFO interface is observed. EDS analysis indicates a homogeneous CFO layer, however, cross-section and plan-view images show the presence of antiphase boundaries (APB) throughout the CFO thin film. Finally, high spatial and energy resolution EELS is used to probe the structure of V[TCNE]x films, which contain high structural disorder and are air-sensitive. It is found that the V has an oxidation state of 2+ that does not change spatially through the film. In addition, splitting on the V L2 peak indicates that the V is in a perfect octahedral environment. EELS analysis of oxidized specimens provides a further understanding of the oxidation mechanism. A reduction of the C K edge 1s-π* peak is observed suggesting that during oxidation the C-C double bond of the TCNE molecule is targeted.
David McComb (Advisor)
Tyler Grassman (Committee Member)
Vicky Doan-Nguyen (Committee Member)
215 p.
Materials Science

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Citations

  • Trout, A. H. (2021). Scanning/Transmission Electron Microscopy of Electronic and Magnetic Two-Dimensional and Layered Materials [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1619124670667218

    APA Style (7th edition)

  • Trout, Amanda. Scanning/Transmission Electron Microscopy of Electronic and Magnetic Two-Dimensional and Layered Materials . 2021. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1619124670667218.

    MLA Style (8th edition)

  • Trout, Amanda. "Scanning/Transmission Electron Microscopy of Electronic and Magnetic Two-Dimensional and Layered Materials ." Doctoral dissertation, Ohio State University, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=osu1619124670667218

    Chicago Manual of Style (17th edition)

osu1619124670667218
162
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