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J Alexander - Dissertation Final.pdf (28.79 MB)

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High-Resolution Electron Energy-Loss Spectroscopy of Beam-Sensitive Functional Materials

Alexander, Jessica Anne
http://rave.ohiolink.edu/etdc/view?acc_num=osu1531309653616002

Abstract Details

2018, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
By making electron energy-loss spectroscopy (EELS) measurements in a scanning transmission electron microscope (STEM), the optoelectronic properties of a material can be determined with nanometer spatial resolution. Since these optoelectronic properties can be related to the electronic structure of a material, STEM-EELS can also probe the local bonding environment at the interface of two materials. Such measurements could be key in developing more efficient P3HT:PCBM bulk-heterojunction organic photovoltaics (OPVs) (P3HT = poly(3-hexylthiophene), PCBM = [6,6] phenyl C61 butyric acid methyl ester), as understanding the local electronic structure at P3HT/PCBM interfaces should provide insight into charge generation/transport. However, organic materials are extremely susceptible to beam-damage when placed under a high energy electron beam, making it difficult to use STEM-EELS to collect reliable data. It was demonstrated that, via a beam damage-minimization EELS acquisition method, reliable high-resolution valence-loss STEM-EELS data could be collected for electron beam-sensitive materials. Using this method, valence-loss EELS spectra were acquired (using an FEI Titan3 60-300 Image-Corrected S/TEM) for thin films of four OPV materials – P3HT, PCBM, CuPc (copper phthalocyanine), and C60. From these valence-loss spectra, the real (e1) and imaginary (e2) parts of the complex dielectric function were extracted and compared to similar spectra obtained via a technique that should not damage these organic materials (variable-angle spectroscopic ellipsometry, VASE), thus proving that the acquisition method developed was suitable for collecting reliable valence-loss EELS spectra of P3HT, PCBM, CuPc, and C60. Valence-loss EELS spectra were then collected for P3HT, PCBM, CuPc, and C60 using a Nion UltraSTEM 100 MC `HERMES’ S/TEM. With this STEM, it was possible to collect valence-loss spectra with higher energy resolutions (35 meV) than what was achievable using the FEI Titan3 60-300 Image-Corrected S/TEM (175 meV). The e1 and e2 spectra obtained from measurements on these two instruments were compared, and it was determined that the improved energy resolution of the Nion UltraSTEM did not result in the acquisition of any new optoelectronic information for these four materials. Using the technique of spectrum imaging, spatially resolved valence-loss EELS data was collected for a CuPc/C60 bilayer structure in order to prove that reliable data could be collected at the interface between two organic materials. These methods were then used to conduct preliminary STEM-EELS measurements of an actual P3HT:PCBM bulk-heterojunction organic photovoltaic. Vanadium tetracyanoethylene (V[TCNE]x~2), was also studied using the methods developed. This organic-based ferrimagnetic semiconductor exhibits desirable magnetic properties, but the optoelectronic properties have not yet been determined. Using STEM-EELS, the complex dielectric function was obtained, from which many other optoelectronic properties can be extracted, and, via core-loss EELS measurements, the oxidation state of vanadium was confirmed to be ~V2+ and homogenous throughout the V[TCNE]x~2 sample. Lastly, low-loss STEM-EELS was used to obtain the optoelectronic properties of two lead-free double halide perovskites – Cs2AgBiBr6 and Cs2AgBiCl6. Comparing these results to density functional theory calculations and correlative VASE data sets suggests that higher (energy) resolution STEM-EELS measurements are required to ensure that all of their optoelectronic properties are extracted.
David McComb (Advisor)
Tyler Grassman (Committee Member)
Vicky Doan-Nguyen (Committee Member)
221 p.
Materials Science
scanning transmission electron microscope; Beam Sensitive Functional Materials; materials science

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Citations

  • Alexander, J. A. (2018). High-Resolution Electron Energy-Loss Spectroscopy of Beam-Sensitive Functional Materials [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1531309653616002

    APA Style (7th edition)

  • Alexander, Jessica. High-Resolution Electron Energy-Loss Spectroscopy of Beam-Sensitive Functional Materials . 2018. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1531309653616002.

    MLA Style (8th edition)

  • Alexander, Jessica. "High-Resolution Electron Energy-Loss Spectroscopy of Beam-Sensitive Functional Materials ." Doctoral dissertation, Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1531309653616002

    Chicago Manual of Style (17th edition)

osu1531309653616002
298
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This open access ETD is published by The Ohio State University and OhioLINK.