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Ultrafast Carrier and Spin Dynamics of Photoexcited Yttrium Iron Garnet

Gajapathy, Harshad
http://orcid.org/0000-0002-8651-3098
http://rave.ohiolink.edu/etdc/view?acc_num=osu1723893494034235

Abstract Details

2024, Doctor of Philosophy, Ohio State University, Chemistry.
Ultrafast carrier and spin dynamics of photoexcited yttrium iron garnet (Y3Fe5O12, YIG) are studied to demonstrate that it is an efficient photocatalyst for oxygen (O2) evolution half-reaction in the electrolysis of water. Producing hydrogen as an alternative fuel to traditional carbon-based fuels requires efficient catalysis of both H2 and O2 production from water. O2 production is the limiting factor in water splitting as it is a 4e- transfer process. An efficient O2 evolution photocatalyst requires a material with proper band alignment and low carrier recombination. Recent advances have shown that a spin polarized catalyst can enhance water splitting to produce paramagnetic O2 from diamagnetic water. A photocatalyst utilizes the carriers excited by light, preferably sunlight, to perform catalysis. Photoexcitation is an ultrafast process occurring on an attosecond time scale whereas the electron transfer in catalysis between the semiconductor and water is a slow process occurring in millisecond time scale. Therefore, understanding carrier and spin dynamics between photoexcitation and catalysis is necessary to identify a potential efficient photocatalyst such as YIG. For this purpose, we used time resolved extreme ultraviolet (XUV) reflection absorption and magnetic circular dichroism (MCD) spectroscopies to study the photoexcited dynamic of YIG. These techniques allow us to probe the surface sensitive, element specific and spin resolved carrier dynamics that occur in YIG after photoexcitation. The details of the instrumentation for these techniques are discussed in detail in Chapter 2. YIG is a ferrimagnetic n-type semiconductor with a valence band containing majority of O 2p and a conduction band with Fe 3d contributions. The Fe atoms occupy tetrahedral (Td) and octahedral (Oh) lattice sites in a 3:2 ratio formed by O atoms. An above bandgap excitation causes electrons to transfer from O 2p to Fe 3d orbitals. These electrons can be probed with linear transient absorption spectroscopy, which shows lattice dependent electron dynamics. The electrons excited to the Oh Fe atoms couple with optical phonons on an ultrafast time scale and trap as small polarons. The electrons excited to the Td lattice do not trap and are mobile due to surface band bending in the n-type semiconductor. The electrons diffuse away from the surface to the bulk leaving behind holes in the valence band. These holes are spin polarized since YIG, as a ferrimagnet, has a spin difference between Td and Oh lattice sites and they are probed using time resolved XUV-MCD spectroscopy. These spin polarized holes are the main driving for water catalysis, enhancing water splitting efficiency. The details of the lattice dependent electron dynamics and spin polarized hole accumulation are discussed in Chapter 3. The kinetics of electron trapping in the Oh lattice and the mobility of electrons in the Td lattice are discussed in Chapter 4. The charge transfer states and the polaron states are simulated using CTM4XAS software, along with reflection absorption simulation, which agrees with the experimental data from the time resolved XUV reflection absorption spectroscopy. Based on this simulation, the kinetic model to obtain charge transfer and polaron time constants is formulated. Based on these kinetics, the mobility of the Td electrons is obtained, assuming an interfacial electric field driving the charge conduction. A minimum spin polarization lifetime is calculated using the excitation fraction from simulation and the observed hole accumulation from MCD spectroscopy. The ultrafast kinetics of accumulated holes show a spin flip in the transient MCD spectroscopy. The spin flip is driven by the photoexcited electron-phonon coupling in the Oh lattice, which causes the hole shared between O and Oh Fe atoms to transiently decouple and forms a new state of hole shared between O and Td Fe. The spin recovers as the phonon thermalizes on a fast time scale. The kinetics of this spin flip due to hole-phonon coupling is discussed in Chapter 5. Finally, the future goals to utilize this spin dependent charge kinetics in YIG are discussed in Chapter 6. with preliminary data from the NiO/YIG and Graphene/YIG systems as examples. The ultrafast spin transfer from YIG to NiO caused by the above band gap pump pulse is seen in the Ni M-edge data from time-resolved XUV-MCD spectroscopy of the interface. The spin transfer from YIG to graphene is shown by spin polarized photoemission spectroscopy of the interface with and without an above band gap excitation pump pulse. These results show that YIG not only has potential future application in O2 evolution reaction but is also influential in interfacial spin control and other spin polarized catalysis.
Robert Baker (Advisor)
Fengyuan Yang (Committee Member)
John Herbert (Committee Member)
89 p.
Chemistry
Water Splitting, Ultrafast Dynamics, Time Resolved, Extreme Ultraviolet Absorption, Magnetic Circular Dichroism, Yttrium Iron Garnet, Magnetic Catalysis, Polarons, Spin Flip

Recommended Citations

Citations

  • Gajapathy, H. (2024). Ultrafast Carrier and Spin Dynamics of Photoexcited Yttrium Iron Garnet [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1723893494034235

    APA Style (7th edition)

  • Gajapathy, Harshad. Ultrafast Carrier and Spin Dynamics of Photoexcited Yttrium Iron Garnet. 2024. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1723893494034235.

    MLA Style (8th edition)

  • Gajapathy, Harshad. "Ultrafast Carrier and Spin Dynamics of Photoexcited Yttrium Iron Garnet." Doctoral dissertation, Ohio State University, 2024. http://rave.ohiolink.edu/etdc/view?acc_num=osu1723893494034235

    Chicago Manual of Style (17th edition)

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