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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2024)

The development of novel materials and electronic devices with distinctive physical

characteristics is revolutionizing the path toward the next major advancement in neuromorphic

computing. Vanadium oxides (VOx) are particularly notable among functional materials as

leading contenders for the future of oxide electronics. This distinction arises from their metal-

insulator transitions (MITs) at or near room temperature, which can be induced by heat, electric

fields, or other external factors. These properties enable the implementation of neuronal or synaptic functionalities, making them essential components for constructing neuromorphic

systems. For technological applications, one of the major challenges in complex oxides is to

obtain material phases with reliable electronic performance in thin films. However, for materials

like vanadium oxides, due to existence of large number of oxidation states, synthesis conditions

for one phase are often detrimental to the stability of the other. Synthesizing thin films that

incorporate multiple vanadium oxide phases is particularly challenging, especially if different

phases must be organized into well-defined spatial patterns for applications. Despite decades of

research, a universal methodology for obtaining high-quality thin films of a specific VOx phase

is still lacking. In Chapter 2 and 3 of dissertation, we proposed and established an efficient solid-

state reaction laser annealing (SRLA) approach to directly write regions of different local

chemical compositions. Using this method, we achieved the controlled local recrystallization of a

uniform V2O3 thin film into VO2, V3O5, and V4O7 regions exhibiting sharp 1st and 2nd order

metal-insulator transitions over a wide range of critical temperatures. We utilized this approach

to pattern spiking oscillators with distinct electrical behavior directly in the vanadium oxide thin

film without employing elaborate lithography fabrication. Our method opens a pathway to

synthesizing a wide range of artificially micropatterned composites, with precision and control

unattainable in the conventional material fabrication methods. In Chapter 4 of dissertation, we

extended our study to superconductivity, and present a rapid thermal hydrogenation approach to

manipulate the critical transition temperature (Tc) of Nb thin films. In analog to the MIT,

superconductor-based neuromorphic computing is another promising aspect of this technological

field. Our works offer fundamental understanding of hydrogen incorporation into superconductor

materials.

    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2008)
    Dicer-like (DCL) enzymes play a pivotal role in RNA silencing in plants, processing the long double-stranded RNA (dsRNA) that triggers silencing into the primary short interfering RNAs (siRNAs) that mediate it. The siRNA population can be augmented and silencing amplified via transitivity, an RNA-dependent RNA polymerase (RDR)-dependent pathway that uses the target RNA as substrate to generate secondary siRNAs. Here we report that Arabidopsis DCL2-but not DCL4-is required for transitivity in cell-autonomous, post-transcriptional silencing of transgenes. An insertion mutation in DCL2 blocked sense transgene-induced silencing and eliminated accumulation of the associated RDR-dependent siRNAs. In hairpin transgene-induced silencing, the dcl2 mutation likewise eliminated accumulation of secondary siRNAs and blocked transitive silencing, but did not block silencing mediated by primary siRNAs. Strikingly, in all cases, the dcl2 mutation eliminated accumulation of all secondary siRNAs, including those generated by other DCL enzymes. In contrast, mutations in DCL4 promoted a dramatic shift to transitive silencing in the case of the hairpin transgene and enhanced silencing induced by the sense transgene. Suppression of hairpin and sense transgene silencing by the P1/HC-Pro and P38 viral suppressors was associated with elimination of secondary siRNA accumulation, but the suppressors did not block processing of the stem of the hairpin transcript into primary siRNAs. Thus, these viral suppressors resemble the dcl2 mutation in their effects on siRNA biogenesis. We conclude that DCL2 plays an essential, as opposed to redundant, role in transitive silencing of transgenes and may play a more important role in silencing of viruses than currently thought.
      Cover page: DICER-LIKE2 Plays a Primary Role in Transitive Silencing of Transgenes in Arabidopsis
      • Article
      • Peer Reviewed
      UC San Diego Previously Published Works (2024)
        Cover page: Coercivity enhancement in hematite/permalloy heterostructures across the Morin transition
        • Article
        • Peer Reviewed
        UC Davis Previously Published Works (2024)
        Electrical triggering of a metal-insulator transition (MIT) often results in the formation of characteristic spatial patterns such as a metallic filament percolating through an insulating matrix or an insulating barrier splitting a conducting matrix. When MIT triggering is driven by electrothermal effects, the temperature of the filament or barrier can be substantially higher than the rest of the material. Using X-ray microdiffraction and dark-field X-ray microscopy, we show that electrothermal MIT triggering leads to the development of an inhomogeneous strain profile across the switching device, even when the material does not undergo a pronounced, discontinuous structural transition coinciding with the MIT. Diffraction measurements further reveal evidence of unique features associated with MIT triggering including lattice distortions, tilting, and twinning, which indicate structural nonuniformity of both low- and high-resistance regions inside the switching device. Such lattice deformations do not occur under equilibrium, zero-voltage conditions, highlighting the qualitative difference between states achieved through increasing temperature and applying voltage in nonlinear electrothermal materials. Electrically induced strain, lattice distortions, and twinning could have important contributions in the MIT triggering process and drive the material into nonequilibrium states, providing an unconventional pathway to explore the phase space in strongly correlated electronic systems.
          Cover page: Local strain inhomogeneities during electrical triggering of a metal-insulator transition revealed by X-ray microscopy.
          • Article
          • Peer Reviewed
          UC San Diego Previously Published Works (2024)
          Manipulating physical properties through ion migration in complex oxide thin films is an emerging research direction to achieve tunable materials for advanced applications. While the reduction of complex oxides has been widely reported, few reports exist on the modulation of physical properties through a direct hydrogenation process. Here, we report an unusual mechanism for hydrogen-induced topotactic phase transitions in perovskite La0.7Sr0.3CoO3 thin films. Hydrogenation is performed upon annealing in a pure hydrogen gas environment, offering a direct understanding of the role that hydrogen plays at the atomic scale in these transitions. Topotactic phase transformations from the perovskite (P) to hydrogenated-brownmillerite (H-BM) phase can be induced at temperatures as low as 220 °C, while at higher hydrogenation temperatures (320-400 °C), the progression toward more reduced phases is hindered. Density functional theory calculations suggest that hydroxyl bonds are formed with the introduction of hydrogen ions, which lower the formation energy of oxygen vacancies of the neighboring oxygen, enabling the transition from the P to H-BM phase at low temperatures. Furthermore, the impact on the magnetic and electronic properties of the hydrogenation temperature is investigated. Our research provides a potential pathway for utilizing hydrogen as a basis for low-temperature modulation of complex oxide thin films, with potential applications in neuromorphic computing.
            Cover page: Hydrogen-Induced Topotactic Phase Transformations of Cobaltite Thin Films
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