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  1. Abstract

    The device concept of ferroelectric-based negative capacitance (NC) transistors offers a promising route for achieving energy-efficient logic applications that can outperform the conventional semiconductor technology, while viable operation mechanisms remain a central topic of debate. In this work, we report steep slope switching in MoS2transistors back-gated by single-layer polycrystalline PbZr0.35Ti0.65O3. The devices exhibit current switching ratios up to 8 × 106within an ultra-low gate voltage window of$$V_{{{\mathrm{g}}}} = \pm \! 0.5$$Vg=±0.5V and subthreshold swing (SS) as low as 9.7 mV decade−1at room temperature, transcending the 60 mV decade−1Boltzmann limit without involving additional dielectric layers. Theoretical modeling reveals the dominant role of the metastable polar states within domain walls in enabling the NC mode, which is corroborated by the relation between SS and domain wall density. Our findings shed light on a hysteresis-free mechanism for NC operation, providing a simple yet effective material strategy for developing low-power 2D nanoelectronics.

  2. The ferrimagnetic inverse spinel NiCo 2 O 4 has attracted extensive research interest for its versatile electrochemical properties, robust magnetic order, high conductivity, and fast spin dynamics, as well as its highly tunable nature due to the closely coupled charge, spin, orbital, lattice, and defect effects. Single-crystalline epitaxial thin films of NiCo 2 O 4 present a model system for elucidating the intrinsic physical properties and strong tunability, which are not viable in bulk single crystals. In this Perspective, we discuss the recent advances in epitaxial NiCo 2 O 4 thin films, focusing on understanding its unusual magnetic and transport properties in light of crystal structure and electronic structure. The perpendicular magnetic anisotropy in compressively strained NiCo 2 O 4 films is explained by considering the strong spin–lattice coupling, particularly on Co ions. The prominent effect of growth conditions reveals the complex interplay between the crystal structure, cation stoichiometry, valence state, and site occupancy. NiCo 2 O 4 thin films also exhibit various magnetotransport anomalies, including linear magnetoresistance and sign change in anomalous Hall effect, which illustrate the competing effects of band-intrinsic Berry phase and impurity scattering. The fundamental understanding of these phenomena will facilitate the functional design of NiComore »2 O 4 thin films for nanoscale spintronic applications.« less
    Free, publicly-accessible full text available July 14, 2023
  3. The inverse spinel ferrimagnetic NiCo 2 O 4 possesses high magnetic Curie temperature T C , high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo 2 O 4 thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo 2 O 4 films on MgAl 2 O 4 substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films down to 1.5 unit cell (1.2 nm), while T C for 3 unit cell and thicker films remains above 300 K. The sign change in the anomalous Hall conductivity [Formula: see text] and its scaling relation with the longitudinal conductivity ([Formula: see text]) can be attributed to the competing effects between impurity scattering and band intrinsic Berry curvature, with the latter vanishing upon the thickness driven metal–insulator transition. Our study reveals the critical role of film thickness in tuning the relative strength of charge correlation, Berry phase effect, spin–orbit interaction, and impurity scattering, providing important material information for designing scalable epitaxial magnetic tunnel junctions and sensing devices using NiCo 2 O 4 .
    Free, publicly-accessible full text available June 13, 2023
  4. The development of multifunctional nanomaterials has received growing research interest, thanks to its ability to combine multiple properties for severing highly demanding purposes. In this work, holmium oxide nanoparticles are synthesized and characterized by various tools including XRD, XPS, and TEM. These nanoparticles are found to emit near-infrared fluorescence (800–1100 nm) under a 785 nm excitation source. Imaging of the animal tissues was demonstrated, and the maximum imaging depth was found to be 2.2 cm. The synthesized nanoparticles also show the capability of facilitating dye (fluorescein sodium salt and rhodamine 6G) degradation under white light irradiation. The synthesized holmium oxide nanoparticles are envisioned to be useful for near-infrared tissue imaging and dye-degradation.
    Free, publicly-accessible full text available June 1, 2023
  5. There is a strong drive behind the quest for thin-film materials that are oxygen-free and polar. Oxygen hinders the integration of ferroelectric oxides with semiconductors, which affects efforts to develop nonvolatile memory—that is, a memory that can sustain its information without power. Ideally, one would use single-crystalline perovskite films to construct these devices so that the polarization can be maximized. However, when depositing crystalline polar perovskite oxides onto silicon or germanium, a nonpolar oxide buffer layer ( 1 ) or a native oxide layer ( 2 ) can be present at the interface, compromising device performance. A nitrogen-based perovskite may overcome this limitation ( 3 ). On page 1488 of this issue, Talley et al. ( 4 ) report the synthesis of lanthanum tungsten nitride (LaWN 3 ) thin films, which marks the first demonstration of polar nitride perovskite. This may lead to oxygen-free integration of functional perovskite on a semiconductor platform.
  6. Free, publicly-accessible full text available December 3, 2023