Ferroelectricity observed in thin-film HfO 2 , either doped with Si, Al, and so forth or in the Hf 0.5 Zr 0.5 O 2 form, has gained great technical significance. While a trilinear coupling between phonon modes could explain its ferroelectric distortion, from a practical perspective, one may be concerned with a theory that is more straightforward to predict similar ferroelectric candidates through some apparent features of HfO 2 and ZrO 2 . In this work, we propose that the 7 cation coordination number of HfO 2 /ZrO 2 lies at the heart of this ferroelectricity, which stems from the proper ionic radii of Hf/Zr compared with O. Among the numerous compounds with a non-centrosymmetric nature, for example, the mm2 point group, HfO 2 and ZrO 2 are special in that they are close to the border of 7 and 8 cation coordination, such that the 8-coordination tetragonal intermediate phase could greatly reduce the switching barrier. Other 7-coordination candidates, including SrI 2 , TaON, YSBr, and YOF, are also studied in comparison to HfO 2 /ZrO 2 , and six switching paths are analyzed in detail for the Pca2 1 phase. A rule of the preferred switching path in terms of the ionic radii ratio and coordination number has been established. We also show the possible route from the ferroelectric Pca2 1 phase to the monoclinic P2 1 /c phase in HfO 2 , which is relevant to the fatigue phenomenon.
We report the synthesis of sulfide nanostructures with trivalent molybdenum, i.e., crystalline Mo2S3 nanorods via a solid-gas reaction between porous Al2O3 impregnated with MoO3 and H2S gas. We show that the introduction of additional 8% H2 gas results in MoS2 nanotubes of the same size. A growth model is proposed for the formation of Mo2S3 nanorods, and the effect of H2 is discussed and demonstrated.
The fast development of high-accuracy neuromorphic computing requires stable analog memristors. While filamentary memory switching is very common in binary oxides, their resistive switching usually involves abrupt changes due to the rupture or reformation of metallic filaments. In this work, we designed a memristor consisting of dual-layer HfOy/HfOx, with different concentrations of oxygen vacancies (y > x). During the electroforming process, both the migration of existing oxygen vacancies in HfOx and the generation of new oxygen vacancies in HfOy occur simultaneously, leaving a semiconducting part close to the HfOy/HfOx interface. The resulting filament is not metallic as a whole, as revealed by first principles calculations. Such a device demonstrates excellent switching uniformity as well as highly gradual resistance change, ideal for neuromorphic computing. Through fine tuning of the filament structure, the device achieves low variation, high speed, gradual SET and RESET processes, and hundreds of stable multi-level state behaviors. The long-term synaptic plasticity was further achieved, showing good linearity and large analog switching window (ΔG as high as 487.5 μS). This works affords a route toward a gradual resistance change in oxide-based memristors.
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