Valence-Ordered Thin-Film Nickelate with Tri-component Nickel Coordination Prepared by Topochemical Reduction.

The metal-hydride-based "topochemical reduction" process has produced several thermodynamically unstable phases across various transition metal oxide series with unusual crystal structures and nontrivial ground states. Here, by such an oxygen (de-)intercalation method we synthesis a samarium nickelate with ordered nickel valences associated with tri-component coordination configurations. This structure, with a formula of Sm9Ni9O22 as revealed by four-dimensional scanning transmission electron microscopy (4D-STEM), emerges from the intricate planes of {303}pc ordered apical oxygen vacancies. X-ray spectroscopy measurements and ab initio calculations show the coexistence of square planar, pyramidal, and octahedral Ni sites with mono-, bi-, and tri-valences. It leads to an intense orbital polarization, charge-ordering, and a ground state with a strong electron localization marked by the disappearance of ligand-hole configuration at low temperature. This nickelate compound provides another example of previously inaccessible materials enabled by topotactic transformations and presents an interesting platform where mixed Ni valence can give rise to exotic phenomena.

Enhanced gain and detectivity of unipolar barrier solar blind avalanche photodetector via lattice and band engineering.

Ga2O3-based solar blind avalanche photodetectors exhibit low voltage operation, optical filter-free and monolithic integration of photodetector arrays, and therefore they are promising to be an alternative to the bulky and fragile photomultiplier tubes for weak signal detection in deep-ultraviolet region. Here, by deliberate lattice and band engineering, we construct an n-Barrier-n unipolar barrier avalanche photodetector consisting of ß-Ga2O3/MgO/Nb:SrTiO3 heterostructure, in which the enlarged conduction band offsets fortify the reverse breakdown and suppress the dark current while the negligible valance band offsets faciliate minority carrier flow across the heterojunction. The developed devices exhibit record-high avalanche gain up to 5.9 × 105 and detectivity of 2.33 × 1016 Jones among the reported wafer-scale grown Ga2O3-based photodetectors, which are even comparable to the commercial photomultiplier tubes. These findings provide insights into precise manipulation of band alignment in avalanche photodetectors, and also offer exciting opportunities for further developing high-performance Ga2O3-based electronics and optoelectronics.

Observation of Moiré Patterns in Twisted Stacks of Bilayer Perovskite Oxide Nanomembranes with Various Lattice Symmetries.

The design and fabrication of novel quantum devices in which exotic phenomena arise from moiré physics have sparked a new race of conceptualization and creation of artificial lattice structures. This interest is further extended to the research on thin-film transition metal oxides, with the goal of synthesizing twisted layers of perovskite oxides concurrently revealing moiré landscapes. By utilizing a sacrificial-layer-based approach, we show that such high-quality twisted bilayer oxide nanomembrane structures can be achieved. We observe atomic-scale distinct moiré patterns directly formed with different twist angles, and the symmetry-inequivalent nanomembranes can be stacked together to constitute new complex moiré configurations. This study paves the way to the construction of higher-order artificial oxide heterostructures based on different materials/symmetries and provides the materials foundation for investigating moiré-related electronic effects in an expanded selection of twisted oxide thin films.

Control of upconversion luminescence by tailoring energy migration in doped perovskite superlattices.

We describe an experimental investigation of photon upconversion (UC) in a series of perovskite BaTiO3/SrTiO3 superlattices doped with different lanthanide compositions. We show that UC emission can be effectively enhanced by precisely incorporating a set of lanthanide ions into separated layers rather than homogeneously distributing the dopant ions in the host lattice. The use of an inert layer in the superlattice can suppress deleterious energy cross-relaxation. Furthermore, UC emission can be rendered by controlling the energy migration mediated by the Yb-doped sublattice. These results demonstrate the opportunity to modulate energy migration and transfer processes through the rational design of superlattice structures.

Mechanically induced enhancement and modulation of upconversion photoluminescence by bending lanthanide-doped perovskite oxides.

We report experimental studies of the bending strain impact on the upconversion processes in Yb3+, Er3+, and Mn2+ co-doped BaTiO3 (BTO) thin films with mica as the flexible substrate. Bending strain induces strong enhancement and modulation of the upconversion emission in doped BTO thin films. Because the unshielded 3d5 configuration of Mn2+ is more susceptible to crystal field changes, the introduction of an Mn2+ ion further promotes the strain-induced modulation effect. The upconversion intensity is amplified by six times at bending strain ε = 1.83% in BTO:Yb3+/Er3+/Mn2+ thin films. These results demonstrate the opportunity of rendering an upconversion emission through integrating lanthanide-doped ferroelectric films with flexible mica, especially by incorporating an Mn2+ ion.

Ultrahigh Gain Solar Blind Avalanche Photodetector Using an Amorphous Ga2O3-Based Heterojunction.

Solar blind photodetectors with a cutoff wavelength within the 200-280 nm region is attracting much attention due to their potential civilian and military applications. The avalanche photodetectors (APDs) formed based on wide-bandgap semiconductor Ga2O3 are expected to meet emerging technological demands. These devices, however, suffer from limitations associated with the quality of as-grown Ga2O3 or the difficulty in alleviating the defects and dislocations. Herein, high-performance APDs incorporating amorphous Ga2O3 (a-Ga2O3)/ITO heterojunction as the central element have been reliably fabricated at room temperature. The a-Ga2O3-based APDs exhibits an ultrahigh responsivity of 5.9 × 104 A/W, specific detectivity of 1.8 × 1014 Jones, and an external quantum efficiency up to 2.9 × 107% under 254 nm light irradiation at 40 V reverse bias. Notably, the gain could reach 6.8 × 104, indicating the outstanding capability for ultraweak signals detection. The comprehensive superior capabilities of the a-Ga2O3-based APDs can be ascribed to the intrinsic carrier transport manners in a-Ga2O3 as well as the modified band alignment at the heterojunctions. The trade-off between low processing temperature and superior characteristics of a-Ga2O3 promises greater design freedom for realization of wide applications of emerging semiconductor Ga2O3 with even better performance since relieving the burden on the integration progress.