for help with the experiments. Reviewer information Nature thanks Hua Zhang and the other anonymous reviewer(s) for their contribution to the peer review of this work.
The field of valleytronics has promised greater control of electronic and spintronic systems with an additional valley degree of freedom. However, conventional and two-dimensional valleytronic systems pose practical challenges in the utilization of this valley degree of freedom. Here we show experimental evidences of the valley effect in a bulk, ambient, and bias-free model system of Tin(II) sulfide. We elucidate the direct access and identification of different sets of valleys, based primarily on the selectivity in absorption and emission of linearly polarized light by optical reflection/transmission and photoluminescence measurements, and demonstrate strong optical dichroic anisotropy of up to 600% and nominal polarization degrees of up to 96% for the two valleys with band-gap values 1.28 and 1.48 eV, respectively; the ease of valley selection further manifested in their non-degenerate nature. Such discovery enables a new platform for better access and control of valley polarization.
Germanium (Ge)-based nanostructures, especially those with germanium dioxide (GeO 2 ), have drawn great interest for applications in lithium (Li)-ion batteries due to their ultrahigh theoretical Li + storage capability (8.4 Li/Ge). However, GeO 2 in conventional Ge(s)/GeO 2 (c) (where (c) means the core and (s) means the shell) composite anodes with Ge shell outside GeO 2 undergoes an irreversible conversion reaction, which restricts the maximum capacity of such batteries to 1126 mAhg −1 (the equivalent of storing 4.4 Li + ). In this work, a porous GeO 2 (s)/Ge(c) nanostructure with GeO 2 shell outside Ge cores are successfully fabricated utilizing the Kirkendall effect and used as a lithium-ion battery anode, giving a substantially improved capacity of 1333.5 mAhg −1 at a current density of 0.1 Ag −1 after 30 cycles and a stable long-time cycle performance after 100 cycles at a current density of 0.5 A g −1 . The enhanced battery performance is attributed to the improved reversibility of GeO 2 lithiation/delithiation processes catalyzed by Ge in the properly structured porous GeO 2 (s)/Ge(c) nanostructure.
Twist-angle-dependent SHG is observed in noncentrosymmetric twisted bilayer graphene The on-resonance susceptibility is comparable with that of a monolayer MoS 2The nonlinear optical property engineering is achieved by the twisting degree of freedom
With a honeycomb single-atomic-layer structure similar to those of graphene and hexagonal boron nitride (hBN), the graphitic phase of ZnO (gZnO) have been predicted to offer many advantages for engineering, including high-temperature stability in ambient conditions and great potential in heterostructure applications. However, there is little experimental data about this hexagonal phase due to the difficulty of synthesizing large-area gZnO for characterization and applications. In this work, we demonstrate a solution-based approach to realize gZnO nanoflakes with thicknesses down to a monolayer and sizes up to 20 μm. X-ray photoelectron spectroscopy, X-ray absorption near-edge spectroscopy, photoluminescence, atomic force microscopy, and electron microscopy characterizations are conducted on synthesized gZnO samples. Measurements show significant changes to the electronic band structure compared to its bulk phase, including an increase of the band gap to 4.8 eV. The gZnO nanosheets also exhibit excellent stability at temperatures as high as 800 °C in ambient environment. This wide band gap layered material provides us with a platform for harsh environment electronic devices, deep ultraviolet optical applications, and a practical alternative for hBN. Our synthesis method may also be applied to achieve other types of 2D oxides.
Engineering the structure of materials endows them with novel physical properties across a wide range of length scales. With high in-plane stiffness and strength, but low flexural rigidity, two-dimensional (2D) materials are excellent building blocks for nanostructure engineering. They can be easily bent and folded to build three-dimensional (3D) architectures. Taking advantage of the large lattice mismatch between the constituents, we demonstrate a 3D heterogeneous architecture combining a basal BiSe nanoplate and wavelike BiTe edges buckling up and down forming periodic ripples. Unlike 2D heterostructures directly grown on substrates, the solution-based synthesis allows the heterostructures to be free from substrate influence during the formation process. The balance between bending and in-plane strain energies gives rise to controllable rippling of the material. Our experimental results show clear evidence that the wavelengths and amplitudes of the ripples are dependent on both the widths and thicknesses of the rippled material, matching well with continuum mechanics analysis. The rippled BiSe/BiTe heterojunction broadens the horizon for the application of 2D materials heterojunction and the design and fabrication of 3D architectures based on them, which could provide a platform to enable nanoscale structure generation and associated photonic/electronic properties manipulation for optoelectronic and electromechanic applications.
Developing cost-effective and non-precious-metal electrocatalysts is a major challenge in water-splitting applications but is important to realize renewable energy systems. Herein, we report an electrocatalyst for the hydrogen evolution reaction (HER) composed of ultrafine cobalt-doped iron sulfide/cobalt sulfide (Co:FeS2/CoS2) heterostructured nanowires (NWs), which were prepared by a facile hydrothermal route. In the HER, the Co:FeS2/CoS2 NWs achieved a low overpotential of 69 mV at 10 mA cm–2 and a Tafel slope of only 46 mV dec–1. The overpotential shifted by about 3 mV over 1000 cycles along with a slight change in morphology of the Co:FeS2/CoS2 NWs. Density functional theory calculations and experimental results revealed that the charge transport kinetics of the FeS2/CoS2 heterostructure contributed substantially to the high performance of this HER electrocatalyst. These ultrafine Co:FeS2/CoS2 NWs represent a promising alternative to platinum-based electrocatalysts for water splitting.
SnS has recently been shown to possess unique valleytronic capability with large polarization degree, where non-degenerate valleys can be accessed using linearly polarized light, bestowed upon by the unique anisotropy and wavefunction symmetry. It is thus of utmost importance to demonstrate the extension of such effects for the IV-VI system in general, thereby elucidating the generality and tunability of such valleytronics. We show the highly tunable valleytronics via gradual compositional control of the Tin(II) Sulfo-Selenide (SnSxSe1-x) alloy system with excellent retainment of symmetry-determined selection rules. We show the presence of both ΓY and ΓX valleys in all alloy compositions via selectivity in absorption and emission of linearly polarized light by optical reflection (R)/transmission (T) and photoluminescence (PL) measurements, and tuned the bandgaps of the valleys within a range of 1.28eV to 1.05eV and 1.48eV to 1.24eV respectively. This simultaneous tuning of non-degenerate valleys agrees well with theoretical calculations. We then fitted the bandgap values in compositional space, obtaining the bowing parameters as a useful database. We further demonstrated the feasibility of using IV-VI valleytronics systems in general by elucidating the retainment of strong polarization degrees of as high as 91% across all compositions. The generalization of such purely symmetry-dependent valleytronics also opens up opportunities for discovery of more multi-functional materials.
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