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.

Preparing transition-metal oxides in their two-dimensional (2D) form is the key to exploring their unrevealed low-dimensional properties, such as the p-type transparent superconductivity, topological Mott insulator state, existence of the condensed 2D electron/hole gas, and strain-tunable catalysis. However, existing approaches suffer from the specific constraint techniques and precursors that limit their product types. Here, we report a solution-based method to directly synthesize KNbO2 in 2D by an out-of-the-pot growth process at low temperature, which is observed directly in real time. The developed method can also be applied to other 2D ternary oxide syntheses, including CsNbO2 and composited Na x K1–x NbO2, and it can be extended to the preparation of self-assembled nanofilms. In addition, We demonstrate the emission of broadband photoluminescence (PL, λ ∼ 350–800 nm) from as-synthesized single-crystal 2D KNbO2 sheets down to a single unit cell thickness. The ultra-broadband emission is ascribed to the self-trapped excitation state (STEs) from the in-phase distortion of the NbO6 octahedrons in 2D NbO2 – layers. Beyond the broader luminescent range and the robust material thermal stability of niobates, the absence of sample size restrictions and the large aspect ratio of the 2D oxide sheets will provide opportunities in miniaturizing and advancing 2D-materials integrated optoelectronic devices.