Jiuxiang Dai

npj 2D Materials and Applications, Published online: 07 August 2023; doi:10.1038/s41699-023-00413-0

Electron transport and scattering mechanisms in ferromagnetic monolayer Fe₃GeTe₂

Nature Materials, Published online: 07 August 2023; doi:10.1038/s41563-023-01625-x

Through the approach of paracrystallization under high-pressure and high-temperature conditions, exceptional toughening has been achieved in oxide glasses by enhancing their crystal-like medium-range order structure. This discovery offers possibilities for the design of more resilient glass materials.

Periodic regular hexagonal-shaped hole arrays in two-dimensional semiconductors is successfully synthesized, and its controlled formation process, specific structural properties are elucidated by a combination of experimental analysis and theoretical calculations. The concept of precisely manipulating structural evolution based on the method of chemical vapor deposition and thermal etching to design hexagonal-shaped hole arrays materials is further demonstrated.

Abstract

The controllable synthesis of complicated nanostructures in advanced two-dimensional (2D) semiconductors, such as periodic regular hole arrays, is essential and remains immature. Here, we report a green, facile, highly controlled synthetic method to efficiently pattern 2D semiconductors, such as periodic regular hexagonal-shaped hole arrays (HHA), in 2D-TMDs. Combining the production of artificial defect arrays through laser irradiation with anisotropic annealing etching, we created HHA with different arrangements, controlled hole sizes, and densities in bilayer WS₂. Atomic force microscopy (AFM), Raman, photoluminescence (PL), and scanning transmission electron microscopy (STEM) characterization show that the 2D semiconductors have high quality with atomical clean and sharp edges as well as undamaged crystals in the unetched region. Furthermore, other nanostructures, such as nanoribbons and periodic regular triangular-shaped 2D-TMD arrays, can be fabricated. This kind of 2D semiconductors fabrication strategy is general and can be extended to a series of 2D materials. Density functional theory (DFT) calculations show that one WS₂ molecule from the edges of the laser-irradiated holed region exhibits a robust etching activation, making selective etching at the artificial defects and the fabrication of regular 2D semiconductors possible.

A stacking-controlled continuous precipitation strategy is proposed to synthesize uniform rBN crystalline films (lateral size of 2 × 1 cm², thickness of >1 µm) on the vicinal FeNi (111) single crystal. The rBN films with parallel stacked layers exhibit ultrahigh nonlinear optical response with the record-high frequency conversion efficiency of 1% in the 2D-material family.

Abstract

Nonlinear optical crystals lie at the core of ultrafast laser science and quantum communication technology. The emergence of 2D materials provides a revolutionary potential for nonlinear optical crystals due to their exceptionally high nonlinear coefficients. However, uncontrolled stacking orders generally induce the destructive nonlinear response due to the optical phase deviation in different 2D layers. Therefore, conversion efficiency of 2D nonlinear crystals is typically limited to less than 0.01% (far below the practical criterion of >1%). Here, crystalline films of rhombohedral boron nitride (rBN) with parallel stacked layers are controllably synthesized. This success is realized by the utilization of vicinal FeNi (111) single crystal, where both the unidirectional arrangement of BN grains into a single-crystal monolayer and the continuous precipitation of (B,N) source for thick layers are guaranteed. The preserved in-plane inversion asymmetry in rBN films keeps the in-phase second-harmonic generation field in every layer and leads to a record-high conversion efficiency of 1% in the whole family of 2D materials within the coherence thickness of only 1.6 µm. The work provides a route for designing ultrathin nonlinear optical crystals from 2D materials, and will promote the on-demand fabrication of integrated photonic and compact quantum optical devices.

Light: Science & Applications, Published online: 02 August 2023; doi:10.1038/s41377-023-01224-0

A single layer of chalcogenide CrSiTe₃ is found to be a 2D ferromagnetic second-order topological insulator. The CrSiTe₃ monolayer exhibits a nontrivial gapped bulk state in the spin-up channel and a trivial gapped bulk state in the spin-down channel. The topological corner states of the CrSiTe₃ monolayer are spin-polarized and pinned at the corners of the sample in real space.

Abstract

2D second-order topological insulators (SOTIs) have sparked significant interest, but currently, the proposed realistic 2D materials for SOTIs are limited to nonmagnetic systems. In this study, for the first time, a single layer of chalcogenide CrSiTe₃—an experimentally realized transition metal trichalcogenide is proposed with a layer structure—as a 2D ferromagnetic (FM) SOTI. Based on first-principles calculations, this study confirms that the CrSiTe₃ monolayer exhibits a nontrivial gapped bulk state in the spin-up channel and a trivial gapped bulk state in the spin-down channel. Based on the higher-order bulk–boundary correspondence, it demonstrates that the CrSiTe₃ monolayer exhibits topologically protected corner states with a quantized fractional charge (e3$\frac{e}{3}$) in the spin-up channel. Notably, unlike previous nonmagnetic examples, the topological corner states of the CrSiTe₃ monolayer are spin-polarized and pinned at the corners of the sample in real space. Furthermore, the CrSiTe₃ monolayer retains SOTI features when the spin–orbit coupling (SOC) is considered, as evidenced by the corner charge and corner states distribution. Finally, by applying biaxial strain and hole doping, this study transforms the magnetic insulating bulk states into spin-gapless semiconducting and half-metallic bulk states, respectively. Importantly, the topological corner states persist in the spin-up channel under these conditions.

The flexible tunability of anomalous-Hal-effect sign and the first discovery of low-conductivity skew-scattering make ferrimagnetic NiCo₂O₄ films be a new platform for the investigation and manipulation of quantum phenomena in carrier transport. Furthermore, their robust perpendicular magnetic anisotropy even in thick films overcomes a long-term challenge in today's spintronics, paving a new way for future spintronics, such as high-density memories.

Abstract

Their high tunability of electronic and magnetic properties makes transition-metal oxides (TMOs) highly intriguing for fundamental studies and promising for a wide range of applications. TMOs with strong ferrimagnetism provide new platforms for tailoring the anomalous Hall effect (AHE) beyond conventional concepts based on ferromagnets, and particularly TMOs with perpendicular magnetic anisotropy (PMA) are of prime importance for today's spintronics. This study reports on transport phenomena and magnetic characteristics of the ferrimagnetic TMO NiCo₂O₄ (NCO) exhibiting PMA. The entire electrical and magnetic properties of NCO films are strongly correlated with their conductivities governed by the cation valence states. The AHE exhibits an unusual sign reversal resulting from a competition between intrinsic and extrinsic mechanisms depending on the conductivity, which can be tuned by the synthesis conditions independent of the film thickness. Importantly, skew-scattering is identified as an AHE contribution for the first time in the low-conductivity regime. Application wise, the robust PMA without thickness limitation constitutes a major advantage compared to conventional PMA materials utilized in today's spintronics. The great potential for applications is exemplified by two proposed novel device designs consisting only of NCO films that open a new route for future spintronics, such as ferrimagnetic high-density memories.

The crystal quality of metal-organic chemical vapor deposition (MOCVD)-grown trilayer MoS₂ film (2 × 2 cm²) is uniformly improved through thermal annealing at certain temperature conditions under ambient air. By in situ spectroscopy, the rapid progress of the oxygen-substitutional repair is observed at the trion dissociation. The point of trion dissociation is considered to be an important indicator in the thermal annealing of 2D transition metal dichalcogenides (TMDs).

Abstract

Overcoming the thermal sensitivity of the van-der Waals layered material, an inevitable high defect concentration in a large area (2 × 2 cm²) vapor phase grown few-layer MoS₂ (2.3 nm thick) is uniformly repaired over the entire area through thermal process. The defect concentration of the healed sample decreased by 3.8 times, and both the PL intensity and field-effect mobility over the entire area increased by more than two times. Through observing in situ photoluminescence (PL) spectra with raising temperature, it is confirmed that the oxygen-substitutional healing proceeds radically from the starting point at which trion dissociation energy is zero, inducing e–h plasma. At this time, for achieving successful healing of the entire area, immediate cooling is required to prevent accelerated oxidation when reaching the critical temperature. Considering the laboratory-dependent crystalline quality of vapor-phase grown transition metal dichalcogenides (TMDs), the methodology to find the optimal thermal healing temperature through in situ PL monitoring will be quite useful.

Nature Materials, Published online: 03 August 2023; doi:10.1038/s41563-023-01626-w

A van der Waals buffer layer of Sb2O3 enables the integration of high-κ dielectric layer with sub-1 nm equivalent oxide thickness on two-dimensional semiconductors, resulting in high performance of two-dimensional field-effect transistors.

Chemical modification opens new applications for polymers in wearables

Nature Reviews Materials, Published online: 04 August 2023; doi:10.1038/s41578-023-00582-w

Although perovskite solar cells now have competitive efficiencies compared with silicon solar cells, their low stability has hindered their commercial application thus far. This Review summarizes the tremendous improvements made over the past decade and offer a perspective on how to reach >25-year stable perovskite solar cells.

The metal phosphorus chalcogenide (MPX₃), has garnered attention in the field of energy storage due to its exceptional electrochemical performance. With high electrical conductivity, relatively large surface area, superb stability, and exceptional energy storage capability, MPX₃ presents itself as a promising material. By employing appropriate regulation strategies, it can be applied to energy storage devices such as alkali metal ion batteries, metal-air batteries, and solid-state batteries.

Abstract

The development of efficient and affordable electrode materials is crucial for clean energy storage systems, which are considered a promising strategy for addressing energy crises and environmental issues. Metal phosphorous chalcogenides (MPX₃) are a fascinating class of two-dimensional materials with a tunable layered structure and high ion conductivity, making them particularly attractive for energy storage applications. This review article aims to comprehensively summarize the latest research progress on MPX₃ materials, with a focus on their preparation methods and modulation strategies. Additionally, the diverse applications of these novel materials in alkali metal ion batteries, metal-air batteries, and all-solid-state batteries are highlighted. Finally, the challenges and opportunities of MPX₃ materials are presented to inspire their better potential in energy storage applications. This review provides valuable insights into the promising future of MPX₃ materials in clean energy storage systems.

MnTe is shown to have the largest spontaneous magnetovolume effect known for any antiferromagnet. An unusual linear scaling between the strain and local magnetic order parameter is observed, justified by a novel trilinear coupling of the strain with short-range-ordered domains of the magnetic order parameter. This provides a route for realizing exceptional magnetostrictive properties in a variety of functional materials.

Abstract

A comprehensive x-ray scattering study of spontaneous magnetostriction in hexagonal MnTe, an antiferromagnetic semiconductor with a Néel temperature of T _N = 307 K, is presented. The largest spontaneous magnetovolume effect known for an antiferromagnet is observed, reaching a volume contraction of |ΔV/V| > 7 × 10⁻³. This can be justified semiquantitatively by considering bulk material properties, the spatial dependence of the superexchange interaction, and the geometrical arrangement of magnetic moments in MnTe. The highly unusual linear scaling of the magnetovolume effect with the short-range magnetic correlations, beginning in the paramagnetic state well above T _N, points to a novel physical mechanism, which is explained in terms of a trilinear coupling of the elastic strain with superposed distinct domains of the antiferromagnetic order parameter. This novel mechanism for coupling lattice strain to robust short-range magnetic order casts new light on magnetostrictive phenomena and also provides a template by which the exceptional magnetostrictive properties of MnTe might be realized in a wide range of other functional materials.

The growth of a high-quality complete graphene layer is successfully achieved in ultra-high vacuum conditions for Ir(111) and Ru(0001) substrates using liquid ethanol as a precursor. The process of graphene formation and its quality are carefully monitored using X-ray photoelectron spectroscopy, low-energy electron diffraction, scanning tunneling microscopy methods, and angular-resolved photoelectron spectroscopy at low temperatures.

Abstract

The growth of a high-quality complete graphene layer is successfully achieved for Ir(111) and Ru(0001) substrates using liquid ethanol as a precursor. Metallic substrates, which are cleaned in ultra-high vacuum conditions, were ex-situ immersed in liquid ethanol followed by the controlled in situ thermal annealing. The process of graphene formation and its quality are carefully monitored using X-ray photoelectron spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy methods. It is found that graphene formation starts at 400 °C via ethanol decomposition and desorption of oxygen from the surface leading to the formation of the high-quality complete graphene layer at 1000 °C. The results of the systematic angular-resolved photoelectron spectroscopy experiments confirm the high quality of the obtained graphene layer, and it concludes that such an approach offers an easy, quick, and reproducible method to synthesize large-scale graphene on different metallic substrates.

2H-VX₂ (X = S, Se, Te) are identified as a two-dimensional second-order topological insulator with a ferromagnetic ground state by first-principles calculation. They also show a topologically protected corner state with spin-polarization. Remarkably, these corner states are robust against symmetry-breaking perturbations, which makes them more easily detectable in experiments.

Abstract

The transition metal dichalcogenides, 2H-VX₂ (X = S, Se, Te), are identified as two-dimensional second-order topological insulator (SOTI) with a ferromagnetic ground state by first-principles calculations. The 2H-VX₂ (X = S, Se, Te) materials have a nontrivial band gap in two spin channels is found and exhibit topologically protected corner states with spin-polarization. These corner states only accommodate the quantized fractional charge (e/3). And the charge is bound at the corners of the nanodisk geometry 2H-VX₂ (X = S, Se, Te) in real space. The corner states are robust against symmetry-breaking perturbations, which makes them more easily detectable in experiments. Further, it is demonstrated that the SOTI properties of 2H-VX₂ (X = S, Se, Te) materials can be maintained in the presence of spin-orbit coupling and are stable against magnetization. Overall, the results reveal 2H-VX₂ (X = S, Se, Te) as an ideal platform for the exploration of magnetic SOTI and suggest its great potential in experimental detection.

Nature Nanotechnology, Published online: 31 July 2023; doi:10.1038/s41565-023-01448-6

Recent experiments demonstrate ultrafast-fluid-like propagation of excitons in monolayers of MoS2.

Nature Physics, Published online: 31 July 2023; doi:10.1038/s41567-023-02163-8

Disordered media with their numerous scattering channels can be used as optical operators. Measurements of the scattering tensor of a second-harmonic medium extend this computing application to the nonlinear regime.

Nature Materials, Published online: 31 July 2023; doi:10.1038/s41563-023-01614-0

An electric field is found to be capable of controlling dislocation movement in semiconducting zinc sulfide, as observed in real time by in situ transmission electron microscopy.

Nature Materials, Published online: 31 July 2023; doi:10.1038/s41563-023-01607-z

Detailed transmission electron microscopy imaging of the dynamics of domain walls in twisted van der Waals ferroelectrics is obtained, capturing the transition to a hysteretic response.

Nature Materials, Published online: 31 July 2023; doi:10.1038/s41563-023-01624-y

The transmission spectrum of single-molecule junctions provides fingerprint information on the charge-transport properties. A technique called single-molecule photoelectron tunnelling spectroscopy has been developed that enables mapping of the transmission spectrum beyond the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap at room temperature and can be used to explore the energy-dependent charge transport through single-molecule junctions.

Plasticity in ceramics, including oxide glasses, is difficult to achieve and typically occurs only at high temperatures. Amorphous aluminum oxide (a-Al₂O₃) makes an exception to the rule by showing substantial ductility at room temperature. The microscale compression samples (micropillars with 2.25–11 µm diameter) are shown to deform without fracture even under fast impact loading resembling for example hammer forging.

Abstract

Oxide glasses are an elementary group of materials in modern society, but brittleness limits their wider usability at room temperature. As an exception to the rule, amorphous aluminum oxide (a-Al₂O₃) is a rare diatomic glassy material exhibiting significant nanoscale plasticity at room temperature. Here, it is shown experimentally that the room temperature plasticity of a-Al₂O₃ extends to the microscale and high strain rates using in situ micropillar compression. All tested a-Al₂O₃ micropillars deform without fracture at up to 50% strain via a combined mechanism of viscous creep and shear band slip propagation. Large-scale molecular dynamics simulations align with the main experimental observations and verify the plasticity mechanism at the atomic scale. The experimental strain rates reach magnitudes typical for impact loading scenarios, such as hammer forging, with strain rates up to the order of 1 000 s⁻¹, and the total a-Al₂O₃ sample volume exhibiting significant low-temperature plasticity without fracture is expanded by 5 orders of magnitude from previous observations. The discovery is consistent with the theoretical prediction that the plasticity observed in a-Al₂O₃ can extend to macroscopic bulk scale and suggests that amorphous oxides show significant potential to be used as light, high-strength, and damage-tolerant engineering materials.