Jiuxiang Dai

Nature Synthesis, Published online: 05 September 2022; doi:10.1038/s44160-022-00149-7

Inverse spin Hall effect (ISHE) is one of the accessible and reliable methods to detect spin current. ISHE signals dependent on the Néel vector in Mn₂Au/[Co/Pd] are observed, and they are much stronger when the converted charge current is parallel to the Néel vector compared with its orthogonal counterpart. The finding enriches the Hall effect family, and makes antiferromagnetic spintronics more flexible.

Abstract

The inverse spin Hall effect (ISHE) is one of the accessible and reliable methods to detect spin current. The magnetization-dependent inverse spin Hall effect has been observed in magnets, expanding the dimension for spin-to-charge conversion. However, antiferromagnetic Néel-vector-dependent ISHE, which has been long time highly pursued, is still elusive. Here, ISHE in Mn₂Au/[Co/Pd] heterostructures is investigated by terahertz emission and spin Seebeck effect measurements, where [Co/Pd] possesses perpendicular magnetic anisotropy for out-of-plane polarized spin current generation and Mn₂Au is a collinear antiferromagnet for the spin-to-charge conversion. The out-of-plane spin polarization (σ _z ) is rotated toward in-plane by the Néel vectors in Mn₂Au, then the spin current is converted into charge current at two staggered spin sublattices. The ISHE signal is much stronger when the converted charge current is parallel to the Néel vector compared with its orthogonal counterpart. The Néel vector and resultant ISHE signals, which is termed as antiferromagnetic inverse spin Hall effect, can be switched. The finding not only adds a new member to the Hall effect family, but also makes antiferromagnetic spintronics more flexible.

MXenes are a quite novel class of 2D inorganic materials that have many possible industrial and biological applications. The review paper describes the synthesis, characterization, and current advancements in using MXenes and their derivatives for anticorrosive application. The review also focuses on challenges and future prospective.

Abstract

MXenes (MXs) (singular; “MXene” (MX); pronounced “Maxenes”) are a relatively new class of 2D inorganic materials that possess numerous potential biological and industrial applications. After the discovery of the first MXene, Ti₃C₂T _x , in 2011 plenty of reports on the synthesis and application of MXenes and their derivatives are published in the literature. Generally, MXenes contain heteroatoms at their surface; therefore, they can act as strong ligands and can form coordination bonding with the metal atoms and their ions. Because of this property, recently MXenes are used as nanofillers in polymer-based coatings. The MXenes possess excellent filler properties. They are extensively used as fillers in epoxy resins and polyurethane-based coatings but their application as fillers in other polymers is highly limited, which opens an area to explore in the future. The anticorrosive application of MXenes-filled polymers coatings is extensively reported. The contemporary review designates the assemblies of various reports on the anticorrosive and filler property of the MXenes and their composites. Previous studies suggest that MXenes and their composites not only affect the performance of polymer coatings but also enhance their durability. The present report also describes the synthesis, characterization, chemical structures, and properties of the MXenes and their composites.

Ultra-thin SiO₂, Al₂O₃, and HfO₂ layers are produced via plasma-enhanced atomic layer deposition, with negatively charged HfO₂ providing excellent passivation. Passivation is temperature and thickness-dependent. Optimum passivation with 2.2–3.3 nm HfO₂ layers annealed at 450 °C gives surface recombination velocities ≤2.5 cm s⁻¹ and J₀ values ≈14 fA cm⁻², thus demonstrating HfO₂'s promise as an ultra-thin passivating layer.

Abstract

Surface passivating thin films are crucial for limiting the electrical losses during charge carrier collection in silicon photovoltaic devices. Certain dielectric coatings of more than 10 nm provide excellent surface passivation, and ultra-thin (<2 nm) dielectric layers can serve as interlayers in passivating contacts. Here, ultra-thin passivating films of SiO₂, Al₂O₃, and HfO₂ are created via plasma-enhanced atomic layer deposition and annealing. It is found that thin negatively charged HfO₂ layers exhibit excellent passivation properties—exceeding those of SiO₂ and Al₂O₃—with 0.9 nm HfO₂ annealed at 450 °C providing a surface recombination velocity of 18.6 cm s⁻¹. The passivation quality is dependent on annealing temperature and layer thickness, and optimum passivation is achieved with HfO₂ layers annealed at 450 °C measured to be 2.2–3.3 nm thick which give surface recombination velocities ≤2.5 cm s⁻¹ and J ₀ values of ≈14 fA cm⁻². The superior passivation quality of HfO₂ nanolayers makes them a promising candidate for future passivating contacts in high-efficiency silicon solar cells.

An unconventional nonlinear-carbon-supply growth strategy for synthesizing hexagonal graphene with specific sizes is presented. The dynamic adjustment of the carbon supply and precise control of the growth and etching processes result in the direct growth of graphene-based tunnel-junction structures and small-gapped quasi-monolayer films. This study provides a foundation for directly synthesizing 2D materials for applications in highly integrated electronics.

Abstract

Controlling the morphology of graphene and other 2D materials in chemical vapor deposition (CVD) growth is crucial because the morphology reflects the crystal quality of as-synthesized nanomaterials in a certain way, and consequently it indirectly represents the physical properties of 2D materials such as bandgap, selective ion transportation, and impermeability. However, precise control of the morphology is limited by the complex formation mechanism and sensitive growth-environment factors of graphene. Therefore, the CVD synthesis of single-crystal hexagonal-shaped graphene islands with specific sizes is challenging. Herein, an unconventional nonlinear-carbon-supply growth strategy is proposed to realize controllable CVD growth of desired hexagonal graphene islands with specific sizes on Cu substrates. Large-area graphene films of isolated islands with desired densities, sizes, and distances between the islands are successfully synthesized. Subsequently, the direct growth of a planar-tunnel-junction structure based on two parallel gapped graphene islands is achieved by specific adjustment of the growth and etching processes of graphene CVD synthesis. It is therefore demonstrated that the nonlinear-carbon-supply growth strategy is a reliable method for the synthesis of high-quality graphene and can facilitate the direct growth of graphene-based nanodevices in the future.

The optical and optoelectronic properties of 2D Xenes and their applications in photonic devices including all-optical modulators, wavelength converters, ultrafast lasers, and photodetectors are comprehensively reviewed. The article mainly focuses on five most extensively studied Xenes: graphene, black phosphorus, antimonene, bismuthene, and tellurenene. The properties and applications of other Xenes are also briefly introduced.

Abstract

In recent years, tremendous attention has been paid to the investigation of single-element 2D materials. These 2D materials mainly consist of elements from group IV and group V such as silicene, phosphorene, and antimonene. Together with other four elements from groups III and VI, they are classified as 2D Xenes and exhibit rich optical and optoelectronic properties such as broadband optical response, strong nonlinearity, ultrafast recovery time, and layer-dependent bandgap. 2D Xenes can be easily integrated with microfibers and other optical platforms. On the basis of their attracting characteristics, 2D Xenes have been utilized in various functional devices. In this review, the optical and optoelectronic properties of the most intensively studied 2D Xenes are introduced. Their applications in photonic devices including all-optical modulators, wavelength converters, ultrafast lasers, and photodetectors are explicitly explored. Finally, the challenges and future perspectives of photonic devices based on 2D Xenes are discussed.

Electron spin resonance studies of a bulk antiferromagnetic semiconductor CrSBr single crystal revel: 2D magnetism, room temperature magnetic anisotropy, spin–spin correlations as well as the formation of potential topological vortex and anti-vortex pairs predicted by the BKT model. These findings together with the chemical stability and semiconducting properties, make CrSBr a promising layered magnet for future magneto- and topological electronic applications.

Abstract

2D magnets represent material systems in which magnetic order and topological phase transitions can be observed. Based on these phenomena, novel types of computing architectures and magnetoelectronic devices can be envisaged. Unlike conventional magnetic films, their magnetism is independent of the substrate and interface qualities, and 2D magnetic properties manifest even in formally bulk single crystals. However, 2D magnetism in layered materials is rarely reported often due to weak exchange interactions and magnetic anisotropy, and low magnetic transition temperatures. Here, the electron spin resonance (ESR) properties of a layered antiferromagnetic CrSBr single crystal are reported. The W-like shape angular dependence of the ESR linewidth provides a signature for room temperature spin–spin correlations and for the XY spin model. By approaching the Néel temperature the arising of competing intralayer ferromagnetic and interlayer antiferromagnetic interactions might lead to the formation of vortex and antivortex pairs. This argument is inferred by modeling the temperature dependence of the ESR linewidth with the topological Berezinskii-Kosterlitz-Thouless phase transition. These findings together with the chemical stability and semiconducting properties, make CrSBr a promising layered magnet for future magneto- and topological-electronics.

3D networks of supramolecular gels provide tuneable spatially confined spaces to drive the growth of large-area 2D fullerene molecular crystals under controlled supersaturation. This strategy is expected to be an efficient and universal method for the growth of organic semiconductors into 2D organic molecular crystals for their potential applications.

Abstract

2D organic molecular crystals (2DOMCs) are promising materials for the fabrication of high-performance optoelectronic devices. However, the growth of organic molecules into 2DOMCs remains a challenge because of the difficulties in controlling their self-assembly with a preferential orientation in solution-process crystallization. Herein, fullerene is chosen as a model molecule to develop a supramolecular gel crystallization approach to grow large-area 2DOMCs by controlling the perfect arrangement on the {220} crystal plane with the assistance of a gelated solvent. In this case, the gel networks provide tuneable confined spaces to control the crystallization kinetics toward the growth of dominant crystal faces by their inhibiting motions of solvent or solute molecules to enable the growth of perfect crystals at appropriate nucleation rates. As a result, a large-area fullerene 2DOMC is produced successfully and its corresponding device on a flexible substrate exhibits excellent bendable properties and ultra-high weak light detection ability (2.9 × 10¹¹ Jones) at a 10 V bias upon irradiation with 450 nm incident light. Moreover, its photoelectric properties remain unchanged after 200 cycles of bending at angles of 45, 90, and 180°. These results can be extended to the growth of other 2DOMCs for potentially fabricating advanced organic (opto)electronics.

The staggered large band offset heterojunction based on MoS₂ and SnSe₂ is an ideal structure for the separation of photogenerated carriers. Combined with doping engineering, a photodetector based on the doped MoS₂/SnSe₂ heterostructure exhibits excellent solar-blind UV photoresponse ability compared to the pure MoS₂/SnSe₂ heterostructure. The rejection ratio is significantly improved by about five orders of magnitude.

Abstract

The intentionally designed band alignment of heterostructures and doping engineering are keys to implement device structure design and device performance optimization. According to the theoretical prediction of several typical materials among the transition metal dichalcogenides (TMDs) and group-IV metal chalcogenides, MoS₂ and SnSe₂ present the largest staggered band offset. The large band offset is conducive to the separation of photogenerated carriers, thus MoS₂/SnSe₂ is a theoretically ideal candidate for fabricating photodetector, which is also verified in the experiment. Furthermore, in order to extend the photoresponse spectrum to solar-blind ultraviolet (SBUV), doping engineering is adopted to form an additional electron state, which provides an extra carrier transition channel. In this work, pure MoS₂/SnSe₂ and doped MoS₂/SnSe₂ heterostructures are both fabricated. In terms of the photoelectric performance evaluation, the rejection ratio R ₂₅₄/R ₅₃₂ of the photodetector based on doped MoS₂/SnSe₂ is five orders of magnitude higher than that of pure MoS₂/SnSe₂, while the response time is obviously optimized by 3 orders. The results demonstrate that the combination of band alignment and doping engineering provides a new pathway for constructing SBUV photodetectors.

The magic-angle twisted trilayer graphene (MATTG) hosts robust superconductivity with unique properties, including the Pauli-limit violating and re-entrant behaviors. A systematic investigation combining nanometer-scale spatially resolved angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy on MATTG revealing the coexistence of the Dirac band and the moiré flat band serves as a stepstone for further understanding of the unconventional superconductivity in MATTG.

Abstract

Moiré superlattices that consist of two or more layers of 2D materials stacked together with a small twist angle have emerged as a tunable platform to realize various correlated and topological phases, such as Mott insulators, unconventional superconductivity, and quantum anomalous Hall effect. Recently, magic-angle twisted trilayer graphene (MATTG) has shown both robust superconductivity similar to magic-angle twisted bilayer graphene and other unique properties, including the Pauli-limit violating and re-entrant superconductivity. These rich properties are deeply rooted in its electronic structure under the influence of distinct moiré potential and mirror symmetry. Here, combining nanometer-scale spatially resolved angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy, the as-yet unexplored band structure of MATTG near charge neutrality is systematically measured. These measurements reveal the coexistence of the distinct dispersive Dirac band with the emergent moiré flat band, showing nice agreement with the theoretical calculations. These results serve as a stepstone for further understanding of the unconventional superconductivity in MATTG.

Epitaxially grown vertical layered heterostructures (VLHs) of mixed 2D layered materials are often thought to align without angular misorientation. However, it is shown that high-mismatch VLHs show discrete and sometimes non-zero twist angles dependent on their natural lattice mismatch value. This opens a pathway for scalable Moire VLH systems.

Abstract

Artificially introduced small twist angles at the interfaces of vertical layered heterostructures (VLHs) have allowed deterministic tuning of electronic and optical properties such as strongly correlated electronic phases and Moiré excitons. But creating a Moiré twist in van der Waals (vdWs) systems by manual stacking is challenging in reproducibility, uniformity, and accuracy of the twist angle, which hinders future studies. Here, it is demonstrated that contrary to the commonly believed 0°-orientation in vdWs epitaxy, these VLHs show small twist angles controlled by the low-order commensurate phase with low energy and local atomic relaxation. A commensurate multilevel map is proposed to predict possible orientations. Remarkably, high-mismatch VLHs show discrete and sometimes non-zero twist angles dependent on their natural mismatch value. Such framework is experimentally confirmed in five epitaxially grown VLHs under high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and can provide significant insights for large-scale engineering of twist angle in VLHs.

Nature Materials, Published online: 31 August 2022; doi:10.1038/s41563-022-01363-6

Our results demonstrate that the electron transfer from a monolayer(1L)-black phosphorus (BP) to MoS₂ occurs quickly within 54 fs. In contrast, hole transfer can only be observed within 1 ps with BP's layer number N ≥ 2. Moreover, the electron and hole transfer time scales exhibit respectively linear and exponential dependence on the layer number N of BP component.

Abstract

Constructing high-performance-2D heterostructures and deciphering the underlying microscopic mechanism of carrier dynamics are crucial in optoelectronic and photovoltaic applications. Here, taking black phosphorus (BP)/MoS₂ heterostructure with type-II band alignment as a prototypical example, the ab initio nonadiabatic molecular dynamics simulations demonstrate that the interlayer carrier dynamics are thickness dependent. Specifically, the electron transfer from a monolayer (1L)-BP to MoS₂ occurs quickly within 54 fs. In contrast, hole transfer can only be observed within 1 ps with BP's layer number N ≥ 2, triggered by the excitation of low-frequency acoustic phonon and interlayer shear and breathing phonon modes within 100 cm^–1 that enhances the interlayer coupling. Particularly, the electron and hole transfer time exhibits respectively linear and exponential dependence on the layer number N of BP component. The present findings shed new light on improving the process of ultrafast carrier dynamics of 2D heterostructures for photoconversion.