zemin zheng

Nature, Published online: 17 November 2021; doi:10.1038/s41586-021-03979-1

This Review discusses the exciton physics of transition metal dichalcogenides, focusing on moiré patterns and exciton many-body physics, and outlines future research directions in the field.

Unique endogenous hetero-MXenes of amorphous MoS₂ coupling with fluoride-free Mo₂CT _x directly from Mo₂Ga₂C MAX is constructed in situ. The obtained hetero-Mo₂C shows extraordinary structural stability and optimized Li⁺ storage mechanism with reversible structural transformation. Moreover, the improved capability of electron and ion transport endows hetero-Mo₂C electrode with superior lithium storage performance, which surpasses all reported Mo₂C MXenes electrode materials.

Abstract

Endogenous heterojunction of 2D MXenes with unique structure shows inspiring potential in energy applications, which is impeded by complex synthesis method and finite MAX materials. Herein, an in situ hydrothermal strategy is implemented to successfully synthesize unique endogenous hetero-MXenes of amorphous MoS₂ coupling with fluoride-free Mo₂CT _x (hetero-Mo₂C) directly from Mo₂Ga₂C MAX. The distinctive morphology and heterojunction structure caused by the introduction of MoS₂ endow the hetero-MXenes with extraordinary structural stability and optimized Li⁺ storage mechanism with improved charge transport and lithium ion adsorption capabilities. As a result, hetero-Mo₂C exhibits excellent electrochemical performance with a high discharge specific capacity of 1242 mAh g^-1 at 0.1 A g⁻¹ and long cycle stability of 683.9 mAh g⁻¹ after 1200 cycling. This work provides new insights into rational design of novel MXenes heterojunctions, practically important for the development of MXenes and their applications in high-performance energy storage systems.

Nature Nanotechnology, Published online: 15 November 2021; doi:10.1038/s41565-021-01004-0

A dual-coupling-guided growth mechanism enables the realization of wafer-scale single-crystal WS2 on vicinal a-plane sapphire.

The potential of fusing 2D materials with silicon technologies, including 2D logic and memory devices, enabling the mitigation of challenges related to silicon integrated circuits (ICs) and even the creation of technologies beyond silicon, is highlighted. The progress of 2D IC applications and the prospects for realizing wafer-scale heterogeneous integration compatible with silicon ICs are also summarized.

Abstract

Continued reduction in transistor size can improve the performance of silicon integrated circuits (ICs). However, as Moore's law approaches physical limits, high-performance growth in silicon ICs becomes unsustainable, due to challenges of scaling, energy efficiency, and memory limitations. The ultrathin layers, diverse band structures, unique electronic properties, and silicon-compatible processes of 2D materials create the potential to consistently drive advanced performance in ICs. Here, the potential of fusing 2D materials with silicon ICs to minimize the challenges in silicon ICs, and to create technologies beyond the von Neumann architecture, is presented, and the killer applications for 2D materials in logic and memory devices to ease scaling, energy efficiency bottlenecks, and memory dilemmas encountered in silicon ICs are discussed. The fusion of 2D materials allows the creation of all-in-one perception, memory, and computation technologies beyond the von Neumann architecture to enhance system efficiency and remove computing power bottlenecks. Progress on the 2D ICs demonstration is summarized, as well as the technical hurdles it faces in terms of wafer-scale heterostructure growth, transfer, and compatible integration with silicon ICs. Finally, the promising pathways and obstacles to the technological advances in ICs due to the integration of 2D materials with silicon are presented.

A transition metal dichalcogenide (TMD)/cavity monolithic cavity is achieved by directly growing single-domain tungsten diselenide (WSe₂) bilayers on single silica microsphere (MS) surfaces. The thermal strain induces bilayer bandgap from indirect to direct, and the cavity confinement factor is also improved, which directly realizes room-temperature whispering-gallery-mode lasing, with a threshold nearly an order of magnitude lower than the existing records.

Abstract

Monolayer transition metal dichalcogenides (TMDs) have emerged as widely accepted 2D gain materials in the field of light sources owing to their direct bandgap and high photoluminescence quantum yield. However, the monolayer medium suffers from weak emission because only a single layer of molecules can absorb the pump energy. Moreover, the material degradation when transferring these fragile materials hinders their cooperation with the optical cavity further. In this study, for the first time, a high-quality monolithic structure is developed by directly growing single-domain tungsten diselenide (WSe₂) bilayers on single silica microsphere (MS) cavities. Such a completely wrapped structure guides the indirect-to-direct bandgap transition of WSe₂ bilayers, leading to a significantly improved photoluminescence intensity by about 60-fold. Moreover, the high-quality monolithic structure enhances the confinement factor of the cavity by more than 20-fold. Based on the above advantages, a bilayer WSe₂/MS microlaser is realized with an ultralow threshold of 0.72 W cm⁻², nearly an order of magnitude lower than the existing records. The results demonstrate the possibility of using multilayer TMD materials as 2D gain media and provide insights into a new ultracompact monolithic platform of TMD material/cavity for lasing devices.

A Roll-to-roll process is developed for dry transfer of large-scale graphene. With peeling speed and tension control, surface resistance of 9.5 kΩ sq⁻¹ is achieved on polyethylene terephthalate/ethylene vinyl acetate, allowing the fabrication of flexible graphene-based field effect transistors with 100 times lower gate leakage current and near-zero doping level compared with those fabricated using wet etched graphene samples.

Abstract

A major challenge for graphene applications is the lack of mass production technology for large-scale and high-quality graphene growth and transfer. Here, a roll-to-roll (R2R) dry transfer process for large-scale graphene grown by chemical vapor deposition is reported. The process is fast, controllable, and environmentally benign. It avoids chemical contamination and allows the reuse of graphene growth substrates. By controlling tension and speed of the R2R dry transfer process, the electrical sheet resistance is achieved as 9.5 kΩ sq⁻¹, the lowest ever reported among R2R dry transferred graphene samples. The R2R dry transferred samples are used to fabricate graphene-based field-effect transistors (GFETs) on polymer. It is demonstrated that these flexible GFETs feature a near-zero doping level and a gate leakage current one to two orders of magnitude lower than those fabricated using wet-chemical etched graphene samples. The scalability and uniformity of the R2R dry transferred graphene is further demonstrated by successfully transferring a 3 × 3 in² sample and measuring its field-effect mobility with 36 millimeter-scaled GFETs evenly spaced on the sample. The field-effect mobility of the R2R dry transferred graphene is determined to be 205 ± 36 cm² V⁻¹.

Single layer β′- and β*-In₂Se₃ films are experimentally validated to host 2D in-plane anti-ferroelectric and ferroelectric order, respectively, with structural and spectroscopic evidences characterized by low-temperature scanning tunneling microscopy (STM)/spectroscopy. An electric field-induced phase transition is also realized via applying an STM tip pulse, demonstrating the manipulation of the ferroelectric polarization with reversible switching.

Abstract

2D ferroelectrics have received wide interest due to the remarkable quantum states of emerging physics at reduced dimensionality, associated with their exotic properties in high-performance and nonvolatile functional devices. Here, by combing molecular beam epitaxy synthesis and scanning tunneling microscopy characterization, two metastable phases of layered In₂Se₃ films: β′- and β*-In₂Se₃ are reported, which develop different types of in-plane spontaneous polarizations, thus resulting in different striped morphologies. The anti-ferroelectric order in β′-In₂Se₃ and ferroelectric order of β*-In₂Se₃ are identified, respectively, down to the 2D limit by comprehensive investigations of structural and spectroscopic signatures, including the lattice distortion, the spatial profile of images, the formation of domain structure, and the electronic band-bending by polarization charges at edges. The ferroelectric switching between those two phases are further controlled via applying an electric field generated from the scanning tunneling microscopy tip in a reversible manner. The intriguing tunability between the (anti-)ferroelectric orders in the 2D limit provides a promising platform for studying the interplay between electronic structure and ferroelectricity in van der Waals materials, and promotes potential development of miniaturized transistors and memory devices based on electric polarizations.

The manipulation of the lattice polarity of quasi-vdW epitaxial GaN on graphene through controlling the interface atomic configuration is reported. This polarity-control rule is not affected by the growth method and is free of either crystalline or non-crystalline substrates. It makes the epitaxy of III-nitrides with preferred lattice polarity possible and improves the ability to fabricate advanced semiconductor devices.

Abstract

Quasi van der Waals epitaxy, a pioneering epitaxy of sp³-hybridized semiconductor films on sp²-hybridized 2D materials, provides a way, in principle, to achieve single-crystal epilayers with preferred atom configurations that are free of substrate. Unfortunately, this has not been experimentally confirmed in the case of the hexagonal semiconductor III-nitride epilayer until now. Here, it is reported that the epitaxy of gallium nitride (GaN) on graphene can tune the atom arrangement (lattice polarity) through manipulation of the interface atomic configuration, where GaN films with gallium and nitrogen polarity are achieved by forming CONGa(3) or COGaN(3) configurations, respectively, on artificial CO surface dangling bonds by atomic oxygen pre-irradiation on trilayer graphene. Furthermore, an aluminum nitride buffer/interlayer leads to unique metal polarity due to the formation of an AlON thin layer in a growth environment containing trace amounts of oxygen, which explains the open question of why those reported wurtzite III-nitride films on 2D materials always exhibit metal polarity. The reported atomic modulation through interface manipulation provides an effective model for hexagonal nitride semiconductor layers grown on graphene, which definitely promotes the development of novel semiconductor devices.

Lattice orientation heredity is shown to be a universal strategy in the transformation of highly oriented 2D van der Waals layered and nonlayered thin films. Different from the classical and direct epitaxial deposition, this process is developed based on the heredity phenomena in material science. It opens a new window for growing textured films on disordered substrates (e.g., GaN-related technology), and provides an effective route for large-area growth of epitaxial 2D MoS₂ thin films.

Abstract

The ability to control lattice orientation is often an essential requirement in the growth of both 2D van der Waals (vdW) layered and nonlayered thin films. Here, a unique and universal phenomenon termed “lattice orientation heredity” (LOH) is reported. LOH enables product films (including 2D-layered materials) to inherit the lattice orientation from reactant films in a chemical conversion process, excluding the requirement on the substrate lattice order. The process universality is demonstrated by investigating the lattice transformations in the carbonization, nitridation, and sulfurization of epitaxial MoO₂, ZnO, and In₂O₃ thin films. Their resultant compounds all inherit the mono-oriented crystal feature from their precursor oxides, including 2D vdW-layered semiconductors (e.g., MoS₂), metallic films (e.g., MXene-like Mo₂C and MoN), wide-bandgap semiconductors (e.g., hexagonal ZnS), and ferroelectric semiconductors (e.g., In₂S₃). Using LOH-grown MoN as a seeding layer, mono-oriented GaN is achieved on an amorphous quartz substrate. The LOH process presents a universal strategy capable of growing epitaxial thin films (including 2D vdW-layered materials) not only on single-crystalline but also on noncrystalline substrates.

The layered superconductor Cu_0.11TiSe₂, which can be obtained by controlled intercalation of TiSe₂ with Cu, has increased electrons and ions transfer rates. It exhibits a superior rate capability and long cycling stability as a cathode of potassium-ion batteries. Cu ions also play a role as a pillar between layers, delivering the highly reversible phase transformations during cycles.

Abstract

The cathode material is one of the main restricting factors for the development of potassium-ion batteries (PIBs). The poor conductivity, sluggish reaction kinetics, and unstable crystal structure of cathode materials have impeded their electrochemical performance. Here, controlled intercalation of TiSe₂ with Cu is used to yield a layered superconductor Cu_0.11TiSe₂, which exhibits increased electrons and ions transfer rates and improved crystal structure stability. The insertion of Cu not only improves the electronic conductivity and reduces the diffusion barrier but also plays a role in crystal structure support, which further leads to a highly reversible charge and discharge process of Cu_0.11TiSe₂. The layered superconductor Cu_0.11TiSe₂ exhibits an excellent cycling performance with a capacity retention of 80% after 300 cycles at a current density of 20 mA g⁻¹ and a superior rate capability with a capacity of 45 mAh g⁻¹ at 1000 mA g⁻¹ (≈8C). Furthermore, a full battery assembled with the Cu_0.11TiSe₂ cathode and graphite anode exhibits a high reversible capacity of 74 mAh g⁻¹ at a current density of 20 mA g⁻¹. This study provides a new path for developing the high-performance cathode material of PIBs and other alkali metal-ion batteries.

Van der Waals (vdW) materials are an indispensable part of functional device technology. Recently, the search for magnetic vdW materials has intensified due to the realization of magnetism in 2D. However, metallic magnetic vdW systems are still uncommon and they rarely show high-mobility charge carriers. Using chemical reasoning, it is found that TaCo₂Te₂ is an air-stable, high-mobility, magnetic vdW material.

Abstract

Van der Waals (vdW) materials are an indispensable part of functional device technology due to their versatile physical properties and ease of exfoliating to the low-dimensional limit. Among all the compounds investigated so far, the search for magnetic vdW materials has intensified in recent years, fueled by the realization of magnetism in 2D. However, metallic magnetic vdW systems are still uncommon. In addition, they rarely host high-mobility charge carriers, which is an essential requirement for high-speed electronic applications. Another shortcoming of 2D magnets is that they are highly air sensitive. Using chemical reasoning, TaCo₂Te₂ is introduced as an air-stable, high-mobility, magnetic vdW material. It has a layered structure, which consists of Peierls distorted Co chains and a large vdW gap between the layers. It is found that the bulk crystals can be easily exfoliated and the obtained thin flakes are robust to ambient conditions after 4 months of monitoring using an optical microscope. Signatures of canted antiferromagntic behavior are also observed at low-temperature. TaCo₂Te₂ shows a metallic character and a large, nonsaturating, anisotropic magnetoresistance. Furthermore, the Hall data and quantum oscillation measurements reveal the presence of both electron- and hole-type carriers and their high mobility.

Through the construction of a W/Cr₂Ge₂Te₆ heterostructure with annealing treatment, the Curie temperature of Cr₂Ge₂Te₆ is raised above 150 K with strong perpendicular magnetic anisotropy, which is attributed to the interfacial orbital hybridization. Due to the weak interlayer coupling, the interfacial enhancement can be effective in long distance. The enhanced ferromagnetism can be controlled by spin-orbit torque with low current density.

Abstract

Ferromagnetic semiconductors discovered in 2D materials open an avenue for highly integrated and multifunctional spintronics. The Curie temperature (T _C) of existing 2D ferromagnetic semiconductors is extremely low and the modulation effect of their magnetism is limited compared with their 2D metallic counterparts. The interfacial effect is found to effectively manipulate the 3D magnetism, providing a unique opportunity for tailoring the 2D magnetism. Here, it is demonstrated that the T _C of a 2D ferromagnetic semiconductor Cr₂Ge₂Te₆ (CGT) can be enhanced by 130% (from ≈65 K to above 150 K) when adjacent to a tungsten layer. The interfacial W–Te bonding contributes to the T _C enhancement with a strong perpendicular magnetic anisotropy, guaranteeing efficient magnetization switching by the spin-orbit torque with a low current density at 150 K. Distinct from the rapid attenuation in conventional magnets, the interfacial effect exhibits a weak dependence on CGT thickness and a long-distance effect (more than 10 nm) due to the weak interlayer coupling inherent to 2D magnets. This work not only reveals a unique interfacial behavior in 2D materials, but also advances the process toward practical 2D spintronics.

Nature Nanotechnology, Published online: 10 November 2021; doi:10.1038/s41565-021-01024-w

2D materials grow large

Two-dimensional (2D) few-layer black phosphorus (BP) with extraordinary electronic and optical properties is an excellent candidate for optoelectronic applications. However, rapid surface oxidation under ambient environment significantly restricts its practicability. Here, we investigate excitonic effect in few-layer BP oxides via first-principle calculation and effective mass approximation. Influence of layer numbers and degree of oxidation on exciton binding energy (EBE) is discussed in detail for the first time. It is found that EBE in BP oxides decreases exponentially with increasing sample thickness and becomes almost oxygen independent over six layers with values similar to that of pristine BP. Instead, oxidation alters excitation probability of excitons in few-layer BP via a direct/indirect bandgap transition.

Monolayers of transition metal dichalcogenides (TMDCs) exhibit attractive properties and are promising for fabricating photonic and optoelectronic devices, while bulk multilayered structures based on the same materials only recently has revealed many properties useful for nanophotonics. In this regard, the combination of monolayer and multilayer properties in one device (on a single flake) is an important and fruitful task that needs to be solved. In this work, we demonstrate the use of local anodic oxidation to improve the optical properties of multilayer MoSe 2 flakes on a gold-covered substrate. Using this method, we fabricated nanostructures demonstrating extraordinarily enhanced photoluminescence (PL), with an intensity up to three orders of magnitude compared to that of the original structure. Low-frequency Raman spectroscopy showed that the nature of this PL enhancement is that the bindings between the layers inside the nanostructures are severely disrupted. Thi...

Proximity effects between layered materials trigger a plethora of novel and exotic quantum transport phenomena. Besides, the capability to modulate the nature and strength of proximity effects by changing crystalline and interfacial symmetries offers a vast playground to optimize physical properties of relevance for innovative applications. In this work, we use large-scale first principles calculations to demonstrate that strain and twist-angle strongly vary the spin–orbit coupling (SOC) in graphene/transition metal dichalcogenide heterobilayers. Such a change results in a modulation of the spin relaxation times by up to two orders of magnitude. Additionally, the relative strengths of valley-Zeeman and Rashba SOC can be tailored upon twisting, which can turn the system into an ideal Dirac–Rashba regime or generate transitions between topological states of matter. These results shed new light on the debated variability of SOC and clarify how lattice deformations can be used as a ...

We study the localization properties of electrons in incommensurate twisted bilayer graphene for small angles, encompassing the narrow-band regime, by numerically exact means. Sub-ballistic states are found within the narrow-band region around the magic angle. Such states are delocalized in momentum-space and follow non-Poissonian level statistics, in contrast with their ballistic counterparts found for close commensurate angles. Transport results corroborate this picture: for large enough systems, the conductance of samples with fixed width decreases with the system size in the longitudinal direction for incommensurate angles within the sub-ballistic regime. Our results show that incommensurability/quasiperiodicity effects are of crucial importance in the narrow-band regime. The incommensurate nature of a general twist angle must therefore be taken into account for an accurate description of magic-angle twisted bilayer graphene.

Modern low-voltage scanning transmission electron microscopes (STEMs) have been invaluable for the atomic scale characterization of two-dimensional (2D) materials. Nevertheless, the observation of intrinsic structures of semiconducting and insulating 2D materials with 60 kV-microscopes has remained problematic due to electron radiation damage. In recent years, ultralow-voltage microscopes have been developed with the prospects of minimizing radiation damage of such 2D materials, however, to date only ultralow-voltage TEM investigations of semiconducting and insulating 2D materials have been reported, but similar results using STEM, despite being more widely adopted, are still missing. Here we report a quantitative analysis of radiation damage and beam-induced defect dynamics in semiconducting 2D WS 2 during 30 kV and 60 kV-STEM imaging, particularly by recording atomic resolution electrostatic potential movies using integrated differential phase contrast to visualize b...

Nature Chemistry, Published online: 11 November 2021; doi:10.1038/s41557-021-00813-z

Several polymorphs of borophene have been synthesized on metal substrates, but typically as monolayers. Now large-size, single-crystalline bilayer borophene has been grown on Cu(111)—a sufficient electron provider to enable the bonding of the second boron layer. The resulting bilayer possesses a metallic character and is less susceptible to oxidation than its monolayer counterpart.

Van der Waals antiferromagnet CrPS₄ undergoes a spin-reorientation transition with magnetic moments realigning from almost parallel to the c axis in the ac plane to along the b axis upon heating. The transition is dramatically suppressed by hydrostatic pressure, offering an effective way to control the Néel vector of antiferromagnets.

Abstract

Controlling the phases of matter is a central task in condensed matter physics and materials science. In 2D magnets, manipulating spin orientation is of great significance in the context of the Mermin–Wagner theorem. Herein, a systematic study of temperature- and pressure-dependent magnetic properties up to 1 GPa in van der Waals CrPS₄ is reported. Owing to the temperature-dependent change of the magnetic anisotropy energy, the material undergoes a first-order spin reorientation transition with magnetic moments realigning from being almost parallel with the c axis in the ac plane to the quasi-1D chains of CrS₆ octahedra along the b axis upon heating. The spin reorientation temperature is suppressed after applying pressure, shifting the high-temperature phase to lower temperatures with the emergence of spin-flop transitions under magnetic fields applied along the b axis. The saturation field increases with pressure, indicating the enhancement of interlayer antiferromagnetic coupling. However, the Néel temperature is slightly reduced, which is ascribed to the suppression of intralayer ferromagnetic coupling. The work demonstrates the control of spin orientation and metamagnetic transitions in layered antiferromagnets, which may provide new perspectives for exploring 2D magnetism and related spintronic devices.