Jiuxiang Dai

Nature Materials, Published online: 28 July 2021; doi:10.1038/s41563-021-01048-6

Two-dimensional (2D) metal oxides that can be exfoliated are produced via direct oxidation of their elemental metals, providing a simple and easy way to incorporate these materials in van der Waals heterostructures.

Nature, Published online: 28 July 2021; doi:10.1038/s41586-021-03646-5

Spectroscopic measurements confirm that when water is adsorbed on drops of an alkali alloy at low pressure a gold-coloured metallic layer forms as electrons rapidly move from the drop into the water.

Nature, Published online: 28 July 2021; doi:10.1038/d41586-021-02065-w

Electrons from a droplet of sodium and potassium turn water into a metallic material that conducts electricity.

Ultrasensitive photodetectors with defect tolerance are demonstrated by interfacing layered-perovskite thin films and MoS₂ grown by chemical vapor deposition. With the pure crystallographic orientation and type-II band alignment, efficient light absorption and exciton transport in layered perovskites, and ultrafast charge transfer sustain a high responsivity of 2.5 × 10⁴ A W⁻¹, and a specific detectivity of 4.1 × 10¹⁴ Jones.

Abstract

Heterostructures for charge-carrier manipulation have laid the foundation of modern optoelectronic devices, such as photovoltaics and photodetectors. High-performance heterostructure devices usually impose stringent requirements on the material quality to sustain efficient carrier transport and charge transfer, thus leading to sophisticated fabrication processes. Here, a simple yet efficient strategy is proposed to develop ultrasensitive photodetectors based on heterostructures of chemical vapor deposition-grown MoS₂ and polycrystalline-layered perovskites. The layered perovskites possess pure crystallographic orientation with conductive edges in contact with MoS₂, which gives rise to efficient light absorption, exciton diffusion, and interfacial charge transfer. In dark state, the mismatch of work functions of two materials facilitates low dark currents by the depletion of electrons in MoS₂. Under light irradiation, efficient exciton diffusion and interfacial charge transfer are realized in the heterostructures with type-II band alignment, which produces drifting electrons in MoS₂ and leaves trapped holes in layered perovskites. The photodetectors present suppress noises and boost photocurrents, yielding a champion device with a responsivity of 2.5 × 10⁴ A W⁻¹, and a specific detectivity of 4.1 × 10¹⁴ Jones. The results demonstrate a scalable approach for the integration of high-performance devices with high tolerance to defects.

Phase-transition-induced growth is effective for synthesizing layer-controlled and electronic-state-modulated MoS₂ films. The energetically unstable amorphous MoS _x O _y is efficiently converted to a thermodynamically stable crystalline MoS₂ film on a wafer scale with excellent uniformity. The number of layers is precisely controlled layer-by-layer from one to eleven layers, which is applicable in the growth of both intrinsic and heteroatom-inserted MoS₂.

Abstract

Wafer-scale growth of transition metal dichalcogenides with precise control over the number of layers, and hence the electronic state is an essential technology for expanding the practical application of 2D materials. Herein, a new growth method, phase-transition-induced growth (PTG), is proposed for the precisely controlled growth of molybdenum disulfide (MoS₂) films consisting of one to eleven layers with spatial uniformity on a 2 in. wafer. In this method, an energetically unstable amorphous MoS _x O _y (a-MoS _x O _y ) phase is effectively converted to a thermodynamically stable crystalline MoS₂ film. The number of MoS₂ layers is readily controlled layer-by-layer by controlling the amount of Mo atoms in a-MoS _x O _y , which is also applicable for the growth of heteroatom-inserted MoS₂. The electronic states of intrinsic and Nb-inserted MoS₂ with one and four layers grown by PTGare are analyzed based on their work functions. The work function of monolayer MoS₂ effectively increases with the substitution of Nb for Mo. As the number of layers increases to four, charge screening becomes weaker, dopant ionization becomes easier, and ultimately the work function increases further. Thus, better electronic state modulation is achieved in a thicker layer, and in this respect, PTG has the advantage of enabling precise control over the film thickness.

Neutral cation vacancy in wide bandgap III-nitrides including AlN, GaN, and AlGaN is predicted to be suitable for single-photon emission, it has zero phonon line that lies within the optimal range for transmission in optical fiber and a moderate radiative rate, thereby render it as a promising single-photon emitter for quantum information applications.

Abstract

Single-photon sources based on solid-state material are desirable in quantum technologies. However, suitable platforms for single-photon emission are currently limited. Herein, a theoretical approach to design a single-photon emitter based on defects in solid-state material is proposed. Through group theory analysis and hybrid density functional theory calculation, the charge-neutral cation vacancy in III-V compounds is found to satisfy a unique 5-electron-8-orbital electronic configuration with T _d symmetry, which is possible for single-photon emission. Furthermore, it is confirmed that this type of single-photon emitter only exists in wide bandgap III-nitrides among all the III-V compounds. The corresponding photon energy in GaN, AlN, and AlGaN lies within the optimal range for transfer in optical fiber, thereby render the charge-neutral cation vacancy in wide-bandgap III-nitrides as a promising single-photon emitter for quantum information applications.

The electrical properties of monolayer hexagonal boron nitride (h-BN) and bilayer h-BN are investigated by atomic force microscopy (AFM) technology. The bilayer h-BN showed quite different electrical characteristics from monolayer h-BN. The mechanism that made this difference is investigated. Besides, the effect of load force on electrical characteristics of h-BN with different layers is also studied.

Abstract

Hexagonal boron nitride (h-BN) is one of the most important 2D materials which attracts tremendous attention for the demonstrated great potential applications in optical and electronic devices. However, whether there are significant differences in the electrical properties of h-BN with different layers and its mechanism is not revealed clearly. Based on the atomic force microscopy (AFM) technology, the electrical properties of monolayer h-BN and bilayer h-BN are investigated. It is found that bilayer h-BN shows quite different electrical characteristics from monolayer h-BN. It is proposed that the difference of work functions between monolayer h-BN and bilayer h-BN contributes to the different electrical characteristics. Meanwhile, the interlayer coupling resistance due to coupling between the layers of h-BN also plays a vital role in electron transport. Besides, the effect of load force on electrical characteristics of h-BN with different layers is also investigated. This work provides a new insight to understand the effect of the different layers on electrical properties of h-BN. It is hoped that this valuable experimental data can offer meaningful suggestions for future studies and applications on h-BN and other 2D nanomaterials in general.

Nature Materials, Published online: 26 July 2021; doi:10.1038/s41563-021-01070-8

Interlayer hybridization in 2D van der Waals materials can change their properties. Here, it is shown that the coupling in CrSBr can be changed from switching the magnetic order from antiferromagnetic to ferromagnetic states.

Nature Electronics, Published online: 26 July 2021; doi:10.1038/s41928-021-00626-5

Magnetic tunnel junctions hit the buffer

2D nanomaterials generally exhibit enhanced physiochemical and biological functions in biomedical applications due to their high surface-to-volume ratio and surface charge. Conventional cancer chemotherapy based on nanomaterials has been hindered by their low drug loading and poor penetration in tumor tissue. To overcome these difficulties, novel materials systems are urgently needed. Hereby, the lanthanide-based porphyrin metal–organic framework (MOF) nanosheets (NSs) with promising cancer imaging/chemotherapy capacities are fabricated, which display superior performance in the drug loading and tumor tissue penetration. The biodegradable PPF-Gd NSs deliver an ultrahigh drug loading (>1500%) and demonstrate the stable and highly sensitive stimuli-responsive degradation/release for multimodal tumor imaging and cancer chemotherapy. Meanwhile, PPF-Gd NSs also exhibit excellent fluorescence and magnetic resonance imaging capability in vitro and in vivo. Compared to the traditional doxorubicin (DOX) chemotherapy, the in vivo results confirm the evident suppression of the tumor growth by the PPF-Gd/DOX drug delivery system with negligible side effects. This work further supports the potential of lanthanide-based MOF nanomaterials as biodegradable systems to promote the cancer theranostics technology development in the future.

The minimized diffusion limitation and completely exposed strong acid sites of the ultrathin zeolites make it an industrially important catalyst especially for converting bulky molecules. However, the structure-controlled and large-scale synthesis of the material is still a challenge. In this work, the direct synthesis of the single-layer MWW zeolite was demonstrated by using hexamethyleneimine and amphiphilic organosilane as structure-directing agents. Characterization results confirmed the formation of the single-layer MWW zeolite with high crystallinity and excellent thermal/hydrothermal stability. The formation mechanism was rigorously revealed as the balanced rates between the nucleation/growth of the MWW nanocrystals and the incorporation of the organosilane into the MWW unit cell, which is further supported by the formation of MWW nanosheets with tunable thickness via simply changing synthesis conditions. The commercially available reagents, well-controlled structure and the high catalytic stability for the alkylation of benzene with 1-dodecene make it an industrially important catalyst.

Under ambient conditions, the only known valence state of calcium ions is +2, and the corresponding crystals with calcium ions are insulating and nonferromagnetic. Here, using cryo-electron microscopy, we report direct observation of two-dimensional (2D) CaCl crystals on reduced graphene oxide (rGO) membranes, in which the calcium ions are only monovalent (i.e. +1). Remarkably, metallic rather than insulating properties are displayed by those CaCl crystals. More interestingly, room-temperature ferromagnetism, graphene–CaCl heterojunction, coexistence of piezoelectricity-like property and metallicity, as well as the distinct hydrogen storage and release capability of the CaCl crystals in rGO membranes are experimentally demonstrated. We note that such CaCl crystals are obtained by simply incubating rGO membranes in salt solutions below the saturated concentration, under ambient conditions. Theoretical studies suggest that the formation of those abnormal crystals is attributed to the strong cation-π interactions of the Ca cations with the aromatic rings in the graphene surfaces. The findings highlight the realistic potential applications of such abnormal CaCl material with unusual electronic properties in designing novel transistors and magnetic devices, hydrogen storage, catalyzers, high-performance conducting electrodes and sensors, with a size down to atomic scale.

A highly ordered organic semiconducting self-assembled monolayer (SAM) with transition metal dichalcogenides is integrated into fully 2D organic–inorganic hybrid van der Waals heterostructures. The ordered nature of the SAM results in efficient exciton dissociation at the interface. The phototransistors show a superior photoresponsivity (475 A W⁻¹), which benefits from the precise confinement of charge in the monolayers and a strong photogating effect.

Abstract

Van der Waals (vdW) heterostructures composing of organic molecules with inorganic 2D crystals open the door to fabricate various promising hybrid devices. Here, a fully ordered organic self-assembled monolayer (SAM) to construct hybrid organic–inorganic vdW heterojunction phototransistors for highly sensitive light detection is used. The heterojunctions, formed by layering MoS₂ monolayer crystals onto organic [12-(benzo[b]benzo[4,5]thieno[2,3-d]thiophen-2-yl)dodecyl)]phosphonic acid SAM, are characterized by Raman and photoluminescence spectroscopy as well as Kelvin probe force microscopy. Remarkably, this vdW heterojunction transistor exhibits a superior photoresponsivity of 475 A W⁻¹ and enhanced external quantum efficiency of 1.45 × 10⁵%, as well as an extremely low dark photocurrent in the pA range. This work demonstrates that hybridizing SAM with 2D materials can be a promising strategy for fabricating diversified optoelectronic devices with unique properties.

In article number 2100428, Sang Wook Han, Soon Cheol Hong, and co-workers uncover that the residual Na cations at the SiO₂ substrate during a NaCl-assisted chemical vapor deposition-growth process induce the n-type doping into the large-scale supported uniform MoS₂ monolayer. Furthermore, the residual Na cations are electrically moved toward the bottom side of the MoS₂ monolayer to cure the interfacial sulfur vacancy defects.

Chiral helimagnetism and 1D magnetic solitons are discovered in Cr-intercalated 2H-TaS₂. The chiral helimagnetic structure has an initial period of ≈25 nm, and it transforms into a chiral soliton lattice in an external magnetic field. The spatial period evolution of this topological spin texture obeys the chiral sine-Gordon theory, revealing great potential for fast-speed racetrack memories.

Abstract

Chiral magnets endowed with topological spin textures are expected to have promising applications in next-generation magnetic memories. In contrast to the well-studied 2D or 3D magnetic skyrmions, the authors report the discovery of 1D nontrivial magnetic solitons in a transition metal dichalcogenide 2H-TaS₂ via precise intercalation of Cr elements. In the synthetic Cr_1/3TaS₂ (CTS) single crystal, the coupling of the strong spin–orbit interaction from TaS₂ and the chiral arrangement of the magnetic Cr ions evoke a robust Dzyaloshinskii–Moriya interaction. A magnetic helix having a short spatial period of ≈25 nm is observed in CTS via Lorentz transmission electron microscopy. In a magnetic field perpendicular to the helical axis, the helical spin structure transforms into a chiral soliton lattice (CSL) with the spin structure evolution being consistent with the chiral sine-Gordon theory, which opens promising perspectives for the application of CSL to fast-speed nonvolatile magnetic memories. This work introduces a new paradigm to soliton physics and provides an effective strategy for seeking novel 2D magnets.

A room-temperature electrically tunable chiral light-emitting diode is realized using strained monolayer transition metal dichalcogenides. The strain effects break the threefold rotational symmetry, which results in different amounts of spin recombination at the K/K’ valleys, to generate chiral light emission driven by electric fields. The results provide a new pathway for practical chiral light sources based on monolayer semiconductors.

Abstract

Room-temperature chiral light sources whose optical helicity can be electrically switched are one of the most important devices for future optical quantum information processing. The emerging valley degree of freedom in monolayer semiconductors allows generation of chiral luminescence via valley polarization. However, relevant valley-polarized light-emitting diodes (LEDs) have only been achieved at low temperatures (typically below 80 K). Here, a room-temperature chiral LED with strained transition metal dichalcogenide monolayers is realized. Spatially resolved polarization spectroscopy reveals that strain effects are crucial to yielding robust valley-polarized electroluminescence. The broken threefold rotational symmetry of strained monolayers induce inequivalent valley drifts at the K/K’ valleys, resulting in different amounts of spin recombination driven by electric fields. Based on this scenario, ideally strained conditions are designed for LEDs on flexible substrates, in which the helicity of room-temperature valley-polarized electroluminescence is electrically tuned. The results provide a new pathway for practical chiral light sources based on monolayer semiconductors.

Phonon chirality and magneto-optical phenomena in CrBr₃ 2D magnet are reported. The superposition of doubly degenerate E_g modes results in chiral phonons with PAM of l = ±1, exhibiting isotropic linearly polarized Raman scattering. The non-degenerate A_g modes display a giant magneto-optical effect under an external out-of-plane magnetic field, rotating the plane of polarization of the scattered linearly polarized light.

Abstract

Phonons with chirality determine the optical helicity of inelastic light scattering processes due to their nonzero angular momentum. Here it is shown that 2D magnetic CrBr₃ hosts chiral phonons at the Brillouin-zone center. These chiral phonons are linear combinations of the doubly-degenerate E_g phonons, and the phonon eigenmodes exhibit clockwise and counterclockwise rotational vibrations corresponding to angular momenta of l = ± 1. Such E_g chiral phonons completely switch the polarization of incident circularly polarized light. On the other hand, the non-degenerate non-chiral A_g phonons display a giant magneto-optical effect under an external out-of-plane magnetic field, rotating the plane of polarization of the scattered linearly polarized light. The corresponding degree of polarization of the scattered light changes from 91% to −68% as the magnetic field strength increases from 0 to 5 T. In contrast, the chiral E_g modes display no field dependence. The results lay a foundation for the study of phonon chirality and magneto-optical phenomena in 2D magnetic materials, as well as their related applications, such as the phonon Hall effect, topological photonics, and Raman lasing.

In article number 2100015, Sanjiv Dhingra and co-workers demonstrate fluorine-free development of a tantalum carbide MXene-tantalum oxide (TTO) hybrid structure through an innovative and facile protocol. TTO as an efficient biocompatible material for supercapacitor electrodes possesses biocompatibility with different types of human cells, and offers excellent volumetric capacitance, energy/power densities, and cyclability when assembled into a symmetric supercapacitor. It offers excellent promise for future biomedical energy storage devices.

Scalable substitutional vanadium doping of 2D WSe₂ is achieved via MOCVD at FEOL and BEOL compatible temperatures. Detailed experimental and the first-principles calculations demonstrate that vanadium dopants introduce discrete acceptor levels that lie within the valance band. Transport measurements further confirm the p-type nature of the vanadium dopants where hole conduction is dominant with increasing the dopant concentration.

Abstract

Scalable substitutional doping of 2D transition metal dichalcogenides is a prerequisite to developing next-generation logic and memory devices based on 2D materials. To date, doping efforts are still nascent. Here, scalable growth and vanadium (V) doping of 2D WSe₂ at front-end-of-line and back-end-of-line compatible temperatures of 800 and 400 °C, respectively, is reported. A combination of experimental and theoretical studies confirm that vanadium atoms substitutionally replace tungsten in WSe₂, which results in p-type doping via the introduction of discrete defect levels that lie close to the valence band maxima. The p-type nature of the V dopants is further verified by constructed field-effect transistors, where hole conduction becomes dominant with increasing vanadium concentration. Hence, this study presents a method to precisely control the density of intentionally introduced impurities, which is indispensable in the production of electronic-grade wafer-scale extrinsic 2D semiconductors.

Author(s): Elena Blundo, Tanju Yildirim, Giorgio Pettinari, and Antonio Polimeni

The formation of gas-filled bubbles on the surface of van der Waals crystals provides an ideal platform whereby the interplay of the elastic parameters and interlayer forces can be suitably investigated. Here, we combine experimental and numerical efforts to study the morphology of the bubbles at eq...

[Phys. Rev. Lett. 127, 046101] Published Fri Jul 23, 2021

Author(s): Benoît Mahault and Hugues Chaté

Working in two space dimensions, we show that the orientational order emerging from self-propelled polar particles aligning nematically is quasi-long-ranged beyond ℓr, the scale associated to induced velocity reversals, which is typically extremely large and often cannot even be measured. Below ℓr, ...

[Phys. Rev. Lett. 127, 048003] Published Fri Jul 23, 2021