zemin zheng

Ultra-thin GeSe/WS₂ vertical heterojunctions are fabricated by a site-controllable transfer method and subsequent GeSe layer thinning. Excellent rectifying behavior and photoresponse characteristics are demonstrated, including high on-off ratio (10³), high photoresponsivity (1.1 A W⁻¹), considerable specific detectivity (1.3×10¹⁰ Jones), and high external quantum efficiency (214.8 %), which reveals the great potential of the GeSe/WS₂ vertical heterojunction for future optoelectronic applications.

Abstract

Heterostructural engineering of atomically thin 2D materials offers an exciting opportunity to fabricate atomically sharp interfaces for optoelectronic devices. Herein, GeSe/WS₂ heterojunction devices composed of 2D WS₂ (n-type) and few-layer GeSe (p-type), are fabricated by transferring mechanically exfoliated GeSe to chemical vapor deposition (CVD)-grown WS₂. Excellent rectification behavior is observed from the I−V characteristics of the GeSe/WS₂ heterojunction devices. The reverse photocurrent increases more rapidly than the forward photocurrent under a 635 nm laser illumination, indicating an effective separation of the photogenerated carriers under a minus bias. A large photocurrent on-off ratio of 10³ at −5 V bias, a high responsivity (R _λ) of 1.1 A W⁻¹, a considerable specific detectivity (D*) of 1.3×10¹⁰ Jones, and a high external quantum efficiency (EQE) of 214.8%, are obtained. Owing to the large built-in potential of the heterojunction, efficient charge transfer is achieved from the abrupt interfaces even though vastly different materials are used in the van der Waals (vdW) heterostructure. A convenient route is demonstrated for the preparation of ultra-thin GeSe/WS₂ vdW heterojunctions. The results reveal great potential of the present GeSe/WS₂ vertical heterojunction for future applications in optoelectronics.

Good-quality centimeter-scale antiferromagnetic semiconductor FeOCl single crystals are controllably synthesized by developing a temperature-oscillation chemical vapor transport method. The semiconducting characteristics, strong in-plane anisotropies, and spin−phonon coupling effect of the 2D FeOCl are explored in detail. This study provides a high-quality low-symmetry van der Waals magnetic candidate for miniaturized spintronic devices.

Abstract

2D van der Waals (vdW) transition-metal oxyhalides with low symmetry, novel magnetism, and good stability provide a versatile platform for conducting fundamental research and developing spintronics. Antiferromagnetic FeOCl has attracted significant interest owing to its unique semiconductor properties and relatively high Néel temperature. Herein, good-quality centimeter-scale FeOCl single crystals are controllably synthesized using the universal temperature-oscillation chemical vapor transport (TO-CVT) method. The crystal structure, bandgap, and anisotropic behavior of the 2D FeOCl are explored in detail. The absorption spectrum and electrical measurements reveal that 2D FeOCl is a semiconductor with an optical bandgap of ≈2.1 eV and a resistivity of ≈10⁻¹ Ω m at 295 K, and the bandgap increases with decreasing thickness. Strong in-plane optical and electrical anisotropies are observed in 2D FeOCl flakes, and the maximum resistance anisotropic ratio reaches 2.66 at 295 K. Additionally, the lattice vibration modes are studied through temperature-dependent Raman spectra and first-principles density functional calculations. A significant decrease in the Raman frequencies below the Néel temperature is observed, which results from the strong spin−phonon coupling effect in 2D FeOCl. This study provides a high-quality low-symmetry vdW magnetic candidate for miniaturized spintronics.

The observation of skyrmions in 2D materials has opened a door to novel tunability of their magnetic structure. Here, it is shown that the chiral Néel skyrmions are stabilized in the metallic 2D ferromagnet Fe₃GeTe₂, as a result of breaking of the inversion symmetry of its crystal structure that is derived from an asymmetric distribution of Fe atoms in alternate layers in the compound. It is found that the skyrmion size in this compound is strongly influenced by the sample thickness and changes from ≈100 to ≈700 nm as the sample thickness is increased from ≈90 nm to ≈2 µm.

Abstract

There is considerable interest in van der Waals (vdW) materials as potential hosts for chiral skyrmionic spin textures. Of particular interest is the ferromagnetic, metallic compound Fe₃GeTe₂ (FGT), which has a comparatively high Curie temperature (150–220 K). Several recent studies have reported the observation of chiral Néel skyrmions in this compound, which is inconsistent with its presumed centrosymmetric structure. Here the observation of Néel type skyrmions in single crystals of FGT via Lorentz transmission electron microscopy (LTEM) is reported. It is shown from detailed X-ray diffraction structure analysis that FGT lacks an inversion symmetry as a result of an asymmetric distribution of Fe vacancies. This vacancy-induced breaking of the inversion symmetry of this compound is a surprising and novel observation and is a prerequisite for a Dzyaloshinskii–Moriya vector exchange interaction which accounts for the chiral Néel skyrmion phase. This phenomenon is likely to be common to many 2D vdW materials and suggests a path to the preparation of many such acentric compounds. Furthermore, it is found that the skyrmion size in FGT is strongly dependent on its thickness: the skyrmion size increases from ≈100 to ≈750 nm as the thickness of the lamella is increased from ≈90 nm to ≈2 µm. This extreme size tunability is a feature common to many low symmetry ferro- and ferri-magnetic compounds.

2D materials have spectacular electrical and optical properties but their device fabrication and integration are arduous and expensive. Optical modification methods offer less detrimental processes that can often be done in one optical setup, sometimes even simultaneously with one another, saving time and money. Additionally, the extreme locality of lasers can be utilized in designable laser direct writing processes.

Abstract

2D materials are under extensive research due to their remarkable properties suitable for various optoelectronic, photonic, and biological applications, yet their conventional fabrication methods are typically harsh and cost-ineffective. Optical modification is demonstrated as an effective and scalable method for accurate and local in situ engineering and patterning of 2D materials in ambient conditions. This review focuses on the state of the art of optical modification of 2D materials and their applications. Perspectives for future developments in this field are also discussed, including novel laser tools, new optical modification strategies, and their emerging applications in quantum technologies and biotechnologies.

2D Janus Monolayers

In article number 2106222, Mete Atature, Sefaattin Tongay, and co-workers use an in situ deterministic plasma technique to enable the synthesis of high-quality excitonic grade 2D SWSe, SMoSe, and other 2D Janus layers. Integrated spectrometers allow for the collection of structural, optical, and phononic properties during the growth. Through time-resolved studies, the team offers the first insights into the growth process and minute control provides the first with excitonic grade Janus layers.

CrSBr is a 2D semiconductor of interest for its magnetic properties. Temperature- and gate-dependent transport exhibit an extreme anisotropy (ca. 10² to 10⁵)—stronger than in any other 2D semiconductor—and the photocurrent shows clear signatures of 1D behavior. This work shows that CrSBr is a quasi-1D electronic system, a unique characteristic in the panorama of known 2D semiconductors.

Abstract

Electronic transport through exfoliated multilayers of CrSBr, a 2D semiconductor of interest because of its magnetic properties, is investigated. An extremely pronounced anisotropy manifesting itself in qualitative and quantitative differences of all quantities measured along the in-plane a and b crystallographic directions is found. In particular, a qualitatively different dependence of the conductivities σ _a and σ _b on temperature and gate voltage, accompanied by orders of magnitude differences in their values (σ _b /σ _a  ≈ 3 × 10² to 10⁵ at low temperature and negative gate voltage) are observed, together with a different behavior of the longitudinal magnetoresistance in the two directions and the complete absence of the Hall effect in transverse resistance measurements. These observations appear not to be compatible with a description in terms of conventional band transport of a 2D doped semiconductor. The observed phenomenology—and unambiguous signatures of a 1D van Hove singularity detected in energy-resolved photocurrent measurements—indicate that electronic transport through CrSBr multilayers is better interpreted by considering the system as formed by weakly and incoherently coupled 1D wires, than by conventional 2D band transport. It is concluded that CrSBr is the first 2D semiconductor to show distinctly quasi-1D electronic transport properties.

Magnetoelectrical Clothing Generator

In article number 2107682, Weilin Xu, Xiaoming Tao, Guangming Tao, Bin Su, and co-workers show scalable-manufactured flexible magnetoelectrical clothing that can convert mechanical energy generated by daily behavior into electrical energy required for electronic devices, solving the problem of continuous power supply of electronic clothing especially in harsh outdoor environments, and promoting sustainable development in wearable electronics.

A high-quality saturable absorber (SA) based on GeAs₂ nanosheets is introduced. The saturation intensity and the modulation depth are measured to be 1.23 GW cm⁻² and 5.2%, respectively. By incorporating this GeAs₂ SA into fiber lasers, a high stable ultrafast fiber laser with a pulse duration of 371 fs and a single-frequency fiber laser with a linewidth of ≈678 Hz are demonstrated, respectively.

Abstract

As a new IV–V group semiconductor, germanium-diarsenide (GeAs₂) compounds have attracted considerable attention due to their outstanding optical and electrical properties, thickness-dependent bandgap, in-plane anisotropy, and excellent optical absorption. However, the potential of GeAs₂ in the field of ultrafast and ultranarrow fiber laser has not been studied. In this article, a high-quality GeAs₂ nanosheets saturable absorber (SA) is successfully prepared by liquid-phase exfoliation. The nonlinear optical characteristics of GeAs₂ nanosheets have been investigated based on a balanced twin-detector measurement system. The modulation depth, nonsaturable loss, and saturation intensity are measured to be 5.2%, 24%, and 1.23 GW cm⁻², respectively. GeAs₂ has been successfully applied as an SA in an ultrafast and single-frequency fiber laser. A stable mode-locked laser pulses operation with a duration as short as 371 fs and a repetition rate of 8.19 MHz at a wavelength of 1560 nm is achieved. Moreover, ultranarrow fiber lasers with a high signal-to-noise ratio of 80 dB and a linewidth of ≈678 Hz are obtained. The findings validate that 2D GeAs₂ can be used as an SA and has promising applications in ultrafast and ultranarrow photonics.

A novel, penguin feather-like MXene aerogel absorber with multifunctional capabilities of excellent thermal insulation and water uptake is developed. It delivers a high energy efficiency of 88.52% and an evaporation rate of 0.92 kg m⁻² h⁻¹ at 0.5-sun irradiance, indicating its great potential for practical solar-powered water desalination and purification under natural sunlight.

Abstract

Solar-powered water evaporation is a straightforward, practical approach to use solar energy for water desalination. Solar absorbers made from photothermal materials capable of effectively confining heat and pumping water to the evaporation surface are essential for a high energy efficiency. However, separate designs of water transport routes and thermal insulating layers are required to simultaneously achieve desired water and thermal managements. This work reports an integrated design for efficient multifunctional capabilities through rational assembly of spectrally modified Ti₃C₂T _x (SM-Ti₃C₂T _x ) nanosheets and polyvinyl alcohol (PVA) into a multiscale 3D aerogel with a feather-like microstructure. The aerogel contains longitudinal struts with transversely parallel ligaments developed at an angle of ≈60° from the struts, resembling the microstructure of down feathers in penguins and thus leading to excellent thermal insulation. The hydrophilic porous ligaments serve as upward water transport channels, pumping the water to the evaporation surface while confining it within the ligaments to avoid oversaturation. These functional features endow the composite aerogel with a high energy efficiency of 88.52% and an evaporation rate of 0.92 kg m⁻² h⁻¹ at a weak solar irradiance of 0.5-sun, indicating its great potential for practical solar-powered water desalination under natural sunlight.

Twist angle induces various Moiré-related properties in 2D materials, but most studies only focus on bilayer systems. Here, via a folding strategy, multilayer MoS₂ Moiré superlattices are fabricated whose interlayer coupling, indirect bandgap, and degree of circular polarization (DOCP) are tunable by twist angle. The highest DOCP for folded bilayer MoS₂ can reach 86% above liquid nitrogen temperature.

Abstract

Twist angle provides a new degree of freedom for 2D material modifications. In principle, the intrinsic properties of twisted multilayers can be regulated by twist angle between each adjacent layer and thus have greater tunability than widely studied bilayer structures. Considering its complexity, it is important to first investigate the simplest twisted multilayers with only one interface twisted. In this work, multilayer Moiré superlattices with only one twisted interface via paraffin-assisted folding of non-twisted stacked (highly symmetrically stacked) multilayer MoS₂ are successfully fabricated, and their twist-angle dependent optical properties are systematically studied. Compared to non-twisted stacked multilayer MoS₂, the one-interface-twisted multilayers show a 2–3.5 times higher PL intensity, and their interlayer coupling, indirect bandgap, and degree of circular polarization (DOCP) are tunable by twist angle. Notably, the DOCP for the one-interface-twisted four-layer (folded bilayer) can reach 86%, which is the highest value ever reported for transition metal dichalcogenide homostructures above liquid nitrogen temperature. This work provides a solid base for understanding twist-angle dependent properties of twisted multilayer 2D-materials.

a) Atomic resolution HADF STEM image showing stacking fault. b) Electrical resistivity fitted by adiabatic small polaron hopping model ρ(T) = ATexp(E _ρ/k _B T) with E _ρ being the activation energy. c) S(T) versus 1000/T curve fitted by polaron model S(T) = (k _B/e)(α + E _S/k _B T) with E _S being the activation energy.

Abstract

Intrinsic 2D ferromagnetic semiconductors are an important class of materials for spin-charge conversion applications. Cr₂Ge₂Te₆ retains long-range magnetic order in the bilayer at cryogenic temperatures and shows complex magnetic interactions with considerable magnetic anisotropy. Here, a series of structural, magnetic, X-ray scattering, electronic, thermal transport and first-principles calculation studies are performed, which reveal that localized electronic charge carriers in Cr₂Ge₂Te₆ are dressed by the surrounding lattice and are involved in polaronic transport via hopping that is observed via magnetocrystalline anisotropy. This opens the possibility for manipulation of charge transport in Cr₂Ge₂Te₆—based devices by electron–phonon- and spin–orbit coupling-based tailoring of polaron properties.

Nature, Published online: 09 March 2022; doi:10.1038/s41586-021-04323-3

Ultra-scaled transistors based on two-dimensional MoS2 with physical gate lengths of 0.34 nm are reported, which show relatively good electrical characteristics and can be switched off.

Nature, Published online: 23 February 2022; doi:10.1038/s41586-021-04337-x

Multiple complementary optical signatures confirm the persistence of ferroelectricity and inversion-symmetry-breaking magnetic order down to monolayer NiI2, introducing the physics of type-II multiferroics into the area of van der Waals materials.

Nature Nanotechnology, Published online: 24 February 2022; doi:10.1038/s41565-022-01072-w

Marginal twisting of 2D semiconductor crystals enables the emergence of room temperature interfacial ferroelectricity.

Using ab initio tight-binding approaches, we investigate Floquet band engineering of the 1T’ phase of transition metal dichalcogenides (MX 2 , M = W, Mo and X = Te, Se, S) monolayers under the irradiation with circularly polarized light. Our first principles calculations demonstrate that light can induce important transitions in the topological phases of this emerging materials family. For example, upon irradiation, Te-based MX 2 undergoes a phase transition from quantum spin Hall (QSH) semimetal to time-reversal symmetry broken QSH insulator with a nontrivial band gap of up to 92.5 meV. On the other hand, Se- and S-based MX 2 undergoes the topological phase transition from the QSH effect to the quantum anomalous Hall effect and into trivial phases with increasing light intensity. From a general perspective, this theoretical work brings further insight into non-equilibrium topological systems.

Nature Communications, Published online: 16 February 2022; doi:10.1038/s41467-022-28542-y

The interplay between reduced dimensionality and interactions in monolayer transition metal dichalcogenides has been of great research interest. Here the authors report an insulating dimer ground state in 1T-IrTe2, driven by the combined effect of the charge density wave instability and local atomic bond formation.