Jing Zhang

Nature Photonics, Published online: 09 January 2023; doi:10.1038/s41566-022-01129-1

Two-dimensional massive and massless Dirac fermions in HgTe/CdHgTe quantum wells yield terahertz Landau emission. The emission frequency is continuously tunable with magnetic field or carrier concentration, over the range from 0.5 to 3 THz.

Nature Communications, Published online: 09 January 2023; doi:10.1038/s41467-022-35616-4

2D metals are promising electrocatalysts due to their potentially large surface area. Here we report double-layer Pt nanosheets derived from exfoliated PtOx nanosheets with higher electrochemically active surface area, oxygen reduction reaction activity, and stability compared to Pt nanoparticles.

Nature Materials, Published online: 10 January 2023; doi:10.1038/s41563-022-01466-0

Transforming atomically thin materials by their magnetic neighbours reveals a surprising asymmetry that allows a versatile control of the valley degrees of freedom and band topology in van der Waals heterostructures.

Nature Materials, Published online: 11 January 2023; doi:10.1038/s41563-022-01462-4

A new spectroscopic technique takes advantage of overlapping electronic bands to probe the strongly correlated states of magic-angle twisted trilayer graphene.

The motivation of this work is to review the inorganic persistent phosphors that are of interest for the dopant and host selection criteria, the afterglow intensity, and multi-color modulating methods, from the perspectives of composition optimization, defect engineering, and coordination environment of hosts. Persistent luminescence imaging and multimodal imaging of persistent phosphors are introduced.

Abstract

Persistent luminescence (PL) is a distinctive optical phenomenon with long-lasting afterglow emissions after the cessation of excitation, thus emerging huge prospects in the applications of anti-counterfeiting, information or data storage, photocatalysis, sensing, and bioimaging. In this regard, it is of great importance to optimize the PL performance. Recently, no comprehensive literature review on inorganic persistent phosphors is reported to aid researchers in focusing on the systematic adjustment of afterglow properties based on reported progress made in recent ten years. Accordingly, the motivation of this work is to review the inorganic persistent phosphors that are of interest for the dopant and host selection criteria, the afterglow intensity, and multi-color modulating methods, from the perspectives of composition optimization, defect engineering, and coordination environment of hosts. Then, X-ray, light emitting diode (LED), ultraviolet (UV), and near-infrared (NIR) excited PL imaging and multimodal imaging of persistent phosphors are introduced. Finally, the summary, challenge, outlook, and future development direction in this field are provided. With the profound influence of the afterglow materials, it is believed that this review will be a guideline for the design, synthesis, and optical performance optimization of persistent materials in the future.

Synthesis of low-dimensional quaternary materials is challenging and remains rare. This is mainly due to themodynamic stability as the most cited reason. Here, potentially stable MM'P₂X₆ quaternary compounds are screened by density functional theory reaction energies. Following the screening, a stable semiconducting layered magnetic material, CdFeP₂Se₆, that exhibits a short-range antiferromagnetic order at T _N  = 21 K with an indirect bandgap of 2.23 eV is synthesized.

Abstract

Recent advances in 2D magnetism have heightened interest in layered magnetic materials due to their potential for spintronics. In particular, layered semiconducting antiferromagnets exhibit intriguing low-dimensional semiconducting behavior with both charge and spin as carrier controls. However, synthesis of these compounds is challenging and remains rare. Here, first-principles based high-throughput search is conducted to screen potentially stable mixed metal phosphorous trichalcogenides (MM^′P₂X₆, where M and M^′ are transition metals and X is a chalcogenide) that have a wide range of tunable bandgaps and interesting magnetic properties. Among the potential candidates, a stable semiconducting layered magnetic material, CdFeP₂Se₆, that exhibits a short-range antiferromagnetic order at T _N = 21 K with an indirect bandgap of 2.23 eV is successfully synthesized . This work suggests that high-throughput screening assisted synthesis can be an effective method for layered magnetic materials discovery.

Strategies for defect engineering and the stability of the 2D magnetic semiconductor CrSBr are explored. Raman spectroscopy is a direct fingerprint of defect modes and the magnetic phase diagram in CrSBr. In the presence of magnetic correlations, pronounced spin-phonon coupling of bulk phonon and defect modes are observed, providing a pathway to engineer and probe magnetic properties in CrSBr.

Abstract

Understanding the stability limitations and defect formation mechanisms in 2D magnets is essential for their utilization in spintronic and memory technologies. Here, defects in mono- to multilayer CrSBr are correlated with structural, vibrational, and magnetic properties. Resonant Raman scattering is used to reveal distinct vibrational defect signatures. In pristine CrSBr, it is shown that bromine atoms mediate vibrational interlayer coupling, allowing for distinguishing between surface and bulk defect modes. Environmental exposure is shown to cause drastic degradation in monolayers, with the formation of intralayer defects. This is in contrast to multilayers that predominantly show bromine surface defects. Through deliberate ion irradiation, the formation of defect modes is tuned: these are strongly polarized and resonantly enhanced, reflecting the quasi--1D electronic character of CrSBr. Strikingly, pronounced signatures of spin-phonon coupling of the intrinsic phonon modes and the ion beam-induced defect modes are observed throughout the magnetic transition temperature. Overall, defect engineering of magnetic properties is possible, with resonant Raman spectroscopy serving as a direct fingerprint of magnetic phases and defects in CrSBr.

Magnetic Heterostructures

In article number 2208528, Cai, Han, Lin and co-workers report that multi-configurational 2D Cr _x Te _y magnetic heterojunctions are visualized by transmission electron microscopy diffraction and atomic scanning transmission electron microscopy imaging. The lateral heterojunction of Cr₂Te₃-Cr₅Te₈ shares the layered CrTe₂ as the backbone, and different arrangements of intercalated Cr in van der Waals gap form distinct magnetic phases. It exhibits a novel magneto-optical behavior of a sharply stepped hysteresis loop with twice spin flips.

This paper reviews the latest research progress on artificial neuronal devices based on emerging volatile switching materials. The neuronal dynamics demonstrated in these devices, the hardware implementation of various artificial neural networks, and their potentials in computational and sensing applications are systematically summarized.

Abstract

Artificial neuronal devices are critical building blocks of neuromorphic computing systems and currently the subject of intense research motivated by application needs from new computing technology and more realistic brain emulation. Researchers have proposed a range of device concepts that can mimic neuronal dynamics and functions. Although the switching physics and device structures of these artificial neurons are largely different, their behaviors can be described by several neuron models in a more unified manner. In this paper, the reports of artificial neuronal devices based on emerging volatile switching materials are reviewed from the perspective of the demonstrated neuron models, with a focus on the neuronal functions implemented in these devices and the exploitation of these functions for computational and sensing applications. Furthermore, the neuroscience inspirations and engineering methods to enrich the neuronal dynamics that remain to be implemented in artificial neuronal devices and networks toward realizing the full functionalities of biological neurons are discussed.

The stacking layer number-dependent electron structure, quantum properties, and circularly (linearly) polarized photogalvanic effect are assessed in the low-dimensional dual topological superlattices (TSLs) (Bi₂Se₃)-(Bi₂/Bi₂Se₃) _N . The results show that the surface electronic structure of the dual TSLs is highly tunable and well-regulated for quantum transport and photoexcitation, which shed light on engineering for opto-electronic/spintronic applications.

Abstract

Dual topological insulators, simultaneously protected by time-reversal symmetry and crystalline symmetry, open great opportunities to explore different symmetry-protected metallic surface states. However, the conventional dual topological states located on different facets hinder integration into planar opto-electronic/spintronic devices. Here, dual topological superlattices (TSLs) Bi₂Se₃-(Bi₂/Bi₂Se₃) _N with limited stacking layer number N are constructed. Angle-resolved photoelectron emission spectra of the TSLs identify the coexistence and adjustment of dual topological surface states on Bi₂Se₃ facet. The existence and tunability of spin-polarized dual-topological bands with N on Bi₂Se₃ facet result in an unconventionally weak antilocalization effect (WAL) with variable WAL coefficient α (maximum close to 3/2) from quantum transport experiments. Most importantly, it is identified that the spin-polarized surface electrons from dual topological bands exhibit circularly and linearly polarized photogalvanic effect (CPGE and LPGE). It is anticipated that the stacked dual-topology and stacking layer number controlled bands evolution provide a platform for realizing intrinsic CPGE and LPGE. The results show that the surface electronic structure of the dual TSLs is highly tunable and well-regulated for quantum transport and photoexcitation, which shed light on engineering for opto-electronic/spintronic applications.

Switchable ferroelectric polarization in Bi₂O₂Se semiconductors is experimentally and theoretically investigated. The interplay between ferroelectricity and semiconducting characteristics of Bi₂O₂Se is explored on device-level operation. Leveraging its ferroelectric polarization, the fabricated device exhibits “smart” photoresponse tunability. This research provides valuable insights into the synergistic effect of ferroelectricity with semiconducting characteristics in 2D ferroelectric semiconductors, toward device simplification and miniaturization.

Abstract

Atomically 2D layered ferroelectric semiconductors, in which the polarization switching process occurs within the channel material itself, offer a new material platform that can drive electronic components toward structural simplification and high-density integration. Here, a room-temperature 2D layered ferroelectric semiconductor, bismuth oxychalcogenides (Bi₂O₂Se), is investigated with a thickness down to 7.3 nm (≈12 layers) and piezoelectric coefficient (d₃₃) of 4.4 ± 0.1 pm V⁻¹. The random orientations and electrically dependent polarization of the dipoles in Bi₂O₂Se are separately uncovered owing to the structural symmetry-breaking at room temperature. Specifically, the interplay between ferroelectricity and semiconducting characteristics of Bi₂O₂Se is explored on device-level operation, revealing the hysteresis behavior and memory window (MW) formation. Leveraging the ferroelectric polarization originating from Bi₂O₂Se, the fabricated device exhibits “smart” photoresponse tunability and excellent electronic characteristics, e.g., a high on/off current ratio > 10⁴ and a large MW to the sweeping range of 47% at V_GS = ±5 V. These results demonstrate the synergistic combination of ferroelectricity with semiconducting characteristics in Bi₂O₂Se, laying the foundation for integrating sensing, logic, and memory functions into a single material system that can overcome the bottlenecks in von Neumann architecture.

Molybdenum disulfide nanoribbons and nanotubes grown from vapor phase are a low-defect-density nanomaterial highly promising for quantum electronic applications. Devices integrating them with bismuth-based contacts show strongly reduced contact resistances and a marked absence of trap states at cryogenic temperatures. This allows for quantum-dot Coulomb blockade measurements. Single-level quantum transport is observed at temperatures below 100mK.

Abstract

Molybdenum disulfide nanoribbons and nanotubes are quasi-1D semiconductors with strong spin–orbit interaction, a nanomaterial highly promising for quantum electronic applications. Here, it is demonstrated that a bismuth semimetal layer between the contact metal and this nanomaterial strongly improves the properties of the contacts. Two-point resistances on the order of 100 kΩ are observed at room temperature. At cryogenic temperature, Coulomb blockade is visible. The resulting stability diagrams indicate a marked absence of trap states at the contacts and the corresponding disorder, compared to previous devices that use low-work-function metals as contacts. Single-level quantum transport is observed at temperatures below 100 mK.

Nature Communications, Published online: 03 January 2023; doi:10.1038/s41467-022-35651-1

Deep level transient spectroscopy (DLTS) is an established characterization technique used to study electrically active defects in 3D semiconductors. Here, the authors show that DLTS can also be applied to monolayer semiconductors, enabling in-situ characterization of the energy states of different defects and their interactions.

Nature Communications, Published online: 06 January 2023; doi:10.1038/s41467-022-35714-3

Here, the authors investigate the Raman spectra of few-layered WS2 when the excitation energy is in resonance with the dark exciton, and observe a Fano resonance between dark excitonsand zone-edge acoustic phonons.

Nature Physics, Published online: 05 January 2023; doi:10.1038/s41567-022-01849-9

The interaction of strong laser fields with tungsten disulfide leads to light-dressed Floquet replica of excitonic states, which manifest as new features in the transient absorption spectrum.

Nature, Published online: 04 January 2023; doi:10.1038/s41586-022-05401-w

A two-dimensional crystalline polymer of C60, termed graphullerene, is synthesized by chemical vapour transport, and mechanically exfoliated to produce molecularly thin flakes with clean interfaces for potential optoelectronic applications.

Nature, Published online: 04 January 2023; doi:10.1038/s41586-022-05393-7

A van der Waals crystal, niobium oxide dichloride, with vanishing interlayer electronic coupling and considerable monolayer-like excitonic behaviour in the bulk, as well as strong and scalable second-order optical nonlinearity, is discovered, which enables a high-performance quantum light source.

Nature, Published online: 04 January 2023; doi:10.1038/s41586-022-05521-3

The authors show a hysteretic behaviour of superconductivity as a function of electric field in bilayer Td-MoTe2, representing observations of coupled ferroelectricity and superconductivity.

Nature Communications, Published online: 29 December 2022; doi:10.1038/s41467-022-35690-8

Binary nanoparticle superlattices exhibit different collective optical, magnetic, and electronic properties. Here, the authors develop an efficient global optimization algorithm for the discovery of periodic 2D architectures forming at fluid interfaces.

Orthogonal α-MoO3 is one of the most common and air-stable compounds of molybdenum, holding the merits of wide bandgap, van der Waals (vdW) structure, biaxial symmetry and recently discovered hyperbolic topological transitions, which has drawn significant attention in developing novel nanophotonic and optoelectronic devices. Herein the broadband optical anisotropy, one of the most fundamental physical characteristics of α-MoO3 crystal, was systematically investigated using a combination of spectroscopic ellipsometry (SE) and reflectance difference spectroscopy (RDS). The centimeter-level high-quality α-MoO3 crystal was grown by modified physical vapor deposition. The optical refractive indices along three crystalline axes were precisely determined by SE in the broad spectral range (400–1600 nm), and then the in-plane and out-plane birefringence was analyzed. Both the intrinsic and resonant cavity modulated optical anisotropy of α-MoO3 was studied by polarization-resolved RDS, from which we find the physical origins of linear dichroism are dominated by electronical transitions along the c-axis. Furthermore, the external photonic cavity of SiO2 enables enhanced sensitivity to view electronical transitions and a high modulation ratio of optical anisotropy reached 30, which provides new opportunities to tune optical anisotropy for polarized photonic devices. Our results can help understand the physical origin of the highly optical anisotropy of α-MoO3 and establish an effective metrological tool to study other types of vdW crystals.

A hot-air-assisted ambient fabrication technique is introduced to prepare 2D layered perovskite films for efficient and stable solar cells. This method is material-saving, humidity-insensitive, large-area-applicable, and adaptable for different 2D perovskites. High-quality 2D perovskite films are obtained with good crystallinity, preferable orientation, and desirable morphology.

Abstract

2D layered perovskites (LPs) have shown great potential to deliver high-performance photovoltaic devices with long-term stability. Despite many signs of progress being made in film quality and device performance, LP films are mainly processed in strict conditions and through non-scalable techniques. Here, the hot-air-assisted ambient fabrication technique is introduced to prepare LP films for efficient and stable solar cells. The high-quality LP films with good crystallinity, preferable orientation and desirable morphology are obtained by balancing the crystal nucleation and growth processes. Employing the synchrotron-based in situ grazing-incidence X-ray diffraction technique, hot air induces the solidification of solutes and forms an intermediate at the air–liquid interface, which transforms into 3D-like perovskite, followed by the growth of the 2D species toward the substrate. The optimal LP film delivers a device power conversion efficiency of 16.36%, the best value for the LP-based solar cells prepared by the non-spin-coating techniques. The solar cell performance is insensitive to the film processing humidity and the device size is upscalable, which promises real-world deployment of LP-based optoelectronic devices.

High-quality and nonlayered WO₂ nanoplates with tunable thickness and lateral dimension by a WSe₂-assisted strategy are reported. Electrical measurements demonstrate that the WO₂ nanoplates exhibit metallic behavior with excellent conductivity, ultrahigh breakdown current density, and strong anisotropic resistance. Magnetotransport studies show quantum-interference-induced weak-localization behavior. These studies demonstrate that 2D metal oxides are promising candidates for interconnecting and other novel electronic devices.

Abstract

2D metal oxides (2DMOs) have stimulated tremendous attention due to their distinct electronic structures and abundant surface chemistry. However, it remains a standing challenge for the synthesis of 2DMOs because of their intrinsic 3D lattice structure and ultrahigh synthesis temperature. Here, a reliable WSe₂-assisted chemical vapor deposition (CVD) strategy to grow nonlayered WO₂ nanoplates with tunable thickness and lateral dimension is reported. Optical microscopy and scanning electron microscopy studies demonstrate that the WO₂ nanoplates exhibit a well-faceted rhombic geometry with a lateral dimension up to the sub-millimeter level (≈135 µm), which is the largest size of 2DMO single crystals obtained by CVD to date. Scanning transmission electron microscopy studies reveal that the nanoplates are high-quality single crystals. Electrical measurements show the nanoplates exhibit metallic behavior with strong anisotropic resistance, outstanding conductivity of 1.1 × 10⁶ S m⁻¹, and breakdown current density of 7.1 × 10⁷ A cm⁻². More interestingly, low-temperature magnetotransport studies demonstrate that the nanoplates show a quantum-interference-induced weak-localization effect. The developed WSe₂-assisted strategy for the growth of WO₂ nanoplates can enrich the library of 2DMO materials and provide a material platform for other property explorations based on 2D WO₂.

A method to scale up freestanding synthesis of MoS₂ sheets on 100 mm-diameter silicon wafers that have hundreds of nanoapertures is proposed. The MoS₂ sheets cover the apertures to form ultrathin membranes that are useful in nanopore sensing.

Abstract

2D materials are ideal for nanopores with optimal detection sensitivity and resolution. Among these, molybdenum disulfide (MoS₂) has gained traction as a less hydrophobic material than graphene. However, experiments using 2D nanopores remain challenging due to the lack of scalable methods for high-quality freestanding membranes. Herein, a site-directed, scaled-up synthesis of MoS₂ membranes on predrilled nanoapertures on 4-inch wafer substrates with 75% yields is reported. Chemical vapor deposition (CVD), which introduces sulfur and molybdenum dioxide vapors across the sub-100 nm nanoapertures results in exclusive formation of freestanding membranes that seal the apertures. Nucleation and growth near the nanoaperture edges is followed by nanoaperture decoration with MoS₂, which proceeds until a critical flake curvature is achieved, after which fully spanning freestanding membranes form. Intentional blocking of reagent flow through the apertures inhibits MoS₂ nucleation around the nanoapertures, promoting the formation of large-crystal monolayer MoS₂ membranes. The in situ grown membranes along with facile membrane wetting and nanopore formation using dielectric breakdown enables the recording of dsDNA translocation events at an unprecedentedly high 1 MHz bandwidth. The methods presented here are important steps toward the development of scalable single-layer membrane manufacture for 2D nanofluidics and nanopore applications.

Nanopatterning

Nanopatterning bridges the microstructure of 2D materials and the integrated chip devices, essentially enabling and prompting their successful industrial application. In article number 2200734, Yuan Li, Tianyou Zhai, and co-workers review the recent development of key nanopatterning technologies for 2D materials with the aim of realizing their large-scale device integration. The contribution offers pioneering reference and guidelines to promote 2D materials from laboratory research to practical use.