Jing Zhang

Nowadays, photodetectors are needed to meet the multiwavelength and high-sensitivity detection requirements in complex application systems. A highly sensitive and broad-spectrum photodetector based on the InSe/ReS₂ vertical heterostructure is reported, in which ReS₂ acts as the transport layer and the top layer InSe serves as the photogate to regulate the channel current.

Abstract

Photogating effect based on vertical structure of 2D materials allows for the realization of a highly sensitive photodetector. A highly sensitive and broad-spectrum (365–965 nm) photodetector is reported based on the indium selenide (InSe)/rhenium disulfide (ReS₂) vertical heterostructure, where the top layer InSe serves as the photogate to regulate the channel current, enabling a large photoconductive gain of 10⁶. The detectivity of the photodetector can reach 6.51 × 10¹³ Jones, making this one of the highest values among reported transition metal dichalcogenide photodetectors. The photodetector represents a high responsivity of 1921 A W⁻¹, an ultrahigh external quantum efficiency (EQE) of 6.53 × 10⁵%, and a fast response time of 21.6 ms. The outstanding properties of the InSe/ReS₂ heterojunction reveal the promising potential in high-efficient, ultrasensitive, broadband photodetectors.

Highly polarized light emission is demonstrated by embedding a WSe₂ monolayer inside a gap-plasmon nanocavity employing a nanowire-on-film geometry. By systematically varying the Ag nanowire width to tune the gap-plasmon resonance and its spectral overlap with the excitonic resonance, light emission with anisotropic ratios above 200, corresponding to degree of polarization exceeding 99%, has been achieved with the optimized geometry.

Abstract

As contemporary star materials, 2D monolayer semiconductors have drawn huge research interests owing to their striking electrical and optical properties, rendering them ideal candidates as building blocks for novel optoelectronic devices. Towards light emitting devices with extended functions, it is necessary to manipulate the polarization of light emission from monolayer semiconductors. However, most of these monolayer semiconductors exhibit no or very limited polarization sensitivity inherited from their structural anisotropy, making it challenging to develop highly polarized light sources. Herein, by embedding monolayer tungsten diselenide (WSe₂) in a nanowire-on-film nanocavity, highly polarized light emission is demonstrated, featuring degree of polarization (DoP) of ≈99%. The highly anisotropic light emission originates from the near-field coupling between the WSe₂ monolayer with the polarization-dependent gap plasmons of the nanowire-on-film nanocavity. Moreover, as the width of the nanowire essentially defines the gap-plasmon resonance of the nanowire-on-film nanocavity, the anisotropy of the highly polarized light emission is systematically tuned by changing the nanowire width. The findings pave the way for engineering anisotropic optical properties of monolayer semiconductors and will boost the development of functional polarization-sensitive 2D optoelectronic devices.

In response to ultraviolet–blue light, Eu²⁺-doped layered double borate phosphor with ultrabroadband near-infrared (NIR) emission peak at 720 nm is reported. This work extends the perception of the d–f transition of Eu²⁺ and provides a strategy to design Eu²⁺ doped phosphors with ultrabroadband long wavelength or NIR emission by occupying irregular and large sites in solids.

Abstract

Ultrabroadband near-infrared (NIR) luminescent materials are important components for compact light sources used in food testing, medical and biosensing applications. A major long-term challenge facing NIR luminescent materials is to design ultrabroadband phosphors activated by the rare-earth ions characterized by the f–d transition. Here, an Eu²⁺-doped Ba₃Lu(BO₃)₃ phosphor, showing peculiar luminescence characteristics such as a long emission wavelength ≈720 nm, a broader full width at half maximum of ≈197 nm, a large Stokes shift of ≈8300 cm^–1, and a longer decay time ≈5.58 µs, is reported. The smaller Eu²⁺ occupy the irregular and larger Ba²⁺ sites further reducing the symmetry, resulting in the increased splitting of the degenerate 5d level, thereby lowering the energy of Eu²⁺ emission. The longer decay time is also associated with the irregular Ba sites. This work improves the NIR mechanism of Eu²⁺ and opens up a new idea for exploring promising broadband NIR luminescent materials.

The chemical vapor deposition-grown hexagonal WS₂ monolayer always exhibits alternating bright and dark photoluminescence domains. The correlation between the patterned photoluminescence emission and the details of defects is explored consistently by experiments and density functional theory calculations. The results indicate that the WS-vacancy is the most likely vacancy that matters.

Abstract

Monolayer transition-metal dichalcogenides grown by chemical vapor deposition (CVD) always contain certain types of defects that dramatically affect their electronic and optical properties. For CVD-grown hexagonal WS₂ monolayer, complex photoluminescence (PL) patterns are commonly observed, but the defect-related optical mechanisms are still not well understood. Here, by combining the optical and structural characterizations and ab initio calculations, the correlation between the patterned PL emission and the details of defects in CVD-grown hexagonal WS₂ monolayer are revealed. The temperature-dependent PL spectra show the correlation between the defect-trapped and band-edge exciton emission. The high-resolution scanning transmission electron microscopy identifies the positive correlation between the density of WS _x -vacancy and PL intensity. In the end, the ab initio calculations and molecule adsorption PL spectra show that the coexistence of p- and n-doping effects, caused by the W and S complex vacancy, weakens the modulation of molecular adsorption on PL intensity. This work gives new insights into the origin of the inhomogeneous PL distribution in WS₂ monolayer, which provides important guidance in the regulation of electronic and optical properties of transition-metal dichalcogenides via defect engineering.

Understanding the coupling between spin and phonons is critical for controlling the lattice thermal conductivity ( κ l ) in magnetic materials, as we demonstrate here for CrX 3 (X = Br and I) monolayers. We show that these compounds exhibit large spin-phonon coupling (SPC), dominated by out-of-plane vibrations of Cr atoms, resulting in significantly different phonon dispersions in ferromagnetic (FM) and paramagnetic (PM) phases. Lattice thermal conductivity calculations provide additional evidence for strong SPC, where particularly large κ l is found for the FM phase. Most strikingly, PM and FM phases exhibit radically different behavior with tensile strain, where κ l increases with strain for the PM phase, and strongly decreases for the FM phase—as we explain through analysis of phonon lifetimes and scattering rates. Taken all together, we uncover the high significance of SPC on the phonon ...

Compared to multilayers, monolayer PtSe₂ exhibits a unique gap opening that enables the detailed characterization of defect induced electronic states. Pt-vacancies whose spin polarized nature has been linked to magnetic ordering are shown to be rare in the material. Instead, a metallization of zigzag edges is discovered with spin polarized edge states that may enable magnetic modifications of PtSe₂ nanomaterials.

Abstract

Edges and point defects in layered dichalcogenides are important for tuning their electronic and magnetic properties. By combining scanning tunneling microscopy (STM) with density functional theory (DFT), the electronic structure of edges and point defects in 2D-PtSe₂ are investigated where the 1.8 eV bandgap of monolayer PtSe₂ facilitates the detailed characterization of defect-induced gap states by STM. The stoichiometric zigzag edge terminations are found to be energetically favored. STM and DFT show that these edges exhibit metallic 1D states with spin polarized bands. Various native point defects in PtSe₂ are also characterized by STM. A comparison of the experiment with simulated images enables identification of Se-vacancies, Pt-vacancies, and Se-antisites as the dominant defects in PtSe₂. In contrast to Se- or Pt-vacancies, the Se-antisites are almost devoid of gap states. Pt-vacancies exhibit defect induced states that are spin polarized, emphasizing their importance for inducing magnetism in PtSe₂. The atomic-scale insights into defect-induced electronic states in monolayer PtSe₂ provide the fundamental underpinning for defect engineering of PtSe₂-monolayers and the newly identified spin-polarized edge states offer prospects for engineering magnetic properties in PtSe₂ nanoribbons.

By trapping active amorphous carbon wear products between transition-metal dichalcogenide (TMDC) flakes, the sandwich structures readily transform into graphene/TMDC heterostructures during running-in stage, based on shear-induced confinement and load-driven graphitization effects. Macroscale structural superlubricity (0.006) is achieved at steady stage by the graphene/TMDC heterostructures assembled in multipoint flake-like tribofilms.

Abstract

Macroscale superlubricity breakdown of lubricating materials caused by substrate surface roughening and mechanochemical modification poses great challenges for their practical tribological applications. Here, a facile way is reported to access robust macroscale superlubricity in a vacuum environment, via the operando formation of graphene/transition-metal dichalcogenide (TMDC) heterostructures at wear-induced rough surfaces. By trapping active amorphous carbon (a-C) wear products between TMDC flakes, the sandwich structures readily transform into graphene/TMDC heterostructures during running-in stage, based on shear-induced confinement and load-driven graphitization effects. Then they assemble into multipoint flake-like tribofilms to achieve macroscale superlubricity at steady stage by reducing contact area, eliminating strong cross-interface carbon–carbon interactions and polishing a-C rough nascent surface. Atomistic simulations reveal the preferential formation of graphene/TMDC heterostructures during running-in stage and demonstrate the superlubric sliding of TMDCs on the graphene. The findings are of importance to achieve robust superlubricity and provide a good strategy for the synthesis of other van der Waals heterostructures.

Nature Nanotechnology, Published online: 20 January 2022; doi:10.1038/s41565-021-01052-6

A cryo-strain device capable of applying large, continuous strains to two-dimensional materials in situ enables the reversible tuning of magnetic order and spin-canting process of the layered magnetic semiconductor CrSBr.

Nature Nanotechnology, Published online: 24 January 2022; doi:10.1038/s41565-021-01061-5

A robotic method enables rapid manufacturing of complex van der Waals solids.

Nature Nanotechnology, Published online: 17 January 2022; doi:10.1038/s41565-021-01059-z

Rhombohedral stacking of two identical non-ferroelectric monolayer transition metal dichalcogenides enables the observation of interfacial ferroelectricity.

An all-solution based cesium lead halide perovskite patterning process with adjustable phases and optical properties is reported. With the patterning process, the high-resolution photodetector array and luminescent patterns are demonstrated. The perovskite patterning process enables the integration of perovskites into image sensors and displays.

Abstract

Perovskite has been actively studied for optoelectronic applications, such as photodetectors and light-emitting diodes (LEDs), because of its excellent optoelectronic properties. However, ionic bonds of the perovskite structure are vulnerable to chemicals, which makes perovskite incompatible with photolithography processes that use polar solvents. Such incompatibility with photolithography hinders perovskite patterning and device integration. Here, an all-solution based cesium lead halide perovskite (Cs _x Pb _y Br _z ) patterning method is introduced in which PbBr₂ is patterned and then synthesized into Cs _x Pb _y Br _z . Each step of the top-down patterning process (e.g., developing, etching, and rinsing) is designed to be compatible with existing photolithography equipment. Structural, chemical, and optical analyses show that the PbBr₂ pattern of (10 µm)² squares is successfully transformed into CsPbBr₃ and Cs₄PbBr₆ with excellent absorption and emission properties. High-resolution photoconductor arrays and luminescent pattern arrays are fabricated with CsPbBr₃ and Cs₄PbBr₆ on various substrates, including flexible plastic films, to demonstrate their potential applications in image sensors or displays. The research provides a fundamental understanding of the properties and growth of perovskite and promotes technological advancement by preventing degradation during the photolithography process, enabling the integration of perovskite arrays into image sensors and displays.

The very recent advances in controlled production of two-dimension transition metal carbides and nitrides (MXenes) crystals by chemical vapor deposition, included several kinds of MXenes crystals and MXenes heterostructures.

Abstract

As a novel family of 2D materials, MXenes have drawn intensive interests owing to its fascinating property profile. The ability to grow high-quality MXenes in a controllable way would in turn further promote the development of fabrication techniques and expand wide advanced applications. Then 2D MXenes crystals are highly desirable and many approaches have been explored to realize the mass production. Chemical vapor deposition (CVD) provides compelling benefits over other alternatives in controllability, uniformity and scalability. In this review, the recent advances in growth of MXenes crystals by CVD method will comprehensively present. Several typical kinds of MXenes crystals are demonstrated to be fabricated with a precise control in terms of size, morphology and thickness. Further, a series of MXenes heterostructures are constructed including vertical and lateral spatial orientations. Then, the properties and applications of MXenes crystals are exhibited, of which superconductivity and electrochemical catalysts will be mainly emphasized. Finally, the authors put forward views on the future development in the synthesis of MXenes. With continuous efforts devoted, a bright future of MXenes crystals prepared by CVD is expected.

The 2D boridene, Mo_4/3B_{2- x} T _z , is investigated with respect to surface terminating species and basic electronic properties, using both experimental and theoretical methods. The theoretical results are consistent with experiments and suggest a possibility for tailoring of the electronic properties. The boridene also shows high catalytic performance for the hydrogen evolution reaction.

Abstract

Recently, a 2D metal boride – boridene – has been experimentally realized in the form of single-layer molybdenum boride sheets with ordered metal vacancies, through selective etching of the nanolaminated 3D parent borides (Mo_2/3Y_1/3)₂AlB₂ and (Mo_2/3Sc_1/3)₂AlB₂. The chemical formula of the boridene was suggested to be Mo_4/3B_{2- x} T _z , where T _z denotes surface terminations. Here, the termination composition and material properties of Mo_4/3B_{2- x} T _z from both theoretical and experimental perspectives are investigated. Termination sites are considered theoretically for termination species T = O, OH, and F, and the energetically favored termination configuration is identified at z = 2 for both single species terminations and binary termination mixes of different stoichiometries in ordered and disordered configurations. Mo_4/3B_{2- x} T _z is shown to be dynamically stable for multiple termination stoichiometries, displaying semiconducting, semimetallic, or metallic behavior depending on how different terminations are combined. The approximate chemical formula of a freestanding film of boridene is attained as Mo_1.33B_1.9O_0.3(OH)_1.5F_0.7 from X-ray photoelectron spectroscopy (XPS) analysis which, within error margins, is consistent with the theoretical results. Finally, metallic and additive-free Mo_4/3B_{2- x} T _z shows high catalytic performance for the hydrogen evolution reaction, with an onset potential of 0.15 V versus the reversible hydrogen electrode.

By “three-in-one” multi-level optimization on atomic, nanoscopic, and macroscopic structures, carbon-coated phosphorous-doped MoS₂ anchored on CNT paper (P-MoS₂@C/CNTP) are designed and prepared. The unique P-MoS₂@C/CNTP exhibits synergetic enhancement on Na⁺ diffusion, electron transport, and structural stability in the whole anode, resulting in enhanced sodium storage capability.

Abstract

Constructing sodium-ion battery anodes with efficient ion/electron transport and high cycling stability is significantly promising for applications but still remains challenging. Here, “three-in-one” multi-level design is performed to develop a carbon-coated phosphorous-doped MoS₂ anchored on carbon nanotube paper (P-MoS₂@C/CNTP). The Na⁺ diffusion and electron transport, as well as the structural stability of the whole anode are simultaneously enhanced through the synergistically optimization of P-MoS₂@C/CNTP at atomic, nanoscopic, and macroscopic levels. Resulted from the multi-level modification, the synergetic mechanism has been demonstrated by electrochemical measurement and theoretical calculation. As a result, the free-standing P-MoS₂@C/CNTP anode presents a high rate performance (150 mA h g⁻¹ at 5 A g⁻¹) and a long cycling life (1 A g⁻¹, 1200 cycles, 249 mA h g⁻¹). This work provides a new approach to the design and fabrication of high-performance conversion-type electrode materials for rechargeable batteries application.

A 2D donor–acceptor covalent organic framework nanosheet, [(TPA)₁(TPhT)₁]_CN, is in situ synthesized in a lead iodide layer to regulate the crystallization process of a perovskite film in a sequential deposition method. A covalent organic framework incorporated perovskite solar cell is endowed with a prominent power conversion efficiency of 22.04% and excellent stability.

Abstract

Poor crystallinity of perovskite and extensive defects around grain boundaries are the acknowledged hindrances to obtaining high efficiency and long-term stability for organic metal halide perovskite solar cells (PSCs). Here, a 2D covalent organic framework (2D COF) nanosheets, [(TPA)₁(TPhT)₁]_CN, is first in situ synthesized in a PbI₂ layer with a highly crystalline structure to precisely regulate the crystallization process of perovskite in the sequential deposition method. The existence of 2D COF nanosheets can decelerate intermolecular interdiffusion and induce perovskite crystals to grow along (110) planes with enlarged grain size. Meanwhile, 2D COF nanosheets distributed around the grain boundaries reduce the defect density and promote carriers transporting in the perovskite film. The superior properties of the perovskite film afford the champion PSC device with a power conversion efficiency of 22.04%, which is over 10% higher than the control device. Moreover, the target PSC also demonstrates outstanding long-term stability. It can maintain over 90% of its initial value after 90 days storage in ambient conditions for unencapsulated devices. This work paves a new path for regulating the crystallization process of perovskites via 2D crystalline materials.

Employing atomically thin and photoluminescent MoS₂ monolayers, a novel 2D sensing interface is developed to detect the diffusion of probe molecules through cellular layers and tissues. In a single cellular layer, breaks of nanoscale cellular junctions less than 40 nm in length are identified that are otherwise impossible to measure with conventional microscopy.

Abstract

The permeability of cell layers plays a critical role in the life sciences and medicine. It remains a long-standing challenge to assess molecular transport through cell layers with subcellular spatial resolution. Herein, a novel sensing platform employing atomically thin and photoluminescent MoS₂ monolayers as 2D sensing interfaces is developed to detect the diffusion of probe molecules through cellular layers and tissues. The 2D form factor enables monolayer MoS₂ to serve as an array of 20 164 optical sensors covering a total area of 420 × 420 µm², being the only nanosensor interface that can spatially image molecular permeability in this way. In a single layer of human umbilical vein endothelial cells (HUVEC), regions with diffusivities ranging from 1 × 10⁻⁹ to 3 × 10⁻⁸ cm² s⁻¹ are found that are in part spatially correlated with the immunofluorescence of vascular endothelial (VE)-cadherin proteins found on the cell membrane. However, the new technique clearly identifies these locations as breaks of nanoscale cellular junctions less than 40 nm in length in intercellular clefts that are otherwise impossible to measure with conventional microscopy. With the ability to measure permeability through various tissues, this 2D sensing interface allows the measurement of biological properties to assist the development of targeted therapeutics and mechanistic models.

Nature, Published online: 22 December 2021; doi:10.1038/s41586-021-04171-1

An electric-field-induced topological phase transition from a Mott insulator to a quantum anomalous Hall insulator in near-60-degree-twisted (or AB-stacked) MoTe2/WSe2 heterobilayers is reported.

Nature, Published online: 22 December 2021; doi:10.1038/d41586-021-03768-w

The inclusion of nitrogen atoms stabilizes the zigzag edges of carbon-based nanoribbons, enabling the ribbons to be decoupled from a substrate and providing a probe for their unconventional magnetism.

Nature, Published online: 22 December 2021; doi:10.1038/s41586-021-04201-y

Decoupling spin-polarized edge states using substitutional N-atom dopants along the edges of a zigzag graphene nanoribbon (ZGNR) reveals giant spin splitting of a N-dopant edge state, and supports the predicted emergent magnetic order in ZGNRs.

The nonlinear optical (NLO) properties of two-dimensional (2D) layered materials have received extensive attention for their promising application prospects in the field of optoelectronics. Among these materials, 2D carbon materials have been pushed to the research hotspot thanks to their unique chemical and electronic structures. However, the exploration of second harmonic generation is currently limited to sp 2 hybridized 2D carbon materials, while the research on multi-hybridized carbon atomic materials is relatively rare. Here, we report the observation of optical second harmonic from tetraphenylethylene-graphdiyne (TPE-GDY), a fully conjugated fluorescent 2D material composed of sp and sp 2 hybridized carbon atoms based on the TPE unit. TPE-GDY is mixed with polyvinylpyrrolidone (PVP) to prepare TPE-GDY/PVP nanocomposite films, which further improves the stability of TPE-GDY. Cotton effect and favorable SHG response have been observed from these composit...

The intrinsically stretchable and patternable ultrathin conformal wrinkled graphene-elastomer composite material is fabricated based on novel strategy under mild conditions, which is free of high temperature, annealing, etching, organic-solvents process. This novel strategy can offer a possibility for the construction of novel graphene or other 2D materials based on intrinsically stretchable soft electronic devices.

Abstract

Tissue-like intrinsically stretchable electronics have attracted ever-increasing study attention in recent years as they can form intimate interfaces with skin, endowing devices monitoring tactile and physiological signals with the negligibly constraining movement of the human body. However, harsh mechanical deformation inevitably leads to degradation of or even destroys the electronic properties of the devices. Strain-insensitive self-powered triboelectric tactile sensor arrays based on wafer-scale patterned and intrinsically stretchable nanoscale thin conformal wrinkled graphene-elastomer composite material are demonstrated here. By regulating the wrinkle structure of the composite, the stretchability performance of the material can be optimized. The fabrication process of the composite can be readily incorporated into photolithography and shadow mask techniques without high temperature, annealing, etching, or organic solvents operating. An intrinsically stretchable semitransparent pressure sensor array is created, which can be stretched to 100% strain without visible signals output degradation. The theoretical modeling points out that the unique conformal wrinkle structure is the key element that attributes to the strain-insensitive property of the device. This work offers an alternative approach for the design of novel graphene-based strain-insensitive stretchable soft electronic devices.

Quantum tunneling in ambipolar multilayered black phosphorus (BP) transistors at the charge neutrality point voltage (V _CNP) is demonstrated without heterojunctions. Under bipolar conduction near the V _CNP, a p-i-n configuration is successfully obtained along the BP channel and negative differential resistance (NDR) is realized via band-to-band tunneling. The fabricated reconfigurable top-gate BP device provides further evidence for the origin of NDR.

Abstract

Negative differential resistance (NDR) is an exotic quantum tunneling phenomenon that is exhibited in narrow p-n junctions with heavy doping concentrations. However, the presence of multiple heterojunctions in a conventional tunneling device often hampers the observance of NDR and a deep understanding of its origin, particularly in 2D van der Waals heterojunctions. Herein, the emergence of quantum tunneling at the charge neutrality point (V _CNP) in ambipolar multilayered black phosphorus (BP) transistors without heterojunctions is reported. The nearly identical electron and hole carrier densities at V _CNP in the presence of a drain bias (V _D) result in a lateral p-i-n configuration inside the BP multilayers similar to that in a tunneling field-effect transistor. The variation of the local carrier density profile and tunneling barrier with V _D at V _CNP drives a sharp enhancement of the activation energy and local resistance, which consequently allows to observe band-to-band tunneling at up to 340 K. The enhancement of the local doping profile along the BP channel and the NDR behavior in the fabricated reconfigurable top-gate BP device with an h-BN top-dielectric provide further evidence for the origin of NDR in 2D ambipolar materials.

Out-of-Plane Resistance Switching

In article number 2105795, Bilu Liu and co-workers reveal the out-of-plane resistance switching of 2D Bi₂O₂Se at the nanoscale. This behavior stems from oxidation-induced formation of hillocks that produce movable Se vacancies as well as Joule heat generated under electric field. The research opens an avenue to explore the applications of emerging Bi₂O₂Se material in highly integrated nanoelectronics with new functionalities.

Diffusive memristors from volatile electrochemical metallization devices are interesting for applications in emerging memories and neuromorphic computing areas. New insights gained from the combined threshold and relaxation kinetics study of Ag/HfO₂/Pt devices outlines the significance of the filament formation and growth process on its relaxation time. This enables an optimization of the operation conditions of diffusive memristors in neuromorphic circuits.

Abstract

Owing to their unique features such as thresholding and self-relaxation behavior diffusive memristors built from volatile electrochemical metallization (v-ECM) devices are drawing attention in emerging memories and neuromorphic computing areas such as temporal coding. Unlike the switching kinetics of non-volatile ECM cells, the thresholding and relaxation dynamics of diffusive memristors are still under investigation. Comprehension of the kinetics and identification of the underlying physical processes during switching and relaxation are of utmost importance to optimize and modulate the performance of threshold devices. In this study, the switching dynamics of Ag/HfO₂/Pt v-ECM devices are investigated. Depending on the amplitude and duration of applied voltage pulses, the threshold kinetics and the filament relaxation are analyzed in a comprehensive approach. This enables the identification of different mechanisms as the rate-limiting steps for filament formation and, consequently, to simulate the threshold kinetics using a physical model modified from non-volatile ECM. New insights gained from the combined threshold and relaxation kinetics study outline the significance of the filament formation and growth process on its relaxation time. This knowledge can be directly transferred into the optimization of the operation conditions of diffusive memristors in neuromorphic circuits.