Jing Zhang

The electrocatalytic performance of the emerging 2D MC₂ structures is explored by performing an unbiased structural search and first‐principles calculations. A series of excellent catalysts are identified, where NbC₂ is used for hydrogen evolution reaction (HER), TaC₂ for bifunctional catalysis in HER/ oxygen reduction reaction (ORR), and MoC₂ for multifunctional catalysis in HER/ oxygen evolution reaction (OER)/ORR. Additionally, the catalytic mechanisms for different reactions are investigated in depth.

Abstract

Searching for highly efficient, stable, and cost‐effective electrocatalysts for water splitting and oxygen reduction reaction (ORR) is critical for renewable energies, yet it remains a great challenge. Here, by performing an unbiased structural search and first‐principles calculations, the electrocatalytic performance of the emerging 2D transitional‐metal carbides, MC₂ (M represents the transition metal of Ti, V, Nb, Ta, and Mo, C is carbon), is systematically investigated. Owing to their super stability and outstanding electronic conductivity, fast charge transfer kinetics is allowed during catalysis. Specifically, NbC₂, TaC₂, and MoC₂ possess excellent hydrogen evolution reaction (HER) performance under the reaction by the Volmer‐Heyrovsky mechanism. Moreover, taking advantage of the dual‐active‐site catalytic mechanism for oxygen evolution reaction (OER) and ORR over traditional single‐active‐site mechanism, TaC₂ presents promising bifunctional electrocatalytic activity with a low overpotential of 0.06 and 0.37 V for HER and ORR, respectively. Meanwhile, the low overpotential endows MoC₂ remarkable multifunctional electrocatalytic performance in overall water splitting (0.001 V for HER, 0.45 V for OER) and ORR (0.47 V). These intriguing results demonstrate the robust applicability of MC₂ monolayers as effective electrocatalysts.

High resolution, sub‐micron patterning of liquid crystalline conjugated polymers (LCCP) is demonstrated using two‐photon laser writing, offering great flexibility for the construction of optical structures for a variety of applications. The image displays a college crest patterned by laser writing on a uniformly aligned photoalignment film on top of which the LCCP has been oriented as a nematic glass.

Abstract

Systematic tuning of chemical and physical structure allows fine control over desired electronic and optical properties, including those of conjugated polymer semiconductors. In the case of physical structure, orientation via liquid crystalline alignment allows access to fundamental optical anisotropies and the associated refractive index modification offers great potential for fabrication of photonic structures. In this paper, photoalignment is used to orient the liquid crystalline conjugated polymer poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT), specifically involving two‐photon infrared laser writing of patterns in an azobenzene sulphonic dye (SD1). These patterns are transferred into the overlying film by thermotropic orientation in the nematic melt, then frozen in place by quenching to a room temperature nematic glass. Optimization of laser power and scan speed allows features with linewidths ≤ 1 µm. Photoluminescence (PL) peak anisotropy values reach PL_II/PL_⊥ = 13 for laser writing, compared with PL_II/PL_⊥ = 9 for polarized ultraviolet light emitting diode exposure of the same SD1 layer. These two approaches also result in different film microstructures; evidenced by characteristic changes in PL spectra. The anisotropic PL spectra provide information on emissive excited states that complements previous studies on non‐oriented F8BT and related copolymers, also suggesting two emissive states.

A strong, unexpected bulk damping‐like spin‐orbit torque is observed within chemically disordered ferromagnetic single layers that have no detectable long‐range asymmetry. This bulk torque, which most likely arises from a net transverse spin polarization associated with the spin Hall effect, increases monotonically with the ferromagnet thickness and is insensitive to the neighbor layers. These results broaden the scope of spin‐orbitronics.

Abstract

Strong damping‐like spin‐orbit torque (τ_DL) has great potential for enabling ultrafast energy‐efficient magnetic memories, oscillators, and logic. So far, the reported τ_DL exerted on a thin‐film magnet must result from an externally generated spin current or from an internal non‐equilibrium spin polarization in non‐centrosymmetric GaMnAs single crystals. Here, for the first time a very strong, unexpected τ_DL is demonstrated from current flow within ferromagnetic single layers of chemically disordered, face‐centered‐cubic CoPt. It is established here that the novel τ_DL is a bulk effect, with the strength per unit current density increasing monotonically with the CoPt thickness, and is insensitive to the presence or absence of spin sinks at the CoPt surfaces. This τ_DL most likely arises from a net transverse spin polarization associated with a strong spin Hall effect, while there is no detectable long‐range asymmetry in the material. These results broaden the scope of spin‐orbitronics and provide a novel avenue for developing single‐layer‐based spin‐torque memory, oscillator, and logic technologies.

An efficient electrochemical exfoliation of MoS₂ is realized through a lateral inward oxidation reaction starting from a typical layer edge with a rapid depth penetration, ultimately forming a stable stacked few‐layer (two/three layers) structure. This stacked atomically thin MoS₂ shows enhanced electrocatalysis performance and surface‐enhanced Raman spectroscopy sensitivity.

Abstract

The production of atomically thin transition‐metal dichalcogenides (TMDs) has been investigated through various top‐to‐down exfoliation methods, such as mechanical and chemical exfoliation, while large‐scale chemical exfoliation is sluggish and needs over ten hours to achieve atomically thin TMDs. Herein, a new strategy is reported for exfoliating bulk MoS₂ into two/three‐layer flakes within tens of seconds through a mild electrochemical treatment. This exfoliation method is driven by a lateral inward oxidation reaction starting from the typical layer edge with a rapid depth penetration, whereby a stacked few‐layer (two/three layers) structure is ultimately formed. This efficient reaction process is monitored based on an individual MoS₂ on‐chip device combined with in situ Raman and cross‐sectional scanning transmission electron microscopy, and the uniformity of thickness is demonstrated. This preferentially initiated method can be also extended to produce few‐layer MoSe₂ and the selective extraction mechanism is assumed to be related to intrinsic layer‐dependent energy band properties. Moreover, the special reassembled few‐layer MoS₂ possesses great performance as functional materials in electrocatalysis (127 mV overpotential for hydrogen evolution reaction) and surface‐enhanced Raman spectroscopy (10⁵ enhancement factor). These results illustrate the broad prospects of the reassembled few‐layer MoS₂ for optics, catalysis, and sensors.

Magnons dominate the magnetic response of ferromagnetic two-dimensional crystals such as CrI 3 . Because of the arrangement of Cr spins in a honeycomb lattice, magnons in CrI 3 bear a strong resemblance with electrons in graphene. Neutron scattering experiments carried out in bulk CrI 3 show the existence of a gap at the Dirac points, conjectured to have a topological nature. We propose a theory for magnons in CrI 3 monolayers based on an itinerant fermion picture, with a Hamiltonian derived from first principles. We obtain the magnon dispersion for 2D CrI 3 with a gap at the Dirac points with the same Berry curvature in both valleys. For CrI 3 ribbons, we find chiral in-gap edge states. Analysis of the magnon wave functions in momentum space confirms their topological nature. Importantly, our approach does not require a spin Hamiltonian, and can be applied to insulating and conducting 2D materials with any type of magnet...

Direct growth method of germanene at interfaces between van der Waals (vdW) materials and Ag(111) is proposed and developed. The grown germanene is stable in air, which enables its handling in air. A vdW interface provides a nanoscale platform for growing germanene similarly to that in vacuum, which cannot be achieved with a typical oxide interface such as Al₂O₃.

Abstract

Germanene, a 2D honeycomb germanium crystal, is grown at graphene/Ag(111) and hexagonal boron nitride (h‐BN)/Ag(111) interfaces by segregating germanium atoms. A simple annealing process in N₂ or H₂/Ar at ambient pressure leads to the formation of germanene, indicating that an ultrahigh‐vacuum condition is not necessary. The grown germanene is stable in air and uniform over the entire area covered with a van der Waals (vdW) material. As an important finding, it is necessary to use a vdW material as a cap layer for the present germanene growth method since the use of an Al₂O₃ cap layer results in no germanene formation. The present study also proves that Raman spectroscopy in air is a powerful tool for characterizing germanene at the interfaces, which is concluded by multiple analyses including first‐principles density functional theory calculations. The direct growth of h‐BN‐capped germanene on Ag(111), which is demonstrated in the present study, is considered to be a promising technique for the fabrication of future germanene‐based electronic devices.

In article number 2000712, Laisheng Li, Jing Wang, and co‐workers discuss 2D MXenes, which demonstrate a superior photothermal conversion property by virtue of their electromagnetic wave absorption capacity and localized surface plasmon resonance effect. This efficient way to utilize solar energy that allows the transformation of solar illumination into thermal energy enables MXene‐based materials to be applied in diverse fields, such as solar steam generation and biomedicals.

In article number 2000883, Chong Min Koo, Aamir Iqbal, and Pradeep Sambyal comprehensively review the recent advancements in MXene‐based electromagnetic interference shielding materials with different structural morphologies and provides an insight into future challenges and guideline for finding material solutions for the next‐generation shielding applications.

MXene‐derived high aspect‐ratio lithium niobate (LiNbO₃) single crystals are successfully synthesized using 2D Nb₂C MXene and LiOH as the niobium and lithium sources, respectively. In addition, trivalent rare‐earth ions are doped in the M‐LiNbO₃ crystals using a new synthesis process, and provide photoluminescent properties to those doped LiNbO₃ crystals.

Abstract

MXenes have recently been used to grow highly textured potassium niobate ferroelectric crystals. Herein, the versatility of MXenes is further demonstrated by growing ferroelectric and luminescent lithium niobate (LiNbO₃) crystals from niobium carbide MXene (Nb₂CT _x ). The formation of high‐aspect‐ratio LiNbO₃ rhombic crystals is confirmed by extensive structural analysis. The ferroelectricity of MXene‐derived LiNbO₃ is verified, using standard ferroelectric and dielectric measurements. In addition, for the first time, Pr³⁺‐doped LiNbO₃ crystals are synthesized with simultaneous visible photoluminescence (PL) from Nb₂CT _x MXene. This work demonstrates that it is possible, by using the 2D character of MXenes, to fabricate high aspect ratio and well‐oriented photoluminescent ferroelectric crystals for advanced optoelectronic applications.

Advanced Functional Materials, Volume 30, Issue 47, November 18, 2020.

Reversible polarity is required to increase the process efficiency of signals. Polarity‐reversible photodiodes based on ambipolar 2D semiconductors are constructed by applying asymmetrically metal‐contacted architectures. The photodiode exhibits reversible rectifying behaviors and photovoltaic response under gate manipulation, which enables it to perform as a gate‐tunable photovoltaic detector or a logic optoelectronic device.

Abstract

A photosensor with an amplitude‐tunable and polarity‐reversible response under gate modulation has potential as a computational photosensor, which can provide more recognition degree of data to enhance signal processing efficiency. Although, the ambipolar 2D semiconductors possess unique gate‐tunable properties, the question of how to utilize this property to design polarity‐reversible photodiodes for intelligent applications remains unanswered. Here, gate‐controllable polarity‐reversible photodiodes based on ambipolar 2D semiconductors with an asymmetrically metal‐contacted architecture are proposed. By controlling the gate‐field, the local carrier type and density profile can be manipulated in the channel due to the partial shielding feature of the asymmetrically metal‐contacted architecture, resulting in a polarity‐reversible photodiode. The reported WSe₂‐based photodiode possesses excellent rectifying behavior with a rectification ratio over 10⁵, photovoltaic performance with 90% external quantum efficiency, and 2.3% power conversion efficiency under gate regulation. Meanwhile, the device exhibits reversible polarity of photovoltage from a negative to positive state under gate control. By utilizing the reversible photovoltage of the WSe₂ photodiode, an optoelectronic switch with a photovoltage polarity signal is demonstrated without a bias voltage. This photovoltage‐reversible homodiode paves the way to develop 2D devices with multiple operation modes for potential applications in high‐efficiency photovoltaics, intelligent vision sensors, and logic optoelectronics.

Nature Nanotechnology, Published online: 21 September 2020; doi:10.1038/s41565-020-0763-9

Both extrinsic and intrinsic factors determine the properties of ferroic materials and are difficult to disentangle. This study on artificial crystals of planar nanomagnets with well-defined, tuneable magnetic interactions unveils the intrinsic correlations between microscopic interactions and macroscopic properties such as the domain size and morphology or the domain-wall mobility.

Nature Nanotechnology, Published online: 28 September 2020; doi:10.1038/s41565-020-0771-9

Holography of incoherent emission from SERS probes allows multiplexed single-particle localization in three dimensions in one shot using a wide-field microscope.

Nature Nanotechnology, Published online: 19 October 2020; doi:10.1038/s41565-020-00779-y

Function implementation and optimization in nanoscale and quantum-electronic devices become increasingly challenging with the growing complexity of the devices. Training a deep neural network with the physical device response and searching for the functionality in the digital device can ease this challenge.

Nature Nanotechnology, Published online: 09 November 2020; doi:10.1038/s41565-020-00795-y

Two-dimensional self-assembled heterostructures of graphene oxide and polyamine macromolecules are used to create membranes with tuneable permeability for water and ions.

Nature Nanotechnology, Published online: 09 November 2020; doi:10.1038/s41565-020-00789-w

A combination of atomistic imaging and spectroscopy reveals that metal substitution into a sulfur vacancy is the underlying mechanism for resistive switching in transition metal dichalcogenide monolayers.

By selectively applying electrostatic doping to a specific channel layer, a p‐WSe₂/n‐WS₂/n‐MoS₂ heterojunction device with both p–n and n–n junctions can modulate the charge carrier balance between heterojunction layers to generate photocurrent upon illumination. The relationship between photocurrent and the charge balance of electrons and holes in van der Waals heterojunctions is investigated.

Abstract

Heterojunction structures using 2D materials are promising building blocks for electronic and optoelectronic devices. The limitations of conventional silicon photodetectors and energy devices are able to be overcome by exploiting quantum tunneling and adjusting charge balance in 2D p–n and n–n junctions. Enhanced photoresponsivity in 2D heterojunction devices can be obtained with WSe₂ and BP as p‐type semiconductors and MoS₂ and WS₂ as n‐type semiconductors. In this study, the relationship between photocurrent and the charge balance of electrons and holes in van der Waals heterojunctions is investigated. To observe this phenomenon, a p‐WSe₂/n‐WS₂/n‐MoS₂ heterojunction device with both p–n and n–n junctions is fabricated. The device can modulate the charge carrier balance between heterojunction layers to generate photocurrent upon illumination by selectively applying electrostatic doping to a specific layer. Using photocurrent mapping, the operating transition zones for the device is demonstrated, allowing to accurately identify the locations where photocurrent generates. Finally, the origins of flicker and shot noise at the different semiconductor interfaces are analyzed to understand their effect on the photoresponsivity and detectivity of unit active area (2.5 µm², λ = 405 nm) in the p‐WSe₂/n‐WS₂/n‐MoS₂ heterojunction device.

Anomalous negative photoresponse is identified in the pristine molybdenum ditelluride (MoTe₂)/amorphous strontium titanium oxide (a‐STO) heterostructure with remarkable photoresponsivity and detectivity. Through tuning the light programming time, its photodetection behavior experiences a dynamic evolution from negative to positive due to the optically controllable modulation of the interfacial states. These results envision the 2D material/a‐STO heterostructure as a potential platform for exploring new heterogeneous integrated optoelectronics.

Abstract

Heterostructures play a vital role in functional devices on the basis of the individual constituents. Non‐conventional heterostructures formed by stacking 2D materials onto structurally distinct materials are of great interest in achieving novel phenomena that are both scientifically and technologically relevant. Here, a heterostructure based on a 2D (molybdenum ditelluride) MoTe₂ and an amorphous strontium titanium oxide (a‐STO) thin film is reported. The heterostructure functions as a high‐performance photodetector, which exhibits anomalous negative photoresponse in the pristine device due to the scattering effect from the light‐induced O^δ‐ ions. The photoresponsivity and the specific detectivity are found to be >10⁴ AW^‐1 and >10¹³ Jones, respectively, which are significantly higher than those in standard MoTe₂ devices. Moreover, through tuning the light programming time, the photodetection behavior of the MoTe₂/a‐STO heterostructure experiences a dynamic evolution from negative to positive. This is due to the optically controllable modulation of the interfacial states, which is further confirmed by the X‐ray photoelectron spectroscopy and photoluminescence measurements. It is envisioned that the 2D material/a‐STO heterostructure could be a potential platform for exploring new functional devices.

A boron nitride (BN) incorporated solid polymer electrolyte is developed with improved ionic conductivity, thermal responses, and mechanical properties for the application of all‐solid‐state Li–S batteries. The thermal and current uniformity derived from BN additive leads to a uniform Li deposition and preferred S conversion. Thus, an all‐solid‐state Li–S battery is successfully demonstrated with outstanding electrochemical performances.

Abstract

Polymer‐based solid‐state electrolytes are shown to be highly promising for realizing low‐cost, high‐capacity, and safe Li batteries. One major challenge for polymer solid‐state batteries is the relatively high operating temperature (60–80 °C), which means operating such batteries will require significant ramp up time due to heating. On the other hand, as polymer electrolytes are poor thermal conductors, thermal variation across the polymer electrolyte can lead to nonuniformity in ionic conductivity. This can be highly detrimental to lithium deposition and may result in dendrite formation. Here, a polyethylene oxide‐based electrolyte with improved thermal responses is developed by incorporating 2D boron nitride (BN) nanoflakes. The results show that the BN additive also enhances ionic and mechanical properties of the electrolyte. More uniform Li stripping/deposition and reversible cathode reactions are achieved, which in turn enable all‐solid‐state lithium–sulfur cells with superior performances.

Nature Nanotechnology, Published online: 10 August 2020; doi:10.1038/s41565-020-0750-1

Interlayer excitons in bilayer MoS2 exhibit both a high oscillator strength and highly tunable energies in an applied electric field.

Growth of two-dimensional metals has eluded materials scientists since the discovery of the atomically thin graphene and other covalently bound 2D materials. Here, we report a two-atom-thick hexagonal copper-gold alloy, grown through thermal evaporation on freestanding graphene and hexagonal boron nitride. The structures are imaged at atomic resolution with scanning transmission electron microscopy and further characterized with spectroscopic techniques. While the 2D structures are stable over months in vacuum, electron irradiation in the microscope provides sufficient energy to cause a phase transformation—atoms are released from their lattice sites with the gold atoms eventually forming face-centered cubic nanoclusters on top of 2D regions during observation. The presence of copper in the alloy enhances sticking of gold to the substrate, which has clear implications for creating atomically thin electrodes for applications utilizing 2D materials. Its practically infinite surfac...

Optical selection rules in monolayers of transition metal dichalcogenides and of their heterostructures are determined by the conservation of the z-component of the total angular momentum—J Z = L Z +S Z – associated with the C 3 rotational lattice symmetry which assumes half integer values corresponding, modulo 3, to distinct states. Here we show, based on polarization resolved and low temperature magneto-optical spectroscopy experiments, that the conservation of the total angular momentum in these systems leads to a very efficient exciton-phonon interaction when the coupling is mediated through chiral phonons. We identify these phonons as the Γ point E” modes which despite carrying angular momentum ± 1 are able to induce an excitonic spin-flip of ##IMG## [http://ej.iop.org/images/2053-1583/7/4/041002/tdmaba567ieqn1.gif] {$\mp2$} thanks to the C 3 symmetry. These experiments reveal ...

sp 2 -hybridized boron nitride is identified as a strategic material for many purposes related to the integration of graphene and two-dimensional materials in devices and the fabrication of van der Waals heterostructures. Thus, it becomes mandatory to have scalable synthesis and characterization procedures for providing suitable and reliable boron nitride material according to these identified needs. We report here on the growth of sp 2 -hybridized boron nitride film on polycrystalline nickel substrate by chemical vapor deposition with borazine as precursor. We propose a complete study of the influence of the underlying nickel grain orientation on the BN structure layers, in terms of thickness, crystallographic orientation, domain size and stacking. We show the heteroepitaxial growth of continuous, single crystalline hexagonal boron nitride multilayer film on nickel (111)-like grains. We highlight its ABC stacking sequence with AB stacking faults and show how i...

Extraordinary optical, electrical and chemical properties of 2D materials have potential to be useful for quick and sensitive detection of pathological diseases. One important example is malaria disease that can progress rapidly and cause death within days. Therefore, fast, accurate and cost-effective malaria diagnosis available at the point of care is urgently needed to facilitate precise treatment. Here we report rapid and highly sensitive malaria detection with an inexpensive graphene-protected copper surface plasmon resonance biosensor. Using phase sensitive surface plasmon resonance technique and a graphene functionalization protocol for attaching end-tethered DNA probes that were complementary to a malaria specific DNA target, we were able to significantly improve the detection limit of the malarial plasmodium parasite. The phase sensitivity of our graphene-enhanced sensors exceeds by two orders of magnitude the sensitivity of analogous optical biosensors. This enhanced se...

Ti₃C₂ MXene thin films with in situ formed multiple Schottky barriers (SBs) are synthesized by employing a solution‐based oxidation method, selectively forming TiO₂ nanocrystals at the edge sites of each individual MXene sheet. Gas sensors based on the TiO₂/Ti₃C₂ heterostructure show a highly enhanced gas response toward nitrogen dioxide gas resulting from SB modulation.

Abstract

The main gas‐sensing mechanisms of 2D materials are surface charge transfer by analytes and Schottky barrier (SB) modulation at the interface between the metallic and semiconducting surfaces. In particular, dramatic differences in the gas‐sensing performances of 2D materials originate from SB modulation. However, SB sites typically exist only at the interface between the semiconducting channel material and the metal electrode. Herein, in situ formed multiple SBs in a single gas‐sensing channel are demonstrated, which are derived from the heterojunction of metallic Ti₃C₂ and semiconducting TiO₂. In stark contrast with previous techniques, edge‐oxidized Ti₃C₂ flakes are synthesized by solution oxidation, allowing the uniform formation of TiO₂ crystals on all flakes that comprise the gas sensing channel. Oxidized colloidal solutions are subjected to vacuum filtration to automatically form SB sites at the multiple inter‐flake junctions in both the outer surface and inner bulk regions of the film. The TiO₂/Ti₃C₂ composite sensor shows 13.7 times higher NO₂ sensitivity as compared with pristine Ti₃C₂ MXene, while the responses of the reducing gases are almost unchanged. The results suggest a new strategy for improving gas‐sensing performance by maximizing the density of SB sites through a simple method.

Lattice defect is a major cause of energy dissipation in conventional electric current due to the drift and diffusion motions of electrons. Different nature of current emerges when noncentrosymmetric materials are excited by light. This current, called the shift current, originates from the change in the Berry connection of electrons’...

2D materials based on main group element compounds exhibit rich stoichiometries and structure motifs. The synthesis of these multiphase materials is controllable using new methodologies. Furthermore, their properties are revealed based on theoretical predictions and experimental exploration, and then developed for various potential applications.

Abstract

2D materials based on main group element compounds have recently attracted significant attention because of their rich stoichiometric ratios and structure motifs. This review focuses on the phases in various 2D binary materials including III–VI, IV–VI, V–VI, III–V, IV–V, and V–V materials. Reducing 3D materials to 2D introduces confinement and surface effects as well as stabilizes unstable 3D phases in their 2D form. Their crystal structures, stability, preparation, and applications are summarized based on theoretical predictions and experimental explorations. Moreover, various properties of 2D materials, such as ferroelectric effect, anisotropic optical and electrical properties, ultralow thermal conductivity, and topological state are discussed. Finally, a few perspectives and an outlook are given to inspire readers toward exploring 2D materials with new phases and properties.

Morphology and structure can significantly influence the electrochemical performance of electrodes used in batteries and supercapacitors. This review discusses recent advances in engineering the structure of electrodes based on 2D MXenes to boost their ionic and electronic transport properties. Electrode fabrication methods and assembly processes are reviewed and compared and future research needs in this area are discussed.

Abstract

MXenes are 2D materials with relatively high surface areas, high electrical conductivities, functional transition metal surfaces, tunable surface chemistries, and solution processability. Due to these properties, 2D MXenes have attracted widespread attention as electrode materials for energy storage devices, including electrochemical capacitors, with high power and energy densities. However, many studies have shown that the electrochemical performance of MXene electrodes is considerably affected by their structure and morphology. These properties are, for the most part, controlled by the method used for the assembly of 2D MXene flakes and the electrode fabrication methods. A successful electrode assembly and fabrication method should address several challenges, such as the restacking of 2D flakes, to achieve electrode structures and morphologies that deliver high ionic transport properties, electrical conductivity, and mechanical stability. This review aims to provide insight into the current state‐of‐the‐art assembly and fabrication methods used to design and fabricate high performance electrodes based on MXenes. The major challenges to be addressed and possible future directions in the fabrication of MXene electrodes for practical energy storage applications are highlighted.