Jiuxiang Dai

Nature Materials, Published online: 28 August 2023; doi:10.1038/s41563-023-01653-7

The authors use low-temperature transport measurements to report superconductivity in a twisted double bilayer graphene system.

The ferroelectric negative capacitor transistors composed of all-2D components are developed, exhibiting ultra-steep-slope and high air-stability enabled by using the strategies of high-k Al₂O₃ passivation and dual-gate modulation. Leveraging the unique negative capacitance effect, it can break the Boltzmann limit of 60 mV dec⁻¹ in conventional metal-oxide-semiconductor field effect transistors, demonstrating tremendous potential in ultra-low-power applications.

Abstract

Ferroelectric negative capacitance transistors (Fe-NCFETs) have emerged as a promising technology for low-power electronics and have the potential to continue Moore's law. However, the existing 2D ferroelectric materials are predominantly sulfides or halides, which are susceptible to oxidation or hydrolysis, thereby hindering their commercial production due to concerns related to performance and stability. To address these obstacles, the authors have optimized the Fe-NCFETs composed of 2D ferroelectric CuInP₂S₆ and semiconductor WS₂ using high-k Al₂O₃ passivation and dual-gate modulation strategy. With atomic layer deposition (ALD) of Al₂O₃, all-2D Fe-NCFETs, operated at a low driven voltage of 0.3 V, achieve much improvement in stability and performance with a high ON/OFF ratio of 10⁹ and minimum subthreshold swing (SS) of 14 mV dec⁻¹, which is attributed to the negative capacitance effect of CuInP₂S₆ and passivation effect of ALD-Al₂O₃. The dual-gate modulation approach is also implemented to maintain the device stability and enable the improved ON/OFF ratio from 10⁵ to 10⁸, minimum SS of 10 mV dec⁻¹, and an average SS of ≈60 mV dec⁻¹ covering more than five orders of magnitude of current. This work provides a facile and effective strategy for designing all-2D Fe-NCFETs with ultra-steep SS and high stability, showing exciting potential for future low-power electronic applications.

Highly flexible and very thin hexagonal-shaped Te and Te–metal nanorope arrays are developed using radiofrequency sputtering at 25 °C and a low deposition power (6 W). The fabricated blue organic light-emitting diodes with 7 nm Te–W anodes exhibit high efficiencies and stable mechanical flexibility. The Te-based nanorope films are a step toward addressing the limitations of typical nanomaterials.

Abstract

Nanomaterials that can be easily processed into thin films are highly desirable for their wide range of applicability in electrical and optical devices. Currently, Te-based 2D materials are of interest because of their superior electrical properties compared to transition metal dichalcogenide materials. However, the large-scale manufacturing of these materials is challenging, impeding their commercialization. This paper reports on ultrathin, large-scale, and highly flexible Te and Te–metal nanorope films grown via low-power radiofrequency sputtering for a short period at 25 °C. Additionally, the feasibility of such films as transistor channels and flexible transparent conductive electrodes is discussed. A 20 nm thick Te–Ni-nanorope-channel-based transistor exhibits a high mobility (≈450 cm² V⁻¹ s⁻¹) and on/off ratio (10⁵), while 7 nm thick Te–W nanorope electrodes exhibit an extremely low haze (1.7%) and sheet resistance (30 Ω sq⁻¹), and high transmittance (86.4%), work function (≈4.9 eV), and flexibility. Blue organic light-emitting diodes with 7 nm Te–W anodes exhibit significantly higher external quantum efficiencies (15.7%), lower turn-on voltages (3.2 V), and higher and more uniform viewing angles than indium-tin-oxide-based devices. The excellent mechanical flexibility and easy coating capability offered by Te nanoropes demonstrate their superiority over conventional nanomaterials and provide an effective outlet for multifunctional devices.

Abstract

The discovery of two-dimensional (2D) semiconductor has opened up new avenues for the development of short-channel field-effect transistors (FETs) with desired electrical performance. Among them, orthorhombic tin-selenide (SnSe) has garnered increasing attention due to its potential applications in a variety of electronic, optoelectronic, and thermoelectric devices. However, the realization of high-performance SnSe FETs with low contact resistance (R_c) remains a challenge. Herein, we systematically investigate the contact of few-layer SnSe FETs through the modulation of native oxide on SnSe by using different metals. It is found that chromium (Cr)-contacted devices possess the best FET performance, such as electron mobility up to 606 cm²/(V·s) at 78 K, current on/off ratio exceeding 10¹⁰, and saturation current of ∼ 550 µA/µm, where a negligible Schottky barrier (SB) of ∼ 30 meV and a low contact resistance of ∼ 425 Ω µm are achieved. X-ray photoelectron spectroscopy (XPS) and cross-sectional electron dispersive X-ray spectroscopy (EDX) results further reveal that the improved contact arises from the Cr-induced reduction of native oxide (SnO_x) to Sn, which thins the tunneling barrier for efficient electron injection. Our findings provide a deep insight into the 2D-metal contact of SnSe and pave the way for its applications in future nanoelectronics.

Author(s): Liu Yang, Shiping Ding, Jinhua Gao, and Menghao Wu

First-principles calculations show that pure graphene multilayers with more than three layers exhibit atypical sliding ferroelectricity.

[Phys. Rev. Lett. 131, 096801] Published Tue Aug 29, 2023

Nature Materials, Published online: 29 August 2023; doi:10.1038/s41563-023-01623-z

Inspired by valley pseudospins in two-dimensional materials, high-quality-factor (high-Q) spin–valley states were created through the photonic Rashba-type spin splitting of a bound state in the continuum. This approach enabled the construction of a coherent and controllable spin-optical laser using monolayer-integrated spin–valley microcavities without requiring magnetic fields or cryogenic temperatures.

Nature Materials, Published online: 29 August 2023; doi:10.1038/s41563-023-01642-w

Using the van der Waals crystal Sb2O3 as a buffer layer enables the growth of high-κ dielectrics on two-dimensional materials via atomic layer deposition.

Nature Materials, Published online: 29 August 2023; doi:10.1038/s41563-023-01638-6

Amorphization can be an additional mechanism to assist plastic deformation in crystalline materials, providing a strategy to improve the load-bearing ability of brittle materials.

A step-nucleation in situ self-repair strategy to fabricate rollable large-area (2400 cm²) ultrathin (210–300 nm) zeolitic imidazolate framework (ZIF) membranes is elucidated. Two spiral-wound membrane modules with an area of 4800 cm² are prepared. Both the membranes and membrane modules show unprecedented high olefin gas permeance and good olefin/paraffin gas selectivity, thus promoting ZIF membrane practical applications.

Abstract

Ultrathin membranes with ultrahigh permeance and good gas selectivity have the potential to greatly decrease separation process costs, but it requires the practical preparation of large area membranes for implementation. Metal–organic frameworks (MOFs) are very attractive for membrane gas separation applications. However, to date, the largest MOF membrane area reported in the literature is only about 100 cm². In the present study, a new step-nucleation in situ self-repair strategy is proposed that enables the preparation of large-area (2400 cm²) ultrathin and rollable MOF membranes deposited on an inexpensive flexible polymer membrane support layer for the first time, combining a polyvinyl alcohol (PVA)‒metal-ion layer and a pure metal-ion layer. The main role of the pure metal-ion layer is to act as the main nucleation sites for MOF membrane growth, while the PVA‒metal-ion layer acts as a slow-release metal-ion source, which supplements MOF crystal nucleation to repair any defects occurring. Membrane modules are necessary components for membrane applications, and spiral-wound modules are among the most common module formats that are widely applied in gas separation. A 4800 cm² spiral-wound membrane module was successfully prepared, demonstrating the practical implementation of large-area MOF membranes.

A new quaternary compound Bi₅O₄S₃Cl, containing the unique BiS₃ layer (in which Bi and S atoms form a six-coordinated octahedron) and Bi₂O₂ layer with a Cl atom at the center of the unit cell, is discovered. A superconducting transition above 3 K is observed. This finding will shed light on the exploration of new BiS-based superconductors.

Abstract

The BiS₂-based layered superconductors with structures similar to those of cuprates and iron-based superconductors have stimulated much research interest. Here, a new quaternary compound is reported, Bi₅O₄S₃Cl, which crystalizes in a tetragonal structure with P4/mmm (No. 123) space group having alternately stacking unique BiS₃ layers and Bi₂O₂ layers along the c-axis with a Cl atom located at the center of the unit cell. A superconducting transition above 3 K is observed for both electrical transport and magnetic measurements. Hall resistivity measurements show its multiband character with a conduction dominated by electron-like charge carriers. The first-principles calculations exhibit that the semiconducting parent phase Bi₅O₄S₃Cl becomes metallic when sulfur vacancies are introduced, which hints the origin of superconductivity in Bi₅O₄S₃Cl. The findings will inspire the exploration of new BiS-based superconductors.

For single-layer 1T NbSe₂ islands with a few nanometers, the charge density wave array can change from a triangular lattice to a local square-like packing, with the robust quantum unit—Star-of-David cluster. Besides, the electronic states keep largely the same.

Abstract

Charge density wave (CDW) is a typical collective phenomenon, and the phase change is generally accompanied by electronic transition with potential device applications. For the continuous miniaturization of devices, it is important to investigate the size effect down to the nanoscale. In this work, single-layer (SL) 1T-NbSe₂ islands provide an ideal research platform to investigate the size effect on CDW arrangement and electronic states. The CDW motifs (Star-of-David [SOD]) at the island border are along the edge, and those at the interior tend to arrange in a triangular lattice for islands as small as 5 nm. Interestingly, in some small islands, the SOD clusters rearrange into a square-like lattice, and each SOD cluster remains robust as a quantum motif, both in the sense of geometry and electronic structures. Moreover, the electronic structure at the center of the small islands is downwards shifted compared to the big islands, explained by the spatial extension of the band bending originating from the edge of the islands. These findings reveal the robust behavior of CDW motifs down to the nanoscale and provide new insights into the size-limiting effect on 2D2D CDW ordering and electronic states down to a few nanometer extremes.

The recent progress including particle growth, phase transformation, self-assembly, core–shell nanostructure growth, and chemical etching. With the late technical advances in transmission electron microscopy (TEM) and liquid cells, liquid-phase TEM is used to characterize many fundamental processes of metal oxides for CO₂ reduction and water-splitting reactions.

Abstract

Metal oxides with diverse compositions and structures have garnered considerable interest from researchers in various reactions, which benefits from transmission electron microscopy (TEM) in determining their morphologies, phase, structural and chemical information. Recent breakthroughs have made liquid-phase TEM a promising imaging platform for tracking the dynamic structure, morphology, and composition evolution of metal oxides in solution under work conditions. Herein, this review introduces the recent advances in liquid cells, especially closed liquid cell chips. Subsequently, the recent progress including particle growth, phase transformation, self-assembly, core–shell nanostructure growth, and chemical etching are introduced. With the late technical advances in TEM and liquid cells, liquid-phase TEM is used to characterize many fundamental processes of metal oxides for CO₂ reduction and water-splitting reactions. Finally, the outlook and challenges in this research field are discussed. It is believed this compilation inspires and stimulates more efforts in developing and utilizing in situ liquid-phase TEM for metal oxides at the atomic scale for different applications.

Nonlinear multi-quantum-well-based metasurfaces for frequency upconversion are enhanced by taking advantage of a pumping scheme coherently coupled with unpopulated upper electron subbands. This pumping scheme, combined with optimized material and metasurface designs, avoids saturation at practical levels of continuous-wave pump intensities, yielding larger upconversion efficiencies than in conventional approaches.

Abstract

Engineered intersubband transitions in semiconductor heterostructures featuring multiple quantum wells (MQWs) are shown to support record-high second-order nonlinear susceptibilities. By integrating these materials in metasurfaces with tailored optical resonances, it is possible to further enhance photonic interactions, yielding giant nonlinear responses in ultrathin devices. These metasurfaces form a promising platform for efficient nonlinear processes, including frequency upconversion of low-intensity thermal infrared radiation and harmonic generation, free of phase-matching constraints intrinsic to bulk nonlinear crystals. However, nonlinear saturation at moderately large pump intensities due to the transfer of electron population into excited subbands facilitated by strongly enhanced light–matter interactions in metasurfaces fundamentally limits their overall efficiency for various nonlinear processes. Here, the saturation limits of nonlinear MQW-based metasurfaces for mid-infrared frequency upconversion are significantly extended by optimizing their designs for excitation with a strong pump coherently coupled with unpopulated upper electron subbands. This counterintuitive pumping scheme, combined with tailored material and photonic engineering of the metasurface, avoids saturation at practical levels of continuous-wave pump intensities, yielding significantly larger upconversion efficiencies than in conventional approaches. The present results open new opportunities for nonlinear metasurfaces, less limited by saturation mechanisms, with important implications for night-vision imaging and compact nonlinear wave mixing systems.

A dual-phase microstructure consisting of cubic boron nitride (cBN) columns surrounded by a turbostratic boron nitride matrix is formed by adding a certain amount of oxygen to the reactive sputtering gas atmosphere. The preferred orientation of cBN grains changes from <220> to <111> with the formation of the dual phase, which is interpreted as a reduction in strain energy induced by the microstructure change.

The cross-sectional and planar microstructures of a cubic boron nitride (cBN) thin film with a <111>-preferential orientation are observed using transmission electron microscopy. The cBN films are deposited by unbalanced magnetron sputtering under the condition that oxygen is added to a 20 sccm Ar–N₂(25%) gas mixture. Thecross-sectional view of the cBN film deposited with the addition of 0.4 sccm of oxygen shows a dual-phase structure: turbostratic boron nitride (tBN) layers are filled between cBN columns, and the planar view shows that cBN crystals were surrounded by a tBN matrix. The films deposited under less than 0.4 sccm of oxygen addition show a single-phase structure with no tBN layers between the cBN columns. The difference between dual- and single-phase structures is that they have preferred <111> and <220> orientations. This texture variation is interpreted as being due to the low residual stress of the dual-phase-structured cBN film and the low surface energy of the cBN (111) plane. The residual stress of the dual-phase structure is significantly lower than that of the single-phase structure. This is attributed to the compressive residual stress relieved by the tBN layers formed between the cBN columns.

Herein, a high underlayer coverage of sub-millimeter bilayer graphene single crystal is achieved successfully, by modifying the gas flow streamline in the circumfluence chemical vapor deposition, which is attributed to the promotion of carbon sources depositing and absorbing on the copper surface under the vertical streamline.

The growth of large-scale bilayer graphene (bi-graphene) is significantly important for graphene-based device fabrications. Chemical vapor deposition is usually used for the synthesis of high-quality and large-scale thin films including various monolayers and bilayers. However, a major challenge for bi-graphene growth is the so-called limited underlayer coverage, i.e., the faster growth of the top layer than the underlayer. Herein, using the circumfluence chemical vapor deposition, it is demonstrated that the underlayer growth can be greatly enhanced via optimizing the streamline, and high-quality AB-stacking sub-millimeter bi-graphene with underlayer coverage over 93% is achieved successfully. Raman spectroscopy and selected area electron diffraction confirm the high crystalline quality and uniformity of the as-grown bilayers. The as-fabricated field-effect transistor using the bi-graphene as the channel layer exhibits typical semiconductor transfer characteristics and a nonzero bandgap which is required for device applications. It is suggested that the optimized streamline design largely allows a reduction of difference in the edge growth kinetics between the top and bottom layers. Thus, in this work, a promising technical route is presented for the growth of large-scale bi-graphene with high underlayer coverage, beneficial for the development of functional graphene devices.

Nature Electronics, Published online: 24 August 2023; doi:10.1038/s41928-023-01027-6

Two-dimensional materials cool down

Nature Electronics, Published online: 24 August 2023; doi:10.1038/s41928-023-01028-5

Metal halide perovskites are of increasing use in applications beyond conventional photovoltaics, from flexible solar cells for wearable devices to field-effect transistors for unconventional computing.

Nature Reviews Materials, Published online: 24 August 2023; doi:10.1038/s41578-023-00585-7

Two-dimensional materials can enable a new generation of flexible and printed electronics suitable for light-weight, low-power, sustainable and cost-effective field-effect transistors. This Review surveys solution-processed transistors based on 2D materials, discussing their performance, limitations and future perspectives.

Single-wall carbon nanotubes embedded in BiCuSeO-based materials, as charge channels and phase interfaces, can greatly enhance the carrier mobility while strengthening the phonon scattering, triggering an enhancement in zT as well as a mechanical property. This work proposes an efficient approach for improving the carrier transport in oxide thermoelectric materials and potentially driving the application of BiCuSeO materials.

Abstract

BiCuSeO oxyselenides possess a highlighted thermoelectric performance among oxides, which originates from their intrinsically low thermal conductivity. However, intrinsic factors causing low thermal transport are also detrimental to carrier transport, leading to ultralow carrier mobility and relatively low electrical transport properties. Here, high-conductivity single-wall carbon nanotubes (SWCNTs) are adopted as the charge channels to be embedded in a BiCuSeO-based matrix, providing a transport pathway for charge carriers. The results show that carrier mobility is increased to 188 cm² V⁻¹ s⁻¹ due to the SWCNTs composited, triggering an enhancement in electrical transport properties. Besides, the SWCNTs embedded in the matrix introduce abundant interfaces, suppressing phonon transport and depressing lattice thermal conductivity. With these achievements, a maximum zT of 0.84 at 818 K is realized in the composite with 0.1 wt% SWCNTs. The mechanical property of the composites is strengthened as well because of the SWCNTs. The work indicates that the SWCNTs, as the charge channels, propose an effective approach for enhancing carrier mobility in BiCuSeO-based materials, finally optimizing the thermoelectric performance as well as the mechanical property.

By constructing a planar field emission (FE)-type photodetector of monolayer WS₂ with microtips, high photoresponsivity and fast response time can be achieved at the same time. This novel planar FE device may shed new light on the fabrication of high-performance 2D material-based photodetectors.

Abstract

Monolayer tungsten disulfide (ML WS₂) is believed as an ideal photosensitive material due to its small direct bandgap, large exciton/trion binding energy, high carrier mobility, and considerable quantum conversion efficiency. Compared with other photosensitive devices, planar field emission (FE)-type photodetectors with a full-plane structure should simultaneously have rapider switching speed and lower power consumption. In this work, ML WS₂ microtips are fabricated by electron beam lithography (EBL) way and used to construct a planar FE-type photodetector. By optimization design, ML WS₂ with three microtips can exhibit the maximum current density as high as  52 A cm⁻² (@300 V µm⁻¹), and the largest photoresponsivity is up to 6.8 × 10⁵ A W⁻¹ under green light irradiation, superior to that of many other ML transition metal dichalcogenide (TMDC) detectors. More interestingly, ML WS₂ devices with microtips can effectively solve the contradictory problem between large photoresponsivity and rapid switching speed. The excellent photoresponse performances of ML WS₂ with microtips should be attributed to their high carrier mobility, sharp emission edge, ultrahigh quantum yield, and unique planar FE device structure. Our research may shed new light on exploring the fabrication technology and photosensitive mechanism of two dimensional (2D) material-based planar FE photodetectors.

GaN-based UV photodetectors (PDs) are a prominent research focus due to their low voltage, durability, and energy efficiency. This study introduces innovative methods to enhance photoconductive gain in GaN-based UV PDs. Approaches include optimizing microwire structure, analyzing strain behavior, achieving high sensitivity, and introducing a trap-assisted energy mechanism. This research shapes future UV PDs with advanced GaN-based materials.

Abstract

GaN-based materials are the hottest research topic in UV photodetectors (PDs) because of their low operating voltage, small volume, long lifetime, high-temperature resistance, and low energy consumption. However, there are still fundamental issues to be overcome, and the most important issue is to get a photoconductive gain. In this paper, the following new approaches are provided to innovatively improve the photoconductive gain of UV PDs in GaN-based materials. First, the aspect ratio of the 1D GaN microwire (MW) structure is dramatically improved by analyzing the pulse growth mechanism using the metal-organic vapor deposition system. Second, the comprehensive strain behavior in the MW epitaxial growth system is successfully analyzed. Third, the fabricated metal-semiconductor-metal-based MW UV PD shows photoresponsivity and sensitivity of 28.365 A W⁻¹ and 93.16%, respectively, at the −2 V bias, which significantly outperforms the conventional structures in the UV region. Finally, a trap-assisted Poole–Frenkel effect-based energy bandgap mechanism, that allows the defect level formed by lattice mismatch between the substrate and GaN to be used as an electron carrier path, is newly defined. This study will present the direction of future UV PDs by providing a new MW structure based on GaN materials, a third-generation semiconductor.

A quasi-2D electron gas of charge carriers is realized in a bulk 3D oxide perovskite. Furthermore, these carriers have magnetically controllable topological spin textures and are of a single orbital character, making it an ideal platform for spintronic devices.

Abstract

2D phases of matter have become a new paradigm in condensed matter physics, bringing in an abundance of novel quantum phenomena with promising device applications. However, realizing such quantum phases has its own challenges, stimulating research into non-traditional methods to create them. One such attempt is presented here, where the intrinsic crystal anisotropy in a “fractional” perovskite, Eu _x TaO₃ (x = 1/3 − 1/2), leads to the formation of stacked layers of quasi-2D electron gases, despite being a 3D bulk system. These carriers possess topologically non-trivial spin textures, indirectly controlled by an external magnetic field via proximity effect, making it an ideal system for spintronics, for which several possible applications are proposed. An anomalous Hall effect with a non-monotonic dependence on carrier density is shown to exist, signifying a shift in band topology with carrier doping. Furthermore, quantum oscillations in charge conductivity and oscillating thermoelectric properties are examined and proposed as routes to experimentally demonstrate the quasi-2D behavior.