Jiuxiang Dai

Nature Electronics, Published online: 25 November 2021; doi:10.1038/s41928-021-00682-x

Intrinsically honeycomb-patterned hydrogenated graphene (HPHG) with millimeter-scale has been successfully synthesized by an epitaxial method and the growth mechanism has been revealed by density-functional-theory (DFT) calculations. The growth mechanism is that the intercalated H layer serves as a template for the double-sided hydrogenation of the graphene layer. DFT calculations further reveal that monolayer HPHG is an antiferromagnetic semiconductor.

Abstract

Since the advent of graphene ushered the era of 2D materials, many forms of hydrogenated graphene have been reported, exhibiting diverse properties ranging from a tunable bandgap to ferromagnetic ordering. Patterned hydrogenated graphene with micron-scale patterns has been fabricated by lithographic means. Here, successful millimeter-scale synthesis of an intrinsically honeycomb-patterned form of hydrogenated graphene on Ru(0001) by epitaxial growth followed by hydrogenation is reported. Combining scanning tunneling microscopy observations with density-functional-theory (DFT) calculations, it is revealed that an atomic-hydrogen layer intercalates between graphene and Ru(0001). The result is a hydrogen honeycomb structure that serves as a template for the final hydrogenation, which converts the graphene into graphane only over the template, yielding honeycomb-patterned hydrogenated graphene (HPHG). In effect, HPHG is a form of patterned graphane. DFT calculations find that the unhydrogenated graphene regions embedded in the patterned graphane exhibit spin-polarized edge states. This type of growth mechanism provides a new pathway for the fabrication of intrinsically patterned graphene-based materials.

2D arsenene and arsenic materials are an essential member of pnictogens. To accommodate the rapid progress of 2D arsenene and arsenic materials, its recent advances from perspectives of fundamental properties, preparation, and applications are reviewed. Finally, current challenges are summarized and a perspective on the further development of 2D arsenene and arsenic materials are made.

Abstract

As emerging 2D materials, arsenene and arsenic materials have attracted rising interest in the past few years. The diverse crystalline phases, exotic electrical characteristics, and widespread applications of 2D arsenene and arsenic bring them great research value and utilization potential. Herein, the recent progress of 2D arsenene and arsenic is reviewed in terms of fundamental properties, preparation, and applications. The fundamental properties of 2D arsenene and arsenic, including the crystal phases, environmental stability, and electrical structure, from theoretical to experimental reports are first summarized. Then, the experimental processes for preparing 2D arsenene and arsenic, along with their respective advantages and disadvantages, are introduced including epitaxial growth, mechanical exfoliation, and liquid-phase exfoliation. Moreover, applications of 2D arsenene and arsenic are discussed, suggesting a wide range of applications of 2D arsenene and arsenic in field-effect transistors, sensors, catalysts, biological applications, and so on. Finally, some perspectives about the challenges and opportunities of promising 2D arsenene and arsenic are provided. This review provides a helpful guidance and stimulates more focus on future explorations and developments of 2D arsenene and arsenic.

Nature Electronics, Published online: 30 November 2021; doi:10.1038/s41928-021-00690-x

Nature Nanotechnology, Published online: 29 November 2021; doi:10.1038/s41565-021-00994-1

npj 2D Materials and Applications, Published online: 29 November 2021; doi:10.1038/s41699-021-00271-8

Nature Reviews Materials, Published online: 26 November 2021; doi:10.1038/s41578-021-00395-9

Unexpected structural and magnetic states transitions in Cr₂O_{3−1.5 δ} N _δ oxynitride films are observed upon nitrogen doping. Substantially, a large and controllable exchange bias is achieved at the synthetic chromia heterointerfaces, demonstrating the advanced anion engineering in oxides exhibits a great potential in spintronic applications.

Abstract

Control of magnetic states by external factors has garnered a mainstream status in spintronic research for designing low power consumption and fast-response information storage and processing devices. Previously, magnetic-cation substitution was the conventional approach to induce ferromagnetism in an intrinsic antiferromagnet. Theoretically, anion doping is proposed to be another means to change magnetic ground states. Here, the authors demonstrate the synthesis of high-quality single-phase chromium oxynitride thin films using in-situ nitrogen doping. Unlike antiferromagnetic monoanionic chromium oxide and nitride phases, chromium oxynitride exhibits a robust ferromagnetic and insulating state, as demonstrated by the combination of multiple magnetization probes and theoretical calculations. With increasing the nitrogen content, the crystal structure of chromium oxynitride transits from trigonal (R3¯c) to tetragonal (4 mm) phase and its saturation magnetization reduces significantly. Furthermore, they achieve a large and controllable exchange bias field in the chromia heterostructures by synthetic anion engineering. This work reflects the anion engineering in functional oxides towards potential applications in giant magnetoresistance and tunnelling junctions of modern magnetic sensors and read heads.

Ultrathin indium phosphorus sulfide (In₂P₃S₉) nanosheets are obtained utilizing a space-confined epitaxy growth approach, which exhibits a high dielectric constant (≈24) and large breakdown voltage (≈8.1 MV cm⁻¹) at room temperature. The excellent insulating properties make it suitable for integration into 2D field-effect transistors as dielectric layer, which can provide an efficient modulation for the carrier density in a semiconducting channel.

Abstract

2D van der Waals (vdW) semiconductors hold great potentials for more-than-Moore field-effect transistors (FETs), and the efficient utilization of their theoretical performance requires compatible high-k dielectrics to guarantee the high gate coupling efficiency. The deposition of traditional high-k dielectric oxide films on 2D materials usually generates interface concerns, thereby causing the carrier scattering and degeneration of device performance. Here, utilizing a space-confined epitaxy growth approach, the authors successfully obtained air-stable ultrathin indium phosphorus sulfide (In₂P₃S₉) nanosheets, the thickness of which can be scaled down to monolayer limit (≈0.69 nm) due to its layered structure. 2D In₂P₃S₉ exhibits excellent insulating properties, with a high dielectric constant (≈24) and large breakdown voltage (≈8.1 MV cm⁻¹) at room temperature. Serving as gate insulator, ultrathin In₂P₃S₉ nanosheet can be integrated into MoS₂ FETs with high-quality dielectric/semiconductor interface, thus providing a competitive electrical performance of device with subthreshold swings (SS) down to 88 mV dec⁻¹ and a high ON/OFF ratio of 10⁵. This study proves an important strategy to prepare 2D vdW high-k dielectrics, and greatly facilitates the ongoing research of 2D materials for functional electronics.

This work presents the direct synthesis of step-like Cr₂S₃ lateral homojunctions via a one-step chemical vapor deposition (CVD) route. The CVD derived Cr₂S₃ lateral homojunctions with thickness gradients allow the integration of diverse conduction polarities. Intriguingly, it is revealed that, the step-like Cr₂S₃ homojunctions can be utilized to significantly ameliorate on-state current density and field-effect mobility of 2D Cr₂S₃ flakes.

Abstract

For expanding the applications of 2D transition metal dichalcogenides (TMDCs), integrating functional devices with diverse conduction polarities in the same parent material is a very promising direction. Improving the contact issue at the metal-semiconductor interface also holds fundamental significance. To achieve these concurrently, step-like Cr₂S₃ vertical stacks with varied thicknesses are achieved via a one-step chemical vapor deposition (CVD) method route. Various types of 2D Cr₂S₃ lateral homojunctions are thus naturally evolved, that is, p_m -ambipolar/n, p/ambipolar, ambipolar/n, and n_m -ambipolar/n junctions, allowing the integration of diverse conduction polarities in single Cr₂S₃ homojunctions. Significantly, on-state current density and field-effect mobility of the thinner 2D Cr₂S₃ flakes stacked below are detected to be ≈5 and ≈6 times increased in the lateral homojunctions, respectively. This work should hereby provide insights for designing 2D functional devices with simpler structures, for example, multipolar field-effect transistors, photodetectors, and inverters, and provide fundamental references for optimizing the electrical performances of 2D materials related devices.

MXene (Ti₃C₂T _x ), a 2D material with high conductivity, mild reflectivity, flexible flake design, and configurable work function, is used as a novel type of back contact material in Sb₂(S,Se)₃ planar solar cells for the first time. Thus, a noble metal and/or hole transport layer-free antimony-based device with a high efficiency of 8.29% is presented.

Abstract

MXene, a class of 2D materials of metal carbide or nitride, has attracted a lot of attention recently due to its excellent optical and electrical properties. In this work, titanium-carbide MXene (Ti₃C₂T _x ) is introduced as a back electrode in Sb₂(S,Se)₃ thin-film solar cells (FTO/CdS/Sb₂(S,Se)₃/MXene) for the first time, which displaces traditional carbon (C) and gold (Au) electrodes entirely. Impressively, thanks to its high conductivity, mild reflectivity, and flexible flake architecture, the MXene-based device performance outperforms typical C and Au electrodes by 153% and 77%, respectively. Specifically, the tunable work function of MXene and a beneficial Sb–O bond formed between Sb₂(S,Se)₃ and MXene efficiently suppress the recombination and enhance charge transport by enjoying the unique merit of the rich terminal groups of MXene. As a result, the best efficiency of 8.29% of MXene-based Sb₂(S,Se)₃ solar device is achieved, which represents the highest performance of noble metal and/or hole transport layer-free derived Sb₂(S,Se)₃ solar cells to date. This result has revealed that MXene is a feasible material to substitute the back electrode in Sb-based solar cells to reach high efficiency, low cost, and high stability.

Nature Electronics, Published online: 25 November 2021; doi:10.1038/s41928-021-00670-1

An anomalous superconducting proximity effect (SPE) is unveiled in topological insulator (TI)/superconductor Bi₂Se₃/FeSe_0.5Te_0.5 heterostructures. In contrast with the normal SPE that decays with thickness, such an effect occurs in momentum space regardless of the TI film thickness, as long as the topologically protected surface states are robust and form a continuous conduction loop.

Abstract

The superconducting proximity effect (SPE) induces a superconductivity transition in otherwise non-superconducting thin films in proximity with a superconductor. The SPE usually occurs in real space and decays exponentially with film thickness. Herein, an abnormal SPE in a topological insulator (TI)/superconductor heterostructure is unveiled, which is attributed to the topologically protected surface state. Surprisingly, such abnormal SPE occurs in momentum space regardless of the TI film thickness, as long as the topological surface states are robust and form a continuous conduction loop. Combining transport measurements and scanning tunneling microscopy/spectroscopy techniques, the SPE in Bi₂Se₃/FeSe_0.5Te_0.5 heterostructures is explored, where Bi₂Se₃ is an ideal 3D topological insulator and FeSe_0.5Te_0.5 a typical iron-based superconductor. As the thickness of the Bi₂Se₃ thin film exceeds 400 nm, there still exists SPE-induced superconductivity on the surface of Bi₂Se₃ thin film with a transition temperature T _c not less than 10 K. Such an extraordinary behavior is induced by the unique properties of topologically protected surface states of Bi₂Se₃. This research deepens the understanding of the important role of topologically protected surface states in the SPE.

A dual-function, flexible, free-standing framework coupling catalytic and lithiophilic 1T′-MoTe₂ nanosheets with carbon nanotubes as an advanced host for both sulfur cathodes and lithium-metal anodes is presented. A full cell with a low negative to positive electrode capacity ratio of only ≈2.5 displays stable cycling over 500 cycles, culminating in a high areal capacity of 7.6 mA h cm⁻².

Abstract

Lithium–sulfur batteries offer the advantage of high energy density at a low cost, but their viability is hindered by the polysulfide shuttle effect, sluggish reaction kinetics, and dendritic Li growth. To address these persistent challenges in a unified manner, a dual-function, flexible, free-standing framework by coupling catalytic and lithiophilic 1T′-MoTe₂ nanosheets with conductive carbon nanotubes (MoTe₂-CNT), which serve as a host for both a sulfur cathode (S/MoTe₂-CNT) and a lithium-metal anode (MoTe₂-CNT/Li) is presented here. MoTe₂-CNT not only guides a uniform growth of lithium within the framework, but also forms a thin, unique sulfide-rich solid-electrolyte interphase (SEI) composed of lithium thiotellurate on the Li surface when paired with a sulfur cathode. This SEI stabilizes Li deposition, suppresses electrolyte decomposition, and prevents Li loss, thereby prolonging cycle life. Full coin cells with a very low negative to positive electrode capacity ratio of ≈2.5 and a high areal capacity of 7.6 mA h cm⁻² display 75% capacity retention after 500 cycles. The pouch cells fabricated with MoTe₂-CNT deliver a high capacity of 1533 mA h g⁻¹ and energy density of 319 Wh kg⁻¹ at a low electrolyte-to-capacity ratio of ≈2.9 µL [mA h]⁻¹ and a low electrolyte-to-sulfur ratio of 4.5 µL mg⁻¹.

A molybdenum disulfide/germanium junction field-effect transistor (JFET) is proposed, which has a low subthreshold swing of ≈88 mV/dec and a high on/off ratio of ≈10⁵. Bidirectional photocurrent is obtained under visible and infrared light of 532 and 1550 nm, respectively. In addition, three controllable current states (−1, 0, and 1) for logic operations are realized based on this JFET device.

Abstract

There has been a growing interest in electronic and optoelectronic devices based on heterostructures between atomically thin 2D and 3D semiconductor materials. This paper proposes a 2D molybdenum disulfide (MoS₂)/3D germanium (Ge) junction field-effect transistor (JFET). Typical electrical characteristics of the JFET are observed, with a low subthreshold swing of ≈88 mV/dec and a high on/off ratio of ≈10⁵. The device exhibits a bidirection photoresponse in which the photocurrent polarity is reversed depending on the wavelength of light. Under visible illumination at 532 nm, the positive photoresponsivity of this device can be modulated by the gate voltage, reaching a peak value of 66 A W⁻¹. In contrast, the device exhibits a tunable negative photoresponse behavior under an infrared illumination of 1550 nm. This is attributed to the competition between the negative photoresponse from the bolometric effect in MoS₂ and the positive photoresponse from photogenerated carriers in Ge. Based on these interesting characteristics in this JFET, three controllable current states (−1, 0, and 1) are realized by changing the gate voltage and infrared light. These results indicate that the device has promising potential as a multifunctional optoelectronic unit, including signal amplification, broadband photodetection, and multilogic calculations.

The chemical interface confinement reduction method is proposed to produce the model catalysts of Bi (001) nanosheets via topotactic transformation of BiOBr (001) nanosheets for CO₂ electroreduction at a scalable large-scale. The formate Faradaic efficiency of 95.2% is achieved on Bi (001) nanosheets due to the small-angle grain boundaries that can significantly lower the free energy barrier for the formation of *OCHO.

Abstract

Electrochemical reduction of carbon dioxide (CO₂) toward chemical and fuel production is a compelling component of the new energy system. Two-dimensional bismuth with a particular surface has been identified as a highly efficient electrocatalyst for converting CO₂ to formate. However, the development of a controllable synthetic strategy for possible large-scale production of such Bi materials remains highly challenging. Herein, a scalable chemical interface confinement reduction method is proposed for topotactic transformation of BiOBr (001) nanosheets to metallic Bi (001) porous nanosheets (PNS). As expected, the Bi (001) PNS exhibits excellent electrochemical performance on CO₂ reduction to formate, with Faradaic efficiency of 95.2% and formate partial current density of 72 mA cm⁻². Density functional theory calculations suggest that Bi PNS selectively exposes (001) surfaces with small-angle grain boundaries can significantly lower the free energy barrier for the formation of *OCHO, which are responsible for the high activity and selectivity toward CO₂-to-formate conversion.

Publication date: January–February 2022

Source: Materials Today, Volume 52

Author(s): Seung Ju Kim, Tae Hyung Lee, June-Mo Yang, Jin Wook Yang, Yoon Jung Lee, Min-Ju Choi, Sol A Lee, Jun Min Suh, Kyung Ju Kwak, Ji Hyun Baek, In Hyuk Im, Da Eun Lee, Jae Young Kim, Jaehyun Kim, Ji Su Han, Soo Young Kim, Donghwa Lee, Nam-Gyu Park, Ho Won Jang

Nature, Published online: 24 November 2021; doi:10.1038/s41586-021-04021-0

Nature, Published online: 24 November 2021; doi:10.1038/d41586-021-02957-x