zemin zheng

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

Transistors made from two-dimensional materials have been around for a decade, but do the devices have a realistic future in integrated circuits?

Nature Nanotechnology, Published online: 29 November 2021; doi:10.1038/s41565-021-01014-y

In moiré superlattice van der Waals magnetic materials, competing interactions emerge and can stabilize new magnetic states. Here, stacking-dependent interlayer exchange interactions in small-twist-angle CrI3 bilayers yield an ordered ground state with coexisting ferromagnetic and antiferromagnetic regions.

Both n- and p-channel multivalue field-effect transistors (FETs) are fabricated using p-MoTe₂/n-MoS₂ heterostack channel architecture, where either p- or n-channel ternary value FET is reproducible by switching the stacking order. For a state-of-the-art device application, a quaternary NAND logic circuit is for the first time demonstrated by integrating two ternary n-channel FETs, and a complementary ternary inverter is also fabricated.

Abstract

Applications of 2D semiconductors have been extensively studied, much oriented to various electron devices. Recently, multivalue field-effect transistors (FETs) are also included among 2D-based electron device studies in consideration that multivalue FETs may resolve power consumption issues in future integrated circuits. Several n-channel devices are thus reported along with a few p-channel devices, while studies to achieve both n- and p-channel multivalue FETs are hardly found. Here, both n- and p-channel multivalue FETs are fabricated using p-MoTe₂/n-MoS₂ heterostack channel architecture, where either p- or n-channel ternary value FET is reproducible by switching the stacking order of p- and n-channel layer. The main ternary value mechanism originates from resonant tunneling type injection and channel inversion, which take place during device operation. For a state-of-the-art device application in 2D electronics, a quaternary NAND logic circuit is for the first time demonstrated by integrating two ternary n-channel FETs, and a complementary ternary inverter is also fabricated by integrating multivalue p-channel and plain n-channel FET.

This review briefly introduces preparation methods, oxidation mechanisms, a comprehensive summarization of factors affecting oxidation and a detailed account of strategies to mitigate MXenes degeneration. It furthermore offers an overview of in situ synthesis MXene derivatives with enhanced performance, and presents perspectives for the challenges and opportunities for the protection and application of MXenes and their derivatives.

Abstract

As an emerging star of 2D nanomaterials, 2D transition metal carbides and nitrides, named MXenes, present a large potential in various research areas owing to their intrinsic multilayer structure and intriguing physico-chemical properties. However, the fabrication and application of functional MXene-based devices still remain challenging as they are prone to oxidative degradation under ambient environment. Within this review, the preparation methods of MXenes focusing on the recent investigations on their thermal structure–stability relationships in inert, oxidizing, and aqueous environments are systematically introduced. Moreover, the key factors that affect the oxidation of MXenes, such as, atmosphere, temperature, composition, microstructure, and aqueous environment, are reviewed. Based on different scenarios, strategies for avoiding or delaying the oxidation of MXenes are proposed to encourage the utilization of MXenes in complicated environments, especially at high temperature. Furthermore, the chemistry of MXene-derived oxides is analyzed, which can offer perspectives on the further design and fabrication of novel 2D composites with the unique structures of MXenes being preserved.

Room-temperature 2D ferromagnetism is successfully realized in topological-insulator-induced van der Waals Fe₃GeTe₂, which is evidenced by ultrafast THz emission spectroscopy. Furthermore, the direction of the external magnetic field is rotated and the sample's front surface is reversed to probe the THz radiation symmetry. The results reveal that the spin-to-charge conversion effect is the dominant THz radiation mechanism.

Abstract

Future information technologies for low-dissipation quantum computation, high-speed storage, and on-chip communication applications require the development of atomically thin, ultracompact, and ultrafast spintronic devices in which information is encoded, stored, and processed using electron spin. Exploring low-dimensional magnetic materials, designing novel heterostructures, and generating and controlling ultrafast electron spin in 2D magnetism at room temperature, preferably in the unprecedented terahertz (THz) regime, is in high demand. Using THz emission spectroscopy driven by femtosecond laser pulses, optical THz spin-current bursts at room temperature in the 2D van der Waals ferromagnetic Fe₃GeTe₂ (FGT) integrated with Bi₂Te₃ as a topological insulator are successfully realized. The symmetry of the THz radiation is effectively controlled by the optical pumping incidence and external magnetic field directions, indicating that the THz generation mechanism is the inverse Edelstein effect contributed spin-to-charge conversion. Thickness-, temperature-, and structure-dependent nontrivial THz transients reveal that topology-enhanced interlayer exchange coupling increases the FGT Curie temperature to room temperature, which provides an effective approach for engineering THz spin-current pulses. These results contribute to the goal of all-optical generation, manipulation, and detection of ultrafast THz spin currents in room-temperature 2D magnetism, accelerating the development of atomically thin high-speed spintronic devices.

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.

Nature Nanotechnology, Published online: 29 November 2021; doi:10.1038/s41565-021-01014-y

In moiré superlattice van der Waals magnetic materials, competing interactions emerge and can stabilize new magnetic states. Here, stacking-dependent interlayer exchange interactions in small-twist-angle CrI3 bilayers yield an ordered ground state with coexisting ferromagnetic and antiferromagnetic regions.

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

A study demonstrates the synthesis and characterization of a two-dimensional van der Waals heterostructure hosting artificial heavy fermions, providing a tunable platform for investigations of heavy-fermion physics.

Nature Communications, Published online: 25 November 2021; doi:10.1038/s41467-021-27218-3

Two-dimensional magnetic semiconductors hold promise for spin- and valleytronic applications. Here, the authors report the realization of light helicity detectors based on graphene/CrI3 van der Waals heterostructures, exhibiting a photocurrent behaviour determined by the magnetic state of CrI3.

Superficial Se modification makes Co₄N nanowire arrays a more effective catalyst. Surface modification of Se on Co₄N can effectively optimize the surface electronic structure with no significant change of bulk phase structure, leading to more efficient hydrogen evolution catalytic activity in alkaline media.

Abstract

Nonmetal heteroatom incorporation into the lattice of host materials is a common way to regulate the surface electronic structure of electrocatalysts to boost their electrocatalytic performance. However, the heteroatom incorporation will inevitably trigger lattice strain and vacancy, which may lead to large changes in the structure of host materials. In this situation, the reconstruction of doped catalysts easily occurs during the catalytic process under harsh alkaline media, which hinders understanding the structure–activity correlation between catalysts and catalytic performance. Herein, taking cobalt nitride as an example, it is promulgated that a low-temperature selenium sublimation strategy can effectively optimize the surface electronic structure in Co₄N nanowire arrays with no significant change of bulk phase structure. Benefiting from surface selenium modification, the optimized SeCo₄N nanowire arrays exhibit a 6.5 times enhancement of catalytic activity with structural phase stability for hydrogen evolution reaction in basic media. These findings may provide a new concept to design stable structured catalysts for energy-related applications.

Nature Nanotechnology, Published online: 18 November 2021; doi:10.1038/s41565-021-01020-0

Dense, short hydrophobic nanochannels have been restacked from two-dimensional quantum sheets to achieve both high areal and volumetric capacitance in thick electrodes under ultrahigh rates.

Nature Communications, Published online: 18 November 2021; doi:10.1038/s41467-021-27041-w

The interfaces between ferromagnets and superconductors receive many attentions due to emergent relativistic spin-orbit coupling. Here, the authors provide possible evidence for spin triplet Andreev reflection at the interface between a van der Waals ferromagnet Fe0.29TaS2 and a s-wave superconductor NbN.

Nature Communications, Published online: 19 November 2021; doi:10.1038/s41467-021-27033-w

Single-photon emitters in 2D semiconductors hold promise for quantum applications, but usually operate in the 500-800 nm wavelength range. Here, the authors report site-controlled creation of quantum emitters in the telecommunication wavelength window by coupling 2D MoTe2 to strain inducing nano-pillar arrays.