Jiuxiang Dai

p-Type NbS₂–MoS₂ lateral heterostructures with a high on/off current ratio are synthesized by a one-step metal–organic chemical vapor deposition (MOCVD) method with Nb dopants present in the monolayer MoS₂. The heterostructure provides a platform to explore the instructive interface of substitutional doped TMDC materials and 2D metal–semiconductor heterojunctions, which paves a prospective way to designing innovative nanoscale devices and complementary metal–oxide–semiconductor (CMOS)-like 2D circuits.

Abstract

Monolayer MoS₂ has attracted significant attention owing to its excellent performance as an n-type semiconductor from the transition metal dichalcogenide (TMDC) family. It is however strongly desired to develop controllable synthesis methods for 2D p-type MoS₂, which is crucial for complementary logic applications but remains difficult. In this work, high-quality NbS₂–MoS₂ lateral heterostructures are synthesized by one-step metal–organic chemical vapor deposition (MOCVD) together with monolayer MoS₂ substitutionally doped by Nb, resulting in a p-type doped behavior. The heterojunction shows a p-type transfer characteristic with a high on/off current ratio of ≈10⁴, exceeding previously reported values. The band structure through the NbS₂–MoS₂ heterojunction is investigated by density functional theory (DFT) and quantum transport simulations. This work provides a scalable approach to synthesize substitutionally doped TMDC materials and provides an insight into the interface between 2D metals and semiconductors in lateral heterostructures, which is imperative for the development of next-generation nanoelectronics and highly integrated devices.

Nature Communications, Published online: 16 January 2023; doi:10.1038/s41467-022-35569-8

Top-down exfoliation is one of the most promising approaches for the scalable production of 2D materials, but the current techniques are limited by low yield of monolayers. Here, the authors report the exfoliation of graphene and other layered materials via viscous-polymer-assisted ball-milling, leading to a production of graphene products with monolayer percentage up to 97.9% at a yield of 78.3%.

Nature Materials, Published online: 16 January 2023; doi:10.1038/s41563-022-01458-0

The authors present evidence suggesting that amorphous Bi2Se3 displays topological properties, signalling a new regime for the pursuit of topological matter.

Nature Physics, Published online: 16 January 2023; doi:10.1038/s41567-022-01894-4

Superconductivity can emerge from a strange-metal state, but the exact relationship between them is unknown. Now, quantitative measurements reveal the dependence of resistivity in the strange metal on the superconducting transition temperature.

Nature Synthesis, Published online: 16 January 2023; doi:10.1038/s44160-022-00220-3

Nanoscale heterostructures are promising for applications in energy and information conversion. Now, a competitive ion-exchange method in which multiple ions diffuse in and out of colloidal nanocrystals provides a route to rapidly synthesize heterostructures on the nanoscale.

The room-temperature operation of spin-valve devices using the van der Waals itinerant ferromagnet Fe₅GeTe₂ in heterostructures with graphene is demonstrated. Lateral spin-valve, Hanle spin precession measurements, and theoretical calculations provide unique insights by probing the Fe₅GeTe₂/graphene interface spintronic properties via spin-dynamics measurements, revealing multidirectional spin polarization.

Abstract

The discovery of van der Waals (vdW) magnets opened a new paradigm for condensed matter physics and spintronic technologies. However, the operations of active spintronic devices with vdW ferromagnets are limited to cryogenic temperatures, inhibiting their broader practical applications. Here, the robust room-temperature operation of lateral spin-valve devices using the vdW itinerant ferromagnet Fe₅GeTe₂ in heterostructures with graphene is demonstrated. The room-temperature spintronic properties of Fe₅GeTe₂ are measured at the interface with graphene with a negative spin polarization. Lateral spin-valve and spin-precession measurements provide unique insights by probing the Fe₅GeTe₂/graphene interface spintronic properties via spin-dynamics measurements, revealing multidirectional spin polarization. Density functional theory calculations in conjunction with Monte Carlo simulations reveal significantly canted Fe magnetic moments in Fe₅GeTe₂ along with the presence of negative spin polarization at the Fe₅GeTe₂/graphene interface. These findings open opportunities for vdW interface design and applications of vdW-magnet-based spintronic devices at ambient temperatures.

In this article, the manipulation of second-order corner states in 2D multiferroics of SbAs and BP₅ monolayers is identified. The charge spatial distribution is well located at the corners of nanoflakes for both SbAs and BP₅ when they possess the in-plane polarization and under a ferroelastic switching from the initial to final states, the spatial distribution of the corner states are effectively rotated by 90°.

Abstract

The understanding and manipulate of the second-order corner states are central to both fundamental physics and future topotronics applications. Despite the fact that numerous second-order topological insulators (SOTIs) are achieved, the efficient engineering in a given material remains elusive. Here, the emergence of 2D multiferroics SOTIs in SbAs and BP₅ monolayers is theoretically demonstrated, and an efficient and straightforward way for engineering the nontrivial corner states by ferroelasticity and ferroelectricity is remarkably proposed. With ferroelectric polarization of SbAs and BP₅ monolayers, the nontrivial corner states emerge in the mirror symmetric corners and are perpendicular to orientations of the in-plane spontaneous polarization. And remarkably the spatial distribution of the corner states can be effectively tuned by a ferroelastic switching. At the intermediate states of both ferroelectric and ferroelastic switchings, the corner states disappear. These finding not only combines exotic SOTIs with multiferroics but also pave the way for experimental discovery of 2D tunable SOTIs.

A terminal group-oriented self-assembly strategy is developed to controllably synthesize a homogeneous layer-by-layer SnSe₂ and MXene heterostructure, which possesses ultrastable lithium storage performance, maintaining 410 mAh g⁻¹ at 5 C even after 16 000 cycles.

Abstract

Heterostructured materials integrate the advantages of adjustable electronic structure, fast electron/ions transfer kinetics, and robust architectures, which have attracted considerable interest in the fields of rechargeable batteries, photo/electrocatalysis, and supercapacitors. However, the construction of heterostructures still faces some severe problems, such as inferior random packing of components and serious agglomeration. Herein, a terminal group-oriented self-assembly strategy to controllably synthesize a homogeneous layer-by-layer SnSe₂ and MXene heterostructure (LBL-SnSe₂@MXene) is designed. Benefitting from the abundant polar terminal groups on the MXene surface, Sn²⁺ is induced into the interlayer of MXene with large interlayer spacing, which is selenized in situ to obtain LBL-SnSe₂@MXene. In the heterostructure, SnSe₂ layers and MXene layers are uniformly intercalated in each other, superior to other heterostructures formed by random stacking. As an anode for lithium-ion batteries, the LBL-SnSe₂@MXene is revealed to possess strong lithium adsorption ability, the small activation energy for lithium diffusion, and excellent structure stability, thus achieving outstanding electrochemical performance, especially with high specific capacities (1311 and 839 mAh g⁻¹ for initial discharge and charge respectively) and ultralong cycling stability (410 mAh g⁻¹ at 5C even after 16 000 cycles). This work conveys an inspiration for the controllable design and construction of homogeneous layered heterostructures.

This study reviews the current state of III-nitride synthesis on different 2D materials for a variety of flexible applications, meanwhile identifying key advances in this rapidly accelerating field. The focus of this study is to provide guidelines on the exfoliation of epitaxial layers using 2D materials, with the aim of fabricating flexible nitride devices.

Abstract

Group III-nitrides have attracted significant attention in recent years for their wide tunable band-gaps and excellent optoelectronic capabilities, which are advantageous for several applications including light-emitting diodes, lasers, photodetectors, and large-size low-cost power electronic devices. However, conventional epitaxy accompanied by the covalent bond formation renders the transfer of nitride epilayers difficult, thereby limiting the application potential of nitrides in wearable and flexible electronics. Furthermore, interfacial covalent bonds also limit substrate selection and hinder the development of heterogeneous integration between nitrides and other material systems. 2D materials can mitigate these problems significantly. On the one hand, due to the weak van der Waals forces between the layers of 2D materials, influences of lattice mismatch can be avoided to improve crystal quality. On the other hand, delamination and transfer of nitride epilayers can be achieved easily. Therefore, this study focuses on providing comprehensive guidelines regarding the exfoliation of epitaxial layers using 2D materials to provide new design freedoms for nitride devices. Different 2D buffers and release layers have also been discussed. Furthermore, the limitations, promising solutions, future directions, and applicability of this strategy to flexible nitride devices are presented.

A versatile protocol for the horizontally-oriented growth of submillimeter-long organic crystalline nanowires on flexible polymer films is proposed. Accordingly, a large number of organic nanowires are integrated into flexible photodetectors directly on their growth substrate. This in situ integration strategy enables the nanowire-based flexible photodetectors to maintain a stable and reliable photoresponse in the Vis-NIR region after repeated severe bending.

Abstract

Various epitaxial mechanisms have been proposed to control the growth orientation of vapor-deposited nanowires, yet the required lattice matching between target nanowires and supporting substrates limits their applicability. In this work, a versatile hot stamping protocol for fabricating parallel hydrophobic nanogrooves on flexible polymer films (e.g., polyimide (PI), polyethylene naphthalate (PEN), polydimethylsiloxane (PDMS)) is proposed. More interestingly, various organic small molecules, including several metal phthalocyanines (MPc, M = Cu, Zn, Fe, Ni, Co), 9,10-bis(phenylethynyl)anthracene (BPEA), 9,10-diphenylanthracene (DPA), and tris-(8-hydroxyquinoline)aluminium (Alq₃), are directly assembled into horizontally-oriented nanowires along the hot-stamped nanogrooves on a flexible PI film, thereby breaking the lattice-matching limitation for oriented nanowire growth. These submillimeter-long horizontally oriented nanowires can be integrated into flexible photodetectors directly on their growth film, eliminating the need for laborious post-growth transfer and alignment steps and the associated structural damage and contamination. Consequently, the in situ integrated flexible photodetector made of aligned CuPc nanowires maintains a stable and fast photoresponse to a spectrum in the region of 405-980 nm even when the detector is bent to a radius of curvature of 2.5 mm and 1000 times. This work will open new opportunities to develop in situ integrated flexible devices based on organic crystalline nanowires for practical applications.

Novel ice-aided transfer, ice-stamp transfer, and ice cleaning methods, completely free from polymer and solvents, are developed to yield ultrahigh quality and exceptional cleanliness in 2D materials, which will contribute to technological revolutions in 2D materials, and their associated structures and devices.

Abstract

The scalable 2D device fabrication and integration demand either the large-area synthesis or the post-synthesis transfer of 2D layers. While the direct synthesis of 2D materials on most targeted surfaces remains challenging, the transfer approach from the growth substrate onto the targeted surfaces offers an alternative pathway for applications and integrations. However, the current transfer techniques for 2D materials predominantly involve polymers and organic solvents, which are liable to contaminate or deform the ultrasensitive atomic layers. Here, novel ice-aided transfer and ice-stamp transfer methods are developed, in which water (ice) is the only medium in the entire process. In practice, the adhesion between various 2D materials and ice can be well controlled by temperature. Through such controlled adhesion of ice, it is shown that the new transfer methods can yield ultrahigh quality and exceptional cleanliness in transferred 2D flakes and continuous 2D films, and are applicable for a wide range of substrates. Furthermore, beyond transfer, ice can also be used for cleaning the surfaces of 2D materials at higher temperatures. These novel techniques can enable unprecedented ultraclean 2D materials surfaces and performances, and will contribute to the upcoming technological revolutions associated with 2D materials.

Journal of Applied Physics, Volume 133, Issue 2, January 2023.
Functionalizing graphene beyond its intrinsic properties has been a key concept since the first successful realization of this archetype monolayer system. While various concepts, such as doping, co-doping, and layered device design, have been proposed, the often complex structural and electronic changes are often jeopardizing simple functionalization attempts. Here, we present a thorough analysis of the structural and electronic properties of co-doped graphene via Raman spectroscopy as well as magneto-transport and Hall measurements. The results highlight the challenges in understanding its microscopic properties beyond the simple preparation of such devices. It is discussed how co-doping with N and B dopants leads to effective charge-neutral defects acting as short-range scatterers, while charged defects introduce more long-range scattering centers. Such distinct behavior may obscure or alter the desired structural as well as electronic properties not anticipated initially. Exploring further the preparation of effective pn-junctions, we highlight step by step how the preparation process may lead to alterations in the intrinsic properties of the individual layers. Importantly, it is highlighted in all steps how the inhomogeneities across individual graphene sheets may challenge simple interpretations of individual measurements.

Nature Reviews Chemistry, Published online: 13 January 2023; doi:10.1038/s41570-022-00457-8

Molecular rectifiers are an essential component for the construction of molecular electronic devices, which are becoming potentially competitive with existing silicon-based devices. This Review provides an overview of molecular rectification and discusses the outlook for the field as well as prospects for commercialization.

Semiconductor Devices

In article 2208698, Janio Venturini, Tom Nilges, and co-workers report on the first position- and direction-flexible diode made solely from Ag₁₈Cu₃Te₁₁Cl₃. A temperature gradient of a few degrees around room temperature is sufficient to switch between p- and n-type conduction, controlling the existence, position, and current flow direction in the device.

Through direct and automated control over the electron beam scan pathway profile in an aberration-corrected scanning transmission electron microscope, metallic 1D–2D edge heterostructures are fabricated with atomic precision in semiconducting MoS₂. This atomic fabrication workflow paves the way for precision controlled atomic fabrication of functional nanomaterials.

Abstract

The ability to deterministically fabricate nanoscale architectures with atomic precision is the central goal of nanotechnology, whereby highly localized changes in the atomic structure can be exploited to control device properties at their fundamental physical limit. Here, an automated, feedback-controlled atomic fabrication method is reported and the formation of 1D–2D heterostructures in MoS₂ is demonstrated through selective transformations along specific crystallographic orientations. The atomic-scale probe of an aberration-corrected scanning transmission electron microscope (STEM) is used, and the shape and symmetry of the scan pathway relative to the sample orientation are controlled. The focused and shaped electron beam is used to reliably create Mo₆S₆ nanowire (MoS-NW) terminated metallic-semiconductor 1D–2D edge structures within a pristine MoS₂ monolayer with atomic precision. From these results, it is found that a triangular beam path aligned along the zig-zag sulfur terminated (ZZS) direction forms stable MoS-NW edge structures with the highest degree of fidelity without resulting in disordering of the surrounding MoS₂ monolayer. Density functional theory (DFT) calculations and ab initio molecular dynamic simulations (AIMD) are used to calculate the energetic barriers for the most stable atomic edge structures and atomic transformation pathways. These discoveries provide an automated method to improve understanding of atomic-scale transformations while opening a pathway toward more precise atomic-scale engineering of materials.

For the development of ferroelectric (FE)-based optoelectronic devices, a better understanding of the fundamental properties of FE materials, quantification of FE polarization, and interface study of heterostructures between FE/low-dimensional (LD) materials are of great importance. This review summarizes recent developments and challenges in FE-enhanced LD optoelectronic devices.

Abstract

Ferroelectric (FE) materials, including BiFeO₃, P(VDF-TrFE), and CuInP₂S₆, are a type of dielectric material with a unique, spontaneous electric polarization that can be reversed by applying an external electric field. The combination of FE and low-dimensional materials produces synergies, sparking significant research interest in solar cells, photodetectors (PDs), nonvolatile memory, and so on. The fundamental aspects of FE materials, including the origin of FE polarization, extrinsic FE materials, and FE polarization quantification are first discussed. Next, the state-of-the-art of FE-based optoelectronic devices is focused. How FE materials affect the energy band of channel materials and how device structures influence PD performance are also summarized. Finally, the future directions of this rapidly growing field are discussed.

Nature Synthesis, Published online: 12 January 2023; doi:10.1038/s44160-022-00210-5

Synthesizing Se-based nanocrystals with large diameters remains challenging. Here, a reactivity-controlled epitaxial growth strategy was demonstrated to synthesize nanocrystals of ZnSe, CdSe and PbSe with average diameters of 35 nm, 76 nm and 87 nm, respectively. The large ZnSe nanocrystals emitted pure blue light, which is important for display technology.

Nature Energy, Published online: 12 January 2023; doi:10.1038/s41560-022-01175-7

Li–S chemistry can provide high-energy-density batteries. Here the authors use lithiated metallic phase 2D materials as a sulfur host for cathodes that leads to high-energy-density Li–S pouch cell batteries.

The uniform Er₂O₃ layer on monolayer MoS₂ with an equivalent oxide thickness of 1.1 nm is achieved by the direct deposition system based on thermal evaporation. The top-gated MoS₂ field-effect transistors (FETs) show a small gate leakage current (<3 µA cm⁻²) and hysteresis (11.4 mV). Furthermore, the interface state density of Er₂O₃/MoS₂ is comparable with that of h-BN/MoS₂ heterostructure.

Abstract

Achieving the direct growth of an ultrathin gate insulator with high uniformity and high quality on monolayer transition metal dichalcogenides (TMDCs) remains a challenge due to the chemically inert surface of TMDCs. Although the main solution for this challenge is utilizing buffer layers before oxide is deposited on the atomic layer, this method drastically degrades the total capacitance of the gate stack. In this work, we constructed a novel direct high-κ Er₂O₃ deposition system based on thermal evaporation in a differential-pressure-type chamber. A uniform Er₂O₃ layer with an equivalent oxide thickness of 1.1 nm was achieved as the gate insulator for top-gated MoS₂ field-effect transistors (FETs). The top gate Er₂O₃ insulator without the buffer layer on MoS₂ exhibited a high dielectric constant that reached 18.0, which is comparable to that of bulk Er₂O₃ and is the highest among thin insulators (< 10 nm) on TMDCs to date. Furthermore, the Er₂O₃/MoS₂ interface (D _it ≈ 6 × 10¹¹ cm⁻² eV⁻¹) is confirmed to be clean and is comparable with that of the h-BN/MoS₂ heterostructure. These results prove that high-quality dielectric properties with retained interface quality can be achieved by this novel deposition technique, facilitating the future development of 2D electronics.