Jiuxiang Dai

Author(s): Shuanhu Wang, Hui Zhang, Jine Zhang, Shuqin Li, Dianbing Luo, Jianyuan Wang, Kexin Jin, and Jirong Sun

Two-dimensional electron gases (2DEGs) at the LaAlO3/SrTiO3 interface have attracted wide interest, and some exotic phenomena are observed, including 2D superconductivity, 2D magnetism, and diverse effects associated with Rashba spin-orbit coupling. Despite the intensive investigations, however, the…

[Phys. Rev. Lett. 128, 187401] Published Wed May 04, 2022

Nature, Published online: 04 May 2022; doi:10.1038/s41586-022-04520-8

Nature, Published online: 04 May 2022; doi:10.1038/s41586-022-04523-5

Nature, Published online: 04 May 2022; doi:10.1038/s41586-022-04514-6

Nature Materials, Published online: 05 May 2022; doi:10.1038/s41563-022-01245-x

Hafnium oxide (HfO₂), a long-standing warrior in the semiconductor battlefield, is the all-purpose oxide. Here, the atomic view of HfO₂ with microscopic properties, followed by the macroscopic electrical features, which further continue to explore the applications of HfO₂ in resistive random access memory, and ferroelectric memory, and finally prospects and challenges are reviewed.

Abstract

Hafnium oxide (HfO₂) is one of the mature high-k dielectrics that has been standing strong in the memory arena over the last two decades. Its dielectric properties have been researched rigorously for the development of flash memory devices. In this review, the application of HfO₂ in two main emerging nonvolatile memory technologies is surveyed, namely resistive random access memory and ferroelectric memory. How the properties of HfO₂ equip the former to achieve superlative performance with high-speed reliable switching, excellent endurance, and retention is discussed. The parameters to control HfO₂ domains are further discussed, which can unleash the ferroelectric properties in memory applications. Finally, the prospect of HfO₂ materials in emerging applications, such as high-density memory and neuromorphic devices are examined, and the various challenges of HfO₂-based resistive random access memory and ferroelectric memory devices are addressed with a future outlook.

Organometal-Halide Perovskites

In article number 2107739, Ángel Barranco, Juan Ramon Sanchez-Valencia, and co-workers report highly aligned organometal-halide perovskite vertical nanowalls using a new vacuum-based approach evolved from the glancing-angle deposition (GLAD) technique. The alignment and separation degree of the supported nanostructures are governed by the angle of deposition, which for glancing angles provides samples with strong optical anisotropic properties such as polarized-photoluminescence or polarization-sensitive photocurrent response.

Slip lines on metal surfaces are found to play a pivotal role in determining the orientation of graphene. The slip-line-guided growth principle is established to explain and predict the orientation of graphene grown on a variety of metal facets, further enabling the controllable synthesis of single-crystal graphene and grain-boundary engineering of bicrystal graphene on designed metal facets.

Abstract

Manipulating the crystal orientation of emerging 2D materials via chemical vapor deposition (CVD) is a key premise for obtaining single-crystalline films and designing specific grain-boundary (GB) structures. Herein, the controllable crystal orientation of graphene during the CVD process is demonstrated on a single-crystal metal surface with preexisting atomic-scale stair steps resulting from dislocation slip lines. The slip-line-guided growth principle is established to explain and predict the crystal orientation distribution of graphene on a variety of metal facets, especially for the multidirectional growth cases on Cu(hk0) and Cu(hkl) substrates. Not only large-area single-crystal graphene, but also bicrystal graphene with controllable GB misorientations, are successfully synthesized by rationally employing tailored metal substrate facets. As a demonstration, bicrystal graphenes with misorientations of ≈21° and ≈11° are constructed on Cu(410) and Cu(430) foils, respectively. This guideline builds a bridge linking the crystal orientation of graphene and the substrate facet, thereby opening a new avenue for constructing bicrystals with the desired GB structures or manipulating 2D superlattice twist angles in a bottom-up manner.

Nature Reviews Materials, Published online: 04 May 2022; doi:10.1038/s41578-022-00440-1

Light: Science & Applications, Published online: 07 May 2022; doi:10.1038/s41377-022-00827-3

1D p-Te/2D n-Bi₂O₂Se heterodiodes with rationally designed nanoscale ultra-photosensitive channels are fabricated via dangling bond-free mixed-dimensional van der Waals integration. Except for the excellent photodetection performance, the type-II diodes exhibit a superlinear photoelectric conversion phenomenon, with a model based on the in-gap trap-assisted recombination proposed for this superlinearity.

Abstract

Continuous miniaturization of semiconductor devices is the key to boosting modern electronics development. However, this downscaling strategy has been rarely utilized in photoelectronics and photovoltaics. Here, in this work, a full-van der Waals (vdWs) 1D p-Te/2D n-Bi₂O₂Se heterodiode with a rationally designed nanoscale ultra-photosensitive channel is reported. Enabled by the dangling bond-free mixed-dimensional vdWs integration, the Te/Bi₂O₂Se type-II diodes show a high rectification ratio of 3.6 × 10⁴. Operating with 100 mV reverse bias or in a self-power mode, the photodiodes demonstrate excellent photodetection performances, including high responsivities of 130 A W⁻¹ (100 mV bias) and 768.8 mA W⁻¹ (self-power mode), surpassing most of the reports of other heterostructures. More importantly, a superlinear photoelectric conversion phenomenon is uncovered in these nanoscale full-vdWs photodiodes, in which a model based on the in-gap trap-assisted recombination is proposed for this superlinearity. All these results provide valuable insights in light–matter interactions for further performance enhancement of photoelectronic devices.

Comprehensive understanding of interaction between photovoltaic and photothermoelectric effects is demonstrated via a time-resolved photocurrent (TRPC) measurement technique. Compared to MoS₂/multilayer WSe₂ p–n junction having a conventional TRPC dip, MoS₂/1L WSe₂ n–n junction processes a distinct TRPC peak, which is attributed to the opposite polarity between photovoltaic and photothermoelectric currents and can be further modulated via an external bias.

Abstract

Self-powered ultrafast 2D photodetectors have demonstrated great potential in imaging, sensing, and communication. Understanding the intrinsic ultrafast charge carrier generation and separation processes is essential for achieving high-performance devices. However, probing and manipulating the ultrafast photoresponse is limited either by the temporal resolution of the conventional methods or the required sophisticated device configurations. Here, van der Waals heterostructure photodetectors are constructed based on MoS₂/WSe₂ p–n and n–n junctions and manipulate the picosecond photoresponse by combining photovoltaic (PV) and photothermoelectric (PTE) effects. Taking time-resolved photocurrent (TRPC) measurements, a TRPC peak at zero time delay is observed with decay time down to 4 ps in the n–n junction device, in contrast to the TRPC dip in the p–n junction and pure WSe₂ devices, indicating an opposite current polarity between PV and PTE. More importantly, with an ultrafast photocurrent modulation, a transition from a TRPC peak to a TRPC dip is realized, and detailed carrier transport dynamics are analyzed. This study provides a deeper understanding of the ultrafast photocurrent generation mechanism in van der Waals heterostructures and offers a new perspective in instruction for designing more efficient self-powered photodetectors.

The influence of host dynamics on the ionic and thermal transport properties in Cu₇PSe₆ is investigated via neutron scattering measurements and machine-learned molecular dynamics simulations. In the absence of host dynamics, the intercluster Cu⁺ hopping is strongly suppressed, reducing long-range Cu⁺ diffusion. The anharmonic low-energy Cu dominated phonon modes are overdamped at high temperatures and lead to ultralow thermal conductivity.

Abstract

The quest for advanced superionic materials requires understanding their complex atomic dynamics, but detailed studies of the interplay between lattice vibrations and ionic diffusion remain scarce. Here inelastic and quasielastic neutron scattering measurements in the superionic argyrodite Cu₇PSe₆ are reported, combined with molecular dynamics (MD) based on ab initio and machine-learned potentials, providing critical insights into the atomistic mechanisms underlying fast ion conduction. The results reveal how long-range Cu diffusion is limited by intercluster hopping, controlled by selective anharmonic phonons of the crystalline framework. Further, the Green–Kubo simulations reproduce the ultralow lattice thermal conductivity and identify contributions from mobile ions, phonons, and their cross-correlations. The mode resolved analysis shows that the thermal conductivity is dominated by low-energy acoustic phonon modes of the overall crystal framework. The analysis of mode-resolved spectral functions further show that vibrational modes with significant Cu contributions are strongly damped, corresponding to the breakdown of associated phonon quasiparticles. These results highlight the importance of strongly anharmonic effects in superionic systems, in which the traditional quasiharmonic phonon picture is insufficient, and pave the way toward combining machine-learning accelerated simulations with neutron scattering experiments to rationalize the complex atomic dynamics underlying ionic and thermal transport.

The in-plane Mo₅N₆-MoS₂ heterojunction nanosheets grown on hollow carbon nanoribbons are designed as Mott-Schottky electrocatalysts for efficient pH-universal hydrogen evolution reaction. The heterointerface produces a built-in electric field and electron redistribution, which effectively activate the inert MoS₂ basal planes to intrinsically increase the electrocatalytic activity, improve the electronic conductivity, and boost the water dissociation activity to enable excellent HER performance.

Abstract

Cost-effective electrocatalysts for the hydrogen evolution reaction (HER) spanning a wide pH range are highly desirable but still challenging for hydrogen production via electrochemical water splitting. Herein, Mo₅N₆-MoS₂ heterojunction nanosheets prepared on hollow carbon nanoribbons (Mo₅N₆-MoS₂/HCNRs) are designed as Mott–Schottky electrocatalysts for efficient pH-universal HER. The in-plane Mo₅N₆-MoS₂ Mott–Schottky heterointerface induces electron redistribution and a built-in electric field, which effectively activates the inert MoS₂ basal planes to intrinsically increase the electrocatalytic activity, improve electronic conductivity, and boost water dissociation activity. Moreover, the vertical Mo₅N₆-MoS₂ nanosheets provide more activated sites for the electrochemical reaction and facilitate mass/electrolyte transport, while the tightly coupled HCNRs substrate and metallic Mo₅N₆ provide fast electron transfer paths. Consequently, the Mo₅N₆-MoS₂/HCNRs electrocatalyst delivers excellent pH-universal HER performances exemplified by ultralow overpotentials of 57, 59, and 53 mV at a current density of 10 mA cm⁻² in acidic, neutral, and alkaline electrolytes with Tafel slopes of 38.4, 43.5, and 37.9 mV dec⁻¹, respectively, which are superior to those of the reported MoS₂-based catalysts and outperform Pt in overall water splitting. This work proposes a new strategy to construct an in-plane heterointerface on the nanoscale and provides fresh insights into the HER electrocatalytic mechanism of MoS₂-based heterostructures.

2D materials show great potential to enable the transistor scaling down below 1 nm node. This review focuses on the introduction of the optimization of 2D materials-based transistor, new mechanisms, design flow, and circuits, which indicate the trend of 2D transistors toward very large-scale integrated circuits.

Abstract

2D transition metal chalcogenide (TMDC) materials, such as MoS₂, have recently attracted considerable research interest in the context of their use in ultrascaled devices owing to their excellent electronic properties. Microprocessors and neural network circuits based on MoS₂ have been developed at a large scale but still do not have an advantage over silicon in terms of their integrated density. In this study, the current structures, contact engineering, and doping methods for 2D TMDC materials for the scaling-down process and performance optimization are reviewed. Devices are introduced according to a new mechanism to provide the comprehensive prospects for the use of MoS₂ beyond the traditional complementary–metal–oxide semiconductor in order to summarize obstacles to the goal of developing high-density and low-power integrated circuits (ICs). Finally, prospects for the use of MoS₂ in large-scale ICs from the perspectives of the material, system performance, and application to nonlogic functionalities such as sensor circuits and analogous circuits, are briefly analyzed. The latter issue is along the direction of “more than Moore” research.

Abstract

When “cut off” continuous and uniform basal plane of two-dimensional (2D) materials, edges appear at cross-sections. Such edges with unique one-dimensional (1D) structures and bound-states significantly alter materials’ local chemical activities and have been extensively investigated as model platforms for investigating structure-property-performance relationships for chemistry. Many interesting phenomena have been discovered in the past decades, highlighting the importance of interactions between active species and edge atoms at the atomic level and making 1D edges as emerging catalysts with high efficiency, promising candidates for battery and electrochemical contacts. Here, this review focuses on the recent progress of edge synthesis and structural engineering methods, understanding of edge structure-activity mechanisms, and potential applications using edge sites. Challenges and prospects are also envisioned.

Abstract

In comparison to monolayer (1L), multilayer (ML) two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) exhibit more application potential for electronic and optoelectronic devices due to their improved current carrying capability, higher mobility, and broader spectral response. However, the investigation of devices based on wafer-scale ML-TMDs is still restricted by the synthesis of uniform and high-quality ML films. In this work, we propose a strategy of stacking MoS₂ monolayers via a vacuum transfer method, by which one could obtain wafer-scale high-quality MoS₂ films with the desired number of layers at will. The optical characteristics of these stacked ML-MoS₂ films (> 2L) indicate a weak interlayer coupling. The stacked ML-MoS₂ phototransistors show improved optoelectrical performances and a broader spectral response (approximately 300–1,000 nm) than that of 1L-MoS₂. Additionally, the dual-gate ML-MoS₂ transistors enable enhanced electrostatic control over the stacked ML-MoS₂ channel, and the 3L and 4L thicknesses exhibit the optimal device performances according to the turning point of the current on/off ratio and the subthreshold swing.

Abstract

The chemical vapor deposition (CVD) method has been widely used to synthesize high-quality two-dimensional (2D) materials. However, just one type of product can be synthesized by general CVD at one time. Here, we demonstrate a one-step CVD method to simultaneously grow two types of products. Importantly, the products can be completely separated by selecting the deposition region. In detail, the controllable and completely separable growth for α-GeTe and Te nanosheets was realized by using one precursor-GeTe powder through the atmospheric pressure CVD (APCVD) approach. High crystal quality of the as-grown α-GeTe nansosheets and Te nanosheets have been proved by the high-resolution transmission electron microscopy (HRTEM) characterization. Further, the field-effect-transistor (FET) based on α-GeTe nanosheet manifests that the as-grown α-GeTe nanosheet is a degenerate semiconductor due to the intrinsic Ge vacancies. Additionally, Te-based FET devices indicate that the good electrical performance of the as-grown Te nanosheet, such as high mobility of 900 cm²V⁻¹s⁻¹ (at room temperature), high on/off ratio of < 10⁶ (at 77 K), and good air-stability. The developed one-step CVD method shows the huge potentials for high-efficiency and high-quality material growth.

Abstract

Two-dimensional (2D) van der Waals (vdW) magnetic materials with reduced dimensionality often exhibit unexpected properties compared to their bulk counterparts. In particular, the mechanical flexibility of 2D structure, enhanced ferromagnetism at reduced layer thickness, as well as robust perpendicular magnetic anisotropy are quite appealing for constructing novel spintronic devices. The vdW vanadium diselenide (VSe₂) is an attractive material whose bulk is paramagnetic while monolayer is ferromagnetic with a Curie temperature (T_c) above room temperature. To explore its possible device applications, a detailed investigation on the thickness-dependent magnetism and strain modulation behavior of VSe₂ is highly demanded. In this article, the VSe₂ nanoflakes were controllably prepared via chemical vapor deposition (CVD) method. The few-layer single VSe₂ nanoflakes were found to exhibit magnetic domain structures at room temperature. Ambient magnetic force microscopy (MFM) phase images reveal a clear thickness-dependent magnetism and the MFM phase contrast is traceable for the nanoflakes of layer thickness below ∼ 6 nm. Moreover, applying strain is found efficient in modulating the magnetic moment and coercive field of 2D VSe₂ at room temperature. These results are helpful for understanding the ferromagnetism of high temperature 2D magnets and for constructing novel straintronic devices or flexible spintronic devices.