Jiuxiang Dai

Nature Materials, Published online: 23 January 2023; doi:10.1038/s41563-022-01422-y

Research on two-dimensional van der Waals ferroelectrics has witnessed an explosion over the past few years. This Perspective formulates a framework by which results can be analysed, reviews recent progress, discusses mechanisms and properties for applications, and outlines challenges to be addressed.

The p-βGa₂O₃/n-GaN heterojunction is constructed, which shows an ultrahigh detectivity in a wide UV spectrum. The p-βGa₂O₃ is realized by a reversed substitution doping process. The high performance of photodetector is attributed to the continuous conduction band in the heterojunction without a potential barrier, which is helpful for photogenerated electron transport from the space charge region to the n-GaN layer.

Abstract

An excellent broad-spectrum (220–380 nm) UV photodetector, covering the UVA-UVC wavelength range, with an ultrahigh detectivity of ≈10¹⁵ cm Hz^1/2 W⁻¹, is reported. It is based on a p-β Ga₂O₃/n-GaN heterojunction, in which p-β Ga₂O₃ is synthesized by thermal oxidation of GaN and a heterostructure is constructed with the bottom n-GaN. XRD shows the oxide layer is (−201) preferred oriented β-phase Ga₂O₃ films. SIMS and XPS indicate that the residual N atoms as dopants remain in β Ga₂O₃. XPS also demonstrates that the Fermi level is 0.2 eV lower than the central level of the band gap, indicating that the dominant carriers are holes and the β Ga₂O₃ is p-type conductive. Under a bias of −5 V, the photoresponsivity is 56 and 22 A W⁻¹ for 255 and 360 nm, respectively. Correspondingly, the detectivities reach an ultrahigh value of 2.7 × 10¹⁵ cm Hz^1/2 W⁻¹ (255 nm) and 1.1 × 10¹⁵ cm Hz^1/2 W⁻¹ (360 nm). The high performance of this UV photodetector is attributed mainly to the continuous conduction band of the p-β Ga₂O₃/n-GaN heterojunction without a potential energy barrier, which is more helpful for photogenerated electron transport from the space charge region to the n-type GaN layer.

Single unit-cell layered Bi₂Fe₄O₉ nanosheet is synthesized for the first time. Its formation mechanism and structural model are given. The ultrathin Bi₂Fe₄O₉ nanosheet exhibits nonlinear thermal expansion behavior. The thermal expansion coefficients of Bi₂Fe₄O₉ nanosheets are first determined for the a-, b-, c-axes, and the cell volume V, respectively, showing an anisotropic thermal expansion behavior.

Abstract

As an important multiferroic material, pure and low-dimensional phase-stable bismuth ferrite has wide applications. Herein, one-pot hydrothermal method was used to synthesize bismuth ferrite. Almost pure Bi₂Fe₄O₉, BiFeO₃, and their mixture were successfully obtained by controlling the KOH concentration in the hydrothermal solutions. The as-prepared Bi₂Fe₄O₉ products were crystalline with Pbam space group, had nanosheet morphology, and tended to aggregate into nanofloret or random stacking. Each Bi₂Fe₄O₉ nanosheet was a single crystal with (001) plane as its exposed surface. Single unit-cell layered Bi₂Fe₄O₉ nanosheets had a uniform thickness of 1 nm. The surface energies of various (100), (010), and (001) planes were 3.6–4.0, 5.6–15.1, and 1.7–3.0 J m⁻², respectively, in the Bi₂Fe₄O₉ crystal. The formation mechanism and structural model of the as-prepared single unit-cell layered Bi₂Fe₄O₉ nanosheets have been given. The growth of Bi₂Fe₄O₉ nanosheets was discussed. Thermal analysis showed that the Bi₂Fe₄O₉ phase was stable up to 1260 K. The thermal expansion behavior of the Bi₂Fe₄O₉ nanosheet was nonlinear. The thermal expansion coefficients of the ultrathin Bi₂Fe₄O₉ nanosheets on the a-, b-, c-axes, and on the unit-cell volume V were determined, showing an anisotropic thermal expansion behavior. This study is helpful for the controllable synthesis of ultrathin Bi₂Fe₄O₉ nanosheets.

The lateral shape and vertical thickness of low-symmetry, weakly coupled 2D ReS₂ are tuned by the growth temperature and substrate symmetry using a CVD method. The substrate rotational symmetry determines the growth anisotropy and domain geometry of polycrystalline monolayer ReS₂. Abundant interlayer stacking configurations are unclosed at the atomic scale, and some are predicted to show great potential in photovoltaics.

Abstract

Morphology significantly affects material's electronic, catalytic, and magnetic properties, especially for 2D crystals. Abundant achievements have been made in the morphology engineering of high-symmetry 2D materials, but for the emerging low-symmetry ones, such as ReS₂, both the morphology control technique and comprehension are lacking. Here, the lateral shape and vertical thickness engineering of 2D ReS₂ by tailoring the growth temperature and the substrate symmetry using chemical vapor deposition, is reported. The temperature increase induces an isotropic-to-anisotropic transition of domain shapes, as well as a monotonic decrease of the domain thickness, which promotes the electrocatalytic performance. The substrate rotational symmetry determines the shape anisotropy of polycrystalline ReS₂ monolayers via a diffusion-limited mechanism, leading to highly oriented square, triangular, and strip-like domains synthesized on the fourfold symmetry SrTiO₃ (001), threefold symmetry c-sapphire, and twofold symmetry a-sapphire substrates, respectively. Various stacking configurations in bilayers are unclosed at the atomic scale. Some are predicted to adopt a type-II band alignment with great potential in photovoltaics. The results give insights into the morphological engineering of a unique class of 2D material with low in-plane lattice symmetry and weak interlayer coupling, which are crucial for their high-quality synthesis and industrial applications.

Organic molecular and two dimensional (2D) materials combined will result into 2D organic materials, which would have unique optical and electrical properties. The free design and large scale synthesis of 2D materials can be realized in principle. This work summarizes status and challenges of 2D organic materials, and the great potential for developing sensors, biomedicine, and electronics.

Abstract

In the past few decades, 2D layer materials have gradually become a central focus in materials science owing to their uniquely layered structural qualities and good optoelectronic properties. However, in the development of 2D materials, several disadvantages, such as limited types of materials and the inability to synthesize large-scale materials, severely confine their application. Therefore, further exploration of new materials and preparation methods is necessary to meet technological developmental needs. Organic molecular materials have the advantage of being customizable. Therefore, if organic molecular and 2D materials are combined, the resulting 2D organic materials would have excellent optical and electrical properties. In addition, through this combination, the free design and large-scale synthesis of 2D materials can be realized in principle. Furthermore, 2D organic materials exhibit excellent properties and unique functionalities along with great potential for developing sensors, biomedicine, and electronics. In this review, 2D organic materials are divided into five categories. The preparation methods and material properties of each class of materials are also described in detail. Notably, to comprehensively understand each material's advantages, the latest research applications for each material are presented in detail and summarized. Finally, the future development and application prospects of 2D organic materials are briefly discussed.

Journal of Applied Physics, Volume 133, Issue 3, January 2023.
Recently, vanadium diselenide (VSe2), a member of transition metal dichalcogenides, has attracted a great deal of interest in spintronic devices and memory devices due to its unique physical properties. However, it is still a challenge to prepare a continuous VSe2 thin film which is critical for its potential application. Here, we report a continuous single-crystalline 1T-VSe2 thin film grown on mica by pulsed laser deposition. Both x-ray diffraction and high-resolution transmission electron microscopy verify the van der Waals epitaxy of the VSe2/mica heterostructure. Free-standing and flexible VSe2 thin films can be obtained and combined with integrated circuit technology, which is of great significance for the application of two-dimensional materials in the field of multifunctional flexible electronic devices.

This review presents a detailed summary of the current progress of broadband photodetectors based on group-10 transition metal dichalcogenides (TMDCs). It is followed by an overview of the structural characteristics and synthesis of group-10 TMDCs used in realizing their heterostructures aimed at broadband photodetectors. Subsequently, this review describes the potential applications of group-10 TMDCs in electronics and optoelectronics.

Abstract

Rapidly evolving group-10 transition metal dichalcogenides (TMDCs) offer remarkable electronic, optical, and mechanical properties, making them promising candidates for advanced optoelectronic applications. Compared to most TMDCs semiconductors, group-10-TMDCs possess unique structures, narrow bandgap, and influential physical properties that motivate the development of broadband photodetectors, specifically infrared photodetectors. This review presents the latest developments in the fabrication of broadband photodetectors based on conventional 2D TMDCs. It mainly focuses on the recent developments in group-10 TMDCs from the perspective of the lattice structure and synthesis techniques. Recent progress in group-10 TMDCs and their heterostructures with different dimensionality of materials-based broadband photodetectors is provided. Moreover, this review accounts for the latest applications of group-10 TMDCs in the fields of nanoelectronics and optoelectronics. Finally, conclusions and outlooks are summarized to provide perspectives for next-generation broadband photodetectors based on group-10 TMDCs.

The 3D monolithic heterogeneous complementary field-effect transistor (CFET) integrated with polycrystalline silicon (Poly-Si) and atomically-thin amorphous indium tungsten oxide (a-IWO) TFTs for a vertically stack architecture realizes high voltage gain and low-power logic circuit, showing the high potential to meet the requirements of next-generation IC technology with a tiny footprint and extremely high chip density.

Abstract

In this work, the authors demonstrate a novel vertically-stacked thin film transistor (TFT) architecture for heterogeneously complementary inverter applications, composed of p-channel polycrystalline silicon (poly-Si) and n-channel amorphous indium tungsten oxide (a-IWO), with a low footprint than planar structure. The a-IWO TFT with channel thickness of approximately 3-4 atomic layers exhibits high mobility of 24 cm² V⁻¹ s⁻¹, near ideally subthreshold swing of 63 mV dec⁻¹, low leakage current below 10⁻¹³ A, high on/off current ratio of larger than 10⁹, extremely small hysteresis of 0 mV, low contact resistance of 0.44 kΩ-µm, and high stability after encapsulating a passivation layer. The electrical characteristics of n-channel a-IWO TFT are well-matched with p-channel poly-Si TFT for superior complementary metal–oxide-semiconductor technology applications. The inverter can exhibit a high voltage gain of 152 V V⁻¹ at low supply voltage of 1.5 V. The noise margin can be up to 80% of supply voltage and perform the symmetrical window. The pico-watt static power consumption inverter is achieved by the wide energy bandgap of a-IWO channel and atomically-thin channel. The vertically-stacked complementary field-effect transistors (CFET) with high energy-efficiency can increase the circuit density in a chip to conform the development of next-generation semiconductor technology.

A polymorphic memtransistor based on atomically thin Mo_0.91W_0.09Te₂ is achieved, where the lateral device channel can be tuned as metallic and semiconducting for diverse neuromorphic and in-memory computing.

Abstract

Modulating semiconducting channel potential has been used for electrical switching in transistors without biological plasticity operations that are critical for energy-efficient neuromorphic computing. To achieve efficient data processing, alternative transport mechanisms, such as tunneling and thermionic emission, have been introduced with 2D materials. Here, a polymorphic memtransistor based on atomically thin Mo_0.91W_0.09Te₂ is presented, where the lattice and electronic structures of the lateral device channel can be tuned as either metallic (1T′) or semiconducting (2H) phases by electrical gating. The structural and electronic phase change of the channel material, optimized in Mo_0.91W_0.09Te₂, is explored using transport and optical measurements at the device scale. Based on the phase transition, the polymorphic memtransistor demonstrates a high on/off ratio (up to 10⁵), low subthreshold swing (down to 80 mV dec⁻¹), and various memristive behaviors, which are distinguished from traditional phase-change memory, transistors, and passive memristors for diverse neuromorphic and in-memory computing.

Nature Photonics, Published online: 19 January 2023; doi:10.1038/s41566-022-01132-6

Quantum recoil is experimentally observed via photon energy shifts in Smith–Purcell radiation. Leveraging van der Waals materials as atomic-scale gratings, the quantum recoil is measured at room temperature on a tabletop platform.

Nature Communications, Published online: 19 January 2023; doi:10.1038/s41467-023-36006-0

The limitation in metal-semiconductor contact has been a major challenge for high-performance organic field-effect transistors. Here, the authors fabricate the contact by transferring platinum electrode on solution-processed organic films, realizing ultralow total contact resistance down to 14 Ω ∙ cm.

Abstract

Tattoo electronics has attracted intensive interest in recent years due to its comfortable wearing and imperceivable sensing, and has been broadly applied in wearable healthcare and human—machine interface. However, the tattoo electrodes are mostly composed of metal films and conductive polymers. Two-dimensional (2D) materials, which are superior in conductivity and stability, are barely studied for electronic tattoos. Herein, we reported a novel electronic tattoo based on large-area Mo₂C film grown by chemical vapor deposition (CVD), and applied it to accurately and imperceivably acquire on-body electrophysiological signals and interface with robotics. High-quality Mo₂C film was obtained via optimizing the distribution of gas flow during CVD growth. According to the finite element simulation (FES), bottom surface of Cu foil covers more stable gas flow than the top surface, thus leading to more uniform Mo₂C film. The resulting Mo₂C film was transferred onto tattoo paper, showing a total thickness of ∼ 3 µm, sheet resistance of 60–150 Ω/sq, and skin-electrode impedance of ∼ 5 × 10⁵ Ω. Such thin Mo₂C electronic tattoo (MCET in short) can form conformal contact with skin and accurately record electrophysiological signals, including electromyography (EMG), electrocardiogram (ECG), and electrooculogram (EOG). These body signals collected by MCET can not only reflect the health status but also be transformed to control the robotics for human—machine interface.

Applied Physics Letters, Volume 122, Issue 3, January 2023.
The stacked two layered materials with a lattice constant mismatch and/or with a twist angle relative to each other can create a moiré pattern, modulating electronic properties of pristine materials. Here, we combine scanning tunneling microscopy/spectroscopy and density functional theory calculations to investigate the moiré potential induced bandgap tuning in an InSe/CuSe vertical heterostructure synthesized by a two-step of molecular beam epitaxy. Scanning tunneling microscopy measurements demonstrate the heterostructure with a superlattice periodicity of ∼3.48 nm and a twist angle of about 11° between the monolayers. Scanning tunneling spectroscopy record on the different stacking sites of the heterostructure reveals the bandgap of the InSe is location-dependent and a variation of 400 meV is observed. Density functional theory calculations reveal that the moiré-induce electric dipole in the monolayer InSe is the key factor for tuning the bandgap. Moreover, charge transfer between CuSe and InSe also contributes to the bandgap variation due to its stacking. We also show that the moiré potential not only can tune the bandgap of InSe but also can vanish the Dirac nodal line of CuSe in some stackings. Our explorations provide valuable information in understanding electronic properties of two-dimensional moiré materials.

Harsh-environment-resistant MoS₂ field-effect transistors are demonstrated by engineering the electrode–channel interfaces with an all-transfer technique of van der Waals electrodes. The intact and defect-free interfaces not only reduce the Schottky barrier height at electrodes, but enable high resistances of the MoS₂ devices to humid, oxidizing, and high-temperature environments, surpassing the devices with other kinds of electrodes.

Abstract

Nanoscale electronic devices that can work in harsh environments are in high demand for wearable, automotive, and aerospace electronics. Clean and defect-free interfaces are of vital importance for building nanoscale harsh-environment-resistant devices. However, current nanoscale devices are subject to failure in these environments, especially at defective electrode–channel interfaces. Here, harsh-environment-resistant MoS₂ transistors are developed by engineering electrode–channel interfaces with an all-transfer of van der Waals electrodes. The delivered defect-free, graphene-buffered electrodes keep the electrode–channel interfaces intact and robust. As a result, the as-fabricated MoS₂ devices have reduced Schottky barrier heights, leading to a very large on-state current and high carrier mobility. More importantly, the defect-free, hydrophobic graphene buffer layer prevents metal diffusion from the electrodes to MoS₂ and the intercalation of water molecules at the electrode–MoS₂ interfaces. This enables high resistances of MoS₂ devices with all-transfer electrodes to various harsh environments, including humid, oxidizing, and high-temperature environments, surpassing the devices with other kinds of electrodes. The work deepens the understanding of the roles of electrode–channel interfaces in nanoscale devices and provides a promising interface engineering strategy to build nanoscale harsh-environment-resistant devices.

Journal of Applied Physics, Volume 133, Issue 2, January 2023.
Two-dimensional GaGeTe flakes with different thicknesses from 80 to 2.2 nm (bilayer) were exfoliated and transferred to a SiO2/Si substrate. A series of samples with different thicknesses were prepared and identified by optical microscopy, atomic force microscopy, and Raman spectrum. Raman modes strongly dependent on the layer thickness and characteristic Raman-active modes for few-layer (FL) GaGeTe flakes are demonstrated. These vibration modes of FL GaGeTe show a linear red-shift phenomenon with increasing temperature and their full width at half maximum of the Raman mode exhibits a weak temperature dependence below 200 K, and then, a linear increase with temperature. The electrical conductivity is 96.48 S/cm for 74 nm flakes and drops exponentially to 2.27 × 10−7 S/cm for 7 nm ones because of the bandgap widening with the decrease of layer thickness, which is evidenced by the work function increase from 4.4 to 4.96 eV, when the thickness decreases from 80 to 2.2 nm. Moreover, the electrical conductivity performs two different temperature dependence behaviors on the thickness, indicating a transition from semimetal for bulk to semiconductor for FL GaGeTe, which agrees well with that of the theoretical calculation.

Applied Physics Letters, Volume 122, Issue 3, January 2023.
In this article, we fabricated amorphous InSnO thin film transistors (TFTs) with exceedingly high mobility and low thermal budget. The device is annealed only at a low temperature of 150 °C, a field-effect mobility ([math]) of 70.53 cm2/V s, a subthreshold swing of 0.25 V/decade, an on/off current ratio over 108, and a reasonable threshold voltage shift under negative bias stress. The influence of thermal annealing on amorphous InSnO TFTs was investigated by systematically analyzing the crystallization, surface morphology, internal chemical state, and energy band relationship of the InSnO thin film. Amorphous InSnO films deposited at room temperature have a sparse and porous loose structure, which has carrier scattering caused by poor film quality, resulting in low mobility and few free carriers in the film. With the increase in the annealing temperature, the In and Sn metal cations are further oxidized, increasing the carrier concentration in the film and forming a dense M–O–M network when annealed at 150 °C. With the further increase in the annealing temperature, a large number of thermally excited free electrons make the device appear metal like conductivity. This paper expands the research on a high electron concentration InSnO material as the active layer and promotes the development of amorphous oxide semiconductors in high mobility and flexible TFTs.

Nature Communications, Published online: 19 January 2023; doi:10.1038/s41467-023-35983-6

Chemical vapor deposition (CVD) is a versatile method to synthesize 2D materials, but usually requires high growth temperatures. Here, the authors report a BiOCl-assisted CVD approach to grow 2D nanosheets from 27 different layered and nonlayered materials at temperatures <500 °C, which are compatible with back-end-of-the-line industrial processes.

Highlights

• This review summarized the state of the art of MoS₂ from their controllable growth and potential application in integrated circuit.

• The influence of promoter, substrate, pressure, catalyst and precursor on the nucleation and growth are discussed.

• The current challenges and future perspectives of wafer-scale MoS₂ are outlined from the materials and device applications.

Nature, Published online: 18 January 2023; doi:10.1038/s41586-022-05592-2

Vertical organic electrochemical transistors demonstrating unprecedented performances in both p- and n-type operation modes have been synthesized from new electro-active and ion-permeable semiconducting polymers by the interface engineering of electro-active blend layers.

Nature, Published online: 18 January 2023; doi:10.1038/s41586-022-05524-0

Geometric confinement on arbitrary substrates promotes, without epitaxial seeding, the layer-by-layer growth of two-dimensional single-crystal monolayers and bilayers of transition metal dichalcogenides.

Nature, Published online: 18 January 2023; doi:10.1038/s41586-022-05461-y

A new exchange-bias effect between two different antiferromagnetic layers enables the fabrication of all-antiferromagnetic structures that have a large room-temperature tunnelling magnetoresistance and potential applications for ultrafast memory technologies.

Nature, Published online: 18 January 2023; doi:10.1038/s41586-022-05503-5

The direct observation of in-plane charged domain walls in BiFeO3 ferroelectric films a few nanometres thick, their deterministic creation, manipulation and annihilation by applied voltage, as well the demonstration of their memristive functionality is reported.

This review aims to draw a comprehensive picture on the recent advancements in the synthesis strategies for modulating the physiochemical, geometrical, and electronic properties of transition metal phosphides and exploration of their unprecedented catalytic potential toward different organic transformation reactions.

Abstract

Transition metal phosphides (TMP) posses unique physiochemical, geometrical, and electronic properties, which can be exploited for different catalytic applications, such as photocatalysis, electrocatalysis, organic catalysis, etc. Among others, the use of TMP for organic catalysis is less explored and still facing many complex challenges, which necessitate the development of sustainable catalytic reaction protocols demonstrating high selectivity and yield of the desired molecules of high significance. In this regard, the controlled synthesis of TMP-based catalysts and thorough investigations of underlying reaction mechanisms can provide deeper insights toward practical achievement of desired applications. This review aims at providing a comprehensive analysis on the recent advancements in the synthetic strategies for the tailored and tunable engineering of structural, geometrical, and electronic properties of TMP. In addition, their unprecedented catalytic potential toward different organic transformation reactions is succinctly summarized and critically analyzed. Finally, a rational perspective on future opportunities and challenges in the emerging field of organic catalysis is provided. On the account of the recent achievements accomplished in organic synthesis using TMP, it is highly anticipated that the use of TMP combined with advanced innovative technologies and methodologies can pave the way toward large scale realization of organic catalysis.

Wafer-scale patterned PtTe₂ arrays for the fabrication of terahertz detector arrays are reported. The unique physical properties of PtTe₂ together with bow-tie antenna enables simultaneous fast response and high sensitivity of the detector in terahertz region, exhibiting a high-resolution transmission terahertz-imaging at 0.28 THz. The preparation of integrated terahertz detector arrays and large-area terahertz array imaging are also explored.

Abstract

As the lastly unexplored electromagnetic wave, terahertz (THz) radiation has been exploited in a plenty of contexts such as fundamental research, military and civil fields. Most recently, representative two-dimensional (2D) topological semimetal, platinum ditelluride (PtTe₂) has attracted considerable research interest in THz detection due to its unique physical properties. However, to achieve practical applications, the low-cost, large-scale, controllable synthesis and efficient patterning of 2D materials are key requirements, which remain a challenge for PtTe₂ and its photodetectors (PDs). Herein, a facile approach is developed to obtain wafer-scale (2-inches) patterned PtTe₂ arrays using one-step tellurium-vapor transformation method and micro-Nano technology. PtTe₂ PD arrays are fabricated with the as-grown PtTe₂ arrays evenly distributed on a 2-inch wafer, exhibiting high conductivity (~2.7 × 10⁵ S m⁻¹) and good electrical consistency. Driven by the Dirac fermions, PtTe₂ PDs achieve a broadband (0.02–0.3 THz) response with a fast response speed (~4.7 μs), a high sensitivity (~47 pW Hz^−1/2) and high-resolution transmission THz-imaging capability, which displays the potential of large-area THz array imaging. These results are one step towards the practical applications of integrated PD arrays based on 2D materials.

Nature Reviews Chemistry, Published online: 17 January 2023; doi:10.1038/s41570-022-00458-7

As the most common derivative of graphene, graphene oxide has emerged as a new frontier material with tremendous applications to photonics, electronics and optoelectronics in the past decade. This Review highlights the state of the art and future prospects for this fast-growing field.