Jiuxiang Dai

Nature Nanotechnology, Published online: 15 June 2023; doi:10.1038/s41565-023-01426-y

Nature Nanotechnology, Published online: 15 June 2023; doi:10.1038/s41565-023-01433-z

Nature Nanotechnology, Published online: 15 June 2023; doi:10.1038/s41565-023-01413-3

Light: Science & Applications, Published online: 16 June 2023; doi:10.1038/s41377-023-01141-2

A low-cost, scalable method is introduced for producing flakes of van der Waals materials. Through a roll-to-roll device, a massively parallel exfoliation process is achieved, providing coatings with a high density of nanosheets featuring a large lateral size. Field-effect transistors and flexible photodetectors are fabricated in batches.

Abstract

A method is presented for scaling up the production of flakes of van der Waals materials via mechanical exfoliation. Using a roll-to-roll setup and an automatized, massive parallel exfoliation process, adhesive tapes with a high density of nanosheets of van der Waals materials are produced. The technique allows for obtaining a good trade-off between large lateral size and excellent area scalability, while also maintaining low cost. The potential of the method is demonstrated through the successful fabrication of field effect transistors and flexible photodetectors in large batches. This low-cost method to produce large area films out of mechanically exfoliated flakes is very general, and it can be applied to a variety of substrates and van der Waals materials and, moreover, it can be used to combine different van der Waals materials on top of each other. Therefore, it is believed that this production method opens an interesting avenue for fabrication of low-cost devices while maintaining a good scalability and performance.

Light: Science & Applications, Published online: 15 June 2023; doi:10.1038/s41377-023-01143-0

A highly efficient ferroelectric transistor with exceptional memory window efficiency is demonstrated, offering a wide sensing margin for neuromorphic computing hardware . The device exhibits remarkable memory window tunability, achieving an impressive 94.4% accuracy in classifying the MNIST dataset. Integration with 2D materials reduces the depolarization field in the gate stack, leading to enhanced retention and endurance. This research highlights the potential for ferroelectric transistors to enable high-density memory and energy-efficient synapses in neuromorphic computing.

Abstract

In-memory computing, particularly neuromorphic computing, has emerged as a promising solution to overcome the energy and time-consuming challenges associated with the von Neumann architecture. The ferroelectric field-effect transistor (FeFET) technology, with its fast and energy-efficient switching and nonvolatile memory, is a potential candidate for enabling both computing and memory within a single transistor. In this study,  the capabilities of an integrated ferroelectric HfO₂ and 2D MoS₂ channel FeFET in achieving high-performance 4-bit per cell memory with low variation and power consumption synapses, while retaining the ability to implement diverse learning rules, are demonstrated. Notably, this device accurately recognizes MNIST handwritten digits with over 94% accuracy using online training mode. These results highlight the potential of FeFET-based in-memory computing for future neuromorphic computing applications.

The profound question of how the laws of quantum mechanics merge into those governing classical physics at macroscopic sizes remains largely unexplored. Here quantum tunneling of the magnetization, a phenomenon quantum by nature, is studied in a magnetic object that is, without any doubt, macroscopic.

Abstract

The recent advent of topological states of matter spawned many significant discoveries. The quantum anomalous Hall (QAH) effect is a prime example due to its potential for applications in quantum metrology, as well as its influence on fundamental research into the underlying topological and magnetic states and into axion electrodynamics. Here, electronic transport studies on a (V,Bi,Sb)₂Te₃ ferromagnetic topological insulator nanostructure in the QAH regime are presented. This allows access to the dynamics of an individual ferromagnetic domain. The domain size is estimated to be in the 50–100 nm range. Telegraph noise resulting from the magnetization fluctuations of this domain is observed in the Hall signal. Careful analysis of the influence of temperature and external magnetic field on the domain switching statistics provides evidence for quantum tunneling (QT) of magnetization in a macrospin state. This ferromagnetic macrospin is not only the largest magnetic object in which QT is observed, but also the first observation of the effect in a topological state of matter.

A method to pattern-grow high-quality GaN thin films is introduced. The GaN photodetectors have excellent ultraviolet (UV) detection performance, include a high responsivity of 4.23 A/W and a respectable specific detectivity of 1.76 × 10¹² Jones. Moreover, the photodetector arrays exhibit the strong homogeneity and repeatability, allowing it to function as a reliable UV image sensor with reasonable spatial resolution.

Abstract

GaN's outstanding physical characteristics allow for a wide range of applications in numerous industries. Although individual GaN-based ultraviolet (UV) photodetectors are the subject of in-depth research in recent decades, the demand for photodetectors array is rising as a result of advances in optoelectronic integration technology. However, as a prerequisite for constructing GaN-based photodetectors array, large-area, patterned synthesis of GaN thin films remains a certain challenge. This work presents a facile technique for pattern growing high-quality GaN thin films for the assembly of an array of high-performance UV photodetectors. This technique uses UV lithography, which is not only very compatible with common semiconductor manufacturing techniques, but also enables precise patterning modification. A typical detector has impressive photo-response performance under 365 nm irradiation, with an extremely low dark current of 40 pA, a high I _light/I _dark ratio over 10⁵, a high responsivity of 4.23 AW⁻¹, and a decent specific detectivity of 1.76 × 10¹² Jones. Additional optoelectronic studies demonstrate the strong homogeneity and repeatability of the photodetectors array, enabling it to serve as a reliable UV image sensor with enough spatial resolution. These outcomes highlight the proposed patterning technique's enormous potential.

To expand industrial applications of 2D-TMD-based photodetectors, a link between oxidation behavior of large-scale MoS₂ layers and their photoelectrical properties via combinatorial spectro-microscopic analyses is profoundly capture. Due to the several oxidation effects of 2D MoS₂ based photodetector, photoelectrical properties are gradually enhanced and rapidly degraded with altered temperature and duration of annealing process.

Abstract

Despite the encouraging properties and research of 2D MoS₂, an ongoing issue associated with the oxidative instability remains elusive for practical optoelectronic applications. Thus, in-depth understanding of the oxidation behavior of large-scale and homogeneous 2D MoS₂ is imperative. Here the structural and chemical transformations of large-area MoS₂ multilayers by air-annealing with altered temperature and time via combinatorial spectro-microscopic analyses (Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy) are surveyed. The results gave indications pertaining to temperature- and time-dependent oxidation effects: i) heat-driven elimination of redundant residues, ii) internal strain stimulated by the formation of MoO bonds, iii) deterioration of the MoS₂ crystallinity, iv) layer thinning, and v) morphological transformation from 2D MoS₂ layers to particles. Photoelectrical characterization of the air-annealed MoS₂ is implemented to capture the link between the oxidation behavior of MoS₂ multilayers and their photoelectrical properties. The photocurrent based on MoS₂ air-annealed at 200 °C is assessed to be 4.92 µA, which is 1.73 times higher than that of pristine MoS₂ (2.84 µA). The diminution in the photocurrent of the photodetector based on MoS₂ air-annealed above 300 °C in terms of the structural, chemical, and electrical conversions induced by the oxidation process is further discussed.

Fast and sensitive photodetectors are critical in various applications. This study demonstrates efficient charge transfer in vanadium(V)-doped WS₂/Bi₂O₂Se heterostructures via Kelvin probe force microscopy (KPFM) photoluminescence (PL), and Raman spectroscopy. The results indicate that combining V-doped WS₂ with Bi₂O₂Se can enhance the performance of photodetectors via charge transfer modulated ultrathin p–n junctions, resulting in better sensitivity and faster response time.

Abstract

The field of photovoltaics is revolutionized in recent years by the development of two–dimensional (2D) type-II heterostructures. These heterostructures are made up of two different materials with different electronic properties, which allows for the capture of a broader spectrum of solar energy than traditional photovoltaic devices. In this study, the potential of vanadium (V)-doped WS₂ is investigated, hereafter labeled V-WS₂, in combination with air-stable Bi₂O₂Se for use in high-performance photovoltaic devices. Various techniques are used to confirm the charge transfer of these heterostructures, including photoluminescence (PL) and Raman spectroscopy, along with Kelvin probe force microscopy (KPFM). The results show that the PL is quenched by 40%, 95%, and 97% for WS₂/Bi₂O₂Se, 0.4 at.% V-WS₂/Bi₂O₂Se, and 2 at.% V-WS₂/Bi₂O₂Se, respectively, indicating a superior charge transfer in V-WS₂/Bi₂O₂Se compared to pristine WS₂/Bi₂O₂Se. The exciton binding energies for WS₂/Bi₂O₂Se, 0.4 at.% V-WS₂/Bi₂O₂Se and 2 at.% V-WS₂/Bi₂O₂Se heterostructures are estimated to be ≈130, 100, and 80 meV, respectively, which is much lower than that for monolayer WS₂. These findings confirm that by incorporating V-doped WS₂, charge transfer in WS₂/Bi₂O₂Se heterostructures can be tuned, providing a novel light-harvesting technique for the development of the next generation of photovoltaic devices based on V-doped transition metal dichalcogenides (TMDCs)/Bi₂O₂Se.

Carbon point (CDs) is widely used in many fields such as photoelectric devices, biomedicine and detection due to its excellent optical properties. However, the common fluorescence quenching phenomenon of CDs in the aggregated state greatly restricts its application potential in various fields. Therefore, it is urgent to review the construction strategy, fluorescence mechanism, property regulation and potential applications of solid-state fluorescent CDs.

Abstract

Carbon dots (CDs)—carbon nanoparticles smaller than 10 nm—have attracted widespread attention owing to their excellent optical properties. However, high-performing CDs often suffer from severe aggregation-induced quenching in the solid state, which limits their commercial applicability. Therefore, CD materials with efficient solid-state luminescence are sought. As research on the structure and photoluminescence mechanisms of CDs has intensified in recent years, strategies to construct fluorescent solid-state CD materials and tune their emissions have broadened. This article reviews recent advances in the strategies and mechanisms for attaining solid-state fluorescence in CDs, describes specific ways to tune the optical properties of this fluorescence, introduces the latest applications of the resulting CDs to optoelectronics, biology, and sensing, and finally considers the prospects for future application and current challenges facing the development of solid-state fluorescence in CDs.

Optoelectronic vision and synaptic devices can help overcome high energy requirements of computation and enable miniaturised, precise, real-time vision systems. Herein, atomically thin layers of Sb doped In₂O₃ are utilised as ultraviolet-active optoelectronic synapses with recognition and prolonged memory capabilities. The material is devised into an imaging array of photo-active pixels capable of pattern recognition and memorization at low power with very few training cycles.

Abstract

Miniaturization and energy consumption by computational systems remain major challenges to address. Optoelectronics based synaptic and light sensing provide an exciting platform for neuromorphic processing and vision applications offering several advantages. It is highly desirable to achieve single-element image sensors that allow reception of information and execution of in-memory computing processes while maintaining memory for much longer durations without the need for frequent electrical or optical rehearsals. In this work, ultra-thin (<3 nm) doped indium oxide (In₂O₃) layers are engineered to demonstrate a monolithic two-terminal ultraviolet (UV) sensing and processing system with long optical state retention operating at 50 mV. This endows features of several conductance states within the persistent photocurrent window that are harnessed to show learning capabilities and significantly reduce the number of rehearsals. The atomically thin sheets are implemented as a focal plane array (FPA) for UV spectrum based proof-of-concept vision system capable of pattern recognition and memorization required for imaging and detection applications. This integrated light sensing and memory system is deployed to illustrate capabilities for real-time, in-sensor memorization, and recognition tasks. This study provides an important template to engineer miniaturized and low operating voltage neuromorphic platforms across the light spectrum based on application demand.

Thermal conductivity imaging via frequency-domain thermoreflectance enables the detection of localized microscale suppression in thermal conductivity at the grain boundaries of thermoelectric SnTe. By employing a Gibbs excess approach, a thermal boundary resistance is extracted from the images, establishing a correlation between misorientation angle and thermal resistance. These findings demonstrate the capability of thermal imaging in extracting structure–property relationships.

Abstract

Grain-boundary engineering is an effective strategy to tune the thermal conductivity of materials, leading to improved performance in thermoelectric, thermal-barrier coatings, and thermal management applications. Despite the central importance to thermal transport, a clear understanding of how grain boundaries modulate the microscale heat flow is missing, owing to the scarcity of local investigations. Here, thermal imaging of individual grain boundaries is demonstrated in thermoelectric SnTe via spatially resolved frequency-domain thermoreflectance. Measurements with microscale resolution reveal local suppressions in thermal conductivity at grain boundaries. Also, the grain-boundary thermal resistance – extracted by employing a Gibbs excess approach – is found to be correlated with the grain-boundary misorientation angle. Extracting thermal properties, including thermal boundary resistances, from microscale imaging can provide comprehensive understanding of how microstructure affects heat transport, crucially impacting the materials design of high-performance thermal-management and energy-conversion devices.

GaN Nanoscale Air Channel Diodes

In article number 2206385 by Mo Li and co-workers, a vertical GaN nanodiode with a 50 nm air channel is reported, fabricated using IC-compatible technologies, with a record field emission current of 11 mA@10 V. Notably, the device displays outstanding stability and fast switching characteristics with a sub-10 ns response time. Additionally, the temperature-dependent performance can guide the design of nano-air-channel-devices for applications in extreme conditions.

Ferroelectric–optoelectronic hybrid system for photodetection has been comprehensively discussed in this review. The fundamentals of optoelectronic and ferroelectric materials and their interactions in the hybrid system are summarized. Especially, the modulation mechanisms, integration strategies, and typical device structures of ferroelectrics-integrated photodetectors are discussed in detail. This review provides a roadmap for materials design and functional applications for ferroelectrics-integrated optoelectronic devices.

Abstract

Photodetectors (PDs), as functional devices based on photon-to-electron conversion, are an indispensable component for the next-generation Internet of Things system. The research of advanced and efficient PDs that meet the diverse demands is becoming a major task. Ferroelectric materials can develop a unique spontaneous polarization due to the symmetry-breaking of the unit cell, which is switchable under an external electric field. Ferroelectric polarization field has the intrinsic characteristics of non-volatilization and rewritability. Introducing ferroelectrics to effectively manipulate the band bending and carrier transport can be non-destructive and controllable in the ferroelectric–optoelectronic hybrid systems. Hence, ferroelectric integration offers a promising strategy for high-performance photoelectric detection. This paper reviews the fundamentals of optoelectronic and ferroelectric materials, and their interactions in hybrid photodetection systems. The first section introduces the characteristics and applications of typical optoelectronic and ferroelectric materials. Then, the interplay mechanisms, modulation effects, and typical device structures of ferroelectric–optoelectronic hybrid systems are discussed. Finally, in summary and perspective section, the progress of ferroelectrics integrated PDs is summed up and the challenges of ferroelectrics in the field of optoelectronics are considered.

Nature, Published online: 14 June 2023; doi:10.1038/s41586-023-06289-w

Author(s): Zhi Tan, Hui Zhang, Xiaojun Wu, Jie Xing, Qiming Zhang, and Jianguo Zhu

High-performance piezoelectrics have been extensively reported with a typical perovskite structure, in which a huge breakthrough in piezoelectric constants is found to be more and more difficult. Hence, the development of materials beyond perovskite is a potential means of achieving lead-free and hi…

[Phys. Rev. Lett. 130, 246802] Published Tue Jun 13, 2023

This review aims at discussing the spin-related performance and functionalities of diverse emerging spintronic material systems. The spin transport/manipulation properties of organic semiconductors, organic–inorganic hybrid perovskites, and 2D materials are discussed in the first place, respectively. Following, device multifunctionalities based on these materials are discussed, including spin-filter effect via chiral-induced spin selectivity, spin-photovoltaic, spin-light-emitting, and spin-transistor functions.

Abstract

The explosive growth of the information era has put forward urgent requirements for ultrahigh-speed and extremely efficient computations. In direct contrary to charge-based computations, spintronics aims to use spins as information carriers for data storage, transmission, and decoding, to help fully realize electronic device miniaturization and high integration for next-generation computing technologies. Currently, many novel spintronic materials have been developed with unique properties and multifunctionalities, including organic semiconductors (OSCs), organic–inorganic hybrid perovskites (OIHPs), and 2D materials (2DMs). These materials are useful to fulfill the demand for developing diverse and advanced spintronic devices. Herein, these promising materials are systematically reviewed for advanced spintronic applications. Due to the distinct chemical and physical structures of OSCs, OIHPs, and 2DMs, their spintronic properties (spin transport and spin manipulation) are discussed separately. In addition, some multifunctionalities due to photoelectric and chiral-induced spin selectivity (CISS) are overviewed, including the spin-filter effect, spin-photovoltaics, spin-light emitting devices, and spin-transistor functions. Subsequently, challenges and future perspectives of using these multifunctional materials for the development of advanced spintronics are presented.

Precision patterning of Cu atoms in twisted bilayer graphene using a top-down and bottom-up approach is presented. The process involves utilizing controlled ejection of carbon atoms from the graphene lattice with an aberration-corrected scanning transmission electron microscope (STEM). The e-beam defines attachment points for Cu atoms, and thermally induced migration of source atoms allows impurities and clusters to be attached with limited human interaction.

Abstract

Atomic-scale engineering typically involves bottom-up approaches, leveraging parameters such as temperature, partial pressures, and chemical affinity to promote spontaneous arrangement of atoms. These parameters are applied globally, resulting in atomic-scale features scattered probabilistically throughout the material. In a top-down approach, different regions of the material are exposed to different parameters, resulting in structural changes varying on the scale of the resolution. In this work, the application of global and local parameters is combined in an aberration-corrected scanning transmission electron microscope (STEM) to demonstrate atomic-scale precision patterning of atoms in twisted bilayer graphene. The focused electron beam is used to define attachment points for foreign atoms through the controlled ejection of carbon atoms from the graphene lattice. The sample environment is staged with nearby source materials such that the sample temperature can induce migration of the source atoms across the sample surface. Under these conditions, the electron-beam (top-down) enables carbon atoms in the graphene to be replaced spontaneously by diffusing adatoms (bottom-up). Using image-based feedback control, arbitrary patterns of atoms and atom clusters are attached to the twisted bilayer graphene with limited human interaction. The role of substrate temperature on adatom and vacancy diffusion is explored by first-principles simulations.

The first sub-ps (≈600 fs) magnetization process in a ferromagnetic quantum well is successfully demonstrated, which utilizes the high carrier coherency and a rapid photo-Demba field effect, using pump-and-probe measurements with an X-ray free-electron laser. This hopefully opens a new way to ultrafast magnetic storage and information processing.

Abstract

Strong spin-charge interactions in several ferromagnets are expected to lead to subpicosecond (sub-ps) magnetization of the magnetic materials through control of the carrier characteristics via electrical means, which is essential for ultrafast spin-based electronic devices. Thus far, ultrafast control of magnetization has been realized by optically pumping a large number of carriers into the d or f orbitals of a ferromagnet; however, it is extremely challenging to implement by electrical gating. This work demonstrates a new method for sub-ps magnetization manipulation called wavefunction engineering, in which only the spatial distribution (wavefunction) of s (or p) electrons is controlled and no change is required in the total carrier density. Using a ferromagnetic semiconductor (FMS) (In,Fe)As quantum well (QW), instant enhancement, as fast as 600 fs, of the magnetization is observed upon irradiating a femtosecond (fs) laser pulse. Theoretical analysis shows that the instant enhancement of the magnetization is induced when the 2D electron wavefunctions (WFs) in the FMS QW are rapidly moved by a photo-Dember electric field formed by an asymmetric distribution of the photocarriers. Because this WF engineering method can be equivalently implemented by applying a gate electric field, these results open a new way to realize ultrafast magnetic storage and spin-based information processing in present electronic systems.