Jing Zhang

Two-dimensional Ferrovalley materials with intrinsic valley polarization are rare but highly promising for valley-based nonvolatile random access memory and valley filter devices. These ferromagnetic materials exhibit valleys at or near the Fermi level with intrinsic magnetism. The strong coupling between magnetism and spin–orbit coupling induces intrinsic valley polarization. Using Kinetically Limited Minimization, an unconstrained crystal structure prediction algorithm, and prototype sampling based on first-principles calculations, we have discovered new Ferrovalley materials, rare-earth iodides RI2, where R is a rare-earth element belonging to Sc, Y, or La-Lu, and I is Iodine. The rare-earth iodides are layered and demonstrate either 2H, 1T, or 1Td phase as the ground state in bulk, analogous to transition metal dichalcogenides (TMDCs). The calculated exfoliation energy of monolayers (MLs) is comparable to that of graphene and TMDCs, suggesting possible experimental synthesis. The MLs in the 2H phase exhibit ferromagnetism due to unpaired electrons in d and f orbitals. Throughout the rare-earth series, d bands have valley polarization at K and points in the Brillouin zone in the vicinity of the Fermi level. Large intrinsic valley polarization in the range of 15–143 meV without external stimuli is observed in these Ferrovalley materials, which can be enhanced further by applying an in-plane bi-axial strain. These valleys can selectively be probed and manipulated for information storage and processing, potentially offering superior performance beyond conventional electronics and spintronics. We further show that the 2H ferromagnetic phase of RI2 MLs possesses non-zero Berry curvature and exhibits anomalous valley Hall effect with considerable anomalous Hall conductivity. Our work will incite exploratory synthesis of the predicted Ferrovalley materials and their application in valleytronics and beyond.

Two-dimensional group-IV monochalcogenides such as GeS, GeSe, SnS, and SnSe were theoretically predicted as multiferroic materials with two or more ferroic properties. However, most of their bulk crystals are stacked layer by layer with an antiferroelectric manner, which lose the macroscopic in-plane ferroelectricity. In this work, we studied SnS in which the layers are stacked in a ferroelectric manner both experimentally and theoretically. We utilized polarization-resolved second harmonic generation (SHG) microscopy to investigate numerous flakes of ferroelectric SnS few layers on mica substrates. We found the SHG polar patterns dramatically varied in the range of 800 nm and 1000 nm due to the frequency-dependent SHG susceptibilities. First-principles calculations have been performed to study the frequency-dependent and layer-dependent SHG susceptibilities in the ferroelectric SnS with AA and AC stacking orders. The variation trend of calculated SHG polar patterns as a function of frequency agrees well with that of the experimental results. Since polarization-resolved SHG is a noncontact and nondestructive technique to determine the crystal orientation, understanding of its properties is important, especially for monitoring the transition of different ferroic phases.

Van der Waals (vdW) magnets are appealing candidates for realising spintronic devices that exploit current control of magnetization (e.g. switching or domain wall motion), but so far experimental demonstrations have been sparse, in part because of challenges associated with imaging the magnetization in these systems. Widefield nitrogen-vacancy (NV) microscopy allows rapid, quantitative magnetic imaging across entire vdW flakes, ideal for capturing changes in the micromagnetic structure due to an electric current. Here we use a widefield NV microscope to study the effect of current injection in thin flakes (∼10 nm) of the vdW ferromagnet Fe3GeTe2 (FGT). We first observe current-reduced coercivity on an individual domain level, where current injection in FGT causes substantial reduction in the magnetic field required to locally reverse the magnetisation. We then explore the possibility of current-induced domain-wall motion, and provide preliminary evidence for such a motion under relatively low current densities, suggesting the existence of strong current-induced torques in our devices. Our results illustrate the applicability of widefield NV microscopy to imaging spintronic phenomena in vdW magnets, highlight the possibility of efficient magnetization control by direct current injection without assistance from an adjacent conductor, and motivate further investigations of the effect of currents in FGT and other vdW magnets.

In this review, the most recent progress on low-dimensional II–VI semiconductors based flexible photodetectors and their application in wearable electronics are summarized, including wearable monitoring sensors, image sensors, and self-powered integrated wearable electronics. Meanwhile, the challenges and outlook of flexible photodetectors in the future integration of wearable electronic are also discussed.

Abstract

Flexible photodetectors exhibit many advantages such as a good bendability, foldability, and even stretchability as well as weight light, which have triggered a widely concerned in wearable electronics including wearable monitoring, wearable image sensing, self-powered integrated electronics, etc. Recently, various II–VI semiconductor nanostructures have become promising candidates in flexible photodetectors due to their unique characteristics, such as direct bandgap semiconductors, excellent optical and electric properties, high quantum efficiency, and inherent mechanical flexibility. Herein, the most recent progress on low-dimensional (0D, 1D, 2D, and related heterostructures) II–VI semiconductors based flexible photodetectors and their application in wearable electronic is reviewed. First, a brief introduction of the main sensing mechanisms and key figures of merits for photodetectors is presented. Then, the recent progresses on flexible photodetectors are provided, in which the functional materials synthesis methods are also discussed. More importantly, the applications of the flexible photodetectors are summarized, including wearable monitoring sensors, image sensors, and self-powered integrated wearable electronics. Finally, the challenges and the future research direction of the flexible photodetectors are discussed, meanwhile the outlook for the development of flexible photodetectors in the future integration of wearable electronic is also provided.

Nature Nanotechnology, Published online: 28 December 2022; doi:10.1038/s41565-022-01259-1

The interaction between distinct excitations in solids is of both fundamental interest and technological importance. The layered magnetic semiconductor CrSBr exhibits strong coupling between excitons and coherently hybridized magnons, where both magnetic fields and strain can tune the coupling precisely.

Nature Communications, Published online: 26 December 2022; doi:10.1038/s41467-022-35588-5

The authors present a planar photonic chip, which operate as a multiple-order analog spatial differentiator. It provides a route for designing fast, power-efficient, compact and low-cost devices used in edge detection and optical image processing, thus expanding the functions of standard microscopes.

Publication date: March 2023

Source: Materials Today, Volume 63

Author(s): Xiaoyue Wang, Chi Liu, Yuning Wei, Shun Feng, Dongming Sun, Huiming Cheng

Nature Communications, Published online: 22 December 2022; doi:10.1038/s41467-022-35633-3

Probing the localized electrocatalytic activity of heterogeneous electrocatalysts is crucial. Here, the authors propose a method of imaging the surface charge density and electrocatalytic activity of single two-dimensional electrocatalyst nanosheets.

Nature Communications, Published online: 23 December 2022; doi:10.1038/s41467-022-35628-0

Designing efficient sensing-memory-computing systems remains a challenge. Here, the authors propose a self-powered vertical tribo-transistor based on MXenes to implement the multi-sensing-memory-computing function and the interaction of multisensory integration.

Nature Materials, Published online: 22 December 2022; doi:10.1038/s41563-022-01428-6

The authors demonstrate a spectroscopic method, based on magnetotransport measurements, to quantitatively measure the size of the correlated gaps in twisted trilayer graphene and infer their topology.

Nature Electronics, Published online: 21 December 2022; doi:10.1038/s41928-022-00890-z

A lithography method that is based on interfacial adhesion energy differences and physical etching processes can be used to fabricate more than 10,000 molybdenum disulfide field-effect transistors on six-inch wafers with a yield of around 100%.

A high-performance lateral photovoltaic device is reported based on the geometrically asymmetric MoS₂ diode, which greatly simplifies the fabrication process of solar cells due to the absence of doping and complicated fabrication steps. This device shows high performance along with superior flexibility, making it an excellent candidate for wearable solar cells and optoelectronic devices.

Abstract

Optoelectronic performance of 2D transition metal dichalcogenides (TMDs)-based solar cells and self-powered photodetectors remain limited due to fabrication challenges, such as difficulty in doping TMDs to form p–n junctions. Herein, MoS₂ diodes based on geometrically asymmetric contact areas are shown to achieve a high current rectification ratio of ≈10⁵, facilitating efficient photovoltaic charge collection. Under solar illumination, the device demonstrates a high open-circuit voltage (V _oc) of 430 mV and a short-circuit current density (J _sc) of −13.42 mA cm⁻², resulting in a high photovoltaic power conversion efficiency (PCE) of 3.16%, the highest reported for a lateral 2D solar cell. The diodes also show a high photoresponsivity of 490.3 mA W⁻¹, and a large photo detectivity of 4.05 × 10¹⁰ Jones, along with a fast response time of 0.8 ms under 450 nm wavelength at zero bias for self-powered photodetection applications. The device transferred on a flexible substrate shows a high photocurrent and PCE retentions of 94.4%, and 88.2% after 5000 bending cycles at a bending radius of 1.5 cm, respectively, demonstrating robustness for flexible optoelectronic applications. The simple fabrication process, superior photovoltaic properties, and high flexibility suggests that the geometrically asymmetric MoS₂ device architecture is an excellent candidate for flexible photovoltaic and optoelectronic applications.

High-quality c-axis-aligned crystalline indium gallium tin oxide (CAAC–IGTO) semiconductor can be achieved by annealing of a-IGTO in O₂ furnace at 700 °C. The O vacancy-rich CAAC-IGTO semiconductor has ferroelectric-like properties. As a result, synaptic CAAC-IGTO TFT can mimic the electrical characteristics of biological synapses using a conventional SiO₂ gate insulator. Therefore, the CAAC-IGTO TFTs can be applied to highly integrated neuromorphic electronics.

Abstract

Artificial synapses are a key component of neuromorphic computing systems. To achieve high-performance neuromorphic computing ability, a huge number of artificial synapses should be integrated because the human brain has a huge number of synapses (≈10¹⁵). In this study, a coplanar synaptic, thin-film transistor (TFT) made of c-axis-aligned crystalline indium gallium tin oxide (CAAC–IGTO) is developed. The electrical characteristics of the biological synapses such as inhibitory postsynaptic current (IPSC), paired-pulse depression (PPD), short-term plasticity (STP), and long-term plasticity at V_DS = 0.1 V, are demonstrated. The measured synaptic behavior can be explained by the migration of positively charged oxygen vacancies (V_o ⁺/V_o ⁺⁺) in the CAAC–IGTO layer. The mechanism of implementing synaptic behavior is completely new, compared to previous reports using electrolytes or ferroelectric gate insulators. The advantage of this device is to use conventional gate insulators such as SiO₂ for synaptic behavior. Previous studies use chitosan, Ta₂O₃, SiO₂ nanoparticles , Gd₂O₃, and HfZrO_x for gate insulators, which cannot be used for high integration of synaptic devices. The metal–oxide TFTs, widely used in the display industry, can be applied to the synaptic transistors. Therefore, CAAC–IGTO synaptic TFT can be a good candidate for application as an artificial synapse for highly integrated neuromorphic chips.

A flexible memristor constructed by 2D cadmium phosphorus trichalcogenide nanosheets emerges with excellent resistive switching characteristics. Essential synaptic plasticities can be successfully mimicked. The applications on decimal operation including the addition, subtraction, multiplication, and division of decimal operation are successfully explored, which demonstrates the promising prospect in artificial electronic synapses of CdPS₃-based memristor.

Abstract

The development of advanced microelectronics requires new device architecture and multi-functionality. Low-dimensional material is considered as a powerful candidate to construct new devices. In this work, a flexible memristor is fabricated utilizing 2D cadmium phosphorus trichalcogenide nanosheets as the functional layer. The memristor exhibits excellent resistive switching performance under different radius and over 10³ bending times. The device mechanism is systematically investigated, and the synaptic plasticity including paired-pulse facilitation and spiking timing-dependent plasticity are further observed. Furthermore, based on the linearly conductance modulation capacity of the flexible memristor, the applications on decimal operation are explored, that the addition, subtraction, multiplication, and division of decimal calculation are successfully achieved. These results demonstrate the potential of metal phosphorus trichalcogenide in novel flexible neuromorphic devices, which accelerate the application process of neuromorphic computing.

Blue Photodetectors

Vertically stacked blue/green/red photodetectors receive great attention for next-generation image sensors, where a high-performance blue device is the most important building block. In article 2206932, Gill Sang Han, Hyun Suk Jung, Sangwook Lee, and co-workers demonstrate a high-performance self-powered blue photodetector based on wide-bandgap halide perovskites. Outstanding external quantum efficiency (84.9%) and responsivity (0.307 A W⁻¹) with improved stability is achieved by compositional engineering.

Artificial intelligence in general and machine learning in particular are gaining ever more importance in science and technology. In materials science, they are taking over tedious screening tasks and helping design and discover materials based on both explicit and tacit previous knowledge possessed by the scientist. Social and humanistic aspects are acquiring tremendous importance, as power and explainability seem to grow at the expense of each other.

Abstract

Artificial intelligence (AI) is gaining strength, and materials science can both contribute to and profit from it. In a simultaneous progress race, new materials, systems, and processes can be devised and optimized thanks to machine learning (ML) techniques, and such progress can be turned into innovative computing platforms. Future materials scientists will profit from understanding how ML can boost the conception of advanced materials. This review covers aspects of computation from the fundamentals to directions taken and repercussions produced by computation to account for the origins, procedures, and applications of AI. ML and its methods are reviewed to provide basic knowledge of its implementation and its potential. The materials and systems used to implement AI with electric charges are finding serious competition from other information-carrying and processing agents. The impact these techniques have on the inception of new advanced materials is so deep that a new paradigm is developing where implicit knowledge is being mined to conceive materials and systems for functions instead of finding applications to found materials. How far this trend can be carried is hard to fathom, as exemplified by the power to discover unheard of materials or physical laws buried in data.

A review of hardware security solutions using novel electronics, optical, and material properties of 2D materials and their corresponding heterostructure device configurations for Internet of Things platforms. The five quintessential security domains for discussion are physically unclonable functions, true random number generators, logic locking, hardware watermarking and anticounterfeiting, and hardware camouflaging.

Abstract

Hardware security is a major concern for the entire semiconductor ecosystem that accounts for billions of dollars in annual losses. Similarly, information security is a critical need for the rapidly proliferating edge devices that continuously collect and communicate a massive volume of data. While silicon-based complementary metal-oxide-semiconductor technology offers security solutions, these are largely inadequate, inefficient, and often inconclusive, as well as resource intensive in time, energy, and cost, leading to tremendous room for innovation in this field. Furthermore, silicon-based security primitives have shown vulnerability to machine learning (ML) attacks. In recent years, 2D materials such as graphene and transition metal dichalcogenides have been intensely explored to mitigate these security challenges. In this review, 2D-materials-based hardware security solutions such as camouflaging, true random number generation, watermarking, anticounterfeiting, physically unclonable functions, and logic locking of integrated circuits (ICs) are summarized with accompanying discussion on their reliability and resilience to ML attacks. In addition, the role of native defects in 2D materials in developing high entropy hardware security primitives is also examined. Finally, the existing challenges for 2D materials, which must be overcome for large-scale deployment of 2D ICs to meet the security needs of the semiconductor industry, are discussed.

p-Type AgSbTe₂ materials stabilized with S and Se co-doping are demonstrated to exhibit an outstanding thermoelectric (TE) figure of merit zT _max of 2.3 and a unicouple device reaching energy conversion efficiencies up to 12.3% at a temperature difference of 370 K. This exceptional performance arises from an enhanced carrier density resulting from a higher concentration of silver vacancies, a vastly improved Seebeck coefficient enabled by the flattening of the valence band maximum and the inhibited formation of n-type Ag₂Te.

Abstract

Thermoelectric (TE) generators enable the direct and reversible conversion between heat and electricity, providing applications in both refrigeration and power generation. In the last decade, several TE materials with relatively high figures of merit (zT) have been reported in the low- and high-temperature regimes. However, there is an urgent demand for high-performance TE materials working in the mid-temperature range (400–700 K). Herein, p-type AgSbTe₂ materials stabilized with S and Se co-doping are demonstrated to exhibit an outstanding maximum figure of merit (zT _max) of 2.3 at 673 K and an average figure of merit (zT _ave) of 1.59 over the wide temperature range of 300–673 K. This exceptional performance arises from an enhanced carrier density resulting from a higher concentration of silver vacancies, a vastly improved Seebeck coefficient enabled by the flattening of the valence band maximum and the inhibited formation of n-type Ag₂Te, and ahighly improved stability beyond 673 K. The optimized material is used to fabricate a single-leg device with efficiencies up to 13.3% and a unicouple TE device reaching energy conversion efficiencies up to 12.3% at a temperature difference of 370 K. These results highlight an effective strategy to engineer high-performance TE material in the mid-temperature range.

Nature Nanotechnology, Published online: 21 December 2022; doi:10.1038/s41565-022-01253-7

Femtosecond electron diffraction and ab initio theory unravel ultrafast lattice dynamics in photoexcited two-dimensional heterostructures during charge transfer.

The past two years have witnessed increased experimental and theoretical efforts toward studying MXenes’ mechanical and tribological properties when used as lubricant additives, reinforcement phases in composites, or solid lubricant coatings. The most promising results in MXene tribology are summarized, future important problems to be pursued further are outlined, and methodological recommendations are provided.

Abstract

The large and rapidly growing family of 2D early transition metal carbides, nitrides, and carbonitrides (MXenes) raises significant interest in the materials science and chemistry of materials communities. Discovered a little more than a decade ago, MXenes have already demonstrated outstanding potential in various applications ranging from energy storage to biology and medicine. The past two years have witnessed increased experimental and theoretical efforts toward studying MXenes’ mechanical and tribological properties when used as lubricant additives, reinforcement phases in composites, or solid lubricant coatings. Although research on the understanding of the friction and wear performance of MXenes under dry and lubricated conditions is still in its early stages, it has experienced rapid growth due to the excellent mechanical properties and chemical reactivities offered by MXenes that make them adaptable to being combined with other materials, thus boosting their tribological performance. In this perspective, the most promising results in the area of MXene tribology are summarized, future important problems to be pursued further are outlined, and methodological recommendations that could be useful for experts as well as newcomers to MXenes research, in particular, to the emerging area of MXene tribology, are provided.

Atomically thin MoS₂ electrical switches are characterized for the first time in operando and in ambient conditions through a non-invasive spectroscopy-based method. The study sheds light on the – controversial and still debated – dynamics driving the switching mechanism. It reveals volatile metallic filaments percolating interatomic spacing in MoS₂ and stresses the impact of transfer residues on the electrical performance of switches.

Abstract

MoS₂ nanoswitches have shown superb ultralow switching energies without excessive leakage currents. However, the debate about the origin and volatility of electrical switching is unresolved due to the lack of adequate nanoimaging of devices in operando. Here, three optical techniques are combined to perform the first noninvasive in situ characterization of nanosized MoS₂ devices. This study reveals volatile threshold resistive switching due to the intercalation of metallic atoms from electrodes directly between Mo and S atoms, without the assistance of sulfur vacancies. A “semi-memristive” effect driven by an organic adlayer adjacent to MoS₂ is observed, which suggests that nonvolatility can be achieved by careful interface engineering. These findings provide a crucial understanding of nanoprocess in vertically biased MoS₂ nanosheets, which opens new routes to conscious engineering and optimization of 2D electronics.

Dual-gated Cr-doped (Bi,Sb)2Te₃ magnetic topological insulator devices are fabricated by combining molecular beam epitaxial growth and 2D transfer methods. The large gate-tunability using mica as the gate dielectric leads to the observation of the reversible transition between two distinct topological phases, namely the quantum anomalous Hall and the anomalous Hall insulator, via electric field tuning.

Abstract

Quantum anomalous Hall phases arising from the inverted band topology in magnetically doped topological insulators have emerged as an important subject of research for quantization at zero magnetic fields. Though necessary for practical implementation, sophisticated electrical control of molecular beam epitaxy (MBE)-grown quantum anomalous Hall matter have been stymied by growth and fabrication challenges. Here, a novel procedure is demonstrated, employing a combination of thin-film deposition and 2D material stacking techniques, to create dual-gated devices of the MBE-grown quantum anomalous Hall insulator, Cr-doped (Bi,Sb)₂Te₃. In these devices, orthogonal control over the field-induced charge density and the electric displacement field is demonstrated. A thorough examination of material responses to tuning along each control axis is presented, realizing magnetic property control along the former and a novel capability to manipulate the surface exchange gap along the latter. Through electrically addressing the exchange gap, the capabilities to either strengthen the quantum anomalous Hall state or suppress it entirely and drive a topological phase transition to a trivial state are demonstrated. The experimental result is explained using first principle theoretical calculations, and establishes a practical route for in situ control of quantum anomalous Hall states and topology.