Jing Zhang

Nature Communications, Published online: 17 October 2022; doi:10.1038/s41467-022-33699-7

Designing efficient Bayesian neural networks remains a challenge. Here, the authors use the cycle variation in the programming of the 2D memtransistors to achieve Gaussian random number generator-based synapses, and combine it with the complementary 2D memtransistors-based tanh function to implement a Bayesian neural network.

Magnetic skyrmions and large topological Hall effect are demonstrated in chromium telluride. The Curie temperature and magnetic anisotropy in Cr_{1+ x} Te₂ can be controlled by the self-intercalate concentration x. In Cr_1.53Te₂, which has a Curie temperature of 295 K and a relatively weak perpendicular magnetic anisotropy, room-temperature skyrmions and topological Hall resistivity as large as ≈106 nΩ cm are observed.

Abstract

Room-temperature magnetic skyrmion materials exhibiting robust topological Hall effect (THE) are crucial for novel nano-spintronic devices. However, such skyrmion-hosting materials are rare in nature. In this study, a self-intercalated transition metal dichalcogenide Cr_{1+ x} Te₂ with a layered crystal structure that hosts room-temperature skyrmions and exhibits large THE is reported. By tuning the self-intercalate concentration, a monotonic control of Curie temperature from 169 to 333 K and a magnetic anisotropy transition from out-of-plane to the in-plane configuration are achieved. Based on the intercalation engineering, room-temperature skyrmions are successfully created in Cr_1.53Te₂ with a Curie temperature of 295 K and a relatively weak perpendicular magnetic anisotropy. Remarkably, a skyrmion-induced topological Hall resistivity as large as ≈106 nΩ cm is observed at 290 K. Moreover, a sign reversal of THE is also found at low temperatures, which can be ascribed to other topological spin textures having an opposite topological charge to that of the skyrmions. Therefore, chromium telluride can be a new paradigm of the skyrmion material family with promising prospects for future device applications.

2D semiconductors represent one of the most promising candidates to extend Moore's law. However, their full potential has been greatly hindered by the limited p-type ones with high performance and stability. This review offers a comprehensive discussion of the fundamentals, current status, and outlook of p-type 2D semiconductors, aiming to inspire relevant research for advanced future electronics.

Abstract

2D semiconductors represent one of the best candidates to extend Moore's law for their superiorities, such as keeping high carrier mobility and remarkable gate-control capability at atomic thickness. Complementary transistors and van der Waals junctions are critical in realizing 2D semiconductors-based integrated circuits suitable for future electronics. N-type 2D semiconductors have been reported predominantly for the strong electron doping caused by interfacial charge impurities and internal structural defects. By contrast, superior and reliable p-type 2D semiconductors with holes as majority carriers are still scarce. Not only that, but some critical issues have not been adequately addressed, including their controlled synthesis in wafer size and high quality, defect and carrier modulation, optimization of interface and contact, and application in high-speed and low-power integrated devices. Here the material toolkit, synthesis strategies, device basics, and digital electronics closely related to p-type 2D semiconductors are reviewed. Their opportunities, challenges, and prospects for future electronic applications are also discussed, which would be promising or even shining in the post-Moore era.

Nature Photonics, Published online: 13 October 2022; doi:10.1038/s41566-022-01085-w

Indirect-bandgap transition lasing, even under continuous-wave excitation at room temperature, is demonstrated in an ultra-thin WS2 disk.

Nature Photonics, Published online: 13 October 2022; doi:10.1038/s41566-022-01083-y

Green OLEDs based on BNSeSe offer high operational efficiency and reduced efficiency roll-off.

Nature Communications, Published online: 10 October 2022; doi:10.1038/s41467-022-33636-8

Metal chalcogenides have shown promising performances for renewable hydrogen evolution and such activities are sensitive to the material electronic structures. Here, authors modulate the electronic properties of molybdenum sulfide in 1T'''-MoS2 for hydrogen evolution electrocatalysis.

Nature Physics, Published online: 13 October 2022; doi:10.1038/s41567-022-01797-4

Making monolayer superconductors creates interesting effects, but often decreases the transition temperature compared to 3D materials. Instead, intercalating molecules into a layered superconductor tailors the superconductivity with fewer trade-offs.

Nature Physics, Published online: 13 October 2022; doi:10.1038/s41567-022-01778-7

The superconducting critical temperature of monolayer materials is often lower than their bulk counterparts. Now, intercalation is shown to induce two-dimensional superconducting properties while maintaining the bulk critical temperature.

Nature Electronics, Published online: 13 October 2022; doi:10.1038/s41928-022-00847-2

A reservoir computing system for multimode and multiscale signal processing can be created using optoelectronic synapses that are based on α-In2Se3 and exploit the tightly coupled ferroelectric and optoelectronic properties of the material.

Exploring the interplay between magnetic and optoelectronic properties and developing spin-optoelectronic devices are potential research strategies for further studies of 2D materials and their applications. A great exchange bias, intrinsic negative magnetoresistance, and broadband photodetection with fast response time and high responsivity are presented in the newly synthesized magnetic Fe_0.75Ta_0.5S₂, demonstrating great potential for advanced magneto-optoelectronic applications.

Abstract

Understanding the interplay between magnetic and optoelectronic properties and developing spin-optoelectronic devices are promising research strategies to further study 2D materials and advance their applications. Here, the broadband photoresponse in the newly synthesized magnetic Fe_0.75Ta_0.5S₂ single crystals is reported. Because the uncompensated magnetic moment of the spin glass state is pinned by the moment of the antiferromagnetic state, a large exchange bias field of ≈1.98 T is found at 2 K when cooled down at a field of 7 T. The as-prepared samples show a large negative magnetoresistance (nMR). The field dependence of nMRs displays a similar trend up to 50 K, which is likely to originate from the significant dependence of the localization length on magnetic field. In addition, a photodetector prepared using Fe_0.75Ta_0.5S₂ flakes exhibits a fast response time (121.7 ms), good stability, high responsivity of 26.1 A W⁻¹, and broadband photodetection, showing application potentials in spin-optoelectronics.

Nucleation from the amorphous phase is probed at the atomic scale through in situ scanning transmission electron microscopy. Three distinct stages, including aggregation of atoms, crystallization to form lattice-expanded nanocrystals, and relaxation of the lattice-expanded nanocrystals to form final nanocrystals, are observed. Unconventional crystal phases, e.g., fcc Ru, hcp Rh, and a new intermetallic IrCo, are formed through nucleation from the corresponding amorphous C-doped metal nanosheets.

Abstract

The nucleation pathway determines the structures and thus properties of formed nanomaterials, which is governed by the free energy of the intermediate phase during nucleation. The amorphous structure, as one of the intermediate phases during nucleation, plays an important role in modulating the nucleation pathway. However, the process and mechanism of crystal nucleation from amorphous structures still need to be fully investigated. Here, in situ aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) is employed to conduct real-time imaging of the nucleation of ultrathin amorphous nanosheets (NSs). The results indicate that their nucleation contains three distinct stages, i.e., aggregation of atoms, crystallization to form lattice-expanded nanocrystals, and relaxation of the lattice-expanded nanocrystals to form final nanocrystals. In particular, the crystallization processes of various amorphous materials are investigated systematically to form corresponding nanocrystals with unconventional crystalline phases, including face-centered-cubic (fcc) Ru, hexagonal-close-packed (hcp) Rh, and a new intermetallic IrCo alloy. In situ electron energy-loss spectroscopy (EELS) analysis unveils that the doped carbon in the original amorphous NSs can migrate to the surface during the nucleation process, stabilizing the obtained unconventional crystal phases transformed from the amorphous structures, which is also proven by density functional theory (DFT) calculations.

This review summarizes the recent development of dielectrics for high-performance 2D devices. Adapting integration protocols of conventional high-κ dielectrics and emerging dielectrics with inert surface can significantly improve the dielectric film and interface quality. The advancement of novel dielectrics boosts overall performance of 2D devices and paves way toward next-generation nanoelectronics.

Abstract

2D semiconductors have emerged both as an ideal platform for fundamental studies and as promising channel materials in beyond-silicon field-effect-transistors due to their outstanding electrical properties and exceptional tunability via external field. However, the lack of proper dielectrics for 2D semiconductors has become a major roadblock for their further development toward practical applications. The prominent issues between conventional 3D dielectrics and 2D semiconductors arise from the integration and interface quality, where defect states and imperfections lead to dramatic deterioration of device performance. In this review article, the root causes of such issues are briefly analyzed and recent advances on some possible solutions, including various approaches of adapting conventional dielectrics to 2D semiconductors, and the development of novel dielectrics with van der Waals surface toward high-performance 2D electronics are summarized. Then, in the perspective, the requirements of ideal dielectrics for state-of-the-art 2D devices are outlined and an outlook for their future development is provided.

Nature Nanotechnology, Published online: 11 October 2022; doi:10.1038/s41565-022-01238-6

First synthesized in 2011, MXenes are two-dimensional materials currently generating a whirlpool of interest.

Manipulating the strength of the interlayer coupling is an effective strategy to induce intriguing properties in layered materials. Recently, enhanced superconductivity has been reported in Weyl semimetal MoTe2 and WTe2 via ionic liquid (IL) cation intercalation. However, how the superconductivity enhancement depends on the interlayer interaction still remains elusive. Here by inserting IL cations with different sizes into MoTe2 through this strategy, we are able to tune the interlayer spacing of the intercalated MoTe2 samples and reveal the dependence of superconducting transition temperature Tc on the interlayer spacing. Our results show that Tc increases with the interlayer spacing, suggesting that the weakened interlayer coupling plays an important role in the superconductivity. Interestingly, the intercalation induced superconductivity shows a high Ginzburg–Landau anisotropy, which suggests a quasi-two-dimensional nature of the superconductivity where the adjacent superconducting layers are coupled through Josephson tunnelling.

Nature Communications, Published online: 24 September 2022; doi:10.1038/s41467-022-33306-9

The topological properties of square-root Weyl semimetals are derived from the square of the Hamiltonian. Here, the authors propose a tight-binding model for a square-root higher-order Weyl semimetal hosting both Fermi-arc surface and hinge states.

Nature Electronics, Published online: 29 September 2022; doi:10.1038/s41928-022-00836-5

An artificial synaptic transistor that uses a stretchable bilayer semiconductor as the channel and an encapsulating elastomer as the dielectric can exhibit both excitatory and inhibitory synaptic behaviour, even when under 50% strain.

Nature Electronics, Published online: 06 October 2022; doi:10.1038/s41928-022-00840-9

Monolithically integrated superconducting single-photon detectors and Josephson junctions can be used to create superconducting optoelectronic synapses with analogue weighting and temporal leaky integration of single-photon presynaptic signals

A time-dependent fluorescent hydrogel driven by the hydrolysis of urea is developed to achieve information encryption. Information is encoded in this material that self-erased with time, and moreover, false information is generated during the erasing process. The correct information can only be recognized at a specific time.

Abstract

Information encryption has become increasingly important in recent years; however, information encryption materials, especially those encrypting on a time scale, are still in fancy. Herein, a “time-lock” information encryption material is developed based on a time-dependent fluorescent hydrogel. The fluorescence color of this hydrogel can be regulated between green and yellow, with distinctive changes in intensity, on a time scale by controlling the concentration of urea/urease and HCl. By taking advantage of this feature, “time-locked” information can be encoded. Such information self-erases with time, and moreover, fake or even opposing information is generated during this process. The correct information can only be recognized at a specified time, i.e., using a “time-key” to decrypt the information. This time-dependent feature endows the material with a higher level of security and provides new insight for information encryption.

Nature Synthesis, Published online: 29 September 2022; doi:10.1038/s44160-022-00165-7

Two-dimensional materials have many desirable properties but controllable synthesis is difficult. Now, a flux-assisted growth approach has been designed to reproducibly prepare high-quality, atomically thin materials. Eighty atomically thin composite flakes have been prepared by this approach.

Nature Electronics, Published online: 23 September 2022; doi:10.1038/s41928-022-00835-6

A monocrystalline native oxide dielectric, β-Bi2SeO5, with a high dielectric constant has been synthesized by oxidizing a two-dimensional (2D) semiconductor, Bi2O2Se. In 2D transistors, the ultrathin β-Bi2SeO5 dielectric demonstrates sub-0.5-nm equivalent oxide thickness and leakage current below the low-power limit, meeting the requirements of the International Roadmap for Devices and Systems.

Thiocyanidomettalate containing Au(I) and hydroxo-bridged Dy(III) ions are synthesized having {Au⋯Au}-related phonons detected through Raman spectroscopy. Thermometers are designed with Raman active vibrations, which are compared with co-existing emission thermometry. The thermometric ability is tuned by varying the vibrational energy in the low-frequency region and through magnetic dilution. The magnetic diluted sample has shown slow magnetic relaxation.

Abstract

Molecular crystals acting as temperature sensors, designed for multiple measurement techniques, can be a promising pathway for self-calibrating thermometers. The molecular assemblies [Dy_x ^IIIY_1–x ^III(phen)₂(μ-OH)₂(H₂O)₂]·[Au^I(SCN)₂]₂·phen·0.5MeCN·0.5H₂O (x = 0, 0.1, 0.02; phen = 1,10-phenanthroline) which contain weakly bonded lanthanide(III) and Au(I) metal complexes are reported. They have Raman scattering in the low-frequency (LF) region with sharp peaks, one of the prerequisites to design Raman thermometers. The Raman thermometric behaviors are characterized by three vibrational bands, and their thermal sensitivity is compared with their emission thermometric ability. The LF phonon linked with the Au···Au vibration helps increase the thermometric sensitivity of emission and Raman thermometers. Additionally, magnetically diluted complexes containing both Dy³⁺ and Y³⁺ ions are prepared to demonstrate the effect of temperature sensing capability through Raman and emission spectroscopy among isostructural mixed-metal materials. Compounds are immersed inside various benchtop solvents to unravel the robustness and functioning of Raman thermometers. Furthermore, they reveal single-molecule magnet properties, which disclose the effect of LF phonons on the spin relaxation process. Therefore, the reported molecular materials are Raman and luminescent thermometers, and they contain a molecular magnetic center with thiocyanidoaurate ions playing a critical role due to their emissive and Raman activities.

The dielectric constant modified by the interfacial geometric factor (interfacial distance and van der Waals gap) can accurately describe the electrostatic screening effect in van der Waals 2D metal/semiconductor junctions. A field-effect transistor with a high on/off ratio can be designed by screening out the 2D metal/semiconductor junction with high electrostatic controllability.

Abstract

The prediction of the dielectric response in 2D metal–semiconductor junctions (MSJs) is challenging since the screening is inhomogeneous. Herein, a generalized model is proposed to uncover the relationship between interfacial electrostatic screening and the modulation of Schottky barrier height (Φ_SB) in 2D MSJs. This model is developed based on the effective dielectric constant, interfacial distance, and van der Waals gap that sheds light on the profound difference between dielectric screening in 2D and conventional MSJs. This model can predict the variation of Φ_SB for a series of 2D MSJs. Combining thermionic field emission theory with the model, several compatible 2D metals are provided for low-dimensional electronics, among which the α-graphyne-based junction exhibits the largest on/off ratio.

Thermoelectric Materials

In article number 2204132, G. Jeffrey Snyder, Yeon Sik Jung, Min-Wook Oh, and co-workers show that charged cation antisites are principal defects forestalling p-type dopability of AgBiSe₂-based thermoelectric materials via saturation annealing under selenium vapor. This finding offers a rational design rule for stabilizing thermoelectric properties of chalcogenide-based materials.

Nature Nanotechnology, Published online: 22 September 2022; doi:10.1038/s41565-022-01214-0

Secondary-ion mass spectrometry shows the presence of oxygen in the carbon sublattice of MXene, demonstrating the existence of oxycarbide MXenes.