Jiuxiang Dai

Using direct magnetic imaging of CrSBr, a van der Waals material 2D antiferromagnet, it is demonstrated that the magnetic anisotropy and moment density are nearly preserved down to the monolayer. These images reveal the formation of Néel magnetic domain walls down to the monolayer. This material shows remarkable stability even for monolayer exposed to air.

Abstract

Recent advancements in 2D materials have revealed the potential of van der Waals magnets, and specifically of their magnetic anisotropy that allows applications down to the 2D limit. Among these materials, CrSBr has emerged as a promising candidate, because its intriguing magnetic and electronic properties have appeal for both fundamental and applied research in spintronics or magnonics. In this work, nano-SQUID-on-tip (SOT) microscopy is used to obtain direct magnetic imaging of CrSBr flakes with thicknesses ranging from monolayer (N = 1) to few-layer (N = 5). The ferromagnetic order is preserved down to the monolayer, while the antiferromagnetic coupling of the layers starts from the bilayer case. For odd layers, at zero applied magnetic field, the stray field resulting from the uncompensated layer is directly imaged. The progressive spin reorientation along the out-of-plane direction (hard axis) is also measured with a finite applied magnetic field, allowing evaluation of the anisotropy constant, which remains stable down to the monolayer and is close to the bulk value. Finally, by selecting the applied magnetic field protocol, the formation of Néel magnetic domain walls is observed down to the single-layer limit.

This work presents a charge-trap memory (CTM) based on MoS₂ where the memory operation arises thanks to electron trapping/detrapping at interface states. The CTM device displays synaptic potentiation/depression induced by pulses applied to the gate or drain terminal with excellent linearity. The synaptic characteristics are used in a reservoir computing system for visual pattern recognition, which makes MoS₂-based CT emerge as one of the most promising technologies for neuromorphic engineering.

Abstract

Novel memory devices are essential for developing low power, fast, and accurate in-memory computing and neuromorphic engineering concepts that can compete with the conventional complementary metal−oxide−semiconductor (CMOS) digital processors. 2D semiconductors provide a novel platform for advanced semiconductors with atomic thickness, low-current operation, and capability of 3D integration. This work presents a charge-trap memory (CTM) device with a MoS₂ channel where memory operation arises, thanks to electron trapping/detrapping at interface states. Transistor operation, memory characteristics, and synaptic potentiation/depression for neuromorphic applications are demonstrated. The CTM device shows outstanding linearity of the potentiation by applied drain pulses of equal amplitude. Finally, pattern recognition is demonstrated by reservoir computing where the input pattern is applied as a stimulation of the MoS₂-based CTMs, while the output current after stimulation is processed by a feedforward readout network. The good accuracy, the low current operation, and the robustness to input random bit flip makes the CTM device a promising technology for future high-density neuromorphic computing concepts.

Hexagonal boron nitride (h-BN) thin films have been grown on silicon carbide substrates exhibiting strong non-linear second-harmonic generation and ultra-low cross-plane thermal conductivity, attributed to the inherent formation of twisted nano-domain edges between the stacked h-BN nanocrystals with random in-plane orientations, as revealed by the first-principles time-dependent density functional theory.

Abstract

Understanding the emergent electronic structure in twisted atomically thin layers has led to the exciting field of twistronics. However, practical applications of such systems are challenging since the specific angular correlations between the layers must be precisely controlled and the layers have to be single crystalline with uniform atomic ordering. Here, an alternative, simple, and scalable approach is suggested, where nanocrystallinetwo-dimensional (2D) film on 3D substrates yields twisted-interface-dependent properties. Ultrawide-bandgap hexagonal boron nitride (h-BN) thin films are directly grown on high in-plane lattice mismatched wide-bandgap silicon carbide (4H-SiC) substrates to explore the twist-dependent structure-property correlations. Concurrently, nanocrystalline h-BN thin film shows strong non-linear second-harmonic generation and ultra-low cross-plane thermal conductivity at room temperature, which are attributed to the twisted domain edges between van der Waals stacked nanocrystals with random in-plane orientations. First-principles calculations based on time-dependent density functional theory manifest strong even-order optical nonlinearity in twisted h-BN layers. This work unveils that directly deposited 2D nanocrystalline thin film on 3D substrates could provide easily accessible twist-interfaces, therefore enabling a simple and scalable approach to utilize the 2D-twistronics integrated in 3D material devices for next-generation nanotechnology.

It is shown that Raman spectroscopy can be used to investigate the silicon-hydrogen (Si─H) bonding configuration of thinhydrogenated amorphous silicon layers directly on silicon heterojunction solar cells. Additionally, a detailed comparison of the Si–H vibrational spectra near the amorphous/crystalline silicon interface measured by Fourier transform infrared and Raman spectroscopy is performed.

Abstract

In silicon heterojunction solar cell technology, thin layers of hydrogenated amorphous silicon (a-Si:H) are applied as passivating contacts to the crystalline silicon (c-Si) wafer. Thus, the properties of the a-Si:H is crucial for the performance of the solar cells. One important property of a-Si:H is its microstructure which can be characterized by the microstructure parameter R based on Si─H bond stretching vibrations. A common method to determine R is Fourier transform infrared (FTIR) absorption measurement which, however, is difficult to perform on solar cells for various reasons like the use of textured Si wafers and the presence of conducting oxide contact layers. Here, it is demonstrated that Raman spectroscopy is suitable to determine the microstructure of bulk a-Si:H layers of 10 nm or less on textured c-Si underneath indium tin oxide as conducting oxide. A detailed comparison of FTIR and Raman spectra is performed and significant differences in the microstructure parameter are obtained by both methods with decreasing a-Si:H film thickness.

MXene-based van der Waals (vdW) heterostructures have recently gained tremendous attention as prospective optical materials that combine an extraordinary optical response with tunable light–matter interactions. This P erspective puts the spotlight on MXenes as facile 2D building blocks to design vertical MXene/2D assemblies. Further exploration of MXene/2D vdW heterostructures is encouraged since they can crosscut the current limitations of light utilization.

Abstract

The vertical integration of distinct 2D materials in van der Waals (vdW) heterostructures provides the opportunity for interface engineering and modulation of electronic as well as optical properties. However, scarce experimental studies reveal many challenges for vdW heterostructures, hampering the fine-tuning of their electronic and optical functionalities. Optically active MXenes, the most recent member of the 2D family, with excellent hydrophilicity, rich surface chemistry, and intriguing optical properties, are a novel 2D platform for optoelectronics applications. Coupling MXenes with various 2D materials into vdW heterostructures can open new avenues for the exploration of physical phenomena of novel quantum-confined nanostructures and devices. Therefore, the fundamental basis and recent findings in vertical vdW heterostructures composed of MXenes as a primary component and other 2D materials as secondary components are examined. Their robust designs and synthesis approaches that can push the boundaries of light-harvesting, transition, and utilization are discussed, since MXenes provide a unique playground for pursuing an extraordinary optical response or unusual light conversion features/functionalities. The recent findings are finally summarized, and a perspective for the future development of next-generation vdW multifunctional materials enriched by MXenes is provided.

Light: Science & Applications, Published online: 15 September 2023; doi:10.1038/s41377-023-01278-0

Nature Reviews Materials, Published online: 15 September 2023; doi:10.1038/s41578-023-00592-8

This paper presents a MoS₂-based asymmetric dual-gate field-effect transistor (ADGFET) with a small top gate and a global bottom gate. Taking advantage of independent gate control and strong interaction between gates, the device demonstrates basic logic function, while enabling nonvolatile memory and tunable photoresponse. The ADGFET shows potential for high-performance and multifunctional applications in future computing architecture.

Abstract

Advanced computing technologies such as distributed computing and the Internet of Things require highly integrated and multifunctional electronic devices. Beyond the Si technology, 2D-materials-based dual-gate transistors are expected to meet these demands due to the ultra-thin body and the dangling-bond-free surface. In this work, a molybdenum disulfide (MoS₂) asymmetric-dual-gate field-effect transistor (ADGFET) with an In₂Se₃ top gate and a global bottom gate is designed. The independently controlled double gates enable the device to achieve an on/off ratio of 10⁶ with a low subthreshold swing of 94.3 mV dec⁻¹ while presenting a logic function. The coupling effect between the double gates allows the top gate to work as a charge-trapping layer, realizing nonvolatile memory (10⁵ on/off ratio with retention time over 10⁴ s) and six-level memory states. Additionally, ADGFET displays a tunable photodetection with the responsivity reaching the highest value of 857 A W⁻¹, benefiting from the interface coupling between the double gates. Meanwhile, the photo-memory property of ADGFET is also verified by using the varying exposure dosages-dependent illumination. The multifunctional applications demonstrate that the ADGFET provides an alternative way to integrate logic, memory, and sensing into one device architecture.

Nature Reviews Materials, Published online: 14 September 2023; doi:10.1038/s41578-023-00602-9

Nature Reviews Materials, Published online: 14 September 2023; doi:10.1038/s41578-023-00600-x

Nature Reviews Materials, Published online: 14 September 2023; doi:10.1038/s41578-023-00599-1

Nature Physics, Published online: 14 September 2023; doi:10.1038/s41567-023-02204-2

Nature Synthesis, Published online: 14 September 2023; doi:10.1038/s44160-023-00388-2

Nature Synthesis, Published online: 14 September 2023; doi:10.1038/s44160-023-00396-2

Nature Reviews Methods Primers, Published online: 14 September 2023; doi:10.1038/s43586-023-00253-8

Raman spectroscopy is a great analysis tool but the spectra are sometimes difficult to interpret due to the occurrence of spectral artefacts. This paper dives into the details of many spurious signals and spectral artefacts that occur in Raman spectra, explains their origin, and provides the tools to identify and avoid them.

Abstract

Micro-Raman spectroscopy is an important analytical tool in a large variety of science disciplines. The technique is suitable for both identification of chemical bonds and studying more detailed phenomena like molecular interactions, material strain, crystallinity, defects, and bond formations. Raman scattering has one major weakness however: it is a very low probability process. The weak signals require very sensitive detection systems, which leads to a high probability of picking up signals from origins other than the sample. This complicates the analysis of the results and increases the risk of misinterpreting data. This work provides an overview of the sources of spurious signals occurring in Raman spectra, including photoluminescence, cosmic rays, stray light, artefacts caused by spectrometer components, and signals from other compounds in or surrounding the sample. The origins of these false Raman peaks are explained and means to identify and counteract them are provided.

Co-crystal assemblies of mononuclear Yb³⁺ and Sm³⁺ complexes enable cooperative sensitization upconversion and energy transfer upconversion from Sm³⁺ at room temperature. This promising system opens powerful horizons for new upconverting material using co-crystal assemblies.

Abstract

Metal-based upconversion luminescence transforming high-energy photons into low-energy photons is an attractive anti-Stokes shift process for fundamental research and promising applications. In this work, we developed the upconversion luminescence in co-crystal assemblies consisting of discrete mononuclear Yb and Sm complexes. The characteristic visible emissions of Sm³⁺ were observed under the excitation of absorption band of Yb³⁺ at 980 nm. A series of co-crystal assemblies were investigated based on mononuclear Yb and Sm complexes, and the strongest luminescence was obtained when the molar concentration between Yb³⁺ and Sm³⁺ is equivalent. The crystal structure was fully characterized by the single crystal X-ray diffraction and upconverting energy transfer mechanisms were verified as cooperative sensitization upconversion and energy transfer upconversion. This is the first example of Sm³⁺-based upconverting luminescence in discrete lanthanide complexes which present as co-crystal assemblies at room temperature.

Nature Communications, Published online: 13 September 2023; doi:10.1038/s41467-023-41433-0

Nature, Published online: 13 September 2023; doi:10.1038/s41586-023-06339-3