Jiuxiang Dai

npj 2D Materials and Applications, Published online: 01 April 2024; doi:10.1038/s41699-024-00466-9

npj 2D Materials and Applications, Published online: 06 April 2024; doi:10.1038/s41699-024-00453-0

This work unifies the study of effects of local strain and disorder on the Raman and photoluminescence spectra of chemical vapor deposition (CVD) grown semiconducting 2D-TMDs, shedding light on their influence on the frequency and intensity of second-order acoustic Raman bands under resonant conditions. This study enables the resonant Raman-based extraction of defect density and Grüneissen parameters in MoS₂ and other TMDs.

Abstract

The development of bottom-up synthesis routes for semiconducting transition metal dichalcogenides (TMDs) and the assessment of their defects are of paramount importance to achieve their applications. TMD monolayers grown by chemical vapor deposition (CVD) can be subjected to significant strain and, here, Raman and photoluminescence spectroscopies are combined to characterize strain in over one hundred MoS₂ monolayer samples grown by CVD. The frequency changes of phonons as a function of strain are analyzed, and used to extract the Grüneisen parameter of both zone-center and edge phonons. Additionally, the intensity of the defect-induced longitudinal acoustic (LA) and transverse acoustic (TA) Raman bands are discussed in relation to strain and electronic doping. The experimental mode-Grüneisen parameters obtained are compared with those calculated by density functional theory (DFT), to better characterize the type of strain and its resulting effects on Grüneisen parameters. The findings indicate that the use of Raman spectra to determine defect densities in 2D MoS₂ must be always conducted considering strain effects. To the best of the authors' knowledge, this work constitutes the first report on double resonance Raman processes studied as a function of strain in 2D-MoS₂. The new approach to obtain the Grüneisen parameter from zone-edge phonons in MoS₂ can also be extended to other 2D semiconducting TMDs.

Cs doped and Cs, S co-doped CuI are high figure of merit p-type transparent conductors. The local coordination around Cu is maintained with Cs doping of CuI as confirmed by XAS and TEM. Ionized impurity scattering and high Seebeck hole effective mass are the main hole mobility limiting mechanisms. The high conductivity, transmittance, and stability promise the applications in transparent electronics.

Abstract

One hindrance in transparent electronics is the lack of high-performance p-type transparent conductors (TCs). The state-of-the-art p-type TC, CuI, has a conductivity two orders of magnitude lower than n-type TCs like ITO. While doping strategies have shown promise in enhancing the hole carrier density in CuI, they often come at the expense of hole mobility. Therefore, understanding how extrinsic dopants affect the mobility of CuI is critical to further improve the performance of CuI-based TCs. Here the structural and electronic properties of Cs-doped CuI are investigated. It is demonstrated that ≈4 at.% Cs doping in CuI increases the carrier density from 2.1 × 10¹⁹ to 3.8 × 10²⁰ cm⁻³ while preserving the film microstructure and local coordination of Cu, as confirmed by HRTEM and XAS analysis. Introducing S as a co-dopant in Cs:CuI boosts the carrier density to 8.2 × 10²⁰ cm⁻³, reaching a stable conductivity of ≈450 S cm⁻¹. In all cases, the enhanced carrier density negatively affects the hole mobility with ionized impurity scattering and increased Seebeck hole effective mass as mobility limiting mechanisms. Nonetheless, the new Cs, S co-doped CuI exhibits high p-type conductivity, Vis–NIR transparency, and stability, presenting an attractive candidate for future transparent electronic devices.

An effective and universal strategy to constructing arbitrary on-demand self-wrinkle patterns on polyurethane elastomers via spatiotemporal photochemistry boundaries is developed. This technology allows for the attainment of a diverse array of controllable patterns, ranging from highly 2D ordered to periodic or intricate designs, providing a versatile basis for applications in anticounterfeiting devices, dynamic light grating, and functional systems.

Abstract

Harnessing the spontaneous surface instability of pliable substances to create intricate, well-ordered, and on-demand controlled surface patterns holds great potential for advancing applications in optical, electrical, and biological processes. However, the current limitations stem from challenges in modulating multidirectional stress fields and diverse boundary environments. Herein, this work proposes a universal strategy to achieve arbitrarily controllable wrinkle patterns via the spatiotemporal photochemical boundaries. Utilizing constraints and inductive effects of the photochemical boundaries, the multiple coupling relationship is accomplished among the light fields, stress fields, and morphology of wrinkles in photosensitive polyurethane (PSPU) film. Moreover, employing sequential light-irradiation with photomask enables the attainment of a diverse array of controllable patterns, ranging from highly ordered 2D patterns to periodic or intricate designs. The fundamental mechanics of underlying buckling and the formation of surface features are comprehensively elucidated through theoretical stimulation and finite element analysis. The results reveal the evolution laws of wrinkles under photochemical boundaries and represent a new effective toolkit for fabricating intricate and captivating patterns in single-layer films.

A 3 × 3 charge density wave (CDW) order and enhanced 2D Ising superconductivity (SC) are demonstrated in (SnS)_1.15TaS₂ van der Waals superlattices, which exhibit monolayer-like electronic characteristics. The observed low-lying sulfur p band rearrangement implies a novel CDW driving force, providing new insights into the long-debated CDW order and 2D SC in monolayer transition-metal dichalcogenides.

Abstract

Noncentrosymmetric transition metal dichalcogenide (TMD) monolayers offer a fertile platform for exploring unconventional Ising superconductivity (SC) and charge density waves (CDWs). However, the vulnerability of isolated monolayers to structural disorder and environmental oxidation often degrade their electronic coherence. Herein, an alternative approach is reported for fabricating stable and intrinsic monolayers of 1H-TaS₂ sandwiched between SnS blocks in a (SnS)_1.15TaS₂ van der Waals (vdW) superlattice. The SnS block layers not only decouple individual 1H-TaS₂ sublayers to endow them with monolayer-like electronic characteristics, but also protect the 1H-TaS₂ layers from electronic degradation. The results reveal the characteristic 3 × 3 CDW order in 1H-TaS₂ sublayers associated with electronic rearrangement in the low-lying sulfur p band, which uncovers a previously undiscovered CDW mechanism rather than the conventional Fermi surface-related framework. Additionally, the (SnS)_1.15TaS₂ superlattice exhibits a strongly enhanced Ising-like SC with a layer-independent T _c of ≈3.0 K, comparable to that of the isolated monolayer 1H-TaS₂ sample, presumably attributed to their monolayer-like characteristics and retained Fermi states. These results provide new insights into the long-debated CDW order and enhanced SC of monolayer 1H-TaS₂, establishing bulk vdW superlattices as promising platforms for investigating exotic collective quantum phases in the 2D limit.

This review synthesizes recent progress in investigating magneto-optical interactions in 2D magnetic materials. It categorizes advancements by interaction type, exploring their insights into the properties like magnetic phase transitions, lattice alterations, and spin dynamics, and examines field modulation of optical signals for each interaction. It offers an outlook on the rapidly evolving field of magneto-optical interactions in 2D magnets.

Abstract

The rapidly emerging field of 2D magnetic materials has garnered significant attention due to its fascinating physical properties and wide-ranging potential applications. This review highlights the importance of magneto-optical interactions as a crucial tool for both studying and modulating 2D magnets. It offers a comprehensive survey of current research concerning magneto-optical interactions in 2D magnetic materials, encompassing the magneto-optical Kerr effect, reflection magnetic circular dichroism, second-harmonic generation, photoluminescence, inelastic light scattering, and time-resolved spectroscopy. This review discusses how these techniques provide insights into the properties of 2D magnets, enabling exploration of magnetic phase transitions, lattice alterations, spin dynamics, as well as their responses to external fields. Moreover, it emphasizes the modulation of magnetic properties by photo-stimulation and offers a brief outlook on this swiftly developing field.

This paper reports a facile chemical strategy for producing atomically thin PdSe₂ nanosheets using electrochemical intercalation. The as-exfoliated PdSe₂ nanosheets undergo structural phase transition after thermal annealing to transform into metallic PdSe_{2- x} . Moreover, controllable phase transformation is demonstrated by van der Waals encapsulation, which leads to the formation of a metal-semiconductor junction. This paper illustrates the potential of phase-change materials in nanoelectronics.

Abstract

2D orthorhombic palladium diselenide is attracting rapidly increased interest by virtue of its fascinating physical properties and feasibility of phase transformation. However, it remains a major challenge to produce ultrathin PdSe₂ through a facile chemical route and control phase transformation because of its anisotropic structure with strong interlayer coupling. Here, the efficient synthesis of few-layer PdSe₂ nanosheets with large sizes using an electrochemical exfoliation approach is reported. Upon thermal annealing at 300–350 °C, the as-exfoliated PdSe₂ nanosheets transform into metallic phase PdSe_{2- x} , as verified by scanning transmission electron microscopy, Raman spectroscopy, and electrical characterizations. Simple encapsulation using hexagonal boron nitride (h-BN) can effectively suppress the Se-loss triggered phase transformation, so that a metal-semiconductor junction is formed by local phase modification. The fabricated PdSe₂ field-effect transistors exhibit p-type transport property, which is in stark contrast to electron-dominated ambipolar transport of pristine PdSe₂ devices. The combination of high-resolution X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy analysis reveals that the modulation of carrier polarity in h-BN encapsulated PdSe₂ should arise from the p-doping effect associated with the impact of interfacial condition. The study opens up a new route for future phase-engineered electronics in PdSe₂ and other 2D noble metal dichalcogenides materials.

Author(s): Xiaoyu Liu, Yuchi He, Chong Wang, Xiao-Wei Zhang, Ting Cao, and Di Xiao

A series of recent experimental works on twisted MoTe2 homobilayers have unveiled an abundance of exotic states in this system. Valley-polarized quantum anomalous Hall states have been identified at hole doping of ν=−1, and the fractional quantum anomalous Hall effect is observed at ν=−2/3 and ν=−3/…

[Phys. Rev. Lett. 132, 146401] Published Wed Apr 03, 2024

Nature Synthesis, Published online: 03 April 2024; doi:10.1038/s44160-024-00520-w

A 2D selector based on Gr/hBN/WSe₂ heterostructure exhibits high bipolar nonlinearity at room temperature. Profit for well-designed band alignment and proper tunneling barrier, the selector undergoes direct tunneling at low bias while Fowler–Nordheim tunneling at high bias. This proof-of-concept demonstration opens up a new paradigm for promoting 2D 1S1R arrays for memory applications.

Abstract

The integration of one-selector-one-resistor crossbar arrays requires the selectors featured with high nonlinearity and bipolarity to prevent leakage currents and any crosstalk among distinct cells. However, a selector with sufficient nonlinearity especially in the frame of device miniaturization remains scarce, restricting the advance of high-density storage devices. Herein, a high-performance memory selector is reported by constructing a graphene/hBN/WSe₂ heterostructure. Within the temperature range of 300–80 K, the nonlinearity of this selector varies from ≈10³ – ≈10⁴ under forward bias, and increases from ≈300 – ≈10⁵ under reverse bias, the highest reported nonlinearity among 2D selectors. This improvement is ascribed to direct tunneling at low bias and Fowler–Nordheim tunneling at high bias. The tunneling current versus voltage curves exhibit excellent bipolarity behavior because of the comparable hole and electron tunneling barriers, and the charge transport polarity can be effectively tuned from N-type or P-type to bipolar by simply changing source-drain bias. In addition, the conceptual memory selector exhibits no sign of deterioration after 70 000 switching cycles, paving the way for assembling 2D selectors into modern memory devices.

Off-grid all-optical ECL is generated with devices based on cheap and small commercial p-i-n Si photodiodes that integrate, in a single component, the photovoltaic junction and the reactive electrodes, integrated directly onto the contact leads by wet simple processes. Because of their size, cost, and ease of use, these systems are promising for remote and portable bioanalysis.

Abstract

Electrochemiluminescence (ECL) is the generation of light induced by an electrochemical reaction, driven by electricity. Here, an all-optical ECL (AO–ECL) system is developped, which triggers ECL by the illumination of electrically autonomous “integrated” photoelectrochemical devices immersed in the electrolyte. Because these systems are made using small and cheap devices, they can be easily prepared and readily used by any laboratories. They are based on commercially available p-i-n Si photodiodes (≈1 € unit⁻¹), coupled with well-established ECL-active and catalytic materials, directly coated onto the component leads by simple and fast wet processes. Here, a Pt coating (known for its high activity for reduction reactions) and carbon paint (known for its optimal ECL emission properties) are deposited at cathode and anode leads, respectively. In addition to its optimized light absorption properties, using the commercial p-i-n Si photodiode eliminates the need for a complicated manufacturing process. It is shown that the device can emit AO–ECL by illumination with polychromatic (simulated sunlight) or monochromatic (near IR) light sources to produce visible photons (425 nm) that can be easily observed by the naked eye or recorded with a smartphone camera. These low-cost off-grid AO-ECL devices open broad opportunities for remote photodetection and portable bioanalytical tools.

Near-infrared (NIR)-light is ideal for studying biological systems. Combined with lanthanide-based nanoparticles, UV or visible light is generated that can be utilized for diagnostic and therapeutic applications. Particle synthesis has advanced significantly, allowing for complex architectures. This article highlights recent achievements, remaining challenges in particle design and surface control, and the potential in biomedicine.

Abstract

Near-infrared (NIR) light is highly suitable for studying biological systems due to its minimal scattering and lack of background fluorescence excitation, resulting in high signal-to-noise ratios. By combining NIR light with lanthanide-based upconversion nanoparticles (UCNPs), upconversion is used to generate UV or visible light within tissue. This remarkable property has gained significant research interest over the past two decades. Synthesis methods are developed to produce particles of various sizes, shapes, and complex core–shell architectures and new strategies are explored to optimize particle properties for specific bioapplications. The diverse photophysics of lanthanide ions offers extensive possibilities to tailor spectral characteristics by incorporating different ions and manipulating their arrangement within the nanocrystal. However, several challenges remain before UCNPs can be widely applied. Understanding the behavior of particle surfaces when exposed to complex biological environments is crucial. In applications where deep tissue penetration is required, such as photodynamic therapy and optogenetics, UCNPs show great potential as nanolamps. These nanoparticles can combine diagnostics and therapeutics in a minimally invasive, efficient manner, making them ideal upconversion probes. This article provides an overview of recent UCNP design trends, highlights past research achievements, and outlines potential future directions to bring upconversion research to the next level.

Nature, Published online: 03 April 2024; doi:10.1038/s41586-024-07209-2

Nature Reviews Methods Primers, Published online: 04 April 2024; doi:10.1038/s43586-024-00301-x

Unusual deformation in <100>-oriented CdTe is discovered through in-situ micropillar compression experiments. Machine-learning (ML) molecular dynamics (MD) simulations reveal a reversible martensitic transformation between face-centered cubic (FCC) and body-centered cubic (BCC) structures in CdTe sublattice, which competes with dislocation motion. This process enables the <100> orientation of CdTe to exhibit over 36% hyper-elastic strain, highlighting its unique mechanical properties.

Cadmium telluride (CdTe) is a highly promising material for photovoltaics (PV) and photodetectors due to its light-absorbing properties. However, efficient design and use of flexible devices require a deep understanding of its atomic-level deformation mechanism. Herein, uniaxial compression deformation of CdTe monocrystalline with varying crystal orientations is investigated using molecular dynamics (MD) with a newly developed machine-learning force field (ML-FF), alongside in-situ micropillar compression experiments. The findings reveal that CdTe bulk deformation is dominated by reversible martensitic phase transformation, whereas CdTe pillar deformation is primarily driven by dislocation nucleation and movement. CdTe monocrystals possess exceptional super-recoverable deformation along the <100> orientation due to hyper-elastic processes induced by martensitic transformation. This discovery not only sheds light on the peculiarities observed in micropillar experimental measurements, but also provides pivotal insights into the fundamental deformation behaviors of CdTe and similar II–VI compounds under various stress conditions. These insights are crucial for the innovative design and enhanced functionality of future flexible electronic devices.

Currently, the methods for preparing phosphorene (such as black phosphorene, blue phosphorene, and violet phosphorene) mainly include mechanical exfoliation, liquid-phase exfoliation, electrochemical exfoliation, chemical vapor deposition (CVD), pulsed laser deposition, thermal thinning and so on. Here, the current development status, advantages, and disadvantages of these phosphorene preparation methods is summarized.

Abstract

Phosphorene, a 2D(two-dimensional) material, holds immense promise in various applications due to its unique properties. Here, a comprehensive overview of their recent synthesis progresses, with a specific focus on three phosphorene varieties: black phosphorene, blue phosphorene, and violet phosphorene is provided. Black phosphorene, given its versatile properties, is extensively studied, but the challenges persist in achieving scalable production with high quality and controllable thicknesses. The synthesis of blue phosphorene involves epitaxial methods but results in relatively small structures, limiting its practical applications. Notably, the successful synthesis of violet phosphorene remains limited, achieved only through mechanical and liquid-phase exfoliation (LPE) methods. Despite significant progression in phosphorene synthesis, a critical need is emphasized for a cost-effective and easily controllable method capable of efficiently managing the thickness of phosphorene. The challenges, such as scalability issues and the presence of impurities, highlight the complexity of phosphorene preparation methods. The review emphasizes the importance of continued scientific engagement to overcome these obstacles and advance the research topic. In essence, while phosphorene shows great potential, unlocking its full range of applications requires further research and innovation in synthesis methodologies.

A novel 2D/3D integrated dual-channel device artificial tetrachromatic synaptic device by stacking WSe₂/h-BN/Graphene on a GaN substrate is proposed to provide a promising avenue for the development of next-generation artificial visual perception systems. Moreover, the device exhibits remarkable performances under electrical and optical stimulation, owing to the enhanced charge-pumping effect of the GaN layer.

Abstract

The development of artificial tetrachromatic vision holds great potential to enhance human color perception and discrimination, thereby enabling more effective navigation in diverse environments. Herein, an artificial tetrachromatic synaptic device is presented built upon 2D-3D vertically stacked semiconductors composed of tungsten diselenide (WSe₂)-gallium nitride (GaN) configuration, forming a dual-channel floating gate transistor (FGT). Under the concerted influence of electrical and optical stimulation, the device successfully mimics fundamental tetrachromatic synaptic behaviors, including short-term potentiation (STP), weak long-term potentiation (wLTP), long-term potentiation (LTP), paired-pulse facilitation (PPF), spike number-dependent plasticity (SNDP), and spike rate-dependent plasticity (SRDP). Notably, the plasticity of the device can be further modulated under ultraviolet (UV) stimulation, providing insights into the modulation of synaptic plasticity through the photogenerated carrier dynamics in the GaN channel. These results imply that WSe₂-GaN-based FGT architecture with dual-channel characteristics seamlessly integrates optical sensing and synaptic simulation functionalities, representing a promising avenue for the development of next-generation artificial visual perception systems (AVPS), with a particular advantage for the pursuit of high-performance artificial tetrachromatic neuromorphic computing applications of the future.

Nature Photonics, Published online: 05 April 2024; doi:10.1038/s41566-024-01414-1

This study explores ultrathin all solid-state electrolyte-gated synaptic transistors (EGSTs) with chemical vapor deposition (CVD) grown molybdenum disulfide (MoS₂) channel and initiated chemical vapor deposition (iCVD) grown organic–inorganic hybrid electrolyte. The fabricated MoS₂-based EGST achieves outstanding synaptics properties, benefitting from sub-30 nm ultrathin electrolyte and atomically thin channel. The approach provides a guide for EGSTs to overcome scaling challenge.

Abstract

Electrolyte-gated synaptic transistors (EGSTs) have attracted considerable attention as synaptic devices owing to their adjustable conductance, low power consumption, and multi-state storage capabilities. To demonstrate high-density EGST arrays, 2D materials are recommended owing to their excellent electrical properties and ultrathin profile. However, widespread implementation of 2D-based EGSTs has challenges in achieving large-area channel growth and finding compatible nanoscale solid electrolytes. This study demonstrates large-scale process-compatible, all-solid-state EGSTs utilizing molybdenum disulfide (MoS₂) channels grown through chemical vapor deposition (CVD) and sub-30 nm organic-inorganic hybrid electrolyte polymers synthesized via initiated chemical vapor deposition (iCVD). The iCVD technique enables precise modulation of the hydroxyl group density in the hybrid matrix, allowing the modulation of proton conduction, resulting in adjustable synaptic performance. By leveraging the tunable iCVD-based hybrid electrolyte, the fabricated EGSTs achieve remarkable attributes: a wide on/off ratio of 10⁹, state retention exceeding 10³, and linear conductance updates. Additionally, the device exhibits endurance surpassing 5 × 10⁴ cycles, while maintaining a low energy consumption of 200 fJ/spike. To evaluate the practicality of these EGSTs, a subset of devices is employed in system-level simulations of MNIST handwritten digit recognition, yielding a recognition rate of 93.2%.