Jiuxiang Dai

Solid state reaction of Cr and Mn with Bi₂Se₃ surface is investigated. Cr reacts with Se to form a 2D Cr_1+δSe₂ adlayer while reducing Bi₂Se₃ to Bi₂. In contrast, precisely controlled monolayer amounts of Mn transforms the surface layer to MnBi₂Se₄. The different interface reactions are due to varying stability of the monolayer selenides and the mixed bismuth selenide phases.

Abstract

Van der Waals (vdW) heterostructures that pair materials with diverse properties enable various quantum phenomena. However, the direct growth of vdW heterostructures is challenging. Modification of the surface layer of quantum materials to introduce new properties is an alternative process akin to solid state reaction. Here, vapor deposited transition metals (TMs), Cr and Mn, are reacted with Bi₂Se₃ with the goal to transform the surface layer to XBi₂Se₄ (X = Cr, Mn). Experiments and ab initio MD simulations demonstrate that the TMs have a high selenium affinity driving Se diffusion toward the TM. For monolayer Cr, the surface Bi₂Se₃ is reduced to Bi₂-layer and a stable (pseudo) 2D Cr_1+δSe₂ layer is formed. In contrast, monolayer Mn can transform upon mild annealing into MnBi₂Se₄. This phase only forms for a precise amount of initial Mn deposition. Sub-monolayer amounts dissolve into the bulk, and multilayers form stable MnSe adlayers. This study highlights the delicate energy balance between adlayers and desired surface modified layers that governs the interface reactions and that the formation of stable adlayers can prevent the reaction with the substrate. The success of obtaining MnBi₂Se₄ points toward an approach for the engineering of other multicomponent vdW materials by surface reactions.

A scalable synthesis for highly efficient thermocatalyst using two methods— continuous microfluidization and single-layer layered double hydroxide synthesis is reported. Owing to the methods, quantitatively scalable two-dimensional precursors are well mixed at the nanoscale, leading to highly dispersed MnCoAl mixed metal oxide nanoparticles on h-BN. The catalyst exhibits the highest turnover frequency among reported Mn-based NH₃-SCR catalysts.

Abstract

The preparation of two-dimensional (2D) materials often requires complicated exfoliation procedures having low yields. The exfoliated nanosheets are prone to thermal aggregation and unsuitable for thermocatalysis. Herein, a scalable approach produces 2D catalyst precursors well-distributed and mixed at the nanoscale. Using continuous microfluidization and single-layer layered double hydroxide (LDH) synthesis, the prepared suspension contained exfoliated hexagonal boron nitride (h-BN) nanosheets and single-layer LDHs. The increased contact area between h-BN and LDHs enables the formation of highly dispersed MnCoAl mixed metal oxide nanoparticles anchored on h-BN nanosheets after calcination. In the selective catalytic reduction of NO _x with NH₃ (NH₃-SCR, a representative thermocatalytic application), this nanocomposite demonstrates a record turnover frequency of 0.772 h⁻¹ among reported Mn-based NH₃-SCR catalysts, with high NO _x conversion and high N₂ selectivity at low temperatures. By creating 2D precursors mixed at the nanoscale, this new synthetic approach can realize the scalable production of highly efficient thermocatalysts.

Nature Communications, Published online: 13 March 2024; doi:10.1038/s41467-024-46222-x

Nature, Published online: 13 March 2024; doi:10.1038/d41586-024-00727-z

Nature, Published online: 13 March 2024; doi:10.1038/d41586-024-00492-z

npj 2D Materials and Applications, Published online: 14 March 2024; doi:10.1038/s41699-024-00458-9

This paper proposes high permittivity and low dielectric loss inorganic materials as triboelectric layers. To prevent charge injection through the surface of inorganic materials caused by air breakdown, an organic polymer is coated, which has high permittivity, big polarizability, none charge traps, and large work function difference with metal electrodes.

Abstract

Although high permittivity of inorganic materials (possibly two orders larger than organic polymers) is theoretically considered as ideal triboelectric materials, their high leakage property and low contact potential difference with metal electrodes lead to failure to obtain high triboelectric charge density. Besides, the internal space charge accumulation as a result of their defect levels reduces the output charge density and causes dielectric loss or even dielectric breakdown under charge injection in charge-excitation triboelectric nanogenerator (CE-TENG). Herein, this study proposes high permittivity and low dielectric loss inorganic materials as triboelectric layers. In order to prevent charge injection passing through the surface of inorganic materials caused by air breakdown under charge excitation, an organic polymer is coated, which has high permittivity, big polarizability, none charge traps, and large work function difference with metal electrodes. After optimizing the high dielectric inorganic layer and the coated organic polymer layer, the output charge of CE-TENG based on 1 mm PZT-5H coated with P(VDF-TrFE-CFE) achieves 2.83 mC m⁻², 6.5 times of CE-TENG based on 1 mm PZT-5H, breaking the historical record for inorganic material TENG. This work clarifies the material selection criteria for CE-TENG and provides a deeper understanding of charge transfer mechanism of inorganic materials.

Nanoscopic domain dynamics and resistive switching behavior in the Al_1–xSc_xN capacitors are revealed by a combination of pulse testing measurements and piezoresponse force microscopy (PFM). Capacitors in the polydomain state exhibit a significant increase in the steady-state conductance due to the conductive domain walls.

Abstract

In this paper, using a combination of pulse testing measurements and piezoresponse force microscopy (PFM), an investigation of the polarization reversal behavior and the accompanying resistive switching in the Al_0.72Sc_0.28N thin film capacitors is reported. The obtained results reveal a transition from the nucleation-limited switching (NLS) in the low field range toward the more uniform switching described by the Kolmogorov–Avrami–Ishibashi (KAI) model in the high field range. It is found that the Al_0.72Sc_0.28N capacitors exhibit an unusually steep change in the switching time– it decreases by five orders of magnitude with a moderate increase of the applied field. This feature is caused by a significantly higher activation field value (≈126 MV cm⁻¹) in comparison with the conventional perovskite ferroelectrics. PFM visualization of the field-induced domain dynamics has allowed the evaluation of the nucleation rate and domain wall velocity. Furthermore, capacitors in the polydomain state generated by partial switching of polarization exhibit a significant (up to two orders of magnitude) increase in the steady-state conductance. This effect is likely caused by the injection of strongly inclined conducting 180° domain walls. Resistance tunability offers additional functionalities to the Al_1-xSc_xN devices where conductive domain walls are used as active elements.

A cost-effective route of combining liquid phase exfoliation and self-assembly technology is proposed for the preparation of large-scale hBN nanosheet films, further, to fabricate vacuum ultraviolet photodetectors with ultralow dark current, high detectivity, and fast response speed. The working mechanism and electron transport model of these photodetectors are clearly analyzed.

Abstract

Hexagonal boron nitride (hBN) is one of the most promising candidates for vacuum-ultraviolet photodetectors (VUV PDs). However, the efficient and low-cost fabrication of large-area hBN-PDs still encounters challenges. Herein, a cost-effective route is proposed for fast and scalable fabrication of high-performance VUV PDs via hBN nanosheet (BNNS) films. BNNSs are peeled from bulk hBN and self-assembled into large-area ordered films. In such PDs, junction barriers are present at the contact interfaces of BNNSs and give the PDs a “light-induced reduction of junction barrier height” working mechanism. The number of junction barriers are qualitatively adjusted by designing the size of BNNSs to optimize the performance of the devices. The performance of ultralow dark current (0.27 pA@80 V), high detectivity (3.42 × 10¹¹ Jones), and fast response speed (20.97/17.69 ms) for 185 nm VUV light is achieved by a fabricated PD. Analysis based on the Schottky contact model proves that the large photoresponse is mainly attributed to the reduction of the barriers and series resistance on illumination. Meanwhile, a physical model is established to describe the working process of such PDs, of which conductivity is dominated by the junction barriers. Besides, a flexible PD is also fabricated, depicting excellent stability, and robustness.

Scanning microwave impedance microscopy (sMIM) is utilized to locally investigate α-phase indium selenide (α-In₂Se₃) on the material, metal oxide semiconductor (MOS) structure, and ferroelectric semiconductor transistor (FeSFET) device levels. The results shed light on understanding the electronic properties of van der Waals ferroelectrics encompassing semiconductivity and ferroelectricity toward device design and optimization.

Abstract

Van der Waals ferroelectric semiconductors, which encompass both ferroelectricity and semiconductivity, have garnered intensive research interests for developing novel non-volatile functional devices. Previous studies focus on ferroelectricity characterization and device demonstration, with little attention paid to the fundamental electronic properties of these materials and their functional structures, which are essential for both device design and optimization. In this study, scanning microwave impedance microscopy (sMIM) is utilized to investigate the ferroelectric semiconductor of α-phase indium selenide (α-In₂Se₃) and its synaptic field effect transistors. α-In₂Se₃ nanoflakes of varying thicknesses are visualized through capacitive signal detection, whose responses are consistent with finite element simulations manifesting dependence on both flake thickness and its semiconductor property. sMIM spectroscopy performed on α-In₂Se₃-based metal-oxide-semiconductor (MOS) structures reveals typical MOS capacitance-voltage characteristics, with additional hysteresis arising from the ferroelectric switching of α-In₂Se₃. The local conductance state changes of synaptic α-In₂Se₃ ferroelectric semiconductor transistors (FeSFET) in response to gate voltage stimuli are effectively detected by in situ sMIM, in good agreement with electrical device transport properties. This work deepens the understanding of ferroelectric semiconductor physics toward their practical device application.

This study focuses on the captivating interplay between the ferromagnetic Fe₃GeTe₂ and the piezoelectric α-In₂Se₃, both 2D van der Waals (vdW) materials. Strained heterojunctions exhibit several compelling transformations: increased domain density, reduced Curie temperature, and emergent magnetostrictive ferromagnetic domains. Using α-In₂Se₃ is a versatile approach to strain-tune vdW materials, including graphite and Te based vdW chalcogenides.

Abstract

The van der Waals interaction enables atomically thin layers of exfoliated 2D materials to be interfaced in heterostructures with relaxed epitaxy conditions, however, the ability to exfoliate and freely stack layers without any strain or structural modification is by no means ubiquitous. In this work, the piezoelectricity of the exfoliated van der Waals piezoelectric α-In₂Se₃ is utilized to modify the magnetic properties of exfoliated Fe₃GeTe₂, a van der Waals ferromagnet, resulting in increased domain wall density, reductions in the transition temperature ranging from 5 to 20 K, and an increase in the magnetic coercivity. Structural modifications at the atomic level are corroborated by a comparison to a graphite/α-In₂Se₃ heterostructure, for which a decrease in the Tuinstra-Koenig ratio is found. Magnetostrictive ferromagnetic domains are also observed, which may contribute to the enhanced magnetic coercivity. Density functional theory calculations and atomistic spin dynamic simulations show that the Fe₃GeTe₂ layer is compressively strained by 0.4%, reducing the exchange stiffness and magnetic anisotropy. The incorporation of α-In₂Se₃ may be a general strategy to electrostatically strain interfaces within the paradigm of hexagonal boron nitride-encapsulated heterostructures, for which the atomic flatness is both an intrinsic property and paramount requirement for 2D van der Waals heterojunctions.

Defects in MoS₂ defects are tailored via pressure-controlled CVD to achieve diverse functionalities. Low-pressure CVD growth yields sulfur-vacancy rich MoS₂, enhancing photoresponsivity, FET characteristics and electrocatalytic activity for HER, while atmospheric pressure CVD creates MoS₂ with oxygen-passivated defects leading to superior photoluminescence. This work demonstrates a single-step CVD approach for defect engineering in MoS₂, with potential applicability to other monolayer 2D materials.

Abstract

Molybdenum disulfide (MoS₂), a two-dimensional (2D) semiconducting material harbors intrinsic defects that can be harnessed to achieve tuneable electronic, optoelectronic, and electrochemical devices. However, achieving precise control over defect formation within monolayer MoS₂, remains a notable challenge. Here, an in-situ defect engineering approach for monolayer MoS₂ using a pressure-dependent chemical vapor deposition (CVD) process is presented. Monolayer MoS₂ grown in a low pressure CVD conditions (LP-MoS₂) produces sulfur vacancy (V _s) induced defect-rich crystals primarily attributed to the oxygen-deficient growth conditions. Conversely, atmospheric pressure CVD-grown MoS₂ (AP-MoS₂) passivates these defects with oxygen from ambient conditions. This disparity in defect profiles profoundly impacts crucial functional properties and device performance.  AP-MoS₂ shows a drastically enhanced photoluminescence, which is significantly quenched in LP-MoS₂ attributed to in-gap electron donor states induced by the V _s defects. However, the n-doping induced in LP-MoS₂ generates enhanced photoresponsivity and detectivity in fabricated photodetectors compared to the AP-MoS₂-based devices. Defect-rich LP-MoS₂ outperforms AP-MoS₂ as channel layers of field-effect transistors (FETs), as well as electrocatalytic material for hydrogen evolution reaction (HER). This work presents a single-step CVD approach for in situ defect engineering in monolayer MoS₂ and presents a pathway to control defects in other monolayer 2D materials.

High-quality ultrathin Mg₂Mo₃O₈ crystals are synthesized using chemical vapor deposition. Mg₂Mo₃O₈ crystals of varying thicknesses exhibit notable out-of-plane ferroelectric properties at room temperature, with ferroelectricity retained even at 2 nm thickness. This work introduces nolanites-type crystals into ultrathin ferroelectrics. The scalable synthesis of stable ultrathin ferroelectric Mg₂Mo₃O₈ expands the scope of ferroelectric materials and may prosper applications of ferroelectrics.

Abstract

The development of ultrathin, stable ferroelectric materials is crucial for advancing high-density, low-power electronic devices. Nonetheless, ultrathin ferroelectric materials are rare due to the critical size effect. Here, a novel ferroelectric material, magnesium molybdenum oxide (Mg₂Mo₃O₈) is presented. High-quality ultrathin Mg₂Mo₃O₈ crystals are synthesized using chemical vapor deposition (CVD). Ultrathin Mg₂Mo₃O₈ has a wide bandgap (≈4.4 eV) and nonlinear optical response. Mg₂Mo₃O₈ crystals of varying thicknesses exhibit out-of-plane ferroelectric properties at room temperature, with ferroelectricity retained even at a 2 nm thickness. The Mg₂Mo₃O₈ exhibits a relatively large remanent polarization ranging from 33 to 52 µC cm^{− 2}, which is tunable by changing its thickness. Notably, Mg₂Mo₃O₈ possesses a high Curie temperature (>980 °C) across various thicknesses. Moreover, the as-grown Mg₂Mo₃O₈ crystals display remarkable stability under harsh environments. This work introduces nolanites-type crystal into ultrathin ferroelectrics. The scalable synthesis of stable ultrathin ferroelectric Mg₂Mo₃O₈ expands the scope of ferroelectric materials and may prosper applications of ferroelectrics.

A locomotion mechanism for magnetic robots based on asymmetrical friction effect induced by magnetic torque has been revealed and defined. A biped robot based on the mechanism is proposed, which not only exhibits rapid locomotion across substrates with varying friction coefficients but also achieves precise motions along patterned trajectories through programmed controlling.

Abstract

Multimodal and controllable locomotion in complex terrain is of great importance for practical applications of insect-scale robots. Robust locomotion plays a particularly critical role. In this study, a locomotion mechanism for magnetic robots based on asymmetrical friction effect induced by magnetic torque is revealed and defined. The defined mechanism overcomes the design constraints imposed by both robot and substrate structures, enabling the realization of multimodal locomotion on complex terrains. Drawing inspiration from human walking and running locomotion, a biped robot based on the mechanism is proposed, which not only exhibits rapid locomotion across substrates with varying friction coefficients but also achieves precise locomotion along patterned trajectories through programmed controlling. Furthermore, apart from its exceptional locomotive capabilities, the biped robot demonstrates remarkable robustness in terms of load-carrying and weight-bearing performance. The presented locomotion and mechanism herein introduce a novel concept for designing magnetic robots while offering extensive possibilities for practical applications in insect-scale robotics.

α-TeO₂ oxide is exploited as an active channel semiconductor for TFTs using a direct evaporation technique. The fabricated TFT devices with 5 nm α-TeO₂ film and a 5 nm passivation layer of Al_xO_y exhibits a remarkable hole mobility of 3.8 cm² V⁻¹ s⁻¹, an on-state current of 966 µA, and an on/off ratio of 3.8 × 10³ at a processing temperature of 50 °C with exceptional stability and reproducibility.

Abstract

In comparison to reports on n-type semiconducting oxides, p-type oxide semiconducting materials are still rare. Scarcely reported p-type oxide transistors demonstrated unsatisfactory environmental stability which still hinders their implementation for all oxide transistors and circuit applications. In this study, for the first time on α-TeO₂ as an active channel material with p-type characteristics accessible by direct evaporation technique. Notably, the fabricated 5 nm α-TeO₂ thin film in connection with an equally thin passivation layer exhibits a remarkable low processing temperature of 50 °C generating a hole mobility of 3.8 cm² V⁻¹ s⁻¹, an on-state current of 966 µA, and an on/off ratio of 3.8 × 10³. Additionally, the reproducibility of these devices confirmed a narrow variation in the TFT metrics, yielding an average hole mobility, on-current, and on/off ratio of 3.59 cm² V⁻¹ s⁻¹, 914 µA, and 3.3 × 10³, respectively. Furthermore, the devices are subjected to extensive stability testing under ambient atmospheric conditions that exhibits a marginal mobility reduction while maintaining a stable on/off ratio over 125-day period, highlighting their robust environmental stability. Notably, the low processing temperatures with both exceptional transistor performance and environmental endurance makes them suitable for the integration onto flexible substrates, particularly bendable/stretchable displays.

Room-temperature oxidation mechanism of a HF-etched Ti₃C₂T _x MXene is revealed by environmental transmission electron microscopy and molecular dynamics. Discovery of the crystal plane-dependent oxidation rate, oxide expansion associated with coordination and charge of superficial Ti atoms and evolution of TiO₂ is critical to the optimal design of microstructure and properties of MXenes based on oxidation strategy.

Abstract

The oxidation chemistry of two-dimensional transition metal carbide MXenes has brought new research significance to their protection and application. However, the oxidation behavior and degradation mechanism of MXenes, in particular with time under oxygen conditions at room temperature, remain largely unexplored. Here, several experimental and theoretical techniques are used to determine a very early stage of the oxidation mechanism of HF-etched Ti₃C₂T _x (a major member of MXenes and T _x  = surface functional groups) in an oxygen environment at room temperature. Aberration-corrected environmental transmission electron microscopy coupled with reactive molecular dynamics simulations show that the crystal plane-dependent oxidation rate of Ti₃C₂T _x and oxide expansion are attributed to differences in the coordination and charge of superficial Ti atoms, and the existence of the channels between neighboring MXene layers on the different crystal planes. The complementary x-ray photoelectron spectroscopy and Raman spectroscopy analyses indicate that the anatase and a tiny fraction of brookite TiO₂ successively precipitate from the amorphous region of oxidized Ti₃C₂T _x , grow irregularly and transform to rutile TiO₂. Our study reveals the early-stage structural evolution of MXenes in the presence of oxygen and facilitates further tailoring of the MXene performance employing oxidation strategy.

Nature Communications, Published online: 11 March 2024; doi:10.1038/s41467-024-46502-6

Light: Science & Applications, Published online: 11 March 2024; doi:10.1038/s41377-024-01389-2

A highly reliable 2D MoS₂/GaPS₄ artificial synapse device displays high memory retention, even at 400 °C. Moreover, the device exhibits remarkable performances under electrical and optical stimulation, owing to the enhanced charge-trapping effect of the GaPS₄ layer.

Abstract

Artificial synaptic devices (ASDs) are attracting widespread attention as highly promising components for use in complex neuromorphic systems, playing crucial roles in addressing the challenges posed by the conventional von Neumann architecture. However, the instabilities of ASDs in high-temperature environments diminish the reliabilities of the device performances, significantly inhibiting their practical application. Herein, a highly reliable 2D MoS₂/GaPS₄ ASD that maintains its functionality even after exposure to 400 °C is proposed. Moreover, due to the enhanced charge-trapping effect of the GaPS₄ layer, the memory window expands from an initial 42 to 55 V, accompanied by a substantial on/off ratio of 10⁵, low off-leakage current of 10⁻¹¹ A, and high number of endurance cycles (10³). The device effectively simulates various biological synaptic functions via electric and light stimulation. Notably, the high electric and light paired-pulse facilitation indices suggest an exceptional synaptic performance. The findings introduce a novel approach to high-temperature neuromorphic applications via defect engineering.

A general overview of ion transport behavior in van der Waals (vdW) gaps of 2D materials is provided here. The 2D nanosheet synthesis, assembly methods of 2D lamellar membranes, ion transport mechanism, responsive regulation, and novel membrane applications are emphasized. This work will be an inspiring contribution to the development of multifunctional lamellar membranes based on 2D materials.

Abstract

2D materials, with advantages of atomic thickness and novel physical/chemical characteristics, have emerged as the vital building blocks for advanced lamellar membranes which possess promising potential in energy storage, ion separation, and catalysis. When 2D materials are stacked together, the van der Waals (vdW) force generated between adjacent layered nanosheets induces the construction of an ordered lamellar membrane. By regulating the interlayer spacing down to the nanometer or even sub-nanometer scale, rapid and selective ion transport can be achieved through such vdW gaps. The further improvement and application of qualified 2D materials-based lamellar membranes (2DLMs) can be fulfilled by the rational design of nanochannels and the intelligent micro-environment regulation under different stimuli. Focusing on the newly emerging advances of 2DLMs, in this review, the common top-down and bottom-up synthesis approaches of 2D nanosheets and the design strategy of functional 2DLMs are briefly introduced. Two essential ion transport mechanisms within vdW gaps are also involved. Subsequently, the responsive 2DLMs based on different types of external stimuli and their unique applications in nanofluid transport, membrane-based filters, and energy storage are presented. Based on the above analysis, the existing challenges and future developing prospects of 2DLMs are further proposed.