Jiuxiang Dai

Abstract

Intrinsic ferroelectric materials play a critical role in the development of high-density integrated device. Despite some two-dimensional (2D) ferroelectrics have been reported, the research on one-dimensional (1D) intrinsic ferroelectric materials remains relatively scare since 1D atomic structures limit their van der Waals (vdW) epitaxy growth. Here, we report the synthesis of 1D intrinsic vdW ferroelectric SbSI nanowires via a confined-space chemical vapor deposition. By precisely controlling the partial vapor pressure of I₂ and reaction temperature, we can effectively manipulate kinetics and thermodynamics processes, and thus obtain high quality of SbSI nanowires, which is determined by Raman spectroscopy and high-resolution scanning transmission electron microscopy characterizations. The ferroelectricity in SbSI is confirmed by piezo-response force microscopy measurements and the ferroelectric transition temperature of 300 K is demonstrated by second harmonic generation. Moreover, the in-plane polarization switching can be maintained in the thin SbSI nanowires with a thickness of 20 nm. Our prepared 1D vdW ferroelectric SbSI nanowires not only enrich the vdW ferroelectric systems, but also open a new possibility for high-power energy storage nanodevices.

Large-area self-doped, monolayer 2D MoS₂ is successfully synthesized via the one-step chemical vapor deposition method without the use of a catalyst. Notably, the self-doped MoS₂ (1T-2H mixed-phase) exhibits significantly enhanced photo responsivity (55 times higher), alongside improved detectivity, photo gain, and response time compared to the 2H phase.

Abstract

Transition metal dichalcogenides (TMDs) exist in two distinct phases: the thermodynamically stable trigonal prismatic (2H) and the metastable octahedral (1T) phase. Phase engineering has emerged as a potent technique for enhancing the performance of TMDs in optoelectronics applications. Nevertheless, understanding the mechanism of phase transition in TMDs and achieving large-area synthesis of phase-controlled TMDs continue to pose significant challenges. This study presents the synthesis of large-area monolayered 2H-MoS₂ and mixed-phase 1T/2H-MoS₂ by controlling the growth temperature in the chemical vapor deposition (CVD) method without use of a catalyst. The field-effect transistors (FETs) devices fabricated with 1T/2H-MoS₂ mixed-phase show an on/off ratio of 10⁷. Photo response devices fabricated with 1T/2H-MoS₂ mixed-phase show ≈55 times enhancement in responsivity (from 0.32 to 17.4 A W⁻¹) and 10² times increase in the detectivity (from 4.1 × 10¹⁰ to 2.48 × 10¹² cm Hz W⁻¹) compare to 2H-MoS₂. Introducing the metallic 1T phase within the 2H phase contributes additional carriers to the material, which prevents the electron-hole recombination and thereby increases the carrier density in the 1T/2H-MoS₂ mixed-phase in comparison to 2H-MoS_2. This work provides insights into the self-doping effects of 1T phase in 2H MoS₂, enabling the tuning of 2D TMDs properties for optoelectronic applications.

Multiple excitonic-series emissions possess dissimilar polarization-sensitive orientations are clearly present in a multilayered quasi-1D ZrS₃ nanoribbon with respect to the nanostripe edge of b axis. The excitonic emissions with E || b and E ⊥ b polarizations are respectively identified. A p-GaSe/n-ZrS₃ stacked-junction solar cell is also fabricated to show a highest polarized efficiency along the ZrS₃ nanoribbon's b axis.

Abstract

Anisotropic optical 2D materials are crucial for achieving multiple-quanta functions within quantum materials, which enables the fabrication of axially polarized electronic and optoelectronic devices. In this work, multiple excitonic emissions owning polarization-sensitive orientations are clearly detected in a multilayered quasi-1D ZrS₃ nanoribbon with respect to the nanostripe edge. Four excitons denoted as A_S1, A_S2, A_S, and A₂ with E ⊥ b polarized direction and one prominent A₁ exciton with E || b polarized emission are simultaneously detected in the polarized micro-photoluminescence (µPL) measurement of 1.9–2.2 eV at 10 K. In contrast to light emission, polarized micro-thermoreflectance (µTR) measurements are performed to identify the polarization dependence and verify the excitons in the multilayered ZrS₃ nanoribbon from the perspective of light absorption. At 10 K, a prominent and broadened peak on the lower-energy side, containing an indirect resonant emission (D_I) observed by µPL and an indirect defect-bound exciton peak (A_Ind) observed by both µPL and µTR, is simultaneously detected, confirming the existence of a quasi-direct band edge in ZrS₃. A van der Waals stacked p-GaSe/n-ZrS₃ heterojunction solar cell is fabricated, which demonstrates a maximum axially-polarized conversion efficiency up to 0.412% as the E || b polarized light incident onto the device.

An artificial nociceptor is developed utilizing ultrathin 2D-GaO_x, featuring low power consumption and quick switching. Electron energy loss spectroscopy reveals that, when a threshold bias is applied, Ag atoms disperse in the active layer, and then reassemble after bias removal, in order to reduce interfacial energy. The heterointerfaced 2D-GaO_x nociceptor ensures stable, precise charge transfer control, enabling long-term atmospheric use.

Abstract

Nociceptors are key sensory receptors that transmit warning signals to the central nervous system in response to painful stimuli. This fundamental process is emulated in an electronic device by developing a novel artificial nociceptor with an ultrathin, nonstoichiometric gallium oxide (GaO_x)-silicon oxide heterostructure. A large-area 2D-GaO_x film is printed on a substrate through liquid metal printing to facilitate the production of conductive filaments. This nociceptive structure exhibits a unique short-term temporal response following stimulation, enabling a facile demonstration of threshold-switching physics. The developed heterointerface 2D-GaO_x film enables the fabrication of fast-switching, low-energy, and compliance-free 2D-GaO_x nociceptors, as confirmed through experiments. The accumulation and extrusion of Ag in the oxide matrix are significant for inducing plastic changes in artificial biological sensors. High-resolution transmission electron microscopy and electron energy loss spectroscopy demonstrate that Ag clusters in the material dispersed under electrical bias and regrouped spontaneously when the bias is removed owing to interfacial energy minimization. Moreover, 2D nociceptors are stable; thus, heterointerface engineering can enable effective control of charge transfer in 2D heterostructural devices. Furthermore, the diffusive 2D-GaO_x device and its Ag dynamics enable the direct emulation of biological nociceptors, marking an advancement in the hardware implementation of artificial human sensory systems.

It presents a suite of techniques that allow to simultaneously measure both electrical and interfacial properties of room-temperature liquid eutectic gallium indium (EGaIn) as a function of applied electric potential, allowing for new insights into the mechanisms, which cause its dramatic decrease in interfacial tension under electrochemical oxidation.

Abstract

When in a pristine state, gallium and its alloys have the largest interfacial tensions of any liquid at room temperature. Nonetheless, applying as little as 0.8 V of electric potential across eutectic gallium indium (EGaIn) placed within aqueous sodium hydroxide (NaOH, or other electrolyte) solution will cause the metal to behave as if its interfacial tension is near zero. The mechanism behind this phenomenon has remained poorly understood because NaOH dissolves the oxide species, making it difficult to directly measure the concentration, thickness, or chemical composition of the film that forms at the interface. In addition, the oxide layers formed are atomically-thin. Here, it presents a suite of techniques that allow to simultaneously measure both electrical and interfacial properties as a function of applied electric potential, allowing for new insights into the mechanisms, which cause the dramatic decrease in interfacial tension. A key discovery from this work is that the interfacial tension displays hysteresis while lowering the applied potential. It combines these observations with electrochemical impedance spectroscopy to evaluate how these changes in interfacial tension arise from chemical, electrical, and mechanical changes on the interface, and close with ideas for how to build a free energy model to predict these changes from first principles.

The development of superconducting materials with high transition temperatures (T_C ) has sparked intense interest. Efforts have been focused on interfaces of materials, including superconductivity induced at non-superconducting material interfaces or augmenting T_C at the interface between superconducting/non-superconducting materials. This review explores interfacial and interface-enhanced superconductivity, and propelling superconductivity research toward practical, high-temperature applications.

Abstract

The development of superconducting materials has attracted significant attention not only for their improved performance, such as high transition temperature (T_C ), but also for the exploration of their underlying physical mechanisms. Recently, considerable efforts have been focused on interfaces of materials, a distinct category capable of inducing superconductivity at non-superconducting material interfaces or augmenting the T_C at the interface between a superconducting material and a non-superconducting material. Here, two distinct types of interfaces along with their unique characteristics are reviewed: interfacial superconductivity and interface-enhanced superconductivity, with a focus on the crucial factors and potential mechanisms responsible for enhancing superconducting performance. A series of materials systems is discussed, encompassing both historical developments and recent progress from the perspectives of technical innovations and the exploration of new material classes. The overarching goal is to illuminate pathways toward achieving high T_C , expanding the potential of superconducting parameters across interfaces, and propelling superconductivity research toward practical, high-temperature applications.

The state-of-the-art WS₂ nano-kirigami structures with various layer combinations and stacking configurations are synthesized by space-confined chemical vapor deposition method. With this obtained special stacking structure and unique etching channel, the metasurface is specially designed and integrated to achieve a photodetector with bidirectional polarization-sensitive detection capability in the infrared spectrum.

Abstract

The assembly and patterning engineering in two-dimensional (2D) materials hold importance for chip-level designs incorporating multifunctional detectors. At present, the patterning and stacking methods of 2D materials inevitably introduce impurity instability and functional limitations. Here, the space-confined chemical vapor deposition method is employed to achieve state-of-the-art kirigami structures of self-assembled WS₂, featuring various layer combinations and stacking configurations. With this technique as a foundation, the WS₂ nano-kirigami is integrated with metasurface design, achieving a photodetector with bidirectional polarization-sensitive detection capability in the infrared spectrum. Nano-kirigami can eliminate some of the uncontrollable factors in the processing of 2D material devices, providing a freely designed platform for chip-level multifunctional detection across multiple modules.

Nature, Published online: 07 August 2024; doi:10.1038/s41586-024-07786-2

By using intercalative oxidation techniques, stable, stoichiometric and atomically thin single-crystalline Al2O3 films can be produced, which can be effectively used as a dielectric in top-gated field-effect transistors based on two-dimensional materials.

Nature Photonics, Published online: 08 August 2024; doi:10.1038/s41566-024-01501-3

A two-dimensional van der Waals material, NbOCl2, that simultaneously exhibits near-unity linear dichroism (~99%) over 100 nm bandwidth in ultraviolet regime and large birefringence (0.26–0.46) within a wide visible–near-infrared transparency window is reported.

npj 2D Materials and Applications, Published online: 08 August 2024; doi:10.1038/s41699-024-00480-x

Multiphase superconductivity at the interface between ultrathin FeTe islands and Bi₂Te₃

Freestanding perovskite oxide membranes are directly integrated on n-type silicon wafers for high-performance ferroelectric tunnel junctions (FTJs). By strain engineering using a bilayer structure (BiFeO₃/PbTiO₃), the ferroelectric polarization is significantly enhanced, resulting in a high ON/OFF ratio (10⁶) and endurance (>10⁶). Furthermore, these FTJ-based synapses exhibit a commendable synaptic plasticity and a high-precision convolutional neural network (CNN) simulation.

Abstract

Perovskite-oxide-based ferroelectric tunnel junctions (FTJs) hold great potential for applications in non-volatile memory and neuromorphic computing due to their unique properties. However, the challenges in synthesizing high crystalline quality perovskite oxides directly on silicon wafer limit the applications of these FTJs in conventional Si-based integrated circuits, let alone the neural networks. Herein, perovskite oxide FTJs with an ON/OFF ratio up to 1.2×10⁶, writing/erasing speed down to 1 nanosecond, and cycling endurance (>10⁶) are achieved by integrating ultrathin freestanding ferroelectric perovskite oxide membranes directly on silicon wafers using a wet-transfer method. Moreover, synapses based on these FTJs exhibit long-term plasticity. For handwritten digits recognition task, the convolutional neural network (CNN) simulation is implemented achieving a recognition accuracy up to 98.9% based on the experimental performance, close to the result of 99.2% by software-floating-point-based CNN. This work sheds light on the integration of ferroelectric perovskite oxides directly on silicon for high-performance FTJ-based non-volatile memory and synaptic devices.

Highlights

• The review provides a comprehensive summary of performance limits of the single two-dimensional transition metal dichalcogenide (2D-TMD) transistor.

• The review details the two logical expressions of the single 2D-TMD logic transistor, including current and voltage.

• The review demonstrates the two calculating methods for dynamic energy consumption of 2D synaptic devices.

Nature Communications, Published online: 09 August 2024; doi:10.1038/s41467-024-51176-1

The authors investigate the twin boundaries in ferroelectric transition metal dichalcogenides, predicting their trap electrons with low densities, enhancing electron-electron interactions and potentially promoting Wigner crystallisation.

Single-atom thulium (Tm) modified porous tubular carbon nitride with carbon vacancies is successfully designed and synthesized. The construction of in-plane Tm sites, interlayer-bridged Tm-N charge transfer channels, and carbon vacancies promoted the transfer/separation of photogenerated carriers, enhanced the CO₂ adsorption/activation, optimized the reaction pathway, and ultimately led to excellent CO₂ photo-reduction.

Abstract

CO₂ reduction photocatalysts are favorable for obtaining renewable energy. Enriched active sites and effective photogenerated-carriers separation are keys for improving CO₂ photo-reduction. A thulium (Tm) single atom tailoring strategy introducing carbon vacancies in porous tubular graphitic carbon nitride (g-C₃N₄) surpassing the ever-reported g-C₃N₄ based photocatalysts, with 199.47 µmol g⁻¹ h⁻¹ CO yield, 96.8% CO selectivity, 0.84% apparent quantum efficiency and excellent photocatalytic stability, is implemented in this work. Results revealed that in-plane Tm sites and interlayer-bridged Tm-N charge transfer channels significantly enhanced the aggregation/transfer of photogenerated electrons thus promoting CO₂ adsorption/activation and contributing to *COOH intermediates formation. Meanwhile, Tm atoms and carbon vacancies both benefit for rich active sites and enhanced photogenerated-charge separation, thus optimizing reaction pathway and leading to excellent CO₂ photo-reduction. This work not only provides guidelines for CO₂ photo-reduction catalysts design but also offers mechanistic insights into single-atom based photocatalysts for solar fuel production.

2D antiferromagnetic (AFM) semiconductors are promising components of opto-spintronic devices due to terahertz operation frequencies and minimal interactions with stray fields. Here, photocurrent spectroscopy is employed to study the 2D AFM semiconductor NiI₂, which enables the detection of optically dark magnetic excitons down to bilayer thickness, revealing linear polarization that is coupled to the underlying helical AFM order of NiI₂.

Abstract

Two-dimensional (2D) antiferromagnetic (AFM) semiconductors are promising components of opto-spintronic devices due to terahertz operation frequencies and minimal interactions with stray fields. However, the lack of net magnetization significantly limits the number of experimental techniques available to study the relationship between magnetic order and semiconducting properties. Here, they demonstrate conditions under which photocurrent spectroscopy can be employed to study many-body magnetic excitons in the 2D AFM semiconductor NiI₂. The use of photocurrent spectroscopy enables the detection of optically dark magnetic excitons down to bilayer thickness, revealing a high degree of linear polarization that is coupled to the underlying helical AFM order of NiI₂. In addition to probing the coupling between magnetic order and dark excitons, this work provides strong evidence for the multiferroicity of NiI₂ down to bilayer thickness, thus demonstrating the utility of photocurrent spectroscopy for revealing subtle opto-spintronic phenomena in the atomically thin limit.

Flexoelectricity has triggered new feasibilities of advancements in functional materials, especially for nanoscale materials. This review offers a comprehensive overview of ongoing advancements of flexoelectric materials and discusses their implementation and practical applications. The mechanisms for the enhancement of flexoelectricity in materials and corresponding design principles are also discussed. Finally, conclusions and future prospects are provided in the review, considering the unsolved issues.

Abstract

Flexoelectricity, a universal electromechanical coupling phenomenon, has triggered new feasibilities of advancements in functional materials, especially for nanoscale materials. The strong flexoelectric response is initially discovered in ceramic materials with high permittivity, and then the past decades have witnessed the expansion of flexoelectricity to a broader range of material systems including semiconductors, polymers, and soft elastomers, which in turn raise emerging applications of flexoelectricity. Moreover, flexoelectricity is demonstrated to be significantly enhanced in thin films and nanostructures where ultra-high strain gradients are easier to achieve, rendering flexoelectricity attractive for modifying the functional properties of advanced materials and devices at the nanoscale. To provide a comprehensive drawing of the above aspects, this review highlights the recent progress of flexoelectricity in diverse materials, covering the characterization of flexoelectricity, the fundamental mechanisms of the enhancement flexoelectric response as well as the multi-functional applications. Finally, some open questions and perspectives are presented, underlining the fascinating future of this field.

Reconfigurable and regenerable magnetic liquid metal microrobots are designed for capturing and removing micro/nanoplastics. The process involves four steps: 1) Preparation of LiquidBots. 2) Magnetic manipulation and transport of micro/nanoplastics using a 3D magnetic field. 3) Desorption of the captured micro/nanoplastics. 4) Reconfigurable of GaIn-Fe.

Abstract

The pervasive presence of micro/nanoplastics in the environment is a significant threat to ecosystems and human health, demanding effective remediation strategies. Traditional methods for extracting these pollutants from water are often inadequate, typically leaving environmentally harmful residues. In response, this work introduces an innovative approach using reconfigurable and regenerable liquid metal microrobots (LiquidBots) that are magnetically driven to actively sequester micro/nanoplastics from aquatic environments. These LiquidBots utilize a coating of gallium oxide for enhanced adhesion and electrostatic interaction to capture over 80% of nanoplastics present in the solution. Additionally, the LiquidBots can be easily regenerated through sonication, which dislodges captured nanoplastics, allowing the microrobots to be reused. This novel technology offers a highly efficient, adaptable, and sustainable solution to combat the micro/nanoplastic pollution crisis.

A self-anchored van-der-Waals stacking growth mechanism is developed to produce transition-metal dichalcogenide nanoplates with desired lateral size and locations. As such, WSe₂ and MoSe₂ nanoplate arrays are produced for the subsequent fabrication of nanoplate-based optoelectronics.

Abstract

Transition-metal dichalcogenide (TMDs) nanoplates exhibit unique properties different from their monolayer counterparts. Controllable nucleation and growth are prerequisite and highly desirable for their practical applications. Here, a self-anchored van-der-Waals stacking growth method is developed, by which the substrate pit induced by precursor etching anchors the source material, impedes the lateral spreading of source droplets and facilitates the in situ stacking growth of high-quality TMD nanoplates with a thickness of tens to hundreds of nanometers at well-defined locations. As such, an array of TMD nanoplates with controlled lateral dimensions are produced and applied in arrayed photodetectors. This study solves the problem of controllable preparation of TMD nanoplates, holding promise for applications in electronics and optoelectronics.

The band alignment of 1L-WSe₂/5L-Bi₂Te_{3- x} Se _x vdWHs can be continuously modulated from Type I (x = 0) to Type III (x = 3) by increasing Se mole fraction, which is crucial for promoting charge separation and transfer in optoelectronic devices, ultimately determining the behavior of photodetectors and LEDs based on vdWHs.

Abstract

Band alignment engineering is crucial for facilitating charge separation and transfer in optoelectronic devices, which ultimately dictates the behavior of Van der Waals heterostructures (vdWH)-based photodetectors and light emitting diode (LEDs). However, the impact of the band offset in vdWHs on important figures of merit in optoelectronic devices has not yet been systematically analyzed. Herein, the regulation of band alignment in WSe₂/Bi₂Te_{3- x} Se _x vdWHs (0 ≤ x ≤ 3) is demonstrated through the implementation of chemical vapor deposition (CVD). A combination of experimental and theoretical results proved that the synthesized vdWHs can be gradually tuned from Type I (WSe₂/Bi₂Te₃) to Type III (WSe₂/Bi₂Se₃). As the band alignment changes from Type I to Type III, a remarkable responsivity of 58.12 A W⁻¹ and detectivity of 2.91×10¹² Jones (in Type I) decrease in the vdWHs-based photodetector, and the ultrafast photoresponse time is 3.2 µs (in Type III). Additionally, Type III vdWH-based LEDs exhibit the highest luminance and electroluminescence (EL) external quantum efficiencies (EQE) among p-n diodes based on Transition Metal Dichalcogenides (TMDs) at room temperature, which is attributed to band alignment-induced distinct interfacial charge injection. This work serves as a valuable reference for the application and expansion of fundamental band alignment principles in the design and fabrication of future optoelectronic devices.