Vapor Deposition of Bilayer 3R MoS2 with Room-Temperature Ferroelectricity.

Two-dimensional ultrathin ferroelectrics have attracted much interest due to their potential application in high-density integration of non-volatile memory devices. Recently, 2D van der Waals ferroelectric based on interlayer translation has been reported in twisted bilayer h-BN and transition metal dichalcogenides (TMDs). However, sliding ferroelectricity is not well studied in non-twisted homo-bilayer TMD grown directly by chemical vapor deposition (CVD). In this paper, for the first time, experimental observation of a room-temperature out-of-plane ferroelectric switch in semiconducting bilayer 3R MoS2 synthesized by reverse-flow CVD is reported. Piezoelectric force microscopy (PFM) hysteretic loops and first principle calculations demonstrate that the ferroelectric nature and polarization switching processes are based on interlayer sliding. The vertical Au/3R MoS2/Pt device exhibits a switchable diode effect. Polarization modulated Schottky barrier height and polarization coupling of interfacial deep states trapping/detrapping may serve in coordination to determine switchable diode effect. The room-temperature ferroelectricity of CVD-grown MoS2 will proceed with the potential wafer-scale integration of 2D TMDs in the logic circuit.

CVD Synthesis of Twisted Bilayer WS2 with Tunable Second Harmonic Generation.

The introduction of rotational freedom by twist angles in twisted bilayer (TB) transition metal dichalcogenides (TMDCs) can tailor the inherent properties of the TMDCs, which provides a promising platform to investigate the exotic physical properties. However, direct synthesis of high-quality TB-TMDCs with full twist angles is significantly challenging due to the substantial energy barriers during crystal growth. Here, a modified chemical vapor deposition strategy is proposed to synthesize TB-WS2 with a wide twist angle range from 0° to 120°. Utilizing a tilted SiO2/Si substrate, a gas flow disturbance is generated in the furnace tube to create a heterogeneous concentration gradient of the metal precursor, which provides an extra driving force for the growth of TB-WS2. The Raman and photoluminescence results confirm a weak interlayer coupling of the TB-WS2. High-quality periodic Moiré patterns are observed in the scanning transmission electron microscopy images. Moreover, owing to the strong correlation between the nonlinear optical response and the twisted crystal structure, tunable second harmonic generation behaviors are realized in the TB-WS2. This approach opens up a new avenue for the direct growth of high-crystalline-quality and pristine TB-TMDCs and their potential applications in nonlinear optical devices.

Reconfiguring nucleation for CVD growth of twisted bilayer MoS2 with a wide range of twist angles.

Twisted bilayer (TB) transition metal dichalcogenides (TMDCs) beyond TB-graphene are considered an ideal platform for investigating condensed matter physics, due to the moiré superlattices-related peculiar band structures and distinct electronic properties. The growth of large-area and high-quality TB-TMDCs with wide twist angles would be significant for exploring twist angle-dependent physics and applications, but remains challenging to implement. Here, we propose a reconfiguring nucleation chemical vapor deposition (CVD) strategy for directly synthesizing TB-MoS2 with twist angles from 0° to 120°. The twist angles-dependent Moiré periodicity can be clearly observed, and the interlayer coupling shows a strong relationship to the twist angles. Moreover, the yield of TB-MoS2 in bilayer MoS2 and density of TB-MoS2 are significantly improved to 17.2% and 28.9 pieces/mm2 by tailoring gas flow rate and molar ratio of NaCl to MoO3. The proposed reconfiguring nucleation approach opens an avenue for the precise growth of TB-TMDCs for both fundamental research and practical applications.

Scalable Manufacturing of Large-Area Pressure Sensor Array for Sitting Posture Recognition in Real Time.

Pressure sensors are considered the key technology for potential applications in real-time health monitoring, artificial electronic skins, and human-machine interfaces. Despite the significant progress in developing novel sensitive materials and constructing unique sensor structures, it remains challenging to fabricate large-area pressure sensor arrays due to the involvement of complex procedures including photolithography, laser writing, or coating. Herein, a scalable manufacturing approach for the realization of pressure sensor arrays with substantially enlarged sensitive areas is proposed using a versatile screen-printing technique. A compensation mechanism is introduced into the printing process to ensure the precise alignment of conductive electrodes, insulation layers, and sensitive microstructures with an alignment error of less than 4 µm. The fully screen-printed sensors exhibit excellent collective sensing performance, such as a reasonable pressure sensitivity of -2.2 kPa-1, a fast response time of 40 ms, and superior durability over 3000 consecutive pressures. Additionally, an integrated 16 × 32 pressure sensor array with a sensing area of 190 × 380 mm2 is demonstrated to precisely recognize the sitting postures and the body weights, showing great potential in continuous and real-time health status monitoring.

Berry Curvature Dipole Induced Giant Mid-Infrared Second-Harmonic Generation in 2D Weyl Semiconductor.

Due to its inversion-broken triple helix structure and the nature of Weyl semiconductor, 2D Tellurene (2D Te) is promising to possess a strong nonlinear optical response in the infrared region, which is rarely reported in 2D materials. Here, a giant nonlinear infrared response induced by large Berry curvature dipole (BCD) is demonstrated in the Weyl semiconductor 2D Te. Ultrahigh second-harmonic generation response is acquired from 2D Te with a large second-order nonlinear optical susceptibility (χ(2) ), which is up to 23.3 times higher than that of monolayer MoS2 in the range of 700-1500 nm. Notably, distinct from other 2D nonlinear semiconductors, χ(2) of 2D Te increases extraordinarily with increasing wavelength and reaches up to 5.58 nm V-1 at ≈2300 nm, which is the best infrared performance among the reported 2D nonlinear materials. Large χ(2) of 2D Te also enables the high-intensity sum-frequency generation with an ultralow continuous-wave (CW) pump power. Theoretical calculations reveal that the exceptional performance is attributed to the presence of large BCD located at the Weyl points of 2D Te. These results unravel a new linkage between Weyl semiconductor and strong optical nonlinear responses, rendering 2D Te a competitive candidate for highly efficient nonlinear 2D semiconductors in the infrared region.

Enhancing the Toughness of Free-Standing Polyimide Films for Advanced Electronics Applications: A Study on the Impact of Film-Forming Processes.

High-quality and free-standing polyimide (PI) film with desirable mechanical properties and uniformity is in high demand due to its widespread applications in highly precise flexible and chip-integrated sensors. In this study, a free-standing PI film with high toughness was successfully prepared using a diamine monomer with ether linkages. The prepared PI films exhibited significantly superior mechanical properties compared to PI films of the same molecular structure, which can be attributed to the systematic exploration of the film-forming process. The exploration of the film-forming process includes the curing procedures, film-forming substrates, and annealing treatments. Additionally, the thickness uniformity and surface homogeneity of free-standing films were crucial for toughness. Increasing the crystallinity of the PI films by eliminating residual stress also contributed to their high strength. The results demonstrate that by adjusting the above-mentioned factors, the prepared PI films possess excellent mechanical properties, with tensile strength and elongation at break of 194.71 MPa and 130.13%, respectively.

Laser-assisted synthesis of two-dimensional transition metal dichalcogenides: a mini review.

The atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted the researcher's interest in the field of flexible electronics due to their high mobility, tunable bandgaps, and mechanical flexibility. As an emerging technique, laser-assisted direct writing has been used for the synthesis of TMDCs due to its extremely high preparation accuracy, rich light-matter interaction mechanism, dynamic properties, fast preparation speed, and minimal thermal effects. Currently, this technology has been focused on the synthesis of 2D graphene, while there are few literatures that summarize the progress in direct laser writing technology in the synthesis of 2D TMDCs. Therefore, in this mini-review, the synthetic strategies of applying laser to the fabrication of 2D TMDCs have been briefly summarized and discussed, which are divided into top-down and bottom-up methods. The detailed fabrication steps, main characteristics, and mechanism of both methods are discussed. Finally, prospects and further opportunities in the booming field of laser-assisted synthesis of 2D TMDCs are addressed.

Quasi-One-Dimensional van der Waals Transition Metal Trichalcogenides.

The transition metal trichalcogenides (TMTCs) are quasi-one-dimensional (1D) MX3-type van der Waals layered semiconductors, where M is a transition metal element of groups IV and V, and X indicates chalcogen element. Due to the unique quasi-1D crystalline structures, they possess several novel electrical properties such as variable bandgaps, charge density waves, and superconductivity, and highly anisotropic optical, thermoelectric, and magnetic properties. The study of TMTCs plays an essential role in the 1D quantum materials field, enabling new opportunities in the material research dimension. Currently, tremendous progress in both materials and solid-state devices has been made, demonstrating promising applications in the realization of nanoelectronic devices. This review provides a comprehensive overview to survey the state of the art in materials, devices, and applications based on TMTCs. Firstly, the symbolic structure, current primary synthesis methods, and physical properties of TMTCs have been discussed. Secondly, examples of TMTC applications in various fields are presented, such as photodetectors, energy storage devices, catalysts, and sensors. Finally, we give an overview of the opportunities and future perspectives for the research of TMTCs, as well as the challenges in both basic research and practical applications.

Large-Scale Ultra-Robust MoS2 Patterns Directly Synthesized on Polymer Substrate for Flexible Sensing Electronics.

Synthesis of large-area patterned MoS2 is considered the principle base for realizing high-performance MoS2 -based flexible electronic devices. Patterning and transferring MoS2 films to target flexible substrates, however, require conventional multi-step photolithography patterning and transferring process, despite tremendous progress in the facilitation of practical applications. Herein, an approach to directly synthesize large-scale MoS2 patterns that combines inkjet printing and thermal annealing is reported. An optimal precursor ink is prepared that can deposit arbitrary patterns on polyimide films. By introducing a gas atmosphere of argon/hydrogen (Ar/H2 ), thermal treatment at 350 °C enables an in situ decomposition and crystallization in the patterned precursors and, consequently, results in the formation of MoS2 . Without complicated processes, patterned MoS2 is obtained directly on polymer substrate, exhibiting superior mechanical flexibility and durability (≈2% variation in resistance over 10,000 bending cycles), as well as excellent chemical stability, which is attributed to the generated continuous and thin microstructures, as well as their strong adhesion with the substrate. As a step further, this approach is employed to manufacture various flexible sensing devices that are insensitive to body motions and moisture, including temperature sensors and biopotential sensing systems for real-time, continuously monitoring skin temperature, electrocardiography, and electromyography signals.

Polarized Cu-Bi Site Pairs for Non-Covalent to Covalent Interaction Tuning toward N2 Photoreduction.

A universal atomic layer confined doping strategy is developed to prepare Bi24 O31 Br10 materials incorporating isolated Cu atoms. The local polarization can be created along the CuOBi atomic interface, which enables better electron delocalization for effective N2 activation. The optimized Cu-Bi24 O31 Br10 atomic layers show 5.3× and 88.2× improved photocatalytic nitrogen fixation activity than Bi24 O31 Br10 atomic layer and bulk Bi24 O31 Br10 , respectively, with the NH3 generation rate reaching 291.1 µmol g-1 h-1 in pure water. The polarized Cu-Bi site pairs can increase the non-covalent interaction between the catalyst's surface and N2 molecules, then further weaken the covalent bond order in NN. As a result, the hydrogenation pathways can be altered from the associative distal pathway for Bi24 O31 Br10 to the alternating pathway for Cu-Bi24 O31 Br10 . This strategy provides an accessible pathway for designing polarized metal site pairs or tuning the non-covalent interaction and covalent bond order.

Metamaterial Absorbers: From Tunable Surface to Structural Transformation.

Since the first demonstration, remarkable progress has been made in the theoretical analysis, structural design, numerical simulation, and potential applications of metamaterial absorbers (MAs). With the continuous advancement of novel materials and creative designs, the absorption of MAs is significantly improved over a wide frequency spectrum from microwaves to the optical regime. Further, the integration of active elements into the MA design allows the dynamical manipulation of electromagnetic waves, opening a new platform to push breakthroughs in metadevices. In the last several years, numerous efforts have been devoted to exploring innovative approaches for incorporating tunability to MAs, which is highly desirable because of the progressively increasing demand on designing versatile metadevices. Here, a comprehensive and systematical overview of active MAs with adaptive and on-demand manner is presented, highlighting innovative materials and unique strategies to precisely control the electromagnetic response. In addition to the mainstream method by manipulating periodic patterns, two additional approaches, including tailoring dielectric spacer and transforming overall structure are called back. Following this, key parameters, such as operating frequency, relative tuning range, and switching speed are summarized and compared to guide for optimum design. Finally, potential opportunities in the development of active MAs are discussed.

Ultra-Robust and Extensible Fibrous Mechanical Sensors for Wearable Smart Healthcare.

Fibrous material with high strength and large stretchability is an essential component of high-performance wearable electronic devices. Wearable electronic systems require a material that is strong to ensure durability and stability, and a wide range of strain to expand their applications. However, it is still challenging to manufacture fibrous materials with simultaneously high mechanical strength and the tensile property. Herein, the ultra-robust (≈17.6 MPa) and extensible (≈700%) conducting microfibers are developed and demonstrated their applications in fabricating fibrous mechanical sensors. The mechanical sensor shows high sensitivity in detecting strains that have high strain resolution and a large detection range (from 0.0075% to 400%) simultaneously. Moreover, low frequency vibrations between 0 and 40 Hz are also detected, which covers most tremors that occur in the human body. As a further step, a wearable and smart health-monitoring system has been developed using the fibrous mechanical sensor, which is capable of monitoring health-related physiological signals, including muscle movement, body tremor, wrist pulse, respiration, gesture, and six body postures to predict and diagnose diseases, which will promote the wearable telemedicine technology.

Solid-Ionic Memory in a van der Waals Heterostructure.

Defect states dominate the performance of low-dimensional nanoelectronics, which deteriorate the serviceability of devices in most cases. But in recent years, some intriguing functionalities are discovered by defect engineering. In this work, we demonstrate a bifunctional memory device of a MoS2/BiFeO3/SrTiO3 van der Waals heterostructure, which can be programmed and erased by solely one kind of external stimuli (light or electrical-gate pulse) via engineering of oxygen-vacancy-based solid-ionic gating. The device shows multibit electrical memory capability (>22 bits) with a large linearly tunable dynamic range of 7.1 × 106 (137 dB). Furthermore, the device can be programmed by green- and red-light illuminations and then erased by UV light pulses. Besides, the photoresponse under red-light illumination reaches a high photoresponsivity (6.7 × 104 A/W) and photodetectivity (2.12 × 1013 Jones). These results highlighted solid-ionic memory for building up multifunctional electronic and optoelectronic devices.

Programmable patterned MoS2 film by direct laser writing for health-related signals monitoring.

The two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising flexible electronic materials for strategic flexible information devices. Large-area and high-quality patterned materials were usually required by flexible electronics due to the limitation from the process of manufacturing and integration. However, the synthesis of large-area patterned 2D TMDs with high quality is difficult. Here, an efficient and powerful pulsed laser has been developed to synthesize wafer-scale MoS2. The flexible strain sensor was fabricated using MoS2 and showed high performance of low detection limit (0.09%), high gauge factor (1,118), and high stability (1,000 cycles). Besides, we demonstrated its applications in real-time monitoring of health-related physiological signals such as radial artery pressure, respiratory rate, and vocal cord vibration. Our findings suggest that the laser-assisted method is effective and capable of synthesizing wafer-scale 2D TMDs, which opens new opportunities for the next flexible electronic devices and wearable health monitoring.

Machine Learning Driven Synthesis of Few-Layered WTe2 with Geometrical Control.

Reducing the lateral scale of two-dimensional (2D) materials to one-dimensional (1D) has attracted substantial research interest not only to achieve competitive electronic applications but also for the exploration of fundamental physical properties. Controllable synthesis of high-quality 1D nanoribbons (NRs) is thus highly desirable and essential for further study. Here, we report the implementation of supervised machine learning (ML) for the chemical vapor deposition (CVD) synthesis of high-quality quasi-1D few-layered WTe2 NRs. Feature importance analysis indicates that H2 gas flow rate has a profound influence on the formation of WTe2, and the source ratio governs the sample morphology. Notably, the growth mechanism of 1T' few-layered WTe2 NRs is further proposed, which provides new insights for the growth of intriguing 2D and 1D tellurides and may inspire the growth strategies for other 1D nanostructures. Our findings suggest the effectiveness and capability of ML in guiding the synthesis of 1D nanostructures, opening up new opportunities for intelligent materials development.

Inversion symmetry broken in 2H phase vanadium-doped molybdenum disulfide.

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have received much attention in nonlinear optical applications due to their unique crystal structures and second harmonic generation (SHG) efficiency. However, SHG signals in TMDs show a layer-dependent behavior, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer) of TMDs. Herein, we synthesized monolayer and bilayer 2H and 3R phase vanadium (V)-doped MoS2 crystal. Raman spectroscopy, XPS, and STEM were used to identify the chemical composition and crystalline structure of as-grown nanoflakes. SHG measurement was used to research the symmetry of V-doped MoS2 crystals with different stacking orders. Significantly, the SHG efficiency in bilayer 2H phase V-doped MoS2 is equivalent to the 3R phase, indicating an inversion symmetry broken lattice structure caused by the in situ V substitute for Mo sites. This study will be conducive to promote the development of promising nonlinear optical devices based on 2D material.

Two-step chemical vapor deposition synthesis of NiTe2-MoS2 vertical junctions with improved MoS2 transistor performance.

The primary challenge for the widespread application of two-dimensional (2D) electronics is to achieve satisfactory electrical contacts because, during the traditional metal integration process, difficulties arise due to inevitable physical damage and selective doping. Two-dimensional metal-semiconductor junctions have attracted attention for the potential application to achieve reliable electrical contacts in future atomically thin electronics. Here we demonstrate the van der Waals epitaxial growth of 2D NiTe2-MoS2 metal-semiconductor vertical junctions where the upper NiTe2 selectively nucleates at the edge of the underlying MoS2. Optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning transmission electron microscope (STEM) studies confirmed that NiTe2-MoS2 metal-semiconductor vertical junctions had been successfully synthesized. The electrical properties of the NiTe2-contacted MoS2 field-effect transistors (FETs) showed higher field-effect mobilities (µ FE) than those with deposited Cr/Au contacts. This study demonstrates an effective pathway to improved MoS2 transistor performance with metal-semiconductor junctions.

Two-step CVD synthesis of NiTe2-MoS2vertical junctions with improved MoS2transistors performance.

The primary challenge for widespread applications of two-dimensional electronics is to achieve satisfactory electrical contacts due to the difficulties in inevitable physical damages and selectively doping during traditional metal integration process. The two-dimensional (2D) metal-semiconductor junctions have attracted captivated attention for potential applications in future atomically thin electronics as perfect candidates for achieving reliable electrical contacts. Here we demonstrate the van der Waals epitaxial growth of 2D NiTe2-MoS2 metal-semiconductor vertical junctions which the upper NiTe2 selectively nucleate at the edge of underlying MoS2. Optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning transmission electron microscope (STEM) studies confirm that NiTe2-MoS2 metal-semiconductor vertical junctions are successfully synthesized. Electrical properties of the NiTe2-contacted MoS2 field-effect transistors (FETs) show higher field-effect mobilities (µFE) than those with deposited Cr/Au contacts. This study demonstrates an effective pathway to improved MoS2 transistors performance with metal-semiconductor junctions.

Terahertz Surface Emission from MoSe2 at the Monolayer Limit.

The surface-charge region of bulk and monolayer MoSe2 is analyzed directly by terahertz (THz) surface emission spectroscopy in a nondestructive way. Both surface nonlinear optical polarization and surface field-induced photocurrent contribute to the THz radiation in both bulk and monolayer MoSe2. The first THz emission mechanism is due to the surface optical rectification and the second one is due to the photogenerated carriers accelerated by the surface depletion field. The THz radiation contribution from the surface optical rectification is basically the same for both bulk and monolayer MoSe2 because of the same symmetry at the surface. However, the contribution from the surface field-induced photocurrent is ∼94.2% in bulk MoSe2 and it goes down to 74.5% in monolayer MoSe2. This is due to the larger surface depletion field in bulk MoSe2 (∼2.54 × 107 V/m) compared with that in monolayer MoSe2 (∼5.42 × 105 V/m), as such THz emission from the bulk is approximately four times larger than that from monolayer MoSe2. This work not only proves the clear THz radiation mechanism from MoSe2 crystals but also affords a THz technology for the surface characterization of two-dimensional materials.

Phase-controllable growth of ultrathin 2D magnetic FeTe crystals.

Two-dimensional (2D) magnets with intrinsic ferromagnetic/antiferromagnetic (FM/AFM) ordering are highly desirable for future spintronic devices. However, the direct growth of their crystals is in its infancy. Here we report a chemical vapor deposition approach to controllably grow layered tetragonal and non-layered hexagonal FeTe nanoplates with their thicknesses down to 3.6 and 2.8 nm, respectively. Moreover, transport measurements reveal these obtained FeTe nanoflakes show a thickness-dependent magnetic transition. Antiferromagnetic tetragonal FeTe with the Néel temperature (TN) gradually decreases from 70 to 45 K as the thickness declines from 32 to 5 nm. And ferromagnetic hexagonal FeTe is accompanied by a drop of the Curie temperature (TC) from 220 K (30 nm) to 170 K (4 nm). Theoretical calculations indicate that the ferromagnetic order in hexagonal FeTe is originated from its concomitant lattice distortion and Stoner instability. This study highlights its potential applications in future spintronic devices.