High-Mobility Transistors Based on Large-Area and Highly Crystalline CVD-Grown MoSe2 Films on Insulating Substrates

AC Characteristics of van der Waals Bipolar Junction Transistors Using an MoS2/WSe2/MoS2 Heterostructure

Two-dimensional layered materials, characterized by their atomically thin thicknesses and surfaces that are free of dangling bonds, hold great promise for fabricating ultrathin, lightweight, and flexible bipolar junction transistors (BJTs). In this paper, a van der Waals (vdW) BJT was fabricated by vertically stacking MoS2, WSe2, and MoS2 flakes in sequence. The AC characteristics of the vdW BJT were studied for the first time, in which a maximum common emitter voltage gain of around 3.5 was observed. By investigating the time domain characteristics of the device under various operating frequencies, the frequency response of the device was summarized, which experimentally proved that the MoS2/WSe2/MoS2 BJT has voltage amplification capability in the 0-200 Hz region. In addition, the phase response of the device was also investigated. A phase inversion was observed in the low-frequency range. As the operating frequency increases, the relative phase between the input and output signals gradually shifts until it is in phase at frequencies exceeding 2.3 kHz. This work demonstrates the signal amplification applications of the vdW BJTs for neuromorphic computing and wearable healthcare devices.

Biomimetic wafer-scale alignment of tellurium nanowires for high-mobility flexible and stretchable electronics

Flexible and stretchable thin-film transistors (TFTs) are crucial in skin-like electronics for wearable and implantable applications. Such electronics are usually constrained in performance owing to a lack of high-mobility and stretchable semiconducting channels. Tellurium, a rising semiconductor with superior charge carrier mobilities, has been limited by its intrinsic brittleness and anisotropy. Here, we achieve highly oriented arrays of tellurium nanowires (TeNWs) on various substrates with wafer-scale scalability by a facile lock-and-shear strategy. Such an assembly approach mimics the alignment process of the trailing tentacles of a swimming jellyfish. We further apply these TeNW arrays in high-mobility TFTs and logic gates with improved flexibility and stretchability. More specifically, mobilities over 100 square centimeters per volt per second and on/off ratios of ~104 are achieved in TeNW-TFTs. The TeNW-TFTs on polyethylene terephthalate can sustain an omnidirectional bending strain of 1.3% for more than 1000 cycles. Furthermore, TeNW-TFTs on an elastomeric substrate can withstand a unidirectional strain of 40% with no performance degradation.

Phase-engineered synthesis of atomically thin te single crystals with high on-state currents

Multiple structural phases of tellurium (Te) have opened up various opportunities for the development of two-dimensional (2D) electronics and optoelectronics. However, the phase-engineered synthesis of 2D Te at the atomic level remains a substantial challenge. Herein, we design an atomic cluster density and interface-guided multiple control strategy for phase- and thickness-controlled synthesis of α-Te nanosheets and β-Te nanoribbons (from monolayer to tens of μm) on WS2 substrates. As the thickness decreases, the α-Te nanosheets exhibit a transition from metallic to n-type semiconducting properties. On the other hand, the β-Te nanoribbons remain p-type semiconductors with an ON-state current density (ION) up to ~ 1527 μA μm-1 and a mobility as high as ~ 690.7 cm2 V-1 s-1 at room temperature. Both Te phases exhibit good air stability after several months. Furthermore, short-channel (down to 46 nm) β-Te nanoribbon transistors exhibit remarkable electrical properties (ION = ~ 1270 μA μm-1 and ON-state resistance down to 0.63 kΩ μm) at Vds = 1 V.

First-principles study of the effect of doping on the optoelectronic properties of defective monolayers of MoSe2

CONTEXT

In this paper, the structural stability, electronic structure, and optical properties of monolayer MoSe2 doped with C, O, Si, S, and Te atoms, respectively, under defective conditions are investigated based on first principles. It is found that the system is more structurally stable when defecting a single Se atom as compared to defecting a single Mo or two Se atoms. The electronic structure analysis of the system reveals that intrinsic MoSe2 is a direct bandgap semiconductor. The bandgap value of the system decreases with a single Se atom defect and introduces two new impurity energy levels in the conduction band. The defective systems doped with C and Si atoms all exhibit P-type doping. The total density of states of intrinsic MoSe2 is mainly contributed by the Mo-d and Se-p orbitals, and new density of state peaks appears near the conduction band after the defects of Se atoms. The total density of states of the defective system doped by each atom is mainly contributed by Mo-d, Se-p, and the result of the p orbital contribution of each dopant atom. By analyzing the dielectric function of each system, it is found that the intrinsic MoSe2 has the lowest static permittivity and the C-doped defect system has the highest static permittivity, which reaches 21.42. The C- and Si-doped defect systems are the first to start absorbing the light, and the intrinsic MoSe2 absorbs the light later, with its absorption edge starting at 1.25 eV. In the visible range, the reflection peaks of the systems move toward the high-energy region and the blue-shift phenomenon occurs. It is hoped that applying modification means to modulate the physical properties of the two-dimensional materials will provide some theoretical basis for broadening the application of monolayer MoSe2 in the field of optoelectronic devices.

METHODS

This study utilizes the first principle computational software package MS8.0 (Materials studio8.0) under density functional theory (DFT). The exchange-correlation potential (GGA-PBE) is described by the Perdew-Burke-Ernzerhof correlation function in CASTEP, and the potential function adopts the ultrasoft pseudopotential in the inverse space formulation. The plane wave truncation energy Ecut is set to 400 eV, the K-point is taken as 5 × 5 × 1, and the force convergence criterion is 0.05 eV/Å. The convergence accuracy of the total energy of the system is less than 1.0 × 10-5 eV/atom, the tolerance shift is less than 0.002 Å, and the stress deviation is less than 0.1 GPa. The vacuum layer is taken as 15 Å, which is intended to minimize the interlayer force. The vacuum layer was set to 15 Å to avoid the effect of layer-to-layer interaction forces in the crystal cell.

High-performance monolayer MoS2nanosheet GAA transistor

In this article, a 0.7 nm thick monolayer MoS2nanosheet gate-all-around field effect transistors (NS-GAAFETs) with conformal high-κmetal gate deposition are demonstrated. The device with 40 nm channel length exhibits a high on-state current density of ~410μAμm-1with a large on/off ratio of 6 × 108at drain voltage = 1 V. The extracted contact resistance is 0.48 ± 0.1 kΩμm in monolayer MoS2NS-GAAFETs, thereby showing the channel-dominated performance with the channel length scaling from 80 to 40 nm. The successful demonstration of device performance in this work verifies the integration potential of transition metal dichalcogenides for future logic transistor applications.

External Fields Assisted Highly Efficient Oxygen Evolution Reaction of Confined 1T-VSe2 Ferromagnetic Nanoparticles

As a clean and effective approach, the introduction of external magnetic fields to improve the performance of catalysts has attracted extensive attention. Owing to its room-temperature ferromagnetism, chemical stability, and earth abundance, VSe2 is expected to be a promising and cost-effective ferromagnetic electrocatalyst for the accomplishment of high-efficient spin-related OER kinetics. In this work, a facile pulsed laser deposition (PLD) method combined with rapid thermal annealing (RTA) treatment is used to successfully confine monodispersed 1T-VSe2 nanoparticles in amorphous carbon matrix. As expected, with external magnetic fields of 800 mT stimulation, the confined 1T-VSe2 nanoparticles exhibit highly efficient oxygen evolution reaction (OER) catalytic activity with an overpotential of 228 mV for 10 mA cm-2 and remarkable durability without deactivation after >100 h OER operation. The experimental results together with theoretical calculations illustrate that magnetic fields can facilitate the surface charge transfer dynamics of 1T-VSe2 , and modify the adsorption-free energy of *OOH, thus finally improving the intrinsic activity of the catalysts. This work realizes the application of ferromagnetic VSe2 electrocatalyst in highly efficient spin-dependent OER kinetics, which is expected to promote the application of transition metal chalcogenides (TMCs) in external magnetic field-assisted electrocatalysis.

Transparent Electronics for Wearable Electronics Application

Recent advancements in wearable electronics offer seamless integration with the human body for extracting various biophysical and biochemical information for real-time health monitoring, clinical diagnostics, and augmented reality. Enormous efforts have been dedicated to imparting stretchability/flexibility and softness to electronic devices through materials science and structural modifications that enable stable and comfortable integration of these devices with the curvilinear and soft human body. However, the optical properties of these devices are still in the early stages of consideration. By incorporating transparency, visual information from interfacing biological systems can be preserved and utilized for comprehensive clinical diagnosis with image analysis techniques. Additionally, transparency provides optical imperceptibility, alleviating reluctance to wear the device on exposed skin. This review discusses the recent advancement of transparent wearable electronics in a comprehensive way that includes materials, processing, devices, and applications. Materials for transparent wearable electronics are discussed regarding their characteristics, synthesis, and engineering strategies for property enhancements. We also examine bridging techniques for stable integration with the soft human body. Building blocks for wearable electronic systems, including sensors, energy devices, actuators, and displays, are discussed with their mechanisms and performances. Lastly, we summarize the potential applications and conclude with the remaining challenges and prospects.

Flexible 2D Materials beyond Graphene: Synthesis, Properties, and Applications

2D materials are now at the forefront of state-of-the-art nanotechnologies due to their fascinating properties and unique structures. As expected, low-cost, high-volume, and high-quality 2D materials play an important role in the applications of flexible devices. Although considerable progress has been achieved in the integration of a series of novel 2D materials beyond graphene into flexible devices, a lot remains to be known. At this stage of their development, the key issues concern how to make further improvements to high-performance and scalable-production. Herein, recent progress in the quest to improve the current state of the art for 2D materials beyond graphene is reviewed. Namely, the properties and synthesis techniques of 2D materials are first introduced. Then, both the advantages and challenges of these 2D materials for flexible devices are also highlighted. Finally, important directions for future advancements toward efficient, low-cost, and stable flexible devices are outlined.

Flexible and Stretchable Bioelectronics

Medical science technology has improved tremendously over the decades with the invention of robotic surgery, gene editing, immune therapy, etc. However, scientists are now recognizing the significance of 'biological circuits' i.e., bodily innate electrical systems for the healthy functioning of the body or for any disease conditions. Therefore, the current trend in the medical field is to understand the role of these biological circuits and exploit their advantages for therapeutic purposes. Bioelectronics, devised with these aims, work by resetting, stimulating, or blocking the electrical pathways. Bioelectronics are also used to monitor the biological cues to assess the homeostasis of the body. In a way, they bridge the gap between drug-based interventions and medical devices. With this in mind, scientists are now working towards developing flexible and stretchable miniaturized bioelectronics that can easily conform to the tissue topology, are non-toxic, elicit no immune reaction, and address the issues that drugs are unable to solve. Since the bioelectronic devices that come in contact with the body or body organs need to establish an unobstructed interface with the respective site, it is crucial that those bioelectronics are not only flexible but also stretchable for constant monitoring of the biological signals. Understanding the challenges of fabricating soft stretchable devices, we review several flexible and stretchable materials used as substrate, stretchable electrical conduits and encapsulation, design modifications for stretchability, fabrication techniques, methods of signal transmission and monitoring, and the power sources for these stretchable bioelectronics. Ultimately, these bioelectronic devices can be used for wide range of applications from skin bioelectronics and biosensing devices, to neural implants for diagnostic or therapeutic purposes.

Enhanced Valley Polarization of Bilayer MoSe2 with Variable Stacking Order and Interlayer Coupling

In two-dimensional transitional metal dichalcogenides, tuning the spin-valley-layer coupling via changing layer numbers and stacking orders remains desirable for their application in valleytronics. Herein, six-point star-like MoSe₂ nanoflakes simultaneously containing different atom registration regions from monolayer to bilayer with 2H and 3R stacking order were fabricated, and the valley polarizations were comparably investigated by circular polarized photoluminescent spectroscopy. The degree of valley polarization was detected to be about 12.5% in the monolayer and 10% in the 2H bilayer, but greatly upgraded to about 40% in the 3R bilayer MoSe₂. This enhancement was attributed to the multiband spin splitting and generation of spin-dependent layer polarization for the 3R MoSe₂ bilayer, which is well evidenced by our ab initio calculations of the energy band structures. Our results demonstrate that preparing TMD crystals with controllable stacking orders and interlayer coupling is a promising route to tune the valley index in TMDs for developing valleytronics technology.

Highly sensitive active pixel image sensor array driven by large-area bilayer MoS2 transistor circuitry

Various large-area growth methods for two-dimensional transition metal dichalcogenides have been developed recently for future electronic and photonic applications. However, they have not yet been employed for synthesizing active pixel image sensors. Here, we report on an active pixel image sensor array with a bilayer MoS2 film prepared via a two-step large-area growth method. The active pixel of image sensor is composed of 2D MoS2 switching transistors and 2D MoS2 phototransistors. The maximum photoresponsivity (Rph) of the bilayer MoS2 phototransistors in an 8 × 8 active pixel image sensor array is statistically measured as high as 119.16 A W-1. With the aid of computational modeling, we find that the main mechanism for the high Rph of the bilayer MoS2 phototransistor is a photo-gating effect by the holes trapped at subgap states. The image-sensing characteristics of the bilayer MoS2 active pixel image sensor array are successfully investigated using light stencil projection.

Thickness-Dependent Elastic Softening of Few-Layer Free-Standing MoSe2

Few-layer van der Waals (vdW) materials have been extensively investigated in terms of their exceptional electronic, optoelectronic, optical, and thermal properties. Simultaneously, a complete evaluation of their mechanical properties remains an undeniable challenge due to the small lateral sizes of samples and the limitations of experimental tools. In particular, there is no systematic experimental study providing unambiguous evidence on whether the reduction of vdW thickness down to few layers results in elastic softening or stiffening with respect to the bulk. In this work, micro-Brillouin light scattering is employed to investigate the anisotropic elastic properties of single-crystal free-standing 2H-MoSe₂ as a function of thickness, down to three molecular layers. The so-called elastic size effect, that is, significant and systematic elastic softening of the material with decreasing numbers of layers is reported. In addition, this approach allows for a complete mechanical examination of few-layer membranes, that is, their elasticity, residual stress, and thickness, which can be easily extended to other vdW materials. The presented results shed new light on the ongoing debate on the elastic size-effect and are relevant for performance and durability of implementation of vdW materials as resonators, optoelectronic, and thermoelectric devices.

Synthesis of large-area monolayer and few-layer MoSe2 continuous films by chemical vapor deposition without hydrogen assistance and formation mechanism

Two dimensional (2D) MoSe2 with a layered structure has attracted extensive research due to its excellent electronic and optical properties. The controlled synthesis of large-scale and high-quality MoSe2 is highly desirable but still remains challenging. Ambient pressure chemical vapor deposition (APCVD) is an excellent method for the synthesis of 2D materials but the inevitable use of hydrogen during the growth and the easy formation of cracks in the ultrathin films still need to be solved. In the present work, we reported the synthesis of large-area continuous MoSe2 films with different layers by the APCVD method without the assistance of hydrogen on SiO2/Si substrates just by raising the reaction temperature of Se. The synthesized continuous MoSe2 films can reach several centimeters, which can be seen clearly by naked eyes, and, more importantly, the size of the monolayer film can reach up to 3 mm. The morphology, structural characteristics, and optical properties of the synthesized MoSe2 films have been investigated, demonstrating good performance and high crystallinity of the films. Raman spectra give the empirical expression of the frequency difference between E2g1 and A1g dependence of the layer number (N = 1-10 L) for CVD grown MoSe2, which is useful in layer number identification. Further, the formation mechanism of the MoSe2 continuous film is of interest as a fundamental scientific problem and needs to be studied. We proposed the wing model, boundary layer theory, and diffusion theory to account quantitatively for the formation behavior of the MoSe2 film. The presented facile growth method and theoretical model are useful to synthesize other ultrathin transition metal dichalcogenide films and understand the formation behaviors of the systems.

Ultrafast Photocurrent Response and High Detectivity in Two-Dimensional MoSe2-based Heterojunctions

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials have garnered great attention on account of their novel properties and potential to advance modern technology. Recent studies have demonstrated that TMDCs can be utilized to create high-performing heterostructures with combined functionality of the individual layers and new phenomena at these interfaces. Here, we report an ultrafast photoresponse within MoSe₂-based heterostructures in which heavily p-doped WSe₂ and MoS₂ flakes share an undoped MoSe₂ channel, allowing us to directly compare the optoelectronic properties of MoSe₂-based heterojunctions with different 2D materials. Strong photocurrent signals have been observed in both MoSe₂-WSe₂ and MoSe₂-MoS₂ heterojunctions with a photoresponse time constant of ∼16 μs, surmounting previous MoSe₂-based devices by three orders of magnitude. Further studies have shown that the fast response is independent of the integrated 2D materials (WSe₂ or MoS₂) but is likely attributed to the high carrier mobility of 260 cm² V^-1 s^-1 in the undoped MoSe₂ channel as well as the greatly reduced Schottky barriers and near absence of interface states at MoSe₂-WSe₂/MoS₂ heterojunctions, which lead to reduced carrier transit time and thus short photocurrent response time. Lastly, a high detectivity on the order of ∼10¹⁴ Jones has been achieved in MoSe₂-based heterojunctions, which supersedes current industry standards. These fundamental studies not only shed light on photocurrent generation mechanisms in MoSe₂-based heterojunctions but also open up new avenues for engineering future high-performance 2D optoelectronic devices.

Type-II Interface Band Alignment in the vdW PbI2-MoSe2 Heterostructure

Energy band alignments at heterostructure interfaces play key roles in device performance, especially between two-dimensional atomically thin materials. Herein, van der Waals PbI₂-MoSe₂ heterostructures fabricated by in situ PbI₂ deposition on monolayer MoSe₂ are comprehensively studied using scanning tunneling microscopy/spectroscopy, atomic force microscopy, photoemission spectroscopy, and Raman and photoluminescence (PL) spectroscopy. PbI₂ grows on MoSe₂ in a quasi layer-by-layer epitaxial mode. A type-II interface band alignment is proposed between PbI₂ and MoSe₂ with the conduction band minimum (valence band maximum) located at PbI₂ (MoSe₂), which is confirmed by first-principles calculations and the existence of interfacial excitons revealed using temperature-dependent PL. Our findings provide a scalable method to fabricate PbI₂-MoSe₂ heterostructures and new insights into the electronic structures for future device design.

Strain robust spin gapless semiconductors/half-metals in transition metal embedded MoSe2monolayer

The realization of spin gapless semiconductor (SGS) and half-metal (HM) behavior in two-dimensional (2D) transition metal (TM) dichalcogenides is highly desirable for their applications in spintronic devices. Here, using density functional theory calculations, we demonstrate that Fe, Co, Ni substitutional impurities can not only induce magnetism in MoSe2monolayer, but also convert the semiconducting MoSe2to SGS/HM system. We also study the effects of mechanical strain on the electronic and magnetic properties of the doped monolayer. We show that for all TM impurities we considered, the system exhibits the robust SGS/HM behavior regardless of biaxial strain values. Moreover, it is found that the magnetic properties of TM-MoSe2can effectively be tuned under biaxial strain by controlling the spin polarization of the 3dorbitals of Fe, Co, Ni atoms. Our findings offer a new route to designing the SGS/HM properties and modulating magnetic characteristics of the TM-MoSe2system and may also facilitate the implementation of SGS/HM behavior and realization of spintronic devices based on other 2D materials.

Thermal Evaporation of Large-Area SnS2 Thin Films with a UV-to-NIR Photoelectric Response for Flexible Photodetector Applications

In addition to device flexibility, the retentivity performance of photoelectric materials after an extreme reverse-bending process is intrinsically important and desirable for next-generation advanced flexible optoelectronics. In this work, we designed and fabricated large-area flexible SnS₂ thin films with a novel nanosheet/amorphous blended structure to achieve an outstanding flexible photoelectric performance via a facile evaporation and post-thermal annealing route. Crystal structure analysis showed that the obtained SnS₂ thin films were constructed with nanosheets oriented parallel to the substrate which were surrounded and connected by the amorphous component with a smooth surface. This nanosheet/amorphous blended structure allowed extreme bending because of the adhesive and strain-accommodation effect that arises from the amorphous components. The assembled SnS₂ flexible photodetectors can bear a small bending radius as low as 1 mm for over 3000 bending-flatting cycles without a drastic performance decay. In particular, over 90% of the initial photoelectric responsivity (40.8 mA/W) was maintained even after 1000 bending-flatting cycles. Moreover, the SnS₂ thin film can convert photons to photocurrent over a wide spectral range from ultraviolet to near infrared. These unique characteristics indicate that the strategy used in this work is attractive for the development of future wearable photoelectric and artificial intelligence applications.

Rapid and mass-producible synthesis of high-crystallinity MoSe2 nanosheets by ampoule-loaded chemical vapor deposition

MoSe₂ is an attractive transition-metal dichalcogenide with a two-dimensional layered structure and various attractive properties. Although MoSe₂ is a promising negative electrode material for electrochemical applications, further investigation of MoSe₂ has been limited, mainly by the lack of MoSe₂ mass-production methods. Here, we report a rapid and ultra-high-yield synthesis method of obtaining MoSe₂ nanosheets with high crystallinity and large grains by ampoule-loaded chemical vapor deposition. Application of high pressure to an ampoule-type quartz tube containing MoO₃ and Se powders initiated rapid reactions that produced vertically oriented MoSe₂ nanosheets with grain sizes of up to ∼100 μm and yields of ∼15 mg h^-1. Spectroscopy and microscopy characterizations confirmed the high crystallinity of the obtained MoSe₂ nanosheets. Transistors and lithium-ion battery cells fabricated with the synthesized MoSe₂ nanosheets showed good performance, thereby further indicating their high quality. The proposed simple scalable synthesis method can pave the way for diverse electrical and electrochemical applications of MoSe₂.

Sub-millimeter size high mobility single crystal MoSe2 monolayers synthesized by NaCl-assisted chemical vapor deposition

Monolayer MoSe₂ is a transition metal dichalcogenide with a narrow bandgap, high optical absorbance and large spin-splitting energy, giving it great promise for applications in the field of optoelectronics. Producing monolayer MoSe₂ films in a reliable and scalable manner is still a challenging task as conventional chemical vapor deposition (CVD) or exfoliation based techniques are limited due to the small domains/nanosheet sizes obtained. Here, based on NaCl assisted CVD, we demonstrate the simple and stable synthesis of sub-millimeter size single-crystal MoSe₂ monolayers with mobilities ranging from 38 to 8 cm² V⁻¹ s⁻¹. The average mobility is 12 cm² V⁻¹ s⁻¹. We further determine that the optical responsivity of monolayer MoSe₂ is 42 mA W⁻¹, with an external quantum efficiency of 8.22%.

Sub-millimeter single crystal MoSe₂ monolayers with a mobility of 38 cm² V⁻¹ s⁻¹ and responsivity of 42 mA W⁻¹ were synthesized by NaCl-assisted chemical vapor deposition.

Synthesis of Monolayer MoSe2 with Controlled Nucleation via Reverse-Flow Chemical Vapor Deposition

Two-dimensional (2D) layered semiconductor materials, such as transition metal dichalcogenides (TMDCs), have attracted considerable interests because of their intriguing optical and electronic properties. Controlled growth of TMDC crystals with large grain size and atomically smooth surface is indeed desirable but remains challenging due to excessive nucleation. Here, we have synthesized high-quality monolayer, bilayer MoSe₂ triangular crystals, and continuous thin films with controlled nucleation density via reverse-flow chemical vapor deposition (CVD). High crystallinity and good saturated absorption performance of MoSe₂ have been systematically investigated and carefully demonstrated. Optimized nucleation and uniform morphology could be achieved via fine-tuning reverse-flow switching time, growth time and temperature, with corresponding growth kinetics proposed. Our work opens up a new approach for controllable synthesis of monolayer TMDC crystals with high yield and reliability, which promote surface/interface engineering of 2D semiconductors towards van der Waals heterostructure device applications.

Rolling up MoSe2 Nanomembranes as a Sensitive Tubular Photodetector

Transition metal dichalcogenides, as a kind of 2D material, are suitable for near-infrared to visible photodetection owing to the bandgaps ranging from 1.0 to 2.0 eV. However, limited light absorption restricts photoresponsivity due to the ultrathin thickness of 2D materials. 3D tubular structures offer a solution to solve the problem because of the light trapping effect which can enhance optical absorption. In this work, thanks to mechanical flexibility of 2D materials, self-rolled-up technology is applied to build up a 3D tubular structure and a tubular photodetector is realized based on the rolled-up molybdenum diselenide microtube. The tubular device is shown to present one order higher photosensitivity compared with planar counterparts. Enhanced optical absorption arising from the multiple reflections inside the tube is the main reason for the increased photocurrent. This tubular device offers a new design for increasing the efficiency of transition metal dichalcogenide-based photodetection and could hold great potential in the field of 3D optoelectronics.

Recent Progress in CVD Growth of 2D Transition Metal Dichalcogenides and Related Heterostructures

In recent years, 2D layered materials have received considerable research interest on account of their substantial material systems and unique physicochemical properties. Among them, 2D layered transition metal dichalcogenides (TMDs), a star family member, have already been explored over the last few years and have exhibited excellent performance in electronics, catalysis, and other related fields. However, to fulfill the requirement for practical application, the batch production of 2D TMDs is essential. Recently, the chemical vapor deposition (CVD) technique was considered as an elegant alternative for successfully growing 2D TMDs and their heterostructures. The latest research advances in the controllable synthesis of 2D TMDs and related heterostructures/superlattices via the CVD approach are illustrated here. The controlled growth behavior, preparation strategies, and breakthroughs on the synthesis of new 2D TMDs and their heterostructures, as well as their unique physical phenomena, are also discussed. Recent progress on the application of CVD-grown 2D materials is revealed with particular attention to electronics/optoelectronic devices and catalysts. Finally, the challenges and future prospects are considered regarding the current development of 2D TMDs and related heterostructures.

Vertically Stacked CVD-Grown 2D Heterostructure for Wafer-Scale Electronics

This paper demonstrates, for the first time, wafer-scale graphene/MoS₂ heterostructures prepared by chemical vapor deposition (CVD) and their application in vertical transistors and logic gates. A CVD-grown bulk MoS₂ layer is utilized as the vertical channel, whereas CVD-grown monolayer graphene is used as the tunable work-function electrode. The short vertical channel of the transistor is formed by sandwiching bulk MoS₂ between the bottom indium tin oxide (ITO, drain electrode) and the top graphene (source electrode). The electron injection barriers at the graphene-MoS₂ junction and ITO-MoS₂ junction are modulated effectively through variation of the Schottky barrier height and its effective barrier width, respectively, because of the work-function tunability of the graphene electrode. The resulting vertical transistor with the CVD-grown MoS₂/graphene heterostructure exhibits a current density exceeding 7 A/cm², a subthreshold swing of 410 mV/dec, and an on-off current ratio exceeding 10³. The large-area synthesis, transfer, and patterning processes of both semiconducting MoS₂ and metallic graphene facilitate construction of a wafer-scale array of transistors and logic gates such as NOT, NAND, and NOR.

Solution-processed thin films of semiconducting carbon nanotubes and their application to soft electronics

Semiconducting single-walled carbon nanotube (SWNT) networks are promising for use as channel materials in field-effect transistors (FETs) in next-generation soft electronics, owing to their high intrinsic carrier mobility, mechanical flexibility, potential for low-cost production, and good processability. In this article, we review the recent progress related to carbon nanotube (CNT) devices in soft electronics by describing the materials and devices, processing methods, and example applications in soft electronic systems. First, solution-processed semiconducting SWNT deposition methods along with doping techniques used to achieve stable complementary metal-oxide-semiconductor devices are discussed. Various strategies for developing high-performance SWNT-based FETs, such as the proper material choices for the gates, dielectrics, and sources/drains of FETs, and methods of improving FET performance, such as hysteresis repression in SWNT-based FETs, are described next. These SWNT-based FETs have been used in flexible, stretchable, and wearable electronic devices to realize functionalities that could not be achieved using conventional silicon-based devices. We conclude this review by discussing the challenges faced by and outlook for CNT-based soft electronics.

Bi2Se3 decorated recyclable liquid-exfoliated MoS2 nanosheets: Towards suppress smoke emission and improve mechanical properties of epoxy resin

Bimetallic compounds have been proved superior suppression effect on smoke emission during combustion of polymers. In this work, MoS₂/Bi₂Se₃ (MB) hybrids were prepared by a facile wet chemical method and showed superior performance on smoke suppression of EP matrix during combustion. N-vinyl pyrrolidone (NVP) was employed to exfoliate molybdenum disulfide (MoS₂) nanosheets in a recyclable method, which showed high efficiency and was recyclable. Exfoliated MoS₂ exhibited large surface area and used as carriers to synthesize MB hybrids. Considering the catalytic effect of bismuth and molybdenum, the hybrids had a great influence on the smoke emission behaviors of EP composites. The smoke production was obviously suppressed during the flaming combustion (more than 22% and 23% decrease obtained from cone calorimeter and steady state tube furnace, respectively) or smolder processes (more than 23% decrease obtained from smoke chamber) at only 1 wt% content of MB hybrids. What's more, due to superior dispersion state, the addition of MB hybrids also enhanced the mechanical properties of EP matrix, including wear resistance and tensile property. This work provided a safe and green exfoliation method of MoS₂ to prepare polymers/MoS₂ composites and also constructed a novel binary hybrids for enhancing combination performances of polymers.

WS(1-x)Sex Nanoparticles Decorated Three-Dimensional Graphene on Nickel Foam: A Robust and Highly Efficient Electrocatalyst for the Hydrogen Evolution Reaction

To find an effective alternative to scarce, high-cost noble platinum (Pt) electrocatalyst for hydrogen evolution reaction (HER), researchers are pursuing inexpensive and highly efficient materials as an electrocatalyst for large scale practical application. Layered transition metal dichalcogenides (TMDCs) are promising candidates for durable HER catalysts due to their cost-effective, highly active edges and Earth-abundant elements to replace Pt electrocatalysts. Herein, we design an active, stable earth-abundant TMDCs based catalyst, WS(1-x)Sex nanoparticles-decorated onto a 3D porous graphene/Ni foam. The WS(1-x)Sex/graphene/NF catalyst exhibits fast hydrogen evolution kinetics with a moderate overpotential of ~-93 mV to drive a current density of 10 mA cm-2, a small Tafel slope of ~51 mV dec-1, and a long cycling lifespan more than 20 h in 0.5 M sulfuric acid, which is much better than WS₂/NF and WS₂/graphene/NF catalysts. Our outcomes enabled a way to utilize the TMDCs decorated graphene and precious-metal-free electrocatalyst as mechanically robust and electrically conductive catalyst materials.

Halide-Induced Self-Limited Growth of Ultrathin Nonlayered Ge Flakes for High-Performance Phototransistors

2D nonlayered materials have attracted intensive attention due to their unique surface structure and novel physical properties. However, it is still a great challenge to realize the 2D planar structures of nonlayered materials owing to the naturally intrinsic covalent bonds. Ge is one of them with cubic structure impeding its 2D anisotropic growth. Here, the ultrathin single-crystalline Ge flakes as thin as 8.5 nm were realized via halide-assisted self-limited CVD growth. The growth mechanism has been confirmed by experiments and theoretical calculations, which can be attributed to the preferential growth of the (111) plane with the lowest formation energy and the giant interface distortion effect of the Cl-Ge motif. Excitingly, a Ge flake-based phototransistor shows excellent performances such as a high hole mobility of ∼263 cm² V^-1 s^-1, a high responsivity of ∼200 A/W, and fast response rates (τ_rise = 70 ms, τ_decay = 6 ms), suggesting its great potential in the applications of electronics and optoelectronics.

Hybrid Characteristics of MoS2 Monolayer with Organic Semiconducting Tetracene and Application to Anti-Ambipolar Field Effect Transistor

An n-type MoS₂ monolayer grown by chemical vapor deposition method was partially hybridized with an organic semiconducting p-type tetracene thin film. The photoluminescence (PL) intensity in the hybrid region of the MoS₂/tetracene is clearly lower than that of pristine tetracene because of the charge-transfer effect, which was confirmed by the decrease in exciton lifetimes. Decrease in the temperature led to blue-shift in the PL peak position of MoS₂ layers and, consequently, the PL intensities of both tetracene and MoS₂ considerably increased owing to the decrease in phonon interaction. The PL spectra of bound excitons in the hybrid region were clearly observed at low temperatures, indicating the formation of trap states. The lateral-type n-p heterojunction field-effect transistors (FETs) using the MoS₂/tetracene hybrid as an active layer showed gate-tunable rectification I- V and anti-ambipolar field-effect characteristics with hysteresis effect. The charge transport characteristics across the n-p heterojunction of the hybrid region of the FET can be explained in terms of the Shockley-Read-Hall trap-intermediated tunneling and Langevin recombination mechanisms. To improve the performance of MoS₂/tetracene-based FET, a dielectric hexagonal boron nitride (h-BN) thin layer was inserted between the SiO₂ surface and the active MoS₂ layer. We observed the decrease in the hysteresis effect and threshold voltage of the h-BN/MoS₂/tetracene-based FETs due to the decrease in the number of traps at the interface. The performance of h-BN/MoS₂/tetracene FET device was also enhanced after the annealing process.

Molecular Beam Epitaxy of Highly Crystalline MoSe2 on Hexagonal Boron Nitride

Molybdenum diselenide (MoSe₂) is a promising two-dimensional material for next-generation electronics and optoelectronics. However, its application has been hindered by a lack of large-scale synthesis. Although chemical vapor deposition (CVD) using laboratory furnaces has been applied to grow two-dimensional (2D) MoSe₂ cystals, no continuous film over macroscopically large area has been produced due to the lack of uniform control in these systems. Here, we investigate the molecular beam epitaxy (MBE) of 2D MoSe₂ on hexagonal boron nitride (hBN) substrate, where highly crystalline MoSe₂ film can be grown with electron mobility ∼15 cm²/(V s). Scanning transmission electron microscopy (STEM) shows that MoSe₂ grains grown at an optimum temperature of 500 °C are highly oriented and coalesced to form continuous film with predominantly mirror twin boundaries. Our work suggests that van der Waals epitaxy of 2D materials is tolerant of lattice mismatch but is facilitated by substrates with similar symmetry.

Flexible PI-Based Plant Drought Stress Sensor for Real-Time Monitoring System in Smart Farm

Plant growth and development are negatively affected by a wide range of external stresses, including water deficits. Especially, plants generally reduce the stomatal aperture to decrease transpiration levels upon drought stress. Advanced technologies, such as wireless communications, the Internet of things (IoT), and smart sensors have been applied to practical smart farming and indoor planting systems to monitor plants’ signals effectively. In this study, we develop a flexible polyimide (PI)-based sensor for real-time monitoring of water conditions in tobacco plants. The stoma response, by which a plant adjusts to drought stress to maintain homeostasis, can be confirmed through the examination of evaporated water. Using a flexible PI-based sensor, a plant’s response variation is translated into an electrical signal. The sensors are integrated with a Bluetooth (BLE) module and a processing module and show potential as smart real-time water sensors in smart farms.

Novel Nano-Materials and Nano-Fabrication Techniques for Flexible Electronic Systems

Recent progress in fabricating flexible electronics has been significantly developed because of the increased interest in flexible electronics, which can be applied to enormous fields, not only conventional in electronic devices, but also in bio/eco-electronic devices. Flexible electronics can be applied to a wide range of fields, such as flexible displays, flexible power storages, flexible solar cells, wearable electronics, and healthcare monitoring devices. Recently, flexible electronics have been attached to the skin and have even been implanted into the human body for monitoring biosignals and for treatment purposes. To improve the electrical and mechanical properties of flexible electronics, nanoscale fabrications using novel nanomaterials are required. Advancements in nanoscale fabrication methods allow the construction of active materials that can be combined with ultrathin soft substrates to form flexible electronics with high performances and reliability. In this review, a wide range of flexible electronic applications via nanoscale fabrication methods, classified as either top-down or bottom-up approaches, including conventional photolithography, soft lithography, nanoimprint lithography, growth, assembly, and chemical vapor deposition (CVD), are introduced, with specific fabrication processes and results. Here, our aim is to introduce recent progress on the various fabrication methods for flexible electronics, based on novel nanomaterials, using application examples of fundamental device components for electronics and applications in healthcare systems.

Large-area synthesis and photoelectric properties of few-layer MoSe2 on molybdenum foils

Compared with MoS₂ and WS₂, selenide analogs have narrower band gaps and higher electron mobilities, which make them more applicable to real electrical devices. In addition, few-layer metal selenides have higher electrical conductivity, carrier mobility and light absorption than the corresponding monolayers. However, the large-scale and high-quality growth of few-layer metal selenides remains a significant challenge. Here, we develop a facile method to grow large-area and highly crystalline few-layer MoSe₂ by directly selenizing the Mo foil surface at 550 °C within 60 min under ambient pressure. The atomic layers were controllably grown with thicknesses between 3.4 and 6 nm, which just met the thickness range required for high-performance electrical devices. Furthermore, we fabricated a vertical p-n junction photodetector composed of few-layer MoSe₂ and p-type silicon, achieving photoresponsivity higher by two orders of magnitude than that of the reported monolayer counterpart. This technique provides a feasible approach towards preparing other 2D transition metal dichalcogendes for device applications.

Interstitial Mo-Assisted Photovoltaic Effect in Multilayer MoSe2 Phototransistors

Thin-film transistors (TFTs) based on multilayer molybdenum diselenide (MoSe₂ ) synthesized by modified atmospheric pressure chemical vapor deposition (APCVD) exhibit outstanding photoresponsivity (103.1 A W^-1 ), while it is generally believed that optical response of multilayer transition metal dichalcogenides (TMDs) is significantly limited due to their indirect bandgap and inefficient photoexcitation process. Here, the fundamental origin of such a high photoresponsivity in the synthesized multilayer MoSe₂ TFTs is sought. A unique structural characteristic of the APCVD-grown MoSe₂ is observed, in which interstitial Mo atoms exist between basal planes, unlike usual 2H phase TMDs. Density functional theory calculations and photoinduced transfer characteristics reveal that such interstitial Mo atoms form photoreactive electronic states in the bandgap. Models indicate that huge photoamplification is attributed to trapped holes in subgap states, resulting in a significant photovoltaic effect. In this study, the fundamental origin of high responsivity with synthetic MoSe₂ phototransistors is identified, suggesting a novel route to high-performance, multifunctional 2D material devices for future wearable sensor applications.

Synthesis and properties of MoCl4complexes with thio- and seleno-ethers and their use for chemical vapour deposition of MoSe2and MoS2films

A new series of Mo(iv) chloride complexes with thioether and seleneoether ligands is reported; [MoCl₄(ⁿBu₂E)₂] (E = S, Se) function as single source precursors for the CVD of MoE₂thin films.

Improving the Stability of High-Performance Multilayer MoS2 Field-Effect Transistors

In this study, we propose a method for improving the stability of multilayer MoS₂ field-effect transistors (FETs) by O₂ plasma treatment and Al₂O₃ passivation while sustaining the high performance of bulk MoS₂ FET. The MoS₂ FETs were exposed to O₂ plasma for 30 s before Al₂O₃ encapsulation to achieve a relatively small hysteresis and high electrical performance. A MoO_x layer formed during the plasma treatment was found between MoS₂ and the top passivation layer. The MoO_x interlayer prevents the generation of excess electron carriers in the channel, owing to Al₂O₃ passivation, thereby minimizing the shift in the threshold voltage (V_th) and increase of the off-current leakage. However, prolonged exposure of the MoS₂ surface to O₂ plasma (90 and 120 s) was found to introduce excess oxygen into the MoO_x interlayer, leading to more pronounced hysteresis and a high off-current. The stable MoS₂ FETs were also subjected to gate-bias stress tests under different conditions. The MoS₂ transistors exhibited negligible decline in performance under positive bias stress, positive bias illumination stress, and negative bias stress, but large negative shifts in V_th were observed under negative bias illumination stress, which is attributed to the presence of sulfur vacancies. This simple approach can be applied to other transition metal dichalcogenide materials to understand their FET properties and reliability, and the resulting high-performance hysteresis-free MoS₂ transistors are expected to open up new opportunities for the development of sophisticated electronic applications.

Large-area and highly crystalline MoSe2 for optical modulator

Transition metal dichalcogenides (TMDs) have been successfully used as broadband optical modulator materials for pulsed fiber laser systems. However, the nonlinear optical absorptions of exfoliated TMDs are strongly limited by their nanoflakes morphology with uncontrollable lateral size and thickness. In this work, we provide an effective method to fully explore the nonlinear optical properties of MoSe₂. Large-area and high quality lattice MoSe₂ grown by chemical vapor deposition method was adopted as an optical modulator for the first time. The large-area MoSe₂ shows excellent nonlinear optical absorption with a large modulation depth of 21.7% and small saturable intensity of 9.4 MW cm^-2. After incorporating the MoSe₂ optical modulator into fiber laser cavity as a saturable absorber, a highly stable Q-switching operation with single pulse energy of 224 nJ is achieved. The large-area MoSe₂ possessing superior nonlinear optical properties compared to exfoliated nanoflakes affords possibility for the larger-area two-dimensional materials family as high performance optical devices.

Large-area highly crystalline WSe2 atomic layers for ultrafast pulsed lasers

Large-area and highly crystalline transition metal dichalcogenides (TMDs) films possess superior saturable absorption compared to the TMDs nanosheet counterparts, which make them more suitable as excellent saturable absorbers (SA) for ultrafast laser technology. Thus far, the nonlinear optical properties of large-scale WSe₂ and its applications in ultrafast photonics have not yet been fully investigated. In this work, the saturable absorption of chemical vapor deposition (CVD) grown WSe₂ films with large-scale and high quality are studied and the use of WSe₂ films as a broadband SA for passively mode-locked fiber lasers at both 1.5 and 2 μm ranges is demonstrated. To enhance the light-material interaction, large-area WSe₂ film is tightly transferred onto the side wall of a microfiber to form a hybrid structure, which realizes strong evanescent wave interaction between light and WSe₂ film. The integrated microfiber-WSe₂ device shows a large modulation depth of 54.5%. Using the large-area WSe₂ as a mode-locker, stable soliton mode-locked pulse generation is achieved and the pulse durations of 477 fs (at 1.5 μm) and 1.18 ps (at 2.0 μm) are demonstrated, which suggests that the large-area and highly crystalline WSe₂ films afford an excellent broadband SA for ultrafast photonic applications.

Impact of Metal Contacts on the Performance of Multilayer HfS2 Field-Effect Transistors

HfS₂ is one of the emerging transition metal dichalcogenides and is very promising for low-power nanoelectronics and high-sensitivity optoelectronic device applications. We studied the band structures of 1T-HfS₂ with different thicknesses by first principles simulation, and the impact of different metal contacts to the HfS₂ device performance has been experimentally studied. Back-gate and top-gate HfS₂ field-effect transistors (FETs) were fabricated, and better electrical characteristics have been achieved with the FETs with the Ti/Au contact as compared with the Pt-contacted FETs. Thin layers of Pt and Ti/Au films were deposited on HfS₂ flakes to investigate the metal/HfS₂ interface by using scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. A smoother Ti/Au film was formed on HfS₂, resulting in higher carrier injection and transport efficiency. The phonon behavior being dominated by the interface chemical bonding at the Ti/Au contact region has been confirmed with the more sensitive A_1g phonon mode from the bilayer HfS₂.

Centimeter-Scale Deposition of Mo0.5W0.5Se2 Alloy Film for High-Performance Photodetectors on Versatile Substrates

Because of their great potential for academic investigation and practical application in next-generation optoelectronic devices, ternary layered semiconductors have attracted considerable attention in recent years. Similar to the applications of traditional layered materials, practical applications of ternary layered semiconductor alloys require the synthesis of large-area samples. Here, we report the preparation of centimeter-scale and high-quality Mo_0.5W_0.5Se₂ alloy films on both a rigid SiO₂/Si substrate and a flexible polyimide (PI) substrate. Then, photodetectors based on these alloy films are fabricated, which are capable of conducting broad-band photodetection from ultraviolet to near-infrared region (370-808 nm) with high performance. The photodetector on SiO₂/Si substrates demonstrates a high responsivity (R) of 77.1 A/W, an outstanding detectivity (D*) of 1.1 × 10¹² Jones, and a fast response time of 8.3 ms. These figures-of-merit are much superior to those of the counterparts of binary material-based devices. Moreover, the photodetector on PI substrates also achieves high performance (R = 63.5 A/W, D* = 3.56 × 10¹² Jones). And no apparent degradation in the device properties is observed even after 100 bending cycles. These results make Mo_0.5W_0.5Se₂ alloy a highly qualified candidate for next-generation optoelectronic applications.

MoSe2 Embedded CNT-Reduced Graphene Oxide Composite Microsphere with Superior Sodium Ion Storage and Electrocatalytic Hydrogen Evolution Performances

Highly porous MoSe₂-reduced graphene oxide-carbon nanotube (MoSe₂-rGO-CNT) powders were prepared by a spray pyrolysis process. The synergistic effect of CNTs and rGO resulted in powders containing ultrafine MoSe₂ nanocrystals with a minimal degree of stacking. The initial discharge capacities of MoSe₂-rGO-CNT, MoSe₂-CNT, MoSe₂-rGO, and bare MoSe₂ powders for sodium ion storage were 501.6, 459.7, 460.2, and 364.0 mA h g^-1, respectively, at 1.0 A g^-1. The MoSe₂-rGO-CNT composite powders had superior cycling and rate performances compared with the MoSe₂-CNT, MoSe₂-rGO composite, and bare MoSe₂ powders. The electrocatalytic activity of MoSe₂-rGO-CNT in the hydrogen evolution reaction (HER) was also compared with that of MoSe₂-CNT, MoSe₂-rGO, and bare MoSe₂. MoSe₂-rGO-CNT composite powders exhibited an overpotential of 0.24 V at a current density of 10 mA cm^-2, which was less than that of MoSe₂-CNT (0.26 V at 10 mA cm^-2), MoSe₂-rGO (0.32 V at 10 mA cm^-2), and bare MoSe₂ (0.33 V at 10 mA cm^-2). Tafel slopes for the MoSe₂-rGO-CNT, MoSe₂-CNT, MoSe₂-rGO, and bare MoSe₂ powders were 53, 76, 86, and 115 mV dec^-1, respectively. Because a large electrochemical surface area and ultrafine MoSe₂ nanocrystals, the MoSe₂-rGO-CNT composite possesses more active sites than the MoSe₂-CNT, MoSe₂-rGO composite, and bare MoSe₂ powders with extensive stacking and large crystalline size, which provide greater catalytic HER activity.

Bionanofiber Assisted Decoration of Few-Layered MoSe2 Nanosheets on 3D Conductive Networks for Efficient Hydrogen Evolution

Molybdenum diselenide (MoSe₂ ) has emerged as a promising electrocatalyst for hydrogen evolution reaction (HER). However, its properties are still confined due to the limited active sites and poor conductivity. Thus, it remains a great challenge to synergistically achieve structural and electronic modulations for MoSe₂ -based HER catalysts because of the contradictory relationship between these two characteristics. Herein, bacterial cellulose-derived carbon nanofibers are used to assist the uniform growth of few-layered MoSe₂ nanosheets, which effectively increase the active sites of MoSe₂ for hydrogen atom adsorption. Meanwhile, carbonized bacterial cellulose (CBC) nanofibers provide a 3D network for electrolyte penetration into the inner space and accelerate electron transfer as well, thus leading to the dramatically increased HER activity. In acidic media, the CBC/MoSe₂ hybrid catalyst exhibits fast hydrogen evolution kinetics with onset overpotential of 91 mV and Tafel slope of 55 mV dec^-1 , which is much more outstanding than both bulk MoSe₂ aggregates and CBC nanofibers. Furthermore, the fast HER kinetics are well supported by theoretical calculations of density-functional-theory analysis with a low activation barrier of 0.08 eV for H₂ generation. Hence, this work highlights an efficient solution to develop high-performance HER catalysts by incorporating biotemplate materials, to simultaneously achieve increased active sites and conductivity.

Flexible and Wavelength-Selective MoS2 Phototransistors with Monolithically Integrated Transmission Color Filters

Color-selective or wavelength-tunable capability is a crucial feature for two-dimensional (2-D) semiconducting material-based image sensor applications. Here, we report on flexible and wavelength-selective molybdenum disulfide (MoS₂) phototransistors using monolithically integrated transmission Fabry-Perot (F-P) cavity filters. The fabricated multilayer MoS₂ phototransistors on a polyarylate substrate exhibit decent electrical characteristics (μ_FE > 64.4 cm²/Vs, on/off ratio > 10⁶), and the integrated F-P filters, being able to cover whole visible spectrum, successfully modulate the spectral response characteristics of MoS₂ phototransistors from ~495 nm (blue) to ~590 nm (amber). Furthermore, power dependence of both responsivity and specific detectivity shows similar trend with other reports, dominated by the photogating effect. When combined with large-area monolayer MoS₂ for optical property enhancement and array processing, our results can be further developed into ultra-thin flexible photodetectors for wearables, conformable image sensor, and other optoelectronic applications.

Booming Development of Group IV-VI Semiconductors: Fresh Blood of 2D Family

As an important component of 2D layered materials (2DLMs), the 2D group IV metal chalcogenides (GIVMCs) have drawn much attention recently due to their earth-abundant, low-cost, and environmentally friendly characteristics, thus catering well to the sustainable electronics and optoelectronics applications. In this instructive review, the booming research advancements of 2D GIVMCs in the last few years have been presented. First, the unique crystal and electronic structures are introduced, suggesting novel physical properties. Then the various methods adopted for synthesis of 2D GIVMCs are summarized such as mechanical exfoliation, solvothermal method, and vapor deposition. Furthermore, the review focuses on the applications in field effect transistors and photodetectors based on 2D GIVMCs, and extends to flexible devices. Additionally, the 2D GIVMCs based ternary alloys and heterostructures have also been presented, as well as the applications in electronics and optoelectronics. Finally, the conclusion and outlook have also been presented in the end of the review.

Facile Synthesis of Single Crystal PtSe2 Nanosheets for Nanoscale Electronics

Ultrathin single crystal platinum diselenide (PtSe₂ ) nanosheets are synthesized using H₂ PtCl₆ and Se as the precursors. The electronic properties are first investigated and exhibit p-type transport behavior with the mobility much larger than 7 cm² V^-1 s^-1 . The further investigation on PtSe₂ /MoS₂ var der Waals p-n junction demonstrated that PtSe₂ could be potentially applied in 2D electronics.

Soluble, Exfoliated Two-Dimensional Nanosheets as Excellent Aqueous Lubricants

Dispersion in water of two-dimensional (2D) nanosheets is conducive to their practical applications in fundamental science communities due to their abundance, low cost, and ecofriendliness. However, it is difficult to achieve stable aqueous 2D material suspensions because of the intrinsic hydrophobic properties of the layered materials. Here, we report an effective and economic way of producing various 2D nanosheets (h-BN, MoS₂, MoSe₂, WS₂, and graphene) as aqueous dispersions using carbon quantum dots (CQDs) as exfoliation agents and stabilizers. The dispersion was prepared through a liquid phase exfoliation. The as-synthesized stable 2D nanosheets based dispersions were characterized by UV-vis, HRTEM, AFM, Raman, XPS, and XRD. The solutions based on CQD decorated 2D nanosheets were utilized as aqueous lubricants, which realized a friction coefficient as low as 0.02 and even achieved a superlubricity under certain working conditions. The excellent lubricating properties were attributed to the synergetic effects of the 2D nanosheets and CQDs, such as good dispersion stability and easy-sliding interlayer structure. This work thus proposes a novel strategy for the design and preparation of high-performance water based green lubricants.

Wafer-Size and Single-Crystal MoSe2 Atomically Thin Films Grown on GaN Substrate for Light Emission and Harvesting

Two-dimensional (2D) atomic-layered semiconductors are important for next-generation electronics and optoelectronics. Here, we designed the growth of an MoSe2 atomic layer on a lattice-matched GaN semiconductor substrate. The results demonstrated that the MoSe2 films were less than three atomic layers thick and were single crystalline of MoSe2 over the entire GaN substrate. The ultrathin MoSe2/GaN heterojunction diode demonstrated ∼850 nm light emission and could also be used in photovoltaic applications.

Exciton formation in monolayer transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides provide a unique platform to study excitons in confined structures. Recently, several important aspects of excitons in these materials have been investigated in detail. However, the formation process of excitons from free carriers has yet to be understood. Here we report time-resolved measurements on the exciton formation process in monolayer samples of MoS2, MoSe2, WS2, and WSe2. The free electron-hole pairs, injected by an ultrashort laser pulse, immediately induce a transient absorption signal of a probe pulse tuned to the exciton resonance. The signal quickly drops by about a factor of two within 1 ps and is followed by a slower decay process. In contrast, when excitons are resonantly injected, the fast decay component is absent. Based both on its excitation excess energy and intensity dependence, this fast decay process is attributed to the formation of excitons from the electron-hole pairs. This interpretation is also consistent with a model that shows how free electron-hole pairs can be about twice more effective than excitons in altering the exciton absorption strength. From our measurements and analysis of our results, we determined that the exciton formation times in these monolayers to be shorter than 1 ps.