Highly flexible MoS2 thin-film transistors with ion gel dielectrics

Viscoelastic change of block copolymer ion gels in a photo-switchable azobenzene ionic liquid triggered by light

A photo-switchable ionic liquid solvent bearing an azobenzene moiety induced a viscoelastic change of block copolymer ion gels by light.

p-MoS2/n-InSe van der Waals heterojunctions and their applications in all-2D optoelectronic devices

A library of 2D semiconductors are prepared providing a new platform for developing high-performance optoelectronic devices. All-2D optoelectronic devices based on type-II p-MoS₂/n-InSe vdWs heterojunctions operate at the near-IR wavelength range.

Investigation of the Energy Band at the Molybdenum Disulfide and ZrO2 Heterojunctions

The energy band alignment at the multilayer-MoS₂/ZrO₂ interface and the effects of CHF₃ plasma treatment on the band offset were explored using x-ray photoelectron spectroscopy. The valence band offset (VBO) and conduction band offset (CBO) for the MoS₂ /ZrO₂ sample is about 1.87 eV and 2.49 eV, respectively. While the VBO was enlarged by about 0.75 eV for the sample with CHF₃ plasma treatment, which is attributed to the up-shift of Zr 3d core level. The calculation results demonstrated that F atoms have strong interactions with Zr atoms, and the valence band energy shift for the d-orbital of Zr atoms is about 0.76 eV, in consistent with the experimental result. This interesting finding encourages the application of ZrO₂ as gate materials in MoS₂-based electronic devices and provides a promising way to adjust the band alignment.

Recent Progress and Future Prospects of 2D-Based Photodetectors

Conventional semiconductors such as silicon- and indium gallium arsenide (InGaAs)-based photodetectors have encountered a bottleneck in modern electronics and photonics in terms of spectral coverage, low resolution, nontransparency, nonflexibility, and complementary metal-oxide-semiconductor (CMOS) incompatibility. New emerging two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and their hybrid systems thereof, however, can circumvent all these issues benefitting from mechanically flexibility, extraordinary electronic and optical properties, as well as wafer-scale production and integration. Heterojunction-based photodiodes based on 2D materials offer ultrafast and broadband response from the visible to far-infrared range. Phototransistors based on 2D hybrid systems combined with other material platforms such as quantum dots, perovskites, organic materials, or plasmonic nanostructures yield ultrasensitive and broadband light-detection capabilities. Notably the facile integration of 2D photodetectors on silicon photonics or CMOS platforms paves the way toward high-performance, low-cost, broadband sensing and imaging modalities.

Fabrication of Sub-Micrometer-Sized MoS2 Thin-Film Transistor by Phase Mode AFM Lithography

The phase mode atomic force microscopy (AFM) lithography and monolayer lift-off process are combined to fabricate electronics based on 2D materials (2DMs), which remove the need for pre-fabricating markers and increase the accuracy of the overlay and alignment. The promising phase mode of AFM lithography eliminates the drawbacks of the conventional force mode such as the over-cut, under-cut, debris effect, and severe tip wear. The planar size of MoS₂ thin-film transistors is shrunken down to sub-micrometer by the proposed method, and the fabricated devices demonstrate n-type characteristics. It offers a more flexible and easier way to fabricate prototypes of sub-micrometer-sized 2DMs based devices, and gives the opportunity to explore the size effect on the performance of 2DMs devices.

Hybrid Architectures of Heterogeneous Carbon Nanotube Composite Microstructures Enable Multiaxial Strain Perception with High Sensitivity and Ultrabroad Sensing Range

Low-dimensional nanomaterials are widely adopted as active sensing elements for electronic skins. When the nanomaterials are integrated with microscale architectures, the performance of the electronic skin is significantly altered. Here, it is shown that a high-performance flexible and stretchable electronic skin can be produced by incorporating a piezoresistive carbon nanotube composite into a hierarchical topography of micropillar-wrinkle hybrid architectures that mimic wrinkles and folds in human skin. Owing to the unique hierarchical topography of the hybrid architectures, the hybrid electronic skin exhibits versatile and superior sensing performance, which includes multiaxial force detection (normal, bending, and tensile stresses), remarkable sensitivity (20.9 kPa^-1 , 17.7 mm^-1 , and gauge factor of 707 each for normal, bending, and tensile stresses), ultrabroad sensing range (normal stress = 0-270 kPa, bending radius of curvature = 1-6.5 mm, and tensile strain = 0-50%), sensing tunability, fast response time (24 ms), and high durability (>10 000 cycles). Measurements of spatial distributions of diverse mechanical stimuli are also demonstrated with the multipixel electronic skin. The stress-strain behavior of the hybrid structure is investigated by finite element analysis to elucidate the underlying principle of the superior sensing performance of the electronic skin.

Dielectric Properties and Ion Transport in Layered MoS2 Grown by Vapor-Phase Sulfurization for Potential Applications in Nanoelectronics

Electronic and dielectric properties of vapor-phase grown MoS₂ have been investigated in metal/MoS₂/silicon capacitor structures by capacitance-voltage and conductance-voltage techniques. Analytical methods confirm the MoS₂ layered structure, the presence of interfacial silicon oxide (SiO _x ) and the composition of the films. Electrical characteristics in combination with theoretical considerations quantify the concentration of electron states at the interface between Si and a 2.5-3 nm thick silicon oxide interlayer between Si and MoS₂. Measurements under electric field stress indicate the existence of mobile ions in MoS₂ that interact with interface states. On the basis of time-of-flight secondary ion mass spectrometry, we propose OH^- ions as probable candidates responsible for the observations. The dielectric constant of the vapor-phase grown MoS₂ extracted from CV measurements at 100 kHz is 2.6 to 2.9. The present study advances the understanding of defects and interface states in MoS₂. It also indicates opportunities for ion-based plasticity in 2D material devices for neuromorphic computing applications.

Photo/thermoresponsive ABC triblock copolymer-based ion gels: photoinduced structural transitions

A photo/thermoresponsive ABC triblock copolymer-based ion gel exhibiting photoinduced structural transitions accompanied by significant rheological changes is newly developed. The ABC triblock copolymer comprises an ionic liquid (IL)-phobic A block, an IL-philic B block, and a photo/thermoresponsive C block containing azobenzene moieties. The IL-phobic A block forms a rigid micellar core in an IL over a wide temperature range and the photo/thermoresponsive C block undergoes upper critical solution temperature (UCST)-type phase transition in ILs. In concentrated polymer solution, the ABC triblock copolymer can form a percolated micellar network at low temperatures through aggregation of A and C blocks as physical crosslinks, bridged by IL-philic B blocks. In contrast, the ion gel undergoes structural transition to jammed micelles at high temperatures due to the disassembly of the thermoresponsive C block, resulting in significant softening of the ion gel. Importantly, the temperature dependences of the viscoelastic properties of the ion gel differ drastically depending on photo-irradiation conditions as the photoinduced isomerization of azobenzene moieties in the C block modulates the affinity between the polymer chain and IL. Utilizing this feature, photoinduced softening/hardening of the ion gel is realized at constant temperature. This study provides a promising strategy to control the rheological properties of nonvolatile soft materials via contactless light irradiation that could be exploited in various applications such as photoresponsive soft actuators and photo-healable soft materials.

Surface-Mediated Aligned Growth of Monolayer MoS2 and In-Plane Heterostructures with Graphene on Sapphire

Aligned growth of transition metal dichalcogenides and related two-dimensional (2D) materials is essential for the synthesis of high-quality 2D films due to effective stitching of merging grains. Here, we demonstrate the controlled growth of highly aligned molybdenum disulfide (MoS₂) on c-plane sapphire with two distinct orientations, which are highly controlled by tuning sulfur concentration. We found that the size of the aligned MoS₂ grains is smaller and their photoluminescence is weaker as compared with those of the randomly oriented grains, signifying enhanced MoS₂-substrate interaction in the aligned grains. This interaction induces strain in the aligned MoS₂, which can be recognized from their high susceptibility to air oxidation. The surface-mediated MoS₂ growth on sapphire was further developed to the rational synthesis of an in-plane MoS₂-graphene heterostructure connected with the predefined orientation. The in-plane epitaxy was observed by low-energy electron microscopy. Transmission electron microscopy and scanning transmission electron microscopy suggest the alignment of a zigzag edge of MoS₂ parallel to a zigzag edge of the neighboring graphene. Moreover, better electrical contact to MoS₂ was obtained by the monolayer graphene compared with a conventional metal electrode. Our findings deepen the understanding of the chemical vapor deposition growth of 2D materials and also contribute to the tailored synthesis as well as applications of advanced 2D heterostructures.

Nonmonotonic thickness-dependence of in-plane thermal conductivity of few-layered MoS2: 2.4 to 37.8 nm

Recent first-principles modeling reported a decrease of in-plane thermal conductivity (k) with increased thickness for few layered MoS2, which results from the change in phonon dispersion and missing symmetry in the anharmonic atomic force constant. For other 2D materials, it has been well documented that a higher thickness could cause a higher in-plane k due to a lower density of surface disorder. However, the effect of thickness on the k of MoS2 has not been systematically uncovered by experiments. In addition, from either experimental or theoretical approaches, the in-plane k value of tens-of-nm-thick MoS2 is still missing, which makes the physics on the thickness-dependent k remain ambiguous. In this work, we measure the k of few-layered (FL) MoS2 with thickness spanning a large range: 2.4 nm to 37.8 nm. A novel five energy transport state-resolved Raman (ET-Raman) method is developed for the measurement. For the first time, the critical effects of hot carrier diffusion, electron-hole recombination, and energy coupling with phonons are taken into consideration when determining the k of FL MoS2. By eliminating the use of laser energy absorption data and Raman temperature calibration, unprecedented data confidence is achieved. A nonmonotonic thickness-dependent k trend is discovered. k decreases from 60.3 W m-1 K-1 (2.4 nm thick) to 31.0 W m-1 K-1 (9.2 nm thick), and then increases to 76.2 W m-1 K-1 (37.8 nm thick), which is close to the reported k of bulk MoS2. This nonmonotonic behavior is analyzed in detail and attributed to the change of phonon dispersion for very thin MoS2 and a reduced surface scattering effect for thicker samples.

Ion Gel Network Formation in an Ionic Liquid Studied by Time-Resolved Small-Angle Neutron Scattering

We report the time-resolved small-angle neutron scattering (SANS) study of tetra-arm poly(ethylene glycol) (TetraPEG) polymer network formation in a typical ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C₂mim][TFSA]). To observe time-dependent SANS profiles, the reaction rate for the AB-type cross-end coupling reaction of TetraPEG macromers was controlled by adding an analogous protic IL, 1-ethylimidazolium TFSA ([C₂imH][TFSA]). At polymer concentrations higher than the overlap concentration ( c*), the SANS profile remained unchanged during the gelation reaction, indicating that the network structure was independent of macromer connectivity in a semidiluted solution. On the other hand, at low polymer concentrations, an increase in the SANS profile intensity was clearly observed. The correlation length (ξ), estimated by a fitting analysis based on the Ornstein-Zernike function, increased as the reaction proceeded. This result indicated that the sparsely dispersed macromers formed clusters during the cross-linking process and polymer size growth followed thereafter. We found that the network formation process and homogeneity of the network structure were strongly dependent on the polymer concentration in IL solutions.

MoS2 Memristors Exhibiting Variable Switching Characteristics toward Biorealistic Synaptic Emulation

Memristors based on 2D layered materials could provide biorealistic ionic interactions and potentially enable construction of energy-efficient artificial neural networks capable of faithfully emulating neuronal interconnections in human brains. To build reliable 2D-material-based memristors suitable for constructing working neural networks, the memristive switching mechanisms in such memristors need to be systematically analyzed. Here, we present a study on the switching characteristics of the few-layer MoS₂ memristors made by mechanical printing. First, two types of dc-programmed switching characteristics, termed rectification-mediated and conductance-mediated behaviors, are observed among different MoS₂ memristors, which are attributed to the modulation of MoS₂/metal Schottky barriers and redistribution of vacancies, respectively. We also found that an as-fabricated MoS₂ memristor initially exhibits an analog pulse-programmed switching behavior, but it can be converted to a quasi-binary memristor with an abrupt switching behavior through an electrical stress process. Such a transition of switching characteristics is attributed to field-induced agglomeration of vacancies at MoS₂/metal interfaces. The additional Kelvin probe force microscopy, Auger electron spectroscopy analysis, and electronic characterization results support this hypothesis. Finally, we fabricated a testing device consisting of two adjacent MoS₂ memristors and demonstrated that these two memristors can be ionically coupled to each other. This device interconnection scheme could be exploited to build neural networks for emulating ionic interactions among neurons. This work advances the device physics for understanding the memristive properties of 2D-material-based memristors and serves as a critical foundation for building biorealistic neuromorphic computing systems based on such memristors.

Voltage Control of Magnetic Anisotropy through Ionic Gel Gating for Flexible Spintronics

In spite of recent rapid development of flexible electronics, voltage-tunable spintronic structures and devices on flexible substrates have been rarely studied. Here, voltage control of magnetic anisotropy (VCMA) is demonstrated via ionic gel (IG) gating on flexible polyimide substrates with a circuit operating voltage of 1.8 V. A reversible, nonvolatile VCMA switching of 114 Oe is achieved in Pt/Fe/Pt multilayer, where the spatial magnetic anisotropy distribution is determined quantitatively by electron spin resonance technique. This IG gating process is repeatable as the substrates are under different bending conditions. The voltage modulation of magnetic anisotropy through IG gating with excellent flexibility proposes potential applications in low-power wearable spintronic devices.

Synthesis of Large-Area InSe Monolayers by Chemical Vapor Deposition

Recently, 2D materials of indium selenide (InSe) layers have attracted much attention from the scientific community due to their high mobility transport and fascinating physical properties. To date, reports on the synthesis of high-quality and scalable InSe atomic films are limited. Here, a synthesis of InSe atomic layers by vapor phase selenization of In₂ O₃ in a chemical vapor deposition (CVD) system, resulting in large-area monolayer flakes or thin films, is reported. The atomic films are continuous and uniform over a large area of 1 × 1 cm² , comprising of primarily InSe monolayers. Spectroscopic and microscopic measurements reveal the highly crystalline nature of the synthesized InSe monolayers. The ion-gel-gated field-effect transistors based on CVD InSe monolayers exhibit n-type channel behaviors, where the field effect electron mobility values can be up to ≈30 cm² V^-1 s^-1 along with an on/off current ratio, of >10⁴ at room temperature. In addition, the graphene can serve as a protection layer to prevent the oxidation between InSe and the ambient environment. Meanwhile, the synthesized InSe films can be transferred to arbitrary substrates, enabling the possibility of reassembly of various 2D materials into vertically stacked heterostructures, prompting research efforts to probe its characteristics and applications.

Molecular chemistry approaches for tuning the properties of two-dimensional transition metal dichalcogenides

Two-dimensional (2D) semiconductors, such as ultrathin layers of transition metal dichalcogenides (TMDs), offer a unique combination of electronic, optical and mechanical properties, and hold potential to enable a host of new device applications spanning from flexible/wearable (opto)electronics to energy-harvesting and sensing technologies. A critical requirement for developing practical and reliable electronic devices based on semiconducting TMDs consists in achieving a full control over their charge-carrier polarity and doping. Inconveniently, such a challenging task cannot be accomplished by means of well-established doping techniques (e.g. ion implantation and diffusion), which unavoidably damage the 2D crystals resulting in degraded device performances. Nowadays, a number of alternatives are being investigated, including various (supra)molecular chemistry approaches relying on the combination of 2D semiconductors with electroactive donor/acceptor molecules. As yet, a large variety of molecular systems have been utilized for functionalizing 2D TMDs via both covalent and non-covalent interactions. Such research endeavours enabled not only the tuning of the charge-carrier doping but also the engineering of the optical, electronic, magnetic, thermal and sensing properties of semiconducting TMDs for specific device applications. Here, we will review the most enlightening recent advancements in experimental (supra)molecular chemistry methods for tailoring the properties of atomically-thin TMDs - in the form of substrate-supported or solution-dispersed nanosheets - and we will discuss the opportunities and the challenges towards the realization of novel hybrid materials and devices based on 2D semiconductors and molecular systems.

Comparison of trapped charges and hysteresis behavior in hBN encapsulated single MoS2 flake based field effect transistors on SiO2 and hBN substrates

Molybdenum disulfide (MoS₂) based field effect transistors (FETs) are of considerable interest in electronic and opto-electronic applications but often have large hysteresis and threshold voltage instabilities. In this study, by using advanced transfer techniques, hexagonal boron nitride (hBN) encapsulated FETs based on a single, homogeneous and atomic-thin MoS₂ flake are fabricated on hBN and SiO₂ substrates. This allows for a better and a precise comparison between the charge traps at the semiconductor-dielectric interfaces at MoS₂-SiO₂ and hBN interfaces. The impact of ambient environment and entities on hysteresis is minimized by encapsulating the active MoS₂ layer with a single hBN on both the devices. The device to device variations induced by different MoS₂ layer is also eliminated by employing a single MoS₂ layer for fabricating both devices. After eliminating these additional factors which induce variation in the device characteristics, it is found from the measurements that the trapped charge density is reduced to 1.9 × 10¹¹ cm^-2 on hBN substrate as compared to 1.1 × 10¹² cm^-2 on SiO₂ substrate. Further, reduced hysteresis and stable threshold voltage are observed on hBN substrate and their dependence on gate sweep rate, sweep range, and gate stress is also studied. This precise comparison between encapsulated devices on SiO₂ and hBN substrates further demonstrate the requirement of hBN substrate and encapsulation for improved and stable performance of MoS₂ FETs.

Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor

Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mechanically flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to its contact, doping and mobility (µ) engineering must be overcome. This paper reviews the important technologically relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solutions employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key experimental and theoretical results are cited to supplement the discussions and provide further insight.

Self-Healing Micellar Ion Gels Based on Multiple Hydrogen Bonding

Ion gels, composed of macromolecular networks filled by ionic liquids (ILs), are promising candidate soft solid electrolytes for use in wearable/flexible electronic devices. In this context, the introduction of a self-healing function would significantly improve the long-term durability of ion gels subject to mechanical loading. Nevertheless, compared to hydrogels and organogels, the self-healing of ion gels has barely investigated been because of there being insufficient understanding of the interactions between polymers and ILs. Herein, a new class of supramolecular micellar ion gel composed of a diblock copolymer and a hydrophobic IL, which exhibits self-healing at room temperature, is presented. The diblock copolymer has an IL-phobic block and a hydrogen-bonding block with hydrogen-bond-accepting and donating units. By combining the IL and the diblock copolymer, micellar ion gels are prepared in which the IL phobic blocks form a jammed micelle core, whereas coronal chains interact with each other via multiple hydrogen bonds. These hydrogen bonds between the coronal chains in the IL endow the ion gel with a high level of mechanical strength as well as rapid self-healing at room temperature without the need for any external stimuli such as light or elevated temperatures.

Semiconductor Nanomembrane Materials for High-Performance Soft Electronic Devices

The development of methods to synthesize and physically manipulate extremely thin, single-crystalline inorganic semiconductor materials, so-called nanomembranes, has led to an almost explosive growth of research worldwide into uniquely enabled opportunities for their use in new "soft" and other unconventional form factors for high-performance electronics. The unique properties that nanomembranes afford, such as their flexibility and lightweight characteristics, allow them to be integrated into electronic and optoelectronic devices that, in turn, adopt these unique attributes. For example, nanomembrane devices are able to make conformal contact to curvilinear surfaces and manipulate strain to induce the self-assembly of various 3D nano/micro device architectures. Further, thin semiconductor materials (e.g., Si-nanomembranes, transition metal dichalcogenides, and phosphorene) are subject to the impacts of quantum and other size-dependent effects that in turn enable the manipulation of their bandgaps and the properties of electronic and optoelectronic devices fabricated from them. In this Perspective, nanomembrane synthesis techniques and exemplary applications of their use are examined. We specifically describe nanomembrane chemistry exploiting high-performance materials, along with precise/high-throughput techniques for their manipulation that exemplify their growing capacities to shape outcomes in technology. Prominent challenges in the chemistry of these materials are presented along with future directions that might guide the development of next generation nanomembrane-based devices.

Light-Emitting GaAs Nanowires on a Flexible Substrate

Semiconductor nanowire-based devices are among the most promising structures used to meet the current challenges of electronics, optics and photonics. Due to their high surface-to-volume ratio and excellent optical and electrical properties, devices with low power, high efficiency and high density can be created. This is of major importance for environmental issues and economic impact. Semiconductor nanowires have been used to fabricate high performance devices, including detectors, solar cells and transistors. Here, we demonstrate a technique for transferring large-area nanowire arrays to flexible substrates while retaining their excellent quantum efficiency in emission. Starting with a defect-free self-catalyzed molecular beam epitaxy (MBE) sample grown on a Si substrate, GaAs core-shell nanowires are embedded in a dielectric, removed by reactive ion etching and transferred to a plastic substrate. The original structural and optical properties, including the vertical orientation, of the nanowires are retained in the final plastic substrate structure. Nanowire emission is observed for all stages of the fabrication process, with a higher emission intensity observed for the final transferred structure, consistent with a reduction in nonradiative recombination via the modification of surface states. This transfer process could form the first critical step in the development of flexible nanowire-based light-emitting devices.

Ionic Gels and Their Applications in Stretchable Electronics

Ionic gels represent a novel class of stretchable materials where ionic conducting liquid is immobilized in a polymer matrix. This review focuses on the design of ionic gel materials and device fabrication of ionic-gel-based stretchable electronics. In particular, recent progress in ionic-gel-based electronic skin (pressure/strain sensors, electric double-layer transistors, etc.), flexible displays, energy storage devices, and soft actuators are summarized, followed by a discussion of challenges in developing ionic-gel-based electronics and suggestions for future research directions that might overcome those challenges.

Monolayer Transition Metal Dichalcogenides as Light Sources

Reducing the dimensions of materials is one of the key approaches to discovering novel optical phenomena. The recent emergence of 2D transition metal dichalcogenides (TMDCs) has provided a promising platform for exploring new optoelectronic device applications, with their tunable electronic properties, structural controllability, and unique spin valley-coupled systems. This progress report provides an overview of recent advances in TMDC-based light-emitting devices discussed from several aspects in terms of device concepts, material designs, device fabrication, and their diverse functionalities. First, the advantages of TMDCs used in light-emitting devices and their possible functionalities are presented. Second, conventional approaches for fabricating TMDC light-emitting devices are emphasized, followed by introducing a newly established, versatile method for generating light emission in TMDCs. Third, current growing technologies for heterostructure fabrication, in which distinct TMDCs are vertically stacked or laterally stitched, are explained as a possible means for designing high-performance light-emitting devices. Finally, utilizing the topological features of TMDCs, the challenges for controlling circularly polarized light emission and its device applications are discussed from both theoretical and experimental points of view.

High- k Gate Dielectrics for Emerging Flexible and Stretchable Electronics

Recent advances in flexible and stretchable electronics (FSE), a technology diverging from the conventional rigid silicon technology, have stimulated fundamental scientific and technological research efforts. FSE aims at enabling disruptive applications such as flexible displays, wearable sensors, printed RFID tags on packaging, electronics on skin/organs, and Internet-of-things as well as possibly reducing the cost of electronic device fabrication. Thus, the key materials components of electronics, the semiconductor, the dielectric, and the conductor as well as the passive (substrate, planarization, passivation, and encapsulation layers) must exhibit electrical performance and mechanical properties compatible with FSE components and products. In this review, we summarize and analyze recent advances in materials concepts as well as in thin-film fabrication techniques for high- k (or high-capacitance) gate dielectrics when integrated with FSE-compatible semiconductors such as organics, metal oxides, quantum dot arrays, carbon nanotubes, graphene, and other 2D semiconductors. Since thin-film transistors (TFTs) are the key enablers of FSE devices, we discuss TFT structures and operation mechanisms after a discussion on the needs and general requirements of gate dielectrics. Also, the advantages of high- k dielectrics over low- k ones in TFT applications were elaborated. Next, after presenting the design and properties of high- k polymers and inorganic, electrolyte, and hybrid dielectric families, we focus on the most important fabrication methodologies for their deposition as TFT gate dielectric thin films. Furthermore, we provide a detailed summary of recent progress in performance of FSE TFTs based on these high- k dielectrics, focusing primarily on emerging semiconductor types. Finally, we conclude with an outlook and challenges section.

Low-temperature synthesis of 2D MoS2 on a plastic substrate for a flexible gas sensor

The efficient synthesis of two-dimensional molybdenum disulfide (2D MoS2) at low temperatures is essential for use in flexible devices. In this study, 2D MoS2 was grown directly at a low temperature of 200 °C on both hard (SiO2) and soft substrates (polyimide (PI)) using chemical vapor deposition (CVD) with Mo(CO)6 and H2S. We investigated the effect of the growth temperature and Mo concentration on the layered growth by Raman spectroscopy and microscopy. 2D MoS2 was grown by using low Mo concentration at a low temperature. Through optical microscopy, Raman spectroscopy, X-ray photoemission spectroscopy, photoluminescence, and transmission electron microscopy measurements, MoS2 produced by low-temperature CVD was determined to possess a layered structure with good uniformity, stoichiometry, and a controllable number of layers. Furthermore, we demonstrated the realization of a 2D MoS2-based flexible gas sensor on a PI substrate without any transfer processes, with competitive sensor performance and mechanical durability at room temperature. This fabrication process has potential for burgeoning flexible and wearable nanotechnology applications.

Low-Temperature Fabrication of Robust, Transparent, and Flexible Thin-Film Transistors with a Nanolaminated Insulator

The lack of reliable, transparent, and flexible electrodes and insulators for applications in thin-film transistors (TFTs) makes it difficult to commercialize transparent, flexible TFTs (TF-TFTs). More specifically, conventional high process temperatures and the brittleness of these elements have been hurdles in developing flexible substrates vulnerable to heat. Here, we propose electrode and insulator fabrication techniques considering process temperature, transmittance, flexibility, and environmental stability. A transparent and flexible indium tin oxide (ITO)/Ag/ITO (IAI) electrode and an Al₂O₃/MgO (AM)-laminated insulator were optimized at the low temperature of 70 °C for the fabrication of TF-TFTs on a polyethylene terephthalate (PET) substrate. The optimized IAI electrode with a sheet resistance of 7 Ω/sq exhibited the luminous transmittance of 85.17% and maintained its electrical conductivity after exposure to damp heat conditions because of an environmentally stable ITO capping layer. In addition, the electrical conductivity of IAI was maintained after 10 000 bending cycles with a tensile strain of 3% because of the ductile Ag film. In the metal/insulator/metal structure, the insulating and mechanical properties of the optimized AM-laminated film deposited at 70 °C were significantly improved because of the highly dense nanolaminate system, compared to those of the Al₂O₃ film deposited at 70 °C. In addition, the amorphous indium-gallium-zinc oxide (a-IGZO) was used as the active channel for TF-TFTs because of its excellent chemical stability. In the environmental stability test, the ITO, a-IGZO, and AM-laminated films showed the excellent environmental stability. Therefore, our IGZO-based TFT with IAI electrodes and the 70 °C AM-laminated insulator was fabricated to evaluate robustness, transparency, flexibility, and process temperature, resulting in transfer characteristics comparable to those of an IGZO-based TFT with a 150 °C Al₂O₃ insulator.

Si Nanocrystal-Embedded SiO x nanofoils: Two-Dimensional Nanotechnology-Enabled High Performance Li Storage Materials

Silicon (Si) based materials are highly desirable to replace currently used graphite anode for lithium ion batteries. Nevertheless, its usage is still a big challenge due to poor battery performance and scale-up issue. In addition, two-dimensional (2D) architectures, which remain unresolved so far, would give them more interesting and unexpected properties. Herein, we report a facile, cost-effective, and scalable approach to synthesize Si nanocrystals embedded 2D SiO_x nanofoils for next-generation lithium ion batteries through a solution-evaporation-induced interfacial sol-gel reaction of hydrogen silsesquioxane (HSiO_1.5, HSQ). The unique nature of the thus-prepared centimeter scale 2D nanofoil with a large surface area enables ultrafast Li⁺ insertion and extraction, with a reversible capacity of more than 650 mAh g⁻¹, even at a high current density of 50 C (50 A g⁻¹). Moreover, the 2D nanostructured Si/SiO_x nanofoils show excellent cycling performance up to 200 cycles and maintain their initial dimensional stability. This superior performance stems from the peculiar nanoarchitecture of 2D Si/SiO_x nanofoils, which provides short diffusion paths for lithium ions and abundant free space to effectively accommodate the huge volume changes of Si during cycling.

Ultrathin Alumina Mask-Assisted Nanopore Patterning on Monolayer MoS2 for Highly Catalytic Efficiency in Hydrogen Evolution Reaction

Nanostructured molybdenum disulfide (MoS₂) has been considered as one of the most promising catalysts in the hydrogen evolution reaction (HER), for its approximately intermediate hydrogen binding free energy to noble metals and much lower cost. The catalytically active sites of MoS₂ are along the edges, whereas thermodynamically MoS₂ favors the presence of a two-dimensional (2-D) basal plane and the catalytically active atoms only constitute a small portion of the material. The lack of catalytically active sites and low catalytic efficiency impede its massive application. To address the issue, we have activated the basal plane of monolayer 2H MoS₂ through an ultrathin alumina mask (UTAM)-assisted nanopore arrays patterning, creating a high edge density. The introduced catalytically active sites are identified by Cu electrochemical deposition, and the hydrogen generation properties are assessed in detail. We demonstrate a remarkably improved HER performance as well as the identical catalysis of the artificial edges and the pristine metallic edges of monolayer MoS₂. Such a porous monolayer nanostructure can achieve a much higher edge atom ratio than the pristine monolayer MoS₂ flakes, which can lead to a much improved catalytic efficiency. This controllable edge engineering can also be extended to the basal plane modifications of other 2-D materials, for improving their edge-related properties.

Thickness-modulated metal-to-semiconductor transformation in a transition metal dichalcogenide

The possibility of tailoring physical properties by changing the number of layers in van der Waals crystals is one of the driving forces behind the emergence of two-dimensional materials. One example is bulk MoS2, which changes from an indirect gap semiconductor to a direct bandgap semiconductor in the monolayer form. Here, we show a much bigger tuning range with a complete switching from a metal to a semiconductor in atomically thin PtSe2 as its thickness is reduced. Crystals with a thickness of ~13 nm show metallic behavior with a contact resistance as low as 70 Ω·µm. As they are thinned down to 2.5 nm and below, we observe semiconducting behavior. In such thin crystals, we demonstrate ambipolar transport with a bandgap smaller than 2.2 eV and an on/off ratio of ~105. Our results demonstrate that PtSe2 possesses an unusual behavior among 2D materials, enabling novel applications in nano and optoelectronics.

Group 6 transition metal dichalcogenide nanomaterials: synthesis, applications and future perspectives

Group 6 transition metal dichalcogenides (G6-TMDs), most notably MoS₂, MoSe₂, MoTe₂, WS₂ and WSe₂, constitute an important class of materials with a layered crystal structure. Various types of G6-TMD nanomaterials, such as nanosheets, nanotubes and quantum dot nano-objects and flower-like nanostructures, have been synthesized. High thermodynamic stability under ambient conditions, even in atomically thin form, made nanosheets of these inorganic semiconductors a valuable asset in the existing library of two-dimensional (2D) materials, along with the well-known semimetallic graphene and insulating hexagonal boron nitride. G6-TMDs generally possess an appropriate bandgap (1-2 eV) which is tunable by size and dimensionality and changes from indirect to direct in monolayer nanosheets, intriguing for (opto)electronic, sensing, and solar energy harvesting applications. Moreover, rich intercalation chemistry and abundance of catalytically active edge sites make them promising for fabrication of novel energy storage devices and advanced catalysts. In this review, we provide an overview on all aspects of the basic science, physicochemical properties and characterization techniques as well as all existing production methods and applications of G6-TMD nanomaterials in a comprehensive yet concise treatment. Particular emphasis is placed on establishing a linkage between the features of production methods and the specific needs of rapidly growing applications of G6-TMDs to develop a production-application selection guide. Based on this selection guide, a framework is suggested for future research on how to bridge existing knowledge gaps and improve current production methods towards technological application of G6-TMD nanomaterials.

Direct Assessment of the Toxicity of Molybdenum Disulfide Atomically Thin Film and Microparticles via Cytotoxicity and Patch Testing

The low toxicity of molybdenum disulfide (MoS₂ ) atomically thin film and microparticles is confirmed via cytotoxicity and patch testing in this report. The toxicity of MoS₂ thin film and microparticles is extensively studied but is still inconclusive due to potential organic contamination in the preparations of samples. Such contamination is avoided here through preparing MoS₂ atomically thin film via direct sulfurization of molybdenum thin film on quartz plate, which permits a direct assessment of its toxicity without any contamination. Six different types of cells, including normal, cancer, and immortal cells, are cultured in the media containing MoS₂ thin film on quartz plates or dispersed MoS₂ microparticles and their viability is evaluated with respect to the concentrations of samples. Detached thin films from the quartz plates are also investigated to estimate the toxicity of dispersed MoS₂ in biological media. Allergy testing on skin of guinea pigs is also conducted to understand their effect on animal skins. By avoiding possible organic contamination, the low toxicity of MoS₂ atomically thin film and microparticles to cells and animal skins paves the way for its applications in flexible biosensing/bioimaging devices and biocompatible coatings.

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.

Interaction between H2O, N2, CO, NO, NO2 and N2O molecules and a defective WSe2 monolayer

In this study, the interaction between gas molecules, including H₂O, N₂, CO, NO, NO₂ and N₂O, and a WSe₂ monolayer containing an Se vacancy (denoted as V_Se) has been theoretically studied. Theoretical results show that H₂O and N₂ molecules are highly prone to be physisorbed on the V_Se surface. The presence of the Se vacancy can significantly enhance the sensing ability of the WSe₂ monolayer toward H₂O and N₂ molecules. In contrast, CO and NO molecules highly prefer to be molecularly chemisorbed on the V_Se surface with the non-oxygen atom occupying the Se vacancy site. Furthermore, the exposed O atoms of the molecularly chemisorbed CO or NO can react with additional CO or NO molecules, to produce C-doped or N-doped WSe₂ monolayers. The calculated energies suggest that the filling of the CO or NO molecule and the removal of the exposed O atom are both energetically and dynamically favorable. Electronic structure calculations show that the WSe₂ monolayers are p-doped by the CO and NO molecules, as well as the C and N atoms. However, only the NO molecule and N atom doped WSe₂ monolayers exhibit significantly improved electronic structures compared with V_Se. The NO₂ and N₂O molecules will dissociate directly to form an O-doped WSe₂ monolayer, for which the defect levels due to the Se vacancy can be completely removed. The calculated energies suggest that although the dissociation processes for NO₂ and N₂O molecules are highly exothermic, the N₂O dissociation may need to operate at an elevated temperature compared with room temperature, due to its large energy barrier of ∼1 eV.

Spin splitting and p-/n-type doping of two-dimensional WSe2/BiIrO3(111) heterostructures

Switching the polarization of a substrate causes charge transfer in WSe₂ which can switch p-/n-type doping and influence spin splitting energy.

The effect of the experimental parameters on the growth of MoS2 flakes

The growth mechanism and optical performance of MoS₂ crystals have been systemically studied by manipulating the growth parameters.

Very fast hot carrier diffusion in unconstrained MoS2on a glass substrate: discovered by picosecond ET-Raman

The currently reported optical-phonon-scattering-limited carrier mobility of MoS₂ is up to 417 cm² V⁻¹ s⁻¹ with two-side dielectric screening: one normal-κ side and one high-κ side. Herein, using picosecond energy transport state-resolved Raman (ET-Raman), we demonstrated very fast hot carrier diffusion in μm-scale (lateral) unconstrained MoS₂ (1.8–18 nm thick) on a glass substrate; this method enables only one-side normal-κ dielectric screening. The ET-Raman method directly probes the diffusion of the hot carrier and its contribution to phonon transfer without contact and additional sample preparation and provides unprecedented insight into the intrinsic D of MoS₂. The measured D values span from 0.76 to 9.7 cm² s⁻¹. A nonmonotonic thickness-dependent D trend is discovered, and it peaks at 3.0 nm thickness. This is explained by the competition between two physical phenomena: with an increase in sample thickness, the increased screening of the substrate results in higher mobility; moreover, thicker samples are subject to more surface contamination, loose substrate contact and weaker substrate dielectric screening. The corresponding carrier mobility varies from 31.0 to 388.5 cm² V⁻¹ s⁻¹. This mobility is surprisingly high considering the normal-κ and single side dielectric screening by the glass substrate. This is a direct result of the less-damaged structure of MoS₂ that is superior to those of MoS₂ samples reported in literature studies that are subjected to various post-processing techniques to facilitate measurement. The very high hot carrier mobility reduces the local carrier concentration and enhances the Raman signal, which is further confirmed by our Raman signal studies and comparison with theoretical studies.

Very high nonmonotonic thickness-dependent hot carrier diffusivity of MoS₂ in a normal-κ dielectric screening environment was discovered by ET-Raman technique.

Block copolymer self-assembly in ionic liquids

Recent developments in block copolymer self-assembly in ionic liquids are reviewed from both fundamental and applied aspects.

Time-evolution of the electrical characteristics of MoS2 field-effect transistors after electron beam irradiation

The mechanisms of threshold voltage shift evolution of MoS₂ FETs after electron beam irradiation were demonstrated experimentally for the first time.

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.

Self-Assembly High-Performance UV-vis-NIR Broadband β-In2Se3/Si Photodetector Array for Weak Signal Detection

The emergence of a rich variety of layered materials has attracted considerable attention in recent years because of their exciting properties. However, the applications of layered materials in optoelectronic devices are hampered by the low light absorption of monolayers/few layers, the lack of p-n junction, and the challenges for large-scale production. Here, we report a scalable production of β-In₂Se₃/Si heterojunction arrays using pulsed-laser deposition. Photodetectors based on the as-produced heterojunction array are sensitive to a broadband wavelength from ultraviolet (370 nm) to near-infrared (808 nm), showing a high responsivity (5.9 A/W), a decent current on/off ratio (∼600), and a superior detectivity (4.9 × 10¹² jones), simultaneously. These figures-of-merits are among the best values of the reported heterojunction-based photodetectors. In addition, these devices can further enable the detection of weak signals, as successfully demonstrated with weak light sources including a flashlight, lighter, and fluorescent light. Device physics modeling shows that their high performance is attributed to the strong light absorption of the relatively thick β-In₂Se₃ film (20.3 nm) and the rational energy band structures of β-In₂Se₃ and Si, which allows efficient separation of photoexcited electron-hole pairs. These results offer a new insight into the rational design of optoelectronic devices from the synergetic effect of layered materials as well as mature semiconductor technology.

Elastic Properties of Few Nanometers Thick Polycrystalline MoS2 Membranes: A Nondestructive Study

The performance gain-oriented nanostructurization has opened a new pathway for tuning mechanical features of solid matter vital for application and maintained performance. Simultaneously, the mechanical evaluation has been pushed down to dimensions way below 1 μm. To date, the most standard technique to study the mechanical properties of suspended 2D materials is based on nanoindentation experiments. In this work, by means of micro-Brillouin light scattering we determine the mechanical properties, that is, Young modulus and residual stress, of polycrystalline few nanometers thick MoS₂ membranes in a simple, contact-less, nondestructive manner. The results show huge elastic softening compared to bulk MoS₂, which is correlated with the sample morphology and the residual stress.

Stable ultra-thin CdTe crystal: a robust direct gap semiconductor

Employing density functional theory based calculations, we investigate structural, vibrational and strain-dependent electronic properties of an ultra-thin CdTe crystal structure that can be derived from its bulk counterpart. It is found that this ultra-thin crystal has an 8-atom primitive unit cell with considerable surface reconstructions. Dynamic stability of the structure is predicted based on its calculated vibrational spectrum. Electronic band structure calculations reveal that both electrons and holes in single layer CdTe possess anisotropic in-plane masses and mobilities. Moreover, we show that the ultra-thin CdTe has some interesting electromechanical features, such as strain-dependent anisotropic variation of the band gap value, and its rapid increase under perpendicular compression. The direct band gap semiconducting nature of the ultra-thin CdTe crystal remains unchanged under all types of applied strain. With a robust and moderate direct band gap, single-layer CdTe is a promising material for nanoscale strain dependent device applications.

A Hybrid Gate Dielectrics of Ion Gel with Ultra-Thin Passivation Layer for High-Performance Transistors Based on Two-Dimensional Semiconductor Channels

We propose a hybrid gate structure for ion gel dielectrics using an ultra-thin Al₂O₃ passivation layer for realizing high-performance devices based on electric-double-layer capacitors. Electric-double-layer transistors can be applied to practical devices with flexibility and transparency as well as research on the fundamental physical properties of channel materials; however, they suffer from inherent unwanted leakage currents between electrodes, especially for channel materials with low off-currents. Therefore, the Al₂O₃ passivation layer was introduced between the metal electrodes and ion gel film as a leakage current barrier; this simple approach effectively reduced the leakage current without capacitance degradation. In addition, we confirmed that a monolayer MoS₂ transistor fabricated with the proposed hybrid gate dielectric exhibited remarkably enhanced device properties compared to a transistor using a normal ion gel gate dielectric. Our findings on a simple method to improve the leakage current properties of ion gels could be applied extensively to realize high-performance electric-double-layer transistors utilizing various channel materials.

Oxygen Vacancy in WO3 Film-based FET with Ionic Liquid Gating

Ionic liquid gating is a versatile method for inducing a metal-insulator transition in field-effect transistor device structures. The mechanism of carrier doping in metal oxide films is under debate. Ionic liquid gating of a WO₃ film-based field effect transistor is discussed in this report. Flat and relatively smooth WO₃ films were deposited on SrTiO₃ substrates by pulsed laser deposition. Swept and constant gate voltage characteristics are measured in both argon and oxygen atmospheres. The results show a clear dependence on the oxygen pressure of the experimental chamber. Metallic behavior in the films is attributed to oxygen vacancy formation in the WO₃ layer induced by the high electric field at the oxide-ionic liquid interface. The density of states of a monoclinic supercell of oxygen deficient WO₃ was studied by density functional theory (DFT). Calculated W and O partial densities of states verify metallic behavior even at dilute oxygen vacancy concentrations and show the role of W and O orbitals in the conductivity.

Flexible Device Applications of 2D Semiconductors

Graphene-like single- or few-layer semiconductors, such as dichalcogenides and buckled nanocrystals, possess direct and tunable bandgaps, and excellent electrical, optical, mechanical and thermal properties. This unique set of desirable properties of 2D semiconductors has triggered great interest in developing ultra-thin 2D flexible electronic devices, which ranges from realizing better material quality and simplified fabrication processes, to improving device performance and expanding the application horizon. The most explored 2D flexible devices based on transition metal dichalcogenides and black phosphorous include field-effect transistors, optoelectronics, electronic sensors and supercapacitors. By taking advantage of a large portfolio of materials and properties of 2D crystals, a new generation of low-cost, high-performance, transparent, flexible and wearable devices looks attractive and promising in advancing flexible electronic technologies.

Progress of Large-Scale Synthesis and Electronic Device Application of Two-Dimensional Transition Metal Dichalcogenides

The recent exploration of semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) with atomic thickness has taken both the scientific and technological communities by storm. Extensively investigated TMD that are accessible by large-scale synthetic methods materials are remarkably stable, such as MoS₂ and WSe₂ . They allow superior gate control due to their 2D nature and favorable electronic transport properties, thus suggesting a bright future for digital and RF electronics. In this review, the latest developments in the controlled synthesis of large scale TMDs are firstly introduced by discussing various approaches. The major obstacles that must be overcome to achieve wafer-scale, uniform, and high-quality TMD films for practical electronic applications are included. Advances in the electronic transport studies of TMDs are presented, such as doping, contact engineering, and mobility improvement, which contribute to overall device performance. A perspective and a look at the future for this field is provided in closing.

Hierarchical Porous Co9S8/Nitrogen-Doped Carbon@MoS2 Polyhedrons as pH Universal Electrocatalysts for Highly Efficient Hydrogen Evolution Reaction

The development of highly active and stable earth-abundant electrocatalysts to reduce or eliminate the reliance on noble-metal based ones for hydrogen evolution reaction (HER) over a broad pH range remains a great challenge. Herein, hierarchical porous Co₉S₈/N-doped carbon@MoS₂ (Co₉S₈/NC@MoS₂) polyhedrons have been synthesized by a facile hydrothermal approach using highly conductive Co/NC polyhedrons composed of cobalt nanoparticles embedded in N-doped carbon matrices as both the structural support and cobalt source. The Co/NC polyhedrons were prepared by direct carbonization of Co-based zeolitic imidazolate framework (ZIF-67) in Ar atmosphere. Benefiting from the prominent synergistic effect of N-doped carbon enhancing the conductivity of the hybrid, MoS₂ and Co₉S₈ providing abundant catalytically active sites as well as the well-defined polyhedral structure promoting mechanical stability, the as-synthesized Co₉S₈/NC@MoS₂ shows excellent HER activity and good stability over a broad pH range, with onset overpotentials of 4, 38, and 45 mV, Tafel slopes of 60.3, 68.8, and 126.1 mV dec^-1, and overpotentials of 67, 117, and 261 mV at 10 mA cm^-2 in 1.0 M KOH, 0.5 M H₂SO₄, and 1.0 M phosphate buffer solution (PBS), respectively. This work provides a general and promising approach for the design and synthesis of inexpensive and efficient pH-universal HER electrocatalysts.

Self-propelled ion gel at air-water interface

We report on a self-propelled gel using ionic liquid as a new type of self-propellant that generates a powerful and durable motion at an air-water interface. The gel is composed of 1-ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) and poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)). A long rectangular ion gel piece placed on the interface shows rapid rotation motion with maximum frequency close to 10 Hz, corresponding to the velocity over 300 mms⁻¹ at an outmost end of the piece. The rotation continues for ca. 10² s, followed by a reciprocating motion (<~10³ s) and a nonlinear motion in long-time observations (>~10³ s). The behaviours can be explained by the model considering elution of EMIM-TFSI to the air-water interface, rapid dissolution into water, and slow diffusion in an inhomogeneous polymer gel network. Because the self-propellants are promptly removed from the interface by dissolution, durable self-propelled motions are observed also at limited interface areas close in size to the gel pieces. A variety of motions are induced in such systems where the degree of freedom in motion is limited. As the ion gel possesses formability and processability, it is also advantageous for practical applications. We demonstrate that the gel does work as an engine.

Junctionless Diode Enabled by Self-Bias Effect of Ion Gel in Single-Layer MoS2 Device

The self-biasing effects of ion gel from source and drain electrodes on electrical characteristics of single layer and few layer molybdenum disulfide (MoS₂) field-effect transistor (FET) have been studied. The self-biasing effect of ion gel is tested for two different configurations, covered and open, where ion gel is in contact with either one or both, source and drain electrodes, respectively. In open configuration, the linear output characteristics of the pristine device becomes nonlinear and on-off ratio drops by 3 orders of magnitude due to the increase in "off" current for both single and few layer MoS₂ FETs. However, the covered configuration results in a highly asymmetric output characteristics with a rectification of around 10³ and an ideality factor of 1.9. This diode like behavior has been attributed to the reduction of Schottky barrier width by the electric field of self-biased ion gel, which enables an efficient injection of electrons by tunneling at metal-MoS₂ interface. Finally, finite element method based simulations are carried out and the simulated results matches well in principle with the experimental analysis. These self-biased diodes can perform a crucial role in the development of high-frequency optoelectronic and valleytronic devices.

Suppressing Nucleation in Metal-Organic Chemical Vapor Deposition of MoS2 Monolayers by Alkali Metal Halides

Toward the large-area deposition of MoS₂ layers, we employ metal-organic precursors of Mo and S for a facile and reproducible van der Waals epitaxy on c-plane sapphire. Exposing c-sapphire substrates to alkali metal halide salts such as KI or NaCl together with the Mo precursor prior to the start of the growth process results in increasing the lateral dimensions of single crystalline domains by more than 2 orders of magnitude. The MoS₂ grown this way exhibits high crystallinity and optoelectronic quality comparable to single-crystal MoS₂ produced by conventional chemical vapor deposition methods. The presence of alkali metal halides suppresses the nucleation and enhances enlargement of domains while resulting in chemically pure MoS₂ after transfer. Field-effect measurements in polymer electrolyte-gated devices result in promising electron mobility values close to 100 cm² V^-1 s^-1 at cryogenic temperatures.