Highly flexible MoS2 thin-film transistors with ion gel dielectrics

A scalable fabrication method for gold nanodisk-upconverting nanoparticle hybrid nanostructures

Plasmonic nanostructures can be used to enhance the efficiency of upconversion nanoparticles (UCNPs) and enable new functionalities. However, the fabrication of these hybrid plasmon-UCNP nanostructures has traditionally relied on either wet chemistry or nanolithography routes that are difficult to control, scale up, or both. In this work, we present a scalable nanofabrication process, capable of producing a massive array of gold-UCNP hybrid nanostructures over a few mm2 area and with excellent uniformity in the photoluminescence intensity. This new approach combines the scalability of the bottom-up self-assembly method and the precision of the top-down nanolithography approach. It provides an efficient alternative route for the production of plasmonically enhanced UCNPs. A detailed discussion on the optimization of the UCNP self-assembly, the gold nanodisk lithography, and the nanopattern transfer processes is presented here. Additionally, we showcase the potential of this new approach for fabricating mechanical force sensors based on the selective plasmonic enhancement of the UCNP emission. This new approach holds great potential in facilitating the production of plasmonically enhanced UCNPs that can be deployed for both imaging and sensing applications.

Experimental and Theoretical Insights into Interfacial Properties of 2D Materials for Selective Water Transport Membranes: A Critical Review

Interfacial properties, such as wettability and friction, play critical roles in nanofluidics and desalination. Understanding the interfacial properties of two-dimensional (2D) materials is crucial in these applications due to the close interaction between liquids and the solid surface. The most important interfacial properties of a solid surface include the water contact angle, which quantifies the extent of interactions between the surface and water, and the water slip length, which determines how much faster water can flow on the surface beyond the predictions of continuum fluid mechanics. This Review seeks to elucidate the mechanism that governs the interfacial properties of diverse 2D materials, including transition metal dichalcogenides (e.g., MoS2), graphene, and hexagonal boron nitride (hBN). Our work consolidates existing experimental and computational insights into 2D material synthesis and modeling and explores their interfacial properties for desalination. We investigated the capabilities of density functional theory and molecular dynamics simulations in analyzing the interfacial properties of 2D materials. Specifically, we highlight how MD simulations have revolutionized our understanding of these properties, paving the way for their effective application in desalination. This Review of the synthesis and interfacial properties of 2D materials unlocks opportunities for further advancement and optimization in desalination.

Chemically Transformed Ag2 Te Nanowires on Polyvinylidene Fluoride Membrane For Flexible Thermoelectric Applications

Flexible thermoelectric devices of nanomaterials have shown a great potential for applications in wearable to remotely located electronics with desired shapes and geometries. Continuous powering up the low power flexible electronics is a major challenge. We are reporting a flexible thermoelectric module prepared from silver telluride (Ag2 Te) nanowires (NWs), which are chemically transformed from uniquely synthesized and scalable tellurium (Te) NWs. Conducting Ag2 Te NWs composites have shown an ultralow total thermal conductivity ~0.22 W/mK surpassing the bulk melt-grown Ag2 Te ~1.23 W/mK at ~300 K, which is attributed to the nanostructuring of the material. Flexible thermoelectric device consisting of 4 legs (n-type) of Ag2 Te NWs on polyvinylidene fluoride membrane displays a significant output voltage (Voc ) ~2.3 mV upon human touch and Voc ~18 mV at temperature gradient, ΔT ~50 K, which shows the importance of NWs based flexible thermoelectric devices to power up the low power wearable electronics.

Ultraflexible, High-Performance Transistors and Logic Circuits on Biocompatible Polylactic Acid (PLA) Substrate

Wearable and implantable devices have gained significant popularity, playing a crucial role in smart healthcare and human-machine interfaces, which necessitates the development of more complex electronic devices and circuits on biocompatible flexible materials. Polylactic acid (PLA) stands out due to its biodegradability, cost-effectiveness, and low immunogenicity. In this study, we utilize a solution-based spin-coating method to produce high-quality PLA thin films, serving as substrates for the fabrication of thin-film transistors (TFTs) in which the dielectric layer material is silicon dioxide, the channel layer material is IGZO, and the gate, drain, and source material is ITO at low temperatures (<40 °C) through a shadow masking technique. The resulting PLA-TFT devices exhibited remarkable flexibility, biocompatibility, and impressive electrical characteristics, including a charge carrier mobility of 27.81 cm2/(V s), a subthreshold swing of 162.8 mV/decade, and an ON/OFF current ratio of up to 1 × 106, and maintained performance under various deformations. We successfully constructed fundamental logic gate circuits using PLA-TFTs, including AND, OR, and NOT gates, which effectively performed logical functions and demonstrated stability under diverse bending conditions. These research findings provide valuable support for future endeavors in fabricating intricate logic circuits and realizing advanced functionalities on biocompatible flexible materials.

Density Functional Theory-Based Approach For Dielectric Constant Estimation of Soluble Polyimide Insulators

Evaluation of the insulating properties of polymers, such as the dielectric constant and dissipation factor, is crucial in electronic devices, including field-effect transistors and wireless communication applications. This study applies density functional theory (DFT) to predict the dielectric constant of soluble polyimides (SPIs). Various SPIs containing trifluoromethyl groups in the backbone with different pendant types, numbers, and symmetries are successfully synthesized, and their dielectric constants are evaluated and compared with the DFT-estimated values. Two types of DFT-optimized SPIs, single-chain and stacked-chain models, are used to describe the local geometries of the SPIs. In addition, to reveal the relationship between the molecular structure and dielectric constant, further investigations are conducted by considering the dielectric constant of composing ionic and electronic components. The DFT-estimated static dielectric constant of the single-chain model accurately reproduces the corresponding experimental value with at least 80% accuracy. Our approach provides a rational and accelerated strategy to evaluate polymer insulators for electronic devices based on cost-effective DFT calculations.

Opto-Electrochemical Synaptic Memory in Supramolecularly Engineered Janus 2D MoS2

Artificial synapses combining multiple yet independent signal processing strategies in a single device are key enabler to achieve high-density of integration, energy efficiency, and fast data manipulation in brain-like computing. By taming functional complexity, the use of hybrids comprising multiple materials as active components in synaptic devices represents a powerful route to encode both short-term potentiation (STP) and long-term potentiation (LTP) in synaptic circuitries. To meet such a grand challenge, herein a novel Janus 2D material is developed by dressing asymmetrically the two surfaces of 2D molybdenum disulfide (MoS2 ) with an electrochemically-switchable ferrocene (Fc)/ ferrocenium (Fc+ ) redox couple and an optically-responsive photochromic azobenzene (Azo). Upon varying the magnitude of the electrochemical stimulus, it is possible to steer the transition between STP and LTP, thereby either triggering electrochemical doping of Fc/Fc+  pair on MoS2  or controlling an adsorption/desorption process of such redox species on MoS2 . In addition, a lower magnitude LTP is recorded by activating the photoisomerization of azobenzene chemisorbed molecules and therefore modulating the dipole-induced doping of the 2D semiconductor. Significantly, the interplay of electrochemical and optical stimuli makes it possible to construct artificial synapses where LTP can be boosted to 4-bit (16 memory states) while simultaneously functioning as STP.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Two-Dimensional Van Der Waals Thin Film and Device

In the rapidly evolving field of thin-film electronics, the emergence of large-area flexible and wearable devices has been a significant milestone. Although organic semiconductor thin films, which can be manufactured through solution processing, have been identified, their utility is often undermined by their poor stability and low carrier mobility under ambient conditions. However, inorganic nanomaterials can be solution-processed and demonstrate outstanding intrinsic properties and structural stability. In particular, a series of two-dimensional (2D) nanosheet/nanoparticle materials have been shown to form stable colloids in their respective solvents. However, the integration of these 2D nanomaterials into continuous large-area thin with precise control of layer thickness and lattice orientation still remains a significant challenge. This review paper undertakes a detailed analysis of van der Waals thin films, derived from 2D materials, in the advancement of thin-film electronics and optoelectronic devices. The superior intrinsic properties and structural stability of inorganic nanomaterials are highlighted, which can be solution-processed and underscor the importance of solution-based processing, establishing it as a cornerstone strategy for scalable electronic and optoelectronic applications. A comprehensive exploration of the challenges and opportunities associated with the utilization of 2D materials for the next generation of thin-film electronics and optoelectronic devices is presented.

Artificial Neuron Devices

Efforts to design devices emulating complex cognitive abilities and response processes of biological systems have long been a coveted goal. Recent advancements in flexible electronics, mirroring human tissue's mechanical properties, hold significant promise. Artificial neuron devices, hinging on flexible artificial synapses, bioinspired sensors, and actuators, are meticulously engineered to mimic the biological systems. However, this field is in its infancy, requiring substantial groundwork to achieve autonomous systems with intelligent feedback, adaptability, and tangible problem-solving capabilities. This review provides a comprehensive overview of recent advancements in artificial neuron devices. It starts with fundamental principles of artificial synaptic devices and explores artificial sensory systems, integrating artificial synapses and bioinspired sensors to replicate all five human senses. A systematic presentation of artificial nervous systems follows, designed to emulate fundamental human nervous system functions. The review also discusses potential applications and outlines existing challenges, offering insights into future prospects. We aim for this review to illuminate the burgeoning field of artificial neuron devices, inspiring further innovation in this captivating area of research.

Synergetic enhancement effect of two-dimensional MoS2 nanosheets and metal organic framework-derived porous ZnO nanorods for photodegradation performance

Two-dimensional transition metal dichalcogenides and semiconductor metal oxides have shown great potential in photocatalysis. However, their stability and efficiency need to be further improved. In this paper, porous ZnO nanorods with high specific surface area were prepared from metal-organic framework ZIF-8 by a simple hydrothermal method. A MoS2/ZnO composite was constructed by loading MoS2 onto the surface of porous ZnO nanorods. Compared with ZnO materials prepared by other methods, MoS2/ZnO prepared in this paper exhibits superior photocatalytic performance. The enhanced photocatalytic activity of the MoS2/ZnO composite can be attributed to the formation of heterojunctions and strong interaction between them, which greatly facilitate the separation of electrons and holes at the contact interface. In addition, due to the wide absorption region of the visible spectrum, MoS2 can greatly broaden the light absorption range of the material after the formation of the composite material, increase the utilization rate of visible light, and reduce the combination of electrons and holes. This study provides a new way to prepare cheap and efficient photocatalysts.

Graphene-like emerging 2D materials: recent progress, challenges and future outlook

Owing to the unique physical and chemical properties of 2D materials and the great success of graphene in various applications, the scientific community has been influenced to explore a new class of graphene-like 2D materials for next-generation technological applications. Consequently, many alternative layered and non-layered 2D materials, including h-BN, TMDs, and MXenes, have been synthesized recently for applications related to the 4th industrial revolution. In this review, recent progress in state-of-the-art research on 2D materials, including their synthesis routes, characterization and application-oriented properties, has been highlighted. The evolving applications of 2D materials in the areas of electronics, optoelectronics, spintronic devices, sensors, high-performance and transparent electrodes, energy conversion and storage, electromagnetic interference shielding, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and nanocomposites are discussed. In particular, the state-of-the-art applications, challenges, and outlook of every class of 2D material are also presented as concluding remarks to guide this fast-progressing class of 2D materials beyond graphene for scientific research into next-generation materials.

Controllable Doping Characteristics for WSxSey Monolayers Based on the Tunable S/Se Ratio

Transition metal dichalcogenides (TMDs) have attracted much attention because of their unique characteristics and potential applications in electronic devices. Recent reports have successfully demonstrated the growth of 2-dimensional MoS_xSe_y, Mo_xW_yS₂, Mo_xW_ySe₂, and WS_xSe_y monolayers that exhibit tunable band gap energies. However, few works have examined the doping behavior of those 2D monolayers. This study synthesizes WS_xSe_y monolayers using the CVD process, in which different heating temperatures are applied to sulfur powders to control the ratio of S to Se in WS_xSe_y. Increasing the Se component in WS_xSe_y monolayers produced an apparent electronic state transformation from p-type to n-type, recorded through energy band diagrams. Simultaneously, p-type characteristics gradually became clear as the S component was enhanced in WS_xSe_y monolayers. In addition, Raman spectra showed a red shift of the WS₂-related peaks, indicating n-doping behavior in the WS_xSe_y monolayers. In contrast, with the increase of the sulfur component, the blue shift of the WSe₂-related peaks in the Raman spectra involved the p-doping behavior of WS_xSe_y monolayers. In addition, the optical band gap of the as-grown WS_xSe_y monolayers from 1.97 eV to 1.61 eV is precisely tunable via the different chalcogenide heating temperatures. The results regarding the doping characteristics of WS_xSe_y monolayers provide more options in electronic and optical design.

Dielectrics for Two-Dimensional Transition-Metal Dichalcogenide Applications

Despite over a decade of intense research efforts, the full potential of two-dimensional transition-metal dichalcogenides continues to be limited by major challenges. The lack of compatible and scalable dielectric materials and integration techniques restrict device performances and their commercial applications. Conventional dielectric integration techniques for bulk semiconductors are difficult to adapt for atomically thin two-dimensional materials. This review provides a brief introduction into various common and emerging dielectric synthesis and integration techniques and discusses their applicability for 2D transition metal dichalcogenides. Dielectric integration for various applications is reviewed in subsequent sections including nanoelectronics, optoelectronics, flexible electronics, valleytronics, biosensing, quantum information processing, and quantum sensing. For each application, we introduce basic device working principles, discuss the specific dielectric requirements, review current progress, present key challenges, and offer insights into future prospects and opportunities.

Impact of Large Gate Voltages and Ultrathin Polymer Electrolytes on Carrier Density in Electric-Double-Layer-Gated Two-Dimensional Crystal Transistors

Electric-double-layer (EDL) gating can induce large capacitance densities (∼1-10 μF cm^-2) in two-dimensional (2D) semiconductors; however, several properties of the electrolyte limit performance. One property is the electrochemical activity which limits the gate voltage (V_G) that can be applied and therefore the maximum extent to which carriers can be modulated. A second property is electrolyte thickness, which sets the response speed of the EDL gate and therefore the time scale over which the channel can be doped. Typical thicknesses are on the order of micrometers, but thinner electrolytes (nanometers) are needed for very-large-scale-integration (VLSI) in terms of both physical thickness and the speed that accompanies scaling. In this study, finite element modeling of an EDL-gated field-effect transistor (FET) is used to self-consistently couple ion transport in the electrolyte to carrier transport in the semiconductor, in which density of states, and therefore quantum capacitance, is included. The model reveals that 50 to 65% of the applied potential drops across the semiconductor, leaving 35 to 50% to drop across the two EDLs. Accounting for the potential drop in the channel suggests that higher carrier densities can be achieved at larger applied V_G without concern for inducing electrochemical reactions. This insight is tested experimentally via Hall measurements of graphene FETs for which V_G is extended from ±3 to ±6 V. Doubling the gate voltage increases the sheet carrier density by an additional 2.3 × 10¹³ cm^-2 for electrons and 1.4 × 10¹³ cm^-2 for holes without inducing electrochemistry. To address the need for thickness scaling, the thickness of the solid polymer electrolyte, poly(ethylene oxide) (PEO):CsClO₄, is decreased from 1 μm to 10 nm and used to EDL gate graphene FETs. Sheet carrier density measurements on graphene Hall bars prove that the carrier densities remain constant throughout the measured thickness range (10 nm-1 μm). The results indicate promise for overcoming the physical and electrical limitations to VLSI while taking advantage of the ultrahigh carrier densities induced by EDL gating.

Measuring Band Modulation of MoS2 with Ferroelectric Gates

Electronic properties of two-dimensional (2D) materials can be significantly tuned by an external electric field. Ferroelectric gates can provide a strong polarization electric field. Here, we report the measurements of the band structure of few-layer MoS₂ modulated by a ferroelectric P(VDF-TrFE) gate with contact-mode scanning tunneling spectroscopy. When P(VDF-TrFE) is fully polarized, an electric field up to ∼0.62 V/nm through the MoS₂ layers is inferred from the measured band edges, which affects the band structure significantly. First, strong band bending in the vertical direction signifies the Franz-Keldysh effect and a large extension of the optical absorption edge. Photons with energy of half the band gap are still absorbed with 20% of the absorption probability of photons at the band gap. Second, the electric field greatly enlarges the energy separations between the quantum-well subbands. Our study intuitively demonstrates the great potential of ferroelectric gates in band structure manipulation of 2D materials.

Flexo-Ferroelectricity and a Work Cycle of a Two-Dimensional-Monolayer Actuator

Well recognized mechanical flexibility of two-dimensional (2D) materials is shown to bring about unexpected behaviors to the recently discovered monolayer ferroelectrics, especially those displaying normal, off-plane polarization. A "ferro-flexo" coupling term is introduced into the energy expression, to account for the connection of ferroelectricity and bending (strain gradient) of the layer, to predict and quantify its spontaneous curvature and how it affects the phase transitions. With InP as a chemically specific representative example, the first-principles calculations indeed reveal strong coupling ∼P·ϰ between the ferroelectric polarization (P) and the curvature of the layer (ϰ ≡ 1/r), having profound consequences for both mechanics and ferroelectricity of the material. Due to flexural relaxation, the spontaneous polarization and the transition barrier rise significantly, leading to large changes in the Curie temperature, coercive field, and domain wall width and energy, based on Monte Carlo simulations. On the other hand, the polarization switching, characteristic to ferroelectrics, does induce an overall layer bending, enabling a conversion of electrical signal to movement as an actuator; its possible work-cycles and maximum work-efficiency are briefly discussed.

Combination of Polymer Gate Dielectric and Two-Dimensional Semiconductor for Emerging Field-Effect Transistors

Two-dimensional (2D) materials are considered attractive semiconducting layers for emerging field-effect transistors owing to their unique electronic and optoelectronic properties. Polymers have been utilized in combination with 2D semiconductors as gate dielectric layers in field-effect transistors (FETs). Despite their distinctive advantages, the applicability of polymer gate dielectric materials for 2D semiconductor FETs has rarely been discussed in a comprehensive manner. Therefore, this paper reviews recent progress relating to 2D semiconductor FETs based on a wide range of polymeric gate dielectric materials, including (1) solution-based polymer dielectrics, (2) vacuum-deposited polymer dielectrics, (3) ferroelectric polymers, and (4) ion gels. Exploiting appropriate materials and corresponding processes, polymer gate dielectrics have enhanced the performance of 2D semiconductor FETs and enabled the development of versatile device structures in energy-efficient ways. Furthermore, FET-based functional electronic devices, such as flash memory devices, photodetectors, ferroelectric memory devices, and flexible electronics, are highlighted in this review. This paper also outlines challenges and opportunities in order to help develop high-performance FETs based on 2D semiconductors and polymer gate dielectrics and realize their practical applications.

Passivated Interfacial Traps of Monolayer MoS2 with Bipolar Electrical Pulse

Heterogeneous integration of monolayers is an emergent route of spatially combining materials with available platforms for unprecedented properties. A long-standing challenge along this route is to manipulate interfacial configurations of each unit in stacking architecture. A monolayer of transition metal dichalcogenides (TMDs) offers an embodiment of studying interface engineering of integrated systems because optoelectronic performances generally trade off with each other due to interfacial trap states. While ultrahigh photoresponsivity of TMDs phototransistors has been realized, a long response time commonly appears and hinders applications. Here, fundamental processes in excitation and relaxation of the photoresponse are studied and correlated with interfacial traps of the monolayer MoS₂. A mechanism for the onset of saturation photocurrent and the reset behavior in the monolayer photodetector is illustrated based on device performances. Electrostatic passivation of interfacial traps is achieved with the bipolar gate pulse and significantly reduces the response time for photocurrent to reach saturated states. This work paves the way toward fast-speed and ultrahigh-gain devices of stacked two-dimensional monolayers.

Defect-Engineering of 2D Dichalcogenide VSe2 to Enhance Ammonia Sensing: Acumens from DFT Calculations

Opportune sensing of ammonia (NH₃) gas is industrially important for avoiding hazards. With the advent of nanostructured 2D materials, it is felt vital to miniaturize the detector architecture so as to attain more and more efficacy with simultaneous cost reduction. Adaptation of layered transition metal dichalcogenide as the host may be a potential answer to such challenges. The current study presents a theoretical in-depth analysis regarding improvement in efficient detection of NH₃ using layered vanadium di-selenide (VSe₂) with the introduction of point defects. The poor affinity between VSe₂ and NH₃ forbids the use of the former in the nano-sensing device's fabrications. The adsorption and electronic properties of VSe₂ nanomaterials can be tuned with defect induction, which would modulate the sensing properties. The introduction of Se vacancy to pristine VSe₂ was found to cause about an eight-fold increase (from -012 eV to -0.97 eV) in adsorption energy. A charge transfer from the N 2p orbital of NH₃ to the V 3d orbital of VSe₂ has been observed to cause appreciable NH₃ detection by VSe₂. In addition to that, the stability of the best-defected system has been confirmed through molecular dynamics simulation, and the possibility of repeated usability has been analyzed for calculating recovery time. Our theoretical results clearly indicate that Se-vacant layered VSe₂ can be an efficient NH₃ sensor if practically produced in the future. The presented results will thus potentially be useful for experimentalists in designing and developing VSe₂-based NH₃ sensors.

Modifying the Power and Performance of 2-Dimensional MoS2 Field Effect Transistors

Over the past 60 years, the semiconductor industry has been the core driver for the development of information technology, contributing to the birth of integrated circuits, Internet, artificial intelligence, and Internet of Things. Semiconductor technology has been evolving in structure and material with co-optimization of performance-power-area-cost until the state-of-the-art sub-5-nm node. Two-dimensional (2D) semiconductors are recognized by the industry and academia as a hopeful solution to break through the quantum confinement for the future technology nodes. In the recent 10 years, the key issues on 2D semiconductors regarding material, processing, and integration have been overcome in sequence, making 2D semiconductors already on the verge of application. In this paper, the evolution of transistors is reviewed by outlining the potential of 2D semiconductors as a technological option beyond the scaled metal oxide semiconductor field-effect transistors. We mainly focus on the optimization strategies of mobility (μ), equivalent oxide thickness (EOT), and contact resistance (R_C ), which enables high ON current (I_on ) with reduced driving voltage (V_dd ). Finally, we prospect the semiconductor technology roadmap by summarizing the technological development of 2D semiconductors over the past decade.

Electronic Structures of Monolayer Binary and Ternary 2D Materials: MoS2, WS2, Mo1-xCrxS2, and W1-xCrxS2 Using Density Functional Theory Calculations

Two-dimensional (2D) materials with binary compounds, such as transition-metal chalcogenides, have emerged as complementary materials due to their tunable band gap and modulated electrical properties via the layer number. Ternary 2D materials are promising in nanoelectronics and optoelectronics. According to the calculation of density functional theory, in this work, we study the electronic structures of ternary 2D materials: monolayer Mo_1-xCr_xS₂ and W_1-xCr_xS₂. They are mainly based on monolayer molybdenum disulfide and tungsten disulfide and have tunable direct band gaps and work functions via the different mole fractions of chromium (Cr). Meanwhile, the Cr atoms deform the monolayer structures and increase their thicknesses. Induced by different mole fractions of Cr material, energy band diagrams, the projected density of states, and charge transfers are further discussed.

Ionic dielectrics for fully printed carbon nanotube transistors: impact of composition and induced stresses

Printed carbon nanotube thin-film transistors (CNT-TFTs) are candidates for flexible electronics with printability on a wide range of substrates. Among the layers comprising a CNT-TFT, the gate dielectric has proven most difficult to additively print owing to challenges in film uniformity, thickness, and post-processing requirements. Printed ionic dielectrics show promise for addressing these issues and yielding devices that operate at low voltages thanks to their high-capacitance electric double layers. However, the printing of ionic dielectrics in their various compositions is not well understood, nor is the impact of certain stresses on these materials. In this work, we studied three compositionally distinct ionic dielectrics in fully printed CNT-TFTs: the polar-fluorinated polymer elastomer PVDF-HFP; an ion gel consisting of triblock polymer PS-PMMA-PS and ionic liquid EMIM-TFSI; and crystalline nanocellulose (CNC) with a salt concentration of 0.05%. Although ion gel has been thoroughly studied, e-PVDF-HFP and CNC printing are relatively new and this study provides insights into their ink formulation, print processing, and performance as gate dielectrics. Using a consistent aerosol jet printing approach, each ionic dielectric was printed into similar CNT-TFTs, allowing for direct comparison through extensive characterization, including mechanical and electrical stress tests. The ionic dielectrics were found to have distinct operational dependencies based on their compositional and ionic attributes. Overall, the results reveal a number of trade-offs that must be managed when selecting a printable ionic dielectric, with CNC showing the strongest performance for low-voltage operation but the ion gel and elastomer exhibiting better stability under bias and mechanical stresses.

Atomistic reaction mechanism of CVD grown MoS2 through MoO3 and H2S precursors

Chemical vapor deposition (CVD) through sulfidation of MoO₃ is one of the most important synthesis techniques to obtain large-scale and high-quality two-dimensional (2D) MoS₂. Recently, H₂S precursor is being used in the CVD technique to synthesize 2D MoS₂. Although several studies have been carried out to examine the mechanism of MoS₂ growth in the presence of sulfur and MoO₃ precursors, the growth of MoS₂ in the presence of H₂S precursor has largely remained unknown. In this study, we present a Reactive molecular dynamics (RMD) simulation to investigate the reaction mechanism of MoS₂ from MoO₃ and H₂S precursors. The intermediate molecules formation, the reason behind those formations, and the surface compositions of MoO_xS_yH_z during the initial steps of CVD have all been quantified. Surprisingly, a sudden separation of sulfur atoms from the surface was observed in the H₂S precursor system due to the substantial oxygen evolution after 1660 K. The sulfur detachments and oxygen evolution from the surface were found to have a linear relationship. In addition, the intermediate molecules and surface bonds of MoS₂ synthesized by MoO₃ and H₂S precursors were compared to those of a system using S₂ and MoO₃ precursors. The most stable subsidiary formation from the H₂S precursor was found to be H₂O, whereas in case of S₂ precursor it was SO. These results provide a valuable insight in the formation of large-scale and high-quality 2D MoS₂ by the CVD technique.

MoS2 Defect Healing for High-Performance Chemical Sensing of Polycyclic Aromatic Hydrocarbons

The increasing population and industrial development are responsible for environmental pollution. Among toxic chemicals, polycyclic aromatic hydrocarbons (PAHs) are highly carcinogenic contaminants resulting from the incomplete combustion of organic materials. Two-dimensional materials, such as transition metal dichalcogenides (TMDCs), are ideal sensory scaffolds, combining high surface-to-volume ratio with physical and chemical properties that are strongly susceptible to environmental changes. TMDCs can be integrated in field-effect transistors (FETs), which can operate as high-performance chemical detectors of (non)covalent interaction with small molecules. Here, we have developed MoS₂-based FETs as platforms for PAHs sensing, relying on the affinity of the planar polyaromatic molecules for the basal plane of MoS₂ and the structural defects in its lattice. X-ray photoelectron spectroscopy analysis, photoluminescence measurements, and transfer characteristics showed a notable reduction in the defectiveness of MoS₂ and a p-type doping upon exposure to PAHs solutions, with a magnitude determined by the correlation between the ionization energies (E_I) of the PAH and that of MoS₂. Naphthalene, endowed with the higher E_I among the studied PAHs, exhibited the highest output. We observed a log-log correlation between MoS₂ doping and naphthalene concentration in water in a wide range (10^-9-10^-6 M), as well as a reversible response to the analyte. Naphthalene concentrations as low as 0.128 ppb were detected, being below the limits imposed by health regulations for drinking water. Furthermore, our MoS₂ devices can reversibly detect vapors of naphthalene with both an electrical and optical readout, confirming that our architecture could operate as a dual sensing platform.

Site-selective growth of two-dimensional materials: strategies and applications

Over the years, there have been major advances in two-dimensional (2D) materials on account of their excellent and unique properties. Among the various strategies for 2D material fabrication, chemical vapor deposition (CVD) is considered as the most promising method to achieve large-area and high-quality 2D film growth. Furthermore, to realize the potential applications of 2D materials in different fields, the integration of 2D materials into functional devices is essential. However, the materials made by common CVD are randomly distributed on substrates, which is disadvantageous for fabricating arrays of devices. To solve this problem, a site-selective growth method was developed to meet the requirement of batch production for practical applications because it achieves control over the locations of products and benefits the subsequent direct integration. Herein, state-of-the-art methods for site-selective synthesis, including seeded growth and patterned growth, are reviewed. Then, the electronic and optoelectronic applications of the as-grown 2D materials are also reviewed. Finally, the remaining challenges and future prospects regarding site-selective methods and applications are discussed.

Raman Spectroscopy-Assisted Characterization of Nanoform MoS2 Thin Film Transistor

In this paper, we report the simple preparation and investigation of electrical transport properties of nanoform MoS₂ thin film transistor (TFT) devices. MoS₂ nanoparticles were synthesized by using the hydrothermal method. The physiochemical characterizations such as UV-vis, Fourier transform infrared, X-ray diffraction, and Raman spectroscopy studies were performed. Spin-coating was used to make the thin film on which silver electrodes were made. We observed nonlinear current-voltage (I-V) characteristics; however, the symmetricity was found in the I-V curve which confirms the no formation of the Schottky barrier between thin film and electrodes. Transistor transfer characteristics reveal that the TFT device is n-doped as more drain current modulation is observed when the positive gate voltage is applied. The relationship between gate-current and gate voltage studies concludes that there is no leakage gate current in the TFT device which further confirms the good reliability of transfer characteristics of a device. The device mobility was calculated as ~10.2 cm²/Vs, and the same was explained with plausible reason supported with Raman spectra analysis.

Photothermally Crumpled MoS2 Film as an Omnidirectionally Stretchable Platform

Molybdenum disulfide (MoS₂ ) is considered a fascinating material for next-generation semiconducting applications due to its outstanding mechanical stability and direct transition characteristics comparable to silicon. However, its application to stretchable platforms still is a challenging issue in wearable logic devices and sensors with noble form-factors required for future industry. Here, an omnidirectionally stretchable MoS₂ platform with laser-induced strained structures is demonstrated. The laser patterning induces the pyrolysis of MoS₂ precursors as well as the weak adhesion between Si and SiO₂ layers. The photothermal expansion of the Si layer results in the crumpling of SiO₂ and MoS₂ layers and the field-effect transistors with the crumpled MoS₂ are found to be suitable for strain sensor applications. The electrical performance of the crumpled MoS₂ depends on the degree of stretching, showing the stable omnidirectional stretchability up to 8% with approximately four times higher saturation current than its initial state. This platform is expected to be applied to future electronic devices, sensors, and so on.

NiO-Based Electronic Flexible Devices

Personal, portable, and wearable electronics have become items of extensive use in daily life. Their fabrication requires flexible electronic components with high storage capability or with continuous power supplies (such as solar cells). In addition, formerly rigid tools such as electrochromic windows find new utilizations if they are fabricated with flexible characteristics. Flexibility and performances are determined by the material composition and fabrication procedures. In this regard, low-cost, easy-to-handle materials and processes are an asset in the overall production processes and items fruition. In the present mini-review, the most recent approaches are described in the production of flexible electronic devices based on NiO as low-cost material enhancing the overall performances. In particular, flexible NiO-based all-solid-state supercapacitors, electrodes electrochromic devices, temperature devices, and ReRAM are discussed, thus showing the potential of NiO as material for future developments in opto-electronic devices.

Local strain and tunneling current modulate excitonic luminescence in MoS2 monolayers

Local strain in monolayer molybdenum disulfide (MoS2) on an evaporated Au surface is studied by scanning tunneling microscopy (STM) induced excitonic luminescence on a length scale of 10 nm.

Recoverable and reusable visible-light photocatalytic performance of CVD grown atomically thin MoS2 films

The decomposition of water pollutants including industrial dyes and chemicals via photocatalytic decontamination is one of the major investigated problems in recent years. Two-dimensional molybdenum disulfide (MoS₂) layers have shown great promise as an efficient visible-light photocatalyst owing to its numerous active sites and large surface area. In this study, atomically thin MoS₂ films of different thicknesses from monolayer to five-layer and ten layers were fabricated on sapphire substrates using chemical vapor deposition (CVD). We demonstrate that these MoS₂ thin films can be used as a photocatalyst to degrade Methylene Blue (MB) dye and can be recovered completely with utmost structural and chemical stability. Under visible-light irradiation, the MB absorption peak completely disappears with ∼95.6% of degradation after 120 min. We also demonstrate the reusability of the MoS₂ thin films without significantly losing the photocatalytic activity even after 5-cycles of degradation studies. The chemical and structural stability of the MoS₂ films after 5-cycles of degradation studies were affirmed using various spectroscopic studies. Our findings suggest that the MB degradation efficiency increases from 19.01% to 98.46% with an increase in pH from 4 to 14. Our approach may facilitate a further design of other transition metal dichalcogenides-based recoverable photocatalysts for industrial applications.

Spontaneous n-Doping in Growing Monolayer MoS2 by Alkali Metal Compound-Promoted CVD

Monolayer MoS₂ has emerged as one of the most promising candidate materials for future semiconductor devices because of its fascinating physical properties and optoelectronic performance. Recently, the utilization of alkali metal compounds as promoters in CVD growth has been demonstrated to be a facile strategy for growing monolayer MoS₂ and other 2D TMDs with large domain sizes. In this work, we systematically investigated the residues derived from alkali metal compounds and the spontaneous n-doping effect on monolayer MoS₂ in alkali metal compound-promoted CVD growth. When using NaOH and other alkali metal compounds as promoters, it is found that the Raman peak of the A_1g mode red shifted with a broadening width and the PL intensity of the A peak decreased with a red shift, which was attributed to the spontaneous n-doping effect during growth. Moreover, the growth using varying amounts of NaOH promoter suggests that the n-doping level could be controlled by the amount of promoter. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) showed the existence of cation-derived residues in the form of a Na-O cluster physiosorbed on top of monolayer MoS₂, which was also confirmed by the transfer experiment. The NaOH treatment experiment and density functional theory (DFT) calculations demonstrate that sodium hydroxide clusters, which could be converted from a combination of Na-O clusters and water vapor, could produce an n-doping effect on monolayer MoS₂. This study provides a facile route to controllably grow monolayer 2D materials with a desired doping level without further treatment.

Flexible Low-Temperature Ammonia Gas Sensor Based on Reduced Graphene Oxide and Molybdenum Disulfide

Owing to harsh working environments and complex industrial requirements, traditional gas sensors are prone to deformation damage, possess a limited detection range, require a high working temperature, and display low reliability, thereby necessitating the development of flexible and low-temperature gas sensors. In this study, we developed a low-temperature polyimide (PI)-based flexible gas sensor comprising a reduced graphene oxide (rGO)/MoS2 composite. The micro-electro-mechanical system technology was used to fabricate Au electrodes on a flexible PI sheet to form a “sandwiched” sensor structure. The rGO/MoS2 composites were synthesized via a one-step hydrothermal method. The gas-sensing response was the highest for the composite comprising 10% rGO. The structure of this material was characterized, and a PI-based flexible gas sensor comprising rGO/MoS2 was fabricated. The optimal working temperature of the sensor was 141 °C, and its response-recovery time was significantly short upon exposure to 50–1500 ppm NH3. Thus, this sensor exhibited high selectivity and a wide NH3 detection range. Furthermore, it possessed the advantages of low power consumption, a short response-recovery time, a low working temperature, flexibility, and variability. Our findings provide a new framework for the development of pollutant sensors that can be utilized in an industrial environment.

Field-effect-transistor-based ion sensors: ultrasensitive mercury(II) detection via healing MoS2 defects

The contamination of water with heavy metal ions represents a harsh environmental problem resulting from societal development. Among various hazardous compounds, mercury ions (Hg²⁺) surely belong to the most poisonous ones. Their accumulation in the human body results in health deterioration, affecting vital organs and eventually leading to chronic diseases, and, in the worst-case scenario, early death. High selectivity and sensitivity for the analyte of choice can be achieved in chemical sensing using suitable active materials capable of interacting at the supramolecular level with the chosen species. Among them, 2D transition metal dichalcogenides (TMDCs) have attracted great attention as sensory materials because of their unique physical and chemical properties, which are highly susceptible to environmental changes. In this work, we have fabricated MoS₂-based field-effect transistors (FETs) and exploited them as platforms for Hg²⁺ sensing, relying on the affinity of heavy metal ions for both point defects in TMDCs and sulphur atoms in the MoS₂ lattice. X-ray photoelectron spectroscopy characterization showed both a significant reduction of the defectiveness of MoS₂ when exposed to Hg²⁺ with increasing concentration and a shift in the binding energy of 0.2 eV suggesting p-type doping of the 2D semiconductor. The efficient defect healing has been confirmed also by low-temperature photoluminescence measurements by monitoring the attenuation of defect-related bands after Hg²⁺ exposure. Transfer characteristics in MoS₂ FETs provided further evidence that Hg²⁺ acts as a p-dopant of MoS₂. Interestingly, we observed a strict correlation of doping with the concentration of Hg²⁺, following a semi-log trend. Hg²⁺ concentrations as low as 1 pM can be detected, being way below the limits imposed by health regulations. Electrical characterization also revealed that our sensor can be efficiently washed and used multiple times. Moreover, the developed devices displayed a markedly high selectivity for Hg²⁺ against other metal ions as ruled by soft/soft interaction among chemical systems with appropriate redox potentials, being a generally applicable approach to develop chemical sensing devices combining high sensitivity, selectivity and reversibility, to meet technological needs.

Colloidal n-Doped CdSe and CdSe/ZnS Nanoplatelets

Colloidal semiconductor nanoplatelets (NPLs) are chemical versions of well-studied quantum wells (QWs). For QWs, gating and carrier doping are standard tools to manipulate their optical, electric, or magnetic properties. It would be highly desirable to use pure chemical methods to dope extra charge carriers into free-standing colloidal NPLs to achieve a similar level of manipulation. Here we report colloidal n-doped CdSe and CdSe/ZnS NPLs achieved through a photochemical doping method. The extra electrons doped into the conduction band edges are evidenced by exciton absorption bleaches recoverable through dedoping and the appearance of new intersub-band transitions in the near-infrared. A high surface ligand coverage is the key to successful doping; otherwise, the doped electrons can be depleted likely by unpassivated surface cations. Large trion binding energies of 20-30 meV are found for the n-doped CdSe NPLs, which, in contrast, are reduced by 1 order of magnitude in CdSe/ZnS core/shell NPLs due to dielectric screening. Furthermore, we identify a long-lived negative trion with a lifetime of 1.5-1.6 ns that is likely dominated by radiative recombination.

Gradual Edge Contact between Mo and MoS2 Formed by Graphene-Masked Sulfurization for High-Performance Field-Effect Transistors

Two-dimensional materials have attracted great attention for their outstanding electronic properties. In particular, molybdenum disulfide (MoS₂) shows great potential as a next-generation semiconductor due to its tunable direct bandgap with a high on-off ratio and extraordinary stability. However, the performance of MoS₂ synthesized by physical vapor deposition has been limited by contact resistance between an electrode and MoS₂, which determines overall device characteristics. Here, in order to reduce the contact resistance, we use in situ sulfurization of Mo by H₂S gas treatment masked by a patterned graphene gas barrier, so that the Mo channel area can be selectively formed, resulting in a gradual edge contact between Mo and MoS₂. Compared with field-effect transistors with a top contact between the Au/Ti electrode and the MoS₂ channel, a gradual edge contact between the Mo electrode and the MoS₂ channel provides a considerably enhanced electrical performance.

Oxidation and Degradation of WS2 Monolayers Grown by NaCl-Assisted Chemical Vapor Deposition: Mechanism and Prevention

The preservation of two-dimensional WS₂ in the environment is a concern for researchers. In addition to water vapor and oxygen, the latest research points out that degradation is directly related to light absorption. Based on the selection rules of nonlinear optics, two-photon absorption is dipole forbidden in the exciton 1s states, but second-harmonic generation (SHG) is allowed with virtual transitions. According to this mechanism, we proved that SHG is an optical detection method with non-photooxidative damage and energy characteristics. With this detection method, we can explore the oxidation and degradation mechanisms of WS₂ grown by NaCl-assisted chemical vapor deposition in its original state. The WS₂ monolayers that use NaCl to assist in growth have undergone different degradation processes, starting to oxidize from random positions in the triangular flake. We use a photocatalytic reaction to explain the photo-induced degradation mechanism with sulfur vacancies. It was further found that WS₂ grown with NaCl assistance is hydrolyzed in a dark and high-humidity environment, which does not occur in pure WS₂. Finally, we demonstrated that changing the direction of the sapphire substrate relative to the gas flow direction to grow NaCl-assisted WS₂ can greatly improve its stability in the ambient atmosphere, even when exposed to light. The optimal geometric structures and ground state energies are investigated by the density functional theory-based calculations. According to the orientation and symmetry of NaCl-assisted WS₂, we can expect that it will have a better growth quality when the gas flow direction is perpendicular to the [112̄0] direction of the sapphire substrate. This contributes to the nucleation and subsequent growth of NaCl-assisted WS₂. This research provides a more stable optical inspection method than other established methods and greatly improves the operational stability of NaCl-assisted WS₂ under environmental conditions.

Advanced Strategies to Improve Performances of Molybdenum-Based Gas Sensors

Molybdenum-based materials have been intensively investigated for high-performance gas sensor applications. Particularly, molybdenum oxides and dichalcogenides nanostructures have been widely examined due to their tunable structural and physicochemical properties that meet sensor requirements. These materials have good durability, are naturally abundant, low cost, and have facile preparation, allowing scalable fabrication to fulfill the growing demand of susceptible sensor devices. Significant advances have been made in recent decades to design and fabricate various molybdenum oxides- and dichalcogenides-based sensing materials, though it is still challenging to achieve high performances. Therefore, many experimental and theoretical investigations have been devoted to exploring suitable approaches which can significantly enhance their gas sensing properties. This review comprehensively examines recent advanced strategies to improve the nanostructured molybdenum-based material performance for detecting harmful pollutants, dangerous gases, or even exhaled breath monitoring. The summary and future challenges to advance their gas sensing performances will also be presented.

Ultrasensitive Photodetection in MoS2 Avalanche Phototransistors

Recently, there have been numerous studies on utilizing surface treatments or photosensitizing layers to improve photodetectors based on 2D materials. Meanwhile, avalanche breakdown phenomenon has provided an ultimate high-gain route toward photodetection in the form of single-photon detectors. Here, the authors report ultrasensitive avalanche phototransistors based on monolayer MoS2 synthesized by chemical vapor deposition. A lower critical field for the electrical breakdown under illumination shows strong evidence for avalanche breakdown initiated by photogenerated carriers in MoS2 channel. By utilizing the photo-initiated carrier multiplication, their avalanche photodetectors exhibit the maximum responsivity of ≈3.4 × 107 A W-1 and the detectivity of ≈4.3 × 1016 Jones under a low dark current, which are a few orders of magnitudes higher than the highest values reported previously, despite the absence of any additional chemical treatments or photosensitizing layers. The realization of both the ultrahigh photoresponsivity and detectivity is attributed to the interplay between the carrier multiplication by avalanche breakdown and carrier injection across a Schottky barrier between the channel and metal electrodes. This work presents a simple and powerful method to enhance the performance of photodetectors based on carrier multiplication phenomena in 2D materials and provides the underlying physics of atomically thin avalanche photodetectors.

Spatial Control of Dynamic p-i-n Junctions in Transition Metal Dichalcogenide Light-Emitting Devices

Emerging transition metal dichalcogenides (TMDCs) offer an attractive platform for investigating functional light-emitting devices, such as flexible devices, quantum and chiral devices, high-performance optical modulators, and ultralow threshold lasers. In these devices, the key operation is to control the light-emitting position, that is, the spatial position of the recombination zone to generate electroluminescence, which permits precise light guides/passes/confinement to ensure favorable device performance. Although various structures of TMDC light-emitting devices have been demonstrated, including the transistor configuration and heterostructured diodes, it is still difficult to tune the light-emitting position precisely owing to the structural device complexity. In this study, we fabricated two-terminal light-emitting devices with chemically synthesized WSe₂, MoSe₂, and WS₂ monolayers, and performed direct observations of their electroluminescence, from which we discovered a divergence in their light-emitting positions. Subsequently, we propose a method to associate spatial electroluminescence imaging with transport properties among different samples; consequently, a common rule for determining the locations of recombination zones is revealed. Owing to dynamic carrier accumulations and p-i-n junction formations, the light-emitting positions in electrolyte-based devices can be tuned continuously. The proposed method will expand the device applicability for designing functional optoelectronic applications based on TMDCs.

Effectiveness of high curvature segmentation on the curved flexible surface plasmon resonance

In this report, we explore a segmentation-based approach for the calculation of surface plasmon resonance (SPR) on the curved surface with high curvature by modeling it as a contiguous array of finite segments. The approach would significantly facilitate the calculation with good accuracy because of the inherent nature that transfer matrix analysis can be used. Using the segmentation model, resonance characteristics at SPR were obtained as the curvature radius was varied. For validation of the segmentation, resonance wavelength (λ_SPR), reflectance at resonance (R_SPR), and resonance width (δλ_SPR) were compared with the finite element method in the parallel and perpendicular light incidence. It was found that the results from the segmentation were in excellent agreement, λ_SPR in particular, while R_SPR and δλ_SPR under parallel incidence showed disparity between the two models due to the short segmentation. Resonance of curved surface on the rigid and flexible substrate was compared and the overall trend was found to be almost identical. The segmentation is expected to provide a simple, fast, and efficient way for studying plasmonic devices with high curvature in flexible and wearable applications.

Hybrid System Combining Two-Dimensional Materials and Ferroelectrics and Its Application in Photodetection

Photodetectors are one of the most important components for a future "Internet-of-Things" information society. Compared to the mainstream semiconductor-based photodetectors, emerging devices based on two-dimensional (2D) materials and ferroelectrics as well as their hybrid systems have been extensively studied in recent decades due to their outstanding performances and related interesting physical, electrical, and optoelectronic phenomena. In this paper, we review the photodetection based on 2D materials and ferroelectric hybrid systems. The fundamentals of 2D and ferroelectric materials as well as the interaction in the hybrid system will be introduced. Ferroelectricity modulated optoelectronic properties in the hybrid system will be discussed in detail. After the basics and figures of merit of photodetectors are summarized, the 2D-ferroelectrics devices with different structures including p-n diodes, Schottky diodes, and field-effect transistors will be reviewed and compared. The polarization of ferroelectrics offers the possibility of the modulation and enhancement of the photodetection in the hybrid detectors, which will be discussed in depth. Finally, the challenges and perspectives of the photodetectors based on 2D ferroelectrics will be proposed. This Review outlines the important aspects of the recent development of the hybrid system of 2D and ferroelectric materials, which could interact with each other and thus lead to photodetectors with higher performances. Such a Review will be helpful for the research of emerging physical phenomena and for the design of multifunctional nanoscale electronic and optoelectronic devices.

Differences and Similarities of Photocatalysis and Electrocatalysis in Two-Dimensional Nanomaterials: Strategies, Traps, Applications and Challenges

Photocatalysis and electrocatalysis have been essential parts of electrochemical processes for over half a century. Recent progress in the controllable synthesis of 2D nanomaterials has exhibited enhanced catalytic performance compared to bulk materials. This has led to significant interest in the exploitation of 2D nanomaterials for catalysis. There have been a variety of excellent reviews on 2D nanomaterials for catalysis, but related issues of differences and similarities between photocatalysis and electrocatalysis in 2D nanomaterials are still vacant. Here, we provide a comprehensive overview on the differences and similarities of photocatalysis and electrocatalysis in the latest 2D nanomaterials. Strategies and traps for performance enhancement of 2D nanocatalysts are highlighted, which point out the differences and similarities of series issues for photocatalysis and electrocatalysis. In addition, 2D nanocatalysts and their catalytic applications are discussed. Finally, opportunities, challenges and development directions for 2D nanocatalysts are described. The intention of this review is to inspire and direct interest in this research realm for the creation of future 2D nanomaterials for photocatalysis and electrocatalysis.

Emerging 2D Memory Devices for In-Memory Computing

It is predicted that the conventional von Neumann computing architecture cannot meet the demands of future data-intensive computing applications due to the bottleneck between the processing and memory units. To try to solve this problem, in-memory computing technology, where calculations are carried out in situ within each nonvolatile memory unit, has been intensively studied. Among various candidate materials, 2D layered materials have recently demonstrated many new features that have been uniquely exploited to build next-generation electronics. Here, the recent progress of 2D memory devices is reviewed for in-memory computing. For each memory configuration, their operation mechanisms and memory characteristics are described, and their pros and cons are weighed. Subsequently, their versatile applications for in-memory computing technology, including logic operations, electronic synapses, and random number generation are presented. Finally, the current challenges and potential strategies for future 2D in-memory computing systems are also discussed at the material, device, circuit, and architecture levels. It is hoped that this manuscript could give a comprehensive review of 2D memory devices and their applications in in-memory computing, and be helpful for this exciting research area.

Electrochemically active site-rich nanocomposites of two-dimensional materials as anode catalysts for direct oxidation fuel cells: new age beyond graphene

Direct oxidation fuel cell (DOFC) has been opted as a green alternative to fossil fuels and intermittent energy resources as it is economically viable, possesses good conversion efficiency, as well as exhibits high power density and superfast charging. The anode catalyst is a vital component of DOFC, which improves the oxidation of fuels; however, the development of an efficient anode catalyst is still a challenge. In this regard, 2D materials have attracted attention as DOFC anode catalysts due to their fascinating electrochemical properties such as excellent mechanical properties, large surface area, superior electron transfer, presence of active sites, and tunable electronic states. This timely review encapsulates in detail different types of fuel cells, their mechanisms, and contemporary challenges; focuses on the anode catalyst/support based on new generation 2D materials, namely, 2D transition metal carbide/nitride or carbonitride (MXene), graphitic carbon nitride, transition metal dichalcogenides, and transition metal oxides; as well as their properties and role in DOFC along with the mechanisms involved.

High-Performance CVD Bilayer MoS2 Radio Frequency Transistors and Gigahertz Mixers for Flexible Nanoelectronics

Two-dimensional (2D) MoS₂ have attracted tremendous attention due to their potential applications in future flexible high-frequency electronics. Bilayer MoS₂ exhibits the advantages of carrier mobility when compared with monolayer mobility, thus making the former more suitable for use in future flexible high-frequency electronics. However, there are fewer systematical studies of chemical vapor deposition (CVD) bilayer MoS₂ radiofrequency (RF) transistors on flexible polyimide substrates. In this work, CVD bilayer MoS₂ RF transistors on flexible substrates with different gate lengths and gigahertz flexible frequency mixers were constructed and systematically studied. The extrinsic cutoff frequency (f_T) and maximum oscillation frequency (f_max) increased with reducing gate lengths. From transistors with a gate length of 0.3 μm, we demonstrated an extrinsic f_T of 4 GHz and f_max of 10 GHz. Furthermore, statistical analysis of 14 flexible MoS₂ RF transistors is presented in this work. The study of a flexible mixer demonstrates the dependence of conversion gain versus gate voltage, LO power and input signal frequency. These results present the potential of CVD bilayer MoS₂ for future flexible high-frequency electronics.

Direct Observation of Photo-Induced Reversible Sol-Gel Transition in Block Copolymer Self-Assembly Containing an Azobenzene Ionic Liquid

Using atomic force microscopy, the photo-induced reversible changes in a block copolymer self-assembly containing an azobenzene ionic liquid, which undergoes sol-gel transition is directly observed. This is the first report on the sol-gel transition of an ABA-type block copolymer consisting of upper critical solution temperature (UCST)-type A blocks in a photoresponsive ionic liquid mixture. The sol-gel transition is accompanied by an order-to-disorder structural change, which subsequently induces a change in the ionic conductivity. Surprisingly, the photo-induced ionic conductivity and rheological changes occurs rapidly (≈30 s) despite the dense (≈80 wt%) polymeric system. The rapid structural change is probably attributable to the fast diffusion of the ionic liquid.

Printable and recyclable carbon electronics using crystalline nanocellulose dielectrics

Electronic waste can lead to the accumulation of environmentally and biologically toxic materials and is a growing global concern. Developments in transient electronics-in which devices are designed to disintegrate after use-have focused on increasing the biocompatibility, whereas efforts to develop methods to recapture and reuse materials have focused on conducting materials, while neglecting other electronic materials. Here, we report all-carbon thin-film transistors made using crystalline nanocellulose as a dielectric, carbon nanotubes as a semiconductor, graphene as a conductor and paper as a substrate. A crystalline nanocellulose ink is developed that is compatible with nanotube and graphene inks and can be written onto a paper substrate using room-temperature aerosol jet printing. The addition of mobile sodium ions to the dielectric improves the thin-film transistor on-current (87 μA mm^-1) and subthreshold swing (132 mV dec^-1), and leads to a faster voltage sweep rate (by around 20 times) than without ions. The devices also exhibit stable performance over six months in ambient conditions and can be controllably decomposed, with the graphene and carbon nanotube inks recaptured for recycling (>95% recapture efficiency) and reprinting of new transistors. We demonstrate the utility of the thin-film transistors by creating a fully printed, paper-based biosensor for lactate sensing.

Two-dimensional WS2/MoS2 heterostructures: properties and applications

The successful fabrication of WS₂/MoS₂ heterostructures provides more possibilities for optoelectronic and thermoelectric applications than graphene because of their direct bandgap characteristics; therefore, scientific investigations on WS₂/MoS₂ heterostructures are more significant and thriving. In this paper, we review the latest research progress in WS₂/MoS₂ heterostructures, and look forward to their properties and applications. Firstly, we analyze the crystal structure and electronic structure of WS₂, MoS₂, and their heterostructures. Secondly, we comprehensively present the widely used methods for preparing heterostructures. Finally, based on the unique physical characteristics of WS₂/MoS₂ heterostructures, we focus on their properties and applications in mechanics, electronics, optoelectronics, and thermoelectronics.

Laser printed two-dimensional transition metal dichalcogenides

Laser processing is a highly versatile technique for the post-synthesis treatment and modification of transition metal dichalcogenides (TMDCs). However, to date, TMDCs synthesis typically relies on large area CVD growth and lithographic post-processing for nanodevice fabrication, thus relying heavily on complex, capital intensive, vacuum-based processing environments and fabrication tools. This inflexibility necessarily restricts the development of facile, fast, very low-cost synthesis protocols. Here we show that direct, spatially selective synthesis of 2D-TMDCs devices that exhibit excellent electrical, Raman and photoluminescence properties can be realized using laser printing under ambient conditions with minimal lithographic or thermal overheads. Our simple, elegant process can be scaled via conventional laser printing approaches including spatial light modulation and digital light engines to enable mass production protocols such as roll-to-roll processing.