Stable n-doping of WSe2 using thin films of SiNx deposited on the surface via plasma-enhanced chemical vapor deposition is presented. Positive fixed charge centers inside SiNx act to dope WSe2 thin ...flakes n-type via field-induced effect. The electron concentration in WSe2 can be well controlled up to the degenerate limit by simply adjusting the stoichiometry of the SiNx through deposition process parameters. For the high doping limit, the Schottky barrier width at the metal/WSe2 junction is significantly thinned, allowing for efficient electron injection via tunneling. Using this doping scheme, we demonstrate air-stable WSe2 n-MOSFETs with a mobility of ∼70 cm2/V s.
Valence tautomerism (VT) refers to equilibria between two or more different tautomers differing in their charge distributions. This contribution shows how molecular VT occurring in metal–dioxolene ...complexes (metal = cobalt, manganese) functionalized with long alkyl chains can be coupled with several types of macroscopic phase transformations, such as solid‐to‐solid and solid‐to‐liquid transitions and the double‐melting phenomenon. Observations of simultaneous transformation of molecular and macroscopic states have led to the concept of synchronic bistability. The central focus in this microreview is on molecular design, thermodynamic basis, and kinetic control of the synchronic bistability. It is demonstrated that a key factor in designing a synchronic transformation is to build up strong interdependent correlations between molecules and their assembled states.
This microreview summarizes molecular strategy directed towards synchronous transformations of molecular states and macroscopic phases. The coexistence of valence‐tautomeric Co– or Mn–dioxolene cores with structurally flexible long alkyl tails leads to rich molecular bistability that synchronously occurs with a macroscopic phase transformation, such as the solid‐to‐liquid phase transition.
Transition metal dichalcogenides (TMDCs) have received attention as atomically thin post‐silicon semiconducting materials. Tuning the carrier concentrations of the TMDCs is important, but their thin ...structure requires a non‐destructive modulation method. Recently, a surface‐charge transfer doping method was developed based on contacting molecules on TMDCs, and the method succeeded in achieving a large modulation of the electronic structures. The successful dopant is a neutral benzyl viologen (BV0); however, the problem remains of how to effectively prepare the BV0 molecules. A reduction process with NaBH4 in water has been proposed as a preparation method, but the NaBH4 simultaneously reacts vigorously with the water. Here, a simple method is developed, in which the reaction vial is placed on a hotplate and a fragment of air‐stable metal is used instead of NaBH4 to prepare the BV0 dopant molecules. The prepared BV0 molecules show a strong doping ability in terms of achieving a degenerate situation of a TMDC, MoS2. A key finding in this preparation method is that a convection flow in the vial effectively transports the produced BV0 to a collection solvent. This method is simple and safe and facilitates the tuning of the optoelectronic properties of nanomaterials by the easily‐handled dopant molecules.
A strong electron dopant molecule, benzyl viologen, is generated with a simple method with air‐stable metals as a reducing agent. The reacted solution shows a strong doping ability for MoS2 to reach a degenerately doped level. The convection flow is found to effectively transport and purify the product dopant from the reactant molecules.
Air-stable doping of transition metal dichalcogenides is of fundamental importance to enable a wide range of optoelectronic and electronic devices while exploring their basic material properties. ...Here we demonstrate the use of benzyl viologen (BV), which has one of the highest reduction potentials of all electron-donor organic compounds, as a surface charge transfer donor for MoS2 flakes. The n-doped samples exhibit excellent stability in both ambient air and vacuum. Notably, we obtained a high electron sheet density of ∼1.2 × 1013 cm–2, which corresponds to the degenerate doping limit for MoS2. The BV dopant molecules can be reversibly removed by immersion in toluene, providing the ability to control the carrier sheet density as well as selective removal of surface dopants on demand. By BV doping of MoS2 at the metal junctions, the contact resistances are shown to be reduced by a factor of >3. As a proof of concept, top-gated field-effect transistors were fabricated with BV-doped n+ source/drain contacts self-aligned with respect to the top gate. The device architecture, resembling that of the conventional Si transistors, exhibited excellent switching characteristics with a subthreshold swing of ∼77 mV/decade.
Printing technologies offer large‐area, high‐throughput production capabilities for electronics and sensors on mechanically flexible substrates that can conformally cover different surfaces. These ...capabilities enable a wide range of new applications such as low‐cost disposable electronics for health monitoring and wearables, extremely large format electronic displays, interactive wallpapers, and sensing arrays. Solution‐processed carbon nanotubes have been shown to be a promising candidate for such printing processes, offering stable devices with high performance. Here, recent progress made in printed carbon nanotube electronics is discussed in terms of materials, processing, devices, and applications. Research challenges and opportunities moving forward from processing and system‐level integration points of view are also discussed for enabling practical applications.
Printed electronics and sensors enable new applications ranging from low‐cost disposable analytical devices to large‐area sensor networks. Solution‐processed carbon nanotubes are an ideal material system for such applications as they offer stable devices with high performance.
The interface between conventional semiconductors and aqueous ionic solutions is an important target in chemistry and materials science. Recently, a wide variety of research has been done on ...transition-metal dichalcogenides (TMDCs) for use as 2D layered semiconductors, and their optoelectronic properties have been widely explored. One representative TMDC, monolayer (1L) MoS2, is known to show a photoluminescence (PL) signal of a direct band gap nature, and the PL intensity is dependent on the carrier concentration. Various methods of 1L MoS2 carrier modulation have been shown to enhance the PL intensity in dry environments. In contrast, enhancement in an aqueous environment is limited, and a strategy to design an interface with aqueous media has not yet been established. One proposed idea was an aqueous acid interface; however, the enhancement of the PL with this method was usually minimal, about 1 order of magnitude. In this study, we demonstrate a method to achieve strong PL enhancement in 1L MoS2 in an aqueous media by incorporating bis(trifluoromethane)sulfonyl anion (TFSI– ion) in an acidic environment. With the addition of the TFSI– ion in an acidic environment, the enhancement factor of the PL in 1L MoS2 is more than 100 times greater than its PL intensity in water. The molecular anion is the key factor, as the TFSI– ion facilitates the oxidation of MoS2. This anionic effect is the additional factor needed to modulate the optoelectronic properties of 2D semiconductors in aqueous media. The proposed idea could have potential applications for biochemical sensors in aqueous situations.
Mechanically deformable devices and sensors enable conformal coverage of electronic systems on curved and soft surfaces. Sensors utilizing liquids confined in soft templates as the sensing component ...present the ideal platform for such applications, as liquids are inherently more deformable than solids. However, to date, liquid-based devices have been limited to metal lines based on a single-liquid component given the difficulty in the fabrication of liquid-based junctions due to intermixing. Here, we demonstrate a robust platform for the fabrication of liquid-liquid 'heterojunction' devices, presenting an important advancement towards the realization of liquid-state electronic systems. The device architecture and fabrication scheme we present are generic for different sensing liquids, enabling demonstration of sensors responsive to different stimuli. As a proof of concept, we demonstrate temperature, humidity and oxygen sensors by using different ionic liquids, exhibiting high sensitivity with excellent mechanical deformability arising from the inherent property of the liquid phase.
Wearable sensor technologies are essential to the realization of personalized medicine through continuously monitoring an individual's state of health. Sampling human sweat, which is rich in ...physiological information, could enable non-invasive monitoring. Previously reported sweat-based and other non-invasive biosensors either can only monitor a single analyte at a time or lack on-site signal processing circuitry and sensor calibration mechanisms for accurate analysis of the physiological state. Given the complexity of sweat secretion, simultaneous and multiplexed screening of target biomarkers is critical and requires full system integration to ensure the accuracy of measurements. Here we present a mechanically flexible and fully integrated (that is, no external analysis is needed) sensor array for multiplexed in situ perspiration analysis, which simultaneously and selectively measures sweat metabolites (such as glucose and lactate) and electrolytes (such as sodium and potassium ions), as well as the skin temperature (to calibrate the response of the sensors). Our work bridges the technological gap between signal transduction, conditioning (amplification and filtering), processing and wireless transmission in wearable biosensors by merging plastic-based sensors that interface with the skin with silicon integrated circuits consolidated on a flexible circuit board for complex signal processing. This application could not have been realized using either of these technologies alone owing to their respective inherent limitations. The wearable system is used to measure the detailed sweat profile of human subjects engaged in prolonged indoor and outdoor physical activities, and to make a real-time assessment of the physiological state of the subjects. This platform enables a wide range of personalized diagnostic and physiological monitoring applications.
Artificial reconstruction of fibre-shaped cellular constructs could greatly contribute to tissue assembly in vitro. Here we show that, by using a microfluidic device with double-coaxial laminar flow, ...metre-long core-shell hydrogel microfibres encapsulating ECM proteins and differentiated cells or somatic stem cells can be fabricated, and that the microfibres reconstitute intrinsic morphologies and functions of living tissues. We also show that these functional fibres can be assembled, by weaving and reeling, into macroscopic cellular structures with various spatial patterns. Moreover, fibres encapsulating primary pancreatic islet cells and transplanted through a microcatheter into the subrenal capsular space of diabetic mice normalized blood glucose concentrations for about two weeks. These microfibres may find use as templates for the reconstruction of fibre-shaped functional tissues that mimic muscle fibres, blood vessels or nerve networks in vivo.
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