Wearable electronics is expected to be one of the most active research areas in the next decade; therefore, nanomaterials possessing high carrier mobility, optical transparency, mechanical robustness and flexibility, lightweight, and environmental stability will be in immense demand. Graphene is one of the nanomaterials that fulfill all these requirements, along with other inherently unique properties and convenience to fabricate into different morphological nanostructures, from atomically thin single layers to nanoribbons. Graphene-based materials have also been investigated in sensor technologies, from chemical sensing to detection of cancer biomarkers. The progress of graphene-based flexible gas and chemical sensors in terms of material preparation, sensor fabrication, and their performance are reviewed here. The article provides a brief introduction to graphene-based materials and their potential applications in flexible and stretchable wearable electronic devices. The role of graphene in fabricating flexible gas sensors for the detection of various hazardous gases, including nitrogen dioxide (NO), ammonia (NH), hydrogen (H), hydrogen sulfide (HS), carbon dioxide (CO), sulfur dioxide (SO), and humidity in wearable technology, is discussed. In addition, applications of graphene-based materials are also summarized in detecting toxic heavy metal ions (Cd, Hg, Pb, Cr, Fe, Ni, Co, Cu, Ag), and volatile organic compounds (VOCs) including nitrobenzene, toluene, acetone, formaldehyde, amines, phenols, bisphenol A (BPA), explosives, chemical warfare agents, and environmental pollutants. The sensitivity, selectivity and strategies for excluding interferents are also discussed for graphene-based gas and chemical sensors. The challenges for developing future generation of flexible and stretchable sensors for wearable technology that would be usable for the Internet of Things (IoT) are also highlighted.

Biosensors with high sensitivity, selectivity and a low limit of detection, reaching nano/picomolar concentrations of biomolecules, are important to the medical sciences and healthcare industry for evaluating physiological and metabolic parameters.

Flexible, stretchable, and bendable materials, including inorganic semiconductors, organic polymers, graphene, and transition metal dichalcogenides (TMDs), are attracting great attention in such areas as wearable electronics, biomedical technologies, foldable displays, and wearable point-of-care biosensors for healthcare. Among a broad range of layered TMDs, atomically thin layered molybdenum disulfide (MoS 2 ) has been of particular interest, due to its exceptional electronic properties, including tunable bandgap and charge carrier mobility. MoS 2 atomic layers can be used as a channel or a gate dielectric for fabricating atomically thin field-effect transistors (FETs) for electronic and optoelectronic devices. This review briefly introduces the processing and spectroscopic characterization of large-area MoS 2 atomically thin layers. The review summarizes the different strategies in enhancing the charge carrier mobility and switching speed of MoS 2 FETs by integrating high-κ dielectrics, encapsulating layers, and other 2D van der Waals layered materials into flexible MoS 2 device structures. The photoluminescence (PL) of MoS 2 atomic layers has, after chemical treatment, been dramatically improved to near-unity quantum yield. Ultraflexible and wearable active-matrix organic light-emitting diode (AM-OLED) displays and wafer-scale flexible resistive random-access memory (RRAM) arrays have been assembled using flexible MoS 2 transistors. The review discusses the overall recent progress made in developing MoS 2 based flexible FETs, OLED displays, nonvolatile memory (NVM) devices, piezoelectric nanogenerators (PNGs), and sensors for wearable electronic and optoelectronic devices. Finally, it outlines the perspectives and tremendous opportunities offered by a large family of atomically thin-layered TMDs. KEYWORDS: molybdenum disulfide (MoS 2 ), flexible electronics, flexible field-effect transistors (FETs), wearable organic light-emitting diode (OLED), flexible memory devices, flexible piezoelectric nanogenerators (PNGs), sensors

Transition metal dichalcogenides (TMDs) are becoming significant because of their interesting semiconducting and photonic properties. In particular, TMDs such as molybdenum disulfide (MoS), molybdenum diselenide (MoSe), tungsten disulfide (WS), tungsten diselenide (WSe), titanium disulfide (TiS), tantalum sulfide (TaS), and niobium selenide (NbSe) are increasingly attracting attention for their applications in solar cell devices. In this review, we give a brief introduction to TMDs with a focus on MoS; and thereafter, emphasize the role of atomically thin MoS layers in fabricating solar cell devices, including bulk-heterojunction, organic, and perovskites-based solar cells. Layered MoS has been used as the hole-transport layer (HTL), electron-transport layer (ETL), interfacial layer, and protective layer in fabricating heterojunction solar cells. The trilayer graphene/MoS/n-Si solar cell devices exhibit a power-conversion efficiency of 11.1%. The effects of plasma and chemical doping on the photovoltaic performance of MoS solar cells have been analyzed. After doping and electrical gating, a power-conversion efficiency (PCE) of 9.03% has been observed for the MoS/h-BN/GaAs heterostructure solar cells. The MoS-containing perovskites-based solar cells show a PCE as high as 13.3%. The PCE of MoS-based organic solar cells exceeds 8.40%. The stability of MoS solar cells measured under ambient conditions and light illumination has been discussed. The MoS-based materials show a great potential for solar cell devices along with high PCE; however, in this connection, their long-term environmental stability is also of equal importance for commercial applications.

The Internet of Things (IoT) has limitless possibilities for applications in the entire spectrum of our daily lives, from healthcare to automobiles to public safety.

Dye-sensitized solar cells (DSSCs) are gaining considerable interest as alternatives to semiconductor-based thin film solar cells. The noble metal platinum (Pt) is conventionally used as a counter electrode (CE) material for fabricating DSSCs, since Pt is expensive and scarce, therefore, new materials have been explored to develop cost-effective Pt-free counter electrodes. Two-dimensional (2D) graphene-based counter electrodes have achieved the highest power conversion efficiency (PCE, h) of 13%, which has stimulated research activities in 2D layered transition metal dichalcogenides (TMDs) for developing Pt-free DSSCs.In this review, progress made on alternative counter electrodes for fabricating low-cost Pt-free DSSCs, based on earth-abundant 2D TMDs including MoS

Among the layered transition metal dichalcogenides (TMDs) that can form stable two-dimensional crystal structures, molybdenum disulfide (MoS) has been intensively investigated because of its unique properties in various electronic and optoelectronic applications with different band gap energies from 1.29 to 1.9 eV as the number of layers decreases. To control the MoS layers, atomic layer etching (ALE) (which is a cyclic etching consisting of a radical-adsorption step such as Cl adsorption and a reacted-compound-desorption step via a low-energy Ar-ion exposure) can be a highly effective technique to avoid inducing damage and contamination that occur during the reactive steps. Whereas graphene is composed of one-atom-thick layers, MoS is composed of three-atom-thick S-Mo-S layers; therefore, the ALE mechanisms of the two structures are significantly different. In this study, for MoS ALE, the Cl radical is used as the adsorption species and a low-energy Ar ion is used as the desorption species. A MoS ALE mechanism (by which the S, Mo, and S atoms are sequentially removed from the MoS crystal structure due to the trapped Cl atoms between the S layer and the Mo layer) is reported according to the results of an experiment and a simulation. In addition, the ALE technique shows that a monolayer MoS field effect transistor (FET) fabricated after one cycle of ALE is undamaged and exhibits electrical characteristics similar to those of a pristine monolayer MoS FET. This technique is also applicable to all layered TMD materials, such as tungsten disulfide (WS), molybdenum diselenide (MoSe), and tungsten diselenide (WSe).

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