Vertically Aligned 2D MoS2 Layers with Strain-Engineered Serpentine Patterns for High-Performance Stretchable Gas Sensors: Experimental and Theoretical Demonstration

First-principles study of the oxidation susceptibility of WS2, WSe2, and WTe2 monolayers

The environmental stability of two-dimensional (2D) transition metal dichalcogenide monolayers is of great importance for their applications in electronic, photonic, and energy storage devices. In this study, we focus on understanding the susceptibility of WS2, WSe2, and WTe2 monolayers to oxygen exposure in the form of atomic oxygen and O2 and O3 molecules, respectively. Calculations based on the van der Waals-corrected density functional theory predicted that O2 and O3 molecules are weakly adsorbed on these monolayers, although atomic oxygen prefers chemisorption accompanied by a significant charge transfer from the surface to oxygen. In the physisorbed molecular configurations consisting of O2 and O3, the partially oxidized monolayers retain their geometrical and electronic structures. The calculated transition path as the oxygen approaches the surface shows a high-energy barrier for all cases, thus explaining the photo-induced formation of the oxidized configurations in the experiments. Furthermore, oxidizing the WS2 monolayer is predicted to modify its electronic structure, reducing the band gap with increasing oxygen coverage on the surface. Overall, the calculated results predict the resilience of WS2, WSe2, and WTe2 monolayers against oxygen exposure, thus ensuring stability for devices fabricated with these monolayers.

2D nanomaterials for realization of flexible and wearable gas sensors: A review

Gas sensors are extensively employed for monitoring and detection of hazardous gases and vapors. Many of them are produced on rigid substrates, but flexible and wearable gas sensors are needed for intriguing usage including the internet of things (IoT) and medical devices. The materials with the greatest potential for the fabrication of flexible and wearable gas sensing devices are two-dimensional (2D) semiconducting nanomaterials, which consist of graphene and its substitutes, transition metal dichalcogenides, and MXenes. These types of materials have good mechanical flexibility, high charge carrier mobility, a large area of surface, an abundance of defects and dangling bonds, and, in certain instances adequate transparency and ease of synthesis. In this review, we have addressed the different 2D nonmaterial properties for gas sensing in the context of fabrication of flexible/wearable gas sensors. We have discussed the sensing performance of flexible/wearable gas sensors in various forms such as pristine, composite and noble metal decorated. We believe that content of this review paper is greatly useful for the researchers working in the research area of fabrication of flexible/wearable gas sensors.

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.

Highly Strain-Stable Intrinsically Stretchable Olfactory Sensors for Imperceptible Health Monitoring

Intrinsically stretchable gas sensors possess outstanding advantages in seamless conformability and high-comfort wearability for real-time detection toward skin/respiration gases, making them promising candidates for health monitoring and non-invasive disease diagnosis and therapy. However, the strain-induced deformation of the sensitive semiconductor layers possibly causes the sensing signal drift, resulting in failure in achievement of the reliable gas detection. Herein, a surprising result that the stretchable organic polymers present a universal strain-insensitive gas sensing property is shown. All the stretchable polymers with different degrees of crystallinity, including indacenodithiophene-benzothiadiazole (PIDTBT), diketo-pyrrolo-pyrrole bithiophene thienothiophene (DPPT-TT) and poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiad-iazolo [3,4-c] pyridine] (PCDTPT), show almost unchanged gas response signals in the different stretching states. This outstanding advantage enables the intrinsically stretchable devices to imperceptibly adhere on human skin and well conform to the versatile deformations such as bending, twisting, and stretching, with the highly strain-stable gas sensing property. The intrinsically stretchable PIDTBT sensor also demonstrates the excellent selectivity toward the skin-emitted trimethylamine (TMA) gas, with a theoretical limit of detection as low as 0.3 ppb. The work provides new insights into the preparation of the reliable skin-like gas sensors and highlights the potential applications in the real-time detection of skin gas and respiration gas for non-invasive medical treatment and disease diagnosis.

Lithium-Induced Reorientation of Few-Layer MoS2 Films

Molybdenum disulfide (MoS2) few-layer films have gained considerable attention for their possible applications in electronics and optics and also as a promising material for energy conversion and storage. Intercalating alkali metals, such as lithium, offers the opportunity to engineer the electronic properties of MoS2. However, the influence of lithium on the growth of MoS2 layers has not been fully explored. Here, we have studied how lithium affects the structural and optical properties of the MoS2 few-layer films prepared using a new method based on one-zone sulfurization with Li2S as a source of lithium. This method enables incorporation of Li into octahedral and tetrahedral sites of the already prepared MoS2 films or during MoS2 formation. Our results discover an important effect of lithium promoting the epitaxial growth and horizontal alignment of the films. Moreover, we have observed a vertical-to-horizontal reorientation in vertically aligned MoS2 films upon lithiation. The measurements show long-term stability and preserved chemical composition of the horizontally aligned Li-doped MoS2.

Wearable chemical sensors based on 2D materials for healthcare applications

Chemical sensors worn on the body could make possible the continuous, noninvasive, and accurate monitoring of vital human signals, which is necessary for remote health monitoring and telemedicine. Attractive for creating high-performance, wearable chemical sensors are atomically thin materials with intriguing physical features, abundant chemistry, and high surface-to-volume ratios. These advantages allow for appropriate material-analyte interactions, resulting in a high level of sensitivity even at trace analyte concentrations. Previous review articles covered the material and device elements of 2D material-based wearable devices extensively. In contrast, little research has addressed the existing state, future outlook, and promise of 2D materials for wearable chemical sensors. We provide an overview of recent advances in 2D-material-based wearable chemical sensors to overcome this deficiency. The structure design, manufacturing techniques, and mechanisms of 2D material-based wearable chemical sensors will be evaluated, as well as their applicability in human health monitoring. Importantly, we present a thorough review of the current state of the art and the technological gaps that would enable the future design and nanomanufacturing of 2D materials and wearable chemical sensors. Finally, we explore the challenges and opportunities associated with designing and implementing 2D wearable chemical sensors.

Gas-Sensitive Characteristics of Graphene Composite Tungsten Disulfide to Ammonia

Two-dimensional materials have outstanding application prospects in gas sensing. By constructing composite structures of various gas-sensitive materials, more-efficient and sensitive gas sensors can be further developed. After graphene is compounded with WS₂, the composite material can improve the gas detection performance. In this work, the adsorption energy and the electronic properties of a graphene/WS₂ structure were calculated by first-principles before and after adsorption of NH₃. The calculation results indicate that the bandgap of the material was appreciably reduced after NH₃ was adsorbed. In addition, a graphene/WS₂ gas sensor was prepared. The response of the sensor to NH₃ at a concentration of 100 ppm was 2.42% and 1.73% at 30 °C and 60 °C, respectively. Combining simulation with experiment, it is feasible to use graphene composite WS₂ to detect NH₃, which provides a new idea for applications of graphene and other composite materials in gas sensing.

Large-area vertically aligned 2D MoS2layers on TEMPO-cellulose nanofibers for biodegradable transient gas sensors

Crystallographically anisotropic two-dimensional (2D) molybdenum disulfide (MoS₂) with vertically aligned (VA) layers is attractive for electrochemical sensing owing to its surface-enriched dangling bonds coupled with extremely large mechanical deformability. In this study, we explored VA-2D MoS₂layers integrated on cellulose nanofibers (CNFs) for detecting various volatile organic compound gases. Sensor devices employing VA-2D MoS₂/CNFs exhibited excellent sensitivities for the tested gases of ethanol, methanol, ammonia, and acetone; e.g. a high response rate up to 83.39% for 100 ppm ethanol, significantly outperforming previously reported sensors employing horizontally aligned 2D MoS₂layers. Furthermore, VA-2D MoS₂/CNFs were identified to be completely dissolvable in buffer solutions such as phosphate-buffered saline solution and baking soda buffer solution without releasing toxic chemicals. This unusual combination of high sensitivity and excellent biodegradability inherent to VA-2D MoS₂/CNFs offers unprecedented opportunities for exploring mechanically reconfigurable sensor technologies with bio-compatible transient characteristics.

Hydrogel- and organohydrogel-based stretchable, ultrasensitive, transparent, room-temperature and real-time NO2 sensors and the mechanism

Highly stretchable, sensitive and room-temperature nitrogen dioxide (NO₂) sensors are fabricated by exploiting intrinsically stretchable, transparent and ion-conducting hydrogels and active metals as the novel transducing materials and electrodes, respectively. The NO₂ sensor exhibits high sensitivity (60.02% ppm^-1), ultralow theoretical limit of detection (6.8 ppb), excellent selectivity, linearity and reversibility at room temperature. Notably, the sensitivity can be maintained even under 50% tensile strain. For the first time, it's found that the metal electrodes significantly impact the sensing performance. Specifically, the sensitivity is boosted from 31.18 to 60.02% ppm^-1 by replacing the anodic silver with copper-tin alloy. Importantly, by applying specially designed sensing tests, and microscopic and composition analyses, we have obtained the inherent NO₂ sensing mechanism: the anodic metal tends to be oxidized and the NO₂ molecules tend to react in the cathode-gel interface. The introduction of glycerol converts the hydrogel into the organohydrogel with remarkably enhanced anti-drying and anti-freezing capacities and toughness, which effectively improved the long-time stability of the sensors. Importantly, we execute sound/light alarms and a wireless smartphone alarm by utilizing a designed circuit board and applet. This work gives an incisive investigation for the preparation, performance improvement, mechanism and application of hydrogel-based NO₂ sensors, promoting the evolution of hydrogel ionotronics.

Peel-and-Stick Integration of Atomically Thin Nonlayered PtS Semiconductors for Multidimensionally Stretchable Electronic Devices

Various near-atom-thickness two-dimensional (2D) van der Waals (vdW) crystals with unparalleled electromechanical properties have been explored for transformative devices. Currently, the availability of 2D vdW crystals is rather limited in nature as they are only obtained from certain mother crystals with intrinsically possessed layered crystallinity and anisotropic molecular bonding. Recent efforts to transform conventionally non-vdW three-dimensional (3D) crystals into ultrathin 2D-like structures have seen rapid developments to explore device building blocks of unique form factors. Herein, we explore a "peel-and-stick" approach, where a nonlayered 3D platinum sulfide (PtS) crystal, traditionally known as a cooperate mineral material, is transformed into a freestanding 2D-like membrane for electromechanical applications. The ultrathin (∼10 nm) 3D PtS films grown on large-area (>cm²) silicon dioxide/silicon (SiO₂/Si) wafers are precisely "peeled" inside water retaining desired geometries via a capillary-force-driven surface wettability control. Subsequently, they are "sticked" on strain-engineered patterned substrates presenting prominent semiconducting properties, i.e., p-type transport with an optical band gap of ∼1.24 eV. A variety of mechanically deformable strain-invariant electronic devices have been demonstrated by this peel-and-stick method, including biaxially stretchable photodetectors and respiratory sensing face masks. This study offers new opportunities of 2D-like nonlayered semiconducting crystals for emerging mechanically reconfigurable and stretchable device technologies.

Temperature-Based Selective Detection of Hydrogen Sulfide and Ethanol with MoS2/WO3 Composite

A sensitive and temperature-based selective sensor toward hydrogen sulfide and ethanol using MoS₂/WO₃ composite as a sensing surface was developed in this work. The MoS₂/WO₃ nanocomposite was successfully obtained using a facile two-step method. Structural analysis revealed the successful formation of the composite. Further, the n-type semiconducting nature as revealed in the initial gas-sensing measurements was also confirmed via Mott-Schottky plots. The composite-based sensor showed preferential detection of ethanol (260 °C) and hydrogen sulfide (320 °C) by simply modulating the temperature of the sensor device. The device also displayed repeatability and long-term stability at respective operating temperatures. Improved sensitivity and selectivity are ascribed to synergistic effects arising from the formation of n-n type heterostructures. The present work indicates the potential use of composite-based heterojunctions to tune the sensing parameters and provide new possibilities to enhance the applications of MoS₂ and metal-oxide semiconductor-based composites.

Adsorption of gas molecules on buckled GaAs monolayer: a first-principles study

The design of sensitive and selective gas sensors can be significantly simplified if materials that are intrinsically selective to target gas molecules can be identified. In recent years, monolayers consisting of group III–V elements have been identified as promising gas sensing materials. In this article, we investigate gas adsorption properties of buckled GaAs monolayer using first-principles calculations within the framework of density functional theory. We examine the adsorption energy, adsorption distance, charge transfer, and electron density difference to study the strength and nature of adsorption. We calculate the change in band structure, work function, conductivity, density of states, and optical reflectivity for analyzing its prospect as work function-based, chemiresistive, optical, and magnetic gas sensor applications. In this regard, we considered the adsorption of ten gas molecules, namely NH₃, NO₂, NO, CH₄, H₂, CO, SO₂, HCN, H₂S, and CO₂, and noticed that GaAs monolayer is responsive to NO, NO₂, NH₃, and SO₂ only. Specifically, NH₃, SO₂ and NO₂ chemisorb on the GaAs monolayer and change the work function by more than 5%. While both NO and NO₂ are found to be responsive in the far-infrared (FIR) range, NO shows better spin-splitting property and a significant change in conductivity. Moreover, the recovery time at room temperature for NO is observed to be in the sub-millisecond range suggesting selective and sensitive NO response in GaAs monolayer.

NH₃, NO₂, and SO₂ chemisorb on the GaAs monolayer. NO adsorption induces a magnetic moment (1.02 μ_B per cell), and significantly changes the conductivity and reflectivity.

Ion-Conductive Hydrogel-Based Stretchable, Self-Healing, and Transparent NO2 Sensor with High Sensitivity and Selectivity at Room Temperature

Here stretchable, self-healable, and transparent gas sensors based on salt-infiltrated hydrogels for high-performance NO₂ sensing in both anaerobic environment and air at room temperature, are reported. The salt-infiltrated hydrogel displays high sensitivity to NO₂ (119.9%/ppm), short response and recovery time (29.8 and 41.0 s, respectively), good linearity, low theoretical limit of detection (LOD) of 86 ppt, high selectivity, stability, and conductivity. A new gas sensing mechanism based on redox reactions occurring at the electrode-hydrogel interface is proposed to understand the sensing behaviors. The gas sensing performance of hydrogel is greatly improved by incorporating calcium chloride (CaCl₂ ) in the hydrogel via a facile salt-infiltration strategy, leading to a higher sensitivity (2.32 times) and much lower LOD (0.06 times). Notably, both the gas sensing ability, conductivity, and mechanical deformability of hydrogels are readily self-healable after cutting off and reconnection. Such large deformations as 100% strain do not deprive the gas sensing capability, but rather shorten the response and recovery time significantly. The CaCl₂ -infiltrated hydrogel shows excellent selectivity of NO₂ , with good immunity to the interference gases. These results indicate that the salt-infiltrated hydrogel has great potential for wearable electronics equipped with gas sensing capability in both anaerobic and aerobic environments.

Force fields for water-surface interaction: is reproduction of the experimental water contact angle enough?

A new protocol based on quantum chemical calculations and molecular dynamics simulations is proposed to revisit water-MoS₂ interfacial force fields (FFs). The accurate reproduction of experimental water contact angles is suggested to be insufficient to ensure reliable FFs for recovering structural properties of the interfacial solvent. As an example, this protocol is used to develop a new set of FF parameters to both capture interfacial structural phenomena at the interface between water and MoS₂ and recover experimental water contact angle data. This approach can be applied to any interface where contact angle data are available.