Transition metal dichalcogenides-based flexible gas sensors

Journal Pre-proof Transition Metal Dichalcogenides-Based Flexible Gas Sensors Rahul Kumar, Neeraj Goel, Mirabbos Hojamberdiev, Mahesh Kumar

PII:

S0924-4247(19)31903-X

DOI:

https://doi.org/10.1016/j.sna.2020.111875

Reference:

SNA 111875

To appear in:

Sensors and Actuators: A. Physical

Received Date:

15 October 2019

Revised Date:

23 January 2020

Accepted Date:

27 January 2020

Please cite this article as: Kumar R, Goel N, Hojamberdiev M, Kumar M, Transition Metal Dichalcogenides-Based Flexible Gas Sensors, Sensors and Actuators: A. Physical (2020), doi: https://doi.org/10.1016/j.sna.2020.111875

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Transition Metal Dichalcogenides-Based Flexible Gas Sensors Rahul Kumar,1 Neeraj Goel,1 Mirabbos Hojamberdiev,2 and Mahesh Kumar1,* 1

Department of Electrical Engineering, Indian Institute of Technology Jodhpur, Jodhpur-342037,

India 2

Fachgebiet Keramische Werkstoffe, Institut für Werkstoffwissenschaften und -technologien,

Technische Universität Berlin, Hardenbergstraße 40, 10623 Berlin, Germany

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*Corresponding author: [email protected]

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Graphical abstract

Highlights 

2D layered materials such as MoS2, MoSe2, WS2, WSe2, VS2, SnS2, etc. utilized for detection of many toxic and combustible inorganic and organic gases on flexible sensing platform.

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Sensitivity, response/recovery time, stability, and selectivity of a sensor are some of the key factors, which are addressed with suitable sensing mechanism for different TMDCs materials.

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Integration of TMDCs materials into e-skin-oriented and e-textiles gas sensor are also highlighted.

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Challenges and future perspectives for TMDCs materials in emerging flexible and wearable gas sensing field are discussed.

Abstract In recent years, two-dimensional (2D) layered materials, such as MoS2, MoSe2, WS2, WSe2, SnS2, etc., have gained enormous interest in sensing applications with low-power consumption owing to

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their unique electrical, chemical, and mechanical properties. Among a broad range of sensors, rapid advances in flexible and wearable sensors have paved the way for smart sensing applications, including electronic skin (e-skin), home security, air- and health-monitoring, etc. This review aims at

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summarizing recent progress in energy-efficient flexible gas sensors by utilizing 2D transition metal dichalcogenides (TMDCs) materials. Main concepts and different approaches are overviewed for

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optimizing the gas sensing characteristics on flexible sensing platforms. The different strategies and challenges for incorporating 2D TMDCs materials into e-skin-oriented and e-textiles gas sensors are

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also highlighted. In addition, this review also includes the challenges and future perspectives for

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TMDCs materials in emerging flexible and wearable gas sensing field.

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Keywords: 2D layered materials; Gas sensors; Flexibility; Wearability; Electronic textiles;

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1. Introduction

Over the last decade, Internet-of-Things (IoT) has registered its exponentially increasing presence with energy-efficient chip-scale devices due to the true realization of powerful computing and analytic capabilities for a variety of applications[1][2][3]. Nowadays, smartwatches, smartphones, smart tablets, and smart glasses on flexible substrates with wireless connectivity are fascinating [4][5][6]. Moreover, chemical sensing applications of IoT have also received significant attention for the real-time detection of environmental pollutants and which is crucial not only in the medical 2

diagnosis of several diseases but also to take preventive measures [7][8][9][10][11]. Despite the enormous potential of flexible and stretchable sensors in wearable technology [5][12], this field still remains less explored, particularly in chemical sensing applications. Conventional semiconductors can also be used in developing flexible electronic sensors. For instance, Si can be used for making flexible devices by thinning it down below a particular value [13]. However, Si poses a severe threat to the reliability of the devices due to its brittle nature [14]. Other approaches including an use of organic semiconductors for making flexible devices were also investigated [15]. However, these technologies also suffer setbacks due to a low value of mobility of

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charge carriers and high operation voltages [16]. The lowered value of mobility reduces the driving capacity of the device, and the large voltage requirement makes the circuitry complex. Interestingly, polycrystalline Si offers a high value of mobility but deteriorates the performance of the device due

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to the considerable value of leakage current and high cost of flexible electronic devices [17]. Thus,

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traditional approaches face several impediments restricting their use by the flexible industry. Since the isolation of graphene in 2004 by Novoselov and Geim, 2D materials have become a

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promising candidate for advanced electronic device applications owing to its excellent electrical, mechanical and chemical properties. And the last decade has witnessed the exploration of several

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properties of atomically thin layered 2D materials particularly for flexible and stretchable devices [18] [19][20]. Their low-power consumption, high mobility of charge carriers, lightweight, high

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stretchability, reliable mechanical robustness, excellent optical transparency, and good environmental stability make them excellent candidates for flexible devices. Recently, several 2D materials,

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including graphene, TMDCs, and MXenes, have shown their potential for fabricating stretchable and flexible devices that can be used in foldable displays, memory devices, chemical sensors, photodetectors, supercapacitors, and lithium-ion batteries [21] [22][23] [24][25][26][27][28]. The large surface-to-volume ratio establishes 2D materials as the torch-bearer in chemical sensing applications.

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Fig. 1. Transition metal dichalcogenides materials are layered materials and an example of 2D materials.

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Schematic representation of the crystalline structure of the layered transition metal dichalcogenides material.

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Graphene registered his strong presence in the field of gas sensing by detecting a single molecule adsorbed on its surface. These exceptionally promising sensing results had never been achieved by using conventional solid-state gas sensors technology. This milestone was achieved due to an

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exceptionally low-noise in graphene [29]. Moreover, Singh et al. [23] have reported chemical and gas sensing behaviour of the graphene on the flexible and wearable platform in review articles. The same

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group has also demonstrated electrochemical and fluorescent biosensors of the graphene and its

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nanocomposites [30]. In the past few years, many 2D materials have shown promising results in chemical and biosensors applications [31] [32] [33] [34][35][36] [37][38][39][40][41]. The key figures of merit, such as ultimate sensitivity and detection at room temperature in chemical sensors could easily be achieved by using atomically thin layered 2D materials. In particular, the detection of target gas molecules at room temperature removes the micro-heater from the conventional gas sensor and making them as energy-efficient materials after reducing the consumption power from several hundreds of mW to µW level. However, the absence of a bandgap in graphene restricts its usage in 4

digital circuits on flexible and wearable integrated circuits platforms for IoT applications. Semiconducting properties are still an open challenge for pristine graphene, and the semiconducting behaviour of a sensing material is a crucial factor for tuning the sensing characteristic of the gas sensor. So, another class of 2D materials, such as transition metal dichalcogenides (TMDCs) (shown in fig. 1) can easily be applied in switching applications due to their intrinsic bandgap[42][43][44]. The bandgap of these materials can easily be tuned between 1.0-2.5 eV by changing the number of layers due to quantum confinement effect [45][46] as well as by numerous other techniques [47]. Several TMDCs, such as MoS2, MoSe2, WS2, WSe2, and VS2 etc., have shown their suitability for

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smart sensors and radiofrequency tags which are widely used for space, defense, security and IoT applications [48][49] [50]. Moreover, the large-area scalable growth of 2D TMDCs materials on plastic and polymeric substrates facilitates the development of cost-effective and mechanically stable

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chemical sensors.

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In this review, we briefly discuss the suitability of different 2D TMDCs materials for energy-efficient flexible electronic devices, targeting the chemical sensors to detect various organic and inorganic air

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pollutants, as illustrated in scheme 1. We have also highlighted the importance of 2D TMDCs material-based battery-power-operated flexible chemical sensors usable for wearable and e-textiles

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sensor applications. Sensor stability, sensitivity, response/recovery time, and selectivity are some of the key factors which are addressed based on strengths and limitations of different materials. We also

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focus on sensing mechanisms and new concepts for 2D TMDCs materials based gas sensors on the flexible and wearable sensing platforms. In addition, we discuss the key challenges and possible

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solutions for the commercial implementation of flexible chemical sensors.

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Scheme 1: A flow chart illustrating the development of a flexible chemiresistive sensor by using 2D TMDCs materials for application in inorganic and organic gas detection.

2. Gas sensing mechanism of the 2D TMDCs materials based gas sensor The gas-sensing performance of a developed gas sensor strictly correlates to its sensing mechanism. Generally, the sensing mechanism of conventional metal oxide gas sensors depends on surface

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reaction performing in between target gas molecules and pre-adsorbed oxygen ions on the surface of metal-oxide [51]. Various kinds of oxygen ions (O2-, O- and O2-) are generated after extracting electrons from the conduction band of metal oxide by adsorbed atmospheric oxygen at different working temperatures [52] [53]. In contrast, the sensing mechanism of TMDCs based gas sensors mainly depends on the charge transfer process in between target gas molecules and TMDCs materials [54] [36] [55] [56]. The direction of charge transfer depends on oxidizing or reducing nature of target gas analytes. In general, the oxidizing gas (NO2) takes the electrons due to having an unpaired electron at its N atom, whereas reducing gas (NH3) gives the electrons to materials because of one loan pair

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[57], as shown in fig. 2. Thereby, the electronic interaction of gas molecules and reactive sites of the material surface perturbs the charge carrier concentration, resulting in a change in conductivity (resistivity) of the TMDCs materials. Such a charge transfer process has already been verified by

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theoretical calculation (DFT) [54] [58] and in-situ photoluminescence PL study [59].

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Besides the direct charge transfer in between gas molecules and reactive sites, modulation of a Schottky barrier as well as built-in potential at the material/metal (electrode) interfaces upon exposure

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to gas also plays a crucial role for enhancing sensing performance [60]. In this case, a Schottky or an ohmic contact is formed at the interface of metal (electrode) and material, and which directly depends

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on the metal work function and electron affinity value of the material. For example, n-type MoS2 (electron affinity ~ 4.2 eV) [61] [62] and Au (work function ~ 5.1 eV) [63] form Schottky junction at

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their interface. Upon exposure to NO2, Fermi level of n-type MoS2 moves towards the valence band because of electron extraction by NO2, thereby, the Schottky barrier increases and built-in potential

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decreases, as shown in fig. 2 and results in decrease conductivity of the device [64]. That is viceversa for the NH3 adsorption.

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Fig. 2. Schematic illustration of the gas sensing mechanism of TMDCs materials based gas sensor. (here, MoS2

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material, a famous member of the TMDCs family is only used for representation perspective) (i) Reproduced with permission from ref. 59. Published by Nature Publishing Group. (ii) Reproduced with permission from ref. 64. Copyright © 2014, American Chemical Society. (iii) Reproduced with permission from ref. 66.

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Copyright © 2018, American Chemical Society.

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On the other hand, incorporation of another material into TMDCs sensing material in terms of a hybrid or nanocomposite or van der Waals heterostructure makes a more complicated sensing mechanism. Apart from the above-mentioned sensing mechanism, integration of another material improves the sensing performance of the sensor through tuning of barrier height at the heterointerface between the constituent materials by adsorption of gas molecules [65]. For instance, Han et al. [66] have proposed the sensing mechanism based on modulation of a developed heterojunction barrier

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potential at the ZnO/MoS2 interface upon exposure to gas. The barrier potential was developed through formed depletion region at the interface, which was formed due to electron-hole diffusion for equilibrating the different Fermi levels of the ZnO and MoS2 during contact formation, as shown in fig. 2. The sensing mechanism of various mixed dimensional heterostructures of the TMDCs materials based on electronic and chemical sensitization can be read in more detail in Ref [65]. Proposed above all gas sensing mechanism for the TMDCs materials based gas sensors have been proved for improving the sensing performance via individual or synergistic effects. On the other hand, the discussion on the gas sensing mechanism of the different TMDCs materials is given in detail in

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the following their material sections.

3. Flexible and wearable gas sensors using TMDCs materials

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3.1. MoS2-based flexible gas sensors

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Layered MoS2 material is a prominent member of the TMDCs family owing to its remarkable electrical, mechanical, and chemical properties. A single layer of MoS2 includes three planes, in

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which one plane of Mo atoms is sandwiched in between two planes of sulfur atoms and in multilayer MoS2, different layers are connected to each other through weak van der Waals forces, while in

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intralayer, Mo atoms are connected with sulfur atoms via strong covalent bonding (fig. 3a)[67] [68][69][70][71]. The crystal structure of the MoS2 is commonly classified into four types as 1H, 1T,

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2H, and 3R on the basis of coordination of Mo atom with six sulfur atoms and orientation of the MoS2 layers in stacking, as shown in fig. 3c [72]. In the name of four polytypes of the MoS2, digits 1, 2,

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and 3 represent a number of the MoS2 layer in per unit cell and characters H, T, and R represent symmetry of the polytypes as hexagonal, tetragonal and rhombohedral, respectively. For example, 1T-MoS2 polytype has one layer per unit cell, tetragonal polytype layered crystal and octahedral coordination of central Mo atom with six sulfur atoms (fig. 3b)[73]. Moreover, the 1T phase is a metastable and metallic phase of the MoS2, which has been potentially used in hydrogen evolution and other applications [74][75][76][77][78]. However, 1H and 2H have one and two-layer per unit

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cell, respectively, hexagonal symmetry and trigonal prismatic arrangement of the six sulfur atoms with central Mo atom (fig. 3b)[79]. Moreover, the 2H phase is a stable and semiconducting and so, 2H-MoS2 has been widely recognized as a potential material for a variety of gas sensing applications [80][81][82][83][84]. The available large surface area promotes high chemical adsorption leading to a significant change in the obtained electrical signals, thus detecting the chemical molecules present even in very low concentration [35] [64]. Besides the excellent gas adsorption, high flexibility and transparency aspects of the MoS2 make it a leading candidate for emerging flexible gas sensing

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technology.

Fig. 3. (a) Top and side view of MoS2. (b) The lattice structure of 2H- and 1T-MoS2. Reproduced with permission from ref. 85. Copyright © 2011, American Chemical Society. (c) Schematic representation of the

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different polytypes of MoS2. Reproduced with permission from ref. 67. Copyright © 2016 Elsevier Ltd.

Several methodologies have been adopted for the synthesis of mono and few-layer MoS2 films, including micromechanical exfoliation [69], wet chemical process [85][86], chemical/physical vapor deposition [87] [88], and pulse laser deposition [89]. The morphology of deposited MoS2 could easily be tailored by changing the synthesis technique. For instance, nanobelts, nanoribbons, nanorods, and nanoflowers were fabricated using various fabrication methods [90][91][92][93]. Burman et al. [94] 10

fabricated large MoS2 film by using a sonication-assisted liquid exfoliation approach and later, a flexible ammonia gas sensor was fabricated after transferring large MoS 2 film on flexible PET substrate. Here, intrinsic n-type behaviour of the MoS2 was changed to a p-type MoS2 because chemisorption of atmospheric oxygen on defects in exfoliated MoS 2 formed the MoOx and thereby, MoOx injected the hole in n-type MoS2 and resulted in p-type MoS2 as a sensing layer of the sensor [93] [95]. They studied gas sensing characteristics of the p-type MoS2 sensor for NH3 gas at different temperatures in a range of 70-90 °C. The flexible sensor exhibited high sensitivity ~ 33% to 400ppm NH3 with a response/recovery time of 158/385 s at 80 °C. Moreover, it was observed that the sensor

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showed slightly increased sensitivity to NH3 in an increased humidity environment, however, recovery of the sensor was deteriorated in the presence of humidity. In addition, the gas response was deteriorated after 25 bending cycles because bending increases the interplaner separation as well as

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the distance between adjacent grain boundaries, which increase the resistance of sensing layer through

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creating an obstacle in the movement of charge carrier. On the other hand, among all fabrication methods, the chemical vapor deposition (CVD) process is the most common process for synthesizing

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highly crystalline pure MoS2 nanosheet [96]. However, a high-temperature requirement for the growth of 2D MoS2 material in the CVD process is the major obstacle in the synthesis of this material

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directly on flexible substrates. So, an additional step is required for transferring CVD grown MoS2 nanosheet from a rigid substrate to a flexible substrate. However, this extra transfer process

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deteriorates the quality of the MoS2 nanosheet through creating micro-cracks, wrinkles, contamination, and defects [97][98]. Moreover, this transfer process is also limited to small scale

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production. In this context, Zhao et al. [99] proposed a CVD method at a low temperature of 200 °C by employing Mo(CO)6 and H2S as precursors. They directly synthesized the MoS2 on a flexible PI substrate, as shown in Fig. 4a, and the as-fabricated flexible sensor exhibited high sensitivity to NO2 and NH3 in a range of 25-500 ppm concentration at room temperature. Moreover, the flexible gas sensor showed a stable gas response up to 6000 bending cycles and even an increased sensitivity up to 4000 bending cycles. The enhancement in sensitivity of the sensor after bending was attributed to

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increased gas adsorption on more active edge sites, which were generated on micro-cracks during

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bending.

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Fig. 4. (a) Schematic illustration of the direct growth of MoS2 on flexible substrate using CVD. (b) Schematic representation, and (c) photograph in bending and without bending condition, of the flexible MoS 2 gas sensor. (d, e) Gas sensing response to NO2 and NH3 gases for different concentrations in bending and without bending

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conditions. Reproduced with permission from ref. 99. Copyright 2009, Published by The Royal Society of Chemistry.

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Further, in order to improve the flexibility, stability as well sensitivity of the MoS2 based gas sensors

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for wearable applications in IoT, metal electrodes of chemiresistive/FET sensor devices could be replaced by metallic 2D materials. In this context, a flexible sensing device was fabricated on a flexible polyimide (PI) substrate after depositing interdigitated graphene electrodes on mechanical exfoliated MoS2 channel (Fig. 5c)[100]. The flexible graphene/MoS2 sensor exhibited similar sensing characteristics to 100 ppm NH3 and 5 ppm NO2 even after 5000 bending cycles at its operating temperature of 150 °C. Moreover, the flexible sensor showed remarkable long term stability after retaining the same response even after 19 months. The sensing mechanism of the graphene/MoS2

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sensor was explained via a resistance model through a change in total resistance value (MoS2 channel resistance and graphene electrode resistance) of the device. Under gas exposure, besides the change in the resistance value of the MoS2 channel via charge transfer mechanism, change in the resistance value of the graphene electrodes also contributed to enhancing the sensitivity of the sensor. In addition, moisture detection by using the 2D materials has also become an efficient tool for realizing noncontact and long-range signal induction to develop advanced flexible and wearable electronic devices. CVD grown monolayer MoS2 has also shown the response to water molecules by changing its carrier concentration through charge transfer mechanism and it was verified via Raman

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and photoluminescence (PL) experimental techniques by Zho et al. [101]. Under varying humidity from 0 to 40 %, water molecules adsorption shifted the position of Raman peaks A1g (out-of-plane vibration) of the MoS2 towards higher energy due to change in electron-phonon interactions [102].

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Moreover, under the same humidity concentration, PL intensity of monolayer MoS2 was enhanced

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with activation of neutral exciton (X0) recombination due to induced new recombination centers after transferring electrons from MoS2 to water molecules[103]. From this context, they also fabricated a

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highly sensitive humidity sensor array by using monolayer MoS2 deposited on soft flexible PDMS substrate for noncontact sensation, as shown in fig. 5a. The sensor array showed excellent noncontact

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humidity sensing property after recognizing the moist object through moisture mapping image. Moreover, the flexible MoS2 sensor array exhibited similar sensing characteristics to water molecules

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in its flat and bent (strain ≈1%) states. In addition, the sensitivity of the MoS2 sensor has been enhanced via changing the physical orientation/structure of constituent MoS2 layers. For example,

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vertically aligned MoS2 exhibited superior sensing performance than horizontally aligned MoS2 based sensors because exposed edge sites of vertically aligned MoS 2 showed stronger binding interaction between gas molecules and active sites due to high d-orbital electron density on edge sites[92][104]. However, the growth of exposing edge sites is usually not easier compared to that of terrace sites on the basal plane of MoS2 because of its higher surface energy[105][106]. Islam et al.[107] reported a new approach for fabricating three dimensional (3D) ordered MoS2 with vertically

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aligned layers and transferring on a flexible substrate and later, it was exploited for humidity sensing application. In this work, they synthesized the vertically aligned MoS 2 on photolithography patterned 3D-ordered SiO2/Si pillars as templates and then, it was transferred on flexible PDMS substrate via water-assisted transfer process, as shown in fig. 5d. The flexible vertically aligned MoS2 with patterned (3D-ordered pillar) exhibited about 7 times higher sensitivity to humidity compared to nonpatterned vertically aligned MoS2 because exposed dangling bonds, as well as drastically increased active sites via the increased surface-to-volume ratio, enhanced the capturing of water molecules. Moreover, the flexible 3D-ordered MoS2 with vertically aligned layers showed a similar sensing

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response after bending with a bending radius of 6 mm, indicating high mechanical flexibility of the sensor. And, it was also observed that a flexible sensor did not show any serious degradation in

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and strong chemical bonding with PDMS substrate.

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sensing response even after one month and it suggests long-terms stability, high structural robustness

Fig. 5. (a) Schematic and a photograph of MoS2 FETs array of the sensor, and (b) humidity sensing response of the MoS2 FET array in flat and bend condition. Reproduced with permission from ref. 101. Copyright © 2017 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. (c) SEM image and photograph of flexible MoS2 sensor with graphene electrode. Reproduced with permission from ref. 100. Copyright © 2015, American Chemical Society. (d) Schematic illustration of the synthesis of 2D MoS2 vertical layers via a water-assisted

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layer transfer method. (e, f) Top and side view SEM images of MoS 2 layer-coated SiO2/Si pillars. (g) Comparison in humidity sensing responses of different flexible MoS 2 sensors for different morphologies. Reproduced with permission from ref. 107. Copyright 2009, Published by The Royal Society of Chemistry.

On the other hand, decoration or functionalization of the sensing layer by using the nanoparticles of noble metal and semiconducting material has been utilized for enhancing the sensitivity and improving the selectivity as well as stability in atmosphere ambient of the sensor [108][109][110]. In this context, He et al. [111] fabricated a flexible thin-film transistor (TFT) array from pristine MoS2

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and Pt nanoparticles functionalized MoS2 on polyethylene terephthalate substrate for NO2 detection. In this work, monolayer MoS2 film was deposited on patterned rGO electrodes via a simple spincoating method. The flexible TFT sensor array showed the same high sensing performance results to

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NO2 gas before and after 5000 bending cycles. Further, it was noted that they had shown a three times improvement in sensitivity by functionalizing the active MoS2 channel with Pt nanoparticles and

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calculated low detection limit as 2 ppb to NO2 gas for MoS2-Pt nanoparticles. The increment in sensitivity was due to a change in the Schottky barrier between MoS2 and Pt nanoparticles upon the

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adsorption of gas molecules. Therefore, the sensitivity of gas sensors against particular gas molecules can easily be increased by functionalizing the deposited 2D film through metal nanoparticles due to

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their synergetic effects. These nanoparticles form the junction and change the carrier concentration at the heterointerface resulting in ultrahigh sensitivity. Besides the high sensitivity, selectivity could

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also be improved using the functionalization of nanoparticles on the sensing layer. Cho et al. [112] fabricated gas sensors by using functionalized MoS2 with different types of Al and Pd nanoparticles

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and it was observed that Al decorated MoS2 exhibited higher sensitivity to NO2 and however, Pd decorated MoS2 showed more sensitivity to NH3 gas. The high sensitivity and selectivity of the metal nanoparticles decorated MoS2 based gas sensor was due to chemical and electronic sensitization mechanisms [113]. Nanoparticles through chemical sensitization increased the active surface area on the MoS2 sensing layer as adsorption sites for interacting more gas molecules. And electronic sensitization effect indicates that different work functions of metal nanoparticles and the MoS2

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perturbed the carrier concentration of the MoS2 sensing layer owing to their electron transfer, and thereby, it turned to selective gas sensing characteristics. Here, high work function Pd and low work function Al metal increased and depleted the hole carriers in the p-type MoS2 device and thereby, sensitivity was increased towards NH3 and NO2, respectively. In addition, the flexible Pd-MoS2 sensor also exhibited an almost similar sensing response to NH3 gas before and after 5000 bending cycles at 150 °C. Recently, Chen et al.[114] examined the selective gas sensing characteristics of the MoS 2 and Au functionalized MoS2 for volatile organic compounds (VOCs) via experimental and theoretical studies.

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They fabricated flexible sensors from the MoS2 and MoS2-Au deposited on the PET substrate, as shown in fig. 6a. The flexible MoS2-Au sensor showed higher sensitivity to VOCs at room temperature compared to that of the pristine MoS2 sensor (Fig. 6b) and it was also observed that

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MoS2-Au sensor showed selective sensing behaviour towards oxygen-based VOCs against to

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hydrocarbon-based VOCs. In addition, the flexible MoS2-Au sensor did not show any serious degradation in sensing response after one month as well as 1000 bending cycles, which suggest high

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stability and flexibility of the sensor, respectively. The selective and higher sensing response of the flexible sensor towards oxygen-based VOCs at room temperature was attributed to the electrons-

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donating effect of Au and synergistic effects between adsorbed oxygen species on MoS2, Au nanoparticles, and oxygen-based VOCs. In the MoS2-Au sensor, donation of electrons from Au

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nanoparticles to MoS2 increased electrons carrier concentration in the MoS2 sensing layer as well as a large number of adsorbed oxygen species and thereby, a large number of chemical interactions in

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between oxygen-based VOCs and adsorbed oxygen species was enhanced, as shown in fig. 6d. Moreover, density functional theory (DFT) simulations also verified that oxygen-based VOCs had stronger interaction on the MoS2-Au sensing layer than that on MoS2 because of higher adsorption energy as well as shortest equilibrium distance in between oxygen-based VOCs and the MoS2-Au.

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Fig. 6. (a) Schematic illustration of the synthesis of MoS2-Au based gas sensor. (b, c) Gas sensing response of the flexible MoS2 and MoS2-Au sensors for different volatile organic compounds. (d) Schematic representation

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of the gas sensing mechanism of the MoS 2 and MoS2-Au sensors. Reproduced with permission from ref. 114. Copyright © 2019, American Chemical Society.

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To improve the flexibility and stability of individual MoS2 based chemical sensors, Kim et al. [115] mixed single-walled carbon nanotubes with 2D MoS2 nanosheets and transferred them on PET

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substrate (Fig. 7a). SEM image and RBM map of the MoS2-SWCNT hybrid are shown in fig. 7b, c, respectively. The nanocomposite sensor showed a stable resistance value even after 105 bending

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cycles, but, the resistance value of the pristine MoS2 sensor was increased to 300 % (Fig. 7d). In addition, the flexible MoS2-SWCNT sensor exhibited a reduction in sensitivity to NO2 and NH3 gases, however, it was a much lower reduction in sensitivity compared to that of the pristine MoS2 sensor, as shown in fig. 7d. These phenomenal results were obtained due to the excellent mechanical flexibility of carbon nanotubes that reduces the cracks and damages occurring in the MoS2 layer while performing the repetitive bending tests. Further, Jung et al. [116] fabricated a highly transparent and

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flexible NO2 gas sensor using patterned MoS2/rGO composite thin film. The first patterned to MoS2/rGO composite film on a rigid substrate (SiO2/Si) by soft lithographic patterning process and later, the patterned MoS2/rGO composite was transferred on a flexible PET substrate. The patterned MoS2/rGO composite based sensor exhibited a change in resistance value after bending test with a bending radius of 7 mm and showed a high transmittance of 93% and which indicates that the patterned MoS2/rGO sensor is a highly flexible and transparent device, respectively. Moreover, the flexible and transparent sensor showed high sensitivity to NO2 gas at 90 °C via the charge transfer mechanism. Li et al. [117] reported a flexible room temperature formaldehyde (HCHO) sensor using

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rGO/MoS2 hybrid deposited on a PEN substrate. In this work, the rGO/MoS2 hybrid was fabricated from two different hydrothermal and chemically exfoliated MoS 2 and hydrothermal MoS2 based rGO/MoS2 hybrid sensor exhibited sensitivity about 1.7 times higher than that of chemically

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exfoliated MoS2 based hybrid because of rich defect sites generation via hydrothermal method.

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Moreover, the flexible sensor showed high flexibility and stability after detecting HCHO of ppm level at room temperature with complete recovery even after 5000 bending cycles and 90 days,

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respectively. In this hybrid sensor, MoS2 served as adsorption centers as well as electron acceptors for HCHO and thereby, two-stage electron transfer through MoS2 decreased the barriers for electrons

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transferring from HCHO to conducting channel rGO and results in improved sensitivity of the sensor. Further, Park et al. [118] fabricated a flexible selective humidity sensor using rGO/MoS 2 composites

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deposited on PET substrate. The rGO/MoS2 composite sensor exhibited selective detection of humidity against NO2, NH3, H2 and C2H5OH at room temperature and the detection limit was

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calculated as 0.01783% RH. Moreover, the flexible sensor also showed a response of 25.01% to humidity in its bent state. High sensing response to humidity at room temperature of the rGO/MoS2 sensor was attributed to synergistic effects of maximized active sites (dangling bond at edges of the MoS2 and oxygen-containing functional groups on rGO surface) and electronic sensitization (modulation of the potential barrier at p-type rGO/n-type MoS2 interface).

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Fig. 7. (a) Schematic illustration of the fabrication of flexible hybrid gas sensor using the MoS2 and CNTs. (b)

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SEM image and (c) RBM map of the MoS2-CNT hybrid. (d) Gas sensing response of flexible MoS2 and hybrid MoS2-CNT sensor to NO2 and NH3 after 104 bending cycles. Reproduced with permission from ref. 115. Copyright 2017, Elsevier. (e) Sensing response of flexible MoS2 sensor to humidity with different strains and

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relative humidity. Reproduced with permission from ref. 124. Copyright 2018, American Chemical Society.

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From the above sensing results, it is noticed that sensing performance of the flexible MoS2 sensor could be improved via surface modification, functionalization, composite and heterostructure of the

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MoS2. However, all these processes or approaches are often increased complexity, cost, time and controllability for developing a highly flexible gas sensor on the commercial platform. In particular, MoS2 FET structure-based humidity sensor requires high gate bias voltage for ultra-high sensitivity, resulting in high power consumption, manufacturing complexity and expense of lifetime [101] [119][111]. To address all these problems, mechanical stimulation has been observed as a simple, economic and stable approach for developing flexible 2D material based sensors because 2D materials have a highly crystalline structure and excellent flexibility[120][121]. In contrast to 19

centrosymmetric structure of the bulk 2D TMDCs materials, non-centrosymmetric structure of monolayer of the 2D TMDCs materials exhibits piezotronics effects [122][123]. In this view, Guo et al. [124] fabricated a flexible humidity sensor by using CVD grown monolayer MoS2. By applying a mechanical strain at the MoS2/Pd junction, strain induces piezo charges at the interface, which modifies the carrier movement through modulating the Schottky barrier height at the interface and it results in an improvement in sensitivity. The piezotronic effect was more dominating to enhance the sensitivity at low humidity than high humidity concentration (Fig. 7e). Further, a high performance flexible MoS2 sensor was reported by the same group [125] for NO2 detection with complete recovery

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at room temperature by exploiting the photogating and piezo-phototronic effects. The flexible sensor showed excellent sensitivity to 671 % to NO2 with a fast response/recovery time of 16/65 s at room temperature by applying 0.67% tensile strain under the illumination of a red (625 nm) light source.

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The excellent sensing performance of the flexible sensor was due to synergistic effects of

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piezoelectricity and photoelectricity along with the charge transfer mechanism at Schottky contact of the MoS2/Pd junction.

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Besides the good flexibility of the gas sensor, stretchability, transparency, and conformability overstretched substrate are also basic requirements of a sensing material for developing a truly

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wearable and human body-attachable chemical sensor. In this regard, Yan et al. [126] demonstrated a transparent and wearable chemical sensor fabricated on a flexible and transparent PDMS substrate

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by using a stretchable polyaniline/MoS2 nanocomposite (Fig. 8a). The SEM image of the polyaniline/MoS2 nanocomposite is shown in fig. 8b. The stretchable sensor exhibited ultrahigh

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sensitivity to NH3 gas as detecting as low as 50 ppb concentration with good recovery at room temperature without using any extra energy sources (UV-light), as shown in fig. 8c. Ultrahigh sensitivity with good reversibility of the sensor for chemical vapours was attributed to higher specific surface area and improved electrons transfer in between polyaniline and MoS2 in the nanocomposite. In addition, the sensor showed reliable selectivity toward NH3, good stability, and high stretchability after exhibiting no serious degradation in sensing performance during 500 stretching cycles test at a

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reversible strain of 30% (Fig. 8 d, e). The exceptional sensing performance of the wearable chemical sensor was achieved by leveraging the synergistic sensing advantages of flexible polyaniline and

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atomically thin MoS2 sensing material.

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Fig. 8. (a) Photographs of PANI/MoS2 nanosheet on PDMS substrate in stretching and relaxing state. (b) SEM image of the PANI/MoS 2 hybrid. (c) Gas response versus NH3 concentration, and (d) selectivity bar diagram, of the stretchable PANI/MoS2 sensor. (e) The response of the sensor for various stretching cycles. Reproduced

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with permission from ref. 126. Copyright © 2017 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

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3.2. MoSe2-based gas sensors

MoSe2 also is a member of layered TMDCs family and its structure similar to the MoS2 consists of

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sandwiched Mo atoms layer in between two Se atoms layers[127]. The MoSe2 has been shown its potential in optical and chemical sensing owing to low direct bandgap and high adsorption energy [128][129]. Guo et al. [130] presented a high-performance epidermal gas sensor using MoSe2 with a serpentine structure. In this work, a gold-assisted mechanical exfoliation method was used for the synthesis of MoSe2 nanosheet that can be used in fabricating an epidermal gas sensor (Fig. 9a). The sensor exhibited a high sensing response to hazardous gases (NO2 and NH3) at room temperature with fast response (250 s) and recovery time (150 s) (Figs. 9c, f). The sensor’s detection limits of NO2 and 21

NH3 gases were calculated as 10 and 20 ppb, respectively. The gas sensing mechanism of the n-type MoSe2 sensor was reported based on the charge transfer mechanism, which have discussed in the sensing mechanism section. The gas sensing response of the fabricated sensor was steady in O2 and H2O environment (Fig. 9e), and no significant deterioration was observed in response for tensile strain up to 30%. The serpentine structure of the device enabled stretchable and conformal, which helped to MoSe2 sensor to successfully integrate on human skin. Moreover, they integrated the epidermal sensor with a smartphone internet of thing interface and successfully stored the gas sensing data on the cloud after collecting the data through a wireless network (Fig. 9b). The entire sensing system

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including sensor and other electronic devices consumed very less power of 0.63 mW with 24 days of the lithium battery lifetime, which is lesser compared the conventional metal oxide gas sensor. It was believed that the low-power flexible epidermal gas sensor could be used for the detection of hazardous

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gas at room temperature, warning to asthma patients and further clinical study/research from uploaded

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cloud data.

Fig. 9. (a) Schematic representation of the fabrication process of the epidermal gas sensor. (b) Wirelessconnected epidermal MoSe2-based gas sensor. (c) Gas sensing response of the fabricated sensor to different NO2 concentrations. (d) Gas sensor and PCB attached in a bandage. (e) Gas sensing response of the sensor to 22

NO2 under different humidity. (f) Gas sensing response of the sensor to different NH3 concentrations. Reproduced with permission from ref. 130. Copyright © 2019 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

Zhang et al. [131] demonstrated a self-powered high-performance NH3 sensor using MoSe2 deposited on a flexible PET substrate. The flexible gas sensor was fabricated by integrating MoS2-based piezoelectric nanogenerator (PENG) with Au-MoSe2 sensor. The Au-MoSe2 nanoflower synthesized by a solvothermal method was deposited on a PET substrate by a screen printing method. The AuMoSe2-based flexible sensor with MoS2 PENG showed a higher sensitivity to NH3 than the pristine

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MoSe2 sensor. The self-activated flexible gas sensor exhibited high selectivity towards NH3 with long-term stability for seven weeks and fast response (18 s) and recovery time (16 s) at room temperature. Au nanoparticles enhanced the sensitivity of the MoSe2 sensor by two factors: (i)

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modulation of the potential barrier at Au/MoS2 interface under gas environment [64] and (ii) as a catalyst providing more active sites on the surface through the dissociation of molecular oxygen

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[132].

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Guo et al.[133] fabricated an artificial intelligent flexible gas sensor using the multilayer MoSe2. In this work, chemical vapor transport (CVT) grown MoSe2 on a rigid SiO2 substrate was transferred on

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flexible and semi-transparent Kapton tape by using the gold-mediated exfoliation method and fabricated flexible chemiresistive sensor. The flexible sensor exhibited a sensitivity of 7.5 and 9.5 %

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to 5 ppm NH3 and 1 ppm NO2 at room temperature, respectively. The sensor also showed relatively stable sensitivity to target gases under the bending test with tensile and compressive strain. They also

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proposed a complete circuit design with an artificial neural network for detecting the location of NO2 toxic gas source in the home. Besides the detection of toxic gases, a flexible MoSe2 sensor was also used for detecting water vapour at room temperature. Zhang et al. [134] developed a flexible humidity sensor using the MoSe2/CuWO4 depositing on a PET substrate. They fabricated MoSe2 and CuWO4 nanoparticles via the hydrothermal method and mixed in wt% ratio in 1:1. The flexible humidity sensor showed high sensitivity in a range of 0-97% RH at 20 °C with a fast response and recovery of the sensor. The sensor with a portable device and smartphone was used for detecting the human 23

breath, water drop and finger-tips without touch through measuring humidity for advanced applications in different fields. The sensing performance of the flexible MoSe 2 sensor was enhanced by the CuWO4 nanoparticles because nanoparticles act as new adsorption centers as well as have a high surface area which helps to contact more water with MoSe2/CuWO4 sensing layer. At low humidity level, water molecules were chemisorbed on Se vacancies on MoSe 2 as well as oxygen vacancies on the surface of CuWO4. Later, high humidity concentration increased the physisorbed water layers on the chemisorbed water molecules and sensitivity was enhanced by proton hopping

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transport in the serial water layer through the Grotthuss mechanism.

3.3. WS2 -based gas sensors

WS2 material also is a attracted member of TMDCs family for its usage in diverse applications such

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as sensors[135][136][137][138], field-effect transistors[139][140], optoelectronics[141][142] and

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water splitting[143][144]. The lattice structure and material properties of the WS2 to a large extent are similar to its MoS2 counterpart. In contrast to Mo atom, a larger size of W atom as a larger cation

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in lattice structure provides an opportunity to change the 2D structure of the WS2 and better flexibility to modulate the properties of the WS2 material by doping through substituting larger atoms with

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smaller atoms [145]. So, manipulating the 2D structure of the WS2 by strain engineering or other processes may be a better utilization in advanced research strategy for futuristic device applications.

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On the other hand, low cost, natural abundance and less toxicity of W atom over the Mo atom [146], as well as currently higher consumption of Mo on the industrial platform make to the WS2 more

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preferable for future commercial applications. Guo et al. [147] demonstrated a high-performance electronic skin (e-skin)-oriented humidity gas sensor using multilayer WS2. In this work, a few-layer WS2 film was fabricated through sulfurization of predeposited W metal film on a solid SiO2/Si substrate, and the as-synthesized WS2 film was transferred on pre-stretched flexible and transparent PDMS substrate, as shown in fig. 10a. The flexible sensor exhibited the increasing sensing response with increasing humidity concentration at

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room temperature. It also showed increased device current for increasing humidity and similar response in both flat and bent state (Fig. 10 b,c). In addition, the sensor was exploited for real-time monitoring of human breath because of its fast response/recovery kinetics. The rapid response and recovery time of the sensor were attributed to the intrinsic hydrophobic properties of the WS2. Further, it was observed that the sensor did not show any deterioration in gas response up to 40% tensile strain,

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as shown in Fig. 10e.

Fig. 10. (a) Schematic representation of the fabrication of a flexible WS2 gas sensor. (b) The current of the sensor with increased humidity, (c) cyclic test of the sensor in a flat and bent state. (d) Photo of the stretchable 25

WS2 sensor and home made equipment for applying tensile strain. (e) Dynamic humidity response of the sensor under different tensile strain. Reproduced with permission from ref. 147. Copyright 2009, Published by The Royal Society of Chemistry.

They further fabricated a transparent e-skin humidity sensor using a transparent patterned graphene electrode in place of Ti/Au. The e-skin-oriented sensor showed a significant response to humidity for different muscle motions. However, the graphene electrode-based device had more background noise compared to that of Ti/Au based device and hence, optimization of the readout electrode contact for the device is further needed. Recently, Lee et al. [148] reported a highly sensitive and excellent deformable gas sensor using the

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CNT/WS2 hybrid deposited on cellulose paper (Fig. 11a). They fabricated this extremely deformable gas sensor within 30 min in minimum cost of ~0.23 $ per sensor by a simple dip-coating of cellulose paper in CNT and WS2 dispersions and later, dried until the formation of percolation path. The

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deformable sensor was tested for NO2 in the range of 0.1 to 10 ppm and exhibited a high sensitivity

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of 4.57% ppm–1 for less than 2 ppm concentration of NO2 (Fig. 11b). Also, an excellent selectivity toward NO2 compared to other organic/inorganic gases was observed at room temperature in the

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ambient atmosphere. However, the sensor showed incomplete recovery to NO2 gas at room temperature because of high adsorption of NO2 gas on CNT and WS2, and UV light irradiation during

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recovery process was used to achieve the complete recovery. Further, to test the stability of the sensor with respect to different mechanical deformation processes, the sensor was bent with a bending radius

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of 0.25 mm, twisted up to 1800° (628.8 radm –1) and crumpled. It was noted that the sensor showed no substantial degradation in resistance value to the same parameter of twisting and bending up to

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1000 cycles. Interestingly, the sensor exhibited comparable and even better gas sensitivity under highly folded, crumpled and bent state than that of in the flat state. The high sensitivity and deformability of the sensor were attributed to a combined effect of CNT, WS2 and cellulose paper. The CNTs and WS2 provide a high sensitivity to NO2 due to their high chemical activity, while the cellulose paper allows deformability through its excellent twist ability and foldability.

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Fig. 11. (a) Photographs of bending, twisting and folding and SEM image of the CNT+WS2 hybrid. (b) Gas sensing response of the CNT and CNT+WS 2 sensor to 10 ppm NO2 at room temperature. Reproduced with permission from ref. 148. Copyright 2019, American Chemical Society. (c) Cross-sectional SEM image, (d) illumination under red light, (e) bending, (f) wrapping on a glass rod, of the ultrathin Si wafer. (g) Schematic representation of the sensor in flat and bending states. (h) Selectivity bar diagram of the flexible Pd-WS2/Si/Al sensor. Reproduced with permission from ref. 152. Copyright 2019, Elsevier.

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Kuru et al. [149] reported a high-performance hydrogen sensor operated at room temperature using WS2-Pd composite deposited on a flexible PI substrate. The flexible sensor showed a higher response of 7.8 than the response of 1.14 of a graphene-Pd composite to 50,000 ppm H2. Moreover, the sensor showed a fats response (119 s) and recovery time (370s) to H2 at room temperature without any extra energy source. The high sensing performance of the sensor was because of a well-known mechanism, upon H2 exposure, the work function of Pd decreases as forming PdHx,[150][151] which changes the carrier concentration of the WS2 through transferring a number of charge carriers. Due to a less carrier density of the WS2, a relative change in carrier concentration upon exposure to H2 was higher than

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that of the graphene-Pd composite. In addition. in order to probe the mechanical stability of the sensor, the sensing response of the sensor was tested to 50000 ppm H2 in flat, bent and 100 bending cycles and stable response without any serious degradation was obtained in all mechanical deformation

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processes. Further, the hydrogen sensing response of the WS2 was enhanced through fabricating a

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flexible heterojunction sensor [152]. The WS2 nanofilm was deposited on ultrathin Si substrate (10 µm) by a sputtering method, and later, a 3-nm Pd nanoparticles layer was sputtered on WS2/Si

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heterostructure, resulting in developing a Pd-WS2/Si heterojunctions sensor (Fig. 11g). The flexible sensor exhibited an excellent sensing response to H2 in a range of 0.1-4% with a fast

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response/recovery time of 44.7/35.1 s and high selectivity toward H2 (Fig. 11h). The high sensing performance of the Pd-WS2/Si heterojunctions sensor was attributed to a joint effect of change in the

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work function of Pd and modulation of barrier potential at WS2/Si interface under H2 exposure. Moreover, higher barrier potential at the interface of WS2 and Si compared to that of other 2D

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materials and Si helped to improve the gas sensing response through a big change in interface potential. In addition, the flexible sensor showed a slight degradation in hydrogen response in increased humidity environment from 25 to 89% RH because adsorbed H2O molecules reduced the contact area on the Pd cap layer for H2 molecules. The flexible sensor indicated excellent mechanical stability by showing stable response up to 50 bending cycles.

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3.4. WSe2-based gas sensors WSe2 is a p-type inorganic 2D material with high hole mobility and also a member of the layered TMDCs family [153][154]. Similar to other TMDCs materials, in most cases, WSe 2 was first synthesized on a rigid substrate and then transferred on a flexible substrate for its utilization in flexible and wearable electronic applications. However, a polymer transfer method degrades the performance of the device due to wrinkles, scratches and a residue of the polymer [155]. Many efforts have been made for the direct synthesis of 2D materials on a flexible substrate. Medina et al. [156] directly fabricated WSe2 on a flexible PI substrate (30×40 cm2) (Figs. 12a, b) and utilized it for NO2 gas

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sensing application. They used an inductively coupled plasma process for selenization at low temperature (250°C) of predeposited 5-nm WOx film on PI substrate and in this process, WSe2 was formed through decomposition of the tungsten oxide in the presence of high-energy selenium ions. It

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was observed that the WOx/WSe2 based sensor showed excellent sensitivity of 20 % to 25 ppb NO2

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at room temperature due to modulation of depletion region at the interface of p-type WSe2 and n-type WOx under NO2 exposure. Moreover, the flexible sensor exhibited to start a slight decrease in

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sensitivity to NO2 gas at room temperature after a bending angle of 45° and operated up to a bending angle of 75°. Further, Li et al. [157] reported a highly sensitive NO2 gas sensor using WSe2 deposited

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on the highly flexible paper substrate by abrasion. After covering the surface as well as filling the gap of cellulose fibers of the flexible substrate paper via the WSe2 flakes, the flexible sensor exhibited a

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1000% sensitivity to 10 ppm NO2 at room temperature in an ambient atmosphere. However, the sensor showed incomplete recovery at room temperature due to the strong adsorption of NO2 on the

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WSe2 sensing film. Besides the high sensitivity to NO2 of the sensor, this simple fabrication process and sensor preparing time less than one minute provide a fast smart way for developing a flexible gas sensor. Moreover, the sensor is very inexpensive without any complex design also reduces electronic waste. On the other hand, Ko et al. [158] demonstrated a high-performance wearable gas sensor using WS2xSe2-2x alloy fabricated on flexible PET substrate. The WS 0.96Se1.04 alloy sensor exhibited about two times higher response to NO2 at room temperature as compared to the pristine WSe2 sensor (Fig.

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12e). The alloy based sensor showed high sensitivity to NO2 gas because of two reasons: First, modulation in controlled hole carriers concentration in p-type WS0.96Se1.04 through electronic

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sensitization mechanism.

Fig. 12. (a) Schematic representation of the low-temperature synthesis of WSe2 by using vertical plasmaassisted selenization process. (b) Photograph of large scale WSe2 film on a flexible PI substrate. Reproduced with permission from ref. 156. Copyright 2018, American Chemical Society (c) Optical image of the flexible WS0.96Se1.04 alloy gas sensor. (d) Gas sensing response of the alloy sensor to different bending cycles. (e) Gas

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sensing response of the WS0.96Se1.04 and WSe2 sensor to 100 ppm NO2 with different bending cycles. (f) Wearable wristband system that comprises flexible WS0.96Se1.04 alloy gas sensor and a light-emitting diode. (g) LEDs show different states for different concentrations (0, 10, 20, 50, and 100 ppm) of NO2. Reproduced with permission from ref.158. Copyright 2018, American Chemical Society.

Under the NO2 exposure, variation in hole concentration of alloy was more compared to WSe 2 due to less hole concentration in the alloy. Second, direct charge transfer between NO2 gas molecule and alloy sensing material was higher. The flexible alloy sensor showed an increased gas response to NO2 by increasing the bending cycles of 10,000 (Fig. 12d). During the mechanical deformation

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process, the increased response of the sensor was attributed to the created reactive edge sites through wrinkles and microcracks by strain and adsorption energy to NO2 gas on edge sites is more as compared to the basal plane of the TMDCs materials. In addition, the sensor showed only 5%

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variation in its gas response after three months, resulting in good stability (Fig. 12e). Furthermore, the alloy sensor was integrated with an LED module and a microcontroller for developing a wearable

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wristband gas sensor, which generates information on NO2 detection through glowing the LED (Figs. 12f, g). This wearable sensor only consumes power of 2-3 µW, which is about 104 times less than

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that of commercial SnO2 gas sensor (> 900 mW).

As presented above, in all flexible and wearable gas sensors based on WSe2, it was observed that

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electrode materials of the flexible sensor were made of rigid or hard metals, such as Au, Pt, Ti, Pd, Cu, etc. However, electrode materials should also be flexible and stretchable for high-performance

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wearable devices. Therefore, a high-performance, flexible, wearable and launderable gas sensor was

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developed using 2D NbSe2 as electrodes and WSe2 as a sensing layer [159]. In this work, they directly selenized the pre-patterned metal oxide WO3 and Nb2O5 materials through the one-step process by the chemical vapor deposition system and transferred on a flexible PET substrate. The NbSe2/WSe2 sensor exhibited higher sensitivity to NO2 at room temperature than the Au/WSe2 sensor because an NbxW1-xSe2 alloy was formed at the interface of an electrode (NbSe2) and sensing layer (WSe2) due to the diffusion of W and Nb metal atoms during the growth process (Figs. 13a, b). The alloy facilitated the transport of charge (induced on the channel by gas adsorption) from the channel to

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electrode through reducing the barrier height at the NbSe2/WSe2 interface. The flexible sensor maintained its gas sensing response as 30% to 5 ppm NO2 and 100 ppm NH3 even after decreasing the bending radius and did not show any degradation in response even after bending test (10000 bending cycles) (Figs. 13f, i). Interestingly, the sensor showed a sensing response to gases even after the conventional laundry process (Fig. 13l). Excellent stability of the sensor in a harsh environment such as chemical (chemical detergent and water) and mechanical, during washing makes it wearable

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and launderable gas sensor.

Fig. 13. (a) Schematic representation of a cross-sectional view of the NbSe2−NbxW1−xSe2−WSe2 heterojunction. (b) Cross-sectional EDS elemental line scan. Optical images of (c) gas sensor and (d) flexible and transparent gas sensor. (e) Flexible sensor with bending radius. (f) Gas sensing response of the sensor to NO2 and NH3 with different bending radius. Photographs of the sensor with bending system in flat (g) and (h) bending state. (i) Gas sensing response of the sensor to NO2 and NH3 with different bending cycles. A stitched 32

sensor on a T-shirt (j) in a laundry machine and (k) after the drying process. (l) Gas sensing response of the sensor to NO2 and NH3 after conventional laundry process. Reproduced with permission from ref. 159. Copyright 2016, American Chemical Society.

3.5. VS2-based gas sensors Vanadium disulfide (VS2) with metallic behaviour owing to zero bandgap in its electronic structure is a five group transition metal dichalcogenides [42]. Side view and top view of a lattice structure of the VS2 are shown in fig. 14a, where, V cations layer is sandwiched in between two layers of S anions and two-layer of VS2 are attached via weak van der Waals forces [160]. However, hybridization of

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V-3d with S-3p orbitals form strong in-plane bonding through covalent bonds [161]. A stable 1T phase of VS2 has been shown its potential in different applications, including sensor[162][163][164], supercapacitor[165][166][167][168], catalysis[169][170], and battery[171][172][173][174] materials

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owing to its metallic nature. In particular, humidity sensor application, it is known that electropositivity of the metal atom in all 2D materials is responsible for improving the sensitivity of

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the sensor[175][176]. In this context, high electropositivity of the V atom as well as high conductivity

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of VS2 makes it a very promising candidate for fabricating humidity sensors.

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Fig. 14. (a) Side and top views of the VS2 lattice structure. Reproduced with permission from ref. 160 Copyright © 2018 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Current versus voltage characteristics of the VS2 sensor in the presence of humidity in a range of 0-100% RH. (c) The dynamic current of the sensor under finger approaching as the indicated finger on and off states, and photo of the sensor in flat and bent states. (d) Cyclic test for flexible and transparent humidity sensor to 10% RH. Reproduced with permission from ref. 161. Copyright © 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

Feng et al. [162] fabricated an ultrasensitive humidity sensor using 2D VS 2 nanosheets on a PET substrate. In this work, VS2 nanosheet (thickness ~ 2 to 5 nm) was synthesized by a modified liquidexfoliation method and specific formamide organic solvent was used for getting high exfoliation

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efficiency because surface energy of formamide and bulk VS 2 is approximately same[177]. The sensor exhibited a change in the slope of I-V characteristic under the humidity in a range of 0 to 100% RH at 25 °C, as shown in fig. 14b and shown fast response and recovery time of 30-40 s and 12-50 s,

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respectively (Fig. 14d). It was also observed that the sensor also shows similar behaviour in both flat

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and bent states, resulting in a high endurance toward mechanical deformation (Fig. 14c). In addition, they also utilized the excellent humidity sensitivity of the sensor for positioning the touchless

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fingertips through the spatial mapping of finger moisture. The sensor showed a large change in resistance value (more than three times) in finger-off and finger-on states. Despite the high sensing

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performance of the VS2 based humidity sensor, the VS2 nanosheet has a long-standing challenges in a humid environment. To address this issue and in order to increase the stability of the VS2 humidity

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sensor, incorporation of other materials into VS 2 could be an excellent option as a new sensing material in terms of hybrids or nanocomposites, which has the unique features from its single-

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constituent counterparts. In this context, Chen et al. [163] fabricated a stable high-performance flexible humidity sensor by VS2@MoS2 hybrid deposited on the PET substrate. They deposited vertically aligned nanosheet of MoS2 on VS2 nanoflowers by using a modified two-step hydrothermal method. The flexible hybrid sensor exhibited very fast response/recovery time as 23/13 s and high sensitivity of 5798.5 was remained the same with slight fluctuation even after 30 days, which results in the long-term stability of the sensor. Humidity detection of the hybrid sensor was higher than the

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pristine VS2 sensor because of large moisture capturing by the MoS2 in the hybrid sensor. The sensing mechanism was proposed that in low humidity concentration, water molecules chemisorbed on the active site of the VS2@MoS2 hybrid and dissociated into hydroxyl groups [178]. Later, at a high concentration of humidity, a stack of water molecules layers formed after adsorbing water molecules physically through single hydrogen bond and thereby, protons transfer through neighbouring water molecules reduced the resistance value of the sensor via Grotthuss chain reaction mechanism[179]. As a result, physicochemical stability provided by the MoS2 through retaining metallicity of the VS2, good structural property as exposed active sites of the vertically MoS 2 and high conductivity of the

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VS2 were responsible to improve the humidity characteristics of the sensor.

3.6. TaS2-based gas sensors

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similar to other layered materials, here Ta cations layer is sandwiched in between two layers of S anions and two-layer of TaS2 are attached via weak van der Waals forces. TaS2 has pressure and

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temperature-dependent very rich phase diagrams [180] and these first time were studied by Jellinek in 1962 [181]. Similar to other TMDCs materials, TaS2 also has various types of polytypes such as

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1T, 2H, 3R, 4H, and 6R because of different stacking arrangements of the S-Ta-S layers. However, among the all, 1T (one layer trigonal), 2H (two-layer hexagonal), and 3R (three-layer rhombohedral)

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polytypes of the TaS2 are very common [181] [182] (structures are shown in fig. 15(a, b, c)) and have been utilized in different applications in past years [183] [184][185] [182][186][187][188][189]. In

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particular, TaS2 similar to VaS2 is a very useful material for developing a humidity sensor owing to its metallic nature. However, TaS2 is more stable in humidity ambient compared to that of the metallic VS2 material. Besides the high conductivity and stability, V metal atom in the VS2 has higher electropositivity compared to that of other metal atoms (C, Mo, and W) in 2D materials (graphene, MoS2 and WS2) and suggests that higher surface energy of the VS2 is responsible for capturing more water molecules.

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Recently, Feng et al. [190] first time fabricated the flexible humidity sensor using TaS2 deposited on ceramic, PET and PDMS substrates. They synthesized the different morphologies of the TaS2 such as microspheres, nanosheets, rectangles, hexagonal and octagonal, etc. by liquid exfoliation and CVD

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Fig. 15. (a, b, c) Lattice structures of the TaS2 in its 1T, 2H and 3R phases. Reproduced with permission from ref. 184. Copyright © 2018, American Chemical Society. (d) SEM image of 3R-TaS2 nanosheets. (e) Humidity

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distribution of the fingertip. (f) Impedance versus finger height graph. (g) The impedance of the flexible sensor under blow-drying, meditation and running conditions. Photograph of the sensor on (h) PET, (i, j) PDMS substrate. (k) Performance of the sensor under human breathing. Reproduced with permission from ref. 190. Copyright 2013, Published by The Royal Society of Chemistry.

Among all these morphologies, 3R-TaS2 nanosheets exhibited good sensitivity to humidity in a range of 11-95% RH with a fast response/recovery time of 0.6/2 s because of its higher specific surface area (more active sites and edges). Moreover, a flexible PET substrate is more useful for improving

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humidity characteristics as compared to that of the other substrates owing to its high hydrophobic behaviour. The sensor was tested in order to check stability up to 10 days in 2 days interval and it showed the same response to humidity with a slight response change of 2%. The high sensing performance of the TaS2 sensor was because of high electropositivity of Ta atom, high surface/volume area, large exposed active sites and impurities as oxygenated functional groups. In addition, flexible TaS2 humidity sensor was also used for non-contact sensation through detecting moisture of fingertip (Fig. 15e), real-time monitoring the moisture on human skin in different condition such as blowdrying, meditation and running conditions (Fig. 15g), and monitoring the human respiration through

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measuring moisture in-breath (Fig. 15k).

3.7. SnS2-based gas sensors

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member of TMDCs family. However, SnS2 is a 2D layered material and also has some aspects similar to TMDCs materials[191][192]. Sn cations are sandwiched in between two hexagonally packed layers

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of S anions and two-layer of SnS2 are attached via weak van der Waals forces. The SnS2 material has been used in different applications such as gas sensing[36][193][194], bio-sensing[195][196], Li-ion

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battery[197][198], field emission[199], and solar energy[200][201]. In particular, it has shown its great potential in gas sensing applications owing to high electronegativity, a large number of

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adsorption sites, stability, and non-toxicity[202]. Ma et al. [203] demonstrated an NH3 gas sensor at room temperature by depositing 2D SnS2 on a flexible polyimide substrate. In this work, a line array

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of SnS2 was printed directly on a PI substrate using a commercial ink-jet printer, and liquid exfoliated SnS2 in ethanol was used as the ink of the printer. The printed gas sensor exhibited a high sensitivity to NH3 with the detection limit as low as 5 ppm and a fast response time of 1 min. The high sensitivity and fast response time of the sensor were achieved due to the thin SnS2 sensing layer and the nanoscale gap between nanosheets because they provide high surface area to interact gas molecules and permit to gas molecules to diffuse fast into and out from nanosheets, respectively. Besides the

37

good sensing performance of the sensor, this flexible sensor fabrication technique is easy to handle, low cost, environment-friendly and has the potential to provide scalable production. Further, Li et al. [204] presented a high-performance NH3 sensor at room temperature using composites of SnO2/SnS2 nanotubes on a flexible PET substrate. They first synthesized SnO2 nanotubes by an electrospinning process and then fabricated the SnO2/SnS2 nanotube using a hydrothermal method (Fig. 16a). The flexible composite sensor exhibited high sensitivity to NH 3 gas with complete recovery at room temperature and also showed an excellent selectivity toward NH3 (Figs. 16b, c). Moreover, the sensor preserved its gas response without any degradation even after different bent (up to 3000 times) (Fig.

ro of

16d) and bending angle (up to 150°). The high sensing performance of the SnO2/SnS2 nanotube sensor was because of the hollow structure and synergistic effects of the SnS2 and SnO2. The hollow structure provides a high aspect ratio for more interaction of NH3 molecules with increased active sites.

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Besides, a depletion and accumulation regions are formed at SnO2/SnS2 interface on the side of SnS2

re

and SnO2, respectively, because during heterojunction formation in order to equilibrate the Fermil level, electron moves towards SnO2 from the conduction band of the SnS2 due to higher work function

lP

of the SnS2. Upon exposure to air, oxygen in air forms oxygen ion after extracting electron from SnS2 and SnO2 and results in increase depletion layer. Thereby, the resistnace value of the device was

na

increased. And upon exposure to NH3 gas, oxygen ions give back the electron to materials after reacting with NH3 and results in a narrow depletion region (Fig. 16e). As a result, heterojunctions at

ur

a SnO2/SnS2 interface enhanced the sensitivity through the modulation of potential barrier upon exposure to NH3. In addition, Zhang et al. [205] have developed a flexible humidity sensor by using

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the layer-by-layer self-assembly technique for fabricating the hybrid of SnS 2 nanoflower and Zn2SnO4 hollow sphere on a PI substrate. The flexible hybrid sensor showed high sensitivity (10709 pF% RH) with fast response (18 s) and recovery time (1 s) in a range of 0 to 97 % RH at room temperature. Because of high sensing performance, the sensor directly detected human breath, palm sweat, urine and water drop and which proved its utilization in the multifunctional application. The excellent sensitivity of the flexible hybrid sensor was because of a large change in free electron

38

concentration of the sensing layer under humidity. Upon exposure to the water, H 2O molecules are adsorbed on pre-chemisorbed oxygen vacancies of Zn2SnO4 as well as adsorbed at hydrophilic groups

ur

na

lP

re

-p

ro of

and active sites of hybrid sensing film.

Fig. 16. (a) SEM image, (b) selectivity bar diagram, (c) transient gas response to different concentrations of

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NH3, (d) response versus bent cycles, and (e) gas sensing mechanism of the SnO 2/SnS2 nanotube composites. Reproduced with permission from ref. 204. Published by The Royal Society of Chemistry.

After increasing the humidity concentration, water molecules form a continuous film and ionized H2O molecules as H3O+ charge carries [206]. According to Grotthuss reaction,[207] proton hopping occurs and which helps to enhance the sensitivity of the sensor at a higher humidity level. Likewise,

39

Zhang et al. [208] fabricated a flexible humidity sensor using SnS2/GO nanoflower on the PET substrate. The flexible sensor exhibited high sensitivity (65396) with fast response (0.9 s) and recovery time (10 s) at 97 %RH at room temperature. The sensor also showed similar sensing response in inward and outward bending condition of the sensor and detected fingertip approaching/retracting behaviour through measuring diffusing moisture from a fingertip. The high sensing performance of the sensor was attributed to the adsorption of more water molecules on many hydrophilic functional groups such as hydroxyl, carboxyl and epoxy groups on the GO and S-vacancy sites on SnS2 [209][210]. Moreover, the sensitivity of the sensor was enhanced under higher humidity

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concentration through proton hopping, as we have discussed above. Recently, Wu et al. [211] fabricated a flexible NO2 gas sensor using the 3D SnS2/RGO heterostructure. In this work, they synthesized SnS2/RGO heterostructure by hydrothermal method and first time employed the flexible

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and porous liquid crystal polymer (LCP) as a substrate for NO2 gas sensing. The flexible sensor

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exhibited a significant sensitivity of 0.56 % to even a very low concentration of 20 ppb NO2 and complete recovery for low concentration of NO2 gas at room temperature. The sensor also showed

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stable gas response without any degradation during mechanical deformation such as 120° flexion. The high sensitivity of the sensor was due to modulation of heterojunction potential at the SnS2/RGO

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interface as well as 3D porous structure, which provides NO2 diffusion and extra charge transfer path through charge hopping [212]. Moreover, high endurance to mechanical deformation processes

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because of employing flexible LCP substrate opens a window for a flexible gas sensor in wearable device applications.

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3.8. Other chalcogenide-based layered materials for gas sensor There is other new chalcogenide-based layered material, metal phosphochalcogenides formulated as MPX3 (where M = metal atoms, such as Fe, Ni, Zn, etc, P = P, and X = chalcogen atoms, such as S, Se, Te), which have also gained an increasing interest for electronic device applications [213][214][215]. Jenjeti et al. [216] presented a highly selective humidity sensor using the few-layer NiPS3 deposited on a flexible polypropylene substrate. The flexible NiPS3 sensor exhibited a higher

40

response of 1.86 × 106 to 98% RH at room temperature than both graphene and MoS2 sensors. The sensor showed an excellent selectivity towards humidity against other gases, such as N2, O2, CO2, ethanol and acetone, and also exhibited a similar response in the flat and bent state without any significant degradation. Moreover, in order to utilize a fast response (1-2 s) and recovery time (2-3 s) of the humidity sensor, they tested the sensor for monitoring the human respiration and detecting touchless fingertips. Similar to MoS2 sensor [217], a humidity sensing mechanism in the NiPS3 sensor was attributed to the synergistic effect of the modulation of charge in the transporting layer of the NiPS3 via electron-donation by H2O molecules and proton (H+) hopping in a continuous layer of

ro of

water molecules.

4. Conclusion and future outlook

In this review, we discussed the recent progress in the emerging flexible and wearable gas sensors by

-p

using fascinating 2D TMDCs materials (MoS2, MoSe2, WS2, WSe2, SnS2, VS2, etc.). In Table 1. some

re

of the most promising and recent flexible gas sensors based on 2D TMDCs materials are described by depicting their figures of merit. In addition, at flexible gas sensing platform, we also compare gas

lP

sensing results published by many research articles and reviews based on different materials (particular

conventional

metal

oxide

semiconductors)

na

[218][219][220][221][222][223][224][225][226][227][228] to that of the 2D TMDCs materials based gas sensors. Interestingly, it is observed that the 2D TMDCs materials based gas sensors exhibit better

ur

gas sensing performance after detecting gases at room temperature. The 2D TMDCs materials have a great potential to substitute the matured metal oxide gas sensor technology on a flexible and

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wearable sensing platform owing to the detection of target gases at room temperature as well as their extraordinary flexible, transparent and stretchable properties. Table 1. 2D transition metal dichalcogenides (TMDCs) materials based flexible gas sensors.

Material

Flexible Substrate

Temp (°C)

Detected Gases

Selective Gas

Con cent ratio n

41

Sensitiv ity or Respon se

Response/ Recovery Time

Limit of detection (LOD) or range

Stability

Ref

PI

RT

NO2, NH3

NO2

25 ppm

97%

3/8 min

25-500 ppm

-

[99]

MoS2

PET

80

NH3, IPA, Toluene, Ethanol

NH3

400 ppm

33%

158/385 s

50 ppm

-

[94]

MoS2

PET

RT

NO2

NO2

400 ppb

671%

16/65 s

20 ppb

-

[125]

MoS2

PDMS

RT

Humidity

Humidity

90% RH

2600

-

30-90% RH

-

[107]

MoS2

PDMS

RT

Humidity

Humidity

35% RH

104

10/60 s

0-35% RH

30 days

[101]

MoS2

PET

RT

Humidity

Humidity

65% RH

2048%

-

5-65% RH

100 hour

[124]

MoS2

PET

RT

Acetone

120 ppm

16.3%

-

-

-

[114]

MoS2-Au

PET

RT

Acetone

10 ppm

~100/385 s

-

30 days

[114]

MoS2 -Pt NPs

PET

RT

Acetone, Ethanol, propanol, Toluene, Hexane, Benzene Acetone, Ethanol, propanol, Toluene, Hexane, Benzene NO2

6%

>30 min

2 ppb

-

[111]

Pd-MoS2

PI

150

NH3, NO2

NH3,

10 ppm

3%

-

-

-

[112]

MoS2 -Cu2S

Cellulose paper

27

Humidity Ethanol

Humidity

2578% RH

40%

-

2%

-

[229]

Graphene/MoS2

PI

150

NO2, NH3

NO2,

5 ppm

7%

-

1.2 ppm

19 months

[100]

RT

NO2, NH3

NO2,

40 ppm

54%

-

1.5 ppm

-

[115]

-p 15%

re 0.5 ppm

lP

na

ur PET

Jo

MoS2 -SWCNT

NO2

ro of

MoS2

PANI/ MoS2

PDMS

RT

NH3, CO2, H2O, C2H5OH

NH3

30 ppm

75.58%

~ 150 s

50 ppb

-

[126]

rGO/ MoS2

PET

25

Humidity NO2, NH3, H2, C2H5OH

Humidity ,

10 % RH

9.03

~30/253 s

178.3 ppm and 10-90% RH

-

[118]

MoS2 /rGO

PET

90

NO2

NO2

5 ppm

~ 25%

-

0.15 ppm

-

[116]

42

PEN

RT

HCHO

HCHO

10 ppm

4.8%

-

-

90 days

[117]

MoSe2

PI

RT

NO2

NO2

2 ppm

~ 20 %

250/150 s

10 ppb

-

[130]

MoSe2

PI

RT

NH3

NH3

5 ppm

~ 45%

250/150 s

20 ppb

-

[130]

MoSe2

Kapton tape

RT

NH3

NH3

5 ppm

7.5%

400/800 s

-

-

[133]

MoSe2

Kapton tape

RT

NO2

NO2

1 ppm

9.5%

500/1000 s

-

-

[133]

Au-MoSe2

PET

25

NH3, Ethanol, Acetone, Benzene, CO

NH3

20 ppm

2.6

18/16 s

-

40 days

[131]

MoSe2/CuWO4

PET

20

Humidity

Humidity

97% RH

31982

79/4 s

WS2

PDMS

RT

Humidity Acetone, Ethanol

Humidity

90% RH

WS2 -Pd

PI

RT

H2

H2

5000 0 ppm

Pd-WS2/Si

Ultrathin Si

27

H2, N2 , CO, O2, NH3, H2O, C2H5OH

H2

CNT/WS2

Cellulose paper

RT

NO2

CNT-WS2

Cellulose paper

RT

-

[134]

2357

5/6 s

20-90% RH

Several months

[147]

7.8

119/370 s

10 ppm

-

[149]

4%

19.8

44.7/35.1 s

50 ppm

30 days

[152]

NO2

10 ppm

~ 20%

-

1 ppm

-

[230]

NO2, CO, NH3, Ethanol, Acetone, Tolune,

NO2

1 ppm

4.57%

-

< 2 ppm

-

[148]

ur

na

lP

re

-p

0-97% RH

Paper

RT

NO2

-

0.5 ppm

238.3 %

-

-

-

[157]

WOx/WSe2

PI

RT

NO

NO

25 ppb

13%

250 s / -

0.3 ppb

-

[156]

NbSe2/WSe2

PET

RT

NO2, NH3, CO, CO2, N2O

NO2

5 ppm

30%

-

0.12 ppm

-

[159]

WS2xSe2-2x

PET

27

NO2, NH3, CO

NO2

100 ppm

1076%

-

-

3 month

[158]

Jo

WSe2

ro of

rGO/ MoS2

43

Humidity

Humidity

VS2@MoS2

PET

RT

Humidity

-

TaS2

PET

25

Humidity

Humidity

SnS2

PI

RT

NH3

SnS2/SnO2

PET

RT

SnS2/Zn2SnO4

PI

RT

SnS2/GO

PET

25

SnS2/RGO

LCP

SnS2/RGO

LCP

90% RH

30

~ 30/12 S

0-100% RH

-

[162]

5798.50

23/13 s

11-95% RH

30 days

[163]

95% RH

201.90.4

0.6/2 s

11-95% RH

-

[190]

-

5 ppm

<1

~ 1/1 min

5-1000 ppm

-

[231]

NH3 acetone, ethanol, toluene,c hlorofor m Human respiratio n, palm sweat, urine, water droplets Humidity

NH3

100 ppm

2.48

21/110 s

1 ppm

-

[204]

H2 O

97% RH

10709

18/1 s

0-97% RH

-

[205]

Humidity

97% RH

65396

0.9/10 s

23-97% RH

5 weeks

[208]

26

NO2 NH3, CO2, ethanol, acetone, toluene,

NO2

8 ppm

49.8%

153/76 s

8.7 ppb

-

[211]

156

NO2, NH3, CO2, ethanol, acetone, toluene,

NO2

-

-

-

[211]

14.7%

lP

8 ppm

ro of

25

-p

PET

re

VS2

na

RT= Room temperature, PI= Polyimide, PET= Polyethylene terephthalate, PEN= Polyethylene naphthalate, PDMS= polydimethylsiloxane, LCP= Liquid crystal polymer. From the energy point of view, the 2D TMDCs materials-based gas sensors consume less power in

ur

µW level compared to the commercially available metal oxide gas sensors (several tens of mW level). Besides reduced power consumption after removing the extra heating element form the sensor, this

Jo

disintegration of the heater makes a simple design for chemiresitive gas sensor through minimizing complexity and also provides a simple and small footprint for the integration of the sensor into IC technology with CMOS compatibility. Moreover, low power consumption helps to reduce the thermal effects on the human body as well as increases long-term operation. So, the 2D TMDCs materials are also suitable for e-skin-oriented and e-textiles nano-gas sensor through the flexible nanotechnology as they offer atomic-scale thickness with a high value of flexibility and transparency. On the other

44

hand, detection of gases at room temperature by the flexible 2D TMDCs materials based gas sensors also reduce the cost of the sensor through reducing the cost of the heater as well as removing complex fabrication technology’ steps. The fabrication of the sensor using 2D TMDCs materials on a common cellulose paper via facile wet-chemical process make it cheaper (~$ 0.02 per sensor) and disposable [229]. Moreover, the replacement of rigid metal electrodes by depositing flexible 2D metallic materials as electrodes in sensor device enhances the sensing performance, transmittance as well as flexibility, and paves the way for e-skin-oriented and e-textiles nano-gas sensor by improving

ro of

conformal contact with the human body and ease integration with textile or single fiber, respectively.

The 2D TMDCs layered materials showed a layer-dependent tuneable semiconducting bandgap. This semiconducting behaviour has raised the utilization of these materials for fabricating the sensor in a

-p

flexible and wearable sensing field for IoT applications. The semiconducting nature of the 2D

re

TMDCs materials has been exploited through fabricating FET structure gas sensor in which gate voltage is utilized to improve the response and recovery times at room temperature. Moreover, light

lP

irradiation also enhances the recovery of the sensor after accelerating the desorption of the gas molecules from the sensing layer at room temperature. So, the integration of LED technology with

na

2D TMDCs materials based sensors can be a good option for developing future commercial flexible gas sensors. On the other hand, the increased operating temperature has also been demonstrated to

ur

improve the response/recovery kinetics of the sensor, however, this elevated temperature rises the thermal and power budget.

Jo

The individual 2D TMDCs material based gas sensor is limited to enhance more its gas sensing performance due to weak adsorption of gas molecules on the surface of sensing material through weak van der Waals interaction. Nevertheless, gas sensing response of the individual TMDCs materials was significantly enhanced by structural or defect engineering due to strong interaction (chemisorption) of gas molecules on deliberately created active sites ( edge sites, vacancy sites). In addition, doping or functionalization of the sensing 2D TMDCs material by using noble

45

metal/semiconductor nanoparticles has also been proved helpful to modulate the gas sensing characteristics of the sensor by electronic and chemical sensitization. Furthermore, incorporation of one 2D material in another 2D/1D material as a new hybrid material enhanced the sensing performance of flexible and wearable sensor due to the synergistic effects of both materials. For instance, the integration of superior mechanical flexible CNT with MoS2 increased the flexibility and stability of the MoS2 flexible gas sensor through reducing micro-cracks or defects in the MoS2 after bending test. Besides the detection of inorganic and organic gases, humidity detection by the TMDCs materials has been shown its potential for touch less sensors and medical applications through

ro of

detecting moisture of human skin and breath. The metallic material of the TMDCs family with higher electropositivity of its metal atom captures more water molecules due to its higher surface energy and exhibits high sensitivity through the Grotthuss reaction mechanism.

-p

These flexible and wearable sensors are at an early age and multiple challenges need to be addressed

re

on various fronts. From growth to sensing properties, such as a large-scale direct growth of 2D materials on flexible substrates, stability in atmospheric ambient, rapid detection of volatile

lP

biomarker in parts-per-trillion range (for particular medical science applications), biocompatible, self-healing (recovery itself after mechanical impairment), the selectivity of the gas sensors faces

na

several technological impediments for their practical realization. Moreover, the use of these chemical sensors for IoT applications also needs more efficient acquisition, processing and communication of

ur

the required data. However, there are several proposed solutions, such as innovation in sensing material through synthesizing a new flexible and stretchable material after incorporating an excellent

Jo

flexible material CNTs with 2D TMDCs materials which improve the flexibility, stretchability and stability, but toxicity and reproducibility of these nanocomposites put a big question mark on their success. Moreover, passivation of the sensing layer of 2D TMDCs materials with other different materials (metal oxide, polymer, etc.) improves the stability in a humid environment and fabrication of sensor array with pattern recognition analysis makes selective after distinguishing one gas from a complex mixture. Progress in battery technologies and energy scavenging techniques (human body

46

heat and motion) could be the best tool to solve technical issues (battery size, miniaturization and long-terms-operation without maintenance), especially in the e-skin-oriented and e-textiles nano-gas sensor. Moreover, a self-powered sensor can be a technological and economic driver for emerging flexible gas sensor industries. An epidermal or skin-like gas sensing system via coupling of piezoelectric properties with gas sensing characteristics may be a good option for fabricating a selfpowered gas sensor because, during body motion, piezoelectric network with intimate contact to the human body would provide a piezoelectric signal for operating gas sensor without the need of external power. Moreover, geometry engineering for designing different geometries (serpentine, wavy,

ro of

horseshoe, buckled and crumpled) of the sensor device has shown and also has great potential for further enhance sensing performance and mechanical endurance. To further enhance the lifetime and reliability of the e-skin sensor after many mechanical deformations over time or accidental fracture,

-p

the flexible e-skin gas sensor is envisaged with inviting self-healing capability. Despite, there are

re

scientific and technological gaps for further improvements to realize their full potential at commercial sensing platform. So, an innovative holistic multi-disciplinary effort is required to fill the existing

lP

scientific gaps faced by flexible and wearable sensors. In addition, the ever-increasing demand for transparent, flexible, and energy-efficient devices has inspired the scientific community to explore

na

the exciting physics of these devices by using ultra-thin functional materials.

ur

Declaration by the Authors

Jo

Authors declare that there is no conflict of interest.

Acknowledgements

MH would like to thank the Alexander von Humboldt (AvH) Stiftung, Germany for the research award.

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ammonia gas sensor, Mater. Res. Express. (2019). doi:10.1088/2053-1591/aae5c4.

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Rahul Kumar received the B.Tech. degree in electronics and communication engineering from Uttar Pradesh Technical University, Lucknow, India, and the M.Tech degree in material science and technology from Indian Institute of Technology (BHU) Varanasi, India. He is currently pursuing the Ph.D. degree in electrical engineering at IIT Jodhpur, India. His research interests include synthesis of TMDCs materials using CVD technique and 2-D materials based gas sensors and photodetectors.

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Neeraj Goel received the B.Tech. degree in electronics and communication engineering from Uttar Pradesh Technical University, Lucknow, India, and the M.Tech. degree in electronics and communication engineering from IIT (ISM) Dhanbad, Dhanbad, India. He is currently pursuing the Ph.D. degree in electrical engineering with IIT Jodhpur, Jodhpur, India. His current research interests include 2-D materials-based photodetectors and gas sensors.

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Mirabbos Hojamberdiev received the M.Sc. and Ph.D. degrees in materials science from the Tashkent Institute of Chemical Technology, Tashkent, Uzbekistan.

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His current research interests include the fabrication, characterization, and application of visible-light-responsive photocatalytic materials for energy and environmental applications.

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Mahesh Kumar has received M.Tech degree in Solid State Materials from IIT Delhi and Ph.D degree from IISc Bangalore. He is working as Associate Professor at the department of Electrical Engineering, Indian Institute of Technology Jodhpur, Jodhpur (India).

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His area of research interest is electronic materials, Semiconductor devices, thin films and nanostructures of wide band gap semiconductors and 2D materials for gas sensing application.

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