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Here, we present a highly sensitive and fast hydrogen (H2) sensor for 1% H2, well below the critical limit of explosion ignite in air, in a temperature range of 28–150 °C by using monolayer MoS2 pyramid structures with enhanced adsorption sites. The monolayer MoS2 pyramid structures is synthesized by modified chemical vapor deposition technique and characterized by field emission scanning electron microscopy, Raman, photoluminescence and atomic force microscopy. The highest sensitivity of 69.1% was achieved at a moderate temperature with a response time of 32.9 s for the monolayer MoS2 pyramid structures. At room temperature (RT), the sensor showed a sensitivity of 6% with a faster response of 11.3 s and recovery time of 125.3 s. The availability of favourable adsorption sites on in-plane MoS2 and edges of MoS2 in monolayer MoS2 structures provide enhanced adsorption sites for gas sensing and resulted in the high sensitivity and low response time compared to that of bare MoS2 and other nanostructures-based H2 sensor. The detailed gas sensing mechanism is proposed in the light of detail surface morphology and density function theory (DFT). This study reveals that tailoring the favourable adsorption sites in 2D materials is helpful to develop the highly sensitive and fast H2 sensor for next generation safety devices for H2 fueled vehicle and clean energy applications.

A design of an advanced sensing material, such as MoS2, is imperative to enhance the sensing performance of a sensor. Because their usage alone for developing a practical sensor is impeditive owing to low gas response and slow response/recovery kinetics. Here, we report a high-performance NO2 gas sensor using a hybrid of temperature-assisted sulfur vacancy within the edge-oriented vertically aligned MoS2 (Sv-MoS2) and crumpled reduced graphene oxide (rGO) particles. Interestingly, the Sv-MoS2 functionalized by optimized rGO concentration exhibited a significant enhancement of response to NO2 (approximately three times higher than that of pristine vertically aligned MoS2) with fast response (< 1 min) and complete recovery. Such a large improvement in the sensing performance could be attributed to controlled electrical/chemical sensitization level of MoS2 through controllable vacancy and interface engineering. The vacancy engineering offers abundant active sites through creating sulfur vacancy in additionally rich edge active sites of vertically oriented MoS2 for more electronic interaction with gas molecules. While interfacing of p-type rGO particles with n-type MoS2 leads to multiple out-of-plane vertical nano-heterojunctions as a sensitizing configuration for boosting the performance of the sensor. This paper opens up a new approach towards improving the sensing activity of a 2D material via a synergistic vacancy and interface engineering.

Semiconductor-based gas sensors are the important family of electronic devices that are widely used in various sciences and industries. The basic principles of this type of sensor are based on the reaction of the gas molecules and the semiconductor surface. Therefore, nanostructured semiconductors are the best candidate for fabricating of this type of gas sensor due to their high surface area/volume ratio. Of these types of semiconductors, metal-oxides semiconductors (MOS) such as zinc oxide (ZnO) and transition metal dichalcogenides (TMD) such as molybdenum disulfide (MoS2) (or moly) are very important. Therefore, in this chapter the recent findings of these types of sensors will be presented and discussed. The capability of MoS2 nanostructures as a member of TMDs to detect different gases such as H2 and NO2 has been presented. We presented that, how hybrid MoS2 shown better gas sensing performance in comparison the pristine MoS2. In the next step, ZnO nanostructures from MOS family have been chosen to study their gas sensing applications to detect ammonia, acetic acid, ethanol, and methanol. In addition, the effects of graphene as a 2-dimensional (2D) structure on gas sensing performance of ZnO nanostructures have been discussed. Furthermore, it has been shown how the gas sensing performance of ZnO nanostructures could be improved by an organic semiconductor such as polyaniline (PAni).

Controlled and tunable growth of chemically active edge sites over inert in-plane MoS2 flakes is the key requirement to realize their vast number of applications in catalytic activities. Thermodynamically, growth of inert in-plane MoS2 is preferred due to fewer active sites on its surface over the edge sites. Here, we demonstrate controlled and tunable growth from in-plane MoS2 flakes to dense and electrically connected edge-enriched three-dimensional (3D) network of MoS2 flakes by varying the gas flow rate using tube-in-tube chemical vapor deposition technique. Field emission scanning electron microscope results demonstrated that the density of edge-enriched MoS2 flakes increase with increase in the gas flow rate. Raman and transmission electron microscopy analyses clearly revealed that the as-synthesized in-plane and edge-enriched MoS2 flakes are few layers in nature. Atomic force microscopy measurement revealed that the growth of the edge-enriched MoS2 takes place from the in-plane MoS2 flakes. On the ...

Toxic gases are produced during the burning of fossil fuels. Room temperature (RT) fast detection of toxic gases is still challenging. Recently, MoS2 transition metal dichalcogenides have sparked great attention in the research community due to their performance in gas sensing applications. However, MoS2 based gas sensors still suffer from long response and recovery times, especially at RT. Considering this challenge, here, we report photoactivated highly reversible and fast detection of NO2 sensors at room temperature (RT) by using mixed in-plane and edge-enriched p-MoS2 flakes (mixed MoS2). The sensor showed fast response with good sensitivity of ∼10.36% for 10 ppm of NO2 at RT without complete recovery. However, complete recovery was obtained with better sensor performance under UV light illumination at RT. The UV assisted NO2 sensing showed improved performance in terms of fast response and recovery kinetics with enhanced sensitivity to 10 ppm NO2 concentration. The sensor performance is also investig...

Two dimensional (2D) van der Waals heterostructures are becoming one of the ascendant research areas for semiconducting device application owing to their remarkable optoelectronic properties, which allows more functioning ability beyond its individual constituent. 2D layered materials can be easily integrated and form heterostructure due to the dangling bond free surfaces. However, for novel optoelectronic device applications, the understanding of charge carrier dynamics at the interface of heterostructures is critical and essential. Here, we demonstrate the charge transport behaviour and energy level band alignment at MoS2-ReS2 heterointerface. Interlayer coupling and charge transport behaviour are investigated by Raman and photoluminescence spectroscopy. The photoelectron spectroscopy confirms type Ⅱ band alignment between MoS2-ReS2 interface, which is required for efficient separation and transportation of charge carriers. As a proof of concept, a highly sensitive, self-biased broadband photodetector is fabricated with a responsivity of 42.61 A/W at a low bias of 1 V under the illumination of 800 nm, which is 16 fold higher than the reference pristine MoS2 photodetector. Moreover, fast rise/decay transient photoresponse (20/19 ms) strongly advocate the spatial separation of charge carriers across the interface. Our proposed work establishes the MoS2 and ReS2 as promising candidates for next-generation broadband photodetector applications.

Two-dimensional layered materials have emerged prominently in the past decade, largely being investigated fundamentally and practically. Their unique layered structure and atomic-scale thickness make them attractive with exclusive electrical and optical properties compared to their bulk counterparts. Molybdenum disulfide (MoS2) is the most widely studied material in the family of transition metal dichalcogenides. The direct and variable bandgap, high carrier mobility, thermal and chemical stability makes it an attractive choice for next-generation photodetector applications. MoS2 heterojunction-based photodetectors offer ultrafast charge transfer and broadband photoresponse, adding more functionality beyond their individual counterparts. Enormous efforts have been devoted to adopting a new strategy that can improve photodetector performance in terms of responsivity and response time. This review briefly discusses the photo-induced current mechanism and performance parameters along with some important aspects to realize better device performance. Here, we critically review the current status and progress made towards MoS2-based photodetectors, followed by a discussion on open challenges and opportunities in their future application.

Here, we report the comparative study of enhanced second harmonic generation using defect engineering in pyramid-like MoS2 (P-MoS2) flakes to vertically aligned MoS2 (VA-MoS2) flakes. P-MoS2 and VA-MoS2 is synthesized via the modified chemical vapor deposition technique. The second harmonic generation measurements on P-MoS2 and VA-MoS2 are performed by sweeping the excitation wavelength from 1200 nm to 1310 nm in identical conditions. The P-MoS2 flakes show a high SHG signal. The high SHG signal in pyramid-like MoS2 is attributed to the broken inversion symmetry and high thickness of grown MoS2 flakes. VA-MoS2 flakes under the identical conditions show a 34% enhanced SHG signal in comparison to P-MoS2. The midgap states generated due to defects in the form of S vacancies in VA-MoS2 are responsible for this enhancement. These midgap states confine the photons and result in enhanced SHG properties. Our study will pave a new path to understand the role of 2D material morphology in fabricating versatile optical and photonics devices.

The p-n junction is an influential trait for efficient charge transport in electronic and optoelectronic devices. Demonstrating p-n junction using 2D TMDCs is inescapable to integrate these materials with matured 3D material technology, as 2D material offers easy integration due to the absence of dangling bonds. However, understanding the electronic properties at the interface is crucial for rational design of heterojunction devices for their novel and unmatched functionality. Present work reports detail interfacial study and band-alignment of two potential materials, 3D Cu2ZnSnS4(CZTS) and 2D-layered MoS2. The detail photoelectron spectroscopy measured the valance band offset (VBO) and conduction band offset (CBO) of −0.23 ± 0.1 eV and −0.29 ± 0.1 eV, respectively and established a favorable type-II band-alignment at CZTS/MoS2 interface. Significant PL quenching at CZTS/MoS2 interface proven to be strong evidence of efficient charge transport. As a proof of concept, a self-driven CZTS/MoS2 heterojunction broadband photodetector was constructed exhibiting pronounced photovoltaic features with high responsivity 141 mA/W, outstanding photoswitching capability (IOn/IOff = 112) and fast response (τr/τd = 81/79 ms). The responsivity was further enhanced to 79A/W at moderate bias (@6V). Additionally, the device showed exceptional stability after 1500 h of operation. This work intends to trigger the research on 3D/2D for high performing optoelectronic devices based on CZTS/MoS2 heterojunctions.

The increased usage of hydrogen as a next generation clean fuel strongly demands the parallel development of room temperature and low power hydrogen sensors for their safety operation. In this work, we report strong evidence for preferential hydrogen adsorption at edge-sites in an edge oriented vertically aligned 3-D network of MoS2 flakes at room temperature. The vertically aligned edge-oriented MoS2 flakes were synthesised by a modified CVD process on a SiO2/Si substrate and confirmed by Scanning Electron Microscopy. Raman spectroscopy and PL spectroscopy reveal the signature of few-layer MoS2 flakes in the sample. The sensor's performance was tested from room temperature to 150 °C for 1% hydrogen concentration. The device shows a fast response of 14.3 s even at room temperature. The sensitivity of the device strongly depends on temperature and increases from ∼1% to ∼11% as temperature increases. A detail hydrogen sensing mechanism was proposed based on the preferential hydrogen adsorption at MoS2 edge ...