Kumar, Mahesh

We report significantly enhanced sensitivity of AlGaN/GaN-based high electron mobility transistor (HEMT) sensor by the targeted synthesis of IT and 2H coexisting phase MoS2 and applying the gate bias voltage. The HEMT structures on Si (111) substrates were used for the detection of Hg2+ ions. The optimum sensitive regime in terms of V GS and V DS of the sensor was investigated by keeping the drain source voltage V DS constant at 2 V and by only varying the gate bias voltage V GS from 0 to 3 V. The strongest sensing response obtained from the device was around 0.547 mA ppb-1 at V GS = 3 V, which is 63.7% higher in comparison to the response achieved at 0 V which shows a sensing response of around 0.334 mA ppb-1. The current response depicts that the fabricated device is very sensitive and selective towards Hg2+ ions. Moreover, the detection limit of our sensor at 3 V was calculated around 6.21 ppt, which attributes to the strong field created between the gate electrode and the HEMT channel due to the presence of 1T metallic phase in synthesized MoS2, indicating that the lower detection limits are achievable in adequate strong fields.

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.

The anisotropic bonding in layered materials crystallize to form different structure such as smooth films, nanotubes, and fullerene-like nanoparticles. Here, the growth of different microstructure of MoS2 via chemical vapor deposition (CVD) method through controlled processing parameters is reported. Scanning electron microscopy and Raman spectroscopy ascertained the MoS2 on insulating substrate (SiO2/Si). It was observed that poor sulfur environment and slow vapor flow were unable to induce complete transition from MoO3-x to MoS2 and formed intermediate MoO2.The MoS2 and MoO2/MoS2 heterostructure were synthesized via single step. In addition, by adjustment of heating rate with temperature of centre zone and vapor flow, flower like structure of MoS2 was achieved.

Nanostructured zinc oxide sensing film was deposited on the Si/SiO2/Pt substrate by the RF magnetron sputtering process. The film was characterized by FESEM (field-emission scanning electron microscope) and XRD (X-ray diffraction) for their morphology and structural analysis. The FESEM results show that the film morphology is in nanophase with an average nanostructure size of ~ 50 nm. XRD results show that the film is polycrystalline. The AFM (atomic force microscopy) and Raman spectroscopy were done to analyze the surface roughness and the structural properties of the film, respectively. FTIR (Fourier-transform infrared spectroscopy) was used to analyze the presence of ZnO. Further, the ZnO nanostructure film has been explored for pH sensing for pH (4–12). The sensitivity of the film was found to be 31.81 mV/pH. The drift characteristics of the film were also done to find out the stability of the film.

Excitonic quasi-particles, excitons/trions/bi-excitons, and their coupling with phonons and charge carriers play a crucial role in controlling the optical properties of atomically thin semiconducting 2D materials. In this study, we revealed the dynamics of excitons/trions and their coupling with phonons and charge carriers in few-layer vertically and horizontally aligned MoS2. We observed the trion signature up to the highest recorded temperature (330 K) in both systems and have shown that the dynamics of excitons/trions and their coupling with phonons and electrons are more affected in vertically aligned MoS2. Homogeneous linewidth broadening was observed with an increase in temperature, which is attributed mainly to acoustic phonons in the low-temperature regime (<100 K). In contrast, acoustic and longitudinal optical phonon contributions to linewidth broadening were observed at high temperature. We also observed the significant effects of interlayer coupling in both systems by understanding their temperature-dependent valence band splitting and trion binding energy. A decrease of ∼22% and 12% in valence band splitting with an increase in temperature was observed for the vertically and horizontally aligned MoS2, respectively, suggesting that the valence band splitting is affected more in the case of vertically than horizontally aligned MoS2. Furthermore, we also observed significant thermal quenching in the intensity of the trion band compared to that of exciton bands, which is attributed to the small binding energy of the trions.

Agricultural cycle is the annual cycle of activities for planting and harvesting. The global warming and climate change currently cause serious impacts on the agricultural cycle leading to reduced crop quantity and quality. In the past decade, the gas sensors and electronic nose (E-nose) technologies have shown great promise and utility in monitoring and prediction of important parameters related to the growth and harvest of a crop. This review summarizes overall the evolution of utilizing gas sensors and E-nose technologies for guideline of agricultural best management practices including applications on cultivation preparation, crop production, harvesting and storage of crops. Advantages and limitations of these technologies in agricultural applications are reviewed to highlight potential research directions.

Two important methods for enhancing gas sensing performance are vacancy/defect and interlayer engineering. Tin sulfide (SnS2) has recently attracted much attention for sensing of the NO2 gas due to its active surface sites and tunable electronic structure. Herein, SnS2 has been synthesized by the chemical vapor deposition (CVD) method followed by nitrogen plasma treatment with different exposure times for fast detection of NO2 molecules. Plasma treatment created a substantial number of surface vacancies on SnS2 flakes, which were controlled by the exposure period to modify the surface of flakes. After 12 min of nitrogen plasma treatment, SnS2 nanoflakes show considerable improvement in NO2 sensing characteristics, including a high sensing response of ∼264% toward 100 ppm NO2 at 120°C. The enhancement in the relative response of the sensor is due to the electronic interaction between NO2 molecules and the S vacancies on the surface of SnS2. Density functional theory (DFT) computations indicate that the S-vacancy defects on the surface dominate the effective NO2 detection and the NO2 adsorption mechanism transition from physisorption to chemisorption. Adsorption kinetics of the NO2 gas over SnS2 nanoflake-based chemiresistor sensors were studied using the Lee and Strano model [ Langmuir 2005, 21(11), 5192−5196 ]. The irreversible rate of the reaction for various NO2 concentrations exposed to the gas sensor is extracted using this model, which also appropriately describes the response curves. The forward rate constant of the irreversible gas sensor increased with the increase of the N2 plasma treatment time and reached the maximum in the 12 min plasma-treated sample. Through defect engineering, this research may open up new vistas for the design and synthesis of 2D materials with enhanced sensing properties.

Two-dimensional materials have gained considerable attention in chemical sensing owing to their naturally high surface-to-volume ratio. However, the poor response time and incomplete recovery at room temperature restrict their application in high-performance practical gas sensors. Herein, we demonstrate ultrafast detection and reversible MoS2 gas sensor at room temperature. The sensor's performance is investigated to NO2 at room temperature, under thermal and photo energy. Incomplete recovery and high response time of ∼249 s of sensor are observed at room temperature. Thermal energy is enough to complete recovery, but it is at the expense of sensitivity. Further, under photo excitation, MoS2 exhibits an enhancement in sensitivity with ultrafast response time of ∼29 s and excellent recovery to NO2 (100 ppm) at room temperature. This significant improvement in sensitivity (∼30%) and response time (∼88%) is attributed to the charge perturbation on the surface of the sensing layer in the context of NO2/MoS2 interaction under optical illumination. Moreover, the sensor shows reliable selectivity toward NO2 against various other gases. These unprecedented results reveal the potential of 2D MoS2 to develop a low power portable gas sensor.

Over the past few years, two-dimensional transition metal dichalcogenides (2D-TMDC) have attracted huge attention due to their high mobility, high absorbance, and high performance in generating excitons (electron and hole pairs). Especially, 2D molybdenum disulfide (MoS2) has been extensively used in optoelectronic and photovoltaic applications. Due to the low photo-to-dark current ratio (Iphoto/dark) and low speed, pristine MoS2-based devices are unsuitable for these applications. So, they need some improvements, i.e., by adding layers or decorating with materials of complementary majority charges. In this work, we decorated pristine MoS2 with reduced graphene oxide (rGO) and got improved dark current, Iphoto/dark, and response time. When we compared the performance of pristine MoS2 based device and rGO decorated MoS2 based device, the rGO/MoS2-based device showed an improved performance of responsivity of 3.36 A W−1, along with an Iphoto/dark of about 154. The heterojunction device exhibited a detectivity of 4.75 × 1012 Jones, along with a very low response time of 0.184 ms. The stability is also outstanding having the same device performance even after six months.

Heavy metal ions are highly toxic, carcinogens, and non-biodegradable in nature and pollute most water resources that lead to severe health-related issues. It is essential to develop highly sensitive, selective, rapid, and accurate approaches for their detection in water. Semiconducting devices and materials with micro and nanostructures have been featured with fast response time, low power, high sensitivity, low detection limit. This review concisely introduces the recent trends in heavy metal ion sensing with semiconductor devices, including ion-sensitive field-effect transistors (ISFETs) and AlGaN/GaN high electron mobility transistors (HEMTs) and semiconductor materials like graphene, two-dimensional metal dichalcogenides, decorated with different nanoparticles with appropriate functionalization.