Showing papers by "Mahesh Kumar published in 2019"

Graphene Oxide–Silver Nanowire Nanocompositesfor Enhanced Sensing of Hg 2+

Abstract: We demonstrate a highly sensitive and selective sensing platform for the electrochemical detection of Hg2+ in aqueous media. A graphene oxide (GO) and silver nanowire (AgNW) nanocomposites modified...

A high-performance hydrogen sensor based on a reverse-biased MoS 2/GaN heterojunction

Abstract: We report a MoS2/GaN heterojunction-based gas sensor by depositing MoS2 over a GaN substrate via a highly controllable and scalable sputtering technique coupled with a post sulfurization process in a sulfur-rich environment. The microscopic and spectroscopic measurements expose the presence of highly crystalline and homogenous few atomic layer MoS2 on top of molecular beam epitaxially grown GaN film. Upon hydrogen exposure, the molecular adsorption tuned the barrier height at the MoS2/GaN interface under the reverse biased condition, thus resulting in high sensitivity. Our results reveal that temperature strongly affects the sensitivity of the device and it increases from 21% to 157% for 1% hydrogen with an increase in temperature (25-150 °C). For a deeper understanding of carrier dynamics at the heterointerface, we visualized the band alignment across the MoS2/GaN heterojunction having valence band and conduction band offset values of 1.75 and 0.28 eV. The sensing mechanism was demonstrated based on an energy band diagram at the MoS2/GaN interface in the presence and absence of hydrogen exposure. The proposed methodology can be readily applied to other combinations of heterostructures for sensing different gas analytes.

MPA-GSH Functionalized AlGaN/GaN High-Electron Mobility Transistor-Based Sensor for Cadmium Ion Detection

Abstract: This paper demonstrates a novel AlGaN/GaN high-electron mobility transistor (HEMT)-based cadmium ion (Cd2+) sensor with mercaptopropionic acid (MPA) and glutathione (GSH) functionalization. The sensing response of the sensor was analyzed by detecting Cd2+ ions at different concentrations. The AlGaN/GaN HEMT sensor exhibits excellent response with the sensitivity of $0.241~\mu \text{A}$ /ppb, a fast response time of ~ 3 s, and a lower detection limit of 0.255 ppb. The observed lower detection limit is significantly lower than the World Health Organization (WHO) standard recommended limit for Cd2+ ions in drinking water. Furthermore, the sensor showed good selectivity of Cd2+ ions toward other heavy metal ions. The results indicate that the binding properties of GSH to Cd and the sensitivity of 2-D electron gas toward the variation of charges at the gate region make the device highly sensitive with rapid detection of Cd2+ ions.

PAN/(PAN-b-PMMA) derived nanoporous carbon nanofibers loaded on ZnO nanostructures for hydrogen detection

Abstract: The present report investigates hydrogen gas sensing properties on PAN/(PAN-b-PMMA) derived electrospun nanoporous carbon nanofibers loaded on ZnO nanostructures. Polymer blend of PAN and PAN-b-PMMA is used to obtain the high surface area porous CNF which improve further sensing performance. Moreover, a facile technique such as drop cast method is used to load the CNF (0.1–0.5 wt%) on Au pattern interdigitated electrodes over ZnO nanostructures. Loading of CNF was optimized to ensure proper connection between the electrodes over ZnO surface, which was measured using I–V characteristics. Initially, current decreased for less amount of CNF (0.1–0.2 wt%), which later increased for high concentration of CNF (0.3–0.5 wt%). It was further observed that 0.2 wt% CNF/ZnO nanostructures based sensor exhibited maximum sensing response (73.54%) as compared to CNF (3.29%) and ZnO (44.51%) for 100 ppm hydrogen at 150 °C. This enhanced sensing response may be attributed to diffusion of hydrogen molecules through the nanoporous CNF, thus enabling the formation of p - n heterojunction at the interface of CNF and ZnO. The presence of oxygen functional groups on CNF surface also contributes to the enhancement of the sensing performance.

NO2 gas sensing performance enhancement based on reduced graphene oxide decorated V2O5 thin films.

Abstract: Here, we demonstrate improved NO2 gas sensing properties based on reduced graphene oxide (rGO) decorated V2O5 thin film. Excluding the DC sputtering grown V2O5 thin film, rGO was spread over V2O5 thin film by the drop cast method. The formation of several p-n heterojunctions was greatly affected by the current-voltage relation of the rGO-decorated V2O5 thin film due to the p-type and n-type nature of rGO and V2O5, respectively. Initially with rGO decoration on V2O5 thin film, current decreased in comparison to the pristine V2O5 thin film, whereas depositing rGO film on a glass substrate drastically increased current. Among all sensors, only the rGO-decorated V2O5 sensor revealed a maximum NO2 gas sensing response for 100 ppm at 150 °C, and it achieved an approximately 61% higher response than the V2O5 sensor. The elaborate mechanism for an extremely high sensing response is attributed to the formation and modulation of p-n heterojunctions at the interface of rGO and V2O5. In addition, the presence of active sites like oxygenous functional groups on the rGO surface enhanced the sensing response. On that account, sensors based on rGO-decorated V2O5 thin film are highly suitable for the purpose of NO2 gas sensing. They enable the timely detection of the gas, further protecting the ecosystem from its harmful effects.

Boosting Sensing Performance of Vacancy-Containing Vertically Aligned MoS 2 Using rGO Particles

Abstract: 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.

Sensitive and Selective Detection of Pb 2+ Ions Using 2,5-Dimercapto-1,3,4-Thiadiazole Functionalized AlGaN/GaN High Electron Mobility Transistor

Abstract: We report sensitive and selective AlGaN/GaN High Electron Mobility Transistor (HEMT)-based sensor for Lead ion (Pb2+) detection. The gate region of the HEMT was functionalized by 2,5-dimercapto-1,3,4-thiadiazole (DMTD). The response of the sensor is observed by monitoring drain to source current ( $\text{I}_{\textsf {DS}}$ ) for different concentrations of Pb2+ ions at a fixed drain to source voltage ( $\text{V}_{\textsf {DS}}$ ). Our sensor reaches the lower detection limit of 0.018 ppb, which is much lower than the standard detection limit recommended by the World Health Organization (WHO) for drinking water. Furthermore, the sensor exhibited a rapid response time of ~4 seconds and high sensitivity of $0.607~\mu \text{A}$ /ppb. Moreover, the selectivity analysis was performed and found that the sensor was highly selective towards Pb2+ ions. The change in electron concentration at 2-dimensional electron gas (2DEG) upon the capture of Pb2+ ions at gate region by DMTD, causes a change in the $\text{I}_{\textsf {DS}}$ , which showed excellent sensing response towards Pb2+ ions. The highly sensitive, selective, and rapid detection of Pb2+ ions paves the way for stable sensing performance based on DMTD functionalized AlGaN/GaN HEMT sensor.

MoS 2 quantum dots-modified porous β-Bi 2 O 3 microspheres with enhanced visible-light-induced photocatalytic activity for Bisphenol A degradation and NO removal

Abstract: In this study, a three dimensional (3D) MoS2/β-Bi2O3 heterostructure with high-performance for photocatalytic removal of pollutants was constructed by a simple deposition of the MoS2 quantum dots on the surface of porous β-Bi2O3 spheres. Compared with pure β-Bi2O3 microspheres, the 1.0% MoS2/β-Bi2O3 composite exhibited excellent photocatalytic activity in the degradation of Bisphenol A (BPA) in aqueous solution and oxidative removal of gaseous NO under visible light irradiation, and the removal efficiency reached 94.0% and 41.4% for BPA and NO within 30 min, respectively. The enhanced photocatalytic activity is attributed to the formation heterojunction between MoS2 quantum dots and β-Bi2O3, which facilitates the separation and transfer efficiency of photogenerated electron–hole pairs, boosts catalytic active sites and hinders an electron–hole recombination rate. The results of trapping experiments indicates that both of O2−∙ and h+ are the reactive species for BPA degradation. The recyclability test confirms that the 1.0% MoS2/β-Bi2O3 composite photocatalyst has a superior stability during recycling cycle. This work demonstrates that a unique 3D photocatalyst system can provides a new perspective and opens up new opportunities on the design of efficient composite photocatalysts.

Spatial variability of available soil nutrients in the Shekhawati region of Thar Desert, India.

Abstract: Based on soil samples collected from 814 geo-coded sites in the rainfed croplands, irrigated croplands, rangelands and protected forests within the Shekhawati region of Thar Desert, Rajasthan, India, a spatial analysis of the distribution pattern of organic carbon (OC) and macronutrients i.e., available phosphorus (P) and available potassium (K) and micronutrients viz., DTPA extractable zinc (Zn), copper (Cu), iron (Fe) and manganese (Mn) was carried out in GIS environment. It revealed that OC is deficient almost throughout the region, while P is deficient in large parts of the dune-covered west. Among others, K, Cu and Mn are adequately supplied in most areas, but Zn and Fe are inadequate in large parts. Irrigated croplands are better endowed than other land uses in respect of OC, P, Zn and Cu; forests in respect of K and Fe, and rainfed croplands in respect of Mn. The distribution pattern of the organic C and nutrients appears to be controlled by both natural and anthropogenic processes. Among the natural processes, fluvial processes appear to control the distribution in the hills and plains in the wetter east, while aeolian processes appear to control the distribution in the drier dune-covered west. Anthropogenic processes, especially through land use decisions and fertilizers input in irrigated tracts, have also influenced the sufficiency or otherwise of the soil nutrients.

Ultrafast Spectroscopy of Bi2Se3 Topological Insulator

Abstract: We investigate the ultrafast transient absorption spectrum of Bi2Se3 topological insulator. Bi2Se3 single crystal is grown through conventional solid-state reaction routevia self-flux method. The structural properties have been studied in terms of high-resolution Powder X-ray Diffraction (PXRD). Detailed Rietveld analysis of PXRD of the crystal showed that sample is crystallized in the rhombohedral crystal structure with a space group of R-3m, and the lattice parameters are a=b=4.14A and c=28.7010A. Scanning Electron Microscopy (SEM) result shows perfectly crystalline structure with layered type morphology which evidenced from surface XRD. Energy Dispersive Spectroscopy (EDS) analysis determined quantitative amounts of the constituent atoms, found to be very close to their stoichiometric ratio. Further the fluence dependent nonlinear behaviour is studied by means of ultrafast transient absorption spectroscopy. The ultrafast spectroscopy also predicts the capability of this single crystal to generate Terahertz (THz) radiations (T-rays).

Scalable Growth of High-Quality MoS 2 Film by Magnetron Sputtering: Application for NO 2 Gas Sensing

Abstract: To address the formidable challenges associated with large-scale and high-quality fabrication of MoS 2 , we reported a two-step process for growing large-scalable MoS 2 films with superb quality. Firstly, we grow different thickness Mo films on SiO 2 /Si substrates by DC magnetron sputtering technique. Secondly, the deposited film was then sulfurized in a sulfur-rich environment. The qualitative analysis confirmed the growth of highly crystalline and continuous few-layer to multi-layer MoS2 films. We have also examined the sensing performance of deposited ML-MoS 2 film upon NO 2 exposure.

Growth of Different Microstructure of MoS 2 through Controlled Processing Parameters of Chemical Vapor Deposition Method

Abstract: 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 MoS 2 via chemical vapor deposition (CVD) method through controlled processing parameters is reported. Scanning electron microscopy and Raman spectroscopy ascertained the MoS 2 on insulating substrate (SiO 2 /Si). It was observed that poor sulfur environment and slow vapor flow were unable to induce complete transition from MoO 3 -x to MoS 2 and formed intermediate MoO 2 .The MoS 2 and MoO 2 /MoS 2 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 MoS 2 was achieved.

Growth of Large-Scale α-MoO 3 on SiO 2 and Its Uses for Efficient Hydrogen Sensing Application

Abstract: MoO 3 is a well-known multifunctional material which can be potentially used in a distinct gas-sensing applications. In this study, we grow the large-scale nanonetwork composed of α-MoO 3 nanosheets by using conventional chemical vapor deposition method. The microscopic and spectroscopic characterizations confirm the continuity and crystallinity of the deposited MoO 3 film. The dynamic response resistance of the device reveals a high sensitivity of 53% for 1% hydrogen at 180 °C. A detailed gas-sensing mechanism was explained by the depletion-layer modulation process.