Showing papers by "Mahesh Kumar published in 2018"

Ultrahigh Performance of Self-Powered β-Ga2O3 Thin Film Solar-Blind Photodetector Grown on Cost-Effective Si Substrate Using High-Temperature Seed Layer

Abstract: We demonstrated an ultrahigh-performance and self-powered β-Ga2O3 thin film solar-blind photodetector fabricated on a cost-effective Si substrate using a high-temperature seed layer (HSL). The polycrystalline β-Ga2O3 thin film deposited with HSL shows high performance in the solar-blind region in comparison to the amorphous Ga2O3 thin film deposited without HSL. The zero-bias digitizing sensor prototype with an HSL produces a digitized output bit with deep UV (DUV) light that exhibits a high on/off (I254 nm/Idark) ratio of >103, a record-low dark current of 1.43 pA, and high stability and reproducibility over 100 cycles even after >2100 h. The photodetector shows minimum persistent photoconductivity and fast response in milliseconds. The photodetector yields a responsivity of 96.13 A W–1 with an external quantum efficiency of 4.76 × 104 at 5 V for 250 nm monochromatic light. The photodetector shows a high response to even a rare weak signal of DUV (44 nW/cm2). These values are the highest reported to date...

Photoactivated Mixed In-Plane and Edge-Enriched p-Type MoS2 Flake-Based NO2 Sensor Working at Room Temperature.

Abstract: 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...

Improved Sensitivity with Low Limit of Detection of a Hydrogen Gas Sensor Based on rGO-Loaded Ni-Doped ZnO Nanostructures.

Abstract: We report enhanced hydrogen-gas-sensing performance of a Ni-doped ZnO sensor decorated with the optimum concentration of reduced graphene oxide (rGO). Ni-doped ZnO nanoplates were grown by radio frequency sputtering, rGO was synthesized by Hummer’s method and decorated by the drop cast method of various concentration of rGO (0–1.5 wt %). The current–voltage characteristics of the rGO-loaded sensor are highly influenced by the loading concentration of rGO, where current conduction decreases and sensor resistance increases as the rGO concentration is increased up to 0.75 wt % because of the formation of various Schottky heterojunctions at rGO/ZnO interfaces. With the combined effect of more active site availability and formation of various p–n heterojunctions due to the optimum loading concentration of rGO (0.75 wt %), the sensor shows the maximum sensing response of ∼63.8% for 100 ppm hydrogen at moderate operating temperature (150 °C). The rGO-loaded sensors were able to detect a minimum of 1 ppm hydrogen...

Efficient hydrogen sensor based on Ni-doped ZnO nanostructures by RF sputtering

Abstract: We demonstrate RF sputtered Ni-doped ZnO nanostructures for detection of extremely low concentration (1 ppm) of hydrogen gas at moderate operating temperature of 75 °C. Structural, morphological, electrical and hydrogen sensing behavior of the Ni-doped ZnO nanostructures strongly depends on doping concentration. Ni doping exceptionally enhances the sensing response and reduces the operating temperature of the sensor as compared to undoped ZnO. The major role of the Ni-doping is to create more active sites for chemisorbed oxygen on the surface of sensor and, correspondingly, to improve the sensing response. The 4 at% of Ni-doped ZnO exhibits the highest response (∼69%) for 1% H 2 at 150 °C, which are ∼1.5 times higher than for the undoped ZnO. This is ascribed to lowest activation energy ∼6.47 KJ/mol. Diminishing of the relative response was observed in 6% Ni- doped ZnO due to separation of NiO phase.

Growth of MoS2–MoO3 Hybrid Microflowers via Controlled Vapor Transport Process for Efficient Gas Sensing at Room Temperature

Abstract: DOI: 10.1002/admi.201800071 thin devices. From the past few years, transition metal dichalcogenides (TMDCs) materials analogous to graphene have received captivated attention because of their unconventional mechanical, electrical, physical, and structural properties.[1,2] MoS2, being the frontrunner of TMDC family, has opened up new avenues because of its tunable band gap, a great degree of flexibility, high surface to volume ratio, and its chemical and mechanical robustness.[3,4] Consequently, MoS2 has attained importance in the developement of transistor,[5] water splitting,[6] photodiode,[7] and sensing.[8] MoS2 has also been extensively used as a potential gas-sensing material because of its high selectivity, low detection limit, and having various reactive sites such as sulfur defects, edge sites, and vacancies.[9,10] Nonetheless, incomplete recovery and slow detection to gases at room temperature restrict to MoS2 for practical gas sensing. Moreover, a poor charge transport in MoS2 and an adverse effect of ambiental oxygen and humidity represent the MoS2 limitations for use in advanced applications and must be mitigated.[11,12] In recent years, hybrid MoS2 structures with different morphologies have received considerable attention due to their high sensing performance, exceptional optoelectronic relevance, and potential use in several other low-power applications. For instance, Yin et al. synthesized MoS2–MoO3 hybrid nanomaterial using lithium-exfoliation and as a proof-of-concept this hybrid nanomaterial was used as an active layer for light-emitting diodes.[13] Chen et al. synthesized a strain-gated field effect transistor (FET) hybrid structure consisting of 2D MoS2 flake and 1D ZnO nanowire.[14] A MoS2/SnO2 nanohybrids sensor was reported by Cui et al. for high-performance stable gas sensing in air. Here, the hole injection from SnO2 to MoS2 resulted in better stability of MoS2 toward ambiental oxygen.[15] Chen et al. synthesized a core–shell MoO3–MoS2 nanowires structure to drive a stable hydrogen evolution reaction through a highly efficient mechanism.[16] A partially reduced MoO3 has high conductivity and MoS2 has poor conductivity along particular crystallographic direction. So, MoO3 mitigate the the deficiencies of MoS2 after design a particular architecture. All these investigations depicts that the morphology of the hybrid structures crucially influence A nucleation controlled one-step process to synthesize MoS2–MoO3 hybrid microflowers using vapor transport process and its application in efficient NO2 sensing at room temperature are reported. The morphology and crystal structure of the microflowers are characterized by scanning electron microscope (SEM), Raman, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy techniques. A cathodoluminence mapping reveals that the core of the microflower consists of MoO3, and the flower petals as well as nanosheet are composed of a few layers of MoS2. Further, the MoS2–MoO3 hybrid microflower sensor exhibits a high sensitivity of ≈33.6% with a complete recovery to 10 ppm NO2 at room temperature without any extra stimulus like optical or thermal source. Unlike many earlier reports on MoS2 sensor, this advanced approach shows that the sensor is exhibited a low response time (≈19 s) with complete recovery at room tepmerature and excellent selectivity toward NO2 against various other gases. The efficient conventional sensing of the sensor is attributed to a combination of high hole injection from MoO3 to MoS2 and modulation of a potential barrier at MoS2–MoO3 interface during adsorption/ desorption of NO2. It is believed that the modified properties of MoS2 by such composite could be used for various advanced device applications. Room Temperature Sensors

High performance NO2 sensor using MoS2 nanowires network

Abstract: We report on a high-performance NO2 sensor based on a one dimensional MoS2 nanowire (NW) network The MoS2 NW network was synthesized using chemical transport reaction through controlled turbulent vapor flow The crystal structure and surface morphology of MoS2 NWs were confirmed by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy Further, the sensing behavior of the nanowires was investigated at different temperatures for various concentrations of NO2 and the sensor exhibited about 2-fold enhanced sensitivity with a low detection limit of 46 ppb for NO2 at 60 °C compared to sensitivity at room temperature Moreover, it showed a fast response (16 s) with complete recovery (172 s) at 60 °C, while sensitivity of the device was decreased at 120 °C The efficient sensing with reliable selectivity toward NO2 of the nanowires is attributed to a combination of abundant active edge sites along with a large surface area and tuning of the potential barrier

Highly selective and reversible NO2 gas sensor using vertically aligned MoS2 flake networks.

Abstract: We demonstrate a highly selective and reversible NO2 resistive gas sensor using vertically aligned MoS2 (VA-MoS2) flake networks. We synthesized horizontally and vertically aligned MoS2 flakes on SiO2/Si substrate using a kinetically controlled rapid growth CVD process. Uniformly interconnected MoS2 flakes and their orientation were confirmed by scanning electron microscopy, x-ray diffraction, Raman spectroscopy and x-ray photoelectron spectroscopy. The VA-MoS2 gas sensor showed two times higher response to NO2 compared to horizontally aligned MoS2 at room temperature. Moreover, the sensors exhibited a dramatically improved complete recovery upon NO2 exposure at its low optimum operating temperatures (100 °C). In addition, the sensing performance of the sensors was investigated with exposure to various gases such as NH3, CO2, H2, CH4 and H2S. It was observed that high response to gas directly correlates with the strong interaction of gas molecules on edge sites of the VA-MoS2. The VA-MoS2 gas sensor exhibited high response with good reversibility and selectivity towards NO2 as a result of the high aspect ratio as well as high adsorption energy on exposed edge sites.

Wafer-scale synthesis of a uniform film of few-layer MoS2 on GaN for 2D heterojunction ultraviolet photodetector

Abstract: Layered transition metal dichalcogenide materials grown over a conventional 3D semiconductor substrate have ignited a spark of interest in the electronics industry. The integration of these 2D layered materials extensively addresses the formidable challenges faced by a new generation of opto-electronic and photovoltaic devices. Herein, we have demonstrated a 2D/3D heterojunction type photodetector by depositing MoS2 on a GaN substrate with a mass-scalable sputtering method. Spectroscopic and microscopic characterizations expose the signature of the highly crystalline, homogeneous and controlled growth of a deposited few-layer MoS2 film. The greater light absorption of few-layer MoS2 results in the high performance of the MoS2/GaN photodetector. Our device shows high external spectral responsivity (~103 A W−1) and detectivity (~1011 Jones) with a very fast response time (~5 ms). Our obtained results are significantly better than previous MoS2- and GaN-based photodetectors. This work unveils a new perspective in MoS2/GaN heterojunctions for high-performance optoelectronic applications.

High-performance photodetector based on hybrid of MoS2 and reduced graphene oxide.

Abstract: 2D materials are a promising new class of materials for next generation optoelectronic devices owing to their appealing optical and electrical properties. Pristine molybdenum disulfide (MoS2) is widely used in next generation photovoltaic and optoelectronic devices, but its low photo-dark current ratio prevents its use in highly efficient photo detection applications. Here, we decorated crumpled reduced graphene oxide (rGO) particles on a large-area vertically aligned MoS2 flake network to enhance the performance of the MoS2-based photodetector by forming multiple nanoscale p–n heterojunctions. The rGO/MoS2 device exhibited a significantly improved photoresponsivity of ~2.10 A W−1 along with a good detectivity of ~5 × 1011 Jones (Jones = cm Hz1/2/W) compared to that of the pristine MoS2 photodetector in ambient atmosphere. Moreover, the rGO/MoS2 photodetector showed a fast response of ~18 ms with excellent stability and reproducibility in ambient air even after three months. The high performance of the photodetector is attributed to enhanced photoexcited carrier density and suppressed photo generated electron–hole recombination due to the strong local built-in electric field developed at the rGO/MoS2 interface. Our results showed that integration of rGO with MoS2 provides an efficient platform for photo detection applications.

Deep UV narrow-band photodetector based on ion beam synthesized indium oxide quantum dots in Al2O3 matrix

Abstract: Semiconductor quantum dots have attracted tremendous attention owing to their novel electrical and optical properties as a result of their size dependent quantum confinement effects. This provides the advantage of tunable wavelength detection, which is essential to realize spectrally selective photodetectors. We report on the fabrication and characterization of a high performance narrow band ultraviolet photodetector (UV-B) based on Indium oxide (In2O3) nanocrystals embedded in aluminium oxide (Al2O3) matrices. The In2O3 nanocrystals are synthesized in an Al2O3 matrix by sequential implantation of In+ and ions and post-implantation annealing. The photodetector exhibits excellent optoelectronic performances with high spectral responsivity and external quantum efficiency. The spectral response shows a band-selective nature with a full width half maximum of ~60 nm, and a responsivity reaching up to 70 A W−1 under 290 nm at 5 V bias. The corresponding rejection ratio to visible region was as high as 8400. The high performance of this photodetector makes it highly suitable for practical applications such as narrow-band spectrum-selective photodetectors. The device design based on ion-synthesized nanocrystals could provide a new approach for realizing a visible-blind photodetector.

Determination of band alignment at two-dimensional MoS2/Si van der Waals heterojunction

Abstract: To understand the different mechanism occurring at the MoS2-silicon interface, we have fabricated a MoS2/Si heterojunction by exfoliating MoS2 on top of the silicon substrate. Raman spectroscopy and atomic force microscopy (AFM) measurement expose the signature of few-layers in the deposited MoS2 flake. Herein, the temperature dependence of the energy barrier and carrier density at the MoS2/Si heterojunction has been extensively investigated. Furthermore, to study band alignment at the MoS2/Si interface, we have calculated a valence band offset of 0.66 ± 0.17 eV and a conduction band offset of 0.42 ± 0.17 eV using X-ray and Ultraviolet photoelectron spectroscopy. We determined a type-II band alignment at the interface which is very conducive for the transport of photoexcited carriers. As a proof-of-concept application, we extend our analysis of the photovoltaic behavior of the MoS2/Si heterojunction. This work provides not only a comparative study between MoS2/p-Si and MoS2/n-Si heterojunctions but also paves the way to engineer the properties of the interface for the future integration of MoS2 with silicon.

Enhanced Carrier Density in a MoS 2 /Si Heterojunction-Based Photodetector by Inverse Auger Process

Abstract: When exploring the charge transport in the MoS2/Si heterojunction, it is imperative to understand the contribution of different carrier interactions and scattering mechanisms in the conduction process. Here, we have demonstrated an ultrasensitive photodetector based on MoS2/Si (for both p-type and n-type Si) van der Walls (vdW) heterojunction, which provides a high photoresponsivity of greater than 103 A/W along with a large detectivity of ≈ 1012 Jones (Jones = $\text{cm} \cdot \text{Hz}^{ {1/2}} \cdot \text{W}^{-1}$ ). This high photoresponsivity is attributed to the high absorption rate of incident photons and large current carrying capacity of a few layer MoS2. The tuning of barrier height at the MoS2/n-Si interface with photoexcitation of different wavelengths was interpreted by using Bardeen’s model. The increase in carrier density due to the photon energy results in a reduction of barrier height at the heterojunction. Because of increase in carrier density with wavelength, it was observed that the inverse Auger process prevails at higher wavelength for the relaxation of photoinduced charge carriers at the MoS2/Si interface over other scattering mechanisms. Our obtained results unveil the great potential of the MoS2/Si vdW heterojunction that may pave the way for new generation photodetectors.

Enhanced Solar-Blind Photodetection Performance of Encapsulated Ga 2 O 3 Nanocrystals in Al 2 O 3 Matrix

Abstract: Gallium oxide nanocrystals encapsulated in Al2O3 film on a Si substrate have been synthesized by using ion implantation followed by thermal annealing at 800 °C or 900 °C. Formation of $\beta $ -Ga2O3 nanocrystals is confirmed by transmission electron microscopy. The photoresponse of a photodetector fabricated on the implanted layer by the deposition of interdigitated electrode pattern is estimated by analyzing the current–voltage characteristics. The ratio of photocurrent at the wavelength of 250 nm and a voltage of 4 V to dark current around ~15 is found for the sample annealed at 900 °C. The spectral dependence of photoresponse for this sample demonstrates excellent solar-blind ultraviolet characteristics in the wavelength range of 250–270 nm with a rejection ratio 42 at the bias of 1 V. At the bias of 4 V, the responsivity of 50 mA/ $\mu \text{W}$ upon illumination at 250 nm and a cutoff at 280 nm are observed for the 900 °C annealing. The sample annealed at 800 °C shows lower ultraviolet responsivity due to the high density of defects. The data are analyzed in relation to the generation of carriers in Ga2O3 nanocrystals and their transport in the damaged Al2O3 matrix. The reported results pave the way toward the fabrication of next generation high sensitive solar-blind photodetectors.

Enhanced sensing response with complete recovery of MoS2 sensor under photoexcitation

Abstract: Molybdenum disulfide (MoS2), a just three atoms thick semiconductor has opened up new avenues in gas sensing because of its high surface–to-volume ratio. However, the low sensitivity, incomplete recovery and poor selectivity at room temperature restricts its uses for practical gas sensors. Here, we report a facile strategy to develop highly sensitive, extremely selective and fully recoverable gas sensor at room temperature through photo excitation. Although, pristine MoS2 gas sensor showed a considerable response and incomplete recovery to NO2 at room temperature. Though after photo excitation, the MoS2 sensor exhibited an improved response along with the complete recovery upon NO2 exposure.

Highly sensitive H2 gas sensor of Co doped ZnO nanostructures

Abstract: In this report, the hydrogen gas sensing properties based on Co doped ZnO nanostructures are explored. The undoped and Co doped nanostructures were grown by RF magnetron sputtering system, and its structural, morphological, and hydrogen sensing behavior are investigated. The maximum relative response was occurred by the 2.5% Co doped ZnO nanostructures among undoped and other doped sensors. The enhancement of relative response might be due to large chemisorbed sites formation on the ZnO surface for the reaction to hydrogen gas.

NO2 sensing at room temperature using vertically aligned MoS2 flakes network

Abstract: To exploit the role of alignment of MoS2 flake in chemical sensing, here, we have synthesized the horizontally and vertically aligned MoS2 flake network using conventional chemical vapor deposition technique. The morphology and number of layers were confirmed by SEM and Raman spectroscopy, respectively. The sensing performance of horizontally aligned and vertically aligned flake network was investigated to NO2 at room temperature. Vertically aligned MoS2 based sensor showed higher sensitivity ~51.54 % and ~63.2 % compared to horizontally aligned MoS2 sensor’ sensitivity of ~35.32 % and ~45.2 % to 50 ppm and 100 ppm NO2, respectively. This high sensitivity attributed to the high aspect ratio and high adsorption energy on the edge site of vertically aligned MoS2.To exploit the role of alignment of MoS2 flake in chemical sensing, here, we have synthesized the horizontally and vertically aligned MoS2 flake network using conventional chemical vapor deposition technique. The morphology and number of layers were confirmed by SEM and Raman spectroscopy, respectively. The sensing performance of horizontally aligned and vertically aligned flake network was investigated to NO2 at room temperature. Vertically aligned MoS2 based sensor showed higher sensitivity ~51.54 % and ~63.2 % compared to horizontally aligned MoS2 sensor’ sensitivity of ~35.32 % and ~45.2 % to 50 ppm and 100 ppm NO2, respectively. This high sensitivity attributed to the high aspect ratio and high adsorption energy on the edge site of vertically aligned MoS2.

High-performance ultraviolet detector employing out-of-plane rGO/MoS 2 PN heterostructure

Abstract: We demonstrated an ultraviolet detector employing an in-plane transport channel of n-type MoS 2 with out-of-plane p-type rGO, which acts as a sensitizer for underlying n-type MoS 2 photodetector. A developed vertical built-in field from vertical p-n nano-heterojunction separates the photo-excited carriers at the rGO/MoS 2 interface. Therefore, the rGO/MoS 2 device showed a notably improved photo-responsivity of $\sim$ 6.92 A)/W and an excellent detectivity of 1.26 $\times$ 1012 Jones under the irradiation of ultraviolet light. Moreover, the device exhibited an excellent reproducibility and stability in ambient environment even after four months.