A high-performance hydrogen sensor based on a reverse-biased MoS2/GaN heterojunction.

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.

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

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.

Quantum Phase Transition in Few-Layer NbSe_{2} Probed through Quantized Conductance Fluctuations.

We present the first observation of dynamically modulated quantum phase transition between two distinct charge density wave (CDW) phases in two-dimensional 2H-NbSe_{2}. There is recent spectroscopic evidence for the presence of these two quantum phases, but its evidence in bulk measurements remained elusive. We studied suspended, ultrathin 2H-NbSe_{2} devices fabricated on piezoelectric substrates-with tunable flakes thickness, disorder level, and strain. We find a surprising evolution of the conductance fluctuation spectra across the CDW temperature: the conductance fluctuates between two precise values, separated by a quantum of conductance. These quantized fluctuations disappear for disordered and on-substrate devices. With the help of mean-field calculations, these observations can be explained as to arise from dynamical phase transition between the two CDW states. To affirm this idea, we vary the lateral strain across the device via piezoelectric medium and map out the phase diagram near the quantum critical point. The results resolve a long-standing mystery of the anomalously large spectroscopic gap in NbSe_{2}.

Effect of Illumination Intensities on the Visible and Infrared Photoresponse of Cu2SnS3 Nanostructures.

We report the synthesis of Cu(2)SnS(3) nanostructures using solvothermal technique and the study of its visible and infrared (IR) photoresponse under different illumination intensities. The CTS nano-crystals were found to have tetragonal crystal structure using X-ray Diffraction (XRD). Both flower and sphere shaped structures of around 1.5 µm were obtained as seen using scanning electron microscopy (SEM). Transmission electron microscopy (TEM) was used to study the crystalline nature as well as the different planes present in the crystal. The band gap of the obtained crystals was found to be 1.4 eV using optical studies. The visible photocurrent increased from 0.25 µA at dark to 0.42 µA at 1.05 suns and 1 V applied bias. The sensitivity increased from 1.25 at 0.88 suns to 1.68 at 1.05 suns. The IR photocurrent increased from 0.13 µA at dark to 0.66 µA at 477.7 mWcm(−2). The device exhibited an increase in the sensitivity, responsivity, external quantum efficiency and specific detectivity from 1.23, 0.10 mA/W, 0.016% and 5.02 × 10(8) Jones respectively at 127.4 mWcm(−2 ) to 4.95, 0.46 mA/W, 0.071% and 2.22×10(9) Jones respectively at 477.7 mWcm(−2). The time response of the photocurrent was measured over different ON-OFF cycles and the cyclic stability of the device was verified.

Solution-Cast Photoconductive Photodetectors Based on CuInSe2 Nanoparticles.

We, herein, report an eco-friendly low temperature route for the gram-scale synthesis of copper indium selenide nanoparticles. We have also shown the possibility of using CuInSe2 nanoparticles in infrared photodetection by maneuvering the photoconductive property. We rationalize the long-lived trap states to be the cause for the observed photoconductive gain. It is worth noting that the photoresponse time of the device was found to be faster than 0.1 s.

Tailoring the band gap and transport properties of Cu3BiS3 nanopowders for photodetector applications.

We report a facile route to synthesize high quality earth abundant absorber Cu3BiS3, tailoring the band gap with the morphology manipulation and thereby analyzed the secondary phases and their role in the transport property. The sample at 48 hours reaction profile showed good semiconducting behavior, whereas other samples showed mostly a metallic behavior. Band gap was varied from 1.86 eV to 1.42 eV upon controling the reaction profile from 8 hours to 48 hours. The activation energy was calculated to be 0.102 eV. The temperature coefficient of resistance (TCR) was found to be 0.03432 K(-1) at 185 K. The IR photodectection properties in terms of photoresponse have been demonstrated. The high internal gain (G = 3.7 x 10(4)), responsivity (R = 3.2 x 10(4) A W(-1)) for 50 mW cm(-2) at 5 V make Cu3BiS3, an alternative potential absorber in meliorating the technological applications as near IR photodetectors.

Spectroscopic studies of In2O3 nanostructures; photovoltaic demonstration of In2O3/p-Si heterojunction.

The thermal oxidation process of the indium nitride (InN) nanorods (NRs) was studied. The SEM studies reveal that the cracked and burst mechanism for the formation of indium oxide (In2O3) nanostructures by oxidizing the InN NRs at higher temperatures. XRD results confirm the bcc crystal structure of the as prepared In2O3 nanostructures. Strong and broad photoluminescence spectrum located at the green to red region with maximum intensity at 566 nm along with a weak ultraviolet emission at 338 nm were observed due to oxygen vacancy levels and free excitonic transitions, respectively. The valence band onset energy of 2.1 eV was observed from the XPS valence band spectrum, clearly justifies the alignment of Fermi level to the donor level created due to the presence of oxygen vacancies which were observed in the PL spectrum. The elemental ratio In:O in as prepared In2O3 was found to be 42:58 which is in close agreement with the stoichiometric value of 40:60. A downward shift was observed in the Raman peak positions due to a possible phonon confinement effect in the nanoparticles formed in bursting mechanism. Such single junction devices exhibit promising photovoltaic performance with fill factor and conversion efficiency of 21% and 0.2%, respectively, under concentrated AM1.5 illumination.

Self-powered, ultrasensitive, room temperature humidity sensors using SnS2 nanofilms.

Humidity monitoring has become extremely vital in various technological fields such as environment control, biomedical engineering, and so on. Therefore, a substantial interest lies in the development of fast and highly sensitive devices with high figures of merit. Self-powered and ultrasensitive humidity sensors based on SnS2 nanofilms of different film thicknesses have been demonstrated in this work. The sensing behavior has been investigated in the relative humidity (RH) range of 2-99%. The observed results reveal a remarkable response and ultrafast detection even with zero applied bias (self-powered mode), with response and recovery times of ~ 10 and ~ 0.7 s, respectively. The self-powered behavior has been attributed to the inhomogeneities and the asymmetry in the contact electrodes. The highest sensitivity of ~ 5.64 × 106% can be achieved at an applied bias of 5 V. This approach of fabricating such highly responsive, self-powered and ultrafast sensors with simple device architectures will be useful for designing futuristic sensing devices.

Negative differential resistance in GaN nanocrystals above room temperature.

Negative differential resistance (NDR) has been observed for the first time above room temperature in gallium nitride nanocrystals synthesized by a simple chemical route. Current-voltage characteristics have been used to investigate this effect through a metal-semiconductor-metal (M-S-M) configuration on SiO2. The NDR effect is reversible and reproducible through many cycles. The threshold voltage is approximately 7 V above room temperature.

Synthesis of one-dimensional ZnO nanostructures from Zn powder/granule.

We report the growth of one-dimensional ZnO nanostructures with different morphologies such as nanoneedles, nanorods, nanobelts from Zn powder/granule. The growth process is different from the conventional vapor-solid mechanism. The advantage of this method is that neither a catalyst nor any gas flow is required for the synthesis of nanostructures. Depending upon the Zn powder or Zn granules as the starting material different nanostructures have been synthesized which demonstrates the versatility of the technique.

Synthesis, structural characterization and formation mechanism of ferroelectric bismuth vanadate nanotubes.

We report the synthesis and structural characterization of ferroelectric bismuth vanadate (Bi2VO5.5) (BVO) nanotubes within the nanoporous anodic aluminum oxide (AAO) templates via sol-gel method. The as-prepared BVO nanotubes were characterized by X-ray powder diffraction (XRD), Scanning Electron Microscope (SEM), High-Resolution Transmission Electron Microscope (HRTEM) and the stoichiometry of the nanotubes was established by energy-dispersive X-ray spectroscopy (EDX). Postannealed (675 degrees C for 1 h), BVO nanotubes were a polycrystalline and the XRD studies confirmed the crystal structure to be orthorhombic. The uniformity in diameter and length of the nanotubes as reveled by the TEM and SEM suggested that these were influenced to a guest extent by the thickness and pore diameter of the nanoporous AAO template. EDX analysis demonstrated the formation of stoichiometric Bi2VO5.5 phase. HRTEM confirmed that the obtained BVO nanotubes were made up of nanoparticles of 5-9 nm range. The possible formation mechanism of nanotubes was elucidated.

Self-Powered, Broad Band, and Ultrafast InGaN-Based Photodetector.

A self-powered, broad band and ultrafast photodetector based on n+-InGaN/AlN/n-Si(111) heterostructure is demonstrated. Si-doped (n+ type) InGaN epilayer was grown by plasma-assisted molecular beam epitaxy on a 100 nm thick AlN template on an n-type Si(111) substrate. The n+-InGaN/AlN/n-Si(111) devices exhibit excellent self-powered photoresponse under UV-visible (300-800 nm) light illumination. The maximum response of this self-powered photodetector is observed at 580 nm for low-intensity irradiance (0.1 mW/cm2), owing to the deep donor states present near the InGaN/AlN interface. It shows a responsivity of 9.64 A/W with rise and fall times of 19.9 and 21.4 µs, respectively. A relation between the open circuit voltage and the responsivity has been realized.

Lead zirconate nanotubes: synthesis, structural characterization and growth mechanism.

This paper reports an experimental study on the synthesis and structural characterization of lead zirconate, PbZrO3, (PZ) nanotubes fabricated by an improved sol-gel method within the nanochannels of anodic aluminum oxide (AAO) templates. The morphology, structure, and composition of the nanotubes were characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), selected-area electron diffraction (SAED), high resolution TEM (HRTEM) and energy-dispersive X-ray spectroscopy (EDX). The results of XRD and SAED indicated that postannealed (700 degrees C for 1 h) PZ nanotubes had an orthorhombic distorted perovskite polycrystalline crystal. SEM and TEM analysis revealed that PZ nanotubes possessed a uniform length and diameter, which were controlled by the thickness and the pore diameter of the applied AAO template, respectively and the thickness of the wall of the PZ nanotubes was about 15 nm. Y-junctions were also observed in the PZ nanotubes. EDX analysis demonstrated that stoichiometric PbZrO3 was formed. HRTEM image and a typical SEM image confirmed that the obtained PZ nanotubes composed of nanoparticles in the range of 4-8 nm. The possible formation mechanism of nanotubes was discussed.

Fabrication, structural characterization and formation mechanism of multiferroic BiFeO3 nanotubes.

Multiferroic BiFeO3 (BFO) nanotubes have been successfully fabricated by the modified sol-gel method within the nanochannels of porous anodic aluminum oxide (AAO) templates. The morphology, structure and composition of the nanotubes were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), selected-area electron diffraction (SAED), high resolution TEM, (HRTEM) and energy-dispersive X-ray spectroscopy (EDX). Postannealed (650 degrees C for 1 h), BFO nanotubes were polycrystalline and X-ray diffraction study revealed that they are of the rhomohedrally distorted perovskite crystal structure. The results of SEM and TEM revealed that BFO nanotubes possessed a uniform length (up to 60 microm) and diameter (about 200 nm), which were controlled by the thickness and the pore diameter of the applied AAO template, respectively and the thickness of the wall of the BFO nanotube was about 15 nm. Y-junctions in the BFO nanotubes were observed. EDX analysis demonstrated that stoichiometric BiFeO3 was formed. HRTEM analysis confirmed that the obtained BFO nanotubes made up of nanoparticles (3-6 nm). The possible formation mechanism of BFO nanotubes was discussed.

Engineering Defects in Graphene Oxide for Selective Ammonia and Enzyme-Free Glucose Sensing and Excellent Catalytic Performance for para-Nitrophenol Reduction.

Recently, extensive attention has been given to developing an active and durable metal-free economical sensor and catalyst. Graphene oxide (GO)-based sensors and catalysts have been considered as a promising candidate in current material science research. However, the sensing and catalytic properties of GO also need to be further improved to satisfy the specific applications, such as gas detection in harsh environments, medical diagnosis based on human breath, blood glucose detection, catalytic activity, and so forth. Therefore, the effect of nitrogen in GO on the performance of glucose and ammonia sensing, and catalytic activity has been investigated. Herein, we propose a practical, high-sensitive sensor and catalyst based on high-quality defect N-enriched GO. One-step, low-cost solvothermal synthesis of N-enriched GO has been exploited for the development of high-performance sensors and excellent catalyst at room temperature. The resultant N-enriched GO (N8GO) has been studied as a promising sensing material for ammonia, glucose, and para-nitrophenol (PNP) reduction. The prevalent outstanding sensing and catalytic performance may be due to the synergistic effect of nitrogen. A probable mechanism for sensing and catalytic reduction of PNP using N8GO has been proposed.

Note: Simultaneous water quality monitoring and degradation of hazardous organic pollutants.

Here, we report a simple technique that uses mesoporous SnO2 to monitor the water quality and degrade the hazardous organic pollutants simultaneously. The technique generates hydroxyl radicals and a voltage that is hindered by the presence of hazardous organic pollutants. Pollutant as low as 1 ppb concentration level can easily be detected. The developed system not only monitors the water quality but also is capable of degrading hazardous dyes (organic pollutants) through its self-power, not relying on any external stimuli such as light, heat, radiation, and current. A simple digital laboratory multimeter is shown to be useful for the overall study. Overall, the study indicates that spectrophotometer generally used to monitor the dye concentration can be avoided.

In-situ deposition of sodium titanate thin film as anode for sodium-ion micro-batteries developed by pulsed laser deposition.

Sodium-ion thin-film micro-batteries form a niche sector of energy storage devices. Sodium titanate, Na2Ti6O13 (NTO) thin films were deposited by pulsed laser deposition (PLD) using solid-state synthesized polycrystalline Na2Ti6O13 compound. The phase-purity and crystallinity of NTO in bulk and thin-film forms were confirmed by Rietveld refinement. Electron microscopy and atomic force microscopy revealed the formation of uniform ∼100 nm thin film with roughness of ∼4 nm consisting of homogeneous nanoscale grains. These PLD-deposited NTO thin-films, when tested in Na-half cell architecture, delivered a near theoretical reversible capacity close to 42 mA h g-1 involving Ti4+/Ti3+ redox activity along with good cycling stability and rate kinetics. Na2Ti6O13 can work as an efficient and safe anode in designing sodium-ion thin-film micro-batteries.

In-Plane Anisotropic Photoconduction in Nonpolar Epitaxial a-Plane GaN.

Nonpolar a-plane GaN epitaxial films were grown on an r-plane sapphire using the plasma-assisted molecular beam epitaxy system, with various nitrogen plasma power conditions. The crystallinity of the films was characterized by high-resolution X-ray diffraction and reciprocal space mapping. Using the X-ray "rocking curve-phi scan", [0002], [1-100], and [1-102] azimuth angles were identified, and interdigitated electrodes along these directions were fabricated to evaluate the direction-dependent UV photoresponses. UV responsivity ( R) and internal gain ( G) were found to be dependent on the azimuth angle and in the order of [0002] > [1-102] > [1-100], which has been attributed to the enhanced crystallinity and lowest defect density along [0002] azimuth. The temporal response was very stable irrespective of growth conditions and azimuth angles. Importantly, response time, responsivity, and internal gain were 210 ms, 1.88 A W-1, and 648.9%, respectively, even at a bias as low as 1 V. The results were validated using the Silvaco Atlas device simulator, and experimental observations were consistent with simulated results. Overall, the photoresponse is dependent on azimuth angles and requires further optimization, especially for materials with in-plane crystal anisotropy.

Sequential Elemental Dealloying Approach for the Fabrication of Porous Metal Oxides and Chemiresistive Sensors Thereof for Electronic Listening.

Highly porous materials, with large surface area and accessible space, variable chemical compositions, and porosity at different length scales, have captivated the attention of researchers in recent years as an important family of functional materials. Here, we report a novel approach to grow porous metal oxides (PMOs) by sequential elemental dealloying in which a highly mobile element gets dealloyed first under the thermal treatment (annealing) and facilitates the formation of PMOs. Subsequently, a chemiresistive sensor based on porous SnO2 was fabricated for humidity sensing at room temperature which shows a high sensitivity of 348 in a fully humid [>99% relative humidity (RH)] atmosphere with an accuracy of 1% RH change. In addition, the sensor is highly durable and reproducible. Eventually, the chemiresistive sensor has been exploited for electronic listening toward speaking, whistling, and breath monitoring. Overall, the results advocate the fabrication of PMOs and the development of resistive humidity sensors for electronic listening as well as for biomedical applications.

Nanocomposite Based Organic-Inorganic Cu3BiS3 High Sensitive Hybrid Photonic Devices.

We report the synthesis and application Cu3BiS3 nanorods in infrared photodectection. Cu3BiS3 nano rods were characterized structurally, optically and electrically. The detailed IR photodectection properties in terms of photo response were demonstrated with IR lamp and 1064 nm laser illuminations. The rapid photocurrent time constants followed by the slower components, resulting due to the defect states. The photo detecting properties for different concentrations of nanorods blended with the conjugate polymer devices were demonstrated. Further the photocurrent was enhanced to threefold increase from 3.47 x 10(-7) A to 2.37 x 10(-3) A at 1 V for 10 mg nanorods embedded in the polymer device. Responsivity of hybrid device was enhanced from 0.0158 A/W to 102 A/W. The detailed trap assisted space charge transport properties were studied considering the different regimes. Hence Cu3BiS3 can be a promising candidate in the nano switchable near IR photodetectors.