Gas Sensing Properties of PLD Grown 2D SnS Film: Effect of Film Thickness, Metal Nanoparticle Decoration, and In Situ KPFM Investigation.

This study employs novel growth methodologies and surface sensitization with metal nanoparticles to enhance and manipulate gas sensing behavior of two-dimensional (2D)SnS film. Growth of SnS films is optimized by varying substrate temperature and laser pulses during pulsed laser deposition (PLD). Thereafter, palladium (Pd), gold (Au), and silver (Ag) nanoparticles are decorated on as-grown film using gas-phase synthesis techniques. X-ray diffraction (XRD), Raman spectroscopy, and Field-emission scanning electron microscopy (FESEM) elucidate the growth evolution of SnS and the effect of nanoparticle decoration. X-ray photoelectron spectroscopy (XPS) analyses the chemical state and composition. Pristine SnS, Ag, and Au decorated SnS films are sensitive and selective toward NO2 at room temperature (RT). Ag nanoparticle increases the response of pristine SnS from 48 to 138% toward 2 ppm NO2, which indicates electronic and chemical sensitization effect of Ag. Pd decoration on SnS tunes its selectivity toward H2 gas with a response of 55% toward 70 ppm H2 and limit of detection (LOD) < 1 ppm. In situ Kelvin probe force microscopy (KPFM) maps the work function changes, revealing catalytic effect of Ag toward NO2 in Ag-decorated SnS and direct charge transfer between Pd and SnS during H2 exposure in Pd-decorated SnS.

Nanoscale Characterization of Growth of Secondary Phases in Off-Stoichiometric CZTS Thin Films.

The presence of secondary phases is one of the main issues that hinder the growth of pure kesterite Cu2ZnSnS4 (CZTS) based thin films with suitable electronic and junction properties for efficient solar cell devices. In this work, CZTS thin films with varied Zn and Sn content have been prepared by RF-power controlled co-sputtering deposition using Cu, ZnS and SnS targets and a subsequent sulphurization step. Detailed TEM investigations show that the film shows a layered structure with the majority of the top layer being the kesterite phase. Depending on the initial thin film composition, either about ~1 µm Cu-rich and Zn-poor kesterite or stoichiometric CZTS is formed as top layer. X-ray diffraction, Raman spectroscopy and transmission electron microscopy reveal the presence of Cu2-xS, ZnS and SnO2 minor secondary phases in the form of nanoinclusions or nanoparticles or intermediate layers.

Optical properties and band alignments in ZnTe nanoparticles/MoS2 layer hetero-interface using SE and KPFM studies.

Integration of a layered two-dimensional (2D) material with a non-2D material provides a platform where one can modulate and achieve the properties desired for various next-generation electronic and opto-electronic applications. Here, we investigated ZnTe nanoparticles/MoS2 hetero-interfaces with the thickness of the MoS2 varying from few to multilayer. High-resolution transmission electron microscopy was used to observe the crystalline behaviour of the ZnTe nanoparticles, while the number of MoS2 layers was investigated using Raman measurements. Spectroscopic ellipsometry (SE) analysis based on the five-layer fitting model was used to analyse the optical behaviour of the heterojunction, where the excitonic features corresponding to the MoS2 layers and absorption features due to the ZnTe nanoparticles are observed. From the Kelvin probe force microscopy (KPFM) measurements, the surface potential (SP) of the ZnTe nanoparticles/MoS2 is found to be different in comparison with the SP of the ZnTe nanoparticles and MoS2, which is indicative of the charge transfer at the ZnTe nanoparticles/MoS2 hetero-interface. Various parameters obtained using SE and KPFM measurements were used to propose energy band alignments at the ZnTe nanoparticles/MoS2 hetero-interface. In addition, an interface photovoltage of 193 mV was obtained by carrying out KPFM measurements under illuminating condition.

Achieving independent control of core diameter and carbon shell thickness in Pd-C core-shell nanoparticles by gas phase synthesis.

Pd-C core-shell nanoparticles with independently controllable core size and shell thickness are grown by gas phase synthesis. First, the core size is selected by electrical mobility values of charged particles, and second, the shell thickness is controlled by the concentration of carbon precursor gas. The carbon shell grows by adsorption of carbon precursor gas molecules on the surface of nanoparticles, followed by sintering. The presence of a carbon shell on Pd nanoparticles is potentially important in hydrogen-related applications operating at high temperatures or in catalytic reactions in acidic/aqueous environments.

A Parametric Study on the Influence of Synthesis and Transfer Conditions on the Quality of Graphene.

In this paper, a systematic and comprehensive study has been carried out to observe the effect of synthesis and transfer conditions on the quality and uniformity of graphene deposition in an atmospheric pressure chemical vapour deposition set up. It was observed that the quality of graphene was highly affected by the synthesis conditions, such as, synthesis temperature, synthesis duration, methane and hydrogen flow rate ratio and total flow rate during deposition and cooling cycles. The quality of graphene was observed to be significantly improved upon increasing the synthesis temperature while increase in methane and hydrogen flow rates beyond a particular limit resulted into degradation in the quality of graphene. From the comparison of scanning electron microscopy images of graphene grown at different times, it was found that the nucleation and growth of graphene domains strongly depend on the growth time. The process of transfer of monolayer graphene was significantly improved by controlling the PMMA concentration using a modified three step technique. Raman spectroscopy and the high mobility (˜8153 cm2V−1s−1) of graphene after transferred onto a SiO2/Si substrate confirm the high quality of monolayer graphene obtained by the optimizations of synthesis and transfer conditions in this study.

A fast and effective approach for reversible wetting-dewetting transitions on ZnO nanowires.

Here, we demonstrate a facile approach for the preparation of ZnO nanowires (NWs) with tunable surface wettability that can be manipulated reversibly in a controlled manner from a superhydrophilic state to a superhydrophobic state. The as-synthesized ZnO NWs obtained by a chemical vapor deposition method are superhydrophilic with a contact angle (CA) value of ~0°. After H2 gas annealing at 300 °C for 90 minutes, ZnO NWs display superhydrophobic behavior with a roll-off angle less than 5°. However, O2 gas annealing converts these superhydrophobic ZnO NWs into a superhydrophilic state. For switching from superhydrophobic to superhydrophilic state and vice versa in cyclic manner, H2 and O2 gas annealing treatment was used, respectively. A model based on density functional theory indicates that the oxygen-related defects are responsible for CA switching. The water resistant properties of the ZnO NWs coating is found to be durable and can be applied to a variety of substrates including glass, metals, semiconductors, paper and even flexible polymers.

Exploring Nanoscale Electrical Properties of CuO-Graphene Based Hybrid Interfaced Memory Device by Conductive Atomic Force Microscopy.

The phenomenon of resistive switching is based on nanoscale changes in the electrical properties of the interface. In the present study, conductive atomic force microscope based nanoscale measurements of copper oxide (CuO-multilayer graphene (MLG) hybrid interface based devices have been carried out to understand changes in the electrical properties during resistive switching of the Ti-CuO/MLG-Cu memory cells having different dimensions fabricated on the same substrate using stencil lithography technique. The dependence of resistive switching characteristics in LRS and HRS and current level of the conductive filaments (CF) on the electrode area have been studied. As the device dimension is reduced, the filamentary contribution is enhanced in comparison to the background contribution, resulting in'an increase in the current density ratio between LRS and HRS. It is also observed that as the device dimension is decreased from 150 to 25 µm, the filament size decreases from 95 nm to 20 nm, respectively, which causes a decrease in the reset current and reset voltage. The results of the nanoscale CAFM measurements have shown a good correlation with the switching parameters obtained by the macroscale pad I-V measurements, thereby, suggesting the origin of resistive switching is due to the formation and rupture of an entity called filament, whose dimension is in nanorange. It is observed that changes in the electrical properties of the overall interface layer along with changes in the electrical conductivity of these filaments contribute towards resistive switching phenomenon. This study suggests that a significant reduction of reset current can be achieved by decreasing the memory device dimensions.

Visible-Light-Driven Photoelectrochemical and Photocatalytic Performance of NaNbO3 /Ag2 S Core-Shell Heterostructures.

Herein, we report the fabrication of visible-light-active NaNbO3 /Ag2 S staggered-gap core-shell semiconductor heterostructures with excellent photoelectrochemical activity toward water splitting, and the degradation of a model pollutant (methylene blue) was also monitored. The heterostructures show a pronounced photocurrent density of approximately 2.44 mA cm(-2) at 0.9 V versus Ag/AgCl in 0.5 m Na2 SO4 and exhibit a positive shift in onset potential by approximately 1.1 V. The high photoactivity is attributed to the efficient photoinduced interfacial charge transfer (IFCT). The core-shell design alleviates the challenges associated with the electron-hole paths across semiconductor junctions and at the electrolyte-semiconductor interface. These properties demonstrate that NaNbO3 /Ag2 S core-shell heterostructures show promising visible-light photoactivity and are also efficient, stable, and recyclable photocatalysts.

Synthesis and Characterization of Bi2Te3 Nanostructured Thin Films.

Thin films of Bi2Te3 were obtained using vacuum evaporation and inert gas evaporation techniques. To study the effect of nanocrystallite size on thermal and electrical properties, deposition temperature and gas pressure were varied and thin films of Bi2Te3 having different crystallite sizes ranging from 7-20 nm were obtained. X-ray Diffraction and scanning electron microscopic studies were carried out to determine phase, crystallite size, strain and surface morphology of nanocrystalline films. Effect of nanocrystallite size on electron transport and thermal properties of Bi2Te3 thin films was studied using Hall effect and Harman's four probe methods. Calculated ZT values were correlated with the carrier concentration, carrier mobility and electrical conductivity of Bi2Te3 thin films. This study shows that strain may influence the electron transport and thermoelectric properties of Bi2Te3 films along with nanocrystallite size.

In-flight gas phase growth of metal/multi layer graphene core shell nanoparticles with controllable sizes.

In this report, we present a general method for a continuous gas-phase synthesis of size-selected metal/multi layer graphene (MLG) core shell nanoparticles having a narrow size distribution of metal core and MLG shell for direct deposition onto any desired substrate kept under clean vacuum conditions. Evolution of MLG signature is clearly observed as the metal-carbon agglomerates get transformed to well defined metal/MLG core shell nanoparticles during their flight through the sintering zone. The growth takes place via an intermediate state of alloy nanoparticle (Pd-carbon) or composite nanoparticle (Cu-carbon), depending upon the carbon solubility in the metal and relative surface energy values. It has been also shown that metal/MLG nanoparticles can be converted to graphene shells. This study will have a large impact on how graphene or graphene based composite nanostructures can be grown and deposited in applications requiring controllable dimensions, varied substrate choice, large area and large scale depositions.

Study of charge separation and interface formation in a single nanorod CdS-Cu(x)S heterojunction solar cell using Kelvin probe force microscopy.

In the present investigation, Kelvin probe force microscopy (KPFM) is used to study the charge separation, shift in Fermi level position and interfacial depletion region formation in a single cadmium sulfide (CdS)-copper sulfide (CuxS) nanorod heterojunction fabricated using hydrothermal synthesis and a topotaxial conversion reaction. A detailed analysis of KPFM images in the dark shows work function (or Fermi energy) values of CdS and CuxS regions consistent with the energy band diagram of the CdS-CuxS junction. Under illumination, Fermi energy levels of both the CuxS and CdS shift away from the vacuum level by 0.2 and 0.4 eV, respectively, which is very different from the behaviour expected in the case of a bulk p-n junction. The existence of interfacial regions topographically placed between ITO-CdS and CdS-CuxS with intermediate work function values as well as the observed narrowing of the work function spread under illumination are important for understanding the fundamental process of charge separation and junction formation in semiconductor nanorod solar cells.

Antireflection properties of graphene layers on planar and textured silicon surfaces.

In this study, theoretical and experimental investigations have been carried out to explore the suitability of graphene layers as an antireflection coating. Microwave plasma enhanced chemical vapor deposition and chemically grown graphene layers deposited on polished and textured silicon surfaces show that graphene deposition results in a large decrease in reflectance in the wavelength range of 300-650 nm, especially in the case of polished silicon. A Si3N4/textured silicon reference antireflection coating and graphene deposited polished and textured silicon exhibit similar reflectance values, with the graphene/Si surface showing lower reflectance in the 300-400 nm range. Comparison of experimental results with the finite difference time domain calculations shows that the graphene along with a SiO2 surface layer results in a decrease in reflectance in the 300-650 nm range, with a reflectance value of <5% for the case of graphene deposited textured silicon surfaces. The monolayer and inert character along with the high transmittance of graphene make it an ideal surface layer. The results of the present study show its suitability as an antireflection coating in solar cell and UV detector applications.

CAFM investigations of filamentary conduction in Cu2O ReRAM devices fabricated using stencil lithography technique.

With the objective of understanding the role of size and current level of filamentary regions on the resistive switching parameters, detailed conductive atomic force microscope investigations of resistive memory cells having different dimensions have been carried out in this study. Cu-Cu(2)O-Ti memory cells having dimensions of 150, 50 and 25 µm have been fabricated on the same substrate using a stencil lithography technique. The dependence of resistive switching parameters on the device dimensions can be directly related to the average size, current level of the filaments and difference in these parameters between the low resistance state (LRS) and high resistance state (HRS). It is observed that the large increase in the ratio of current in the two states in cells having lower dimensions is mainly due to the smaller number of conducting regions in the HRS, indicating efficient switching from the LRS to the HRS at lower dimensions.

Size and oxygen passivation induced reversal of photoconducting behaviour in CdS nanorods.

The effect of oxygen adsorption and desorption on the photoconducting gain, spectral dependence of quantum efficiency and optical switching was studied in CdS nanorods with diameters of 20, 50 and 100 nm. These were found to have an increasing degree of crystallinity and consequently a decreasing overall density of defects leading to better stoichiometry being maintained. These properties, along with the complete depletion of electrons from the nanorod volume and oxygen induced passivation of defects, resulted in: (i) a large difference in photoconducting gain, (ii) reversal of the photoconducting behaviour on annealing in oxygen and a vacuum, and (iii) onset of an absorption edge in the spectral dependence of quantum efficiency on oxygen annealing in 20 nm diameter nanorods in comparison to the normal photoconducting behaviour expected from an n-type semiconductor observed in 50 and 100 nm diameter nanorods. Single CdS nanorod devices show stable I-V characteristics in dark and light conditions under a wide temperature range and the effect of oxygen and vacuum annealing can be clearly observed. The oxygen induced defect passivation observed in this study is important for the application of compound semiconductor nanorods in optoelectronic devices.

Fast response and recovery of hydrogen sensing in Pd-Pt nanoparticle-graphene composite layers.

This study reports the fast response and recovery of hydrogen sensing in nanoparticle-graphene composite layers fabricated using chemical methods and comprising of isolated Pd alloy nanoparticles dispersed onto graphene layers. For 2% hydrogen at 40 °C and 1 atm pressure, a response time of <2 s and a recovery time of 18 s are observed. The fast response and recovery observed during sensing are due to hydrogen-induced changes in the work function of the Pd alloy and modification in the distribution of defect states in the graphene band gap due to gas adsorption. The results of hydrogen sensing in the new class of Pd-Pt nanoparticle-graphene composite material are important for understanding the effect of gas adsorption on electronic conduction in graphene layers and for developing a new type of gas sensor based on changes in the electronic properties of the interface.

Biosynthesis of Silver Nanoparticles from Desmodium triflorum: A Novel Approach Towards Weed Utilization.

A single-step environmental friendly approach is employed to synthesize silver nanoparticles. The biomolecules found in plants induce the reduction of Ag(+) ions from silver nitrate to silver nanoparticles (AgNPs). UV-visible spectrum of the aqueous medium containing silver ions demonstrated a peak at 425 nm corresponding to the plasmon absorbance of silver nanoparticles. Transmission electron microscopy (TEM) showed the formation of well-dispersed silver nanoparticles in the range of 5-20 nm. X-ray diffraction (XRD) spectrum of the AgNPs exhibited 2θ values corresponding to the silver nanocrystal. The process of reduction is extracellular and fast which may lead to the development of easy biosynthesis of silver nanoparticles. Plants during glycolysis produce a large amount of H(+) ions along with NAD which acts as a strong redoxing agent; this seems to be responsible for the formation of AgNPs. Water-soluble antioxidative agents like ascorbic acids further seem to be responsible for the reduction of AgNPs. These AgNPs produced show good antimicrobial activity against common pathogens.

Photovoltaic response of a topotaxially formed CdS-Cu(x)S single nanorod heterojunction.

In the present study, a combination of a hydrothermal route and a topotaxial conversion reaction has been used to grow a cadmium sulfide-copper sulfide (CdS-Cu(x)S) single nanorod heterojunction. The J-V characteristics of the CdS nanorods show Shockley behaviour consistent with the energy band diagram of the platinum conducting atomic force microscope (CAFM) probe-CdS nanorod combination. The photovoltaic response measured on the CdS-Cu(x)S nanorods using a CAFM probe shows the formation of a heterojunction with an open circuit voltage of 320 mV, a short circuit current density of 5.5 mA cm⁻² and a crossover of dark and light J-V curves related to the photoconductivity of the interfacial CdS layer. The lengthwise heterojunction fabricated in the present study has many potential advantages in comparison to other single nanorod junctions.

Resistive switching in copper oxide nanorods: a bottom up approach applicable for enhanced scalability.

Reversible, stable and reproducible resistive switching in a parallel network of Cu2O nanorods, observed in the present study, highlights the advantages of using nanorods in comparison to normally used thin films. Unipolar and symmetric current-voltage characteristics of the metal/insulator/metal structure consisting of Hg top contact/Copper oxide (Cu2O) nanorods/Ag bottom contact in a sandwich configuration shows electroforming at about 11 V, reproducible reset and set points at 0.53 +/- 0.03 and 4.2 +/- 0.02 V and a high OFF/ON resistance ratio > 10(3). Slope of current-voltage characteristics and current contrast in CAFM mapping indicate that filamentary conduction mechanism is responsible for resistive switching. This study sets the foundation for fabricating a nanorods based resistive random access memory device and thus a manifold increase in the device scalability.

Effect of induced shape anisotropy on magnetic properties of ferromagnetic cobalt nanocubes.

We report on the synthesis of ferromagnetic cobalt nanocubes of various sizes using thermal pyrolysis method and the effect of shape anisotropy on the static and dynamic magnetic properties were studied. Shape anisotropy of approximately 10% was introduced in nanocubes by making nanodiscs using a linear chain amine surfactant during synthesis process. It has been observed that, ferromagnetism persisted above room temperature and a sharp drop in magnetic moment at low temperatures in zero-field cooled magnetization may be associated with the spin disorder due to the effective anisotropy present in the system. Dynamic magnetic properties were studied using RF transverse susceptibility measurements at different temperatures and the singularities due to anisotropy fields were probed at low temperatures. Symmetrically located broad peaks are observed in the frozen state at the effective anisotropy fields and the peak structure is strongly affected by shape anisotropy and temperature. Irrespective of size the shape anisotropy gave rise to higher coercive fields and larger transverse susceptibility ratio at all temperatures. The role of shape anisotropy and the size of the particles on the observed magnetic behaviour were discussed.

Rapid synthesis of silver nanoparticles using dried medicinal plant of basil.

Plants respond to heavy metal stress by metal complexation process like production of phytochelations or by other metal chelating peptides. In this paper we report the synthesis of silver nanoparticles (AgNPs) from the room dried stem and root of Ocimum sanctum. The broth of the plant is used as a reducing agent for the synthesis of Ag nanoparticles at room temperature. The reaction process was simple and was monitored by ultraviolet-visible spectroscopy (UV-vis). There was formation of highly stable silver nanoparticles in the solution. The morphology and crystalline phase of the NPs were determined from transmission electron microscopy (TEM), selected area electron diffraction (SAED) and X-ray diffraction (XRD) spectra. Transmission Electron Microscopy studies showed that the silver nanoparticles obtained from roots and stem were of sizes 10+/-2 and 5+/-1.5 nm, respectively. The various phytochemicals present within the ocimum plant result in effective reduction of silver salts to nanoparticles but their chemical framework is also effective at wrapping around the nanoparticles to provide excellent robustness against agglomeration.