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.

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.

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.

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.

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.

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.

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.

Interface quantum trap depression and charge polarization in the CuPd and AgPd bimetallic alloy catalysts.

The ability of a catalyst to accept or donate charge is the key to the process of catalytic reaction. However, the determination of the catalytic nature of a specimen as yet remains a great challenge. Here we report an effective yet simple method for this purpose based on the tight binding theory considerations and XPS monitoring of the evolution of valence and core electrons upon alloy formation. Firstly, we measured the valence and core band charge density of the constituent elements of Cu, Ag, and Pd and then the respective states upon alloy formation. A subtraction of the resultant spectrum of the alloy by the composed elemental spectra gives the residual that shows clearly the occurrence of charge trapping or polarization. We found that the valence and the core electrons of the CuPd alloy shift positively to deeper energies, opposite to the occurrences in the AgPd alloy. Findings clarify for the first time that CuPd serves as an acceptor due to quantum trapping and the AgPd as a donor because of charge polarization, which also explain why AgPd and CuPd perform very differently as important catalysts.

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.

Mechanical characteristics of silicon nanostructures using force distance spectroscopy.

Experimental studies were undertaken to determine mechanical stiffness of Si chevron nanostructures grown by glancing angle deposition. Atomic force microscope based force-distance spectroscopy was performed on two types of chevron structures. The average stiffness of four-armed chevrons was found to be 381 +/- 16 Nm(-1), while that of five-armed chevrons was determined to be 375 +/- 23 Nm(-1). Simulations using finite element modeling were carried out to understand the mechanical characteristics of chevrons. For the nanostructures investigated in the present study, the simulation results indicate that while five-armed chevrons behave as springs, the four-armed chevrons act as cantilevers. It is shown that the position of loading point, physical dimensions and the geometry of the chevron control the overall mechanical response of chevron structures when subjected to an external load. It is proposed that by controlling the deposition parameters in glancing angle deposition, the topography of the structures and hence the position of loading points can be manipulated to generate a desirable mechanical response.

Racial disparities in pre-operative pain, function and disease activity for patients with rheumatoid arthritis undergoing Total knee or Total hip Arthroplasty: a New York based study.

BACKGROUND: Black and Hispanic patients with osteoarthritis have more pain and worse function than Whites at the time of arthroplasty. Whether this is true for patients with rheumatoid arthritis (RA) is unknown. METHODS: This cross-sectional study used data on RA patients acquired between October 2013 and November 2018 prior to elective total knee (TKA) or hip arthroplasty (THA). Pain, function, and disease activity were assessed using the visual analogue scale (VAS), the Multidimensional Health Assessment Questionnaire (MDHAQ), and the Disease Activity Score (DAS28-ESR). We linked the cases to census tracts using geocoding to determine the community poverty level. Race, education, income, insurance and medications were collected via self-report. Using multivariable linear and logistic models we examined whether minority status predicted pain, function and RA disease activity at the time of arthroplasty. RESULTS: Thirty seven (23%) of the 164 patients were Black or Hispanic (minorities). The MDHAQ and DAS28-ESR were not significantly worse while VAS pain score was significantly worse in minority patients (p = 0.03). There was no significant difference in education between the groups. Insurance varied significantly; 29% of minority patients had Medicaid vs. 0% of Whites (p < 0.0001). In the multivariable analyses minority status was not significantly associated with DAS28-ESR [p = 0.66], MDHAQ [p = 0.26], or VAS pain [p = 0.18]. CONCLUSIONS: For Black and/or Hispanic patients with RA undergoing THA or TKA at a high-volume specialty hospital, unlike Black or Hispanic patients with osteoarthritis (OA), there was no association with worse pain, function, or RA disease activity at the time of elective arthroplasty.

Concentration-specific hydrogen sensing behavior in monosized Pd nanoparticle layers.

In this study, H2 sensing behavior of monosized and monocrystalline Pd nanoparticles has been studied as a function of H2 concentration and measurement temperature. A unique concentration-specific H2 sensing behavior with a 'pulsed' response at larger H2 concentrations and 'saturated' response at lower concentrations has been observed. The threshold concentration required for transition from 'saturated' to 'pulsed' response is very sensitive to measurement temperature. The characteristic change in the sensing behavior can be used to develop a novel sensor capable of determining H2 concentration level having high sensitivity and fast response. This study demonstrates that electrical and gas sensing properties of the nanoparticle layer depend critically on interparticle gaps.

Tunable synthesis of indium oxide octahedra, nanowires and tubular nanoarrow structures under oxidizing and reducing ambients.

We report the effect of oxidizing and reducing ambients on the growth of indium oxide (IO) nanostructures in the vapor phase evaporation method. Our results show that the oxidizing reagent, water, results in the growth of IO nanowires and preserves the In/O stoichiometry throughout the length of the nanowires. The reducing reagent, ethanol, makes the growth environment indium rich, resulting in the growth of indium-filled IO tubular nanoarrow structures. The growth of solid IO nanowires is attributed to a vapor-liquid-solid mechanism, whereas for indium-filled tubular nanoarrow structures a modified bottom-vapor-solid growth mechanism is proposed. The tunable synthesis of IO nanostructures in different morphologies with correction of their stoichiometry may have potential applications in future nanodevices.

Nitrogen ion induced 2D-GaN layer formation of GaAs (001) surface.

This study demonstrates the formation of two-dimensional GaN on GaAs (001) surface by bombardment of nitrogen ions at room temperature. In this work the ion induced nitridation of GaAs (001) surface using nitrogen ion beam of different energies (range from 250 eV to 5 keV) has been investigated using in-situ X-ray Photoelectron Spectroscopy (XPS). A Ga rich surface produced by Art ion etching, promotes initial nitridation. Using nitrogen ion of different energies of constant fluence performs the nitridation. The nitridation suggests that the degree of nitridation increase as the nitrogen ion energy increases up to 3 keV and then attains saturation. The core level and valance band spectra were monitored to observe the chemical and electronic changes as a function of nitrogen ion beam energy. It is observed that Ga(3d) core level peak shifts during nitridation and N(1s) core level spectra shows that the intensity of the nitrogen peak increases and the Ga (LMM) auger peak shifts towards the higher binding energy, reveal the forming of N bonds with Ga by replacing the Ga-As bonds, forming GaN.