Graphene-based Schottky Device Detecting NH3 at ppm level in Environmental Conditions

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Tin Dioxide-Graphene Based Chemi-Device for NO2 Detection in the Sub ppm Range

Chemical nanodevices based on tin dioxide, graphene and a mixture of both materials were developed and characterized for NO2 detection at low concentrations. The chemiresistors were prepared by both electrospinning and drop casting. The films morphologies were investigated by scanning electron microscopy (SEM). The devices response to sub-ppm NO2 concentrations was measured from room temperature up to 300 °C. An improvement in the performance in terms of sensitivity and response time, as well as higher responses at room temperature, was obtained when a mixture of these materials is used.Authors want to thank Spanish Ministry of Economy and Competitiveness for supporting the project TEC2013-48147-C

Low-humidity sensing properties of multi-layered graphene grown by chemical vapor deposition

Humidity sensing is fundamental in some applications, as humidity can be a strong interferent in the detection of analytes under environmental conditions. Ideally, materials sensitive or insensitive towards humidity are strongly needed for the sensors used in the first or second case, respectively. We present here the sensing properties of multi-layered graphene (MLG) upon exposure to different levels of relative humidity. We synthesize MLG by chemical vapor deposition, as shown by Raman spectroscopy, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Through an MLG-based resistor, we show that MLG is scarcely sensitive to humidity in the range 30%–70%, determining current variations in the range of 0.005%/%relative humidity (RH) well below the variation induced by other analytes. These findings, due to the morphological properties of MLG, suggest that defective MLG is the ideal sensing material to implement in gas sensors operating both at room temperature and humid conditions.</p

Analysis of a calibration method for non-stationary CVD multi-layered graphene-based gas sensors

Limitations such as lack of detected stationary signal and slow signal recovery after detection currently affect graphene-based chemi-sensors operating at room temperature. In this work, we model the behavior of a sensor in a test chamber having limited volume and simulating the environmental conditions. From this model, we mathematically derive the calibration method for the sensor. The approach, focused on the time differential of the signal output, is tested on multi-layered graphene (MLG)-based sensors towards the chosen target gas (nitrogen dioxide) in the range from 0.12 to 1.32 ppm. MLG acting as sensing layer is synthesized by chemical vapor deposition. Our study paves the route for a wider applicability of the analysis to calibrate the class of devices affected by non-stationary and recovery issues

Harmonic Susceptibilities and Pinning Properties of MgB<SUB>2</SUB> Bulk Superconductors

The fundamental and third harmonics of the ac magnetic susceptibility have been studied on MgB2 bulk superconductors, obtained by reactive liquid infiltration. In particular, measurements performed as a function of temperature, dc magnetic field, ac field amplitude and frequency have been compared with susceptibility curves obtained by means of numerical simulations of the non-linear diffusion equation for the magnetic field. The experimental frequency dependence of the χ1″(T) and |χ3|(T) peak amplitude cannot be explained by frequency dependent critical state models. On the contrary, we have shown that the measured curves are correctly derived using a diffusion coefficient dominated by a creep process within the framework of the vortex glass approach

<title>Low-loss small-cross-section silicon-on-silicon rib waveguides with high-confining ion-implanted lower cladding</title>

The realization of single-mode rib waveguides in standard epitaxial silicon layer on lightly-doped silicon substrate, using ion-implantation to form the lower cladding, is reported. We exploited a standard microelectronic process step, followed by a calibrated thermal treatment in order to activate and drive-in the implanted impurities, so obtaining a spatially confined lower cladding. The implanted buffer layer enhances the vertical confinement and improves the propagation characteristics. The waveguides were designed with a cross-section comparable in size to the mode-field- diameter of standard single-mode optical fiber, so reducing the fiber-waveguide coupling losses. Propagation losses of about 1.2 dB/cm, for (lambda) equals 1.3 micrometers , in the single mode regime, have been measured. This attenuation is about one order of magnitude lower respect to similar standard all-silicon waveguides. This is the best value of attenuation, to our knowledge, for all-silicon single-mode small-cross-section waveguides reported in literature. A numerical analysis has been performed to evaluate the theoretical attenuation and the transverse optical field profiles, both for (lambda) equals 1.3 micrometers and (lambda) equals 1.55 micrometers . As a result of the presence of the ion implanted buffer layer, a strong reduction of propagation losses and an increase of the fundamental mode confinement have been shown. This results in a great enhancement of the coupling efficiency with standard single-mode optical fibers. Moreover, the proposed technique is low-cost, fully compatible with standard VLSI processes, and allows a great flexibility in the integration of guided-wave devices and electronic circuits. Finally, the very high thermal conductivity characterizing these waveguides makes them attractive host-structures for electrically and thermally- controlled active optical devices

A study on the physicochemical properties of hydroalcoholic solutions to improve the direct exfoliation of natural graphite down to few-layers graphene

Straightforward methods to produce pristine graphene flakes in large quantities are based on the liquid-phase exfoliation processes. These one-step physical transformations of graphite into graphene offer many unique advantages. To date, a large number of liquids have been employed as exfoliation media exploiting their thermodynamic and chemical features as compared to those of graphene. Here, we pursued the goal of realizing water based mixtures to exfoliate graphite and disperse graphene without the aid of surfactants. To this aim, aqueous mixtures with suitable values of surface tension and Hansen solubility parameters (HSPs), were specifically designed and used. The very high water surface tension was decreased by the addition of solvents with lower surface tensions such as alcohols, obtaining, in this way, more favourable HSP distances. The specific role of each of these thermodynamic features was finally investigated. The results showed that the designed hydroalcoholic solutions were effective in both the graphite exfoliation and dispersion without the addition of any surfactants or other stabilizing agents. Stable graphene suspensions were obtained at concentration comparable to those produced with low-boiling solvents and water/surfactants

Low-humidity sensing properties of multi-layered graphene grown by chemical vapor deposition

Humidity sensing is fundamental in some applications, as humidity can be a strong interferent in the detection of analytes under environmental conditions. Ideally, materials sensitive or insensitive towards humidity are strongly needed for the sensors used in the first or second case, respectively. We present here the sensing properties of multi-layered graphene (MLG) upon exposure to different levels of relative humidity. We synthesize MLG by chemical vapor deposition, as shown by Raman spectroscopy, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Through an MLG-based resistor, we show that MLG is scarcely sensitive to humidity in the range 30%–70%, determining current variations in the range of 0.005%/%relative humidity (RH) well below the variation induced by other analytes. These findings, due to the morphological properties of MLG, suggest that defective MLG is the ideal sensing material to implement in gas sensors operating both at room temperature and humid conditions.Electronic Components, Technology and Material

CVD transfer-free graphene for sensing applications

The sp2 carbon-based allotropes have been extensively exploited for the realization of gas sensors in the recent years because of their high conductivity and large specific surface area. A study on graphene that was synthetized by means of a novel transfer-free fabrication approach and is employed as sensing material is herein presented. Multilayer graphene was deposited by chemical vapour deposition (CVD) mediated by CMOS-compatible Mo. The utilized technique takes advantage of the absence of damage or contamination of the synthesized graphene, because there is no need for the transfer onto a substrate. Moreover, a proper pre-patterning of the Mo catalyst allows one to obtain graphene films with different shapes and dimensions. The sensing properties of the material have been investigated by exposing the devices to NO2, NH3 and CO, which have been selected because they are wellknown hazardous substances. The concentration ranges have been chosen according to the conventional monitoring of these gases. The measurements have been carried out in humid N2 environment, setting the flow rate at 500 sccm, the temperature at 25 °C and the relative humidity (RH) at 50%. An increase of the conductance response has been recorded upon exposure towards NO2, whereas a decrease of the signal has been detected towards NH3. The material appears totally insensitive towards CO. Finally, the sensing selectivity has been proven by evaluating and comparing the degree of adsorption and the interaction energies for NO2 and NH3 on graphene. The direct-growth approach for the synthesis of graphene opens a promising path towards diverse applicative scenarios, including the straightforward integration in electronic devices.</p