Ahalapitiya Hewage Jayatissa

Adsorption kinetics of ammonia sensing by graphene films decorated with platinum nanoparticles

Ammonia sensing behavior of graphene synthesized by a chemical vapor deposition on a copper substrate using a methane and hydrogen gas mixture has been investigated. The Raman spectroscopy is used to monitor the quality of graphene films transferred onto SiO2/Si substrates. The sensitivity and the recovery time of the device are enhanced by the decoration of platinum nanoparticles on the surface of graphene. The effects of the operating temperature on the sensing response have been studied. The adsorption and desorption curves have been analyzed using Langmuir kinetic theory for the adsorption of ammonia, and the influence of surface inhomogeneity was analyzed by Freundlich isotherm. The activation energy of adsorption and the heat of adsorption estimated from the above theories indicate that the platinum decorated surface has two different adsorption sites whereas bare graphene has only one adsorption site. This effect caused 80%–85% enhancement of sensor response for platinum decorated surface compared ...

Graphene based field effect transistor for the detection of ammonia

Graphene synthesized by chemical vapor deposition has been used to fabricate the back-gated field effect transistor to study the sensing of ammonia (NH3) in ppm levels. Graphene has been synthesized directly on a target substrate using a thin Cu film as a catalyst, which has several advantages over deposition of graphene on Cu foil followed by a transferring process to another substrate. Raman spectroscopy was used to monitor the quality of the deposited graphene films on SiO2/Si substrates. The adsorption/desorption behavior of NH3 on graphene in dry air was analyzed from the progressive shift of the Dirac peak at smaller/larger gate voltages based on different time exposures to different concentrations of NH3. The relative change in the shift of the Dirac peak was consistent with a small charge transfer (0.039 ± 0.001 electrons per molecule at room temperature). The response of the device was found to increase with increasing NH3 concentrations and operating temperatures. The dependence of device respon...

Dye-sensitised solid-state photovoltaic cells

A thin layer of dye sandwiched between p-CuCNS deposited on a copper substrate and n-SnO2 coated transparent glass was found to yield photocurrents resulting from light absorption in the dye. The mechanism of photocurrent generation is discussed.

Detection of organic vapors by graphene films functionalized with metallic nanoparticles

Graphene synthesized by chemical vapor deposition has been used to study the sensing behavior of graphene for different organic vapors in ppm level. All the measurements were carried out at a room temperature with dry air as the background gas. Synthesis of graphene was carried out on a copper foil using methane gas as a precursor. The Raman spectroscopy was used to monitor the quality of graphene films transferred to SiO2/Si substrates, revealing the sensing channel to be mostly monolayer, bilayer, or tri-layer graphene. Graphene surface was functionalized with gold and platinum nanoparticles to enhance the gas sensing behavior. The adsorption/desorption behavior of organic vapors in dry air was analyzed based on the change in the conductivity with the different exposure times in different concentrations. Among the vapors tested, the sensitivity was in the order: acetic acid > ethanol > acetone from all surfaces (bare graphene, graphene decorated with gold and platinum nanoparticles). The devices were su...

Empirical dielectric function of amorphous materials for spectroscopic ellipsometry

An empirical dielectric function (EDF) is proposed for optical characterization of amorphous materials. The EDF consists of the sum of damped harmonic oscillator terms whose square root amplitudes are distributed according to a hyperbolic function of photon energy connected to an exponential function. The usefulness of EDF is demonstrated by fitting it to the table values of dielectric constants for a‐Si and a‐Si3N4 and by applying it to the spectro‐ellipsometric analysis of a‐Si films deposited by vacuum evaporation and rf glow discharge methods.

Preparation of Polycrystalline Silicon Thin Films by Cathode-Type RF Glow Discharge Method

Polycrystalline silicon films were deposited by the cathode-type rf glow discharge method where a protective mesh against ions was attached to the cathode using 0.5% SiH4 diluted with hydrogen at a pressure of 0.09 Torr. The deposited films were found to be polycrystalline for a wide range of applied rf power (0.05-1 W cm-2) and substrate temperature above 200°C. The crystallization mechanism was suggested to be the surface reaction in the chemical equilibrium state between growth and etching of weak Si-SiHx bonds on the growing surface caused by hydrogen radicals in the plasma. Electrical and optical properties of the film suggested that substrate temperatures above 350°C and deposition power densities above 0.5 W cm-2 are suitable for formation of polycrystalline silicon.

Enhanced field emission current from diamond-like carbon films deposited by laser ablation of C60 fullerene

Carbon films deposited by pulsed ArF laser ablation of a C60 fullerene target were found to have a higher electric field emission current than those films deposited using graphite. The emission current density of the best films produced was 250 mA cm-2 at a field of 50 V µm-1 and yet the films were highly stable. These electron emission characteristics are discussed in terms of the structural properties of films. The effective work function of the electron emission is interpreted as due to the tunnelling length of electrons from the Fermi level of sp2 clusters to the conduction band of sp3 sites.

Thickness change in an annealed amorphous silicon film detected by spectroscopic ellipsometry

Abstract The annealing process of an amorphous silicon (a-Si) film has been studied by means of spectroscopic ellipsometry (SE). Measured SE spectra were analyzed using an empirical dielectric function (EDF) proposed in a previous paper and a refractive index function proposed by Forouhi and Bloomer (FB). We found several remarkable facts: (1) our EDF fits to measured spectra much better than FB; (2) the FB refractive index function does not satisfy Kramers-Kronig relations; (3) an adsorption layer should be taken into account to improve the fitting of SE spectra measured in air; (4) the thickness of a-Si deposited by r.f. glow discharge at 250 °C decreases more than 10% by annealing at 550 °C due to hydrogen evolution; (5) peak position and peak height of the dielectric function changes systematically with the annealing so that an analysis using the effective medium approximation (EMA) with a-Si and voids cannot explain these changes; (6) the EMA interface layer model cannot explain the optical behavior of a rough surface.

Structural properties of carbon films deposited by pulsed ArF laser ablation: effects of substrate temperature, bias and H2 pressure

The structural properties of carbon films grown at different conditions using ArF pulsed laser deposition are reported. Diamond-like carbon (DLC) films were grown by laser ablation of graphite in a H2 atmosphere and a substrate temperature range of 30–500°C with a substrate bias range of +200 to −500 V. The structural properties, crystallinity and morphology of these DLC films were investigated by means of RHEED, Raman and atomic force microscopy (AFM) spectroscopies, and spectroscopic ellipsometry. The films deposited under optimal conditions contained nano-size diamond crystals with a lonsdaleite structure and amorphous carbon. AFM indicated that the surface morphology was considerably influenced by the H2 pressure, the substrate temperature and the substrate bias. The size of the carbon clusters was reduced and the film uniformity was improved by the applications of the substrate bias. These results suggested that the substrate bias enhanced the surface mobility of the adspecies due to the high kinetic energy of the ionic carbon species. It was also found that the formation of the diamond phase was not significantly enhanced by varying the deposition conditions except for the substrate temperature. The optical constants and band gap of these films are also discussed in terms of the deposition conditions.

Microcrystalline structure of poly-Si films prepared by cathode-type r.f. glow discharge

Abstract X-ray diffraction (XRD) and spectroscopic ellipsometry measurements have been performed to study the crystalline structure of polycrystalline silicon films deposited by cathode-type r.f. glow discharge. Ratios of XRD peak intensities are used to describe the degree of crystallinity of samples. For ellipsometric data analysis, a multilayer structure model is used based on the effective medium approximation. It is found that the crystalline fractions of the deposited films increase with the increase of power density and substrate temperature. The film compositions of as-deposited samples are compared with those of annealed samples and films prepared by thermal pyrolysis of silane. The crystallinity estimated from the ellipsometric measurements compares well with that obtained from XRD measurements.