Fabrication of infrared photodetectors

Abstract

This paper presents the fabrication of an integrated optoelectronic circuit consisting of a waveguide and photodetector. Fabrication of the waveguides took place in a RIBE system with 1·5 sccm CH₄/H₂ (60:40) and 0·5 sccm Ar. Wet etching defines the photodetector regions. The detection of surface damage is minimal, using a novel differential optical reflectance technique.     

Surface-enhanced Raman spectroscopy of soft-landed polyatomic ions and molecules

Surface-enhanced Raman spectroscopy (SERS) was used to detect and characterize polyatomic cations and molecules that were electrosprayed into the gas phase and soft-landed in vacuum on plasma-treated silver substrates. Organic dyes such as crystal violet and Rhodamine B, the nucleobase cytosine, and nucleosides cytidine and 2'-deoxycytidine were immobilized by soft landing on plasma-treated metal surfaces at kinetic energies ranging from near thermal to 200 eV. While enhancing Raman scattering 10(5)-10(6)-fold, the metal surface effectively quenches the fluorescence that does not interfere with the Raman spectra. SERS spectra from submonolayer amounts of soft-landed compounds were sufficiently intense and reproducible to allow identification of Raman active vibrational modes for structure assignment. Soft-landed species appear to be microsolvated on the surface and bound via ion pairing or pi-complexation to the Ag atoms and ions in the surface oxide layer. Comparison of spectra from soft-landed and solution samples indicates that the molecules survive soft landing without significant chemical damage even when they strike the surface at hyperthermal collision energies.     

Investigating the Potential for Ongoing Pollution from an Abandoned Pyrite Mine

Davis Mine was the largest working pyrite mine in the state of Massachusetts during its lifetime between 1882 and 1911. Since abandonment, a highly-polluting mine water discharge has emerged from the site of an old mine shaft and a waste rock pile and is contaminating the nearby Davis Mine Brook. During the past 90 years, no attempt has been made to implement any pollution abatement measures. This paper assesses the likely current volume of mine waste on the site and compares this figure with the amount of mine waste produced during the lifetime of the mine based on old mine plans and production figures. A simple mass balance model allowed us to compute the loadings of contaminants into Davis Mine Brook and to calculate the ratio of loadings from different sources of pollution, namely the mine shaft discharge and ground water discharge from the waste pile. Results for 2004 indicated that the proportion of mine water flowing from the shaft varies seasonally, with the greatest discharge in spring and lowest in summer. These results allow us to assess the potential lifetime of the discharge if left untreated and determine what flow pathways are important if a treatment scheme were to be implemented at the site.     

Oxidation state-differentiated measurement of aqueous inorganic arsenic by continuous flow electrochemical arsine generation coupled to gas-phase chemiluminescence detection

The electrochemical reduction of inorganic As on a graphite cathode depends on the current density. We observed that while only inorganic As(III) is reduced to AsH(3) at low current densities, at high current densities both forms of inorganic As are reduced. We describe a unique electrochemical reactor in which the cylindrical anode compartment is isolated from the outer concentric cathode compartment by a Nafion tube in which a hole is deliberately made and the entire anode compartment is inside the cylindrical cavity of a small volume (～115 μL) cathode chamber. The evolved arsine is then quantitated by gas-phase chemiluminescence (GPCL) reaction with ozone; the latter is generated from oxygen formed during electrolysis. For the dimensions used, inorganic As(III) can be selectively determined at a current of 0.1 A while total inorganic As (both As(III) and As(V)) respond equally at an applied electrolysis current at 0.85 A, without any sample treatment. For a 1-mL sample, the system provides a limit of detection (LOD, S/N = 3) of 0.09 μg/L for total As (i = 0.85 A) and an LOD of 0.76 μg/L for As(III) (i = 0.10 A); As(V) is obtained by difference. Comparison of ICP-MS results for total As in groundwater samples that span a large range of concentration and total inorganic As determined by the present method showed a high correlation (r(2) = 0.9975) and a near unity slope. The basic electrochemical arsine generation technique and current-differentiated oxidation state speciation should be applicable as the front end to many other arsenic measurements techniques, including atomic spectrometry.     

Sedimentation of helminth eggs in water

Helminth parasite eggs in low quality water represent health risks when used for irrigation of crops. The settling velocities of helminth eggs (Ascaris suum, Trichuris suis, and Oesophagostomum spp.) and wastewater particles were experimentally determined in tap water and in wastewater using Owen tubes. The settling velocities of eggs in tap water was compared with theoretical settling velocities calculated by Stoke’s law using measurements of size and density of eggs as well as density and viscosity of tap water. The mean settling velocity in tap water of 0.0612 mm s⁻¹ found for A. suum eggs was significantly lower than the corresponding values of 0.1487 mm s⁻¹ for T. suis and 0.1262 mm s⁻¹ for Oesophagostomum spp. eggs. For T. suis and Oesophagostomum spp. eggs the theoretical settling velocities were comparable with the observed velocities in the Owen tubes, while it was three times higher for A. suum eggs. In wastewater, the mean settling velocity for A. suum eggs (0.1582 mm s⁻¹) was found to be different from T. suis (0.0870 mm s⁻¹), Oesophagostomum spp. (0.1051 mm s⁻¹), and wastewater particles (0.0474 mm s⁻¹). This strongly indicates that in low quality water the eggs are incorporated into particle flocs with different settling velocities and that the settling velocity of eggs and particles is closely associated. Our results document that there is a need to differentiate the sedimentation of different types of helminth eggs when assessing the quality of low quality water, e.g. for irrigation usage. The results can also be used to improve existing models for helminth egg removal.     

Detection of drugs of abuse in saliva by surface-enhanced Raman spectroscopy (SERS)

Eighty drugs of abuse and metabolites were successfully measured by surface-enhanced Raman spectroscopy (SERS) using gold- and silver-doped sol-gels immobilized in glass capillaries. A method was developed that provided consistent detection of 50 ppb cocaine in saliva in a focused study. This general method was successfully applied to the detection of a number of additional drugs in saliva, such as amphetamine, diazepam, and methadone.     

Fundamental Understanding of Fouling Mechanisms During Microfiltration of Bitter Gourd (<Emphasis Type="Italic">Momordica charantia</Emphasis>) Extract and Their Dependence on Operating Conditions

Microfiltration of bitter gourd (Momordica charantia) extract using hollow fiber membrane module was carried out in the present study. To identify the dominant fouling mechanism, flux decline behavior was examined using Field model. At lower transmembrane pressure, pore blocking mechanism was found to be more important, while cake filtration was dominant at higher pressure. Higher cross flow rate reduced filtration constant indicating slower rate of membrane fouling. Additionally, surface and particle size analyses were undertaken to validate the findings of modeling. Scanning electron microscope analysis clearly showed prevalence of pore blocking mechanism at lower transmembrane pressure drop, whereas cake filtration was dominant fouling mechanism at higher pressure. Fourier transform infrared spectroscopy analysis supported the role of cake layer as a secondary membrane retaining some amount of polyphenols. Analysis of flux decline ratio also confirmed that for transmembrane pressure of 104 kPa and beyond, cake layer became compact, and hence, increase in cross flow rate was unable to influence the improvement of permeate flux. The current study provides an insight into the fouling mechanism involved in scaling up of clarification of bitter gourd extract for successful processing of this medicinal herb.     

Effect of LiCl as an additive in the polymerization reaction of aniline and its influence on the structural and electrical property of polyaniline

Doped polyaniline (PANI) is synthesized in aqueous HCl medium by ammonium persulphate (APS) as oxidant. We used LiCl during in situ polymerization of aniline to explore any improvement of electrical properties of PANI due to Li⁺ and HCl incorporation. We varied the HCl and LiCl molarity in the polymerization medium keeping the total Cl⁻ concentration constant. The level of Li⁺ loading was measured by fluorescence spectroscopy of doped PANI. The PANI was characterized by FTIR and UV–visible spectroscopy, XRD analysis, intrinsic viscosity measurement and optical microscopy. The conductivity of doped PANI was measured by four probe method. Hall effect measurement was also done to measure the resistivity and the charge carrier density of the doped polyaniline.     

Effect of zirconia addition on pressureless sintering of boron carbide

This paper presents the results of experiments on pressureless sintering of boron carbide with varying addition of zirconia (ZrO₂: 0–30 wt.%). Green pellets were densified by sintering at 2275 °C in vacuum for 60 min and characterized by measurement of density, hardness, thermal conductivity and microstructure. Samples prepared with the addition of ≥5 wt.% ZrO₂ showed higher densities in the range of 93–96% ρth, compared to 86.63% ρth for boron carbide only. Addition of ZrO₂ was found to increase the hardness of sintered samples and regardless of ZrO₂ content, the hardness values ranged between 30 and 31.5 GPa. XRD of the sintered pellets showed the presence of ZrB₂. Optical microscope as well as electron probe microanalysis (EPMA) showed the presence of two phases, grey matrix with white precipitates. EPMA analysis of second phase revealed the presence of Zirconium in this phase. Fractography of boron carbide with 25% ZrO₂ showed the failure to be by mixed fracture (transgranular and intergranular). Thermal conductivity values of the samples measured in the temperature range of 400–1000 °C were marginally higher with the addition of ZrO₂.     

Adaptive laboratory evolution of the fast-growing cyanobacterium Synechococcus elongatus PCC 11801 for improved solvent tolerance

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