Showing papers by "Nathan S. Lewis published in 2001"

Electron Transfer Dynamics in Nanocrystalline Titanium Dioxide Solar Cells Sensitized with Ruthenium or Osmium Polypyridyl Complexes

Abstract: The electron transfer dynamics in solar cells that utilize sensitized nanocrystalline titanium dioxide photoelectrodes and the iodide/triiodide redox couple have been studied on a nanosecond time scale. The ruthenium and osmium bipyridyl complexes Ru(H2L‘)2(CN)2, Os(H2L‘)2(CN)2, Ru(H2L‘)2(NCS)2, and Os(H2L‘)2(NCS)2, where H2L‘ is 4,4‘-dicarboxylic acid 2,2‘-bipyridine, inject electrons into the semiconductor with a rate constant >108 s-1. The effects of excitation intensity, temperature, and applied potential on the recombination reaction were analyzed using a second-order kinetics model. The rates of charge recombination decrease with increasing driving force to the oxidized sensitizer, indicating that charge recombination occurs in the Marcus inverted region. The electronic coupling factors between the oxidized sensitizer and the injected electrons in TiO2 and the reorganization energies for the recombination reaction vary significantly for the different metal complexes. The charge recombination rates a...

Light work with water.

Abstract: Sunlight can be harnessed by semiconductors to generate a fuel, hydrogen gas, from water. This approach will be impracticable until certain materials-related constraints are overcome: photochemists are on the case.

Spectroscopic Studies of the Modification of Crystalline Si(111) Surfaces with Covalently-Attached Alkyl Chains Using a Chlorination/Alkylation Method

Abstract: A two-step procedure, involving radical-initiated chlorination of the Si surface with PCl5 followed by reaction of the chlorinated surface with alkyl-Grignard or alkyl-lithium reagents, has been developed to functionalize crystalline (111)-oriented H-terminated Si surfaces. The surface chemistry that accompanies these reaction steps has been investigated using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), temperature programmed desorption spectroscopy (TPDS), high-resolution electron energy loss spectroscopy (HREELS), infrared (IR) spectroscopy in both glancing transmission (TIR) and attenuated total multiple internal reflection (ATR) modes, ellipsometry, and contact angle goniometry. The XPS data show the appearance of the Cl signal after exposure to PCl5 and show its removal, and concomitant appearance of a C 1s signal, after the alkylation step. Auger electron spectra, in combination with TPD spectroscopy, demonstrate the presence of Cl after the chlorination process and it...

Use of an array of polymeric sensors of varying thickness for detecting analytes in fluids

Abstract: Chemical sensors for detecting the activity of a molecule or analyte of interest is provided. The chemical sensors comprise and array or plurality of sensors that are capable of interacting with a molecule of interest, wherein the interaction provides a response fingerprint. The fingerprint can be associated with a library of similar molecules of interest to determine the molecule's activity and diffusion coefficient.

Formation of Covalently Attached Polymer Overlayers on Si(111) Surfaces Using Ring-Opening Metathesis Polymerization Methods

Abstract: We describe a method for growing uniform, covalently attached polymer onto crystalline Si(111) surfaces H-Terminated Si was first chlorinated, and the surface-bound chlorine was then replaced by a terminal olefin using a Grignard reaction A ruthenium ring-opening metathesis polymerization catalyst was then crossed onto the terminal olefin, and the resulting surface was subsequently immersed into a solution of monomer to produce the desired surface-attached polymer The method provides a direct linkage between the polymer and the Si without the presence of an electrically defective oxide layer Growth of the polymeric layer could be controlled by varying the concentration of monomer in solution, and polynorbornene films between 09 and 5500 nm in thickness were produced through the use of 001−244 M solutions of norbornene

Spatiotemporal and geometric optimization of sensor arrays for detecting analytes in fluids

Abstract: Sensor arrays and sensor array systems for detecting analytes in fluids. Sensors configured to generate a response upon introduction of a fluid containing one or more analytes can be located on one or more surfaces relative to one or more fluid channels in an array. Fluid channels can take the form of pores or holes in a substrate material. Fluid channels can be formed between one or more substrate plates. Sensor can be fabricated with substantially optimized sensor volumes to generate a response having a substantially maximized signal to noise ratio upon introduction of a fluid containing one or more target analytes. Methods of fabricating and using such sensor arrays and systems are also disclosed.

Detection and classification characteristics of arrays of carbon black/organic polymer composite chemiresistive vapor detectors for the nerve agent simulants dimethylmethylphosphonate and diisopropylmethylphosponate.

Abstract: Arrays of conducting polymer composite vapor detectors have been evaluated for performance in the presence of the nerve agent simulants dimethylmethylphosphonate (DMMP) and diisopropylmethylphosponate (DIMP). Limits of detection for DMMP on unoptimized carbon black/organic polymer composite vapor detectors in laboratory air were estimated to be 0.047−0.24 mg m^(-3). These values are lower than the EC_50 value (where EC_50 is the airborne concentration sufficient to induce severe effects in 50% of those exposed for 30 min) for the nerve agents sarin (methylphosphonofluoridic acid, 1-methylethyl ester) and soman (methylphosphonofluoridic acid, 1,2,2-trimethylpropyl ester), which has been established as ∼0.8 mg m^(-3). Arrays of these vapor detectors were easily able to resolve signatures due to exposures to DMMP from those due to DIMP or due to a variety of other test analytes (including water, methanol, benzene, toluene, diesel fuel, lighter fluid, vinegar, and tetrahydrofuran) in a laboratory air background. In addition, DMMP at 27 mg m^(-3) could be detected and differentiated from the signatures of the other test analytes in the presence of backgrounds of potential interferences, including water, methanol, benzene, toluene, diesel fuel, lighter fluid, vinegar, and tetrahydrofuran, even when these interferents were present in much higher concentrations than that of the DMMP or DIMP being detected.

Investigation of the size-scaling behavior of spatially nonuniform barrier height contacts to semiconductor surfaces using ordered nanometer-scale nickel arrays on silicon electrodes

Abstract: Nanosphere lithography has been used to prepare a series of ordered, periodic arrays of low barrier height nanometer-scale n-Si/Ni contacts interspersed among high barrier height n-Si/liquid contacts To form the arrays, ordered bilayers of close-packed polystyrene spheres were deposited onto (100)-oriented n-type single crystal Si surfaces The spheres formed a physical mask through which Ni was evaporated to produce regularly spaced and regularly sized Si/Ni contacts By varying the diameter of the latex spheres from 174 to 1530 nm, geometrically self-similar Si/Ni structures were produced having triangular Si/Ni regions with edge dimensions of 100−800 nm The resulting Si surfaces were used as electrodes in contact with a methanolic solution of LiClO_4 and 1,1‘-dimethylferrocene^(+/0) The current−voltage and photoresponse properties of these mixed barrier height contacts were strongly dependent on the size of the Ni regions, even though the fraction of the Si surface covered by Ni remained constant Electrodes formed from large-dimension Si/Ni and Si/electrolyte contacts behaved as expected for two area-weighted Schottky diodes operating independently and in parallel, whereas electrodes having nanoscale Si/Ni regions surrounded by Si/liquid contacts behaved in accord with effective barrier height theories that predict a “pinch-off” effect for mixed barrier height systems of sufficiently small physical dimensions

Nanoparticle-based sensors for detecting analytes in fluids

Abstract: Sensors, sensor arrays and sensing methods provide for detection of a chemical analyte in a fluid. Sensors include a plurality of conductive and nonconductive regions in contact with a measuring apparatus. One or more sensors include a plurality of particles that include a metallic core. Preferably, the particles also include one or more capping ligands coupled to the metallic core. Exposure of the sensors to a fluid containing a chemical analyte causes the analyte to react with the metal core, preferably by displacing one or more of the capping ligands. The chemical analyte can be detected based on a change in electrical or optical properties of the sensors.

Comparison of odor detection thresholds and odor discriminablities of a conducting polymer composite electronic nose versus mammalian olfaction

Abstract: Response data from an array of conducting polymer composite vapor detectors that form an electronic nose were collected for the purpose of comparing selected, quantitatively measurable, phenomena in odor detection and classification to the olfactory characteristics of monkeys and humans. Odor detection thresholds and discriminability between structurally similar pairs of odorants were the two primary quantities evaluated for this comparison. Comparisons were only made for volatile organic vapors as opposed to aroma active odorant vapors. Electronic nose detection thresholds for a homologous series of n-alkane and 1-alcohol odorants were determined and the results were compared to literature values for the mean olfactory detection thresholds observed in psychophysical experiments on humans exposed to these same vapors. The trends in odor detection thresholds of the electronic nose towards the tested analytes were very similar to those exhibited by humans. The discrimination performance of the electronic nose for distinguishing between pairs of odorants within incrementally varying series of esters, carboxylic acids and alcohols were also compared to the published data of Laska and co-workers on the psychophysical performance of humans and monkeys for these same odorant pairs. Similar trends were generally observed between the humans, monkeys, and the electronic nose in that discrimination performance increased as the compounds of an odorant pair became more structurally dissimilar. With use of the Fisher linear discriminant algorithm for classification of these test pairs of odorants, the electronic nose exhibited significantly better discriminability than humans or monkeys for the odorant pairs evaluated in this work under the test conditions for which the discriminability was evaluated. # 2001 Elsevier Science B.V. All rights reserved.

Assessing the ability to predict human percepts of odor quality from the detector responses of a conducting polymer composite-based electronic nose

Abstract: The responses of a conducting polymer composite “electronic nose” detector array were used to predict human perceptual descriptors of odor quality for a selected test set of analytes. The single-component odorants investigated in this work included molecules that are chemically quite distinct from each other, as well as molecules that are chemically similar to each other but which are perceived as having distinct odor qualities by humans. Each analyte produced a different, characteristic response pattern on the electronic nose array, with the signal strength on each detector reflecting the relative binding of the odorant into the various conducting polymer composites of the detector array. A “human perceptual space” was defined by reference to English language descriptors that are frequently used to describe odors. Data analysis techniques, including standard regression, nearest-neighbor prediction, principal components regression, partial least squares regression, and feature subset selection, were then used to determine mappings from electronic nose measurements to this human perceptual space. The effectiveness of the derived mappings was evaluated by comparison with average human perceptual data published by Dravnieks. For specific descriptors, some models provided cross-validated predictions that correlated well with the human data (above the 0.60 level), but none of the models could accurately predict the human values for more than a few descriptors.

Electrical passivation of silicon-containing surfaces using organic layers

Abstract: Electrical structures and devices may be formed and include an organic passivating layer that is chemically bonded to a silicon-containing semiconductor material to improve the electrical properties of electrical devices. In different embodiments, the organic passivating layer may remain within finished devices to reduce dangling bonds, improve carrier lifetimes, decrease surface recombination velocities, increase electronic efficiencies, or the like. In other embodiments, the organic passivating layer may be used as a protective sacrificial layer and reduce contact resistance or reduce resistance of doped regions. The organic passivation layer may be formed without the need for high-temperature processing.

Comparison of the Performance of Different Discriminant Algorithms in Analyte Discrimination Tasks Using an Array of Carbon Black−Polymer Composite Vapor Detectors

Abstract: An array of 20 compositionally different carbon black−polymer composite chemiresistor vapor detectors was challenged under laboratory conditions to discriminate between a pair of extremely similar pure analytes (H_(2)O and D_(2)O), compositionally similar mixtures of pairs of compounds, and low concentrations of vapors of similar chemicals. Several discriminant algorithms were utilized, including k nearest neighbors (kNN, with k = 1), linear discriminant analysis (LDA, or Fisher's linear discriminant), quadratic discriminant analysis (QDA), regularized discriminant analysis (RDA, a hybrid of LDA and QDA), partial least squares, and soft independent modeling of class analogy (SIMCA). H_(2)O and D_(2)O were perfectly classified by most of the discriminants when a separate training and test set was used. As expected, discrimination performance decreased as the analyte concentration decreased, and performance decreased as the composition of the analyte mixtures became more similar. RDA was the overall best-performing discriminant, and LDA was the best-performing discriminant that did not require several cross-validations for optimization.

Medical applications of artificial olfactometry

Abstract: The present invention provides methods for detecting the presence of an analyte indicative of various medical conditions, including halitosis, periodontal disease and other diseases are also disclosed.

Low power, lightweight vapor sensing using arrays of conducting polymer composite chemically-sensitive resistors.

Abstract: Arrays of broadly responsive vapor detectors can be used to detect, identify, and quantify vapors and vapor mixtures. One implementation of this strategy involves the use of arrays of chemically-sensitive resistors made from conducting polymer composites. Sorption of an analyte into the polymer composite detector leads to swelling of the film material. The swelling is in turn transduced into a change in electrical resistance because the detector films consist of polymers filled with conducting particles such as carbon black. The differential sorption, and thus differential swelling, of an analyte into each polymer composite in the array produces a unique pattern for each different analyte of interest, Pattern recognition algorithms are then used to analyze the multivariate data arising from the responses of such a detector array. Chiral detector films can provide differential detection of the presence of certain chiral organic vapor analytes. Aspects of the spaceflight qualification and deployment of such a detector array, along with its performance for certain analytes of interest in manned life support applications, are reviewed and summarized in this article.

Mining the Detector Responses of a Conducting Polymer Composite-Based Electronic Nose.

Abstract: Arrays of polymer films embedded with conductive or resistive material have attracted significant attention as “electronic noses”. Sorption of a vapor into the polymer films causes physical swelling, which leads to a change in the DC electrical resistance of the film. Like the receptors of the mammalian olfactory system, each polymer-based detector responds to more than one analyte and each analyte elicits a response from more than one detector. The DC resistance across the array of detectors is sampled versus time producing a multivariate time-series. Except when rapid response is critical, this multivariate time-series can be converted into a vector representation by focusing on the steady-state behavior and calculating the relative change in resistance in each channel relative to an initial baseline. In this paper, we provide an overview of our efforts to mine useful information from electronic nose data. Four case studies are presented: rapid detection of DNT (dinitrotoluene) vapors at very low concentrations (< 1 part-per-billion), pairwise discrimination between chemically similar analytes (e.g., H2O and D2O; heptane and hexane), prediction of human percepts of odor quality from electronic nose detector responses, and classification of vapors into the appropriate chemical family (e.g., saying methanol is an alcohol without having previously smelled methanol). keywords: electronic nose, scientific datasets, case study/application, pattern recognition, classification, statistical methods ∗1. JPL, M/S 126-347, 4800 Oak Grove Drive, Pasadena, CA 91109, burl@aig.jpl.nasa.gov †2. WUSTL, Campus Box 1134, St. Louis, MO 63130, vaid@wuchem.wustl.edu ‡3. Caltech, M/S 127-72, Pasadena, CA 91125, nslewis@its.caltech.edu 1 Copyright © by SIAM. Unauthorized reproduction of this article is prohibited burlm re 2001/2/17 page 2 ✐ ✐

Array-based carbon black-polymer composite vapor detectors for detection of DNT in environments containing complex analyte mixtures

Abstract: Thin films of carbon black-organic polymer composites have been deposited across two metallic leads, with sorption of vapors producing swelling-induced resistance changes of the detector films. To identify and classify vapors, arrays of such vapor sensing elements have been constructed in which each element of the array contains a different polymer as the insulating phase and a common conductor, carbon black, as the conducting phase. The differing gas-solid partition coefficients for the various polymers of the detector array produce a pattern of differential resistance changes that is used to classify vapors and vapor mixtures. The performance of this detector array system towards 2,4-dinitrotoluene, the predominant signature in the vapor phase above land mines, in the presence high concentrations of water or of acetone has been evaluated.

Array Based Carbon Black-Polymer Composite Vapor Detectors for Detection of DNT in Environments Containing Complex Analyte Mixtures

Abstract: Thin films of carbon black-organic polymer composites have been deposited across two metallic leads, with sorption of vapors producing swelling-induced resistance changes of the detector films. To identify and classify vapors, arrays of such vapor sensing elements have been constructed in which each element of the array contains a different polymer as the insulating phase and a common conductor, carbon black, as the conducting phase. The differing gas-solid partition coefficients for the various polymers of the detector array produce a pattern of differential resistance changes that is used to classify vapors and vapor mixtures. The performance of this detector array system towards 2,4-dinitrotoluene, the predominant signature in the vapor phase above land mines, in the presence high concentrations of water or of acetone (as a selected volatile organic carbon vapor), has been evaluated.

A Conducting Polymer-Based Electronic Nose for Landmine Detection

Abstract: : This program was part of DARPA's 'Dog Nose' initiative to develop landmine detection technology based upon the chemical signature of the mine explosive charge. The focus of this DARPA-sponsored project was to exploit the exciting breakthrough technology developed recently at Caltech that forms the basis for a low power, simple , manufacturable 'electronic nose'. This nose-on-a-chip involves chemically sensitive resistors, whose signals reveal the identification and concentration of vapors in a fashion analogous to that of the mammalian olfactory system. This technology has been developed into a landmine detection system, based on the characteristic chemical signature of mines, that operates in real-time through a VLSI-compatible Si process.