Showing papers by "Nathan S. Lewis published in 2003"

Comparison of the Electrical Properties and Chemical Stability of Crystalline Silicon(111) Surfaces Alkylated Using Grignard Reagents or Olefins with Lewis Acid Catalysts

Abstract: Four methods were used to functionalize crystalline Si(111) surfaces with alkyl groups (C_nH_(2n+1), n = 1, 2, 6, 8): chlorination with PCl_5 followed by alkylation with C_nH_(2n+1)MgX (X = Cl, Br), chlorination with Cl_2(g) followed by alkylation with C_nH_(2n+1)MgX, Lewis acid-mediated reduction of a terminal alkene, and anodization in diethyl ether containing 3.0 M CH_3MgI. The chemical properties of each surface were characterized as a function of time exposed to air using X-ray photoelectron spectroscopy, and the electrical properties of the various surfaces were probed using time-resolved radio frequency (rf) photoconductivity decay methods. Both chlorination/alkylation routes produced alkylated Si surfaces that displayed low ( 600 h of exposure to air. Surfaces functionalized through this route also displayed a significantly lower rate of oxidation than did unalkylated, H-terminated or Cl-terminated Si(111) surfaces. In contrast, surfaces modified by the Lewis acid-catalyzed reduction of 1-hexene and 1-octene exhibited high S values (S > 400 cm s^(-1)) when initially exposed to air and oxidized as rapidly as H-terminated Si(111) surfaces. Methyl-terminated Si(111) surfaces functionalized by anodization in a solution of CH_3MgI in ether exhibited stable, albeit high, S values (460 cm s^(-1)), indicating that the surface had been partially modified by the anodization process. The fractional monolayer coverage of oxide on the alkylated surface after exposure to air was determined for each functionalization technique. Although all four of the functionalization routes studied in this work introduced alkyl groups onto the Si surface, subtle changes in the extent and quality of the alkyl termination are significant factors in determining the magnitude and degree of chemical and electrical passivation of the resulting functionalized Si surfaces.

Methods for remote characterization of an odor

Abstract: Provided are compositions and systems useful in remote monitoring of chemical hazards, air quality, and medical conditions, for example, robotic systems to search for and detect explosives, mines, and hazardous chemicals. In addition, the methods, systems and compositions of the invention provide the ability to mine data from a database containing a plurality of chemical fingerprints.

Effects of bridging ligands on the current-potential behavior and interfacial kinetics of ruthenium-sensitized nanocrystalline TiO2 photoelectrodes

Abstract: We have shown that Ru^(II)(bpy)_2(bpy-4-(xylyl)_x-≡-phenyl-COOH)(PF_6)_2 (abbreviated Rux, where x = 0, 1 or 2 xylyl groups; bpy = 2,2‘-bipyridine) dyes can act as sensitizers for nanocrystalline TiO_2 in functional photoelectrochemical cells under simulated solar illumination, albeit with low efficiencies. Both the short-circuit photocurrent density and the open-circuit voltage decreased as x was increased. Electron injection (10^6−10^8 s^(-1)) was slightly faster for the x = 0 dye, but both recombination (10^(-15)−10^(-13) cm^3 s^(-1)) and regeneration (10^4−10^6 s^(-1) for 10 mM I^-) were slightly faster for the x = 2 dye. We suggest that the lack of distance dependence is due to the flexible one-carboxyl attachment to the surface resulting in the Ru−TiO_2 electron-tunneling distance being very similar for x = 0, 1, and 2. For all of the Rux sensitizers, a relatively small potential was needed for generation of current in the dark, indicating that the reaction between electrons in TiO_2 and the I_3^-/I^- electrolyte solution is as favorable for the Rux sensitizers as for unmodified TiO_2 electrodes.

Characterization of the temporal response profile of carbon black-polymer composite detectors to volatile organic vapors

Abstract: The relative differential resistance responses of carbon black−poly(ethylene-co-vinyl acetate) (PEVA) composite vapor detectors were evaluated in response to short rise time (<2 ms for a 17 ms pulse length) square pulses of acetone, n-hexane, methanol, 2-propanol, or toluene, in a background of synthetic air. The use of ultrathin films, along with a rapid vapor delivery system, facilitated measurement of the rapid time response available from this exemplary carbon black−polymer composite chemiresistive film for the detection of common organic vapors. Detectors formed from very thin (<200 nm) PEVA−carbon black composites produced steady-state responses within 17 ms upon exposure to methanol and produced steady-state responses within 90 ms upon exposure to toluene, acetone, and n-hexane. In accord with Fickian diffusion, the response times of the relative differential resistance of PEVA−carbon black detectors to analyte exposures were proportional to the square of the film thickness, l, in the range 510 ≤ l ≤ 5700 nm. Additionally, the relative differential resistance versus time profiles of PEVA−carbon black detectors were well fit by a simple finite difference model based on Fickian analyte diffusion, using a single analyte diffusion coefficient, for a variety of different film thicknesses and analyte concentrations.

Enhanced Sensitivity to and Classification of Volatile Carboxylic Acids Using Arrays of Linear Poly(ethylenimine)−Carbon Black Composite Vapor Detectors

Abstract: Vapor detectors formed from composites of conductors and insulating organic polymers have been tailored to produce increased sensitivity toward specific classes of analyte vapors. Upon exposure to acetic acid at 1% of its vapor pressure, detectors consisting of linear poly(ethylenimine) (l-PEI)−carbon black composites showed an ∼103 increase in signal/noise relative to the performance of typical insulating organic polymer−carbon black composite vapor detectors. Compositional diversity in an array of such vapor detectors was obtained by varying the degree of plasticization of the l-PEI films. The resulting vapor detector array produced sensitive detection of, and robust discrimination between, various volatile organic acids and relatively little response from nonacidic organic vapors or from water vapor. Measurements of the mass uptake, thickness change, and electrical conductivity of such composites indicate that swelling of the polymer film, and thus its normalized resistance response, is beyond that exp...

Estimation of chemical and physical characteristics of analyte vapors through analysis of the response data of arrays of polymer-carbon black composite vapor detectors

Abstract: Analysis of the signals produced by a collection of organic polymer-carbon black composite vapor detectors has been performed to assess the ability to estimate various chemical and physical properties of analyte vapors based on information contained in the response patterns of the detector array. A diverse array of composite chemiresistive vapor detectors was exposed to a series of 75 test analytes that had been selected from among five different chemical classes: alcohols, halogenated hydrocarbons, aromatics, unsubstituted hydrocarbons, and esters. The algorithmic task of interest was to use the resulting array of response data to assign one of the five chemical class labels to a test analyte, despite having left that analyte out of the model used to generate the class labels. Algorithms evaluated for this purpose included principal components analysis (PCA) and k-nearest neighbor (k-NN) analysis employing either Euclidean or Mahalanobis distance calculations. Each data cluster that was produced by replicate exposures to an individual analyte was well resolved from all of the other 74 analyte clusters. Furthermore, with the exception of the halide cluster, the analyte response clusters could be robustly grouped into supersets such that each of the five individual chemical classes was well-separated from every other class of analytes in principal component space. Accordingly, using either of the k-nearest neighbor algorithms, in excess of 85% of the non-halide test analyte exposures were correctly assigned to their chemical classes, and halides were only routinely confused with aromatics or esters but not with alcohols or hydrocarbons. The detector array response data also was found to contain semi-quantitative information regarding physicochemical properties of the members of the test analyte series, such as the degree of unsaturation of the carbon chain, the dipole moment, the molecular weight, the number of halogen atoms, and type of aromatic ring in the test analytes. The performance in these types of tasks is relevant for applications of a semi-selective array of vapor detectors in situations when no prior knowledge of the analyte identity is available and when there is no assurance that the test analyte will have been contained in the training set database produced by a compiling a library of responses from the detector array.

Mechanism of enhanced sensitivity of linear poly(ethylenimine)-carbon black composite detectors to carboxylic acid vapors

Abstract: Enhanced sensitivity towards volatile carboxylic acid vapors is obtained when the basic, amine-containing polymer, linear poly(ethylenimine), l -PEI, is used as the insulating component in a carbon black-polymer composite vapor detector. Specifically, at a partial pressure of analyte corresponding to 1% of its vapor pressure at room temperature, the signal-to-noise ratio for detection of acetic acid is 10 3 times larger than that for non-acidic organic vapors. Measurements of the mass uptake, thickness change, and electrical conductivity of such composites have been performed to elucidate the mechanism of this sensitivity enhancement towards volatile carboxylic acid vapors. These data have allowed quantification of the relative contributions of electrical percolation effects, increases in analyte sorption, and charge-induced swelling effects in determining the response characteristics of l -PEI composites.

Use of basic polymers in carbon black composite vapor detectors to obtain enhanced sensitivity and classification performance for volatile fatty acids

Abstract: Provided are sensors, sensor arrays, and systems for detecting an analyte. A sensor provided by the disclosure comprises an amine-containing material. Such sensors are useful to detect carboxylic-containing analytes such as fatty acids.

Controlled Passivation and Luminescence Blue Shifts of Isolated Silicon Nanocrystals

Abstract: We have performed a comparative study of oxide- and nonoxide-passivated silicon nanocrystals to probe the role of the silicon/oxygen interface in low coverage, non-interacting silicon nanocrystal systems. Ensembles of Si nanocrystals characterized by a narrow distribution and diameters of 2–5 nm were synthesized by ion implantation into SiO 2 films followed by a high-temperature anneal in Ar. The nanocrystals were removed from the SiO 2 film matrix and deposited on Si substrates using a chemical etch in HF, leaving a hydrogen-terminated surface. A natural oxide layer grows on these surfaces in air. We characterized the morphology of the samples with atomic force microscopy (AFM) and the spectroscopic properties with photoluminescence (PL) and X-Ray photoelectron spectroscopy. We found that the PL energy of Si nanocrystals can be shifted by particle size reduction and hydrogen or oxygen termination. Further, PL peak energy shifts upon etching and oxidation were consistent with the model of Wolkin et al . that proposes that for very small radii, a silicon-oxygen double bond will produce deep interface states which red shift the luminescence.

Classifier comparison and sensor selection for e-noses

Abstract: The authors analyze 4 datasets produced by an e-nose based on 20 compositionally distinct polymer-carbon black sensors exposed to laboratory designed gas mixtures and one dataset produced by a second e-nose based on five thin films metal oxide sensors exposed to the headspace of seven different coffee blends. We aim at selecting the best 5 sensors (for the e-nose which has 20) using as FS criterion the test performance of Fisher linear discriminant analysis (LDA) and multilayer perceptrons (MLP). The datasets were chosen to give classification problems of varying hardness: from the discrimination of two almost Gaussian distributed analytes to the discrimination of two analytes in the presence of interferents at different concentration levels. The performance of the best 5-sensors subset selected with MLP was found to be better - but not significantly better - than the performance of the LDA-selected subsets. This is true also for classes with a strong multimodal distribution. In only one case the test set performance distribution over all 5-sensors subsets was found to be clearly better with MLP than with LDA.