Showing papers by "Nathan S. Lewis published in 1996"

Alkylation of Si Surfaces Using a Two-Step Halogenation/Grignard Route

Abstract: Despite the fact that H-terminated, HF-etched Si crystals are the starting point for construction of most contemporary electronic devices,1 little is known about the chemical reactions of H-terminated Si surfaces under ambient temperature and pressure.2,3 Functionalization of Si without partial oxidation and/or formation of electrical defects is potentially important in fabricating improved electronic devices4,5 as well as in measurement of charge transfer rate constants at semiconductor/ liquid contacts.6 One recently described approach involves the reaction of HF-etched Si(111) with olefins and organic diacyl peroxides, in which formation of a self-assembled (near)monolayer of Si-alkyls was hypothesized.2 We report here an alternative strategy to functionalize HF-etched Si surfaces involving halogenation and subsequent reaction with alkyl Grignard or alkyl lithium reagents. We report vibrational spectroscopic and temperature programmed desorption data which confirm that the alkyl groups are bonded covalently to the Si surface, and we demonstrate that such overlayer formation can impede the undesirable oxidation of Si in a variety of environments while providing surfaces of high electrical quality. The H-terminated Si surface7 was first exposed to PCl5 for 20-60 min at 80-100 °C, in chlorobenzene with benzoyl peroxide as the radical initiator.8,9 Upon chlorination, the XP survey spectra (Figure 1) showed peaks at 270.2 ( 0.4 binding electron volts, BeV, (Cl 2s) and 199.3 ( 0.4 BeV (Cl 2p), indicating that this procedure yielded Cl on the surface. The high-resolution XP spectrum of the Si 2p peak of this surface displayed, in addition to the substrate Si signal, an additional peak located at 0.98 ( 0.12 BeV higher in binding energy (Figure 2) whose position and intensity was consistent with the formation of a surface Si-Cl bond.10 Auger electron spectra

Sensor arrays for detecting analytes in fluids

Abstract: Chemical sensors for detecting analytes in fluids comprise first and second conductive elements (e.g. electrical leads) electrically coupled to and separated by a chemically sensitive resistor which provides an electrical path between the conductive elements. The resistor comprises a plurality of alternating nonconductive regions (comprising a nonconductive organic polymer) and conductive regions (comprising a conductive material) transverse to the electrical path. The resistor provides a difference in resistance between the conductive elements when contacted with a fluid comprising a chemical analyte at a first concentration, than when contacted with a fluid comprising the chemical analyte at a second different concentration. Arrays of such sensors are constructed with at least two sensors having different chemically sensitive resistors providing dissimilar such differences in resistance. Variability in chemical sensitivity from sensor to sensor is provided by qualitatively or quantitatively varying the composition of the conductive and/or nonconductive regions. An electronic nose for detecting an analyte in a fluid may be constructed by using such arrays in conjunction with an electrical measuring device electrically connected to the conductive elements of each sensor.

Surface Structures of 4-Chlorocatechol Adsorbed on Titanium Dioxide

Abstract: TiO_2 has been extensively studied as a photocatalyst for the complete oxidation of a variety of organic pollutants commonly found in groundwater. In this paper, we investigate the surface structures formed between an organic substrate and TiO_2 in the context of understanding how these specific surface interactions affect photoreactivity. The surface complexes formed by 4-chlorocatechol (CT) sorbed on TiO_2 are investigated as a function of concentration and of pH. Singular-value decomposition of the IR spectra of CT adsorbed on TiO_2 indicates that a single bidentate chemisorbed species is present over the pH range of 2−10. The surface-bound species appears to have 40% covalent and 60% ionic bond character. The pH dependence of the adsorption isotherms is modeled using a generalized electric double-layer approach. The data are consistent with the formation of a bidentate binuclear surface group for solution CT concentrations below 50 μM followed by nonspecific multilayer partitioning at concentrations above 100 μM.

Rate Constants for Charge Transfer Across Semiconductor-Liquid Interfaces

Abstract: Interfacial charge-transfer rate constants have been measured for n-type Si electrodes in contact with a series of viologen-based redox couples in methanol through analyses of the behavior of these junctions with respect to their current density versus potential and differential capacitance versus potential properties. The data allow evaluation of the maximum rate constant (and therefore the electronic coupling) for majority carriers in the solid as well as of the dependence of the rate constant on the driving force for transfer of delocalized electrons from the n-Si semiconducting electrode into the localized molecular redox species in the solution phase. The data are in good agreement with existing models of this interfacial electron transfer process and provide insight into the fundamental kinetic events underlying the use of semiconducting photoelectrodes in applications such as solar energy conversion.

Theoretical and Experimental Upper Bounds on Interfacial Charge-Transfer Rate Constants between Semiconducting Solids and Outer-Sphere Redox Couples

Abstract: Theoretical expressions for the charge-transfer rate constant at a semiconductor/liquid junction have been modified to include the effects of adiabaticity and the existence of a Helmholtz layer at the solid/liquid interface. These expressions have yielded an estimate of the maximum interfacial charge-transfer rate constant, at optimal exoergicity, for a semiconductor in contact with a random distribution of nonadsorbing, outer-sphere redox species. An experimental upper bound on this interfacial charge-transfer rate constant has been obtained through the determination of key energetic and kinetic properties for stable semiconductor electrodes in contact with outer-sphere redox species. For this purpose, n-Si/CH3OH−dimethylferrocenium−dimethylferrocene, n-GaAs/CH3CN−ferrocenium−ferrocene, and p-InP/CH3CN−cobaltocenium−cobaltocene contacts were investigated using a combination of current density-potential and differential capacitance-potential methods. The upper limits for the interfacial charge-transfer ra...