Showing papers by "Nathan S. Lewis published in 2005"

Comparison of the device physics principles of planar and radial p-n junction nanorod solar cells

Abstract: A device physics model has been developed for radial p-n junction nanorod solar cells, in which densely packed nanorods, each having a p-n junction in the radial direction, are oriented with the rod axis parallel to the incident light direction. High-aspect-ratio (length/diameter) nanorods allow the use of a sufficient thickness of material to obtain good optical absorption while simultaneously providing short collection lengths for excited carriers in a direction normal to the light absorption. The short collection lengths facilitate the efficient collection of photogenerated carriers in materials with low minority-carrier diffusion lengths. The modeling indicates that the design of the radial p-n junction nanorod device should provide large improvements in efficiency relative to a conventional planar geometry p-n junction solar cell, provided that two conditions are satisfied: (1) In a planar solar cell made from the same absorber material, the diffusion length of minority carriers must be too low to allow for extraction of most of the light-generated carriers in the absorber thickness needed to obtain full light absorption. (2) The rate of carrier recombination in the depletion region must not be too large (for silicon this means that the carrier lifetimes in the depletion region must be longer than ~10 ns). If only condition (1) is satisfied, the modeling indicates that the radial cell design will offer only modest improvements in efficiency relative to a conventional planar cell design. Application to Si and GaAs nanorod solar cells is also discussed in detail.

Basic Research Needs for Solar Energy Utilization: report of the Basic Energy Sciences Workshop on Solar Energy Utilization, April 18-21, 2005

Abstract: This report of the Basic Energy Sciences Workshop on Solar Energy Utilization identifies the key scientific challenges and research directions that will enable efficient and economic use of the solar resource to provide a significant fraction of global primary energy by the mid 21st century. The report reflects the collective output of the workshop attendees, which included 200 scientists representing academia, national laboratories, and industry in the United States and abroad, and the U.S. Department of Energy’s Office of Basic Energy Sciences and Office of Energy Efficiency and Renewable Energy. Solar energy conversion systems fall into three categories according to their primary energy product: solar electricity, solar fuels, and solar thermal systems. Each of the three generic approaches to exploiting the solar resource has untapped capability well beyond its present usage. Workshop participants considered the potential of all three approaches, as well as the potential of hybrid systems that integrate key components of individual technologies into novel cross-disciplinary paradigms.

Enhanced Radiative Emission Rate and Quantum Efficiency in Coupled Silicon Nanocrystal-Nanostructured Gold Emitters

Abstract: We report local-field-enhanced light emission from silicon nanocrystals close to a film of nanoporous gold. We resolve photoluminescence as the gold-Si nanocrystal separation distance is varied between 0 and 20 nm and observe a fourfold luminescence intensity enhancement concomitant with increases in the coupled silicon nanocrystal/nanoporous gold absorbance cross section and radiative decay rate. A detailed analysis of the luminescence data indicated a local-field-enhanced quantum efficiency of 58% for the Si nanocrystals coupled to the nanoporous gold layer.

Chemical and electronic characterization of methyl-terminated Si(111) surfaces by high-resolution synchrotron photoelectron spectroscopy

Abstract: The chemical state, electronic properties, and geometric structure of methyl-terminated Si(111) surfaces prepared using a two-step chlorination/alkylation process were investigated using high-resolution synchrotron photoelectron spectroscopy and low-energy electron diffraction methods. The electron diffraction data indicated that the methylated Si surfaces maintained a (1×1) structure, where the dangling bonds of the silicon surface atoms were terminated by methyl groups. The surfaces were stable to annealing at 720 K. The high degree of ordering was reflected in a well-resolved vibrational fine structure of the carbon 1s photoelectron emission, with the fine structure arising from the excitation of C-H stretching vibrations having hnu=0.38±0.01 eV. The carbon-bonded surface Si atoms exhibited a well-defined x-ray photoelectron signal having a core level shift of 0.30±0.01 eV relative to bulk Si. Electronically, the Si surface was close to the flat-band condition. The methyl termination produced a surface dipole of –0.4 eV. Surface states related to piCH3 and sigmaSi-C bonding orbitals were identified at binding energies of 7.7 and 5.4 eV, respectively. Nearly ideal passivation of Si(111) surfaces can thus be achieved by methyl termination using the two-step chlorination/alkylation process.

Chemical Control of Charge Transfer and Recombination at Semiconductor Photoelectrode Surfaces

Abstract: Semiconductor/liquid contacts provide very efficient systems for converting sunlight into electrical and/or chemical energy. Until recently, relatively little was understood about the factors that control the rates of interfacial charge transfer in such systems. This Forum Article summarizes recent results that have elucidated the key factors that control such charge-transfer rates, including verification of the Marcus inverted region, identification of the maximum charge-transfer rate constant for outer-sphere, nonadsorbing redox couples at optimal exoergicity, the role of nuclear reorganization on the value of the interfacial charge-transfer rate constant at semiconductor electrodes, and the effects of pH-induced changes in the driving force on the rates of such systems. In addition, we discuss methods for using main group inorganic chemistry to control the electrical properties of surfaces of important semiconductors for solar energy conversion, with specific emphasis on alkylation of the (111)-oriented surface of Si. Control of the rates at which carriers cross such interfaces, along with control of the rates at which carriers recombine at such interfaces, forms the basis for exerting chemical control over the key solar energy conversion properties of semiconductor photoelectrode-based devices.

Detection and Classification of Volatile Organic Amines and Carboxylic Acids Using Arrays of Carbon Black-Dendrimer Composite Vapor Detectors

Abstract: Carbon black-insulator composite chemiresistive vapor detectors have been prepared using dendrimers as the polymeric constituent of the composite. Amino-terminated dendrimer-carbon black composites exhibited an enhancement in detection sensitivity of ∼103 for volatile carboxylic acids as compared to nondendrimeric insulating polymer-carbon black composites. Similarly, protonated carboxylato-terminated and protonated amino-terminated dendrimer-carbon black composites showed an ∼103−104 increase in sensitivity for detection of volatile amines relative to the response of nondendrimeric insulating polymer-carbon black composites. The protonated amino-terminated dendrimer carbon black composite detectors exhibited a signal-to-noise ratio (S/N) of 22.4 ± 0.9 upon exposure to 2.7 ppb of butylamine in air, whereas poly(ethylene oxide)-carbon black composites exhibited a S/N of 3.5 ± 1.2 at 54 ppm of butylamine. The protonated amino-terminated dendrimer-carbon black detectors additionally exhibited relatively smal...

Low-Temperature STM Images of Methyl-Terminated Si(111) Surfaces

Abstract: Low-temperature scanning tunneling microscopy (STM) has been used to image CH3-terminated Si(111) surfaces that were prepared through a chlorination/alkylation procedure. The STM data revealed a well-ordered structure commensurate with the atop sites of an unreconstructed 1 × 1 overlayer on the silicon (111) surface. Images collected at 4.7 K revealed bright spots, separated by 0.18 ± 0.01 nm, which are assigned to adjacent H atoms on the same methyl group. The C−H bonds in each methyl group were observed to be rotated by 7 ± 3° away from the center of an adjacent methyl group and toward an underlying Si atom. Hence, the predominant interaction that determines the surface structure arises from repulsions between hydrogen atoms on neighboring methyl groups, and secondary interactions unique to the surface are also evident.

High-Resolution X-ray Photoelectron Spectroscopic Studies of Alkylated Silicon(111) Surfaces

Abstract: Hydrogen-terminated, chlorine-terminated, and alkyl-terminated crystalline Si(111) surfaces have been characterized using high-resolution, soft X-ray photoelectron spectroscopy from a synchrotron radiation source. The H-terminated Si(111) surface displayed a Si 2p_(3/2) peak at a binding energy 0.15 eV higher than the bulk Si 2p_(3/2) peak. The integrated area of this shifted peak corresponded to one equivalent monolayer, consistent with the assignment of this peak to surficial Si−H moieties. Chlorinated Si surfaces prepared by exposure of H-terminated Si to PCl_5 in chlorobenzene exhibited a Si 2p_(3/2) peak at a binding energy of 0.83 eV above the bulk Si peak. This higher-binding-energy peak was assigned to Si−Cl species and had an integrated area corresponding to 0.99 of an equivalent monolayer on the Si(111) surface. Little dichloride and no trichloride Si 2p signals were detected on these surfaces. Silicon(111) surfaces alkylated with C_nH_(2n+1)^− (n = 1 or 2) or C_6H_5CH_2^− groups were prepared by exposing the Cl-terminated Si surface to an alkylmagnesium halide reagent. Methyl-terminated Si(111) surfaces prepared in this fashion exhibited a Si 2p_(3/2) signal at a binding energy of 0.34 eV above the bulk Si 2p_(3/2) peak, with an area corresponding to 0.85 of a Si(111) monolayer. Ethyl- and C_6H_5CH_2-terminated Si(111) surfaces showed no evidence of either residual Cl or oxidized Si and exhibited a Si 2p_(3/2) peak ∼0.20 eV higher in energy than the bulk Si 2p_(3/2) peak. This feature had an integrated area of ∼1 monolayer. This positively shifted Si 2p_(3/2) peak is consistent with the presence of Si−C and Si−H surface functionalities on such surfaces. The SXPS data indicate that functionalization by the two-step chlorination/alkylation process proceeds cleanly to produce oxide-free Si surfaces terminated with the chosen alkyl group.

Chlorination of hydrogen-terminated silicon (111) surfaces

Abstract: Infrared absorption spectroscopy was used to investigate the chlorination of hydrogen-terminated Si(111) surfaces by three different methods: (a) exposure to a saturated solution of phosphorus pentachloride (PCl5) in chlorobenzene; (b) exposure to chlorine gas, Cl2(g), and (c) exposure to Cl2(g) under UV illumination. X-ray photoelectron spectroscopy and first principles model (clusters) calculations were used to explore the structure and dynamics of these surfaces. The infrared spectra exhibited sharp chlorine-related vibrations at 586 and 527 cm^–1. The narrow full width at half maximum of these vibrations for all three preparation methods indicated that all functionalization schemes produced a nearly complete monolayer of Cl with little surface roughening or introduction of step edges. The 527 cm^–1 mode was at a much higher frequency than might be expected for the bending vibration of Si monochloride. Theoretical calculations show, however, that this vibration involves the displacement of the top Si atom parallel to the surface, subject to a relatively stiff potential, shifting its frequency to a value fairly close to that of the Si–Cl stretching mode on a Si(111) surface.

High-resolution x-ray photelectron spectroscopic studies of alkylated silicon(111) surfaces

Abstract: Hydrogen-terminated, chlorine-terminated, and alkyl-terminated crystalline Si(111) surfaces have been characterized using high-resolution, soft X-ray photoelectron spectroscopy from a synchrotron radiation source. The H-terminated Si(111) surface displayed a Si 2p(3/2) peak at a binding energy 0.15 eV higher than the bulk Si 2p(3/2) peak. The integrated area of this shifted peak corresponded to one equivalent monolayer, consistent with the assignment of this peak to surficial Si-H moieties. Chlorinated Si surfaces prepared by exposure of H-terminated Si to PCl5 in chlorobenzene exhibited a Si 2p(3/2) peak at a binding energy of 0.83 eV above the bulk Si peak. This higher-binding-energy peak was assigned to Si-Cl species and had an integrated area corresponding to 0.99 of an equivalent monolayer on the Si(111) surface. Little dichloride and no trichloride Si 2p signals were detected on these surfaces. Silicon(111) surfaces alkylated with CnH(2n+1)- (n = 1 or 2) or C6H5CH2- groups were prepared by exposing the Cl-terminated Si surface to an alkylmagnesium halide reagent. Methyl-terminated Si(111) surfaces prepared in this fashion exhibited a Si 2p(3/2) signal at a binding energy of 0.34 eV above the bulk Si 2p(3/2) peak, with an area corresponding to 0.85 of a Si(111) monolayer. Ethyl- and C6H5CH2-terminated Si(111) surfaces showed no evidence of either residual Cl or oxidized Si and exhibited a Si 2p(3/2) peak approximately 0.20 eV higher in energy than the bulk Si 2p(3/2) peak. This feature had an integrated area of approximately 1 monolayer. This positively shifted Si 2p(3/2) peak is consistent with the presence of Si-C and Si-H surface functionalities on such surfaces. The SXPS data indicate that functionalization by the two-step chlorination/alkylation process proceeds cleanly to produce oxide-free Si surfaces terminated with the chosen alkyl group.

Measurement of the Free-Energy Dependence of Interfacial Charge-Transfer Rate Constants using ZnO/H2O Semiconductor/Liquid Contacts

Abstract: The dependence of electron-transfer rate constants on the driving force for interfacial charge transfer has been investigated using n-type ZnO electrodes in aqueous solutions. Differential capacitance versus potential and current density versus potential measurements were used to determine the energetics and kinetics, respectively, of the interfacial electron-transfer processes. A series of nonadsorbing, one-electron, outer-sphere redox couples with formal reduction potentials that spanned approximately 900 mV allowed evaluation of both the normal and Marcus inverted regions of interfacial electron-transfer processes. All rate processes were observed to be kinetically first-order in the concentration of surface electrons and first-order in the concentration of dissolved redox acceptors. The band-edge positions of the ZnO were essentially independent of the Nernstian potential of the solution over the range 0.106−1.001 V vs SCE. The rate constant at optimal exoergicity was observed to be approximately 10-1...

Measurement of the Dependence of Interfacial Charge-Transfer Rate Constants on the Reorganization Energy of Redox Species at n-ZnO/H2O Interfaces

Abstract: The interfacial energetic and kinetics behavior of n-ZnO/H2O contacts have been determined for a series of compounds, cobalt trisbipyridine (Co(bpy)33+/2+), ruthenium pentaamine pyridine (Ru(NH3)5py3+/2+), cobalt bis-1,4,7-trithiacyclononane (Co(TTCN)23+/2+), and osmium bis-dimethyl bipyridine bis-imidazole (Os(Me2bpy)2(Im)23+/2+), which have similar formal reduction potentials yet which have reorganization energies that span approximately 1 eV. Differential capacitance vs potential and current density vs potential measurements were used to measure the interfacial electron-transfer rate constants for this series of one-electron outer-sphere redox couples. Each interface displayed a first-order dependence on the concentration of redox acceptor species and a first-order dependence on the concentration of electrons in the conduction band at the semiconductor surface, in accord with expectations for the ideal model of a semiconductor/liquid contact. Rate constants varied from 1 × 10-19 to 6 × 10-17 cm4 s-1. T...

Radial pn junction nanorod solar cells: device physics principles and routes to fabrication in silicon

Abstract: We have developed quantitative device-physics models for a radial pn junction nanorod solar cell, that is, a cell which consists of densely packed nanorods attached to a conducting substrate, each nanorod with a pn junction in the radial direction. It is found that this novel design shows large improvements over the planar geometry so long as two conditions are satisfied: a) a planar solar cell made from the same material is collection limited, i.e. the diffusion length of minority carriers is too low to allow for collection of most or all of the light-generated carriers in the conventional planar geometry, and b) recombination in the depletion region is not too high, or, equivalently, the lifetime of carriers in the depletion region is not too short. In order to experimentally validate this concept, the vapor-liquid-solid (VLS) growth of silicon (Si) nanorods has been explored using metal catalyst particles that are not as deleterious to the minority carrier lifetime of Si as gold (Au), the most commonly used wire growth catalyst.

Comparison of analytical methods and calibration methods for correction of detector response drift in arrays of carbon black-polymer composite vapor detectors

Abstract: The responses of 15 carbon black-polymer composite chemiresistors have been analyzed during exposure to eight different analytes (n-hexane, tetrahydrofuran, ethanol, ethyl acetate, cyclohexane, n-heptane, n-octane, and isooctane) in random order at low concentration (0.5% of the vapor pressure of analyte at room temperature) over 4 months (8000 total analyte exposures) of data collection. Data were collected for periods during which the array was continuously exposed periodically to analytes and after long periods during which no analyte exposures had been performed. All but the most difficult separation tasks (for example, discrimination between low concentrations of straight-chain hydrocarbons) could be performed robustly over the entire 4 month time period based only on the use of a decision boundary formulated from an initial training set of 200 exposures, indicating the sensor drift had minimal effect on system performance in such classification tasks. For the remaining classification tasks, modeling the dynamics of sensor drift either through a linear regression or Fourier transform decomposition of the individual relative differential resistance responses versus time of each sensor yielded little improvement in classification performance, indicating that external events were largely responsible for changes in sensor response versus time. Six analytes that were not treated as unknowns for a binary separation task were individually treated as calibrants whose response was intermittantly used to renormalize the response of the sensor array. A simple linear sensor-by-sensor calibration scheme proved effective at restoring the classification performance of difficult binary separation tasks to the performance that was observed in the initial training set period. Calibrants that were mutually similar to the analytes being differentiated tended to be more effective than calibrants that were very chemically different from the analytes of interest. Evaluation of various calibration protocols indicated that an optimal tradeoff existed between the number of calibration exposures and the frequency of calibration periods. Condition-based calibration, in which calibration was only performed when the classification model exhibited a decline in classification performance below a predetermined threshold value, was observed to be superior to a time-based calibration approach or to interval-based, cyclic calibration protocols for this set of analytes exposed under the chosen analysis conditions.

Chemical Control of Charge Transfer and Recombination at Semiconductor Photoelectrode Surfaces

Abstract: Semiconductor/liquid contacts provide very efficient systems for converting sunlight into electrical and/or chemical energy. Until recently, relatively little was understood about the factors that control the rates of interfacial charge transfer in such systems. This Forum Article summarizes recent results that have elucidated the key factors that control such charge-transfer rates, including verification of the Marcus inverted region, identification of the maximum charge-transfer rate constant for outer-sphere, nonadsorbing redox couples at optimal exoergicity, the role of nuclear reorganization on the value of the interfacial charge-transfer rate constant at semiconductor electrodes, and the effects of pH-induced changes in the driving force on the rates of such systems. In addition, we discuss methods for using main group inorganic chemistry to control the electrical properties of surfaces of important semiconductors for solar energy conversion, with specific emphasis on alkylation of the (111)-oriented surface of Si. Control of the rates at which carriers cross such interfaces, along with control of the rates at which carriers recombine at such interfaces, forms the basis for exerting chemical control over the key solar energy conversion properties of semiconductor photoelectrode-based devices.