Showing papers by "Nathan S. Lewis published in 2007"

Toward Cost-Effective Solar Energy Use

Abstract: At present, solar energy conversion technologies face cost and scalability hurdles in the technologies required for a complete energy system. To provide a truly widespread primary energy source, solar energy must be captured, converted, and stored in a cost-effective fashion. New developments in nanotechnology, biotechnology, and the materials and physical sciences may enable step-change approaches to cost-effective, globally scalable systems for solar energy use.

Solar energy conversion.

Abstract: If solar energy is to become a practical alternative to fossil fuels, we must have efficient ways to convert photons into electricity, fuel, and heat. The need for better conversion technologies is a driving force behind many recent developments in biology, materials, and especially nanoscience.

Growth of vertically aligned Si wire arrays over large areas (>1 cm^2) with Au and Cu catalysts

Abstract: Arrays of vertically oriented Si wires with diameters of 1.5 µm and lengths of up to 75 µm were grown over areas >1 cm^2 by photolithographically patterning an oxide buffer layer, followed by vapor-liquid-solid growth with either Au or Cu as the growth catalyst. The pattern fidelity depended critically on the presence of the oxide layer, which prevented migration of the catalyst on the surface during annealing and in the early stages of wire growth. These arrays can be used as the absorber material in novel photovoltaic architectures and potentially in photonic crystals in which large areas are needed.

Powering the Planet

Abstract: The following article is an edited transcript based on the plenary presentation given by Nathan S. Lewis (California Institute of Technology) on April 11, 2007, at the Materials Research Society Spring Meeting in San Francisco.

High aspect ratio silicon wire array photoelectrochemical cells.

Abstract: In an effort to develop low-cost solar energy conversion techniques, high uniformity vertically oriented silicon wire arrays have been fabricated. These arrays, which allow for radial diffusion of minority charge carriers, have been measured in a photoelectrochemical cell. Large photovoltages (∼400 mV) have been measured, and these values are significantly greater than those obtained from the substrate alone. Additionally, the wire array samples displayed much higher current densities than the underlying substrate, demonstrating that significant energy conversion was occurring due to the absorption and charge-carrier transport in the vertically aligned Si wires. This method therefore represents a step toward the use of collection-limited semiconductor materials in a wire array format in macroscopic solar cell devices.

Plasmon-Enhanced Photoluminescence of Silicon Quantum Dots: Simulation and Experiment

Abstract: The enhancement of photoluminescence emission from silicon quantum dots in the near field of cylindrical silver particles has been calculated using finite integration techniques. This computational method permitted a quantitative examination of the plasmon resonance frequencies and locally enhanced fields surrounding coupled arrays of silver particles having arbitrary shapes and finite sizes. We have studied Ag nanoparticles with diameters in the 50−300 nanometer range and array pitches in the range of 50−800 nm, near a plane of optical emitters spaced 10−40 nm from the arrays. The calculated and experimental plasmon resonance frequencies and luminescence enhancements are in good agreement. In the tens-of-nanometers size regime, for the geometries under investigation, two competing factors affect the photoluminescence enhancement; on one hand, larger field enhancements, which produce greater emission enhancements, exist around smaller silver particles. However, as the spacing of such particles is decrease...

Electrical Properties of Junctions between Hg and Si(111) Surfaces Functionalized with Short-Chain Alkyls

Abstract: Metal−semiconductor junctions between Hg and chemically modified n- and p-Si(111) surfaces have been prepared and analyzed using current−voltage and differential capacitance−voltage methods. To understand the role of the interfacial dipole on interfacial charge transfer, silicon surfaces were modified with either nonstoichoimetric oxide (SiO_x), terminal monohydride, short (CnH_(2n+1)−, n = 1, 2, 3) saturated alkyl chains, or propynyl (CH_3−C≡C−) groups. X-ray photoelectron spectra of the modified Si electrode surfaces taken before and after exposure to Hg contacts showed no evidence of irreversible chemical interactions between the Si and the Hg. Hg/Si contacts made using H-terminated Si(111) surfaces exhibited Schottky junctions having barrier heights (Φ_b) that were consistent with the known surface electron affinity of Si and the work function of Hg. In contrast, Si coated with a thin, chemically grown oxide formed Hg/Si junctions having barrier heights suggestive of Fermi level pinning. Si(111) surfaces modified with methyl groups yielded Hg junctions having barrier heights in accord with expectations based on the electron affinity (3.67 eV) and surface dipole (0.38 eV) measured on such surfaces by photoemission spectroscopy, attesting to the degree of chemical control that can be exerted over the barrier heights of such systems by surface functionalization methods. Incomplete coverages of functional groups produced by alkylation with ethyl or iso-propyl groups did not greatly impact the observed values of Φ_b relative to Φ_b values observed for CH_3-terminated Si(111) surfaces. However, the observed variation in Φ_b between nominally identical samples increased as the number of carbons in the functionalizing alkyl group increased. Junctions between Hg and Si(111) surfaces modified with propynyl groups showed nearly identical behavior to that of CH_3−Si(111)/Hg contacts, both in average Φ_b values and standard deviation between samples. The behavior of Si/Hg interfaces modified with short organic functional groups is consistent with the efficacy and utility of passivated surfaces in modifying the properties of surface-based Si devices.

High-Resolution Synchrotron Photoemission Studies of the Electronic Structure and Thermal Stability of CH3- and C2H5-Functionalized Si(111) Surfaces

Abstract: The relative coverage, thermal stability, and electronic properties of CH_(3-) and C_2H_(5-)functionalized Si(111) surfaces prepared by a two-step chlorination/alkylation procedure have been compared using high-resolution synchrotron photoemission spectroscopy. Whereas the CH_(3-) terminated Si(111) surface showed only one C 2s peak for the occupied σ orbitals, the C 2s spectra of C_2H_(5-)terminated Si(111) surfaces showed a symmetric splitting of the occupied σ orbitals, as expected for an ethyl moiety bonded to the surface. The C_2H_5 termination resulted in an unpinning of the Si surface Fermi level, with a band bending of ∼0.2 eV, and produced a surface dipole potential step of −0.23(15) eV. The observed close-to-flat-band condition is similar to that of CH_3−Si(111) and is consistent with H termination of the non-alkylated Si atop sites in the two-step chlorination/alkylation process. The C_2H_(5-)functionalized Si(111) surfaces decomposed at temperatures >300 °C, whereas CH_3−Si(111) surfaces were stable up to at least 440 °C. The data clearly highlight the similarities and identify some significant differences between the behavior of the CH_3- and C_2H_(5-)functionalized Si(111) surfaces.

Investigation of the Reactions during Alkylation of Chlorine-Terminated Silicon (111) Surfaces

Abstract: Absorption infrared spectroscopy (IRAS) and Rutherford backscattering (RBS) have been used to investigate the reaction of chlorine-terminated Si(111) surfaces with organometallic molecules (Grignard reagents). Although the predominant reaction leads to alkylation, with formation of covalent Si−C bonds, evidenced by a 678 cm^(-1) feature assigned to the Si−C stretch mode, solvents typically used during alkylation (tetrahydrofuran and methanol) can also react with Cl/Si(111) surfaces, either during the alkylation reaction or during the rinsing/cleaning process to form Si−OC_nH_(2n+1) as observed by the presence of a SiO−C stretch mode at 1090 cm^(-1). We also address the origin of some silicon oxidation observed after the methylation or ethylation reactions.

Use of spatiotemporal response information from sorption-based sensor arrays to identify and quantify the composition of analyte mixtures

Abstract: Linear sensor arrays made from small molecule/carbon black composite chemiresistors placed in a low-headspace volume chamber, with vapor delivered at low flow rates, allowed for the extraction of new chemical information that significantly increased the ability of the sensor arrays to identify vapor mixture components and to quantify their concentrations. Each sensor sorbed vapors from the gas stream and, thereby, as in gas chromatography, separated species having high vapor pressures from species having low vapor pressures. Instead of producing only equilibrium-based sensor responses that were representative of the thermodynamic equilibrium partitioning of analyte between each sensor and the initial vapor phase, the sensor responses varied depending on the position of the sensor in the chamber and the time since the beginning of the analyte exposure. The concomitant spatiotemporal (ST) sensor array response therefore provided information that was a function of time, as well as of the position of the sensor in the chamber. The responses to pure analytes and to multicomponent analyte mixtures comprised of hexane, decane, ethyl acetate, chlorobenzene, ethanol, and/or butanol were recorded along each of the sensor arrays. Use of a non-negative least-squares (NNLS) method for analysis of the ST data enabled the correct identification and quantification of the composition of two-, three-, four-, and five-component mixtures from arrays using only four chemically different sorbent films. In contrast, when traditional time- and position-independent sensor response information was used, these same mixtures could not be identified or quantified robustly. The work has also demonstrated that, for ST data, NNLS yielded significantly better results than analyses using extended disjoint principal components modeling. The ability to correctly identify and quantify constituent components of vapor mixtures through the use of such ST information significantly expands the capabilities of such broadly cross-reactive arrays of sensors.

Relationships between nonadiabatic bridged intramolecular, electrochemical, and electrical electron-transfer processes.

Abstract: Fermi's golden rule is used to develop relationships between rate constants for electron transfer in donor−bridge−acceptor and electrode−bridge−acceptor systems and resistances across metal−bridge−electrode and metal−bridge−tip junctions. Experimental data on electron-transfer rates through alkanethiolate, oligophenylene, and DNA bridges are used to calculate the electronic coupling matrix element per state through these moieties. The formulation is then used to predict the resistance of these bridges between two gold contacts. This approach provides a straightforward method for experimentalists to assess the self-consistency between intramolecular electron-transfer rate constants and low-bias resistances measured for molecularly bridged junctions between two metallic contacts. Reported resistances for alkanethiolate bridges vary by a factor of 20, with predicted resistances falling within this range. However, comparisons between carboxylato and directly linked alkanethiolate bridges suggest differences between the coupling at the interface to either the redox center or the gold electrode in such systems. Calculated resistances for oligophenylene bridges are close to those measured experimentally in a similar oligophenylene system.

Interfacial Energetics of Silicon in Contact with 11 M NH4F(aq), Buffered HF(aq), 27 M HF(aq), and 18 M H2SO4

Abstract: Open-circuit impedance spectra, channel impedance spectroscopy on solution-gated field-effect devices, and differential capacitance vs potential (Mott−Schottky) measurements were used to determine the energetics of n-Si(111), n-Si(100), and p-Si(111) electrodes in contact with aqueous 11 M (40% by weight) NH_4F, buffered HF (BHF), 27 M (48%) HF(aq), and concentrated (18 M) H_2SO_4. A Mott−Schottky analysis of A_s^2C_(sc)^(-2)-vs-E (where As is the interfacial area, and C_(sc) is the differential capacitance as a function of the electrode potential, E) data yielded reliable barrier heights for some silicon/liquid contacts in this work. Performing a Mott−Schottky analysis, however, requires measurement of the differential capacitance under reverse bias, where oxidation or etching can occur for n-Si and where electroplating of metal contaminants can occur for p-Si. Hence, open-circuit methods would offer desirable, complementary approaches to probing the energetics of such contacts. Accordingly, open-circuit, near-surface channel conductance measurements have been performed using solution-gated n^+-p-Si(111)-n^+ and p^+-n-Si(100)-p^+ devices. Additionally, open-circuit impedance spectra were obtained for silicon electrodes in contact with these solutions. The combination of the three techniques indicated that the surfaces of n-Si(111) and n-Si(100) were under accumulation when in contact with either 11 M NH_4F(aq) or BHF(aq). The barrier heights for n-Si(111) and n-Si(100) in 11 M NH_4F(aq) were −0.065 ± 0.084 V and −0.20 ± 0.21 V, respectively, and were −0.03 ± 0.19 V and −0.07 ± 0.24 V, respectively, for these surfaces in contact with buffered HF(aq). Consistently, p-Si(111) surfaces were determined to be in inversion in contact with these electrolytes, exhibiting barrier heights of 0.984 ± 0.078 V in contact with 11 M NH_4F(aq) and 0.97 ± 0.22 V in contact with buffered HF(aq). In contact with 27 M HF(aq), n-Si(111) and n-Si(100) were in depletion, with barrier heights of 0.577 ± 0.038 V and 0.400 ± 0.057 V, respectively, and p-Si(111) was under inversion with a barrier height of 0.856 ± 0.076 V. Measurements performed in 18 M H_2SO_4 revealed barrier heights of 0.75 ± 0.11 V, 0.696 ± 0.043 V, and 0.889 ± 0.018 V for n-Si(111), n-Si(100), and p-Si(111), respectively, demonstrating that in 18 M H_2SO_4, the band edge positions of Si were different for different doping types. The barrier height data demonstrate that the observed low recombination rates of silicon in contact with 11 M NH_4F, BHF, or 18 M H_2SO_4 cannot necessarily be attributed to a reduction in the number of surface trap states. In part, low surface recombination rates are expected for such systems because the very large or very small barrier height for silicon in contact with these liquids provides a potential barrier that prevents one type of photogenerated carrier (either electrons or holes) from reaching the surface, thereby producing a low steady-state surface recombination rate.

Near-surface channel impedance measurements, open-circuit impedance spectra, and differential capacitance vs potential measurements of the fermi level position at Si/ CH3CN contacts

Abstract: Near-surface channel impedance measurements, open-circuit impedance spectra, and differential capacitance vs potential measurements have been used to determine the barrier height of liquid contacts formed with n-type and p-type Si electrodes. Barrier heights were measured as the redox potential, E(A/A^-), of a metallocene-based, one-electron, outer-sphere, acceptor/donor (A/A^-) pair was varied in CH_3CN solvent. The barrier heights of p-Si(111) electrodes in contact with CH_3CN−Me_(10)Fc^(+/0) (where Me_(10)Fc is decamethylferrocene) or CH_3CN-CoCp_2^(+/0) (where CoCp_2 is cobaltocene) were 0.69 ± 0.1 and 1.1 ± 0.1 V respectively. In contrast, barrier heights for n-Si(111)/CH_3CN−Me_(10)Fc^(+/0) and n-Si(111)/CH_3CN-CoCp_2^(+/0) contacts were 0.66 ± 0.1 and 0.09 ± 0.01 V, respectively. These measurements indicate that the barrier heights closely track changes in the electrochemical potential of the contact, instead of being relatively invariant to changes in the Fermi level of the contacting phase, as is observed for Si/metal Schottky barriers. These measurements also demonstrate that the low effective surface recombination velocity, S, for silicon in contact with CoCp_2^(+/0) is primarily the result of an accumulation layer rather than solely being due to a low density of surface electrical defects.

Powering the Planet [2007 MRS Spring Meeting Plenary Address]

Abstract: I am humbled and honored to be here to tell you about a topic that is dear to everyone’s heart — and vital to the future of our planet. My colleague, Richard Smalley, gave a presentation1 on this topic several years ago, at a similar MRS plenary session. Over the last few years of Dr. Smalley’s life, he and I worked together, traveling across our country to deliver a message about a subject that we — like many others, both scientists and lay people — have come to believe is unequivocally the most important technological problem in the world: our global energy future. That is an incredibly powerful statement, one that during the next hour I hope to ably defend.