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Showing papers by "Richard J. Saykally published in 2005"


Journal ArticleDOI
TL;DR: This work introduces a version of the dye-sensitized cell in which the traditional nanoparticle film is replaced by a dense array of oriented, crystalline ZnO nanowires, which features a surface area up to one-fifth as large as a nanoparticle cell.
Abstract: Excitonic solar cells1—including organic, hybrid organic–inorganic and dye-sensitized cells (DSCs)—are promising devices for inexpensive, large-scale solar energy conversion. The DSC is currently the most efficient2 and stable3 excitonic photocell. Central to this device is a thick nanoparticle film that provides a large surface area for the adsorption of light-harvesting molecules. However, nanoparticle DSCs rely on trap-limited diffusion for electron transport, a slow mechanism that can limit device efficiency, especially at longer wavelengths. Here we introduce a version of the dye-sensitized cell in which the traditional nanoparticle film is replaced by a dense array of oriented, crystalline ZnO nanowires. The nanowire anode is synthesized by mild aqueous chemistry and features a surface area up to one-fifth as large as a nanoparticle cell. The direct electrical pathways provided by the nanowires ensure the rapid collection of carriers generated throughout the device, and a full Sun efficiency of 1.5% is demonstrated, limited primarily by the surface area of the nanowire array.

5,308 citations


Journal ArticleDOI
TL;DR: It is shown here that many of the features of the Raman spectrum that are considered to be hallmarks of a multistate system, including the asymmetric band profile, the isosbestic (temperature invariant) point, and van't Hoff behavior, actually result from a continuous distribution.
Abstract: The unique chemical and physical properties of liquid water are a direct result of its highly directional hydrogen-bond (HB) network structure and associated dynamics. However, despite intense experimental and theoretical scrutiny spanning more than four decades, a coherent description of this HB network remains elusive. The essential question of whether continuum or multicomponent (“intact,” “broken bond,” etc.) models best describe the HB interactions in liquid water has engendered particularly intense discussion. Most notably, the temperature dependence of water's Raman spectrum has long been considered to be among the strongest evidence for a multicomponent distribution. Using a combined experimental and theoretical approach, we show here that many of the features of the Raman spectrum that are considered to be hallmarks of a multistate system, including the asymmetric band profile, the isosbestic (temperature invariant) point, and van't Hoff behavior, actually result from a continuous distribution. Furthermore, the excellent agreement between our newly remeasured Raman spectra and our model system further supports the locally tetrahedral description of liquid water, which has recently been called into question [Wernet, P., et al. (2004) Science 304, 995-999].

432 citations


Journal ArticleDOI
TL;DR: A comparative second harmonic generation study of aqueous solutions of hydriodic acid and alkali iodides establishes lower limits of 55% and 34% larger surface iodide concentrations for HI solutions relative to NaI and KI solutions, respectively, which implies that hydronium ions must exist in much higher density near the liquid surface than do the alkali ions.
Abstract: The recent demonstration of enhanced surface anion concentrations for aqueous electrolyte solutions strongly contrasts current textbook descriptions. Small cations are still expected to be repelled from the surface, but recent simulations predict that hydronium (H3O+) cations are instead preferentially adsorbed at the interface. Here we describe a comparative second harmonic generation (SHG) study of aqueous solutions of hydriodic acid (HI) and alkali iodides (NaI and KI), which establish lower limits of 55% and 34% larger surface iodide concentrations for HI solutions relative to NaI and KI solutions, respectively. This result implies that hydronium ions must exist in much higher densities near the liquid surface than do the alkali ions, in support of the theoretical predictions.

228 citations


Journal ArticleDOI
TL;DR: It is found that nanoribbons function efficiently as waveguides in liquid media and provide a unique means for probing molecules in solution or in proximity to the waveguide surface and presage the use of nanowire waveguide in microfluidics and biology.
Abstract: The manipulation of photons in structures smaller than the wavelength of light is central to the development of nanoscale integrated photonic systems for computing, communications, and sensing. We assemble small groups of freestanding, chemically synthesized nanoribbons and nanowires into model structures that illustrate how light is exchanged between subwavelength cavities made of three different semiconductors. The coupling strength of the optical linkages formed when nanowires are brought into contact depends both on their volume of interaction and angle of intersection. With simple coupling schemes, lasing nanowires can launch coherent pulses of light through ribbon waveguides that are up to a millimeter in length. Also, interwire coupling losses are low enough to allow light to propagate across several right-angle bends in a grid of crossed ribbons. The fraction of the guided wave traveling outside the wire/ribbon cavities is used to link nanowires through space and to separate colors within multiribbon networks. In addition, we find that nanoribbons function efficiently as waveguides in liquid media and provide a unique means for probing molecules in solution or in proximity to the waveguide surface. Our results lay the spadework for photonic devices based on assemblies of active and passive nanowire elements and presage the use of nanowire waveguides in microfluidics and biology.

223 citations


Journal ArticleDOI
TL;DR: In this paper, a combined experimental and theoretical investigation of the liquid/air interface of aqueous sodium thiocyanide at varying salt concentrations is presented, which is in accord with molecular dynamics simulations in slab geometry, which predict an appreciable surface enhancement of SCN-.
Abstract: Contrasting current textbook descriptions, a consistent picture of substantial concentration enhancement of highly polarizable anions at the surface of aqueous electrolyte solutions is emerging. Such enhancement may have important implications for chemistry occurring on aqueous aerosols and ocean surfaces. Here we present a combined experimental and theoretical investigation of the liquid/air interface of aqueous sodium thiocyanide at varying salt concentrations. Normalized second harmonic generation intensities fitted to Langmuir isotherms yield a Gibbs free energy of adsorption of −1.80 kcal/mol. These results are in accord with molecular dynamics simulations in slab geometry, which predict an appreciable surface enhancement of SCN-.

179 citations


Journal ArticleDOI
TL;DR: Density functional theory calculations of the XAS indicate that the observed red shift of the water transitions as a function of salt concentration arises from a strong, direct perturbation of the unoccupied molecular orbitals on water by anions, and does not require significant distortion of the hydrogen bond network beyond the first solvation shell.
Abstract: Measurements of the oxygen K-edge X-ray absorption spectrum (XAS) of aqueous sodium halide solutions demonstrate that ions significantly perturb the electronic structure of adjacent water molecules. The addition of halide salts to water engenders an increase in the preedge intensity and a decrease in the postedge intensity of the XAS, analogous to those observed when increasing the temperature of pure water. The main-edge feature exhibits unique behavior and becomes more intense when salt is added. Density functional theory calculations of the XAS indicate that the observed red shift of the water transitions as a function of salt concentration arises from a strong, direct perturbation of the unoccupied molecular orbitals on water by anions, and does not require significant distortion of the hydrogen bond network beyond the first solvation shell. This contrasts the temperature-dependent spectral variations, which result primarily from intensity changes of specific transitions due to geometric rearrangement of the hydrogen bond network.

164 citations


Journal ArticleDOI
TL;DR: Branched nanocrystal heterostructures synthesized from CdSe and CdTe exhibit a type II band structure alignment that induces separation of charge upon photoexcitation and localizes carriers to different regions of the tetrahedral geometry.
Abstract: Branched nanocrystal heterostructures synthesized from CdSe and CdTe exhibit a type II band structure alignment that induces separation of charge upon photoexcitation and localizes carriers to different regions of the tetrahedral geometry The dynamics of carrier relaxation examined with femtosecond pump-probe spectroscopy showed heterostructures having rise times and biexponential decays longer than those of nanorods with similar dimensions This is attributed to weaker interactions with surface states and nonradiative relaxation channels afforded by the type II alignment

152 citations


Journal ArticleDOI
TL;DR: New resonance-enhanced second harmonic generation (SHG) experiments demonstrating surface enhancement of simple inorganic anions in the same concentration range show that the quadruply charged ferrocyanide, Fe(CN)(6)(4-), anion is not surface active at high concentrations, as expected, but at dilute concentrations, the anions are strongly attracted to the interface with a Gibbs free energy of adsorption.
Abstract: The controversial observation of a minimum in the surface tension of dilute aqueous electrolyte solutions by Jones and Ray in the 1930s is confirmed by new resonance-enhanced second harmonic generation (SHG) experiments demonstrating surface enhancement of simple inorganic anions in the same concentration range. New experiments show that the quadruply charged ferrocyanide, Fe(CN)64-, anion is not surface active at high concentrations, as expected, but at dilute concentrations, the anion is strongly attracted to the interface with a Gibbs free energy of adsorption of −6.8 kcal/mol. Using this value, the original Jones and Ray data are fit to a simple model of the surface tension with qualitative agreement, although better agreement is found for all 13 Jones and Ray salts with an even stronger surface adsorption.

129 citations


Journal ArticleDOI
06 May 2005-Science
TL;DR: A strong temperature dependence of oxygen K-edge x-ray absorption fine structure features was observed for supercooled and normal liquid water droplets prepared from the breakup of a liquid microjet, consistent with the distribution of hydrogen bond strengths obtained for the "overstructured" ST2 model of water.
Abstract: A strong temperature dependence of oxygen K-edge x-ray absorption fine structure features was observed for supercooled and normal liquid water droplets prepared from the breakup of a liquid microjet. Analysis of the data over the temperature range 251 to 288 kelvin (-22 degrees to +15 degrees C) yields a value of 1.5 +/- 0.5 kilocalories per mole for the average thermal energy required to effect an observable rearrangement between the fully coordinated (\"ice-like\") and distorted (\"broken-donor\") local hydrogen-bonding configurations responsible for the pre-edge and post-edge features, respectively. This energy equals the latent heat of melting of ice with hexagonal symmetry (ice Ih) and is consistent with the distribution of hydrogen bond strengths obtained for the \"overstructured\" ST2 model of water.

117 citations


Journal ArticleDOI
TL;DR: In this article, the carbon, nitrogen and oxygen K-edge spectra were measured for aqueous solutions of glycine by total electron yield near-edge X-ray absorption fine structure (TEY NEXAFS) spectroscopy.
Abstract: The carbon, nitrogen, and oxygen K-edge spectra were measured for aqueous solutions of glycine by total electron yield near-edge X-ray absorption fine structure (TEY NEXAFS) spectroscopy. The bulk solution pH was systematically varied while maintaining a constant amino acid concentration. Spectra were assigned through comparisons with both previous studies and ab initio computed spectra of isolated glycine molecules and hydrated glycine clusters. Nitrogen K-edge solution spectra recorded at low and moderate pH are nearly identical to those of solid glycine, whereas basic solution spectra strongly resemble those of the gas phase. The carbon 1s → π*CO transition exhibits a 0.2 eV red shift at high pH due to the deprotonation of the amine terminus. This deprotonation also effects a 1.4 eV red shift in the nitrogen K-edge at high pH. Two sharp preedge features at 401.3 and 402.5 eV are also observed at high pH. These resonances, previously observed in the vapor-phase ISEELS spectrum of glycine,1 have been rea...

94 citations


Journal ArticleDOI
TL;DR: Comparison with DFT computed spectra of model methanol clusters indicates that the bulk liquid comprises long chains and rings of hydrogen-bonded monomers.
Abstract: We have measured the X-ray absorption (XA) spectrum of liquid (298 K) methanol at the oxygen and carbon K edges. The 4a1 orbital at the O K edge exhibits a pronounced sensitivity to the formation of intermolecular hydrogen bonds, with significant differences observed between the vapor and bulk spectra, whereas the C K edge reveals only subtle corresponding spectral changes. Comparison with DFT computed spectra of model methanol clusters indicates that the bulk liquid comprises long chains (n > 6) and rings of hydrogen-bonded monomers.


Journal ArticleDOI
TL;DR: The NEXAFS spectrum of zwitterionic lysine solution was also measured, exhibiting features similar to those of both anionic and zwingerionic glycine, and leading us to conclude that the alpha amine group is present in an acceptor-only configuration, while the end of the butylammonium side chain is fully solvated.
Abstract: The nitrogen K-edge spectra of aqueous proline and diglycine solutions have been measured by total electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at neutral and high pH. All observed spectral features have been assigned by comparison to the recently reported spectrum of aqueous glycine and calculated spectra of isolated amino acids and hydrated amino acid clusters. The sharp preedge resonances at 401.3 and 402.6 eV observed in the spectrum of anionic glycine indicate that the nitrogen terminus is in an “acceptor-only” configuration, wherein neither amine proton is involved in hydrogen bonding to the solvent, at high pH. The analogous 1s → σ*NH preedge transitions are absent in the NEXAFS spectrum of anionic proline, implying that the acceptor-only conformation observed in anionic glycine arises from steric shielding induced by free rotation of the amine terminus about the glycine CN bond. Anionic diglycine solutions exhibit a broadened 1s → π*CN resonance at 401.2 eV and a ...

Journal ArticleDOI
TL;DR: Measurements of isotope fractionation occurring during free molecular evaporation from liquid microjets show that the isotope ratios of evaporating molecules exhibit dramatic differences from equilibrium vapor values, strong variations with the solution deuterium mole fraction, and a clear temperature dependence.
Abstract: The microscopic events engendering liquid water evaporation have received much attention over the last century, but remain incompletely understood. We present measurements of isotope fractionation occurring during free molecular evaporation from liquid microjets and show that the isotope ratios of evaporating molecules exhibit dramatic differences from equilibrium vapor values, strong variations with the solution deuterium mole fraction, and a clear temperature dependence. These results indicate the existence of an energetic barrier to evaporation and that the evaporation coefficient of water is less than unity. These new insights into water evaporation promise to advance our understanding of the processes that control the formation and lifetime of clouds in the atmosphere.

Journal ArticleDOI
TL;DR: In this paper, the authors present results of gas phase cluster and liquid water simulations from the recently determined VRT(ASP-W)III water dimer potential energy surface (the third fitting of the Anisotropic Site Potential with Woermer dispersion to vibration-rotation-tunnelling data).
Abstract: We present results of gas phase cluster and liquid water simulations from the recently determined VRT(ASP-W)III water dimer potential energy surface (the third fitting of the Anisotropic Site Potential with Woermer dispersion to vibration–rotation– tunnelling data). VRT(ASP-W)III is shown to not only be a model of high ‘spectroscopic’ accuracy for the water dimer, but also makes accurate predictions of vibrational ground-state properties for clusters up through the hexamer. Results of ambient liquid water simulations from VRT(ASP-W)III are compared with those from ab initio molecular dynamics, other potentials of ‘spectroscopic’ accuracy and with experiment. The results herein represent the first time to the authors’ knowledge that a ‘spectroscopic’ potential surface is able to correctly model condensed phase properties of water.

Journal ArticleDOI
TL;DR: It is concluded, through careful analysis of five new (D2O)5 bands, that at least three torsional-puckering manifolds have been probed and that the lowest-energy manifold is highly compacted.
Abstract: We present the measurement and analysis of five new (D2O)5 bands via vibration−rotation-tunneling (VRT) spectroscopy as well as a preliminary description of a second (H2O)5 band. The vibrationally averaged rotational constants of all five fitted bands agree well with those from the two previously observed (D2O)5 bands and confirm that the pentamer averages to a symmetric, quasi-planar structure on the time scale of our experiment. While the spectrum of the first two bands, located at 50.7 cm-1 (1.52 THz) and 27.3 cm-1 (0.82 THz) are indicative of unperturbed oblate rotors, the three remaining (D2O)5 bands centered at 47.7 cm-1 (1.43 THz), 45.4 cm-1 (1.36 THz), and 45.0 cm-1 (1.35 THz) are severely perturbed by first-order Coriolis coupling. This represents the first observation of this perturbation in the perdeuterated water pentamer, as well as the first observation of transitions between degenerate states of the torsional-puckering manifold. Unlike transitions from the (H2O)5 band observed by Brown et a...

Journal ArticleDOI
TL;DR: Velocity modulation spectroscopy has been investigated in the terahertz region, employing pure rotational transitions of ArH + and rotation-tunneling transitions of H3O + to study the competition between pressure broadening and Doppler broadening on the lineshapes, neutral suppression, and modulation efficiency as mentioned in this paper.

Journal ArticleDOI
01 Jan 2005
TL;DR: In this article, the authors present dissociative recombination measurements, using the CRYRING ion storage ring, of H3+ ions produced in a supersonic expansion discharge source.
Abstract: We present dissociative recombination measurements, using the CRYRING ion storage ring, of H3+ ions produced in a supersonic expansion discharge source. Before and after the CRYRING measurements, the ion source was characterized in Berkeley using infrared cavity ringdown spectroscopy, and was found to exhibit a typical rotational temperature of ~30 K. Our measurement of the dissociative recombination cross section using this ion source revealed resonances that had not been observed clearly in previous experiments that used rotationally hot ion sources. Based on the present measurements, we infer a thermal dissociative recombination rate coefficient for ions at interstellar temperatures of ~ 2.6 × 10−7 cm3s−1. Our results are in general agreement with theoretical calculations of the dissociative recombination cross section by Kokoouline and Greene. We will review the enigma of the abundance of H3+ in the diffuse interstellar medium, and discuss the impact of these experiments, especially in the context of the recent observation of H3+ towards ζ Persei.