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Caltech Authors (166.781 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 81 - 100 de 157

  1. Solar fuels photoanode materials discovery by integrating high-throughput theory and experiment

    Yan, Qimin; Yu, Jie; Suram, Santosh K.; Zhou, Lan; Shinde, Aniketa; Newhouse, Paul F.; Chen, Wei; Li, Guo; Persson, Kristin A.; Gregoire, John M.; Neaton, Jeffrey B.
    The limited number of known low-band-gap photoelectrocatalytic materials poses a significant challenge for the generation of chemical fuels from sunlight. Using high-throughput ab initio theory with experiments in an integrated workflow, we find eight ternary vanadate oxide photoanodes in the target band-gap range (1.2–2.8 eV). Detailed analysis of these vanadate compounds reveals the key role of VO_4 structural motifs and electronic band-edge character in efficient photoanodes, initiating a genome for such materials and paving the way for a broadly applicable high-throughput-discovery and materials-by-design feedback loop. Considerably expanding the number of known photoelectrocatalysts for water oxidation, our study establishes ternary metal...

  2. Experimental and Ab Initio Ultrafast Carrier Dynamics in Plasmonic Nanoparticles

    Brown, Ana M.; Sundararaman, Ravishankar; Narang, Prineha; Schwartzberg, Adam M.; Goddard, William A., III; Atwater, Harry A.
    Ultrafast pump-probe measurements of plasmonic nanostructures probe the nonequilibrium behavior of excited carriers, which involves several competing effects obscured in typical empirical analyses. Here we present pump-probe measurements of plasmonic nanoparticles along with a complete theoretical description based on first-principles calculations of carrier dynamics and optical response, free of any fitting parameters. We account for detailed electronic-structure effects in the density of states, excited carrier distributions, electron-phonon coupling, and dielectric functions that allow us to avoid effective electron temperature approximations. Using this calculation method, we obtain excellent quantitative agreement with spectral and temporal features in transient-absorption measurements. In both our...

  3. Experimental and Ab Initio Ultrafast Carrier Dynamics in Plasmonic Nanoparticles

    Brown, Ana M.; Sundararaman, Ravishankar; Narang, Prineha; Schwartzberg, Adam M.; Goddard, William A., III; Atwater, Harry A.
    Ultrafast pump-probe measurements of plasmonic nanostructures probe the nonequilibrium behavior of excited carriers, which involves several competing effects obscured in typical empirical analyses. Here we present pump-probe measurements of plasmonic nanoparticles along with a complete theoretical description based on first-principles calculations of carrier dynamics and optical response, free of any fitting parameters. We account for detailed electronic-structure effects in the density of states, excited carrier distributions, electron-phonon coupling, and dielectric functions that allow us to avoid effective electron temperature approximations. Using this calculation method, we obtain excellent quantitative agreement with spectral and temporal features in transient-absorption measurements. In both our...

  4. The Reaction Mechanism with Free Energy Barriers at Constant Potentials for the Oxygen Evolution Reaction at the IrO_2 (110) Surface

    Ping, Yuan; Nielsen, Robert J.; Goddard, William A.
    How to efficiently oxidize H_2O to O_2 (Oxygen Evolution Reaction –OER) in photoelectrochemical cells (PEC) is a great challenge due to its complex charge transfer process, high overpotential, and corrosion. So far no OER mechanism has been fully explained atomistically with both thermodynamic and kinetics. IrO_2 is the only known OER catalyst with both high catalytic activity and stability in acidic conditions. This is important because PEC experiments often operate at extreme pH conditions. In this work we performed first principles calculations integrated with implicit solvation at constant potentials to examine the detailed atomistic reaction mechanism of OER at the...

  5. Automated Phase Mapping with AgileFD and its Application to Light Absorber Discovery in the V-Mn-Nb Oxide System

    Suram, Santosh K.; Xue, Yexiang; Bai, Junwen; LeBras, Ronan; Rappazzo, Brendan H.; Bernstein, Richard; Bjorck, Johan; Zhou, Lan; van Dover, R. Bruce; Gomes, Carla P.; Gregoire, John M.
    Rapid construction of phase diagrams is a central tenet of combinatorial materials science with accelerated materials discovery efforts often hampered by challenges in interpreting combinatorial x-ray diffraction datasets, which we address by developing AgileFD, an artificial intelligence algorithm that enables rapid phase mapping from a combinatorial library of x-ray diffraction patterns. AgileFD models alloying-based peak shifting through a novel expansion of convolutional nonnegative matrix factorization, which not only improves the identification of constituent phases but also maps their concentration and lattice parameter as a function of composition. By incorporating Gibbs’ phase rule into the algorithm, physically meaningful phase maps are...

  6. Millivolt Modulation of Plasmonic Metasurface Optical Response via Ionic Conductance

    Thyagarajan, Krishnan; Sokhoyan, Ruzan; Zornberg, Leonardo Z.; Atwater, Harry A.
    A plasmonic metasurface with an electrically tunable optical response that operates at strikingly low modulation voltages is experimentally demonstrated. The fabricated metasurface shows up to 30% relative change in reflectance in the visible spectral range upon application of 5 mV and 78% absolute change in reflectance upon application of 100 mV of bias. The designed metasurface consists of nanostructured silver and indium tin oxide (ITO) electrodes which are separated by 5 nm thick alumina. The millivolt-scale optical modulation is attributed to a new modulation mechanism, in which transport of silver ions through alumina dielectric leads to bias-induced nucleation and growth...

  7. Millivolt Modulation of Plasmonic Metasurface Optical Response via Ionic Conductance

    Thyagarajan, Krishnan; Sokhoyan, Ruzan; Zornberg, Leonardo Z.; Atwater, Harry A.
    A plasmonic metasurface with an electrically tunable optical response that operates at strikingly low modulation voltages is experimentally demonstrated. The fabricated metasurface shows up to 30% relative change in reflectance in the visible spectral range upon application of 5 mV and 78% absolute change in reflectance upon application of 100 mV of bias. The designed metasurface consists of nanostructured silver and indium tin oxide (ITO) electrodes which are separated by 5 nm thick alumina. The millivolt-scale optical modulation is attributed to a new modulation mechanism, in which transport of silver ions through alumina dielectric leads to bias-induced nucleation and growth...

  8. Detecting structural variances of Co_3O_4 catalysts by controlling beam-induced sample alterations in the vacuum of a transmission electron microscope

    Kisielowski, C.; Frei, H.; Specht, P.; Sharp, I. D.; Haber, J. A.; Helveg, S.
    This article summarizes core aspects of beam-sample interactions in research that aims at exploiting the ability to detect single atoms at atomic resolution by mid-voltage transmission electron microscopy. Investigating the atomic structure of catalytic Co_3O_4 nanocrystals underscores how indispensable it is to rigorously control electron dose rates and total doses to understand native material properties on this scale. We apply in-line holography with variable dose rates to achieve this goal. Genuine object structures can be maintained if dose rates below ~100 e/Å^2s are used and the contrast required for detection of single atoms is generated by capturing large image series....

  9. Near-Unity Unselective Absorption in Sparse InP Nanowire Arrays

    Fountaine, Katherine T.; Cheng, Wen-Hui; Bukowsky, Colton R.; Atwater, Harry A.
    We experimentally demonstrate near-unity, unselective absorption, broadband, angle-insensitive, and polarization-independent absorption, in sparse InP nanowire arrays, embedded in flexible polymer sheets via geometric control of waveguide modes in two wire motifs: (i) arrays of tapered wires and (ii) arrays of nanowires with varying radii. Sparse arrays of these structures exhibit enhanced absorption due to strong coupling into the first order azimuthal waveguide modes of individual nanowires; wire radius thus controls the spectral region of the absorption enhancement. Whereas arrays of cylindrical wires with uniform radius exhibit narrowband absorption, arrays of tapered wires and arrays with multiple wire radii expand this...

  10. High Throughput Light Absorber Discovery, Part 2: Establishing Structure–Band Gap Energy Relationships

    Suram, Santosh K.; Newhouse, Paul F.; Zhou, Lan; Van Campen, Douglas G.; Mehta, Apurva; Gregoire, John M.
    Combinatorial materials science strategies have accelerated materials development in a variety of fields, and we extend these strategies to enable structure–property mapping for light absorber materials, particularly in high order composition spaces. High throughput optical spectroscopy and synchrotron X-ray diffraction are combined to identify the optical properties of Bi–V–Fe oxides, leading to the identification of Bi_4V_(1.5)Fe_(0.5)O_(10.5) as a light absorber with direct band gap near 2.7 eV. The strategic combination of experimental and data analysis techniques includes automated Tauc analysis to estimate band gap energies from the high throughput spectroscopy data, providing an automated platform for identifying new optical materials.

  11. High Throughput Light Absorber Discovery, Part 1: An Algorithm for Automated Tauc Analysis

    Suram, Santosh K.; Newhouse, Paul F.; Gregoire, John M.
    High-throughput experimentation provides efficient mapping of composition–property relationships, and its implementation for the discovery of optical materials enables advancements in solar energy and other technologies. In a high throughput pipeline, automated data processing algorithms are often required to match experimental throughput, and we present an automated Tauc analysis algorithm for estimating band gap energies from optical spectroscopy data. The algorithm mimics the judgment of an expert scientist, which is demonstrated through its application to a variety of high throughput spectroscopy data, including the identification of indirect or direct band gaps in Fe_2O_3, Cu_2V_2O_7, and BiVO_4. The applicability of the algorithm...

  12. Surface reconstruction of pure-Cu single-crystal electrodes under Co-reduction potentials in alkaline solutions: A study by seriatim ECSTM-DEMS

    Kim, Youn-Geun; Javier, Alnald; Baricuatro, Jack H.; Torelli, Daniel; Cummins, Kyle D.; Tsang, Chu F.; Hemminger, John C.; Soriaga, Manuel P.
    Quasi-operando electrochemical scanning tunneling microscopy (ECSTM) recently showed that a polycrystalline Cu electrode kept in 0.1 M KOH at − 0.9 V (SHE), a potential very close to that for electrochemical CO reduction, underwent a two-step surface reconstruction, initially to Cu(111), or Cu(pc)-[Cu(111)], and terminally to Cu(100), or Cu(pc)-[Cu(100)]. When subjected to monolayer-limited Cu_((s)) ↔ Cu_2O_((s)) oxidation-reduction cycles (ORC), the Cu(pc)-[Cu(100)] surface was further transformed to Cu(pc)-[Cu(511)] that produced C_2H_5OH exclusively, as detected by differential electrochemical mass spectrometry, at an overvoltage lower by 645 mV relative to that for the formation of hydrocarbons. In this paper, results are presented from...

  13. Resonant Thermoelectric Nanophotonics

    Mauser, Kelly W.; Kim, Seyoon; Mitrovic, Slobodan; Fleischman, Dagny; Pala, Ragip; Schwab, K. C.; Atwater, Harry A.
    Photodetectors are typically based either on photocurrent generation from electron–hole pairs in semiconductor structures or on bolometry for wavelengths that are below bandgap absorption. In both cases, resonant plasmonic and nanophotonic structures have been successfully used to enhance performance. Here, we show subwavelength thermoelectric nanostructures designed for resonant spectrally selective absorption, which creates large localized temperature gradients even with unfocused, spatially uniform illumination to generate a thermoelectric voltage. We show that such structures are tunable and are capable of wavelength-specific detection, with an input power responsivity of up to 38 V W^(–1), referenced to incident illumination, and bandwidth of nearly 3...

  14. Enhanced Absorption and <1% Spectrum-and-Angle-Averaged Reflection in Tapered Microwire Arrays

    Yalamanchili, Sisir; Emmer, Hal S.; Fountaine, Katherine T.; Chen, Christopher T.; Lewis, Nathan S.; Atwater, Harry A.
    We report ordered, high aspect ratio, tapered Si microwire arrays that exhibit an extremely low angular (0° to 50°) and spectrally averaged reflectivity of <1% of the incident 400–1100 nm illumination. After isolating the microwires from the substrate with a polymer infill and peel off process, the arrays were found to absorb 89.1% of angular averaged incident illumination (0° to 50°) in the equivalent volume of a 20 μm thick Si planar slab, reaching 99.5% of the classical light trapping limit between 400 and 1100 nm. We explain the broadband absorption by enhancement in coupling to waveguide modes due to...

  15. The role of the CeO_2/BiVO_4 interface in optimized Fe-Ce oxide coatings for solar fuels photoanodes

    Shinde, A.; Li, G.; Zhou, L.; Guevarra, D.; Suram, S. K.; Toma, F. M.; Yan, Q.; Haber, J. A.; Neaton, J. B.; Gregoire, J. M.
    Solar fuel generators entail a high degree of materials integration, and efficient photoelectrocatalysis of the constituent reactions hinges upon the establishment of highly functional interfaces. The recent application of high throughput experimentation to interface discovery for solar fuels photoanodes has revealed several surprising and promising mixed-metal oxide coatings for BiVO_4. Using sputter deposition of composition and thickness gradients on a uniform BiVO_4 film, we systematically explore photoanodic performance as a function of the composition and loading of Fe–Ce oxide coatings. This combinatorial materials integration study not only enhances the performance of this new class of materials but also identifies CeO_2...

  16. Gate-Tunable Conducting Oxide Metasurfaces

    Huang, Yao-Wei; Lee, Ho Wai Howard; Sokhoyan, Ruzan; Pala, Ragip A.; Thyagarajan, Krishnan; Han, Seunghoon; Tsai, Din Ping; Atwater, Harry A.
    Metasurfaces composed of planar arrays of subwavelength artificial structures show promise for extraordinary light manipulation. They have yielded novel ultrathin optical components such as flat lenses, wave plates, holographic surfaces, and orbital angular momentum manipulation and detection over a broad range of the electromagnetic spectrum. However, the optical properties of metasurfaces developed to date do not allow for versatile tunability of reflected or transmitted wave amplitude and phase after their fabrication, thus limiting their use in a wide range of applications. Here, we experimentally demonstrate a gate-tunable metasurface that enables dynamic electrical control of the phase and amplitude of the...

  17. Selectivity for HCO_2^– over H_2 in the Electrochemical Catalytic Reduction of CO_2 by (POCOP)IrH_2

    Johnson, Samantha I.; Nielsen, Robert J.; Goddard, William A., III
    It has been demonstrated experimentally that electrochemical CO_2 reduction catalyzed by (POCOP)IrH_2 ([C_6H_3-2,6-[OP(tBu)_2]_2]IrH_2) produces formate without significant H_2. We use first-principles density functional theory (M06) including Poisson–Boltzmann solvation to determine the detailed atomistic mechanism and illuminate strategies for designing formate-selective catalysts. A mechanism involving hydride transfer from Ir^(III) dihydride explains the selectivity for formate over H_2 and is corroborated by reduction potential (irreversible reduction of (POCOP)Ir(H)(NCMe)_2^+ at ca. −1.3 V vs NHE, in comparison to −1.31 V vs NHE calculated for one-electron reduction of Ir^(III)(H)(NCMe)_2^+) and turnover frequency. We find that several thermodynamically favorable pathways exist for the hydrogen evolution...

  18. Selectivity for HCO_2^– over H_2 in the Electrochemical Catalytic Reduction of CO_2 by (POCOP)IrH_2

    Johnson, Samantha I.; Nielsen, Robert J.; Goddard, William A., III
    It has been demonstrated experimentally that electrochemical CO_2 reduction catalyzed by (POCOP)IrH_2 ([C_6H_3-2,6-[OP(tBu)_2]_2]IrH_2) produces formate without significant H_2. We use first-principles density functional theory (M06) including Poisson–Boltzmann solvation to determine the detailed atomistic mechanism and illuminate strategies for designing formate-selective catalysts. A mechanism involving hydride transfer from Ir^(III) dihydride explains the selectivity for formate over H_2 and is corroborated by reduction potential (irreversible reduction of (POCOP)Ir(H)(NCMe)_2^+ at ca. −1.3 V vs NHE, in comparison to −1.31 V vs NHE calculated for one-electron reduction of Ir^(III)(H)(NCMe)_2^+) and turnover frequency. We find that several thermodynamically favorable pathways exist for the hydrogen evolution...

  19. Cubic Nonlinearity Driven Up-Conversion in High-Field Plasmonic Hot Carrier Systems

    Narang, Prineha; Sundararaman, Ravishankar; Jermyn, Adam S.; Goddard, William A., III; Atwater, Harry A.
    Surface plasmon resonances confine electromagnetic fields to the nanoscale, producing high field strengths suitable for exploiting nonlinear optical properties. We examine the prospect of detecting and utilizing one such property in plasmonic metals: the imaginary part of the cubic susceptibility, which corresponds to two plasmons decaying together to produce high energy carriers. Here we present ab initio predictions of the rates and carrier distributions generated by direct interband and phonon-assisted intraband transitions in one and two-plasmon decay. We propose detection of the higher energy carriers generated from two-plasmon decays that are inaccessible in one-plasmon decay as a viable signature of...

  20. Cubic Nonlinearity Driven Up-Conversion in High-Field Plasmonic Hot Carrier Systems

    Narang, Prineha; Sundararaman, Ravishankar; Jermyn, Adam S.; Goddard, William A., III; Atwater, Harry A.
    Surface plasmon resonances confine electromagnetic fields to the nanoscale, producing high field strengths suitable for exploiting nonlinear optical properties. We examine the prospect of detecting and utilizing one such property in plasmonic metals: the imaginary part of the cubic susceptibility, which corresponds to two plasmons decaying together to produce high energy carriers. Here we present ab initio predictions of the rates and carrier distributions generated by direct interband and phonon-assisted intraband transitions in one and two-plasmon decay. We propose detection of the higher energy carriers generated from two-plasmon decays that are inaccessible in one-plasmon decay as a viable signature of...

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