Recursos de colección

Caltech Authors (147.820 recursos)

Repository of works by Caltech published authors.

Group = Resnick Sustainability Institute

Mostrando recursos 1 - 20 de 176

  1. Outstanding hydrogen evolution reaction catalyzed by porous nickel diselenide electrocatalysts

    Zhou, Haiqing; Yu, Fang; Liu, Yuanyue; Sun, Jingying; Zhu, Zhuan; He, Ran; Bao, Jiming; Goddard, William A., III; Chen, Shuo; Ren, Zhifeng
    To relieve our strong reliance on fossil fuels and to reduce greenhouse effects, there is an ever-growing interest in using electrocatalytic water splitting to produce green, renewable, and environment-benign hydrogen fuel via the hydrogen evolution reaction. For commercially feasible water electrolysis, it is imperative to develop electrocatalysts that perform as efficiently as Pt but using only earth-abundant commercial materials. However, the highest performance current catalysts consist of nanostructures made by using complex methods. Here we report a porous nickel diselenide (NiSe_2) catalyst that is superior for water electrolysis, exhibiting much better catalytic performance than most first-row transition metal dichalcogenide-based catalysts,...

  2. Experiments on imperfection insensitive axially loaded cylindrical shells

    Ning, Xin; Pellegrino, Sergio
    This paper presents an experimental study of imperfection insensitive composite wavy cylindrical shells subject to axial compression. A fabrication technique for making cylindrical shells with intricate shape of cross-sections has been developed. A photogrammetry technique to measure the geometric imperfections has also been developed. The behavior of the wavy shells under axial compression was predicted through simulations and measured through compression tests. Both the analyses and experiments have confirmed that the wavy shells are imperfection insensitive. Comparisons between the wavy shells and circular shells have also confirmed that introducing optimal symmetry-breaking wavy cross-sections can significantly reduce the imperfection sensitivity and...

  3. Application of Bottlebrush Block Copolymers as Photonic Crystals

    Liberman-Martin, Allegra L.; Chu, Crystal K.; Grubbs, Robert H.
    Brush block copolymers are a class of comb polymers that feature polymeric side chains densely grafted to a linear backbone. These polymers display interesting properties due to their dense functionality, low entanglement, and ability to rapidly self-assemble to highly ordered nanostructures. The ability to prepare brush polymers with precise structures has been enabled by advancements in controlled polymerization techniques. This Feature Article highlights the development of brush block copolymers as photonic crystals that can reflect visible to near-infrared wavelengths of light. Fabrication of these materials relies on polymer self-assembly processes to achieve nanoscale ordering, which allows for the rapid preparation...

  4. Atomic-scale Structural and Chemical Characterization of Hexagonal Boron Nitride Layers Synthesized at the Wafer-Scale with Monolayer Thickness Control

    Lin, Wei-Hsiang; Brar, Victor W.; Jariwala, Deep; Sherrott, Michelle C.; Tseng, Wei-Shiuan; Wu, Chih-I; Yeh, Nai-Chang; Atwater, Harry A.
    Hexagonal boron nitride (h-BN) is a promising two-dimensional insulator with a large band gap and low density of charged impurities that is isostructural and isoelectronic with graphene. Here we report the chemical and atomic-scale structure of CVD-grown wafer-scale (~25 cm2) h-BN sheets ranging in thickness from 1-20 monolayers. Atomic-scale images of h-BN on Au and graphene/Au substrates obtained by scanning tunneling microscopy (STM) reveal high h-BN crystalline quality in monolayer samples. Further characterization of 1-20 monolayer samples indicates uniform thickness for wafer-scale areas; this thickness control is a result of precise control of the precursor flow rate, deposition temperature and...

  5. Density-based errors in mixed-basis mean-field electronic structure, with implications for embedding and QM/MM methods

    Lee, Sebastian J.R.; Miyamoto, Kaito; Ding, Feizhi; Manby, Frederick R.; Miller, Thomas F., III
    We consider mean-field electronic structure calculations with subsystems that employ different atomic-orbital basis sets. A major source of error arises in charge-manifestation reactions (including ionization, electron attachment, or deprotonation) due to electronic density artifacts at the subsystem interface. The underlying errors in the electronic density can be largely eliminated with Fock-matrix corrections or by avoiding the use of a minimal basis set in the low-level region. These corrections succeed by balancing the electronegativity of atoms at the subsystem interface, much as link-atoms in QM/MM calculations rely upon balancing the electronegativity of atoms in the truncated QM region.

  6. Experimental Demonstration of >230° Phase Modulation in Gate-Tunable Graphene-Gold Reconfigurable Mid-Infrared Metasurfaces

    Sherrott, Michelle C.; Hon, Philip W. C.; Fountaine, Katherine T.; Garcia, Juan C.; Ponti, Samuel M.; Brar, Victor Watson; Sweatlock, Luke A.; Atwater, Harry A.
    Metasurfaces offer significant potential to control far-field light propagation through the engineering of amplitude, polarization, and phase at an interface. We report here phase modulation of an electronically reconfigurable metasurface and demonstrate its utility for mid-infrared beam steering. Using a gate-tunable graphene-gold resonator geometry, we demonstrate highly tunable reflected phase at multiple wavelengths and show up to 237° phase modulation range at an operating wavelength of 8.50 μm. We observe a smooth monotonic modulation of phase with applied voltage from 0° to 206° at a wavelength of 8.70 μm. Based on these experimental data, we demonstrate with antenna array calculations...

  7. Potassium tert-Butoxide-Catalyzed Dehydrogenative C–H Silylation of Heteroaromatics: A Combined Experimental and Computational Mechanistic Study

    Liu, Wen-Bo; Schuman, David P.; Yang, Yun-Fang; Toutov, Anton A.; Liang, Yong; Klare, Hendrik F. T.; Nesnas, Nasri; Oestreich, Martin; Blackmond, Donna G.; Virgil, Scott C.; Banerjee, Shibdas; Zare, Richard N.; Grubbs, Robert H.; Houk, K. N.; Stoltz, Brian M.
    We recently reported a new method for the direct dehydrogenative C–H silylation of heteroaromatics utilizing Earth-abundant potassium tert-butoxide. Herein we report a systematic experimental and computational mechanistic investigation of this transformation. Our experimental results are consistent with a radical chain mechanism. A trialkylsilyl radical may be initially generated by homolytic cleavage of a weakened Si–H bond of a hypercoordinated silicon species as detected by IR, or by traces of oxygen which can generate a reactive peroxide by reaction with (KOt-Bu)_4 as indicated by density functional theory (DFT) calculations. Radical clock and kinetic isotope experiments support a mechanism in which the...

  8. Regulating Top-Surface Multilayer/Single-Crystal Graphene Growth by “Gettering” Carbon Diffusion at Backside of the Copper Foil

    Abidi, Irfan H.; Liu, Yuanyue; Pan, Jie; Tyagi, Abhishek; Zhuang, Minghao; Zhang, Qicheng; Cagang, Aldrine A.; Weng, Lu-Tao; Sheng, Ping; Goddard, William A., III; Luo, Zhengtang
    A unique strategy is reported to constrain the nucleation centers for multilayer graphene (MLG) and, later, single-crystal graphene domains by gettering carbon source on backside of the flat Cu foil, during chemical vapor deposition. Hitherto, for a flat Cu foil, the top-surface-based growth mechanism is emphasized, while overlooking the graphene on the backside. However, the systematic experimental findings indicate a strong correlation between the backside graphene and the nucleation centers on the top-surface, governed by the carbon diffusion through the bulk Cu. This understanding steers to devise a strategy to mitigate the carbon diffusion to the top-surface by using a...

  9. Control of Grafting Density and Distribution in Graft Polymers by Living Ring-Opening Metathesis Copolymerization

    Lin, Tzu-Pin; Chang, Alice B.; Chen, Hsiang-Yun; Liberman-Martin, Allegra L.; Bates, Christopher M.; Voegtle, Matthew J.; Bauer, Christina A.; Grubbs, Robert H.
    Control over polymer sequence and architecture is crucial to both understanding structure–property relationships and designing functional materials. In pursuit of these goals, we developed a new synthetic approach that enables facile manipulation of the density and distribution of grafts in polymers via living ring-opening metathesis polymerization (ROMP). Discrete endo,exo-norbornenyl dialkylesters (dimethyl DME, diethyl DEE, di-n-butyl DBE) were strategically designed to copolymerize with a norbornene-functionalized polystyrene (PS), polylactide (PLA), or polydimethylsiloxane (PDMS) macromonomer mediated by the third-generation metathesis catalyst (G3). The small-molecule diesters act as diluents that increase the average distance between grafted side chains, generating polymers with variable grafting density....

  10. Hyper-Selective Plasmonic Color Filters

    Fleischman, Dagny; Sweatlock, Luke A.; Murakami, Hirotaka; Atwater, Harry
    The subwavelength mode volumes of plasmonic filters are well matched to the small size of state-of-the-art active pixels (~ 1 µm) in CMOS image sensor arrays used in portable electronic devices. Typical plasmonic filters exhibit broad (> 100 nm) transmission bandwidths. Dramatically reducing the peak width of filter transmission spectra would allow for the realization of CMOS hyperspectral imaging arrays, which demand the FWHM of transmission peaks to be less than 30 nm. We find that the design of 5 layer metal-insulator-metal-insulator-metal structures gives rise to multi-mode interference phenomena that suppresses spurious transmission features gives rise to a single narrow...

  11. Directed Evolution of a Bright Near-Infrared Fluorescent Rhodopsin Using a Synthetic Chromophore

    Herwig, Lukas; Rice, Austin J.; Bedbrook, Claire N.; Zhang, Ruijie K.; Lignell, Antti; Cahn, Jackson K. B.; Renata, Hans; Dodani, Sheel C.; Cho, Inha; Cai, Long; Gradinaru, Viviana; Arnold, Frances H.
    By engineering a microbial rhodopsin, Archaerhodopsin-3 (Arch), to bind a synthetic chromophore, merocyanine retinal, in place of the natural chromophore all-trans-retinal (ATR), we generated a protein with exceptionally bright and unprecedentedly red-shifted near-infrared (NIR) fluorescence. We show that chromophore substitution generates a fluorescent Arch complex with a 200-nm bathochromic excitation shift relative to ATR-bound wild-type Arch and an emission maximum at 772 nm. Directed evolution of this complex produced variants with pH-sensitive NIR fluorescence and molecular brightness 8.5-fold greater than the brightest ATR-bound Arch variant. The resulting proteins are well suited to bacterial imaging; expression and stability have not been...

  12. Non-conventional fluorescent biogenic and synthetic polymers without aromatic rings

    Ye, Ruquan; Liu, Yuanyue; Zhang, Haoke; Su, Huifang; Zhang, Yilin; Xu, Liguo; Hu, Rongrong; Kwok, Ryan T. K.; Wong, Kam Sing; Lam, Jacky W. Y.; Goddard, William A., III; Tang, Ben Zhong
    Non-conventional fluorescent materials without aromatic structures have attracted much research attention in recent years. However, the working mechanism responsible for their fluorescence remains mysterious. Here we decipher the origin of fluorescence by studying the photophysical properties of a series of non-aromatic biogenic and synthetic peptides. An experimental study suggests that the turn-on fluorescence in the aggregation state/condensed phase is associated with the communication of amide groups, where hydrogen bonds are playing a critical role in bringing these functionalities into close proximity. This explanation is further justified by the study of the hierarchical influence on fluorescence and applied to biomimetic polymers...

  13. 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...

  14. A potassium tert-butoxide and hydrosilane system for ultra-deep desulfurization of fuels

    Toutov, Anton A.; Salata, Mike; Fedorov, Alexey; Yang, Yun-Fang; Liang, Yong; Cariou, Romain; Betz, Kerry N.; Couzijn, Erik P. A.; Shabaker, John W.; Houk, Kendall N.; Grubbs, Robert H.
    Hydrodesulfurization (HDS) is the process by which sulfur-containing impurities are removed from petroleum streams, typically using a heterogeneous, sulfided transition metal catalyst under high H_2 pressures and temperatures. Although generally effective, a major obstacle that remains is the desulfurization of highly refractory sulfur-containing heterocycles, such as 4,6-dimethyldibenzothiophene (4,6-Me_2DBT), which are naturally occurring in fossil fuels. Homogeneous HDS strategies using well-defined molecular catalysts have been designed to target these recalcitrant S-heterocycles; however, the formation of stable transition metal sulfide complexes following C–S bond activation has largely prevented catalytic turnover. Here we show that a robust potassium (K) alkoxide (O)/hydrosilane (Si)-based (‘KOSi’)...

  15. Convexification of AC optimal power flow

    Gan, Lingwen; Low, Steven H.
    This overview paper summarizes the key elements of semidefinite relaxations of the optimal power flow problem, and discusses several open challenges.

  16. Optimal decentralized primary frequency control in power networks

    Zhao, Changhong; Low, Steven
    We augment existing generator-side primary frequency control with load-side control that are local, ubiquitous, and continuous. The mechanisms on both the generator and the load sides are decentralized in that their control decisions are functions of locally measurable frequency deviations. These local algorithms interact over the network through nonlinear power flows. We design the local frequency feedback control so that any equilibrium point of the closed-loop system is the solution to an optimization problem that minimizes the total generation cost and user disutility subject to power balance across entire network. With Lyapunov method we derive a sufficient condition for any...

  17. Enhancing Cation Diffusion and Suppressing Anion Diffusion via Lewis-Acidic Polymer Electrolytes

    Savoie, Brett M.; Webb, Michael A.; Miller, Thomas F., III
    Solid polymer electrolytes (SPEs) have the potential to increase both the energy density and stability of lithium-based batteries, but low Li^+ conductivity remains a barrier to technological viability. SPEs are designed to maximize Li^+ diffusivity relative to the anion while maintaining sufficient salt solubility. It is thus remarkable that poly(ethylene oxide) (PEO), the most widely used SPE, exhibits Li^+ diffusivity that is an order of magnitude smaller than that of typical counterions at moderate salt concentrations. We show that Lewis-basic polymers like PEO favor slow cation and rapid anion diffusion, while this relationship can be reversed in Lewis-acidic polymers. Using...

  18. An Efficient and Incentive Compatible Mechanism for Wholesale Electricity Markets

    Xu, Yunjian; Low, Steven H.
    Being widely used in many deregulated wholesale electricity markets, the locational marginal pricing (LMP) mechanism is known to achieve social optimality in a competitive market. When profit-maximizing generators act strategically to manipulate prices; however, LMP may lead to high loss of economic efficiency. In this paper, we apply the Vickrey-Clarke–Groves (VCG) mechanism to wholesale electricity markets. We show that the VCG mechanism minimizes the total cost at a truth-telling dominant strategy equilibrium. We establish an important comparative result that the VCG mechanism always results in higher per-unit electricity prices than the LMP mechanism under any given set of reported supply...

  19. Environment-economy tradeoff for Beijing-Tianjin-Hebei’s exports

    Zhao, Hongyan; Zhang, Qiang; Huo, Hong; Lin, Jintai; Liu, Zhu; Wang, Haikun; Guan, Dabo; He, Kebin
    The trade of goods among regions or nations associated with large environmental consequences. Yet balancing economic gains and environmental consequences induced by trade is still hindered by a lack of quantification of these two factors, especially for the environmental problems those are more locally oriented, such as the atmospheric pollution. Based on an environmental input–output analysis for 2010, we contrast economic gains (value added) against atmospheric pollutant emissions (sulfur dioxide (SO_2), nitric oxide (NO_x), primary fine particulate matter (PM_(2.5)) and non-methane volatile organic compounds (NMVOC)) and the widely concerned CO2 emissions associated with international and interprovincial exports from Beijing–Tianjin–Hebei (BTH),...

  20. Understanding the energy consumption and greenhouse gas emissions and the implication for achieving climate change mitigation targets

    Liu, Zhu; Feng, Kuishuang; Davis, Steven J.; Guan, Dabo; Chen, Bin; Hubacek, Klaus; Yan, Jinyue
    [No abstract]

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