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Yimer Y.Y.,University of Akron | Yimer Y.Y.,University of Washington | Yang B.,Harker School | Yang B.,Stanford University | And 2 more authors.
Chemical Physics Letters | Year: 2015

The wetting behavior of water on different types of poly(3-hexylthiophene)(P3HT) surfaces is studied using molecular dynamics simulations. All the different P3HT surfaces are found to be hydrophobic with water contact angles greater than 90°. The water contact angle on hexyl exposed crystalline P3HT films is about 20° larger than its contact angle on backbone-exposed crystalline P3HT films. However, all the films show comparable interfacial surface energies at the P3HT/water interface. The simulation results are in very good agreement with experimentally measured water contact angles on P3HT films and also interfacial tensions. © 2015 Elsevier B.V. All rights reserved.

Liu C.,Harker School | Wang R.K.,University of Washington
Progress in Biomedical Optics and Imaging - Proceedings of SPIE | Year: 2014

Choroidal thickness (ChT), defined as the distance between the retinal pigment epithelium (RPE) and the choroid-sclera interface (CSI), is highly correlated with various ocular disorders like high myopia, diabetic retinopathy, and central serous chorioretinopathy. Long wavelength Optical Coherence Tomography (OCT) has the ability to penetrate deep to the CSI, making the measurement of the ChT possible. The ability to accurately segment the CSI and RPE is important in extracting clinical information. However, automated CSI segmentation is challenging due to the weak boundary in the lower choroid and inconsistent texture with varied blood vessels. We propose a K-means clustering based automated algorithm, which is effective in segmenting the CSI and RPE. The performance of the method was evaluated using 531 frames from 4 normal subjects. The RPE and CSI segmentation time was about 0.3 seconds per frame, and the average time was around 0.5 seconds per frame with correction among frames, which is faster than reported algorithms. The results from the proposed method are consistent with the manual segmentation results. Further investigation includes the optimization of the algorithm to cover more OCT images captured from patients and the increase of the processing speed and robustness of the segmentation method. © 2014 SPIE.

Yang L.,Peking University | Kirby E.N.,California Institute of Technology | Guhathakurta P.,University of California at Santa Cruz | Peng E.W.,Peking University | Cheng L.,Harker School
Astrophysical Journal | Year: 2013

The ability to measure metallicities and α-element abundances in individual red giant branch (RGB) stars using medium-resolution spectra (R ≈ 6000) is a valuable tool for deciphering the nature of Milky Way dwarf satellites and the history of the Galactic halo. Extending such studies to more distant systems like Andromeda is beyond the ability of the current generation of telescopes, but by co-adding the spectra of similar stars, we can attain the necessary signal-to-noise ratio (S/N) to make detailed abundance measurements. In this paper, we present a method to determine metallicities and α-element abundances using the co-addition of medium-resolution spectra. We test the method of spectral co-addition using high-S/N spectra of more than 1300 RGB stars from Milky Way globular clusters and dwarf spheroidal galaxies obtained with the Keck II telescope/DEIMOS spectrograph. We group similar stars using photometric criteria and compare the weighted ensemble average abundances ([Fe/H], [Mg/Fe], [Si/Fe], [Ca/Fe], and [Ti/Fe]) of individual stars in each group with the measurements made on the corresponding co-added spectrum. We find a high level of agreement between the two methods, which permits us to apply this co-added spectra technique to more distant RGB stars, like stars in the M31 satellite galaxies. This paper outlines our spectral co-addition and abundance measurement methodology and describes the potential biases in making these measurements. © 2013. The American Astronomical Society. All rights reserved.

Trump J.R.,University of California at Santa Cruz | Hsu A.D.,Harker School | Fang J.J.,University of California at Santa Cruz | Faber S.M.,University of California at Santa Cruz | And 2 more authors.
Astrophysical Journal | Year: 2013

We present the first quantified, statistical map of broad-line active galactic nucleus (AGN) frequency with host galaxy color and stellar mass in nearby (0.01 < z < 0.11) galaxies. Aperture photometry and z-band concentration measurements from the Sloan Digital Sky Survey are used to disentangle AGN and galaxy emission, resulting in estimates of uncontaminated galaxy rest-frame color, luminosity, and stellar mass. Broad-line AGNs are distributed throughout the blue cloud and green valley at a given stellar mass, and are much rarer in quiescent (red sequence) galaxies. This is in contrast to the published host galaxy properties of weaker narrow-line AGNs, indicating that broad-line AGNs occur during a different phase in galaxy evolution. More luminous broad-line AGNs have bluer host galaxies, even at fixed mass, suggesting that the same processes that fuel nuclear activity also efficiently form stars. The data favor processes that simultaneously fuel both star formation activity and rapid supermassive black hole accretion. If AGNs cause feedback on their host galaxies in the nearby universe, the evidence of galaxy-wide quenching must be delayed until after the broad-line AGN phase. © 2013. The American Astronomical Society. All rights reserved..

Deming C.P.,University of California at Santa Cruz | Mercado R.,University of California at Santa Cruz | Gadiraju V.,Harker School | Sweeney S.W.,University of California at Santa Cruz | And 2 more authors.
ACS Sustainable Chemistry and Engineering | Year: 2015

Graphene quantum dots (GQDs)-supported palladium nanoparticles were synthesized by thermolytic reduction of PdCl2 in 1,2-propanediol at 80 °C in the presence of GQDs and then were subject to hydrothermal treatment at an elevated temperature within the range of 140 to 200 °C. Transmission electron microscopic measurements showed a raspberry-like morphology for the samples before and after hydrothermal treatment at temperatures ≤160 °C, where nanoparticles of ca. 8 nm in diameter formed large aggregates in the range of 50 to 100 nm in diameter, and at higher hydrothermal temperatures (180 and 200 °C), chain-like nanostructures were formed instead. X-ray photoelectron and Raman spectroscopic measurements revealed that the GQD structural defects were readily removed by hydrothermal treatments, and the defect concentrations exhibited a clear diminishment with increasing hydrothermal temperature, as indicated by the loss of oxygenated carbons in XPS and a drop in the D to G band ratio in Raman measurements. Voltammetric studies showed apparent electrocatalytic activity toward oxygen reduction, with a volcano-shaped variation of the activity with GQD defect concentration, and the peak activity was observed for the sample prepared at 180 °C with a mass activity of 23.9 A/gPd and specific activity of 1.08 A/m2 at +0.9 V vs RHE. This peak activity is attributed to optimal interactions between Pd and GQD where the GQD defects promoted charge transfer from metal to GQDs and hence weakened interactions with oxygenated intermediates, leading to enhanced ORR activity. The corresponding defect concentration was higher than that identified with the platinum counterparts due to the stronger affinity of oxygen to palladium. © 2015 American Chemical Society.

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