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Recent Publications

Savas, Kaya

Savas Kaya

Title: AFM-based approach to study blending between RAP and virgin asphalt binders
Authors: AbuQtaish, L; Nazzal, MD; Kaya, S; Kim, SS; Abbas, A; Abu Hassan, Y
Source: JOURNAL OF MATERIALS IN CIVIL ENGINEERING, Volume 30, Issue 3; DOI: 10.1061/(ASCE)MT.1943-5533.0002182
Published: March 2018

Abstract: In this paper, atomic force microscopy (AFM) is used to evaluate micromechanical properties of the interfacial blending zone between reclaimed asphalt pavement (RAP) and virgin asphalt binders. Three virgin asphalt binders with different performance grades (PG 58-28, PG 64-28, and PG 64-22) and two sources of RAP binder are used to evaluate the effects of the different RAP and virgin asphalt binder properties on the stiffness and adhesive properties of the blending zone between these binders. A new procedure is developed to simulate the interaction between RAP and virgin asphalt binders that occurs in the drum in an asphalt plant. The AFM tests indicate that blending occurred between RAP and virgin binder for all six tested RAP-virgin asphalt binder combinations. The reduced modulus for the blending zone depended generally on the virgin binder grade and RAP source. The highest stiffness was achieved from the combination of RAP and PG 64-22, whereas the lowest stiffness was obtained from the combination of RAP and PG 58-28. The bonding energy results indicate that adhesive properties of the blending zone were adversely affected by the presence of RAP within the zone. However, the adhesive properties of the blending zone were significantly better than those for the RAP binders. The variation in the blending zone between both RAP binders and PG 58-28/PG 64-28 indicate that the blending zone might not be homogenous. Further analysis of the concentration of RAP binder within the blending zone revealed dependence on RAP stiffness characteristics. However, the adhesive properties of the blending zone were primarily controlled by the virgin binder properties.



Jixin Chen\

Jixin Chen

Title: Photobleaching of YOYO-1 in super-resolution single DNA fluorescence imaging
Authors: Pyle, JR; Chen, JX
Source: BEILSTEIN JOURNAL OF NANOTECHNOLOGY, Volume 9, Issue 74, p.809-811; DOI:10.3762/bjnano.9.74
Published: March 2018

Abstract: Super-resolution imaging of single DNA molecules via point accumulation for imaging in nanoscale topography (PAINT) has great potential to visualize fine DNA structures with nanometer resolution. In a typical PAINT video acquisition, dye molecules (YOYO-1) in solution sparsely bind to the target surfaces (DNA) whose locations can be mathematically determined by fitting their fluorescent point spread function. Many YOYO-1 molecules intercalate into DNA and remain there during imaging, and most of them have to be temporarily or permanently fluorescently bleached, often stochastically, to allow for the visualization of a few fluorescent events per DNA per frame of the video. Thus, controlling the fluorescence on–off rate is important in PAINT. In this paper, we study the photobleaching of YOYO-1 and its correlation with the quality of the PAINT images. At a low excitation laser power density, the photobleaching of YOYO-1 is too slow and a minimum required power density was identified, which can be theoretically predicted with the proposed method in this report.


Gang Chen

Gang Chen

Title: Transparent oxyfluoride glass-ceramics with NaGdF4 nanocrystals doped with Pr3+ and Pr3+-Yb3+
Authors: Velazquez, JJ; Balda, R; Fernandez, J; Gorni, G; Pascual, L; Chen, G; Sundararajan, M; Duran, A; Pascual, MJ
Source: JOURNAL OF LUMINESCENCE, Volume 193, p. 61-69; DOI: 10.1016/j.jlumin.2017.07.034 
Published: January 2018

Abstract: Transparent oxyfluoride nano-glass-ceramics (GCs) containing NaGdF4 nanocrystals doped with 0.1Pr3+and 0.5Pr3+, and co-doped with 0.5Pr3+-2Yb3+ ions (mol%) were obtained by melting-quenching followed by heat treatments at temperatures near Tg. The addition of rare-earths (RE) ions affects the crystallization kinetics due to the progressive increase of Tg as RE concentration increases. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) show the precipitation of NaGdF4 nanocrystals with sizes between 9 and 30 nm. The crystal size increases with the increasing amount of dopant. Energy dispersive X-ray (EDX) analysis confirms the incorporation of the RE ions in the fluoride nanocrystals in the glass-ceramics. This incorporation is also supported by optical characterization. Photoluminescence measurements shows a better resolved structure together with a narrowing of the Pr3+ emission and excitation spectra in the glass–ceramics compared to the precursor glass. The emission spectra and fluorescence decay curves from the 3P0 level of Pr3+ in the codoped samples support the existence of a Pr3+ → Yb3+ energy transfer with slight enhancement of the energy transfer efficiency in the GC sample.


Govorov, AO

Alexander Govorov

Title: Understanding Hot-Electron Generation and Plasmon Relaxation in Metal Nanocrystals: Quantum and Classical Mechanisms
Authors: Besteiro, LV; Kong, XT; Wang, ZM; Hartland, G; Govorov, AO
Source: ACS PHOTONICS, Volume 4, Issue 11, p. 2759-2781; DOI: 10.1021/acsphotonics.7b00751 
Published: November 2017

Abstract: Generation of energetic (hot) electrons is an intrinsic property of any plasmonic nanostructure under illumination. Simultaneously, a striking advantage of metal nanocrystals over semiconductors lies in their very large absorption cross sections. Therefore, metal nanostructures with strong and tailored plasmonic resonances are very attractive for photocatalytic applications in which excited electrons play an important role. However, the central questions in the problem of plasmonic hot electrons are the number of optically excited energetic electrons in a nanocrystal and how to extract such electrons. Here we develop a theory describing the generation rates and the energy distributions of hot electrons in nanocrystals with various geometries. In our theory, hot electrons are generated due to surfaces and hot spots. As expected, the formalism predicts that large optically excited nanocrystals show the excitation of mostly low-energy Drude electrons, whereas plasmons in small nanocrystals involve mostly high-energy (hot) electrons. We obtain analytical expressions for the distribution functions of excited carriers for simple shapes. For complex shapes with hot spots and for small quantum nanocrystals, our results are computational. By looking at the energy distributions of electrons in an optically excited nanocrystal, we see how the quantum many-body state in small particles evolves toward the classical state described by the Drude model when increasing nanocrystal size. We show that the rate of surface decay of plasmons in nanocrystals is directly related to the rate of generation of hot electrons. On the basis of a detailed many-body theory involving kinetic coefficients, we formulate a simple scheme describing how the plasmon in a nanocrystal dephases over time. In most nanocrystals, the main decay mechanisms of a plasmon are the Drude friction-like process and the interband electron–hole excitation, and the secondary path comes from generation of hot electrons due to surfaces and electromagnetic hot spots. The hot-electron path strongly depends on the material system and on its shape. Correspondingly, the efficiency of hot-electron production in a nanocrystal strongly varies with size, shape, and material. The results in the paper can be used to guide the design of plasmonic nanomaterials for photochemistry and photodetectors.


Title: Near-Infrared Plasmonic Copper Nanocups Fabricated by Template-Assisted Magnetron Sputterings
Authors: Qin, YX; Kong, XT; Wang, M; Govorov, AO; Kortshagen, UR
Source: ACS PHOTONICS, Volume 4, Issue 11, p. 2881-2890; DOI: 10.1021/acsphotonics.7b00866 
Published: November 2017

Abstract: In this article we experimentally and theoretically study the plasmonic properties of discrete copper nanocups fabricated by magnetron sputtering on ordered, non-close-packed colloidal templates. Wide tunability of the main plasmon resonance peak between 900 and 1500 nm, extending the typical plasmon resonance range previously reported for other copper nanostructures between 600 and 1000 nm, is achieved by varying shell thickness and particle size in the colloidal template. The nature of the plasmon resonance peaks is revealed from calculated charge maps and electromagnetic field intensity maps. Good agreements are found between experimental and calculated extinction spectra, which validates the geometry model and suggests that the nanocups have a well-defined shape. The main plasmon resonance peak exhibits a minor red-shift and attenuation after 3 days of oxidation and eventually stabilizes after 13 days. We also demonstrate that a potentially useful optical material that blocks near-infrared but transmits visible light can be constructed by mixing copper nanocups of three different sizes at appropriate ratios.


Drabold DA

David Drabold

Title: Refining glass structure in two dimensions
Authors: Sadjadi, M; Bhattarai, B; Drabold, DA; Thorpe, MF; Wilson, M
Source: PHYSICAL REVIEW B, Volume 96, Issue 20; DOI: 10.1103/PhysRevB.96.201405 
Published: November 2017

Abstract: Recently determined atomistic scale structures of near-two dimensional bilayers of vitreous silica (using scanning probe and electron microscopy) allow us to refine the experimentally determined coordinates to incorporate the known local chemistry more precisely. Further refinement is achieved by using classical potentials of varying complexity; one using harmonic potentials and the second employing an electrostatic description incorporating polarization effects. These are benchmarked against density functional calculations. Our main findings are that (a) there is a symmetry plane between the two disordered layers; a nice example of an emergent phenomenon, (b) the layers are slightly tilted so that the Si-O-Si angle between the two layers is not 180∘ as originally thought but rather 175±2∘ and (c) while interior areas that are not completely imagined can be reliably reconstructed, surface areas are more problematical. It is shown that small crystallites that appear are just as expected statistically in a continuous random network. This provides a good example of the value that can be added to disordered structures imaged at the atomic level by implementing computer refinement.



Sergio E. Ulloa

Title: Minimal geometry for valley filtering in graphene
Authors: Asmar, MM; Ulloa, SE
Source: PHYSICAL REVIEW B, Volume 96, Issue 20; DOI: 10.1103/PhysRevB.96.201407
Published: November 2017

Abstract: The possibility to effect valley splitting of an electronic current in graphene represents the essential component in the new field of valleytronics in such two-dimensional materials. Based on a symmetry analysis of the scattering matrix, we show that if the spatial distribution of multiple potential scatterers breaks mirror symmetry about the axis of incoming electrons, then a splitting of the current between two valleys is observed. This leads to the appearance of the valley Hall effect. We illustrate the effect of mirror symmetry breaking in a minimal system of two symmetric impurities, demonstrating the splitting between valleys via the differential cross sections and non-vanishing skew parameter. We further discuss the role that these effects may play in transport experiments.



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