Key role of molecular kinetic energy in early stages of pentacene island growth

Organic molecular beam deposition is studied systematically at thermal and hyperthermal regimes aiming at investigating the role of molecular kinetic energy on the growth mechanism of pentacene submonolayers on SiO _x /Si. We show that the kinetic energy of the impinging molecule (E _k ) plays a crucial role in determining island structure and shape, distribution of island sizes, the crystalline quality of the first monolayer, and even the growth mode of subsequent layers. With increasing E _k , the island structure changes from fractal to nonfractal, the shape becomes more anisotropic and the island size more uniform, pointing to correlated island growth. Moreover, while 3D island growth is observed for thermal organic molecular beam deposition, supersonic molecular beam deposition gives rise to layer-by-layer growth, at least for the first two layers. When E _k ≥5.0 eV, the first monolayer is composed of large single crystalline domains which can extend over up to 10 μm, inferred from comparing atomic force micrographs of height and net transverse shear force. In these growth conditions both the high surface diffusivity and energy redistribution play a major role. We propose a mechanism where the energy dissipation occurring during the molecule–surface collision leads to the reorientation of whole islands during island coalescence, resulting in the elimination of grain boundaries.     

Ultra-low background measurements of decayed aerosol filters

Aerosol samples collected on filter media were analyzed using HPGe detectors employing varying background-reduction techniques in order to experimentally evaluate the opportunity to apply ultra-low background measurement methods to samples collected, for instance, by the Comprehensive Test Ban Treaty International Monitoring System (IMS). In this way, realistic estimates of the impact of low-background methodology on the sensitivity obtained in systems such as the IMS were assessed. The current detectability requirement of stations in the IMS is 30 μBq/m³ of air for ¹⁴⁰Ba, which would imply ~10⁶ fissions per daily sample. Importantly, this is for a fresh aerosol filter. One week of decay reduces the intrinsic background from radon daughters in the sample allowing much higher sensitivity measurement of relevant isotopes, including ¹³¹I. An experiment was conducted in which decayed filter samples were measured at a variety of underground locations using Ultra-Low Background (ULB) gamma spectroscopy technology. The impacts of the decay and ULB are discussed.     

Ionic liquid-mediated sol–gel coatings for capillary microextraction

Ionic liquid (IL)-mediated sol–gel hybrid organic–inorganic materials present enormous potential for effective use in analytical microextraction. This opportunity, however, has not yet been explored. One obstacle to materializing this prospect arises from high viscosity of ILs significantly slowing down sol–gel reactions. In this work, we developed a method that overcomes this hurdle and provides IL-mediated advanced sol–gel materials for capillary microextraction (CME). We examined two different ILs: (a) a phosphonium-based IL, trihexyltetradecylphosphonium tetrafluoroborate, and (b) a pyridinium-based ionic liquid, N-butyl-4-methylpyridinium tetrafluoroborate. These ILs were evaluated in conjunction with two types of hydroxy-terminated polymers: (a) two Si–OH terminated polymers (PDMS and BMPO), and (b) two C–OH terminated polymers (PEG and polyTHF) that differ in their sol–gel reactivity. Scanning electron microscopy results demonstrate that ILs can serve as porogenic agents in sol–gel reactions. The IL-mediated sol–gel coatings prepared with silanol-terminated polymers provided up to 28 times higher extractions in off-line CME-GC compared to analogous sol–gel coatings prepared without any IL in the sol solution. Contrary to this, the IL-mediated sol–gel coatings prepared with C–OH terminated polymers provided lower extraction efficiencies compared to their IL-free counterparts. These observations were explained by (a) lower sol–gel reactivity of C–OH groups in PEG and polyTHF compared to Si–OH groups in PDMS and in hydrolyzed alkoxysilane precursors and (b) extremely high viscosity of ionic liquids. This study shows that IL-generated porous morphology alone is not enough to provide effective extraction media: careful choice of the organic polymer and the precursor with close sol–gel reactivity must be made to ensure effective chemical bonding of the organic polymer to the created sol–gel material to be able to provide the desired sorbent characteristics. Additionally, IL-mediated sol–gel PDMS coatings provided run-to-run RSD values of 4.2–5.0% and detection limits ranging from 3.2 ng/L to 17.4 ng/L. PDMS sol–gels prepared without ILs provided RSD values of 2.8–14.1%, and detection limits ranging from 4.9 ng/L to 487.0 ng/L.     

Activated hopping and dynamical fluctuation effects in hard sphere suspensions and fluids

Single particle Brownian dynamics simulation methods are employed to establish the full trajectory level predictions of our nonlinear stochastic Langevin equation theory of activated hopping dynamics in glassy hard sphere suspensions and fluids. The consequences of thermal noise driven mobility fluctuations associated with the barrier hopping process are determined for various ensemble-averaged properties and their distributions. The predicted mean square displacements show classic signatures of transient trapping and anomalous diffusion on intermediate time and length scales. A crossover to a stronger volume fraction dependence of the apparent nondiffusive exponent occurs when the entropic barrier is of order the thermal energy. The volume fraction dependences of various mean relaxation times and rates can be fitted by empirical critical power laws with parameters consistent with ideal mode-coupling theory. However, the results of our divergence-free theory are largely a consequence of activated dynamics. The experimentally measurable alpha relaxation time is found to be very similar to the theoretically defined mean reaction time for escape from the barrier-dominated regime. Various measures of decoupling have been studied. For fluid states with small or nonexistent barriers, relaxation times obey a simple log-normal distribution, while for high volume fractions the relaxation time distributions become Poissonian. The product of the self-diffusion constant and mean alpha relaxation time increases roughly as a logarithmic function of the alpha relaxation time. The cage scale incoherent dynamic structure factor exhibits nonexponential decay with a modest degree of stretching. A nearly universal collapse of the different volume fraction results occurs if time is scaled by the mean alpha relaxation time. Hence, time-volume fraction superposition holds quite well, despite the presence of stretching and volume fraction dependent decoupling associated with the stochastic barrier hopping process. The relevance of other origins of dynamic heterogeneity (e.g., mesoscopic domains), and comparison of our results with experiments, simulations, and alternative theories, is discussed.     

New mixed-valence chromium structure type: NH4Cr(CrO4)2

Synthesis and crystal structure of a new structure type of mixed Cr(III)/Cr(VI) chromates is reported. NH₄Cr(CrO₄)₂ was prepared from CrO₃ in the presence of (NH₄)₂Ce(NO₃)₆. Since this is the first preparation of mixed valence ternary chromium oxides from aqueous solution, a reaction pathway for this synthesis is suggested. The crystal structure of NH₄Cr(CrO₄)₂ has been determined from three-dimensional X-ray data collected at low temperature, 173 K. The structure belongs to the orthorhombic space group Pnma, with a=14.5206(10), b=5.4826(4), and Z=4. The title compound consists of corner-sharing chromium(III) octahedra and chromium(VI) tetrahedra forming a three-dimensional network with the composition [Cr(CrO₄)₂]_n^n-, containing channels in which zigzag rows of ammonium ions balance the net charge.     

Chemoselective N-acylation of indoles and oxazolidinones with carbonylazoles

Unique reactivity: In the presence of more reactive amine and alcohol functional groups and of carboxylic acids, the chemoselective N-acylation of indoles and oxazolidinones is achieved by taking advantage of the unique reactivity of carbonylazole acylating agents with catalytic amounts of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).     

Continuous operation of a hybrid solid-liquid state reconfigurable photonic system without resupply of liquids

Optofluidics offers a number of potentially transformative advantages for photonic systems. At present however there are a number of technological roadblocks that prevent the practical integration of liquid-state elements into traditional high-speed solid-state photonic systems. Two of the most important of these are the need for continuous resupply of liquids and the difficulty in shuttling light between the liquid- and solid-states. In this paper we present an integrated system that solves both these problems. For the first time we demonstrate direct evanescent and end-fire coupling between liquid- and solid-state waveguides and an on-chip fluid core/cladding separation and recirculation system that reduces the consumption of liquids more than 200 fold over the state of the art. The device is operated continuously for over 20 h without performance degradation or requiring the replenishment of liquids. We believe that our system represents an important step towards the development of practical optofluidically enabled photonic systems.     

Slab waveguide photobioreactors for microalgae based biofuel production

Microalgae are a promising feedstock for sustainable biofuel production. At present, however, there are a number of challenges that limit the economic viability of the process. Two of the major challenges are the non-uniform distribution of light in photobioreactors and the inefficiencies associated with traditional biomass processing. To address the latter limitation, a number of studies have demonstrated organisms that directly secrete fuels without requiring organism harvesting. In this paper, we demonstrate a novel optofluidic photobioreactor that can help address the light distribution challenge while being compatible with these chemical secreting organisms. Our approach is based on light delivery to surface bound photosynthetic organisms through the evanescent field of an optically excited slab waveguide. In addition to characterizing organism growth-rates in the system, we also show here, for the first time, that the photon usage efficiency of evanescent field illumination is comparable to the direct illumination used in traditional photobioreactors. We also show that the stackable nature of the slab waveguide approach could yield a 12-fold improvement in the volumetric productivity.     

Synthesis and Photophysical Properties of a Series of Cyclopenta[b]naphthalene Solvatochromic Fluorophores

The synthesis and photophysical properties of a series of naphthalene-containing solvatochromic fluorophores are described within. These novel fluorophores are prepared using a microwave-assisted dehydrogenative Diels-Alder reaction of styrene, followed by a palladium-catalyzed cross coupling reaction to install an electron donating amine group. The new fluorophores are structurally related to Prodan. Photophysical properties of the new fluorophores were studied and intriguing solvatochromic behavior was observed. For most of these fluorophores, high quantum yields (60-99%) were observed in methylene chloride in addition to large Stokes shifts (95-226 nm) in this same solvent. As the solvent polarity increased, so did the observed Stokes shift with one derivative displaying a Stokes shift of ～300 nm in ethanol. All fluorophore emission maxima, and nearly all absorption maxima were significantly red-shifted when compared to Prodan. Shifting the absorption and emission maxima of a fluorophore into the visible region increases its utility in biological applications. Moreover, the cyclopentane portion of the fluorophore structure provides an attachment point for biomolecules that will minimize disruptions of the photophysical properties.