Laser-induced nucleation promotes crystal growth of anhydrous sodium bromide

We report on a study of crystal hydrate formation in supersaturated aqueous sodium bromide using different methods to induce nucleation: mechanical shock-induced nucleation (MSIN), nucleation by ultrasound (sonocrystallization), non-photochemical laser-induced nucleation (NPLIN) and laser-trapping nucleation. The most stable crystal form at room temperature is known to be sodium bromide dihydrate (DH) and this form was favoured (>95%) through spontaneous nucleation or mechanical shock. Sonocrystallization favoured DH crystals (74%). Remarkably both laser-induced nucleation methods showed a strong preference (>90%) for anhydrous (AH) crystals. The nucleation mechanisms are discussed with reference to the solution–solid phase diagram. For laser-trapping nucleation, the results are consistent with previous studies showing that nucleation is preceded by formation of a localised volume of increased solute concentration. The common mechanistic feature linking sonocrystallization, MSIN and NPLIN is cavitation. The preference for AH sodium bromide suggests that nanosecond laser pulses produce cavitation events with more thermal energy compared to the other methods. The results demonstrate the value of laser-induced nucleation in controlling crystal hydrate growth and provide new understanding of the nucleation mechanisms

Using time-resolved fluorescence to investigate exciton harvesting in organic photovoltaic blends

This thesis is an investigation of the photophysical processes that occur in organic photovoltaic blends in the time between light being absorbed and free charges being generated. The purpose of all solar cells is to generate a photocurrent. The free charges, as they flow out of the device, make up the photocurrent, so understanding the processes by which they are created is vitally important to organic photovoltaic research. The main experimental method used was time-resolved fluorescence spectroscopy. This technique was used to probe the exciton population with respect to time for a variety of blends of organic semiconductors, including the high performance photovoltaic materials PCDTBT, PTB7, C71-PCBM and P3HT. The main goal of the work was to characterise the exciton diffusion lengths of these materials by developing a technique called volume quenching. Volume quenching involves blending a small quantity of quenching material into a thin film of semiconducting material. These introduced quenching sites render excitons unemissive on contact. Thus, from the drop in fluorescence compared with the ‘unquenched’ material, it was possible to work out what proportion of the initial excitons have encountered a quenching site in the blends. The results can then be fitted to quantify how diffusive the excitons are -i.e. how far they move. By looking at the rate constant of the quenching process and how it varies with respect to time, quencher concentration and quencher type, it was possible to generate a wealth of additional information, not just about exciton diffusion, but about all the inter-related processes that contribute to exciton harvesting. These processes included the measurement of long-range energy transfer from the donor to the acceptor, electron transfer at the interface with the acceptor and the understanding of nanomorphology of donor-acceptor heterojunctions

IDO1 is an Integral Mediator of Inflammatory Neovascularization.

The immune tolerogenic effects of IDO1 (indoleamine 2,3-dioxygenase 1) have been well documented and genetic studies in mice have clearly established the significance of IDO1 in tumor promotion. Dichotomously, the primary inducer of IDO1, the inflammatory cytokine IFNγ (interferon-γ), is a key mediator of immune-based tumor suppression. One means by which IFNγ can exert an anti-cancer effect is by decreasing tumor neovascularization. We speculated that IDO1 might contribute to cancer promotion by countering this anti-neovascular effect of IFNγ, possibly through IDO1-potentiated elevation of the pro-tumorigenic inflammatory cytokine IL6 (interleukin-6). In this study, we investigated how genetic loss of IDO1 affects neovascularization in mouse models of oxygen-induced retinopathy and lung metastasis. Neovascularization in both models was significantly reduced in mice lacking IDO1, was similarly reduced with loss of IL6, and was restored in both cases by concomitant loss of IFNγ. Likewise, the lack of IDO1 or IL6 resulted in reduced metastatic tumor burden and increased survival, which the concomitant loss of IFNγ abrogated. This insight into IDO1\u27s involvement in pro-tumorigenic inflammatory neovascularization may have important ramifications for IDO1 inhibitor development, not only in cancer where clinical trials are currently ongoing, but in other disease indications associated with neovascularization as well

The role of cavitation and gas bubbles in the non-photochemical laser-induced nucleation of sodium acetate

An experimental study of the effects of the sodium salt of poly(methacrylic acid) (Na-PMAA) on non-photochemical laser-induced nucleation (NPLIN) of sodium acetate crystals is presented. Seeding of supersaturated aqueous solutions with anhydrous (AH) seeds always produced trihydrate (TH) crystals, with or without polymer additive. Using NPLIN, with no Na-PMAA and at low Na-PMAA concentrations (0.25% w/w) AH sodium acetate was produced, firstly as plate-like form IV, but subsequently growing needles, likely to be form I. At high Na-PMAA concentrations (0.73% w/w) we observe formation mostly of stable bubbles. In all samples at low laser peak power densities (<26 MW cm−2) we show for the first time using NPLIN that both crystals and bubbles can be nucleated with a single laser pulse. Measurements of the dependence of bubble or crystal count on laser pulse power indicate a common mechanistic origin for nucleation, which is cavitation due to laser heating of impurity nanoparticles. The bubbles observed are attributed to laser heating of the nanoparticles to high temperatures, resulting in gas formed by thermochemical reactions or gas that was previously dissolved in the solution. Our results provide new insight into the particle-heating mechanism for NPLIN, but whether stable bubbles play a defining role in the nucleation of crystals remains to be resolved

Role of Impurity Nanoparticles in Laser-Induced Nucleation of Ammonium Chloride

Results of experiments on laser-induced nucleation (LIN) in supersaturated (120%) aqueous ammonium chloride solutions are presented. Measurement of the particle-size distribution in unfiltered solutions near saturation (95%) indicates a population of nanometer-scale species with a mean hydrodynamic diameter of 750 nm, which is almost entirely removed by single-pass filtration through a poly­(ether sulfone) membrane (0.2 μm pores). Analysis of filter residues reveals iron and phosphate as major impurities in the solute. Experiments show that the number of nuclei induced by LIN can be reduced substantially by preprocessing (filtering or long-term exposure to laser pulses) and that this reduction can be reversed by intentional doping with iron-oxide (Fe₃O₄) nanoparticles. The use of surfactant to assist dispersion of the nanoparticles was found to increase the number of laser-induced nuclei. We discuss the results with reference to mechanisms of non-photochemical laser-induced nucleation

Effect of a high boiling point additive on the morphology of solution-processed P3HT-fullerene blends

Funding: UK Engineering and Physical Sciences Research Council (EPSRC) EP/L505079/1, EP/G03673X/1, EP/J009016/1The use of high boiling point additives in solution processing has been widely employed to control the active layer morphology in bulk heterojunction organic solar cells. The morphology of the heterojunction is crucial in controlling charge separation and extraction by the electrodes, and therefore the power conversion efficiency (PCE) of the device. This paper presents a study of time-resolved fluorescence quenching in blends of P3HT containing varying concentrations of the fullerenes PC61BM or PC71BM. The relationship between the fluorescence quenching rate and fullerene concentration indicates that the fullerene molecules are dispersed within the P3HT film for up to 5% by mass of fullerene. For higher fullerene concentrations, the additional fullerene molecules aggregate and form fullerene domains. The high degree of phase segregation observed in these blends is beneficial for solar cell performance because the segregated fullerene phase provides electron percolation pathways through the blend. The addition of 1,8-diiodooctane (DIO) to the solutions for spin coating into films changes the scale of fullerene segregation when the ratio by mass of fullerene exceeds 20%. At high fullerene concentrations the rate of fluorescence quenching decreases in P3HT:PC61BM blends when prepared with DIO indicating a larger scale phase separation. The effect of DIO on the morphology of P3HT:PC71BM blends is the opposite in that it causes faster quenching in the blends. Overall the results show that DIO can be used to control the morphology of photovoltaic blends of P3HT with fullerenes.Peer reviewe

Controlling exciton diffusion and fullerene distribution in photovoltaic blends by side chain modification

The influence of crystallinity on exciton diffusion and fullerene distribution was investigated by blending amorphous and semicrystalline copolymers. We measured exciton diffusion and fluorescence quenching in such blends by dispersing fullerene molecules into them. We find that the diffusion length is more than two times higher in the semicrystalline copolymer than in the amorphous copolymer. We also find that fullerene preferentially mixes into disordered regions of the polymer film. This shows that relatively small differences in molecular structure are important for exciton diffusion and fullerene distribution

On the distribution of career longevity and the evolution of home run prowess in professional baseball

Statistical analysis is a major aspect of baseball, from player averages to historical benchmarks and records. Much of baseball fanfare is based around players exceeding the norm, some in a single game and others over a long career. Career statistics serve as a metric for classifying players and establishing their historical legacy. However, the concept of records and benchmarks assumes that the level of competition in baseball is stationary in time. Here we show that power-law probability density functions, a hallmark of many complex systems that are driven by competition, govern career longevity in baseball. We also find similar power laws in the density functions of all major performance metrics for pitchers and batters. The use of performance-enhancing drugs has a dark history, emerging as a problem for both amateur and professional sports. We find statistical evidence consistent with performance-enhancing drugs in the analysis of home runs hit by players in the last 25 years. This is corroborated by the findings of the Mitchell Report [1], a two-year investigation into the use of illegal steroids in major league baseball, which recently revealed that over 5 percent of major league baseball players tested positive for performance-enhancing drugs in an anonymous 2003 survey.Comment: 5 pages, 5 figures, 2-column revtex4 format. Revision has change of title, a figure added, and minor changes in response to referee comment