Charge transport in composites of inorganic nanorods and a conjugated polymer is investigated using a photovoltaic device structure. We show that t he current-voltage (I-V) curves in the dark can be modelled using the Shockley equation modified to include series and shunt resistance at low current levels, and using an improved model that incorporates both the Shockley equation and the presence of a space charge limited region at high currents. Under illumination the efficiency of photocurrent generation is found to be dependent on applied bias.Furthermore, the photocurrent-light intensity dependence was found to be sublinear. An analysis of the shunt resistance as a function of light intensity suggests that the photocurrent as well as the fill factor is diminished as a result of increased photoconductivity of the active layer at high light intensity. By studying the intensity dependence of the open circuit voltage for nanocrystals with different diameters and thus band gaps, it was inferred that Fermi-level pinning occurs at the interface between the aluminum electrode and the nanocrystal.3
A microfluidic four-roll mill device that can cover the entire spectrum of flow types including purely rotational flow was designed using pseudo-three-dimensional simulations. In experiments using high aspect ratio devices etched in silicon, the authors observed the whole range of flow type by changing only the flow rate ratio. This microfluidic four-roll mill device can be applied to examining microdrop deformation and the dynamics of single molecules in a mixed flow or to enhancing mixing efficiency by sinusoidal changes of the inlet flow rate.
Understanding the dynamics of biopolymers in complex flows is critical for the successful design of lab-on-a-chip devices. In this paper, we demonstrate the first direct comparison of experiments and simulations of DNA transport in a pressure-driven post array flow. High aspect ratio ordered silicon posts arrays were microfabricated, and single molecule experiments were employed to examine the dynamics of DNA traversing through arrays of ordered obstacles. Three different geometries with varying post spacing were tested, and probability distributions of DNA hooking location and fractional DNA lengths were generated. It is demonstrated that the appropriate design of post array geometry can be used to control DNA conformation and guide the location of hooking events. Finally, agreement between experimental and Brownian dynamics simulation results is good, validating the simulations that can be used to guide future array designs.
Understanding the fate and transport of biological agents into buildings will be critical to recovery and restoration efforts after a biological attack in an urban area. As part of the Interagency Biological Restoration Demonstration (IBRD), experiments were conducted in Fairfax County, VA, to study whether a biological agent can be expected to infiltrate into buildings following a wide-area release. Bacillus thuringiensis var. kurstaki is a common organic pesticide that has been sprayed in Fairfax County for a number of years to control the gypsy moth. Because the bacterium shares many physical and biological properties with Bacillus anthracis, the results from these studies can be extrapolated to a bioterrorist release. In 2009, samples were collected from inside buildings located immediately adjacent to a spray block. A combined probabilistic and targeted sampling strategy and modeling were conducted to provide insight into likely methods of infiltration. Both the simulations and the experimental results indicate sampling entryways and heating, ventilation, and air conditioning (HVAC) filters are reasonable methods for "ruling in" a building as contaminated. Following a biological attack, this method is likely to provide significant savings in time and labor compared to more rigorous, statistically based characterization. However, this method should never be used to "rule out," or clear, a building.
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