The energy relaxation of the electrons in the conduction band of 12 and 30 nm diameter copper nanoparticles in colloidal solution was investigated using femtosecond time-resolved transient spectroscopy. Experimental results show that the hot electron energy relaxation is faster in 12 nm copper nanoparticles (0.37 ps) than that in 30 nm copper nanoparticles (0.51 ps), which is explained by the size-dependent electron-surface phonon coupling. Additional mechanisms involving trapping or energy transfer processes to the denser surface states (imperfection) in the smaller nanoparticles are needed to explain the relaxation rate in the 12 nm nanoparticles. The observed fluorescence quantum yield from these nanoparticles is found to be enhanced by roughly 5 orders of magnitude for the 30 nm nanoparticles and 4 orders of magnitude for the 12 nm nanoparticles (relative to bulk copper metal). The increase in the fluorescence quantum yield is attributed to the electromagnetic enhancement of the radiative recombination of the electrons in the s-p conduction band below the Fermi level with the holes in the d bands due to the strong surface plasmon oscillation in these nanoparticles.

Diffusive transport within complex environments is a critical piece of the chemistry occurring in such diverse membrane systems as proton exchange and bilayer lipid membranes. In the present study, fluorescence correlation spectroscopy was used to evaluate diffusive charge transport within a strong polyelectrolyte polymer brush. The fluorescent cation rhodamine-6G was used as a counterion probe molecule, and the strong polyelectrolyte poly(styrene sulfonate) was the polymer brush. Such strong polyelectrolyte brushes show promise for charge storage applications, and thus it is important to understand and tune their transport efficiencies. The polymer brush demonstrated preferential solvation of the probe counterion as compared to solvation by the aqueous solvent phase. Additionally, diffusion within the polymer brush was strongly inhibited, as evidenced by a decrease in diffusion constant of 4 orders of magnitude. It also proved possible to tune the transport characteristics by controlling the solvent pH, and thus the ionic strength of the solvent. The diffusion characteristics within the charged brush system depend on the brush density as well as the effective interaction potential between the probe ions and the brush. In response to changes in ionic strength of the solution, it was found that these two properties act in opposition to each other within this strong polyelectrolyte polymer brush environment. A stochastic random walk model was developed to simulate interaction of a diffusing charged particle with a periodic potential, to show the response of characteristic diffusion times to electrostatic field strengths. The combined results of the experiments and simulations demonstrate that responsive diffusion characteristics in this brush system are dominated by changes in Coulombic interactions rather than changes in brush density. More generally, these results support the use of FCS to evaluate local charge transport properties within polyelectrolyte brush systems, and demonstrate that the technique shows promise in the development of novel polyelectrolyte films for charge storage/transport materials.

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In surfactant flooding for enhanced oil recovery, adsorption of surfactants on the porous media of an oil reservoir is a major concern. It weakens the efficacy of the injected surfactant in reducing oil–water interfacial tension (IFT) and makes the oil recovery process uneconomical. Colloidal silica nanoparticles were found to adsorb at a lower rate than surfactant in porous media because of their charge density and high surface area. Silica nanoparticle surfaces with a negative surface charge are expected to adsorb onto the same active sites in the reservoir as anionic surfactant molecules used in enhanced oil recovery (EOR) applications. Experiments conducted in sandpack demonstrated that pre-treatment of the sandpack with silica nanoparticles at 80°C reduced surfactant adsorption by a factor of three when using artificial seawater as the injection fluid.

The optical gain dynamics has been studied for two CdS quantum dot samples dispersed in toluene at room temperature. This was carried out by using femtosecond transient absorption technique with an excitation at 400nm and gain measurement was studied at the fluorescence maxima (440 and 460nm). The optical gain lifetime was found to be as long as 20ps under pump fluence as low as 0.77mJ∕cm2. The low threshold is the result of long lifetime of electrons and holes and narrow emission bandwidth. These results suggest that CdS quantum dots in solution are excellent gain media for optically pumped high power blue lasers.

We discovered a novel nanoparticle (NP)–crude oil interaction and propose a mechanism of NP-based enhanced oil recovery. This NP–crude oil interaction and its effects on oil recovery are systematically investigated by conducting microfluidic experiments in both single-pore scale and “reservoir-on-a-chip” scale. It is confirmed that hydrophilic silica NPs in an aqueous phase could lead to dramatic swelling, dewetting, and disjoining of crude oil. The swelling ratio increased with decreased aqueous phase salinity and with increased concentrations of negative charging of NPs. Natural polar components in crude oil is shown to play a very important role. From a pore-scale perspective, this oil swelling and dewetting increased the flow resistance in the swept region and redirected flooding liquid toward the unswept region. From a reservoir perspective, the mobility ratio was reduced because oil swelling and dewetting modified the relative permeabilities. This improvement in sweep efficiency resulted in approximately 11% incremental oil recovery in a completely homogeneous porous micromodel, with 2000 ppm of NPs suspended in seawater.