Solvation dynamics in a neat ionic liquid, 1-pentyl-3-methyl-imidazolium tetra-flouroborate ([pmim][BF4]) and its microemulsion in Triton X-100 (TX-100)/benzene is studied using femtosecond up-conversion. In both the neat ionic liquid and the microemulsion, the solvation dynamics is found to depend on excitation wavelength (lambda(ex)). The lambda(ex) dependence is attributed to structural heterogeneity in neat ionic liquid (IL) and in IL microemulsion. In neat IL, the heterogeneity arises from clustering of the pentyl groups which are surrounded by a network of cation and anions. Such a nanostructural organization is predicted in many recent simulations and observed recently in an X-ray diffraction study. In an IL microemulsion, the surfactant (TX-100) molecules aggregate in form of a nonpolar peripheral shell around the polar pool of IL. The micro-environment in such an assembly varies drastically over a short distance. The dynamic solvent shift (and average solvation time) in neat IL as well as in IL microemulsions decreases markedly as lambda(ex) increases from 375 to 435 nm. In a [pmim][BF4]/water/TX-100/benzene quaternary microemulsion, the solvation dynamics is slower than that in a microemulsion without water. This is ascribed to the smaller size of the water containing microemulsion. The anisotropy decay in an IL microemulsion is found to be faster than that in neat IL.

Fluorescence correlation spectroscopy (FCS) has been used to study translational diffusion of three fluorescent dyes in a micelle and a gel. It was demonstrated that a highly hydrophobic dye, DCM, remains confined to a particular micelle during the passage of the micellar aggregation through the confocal volume. As a result, DCM exhibits slow diffusion of the large micellar aggregate with a diffusion coefficient (D(t)) approximately 25 times slower compared with that of water. In contrast, a hydrophilic probe (C343 or C480) occasionally diffuses out of the micelle into bulk water and displays a large D(t) (twofold smaller in F127 and approximately six times smaller in the P123 micelle compared with that in bulk water). In a gel, diffusion of the individual micelles is completely arrested and hence, the autocorrelation in FCS arises solely from the diffusion of the dye in the gel. In this case, all the three dyes exhibit extremely slow diffusion (300, 45, and 20 times slower than that in water for DCM, C480, and C343 in F127 gel, respectively). In a P123 and F127 gel, diffusion of DCM is respectively, seven and 29 times slower compared with that of the ionic probe C343. The relatively small value of red-edge excitation shift (REES) of the emission maximum, suggests that DCM is confined within the core of the triblock copolymer micelles and gels. The hydrophilic probes (C343 or C480) exhibit fast diffusion in the micelles and gels. However, their REES is very different. The large REES of C480 suggests that it is distributed over a large region of the micelle, whereas the low REES of C343 indicates that it is located primarily in the peripheral corona region.

The triblock copolymer (PEO)20-(PPO)70-(PEO)20 (P123) forms a supramolecular aggregate with sodium dodecyl sulfate (SDS). The solvation dynamics and anisotropy decay of coumarin 480 (C480) in different regions of a P123-SDS aggregate are studied through variation of the excitation wavelength (lambdaex) using femtosecond upconversion. In a P123 micelle, because of the drastic differences in polarity between the hydrophilic corona region (PEO block) and the hydrophobic PPO core, C480 exhibits a pronounced red edge excitation shift (REES) of emission maximum by 24 nm. In the P123-SDS aggregate, SDS penetrates the core of the P123 micelle. This increases the polarity of the core and reduces the difference in the polarity between the core and the corona region. In a P123-SDS aggregate, the REES is much smaller (5 nm) which suggests a reduced difference between the core and the corona. Solvation dynamics in a P123 micelle displays a bulklike ultrafast component (<0.3 and 1 ps) in the PEO corona region, a 200 ps component arising from dynamics of polymer segments, and a very long component (5000 or 3000 ps) due to the highly restricted PPO core. In a P123-SDS aggregate, at lambdaex = 375 and 405 nm, the solvation dynamics is found to be faster than that in P123 micelle. In this case, the component (3000 ps) arising from the core region is faster than that (5000 ps) in P123 micelle. In both P123 micelle and P123-SDS aggregate, the relative contribution of the core region decreases and that of the corona region increases with an increase in lambdaex. At lambdaex = 435 nm, which probes the hydrophilic corona, the solvation dynamics for both P123 micelle and P123-SDS aggregate are almost similar.

Fluorescence resonance energy transfer (FRET) from coumarin 480 (C480) to rhodamine 6G (R6G) is studied in a room temperature ionic liquid (RTIL) microemulsion by picosecond and femtosecond emission spectroscopy. The microemulsion is comprised of the RTIL 1-pentyl-3-methylimidazolium tetraflouroborate, [pmim][BF4], in TX-100/ benzene. We have studied the microemulsion with and without water. The time constants of FRET were obtained from the risetime of the acceptor (R6G) emission. In the RTIL microemulsion, FRET occurs on multiple time scales: 1, 250, and 3900 ps. In water containing RTIL microemulsion, the rise components are 1.5, 250, and 3900 ps. The 1 and 1.5 ps components are assigned to FRET at a close contact of donor and acceptor (RDA approximately 12 A). This occurs within the highly polar (RTIL/water) pool of the microemulsion. With increase in the excitation wavelength (lambdaex) from 375 to 435 nm, the relative contribution of the ultrafast component of FRET (1 ps) increases from 4% to 100% in the RTIL microemulsion and 12% to 100% in the water containing RTIL microemulsion. It is suggested that at lambdaex = 435 nm, mainly the highly polar RTIL pool is probed where FRET is very fast due to the close proximity of the donor and the acceptor. The very long 3900 ps (RDA approximately 45 A) component may arise from FRET from a donor in the outer periphery of the microemulsion to an acceptor in the polar RTIL pool. The 250 ps component (RDA approximately 29 A) is assigned to FRET from a donor inside the surfactant chains.

Managing trap states and understanding their role in ultrafast charge-carrier dynamics, particularly at surface and interfaces, remains a major bottleneck preventing further advancements and commercial exploitation of nanowire (NW)-based devices. A key challenge is to selectively map such ultrafast dynamical processes on the surfaces of NWs, a capability so far out of reach of time-resolved laser techniques. Selective mapping of surface dynamics in real space and time can only be achieved by applying four-dimensional scanning ultrafast electron microscopy (4D S-UEM). Charge carrier dynamics are spatially and temporally visualized on the surface of InGaN NW arrays before and after surface passivation with octadecylthiol (ODT). The time-resolved secondary electron images clearly demonstrate that carrier recombination on the NW surface is significantly slowed down after ODT treatment. This observation is fully supported by enhancement of the performance of the light emitting device. Direct observation of surface dynamics provides a profound understanding of the photophysical mechanisms on materials' surfaces and enables the formulation of effective surface trap state management strategies for the next generation of high-performance NW-based optoelectronic devices.