This PDF file contains the front matter associated with SPIE Proceedings Volume 6643, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

Polyaromatic compounds, with terminal functional groups, can be non-covalently bonded to the sidewall of carbon nanotubes. This architecture preserves the structural, mechanical, electrical, and electromechanical properties of the CNTs and ensures that an unhindered functional group is available to bond with an extended polymer matrix. Spectroscopic measurements and high resolution imaging are used to confirm the functionalization and incorporation of functionalized MWNTs into a nylon 12 matrix.

We report that dispersions and functionalizations of single-walled carbon nanotubes (SWNTs) with different dispersing methods and dispersing agents result in SWNTs with different electronic structures and surface chemistries. By In-situ polymerizing conducting polymer (here, polyaniline boronic acid (PABA)) in the presence of these SWNTs, we obtained composites with different chemical structures of PABA, different arrangement and distribution of the SWNTs, and dramatically different conductive properties. We also found the polymerization rates are very different depending on the electronic fingerprint and surface chemistry of the carbon nanotubes. We applied a series of techniques to characterize the produced composites and studying the electronic and molecular interactions in the composites to understand these remarkable effects.

We studied hole injection from the conducting polymer blend poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) by optical spectroscopy and characterization of organic light-emitting diodes (OLEDs). Electroabsorption (EA) spectroscopy was used to measure the built-in potential of polyfluorene-based OLEDs with indium tin oxide (ITO) or poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) anodes. Although the work function of PEDOT:PSS is 5.1 eV, the inferred anode work function matches the ionization potential of the emitting polymer. We conclude that the Fermi level at the PEDOT:PSS/polyfluorene interface is pinned to the highest-occupied molecular orbital (HOMO) of the emitting polymer, permitting efficient hole injection. To test this hypothesis, we fabricated OLEDs using the archetypical molecular semiconductor, tris(8-hydroxyquinoline) aluminum (III) (Alq₃). Although the anticipated hole injection barrier is 0.7 eV, OLEDs with Alq₃ deposited onto PEDOT:PSS operate at a lower bias and higher power efficiency than OLEDs with a hole transport layer. The quantum efficiency of single layer Alq₃ and rubrene-doped Alq₃ devices is equal to that of multi-layer devices, showing that EL is not quenched by PEDOT:PSS.

A novel approach to studying interfacial processes in dye-sensitized solar cells is presented. In order to reduce the complexities of heterogeneity at the heterojunction in such cells, charge transfer is investigated from single fluorescent molecules (alkyl-perylene bisimide) to a highly defined single-crystalline wide-bandgap semiconductor (GaN) using confocal fluorescence microscopy under ultrahigh vacuum conditions. We report detailed studies on the energy level alignment between the perylene bisimide and GaN, characterize the nature of the surfaces involved and demonstrate confocal fluorescence microscopy in an ultrahigh vacuum set-up. The results reported here indicate that the excited state in the chromophore lies at 0 ± 100 meV with respect to the bulk conduction band minimum of GaN.

Time-resolved fluorescence is a direct measure for excited states lifetimes, decay channels and corresponding rates. Hitherto, investigations on systems exhibiting fluorescence lifetimes below approximately 10 ps have been restricted to ensemble measurement. Ensemble measurements bear the disadvantage of averaging sample inhomogeneities and complex distributions. However, the latter problem can be circumvented by single-molecule experiments, without the restriction to special, typically simple systems that can be prepare with very high homogeneity. Time-resolved single-molecule microscopy is especially powerful as it allows one to probe the spatial, temporal and spectral inhomogeneities. At present, its most common implementation, the scanning confocal time correlated single photon counting (TCSPC), is limited to a time resolution of 20 ps. In the wide-field epifluorescence microscopy temporal resolution is achieved by the use of intensified CCD cameras, the fastest of which reach resolution of 80 ps. Here we present a Kerr-gated microscope setup capable of collecting diffraction limited 2D fluorescence images with approximately 100 fs time resolution. The concept is based on the insertion of an optical Kerr gate into a standard wide-field microscope. In addition to the considerably improved temporal resolution, the wide-field design will allow simultaneous tracking of several molecules or nanoparticles and ultrafast fluorescence lifetime imaging of doped and heterogeneous surfaces. Preliminary measurements to demonstrate the performance of the setup are presented.

Two dye sensitized solar cells (DSC) can be joined to form a tandem cell with two separate absorption ranges for the two different absorber materials. This can enhance the solar conversion efficiency and in particular the photovoltage of the DSC. Water splitting appears as a realistic long term target. The DSC tandem can be realized as n-n junction employing known dye molecules with optimal absorption spectra. Dye molecules with elongated shapes can be realized by covalently attaching a conducting bridge group terminated by an anchor group to a desired chromophore. Due to the long conducting bridge group separating the hole state of the dye from the surface of the semiconductor recombination is slowed down. The ordered molecular structure can be self-assembled on the recently introduced rod or cylinder shaped oxide electrodes but will not slow down recombination in the nm-cavities of the conventional TiO₂ Graetzel electrode.

Ultrafast heterogeneous electron transfer (HET) from the excited singlet state of the organic chromophore perylene into the inorganic semiconductor rutile TiO₂ was investigated with femtosecond time-resolved two-photon photoemission (2PPE). With 2PPE one can address adsorbates at coverages far below a monolayer on single crystal surfaces. With the same chromophore perylene fixed with different anchor and bridge groups at the surface of rutile TiO₂(110) the corresponding 2PPE transients revealed the relevant parameters that characterize the contributing processes. Instantaneous optical injection on one hand and slow injection over a long distance on the other hand were realized. Direct optical charge transfer was realized with the chromophore catechol that is known to form a charge transfer complex with Ti atoms on the surface of TiO₂. The slow injection cases were realized by inserting rigid molecular bridges. Comparison of the different 2PPE signals with corresponding transient absorption (TA) signals for the identical systems revealed the physical processes and time scales that control the 2PPE transients. On the surface of the single crystals only one long time constant was measured via 2PPE also in the case of a long rigid bridge/anchor group in contrast to a broad distribution of time constants observed for the same molecules anchored in the nm-size cavities of an anatase TiO₂ film measured via TA. The broad distribution of time constants in the latter measurements can be attributed to different microscopic environments giving rise to different distances between the chromophore and the nearest TiO₂ wall.

The transient absorption of nanocrystalline TiO₂ films in the visible-to-IR wavelength region was measured under UV excitation conditions at different wavelengths of 266 nm and 355 nm. Under weak 355 nm excitation, the generated charge carrier density could be reduced as low as the second-order electron-hole recombination process could be ignored as we reported previously (Y. Tamaki et al. Phys. Chem. Chem. Phys. 9, 1453-1460 (2007)). The result was compared with data obtained under 266 nm excitation, where the band-gap exaction was strong and efficient electron-hole recombination occurred due to the high charge carrier density. Taking into account the dynamics of the electrons and holes in the femtosecond to picosecond time range, such as ultrafast charge carrier trapping and slow deep trapping of electrons, intra-band relaxation in the conduction and the valence bands and intra-particle diffusion of electrons in the shallow trap levels were revealed.

Multilayer of SrTiO₃(STO)-YBa₂Cu₃O_7-δ(YBCO) was fabricated by laser molecular beam epitaxy (LMBE). The properties of multilayer in terms of growth modes, strain and interface structures were characterized by the in situ reflective high energy electron diffraction (RHEED) pattern, and ex situ measurements, such as atomic force microscope (AFM). By controlling growth and processing conditions, we observed a change of different growth modes of thin films. Furthermore, we also demonstrate a strong dependence of growth modes in YBCO films on the growth fashion of STO films, which could be explained in terms of the stress effect at the interface. The dependence of interface stress on thickness and growth condition was determined with AFM. These results provide an understanding and manipulating growth mechanism of the films.