The influence of surface hydration on the time-scales and mechanisms of interfacial electron transfer from rhodamine B into SnO₂ is investigated. We combine molecular dynamics simulations and quantum dynamics propagation of transient electronic excitations to analyze the regulatory role of water molecules affecting the adsorbate-semiconductor interactions and the underlying electronic couplings that determine the electron injection times. The reported results are essential to advance our understanding of interfacial electron transfer dynamics in dye sensitized semiconductor surfaces at the molecule level, including fundamental interactions that affect the efficiency of interfacial electronic processes in dye-sensitized solar cells as well as in a wide range of other technological applications.

The carbon nanotube photoexcitation spectrum is dominated by excitonic transitions, rather than interband transitions between continuum states. There are eight distinct excitonic transitions (four singlet and four triplet), each with two-fold degeneracy. Because the triplet excitons are spin polarized with electron and hole spins both pointing in the same direction, they are optically inactive, and optical spectroscopy has revealed no evidence for their existence. Here, we show that by the interaction with a spin filter ferromagnetic semiconductor, photoexcitation of the carbon nanotube triplet exciton is possible, and its contribution to the photocurrent can be detected. The perturbation provided by the spin filter allows for inter-system mixing between the singlet and triplet excitonic states, and relaxes the spin selection rules. This supplies the first evidence for the existence of the triplet exciton, and provides an avenue for the optical excitation of spin polarized carriers in carbon nanotubes.

An inorganic silsesquioxane and organic 4-vinyl biphenyl chromophore based dendrimer was synthesized and the steric hindrance of the dendrons was used as a trigger to control the photophysical properties in the near-UV and blue spectral ranges. Consistent photoluminescence quantum yields and time resolved fluorescence were measured in solution, confirming that molecular engineering of the dendrons together with confinement around the inorganic core allows the design of more efficient photoluminescent dendrimers. Low temperature photoluminescent studies were completed to demonstrate the stability of the dendrimer photophysical properties. A very general strategy is then presented which uses stable chemistry to control the emission spectral range by changing the chromophore, and gives control of photoluminescence efficiency by grafting side-groups onto the chromophores.

Photoemission spectra of self-assembled monolayers of para-phenylene-ethynylene thiols chemisorbed on gold have been measured. Three compounds were studied: 4,4'-bis(phenylethynyl)benzenethiol, 4-(phenylethynyl)benzenethiol, and benzenethiol. The monolayer spectra were interpreted with the aid of gas-phase photoemission spectra of 4,4'- bis(phenylethynyl)benzenethiol and benzenethiol. The work function of the monolayer-covered surface and alignment of the highest occupied π-state of the monolayer relative to the Fermi level of the substrate were determined from the monolayer spectra. The work function of the monolayer-covered surface decreased relative to that of the bare substrate by about an electronvolt. The shift is attributed to changes in the charge distribution at the interface associated with chemisorption. No statistically significant trend in the work function shift was observed with respect to oligomer length. This observation points to a near-constant interface dipole and the role of the gold-sulfur bond in determining energy-level alignment.

We present experimental results on changing the fluorescence spectrum of a single molecule by embedding it within a tunable optical microresonator with subwavelength spacing. The cavity length is reversibly changed across the entire visible range with nanometer precision by using a piezoelectric actuator. By varying its length, the local mode structure of the electromagnetic field is changed together with the radiative coupling of the emitting molecule to the field. Since mode structure and coupling are both frequency dependent, this leads to a renormalization of the emission spectrum of the molecule. Moreover, we use doughnut laser modes in the tunable microcavity to determine the longitudinal position of an isotropic emitter. By analyzing the excitation patterns resulting from the illumination of a single fluorescent bead in the focus of a radially polarized doughnut mode laser beam we can determine the longitudinal position of this bead in the microcavity with an accuracy of a few nanometers.

The demand for novel optoelectronic and photonic technologies has fueled an intense research effort to synthesize and characterize nanostructured semiconductor materials with unique properties that lend themselves to technological innovation. Zinc Oxide has emerged as an attractive candidate for a variety of applications, due in part to a large second order nonlinear susceptibility, its wide band-gap and large exciton binding energy. We have used time-resolved nonlinear two-photon emission and second harmonic generation microscopy to characterize the optical properties and excited state dynamics of individual rods. Ultrafast emission microscopy is used to follow the trapping dynamics of photoexcited charge carriers. Our results show a time-dependent red-shift in the trap emission band that is interpreted as arising from carrier percolation through trap states. In a second series of experiments, second harmonic generation (SHG) microscopy illustrates the connection between the optical mode structure of the object and its nonlinear mixing efficiency. Images show a periodic modulation in the SHG efficiency that is symmetrically situated relative to the rod midpoint. This phenomenon arises when the fundamental optical field couples into standing wave resonator modes of the structure and is a direct manifestation of the tapered shape of the rod.

The surface properties of Si(111) : H can be modeled by means of a Si slab with increasing number of layers. A slab is modeled here with a finite periodic potential for electrons, parameterized with information about atomic radii and electron binding energies. The model is then solved numerically to obtain electronic energy levels and the shape of layer orbitals. The procedure provides trends in confinement and optical absorption intensities. Results include electronic band gap excitation energies, and intensities of absorption as function of light frequency, α(ω), from calculated electric transition dipoles and density of states. Electronic orbitals obtained here and from previous ab initio calculations show patterns of periodicity due to confinement effects. These effects influence the optical properties of the surface when it is excited by visible light as described by means of absorption selection rules. Our results for the absorption coefficient are compared with experimental curves showing the same pattern of stepwise increases with increasing photon energies.

The adsorption of a submonolayer of catechol (C₆H₆O₂) on the rutile TiO₂(110)-1×1 surface has been investigated by Scanning Tunneling Microscopy (STM). The catechol molecules are preferentially adsorbed on the surface 5-fold coordinated Ti⁴⁺ sites, and occupy two neighboring lattice Ti sites. No preference for adsorption at surface step edges is observed at room temperature. A statistical analysis of intermolecular distances demonstrates that the interaction between the molecules strongly depends on the surface crystallographic direction: catechol molecules exhibit attractive interaction along [1-1 0], while they repel each other along the [001] direction. The attractive interaction is proposed to be caused by the coupling of π bonding electrons and the repulsive interaction is possibly mediated by substrate.

Interfacial electronic states and charge transfer play an important role in many organic devices, but it has been technically challenging to probe surface electronic states under ambient conditions or at buried interfaces. Developments in two-dimensional (2D) IR-visible sum frequency generation (SFG) spectroscopy have made it possible to study the optical and electronic properties of organic molecules located at a buried interface. We present studies of poly[2- methoxy, 5-ethyl (2'-hexyloxy) para-phenylenevinylene] (MEH-PPV) surfaces using 2D SFG. Surface SFG electronic spectra were obtained by scanning the frequencies of both incident visible and IR beams, and used to study the electronic transitions associated with the C-C stretching of benzene rings. Assuming an oligomer model and a Gaussian conjugation-length distribution, the average conjugation lengths were estimated to be 5.8 monomer units at the MEHPPV/ solid interface and 5.1 monomer units at the air/polymer interface.

Ultrafast visible pump - infrared probe spectroscopy is used to examine the dynamics of free carrier formation following photoinduced electron transfer in an organic photovoltaic polymer blend. The carbonyl (C=O) stretch of the functionalized fullerene, PCBM, is probed as a local vibrational reporter of the dynamics in a blend with a conjugated polymer, CN-MEH-PPV. It has been determined that PCBM molecules at the interfaces of PCBM and polymer phases possess higher frequency carbonyl vibrational modes while molecules in the centers of PCBM domains have lower frequency modes. The shift in frequency of the carbonyl stretching mode is used to directly resolve the dynamics of free carrier formation that occur on the few picosecond timescale. The fast dynamics suggest that the presence of an interfacial dipole causes the charge carriers to experience a smaller Coulombic potential than that which would be predicted from the dielectric properties of the materials. The free carrier formation dynamics are temperature independent indicating that excess vibrational energy remaining for a short period of time after the

Adsorption was considered as on the atomically-rough surface (near the step and step fracture) as on the smooth surface. The electron structure of AgCl nanocrystals with the adsorbed silver ion were calculated by semiempirical tight-binding method relying on a self-consistent approach for the effective charges and dipole moments of the ions and in frame of DFT method B3LYP/HW. The quantum transitions were investigated in semiempirical approach. Visualization of the wave functions was performed for the localized states. Basing on the obtained data a conclusion is specically made that one can expect an enhancement of photoelectron localization with a decrease of the anions number in the substrate nearest to the adsorbed ion. It means that the most ecient trapping of photoelectron should occur under adsorption on a smooth surface rather than near the steps and their fractures as it was assumed previously. Also probabilities of quantum transitions for AgCl : J nanocrystal with the adsorbed silver ion were been discussed. This work is a continuation of paper [1].

Sol-gel preparation of amorphous titanium oxide (TiO_x) thin films with distinct morphological properties on the hydrophobic substrate was obtained by solution spin coating method. The TiO_x thin films were deposited by three precursors using 2-methoxyethanol (2MOE), isopropanol (IPA) and mixture of 2MOE and hexane as solvents. We demonstrate evidence that the morphology of TiO_x thin film is strongly related to the employment of dissimilar solvent. Among these three solvents, TiO_x film obtained from 2MOE/hexane mixed solvent is a superior choice for the preparation of TiO_x thin film on the hydrophobic substrate because of its smooth surface morphology.