Many-body effects in single-walled carbon nanotubes (SWNTs) were investigated using multi-color transient absorption spectroscopy. A population of excitons in isolated SWNTs was created by resonantly exciting the lowest allowed excitonic transition. With low pump fluence, photo-induced bleaching was observed regardless of the probe energy, as expected. However, at high pump fluence, photo-induced absorption was observed when probing at a higher energy than the pump. These observations are consistent with a blue-shifted exciton absorption due to phase-space filling effects at high exciton densities. As a result of the rapid loss of excited state population through Auger recombination, the blue shift recovered with a time constant of less than 1 ps.

Molecular-resolved real-space images of self-assembled structures of the conductive polymer regioregular poly(3- hexylthiophene) (rrP3HT) on single-walled carbon nanotubes (SWNT) were obtained using scanning tunneling microscopy (STM). The STM images revealed that the adsorbed polymer typically formed a 10 nm thick coating on SWNT's. This is in agreement with transmission electron microscopy (TEM) results for drop-cast composite films that provided strong evidence that SWNTs were isolated in a polymer matrix and coated with rrP3HT multilayers. A 10 nm thick deposit corresponds to a coating of ~25 layers of polymer on SWNT, assuming that π-π interactions between rrP3HT layers determine deposition and that the underlying SWNT directs the polymer self-assembly process. STM measurements of adsorbed monolayers and multilayers of rrP3HT on SWNT surfaces were compared to rrP3HT monolayer and multilayer deposition on highly ordered pyrolytic graphite (HOPG) surfaces. The average inter-lamellar distances of adsorbed polymer was greater for both rrP3HT monolayer and multilayer films adsorbed onto the curved surfaces of SWNTs than on the flat surfaces of HOPG samples. Analysis of STM images yielded the interchain spacings of adsorbed macromolecules, d_cc = 1.55 - 1.68 ± 0.02 nm. The polymer was observed to wrap around some SWNTs at an angle with respect to the SWNT long-axis, which indicated that the rrP3HT self-assembly is hierarchical. The conductive polymer's deposition appears to occur with epitaxy and is directed by the underlying SWNT chiral structure.

Exciton dynamics in nanoparticles and nanotubes is important in application as well as from the fundamental point of view. The numbers of excitons in these systems are usually very small, and fluctuations in the number are comparable to the average number. In this situation the conventional deterministic approach that considers only the average density of excitons is not sufficient to describe the reaction kinetics. Exciton dynamics in nanoparticles and nanotubes is analyzed on the basis of stochastic models and compared with experiment.

Irradiating metal particles by an ultrafast laser pulse produces rapid heating of the lattice. This can lead to coherent excitation of the vibrational modes of the particle that correlate with the expansion co-ordinates. By comparing the measured periods to continuum mechanics calculations, these experiments can provide information about the elastic constants of the particle if the size and shape are known. In this paper recent results are presented for particles with cubic symmetry, specifically, nanocubes, nanoboxes (hollow cubes) and nanocages (nanoboxes with holes on the corners and/or facets of the box). The way the vibrational modes are assigned, and the information content of the experiments will be discussed, as well as the energy relaxation dynamics of the particles. Energy relaxation is important for the proposed use of the nanocages in phothermal therapy, where heat dissipation following laser excitation is used to selectively kill cells.

At the nanometer scale several physical principles govern the growth of nanoparticles through redox reactions in solution. These include the competition between diffusion and electron transfer for a redox agent at the surface of a very small particle, the dependence of electron transfer kinetics upon the size of the nanoparticle, and a coulomb charging of the nanoparticle which affects the kinetics of the reaction. Using the growth of a metal nanoparticle as an example, mathematical models describing these principles have been formulated, and rates of growth predicted as a function of particle size, electrochemical potential of the redox agent, and the rate constant for electron transfer. The growth rate is seen to be nonlinear with time.

Femtosecond polarized transient absorption results are obtained for InSe and GaSe nanoparticles. The results indicate that the transient absorption spectrum of large GaSe particles is dominated by a size-independent, z-polarized hole intraband transition. The small particle spectra exhibit the same z-polarized hole transition and a much more intense x,y-polarized absorption that is assigned to a charge transfer transition from the conduction band to particle surface (edge) states. The intensity of this transition depends on the momentum state (Γ or M) of the electron, and Γ to M electron momentum relaxation results in a 15 ps absorption decay. These results are used to interpret analogous results obtained for mixed GaSe-InSe nanoparticle aggregates, also in the solution phase. The static absorption spectrum of the mixed aggregates exhibits a strong interparticle charge transfer absorption band at an energy slightly higher than the InSe bandgap. Photoexcitation of this band results in a polarized transient absorption spectrum and transient absorption kinetics characteristic of InSe valence band holes and GaSe conduction band electrons. This result indicates that with small GaSe particles, direct InSe to GaSe electron transfer occurs upon photoexcitation.

This paper describes the challenges faced by the microelectronics community in growing ultra-thin films using Atomic Layer Deposition and summarizes how mechanistic information derived from in situ infrared absorption spectroscopy studies can guide the growth of sub-nanometer films. Examples are drawn from the growth of high-k dielectrics (e.g. HfO₂ ) on oxide-free silicon surfaces to achieve the lowest effective oxide thickness.

VUV-laser-induced oxidation of Si(111)-(1×1):H, Si(100):H, and a-Si:H at 157 nm (F₂ laser) in pure O₂ and pure H₂O atmospheres was studied between 30°C and 250°C. The oxidation process was monitored in real time by spectroscopic ellipsometry (NIR-UV) and FTIR spectroscopy. The ellipsometric measurements could be simulated with a three-layer model, providing detailed information on the variation of the suboxide interface with the nature of the silicon substrate surface. Besides the silicon-dioxide and suboxide layer, a dense, disordered, roughly monolayer thick silicon layer was included, as found previously by molecular dynamics calculations. The deviations from the classical Deal-Grove mechanism and the self-limited growth of the ultrathin dioxide layers (<6 nm) are described by different kinetic models for O₂ and H₂O. The tailored modification of silicon surfaces by functionalization with organic end groups was studied by silylation of oxidized silicon surfaces with terminating trimethylsilyl (TMS) groups and n-alkylthiol monolayers on gold-coated silicon. The C-H stretching vibrations of the methylene and methyl groups could be identified by FTIR spectroscopy and IR ellipsometry.

The structure of indium phosphide surfaces and interfaces has been investigated during the growth of nanomaterials by metalorganic chemical vapor deposition. In the phosphorus-rich environment of the MOCVD process, the InP (001) surface forms a (2x1) reconstruction that is terminated with a monolayer of phosphorus dimers. Each of these dimers is stabilized by the adsorption of hydrogen onto one of the dangling bonds. Upon switching from the growth of InP to InGaAs or InSb, the surface can change dramatically depending on the interactions among the group V elements. When InP is exposed to tertiarybutylarsine above 500 °C, arsenic atoms diffuse into the bulk, creating strained InAsP films with a roughness exceeding 1.0 nm. By contrast, when InP is exposed to trimethylantimony between 450 and 600 °C, a thin layer of InSb, ~6.8 Å thick, is deposited. This layer is strained in only one direction, and forms an interesting one-dimensional, ridge-shaped structure.

The reconstruction of P-rich InP(100) requires at least a (2x4) surface unit cell to stay semiconducting and uncharged (electron counting rule). Recently it has been shown that the much smaller (2x2) unit cell obtained from MOCVD (metalorganic vapor deposition) growth contains P-H bonds. Orientation and polarization dependent Fourier Transform Infrared Spectroscopy (FTIR) of the P-H bonds in the Attenuated Total Reflection (ATR) mode have confirmed the specific form of the (2x2) surface unit cell (T. Letzig et al., Phys. Rev. B 71 (2005) 033308) earlier proposed by W.G. Schmidt and coworkers (W.G. Schmidt et al., Phys. Rev. Lett. 90 (2003) 126101). Surface unit cells with a higher concentration of P-H bonds also obey the electron counting rule. A c(2x2) LEED image and two matching FTIR peaks were observed when the (2x2) reconstructed surface was exposed to atomic hydrogen. The corresponding c(2x2)-2P-3H surface unit cell can be shown to form a stable surface phase (T. Letzig et al., Phys. Rev. B, submitted). The complete transformation of the (2x2) surface to this new phase is not observed since the surface deteriorates when exposed to a higher dose of atomic hydrogen.

We have constructed a laser-based microspot photoemission spectrometer which achieved lateral resolution of 0.3 μm and energy resolution of 30 meV. The light source at wavelength of 140 nm (photon energy of 8.86 eV) was generated as the 6-th harmonics of a titanium sapphire laser of 100 fs pulse duration. The high-energy resolution is the characteristics of our apparatus. The space-charge effect which affects the energy resolution and limits the sensitivity was described. The apparatus was applied to reveal spatial inhomogeneity of copper phthalocyanine films caused by intermolecular interaction and inter-layer interaction. We found that the highest occupied molecular orbital is peaking at the binding energies of 1.13, 1.23, and 1.38 eV depending on the film thickness and the sample positions. The peaks were assigned to originate from isolated molecules, ordered monolayer, and the second layer, respectively. The apparatus can also be operated as microspot two-photon photoemission spectrometer which probes unoccupied electronic states. A surface image due to the unoccupied first image potential state of Cu(111) facets showed that the lateral resolution for two photon photoemission with a light of 280 nm wavelength was 0.4 μm, smaller by 1/√2 than the diffraction limited spot size.

Ultrafast heterogeneous electron transfer (HET) between a molecule attached to a semiconductor nanocluster and the band states of the cluster is discussed theoretically with emphasis on the perylene TiO₂ system. The whole approach has been formulated in such a way to be ready to describe different optical excitation and detection processes. Therefore, a model is introduced which accounts for the specialty of the molecule i.e. its particular electronic level scheme together with its vibrational degrees of freedom as well as the band continuum of the semiconductor. Calculations of the steady state linear absorption spectra are used to adjust the parameters of perylene attached to nano-structured TiO₂ via different bridge-anchor groups. These data are used to compute the temporal evolution of the energetic distribution of the injected electron. Finally, it is demonstrated that a two-photon photon emission spectrum carries signatures of the molecular vibrations.

Quantum chemical calculations providing detailed information of dye-sensitized semiconductor nanocrystals are presented. The calculations are used to elucidate both structural and electronic properties of photoelectrochemical devices, such as environmentally friendly Dye-Sensitized Solar Cells (DSSCs), at the molecular level. Quantum chemical calculations have recently been performed on both organic and organometallic dye molecules attached to titanium dioxide (TiO₂) nanocrystals via different anchor and spacer groups. Strategies to make accurate quantum chemical calculations, e.g. at the DFT level of theory, on increasingly realistic models of such dye-sensitized semiconductor interfaces are presented. The ability of different anchor and spacer groups to act as mediators of ultrafast photo-induced electron injection from the dye molecules into the semiconductor nanocrystals is, in particular, discussed in terms of calculated electronic coupling strengths, and direct comparisons with experimental information are made whenever possible. Progress in the development of multi-scale simulation techniques using so called reactive force fields is illustrated for dye-sensitized solar cell systems.

In this work, we investigate how and to which extent a quantum system can be driven along a prescribed path in space by a suitably tailored laser pulse. The laser field is calculated with the help of quantum optimal control theory employing a time-dependent formulation for the control target. Within a two-dimensional (2D) model system we have successfully optimized laser fields for two distinct charge transfer processes. The resulting laser fields can be understood as a complicated interplay of different excitation and de-excitation processes in the quantum system.

Force field parameters for large scale computational modeling of sensitized TiO₂-anatase surfaces are developed from ab initio molecular dynamics simulations and geometry optimization based on Density Functional Theory (DFT). The resulting force field, composed of Coulomb, van der Waals and harmonic interactions, reproduces the ab initio structures and the phonon spectra density profiles of TiO₂-anatase nanostructures functionalized with catechol, a prototype of an aromatic linker commonly used to sensitize TiO₂ nanoparticles with Ru(II)-polypyridyl dyes. In addition, simulations of interfacial electron injection and electron-hole relaxation dynamics demonstrate the capabilities of the resulting molecular mechanics force-field, as applied in conjunction with mixed quantum-classical methods, for modeling quantum processes that are critical for the overall efficiency of sensitized-TiO₂ solar cells.

Hot electron injection from the excited electronic singlet state of perylene chromophores into the (110) surface of rutile TiO₂ single crystals was measured with femtosecond two-photon photoemission (2PPE) for different anchor/bridge groups attached to the perylene chromophore. Femtosecond 2PPE probes the time and energy dependence of the population of firstly the excited state of the chromophore and secondly of the hot electrons injected into the surface layer of the semiconductor. Measuring both these contributions gives a complete picture of the ultrafast photo-induced injection process and bridges the gap to conventional measurements of the rise time of the corresponding photocurrent. Studying the system in ultra-high-vacuum (UHV) makes all the tools of surface science available. Impurities on the surface were studied with XPS, the alignment of the occupied and unoccupied electronic levels at the interface with UPS and with 2PPE, respectively. The orientation of the elongated chromophores with respect to the crystal surface was deduced from angle and polarization dependent 2PPE signals making use of the known orientation of the dipole moment for the optical transition, the energy distribution of the injected hot electrons was determined with 2PPE from the energy distribution of the photoemitted electrons, and finally the escape of the injected electrons from the surface to bulk states of the semiconductor was obtained from femtosecond 2PPE transients.

The effect of electronic and nuclear factors on the dynamics of dye-to-semiconductor electron transfer was studied employing Ru^II(terpy)(NCS)₃ sensitizers grafted onto transparent films made of titanium dioxide nanoparticles. Various approaches were strived to understand the dependence of the kinetics of charge injection and recombination processes upon the distance separating the dye molecules and the redox active surface. A series of bridged sensitizers containing p-phenylene spacers of various lengths and phosphonic anchoring groups were adsorbed onto TiO₂ films. The kinetics of interfacial charge transfer was recorded by use of time-resolved spectroscopy in the fs-ps domain. The electron injection process was found to be biphasic with a clear exponential distance dependence of the fast kinetic component. The slower part of the kinetics was essentially unaffected by the length of the spacer bridge and was attributed to sensitizer molecules that are weakly bound to the surface with no direct contact of the anchoring group with the semiconductor. In a second approach, the kinetics of both forward- and back-electron transfer across a layer of insulating Al₂O₃ deposited onto TiO₂ nanocrystalline particles was investigated. Efficient charge injection was observed over distances up to 3 nm.

At metal-oxide/protic-solvent interfaces, partially hydrated or "wet electron" states represent the lowest energy pathway for electron transfer. Here we study the photoinduced charge transfer at the H₂O/TiO₂(110) interface by means of timeresolved two-photon photoemission spectroscopy and electronic structure theory. At ~1 monolayer coverage of H₂O on partially hydroxylated TiO₂ surfaces we find an unoccupied electronic state 2.4±0.1 eV above the Fermi level. Density functional theory shows this to be a two-dimensional "wet electron" state, which is distinct from hydrated electrons observed on water-covered metal surfaces. The decay of electrons from the wet electron state by the resonant charge transfer to the conduction band of TiO₂ occurs in ≤15 femtoseconds. Similar unoccupied electronic structure is observed for CH₃OH covered TiO₂(110) surfaces; however, the electron dynamics are considerably more complex. The wet electron state dynamics of CH₃OH/TiO₂ exhibit both energy and population decay. The excited state lifetime is strongly coverage dependent increasing to >100 fs range above 1 ML CH₃OH coverage. Significantly, a pronounced deuterium isotope effect (CH₃OD) indicates a strong correlation between the interfacial electron transfer and the motion of protons in the molecular overlayer.

Pump-probe experiments show that electron injection from a dye into mesoporous TiO₂ is as fast as 1×10¹³ s^-1. However, the same materials exhibit residual dye emission with lifetimes in the long nanosecond range. This inhomogeneity of e- injection rates was addressed in fluorescence lifetime microscopy experiments. The residual emission of continuous films of TiO₂ was compared with that of individual anatase nanoparticles that had undergone extensive dialysis. The films produce intense emission with multiexponential decay. The mesoporous film contains physisorbed and trapped dye, which is the dominant source of the emission. The distribution of emission lifetimes may reflect the mean free paths experienced by the dye molecules diffusing within the porous TiO₂. The intensity of emission from individual nanoparticles from which the loose dye was removed is orders of magnitude lower. The lifetimes are much shorter, with the primary components on subnanosecond time scale. The presence of residual emission with a ~200 ps lifetime shows that even on dialyzed nanoparticles a fraction of dye does not inject electrons with the same rate as observed in ultrafast pump-probe experiments. It is likely that the residual emission originates from the dye bound to defects.

A transient molecular probe for characterization of the surface properties of TiO₂ nanoparticles in colloidal solution has been developed recently in our laboratory. The probe molecule is all-trans-retinoic acid (ATRA) adsorbed on the TiO2 nanoparticle surface. After photoexcitation, the photoinduced interfacial charge recombination would generate ATRA triplet state (ATRA^T) with a substantial quantum yield. While the quantum yield of triplet ATRA generated in the solution phase is substantially low, which renders the interfacial-charge-recombination generated triplet ATRA being a transient probe molecule specific only to the interface. It is found that the triplet-triplet absorption spectrum of ATRA adsorbed molecule is sensitive to its binding form with the surface Ti atom through the carboxylic group, as well as to the polarity of the medium. Especially the apparent lifetime of ATRA^T at the TiO₂ surface changes substantially when the local polarity around the TiO₂ nanoparticle changes. We found that the ATRAT monolayer adsorbed at the TiO₂ surface can be used as a transient molecular probe for the surface binding forms, coordination state of the surface Ti atoms and the light-induced wettability change of the TiO₂ nanoparticle. TiO₂ nanoparticle, all-trans-retinoic acid, molecular probe, interfacial charge recombination, surface binding form, light-induced wettability change.