The electronic structure of SWNTs was investigated using the complementary techniques of single molecule photoluminescence spectroscopy and ultrafast optical spectroscopy. We found that photoexcited electrons in SWNTs isolated in surfactant micelles decay through many channels exhibiting a range of decay times (~200 fs to ~ 120 ps). The magnitude of the longest-lived component in the ultrafast signal specifically depends on resonant excitation, thus suggesting that this lifetime corresponds to the band-edge relaxation time. Fluorescence spectra from single SWNTs are well described by a single, Lorentzian lineshape. However, nanotubes with identical structure fluoresce over a distribution of peak positions and line widths not observed in ensemble studies, caused by localized defects and electrostatic perturbations. Unlike for most other single molecules, for SWNTs the photoluminescence unexpectedly does not show any intensity or spectral fluctuations at 300K. This lack of photoluminescence intensity blinking or bleaching demonstrates that SWNTs have the potential to provide a stable, single molecule infrared photon source, allowing for the exciting possibility of single nanotube integrated photonic devices and biophotonic sensors.

Electronic and mechanical studies of metal-molecules-metal junction have been accomplished to evaluate how bias-induced adhesion forces influence the charge transport efficiency of these junctions. The conducting probe atomic force microscope (CP-AFM) measures the current through an organic film sandwiched between two metal electrodes as a function of the tip-sample separation simultaneously with detection of the force between the probe and the surface. By applying a voltage between the sample and the tip, an attractive electrostatic capacitance force is added to the adhesion force. This paper describes probe-sample capacitance forces in conducting probe (CP) microscopy of polythiophene and alkanethiol monolayers, both theoretically and experimentally. The importance of taking into account an offset in the interaction force determined by the bias-induced adhesion force in the electronic measurements is demonstrated using current-voltage (I-V) characteristics of the polythiophene monolayer and the dependence of the adhesion force as a function of applied tip bias.

The extinction spectra of silver nanoparticle arrays are studied using the couple dipole (CD) method, with emphasis on determined the array pattern and particle spacing which produces the narrowest plasmon resonances. All calculations refer to one and two dimensional arrays of spherical particles having radii 50 nm (or of nonspherical particles with the equivalent effective volume), and only particle spacings much larger than the particle radius are considered so that the dipole approximation is accurate. The narrowest lines in all cases occur when the incident wave vector is perpendicular to the plane of the array while the polarization vector is in the plane and along a symmetry axis which depends on the array structure. We find that the narrowest plasmon bands for square and hexagonal arrays have about the same width (about 100 meV), but that the array spacing for the square array where this occurs is smaller than that for the hexagonal array. The comparison at constant array density is closer. Much smaller widths (20 meV) occur for one dimensional arrays than for two dimensional arrays. For rectangular lattices, we find that the array spacings perpendicular to the polarization vector play a much more important role in determining the plasmon wavelength and width than do spacings parallel to the polarization vector. The evolution of spectra from a two dimensional array to a one dimensional chain is studied by considering rectangular arrays in which one spacing is very large. We find that when the large spacing is 5000 nm or more, the interactions between rows of particles is weak and extinction spectrum has a narrow peak that matches what is seen for the equivalent one dimensional chain.

The relaxation dynamics of Cu₂O and Cu_1.8S quantum dots (QDs) are compared via time-resolved femtosecond pump probe experiments. It is found that Cu₂O shows extremely long-lived excited states on the microsecond time scale and Cu_1.8S exhibits much shorter lifetimes in the picosecond time regime. While copper sulfide systems are described in the literature as p-type direct band gap materials, the Cu₂O system is direct band gap, however it has a forbidden lowest-energy state. These differences are expressed in the different lifetimes displayed in the time-resolved femtosecond and nanosecond measurements. Moreover, it is confirmed by photoluminescence spectroscopy that reveals that only the Cu_1.8S QDs show efficient PL and the Cu₂O QDs do not luminescence. In all of the systems, carrier trapping is probably the lifetime limiting process for the conduction band edge depopulation.

Organic monolayer-coated single-crystal germanium (Ge) nanowires ranging from 8 to 80 nm in diameter were synthesized by the gold nanocrystal-seeded supercritical-fluid-liquid-solid (SFLS) approach and studied by energy loss spectroscopy (ELS) on a scanning transmission electron microscope (STEM). Energy losses from a ~0.5 nm diameter electron probe due to scattering from volume and surface plasmons vary with respect to the probe position relative to the nanowire surface, and the volume plasmon energy increases with decreasing diameter for nanowires narrower than 24 nm. Below 24 nm, the organic monolayer-coated nanowires also exhibit size-dependent Ge 3d core ionization spectra that shift to higher energy with reduced diameter that are independent of probe position relative to the surface.

Ultrafast excitation of metal particles in solution coherently excites the phonon modes that correlate with the expansion coordinate of the particle. The period of the modulations yields information about the average size of the particles if their elastic constants and shape are known, or information about the elastic constants if the average size and shape is known. In this paper we describe recent experiments where we have used time-resolved spectroscopy to examine the elastic constants of: (i) gold nanorods with aspect ratios between 2 and 5; and (ii) spherical gold particles in aqueous solution at very high pump excitation levels. The first set of experiments shows that the elastic moduli (Young's modulus, and the bulk and shear modulus) of gold nanorods are significantly smaller than those of bulk gold. This is attributed to the structure of the nanorods, specifically, that they grown with a five-fold twinned structure. In the second set of experiments essentially the change in the elastic moduli with laser intensity is used to estimate the temperature of the particles. The results show that the particles can reach very high lattice temperatures (approaching the melting point of the metal). Examination of the transient absorption data suggests that the hot particles produce explosive boiling of the solvent in these experiments.

Interfacial electron transfer dynamics of dye sensitised metal oxide films have been widely studied by transient optical techniques. In this paper, we extend such studies to complete dye sensitised solar cells, and show how such transient optical studies can be correlated both with transient photovoltage studies and current / voltage analyses of device photovoltaic performance.

The adsorption and ultraviolet photolysis at 300 K of trimethylacetate, (CH₃)₃CCOO-, on TiO₂(110) has been explored by combining scanning tunneling microscopy, isothermal reaction mass spectrometry and electron energy loss spectroscopy. The photolysis, initiated by forming excited electrons and holes in the oxide, is dominated by ejection of CO₂ and C₄H₈ with no evidence for retention of carbon-containing species. The chemistry is the result of holes extracting electron density from the pi-orbital of a carboxyl group leading to decarboxylation. The accompanying electron is trapped as Ti³⁺ at the surface. These trapped species compete for holes and inhibit the hole-mediated decarboxylation. The trapped electrons can be removed by exposure to O₂ at 300 K leading to a transient acceleration of the reaction rate after resuming photolysis. Under aerobic conditions, arriving O₂ scavenges trapped electrons as they are formed reducing the degree of quenching and increasing the rate of trimethylacetate consumption without changing the products.

We study the processes of charge transfer and recombination at the interface between semiconductor nanoparticles and conjugated polymers. These processes are crucial in determining the performance of photovoltaic devices based on these materials. Using femtosecond transient absorption we are able to follow the charge separation on picosecond timescales in blends of spherical CdSe nanocrystals with a poly(p-phenylenevinylene) derivative. Charge separation occurs on timescales of greater than 15 ps, indicating that it is limited by the diffusion of excitons to the nanoparticle interface. We also use time-resolved photoluminescence and quasi-steady-state photoinduced absorption measurements to study the vertical structure in films containing conjugated polymers and semiconductor tetrapods. Finally, we demonstrate that use of slow-evaporating solvents allows the formation of fibrilar structures in poly(3-hexylthiophene) films, and that this is correlated with improved performance in photovoltaic devices containing poly(3-hexylthiophene) and CdSe nanorods.

This proceeding is a summary of our progress in both fundamental studies of surface enhanced Raman scattering (SERS) active surfaces and the design and characterization of nanostructured SERS-active surfaces. Based on the prior demonstration of single molecule SERS (smSERS)-like behavior from vapor deposited thin silver films, we've focused on these substrates as model systems for fundamental studies of the "blinking" phenomenon. Preliminary studies suggest that Stokes-shifted emission "blinking" is more directly associated with metal nanofeatures and less dependent on the nature of the adsorbate. It is anticipated that the insight provided by these fundamental studies will eventually lead to the rational design of nanostructured surfaces capable of smSERS. Toward that goal, preliminary characterization of the optical properties of nanoaperture arrays in silver suggests that these surfaces may exhibit SERS enhancement greater than that of the overlaying thin silver film.

We have proposed using single molecule fluorescence resonant energy transfer (SM-FRET) to investigate the induction of secondary structure in model, surface-active peptides upon binding at an interface. The ability for SM-FRET to distinguish structural heterogeneity will offer a distinct advantage over traditional biophysical methods in these types of studies. Ensemble methods mask heterogeneity and only provide an average measure of secondary structural features. Because secondary structure contributes greatly to the energetics of dehydrating the amide backbone, detailed information of conformational distributions is crucial to the understanding of the thermodynamic cycle involved. Here we present results from our first efforts at using SM-FRET to study an amphipathic α-helix forming peptide immobilized at the solid-liquid interface between an aqueous solution and an octadecylsilane modified glass surface. This system serves as a model for future studies of peptide partitioning to lipid bilayers and other relevant interfaces.

Noble metal thin films have been used for surface enhanced Raman scattering (SERS) nearly since its inception, but only recently has single molecule detection (indicated by blinking of the Raman signal) been demonstrated on these types of films. It has been widely accepted that thin metal films provide an average enhancement of the Raman signal of only 106. However, with the combination of the use of high magnification objectives and sensitive detection a new view of thin metal films as a SERS substrate is emerging. Bolstered by these results, our lab has endeavored to further study the optical properties of vapor deposited Ag films. A Stokes-shifted blinking optical response has been observed in our lab in the absence of any specific adsorbate on a silver thin metal film surface. The origin of blinking behavior on Ag this films in the presence and absence of adsorbate was investigated under various environmental conditions. It is anticipated that this system will help elucidate the mechanistic relationship between blinking and in SERS.

The solution phase synthesis of narrow diameter (< 10 nm) CdSe NWs is described. Crystalline NWs with lengths between 1-10 mm are obtained using a seeded solution approach, whereby NW growth is catalyzed by Au/Bi core/shell NPs. A gold biphasic reduction step results in 1.5 (3) nm diameter Au NPs and is followed by the thermolysis of trialkylbismuthines to yield low melting, bimetallic particles with diameters less than 3 nm. These Au/Bi NPs are catalytically active towards the growth of similar diameter CdSe NWs (~7 nm) that exhibit unique quantum confinement effects since the bulk exciton Bohr radius of CdSe is 5.6 nm. Manipulating the Cd:Se ratio results in a transition from straight to branched NWs, yielding v-shapes, tripods, y-shapes, as well as higher order structures. Structural characterization shows NW growth along either the [111] or [0001] directions of zinc blende (ZB) and wurtzite (W) phases respectively for both straight and branched NWs. High resolution TEM imaging reveals that the NWs alternate between ZB and W along their length. A similar reaction scheme can be used to produce PbSe NWs with diameters less than 5 nm, demonstrating the generality of the technique.

Buffer Layer Assisted Laser Patterning (BLALP) method is presented, for patterning metallic layers on surfaces, using laser desorption of a physisorbed buffer layer, e.g. Xe, CO₂ or H₂O. This technique is based on the utilization of a low power laser pulse used as the photolithographic printer of a metallic thin film. Using a weakly bound buffer material as the template for laser patterning, led to the development of two complementary procedures, 'positive' and 'negative' BLALP. It is discussed as a potential alternative for standard photo-lithography, promising a cleaner, more cost effective, better resolution and more environmentally friendly procedure.

We investigated the engineered bioconjugate of cadmium selenide core/zinc sulfide shell, (CdSe)ZnS, quantum dots (QDs) with genetically modified proteins using near-field scanning optical microscopy (NSOM). A genetically engineered protein polymer was expressed and purified from E. coli. The protein polymer was allowed to self-assemble to the bacterial microcrystalline cellulose surface through the cellulosic binding domain. QDs were then conjugated to the protein/cellulose assembly through interaction with the 6x-histidine tag on the protein. The transmitted near-field optical signals are collected and detected by both a PMT (near-field scanning optical microscopy, NSOM) and a spectrometer (near-field scanning optical spectroscopy, NSOS). Results from the sample containing the QDs/protein/cellulose assemblies suggest that QDs were arrayed along the cellulose surface. The near-field spectroscopic study also showed that the slight change of spectroscopic properties of the QDs upon bioconjugation, indicating the strong interaction between the constructed protein and QDs.

The adsorption of organics from the gas phase to solid and liquid surfaces is of great interest in the environment. In this study, the surface vibrational spectra of piperidine at the air/liquid interface, air/alumina (a-Al2O3, corundum) (0001) interface and air/a-alumina powder interface were obtained by using vibrational broad bandwidth sum frequency generation (SFG) spectroscopy. The interfacial vibrational signatures in the C-H stretching region of piperidine at the air/liquid and air/solid interfaces are shown to be sensitive spectroscopic-probes revealing the dominating interfacial species. Results indicated that the interfacial species at the air/water interface is mainly hydrogen-bonded piperidine, the interfacial species at the air/alumina (0001) interface is mainly protonated piperidine (piperidium) and the interfacial species at the air/alumina powder interface is mainly liquid-like piperidine.

Two-photon photemission (2PPE) spectroscopy was employed to investigate the unoccupied electronic states at surfaces of Cu(111) dosed with 0₂ at 400 K and then exposed to styrene at 90 K. Without styrene, the spectrum after 100 L 0₂ exhibits an occupied Cu-derived surface state and an unoccupied state at 2.8 eV above the Fermi level. Consistent with polarization results, we attribute the latter to strong hybridization and covalent bonding between the 2p states of oxygen atoms located in three-fold hollow sites and the d_z2 states of the Cu atoms in the second layer. As styrene is added, the O-induced unoccupied state disappears and a new unoccupied state appears 3.3 eV above the Fermi level. For styrene on clean Cu(111), a different state appears at 3.5 eV above the Fermi level and is attributed to antibonding orbitals formed by hybridization of copper and styrene orbitals. Thermal desorption provides evidence that the chemisorbed oxygen and styrene react. For a 1000 L 0₂ exposure, the occupied Cu-based surface state vanishes, and there is a broad unoccupied state located at 2.8 eV above the Fermi level. These results are consistent with a surface structure that is a precursor to Cu₂O. As styrene is added, no new features appear in 2PPE and there is no evidence for chemical reaction in thermal desorption.`

Aromatic molecules, such as pentacene, sexiphenyl and naphtho[2,3-a]pyrene, adsorbed on Au(111) surfaces are studied with x-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), temperature programmed desorption (TPD) and scanning tunneling microscopy (STM). All the molecules developed a strong interface dipole indicative of charge transfer from the molecule to the substrate. At monolayer coverages or less a variety of two-dimensional ordered structures were discovered. Naphtho[2,3-a]pyrene, a 2D chrial adsorbate, forms chiral domains rater than a 2D racemic unit cell.

Applicability of Atomic Force Microscopy (AFM) for structural characterization of nanocrystal superlattices is demonstrated on high-resolution imaging of superlattices formed by thiol stabilized gold nanoparticles on carbon coated and hydrophobic supports. Thin (<1nm) uniform coating of the samples with metal film before imaging was found to eliminate the undesirable effects of tip-sample interaction. Size and interparticle spacing are in excellent agreement with transmission electron microscopy results. AFM can be used as a complementary technique for nanocrystal superlattice structural characterization providing possibilities for crystal growth investigation on a variety of supports of practical interest and high resolution of the surface structure of superlattice structures.

Scanning tunneling microscopy was used to characterize the structure of partial monolayers of C60 and C70 on the Au(111) surface. Both 2√3 × 2√3 R30° and 7 × 7 lattice symmetries were observed for C60 monolayers, in accordance with previous results. For C70 monolayers, structures are observed with rotation angles of 0°, 30°, and 14° with respect to the underlying substrate; we propose a previously unreported √13 × √13 R13.9° lattice structure to explain this last observation. Time sequences of STM images show that while fullerene monolayers are largely stable, molecular motion can be observed on the timescale of minutes or hours. For C60, thermal diffusion is the predominant cause of this motion, and STM perturbation of the sample is negligible. In contrast, C70 is observed to diffuse far more slowly; under normal scanning conditions, tip-induced motion is the major effect.

The structures of the thin films of pentacene (C₂₂H₁₄) grown by organic molecular beam deposition on Au(111) have been investigated by scanning tunneling microscopy (STM). In the coverage range of 1-4 monolayers, pentacene molecules arrange into striped structure where each row consists of pentacene molecules aligned side-by-side. Based on the two dimensional (2D) unit cell as well as step height of the molecular island, we conclude that pentacene molecules form close-packed layers with their long molecular axis parallel to the surface. Although there is a possibility for the molecules to be tilted around their short molecular axis in this form, there is little difference between monolayer and multiplayer structure. Within the molecular layer and between adjacent layers, there is a broad distribution of relative angles in different domaims. The relative interaction strength between pentacene molecules in the same layer and in adjacent layers is discussed.

Networked films comprised of Au-nanoparticles and organic dithiols were deposited onto silicon or glass substrates via repetitive self-assembly from solution. The substrates were equipped with interdigitated electrode structures for electrically addressing the films. Dodecylamine stabilized Au-nanoparticles with a core diameter of 4 (+/-0.8) nm were used for film assembly. The metal cores were networked through 1,n-alkylenedithiols with chain lengths varying from 6 to 20 methylene units. With increasing number of methylene units, the conductivity of the films decreased by several orders of magnitude without following a monoexponential decay. When dosing the films with organic vapors (toluene, 1-propanol, 4-methyl-2-pentanone) their resistance increased reversibly. The amplitudes of this response increased strongly with increasing length of the linker molecules. In contrast, the sensitivity to water vapor was marginal for all alkylenedithiol-linked films. However, insertion of polar amide groups into the backbone of the linker decreased the sensitivity to toluene and significantly enhanced the sensitivity to water. The deposition of sensor films on chip from organic solvents could be directed by using a patterned CaO mask. After film deposition, the lift-off of nanoparticles from protected parts of the substrate was achieved by dissolving the mask in aqueous solution at ambient temperature.

Sodium sulfide used to produce gold nanoparticle aggregates has been shown to require aging, however, until this work, few studies have attempted to ascertain the nature of this aging effect. UV-vis spectroscopy and other experimental evidence suggest that chemical changes take place during the aging process. NEXAFS has helped determine that sodium sulfide is oxidized over time to form, at least partially, sodium thiosulfate. Experiments performed with sodium thiosulfate yield similar results spectroscopically, however, in SERS experiments it is apparent that the surface chemistry is substantially different. This is likely due to both a lack of sodium sulfide and an increase in thiosulfate concentration. It has also been found that thiosulfate can be utilized for the reduction of several other metal salts into metal and, in some cases, metal sulfide nanoparticles, including copper, platinum, palladium, and silver.

Photoluminescence (PL) enhancements of CdTe nanoparticles (NPs) and nanowires (NWs) are observed with the bioconjugation of metal NPs using d-biotin and streptavidin in solution. In the presence of metallic nanoparticles, strong enhancement of excitonic light emission of nanoparticles is observed. The enhancement effect is explained in terms of plasmon-assisted absorption of incident light and plasmon-induced increase of nanoparticles dipole moments. This supermolecules and bioconjugates system can be useful to build further efficient photonic devices and biological sensors.

We report on the wet-chemical synthesis of ZnO nanoparticles and their functionalization with metal colloids by photocatalytic reduction of metal ions. Different morphologies of ZnO nanoparticles were prepared by using different precursor concentrations and zinc sources such as zinc acetate, zinc propanoate and zinc decanoate. Spherical ZnO nanoparticles were produced at low concentrations and with zinc precursors having long alkylchains. The formation of elongated particles was achieved by using zinc acetate and high precursor concentrations. We found that ZnO nanorods were grown via oriented attachment of pre-formed quasi-spherical particles. This growth mechanism occurs at almost ambient temperature and in the first step, pearl chain like structures of 5 nm particles are formed, which coarse by condensation and finally grow - assisted by Ostwald ripening - to almost perfect single crystalline rods with length up to 300 nm. These nanorods were metallizied with silver and platinum by photocatalytic reduction of the appropriate metal ions on pre-formed ZnO nanorods. The deposition of metal took place at different locations of the ZnO nanorods and depended on the metal source. Positively charged silver ions were preferentially reduced to silver colloids at one end of the ZnO nanorods and led to anisotropic functionalized nanoparticles. Using a negatively charged platinum complex instead of silver ions generated a statistical coverage of the ZnO nanorods.

Successful preparation of the Ce-Zr-Al oxide thin films and glassy products by a newly developed organic-free modification of the sol-gel technique is reported. The structural composition and some properties of the samples obtained were investigated by TEM, XRD, FTIR, ESR, UV-Vis, PL and XPS. The optical investigation of the obtained films together with ESR data indicate the appearance of the bulk Ce³⁺-defects (g⊥ = 1.962-1.967, g// = 1.938-1.940, assigned to 4f¹ state, with concentration ~2•1018spin/g). The significant PL intensity rising at elevated temperature was related to spontaneous increasing of Ce³⁺ concentration in sol-gel samples under thermal dehydration. Also, an unexpected formation of intra-band gap states during thermal treatment of xerogels was manifested in UV-Vis spectra. This intra-band-gap states was attributed to the oxygen related defects that contribute to PL signal.

This manuscript is a summary of our progress toward the development of nanoaperture arrays as surface enhanced Raman scattering (SERS) active surfaces. Nanopatterned substrates have been fabricated using electron beam lithography. The substrates were metallized by thermal vapor deposition of silver. The resulting silver films exhibited interesting optical transmission and preliminary results with respect to SERS are encouraging.