S. J. Sibener

Bound level resonances in rotationally inelastic HD/Pt (111) surface scattering

The rotationally inelastic scattering of HD(J = 0→, 1, 2, 3) from a clean Pt(111) surface is reported. Discrete peaks for each final J state are resolved in the in‐plane angular distributions, with the inelastic peaks typically being large compared to the elastic peak. Sharp modulations of these inelastic transition probabilities were found as the incident angle was varied, suggesting bound level resonances. These resonances are inconsistent with G‐vector mediated (elastic) selective adsorption, but are consistent with rotationally inelastic selective adsorption into the bound levels of the physisorption potential. Observation of bound level resonances can be used to determine molecule–surface potentials. However, unlike elastic selective adsorption, highly probable rotationally inelastic HD selective adsorption is applicable to metallic surfaces of low corrugation.

Improved Hybrid Solar Cells via in situ UV Polymerization

One approach for making inexpensive inorganic-organic hybrid photovoltaic (PV) cells is to fill highly ordered TiO(2) nanotube (NT) arrays with solid organic hole conductors such as conjugated polymers. Here, a new in situ UV polymerization method for growing polythiophene (UV-PT) inside TiO(2) NTs is presented and compared to the conventional approach of infiltrating NTs with pre-synthesized polymer. A nanotubular TiO(2) substrate is immersed in a 2,5-diiodothiophene (DIT) monomer precursor solution and then irradiated with UV light. The selective UV photodissociation of the C--I bond produces monomer radicals with intact pi-ring structure that further produce longer oligothiophene/PT molecules. Complete photoluminescence quenching upon UV irradiation suggests coupling between radicals created from DIT and at the TiO(2) surface via a charge transfer complex. Coupling with the TiO(2) surface improves UV-PT crystallinity and pi-pi stacking; flat photocurrent values show that charge recombination during hole transport through the polymer is negligible. A non-ideal, backside-illuminated setup under illumination of 620-nm light yields a photocurrent density of approximately 5 microA cm(2)-surprisingly much stronger than with comparable devices fabricated with polymer synthesized ex situ. Since in this backside architecture setup we illuminate the cell through the Ag top electrode, there is a possibility for Ag plasmon-enhanced solar energy conversion. By using this simple in situ UV polymerization method that couples the conjugated polymer to the TiO(2) surface, the absorption of sunlight can be improved and the charge carrier mobility of the photoactive layer can be enhanced.

HD scattering from Pt(111): Rotational excitation probabilities

We measured the rotationally inelastic diffractive scattering probabilities for an HD beam colliding with a smooth Pt(111) surface. These large T yield R inelastic probabilities were measured as a function of incidence angle for a 110 meV beam energy. The results are in excellent agreement with a simple physical model of an eccentrically weighted sphere colliding with a hard wall with an attractive well depth of 55 + or - 10 meV. The numerical GR method of Garcia and co-workers was found superior to an eikonal method in solving this rotational quantum boundary value problem.

Defect evolution in ultrathin films of polystyrene-block-polymethylmethacrylate diblock copolymers observed by atomic force microscopy

We track individual defects in the microdomain patterns of cylinder-forming polystyrene-block-polymethylmethacrylate films with atomic force microscopy to elucidate the evolution of diblock domain topology during annealing. This evolution takes place through relinking, joining, clustering, and annihilation of defects. Such processes form the basis for predicting structural change in polymer films.

Ultraviolet laser photodesorption of NO from condensed films: Translational and internal energy distributions

In this paper we report that ultraviolet laser induced desorption from the surface of a thin nitric oxide film proceeds via two mechanisms which are present simultaneously. One mechanism is attributed to laser induced thermal desorption while the other is due to a nonthermal, single photon process. A film of 1–2000 ML (layers) of NO condensed on a Ag(111) substrate under UHV conditions at 25–50 K was irradiated by 5 ns pulses of 220–270 nm laser light (4.6–5.5 eV) with 0.5–5 mJ/cm2 (0.1–1 MW/cm2 ) power density at the surface. Translational energies of desorbed molecules were measured from time‐of‐flight (TOF) spectra taken with a mass spectrometer, while the internal energy distribution of molecules desorbed in the nonthermal channel was determined by a (1+1) resonance enhanced multiphoton ionization (REMPI) probe. NO monomer in the 2Π3/2,1/2 electronic ground states was the only significant product. There were two distinct characteristic TOF components, which we associate with different desorption mecha...

Experimental and simulation study of neon collision dynamics with a 1-decanethiol monolayer

A study of the energy accommodation of neon colliding with a crystalline self-assembled 1-decanethiol monolayer adsorbed on Au(111) is presented. The intensity and velocity dependencies of the scattered neon as a function of incident angle and energy were experimentally measured. Scattering calculations show good agreement with these results, which allows us to examine the detailed dynamics of the energy and momentum exchange at the surface. Simulation results show that interaction times are, at most, a few picoseconds. Even for these short times, energy exchange with the surface, both normal and in-plane, is very rapid. An important factor in determining the efficiency of energy exchange is the location at which the neon collides with the highly corrugated and structurally dynamic unit cell. Moreover, our combined experimental and theoretical results confirm that these are truly surface collisions in that neon penetration into the organic boundary layer does not occur, even for the highest incident energies explored, 560 meV.

Time-resolved atomic force microscopy imaging studies of asymmetric PS-b-PMMA ultrathin films: Dislocation and disclination transformations, defect mobility, and evolution of nanoscale morphology

Time-sequenced atomic force microscopy (AFM) studies of ultrathin films of cylinder-forming polystyrene-block-polymethylmethacrylate (PS-b-PMMA) copolymer are presented which delineate thin film mobility kinetics and the morphological changes which occur in microphase-separated films as a function of annealing temperature. Of particular interest are defect mobilities in the single layer (L thick) region, as well as the interfacial morphological changes which occur between L thick and adjacent 3L/2 thick layers, i.e., structural changes which occur during multilayer evolution. These measurements have revealed the dominant pathways by which disclinations and dislocations transform, annihilate, and topologically evolve during thermal annealing of such films. Mathematical combining equations are given to better explain such defect transformations and show the topological outcomes which result from defect–defect encounters. We also report a collective, Arrhenius-type flow of defects in localized L thick region...

Calculation of rotationally mediated selective adsorption in molecule surface scattering: HD on Pt(111)☆

Abstract Rotationally mediated selective adsorption of HD on Pt(111) is examined theoretically using R -matrix scattering techniques. With a laterally averaged surface-molecule Morse potential interaction and for an anisotropic potential term transformed from H 2 , excellent agreement is obtained between the resonances and the first-order perturbed bound vibration-free rigid rotor energies.

Experiments and simulations of Ar scattering from an ordered 1-decanethiol–Au(111) monolayer

A study of the scattering of Ar from a well-ordered standing-up phase of 1-decanethiol adsorbed on Au(111) at surface temperatures from 110 to 185 K is presented. The final energies and intensities were measured as a function of incident polar and azimuthal angles using incident energies from 65 to 600 meV. These experimental results are compared to classical trajectory calculations. Scattering shows two distinct exit channels. The higher energies are due to direct inelastic scattering and have the greatest intensities at glancing incident and final angles. The lower energy channel is due to trapping-desorption; it has a Maxwell–Boltzmann energy distribution at the surface temperature and a cosine angular intensity profile. The simulations show that the timescale for normal momentum accommodation is very fast. The parallel momentum accommodation takes slightly longer, dependent on the initial conditions, but is still complete within only a few picoseconds. The result is that much of the Ar undergoes trapp...

Silicon nitride membrane substrates for the investigation of local structure in polymer thin films

Abstract The fabrication of silicon nitride membrane substrates and their use in studies of polymer thin films are described. As an integral part of a wafer, these membranes are both self-supporting and transparent for transmission electron microscopy (TEM). Therefore, the same polymer film can be spin-cast on the substrate and, without being removed, studied by a variety of techniques, including TEM, and atomic force microscopy (AFM). To demonstrate the utility of these substrates in characterizing both global and local film morphology, experimental results are presented on polystyrene–polymethylmethacrylate diblock copolymers in the ultrathin film limit, using optical microscopy together with combinations of AFM and TEM at the same location. The addition of microfabricated structures to these substrates, such as planar electrodes is also discussed.