Ulrich G. Volkmann

High-resolution ellipsometric study of an n-alkane film, dotriacontane, adsorbed on a SiO2 surface

Using high-resolution ellipsometry and stray light intensity measurements, we have investigated during successive heating-cooling cycles the optical thickness and surface roughness of thin dotriacontane (n-C32H66) films adsorbed from a heptane (n-C7H16) solution onto SiO2-coated Si(100) single-crystal substrates. Our results suggest a model of a solid dotriacontane film that has a phase closest to the SiO2 surface in which the long-axis of the molecules is oriented parallel to the interface. Above this “parallel film” phase, a solid monolayer adsorbs in which the molecules are oriented perpendicular to the interface. At still higher coverages and at temperatures below the bulk melting point at Tb=341 K, solid bulk particles coexist on top of the “perpendicular film.” For higher temperatures in the range Tb Ts, a uniformly thick fluid film wets to the parallel film phase. This struc...

Optical properties of Pd thin films exposed to hydrogen studied by transmittance and reflectance spectroscopy

The optical properties of hydrogen exposed palladium (Pd) and palladium oxide (PdO) thin films are calculated from reflectance and transmittance measurements in the visible and near infrared range (400–900nm). The time evolution of the films’ optical constants when exposed to hydrogen is shown. The real part of palladium’s refractive index increases about 5%, whereas the imaginary part decreases nearly 15% after hydrogen absorption. The Pd films’ resistance also increased upon hydrogen absorption, as expected. Pd oxide reduces to metallic palladium upon hydrogenation, showing a dramatic change in all the properties. The palladium films’ resistance initially decreases after a first exposure to 5Torr of hydrogen, and this is tentatively explained by the reaction of hydrogen with impurities trapped in the films.

A novel growth mode of alkane films on a SiO2 surface

Abstract Synchrotron X-ray specular scattering measurements confirm microscopically a structural model recently inferred by very-high-resolution ellipsometry of a solid dotriacontane ( n -C 32 H 66 or C32) film formed by adsorption from solution onto a SiO 2 surface. Sequentially, one or two layers adsorb on the SiO 2 surface with the long-axis of the C32 molecules oriented parallel to the interface followed by a C32 monolayer with the long-axis perpendicular to it. Finally, preferentially oriented bulk particles nucleate having two different crystal structures. This growth model differs from that found previously for shorter alkanes deposited from the vapor phase onto solid surfaces.

Inclusion effect of gold and copper particles in a poly(amide) matrix that contains a thiophene moiety and Si or Ge atoms in the main chain

Soluble pure silicon or germanium-containing poly(amide)s and their metallic composites (Cu or Au) were synthesized and characterized. Optical band gaps of pure polymer were comparable to an insulator behavior; however, the conductivity of some composites at several concentrations shows a diode-like behavior. Samples exhibit a monoclinic lattice mixed with amorphous structures. Specifically, polymer–Au composites showed distortion of this unit cell, associated with an increase in the conductance. This effect would be related to the coordination of the central atom (Si or Ge) to the incorporated metal, producing a homogeneous distribution of metallic particles within the system.

Atomic force microscopy measurements of topography and friction on dotriacontane films adsorbed on a SiO2 surface.

We report comprehensive atomic force microscopy (AFM) measurements at room temperature of the nanoscale topography and lateral friction on the surface of thin solid films of an intermediate-length normal alkane, dotriacontane (n-C32H66), adsorbed onto a SiO2 surface. Our topographic and frictional images, recorded simultaneously in the contact mode, reveal a multilayer structure in which one to two layers of molecules adsorb adjacent to the SiO2 surface oriented with their long axis parallel to the interface followed by partial layers of molecules oriented perpendicular to the surface. The thicknesses of the parallel and perpendicular layers that we measured with the AFM agree with those inferred from previous x-ray specular reflectivity measurements on similarly prepared samples. We also observe bulk dotriacontane particles and, in contrast with our previous measurements, are able to determine their location. Above a minimum size, the bulk particles are separated from islands of perpendicularly oriented molecules by regions of exposed parallel layers that most likely extend underneath the particles. We find that the lateral friction is sensitive to the molecular orientation in the underlying crystalline film and can be used effectively with topographic measurements to resolve uncertainties in the film structure. We measure the same lateral friction on top of the bulk particles as on the perpendicular layers, a value that is about 2.5 times smaller than on a parallel layer. Scans on top of parallel layers indicate a constant height but reveal domains having different sublevels of friction. We explain this by the domains having different azimuthal orientations of the molecules.

Synthesis of Oligomeric Silicon-containing Poly(imide-amide)s Derived from Trimellitic Anhydride and Amino-Acids. Vibration Spectral, Optical, Thermal and Morphological Characterization

Poly(imide-amide)s (PIAs) were synthesized from diacids, which were obtained by trimellitic anhydride and glycine, L-alanine, L-phenylalanine, L-valine, L-leucine, L-isoleucine and p-aminobenzoic acid, and bis(4-aminophenyl)diphenylsilane. Compounds were characterized by elemental analysis, optical activity, IR and NMR spectroscopies. Yields were good, but ηinh values were low, showing materials of oligomeric nature, confirmed by MALDI-TOF spectrometry. PIAs were soluble in polar aprotic solvents, while those derived from amino acids with bulky side groups were soluble in CHCl3 and acetone. The wavenumber of vibrational bands were compared by Infrared and Raman spectroscopy. However, the high fluorescence emitted for PIAs, difficult to predict the vibrations of certain functional groups by Raman spectroscopy. The incorporation of chiral, amide and imide moieties interrupt the conjugation and increase the flexibility of the system. In addition, the adjacent phenyl units from the silicon-containing diamine, disfavors the planar conformation, which affects the optical properties showing an insulator behavior. DSC analyses showed that PIA-VII of aromatic nature had the higher glass transition temperature (Tg) value. For the other PIAs the Tg values decreased when volume of the side groups was increased, with the exception of PIA-VI derived from L-isoleucine. The thermal decomposition temperatures showed that only PIA-VII derived from p-aminobenzoic acid was thermostable. For the other PIAs it was possible to see a decrease of the TDT values when the size of the side chain is increased. The size and shape of the polymeric particles were studied by Scanning Electronic Microscopy (SEM), showing a relation between structure, incorporation of rigid aromatic rings and voluminous group into the system.

Nanoscale observation of delayering in alkane films

Tapping-mode Atomic Force Microscopy and synchrotron X-ray scattering measurements on dotriacontane (n-C32H66 or C32) films adsorbed on SiO2-coated Si(100) wafers reveal a narrow temperature range near the bulk C32 melting point Tb in which a monolayer phase of C32 molecules oriented perpendicular to surface is stable. This monolayer phase undergoes a delayering transition to a three-dimensional (3D) fluid phase on heating to just above Tb and to a solid 3D phase on cooling below Tb. An equilibrium phase diagram provides a useful framework for interpreting the unusual spreading and receding of the monolayer observed in transitions to and from the respective 3D phases.

Position resolution and efficiency measurements with large scale Thin Gap Chambers for the super LHC

New developments in Thin Gap Chambers (TGC) detectors to provide fast trigger and high precision muon tracking under sLHC conditions are presented. The modified detectors are shown t o stand a high total irradiation dose equivalent to 6 Coulomb/cm of wire, without showing any deterioration in their performan ce. Two large (1.2× 0.8 m 2 ) prototypes containing four gaps, each gap providing pad, strips and wires readout, with a total thickn ess of 50 mm, have been constructed. Their local spatial resolution has been measured in a 100 GeV/c muon test beam at CERN. At perpendicular incidence angle, single gap position resolution better than 60µm has been obtained. For incidence angle of 20 o resolution of less than 100µm was achieved. TGC prototypes were also tested under a flux of 10 5 Hz/cm 2 of 5.5-6.5 MeV neutrons, showing a high effi ciency for cosmic muons detection.

Structure and Growth of Vapor-Deposited n-Dotriacontane Films Studied by X-ray Reflectivity

We have used synchrotron X-ray reflectivity measurements to investigate the structure of n-dotriacontane (n-C(32)H(66) or C32) films deposited from the vapor phase onto a SiO(2)-coated Si(100) surface. Our primary motivation was to determine whether the structure and growth mode of these films differ from those deposited from solution on the same substrate. The vapor-deposited films had a thickness of approximately 50 A thick as monitored in situ by high-resolution ellipsometry and were stable in air. Similar to the case of solution-deposited C32 films, we find that film growth in vacuum begins with a nearly complete bilayer adjacent to the SiO(2) surface formed by C32 molecules aligned with their long axis parallel to the interface followed by one or more partial layers of perpendicular molecules. These molecular layers coexist with bulk particles at higher coverages. Furthermore, after thermally cycling our vapor-deposited samples at atmospheric pressure above the bulk C32 melting point, we find the structure of our films as a function of temperature to be consistent with a phase diagram inferred previously for similarly treated solution-deposited films. Our results resolve some of the discrepancies that Basu and Satija (Basu, S.; Satija, S. K. Langmuir 2007, 23, 8331) found between the structure of vapor-deposited and solution-deposited films of intermediate-length alkanes at room temperature.

Crystalline-to-plastic phase transitions in molecularly thin n-dotriacontane films adsorbed on solid surfaces

Crystalline-to-rotator phase transitions have been widely studied in bulk hydrocarbons, in particular in normal alkanes. But few studies of these transitions deal with molecularly thin films of pure n-alkanes on solid substrates. In this work, we were able to grow dotriacontane (n-C(32)H(66)) films without coexisting bulk particles, which allows us to isolate the contribution to the ellipsometric signal from a monolayer of molecules oriented with their long axis perpendicular to the SiO(2) surface. For these submonolayer films, we found a step in the ellipsometer signal at approximately 331 K, which we identify with a solid-solid phase transition. At higher coverages, we observed additional steps in the ellipsometric signal that we identify with a solid-solid phase transition in multilayer islands ( approximately 333 K) and with the transition to the rotator phase in bulk crystallites ( approximately 337 K), respectively. After considering three alternative explanations, we propose that the step upward in the ellipsometric signal observed at approximately 331 K on heating the submonolayer film is the signature of a transition from a perpendicular monolayer phase to a denser phase in which the alkane chains contain on average one to two gauche defects per molecule.