M. Rei Vilar

Adsorption of monoethanolamine on clean, oxidized and hydroxylated aluminium surfaces : a model for amine-cured epoxy/aluminium interfaces

Abstract The interaction of 1-amino-2-hydroxyethane (monoethanolamine) with clean Al(100), in-situ oxidized Al(100) and in-situ hydroxylated Al(100) has been investigated using Auger electron spectroscopy (AES) and high-resolution electron energy-loss spectroscopy (HREELS). It has been shown that, on all these surfaces, adsorption of monoethanolamine can be interpreted in the framework of the Hard and Soft Acid Base (HSAB) principle. However, the detailed mechanism differs for each surface: (i) monoethanolamine interacts with clean Al(100) through its amine termination via a Lewis-like acid-base reaction; (ii) monoethanolamine interacts with oxidized Al(100) through its alcohol termination via a Lewis-like acid-base reaction; (iii) monoethanolamine interacts with hydroxylated Al(100) through its alcohol termination via a Bronsted-like acid-base reaction.

Study of polymer film surfaces by EELS using selectively deuterated polystyrene

HREELS spectra of selectively deuterated polystyrene films show loss energies corresponding to the excitation of vibrational modes of the surface region of the organic medium. The thickness of the analyzed surface region can be varied by changing the incident electron energy and/or the scattering geometry. Selective labelling allows to better separate the contributions of main chain and of pendant-groups, and to eliminate the influence of contaminants. It is shown for instance that isotactic and atactic polymer surfaces differ, the former being mainly composed of phenyl rings. End group segregation at the surface may also be important. Multiple losses are much more intense than the corresponding combination modes in IR: the theory of interaction of low energy electrons with polymer films should be further investigated.

High-resolution electron energy loss spectroscopy of thin crystalline highly oriented films of poly(tetrafluoroethylene)

High-resolution electron energy loss spectroscopy (HREELS) spectra of highly oriented films of poly(tetrafluoroethylene) (PTFE) are reported. With one exception, all peaks in the spectra correspond to IR active vibrations. They are well resolved, and with a remarkably high intensity, more than two orders of magnitude greater than we have observed on any other polymer in HREELS. The angular distributions of the elastic peak, and of the vibrational peaks are very narrow, which indicates both a well ordered system and a dipolar scattering behaviour. No evidence of amorphous regions in these films is found. A Raman active mode can be observed in off-specular geometry, using an incident electron beam coplanar with the PTFE fibers direction. This corresponds to resonance excitation of a transient negative ion state, with a maximum cross-section at an incident electron kinetic energy of about 4 eV.

Controlled growth of Cu2O nanoparticles bound to cotton fibres

A green, safe and fast procedure is presented for in situ generation of nanoparticles (NPs) of cuprous oxide (Cu2O) onto cotton fibres at room temperature using water as a solvent. The method is based on a mild surface oxidation of cellulose fibres to generate in a controlled way carboxylic groups acting as a binding site for the adsorption of Cu(2+) via electrostatic coordination. Then, the adsorbed Cu(2+) ions were readly converted into Cu2O by dipping the treated cotton fibres into a aqueous solution of a reducing agent. Field-emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), as well as UV-vis absorption and emission spectroscopic methods were used to analyse the size, morphology, chemical composition and the crystalline structure of the generated nanoparticles on the fabrics. The morphology of the ensuing Cu2O nanoparticles was shown to be dependent on the reduycing agent used. Antibacterial properties of the modified fibres were also investigated.

Oligothiophene films under electron irradiation: electron mobility and contact potentials

Abstract This work compares mobility in various films of α-oligothiophenes with chainlengths ranging from four to six thiophene rings (4T, 5T and 6T), which were studied by high-resolution electron energy-loss spectroscopy (HREELS) in the same conditions of beam intensity and geometrical configuration. The increase of electron mobility going from 4T to 6T was estimated from an analysis of the energy extent in the complete spectra of the electrons backscattered from the sample surface. The same method was still applied to two films of 6T deposited on different substrates, gold and highly oriented pyrolitic graphite (HOPG), where a change in the molecular orientation is expected. The same value was found for the electron mobility in both films. However, different contact potentials were observed and no charge accumulation was detected. On the other hand, work functions were estimated for both films using the work functions values found in the literature for gold and HOPG, which are 4.6 and 4.5 eV, respectively. In this case, it was not possible to conclude if a different molecular orientation produced a change in the work function or not, as the difference between the work function values of the substrates given in the literature is larger than 0.1 eV.

Electronic excitation and secondary electron emission studies by low-energy electrons backscattered from thin polystyrene film surfaces

Thin atactic polystyrene films were studied by backscattering of low-energy electrons. Energy losses corresponding to the excitation in the film of triplet and singlet electronic excitation of the pendant groups are observed. The variation of the corresponding cross sections with incident electron energy are in good agreement with theoretical calculations for benzene. The variation of the backscattered intensity integrated over electron energy in a given direction, yields a ionization threshold of about 7 eV and shows evidence for structure of the valence band.

SAMs functionalised by α-quaterthiophene as model for polythiophene grafting

Abstract A model of polythiophene grafting on a SAM functionalised by α-quaterthiophene is presented. The elaboration and characterisation of the dodecanethiol monolayer terminated by α-quaterthiophene are reported. Blocking level of substrates, structure of the alkyl chains and organisation of the quaterthiophene units as well as their chemical behaviour are analysed. HREELS spectra showed that the extreme surface of the monolayers is entirely covered by quaterthiophene units after typical adsorption times of 12 h. PM-IRRAS spectra allow to follow the orientation of molecules within the monolayer versus adsorption time. It is shown that the long axis of quaterthienyl groups stand slightly tilted with respect to the perpendicular. In addition, interfacial reactions of the SAM layer with a model molecule, the end-capped terthiophene, were electrochemically induced via the formation of radical cations. The number of grafted molecules that depends on the 4T film structure is discussed in regard to the 4T moiety reactivity.

High-resolution electron energy loss spectroscopy studies on the electronic structure of quinquethiophene (5T) and sexithiophene (6T) films

High-resolution electron energy loss spectroscopy (HREELS) and optical absorption spectra were recorded for 5- and 6-thiophene films. These compounds are characterized by a large excitonic band of u symmetry. Optical transitions are only allowed to the high-energy edge (top) of this band. Due to different selection rules, HREELS spectra reveal transitions to the low-energy edge (bottom) of the exciton band, and allow a precise determination of the excitonic gap. Furthermore, a second threshold above 3 eV is tentatively assigned to the electronic gap.

Characterization of oligothiophene films by high resolution electron energy loss spectroscopy

Electronic structures of quinquethiophene (5T) and sexithiophene (6T) polycrystalline films deposited by evaporation on gold substrates were studied by high resolution electron energy loss spectroscopy (HREELS). Vibrational spectra were compared to infrared spectroscopy and Raman scattering results. They reveal that surfaces are not rough and are free from contaminants. Energy gap values of 2.38 ∠ 0.02 and 2.27 ∠ 0.02 eV for 5T and 6T films, respectively, were evaluated from electronic spectra. Comparison with optical spectroscopy enabled the assignment of losses corresponding to optically allowed and forbidden transitions. Some of the latter ones located at 1.2, 1.7 and 2.0 eV were observed in 5T films and were assigned to S 0 φι Tn transitions. Differential cross section analysis reveals the presence of resonance processes in the excitation mechanism. Ionization potentials were estimated to be 4.7 ∠ 0.5 eV for both films.© 1998 Elsevier Science S.A. All rights reserved

HREELS characterization of surfaces and interfaces in self-assembled molecular monolayers

Abstract High resolution electron energy loss spectroscopy ( HREELS ) is used to characterize the extreme surface composition and the interfaces of silane and thiol monolayers deposed on silicon and platinum crystals, respectively. Energy losses due to impact interactions are very sensitive to the molecular groups present in the extreme surface of the film. Long range dipole interactions can supply information about buried interfacial bonds. Silicon substrates were characterized before and after molecular interactions. Energy losses assigned to interfacial siloxane bonds clearly vary with previous substrate annealing. Spectra of adsorbed thiols are dominated by losses corresponding to functional groups attached to the chain-end. An energy loss at 360 cm−1 was identified as corresponding to an interfacial S–Pt bond.