E. A. Sánchez

Self-assembly of alkanedithiols on Au(111) from solution: effect of chain length and self-assembly conditions.

A comparative study on the adsorption of buthanedithiol (BDT), hexanedithiol (HDT), and nonanedithiol (NDT) on Au(111) from ethanolic and n-hexane solutions and two different preparation procedures is presented. SAM characterization is based on reflection-absorption infrared spectroscopy, electrochemistry, X-ray photoelectron spectroscopy, and time of flight direct recoil spectroscopy. Results indicate that one can obtain a standing-up phase of dithiols and that the amount of the precursor lying-down phase decreases from BDT to NDT, irrespective of the solvent and self-assembly conditions. A good ordering of the hydrocarbon chains in the standing-up configuration is observed for HDT and NDT when the system is prepared in degassed n-hexane with all operations carried out in the dark. Disulfide bridges at the free SH terminal groups are formed for HDT and to a lesser extent for NDT prepared in ethanol in the presence of oxygen, but we found no evidence of ordered multilayer formation in our experiments. No disulfides were observed for BDT that only forms the lying-down phase. Our results demonstrate the key role of the chain length and the procedure (solvent nature and oxygen presence) in controlling the surface structure and chemistry of SAMs dithiols on Au(111).

Energy loss and angular dispersion of 2-200 keV protons in amorphous silicon

Abstract The energy loss of 2–200 keV protons in thin amorphous silicon foils has been measured for projectiles transmitted in the forward direction and as a function of the exit angle. At the lowest energies, differences of up to 30% with recently published values are observed. Angular effects in the energy loss, at low and high energies, have been investigated. The low-energy results are reproduced by model calculations and Monte Carlo simulations, which indicate that the inelastic energy loss does not show a dependence upon the impact parameter in the low energy region. A fitting formula for the present energy loss values is provided.

Stopping power of fluorides for low-velocity protons

A combined experimental and theoretical study of the energy loss of protons in fluorides is presented. The measurements were performed in fresh AlF{sub 3} evaporated in situ on self-supported C foils, extending the low-energy range from 25 down to 0.7 keV. The transmission method is used in combination with time-of-flight spectrometry. The experimental stopping power increases almost linearly with the mean projectile velocity, showing a velocity threshold at about 0.1 a.u. These features are well reproduced by a model based on quantum scattering theory that takes into account the velocity distribution and the excitation of the active 2p electrons in the F{sup -} anions, and the properties of the electronic bands of the insulators. The model also reproduces the threshold behavior measured previously on LiF.

Adsorption kinetics and surface unrelaxation in H:GaAs(110) studied by time-of-flight scattering and recoiling spectrometry

Time-of-flight scattering and recoiling spectrometry is used to study the chemisorption of H on a GaAs(110) surface. Monitoring of the neutral plus ion intensity for both the H direct recoils and the Ne and Ar backscattered projectiles as a function of the H exposure, the surface temperature, and the incident direction of the projectiles gives information about the substrate unrelaxation and its dependence on the adsorption kinetics. It is found that the adsorption proceeds with a sticking coefficient that decreases strongly with the coverage. At low exposures, the unrelaxed fraction of the surface increases linearly with the coverage. For coverages of the order of one monolayer there is an important fraction of the surface that remains relaxed as in the clean surface. The H recoil intensity is almost independent of the crystallographic sample orientation, suggesting that an important fraction of the H atoms are not adsorbed in well ordered sites.

Convoy-electron-yield dependence on controlled surface properties by Na deposition

We present double differential distributions of electrons emitted in the direction of a 60 keV proton beam traversing a 10 μg/cm2 Al-foil placed in ultrahigh vacuum. We observe a very strong dependence of the convoy electron yield when the downstream surface is modified by controlled Na deposition. From the secondary electron emission it can be qualitatively concluded that an increasing yield is observed for larger work-function values.

Assembly and thermal stability of thin EP-PTCDI films on Ag(111)

We report a multi-technique study of the adsorption kinetics and self-assembly characteristics of EP-PTCDI grown on Ag(111) at UHV conditions and room temperature. Changes in the valence band characteristics for the mono and the multilayer film and the stability as a function of the annealing temperature are discussed. The results show that the molecules start to adsorb on the step edges forming ordered islands that grow to fully cover a monolayer. Further exposure results in the stacking of similarly ordered islands of several layers. The desorption experiment shows that the film is stable up to 150 degrees C where a rapid desorption of the multilayer takes place, followed by a decomposition of the molecules for temperatures higher than 170 degrees C.

Interaction of keV ions with insulator films at grazing incidence: growth characterization and electron emission

We present a study of the growth of AlF3 thin films on Al(1 1 1) surface, together with the electron emission produced in the scattering of 60 keV protons from these films. The growth of the AlF3 films at room temperature, from submonolayer coverage up to several layers, was characterised by means of Auger electron spectroscopy and electron energy loss spectroscopy. We found that from the beginning of the evaporation the AlF3 molecules adsorb stoichiometrically, and layer-by-layer. The electron emission induced by grazing proton bombardment was measured as a function of the film thickness. In the forward direction, the most prominent structure can be related with convoy electron emission. For the case of the metallic surface, the maximum of this peak is located at energies above the corresponding one to electron transfer to projectile continuum states in gas-phase collisions, and shifts to lower values for sufficiently thick films. This result is discussed in terms of the competition between track and polarisation potentials generated in the insulator film, and image potentials induced in the metallic substrate. 2003 Elsevier Science B.V. All rights reserved.

Adsorption of potassium on GaAs(110)

The adsorption of potassium on the GaAs(110) surface has been studied by ion scattering and recoiling spectroscopy (SARS) and Auger electron spectroscopy (AES). The SARS measurements indicate that a major part of the potassium atoms adsorb along the [001] gallium rows, in a region close to the sites of a new arsenic layer, and that only a minor part of the potassium atoms adsorb along the [001] arsenic rows. In agreement with this suggestion, the energy shifts in the substrate Auger peaks indicate that, at the beginning of the adsorption, the potassium atoms react preferentially with gallium atoms.

Ion fractions in 6 keV Ne+, Ar+ and Na+ scattering from a GaAs(110) surface

Abstract We used Ion Scattering Spectrometry (ISS) with Time-of-Flight (TOF) analysis to study the neutralization of 6 keV Ne+, Ar+ and Na+ backscattering from a flat and ordered GaAs(110) surface. At low incident angles an important contribution to the total (neutral + ion) backscattering spectra comes from quasi single collisions with As and Ga first layer surface atoms. We observed that the ion fraction in this contribution is strongly dependent on both the incident projectile and the target atom, suggesting that the violent collision plays an important role in the neutralization process. In order to interpret these dependencies we performed a calculation that discriminates the interaction of the projectile with the extended and localized states of the solid.