L. Houssiau

ToF-SIMS study of organosilane self-assembly on aluminum surfaces

In order to understand the adsorption and self-assembly of organosilanes, we have studied the adsorption of octyltrichlorosilane (OS) and ocradecyltrichlorosilane (OTS) on an aluminum surface, naturally oxidized. The self-assembled monolayer (SAM) formation was followed with the time-of-flight-secondary ion mass spectrometry (ToF-SIMS) technique. The adsorption is readily detected by means of the Si-containing ions, such as SiH+, SiH-, SIOH or SiO2-. Heavier ions that carry information on the surface structure were also identified, such as C8H17SiO2- for an OS adsorption or C18H37SiO2- for an OTS adsorption, The adsorption kinetics can be easily followed by monitoring several characteristic fragments as a function of the adsorption time. The OS adsorption on aluminum appears to have an oscillatory behavior, with a fast adsorption during the first minute followed by a desorption, then a slow adsorption again. All the Si-containing peaks reflect this trend. A possible model that explains the oscillation is given. The aluminum oxide fragments of the type AlxOyHz- exhibit a dramatic decrease with increased coverage, due to the reaction of the active surface sites with the: silanols. Interestingly. the OTS adsorption appears to he monotonic, with a coverage slowly increasing with time. The final achieved coverage is more complete with OTS than with OS. The ToF-SIMS technique appears to be well suited to study the chemical changes and the degree of surface coverage during the adsorption

A round robin experiment of elemental sensitivity factors in low-energy ion scattering

In a round robin experiment a set of five polycrystalline, metallic samples is studied by low-energy ion scattering (LEIS) in five different laboratories, The energy range is 0.6-3.5 keV and He and Ne ions are used. Even though different experimental setups are used the evaluated elemental sensitivity factors agree within +/-20%. Reproducibility within single laboratories is better than 10%. In an additional study carried out in three laboratories the surface composition of an alloy, Cu55Pd45, was determined, using in situ calibration standards, These surface composition measurements agreed within +/-3 at% demonstrating that quantitative composition determination is possible using this procedure

TOF-SIMS study of alkanethiol adsorption and ordering on gold

A systematic study of alkanethiol self-assembly on gold was undertaken with the time-of-flight-secondary ion mass spectrometry (TOF-SIMS) technique. Following adsorption, the TOF-SIMS spectra exhibit sulfur-containing peaks (specially AuxSx clusters) and a rise of all hydrocarbon peaks (CxHy). But the most specific peaks are those directly related to the adsorbed molecules (M) such as deprotonated monomers (M-H)(-), thiolate (MAu-). dimers (M2Au-), trimers (M3Au2-) and even tetramers (M4Au3-) By examining the ion yield of the most relevant fragments as a function of adsorption time, it is possible to follow the self-assembly kinetics for all the alkanethiols chosen. The hydrocarbon and Au,S, peak intensities quickly rise and reach a saturation level after typically an adsorption period of a few minutes. However, the oligomer fragments exhibit a much slower rise and reach saturation only after a few hours. This observation corroborates the widely accepted theory of alkanethiol adsorption on gold, starting with a fast adsorption of a disordered layer followed by a slower phase of monolayer self-assembly leading to a high packing of the alkane chains. The oligomer ions clearly reflect, to some extent, the degree of organization of the monolayer, whereas hydrocarbon and AuxSy, clusters just indicate the chemical changes at the surface

Direct observation of the rippling and the order-disorder transition at the Cu3Au(100) surface by TOF-ion scattering

Low energy ion scattering provides a direct measurement of surface rippling at room temperature, i.e. a relaxation of Au atoms 0.12 Angstrom above the Cu atoms in the outermost layer, which is unambiguously terminated by a mixed and ordered CuAu layer. It is shown that the technique, which is sensitive to the locations of atoms at the first monolayer, is also sensitive to the order-disorder transition in this layer. The occurrence of disorder modifies the arrangement of the atomic rows and then changes the double scattering conditions for low incident angles. As a result, the Au signal decreases significantly at the transition temperature. This effect is continuous and starts at about 50 degrees C below the bulk transition temperature (663 K).

Observation of surface rippling on Cu3Au(100) with ToF-ISS

We have investigated a Cu3Au(100) surface with ToF-ISS. The crystal was accurately oriented by use of azimuthal scans taken at grazing incidence. The polar scans measured with 2 keV Ne+ ions prove that the surface is unambiguously terminated by a mixed and ordered CuAu layer at room temperature. Along the [100] azimuth, the ion beam analyses only the rows of the first monolayer, which are either pure Cu rows or pure Au rows. Along the [110] azimuth, the beam analyses rows of the first plane, which are alternated Cu-Au chains, but also the pure Cu rows in the second plane. This implies that the Cu signal is more intense in this direction than in others. The most striking result along the [110] rows is the direct measurement of rippling on the first atomic layer. We show that Au atoms lie 0.12 Angstrom above the Cu atoms, which is in pretty good agreement with theoretical predictions.

Order-disorder phase transition of the Cu3Au(100) surface studied by ToF-ion scattering

Cu3Au(100) surface has been investigated with ToF-ISS at different temperatures below and above the bulk order-disorder transition temperature (T-c = 663 K). Grazing angle azimuthal scans on the surface reveal the crystallinity of the first monolayer. The polar scans measured by 2 keV Ne ion scattering aligned along the major orientations, i.e. (100) and (110), show that the surface is terminated at room temperature by an ordered and rippled CuAu layer. The (100) rows are alternatively pure Cu rows and pure Au rows while the (110) rows are made by an alternated sequence of Cu and Au atoms. When increasing the temperature close to T,, the Au focusing peak at low incidence, along the (110) rows, broadens in a first time owing to thermal vibrations, then shifts rapidly to higher incident angles when we approach the transition. This effect is interpreted as the result of the order-disorder transition at the surface, which modifies the ordered atomic sequence constituting the rows, By plotting the Au scattering yield vs. temperature at fixed azimuthal and incident angles, a strong decrease of the signal near the transition is observed. However, the transition is not abrupt, indicating possibly a second-order phase transition at the first monolayer, in opposition to the first-order bulk transition.

TOF-SIMS study of organosilane adsorption on model hydroxyl terminated surfaces

In order to further understand organosilane oscillatory adsorption, we have adsorbed octyltrichlorosilane (C8H17SiCl3 or OS) on model hydroxyl terminated surfaces consisting of a thiol-alcohol self-assembled on gold. The adsorption kinetics was followed with the time-of-flight secondary ion mass spectrometry (TOF-SIMS) technique. The OS adsorption is readily detected by means of a variety of silicon containing ions, some non-specific (e.g. SiH-, SiO2-) and others specific of the OS (C8H17SiO2- or dimers C16H35Si2O4-). Interestingly, a complex non-monotonic adsorption pattern is observed. Moreover, carefully selected ions exhibit different intensity variations with the adsorption time, providing some insight not only on adsorption kinetics, but also on film cross-linking, adsorption sites occupation and even on HCl production due to the trichlorosilane hydrolysis. Small, non-specific fragments (SiH-) are believed to indicate the OS surface coverage whereas large specific fragments are strongly dependent on the interface cross-linking, which reduces their probability of sputtering

Real-space surface crystallography: Experimental stereographic projections from ion scattering

Scattering and recoiling imaging spectrometry (SARIS) in the blocking configuration is used to obtain experimental two-dimensional stereographic projections of the Ni(110) and Pt(111) surfaces. The development of this technique as an element-specific real-space surface crystallography that is sensitive to interatomic spacings in the surface and subsurface layers of a crystal is described. This projection imaging method is based on the blocking of atomic trajectories scattered from subsurface layers by atoms in layers nearer to the surface. The resulting magnification of the blocking cones, as captured by a gated position-sensitive microchannel plate detector, is similar to 10(9). The images of the blocking patterns provide direct information on interatomic spacings and surface symmetry and structure. Classical ion trajectory simulations using the three-dimensional scattering and recoiling imaging code (SARIC) are used to simulate the stereographic projections and blocking patterns and to provide quantitative interpretations. The method is sensitive to interatomic spacings in the surface and subsurface layers. The physical properties of the blocking process are derived from analysis of the data and a simplified, approximate, two-atom model of the scattering/blocking process is developed

MARLOWE simulations of He and Ne ion scattering on Cu3Au(100) and comparison with TOF-ISS experiments

We investigated the Cu/sub 3/Au(100) surface structure with TOF-ISS. This provided evidence for a bulk truncated structure, with a first layer containing Cu and Au in equal proportions and a second layer containing only Cu. The topmost layer is rippled, with Au atoms lying 0.12 A above the Cu atoms. A better understanding of the signal origin requires the use of the MARLOWE simulation code. The spectra are first simulated and then decomposed into their layer contributions. The agreement between experimental and simulated spectra is pretty good. The large differences in the 1 keV He spectrum shape along the and the directions are reproduced by simulation. With 2 keV Ne the simulations clearly demonstrate that the second layer is directly reached by the beam along the direction but not along the direction. The surface model proposed from the experiments is then fully confirmed by the simulations.

Surface structure analysis by time-of-flight ISS: influence of primary ion nature and energy for scattering on Cu(110)

The influence of primary ion nature and energy on ISS polar plots is studied experimentally and by simulation for scattering on a Cu(110) surface. The scattering of He and Ne ions in the energy range 1-3 keV is determined by the sampling depth of these ions. Simulations performed with the MARLOWE code show that helium ions analyse a large depth into the crystal while neon ions analyse the very first few atomic layers only.