Insight in the outside

Modern low energy ion scattering (LEIS) can provide new information on the atomic composition of both outermost surface and sub-surface layers. The wide scope of new possibilities is illustrated with results on ceramics, polymers and ultra-thin layers. Most of the results presented are difficult or even impossible to obtain otherwise. The combination of LEIS and isotopic oxygen exchange reactions was used to investigate the surface of yttria stabilised zirconia (YSZ) that is used in the solid oxide fuel cell. It is found that under working conditions the surface is covered by a monolayer of contamination, followed by an yttria segregated layer. The quantification of LEIS is demonstrated for fluorinated self-assembled monolayers, while also the conformation of the endgroups is determined. The use of LEIS to follow the growth of ultra-thin layers is discussed for a TiN barrier layer on silicon

On the correlation between Si+ yields and surface oxygen concentration using in situ SIMS-LEIS

To study the oxygen dependence of ionisation processes during O2+ bombardment, low-energy ion scattering was used as a surface analysis technique since it has very high surface sensitivity. An LEIS instrument was reconfigured to collect secondary ions simultaneously with the compositional analysis based on LEIS. The gradual conversion of the Si surface into an oxidised surface under oxygen bombardment has been studied. A correlation between the Si+ intensity and the oxygen concentration could be established, i.e. I(Si+)¿(CO/CSi)2.9. It was observed that, at the stage where the oxygen concentration no longer increases, the amount of visible (for LEIS) Si atoms decreases further, implying a further structural change. This change coincides with the steep increase in the Si+ ion intensity near complete oxidation and can be interpreted as the main cause for this change in ionisation probability

On the correlation between Si+ yields and surface oxygen concentration using in situ SIMS-LEIS

To study the oxygen dependence of ionisation processes during O2+ bombardment, low-energy ion scattering was used as a surface analysis technique since it has very high surface sensitivity. An LEIS instrument was reconfigured to collect secondary ions simultaneously with the compositional analysis based on LEIS. The gradual conversion of the Si surface into an oxidised surface under oxygen bombardment has been studied. A correlation between the Si+ intensity and the oxygen concentration could be established, i.e. I(Si+)¿(CO/CSi)2.9. It was observed that, at the stage where the oxygen concentration no longer increases, the amount of visible (for LEIS) Si atoms decreases further, implying a further structural change. This change coincides with the steep increase in the Si+ ion intensity near complete oxidation and can be interpreted as the main cause for this change in ionisation probability

Surface segregation of silicon impurities in organic materials

The present revolution in novel organic materials is driven by the synthesis of new materials exhibiting specific functional properties. Traces of silicon compounds are often present in these materials and, although the bulk concentrations of these impurities may be low, segregation can seriously modify the surface composition. Surfaces and interfaces play an important role in many applications, and the intrinsic properties of the materials are thus often obscured by the presence of segregated impurities. By studying silicon impurity segregation in poly-dialkoxy phenylenevinylene (PPV), polycarbonate and dendrimer macromolecules, we demonstrate how low-energy ion scattering may be used to determine the surface impurity fraction and to observe which groups at the surface are shielded by the segregated species. We demonstrate that the performance of PPV- based light-emitting diodes is significantly reduced for submonolayer coverages of siloxanes. We find that the kinetics of the segregation process depend strongly on the materials and the sample preparation conditions. We find that the presence of solvents is needed to enable segregation at room temperature. Heating does enable siloxane impurity segregation in polycarbonate in the solid phase, whereas for polydimethylsiloxane in PPV films we find that segregation in the solid phase does not occur up to 200°C. The siloxane molecules are found to segregate to preferential sites at the surface, shielding the polar groups. Finally, we demonstrate that purification of the surface is often possible through simple procedures that provide an easy way to study the intrinsic properties of the materials

Thermodynamically controlled surface segregation of perfluoroalkyl-end-capped PMMA as investigated by low energy ion scattering (LEIS)

Low MW PMMA prepd. by atom transfer radical polymn. (ATRP) was blended with a perfluoroalkyl-end-capped PMMA (Rf-PMMA) synthesized also by ATRP. Thin films were prepd. by spin casting the polymer blend solns. in toluene. Fluorinated species became enriched at the film surface, evidenced by increased contact angles for both water and hexadecane. The temp.-dependent fluorine concn. in the outermost one layer was quant. detd. by low energy ion scattering (LEIS). The fluorine concn. decreased significantly upon the increase of the temp. when the temp. was above the glass transition temp. of polymers. The surface segregation of fluorinated species at temps. above the Tg of polymers appears to be thermodynamically controlled. [on SciFinder (R)

Quantification of fluorine density in the outermost atomic layer

The outermost at. layer of perfluorinated thiol monolayers on gold and poly(tetrafluoroethylene) (PTFE) is analyzed by low-energy ion scattering. Abs. quantification of fluorine d. in this layer was achieved after calibrating the fluorine signal with a freshly cleaved LiF(100) single crystal. The fluorine d. of monolayers of a C8F17-thiol on gold was 1.48 * 1015 F atoms/cm2, whereas for PTFE a value of 1.24 * 1015 F atoms/cm2 was obsd. This difference was explained by the different tilt angles of the thiol on gold and PTFE chains with respect to the surface normal. Both a configurational and a mol. interpretation on the perfluorinated thiol monolayer on gold are given

Surface studies of partially fluorinated polymethacrylates: a combined XPS and LEIS analysis

Thin films of copolymers of 1,1-dihydroperfluoroheptyl methacrylate (FHMA) and methyl methacrylate (MMA) were investigated with respect to both their molecular and macroscopic surface properties. Introduction of FHMA entities in copolymer decreases the surface energy to a large extent. Angle resolved X-ray photoelectron spectroscopy (XPS) measurements show an increased average fluorine concentration in a surface layer thickness of a few nanometers, when compared to the fluorine concentration in the bulk. Static LEIS experiments, which selectively probe the outermost atomic layer, show an even stronger enrichment of fluorine atoms. (C) 2002 Elsevier Science B.V. All rights reserve

The outermost atomic layer of thin films of fluorinated polymethacrylates

The surface properties of a series of copolymers of perfluoroalkyl methacrylates (CH2=C(CH3)COOCH2CnF2n+1, n = 1, 6, or 10) and Me methacrylate (MMA) and of blends of perfluorooctyl-end-capped poly(Me methacrylate) (PMMA) and pure PMMA were studied. The introduction of perfluoroalkyl groups significantly lowers the surface energy of fluoropolymers as detd. by the acid-base approach. The XPS data confirm a higher fluorine concn. in the surface region (the outer 3.8 nm) as compared to the bulk. The fluorine d. in the outermost at. layer is quant. detd. by low-energy ion scattering (LEIS). A linear relation is found between the fluorine d. in the outermost at. layer and the surface energy of the partially fluorinated polymethacrylates, irresp. of the length of the perfluoroalkyl chain. This linearity confirms Langmuir's principle of independent surface action. Deviation from this linear relation exists for both highly and sparsely fluorinated polymethacrylates and can be ascribed to the local (surface) ordering of the fluorinated tails and MMA units, resp. The study contributes to understanding the correlations between macroscopic surface properties and microscopic surface chem. compn. [on SciFinder (R)

Insight in the outside: New applications of low-energy ion scattering

Low energy ion scattering is used to selectively probe the outermost atomic layer of the surface. The development of double toroidal analyzers has improved the detection sensitivity of low energy ion scattering by orders of magnitude. The features of these analyzers are discussed. It is shown that the absence of matrix effects makes it possible to quantify the surface density of fluorine in polymers with a LiF(1 0 0) surface. The extreme surface sensitivity of LEIS also enables one to study intramolecular segregation processes. As an example the aging of a polypropylene surface that has been activated with atomic oxygen is described. As an example of the LEIS analysis of highly dispersed isolating surfaces, the formation of coke on a commercial three-way catalyst is discussed

The outermost atomic layer of thin films of fluorinated polymethacrylates

The surface properties of a series of copolymers of perfluoroalkyl methacrylates (CH2=C(CH3)COOCH2CnF2n+1, n = 1, 6, or 10) and Me methacrylate (MMA) and of blends of perfluorooctyl-end-capped poly(Me methacrylate) (PMMA) and pure PMMA were studied. The introduction of perfluoroalkyl groups significantly lowers the surface energy of fluoropolymers as detd. by the acid-base approach. The XPS data confirm a higher fluorine concn. in the surface region (the outer 3.8 nm) as compared to the bulk. The fluorine d. in the outermost at. layer is quant. detd. by low-energy ion scattering (LEIS). A linear relation is found between the fluorine d. in the outermost at. layer and the surface energy of the partially fluorinated polymethacrylates, irresp. of the length of the perfluoroalkyl chain. This linearity confirms Langmuir's principle of independent surface action. Deviation from this linear relation exists for both highly and sparsely fluorinated polymethacrylates and can be ascribed to the local (surface) ordering of the fluorinated tails and MMA units, resp. The study contributes to understanding the correlations between macroscopic surface properties and microscopic surface chem. compn. [on SciFinder (R)