M.A Reijme

Indium diffusion in model polymer light-emitting diodes

The diffusion of indium into poly-(phenylenevinylene) (PPV) in model polymer light-emitting diodes (p-LEDs) was studied with Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (LEIS) and particle induced X-ray emission (PIXE). The model p-LEDs consisted of a glass substrate, an indium–tin-oxide (ITO) electrode, a PPV layer obtained by thermal conversion of sulfonium precursor PPV, and a patterned aluminium electrode. From RBS measurements it was concluded that about 0.01 at.% indium was present in the PPV, homogeneously distributed in depth. Annealing at 230°C for 19 h caused the amount of In in the PPV layer to increase by roughly an order of magnitude. Under the patterned aluminium electrode, the annealing treatment resulted in accumulation of In at the PPV/Al interface, whereas the depth distribution of In remained homogeneous in the uncovered region of the model LEDs. XPS spectra on annealed model LEDs show that In was present in the near surface region of the PPV films, although LEIS analysis showed that In was not situated in the outermost atomic layer. LEIS measurements on as-prepared model LEDs showed that the patterned Al electrode had caused surface contamination of the uncovered PPV film with Al, which can have impact on the diffusion of In to the outermost surface during annealing treatments.

Domain structure, segregation and morphology of the Pt3Sn(111) surface

Abstract Using spot-profile analysis low-energy electron diffraction, we have studied the domain structure and morphology of the Pt3Sn(111) surface after sputtering at room temperature and subsequent anneal treatments which result in the formation of the previously reported ( 3 × 3 ) R30° and (2×2) structures. In addition, we present new results on the temperature-dependent composition of the outermost layer and the subsurface region that is obtained from quantitative analysis of low-energy ion scattering and Auger electron spectroscopy experiments. The combination of these results and previous experiments yields a detailed picture of the evolution of the surface region during the anneal treatments. Finally, we propose the existence of a phase transition at high temperatures between the (2×2) phase and a (2×2)′ phase (not previously observed) on the basis of reversible changes in the SPA–LEED, LEIS and AES data.

Intermolecular segregation of siloxane in P3HT: surface quantification and molecular surface-structure

Abstract Surfaces of spin-coated and solution-cast poly(3-hexylthiophene) (P3HT) films are analysed by X-ray Photoelectron Spectroscopy and Low Energy Ion Scattering. Here, we use the P3HT–siloxane system with only 2% siloxane monomers in the bulk as a model system to study segregation and surface orientation of molecules in polymers. The surfaces are enriched in siloxane due to the intermolecular segregation of the siloxanes present in P3HT. The siloxane coverage fraction was found to depend on the preparation parameters such as spinspeed and solution-concentration, and ranges from 25 to 100%. The molecular orientation of segregated siloxanes on P3HT was found to resemble that on pure PDMS. Furthermore, siloxane molecules prefer specific sites on P3HT, such that sulphur atoms are screened from being at the outermost surface to lower the surface free energy. The results presented here demonstrate clearly the unique ability of LEIS to quantify the composition of the outermost atomic layer, and to obtain detailed information on the surface structure.

A combined time-of-flight and electrostatic analyzer for low-energy ion scattering

A new instrument for low-energy ion scattering (LEIS) is presented which employs a combination of electrostatic and time-of-flight analysis. In this instrument, electrostatic analysis is used to determine the kinetic energy of the ions, and flight-time analysis is used to select the mass of the ions. The combination allows us to discriminate the signals resulting from particles with a different mass than the primary ions, resulting in a very efficient suppression of signals caused by sputtered particles. The suppression of signals from sputtered particles enables more accurate determination of LEIS signals, especially for light elements. This technique is especially suited for the study of polymers and oxides, but can also be valuable for determination of low concentration of heavier elements. In this article the design of the instrument is presented and the method is demonstrated by some examples of LEIS spectra.

In situ surface analysis by low energy ion scattering

We discuss the possibilities to apply low energy ion scattering (LEIS) for in situ surface analysis of devices and functional materials. First we discuss the limitations imposed by the technique such as pressure and spatial resolution. Next, we present a new method to suppress backgrounds in LEIS spectra due to the presence of light elements by using a combined electrostatic and flight-time analysis. Examples of in situ analysis on thermionic cathodes are presented which shed light on the surface structure and composition of these cathodes, and also allow us to study the resistivity of such cathodes to ion bombardement. Finally, a design is presented for a pressure cell for in situ surface analysis of catalysts during catalytic reaction.

Molecular surface structure of poly(3-hexylthiophene) studied by low energy ion scattering

Low energy ion scattering and X-ray photoelectron spectroscopy were used to study the surface of thin spin-coated poly(3-hexylthiophene) (P3HT) films on silica. We found that the composition of the outermost surface differs from that of the bulk due to the surface molecular structure: the sulphur atoms are screened from being at the outermost surface, presumably by the hexyl side-chains. The influence of this intramolecular segregation phenomenon on the composition is limited to the outermost surface. Comparison of the sputter-profile of P3HT with that taken on a polycrystalline α-quaterthiophene film shows that after sputtering the sulphur to carbon atomic ratio of bulk P3HT is observed.

Intramolecular segregation in polymers and macromolecules studied by low-energy ion scattering

Abstract The intramolecular segregation in several generations of poly(propylene imine) dendrimers and oxygen plasma modified HDPE was studied with low-energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). Although the different generations of poly(propylene imine) dendrimers contain almost the same atomic carbon to nitrogen (C/N) ratio, it is shown that this ratio at the outermost surface depends strongly on the generation of the dendrimer. The conformation of a dendrimer is probably related to the flexibility of the bis(3-aminopropyl)amine endgroups, which becomes lower for higher generations. Furthermore, the formation of a metallodendrimer, by complexation of CuCl2 with the amine endgroups of a fourth generation dendrimer (D4), was followed with LEIS. Compared to pure CuCl2, a relatively high copper concentration and a high atomic copper to chlorine ratio was found at the surface of the metallodendrimer. The high copper concentration and the reduced sterical hindrance by chlorine demonstrate the high potential of the metallodendrimers as catalytic material. The hydrophobic surface of HDPE can be made hydrophilic by treating it in an oxygen plasma. During such a treatment, oxygen containing functional groups are introduced at the surface, which improves wetting and thus adhesion. Unfortunately, the effect of an oxygen treatment is only temporary, an effect referred to as ageing or hydrophobic recovery. LEIS has been used to study this ageing process. We were able to follow the surface oxygen concentration as a function of the time between the plasma treatment and the LEIS analysis. Combined information obtained from LEIS and XPS measurements indicates that the ageing is mainly confined to the outermost atomic layers. Moreover, the ageing process depends strongly on the exact experimental conditions.

Molecular orientation and morphology of contact printed α-quaterthiophene films on gold

We present contact printing as a technique to deposit α-quaterthiophene (α-4T) films from the solid phase onto gold. The molecular orientation and morphology of both the printed film and the original polycrystalline α-4T film on silica were investigated with low energy ion scattering (LEIS), atomic force microscopy (AFM), X-ray diffraction (XRD) and optical microscopy. We show that the strong interaction between clean gold and α-4T induces a drastic change in the molecular orientation and morphology of a 380 nm thick α-4T film. On gold the α-4T molecules are orientated with the thiophene rings parallel to the substrate and form rod-like crystallites (typically ∼13×1.3 μm2), whereas on silica α-4T molecules stand almost upright and form large cobblestone-like crystallites (typical diameter ∼10 μm). Exposure of α-4T to a low energy ion beam (dose <1×1014 3 keV 3He+ ions/cm2) prior to printing causes polymerisation, which decreases the ability to print and alters the morphology of the printed film.

The distribution of InClx compounds in model polymeric LEDs: A combined low and high-energy ion beam analysis study

Abstract A combination of low- and high-energy ion beam analysis techniques was used to determine the distribution of indium chloride compounds in model polymeric light-emitting diodes (p-LEDs). Parts of polymeric LEDs (polydialkoxyphenylenevinylene (OC 1 C 10 -PPV) on indium-tin-oxide (ITO) substrates) were exposed to a HCl/Ar flow to simulate the processes occurring during conversion of precursor PPVs and acid treatment of polymers. Samples with variable exposure times as well as pristine samples were studied with Rutherford backscattering spectrometry (RBS), low energy ion scattering (LEIS), X-ray photoelectron spectroscopy (XPS) and particle induced X-ray emission (PIXE). The RBS measurements show that after HCl exposure indium is distributed throughout the OC 1 C 10 -PPV layer. LEIS and XPS measurements indicate that the indium and chlorine are present at the outermost surface of the OC 1 C 10 -PPV layer. PIXE measurements in combination with the RBS data demonstrate that the indium chloride in the OC 1 C 10 -PPV layer is distributed in blisters (∅=50 μm) which are orientated perpendicular to the sample surface. To study the OC 1 C 10 -PPV/ITO interface, samples were delaminated by peeling the polymeric layer of the ITO using Scotch tape. LEIS measurements on the fracture planes demonstrate the formation of an interfacial layer between the ITO and the PPV. The implications for p-LEDs are discussed.

IBA on functional polymers

The analysis of element distributions in polymer-based structures using IBA techniques offers the possibility to study a variety of interesting problems, in particular diffusion and reaction phenomena. Indium diffusion in model polymer light emitting diodes (p-LEDs) consisting of a stack Al/poly-(phenylenevinylene)/indium-tin-oxide/glass has been studied with Rutherford backscattering spectrometry (RBS), particle induced X-ray emission (PIXE), X-ray photoelectron spectroscopy (XPS), and low energy ion scattering (LEIS). A second example is provided by the analysis of organic optical gratings, in which the diffusion of labeled monomers during holographic photo-polymerization of photo-reactive monomer mixtures has been studied with μPIXE using a scanning proton microprobe. Since polymers are sensitive to ion irradiation, a new RBS/ERDA set-up has been constructed that is equipped with a sample holder mounted on a closed cycle helium refrigerator, which enables the cooling of samples to cryogenic temperature...