Thomas Andreas Barberka

Investigation of the in-plane structure of Pb and Ni stearate multilayers by means of grazing incidence X-ray diffraction

Abstract Multilayers of fatty acid salts prepared by the Langmuir-Blodgett technique on solid supports build structures that are well ordered in growth direction perpendicular to the surface of the substrate. Open questions remain concerning the arrangement of molecules within the layers and their stacking from layer to layer. We have investigated Pb and Ni stearate multilayers on an Si support by means of grazing-incidence diffraction using X-rays from a synchroton source. For Pb stearate, six in-plane Bragg peaks (0 x A −1 were detected, whereas only one peak was found for Ni stearate. The intensity distributions along the surface normal (rod scan) were recorded at each Bragg peak for 0 z A −1 . Based on the experimental data a structure model of the three-dimensional arrangement of molecules within the multilayer domains is proposed which prefers interlayer chain coupling between neighbouring monolayers.

Thermally-induced phase transitions in LB multilayers of lead stearate

Abstract Pb-stearate multilayers prepared by the Langmuir-Blodgett (LB) technique on silicon show a change of layer structure just below the melting point. At temperatures between 98 °C and 112 °C the samples have been investigated by energy dispersive X-ray scattering in specular and diffuse scattering geometry. The time constants of the transition at fixed temperature was determined. After the disappearance of the old structure, new Bragg peaks grow out of an amorphous intermediate phase corresponding to a 20% smaller layer distance compared with the initial LB structure. At fixed temperature the intensity of the respective Bragg peaks increases and, after reaching a maximum, it decreases again until the new peaks disappear. Maximum Bragg intensity was observed at a transition temperature of 106 °C. The new phase is instable in time. Our results are compared with related effects of Cd stearate, which have been interpreted as a transformation into a two-dimensional ordered hexagonal structure of long columns.

Investigation of the chain-chain interface in a lead-stearate multilayer using neutron reflectivity

A partially deuterated Pb-stearate multilayer prepared by means of the Langmuir-Blodgett technique on a Si support was investigated by X-ray and neutron reflectometry. It consists of a superstructure built by two protonated and two deuterated hydrocarbon chains. The X-ray reflectivity is sensitive to the gradient of the electron density between the hydrocarbon chains and the sheets of metallic counter-ions whereas the neutron reflectometry measures the gradient of scattering length density which is a maximum at the interface between the deuterated and protonated chains. Sharp Bragg peaks were found which are due to the periodic distance between the metallic sheets whereas the peaks responsible for the interface between deuterated and protonated chains are smeared out. The intensities of these peaks can only be explained assuming a partial exchange of deuterated and protonated chains between the sublayers.

Structure of thermally treated oxadiazoleamide Langmuir-Blodgett films

Abstract The thermal treatment of Y-type Langmuir-Blodgett (LB) films formed from the amphiphilic derivative of 2,5-diphenyl-1,3,4-oxadiazole 1 results in changes of the molecular packing. These changes have been analysed by a combination of X-ray specular reflectivity data, X-ray grazing incidence diffraction data and scanning force microscopy images. On the basis of these experimental data we have simulated possible supramolecular structures. These simulations provide insight into the intermolecular interactions giving rise to the observed structural transitions. The crystalline structure induced by thermal treatment of the LB films is chracterized by a uniaxial texture, which is correlated with the dipping direction during deposition of the LB film.

X-ray diffuse scattering from lead stearate multilayers☆

Abstract We give an account of X-ray diffuse scattering (XDS) as a non-destructive technique to characterize the defect structure of organic multilayers. As well as the interface roughness we especially investigated the lateral correlation length τ of scattering aggregates using the angle-dispersive as well as the energy-dispersive scattering from a Pb stearate film. Although the present theoretical model is not satisfactory, it is possible to relate τ to the in-plane dimension of the microcrystalline domains. The XDS and the X-ray specular reflectivity (XSR) depend in a different manner on the interface roughness and the domain size. Therefore the ratio of XDS and XSR measured at special points of reciprocal space can be used to observe the changes in these real structure parameters. This is demonstrated in the case of thermally induced melting of domains.

Do perfluorinated chains always have to be twisted

Short perfluorinated carbon chains do not take on a helical structure when they are closely packed.

The domain structure of LB multilayers prepared from fatty acid salts

Abstract Langmuir-Blodgett (LB) multilayers from fatty acid salts can be characterized by finite sized scattering aggregates. High-resolution atomic force microscopy maps reveal a substructure of molecular ordering. These blocks are to be considered as 3D domains. Furthermore, macroscopic holes and scratches appear at the surface. These defect structures have to be taken into account when interpreting observed X-ray and neutron reflectivity curves. The density loss due to the holes can be detected from the angular shift of the critical angle of total external reflection. Offdiagonal scans demonstrate a slight inclination of domains relative to each other. Neutron reflectivity patterns of sequentially deuterated and hydrogenated carbon chains provide information about the disorder of the organic chains. In the second part of the paper a structure model of the molecular order within the domains is developed using 16 in-plane Bragg peaks and their truncation rods measured by X-ray grazing incidence diffraction. The refined 3D unit cell contains at least four monolayers in a herringbone arrangement of the hydrocarbon chains. Its stability is given by the inter-chain coupling between two neighbouring monolayers.

X-ray scattering from thin organic films and multilayers

SummaryThe real structure of LB-multilayers prepared with fatty-acid salts is dominated by finite-sized scattering aggregates. Their different length scales become visible using AFM. It shows that not the whole substrate is wetted by the film. The molecular order is restricted into domains. These micrometer domains are not homogeneous. They contain mesoscopic subdomains of different heights which vary in steps of double layers. Finally high-resolution AFM-maps display a nearly hexagonal arrangement of molecules within subgrains with a diameter of several 10 nm. This domain structure has to be taken into account when interpreting X-ray diffraction data. The size of the crystalline aggregates is obtained by means of X-ray grazing incidence diffraction. On the mesoscopic scale the domain size is determined by X-ray diffuse scattering experiments. Because Sinha’s model fails for the present kind of multilayers, we used another approach for data analysis. The lateral correlation length caused by height fluctuations is estimated without knowledge of a definite correlation function. Additionally the mosaicity of the domain orientation can be taken into account.

Multilayers of perfluorinated fatty acids

Abstract We have formed Y layers of perfluorododecanoic acid CF 3 (CF 2 ) 10 COOH by thermal evaporation in vacuo and of perfluorotetradecanoic acid CF 3 (CF 2 ) 12 COOH by thermal evaporation and by the Langmuir–Blodgett (LB) technique. We have obtained the bilayer spacing of both these materials by X-ray diffraction and have also studied the in-plane structure of these materials by means of grazing incidence diffraction (GID). Computer modelling was used to interpret the results obtained. For the perfluorododecanoic acid, we find two stable untwisted phases at 25°C and a combination of these two predicts both the Bragg peaks arising from the layer structure and the GID results. Our experimental results show that the perfluorotetradecanoic acid exists in the generally accepted helical structure. Computer modelling leads to the conclusion that closely packed perfluorinated chains with 12 or less carbon atoms should exist in an untwisted state while molecules having more than 12 carbon atoms show the onset of the helical conformation.