Determination of molecular orientation in ultrathin liquid crystal. films on solid substrates using X-ray absorption spectroscopy

42 H₆₀O₆, HBT) deposited on a (111)-oriented Au single crystal and a polycrystalline indium tin oxide (ITO) substrate has been carried out using X-ray absorption spectroscopy. Films of thicknesses between 2 nm and 15 nm were prepared in UHV by evaporation of HBT [which exhibits a discotic liquid crystalline (LC) bulk phase] from a Knudsen cell. Thickness and composition of the HBT films were determined using X-ray photoelectron spectroscopy (XPS). For the thinnest films with thicknesses in the monolayer regime, the orientational analysis reveals a pronounced orientation of the disc-shaped HBT molecules parallel to the Au surface. For thicker films, a significantly reduced anisotropy is observed with the molecular plane oriented more normal to the Au surface. In the case of the ITO-substrates, no significant differences were observed between different thickn esses and the average orientation of the molecular planes was predominantly normal to the surfaces, as for the thicker films on the Au substrate. Received: 1 March 1997/Accepted: 24 April 1997     

Studies of thin smectic C films by X-ray reflectivity

The smectic order in thin and ultra thin films (150–600Å) of the chiral ferroelectric liquid crystal mixture ZLI-3654 is studied using the X-ray reflectivity technique. The spin cast films on various substrates (float glass, Si wafer, polymer coated glass, etc.) order spontaneously with smectic layering parallel to the substrate surface. A simple model which assumes a sinusoidal density modulation can describe well the experimental reflectivity profiles. The X-ray reflectivity provides a method to evaluate the phases of the structure factor. We demonstrate, for the first time, that is possible to extract the molecular tilt angle, , in ferroelectric liquid crystals from X-ray reflectivity measurements of ultra thin films. The temperature dependence of the tilt angle in the smectic C^* phase are almost independent of the film thickness (down to 200 Å) and are similar to those in the bulk.     

Rapid X-ray reflectivity measurement using a new liquid interface reflectometer at SPring-8

X-ray reflectivity measurements of liquid surfaces were demonstrated using a recently developed liquid interface reflectometer at SPring-8. The reflectometer, equipped with a two-dimensional hybrid pixel array detector (PILATUS), achieved x-ray reflectivity towards 10^-9 with an integration time at each angle of only 1 sec, offering enormous potential for rapid measurements. Time-resolved measurements at a time resolution of 1 min were performed on the adsorption process of a globular protein lysozyme on a water/air interface.     

Analysis of mesoporous thin films by X-ray reflectivity, optical reflectivity and grazing incidence small angle X-ray scattering

It is well-established that X-ray reflectivity (XR) is an invaluable tool to investigate the structure of thin films. Indeed, this technique provides under correct analysis, the electron density profile of thin films in the direction perpendicular to the substrate. For thin films that exhibit lateral ordering at the nanometer scale, grazing incidence small angle X-ray scattering (GISAXS) ideally complements the XR technique to measure the scattering in off-specular directions. As typical examples, XR and GISAXS data of mesoporous silica thin films and porous materials are presented. The analysis of the XR curve allows to determine the porosity of the film. We also show that the combination of X-ray and visible optical reflection provides information about the index of refraction of thin films. Finally we report how capillary condensation of water can be monitored by XR and GISAXS.     

X-ray reflectivity study on gold films during sputter deposition

We performed in-situ X-ray reflectivity measurements of gold films during sputter deposition on polished silicon substrates. The measurements were performed at several substrate temperatures and under two argon pressures. The gold surfaces were also examined by scanning tunneling microscopy after deposition to obtain their real-space topographic images. These images were used to complement the X-ray reflectivity measurements in determining the effect of argon pressure on the gold surface and its height-height difference functions. An approximation for height-height difference functions was employed to analyze the X-ray reflectivity data. The measured interface width during growth followed a simple power law, consistent with recent theoretical results of dynamic scaling behavior. The scaling exponents, however, do not agree well with predictions based on some models in 2 + 1 dimensions.     

Spectral investigations of surface ordering in ultrathin molecular films

Surface molecular ordering in ultrathin molecular films is investigated. The optical transmission spectra of molecular films ranging in thickness from 2 to 13 smectic layers (6.7–43 nm) in the region of the electronic absorption bands in the smectic A phase of cyanobiphenyl CB9 are measured. The thickness and temperature dependences of the permittivity are determined. It is found that the orientational ordering of the molecules depends on the film thickness. The penetration depth of the surface molecular orientational order does not exceed two smectic layers (<7 nm). Zh. éksp. Teor. Fiz. 115, 1833–1842 (May 1999)     

有序组装超薄膜热释电性能的优化研究

报道两亲性染料半花菁(DAEP)与隔层材料氮冠醚(NC)交替LB膜的热释电效应以及掺杂金属离子（Ba2+）对LB膜热释电性能的影响.发现所测样品的热释电系数p高达58μCm-2K-1；在频率为1kHz—100kHz的范围内，其εr和tanδ的数值分别为2.34—1.96和0.08—0.04.并讨论了不同成膜方式和掺杂金属离子Ba2+对LB膜热释电性能影响的物理机理.     

Dielectric properties of ultrathin films of molecular crystals

Dielectric properties of thin films (TF) of molecular crystals, including the effect of size and the boundary surfaces were analysed using Green's function method. Polarisability of molecules in various film layers and dielectric susceptibility of TF were calculated. A comparison with crystal bulk has shown that dielectric properties of TF are strongly influenced both by the sample dimensions and by the boundary conditions.The frequency dependence of the dielectric susceptibility has also been derived. One obtains the monotonous variation for the frequencies above and below exciton band. However, for the frequencies within the exciton band there appears to be complicated, non-monotonous dependence. The relationship between the dielectric susceptibility and the film width for the same frequency shows a complex, oscillatory behaviour. Furthermore, the amplitude of these oscillations increases with increasing film width, demanding the introduction of a damping factor. Finally, the thickness dependence of dielectric susceptibility was analysed in the Cole-Cole plot.     

X-ray photoelectron diffraction study of ultrathin PbTiO3 films

Full hemispherical X-ray photoelectron diffraction (XPD) experiments have been performed to investigate at the atomic level ultrathin epitaxial c-axis oriented PbTiO₃ (PTO) films grown on Nb-doped SrTiO₃ substrates. Comparison between experiment and theory allows us to identify a preferential ferroelectric polarization state in a 60 ? -thick PTO film. Multiple scattering theory based on a cluster-model [ Phys. Rev. B , 075404 (2001)] is used to simulate the experiments.     

X-ray and neutron reflectivity analysis of thin films and superlattices

X-ray and neutron reflectivity measurements provide a wealth of information on thickness and interfacial properties on the nanometer scale. This method is therefore well suited for the study of nano-structured thin films and superlattices. Neutrons provide a different contrast between the elements than X-rays and, in addition, are sensitive to the magnetization in the sample. Using polarized neutrons, magnetic as well as chemical profiles can be probed. In this review a basic introduction into the theory of X-ray and neutron reflectivity is provided along with some recent examples including the oxidation of Fe films and the structural and magnetic properties of Co/Cu superlattices.     

Accelerated aging in ultrathin films of a molecular glass former

We report the thermodynamic measurement of the enthalpy released during the aging of supported films of a molecular glass former, toluene, at temperatures well below the glass transition temperature. By using microfabricated devices with very short equilibration times (below 1 s), we evidence a remarkable variation of the relaxation rate on decreasing film thickness from 100 nm down to a 7 nm thick film. Our results demonstrate that surface atoms are more efficient than bulk atoms in attaining low energy configurations within the potential energy landscape.     

Electroosmotic flow of ultrathin liquid films and their stability

The Debye-Huckel approximation is generalized to the case of two-phase flows. A one-dimensional steady solution describing electroosmotic flow is found, and, for the first time, its linear stability is investigated analytically.