Structural characterization of thin hydroxypropylcellulose films. X-ray reflectivity studies

Thin solid films of hydroxypropylcellulose (HPC) have been investigated using synchrotron X-ray reflectivity. Evidence of preferential alignment of HPC molecules at the substrate surface is obtained. In the surface region the liquid crystalline domains ofHPC are preferentially oriented parallel to the substrate, whereas in the bulk they are mostly distributed randomly. Incorporation of colloidal particles in the film-substrate region destroys the preferential alignment. It is also found that in conditions of restricted geometry (very thin films), a minimum film thickness is required to produce lateral packing order perpendicular to the substrate surface.     

Structure of thin block copolymer films studied by X-ray reflectivity

Summary X-ray reflectivity may be used to determine the internal structure of thin polymer films. An electron density difference of 10% for polystyrene and polyisoprene is sufficient to distinguish between a random distribution of lamellae, complete orientation parallel to the substrate surface and a surface induced formation of lamellae. The disappearance of the lamellar Bragg-peaks, with heating of the film, shows the transition into the disordered state.     

X-ray reflectivity study of ultrathin liquid films of diphenylsiloxane-dimethylsiloxane copolymers

Using X-ray reflectivity, we observe drastic differences in the interfacial structure and molecular ordering of diphenylsiloxane-dimethylsiloxane copolymer thin films deposited on hydroxylated versus H-terminated (etched) silicon wafers. We find that substrate type and comonomer ratio determine the conformational arrangements in these liquid films. High-energy bonding between the substrate and the molecules and an increase in rigidity of the molecules due to replacement of methyl groups by phenyl groups leads to a specific molecular ordering at the liquid/solid interface and pronounced density oscillations in this region. The observed structural reorganizations are explained by the interplay and the established balance between the chain flexibility and the polymer-substrate interactions.     

A facile method for the density determination of ceramic thin films using X-ray reflectivity

A fast and non-destructive method based on X-ray reflectivity was developed to determine the density of sol–gel derived ceramic thin films, without prior assumptions on the microstructure of the system. The thin film density is calculated from the critical angle θ_c, i.e. the maximum angle at which total external reflection is still observed, which becomes increasingly difficult for imperfect films. We propose a simple numerical approach, instead of laborious fitting procedures, to determine the thin film density. A pseudo-critical angle, θ_pc, was defined by the first minimum in the 3rd derivative of the reflectivity curves. The measured samples were compared with calibration curves obtained from simulations with changing film densities. Although the absolute positions of θ_c and θ_pc are different, similar shifts are observed with changing density. The accuracy of the described method was validated by determining the density of single crystal substrates (ρ_rel = 100 %) and by Rutherford backscattering spectroscopy in combination with scanning electron microscopy. Varying sample size, film thickness, and film/interface roughness of yttria-stabilized zirconia films were found to have no influence on the final calculated density.     

In-situ X-ray reflectivity study of alkane films grown from the vapor phase

We carried out in-situ X-ray reflectivity study of nine n-alkane chains (CnH2n+2) on Si substrate, n in the range of 17-30. These films formed under vacuum at equilibrium vapor pressure of the respective alkane molecule. For all the alkanes studied we found a bilayer structure on the substrate, a higher density vertical layer at the air-film interface with the layer thickness equal to the all-trans length of the respective molecule, and a lower density layer below it with the molecules lying horizontal on the substrate. This model was earlier proposed for C32 films on Si by Volkmann et al.11 We observe that this model can fit the entire range of data from C17 to C30 in our experiments.     

Neutron reflectivity study of lipid membranes assembled on ordered nanocomposite and nanoporous silica thin films

Single bilayer membranes of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were formed on ordered nanocomposite and nanoporous silica thin films by fusion of small unilamellar vesicles. The structure of these membranes was investigated using neutron reflectivity. The underlying thin films were formed by evaporation induced self-assembly to obtain periodic arrangements of silica and surfactant molecules in the nanocomposite thin films, followed by photocalcination to oxidatively remove the organics and render the films nanoporous. We show that this platform affords homogeneous and continuous bilayer membranes that have promising applications as model membranes and sensors.     

Effect of water contact on the density distributions of thin supported polymer films investigated by an X-ray reflectivity method

The diffusion processes of water molecules into polymer films (PMMA/PS homopolymers and random copolymers) in contact with liquid water were investigated using gravimetric methods and X-ray reflectivity (XRR) analysis. Methods of water contact and XRR measurement were designed for studying the systems in the nonequilibrium state of diffusion. Gravimetric measurements confirmed the Fickian diffusion behavior of films in contact with water. Vertical density distributions in PMMA and methylmethacrylate-rich copolymer films demonstrate the existence of a water-rich layer at the interface. However, with further absorption of water into the film, the overall density increased throughout the film. The results suggest that the diffusion of water into the polymer film occurs to recover density uniformity with a high concentration of water molecules at the surface. Some XRR data for the PS- and styrene-rich copolymer films could not be fit and converted to a vertical density distribution because of their huge diffusion coefficients. However, the reflectivity curves for these films and the vertical density distribution after sufficient water contact suggested that the surfaces of these films were commonly diffused after water contact. Atomic force microscopy (AFM) analysis demonstrated that the surface roughness of these films actually increased with water content.     

X-ray and Raman study of spray pyrolysed vanadium oxide thin films

Vanadium oxide thin films were prepared by spray pyrolysis using solutions of vanadium chloride (VCl₃) with different concentrations on glass substrates heated at 200 and 250 °C. The influence of substrate temperature (T_s) and solution concentration (molarity) on structural and vibrational properties is discussed by using X-ray diffraction and Raman spectroscopy. The results revealed that at 0.05 M and T_s = 200 °C, V₄O₉ thin films are obtained. At 250 °C, V₂O₅ phases with preferential orientation are observed and the films become polycrystalline when the molarity increases.     

Three-dimensional molecular packing of thin organic films of PTCDI-C8 determined by surface X-ray diffraction

We have determined the full molecular 3D packing of thin organic films of the archetypical organic n-type semiconductor N, N'-dioctyl-3,4:9,10-perylene tetracarboxylic diimide (PTCDI-C 8) by surface X-ray crystallography. We show that PTCDI-C 8 forms smooth layered films on Al 2O 3 (11-20) with an outstanding degree of molecular order. The thin-film structure is found to consist of a triclinic unit cell with the plane of the aromatic core tilted by 67 +/- 2 degrees with respect to the surface plane, which differs significantly from the bulk structure. The 3D crystallites extend with vertical coherent order across the entire film thickness.     

Characterization of Langmuir-Blodgett organoclay films using X-ray reflectivity and atomic force microscopy

Monolayers of organoclay platelets were formed at the air/water interface using the Langmuir technique and were then investigated either by in situ or lifted onto Si wafers and studied ex situ, using X-ray reflectivity (XR) methods. The XR data showed that the surfactant molecules on the clay platelets formed a dense, self-assembled monolayer where the molecules were tilted at an angle of 35 degrees +/-6 degrees from the normal to the dry clay surface. The surfactant layers only covered a fraction of the clay platelet surface area, where the fractional surface coverage for the three clays studied (C6A, C15A, and C20A) was found to be 0.90, 0.86, and 0.73, respectively. These values were significantly higher than those estimated from the cation exchange capacity (CEC) values. Rather than being uniformly distributed, the surfactant was clustered in patchy regions, indicating that the surface of the clay platelets had both polar and non-polar segments. This heterogeneity confirmed the hypothesis which was previously invoked to explain the distribution of the clay platelets in melt mixed homopolymer and polymer blend nanocomposites.     

X-ray reflectivity measurements of layer-by-layer films at the solid/liquid interface

In this Letter, we present a method for the decoration of layer-by-layer (LbL) structures by heavy metal ions, which allows X-ray reflectivity (XRR) measurements at the solid/water interface. The improved contrast has allowed us to obtain well-structured X-ray reflectivity curves from samples at the liquid/solid interface that can be used for the film structure modeling. The developed technique was also used to follow the formation of complexes between DNA and the LbL multilayer. The XRR data are confirmed by independent null-ellipsometric measurements at the solid/liquid interface on the very same architectures.     

Simultaneous calorimetric and optical reflectivity measurements of extremely thin liquid crystal films

Abstract

An extremely sensitive measuring system has been developed to obtain simultaneously heat capacity and optical reflectivity data from thin free-standing liquid crystal films. Results from 3(10)OBC near the smectic A to hexatic B transition will be reported and discussed in the context of recent theory regarding phase transitions in two dimensions.     

Nonlinear optical properties of thin organic films

A brief survey is given on our recent work on the linear and nonlinear optical properties of thin films of polymers with a conjugated π–electron system. Third harmonic generation and degenerate four wave mixing experiments are used to study magnitude and response time of the third order nonlinear optical susceptibility of poly(p-phenylenevinylene) and poly(phenyl acetylene). Phthalocyanine derivatives show a strong influence of the intermolecular electronic coupling on the response time of transient gratings. The influence of energy transfer on the relaxation process is discussed.     

X-ray reflectivity study of polymer assembly at air-water interface

X-ray reflectivity (XR) measurements were carried out for amphiphilic diblock copolymer monolayers on water surface. From XR data, the layer thickness, and surface and interface roughnesses could be determined as a function of surface pressure. The XR experiments were performed using an ‘Air–Water Interface X-ray Reflectometer’ with conventional X-ray source (Cu target) in our laboratory. LB trough was equipped to the reflectometer, so the in situ measurements could be carried out for spread monolayers on water surface at different surface pressures. Kiessig fringes were observed for specular measurement for amphiphilic diblock copolymer, poly(α-methylstyrene)-poly(decyl 4-vinylpyridine) (P(α MSt)-b-P(4VP-C₁₀H₂₁I)) monolayers on water surface. It was observed that the thickness of the monolayer became thicker with increasing surface pressure. By curve fitting for the data obtained for the monolayer at surface pressure of 37 mN m^-1, the thicknesses of P(αMSt)₅₀ and P(4VP-C₁₀H₂₁I)₅₀ layers was determined to be 21 and 22 Å, respectively. Because the chain length of P(αMSt)₅₀ is calculated to be 126 Å in all-trans conformation, it was indicated that the molecules did not get aligned even when they were pressedto become such a dense state.     

X-ray diffraction study of TiO2 thin films on mica

Thin layers of polycrystalline TiO₂ were deposited on flaky muscovite and phlogopite particles. Deposition was performed in water slurry using aqueous TiCl₄ as titanium source. The effect of heat treatments on the structure of TiO₂ thin films was investigated at different layer thicknesses. On muscovite mica the films crystallized as anatase TiO₂. On phlogopite mica the increasing layer thickness favored partial rutile formation at higher calcination temperatures, although the films could not be converted to pure rutile. The preferred orientation of the TiO₂ films was evident. The most intense reflections were measured from (004) and (105) planes. The crystallite size of TiO₂ was strongly dependent on calcination temperature and TiO₂ layer thickness. It varied between 15 and 47 nm for films deposited on phlogopite, and 15 and 57 nm for films deposited on muscovite.     

X-ray study of CVD aluminium thin films deposited on silicon and quartz substrates

The structure of aluminium films deposited by chemical vapour deposition (CVD) method from aluminium triethyl on amorphous quartz and monocrystalline silicon substrates has been studied. X-ray data processing shows that the broadening of the diffraction maxima is due both to the reduced dimensions of the crystallites and microtensions appearing in the thin films. The deposition temperature influences the surface of the thin film and the value of lattice parameter. A simple model is proposed to explain cracking of Al thin films. Aluminium thin films deposited by CVD method show a smoother surface than Al films obtained by vacuum evaporation as revealed by X-ray diffraction topography patterns.     

X-ray photoelectron spectroscopy study of neodymium niobate and tantalate precursors and thin films

Neodymium niobate NdNbO₄ (NNO) and tantalate NdTaO₄ (NTO) thin films (~100 nm) were prepared by sol-gel/spin-coating process on Al₂O₃ substrate with LaNbO₄/PbZrO₃ interlayer and annealing at 1000°C. Surface chemistry was investigated by X-ray photoelectron spectroscopy (XPS). The core-level XPS studies of sol-gel NNO and NTO were performed for the first time. The binding energy differences Δ(O―Nb) and Δ(O―Ta) were used to characterize average energies of Nb―O bonding in NNO (322.9 eV) and Ta―O bonding in NTO (504.2 eV). The XPS demonstrated single valence state of Nd (Nd³⁺) in precursors. Nd concentration (at. %) decreases from 22% in precursors to 7% in films considering the substrate contains C, Al, Si, Pb, and Zr elements (37%) at Nb or Ta (5%) and O (51%). The X-ray diffraction analyses verified formation of the monoclinic (M-NdNbO₄ or M′-NdTaO₄), orthorhombic (O-NdNbO₄) and tetragonal (T-NdTaO₄) phases in precursors and films. Single valence state of Nd³⁺ was confirmed in these films designed for the application in environmental electrolytic thin film devices.     

A low-temperature X-ray diffraction study of the Cu2ZnSnSe4 thin films

Low-temperature XRD measurements were performed to confirm the phase composition and structural parameters of the electrochemically deposited Cu₂ZnSnSe₄ thin films on flexible metal substrates.     

Photoelectrochemical study of organic–inorganic hybrid thin films via electrostatic layer-by-layer assembly

The stepwise assembly of negatively charged organic molecules (poly(sodium 4-styrenesulfonate) (PSS) or tetrasodium-meso-tetra(4-sulfonatophenyl) porphine (TPPS)) and positively charged TiO₂ colloids on pretreated substrate surfaces utilizing the layer-by-layer (LbL) approach was investigated. The step-by-step formation of these films was studied by UV–vis spectrophotometry and electrochemistry. Photocurrent was generated upon light irradiation of the hybrid thin films assembled on fluorine-doped tin oxide (FTO) conducting glass, which increased linearly as the deposited bilayers increased. In addition, compared to PSS/TiO₂ hybrid thin films, the enhancement of the generated photocurrent and the photocurrent response within the wavelength range from 400 to 450 nm were observed in the TPPS/TiO₂ hybrid thin films. This was attributed to the dye-sensitized effect of the layered TPPS molecules. It was demonstrated that electrostatic LbL films were attractive systems for the photoelectrochemical investigation, and the control of the generated photocurrent could be achieved by the structure of the multilayered films.