Formation of Langmuir-Blodgett films in solutions of comblike polymers

The structures have been analyzed of the monolayers of comblike precursor polymers of polyimides and mixed cellulose esters formed at the water/air interface and of the Langmuir-Blodgett films obtained by transfer of these condensed monolayers onto solid substrates. The important factors that ensure the structure control and supramolecular organization of these monolayers and films are established.     

NMR and X-ray Investigations of a Plastic Crystalline Phase in MBBA

Abstract

Wideline and high resolution NMR studies have been carried out on MBBA in its isotropic, nematic and solid phases. Isotropic and nematic phase spectra correspond to what has been reported earlier. In the solid phase, contrary to expectations, very intense narrow signals similar to signals of the isotropic phase have been observed for the first time at temperatures close to the solid ? nematic phase transition temperature. This indicates rapid reorientational or translational motion in the system. The X-ray results however confirm the existence of translational order. The results are interpreted as indicative of the existence of a plastic crystalline phase in MBBA.     

Highly Electrical Conductivity of Hybrid Langmuir-Blodgett Films of Transition Metal Dichalcogenide and Amphiphilic Compounds

Abstract

Hybrid alternate layered films of transition metal dichalcogenides and amphiphilic compounds were prepared by Langmuir-Blodgett (LB) technique. The conductivity at room temperature depended on the transition metal dichalcogenide species, showing the highest value for the hybrid LB films of MoS₂ system. This methodology was successfully applied to the fabrication of the hybrid LB films using various organic amphiphiles such as cyanine dyes, phthalocyanines, azobenzene, and ferrocene derivatives. The conductivity depended on the interlayer spacing: the conductivity decreased with increasing interlayer spacing of the film. The highest electrical conductivity of the hybrid LB films was over 100 Scm^?.     

Orientation of Small Molecules in a Nematic Liquid Crystal as Studied by Electron Spin Resonance

Based on the measurements of hyperfine splitting constants (hfsc) of small aminoxyl (nitroxide) radicals in the nematic phase of N-4-methoxybenzylidene-4'-n-butylaniline (MBBA) we discussed the relation between the molecular orientation and the molecular shapes, especially the contribution of the phenyl groups to the molecular alignments.     

Two-dimensional crystallization of proteins on lipid monolayers at the air–water interface and transfer to an electron microscopy grid

The two-dimensional (2-D) crystallization of proteins on lipid monolayers at the air–water interface is a well established method for crystallizing soluble proteins. The transfer of 2-D crystals from the air–water interface to an electron microscopy (EM) grid constitutes a critical and ill-controlled step in the whole procedure, which is likely to be responsible for the high variability of results obtained with this method. In this paper, we address the following questions: (1) does the material observed on EM grids constitute a true representation of the material present at the air–water interface? (2) is there an optimal method of transfer to obtain well-ordered protein 2-D crystals? To answer these questions, we combine data obtained on three different protein systems, annexin V, streptavidin and cholera toxin, using two types of EM grids, coated with either holey carbon films or continuous carbon films. These combined observations help us draw a coherent picture of the state of the interfacial films at the air–water surface and provide new insight into the perturbing influence of the transfer step. The main conclusions are: (1) both annexin V and streptavidin form crystalline monolayers at the air–water interface, which are well preserved when transfer is performed by means of holey carbon films; (2) a major reorganization of the material present at the water surface accompanies transfer with continuous carbon films; the basal monolayer is extensively damaged, transforming into domains and vesicular structures, which do not pre-exist at the water surface; with the three protein systems studied here, these domains are often crystalline; (3) the most striking structural reorganization induced by transfer with continuous carbon films is observed with annexin V, for which the native p6 crystalline assembly is transformed into another crystal form more ordered, with p3 symmetry. It is most probable that these conclusions also apply to other protein 2-D crystals formed by the lipid monolayer method. The recent in situ observation of 2-D crystals of annexin V formed on solid-supported bilayers, by atomic force microscopy, supports our interpretation that monolayers transferred with holey carbon films represent the genuine material pre-exisiting at the air–water interface.     

Epitaxial growth of magnetic semiconductor EuO on silicon by molecular beam epitaxy

Functional oxides demonstrate a wide range of magnetic, optical and transport properties. Their integration with silicon promises significant advances in electronics. An important key in enabling brand‐new oxide technologies is the utilization of silicon/oxide epitaxy, thus making quality of the interface a critical issue. The progress depends on our ability to avoid formation of impurity phases at the interface and to tackle structural mismatch of the oxide and Si. We design a novel chemical protection of Si (001) surface on the submonolayer scale based on the surface metal silicide with the (1×5) reconstruction. This new technique is applied to the long‐standing problem of integration of a ferromagnetic semiconductor with Si. Direct epitaxial growth of EuO on Si without any buffer layer, so far inaccessible, is achieved by molecular beam epitaxy. The nucleation step, comprising first 10 monolayers of EuO, is followed by a distillation‐controlled growth. An alternative to standard capping procedures for EuO, based on controlled formation of an amorphous Eu₂O₃ layer, is devised. Crystal perfection of the films is established ex situ by x‐ray diffraction and Rutherford backscattering. Magnetic properties of the EuO films match those of the bulk.     

Birefringence of binary liquid crystalline mixtures of MBBA and PPMAECOBA in TCM,interferometric assessment

Abstract

The mutual influence of two mesogenic components, namely a nematic liquid crystal N-(p-methoxy-bezylidene)-butylaniline (MBBA) and a lyotropic liquid crystal poly-(phenyl-methacrylic) ester of cetyloxybenzoic acid (PPMAECOBA) in tetra-chloromethane (TCM) is studied in this paper. The ordinary and extraordinary refractive indices of the mixtures, nematic liquid crystal and lyotropic liquid crystal were measured at 589.3?nm with a Rayleigh interferometer and some electro-optical parameters were computed. The main refractive indices, the birefringence, the main normalized polarizabilities and their difference are dependent on the mixture volumetric concentrations, proving the existence of the collective orientational interactions between the two types of liquid crystals.     

Linear Optical Properties in Multilayers of Polydiacetylenes

Abstract

The linear optical properties in multilayers of polydiacetylenes (PDA) have been investigated by studying both the transmission/absorption coefficients and the waveguide properties of the thin PDA films (?5000 A). The PDA films were deposited one monolayer at a time by the Langmuir Blodgett technique. The waveguide modes of the PDA waveguide (in an air - PDA film - silver configuration) were coupled via a grating at the air-PDA and/or PDA-Ag interface. Since the diacetylene chains are parallel to the interface, the TE wave/ waveguide modes (i.e., the electronic field is parallel to the PDA film), the system can be treated simply as an isotropic case. For TM wave/waveguide modes, the system has to be treated as an anisotropic one. The results of wavelength dependent linear optical property of PDA films and their impact to the waveguide properties as well as the grating coupling effects in the waveguide system will be discussed.     

Photocatalytic properties of TiO2/ZnO thin film

ABSTRACT

TiO₂, ZnO and ZnO/TiO₂ thin films have been prepared by radio frequency magnetron sputtering method under different temperatures. Their photo catalytic activities have been investigated. The structural of the thin films were characterized by X-ray diffraction and Raman spectroscopy. The photo catalytic activities of TiO₂ and ZnO/TiO₂ samples were evaluated by the photo decomposition of methylene blue. We note that the structural proprieties of the thin films showed a perfect crystallization along the (002) for ZnO, Rutile (110) for TiO₂ and Anatase (101) for TiO₂. The experimental results show that the bilayer ZnO/TiO₂ were the most efficient photo catalysts compared to the layer of TiO₂. This increased catalytic effect can attributed to the interface between the ZnO layer and the TiO₂ one, which modify significantly the chemical potential of the bilayer.     

Behavior of poly(ε-caprolactone)-based diblock copolymers in a bulk state and in solution

Abstract

A series of amphiphilic MOPEO-b-PCL copolymers (DBCs), based on biocompatible methoxypoly(ethylene oxide) with M_n=4.5?kDa and poly(ε-caprolactone) of a variable chain length, was synthesized by an anionic ring-opening block copolymerization. The structural investigations, performed by DSC and WAXS methods, revealed the microphase separation in DBC bulk structure and the existence of separate amorphous regions and microcrystalline domains of MOPEO and PCL blocks. Spectrophotometry and SLS were used to study the self-assembling of DBC macromolecules in selective dioxane/aqueous solution and to determine the main micellization parameters (CMC and -ΔG°). The DBC micelles morphology and their specific aggregation in mixed solvent were shown.     

Photostimulated changes of properties of CdTe films

The effect of illumination during the close‐spaced sublimation (CSS) growth on composition, structural, electrical, optical and photovoltaic properties of CdTe films and CdTe/CdS solar cells were investigated. Data on comparative study by using X‐ray diffraction (XRD), scanning electron microscopy (SEM), absorption spectra and conductivity‐temperature measurements of CdTe films prepared by CSS method in dark (CSSD) and under illumination (CSSI) were presented. It is shown that the growth rate and the grain size of CdTe films grown under illumination is higher (by factor about of 1.5 and 3 respectively) than those for films prepared without illumination. The energy band gap of CdTe films fabricated by both technology, determined from absorption spectra, is same (about of 1.50 eV), however conductivity of the CdTe films produced by CSSI is considerably greater (by factor of 10⁷) than that of films prepared by CSSD. The photovoltaic parameters of pCdTe/nCdS solar cells fabricated by photostimulated CSSI technology (J_sc = 28 mA/cm², V_oc =0.63 V) are considerably larger than those for cells prepared by CSSD method (J_sc = 22 mA/cm², V_oc = 0.52 V). A mechanism of photostimulated changes of properties of CdTe films and improvement of photovoltaic parameters of CdTe/CdS solar cells is suggested. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Morphological,structural and electrical investigations on non-polar a-plane ZnO epilayers

We report on the growth of non-polar a-plane ZnO by CVD on r-plane-sapphire-wafers, a-plane GaN-templates and a-plane ZnO single-crystal substrates. Only the homoepitaxial growth approach leads to a Frank–van-der–Merwe growth mode, as shown by atomic force microscopy. The X-ray-diffraction spectra of the homoepitaxial thin films mirror the excellent crystalline quality of the ZnO substrate. The morphological and the structural quality of the homoepitaxial films is comparable to the best results for the growth on c-plane ZnO-substrates. The impurity incorporation, especially of group III elements, seems to be reduced when growing on the non-polar a-plane surface compared to the c-plane films as demonstrated by secondary ion mass spectrometry (SIMS). Optical properties have been investigated using low temperature photoluminescence measurements. We employed capacitance–voltage measurements (C–V) to measure the background carrier density and its profile from substrate/film interface throughout the film to the surface. In thermal admittance spectroscopy (TAS) specific traps could be distinguished, and their thermal activation energies and capture cross sections could be determined.     

Generation and intermolecular trapping of a cage-annulated cycloalkylidenecarbene

Base-promoted reaction of 11-methylenepentacyclo[5.4.0.0^2,6.0^3,10.0^{5, 9}]undecan-8-one (5) with diethyl diazomethylphosphonate when performed in the presence of excess cyclohexene, resulted in the formation of the corresponding cycloalkylidenecarbene, 6, which subsequently was trapped in situ to afford 8-methylene-11-(7-bicyclo[4.1.0]heptylidene)pentacyclo-[5.4.0.0^{2, 6}.0^{3, 10}.0^{5, 9}]undecane (7, obtained in 44% yield as a mixture of exo, endo isomers). Subsequent reaction of 7 with dichlorocarbene (generated under phase transfer catalytic conditions) produced the corresponding mono- and di-:CCl₂ adducts [i.e., 8 (64% yield) and 9 (5% yield), respectively]. The structure of 9 was established unequivocally via application of single crystal X-ray analysis: Triclinic, P1¯, a = 6.276(2), b = 8.700(2), c = 18.550(3) Å, = 76.52(3), = 87.59(3), = 70.88(4)° Z = 2; D _calc 1.486 g cm^–3.     

Electronic states in a-Si:H/c-Si heterostructures

We have investigated PECVD-deposited ultrathin intrinsic a-Si:H layers on c-Si substrates using UV-excited photoemission spectroscopy (hν = 4–8 eV) and surface photovoltage measurements. For samples deposited at 230 °C, the Urbach energy is minimal, the Fermi level closest to midgap and the interface recombination velocity has a minimum. The a-Si:H/c-Si interface density of states is comparable to that of thermally oxidized silicon interfaces. However, the measured a-Si:H dangling bond densities are generally higher than in thick films and not correlated with the Urbach energy. This is ascribed to additional disorder induced by the proximity of the a-Si:H/c-Si interface and H-rich growth in the film/substrate interface region.     

Nematic-Isotropic Transition in MBBA and its Mixtures with 4-Methoxybenzaldehyde: Applications to the Purity Control of Liquid Crystals

The nematic-isotropic transition temperatures of MBBA(1) + 4-methoxybenzaldehyde(2) mixtures (where MBBA=N(4-methoxybenylidene)-4-n-butylaniline) for 0  x₂  0.06 and additionally the transition enthalpy of pure MBBA were measured by a calorimetric method. Straight lines were found for the T¹(x₂) and Tⁿ(x₂) curves respectively where T¹ and Tⁿ are the upper and the lower limiting temperatures of the transition range for a given x₂. The results are compared with literature data on similar systems and confirm the wellknown rule that the decrease of the nematic-isotropic transition temperature with increasing x₂ depends more strongly on the size and the shape than on the polarity of component 2. From a plot of δT [tbnd] T¹ - Tⁿ against x₂ the mole fraction of impurity in the original sample can be extrapolated.     

Further Experimental Evidence for the Wall Structure of the Flexoelectric Domains in Symmetrically Weakly-Anchored MBBA Layers

An attempt for measuring of the penetration non-critical length of MBBA flexoelectric surface-induced domains is made. The disappearance of the domains into subcritical regions is related to the strong nonhomogeneity of the electric field around the gap which is able to inhibit the flexoelectric domain formation. The disappearance of the electrohydrodynamic domains generated from the flexoelectric ones, on the other hand, shows their secondary character as well. The existence of the penetration non-critical length of the flexoelectric domains is further confirmed with the observation of the corresponding supercritical influence length of side walls or air bubbles.

The behavior of the flexoelectric domains in an additionally applied magnetic field reveals their wall structure and permits the determination of the value and sign of the two MBBA flexoelectric coefficients of bend e _3x and splay e _1z, when the value and sign of the total flexoelectric coefficient ( e _z, + e _3x) are known.     

CROSSOVER BETWEEN FLEXOELECTRIC STRIPE PATTERNS AND ELECTROCONVECTION IN HYBRID ALIGNED NEMATICS

Abstract

We report experimental and theoretical results on the flexoelectric instability and crossover between flexoelectric domains and electroconvection in a hybrid aligned nematic MBBA under d.c. voltage. At threshold a spatially periodic flexoelectric deformation in the form of longitudinal domains (along the planar director) was observed. With increasing voltage the director deviation out of the initial plane becomes clearly detectable. Observations of tracers show that hydrodynamic flow develops inside the flexoelectric stripe pattern in the form of circular orbits perpendicular to the domain stripes. With further increasing d.c. voltage electroconvection sets in in the form of drifting oblique rolls inside the flexoelectric domains.     

Electrical characterisation of SrTiO3/Si interfaces

Deposition of SrTiO₃ (STO) thin films by ultra-high vacuum rf magnetron sputtering was performed in order to produce high-quality STO/p-Si (1 0 0) interfaces and STO insulator layers with high dielectric constants. The deposition temperatures were in the range from room temperature to 550 °C. Capacitance-voltage (C-V) and conductance-frequency measurements showed that the dielectric constant of the films ranges from 55 to 120. C-V measurements on Al/STO/p-Si structures clearly revealed the creation of metal-insulator-semiconductor diodes. The interface state densities (D_it) at the STO/p-Si interfaces were obtained from admittance spectroscopy measurements. The samples deposited at lower temperatures revealed values of D_it between 2×10¹¹ and 3.5×10¹² eV⁻¹ cm⁻² while the higher temperature deposited samples had a higher D_it ranging between 1×10¹¹ and 1×10¹³ eV⁻¹ cm⁻². The above results were also well correlated to X-ray diffraction measurements, Rutherford backscattering spectroscopy, and spectroscopic ellipsometry.     

Photoelectrical properties of heterojunction devices based on transparent oxide semiconductors on silicon

In this study photoelectrical properties of heterojunctions based on transparent oxide semiconductors thin films are outlined. The structures consisted of thin films of TiO₂ doped with V and Pd (n-type semiconductor) and TiO₂ with Co and Pd (p-type semiconductor) on silicon were examined by means of current–voltage (I–V) measurements and the optical beam induced current (OBIC) method. I–V characteristics displayed a strong non-linear (diode-like) behavior of prepared heterojunctions. The OBIC examinations enable for the comprehensive analysis of photocurrent generated at the microregion of electrically active areas at the interface of fabricated heterojunctions.     

VERTICALLY SUSPENDED SMECTIC FILMS WITH IN-PLANE TEMPERATURE GRADIENTS

Abstract

We present a study of vertically suspended smectic films under the influence of temperature gradients in the film plane. It is shown that such gradients lead to the transport of smectic material, even against the action of the gravitation forces, from the hot to the cold film edge. In addition, we observe thermally driven convection in these films, and it is demonstrated that the surrounding air plays an essential role for this instability. We compare this result with convective patterns in soap films reported by Martin and Wu in Phys. Rev. Lett. 80 1892 (98) and give some evidence that their interpretation has to be modified, the contact with surrounding air has to be taken into account.