Radoslav Ionov

Preparation and characterization of amorphous SeTe/CdSe superlattices and their constituent thin layers

Abstract A method for the preparation of amorphous Se0.85Te0.15/CdSe chalcogenide superlattices by means of vacuum evaporation under 1.33 × 10-4 Pa is described. The small and large angle X-ray diffraction measurements, Raman spectroscopy, electron microscopy and diffraction are used for their characterization. The constituent layers are well defined with interfaces about two atomic layers thick. It is shown for the first time that the thermally evaporated CdSe layer converts into amorphous when two critera—for the layer to be sufficiently thin and to be enclosed between two amorphous Se0.85Te0.15 layers—are satisfied.

Control of the structure of Langmuir-Blodgett films of a discotic liquid crystalline compound via the subphase composition and the adjacent molecular environment

Abstract Changes of the ,olecular arrangement that can be induced by means of the LB technique in the multilayers of a disc-shaped multialkynyl amphiphile are monitored by means of small angle X-ray diffraction. Studies of the monolayers at the air-water interface reveal “edge-on” orientation of the discs. Specific effects of the counter-ions (Na + , Cd 2+ , Pb 2+ and Ba 2+ ) and sub-solution pH on the monolayer collapse pressure, transfer efficiency and molecular order in the multilayers are found. A correlation between the monolayer properties and the ability for formation of periodic discotic structures in the presence of divalent counterions is established. The discotic molecules retain their “edge-on” arrangement in the highly compressed transferred films with slight irregular interdigitation of the flexible wings and inclination to the substrate normal. The tilt and the interdigitation are reduced when the discotic monolayers are deposited in alternating LB films with barium arachidate spacer layers.

Head-to tail or head-to-head structure of Langmuir-Blodgett long chain ester multilayers?

Abstract The effect of the deposition velocity on the type of transfer (X-, Y- or XY-) of methylarachidate and methylbehenate monolayers is investigated. To provide conditions for deposition of monolayers, but not of three-dimensional aggregates, the effect of the temperature on monolayer stability at constant surface pressure is followed at first. It is found that the gradual increase of the deposition velocity results in a transition from X- to Y-type transfer of the layers. Transition from X- to Y-type deposition is also observed on increase of the number of deposited monolayers. Small angle X-ray diffraction investigation reveals repeated spacings of a bilayer (head-to-head) structure for Y-type deposition. Structural similarity is found for multilayers built up by either XY- or X-type transfer. The specific electron density profile of the methyl ester films might result in a misleading conclusion about head-to-tail molecular arrangement in the multilayers.

Structural properties of amorphous chalcogenide SeTe/CdSe superlattices

Abstract Structural perfection, periodicity and stability of novel amorphous chalcogenide SeTe/CdSe superlattices are investigated by means of small-angle X-ray diffraction. Interfacial roughness of about two atomic layers is estimated. Conclusions about the SeTe sublayer structures are deduced from the study of radial distribution function and Raman spectra of the superlattices.

Swelling of bilayer lipid membranes

Abstract Short-range repulsive (“hydration”) forces dominate the interactions of many bilayer membranes, as they approach a close contact, and play an important role in the membrane swelling. The physical origin of these forces has still not been completely understood, especially for ionic bilayers. This study presents an extension of the osmotic stress method for evaluation of the bilayer membrane swelling in humid environment. Zwitterionic dimyristoylphosphocholine (DMPC) and ionic dioctadecyldimethylammonium bromide (DODAB) multilayers are studied by means of low-angle X-ray diffraction and Fourier transform infrared spectroscopy. The effect of humidity on the swelling and organization of the DMPC and DODAB multilayers is compared. The physical origin of the established strong short-range repulsive forces in both lipid systems is discussed.

Equilibrium surface properties of lipid mixtures of retinal, phosphatidylcholine and fatty acid derivatives at the air/water interface

Abstract The miscibilities of phosphatidylcholine, retinal and saturated fatty acid derivatives in surface phases at the air/water interface are investigated on the basis of the thermodynamic two-dimensional phase rule. The latter is applied to the ‘collapse’ pressure and the equilibrium surface pressure characteristics of binary lipid monolayers or spread amphiphilic mixtures, respectively. The equilibrium surface pressures (ESPs), at which insoluble lipid monolayers are in equilibrium with three-dimensional lipid phases, are determined by spreading of single-component or binary solutions of lipids in organic solvent up to supersaturation at the air/water interface. The kinetics of establishment of steady surface pressure values at supersaturation is followed depending on the nature of the lipid samples. ESPs and ‘collapse’ pressures of mixtures of dilaur-oylphosphatidylcholine (DLPC), all- trans retinal ( t -R) and lauric acid (LA) are studied at various lipid molar ratios. The compositional phase diagrams of the ESPs and ‘collapse’ pressures, obtained at a constant temperature, indicate that the interfacial miscibilities of both DLPC and t -R and DLPC and LA are non-ideal. Owing to its ‘bulky’ molecular structure and the tendency towards self-aggregation, dominated by intermolecular π-π interactions, the t -R component could be accommodated in the hydrophobic portion of the phospholipid membrane at mole fractions less than 0.5. The accommodation of the other neutral, rod-like fatty acid component in the DLPC matrix is probably favoured by the formation of intermolecular hydrogen bonding. Phase separation between DLPC and LA is evident from the thermodynamic results at high LA mole fractions (> 0.75) in the surface mixtures.

Does the tail-to-head molecular arrangement exist in asymmetrically deposited fatty acid alkylester Langmuir-Blodgett multilayers?

Abstract A major problem in the X-ray diffraction investigations of asymmetrically deposited fatty acid alkylester Langmuir-Blodgett (LB) films is whether the molecular overturning to a symmetrical multi-bilayer arrangement could always be detected. The conclusion for the occurrence of molecular overturning is related to the specificity of the electron density profiles (EDP) of the ester multilayers. The problem arises from the similarity of the EDP of the “tail-to-head” and the “head-to-head” arranged alkylester films. To establish the true structural arrangement, two approaches are applied in the present study for modification of fatty acid alkylester film EDP: (i) by using a ω-bromopentadecanoic acid methyl ester (BP) as a “label” of the molecular arrangement in propyl stearate (PS) and methyl arachidate (MA) multilayers, and (ii) by mixing of PS and MA. The structural arrangements, normal to the solid substrate, of X-type deposited pure and mixed LB films of PS, MA, and BP are studied by means of low-angle X-ray diffraction. The pure PS and MA films are characterized by a same single-layer periodicity. The X-ray patterns of mixed MA/PS films and of PS films, labeled by relatively low BP molar fractions, evidence a bilayer periodicity (head-to-head arrangement) of these fatty acid alkylester multilayers. Therefore, the multilayers studied possess symmetrical multi-bilayer structures, which could be detected by means of X-ray diffraction only under particular conditions.

Interface-induced structural changes in amorphous chalcogenide superlattices

Abstract Phonon modes of amorphous selenium and their corresponding wavenumbers are systematized. The structural modifications induced by the interfaces and tellurium additives in the sublayers of SeTe CdSe superlattices are studied by means of IR and Raman spectroscopy. The chain orientation in SeTe sublayers and the influence of CdSe sublayer amorphization on the longitudinal optical phonon band width are examined.

Comment on “Molecular organization of amphiphilic disc-shaped penta-alkynes in LB mono- and multilayers”, D. Janietz, D. Hofmann and J. Reiche, Thin Solid Films, 244 (1994) 794

Abstract The molecular arrangement of disc-like penta-alkynes in Langmuir-Blodgett films is discussed on the basis of the deposition conditions and the X-ray diffraction data. It is concluded that the films deposited at low transfer ratios exhibit the same crystalline structure as the bulk discotic materials, while those obtained at transfer ratios higher than 0.5 are of a bilayer arrangement.

A new type of arrangement of disc-shaped molecules in Langmuir-Blodgett films

Abstract The molecular arrangement induced in Langmuir-Blodgett films of a disc-like mesogene is studied by means of various structural and optical methods. The disc-like molecules show an “edge-on” arrangement with a bilayer periodicity normal to the solid substrate and a nematic arrangement in the monolayers planes.