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Magnetite (Fe 3 O 4 ) nanostructures with different morphologies including uniform nanoparticles, magnetic beads and nanorods were synthesized via a co-precipitation method. The synthesis process was performed at various temperatures in the presence of polyvinyl alcohol (PVA) at different concentrations. It is shown that small amounts of PVA act as a template in hot water (70 °C), leading to the oriented growth of Fe 3 O 4 nanorods, which was confirmed by selected area electron diffraction. Individually coated magnetite nanoparticles and magnetic beads were formed at a relatively lower temperature of 30 °C in the folded polymer molecules due to the thermo-physical properties of PVA. When a moderate temperature ( i.e. 50 °C) was used, nanorods and nanobeads co-existed. At higher concentrations of PVA (polymer/iron mass ratio of 5), however, the formation of magnetic beads was favored. The nanorods were shown to be unstable upon exposure to electron beams. Freezing/thawing process was applied post synthesis as temperature programming to fabricate stable nanorods with rigid walls.

Iron oxide γ -Fe 2 O 3 nanoparticles were synthesized in aqueous solutions and the equilibrium pressure–area isotherms of the Langmuir layers of stearic acid/ γ -Fe 2 O 3 nanoparticle complexes at the air–water interface were studied. In comparison to a pure water subphase, the surface pressure versus the specific area isotherm of the stearic acid was significantly expanded in the presence of the iron oxide nanoparticles in the subphase indicating the existence of interactions between the stearic acid monolayer and the iron oxide nanoparticles. Stearic acid/ γ -Fe 2 O 3 nanoparticle complex monolayers were transferred onto ZnSe and quartz plate substrates from the air–water interface using the Langmuir–Blodgett (LB) technique and the LB films were characterized with FTIR and UV-vis spectroscopy. From the FTIR studies it was observed that complexation between γ -Fe 2 O 3 nanoparticles and stearic acid occurred by electrostatic interactions between the carboxylate ions of stearate and the nanoparticle surface. Using UV-vis spectroscopy, the optical absorbance increased linearly with the number of complex LB layers, which indicated that the multilayer complex LB films were deposited uniformly.

Polyion complexes of arachidic acid and poly (ethyleneimine) were deposited onto Celgard 2400 microporous support membranes and the permeabilities of carbon dioxide and nitrogen through these composite membranes were measured at elevated temperatures. While nitrogen is relatively inert, carbon dioxide tends to have a high solubility in the hydrophilic polymer and the resulting difference in the permeabilities of carbon dioxide and nitrogen can be used to characterize solution-diffusion permeation. Room-temperature gas permeabilities were consistent with Knudsen diffusion through holes or defects. At elevated temperatures the permeabilities of carbon dioxide increased sharply while those of nitrogen increased only moderately indicating that the solution-diffusion mechanism became competitive with Knudsen diffusion. Room-temperature reflection and transmission IR spectra of films on separate substrates indicated that the alkyl tails of the fatty acid were oriented parallel with the surface normal and were packed hexagonally. At elevated temperatures, the tails and the carboxylate head groups disordered with increasing temperature. The disordering was correlated with the sharp increase in the carbon dioxide permeability.

Asymmetric membranes for gas separation were fabricated using the layer-by-layer adsorption process based on the spontaneous self-assembly of alternating layers of cationic and anionic polymers on porous and solid support membranes. The support membranes were first dipped in a dilute solution of poly (allylamine) (a polycation) followed by dipping in a dilute solution of poly (styrenesulfonate) (a polyanion). Repeating this process, 40 polymer layers were deposited on porous poly(propylene) membranes (Celgard 2400) and 100 layers on solid dimethyl silicone membranes. Gas-transfer experiments at several temperatures indicated reduced mass permeabilities due to the adsorbed films. Permeabilities of pure CO2 and N2 through coated Celgard samples were virtually equal at all temperatures indicating dominance of Knudsen diffusion through micropores in the film. Permeabilities in coated silicone membranes indicate higher CO2/N2 selectivity than the bare membrane at elevated temperatures. Microphotography indicated the presence of breaks in the polyion complex coating on the silicone membrane. Fabrication of gas transfer membranes via self-assembly of ionic polymers offers the possibility of designing highly selective membranes as well as the ability to control the thickness and architecture of films at the molecular level.

In this study the stability of Langmuir layers of tetradecyl benzoic acid (TDBA) interacting with poly (ethyleneimine) (PEI) in the subphase is analyzed as a function of PEI concentration, PEI molecular weight and surface pressure. Stable Gegenion complex, Langmuir monolayers were obtained in the PEI concentration range of 0.75 to 1.5 mM PEI monomer, and the surface pressure vs. specific area isotherms were independent of the PEI concentration. The isotherms were expanded indicating significant molecular interactions between TDBA and PEI. No effect of the molecular weight of PEI on the isotherms was observed. Outside this concentration range the Gegenion complex, Langmuir monolayers were either unstable or the isotherms were dependent on the PEI concentration. The structure of the Gegenion complex, Langmuir-Blodgett films were investigated using IR spectroscopy.

Monolayers of surface-active hemicyanine dyes have been extensively studied owing in part to their potential importance for fabrication into non-linear optical materials. In an effort to increase the dipole moment to a value larger than that present in the typical hemicyanine, a neew compound was characterized, having the structure: (CH 3 ) 2 NC 6 H 4 CHCHCHCHC 6 H 4 N + C 22 H 45 B − which contains a more extended π-bonded system compared with some more commonly studied hemicyanines. It is known that hemicyanines may be present in various aggregated states as well as species having varying degrees of order when in monolayer form. For the new compound, monolayers could be produced having both ordered and disordered structures. On a pure water subphase, the new hemicyanine in a highly compressed monolayer had a narrow, blue-shifted absorption band with a maximum at 435 nm. Such a spectrum is characteristic of H-aggregate species. When the subphase contained cadmium chloride (1 × 10 −3 M) the optical absorption band was similar to that seen in chloroform solution for disaggregated species, with a broad absorption band at 485 nm, shifting only slightly as the monolayer was progressively compressed. Atomic force microscopy visualizing the two different films indicated that monolayers transferred from pure water subphase appeared inhomogeneous, discontinuous and indicative of island structures possibly in the original spread monolayer, or formed during transfer. Monolayers formed on Cd 2+ -containing subphases were uniform except for some small surface defects.

Ferric stearate and ferric complexes were successfully deposited onto porous support membranes. Langmuir-Blodgett (LB) films of ferric stearate greatly reduced the permeabilities of carbon dioxide and nitrogen through the membrane. Above 65°C, a time-dependent change in the LB film leads to a permeability increase. Permeation through ferric stearate alone was mostly due to a Knudsen diffusion mechanism. LB films selective for carbon dioxide were made by co-depositing poly(2-vinylpyridine) with ferric stearate and with the ferric salt of poly(maleic anhydride 1-octadecene).

Second harmonic generation (SHG) is tested as an in situ technique for monitoring the properties of the Langmuir layer and the Langmuir-Blodgett deposition process. From compression-expansion of the Langmuir layer the SHG for a pure hemicyane dye increased sharply near a surface pressure of 20 mN m−1. For mixtures of hemicyanine with behenic acid the SHG increased sharply before any significant surface pressure was detected, indicating that the hemicyanine molecules are aligning at nearly zero surface pressure. Compression-expansion cycles of the Langmuir layer with hemicyanine suggest hemicyanine aggregate formation or two-dimensional crystal realignment. During Langmuir-Blodgett deposition the flow of the monolayer can be detected by both a surface pressure decrease and an SHG decrease.

Gegenion complex, Langmuir monolayers at the air-water interface, composed of poly(ethyleneimine) (PEI70000) and arachidic acid, ranging between pH 2 and pH 12, were prepared and the effect of the pH on the forming of the complex monolayer was investigated. Gegenion complex, Langmuir-Blodgett film depositions of varying pH on ZnSe wafers and transmission IR of the samples allowed analysis of the composition of the Langmuir-Blodgett film. The surfactant and polymer adhere to one another by means of protonation and deprotonation of the charged head group of arachidic acid and amine groups of PEI70000. By analyzing how the pH affects the interaction between the surfactant and the polymer, an optimum pH near 8 was determined, in which both the fatty salt and the polymer form the Gegenion complex, Langmuir layer at the air-water interface. For pH greater or less than 8, IR results of Gegenion complex, LB films show decreased adhesion of the polymer to the surfactant.

Polymeric ultrathin film systems need to be developed in the context of applications where their unique combinations of properties promise revolutionary improvements in performance or cost effectiveness. The applications examined include electron beam resists for microlithography and nanolithography, insulating films in semiconductor devices, non-linear optical elements and coatings for communications and computing, as well as highly permselective membranes in biotechnology. In this paper, we will review some of the most appealing suggestions and evaluate their current status. Improvements in film characterization and deposition based on Langmuir-Blodgett techniques are also suggested.

Second-harmonic generation (SHG) is used to probe the structure and order in monolayer and multilayer Langmuir-Blodgett (LB) films of highly polarizable dye molecules deposited on glass. Optical measurements on LB monolayers of hemicyanine-poly(octadecyl methacrylate) (PODMA) and hemicyanine-behenic acid mixtures in conjuction with data from compression isotherms show that the hemicyanine-PODMA system is immiscible and that the hemicyanine-behenic acid system is miscible. Incomplete Y-type LB multilayers of 54 mol.% hemicyanine in PODMA exhibited an erratic increase, which was less than linear, in the SHG with the number of layers deposited on the upstroke. Multilayer films of hemicyanine interleaved with behenic acid showed a slightly greater than quadratic increase in SHG with the number of hemocyanine layers deposited.

Asymmetric membranes were fabricated by depositing Langmuir-Blodgett (LB) films of polymeric lipids on porous supports. Three polymeric lipids were used for deposition. Up to 50 Y-type layers were deposited on one side of the porous supports of polypropylene (Celgard) and polytetrafluoroethylene (GORE-TEX) membranes. A marked decrease in gas transfer with increasing number of LB layers was observed for the asymmetric membranes fabricated with the porous polypropylene. Gas transfer in the asymmetric membranes using porous polytetrafluoroethylene remained unchanged with the number of polymeric layers. The LB multilayers on polypropylene showed no evidence for pores or cracks as determined by scanning electron microscopy (at a magnification of 40 000x), but the LB multilayers on polytetrafluoroethylene revealed large cracks. The gas permeabilities of nitrogen, methane and carbon dioxide in the polypropylene-based asymmetric membranes were a function of the molecular weight of the gas for two of the three polymers which suggests that micropores are present in the LB films. Heat annealing decreased the gas permeabilities from 0% to 40%, but only one polymer showed a change in selectivity.

We report second harmonic generation (SHG) in structures composed of optical fiber cores coated with Langmuir-Blodgett non-linear films. Such structures may serve as attractive alternatives to currently available coated slab non-linear waveguides. The fabrication of frequency doublers, couplers and modulators on optical fibers would eliminate the difficult problems of coupling from planar signal conditioners, sensors and detectors onto fiber transmission systems. In addition, the small size of fibers allows for the high energy densities required for non-linear interactions to exist over large distances. Our results confirm that fibers can be coated and the deposition ratio monitored, provided that a sufficient number are dipped simultaneously. The dependence of SHG on distance as well as the variation with number of layers for a hemicyanine dye deposited on multimode fiber cores are discussed.

The Proceedings of the National Science Foundation Workshop on the Molecular Engineering of Ultrathin Polymeric Films, published in this issue of Thin Solid Films, are reviewed to assess the current state of the art and future research needs. Based on the discussions at the workshop, a list of research needs in the molecular engineering of ultrathin organic and polymeric films is presented.

Langmuir-Blodgett (LB) multilayers of a hemicyanine dye in the Z-type configuration (head group towards substrate) have been made with and without interleaving layers of perdeuterated arachidic acid or cadmium arachidate. Polarized IR spectroscopy studies show that LB layers of hemicyanine without interleaving spacer layers lose their registration with the surface of the substrate after only a few layers are deposited. Hemicyanine layers interleaved with spacer layers maintain an approximately fixed orientation with respect to the surface. Deuteration of the interleaving layers permits IR differentiation of the hydrocarbon tails of the dye and the fatty acid or salt so that the orientation and order of each molecule can be separately measured. The choice of interleaving spacer layer, arachidic acid or cadmium arachidate, can be used to change the orientation of hemicyanine with respect to the surface.

Asymmetric membranes consisting of multilayers of polymeric lipids on a porous Teflon substrate were fabricated by the Langmuir-Blodgett technique. Fourier transform IR transmission spectroscopy was used to measure the level of molecular order in the n-alkyl side-chains of the polymeric lipids. The level of orientational order was monitored as a function of heat treatment temperature. We find that in the two polymer lipids studied, annealing below the bulk melting temperature leads to only a small decrease of order in the n-alkyl tails while removing a significant percentage of macroscopic defects. Heating to above the melting point results in an irreversible disordering of the alkyl chains.

Prototype capacitance sensors, composed of interdigitated contacts, have been fabricated and evaluated. Measurements during deposition in a Langmuir-Blodgett trough and measurements subsequent to multilayer deposition confirm that the technique is capable of monitoring film growth in real-time. Changes of 0.01 pF per monolayer of poly(methyl methacrylate) deposited onto glass were measured against a background of 58 pF. Identical results were obtained using both a capacitance meter and a spectrum analyzer. It appears that a factor of 10 in sensitivity can be readily obtained by reduction of the width of the sensor lines from 50 to 5 μm. This will permit the use of several sensors to monitor differential effects of deposition over the substrate. In particular, this technique is expected to permit accurate evaluation of the behavior of water during deposition and drying.

Multilayer Langmuir-Blodgett (LB) films of a new dye-substitutted polymer, dialkylaminostilbazolium poly(epichlorohydrin) are studied with second harmonic generation (SHG) and UV-visible absorption techniques. The synthesis resulted in a nearly quantitatively substituted stilbazolium chloride polyether (stilbazolium-PECH) with a “comb” structure. The dye can be compressed up to 47 mNm −1 to form a stable monolayer that is easily transferred to glass supports, and can be interleaved with behenic acid to form non-centrosymmetric structures. The second-order susceptibility of the stilbazolium-PECH is comparable in magnitude with that of a well-known hemicyanine dye and is thus potentially a useful material for fabricating films for non-linear optics. Multilayers of stilbazolium-PECH interleaved with behenic acid display enhancement of the SHG efficiency, although not quadratic. Unlike the hemicyanine dye, stilbazolium-PECH does not appear to form H aggregates in LB films. We have also investigated the effect of varying the number of behenic acid spacer layers between each pair of dye layers on SHG intensity and absorbance.

Deposition studies of mixed monolayers of poly(methylmethacrylate) (PMMA) and merocyanine chromophore (M22) were carried out by the Langmuir-Blodgett technique. The equilibrium surface pressure vs. specific area of the monolayers at the air-water interface was measured for several mixtures. The specific area vs. film composition at a fixed surface pressure deviated from linear behavior. The deposition of the monolayer onto a glass substrate was Z type for mixtures with high PMMA content, while Y type was preferred for mixtures with high dye content. The former structure exhibited second harmonic generation and should be of considerable interest in exploring the non-linear optical effects in Langmuir-Blodgett films. When prepared under a class 100 modular laminar flow hood and observed with reflection microscopy, the films showed no evidence of aggregate formation, suggesting that the PMMA-M22 mixtures were miscible.

An on-line capacitance technique has been employed for the detection of entrainment of the aqueous subphase during Langmuir-Blodgett deposition. This technique exploits the high dielectric constant of the subphase relative to air and to deposited monolayers. An interdigitated electrode structure was patterned on a substrate, and the capacitance was recorded throughout a dipping cycle. Electrodes were oriented parallel and perpendicular to the air-water interface during dipping. Subphase entrainment was detected by means of two indicators, drying time and the slope of the capacitance vs. time trace. Entrainment occurred during the deposition of all the monolayer materials employed, namely arachidic acid and n-docosyl homologues of merocyanine and hemicyanine. Comparable levels of entrainment observed for both orientations of the electrode indicate that entrainment is independent of topography. The dyes had characteristics different from those of arachidic acid. Poor deposition was obtained on the downstroke with partially dry substrates, indicating the strong influence of entrainment on the quality of deposition. This emphasizes the need for entrainment studies, and also the applicability of the capacitance technique in determining the necessary dwell times.