Films Of Cadmium Arachidate Research Articles

FTIR Study of the Changes in Orientation and Chemistry of Langmuir-Blodgett Films of Cadmium Arachidate during the Formation of Q-State CdS Particles

FTIR Study of the Changes in Orientation and Chemistry of Langmuir-Blodgett Films of Cadmium Arachidate during the Formation of Q-State CdS Particles

Photoelectron and NEXAFS microscopy of radiation induced changes in cadmium arachidate films

The radiation induced changes caused by both low energy Ar + ion bombardment and high energy H + ion bombardment on Langmuir-Blodgett (LB) films of cadmium arachidate were investigated using a commercial photoelectron microscope installed at the German synchrotron facility BESSY. For the low energy situation, XPS imaging was performed using damage induced differential charging as the contrast mechanism. The high energy H + bombardment produced less damage and no resolvable charging shift, so this contrast mechanism cannot be used for imaging. However, chemical shifts in the C NEXAFS (near edge X-ray absorption fine structure), caused by the H + bombardment, are sufficiently large to use as a contrast mechanism. NEXAFS, of course, requires tunable radiation and hence spectromicroscopy in conjunction with synchrotron radiation offers the possibility for imaging chemical changes in organic films which are not observable using fixed energy laboratory sources. In principle, molecular orientation changes are also imageable, as the relative intensities of different features in a NEXAFS spectrum often vary with orientation.

Liquid to hexatic to crystalline order in Langmuir-Blodgett films.

Atomic force microscope images of zinc arachidate (ZnA2) Langmuir-Blodgett films show that three- and five-layer films are "hexatic," with long-range bond-orientational order and short-range positional correlations of three to five lattice repeats. The monolayer in contact with the substrate is disordered. Films of seven or more layers of ZnA2 are crystalline. A population of dislocations, most likely originating at the substrate, disrupts the positional but not the orientational order of the lattice, leading to hexatic layers intermediate between crystal and liquid. The influence of the substrate propagates farther into ZnA2 films than into cadmium arachidate films because the molecular cohesion is much weaker in ZnA2 than in cadmium arachidate, as evidenced by a less dense molecular packing.

Studies of the Formation and Growth of Q-State Cadmium Selenide Particles in Cadmium Arachidate Langmuir-Blodgett Films

The formation and growth of Q-state CdSe particles during exposure of Langmuir-Blodgett films of cadmium arachidate (CdAr) to hydrogen selenide has been studied using UV-visible absorption, grazing angle FTIR, X-ray photoelectron spectroscopy (XPS), and quartz crystal microbalance (QCM) measurements. UV-visible absorbance measurements showed an optical blue shift typical of particles between 1.1 and 1.3 nm in radius. In addition to the formation of Q-state CdSe particles, elemental selenium (formed by the decomposition of H 2 Se) was present in the films. Grazing angle FTIR and XPS measurements (after subtraction of the contributions due to elemental selenium) were consistent with Q-state particle formation involving quantitative conversion of the Cd 2+ ions in the films to CdSe and the corresponding conversion of CdAr to arachidic acid. Changes in the orientation of the surfactant molecules were also found to be a consequence of particle growth in the films. QCM measurements on films exposed to H 2 Se showed large apparent mass uptakes which were inconsistent with the trends observed using other techniques. This behavior was ascribed to anomalous interactions between H 2 Se gas and the QCM electrodes

Substrate Effects on the Structure of Langmuir-Blodgett Monolayers of Cadmium Arachidate Using X-ray Absorption Spectroscopy

NEXAFS (near edge X-ray absorption fine structure) spectroscopy at the C Is edge has been employed to study the molecular orientations in Langmuir-Blodgett (LB) films of cadmium arachidate deposited onto various substrates, silver, hydrogen terminated silicon, and silicon oxide. In contrast to other techniques (like infrared spectroscopy) NEXAFS spectroscopy allows for a direct comparison of molecular orientations in LB films deposited on the different substrate materials and reveals that the different substrates induce strongly different structural properties ofthe deposited films, although preparation conditions were identical. Whereas for the Ag substrate the monolayers show a virtually perpendicular orientation of the alkyl chains with respect to the surface, tilted and/or disordered chains were found for the differently pretreated silicon substrates. Films with three or more layers, however, show virtually perpendicular orientation of the alkyl chains in the outermost layers irrespective of the substrate, indicating that substrate effects are mainly limited to the first LB layer.

The fabrication of titania Q-state particles in Langmuir–Blodgett films

Optical absorption and XPS analyses have shown that titania Q-state particles (<10 nm diameter) can be fabricated within arachidic (icosanoic) acid and cadmium arachidate Langmuir–Blodgett (LB) films by sequential exposure to gaseous titanium tetrachloride and water vapour; when a cadmium arachidate film was subsequently gassed with H2S, sulfur was not incorporated into the film; CdS was not formed.

The two-dimensional hexatic-B phase of single Langmuir-Blodgett monolayers observed by atomic force microscopy

Molecular resolution with the atomic force microscope has been achieved, on single Langmuir-Blodgett monolayers, using films of cadmium arachidate on mica (see also T. Kajiyama et al., Langmuir, 10 (1994) 1297). The images clearly show the molecular organization that has been postulated for the two-dimensional hexatic-B phase; the molecular chains are packed hexagonally with their long axes normal to the substrate surface and in small domains with their hexagonal patterns oriented in the same direction (known as bond orientational order) separated by regions of relative disorder. For the domains, the dimensions of the unit cell are in excellent agreement with data reported from X-ray and electron diffraction. There is some variation in film thickness, with the highly ordered hexagonal domains being slightly thicker than the regions showing dislocations and disorder. The monolayer films show a mismatch of about 10% with the mica lattice, indicating that the substrate structure does not determine the structure of the deposited monolayer.

X-ray photoelectron spectroscopy, attenuated total reflectance Fourier transform IR spectroscopy and transmission electron diffraction studies of oriented cadmium iodide prepared in a Langmuir-Blodgett template

Oriented CdI 2 has been prepared in a deposited Langmuir-Blodgett (LB) film by the reaction of gaseous HI with a 20-layer LB film of cadmium arachidate. Attenuated total reflectance Fourier transform IR spectroscopy shows that the conversion goes to completion, while X-ray photoelectron spectroscopy analysis of the HI-exposed cadmium arachidate film reveals a Cd:I ratio consistent with the CdI 2 stoichiometry. Electron diffraction shows that the CdI 2 is oriented exclusively with its [001] axis normal to the LB film basal plane and for domains ranging up to several microns has a preferred alignment about the [001] axis. The results suggest that the organic template plays a role in orienting the CdI 2 within the LB plane and that some degree of lattice matching exists between the LB template and the (001) face of CdI 2. The oriented synthesis described here is in contrast with previous attempts at template-directed syntheses using deposited LB films which have resulted in the mediation of particle size only and not particle orientation. The difference here may be that CdI 2 has a layered structure in the bulk, crystalline phase which complements the LB film structure.

Growth of cadmium sulfide particles in cadmium arachidate films: monitoring by surface plasmon resonance, UV-visible absorption spectroscopy, and quartz crystal microgravimetry

The growth of Q-state CdS particles during exposure of Langmuir-Blodgett films of cadmium arachidate to hydrogen sulfide has been monitored by surface plasmon resonance (SPR), UV-visible absorption, crystal microbalance (QCM) measurements. The growth kinetic profile, expressed in terms of increasing refractive index (SPR), absorption or mass increase (QCM), was the same when films were gassed under similar conditions. Microgravimetry showed the quantitative conversion of Cd 2+ to CdS in such systems; UV-visible absorption exhibited the blue shift typical of Q-state particles of 2-3-nm diameter

Electron energy loss and x-ray photoemission study of electron inelastic scattering in cadmium arachidate Langmuir Blodgett films

The inelastic scattering of electrons in Langmuir–Blodgett films of cadmium arachidate [(C19H39COO)2Cd] deposited on glass substrates has been studied by reflection electron energy loss spectroscopy (REELS) and by monitoring the loss features of the C 1s and O 1s core levels by x-ray photoemission spectroscopy (XPS). REELS studies with a primary beam of energy 95 eV revealed some vibrational modes of the cadmium arachidate molecules and features due to transition of valence electrons to the conduction band. Similar structures were also identified in the C 1s and O 1s loss spectra and interpreted as reflecting the density of states of the conduction band, in agreement with earlier findings using secondary electron emission spectroscopy. In contrast to the monolayer film, the 5 ML film showed reduced order as inferred from the lack of features in the loss spectra.

Quantitative lattice measurement of thin Langmuir-Blodgett films by atomic-force microscopy.

We have quantitatively determined the nature of the surface order of the outermost layer of monolayer and multilayer Langmuir-Blodgett films of cadmium arachidate, in air and under water, using atomic-force microscopy. Molecular-resolution images of 1200-${\mathrm{nm}}^{2}$ areas show that the alkyl chains of 2--5-layer films have a noncentered rectangular lattice with lattice constants of 0.482\ifmmode\pm\else\textpm\fi{}0.004 and 0.748\ifmmode\pm\else\textpm\fi{}0.006 nm, in good agreement with diffraction measurements of bulk aliphatic systems. We show unambiguously that the unit cell consists of two molecules. In contrast with the highly ordered multilayers, monolayer films have a disordered surface. Images taken under water of the head-group surface show that the bilayer surface is disordered, while thicker films show order of the alkyl chains. The presence of an underlying head-group--head-group interface with its associated cadmium ion bridging is shown to be the critical feature needed to obtain a stable and ordered surface layer.

Crystallinity of the Double Layer of Cadmium Arachidate Films at the Water Surface

A crystalline counterionic layer at the interface between an electrolyte solution and a charged layer of insoluble amphiphilic molecules was observed with grazing incidence synchrotron x-ray diffraction. Uncompressed arachidic films spread over 10(-3) molar cadmium chloride solution (pH 8.8) spontaneously form crystalline clusters with coherence lengths of approximately 1000 angstroms at 9 degrees C. Ten distinct diffraction peaks were observed, seven of which were attributed to scattering only from a crystalline Cd(2+) layer and the other three to scattering primarily from the arachidate layer. The reflections from the Cd(2+) layer were indexed according to a 2 x 3 supercell of the arachidate lattice with three Cd(2+) ions per cadmium unit cell.

SFG study of rotational anisotropy of cadmium archidate Langmuir—Blodgett films

Anisotropic orientation of Langmuir—Blodgett (LB) films of cadmium arachidate on fused silica is studied by infrared—visible sum-frequency generation (SFG). The results of rotational anisotropy in the SFG signal show that the terminal methyl groups are tilted along the withdrawal direction for the 9- and 3-monolayer films but not for the 1-monolayer film.

Modeling of fluorescence emission from cyanine-dye-impregnated Langmuir-Blodgett films deposited on the surface of an optical fiber

Fluorescence emission was collected from cyanine-dye-impregnated Langmuir-Blodgett (LB) films of cadmium arachidate deposited on the surface of a quartz multimode optical fiber. A Y-type fiber optic coupler system was used to allow remote, distal-end spectral excitation and collection as the fiber tip was repeatedly dipped into the LB trough. Built-up LB films of up to 400 LB layers were examined. Fluorescence emission intensities were modeled as a function of the number of layers, or LB film thickness, using a simplified theory based on planar thin film optics. The fit of theory to experiment was qualitatively satisfying, indicating that LB films of reasonable optical quality can be deposited on the surface of cylindrical optical fibers.

Grazing Incidence X-Ray Reflectometry Studies of Cadmium Arachidate Langmuir-Blodgett Films

ABSTRACTGrazing incidence X-ray reflectometry has been used to characterize Langmuir-Blodgett films of cadmium arachidate deposited on silicon substrates. The agreement between layer parameters deduced from the interference fringe period and low angle Bragg peak positions was excellent. Good agreement was found between experimental and simulated reflectivity profiles only when interface roughness and a varying molecular layer thickness was included. Inclusion of interface roughness alone results in a substantial enhancement in the intensity of the Bragg peaks. This effect is identified as being equivalent to the reduction in extinction found in classical X-ray diffraction due to crystal imperfections.

Molecular-resolution images of Langmuir–Blodgett films using atomic force microscopy

THE ability to prepare thin films of amphiphilic molecules (Langmuir–Blodgett (LB) films) is valuable to many areas of research. In biology they provide models for ideal membranes; the two-dimensional behaviour and structural phase transitions are of fundamental interest in surface physics; and their tribological characteristics suggest potential engineering applications. For determining the structure of these films, the common techniques such as X-ray and neutron scattering are limited to thick ( ≳200 A) multilayers. Thinner films can be studied by transmission electron microscopy and low-energy electron diffraction1,2, but these electron-beam techniques tend to damage thin films. More recently, the scanning tunnelling microscope3 has provided a non-destructive means of investigating the structures of LB films4–6, but as the films are insulating, the interpretation of such images has been controversial. The atomic force microscope7 is not plagued with these ambiguities, as it does not require a conductive sample. Here we present images, with molecular resolution, of LB films of cadmium arachidate deposited on an amorphous silicate substrate. Despite the disorder in the substrate, the films display a periodic structure over large distances (several hundreds of angstroms). This suggests that the adsorbed molecules near the interface are driven to self-assemble primarily, if not solely, by intermolecular forces rather than by dependence on substrate periodicity.

Direct Observation of Cadmium Arachidate Thin Films with Lateral and Normal Molecular Orientations by Superconducting Cryo-Electron Microscopy

Thin films of cadmium arachidate with two different molecular orientations were prepared by controlling the deposition conditions and were directly observed by a superconducting cryo-electron microscope (JEOL JEM-2000SCM). In the case of the rod-like crystals consisting of molecules parallel to the surface, the lattices of 5 nm in width which correspond to the (001) spacing were clearly observed at an electron dose of 1 × 10 3 e nm −2 with specimen cooling to 4.2 K; the plate-like crystals with molecules orienting normal to the substrate surface, after being irradiated by electrons of 1.0 × 10 3 e nm −2 at 4.2 K, changed from orthorhombic phase to hexagonalo one and then above electron dose of 1.7 × 10 3 enu −2 became amorphous states

Atomic force microscope studies of lubricant films on solid surfaces

The tribological properties of Langmuir-Blodgett films of cadmium arachidate deposited on silicon wafers have been studied as a model boundary lubricant. An Atomic Force Microscope (AFM) was used to measure the effect of a single molecular layer on the frictional force. Wear experiments on multilayers of cadmium arachidate indicate that the upper layers are easily worn away with the AFM tip, but the layer bonded to the SiO x-Si(111) substrate is not removed under the loads (< 2 x 10 −7N) used in our experiments.

Surface decoration of Langmuir-Blodgett films with evaporated silver islands

Silver was evaporated on Langmuir-Blodgett (LB) films of cadmium arachidate and phthalocyaninato-polysiloxane. Electron microscopy and UV-visible- near IR absorption spectroscopy show that the island formation and the sticking probability of silver depend on the thickness of the cadmium arachidate films. The surface coverage and sticking probability of silver on the polymeric phthalocyanine is considerably higher on and independent of the LB film thickness. A qualitative explanation is based on the different film structures and morphologies. Therefore the evaporation of silver on various surfaces and the formation of metal islands with their characteristic optical absorptions can be used to visualize different surface properties.

Electron affinity and structure of Langmuir-Blodgett films of cadmium arachidate by means of low-energy electron transmission

The results of low-energy electron transmission experiments on Langmuir-Blodgett (LB) films of cadmium arachidate deposited on copper substrates precoated with ferric stearate are described. The energy location of the bottom of the conduction band ( V 0) for the ordered alkyl tails in the LB film was directly determined to be 0.46±0.02 eV above the vacuum level at −120 to −130°C, that is, the electron affinity ( V A = − V 0) is −0.46±0.02eV. Further, it is demonstrated that the band structure is significantly blurred by the thermal excitation of molecular vibrations and by poor crystallinity. The crystallinity of the films is dependent on the number of layers deposited, and the results are consistent with those obtained by angle-resolved photoemission experiments. The present results strongly suggest that the electron affinities (2.25 and 2.3 eV) obtained by other workers for a metal/monolayer/metal junction, using fatty acid salts, are not those of the ordered parts of the alkyl tails.