Monolayers Of Cadmium Arachidate Research Articles

Polarization Modulation Infrared Reflection Absorption Spectroscopy of a Cadmium Arachidate Monolayer at the Air/Water Interface

A polarization modulation infrared reflection absorption spectrum (PM-IRRAS) of a Langmuir monolayer on the water surface was constructed. The optimal incidence angle, the surface selection rule, a...

X-ray Scattering Studies of Mixed Monolayers of Tetrakis(3,5-di-tert-butylphenyl)porphinatocopper(II) with Cadmium Arachidate at the Air/Water Interface

The structures of mixed monolayers of cadmium arachidate (CdAr) with tetrakis(3,5-di-tert-butylphenyl)porphinatocopper(II) (TDBPC) (0.13 mole fraction) at the air/water interface have been studied by grazing incidence synchrotron X-ray diffraction (GIXD) and specular X-ray reflectivity. Only the hexagonal structure of CdAr was observed in the GIXD measurements, with d11 = 4.16 Å, giving an area per chain of 20.0 Å2. The chains are perpendicular to the subphase surface and the positional correlation length is 45 Å. For comparison, GIXD measurements of CdAr alone show a hexagonal in-plane structure with d11 = 4.13 Å and an area per chain of 19.7 Å2. The acyl chains in the film are perpendicular to the surface of the subphase and have a correlation length of 140 Å. Reflectivity measurements on the monolayer of arachidic acid on a CdCl2 solution confirm that the chains are normal to the surface of the subphase and suggest that the headgroups have the composition −COOCdOH. For the mixed film reflectivity measurements indicate that, at a surface pressure of 25 mN m-1, the porphyrin rings have a diameter of about 15.2 Å and are perpendicular to the surface. At 2 mN m-1 the rings are either tilted from the normal by about 28°, or normal to the surface leaving some areas of low density in the film. The two compounds are immiscible in the mixed monolayer, but the formation of CdAr domains is hindered by the presence of the porphyrin.

Evaluation of Anisotropic Photoresponses in Ultrathin Organic films by a Highly Sensitive Polarized Optical Waveguide Method

Highly sensitive polarized absorption measurement by the optical waveguide (OWG) detection system was made to analyze the orientation of photogenerated radicals in Langmuir-Blodgett (LB) films. The extinction coefficient changes upon photoexcitation of specific ion-pair charge-transfer complexes between two kinds of amphiphilic 4,4′-bipyridinium and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate were estimated by this system. Amphiphilic 4,4′-bipyridinium ions in LB films deposited directly on an OWG showed different anisotropy from those deposited on an OWG covered with three monolayers of cadmium arachidate. The structure of substituents also affected the orientation.

Organization in pure and alternate deuterated cadmium arachidate monolayers on solid substrates and at the air/water interface studied by conventional and differential Fourier transform infrared spectroscopies

The molecular organization in monolayers of deuterated cadmium arachidate deposited on solid metallic and dielectric substrates or spread at the air/water interface is investigated using different techniques of Fourier transform infrared spectroscopy. For metals, the polarization modulated reflectivity shows that the molecules have a high molecular order and are oriented quasi perpendicularly with respect to the surface. The in-plane organization is determined by complementary transmission experiments on CaF2 plates. The CD2 bending vibration in mono- and multilayer Langmuir–Blodgett stacks reveals the hexagonal packing of a single monolayer irrespective of the hydrophilic or hydrophobic character of the substrate. Probing the stack organization using selective monolayer deuteration in hydrogenated Langmuir–Blodgett films shows that the first deposited monolayer of an odd-numbered stack keeps its hexagonal structure while additional bilayers adopt an orthorhombic packing induced by the strong polar head/head interactions. However a monolayer may have an orthorhombic arrangement either if it is deposited on an orthorhombic commensurable substrate or if it is at a low temperature in which case intermolecular interactions are strengthened. Finally, differential reflectivity allows a single monolayer spread at the air/water interface to be studied. It is shown that such a Langmuir film is arranged in a two-dimensional hexagonal packing with its alkyl chains oriented perpendicularly with respect to the surface and its COO− polar groups symmetrically anchored at the interface.

Structure of Relaxed Monolayers of Arachidic Acid Studied by X-ray Reflectometry and Atomic Force Microscopy

Following the constant pressure relaxation on different subphases and the deposition on solid substrates, the structure and morphology of arachidic acid monolayers were studied by X-ray reflectometry and atomic force microscopy. The growth of three-dimensional (3D) phase nuclei was observed from the first minutes of the relaxation process in a monolayer relaxed on a subphase of low pH ≤ 3 with or without metal ions. With an increase in the relaxation time, the granule-like nuclei were transformed into platelike multilayer islands. The structure of 3D islands was commensurate with the B and C forms of bulk arachidic acid, with the B phase being prevalent after a long period of relaxation. During the storage of the samples in air, the granule-like islands were unstable and, after 2 months, transformed into the more stable platelike islands. The relaxation process was much slower in the case of binding the monolayer to cadmium or lead cations in a subphase of pH 5.3. After about 1 h of relaxation, only low folds arrayed with a hexagonal symmetry were observed on the monolayer of cadmium arachidate. On monolayers of lead arachidate, 1-2 bilayer islands emerged with hexagonal boundaries after a relaxation of some 3 h.

Grazing Incidence X-ray Diffraction Studies of Thin Films Using an Imaging Plate Detection System

First results of the grazing incidence X-ray diffraction patterns of cadmium arachidate Langmuir−Blodgett (LB) films measured using imaging plate detection are presented and discussed. The use of imaging plates has made possible the observation of diffraction peaks at far higher values of wavevector transfer than have been observed previously with other methods of detection, with a dramatic increase in signal-to-noise ratio. Values of in-plane d-spacing, area per molecule, and positional correlation length of the hexatic-B phase of a monolayer of cadmium arachidate are in excellent agreement with previous measurements. Multilayer LB films showed, in addition to the in-plane spots Q110 and Q200, sets of off-plane diffraction spots (Qhkl, l > 0) which are observed up to a Qz value of 3.3 Å-1. Spots with both odd and even values of l are observed.

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.

Polarization modulation FTIR spectroscopy at the air-water interface

Mid-infrared spectra of monolayers spread at the air-water interface have been obtained, completely devoid of strong water vapor absorptions, using polarization modulation infrared reflexion absorption spectroscopy (PM-IRRAS). On normalized difference (covered vs. uncovered water) PM-IRRAS spectra, the monolayer absorption bands appear upwards or downwards depending on the orientation of their transition moment with respect to the water surface. Vibrational modes of the water subphase contribute to these difference spectra as broad dips. Study of a monolayer of cadmium arachidate has allowed observation of the vibrational modes of the polar heads and provides some evidence of their symmetrical anchoring at the water surface. Under surface compression, a monolayer of deuterated arachidic acid undergoes a molecular reorganization leading to a better ordering of the deuterated chains.

Probing toward atomic resolution in molecular topography.

Probing toward atomic resolution in molecular topography.

Photovoltaic and electrical properties of aluminum/Langmuir-Blodgett films/silver sandwich cells incorporating either chlorophyll a, chlorophyll b, or zinc porphyrin derivative

The photovoltaic properties of sandwich cells of Langmuir-Blodgett films of chlorophyll a, chlorophyll b, and zinc porphyrin derivative have been measured. The rectifying devices were prepared by interposing the pigment multilayers between two semitransparent metal electrodes, i.e., aluminum and silver with work functions ([phi]) such that [phi][sub Al] < [phi][sub pigment] < [phi][sub Ag]. All sandwich cells exhibit small short-circuit photocurrents and large open-circuit photovoltages (as high as 1.1 V). The low-power conversion efficiency of all three pigments (0.001--0.030%) is related to the large internal resistance of the photovoltaic cells (30 to more than 1,000 M[Omega]). This suggests that the poor photovoltaic properties of the sandwich cells are extrinsic to the molecular structure of pigments and should rather be related to the presence of the insulating layers of aluminum and chrome oxide and of the monolayer of cadmium arachidate. In addition, a theoretical treatment of the action spectra is presented. The multiple internal reflections of light between the semitransparent metal electrodes strongly modify the photovoltaic properties of the sandwich cells. The characterization of the metal/semiconductor junction was carried out with the low-frequency oscillographic method. Large variations in the relations between 1/C[sup 2] and V were observed as a function ofmore » the light intensity, the frequency, and the amplitude of the applied bias. These variations have been related to the presence of the insulating layer which modifies the voltage applied to the junction. 49 refs., 11 figs., 1 tab.« less

Dansyldihexadecylamine as a polarity sensitive probe in air-water monolayers and Langmuir-Blodgett films immersed in aqueous solution

The emission spectrum of the polarity sensitive probe, N-(5-dimethylaminonaphthalene-1-sulfonyl)dihexa-decylamine, more commonly referred to as dansyldihexadecylamine (DDHA), was studied in several air-water monolayers and in Langmuir-Blodgett (LB) films submerged in aqueous solution. It was found that DDHA in air-water monolayers of cadmium arachidate (CdAa) existed in an aggregated state, whereas in oleic acid (OA) and dioleoylphosphatidylcholine (DOPC), it was homogeneously distributed with the dansyl headgroup residing in a polar environment. In contrast, the DDHA fluorescence spectrum of the LB films submerged in aqueous solution was significantly different to that observed in the respective air-water mixed monolayers. This indicates that the deposition process changed the environment sensed by the dansyl headgroup considerably. The effect of deposition speed and position of a fixed or freely suspended substrate on the environment surrounding the dansyl headgroup in the LB films was also investigated. The Förster energy transfer technique was employed to demonstrate translayer migration of DDHA through a CdAa multilayer structure immersed in an aqueous medium.

Characterization of Langmuir-Blodgett monolayers using polarization modulated FTIR spectroscopy

Structural and conformational analysis of cadmium arachidate monolayers deposited on a gold substrate has been performed on the basis of the well-defined spectra obtained by polarization modulated infrared reflection spectroscopy. This quantitative analysis gives accurate values for the tilt and twist orientations of the arachidate chains relative to the metallic substrate in agreement with a high organization of the LB molecules that decreases as the film goes to a single layer and gauche conformations appear.

Angular-Resolved ESCA Studies of Cadmium Arachidate Monolayers on Si (100): Inelastic Mean-Free Path and Depth Profile Analysis

AbstractAngular-resolved ESCA was used to study single cadmium arachidate monolayers transferred to Si (100) wafers by the Langmuir-Blodgett technique. We find the monolayers to be of high integrity with respect to those defects which enhance the escape probability of substrate photoelectrons through the overlayer. The inelastic mean-free pathlengths of Si (2p) and C (1s) electrons were calculated to be 49±6 Å and 45±6 Å for the kinetic energies of 1388 eV and 1202 eV, respectively. The overall ordering of the hydrocarbon chains is less than for alkane thiols assembled on noble metals. We find that the precision of the Tyler algorithm to deconvolute angular-resolved ESCA data into depth profiles is accurate within 10% for predicting the thickness of the hydrocarbon overlayer but less precise for intermediate layers.

InGaAs shallow junction fabrication using Langmuir–Blodgett film diffusion source

A new source of cadmium diffusion in In0.53Ga0.47As has been developed. Langmuir–Blodgett (LB) deposited monolayers of cadmium arachidate have been used as a source of cadmium. The LB film has been characterized by grazing incidence infrared spectroscopy and Auger electron spectroscopy. Acceptor profiles obtained by differential Hall technique are presented. Highly doped (NA =2×1019 cm−3) shallow (xj ≊0.1–0.4 μm), p+-n junctions are obtained. Mesa-type p-i-n diodes with 125 μm diameter, ideality factor =1.3, Idark =5 nA at 20 V reverse bias, and Vbreakdown =30 V have been fabricated.

Dispersion and localization of guided acoustic modes in a Langmuir-Blodgett film studied by surface-plasmon-polariton-enhanced Brillouin scattering.

We present the results of our surface-Brillouin-scattering measurements on a film of 18 monolayers of cadmium arachidate deposited on a Ag film by the Langmuir-Blodgett (LB) dipping technique. By using the Kretschmann attenuated-total-reflection geometry, the resonant-Brillouin-scattering measurements on the prism side are almost exclusively sensitive to guided acoustic modes that ripple the active interface between the Ag film and LB film. This is in contrast to conventional Brillouin measurements taken with light impinging directly onto the LB film, for which there are strong interference effects between the ripple and elasto-optic scattering mechanisms. The resonant Brillouin scattering is therefore used to investigate the localization of guided acoustic modes at the CdA/Ag interface. The hybridization of the dispersion relations between the guided modes of the LB film and the Rayleigh-Sezawa modes of the supported Ag film is investigated both experimentally and theoretically. We also study for the first time the angular dependence of the intensity of the surface-plasmon-polariton-enhanced Brillouin scattering for a dielectric-metal-prism system. The enhancement factor defined as the ratio of the Brillouin signal obtained at optimum coupling to surface plasmons to the signal away from the resonance is found to be greater than 15.

Temperature-dependent electron diffraction studies of cadmium arachidate monolayers and multilayers

Order and phase transitions of cadmium arachidate monolayers and multilayers were studied by electron diffraction. Both the monolayer and the 21-layer film undergo a thermotropic disordering at temperatures below the bulk melting temperatures known from differential scanning calorimetry measurements. This transition is marked by the disappearance of the peak intensities with the hexagonal diffraction pattern geometry remaining unchanged. The hexagonal diffraction pattern of the 21-layer film confirms epitactic deposition, and “hexagonal twinning” of the diffraction spots indicates chain tilt. The loss of intensity on heating is reversible for both the monolayer and the multilayers up to temperatures where the peaks fully disappear. Heating above these temperatures reorders the films partly into a polycrystalline structure.

Sensitive detection of protein adsorption to supported lipid bilayers by frequency-dependent capacitance measurements and microelectrophoresis

In the first part, we report experiments which enable the sensitive detection of protein adsorption to lipid bilayers deposited onto chromium electrodes on glass substrates by frequency-dependent capacitance measurements. The sensitivity of the present type of sensor (better than 0.3 nm average protein layer thickness) is at least equivalent to that of ellipsometry. A high specific resistance of the supported bilayer of (1-5).10(5) omega.cm2 is achieved by deposition of a tightly packed (crystalline) cadmium arachidate monolayer in contact with the substrate, whereas the outer monolayer can be more loosely packed (fluid phase or state of fluid-solid coexistence) which is essential for the incorporation of receptors. In the present work, charged lipids are incorporated as nonspecific receptors for polylysine and cytochrome c. The capacitance measurements provide a very sensitive test of the tightness and the long-time stability of the supported bilayers and, in combination with ellipsometric thickness measurements, enable estimations of dielectric properties of protein layers (such as the permittivity). In the second part, we report first electrophoresis experiments in asymmetric bilayers on substrates which enable simultaneous measurements of lateral diffusion coefficients and frictional coefficients between monolayers. The potential application of the electrophoretic effect for the differentiation between different receptors and the amplification of signals in biosensors is discussed.

Near-edge X-ray absorption fine structure studies of oriented molecular chains in polyethylene and Langmuir-Blodgett monolayers on Si(111)

C K shell near-edge X-ray absorption fine structure (NEXAFS) spectra of oriented hydrocarbon chains were measured in polyethylene and single Langmuir-Blodgett monolayers of arachidic acid, the calcium salt of arachidic acid, the cadmium salts of arachidic, octadecylfumaric and tetradecylbenzoic acids and a semifluorinated fatty acid on oxidized Si(111) surfaces. The observed NEXAFS resonances have been assigned to transitions to excited states localized on individual bonds, the presence of functional groups has been detected and the orientation of molecular segments relative to the surface has been determined. From a detailed analysis of the angular dependence of the various spectral peaks, the hydrocarbon chains of the cadmium arachidate monolayer were estimated to lie within 15° of the surface normal and the hydrocarbon chains of the calcium arachidate monolayer were estimated to be tilted by 33° ± 5° from the surface normal, while the arachidic acid film was not ordered at all. The determined chain orientations are discussed in terms of the surface bonding geometry of the terminal carboxylate group and the lateral interactions between the zigzag hydrocarbon chains. Similarly, a fumarate monolayer was found to be oriented perpendicular to the substrate while the benzoate and the semifluorinated fatty acid salt were more tilted.

Fourier transform IR spectroscopy of protonated cadmium arachidate layers in different positions of a perdeuterated multilayer

The grazing incidence reflection IR spectra of protonated cadmium arachidate monolayers and bilayers at different positions in a deuterated multilayer on a gold surface were studied. In this way the degree of disorder in a single layer can be determined qualitatively as a function of distance from the substrate surface. The characteristic ratios of the CH stretching band intensities are interpreted in terms of increased disorder or tilt of the first monolayer on the substrate and the uppermost layers of the stack.

The orientation of Langmuir–Blodgett monolayers using NEXAFS

Carbon K-shell NEXAFS (near edge x-ray absorption fine structure) spectra of oriented hydrocarbon chains in Langmuir–Blodgett (LB) monolayers were measured and used to study the orientation of these molecules. The LB monolayers were assembled from arachidic acid or cadmium or calcium arachidate on the oxidized Si(111) surface. The observed NEXAFS resonances are assigned to transitions to excited states which are localized on individual CH2 groups or C–C bonds. From a detailed analysis using curve-fitting techniques of the angular dependence of the various spectral peaks, the hydrocarbon chains of the cadmium arachidate monolayer is estimated to lie within 15° of the surface normal, the hydrocarbon chains of the calcium arachidate monolayer is estimated to be tilted by 33±5° from the surface normal, while the arachidic acid monolayer is not ordered at all. The determined chain orientations are discussed in terms of a microscopic model involving lateral interactions between the zig–zag hydrocarbon chains.