Cadmium Arachidate Multilayers Research Articles

Growth and Structural Study of Cadmium Arachidate Multilayers Deposited by LB Technique

Langmuir–Blodgett (LB) multilayer films of cadmium arachidate acid (CdAA) have been studied by X-ray diffraction and X-ray reflectivity techniques, which reveal uniform ordered packing of CdAA molecules with a small tilt angle of long alkyl chain equal to 6.8º. Electron density profiles reveal that the coverage of films remains almost constant with increase in bilayer thickness, which indicates very little presence of pinhole defects. FTIR studies confirm orthorhombic packing in CdAA LB films and the formation of unidentate bridging metal-carboxylate coordination type headgroups consistent with one cadmium metal ion between two carboxylic (COO) groups in each headgroup.

Crystalline Growth of Rubrene Film Enhanced by Vertical Ordering in Cadmium Arachidate Multilayer Substrate

The growth of highly crystalline rubrene thin films for organic field effect transistor (OFET) application remains a challenge. Here, we report on the vapor-deposited growth of rubrene films on the substrates made of cadmium arachidate (CdA) multilayers deposited onto SiO2/Si(100) via the Langmuir-Blodgett technique. The CdA films, containing 2n+1 layers, with integer n ranging from 0 to 4, are surface-terminated identically by the methyl group but exhibit the thickness-dependent morphology. The morphology and structure of both CdA and rubrene films are characterized by X-ray reflectivity (XRR), X-ray diffraction (XRD), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and atomic force microscopy (AFM). Crystalline rubrene films, evidenced by XRD and marked by platelet features in AFM images, become observable when grown onto the CdA layer thicker than 5L. XRD data show that vertical ordering, that is, ordering along surface normal, of CdA multilayer substrates exerts a strong influence in promoting the crystalline growth of rubrene films. This promoted growth is not due to the surface energy of CdA layer but derived from the additional interaction localized between rubrene and CdA island sidewall and presumably strengthened by a close dimensional match between the a-axis of rubrene lattice and the layer spacing of CdA multilayer. The best OFET mobility is recorded for 9L CdA substrate and reaches 6.7 × 10(-2) cm(2) V(-1) s(-1), presumably limited by the roughness of the interface between CdA and rubrene films.

CdS/ZnS core–shell nanoparticles in arachidic acid LB films

Core–shell CdS/ZnS nanoparticles in arachidic acid film were prepared through a novel Langmuir–Blodgett (LB) approach. Post-deposition treatment of the precursor LB multilayers of cadmium arachidate with H 2S gas followed by intercalation of Zn 2+ ions and further sulfidation result in the formation of CdS/ZnS nanoparticles in the LB film. The formation of these nanoparticles and resulting changes in layered structures were studied by FTIR and X-ray reflection measurements. The optical properties were studied using UV–vis absorption and photoluminescence spectroscopy. A red-shift in the absorption spectrum and enhancement of CdS excitonic emission together with reduction of surface states emission suggest that after the intercalation step, a thin layer of ZnS surrounds the CdS nanoparticles, thus forming a core–shell structure. Subsequent to the second sulfidation, a further red-shift in absorption suggests the formation of a thicker ZnS coating on CdS. Electron diffraction of CdS nanoparticles coated with thicker ZnS showed the diffraction patterns of only ZnS, as expected for core–shell structures.

Grazing incidence diffraction of cadmium arachidate multilayers at the solid-liquid interface

Abstract Using complementary X-ray reflectivity (XR) and grazing incidence X-ray diffraction (GIXD), we report structural studies of supported thin-organic layers in contact with water and air. Using a monochromatic synchrotron beam to penetrate 10 mm of liquid, we have characterized buried films composed of 12.5 repeating bilayers of arachidic acid (C20H40O2) complexed with cadmium ions (cadmium arachidate, CdAr2). We found that the layered structure of the CdAr2 multilayers do not exhibit rearrangement after exposure to water with negligible water penetration into the inner layers when compared to their dry state. These findings are consistent with the formation of extremely robust CdAr2 multilayers that can withstand multiple rinses in strong organic solvents, acid, and mechanical wear. The second goal was to demonstrate the feasibility of similar experiments to study much thinner single bilayer bio-membranes. These studies are the first successful GIXD experiments of ultra thin-organic film composed of a few layers at the solid-liquid interface.

Grazing incidence diffraction of cadmium arachidate multilayers at the solid-liquid interface

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Cd xZn 1− xS–arachidic acid composite LB films

Mixed LB multilayers of cadmium arachidate–zinc arachidate (CdA–ZnA) have been exposed to H 2S to form Cd x Zn 1− x S alloy nanoclusters within the multilayers. FTIR and UV–vis spectra of H 2S exposed multilayers showed that the conversion of arachidate salt into arachidic acid (AA) and sulphide formation begins in the first 5 min and is completed in ∼3 h of H 2S exposure. X-ray reflection studies and the nature of CH 2 scissoring band of the composite multilayers show that the formation of sulphide nanoclusters is accompanied by the formation of molecular chain domains with different polymorphic phases of AA, depending upon the relative proportions of CdA and ZnA in the precursor multilayer. Depending upon the CdS and ZnS content, the absorption edge is found to continuously shift from that of pure CdS to that of pure ZnS nanoclusters, indicating the formation of Cd x Zn 1− x S alloy nanoclusters in the arachidic acid matrix. The nanocluster formation is more facile in the mixed archidate multilayers than in pure CdA or ZnA multilayers.

Structure of CdS–arachidic acid composite LB multilayers

Abstract Langmuir–Blodgett (LB) multilayers of cadmium arachidate were used as precursors to grow semiconducting CdS nanoclusters. The formation of CdS in the multilayers was determined by Fourier transform-infrared (FT-IR), ultraviolet–visible (UV–vis) and Raman spectroscopy. The structural changes occurring as a consequence of CdS formation have been characterized using X-ray reflection (XR) and grazing incidence X-ray diffraction (GIXD) techniques. The CdS containing composite multilayers exhibit the presence of two types of molecular domains, one with close packed herringbone arrangement and the other with tilted molecular chains with no in-plane order. The structural and spectroscopic evidences together suggest that the CdS nanoclusters formed within the arachidic acid LB matrix are quasi two-dimensional in nature with lateral dimension ∼5–10 nm and thickness ∼1.1 nm.

Molecular packing in cadmium and zinc arachidate LB multilayers

Abstract A combination of grazing incidence X-ray reflection/diffraction (GIXR/GIXD) and atomic force microscopy have been used to study the 3D structure in cadmium arachidate (CdA) and single phase zinc arachidate (ZnA) LB multilayers. The CdA multilayers have an ideal close packed herringbone structure. The molecules have a non-centred in-plane arrangement with specific orientational relation between the central and the corner molecules. In contrast, the ZnA multilayers have a hexagonal layer plane packing with tilted molecules. The molecules are loosely packed with rotational freedom about chain axis as observed in ‘rotator’ phases.

Influence of temperature on hemicyanine Langmuir–Blodgett multilayer films

Molecular structure and optical nonlinearity in interleaving hemicyanine/arachidic acid or cadmium arachidate multilayers, and Z-type pure hemicyanine multilayers were investigated as a function of temperature by means of second harmonic generation (SHG) and linear absorption. The SHG intensity shows a maximum at about 45°C in the multilayers. An inflection was observed in the temperature dependence of SHG intensity in the interleaving hemicyanine/arachidic acid and hemicyanine/cadmium arachidate multilayers, and was attributed to the melting of arachidic acid or cadmium arachidate. The absorption spectra show that the molecules in the multilayers formed H-aggregates which could be dissociated by heating.

Polyaniline–CdS Composite Films Obtained from Polyaniline–Cadmium Arachidate Multilayers

Polyaniline–CdS Composite Films Obtained from Polyaniline–Cadmium Arachidate Multilayers

Stability of Cadmium Arachidate Langmuir−Blodgett Multilayers as Determined by Neutron and X-ray Reflectivity

The thermal stability of Langmuir−Blodgett multilayers of cadmium arachidate and perdeuterated cadmium arachidate layers deposited on hydrophobic silicon is investigated by neutron and X-ray reflectometry. Using this pair of techniques allows one to depth resolve changes induced by annealing and also to discriminate between the behavior of the Cd head lattice and that of the aliphatic tails. All samples initially have ill-defined interfaces between the tails of adjoining protonated and deuterated layers, and some interchange of chains among layers occurs during the deposition process itself. The form of the multilayer structure is thermally stable at 70 and 84 °C, even though aliphatic tails are lost gradually due to ablation, leading to noticeable losses when annealing for 1 week at 84 °C. These losses occur almost exclusively from the layers closest to air, suggesting that the rate of ablation is considerably higher than that of interdiffusion. There appears to be little interlayer interdiffusion of chains when annealing for several hours at 84 °C. We conjecture that the type of interaction anchoring the two layers adjoining an interface may influence the stability of that interface to molecular exchange across it.

In-plane organization of LB monolayers from FTIR spectroscopy

The in-plane organization of monolayers and multilayers of cadmium arachidate deposited, by the conventional Langmuir-Blodgett technique, onto dielectric substrates has been studied by transmittance FTIR spectroscopy. Examination of the bending CH 2 band associated with the hydrocarbon chains suggests that the first monolayer has a hexagonal subcell packing whether the interaction with the supporting substrate is hydrophilic or hydrophobic. Conversely, even-numbered multilayer stacks are arranged in bilayer structures, and their hydrocarbon chains present an orthorhombic packing with their crystallographic a axis preferentially oriented parallel to the withdrawal direction. Odd-numbered multilayer stacks are composed of a first hexagonal monolayer followed by orthorhombic bilayers.

Surface Order and Stability of Langmuir-Blodgett Films

Angstrom-resolution atomic force microscope images of Langmuir-Blodgett monolayers and multilayers of cadmium arachidate in air and under water show a dramatic change from a disordered arrangement to a crystalline lattice by the addition or removal of a single layer of molecules. The disordered surface is less stable than the ordered one to mechanical stresses such as atomic force microscopy tip forces or at the air-water contact line during contact angle measurements. The difference in the degree of order in the alkyl chains is attributed to the strong attractive interaction between headgroups in the presence of the divalent cation.

Comparative optical reflection and mass spectrometry analysis of thermodesorption of Langmuir-Blodgett films

Thermodesorption of cadmium arachidate multilayers is studied by optical surface analysis and by mass spectrometry measurements. The optical reflection technique has been improved to discriminate signal contributions from desorption and light scattering. The scattering arises from film heterogeneities that are also observed by Nomarsky microscopy. The assessment of these heterogeneities is important to understand mass spectrometry data. Both the optical technique and mass spectrometry are sensitive to observing the multilayer phase transition at 110 °C and the desorption near 200 °C (at the heating rate applied). The mass spectrometry analysis yields detailed information on the desorbing species and hence on the desorption mechanism, whereas the optical reflection technique, because of its ease of application, can be used for screening materials and processes. In addition, it can be used to calibrate the mass spectrometry signal as a rate of thickness changes.

Ultrathin metal films and inorganic clusters via thermodesorption of LB films

AbstractLangmuir‐Blodgett multilayers of cadmium arachidate and ferric stearate were investigated during heating in order to derive information on processes involved in thermodesorption and on the inorganic microstructures left on the substrate after thermodesorption. We studied these processes by X‐ray induced photoelectron spectroscopy (XPS), interference enhanced reflection (IeR), Nomarsky microscopy, electron microprobe analysis and in the case of Fe57‐ions by conversion electron Mössbauer spectroscopy (CEMS).Two different mechanisms of desorption were observed. In the case of cadmium arachidate multilayers it is shown that before desorption the film ruptures laterally and then forms droplets. At temperatures between 250°C and 350°C the aliphatic molecular part is desorbed and the inorganic material (Cd) remains on the surface. These threedimensional Cd‐clusters have a rather uniform lateral distribution and dimension (0.5–2μm) and normal extension larger than 1000Å. The size may be controlled via nucleation and growth kinetics.In contrast to this thermodesorption of the aliphatic chains of ferric stearate multilayers leads to a rather uniform continuous oxidic layer. By means of CEMS it could be shown that during desorption under H2‐atmosphere the iron was reduced (e.g. 40% metallic iron). The degree of oxidation can be controlled via temperature and time of reduction.

Interaction forces of a sharp tungsten tip with molecular films on silicon surfaces.

An atomic-force microscope was used to measure the surface forces between a sharp tungsten tip and several different types of molecularly thin organic films on a silicon substrate. The forces are dramatically influenced by the molecular films covering the surfaces. The films investigated fall into three categories: liquid for unbound films of perfluoropolyether, intermediate for bonded films of perfluoropolyether, and soft solid for multilayers of cadmium arachidate. Molecular-level origins of these forces are discussed.

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.

Structural studies of Langmuir-Blodgett multilayers by means of soft X-ray diffraction

The specularly reflected intensity from cadmium arachidate multilayers has been measured as a function of incidence angle. The films were prepared by the Langmuir-Blodgett technique and ranged from three to 21 layers in thickness. Monochromatic synchrotron radiation of 1560 eV photon energy (wavelength 7.95 Å) was used in combination with a high vacuum compatible triple-axis diffractometer. The measured diffraction profiles were fitted with a theory based on the exact solution of the Fresnel-Bragg equations for multiple reflections including absorption. The optical parameters for the layers were taken from tabulated values, and the layers were constructed using a formulation similar to that employed previously by Pomerantz and Segmüller. The individual layer spacing as well as the total thickness of the film could be derived to within an accuracy of 1% or 1 Å (whichever is larger).

Aligned textures in multilayers of cadmium arachidate as seen by electron microscopy

Platinum/carbon replicas were prepared of multilayers of cadmium arachidate on glass. In the electron microscope, domains were observed with a texture of parallel streaks 15 nm apart along two directions at an angle of 120°, symmetrical to the direction of dipping. A dumbell shaped Fourier map suggests that the texture is caused by shadowing. The pattern disappears with the addition of methylstearate and cyanine dye.

Structure and dynamics in a Langmuir-Blodgett film at elevated temperatures

By a combination of waveguide Raman scattering, scanning microellipsometry and interference contrast microscopy some new structural details of Langmuir-Blodgett (LB) multilayers of cadmium arachidate on glass substrates have been determined. Raman measurements gave orientation and order as a function of temperature. Spectral changes, similar to those in the bulk, could be attributed to a progressive increase in librations of the hydrocarbon chain as the temperature was increased. Scanning microellipsometry on bilayer steps gave an upper limit for the lateral diffusion ( D⩽10 −10 cm 2 s −1), indicative of a solid phase at a temperature as high as 100°C. The optical thickness of a given multilayer decreased considerably on a time scale of 10 min, after an instantaneous temperature rise to 100°C. The initial loss of material apparently reflects the creation of voids needed to provide the additional space for the hydrocarbon chains to disorder. It was followed by a slower constant thermodesorption on a time scale of hours. This picture was supported by observations with an interference contrast microscope. Although films remained homogeneous upon heating to about 95°C, cracks were formed upon cooling to room temperature, probably a consequence of some ablation of chains at higher temperatures. Interestingly, the cracks were oriented mainly perpendicularly to the dipping direction, indicating some induced lateral structure.