Cadmium Arachidate Langmuir-Blodgett Films Research Articles

Highly sensitive gas sensor using hierarchically self-assembled thin films of graphene oxide and gold nanoparticles

In this study, we fabricated hierarchically self-assembled thin films composed of graphene oxide (GO) sheets and gold nanoparticles (Au NPs) using the Langmuir–Blodgett (LB) and Langmuir–Schaefer (LS) techniques and investigated their gas-sensing performance. First, a thermally oxidized silicon wafer (Si/SiO2) was hydrophobized by depositing the LB films of cadmium arachidate. Thin films of ligand-capped Au NPs and GO sheets of the appropriate size were then sequentially transferred onto the hydrophobic silicon wafer using the LB and the LS techniques, respectively. Several different films were prepared by varying the ligand type, film composition, and surface pressure of the spread monolayer at the air/water interface. Their structures were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM), and their gas-sensing performance for NH3 and CO2 was assessed. The thin films of dodecanethiol-capped Au NPs and medium-sized GO sheets had a better hierarchical structure with higher uniformity and exhibited better gas-sensing performance.

Role of metal ions of Langmuir–Blodgett film in hydrophobic to hydrophilic transition of HF-treated Si surface

Hydrophobic to hydrophilic transition of HF-treated Si surface strongly depends upon the metal ions, which are present in the headgroups of the deposited Langmuir–Blodgett (LB) film. Structure of LB films studied by X-ray reflectivity technique and chemical analysis of LB film–substrate interfaces studied by X-ray photoelectron spectroscopy combinedly suggest that the partial transition or partial oxidation of the HF-treated Si surface takes place under the subphase water but further transition or oxidation is possible only in the presence of metal ions. Electrovalent and covalent natures of the metal ions tune this transition or oxidation. Ni ions, for which bonding with headgroups are electrovalent in nature, are favorable for such transition/oxidation and as a result, complete transition/oxidation takes place when nickel arachidate LB film is deposited. On the other hand, Cd ions, for which bonding with headgroups show covalent nature, is not favorable for such transition and can not oxidize the underlying H-passivated Si substrate totally when cadmium arachidate LB film is deposited on such HF-treated Si surface. This ion-specific hydrophobic to hydrophilic transition is visualized by X-ray reflectivity, contact angle and X-ray photoelectron spectroscopy measurements.

Local field anisotropy in Langmuir-Blodgett films of cadmium arachidate

Lorentz tensor and local field tensor components for uniaxial Langmuir-Blodgett films of cadmium arachidate from 3 to 100 nm thick on silver, polymethyl methacrylate, and glass substrates have been experimentally determined using the data on the film refractive index dispersion in the visible range. The constraints from below on the mean value and anisotropy of the molecular polarizability caused by the intermolecular interactions in the film have been established.

Effect of water and air–water interface on the structural modification of Ni-arachidate Langmuir–Blodgett films

Nickel arachidate (NiA) Langmuir-Blodgett (LB) films have been deposited on hydrophilic Si(0 0 1) substrates by three (up-down-up) and five (up-down-up-down-up) strokes. During deposition, substrates were kept inside the water subphase for different times after each down stroke. Structural information of all the LB films have been obtained from X-ray reflectivity (XRR) studies. One and two symmetric monolayer (SML) was deposited on top of the asymmetric monolayer (AML) in three and five stokes respectively. All the preformed LB films were then used to go through the air-water interface with the same speed that was used at the time of film deposition. Structural information obtained from the XRR studies show that mainly the top layer density decreases after passing through the air-water interface but the layered structure remains the same. Information obtained from both the XRR and atomic force microscopy (AFM) studies suggest that molecules peeled from the top SML layer do not reincorporate with the LB film through tail-tail hydrophobic interaction. Our study shows that NiA LB film has better stability compared with cadmium arachidate LB film inside the water subphase without forming any out-of-plane molecular reorganization.

Quantification and IMFP determination of multilayer Langmuir–Blodgett films by AFM and XPS measurements

AbstractXPS spectra of cadmium arachidate Langmuir–Blodgett (LB) multilayer films have been investigated by quantitative peak shape and intensity analysis (the Tougaard method). The structure and film thickness of cadmium arachidate multilayer (1–21 layers) LB films on silicon wafers were observed by atomic force microscopy (AFM). The monolayer and bilayer thickness of cadmium arachidate LB film was determined to be 2.3 and 5.5 nm from AFM images of partially scratched multilayer LB films. The angle‐resolved XPS (ARXPS) spectra of these LB films with well‐defined thickness were analyzed by the Tougaard method. The inelastic mean free paths (IMFPs) in the LB film were determined to be 35.5 Å at 1382 eV and 29.1 Å at 1078 eV from analysis of ARXPS spectra taken at several emission angles (10°, 32° and 45° from the surface normal). These values are in good agreement with the reported values of bulk organic materials. Copyright © 2006 John Wiley & Sons, Ltd.

Multiple Ordered Phases in Langmuir−Blodgett Films of Cadmium Arachidate at Elevated Temperatures

Using synchrotron X-ray grazing incidence diffraction, superlattice structures have been observed to develop in Langmuir-Blodgett films of cadmium arachidate as the temperature is raised. The previously reported superstructure in the stacked lamellae at room temperature changes at about 70 degreesC and there are further changes at about 90 and 103 degreesC before the major phase transition from stacked lamellae to hexagonally packed rods occurs at 107 degreesC (Langmuir 1997, 13, 1602). Between 70 and 103 degreesC there is a 1 x 10 one-dimensional in-plane superstructure, which is commensurate with the local structure and has an interlayer shift along [01] by a distance of b (of the local structure) at lower temperatures, and a further shift at about 90 degreesC. At lower (<90 degreesC) temperatures, the local in-plane structure retains rectangular symmetry with the chains perpendicular to the film plane (d(001) = 55.4 Angstrom). At temperatures above 90 degreesC, an oblique local structure develops in the film plane, but the value of d(001) is unchanged. At about 103 degreesC, the superstructure evolves into an oblique net covering 20 x 10 molecules, commensurate with the local structure, and the value of d(001) reduces to 53.0 Angstrom. Rearrangement of the cadmium cations in the film is suggested for the development of the superstructures.

Robustness of monolayers on the solids; comparative studies on thermal, solvent, pH, and mechanical resistance among 1-layer LB films of cadmium arachidate and alkylchlorosilanes

The resistance of Langmuir–Blodgett (LB) monolayers of n-alkylchlorosilane compounds against the external disturbances has been investigated. The silane compounds used as film materials are trichloro(octadecyl)silane (TCOS), dichloro(methyl)(octadecyl)silane (DCMOS), and chloro(dimethyl)(octadecyl)silane (CDMOS). Cadmium arachidate (CdA) monolayers are also employed for comparison. FT-IR transmission and reflection absorption spectroscopic analysis of 5-layer LB films of the silanes indicates that the film molecules have residual silanol groups. The results reveal that the silane molecules can be fixed on the hydroxylic solid surfaces with covalent bonds after the monolayers are transferred on such substrates. Thermal stability and dissolution resistance of the LB monolayers on glass slides are investigated by the change of wavenumbers and intensities of CH 2 stretching bands in FT-IR transmission spectra. It becomes clear that the TCOS monolayers are very stable against heating, organic solvents and acid, whereas the DCMOS and CDMOS monolayers are very weak. The mechanical durability of the monolayers is investigated by following friction coefficient profiles. The results reveal that the friction coefficients for the silane monolayers begin to increase after a few transits under high normal loads although those for the CdA monolayers maintain lower values. These results can be interpreted from the view point of monolayer structures.

Growth Mechanism of Langmuir-Blodgett Films

Langmuir-Blodgett (LB) deposition is an astonishingly simple technique to grow well-ordered correlated metal-organic multilayers. To understand this growth mechanism, we have performed x-ray scattering and atomic force microscopic (AFM) studies on cadmium arachidate LB films exhibiting self-affine and logarithmic in-plane correlation at the interfaces. Using linear stochastic theory for interface evolution, it is proposed that a 1D deposition followed by a 2D desorption process is the growth mechanism of LB films. X-ray and AFM measurements confirm the crossover between these two growth regimes. {copyright} {ital 1999} {ital The American Physical Society}

Analysis of defects in CdA Langmuir–Blodgett film using synchrotron X-ray radiation

The deposition defects, known as a transition from Y- to X-type deposition, during the Langmuir–Blodgett (LB) film deposition were analyzed by the measurement of the reflectivity of small angle X-rays using synchrotron radiation. The reflectivity profile of the cadmium arachidate LB film over a wide range of grazing angles was obtained. By fitting the location and dispersion of the subsidiary maxima between the Bragg peaks of the experimentally measured reflectivity profile with that of the calculated reflectivity profile, the average thickness and the distribution of layer thickness were evaluated. A Monte Carlo simulation was performed for the transition of Y- to X- type deposition by using a probability of surface molecule detachment accounting between neighboring molecules. The genetic algorithm was incorporated into Monte Carlo simulation to get the optimum value of the probability factors. The distribution of layers having different thickness predicted by the simulation correlates well with that obtained from the X-ray reflectivity analysis. Based on the morphology measurement with an atomic force microscopy (AFM), the domain structure and mean roughness of LB films were obtained. The mean roughness value calculated based on the number of layer distribution is consistent with that obtained from X-ray reflectivity analysis.

Evidence for structural and electronic anisotropy in CdS nanocrystal layers from langmuir-blodgett film precursors

Ultrathin semiconductor layers made of CdS nanocrystals synthesized in cadmium arachidate Langmuir-Blodgett films have been investigated by means of polarized optical and infrared absorption spectroscopy, resonance Raman scattering and X-ray diffraction during two different phases of the formation process (before and after the removal of the organic precursor matrix). Optical spectroscopy revealed that CdS nanocrystals are anisotropic, likely ellipsoidal in shape, laying with their major axes in the film plane. Resonance Raman scattering confirmed these results by monitoring, in two different experimental geometries and after different annealing conditions, the coupling to the lattice of the lowest electronic excited state. The reported data allow to describe these samples as made of anisotropic CdS nanocrystals surrounded by a small residual amount of fatty acids. Finally, we have demonstrated the possibility of varying the CdS crystal size from 3 to 10nm upon the removal of the organic matrix and the annealing procedure.

A control system for Langmuir-Blodgett film deposition set-up based on microstepping

A control system for Langmuir-Blodgett (LB) film set-up has been developed based on the principle of microstepping of the stepper motors to drive the barriers and the substrate with precision and smooth motion. The stepper motors and the Wilhelmy plate used to measure the monolayer surface pressure are interfaced to a microcomputer for automatic operation. This home-made LB film set-up has been successfully used to fabricate LB films of cadmium arachidate of high crystalline quality.

Capillary Waves in Langmuir-Blodgett Interfaces and Formation of Confined CdS Layers

Nanometer sized CdS particles can be formed by exposing cadmium arachidate Langmuir-Blodgett films to hydrogen sulphide environment. X-ray scattering studies were performed to understand the formation of these particles that exhibit the quantum confinement effect in band structure. It was observed that the CdS layers remain confined within a region of $\ensuremath{\sim}14\AA{}$ around metal interfaces in these films. The sharp interfaces and the multilayer molecular stack of the films remains intact in the process of CdS formation. Logarithmic in-plane interfacial correlation, characteristic of capillary waves on liquid surfaces, was observed in both exposed and unexposed films.

Template-assisted electrochemical deposition of ultrathin films of cadmium sulfide

Uniform thin films of cadmium sulfide were prepared by template-assisted electrochemical deposition on gold electrodes covered with Langmuir-Blodgett (LB) films. An electrolytic cell was used for the production of low concentrations of S − to effect heterogeneous crystallization on LB films, while preventing homogeneous precipitation. 3-, 4-, and 5-layer LB films of cadmium arachidate were investigated. Characterization of deposited films by optical microscopy, atomic force microscopy and energy dispersive X-ray analysis of film composition indicated that a variety of different types of CdS films can be produced depending on the type of LB film used, the electrode at which template-assisted deposition is carried out, the magnitude and frequency of alternating potentials used, and the deposition time used. In general, hydrophilic organic films that present cadmium carboxylate groups to the deposition surface (i.e. 3- and 5-layer LB films) supported crystallization of CdS, while those that present hydrophobic, methyl groups (i.e. 4-layer LB films and self-assembled monolayers of alkanethiolates on gold) did not. The best methods for the rapid deposition of conformal, ultrathin films (< 100 nm) of CdS used highly-ordered 5-layer LB films at grounded electrodes. The data presented suggest that templating with LB films, when used together with electrochemical deposition, has potential for the convenient production of oriented crystalline thin films of cadmium sulfide and similar materials.

RBS analysis of Langmuir-Blodgett films bearing quantum-sized CdS particles

Rutherford backscattering spectrometry is applied to an analysis of the structure of Langmuir-Blodgett (LB) films bearing quantum-sized cadmium sulfide particles (Q-CdS). Cadmium arachidate LB films before and after exposure to hydrogen sulfide are analyzed using a beam of 2 MeV 4He 2+. The change in the Cd-edge is ascribed to the Cd migration toward the substrate, which suggests that a well-ordered layer structure of the original film is disordered during Q-CdS formation. This finding indicates that the Q-CdS produced in hydrophilic interlayers cannot be two-dimensional platelets but are spherical particles.

Room-temperature single-electron junction.

The design, realization, and test performances of an electronic junction based on single-electron phenomena that works in the air at room temperature are hereby reported. The element consists of an electrochemically etched sharp tungsten stylus over whose tip a nanometer-size crystal was synthesized. Langmuir-Blodgett films of cadmium arachidate were transferred onto the stylus and exposed to a H2S atmosphere to yield CdS nanocrystals (30-50 angstrom in diameter) imbedded into an organic matrix. The stylus, biased with respect to a flat electrode, was brought to the tunnel distance from the film and a constant gap value was maintained by a piezo-electric actuator driven by a feedback circuit fed by the tunneling current. With this set-up, it is possible to measure the behavior of the current flowing through the quantum dot when a bias voltage is applied. Voltage-current characteristics measured in the system displayed single-electron trends such as a Coulomb blockade and Coulomb staircase and revealed capacitance values as small as 10(-19) F.

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

Medium energy ion beam analysis and modification of Langmuir-Blodgett thin films

The erosion behavior of cadmium arachidate Langmuir-Blodgett films is studied under 270 keV He and Ar ion irradiation. Initial characterization by medium energy time-of-flight backscattering yields a thickness and stoichiometry consistent with an unmodified film. In addition, time-of-flight elastic recoil detection is used to monitor the hydrogen evolution from the films as a function of the cumulative dose. After a dose of 5 × 10 14 Ar ions/cm 2, the film is reduced to less than 50% of its initial thickness and depleted in hydrogen. Smaller losses are measured for He at a dose of nearly 1 × 10 15 ions/cm 2. Dramatic changes are observed in the interference color of 50-layer films under ion erosion. Measurements by ellipsometry yield thicknesses which are consistent with the backscattering estimates and indicate an increase in the refractive index of the film. The response of cadmium arachidate to energetic ions is compared with the cadmium salt of ω-tricosenoic acid, which can polymerize due to the presence of a CC double bond.

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

Quartz crystal microbalance and UV-vis absorption study of Q-State CdS particle formation in cadmium arachidate Langmuir-Blodgett films

Quartz crystal microbalance and UV-vis absorption study of Q-State CdS particle formation in cadmium arachidate Langmuir-Blodgett films

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