Cadmium Arachidate Layers Research Articles

Large scale solvothermal synthesis and a strategy to obtain stable Langmuir–Blodgett film of CoFe2O4 nanoparticles

Nearly monodisperse oleic acid coated cobalt ferrite nanoparticles were synthesized in large scale by a simple solvothermal method utilizing N-methyl 2-Pyrrolidone (NMP) as a high boiling solvent. The magnetic oxide was further investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and vibrating sample magnetometer (VSM). Langmuir–Blodgett (LB) technique is discussed to obtain a 2D assembly of oleic acid coated CoFe2O4 nanoparticles over a large area. We describe a method to obtain stable, condensed three layers of cadmium arachidate on a piranha treated glass substrate. The hydrophobic surface thus obtained was subsequently used for forming a stable monolayer of oleic acid stabilized cobalt ferrite nanoparticles at the air–water interface. The stability of the LB films at the air–water interface was studied by pressure–area isotherm curves and atomic force microscopy (AFM) cross sectional height profile analysis. 2D organization of the magnetic nanoparticles at different surface pressures was studied by TEM. Preparation of large area LB films of CoFe2O4 nanoparticles is reported for a surface pressure more than 15mN/m.

Structural and surface morphological studies of long chain fatty acid thin films deposited by Langmuir–Blodgett technique

Abstract In the present work we aim to study the structural and surface morphological characteristics of divalent cation (cadmium ion, Cd2+) induced thin mono- to multilayer films of fatty acids such as arachidic acid and stearic acid prepared by the Langmuir–Blodgett (LB) technique. These ultra thin films of various numbers of layers were studied by X-ray diffraction (XRD), X-ray reflectivity (XRR) and Atomic Force Microscopy (AFM). In this specific Y-type deposition, it was found that as the individual layer thickness increases, the corresponding layer by layer interfacial electron density of the thin films decreases. Since the fatty acid chain tries to maintain its minimum value of cross-sectional area, tilting occurs with respect to its nearest neighbor. The tilt angle calculated for 9 layers of cadmium arachidate (CdA2) and cadmium stearate (CdSt2) are 18° and 19.5°, respectively. An asymmetric air gap of thickness ∼3 A was also seen between the tail parts of 2 molecular chains. The RMS roughness and average height factors calculated through AFM studies show non-uniform surface morphology of both CdA2 and CdSt2, although the calculated topographic variations were found to have more irregularity in case of CdSt2 than in case of CdA2.

Effect of Cadmium Arachidate Layers on the Growth of Pentacene and the Performance of Pentacene-Based Thin Film Transistors

The effect of cadmium arachidate (CdA) layers deposited by Langmuir-Blodgett technique on the growth of pentacene thin films and the performance of pentacene-based thin film transistors has been investigated. The hydrophobicity of the SiO2 gate dielectric surface was increased (surface energy reduced) with the deposition of CdA layers as a result of the presence of long hydrophobic alkyl chains attached to the cadmium atoms. The change in surface wetting properties of SiO2 strongly influenced the growth mechanism of pentacene thin films. The grain size and root-mean-square surface roughness of pentacene was decreased with an increase in the number of CdA layers compared to the pentacene deposited on a bare SiO2 surface. Organic thin film transistors (OTFTs) with seven layers of CdA on SiO2 showed the highest mobility of 0.27 cm2/Vs and the lowest subthreshold slope of 2.4 V/dec. The enhanced electrical properties of the OTFTs with SiO2/CdA as the dielectric is attributed to the better intermolecular connection, tight packing, and improved surface quality of the pentacene, as evident from the X-ray diffraction (XRD) and atomic force microscopy (AFM) results.

Structural study of meso-octaethylcalix[4]pyrrole Langmuir–Blodgett films used as gas sensors

Monolayers of meso-octaethylcalix[4]pyrrole (MOP) have been prepared at the air/water interface by the Langmuir–Blodgett (LB) technique. LB isotherms indicate that during the barrier compression the MOP monolayer slowly goes towards a closely packed system consisting of molecules whose molecular area is consistent with a calix-like shape. A significant molecular area loss, suggesting the occurrence of self-assembling phenomena, has been observed by: (a) increasing the molecular concentration; (b) performing cyclic compression–decompression curves; (c) recording constant-pressure relaxation curves. By means of the LB dipping method, MOP thin films have been built-up by transferring four and eight layers on solid surfaces. Model-flat silicon oxide and nanostructured silver surfaces, both hydrophobized by three LB layers of cadmium arachidate, have been used as substrates. Scanning force microscopy (SFM) images performed on these systems showed surfaces constituted by the ensemble of MOP ellipsoidal aggregates tens of nanometers in size. The films transferred on silver have been found to be very useful for the molecular sensing of alcohol vapours by using the surface plasmon resonance (SPR) spectroscopy as transduction method. The film structure has been correlated to the SPR performance.

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.

Förster energy transfer between Langmuir-Blodgett film layers in aqueous solution

The Förster energy transfer between chromophores in several different types of Langmuir-Blodgett (LB) films exposed to aqueous conditions was examined. The donor and acceptor chromophores were either contained in a monolayer of a preformed copolymer incorporating amphiphilic side chains and hydrophilic spacer groups (dansyldihexadecylamine as the donor and 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate as the acceptor) or grafted to a monolayer of such a copolymer (anthracene as the donor and 7-nitrobenz-2-oxa-1,3-diazole as the acceptor). The monolayers containing donor or acceptor chromophores were separated by neutral spacer layers of cadmium arachidate or copolymer material. It was found that the unattached chromophores diffused through the LB films during the experimental procedure and for several days afterwards. LB films containing chromophores that were grafted to copolymer and copolymer material spacer layers possessed high translayer structural order. There was evidence, however, that indicated that the structure relaxed with time.

Characterization of long-periodic layered structures by X-ray diffraction III: Structure of a Langmuir-Blodgett film of cadmium arachidate at elevated temperatures

The thermal behavior of a Langmuir-Blodgett (LB) multilayer of cadmium arachidate (CdA) was investigated by small-angle X-ray diffraction. Before melting, the LB multilayer underwent a three-step structural change: (1) from room temperature to 60°C, rearrangement of the CdA layers; (2) between 60 and 80°C, onset of the pretransitional disordering; (3) above 80°C, conformational change from trans to gauche state in the hydrocarbon chain. The 001 reflection intensity, a measure of the short-range disorder of the multilayer structure, is sensitive to these changes. In contrast, the lattice distortion parameter tends to increase monotonously up to 100°C. This indicates that the long-range order of the multilayer structure is well preserved at temperatures close to the melting point. On cooling down to room temperature after melting, the crystallites of a periodic lamellar structure grow. Their lattice distortion and dimension in the [001] direction are much larger than those of the multilayer before melting.

Electrical properties of polyimide Langmuir-Blodgett films sandwiched between superconducting electrodes: detection of microwaves

We have examined the electrical properties of ultrathin insulating layers of cadmium arachidate (C 20), polyamic acid long alkyl amine salts (PAA) and polyimide (PI) by using tunnel junctions with structures of (Pb-Bi)/PAA (or C 20)/(Pb-Bi), Au/PI/(Pb-Bi) and Au/PI/Au. Typical current-voltage ( I-V) characteristics of weakly coupled superconductors were obtained for (Pb-Bi)/PAA (or C 20)/(Pb-Bi) junctions with one, two or three deposited layers at a temperature below the critical temperature of a Pb-Bi alloy. On the contrary, typical I-V characteristics of tunnel junctions were obtained for Au/PI/(Pb-Bi) and Au/PI/Au junctions, when the number of deposited layers is greater than about 30. Finally, we concluded that the efficiency of detecting microwaves is excellent in (Pb-Bi)/PAA (or C 20)/(Pb-Bi) junctions, although bridging filaments exist in the PAA (or C 20) Langmuir-Blodgett (LB) layers, and that the PI LB layers become good electrical insulating spacers when the thickness of deposited layers is more than about 11 nm.

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.

Standing light wave in front of a silver mirror investigated by photothermal spectroscopy

Photothermal spectra of an isocyanine dye monolayer, separated from a silver mirror by a varying number of cadmium arachidate layers, are recorded with a pyroelectric calorimeter. The spatial dependence of the light intensity in front of the silver mirror is observed. The phase angle of a light wave upon reflection from the silver surface is determined to be 124±5° at a wavelength of 578 nm. The quantum yield for radiationless deexcitation at that wavelength is found to be 33±7%.

Transient photocurrents in monolayer assemblies containing amphipathic porphyrin upon excitation with a dye laser

Steady-state and transient photoelectric responses were measured for monolayer assemblies composed of amphipathic porphyrin layers deposited on cadmium arachidate layers. Transient photocurrents due to excitation of chromophores in monolayer assemblies were observed for the first time, which showed a fast rise and a slow decay. Transient photocurrents decayed exponentially except for a small fraction at the initial stage. The decay constant of transient photocurrents and the value of the steady-state photocurrents decreased exponentially with the number of arachidate layers giving a characteristic decay length of 180-208 A. The decay of photocarriers in arachidate layers was strongly suggested.

Ellipsometry and reflection, luminescence and Raman spectroscopies of monolayer assemblies on solid substrates

Long-chain fatty acid monolayer assembles were built up on metal and semiconductor surfaces by the Langmuir-Blodgett dipping technique. Ellipsometry was used to characterize the multilayers. At the air-metal interface of silver gratings, plasmon surface polaritons (PSPs) can be excited directly. The dispersion of the PSP modes and its dependence on the overcoatings was determined by reflection and luminescence measurements. The high field of the PSP was used to enhance the Raman spectra of cadmium arachidate layers. It is shown how the different spectral regions are individually enhanced by selecting the appropriate scattering angle. The dependence on film thickness of the intensities of the Raman vibrational bands and the PSP dispersion parameters such as the position and width of the gap at the Brillouin zone centre are presented. Raman peak intensity ratios can be used to evaluate an order parameter for the different layers. It is shown that this parameter decreases with increasing layer thickness. If dye layers are intercalated into the monolayer assembly, weak flourescence from the dye can be detected. In addition, emmission of light occurs from all the PSP modes of energy lower than that of the exciting light. Thi emmission is identified by its characteristic polarization and the dependence of its wavelength on the angle of emission. At energies within the absorption bands of the dye monolayer, this emission is greatly enhanced.

Plasmon surface polariton luminescence from periodic metal gratings

When a gold grating is illuminated at an appropriate energy, polarization, and angle to excite a plasmon surface polariton (SP) all lower energy radiative SP states emit light. This enables the dispersion of SP states to be mapped by luminescence providing an alternative to reflectivity measurements. A similar effect has been observed for bare silver gratings, and silver gratings coated with several layers of cadmium arachidate by Langmuir-Blodgett technique. Two additional effects have been observed for Au gratings. First, one can obtain emission from radiative SP states by exciting the interband transition directly with photons of energy greater than approximately 2.4 eV. At these energies, SP states do not exist because the metal dielectric function has a large imaginary part. This is significant because it suggests that the degradation pathway involves electron-hole pair states as intermediates. Second, some gold gratings show surface enhanced Raman of molecular species superimposed on the SP luminescence emission.

Photoelectric properties of molecular layers of a fatty acid mixed with cyanine dyes

Photocurrents in Langmuir multimolecular layers of cadmium arachidate mixed with cyanine dyes between metal electrodes were studied. The photocurrent efficiency was found to depend on the applied voltage, the temperature and the distance between the dye molecules and the electrode. All samples exhibited non- ohmic behavior of the photocurrent. It was concluded that the photocurrent was initiated by the thermionic emission of electrons from the excited dyes. The activation energy necessary for the electron emission from the excited dye in the mixed layers was determined to be 0.52 eV. Absorption spectra were measured for layers deposited on an aluminum film and on a glass slide.

Surface plasmon spectroscopy of organic monolayer assemblies

Because the surface plasmon resonance is a sensitive probe of metallic surfaces we have measured quantitatively the changes that occur in surface plasmon resonances from coatings of thin layers of cadmium arachidate of different but known thicknesses on silver films. An ATR (attenuated total reflection) experimental arrangement was used. The dispersion of the refractive index of the organic layer was derived by a least square fit of the measured reflectivity curves and it agrees well with other data obtained by different methods. The influence of a possible intermediate layer between the silver surface and the first organic monolayer due to the preparation process is also discussed.

The effect of thin organic films on the surface plasma resonance on gold

Optical constants of organic monolayers on gold are calculated from shifts in the surface plasmon resonance. Two to several organized layers of cadmium arachidate (each known independently to be 2.68 nm thick) were deposited onto gold films. The angle of minimum attenuated total reflectance changed by about 0.5 degree, corresponding to a Δ k sp of approximately 900 cm -1 for each additional monolayer. Because of the large angular shifts and the simplicity of the ATR method, this technique appears to be an easy and accurate way to determine optical constants and monitor changes in thin (order of nm) interfacial layers on metals.

Contact angles during monolayer deposition

Preliminary results are reported in a study of the contact angles and adhesional energy changes during the transfer of monolayers from the air-water interface to solid (hydrophobic glass) supports. The technique used is the vertical-plate method of Neumann, as modified for this application by Clint and Walker. In building up cadmium arachidate layers, it is found that the hydrophobic (immersion) adhesion energy depends slightly on pH and concomitant extent of soap formation in the film, suggesting that polar group interactions, while small, are not negligible. (The narrow pH range of condensation for cadmium arachidate requires the use of a stable buffer system such as acetate, rather than bicarbonate, for adequate pH control.) The acute contact angles observed during the withdrawal step in layer buildup change slowly after motion is stopped. While no quantitative interpretation is yet possible, it is suggested that this may reflect water and ion redistribution in the interfacial region; such redistribution would also contribute previously ignored surface excess terms to the interfacial energies involved.

Instability of Langmuir-Blodgett layers of barium stearate, cadmium arachidate and tripalmitin, studied by means of electron microscopy and infrared spectroscopy.

Results of an investigation of the stability of n-layers of barium stearate, cadmium arachidate and tripalmitin by means of electron microscopy and attenuated total reflection infrared spectroscopy are reported. Odd and even numbered barium starate n-layers with n - 1,2,3,4,5 are found to rearrange spontaneously from a regular film into ultrastructures of irregular, flat islands of varying thickness. The kinetics of the phase transformation of the first layer depends on the substrate, that of n-layers appears to be dependent on n, the temperature, and the surrounding medium. The kinetic behaviour of odd and even numbered layers is distinctly different. Similar studies on cadmium arachidate layers reveal much slower kinetics of the rearrangement process. In the case of tripalmitin n-layers it is shown that electron microscopy and infrared spectroscopy yield valuable complementary information about ultrastructure and molecular structure of the layers in correlation with the rearrangement process, which also occurs with this system. Consequences of the results of this paper for work published in various fields are briefly discussed.

Variation of the Fluorescence Decay Time of a Molecule in Front of a Mirror

AbstractThe out‐going wave‐front of a radiating oscillator (e. g. a Hertzian dipole) is disturbed by a reflecting surface. The resulting interference phenomenon affects the radiated power, which thus depends on the distance between oscillator and mirror. If this concept is applied to the emission of light by molecules, a dependence of the fluorescence decay time on the distance between excited molecule and mirror is to be expected.This effect has been investigated for the case of monomolecular layers of an europium dibenzoylmethane complex, which are kept in a definite distance from an evaporated metal surface (gold, silver or aluminum) by cadmiumarachidate layers. Exciting with a flash of 366 nm light the intense red fluorescence (λ = 612 nm) of the europium ion is observed. The fluorescence follows an exponential decay law over at least two decades with a decay time of roughly 1 msec. The expected variation of the decay time with distance from the mirror is verified by the experiment. The decay time is lengthened or shortened in close agreement with the above theory, except for small distances, where energy transfer processes occur.The mirror affects only the radiating process, while the probabilities for non‐radiating desactivation processes remain unaltered. Thus the quantum yield of fluorescence of the emitting state can be determined, which is an otherwise hardly accessible quantity.