Arachidate Layers Research Articles

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.

Sum Frequency Generation from Langmuir−Blodgett Multilayer Films on Metal and Dielectric Substrates

Sum frequency generation (SFG) vibrational spectra of cadmium arachidate multilayer films adsorbed on a substrate with high nonresonant susceptibility, i.e., gold, and on a low nonresonant susceptibility substrate, i.e., fused quartz, have been investigated in the C-H stretching region in air. The films were formed by Langmuir-Blodgett (LB) deposition and their spectra recorded using SFG spectrometers employing both 532-nm nanosecond and 800-nm femtosecond lasers, with counter-propagating and co-propagating beam geometries, respectively. Both kinds of substrate were rendered hydrophobic by coating them with per-deuterated octadecanethiol (gold) or per-deuterated cadmium arachidate (fused quartz) monolayers. Single per-protonated arachidate layers in otherwise per-deuterated 10-layer films were used to show that the SFG resonances arise only from the topmost and lowermost layers in a LB film comprised of an even number of per-protonated layers, although the SFG spectra from the two hydrophobic substrates are different from each other. The differences in the spectra from the same ten-layer per-protonated films deposited on the two types of hydrophobic substrate have been explained in terms of a simple model that accounts for resonant and nonresonant contributions.

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.

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.

Arachidate Layers on Ag and Au Substrates Detected by the A T R Method

Monolayers of cadmium arachidate (thickness 26.8 A) are deposited on Ag and Au substrates. They can be detected by the resonance excitation of surface plasmons (ATR method). The number of monolayers is correlated with the displacement of the reflection minimum. The observed values of the position of the minima as well as the shape of the reflection curve are in agreement with the calculated ones, using a refractive index of cadmium arachidate of 1.52 in agreement with other measurements

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.

Potential model of dye molecule from measurements of the photocurrent in monolayer assemblies

A semitransparent Al layer is coated with several arachidate layers, one monolayer of quinquethienyl (mixed with arachidate), and some layers of arachidate; the arrangement is contacted with mercury. A bias voltage is applied and the photocurrent is measured as a function of the applied voltage and the temperature for different positions of the dye within the layer assembly. The photocurrent depends on the detailed assemblage, demonstrating the possibility of constructing photoelectric devices based on a planned molecular architecture. The results are in agreement with a simple theoretical model. A reasonably detailed description is obtained for the shape of the potential profile of the excited π electron of the dye responsible for the photocurrent.

Model systems for photosynthesis IV. Photosensitization by chlorophyllamonolayers at a lipid/water interface

Multilayers of chlorophyllaand plant quinones have been constructed on solid surfaces and their absorption spectra have been characterized. Chlorophyllafluorescence quenching by quinones was studied by using a Cd2+1:1 stearate:oleate layer diluent and Cd2+arachidate layers as spacers. The linear dependence of fluorescence yield on separation of the pigments indicates that the quinone can diffuse through arachidate layers. Photosensitized redox reactions have been studied by means of ascorbate-tetrazolium blue and ascorbate-methyl red pairs incorporated in an aqueous solution adjacent to chlorophyllamonolayers. Photoreduction of the dye occurred with light absorbed only by the chlorophyll. A quinone layer adjacent to the chlorophyll layer enhanced the rate of photoreduction by a factor of three, whereas incorporation of quinone in the chlorophyll layer inhibited the photoreduction.