Langmuir-Blodgett Film Deposition Research Articles

Deposition and characterization of polyimide Langmuir-Blodgett films

Deposition and characterization of polyimide Langmuir-Blodgett films

Control of the homogeneity of Langmuir-Blodgett films using the deposition-induced flow in the monolayer

The flow in the floating monolayer that is associated with the deposition of Langmuir-Blodgett (LB) films in the conventional vertical dipping method leads to a gradual change in the properties of subsequently deposited layers. It is found that this change is especially marked during a short induction period, after which a steady state is achieved where changes become very small on a film balance of suitable size and geometry. To fabricate homogeneous LB films that consist of only these pre-flow modified, steady state layers, the monolayer is subjected to flow before the first layer is deposited. These homogeneous samples exhibit properties that are markedly different from those of conventional films where the properties of the first individual layers change. The method is especially suitable for large-scale, continuous production of L film coatings, but may be used on a laboratory scale after minor modification of the standard deposition procedure. Several applications for basic research are obvious.

Preparation of polyimide Langmuir-Blodgett films containing [Ru(bpy) 3] 2+ complex

The preparation of polyimide Langmuir-Blodgett (LB) films containing [Ru(bpy) 3] 2+ (bpy ≡ 2,2'-bipyridine) is described. Newly synthesized [Ru(5,5'-diamino-bpy)(bpy) 2] 2+ was found to undergo a direct polymerization reaction with acid dianhydrides, which affords the corresponding polyamic acids (potential precursors of polyimide LB films). For LB film deposition, the copolymerization approach using flexible 4,4'-diaminodiphenyl ether was adopted, because of the insufficient stability of a monolayer of the polyamic acid long alkyl amine salt from only [Ru(5,5'-diamino ≡ bpy) (byp) 2] 2+. Thus obtained polyamic acid LB films using 1,2,3,4,-cyclobutanetetrarboxylic dianhydride were finally converted into the polyimide LB films containing [Ru(bpy) 3] 2+. The photoelectrochemical characteristics of the polyimide LB films on indium tin oxide electrodes were also studied.

Flow Orientation in Langmuir-Blodgett Films of a Charge-Transfer Complex (TMTTF)3(C14TCNQ)2

The occurrence of flow orientation during the deposition of Langmuir-Blodgett films of (TMTTF)3(C14TCNQ)2 is shown through studies of in-plane molecular orientation in films deposited by two types of the vertical dipping method, with the substrates parallel and perpendicular to the barrier, using ESR and polarized UV-visible spectroscopies. Larger in-plane anisotropy is observed toward the edge of the substrates, which is consistent with the prediction of the recent theory of flow orientation during the deposition process. Furthermore, in-plane anisotropy in the films deposited by the horizontal lifting method is detected, suggesting the existence of a compression orientation at the air-water interface.

Effects of insoluble surfactants on dip coating

Dip coating flow at a low capillary number has been studied in which a flat plate is withdrawn from a large liquid trough covered with an insoluble surfactant. If the surface concentration of the surfactant is relatively high, the coating flow may be relevant to the process of preparing Langmuir-Blodgett (LB) films. If it is low, on the other hand, the flow may correspond to a situation of inadvertent surface contamination by a small amount of impurities. By the method of matched asymptotic expansions, the thickness of the liquid film entrained onto the plate as affected by the surfactant and the surfactant concentration along the surface is determined. The spatial distribution of the surfactant is of particular interest from the viewpoint of the LB film deposition process as it determines the amount of the surfactant transferred from the trough surface to the plate (i.e., transfer efficiency). It is shown that the influence of surfactant on the flow mechanics is significant for a specific range of capillary numbers. When the capillary number is within the range, surface elasticity increases liquid entrainment, thus forming a thicker film compared with the Landau-Levich solution for a clean liquid surface. The increase in film thickness is shown to be limited by a maximum factor of 4 2 2 in the limit of large surface elasticity or small surfactant diffusivity. If the capillary number is outside the range, either the viscous stress or the surface diffusion is dominant and the Landau-Levich solution is recovered.

A simulation for the transitional deposition of Langmuir-Blodgett films from Y to X type

A Monte Carlo deposition simulation is presented for a Langmuir-Blodgett multilayer film of cadmium arachidate to explain the experimentally observed inhomogeneous film thickness. The simulation is based on a model for a deposition defect known as a “transition from Y- to X-type deposition”. The transition is simulated by introducing a probability of surface molecule detachment considering interaction between neighbouring molecules. A rough surface derived from an inhomogeneous number of layers is predicted. The result is consistent with the measured distribution of the number of layers using soft X-ray reflection.

Measurement of contact angle relaxation during the deposition of Langmuir-Blodgett films of cadmium stearate and valinomycin

We studied the contact angle relaxation during the Langmuir-Blodgett deposition of cadmium stearate and valinomycin films. For cadmium, we found that the relaxation time of the receding contact angle depends strongly on the deposition type; the static value that the receding angle relaxes to depends on the concentration of Cd 2+ ions in the subphase. For 4.0 × 10 -2 M Cd 2+ subphase, the deposition begins and remains Y-type. However, at a lower concentration of 2.5 × 10 -3 M the deposition changes from Y- to X-type. Prior to this transition, the static receding angles for the two different Cd 2+ concentrations are different but the relaxation times are about the same. After the transition, the relaxation process is more than 20 times faster. For valinomycin, the deposition is predominantly Z-type.

The deposition and characterization of phosphatidic acid Langmuir-Blodgett films

The deposition of Langmuir-Blodgett films of the calcium salt of the phospholipid 1,2-dipalmitoyl- sn-glycero-3-phosphatidic acid is described. The layers have been investigated using the techniques of IR spectroscopy and ellipsometry. These methods reveal reproducible film deposition, up to 80 monolayers, with an average thickness per layer of 2.7 nm. The use of the multilayer structure as a host for molecules of the ionophore valinomycin is also briefly discussed.

Electrochromism in ytterbium bisphthalocyanine films deposited by the Langmuir-Blodgett technique

The surface pressure versus area isotherm of a Langmuir film of ytterbium bisphthalocyanine on water subphases of pH 2.1 to 9.2 has been investigated and an average area per molecule of 70 AA2 measured. The monolayer was found to collapse at a rate of 29%/hour. A dipping regime has been established for the deposition of Langmuir-Blodgett films with transfer ratios of unity. These deposited films were found to be electrochromic and their spectra are discussed.

Mobile ion drift in Langmuir-Blodgett films

The temperature dependence of the drift of polymerized 10,12-nonacosadiyinoic acid based metal-Langmuir-semiconductor field effect transistors (MLSFETs) is examined. It was concluded that the drift was due to metal ions, probably in the Langmuir-Blodgett (LB) film. It was deduced through elimination that the ions were introduced by the use of low purity (less than 500 kΩ resistivity) water for solid state processing and LB film deposition. The devices used to monitor the drift were n-channel silicon MLSFETs with the gate and drain shorted together to ensure operation in the saturation region. The drain current as a function of time under constant applied bias was measured and used to extract information about the polarity and activation energy of the drifting ions. The results were found to be best modeled by the first-order drift of two species of positive ions. The activation energies were found to be approximately 0.22 eV for the dominant species and 0.15 eV for the second species. These activation energy values are quite different from known values for sodium and potassium ions in SiO 2, leading to the suspicion that most of the drift is occurring in the polymer film. The final proof of the drift's being associated with mobile ions is the fact that by switching to high purity water for the processing (greater than 7 MΩ cm resistivity) and deposition (better than 14 MΩ cm resistivity) the drift was entirely eliminated.

Aspects of Langmuir trough design in relation to the study of molecular monolayers of polymers and condensed films

The merits are considered of Langmuir trough designs that minimize shear in monolayers by enclosing them within a frame that moves at the same speed as the monolayer. Use of such systems with Langmuir-Blodgett (LB) plates as wide as the frame would appear to be the logical way to transport monolayers for LB film deposition. However, for studies of condensed monolayers at the air-water interface over a range of areas and pressures it is important to compress the monolayers homogeneously. The use of a folding rectangular frame in the interface, designed for this purpose, is described. It is shown that when a polymer monolayer is compressed in this apparatus its properties become anisotropic and these can be detected by a Wilhelmy plate. Other applications suggested include its use to study the kinetics of monolayer collapse and in preparing oriented polymer specimens for research and technical applications.

Optical properties of highly ordered perylene multilayers

The deposition and characterization of Langmuir-Blodgett films containing the organic semiconductor perylene are described. By itself, perylene was found not to form good Langmuir-Blodgett films; however, when mixed with a fatty acid it was possible to build up high quality multilayers. These have been investigated using a number of techniques, including absorption and fluorescence spectroscopy, surface plasmon resonance and ellipsometry.

Deposition and structure of Langmuir-Blodgett films of ferric hydroxystearate

Langmuir-Blodgett films of ferric hydroxystearate (FeSt 2OH) were deposited onto silicon substrates by spreading directly on an aqueous subphase of pH 3.4–3.6. The effect of pH on the π- A curves of FeSt 2OH was studied. Auger spectroscopy, IR spectroscopy and X-ray diffraction were used to characterize the composition and the period structure of the film. The results show that the ferric ions of the adjacent layers interleave to form a planar structure.

Forced deposition of polar Z-type Langmuir-Blodgett films

Polar Z-type Langmuir-Blodgett films have been formed by UV-radiation-induced surface modifications of 10,12-nonacosadiyinoic acid. Metal-insulator-metal (MIM) and metal-insulator-semiconductor (MIS) capacitors were fabricated, tested and compared. Calculated polarization charge densities for the MIM devices were approximately 6.3 × 10 −7 C cm −2. This leads to a molecular dipole moment (assuming the charge separation to be equal to the molecule's length of 3.5 nm) of 9.80 × 10 −28 C cm (2.92 D). A shift in the flat-band voltage of MIS devices due to the built-in polarization field was observed. Using the difference from the flat-band voltage of a non-Z-type MIS capacitor the dipole moment was calculated for comparison with the MIM value. The difference in the dipole moments between the MIM and MIS devices was found to be under 20%.

The flow of molecular monolayers in relation to the design of Langmuir troughs and the deposition of Langmuir-Blodgett films

The flow of condensed molecular monolayers of barium stearate has been examined as a model system in relation to the deformation of monolayers during experimental procedures. It is shown that when a monolayer is transferred at constant area along the surface in a Langmuir trough the movement of the monolayer in relation to the boundaries sets up a circulation in the surface. This can be eliminated by a simple modification to a conventional trough. A new method is described to follow the progressive distortion of the monolayer during deposition of Langmuir-Blodgett films. This is applied to show how distortion can be minimised in a conventional Langmuir trough and to produce designs using a flexible belt to compress the monolayer. A belt profile is proposed which can be used to deposit the monolayer with minimal distortion.

The deposition and characterization of multilayers of the ionophore valinomycin

The deposition and characterization of Langmuir-Blodgett films of the ionophore valinomycin on single-crystal silicon and quartz substrates are described. To enable good monolayer transfer the substrates first need to be coated with a few fatty acid layers. The valinomycin is found to deposit onto either the hydrophobic or the hydrophilic surfaces of the fatty acid film. The valinomycin multilayers have been investigated using IR and UV spectroscopy. These techniques show that up to 60 layers of the ionophore may be built up with good reproducibility. A detailed investigation by ellipsometry has revealed a thickness gradient in the multilayers in the direction of dipping; the average thickness per monolayer is between 0.40 and 0.66 nm.

Second-order non-linear optical effects in Langmuir-Blodgett films

The synthesis of a new series of amphiphilic dyes of the form R 1 R 2 NC 6 H 4 CH CHC 5 H 4 + NCH 2 CH 2 COO - ( R 1= R 2= C 6 H 13, C 10 H 21, C 14 H 29 and C 18 H 37) is described. These dyes have two alkyl chains per chromophore so as to facilitate fast deposition of Langmuir-Blodgett films. We report on the fabrication of monolayer films of these materials and the measurement of optical second harmonic generation in them. We deduce information about molecular tilt orientations and hyperpolarizabilities and compare the latter with previously published values of the electro-optic coefficients.

Langmuir-Blodgett deposited cadmium gate inverted InP-GaInAs modulation-doped field-effect transistors

We report on the Langmuir-Blodgett film deposition and plasma etching of cadmium distearate on n-Gao.47Ino.53As to form a high-barrier-height Schottky barrier. Using this technique to form the gate electrode, we fabricated a 1?m-gate-length inverted InP-GalnAs modulation-doped field-effect transistor (MODFET) with an extrinsic transconductance of 170mS/mm and a cutoff frequencyfT of 19 GHz.

Opportunities for Langmuir-Blodgett films in piezoelectric and pyroelectric devices

Oriented films possessing dipole moments have seen increasing use of their piezoelectric and pyroelectric properties in pressure, acoustic, thermal, and optical devices. The performance of these devices in many applications is enhanced by thin film technology. The current thin film technology and possible opportunities for the developing Langmuir-Blodgett thin film deposition technique are reviewed with an emphasis on applications.

Deposition of Langmuir-Blodgett films of ferric stearate

Deposition of Langmuir-Blodgett films of ferric stearate