Langmuir-Blodgett Film Deposition Research Articles

Langmuir and Langmuir–Blodgett (LB) film properties of poly(9,9-dioctylfluorene)

The Langmuir (L) and Langmuir–Blodgett (LB) properties of a prototypical member of the polyfluorene family of commercially viable organic light emitting diode (OLED) materials, namely poly(9,9-dioctylfluorene) (PFO) are reported. The Langmuir monolayer was characterised by π-A isotherms. UV–Visible absorption spectroscopy demonstrated the existence of PFO within the β-phase conformation (exhibiting extended conjugation lengths) in the Langmuir monolayer on the water surface, as evidenced by the presence of a well-resolved peak at 435 nm. A high boiling point solvent (xylene) was used in the monolayer to facilitate LB film deposition, which was performed on glass and silicon substrates. Langmuir–Blodgett films of PFO were deposited at a range of pressures (2–15 mN m −1) and deposition speeds (100–1000 mm min −1). The LB films were characterised by photoluminescence spectroscopy and decay dynamics, by fluorescence microscopy, and by UV–Visible absorption spectroscopy. The latter measurements demonstrated that the β-phase component remained present after film deposition.

Moving contact lines and Langmuir–Blodgett film deposition

The objective of this paper is to point out the close relationship between contact line dynamics and LB film depositions, and it is designed to serve as a blueprint for future analysis of the LB technique. Moving contact lines and contact angles play a major role in Langmuir–Blodgett ultrathin film depositions. Although the effect of contact angles has been recognized for many years, a fundamental and comprehensive explanation of the phenomena taking place at the contact line has not been formulated before. Our understanding of contact line dynamics has improved thanks to careful experiments and new theoretical developments. Flow patterns depend on dynamic contact angle and the ratio of viscosities of the gas and liquid phases. More recently dynamic contact angles—and flow patterns—have been linked to forces of molecular and double-layer origin. The dynamic relationship of flow patterns to interfacial and transport properties can be used to explain seemingly contradictory experimental results reported by researchers during more than 60 years of experience with the L–B technique. Windows of operability can be defined for X-type and Z-type depositions that are useful in the design of experimental and industrial L–B deposition equipment.

Formation and Deposition of Langmuir-Blodgett Films with Thiol-Stabilized CdTe Nanoparticles

Incorporation of the thiol-stabilized CdTe nanoparticles into mono- and multilayer films produced by Langmuir-Blodgett (LB) technique was investigated using “pressure-area” isotherms, Quartz Crystal Microbalance, FTIR, UV/Vis absorption and photoluminescence spectroscopy. LB films were formed from cationic (octadecyltrinonylammonium iodide—ODTNA) and anionic (behenic acid) surfactants. Both noticeable increase of limiting surface area of behenic acid monolayer and simultaneous decrease of film strength have been established when the aqueous solution of CdTe nanoparticles was used as a subphase. LB films deposition on solid substrates depends on the type of stabilizing ligands (1-thioglycerol or 2-mercaptoethanol) for CdTe nanoparticles. Optical absorption and photoluminescence properties of the films obtained from behenic acid monolayers that has thioglycerol-stabilized CdTe nanoparticles incorporated into them acquire the same properties as the size-quantized CdTe nanoparticles. The bonding of the cadmium ions, that released as a result of a partial dissociation of ligand-stabilized CdTe nanoparticles with carboxylic group, is considered to be responsible for the incorporation in case of behenic acid surfactant, and in case of ODTNA the incorporation proceeds by means of a weak “nanoparticle-LB matrix” interaction.

Hydrodynamics of subphase entrainment during Langmuir–Blodgett film deposition

The phenomenon of subphase entrainment that accompanies Langmuir–Blodgett film deposition is examined in terms of the hydrodynamics of dip coating coupled with mass transport of the depositing monolayer. The governing equations for film thickness and for concentration of the monolayer material have been derived based on the lubrication approximation theory for steady-state conditions, subject to viscous, surface tension, gravity, intermolecular and Marangoni forces. The equations have been solved by numerical methods. In the absence of the monolayer, the model reduces to various forms of the case of substrate withdrawal from a pure liquid — the well known dip coating problem. Effect of several parameters related to material properties and operating conditions have been examined. We consider two mechanisms for subphase entrainment: the flow driven by the motion of the solid substrate, and Marangoni driven flow caused by surface tension gradient due to surface concentration gradient. Attractive van der Waals forces and Marangoni flow are responsible for thickening of the entrained subphase film. Gravity drainage and repulsive van der Waals forces thin the entrained subphase film. The entrained subphase film thickness measurement is done by using a dual channel spectrometer in conjunction with a software to obtain thickness from spectral information. Our numerical results are in good agreement with the extent of entrainment predicted by the model.

An indigenously developed electronic control system for Langmuir-Blodgett film deposition set-up

An indigenous and simple electronic control system for Langmuir-Blodgett (LB) film deposition set-up has been developed. This set-up consists of a microstepping circuit to drive the stepper motors with precision and smooth motion, essential for controlled movement of the barriers and substrate in the LB set-up. Linear variable differential transformer (LVDT)-based displacement measuring device has been developed and used to measure the surface pressure of the monolayer material spread on the water surface. A control program is written which incorporates all operational modes required to drive the set-up and to acquire the datain situ using a set of user-friendly commands. This control set-up has been successfully used to plot the pressure-area isotherm of various amphiphilic compounds such as ferric stearate, zinc arachidate etc. and for deposition of ordered LB films of ferric stearate.

Langmuir–Blodgett film forming properties of substituted TCNQ molecules

The synthesis of the zwitterionic D- π-A TCNQ adduct Z- β-(1-hexadecyl-2-quinolinium)-α-cyano-4-styryldicyanomethanide (C 16H 33(2)-Q3CNQ) has been performed for the Langmuir–Blodgett film deposition. The molecules in the floating Langmuir film are believed to lie at an angle of 19° to the water subphase. Optical absorption in the Langmuir–Blodgett films shows that the material is solvatochromic. The angle of molecular stacking in the Langmuir–Blodgett film is found to be 32° from surface plasmon measurements.

Transient electroluminescence from rubrene light-emitting diodes using double voltage pulse

Light-emitting diodes using a dye-insulating matrix blend of rubrene and arachidic acid have been fabricated using Langmuir–Blodgett film deposition technique. Transient electroluminescence (EL) measurements were performed by applying two consecutive square-wave voltage pulses separated by a short delay. This enabled us to study the effect of accumulated charges during the first pulse separated from that of the injected carriers, which are predominant during the second pulse. During the first voltage pulse, intrinsically accumulated charges recombine to generate an instantaneous EL peak, which is absent during the second pulse. A constant EL was observed during both the pulses. We also have observed an overshoot in EL in the falling edges of mainly negative voltage pulses.

Langmuir−Blodgett Films of a Manganese Corrole Derivative

The first example of deposition of Langmuir-Blodgett (LB) films of a manganese complex of corrole is described. Characterization of the morphological and molecular structure of these LB films has been carried out by means of different microscopic and spectroscopic methods. Depositions onto different substrates afford very uniform films of determined thickness; all the experiments carried out suggest that manganese correlates in LB films are normally oriented to the substrate, with a face-to-face packing of the macrocycles, due to pi-pi interactions. The interactions of these LB films with different volatile organic compounds (VOCs) have been investigated using the quartz microbalance (QMB) technique.

Study of horseradish peroxidase activity in alternate-layer Langmuir–Blodgett films

A previously proposed method of alternate-layer deposition, which enables the inclusion into the film of adsorbed proteins, was applied to the deposition of LB films with the structure (monolayer 1/adsorbed layer of horseradish peroxidase (HRP)/monolayer 2). Various combinations of the monolayers 1 and 2 were tested with regard to the following goals: to obtain a high efficiency of protein adsorption with retained enzymatic activity; to avoid detachment of protein molecules from the substrate in aqueous solutions; to increase duration of the activity preservation of HRP during sample storage; and to provide easy removal of protective monolayer 2 when the film is used for measurements. Creation of films with such characteristics is important for their successful bioanalytical applications. Spectrophotometric measurements of the HRP activity as well as quartz crystal microbalance studies reveal an interconnection between the properties of the deposited biologically active media, experimental conditions of their preparation, and film composition. To increase structural stability of films, electron beam sensitive polymer monolayers were used for the fabrication of some variants of the alternate-layer Langmuir–Blodgett (LB) films and enzymatic activity of irradiated samples was studied as well.

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.

On the deposition of Langmuir-Blodgett films of Q-state CdS nanoparticles through electrostatic immobilization at the air-water interface

The deposition of ordered multilayer Langmuir-Blodgett films of carboxylic acid derivatized Q-state CdS nanoparticles through electrostatic interaction with octadecyl amine Langmuir monolayers is demonstrated in this communication. The degree of nanoparticle incorporation into the Langmuir monolayer can be controlled by varying the charge on the amine molecules through the colloidal subphase pH. Pressure-area isotherms were used to follow the adsorption of the Q-state colloidal CdS particles at the amine surface while quartz crystal microgravimetry (QCM), optical absorption spectroscopy and ellipsometry were used to characterize the multilayer LB films. The films were found to grow in the classical γ-type mode. Ellipsometry measurements indicated a bilayer thickness of 8.2 nm which yields a cluster size of 3.2 nm, in reasonable agreement with a mean cluster size of ∼3.5 nm determined from the position of the band edge obtained from optical absorption measurements. Incorporation of clusters into inorganic matrices as outlined in the communication shows promise for the growth of superlattice structures, mixed cluster systems, etc., which are not realizable by currently-used experimental methods.

Ultrathin Organic Films Grown by Organic Molecular Beam Deposition and Related Techniques.

During the past decade, enormous progress has been made in growing ultrathin organic films and multilayer structures with a wide range of exciting optoelectronic properties. This progress has been made possible by several important advances in our understanding of organic films and their modes of growth. Perhaps the single most important advance has been the use of ultrahigh vacuum (UHV) as a means to achieve, for the first time, monolayer control over the growth of organic thin films with extremely high chemical purity and structural precision.1-3 Such monolayer control has been possible for many years using well-known techniques such as Langmuir-Blodgett film deposition,4 and more recently, self-assembled monolayers from solution have also been achieved.5 However, ultrahighvacuum growth, sometimes referred to as organic molecular beam deposition (OMBD) or organic molecular beam epitaxy (OMBE), has the advantage of providing both layer thickness control and an atomically clean environment and substrate. When combined with the ability to perform in situ highresolution structural diagnostics of the films as they are being deposited, techniques such as OMBD have provided an entirely new prospect for understanding many of the fundamental structural and optoelectronic properties of ultrathin organic film systems. Since such systems are both of intrinsic as well as practical interest, substantial effort worldwide has been invested in attempting to grow and investigate the properties of such thin-film systems. This paper is a review of recent progress made in organic thin films grown in ultrahigh vacuum or using other vapor-phase deposition methods. We will describe the most important work which has been published in this field since the emergence of OMBD in the mid-1980s. Both the nature of thin-film growth and structural ordering will be discussed, as well as some of the more interesting consequences to the physical properties of such organic thin-film systems will be considered both from a theoretical as well as an experimental viewpoint. Indeed, it will 1793 Chem. Rev. 1997, 97, 1793−1896

Structure and stability of Langmuir monolayers and Langmuir-Blodgett films of bisaroyl azide bolaamphiphiles

Langmuir-Blodgett (LB) films of bolaamphiphiles with reactive head groups can be used for the surface modification of composite membranes for gas-separation processes. The scope of our investigations was to gain a first insight into the monolayer behaviour and LB film structure of previously synthesized bisaroyl azide bolaamphiphiles. The layers have been analysed by means of surface potential measurements and Brewster angle microscopy. Furthermore, parameters for successful deposition of LB films were found. As expected for a molecule with two hydrophilic ends, the transfer ratio on upstroke was close to one and no transfer occurred on downstroke. The multilayer structure was analysed by scanning force microscopy (SFM) and X-ray reflectivity measurements. The SFM images revealed a periodic in-plane structure on the molecular level. Based on a combination of the X-ray data with the results of other methods, two possible models of the multilayer structure are presented.

Electrochemical and optical characteristics of conducting poly( o-toluidine) films

Conducting poly( o-toluidine) (POT) films have been prepared on indium-tin-oxide (ITO) glass plate by electrochemical, dip-coating and Langmuir-Blodgett (LB) film deposition techniques. The electrochemical and optical properties of poly( o-toluidine) films prepared by different methods have been compared using cyclic voltammetry, UV-visible and Fourier transform infrared spectrophotometry techniques.

Alternating-Current Light-Emitting Diodes and their Transient Characteristics: Response Time and Carrier Transport

AbstractWe report the fabrication and characterization of alternating current light-emitting diodes (LEDs) with quinquethiophene as the emitting material. We have obtained equal electroluminescence intensity in both bias sections. From the frequency response of the LEDs, we have estimated the device response times and compared them with the response times obtained from the transient response of dc LEDs. Langmuir-Blodgett film deposition technique has been employed to control the thickness of the emitting layer on the molecular scale. We have shown that the response times originate from the accumulation rather than the transit of charge carriers. We have compared the photo- and electroluminescence spectra of QT LEDs.

Instrument for depositing Langmuir–Blodgett films composed of alternating monolayers using a protective layer of water

A novel instrument has been developed for depositing Langmuir–Blodgett (LB) films composed of alternating molecular monolayers of amphiphilic molecules and adsorbed layers of soluble compounds. The main feature of the deposition method is the possibility to prevent the transfer of the monolayer onto the substrate when dipping up. This can be achieved by keeping the external boundary of the deposited monolayer in contact with the aqueous solution by means of a protecting hydrophilic plate which covers the substrate with the film while still immersed in water. After that, the plate is fixed to the substrate parallel to its surface and both can be transferred to other compartments of the instrument for adsorption of soluble compounds onto the active hydrophilic surface or for deposition of monolayers of other types. Principal design elements, electronics, and software of the LB instrument based on this principle are described. Results on the deposition of LB films composed of alternating monolayers of amphiphilic donor molecules of hexadecylbis(ethylenedithio)tetrathiafulvalene and adsorbed layers of the soluble protein cytochrome P 450scc are reported as an example of the device functioning.

Composite gas-separating membranes with polymerized LB films

Composite gas-separating membranes were prepared by deposition of multibilayer diacetylene Langmuir-Blodgett (LB) films onto a poly(dimethylsiloxane) membrane. Local measurements of gas permeability with the surface resolution of 0.2 cm 2 show that LB modification increases both membrane permeability to O 2 and He gases and dispersion of local permeability over the membrane surface. A poly (diacetylene) LB film increases the selectivity of O 2 N 2 and He N 2 separation by 1.5 and 1.2 times, respectively. The potential of LB films in the improvement of gas separation is discussed.

Deposition and enzymatic activity of Langmuir-Blodgett films of alkaline phosphatase

Langmuir-Blodgett (LB) films of alkaline phosphatase were deposited. The dependence of the enzymatic activity on the conditions of monolayer formation and film transfer onto a solid support was studied. Alkaline phosphatase molecules are very hydrophilic and it is impossible to obtain a significant amount of protein at the surface of pure water. To form a stable monolayer, aqueous solutions of high ionic strength were used. High surface pressures were obtained when 3 M solutions of NaCl or (NH 4) 2SO 4 were used as the subphase. The protein films deposited under such conditions possess an enzymatic activity which increases with deposition pressure. The films were characterized by circular dichroism (CD) spectroscopy, ellipsometry and microgravimetry techniques. Alkaline phosphatase molecules in the film are more thermostable than those in solution. The CD spectra of the films remain unchanged up to 200 °C. The activity of the protein is retained after heating up to 80 °C.

C-nitroso compounds as novel promising substances for the deposition of Langmuir-Blodgett films

C-nitroso compounds and their complexes with transition metals have never been used before in Langmuir experiments. Some isotherms have been registered for several C-nitroso derivatives and for two metal complexes with Pd(II) and Rh(I) thin films were deposited by the standard vertical dipping method. UV-Vis spectroscopy has been performed in order to analyse the absorption bands of the deposited films. Measurements of the dependence of the conductivity were carried out between 300 and 450 K. All samples display an activated conductivity at high temperature and related activation energies were determined. A decrease of the activation energy is reported for the palladium complex thin films into a flux of air and ppm NO 2.

Structure and polymerisation behaviour of cadmium 10,12-pentacosadiynoate Langmuir-Blodgett films

Two different types of Y-films (a Y-film from Z-type deposition and a Y-film from Y-type deposition) have been observed during the Langmuir-Blodgett (LB) film deposition of metal diynoates, but their structural characterisation has not received much attention. In the present work, a study of LB film deposition of cadmium 10,12-pentacosadiynoate, for evaluating the effects of monolayer aging and concentration of spreading solution, has been carried out and three different types of Y-films of cadmium 10,12-pentacosadiynoate have been obtained. Structural characterisation and study of the topochemical polymerisation of different types of films show remarkably large differences in their bilayer spacing and diacetylene polymerisation behaviour. This study shows that Y-films obtained by overturning in the Z-type build-up process are structurally quite different from the Y-films obtained from the Y-type build-up process.