Multilayer LB Films Research Articles

Role of initial morphology in H 2S treated cadmium arachidate Langmuir - Blodgett films

To examine the issues like the shape of the CdS clusters, time and thickness dependence of chemical conversion and to study the correlation between the pre and post sulfidation structure of the LB films on CdS formation, a set of multilayer cadmium arachidate LB films grown with different morphology and thickness have been studied using AFM, TEM, X-ray reflectivity and XPS. Microscopic evidence of formation of nearly uniform CdS nanoparticles of around 4.5 nm in the LB matrix was observed. In relatively compact films, mount like structures were found to develop due to out-of-plane molecular motion, whereas, for non compact films, expansion and molecular rearrangement appears to reduce the height non-uniformity and smooth out the surface as a result of sulfidation. Full conversion of cadmium arachidate to CdS was not achieved in the LB matrix even after a long exposure and the time required for conversion was found to be independent of film thickness, both contrary to the earlier observation. A partial conversion of a maximum of around 60% could be achieved in the films of various thickness and compactness indicating that partial survival of the layered structure after sulfidation may be possible in initially compact films.

Study on Conductivity Properties of PEDOT Prepared through a Modified LB Film Method

AbstractAdopting LB film method, an arachidic acid (AA)/PEDOT multilayer LB film was chosen, and polymerized EDOT monomers in hydrophilic group of LB to prepare arachidic acid (AA)/PEDOT multilayer LB film. UV‐Vis, FT‐IR and XPS analyses implied that EDOT was effectively polymerized in film, and thus PEDOT conducting polymer was produced. Analyses of XRR and SIMS indicated that film had a well‐arranged lamella structure, and further research showed that polymerization of EDOT in AA film destroyed the orderliness of the original LB film. This phenomenon could be related to the destructive effect of polymerization on layered structure. We used four‐point probe and semiconductor instrument to study the conductivity property of the film, and observed that the conductivity of AA/PEDOT film had sudden changes with the processing time of changes in effective conduction network. That was caused by "permeability" in conducting channel of multilayer film. The test results also indicated that the conductivity of AA/PEDOT film was obviously better than that of spin‐coating PEDOT/PSS film or that of ODA‐SA/PEDOT‐PSS film, which was due to the higher π structure of PEDOT structure and ordered film structure.

Lateral 2D−3D Phase Segregation in Fatty Acid/Fatty Amine Monolayers Induced by Langmuir−Blodgett Deposition

We describe the formation of lateral 2D-3D patterns in mixed multilayer LB films of stearic acid (SA) and octadecylamine (ODA) deposited from aqueous subphases at a basic pH. The 3D particles of SA constituting the micrometer-scale linear assemblies in the LB film are assumed to segregate at the three-phase contact line in the course of film deposition. This 2D-3D phase separation of the two-component system presumably originates from the substrate-induced lowering of the collapse point of SA that leads to spontaneous 3D condensation of an acid on a solid support. The morphology of SA/ODA LB patterns is sensitively influenced by the deposition speed and surface pressure, while the chemistry of the solid support does not affect the resulting structures. The possible mechanism that controls the specific orthogonal arrangement of the 3D phase of SA in the LB film through wettability oscillations is suggested.

Supramolecular self-assembly study of a flexible perylenetetracarboxylic diimide dimer in Langmuir and Langmuir–Blodgett films

A novel perylenetetracarboxylic diimide molecule ( 2PDI-TAZ), which contains two perylenetetracarboxylic diimide (PDI) attached to a melamine headgroup, was designed and synthesized. Supramolecular self-assemblies were studied in Langmuir and Langmuir–Blodgett films. Surface pressure–area isotherm measurements and the spectroscopic studies indicate that the 2PDI-TAZ molecules adopted a face-to-face configuration and edge-on orientation in Langmuir or the multilayer LB films. The presence of the barbituric acid in subphase change the hydrophilicity of 2PDI-TAZ due to the hydrogen bonding between melamine and barbituric acid, which has been revealed by the π – A isotherms and the FT-IR spectra. Transmission electron microscopy images of the LB films deposited from the barbituric acid solution revealed uniform nanowire morphology while the X-ray diffraction studies indicate that the molecules in the solid film packed with high order. The strong excimer emission of 2PDI-TAZ in LB films suggests enforced face-to-face configuration for the PDI unites in LB films in relative to that in solution.

Fabrication of a novel 1,3-bis(p-hydrazonobenzoicacid)indane Langmuir–Blodgett film and organic vapor sensing properties

Abstract A novel 1,3-bis( p -hydrazonobenzoicacid)indane (HBI) molecule was synthesised for the study of monolayer properties at the air–water interface, the fabrication of a Langmuir–Blodgett film, and the investigation of organic vapor sensing properties. Langmuir properties of HBI were studied by taking an isotherm graph. Two different surface pressures were selected for the fabrication of multilayer LB films that were deposited onto glass and quartz crystal substrates. UV–vis spectroscopy and a quartz crystal microbalance (QCM) measurement system were employed to monitor the deposition quality under the two selected deposition pressures. Our results showed that a uniform and high quality LB film with a transfer ratio of 0.96 occurred using the HBI material at a deposition pressure of 22.5 mN m −1 . The LB film deposited onto the quartz crystal was used to study the interaction of the HBI material with different organic vapors such as chloroform, benzene, toluene, ethyl alcohol and isopropyl alcohol. Our results show that the HBI material is suitable to produce an organic thin LB film. Our results indicate that the HBI LB film is highly sensitive to chloroform vapor with a fast and reversible response. This new HBI material could be used as a vapor sensing material.

Electron transfer studies on cholesterol LB films assembled on thiophenol and 2-naphthalenethiol self-assembled monolayers

We have formed the cholesterol monolayer and multilayer LB films on the self-assembled monolayers of 2-naphthalenethiol (2-NT) and thiophenol (TP) and studied the electrochemical barrier properties of these composite films using cyclic voltammetry and electrochemical impedance spectroscopy. We have also characterized the cholesterol monolayer film using grazing angle FTIR, scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Cholesterol has a long hydrophobic steroid chain, which makes it a suitable candidate to assemble on the hydrophobic surfaces. We find that the highly hydrophobic surface formed by the self-assembled monolayers (SAM) of 2-NT and TP act as effective platforms for the fabrication of cholesterol monolayer and multilayer films. The STM studies show that the cholesterol monolayer films on 2-NT form striped patterns with a separation of 1.0 nm between them. The area per cholesterol molecule is observed to be 0.64 nm2 with a tilt angle of about 28.96 degrees from the surface normal. The electrochemical studies show a large increase in charge transfer resistance and lowering of interfacial capacitance due to the formation of the LB film of cholesterol. We have compared the behavior of this system with that of cholesterol monolayer and multilayers formed on the self-assembled monolayer of thiophenol.

Langmuir monolayers and Langmuir–Blodgett films of ferritin prepared by using a surfactant mixture of eicosylamine (EA) and methyl stearate (SME)

Magnetic Langmuir–Blodgett films of ferritin have been prepared by using the adsorption properties of a 1/4 mixed monolayer of eicosylamine (EA) and methyl stearate (SME). BAM images show that a more homogeneous distribution of ferritin at the air–water interface is achieved by using this mixture of surfactants instead of the DODA/SME mixed matrix of a previous work. Transfer of the monolayer onto different substrates allowed the preparation of multilayer LB films. Infrared and UV–Vis spectroscopies indicate that ferritin molecules are incorporated within the LB films. Furthermore, UV–Vis spectroscopy measurements reveal that the amount of ferritin incorporated into these LB film has been increased with respect to that in the DODA/SME LB films. Finally magnetic measurements confirm that the superparamagnetic properties of this molecule are preserved in the LB films.

Magnetic Langmuir−Blodgett Films of Ferritin with Different Iron Contents

Magnetic Langmuir-Blodgett films of four ferritin derivatives with different iron contents containing 4220, 3062, 2200, and 1200 iron atoms, respectively, have been prepared by using the adsorption properties of a 6/1 mixed monolayer of methyl stearate (SME) and dioctadecyldimethylammonium bromide (DODA). The molecular organization of the mixed SME/DODA monolayer is strongly affected by the presence of the water-soluble protein in the subphase as shown by pi-A isotherms, BAM images, and imaging ellipsometry at the water-air interface. BAM images reveal the heterogeneity of this mixed monolayer at the air-water interface. We propose that the ferritin is located under the mixed matrix in those regions where the reflectivity is higher whereas the dark regions correspond to the matrix. Ellipsometric angle measurements performed in zones of different brightness of the mixed monolayer confirm such a heterogeneous distribution of the protein under the lipid matrix. Transfer of the monolayer onto different substrates allowed the preparation of multilayer LB films of ferritin. Both infrared and UV-vis spectroscopy indicate that ferritin molecules are incorporated within the LB films. AFM measurements show that the heterogeneous distribution of the ferritin at the water-air interface is maintained when it is transferred onto solid substrates. Magnetic measurements show that the superparamagnetic properties of these molecules are preserved. Thus, marked hysteresis loops of magnetization are obtained below 20 K with coercive fields that depend on the number of iron atoms of the ferritin derivative.

Langmuir–Blodgett films of poly(phenylacetylene) derivatives

LB films of poly(alkoxyphenyl)acetylenes were fabricated, and the higher order structure in the bidimensional field was examined. The polymers formed monolayer films on water surface, though these polymers have no hydrophilic group. For the preparation of the multilayer LB film the horizontal lifting method was more effective than the vertical dipping. The type of substituent affected the aggregated structure and orientation of the film. Area per monomer unit decreased when a chiral alkoxyl group was introduced into para-position. The LB films for the meta-substituted polymer showed some anisotropy in the polarized ultraviolet visible absorption spectrum, indicating that in-plane orientation was induced by compression. The absorbance observed in the perpendicular to direction of compression was stronger than that of parallel one.

Aggregation Behavior of Heteroleptic Tris(phthalocyaninato) Dysprosium Complexes with Different Alkoxy Chains in Monolayer or Multilayer Solid Films

Three heteroleptic tris(phathlocyaninato) dysprosium triple-decker complexes with different alkoxy groups at the peripheral positions of the medium phthalocyanine ligand (Pc)Dy[Pc(OCnH(2n+1))8]Dy(Pc) (n = 4, 8, 16) (I-III) {Pc = unsubstituted phthalocyaninate; Pc(OC4H9)8 = 2,3,9,10,16,17,23,24-octakis(butyloxy)phthalocyaninate; Pc(OC8H17)8 = 2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninate; Pc(OC16H33)8 = 2,3,9,10,16,17,23,24-octakis(hexadecyloxy)phthalocyaninate} have been synthesized, and their aggregate behaviors in monolayer and multilayer solid films have been comparatively studied. The pure compounds and their 1:4 mixtures with stearic acid (SA) have been found to form a stable monolayer at the air/water interface with a tilted edge-on orientation of (Pc)Dy[Pc(OCnH(2n+1))8]Dy(Pc) molecules. In the pure monolayers of the three triple-decker compounds, wirelike molecular aggregates were observed by high-resolution transmission electron microscopy (HRTEM). Adding SA has been found to prevent triple-decker compounds (Pc)Dy[Pc(OC4H9)8]Dy(Pc) (I) and (Pc)Dy[Pc(OC8H17)8]Dy(Pc) (II) from forming large aggregates, and small domains with a diameter of ca. 10 nm were observed in the mixed monolayers. HRTEM studies revealed that two crystalline phases with rectangular and hexagonal lattice structure are present in the small domains. However, both pi-A isotherms and HRTEM studies indicated that the mixed monolayer of compound (Pc)Dy[Pc(OC16H33)8]Dy(Pc) (III) with SA did not show a difference from the corresponding pure monolayer. The SA molecules were pressed into the cavity above the phthalocyanine ring formed by the eight long hexadecyloxy side chains of the medium macrocycle ligand in III. The multilayer LB films of all of these triple deckers fabricated by the vertical dipping method showed very good layered structure as revealed by the multiple-order diffraction peaks in low-angle X-ray diffraction (LAXRD) patterns.

Comparison of molecular orientation and phase transition behaviors in the two kinds of ordered ultrathin films of reversed duckweed polymer ES-3 studied by infrared grazing reflection–absorption spectroscopy

A multilayer LB film and a casting film of reversed duckweed polymer ES-3 on Au-evaporated glass slides were investigated by Fourier Transform infrared grazing reflection–absorption spectroscopy. It is found that the two kinds of ordered ultrathin films have different orientation of alkyl chains, nearly perpendicular to the substrate surface for the LB film while rather tilted for the casting film. The studies on their thermal transition behaviors indicate that both of the films have three phase transition processes, respectively, occurring near 65, 105 and 140 °C for the former while near 80, 105 and 140 °C for the latter, but show different transition behavior in the each corresponding transition process. It is referred that at room temperature there are island-like domain structures formed in the LB film, but no ones in the casting film; however, the latter can form the domain structures between the first two transition points due to the desorption of solvents. The formation of domain structure seems to play two important roles, one of which is to make alkyl chains more perpendicular to the substrate surface, and the other to make alkyl chains more packed closely. Thermal cyclic experiments reveal that neither of the films could return to its original state after thermal cyclic treatment up to the temperature, which is above the third transition point, although its alkyl chain becomes highly ordered again.

The characteristics and gas-sensing property of bis[phthalocyaninato] rare earth complexes based charge-flow transistor

A new kind of sandwich-like bis[2,3,9,10,16,17,23,24-octakis(octyloxy)phthalocyaninato] rare earth complexes RE[Pc ∗] 2 ( Pc ∗= Pc( OC 8 H 17) 8 , RE=Sm, Pr, Er) are used to film-forming material. Pure RE[Pc ∗] 2 and mixture of RE[Pc ∗] 2 and auxiliary film-forming solvents—stearic acid (SA) and octadecanol (OA) deposited from both pure water and 10 −4 M Cd 2+ subphases are investigated. It is found that a mixture of Sm[Pc ∗] 2 and OA forms an excellent material for the fabrication of gas-sensing LB film by studying the film-forming characteristics. A new gas sensor has been fabricated by incorporating the different multilayer LB films into the gate electrode of a MOSFET, forming an array of charge-flow transistor (CFT). On the application of a gate voltage ( V GS), greater than the threshold voltage ( V TH), a delay is observed in the response of the drain current. It is due to the time taken for the resistive gas-sensing film to charge up to V GS. This delay characteristic has been found to depend on the concentrations of NO 2, suggesting that the CFT device can be used as an effective gas sensor. Results are presented of studies the dc operating and turn-on characteristics as well as gas-sensing property, showing that the device can detect reversibly concentration of NO 2 gas down to 5 ppm at room temperature.

Electrical properties of bis(4-diethyannodithiobenzil)nickel and stearyl alcohol mixed Langmuir–Blodgett films

Direct current (d.c.) electrical properties of bis(4-diethyannodithiobenzil)nickel and stearyl alcohol mixed Langmuir–Blodgett films (BDN-SA LB films) were measured using multilayer LB films deposited on planar interdigital electrodes under various pressure and temperature conditions. The results show that the conductivity of the LB films is strongly influenced by the pressure and temperature. The activation energy is estimated to be approximately 0.56 eV, close to the 0.50 eV value of the bulk material. The correlation between the electrical conductivity of the LB films with atmosphere pressure is discussed.

Design of new fluorinated bridged push–pull stilbenes and preparation of LB films for second harmonic generation in the blue domain

In order to prepare alternating Y-type multilayers involving push–pull stilbenes giving second harmonic generation (SHG) in the blue domain, we have synthesized two new bridged compounds, 1 and 2, with the hydrophobic chain grafted onto the donor group. The photophysical properties of these dyes in both solution and LB films are reported. The SHG signal of monolayer LB films containing 1 and 2 are close to that of the analog 3 and 4 having the hydrophobic chain grafted onto the acceptor group. Finally, multilayer LB films based on one, two and four bilayers of 1/3 or 2/4 were prepared and the SHG measurements show a regular noncentrosymmetric arrangement for 2/4 films only.

A Study on Two-Dimensional Phase Transitions in Langmuir Monolayers of a Carboxylic Acid with a Symmetrical Triphenylbenzene Ring System

A detailed investigation on phase transitions in Langmuir monolayers of 5‘-phenyl-1,1‘:3‘1‘ ‘-terphenyl-4-carboxylic acid (PTCA) at the air−water interface is presented. Surface pressure and surface potential isotherms were obtained using varying experimental parameters such as spreading volume, subphase pH, ionic strength, subphase temperature, and compression speed. The isotherms exhibit two liquid-condensed regions separated by a broad plateau with a 2-fold decrease in area/molecule. PTCA monolayers were visualized by Brewster angle microscopy, transferred as multilayer LB films and characterized by Fourier transform infrared spectroscopy, X-ray diffraction (XRD), and surface potential measurements. On the basis of the analysis of surface pressure and surface potential isotherms, it was not possible to distinguish between bilayer formation and conformational changes of the triphenylbenzene ring system upon compression as the origin of the plateau. The LB transfer from monolayers in the plateau was unsuccessful and thus characterization of LB films does not help in elucidating the possible cause for the transition.

Surface Pressure and Surface Potential Isotherms of Ytterbium Bisphthalocyanine Langmuir Monolayers

A systematic study is reported on the monolayer characteristics of ytterbium bisphthalocyanine (YbPc2) at the air−water interface, using surface pressure and surface potential isotherms. The influence of various experimental parameters was analyzed, including compression speed, spreading volume, position of the pressure sensor, and subphase temperature. The area per molecule decreased with increasing spreading volumes. Interestingly, the position of the Wilhelmy plate affected the pressure−area isotherms, which does not occur for traditional aliphatic compounds. No significant influence from the subphase temperature was detected. Upon varying the compression speed, the surface potential curve and the collapse pressure were affected, even though the area per molecule remained practically the same. When these monolayers with distinct dipole moments were transferred onto a gold-coated glass substrate, the resulting multilayer LB films exhibited the same surface potential, probably due to molecular rearrangement upon transfer.

Terbium Luminescence in Langmuir−Blodgett Films of Octafunctionalized Calix[4]resorcinarenes

Calix[4]resorcinarenes functionalized with pendant α-acetates or α-(diethyl acetamides) [(n-C11H23)4−C[4]R−(OCH2COX)8; 1, X = OMe; 2, X = NEt2] have been shown to incorporate terbium(III) ions into Langmuir−Blodgett films. Energy-transfer luminescence from a single monolayer on quartz has been demonstrated. An excitation wavelength of 280 nm, corresponding to the chromophore of the ligands aromatic rings, was used. Emission was observed at 489, 545.5, 586, and 620 nm, corresponding to the known transitions of terbium (5D4 → 7F6, 5D4 → 7F5, 5D4 → 7F4, and 5D4 → 7F3, respectively). It was demonstrated that a multilayer LB film of 1 and a monolayer film of 2 readily abstract terbium(III) from 2 × 10-4 M aqueous solutions of TbCl3 and that energy-transfer luminescence was subsequently exhibited.

Variation of monolayer behaviour and molecular packing in zinc arachidate LB films with subphase pH

Monolayer characteristics and multilayer LB films of zinc arachidate (ZnA) have been studied as a function of subphase pH in the range 4.9–8.0. The monolayer stability was found to be strongly dependent on the subphase pH. Measurement of bilayer spacings in transferred multilayers using X-ray diffraction revealed that depending on subphase pH, three types of molecular packing arrangements corresponding to alkyl chain tilt angles of 0°, 19° and 31° are possible in ZnA multilayers. In certain ranges of subphase pH, more than one type of packing arrangements were found to coexist. These results suggest that the packing arrangement in ZnA multilayers is significantly affected by the properties of the monolayer at the air–water interface and may not be simply related to metal ion electronegativity.

Langmuir–Blodgett Film Formation of a Fullerene Dicarboxylic Acid Derivative C60(HOOCCHNHCHCOOH) and its Photocurrent Generation

The Langmuir films of a fullerene dicarboxylic acid derivative I, C60(HOOCCHNH2CHCOOH)CF3SO3H (FDA), at the air/water interface have been investigated. FDA formed a stable monolayer on pure water when dilute solution (1.04 × 10−5mol/L) was used as the spreading solution. The monolayer formation on various subphases was investigated, including solutions containing different monovalent (Na+, Ag+) and divalent (Ca2+, Cd2+, Cu2+) cations. The introduction of these cations makes the monolayer more stable. The limiting molecular area is also dependent on the pH of the subphase. The surface pressure versus area isotherms at different temperature clearly indicate that FDA molecules easily aggregate at high temperature. The monolayer and multilayer LB films of FDA were deposited on quartz and glass substrates from Ca2+, Cd2+, Cu2+, or Cu2+/Phen solution subphase, and their UV–vis and XPS spectra were obtained. The data showed that for Cd2+and Cu2+/Phen subphase the cation can be transferred onto the substrates together with FDA but not the other two. The photoelectrochemical behavior of FDA on an ITO electrode was investigated, which showed that the photocurrent is cathodic. Oxygen and negative bias are beneficial factors for the photocurrent generation. The external quantum yield is ca. 4%.

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.