Magnetic volcanos in gadolinium Langmuir–Blodgett films

Magnetic, structural and electronic properties of Langmuir–Blodgett films with incorporated Gd³⁺ ions has been detected using a scanning DC SQUID microscope, scanning electron microscope and X-ray diffraction. The magnetic images of 28 and 50 layer thick films at 77 K have been obtained after in-plane and out-of-plane pre-magnetization in a field of 1.4 T at 300 K. Randomly placed “magnetic volcanos” with a remanent magnetic moment of the order of 10⁻¹³ A m² was observed. A decay of the remanent magnetization with a characteristic time of about 120 h was observed. It is suggested that the magnetic order is relatively long ranged, and that topological defects (vortices) lead to the observed out-of-plane field lines, and are responsible for the magnetic volcanos. Finally, it is hypothesized that a similar topology of field lines is responsible for superconductivity as observed in ceramic high-T_C superconductors.     

Optical investigation of CdS quantum dots in Langmuir–Blodgett films

Structures with CdS quantum dots produced by the Langmuir–Blodgett (LB) technique were investigated by Raman, IR, and UV spectroscopies. The confinement effect of longitudinal optical (LO) phonons in CdS quantum dots was investigated by Raman spectroscopy. Surface vibrational modes of CdS quantum dots were observed in IR spectra. It was shown experimentally that the frequency of the surface vibrational modes depends on the properties of the surrounding media. An average size of CdS quantum dots of about 3–6.4 nm was obtained from the analysis of UV measurements. Received: 1 February 1999 / Accepted: 1 April 1999 / Published online: 19 May 1999     

Ordering and growth of Langmuir–Blodgett films: X-ray scattering studies

The interplay of ordering, confinement and growth in ultrathin films gives rise to various interesting phenomena not observed in bulk materials. The nature of ordering and interfacial morphology present in these films, in turn, depends on their growth mechanism. Well-ordered metal–organic films, deposited using an enigmatic Langmuir–Blodgett (LB) technique, are not only ideal systems for understanding the interplay between growth and structure of ultrathin films but also for studying chemical reactions and phase transitions in confined geometries. Studies on these LB films also enhance our understanding of the fundamental interactions of amphiphilic molecules important for biological systems. Advent of grazing incidence X-ray scattering techniques has enabled us to probe the interfacial structure of these multilayer films at very high resolution and as a result has improved our knowledge about the mechanism of growth processes and about physical/chemical properties of ultrathin films. In this review we will focus our attention on recent results obtained using these X-ray scattering techniques to understand the mechanism of growth leading to formation of remarkably well-ordered LB films after giving a brief outline of these scattering techniques. In addition, we also review recent results on growth and structure of nanoparticles formed by suitable chemical processes within the ordered matrix of LB films. Finally, we will discuss the work done on melting of LB films and its implications in our understanding of melting process in lower dimensions. In all these studies, especially those on as-deposited LB films results of atomic force microscopy measurements have provided important complementary morphological information.     

Preparation and probe analysis of Langmuir–Blodgett films with metal-containing dendritic and cluster structures

We aim to reveal the influence of?pH values of the working solution on spatial arrangement of metal-containing inclusions in the monolayer coatings prepared by Langmuir?CBlodgett (LB) technology and transferred to solid substrates with consequent modification of surface electrical properties. Consequently, films with inclusions in the form of dendrites and cluster shapes have been obtained. Submicron and micron lead-containing formations in Langmuir?CBlodgett films have been characterized by using the atomic force microscopy, Kelvin probe microscopy, and scanning electron microscopy methods. The results show that increasing?pH value of the subphase has caused the significant changes in the shape and composition of the inclusions in Langmuir?CBlodgett films. The synthesized inclusions on the solid substrate surface resulted in formation of the regions with the local electric fields and, as consequence, to significant modification of the structure and electrical properties. The location and length of the regions with surface potential disturbance depend on the shape of inclusions, and consequently, on?pH value of the subphase used in LB technology.     

X-ray standing wave studies of metal ions incorporation in Langmuir–Blodgett films

The molecular organization in Langmuir–Blodgett films of phospholipids and metallomesogen deposited on silicon has been studied by the X-ray standing-wave method at the synchrotron radiation source BESSY (Germany). Analysis of the experimental data obtained made it possible to determine the composition of the organic multilayer nanosystems and localize the position of metal ions incorporated in organic layers from the aqueous subphase during film deposition.     

Interfacial electrostatic phenomena in polyimide Langmuir–Blodgett films in electron tunneling devices

Nanometric electrostatic space charges exist at the metal/molecular film interface due to the displacement of excessive electrons from metal to molecular films. As a result, surface potential is built across the films on metal. The distributions of the displaced electrons and the electronic density of states in films can be determined using the surface potential built. The current–voltage characteristic of tunneling device using polyimide Langmuir–Blodgett films is discussed with taking into account the space charge.     

Effects of nanoparticle doping on the phase transitional behaviour of ferroelectric liquid crystal Langmuir–Blodgett composite films

Langmuir–Blodgett films of ferroelectric liquid crystals (FLCs) doped with a low concentration of functionalized Al: ZnO (AZO) nanoparticles were prepared and characterized. Pressure–area isotherms show that the nanoparticles as well as FLC composite systems have the capability to form stable monolayers at the air–water interface. The molecular interaction between nanoparticles and FLC molecules increased during barrier compression, which resulted in increased surface pressure. We observed various phases in isotherms with increasing concentration of nanoparticles in the FLC matrix. An X-ray diffraction profile at a low angle confirmed that FLCs retain their layer structure at a low concentration doping of AZO nanoparticles in the FLC matrix. Atomic force microscopy images indicate that low wt% composites are uniformly deposited without disturbing the translation behaviour of SmC* liquid crystals.     

Immobilization of glucose oxidase onto Langmuir–Blodgett films of poly-3-hexylthiophene

Monolayers of poly(3-hexyl thiophene) (P3HT) were obtained by dispensing a solution of P3HT with stearic acid (SA) in chloroform at air-water interface using Joyce–Loebl LB trough. These films were transferred onto indium-tin-oxide coated glass plate by vertical dipping method. Enzyme glucose oxidase (GOX) was immobilized on the films using LB technique. These films were characterized by X-ray diffraction and atomic force microscopic techniques. Photometric response of these P3HT/SA/GOX films was obtained as a function of glucose concentration. These P3HT/SA/GOX LB films show a linearity from 100 to 500 mg/dl of glucose concentration.     

Langmuir–Blodgett films and optical second-harmonic generation of a crowned [60]fulleropyrrolidine

The Langmuir and Langmuir–Blodgett (LB) films of a novel crowned [60]fullero-pyrrolidine (CFP) were produced in different conditions. Macroscopic second-harmonic generation of the LB film was investigated by means of AM1 Hamiltonian as well as experiments. The monolayer LB film displayed a periodic fringe pattern. A linear dependence of second-harmonic intensity on the number of layers was observed. The second-order molecular susceptibility χ⁽²⁾ and hyperpolarizability β were evaluated to be 3.2×10⁻⁸ and 8.3×10⁻²⁹ esu. Received: 21 October 1999 / Revised version: 30 March 2000 / Published online: 2 August 2000     

Nonlinear optical properties of nanometer-size silver coated polydiacetylene composite vesicles and resulting Langmuir–Blodgett films

Noble metal-coated PDA composite vesicles were expected to increase the effective third-order nonlinear optical susceptibility χ ⁽³⁾(ω), due to the enhancement of the optical electric field induced by localized surface plasmon resonance. Different size (20, 50 and 80 nm) Ag colloidal nanoparticles were coated on the outer surface of polydiacetylene (PDA) vesicles to form PDA/Ag nanocomposite vesicles and the size-dependent effect of Ag colloidal nanoparticles on NLO properties enhancement has been explored. The explanation based on the competition of a size-dependent light-confinement effect and a size-dependent dielectric constant of Ag particles had been presented. Furthermore, these PDA/Ag composite vesicles were successfully immobilized onto the solid substrate by the Langmuir–Blodgett (LB) method and their linear and nonlinear optical properties were characterized, respectively. Obviously, PDA/Ag composite vesicles Langmuir–Blodgett (LB) films promoted the enhancement of the third-order NLO properties.     

The effect of orientation and distance between donor and acceptor molecules on the efficiency of singlet–singlet energy transfer in Langmuir–Blodgett films

Singlet–singlet energy transfer between molecules of fluorescein and oxazine dyes in Langmuir–Blodgett films is studied experimentally. The dependence of the energy-transfer efficiency on the distance shows that the quenching of the donor fluorescence is the most efficient when the layers of the donor and acceptor molecules are in a direct contact. An increase in the distance between the donor and acceptor layers leads to a decrease in the energy-transfer efficiency. To establish the mutual orientation of the donor and acceptor molecules, quantum-chemical calculations of the energy transfer process in the donor–acceptor pair are carried out. The calculations show that the best correlation of the experimental and calculated values of the energy-transfer efficiency is observed when the interacting particles are shifted relative to each other by about ~0.12 nm in parallel planes. The presented approach can be used to estimate the relative orientation of interacting particles in multimolecular ensembles.     

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

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 Å 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.     

Formation of nanoscale aggregates of a coumarin derivative in Langmuir–Blodgett film

In the present communication, we report the formation of organized nanoscale aggregates of a coumarin derivative 7 Hydroxy-N-Octadecyl Coumarin-3-Carboxamide (7HNO3C) at the air–water interface and in Langmuir–Blodgett (LB) films in the presence and absence of stearic acid (SA). A pressure-area isotherm reveals that the 7HNO3C form stable monolayer at the air–water interface. However, the stability can be improved by mixing it with a fatty acid stearic acid (SA). The miscibility study shows that the nature of interaction is strongly dependent on the mixing ratio and surface pressure. At a mole fraction of 0.4 of 7HNO3C in SA, the attractive and repulsive interaction between these two molecules balance each other forming a stable film with nanoscale aggregates. UV-Vis absorption spectroscopic studies reveal the nature of the aggregates in LB films. Scanning electron microscopy gives compelling visual evidence of formation of nanoscale aggregates in the mixed LB films.     

Nature of luminescence of PbS quantum dots synthesized in a Langmuir–Blodgett matrix

We calculate current (shot) noise in a metallic diffusive conductor generated by spin imbalance in the absence of a net electric current. This situation is modeled in an idealized three-terminal setup with two biased ferromagnetic leads (F-leads) and one normal lead (N-lead). Parallel magnetization of the F-leads gives rise to spin-imbalance and finite shot noise at the N-lead. Finite spin relaxation results in an increase in the shot noise, which depends on the ratio of the length of the conductor (L) and the spin relaxation length (l _s). For L >> l _s the shot noise increases by a factor of two and coincides with the case of the antiparallel magnetization of the F-leads.     

IR Dichroism of Langmuir–Blodgett Films Based on Fatty Acids

We propose a method of measurement and quantitative estimation of dichroic ratios of submicron laminated Langmuir–Blodgett films by the IR spectra of multiple attenuated internal reflection. The method has been used in calculating the angles of orientation of molecular chains of stearic and palmitic acids in the process of their molecular deposition on an optical substrate. It has been found that in the process of structural self-organization of ensembles the degree of orientation of macromolecules is the higher, the greater the number of links entering into the aliphatic chain.     

Structure-dependent optical anisotropy of porphyrin Langmuir–Schaefer films

We have studied the optical anisotropy of Langmuir–Schaefer layers of PdC₁₀OAP porphyrin, deposited onto gold substrates with thickness in the range 0–16 monolayers (ML). Deposition has been carried out at two values of the surface pressure Π, corresponding to different layer structures. In one case (Π=30 mN/m), molecules are well ordered in stacks oriented edge-on with respect to the substrate. In the other (Π=10 mN/m), a complex reorganization of the system happens several days after deposition, to form a mesoscopic two-dimensional lattice. The spectra measured by reflectance anisotropy spectroscopy (RAS) in two cases are clearly characterized. In the former, the line shape is dominated by a characteristic, large structure appearing in coincidence with the Soret band of the molecule, the development of which from a “peak-like” to a “derivative-like” appearance occurs at a well-defined critical thickness Θ_c (8 ML). In the latter, the line shape is always “peak-like”. We explain both line shapes in terms of morphological characteristics of the layer, occurring at different thickness values. The present results clearly show the potential of RAS to characterize efficiently the deposition of organic materials, and suggest that in short time it will be used as an in situ and real time spectroscopy, as already done in inorganic growth.     

Exponential temperature dependence and low-bias conductance anomaly in transport through Langmuir–Blodgett monolayer devices

Current–voltage characteristics are reported as a function of temperature (2–300 K) for 2.8-nm-thick eicosanoic acid (C₂₀) Langmuir–Blodgett organic monolayers sandwiched between planar platinum electrodes of area 5–200 m². An exponential temperature dependence observed between 60 and 300 K does not fit standard activated conduction, but can be described by thermionic field emission through a thin 0.1-eV barrier. A second model of tunneling through a vibrationally excited harmonic oscillator barrier also fits the data. A broad 200 meV dip in conductance at V=0 suggests strong inelastic tunneling, supporting the vibrational model. PACS 73.40.Gk; 68.47.Pe; 73.63.-b     

Structural and compositional mapping of a phase-separated Langmuir–Blodgett monolayer by X-ray photoelectron emission microscopy

The structure and composition of phase-separated Langmuir–Blodgett monolayer films comprised of mixtures of arachidic acid (C₁₉H₃₉COOH) and perfluorotetradecanoic acid (C₁₃F₂₇COOH) were characterized using a combination of X-ray photoelectron emission microscopy (X-PEEM), secondary electron emission microscopy (SEEM) and atomic force microscopy (AFM). X-PEEM provides high lateral spatial resolution and is directly sensitive to the elemental and chemical (functional group) composition of these ultrathin films through the chemical sensitivity of NEXAFS spectroscopy; AFM provides high-resolution imaging, both in terms of lateral and vertical (height) film topography. SEEM provides additional structural and electronic information through work function and electron scattering effects. The combination is used for chemical mapping of the phase-separated domains in the monolayer film. Our results directly confirm previous AFM measurements that suggested that the discontinuous domains are enriched in arachidic acid, whereas the surrounding continuous domain is a mixture of both arachidic acid and perfluorotetradecanoic acid.     

Aqueous polymer–nitrate solution deposition of YBCO films

High critical current density YBa₂Cu₃O_7?x (YBCO) films were prepared by solution deposition of aqueous non-fluorine precursors. Non-fluorine polymer-assisted deposition (PAD) processes utilizing rheology modifiers and chelating agents were used to produce 50 nm films with a critical current density (J_c) over 3 MA/cm² and 400 nm films with J_c > 1 MA/cm²_. T_c measurements indicated that films have T_c values near 90 K. The total heat treatment time to produce these high performance films was less than 4 h. Rheology modifiers such as polyvinyl alcohol (PVA) and hydroxyethyl cellulose (HEC) were used to increase the thickness of deposited films independent of the solution cation concentration. Chelating agents such as polyethylene glycol (PEG) and sucrose increased the barium ion solubility. Nitrate crystallization during deposition was controlled through rapid drying with vacuum and coating with hot solutions.     

Atomic composition and stability of Langmuir–Blodgett monolayers based on siloxane dimer of quaterthiophene on the surface of polycrystalline gold

Physics of the Solid State - Atomic composition of monolayers based on siloxane dimer of quaterthiophene deposited by Langmuir–Blodgett technique on a silicon dioxide surface partially...