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.     

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     

Photochemical behaviour of an acid-terminated azopolymer in solution and in Langmuir–Blodgett films

Trans–cis–trans isomerization processes of an acid-terminated azopolymer have been investigated in mono and multilayered Langmuir–Blodgett (LB) films and the results compared with the behaviour of this azopolymer in solution. UV–vis spectroscopy was used to determine the apparent extent of photoisomerization at the photostationary state, as well as the order and rate constant of the cis → trans thermal isomerization, whilst structural changes of the LB films during photoisomerization have been monitored by AFM microscopy. From these studies it was concluded that the trans → cis photoisomerization extent largely depends on the free volume around the azobenzene chromophores, with the smaller apparent extent of photoisomerization obtained for a well-organized 10-layer LB film. Data also revealed a first-order kinetics for the thermal cis → trans isomerization process of the azopolymer both in solution and in LB films. The cis → trans isomerization process depends on the balance between the stress produced by the larger cross-section of cis-isomers inside the system and the interactions between cis-isomers and neighbouring molecules. The slowest cis → trans isomerization process takes place in a 35-layer LB film, whilst LB films with well-packed azobenzene layers show faster cis → trans isomerization processes.     

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.     

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.     

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     

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.     

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.     

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.     

Formation mechanism of ordered stress-relief patterns in a free sustained Cu film system

A nearly free sustained copper （Cu） film system has been successfully fabricated by thermal evaporation deposition of Cu atoms on silicone oil surfaces, and a characteristic ordered pattern has been systematically studied. The ordered pattern, namely, band, is composed of a large number of parallel key-formed domains with different width w but nearly uniform length L; its characteristic values of w and L are very susceptible to the growth period, deposition rate and nominal film thickness. The formation mechanism of the ordered patterns is well explained in terms of the relaxation of the internal stress in the films, which is related to the nearly zero adhesion of the solid-liquid interface. By using a two-time deposition method, it is confirmed that the ordered patterns really form in the vacuum chamber.     

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     

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.     

Formation of linear momentum in a rod during a laser pulse–matter interaction

Recently, we developed an optodynamic experimental technique that makes it possible to measure the linear momentum obtained by a metal target sample in the shape of a rod during a nanosecond laser pulse interaction in the ablative regime. The height of the rod’s rear end axial step-like displacement, caused by the first reflection of the laser-generated ultrasonic wave, is proportional to the linear momentum acquired by the rod. In comparison with commonly used ballistic methods, we can determine the acquired momentum on a much shorter time scale corresponding to the wave transition time, from the front to the rear end of the rod. Using this method we investigated the ambient air pressure dependence on the formation of linear momentum over a laser intensity range, from the ablation threshold to values about ten times higher. Steel rods of various diameters were used to demonstrate the effect of an expanding blast wave which delivers additional momentum to the target, when the laser beam on the target surface is smaller than the target itself. The typical value of the acquired target momentum is on the order of μN s and 10 μN s/J for the momentum coupling coefficient.     

Investigation of V-shaped extended defects in a 4H–SiC epitaxial film

Abstract

We investigated two types of V-shaped extended defects on the basal plane in epitaxial 4H-SiC by synchrotron X-ray topography, photoluminescence imaging/spectroscopy and transmission electron microscopy (TEM). One is the (2, 5) stacking fault (in Zhdanov notation) bounded by two partial dislocations with the Burgers vector b ± 1/4[0?0?0?1]; the other is the (2, 3, 3, 5) stacking fault bounded by partial dislocations with b = ±1/4[0?0?0?1]. The core of the partial dislocations associated with the (2, 3, 3, 5) fault has an out-of-plane component (Frank component) and three in-plane components (Shockley components); the three Shockley components are cancelled out in total. The electronic structures of the (2, 5) and (2, 3, 3, 5) stacking faults were further examined by photoluminescence spectroscopy and first-principles calculations. It is suggested that the (2, 5) and (2, 3, 3, 5) stacking faults both have an interband state at a similar energy level, although they differ structurally.     

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.     

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.     

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.