A phase-field/Monte-Carlo model describing organic crystal growth from solution

In this paper work we present a phase-field/Monte-Carlo hybrid algorithm for the simulation of solutal growth of organic crystals. The algorithm is subsequently used for an investigation of diffusion effects on the growth mechanisms. This method combines a two-scale phase-field model of the liquid phase epitaxial growth and a Monte-Carlo algorithm of the 2D nucleation and thus is faster than previous purely Monte Carlo simulations of crystal growth. The inclusion of supersaturation and diffusion in the method allows the study of crystal growth under various growth conditions. Parameters used in the hybrid algorithm are bound to the energetic parameters of crystal faces, which can be estimated from a detailed study of the actual crystal structure based on a connected nets analysis, which allows the prediction of the shape and morphology of real crystals. The study of the diffusion effect is carried out based on an example of a hydroquinone crystal, which grows from the water solution at various supersaturations. The dependencies of the growth rate and the nucleation rate on the supersaturation indicate the change of the growth mechanism from spiral growth to 2D nucleation. The difference in the growth rate for various faces is in agreement with the crystal morphologies derived from the attachment energy method and observed experimentally. The main result of the simulation is the evaluation of engineering limits for choosing appropriate external process conditions.     

Hydrogen storage characteristics of nanograined free-standing magnesium–nickel films

Free-standing magnesium–nickel (Mg–Ni) films with extensive nanoscale grain structures were fabricated using a combination of pulsed laser deposition and film delaminating processes. Hydrogen sorption and desorption properties of the films, free from the influence of substrates, were investigated. Oxidation of the material was reduced through the use of a sandwiched free-standing film structure in which the top and bottom layers consist of nanometer-thick Pd layers, which also acted as a catalyst to promote hydrogen uptake and release. Hydrogen storage characteristics were studied at three temperatures, 296, 232, and 180°C, where multiple sorption/desorption cycles were measured gravimetrically. An improvement in hydrogen storage capacity over the bulk Mg–Ni target material was found for the free-standing films. As shown from a Van’t Hoff plot, the thermodynamic stability of the nanograined films is similar to that of Mg₂Ni. These results suggest that free-standing films, of which better control of material compositions and microstructures can be realized than is possible for conventional ball-milled powders, represent a useful materials platform for solid-state hydrogen storage research.     

Nonlinear interferometry and phase measurements for surface second-harmonic generation in a dispersive geometry

Direct measurement of the phase of the surface nonlinear susceptibility is based on the interference of nonlinear optical signals. Up to now, this has not been possible for Second-Harmonic Generation (SHG) in geometries such as Total Internal Reflection (TIR) due to the refractive dispersion of harmonic and fundamental light created in TIR. We demonstrate two schemes which enable us to overcome this dispersion, leading to interference between two second-harmonic signals generated consecutively by the same laser. The advantages and limitations of the two approaches are discussed. We use this technique to check the theoretical predictions for the nonlinear Fresnel factors for SHG in the TIR geometry.Paper presented at the 129th HE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Light-induced ejection of calcium atoms from polymer surfaces

Laser-induced fluorescence (LIF) of calcium atoms at room temperature has been observed in a polydimethylsiloxane (PDMS) coated cell when the walls are illuminated with non resonant visible light. Ca atomic density in the gas phase, monitored by the LIF, is much higher than normal room-temperature vapour pressure of calcium. In past years photon-stimulated desorption (PSD) was observed for several alkali metals that adsorbed to solid films of PDMS polymers. High yields of photo-desorbed atoms (and molecules in the case of sodium) can be induced, at room temperature and below, by weak intensity radiation. The desorption is characterised by a frequency threshold, whereas any power threshold is undetectable. The calcium photo-ejection is characterised both by a frequency threshold (about 18 500 cm^-1) and by an observable power threshold (whose value becomes lower when the photo-ejecting light wavelength decreases).     

Application of MEH-PPV/SnO<Subscript>2</Subscript> bilayer as hybrid solar cell

The effects of oxygen content in the sputtering gas on the crystallographic and optoelectronic properties of 210 nm-thick Zr–doped In₂O₃ (Zr–In₂O₃) films by rf magnetron sputtering were initially studied. The results of X-ray diffraction show that the Zr–In₂O₃ films grown on glass substrates exhibit mixed crystallographic orientations. Moreover, the Zr–In₂O₃ film grown in an Ar atmosphere promotes the appearance of crystallographic orientation of (222). The surface of the Zr–In₂O₃ film becomes rougher as the oxygen content in the sputtering gas decreases; the current images obtained by conductive atomic force microscopy reveal that the surfaces of the Zr–In₂O₃ films exhibit a distribution of coexisting conducting and nonconducting regions, and that the area of the nonconducting surface increases with the oxygen content in the sputtering gas. The resistivity is minimized to 3.51×10⁻⁴ Ω cm when the Zr–In₂O₃ film is grown in an Ar atmosphere and the average transmittance in the visible light region is ∼85%. The optical band gap decreases as the oxygen content in the sputtering gas increases.     

Toward a description of reactions of CO and H2 on Fe surfaces based on SCF hybrid AOs and electronegativity equalization

Maximum localization hybrids and electronegativity equalization have been used to obtain a simple reliable SCF picture of directional and charge-transfer effects determining the state and reactivity of CO adsorbed on iron. The hybridization procedure has been applied to H₂ adsorbed on FeCO, so as to describe the evolution of the complex toward a final geometry, which corresponds to a possible chemisorbed =CH−OH species, with the CO bond strongly inclined toward the surface. Abbiamo usato ibridi di massima localizzazione e la condizione di uguaglianza delle elettronegatività per ottenere una descrizione semplice e al tempo stesso affidabile degli effetti direzionali e di trasferimento di carica che determinano lo stato e la reattività del CO adsorbito su ferro. Il procedimento di ibridazione è stato applicato a H₂ adsorbito su FeCO in modo tale da descrivere l'evoluzione del complesso verso una geometria finale, la quale corrisponde ad una possibile specie chemisorbita =CH−OH, in cui il legame CO è fortemente inclinato verso la superficie. Используются смешанные состояния с максимальной локализацией и уравнивание электроотрицательности для получения простой SCF картины для ориентационных эффектов и эффектов переноса заряда, которые определяют состояние и реактивность адсорбированного CO на железе. Применяется процедура гибридизации к адсорбированной молекуле H₂ на FeCO. Описывается эволюция комплекса в сторону конечной геометрии, которая соответствует возможной группе =CH−OH с связью CO, которая сильно наклонена по отношению к поверхности.     

SBN thin films growth by RF plasma beam assisted pulsed laser deposition

SBN thin films were grown on MgO and Silicon substrates by PLD and RF-PLD (radiofrequency assisted PLD) starting from single crystal Sr_0.6Ba_0.4Nb₂O₆ and ceramic Sr_0.5Ba_0.5Nb₂O₆ stoichiometric targets. Morphological and structural analyses were performed on the SBN layers by AFM and XRD and optical properties were measured by spectroellipsometry. The films composition was determined by Rutherford Backscattering Spectrometry. The best set of experimental conditions for obtaining crystalline, c-axis preferential texture and with dominant 31° in-plane orientation relative to the MgO (100) axis is identified.     

Adsorption of carboxymethylester-azobenzene on copper and?gold?single crystal surfaces

The adsorption of 3,3′-di(methoxycarbonyl)azobenzene (CMA) on Au(111) and on Cu(001) substrates was studied by X-ray absorption spectroscopy measurements at the C, N, and O K edges. We find the molecules physisorbed in a planar conformation flat on the Au(111) surface. At higher coverages, a molecular crystal is formed wherein the molecules have the same flat geometry. On Cu(001), additional chemical bonds are formed between the molecules and the surface via the nitrogen atoms. Here the methyl benzoate moieties are tilted out of the surface plane.     

<Emphasis Type="Italic">Para</Emphasis>-hexaphenyl nanofiber growth on Au-coated porous alumina templates

The availability of easy and reproducible methods for formation of periodically nanostructured surfaces allows one to study the quasi-epitaxial growth of nanowires on such artificially modified substrates. Here, we demonstrate that crystalline nanofibers of para-hexaphenyl can grow on porous alumina templates with high pore regularity once the templates have been coated by a thin Au film. Nanofibers of that kind represent a model system for the general class of organic nanowires which have chemically changeable optoelectronic properties and hence are of interest for integration into future optoelectronic devices.     

Molecular dynamics study of groove fabrication process using AFM-based nanometric cutting technique

Three-dimensional molecular dynamics simulation of groove fabrication using atomic force microscopy (AFM)-based nanometric cutting technique is set up, fabrication processes of grooves with two types (line, and folder line) and five folder angles (0°, 30°, 45°, 60°, 90°) are simulated to investigate the effect of groove geometry on the fabrication process. The results show that the Normal force, Lateral force, and Resultant forces are almost symmetric with respect to the critical folder angle of 45°. The best surface quality of fabricated groove can be obtained at the folder angle of 45°. It reveals that the groove geometry has a significant effect on the groove fabrication process due to the material anisotropy on the atomic scale.     

Investigations of electron beam characteristics using an accelerator based on a magnetron gun with a secondary-emission cathode

The reliability and service life of accelerating installations are substantially determined by the lifetime of electron sources. The accelerator under consideration has a magnetron gun with a channel-free cold secondary-emission cathode in crossed fields as an electron source [1, 2]. In the present work, the data of the electron beam parameters obtained in the accelerator based on the magnetron gun with a secondary-emission cathode are given, and possible secondary uses are suggested. The text was submitted by the authors in English.     

A comprehensive study of the effect of <Emphasis Type="Italic">in situ</Emphasis> annealing at high growth temperature on the morphological and optical properties of self-assembled InAs/GaAs QDs

We investigate the effect of in situ annealing during growth pause on the morphological and optical properties of self-assembled InAs/GaAs quantum dots (QDs). The islands were grown at different growth rates and having different monolayer coverage. The results were explained on the basis of atomic force microscopy (AFM) and photo-luminescence (PL) measurements. The studies show the occurrence of ripening-like phenomenon, observed in strained semiconductor system. Agglomeration of the self-assembled QDs takes place during dot pause leading to an equilibrium size distribution. The PL properties of the QDs are affected by the Indium desorption from the surface of the QDs during dot pause annealing at high growth temperature (520°C) subsiding the effect of the narrowing of the dot size distribution with growth pause. The samples having high monolayer coverage (3.4 ML) and grown at a slower growth rate (0.032 ML s⁻¹) manifested two different QD families. Among the islands the smaller are coherent defect-free in nature, whereas the larger dots are plastically relaxed and hence optically inactive. Indium desorption from the island surface during the in situ annealing and inhomogeneous morphology as the dots agglomerate during the growth pause, also affects the PL emission from these dot assemblies.     

Surface morphology of ZnO buffer layer and its effects on the growth of GaN films on Si substrates by magnetron sputtering

ZnO thin films were first prepared on Si(111) substrates using a radio frequency magnetron sputtering system. Then the as-grown ZnO films were annealed in oxygen ambient at temperatures of 700, 800, 900, and 1000°C , respectively. The morphologies of ZnO films were studied by an atom force microscope (AFM). Subsequently, GaN epilayers about 500 nm thick were deposited on the ZnO buffer layers. The GaN/ZnO films were annealed in NH₃ ambient at 900°C. The microstructure, morphology and optical properties of GaN films were studied by x-ray diffraction (XRD), AFM, scanning electron microscopy (SEM) and photoluminescence (PL). The results are shown, their properties having been investigated particularly as a function of the ZnO layers. For better growth of the GaN films, the optimal annealing temperature of the ZnO buffer layers was 900°C.     

Self-organization in a chromium thin film under laser irradiation

Self-organization of chromium on glass was observed during laser ablation of the metal film with partially overlapping laser pulses. The beam of a nanosecond pulse laser tightly focused to a line was applied to the back-side ablation of the chromium thin film on a glass substrate. While the line ablated with a single laser pulse had sharp edges on both sides with ridges of the melted metal, the use of partially overlapping pulses formed a complicated structure made of the metal remaining from the ridges. Regular structures of ripples were developed in a certain range of laser fluence and pulse overlap. The ripple period could be controlled from 2.5 to 4 μm by variation of the processing parameters. Various experimental techniques were applied to test the structures, and different models of the ripple formation in the thin metal film were considered. The initial quasi-periodical formation started because of dewetting of thin liquid metal films on the glass substrate after its melting. Similar to the evaporation of liquid films, the small perturbation in the ridge thickness was able to induce instability in evaporation of the thin melted metal film. Freezing of the nonequilibrium state between laser pulses was one of the stabilizing factors in self-organization of the metal.     

A study of KCL lattice defects by thermoluminescence experiments

The thermoluminescent emission of X-irradiated potassium chloride is recorded simultaneouslyvs. temperature and wavelength. Samples of different origin and prepared through different processes, including thermal treatments, are examined. Most records show essentially two glow peaks, the wavelength of the one at higher temperature being slightly shifted toward the red. On the whole, experimental results suggest that luminescent centres are originated by potassium ion vacancies lying at a variable distance from interstitial potassium ions. The observed red shift is ascribed to the Coulomb energy of the pairs of these point defects of opposite charges.     

Thickness dependence of microstructure and magnetic properties in FePt/B<Subscript>4</Subscript>C multilayer thin films

FePt/B₄C multilayer films with different single FePt layer thickness were prepared by magnetron sputtering and subsequently annealing in vacuum. Influence of single FePt layer thickness on microstructure and magnetic property of FePt/B₄C films is investigated. Experimental results suggest that the Fe and Pt rich regions will appear in the interior of single FePt layer. The increasing of FePt layer thickness leads to the increase of grain size and volume fraction of order phase f ₀, which eventually induce satisfied coercivity (5.8 kOe).     

Highly improved electroluminescence from double-layer devices based on a carbazole-functionalized europium<Superscript>3+</Superscript> complex

A novel europium complex Eu(TTA)₃(CPPO)₂ (1) (TTA=thenoyltrifluoroacetone, CPPO=9-[4-(diphenyl-phosphinoyl)-phenyl]-9H-carbazole) based on the phosphine oxide ligand with bipolar structure was used to fabricate double-layer devices. The strong hole injection and transport ability of 1 was proved. The luminance of 414 cd m⁻² was achieved with the device configuration ITO/Eu(TTA)₃(CPPO)₂(40 nm)/BCP (30 nm)/Mg:Ag (BCP = 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline), which is favorable among double-layer organic light emitting devices based on small molecular Eu³⁺ complexes. The maximum current efficiency of 2.44 cd A⁻¹ and external quantum efficiency of 1.55% demonstrate the potential application of 1 as a promising candidate for high-efficiency, simple-structure and pure red-emitting devices.     

First-principles study for transport properties of defective carbon nanotubes with oxygen adsorption

Oxygen gas usually presents in carbon nanotube (CNT) based devices and can affect their transport properties. Here, we perform simulations for O₂ adsorption on a (5, 5) CNT with a double vacancy. We first use first-principles plane-wave calculation to optimize the structures and then use single-particle Green function method to study their transport properties. It is found that an O₂ can be either physisorbed or chemisorbed on the defective CNT. The physisorption has only minor effects on the transport while the chemisorption can improve it and the resulting conductance is affected by the orientation of the O₂ bonding.     

Real-time X-ray diffraction measurements of structural dynamics and polymorphism in diindenoperylene growth

We investigate the temperature-dependent polymorphs in diindenoperylene (DIP) thin films on sapphire and silicon oxide substrates using in situ X-ray scattering. On both substrates the DIP unit cell is very similar to the high-temperature phase of bulk crystals, with the substrate stabilising this structure well below the temperature where a phase transition to a low-temperature phase is observed in the bulk. Lowering the substrate temperature for DIP growth leads to a change in molecular orientation and an additional polymorph appears, with both these effects being more pronounced on sapphire as compared to silicon oxide. Using real-time reciprocal-space mapping we observe an expansion of the in-plane unit cell during DIP growth, which may be due to changes in molecular orientation as well as strain in the first monolayers. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.