ToF-SIMS imaging of the nanoscale phase separation in polymeric light emitting diodes: effect of nanostructure on device efficiency

The nanostructure of the light emissive layer (EL) of polymer light emitting diodes (PLEDs) was investigated using force modulation microscopy (FMM) and scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) excited with focused Bi(3)(2+) primary beam. Three-dimensional nanostructures were reconstructed from high resolution ToF-SIMS images acquired with different C(60)(+) sputtering times. The observed nanostructure is related to the efficiency of the PLED. In poly(9-vinyl-carbazole) (PVK) based EL, a high processing temperature (60 °C) yielded less nanoscale phase separation than a low processing temperature (30 °C). This nanostructure can be further suppressed by replacing the host polymer with poly[oxy(3-(9H-9-carbazol-9-ilmethyl-2-methyltrimethylene)] (SL74) and poly[3-(carbazol-9-ylmethyl)-3-methyloxetane] (RS12), which have similar chemical structures and energy levels as PVK. The device efficiency increases when the phase separation inside the EL is suppressed. While the spontaneous formation of a bicontinuous nanostructure inside the active layer is known to provide a path for charge carrier transportation and to be the key to highly efficient polymeric solar cells, these nanostructures are less efficient for trapping the carrier inside the EL and thus lower the power conversion efficiency of the PLED devices.     

A Roe-type scheme with low Mach number preconditioning for a two-phase compressible flow model with pressure relaxation

We describe two-phase compressible flows by a hyperbolic six-equation single-velocity two-phase flow model with stiff mechanical relaxation. In particular, we are interested in the simulation of liquid-gas mixtures such as cavitating flows. The model equations are numerically approximated via a fractional step algorithm, which alternates between the solution of the homogeneous hyperbolic portion of the system through Godunov-type finite volume schemes, and the solution of a system of ordinary differential equations that takes into account the pressure relaxation terms. When used in this algorithm, classical schemes such as Roe’s or HLLC prove to be very efficient to simulate the dynamics of transonic and supersonic flows. Unfortunately, these methods suffer from the well known difficulties of loss of accuracy and efficiency for low Mach number regimes encountered by upwind finite volume discretizations. This issue is particularly critical for liquid-gasmixtures due to the large and rapid variation in the flow of the acoustic impedance. To cure the problem of loss of accuracy at low Mach number, in this work we apply to our original Roe-type scheme for the two-phase flow model the Turkel’s preconditioning technique studied by Guillard–Viozat [Computers & Fluids, 28, 1999] for the Roe’s scheme for the classical Euler equations.We present numerical results for a two-dimensional liquid-gas channel flow test that show the effectiveness of the resulting Roe-Turkel method for the two-phase system.     

A fluid-mixture type algorithm for barotropic two-fluid flow problems

Our goal is to present a simple interface-capturing approach for barotropic two-fluid flow problems in more than one space dimension. We use the compressible Euler equations in isentropic form as a model system with the thermodynamic property of each fluid component characterized by the Tait equation of state. The algorithm uses a non-isentropic form of the Tait equation of state as a basis to the modeling of the numerically induced mixing between two different barotropic fluid components within a grid cell. Similar to our previous work for multicomponent problems, see [J. Comput. Phys. 171 (2001) 678] and references cited therein, we introduce a mixture type of the model system that consists of the full Euler equations for the basic conserved variables and an additional set of evolution equations for the problem-dependent material quantities and also the approximate location of the interfaces. A standard high-resolution method based on a wave-propagation formulation is employed to solve the proposed model system with the dimensional-splitting technique incorporated in the method for multidimensional problems. Several numerical results are presented in one, two, and three space dimensions that show the feasibility of the method as applied to a reasonable class of practical problems without introducing any spurious oscillations in the pressure near the smeared material interfaces.     

In situ reversible conversion of porphyrin aggregate morphology

A porphyrin derivative possessing orthogonal self-assembly units displays in situ reversible transformation of aggregate morphology between nano-rods and hollow spheres upon exposure to different solvents.     

Acid-base properties and zeta potentials of self-assembled monolayers obtained via in situ transformations

Siloxane-anchored, self-assembled monolayers (SAMs) on single crystal Si were prepared with a variety of surface functional groups using a single commercially available surfactant (1-bromo-11-(trichlorosilyl)undecane) followed by in situ transformations. Polar (thioacetate and thiol), nonpolar (methyl), acidic (sulfonic and carboxylic), basic (various amines), and ionic (alkylammonium) surface functionalities were prepared. For primary amine and sulfonate surfaces, the degree of surface charge as a function of pH was determined ex situ using X-ray photoelectron spectroscopy (XPS). Sulfonate SAMs exhibited much higher effective pKa (approximately 2) than dilute sulfonic acid (-5 to -6), and amine SAMs exhibited much lower pKa (approximately 3) than dilute organic amines (approximately 10). This is attributed to the stabilization of nonionized groups by adjacent ionized groups in the SAM. Zeta potentials of these SAMs as a function of pH were consistent with the XPS results and indicated that ionizable SAM surfaces can generate surface potentials much higher than those of nonionic SAMs (thioacetate, methyl) and typical oxide surfaces.     

The Reaction Studies of α‐Chloroformylarylhydrazines with Thiols,Thioureas and α‐Cyclodiketones

The reactions of α‐chloroformylarylhydrazines 1 with various types of mercaptan, thiourea and α‐cyclodiketone have been studied intensively. 1‐Arylhydrazinecarbothioates 2 were obtained via thioesterization when α‐chloroformylarylhydrazines reacted with thiols. On the other hand, compounds 3 were obtained when α‐chloroformylarylhydrazines reacted with thio‐containing heterocyclic compounds, which suggested a totally different mechanism in these types of reactions. Further studies on the reaction of α‐chloroformylarylhydrazines 1 with thiourea compounds confirmed a novel cyclization and de‐cyclization mechanism, which led to give 2‐arylhydrazinecarboximidamides 5 and 1,3,4‐thiadiazolin‐5‐ones 6 . In addition, various 1,3,4‐oxadiazines 9 were obtained by reacting α‐chloroformylarylhydrazines with α‐cyclodiketones, showing ring cyclization was involved in this type of reaction.     

Thermal stabilities of delithiated olivine MPO4 (M = Fe,Mn) cathodes investigated using first principles calculations

We present an analysis of the thermal reduction of delithiated LiMnPO₄ and LiFePO₄ based on the quarternary phase diagrams as calculated from first principles. Our results confirm the recent experimental findings that MnPO₄ decomposes at a much lower temperature than FePO₄, thereby potentially posing larger safety issues for LiMnPO₄ cathodes. We find that while substantial oxygen is released as MnPO₄ reduces to Mn₂P₂O₇, the mixed valence phases that form in the decomposition process of FePO₄ limit the amount of oxygen evolved.     

Parallel detection,quantification, and depth profiling of peptides with dynamic-secondary ion mass spectrometry (D-SIMS) ionized by C60–Ar co-sputtering

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) using pulsed C₆₀⁺ primary ions is a promising technique for analyzing biological specimens with high surface sensitivities. With molecular secondary ions of high masses, multiple molecules can be identified simultaneously without prior separation or isotope labeling. Previous reports using the C₆₀⁺ primary ion have been based on static-SIMS, which makes depth profiling complicated. Therefore, a dynamic-SIMS technique is reported here. Mixed peptides in the cryoprotectant trehalose were used as a model for evaluating the parameters that lead to the parallel detection and quantification of biomaterials. Trehalose was mixed separately with different concentrations of peptides. The peptide secondary ion intensities (normalized with respect to those of trehalose) were directly proportional to their concentration in the matrix (0.01–2.5 mol%). Quantification curves for each peptide were generated by plotting the percentage of peptides in trehalose versus the normalized SIMS intensities. Using these curves, the parallel detection, identification, and quantification of multiple peptides was achieved. Low energy Ar⁺ was used to co-sputter and ionize the peptide-doped trehalose sample to suppress the carbon deposition associated with C₆₀⁺ bombardment, which suppressed the ion intensities during the depth profiling. This co-sputtering technique yielded steadier molecular ion intensities than when using a single C₆₀⁺ beam. In other words, co-sputtering is suitable for the depth profiling of thick specimens. In addition, the smoother surface generated by co-sputtering yielded greater depth resolution than C₆₀⁺ sputtering. Furthermore, because C₆₀⁺ is responsible for generating the molecular ions, the dosage of the auxiliary Ar⁺ does not significantly affect the quantification curves.     

Manipulating the nanostructure of organogels generated from molecules with a 3-dimensional truxene core

Two new truxene-based 3-dimensional (3-D) molecules self-assembled in cyclohexane to give organogels with vesicular and fibrillar nano-morphologies governed by the substitution pattern of the peripheral alkyl amido side-chains grafted onto the rigid 3-D core.