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.     

Waveform control of orientation-dependent ionization of DCl in few-cycle laser fields

Strong few-cycle light fields with stable electric field waveforms allow controlling electrons on time scales down to the attosecond domain. We have studied the dissociative ionization of randomly oriented DCl in 5 fs light fields at 720 nm in the tunneling regime. Momentum distributions of D(+) and Cl(+) fragments were recorded via velocity-map imaging. A waveform-dependent anti-correlated directional emission of D(+) and Cl(+) fragments is observed. Comparison of our results with calculations indicates that tailoring of the light field via the carrier envelope phase permits the control over the orientation of DCl(+) and in turn the directional emission of charged fragments upon the breakup of the molecular ion.     

Giant anisotropy of Zeeman splitting of quantum confined acceptors in Si/Ge

Shallow acceptor levels in Si/Ge/Si quantum well heterostructures are characterized by resonant-tunneling spectroscopy in the presence of high magnetic fields. In a perpendicular magnetic field we observe a linear Zeeman splitting of the acceptor levels. In an in-plane field, on the other hand, the Zeeman splitting is strongly suppressed. This anisotropic Zeeman splitting is shown to be a consequence of the huge light-hole--heavy-hole splitting caused by a large biaxial strain and a strong quantum confinement in the Ge quantum well.     

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.     

Exploration of a hybrid feature selection algorithm

In the Knowledge Discovery Process, classification algorithms are often used to help create models with training data that can be used to predict the classes of untested data instances. While there are several factors involved with classification algorithms that can influence classification results, such as the node splitting measures used in making decision trees, feature selection is often used as a pre-classification step when using large data sets to help eliminate irrelevant or redundant attributes in order to increase computational efficiency and possibly to increase classification accuracy. One important factor common to both feature selection as well as to classification using decision trees is attribute discretization, which is the process of dividing attribute values into a smaller number of discrete values. In this paper, we will present and explore a new hybrid approach, ChiBlur, which involves the use of concepts from both the blurring and χ²-based approaches to feature selection, as well as concepts from multi-objective optimization. We will compare this new algorithm with algorithms based on the blurring and χ²-based approaches.     

Vacuum ultraviolet pulsed-field ionization-photoelectron study of H2S in the energy range of 10-17 eV

Vacuum ultraviolet pulsed-field ionization-photoelectron (PFI-PE) spectra of H(2)S have been recorded at PFI-PE resolutions of 0.6-1.0 meV in the energy range of 10-17 eV using high-resolution synchrotron radiation. The PFI-PE spectrum, which covers the formation of the valence electronic states H(2)S(+) (X (2)B(1), A (2)A(1), and B (2)B(2)), is compared to the recent high-resolution He I photoelectron spectra of H(2)S obtained by Baltzer et al. [Chem. Phys. 195, 403 (1995)]. In addition to the overwhelmingly dominated origin vibrational band, the PFI-PE spectrum for H(2)S(+)(X (2)B(1)) is found to exhibit weak vibrational progressions due to excitation of the combination bands in the nu(1) (+) symmetric stretching and nu(2) (+) bending modes. While the ionization energy (IE) for H(2)S(+)(X (2)B(1)) obtained here is in accord with values determined in previously laser PFI-PE measurements, the observation of a new PFI-PE band at 12.642+/-0.001 eV suggests that the IE for H(2)S(+)(A (2)A(1)) may be 0.12 eV lower than that reported in the He I study. The simulation of rotational structures resolved in PFI-PE bands shows that the formation of H(2)S(+)(X (2)B(1)) and H(2)S(+)(A (2)A(1)) from photoionization of H(2)S(X (1)A(1)) is dominated by type-C and type-B transitions, respectively. This observation is consistent with predictions of the multichannel quantum defect theory. The small changes in rotational angular momentum observed are consistent with the dominant atomiclike character of the 2b(1) and 5a(1) molecular orbitals of H(2)S. The PFI-PE measurement has revealed perturbations of the (0, 6, 0) K(+)=3 and (0, 6, 0) K(+)=4 bands of H(2)S(+)(A (2)A(1)). Interpreting that these perturbations arise from Renner-Teller interactions at energies close to the common barriers to linearity of the H(2)S(+) (X (2)B(1) and A (2)A(1)) states, we have deduced a barrier of 23,209 cm(-1) for H(2)S(+)(X (2)B(1)) and 5668 cm(-1) for H(2)S(+)(A (2)A(1)). The barrier of 23 209 cm(-1) for H(2)S(+)(X (2)B(1)) is found to be in excellent agreement with the results of previous studies. The vibrational PFI-PE bands for H(2)S(+)(B (2)B(2)) are broad, indicative of the predissociative nature of this state.     

Atomistic view of the recombinative desorption of H2 from H/Si(100)

Scanning tunneling microscopy is employed to investigate the recombinative desorption of H2 from hydrogenated Si(100) surfaces consisting of dihydride (SiH2) and monohydride (SiH) surface species organized in (1 x 1), (3 x 1), and (2 x 1) configurations. The results show that desorption from dihydrides involves a pair of neighboring dihydrides linked along the tetrahedral bond direction. Dihydrides in (3 x 1) domains are separated in the same direction by monohydrides, and desorption from a pair is geometrically impossible. The same desorption mechanism nevertheless applies via first a position switching of dihydrides with neighboring monohydrides.     

Bombardment induced ion transport--part II. Experimental potassium ion conductivities in borosilicate glass

A new experimental approach for measuring the ionic conductivity of solid materials is proposed. The experiment is based on bombarding an ion conducting sample with an alkali ion beam. This generates a well defined surface potential which in turn causes ion transport in the material. The ion transport is measured at the back side of the sample. The viability of the concept is demonstrated by measuring the temperature dependence of the potassium ion conductivity of a potassium borosilicate glass. The activation energy for the potassium transport is 1.04 eV ± 0.06 eV. For comparison, conductivity data obtained by impedance spectroscopy are presented, which support the bombardment induced data.     

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.