bluuebunny/crossref_metadata_2025 huggingface dataset:
A joint theoretical and experimental study on the electronic properties of phenyl-capped 3,4-ethylenedioxythiophene oligomers
Authors: Osikowicz W. , Denier van der Gon A. W. , Crispin X. , de Jong M. P. , Friedlein R. , Groenendaal L. , Fahlman M. , Beljonne D. , Lazzaroni R. , Salaneck W. R.
Published: October 2003

The electronic structure of a series of phenyl-capped 3,4-ethylenedioxythiophene oligomers has been studied using ultraviolet photoelectron spectroscopy (UPS) in combination with theoretical calculations. The calculations were performed for isolated oligomers within the framework of density-functional theory, using the Becke+Lee–Yang–Parr exchange-correlation functional. Excellent agreement between the UPS spectra and the quantum-chemical calculations allowed for unambiguous interpretation of the results. We use the asymptotic extension of electronic properties of oligomers to discuss the electronic structure of neutral poly(3,4-ethylenedioxythiophene). In addition, experimentally determined ionization potentials for thin films prepared by vapor deposition and spin coating are reported, and are found to depend on the preparation method. The discrepancy is assigned to differences in the molecular packing.
Cavity formation in the superheated Lennard-Jones liquid and its connection to homogeneous bubble nucleation: A density-functional theory study
Authors: Punnathanam Sudeep , Corti David S.
Published: October 2003

Recent Monte Carlo simulation studies of a model superheated liquid [Punnathanam and Corti, Ind. Eng. Chem. Res. 41, 1113 (2002)] suggest that cavity formation plays an important role in the process of homogeneous bubble nucleation. These simulations revealed that when spherical cavities beyond some certain size, i.e., the so-called critical cavity, were placed inside the superheated Lennard-Jones liquid, an instability was generated that led to phase separation towards the stable vapor phase. In this paper, we explore further the relevance of cavities, and the critical cavity in particular, to the molecular mechanism of homogeneous bubble nucleation. Density-functional theory (DFT) calculations are used to verify the existence of the critical cavity within the superheated Lennard-Jones liquid. In addition, DFT reveals that the critical cavity represents a limit of thermodynamic stability, further strengthening the connection between cavities and bubble nucleation. The DFT calculations also show that the size of the critical cavity is a lower bound to the size of the critical bubble and the work of formation of the critical cavity is a tight upper bound to the work of formation of the critical bubble. These results suggest that the free energy surface of bubble nucleation is influenced by the properties of the critical cavity, thereby possibly leading to a new picture of the molecular mechanism of bubble formation in superheated liquids.
Computer modeling of melting of ionized ice microcrystals
Authors: Brodskaya Elena N. , Egorov Andrei V. , Lyubartsev Alexander P. , Laaksonen Aatto
Published: October 2003

Ionized water clusters, OH−(H2O)N and H3O+(H2O)N, of different sizes (N=19 and 26) have been studied at temperatures ranging from 10 to 200 K using molecular dynamics simulations. The solid–liquid phase transitions are investigated to estimate the effects of the presence of an ion on the melting temperature of the clusters. It was found that the behavior of the aggregates during the melting is determined mainly by water–water interactions. Compared to corresponding pure water clusters, the observed changes in the melting temperature, Tm, are small and within the statistical uncertainty of the simulations. A weak trend can be observed with the hydroxyl ion reducing the Tm, while there is a slight tendency for an increase of Tm for clusters containing the hydronium ion. In general, the ions disrupt the hydrogen bond network and at the same time, the formation of a strong hydration shell contributes to a decrease of the mobility of the molecules. These two phenomena affect the solid–liquid phase transition temperature to opposite directions and seem to largely compensate each other in the simulated clusters.
An ab initio investigation on the vertical electron detachment energies of semiconductor-alkali binary anions EnA− (A=K, Na, Li; E=Ge, Si; n=1–10)
Authors: Li Si-Dian , Ren Guang-Ming , Jin Zhi-Hao
Published: October 2003

Ground-state structures of semiconductor-alkali binary anions EnA− (E=Ge, Si; A=K, Na, Li; n=1–10) have been optimized using the second-order Møller–Plesset perturbation (MP2) method and their vertical detachment energies predicted employing the outer valence Green’s function (OVGF) procedure. The calculated lowest detachment energies of EnA− anions composed of En2− semiconductor Zintl anions and face-capping or edge-bridging A+ alkali cations exhibit maximums around n=2, 5, 9, and 10 and minimums at n=3 and 7, in overall agreement with the magic numbers observed for GenK− in time-of-flight mass spectroscopy. The lowest detachment energies of EnA− series increase from A=K, Na, to Li with increasing electronegativities of the alkali atoms and decreased E–A ionic bond lengths. Small ternary GemSinK− (m+n⩽6) exhibit similar behavior to binary EsK− at the same sizes (s=m+n).
Nuclear spin polarization transfer across an organic-semiconductor interface
Authors: Goehring Lucas , Michal Carl A.
Published: October 2003

Motivated by Tycko’s proposal to harness optically pumped nuclear spin polarization for the enhancement of nuclear magnetic resonance (NMR) signals from biological macromolecules, we investigate the transfer of thermal nuclear spin polarization between H1 or F19 in an organic overlayer and P31 at the surface of micron-sized InP particles by Hartmann–Hahn cross polarization. Comparison with analytic and numerical models indicates that the total quantity of polarization transferred across the semiconductor-organic interface is limited by the relatively short room-temperature H1 T1ρ (11 ms) and the slow diffusion of nuclear spin polarization in the semiconductor. Models and spin-counting experiments indicate that we are able to transfer approximately 20% of the total nuclear spin polarization originating in the organic overlayer to the semiconductor, supporting the feasibility of transferred optically pumped NMR.
Silicon and metal nanotemplates: Size and species dependence of structural and electronic properties
Authors: Gueorguiev G. K. , Pacheco J. M.
Published: October 2003

We utilize first-principles computer simulations to study the dependence on size (n) and species (M) of structural and electronic properties of clusters with stoichiometry M Sin. We investigate a total of 168 clusters comprising from 1 to 14 silicon atoms together with one transition metal atom among 12 different elements. It is found that all elements exhibit a very similar size-dependence for the cohesive energy, in which clusters with n=7, 12 appear as local maxima, with shapes which are found to be essentially independent of the transition metal atom. It is also found that the electronic properties of structurally equivalent clusters depend sensitively on the transition metal atom involved, providing the means to tailor specific properties when designing cluster assembled materials.
Low energy electron interaction with free and bound SF5CF3: Negative ion formation from single molecules, clusters and nanofilms
Authors: Balog Richard , Stano Michal , Limão-Vieira Paulo , König Constanze , Bald Ilko , Mason Nigel J. , Illenberger Eugen
Published: October 2003

The interaction of free electrons with the potent greenhouse molecule SF5CF3 is studied under different degrees of aggregation: single molecules at collision free conditions, clusters within a supersonic molecular beam and condensed molecules. Electron collisions with single molecules are dominated by SF5− formation produced via dissociative electron attachment (DEA) within a resonance located below 2 eV. In clusters, undissociated parent anions SF5CF3− (and larger complexes containing undissociated anions) are observed in addition to the fragment ions. This indicates that (i) SF5CF3 possesses a positive adiabatic electron affinity and (b) low energy attachment is partly channeled into nondissociative processes when the molecule is coupled to an environment. Electron impact to condensed phase SF5CF3 exhibits a remarkably strong F− desorption signal appearing from a pronounced resonance located at 11 eV while in the gas phase at 11 eV only a weak DEA signal is observed. Electron induced desorption from sub-monolayers of SF5CF3 on an amorphous H2O ice surface is found to be more efficient compared to desorption of SF5CF3 from a Xe surface. The implications of these results for the heterogeneous photochemistry of SF5CF3 adsorbed on ice or dust particles in the Earth’s atmosphere are discussed.
Predictions of molecular chirality and helical twisting powers: A theoretical study
Authors: Earl David J. , Wilson Mark R.
Published: October 2003

A theoretical study of a number of chiral molecules has been undertaken using a molecular Monte Carlo simulation approach coupled with calculations of molecular chirality based on a chirality order parameter. Results for a variety of TADDOL (α,α,α′,α′-tetraaryl-1,3-dioxolan-4,5dimethanol) derivatives show good agreement with experimental findings for the sign, magnitude, and the temperature dependence of the helical twisting power (HTP). For a photochromic chiral dopant with variable HTP we are able to model the reduction in the HTP when photoisomerization occurs. Our studies on a liquid crystalline material with a single chiral center have reproduced a temperature dependent twist inversion in the material. We discuss the temperature and solvent dependence of the helical twisting power and argue that in all the systems studied here, preferential selection of certain molecular conformations at different temperatures and in different solvents are able to explain the observed experimental behavior of the HTP.
A method to measure the two-dimensional image of magneto-optical Kerr effect
Authors: Zhu Xiao-Song , Zhao Hai-Bing , Zhou Peng , Xia Guo-Qiang , You Hai-Yang , Zhang Rong-Jun , Li Jing , Wang Song-You , Ni Wei-Ming , Chen Liang-Yao
Published: October 2003

A two-dimensional (2D) image of magneto-optical Kerr effect measurement system has been designed and constructed. The magnetic field is provided by two cylindrical permanent magnets in the 0–8000 G range. To avoid beam deviation and other problems in the measurement, an electronically controlled liquid crystal retarder with a fixed polarizer was used to detect the polarization change of the light at the 546.1 nm wavelength. The Fourier transformation method was used to retrieve the Kerr signal in data analysis. The Kerr image signal was measured by using a 2D charge coupled diode (CCD) array detector with 1536×1024 pixels. In terms of a telescope with 1:1 optical magnification, the CCD pixel detects the Kerr signal that is produced by the microsurface of the sample with the area size equaling to that of the pixel. The image Kerr effect of GdFe film sample was measured with the results to show that the 2D Kerr pattern can map the in-plane distribution of the magnetic property of the sample surface.
Complete analysis of the angular momentum distribution of molecules desorbing from a surface
Authors: Borowski S. , Klüner T. , Freund H.-J.
Published: October 2003

We theoretically analyze the angular momentum distribution of molecules desorbing from a surface. The adsorbate is shown to possess the complete point symmetry of the surface if the molecules adsorb in statistically equilibrated local order. Considering the symmetry of the angular momentum distribution, this point symmetry adopted to the desorbate is particularly taken into account. From the symmetry analysis, the cancellation of several terms in the angular momentum distribution is concluded. The symmetry related sparseness of the angular momentum distribution strongly simplifies the interpretation of experimental data and thus enables an appropriate comparison between experiment and theory. The benefit of this simplified picture is demonstrated by the example of the laser induced desorption of CO molecules from a Cr2O3(0001) surface.
Phase field theory of crystal nucleation in hard sphere liquid
Authors: Gránásy László , Pusztai Tamás , Tóth Gyula , Jurek Zoltán , Conti Massimo , Kvamme Bjørn
Published: October 2003

The phase field theory of crystal nucleation described in L. Gránásy, T. Börzsönyi, and T. Pusztai, Phys. Rev. Lett. 88, 206105 (2002) is applied for nucleation in hard-sphere liquids. The exact thermodynamics from molecular dynamics is used. The interface thickness for phase field is evaluated from the cross-interfacial variation of the height of the singlet density peaks. The model parameters are fixed in equilibrium so that the free energy and thickness of the (111), (110), and (100) interfaces from molecular dynamics are recovered. The density profiles predicted without adjustable parameters are in a good agreement with the filtered densities from the simulations. Assuming spherical symmetry, we evaluate the height of the nucleation barrier and the Tolman length without adjustable parameters. The barrier heights calculated with the properties of the (111) and (110) interfaces envelope the Monte Carlo results, while those obtained with the average interface properties fall very close to the exact values. In contrast, the classical sharp interface model considerably underestimates the height of the nucleation barrier. We find that the Tolman length is positive for small clusters and decreases with increasing size, a trend consistent with computer simulations.
Influence of collision energy on the N(2D)+O2→O(3P)+NO reaction dynamics: A quasiclassical trajectory study involving four potential energy surfaces
Authors: Miquel Irene , Hernando Jordi , Sayós R. , González Miguel
Published: October 2003

The influence of collision energy (ET) on the dynamics of the N(2D)+O2→O(3P)+NO atmospheric reaction was studied by means of the quasiclassical trajectory method. The four lowest potential energy surfaces (PESs) involved in the process were used in the calculations (2 2A′, 3 2A′, 1 2A″, and 2 2A″ PESs), and the nonadiabatic couplings between them were neglected. The dependence of the scalar and two-vector properties of the reaction with ET was analyzed. Moreover, the different modes of reaction taking place were investigated. Although only one type of microscopic mechanism (abstraction) was found for the 2 2A′, 3 2A′, and 2 2A″ PESs, two different modes of reaction (abstraction and insertion) were observed to coexist for the 1 2A″ PES. For this PES, the abstraction mechanism is the most important one at room temperature, while the insertion mechanism increases its contribution to reactivity with ET (it accounts for about half of the reactivity above 0.5 eV).
Zero temperature quantum properties of small protonated water clusters (H2O)nH+ (n=1–5)
Authors: Mella Massimo , Clary David C.
Published: October 2003

The study of the energetics and structure of small protonated water clusters (H2O)nH+ (n=1–5) has been carried out employing the OSS3 potential energy surface developed by Ojamae, Singer, and Shavitt [J. Chem. Phys. 109, 5547 (1998)]. By comparing it with accurate ab initio MP2 calculations for (H2O)nH+, this all-atom potential is also shown to reproduce quantitatively the geometry and the relative energetics of small neutral and protonated water clusters containing up to five molecules. To correct the total and binding energy for vibrational motion, the zero point energy of the clusters has been calculated by means of the harmonic approximation and by simulating the exact ground state using the diffusion Monte Carlo method. From these 0 K results, it appears that the anharmonicity accounts for a decrease (increase) of 1.5–5.5 mhartree (1.0–3.5 kcal/mol) in the total (binding) energy of the protonated clusters. Moreover, we found all the cyclic isomers of (H2O)4H+ and (H2O)5H+ to be unstable during the diffusion Monte Carlo simulations, and to convert into treelike or linear isomers. Employing the same interaction potential, we also simulated the ground state of (H2O)n (n=1–5) to compute the proton binding energy to a water cluster. This quantity is decreased by roughly 12 mhartree (7.5 kcal/mol) by including the zero point energy correction to the total energy. The relevance of these findings with respect to the experimental detection and probing of the protonated water clusters is discussed.
A density functional theory investigation of 1,1-diamino-2,2-dinitroethylene dimers and crystal
Authors: Ju Xue-Hai , Xiao He-Ming , Xia Qi-Ying
Published: October 2003

The density functional method with different basis sets was applied to the study of the highly efficient and low sensitive explosive 1,1-diamino-2,2-dinitroethylene in both gaseous dimer and its bulk state. The binding energies have been corrected for the basis set superposition errors. Four stable dimers (I, II, III, and IV) were located. The corrected binding energy of the most stable dimer IV is predicted to be −38.15 kJ/mol at the B3LYP/6-311++G** level. It was found that the structure of the most stable dimer is just the basic packing pattern in the wave-shaped layer of 1,1-diamino-2,2-dinitroethylene solid phase. Vibrational modes associated with the N–C–N rocking exhibits blueshifts with large intensities as the results of large dipole moment changes, whereas those assigned to the stretching of N–H, which is bound by another submolecule, exhibit large redshifts (over −21 cm−1) with respect to those of the monomer. The changes of Gibbs free energies (ΔG) in the processes from the monomer to the dimers at 298.15 K are 16.46, 16.01, 11.85, and −1.78 kJ/mol for dimers I, II, III, and IV, respectively. Dimer IV can be spontaneously produced from the isolated monomer at room temperature. The calculated lattice energy is −105.81 kJ/mol, and this value decreases to −114.06 kJ/mol when a 50% correction of the basis set superposition error is adopted, which is in good agreement with the theoretical values already reported. The frontier bands are quite flat. Judging from the value of the band gap of 4.0 eV, it may be predicted that 1,1-diamino-2,2-dinitroethylene is an insulator. The frontier crystalline orbitals are formed by contributions from the C, N, and O atoms of the C–NO2 group, indicating that there exists a strong conjugation in the molecule and that the C–NO2 group is the most reactive part of it. The population of the C–NO2 bond is much less than those of the other bonds and the detonation may be initiated by the breakdown of this bond.
Dielectric functions of ferroelectric Bi3.25La0.75Ti3O12 thin films on Si(100) substrates
Authors: Hu Z. G. , Ma J. H. , Huang Z. M. , Wu Y. N. , Wang G. S. , Chu J. H.
Published: October 2003

Ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films were deposited directly on Si(100) substrates under annealing temperatures of 590 and 700 °C. The optical properties of the BLT films have been investigated using spectroscopic ellipsometry at room temperature in the 0.73–6 eV energy range. To model the dielectric functions of the BLT films, the double Tauc–Lorentz dispersion relation was successfully adopted. A four-layer model was used to fit the measured pseudodielectric function in order to deduce the complex dielectric functions. The results show that the annealing temperature mainly affects the dielectric functions of the BLT films beyond the fundamental band gap energy. The volume fraction of air present in the surface rough layer increases with increasing annealing temperature. The fundamental band gap was observed to shift slightly to a higher energy at a high annealing temperature. The difference of the optical properties due to the structure changes testifies the x-ray diffraction spectral results.
Precision Lamb-shift polarimeter for polarized atomic and ion beams
Authors: Engels R. , Emmerich R. , Ley J. , Tenckhoff G. , Paetz gen. Schieck H. , Mikirtytchiants M. , Rathmann F. , Seyfarth H. , Vassiliev A.
Published: October 2003

The Lamb-shift polarimeter described here enables the polarization of a beam of hydrogen (deuterium) atoms, or of a slow proton (deuteron) beam, to be measured with an absolute precision better than 1% within a few seconds. The polarimeter measures the intensity ratios of Lyman-α transitions after Stark quenching of metastable hyperfine substates that were selected in a spin filter. For that purpose the hydrogen (deuterium) atoms are ionized in a Glavish-type ionizer. By charge exchange of the protons (deuterons) in cesium vapor, atoms in the metastable 2S state are produced. For a hydrogen beam of 3×1016 atoms/s, ∼3×106 photons/s are registered in a photomultiplier, i.e., the polarimeter efficiency is about 10−10. The signal-to-background ratio in the Lyman-α spectrum is excellent, thus beam intensities of one to two orders of magnitude less would still be sufficient to carry out a precise measurement. The different components of the polarimeter affect the measured polarization in several ways. It was, therefore, crucial to determine precisely the associated correction factors in order to derive the nuclear polarization from the measured Lyman-α spectra.
Fully automated, gas sensing, and electronic parameter measurement setup for miniaturized nanoparticle gas sensors
Authors: Kennedy M. K. , Kruis F. E. , Fissan H. , Mehta B. R.
Published: October 2003

In this study, a measurement setup has been designed and fabricated for the measurement of gas sensor characteristics and electronic parameters of nanostructured thin layers in the temperature range from room temperature to 450 °C in controlled gas environments. The setup consists of: (i) a gas environment chamber, (ii) a specially designed substrate and substrate holder, and (iii) control, supply, and measurement electronics. The buried geometry of the contacts is specially designed for the deposition of nanoparticles from the gas phase to guarantee uniform thin layers, and the setup can be used to make measurement on high resistivity (1010 Ω cm) nanoparticle samples. The gas inlet, operating temperature, and electronic control of the measurement system are automated by means of a personal computer. Coupling the measurements of interdependent gas sensing and electronic parameters at identical conditions, in a single setup encompassing a wide range of sensing gas levels and substrate temperatures, makes this system ideally suited for carrying out multiple measurements required for optimizing sensor configuration and understanding the size-dependent properties of nanoparticle sensors.
Mechanism for the increased light transmission through Ni/Au/ZnO contacts on p-GaN for high power optoelectronic devices
Authors: Tseng C. L. , Youh M. J. , Moore G. P. , Hopkins M. A. , Stevens R. , Wang W. N.
Published: October 2003

The mechanisms responsible for the transparency of Ni/Au and Ni/Au/ZnO on p-GaN has been investigated. It was found that the optical transmission of Ni/Au contacts is dominated by the film thickness and morphology. A change in film thickness, induced by thermal annealing, results in a more transparent contact material. In addition a ZnO film was used as an antireflection layer on top of a Ni/Au contact. The Ni/Au/ZnO film was found to have an increased light transmission of 15% compared with an annealed Ni/Au contact. The maximum optical transmission measured through the Ni/Au/ZnO contact was 90%.
Measurement of the infrared magneto-optic properties of thin-film metals and high temperature superconductors
Authors: Černe J. , Schmadel D. C. , Rigal L. B. , Drew H. D.
Published: October 2003

A sensitive polarization modulation technique uses photoelastic modulation and heterodyne detection to simultaneously measure the Faraday rotation and induced ellipticity in light transmitted by semiconducting and metallic samples. The frequencies measured are in the midinfrared and correspond to the spectral lines of a CO2 laser. The measured temperature range is continuous and extends from 35 to 330 K. Measured samples include GaAs and Si substrates, gold and copper films, and YBCO and BSCCO high temperature superconductors.
Electrochemical cell for in situ magneto-optic Kerr effect measurements
Authors: Poduska Kristin M. , Morin Sylvie
Published: October 2003

A unique electrochemical cell allows in situ magneto-optic Kerr effect measurements for magnetic characterization of ultrathin films, concurrently with electrochemical control. This durable, compact, and easy to assemble cell is mounted on a rotatable base which enables magnetic measurements in both the longitudinal (in plane) and polar (perpendicular to plane) configurations. Its utility in the reproducible preparation and in situ magnetic characterization of thin films is demonstrated with electrochemical and ferromagnetic hysteresis data for ultrathin Ni films (⩽15 monolayers) electrodeposited on Ag(111) and Au(111) single crystal substrates.
Effect of solvent polarity on the photophysical properties of coumarin-1 dye
Authors: Barik Atanu , Nath Sukhendu , Pal Haridas
Published: October 2003

Effects of solvent polarity on the photophysical properties of coumarin-1 (C1; 7-NEt2-4-CH3-1,2-benzopyrone) dye have been investigated using steady-state and time-resolved fluorescence measurements. In moderate to higher polarity solvents the properties like Stokes’ shifts (Δν̄), fluorescence quantum yields (Φf), fluorescence lifetimes (τf), radiative rate constants (kf), and nonradiative rate constants (knr) follow more or less linear correlation with the solvent polarity function Δf [={(D−1)/(2D+1)}−{(n2−1)/(2n2+1)}]. In nonpolar solvents, namely, hexane, cyclohexane, methylcyclohexane, 3-methylpentane, and decalin, however, all the above-mentioned properties show unusual deviation in comparison to the trend observed in moderate to higher polarity solvents. Thus, the Δν̄ and knr values are unusually lower and the Φf, τf, and kf values are unusually higher in nonpolar solvents. Comparing the results of C1 with those of its lower analogue, namely, coumarin-120 (C120; 7-NH2-4-CH3-1,2-benzopyrone), it has been inferred that like C120, the dye C1 also exists in different structural form in nonpolar solvents than in other solvents of moderate to higher polarities. It is proposed that in nonpolar solvents the dye exists in a nonpolar structure, where the 7-NEt2 group of the dye adopts a pyramidal configuration and consequently out of plane of the 1,2-benzopyrone moiety. In moderate to higher polarity solvents, the dye is supposed to exist in a polar intramolecular charge transfer structure with its 7-NEt2 group in resonance with the 1,2-benzopyrone moiety. The observed differences in some of the properties of C1 and C120 dyes in nonpolar solvents have been rationalized based on the relative rates of the flip-flop motions of the 7-amino groups of the two dyes. It is understood that where a very fast flip-flop motion of the 7-NH2 group in C120 introduces an activation-controlled nonradiative deexcitation channel for the dye excited state in nonpolar solvents, the very slow flip-flop motion of the 7-NEt2 group in C1 cannot introduce any such nonradiative deexcitation channel for the dye.
Oxidation rate enhancement of SiGe epitaxial films oxidized in dry ambient
Authors: Spadafora M. , Privitera G. , Terrasi A. , Scalese S. , Bongiorno C. , Carnera A. , Di Marino M. , Napolitani E.
Published: October 2003

We present a study on thin oxides obtained by rapid thermal oxidation of Si1−xGex epitaxial layers. The oxidation processes were performed in dry O2 at 1000 °C for times up to 600 s. Our data show an oxide growth rate enhancement with respect to pure Si. Except for a very small amount of GeO2 that is found at the surface, all the Ge is rejected towards the SiO2/SiGe interface, forming a Ge-enriched layer free of extended defects. The comparison of our results for dry processes with those reported in the literature for wet ambient supports the idea that the kinetics of SiGe oxidation is controlled by similar mechanisms in both cases, in contrast with models and interpretations so far proposed.
Engineering carrier confinement potentials in 1.3-μm InAs/GaAs quantum dots with InAlAs layers: Enhancement of the high-temperature photoluminescence intensity
Authors: Liu H. Y. , Sellers I. R. , Hopkinson M. , Harrison C. N. , Mowbray D. J. , Skolnick M. S.
Published: October 2003

We describe an optical study of structures consisting of an InAlAs-GaAs strained buffer layer and an InAlAs-InGaAs composite strain-reducing layer designed to modify the confining potential of 1.3-μm InAs/GaAs quantum dots (QDs). With increasing (decreasing) InAlAs (InGaAs) thickness in the strain-reducing layer grown above the QDs, the integrated photoluminescence (PL) intensity of the QD ground-state transition increases dramatically and the emission wavelength decreases slightly from 1.36 to 1.31 μm. The enhancement of PL efficiency is temperature dependent, being much greater above 200 K. A maximum enhancement of 450 is achieved at room temperature. This improvement of the high-temperature PL efficiency should lead to a significant improvement in the characteristics of 1.3-μm InAs/GaAs QD lasers.
Evolution of shape, height, and in-plane lattice constant of Ge-rich islands during capping with Si
Authors: Zhong Zhenyang , Stangl J. , Schäffler F. , Bauer G.
Published: October 2003

The surface morphology of Ge-rich islands on Si (001) substrates capped with 0 to 10 monolayers (MLs) of Si at 550 °C was investigated by atomic force microscopy. An evolution of the island shape from domes to pyramids was observed, which coincides with a dramatic decrease of the island height during overgrowth. The average lateral lattice constant 〈a∥〉 of the Ge-rich islands for a series of samples was obtained from grazing incidence x-ray diffraction. 〈a∥〉 decreases appreciably with deposition of the Si cap layer, even for a cap thickness as low as 1.3 MLs. At the beginning of overgrowth, Si incorporation promotes the shape evolution and the size variation of the islands.
Theoretical study of radiation induced electromotive force effects on mineral insulated cables
Authors: Vermeeren L. , Van Nieuwenhove R.
Published: October 2003

A detailed Monte Carlo model was developed for the calculation of neutron- and gamma-induced currents in mineral insulated cables. Dedicated calculations for selected cables in selected environments were carried out in order to investigate the influence of key parameters on the induced currents. As the concomitant induced voltages depend on the insulation resistance of the cables, the radiation and temperature dependence of the insulation resistance is also studied.
Radical–radical reaction dynamics: The OH formation in the reaction of O(3P) with propargyl radical, C3H3
Authors: Lee Hohjai , Joo Sun-Kyu , Kwon Lee-Kyoung , Choi Jong-Ho
Published: October 2003

The radical–radical reaction dynamics of ground-state atomic oxygen [O(3P)] with propargyl radicals (C3H3) has first been investigated by applying laser induced fluorescence spectroscopy in a crossed beam configuration, together with ab initio calculations. A new exothermic channel of O(3P)+C3H3→C3H2+OH was identified and the nascent distributions of OH reaction product in the ground vibrational state (X 2Π:υ″=0) showed substantial rotational excitations with an unusual bimodal feature composed of the low- and high-N″ components. No spin–orbit propensities were observed, whereas the averaged ratios of Π(A′)/Π(A″) were determined to be 0.60±0.28. With the aid of the CBS-QB3 level of ab initio theory it is predicted that on the lowest doublet potential energy surface the reaction proceeds through the addition complexes formed through the barrierless addition of O(3P) to C3H3, and that the counterpart of C3H2 of the probed OH product is cyclopropenylidene (1c-C3H2). On the basis of the comparison with statistical prior and rotational surprisal analyses, the ratio of population partitioning for the low- and high-N″ regimes is estimated to be about 1:2 and the reaction is described in terms of two competing addition-complex mechanisms: a major short-lived dynamic complex in the high-N″ regime and a minor long-lived statistical complex in the low-N″ regime.
Planar nanocontacts with atomically controlled separation
Authors: Kervennic Y. V. , Vanmaekelbergh D. , Kouwenhoven L. P , Van der Zant H. S. J
Published: October 2003

We have developed a technology to reproducibly make gaps with distance control on the single-atom scale. The gold contacts are flat on the nanometer scale and are fabricated on an oxidized aluminum film that serves as a gate. We show that these contacts are clean and can be stabilized via chemical functionalization. Deposition of conjugated molecules leads to an increase in the gap conductance of several orders of magnitude. Stable current–voltage characteristics at room temperature are slightly nonlinear. At low temperature, they are highly nonlinear and show a clear gate effect.
Thermal detrapping analysis of pumping-related defects in diamond
Authors: Marinelli Marco , Milani E. , Morgada M. E. , Pucella G. , Rodriguez G. , Tucciarone A. , Verona-Rinati G. , Angelone M. , Pillon M.
Published: October 2003

A method is presented to selectively characterize the traps involved in the pumping procedure of diamond films. The pumping process strongly reduces the concentration of active carrier trapping centers, leading to an enhancement of electronic properties of such material, and is obtained by irradiating the diamond films with ionizing radiation. Since the improved transport properties lead to an increased efficiency when diamond films are used as radiation detectors, valuable information on this process can be obtained by analyzing the response of diamond based particle detectors. For this purpose a high-quality diamond film was grown by microwave chemical vapor deposition and a particle detector was realized. Its response to a 5.5-MeV Am241 α-particles was measured after successive annealing steps performed at different temperatures in the 180–228 °C range. Before each annealing curve at a given temperature, the detector was driven to the pumped state through β-particle irradiation. The efficiency versus annealing time curves evidence a thermally induced detrapping, confirming the pumping mechanism as a filling and consequent passivation of defects. The analysis of the decay time of the detector efficiency as a function of temperature allows the determination of the activation energy of these defects, which is Ea=1.62±0.15 eV.
Square-lattice photonic crystal microcavities for coupling to single InAs quantum dots
Authors: Hennessy K. , Reese C. , Badolato A. , Wang C. F. , Imamoǧlu A. , Petroff P. M. , Hu E. , Jin G. , Shi S. , Prather D. W.
Published: October 2003

We have observed optical emission from self-assembled InAs/GaAs quantum dots (QDs) embedded within the single-hole-defect, square-lattice (S1) photonic crystal microcavity. Cavities were measured to have quality factors as high as 4000. Finite-difference time-domain (FDTD) calculations were used to determine the specific S1 geometry that is resonant at the center of our ensemble QD spectrum. Extensive, systematic measurements fully confirmed the FDTD simulations and mapped resonant wavelengths as a function of varying lattice constant and hole radius of the photonic crystal structures.
Correlation between the superconducting and structural properties in MgB2 thin films prepared by molecular-beam epitaxy
Authors: Yamazaki Hiroki , Hikita Yasuyuki , Hori Hiroki , Takagi Hidenori
Published: October 2003

Superconducting MgB2 films were prepared on an Al2O3(0001) substrate by codeposition of Mg and B using the molecular-beam epitaxy technique and the effects of the growth conditions on the structural and the superconducting properties were extensively investigated. The superconducting transition temperature Tc shows a peak as a function of the deposition-rate ratio of Mg to B2 and the stoichiometric composition is likely realized at the Tc peak. The optimum substrate temperature Ts was found to be 330 °C. Detailed structural characterizations indicate the strong suppression of structural coherence in the stoichiometric and Mg-rich films. This accounts for the fact that optimized Tc (26.6 K) is still lower than bulk Tc like in previous molecular-beam epitaxy studies. We argue that such structural disorder originates from epitaxial strain due to the lattice mismatch between the film and the substrate.
Low temperature Si layer transfer by direct bonding and mechanical ion cut
Authors: Cho Y. , Cheung N. W.
Published: October 2003

Edge initiated crack propagation was employed to transfer a thin Si layer from a hydrogen implanted donor wafer to Si and oxidized Si receptor wafers. Referred to as mechanical ion cut, successful Si layer transfer was achieved after directly bonding a Si donor wafer implanted with a hydrogen dose of 8×1016/cm2 at H+ energy of 28 keV with Si and SiO2 receptor wafers and annealing above 105 and 170 °C, respectively. The mechanical cut enables Si layer transfer without annealing at 400–600 °C as in thermal ion cut. Our data verify the condition for successful layer transfer to be that mechanical strength of the bonding interface is greater than that of the cutting interface.
Determination of size and composition of optically active CdZnSe/ZnBeSe quantum dots
Authors: Gu Y. , Kuskovsky Igor L. , Fung J. , Robinson R. , Herman I. P. , Neumark G. F. , Zhou X. , Guo S. P. , Tamargo M. C.
Published: October 2003

The size and composition of optically active CdxZn1−xSe/Zn0.97Be0.03Se quantum dots (QDs) are determined using photoluminescence, photoluminescence excitation, and Raman scattering spectroscopies combined with a model of photoluminescence and LO phonon energies. The diameters of optically active QDs range from 5.1 to 8.0 nm with Cd composition in the range of 47%–54%, corresponding to the “small” QDs group. Additionally, surface phonons from QDs are observed in this system.
Effect of quantum-well confinement on acceptor state lifetime in δ -doped GaAs/AlAs multiple quantum wells
Authors: Zheng W. M. , Halsall M. P. , Harrison P. , Wells J.-P. R. , Bradley I. V. , Steer M. J.
Published: October 2003

Using far-infrared time-resolved spectroscopy, we have investigated the effect of quantum-well confinement on the lifetime of shallow acceptor states in GaAs/AlAs multiple quantum wells with Be δ-doping at the well center. Low-temperature far-infrared absorption measurements clearly show three principal absorption lines due to transitions of Be acceptor states from the ground state to the first three odd-parity excited states, respectively. It is found that the lifetime of excited states monotonically reduces with decreasing quantum-well width, from 350 ps in bulk to 55 ps in a 100 Å well. We suggest that the effect of quantum-well confinement on zone-fold acoustic-phonon modes increases the intra-acceptor scattering rate of acoustic-phonon-assisted relaxation.
State-to-state photodissociation dynamics of OH radical via the A 2Σ+ state: Fine-structure distributions of the O(3PJ) product
Authors: Zhou Weidong , Yuan Yan , Zhang Jingsong
Published: October 2003

Photo-predissociation dynamics of rotational states (N′) of OH (A 2Σ+,ν′=3 and 4) are studied using high-n Rydberg atom time-of-flight technique. Spin–orbit branching fractions of O(3PJ=2,1,0) are measured: 0.676±0.010:0.138±0.013:0.186±0.017 for N′=0 in ν′=3 and 0.873±0.026:0.102±0.028:0.025±0.005 for N′=0 in ν′=4, in excellent agreement with the recent theory. While OH A 2Σ+ ν′=3 predissociates predominantly via a single  4Σ− state, ν′=4 decays via the Σ−4, Σ−2, and Π4 states, and interferences of these dissociation pathways strongly influence the O(3PJ) fine-structure distributions. D0(O–H)=35 565±30 cm−1 is obtained.
Growth mechanism and characterization of zinc oxide hexagonal columns
Authors: Baxter Jason B. , Wu Feng , Aydil Eray S.
Published: October 2003

We report on the growth mechanism, structure, and luminescence properties of ZnO hexagonal columns grown from Zn vapor and air plasma. Single-crystal ZnO columns grow in the [0001] direction through repeated nucleation and growth of epitaxial hexagonal pyramids on the c-planes. Homoepitaxial nucleation of three-dimensional ZnO pyramids is most likely due to the Ehrlich–Schwoebel effect. This mechanism produces columns that are a few hundred nanometers in diameter and up to 2 μm in length. Convergent beam electron diffraction shows that the columns grow with Zn polarity in the [0001] direction. Cathodoluminescence and photoluminescence measurements show near-bandedge emission (3.29 eV) with no emission associated with oxygen vacancies at 2.5 eV.
Dynamics of electric field induced particle alignment in nonpolar polymer matrix
Authors: Tai Xiangyang , Wu Guozhang , Yui Hiroshi , Asai Shigeo , Sumita Masao
Published: October 2003

The dynamics of electric field induced particle alignment in nonpolar polymer matrix to build one-dimensional conductive materials was investigated. The influence of electric field on particle alignment was real-time traced by dynamic percolation measurement using carbon black (CB) filled polyethylene as a model system. The activation energy of the continuous CB path formation was calculated and found to be unchanged with CB alignment. The critical percolation concentration at thermodynamic equilibrium state φc* was deduced to characterize the anisotropism of network structure, by which the thermodynamic prerequisite electric field E* for the transition from three-dimensional isotropic network to one-dimensional chain could be easily found out.
A THz transverse electromagnetic mode two-dimensional interconnect layer incorporating quasi-optics
Authors: Coleman S. , Grischkowsky D.
Published: October 2003

We report the demonstration of a planar THz interconnect layer capable of transmitting subpicosecond pulses in the transverse electromagnetic (TEM) mode over arbitrarily long paths with low absorption and no observable group velocity dispersion. Quasioptical elements are incorporated within the interconnect layer forming a configurable THz bandwidth TEM-mode planar interconnect with negligible group velocity dispersion and low loss. For a 146 mm guided path length, including four reflections, the pulses are broadened by the frequency dependent absorption of the interconnect layer from 0.28 to 0.32 ps, and attenuated by the factor 0.2.
Self-induced transparency in InGaAs quantum-dot waveguides
Authors: Schneider S. , Borri P. , Langbein W. , Woggon U. , Förstner J. , Knorr A. , Sellin R. L. , Ouyang D. , Bimberg D.
Published: October 2003

We report the experimental observation and the theoretical modeling of self-induced-transparency signatures such as nonlinear transmission, pulse retardation and reshaping, for subpicosecond pulse propagation in a 2-mm-long InGaAs quantum-dot ridge waveguide in resonance with the excitonic ground-state transition at 10 K. The measurements were obtained by using a cross-correlation frequency-resolved optical gating technique which allows us to retrieve the field amplitude of the propagating pulses.
Investigation of transport properties in polymer/fullerene blends using time-of-flight photocurrent measurements
Authors: Choulis S. A. , Nelson J. , Kim Y. , Poplavskyy D. , Kreouzis T. , Durrant J. R. , Bradley D. D. C.
Published: October 2003

The electron and hole transport properties of blends of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1-4-phenylene vinylene], (MDMO-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) are investigated at room temperature using time-of-flight photocurrent measurements as a function of blend composition and laser excitation intensity. The experimental results are consistent with the notion of a mobility edge for holes in this material that delineates long-lived trapping and mobile carrier states. Electron transport in PCBM appears to be less dispersive than that of holes in MDMO-PPV in the blend, with electron mobilities in the former exceeding hole mobilities in the latter by up to two orders of magnitude. Photocurrent generation in corresponding solar cell devices may therefore be limited by the low hole mobility of MDMO-PPV.
Line spectra from doped nano-oxide: A design for nanooptics
Authors: Altman Igor S. , Pikhitsa Peter V. , Choi Mansoo , Jeong Jae In , Song Ho-Jun , Agranovski Igor E. , Bostrom Thor E.
Published: October 2003

We report the photoluminescence (PL) observation of extremely sharp zero-phonon lines originating from transitions of Mn2+ and Cr3+ ions embedded into MgO matrix of nanoscale (nanocrystals and films). The excellent performance of PL compared to that from quantum dots makes our finding attractive for applications.
Efficient, deep-blue organic electrophosphorescence by guest charge trapping
Authors: Holmes R. J. , D’Andrade B. W. , Forrest S. R. , Ren X. , Li J. , Thompson M. E.
Published: October 2003

We demonstrate efficient, deep-blue organic electrophosphorescence using a charge-trapping phosphorescent guest, iridium(III) bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) doped in the wide-energy-gap hosts, diphenyldi(o-tolyl)silane (UGH1) and p-bis(triphenylsilyly)benzene (UGH2), where exciton formation occurs directly on the guest molecules. Charge trapping on the guest is confirmed by the dependence of the drive voltage and electroluminescence spectrum on guest concentration. Ultraviolet photoemission spectroscopy measurements establish the relative highest occupied molecular orbital positions of FIr6 in UGH1 and UGH2. Peak quantum and power efficiencies of (8.8±0.9)% and (11.0±1.1) lm/W in UGH1 and (11.6±1.2)% and (13.9±1.4) lm/W in UGH2 are obtained, while the emission in both cases is from FIr6 and is characterized by Commission Internationale de l’Eclairage coordinates of (x=0.16, y=0.26) in UGH2.
Dielectrophoresis and electrohydrodynamics-mediated fluidic assembly of silicon resistors
Authors: Lee S. W. , Bashir R.
Published: October 2003

In this letter, we present techniques, utilizing dielectrophoresis and electrohydrodynamics, which can be used for assembling single-crystal silicon devices suspended in a solution onto a binding site on a heterogeneous substrate. Silicon resistors with gold/chromium layers located at the end of the resistors and silicon blocks without metal were fabricated on bonded and etched-backed silicon-on-insulator wafers and successfully released into deionized water. The devices were subsequently assembled on a different substrate at specific binding sites by dielectrophoretic and electrohydrodynamic forces with submicron precision. Current–voltage measurements of the assembled resistors exhibited low contact resistance after the solution was completely evaporated and the contacts were annealed.
Efficiency of local light-plasmon coupling
Authors: Ditlbacher H. , Krenn J. R. , Hohenau A. , Leitner A. , Aussenegg F. R.
Published: October 2003

We investigate quantitatively the local coupling efficiency of a strongly focused laser beam to surface plasmon polaritons on a gold thin film. The coupling is mediated by gold ridges with nanoscale cross section. The coupling efficiency is determined by measuring the leakage radiation emitted by the surface plasmon polaritons into the glass substrate supporting the thin film.
Double-scaled potential profile in a group-III nitride alloy revealed by Monte Carlo simulation of exciton hopping
Authors: Kazlauskas K. , Tamulaitis G. , Žukauskas A. , Khan M. A. , Yang J. W. , Zhang J. , Simin G. , Shur M. S. , Gaska R.
Published: October 2003

The temperature dependences of the peak position and width of the photoluminescence band in Al0.1In0.01Ga0.89N layers were explained by Monte Carlo simulation of exciton localization and hopping. The introduction of a doubled-scaled potential profile due to inhomogeneous distribution of indium allowed obtaining a good quantitative fit of the experimental data. Hopping of excitons was assumed to occur through localized states distributed on a 16 meV energy scale within the In-rich clusters with the average energy in these clusters dispersed on a larger (42 meV) scale.