bluuebunny/crossref_metadata_2025 huggingface dataset:
Comment on “Precipitate pattern formation in fluctuating media” [J. Chem. Phys. 120, 1837 (2004)]
Authors: Lagzi István , Izsák Ferenc , Müller Stefan C. , Ross John
Published: August 2004

This comment contains critical remarks on simulations on the formation of Liesegang precipitation patterns in media with low concentration gradients, as presented in the article “Precipitate pattern formation in fluctuating media” by F. Iszák and I. Lagzi, which appeared in J. Chem. Phys. 120, 1837 (2004).
Maximum entropy formulation of the Kirkwood superposition approximation
Authors: Singer A.
Published: August 2004

Using a variational formulation, we derive the Kirkwood superposition approximation for systems at equilibrium in the thermodynamic limit. We define the entropy of the triplet correlation function and show that the Kirkwood closure brings the entropy to its maximal value. This approach leads to a different interpretation for the Kirkwood closure relation, usually explained by probabilistic considerations of dependence and independence of particles. The Kirkwood closure is generalized to finite volume systems at equilibrium by computing the pair correlation function in finite domains. Closure relations for high order correlation functions are also found using a variational approach. In particular, maximizing the entropy of quadruplets leads to the high order closure g1234=g123g124g134g234/[g12g13g14g23g24g34] used in the Born-Green-Yvon 2 equations which are a pair of integral equations for the triplet and pair correlation functions.
Theoretical study on isomeric stabilities of C2H2Si and its ionization potentials and electron affinities
Authors: Ikuta Shigeru , Saitoh Toshiaki , Wakamatsu Souichi
Published: August 2004

The geometric structures and isomeric stabilities of various stationary points in C2H2Si neutral and its cation and anion are investigated at the coupled-cluster singles, doubles (triples) [CCSD(T)] level of theory. For the geometrical survey, the basis sets used are of the Dunning’s correlation consistent basis sets of triple-ζ quality (cc-pVTZ) for the neutral and cation. For the anions, the cc-pVTZ basis sets with diffuse functions (aug-cc-pVTZ) are used. The final energies are calculated by the use of the CCSD(T) level of theory with the aug-cc-pVTZ basis set at their optimized geometries. To lower lying neutrals and cations, the Dunning’s correlation consistent basis sets of quadruple-ζ quality (cc-pVQZ) are also applied. Both the global minima of the C2H2Si neutral and cation, N-1 (C2v:1A1) and C-1 (C2v:2B2), respectively, are silacyclopropenylidene conformers, having a CCSi ring with a C=C double bond. No competitive stable isomers exist in the present C2H2Si neutral. In the cation, however, the second lowest lying isomer C-2 lies 10.8 kJ/mol above the most stable C-1. The vertical and adiabatic ionization potentials from the lowest lying neutral N-1 are 9.83 and 8.97 eV, respectively, at the CCSD(T)/cc-pVQZ level of theory. The electron addition to the N-1 does not result in the anion with positive (real) electron affinities. On the other hand, the electron addition to the N-2 isomer produces the global minimum anion A-1 (C2v:2B1) with the positive electron affinities of 1.13 eV. The second lowest lying anion isomer A-2 with silylenylacetylene conformer, produced from an electron addition to the N-3 neutral, very well competes with the A-1 after the zero-point vibrational energy corrections. The energy difference between the two lowest lying isomers of the neutral and its anion, N-1 and A-1, is only 0.39 eV.
First principles resonance widths for Li near an Al(001) surface: Predictions of scattered ion neutralization probabilities
Authors: Niedfeldt Keith , Carter Emily A. , Nordlander P.
Published: August 2004

By combining a first-principles periodic density functional theory calculation of adsorbate resonance widths with a many-body dynamical theory of charge transfer, we assess charge-transfer rates for ions scattering off metal surfaces. This goes beyond previous approaches, which have been limited to modeling the surfaces with either static potentials or finite clusters. Here we consider Li+ scattering from an Al(001) surface. We show how the Li 2s orbital hybridizes with metal valence bands, near the surface, increasing the width of the 2s energy level. This in turn affects the charge-transfer rates between the ion and the metal surface. Our predictions for Li+-Al(001) scattering yield the correct angular dependence of the fraction of neutral Li atoms formed when compared to experiment.
Microcanonical unimolecular rate theory at surfaces. II. Vibrational state resolved dissociative chemisorption of methane on Ni(100)
Authors: Abbott H. L. , Bukoski A. , Harrison I.
Published: August 2004

A three-parameter microcanonical theory of gas-surface reactivity is used to investigate the dissociative chemisorption of methane impinging on a Ni(100) surface. Assuming an apparent threshold energy for dissociative chemisorption of E0=65 kJ/mol, contributions to the dissociative sticking coefficient from individual methane vibrational states are calculated: (i) as a function of molecular translational energy to model nonequilibrium molecular beam experiments and (ii) as a function of temperature to model thermal equilibrium mbar pressure bulb experiments. Under fairly typical molecular beam conditions (e.g., Et⩾25 kJ mol−1, Ts⩾475 K, Tn⩽400 K), sticking from methane in the ground vibrational state dominates the overall sticking. In contrast, under thermal equilibrium conditions at temperatures T⩾100 K the dissociative sticking is dominated by methane in vibrationally excited states, particularly those involving excitation of the ν4 bending mode. Fractional energy uptakes fj defined as the fraction of the mean energy of the reacting gas-surface collision complexes that derives from specific degrees of freedom of the reactants (i.e., molecular translation, rotation, vibration, and surface) are calculated for thermal dissociative chemisorption. At 500 K, the fractional energy uptakes are calculated to be ft=14%, fr=21%, fv=40%, and fs=25%. Over the temperature range from 500 K to 1500 K relevant to thermal catalysis, the incident gas-phase molecules supply the preponderance of energy used to surmount the barrier to dissociative chemisorption, fg=ft+fr+fv≈75%, with the highest energy uptake always coming from the molecular vibrational degrees of freedom. The predictions of the statistical, mode-nonspecific microcanonical theory are compared to those of other dynamical theories and to recent experimental data.
Confinement free energy and chain conformations of homopolymers confined between two repulsive walls
Authors: Wang Yongmei
Published: August 2004

Lattice Monte Carlo simulations of polymer solutions confined between two parallel plates were performed. The confinement free energy Δμconf per chain and the radius of gyrations of the chains parallel and perpendicular to the plates were obtained. When the concentration of the confined solution is above the overlap concentration, Δμconf is found to scale with Na/D in a power law, βΔμconf∼(Na/D)m, with an exponent m=1.10±0.02 for athermal walls where N is the number of monomers in a chain, D is the slit width, and a is the lattice spacing. The presence of a weak attractive polymer/wall interaction εw does not change the scaling variable, but the exponent m increases slightly. Extrapolating the results to melt would suggest that the predictions made by de Gennes [C. R. Acad. Sci. Paris II 305, 1181 (1987)] about the confinement free energy cost per chain in polymer melt is correct as far as the scaling variable is concerned, but is incorrect about the exponent m observed. The implication of this result on the predicted force between plates immersed in polymer melt is discussed. The parallel dimensions of the confined chain is expanded when the slit width D is narrow, however, the expansion is reduced at high concentration. It is conceivable that in melt the chain is not expanded when confined in a repulsive slit.
Thermodynamics and partitioning of homopolymers into a slit-A grand canonical Monte Carlo simulation study
Authors: Jiang Wenhua , Wang Yongmei
Published: August 2004

Grand canonical ensemble Monte Carlo simulation (GCMC) combined with the histogram reweighting technique was used to study the thermodynamic equilibrium of a homopolymer solution between a bulk and a slit pore. GCMC gives the partition coefficients that agree with those from canonical ensemble Monte Carlo simulations in a twin box, and it also gives results that are not accessible through the regular canonical ensemble simulation such as the osmotic pressure of the solution. In a bulk polymer solution, the calculated osmotic pressure agrees very well with the scaling theory predictions both for the athermal polymer solution and the theta solution. However, one cannot obtain the osmotic pressure of the confined solution in the same way since the osmotic pressure of the confined solution is anisotropic. The chemical potentials in GCMC simulations were found to differ by a translational term from the chemical potentials obtained from canonical ensemble Monte Carlo simulations with the chain insertion method. This confirms the equilibrium condition of a polymer solution partition between the bulk and a slit pore: the chemical potentials of the polymer chain including the translational term are equal at equilibrium. The histogram reweighting method enables us to obtain the partition coefficients in the whole range of concentrations based on a limited set of simulations. Those predicted bulk-pore partition coefficient data enable us to perform further theoretical analysis. Scaling predictions of the partition coefficient at different regimes were given and were confirmed by the simulation data.
Electrical and optical properties of Bi2Ti2O7 thin films prepared by metalorganic decomposition method
Authors: Hou Yun , Lin Tie , Huang Zhiming , Wang Genshui , Hu Zhigao , Chu Junhao , Xu Xiaohong , Wang Min
Published: August 2004

Highly (111) oriented Bi2Ti2O7 thin films have been grown on Pt∕Ti∕SiO2∕Si and Al2O3 substrates by metalorganic decomposition method at 550°C. The structural properties of the films were examined by x-ray diffraction. The Bi2Ti2O7 films exhibit good insulating property and the leakage current density of the film on Pt∕Ti∕SiO2∕Si is only about 1.56×10−8A∕cm2 at 200kV∕cm. The refractive index and extinction coefficient of Bi2Ti2O7 thin films were determined by fitting the infrared spectroscopic ellipsometric data using a classical dielectric function formula. As the wavelength increases, the refractive index decreases, while the extinction coefficient increases. And the band-gap energy Eg was obtained from the optical transmission spectra of Bi2Ti2O7 thin films.
Bright white light electroluminescent devices based on a dye-dispersed polyfluorene derivative
Authors: Kim Joo Hyun , Herguth Petra , Kang Mun-Sik , Jen Alex K-Y. , Tseng Ya-Hsien , Shu Ching-Fong
Published: August 2004

A series of efficient and bright white light-emitting diodes were fabricated using the blends of two fluorene-derived fluorescent dyes, (4,7-bis-(9,9,9′,9′-tetrahexyl-9H,9′H-[2,2′]bifluoren-7-yl)-benzo[1,2,5]thiadiazole) (FFBFF-emits green) and (4,7-bis-[5-(9,9-dihexyl-9H-fluoren-2-yl)-thiophen-2-yl]-benzo[1,2,5]thiadiazole) (FTBTF-emits red) in an efficient blue-emitting polyfluorene-derived copolymer (poly[(9,9-bis(4-di(4-n-butylphenyl)aminophenyl))]-stat-(9,9-bis(4-(5-(4-tert-butylphenyl)-2-oxadiazolyl)-phenyl))-stat-(9,9-di-n-octyl)fluorene) (PF-TPA-OXD). The resulting white light-emitting device reaches a maximum external quantum efficiency of 0.82% and a maximum brightness of 12900cd∕m2 at 12V. The Commission Internationale d’Énclairage chromaticity coordinates of the device remain very close to that of pure white emission at a relatively broad bias range from 6V(x=0.36,y=0.37)to12V(x=0.34,y=0.34).
Variation of sidewall roughness of polymeric waveguides during reactive ion etching
Authors: Pani S. K. , Wong C. C. , Sudharsanam K.
Published: August 2004

Sidewall roughness (SWR) of fluorinated polyether waveguides fabricated using reactive ion etching was directly measured using atomic force microscopy. We confirmed the pressure dependence of SWR for shallow structures and discovered an additional etch depth dependence for deeper structures which counteracts the pressure dependence. Lower O2 pressure etching produces SWR which increases with depth while higher O2 pressure etching produces declining SWR with depth. Addition of N2 to the etch ambient suppresses SWR for high pressure etches at intermediate depths. The role of N2 is possibly related to the formation of a N-containing passivating film, confirmed by secondary ion mass spectroscopy measurements.
Control of photoluminescence properties of Si nanocrystals by simultaneously doping n- and p-type impurities
Authors: Fujii Minoru , Yamaguchi Yasuhiro , Takase Yuji , Ninomiya Keiichi , Hayashi Shinji
Published: August 2004

The effects of B and P codoping on photoluminescence (PL) properties of Si nanocrystals (nc-Si) are studied systematically. It is shown that the PL intensity of codoped nc-Si is always higher than that of either P- or B-doped nc-Si. The intensity is sometimes even higher than that of pure nc-Si at relatively low P and B concentrations and low annealing temperatures. By doping P and B simultaneously to very high concentrations, the PL peak shifts below the band gap of bulk Si.
Birefringence in optical waveguides made by silicon nanocrystal superlattices
Authors: Riboli F. , Navarro-Urrios D. , Chiasera A. , Daldosso N. , Pavesi L. , Oton C. J. , Heitmann J. , Yi L. X. , Scholz R. , Zacharias M.
Published: August 2004

We investigate the optical properties of planar waveguides where the core layer is formed by a silicon nanocrystals (Si-nc)∕SiO2 superlattice. M-line measurements of the different waveguides yield the mode indices, which can be modeled by assuming anisotropic optical properties of the core layer. This anisotropy is related to the superlattice, i.e., it is a form birefringence. By modeling the m-line measurements with the structural data obtained by transmission electron microscopy analysis, we determine for each waveguide the value of the form birefringence, an upper limit of the nanocrystals size and their refractive index. Values of the form birefringence as high as 1% have been found.
High-performance photorefractive polymer operating at 975nm
Authors: Eralp Muhsin , Thomas Jayan , Tay Savaş , Li Guoqiang , Meredith Gerald , Schülzgen Axel , Peyghambarian N. , Walker Gregory A. , Barlow Stephen , Marder Seth R.
Published: August 2004

A family of photorefractive polymer composites has been developed that enable high-performance device operation at a wavelength of 975nm. This constitutes a major extension into the near-infrared spectral region for the operation of all-organic photorefractive devices. Utilizing our photorefractive materials, we demonstrate large net two-beam coupling gain of more than 100cm−1, 60% diffraction efficiency in four-wave mixing experiments, and a fast response time of 33ms, at an irradiance of 1W∕cm2.
Neutron holographic study of palladium hydride
Authors: Cser László , Török Gyula , Krexner Gerhard , Prem Manfred , Sharkov Ivan
Published: August 2004

A holographic image of the atomic arrangement in a PdH0.78 single crystal was recorded using spherical neutron waves generated by incoherent neutron-proton scattering (“internal source concept”). The resolution is sufficient to show the positions of single atomic nuclei on their respective lattice sites despite the delocalization of protons due to the weak bonding of hydrogen dissolved in palladium metal. Ways to overcome problems in the reconstruction of atomic positions related to the highly unfavorable sample shape are discussed. The technique offers possibilities for the investigation of various materials containing substantial amounts of hydrogen.
Fabrication of single nanofluidic channels in poly(methylmethacrylate) films via focused-ion beam milling for use as molecular gates
Authors: Cannon Donald M. , Flachsbart Bruce R. , Shannon Mark A. , Sweedler Jonathan V. , Bohn Paul W.
Published: August 2004

Focused-ion beam (FIB) milling provides rapid fabrication of individual cylindrical submicrometer channels with reproducible dimensions (±5% diameters) through 8-μm thick poly(methylmethacrylate) (PMMA) films. PMMA films are spincast on sacrificial Si carriers and sputter-coated with Au before the 30-kV gallium FIB milling process. By adding a trace amount of poly(ethyleneoxide) and poly(dimethylsiloxane) to the PMMA solution before casting, the films can be released for subsequent mounting in microfluidic devices to create hybrid microfluidic-nanofluidic multilevel architectures. In situ FIB sectioning demonstrates the smooth cylindrical surface within the pore. Placing a milled film in contact with an aqueous fluorescein solution fills the channel by capillary action, as verified by confocal fluorescence microscopy. Confocal fluorescence of dyed films reveals that the pores span the thickness of the PMMA film. Small arrays of channels with a defined number and density and arbitrary in-plane spatial arrangement are fabricated with this process, allowing a unique testbed for high aspect ratio nanofluidic devices.
Room temperature synthesis of carbon nanofibers containing nitrogen by plasma-enhanced chemical vapor deposition
Authors: Minea T. M. , Point S. , Granier A. , Touzeau M.
Published: August 2004

This letter reports low-pressure, room-temperature growth of carbon nanofibers containing nitrogen by plasma chemical vapor deposition arrangement. By alternating pure acetylene plasma and afterglow pure nitrogen high dense plasma, a fine control of the fibers growth kinetic is obtained. This layer-by-layer deposition technique takes advantage of nitrogen chemical etching effects during the growth of nitrogen-doped carbon nanofibers.
Resonance-cone focusing in a compensating bilayer of continuous hyperbolic microstrip grids
Authors: Siddiqui Omar , Eleftheriades George V.
Published: August 2004

We propose a periodic structure synthesized by arranging continuous transmission-line segments in a two-dimensional planar grid over a ground plane. The periodic grid exhibits hyperbolic spatial dispersion characteristics, leading to the formation of highly directive beams called “resonance cones.” Simulations and experiments at 10GHz show negative refraction and frequency-dependent spatial focusing of the resonance cones, when two such grids with compensating phase properties are interfaced to form a bilayer. The proposed structures are deprived of loading elements or vias, thus leading to ease of fabrication and scalability with frequency for applications, such as spatial filters, multiplexers, and demultiplexers.
Piezoelectric d33 coefficient of cellular polypropylene subjected to expansion by pressure treatment
Authors: Zhang X. , Hillenbrand J. , Sessler G. M.
Published: August 2004

The piezoelectric activity of charged cellular polypropylene films of originally 40 and 50μm thickness can be significantly increased by thickness expansion due to an exposure to high pressure for time periods of the order of hours. After such a treatment, the d33 coefficient, measured by quasistatic and interferometric methods, is found to be as high as 1200pC∕N at 0.001Hz, decreasing to 350pC∕N just below resonance. At the resonance, which is located in the range of 150–400kHz for differently treated samples, d33 coefficients up to 1400pC∕N are found. The high d33 coefficients result from a decrease of Young’s modulus Y and an increase of the chargeability of the material due to the expansion, while the decrease of d33 with frequency up to resonance is related to a corresponding increase of Y. Static pressures up to 10kPa have little influence on d33, but higher pressures result in a reversible decrease.
Electrical transport in amorphous semiconducting AlMgB14 films
Authors: Tian Y. , Li G. , Shinar J. , Wang N. L. , Cook B. A. , Anderegg J. W. , Constant A. P. , Russell A. M. , Snyder J. E.
Published: August 2004

The electrical transport properties of semiconducting AlMgB14 films deposited at room temperature and 573K are reported in this letter. The as-deposited films are amorphous, and they exhibit high n-type electrical conductivity, which is believed to stem from the conduction electrons donated by Al, Mg, and/or Fe impurities in these films. The film deposited at 573K is less conductive than the room-temperature-deposited film. This is attributed to the nature of donor or trap states in the band gap related to the different deposition temperatures.
Delay-time investigation of electromagnetic waves through homogeneous medium and photonic crystal left-handed materials
Authors: Moussa R. , Foteinopoulou S. , Soukoulis C. M.
Published: August 2004

Results of the delay time for the electromagnetic wave to reach its final direction through both photonic crystal and homogeneous medium are presented. The delay- or transient-time results, based on different cases and different incoming wave angles, show that the diffracted beam is trapped at the interface. This delay time is longer for the negative refractive index photonic crystal and is almost twice the duration of the delay time for the positive one. For the homogeneous medium, we also find that at the interface between a right- and left-handed medium the delay time is even longer than in the photonic crystal case. A comparison between left-handed behavior in photonic crystals and homogeneous media is reported.
Optically patterned nuclear doughnuts in GaAs∕MnAs heterostructures
Authors: Stephens J. , Berezovsky J. , Kawakami R. K. , Gossard A. C. , Awschalom D. D.
Published: August 2004

We demonstrate a scheme for optically patterning nuclear-spin polarization in semiconductor∕ferromagnet heterostructures. A scanning time-resolved Kerr rotation microscope is used to image the nuclear-spin polarization that results when GaAs∕MnAs epilayers are illuminated with a focused laser having a Gaussian profile. Rather than tracking the intensity profile of the laser spot, these images reveal that the nuclear polarization forms an annular lateral structure having circular symmetry with a dip rather than a peak at its center.
Optical and electrical transport properties in silicon carbide nanowires
Authors: Seong Han-Kyu , Choi Heon-Jin , Lee Sang-Kwon , Lee Jung-Il , Choi Doo-Jin
Published: August 2004

We report on the optical and electrical transport properties of single-crystalline silicon carbide nanowires (SiC NWs). The NWs were fabricated by a chemical vapor deposition process, and had diameters of <100nm and lengths of several μm. X-ray diffraction and transmission electron microscopy analysis showed the single-crystalline nature of NWs with a growth direction of ⟨111⟩. Photoluminescence characterization showed blue emission at room temperature. The electrical measurements from a field effect transistor structure on individual NWs showed n-type semiconductor characteristics. The resistivity and estimated electron mobility on the NWs are 2.2×10−2Ωcm for 0V of gate voltage and 15cm2∕(Vs), respectively. Our low-resistivity SiC NWs could be applied to a high-temperature operation sensor and actuator due to its own excellent electrical and optical properties.
Pattern formation in a polymer thin film induced by an in-plane electric field
Authors: Salac David , Lu Wei , Wang Chia-Wei , Sastry Ann Marie
Published: August 2004

This letter reports experimental work involving use of an in-plane electric field to induce morphological patterns in a thin polymer film. The film was first spin coated onto a glass wafer. Then, it was heated to above its glass transition temperature to achieve mobility in the fluid. An in-plane electric field was applied using two parallel electrodes, spaced 10mm apart, whereupon the initially flat polymer∕air interface lost stability and formed islands. The self-assembled islands exhibited a narrow size distribution and demonstrated spatial ordering. We attribute the pattern formation to a combined mechanism of minimization of combined interface energy and electrostatic energy.
Tunable diffraction grating using ultraviolet-light-induced spatial phase modulation in dual-frequency liquid crystal
Authors: Lin Pao-Tai , Liang Xiao , Ren Hongwen , Wu Shin-Tson
Published: August 2004

An electrically tunable diffraction phase grating using ultraviolet (UV)-light-induced spatial dielectric modulation of a dual-frequency liquid crystal (DFLC) cell is demonstrated. A photomask with transparent and opaque stripes was used for fabricating the grating. In the UV-exposed stripes, the negative dielectric anisotropy (Δε) tolane compound of the DFLC mixture is partially polymerized resulting in a decreased threshold voltage as compared to that of the unexposed region. Upon applying a constant voltage, the phase difference between the adjacent pixels is produced. The first-order diffraction efficiency reaches ∼60% which agrees well with the simulation results. Due to the dual-frequency addressing at 30Vrms, the response time of the DFLC phase grating was measured to be ∼1ms at room temperature.
Spin-transfer effects in nanoscale magnetic tunnel junctions
Authors: Fuchs G. D. , Emley N. C. , Krivorotov I. N. , Braganca P. M. , Ryan E. M. , Kiselev S. I. , Sankey J. C. , Ralph D. C. , Buhrman R. A. , Katine J. A.
Published: August 2004

We report measurements of magnetic switching and steady-state magnetic precession driven by spin-polarized currents in nanoscale magnetic tunnel junctions with low-resistance, <5Ωμm2, barriers. The current densities required for magnetic switching are similar to values for all-metallic spin-valve devices. In the tunnel junctions, spin-transfer-driven switching can occur at voltages that are high enough to quench the tunnel magnetoresistance, demonstrating that the current remains spin polarized at these voltages.
Enhancement of optical properties of CdSe pillars fabricated by the combination of electron-beam lithography and electrochemical deposition
Authors: Chen Y. L. , Chen C. C. , Jeng J. C. , Chen Y. F.
Published: August 2004

Combining electron-beam lithography and electrochemical deposition, CdSe pillars can be fabricated in a specific shape and size in a well controlled manner. This simple technique provides an excellent opportunity to probe the change of optical properties of submicron structures due to size variation. Quite interestingly, it is found that the intensities of micro-photoluminescence as well as Raman scattering show an oscillatory behavior and can be enhanced at some particular diameters of CdSe pillars. This phenomenon can be well explained in terms of the resonant effects, in which the CdSe pillars can act as resonance cavities. Once the wavelengths of incident laser or the luminescence emitted from CdSe pillars are consistent with the predicted Fabry–Perot resonance, the optical properties can be greatly enhanced.
Free-standing subnanometer graphite sheets
Authors: Wang J. J. , Zhu M. Y. , Outlaw R. A. , Zhao X. , Manos D. M. , Holloway B. C. , Mammana V. P.
Published: August 2004

Free-standing graphite sheets with thickness less than 1nm, “carbon nanosheets,” were synthesized on a variety of substrates by radio-frequency plasma-enhanced chemical vapor deposition without any catalyst or special substrate treatment. The nanosheets consist of one to three graphene layers with a large smooth surface topography, standing roughly vertical to the substrate. Due to the atomic thickness and corrugated nature of nanosheets, low-energy vibrational modes are present in the Raman spectra. The low turn-on field of 4.7 V/μm for electron field emission suggests that the carbon nanosheets could be used as a potential edge emitter.
Plasma composition during plasma-enhanced chemical vapor deposition of carbon nanotubes
Authors: Bell M. S. , Lacerda R. G. , Teo K. B. K. , Rupesinghe N. L. , Amaratunga G. A. J. , Milne W. I. , Chhowalla M.
Published: August 2004

Neutral species and positive ions were extracted directly from a C2H2:NH3 plasma used to grow vertically aligned carbon nanotubes (CNTs) and analyzed by mass spectrometry. We observe that NH3 suppresses C2H2 decomposition and encourages CNT formation. We show that the removal of excess carbon, essential for obtaining nanotubes without amorphous carbon deposits, is achieved through gas phase reactions which form mainly HCN. We determine an optimum C2H2:NH3 gas ratio which is consistent with previous observations based upon postdeposition analysis. We find, in contrast to thin film growth by plasma-enhanced chemical vapor deposition, that the optimum condition does not correspond to the highest level of ionization. We also provide evidence that C2H2 is the dominant precursor for CNTs in our experiments.
Random lasing in human tissues
Authors: Polson Randal C. , Vardeny Z. Valy
Published: August 2004

A random collection of scatterers in a gain medium can produce coherent laser emission lines dubbed “random lasing.” We show that biological tissues, including human tissues, can support coherent random lasing when infiltrated with a concentrated laser dye solution. To extract a typical random resonator size within the tissue we average the power Fourier transform of random laser spectra collected from many excitation locations in the tissue; we verified this procedure by a computer simulation. Surprisingly, we found that malignant tissues show many more laser lines compared to healthy tissues taken from the same organ. Consequently, the obtained typical random resonator was found to be different for healthy and cancerous tissues, and this may lead to a technique for separating malignant from healthy tissues for diagnostic imaging.
Chemically resolved electrical measurements using x-ray photoelectron spectroscopy
Authors: Cohen Hagai
Published: August 2004

Noncontact chemically resolved electrical measurements are presented, capable of probing selected regions within fine heterostructures. Using a slightly modified x-ray photoelectron spectrometer, an effective means is demonstrated for measuring I–V curves of molecular layers, free of substrate and contact contributions. The concept is simple and general, revealing unique details on electrical response mechanisms. Realized with commonly available equipment, it should be effective for a broad range of heterostructured systems.
Influence of manganese contamination on high-mobility GaAs∕AlGaAs heterostructures
Authors: Wagenhuber K. , Tranitz H.-P. , Reinwald M. , Wegscheider W.
Published: August 2004

Photoluminescence and magnetotransport measurements have been performed to assess the quality of modulation-doped GaAs∕AlGaAs heterostructures. The temporal evolution of the low-temperature electron mobility of samples prepared in a molecular-beam-epitaxy chamber containing manganese as a source material was studied. Mn contamination was identified to be responsible for the reduction of the electron mobility to 1×106cm2∕Vs and the appearance of a distinct photoluminescence band. In contrast, structures in which this signal is absent reach mobility values of 5.4×106cm2∕Vs. This directly proves that the epitaxy of high-mobility electron systems and structures containing GaMnAs layers, in principle, can be combined in one growth chamber.
Near-infrared femtosecond laser-induced crystallization of amorphous silicon
Authors: Shieh Jia-Min , Chen Zun-Hao , Dai Bau-Tong , Wang Yi-Chao , Zaitsev Alexei , Pan Ci-Ling
Published: August 2004

Amorphous silicon (a-Si) was crystallized by femtosecond laser annealing (FLA) using a near-infrared (λ≈800nm) ultrafast Ti:sapphire laser system. The intense ultrashort laser pulses lead to efficient nonlinear photoenergy absorption and the generation of very dense photoexcited plasma in irradiated materials, enabling nonlinear melting on transparent silicon materials. We studied the structural characteristics of recrystallized films and found that FLA assisted by spatial scanning of laser strip spot constitutes superlateral epitaxy that can crystallize a-Si films with largest grains of ∼800nm, requiring laser fluence as low as ∼45mJ∕cm2, and low laser shots. Moreover, the optimal annealing conditions are observed with a significant laser-fluence window (∼30%).
Enhanced transmission of microwave radiation in one-dimensional metallic gratings with subwavelength aperture
Authors: Akarca-Biyikli S. Sena , Bulu Irfan , Ozbay Ekmel
Published: August 2004

We report a theoretical and experimental demonstration of enhanced microwave transmission through subwavelength apertures in metallic structures with double-sided gratings. Three different types of aluminum gratings (sinusoidal, symmetric rectangular, and asymmetric rectangular shaped) are designed and analyzed. Our samples have a periodicity of 16mm, and a slit width of 2mm. Transmission measurements are taken in the 10–37.5GHz frequency spectrum, which corresponds to 8–30mm wavelength region. All three structures display significantly enhanced transmission around surface plasmon resonance frequencies. The experimental results agree well with finite-difference-time-domain based theoretical simulations. Asymmetric rectangular grating structure exhibits the best results with ∼50% transmission at 20.7mm, enhancement factor of ∼25, and ±4° angular divergence.
Dispersion-controlled optical group delay device by chirped photonic crystal waveguides
Authors: Mori Daisuke , Baba Toshihiko
Published: August 2004

Previously, we proposed and demonstrated chirped photonic crystal waveguides, in which some structural parameters are gradually changed so that the photonic band characteristic is smoothly shifted. In this letter, we discuss an optical delay line composed of two index-chirped waveguides in a directional coupler structure. A large delay is realized by a low average group velocity of <c∕350 near the band-edge with almost perfect dispersion compensation even for a short optical pulse. Finite difference time domain simulation demonstrates that such a device is possible in a practical design.
Zn
0.9
Mg
0.1
O
∕
ZnO
p
–
n
junctions grown by pulsed-laser deposition
Authors: Ip K. , Heo Y. W. , Norton D. P. , Pearton S. J. , LaRoche J. R. , Ren F.
Published: August 2004

The electrical characteristics of Zn0.9Mg0.1O∕ZnOp–n junctions grown by pulsed-laser deposition on bulk, single-crystal ZnO substrates are reported. The forward turn-on voltage of the junctions was in the range 3.6–4V for Pt∕Au metallization used for the p-Ohmic contact on Zn0.9Mg0.1O.The reverse breakdown voltage is as high as 9V, but displays a small negative temperature coefficient of −0.1–0.2VK−1 over the range 30–200°C. The achievement of acceptable rectification in the junctions required growth of an n-type ZnO buffer on the ZnO substrate prior to growth of the p-type, phosphorus-doped Zn0.9mg0.1O.Without this buffer, the junctions showed very high leakage current.
Improvement of the beam quality of a broad-area diode laser using double feedback from two external mirrors
Authors: Chi Mingjun , Bøgh Niels-Søren , Thestrup Birgitte , Petersen Paul Michael
Published: August 2004

In this letter, a symmetric double-feedback configuration, to improve the beam quality of broad-area diode lasers is demonstrated. With this configuration, a symmetric double-lobed far field can be obtained, and this configuration leads to good beam quality. The beam quality factor M2 of a diode laser with the emitting area 1μm×200μm is improved by using both the asymmetric single feedback and the symmetric double feedback. M2 values of 4.3 for the asymmetric single-feedback laser system and 3.3 for the symmetric double-feedback laser system are obtained, whereas the M2 value of the freely running laser is 42. The far and the near fields are also measured and compared for the three conditions.
Uniform field-induced strain in a∕b-axes-oriented Bi3.9Pr0.1Ti3O12 thick films on IrO2∕Si substrates for lead-free piezoelectric microdevice applications
Authors: Matsuda Hirofumi , Ito Sachiko , Iijima Takashi
Published: August 2004

We grew 1.5-μm-thick Bi3.9Pr0.1Ti3O12 (BPT) polycrystalline films with a∕b-axes orientation on IrO2∕Si substrates from chemical solutions and measured electric-field-induced strain ε by a double-beam laser doppler vibrometer and atomic-force microscopy (AFM). The results reflected uniform film growth and elimination of elastic deformation of the substrate, and the microscopic piezoelectric response in AFM measurements agreed well with the macroscopic response evaluated by laser vibrometer. A large longitudinal ε=0.3% was observed at 400kV∕cm and 10Hz, and the piezoelectric coefficient d33=dε∕dE∣0=60pm∕V was calculated. X-ray diffraction analyses revealed that the (200)∕(020) peak of BPT was comprised of 40% a domain and led to the reduced values of ferroelectric polarizations of 2Pr=35 and Psat=23μC∕cm2. This suggests that applying an electric field did not cause fraction reconfiguration of a- and b-domains, and, therefore, the measured large strain was achieved without contribution from the 90°-domain-wall motion in a∕b-axes-oriented BPT thick films.
Multilevel memory using an electrically formed single-electron box
Authors: Nishiguchi Katsuhiko , Inokawa Hiroshi , Ono Yukinori , Fujiwara Akira , Takahashi Yasuo
Published: August 2004

A multilevel dynamic random-access memory using a single-electron box (SEB) and single-electron transistor (SET) is fabricated on a silicon-on-insulator substrate. A one-dimensional field-effect transistor (FET), which is connected to the SEB, modulates a barrier potential to precisely control the number of electrons one by one in the SEB by means of the Coulomb-blockade phenomenon. At room temperature and 26K, we demonstrate a multilevel memory, in which each interval between the levels is given by a single electron, by using the SET electrometer coupled capacitively to the SEB. The control of stored electrons by the FET assures long-retention time and high-speed write/erase operation.
Is a “homogeneous” description of dynamic heterogeneities possible?
Authors: Szamel Grzegorz
Published: August 2004

We study the simplest model of dynamic heterogeneities in glass forming liquids: one-spin facilitated kinetic Ising model introduced by Fredrickson and Andersen [G. H. Fredrickson and H. C. Andersen, Phys. Rev. Lett. 53, 1244 (1984); J. Chem. Phys. 83, 5822 (1985)]. We show that the low-temperature, long-time behavior of the density autocorrelation function predicted by a scaling approach can be obtained from a self-consistent mode-couplinglike approximation.
Type-I Ge∕Ge1−x−ySixSny strained-layer heterostructures with a direct Ge bandgap
Authors: Menéndez J. , Kouvetakis J.
Published: August 2004

The electronic properties of Ge∕Ge1−x−ySixSny strained-layer heterostructures are predicted theoretically. It is found that a lattice-matched system with fully strained Ge layers and relaxed Ge1−x−ySixSny alloys can have a direct fundamental bandgap with spatial localization in the Ge layers (type I). The Si and Sn concentrations for which such a direct bandgap obtains are close to those that have already been experimentally demonstrated [M. Bauer, C. Ritter, P. A. Crozier, J. Ren, J. Menendez, G. Wolf, and J. Kouvetakis, Appl. Phys. Lett. 83, 2163 (2003)]. The required level of tensile strain in the Ge layers is compatible with Si–Ge technology. The predicted direct bandgap values are as high as 0.6eV.
Comb-like supercontinuum generation in bulk media
Authors: Tortora Alessandra , Corsi Chiara , Bellini Marco
Published: August 2004

We report the generation of sequences of phase-coherent white-light pulses in bulk media. By passing the pulses from an amplified femtosecond laser through an etalon cavity, we produce an equally spaced time sequence of phase-locked pulses that serve as the pump for the generation of supercontinuum. The mutual coherence among the white-light pulses is probed by studying their spectral interference patterns for varying pump energy levels.
Ion-beam-induced sharpening of ZnO nanotips
Authors: Wu Pan , Saraf Gaurav , Lu Yicheng , Hill David H. , Bartynski Robert A. , Arena Dario A. , Ryu Mee-Yi , Raley Jeremy A. , Yeo Yung Kee
Published: August 2004

Fe ions of dose 5×1016cm−2 were implanted at 200keV into ZnO nanotips at room temperature. Scanning electron microscopy images show that the implanted ZnO tips are much sharper than the as-grown ones. The sharpening effect of the ZnO nanotips is attributed to the interplay between ion sputtering and ion-induced surface diffusion. X-ray diffraction indicates that ZnO crystallinity was preserved during implantation. Photoluminescence measurements show development of a broad blue band and severe UV quenching upon ion bombardment, and partial recovery after annealing. X-ray absorption spectroscopy is consistent with Fe ions that are substitutional for Zn in the ZnO wurtzite lattice.
Enhanced luminescence in top-gate-type organic light-emitting transistors
Authors: Park Byoungchoo , Takezoe Hideo
Published: August 2004

We have studied the structure and operating characteristics of top-gate-type organic light-emitting transistors (OLETs). The OLET is composed of an anode (source), organic layers including an organic electroluminescent (EL) layer, a cathode (drain) being opposed to the anode, and a gate, formed outside of the region between the anode and the cathode. EL characteristics of the OLET show that the luminescent intensity from the organic layers can be balanced by applying the bias voltage of the gate electrode.
Formation and optical properties of stacked GaN self-assembled quantum dots grown by metalorganic chemical vapor deposition
Authors: Hoshino K. , Kako S. , Arakawa Y.
Published: August 2004

Multiple-layer stacked GaN quantum dots (QDs) with intense photoluminescence (PL) have been successfully grown by the Stranski-Krastanow growth mode in metalorganic chemical vapor deposition. Transmission electron microscopy (TEM) and scanning TEM analyses showed vertically aligned QDs resulting from a strain field induced by the buried islands. A redshift and linewidth narrowing of the PL spectra for the multilayered structures indicate that the size of the QDs becomes larger and more uniform throughout the layering process. The PL intensity increased with increasing the number of stacked layers, indicating that the QDs can be stacked without introducing new nonradiative recombination centers.
The TG5 thyroglobulin gene test for a marbling quantitative trait loci evaluated in feedlot cattle
Authors: Barendse W. , Bunch R. , Thomas M. , Armitage S. , Baud S. , Donaldson N.
Published: August 2004

The TG5 (thyroglobulin 5′ leader sequence) single nucleotide polymorphism has been associated with marbling in cattle fed for periods longer than 250 days. To test whether the association could be detected in diverse cattle, fed for less than 250 days, and to measure the size of the effect, we sampled 1750 cattle from the AMH Toowoomba feedlot. These cattle were sampled on 28 separate days, over 9 months. Their marbling scores covered the complete range. We found that the TG5 single nucleotide polymorphism was associated with marbling scores (P<0.05) and estimated that TG5 genotypes explained 6.5% of the residual deviance for the marbling phenotype. We also found that the '3' allele was more frequent in animals with higher marbling scores. The consistency of the allelic association between studies and, in particular, the association found in diverse cattle, indicate that the TG5 polymorphism can be used as a breeding tool and possibly a feedlot entry tool. To estimate the size of the genetic region in which the marbling quantitative trait loci are located, we tested the nearby DNA markers CSSM66 and BMS1747. These do not show allelic associations to marbling. The consistency of the allelic association between studies, the lack of association to nearby DNA markers and the complementary information on gene action of genes near Thyroglobulin suggest that DNA sequence variations, in or near the Thyroglobulin gene sequence, are the likely causes for the marbling quantitative trait loci. Further studies of single nucleotide polymorphism in and near the Thyroglobulin DNA sequence should allow causal mutations for the effect to be identified.
X-ray structure analysis of a designed oligomeric miniprotein reveals a discrete quaternary architecture
Authors: Ali Mayssam H. , Peisach Ezra , Allen Karen N. , Imperiali Barbara
Published: August 2004

The x-ray crystal structure of an oligomeric miniprotein has been determined to a 1.2-Å resolution by means of multiwavelength anomalous diffraction phasing with selenomethionine analogs that retain the biophysical characteristics of the native peptide. Peptide 1, comprising α and β secondary structure elements with only 21 aa per monomer, associates as a discrete tetramer. The peptide adopts a previously uncharacterized quaternary structure in which α and β components interact to form a tightly packed and well defined hydrophobic core. The structure provides insight into the origins of the unusual thermal stability of the oligomer. The miniprotein shares many characteristics of larger proteins, including cooperative folding, lack of 1-anilino-8-naphthalene sulfonate binding, and limited deuterium exchange, and possesses a buried surface area typical of native proteins.
Nucleic acid transport through carbon nanotube membranes
Authors: Yeh In-Chul , Hummer Gerhard
Published: August 2004

We study the electrophoretic transport of single-stranded RNA molecules through 1.5-nm-wide pores of carbon nanotube membranes by molecular dynamics simulations. From ≈170 individual RNA translocation events analyzed at full atomic resolution of solvent, membrane, and RNA, we identify key factors in membrane transport of biopolymers. RNA entry into the nanotube pores is controlled by conformational dynamics, and exit by hydrophobic attachment of RNA bases to the pores. Without electric field, RNA remains hydrophobically trapped in the membrane despite large entropic and energetic penalties for confining charged polymers inside nonpolar pores. Differences in RNA conformational flexibility and hydrophobicity result in sequence-dependent rates of translocation, a prerequisite for nanoscale separation devices.
Prions can infect primary cultured neurons and astrocytes and promote neuronal cell death
Authors: Cronier Sabrina , Laude Hubert , Peyrin Jean-Michel
Published: August 2004

Transmissible spongiform encephalopathies arise as a consequence of infection of the central nervous system by prions, where neurons and glial cells are regarded as primary targets. Neuronal loss and gliosis, associated with the accumulation of misfolded prion protein (PrP), are hallmarks of prion diseases; yet the mechanisms underlying such disorders remain unclear. Here we introduced a cell system based on primary cerebellar cultures established from transgenic mice expressing ovine PrP and then exposed to sheep scrapie agent. Upon exposure to low doses of infectious agent, such cultures, unlike cultures originating from PrP null mice, were found to accumulate
de novo
abnormal PrP and infectivity, as assessed by mouse bioassay. Importantly, using astrocyte and neuron/astrocyte cocultures, both cell types were found capable of sustaining efficient prion propagation independently, leading to the production of proteinase K-resistant PrP of the same electrophoretic profile as in diseased brain. Moreover, contrasting with data obtained in chronically infected cell lines, late-occurring apoptosis was consistently demonstrated in the infected neuronal cultures. Our results provide evidence that primary cultured neural cells, including postmitotic neurons, are permissive to prion replication, thus establishing an approach to study the mechanisms involved in prion-triggered neurodegeneration at a cellular level.