bluuebunny/crossref_metadata_2025 huggingface dataset:
Time of flight photoelectron spectroscopy with a laser-plasma x-ray source
Authors: Kondo Hiroyuki , Tomie Toshihisa , Shimizu Hideaki
Published: July 2002

We are proposing a very compact photoelectron microspectroscopy system with a laser-plasma x-ray source, which should give reasonably short spectrum acquisition time. In an experiment using a Q-switched YAG laser of 0.4 J pulse energy for the production of a laser plasma, it is confirmed that as large number of photoelectrons as expected theoretically is detected, and that the spectral resolution with time of flight method is improved by applying a retardation field. The detected number of Si 2p photoelectrons was 85/pulse in 15 eV bandwidth, and the observed photoelectron spectral resolution was 6 eV close to the bandwidth of the employed Cr/C x-ray mirror.
Effects of hydrogen surface pretreatment of silicon dioxide on the nucleation and surface roughness of polycrystalline silicon films prepared by rapid thermal chemical vapor deposition
Authors: Hu Y. Z. , Zhao C. Y. , Basa C. , Gao W. X. , Irene E. A.
Published: July 2002

It is well known that Si surface treatment is crucial for low-temperature Si epitaxy. Although considerable work exists which is aimed at elucidating the effects of Si surface pretreatments on Si epitaxy, little is known about the effects of SiO2 surface pretreatments for polycrystalline silicon (poly-Si) growth. We report on a study of SiO2 surface pretreatment effects on poly-Si nucleation and film surface roughness using a low energy hydrogen ion beam (200 eV) and H2 gas annealing (850 °C) in a rapid thermal chemical vapor deposition system. In situ real-time ellipsometry was used to monitor the surfaces during pretreatment and observe the nucleation. The microstructure and surface roughness of the deposited poly-Si films are determined by analysis of in situ spectroscopic ellipsometry (SE) and atomic force microscopy (AFM) measurements. Hydrogen ion beam pretreatment was found to produce higher nuclei density and a smoother poly-Si surface than nonpretreated substrates, and the opposite was found for hydrogen gas annealing giving lower nuclei density and rougher poly-Si.
Intense violet-blue photoluminescence in as-deposited amorphous Si:H:O films
Authors: Tong Song , Liu Xiang-na , Gao Ting , Bao Xi-mao
Published: July 2002

Three photoluminescence (PL) bands at 340–370, 400–430, and 740 nm were observed at room temperature in a-Si:H:O films fabricated by plasma enhanced chemical vapor deposition without any postprocessing. The violet-blue emission is very strong and stable, and its intensity is closely related to the oxygen content in the films, which can be controlled by the applied dc biases on the sample substrates during deposition. The first two PL peaks are ascribed to Si–O related species, and the last one to the quantum size effect of the nanocrystallites embedded in the a-Si:H:O matrix.
Collapse of irreversible field of superconducting Bi2Sr2Ca2Cu3O10+δ/Ag tapes with columnar defects
Authors: Li Qiang , Suenaga M. , Kaneko T. , Sato K. , Simmon Ch.
Published: July 2002

We found that the irreversible field of the c-axis oriented superconducting Bi2Sr2Ca2Cu3O10+δ (Bi-2223) tapes irradiated with 5.8 GeV Pb ions collapsed at temperatures approximately 75% of Tc regardless of fluence. This result may indicate an ultimate limit for the field application of Bi-2223/Ag tapes at liquid nitrogen temperature (77 K), since the columnar defects produced by heavy ions are the most efficient pinning centers known so far for the flux lines in a layered superconducting system.
Single crystal Fe films grown on InAs(100) by molecular beam epitaxy
Authors: Xu Y. B. , Kernohan E. T. M. , Tselepi M. , Bland J. A. C. , Holmes S.
Published: July 2002

Thin Fe films have been grown on InAs(100) by molecular beam epitaxy, and studied using in situ magneto-optical Kerr effect (MOKE), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Despite the large lattice mismatch between Fe and InAs, the growth of Fe on InAs at 175 °C was found to be epitaxial with the orientation relationship Fe(100)〈001〉‖InAs(100)〈001〉, as evidenced by LEED. STM images indicate that growth proceeds via a 3D Volmer–Weber mode. The magnetic hysteresis loops measured using in situ MOKE show a distinct cubic anisotropy with the easy axis along 〈001〉, the easy axis of bulk bcc Fe, which further confirms that well ordered single crystal Fe films have been stabilized on the InAs(100) substrate. Current–voltage measurements in the temperature range of 2.5–304 K show that Fe forms an ohmic contact on InAs. We propose that Fe/InAs is a suitable heterostructure for magnetoelectronic devices as, unlike Fe/GaAs, there is no Schottky barrier to electron transport.
Coulomb blockade in Sb nanocrystals formed in thin, thermally grown SiO2 layers by low-energy ion implantation
Authors: Nakajima Anri , Nakao Hiroshi , Ueno Hiroaki , Futatsugi Toshiro , Yokoyama Naoki
Published: July 2002

Sb nanocrystals were formed in thin, thermally grown SiO2 layers using low-energy ion implantation followed by thermal annealing. These Sb nanocrystals have good size and position uniformity. Both the narrow as-implanted profile and the compressive strain that exists near the SiO2/Si interface supposedly contribute to the uniformity. The I–V characteristics of the diode structure show a Coulomb blockade region around 0 V and a Coulomb staircase at 4.2 K. The Coulomb blockade region was observed up to a temperature of 100 K. The technique offers the possibility of developing practical Si-based single-electron devices.
Intense visible light emission from Sr3Al2O6:Eu,Dy
Authors: Akiyama Morito , Xu Chao-nan , Nonaka Kazuhiro , Watanabe Tadahiko
Published: July 2002

The triboluminescence intensity from stress-activated Sr3Al2O6:Eu,Dy (SAO-ED) was so strong that we could see it with the naked eye in the atmosphere. The luminescence integrated intensity was about five hundred times as high as that of crystal sugar. We think that the light emission is due to the movement of dislocations and the 4f7–4f65d transition in the doped Eu2+ ions from the analysis of the emitted light. Furthermore, we have discovered the new phenomenon that the luminescence intensity of the SAO-ED is recovered by the irradiation of ultraviolet light.
Charge trap dynamics in a SiO2 layer on Si by scanning capacitance microscopy
Authors: Kang C. J. , Buh G. H. , Lee S. , Kim C. K. , Mang K. M. , Im C. , Kuk Y.
Published: July 2002

Trapped electrons and holes, and their dynamics, were visualized from spatially resolved capacitance–voltage (C–V) curves and dC/dV images using scanning capacitance microscopy. A trapped charge of 10−16–10−18 C, localized within 2 μm diam circular test structures, was imaged. The detrapping process of the trapped electrons can be explained with a quantum-mechanical tunneling model.
Bulk ferromagnetic glasses prepared by flux melting and water quenching
Authors: Shen T. D. , Schwarz R. B.
Published: July 2002

Several ferromagnetic bulk amorphous alloys of the type Fe–(Co, Cr, Mo, Ga, Sb)–P–B–C, containing between 62 and 71 at. % Fe, have been prepared in the form of 4-mm-diam rods. The glass synthesis consists of mechanically alloying the constituents, purifying the melts in B2O3 flux inside fused silica tubes, and quenching the melts at cooling rates on the order of 100 K/s. All these glasses have a large supercooled region Tx−Tg, ranging from 35 to 61 K, within which the glass can be shaped under a relatively small applied load. It is shown that the value of Tx−Tg depends strongly on the metalloid composition. These bulk metallic glasses have very low coercivity and low hysteresis losses.
Highly confined waveguides and waveguide bends in three-dimensional photonic crystal
Authors: Chutinan Alongkarn , Noda Susumu
Published: July 2002

We demonstrate a method to create a highly confined waveguide with sharp bend in three-dimensional photonic crystal. By theoretical calculation, we show that a single-mode waveguide can be created in the stacked-bar structure by removing one stripe. Crossing waveguides of the adjacent layers can create the waveguide bend and by this way, the guided modes after and before the bend remain the same so that the transmission through the bend is optimized. These results provide very important guidelines for designing a three-dimensional photonic crystal waveguide.
Mechanical property measurement of thin polymeric-low dielectric-constant films using bulge testing method
Authors: Zheng D. W. , Xu Y. H. , Tsai Y. P. , Tu K. N. , Patterson P. , Zhao Bin , Liu Q.-Z. , Brongo Maureen
Published: July 2002

We report here the bulge testing results of both dense and porous poly-arylethers (PAE) polymeric low dielectric constant thin films. These films were prepared on a 4.7-μm-thick Si membrane window of 1600×1600 μm2. The Si membrane was subsequently removed in a pulsed XeF2 etcher and the remaining thin film became the bulge test sample. The biaxial moduli of both the dense and 40% porosity low-k PAE thin films were measured to be 6.40±0.35 and 3.53±0.70 GPa, respectively. These biaxial moduli, when combined with the thermal stress data of these films, enabled us to estimate the thermal expansion coefficients in the range of 26.8–32.6 and 56.1–72.5 ppm/°C for the dense and porous PAE films, respectively.
Optical properties of PbZrxTi1−xO3 on platinized silicon by infrared spectroscopic ellipsometry
Authors: Huang Zhiming , Meng Xiangjiang , Yang Pingxiong , Zhang Zhanhong , Chu Junhao
Published: July 2002

A method of analyzing infrared spectroscopic ellipsometry (IRSE) measurement data is proposed for lead zirconate titanate PbZrxTi1−xO3 (PZT) thin films grown on Pt/Ti/SiO2/Si substrates with x=0.3 and 0.5. The IRSE data measured at an angle of incidence 75° for x=0.3 and 70° for x=0.5 are fitted by a dielectric function formula. The refractive index and extinction coefficient of PZT with x=0.3 and 0.5 are determined in the spectral range of 2.5–12.5 μm. As the wavelength increases, the refractive index decreases, on the contrary, the extinction coefficient increases. The absorption coefficient for x=0.5 is greater than that for x=0.3 by a factor of 1.5. The effective static ionic charges have also been derived, which are smaller than they would be in a purely ionic material for PZT thin films.
Room-temperature luminescence of excitons in ZnO/(Mg, Zn)O multiple quantum wells on lattice-matched substrates
Authors: Makino T. , Chia C. H. , Tuan Nguen T. , Sun H. D. , Segawa Y. , Kawasaki M. , Ohtomo A. , Tamura K. , Koinuma H.
Published: July 2002

We report on the optical properties of ZnO/(Mg, Zn)O multiple quantum wells (MQWs) on lattice-matched ScAlMgO4 substrates fabricated by laser molecular-beam epitaxy. As the well layer thickness decreased down to 7 Å, the photoluminescence (PL) and absorption peaks showed a systematic blueshift, consistent with the quantum-size effect. Moreover, a bright PL of free excitons could be observed even at room temperature. As a result, the PL could be tuned in the energy range of 3.3–3.6 eV by choosing the appropriate barrier height and well layer thickness. The widest tunability on the room-temperature luminescence of the excitons could be attained on the basis of the ZnO quantum structure. These favorable properties could not be attained in the MQWs on lattice-mismatched sapphire substrates.
Modeling the free energy surfaces of electron transfer in condensed phases
Authors: Matyushov Dmitry V. , Voth Gregory A.
Published: July 2002

We develop a three-parameter model of electron transfer (ET) in condensed phases based on the Hamiltonian of a two-state solute linearly coupled to a harmonic, classical solvent mode with different force constants in the initial and final states (a classical limit of the quantum Kubo–Toyozawa model). The exact analytical solution for the ET free energy surfaces demonstrates the following features: (i) the range of ET reaction coordinates is limited by a one-sided fluctuation band, (ii) the ET free energies are infinite outside the band, and (iii) the free energy surfaces are parabolic close to their minima and linear far from the minima positions. The model provides an analytical framework to map physical phenomena conflicting with the Marcus–Hush two-parameter model of ET. Nonlinear solvation, ET in polarizable charge-transfer complexes, and configurational flexibility of donor-acceptor complexes are successfully mapped onto the model. The present theory leads to a significant modification of the energy gap law for ET reactions.
Time-dependent density-functional determination of arbitrary singlet and triplet excited-state potential energy surfaces: Application to the water molecule
Authors: Cai Zheng-Li , Tozer David J. , Reimers Jeffrey R.
Published: July 2002

Over the past few years a large number of density-functional schemes have been developed for molecular excited states, many of which have been shown to produce poor results for water. We apply the time-dependent density-functional method using hybrid and asymptotically corrected functionals to evaluate the vertical excitation energies, C2v-relaxation energies and vibration frequencies, and dissociation pathways for up to eight singlet and six triplet excited states of water. The results are compared to experimental data as well as ab initio calculated data obtained using direct and equations-of-motion coupled-cluster techniques, as well as multireference configuration-interaction techniques. For most properties, the asymptotically corrected density-functional method produces results of comparable quality to those produced by the ab initio methods. However, the time-dependent methods produce very poor results for systems involving molecular dissociation. In fact, only the multireference approaches produce good descriptions of molecular dissociation, but in the regions of the (C2v) minima of the potential energy surfaces single-reference techniques are found to be both more accurate and much more robust.
New scheme for solving the amplitude equations in the state-specific coupled cluster theory with complete active space reference for ground and excited states
Authors: Ivanov Vladimir V. , Adamowicz Ludwik
Published: July 2002

A new scheme for solving the coupled cluster (CC) amplitude equations for ground and excited electronic states in the state-selective multireference (MR) CC method based on the complete active space (CAS) reference wave function (CASCC) is proposed. The CASCC wave function is generated using a single formal reference determinant, which is one of the CAS determinants, as the origin of the configuration expansion. Some single and double excitations from other CAS determinants may be triple, quadruple, and higher excitations from the reference determinant. We show that one may include the contribution from these higher excitations indirectly by modifying the coupled cluster amplitude equations corresponding to the single and double excitations. The modification involves including projections against the higher excitations in the equations for the singles and doubles. Test calculations for the ground and the first excited state of the H8-model system and for the singlet–triplet splitting of the CH2-biradical shows that the results improve with the use of the new scheme and become to the FCI benchmarks.
Role of dye structure in photoinduced reorientation of dye-doped liquid crystals
Authors: Marrucci L. , Paparo D. , Vetrano M. R. , Colicchio M. , Santamato E. , Viscardi G.
Published: July 2002

The giant optical nonlinearity associated with photoinduced molecular reorientation of dye-doped nematic liquid crystals has been measured for a homologous set of dyes belonging to the anthraquinone family, dissolved both in a polar and in a nonpolar liquid crystal host. We found a strong sensitivity of the nonlinearity merit figure to the detailed structure of the dye substituent groups. Our results provide some insight into the molecular mechanisms underlying this phenomenon, whose full understanding will finally require a detailed picture of guest–host intermolecular interactions and their dependence on the molecule electronic state.
Direct observation of a local structural transition for molecular recording with scanning tunneling microscopy
Authors: Shi D. X. , Song Y. L. , Zhang H. X. , Jiang P. , He S. T. , Xie S. S. , Pang S. J. , Gao H.-J.
Published: July 2002

We present a direct observation of a structural transition at molecular scale in an organic p-nitrobenzonitrile (PNBN) thin film using scanning tunneling microscopy (STM). STM images clearly show an ordered molecular structure of unrecorded regions in the films, while the PNBN molecular arrangements are disordered in the recorded regions. The current–voltage (I–V) measurements from the STM demonstrate a great increase of the conductance transition in the recorded regions after the voltage pulses. Those results suggest that the recording mechanism may be due to a crystalline structural transition, which is consistent with our previously proposed mechanism in another organic system for ultrahigh density data storage.
Experimental demonstration of a homogeneous two-scale dynamo
Authors: Stieglitz R. , Müller U.
Published: July 2002

It has been shown theoretically that homogeneous kinematic dynamo action is possible for many unconfined and confined velocity fields, but a rigorous experimental validation is still lacking. G. O. Roberts [Philos. Trans. R. Soc. London, Ser. A 266, 535 (1970)] proposed a spatially periodic velocity field capable to generate a dynamo, which Busse [Geophys. J. R. Astron. Soc. 42, 437 (1975)] modified by introducing a second length scale larger in order to obtain a solution for a finite domain. Based on a scale separation approach Busse [Springer Proceedings in Physics, Vol. 69 (Springer Verlag, New York, 1992)] proposed a conceptual design for an experimental homogeneous dynamo. An engineering design was developed and a test facility has been set up. This test facility is described and first experimental results confirming dynamo action are presented.
Optical pumping studies of vibrational energy transfer in high-pressure diatomic gases
Authors: Lee Wonchul , Adamovich Igor V. , Lempert Walter R.
Published: July 2002

Spontaneous Raman scattering is used to experimentally determine the vibrational distribution functions of diatomic species in N2/CO and N2/CO/O2 gas mixtures optically pumped by a CO laser in the pressure range 410–760 torr. In N2/CO mixtures, as many as 38 vibrational levels of CO are observed, in addition to six levels of N2. The CO vibrational distribution function is highly non-Boltzmann, exhibiting the well-known Treanor plateau. In N2/CO/O2 mixtures, up to 13 vibrational levels of O2 are observed, which also exhibit a highly non-Boltzmann distribution. Experimental data are compared to predictions of a master equation kinetic model, which incorporates absorption of the laser radiation, species, and quantum state-specific vibration–vibration and vibration–translation energy exchange, as well as diffusion of vibrationally excited species out of the laser-excited volume. It is shown for the first time that modest power continuous wave lasers can be used to establish highly excited steady-state vibrational distributions of all three major diatomic species in CO-seeded atmospheric pressure dry air. This has implications for the energy-efficient creation of low-temperature, high-pressure air plasmas, in which the principal free electron loss mechanism is known to be three-body attachment to molecular oxygen.
Thermal dynamics of VO2 films within the metal–insulator transition: Evidence for chaos near percolation threshold
Authors: de Almeida L. A. L. , Deep G. S. , Lima A. M. N. , Neff H.
Published: July 2002

The thermal dynamics of thin vanadium dioxide films at the martensitic metal–insulator phase transition has been evaluated experimentally by thermal excitation spectroscopy. Over the transition region, the device becomes highly nonlinear, and its bolometric performance is affected. At low thermal cycling rates for a temperature around the percolation threshold, the device stochastically switches into an unusual pattern. The originally smooth and monotonic shape of the R(T) curve for minor loops suddenly becomes unstable and unpredictable. By direct observation of at least two strange attractors, the phenomenon clearly has been identified as chaotic. Bolometric performance of VO2 based devices in the transition region may suffer strong degradation for low thermal cycling rates. In this region, sensor responsivity for periodic thermal excitation is significantly reduced. Resistance noise is 1/f-type and self-generated oscillations were observed at frequency <10−2 Hz.
Dipole moments of adiabatic excited states using the Fock space multireference coupled-cluster analytic response approach
Authors: Ajitha D. , Pal Sourav
Published: July 2002

Theoretical evaluation of molecular properties of excited states is extremely necessary as the lifetime of the excited states is too low for experimental probing. High level of theoretical treatment is an attractive option for the study of such states. However, theoretical calculation of this is a challenging task and in this paper we have presented such calculation using the analytic Fock space multireference coupled cluster linear response approach in a one hole–one particle model space. The results for the dipole moments of the adiabatic excited states of the water molecule are presented in this paper. The Sadlej basis set optimized for the property evaluation is used for the calculation and we address the adiabatic singlet B1 and triplet B1 excited states. The results using both the finite-field and the analytic response approach are presented. Full effective Hamiltonian including the three body terms in a singles and doubles approximation has been used for the calculation.
The effect of the first GaN monolayer on the nitridation damage of molecular beam epitaxy grown GaN on GaAs(001)
Authors: Zsebök O. , Thordson J. V. , Gunnarsson J. R. , Zhao Q. X. , Ilver L. , Andersson T. G.
Published: July 2002

The initial molecular beam epitaxy growth of GaN on GaAs(001) was studied by real-time monitoring of the (3×3) surface reconstruction and its transition to an unreconstructed (1×1). Various growth conditions were established by variation of the V/III ratio, i.e., the Ga flux. We characterized the effect of the first two strained GaN monolayers: a N-terminated GaN (3×3) monolayer and a second unreconstructed (1×1) monolayer. A series of samples were grown under N-rich, Ga-rich, and near-stoichiometric growth conditions. The resulting morphology of the interface region was analyzed by high-resolution scanning electron microscopy, Auger-electron spectroscopy, and double crystal x-ray diffractometry. The N-rich and Ga-rich conditions resulted in extensive defect formation due to the nitridation damage of the GaAs substrate. The extent of this was found to be determined by the properties of the first GaN monolayer. The surface roughness under optimum growth conditions could be as low as ∼20 nm, defined by nanocrystalline grains, showing no observable nitridation damage.
Rigorous optical modeling of multilayer organic light-emitting diode devices
Authors: Kahen K. B.
Published: July 2002

We present an exact classical solution to the problem of dipole emission in a planar multilayer light-emitting device. The inputs to the model are the photoluminescence and quantum yield of the emitter material, and the device layer thicknesses and indices of refraction. The results of the model are applied to predicting the radiant intensity of organic light-emitting diodes as a function of varying device layer thickness. It is shown that the predicted radiances are in excellent agreement with the data. We also present results for the Poynting power distribution from a randomly aligned dipole for positions both internal and external to the diodes.
Transconductance improvement in surface-channel SiGe p-metal-oxide-silicon field-effect transistors using a ZrO2 gate dielectric
Authors: Ngai T. , Qi W. J. , Sharma R. , Fretwell J. L. , Chen X. , Lee J. C. , Banerjee S. K.
Published: July 2002

Silicon and surface-channel SiGe p-metal-oxide-silicon field-effect transistors (p-MOSFETs) using a ZrO2 gate dielectric with equivalent oxide thickness (EOT) less than 20 Å was fabricated. These p-MOSFETs show similar behavior to that of other high-k gate dielectric p-MOSFETs reported in the literature, and mobility enhancement is observed in the surface-channel SiGe p-MOSFETs.
Low temperature synthesis and electrical properties of epitaxial Sr0.8Bi2.2Ta2O9 thin films
Authors: Chattopadhyay Soma , Kvit A. , Kumar D. , Sharma A. K. , Sankar J. , Narayan J. , Knight V. S. , Coleman T. S. , Lee C. B.
Published: July 2002

High quality epitaxial thin films of Sr0.8Bi2.2Ta2O9 (SBT) were grown on LaNiO3 (LNO) bottom electrodes. The SBT/LNO heterostructure was fabricated on (001) oriented LaAlO3 substrates by pulsed laser ablation. X-ray diffraction and high resolution transmission electron microscopy revealed epitaxial growth of SBT and LNO layers along the (001) direction and sharp interfaces between the epilayers. The SBT films exhibited a dielectric constant of ∼270 and the loss tangent varied from 0.02 to 0.04. The dielectric constant measured as a function of bias field revealed that the films were not ferroelectric in nature. The room temperature frequency response of the dielectric constant was observed to obey Curie–von Schweidler power law with an exponent of 0.02 in the range of 10 kHz–1 MHz.
Dynamical equilibrium of avalanches on a rough plane
Authors: Daerr Adrian
Published: July 2002

We present experimental results on avalanches which are triggered in a metastable static layer on a rough inclined plane. We observe that despite the continuous increase in mass of a perturbation while it runs down, the flow reaches a dynamical equilibrium where the fronts evolve at constant speed and the mass is spread. The head front is found to be a shock wave with a crest, and its transient acceleration phase is studied. In contrast, the rear front grows in a self-similar manner from the beginning. An instability is observed which breaks thin flowing layers down into solitary waves.
Slow capacitance relaxation in (BaSr)TiO3 thin films due to the oxygen vacancy redistribution
Authors: Boikov Yu. A. , Goltsman B. M. , Yarmarkin V. K. , Lemanov V. V.
Published: July 2002

Capacitance relaxation in Ba0.8Sr0.2TiO3 thin film structures has been investigated. Slow decrease of the capacitance after a bias voltage switching on is explained by suppressing the film dielectric permittivity by the field of a p-n junction originated as a result of the oxygen vacancy migration in the bias field. After the bias switching off, the p-n junction gradually disappears due to the vacancy diffusion, and the capacitance increases. The relation determining capacitance increase after the bias switching off has been obtained in agreement with experimental data. The proposed relaxation mechanism is considered as a certain type of the size effect when the p-n junction depleted region spreads over the film thickness.
Quantum cascade lasers with a heterogeneous cascade: Two-wavelength operation
Authors: Gmachl Claire , Sivco Deborah L. , Baillargeon James N. , Hutchinson Albert L. , Capasso Federico , Cho Alfred Y.
Published: July 2002

A quantum cascade (QC) laser with a heterogeneous cascade containing two substacks previously optimized to emit at 5.2 μm and 8.0 μm wavelengths, respectively, is presented. The low-temperature performance of the two-wavelength laser is comparable to the respective homogeneous stack lasers, indicating no penalty from the heterogeneity of the cascade. Each substack is apportioned the optimum fraction of the applied bias. This demonstrates the general applicability of this scheme. In addition, an etch-stop layer inserted between the two substacks allowed fabrication of a “tap” into the cascade. The latter was used to selectively manipulate the laser threshold of one substack, turning the 8.0 μm laser on and off while the adjacent 5.2 μm QC laser was operating undisturbed.
Self-assembly of ink molecules in dip-pen nanolithography: A diffusion model
Authors: Jang Joonkyung , Hong Seunghun , Schatz George C. , Ratner Mark A.
Published: July 2002

The self-assembly of ink molecules deposited using dip-pen nanolithography (DPN) is modeled as a two-dimensional diffusion with a source (tip). A random walk simulation and simple analytic theory are used to study how the diffusion dynamics affects patterns generated in DPN. For a tip generating a constant flux of ink molecules, circles, lines, and letters are studied by varying the deposition rate of ink molecules and the tip scan speed. Even under the most favorable condition studied here, peripheries of patterns fluctuate from perfect circles or lines, due to the random, diffusional nature of self-assembly. The degree of fluctuation is quantified for circles and lines. Circles generated by fixing the tip position do not depend on the deposition rate if the same amount of ink is deposited. For a moving tip, patterns change drastically depending on tip speed and deposition rate. Overall, fast scan or slow deposition relative to the diffusion time scale makes lines narrower. When the tip deposits ink too slowly or scans too fast, patterns become incoherent, making molecules in patterns separated from each other. Therefore, there seems to be an optimal choice of the deposition rate and tip speed that gives both narrow and coherent patterns. We also explore the consequences of varying the relative rates of diffusion of ink molecules on bare surface and on previously deposited molecules.
Frequency domain instrument for measuring phosphorescence lifetime distributions in heterogeneous samples
Authors: Vinogradov Sergei A. , Fernandez-Searra Maria A. , Dugan Benjamin W. , Wilson David F.
Published: July 2002

The luminescence lifetime distribution can be used to determine the distribution of quencher concentrations in a heterogeneous sample. We describe a frequency domain instrument for real-time measurements of phosphorescence lifetime distributions in microheterogeneous objects. In this system (1) an array of harmonics (typically 100–200 frequencies) is used to modulate the excitation source, a light emitting diode. Due to the relatively long triplet state lifetimes, the frequencies required for the modulation are typically below 40 000 kHz, which allows direct digitization of both excitation and emission signals. (2) The dependence of the phase/amplitude factor on the modulation frequency is determined by linear least-squares analysis of the emission signal, which is sampled and summed over the multiple excitation cycles. (3) The phase/amplitude relationship obtained is analyzed in real time using a “light” version of the maximum entropy algorithm, which provides a complete phosphorescence lifetime distribution. (4) The lifetime distribution is converted into the distribution of quencher concentrations using an appropriate model of quenching. The instrument is also capable of measuring phosphorescence in “single-frequency” mode, which is useful for rapid evaluation of apparent luminescence lifetimes. In this mode, a correction for an in-phase signal, which is due to backscattering and fluorescence, is applied to improve the accuracy of lifetime measurements. The instruments were tested in Stern–Volmer calibrations of Pd-porphyrin based phosphors for oxygen measurements and used for preliminary evaluation of oxygen distributions in rat tumor tissues. The instruments were found to be capable of accurate determination of lifetimes in the range of 10–3000 μs. The average duration of a single lifetime distribution measurement was about 15 s, depending on the sample and on the density of the lifetime grid in the maximum entropy method analysis. In the single-frequency mode, the measurement time was reduced to about 0.2–0.5 s. The instruments provide complete correction for the in-phase signal of up to 40% of the total emission intensity.
Ferroelectric characteristics of oriented Pb(Zr1−xTix)O3 films
Authors: Chen San-Yuan , Sun Chia-Liang
Published: July 2002

Pb 1.2 Zr 1−x Ti x ) O 3 (PZT) films with a variety of compositions were prepared by spin coating on Pt/Ti/SiO2/Si substrate with sol–gel processing. The roles of composition (phase) and orientation in ferroelectric properties of PZT films have been determined. The Zr-rich PZT films with (111)-oriented PZT films have a higher remanent polarization but also show a higher fatigue rate as compared to (100)-oriented films in both Zr-rich(65/35) and Ti-rich(35/65) PZT compositions. The lower fatigue rate of (100)-oriented film can be attributed to its easier reversible domain-wall motions compared to (111)-oriented PZT films due to the absence of internal field stress and less dependence on electrical field. A mode based on domain-wall contribution instead of film–electrode interface is favored to elucidate the role of orientation in fatigue characteristics of PZT films.
Wide-range-tunable laterally coupled distributed feedback lasers based on InGaAsP–InP
Authors: Müller M. , Kamp M. , Forchel A. , Gentner J.-L.
Published: July 2002

We have investigated tunable distributed feedback (DFB) lasers based on InGaAsP quantum wells grown by molecular-beam epitaxy. Two-section tunable DFB lasers were fabricated by patterning laterally gain coupling binary superimposed gratings perpendicular to the ridge waveguide. Side mode suppression ratios of up to 42 dB have been achieved. The tuning range covers 25 nm.
Scaling of local density correlations in a fluid close to freezing
Authors: Saija F. , Prestipino S. , Giaquinta P. V.
Published: July 2002

We simulated the equilibrium properties of some reference model fluids, with hard-core, Yukawa and Lennard-Jones interactions, and compared their local density profiles in thermodynamic states where the residual multiparticle entropy (RMPE), an established and rather sensitive indicator of the incipient ordering of the fluid into a more structured phase, happens to vanish. We found that, once interparticle distances have been referred to the average separation between nearest-neighbor particles, the radial distribution functions (RDF) coalesce—from the second coordination shell onwards—onto nearly the same spatial profile. This property was ascertained for different model systems in different thermodynamic conditions but for the shared zero-RMPE constraint. The emergence of a scaling relation for the RDF’s in the fluid phase further enlightens the nature of the structural condition that is singled out by the vanishing of the RMPE.
Comparison of thin film and bulk forms of the transparent conducting oxide solution Cd1+xIn2−2xSnxO4
Authors: Kammler D. R. , Mason T. O. , Young D. L. , Coutts T. J. , Ko D. , Poeppelmeier K. R. , Williamson D. L.
Published: July 2002

Physical and structural properties of thin films prepared via rf magnetron sputtering of the transparent conducting oxide spinel Cd1+xIn2−2xSnxO4 are compared to those reported for bulk specimens (prepared via high-temperature solid state reaction at 1175 °C). Optical band gaps measured on thin films of Cd1+xIn2−2xSnxO4 were 3.5, 3.70, and 3.65 eV for x=0.15, 0.45, and 0.70, which where 0.57, 0.94, and 0.95 eV higher than their bulk counterparts. Thin film Seebeck coefficients were −18.0, −15.5, and −15.5 μV/K for x=0.15, 0.45, and 0.70, respectively, which were 27, 24, and 19 μV/K smaller in magnitude than their bulk counterparts. Sn-Mössbauer spectroscopy revealed isomer shifts that averaged 0.2 mm/s for both bulk and thin films specimens. The presence of quadrupole splitting, which averaged near 0.48 mm/s for film specimens and 0.39 mm/s for bulk specimens, suggests that Sn+4 in all specimens is in octahedral coordination. The difference in quadrupole splitting suggests that thin films have a different cation distribution than their bulk counterparts. The effective mass at the base of the conduction band, measured via the method-of-four-coefficients, was found to be 0.25, 0.18, 0.21, and 0.22 me for x equal to 0.15, 0.45, 0.70, and 1.0, respectively. A model that explains the changes in optical gap and thermopower as a result of differences in the fundamental band gap (resulting from a changing cation distribution), conduction band curvature, and carrier density is presented.
Anisotropic valence→core x-ray fluorescence from a [Rh(en)3][Mn(N)(CN)5]⋅H2O single crystal: Experimental results and density functional calculations
Authors: Bergmann U. , Bendix J. , Glatzel P. , Gray H. B. , Cramer S. P.
Published: July 2002

High resolution x-ray fluorescence spectra have been recorded for emission in different directions from a single crystal of the compound [Rh(en)3][Mn(N)(CN)5]⋅H2O. The spectra are interpreted by comparison with density functional theory (DFT) electronic structure calculations. The Kβ″ line, which is strongly polarized along the Mn–N axis, can be viewed as an N(2s)→Mn(1s) transition, and the angular dependence is understood within the dipole approximation. The so-called Kβ2,5 region has numerous contributions but is dominated by Mn(4p) and C(2s)→Mn(1s) transitions. Transition energy splittings are found in agreement with those of calculated occupied molecular orbitals to within 1 eV. Computed relative transition probabilities reproduce experimentally observed trends.
Microcantilever charged-particle flux detector
Authors: Stephan A. C. , Gaulden T. , Brown A.-D. , Smith M. , Miller L. F. , Thundat T.
Published: July 2002

A micromechanical sensor for charged-particle flux detection utilizing the extreme force sensitivity of microcantilevers is demonstrated with alpha particles. The latter create a charge buildup on an electrically isolated collection plate, a metallic sphere. The cantilever detector measures the electric field emanating from the charge collection sphere using variations in mechanical parameters. Results are compared with responses due to fixed voltages on the sphere. Parameters investigated include cantilever deflection due to total charge and frequency and damping rate variation due to electrostatic force gradient. The minimum detectable particle fluence using this technique was calculated to be around 1000 particles in air, or about 3.2×10−4 pC.
Enhanced performance of organic light-emitting devices by atmospheric plasma treatment of indium tin oxide surfaces
Authors: Chan I-Min , Cheng Weng-Cheng , Hong Franklin C.
Published: July 2002

Atmospheric plasma treatment of indium tin oxide (ITO) surfaces has been studied and demonstrated to be the most efficient method in improving the performance of vacuum-deposited double-layer organic light-emitting diode devices, among various plasma treatment methods including low-pressure Ar plasma and low-pressure O2 plasma treatment. Although with a current–voltage characteristic close to low-pressure O2 plasma treatment, the atmospheric plasma treatment exhibits a 40% increase of electroluminescence efficiency. X-ray photoelectron spectroscopy results show that the atmospheric plasma treatment increases the work function and reduces the carbon contamination of ITO surfaces. Our results suggest that atmospheric plasma treatment is a cheaper, more convenient, and more efficient method than low-pressure O2 plasma treatment for improving device performance.
Experimental and theoretical investigation of microsolvation of Na+-ions in the gas phase by high resolution mass spectrometry and global cluster geometry optimization
Authors: Hartke B. , Charvat A. , Reich M. , Abel B.
Published: July 2002

The hydration of gas phase Na+ ions produced by electrospray ionization was investigated via high resolution time-of-flight mass spectrometry. Water clusters with up to 80 H2O units have been observed with attention to special peculiarities of mass peaks and structures in the envelope of the mass spectra. For solvated Na+-ions no particularly prominent peaks corresponding to “magic numbers” have been observed in the spectra. In addition, the mass spectra of Na+(H2O)n aggregates are compared with those of the H3O+(H2O)n system. As a very first step towards a theoretical understanding of the intensities of the experimental mass spectra we have calculated global minimum structures for the Na+(H2O)n system with n=4–25, using global geometry optimization methods and a simple model potential for this system. Structural and energetic trends and the systematic build-up of solvent shells of the clusters were studied. Within our simple theoretical model we have not observed (symmetric) clathratelike structures with a central Na+-ion inside as global minimum structures. Instead, a structural transition for n between 17 and 18 water units was found, in agreement with the experimental observations. The unimportance of clathratelike structures and the competition between the two structural principles discovered in this work are proposed as an explanation for the notorious lack of “magic” numbers in the mass spectrum of Na+(H2O)n clusters.
Advanced laminated antiferromagnetically coupled recording media with high thermal stability
Authors: Pang S. I. , Piramanayagam S. N. , Wang J. P.
Published: July 2002

A layered structure to improve the antiferromagnetic exchange coupling constant (J) in laminated antiferromagnetically coupled (LAC) media is proposed. This structure has a much larger thermal stability in comparison to simple LAC structures. It also improves the coercivity and decreases the remnant moment-thickness product to achieve a high resolution and a low pulse width. The recording measurements indicate that this structure has a slightly larger noise than the simple LAC structures. However, its overall signal-to-noise ratio (SNR) is comparable to that of the simple LAC structures. High SNR and much higher thermal stability make this structure suitable for ultrahigh areal densities.
Pinchoff and satellite formation in surfactant covered viscous threads
Authors: Craster R. V. , Matar O. K. , Papageorgiou D. T.
Published: July 2002

The breakup of viscous liquid threads covered with insoluble surfactant is investigated here; partial differential equations governing the spatio-temporal evolution of the interface and surfactant concentrations are derived in the long wavelength approximation. These one-dimensional equations are solved numerically for various values of initial surfactant concentration, surfactant activity and the Schmidt number (a measure of the importance of momentum, i.e., kinematic viscosity, to surfactant diffusion). The presence of surfactant at the air–liquid interface gives rise to surface tension gradients and, in turn, to Marangoni stresses, that drastically affect the transient dynamics leading to jet breakup and satellite formation. Specifically, the size of the satellite formed during breakup decreases with increasing initial surfactant concentration and surfactant activity. The usual self-similar breakup dynamics found in the vicinity of the pinchoff location for jets without surfactant [Eggers, Phys. Rev. Lett. 71, 3458 (1993)], however, are preserved even in the presence of surfactant; this is confirmed via numerical solutions of the initial boundary value problem.
Single magnetic microsphere placement and detection on-chip using current line designs with integrated spin valve sensors: Biotechnological applications
Authors: Graham D. L. , Ferreira H. , Bernardo J. , Freitas P. P. , Cabral J. M. S.
Published: July 2002

Superparamagnetic labels, 400 nm dextran iron oxide particles and 2 μm polymer encapsulated iron oxide microspheres, with biomolecules immobilized on the surface, e.g., the enzyme horseradish peroxidase (20–40 molecules per label) were controllably placed on chip sites (5×15 μm2) using tapered Al current lines (10–20 mA current) and moved to and from adjacent spin valve sensors [2×6 μm,2, magnetoresistance (MR) ∼5%]. Average MR signals of 1.2 and 0.6 mV were obtained for the detection of bulk numbers of 400 nm and 2 μm labels respectively using an on-chip field of 15 Oe and a sense current of 5 mA. The moment per label was calculated at 5×10−13 emu for the 400 nm labels and 5×10−12 emu for the 2 μm labels, illustrating the higher density of the 400 nm particles. MR signals of ∼100 μV were obtained for single 2 μm labels positioned over the spin valve sensor using an on-chip field of 15 Oe and 8 mA sense current. The corresponding sensor saturation occurred at ∼1 mV, with a noise level of ∼10 μV. The estimated maximum MR signal for one 2 μm label directly on top of the sensor was ∼400 μV. Biotechnological applications include high sensitivity biosensors and biochips for protein and DNA screening.
Nanostructure and magnetic properties of polycrystalline FePdPt/MgO thin films
Authors: Jeong Sangki , Roy Anup G. , Laughlin D. E. , McHenry M. E.
Published: July 2002

Atomic ordering, nanostructure and the magnetic properties of polycrystalline FePdPt thin films deposited with MgO underlayers have been investigated. The film thickness was fixed at 5 nm in all films, a thickness where perpendicular anisotropy is dominant. Films deposited at ambient temperature were postannealed at various temperatures to study the kinetics of atomic ordering. The Pd additions to FePt were effective in reducing the temperature needed for atomic ordering. The mean grain size determined by transmission electron microscopy is about 10 nm in annealed films. Within the limits of our observations, FePdPt films do not show phase separation into more than one phase of the L10 structure. The maximum coercivity (Hc) for annealed FePdPt films is about ∼3000–4000 Oe. In situ ordered FePdPt films at 530 °C show no indication of a lower temperature for atomic ordering.
On the virtual aeroshaping effect of synthetic jets
Authors: Mittal R. , Rampunggoon P.
Published: July 2002

The ability of synthetic jets to form large mean recirculation zones in a crossflow is investigated using numerical simulations. It has been suggested that this so-called virtual aeroshaping effect is one mechanism through which synthetic jets affect separation reduction. Here the interaction of a two-dimensional synthetic jet with a flat-plate Blasius boundary layer is simulated and we examine the effect that key flow parameters have on the formation of these mean recirculation zones. The current simulations also suggest a simple scaling for this effect which could prove useful in the design and deployment of these devices.
Weakly nonlinear analysis of boundary layer receptivity to free-stream disturbances
Authors: Brandt L. , Henningson D. S. , Ponziani D.
Published: July 2002

The intent of the present paper is to study the receptivity of a zero pressure gradient boundary layer to free-stream disturbances with the aim to isolate the essential features involved in the generation of streamwise streaks. A weakly nonlinear formulation based on a perturbation expansion in the amplitude of the disturbance truncated at second order is used. It is shown that the perturbation model provide an efficient tool able to disentangle the sequence of events in the receptivity process. Two types of solutions are investigated: the first case amounts to the receptivity to oblique waves generated by a wave-like external forcing term oscillating in the free stream, the second the receptivity to free-stream turbulence-like disturbances, represented as a superposition of modes of the continuous spectrum of the Orr–Sommerfeld and Squire operators. A scaling property of the governing equations with the Reynolds number is also shown to be valid. The relation between this nonlinear receptivity process and previously investigated linear ones is also discussed.