bluuebunny/crossref_metadata_2025 huggingface dataset:
Interplay between thermoelectric and thermionic effects in heterostructures
Authors: Zeng Taofang , Chen Gang
Published: August 2002

When electrons sweep through a double-heterojunction structure, there exist thermionic effects at the junctions and thermoelectric effects in the film. While both thermoelectric and thermionic effects have been studied for refrigeration and power generation applications separately, their interplay in heterostructures is not understood. This paper establishes a unified model including both thermionic and thermoelectric processes based on the Boltzmann transport equation for electrons and the nonequilibrium interaction between electrons and phonons. Approximate solutions are obtained, leading to the electron temperature and Fermi level distributions inside heterostructures. These quantities are discontinuous at the interfaces as a consequence of the highly nonequilibrium transport when the film thickness is much smaller than the electron mean free path. The electron temperature and Fermi level discontinuities at the interfaces determine the thermionic effect, while their gradients inside the film govern the thermoelectric effect.
Photoluminescence of an assembly of size-distributed self-assembled InAs quantum dots
Authors: Fu Y. , Ferdos F. , Sadeghi M. , Wang S. M. , Larsson A.
Published: August 2002

We have performed experimental and theoretical studies of the effects of inhomogeneous broadening on the luminescence properties of a self-assembled InAs quantum dot (QD) assembly. From atomic force microscopic (AFM) images the InAs QD assembly is found to have an average lateral size of 20–22 nm and a height of 10–12 nm, and the dot density is in the range of 1–2×1010 cm−2. Using the statistical distribution of the QD size from AFM measurements and the results from the theoretical analysis of the photoluminescene (PL) spectrum, it is found that the distance between QDs is larger than 30 nm (the average distance is about 100 nm), the penetration of the ground-state wave function into the GaAs barrier is negligible, and the calculated PL spectrum agrees well with that measured when the carriers in each QD are assumed to be at a local thermal equilibrium state, resulting in the conclusion that the QDs are physically independent. The width of the PL peak is determined by the inhomogeneous QD size.
Single electron tunneling of nanoscale TiSi2 islands on Si
Authors: Oh Jaehwan , Meunier Vincent , Ham Hoon , Nemanich R. J.
Published: August 2002

Nanoscale TiSi2 islands are formed by electron beam deposition of a few monolayers of titanium on an atomically clean silicon surface followed by in situ annealing at high temperatures (800–1000 °C). The lateral diameter of typical islands are ∼5 nm, and they form a nanoscale metal–semiconductor interface. Direct probing of the electrical characteristics of these islands on both p- and n-type Si substrates was performed using ultrahigh vacuum scanning tunneling microscopy and scanning tunneling spectroscopy. With the vacuum between the tip and the island as a second tunnel junction, we thus form a double-junction system for observation of single electron tunneling (SET) effects. Moreover, the small dimensions of the system allow room temperature observation. The results showed features in the I–V spectra attributed to single electron tunneling. Features were more evident when the island–Si junction was in reverse bias. For substrates with a thin epitaxial layer of intrinsic Si, the tunneling related features were enhanced for both doping types. The experimental results are compared with the standard theory and numerical values from the fitting are in agreement with the experimental structures. The results indicate that the nanoscale Schottky barrier of the island–substrate interface can be employed as a tunnel barrier in SET structures.
Tapping mode atomic force microscopy in liquid with an insulated piezoelectric microactuator
Authors: Rogers B. , York D. , Whisman N. , Jones M. , Murray K. , Adams J. D. , Sulchek T. , Minne S. C.
Published: August 2002

Tapping mode atomic force microscopy in liquids is enhanced using an insulated cantilever with an integrated piezoelectric microactuator. When vibrating the cantilever via direct force modulation by the actuator, a single resonance peak appears in the plot of rms cantilever amplitude versus excitation frequency, eliminating the spurious resonances typical of acoustic excitation in a liquid medium. This simplifies selection of the cantilever’s natural resonance frequency for improved tuning accuracy and speed. Acoustic excitation can excite cantilever modes that do not displace the tip of the cantilever but vibrate the microscope’s detection system and create unwanted liquid-coupled acoustic waves between the liquid-cell and the sample. These modes are eliminated by directly forcing the cantilever. Insulated microactuated probes offer a simple and more direct alternative solution to recently presented magnetic tuning methods.
High bandwidth nano-positioner: A robust control approach
Authors: Salapaka S. , Sebastian A. , Cleveland J. P. , Salapaka M. V.
Published: August 2002

This article presents the design, identification, and control of a nano-positioning device suited to image biological samples as part of an atomic force microscope. The device is actuated by a piezoelectric stack and its motion is sensed by a linear variable differential transformer. It is demonstrated that the conventional proportional-integral control architecture does not meet the bandwidth requirements for positioning. The design and implementation of an H∞ controller demonstrates substantial improvements in the positioning speed and precision, while eliminating the undesirable nonlinear effects of the actuator. The characterization of the resulting device in terms of bandwidth, resolution, and repeatability provided illustrates the effectiveness of the modern robust control paradigm.
Externally controlled nanosecond Xe discharge lamp equipped with a synchronous high-voltage power supply using an automobile ignition coil
Authors: Iwata Tetsuo , Tanaka Toshiki , Araki Tsutomu , Uchida Teruo
Published: August 2002

We have constructed two kinds of nanosecond discharge lamps using an ignition coil for an automobile, in which a commercially available Xe lamp that is normally operated at a direct-current (dc) mode is employed. The first one is a free-running uncontrolled-driven Xe discharge lamp, where a positive high-voltage pulse obtained from the ignition coil is applied directly to an anode electrode of the Xe lamp. The second one refined is a synchronized-driven Xe discharge lamp, in which a capacitance C attached on the anode electrode of the lamp is charged up in a stepwise fashion to the breakdown voltage of the lamp and then an external control signal for triggering the lamp discharge is fed on a high-voltage semiconductor switch connected between the cathode electrode of the lamp and the ground. Unlike the conventional discharge lamp, no high-voltage dc power supply is required and, therefore, it is free from undesirable arc discharge problems. To increase the repetition frequency of the pulsed emission, two ignition coils whose operating phases are shifted by 180° with respect to each other are incorporated. When C=10 pF, the pulse width, peak power, and repetition frequency were 8.0 ns, 2.0 W, and 125 Hz, respectively, with a time jitter of 60 ns.
Photon detection with n-propanol and C2H6O isomers
Authors: Lipton-Duffin J. A. , Mark A. G. , McLean A. B.
Published: August 2002

We demonstrate that a Geiger–Müller-type bandpass photon detector, suitable for inverse photoemission experiments, can be constructed from a MgF2 entrance window that has a high-energy transmission threshold of 10.97 eV, and ethanol, a detection gas with an ionization potential of 10.48 eV. The photon detector has a mean detection energy of ℏωd=(10.89±0.07) eV and a bandpass of Δℏωd=(0.37±0.05) eV. A photon detector can also be constructed from n-propanol/MgF2 with a mean detection energy of ℏωd=(10.76±0.07) eV and a bandpass Δℏωd=(0.41±0.05) eV. These two new detection gas/window combinations have a higher detection energy and a narrower bandpass than the dimethylether/MgF2 detector [Δℏωd=(0.71±0.04) eV and ℏωd=10.60 eV]. Since all three detectors utilize a MgF2 entrance window, the photon bandpass can be changed straightforwardly by changing the detection gas. For systems that can be easily damaged by electron beams, having the freedom to open up the detector bandpass is an advantage because it can reduce the total electron exposure time.
Effect of the x-ray beam collimation on the resolution of an energy dispersive diffractometer
Authors: Rossi Albertini Valerio , Paci Barbara
Published: August 2002

Although the energy dispersive x-ray diffraction (EDXD) technique has proved to have several merits in comparison with its conventional angular dispersive counterpart, it has the serious drawback that its resolution is intrinsically lower than that of the latter. This makes EDXD unsuitable each time that high-resolution measurements are needed. However, a wide class of samples does not require high resolution since the diffraction peaks they produce are so wide that the further broadening due to the use of EDXD is negligible. Amorphous solids, liquids, semicrystalline materials, and nanocrystalline powders belong to such a category. In this case, it is not worth performing sophisticated simulations to calculate the angular transfer function of the diffractometer because a simplified model is sufficient to describe the effect of the angular divergence of the x-ray beam on peak broadening. The aim of the present work is to obtain an analytic function that can be used for this purpose, allowing the collimation system to be properly set up.
Cross talk between friction and height signals in atomic force microscopy
Authors: Piner Richard , Ruoff Rodney S.
Published: August 2002

Atomic force and lateral force microscopes use a four quadrant p-i-n detector to measure the motion of a laser beam reflected from the top of a cantilever. If the detector is rotated slightly in the plane of the p-i-n diode, this will cause frictional forces to be detected as a false height signal. In this article, we will show how this coupling between friction and height signals can adversely affect the measurement of topology at height scales below 10 nm. This will be demonstrated with contact mode images of single-walled carbon nanotubes. We will also show how to detect this effect and possible ways to correct for it.
System for inducing and characterizing electron beam-induced surface reactions at pressures up to 10−4 Torr
Authors: Jungwirthová I. , Eklund J. , Sun Y. , White J. M.
Published: August 2002

A system for studying electron-induced surface reactions at pressures up to 10−4 Torr has been constructed. It uses the electron source of a single pass cylindrical mirror electron energy analyzer to both stimulate surface reactions and to analyze, by Auger electron spectroscopy, the surface changes brought about by those reactions. The centerpiece of the design is a small separately pumped fixed position chamber lying within the main vacuum chamber. From below, sample surfaces are brought into this chamber through a Viton-sealed opening and are positioned at the focal plane of the Auger analyzer using a rotatable z-motion manipulator. During electron-induced surface reactions at elevated pressures, the hot filament of the electron source is protected via a small 0.5-mm-diam aperture between the electron source operating at 10−8 Torr and the reaction cell operating at ∼10−4 Torr. In a second configuration, selected after the reaction cell has been pumped out, a larger aperture suitable for collecting Auger electrons is used. The sample is mounted on a rotatable off-axis stage that allows several different irradiation experiments for a single sample preparation. Test results for e-beam induced carbon deposition and etching are presented.
Current–voltage and imaging of TiSi2 islands on Si(001) surfaces using conductive-tip atomic force microscopy
Authors: Oh Jaehwan , Nemanich R. J.
Published: August 2002

Using conducting tip atomic force microscopy (c-AFM), we have measured the current–voltage (I–V) characteristics of individual submicron islands of TiSi2 on Si(100) surfaces, and we have developed an imaging approach that distinguishes the electrical properties of the islands. The Schottky barrier height (SBH) of the submicron TiSi2 islands was deduced from the I–V measurements. The results indicate that there is a significant variation of SBH among the islands on the same surface. The measurements employ a conventional AFM with a heavily B-doped diamond tip to obtain the current–voltage relations. In contact mode AFM, electrical signals are extracted independently from the topographic image. In addition, we have modified the imaging method to probe the local electrical properties of a surface with regions of different conductivity. Using a lock-in technique both phase and amplitude images were obtained, and the resultant image is essentially a map of the differential surface conductivity. Using this method, TiSi2 islands on a Si(100) surface were imaged. This approach can be readily extended to other materials systems.
Effect of magnetic islands on the local plasma behavior in a tokamak experiment
Authors: Taylor E. D. , Cates C. , Mauel M. E. , Maurer D. A. , Nadle D. , Navratil G. A. , Shilov M.
Published: August 2002

Experiments provide simultaneous, local measurements of the pressure and ion velocity perturbations from rotating m/n=2/1 magnetic islands using Mach probes in a tokamak. Measurements were made both inside and around the islands. Pressure perturbations followed the magnetic island motion for both naturally rotating and actively controlled islands. The toroidal ion velocity profile was sharply peaked near the center of the 2/1 magnetic island, and the magnitude of this peak was ∼30% of the magnetic island velocity. Active rotation control experiments also successfully changed the ion fluid velocity. The acceleration of the ion fluid was ∼20% of that experienced by the magnetic islands. Understanding the effect of magnetic islands on the pressure and ion velocity profiles is crucial for both fundamental plasma studies and the development of more efficient tokamak using advanced tokamak concepts.
Theory of negative energy holes in current carrying plasmas
Authors: Grießmeier J.-M. , Luque A. , Schamel H.
Published: August 2002

The theory of hole structures or phase-space vortices in a one-dimensional current-carrying plasma is extended, focusing on the energy of trapped-particle modes in comparison to a homogeneous plasma. It is shown how the energy expression presented in [J.-M. Grießmeier and H. Schamel, Phys. Plasmas 9, 2462 (2002)] is obtained for small amplitude structures. Parameter regimes admitting negative energy solutions are given. It is demonstrated how negative energy structures can be found analytically for the case of a generalized solitary electron hole (where ion trapping is shown to further lower the critical drift velocity), for a generalized solitary ion hole (where the influence of electron trapping increases the critical drift velocity) and for a harmonic (monochromatic) structure. Consequently, a plasma may become nonlinearly unstable well below linear threshold already for infinitesimal wave amplitudes.
Simplified Onsager theory for isotropic–nematic phase equilibria of length polydisperse hard rods
Authors: Speranza Alessandro , Sollich Peter
Published: August 2002

Polydispersity is believed to have important effects on the formation of liquid crystal phases in suspensions of rodlike particles. To understand such effects, we analyze the phase behavior of thin hard rods with length polydispersity. Our treatment is based on a simplified Onsager theory, obtained by truncating the series expansion of the angular dependence of the excluded volume. We describe the model and give the full phase equilibrium equations; these are then solved numerically using the moment free energy method which reduces the problem from one with an infinite number of conserved densities to one with a finite number of effective densities that are moments of the full density distribution. The method yields exactly the onset of nematic ordering. Beyond this, results are approximate but we show that they can be made essentially arbitrarily precise by adding adaptively chosen extra moments, while still avoiding the numerical complications of a direct solution of the full phase equilibrium conditions. We investigate in detail the phase behavior of systems with three different length distributions: a (unimodal) Schulz distribution, a bidisperse distribution, and a bimodal mixture of two Schulz distributions which interpolates between these two cases. A three-phase isotropic-nematic-nematic coexistence region is shown to exist for the bimodal and bidisperse length distributions if the ratio of long and short rod lengths is sufficiently large, but not for the unimodal one. We systematically explore the topology of the phase diagram as a function of the width of the length distribution and of the rod length ratio in the bidisperse and bimodal cases.
Methylation effects in state-resolved quenching of highly vibrationally excited azabenzenes (Evib∼38 500 cm−1). II. Collisions with carbon dioxide
Authors: Park Jeunghee , Shum Lawrence , Lemoff Andrew S. , Werner Kathryn , Mullin Amy S.
Published: August 2002

To explore the role of molecular structure in collisions that relax highly excited polyatomic molecules, we have studied collisional deactivation of a series of highly vibrationally excited methylated pyridines (Evib∼38 500 cm−1) in a CO2 bath. Complementary studies that investigated quenching by H2O have been presented in Part I of this series [M. S. Elioff, M. Fang, and A. S. Mullin, J. Chem. Phys. 115, 6990 (2001)]. We have used high-resolution transient infrared absorption probing to measure rotational and translational energy gain in individual quantum levels of CO2 (0000) that are populated via collisions with vibrationally excited picoline (2-methylpyridine) and lutidine (2,6-dimethylpyridine). Vibrationally excited picoline and lutidine were prepared by absorption of pulsed λ=266 nm light and fast internal decay to the ground electronic state. The nascent distribution of CO2 (0000) rotational states was measured for J=60–78. Translational energy gain distributions were determined for the J=60–80 states of CO2 (0000) using Doppler-broadened linewidth measurements. Energy transfer probabilities were determined by measuring absolute energy transfer rate constants for energy gain into specific CO2 quantum states. These results are compared to previous single-collision energy transfer studies on hot pyridine [M. C. Wall, B. Stewart, and A. S. Mullin, J. Chem. Phys. 108, 9658 (1998)] and hot pyrazine [M. C. Wall and A. S. Mullin, J. Chem. Phys. 108, 9658 (1998)] initially excited with 266 nm light and quenched via collisions with CO2. We find that donor methylation reduces the amount of translational and rotational energy imparted to the CO2 (0000) high-J states, but that the cross section for exciting the high-J states of CO2 (0000) increases upon donor methylation. Fermi’s golden rule is used to describe the relaxation process, and the energy transfer distribution functions for ΔE>4000 cm−1 are found to correlate remarkably well to the energy dependence of the density of states of the hot donor molecule. This analysis is also successfully applied to earlier quenching data for vibrationally excited C6F6 [C. A. Michaels et al., J. Chem. Phys. 106, 7055 (1997)], suggesting that this may be a general approach for describing relaxation of highly excited molecules.
Tunable diode laser study of the ν2 infrared band of fluoroketene: Confirmation of anomalously low C=O stretching frequency
Authors: Russell Douglas K. , Wesendrup Ralf
Published: August 2002

The short-lived molecule fluoroketene (FHCCO) was generated by laser pyrolysis of fluoroacetyl chloride in a static multipass cell to record the first high-resolution infrared spectrum of its ν2 fundamental band. More than 400 transitions between 2143 and 2160 cm−1 were assigned and analyzed to yield very accurate rotational and centrifugal distortion constants for the upper vibrational state. The ν2 band origin of fluoroketene lies at 2147.826 93 cm−1, which is surprisingly low compared to the band origins of other ketenes. This result is confirmed by matrix isolation Fourier transform infrared spectroscopy of all four monohaloketenes.
Characteristics of deep levels in As-implanted GaN films
Authors: Lee L. , Lee W. C. , Chung H. M. , Lee M. C. , Chen W. H. , Chen W. K. , Lee H. Y.
Published: August 2002

Hall, current–voltage and deep level transient spectroscopy measurements were used to characterize the electric properties of n-type GaN films implanted with As atoms. After 800 °C thermal annealing for 60 min, one additional deep level located at EC−0.766 eV was found in the films. We presume this induced trap is an arsenic-related point defect, most likely antisite in nature.
Structural and morphological modifications in double layer heterostructures containing H2Pc, perylene-3, 4, 9, 10-tetracarboxylic dianhydride and Alq3
Authors: Heutz S. , Jones T. S.
Published: August 2002

The morphological, structural and spectroscopic properties of molecular double layers grown by organic molecular beam deposition have been investigated using atomic force microscopy, Nomarski microscopy, powder x-ray diffraction and electronic absorption spectroscopy. The structures, based on metal-free phthalocyanine (H2Pc), perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) and aluminum-tris-quinolate (Alq3), were deposited on glass substrates, with individual layer thicknesses ranging from 7 to 380 nm. Alq3 has no influence on the subsequent growth of either H2Pc or PTCDA, and its properties are not affected by a first molecular layer. By contrast, significant structural modifications are observed in double layers based on the two crystalline materials H2Pc and PTCDA. H2Pc deposited onto a PTCDA first layer departs from its herringbone structure, and adopts a layered structure with an interplanar spacing of 3.33 Å and an intermolecular shift of 1.7 Å. The structure of a PTCDA film deposited onto a α-H2Pc layer is completely disrupted. When PTCDA is deposited onto a β-H2Pc first layer, a morphological templating is observed and this annihilates any structural disruption. This is consistent with the growth of a microcrystalline PTCDA film on top of a α-H2Pc first layer.
Temperature selective growth of carbon nanotubes by chemical vapor deposition
Authors: Ducati Caterina , Alexandrou Ioannis , Chhowalla Manish , Amaratunga Gehan A. J. , Robertson John
Published: August 2002

Randomly oriented and vertically aligned carbon nanotubes were synthesized by chemical vapor deposition varying the growth temperature between 550 °C and 850 °C. Transmission electron microscopy (TEM) was used to investigate the growth process and in particular the role of the Ni catalyst. It was found that a catalytic particle was always situated at the tip of the tube thus implying a tip growth mechanism. The TEM analysis revealed a strong correlation between the size and shape of the Ni particle encapsulated at the tip of the tubes and the growth temperature, suggesting that temperature is an influential process parameter in determining an optimum size and shape for the catalytic particle and hence select the diameter, and the wall thickness of the growing tubes.
Optical properties of low-temperature grown GaAs on Bragg reflectors
Authors: Giehler M. , Herfort J. , Ulrici W. , Däweritz L. , Ploog K. H.
Published: August 2002

Near-infrared reflectance spectra of 5 μm thick low-temperature (LT) GaAs films grown by molecular-beam epitaxy at different substrate temperatures TG on GaAs/AlAs Bragg reflectors (BR’s) have been studied. With decreasing TG (increasing absorption coefficient αf of the film), the reflectance of the stop band of the BR decreases monotonously, while the amplitudes of the interference fringes due to the LT-GaAs layer inside the stop band initially increase followed by a decrease for large αf values. This unusual behavior is explained by a particular optical interference effect of a thick absorbing layer on a strongly reflecting structure. This effect remarkably improves the determination of αf and enables the detection of As-antisite defects in LT-GaAs films for concentrations as low as 1×1018 cm−3, where other methods fail.
Mass sensitivity of acoustic wave devices from group and phase velocity measurements
Authors: McHale G. , Martin F. , Newton M. I.
Published: August 2002

The effect of dispersion on acoustic wave sensors is considered. The discussion is focused upon layer guided surface acoustic waves (Love waves), which obtain their high mass sensitivity for the first Love wave mode by optimizing the guiding layer thickness, d, such that d∼λl/4; the wavelength in the layer is given by λl=f/vl where f is the operating frequency and vl is the shear acoustic speed of the guiding layer. We show that this optimization of guiding layer thickness corresponds to strong dispersion so that the phase and group velocities can be quite different. From the definition of the phase velocity mass sensitivity, we show that it can be determined from either the slope of the curve of phase velocity with normalized guiding layer thickness, z=d/λl, or from the phase and group velocities measured for a given guiding layer thickness. Experimental data for a poly(methylmethacrylate) polymer guiding layer on 36° XY Lithium Tantalate is presented. Measurements of phase velocity and group velocity determined by a network analyzer were obtained for systematically increasing guiding layer thicknesses; a pulse transit experiment was also used to provide independent confirmation of the group velocity data. Two independent estimates of the mass sensitivity are obtained for z=d/λl<0.22 from (i) the slope of the phase velocity curve and (ii) the measurements of the group and phase velocity. These two estimates are shown to be consistent and we, therefore, conclude that it is possible to determine the mass sensitivity for a Love wave device with a given guiding layer thickness from measurements of the phase and group velocities. Moreover, we argue that the formula using group velocity to determine phase velocity mass sensitivity can be extended to a wide range of other acoustic wave sensors. In addition, we suggest that variations in the group velocity due to deposited mass may be a more sensitive parameter than variations in the phase velocity.
Modifying the nanocrystalline characteristics—structure, size, and surface states of copper oxide thin films by high-energy heavy-ion irradiation
Authors: Balamurugan B. , Mehta B. R. , Avasthi D. K. , Singh Fouran , Arora Akhilesh K. , Rajalakshmi M. , Raghavan G. , Tyagi A. K. , Shivaprasad S. M.
Published: August 2002

In the present study, x-ray diffraction, Raman spectroscopy, spectroscopic ellipsometry, photoluminescence, and x-ray photoelectron spectroscopy techniques were used to study the effect of 120 MeV Ag9+107 ion irradiation on nanocrystalline Cu2O thin films grown by the activated reactive evaporation technique. The influence of dense electronic excitations during ion irradiation on the structural and optical properties of the Cu2O thin films was studied. Experimental results demonstrate that the phase and the size of nanocrystallites in the Cu2O thin films as well as associated surface states can be tailored by controlling ion fluence. The Cu2O higher symmetry cubic phase is observed to be quite stable under a higher temperature and irradiation-induced thermal spikes, which accompanies ion irradiation.
High-temperature contactless viscosity measurements by the gas–film levitation technique: Application to oxide and metallic glasses
Authors: Haumesser Paul-Henri , Bancillon Jacky , Daniel Michel , Perez Michel , Garandet Jean-Paul
Published: August 2002

In the field of thermophysical characterization of materials at high temperature, a crucial issue is to limit the effect of chemical or physical phenomena occurring at the interface between the sample and the container. Therefore, contactless techniques are well adapted to high-temperature measurements. The gas–film levitation method has recently proved to be applicable to viscosity measurements. Our purpose in this article is to derive viscosity values by the observation of the dynamical response of a perturbed levitating drop. We present here recent improvements in this technique, with particular attention paid to measurement accuracy issues and temperature calibration problems. Viscosity measurements performed on oxide and metallic glasses reveal that a gas–film levitation based viscometer is able to provide measurements with good accuracy (±10%) in a wide viscosity range, from very viscous (up to several kPa s, aperiodic relaxation of the drop) to fluid liquids (a few mPa s, damped oscillation of the drop).
Sum-frequency generation microscope for opaque and reflecting samples
Authors: Hoffmann D. M. P. , Kuhnke K. , Kern K.
Published: August 2002

We report on the performance of a microscope setup, which has been developed for the imaging of sum-frequency generation (SFG) from reflecting, nontransparent samples. In order to maximize the SFG intensity the sample has to be observed from one side at an angle near 60° with respect to the surface normal. The setup is designed (a) to keep focus over the full image field and (b) to compensate for the distortion of the field-of-view, both by means of a blazed grating. In contrast to “specular” SFG spectroscopy, the incident beams reflected from the sample and the generated SF light cannot be separated by angular filtering. In this setup the separation thus relies on spectral filtering only.
Amplified spontaneous emission-based technique for simultaneous measurement of temperature and strain by combining active fiber with fiber gratings
Authors: Lai Y. C. , Qiu G. F. , Zhang W. , Zhang L. , Bennion I. , Grattan K. T. V.
Published: August 2002

This article reports on an optical fiber-based sensing system for carrying out the simultaneous measurement of temperature and strain. The sensor design is based on the combination of active doped optical fiber and fiber gratings. Erbium/ytterbium co-doped fiber is used to meet the requirements for both high temperature responsivity and small sensor size to address smart structure applications. The temperature dependence of the amplified spontaneous emission power under different pump wavelengths is used as the basis of the sensor and an optical reference to enhance the measurement resolution is discussed. The feasibility of this technique for simultaneous measurement of temperature and strain is demonstrated.
Comparison of the photothermal sensitivity of an interferometric optical fiber probe with pulsed photothermal radiometry
Authors: Laufer J. G. , Beard P. C. , Mills T. N.
Published: August 2002

An interferometric optical fiber probe for making photothermal measurements of tissue optical and thermal properties is compared to pulsed photothermal radiometry in terms of its overall thermal sensitivity, linearity, and response time. The principles of operation of the probe are described and its performance as a low finesse Fabry–Perot interferometer is discussed. A probe with a 12 μm sensing film is characterized by a thermal noise floor of 50 mK and a response time of 850 μs. The sensitivities to the optical and thermal coefficients of the two techniques have been analyzed. As a result of the different source geometries, the optical fiber probe was found to be more sensitive to the thermal coefficients of tissue than the optical coefficients while pulsed photothermal radiometry provided maximum sensitivity to the optical coefficients.
Heater cell for materials synthesis and crystal growth in the large volume high pressure apparatus at 10 GPa
Authors: Kanke Yasushi , Akaishi Minoru , Yamaoka Shinobu , Taniguchi Takashi
Published: August 2002

To realize materials synthesis and crystal growth in a large volume of high pressure apparatus in the region of 10 GPa, a heater cell was developed for a belt-type high pressure apparatus with a bore diameter of 32 mm. The cell consists of a cylindrical electrode made of TiC powders located between a pair of graphite heaters with a disk shape. High temperature of 1500 °C at nominal pressure of 9.7 GPa (9.35 GPa corrected) was successfully generated for 1 h using the cell. Synthesis and growth of stishovite was performed at high pressure and high temperature conditions based on their calibrations. Experimental results suggest that the materials synthesis and crystal growth above 10 GPa can be realized in a large volume of belt-type apparatus using the heater cell.
Notch flexure hinges: An effective theory
Authors: Tseytlin Yakov M.
Published: August 2002

This article presents effective tractable equations for rotational compliance (stiffness) of a simple monolithic flexure hinge with circular (radius R and crosspiece thickness t), elliptical [at semiaxis ax, ay, elliptical ratio ε (=ax/ay)] and other cross sections. These equations and the method by inverse conformal mapping of circular approximating contour used to derive them are different from the known and widely used theoretical equations originally derived in 1965 by Paros and Weisbord for circular notch hinges. Later it was found that the circular hinge represents the worst case error between known theoretical and finite element models. The conformal mapping equations data presented in this article are likely to be much closer (within less than 10%) to the finite element analysis and experimental data than other theoretical equations. In particular this is the case for circular notch hinges at relative thickness β(=t/2R) in the range 0.01 to 0.3 and for elliptical hinges at the elliptical ratio ε=1 to 10. The derived general equation is common for all types of notch hinges whose profiles can be approximated by two shifted contiguous circles and includes material parameters with Young’s modulus and Poisson’s ratio. The latter is totally omitted in known theoretical solutions by other authors. New tractable equations are derived from the general equation on the basis of trigonometric functions’ simplified series expansion in certain ranges of hinge crosspiece relative thickness. The corresponding graphs are presented. Experimental data were received by holographic interference and autocollimator measurement.
A heuristic penumbral imaging technique for measurements of laser-produced plasma density profile
Authors: Nozaki Shinya , Chen Yen-wei , Nakao Zensho , Fujioka Shinsuke , Shiraga Hiroyuki
Published: August 2002

Penumbral imaging is a powerful technique for imaging of penetrating radiations such as neutrons and hard x ray with a high spatial resolution. Recently, the technique has also been used for measurement of laser-produced plasma density profile. The limitation is that the straightforward image or signal reconstruction will introduce a significant distortion if the penumbral image is distorted by the noise. In this article, we propose a heuristic method for reconstruction of penumbral images. Both simulation and experimental results demonstrated that the proposed method is robust and tolerant to the noise.
Development of laser assisted nanometric resolution scanning tunneling microscopy time-of-flight mass analyzer system
Authors: Ding Y. , Micheletto R. , Hanada H. , Nagamura T. , Okazaki S.
Published: August 2002

This study describes a ground-breaking process that provides a direct highly localized measurement of the atomic mass on surfaces at room temperature. Employing an original system that joins a scanning tunneling microscopy (STM) device and a time-of-flight (TOF) mass analyzer, we could locally ionize surface atoms by the combination of an optical laser pulse and an appropriate voltage variation between the sample and the STM tip. Desorbed ions were accelerated and detected by a TOF chamber. Detection and discrimination of single atomic species from nanolocalized area have been demonstrated for the first time.
Forward dynamics of water-plasmas produced by femtosecond laser pulses
Authors: Li Y. T. , Zhang J. , Lu X. , Teng H. , Li Y. J. , Peng X. Y. , Jin Z. , Zhao L. Z.
Published: August 2002

The evolution of the forward density front of water-plasmas generated by ultrashort laser pulses on an air–water interface is investigated with optical diagnostics. The density front is found to move with supersonic speed at the beginning, as predicted by a hydrodynamics simulation. This plasma-expansion process is followed by a fluid mechanical process. The expansion rate gradually reduces to the acoustic speed in water.
Lagrangian neoclassical transport theory applied to the region near the magnetic axis
Authors: Satake Shinsuke , Okamoto Masao , Sugama Hideo
Published: August 2002

Neoclassical transport theory around the magnetic axis of a tokamak is studied, in which relatively wide “potato” orbits play an important role in transport. Lagrangian formulation of transport theory, which has been investigated to reflect finiteness of guiding-center orbit widths to transport equations, is developed in order to analyze neoclassical transport near the axis for a low-collisionality plasma. The treatment of self-collision term in Lagrangian formulation is revised to retain momentum conservation property of it. By directly reflecting the orbital properties of all the types of orbits in calculation, the ion thermal conductivity around the axis is found to decrease from that predicted by conventional neoclassical theory. This result supports recent numerical simulations which show the reduction of thermal conductivity near the magnetic axis.
Spectroscopy of Nin(benzene)m anion complexes
Authors: Rao B. K. , Jena P.
Published: August 2002

Total energy calculations based on the generalized gradient approximation to the density functional theory reveal that the Ni(benzene) and Ni(benzene)2 anions are unstable against autodetachment of the additional electron while other anion complexes containing more than one Ni atom are stable. Although the adiabatic electron affinities increase with Ni content, they are significantly smaller than those in pure Ni clusters containing the same number of Ni atoms. The difference between adiabatic electron affinities and vertical detachment energies are around 0.2 eV in most cases, indicating that the equilibrium geometries of Nin(benzene)m− are not significantly altered from their corresponding neutral geometries. The vertical transitions from the anion to the neutral provide new insight into the magnetic moment of these organometallic complexes.
On the definition of the spin-free cumulant of the second-order reduced density matrix
Authors: Lain Luis , Torre Alicia , Bochicchio Roberto
Published: August 2002

This note deals with the appropriate definition for the spin-free cumulant of the second-order reduced density matrix. Our approach leads to a direct derivation of one of the proposals reported by Kutzelnigg and Mukherjee [J. Chem. Phys. 116, 4787 (2002)] and it points out its suitability.
Characterization of coupled pore systems from the diffusion eigenspectrum
Authors: Zielinski L. J. , Song Y.-Q. , Ryu S. , Sen P. N.
Published: August 2002

Complex structures often consist of many interconnected or “coupled” simpler regions. The problem frequently arises of determining the geometry of these individual subregions within the larger structure. We consider a simple model to argue that the high eigenmodes of the diffusion equation can be used to probe their geometry. We find that for a wide range of coupling, certain high eigenmodes preferentially remain within a particular subregion, thereby allowing the association of the corresponding eigenvalue with that subregion. We discuss an application of these results to the characterization of internal structure of porous media.
Electrical and optical properties of GaN films implanted with Mn and Co
Authors: Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Pashkova N. Y. , Kim J. , Ren F. , Overberg M. E. , Thaler G. T. , Abernathy C. R. , Pearton S. J. , Wilson R. G.
Published: August 2002

Optical transmission spectra, microcathodoluminescence spectra, capacitance–voltage and capacitance–frequency curves, temperature dependence of resistivity and deep level spectra with both electrical and optical injection were measured on n-GaN samples implanted with high doses of Mn (3×1016 and 4×1016 cm−2) and Co (4×1016 cm−2). From optical transmission it was found that Mn forms a deep acceptor near Ev+1.8 eV while the Co acceptor is about 0.1 eV deeper. In addition, Mn and Co form complexes with native defects and these complexes are deep electron traps with a level near Ec−0.5 eV. Such complexes are most likely responsible for a strong blue luminescence band with energy near 2.9 eV. Adjacent to the implanted region a defect region about 1 μm deep is formed, most likely by out-diffusion of point defects from the implanted zone during the 700 °C annealing used to partially remove the radiation damage. This region is characterized by a high density of electron traps at Ec−0.25 eV and Ec−0.7 eV and hole traps at Ev+0.2 eV, Ev+0.35 eV and Ev+0.45 eV.
Surface modified silica mesoporous films as a low dielectric constant intermetal dielectric
Authors: Yu Suzhu , Wong Terence K. S. , Pita Kantisara , Hu Xiao , Ligatchev Valeri
Published: August 2002

Silica mesoporous films with low dielectric constant were successfully fabricated by a multiple-step sol–gel process. Various surface modifications were conducted to make the surface of the films hydrophobicity, which was proved very effective to maintain the low dielectric properties of the films. The basic properties of the silica films were evaluated by atomic force microscopy, specular x-ray reflectivity, Fourier transform infrared, and thermal gravimetric and differential thermal analysis. An inherent low dielectric constant of around 2.0 was realized for about 56% porosity of the silica film with pore size less than 40 nm and the leakage current was at a level of 10−6 A/cm2 after two months of fabrication. Preliminary results of the silica films prepared here present a very positive prospective to intermetal dielectric applications.
Ultraviolet excitation fine tuning of luminescence bands of oxygen-deficient centers in silica
Authors: Anedda A. , Carbonaro C. M. , Clemente F. , Corpino R.
Published: August 2002

The photoluminescence (PL) activity of oxygen deficient centers in commercial silica samples has been reconstructed by exciting in the spectral range of their optical absorption band around 5 eV. The fine tuning of the excitation was provided by a tunable pulsed optical parametric oscillator seeded by a Nd:yttrium–aluminum–garnet laser. The experimental results, analyzed in the spectral moment framework, indicate the presence of different centers whose spectral features are modulated by structural disorder typical of the glassy matrix. The photoluminescence excitation spectra at different emission energies evidence the composite structure of the luminescence bands around 3.1 and 4.2 eV, usually ascribed to triplet and singlet transitions of the same oxygen deficient center. Besides the well-known emission bands centered at 3.15 and 4.20 eV, the contribution of a new PL band below 3 eV is reported. Time resolved photoluminescence measurements of the low energy band give an estimate of its spectral characteristics, with peak position around 2.8 eV, full width at half maximum of about 1 eV and exponential lifetime of 840 ns.
Growth model and the effect of CuO nanocrystallites on the properties of chemically derived epitaxial thin films of YBa2Cu3O7−x
Authors: Araki Takeshi , Niwa Toshiharu , Yamada Yutaka , Hirabayashi Izumi , Shibata Junko , Ikuhara Yuichi , Kato Kazumi , Kato Takeharu , Hirayama Tsukasa
Published: August 2002

In metalorganic deposition using trifluoroacetates (TFA-MOD), CuO nanocrystallites in calcined film, which influence the critical current density (Jc) of the resulting film, have seldom been considered before. CuO nanocrystallites ripen and grow during the calcining process at 200–250  °C and grow into large CuO grains in the YBa2Cu3O7-x (YBCO) film. The final diameter of the grains is about 0.1 μm. When highly purified coating solution is used, suppressing the creation of nanocrystallites in the precursor film is an effective way to obtain high-Jc YBCO film because the nanocrystallites consist only of CuO. We obtained the highest-Jc YBCO film on LaAlO3 single crystal with a calcining process of 9h43m at 200–250 °C. It was 141 nm thick and had a Jc of 7.5 MA/cm2 (77 K, 0 T), as measured by the four-probe method. The firing process of TFA-MOD still has several unsolved problems; why can H2O and HF gas diffuse quickly within the film? These phenomena can be explained by a model with a quasiliquid consisting of Y, Ba, Cu, O, H, and F. The H2O and HF can move quickly in the quasiliquid network within the film during the firing process. According to the model, we can get more conversion of CuO nanocrystallites into quasiliquid by increasing the amount of water vapor. Jc of the film was increased from 3.3 to 4.5 MA/cm2 (77 K, 0 T) by increasing the humidity from 4.2% to 12.1% during the long calcining profile of 66h40m at 200–250 °C. These results confirm the above model. CuO nanocrystallites in precursor films induce nonstoichiometric metal contents in the quasiliquid. Isolated CuO grains in YBCO film indirectly cause nonstoichiometric quasiliquid in other areas. Such nonstoichiometric quasiliquid leads to non-YBCO materials such as BaO, Y2O3, and Y2Cu2O5. Thus, films derived from non-highly purified coating solution and ones prepared with an inappropriate calcining process both have lower Jc values. Micrographs obtained by transmission electron microscopy and Jc results support this hypothesis.
Electron field emission properties of gamma irradiated microcrystalline diamond and nanocrystalline carbon thin films
Authors: Gupta S. , Weiss B. L. , Weiner B. R. , Pilione L. , Badzian A. , Morell G.
Published: August 2002

Microcrystalline diamond and nanocrystalline carbon thin films prepared by microwave plasma assisted chemical vapor deposition were submitted to gamma radiation in order to study the radiation-induced structural transformation and the corresponding changes in the electron field emission properties. Electron field emission measurements and microstructural characterizations, which included Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy, were performed before and after gamma radiation doses of 1, 5, and 20 Mrads. Microcrystalline diamond showed a dramatic improvement in the emission properties only after a cumulative dose of 26 Mrad, while nanocrystalline carbon showed a relatively small but systematic decrease in turn-on field from 9 to 4 V/μm with increasing gamma radiation dose. The enhancement in emission characteristics associated with the radiation-induced microstructural transformation shows the critical role of defects with their associated electronic defect states, and of sp2-bonded carbon channels in the electron field emission mechanism of nanocomposite carbon materials. The results also indicate that nanocrystalline carbon tends to reach a state of damage saturation when submitted to Mrad doses of gamma radiation, suggesting the possibility of fabricating radiation buffer materials that would undergo internal sp3–sp2 interconversions while absorbing ionizing radiation without changing their average microstructure, which can be employed for preventing radiation from reaching sensitive materials.