1,721,952 research outputs found
Thermal characteristics of InGaP/GaAs HBT ballasted with extended ledge
No full textA InGaP/GaAs heterojunction bipolar transistor structure is proposed in which the base epi-layer underneath the extended ledge works as a base ballast resistor. The structure eliminates the critical alignment for a passivation ledge formation as well as additional process steps for external base ballast resistor. Both ballasted and unballasted devices were fabricated and compared. Small signal equivalent circuit gives us the magnitude of the effective ballast resistance. The thermal characteristics, including gain-collapsed I-V and Vb, regression curve are shown and modeled. The temperature dependency of base sheet resistance and its influence on the device performance are also discussed.The author would like to thank Dr. H. Koo and Mr. S. Cho of the
Hanmirotek Corp. for their valuable suggestions and participation in
the fabrication of devices
Nanomechanical Thermal Analysis with Silicon Cantilevers of the Mechanical Properties of Poly(vinyl acetate) near the Glass Transition Temperature
No full textPoly(vinyl acetate) (PVAc) was coated onto one side of a silicon cantilever, and the variations with temperature in the deflection, resonance frequency, and Q factor of the PVAc-coated silicon cantilever were measured simultaneously. The changes in the resonance frequency and the inverse of the Q factor of the cantilever are related to the variations in the elastic modulus and the loss tangent of PVAc, respectively. The loss modulus of PVAc was calculated from the resonance frequency and Q factor and used to determine the glass transition temperature (T-g) of PVAc (66 degrees C), which was found to be higher than that obtained from the cantilever deflection (43 degrees C); this difference is attributed to the high resonance frequency of the cantilever (or the fast rate of the applied stress). A series of measurements at various frequencies were carried out to obtain the accurate T-g and apparent activation energy for the glass transition.X11214sciescopu
An electrodeless quartz crystal resonator integrated with UV/Vis spectroscopy for the investigation of the photodecomposition of methylene blue
No full textThe photodecomposition of organic molecules (methylene blue) was investigated using an integrated system consisting of a UV-vis spectrometer and an electrodeless quartz crystal microbalance (EL-QCM). ZnO nanorods were directly grown on a bare quartz crystal and methylene blue was coated onto the nanorods by drop-casting. Ring-type remote electrodes were used to enable the transmission of light. As the coated methylene blue was photo decomposed by UV light, the changes in mass and light transmittance were measured with the EL-QCM and UV-vis spectrometer respectively. These simultaneous measurements revealed that the rate of change in optical transmission is greater than that of the mass in the early stage of photodecomposition, which indicates that the structural degradation of methylene blue is dominant in the early stage. In contrast, the changes in light transmittance were linearly proportional to those in mass during the later stages, which is attributed to the mineralization of methylene blue. (C) 2013 Elsevier B.V. All rights reserved.X1199sciescopu
Direct synthesis of Cu-BDC frameworks on a quartz crystal microresonator and their application to studies of n-hexane adsorption
No full textWe developed a facile route for synthesizing Cu-BDC frameworks using metallic copper as a metal ion source. A thin film of copper was vacuum deposited onto a quartz crystal microresonator (QCM) and converted to Cu-BDC frameworks via a solvothermal reaction. The initially superhydrophilic Cu-BDC surface became superhydrophobic upon being treated with octadecyltrichlorosilane (ODTS). Exposure of the Cu-BDC-coated quartz crystal microresonator (CuBDC-QCM) to various concentrations of n-hexane vapor induced changes in the resonance frequency and Q factor of the resonator that were related to the adsorbed mass of n-hexane and the modulus of the Cu-BDC layer, respectively. The mass of n-hexane vapor adsorbed on the superhydrophobic Cu-BDC layer was found to be three times that on the superhydrophilic Cu-BDC layer. Furthermore, the adsorption of n-hexane on the superhydrophobic Cu-BDC layer induced an increase in the modulus of the framework whereas the adsorption on the superhydrophilic layer induced a decrease in the modulus of the framework. These opposite changes were attributed to differences in the binding sites of n-hexane vapor inside the framework.open1132sciescopu
A POLYMER'S DIELECTRIC NORMAL MODES DEPEND ON ITS FILM THICKNESS WHEN CONFINED BETWEEN NONWETTING SURFACES
No full textThe dielectric loss peaks of both normal-mode relaxation (fluctuations of the end-to-end dipole vector perpendicular to the confining surfaces) and segmental motion (fluctuations perpendicular to the chain backbone) of cis-polyisoprene were measured with special attention paid to contrast between responses of the bulk samples and films approximate to 100 nm thick. The polymers, narrow-distribution samples with number-average molecular weight M-n = 2600, 6000, and 10 000 g mol(-1), were spin-cast onto atomically smooth mica, coated with a second mica sheet, and quenched to temperatures at which the resulting sandwich geometry was kinetically stable although the polymer films dewet these surfaces at equilibrium. The segmental relaxation process was the same for bulk and thin films, but the normal mode (the end-to-end dipole vector relaxation) slowed down, more so as temperature decreased. This loss mode in the capacitance, C " (f), did not for thin films display the expected terminal tail observed in the bulk samples (C " proportional to f(m) with m < 1 at low frequency f). The power m decreased from 0.9 to 0.5 as temperature was lowered from 260 to 235 K. The inability to quantitatively define the average frequency of this apparently inhomogeneous process led us to analyze the temperature dependence of the frequency at peak of the normal mode. In studies of its temperature dependence, the activation energy of the thin films was found to exceed by 10-20% that for bulk samples and, unlike the bulk state for samples in this range of relatively low molecular weight, to be independent of molecular weight. We interpret these results to indicate that the normal mode not only slowed down but also became more inhomogeneous in this temperature range of 100-30 K above the bulk glass transition temperature, Tg. The contrasting thickness and temperature dependence of the normal-mode and segmental relaxation modes indicates strong breakdown of time-temperature superposition.X111516sciescopu
RIGID DNA CHAINS NEAR NANOPARTICLES
No full textTwo-photon excitation time-resolved fluorescence anisotropy and lifetime measurements were used to study the conformation of DNA near oppositely charged nanoparticles. Negatively-charged rhodamine-labeled DNA was allowed to adsorb onto positively-charged nanoparticles in deionized water and surface-induced fluorescence quenching was observed by measuring the fluorescence lifetime. Fluorescence quenching decreased with the addition of NaCl to the DNA-nanoparticle complex, which implies that the separation of DNA from the nanoparticles increased. However, comparison of fluorescence lifetime decays between free labeled-DNA and the DNA-nanoparticle complex shows that the adsorbed DNA remained less flexible than free labeled-DNA. (C) 2004 Elsevier B.V. All rights reserved.X113sciescopu
INSTANT CURVATURE MEASUREMENT FOR MICROCANTILEVER SENSORS
No full textA multiple-point deflection technique has been developed for the instant measurement of microcantilever curvature. Eight light-emitting diodes are focused on various positions of a gold-coated silicon cantilever through optical fibers, and temperature change or chemical adsorption induces cantilever bending. The deflection at each point on the cantilever is measured with subnanometer precision by a position-sensitive detector, and thus the curvature of the cantilever is obtained. (C) 2004 American Institute of Physics.open112627sciescopu
POLYSTYRENE LATEX NANOPARTICLES SHRINK WHEN POLYELECTROLYTE OF THE SAME CHARGE IS ADDED
No full textReversible deswelling of latex nanoparticles, induced by electrostatic repulsion after adding polyelectrolyte (polystyrenesulfonate, PSS) chains of the same charge to suspensions of the particles in deionized water, is shown using time-resolved fluorescence depolarization of fluorescent dyes (Lumogen 5 Yellow 083) impregnated within the nanoparticles, using methods of two-photon excitation. Shrinkage of the particles produced closer spacing between dyes, thus enhancing fluorescence quenching and homo energy transfer, which depolarizes fluorescence emission. Thus, it was observed that the higher the PSS concentration the more rapid decay of fluorescence depolarization on the subnanosecond time scale. When NaCl was subsequently added, screening caused the original particle size to be recovered. Tentatively, the diameter increase was roughly 7%. The spatial resolution provided by two-photon excitation implies that this method can be used to estimate the size changes of particles in situ in concentrated semiturbid solutions.X1111sciescopu
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
- …
