Volume 113, Issue 24, 28 June 2013
Index of content:
The progressive scaling in semiconductor technology allows for advanced miniaturization of intelligent systems like implantable biosensors for low-molecular weight analytes. A most relevant application would be the monitoring of glucose in diabetic patients, since no commercial solution is available yet for the continuous and drift-free monitoring of blood sugar levels. We report on a biosensor chip that operates via the binding competition of glucose and dextran to concanavalin A. The sensor is prepared as a fully embedded micro-electromechanical system and operates at GHz frequencies. Glucose concentrations derive from the assay viscosity as determined by the deflection of a 50 nm TiN actuator beam excited by quasi-electrostatic attraction. The GHz detection scheme does not rely on the resonant oscillation of the actuator and safely operates in fluidic environments. This property favorably combines with additional characteristics—(i) measurement times of less than a second, (ii) usage of biocompatible TiN for bio-milieu exposed parts, and (iii) small volume of less than 1 mm3—to qualify the sensor chip as key component in a continuous glucose monitor for the interstitial tissue.
- Lasers, Optics, and Optoelectronics
113(2013); http://dx.doi.org/10.1063/1.4811447View Description Hide Description
We investigate the detuning of whispering gallery modes (WGMs) in solid polystyrene microspheres (PM) as a function of axisymmetric stress applied to two antipodal points of the microsphere we call poles. We specifically investigate WGMs passing close to these poles, so-called polar WGMs. The applied uniaxial pressure reduces the geometrical circumference of the PM but also increases locally the refractive index at the flattened poles. Our experiments show that the WGMs shift to higher frequencies with increasing pressure and that the magnitude of the strain-induced shift depends on the radial mode number n. Furthermore an energy splitting between azimuthal modes linearly increasing with the pressure is observed. A theoretical model based on a classical ray optics approach is presented which reproduces the main results of our experimental observations.
113(2013); http://dx.doi.org/10.1063/1.4811681View Description Hide Description
We have investigated the carrier dynamics in multilevel intermediate-band solar cells (IBSCs) by solving the Poisson equation, the continuity equations of electrons and holes, and the balance equation of IBs self-consistently. The efficiencies of 6-level IBSCs have stronger dependence on the doping concentration than those of 3-level IBSCs. For non-optimal doping conditions under 1 sun, the efficiencies of 6-level IBSCs can be inferior to those of 3-level IBSCs and even single junction solar cells (i.e., 2-level IBSC). The reasons for this are that multiple IBs in 6-level IBSCs limit their ability to produce currents and the combinations of the energy bandgaps are not optimized for doping concentrations. On the other hand, at around half occupation of electrons in the IBs, the energy conversion efficiencies of IBSCs are maximized under any sun concentrations. The efficiency of 6-level IBSCs has a maximum (66% under 1000 suns) approaching the thermodynamic upper limit, which is similar to the case of 3-level IBSCs. These results indicate the importance of optimizing the doping concentrations in the IB regions of the 6-level IBSCs.
Tuning the effective refractive index of a thin air gap region sandwiched by metallic metamaterials by lateral displacements113(2013); http://dx.doi.org/10.1063/1.4812384View Description Hide Description
We theoretically and experimentally investigate the terahertz transmission of two metallic slabs with periodic cut-through slit arrays. It is revealed that the air gap region between the metallic slabs can be modeled as a medium with an effective refractive index depending on the slab arrangement, that is, depending on the vertical distance and the lateral displacement between the slabs. This property is interpreted in terms of the optical length of the electromagnetic waves propagating in the narrow slit between the two slabs. The result provides a new guide for designing the structure of metallic slab systems.
113(2013); http://dx.doi.org/10.1063/1.4812464View Description Hide Description
Light extraction efficiency enhancement of bulk GaN light-emitting diodes (LEDs) in the shape of truncated-pyramid has been investigated. Compared with the reference LEDs, an enhancement of up to 46% on the light output power from rectangle-shaped LEDs chip with the inclination angle (∼44°) has been observed. Compared with the common triangle-shaped and hexagon-shaped LEDs, large size of conventional rectangular LEDs shaped with truncated-pyramid shows more obvious enhancement in light extraction efficiency. In addition, the ray-tracing simulations results show that light extraction efficiency was influenced not only by inclination angle but also by dimension size.
- Plasmas and Electrical Discharges
Scaling of maximum velocity, body force, and power consumption of dielectric barrier discharge plasma actuators via particle image velocimetry113(2013); http://dx.doi.org/10.1063/1.4811225View Description Hide Description
This study presents Particle Image Velocimetry (PIV) measurements of the induced flow characteristics generated by single dielectric barrier discharge (DBD) actuators in quiescent conditions. The primary aim is to establish accurate empirical trends for model development on both the maximum induced velocity and body force with voltage and consumed power. The results reveal a power law variation for the maximum velocity at low voltages which is followed by an asymptotic behavior. In contrast, the body force is characterized by two power law regions. The power law exponent is shown to be a function of the dielectric thickness, frequency and dielectric constant. Reducing the former or increasing the latter two result in a higher coefficient and lower voltage at which the trend changes. The onset of the second region occurs at a Re ∼ 100 (based on the maximum velocity, um , and corresponding half height, ) and is characterized by a velocity profile which no longer agrees with the laminar profile of Glauert whilst moving increasingly towards the turbulent case. Phase locked PIV measurements show that as the voltage increases the peak momentum transfer shifts from the middle of the AC cycle to the latter end of the forward stroke. Lissajous plots of against the corresponding x location and plasma length demonstrate that the peak momentum transfer remains relatively fixed in space as the voltage and plasma length increase.
113(2013); http://dx.doi.org/10.1063/1.4809975View Description Hide Description
Two-dimensional numerical simulation of ion transport and flow around a single dielectric barrier discharge plasma actuator (PA) is performed. Spatial distributions of ions and electrons as well as their time evolution are obtained by solving the transport equations of monovalent positive ions, monovalent negative ions, and electrons. Voltage and frequency of the driving alternating-current signal are assumed to be 8 kV and 5 kHz, respectively. Special focus is laid upon the effect of voltage gradient dV/dt on the magnitude of the body force. The validity of steady force models often used in flow simulation is also examined. The simulation results show that the magnitude of the body force induced by the PA increases as the voltage gradient dV/dt increases and its increase rate becomes milder at higher voltage. The mechanism of body force generation is explained from the time evolution of number density fields of ions and electrons. A comparison between flow simulations using a time-resolved body force and its time-averaged counterpart demonstrates that the time-averaged model gives sufficiently accurate results when the time scale of the flow is more than 30 times greater than that of the PA.
113(2013); http://dx.doi.org/10.1063/1.4811714View Description Hide Description
The present work is devoted to the study of the properties of the shock wave, appearing in a fast cylindrical discharge (dI/dt about 1012 A/s). The phenomenon under study is the influence of the preionization current (about 500 A) upon the evolution of the shock wave. The discharge was produced in an alumina tube with inner diameter 6 mm, filled with argon to pressures ranging from 80 to 320 Pa. The shock wave was detected from the vacuum ultra violet emission of the hot plasma behind the front, using a pin-hole camera and a microchannel plate detector with a gating time of 10 ns. The measured velocities of the shock wave front are in the range (1–4) × 106 cm/s. The main effect of the preionization is the earlier start of the shock wave from the inner surface of the alumina discharge tube and about 1.5 times increase of the velocity of the front. The time delay of the shock wave cumulation in the absence of the preionization results in the appearance of the eroded wall material in the plasma.
The Influence of spot size on the expansion dynamics of nanosecond-laser-produced copper plasmas in atmosphere113(2013); http://dx.doi.org/10.1063/1.4812580View Description Hide Description
Laser produced copper plasmas of different spot sizes in air were investigated using fast photography and optical emission spectroscopy (OES). The laser energy was 33 mJ. There were dramatic changes in the plasma plume expansion into the ambient air when spot sizes changed from ∼0.1 mm to ∼0.6 mm. A stream-like structure and a hemispherical structure were, respectively, observed. It appeared that the same spot size resulted in similar expansion dynamics no matter whether the target was located in the front of or behind the focal point, although laser-induced air breakdown sometimes occurred in the latter case. Plasma plume front positions agree well with the classic blast wave model for the large spot-size cases, while an unexpected stagnation of ∼80 ns occurred after the laser pulse ends for the small spot size cases. This stagnation can be understood in terms of the evolution of enhanced plasma shielding effects near the plasma front. Axial distributions of plasma components by OES revealed a good confinement effect. Electron number densities were estimated and interpreted using the recorded Intensified Charge Coupled Device (ICCD) images.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
113(2013); http://dx.doi.org/10.1063/1.4811686View Description Hide Description
Micromachined nanocalorimetry sensors have shown excellent performance for high-temperature and high-scanning rate calorimetry measurements. Here, we combine scanning AC nanocalorimetry with in-situ x-ray diffraction (XRD) to facilitate interpretation of the calorimetry measurements. Time-resolved XRD during in-situ operation of nanocalorimetry sensors using intense, high-energy synchrotron radiation allows unprecedented characterization of thermal and structural material properties. We demonstrate this experiment with detailed characterization of the melting and solidification of elemental Bi, In, and Sn thin-film samples, using heating and cooling rates up to 300 K/s. Our experiments show that the solidification process is distinctly different for each of the three samples. The experiments are performed using a combinatorial device that contains an array of individually addressable nanocalorimetry sensors. Combined with XRD, this device creates a new platform for high-throughput mapping of the composition dependence of solid-state reactions and phase transformations.
The effects of microstructural defects on hot spot formation in cyclotrimethylenetrinitramine-polychlorotrifluoroethylene energetic aggregates113(2013); http://dx.doi.org/10.1063/1.4811684View Description Hide Description
Shock initiation due to hot spot formation has been investigated in energetic aggregates subjected to dynamic thermo-mechanical loading conditions. A dislocation-density based crystalline plasticity and specialized finite-element formulations were used to predict hot spot formation due to dynamic thermo-mechanical loading conditions in cyclotrimethylenetrinitramine-polymer energetic aggregates. The effects of grain boundary misorientations, porosity, grain morphology, dislocation densities, and crystal-binder interactions were coupled with adiabatic plasticity heating, thermal decomposition, and dissipated heat to analyze hot spot formation. The predictions indicate that hot spot formation occurs when temperatures become unbounded in localized regions between voids. The time to hot spot formation decreases with increases in dynamic pressure loads, which is consistent with experimental results.
The role of graphene in enhancing the stiffness of polymeric material: A molecular modeling approach113(2013); http://dx.doi.org/10.1063/1.4812275View Description Hide Description
Amorphous epoxy is considered for investigating the role of graphene in enhancing elastic stiffness of polymers. Graphene is incorporated in the amorphous epoxy in order to develop graphene-epoxy systems. The mechanical properties of crosslinked graphene-epoxy (G-Ep) nanocomposites have been investigated using molecular mechanics (MM) and molecular dynamics (MD) simulations. The influences of graphene nanoplatelet weight concentrations, aspect ratios, and dispersion on elastic constants were studied. Both randomly oriented and stacked graphene-epoxy nanocomposites were considered. A polymer consistent force field (pcff) was used in the analysis. The G-Ep nanocomposites system underwent MD equilibration followed by uniform deformation. The stress-strain responses were evaluated in order to determine Young's modulus. MM simulation was also used to calculate the Young's modulus and shear modulus at 0 K. The results from MD and MM simulation showed reasonable improvement in Young's modulus and shear modulus for G-Ep system in comparison to neat epoxy resin. The graphene concentrations in the range of 1%-3% and graphene with high aspect ratio are seen to improve the Young's modulus by 82% approximately. The results from the simulations were compared with the results from micromechanics based analysis and nanoindentation tests. It was observed from both the atomistic scale simulation and nanoindentation tests that incorporation of graphene in neat epoxy at low weight concentration improves the elastic properties. Using similar MD scheme, it was also seen that the dispersed graphene-epoxy system possesses enhanced in-plane elastic modulus compared to the agglomerated graphene-epoxy system.
113(2013); http://dx.doi.org/10.1063/1.4812328View Description Hide Description
Molecular dynamics simulations are performed on monolayer gallium nitride to study their mechanical behavior at various temperatures in the range of 10 to 1700 K. The transition from brittle to ductile nature has been illustrated from the evolution of fracture at two different temperatures of 700 and 1300 K. Brittle to ductile transition temperatures TBDT are obtained from the plots of logarithm of yield stress and inverse temperature at different strain rates and compared qualitatively with the same system in the presence of single and diatomic vacancies. Logarithm of strain rate against inverse of TBDT thus obtained represents an Arrhenius plot, the slope of which corresponds to the activation energy of dislocation glide that is found to be approximately 2.0 ± 0.05 eV for the present case. This suggests that the brittle to ductile transition is controlled by the dislocation mobility as in the case of other semiconductors like silicon and germanium. This behavior is found to be consistent with the presented underlying models. In addition, thermal conductivities are obtained over a temperature range of 300 to 2000 K from the equilibrium Green-Kubo formulations and compared with the (25,0) nanotube that is generated from the same system of monolayer GaN. The values are found to be decreased in both the cases as compared to the bulk gallium nitride, and the reduction in the values of thermal conductivity can be attributed to the finite size effects, increased surface inelastic scattering, and change of phonon spectrum at low dimensions, respectively.
113(2013); http://dx.doi.org/10.1063/1.4812277View Description Hide Description
Typically only dilute (up to ∼10%) highly mismatched alloys can be grown due to the large differences in atomic size and electronegativity of the host and the alloying elements. We have overcome the miscibility gap of the GaN 1−xAsx system using low temperature molecular beam epitaxy. In the intermediate composition range (0.10 < x < 0.75), the resulting alloys are amorphous. To gain a better understanding of the amorphous structure, the local environment of the As and Ga atoms was investigated using extended x-ray absorption fine structure (EXAFS). The EXAFS analysis shows a high concentration of dangling bonds compared to the crystalline binary endpoint compounds of the alloy system. The disorder parameter was larger for amorphous films compared to crystalline references, but comparable with other amorphous semiconductors. By examining the Ga local environment, the dangling bond density and disorder associated with As-related and N-related bonds could be decoupled. The N-related bonds had a lower dangling bond density and lower disorder.
113(2013); http://dx.doi.org/10.1063/1.4811715View Description Hide Description
Electrical conductivity of Sr2-xVMoO6-y (x = 0.0, 0.1, 0.2) double perovskites has been investigated in a reducing atmosphere at temperatures up to 800 °C. This material has a key application in solid oxide fuel cell anodes as a mixed ion and electron conductor. A solid state synthesis technique was used to fabricate materials and crystal structure was verified through x-ray diffraction. Subsequent to conventional sintering in a reducing environment, elemental valence states were indentified through x-ray photoemission spectroscopy on the double perovskite material before and after annealing in a hydrogen environment. Samples exhibited metallic like conduction with electrical conductivities of 1250 S/cm (Sr2VMoO6-y′), 2530 S/cm (Sr1.8VMoO6-y″), and 3610 S/cm (Sr1.9VMoO6-y‴) at 800 °C in 5% H2/95% N2, with a substantial increase in conductivity upon cooling to room temperature. Room temperature electrical conductivity values for Sr1.9VMoO6-y‴ make it a candidate as the highest electrically conductive oxide known. Highly insulating secondary surface phases, Sr3V2O8, and SrMoO4, begin to reduce at 400 °C in a hydrogen environment, as confirmed by X-ray photoemission and thermal gravimetric analysis. This reduction, from V5+ and Mo 6+ to lower valence states, leads to a large increase in sample electrical conductivity.
Dislocation-templated amorphization of Ge2Sb2Te5 nanowires under electric pulses: A theoretical model113(2013); http://dx.doi.org/10.1063/1.4812367View Description Hide Description
Owing to their unique phase change property, GeSbTe alloys hold promise for applications as a candidate material for nonvolatile electronic data storage. In this paper, we theoretically investigate the dislocation mechanisms underlying the phase change phenomenon of GeSbTe alloys under electric pulses. On the basis of the recent experiments by Nam et al. (Science 336, 1561–1566 (2012)), a theoretical model is presented to rationalize the dislocation-templated amorphization process under the action of electric pulses. The physical mechanisms of the nucleation, movement, and multiplication of dislocations in the electric field are analyzed. Using the model, the evolutions of temperature and dislocation density in a Ge2Sb2Te5 nanowire under electric pulses are computed and the critical voltage of amorphization is predicted.
Villain's fractal growth of poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt] J-aggregates onto layer-by-layer films and its effect on film absorbance spectrum113(2013); http://dx.doi.org/10.1063/1.4812381View Description Hide Description
Morphology of poly(allylamine hydrochloride) and poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) layer-by-layer (LBL) films is shown to influence the orientation of PAZO chromophores with respect to solid support surface, which in turn is related with observed red-shifts changes of the chromophore absorbance peak position relative to that of solution spectrum, as the bilayers are being deposited. For the first bilayers, an increase of red shift values is observed, while roughness and grain radius are kept practically constant; after the 5th bilayer, the red-shift values decrease, while grain sizes increase and the number of grains decreases. This behavior is consistent with adsorption of coiled PAZO molecules, treated as pseudo-particles, with the chromophores head-to-head oriented-J aggregates. These aggregates adsorb perpendicularly to the substrate surface for the first layers and, as roughness and grain radius increase, the adsorption of the J aggregates takes place parallel to the solid support surface, which gives rise to a decrease in the red shift value. Moreover, the adsorption of these pseudo-particles follows a fractal growth characterized by a scaling exponent of α = 0.80 ± 0.02 and a temporal growth exponent of β = 0.17 ± 0.02. These values suggest a layer growth according with Villain model, which accounts for the interactions between deposited particles and the surface. This is in accordance with the electrostatic forces driving LbL film formation and accounts for the observed morphology behavior for the different number of layers.
Optical constants and spatial uniformity of thermally grown oxide layer of custom, induced-junction, silicon photodiodes for a predictable quantum efficient detector113(2013); http://dx.doi.org/10.1063/1.4812497View Description Hide Description
We have investigated the optical properties of self-induced inversion-layer silicon photodiodes using spectroscopic ellipsometric measurement techniques. We report a self-consistent data set and dispersion relation for the optical constants of the thermally grown oxide layer. The oxide layer thickness and spatial uniformity of a series of custom manufactured 22 mm × 11 mm rectangular diodes are evaluated. These photodiodes are used in a light trapping arrangement and exhibit predictable quantum efficiency and thus, predictable spectral responsivity. For comparison, we report measurements of the absolute spatial uniformity of the oxide layer on commercial “S6337” and “S1337” radiometric quality photodiodes.
- Electronic Structure and Transport
113(2013); http://dx.doi.org/10.1063/1.4812288View Description Hide Description
The electrical activity of Ga impurities in a high concentration range in B and P highly compensated Si co-doped with Ga for photovoltaic applications has been investigated through the analysis of donor-acceptor pair luminescence. We have identified the fine structure due to the pair luminescence between P-donors and Ga-acceptors based on a comparison with a theoretical spectrum using their generally accepted ionization energies in low concentration ranges in uncompensated Si. The fine structure showed no dependence on dopant concentrations in the P and Ga concentration ranges from 7 × 1016 to 4 × 1017 cm−3 and from 5 × 1016 to 3 × 1017 cm−3, respectively, which leads us to suggest that both P and Ga impurities act as isolated donors and acceptors without noticeable reduction of ionization energies due to high doping.
- Magnetism and Superconductivity
113(2013); http://dx.doi.org/10.1063/1.4812282View Description Hide Description
Among the magnetic materials, those with ferromagnetic character are, by far, the most studied in what concerns applications of the magnetocaloric effect. However, recently, diamagnetic materials received due attention never received before, and an oscillatory behavior, analogous to the de Haas-van Alphen effect, has been found. The present effort describes in details the magnetocaloric properties of a 2D non-relativistic material (a gold thin film, for instance), where oscillations, depending on the reciprocal magnetic field 1/B, are found. A comparison of the magnetic entropy change per electron for some cases is presented, and we found (at 109.3 K) for graphenes, (at 0.7 K) for 2D gold, and (at 0.7 K) for 3D gold.
Giant magneto-impedance effect in amorphous ferromagnetic wire with a weak helical anisotropy: Theory and experiment113(2013); http://dx.doi.org/10.1063/1.4812278View Description Hide Description
An adequate description of the results of experimental measurement of both diagonal and off-diagonal Giant magneto-impedance (GMI) components has been obtained for Co-rich amorphous microwire at moderate frequencies assuming the existence of a small off-diagonal tensor component of the residual quenching stress. The latter is the origin of a weak helical anisotropy of amorphous microwire. The micromagnetic simulation of the magnetization reversal process in the microwire under the influence of the applied magnetic field and dc bias current has been carried out. It is shown that due to the influence of the magneto–elastic interaction in a wire with a weak helical anisotropy, the behavior of the longitudinal and circular magnetization components is significantly correlated. Namely, the change of the sign of the longitudinal magnetization component under the influence of the axial magnetic field leads to a subsequent jump of the circular magnetization component at some critical value of the applied magnetic field. As a result of the jump of the circular magnetization, the off-diagonal GMI component also changes sign during the wire magnetization reversal. This effect is confirmed experimentally for a Co-rich wire with a small negative magnetostriction. It is also shown that the jump of the circular magnetization can be eliminated by a circular magnetic field of a weak dc bias current flowing along the wire. This effect allows one to design sensitive magnetic field sensor based on the measurement of the off-diagonal GMI component.