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An accurate model for intrinsic dielectric loss in sapphire (α-Al2O3) is presented. The proposed model is based on a theory of electromagnetic field absorption governed by multiphonon relaxation processes influenced by a finite lifetime of thermal phonons. Expressions for anisotropic sapphire dielectric loss tangent are derived utilizing sapphire dielectric function given in terms of the quasiharmonic approximation. The frequency-dependent two- and three-phonon damping constants of each fundamental transverse optical mode were determined numerically employing the accurate anharmonic coefficient computations based on sapphire lattice-dynamical data. The results of the dielectric loss tangent calculations in microwave and millimeter-wave regions are presented. The developed sapphire dielectric loss model offers excellent agreement between the experimental and calculated magnitudes of the tan δ components and also accurately reproduces their frequency and temperature dependencies. [ABSTRACT FROM AUTHOR]

In this paper we describe the temperature dependence of buried channel (BC) ion sensitive field effect transistor (ISFET). The device response depends on the temperature; hence, temperature variations can cause erroneous readings. A theoretical model describing the temperature dependence of BC-ISFET and a theoretical solution to eliminate the signal variations due to temperature changes are presented here. The suggested solution is based on an inverter containing n-BC-ISFET and p-BC-ISFET. The influence of various parameters on the operation of the inverter and its sensitivity are investigated. We discuss the influence of self-assembled monolayers on the operation of the inverter. [ABSTRACT FROM AUTHOR]

Magnetization and Mossbauer spectroscopy measurements are performed at low temperature under high field, on nanoparticles with a nickel ferrite core and a maghemite shell. These nanoparticles present finite size and surface effects, together with exchange anisotropy. High field magnetization brings the evidences of a monodomain ordered core and surface spins freezing in disorder at low temperature. Mossbauer spectra at 4.2 K present an extra contribution from the disordered surface which is field dependent. Field and size dependences of this latter show a progressive spin alignment along the ferrite core which is size dependent. The weak surface pinning condition of the nanoparticles confirms that the spin disorder is localized in the external shell. The underfield decrease in the mean canting angle in the superficial shell is then directly related to the unidirectional exchange anisotropy through the interface between the ordered core and the disordered shell. The obtained anisotropy field HEa scales as the inverse of the nanoparticle diameter, validating its interfacial origin. The associated anisotropy constant KEa equals 2.5×10-4 J/m2. [ABSTRACT FROM AUTHOR]

We present a novel scheme for generation of broadband mid-infrared (mid-IR) radiation using a quasiphase-matched difference-frequency-generation (DFG) process in a planar high index core symmetric Bragg reflection waveguide (BRW) geometry based on GaN/AlxGa1-xN system. By suitably tailoring the dispersion properties of the BRW so as to maintain the phase-matching condition over a broad range of pump wavelengths, we show that the pump acceptance bandwidth could be enhanced by nine to ten times for a DFG process. This manifests into a broadband mid-IR difference-frequency idler with greater than 280 nm bandwidth close to 4.35 μm wavelength. Since the design presented here exploits the dispersive features of the BRW; we can shift the broadband characteristics to any desired region of the mid-IR spectrum within the constraints imposed by transparency and nonlinearity of the constituent materials. The design also facilitates pumping by standard high-power solid-state laser sources. [ABSTRACT FROM AUTHOR]

Platinum (Pt) Schottky contacts (SCs) on hydrothermal grown Zn-terminated (0001) ZnO substrates with the different hydrogen peroxide (H2O2) treatment time are investigated. Under the treatment in room temperature, effective SCs are made for 45 min and longer time and the electrical characteristics show the dependence on treatment time. The irregular humps on ZnO surface with roughness measured by atomic force microscope differ as the treatment time and roughness exhibits the large variation between 0.368 and 3.566 nm, indicating the etching effect and near-surface defects related to the lattice imperfections. The evaluated barrier height has the value at 0.89–0.96 eV with the saturation current density in the range of 2.21×10-10–3.31×10-9 A/cm2. The effective donor concentration calculated from capacitance-voltage (C-V) measurement decreases as treatment time, implying the widening of the space charge region after H2O2 treatment. The improved SC characteristics are attributed to the product of the wider bandgap and low-conductivity ZnO2 via grazing-incidence x-ray diffraction analysis. The rectifying behavior has the threshold dependence on the treatment time and has the reverse trend toward the surface asperities. [ABSTRACT FROM AUTHOR]

This paper presents a procedure to evaluate the charge density in the low conductivity regions between the metal and the accumulated intrinsic channel of an organic thin film transistor (OTFT). This charge links different physical mechanisms in the contacts of OTFTs. The charge density is evaluated in transistors with different metal-organic barriers to study its dependence with the voltage, temperature and the materials forming the contact. [ABSTRACT FROM AUTHOR]

Within the framework of the effective-mass approximation, the barrier width dependence of the donor binding energy of hydrogenic impurity in a cylindrical wurtzite (WZ) InGaN/GaN strained quantum dot (QD) is calculated by means of a variational procedure, considering the strong built-in electric field effect due to the spontaneous and piezoelectric polarizations. Numerical results show that the built-in electric field and the donor binding energy of the impurity located at any growth direction position are obviously dependent on the barrier width in WZ In0.1Ga0.9N/GaN strained QD with a small barrier width (<8 nm). However, the built-in electric field and the donor binding energy of the impurity located at any growth direction position are insensitive to the barrier width in WZ In0.1Ga0.9N/GaN strained QD with a large barrier width (>8 nm). Moreover, the donor binding energy of the impurity located at the right boundary of the QD is independent of the barrier width with any dot height and indium composition when the barrier width is large (>8 nm). [ABSTRACT FROM AUTHOR]

Using first-principles density functional calculations, we study the electronic structure and magnetic properties of Mn-doped ZnO, wurtzite crystal structure, with various defects. This allows to understand and to explain the half-metallicity and the ferromagnetism stability, observed in Mn-doped ZnO with acceptor defects like Zn vacancies. The calculations were performed using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation. Hydrogenation effects in (Zn,Mn)O and (Zn,Mn)(O,N) is also investigated with and without defects. This work presents detailed information about total, atom, and vacancy projected density of states functions, and magnetic moment for different atoms and defects in Mn-doped ZnO and N-codoped (Zn,Mn)O. The Curie temperature TC is evaluated by using the mean field approximation. We show also that higher values of TC are attained for high concentration of vacancy defects sites in (Zn,Mn)O and for small concentration of vacancy defects sites in (Zn,Mn)(O,N). Mechanism of exchange interaction between magnetic ions in Mn-doped ZnO and N-codoped(Zn,Mn)O with and without defects is also investigated. Finally, we propose a model which describes the origin of strong ferromagnetism stability observed in p-type ZnO. [ABSTRACT FROM AUTHOR]

We calculate the band structure of a two-dimensional GaAs/AlGaAs superlattice and estimate the ultimate efficiency of solar cells using this type of structure for solar energy conversion. The superlattice under consideration consists of gallium arsenide rods forming a square lattice and embedded in aluminum gallium arsenide. The ultimate efficiency is determined versus structural parameters including the filling fraction, the superlattice constant, the rod geometry, and the concentration of Al in the matrix material. The calculated efficiency of the superlattice proves to exceed the efficiency of each component material in the monolithic state in a wide range of parameter values. [ABSTRACT FROM AUTHOR]

Electronic structure calculations are carried out for investigating the formation of a stable atomic gold wire on the hydrogen terminated Si(001):3×1 [H–Si(001):3×1] surface. The H–Si(001):3×1 surface is patterned in various ways by removing desired hydrogen atoms from the surface. Gold atoms are adsorbed on the patterned H–Si(001):3×1 surface at various submonolayer coverage. However, zigzag continuous Au chains are found to be favorable on patterned H–Si(001):3×1 surfaces at a 4/9 monolayer (ML) coverage. The stabilities of continuous atomic gold chains on the patterned H–Si(001):3×1 surfaces are compared. In addition, the Au chains on the patterned H–Si(001):3×1 surfaces are compared with the existing results of the favorable Au chain structures on patterned H–Si(001):1×1 surfaces [Konar et al., Phys. Rev. B 77, 245411 (2008)], and it is found that a continuous Au chain on the H–Si(001):3×1 surface is energetically more favorable. Our calculations also reveal that beyond the 4/9 ML coverage, the additional Au atoms take part in the formation of Au clusters on the surface. However, beyond the 4/9 ML coverage, the gold structures on the surface are found to be less favorable compared to the chain structures at the 4/9 ML coverage. The continuous atomic gold chains supported by a substrate may be used as atomic scale devices, and therefore, experiments may be designed in this direction to produce a continuous atomic Au chain on the patterned H–Si(001):3×1 surface. [ABSTRACT FROM AUTHOR]

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise. [ABSTRACT FROM AUTHOR]

Thermal agitation induces the rotational motion of the two positively charged hydrogens around the oxygen nucleus in a water molecule, which forms an electric current with a short duration, generating a Lorentz force in a uniform static magnetic field. Since the center of gravity of a water molecule is approximately located at the oxygen nucleus, the Lorentz force has two components that rotate the hydrogens and push the oxygen nucleus. The component of the Lorentz force that pushes the oxygen nucleus parallel to the magnetic field has a strength that is half that of the component that pushes the oxygen nucleus perpendicular to the magnetic field. Since the Lorentz force causes water molecules to carry out a random walk, magnetic-field transfer, that is, a cumulative effect of random walks, is expected. The ratio of the diffusional coefficient for the magnetic-field transfer of D2O to that of H2O is 4:1 because the ratio of the interval between each random walk of D2O to that of H2O, which is also the ratio of the duration of one rotation, is 2:1. The diffusional displacement of the magnetic-field transfer is proportional to the magnetic field and reaches 0.62 Å/day for liquid water at 37 °C in a magnetic field of 2.0 T. Magnetic-field transfer is predicted to occur in polarized molecules with rotational freedom and restricted translational freedom. [ABSTRACT FROM AUTHOR]

A method is proposed to calculate the magnetic energy barrier of nanostructured cells of synthetic ferrimagnets. An important feature of the method is the use of an equation for the total energy that contains the magnetostatic fields at the saddle point as parameters of the energy equation. With no suitable methods of accessing the magnetic configuration at the unstable saddle point, it is difficult to obtain the saddle point magnetostatic fields. This difficulty is overcome with the use of equations that link the magnetostatic fields at the saddle point and critical fields, which are readily obtained by micromagnetic simulation. The present method is essentially based on the micromagnetic simulation, and, therefore, it should provide accurate results for the magnetic energy barrier. A contour diagram showing the thermal stability parameter is constructed as a function of the cell geometry and the thickness asymmetry, and the result should be of great value in designing magnetic cells for high density magnetic random access memory. [ABSTRACT FROM AUTHOR]

The dynamics of microcantilever beams can be strongly affected by immersion in fluid. While the importance of viscosity for devices of microscale dimensions is well established, the significance of fluid compressibility has not been investigated in detail. Here, we present a rigorous theoretical model for the frequency response of a rectangular cantilever beam that is executing normal and torsional oscillations, and is immersed in a compressible fluid. Both the viscous case and the inviscid limit are considered, and the model is valid for arbitrary mode number. We find that compressibility becomes increasingly important as the mode number rises. This is particularly relevant for gases, where compressibility is found to be important for high mode numbers of practical interest. [ABSTRACT FROM AUTHOR]

Measurements of the deflection induced by thermal noise have been performed on a rectangular atomic force microscope cantilever in air. The detection method, based on polarization interferometry, can achieve a resolution of [formula] in the frequency range 1–800 kHz. The focused beam from the interferometer probes the cantilever at different positions along its length, and the spatial modes’ shapes are determined up to the fourth resonance, without external excitation. Results are in good agreement with theoretically expected behavior. From this analysis accurate determination of the elastic constant of the cantilever is also achieved. [ABSTRACT FROM AUTHOR]

Density functional theory is used to calculate defect structures that can accommodate nonstoichiometry in hafnium nitride: HfN1-x, 0≤×≤0.25. It is predicted that a mechanism assuming simple distributions of nitrogen vacancies can accurately describe the variation in the experimentally observed lattice parameter with respect to the nitrogen nonstoichiometry. Although the lattice parameter changes are remarkably small across the whole nonstoichiometry range, the variations in the bulk modulus are much greater. [ABSTRACT FROM AUTHOR]

A theoretical model for equation of state (EOS) of aluminum with helium bubbles is presented. Based on this EOS, the influence of helium bubbles on shock loading is examined. The Hugoniot curve (temperature versus pressure as well as shock velocity versus particle velocity) for aluminum containing bubbles is calculated for various bubbles mass, bubbles percentage, and helium EOS models. The bubble mass and concentration seem to affect the measurably Hugoniot curve. The EOS model, implied for the helium in the bubbles, has minor significance, which means our model is not sensitive to the details of the helium EOS. Our findings are consistent with experiments available in the literature. [ABSTRACT FROM AUTHOR]

In the present paper, we extensively study the optical diffraction in two- and three-dimensional vector holograms and demonstrate the reconstruction of polarized optical images recorded in azobenzene-containing amorphous polymers (AP) and polymer-dissolved liquid-crystalline composites (PDLCC). The polarization states of the interference light are not modulated in the isotropic AP films, while modulated in the anisotropic PDLCC films. The information of the polarized optical image is recorded as the polarization induced anisotropy in the AP and PDLCC medium and is reconstructed as the polarized optical images. The theoretical consideration well explained the characteristics of the reconstructed polarized optical images from both two- and three-dimensional vector holograms. [ABSTRACT FROM AUTHOR]

The atomistic pseudopotential quantum mechanical calculations are used to study the transport in million atom nanosized metal-oxide-semiconductor field-effect transistors. In the charge self-consistent calculation, the quantum mechanical eigenstates of closed systems instead of scattering states of open systems are calculated. The question of how to use these eigenstates to simulate a nonequilibrium system, and how to calculate the electric currents, is addressed. Two methods to occupy the electron eigenstates to yield the charge density in a nonequilibrium condition are tested and compared. One is a partition method and another is a quasi-Fermi level method. Two methods are also used to evaluate the current: one uses the ballistic and tunneling current approximation, another uses the drift-diffusion method. [ABSTRACT FROM AUTHOR]

The AgInSbTe phase change thin films are very important as optical recording and the super-resolution mask materials in high density optical information storage. In this work, the effective nonlinear absorption coefficients of amorphous and crystalline AgInSbTe thin films were measured by the open-mode Z-scan method and no evidence of nonlinear refraction was found in the closed-mode Z-scan measurement. The effective nonlinear absorption coefficient βeff of amorphous AgInSbTe thin films is 7.53×10-3 m/W and the effective photon-absorption number n is 1.722; βeff of crystalline AgInSbTe thin films is 3.5×10-2 m/W, which is of an order lager than that of amorphous state, and the n value is 1.7011. The giant nonlinearity of AgInSbTe results from the free carrier absorption in the nanosecond time scale and this characteristic should be responsible for the mechanism of optical recording as well as the readout of super-resolution disk. [ABSTRACT FROM AUTHOR]

We show that admittance spectroscopy (AS) is a better technique than time of flight (TOF) to study the charge transport properties in dispersive materials. The hole transport properties of N,N′-diphenyl-N,N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′-diamine (NPB) doped with different traps were evaluated by AS and TOF techniques. It was found that both techniques can show clear signals for measuring the mobility of NPB doped with shallow traps. When NPB was doped with deep traps, the AS signals were still clear for mobility extraction. In sharp contrast, the TOF transients become featureless and the carrier transit time cannot be determined. The validity of AS in mobility determination was demonstrated by comparing the extracted AS to TOF mobilities. Generally, the hole mobilities extracted by these two techniques were in excellent agreement. In addition, we will demonstrate that AS can be employed to measure carrier dispersion. [ABSTRACT FROM AUTHOR]

In this paper we are presenting the full characterization of a supersonic flowing CO2/N2/Ar discharge at static pressures of 1–20 Torr and Mach number 2.15. In all aspects besides the flow speed and gas temperature, these conditions correspond to Martian entry plasma. Plasma parameters were determined by optical emission spectroscopy techniques. The gas and vibrational temperature were found from analysis of the rotational and vibrational spectra of the CO B 1Σ+-A 1Π Ångstrom system, respectively. The electron density was determined from the absolute intensity of the N2 C 3Πu-B 3Πg second positive system. A kinetic model for the discharge was developed to calculate the electron density and compared with experimental data. [ABSTRACT FROM AUTHOR]

Density functional theory is used to calculate defect structures that can accommodate nonstoichiometry in hafnium nitride: HfN1-x, 0≤×≤0.25. It is predicted that a mechanism assuming simple distributions of nitrogen vacancies can accurately describe the variation in the experimentally observed lattice parameter with respect to the nitrogen nonstoichiometry. Although the lattice parameter changes are remarkably small across the whole nonstoichiometry range, the variations in the bulk modulus are much greater. [ABSTRACT FROM AUTHOR]

Derivations for equations for calculating the potential and field strength in both single-crystal and two-crystal pyroelectric accelerators are presented. Such expressions for the single-crystal system are well established in the literature, but with cursory derivations. We provide a rigorous derivation of the single-crystal system and expand upon this physical understanding to derive expressions for the potential and field in a two-crystal system. The expressions are verified with finite element modeling and compared with experimental results. This allows for better understanding of pyroelectric accelerators. [ABSTRACT FROM AUTHOR]

In this paper, we conduct a quantitative study on the physical mechanism of electrons dynamics near the nanostructured metal film surfaces, as well as the efficiency of generated terahertz radiation associated with different types of nanostructures. The simulation results show that although the oscillating motion of emitted electrons outside the metal surface may affect the terahertz generation efficiency to some extent, this efficiency is predominantly determined by the electric field magnitude inside the metal film associated with nanostructure enhanced plasmonic excitations. Due to the field enhancement effect of the nanostructure, an appropriately designed nanostructured surface could greatly enhance the strength of generated terahertz signal via the increased nonlinear interactions between the light and the nanostructures. [ABSTRACT FROM AUTHOR]

We investigate the electron spin relaxation of n-type InAs quantum wires by numerically solving the fully microscopic kinetic spin Bloch equations with the relevant scattering explicitly included. We find that the quantum-wire size and the growth direction influence the spin relaxation time by modulating the spin-orbit coupling. Due to intersubband scattering in connection with the spin-orbit interaction, spin-relaxation in quantum wires can show different characteristics from those in bulk or quantum wells and can be effectively manipulated by various means. [ABSTRACT FROM AUTHOR]

Strain distributions in oxidized Si nanowires fabricated on a (001)-oriented silicon-on-insulator wafer have been determined by analyzing intensity profiles of the diffraction, caused by the nanowire periodicity, around the 111 Bragg point. In this analysis, theoretical diffraction curves, calculated by a kinematical treatment, are fitted to experimental ones, examining positions of the central and fringe maxima and their intensity ratios. Strains in oxidized samples are shown to be negative at the bottom surface and positive at the top surface of nanowires changing with depth in a concave way. The magnitudes of the strains at the bottom surface and at the top surface increase monotonically with increasing the oxidation time. The determined strain of a sample oxidized at 850 °C for 5 h is 0.50% at the top surface and -0.11% at the bottom surface. [ABSTRACT FROM AUTHOR]

We report the results of the optical spectra modeling for hydrogenated amorphous silicon (a-Si:H) thin films produced for photovoltaic cell applications, which allowed us to accurately determine the material’s optical gap (Eg). While for thick films of a-Si:H, as well as for other amorphous semiconductors, Eg is normally estimated from a so called Tauc plot, this is hardly possible for thin films because of the interference effects. We developed a physically founded semianalytical model for the complex dielectric function of a-Si:H, valid below and above the optical gap and containing a small number of adjustable meaningful parameters, including Eg and the characteristic energy scales of the optical transition matrix element distribution and the joint density of states in the absorption tail region. With this model and using the transfer matrix formalism for multilayer optics, we have achieved a good agreement between the calculated and experimental transmittance spectra, which allowed us to self-consistently determine the values of the above parameters. We found that both Eg and the characteristic scale of the subgap absorption tail increase with the hydrogen addition. We have also determined the absorption rate spectra of the films, relevant to any optical spectroscopy of subgap states, such as the photothermal deflection spectroscopy and constant photocurrent measurements. [ABSTRACT FROM AUTHOR]

The efficiency of a photovoltaic device is limited by the portion of solar energy that can be captured. We discuss how to measure the optical properties of the various layers in solid-state dye-sensitized solar cells (SDSC). We use spectroscopic ellipsometry to determine the complex refractive index of each of the various layers in a SDSC. Each of the ellipsometry fits is used to calculate a transmission spectrum that is compared to a measured transmission spectrum. The complexities of pore filling on the fitting of the ellipsometric data are discussed. Scanning electron microscopy and energy dispersive x-ray spectroscopy is shown to be an effective method for determining pore filling in SDSC layers. Accurate effective medium optical constants for each layer are presented and the material limits under which these optical constants can be used are discussed. [ABSTRACT FROM AUTHOR]

The influence of an antireflection coating on the input-output power characteristics and the switch-on dynamics of a vertical-external-cavity surface-emitting laser is investigated numerically with an effective rate-equation simulation based on microscopic many-body calculations for the optical gain/absorption and carrier losses. The simulations show a significantly higher threshold and a slower switch-on dynamics for the antireflection coated devices. These differences are caused by a reduction in the mode amplitude at the position of the active medium due to a less pronounced subcavity. [ABSTRACT FROM AUTHOR]

Using terahertz time-domain spectroscopy, we measured the complex conductivity and dielectric function of n-type GaN with various carrier concentrations on sapphire substrate. The measured complex conductivity, which is due to the free carriers, is well fitted by simple Drude model. The contribution from the lattice vibration to the complex dielectric function increases with the decrease in free carrier concentration. A better fitting of the frequency-dependent complex dielectric response was obtained by considering both of the Drude and the classical damped oscillator model. [ABSTRACT FROM AUTHOR]

This paper reviews the concept of high-resolution imaging of a negative index material (NIM) slab and compares its performance with the structure made based on the idea of coupled surface-modes layers. Fourier spectrum theoretical model and finite difference time domain numerical approach are applied to comprehensively characterize the structures and demonstrate the characteristics. It is highlighted that if the loss is small, a NIM slab can provide a better performance at a farther distance than the layered structure with the same thickness. However, considering a realistic design with relatively large loss, the later will offer a more promising performance to the loss and the image can be reconstructed in a farther distance from the object cascading more number of thin layers. [ABSTRACT FROM AUTHOR]

Rate-limiting step, as well as self-limited oxidation of SiGe alloys is so far under controversy. Contrasting to the monoparabolic growth mode for oxidation of Si, a parabolic growth mode and self-limited oxidation of SiGe alloys at different temperature are clearly observed depending on the oxidation time. With modified Deal–Grove model, we extract the parabolic rate constants related to the oxygen diffusion at different temperature and the activation energy of oxygen diffusivity finding that oxygen diffusion is still the rate-limiting step. We attribute this oxidation behavior to the strain effects associated with the volume change in converting Si/SiGe to SiO2/mixed oxide at different oxidation stages. [ABSTRACT FROM AUTHOR]

Molecular dynamics is employed to investigate the collective motion of carbon atoms of a (10,10) single walled carbon nanotube under tensile loading. The “slip vector” parameter is used to study the collective motion of a group of atoms and deformation behavior in three different directions (axial, tangential, and radial directions). According to the variations in the slip vectors in different directions and the stress-strain profile, the axial tensile process can be divided into three stages. In tensile loading process, two local deformation types (symmetrical and nonsymmetrical deformations) are observed before reaching the maximum stress. The variations in the slip vector values of each atom indicate a symmetrical rupture which is oriented 45° to the axis of the nanotube after reaching the maximum stress. By tensile loading, chainlike structures can be formed before the breaking of the nanotube. [ABSTRACT FROM AUTHOR]

The equation describing the capacitance of capacitors is determined. It is shown that by optimizing the material of the conducting electrodes, the capacitance of capacitors reaching the quantum regime can be substantially enhanced or reduced. Dielectric capacitors with negative total capacitances are suggested and their properties analyzed. Resulting perspectives to enhance the performance of electronic devices are discussed. [ABSTRACT FROM AUTHOR]

Current-voltage characteristics were measured, electrically modeled, and calculated for gate oxides, which contain nanocrystals (NCs) in different distributions, sizes, and densities. Ge and Si NCs were synthesized embedded in separate thin SiO2 layers by ion implantation at different fluences and subsequent annealing. It was found that the currents through the NC containing thin gate oxides are strongly related to the NCs’ location and are not driven by ion implantation induced oxide defects. Charging of the NCs determines the internal electrical fields, which is confirmed by simultaneous current and capacitance measurements. Depending on the implanted fluence the Ge NCs were mainly detected in the oxide center or close to the Si/SiO2 interface. The Si NCs were fabricated in the oxide center sandwiched between two oxide regions denuded of NCs. The processes of Si NC formation, growth and dissolution are discussed by means of kinetic lattice Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

Joule heating in single-walled carbon nanotubes (CNTs) using a quantum mechanical approach is presented in this paper. The modeling is based on the energy transfer between the electrons and both acoustic and optical phonons. In this formulation, only the knowledge of the full energy dispersion relation, phonon dispersion relation, and the electron-phonon coupling potential is required for the calculations. For verification of the proposed model, the current-voltage relation for extremely long nanotubes is calculated and the results are compared with the experimental data. The electric field dependence of the amount of energy generated by Joule heating is plotted. Moreover the effect of the thermal environment on the behavior of Joule heating is studied. The formulation proposed in this paper can also be used for structures other than CNTs. Computations indicate that, contrary to popular opinion, metallic CNT does not follow Joule’s law of P=IV. Joule heating in CNT is significantly less than what is predicted with Joule law (P=IV), which would make it a perfect candidate to replace copper as interconnect material in electronics. [ABSTRACT FROM AUTHOR]

An approach to develop room temperature detectors is to use transitions between the light/heavy hole bands and the split-off hole band to produce enhanced response at high temperature. Results are presented on a theoretical model to predict the response in these split-off detectors. The model calculates the dark and illuminated currents from the photoabsorption, carrier escape, and transport, explaining the experimental response. The variation in dark current, responsivity, and D* with the detector parameters is presented. [ABSTRACT FROM AUTHOR]

In this work we studied the structural and optical properties of lithium tantalate (LiTaO3) powders doped with Eu3+ ions. We have examined the different sites occupied by the rare earth ion through the correlation of the DRX data analyzed with the Rietveld method and some spectroscopic parameters derived from the Eu3+ luminescence. A direct relation was established between the lattice parameters and the “occupation fraction” of Eu3+ in each LiTaO3 site. The occupation fraction was set as the relative population of Eu3+ ions for each site obtained by means of the intensity, baricenter, and the spontaneous emission coefficients of the 5D0→7F0 transitions. We concluded that the unit cell parameter a presents the same behavior of the Eu3+ occupation fraction in Ta5+ sites as a function of the Eu3+ content in LiTaO3. The same was observed for the variation in Eu3+ occupation fraction in the Li+ site and the unit cell parameter c with the Eu3+ content. [ABSTRACT FROM AUTHOR]

The effect of plasmons on the bound photon modes has been studied in metallic photonic quantum nanowires. Nanowires are made from a metallic photonic crystal and a dielectric photonic crystal where the former is embedded into the latter. Photons are localized within the metallic crystal and are reflected from the dielectric crystal. The nanowire is doped with nanoparticles, which are interacting with the localized photons. It is found that number of bound photons states within the wire depends on the plasmon frequency. The bound photon energy decreases as the plasmon energy increases. Numerical simulations of the absorption coefficient predict that the position of a transparent state can be switched by changing the frequency of plasmons. The present study can be used to make new types of optoelectronic devices. [ABSTRACT FROM AUTHOR]

Z1/2 defect centers were produced by irradiating 4H-SiC bulk samples with 1 MeV electrons at room temperature. The emission rate dependence on the electric field in the depletion region was measured using deep level transient spectroscopy and double-correlation deep level transient spectroscopy. It is found that the Z1/2 defect level shows a strong electric field dependence with activation energy decreasing from Ec-0.72 eV at zero field to Ec-0.47 eV at 6.91×105 V/cm. The phonon assisted tunneling model of Karpus and Perel [Sov. Phys. JETP 64, 1376 (1986)] completely describes the experimental data. This model describes the dependence of the emission rate on electric field F as en(F)=eno exp(F2/Fc2), where Fc is the characteristic field that depends on the phonon assisted tunneling time τ2. The values of Fc and τ2 were determined and the analysis of the data leads to the suggestion that Z1/2 may be a substitutional point defect. [ABSTRACT FROM AUTHOR]

This paper deals with the formation of nanoparticles in a pulsed discharge. Experiments are performed in a capacitively coupled discharge operated in a mixture of argon and acetylene. The paper focuses especially on the influence of the pulse frequency on the dust formation. The experiments reveal the existence of a rather narrow frequency band that separates a frequency region with no dust formation from a frequency region where dust formation occurs. The decisive point in the observations is that a small change in the pulse frequency (from 700 to 725 Hz) is enough to induce or, respectively, suppress the formation of dust particles. The experimental results are discussed by means of a simple model that allows one to calculate the density of negative ions (C2H-, C4H-, etc.) as a function of the pulse frequency. [ABSTRACT FROM AUTHOR]

Various theoretical formulations are proposed for investigating the carrier transport in nanoscale electronic devices. In this paper, a discrete formulation of the Wigner transport equation (WTE) for the self-consistent simulation of phase-coherent quantum transport in silicon nanowire metal-oxide-semiconductor field-effect transistor (MOSFET) devices is presented. The device is simulated using an effective-mass Hamiltonian within the mode-space approximation. The numerical scheme proposed in this work solves self-consistently three dimensional Poisson’s equation, two dimensional Schrödinger’s equation in each cross-sectional plane of the nanowire, and the steady-state one dimensional WTE for each conduction mode to handle the quantum transport along the channel. Details on numerical implementation of the Wigner function method are given, and the results are compared with those of the nonequilibrium Green’s function (NEGF) method in the ballistic limit. The calculations of current-voltage electrical characteristics of surround-gated silicon nanowires are performed using both NEGF and WTE formulations. The good agreement observed between these approaches means that a direct solution of the WTE is an accurate simulation method for modeling the ballistic quantum transport in silicon nanowire MOSFETs. [ABSTRACT FROM AUTHOR]

Different screening models in quantum cascade lasers are compared by calculating the contribution of intrasubband impurity scattering to the optical linewidth as a function of temperature. We find a strong impact of impurity scattering, which increases substantially with temperature. A simple isotropic bulk screening model works well if the screening length is of the order of or longer than the period length of the cascade structure. [ABSTRACT FROM AUTHOR]

Free vibrations of zigzag single-wall carbon nanotubes (SWCNTs) of aspect ratio (length/diameter) ∼6 and with ends traction-free have been studied using molecular mechanics (MM) simulations with the MM3 potential. It is found that the frequencies of inextensional (the Love and the Rayleigh) modes of an (n, 0) SWCNT saturate at the circumferential wave number of either (n-1)/2 or n/2 where n is odd or even. This is explained in terms of its molecular structure. Since the frequencies of the inextensional modes of vibration of a thin cylinder made of an isotropic linear elastic material do not saturate with an increase in the circumferential wave number, a continuum structure cannot represent all modes of vibration of a zigzag SWCNT. This result is independent of the value assigned to the wall thickness of the SWCNT. We have also found values of material and geometric parameters of a shell and a hollow cylinder by equating their frequencies of the inextensional, the radial breathing, the axial and the torsional modes of vibrations to the corresponding ones of a zigzag SWCNT, and by taking their mean diameter and length equal to those of the SWCNT. The frequencies of the extensional modes of oscillations of the two continuum structures for various axial half wave numbers and circumferential wave numbers are found to match well with those of the SWCNT obtained from the MM simulations. However, the frequencies of the inextensional modes of the continuum structures deviate noticeably from those of the SWCNT, and this deviation increases with an increase in the circumferential wave number. [ABSTRACT FROM AUTHOR]

We present a simple structure of multilayered film that possesses left-handedness-like and negative refraction at infrared frequencies. It periodically consists of antiferromagnetic (AF) and ionic-crystal (IC) layers, where IC material has a transverse optical phonon frequency near and below the AF resonant frequency. We calculated the angle between the energy flow and wave vector of a plane wave in the film and its refraction angle, and found an analytical condition under which both left-handedness and negative refraction appear in the multilayered film, and the negative refraction phenomenon disappears when AF layers are thinner than IC. Numerical simulation results verify these analytical findings. [ABSTRACT FROM AUTHOR]

We study the enhanced transmission through compound subwavelength rectangular hole arrays in perfect electronically conductive films. It is shown that phase effects become important in the enhanced transmission when extra rectangular holes are added per period. There are two types of resonance mechanisms due to the phase effects: one is the localized waveguide mode phase resonance and the other is the surface mode phase resonance. The localized waveguide mode phase resonances, which are related to the number of holes per period and their relative positions, split the waveguide resonance peak into several peaks. The surface mode phase resonances, which appear evidently only for some special arrangements of holes in a unit cell, lead to the suppression of some surface mode resonance peaks. Our approach is advantageous to modulate the enhanced transmission in metallic hole arrays in the terahertz or microwave frequency regime. [ABSTRACT FROM AUTHOR]

Increasing requirements concerning the powder quality for fine-grained ferroelectric ceramics or for low temperature coating technologies provoke further improvements in particle characterization methods. So far, a correct quantitative measurement of the ferroelectric properties for as synthesized particles is lacking and only already sintered ceramics can be measured precisely. We report on a measurement setup to characterize films made of as synthesized ferroelectric particles. The chosen measurement setup enables the definite determination of ferroelectricity in such nondensified particle-based structures. A centrifugally casted BaTiO3 particulate film with 50% porosity is used for characterization. The correct value of the remanent polarization for this porous film obtained by the new method is almost half the value estimated by conventional measurement setups. Compared to a sintered BaTiO3 ceramic the achieved remanent polarization is much lower (10%). At the same time a higher maximum field has to be applied to the sample as lower fields act on the individual particles because of the porosity of this film. [ABSTRACT FROM AUTHOR]

It was found that in twisted nematic liquid crystal displays (TN-LCDs) in which the chiral direction of the liquid crystalline (LC) material is opposite to the twist direction determined by the rubbing directions, which are referred to as a reverse twisted nematic LCDs, LC molecules tilt largely without the application of an electric field. Reducing the pitch length of LC materials increases the polar angles of the LC molecules. This phenomenon can be explained by assuming that the stress generated by the helical power being in the opposite direction to the twist direction can be released by increasing the polar angle. The resultant molecular arrangement resembles that of TN-LCDs to which a voltage is applied. By using this LC molecular arrangement, a drastic reduction in the driving voltage of TN-LCDs could be realized. We believe that this method is capable of reducing the driving voltage of TN-LCDs more than other methods. [ABSTRACT FROM AUTHOR]

The discharge characteristics of He–Ne–Xe gas mixture in the plasma display panel were investigated using a two-dimensional numerical simulation to understand the effects of adding He and varying the Xe contents in the gas mixture, and also varying sustain electrode gap. With 5% Xe content and 60 μm sustain electrode gap, decreased ionization led to the improvement of the vacuum ultraviolet (vuv) efficacy at increasing He mixing ratios. However, at 20% Xe content and 60 μm sustain electrode gap, increased electron heating improved the vuv efficacy until the He mixing ratio reached 0.7, but the efficacy decreased beyond the ratio of 0.7 due to the increased ionization of Xe atoms. At 5% Xe content and 200 μm sustain electrode gap, the vuv efficacy increased as a result of increased electron heating at the gap space at increasing He mixing ratios. [ABSTRACT FROM AUTHOR]