Showing papers in "Physica E-low-dimensional Systems & Nanostructures in 2012"

Ultrafast lasers mode-locked by nanotubes and graphene

Abstract: Ultrafast lasers play an increasingly important role in many applications. Nanotubes and graphene have emerged as promising novel saturable absorbers for passive mode-locking. Here, we review recent progress on the exploitation of these two carbon nanomaterials in ultrafast photonics.

Quantum oscillations in a topological insulator Bi 2 Te 2 Se with large bulk resistivity (6 Ω cm)

Abstract: We report the observation of prominent Shubnikov–de Haas oscillations in a Topological Insulator, Bi2Te2Se, with large bulk resistivity ( 6 Ω cm at 4 K). By fitting the SdH oscillations, we infer a large metallicity parameter k F l =41, with a surface mobility ( μ s ∼ 2800 cm 2 / V s ) much larger than the bulk mobility ( μ b ∼ 50 cm 2 / V s ). The plot of the index fields B ν vs. filling factor ν shows a 1 2 - shift , consistent with massless, Dirac states.

Transport in three-dimensional topological insulators: Theory and experiment

Abstract: This paper reviews recent theoretical and experimental work on transport due to the surface states of three-dimensional topological insulators. The theoretical focus is on longitudinal transport in the presence of an electric field, including Boltzmann transport, quantum corrections and weak localization, as well as longitudinal and Hall transport in the presence of both electric and magnetic fields and/or magnetizations. Special attention is paid to transport at finite doping, and to the pi-Berry phase, which leads to the absence of backscattering, Klein tunneling and half-quantized Hall response. Signatures of surface states in ordinary transport and magnetotransport are identified. The review also covers transport experiments of the past years, tracing its evolution from the initial obscuring of surface transport by bulk transport to the increasing success of experimental work in identifying transport due to the surface states. Current and likely future experimental challenges are given prominence and the present status of the field is assessed. (C) 2011 Elsevier B.V. All rights reserved.

Investigation of tensile response and thermal conductivity of boron-nitride nanosheets using molecular dynamics simulations

Abstract: In this paper, we employed classical molecular dynamics simulations using the Tersoff potential for the evaluation of thermal conductivity and tensile response of single-layer boron-nitride sheets (SBNS). By carrying out uniaxial tension simulations, the elastic moduli of SBNS structures are predicted to be close to those of boron-nitride nanotubes in a range between 0.8 and 0.85 TPa for different chirality directions. Performing non-equilibrium molecular dynamics simulations, the thermal conductivity of SBNS is predicted to be around 80 W/m-K, which is shown to be independent of chirality directions.

Facile one-pot synthesis of gold nanoparticles using tannic acid and its application in catalysis

Abstract: The paper reports a simple and efficient method for the synthesis of stable, nearly spherical gold nanoparticles using tannic acid as both the reducing and stabilizing agent. The nanoparticles are characterized by UV–visible spectroscopy, transmission electron microscopy (TEM), EDX and X-ray diffraction (XRD) analysis. The influence of tannic acid on the control of size and shape of gold nanoparticles is reported. Upon an increase in the concentration of tannic acid, there is a shift in the shape of nanoparticles as evidenced by the change in bandwidth and peak position of the surface plasmon resonance (SPR) band. Also, it is found that tannic acid ceases to act as a reducing agent beyond the limit of 10 mL (6×10−3 M) for 30 mL of HAuCl4 (1.3×10−3 M). On increasing the quantity of tannic acid, nucleation is favored in the initial stages and thereafter growth supersedes nucleation. The stable colloids obtained by this method are found to consist of nanoparticles with average size 8 and 12 nm. The crystallinity of the sample with fcc phase is observed from TEM, SAED and XRD pattern. Involvement of carboxylic acid group in capping of gold nanoparticles is evident from the FTIR spectrum. The application of the synthesized nanoparticles as catalyst in the reduction of 4-Nitrophenol to 4-Aminophenol is also reported.

Atomistic finite element model for axial buckling and vibration analysis of single-layered graphene sheets

Abstract: In this article, an atomistic model is developed to study the buckling and vibration characteristics of single-layered graphene sheets (SLGSs). By treating SLGSs as space–frame structures, in which the discrete nature of graphene sheets is preserved, they are modeled using three-dimensional elastic beam elements for the bonds. The elastic moduli of the beam elements are determined via a linkage between molecular mechanics and structural mechanics. Based on this model, the critical compressive forces and fundamental natural frequencies of single-layered graphene sheets with different boundary conditions and geometries are obtained and then compared. It is indicated that the compressive buckling force decreases when the graphene sheet aspect ratio increases. At low aspect ratios, the increase of aspect ratios will result in a significant decrease in the critical buckling load. It is also indicated that increasing aspect ratio at a given side length results in the convergence of buckling envelops associated with armchair and zigzag graphene sheets. The influence of boundary conditions will be studied for different geometries. It will be shown that the influence of boundary conditions is not significant for sufficiently large SLGSs.

A first-principles study of H2S adsorption and dissociation on the AlN nanotube

Abstract: Adsorption of hydrogen sulfide (H2S) onto AlN nanotube surface was investigated using density functional theory. It was found that the molecule is either physically adsorbed on the surface of the tube or chemically dissociated into –H or –SH fragments. The physical adsorptions are barrierless, while the dissociations have to overcome small activation energies of 2.2–3.1 kcal/mol, suggesting that the tube might be a potential catalyst for dehydrogenation of the H2S molecule. All the adsorptions are electronically harmless processes and have negligible effects on the electronic properties especially on the HOMO/LUMO energy gap of the AlN nanotube.

Can aluminum nitride nanotubes detect the toxic NH3 molecules

Abstract: The sensitivity of aluminum nitride nanotubes (AlNNTs) to NH3 molecules was investigated using DFT calculations. It was found that NH3 molecule cannot be detected by pristine AlNNTs, since it cannot change the HOMO–LUMO energy gap (Eg) of the tube upon adsorption process. Our results demonstrated that doping an oxygen atom in the vicinity of adsorption site makes the AlNNT electrical conductivity strongly sensitive to the NH3 molecule. It suggests that O-doped AlNNT would be a potential candidate for the NH3 molecule detection. The present results provide guidance to experimental scientists in developing nanotube-based chemical sensors.

Small-scale effect on the vibration of non-uniform carbon nanotubes conveying fluid and embedded in viscoelastic medium

Abstract: Vibration characteristics of non-uniform single-walled carbon nanotubes (SWCNTs) conveying fluid embedded in viscoelastic medium are investigated using nonlocal Euler–Bernoulli beam theory. The governing differential equations are solved with the finite element method and the frequencies are obtained by solving a quadratic eigenvalue problem. The effects of taper ratio, small-scale parameter and viscoelastic medium on resonant frequencies and critical steady flow velocity are discussed. It is shown that by increasing the taper ratio, the critical flow velocity decreases and the combined mode observed for uniform SWCNTs in the previous works does not occur when the taper ratio is non-zero.

Bending and buckling of nanowires including the effects of surface stress and nonlocal elasticity

Abstract: This paper presents the bending deformation due to uniformly distributed load and buckling load of nanowires with various boundary conditions including surface stress and nonlocal elasticity effects. The analytical solutions for static displacement and buckling load of nanowires are derived and compared with those of numerical results obtained by the finite element method. The results indicate that the influences of surface stress and nonlocal elasticity affects directly both the displacement and buckling load. The effect of surface stress is to increase the stiffness of nanowires. Therefore, the nanowires for all boundary conditions show stiffer behavior in comparison with classical Euler beam. The effect of nonlocal elasticity can result in either stiffer or softer behaviors of nanowires depending on the boundary conditions. In case of buckling loads, the surface stress increases the buckling load significantly while the nonlocal elasticity is affected slightly. For nanowires including both effects, the results show that the buckling load is between one of the nanowires with surface stress and the one with nonlocal elasticity.

Tunable band gaps of mono-layer hexagonal BNC heterostructures

Abstract: We present an ab initio density functional theory (DFT)-based study of h-BN domain size effect on band gap of mono-layer h-BNC heterostructure modeled as (B3N3)x(C6)1� x. The atomic structures, electronic band structures, density of states and electron localization functions of h-BNC are examined as h-BN concentration ranged from 0% to 100%. We report that the electronic band gap energy of h-BNC can be continuously tuned in full range between that of two phases, graphene and h-BN, as a function of h-BN concentration. The origin of the tunable band gap in these heterostructures are due to the change in the electron localization with h-BN concentration.

Nonlinear optical properties of a Pöschl–Teller quantum well under electric and magnetic fields

Abstract: The nonlinear optical properties of a Poschl–Teller Quantum well (PTQW) under electric and magnetic fields are studied. The salient feature of this potential is its flexibility. It can be made asymmetrical by a proper choice of its two parameters. Optical rectification, second and third-harmonic generation susceptibilities are calculated using the density matrix formalism. We study the effects of quantum confinement, electric and magnetic fields on all of these optical coefficients.

Theory of the nonlinear optical frequency mixing effect in graphene

Abstract: A quantum theory of the frequency mixing effect in graphene is developed. A graphene layer is assumed to be irradiated by two monochromatic waves with the frequencies ω 1 and ω 2 and the nonlinear third order electromagnetic response at the mixed frequency ω e = 2 ω 1 − ω 2 is calculated. The corresponding third order nonlinear conductivity of graphene is calculated as a function of frequency, charge carrier density, as well as the intensity and the polarization of the incident waves. The intensity of the nonlinear optical signal is also calculated as a function of the number N of graphene layers in a multi-layered system and it is shown that the best performance of the graphene/graphite based nonlinear optical mixer or frequency multiplier can be achieved in thin graphite layers with N ≃ 25 − 30 .

Synthesis and characterization of single-crystal CdS nanosheet for high-speed photodetection

Abstract: One-dimensional nanostructures have several unique advantages over bulk material and thin films, which can be exploited for high-speed photodetection. Furthermore, as bulk CdS has a high photosensitivity and quantum efficiency, there is considerable potential for the use of CdS nanostructures in advanced devices. In this study, single-crystal CdS nanosheets were grown by thermal evaporation and fully characterized to determine their potential for application in high-speed photodetectors. A high-quality nanosheet was confirmed to have a smooth surface with no extraneous particles and a strong orientation to the (110) plane of the wurtzite (hexagonal) phase of CdS. The Cd/S ratio was found to be nearly stoichiometric at 1.09. Photoluminescence measurement of a single-crystal CdS nanosheet showed a high emission intensity at a wavelength of 493 nm. The current–voltage characteristics of the CdS nanosheet on Al thin film indicated an Ohmic contact in dark and under illumination by ambient, 365-nm, 405-nm, and 460-nm light. The light responsivity showed a peak at 460 nm. Under 365-nm, 405-nm, and 460-nm chopped light, at a bias voltage of 1, 3, and 5 V, the photocurrent rise and decay times were investigated. The device showed faster response times for 460-nm light. This fast response was attributed to the high quality of the single crystal, the absence of defect states, and the high surface/volume ratio. The device showed a high quantum efficiency of 22.3×10 3 % when it was illuminated by 365-nm light under a bias of 5 V; this efficiency increased to 36.3×10 3 % and 40.5×10 3 % when the device was illuminated by 405-nm and 460-nm light, respectively.

Benincasa hispida seed mediated green synthesis of gold nanoparticles and its optical nonlinearity

Abstract: The paper reports the biosynthesis of stable and nearly spherical gold nanoparticles using the extract of Benincasa hispida seeds as reducing and capping agents. The particle size could be easily tuned by the reaction conditions including quantity of extract, temperature and pH. Gold nanoparticles having different sizes in the range from 10 to 30 nm could be obtained by controlling the synthesis parameters. The nanoparticles were characterized by UV–vis spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD) and FTIR analysis. The high crystallinity of nanoparticles is evident from clear lattice fringes in the HRTEM images, bright circular spots in the SAED pattern and peaks in the XRD pattern. FTIR spectrum indicates the presence of different functional groups present in the biomolecule capping the nanoparticles. The possible mechanism leading to the formation of gold nanoparticles is suggested. The optical nonlinearity of the samples is studied by open aperture Z-scan technique. The as-prepared gold nanoparticles shows good optical limiting behavior and the two-photon absorption coefficient is found to decrease with increase in particle size.

A first principle study of interband transitions and electron energy loss in mono and bilayer graphene: Effect of external electric field

Abstract: We present comparative electronic and dielectric response of mono and bilayer graphene using the first principle pseudopotential and localized basis set based approach. It is found that interband transitions show negligible change in bilayer graphene with respect to the monolayer graphene except π → σ ⁎ transitions for which 1.7 eV blue shift have been found. EEL spectra show variable blue shift ranging between 0.1 eV and 7.9 eV for bilayer graphene with respect to the monolayer graphene for both in-plane and out-of-plane polarization. Electronic band structure and dielectric response in the infrared region up to 1 eV of bilayer graphene on the application of externally applied electric field between 0.5 V/nm to 3.0 V/nm have also been investigated. We have obtained the induced band gap that is in good agreement with the experiment [4] when the applied electric field is less than 2.0 V/nm. Plasmonic excitations have been found to vanish on increasing the intensity of external electric field. Structures in imaginary part of dielectric function ( ϵ 2 ) at different electric fields are found to be consistent with the induced band gap and are most likely due to excitonic transition.

Transverse wave propagation in elastically confined single-walled carbon nanotubes subjected to longitudinal magnetic fields using nonlocal elasticity models

Abstract: Lateral wave propagation in an elastically confined single-walled carbon nanotube (SWCNT) experiences a longitudinal magnetic field is examined using nonlocal Rayleigh, Timoshenko, and higher-order beam theories. The SWCNT is modeled via an equivalent continuum structure (ECS) and its interaction with the surrounding elastic medium is simulated via lateral and rotational continuous springs along its length. For the proposed models, the dimensionless governing equations describing transverse vibration of the SWCNT are constructed. Assuming harmonic solutions for the propagated sound waves, the dispersion equation associated with each model is obtained. Subsequently, the explicit expressions of the frequencies as well as the corresponding phase and group velocities, called characteristics of the waves, are derived for the proposed models. The influences of the slenderness ratio, the mean radius of the ECS, the small-scale parameter, the longitudinal magnetic field, the lateral and rotational stiffness of the surrounding matrix on the characteristics of flexural and shear waves are explored and discussed.

Large displacement of a static bending nanowire with surface effects

Abstract: Nanowires are widely used as building blocks of micro/nano devices, such as micro-sensors, probes, transistors and actuators in micro/nano-electro-mechanical systems (M/NEMS) and biotechnology. In this study, we investigated the large deformation behavior of a nanowire in consideration of its surface effects (surface elasticity and residual surface stress). For nanowires of large displacements with different boundary conditions, we established the governing equation set in combination with the residual surface stress and surface elasticity. Then a computer program of shooting method by using the commercial software MathCAD was developed to solve the boundary value problem numerically. Furthermore, the influences of surface effects on the large and infinitesimal deformation of the nanowires were quantitatively compared. These findings are beneficial to understanding the mechanism of the surface effects, and can also provide some inspirations to characterize the mechanical properties of nano-materials, and engineer new micro/nano-scaled devices.

Nonlocal Timoshenko beam theory for vibration of carbon nanotube-based biosensor

Abstract: This article studies vibration of carbon nanotube (CNT)-based biosensor A CNT-based biosensor is modeled as a nonlocal Timoshenko beam made of multiwall CNT carrying a spherical nanoscale bio-object at the free end, and the influence of the rotary inertia of the bio-object itself is considered The fundamental frequencies are computed via the transfer function method The effects of the attached spherical bio-object's rotary inertia and mass, the length-to-diameter of the CNT on the natural frequencies are discussed If the nonlocal parameter is neglected, the frequencies for four possible cases are compared Obtained results show that the rotary inertia decreases the fundamental frequency, while an increase in the diameter of the attached bio-object reduces the natural frequency, but causes frequency shift to rise The mass sensitivity of biosensor can be improved for short CNTs used The rotary inertia of the attached bio-object has a strong effect on the natural frequencies and cannot be simply neglected The nonlocal Timoshenko beam model is more adequate than the nonlocal Euler-Bernoulli beam model for short CNT biosensors Obtained results are helpful to the design of micro-cantilevered resonator as atomic-resolution mass sensor or biosensor

Effects of nonlocal elasticity and Knudsen number on fluid–structure interaction in carbon nanotube conveying fluid

Abstract: In this paper, we investigate the effect of nano-size of both fluid flow and elastic structure simultaneously on the vibrational behavior of a pinned–pinned and a clamped–clamped nanotube conveying fluid, using both Knudsen number (Kn) and nonlocal continuum theory. Euler–Bernoulli plug flow (EBPF) theory is used for modeling fluid–structure interaction (FSI). It is observed that nonlocal parameter has more effect than Kn on the reduction of critical velocities of a liquid nano-flow. This effect has considerable impact on the reduction of critical velocities for a clamped–clamped beam in comparison with a pinned–pinned one. We concluded that the dimensionless nonlocal parameter, had more impressive effect on the dimensionless critical flow velocity of the second mode divergence and coupled mode flutter instabilities. However, in a gas nano-flow, the situation is totally different and Kn causes more reduction in critical velocities. Furthermore, it is emphasized that ignoring nano-size effects on liquid and gas nano-flow might cause non-conservative design of nano-devices.

Electro-thermo-mechanical nonlinear nonlocal vibration and instability of embedded micro-tube reinforced by BNNT, conveying fluid

Abstract: Nonlinear vibration and stability of a smart composite micro-tube made of Poly-vinylidene fluoride (PVDF) reinforced by Boron-Nitride nanotubes (BNNTs) embedded in an elastic medium under electro-thermal loadings is investigated. The BNNTs are considered to be long straight fibers and the composite used in this study is in the category of piezoelectric fiber reinforced composites (PEFRC). The micro-tube is conveying a fully developed isentropic, incompressible and irrotational fluid flow. The smart micro-tube is modeled as a thin shell based on the nonlinear Donnell's shell theory. Effects of mean flow velocity, fluid viscosity, elastic medium modulus, temperature change, imposed electric potential, small scale, aspect ratio, volume percent and orientation angle of the BNNTs on the vibration behavior of the micro-tube are taken into account. The results indicate that increasing mean flow velocity considerably increases the nonlinearity effects so that small scale and temperature change effects become negligible. It has also been found that stability of the system is strongly dependent on the imposed electric potential and the volume percent of BNNTs reinforcement. The system studied in this article can be used as sensor and actuator in the sensitive applications.

Thermal vibration analysis of orthotropic nanoplates based on nonlocal continuum mechanics

Abstract: This paper presents the thermal vibration analysis of orthotropic nanoplates such as graphene, using the two variable refined plate theory and nonlocal continuum mechanics for small scale effects. The nanoplate is modeled based on two variable refined plate theory and the axial stress caused by the thermal effects is also considered. The two variable refined plate theory takes account of transverse shear effects and parabolic distribution of the transverse shear strains through the thickness of the plate, hence it is unnecessary to use shear correction factors. Nonlocal governing equations of motion for the nanoplate are derived from the principle of virtual displacements. The closed form solution for thermal-vibration frequencies of a simply supported rectangular nanoplate has been obtained by using Navier's method of solution. Numerical results obtained by the present theory are compared with available solutions in the literature and the molecular dynamics results. The influences of the small scale coefficient, the room or low temperature, the high temparature, the half wave number and the aspect ratio of nanoplate on the natural frequencies are considered and discussed in detail. It can be concluded that the present theory, which does not require shear correction factor, is not only simple but also comparable to the first-order and higher order shear deformation theory. The present analysis results can be used for the design of the next generation of nanodevices that make use of the thermal vibration properties of the nanoplates.

Zeptogram sensing from gigahertz vibration: Graphene based nanosensor

Abstract: We develop the mathematical framework for using single layer graphene sheet as nanoscale label-free mass sensors. Graphene resonators are assumed to be in the cantilevered configuration. Four types of mass loadings are considered and closed-form equations are derived for the frequency shift due to the added mass. Using the potential and kinetic energy of the mass loaded graphene sheets, generalised non-dimensional calibration constants are proposed for an explicit relationship between the added mass and the frequency shift. These equations in turn are used for sensing the added mass. Numerical results illustrate that the sensitivity of graphene sensors is in the order of gigahertz/zeptogram. We show that the performance of the sensor depends on the spatial distribution of the attached mass on the graphene sheet.

Synthesis, structural and optical properties of nanocrystalline vanadium doped zinc oxide aerogel

Abstract: We report the synthesis of vanadium-doped ZnO nanoparticles prepared by a sol–gel processing technique. In our approach, the water for hydrolysis was slowly released by esterification reaction followed by a supercritical drying in ethyl alcohol. Vanadium doping concentration of 10 at% has been investigated. After treatment in air at different temperatures, the obtained nanopowder was characterized by various techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and photoluminescence (PL). Analysis by scanning electron microscopy at high resolution shows that the grain size increases with increasing temperature. Thus, in the case of thermal treatment at 500 °C in air, the powder with an average particle size of 25 nm shows a strong luminescence band in the visible range. The intensity and energy position of the obtained PL band depends on the temperature measurement increase. The mechanism of this emission band is discussed.

Green synthesis of gold nanoparticles using gum polysaccharide of Cochlospermum religiosum (katira gum) and study of catalytic activity

Abstract: A green synthesis of gold nanoparticles (Au NPs) using aqueous solution of a hetero-polysaccharide, extracted from the gum of Cochlospermum religiosum (katira gum), has been demonstrated in this work. The hetero-polysaccharide plays the role of both reducing and stabilizing agent. The synthesized Au NPs were characterized by UV–vis spectroscopy, HR-TEM, XRD and FT-IR experiments. The surface plasmon resonance (SPR) band of UV–vis spectrum around 528 nm confirmed the formation of Au NPs. Transmission electron microscopic analysis showed an average size of Au NPs of 6.9 nm. The fcc crystalline nature of these particles was identified by XRD analysis and SAED pattern. These Au NPs also function as an efficient heterogeneous catalyst in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The reduction of 4-NP follows pseudo-first-order kinetics with rate constant 2.67×10 −2 min −1 .

Modeling of graphene-based NEMS

Abstract: The possibility of designing nanoelectromechanical systems based on relative motion or vibrations of graphene layers is analyzed. Ab initio and empirical calculations of the potential relief of the interlayer interaction energy of bilayer graphene are performed. A new potential based on the density functional theory calculations with the dispersion correction is developed to reliably reproduce the potential relief of the interlayer interaction energy of bilayer graphene. Telescopic oscillations and small relative vibrations of graphene layers are investigated using molecular dynamics simulations. It is shown that these vibrations are characterized with small Q-factor values. The perspectives of nanoelectromechanical systems based on relative motion or vibrations of graphene layers are discussed.

Size-dependent optical and dielectric properties of BiVO4 nanocrystals

Abstract: Optical and dielectric properties of BiVO 4 nanocrystals with different particle sizes have been investigated. BiVO 4 nanocrystals with different particle sizes were synthesized through a solid-state reaction method followed by mechanical ball milling for different time durations. The samples were characterized by X-ray diffraction, UV–Vis spectroscopy, field emission scanning electron microscopy, transmission electron microscopy and photoluminescence spectroscopy. Direct and indirect band gap energies were found to vary in the range 4.04–4.16 eV and 3.51–3.67 eV respectively, for different particle sizes. The band gap energies are higher with respect to their values in the bulk BiVO 4 due to quantum confinement effects. Dielectric properties of the BiVO 4 nanocrystals were investigated and it was found that the dielectric constant increased from 32 to 41 for the reduction of particle size from 29 to 7 nm.

Large amplitude vibration of a bilayer graphene embedded in a nonlinear polymer matrix

Abstract: In the present article, nonlinear free and forced vibration of a bilayer graphene embedded in a polymer medium is studied based on the nonlocal elasticity theory. As a nonlinear function of deflection of graphene sheets, a refined pressure expression is established to describe the van der Waals (vdW) interactions between graphene layers and polymer medium. Assuming the large displacements and anisotropic model for graphene layers, the nonlinear couple partial differential equations of a double layered graphene sheet (DLGS) are obtained. The in-phase and out of phase nonlinear to linear natural frequencies are shown for both zigzag and armchair geometries. The effects of small scale parameter, nonlinear coefficients of vdW between two layers, nonlinear factor of polymer matrix and geometric properties on the nonlinear vibrational behavior of a DLGS are discussed in detail. It is found that the nonlinear vdW coefficient has a significant effect on the out of phase frequencies while the nonlinear polymer coefficient has considerable influence on in-phase frequencies.

Second harmonic generation in asymmetric double semi-parabolic quantum wells: Effects of electric and magnetic fields, hydrostatic pressure and temperature

Abstract: The second harmonic generation (SHG) of a typical GaAs / Ga 1 − x Al x As asymmetric double semi-parabolic quantum wells (DSPQWs) is investigated numerically. The SHG is obtained using the compact density matrix approach. In this work, the effects of the structure parameters, the applied electric and magnetic fields, the hydrostatic pressure and temperature on the SHG coefficient of the asymmetric DSPQWs are studied. Our calculations show that the resonant peaks of the SHG experience an obviously blue-shift and decrease in the magnitude with increasing barrier height. The major-resonant peak value of the SHG is a non-monotonic function of the barrier width and decreases with increase in the right-well width. The results show that the applied electric and magnetic fields affect the magnitude and position of the resonant peaks. Moreover, the resonant peaks of the SHG experience a red-shift (blue-shift) and decrease (increase) in magnitude monotonically with increasing pressure (temperature).

Zinc oxide nanowires on carbon microfiber as flexible gas sensor

Abstract: In the past years, zinc oxide nanowires (ZnO NWs) have been proven to be an excellent material for gas sensors. In this work, we used ZnO nanowires in a novel architecture integrated on a carbon microfiber (μC) textile. This innovative design permits us to obtain mechanical flexibility, while the absence of any lithographic technique allows a large-area and low-cost fabrication of gas sensors. The performances of the devices are investigated for both oxidizing and reducing gases. The nano-on-micro structure of the sensor provides a high surface-to-volume ratio, leading to a fast and intense response for both oxygen (O 2 ) and hydrogen (H 2 ) gases. The sensor response has an optimum temperature condition at 280 °C with a response value of 10 for oxygen and 11 for hydrogen. The limit of detection (LoD) has been found to be 2 and 4 ppm for oxygen and hydrogen, respectively. Additionally, the sensor response and recovery time is small being less than 10 s for both O 2 and H 2 .