bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface strain engineering through Tb doping to study the pressure dependence of exciton-phonon coupling in ZnO nanoparticles
Authors: Sharma A. , Dhar S. , Singh B. P. , Nayak C. , Bhattacharyya D. , Jha S. N.
Published: December 2013

A compressive hydrostatic strain has been found to develop in the ZnO lattice as a result of accumulation of Tb ions on the surface of the nanoparticles for Tb mole-fraction less than 0.04. This hydrostatic strain can be controlled up to ≈14 GPa by varying the Tb mole-fraction. Here, we have utilized this novel technique of surface strain engineering through Tb doping for introducing hydrostatic compressive strain in the lattice to study the pressure dependent electronic and vibrational properties of ZnO nanoparticles. Our study reveals that when subjected to pressure, nanoparticles of ZnO behave quite differently than bulk in many aspects. Unlike bulk ZnO, which is reported to go through a wurtzite to rock-salt structural phase transition at ≈8 GPa, ZnO nanoparticles do not show such transition and remain in wurtzite phase even at 14 GPa of pressure. Furthermore, the Grüneisen parameters for the optical phonon modes are found to be order of magnitude smaller in ZnO nanoparticles as compared to bulk. Our study also suggests an increase of the dielectric constant with pressure, which is opposite to what has been reported for bulk ZnO. Interestingly, it has also been found that the exciton-phonon interaction depends strongly upon pressure in this system. The exciton-phonon coupling has been found to decrease as pressure increases. A variational technique has been adopted to theoretically calculate the exciton-LO phonon coupling coefficient in ZnO nanoparticles as a function of pressure, which shows a good agreement with the experimental results. These findings imply that surface engineering of ZnO nanoparticles with Tb could indeed be an efficient tool to enhance and control the optical performance of this material.
Tunneling processes in asymmetric double barrier magnetic tunnel junctions with a thin top MgO layer
Authors: Li D. L. , Feng J. F. , Yu G. Q. , Guo P. , Chen J. Y. , Wei H. X. , Han X. F. , Coey J. M. D.
Published: December 2013

Dynamic conductance dI/dV and inelastic electron tunneling spectroscopy (IETS) d2I/dV2 have been measured at different temperatures for double barrier magnetic tunnel junctions with a thin top MgO layer. The resistance in the antiparallel state exhibits a normal tunnel-like behavior, while the resistance in the parallel state shows metallic-like transport, indicating the presence of pinholes in the thin top MgO layer. Three IETS peaks are the zero-bias anomaly, interface magnons, and barrier phonons in both the parallel and antiparallel states. The zero-bias anomaly is the strongest peak in the parallel state and its intensity decreases with temperature. The magnon has the largest intensity in the antiparallel state and its intensity also decreases with temperature. The origins of the dips and peaks in the dI/dV-V curve are also discussed.
Anomalous optical switching and thermal hysteresis during semiconductor-metal phase transition of VO2 films on Si substrate
Authors: Leahu G. , Li Voti R. , Sibilia C. , Bertolotti M.
Published: December 2013

We present a detailed infrared study of the semiconductor-to-metal transition (SMT) in a vanadium dioxide (VO2) film deposited on silicon wafer. The VO2 phase transition is studied in the mid-infrared region by analyzing the transmittance and the reflectance measurements, and the calculated emissivity. The reflectance has been measured in two configurations: from the side of the VO2 film and from that of Si wafer. The results show a strong asymmetry between the emissivity in the two configurations, and the fact that the emissivity dynamic range from the silicon side is twice as large than that from the VO2 side. The temperature behaviour of the emissivity during the SMT put into evidence the phenomenon of the anomalous absorption in VO2, which has been explained by applying the Maxwell Garnett effective medium approximation theory.
Nanoscale texture development of C-S-H gel: A computational model for nucleation and growth
Authors: González-Teresa R. , Dolado J. S. , Ayuela A. , Gimel Jean-Christophe
Published: December 2013

The development of C-S-H (Calcium-Silicate-Hydrate) gels during cement hydration is often investigated by nucleation and growth models which fit reasonably well with the calorimetric measurements but predict hydration degrees which grossly exceed the experimental values. Here, a computational model is presented which explicitly considers the intrinsic nanoparticulate nature of C-S-H gel. Based on a nucleation and growth algorithm the model reproduces the experimental calorimetric and hydration degree measurements without invoking to any diffusion mechanism. The model also suggests that the peak in the calorimetric curves can be ascribed to the percolation point of the hydrates themselves within the interstitial pore volume.
Absolute quantum cutting efficiency of Tb3+-Yb3+ co-doped glass
Authors: Duan Qianqian , Qin Feng , Zhao Hua , Zhang Zhiguo , Cao Wenwu
Published: December 2013

The absolute quantum cutting efficiency of Tb3+-Yb3+ co-doped glass was quantitatively measured by an integrating sphere detection system, which is independent of the excitation power. As the Yb3+ concentration increases, the near infrared quantum efficiency exhibited an exponential growth with an upper limit of 13.5%, but the visible light efficiency was reduced rapidly. As a result, the total quantum efficiency monotonically decreases rather than increases as theory predicted. In fact, the absolute quantum efficiency was far less than the theoretical value due to the low radiative efficiency of Tb3+ (<61%) and significant cross-relaxation nonradiative loss between Yb3+ ions.
Origin of thickness dependent spin reorientation transition of B2 type FeCo alloy films
Authors: Kim Dongyoo , Hong Jisang
Published: December 2013

We have investigated the origin of thickness dependent spin reorientation transition (SRT) of B2 type FeCo alloy using the full potential linearized augmented plane wave method. It has been reported that FeCo alloy films on various substrates show a SRT from perpendicular to in-plane magnetization at an approximate thickness of 15 monolayers (MLs). The enhanced perpendicular magnetic anisotropy in bulk FeCo is attributed to a tetragonal distortion. However, we have found that the tetragonal distortion tends to suppress the magnetocrystalline anisotropy (MCA) energy at increasing film thickness in two-dimensional structure. In contrast, the magnitude of the shape anisotropy energy increases at increasing FeCo film thickness. Interestingly, the shape anisotropy overcomes the MCA and the SRT, from perpendicular anisotropy to in-plane magnetization, which occurs at a thickness of 15 ML. Consequently, we are able to clearly understand the physical mechanism of the thickness dependent SRT in terms of the competing reactions of these two counteracting contributions.
Dispersion characteristics of surface plasmon polariton modes in a metallic slab waveguide with nonlinear magnetic cladding
Authors: Ajith R. , Mathew Vincent
Published: December 2013

We investigate the propagation characteristics of surface plasmon polaritons along a slab waveguide with a nonlinear magnetic cladding characterized by permeability μ =μl+μnlH2 and a linear substrate. The dispersion relation for the surface plasmon polariton modes supported by the structure was derived and numerically solved. The dependance of mode behavior on input power was studied. As the input power increases, the propagation constant of the modes increases. The effect of self focussed peak on the mode confinement near the interface and the resulting propagation characteristics of the plasmon modes were investigated. Increase in metal film thickness lifts the degeneracy of nonlinear plasmon modes supported by the waveguide. It has been observed that the propagation length is not a single valued function of input power which is crucial in understanding optical nano-switches.
A force-decoupled compound parallel alignment stage for nanoimprint lithography
Authors: Sun Xiantao , Chen Weihai , Zhou Rui , Chen Wenjie , Zhang Jianbin
Published: December 2013

This paper presents the development of a force-decoupled compound parallel alignment stage for nanoimprint lithography. Parallel alignment stage is a critical component of nanoimprint machine to implement the uniform surface contact between the template with predefined micro/nano patterns and the substrate that accepts the patterns. A combination of a high-stiffness spherical air bearing and a multi degree-of-freedom flexure-based mechanism is adopted in the parallel alignment stage. Apart from the parallel alignment function, the proposed stage can also endure a large imprinting force (more than 1000 N) but does not cause any damage to the delicate flexure-based mechanism. The stage performance is evaluated to satisfy the alignment requirement through the theoretical modeling and finite element analysis. Experiments are conducted on the parallel alignment stage to verify its performance on the transferred grating patterns with linewidth of 2.5 μm. This result demonstrates that the proposed approach can enhance the load capacity of the parallel alignment stage without degrading its alignment accuracy for nanoimprint lithography.
Influence of metastable atoms in the simulation of hollow cathode discharge
Authors: He Shoujie , jing Ha , Liu Shumin , Ouyang Jiting
Published: December 2013

The characteristics of hollow cathode discharge are investigated by using two-dimensional fluid model combined with a transport model for metastable atoms (F-M model) in argon. It shows that the stepwise ionization is one of main important mechanism for electrons production. The distribution of electric potential, density of electrons, ions, and metastable atoms are calculated with a pressure of 10 Torr and a voltage of 250 V. The peak density of electron and ion is 1.2×1013 cm−3, and the peak density of metastable atoms is 3.5×1013 cm−3. The results obtained in F-M model are compared with that in fluid model (without metastable atoms involved). Metastable atoms are found to play an important role in the discharge. In addition, with the increase of pressure and voltage, the percentage of stepwise ionization in the total ionization increase, and the difference of discharge characteristics simulated by these two kinds of models rises.
The high-energy-density counterpropagating shear experiment and turbulent self-heating
Authors: Doss F. W. , Fincke J. R. , Loomis E. N. , Welser-Sherrill L. , Flippo K. A.
Published: December 2013

The counterpropagating shear experiment has previously demonstrated the ability to create regions of shock-driven shear, balanced symmetrically in pressure, and experiencing minimal net drift. This allows for the creation of a high-Mach-number high-energy-density shear environment. New data from the counterpropagating shear campaign is presented, and both hydrocode modeling and theoretical analysis in the context of a Reynolds-averaged-Navier-Stokes model suggest turbulent dissipation of energy from the supersonic flow bounding the layer is a significant driver in its expansion. A theoretical minimum shear flow Mach number threshold is suggested for substantial thermal-turbulence coupling.
Kinetics of complex plasma with liquid droplets
Authors: Misra Shikha , Mishra S. K. , Sodha M. S.
Published: December 2013

This paper provides a theoretical basis for the reduction of electron density by spray of water (or other liquids) in hot plasma. This phenomenon has been observed in a hypersonic flight experiment for relief of radio black out, caused by high ionization in the plasma sheath of a hypersonic vehicle, re-entering the atmosphere. The analysis incorporates a rather little known phenomenon for de-charging of the droplets, viz., evaporation of ions from the surface and includes the charge balance on the droplets and number cum energy balance of electrons, ions, and neutral molecules; the energy balance of the evaporating droplets has also been taken into account. The analysis has been applied to a realistic situation and the transient variations of the charge and radius of water droplets, and other plasma parameters have been obtained and discussed. The analysis through made in the context of water droplets is applicable to all liquids.
Quantum Hall effect in graphene decorated with disordered multilayer patches
Authors: Nam Youngwoo , Sun Jie , Lindvall Niclas , Jae Yang Seung , Kireev Dmitry , Rae Park Chong , Woo Park Yung , Yurgens August
Published: December 2013

Quantum Hall effect (QHE) is observed in graphene grown by chemical vapour deposition using platinum catalyst. The QHE is even seen in samples which are irregularly decorated with disordered multilayer graphene patches and have very low mobility (<500 cm2V−1s−1). The effect does not seem to depend on electronic mobility and uniformity of the resulting material, which indicates the robustness of QHE in graphene.
Thermal conductivity of carbon nanocoils
Authors: Zhao Junhua , Wu Jianyang , Jiang Jin-Wu , Lu Lixin , Zhang Zhiliang , Rabczuk Timon
Published: December 2013

The extreme reduction of the thermal conductivity by defects and folds in carbon nanocoils (CNCs) are first demonstrated using nonequilibrium molecular dynamics simulations. The thermal conductivity for two different defect types with five different folds in the CNCs is extensively studied and the maximum reduction can be up to 70% at both room temperature and 600 K by comparison of the corresponding straight single-walled carbon nanotubes. We reveal that the phonon scattering by coupled defects and folds can govern the reduction of the thermal conductivity by calculating phonon polarization vectors.
Electromechanically tuned resistive switching device
Authors: Li Lijie
Published: December 2013

Hysteresis I-V is observed in our Cu-ZnO nanowire-Cu devices, the dynamic characteristics of which are measured across a very wide frequency band. The devices demonstrate a strong frequency dependent I-V. A working mechanism based on that of electromechanically tunable varistors is postulated by analyzing the experimental results. Electrostatic force induced by the external voltage was able to alter the location of the nanowires and in turn change the nonlinearity of the varistor. The theory also well elucidates the behavior of our devices driven at higher frequencies.
Graphene/gallium arsenide-based Schottky junction solar cells
Authors: Jie Wenjing , Zheng Fengang , Hao Jianhua
Published: December 2013

Chemical-vapor-deposited single- and bi-layer graphene sheets have been transferred onto n-type GaAs substrates. The rectifying characteristics and photovoltaic behaviors of graphene/GaAs junctions have been systematically investigated. The graphene sheets can be combined with the underlying n-type GaAs substrates to form Schottky junctions. For bilayer graphene, the Schottky junction shows photovoltaic effects with the open-circuit voltage of 0.65 V and the short-circuit current density of 10.03 mA/cm2, yielding a power conversion efficiency of 1.95%, which are superior to single-layer one. Such performance parameters are comparable to those of other pristine graphene/semiconductor junction-based devices.
F-actin crosslinker: A key player for the mechanical stability of filopodial protrusion
Authors: Li Tong , Oloyede Adekunle , Gu Y. T.
Published: December 2013

Filopodial protrusion initiates cell migration, which decides the fate of cells in biological environments. In order to understand the structural stability of ultra-slender filopodial protrusion, we have developed an explicit modeling strategy that can study both static and dynamic characteristics of microfilament bundles. Our study reveals that the stability of filopodial protrusions is dependent on the density of F-actin crosslinkers. This cross-linkage strategy is a requirement for the optimization of cell structures, resulting in the provision and maintenance of adequate bending stiffness and buckling resistance while mediating the vibration. This cross-linkage strategy explains the mechanical stability of filopodial protrusion and helps understand the mechanisms of mechanically induced cellular activities.
Leaf image segmentation method based on multifractal detrended fluctuation analysis
Authors: Wang Fang , Li Jin-Wei , Shi Wen , Liao Gui-Ping
Published: December 2013

To identify singular regions of crop leaf affected by diseases, based on multifractal detrended fluctuation analysis (MF-DFA), an image segmentation method is proposed. In the proposed method, first, we defend a new texture descriptor: local generalized Hurst exponent, recorded as LHq based on MF-DFA. And then, box-counting dimension f(LHq) is calculated for sub-images constituted by the LHq of some pixels, which come from a specific region. Consequently, series of f(LHq) of the different regions can be obtained. Finally, the singular regions are segmented according to the corresponding f(LHq). Six kinds of corn diseases leaf's images are tested in our experiments. Both the proposed method and other two segmentation methods—multifractal spectrum based and fuzzy C-means clustering have been compared in the experiments. The comparison results demonstrate that the proposed method can recognize the lesion regions more effectively and provide more robust segmentations.
X-ray irradiation induced changes in electron transport in stabilized a-Se photoconductors
Authors: Walornyj M. , Kasap S. O.
Published: December 2013

We have examined the effect of high-dose x-ray irradiation on electron transport in stabilized amorphous selenium (a-Se) x-ray photoconductive films (of the type used in x-ray image detectors) by measuring the electron lifetime τe through interrupted-field time-of-flight experiments. X-ray induced effects have been examined through two types of experiments. In recovery experiments, the a-Se was preirradiated with and without an applied field (5 V/μm) during irradiation with sufficient dose (typically ∼20 Gy at 21 °C) to significantly reduce the electron lifetime by ∼50%, and then the recovery of the lifetime was monitored as a function of time at three different temperatures, 10 °C, 21 °C, and 35 °C. The lifetime recovery kinetics was exponential with a relaxation time τr that is thermally activated with an activation energy of 1.66 eV. τr is a few hours at 21 °C and only a few minutes at 35 °C. In experiments examining the irradiation induced effects, the a-Se film was repeatedly exposed to x-ray radiation and the changes in the drift mobility and lifetime were monitored as a function of accumulated dose D. There was no observable change in the drift mobility. At 21 °C, the concentration of x-ray induced deep traps (or capture centers), Nd, increases linearly with D (Nd ∼ D) whereas at 35 °C, the recovery process prevents a linear increase in Nd with D, and Nd saturates. In all cases, even under high dose irradiation (∼50 Gy), the lifetime was recoverable to its original equilibrium (pre-exposure) value within a few relaxation times.
Generative model selection using a scalable and size-independent complex network classifier
Authors: Motallebi Sadegh , Aliakbary Sadegh , Habibi Jafar
Published: December 2013

Real networks exhibit nontrivial topological features, such as heavy-tailed degree distribution, high clustering, and small-worldness. Researchers have developed several generative models for synthesizing artificial networks that are structurally similar to real networks. An important research problem is to identify the generative model that best fits to a target network. In this paper, we investigate this problem and our goal is to select the model that is able to generate graphs similar to a given network instance. By the means of generating synthetic networks with seven outstanding generative models, we have utilized machine learning methods to develop a decision tree for model selection. Our proposed method, which is named “Generative Model Selection for Complex Networks,” outperforms existing methods with respect to accuracy, scalability, and size-independence.
Pulse-induced resistive and capacitive switching in TiO2 thin film devices
Authors: Salaoru Iulia , Khiat Ali , Li Qingjiang , Berdan Radu , Prodromakis Themistoklis
Published: December 2013

In this study, we exploit the non-zero crossing current–voltage characteristics exhibited by nanoscale TiO2 based solid-state memristors. We demonstrate that the effective resistance and capacitance of such two terminal devices can be modulated simultaneously by appropriate voltage pulsing. Our results prove that both resistive and capacitive switching arise naturally in nanoscale Pt/TiO2/Pt devices under an external bias, this behaviour being governed by the formation/disruption of conductive filaments through the TiO2 thin film.
Electric field activated nonlinear anisotropic charge transport in doped polypyrrole
Authors: Varade Vaibhav , Anjaneyulu P. , Suchand Sangeeth C. S. , Ramesh K. P. , Menon R.
Published: December 2013

Electric field activated nonlinear transport is investigated in polypyrrole thin film in both in-plane and out-of-plane geometries down to 5 K and strong anisotropy is observed. A morphological model is suggested to explain the anisotropy through inter-chain and intra-chain transport. The deviation from the variable range hopping at low temperature is accounted by fluctuation assisted transport. From Zabrodaskii plots, it is found that electric field can tune the transport from insulating to metallic regime. Glazman–Matveev model is used to describe the nonlinear conduction. Field scaling analysis shows that conductance data at different temperature falls on to a single curve. Nonlinearity exponent, mT and characteristic length, LE are estimated to characterize the transport in both the geometries.
Hugoniot and mean ionization of laser-shocked Ge-doped plastic
Authors: Huser G. , Ozaki N. , Sano T. , Sakawa Y. , Miyanishi K. , Salin G. , Asaumi Y. , Kita M. , Kondo Y. , Nakatsuka K. , Uranishi H. , Yang T. , Yokoyama N. , Galmiche D. , Kodama R.
Published: December 2013

Pressure, density, temperature, and reflectivity measurements along the principal Hugoniot of Ge-doped plastics used in Inertial Confinement Fusion capsules surrogates were obtained to pressures reaching up to 7 Mbar and compared to Quotidian Equation of State models. The experiment was performed using the GEKKO XII laser at the Institute of Laser Engineering at Osaka University in Japan. High precision measurements of pressure and density were obtained using a quartz standard and found to be in good agreement with theoretical Hugoniot curves. Modeling of reflectivity measurements show that shocked samples can be described as poor metals and that mean ionization calculated within the frame of QEOS is overestimated. Similarly, shock temperatures were found to be below theoretical Hugoniot curves.
Cavitation in dielectric fluid in inhomogeneous pulsed electric field
Authors: Shneider M. N. , Pekker M.
Published: December 2013

This paper proposes a method for studying the early stages of the cavitation development in arbitrary, non-stationary conditions. This method is based on the comparison of the results of calculations in the framework of a theoretical model of the liquid dielectrics motion in a strong non-uniform electric field and experiments with controlled parameters. This approach allows us to find the critical negative pressure, at which cavitation begins to develop, and to determine the values of the constants in the classical models of cavitation.
Improved power capacity in a high efficiency klystron-like relativistic backward wave oscillator by distributed energy extraction
Authors: Xiao Renzhen , Chen Changhua , Cao Yibing , Sun Jun
Published: December 2013

With the efficiency increase of a klystron-like relativistic backward wave oscillator, the maximum axial electric field and harmonic current simultaneously appear at the end of the beam-wave interaction region, leading to a highly centralized energy exchange in the dual-cavity extractor and a very high electric field on the cavity surface. Thus, we present a method of distributed energy extraction in this kind of devices. Particle-in-cell simulations show that with the microwave power of 5.1 GW and efficiency of 70%, the maximum axial electric field is decreased from 2.26 MV/cm to 1.28 MV/cm, indicating a threefold increase in the power capacity.
Impact of the hydrogen content on the photoluminescence efficiency of amorphous silicon alloys
Authors: Kistner J. , Schubert M. B.
Published: December 2013

This paper analyzes the impact of hydrogen on the photoluminescence (PL) efficiency of the three wide gap silicon alloys: silicon carbide (a-SiCx), silicon nitride (a-SiNx): silicon oxide (a-SiOx). All three materials behave similarly. The progression of the PL efficiency over the Si content splits into two regions. With decreasing Si content, the PL efficiency increases until a maximum is reached. With a further decrease of the Si content, the PL efficiency declines again. A comprehensive analysis of the sample structure reveals that the PL efficiency depends on the degree of passivation of Si and Y atoms (Y = C, N, O) with hydrogen. For samples with a high Si content, an effective passivation of incorporated Y atoms gives rise to an increasing PL efficiency. The PL efficiency of samples with a low Si content is limited due to a rising amount of unpassivated Si defect states. We find that a minimum amount of 0.2 H atoms per Si atom is required to maintain effective luminescence.
Large magnetoresistance of paracyclophane-based molecular tunnel junctions: A first-principles study
Authors: Tao L. L. , Liang S. H. , Liu D. P. , Han X. F.
Published: December 2013

We report a theoretical study of magnetoresistance and spin-polarized transport of a series of paracyclophane-based molecular tunnel junctions. We predict that the molecular tunnel junction using [2.2]-paracyclophane barrier has the desired low resistance area product in combination with high magnetoresistance ratio. In addition, we find the spin-polarized conductance decreases exponentially with increasing the molecular length, indicating a nonresonant tunneling mechanism. In particular, the characteristic decay constant can be theoretically evaluated from the complex band structure of periodic paracyclophane molecule. The spin-polarized transport mechanism is systematically analyzed.
Ion velocities in direct current arc plasma generated from compound cathodes
Authors: Zhirkov I. , Eriksson A. O. , Rosen J.
Published: December 2013

Arc plasma from Ti-C, Ti-Al, and Ti-Si cathodes was characterized with respect to charge-state-resolved ion energy. The evaluated peak velocities of different ion species in plasma generated from a compound cathode were found to be equal and independent on ion mass. Therefore, measured difference in kinetic energies can be inferred from the difference in ion mass, with no dependence on ion charge state. The latter is consistent with previous work. These findings can be explained by plasma quasineutrality, ion acceleration by pressure gradients, and electron-ion coupling. Increasing the C concentration in Ti-C cathodes resulted in increasing average and peak ion energies for all ion species. This effect can be explained by the “cohesive energy rule,” where material and phases of higher cohesive energy generally result in increasing energies (velocities). This is also consistent with the here obtained peak velocities around 1.37, 1.42, and 1.55 (104 m/s) for ions from Ti0.84Al0.16, Ti0.90Si0.10, and Ti0.90C0.10 cathodes, respectively.
Evidence for hybrid surface metallic band in (4 × 4) silicene on Ag(111)
Authors: Tsoutsou D. , Xenogiannopoulou E. , Golias E. , Tsipas P. , Dimoulas A.
Published: December 2013

The electronic band structure of monolayer (4 × 4) silicene on Ag(111) is imaged by angle resolved photoelectron spectroscopy. A dominant hybrid surface metallic band is observed to be located near the bulk Ag sp-band which is also faintly visible. The two-dimensional character of the hybrid band has been distinguished against the bulk character of the Ag(111) sp-band by means of photon energy dependence experiments. The surface band exhibits a steep linear dispersion around the K¯Ag point and has a saddle point near the M¯Ag point of Ag(111) resembling the π-band dispersion in graphene.
Non-equilibrium origin of high electrical conductivity in gallium zinc oxide thin films
Authors: Zakutayev Andriy , Perry Nicola H. , Mason Thomas O. , Ginley David S. , Lany Stephan
Published: December 2013

Non-equilibrium state defines physical properties of materials in many technologies, including architectural, metallic, and semiconducting amorphous glasses. In contrast, crystalline electronic and energy materials, such as transparent conductive oxides (TCO), are conventionally thought to be in equilibrium. Here, we demonstrate that high electrical conductivity of crystalline Ga-doped ZnO TCO thin films occurs by virtue of metastable state of their defects. These results imply that such defect metastability may be important in other functional oxides. This finding emphasizes the need to understand and control non-equilibrium states of materials, in particular, their metastable defects, for the design of novel functional materials.
Using field emission to control the electron energy distribution in high-pressure microdischarges at microscale dimensions
Authors: Li Yingjie , Go David B.
Published: December 2013

Particle simulations of high-pressure microdischarges at gaps below 10 μm show that the electron energy distribution becomes non-continuous, with discrete peaks corresponding to specific inelastic collisions. The relative magnitude of these peaks and shape of the energy distribution can be directly controlled by the parameter pressure times distance (pd) and the applied potential across the gap. These parameters dictate inelastic collisions experienced by electrons and as both increase the distribution smooths into a Maxwellian-like distribution. By capitalizing on field emission at these dimensions, it is possible to control the energy distribution of free electrons to target specific, energy dependent reactions.
Analysis and optimization of carbon nanotubes and graphene sensors based on adsorption-desorption kinetics
Authors: Liang Sang-Zi , Chen Gugang , Harutyunyan Avetik R. , Cole Milton W. , Sofo Jorge O.
Published: December 2013

Single-walled carbon nanotubes mats and graphene have shown great potential as gas sensors. We analyze NO adsorption/sensing experiments with the kinetic Langmuir model adapted to include adsorption sites from which the molecule does not desorb. The model reproduces the available experimental data. Its fitting parameters provide information on the microscopic phenomena governing adsorption, and variation of these parameters allows the optimization of the sensitivity, detection limit, and time response of the sensors. The result reveals an optimal operating temperature before thermal desorption becomes dominant at high temperature, the potential improvement of selectivity by tuning the gate voltage in a field effect transistor configuration, and quantifies the benefits of reducing the density of defects in the sensing materials.
Influence of growth temperature and temperature ramps on deep level defect incorporation in m-plane GaN
Authors: Armstrong A. M. , Kelchner K. , Nakamura S. , DenBaars S. P. , Speck J. S.
Published: December 2013

The dependence of deep level defect incorporation in m-plane GaN films grown by metal-organic chemical vapor deposition on bulk m-plane GaN substrates as a function of growth temperature (Tg) and Tg ramping method was investigated using deep level optical spectroscopy. Understanding the influence of Tg on GaN deep level incorporation is important for InGaN/GaN multi-quantum well (MQW) light emitting diodes (LEDs) and laser diodes (LDs) because GaN quantum barrier (QB) layers are grown much colder than thin film GaN to accommodate InGaN QW growth. Deep level spectra of low Tg (800 °C) GaN films grown under QB conditions were compared to deep level spectra of high Tg (1150 °C) GaN. Reducing Tg, increased the defect density significantly (>50×) through introduction of emergent deep level defects at 2.09 eV and 2.9 eV below the conduction band minimum. However, optimizing growth conditions during the temperature ramp when transitioning from high to low Tg substantially reduced the density of these emergent deep levels by approximately 40%. The results suggest that it is important to consider the potential for non-radiative recombination in QBs of LED or LD active regions, and tailoring the transition from high Tg GaN growth to active layer growth can mitigate such non-radiative channels.
SU-8 doped and encapsulated n-type graphene nanomesh with high air stability
Authors: Al-Mumen Haider , Dong Lixin , Li Wen
Published: December 2013

N-type doping of graphene with long-term chemical stability in air represents a significant challenge for practical application of graphene electronics. This paper reports a reversible doping method to achieve highly stable n-type graphene nanomeshes, in which the SU-8 photoresist simultaneously serves as an effective electron dopant and an excellent encapsulating layer. The chemically stable n-type characteristics of the SU-8 doped graphene were evaluated in air using their Raman spectra, electrical transport properties, and electronic band structures. The SU-8 doping does minimum damage to the hexagonal carbon lattice of graphene and is completely reversible by removing the uncrosslinked SU-8 resist.
Extended electrical tuning of quantum cascade lasers with digital concatenated gratings
Authors: Slivken S. , Bandyopadhyay N. , Bai Y. , Lu Q. Y. , Razeghi M.
Published: December 2013

In this report, the sampled grating distributed feedback laser architecture is modified with digital concatenated gratings to partially compensate for the wavelength dependence of optical gain in a standard high efficiency quantum cascade laser core. This allows equalization of laser threshold over a wide wavelength range and demonstration of wide electrical tuning. With only two control currents, a full tuning range of 500 nm (236 cm−1) has been demonstrated. Emission is single mode, with a side mode suppression of >20 dB.
Role of an ultra-thin AlN/GaN superlattice interlayer on the strain engineering of GaN films grown on Si(110) and Si(111) substrates by plasma-assisted molecular beam epitaxy
Authors: Shen X. Q. , Takahashi T. , Rong X. , Chen G. , Wang X. Q. , Shen B. , Matsuhata H. , Ide T. , Shimizu M.
Published: December 2013

We investigate the role of an ultra-thin AlN/GaN superlattice interlayer (SL-IL) on the strain engineering of the GaN films grown on Si(110) and Si(111) substrates by plasma-assisted molecular beam epitaxy. It is found that micro-cracks limitted only at the SL-IL position are naturally generated. These micro-cracks play an important role in relaxing the tensile strain caused by the difference of the coefficient of thermal expansion between GaN and Si and keeping the residual strain in the crack-free GaN epilayers resulted from the SL-IL during the growth. The mechanism understanding of the strain modulation by the SL-IL in the GaN epilayers grown on Si substrates makes it possible to design new heterostructures of III-nitrides for optic and electronic device applications.
Inversion by metalorganic chemical vapor deposition from N- to Ga-polar gallium nitride and its application to multiple quantum well light-emitting diodes
Authors: Hosalli A. M. , Van Den Broeck D. M. , Bharrat D. , El-Masry N. A. , Bedair S. M.
Published: December 2013

We demonstrate a metalorganic chemical vapor deposition growth approach for inverting N-polar to Ga-polar GaN by using a thin inversion layer grown with high Mg flux. The introduction of this inversion layer allowed us to grow p-GaN films on N-polar GaN thin film. We have studied the dependence of hole concentration, surface morphology, and degree of polarity inversion for the inverted Ga-polar surface on the thickness of the inversion layer. We then use this approach to grow a light emitting diode structure which has the MQW active region grown on the advantageous N-polar surface and the p-layer grown on the inverted Ga-polar surface.
Metallic conductance at the interface of tri-color titanate superlattices
Authors: Kareev M. , Cao Yanwei , Liu Xiaoran , Middey S. , Meyers D. , Chakhalian J.
Published: December 2013

Ultra-thin tri-color (tri-layer) titanate superlattices ([3 u.c. LaTiO3/2 u.c. SrTiO3/3 u.c. YTiO3], u.c. = unit cells) were grown in a layer-by-layer way on single crystal TbScO3 (110) substrates by pulsed laser deposition. High sample quality and electronic structure were characterized by the combination of in-situ photoelectron and ex-situ structure and surface morphology probes. Temperature-dependent sheet resistance indicates the presence of metallic interfaces in both [3 u.c. LaTiO3/2 u.c. SrTiO3] bi-layers and all the tri-color structures, whereas a [3 u.c. YTiO3/2 u.c. SrTiO3] bi-layer shows insulating behavior. Considering that in the bulk YTiO3 is ferromagnetic below 30 K, the tri-color titanate superlattices provide an opportunity to induce tunable spin-polarization into the two-dimensional electron gas with Mott carriers.
Near-ideal strength in metal nanotubes revealed by atomistic simulations
Authors: Sun Mingfei , Xiao Fei , Deng Chuang
Published: December 2013

Here we report extraordinary mechanical properties revealed by atomistic simulations in metal nanotubes with hollow interior that have been long overlooked. Particularly, the yield strength in [1 1 1] Au nanotubes is found to be up to 60% higher than the corresponding solid Au nanowire, which approaches the theoretical ideal strength in Au. Furthermore, a remarkable transition from sharp to smooth yielding is observed in Au nanotubes with decreasing wall thickness. The ultrahigh tensile strength in [1 1 1] Au nanotube might originate from the repulsive image force exerted by the interior surface against dislocation nucleation from the outer surface.
Surface states controlled broadband enhancement of two-photon absorption
Authors: Zhou Zhiqiang , Lu Changgui , Xu Shuhong , Jiang Yuan , Yun Binfeng , Wang Chunlei , Cui Yiping
Published: December 2013

We demonstrate the controllable broadband enhancement of two-photon absorption in a wide spectral range from 710 nm to 960 nm by controlling the surface states of aqueous Co2+ doped CdTe quantum dots, which is consistent with the measurement results of surface potential and fluorescence decay. The enhancement can be tuned in the range between 1 and 1.7 by changing the dopant concentrations that determine the surface states.
Enhanced radiation tolerance of non-polar-terminated ZnO
Authors: Charnvanichborikarn S. , Myers M. T. , Shao L. , Kucheyev S. O.
Published: December 2013

Room-temperature heavy-ion bombardment of polar (0001) ZnO leads to the formation of intermediate peak and step features in damage–depth profiles measured by ion channeling. Here, we show that these anomalous disorder effects are strongly suppressed for crystals with (112¯0) and (101¯0) non-polar surface terminations. Possible defect interaction scenarios responsible for the enhanced radiation tolerance of non-polar-terminated ZnO are discussed.
Phase transition in stoichiometric GaSb thin films: Anomalous density change and phase segregation
Authors: Putero Magali , Coulet Marie-Vanessa , Ouled-Khachroum Toufik , Muller Christophe , Baehtz Carsten , Raoux Simone
Published: December 2013

The crystallization of stoichiometric GaSb thin films was studied by combined in situ synchrotron techniques and static laser testing. It is demonstrated that upon crystallization, GaSb thin films exhibit an unusual behaviour with increasing thickness and concomitant decreasing mass density while its electrical resistance drops as commonly observed in phase change materials. Furthermore, beyond GaSb amorphous-to-crystalline phase transition, an elemental segregation and a separate crystallization of a pure Sb phase is evidenced.
Enhancement of light-extraction efficiency in AlGaInP light-emitting diodes using evanescent wave coupling effect
Authors: Hao Guo-Dong , Wang Xue-Lun
Published: December 2013

Sub-wavelength-sized ridge structures that satisfy evanescent wave coupling effect requirements at the emission wavelength were fabricated through the photolithography and wet chemical etching of the light-extraction surface of a thin-film type AlGaInP light-emitting diode (LED). By doing this, the light output power relative to a reference LED with a flat light-extraction surface was increased by a factor of approximately 3.8, corresponding to an equivalent enhancement in light-extraction efficiency that is possibly attributable to an evanescent wave coupling effect occurring on the surfaces of the minute ridge structures.
Unification of the negative electrocaloric effect in Bi1/2Na1/2TiO3-BaTiO3 solid solutions by Ba1/2Sr1/2TiO3 doping
Authors: Uddin Sarir , Zheng Guang-Ping , Iqbal Yaseen , Ubic Rick , Yang Junhe
Published: December 2013

The microscopic mechanisms of the negative electrocaloric effect (ECE) of the single-phase (1−x)(0.94Bi1/2Na1/2TiO3-0.06BaTiO3)-xBa1/2Sr1/2TiO3 (BNT-BT-BST) perovskite solid solutions fabricated via the sol-gel technique are explored in this study. Dielectric and mechanical relaxation analyses are employed to investigate the ferroelectric and structural transitions of the samples. The electrocaloric properties of the samples were measured by thermodynamics Maxwell relations. The difference between the depolarization temperature (Td) and the maximum dielectric constant temperature (Tm) was found to decrease with increasing BST content. Doping with BST stabilized the ferroelectric phase along with unifying the EC temperature changes (ΔT) to only negative values. The origin of the uniform negative ECE of BNT-BT-BST is discussed.
Temperature and damping effects on the frequency dependence of electrostatic force microscopy force gradients
Authors: Arinero R. , Trasobares J. , Girard P. , Ramonda M. , Clément N.
Published: December 2013

In this paper we demonstrate that depending on the surrounding ambient, which may be manifested by different damping conditions, a “bump” in frequency response of Electric Force Microscope (EFM) cantilevers may be observed, either in electrical amplitude or phase signals of AC gradients. This bump is present in air and at room temperature when the cantilever oscillates at fundamental frequency with high excitation amplitude but disappears at increasing temperature. This effect can be explained by a significant temperature decrease of the Q factor of coated cantilevers. At a constant value of AC voltage, the relevant parameter is the ratio of Q factor by cantilever stiffness (of the nth oscillation mode) from which we can predict bump observation. This study highlights significant effects for quantitative EFM studies under different atmospheres and at different temperatures.
Total absorption of light in sub-wavelength metallic waveguides
Authors: Ghossoub Marc G. , Sinha Sanjiv
Published: December 2013

Resonance cavity modes enhance optical transmission through sub-wavelength metallic apertures but their role in absorption remains unclear. Here, we use full field simulation and a semi-analytical model to report absorption and transmission enhancement in transmission gratings under transverse electric illumination. The fundamental cavity mode of the sub-wavelength grating cavities is the major contributor to absorption. We demonstrate the possibility of tailoring such cavity resonances to induce total absorption in reflection gratings. Our method advances the understanding of transmittance and absorption enhancing mechanisms in metallic nanostructures which constitute fundamental components in energy harvesting, sensing, and spectroscopic applications.
The effects of three-dimensional shaping of vertically aligned carbon-nanotube contacts for micro-electro-mechanical switches
Authors: Dahmardeh Masoud , Vahdani Moghaddam Mehran , Hian Tee Min , Nojeh Alireza , Takahata Kenichi
Published: December 2013

A micro-electro-mechanical switch integrated with vertically aligned carbon nanotubes (CNTs) as the contact material is presented. Arrays of the CNTs are three-dimensionally micropatterned using a pulsed micro-discharge process to have tapered contact surfaces with controlled angles, achieving maximized contact areas, while providing contact resistances in the 10 Ω range with an enhanced current capacity. A shape-memory-alloy actuator is integrated to demonstrate stable switching for ∼1.4 × 106 ON-OFF cycles with no sign of damage. The results prove that post-growth micropatterning of CNTs is a promising path to improved and reliable micro contact switches enabled by arrayed CNT contacts for high-power applications.
Is it viable to improve light output efficiency by nano-light-emitting diodes?
Authors: Wang Chao-Hung , Huang Yu-Wen , Wu Shang-En , Liu Chuan-Pu
Published: December 2013

Nanopillar arrays with InGaN/GaN multiple-quantum-disks (MQDs) are fabricated by focused-ion-beam milling with surface damage layer removed by KOH wet etching. Nano-light-emitting diodes (Nano-LEDs) made of the InGaN/GaN MQD nanopillars are found to have 19.49% less output power than that of a conventional LED. The reasons are analyzed in detail and considering their current-voltage and electroluminescence characteristics, internal quantum efficiency, external quantum efficiency, light extraction, and wall-plug efficiency. Our results suggest that nanopillar-LED can outperform if the density can be increased to 2.81 × 109 cm−2 with the size unchanged or the size can be increased to 854.4 nm with the density unchanged.
Temperature-dependent mechanical properties of single-layer molybdenum disulphide: Molecular dynamics nanoindentation simulations
Authors: Zhao Junhua , Jiang Jin-Wu , Rabczuk Timon
Published: December 2013

The temperature-dependent mechanical properties of single-layer molybdenum disulphide (MoS2) are obtained using molecular dynamics (MD) nanoindentation simulations. The Young's moduli, maximum load stress, and maximum loading strain decrease with increasing temperature from 4.2 K to 500 K. The obtained Young's moduli are in good agreement with those using our MD uniaxial tension simulations and the available experimental results. The tendency of maximum loading strain with different temperature is opposite with that of metal materials due to the short range Stillinger-Weber potentials in MoS2. Furthermore, the indenter tip radius and fitting strain effect on the mechanical properties are also discussed.