bluuebunny/crossref_metadata_2025 huggingface dataset:
Levelized costs of electricity and direct-use heat from Enhanced Geothermal Systems
Authors: Beckers Koenraad F. , Lukawski Maciej Z. , Anderson Brian J. , Moore Michal C. , Tester Jefferson W.
Published: February 2014

GEOPHIRES (GEOthermal energy for the Production of Heat and Electricity (“IR”) Economically Simulated) is a software tool that combines reservoir, wellbore, and power plant models with capital and operating cost correlations and financial levelized cost models to assess the technical and economic performance of Enhanced Geothermal Systems (EGS). It is an upgrade and expansion of the “MIT-EGS” program used in the 2006 “Future of Geothermal Energy” study. GEOPHIRES includes updated cost correlations for well drilling and completion, resource exploration, and Organic Rankine Cycle (ORC) and flash power plants. It also has new power plant efficiency correlations based on AspenPlus and MATLAB simulations. The structure of GEOPHIRES enables feasibility studies of using geothermal resources not only for electricity generation but also for direct-use heating, and combined heat and power (CHP) applications. Full documentation on GEOPHIRES is provided in the supplementary material. Using GEOPHIRES, the levelized cost of electricity (LCOE) and the levelized cost of heat (LCOH) have been estimated for 3 cases of resource grade (low-, medium-, and high-grade resource corresponding to a geothermal gradient of 30, 50, and 70 °C/km) in combination with 3 levels of technological maturity (today's, mid-term, and commercially mature technology corresponding to a productivity of 30, 50, and 70 kg/s per production well and thermal drawdown rate of 2%, 1.5%, and 1%). The results for the LCOE range from 4.6 to 57 ¢/kWhe and for the LCOH from 3.5 to 14 $/MMBTU (1.2 to 4.8 ¢/kWhth). The results for the base-case scenario (medium-grade resource and mid-term technology) are 11 ¢/kWhe and 5 $/MMBTU (1.7 ¢/kWhth), respectively. To account for parameter uncertainty, a sensitivity analysis has been included. The results for the LCOE and LCOH have been compared with values found in literature for EGS as well as other energy technologies. The key findings suggest that given today's technology maturity, electricity and direct-use heat from EGS are not economically competitive under current market conditions with other energy technologies. However, with moderate technological improvements, electricity from EGS is predicted to become cost-effective with respect to other renewable and non-renewable energy sources for medium- and high-grade geothermal resources. Direct-use heat from EGS is calculated to become cost-effective even for low-grade resources. This emphasizes that EGS for direct-use heat may not be neglected in future EGS development.
Phase-constant-nonreciprocal composite right/left-handed metamaterials based on coplanar waveguides
Authors: Porokhnyuk Andrey , Ueda Tetsuya , Kado Yuichi , Itoh Tatsuo
Published: February 2014

The purely phase-constant-nonreciprocal composite right-left handed metamaterial structure is proposed based on coplanar waveguides loaded with a ferrite layer. The structure exhibits considerably large nonreciprocity in phase constant which depends on the effective magnetization and whose magnitude can remain in leaky wave region of wavenumbers or can overcome a boundary to slow wave region. The nonreciprocity in amplitude of transmission coefficients, on the other hand, is effectively reduced by using a cavity-backed design to prevent undesired nonreciprocal radiation loss.
Raindrops push and splash flying insects
Authors: Dickerson Andrew K. , Shankles Peter G. , Hu David L.
Published: February 2014

In their daily lives, flying insects face a gauntlet of environmental challenges, from wind gusts to raindrop impacts. In this combined experimental and theoretical study, we use high-speed videography to film raindrop collisions upon both flying insects and dynamically scaled spherical mimics. We identify three outcomes of the collision based upon the insect's mass and characteristic size: drops push the insect while remaining intact, coat the insect, and splash. We present a mathematical model that predicts impact force and outcome consistent with those found in experiments. Small insects such as gnats and flies are pushed by raindrops that remain intact upon impact; conversely, large flyers such as locusts and micro-aerial vehicles cause drops to splash. We identify a critical mass of 0.3 g for which flyers achieve both peak acceleration (100 g) and applied force (104 dyn) from incoming raindrops; designs of similarly massed flying robots should be avoided.
An experimental and numerical investigation of the dependency on the initial conditions of the Richtmyer-Meshkov instability
Authors: Vandenboomgaerde Marc , Souffland Denis , Mariani Christian , Biamino Laurent , Jourdan Georges , Houas Lazhar
Published: February 2014

The nonlinear evolution of 2D single-mode Richtmyer-Meshkov instabilities is investigated through experiments in shock tube and numerical simulations. In our shock tube, the interface is materialized by a thin membrane attached to a stereo-lithographed grid. The purpose of this study is to compare experimental and numerical results, verify that using a higher Mach number for the incident shock wave (Misw) than in a previous study [C. Mariani, M. Vandenboomgaerde, G. Jourdan, D. Souffland, and L. Houas, “Investigation of the Richtmyer-Meshkov instability with stereolithographed interfaces,” Phys. Rev. Lett. 100, 254503 (2008)] drastically reduces the deleterious effects of the membrane remnants, explore the effect of a high initial amplitude at the interface on the growth of the perturbation, and understand the lack of roll-up structures in the nonlinear phase of the instability. Using grayscale gradient rather than gray level, a new processing of the raw pictures is developed. Numerical simulations run with the TRICLADE code show that adding short-wavelength (swl) perturbations to the single-mode interface is necessary to understand the morphology of the experimental pictures. Through these comparisons, a minimal description of the swl perturbations is proposed. The experimental growth rates are correctly predicted by the TRICLADE code. Positive agreements are also obtained by models only if they take into account the effect of a high initial amplitude. A first result is the agreement that we obtain between experiment, simulations, and models indicates that the membrane remnants no longer influence the global dynamics of the instability. This is due to the increase of Misw from 1.15 to 1.45. Another result of this investigation is that swl perturbations disrupt the spikes which grow at the interface and prevent the appearance of roll-up structures. Numerical simulations show the importance of a diffuse transition layer at the initial interface as a stabilizing process of the swl perturbations. In our case, only such a layer would allow the roll-up structures to grow. Finally, our analysis confirms that, when they are present at the same time, the swl perturbations distort the long wavelength perturbations, and that the latter decreases the growth of the former.
III-V/Si on silicon-on-insulator platform for hybrid nanoelectronics
Authors: Prucnal Slawomir , Zhou Shengqiang , Ou Xin , Facsko Stefan , Oskar Liedke Maciej , Bregolin Felipe , Liedke Bartosz , Grebing Jochen , Fritzsche Monika , Hübner Rene , Mücklich Arndt , Rebohle Lars , Helm Manfred , Turek Marcin , Drozdziel Andrzej , Skorupa Wolfgang
Published: February 2014

The unique properties of SOI wafers enable the integration of heterogeneous materials with distinct functionalities in different layers. In particular, III-V compound semiconductors are very attractive for low-noise and high-speed electronic and photonic components integrated on a single chip. We have developed a CMOS compatible and fully integrated solution for the integration of III-V compound semiconductors with silicon technology for optoelectronic applications. InAs compound semiconductor nanostructures are synthesized in SOI wafers using the combined ion beam implantation and millisecond liquid-phase epitaxial growth. Optoelectronic and microstructural investigations carried out on implanted, annealed, and selectively etched samples confirm the formation of high-quality III-V compound semiconductor nanostructures.
Quadrupole terms in the Maxwell equations: Born energy, partial molar volume, and entropy of ions
Authors: Slavchov Radomir I. , Ivanov Tzanko I.
Published: February 2014

A new equation of state relating the macroscopic quadrupole moment density $\hbox{\sf\bfseries\itshape Q}$Q to the gradient of the field ∇E in an isotropic fluid is derived: $\hbox{\sf\bfseries\itshape Q}$Q = αQ(∇E − $\hbox{\sf\bfseries U}$U∇·E/3), where the quadrupolarizability αQ is proportional to the squared molecular quadrupole moment. Using this equation of state, a generalized expression for the Born energy of an ion dissolved in quadrupolar solvent is obtained. It turns out that the potential and the energy of a point charge in a quadrupolar medium are finite. From the obtained Born energy, the partial molar volume and the partial molar entropy of a dissolved ion follow. Both are compared to experimental data for a large number of simple ions in aqueous solutions. From the comparison the value of the quadrupolar length LQ is determined, LQ = (αQ/3ɛ)1/2 = 1-4 Å. Data for ion transfer from aqueous to polar oil solution are analyzed, which allowed for the determination of the quadrupolarizability of nitrobenzene.
Magnetic properties of Zn doped Co2Y hexaferrite by using high-field Mössbauer spectroscopy
Authors: Tae Lim Jung , Sung Kim Chul
Published: February 2014

The polycrystalline samples of Ba2Co2−xZnxFe12O22 (x = 0.5, 1.0, 1.5) were synthesized by using solid-state-reaction method. From the XRD patterns, analyzed by Rietveld refinement, the prepared samples are found to be single-phased with rhombohedral structure (R-3m). The magnetic properties of samples were investigated with vibrating sample magnetometer, and high-field Mössbauer spectrometer. From the zero-field-cooled curves under 100 Oe between 4.2 and 740 K, we observe that the samples show spin transition from helicalmagnetic to ferrimagnetic order. With increasing Zn ion concentration, the spin transition temperature (Ts) and Curie temperature (TC) decrease linearly. We have obtained Zero-field Mössbauer spectra of all samples at various temperatures ranging from 4.2 to 650 K, and analyzed the spectra below TC as six-sextets for Fe sites. From the temperature dependence of hyperfine field (Hhf), we have noticed an abrupt change in Hhf at Ts. In addition, Mössbauer spectra of all samples at 4.2 K were taken with applied field ranging from 0 to 50 kOe, indicating the canting angle between applied field and Hhf decreased with increasing Zn concentration.
Lattice distortion in single crystal rare-earth arsenide/GaAs nanocomposites
Authors: Young A. J. , Schultz B. D. , Palmstrøm C. J.
Published: February 2014

Epitaxial single crystal nanocomposites comprised of rare-earth arsenide nanoparticles embedded in GaAs (001) layers produce a larger change in lattice parameter than expected from the lattice parameters of relaxed films. Despite similar cubic structures and lattice parameters, elongation of the interfacial bond length between the two materials induces additional strain causing an expansion in the nanocomposite lattice. The interface bond length is material dependent with an average atomic layer spacing at the ErAs:GaAs interface of 1.9 Å while the spacing at the ScAs:GaAs interface is only 1.4 Å. Implications for lattice matching various single crystal epitaxial nanostructures in semiconductors are discussed.
Interface and nanostructure evolution of cobalt germanides on Ge(001)
Authors: Grzela T. , Koczorowski W. , Capellini G. , Czajka R. , Radny M. W. , Curson N. , Schofield S. R. , Schubert M. A. , Schroeder T.
Published: February 2014

Cobalt germanide (CoxGey) is a candidate system for low resistance contact modules in future Ge devices in Si-based micro and nanoelectronics. In this paper, we present a detailed structural, morphological, and compositional study on CoxGey formation on Ge(001) at room temperature metal deposition and subsequent annealing. Scanning tunneling microscopy and low energy electron diffraction clearly demonstrate that room temperature deposition of approximately four monolayers of Co on Ge(001) results in the Volmer Weber growth mode, while subsequent thermal annealing leads to the formation of a Co-germanide continuous wetting layer which evolves gradually towards the growth of elongated CoxGey nanostructures. Two types of CoxGey nanostructures, namely, flattop- and ridge-type, were observed and a systematic study on their evolution as a function of temperature is presented. Additional transmission electron microscopy and x-ray photoemission spectroscopy measurements allowed us to monitor the reaction between Co and Ge in the formation process of the CoxGey continuous wetting layer as well as the CoxGey nanostructures.
Anisotropic picosecond photoconductivity caused by optical alignment of electron momenta in cubic semiconductors
Authors: Malevich Y. V. , Adomavičius R. , Krotkus A. , Malevich V. L.
Published: February 2014

Transient photoconductivity in cubic semiconductors InGaAs and InAs excited by a femtosecond laser pulse in the presence of a uniform dc electric field has been studied with the use of the Monte Carlo simulation by taking into account optical alignment of photoexcited electrons over their momenta. Simulations show that due to the optical alignment effect and energy dependence of the electron mobility, the transient photoconductivity in cubic semiconductors becomes anisotropic during the first few picoseconds after optical excitation. The magnitude of this anisotropy reaches its peak when the excess energy of the optically excited electrons approaches the threshold for the intervalley transfer. It has also been found that when the electrons are excited near the threshold energy for the intervalley transfer, the component of the transient photocurrent directed along the dc field for a short time after the end of the femtosecond optical pulse can become negative. The anisotropy of the transient photoconductivity has been investigated experimentally on (001) InGaAs sample by the optical pump - terahertz-probe technique. Optically induced changes in terahertz pulse amplitude were found to be dependent on the direction of terahertz field relative to the polarization of the optical pump pulse and to the crystallographic axes of the semiconductor. Experimental data have been explained in terms of the transient anisotropic photoconductivity and correlate with the results of the Monte Carlo simulation.
Simulation on near-field light generated by Au nano-dot on GaAs substrate for heat assisted magnetic recording heat source
Authors: Katayama Ryuichi
Published: February 2014

Localized surface plasmons generated by a Au dot in a novel device for a heat source of heat assisted magnetic recording were analyzed using a simple model in which a Au hemisphere was formed on a GaAs substrate and a quasi-electrostatic approximation. The scattering and absorption efficiencies as well as the enhancement factor were investigated. As a result, their dependence on the wavelength and the polarization direction of the incident light was clarified.
Structural, magnetic, and electron transport properties of Mn3−xPtxSn (x = 0, 0.5, 1) nanomaterials
Authors: Nelson A. , Huh Y. , Kharel P. , Shah V. R. , Skomski R. , Sellmyer D. J.
Published: February 2014

The structural, magnetic, and electron-transport properties of Mn3−xPtxSn (x = 0, 0.5, 1) nanomaterials prepared by arc-melting, melt-spinning, and annealing were investigated. It was found that the hexagonal structure is the most stable structure for Mn3Sn and the samples show an antiferromagnetic spin order at room temperature. The Pt-containing samples are mainly tetragonal but contain a small amount of other impurity phases, including hexagonal Mn3Sn and Mn2Sn. At room temperature, the Pt-containing samples show ferri- or ferromagnetic spin order with a Curie temperature of about 370 K. The measured high-field magnetization and anisotropy constant for Mn2PtSn are 405 emu/cm3 and 2.6 Mergs/cm3, respectively. The samples are all highly conducting with metallic electron transport and show a substantial negative magnetoresistance.
On the origin of the electron blocking effect by an n-type AlGaN electron blocking layer
Authors: Zhang Zi-Hui , Ji Yun , Liu Wei , Tiam Tan Swee , Kyaw Zabu , Ju Zhengang , Zhang Xueliang , Hasanov Namig , Lu Shunpeng , Zhang Yiping , Zhu Binbin , Wei Sun Xiao , Volkan Demir Hilmi
Published: February 2014

In this work, the origin of electron blocking effect of n-type Al0.25Ga0.75N electron blocking layer (EBL) for c+ InGaN/GaN light-emitting diodes has been investigated through dual-wavelength emission method. It is found that the strong polarization induced electric field within the n-EBL reduces the thermal velocity and correspondingly the mean free path of the hot electrons. As a result, the electron capture efficiency of the multiple quantum wells is enhanced, which significantly reduces the electron overflow from the active region and increases the radiative recombination rate with holes.
Effect of implanted species on thermal evolution of ion-induced defects in ZnO
Authors: Azarov A. Yu. , Hallén A. , Du X. L. , Rauwel P. , Kuznetsov A. Yu. , Svensson B. G.
Published: February 2014

Implanted atoms can affect the evolution of ion-induced defects in radiation hard materials exhibiting a high dynamic annealing and these processes are poorly understood. Here, we study the thermal evolution of structural defects in wurtzite ZnO samples implanted at room temperature with a wide range of ion species (from 11B to 209Bi) to ion doses up to 2 × 1016 cm−2. The structural disorder was characterized by a combination of Rutherford backscattering spectrometry, nuclear reaction analysis, and transmission electron microscopy, while secondary ion mass spectrometry was used to monitor the behavior of both the implanted elements and residual impurities, such as Li. The results show that the damage formation and its thermal evolution strongly depend on the ion species. In particular, for F implanted samples, a strong out-diffusion of the implanted ions results in an efficient crystal recovery already at 600 °C, while co-implantation with B (via BF2) ions suppresses both the F out-diffusion and the lattice recovery at such low temperatures. The damage produced by heavy ions (such as Cd, Au, and Bi) exhibits a two-stage annealing behavior where efficient removal of point defects and small defect clusters occurs at temperatures ∼500 °C, while the second stage is characterized by a gradual and partial annealing of extended defects. These defects can persist even after treatment at 900 °C. In contrast, the defects produced by light and medium mass ions (O, B, and Zn) exhibit a more gradual annealing with increasing temperature without distinct stages. In addition, effects of the implanted species may lead to a nontrivial defect evolution during the annealing, with N, Ag, and Er as prime examples. In general, the obtained results are interpreted in terms of formation of different dopant-defect complexes and their thermal stability.
Single particle transport in two-dimensional heterojunction interlayer tunneling field effect transistor
Authors: Li Mingda (Oscar) , Esseni David , Snider Gregory , Jena Debdeep , Grace Xing Huili
Published: February 2014

The single particle tunneling in a vertical stack consisting of monolayers of two-dimensional semiconductors is studied theoretically, and its application to a novel Two-dimensional Heterojunction Interlayer Tunneling Field Effect Transistor (Thin-TFET) is proposed and described. The tunneling current is calculated by using a formalism based on the Bardeen's transfer Hamiltonian, and including a semi-classical treatment of scattering and energy broadening effects. The misalignment between the two 2D materials is also studied and found to influence the magnitude of the tunneling current but have a modest impact on its gate voltage dependence. Our simulation results suggest that the Thin-TFETs can achieve very steep subthreshold swing, whose lower limit is ultimately set by the band tails in the energy gaps of the 2D materials produced by energy broadening. The Thin-TFET is thus very promising as a low voltage, low energy solid state electronic switch.
High frequency study of core–shell reusable CoFe2O4–ZnO nanospheres
Authors: Kuanr Bijoy K. , Veerakumar V. , Mishra S. R. , Wilson Armstrong M. , Kuanr Alka V. , Camley R. E. , Celinski Z.
Published: February 2014

Recently, coated nanoshells combined with magnetic nanoparticles and cancer-cell-specific antibodies have been used to develop a multifunctional platform for simultaneously diagnosing and treating cancer, via magnetic resonance imaging and photothermal therapy. For this application, core-shell nano-spheres with a low resonance frequency (low GHz range) in a strong applied field are required. In this aspect, ferromagnetic resonance experiment is an important tool for determining the dynamic properties of nano-materials. Magnetic field dependence of resonance frequency (fr) and linewidths (Δƒ and ΔH) for both the ZnO coated and uncoated CoFe2O4 hollow spheres are studied using a vector network analyzer. As compared to uncoated CoFe2O4 hollow sphere, ZnO coated CoFe2O4 showed reduced resonance frequency, larger Δƒ and ΔH, reduced gyromagnetic ratio and effective fields. The experimental results are confirmed with the effective medium theory.
Perpendicular magnetic anisotropic Pr-Fe-B thin films on glass substrates
Authors: Huang Chuan-Fa , Sun An-Cheng , Chang H. W. , Yuan Fe-Te , Chang S. T. , Mei Jui-Kuo , Hsiao S. N. , Lee H. Y. , Lu Hsi-Chuan , Wang Sea-Fue , Chang W. C.
Published: February 2014

The correlation between microstructures and magnetic properties of Pr-Fe-B thin films with thickness in the range of 25–150 nm sputtered on Ta underlayer buffered glass substrates was investigated. The films with thickness of 50–125 nm showed Pr2Fe14B phase with (004) preferred orientation. Perpendicular magnetic properties, including perpendicular magnetic anisotropy, coercivity ((Hc⊥)2T), remanence ratio ((Mr/M2T)⊥), and energy product [(BH)max⊥]2T were improved with increment of magnetic layer thickness to 150 nm. Higher (Hc⊥)2T of 10.3 kOe and energy product [(BH)max⊥]2T of 19.8 MGOe were attained in the film with the thicker thickness, in turn resulted from fine microstructure and impeded domain wall motion due to nonmagnetic Pr-rich grain boundary phase as a pinning site.
Spin caloritronics in graphene with Mn
Authors: Torres Alberto , Lima Matheus P. , Fazzio A. , da Silva Antônio J. R.
Published: February 2014

We show that graphene with Mn adatoms trapped at single vacancies features spin-dependent Seebeck effect, thus enabling the use of this material for spin caloritronics. A gate potential can be used to tune its thermoelectric properties in a way it presents either a total spin polarized current, flowing in one given direction, or currents for both spins flowing in opposite directions without net charge transport. Moreover, we show that the thermal magnetoresistance can be tuned between −100% and +100% by varying a gate potential.
High-frequency permeability of Ni and Co particle assemblies
Authors: Sakharov V. K. , Booth R. A. , Majetich S. A.
Published: February 2014

A coaxial transmission line was constructed, characterized, and calibrated for the frequency dependent measurement of complex relative permeability (μr) and complex permittivity (εr). The permeability of Ni powder with a grain size of < 1 μm was measured as a function of packing density to verify the system performance. 8–10 nm diameter Co nanoparticles were synthesized, dried to a powder, and measured. The real part of the permeability for the Co nanoparticles decreased over time as a result of oxidation, and decreased the overall magnetic volume due to the formation of an antiferromagnetic CoO shell. Similarly, the imaginary part of the permeability decreased as a function of oxidation. This was attributed to the insulating CoO shell reducing eddy current losses in the nanoparticle composite.
New opportunities in microwave electronics with ferromagnetic nanowires
Authors: Sharma Monika , Kuanr Bijoy K. , Sharma Manish , Basu Ananjan
Published: February 2014

A series of monolithic tunable microwave band-stop filters and phase shifters have been designed, fabricated, and characterized on arrays of ferromagnetic nanowires (FMNWs) constituted of Ni, Co, CoFeB, and NiFe alloy embedded in anodic alumina templates (AAO). The aim of the present investigation is to significantly boost the operating frequency of devices in very low applied magnetic field. The stop-band response can cover from 8 to 40 GHz with an applied magnetic field up to 5 kOe using these materials. In addition, we observed a substantial increase in frequency tunability (>110%) for Ni and NiFe alloy based filter. Frequency bandwidth/linewidth can be tuned with the proper choice of magnetic material, broad band for Co nanowires to narrow band for CoFeB nanowires. The amorphous nature of CoFeB reduces linewidth considerably and hence increases resonance absorption. It is also observed that the frequency linewidth (Δf) decreases with increasing applied magnetic field. The same filter can be used as a tunable phase shifter. For Ni nanowire based phase shifter, a maximum shift of 75°/cm at 4 kOe was observed.
Extraction of Schottky barrier at the F-doped SnO2/TiO2 interface in Dye Sensitized solar cells
Authors: Pillai P. B. , Corpus Mendoza A. N. , De Souza M. M. , Bree G. , Jeng D.
Published: February 2014

The Schottky barrier between the nanoporous TiO2/F–doped SnO2 interface is extracted via experiment for the first time using two independent approaches. In the first approach, Ohmic contacts using Ti and Al on nanoporous TiO2 film are made to facilitate temperature dependant IV measurements. An extracted barrier height of ∼0.6 eV at the TiO2/F–doped SnO2 assuming a theoretical value of the Richardson's constant is obtained. The barrier height is incorrectly deduced to be 0.31 eV if a Richardson constant extracted from experimental data fitted to thermionic emission theory is employed. In the second approach, a voltage dependent barrier height of 0.53 eV at T = 285 K is extracted from the dark current characteristics of a dye sensitized solar cell contacted to the nanoporous TiO2 film via the dye, electrolyte as normal. The analysis shows that in dye sensitized solar cell, the impact of such a barrier is dominant only beyond open circuit voltage conditions in the range above 1–1.2 V.
Conductive SnO2:Sb nanobelts as electrodes for detection of NO2 in ppb level with ultrahigh sensitivity
Authors: Cai Bin , Zhao Xiaoli , Pei Tengfei , Toninelli Ermes , Tang Qingxin , Tong Yanhong , Liu Yichun
Published: February 2014

We report a SnO2 nanobelt field effect transistor sensor, with the SnO2:Sb nanobelts serving as the source and drain electrodes. An anomalous feature of the device is that the threshold voltage shows the negative shift upon exposure to NO2. The comparative results from the other two types of devices, including SnO2 nanobelt with metal film electrodes and SnO2:Sb nanobelt with metal film electrodes, reveal that the interface between the SnO2:Sb nanobelt electrodes and the SnO2 nanobelt is responsible for the improved carrier injection efficiency and the negative shift in the threshold voltage. Such a response mechanism results in the detection limit for NO2 down to 10 ppb, with a sensitivity as high as 7.16 × 105 % at room temperature.
Ge quantum dots encapsulated by AlAs grown by molecular beam epitaxy on GaAs without extended defects
Authors: Qi Meng , Stephenson Chad A. , Protasenko Vladimir , O’Brien William A. , Mintairov Alexander , Xing Huili (Grace) , Wistey Mark A.
Published: February 2014

We demonstrate nearly spherical, strain-free, self-assembled Ge quantum dots (QDs) fully encapsulated by AlAs, grown on (100) GaAs by molecular beam epitaxy. The QDs were formed without a wetting layer using a high temperature, in situ anneal. Subsequent AlAs overgrowth was free from anti-phase domains and threading dislocations in cross section transmission electron microscopy. The straddling band alignment for Ge in AlAs promises strong and tunable confinement for both electrons and holes. The reflection high-energy electron diffraction pattern changed from 2 × 3 to 2 × 5 with anneal, which can be explained by surface reconstructions based on the electron-counting model.
Design and experiment of perfect relay lens based on the Schwarz-Christoffel mapping
Authors: Yang Fan , Mei Zhong Lei , Cui Tie Jun
Published: February 2014

We propose and experimentally demonstrate a planar perfect relay lens using the modified Schwarz-Christoffel (SC) mapping. Based on the SC mapping, a Maxwell fisheye lens is conformally transformed into a slab exactly, realizing a perfect relay lens. Both geometrical optics and wave optics are utilized to verify the excellent imaging performance from surface to surface. The proposed relay lens is implemented using fractal metamaterials with the H-shaped particles and the experimental result shows good agreement to the numerical simulation. The proposed design has many potential applications, such as the night vision system, endoscopes, microscopes, and photocopiers.
Analysis of bias voltage dependent spectral response in Ga0.51In0.49P/Ga0.99In0.01As/Ge triple junction solar cell
Authors: Sogabe Tomah , Ogura Akio , Okada Yoshitaka
Published: February 2014

Spectral response measurement plays great role in characterizing solar cell device because it directly reflects the efficiency by which the device converts the sunlight into an electrical current. Based on the spectral response results, the short circuit current of each subcell can be quantitatively determined. Although spectral response dependence on wavelength, i.e., the well-known external quantum efficiency (EQE), has been widely used in characterizing multijunction solar cell and has been well interpreted, detailed analysis of spectral response dependence on bias voltage (SR −Vbias) has not been reported so far. In this work, we have performed experimental and numerical studies on the SR −Vbias for Ga0.51In0.49P/Ga0.99In0.01As/Ge triple junction solar cell. Phenomenological description was given to clarify the mechanism of operation matching point variation in SR −Vbias measurements. The profile of SR−Vbias curve was explained in detail by solving the coupled two-diode current-voltage characteristic transcend formula for each subcell.
Multi-wavelength Raman scattering of nanostructured Al-doped zinc oxide
Authors: Russo V. , Ghidelli M. , Gondoni P. , Casari C. S. , Li Bassi A.
Published: February 2014

In this work we present a detailed Raman scattering investigation of zinc oxide and aluminum-doped zinc oxide (AZO) films characterized by a variety of nanoscale structures and morphologies and synthesized by pulsed laser deposition under different oxygen pressure conditions. The comparison of Raman spectra for pure ZnO and AZO films with similar morphology at the nano/mesoscale allows to investigate the relation between Raman features (peak or band positions, width, relative intensity) and material properties such as local structural order, stoichiometry, and doping. Moreover Raman measurements with three different excitation lines (532, 457, and 325 nm) point out a strong correlation between vibrational and electronic properties. This observation confirms the relevance of a multi-wavelength Raman investigation to obtain a complete structural characterization of advanced doped oxide materials.
Metal gate work function tuning by Al incorporation in TiN
Authors: Lima L. P. B. , Dekkers H. F. W. , Lisoni J. G. , Diniz J. A. , Van Elshocht S. , De Gendt S.
Published: February 2014

Titanium nitride (TiN) films have been used as gate electrode on metal-oxide-semiconductor (MOS) devices. TiN effective work function (EWF) values have been often reported as suitable for pMOS. For nMOS devices, a gate electrode with sufficient low EWF value with a similar robustness as TiN is a challenge. Thus, in this work, aluminum (Al) is incorporated into the TiN layer to reduce the EWF values, which allows the use of this electrode in nMOS devices. Titanium aluminum (TiAl), Al, and aluminum nitride (AlN) layers were introduced between the high-k (HfO2) dielectric and TiN electrode as Al diffusion sources. Pt/TiN (with Al diffusion) and Pt/TiN/TiAl/TiN structures were obtained and TiN EWF values were reduced of 0.37 eV and 1.09 eV, respectively. The study of TiN/AlN/HfO2/SiO2/Si/Al structures demonstrated that AlN layer can be used as an alternative film for TiN EWF tuning. A decrease of 0.26 eV and 0.45 eV on TiN EWF values were extracted from AlN/TiN stack and AlN/TiN laminate stack, respectively. AlN/TiN laminate structures have been shown to be more effective to reduce the TiN work function than just increasing the AlN thickness.
Nonlinear vs. bolometric radiation response and phonon thermal conductance in graphene-superconductor junctions
Authors: Vora Heli , Nielsen Bent , Du Xu
Published: February 2014

Graphene is a promising candidate for building fast and ultra-sensitive bolometric detectors due to its weak electron-phonon coupling and low heat capacity. In order to realize a practical graphene-based bolometer, several important issues, including the nature of radiation response, coupling efficiency to the radiation and the thermal conductance need to be carefully studied. Addressing these issues, we present graphene-superconductor junctions as a viable option to achieve efficient and sensitive bolometers, with the superconductor contacts serving as hot electron barriers. For a graphene-superconductor device with highly transparent interfaces, the resistance readout in the presence of radio frequency radiation is dominated by non-linear response. On the other hand, a graphene-superconductor tunnel device shows dominantly bolometric response to radiation. For graphene devices fabricated on SiO2 substrates, we confirm recent theoretical predictions of T2 temperature dependence of phonon thermal conductance in the presence of disorder in the graphene channel at low temperatures.
Transport level in disordered organics: An analytic model and Monte-Carlo simulations
Authors: Nikitenko V. R. , Strikhanov M. N.
Published: February 2014

Transport level concept is known as a promising tool which provides great simplification in analytic description of hopping transport in organics. However, quantitative modeling of mobility and diffusion coefficient by the use of this concept is extremely rare up to the moment. Monte-Carlo modeling of transport level and related quantities in the framework of Gaussian disorder model is carried out in this work. Methodology of this modeling is discussed and physical essence of various approaches to transport level is clarified. It is shown that an analytic model, which considers the transport level as the average energy of states from which a carrier can be released by means of energetically upward and downward jumps with equal probability, is applicable for quantitative modeling of temperature dependence of mobility and coefficient of field-stimulated diffusion. Simple analytic expressions for these transport coefficients are obtained.
Pyroelectric properties of Mn-doped 94.6Na0.5Bi0.5TiO3-5.4BaTiO3 lead-free single crystals
Authors: Sun Renbing , Wang Jinzhi , Wang Fang , Feng Tangfu , Li Yanlong , Chi Zhenhua , Zhao Xiangyong , Luo Haosu
Published: February 2014

We investigated the pyroelectric properties of ⟨001⟩, ⟨110⟩, and ⟨111⟩-oriented Mn doped 94.6Na0.5Bi0.5TiO3-5.4BaTiO3 (94.6NBT-5.4BT) crystals. Among the above three crystallographic orientations, the ⟨111⟩-oriented crystals possess the highest pyroelectric coefficients and the largest figures of merit. At 20 °C, the values of p, Fv, and Fd reach as high as 5.88 × 10−4 C/m2 K, 29.8 μPa−1/2, and 0.08 m2/C for ⟨111⟩-oriented crystals, respectively, which are superior to other lead-free ferroelectric materials. The crystals also demonstrate a high depolarization temperature and relatively stable thermal pyroelectric property. These results indicate the potential of Mn-doped 94.6NBT-5.4BT crystals for lead-free infrared pyroelectric detectors.
Bias and temperature dependent charge transport in flexible polypyrrole devices
Authors: Taunk Manish , Chand Subhash
Published: February 2014

The temperature dependent current density-voltage (J-V) characteristics of flexible polypyrrole film sandwiched between metal electrodes (Ag, Al) are investigated in the wide temperature range of 10–300 K. At all temperatures and low bias voltage, charge transport is governed by bulk limited processes with a bias dependent crossover from Ohmic (J ∼ V) to trap free space charge limited conduction (J ∼ V2). At high bias voltage (>3 V) and low temperatures, the conduction becomes space charge limited with exponential distribution of traps. We estimated an average trap energy 29.9 eV and trap density of the order of 1012 cm−3.
Theoretical and electron paramagnetic resonance studies of hyperfine interaction in nitrogen doped 4H and 6H SiC
Authors: Szász K. , Trinh X. T. , Son N. T. , Janzén E. , Gali A.
Published: February 2014

Motivated by recent experimental findings on the hyperfine signal of nitrogen donor (NC) in 4 H and 6 H SiC, we calculate the hyperfine tensors within the framework of density functional theory. We find that there is negligible hyperfine coupling with 29Si isotopes when NC resides at h site both in 4 H and 6 H SiC. We observe measurable hyperfine coupling to a single 29Si at k site in 4 H SiC and k1 site in 6 H SiC. Our calculations unravel that such 29Si hyperfine coupling does not occur at k2 site in 6 H SiC. Our findings are well corroborated by our new electron paramagnetic resonance studies in nitrogen doped 6 H SiC.
Dramatically enhanced self-assembly of GeSi quantum dots with superior photoluminescence induced by the substrate misorientation
Authors: Zhou Tong , Zhong Zhenyang
Published: February 2014

A dramatically enhanced self-assembly of GeSi quantum dots (QDs) is disclosed on slightly miscut Si (001) substrates, leading to extremely dense QDs and even a growth mode transition. The inherent mechanism is addressed in combination of the thermodynamics and the growth kinetics both affected by steps on the vicinal surface. Moreover, temperature-dependent photoluminescence spectra from dense GeSi QDs on the miscut substrate demonstrate a rather strong peak persistent up to 300 K, which is attributed to the well confinement of excitons in the dense GeSi QDs due to the absence of the wetting layer on the miscut substrate.
Calculation of the lattice thermal conductivity in granular crystals
Authors: Kazan M. , Volz S.
Published: February 2014

This paper provides a general model for the lattice thermal conductivity in granular crystals. The key development presented in this model is that the contribution of surface phonons to the thermal conductivity and the interplay between phonon anharmonic scattering and phonon scattering by boundaries are considered explicitly. Exact Boltzmann equation including spatial dependence of phonon distribution function is solved to yield expressions for the rates at which phonons scatter by the grain boundaries in the presence of intrinsic phonon scattering mechanisms. The intrinsic phonon scattering rates are calculated from Fermi's golden rule, and the vibration parameters of the model are derived as functions of temperature and crystallographic directions by using a lattice dynamics approach. The accuracy of the model is demonstrated with reference to experimental measurements regarding the effects of surface orientation and isotope composition on the thermal conductivity in single crystals, and the effect of grains size and shape on the thermal conductivity tensor in granular crystals.
Band gap tunning in BN-doped graphene systems with high carrier mobility
Authors: Kaloni T. P. , Joshi R. P. , Adhikari N. P. , Schwingenschlögl U.
Published: February 2014

Using density functional theory, we present a comparative study of the electronic properties of BN-doped graphene monolayer, bilayer, trilayer, and multilayer systems. In addition, we address a superlattice of pristine and BN-doped graphene. Five doping levels between 12.5% and 75% are considered, for which we obtain band gaps from 0.02 eV to 2.43 eV. We demonstrate a low effective mass of the charge carriers.
Core-halo issues for a very high intensity beam
Authors: Nghiem P. A. P. , Chauvin N. , Simeoni W. , Uriot D.
Published: February 2014

The relevance of classical parameters like beam emittance and envelope used to describe a particle beam is questioned in case of a high intensity accelerator. In the presence of strong space charge effects that affect the beam differently following its density, the much less dense halo part behaves differently from the much denser core part. A method for precisely determining the core-halo limit is proposed, that allows characterizing the halo and the core independently. Results in 1D case are given and discussed. Expected developments extending the method to 2D, 4D, or 6D phase spaces are examined.
Elimination of columnar microstructure in N-face InAlN, lattice-matched to GaN, grown by plasma-assisted molecular beam epitaxy in the N-rich regime
Authors: Ahmadi Elaheh , Shivaraman Ravi , Wu Feng , Wienecke Steven , Kaun Stephen W. , Keller Stacia , Speck James S. , Mishra Umesh K.
Published: February 2014

The microstructure of N-face InAlN layers, lattice-matched to GaN, was investigated by scanning transmission electron microscopy and atom probe tomography. These layers were grown by plasma-assisted molecular beam epitaxy (PAMBE) in the N-rich regime. Microstructural analysis shows an absence of the lateral composition modulation that was previously observed in InAlN films grown by PAMBE. A room temperature two-dimensional electron gas (2DEG) mobility of 1100 cm2/V s and 2DEG sheet charge density of 1.9 × 1013 cm−2 was measured for N-face GaN/AlN/GaN/InAlN high-electron-mobility transistors with lattice-matched InAlN back barriers.
Fabrication of polycrystalline Cu2ZnSnSe4 layers with strongly preferential grain orientation via selenization of Sn/Cu/ZnSe(001)/GaAs(001) structures
Authors: Krämmer Christoph , Sachs Johannes , Lang Mario , Pfaffmann Lukas , Gao Chao , Gerthsen Dagmar , Kalt Heinz , Powalla Michael , Hetterich Michael
Published: February 2014

We report on a simple approach for the fabrication of polycrystalline Cu2ZnSnSe4 films on GaAs with strongly preferential grain orientation. Such layers are important for both fundamental spectroscopic investigations as well as studies of the impact of grain boundaries on the performance of solar cells. Cu2ZnSnSe4 (CZTSe) layers were fabricated by growing a Sn/Cu/ZnSe(001) stack on a GaAs(001) substrate in a molecular-beam epitaxy system followed by selenization in a tube furnace. Raman spectroscopy as well as X-ray diffraction measurements combined with a microstructural investigation indicate the presence of the kesterite phase. Further, X-ray analysis such as rocking curves and ϕ-scans proves a preferential grain orientation of the obtained CZTSe films in all three dimensions.
Communication: The dark singlet state as a doorway state in the ultrafast and efficient intersystem crossing dynamics in 2-thiothymine and 2-thiouracil
Authors: Pollum Marvin , Crespo-Hernández Carlos E.
Published: February 2014

Femtosecond broadband transient absorption experiments are reported for 2-thiothymine and 2-thiouracil in aqueous buffer solution and in acetonitrile. It is shown that the S1(nπ*) state acts as a doorway state in the ultrafast and efficient population of the T1(ππ*) state upon 316 nm excitation. A sequential kinetic model is presented to explain the excited-state dynamics in 2-thiothymine and 2-thiouracil upon UVA excitation: S2(ππ*) → S1(nπ*) → T1(ππ*). The experimental results are also used to scrutinize the excited-state relaxation pathways recently predicted for 2-thiouracil at the CASPT2//CASSCF level of theory [G. Cui and W. Fang, J. Chem. Phys. 138, 044315 (2013)]. The efficient population of the T1(ππ*) state for both 2-thiothymine and 2-thiouracil in a few hundreds of femtoseconds lends further support to the emerging idea that thiobase derivatives exhibit photo-toxic properties that can be effectively harnessed in photo-chemotherapeutic applications.
Intrinsic domain wall pinning in rapidly solidified amorphous nanowires
Authors: Óvári T.-A. , Chiriac H.
Published: February 2014

Magnetoelastic anisotropy calculations and experimental switching field data are used to investigate the origin of the domain wall pinning in rapidly solidified amorphous glass-coated nanowires and submicron wires. The correlation of experimental and calculated results have shown that the magneto-mechanical coupling between internal stresses induced during preparation and magnetostriction is responsible for the pinning of the domain walls in amorphous wires with diameters below 300 nm, whilst in thicker submicron wires demagnetization is also important. The results are important for fully understanding and controlling magnetic switching in rapidly solidified amorphous nanowires for future applications.
Focused ion beam high resolution grayscale lithography for silicon-based nanostructures
Authors: Erdmanis M. , Tittonen I.
Published: February 2014

Nanofabrication techniques providing a fine control over the profile of silicon structures are of great importance for nanophotonics, plasmonics, sensing, micro- and nano fluidics, and biomedical applications. We report on the applicability of focused ion beam for the fine grayscale lithography, which yields surface profiles that are customized at nanoscale. The approach is based on a correlation between the ion beam irradiation dose of inorganic resist and the mask etching rate in the reactive ion etching. An exceptional property of this method is the number of gray tones that are not limited by the resist characteristics. We apply the process to fabricate unique periodic nanostructures with a slope angle varying across the structure and a period as small as 200 nm.
Note: Surface acoustic wave resonators for detecting of small changes of temperature: A thermometric “magnifying glass”
Authors: Kryshtal R. G. , Medved A. V.
Published: February 2014

Application of surface acoustic wave resonators with a phase format of an output signal as the thermometric “magnifying glass” is suggested. Possibilities of monitoring and measuring of small changes of temperature from 0.001 K to 0.3 K of objects having thermal contact with the resonator's substrate are shown experimentally.
Note: Sensitivity multiplication module for quartz crystal microbalance applications
Authors: Burda Ioan , Silaghi Andreea , Tunyagi Arthur , Simon Simion , Popescu Octavian
Published: February 2014

In this Note, a novel sensitivity multiplication module was added to classical quartz crystal microbalance (QCM). The purpose is to increase QCM frequency shift without changing nominal frequency of the quartz crystal resonator or nominal frequency value delivered to the frequency counter. Allan deviance measurement confirms that the multiplication of the frequency shift is limited by the quartz crystal loads with direct effect in quartz crystal quality factor and oscillator stability. An experimental implementation of this new sensitivity multiplication module that can increase up to six times the frequency shift of the QCM was experimentally investigated using different load conditions.