bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamics of polymer translocation into a circular nanocontainer through a nanopore
Authors: Zhang Kehong , Luo Kaifu
Published: May 2012

Using Langevin dynamics simulations, we investigate the dynamics of polymer translocation into a circular nanocontainer through a nanopore under a driving force F. We observe that the translocation probability initially increases and then saturates with increasing F, independent of ϕ, which is the average density of the whole chain in the nanocontainer. The translocation time distribution undergoes a transition from a Gaussian distribution to an asymmetric distribution with increasing ϕ. Moreover, we find a nonuniversal scaling exponent of the translocation time as chain length, depending on ϕ and F. These results are interpreted by the conformation of the translocated chain in the nanocontainer and the time of an individual segment passing through the pore during translocation.
Vertical conduction mechanism of the epitaxial graphene/n-type 4H-SiC heterojunction at cryogenic temperatures
Authors: Tadjer M. J. , Anderson T. J. , Hobart K. D. , Nyakiti L. O. , Wheeler V. D. , Myers-Ward R. L. , Gaskill D. K. , Eddy C. R. , Kub F. J. , Calle F.
Published: May 2012

Vertical diodes of epitaxial graphene on n− 4H-SiC were investigated. The graphene Raman spectra exhibited a higher intensity in the G-line than the 2D-line, indicative of a few-layer graphene film. Rectifying properties improved at low temperatures as the reverse leakage decreased over six orders of magnitude without freeze-out in either material. Carrier concentration of ∼1016 cm−3 in the SiC remained stable down to 15 K, while accumulation charge decreased and depletion width increased in forward bias. The low barrier height of 0.08 eV and absence of recombination-induced emission indicated majority carrier field emission as the dominant conduction mechanism.
Electrical conductivity and crystallization kinetics in Te-Se glassy system
Authors: Barták Jaroslav , Svoboda Roman , Málek Jiří
Published: May 2012

Non-isothermal measurements of dc conductivity were used to study crystallization in bulk glasses from the TexSe100−x system (x = 10, 20, and 30). In combination with infrared microscopy, it provides a very good qualitative picture of the complex crystallization processes, as the formation of conductive paths proceeds more or less separately for each involved process. In addition, due to the pronounced signal corresponding to the surface crystallization, the dc conductivity measurements seem to bear great potential for crystallization kinetic studies. Based on identified characteristic states corresponding to the particular crystallization mechanisms, the activation energies were calculated for surface and bulk processes and compared to the differential scanning calorimetry (DSC) results reported earlier. Furthermore, the conversion rate α was calculated from the conductivity data by using the Odolevsky equation, the obtained dependence is in a good agreement with the results from residual enthalpies measurements performed by DSC. In a supplemental study a thorough kinetic analysis was applied to the DSC measurements of residual enthalpies. The observed decrease of the Johnson-Mehl-Avrami kinetic exponent m was explained by decrease in dimensionality of the bulk crystallization process.
Silicon nanocluster sensitization of erbium ions under low-energy optical excitation
Authors: Prtljaga Nikola , Navarro-Urrios Daniel , Pitanti Alessandro , Ferrarese-Lupi Federico , Garrido Blas , Pavesi Lorenzo
Published: May 2012

The sensitizing action of amorphous silicon nanoclusters on erbium ions in thin silica films has been studied under low-energy (long wavelength) optical excitation. Profound differences in fast visible and infrared emission dynamics have been found with respect to the high-energy (short wavelength) case. These findings point out to a strong dependence of the energy transfer process on the optical excitation energy. Total inhibition of energy transfer to erbium states higher than the first excited state (4I13/2) has been demonstrated for excitation energy below 1.82 eV (excitation wavelength longer than 680 nm). Direct excitation of erbium ions to the first excited state (4I13/2) has been confirmed to be the dominant energy transfer mechanism over the whole spectral range of optical excitation used (540 nm–680 nm).
Conduction mechanism of resistive switching films in MgO memory devices
Authors: Chiu Fu-Chien , Shih Wen-Chieh , Feng Jun-Jea
Published: May 2012

In this work, nonpolar resistance switching behavior was demonstrated in Pt/MgO/Pt structure. The resistance ratio of high resistance state (HRS) and low resistance state (LRS) is about on the order of 105 for the compliance current (Icomp) of 1 mA at 300 K. Using enough Icomp (≥0.5 mA) during SET processes, the LRS resistances reach a minimum of about 102–103 Ω and the RESET currents reach a maximum of about 10−4–10−3 A. Experimental results indicate that the conduction mechanism in MgO films is dominated by the hopping conduction and the Ohmic conduction in HRS and LRS, respectively. Therefore, the electrical parameters of trap energy level, trap spacing, Fermi level, electron mobility, and effective density of states in conduction band in MgO films were obtained.
Heterophase states and domain effects in solid solutions of (1 − x)BiFeO3 − xPbTiO3
Authors: Topolov V. Yu.
Published: May 2012

Examples of heterophase states in multiferroic solid solutions of (1 − x)BiFeO3 − xPbTiO3 with the perovskite-type structure are studied taking into account the features of elastic matching of polydomain (twinned) ferroelectric phases near the morphotropic phase boundary and in a wide temperature range. Conditions for the complete stress relief are examined in the heterophase states P4mm–P4mm and P4mm–Cc to interpret variations of the phase content [S. Bhattacharjee et al., Phys. Rev. B 84, 104116 (2011)] [S. Bhattacharjee and D. Pandey, J. Appl. Phys. 110, 084105 (2011)] in (1−x)BiFeO3−xPbTiO3. The key role of some 90° domain types of the low-temperature ferroelectric P4mm phase in the stress relief at the phase coexistence is emphasized. An agreement between the evaluated and experimental data on the volume fraction of the low-temperature P4mm phase in heterophase samples is observed.
Probing the structural and electronic properties of AgnH− (n = 1–3) using photoelectron imaging and theoretical calculations
Authors: Xie Hua , Xing Xiaopeng , Liu Zhiling , Cong Ran , Qin Zhengbo , Wu Xia , Tang Zichao , Fan Hongjun
Published: May 2012

Structural and electronic properties of silver hydride cluster anions (AgnH−; n = 1–3) have been explored by combining the negative ion photoelectron imaging spectroscopy and theoretical calculations. The photoelectron spectrum of AgH− exhibits transitions from AgH− 2Σ+ to AgH 1Σ+ and AgH 3Σ+, with the electron affinity (EA) 0.57(3) eV. For Ag2H−, the only observed transition is from Ag2H− (C∞v) 1Σ+ to Ag2H (C2v) 2A′ and the electron affinity is 2.56(5) eV. Two obvious electron bands are observed in photoelectron imaging of Ag3H−, which are assigned to the transitions from Ag3H− (C2v-T, which means C2v geometry with top site hydrogen) 2B2 to Ag3H (C2v-T) 1A1 and Ag3H (C2v-T) 3B2. The electron affinity is determined to be 1.61(9) eV. The Ag-H stretching modes in the ground states of AgH and Ag2H are experimentally resolved and their frequencies are measured to be 1710(80) and 1650(100) cm−1, respectively. Aside from the above EAs and the vibrational frequencies, the vertical detachment energies to all ground states and some excited states of AgnH (n = 1–3) are also obtained. Theoretical calculations reproduce the experimental energies quite well, and the results are used to assign the geometries and electronic states for all related species.
Electromechanical strain in Bi(Zn1/2Ti1/2)O3–(Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3 solid solutions
Authors: Patterson Eric A. , Cann David P. , Pokorny Jan , Reaney Ian M.
Published: May 2012

Solid solutions ceramics of the Bi(Zn0.5Ti0.5)O3–(Bi0.5K0.5)TiO3–(Bi0.5Na0.5)TiO3 ternary system for <20 mol. % BZT were created and confirmed to be single phase using x-ray diffraction. The dielectric dispersion showed decreasing Tmax of the dielectric spectrum with a broadening of the transition with increasing BZT content. At 2.5BZT–40BKT–57.5BNT, a secondary transition commonly observed for morphotropic phase boundary (MPB) BNT–BKT was observed. The ferroelectric behavior of the system was characterized by a transition where the polarization hysteresis showed a severe pinching effect on remanent polarization (20.8 μC/cm2 at 2.5% BZT) as BZT contents was increased (Pr = 2.3 μC/cm2 at 20% BZT). Similarly, as the temperature increased to 175 °C, the remanent polarization of the 2.5% BZT composition significantly reduced to 2.1 μC/cm2. The onset of this transition corresponds to the lower temperature frequency dispersion observed in the dielectric spectrum. The strain hysteresis experienced analogous transition to the polarization, with a change in shape from typical ferroelectric butterfly to a complete loss of negative strain as BZT concentration increased. Maximum strain values of 0.33% were observed at 5-40-55 accompanied by a large d33*=547pm/V.
The role of glass-viscosity on the growth of semiconductor quantum dots in glass matrices
Authors: Kumar Mishra Rakesh , Vedeshwar Agnikumar G. , Tandon R. P.
Published: May 2012

The dependence of glass viscosity on the growth of semiconductor (CdS) quantum dots in borosilicate glass matrices is studied systematically. The glass viscosity was varied (3000–260 000 Pa s) by varying the glass compositions of constituent oxides and growth temperatures. Thus, grown dot sizes at various growth times (2–48 h) were in the range 2–30 nm. The linear variation of dot size with cube root of growth time confirms the diffusion controlled growth. The growth of dot size was found to vary linearly with the inverse cube root of glass viscosity. The excitonic peak energy of the dots was found to vary linearly with inverse square of the dot size consistent with the quantum confinement effect. This analysis yields the bulk value of energy gap Eg (bulk) = 2.47 eV and the reduced effective masses as μ = 0.138 for CdS consistent with the values available in the literature. The emission (photoluminescence) spectra further support the absorption results.
Boron distribution in the core of Si nanowire grown by chemical vapor deposition
Authors: Chen Wanghua , Dubrovskii Vladimir G. , Liu Xiaolong , Xu Tao , Lardé Rodrigue , Philippe Nys Jean , Grandidier Bruno , Stiévenard Didier , Patriarche Gilles , Pareige Philippe
Published: May 2012

The boron dopant distribution in Si nanowires grown by the Au-catalyzed chemical vapor deposition is characterized by laser-assisted atom probe tomography. A convenient and an effective method for performing the atom probe tomography of an individual nanowire is developed. Using this technique, we demonstrate that when Si nanowires are doped with boron at high silane partial pressure, the radial distribution of boron atoms is rather inhomogeneous. Much more boron atoms incorporate at the periphery than in the center, with the concentration increasing by an order of magnitude as the distance from the nanowire axis increases from zero to only 15 nm. A theoretical model is presented that is capable of describing the observed spatial inhomogeneity of boron dopant. We also consider different kinetic pathways of boron incorporation and discuss the values of diffusion length and diffusion coefficients obtained by fitting the experimental data.
Equivalence between the mechanical model and energy-transfer theory for the classical decay rates of molecules near a spherical particle
Authors: Chung H. Y. , Leung P. T. , Tsai D. P.
Published: May 2012

In the classical modeling of decay rates for molecules interacting with a nontrivial environment, it is well known that two alternate approaches exist which include: (1) a mechanical model treating the system as a damped harmonic oscillator driven by the reflected fields from the environment; and (2) a model based on the radiative and nonradiative energy transfers from the excited molecular system to the environment. While the exact equivalence of the two methods is not trivial and has been explicitly demonstrated only for planar geometry, it has been widely taken for granted and applied to other geometries such as in the interaction of the molecule with a spherical particle. Here we provide a rigorous proof of such equivalence for the molecule-sphere problem via a direct calculation of the decay rates adopting each of the two different approaches.
Topological asymmetry induced electronic transport in three terminal graphene nanoribbon structure
Authors: Ye En-Jia , Sui Wen-Quan , Zhao Xuean
Published: May 2012

Topological configurations play an important role in graphene nanoribbon based devices. In this work, the Büttiker’s ac transport theory is used to study an asymmetric three terminal graphene nanoribbon structure. With the help of Green’s function and related parameters, we show that the topological asymmetries can form capacitive and inductive junctions in this three terminal structure. The transport properties are sensitive to the geometric features of the branches of the junctions and the coupling positions. It is believed that this kind of structure can be useful for the future nanoelectronic devices.
Surface oxide on thin films of yttrium hydride studied by neutron reflectometry
Authors: Mongstad T. , Platzer-Björkman C. , Mæhlen J. P. , Hauback B. C. , Karazhanov S. Zh. , Cousin F.
Published: May 2012

The applicability of standard methods for compositional analysis is limited for H-containing films. Neutron reflectometry is a powerful, non-destructive method that is especially suitable for these systems due to the large negative scattering length of H. In this work, we demonstrate how neutron reflectometry can be used to investigate thin films of yttrium hydride. Neutron reflectometry gives a strong contrast between the film and the surface oxide layer, enabling us to estimate the oxide thickness and oxygen penetration depths. A surface oxide layer of 5–10 nm thickness was found for unprotected yttrium hydride films.
Model for direct-transition gain in a Ge-on-Si laser
Authors: Chow Weng W.
Published: May 2012

This paper describes a laser gain model for the direct, Γ-point transition in bulk Ge. The model allows calculation of gain and spontaneous-emission spectra for arbitrary strain, as well as doping and injected carrier densities. Many-body effects are included at the level of the screened Hartree-Fock approximation to account for energy renormalization modifications, especially to the impact of n-doping on magnitude and spectral extension of laser gain. Application of the model is illustrated by predicting the net material peak gain versus injected current density under different combinations of tensile strain and n-doping density.
Anomalous enhancement of the thermoelectric figure of merit by V co-doping of Nb-SrTiO3
Authors: Ozdogan K. , Upadhyay Kahaly M. , Alshareef H. N. , Schwingenschlögl U.
Published: May 2012

The effect of V co-doping of Nb-SrTiO3 is studied by full-potential density functional theory. We obtain a stronger increase of the carrier density for V than for Nb dopants. While in Nb-SrTiO3 a high carrier density counteracts a high thermoelectric figure of merit, the trend is inverted by V co-doping. The mechanism leading to this behavior is explained in terms of a local spin-polarization introduced by the V ions. Our results indicate that magnetic co-doping can be a prominent tool for improving the thermoelectric figure of merit.
A model determining optimal doping concentration and material’s band gap of tunnel field-effect transistors
Authors: Vandenberghe William G. , Verhulst Anne S. , Kao Kuo-Hsing , Meyer Kristin De , Sorée Bart , Magnus Wim , Groeseneken Guido
Published: May 2012

We develop a model for the tunnel field-effect transistor (TFET) based on the Wentzel-Kramer-Brillouin approximation which improves over existing semi-classical models employing generation rates. We hereby introduce the concept of a characteristic tunneling length in direct semiconductors. Based on the model, we show that a limited density of states results in an optimal doping concentration as well as an optimal material’s band gap to obtain the highest TFET on-current at a given supply voltage. The observed optimal-doping trend is confirmed by 2-dimensional quantum-mechanical simulations for silicon and germanium.
Influence of 3d transition metals on the stability and electronic structure of MgH2
Authors: Zeng X. Q. , Cheng L. F. , Zou J. X. , Ding W. J. , Tian H. Y. , Buckley C.
Published: May 2012

The effect of minor addition of 3d transition metals on the formation enthalpy (△H) and electronic structure of MgH2 have been studied using first-principle calculations, and considering the phonon-calculated zero point energy. The results indicate that the partial substitution of Mg atoms by 3d transition metal atoms increases the formation enthalpy of MgH2. Both formation enthalpy and Mulliken population analysis showed that the ability to destabilize MgH2 generally increases with the atomic number, except Mn and Zn, which have half-filled and completely filled 3d orbital states. The destabilization of MgH2 by partially alloying 3d elements was due to relatively stronger covalent bonds between 3d elements and the H atom, and a weaker ionic bond between Mg and H in the alloyed material with respect to pure MgH2. Based on electronic structure analyses, MgH2 and MgH2 alloyed with Ti, Fe, and Zn show no spin magnetism, while MgH2 alloyed with Sc, V, Cr, Mn, Co, Ni, and Cu show spin magnetism. In the MgH2-3d metal system except Zn, the bonding peak near the Fermi energy is mainly contributed by 3d electrons of transition metals and weak H (s) states. The bonding nature of MgH2 is ionic, and the bonding nature of MgH2-3d metal systems is mainly ionic with covalent bonds between 3d metal atoms and their neighbor H atoms.
Electric-field induced phase transitions of dielectric colloids: Impact of multiparticle effects
Authors: Wood Jeffery A. , Docoslis Aristides
Published: May 2012

The thermodynamic framework for predicting the electric-field induced fluid like-solid like phase transition of dielectric colloids developed by Khusid and Acrivos [Phys. Rev. E. 54, 5428 (1996)] is extended to examine the impact of multiscattering/multiparticle effects on the resulting phase diagrams. This was accomplished using effective permittivity models suitable both over the entire composition region for hard spheres (0≤c<cmax) and for multiple types of solid packing structures (random close-packed structure, FCC, BCC). The Sihvola-Kong model and the self-consistent permittivity model of Sen et al. [Geophysics 46, 781 (1981)] were used to generate the coexistence (slow phase transition) and spinodal (rapid phase transition) boundaries for the system and compared to assuming Maxwell-Garnett permittivity. It was found that for larger dielectric contrasts between medium and particle that the impact of accounting for multiscattering effects increased and that there was a significant shift in the resulting phase diagrams. Results obtained for model colloidal systems of silica-dimethylsulfoxide and silica-isopropanol showed that critical electric field strength required for phase transitions could rise by up to approximately 20% when considering multiparticle effects versus the isolated dipole case. The impact of multiparticle effects on the phase diagrams was not only limited purely to the direct effect of volume fraction on permittivity and particle dipoles but also on the curvature of the volume fraction dependence. This work stresses the importance of accounting for particle effects on the polarization of colloidal suspensions, which has large implications for predicting the behavior of electrorheological fluids and other electric-field driven phenomena.
Adsorption kinetics of ammonia sensing by graphene films decorated with platinum nanoparticles
Authors: Gautam Madhav , Jayatissa Ahalapitiya H.
Published: May 2012

Ammonia sensing behavior of graphene synthesized by a chemical vapor deposition on a copper substrate using a methane and hydrogen gas mixture has been investigated. The Raman spectroscopy is used to monitor the quality of graphene films transferred onto SiO2/Si substrates. The sensitivity and the recovery time of the device are enhanced by the decoration of platinum nanoparticles on the surface of graphene. The effects of the operating temperature on the sensing response have been studied. The adsorption and desorption curves have been analyzed using Langmuir kinetic theory for the adsorption of ammonia, and the influence of surface inhomogeneity was analyzed by Freundlich isotherm. The activation energy of adsorption and the heat of adsorption estimated from the above theories indicate that the platinum decorated surface has two different adsorption sites whereas bare graphene has only one adsorption site. This effect caused 80%–85% enhancement of sensor response for platinum decorated surface compared with the bare graphene surface throughout the measured temperature range for ammonia concentrations of 15–58 ppm.
Mechanism and kinetics of near-surface dopant pile-up during post-implant annealing
Authors: Gorai Prashun , Kondratenko Yevgeniy V. , Seebauer Edmund G.
Published: May 2012

Dopant pile-up within 1-2 nm of Si/SiO2 interfaces during post-implant annealing can influence the performance of microelectronic devices using silicon-on-insulator technology or super-steep retrograde channels. Pile-up results from changes in the dopant interstitial charge state induced by band bending at the interface. But, there exists little mechanistic understanding of the specific conditions needed for pile-up or of the kinetics of temporal evolution. The present work uses continuum simulations coupled with experiments in the case of B implanted into Si to show that pile-up requires a zone near the interface wherein the Fermi level exceeds the ionization level for dopant interstitials to change their charge state. The spatial extent of pile-up corresponds closely to the width of this zone unless the annihilation probability of defects at the interface is large. The time and temperature dependences of pile-up closely track those of the free dopant interstitials concentration.
Performance of dissipative dielectric elastomer generators
Authors: Chiang Foo Choon , Jin Adrian Koh Soo , Keplinger Christoph , Kaltseis Rainer , Bauer Siegfried , Suo Zhigang
Published: May 2012

Dielectric elastomer generators are high-energy-density electromechanical transducers. Their performance is affected by dissipative losses. This paper presents a theoretical analysis of a dielectric elastomer generator with two dissipative processes: viscoelasticity and current leakage. Conversion cycles are shown to attain steady-state after several cycles. Performance parameters such as electrical energy generated per cycle, average power, and mechanical to electrical energy conversion efficiency are introduced. Trade-offs between large electrical energy and power output and poor conversion efficiency are discussed. Excessive current leakage results in negative efficiency—the dielectric elastomer generator wastes energy instead of generating it. The general framework developed in this paper helps in the design and assessment of conversion cycles for dissipative dielectric elastomer generators.
Effect of pulse profile and chirp on a laser wakefield generation
Authors: Zhang Xiaomei , Shen Baifei , Ji Liangliang , Wang Wenpeng , Xu Jiancai , Yu Yahong , Yi Longqing , Wang Xiaofeng , Hafz Nasr A. M. , Kulagin V.
Published: May 2012

A laser wakefield driven by an asymmetric laser pulse with/without chirp is investigated analytically and through two-dimensional particle-in-cell simulations. For a laser pulse with an appropriate pulse length compared with the plasma wavelength, the wakefield amplitude can be enhanced by using an asymmetric un-chirped laser pulse with a fast rise time; however, the growth is small. On the other hand, the wakefield can be greatly enhanced for both positively chirped laser pulse having a fast rise time and negatively chirped laser pulse having a slow rise time. Simulations show that at the early laser-plasma interaction stage, due to the influence of the fast rise time the wakefield driven by the positively chirped laser pulse is more intense than that driven by the negatively chirped laser pulse, which is in good agreement with analytical results. At a later time, since the laser pulse with positive chirp exhibits opposite evolution to the one with negative chirp when propagating in plasma, the wakefield in the latter case grows more intensely. These effects should be useful in laser wakefield acceleration experiments operating at low plasma densities.
Fullerene as electrical hinge
Authors: Wan Neng , Perriat Pascal , Sun Li-Tao , Huang Qing-An , Sun Jun , Xu Tao
Published: May 2012

The current study demonstrates that fullerenes with sizes between 1.9 nm and 3.3 nm serve as an efficient electrical hinges for interconnecting sub-5 nm carbon nanotubes (CNTs). Three modes of such “soft connections” were validated via transmission electron microscopy employing in-situ nano-manipulation and electrical transportation measurement. A high stability of the electrical connectivity caused by the forces acting between the fullerene and nanotubes was detected. Current densities as high as ∼108 A/cm2, which almost reached the current carrier capability of CNTs, were observed within the nanocontact and a low contact resistivity (∼10−8 Ω·cm2) was observed between the fullerene and nanotubes.
Encapsulating light-emitting electrochemical cells for improved performance
Authors: Asadpoordarvish Amir , Sandström Andreas , Tang Shi , Granström Jimmy , Edman Ludvig
Published: May 2012

We present a functional and scalable encapsulation of light-emitting electrochemical cells (LECs), which results in a measured ambient operation of >400 h at a brightness of >300 cd/m2 with a maximum efficacy of 6 lm/W, and a linearly extrapolated ambient operation of ∼5600 h at >100 cd/m2. Our findings suggest that previous studies have underestimated the practical stability of appropriately encapsulated LECs. We also report that the dominant ambient degradation for non-encapsulated LECs is water-induced delamination of the cathode from the active layer, while encapsulated LECs in contrast are found to decay from spatial variations in the active layer composition.
Guided-mode-resonance-coupled plasmonic-active SiO2 nanotubes for surface enhanced Raman spectroscopy
Authors: Xu Xiaobin , Hasan Dihan , Wang Lei , Chakravarty Swapnajit , Chen Ray T. , Fan D. L. , Wang Alan X.
Published: May 2012

We demonstrate a surface enhanced Raman scattering (SERS) substrate by integrating plasmonic-active SiO2 nanotubes into Si3N4 gratings. First, the dielectric grating that is working under guided mode resonance (GMR) provides enhanced electric field for localized surface plasmon polaritons on the surface of metallic nanoparticles. Second, we use SiO2 nanotubes with densely assembled silver nanoparticles to provide a large amount of “hot spots” without significantly damping the GMR mode of the grating. Experimental measurement on Rhodamine-6G shows a constant enhancement factor of 8 ∼ 10 in addition to the existing SERS effect across the entire surface of the SiO2 nanotubes.
Cd-free buffer layer materials on Cu2ZnSn(SxSe1−x)4: Band alignments with ZnO, ZnS, and In2S3
Authors: Barkhouse D. Aaron R. , Haight Richard , Sakai Noriyuki , Hiroi Homare , Sugimoto Hiroki , Mitzi David B.
Published: May 2012

The heterojunctions formed between Cu2ZnSn(SxSe1−x)4 (CZTSSe) and three Cd-free n-type buffers, ZnS, ZnO, and In2S3, were studied using femtosecond ultraviolet photoemission and photovoltage spectroscopy. The electronic properties including the Fermi level location at the interface, band bending in the CZTSSe substrate, and valence and conduction band offsets were determined and correlated with device properties. We also describe a method for determining the band bending in the buffer layer and demonstrate this for the In2S3/CZTSSe system. The chemical bath deposited In2S3 buffer is found to have near optimal conduction band offset (0.15 eV), enabling the demonstration of Cd-free In2S3/CZTSSe solar cells with 7.6% power conversion efficiency.
GaN epitaxy on Cu(110) by metal organic chemical vapor deposition
Authors: Li Qiming , Figiel Jeffery , Wang George , Xu Huiwen , Balakrishnan Ganesh
Published: May 2012

We demonstrate that Cu can be a suitable substrate material for c-plane GaN epitaxy using metal organic chemical vapor deposition. By using a low temperature AlN buffer layer, Ga and Cu alloying can be prevented so that GaN layer can be grown on Cu at a temperature of 1000 °C. An epitaxial relation of GaN (0001)//Cu(110) is observed using cross-section transmission electron microscopy and electron back scatter diffraction studies. The single crystalline GaN epilayer shows a threading dislocation density of 3 × 109 cm−2 and strong band edge emission at room temperature. The site alignment between GaN (0001) and Cu(110) shows a mesh ratio of 4/3 and 5/3 in GaN [10-10] and GaN [-1100] directions, which is attributed to the epitaxial relation observed.
High-efficiency InAs/GaAs quantum dot solar cells by metalorganic chemical vapor deposition
Authors: Tanabe Katsuaki , Guimard Denis , Bordel Damien , Arakawa Yasuhiko
Published: May 2012

We fabricate a high-efficiency InAs/GaAs quantum dot (QD) solar cell. It contains five layers of high-density self-assembled InAs QDs grown by metalorganic chemical vapor deposition suppressing open-circuit-voltage (VOC) degradation. We develop a dual-layer anti-reflection coating of optimum thicknesses. The resulting cell exhibits efficiencies of 18.7% under AM1.5 G for 1 sun and 19.4% for 2 suns. Concentrator measurements demonstrate the advantage of QD use under concentrated illumination, owing to the significant increase in VOC. We also find a VOC offset of 0.3 V from the QD ground-state transition energies for QD cells, in contrast to 0.4 V for state-of-the-art bulk semiconductor cells.
Strained In0.53Ga0.47As metal-oxide-semiconductor field-effect transistors with epitaxial based biaxial strain
Authors: Kim SangHyeon , Yokoyama Masafumi , Taoka Noriyuki , Nakane Ryosho , Yasuda Tetsuji , Ichikawa Osamu , Fukuhara Noboru , Hata Masahiko , Takenaka Mitsuru , Takagi Shinichi
Published: May 2012

We have demonstrated epitaxial-based biaxially strained In0.53Ga0.47As metal-oxide-semiconductor field-effect transistors (MOSFETs). Tensile strained In0.53Ga0.47As MOSFETs shows a high peak mobility of 2150 cm2/Vs with the enhancement factor of 3.7 against Si MOSFETs. Furthermore, we have investigated the mobility enhancement mechanisms in the tensile strained In0.53Ga0.47As MOSFETs. It has been found that biaxial tensile strain is effective to enhance the electron mobility in InGaAs MOSFETs by an increase of inversion carrier density, which is caused by the modulation of conduction band minimum.
Fast switching of a ground state of a reconfigurable array of magnetic nano-dots
Authors: Verba Roman , Melkov Gennadiy , Tiberkevich Vasil , Slavin Andrei
Published: May 2012

We show numerically that a ground state (ferromagnetic or chessboard antiferromagnetic) and microwave absorption frequency of a dipolarly coupled two-dimensional array of axially magnetized magnetic nano-dots can be switched by application of bias magnetic field pulses (duration 30–70 ns). Switching to the ferromagnetic state can be achieved by applying a rectangular field pulse along the dot axis while switching to the antiferromagnetic state requires the field pulse oriented in the dot plane and having a sufficiently long trailing edge (tail). Our results prove that arrays of magnetic nano-dots can be used as materials having rapidly reconfigurable magnetic and microwave properties.
A universal noncontact flowmeter for liquids
Authors: Wegfrass André , Diethold Christian , Werner Michael , Fröhlich Thomas , Halbedel Bernd , Hilbrunner Falko , Resagk Christian , Thess André
Published: May 2012

Lorentz force velocimetry (LFV) is a noncontact electromagnetic flow measurement technique for liquid metals that is currently used in fundamental research and metallurgy. Up to now, the application of LFV was limited to the narrow class of liquids whose electrical conductivity is of the order 106 S/m. Here, we demonstrate that LFV can be applied to liquids with conductivities up to six orders of magnitude smaller than in liquid metals. We further argue that this range can be extended to 10−3 S/m under industrial and to 10−6 S/m under laboratory conditions making LFV applicable to most liquids of practical interest.
Real-time observation of Escherichia coli cells under irradiation with a 2-MeV H+ microbeam
Authors: Kato Mikio , Meissl Walter , Umezawa Kenji , Ikeda Tokihiro , Yamazaki Yasunori
Published: May 2012

A high-energy H+ microbeam generated by tapered glass capillary optics was applied to a single Escherichia coli cell, in order to evaluate the effects of irradiation on the activity of the flagellar motor and cell growth in real time. The flagellar motor of the tethered cells was stopped by irradiation with an average ion fluence of 2.0 × 1012 protons/cm2. When a lower dose was applied to the cells attached to the substrate, an elongated cell, which seemed ready to divide, divided into two daughter cells; however, the daughter cells did not elongate, neither did further cell division occur.
Simultaneous visualization of oxygen vacancies and the accompanying cation shifts in a perovskite oxide by combining annular imaging techniques
Authors: Kobayashi Shunsuke , Findlay Scott D. , Shibata Naoya , Mizoguchi Teruyasu , Sato Yukio , Okunishi Eiji , Ikuhara Yuichi , Yamamoto Takahisa
Published: May 2012

The defect structures around oxygen vacancies in perovskite-type oxides play very important roles in determining material properties through the change of the electronic state. Therefore, from the viewpoint of developing condensed matter physics and technological applications, it is crucial to obtain direct images of these structures. We report the simultaneous visualization of oxygen vacancies and the cation shifts around them in perovskite-type manganite by scanning transmission electron microscopy. The defective structure of the material makes it amenable to using Fourier analysis to obtain important structural information like the oxygen vacancy locations and the shift of nearby cation columns.
Possible frictionless transport of amorphous matter in confined microdomains
Authors: Kwang-Hua Chu Zotin
Published: May 2012

Almost frictionless transport of amorphous (polymeric) matter in confined microdomains at room temperature regime is investigated by using the verified transition-rate approach which has been successfully adopted to study the critical transport of amorphous solid helium in confined environment. The critical temperatures related to the nearly frictionless transport of polymeric fluids were found after selecting specific activation energies.
Stretch-induced softening of bending rigidity in graphene
Authors: Shi Xinghua , Peng Bo , Pugno Nicola M. , Gao Huajian
Published: May 2012

First principle calculations are performed to show that the bending rigidity of graphene can be softened considerably with in-plane stretching. This phenomenon can be attributed to stretch-induced loosening of atomic packing and should be of fundamental significance for graphene-based structures and devices.
Strong asymmetrical doping properties of spinel CoAl2O4
Authors: Feng Chunbao , Yin Wan-Jian , Nie Jinlan , Zu Xiaotao , Huda Muhammad N. , Wei Su-Huai , Al-Jassim Mowafak M. , Turner John A. , Yan Yanfa
Published: May 2012

Using first-principles density-functional theory, we have investigated the intrinsic and extrinsic doping properties of CoAl2O4 by calculating the transition energies and formation energies of intrinsic and extrinsic defects. We find that CoAl2O4 exhibits strong asymmetrical doping properties: Although excellent p-type conductivity can be achieved by Li or Na doping at O-rich growth condition, n-type conductivity cannot be achieved by any intrinsic or extrinsic dopants at any growth conditions. These asymmetrical doping properties are attributed to the formation of intrinsic defects, particularly AlCo, which has very low formation energy at all growth conditions. Our results suggest that for better use of CoAl2O4, the electronic devices should require CoAl2O4 to exhibit p-type conductivity.
Impulse breakdown delay in liquid dielectrics
Authors: Jadidian Jouya , Zahn Markus , Lavesson Nils , Widlund Ola , Borg Karl
Published: May 2012

Theoretical images of streamers, revealing the mechanisms behind impulse breakdown in liquid dielectrics, are presented. Streamers lead to electrical breakdown by forming paths, capable of carrying large current amplitudes between electrodes. Breakdown delays and terminal currents are calculated for various electrode geometries (40 μm needle and 6.35 mm sphere) and gap distances (up to 10 mm). Modeling results indicate that the breakdown in needle-needle electrodes requires higher impulse voltage amplitudes than in needle-sphere electrodes for the same gap distances. Streamers in needle-sphere geometries are about 50% thicker than streamers propagating in needle-needle geometries under similar impulse voltage amplitudes.
How dominant is the most efficient pathway through the potential energy landscape of a slowly diffusing disordered system?
Authors: Nguyen Crystal N. , Isaacson Joseph I. , Beth Shimmyo Kayoko , Chen Andersen , Stratt Richard M.
Published: May 2012

It has been suggested that the most-efficient pathway taken by a slowly diffusing many-body system is its geodesic path through the parts of the potential energy landscape lying below a prescribed value of the potential energy. From this perspective, slow diffusion occurs just because these optimal paths become particularly long and convoluted. We test this idea here by applying it to diffusion in two kinds of well-studied low-dimensional percolation problems: the 2d overlapping Lorentz model, and square and simple-cubic bond-dilute lattices. Although the most efficient path should be at its most dominant with the high-dimensional landscapes associated with many-body problems, it is useful to examine simpler, low-dimensional, constant-potential-energy problems such as these ones, both because the simpler models lend themselves to more accurate geodesic-path-finding approaches, and because they offer a significant contrast to many of the models used in the traditional energy-landscape literature. Neither the continuum nor the lattice percolation examples are adequately described by our geodesic-path formalism in the weakly disordered (relatively-fast-diffusion) limit, but in both cases the formalism successfully predicts the existence of the percolation transition and (to a certain extent) the slow diffusion characteristic of near-percolation behavior. The numerical results for these models are not nearly accurate enough near their transitions to describe critical exponents, but the models do showcase the qualitative validity of the geodesic perspective in that they allow us to see explicitly how tortuous and sparse the optimal pathways become as the diffusion constants begin to vanish.
On the origin of the 265 nm absorption band in AlN bulk crystals
Authors: Collazo Ramón , Xie Jinqiao , Gaddy Benjamin E. , Bryan Zachary , Kirste Ronny , Hoffmann Marc , Dalmau Rafael , Moody Baxter , Kumagai Yoshinao , Nagashima Toru , Kubota Yuki , Kinoshita Toru , Koukitu Akinori , Irving Douglas L. , Sitar Zlatko
Published: May 2012

Single crystal AlN provides a native substrate for Al-rich AlGaN that is needed for the development of efficient deep ultraviolet light emitting and laser diodes. An absorption band centered around 4.7 eV (∼265 nm) with an absorption coefficient above 1000 cm−1 is observed in these substrates. Based on density functional theory calculations, substitutional carbon on the nitrogen site introduces absorption at this energy. A series of single crystalline wafers were used to demonstrate that this absorption band linearly increased with carbon, strongly supporting the model that CN- is the predominant state for carbon in AlN.
Quantitative measurement of hard x-ray spectra for high intensity laser produced plasma
Authors: Zhang Z. , Nishimura H. , Namimoto T. , Fujioka S. , Arikawa Y. , Nishikino M. , Kawachi T. , Sagisaka A. , Hosoda H. , Orimo S. , Ogura K. , Pirozhkov A. , Yogo A. , Okano Y. , Kiriyama H. , Kondo K. , Ohshima S. , Azechi H.
Published: May 2012

X-ray line spectra ranging from 17 to 77 keV were quantitatively measured with a Laue spectrometer, composed of a cylindrically curved crystal and a detector. Either a visible CCD detector coupled with a CsI phosphor screen or an imaging plate can be chosen, depending on the signal intensities and exposure times. The absolute sensitivity of the spectrometer system was calibrated using pre-characterized laser-produced x-ray sources and radioisotopes. The integrated reflectivity for the crystal is in good agreement with predictions by an open code for x-ray diffraction. The energy transfer efficiency from incident laser beams to hot electrons, as the energy transfer agency for specific x-ray line emissions, is derived as a consequence of this work.
Electrical contact resistance in filaments
Authors: Wu Xiang-Fa , Zhou Zhengping , Zhou Wang-Min
Published: May 2012

Electrical contact resistance (ECR) influences the electrochemical performance of porous electrodes made of stacked discrete materials (e.g., carbon nanotubes, nanofibers, etc.) for use in supercapacitors and rechargeable batteries. This study establishes a simple elasticity-conductivity model for the ECR of filaments in adhesive contact. The elastic deformation and size of electrical contact zone of the filaments are determined by using an adhesive contact model of filaments, and the ECR of adhesive filaments is obtained in explicit form. Dependencies of the ECR upon the filament geometries, surface energy, and elasticity are examined.
Engineering direct-indirect band gap transition in wurtzite GaAs nanowires through size and uniaxial strain
Authors: Copple Andrew , Ralston Nathaniel , Peng Xihong
Published: May 2012

Electronic structures of wurtzite GaAs nanowires in the [0001] direction were studied using first-principles calculations. It was found that the band gap of GaAs nanowires experiences a direct-to-indirect transition when the diameter of the nanowires is smaller than ∼28 Å. For those thin GaAs nanowires with an indirect band gap, it was found that the gap can be tuned to be direct if a moderate external uniaxial strain is applied. Both tensile and compressive strain can trigger the indirect-to-direct gap transition. The critical strains for the gap-transition are determined by the energy crossover of two states in conduction bands.
Detecting giant electrocaloric effect in SrxBa1−xNb2O6 single crystals
Authors: Aly Hamad Mahmoud
Published: May 2012

PbZr0.95Ti0.05O3 thin film is the best electrocaloric effect material which has cooling ΔT per unit field (MV m−1) about 0.254 under electric field shift ΔE = 30 kV cm−1 at Curie temperature Tc = 222 °C [A. S. Mischenko, Q. Zhang, J. F. Scott, R. W. Whatmore, N. D. Mathur, Science 311, 1270 (2006)]. Here, it is shown that an EC effect due to applied electric field shift parallel to the c-axis in SrxBa1−xNb2O6 single crystals thin film has a value of cooling per unit field larger than PbZr0.95Ti0.05O3 thin film. Furthermore, SrxBa1−xNb2O6 single crystals have Curie temperature Tc closer to room temperature.
Modulation effect of hydrogen and fluorine decoration on the surface work function of BN sheets
Authors: Jiao N , He Chaoyu , Zhang C. X. , Peng Xiangyang , Zhang K. W. , Sun L. Z.
Published: May 2012

Using first-principles calculations within the framework of density-functional theory, we studied the modulation effect of hydrogen/fluorine chemical decoration on the surface work function of BN sheets. We found that the difference in the work function (ΔWBN) between two surfaces of the chair structure varies with the different decoration. Geometric distortion and chemical effects cause opposite modulation effects, and the chemical effect plays a leading role by inducing charge redistribution in the system.
Variability in encoding precision accounts for visual short-term memory limitations
Authors: van den Berg Ronald , Shin Hongsup , Chou Wen-Chuang , George Ryan , Ma Wei Ji
Published: May 2012

It is commonly believed that visual short-term memory (VSTM) consists of a fixed number of “slots” in which items can be stored. An alternative theory in which memory resource is a continuous quantity distributed over all items seems to be refuted by the appearance of guessing in human responses. Here, we introduce a model in which resource is not only continuous but also variable across items and trials, causing random fluctuations in encoding precision. We tested this model against previous models using two VSTM paradigms and two feature dimensions. Our model accurately accounts for all aspects of the data, including apparent guessing, and outperforms slot models in formal model comparison. At the neural level, variability in precision might correspond to variability in neural population gain and doubly stochastic stimulus representation. Our results suggest that VSTM resource is continuous and variable rather than discrete and fixed and might explain why subjective experience of VSTM is not all or none.
Subpixel colocalization reveals amyloid precursor protein-dependent kinesin-1 and dynein association with axonal vesicles
Authors: Szpankowski Lukasz , Encalada Sandra E. , Goldstein Lawrence S. B.
Published: May 2012

Intracellular transport of vesicles and organelles along microtubules is powered by kinesin and cytoplasmic dynein molecular motors. Both motors can attach to the same cargo and thus must be coordinated to ensure proper distribution of intracellular materials. Although a number of hypotheses have been proposed to explain how these motors are coordinated, considerable uncertainty remains, in part because of the absence of methods for assessing motor subunit composition on individual vesicular cargos. We developed a robust quantitative immunofluorescence method based on subpixel colocalization to elucidate relative kinesin-1 and cytoplasmic dynein motor subunit composition of individual, endogenous amyloid precursor protein (APP) vesicles in mouse hippocampal cells. The resulting method and data allow us to test a key in vivo prediction of the hypothesis that APP can recruit kinesin-1 to APP vesicles in neuronal axons. We found that APP levels are well-correlated with the amount of the light chain of kinesin-1 (KLC1) and the heavy chain of cytoplasmic dynein (DHC1) on vesicles. In addition, genetic reduction of APP diminishes KLC1 and DHC1 levels on APP cargos. Finally, our data reveal that reduction of KLC1 leads to decreased levels of DHC1 on APP vesicles, suggesting that KLC1 is necessary for the association of DHC1 to these cargos, and help to explain previously reported retrograde transport defects generated when kinesin-1 is reduced.
Farmer Participation in Supermarket Channels, Production Technology, and Efficiency: The Case of Vegetables in Kenya
Authors: Rao Elizaphan J. O. , Brümmer Bernhard , Qaim Matin
Published: May 2012

Supermarkets are gaining ground in agrifood systems of many developing countries. While recent research has analyzed income effects in the small farm sector, impacts on productivity and efficiency have hardly been studied. We use a meta‐frontier approach and combine this with propensity score matching to estimate treatment effects among vegetable farmers in Kenya. Participation in supermarket channels increases farm productivity in terms of meta‐technology ratios by 45%. We also find positive and significant impacts on technical efficiency and scale efficiency. Supermarket expansion therefore presents opportunities for agricultural growth in the small farm sector, which is crucial for poverty reduction in Africa.