bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of activation behavior in annealed Al–N codoped ZnO Films
Authors: Yao Shiau-Lu , Hong Jhen-Dong , Lee Ching-Ting , Ho Chung-Yen , Liu Day-Shan
Published: May 2011

Al–N codoped ZnO films prepared via cosputtering technology were postannealed at 450 °C for 30 min under ambient vacuum and nitrogen. The extrinsic impurities in these annealed samples, resulting in evolutions on the carriers and radiation emissions, were investigated through their photoluminescence spectra and Hall-effect measurements. It was found that the donor-acceptor-pair emission was related to the VZn–AlZn transition at 2.86 eV and predominated over the defect-transition luminescence in the room-temperature photoluminescence (RTPL) spectrum of the vacuum-annealed sample, which possessed a high electron carrier concentration. With the help of temperature-dependent Hall measurements, a shallow donor level corresponding to Al on the Zn site (AlZn) was derived as EC − (51 ± 4) meV. In contrast, the defect-transition luminescence in the RTPL spectrum of the nitrogen-annealed Al–N codoped ZnO film, showing p-type conduction with a hole concentration of 1018 cm−3, was dominated by the VO–NO deep level emission at approximately 1.87 eV. The estimated acceptor level corresponding to the N on the O site (NO) was EV + (149 ± 6) meV. The binding energy and the activation energy associated with the NO acceptor were also determined by the low-temperature photoluminescence and temperature-dependent PL spectra.
The investigation on carrier distribution in InGaN/GaN multiple quantum well layers
Authors: Zhu J. H. , Zhang S. M. , Wang H. , Zhao D. G. , Zhu J. J. , Liu Z. S. , Jiang D. S. , Qiu Y. X. , Yang H.
Published: May 2011

The carrier distribution and recombination dynamics of InGaN/GaN multiple quantum well (MQW) light-emitting diode structure are investigated. Two emission peaks were observed in the low temperature photoluminescence spectra of an InGaN/GaN MQW structure, but only one peak was observed in the electroluminescence (EL) spectra. Combined with the spatially resolved cathodoluminescence (CL) measurements, it is found that the electrically injected carrier distribution is governed by hole transport and diffusion in InGaN/GaN MQW structure due to the much lower mobility of hole. And the electron and hole recombination of EL occurs predominantly in the QWs that are located closer to the p-GaN layer.
Hydrogen-induced self-assembly of helical carbon nanostructures from ethanol over SiO2 catalysts
Authors: Bachmatiuk Alicja , Börrnert Felix , Hoffmann Volker , Lindackers Dirk , Lin Jarrn-Horng , Büchner Bernd , Rümmeli Mark H.
Published: May 2011

Helical carbon nanostructures from ethanol over silica are successfully prepared through the addition of hydrogen during chemical vapor deposition. The helical structure formation appears to occur through hydrogen etching reactions enabling graphitic caps between neighboring tubes to merge, triggering the self-assembly of a helical structure. The experimental data is supported by thermodynamic calculations which correlate the hydrogen radical concentration with the observed efficiency of helical structure formation. Moreover, the calculations show an increase in water vapor content as one increases hydrogen addition. This can account for the improved crystallinity of the samples with higher hydrogen doses.
Structure-induced metamorphoses of eigen modes in photonic atoll resonators
Authors: Nojima S. , Nakahara S.
Published: May 2011

An optical resonator—called a photonic atoll (PA)—is presented and investigated theoretically, which consists of microstructures arranged periodically to make a closed chain loop and permits a photon trapped there to stay in it for an amazingly long time. The systematic investigation for a variety of elliptical PAs reveals that the symmetric (or circular) resonator enables us to attain the maximum lifetime. This structural modification exhibits intriguing variations of eigen-modes, as given below. The optical eigen-modes in the PAs—their degeneracy gets lifted by this deformation—show the splitting widths that strongly depend on the mode position in the photonic band. Moreover, those modes split near the bandedge display a striking anisotropy of excitations. These phenomena should be discussed taking into account the quasi-one-dimensionality of the structure as well as considering them from the group-theoretic standpoint. We have thus clarified the eigen-modes metamorphoses caused by the modification of the PA structure.
Enhanced Ge/Si(001) island areal density and self-organization due to P predeposition
Authors: Cho B. , Bareño J. , Petrov I. , Greene J. E.
Published: May 2011

The predeposition of P, with coverages θP ranging from 0 to 1 ML, on Si(001) significantly increases both the areal density and spatial self-organization of Ge islands grown by gas-source molecular beam epitaxy from hydride precursors. The Ge island density ρGe initially increases with θP, reaching a maximum of 1.4 × 1010 cm−2 at θP = 0.7 ML, a factor of four times higher than on bare Si(001) under the same deposition conditions, before decreasing at higher P coverages. The increase in ρGe(θP) is due to a corresponding decrease in Ge adatom mean free paths resulting from passivation of surface dangling bonds by adsorbed pentavalent P atoms which, in addition, leads to surface roughening and, therefore, higher Ge coverages at constant Ge2H6 dose. As θP (and hence, ρGe) increases, so does the degree of Ge island ordering along 〈100〉 directions due to the anisotropic strain field surrounding individual islands. Similar results are obtained for Ge island growth on P−doped Si(001) layers where strong P surface segregation provides partial monolayer coverage prior to Ge deposition.
Magnetic and electronic properties of Eu0.9Ca0.1BaCo2O5.5+δ with the variation of oxygen stoichiometry
Authors: Motin Seikh Md. , Kundu Asish K. , Caignaert V. , Pralong V. , Raveau B.
Published: May 2011

The effect of the oxygen content on the magnetic and transport properties of the ferromagnetic Eu0.9Ca0.1BaCo2O5.5+δ has been investigated. Unlike the increase in TC with calcium content, paradoxically the TC value decreases as the oxygen content increases, as observed in the undoped phase. Assuming the Kanamori-Goodenough rules apply to this very complex magnetic system, these experiments suggest that the Co3+ disproportionation into Co4+ and Co2+, plays an important role in the appearance of ferromagnetism. The magnetic phase separation scenario of ferromagnetic domains embedded in an antiferromagnetic matrix play an important role in the ferromagnetism of the calcium doped “112” cobaltites. All the samples covering a wide range of oxygen content, exhibit p-type conductivity.
Adaptive integration grids in instanton theory improve the numerical accuracy at low temperature
Authors: Rommel Judith B. , Kästner Johannes
Published: May 2011

The instanton method allows to accurately calculate tunneling rates down to very low temperature. However, with lowering the temperature, the computational effort steeply increases as many more discretization points are required. This is caused in practical applications by the majority of the discretization points accumulating at a very small region in configuration space. Here, we describe a method to flexibly discretize the instanton path adapted to the temperature. Chosen appropriately, the discretization leads to a much more uniform distribution of the images (control points) along the path which reduces the number of required images by about a factor of two. Combined with a modified Newton–Raphson optimizer and successive updates of the Hessians, the proposed method provides converged reaction rates at computational costs reduced by more than an order of magnitude. We show the success of the method on analytic test potentials and on molecules with energies directly obtained from density functional theory calculations.
Multiple isomers in the photoelectron spectra of small mono-niobium carbide clusters
Authors: Iordanov Ivan , Sofo Jorge O.
Published: May 2011

We calculate the photoelectron spectrum of small mono-niobium carbide clusters (NbCn) using density functional theory for clusters with n = 2–7 and the symmetry adapted cluster configuration interaction method for the smallest clusters (n = 2–4). Theoretical spectra of a single structure cannot explain all peaks present in the spectrum measured by Zhai et al. [J. Chem. Phys. 115, 5170 (2001)]. However, we can match all peaks in the experimental spectra if we assume that the beam contains a combination of cyclic and linear structures. This finding is even more surprising given the fact that some of the excited metastable geometries have energies as large as 0.5 eV above the ground state. Our result is confirmed by both theoretical approaches. We suggest further experiments, using additional beam cooling, to corroborate this observation.
Ion optical design of a collinear laser-negative ion beam apparatus
Authors: Diehl C. , Wendt K. , Lindahl A. O. , Andersson P. , Hanstorp D.
Published: May 2011

An apparatus for photodetachment studies on atomic and molecular negative ions of medium up to heavy mass (M ≃ 500) has been designed and constructed. Laser and ion beams are merged in the apparatus in a collinear geometry and atoms, neutral molecules and negative ions are detected in the forward direction. The ion optical design and the components used to optimize the mass resolution and the transmission through the extended field-free interaction region are described. A 90° sector field magnet with 50 cm bending radius in combination with two slits is used for mass dispersion providing a resolution of M/ΔM≅800 for molecular ions and M/ΔM≅400 for atomic ions. The difference in mass resolution for atomic and molecular ions is attributed to different energy distributions of the sputtered ions. With 1 mm slits, transmission from the source through the interaction region to the final ion detector was determined to be about 0.14%.
The central cell model: A mesoscopic hopping model for the study of the displacement autocorrelation function
Authors: Pazzona F. G. , Gabrieli A. , Pintus A. M. , Demontis P. , Suffritti G. B.
Published: May 2011

On the mesoscale, the molecular motion in a microporous material can be represented as a sequence of hops between different pore locations and from one pore to the other. On the same scale, the memory effects in the motion of a tagged particle are embedded in the displacement autocorrelation function (DACF), the discrete counterpart of the velocity autocorrelation function (VACF). In this paper, a mesoscopic hopping model, based on a lattice-gas automata dynamics, is presented for the coarse-grained modeling of the DACF in a microporous material under conditions of thermodynamic equilibrium. In our model, that we will refer to as central cell model, the motion of one tagged particle is mimicked through probabilistic hops from one location to the other in a small lattice of cells where all the other particles are indistinguishable; the cells closest to the one containing the tagged particle are simulated explicitly in the canonical ensemble, whereas the border cells are treated as mean-field cells in the grand-canonical ensemble. In the present paper, numerical simulation of the central cell model are shown to provide the same results as a traditional lattice-gas simulation. Along with this a mean-field theory of self-diffusion which incorporates time correlations is discussed.
Accurate surface tension measurement of glass melts by the pendant drop method
Authors: Chang Yao-Yuan , Wu Ming-Ya , Hung Yi-Lin , Lin Shi-Yow
Published: May 2011

A pendant drop tensiometer, coupled with image digitization technology and a best-fitting algorithm, was built to accurately measure the surface tension of glass melts at high temperatures. More than one thousand edge-coordinate points were obtained for a pendant glass drop. These edge points were fitted with the theoretical drop profiles derived from the Young-Laplace equation to determine the surface tension of glass melt. The uncertainty of the surface tension measurements was investigated. The measurement uncertainty (σ) could be related to a newly defined factor of drop profile completeness (Fc): the larger the Fc is, the smaller σ is. Experimental data showed that the uncertainty of the surface tension measurement when using this pendant drop tensiometer could be ±3 mN/m for glass melts.
Zero-power autonomous buoyancy system controlled by microbial gas production
Authors: Wu Peter K. , Fitzgerald Lisa A. , Biffinger Justin C. , Spargo Barry J. , Houston Brian H. , Bucaro Joseph A. , Ringeisen Bradley R.
Published: May 2011

A zero-power ballast control system that could be used to float and submerge a device solely using a gas source was built and tested. This system could be used to convey sensors, data loggers, and communication devices necessary for water quality monitoring and other applications by periodically maneuvering up and down a water column. Operational parameters for the system such as duration of the submerged and buoyant states can be varied according to its design. The gas source can be of any origin, e.g., compressed air, underwater gas vent, gas produced by microbes, etc. The zero-power ballast system was initially tested using a gas pump and further tested using gas produced by Clostridium acetobutylicum. Using microbial gas production as the only source of gas and no electrical power during operation, the system successfully floated and submerged periodically with a period of 30 min for at least 24 h. Together with microbial fuel cells, this system opens up possibilities for underwater monitoring systems that could function indefinitely.
Explicitly correlated multireference configuration interaction with multiple reference functions: Avoided crossings and conical intersections
Authors: Shiozaki Toru , Werner Hans-Joachim
Published: May 2011

We develop an explicitly correlated multireference configuration interaction method (MRCI-F12) with multiple reference functions. It can be routinely applied to nearly degenerate molecular electronic structures near conical intersections and avoided crossings, where the reference functions are strongly mixed in the correlated wave function. This work is a generalization of the MRCI-F12 method for electronic ground states, reported earlier by Shiozaki et al. [J. Chem. Phys. 134, 034113 (2011)]10.1063/1.3528720. The so-called F12b approximation is used to arrive at computationally efficient formulas. The doubly external part of the wave function is expanded in terms of internally contracted configurations generated from all the reference functions. In addition, we introduce a singles correction to the CASSCF reference energies, which is applicable to multi-state calculations. As examples, we present numerical results for the avoided crossing of LiF, excited states of ozone, and the H2 + OH (A2Σ+) reaction.
Angular dependence of electron induced surface plasmon excitation
Authors: Werner Wolfgang S. M. , Smekal Werner , Salvat-Pujol Francesc , Halavani Zahra , Pfleger Stephan , Rastl Johannes , Eisenmenger-Sittner Christoph
Published: May 2011

The angular dependence of the probability for electron induced surface plasmon excitation has been measured on semi-infinite planar polycrystalline Al and Au surfaces for energies between 500 and 4000 eV. The results agree accurately with the simple model in which the surface excitation probability is proportional to the surface penetration time. However, the penetration time differs from the rectilinear motion model due to deflections during elastic collisions within the surface scattering zone. A simple formula to account for the effect of elastic scattering in the surface scattering zone is given.
Total funneling of light in high aspect ratio plasmonic nanoresonators
Authors: Bouchon Patrick , Pardo Fabrice , Portier Benjamin , Ferlazzo Laurence , Ghenuche Petru , Dagher Gulnar , Dupuis Christophe , Bardou Nathalie , Haïdar Riad , Pelouard Jean-Luc
Published: May 2011

We demonstrate the total extinction of the reflectivity for a transverse magnetic polarized wave on a gold surface etched on 6% of its area by both narrow (150 nm) and deep (2 μm) grooves. These high aspect ratio metallic grooves were fabricated using a mold cast technique based on an electrolytic growth of gold. They exhibit two resonance peaks corresponding to the first and second cavity modes inside the grooves. We also evidence the incidence-invariance of their spectral response, which undoubtedly shows the localized nature of the resonances. These experimental results confirm the prediction of total funneling of light in very narrow grooves.
Designing evanescent optical interactions to control the expression of Casimir forces in optomechanical structures
Authors: Rodriguez Alejandro W. , Woolf David , Hui Pui-Chuen , Iwase Eiji , McCauley Alexander P. , Capasso Federico , Loncar Marko , Johnson Steven G.
Published: May 2011

We propose an optomechanical structure consisting of a photonic-crystal (holey) membrane suspended above a layered silicon-on-insulator substrate in which resonant bonding/antibonding optical forces created by externally incident light from above enable all-optical control and actuation of stiction effects induced by the Casimir force. In this way, one can control how the Casimir force is expressed in the mechanical dynamics of the membrane, not by changing the Casimir force directly but by optically modifying the geometry and counteracting the mechanical spring constant to bring the system in or out of regimes where Casimir physics dominate. The same optical response (reflection spectrum) of the membrane to the incident light can be exploited to accurately measure the effects of the Casimir force on the equilibrium separation of the membrane.
Reliability of bottom-gate graphene field-effect transistors prepared by using inductively coupled plasma-chemical vapor deposition
Authors: Lee Jung-Kyu , Chung Hyun-Jong , Heo Jinseong , Seo Sunae , Cho Il Hwan , Kwon Hyuck-In , Lee Jong-Ho
Published: May 2011

Stability degradation in bottom-gate graphene field-effect transistors prepared by using inductively coupled plasma-chemical vapor deposition method are investigated under bias, temperature and illumination stress. The stretched-exponential time dependence model, which can be derived based on the trapping/detrapping of charges to/from existing traps and continuous redistribution of charges in bulk dielectrics, is well applied in fitting the time dependence of the transfer curve shifts in all stress conditions. The stress wavelength is considered as important factors as well as the bias and temperature stress in the transfer characteristic instabilities of graphene FEFs.
Molecular dynamics simulations for CO2 spectra. III. Permanent and collision-induced tensors contributions to light absorption and scattering
Authors: Hartmann J.-M. , Boulet C.
Published: May 2011

Classical molecular dynamics simulations have been performed for gaseous CO2 starting from an accurate anisotropic intermolecular potential. Through calculations of the evolutions of the positions and orientations of a large number of molecules, the time evolutions of the permanent and collision-induced electric dipole vector and polarizability tensor are obtained. These are computed from knowledge of static molecular parameters taking only the leading induction terms into account. The Laplace transforms of the auto-correlation functions of these tensors then directly yield the light absorption and scattering spectra. These predictions are, to our knowledge, the first in which the contributions of permanent and collision-induced tensors are simultaneously taken into account for gaseous CO2, without any adjusted parameter. Comparisons between computations and measurements are made for absorption in the region of the ν3 infrared band and for depolarized Rayleigh scattering in the roto-translational band. They demonstrate the quality of the model over spectral ranges from the band center to the far wings where the spectrum varies by several orders of magnitude. The contributions of the permanent and interaction-induced (dipole and polarizability) tensors are analyzed for the first time, through the purely permanent (allowed), purely induced, and cross permanent/induced components of the spectra. It is shown that, while the purely induced contribution is negligible when compared to the collision-broadened allowed component, the cross term due to interferences between permanent and induced tensors significantly participates to the wings of the bands. This successfully clarifies the long lasting, confusing situation for the mechanisms governing the wings of the CO2 spectra considered in this work
Efficient first-principles electronic dynamics
Authors: Liang Wenkel , Chapman Craig T. , Li Xiaosong
Published: May 2011

An efficient first-principles electronic dynamics method is introduced in this article. The approach we put forth relies on incrementally constructing a time-dependent Fock/Kohn-Sham matrix using active space density screening method that reduces the cost of computing two-electron repulsion integrals. An adaptive stepsize control algorithm is developed to optimize the efficiency of the electronic dynamics while maintaining good energy conservation. A selected set of model dipolar push-pull chromophore molecules are tested and compared with the conventional method of direct formation of the Fock/Kohn-Sham matrix. While both methods considered herein take on identical dynamical simulation pathways for the molecules tested, the active space density screening algorithm becomes much more computationally efficient. The adaptive stepsize control algorithm, when used in conjunction with the dynamically active space method, yields a factor of ∼3 speed-up in computational cost as observed in electronic dynamics using the time dependent density functional theory. The total computational cost scales nearly linear with increasing size of the molecular system.
Comparing modern density functionals for conjugated polymer band structures: Screened hybrid, Minnesota, and Rung 3.5 approximations
Authors: Janesko Benjamin G.
Published: May 2011

Semiconducting polymers with π-conjugated backbones show promise in fields such as photovoltaics. Practical applications of conjugated polymers require precise control over the polymer's electronic band structure. Several new classes of density functional approximation, including screened hybrids, semilocal Minnesota functionals, and Rung 3.5 functionals, show potential for improved predictions of conjugated polymer band structures. This work compares these methods to standard global hybrid density functionals for bandgaps and band structures of representative conjugated polymers. The new methods exhibit particular promise for modeling three-dimensionally periodic bulk polymers, which can be problematic for global hybrids.
A simple molecular mechanics potential for μm scale graphene simulations from the adaptive force matching method
Authors: Wei Dongshan , Song Yang , Wang Feng
Published: May 2011

A simple molecular mechanics force field for graphene (PPBE-G) was created by force matching the density functional theory Perdew-Burke-Ernzerhof forces using the adaptive force matching method recently developed in our group. The PPBE-G potential was found to provide significantly more accurate forces than other existing force fields. Several properties of graphene, such as Young's modulus, bending rigidity, and thermal conductivity, have been studied with our potential. The calculated properties are in good agreement with corresponding density functional theory and experimental values. The thermal conductivity calculated with reverse non-equilibrium molecular dynamics depends sensitively on graphene size thus requiring the simulation of large sheets for convergence. Since the PPBE-G potential only contains simple additive energy expressions, it is very computationally efficient and is capable of modeling large graphene sheets in the μm length scale.
Experimental and theoretical studies of ammonia decomposition activity on Fe-Pt, Co-Pt, and Cu-Pt bimetallic surfaces
Authors: Hansgen Danielle A. , Thomanek Lisa M. , Chen Jingguang G. , Vlachos Dionisios G.
Published: May 2011

We investigate the decomposition of ammonia on bimetallic surfaces prepared by the deposition of a monolayer of Fe, Co, or Cu on a Pt(111) surface computationally and experimentally. We explore the correlation between predicted activities based on the nitrogen binding energies with experimental decomposition activity on these bimetallic and corresponding monometallic surfaces. Through density functional theory calculations and microkinetic modeling, it is predicted that the Fe-Pt-Pt(111) and Co-Pt-Pt(111) surfaces, with a monolayer of Fe or Co on top of Pt(111), are active toward decomposing ammonia. In contrast, the corresponding subsurface configurations, Pt-Fe-Pt(111) and Pt-Co-Pt(111) are inactive. These predictions were confirmed experimentally through temperature programmed desorption experiments. Decomposition was seen at temperatures below 350 K for the Fe-Pt-Pt(111) and Co-Pt-Pt(111) surfaces. For the Cu/Pt(111) system, the surface, subsurface and parent metals were each predicted to be inactive, consistent with experiments, further validating the model predictions. The stability of these bimetallic surfaces in the presence of adsorbed nitrogen is also discussed.
Ultraviolet-visible absorption of small silver clusters in neon: Agn (n = 1–9)
Authors: Lecoultre S. , Rydlo A. , Buttet J. , Félix C. , Gilb S. , Harbich W.
Published: May 2011

We present optical absorption and fluorescence spectra in the UV-visible range of size selected neutral Agn clusters (n = 1-9) in solid neon. Rich and detailed optical spectra are found with linewidths as small as 50 meV. These spectra are compared to time dependent density functional theory implemented in the TURBOMOLE package. Excellent agreement between theory and experiment is achieved in particular for the dominant spectroscopic features at photon energies below 4.5 eV. This allows a clear attribution of the observed electronic transitions to specific isomers. Optical transitions associated to the s-electrons are concentrated in the energy range between 3 and 4 eV and well separated from transitions of the d-electrons. This is in contrast to the other coinage metals (Au and Cu) which show a strong coupling of the d-electrons.
Thermal conductivity degradation induced by point defects in irradiated silicon carbide
Authors: Crocombette Jean-Paul , Proville Laurent
Published: May 2011

Irradiations are known to decrease the thermal conductivity of ceramics. This phenomenon is tackled by molecular dynamics simulation of the thermal resistance of point defects in cubic silicon carbide. The additional thermal resistivity due to point defects proves to vary linearly with their concentration. Large variations in the proportionality coefficient with the nature of the defects are observed. From these calculations, an approximate scale for the concentration of vacancies in irradiated SiC is built.
Imaging resonant modes in photonic crystal nanocavity by atomic force microscope nano-oxidation
Authors: Chen W.-Y. , Chen M.-J. , Cheng C.-C. , Hsu T. M. , Wang C.-J. , Chyi J.-I.
Published: May 2011

Electric field distributions of resonant modes in a photonic crystal nanocavity were imaged using atomic force microscope nano-oxidation. A grid pattern of nanosize oxides was grown on the nanocavity to perturb the resonant modes. The perturbation caused a shift in the resonant wavelength that was proportional to the local electric field intensity of the resonant mode. The experimentally obtained field intensity images agreed excellently with the finite-difference time-domain calculations. The measured resonant mode images had high spatial resolution and image contrast, owing to the extremely local perturbation of the atomic force microscope oxidation technique.
State-to-state quantum dynamics of the H + HBr reaction: Competition between the abstraction and exchange reactions
Authors: Xie Changjian , Jiang Bin , Xie Daiqian
Published: May 2011

Quantum state-to-state dynamics for the H + HBr(υi = 0,ji =0) reaction was studied on an accurate ab intio potential energy surface for the electronic ground state of BrH2. Both the H + HBr → H2 + Br abstraction reaction and the H′ + HBr → H′Br + H exchange reaction were investigated up to a collision energy of 2.0 eV. It was found that the abstraction channel is dominant at lower collision energies, while the exchange channel becomes dominant at higher collision energies. The total integral cross section of the abstraction reaction at a collision energy of 1.6 eV was found to be 1.37 Å2, which is larger than a recent quantum mechanical result (1.06 Å2) and still significantly smaller than the experimental value (3 ± 1 Å2). Meanwhile, similar to the previous theoretical study, our calculations also predicted much hotter product rotational state distributions than those from the experimental study. This suggests that further experimental investigations are highly desirable to elucidate the dynamic properties of the title reactions.
The effects of charge transfer on the properties of liquid water
Authors: Lee Alexis J. , Rick Steven W.
Published: May 2011

A method for treating charge transfer interactions in classical potential models is developed and applied to water. In this method, a discrete amount of charge is transferred for each hydrogen bond formed. It is designed to be simple to implement, to be applicable to a variety of potential models, and to satisfy various physical requirements. The method does not transfer charge at large intramolecular distances, it does not result in a conductive liquid, and it can be easily parameterized to give the correct amount of charge transfer. Two charge transfer models are developed for a polarizable and a non-polarizable potential. The models reproduce many of the properties of liquid water, including the structure, the diffusion constant, and thermodynamic properties over a range of temperatures.
Hall mobility of cuprous oxide thin films deposited by reactive direct-current magnetron sputtering
Authors: Lee Yun Seog , Winkler Mark T. , Siah Sin Cheng , Brandt Riley , Buonassisi Tonio
Published: May 2011

Cuprous oxide (Cu2O) is a promising earth-abundant semiconductor for photovoltaic applications. We report Hall mobilities of polycrystalline Cu2O thin films deposited by reactive dc magnetron sputtering. High substrate growth temperature enhances film grain structure and Hall mobility. Temperature-dependent Hall mobilities measured on these films are comparable to monocrystalline Cu2O at temperatures above 250 K, reaching 62 cm2/V s at room temperature. At lower temperatures, the Hall mobility appears limited by carrier scattering from ionized centers. These observations indicate that sputtered Cu2O films at high substrate growth temperature may be suitable for thin-film photovoltaic applications.
The Pt2 (1,0) band of System VI in the near infrared by intracavity laser absorption spectroscopy
Authors: O'Brien Leah C. , O’Brien James J.
Published: May 2011

Intracavity laser absorption spectroscopy has been used to record rotationally resolved electronic spectra of Pt2 in the near infrared. The metal dimers were created using a 50 mm-long, platinum-lined hollow cathode plasma discharge. The observed transition at 12 937 cm−1 is identified as the (1,0) band of System VI, with state symmetries Ω = 0 – X Ω = 0.
Effects of tethering a multistate folding protein to a surface
Authors: Wei Shuai , Knotts Thomas A.
Published: May 2011

Protein/surface interactions are important in a variety of fields and devices, yet fundamental understanding of the relevant phenomena remains fragmented due to resolution limitations of experimental techniques. Molecular simulation has provided useful answers, but such studies have focused on proteins that fold through a two-state process. This study uses simulation to show how surfaces can affect proteins which fold through a multistate process by investigating the folding mechanism of lysozyme (PDB ID: 7LZM). The results demonstrate that in the bulk 7LZM folds through a process with four stable states: the folded state, the unfolded state, and two stable intermediates. The folding mechanism remains the same when the protein is tethered to a surface at most residues; however, in one case the folding mechanism changes in such a way as to eliminate one of the intermediates. An analysis of the molecular configurations shows that tethering at this site is advantageous for protein arrays because the active site is both presented to the bulk phase and stabilized. Taken as a whole, the results offer hope that rational design of protein arrays is possible once the behavior of the protein on the surface is ascertained.
Structural atomic-scale analysis of GaAs/AlGaAs quantum wires and quantum dots grown by droplet epitaxy on a (311)A substrate
Authors: Keizer J. G. , Jo M. , Mano T. , Noda T. , Sakoda K. , Koenraad P. M.
Published: May 2011

We report the structural analysis at the atomic scale of GaAs/AlGaAs quantum wires and quantum dots grown by droplet epitaxy on a (311)A-oriented substrate. The shape, interfaces, and composition of these nanostructures and their surrounding matrix are investigated. We show that quantum wires can be created by annealing uncapped quantum dots. Substantial interface fluctuations, attributed to interface instability induced by the liquid Ga droplet, are observed. Despite the interface fluctuations, no intermixing of Al was found in either the quantum wires or quantum dots. A wetting layer connecting the quantum dots could not be observed.
Enhanced optical performance of amber emitting quantum dots incorporated InGaN/GaN light-emitting diodes with growth on UV-enhanced electrochemically etched nanoporous GaN
Authors: Soh C. B. , Liu W. , Hartono H. , Ang Norman S. S. , Chua S. J. , Chow S. Y. , Tay C. B. , Vajpeyi A. P.
Published: May 2011

Phosphor-free amber light emitting quantum dots with bimodal size distribution have been effectively incorporated in InGaN/GaN light emitting diodes (LEDs). With overgrowth of the LEDs structure on electrochemically etched nanoporous GaN templates, a reduction in density of threading dislocations and lower biaxial stress are achieved. A higher density of smaller quantum dots ∼4.5×109 cm−2 is incorporated in the multiple quantum well of LEDs on the nanoporous GaN template, generating higher energy emission, and enhanced light output by 1.45 times over LEDs grown on conventional GaN. The carrier capture kinetic by the bimodal distributed quantum dots is discussed with its energy band profile.
Room temperature ferromagnetism in partially hydrogenated epitaxial graphene
Authors: Xie Lanfei , Wang Xiao , Lu Jiong , Ni Zhenhua , Luo Zhiqiang , Mao Hongying , Wang Rui , Wang Yingying , Huang Han , Qi Dongchen , Liu Rong , Yu Ting , Shen Zexiang , Wu Tom , Peng Haiyang , Özyilmaz Barbaros , Loh Kianping , Wee Andrew T. S. , Ariando  , Chen Wei
Published: May 2011

We report room temperature ferromagnetism in partially hydrogenated epitaxial graphene grown on 4HSiC(0001). The presence of ferromagnetism was confirmed by superconducting quantum interference devices measurements. Synchrotron-based near-edge x-ray absorption fine structure and high resolution electron energy loss spectroscopy measurements have been used to investigate the hydrogenation mechanism on the epitaxial graphene and the origin of room temperature ferromagnetism. The partial hydrogenation induces the formation of unpaired electrons in graphene, which together with the remnant delocalized π bonding network, can explain the observed ferromagnetism in partially hydrogenated epitaxial graphene.
In-plane manipulation of quantum dots in high quality laterally contacted micropillar cavities
Authors: Beetz J. , Kistner C. , Lermer M. , Schneider C. , Reitzenstein S. , Höfling S. , Kamp M. , Forchel A.
Published: May 2011

We report on the realization and study of laterally contacted quantum dot (QD)-micropillars. The lateral contacts are formed by ion beam induced deposition and allow for the application of in-plane electric fields. The processing was optimized to preserve high optical quality of the QD-micropillars and to minimize detrimental leakage currents while providing a good electrical control of the QD properties. Under variation of the applied voltage we are able to Stark-tune the emission energy of the QDs up to 0.12 meV and to reduce their fine structure splitting by up to a factor of 5.
Anomalous transmission of terahertz waves in arrays of double-ring resonators induced by a 400 nm pump pulse
Authors: Shi Yulei , Zhou Qing-li , Liu Wei , Liu Jianfeng , Zhang Cunlin
Published: May 2011

In this letter, we describe detailed femtosecond-pump–terahertz-probe studies of the electromagnetic response of arrays of double-ring and complementary double-ring resonators. A surprising result is that an increase in the terahertz transmission is observed when a double-ring structure is excited by a 400 nm pump pulse. To explain this anomalous transmission, we propose a mechanism in which the photogenerated carriers play two competing roles. In other cases, for instance, when the wavelength of the pump pulse is 800 nm or in the case of a complementary double-ring structure, the normal transmission can also be understood in terms of this mechanism.
Conversion of strain state from biaxial to uniaxial in strained silicon
Authors: Ma Fei , Zhang Tian-Wei , Xu Ke-Wei , Chu Paul K.
Published: May 2011

The Raman shift of Δω3 in (001) strained silicon is found to be independent of the azimuthal angle of the patterned structures but exhibits shape dependence in strain relaxation. The tensile strain is reduced from 0.85% in the unpatterned thin film to 0.16% in the cylindrical pillars showing 82% relaxation. It becomes more significant along the width direction of the patterned gratings due to Poisson’s effect and only a tensile strain of 0.07% remains. Consequently, the strain state changes from biaxial into uniaxial and is expected to enhance the carrier mobility. Finite element analysis is conducted to elucidate the mechanism.
Spin-dependent magnetotransport in a p-InMnSb/n-InSb magnetic semiconductor heterojunction
Authors: Peters J. A. , Rangaraju N. , Feeser C. , Wessels B. W.
Published: May 2011

The spin-dependent transport properties in p-InMnSb/n-InSb magnetic semiconductor heterojunctions are presented. A positive junction giant magnetoresistance is observed from 75 to 298 K. The magnetoresistance is attributed to conduction via two spin channels resulting from p-d exchange interaction. The magnetoconductance of the heterojunction and its magnetic field dependence are well-described by a two-band model where the bands are spin-polarized. At 75 K and zero field, the spin polarization in the alloy is 90% and decreases to 48% at 298 K. The large spin polarization indicates that InMnSb should be suitable for spin-based transistors that operate at room temperature.
Stability control of nonlinear micromechanical resonators under simultaneous primary and superharmonic resonances
Authors: Kacem N. , Baguet S. , Dufour R. , Hentz S.
Published: May 2011

Fast effects of a slow excitation on the main resonance of a nonlinear micromechanical resonator are analytically and experimentally investigated. We show, in particular, how the bifurcation topology of an undesirable unstable behavior is modified when the resonator is simultaneously actuated at its primary and superharmonic resonances. A stabilization mechanism is proposed and demonstrated by increasing the superharmonic excitation.
Molecular beam deposition of Al2O3 on p-Ge(001)/Ge0.95Sn0.05 heterostructure and impact of a Ge-cap interfacial layer
Authors: Merckling C. , Sun X. , Shimura Y. , Franquet A. , Vincent B. , Takeuchi S. , Vandervorst W. , Nakatsuka O. , Zaima S. , Loo R. , Caymax M.
Published: May 2011

We investigated the molecular beam deposition of Al2O3 on Ge0.95Sn0.05 surface with and without an ultra thin Ge cap layer in between. We first studied the atomic configuration of both Ge1−xSnx and Ge/Ge1−xSnx surfaces after deoxidation by reflection high-energy electron diffraction and resulted, respectively, in a c(4×2) and (2×1) surface reconstructions. After in situ deposition of an Al2O3 high-κ gate dielectric we evidenced using time-of-flight secondary ion mass spectroscopy analyses that Sn diffusion was at the origin of high leakage current densities in the Ge1−xSnx/Al2O3 gate stack. This damage could be avoided by inserting a thin 5-nm-thick Ge cap between the oxide and the Ge1−xSnx layer. Finally, metal-oxide-semiconductor capacitors on the Ge capped sample showed well-behaved capacitance-voltage (C-V) characteristics with interface trap density (Dit) in the range of 1012 eV−1 cm−2 in mid gap and higher close to the valence band edge.
Magnetic properties of ion implanted Ge1−xMnx thin films solidified through pulsed laser melting
Authors: Dolph Melissa Commisso , Kim Taeseok , Yin Wenjing , Recht Daniel , Fan Wenbin , Yu Jiani , Aziz Michael J. , Lu Jiwei , Wolf Stuart A.
Published: May 2011

Ge1−xMnx thin films with an average Mn concentration of 0.64 at. % were fabricated through Mn ion implantation into crystalline germanium-on-insulator wafers. Implantation damage was removed and crystallinity restored by pulsed laser melting from a single 30-ns 308-nm XeCl+ excimer laser pulse. Resolidified films demonstrated higher Curie temperatures but smaller saturation magnetizations than those of both as-implanted films and implanted films subjected to rapid thermal annealing. These findings are attributed to the redistribution of Mn during solidification.
Effect of magnetic field on energy and entanglement of an exciton in coupled quantum dots
Authors: Shen Man , Liu Jian-Jun
Published: May 2011

Using the one-dimensional effective potential method, we theoretically study the properties of an exciton in vertically coupled quantum dots (VCQDs) under the influence of an applied magnetic field. The ground state transition energies of the heavy-hole exciton are calculated in a single quantum dot and in VCQDs, respectively. Our results are in good agreement with those of experiments. In symmetrical VCQDs, the entanglement of the exciton state monotonously increases with increasing barrier width, and can even attain the value of the maximally entangled state. When the symmetry of the VCQDs is broken, the entanglement of the exciton state almost entirely breaks down under the combined influences of tunneling and the Coulomb interactions.
High pressure Hugoniot measurements using converging shocks
Authors: Brown J. L. , Ravichandran G. , Reinhart W. D. , Trott W. M.
Published: May 2011

Plate impact experiments are a powerful tool in equation of state development, but are inherently limited by the range of impact velocities accessible to the facility. In an effort to dramatically increase the range of pressures which can be studied with available impact velocities, a new experimental technique is examined. The target plate is replaced by a composite assembly consisting of two concentric cylinders. The target is designed such that the initial shock velocity in a well-characterized outer cylinder is higher than in the inner cylinder material of interest. Conically converging shocks will be generated at the interface due to the impedance mismatch between the two materials and axisymmetric geometry. Upon convergence, an irregular reflection occurs and the conical analog of a Mach reflection develops. This Mach reflection grows until it reaches a steady state, at which point the high pressure state in the Mach disk can be measured using velocity interferometry and impedance matching techniques. The technique is demonstrated by studying the shock response of copper. A strong confinement setup utilizes a low impedance 6061-T6 aluminum outer cylinder and the weak confinement case features a higher impedance molybdenum outer cylinder. The experimental results for copper are in good agreement with a simple analytical model, numerical simulations, and data in the literature. The possibility of utilizing full field measurements to make multiple Hugoniot measurements is also examined.
Enhancement of Raman scattering by two orders of magnitude using photonic nanojet of a microsphere
Authors: Dantham V. R. , Bisht P. B. , Namboodiri C. K. R.
Published: May 2011

The enhancement of Raman signal is observed on excitation through a single microsphere. The dependence of the enhancement ratio (ER) on various parameters viz., numerical aperture (NA) of the microscopic objective lens, pump wavelength, size and refractive index of the microsphere has been studied. The enhancement has been explained due to interaction of the increased field of the photonic nanojet emerging from the single microsphere. The photonic nanojet induced ER of Raman peaks of silicon wafer and cadmium ditelluride is reported here. It is observed for the first time that by suitable selection of the experimental parameters, it is possible to enhance the Raman signal by approximately two orders of magnitude.
Tuneable magnetic patterning of paramagnetic Fe60Al40 (at. %) by consecutive ion irradiation through pre-lithographed shadow masks
Authors: Varea A. , Menéndez E. , Montserrat J. , Lora-Tamayo E. , Weber A. , Heyderman L. J. , Deevi S. C. , Rao K. V. , Suriñach S. , Baró M. D. , Buchanan K. S. , Nogués J. , Sort J.
Published: May 2011

Arrays of ferromagnetic circular dots (with diameters ranging from 225 to 420 nm) have been prepared at the surface of atomically ordered paramagnetic Fe60Al40 (at. %) sheets by means of ion irradiation through prelithographed poly(methyl methacrylate) (PMMA) masks. The cumulative effects of consecutive ion irradiation (using Ar+ ions at 1.2 × 1014 ions/cm2 with 10, 13, 16, 19 and 22 keV incident energies) on the properties of the patterned dots have been investigated. A progressive increase in the overall magneto-optical Kerr signal is observed for increasingly larger irradiation energies, an effect which is ascribed to accumulation of atomic disorder. Conversely, the coercivity, HC, shows a maximum after irradiating at 16–19 keV and it decreases for larger irradiation energies. Such a decrease in HC is ascribed to the formation of vortex states during magnetization reversal, in agreement with results obtained from micromagnetic simulations. At the same time, the PMMA layer, with an initial thickness of 90 nm, becomes progressively thinned during the successive irradiation processes. After irradiation at 22 keV, the remaining PMMA layer is too thin to stop the incoming ions and, consequently, ferromagnetism starts to be generated underneath the nominally masked areas. These experimental results are in agreement with calculations using the Monte-Carlo simulation Stopping Range of Ions in Matter software, which show that for exceedingly thin PMMA layers Ar+ ions can reach the Fe60Al40 layer despite the presence of the mask.
Self-organization of mesoscopically ordered parallel Gd-silicide nanowire arrays on a Si(110)-16×2 surface: A massively parallel active architecture
Authors: Hong Ie-Hong , Tsai Yung-Feng , Chen Tsung-Ming
Published: May 2011

A perfect self-organization of large-area, highly regular parallel arrays, consisting of uniformly spaced, epitaxial Gd-silicide nanowires with an identical width and a typical periodicity as small as ∼4 nm and ∼7.2 nm, respectively, and an average length exceeding 1 μm, has been achieved through the heteroepitaxial growth of Gd silicides on a Si(110)-16×2 surface. Scanning tunneling microscopy/spectroscopy studies clearly show that each metallic Gd-silicide nanowire consists of three atomically precise chain structures with a peculiar charge arrangement of alternating filled and empty states. This unique, massively parallel active architecture of well-ordered Gd-silicide NWs with exotic electronic properties can be exploited in nanoelectronic devices.