Volume 114, Issue 21, 07 December 2013
Index of content:
2 nanoribbons" title="Effects of edge hydrogenation on structural stability, electronic, and magnetic properties of WS2 nanoribbons" />
Using density functional theory based first-principles, we have investigated the structural stability, electronic, and magnetic properties of tungsten disulfide nanoribbons (WS2NRs). When the edges are bare, Zigzag-edge WS2 nanoribbons (ZWS2NRs) and Armchair-edge WS2 nanoribbons (AWS2NRs) are ferromagnetic metal and nonmagnetic semiconductor, respectively. After edge hydrogenation, WS2NRs exhibit different structural stabilities and electronic structures according the patterns of edge hydrogenation. Hydrogenated ZWS2NRs keep ferromagnetic and metallic while AWS2NRs convert from nonmagnetic to magnetic when at least one edge is partially hydrogenated. The transition of AWS2NRs is contributed to the unpaired valence electrons. With the change of nanoribbon width n, magnetic moment of edge fully hydrogenated ZWS2NRs shows nearly periodical variation, the band gap of bare AWS2NRs oscillates like three distinct families and owing to the ever-present edge effect it converges to 0.5 eV as n increases. Compared with bare AWS2NRs, edge fully hydrogenated AWS2NRs have the same scaling rule of band-gap variation when n , while the band gap oscillates up and down when n > 12 because of the more obvious edge asymmetric effect than quantum confinement effect. These findings are essential for applications of WS2NRs in nanoelectronics and spintronics.
- Lasers, Optics, and Optoelectronics
Ultrasensitive mass sensing with nonlinear optics in a doubly clamped suspended carbon nanotube resonator114(2013); http://dx.doi.org/10.1063/1.4838936View Description Hide Description
Nanomechanical resonator makes itself as an ideal system for ultrasensitive mass sensing due to its ultralow mass and high vibrational frequency. The mass sensing principle is due to the linear relationship of the frequency-shift and mass-variation. In this work, we will propose a nonlinear optical mass sensor based on a doubly clamped suspended carbon nanotube resonator in all-optical domain. The masses of external particles (such as nitric oxide molecules) landing onto the surface of carbon nanotube can be determined directly and accurately via using the nonlinear optical spectroscopy. This mass sensing proposed here may provide a nonlinear optical measurement technique in quantum measurements and environmental science.
Spatially addressable design of gradient index structures through spatial light modulator based holographic lithography114(2013); http://dx.doi.org/10.1063/1.4837635View Description Hide Description
In this paper, we present an achievable gradient refractive index in bi-continuous holographic structures that are formed through five-beam interference. We further present a theoretic approach for the realization of gradient index devices by engineering the phases of the interfering beams with a pixelated spatial light modulator. As an example, the design concept of a gradient index Luneburg lens is verified through full-wave electromagnetic simulations. These five beams with desired phases can be generated through programming gray level super-cells in a diffractive spatial light modulator. As a proof-of-concept, gradient index structures are demonstrated using synthesized and gradient phase patterns displayed in the spatial light modulator.
114(2013); http://dx.doi.org/10.1063/1.4837615View Description Hide Description
We present the band parameters such as band gap, spin-orbit splitting energy, band offsets and strain of InGaAsBi on InP based on recent experimental data. It is shown that InGaAsBi is promising for near- and mid-infrared photonic devices operating from 0.3–0.8 eV (1.5–4 μm) on conventional InP substrates. We also show how bismuth may be used to form alloys whereby the spin-orbit splitting energy (ΔSO) is large and controllable and can, for example, be made larger than the band gap (E g) thereby providing a means of suppressing non-radiative hot-hole producing Auger recombination and inter-valence band absorption both involving the spin-orbit band. This is expected to improve the high-temperature performance and thermal stability of light emitting devices.
114(2013); http://dx.doi.org/10.1063/1.4837661View Description Hide Description
Effects of tin doping on crystallization of amorphous silicon were studied using Raman scattering, Auger spectroscopy, scanning electron microscopy, and X-ray fluorescence techniques. Formation of silicon nanocrystals (2–4 nm in size) in the amorphous matrix of Si1−xSnx, obtained by physical vapor deposition of the components in vacuum, was observed at temperatures around 300 °C. The aggregate volume of nanocrystals in the deposited film of Si1−xSnx exceeded 60% of the total film volume and correlated well with the tin content. Formation of structures with ∼80% partial volume of the nanocrystalline phase was also demonstrated. Tin-induced crystallization of amorphous silicon occurred only around the clusters of metallic tin, which suggested the crystallization mechanism involving an interfacial molten Si:Sn layer.
- Plasmas and Electrical Discharges
Improved power capacity in a high efficiency klystron-like relativistic backward wave oscillator by distributed energy extraction114(2013); http://dx.doi.org/10.1063/1.4840956View Description Hide Description
With the efficiency increase of a klystron-like relativistic backward wave oscillator, the maximum axial electric field and harmonic current simultaneously appear at the end of the beam-wave interaction region, leading to a highly centralized energy exchange in the dual-cavity extractor and a very high electric field on the cavity surface. Thus, we present a method of distributed energy extraction in this kind of devices. Particle-in-cell simulations show that with the microwave power of 5.1 GW and efficiency of 70%, the maximum axial electric field is decreased from 2.26 MV/cm to 1.28 MV/cm, indicating a threefold increase in the power capacity.
114(2013); http://dx.doi.org/10.1063/1.4841135View Description Hide Description
Arc plasma from Ti-C, Ti-Al, and Ti-Si cathodes was characterized with respect to charge-state-resolved ion energy. The evaluated peak velocities of different ion species in plasma generated from a compound cathode were found to be equal and independent on ion mass. Therefore, measured difference in kinetic energies can be inferred from the difference in ion mass, with no dependence on ion charge state. The latter is consistent with previous work. These findings can be explained by plasma quasineutrality, ion acceleration by pressure gradients, and electron-ion coupling. Increasing the C concentration in Ti-C cathodes resulted in increasing average and peak ion energies for all ion species. This effect can be explained by the “cohesive energy rule,” where material and phases of higher cohesive energy generally result in increasing energies (velocities). This is also consistent with the here obtained peak velocities around 1.37, 1.42, and 1.55 (104 m/s) for ions from Ti0.84Al0.16, Ti0.90Si0.10, and Ti0.90C0.10 cathodes, respectively.
Controlled deposition of sulphur-containing semiconductor and dielectric nano-structured films on metals in SF6 ion-ion plasma114(2013); http://dx.doi.org/10.1063/1.4842915View Description Hide Description
In the present paper, the deposition processes and formation of films in SF6 ion-ion plasma, with positive and negative ion flows accelerated to the surface, are investigated. The PEGASES (acronym for Plasma Propulsion with Electronegative GASES) source is used as an ion-ion plasma source capable of generating almost ideal ion-ion plasma with negative ion to electron density ratio more than 2500. It is shown that film deposition in SF6 ion-ion plasma is very sensitive to the polarity of the incoming ions. The effect is observed for Cu, W, and Pt materials. The films formed on Cu electrodes during negative and positive ion assisted deposition were analyzed. Scanning electron microscope analysis has shown that both positive and negative ion fluxes influence the copper surface and leads to film formation, but with different structures of the surface: the low-energy positive ion bombardment causes the formation of a nano-pored film transparent for ions, while the negative ion bombardment leads to a continuous smooth insulating film. The transversal size of the pores in the porous film varies in the range 50–500 nm, and further analysis of the film has shown that the film forms a diode together with the substrate preventing positive charge drain, and positive ions are neutralized by passing through the nano-pores. The film obtained with the negative ion bombardment has an insulating surface, but probably with a multi-layer structure: destroying the top surface layer allows to measure similar “diode” IV-characteristics as for the nano-pored film case. Basing on results, practical conclusions for the probes and electrodes cleaning in ion-ion SF6 plasmas have been made. Different applications are proposed for the discovered features of the controlled deposition from ion-ion plasmas, from Li-sulphur rechargeable batteries manufacturing and nanofluidics issues to the applications for microelectronics, including low-k materials formation.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
114(2013); http://dx.doi.org/10.1063/1.4836855View Description Hide Description
Scandium fluoride displays isotropic negative thermal expansion (NTE) from at least 10 to 1100 K and retains a cubic ReO3-type structure over this range; the NTE is most pronounced at low temperatures. Control of thermal expansion was explored by forming Sc1– x Y x F3 , which were characterized with synchrotron powder diffraction at ambient pressure from 100 to 800 K. The behavior of the solid solutions under pressure (≤0.276 GPa) was also examined while heating from 298 to 523 K. Insertion of the relatively large Y3+ ion into ScF3 results in a cubic-to-rhombohedral phase transition upon cooling from ambient temperature to 100 K, even at low substitution levels (5%). The coefficient of thermal expansion (CTE) of the solid solutions in the rhombohedral phase is strongly dependent on both composition and temperature; however, above 400 K, where all samples are cubic, the CTE appears to be largely independent of composition. The isothermal bulk modulus and CTE of ScF3, but not those of the solid solutions, are independent of temperature and pressure, respectively. Yttrium substitution lowers the bulk modulus, even at temperatures where the samples are cubic. Finally, the solid solutions stiffen upon heating.
114(2013); http://dx.doi.org/10.1063/1.4836896View Description Hide Description
We report on a method of synthesizing Mn4+-activated ZnSiF6·6H2O hydrate phosphor by the chemical reaction in a Teflon beaker. The structural and optical properties of ZnSiF6·6H2O:Mn4+ are investigated using x-ray diffraction measurement, photoluminescence (PL) analysis, PL excitation spectroscopy, diffuse reflectance measurement, and Raman scattering spectroscopy. The synthesized phosphor exhibits a series of sharp red emission peaks at ∼630 nm, which is characteristic for Mn4+ ions. The luminescent study is focused on the thermal quenching phenomenon above ∼300 K and is found to be caused by thermal decomposition of the ZnSiF6·6H2O host. Degradation in the PL intensity under near-UV light illumination has also been observed to occur in this phosphor.
114(2013); http://dx.doi.org/10.1063/1.4834575View Description Hide Description
When the plastic deformation is applied to neat polymer, the polymer chains are aligned and the thermal conductivity of neat polymer increases linearly along the loading direction. However, the thermal conductivity change of nanocomposites consisting of polymer matrix and nanofillers during plastic deformation is not simple. The volume fraction and size of nanofillers scarcely affect the structural change of polymer chains during the plastic deformation. In this study, the structural change of polymeric materials according to the mechanical loading and its effect on the thermal transport properties are investigated through a molecular dynamics simulation. To investigate the effects of nanofiller, its volume fraction, and size on the thermal transport properties, the unit cells of neat amorphous nylon 6 and nanocomposites consisting of amorphous nylon 6 matrix and spherical silica particles are prepared. The molecular unit cells are uniaxially stretched by applying constant strain along the loading directions. Then, non-equilibrium molecular dynamics (NEMD) simulations are performed to estimate the thermal conductivities during plastic deformation. The alignment of polymer chains is analyzed by tracing the orientation correlation function of each polymer molecule and the free volume change during the mechanical loading is also analyzed.
114(2013); http://dx.doi.org/10.1063/1.4831669View Description Hide Description
Amorphous carbon and amorphous materials in general are of particular importance for high resolution electron microscopy, either for bulk materials, generally covered with an amorphous layer when prepared by ion milling techniques, or for nanoscale objects deposited on amorphous substrates. In order to quantify the information of the high resolution images at the atomic scale, a structural modeling of the sample is necessary prior to the calculation of the electron wave function propagation. It is thus essential to be able to reproduce the carbon structure as close as possible to the real one. The approach we propose here is to simulate a realistic carbon from an energetic model based on the tight-binding approximation in order to reproduce the important structural properties of amorphous carbon. At first, we compare this carbon with the carbon obtained by randomly generating the carbon atom positions. In both cases, we discuss the limit thickness of the phase object approximation. In a second step, we show the influence of both carbons models on (i) the contrast of Cu, Ag, and Au single atoms deposited on carbon and (ii) the determination of the long-range order parameter in CoPt bimetallic nanoalloys.
Hydrogenated TiO2 film for enhancing photovoltaic properties of solar cells and self-sensitized effect114(2013); http://dx.doi.org/10.1063/1.4832783View Description Hide Description
Hydrogenated TiO2 film was obtained by annealing TiO2 film at 350 °C for 2 h with hydrogen, and TiO2 films were prepared by screen printing on fluorine-doped tin oxide glass. Structural characterization by X-ray diffraction and electron microscopy did not show obvious difference between hydrogenated TiO2 film and pristine TiO2 film. Through optical and electrochemical characterization, the hydrogenated TiO2 film showed enhanced absorption and narrowed band gap, as well as reduced TiO2 surface impedance and dark current. As a result, an obviously enhanced photovoltaic effect was observed in the solar cell with hydrogenated TiO2 as photoanode without adding any dye due to the self-sensitized effect of hydrogenated TiO2 film, which excited electrons injecting internal conduction band of TiO2 to generate more photocurrent.
114(2013); http://dx.doi.org/10.1063/1.4837598View Description Hide Description
A three-dimensional molecular-dynamics model of with order was developed and found to support the excitation of discrete breathers (DBs) and energy localization on the Al sublattice. For an initial lattice temperature of 0 K, large-amplitude DBs polarized along  are found to be very weakly damped, retaining most of their initial energy for more than 1000 cycles, while DBs polarized along  damped out over ∼15 cycles. Because the DBs and their dissipation channels are confined to the Al sublattice, long-lived nonequilibrium states with large energy differences between the Al and Pt sublattices occur. Since collisions during irradiation more efficiently generate lattice vibrations in light atoms than heavy atoms, such nonequilibrium states may occur and alter the relaxation processes occurring during radiation damage.
114(2013); http://dx.doi.org/10.1063/1.4837617View Description Hide Description
The explosion hardening tests of high manganese steel were carried out by using two kinds of explosives of the same composition but different density, respectively. The detonation velocities were tested and the relevant mechanical properties were studied. The results show that the stronger single impulse acting on the specimen, the more hardness of surface increases and the more impact toughness decreases. Compared with the explosive of 1.48 g/cm3 density, the hardness, elongation rate, and impact toughness of the sample for triple explosion with explosive of 1.38 g/cm3 density are larger at the same hardening depth. In addition, the tensile strength of the sample for triple explosion with density of 1.38 g/cm3 is higher from the surface to 15 mm below the surface hardened.
114(2013); http://dx.doi.org/10.1063/1.4837655View Description Hide Description
Boron-carbon-nitrogen films were grown by RF reactive sputtering from a B4C target and N2 as reactive gas. The films present phase segregation and are mechanically softer than boron carbide films (a factor of more than 2 in Young's modulus). This fact can turn out as an advantage in order to select buffer layers to better anchor boron carbide films on substrates eliminating thermally induced mechanical tensions.
Determination of lattice parameters, strain state and composition in semipolar III-nitrides using high resolution X-ray diffraction114(2013); http://dx.doi.org/10.1063/1.4834521View Description Hide Description
In group-III-nitride heterostructures with semipolar or nonpolar crystal orientation, anisotropic lattice and thermal mismatch with the buffer or substrate lead to a complex distortion of the unit cells, e.g., by shearing of the lattice. This makes an accurate determination of lattice parameters, composition, and strain state under assumption of the hexagonal symmetry impossible. In this work, we present a procedure to accurately determine the lattice constants, strain state, and composition of semipolar heterostructures using high resolution X-ray diffraction. An analysis of the unit cell distortion shows that four independent lattice parameters are sufficient to describe this distortion. Assuming only small deviations from an ideal hexagonal structure, a linear expression for the interplanar distances dhkl is derived. It is used to determine the lattice parameters from high resolution X-ray diffraction 2ϑ-ω-scans of multiple on- and off-axis reflections via a weighted least-square fit. The strain and composition of ternary alloys are then evaluated by transforming the elastic parameters (using Hooke's law) from the natural crystal-fixed coordinate system to a layer-based system, given by the in-plane directions and the growth direction. We illustrate our procedure taking an example of (11 2) Al κGa1−κN epilayers with Al-contents over the entire composition range. We separately identify the in-plane and out-of-plane strains and discuss origins for the observed anisotropy.
114(2013); http://dx.doi.org/10.1063/1.4839835View Description Hide Description
Theoretical model for optically excited two-layer elastic plate, which includes plasmaelastic, thermoelastic, and thermodiffusion mechanisms, is given in order to study the dependence of the photoacoustic (PA) elastic bending signal on the optical, thermal, and elastic properties of thin film—substrate system. Thin film-semiconductor sample (in our case Silicon) is modeled by simultaneous analysis of the plasma, thermal, and elastic wave equations. Multireflection effects in thin film are included in theoretical model and analyzed. Relations for the amplitude and phase of electronic and thermal elastic bending in the optically excited two-layer mechanically-supported circular plate are derived. Theoretical analysis of the thermodiffusion, plasmaelastic, and thermoelastic effects in a sample-gas-microphone photoacoustic detection configuration is given. Two normalization procedures of the photoacoustic elastic bending signal in function of the modulation frequency of the optical excitation are established. Given theoretical model can be used for various photoacoustic detection configurations, for example, in the study of optical, thermal, and elastic properties of the dielectric-semiconductor or metal-semiconductor structure, etc., Theoretical analysis shows that it is possible to develop new noncontact and nondestructive experimental method—PA elastic bending method for thin film study, with possibility to obtain the optical, thermal, and elastic parameters of the film thinner than 1 μm.
114(2013); http://dx.doi.org/10.1063/1.4837999View Description Hide Description
Solid state 93 Nb nuclear magnetic resonance spectroscopy has been employed to investigate the atomic and electronic structures in Ni-Nb based metallic glass (MG) model system. 93 Nb nuclear magnetic resonance (NMR) isotropic metallic shift of Ni 60 Nb 35Sn5 has been found to be ∼100 ppm lower than that of Ni 60 Nb 35Zr5 MG, which is correlated with their intrinsic fracture toughness. The evolution of 93 Nb NMR isotropic metallic shifts upon alloying is clearly an electronic origin, as revealed by both local hyperfine fields analysis and first-principle computations. This preliminary result indicates that, in addition to geometrical considerations, atomic form factors should be taken into a description of atomic structures for better understanding the mechanical behaviors of MGs.
114(2013); http://dx.doi.org/10.1063/1.4842116View Description Hide Description
Room-temperature heavy-ion bombardment of polar (0001) ZnO leads to the formation of intermediate peak and step features in damage–depth profiles measured by ion channeling. Here, we show that these anomalous disorder effects are strongly suppressed for crystals with and non-polar surface terminations. Possible defect interaction scenarios responsible for the enhanced radiation tolerance of non-polar-terminated ZnO are discussed.
114(2013); http://dx.doi.org/10.1063/1.4838435View Description Hide Description
The absolute quantum cutting efficiency of Tb3+-Yb3+ co-doped glass was quantitatively measured by an integrating sphere detection system, which is independent of the excitation power. As the Yb3+ concentration increases, the near infrared quantum efficiency exhibited an exponential growth with an upper limit of 13.5%, but the visible light efficiency was reduced rapidly. As a result, the total quantum efficiency monotonically decreases rather than increases as theory predicted. In fact, the absolute quantum efficiency was far less than the theoretical value due to the low radiative efficiency of Tb3+ (<61%) and significant cross-relaxation nonradiative loss between Yb3+ ions.