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CO-gas-sensing response of individual metal–oxide–metal (MOM) heterojunction nanowires (∼250 nm diameter) in the Au–NiO–Au system, where a nanoscale segment of NiO oxide (∼250 nm long) is sandwiched axially between Au nanowires, and between individual all-oxide NiO nanowires (∼250 nm diameter, ∼5 μm long), has been measured at 150 °C. While the all-oxide nanowires are highly resistive and show no measurable response, the MOM nanowires show ∼25% decrease in current with the introduction of 800 ppm CO. The response is reversible, but it is slow due to the low temperature used. MOM nanowires, by virtue of the nanoscale-dimension, low-resistance oxide segment and the high-quality “end-on” metal–oxide contact, are an attractive class of nanowire gas-sensing elements for potential use in multifunctional “bottom up” nanoelectronics.
We have observed efficient damage recovery in large-scale molecular dynamics simulations of 30 keV Zr recoils in pure zirconia and yttria-stabilized zirconia, which is in stark contrast to radiation damage accumulation in zircon. Dynamic annealing is highly effective in zirconia during the first 5 ps of damage evolution, especially in the presence of oxygen structural vacancies. This results in near-complete recovery of damage. Damage recovery on the cation sublattice is assisted by the anion sublattice recovery, which explains the remarkable radiation tolerance of stabilized zirconia. Ceramics engineered to heal themselves in this fashion hold great promise for use in high-radiation environments or for safely encapsulating high-level radioactive waste over geological time scales.
We describe the synthesis of nitrides of iridium and palladium using the laser-heated diamond anvil cell. We have used the in situ techniques of x-ray powder diffraction and Raman scattering to characterize these compounds and have compared our experimental findings where possible to the results of first-principles theoretical calculations. We suggest that palladium nitride is isostructural with pyrite, while iridium nitride has a monoclinic symmetry and is isostructural with baddeleyite.
Giant “pop-in” displacements are observed in crystalline silicon and germanium during high-load nanoindentation with a spherical diamond tip. These events are consistent with material removal triggered by lateral cracking during loading, which poses a hazard to microelectromechanical systems (MEMS) operation. We examine the scaling of the pop-in displacements as a function of peak indentation load and demonstrate a correlation with the depth of the plastic contact zone. We argue that giant pop-ins may occur in a broad range of highly brittle materials.
Crystal lines consisting of nonlinear optical fresnoite-type Ba2TiX2O8 (X = Si, Ge) crystals are patterned on the surface of CuO (1 mol%)-doped 33.3BaO–16.7TiO2–50SiO2 or –50GeO2 glasses by continuous-wave (cw) neodymium:yttrium aluminum garnet (Nd:YAG) laser (wavelength 1064 nm) irradiation. It is confirmed from polarized micro-Raman scattering spectra that the c axis of Ba2TiX2O8 crystals in the lines are oriented along the laser-scanning direction (i.e., parallel to the lines). The azimuthal dependences of second-harmonic (SH) intensities for the crystal lines indicate unique fringe patterns, and the c-axis orientation of the crystals is supported from the analyses of fringe patterns. The value of the d31/d33 ratio is found to be ∼23 for Ba2TiSi2O8 (BTS) crystal lines, and ∼13 for Ba2TiGe2O8 (BTG) crystal lines, where d31 and d33 are the principal d tensors for the second-order optical nonlinearity of fresnoite-type crystals.
This study examines the size-dependent deformation response of pure LiF single crystals using microcompression testing. Microcrystals with an 〈001〉 orientation and sample diameter D ranging from 1 to 20 μm were fabricated by focused ion beam (FIB)-milling from bulk crystals having a low initial dislocation density. Both as-grown and γ-irradiated crystals were examined to characterize the effect of an increased point defect density on the size-affected plastic flow response. Similar to previously studied face-centered cubic (FCC)-derivative metals, both types of LiF microcrystals exhibit typical size-dependent plastic flow behavior: a dramatic size-dependent and statistically varying flow stress, atypically high strain hardening rates at small plastic strains, and fast intermittent strain bursts. The size-dependent strengthening obeys a power law, σ ∼ D−m, where m ≈ 0.8, and this rapid hardening results in engineering flow stresses of 650 MPa in 1-μm samples. The findings are evaluated against possible dislocation mechanisms that could be responsible for the observed size effects.
Due to events of the past two decades, there has been new and increased usage of radiation-detection technologies for applications in homeland security, nonproliferation, and national defense. As a result, there has been renewed realization of the materials limitations of these technologies and greater demand for the development of next-generation radiation-detection materials. This review describes the current state of radiation-detection material science, with particular emphasis on national security needs and the goal of identifying the challenges and opportunities that this area represents for the materials-science community. Radiation-detector materials physics is reviewed, which sets the stage for performance metrics that determine the relative merit of existing and new materials. Semiconductors and scintillators represent the two primary classes of radiation detector materials that are of interest. The state-of-the-art and limitations for each of these materials classes are presented, along with possible avenues of research. Novel materials that could overcome the need for single crystals will also be discussed. Finally, new methods of material discovery and development are put forward, the goal being to provide more predictive guidance and faster screening of candidate materials and thus, ultimately, the faster development of superior radiation-detection materials.
Structures containing stacked layers of silicon-rich silicon nitride (green-blue luminescence) and oxide (red luminescence) fabricated by ion implantation are reported, and it is shown how a Si-based material can be engineered to emit over a broad range. To study in depth the emission from implanted SiNx matrices, single nitride layers have been also fabricated by the first time. Si excess variation and the relative thickness of nitride and oxide provide the intensity and position variation of the peaks, and thus open the way to engineer a stack with desired emission properties over the whole visible spectrum.
Palladium nanoparticles supported by alumina nanofibers have been successfully synthesized by electrospinning using palladium chloride incorporated into a solution of polyvinyl pyrrolidone and aluminum acetate. Palladium agglomerate sizes and the surface morphology of the electrospun nanofibers were determined by transmission electron microscopy. Palladium nanoparticles appeared to be well dispersed within the electrospun nanofiber structure. X-ray diffraction, x-ray photoelectron spectroscopy, and Raman scattering spectroscopy techniques were used to identify the crystalline form and distinguish between oxidized and metallic palladium particles after heating and hydrogenation.
We report on the effect of sample non-uniformity on the results of Hall-effect measurements. False positive Hall coefficients were obtained from an evidently n-type ZnO single crystal, although four electrodes with low contact resistance were made and the Van der Pauw parameter for this electrode configuration was close to 1.00. Further position-sensitive characterization revealed that the false positive Hall coefficient was due to non-uniform electrical properties of the sample. To demonstrate a false positive sign of the Hall coefficient due to sample non-uniformity, we devised a model structure made from evident n-type ZnO thin film and successfully reproduced a false positive Hall coefficient from n-type ZnO.
We used x-ray diffractometry (XRD), x-ray photoelectron spectrometry (XPS), and secondary-ion mass spectrometry (SIMS) to investigate the mechanism of the interfacial room-temperature (RT) chemical reaction between cation-deficient La0.56Li0.33TiO3 solid electrolytes and metallic lithium anodes in all-solid-state lithium batteries. A stoichiometric mixture of La2O3, Li2CO3, and TiO2 powders was calcined at 1250 °C for 8 h to obtain a single perovskite structure of La0.56Li0.33TiO3. When this La0.56Li0.33TiO3 sample and lithium were placed in contact at room temperature for 24 h, the phase of the La0.56Li0.33TiO3 remained unchanged. The XPS results indicate that 12% of the tetravalent Ti4+ ions were converted into trivalent Ti3+ ions. The valence conversion and degree of conversion were limited by the structural rigidity of the host crystal. Our SIMS analysis suggests the existence of a local electric field near the contact surface and indicates that the 6Li+ isotope ions were inserted into the specimen through the effect of this field. The change in the electrical properties of La0.56Li0.33TiO3 supports this mechanism for the interfacial reaction. The ionic conductivities of the grain and total grain boundary decreased and increased, respectively, after the insertion of Li+, and the total electronic conductivity increased as a result of the presence of intervalence electron hopping between mixed Ti3+/Ti4+ states. The mechanism of the lithium-activated RT interfacial reaction is associated with the reduction of Ti4+ transition metal ions from tetravalent to trivalent states and the local-electric-field-induced Li+ insertion into La3+/Li+-site vacancies of La0.56Li0.33TiO3.
In this article, we present the spectral and nonlinear optical properties of ZnO–Cu nanocomposites prepared by colloidal chemical synthesis. The emission consisted of two peaks. The 385-nm ultraviolet (UV) peak is attributed to ZnO and the 550-nm visible peak is attributed to Cu nanocolloids. Obvious enhancement of UV and visible emission of the samples is observed and the strongest UV emission of a typical ZnO–Cu nanocomposite is over three times stronger than that of pure ZnO. Cu acts as a sensitizer and the enhancement of UV emission are caused by excitons formed at the interface between Cu and ZnO. As the volume fraction of Cu increases beyond a particular value, the intensity of the UV peak decreases while the intensity of the visible peak increases, and the strongest visible emission of a typical ZnO–Cu nanocomposite is over ten times stronger than that of pure Cu. The emission mechanism is discussed. Nonlinear optical response of these samples is studied using nanosecond laser pulses from a tunable laser in the wavelength range of 450–650 nm, which includes the surface plasmon absorption (SPA) band. The nonlinear response is wavelength dependent and switching from reverse saturable absorption (RSA) to saturable absorption (SA) has been observed for Cu nanocolloids as the excitation wavelength changes from the low absorption window region to higher absorption regime near the SPA band. However, ZnO colloids and ZnO–Cu nanocomposites exhibit induced absorption at this wavelength. Such a changeover in the sign of the nonlinearity of ZnO–Cu nanocomposites, with respect to Cu nanocolloids, is related to the interplay of plasmon band bleach and optical limiting mechanisms. The SA again changes back to RSA when we move over to the infrared region. The ZnO–Cu nanocomposites show self-defocusing nonlinearity and good nonlinear absorption behavior. The nonlinear refractive index and the nonlinear absorption increases with increasing Cu volume fraction at 532 nm. The observed nonlinear absorption is explained through two-photon absorption followed by weak free-carrier absorption and interband absorption mechanisms. This study is important in identifying the spectral range and composition over which the nonlinear material acts as a RSA-based optical limiter. ZnO–Cu is a potential nanocomposite material for the light emission and for the development of nonlinear optical devices with a relatively small limiting threshold.
This article focuses on the modeling and simulation of thin-film silicon solar cells to obtain increased efficiency. Computer simulations were used to study the performance limits of tandem and triple-junction, silicon-based solar cells. For the analysis, the optical simulator SunShine, which was developed at Ljubljana University, and the optoelectrical simulator ASA, which was developed at Delft University of Technology, were used. After calibration with realistic optical and electrical parameters, we used these simulators to study the scattering properties required, the absorption in nonactive layers, antireflective coatings, and the crucial role of the wavelength-selective intermediate reflector on the performance of the solar cells. Careful current matching was carried out to explore whether a high photocurrent [i.e., more than 15 mA/cm2 for a tandem hydrogenated amorphous silicon (a-Si:H)/hydrogenated microcrystalline silicon (μc-Si:H) solar cell and 11 mA/cm2 for a triple-junction a-Si:H/amorphous silicon germanium (a-SiGe:H)/μc-Si:H solar cell] could be obtained. In simulations, the extraction of the charge carriers, the open-circuit voltage, and the fill factor of these solar cells were improved by optimizing the electrical properties of the layers and the interfaces: a p-doped, a-SiC layer with a larger band gap (EG > 2 eV) and buffer layers at p/i interfaces were used. Simulations demonstrated that a-Si:H/μc-Si:H solar cells could be obtained with a conversion efficiency of 15% or higher, and triple-junction a-Si:H/a-SiGe:H/μc-Si:H solar cells with an efficiency of 17%.
A new technique called energy-loss magnetic chiral dichroism (EMCD) has recently been developed [P. Schattschneider, et al. Nature441, 486 (2006)] to measure magnetic circular dichroism in the transmission electron microscope (TEM) with a spatial resolution of 10 nm. This novel technique is the TEM counterpart of x-ray magnetic circular dichroism, which is widely used for the characterization of magnetic materials with synchrotron radiation. In this paper we describe several experimental methods that can be used to measure the EMCD signal [P. Schattschneider, et al. Nature441, 486 (2006); C. Hébert, et al. Ultramicroscopy108(3), 277 (2008); B. Warot-Fonrose, et al. Ultramicroscopy108(5), 393 (2008); L. Calmels, et al. Phys. Rev. B76, 060409 (2007); P. van Aken, et al. Microsc. Microanal.13(3), 426 (2007)] and give a review of the recent improvements of this new investigation tool. The dependence of the EMCD on several experimental conditions (such as thickness, relative orientation of beam and sample, collection and convergence angle) is investigated in the transition metals iron, cobalt, and nickel. Different scattering geometries are illustrated; their advantages and disadvantages are detailed, together with current limitations. The next realistic perspectives of this technique consist of measuring atomic specific magnetic moments, using suitable spin and orbital sum rules, [L. Calmels, et al. Phys. Rev. B76, 060409 (2007); J. Rusz, et al. Phys. Rev. B76, 060408 (2007)] with a resolution down to 2 to 3 nm.
Aramid-based nanocomposites were prepared by solution intercalation techniques using p-aminobenzoic acid-modified montmorillonite. Polyamide was synthesized by reacting 4,4′-oxydianiline with isophthaloyl chloride in dimethyl acetamide. To create chemical interactions between the two phases for better dispersion of organoclay, aramid chains were selectively amine end-capped. The influence of organically modified clay on the morphology was investigated by x-ray diffraction (XRD), polarized optical microscopy (POM), and transmission electron microscopy (TEM). Mechanical, thermal, and water uptake measurements were carried out to further verify other physical properties of the nanocomposites. Tensile strength, modulus, elongation at break, and toughness were improved relative to pure polymer with the addition of 6 wt% organoclay. Thermal-decomposition temperatures of the nanocomposites were in the range 300–450 °C. Water uptake of neat aramid film was rather high (5.7%) and decreased with augmenting organoclay. DSC exhibited increase in the glass transition temperature (118 °C) up to addition of 16 wt% of organoclay.
A new organic–inorganic hybrid synthesized through a sol-gel process starting from alkoxysilane and chitosan is reported. Functionalization of the hybrid was effected through in situ hydrolysis–condensation reaction of methyltrimethoxysilane (MTMS) and vinyltrimethoxysilane (VTMS) in the reaction medium. The process yields highly transparent and hydrophobic silica–chitosan hybrids. The hybrid gel was investigated with respect to chemical modification, thermal degradation, hydrophobicity, and transparency under the ultraviolet-visible region. The extent of hydrophobicity had been tailored by varying the precursor ratio. SiO2–chitosan–MTMS hybrids showed a higher thermal stability than SiO2–chitosan–VTMS (SCV) hybrids with respect to hydrophobicity. Condensation of silsesquioxanes generated from the hydrolysis of MTMS and VTMS over the silica-chitosan particles impart hydrophobicity to the hybrid. The coatings of functionalized SiO2–chitosan precursor sol on glass substrates showed nearly 100% optical transmittance in the visible region. The present hybrid material may find application in optics and other industries.
Ultrathin films (6–10 nm) of silver and nickel were deposited by pulsed laser deposition (PLD) in high vacuum (1 × 10−6 mbar). Microstructural evolution of these films as function of incident laser energy, substrate temperature, substrate material [borosilicate glass, fused silica, MgO(100) and Si (311)] and target–substrate distance was studied in detail using dynamic force microscopy. It is shown that with increase in laser energy incident on the target, there is a substantial increase in particle size in the film. The effect of increased laser energy on microstructure is much more drastic than that for the increase of substrate temperature. In general, denser packing of nanoparticles and increased clustering have been observed at elevated substrate temperature. Increase in laser energy gives rise to higher average grain size, packing density, and clustering in comparison to substrate temperature. It is observed that the aspect ratio of grains is dependent on incident laser fluence and substrate temperature, but more drastically on the substrate material. Substrate coverage and aspect ratio of the grains are particularly dependent on the nature of crystallinity of the substrates. It is demonstrated that PLD provides a greater degree of microstructural manipulation than other physical vapor deposition techniques.
The electric-pulse–induced resistance switching of the Ag–La0.7Ca03MnO3(LCMO)–Pt heterostructures was studied. The multilevel resistance switching (MLRS), in which several resistance states can be obtained, was observed in the switching from high to low resistance state (HRS → LRS) by applying electric pulse with various pulse voltages. The threshold pulse voltages of MLRS are related to the initial resistance values as well as the switching directions. On the other hand, the resistance switching behavior from low to high resistance states (LRS → HRS) shows unobvious MLRS. According to the resistance switching behavior in serial and parallel modes, MLRS was explained by the parallel effect of multifilament forming/rupture in the Ag–LCMO interface layer. The present results suggest a possible application of Ag–LCMO–Pt heterostructures as multilevel memory devices.
We report a novel finding of slither propagation of shear bands on the fracture surface of a Cu47.5Zr47.5Al5 bulk metallic glass (BMG). The nanoscale heterogeneities in the as-cast state are aggregated along shear bands with irregular morphology. Such heterogeneities create a fluctuating stress field during shear band propagation leading to a slither propagation mode. The slither propagation of 10 to 15 nm wide shear bands is effective to improve both the plasticity and the “work-hardening-like” behavior of BMGs if the size, the morphology, and the elastic properties of the heterogeneities are intimately intercalated during solidification.