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The outstanding radiation damage stability of an NiCoCrFePd high entropy alloy (HEA) as compared to conventional alloys poses the question for the mechanism of an ion–matter interaction. The positron annihilation lifetime spectroscopic and TEM (transmission electron microscopic) measurements are implemented to trace different kinds of defects produced by 120 MeV Au+9 ion irradiation and their evolution as a function of ion fluence. The variation of lifetimes and corresponding intensities with the ion fluence indicates the formation of dislocation-type defects at a lower ion fluence and vacancy clusters at a higher ion fluence caused by coalescence or agglomeration of dislocation defects. Formation of different types of defects in turn modulates the strain development inside the crystal. Additionally, the HR-TEM investigation of NiCoCrFePd HEA also exhibits the formation of dislocation and vacancy clusters with the average size of vacancy clusters increases from ∼2.9 ± 0.1 to ∼3.8 ± 0.1 nm with the increases in the ion fluence. Surprisingly, the average defect cluster size in NiCoCrFePd HEA is suppressed compared to NiCoCrFe MEA, thereby showing the enhanced radiation stability on Pd incorporation due to the high defect recombination caused by reduced thermal conductivity and high lattice distortion. Nano-indentation measurement shows that the radiation hardening behavior of the NiCoCrFePd HEA responded slowly owing to its damage suppression property as compared to the NiCoCrFe MEA. Additionally, softening behavior also appeared at an early fluence in NiCoCrFe MEA compared to the NiCoCrFePd HEA signifying its excellent resistance to defect accumulation. [ABSTRACT FROM AUTHOR]

We present an electron microscopy and diffraction study of a Ca x Ba 1 − x Nb 2 O 6 ceramic with x = 0.28 (CBN28), a ferroelectric material with a partially filled tetragonal tungsten-bronze structure. The microstructure has strong similarities to that of Sr x Ba 1 − x Nb 2 O 6 , with an average orthorhombic symmetry and an intimate intermixture of merohedral twin variants at a length scale of tens of nanometers. Superstructure spots in diffraction patterns are displaced by a one-dimensional incommensurate modulation, characterized by a propagation vector δ δ 0. Heating experiments show that δ is strongly coupled to ferroelectric polarization, decreasing as 180 ° ferroelectric domains become more finely spaced and needle-like as the Curie temperature T C is approached during heating and increasing once more above T C. No change in symmetry is observed at T C , consistent with a transition from ferroelectric to antiferroelectric (or ferrielectric) relaxor properties. The superstructure spots and incommensurate modulation disappear ∼ 250 ° C above T C , consistent with polar regions in the material becoming fully transient. [ABSTRACT FROM AUTHOR]

Tailoring the blue photoluminescence (PL) in Al-doped ZnO (AZO)/TiO2 heterostructures is demonstrated by a controlled induction of shallow defect centers by 50 keV Ar+-ions. This is established by a combination of temperature dependent PL and electron paramagnetic resonance spectroscopy. The dominant blue-violet PL in an as-grown sample comprises a near band-edge emission, along with a peak associated with a radiative recombination of the electrons in shallow donor levels (Zn interstitials) and the holes from the valence band. However, the evolution of an additional yellow-green PL band at a fluence of 1 × 1015 ions/cm2 is governed by deep donor levels, particularly ionized oxygen vacancies. Irradiation at 1 × 1016 ions/cm2 further leads to the formation of Zn vacancies (shallow acceptors) owing to the development of an O-rich surface. The structural modifications of these samples have been investigated by field-emission scanning electron microscopy , transmission electron microscopy, and Rutherford backscattering. While small micro-cracks are found at a fluence of 2 × 1016 ions/cm2, the formation of graded layers is obtained at the highest fluence of 5 × 1016 ions/cm2 owing to ballistic intermixing and diffusion of the constituents. Detailed investigation suggests that a significant amount of Ti atoms is diffused in AZO by a complete deterioration of the AZO/TiO2 matrix at the highest fluence. [ABSTRACT FROM AUTHOR]

Electronic structure, optical, and photocatalytic properties of P, BSe, and SiC monolayers and their van der Waals heterostructures are investigated by (hybrid) first-principle calculations. The stability of the heterostructures and their corresponding induced-strain/unstrain monolayers are confirmed by the phonon spectra calculations. Similar to the corresponding parent monolayers, P-BSe (BSe-SiC) heterostructures are indirect type-II (type-I) bandgap semiconductors. A tensile strain of 10% (2%) transforms P-BSe (BSe-SiC) to type-I (type-II) direct bandgap nature. Interestingly, irrespective of the corresponding monolayers, the P-SiC heterostructure is a direct bandgap (type-II) semiconductor. The calculated electron and hole carrier mobilities of these heterostructures are in the range of 1.2 × 10 4 cm 2 / Vs to 68.56 × 10 4 cm 2 / Vs. Furthermore, absorption spectra are calculated to understand the optical behavior of these systems, where the lowest energy transitions are dominated by excitons. The valence and conduction band edges straddle the standard redox potentials in P-BSe, BSe-SiC, and P-SiC (strained) heterostructures, making them promising candidates for water splitting in the acidic solution. An induced compressive strain of 3.5% makes P suitable for water splitting at pH = 0. [ABSTRACT FROM AUTHOR]

The flexibility of amorphous anodized alumina (AAO) in developing radiation dosimeter for hadron therapy is reported by controlled carbon ion implantation, followed by thermoluminescence (TL) measurements. The efficacy of amorphous AAO in controlling TL sensitivity is found to be governed by an increase in F+ defect centers as a function of carbon concentration, as revealed from the close resemblance of the trend in photoluminescence intensity. Moreover, its nanoporous structure is demonstrated to be advantageous for defect engineering due to the increase in the surface-to-volume ratio. Detailed X-ray photoelectron spectroscopy analysis suggests the formation of F+ centers by substituting Al3+ ions with C2+ in the vicinity of oxygen vacancies, where depth-dependent study showed the evolution of conducting channels owing to sp2 hybridized C–C bonding, leading to a differential charging effect. This work provides a direction to tune nanoporous AAO in its amorphous form for future ion beam dosimetry. [ABSTRACT FROM AUTHOR]

Stretchable electronics are commonly used as a diverse class of interconnected architectures, which accommodate large strain during stretching. A systematic understanding of the underlying mechanism of these interconnects, i.e., stress/strain states is essential to optimize the spiral designs. Here, we demonstrate the in-depth structural response of the spiral-island system when subjected to in-plane and out-of-plane stretchings. We use numerical modeling to simulate the stresses and strains along the arm of the spiral when stretched at a prescribed displacement of 1000 μm. We show the strain contours for spirals connected in-series. Our results show that the additional spirals connected in-series share the prescribed displacement equally and thus lower the von Mises stresses and principal strains. We also compare the stress generated in arms for single spiral and triangular configurations, especially when we stretch out these configurations in-plane and out-of-plane. The evolved stress depends on the angular position of spirals for triangular configuration. For the out-of-plane case, we stretch the spirals vertically and diagonally outward, i.e., along the z direction and x-z directions, respectively. Our results show that spirals experience higher stress during stretching along the x-z direction. However, for the out-of-plane z direction stretching, the spiral's end, which is connected to the island, experiences higher stress as compared to that of x-z direction case. We use 3D printing to additively manufacture the replica for single spiral and triangular configuration and perform the tensile out-of-plane stretching. Our experimental results for elongations corroborate with numerical calculations. [ABSTRACT FROM AUTHOR]

We present an electron microscopy and diffraction study of a Ca x Ba 1 − x Nb 2 O 6 ceramic with x = 0.28 (CBN28), a ferroelectric material with a partially filled tetragonal tungsten-bronze structure. The microstructure has strong similarities to that of Sr x Ba 1 − x Nb 2 O 6 , with an average orthorhombic symmetry and an intimate intermixture of merohedral twin variants at a length scale of tens of nanometers. Superstructure spots in diffraction patterns are displaced by a one-dimensional incommensurate modulation, characterized by a propagation vector δ δ 0. Heating experiments show that δ is strongly coupled to ferroelectric polarization, decreasing as 180 ° ferroelectric domains become more finely spaced and needle-like as the Curie temperature T C is approached during heating and increasing once more above T C. No change in symmetry is observed at T C , consistent with a transition from ferroelectric to antiferroelectric (or ferrielectric) relaxor properties. The superstructure spots and incommensurate modulation disappear ∼ 250 ° C above T C , consistent with polar regions in the material becoming fully transient. [ABSTRACT FROM AUTHOR]

Using macroscopic quantum hydrodynamic formulation, we study the dispersion properties of electrostatic electron plasma oscillations in single-walled carbon nanotubes. The electrons and ions are considered uniformly distributed over the cylindrical surface of a nanotube thus forming a twocomponent (electron-ion) quantum plasma system. Electron degeneracy via Fermi-Dirac statistics as well as electron exchange and correlation effects is taken into account. It is found that the quantum (Bohm) potential arising due to fermionic nature of electrons and exchange-correlations effects has significant impact on the wave. The frequency of wave is influenced by variation in azimuthal index and radius of the nanotube. The results are analyzed numerically for typical systems for relatively longer wavelength waves and possible consequences are discussed. The results can be important in general understanding of the role of exchange-correlation potential in quantum hydrodynamic treatment of charge-carriers in nanotubes. [ABSTRACT FROM AUTHOR]

Low temperature (250-350ºC) hydrogen plasma annealing (HPA) treatments have been performed on amorphous hydrogenated silicon nitride (a-SiN x:H) thin films having a range of compositions and subsequent modification of photoluminescence (PL) is investigated. The PL spectral shape and peak positions for the as deposited films could be tuned with composition and excitation energies. HPA induced modification of PL of these films is found to depend on the N/Si ratio (x). Upon HPA, the PL spectra show an emergence of a red emission band for x ≤ 1, whereas an overall increase of intensity without change in the spectral shape is observed for x > 1. The emission observed in the Si rich films is attributed to nanoscale a-Si:H inclusions. The enhancement is maximum for off-stoichiometric films (x~1) and decreases as the compositions of a-Si (x = 0) and a-Si 3N 4 (x = 1.33) are approached, implying high density of non-radiative defects around x = 1. The diffusion of hydrogen in these films is also analyzed by Elastic Recoil Detection Analysis technique. [ABSTRACT FROM AUTHOR]

Origin of the Raman mode in nanocrystalline zinc oxide in the vicinity of A1 (LO) phonon mode induced by energetic heavy ions is reported. The evolution of this mode in the irradiated films is ascribed to the effect of disorder and the high density of lattice defects induced by irradiation. The presence of such defects is confirmed by the reduction in the intensity of E2 (high) mode and band bending of the near band edge absorption. A softening of the evolved Raman mode with increasing in ion fluence is also observed. This softening cannot be attributed to spatial confinement of phonons, as the sizes of the crystallites are large. Therefore, it is explained in terms of the combined effects of phonon localization by lattice defects and the structural strain in the lattice induced by electronic energy loss transferred by energetic heavy ions. [ABSTRACT FROM AUTHOR]

We report on the resistance switching induced by swift heavy ion (SHI) irradiation in lithium doped nanostructured NiO thin films grown on MgO (100) substrates by chemical solution deposition. Hysteresis in current-voltage curves were observed for the Ag/Li:NiO/Ag planner structures irradiated with 100 MeV Ag+14 ions, whereas pristine samples showed only linear I-V characteristics. No preferential oxygen loss from the film surface has been detected in on-line elastic recoil detection analysis. This suggests that change in the defect density created by SHI irradiation that may contribute to the metallic filaments play a major role as compared to the interfacial oxygen vacancies in resistance switching of NiO. [ABSTRACT FROM AUTHOR]

The topography evolution using the high energy heavy ion irradiation revealed the fact that ion bombardment produces self-affine nanostructures, creating peculiar surface morphologies and regular structures on the surface of the ZnO thin film at certain fluences. The self-affine nanopatterns produced on the surface of ZnO thin film under swift heavy ion irradiation are different types of nanostructures such as nanodimensional grains aligned like a linear array. Scanning probe microscopy is used for investigating the ZnO surfaces and UV-visible spectroscopy for studying the effect of surface modification on optical properties. The two-dimensional power spectral density of the irradiated ZnO thin films have been evaluated for each image to extract the value of growth factor (β) and roughness exponent (α). The exponent n increases from 2.2 to 4.0 up to a critical value of fluence and beyond which it decreases. These values suggest that ion assisted/induced diffusion process plays a crucial role in the evolution of self-affine nanostructures on ZnO surface. [ABSTRACT FROM AUTHOR]

γ-In2Se3 thin films prepared at different annealing temperatures ranging from 100 to 400 °C were irradiated using 90 MeV Si ions with a fluence of 2×1013 ions/cm2. X-ray diffraction analysis proved that there is no considerable variation in structural properties of the films due to the swift heavy ion irradiation. However, photosensitivity and sheet resistance of the samples increased due to irradiation. It was observed that the sample, which had negative photoconductivity, exhibited positive photoconductivity, after irradiation. The negative photoconductivity was due to the combined effect of trapping of photoexcited electrons, at traps 1.42 and 1.26 eV, above the valence band along with destruction of the minority carriers, created during illumination, through recombination. Photoluminescence study revealed that the emission was due to the transition to a recombination center, which was 180 meV above the valence band. Optical absorption study proved that the defects present at 1.42 and 1.26 eV were annealed out by the ion beam irradiation. This allowed photoexcited carriers to reach conduction band, which resulted in positive photoconductivity. Optical absorption study also revealed that the band gap of the material could be increased by ion beam irradiation. The sample prepared at 400 °C had a band gap of 2 eV and this increased to 2.8 eV, after irradiation. The increase in optical band gap was attributed to the annihilation of localized defect bands, near the conduction and valence band edges, on irradiation. Thus, by ion beam irradiation, one could enhance photosensitivity as well as the optical band gap of γ-In2Se3, making the material suitable for applications such as window layer in solar cells. [ABSTRACT FROM AUTHOR]

Electronic sputtering in polycrystalline LiF thin film by 120 MeV Ag25+ is investigated. The sputter yields of Li and F for the different thicknesses (10–265 nm) of films are measured with online elastic recoil detection analysis technique. A reduction in sputter yield, from ∼2.3×106 to 2.2×104 atoms/ion, is observed with increase in the film thickness. The trend in the experimental results can be explained in terms of size effect in thin film following inelastic thermal spike model. The confinement of energy in the film having smaller grains and lower thickness results in higher temperature causing higher sputtering yield. [ABSTRACT FROM AUTHOR]

Highly transparent, conducting, highly oriented, and almost single phase ZnO films have been deposited by simple e-beam evaporation method, and the deposition parameters were optimized. The films were prepared by (a) evaporation of ZnO at different substrate temperatures and (b) evaporation of ZnO at room temperature and subsequent annealing of the films in oxygen ambient at different temperatures. The characterizations of the film were performed by optical absorption spectroscopy (UV-visible), Fourier transform infrared spectroscopy, resistivity measurement, transmission electron microscopy (TEM), photoluminescence, and x-ray diffraction measurement. Absorption spectra revealed that the films were highly transparent and the band gap of the pre- and postannealed films was in good agreement with the reported values. The band gap of the films increases on increasing the substrate temperature as well as annealing temperature, whereas the resistivity of the film decreases with substrate temperature and increases with annealing temperature. Fourier transform infrared spectroscopy of ZnO films confirms the presence of Zn–O bonding. X-ray diffraction, electron diffraction, and TEM images with high resolution and Raman spectra of the films showed the formation of crystalline ZnO having wurtzite structure. [ABSTRACT FROM AUTHOR]

Deep levels have been characterized in n-type GaAs crystalline films grown by metalorganic chemical vapor deposition, doped in situ with 4d transition metal, rhodium, using the deep level transient spectroscopy (DLTS) technique. Two prominent broad bands of deep levels are found to be associated with Rh impurity, one in the upper half and the other in the lower half of the band gap. The band of electron emitting deep levels is shown to extend from 0.57–0.67 eV below the conduction band edge while the hole emitting deep levels band peaks at E[sub v]+0.42 eV. Detailed double-correlation DLTS measurements are performed to investigate the characteristics of the observed Rh-related deep levels. Both deep level bands are found to exhibit significant dependence of the thermal emission rates on the electric field from these measurements. The observed variation of the peak profile with the capture time for the electron emission band and theoretical analysis of the field dependence data support a band-like-extended-defect nature of this deep level band. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We study the spectroscopic properties of luminescent liquid crystals which show a glassy nematic phase at room temperature and then form polymer networks by polymerization using ultraviolet light. The reactive mesogens possess fluorene-based aromatic cores with either diene or acrylate photoreactive groups at the end of aliphatic spacers. The photoluminescence quantum efficiency is enhanced when a rigid polymer backbone is formed by crosslinking of the diene endgroups. Spectral shifts of the vibronic transitions confirm an increase in the viscosity of the matrix following photopolymerization. Continuous and time-resolved photoluminescence measurements show that the quantum efficiency is limited by exciton diffusion to traps. Either the diffusion constant or the density of traps is reduced by photopolymerization. [ABSTRACT FROM AUTHOR]