Characteristic Fine Structure Research Articles

Fast photochromism of helicene-bridged imidazole dimers.

The unique optical and magnetic properties of organic biradicaloids on polycyclic aromatic hydrocarbons are of fundamental interest in the development of novel organic optoelectronic materials. Open-shell π-conjugated molecules with helicity have recently attracted a great deal of attention due to the magnetic-field-dependence and spin-selectivity arising from the combination of helical chirality and electron spins. However, the molecular design for helical biradicaloids is limited due to the thermal instability and high reactivity. Herein, we achieved fast photochromic reactions and reversible photo-generation of biradical species using helicene-bridged imidazole dimers. A [9]helicene-bridged imidazole dimer exhibits reversible photochromism upon UV light irradiation. The transient species produced reversibly by UV light irradiation exhibited ESR spectra with a fine structure characteristic of a triplet radical pair, indicating the reversible generation of the biradical. The half-life of the thermal recombination reaction of the biradical was estimated to be 29 ms at 298 K. Conversely, a substantial activation energy barrier was confirmed for the intramolecular recombination reaction in the [7]helicene-bridged imidazole dimer, attributed to the extended pitch length of [7]helicene. The temperature dependence of the thermal back reactions revealed that the [7]helicene and [9]helicene moieties functioned as 'soft' and 'hard' molecular bridges, respectively. These findings pave the way for future advances in the development of photoswitchable helical biradicaloids.

Fine structure of another blue luminescence band in undoped GaN

A blue luminescence band, labeled BL3, has been found in undoped GaN samples grown by hydride vapor phase epitaxy. It has a maximum at 2.8 eV and a phonon-related fine structure at its high-energy side. The zero-phonon line of this band consists of a duplet with two sharp lines at 3.0071 and 3.0147 eV. Three phonon modes, including the LO mode with an energy of 91.3 meV and two pseudolocal phonon modes with energies of 39.6 and 68.2 meV, form the characteristic fine structure of the BL3 band. The BL3 band is attributed to internal transitions from excited states located near the conduction band to the 0/+ transition level of unknown defect. The defect is preliminarily identified as the RY3 center, which is also responsible for bright red-yellow luminescence bands in the studied samples.

The significant influence of ionic concentrations and immersion temperatures on deposition behaviors of hydroxyapatite on alkali- and heat-treated titanium in simulated body fluid

Biomimetic deposition of apatite in simulated body fluid (SBF) in vitro has been widely used to evaluate the bioactivity of titanium (Ti) and prepare a functional coating on biomedical metallic substrates. In this study, the deposition behaviors of hydroxyapatite (HA) on Ti samples treated by alkali and heat treatment (AH) in SBFs (1×, 2×, 3× SBF) at 37, 47 and 57°C were researched. The electrochemical results confirmed the characteristic fine and porous network structure on Ti substrate after AH resulted in the decrease of corrosion resistance. SEM images showed that the surface structure and morphology of hydroxyapatite (HA) coatings varied markedly under different soaking conditions. Particularly, high SBF concentrations and immersion temperatures created more homogeneous and dense deposited layers with uniform large spherical HA agglomerates. The results of XRD and FTIR demonstrated that brushite (the precursor of HA) preferentially formed and gradually transformed into HA in SBF with high ion concentration and temperature. Thermodynamic analysis in terms of nucleation and crystal growth mechanism was proposed to fundamentally explain HA deposition process in vitro. This work provides a potential method to rapidly evaluate the bioactivity of Ti-based biomaterials and prepare controlled bioactive coating on the implants.

Transesterification in the Solid State of Cyclic and Linear Poly(l‐lactide)s

Poly(l‐lactide)s are synthesized and annealed at 120 °C and changes of the molecular weight distribution (MWD) are monitored by matrix‐assited laser desorption/ionization time‐of‐flight (MALDI‐TOF) mass spectrometry. For example, benzyl alcohol+SnOct2 causes equilibration of odd‐ and even‐numbered chains and the final goal of the transesterification is the most probable distribution. The underlying intermolecular transesterification is even observed at 100 and 80 °C in the solid state. However, cyclic tin mercaptide catalysts transform the initial most probable distribution into a MWD with maxima, which display a conspicuous fine structure due to a preferential crystallization of certain ring sizes. The optimum ring sizes for the crystallization are provided by ring‐ring equilibration. The gradual formation of a special morphology shifts the melting temperature to values up to 187 °C. Annealing of commercial poly(l‐lactide) with a cyclic tin catalyst also yields a distribution of mass peaks with a maximum showing the characteristic fine structure. image

Lateral Electron Confinement with Open Boundaries: Quantum Well States above Nanocavities at Pb(111).

We have studied electron states present at the Pb(111) surface above Ar-filled nanocavities created by ion beam irradiation and annealing. Vertical confinement between the parallel crystal and nanocavity surfaces creates a series of quantum well state subbands. Differential conductance data measured by scanning tunneling spectroscopy contain a characteristic spectroscopic fine structure within the highest occupied subband, revealing additional quantization. Unexpectedly, reflection at the open boundary where the thin Pb film recovers its bulk thickness gives rise to the lateral confinement of electrons.

On the propagation and mode conversion of auroral medium frequency bursts

AbstractAuroral medium frequency (MF) bursts are broadband, impulsive radio emissions associated with local substorm onsets. MF bursts consist of a characteristic fine structure whereby the higher frequencies arrive 10–100 ms before the lower frequencies. LaBelle (2011a) proposed that MF bursts originate as Langmuir/Z mode waves on the topside of the ionosphere that mode‐convert to LO mode waves and propagate to ground level, with the fine structure resulting by propagation delays due to the topside ionospheric density profile. We investigate three aspects of this mechanism. First, full‐wave calculations are used to simulate the MF burst fine structure using a realistic ionospheric density profile. The delay between the highest and lowest frequencies is 21 ms. This value is smaller than the experimentally determined delays of ∼100 ms presented in Bunch and LaBelle (2009), but differences between the topside electron number density profile used in the simulations and the number density profile during disturbed conditions make comparisons only approximate. Second, the Landau damping of Langmuir/Z mode waves on the topside ionosphere is calculated, assuming the electron distribution function consists of a cold background population (ne0) and a warm secondary population (nse). The Landau damping is small when nse/ne0 = 0.04% (consistent with Maggs and Lotko (1981)) but is significant when nse/ne0 > 0.4%. Finally, full‐wave calculations are used to determine the mode conversion efficiency from Langmuir/Z mode waves to LO mode waves. These imply that waves would suffer an attenuation of wave energy density of approximately 5–10% if they are generated with their wave vectors in a narrow cone centered around the local magnetic field. Taken together, these calculations suggest that for small values of nse/ne0 <0.4%, the mechanism proposed by LaBelle (2011a) is a plausible explanation for the origin of MF bursts.

The (111) surface of NaAu2: structure, composition, and stability.

The (111) surface of single-crystal NaAu(2) is a model for catalytically active, powdered NaAu(2). We prepare and characterize this surface with a broad suite of techniques. Preparation in ultrahigh vacuum consists of the traditional approach of ion bombardment (to remove impurities) and thermal annealing (to restore surface order). Both of these steps, however, cause loss of sodium (Na), and repeated treatments eventually trigger conversion of the surface and near-surface regions to crystalline gold. The bulk has a limited ability to repopulate the surface Na. Under conditions where Na depletion is minimized, electron diffraction patterns are consistent with the bulk-terminated structure, and scanning tunneling microscopy reveals mesa-like features with lateral dimensions of a few tens of nanometers. The tops of the mesas do not possess fine structure characteristic of a periodic lattice, suggesting that the surface layer is disordered under the conditions of these experiments.

Electron Microscopy Study of Binary Nanocolloidal Crystals with ico-AB13 Structure Made of Monodisperse Silica Nanoparticles

Binary nanocolloidal crystals (BNCCs) made of two different nanoparticles are a promising new class of advanced functional materials with synergetic and collective properties. These materials have been shown to exhibit excellent structural diversity and a wide range of different stoichiometries. In this study, BNCCs with an ico-AB13 structure were prepared as inorganic bulk materials by simple solvent evaporation of silica nanocolloidal aqueous solutions. Systematic analysis of Fourier diffractograms of high-resolution transmission electron microscopy (HRTEM) images shows the reflection conditions of the space group Fm3̅c without any ambiguities, which is indicative of the ico-AB13 structure. Simulated HRTEM images based on the ideal ico-AB13 structure were also calculated under the kinematical scattering approximation. These images showed the characteristic fine structure of the ico-AB13 structure along the high-symmetry axes. The agreement between the observed and simulated HRTEM images indicates the validity of the ideal ico-AB13 model structure. This result shows that HRTEM together with a quantitative image simulation is a versatile method for the characterization of the BNCCs at the nanoscale and can be routinely applied for any nanocolloidal crystals, even for complex structures.

Fine structure and interface characteristic of α-Al2O3 in molybdenum alloy

Molybdenum alloy reinforced with 10vol.% alumina (α-Al2O3) was prepared using the sol–gel process combined with the powder metallurgy method. The fine structure of α-Al2O3 and the interface structure of α-Al2O3/Mo are analyzed using high-resolution electron microscopy. Systematic analysis reveals that the α-Al2O3, exhibiting polyhedron shape, had a stacking fault substructure. The fine structure of α-Al2O3 grains is characterized as the orderly stacking of several amorphous and crystalline α-Al2O3 nanograins. It is observed that an interfacial bonding zone of dimension 3–5nm existed between α-Al2O3 and molybdenum grain. In the bonding zone, the atomic arrangement was similar to the structure of α-Al2O3, yet it is relatively disordered. The crystallographic orientations at the interface can be illustrated as α-Al2O3 [122]//Mo [11(—)0] and α-Al2O3 (201(—))//Mo (110).

Physical and Chemical Response of FeCl3/FeCl4– Spin Probes on the Functionalizing of Ionic Liquids

Fe(III) compounds FeCl3 and FeCl4(-) have been used as ESR spin probes in ionic liquids (ILs) at 293 and 77 K for the first time. They showed characteristic spectral patterns, which could be separated from each other by simulation. The largest contribution originates from aggregated FeCl4(-) and other exchange coupled species at g' ≈ 2.6. FeCl4(-) has been shown to be an identifiable, changeable, customizable, transferable, and extractable probe with contributions to a characteristic ESR fine structure. For simulation a spin Hamiltonian with up to fourth order and statistic distributions of spin coupling parameters has been used. The different Fe(III) signals coexist being dependent on the functionalization of the IL, i.e., on changing the chain length of the substituent at the imidazolium cation as well as varying the respective anion (BF4(-), PF6(-), Cl(-), and FeCl4(-)). From the molecular structure and occurrence of the Fe(III) species conclusions could be drawn concerning their locations and reactions in polar and nonpolar compartments of the ILs. Their contributions could be purposefully adjusted via the molecular control of the properties of the ILs. The conversion of FeCl3 into FeCl4(-) and [FeCl4X2](3-) species could be observed to be dependent on the formation of polar and nonpolar domains in ILs.

Universal behavior in the scattering of heavy, weakly interacting dark matter on nuclear targets

Particles that are heavy compared to the electroweak scale (M≫mW), and that are charged under electroweak SU(2) gauge interactions display universal properties such as a characteristic fine structure in the mass spectrum induced by electroweak symmetry breaking, and an approximately universal cross section for scattering on nuclear targets. The heavy particle effective theory framework is developed to compute these properties. As illustration, the spin independent cross section for low-velocity scattering on a nucleon is evaluated in the limit M≫mW, including complete leading-order matching onto quark and gluon operators, renormalization analysis, and systematic treatment of perturbative and hadronic-input uncertainties.

Ultrafast carrier dynamics of near-band-edge emission in single-crystal ZnO nanorods

We report on rational synthesis and optical characteristics of highly crystallined ZnO nanorods which were grown by a facile chemical vapor transport method. Temperature-dependent photoluminescence spectra of as-fabricated ZnO nanorods are dominated by near-band-edge emission with a characteristic fine structure due to high crystallinity. Furthermore, the recombination emission involving carrier dynamics of near-band-edge emission in ZnO nanorods was systematically investigated by temperature-dependent time-resolved photoluminescence spectroscopy. Recombination peaks pertaining to the exciton emissions are monitored and resolved in both temporal and spatial regimes.

Characterization and corrosion study of NiTi laser surface alloyed with Nb or Co

The interest in NiTi alloys for medical applications has been steadily growing in recent years because of its biocompatibility, superelasticity and shape memory characteristics. However, the high Ni content in NiTi alloys is still a concern for its long-term applications in the human body. The release of Ni ion into the human body might cause serious problems, as Ni is capable of eliciting toxic and allergic responses. In view of this, surface modification to reduce the surface content of Ni and to improve the corrosion resistance, both of which would reduce Ni release, is an important step in the development of NiTi implants. In the present study, NiTi was surface alloyed with Nb or Co by laser processing. The fine dendritic structure characteristic of laser processing has been described in terms of rapid solidification. The amount of surface elemental Ni was reduced to 10% and 35% for the Nb-alloyed and Co-alloyed layer, respectively. The corrosion resistance in Hanks’ solution (a simulated body fluid) was increased as evidenced by a reduced passive current density and a higher pitting potential for both the Nb- and Co-alloyed specimens. The composition and hardness profiles along the depth of the modified layer were correlated with the distribution of the dendrites. The microhardness of the alloyed layers was around 700–800Hv, which was about four times that of the untreated NiTi specimens.

Investigation of low-temperature excitonic and defect emission from Ni-doped ZnO nanoneedles and V-doped ZnO nanostructured film

We report the growth and low-temperature photoluminescent characteristics of well-aligned Ni-doped ZnO nanoneedles and V-doped ZnO nanostructured thin film grown by a modified pulsed laser deposition technique. Low-temperature photoluminescence spectra of the as-grown films show the presence of free excitonic as well as bound excitonic transitions, whose relative intensity changes with increasing temperature. Ni-doped ZnO films show a characteristic fine structure in the visible range (2.6–2.9 meV) attributed to either exciton–polariton longitudinal–transverse splitting or the splitting caused by electron–hole exchange interaction. The excitonic and visible region emission can be clearly seen as can the phonon replicas produced from longitudinal optical phonons. Different possible attributions of the various peaks in the emission band at low temperature have been discussed. The as-grown nanostructures of Ni- and V-doped ZnO thin films also clearly show the effect of doping on the microstructure of ZnO.

Resonant and Kondo tunneling through molecular magnets

Transport through molecular magnets is studied in the regime of strong coupling to the leads. We consider a resonant-tunneling model where the electron spin in a quantum dot or molecule is coupled to an additional local, anisotropic spin via exchange interaction. The two opposite regimes dominated by resonant tunneling and by Kondo transport, respectively, are considered. In the resonant-tunneling regime, the stationary state of the impurity spin is calculated for arbitrarily strong molecule-lead coupling using a master-equation approach, which treats the exchange interaction perturbatively. We find that the characteristic fine structure in the differential conductance persists even if the hybridization energy exceeds thermal energies. Transport in the Kondo regime is studied within a diagrammatic approach. We show that magnetic anisotropy gives rise to the appearance of two Kondo peaks at nonzero bias voltages.

Superconducting weak link generated on a linear array of substrate steps in ultrathin films ofYBa2Cu3O7−δ

We have studied a three-unit-cell-thick YBa2Cu3O7-x (YBCO) superconducting film grown on a slightly vicinal SrTiO3 substrate that was polished from the (001) plane by a small miscut angle (less than 0.5 degrees). This produced, on its surface, a linear array of single-unit-cell-height steps with a period larger than 40 nm. A characteristic fault, with a displacement corresponding to one-third of the c-axis length of YBCO, is generated in the film on each substrate step, as observed by atomic force microscopy. Transport measurements with current flows parallel and perpendicular to the fault show that the fault serves as a superconducting weak link, as well as a unique one-dimensional flux-pinning center in an applied magnetic field. In addition, a characteristic fine structure is found in the flux-flow resistivity as a function of magnetic field, the origin of which may be ascribed to a matching effect between the periodically aligned faults and the flux lattice.

LOCAL ELECTRONIC STATES ON TWO-DIMENSIONAL NANOSCALE ISLAND OF Si AND Ge FABRICATED ON Si(111) 7 × 7 SUBSTRATE

Nanoscale islands on semiconductor are a strong candidate as building block in nanodevices. In the nanoisland, some local deformation is induced by the surface tension, which has a great influence on the electronic property of the nanoislands. To study the electronic structure of two-dimensional (2D) nanoislands of Si and Ge on the Si(111) 7 × 7 surface, we formed nanoislands of the same size and measured with angle resolved ultraviolet photoelectron spectroscopy (ARUPS), scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). We found that the nanoisland shows a characteristic fine structure due to some strain. We also calculated a relation between the strain and electronic state based on the density-functional theory. In the calculation, the dangling-bond state at the strained adatom on the nanoisland (SR state) shifts to lower energy, which has liner dependence with the height of the adatoms. The ARUPS spectrum and the STS show characteristic peaks corresponding to the SR state, whose energy depends on the deformation of the adatom. The height of the adatom on the nanoisland estimated from the energy difference is consistent with a result of the STM measurement. The strain of adatoms can be estimated from the electronic structure.

Direct observation of defects in hexagonal boron nitride by near-edge X-ray absorption fine structure and X-ray photoemission spectroscopy

Formation of defects in hexagonal boron nitride (h-BN) under low-energy argon or nitrogen ion bombardment has been studied by near-edge X-ray absorption fine structure (NEXAFS) around boron and nitrogen K-edges and X-ray photoemission spectroscopy (XPS) from B1s and N1s core levels. Breaking of B–N bonds and formation of nitrogen vacancies have been identified in the B K-edge NEXAFS and B1s XPS measurements, followed by the formation of molecular nitrogen, N 2, at interstitial positions for both argon and nitrogen bombardments. The formation of N 2 produces a sharp resonance in the low-resolution NEXAFS spectra around the N K-edge, showing characteristic vibrational fine structure in high-resolution measurements.

Formation of defects in boron nitride by low energy ion bombardment

Formation of defects in hexagonal and cubic boron nitride (h-BN and c-BN, respectively) under low-energy argon or nitrogen ion-bombardment has been studied by near-edge x-ray absorption fine structure (NEXAFS) around boron and nitrogen K-edges. Breaking of B–N bonds for both argon and nitrogen bombardment and formation of nitrogen vacancies, VN, has been identified from the B K-edge of both h-BN and c-BN, followed by the formation of molecular nitrogen, N2, at interstitial positions. The presence of N2 produces an additional peak in photoemission spectra around N 1s core level and a sharp resonance in the low-resolution NEXAFS spectra around N K-edge, showing the characteristic vibrational fine structure in high-resolution measurements. In addition, several new peaks within the energy gap of BN, identified by NEXAFS around B and N K-edges, have been assigned to boron or nitrogen interstitials, in good agreement with theoretical predictions. Ion bombardment destroys the cubic phase of c-BN and produces a phase similar to a damaged hexagonal phase.

Modeling of XANES-spectra with the FEFF-program

The aim of this project is the calculation of the absorption coefficient μ of x-ray absorption spectra as a function of energy and a comparison with experimental data. A characteristic fine structure can be found in x-ray absorption spectra (XAS) consisting of the XANES (X-Ray Absorption Near Edge Structure) and the EXAFS (Extended X-Ray Absorption Fine Structure) region. XANES is characterized by multiple scattering and provides information about coordination chemistry and bonding angles of the irradiated sample. The program FEFF 8.4 was used for the calculations of the absorption K-edge spectra. FEFF was generated for ab initio multiple scattering calculations of X-ray Absorption Fine Structure (XAFS) of atom-clusters. The code yields scattering amplitudes, phases and other quantities. We computed μ at the K-edge of several elements like Ti, S and Fe. For this purpose, clusters of Na2SO4, Ba2TiO4, FeS2, CaSO2 2(H 2O) were used, working with several space groups. Some of the calculations are consistent with the results of the experiments, but others show energy shifts in the range of some eV. In summary, the FEFF calculations and the experimental data exhibit similarities as well as deviations. By using trimming parameters, deviations could be eliminated to a certain extent, which will be presented.