Theoretical Study on Inner Shell Electron Impact Excitation of Lithium

Cross sections for electron impact excitation of lithium from the ground state 1s^22s to the excited states 1s2s^2, 1s2p^2, 1s2snp （n = 2-5）, 1s2sns （n = 3 5）, 1s2pns （n = 3-5）, and 1s2pnp （n = 3-5） are calculated by using a full relativistic distorted wave method. The latest experimental electron energy loss spectra for inner-shell electron excitations of lithium at a given incident electron energy of 2500 eV [Chin. Phys. Lett. 25 （2008）3649] have been reproduced by the present theoretical investigation excellently. At the same time, the structures of electron energy loss spectra of lithium at low incident electron energy are also predicted theoretically, it is found that the electron energy loss spectra in the energy region of 55-57eV show two-peak structures.     

Single-crystalline ZnS tubular structures: preparation and characterization

Single-crystalline ZnS tubular structures have been successfully synthesized via thermal evaporation of ZnS powders mixed with a small amount of SnO₂ at 1180 °C in a high-temperature tube furnace. Structure and microstructure of the ZnS tubes have been characterized by means of scanning electron microscopy and various techniques in a transmission electron microscope. It is found that the as-synthesized ZnS tubes are straight and uniform with lengths of several hundred micrometers. The outer diameters of these tubular structures range from several hundred nanometers to a few micrometers. As a part of the as-synthesized product, ZnS/Zn₂SnO₄ cables are also observed. A growth mechanism of these novel structures is proposed on the basis of transmission electron microscopy. PACS 81.07.De; 68.37.Lp; 81.15.Gh; 68.37.Hk; 81.70.Jb     

Characterization of periodic domain structures in lithium niobate crystals by scanning electron microscopy and X-ray diffraction analysis

Congruent lithium niobate crystals with periodic domain structures formed by the method of thermoelectric postgrowth treatment have been investigated. Periodic domain structures in the samples of polar ZY-, YZ-, and YX-cuts were characterized by scanning electron microscopy, high resolution x-ray diffractometry, and topography. The evaluation and comparison of the secondary electron (SE) signal cycling, electron emission coefficient, and charge value to equalize the differences in the SE signal have allowed us to specify the features of ZY-and YZ-cut structures. A correlation between the crystal lattice distortions near domain boundaries and the type of domain walls has been found. The domain walls separating the areas with the tail-to-tail P _s vectors not only cause stronger crystal lattice distortions near them, but also are charged less quickly under electron irradiation.     

Effect of nitrogen flow ratios on the structure and mechanical properties of (TiVCrZrY)N coatings prepared by reactive magnetron sputtering

This study reports the influence of growth conditions on the characteristics of (TiVCrZrY)N coatings prepared by reactive magnetron sputtering at various N₂-to-total (N₂ + Ar) flow ratio, which is R_N. The crystal structures, microstructure, and mechanical properties for different R_N were characterized by electron spectroscopy for chemical analysis, X-ray diffraction, atomic force microscopy, field-emission-scanning electron microscopy, transmission electron microscopy, and nanoindentation. The results indicate that the TiVCrZrY alloy and nitride coatings have hexagonal close-packed (hcp)-type and sodium chloride (NaCl)-type solid-solution structures, respectively. The voids in the coatings are eliminated and the growth of the columnar crystal structures is inhibited along with an increasing R_N. As a consequence, highly packed equiaxed amorphous structures with smooth surfaces are formed. The coatings accordingly achieved a pronounce hardness of 17.5 GPa when R_N = 100%.     

Amorphous helical carbon nanofibers synthesized at low temperature and their elasticity and processablity

Helical carbon nanofibers are synthesized by thermal chemical vapor deposition using copper nanocrystals as a catalyst and acetylene as a source gas at the low temperature of 195 °C. These nanocoils are symmetrically grown on copper nanocrystals in the form of twin helices. They contain high content of hydrogen. Their molecular structures are different from polyacetylene. They can be classified as a new type of amorphous carbon nanofibers. They present novel elasticity and interesting processability. The twin helices exhibit not only the reversible extension of the nanocoil itself but also the reversible angle change between the two nanocoils. They can be easily separated into two by electron beam heating. However, if a certain section of the nanocoil is heated by electron beam in a scanning mode, it will adjust its structures along the scanning line of the electron beam.     

Observation of fine structures in laser-driven electron beams using coherent transition radiation

We have measured the coherent optical transition radiation emitted by an electron beam from laser-plasma interaction. The measurement of the spectrum of the radiation reveals fine structures of the electron beam in the range 400-1000 nm. These structures are reproduced using an electron distribution from a 3D particle-in-cell simulation and are attributed to microbunching of the electron bunch due to its interaction with the laser field. When the radiator is placed closer to the interaction point, spectral oscillations have also been recorded, signature of the interference of the radiation produced by two electron bunches delayed by 74 fs. The second electron bunch duration is shown to be ultrashort to match the intensity level of the radiation. Whereas transition radiation was used at longer wavelengths in order to estimate the electron bunch length, this study focuses on the ultrashort structures of the electron beam.     

双stub介观环结构中的电子输运特征

采用量子波导理论研究了双stub介观环结构中电子输运特性。结果表明电子透射系数随stub的长度和环的大小而周期地振动,对环的周长和stub的长度做适当的调整,能使电子输运达到100%。并且比较了单stub介观环结构和双stub介观环结构对电子输运的影响,发现双stub的介观环结构对电子输运调制要比单stub介观环结构好。理论研究不仅对基础物理而且对量子器件研究均有重要意义。     

类椭球氧化锌纳米结构的制备与表征

"用普通氧化锌晶须作为原材料, 采用高压釜腐蚀法和高温(600 ℃)裂解结合的方法合成纳米棒构成的氧化锌类椭球结构．XRD、SEM和TEM结果表明这种类椭球结构是单晶的,纳米棒取向一致,自组装生长而成．"     

Synthesis of ZnO hollow spherical structures with different surface-to-volume ratios

ZnO hollow spherical structures with different surface-to-volume ratios were prepared using solid Zn microspheres as template via a simple oxygen-controlled thermal evaporation approach. The results of scanning electron microscopy testify that ZnO hollow spherical structures with different morphologies can be realized by changing oxygen supply. The corresponding transmission electron microscopy images further reveal that the prepared spherical structures are hollow, and nanorods epitaxially grow from the surface of the sphere shell along the [0001] direction. A series of experiments indicates that the formation of hollow spherical structures involves the oxidization on the surface of Zn microspheres, sublimation of Zn, and growth of ZnO nanorods.     

Quasimolecular or kinematical broadening of autoionization electron spectra from Li+/He collisions? a coincidence study

We have measured energy- and angular distributions of autoionization electrons from Li⁺/He collisions in coincidence with the scattered Li⁺ projectiles. The coincident energy spectra do not exhibit the broad structures that are observed in the non coincident spectra and which in the past were attributed to quasimolecular autoionization of the (LiHe)⁺ system. We show that these broad structures are caused by kinematical shifts of electron spectra from pure atomic autoionization following different scattering events. With an exploitation of published inelastic differential scattering cross sections and with the assumption that the angular electron distribution with respect to the asymptotic internuclear line is nearly independent of the scattering angle we are able to reproduce the non coincident electron spectra satisfactorily.     

Potential Images of Ferroelectric Domain Structures Formed by Electron Beam in Lithium Niobate Crystals

Ferroelectric domain structures formed by an electron beam in lithium niobate crystals are studied using low-voltage SEM microscopy. The structures are formed in crystals with different conductivity, including samples with high-resistance congruent composition (CLN) and samples with conductivity increased by reductive annealing (RLN). The potential nature of the contrast of the domain structures observed in the secondary electron mode depending on the conductivity of the samples and the direction of spontaneous polarization of the domains is analyzed. It is assumed that the domain contrast in CLN crystals is associated with long-lived charges localized near domain walls and in the irradiated areas. The recorded domain structures in the CLN crystals are visualized on polar and nonpolar cuts. In the RLN crystals with improved conductivity compared to CLN, the potential contrast of the periodic domain structures is found only on the polar cuts, where vector P_s of the domains is perpendicular to the irradiated surface. This contrast is likely because the field of the spontaneous electric polarization charges influences the secondary electrons.     

Intermediate stages of a transformation between a quasicrystal and an approximant including nanodomain structures in the Al-Ni-Co system

Transition states between decagonal quasicrystal and periodic approximants are studied in the Al-Ni-Co system at a measured composition of Al_71.3Ni_11.3Co _17.4 by high-resolution transmission electron microscopy and electron diffraction. The nanodomain structures appearing after annealing at 1270 K show periodic fluctuations coherently embedded in domains with the coarse order of a one-dimensional quasicrystal. Further annealing at lower temperatures changes the features of nanodomain structures and results in an increase in more periodic structures. These can be strongly disordered and full of defects but tiling analysis and electron diffraction patterns show that they correspond to locked phason strain values of two closely related periodic approximants. We conclude that the periodic approximants do not result from a continuous increase in phason strain but from the growth of seeds with a locked phason strain.     

High-resolution electron microscopy and X-ray diffraction study of intergrowth structures in α- and β-type YbAlB4 single crystals

Structural features of layered boride YbAlB₄ single crystals with YCrB₄-type (α-type) and ThMoB₄-type (β-type) phases derived from a hexagonal AlB₂-type structure were investigated by electron diffraction, high-resolution electron microscopy and X-ray diffraction. X-ray diffraction experiments indicate the existence of some structural motifs. High-resolution images clearly show that the structural motifs build the intergrown lamellar structures in the matrix. The lamellar structures can be characterized by a coherent tiling of deformed Yb hexagons, which are a common structure unit in the α- and β-type structures. The characteristic intergrown nanostructure is similar to that observed in the β-type TmAlB₄ polycrystalline sample.     

Two-dimensional ‘hat-scratch’ periodic structures induced by a single femtosecond laser pulse on silica glass

Two-dimensional ‘hat-scratch’ structures are fabricated on silica glass by the interference of three non-coplanar beams originating from a single femtosecond laser pulse. The scanning electron microscope (SEM) characterizations show that the as-formed structures are composed of hat holes and scratch marks. The experimental results indicate that the structures are dependent on the intensity of laser beam. The formation of the two-dimensional ‘hat-scratch’ structures is mainly due to the combined laser ablation effects including ionization, shock wave, plasma expansion, and phase explosion.     

Small amplitude ion-acoustic double layers with cold electron beam and q-nonextensive electrons

Small amplitude ion-acoustic double layers in an unmagnetized and collisionless plasma consisting of cold positive ions, q-nonextensive electrons, and a cold electron beam are investigated. Small amplitude double layer solution is obtained by expanding the Sagdeev potential truncated method. The effects of entropic index q, speed and density of cold electron beam on double layer structures are discussed.     

Electron-phonon interaction effects on the surface states in wurtzite nitride semiconductors

A variational approach is used to study the surface states of an electron in a semi-infinite wurtzite nitride semiconductor. The surface-state energy of the electron is calculated, by taking the effects of the electron-surface optical phonon interaction and structure anisotropy into account. The numerical computation has been performed for the energies of the electronic surface states as a function of the surface potential V₀ for wurtzite GaN, AlN, and InN, respectively. The results show that the electron-phonon interaction lowers the surface state energy. It is also found that the energies of the electronic surface-state in wurtzite structures are lower than that in the zinc-blende structures by hundreds of meV for the materials calculated. The influence of e-p-interactions on the surface state of electron cannot be neglected.     

Fabrication of specified-geometry metal pattern by proton irradiation through a single-layer mask

Results on the determination of protective properties of a ZEP520 electron resist film and geometrical characteristics of Mo metal test structures fabricated by local reduction of MoO₃ in protective mask windows under irradiation by the beams of protons are presented. The advantage of the method for fabricating metal structures based on local selective removal of oxygen atoms from oxides are discussed.     

Reduction of Electron Leakage of AlGaN-Based Deep Ultraviolet Laser Diodes Using an Inverse-Trapezoidal Electron Blocking Layer

To improve the optical and electrical properties of AlGaN-based deep ultraviolet lasers,an inverse-trapezoidal electron blocking layer is designed.Lasers with three different structural electron blocking layers of rectangular,trapezoidal and inverse-trapezoidal structures are established.The energy band,electron concentration,electron current density,P-I and V-I characteristics,and the photoelectric conversion efficiency of different structural devices are investigated by simulation.The results show that the optical and electrical properties of the inversetrapezoidal electron blocking layer laser are better than those of rectangular and trapezoidal structures,owing to the effectively suppressed electron leakage.     

Remotely-doped graded potential well structures

We present a new method to obtain high-mobility three-dimensional electron gas systems. We have achieved control of carrier density and of carrier profile by growth of the first remotely-doped parabolic potential well structures. Computer-controlled molecular beam epitaxy is used to grow a layer of ultra-fine superlattices with a programmable composition gradient. This produces conduction-band potentials which, in the absence of doping, are equivalent to the potential profiles of fixed charge distributions. When conduction electrons are introduced into these graded wells through remote doping of the barrier regions, they distribute themselves in such a way as to produce a uniform chemical potential at thermal equilibrium. We illustrate through computer simulations employing Fermi statistics that electrons introduced into a wide parabolic potential well distribute themselves uniformly. More significantly, the carrier distribution in the well is remarkably insensitive to the dopant sheet charge in the barrier, the more so at lower temperatures. We have fabricated remotely-doped graded potential well structures of the proposed type by molecular beam epitaxy. These structures exhibit the above effects. Measured mobilities of such three-dimensional electron gases grown using the GaAs/Al_xGa_1−xAs system are higher than those of bulk-doped GaAs doped to give the same uniform electron concentration.     

Investigations of backscattering peaks and of the nature of the confining potential in open quantum dots

The properties of open quantum dots are examined in magneto-transport. The quantum dots are prepared from a two-dimensional electron system (2DES) in AlGaAs/GaAs by lateral gate structures. These quantum dots are open, i.e. they are still connected to the surrounding 2DES regions. The low magnetic field magnetoresistance shows peak structures. These structures can be related to semi-classical ballistic trajectories in the confining potential of a dot. The calculations of different confining potentials (abrupt “hard-wall” and parabolic “soft-wall”) are compared with the experimental results. The experiments are better described by a soft-wall potential.