Nanostructuring and hardening of LiF crystals irradiated with 3–15 MeV Au ions

Modifications of the structure and mechanical properties in LiF crystals irradiated with MeV-energy Au ions have been studied using nanoindentation, atomic force microscopy and optical spectroscopy. The nanostructuring of crystals under a high-fluence irradiation (above 10¹³ ions/cm²) was observed. Nanoindentation tests show a strong ion-induced increase of hardness (up to 150–200%), which is related to the high volume concentration of complex color centers, defect aggregates, dislocation loops and grain boundaries acting as strong barriers for dislocations. From the depth profiling of the hardness and energy loss it follows that both nuclear and electronic stopping mechanisms of MeV Au ions contribute to the creation of damage and hardening. Whereas the electronic stopping is dominating in the near-surface region, the effect of elastic displacements prevails in deeper layers close to the projectile range.     

Preparation of smooth Fe (001) on MgO (001)

The high mobility of metal adsorbates on perfect oxide surfaces favours an island-growth mode, which results in a granular morphology for metallic thin films. At low temperature, a fast substrate coverage is achieved due to the reduced mobility which, on the other hand, inhibits the development of an atomically smooth surface. In this study we used a controlled procedure to grow smooth epitaxial films of a few nanometres of Fe on MgO (001). The development of the dynamic conductivity as a measure of morphological roughness was observed during metal deposition by means of in-situ infrared spectroscopy. At various steps of thin-film preparation we used low-energy electron diffraction (LEED) for a characterisation of the crystalline surface quality. With ex situ atomic force microscopy (AFM) we investigated the surface topology of the prepared films with respect to smoothness. For example, for a controlled-growth 2-nm film, we observed a sharp LEED pattern and a metallic dynamic conductivity, and we did not find the distinct grooves which are characteristic of a granular structure. Received: 23 June 2000 / Accepted: 19 December 2000 / Published online: 21 March 2001     

Effect of 100 MeV swift Si8+ ions on structural and thermoluminescence properties of Y2O3:Dy3+nanophosphor

Nanoparticles of Y₂O₃:Dy³⁺ were prepared by the solution combustion method. The X-ray diffraction pattern of the 900°C annealed sample shows a cubic structure and the average crystallite size was found to be 31.49?nm. The field emission scanning electron microscopy image of the 900°C annealed sample shows well-separated spherical shape particles and the average particle size is found to be in a range 40?nm. Pellets of Y₂O₃:Dy³⁺ were irradiated with 100?MeV swift Si⁸⁺ ions for the fluence range of 3?×?10^11_3?×?10¹³ ions cm^?2. Pristine Y₂O₃:Dy³⁺ shows seven Raman modes with peaks at 129, 160, 330, 376, 434, 467 and 590?cm^?1. The intensity of these modes decreases with an increase in ion fluence. A well-resolved thermoluminescence glow with peaks at ～414?K (T_m1) and ～614?K (T_m2) were observed in Si⁸⁺ ion-irradiated samples. It is found that glow peak intensity at 414?K increases with an increase in the dopant concentration up to 0.6?mol% and then decreases with an increase in dopant concentration. The high-temperature glow peak (614?K) intensity linearly increases with an increase in ion fluence. The broad TL glow curves were deconvoluted using the glow curve deconvoluted method and kinetic parameters were calculated using the general order kinetic equation.     

Surface morphology of siloxane block copolymer crystals: (Tetramethyl-p-silphenylenejsiloxane—dimethylsiloxane

The metal decoration technique was applied to solution grown crystals and cast thin films of (tetramethyl-p-silphenylene) siloxane and dimethylsiloxane (DMS) block copolymers in order to elucidate their surface textures as a function of copolymer composition. Electron diffraction and microscopy studies were used. Metal-free zones are observed at surface steps of single crystals, and decorating particles occasionally form spiral growths in the inner regions of crystals. However, the decorating particles form regular patterns, particularly on the copolymer crystal surfaces. In the case of solution-cast thin films, deposition of metallic particles occurs along lamellar crystal edges or where a fibrillar morphology occurs. The nucleation density of metallic particles depends on the copolymer composition and increases with increasing crystallinity of the samples. Probable models are suggested to account for this surface texture. The propensity for screw-dislocations crystal growths on the crystal surfaces decreases as the noncrystallizable DMS component increases.     

Electron spin relaxation of exchange coupled pairs of transition metal ions in solids. Ti2+–Ti2+ pairs and single Ti2+ ions in SrF2 crystals

EPR (X- and Q-band) and electron spin relaxation measured by electron spin echo method (X-band) were studied for ${\text{Ti}}^{2+}(S=1)$ and ${\text{Ti}}^{2+}''–''{\text{Ti}}^{2+}$ pairs in ${\text{SrF}}_2$ crystal at room temperature and in the temperature range 4.2–115 K. EPR spectrum consists of a strong line from ${\text{Ti}}^{2+}$ and quartets 2:3:3:2 from titanium pairs $(S=2)$. Spin-Hamiltonian parameters of the pairs are $g_{\parallel }=1.883$, $g_{\perp }=1.975$ and $D=0.036{\text{cm}}^{-1}$. Temperature behavior of the dimer spectrum indicates ferromagnetic coupling between ${\text{Ti}}^{2+}$. Spin–lattice relaxation of individuals ${\text{Ti}}^{2+}$ is dominated by the ordinary two-phonon Raman process involving the whole phonon spectrum up to the Debye temperature ${\Theta }_{\text{D}}=380\text{K}$ with spin–phonon coupling parameter equal to $215{\text{cm}}^{-1}$. Important contribution to the relaxation arises from local mode vibrations of energy $133{\text{cm}}^{-1}$. The pair relaxation is faster due to the exchange coupling modulation mechanism with the relaxation rate characteristic for ferromagnetic ground state of the pairs $1/T_1\propto [\exp (2J/\mathit{kT})-1{]}^{-1}$ which allowed to estimate the exchange coupling $J=36{\text{cm}}^{-1}$. The theories of electron–lattice relaxation governed by exchange interaction are outlined for extended spin systems, for clusters and for individual dimers. Electron spin echo decay is strongly modulated by coupling with surrounding ¹⁹F nuclei. FT-spectrum of the modulations shows a dipolar splitting of the fluorine lines, which allows the evaluation of the off-center shift of ${\text{Ti}}^{2+}$ in pair as 0.132 nm. The electron spin echo dephasing is dominated by an instantaneous diffusion at low temperatures and by the spin–lattice relaxation processes above 18 K.     

Molecular dynamics simulation for the sputtering of an Al2O3 sample bombarded with MeV Si ions

Sputtering yield and kinetic energy distribution (KED) of Al particles from an Al₂O₃ sample bombarded with 1－5 MeV Si ions have been simulated using the molecular dynamics method. These have also been measured experimentally with a conventional time-of-flight facility. In the simulation, a new interatomic potential specific to the Al₂O₃ target was developed, and both the nuclear energy loss S_n and electronic energy loss S_e were taken into consideration. By carefully adjusting the simulation parameters, the simulated sputtering yields fit well with the experimental results, and the simulated KED of Al particles also fits roughly with the experimental KED after being modified theoretically.     

Structural and magnetic properties of Si1−xGex thin films implanted with Fe ions

A series of Si_1?xGe_x (x = 1, 0.848, 0.591, 0.382, 0.209, 0.064, 0) thin films prepared by ion beam sputtering were implanted with Fe ions to different doses using the metal vapor vacuum arc technique. X-ray absorption fine structure (XAFS) was used to characterize the local microstructure around the Fe atoms in Fe-doped Si_1?xGe_x samples. Structural analysis showed that for annealed samples of Ge-rich thin films (including pure Ge) implanted with low doses of Fe ions, almost all the Fe ions substituted at Ge sites. However, an anti-ferromagnetic Fe₆Ge₅ impurity phase existed in the annealed samples implanted with high doses of Fe. It was also found that the solubility of Fe ions was highest in pure Ge films and that with increasing Si concentration, the solubility decreased. Magnetic analysis showed that for the as-implanted and annealed samples of Ge-rich thin films implanted with Fe ions, room-temperature ferromagnetism was strongest in the pure Ge series of samples and that as the Ge concentration decreased, the ferromagnetism at room temperature weakened. In addition, annealing could increase the number of Fe ions at substitution sites, which resulted in the observed increase in the saturated magnetization after annealing. Experiment and theoretical analysis showed that the ferromagnetism of Fe-doped Ge-rich Si_1?xGe_x thin films samples originated from the s, p–d exchange interactions between the Si_1?xGe_x matrix and those Fe ions which substituted at Ge sites and that the ferromagnetism was mediated by carriers.     

Determination of charge-compensated C3v (Ⅱ) centers for Er3+ ions in CdF2 and CaF2 crystals

A unified theoretical method is established to determine the charge-compensated C3v(Ⅱ)centers of Er3+ions in CdF2 and CaF2 crystals by simulating the electron paramagnetic resonance(EPR)parameters and Stark energy levels.The potential(Er³⁺–F^?–O₄^2?)and(Er³⁺-F₇^?-O₄^2?)structures for theC3v(Ⅱ)centers of Er3+ions in CdF2 and CaF2 crystals are checked by diagonalizing 364×364 complete energy matrices in the scheme of superposition model.Our studies indicate that the C3v(Ⅱ)centers of Er3+ions in CdF2 and CaF2 may be ascribed to the local(Er³⁺-F^?-O₄^2?)structure,where the upper ligand ion F?undergoes an off-center displacement by?Z≈0.3?A for CdF2 and?Z≈0.29?A for the CaF2 along the C3 axis.Meanwhile,a local compressed distortion of the(ErFO4)6?cluster is expected to be?R≈0.07?A for CdF2:Er3+and?R≈0.079?A for CaF2:Er3+.The considerable g-factor anisotropy for Er3+ions in each of both crystals is explained reasonably by the obtained local parameters.Furthermore,our studies show that a stronger covalent effect exists in the C3v(Ⅱ)center for Er3+in CaF2 or CaF2,which may be due to the stronger electrostatic interaction and closer distance between the central Er3+ion and ligand O2?with the(Er³⁺-F^?-O₄^2?)structure.     

Effect of 100 MeV Si7+ ions’ irradiation on Pd/n-GaAs Schottky diodes

Pd/n-GaAs realized devices (junction made on a virgin substrate prior to irradiation) and Pd/n-GaAs fabricated devices (junction realized after the virgin substrate irradiation) have been irradiated with 100?MeV Si⁷⁺ ions for the varying fluence of 10¹²–10^13?ions/cm². The devices have been characterized by I–V and C–V techniques for an electrical response. The electrical characterization of these devices shows the presence of interfacial layer. Moreover, the C–V characteristics show strong frequency dependence behavior, which indicates the involvement of interfacial charge layer with deep electron states. The hydrogenation of these devices has not caused any significant change in the electrical (I–V and C–V) characteristics. The observed results have been discussed in the realm of radiation-induced defects, which cause the carrier removal and compensation phenomena to cause the observed high resistivity and filling and unfilling of these traps’ level to cause strong frequency dependence behavior.     

Production of projectile and target-vacancy in near-symmetric collisions of 60–100 MeV Cu~(9+) ions with thin Zn target

We report studies on both target and projectile K-shell ionization by collisions of Cu~(9+)ions on the thin Zn target in the energy range of 60–100 Me V. In this work, the relative ratio for the production of the target to projectile K-vacancy is measured. The result shows that it almost remains stable over this energy range and has good consistency with the predictions by vacancy transfer via the 2pσ–1sσ rotational coupling, which gives experimental evidence for K-vacancy sharing between two partners. Furthermore, the discussion for comparisons between the experimental ionization cross sections and the possible theoretical estimations is presented. These comparisons suggest that the experimental data agree well with those predicted by the Binary–Encounter approximation(BEA) model but are not in good agreement with the modified BEA calculations. It allows us to infer that the direct ionization(and/or excitation) is of importance to initial K-vacancy production before 2pσ–1sσ transitions in the present collision condition.     

Synthesis and electrochemical performance of α-nickel hydroxide codoped with Al3+ and Ca2+

α-Nickel hydroxide codoped with Al³⁺ and Ca²⁺ was prepared by chemical coprecipitation method. The phase structure and surface morphology of the prepared samples were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM). The electrochemical performances of the prepared samples were analyzed by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge/discharge tests. XRD and SEM tests reveal that the Al³⁺/Ca²⁺ codoped α-nickel hydroxide has a relatively good crystallization and a very coarse surface. Electrochemical tests show that the Al³⁺/Ca²⁺ codoped α-nickel hydroxide has higher proton diffusion coefficient, lower electrochemical reaction resistance, and higher discharge capacity (395.3 mAh g^?1 at 0.2 C) than the Al³⁺ singly doped α-nickel hydroxide, which indicates its potential application as an electrode material for secondary alkaline batteries.     

Luminescence of yttrium–aluminum garnet crystals with Eu<Superscript>2+</Superscript> impurity ions

Luminescence of yttrium-aluminum garnet Y₃Al₅O₁₂: Eu,Si samples is studied. It is found that luminescence is associated with Eu²⁺ ions, but does not correspond to intracenter 4f ⁶5d ¹–4f ⁷ transitions. Measurements of the excited state lifetime point to the luminescence mechanism involving charge-transfer states.     

Numerical investigation of the submonolayer growth and relaxation of stepped Ag (110) and Au (110) surfaces

The terrace width distribution (TWD) as well as the adatom and island densities for some stepped fcc?(110) surfaces with parallel and equidistant steps of equal stiffness are studied by means of kinetic Monte Carlo simulations. Reliable energy barriers for surface diffusion are used. They have been calculated by means of many-body potentials derived within the second moment approximation to the tight-binding model. The effects of the temperature, atom deposition flux and surface diffusion on quantities of interest in the early stage of the surface evolution process have been singled out. In most cases, linear, logarithmic and power-law behaviors are recovered for the evolution of the step width and terrace defects. The TWD is well described by the Gaussian distribution (GD) in the domain of temperature investigated but deviates from the Generalized Wigner Distribution (GWD).     

Investigation of the fusion reaction27Al+236U→263105 at excitation energies of 57 MeV and 65 MeV

The neutron-deficient isotopes^257,258105 were produced in the reaction²⁷Al+²³⁶u in 6n and 5n evaporation channels, respectively. The evaporation residues emerging from the target were separated in-flight from the projectiles and from products of different nuclear reactions by the electrostatic separator VASSILISSA [1]. The isotopes were then implanted into position-sensitive silicon detectors and identified using the --correlation method. The measured production cross-section is (5n)=(0.45±0.20)nb atE _P =154 MeV and (6n)=(0.075±0.055) nb atE _P =163 MeV. These cross-sections are compared with data measured for the same isotopes in the more symmetrical reaction⁵⁰Ti+²⁰⁹Bi.     

Photoconductivity and photodielectric effect in LiY1 − x Lu x F4 crystals doped with Ce3+ and Yb3+ ions

Time and spectral dependences of the dielectric permittivity of the LiY_{1 ? x} Lu _x F₄ (x = 0, 0.5, and 1) crystals doped with Ce³⁺ and co-doped with Yb³⁺ ions under UV laser excitation were studied by the 8-mm microwave resonant technique at room temperature. The obtained photoconductivity spectrum in 240–310 nm spectral range was interpreted as a stepwise photoionization spectrum of the Ce³⁺ ions due to sequential 4f–5d and 5d–6s transitions. Average lifetimes of free and defect trapped (color centers) charge carriers were estimated.     

Synthesis and optical properties of Co2+ and Ni2+ ions doped β-BaB2O4 nanopowders

Co²⁺ and Ni²⁺ ions doped β-BaB₂O₄ nanopowders have been prepared by co-precipitation method and their structural properties are studied by spectroscopic techniques. Powder XRD data reveals that the crystal structure belongs to monoclinic and the average crystallite size is calculated. Optical absorption spectra data reveal octahedral site symmetry for Co²⁺ and Ni²⁺ ions. Crystal field (Dq) and inter-electron repulsion (B and C) parameters are evaluated for Co²⁺ doped β-BaB₂O₄ nanopowders as Dq=960, B=900 and C=3850 cm^?1 and for Ni²⁺ doped β-BaB₂O₄ nanopowders, Dq=900, B=850 and C=3500 cm^?1. FT-IR spectra showed the characteristic vibrational bands related to BO₃ and BO₄ molecules. Photoluminescence spectra contain the emission bands in ultraviolet and blue regions.     

Quasichanneling of ions in crystals. I. Elastic scattering of ions on atomic “chains” and planes

The scattering of quasichanneled particles on atomic chains and planes is studied by computer simulation for the crystals of chemical compounds. It is shown that quasichanneling is the specific regime of ion motion in the lattice. Its main features are: (1) a sharp increase of the backscattering probability; (2) transition of the ions from the quasichanneled component into a random one; (3) high sensitivity to the structure and the composition of the atomic “chain”; (4) dependence on the value and the direction of the atomic displacements.     

Carbon (70 MeV) and copper (120 MeV) ions irradiation effects in Makrofol-N

Makrofol-N polycarbonate was irradiated with carbon (70 MeV) and copper (120 MeV) ions to analyze the induced effects with respect to optical and structural properties. In the present investigation, the fluence for carbon and copper beams was kept in the range of 1×10¹¹– 1×10¹³ ions/cm² to study the swift heavy ion induced modifications. UV–VIS, FTIR and XRD techniques were utilized to study the induced changes. The analysis of UV–VIS absorption studies revealed that the optical energy gap was reduced by 17% on carbon irradiation, whereas the copper beam leads to a decrease of 52% at the highest fluence of 1×10¹³ ions/cm². The band gap can be correlated to the number of carbon atoms, N, in a cluster with a modified Robertson's equation. In copper (120 MeV) ions irradiated polycarbonate, the number of carbon atoms in a cluster was increased from 63 to 269 with the increase of ion fluence from 0 to 1×10¹³ ions/cm², whereas N is raised only up to 91 when the same polymer films were irradiated with carbon (70 MeV) ions under similar conditions. FTIR analysis showed a decrease in almost all characteristic absorption bands under irradiation. The formation of hydroxyl (? OH) and alkene (C?C) groups were observed in Makrofol-N at higher fluence on irradiation with both types of ions, while the formation alkyne end (R? C≡ CH) group was observed only after copper ions irradiation. The radii of the alkyne production of about 3.3 nm were deduced for copper (120 MeV) ions. XRD measurements show a decrease in intensity of the main peak and an increase of the average intermolecular spacing with the increase of ion fluence, which may be attributed to the structural degradation of Makrofol-N on swift ion irradiation.     

Surface phonon dispersion of Cu(100)c(2×2)N

The Cu(100) surface covered with atomic nitrogen has been studied with low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and inelastic electron scattering (EELS). Atomic nitrogen, formed by thermal dissociation of NH₃ adsorbed at 100 K, forms a c(2×2) overlayer on the Cu(100) surface. The dispersion of adsorbate and substrate associated modes of the Cu(100) surface covered with a c(2×2) nitrogen overlayer has been measured along the two dimensional Brillouin zone in the - by inelastic electron scattering. The experimental data are compared to a lattice dynamical slab calculation. The Rayleigh-mode (S₄-phonon) is only slightly changed by the N-overlayer. An optimum fit for the perpendicular and parallel nitrogen modes (=320 cm^–1 and =740 cm^–1 at =0.1) is obtained when the nitrogen atom is placed 0.6 Å above the first copper layer.