Structure and properties of molybdenum implanted with high-flux carbon ion

High purity molybdenum was implanted by C ion in a metal vapour vacuum arc (MEVVA) implanter. The influence of implantation parameters was studied by varying ion fluence and ion current density. The samples were implanted by 45 keV C ion with fluences from 1×10¹⁵ to 1×10¹⁸ ions/cm², respectively. The as-implanted samples were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and nanoindenter. Different morphologies were observed on the surfaces of the as-implanted samples due to irradiation damage, and clearly related to implantation parameters. XRD spectra confirm formation of β-Mo₂C with ion fluences equal to or larger than 1×10¹⁶ ions/cm², and formation of -Mo₂C with ion fluence of 1×10¹⁸ ions/cm². The maximum nanohardness and maximum modulus of the as-implanted samples increased gradually with increasing ion fluence, and reached the corresponding maximum values with ion fluence of 1×10¹⁸ ions/cm². The experimental results suggest that the structure and properties of the as-implanted Mo samples exhibited strong dependence on implantation parameters.     

Study of the effect of swift heavy Ni ion irradiation on ruby single crystal by using the XANES and EXAFS techniques

We have made the XAFS measurements at the Cr–K-edge on natural Indian ruby single crystals (corundum) and its two irradiated samples with fluence 1×10¹² Ni⁶⁺ and 5×10¹² Ni⁶⁺ ions/cm². Irradiated samples show interesting changes in their physical appearance. XANES measurements show progressive decrease in Δ_oct value on increase of Ni fluence in irradiated samples. EXAFS measurements on these samples show decrease in Cr–O distance on increase of Ni fluence. Lowering of Δ_oct value is correlated with the increase of Cr–O distance.     

Surface defects created by low energy (20 < E < 240 eV) ion bombardment of Ge(001)

Surface defects created on Ge(001) exposed to low energy Xe ions are characterized by in situ scanning tunneling microscopy (STM). The temperature of the sample during ion bombardment is 165 C and ion energies range from 20 to 240 eV. The ion collisions create defects (vacancies and adatoms) which nucleate and form vacancy and adatom islands. For fixed total vacancy creation, the vacancy island number density increases with increasing ion energy: the vacancy island number density is 1.6 × 10⁻²⁰ cm⁻² for 40 eV ion bombardment and increases to 4.4 × 10⁻²⁰ cm⁻² for 240 eV ion bombardment. The increased nucleation rate for vacancies is attributed to clustering of defects. The sputtering yield of Ge(001) is also measured by STM. The sputtering yield for 20 eV ions is approximately 10⁻³ per ion but the net yield for surface defects (sum of adatoms and vacancies) is an order of magnitude higher, 10⁻², due to adatom-vacancy pair creation.     

Comparison between MeV Ni and He ion-implanted planar optical waveguides in quartz

Optically polished crystalline quartz samples were implanted at room temperature by 2.6 MeV Ni⁺ ions with a dose of 9×10¹⁴ ions/cm² and 2.0 MeV He⁺ ions with a dose of 1.5×10¹⁶ ions/cm², respectively. A comparison of the MeV Ni⁺ ion-implanted planar waveguide formation was made with the MeV He⁺ ion-implanted one. The prism-coupling method was carried out to measure the dark modes in the quartz waveguides by using model 2010 prism coupler. Five modes were observed in the Ni⁺ implanted waveguide while 15 modes were found in the He⁺ ion-implanted one. Reflectivity calculation method was applied to fitting the refractive index profile. TRIM’98 (transport of ions in matter) code was used to simulate the damage profile in quartz by MeV Ni⁺ and He⁺ ions implantation, respectively. It is found that the refractive index profile in MeV Ni⁺ ions implanted waveguide is somewhat different in shape from that in MeV He⁺ ions implanted waveguide.     

ESDIAD studies of the structure of TiO2(110)(1 × 1) and (1 × 2) surfaces and interfaces in conjunction with LEED and STM

The TiO₂(110)-(1 × 1) surface and its reconstruction as a (1 × 2) form have been studied with low energy electron diffraction (LEED), electron stimulated desorption ion angular distribution (ESDIAD) and scanning tunnelling microscopy (STM). Oxygen ion desorption occurs within a lobe perpendicular to the (1 × 1) surface, changing to two off-normal lobes for the (1 × 2) reconstruction. This transformation in the ESDIAD pattern is consistent with the added Ti₂O₃ row model of the (1 × 2) reconstruction proposed by Onishi and Iwasawa. STM studies of the stoichiometric and electron irradiated surfaces reinforce the association of the O⁺ ESD contribution with majority sites at the surface. Adsorption of acetic acid on the (1 × 1) surface produces a (2 × 1) overlayed and induces a reconstruction of the underlying substrate. ESDIAD reveals H⁺ ions emitted off-normally from dissociatively adsorbed acetate, and along the surface normal from surface hydroxyls. Adsorption of acetic acid on the (1 × 2) surface does not modify the LEED pattern, but ESDIAD reveals H⁺ desorption with a weaker off-normal contribution consistent with the Ti₂O₃ model of the reconstruction.     

Effect of secondary electrons from latent tracks created in YBCO by swift heavy ion irradiation

Swift heavy ions (SHI) with electronic energy loss exceeding a value of 14.4 keVnm⁻¹ create amorphized latent tracks in YBCO type superconductors. In the low fluence regime of an ion beam where tracks do not overlap, a decrease of the superconducting transition temperature as probed through resistivity studies, is not expected due to availability of percolating current paths. The present study however shows T_c decrease by about 1–3 K in thin films of YBCO when irradiated by 250 MeVAg ions at 79 K at a fluence of 5×10¹⁰–1×10¹² ionscm⁻². The highest fluence used in the present study is three times less than the fluence where track overlapping becomes significant. The T_c tends to increase towards the preirradiation value on annealing the films at room temperature. To explain this unusual result, we consider the effect of ion irradiation in inducing materials modification not only through creation of amorphized latent tracks along the ion path, but also through creation of atomic disorder in the oxygen sublattice in the Cu–O chains of YBCO by the secondary electrons. These electrons are emitted radially from the tracks during the passage of the SHI. Considering the correlation between the charge state of copper and its oxygen coordination, we show in particular that the latter process is a consequence of the inelastic interaction of the SHI induced low-energy secondary electrons with the YBCO lattice, which result in chain oxygen disorder and T_c decrease.     

Study of the effect of the oxygen partial pressure on the properties of rf reactive magnetron sputtered tin-doped indium oxide films

The influence of the oxygen partial pressure on the properties of indium tin oxide films deposited by rf reactive magnetron sputtering has been studied. The oxygen partial pressure was varied from 3.2 × 10⁻⁴ to 1.0 × 10⁻³ mbar. It has been found that the 4 × 10⁻⁴ mbar of oxygen partial pressure is a critical point. When the oxygen partial pressure is lower than 4 × 10⁻⁴ mbar, the deposition rate of the films is high; the films have low transmittance and electrical resistivity; the X-ray diffraction shows that the films have a random orientation and the images of the scanning electron microscopy show that the films surface are smooth without structure. When the pressure is higher than 4 × 10⁻⁴ mbar, the deposition rate is low and does not change as the oxygen partial pressure is further increased; the transmittance and the electrical resistivity are both high; the films show the preferred orientation along the (440) direction; the films surface show a clear structure and as the pressure is increased further, the films become porous. Considering both the factor of transmittance and resistivity, the optimum oxygen partial pressure will be 3.6 × 10⁻⁴ mbar. The films prepared at this pressure have 80% transmittance and 9 × 10⁻⁴ Ω cm resistivity.     

Electronic conduction properties of Au/C60/p-Si and C60/Au/p-Si sandwich structures: I–V and transducer characteristics

Gold-fullerite [C₆₀]-silicon (p-type) sandwich structures have been fabricated in order to investigate intrinsic cross-sectional and planar electronic conductive properties, in particular the C₆₀/p-Si p–n heterojunction. The turn-on voltage of this p–n heterojunction lies in the range 0.25–0.27 V. The I–V characteristics of the Au/C₆₀/p-Si structure are mostly defined by the bulk specific resistance of the fullerite crystal film itself (6×10⁷ Ω cm). I–V curves in the C₆₀/Au/p-Si structure are shown to be ohmic. Au/C₆₀/p-Si sandwiches irradiated with swift (300 MeV) heavy ions, (⁸⁴Kr¹⁴⁺) to a total fluence 10¹⁰ ion/cm² yield structures which are sensitive to ambient air pressure, specifically in the case of a transverse contact configuration, and if one of the contacts is located on the irradiated part of the fullerite film. The sandwich-structure sensitivity to pressure is 5×10⁻⁶ Pa⁻¹. This exceeds the sensitivity of conventional silicon pressure transducers by almost three orders of magnitude.     

Ionizing particles may create shockwaves

To find a model that describes the gas diffusion on irradiated polymers (Makrofol KG polycarbonate) the diffusion constants have been measured with argon as diffusion gas. The polymers were irradiated with uranium, gold and lead ions of about 10 MeV/u and ion fluences between 1×10¹⁰ and 4×10¹¹ ions/cm². The ion irradiated probes show two quite different dependencies of the diffusion constant on the ion energy loss. These effects are strongly related to the fluence of the irradiation. In case of low ion fluences, the diffusion constant is up to 8 times higher than that of pristine material. In the probes with high ion fluences we observe a decrease of diffusion constant to half the value of the pristine material. To understand the dependence of the diffusion constant on ion fluences we apply a model of compacting. This model describes the compacting ability of shockwaves arising from latent tracks. A track formation model is suggested. When an ion penetrates the foil it creates shockwaves around its path. These shockwaves put compacting forces on earlier created latent tracks in the same foil.     

Doping of latent tracks in polyethylene by iodine diffusion

Anomalous iodine diffusion was observed on polyethylene implanted with 150 keV F⁺ and As⁺ ions (1×10¹¹−1×10¹⁵cm⁻²) and exposed to iodine vapours at 90°C.     

Electron irradiation of polyesterurethane: study of chemical and structural modifications using FTIR, UV spectroscopy and GPC

To understand the electron irradiation effects on polymers, a polyesterurethane (ESPU) was irradiated by a 200 keV electron beam perpendicular to the surface under a vacuum of 0.04 Torr with electron fluxes of 0.25–1.25×10¹³ s⁻¹ on the irradiated zone. To perform a study of ESPU degradation versus the electron's penetration depth, a stratified structure was made with seventeen 30-μm-thick films. The irradiation was performed at both room temperature (293 K) and 77 K. The applied fluence was in the range of 10¹⁴–10¹⁷ cm⁻². Chemical transformations, such as degradation and oxidation, are studied by FTIR, by following NH and OH bond evolution, and by UV spectroscopy, by following the absorbance shift towards the visible region. These effects are analyzed versus depth, fluence and electron flux. Structural transformations are also characterized by GPC for soluble samples. An increase of crosslinking rate of the polymer is observed and analyzed.     

Measurement of diffusion coefficients using a quick echo split NMR imaging technique

A new method for the ultrafast generation of diffusion-weighted images is reported. The technique combines a quick echo split NMR imaging sequence with the principle of Stejskal and Tanner. It allows to determine the diffusion constant with nearly the same accuracy as the conventional spin-echo technique, requiring only a fraction of the time. The determined values for water doped with 1 g Cu(NO₃)₂ per liter of H₂O and pure acetone were D_water = (1.95 ± 0.02) × 10⁻⁹ m²/s and D_acetone = (4.05 ± 0.02) × 10⁻⁹ m²/s at 18.5°C. They are in good agreement both with literature and our own reference measurements using a diffusion-weighted spin-echo sequence. In addition, the temperature dependence of D_water was measured in the range of 18.5–45.9°C and a good correspondence with reported data was found.     

Scanning tunneling microscopy of N2H4 on silicon surfaces

The chemistry of N₂H₄ on Si(100)2 × 1 and Si(111)7 × 7 has been studied using scanning tunneling microscopy. At low coverages on Si(100)2 × 1 at room temperature the adsorption sites are distributed randomly on the surface and are imaged as dark spots in the dimer row by the STM. Upon annealing the substrate at 600 K, both isolated reaction products, as well as clusters of reaction products are formed on the surface. The STM images show that the majority of the isolated reaction products are adsorbed symmetrically across the dimers. Based on previous HREELS data, these are most likely NH_x groups. However, the clusters are not well resolved. Because of this we speculate that they are not simply symmetrically adsorbed NH_x groups, but likely have a more complicated internal structure. At higher coverages, the STM images show that the predominant pathway for adsorption is with the N---N bond parallel to the surface, in agreement with HREELS studies of this system. On Si(111)7 × 7, the molecule behaves in a manner which is similar to NH₃. That is, at low coverages the molecule adsorbs preferentially at center adatoms due to the greater reactivity of these sites, while at higher coverages it also reacts with the corner adatoms.     

Defect versus nanocrystal luminescence emitted from room temperature and hot-implanted SiO2 layers

Silicon nanocrystals have been synthesized in SiO₂ matrix using Si ion implantation. Si ions were implanted into 300-nm-thick SiO₂ films grown on crystalline Si at energies of 30–55 keV, and with doses of 5×10¹⁵, 3×10¹⁶, and 1×10¹⁷ cm⁻². Implanted samples were subsequently annealed in an N₂ ambient at 500–1100°C during various periods. Photoluminescence spectra for the sample implanted with 1×10¹⁷ cm⁻² at 55 keV show that red luminescence (750 nm) related to Si-nanocrystals clearly increases with annealing temperature and time in intensity, and that weak orange luminescence (600 nm) is observed after annealing at low temperatures of 500°C and 800°C. The luminescence around 600 nm becomes very intense when a thin SiO₂ sample is implanted at a substrate temperature of 400°C with an energy of 30 keV and a low dose of 5×10¹⁵ cm⁻². It vanishes after annealing at 800°C for 30 min. We conclude that this luminescence observed around 600 nm is caused by some radiative defects formed in Si-implanted SiO₂.     

Surface chemical disorder and lattice strain of GaN implanted by 3-MeV Fe10+ ions

Chemical disorder on the surface and lattice strain in GaN implanted by Fe¹⁰⁺ ions are investigated. In this study, 3-MeV Fe¹⁰⁺ ions fluence ranges from 1×10¹³ ions/cm² to 5×10¹⁵ ions/cm² at room temperature. X-ray photoelectron spectroscopy, high-resolution x-ray diffraction, and high-resolution transmission electron microscopy were used to characterize lattice disorder. The transition of Ga-N bonds to oxynitride bonding is caused by ion sputtering. The change of tensile strain out-of-plane with fluence was measured. Lattice disorder due to the formation of stacking faults prefers to occur on the basal plane.     

Study of cluster decay of Cm using SSNTD

Cluster decay of ²⁴²Cm with emission of ³⁴Si-particles was measured using track-recording phosphate glass detectors. The corresponding partial half-life is (1.4±0.3)×10²³ s. The branching ratio relative to decay is 1.0×10⁻¹⁶ and relative to spontaneous fission 1.6×10⁻⁹.     

Effects of 230 MeV Au irradiation on Bi2Sr2CaCu2Ox

Bi₂Sr₂CaCu₂O_x tapes were irradiated using 230 MeV Au¹⁴⁺ ions. Columnar defects were presumably produced due to irradiation. Zero-field-cooling (ZFC) magnetization increased up to a fluence of 1.6 × 10¹¹ Au⁺/cm², but field-cooling (FC) magnetization decreased, indicating the strong pinning effects resulting from the columnar defects. The critical current density as well as the irreversibility field, obtained from the hysteresis loops, were enhanced. Irreversibility fields are fitted by H_irr = A exp(−T/T_A). An effective activation energy for flux motion was obtained from the measurements of magnetization relaxation. The features of flux pinning as a result of the columnar defects were compared with those of point defects brought about by 120 MeV O⁷⁺ irradiation.     

Study of scanning tunneling microscopy images and probable relaxations of the SrTiO3(100) surface by electronic structure calculations

Partial electron density plots were calculated for a model SrTiO₃(100) surface with √5 × √5 ordered oxygen vacancy to examine why the bright spots of the scanning tunneling microscopy (STM) images of SrTiO₃(100) observed in ultrahigh vacuum (UHV) correspond to the oxygen vacancy sites. Possible dependence of the image on the polarity and magnitude of the bias voltage was also discussed on the basis of partial electron density plot calculations. Our study strongly suggests that the UHV STM imaging involves the lowest-lying d-block level of every two Ti³⁺ centers adjacent to an oxygen vacancy, the tip-sample distance involved in the UHV STM experiments is substantially larger than that involved in typical ambient-condition STM imaging, and the Ti⁴⁺ and Ti³⁺ sites of SrTiO₃(100) are reconstructed.     

Characterization of optical and nonlinear optical properties of silver nanoparticles prepared by laser ablation in various liquids

The optical, structural, and nonlinear optical properties of silver nanoparticles prepared by laser ablation in various liquids were investigated at 397.5, 532, and 795 nm. The TEM and spectral measurements have shown temporal dynamics of size distribution of Ag nanoparticles in solutions. The thermal-induced self-defocusing dominated in the case of high pulse repetition rate as well as in the case of nanosecond pulses. In the case of low pulse repetition rate, the self-focusing (γ = 3 × 10⁻¹³ cm² W⁻¹) and saturated absorption (β = −1.5 × 10⁻⁹ cm W⁻¹) of picosecond and femtosecond radiation were observed in these colloidal solutions. The nonlinear susceptibility of Ag nanoparticles ablated in water was measured to be 5 × 10⁻⁸ esu (at λ = 397.5 nm).     

The effects of hydrogen dose and thermal treatment on the formation of microsplits in hydrogen implanted GaAs

Transmission electron microscopy has been used to study the effect of thermal treatment on the formation of microsplits and damage rafts in hydrogen implanted (1 × 10¹⁶ and 5 × 10¹⁶ H⁺ cm^-2) and annealed (700 and 800°C) GaAs. The results show that microsplit and damage raft formation in implanted samples may be prevented if the samples are first given a pre-anneal heat treatment for 15 minute intervals at 300, 350, 400 and 450°C, during which the hydrogen concentration is reduced without allowing vacancy coalescence to take place.