Hideoki Murakami

Positron annihilation in porous silicon

Three lifetime components, one of which is extremely long (25±2 ns), have been observed in experimental studies of positron annihilation in porous silicon, made by anodization in hydrofluoric acid. The Doppler‐broadened spectrum of the porous silicon is sharp compared with that of crystal silicon and becomes even narrower in an applied magnetic field. The positronium yield in the porous silicon therefore is concluded from the long lifetime, narrow Doppler spectrum and its narrowing in a magnetic field. The porous structure is the cause of positronium formation.

Chemisorption of hydrogen into a graphite–potassium intercalation compound C8K studied by means of positron annihilation

The positron‐annihilation spectrum for C8K obtained from the Doppler broadening of the 511 keV annihilation radiation is composed of two Gaussian bands: one due to annihilation with electrons in the interlayer region, the other mainly due to annihilation with electrons in the graphitic layers. Upon introduction of hydrogen to C8K, both bands sharpened, which indicates a conversion of hydrogen atoms to hybride ions through charge transfer from the graphitic layers and the formation of positron–hydride ion complexes in the interlayer region. (AIP)

Structure and thermal stability of nanocrystalline silver studied by transmission electron microscopy and positron annihilation spectroscopy

Abstract Positron annihilation spectroscopy was carried out on nanocrystalline silver. Both lifetime and Doppler broadening were measured. High-resolution electron microscopy observations were also performed. Diffuse vacancy clusters, the sizes of which correspond to two to four vacancies, and voids of 1–5 nm diameter, were found to reduce the average atomic density of the grain boundary. Some grains grew to a diameter larger than about 70 nm during annealing the specimen from 50 to 100°C. A large fraction of the diffuse vacancy clusters and voids in the boundaries remained after annealing at 400°C. They were stabilized during annealing probably by gaseous atoms.

Positron annihilation in a hydrogen-physisorbed graphite-caesium intercalation compound

Abstract The annihilation spectrum for CsC 24 obtained from the Doppler broadening of 511 keV γ-radiation consists of a narrow band superimposed on a broad band. Upon hydrogen physisorption at 77 K, the spectral height decreased. These facts are explained in terms of an interlayer state, the electrons of which are blocked to interact with positrons through the hydrogen physisorption.

Positron annihilation in graphite-alkali metal-hydrogen ternary systems

Abstract Chemisorption and physisorption of hydrogen into the graphite-alkali metal intercalation compounds, KC 8 and CsC 24 , were studied by means of positron-annihilation. The changes of the annihilation spectra obtained from the Doppler-broadening, upon hydrogen-introduction to the compounds, were explained in terms of changes in the positron-electron-annihilation probability in the intercalate layers of graphite.

Defect study of proton-irradiated liquid-encapsulated Czochralski GaAs using the positron-annihilation technique

The positron lifetime of undoped Liquid-Encapsulated Czochralski (LEC)-GaAs and Si-doped (1.3×1018 cm−3) LEC-GaAs was measured before and after irradiation with protons (dose 1×1015/cm2, 15 MeV). In Si-doped GaAs, the decrease of positron lifetime at temperatures between 10 and 300 K are due to the decrease of the positron-diffusion length and the increase of the effective shallow traps such as antisite GaAs. The annealing stage of the proton-irradiation-induced defects which show the different behavior from that of electron-irradiation-induced defects suggests that proton irradiation creates more complicated defect complexes, containing vacancies rather than isolated vacancy-type defects or simple complexes which have been observed during electron-irradiation processes. Above 700 K, proton-irradiation-induced defects such as vacancy-type defects and simple vacancy complexes are almost annealed out, while Si-induced defects such as SiGa-VGa complexes cannot be annealed out above 973 K.

Positron annihilation in potassium-intercalated graphite

Abstract The peak of the Doppler broadening spectrum of KC 24 is depressed by ≈10% upon hydrogen physisorption into its interstices. The depression shows that positrons are distributed in the potassium layers. The lifetime of positrons in KC 8 is measured to be 290 ps which is longer than the lifetime in graphite. It is, therefore, deduced that the positrons are not confined in a special area, but distributed over negatively charged graphitic carbon layers and also positively charged potassium layers in KC 24 and KC 8 . The narrow component of the spectrum is assigned to the annihilation of positrons with electrons in potassium layers. Imperfections included in the intercalant layers of KC 8 are detected as defects in which the positrons have a long lifetime of ≈ = 530 ps.

Nanocrystallization mechanism of amorphous Fe78B13Si9

The nanocrystallization mechanism of an amorphous alloy is discussed based on the kinetics of open nanospaces in Fe78B13Si9. There already exists a high concentration of Fe-enriched fluctuated sites with open nanospaces in the amorphous matrix. The structural and compositional fluctuation helps transient short-range Fe diffusion in the metastable amorphous matrix with an increase of temperature, triggering highly concentrated α-Fe nucleation. Along with the growth of α-Fe nucleus, Fe atoms are transferred from the intergranular amorphous phase to Fe-based nanocrystallites. The nanocrystallization of α-Fe is achieved through nucleation by short-range Fe diffusion and its growth by nanovoid-mediated long-range Fe diffusion.

Formation of Positronium in Cup-Stacked Carbon Nanofibers

The positron-lifetime spectrum for cup-stacked carbon nanofibers (CNFs) is composed of three components: 0.125 ns (fixed) (6%), 0.345 ns (75%), 1.21 ns (19%). The longest-lived component assigned to ortho-positronium (o-Ps) verifies a prominent yield of o-Ps, which is in contrast with the fact that no o-Ps is detected for uncapped cylindrical multi-walled carbon nanotubes. CNFs heat-treated to 1073 K in vacuo showed a positron-lifetime spectrum the same as that of untreated CNFs, which demonstrates that the thermal detachment of some functional groups from the surface of the CNFs does not change the Ps yield. The yield is found almost independent of temperature between 10 and 280 K. A single CNF, ca. 50 nm in outer diameter, observed by transmission electron-microscopy, comprises stacked cups of 9~12 truncated conical graphene sheets at ca. 20 o with respect to the fiber axis, so that all edges of graphene sheets are found on the zigzag outeras well as inner-surfaces of the fiber. The Raman spectrum for CNFs exhibits a band of a disorder-induced mode at 1349 cm -1 (D-band) and a band of the E2g2 in-plane mode at 1577 cm -1 (G-band). The intensity-ratios of a D-band to a G-band are 0.17, 0.25 and 0 for a mat of CNFs, the edge plane and the basal plane of a highly oriented pyrolitic graphite block, respectively. For graphite materials, Ps is formed from the positrons trapped in the defects originating from edges of graphene sheets. Introduction Graphite has been widely investigated by means of positron annihilation, and its lifetime spectrum has been reported to consist of three components. The intensity of a long-lived component (1.3~2.0 ns) attributed to orthoositronium (o-Ps) has been shown to be 1.5% for pyrolytic graphite, but 15.6% for graphite powder [1]. Graphite is composed of graphene sheets (hexagonal carbon networks) piling up along the c-axis, and a block of highly oriented pyrolytic graphite (HOPG) consists of crystallites with the well-oriented c-axis and random orientation in the basal plane. The crystallite size is ca. 25 μm along the a-axis and larger than 7 μm along the c-axis. Recently graphite materials of structure different from graphite have been prepared: multi-walled carbon nanotubes (MNTs) and cup-stacked carbon nanofibers (CNFs). MNTs consist of concentric microtubules of cylindrical graphene sheets, a single MNT being a few tens of nanometers in diameter and ca. 1 μm in length [2]. CNFs have a stacking form of truncated conical graphene sheets with a long hollow core around the fiber axis [3]. The positron-lifetime spectrum for uncapped MNTs is fitted with a single exponential term of 0.382 ns with 2 1.1, but no term assignable to o-Ps is separated [4]. Here we studied the annihilation of positrons in CNFs in order to elucidate the formation of Ps in graphite materials. Experimental CNFs (Carbere 24HT, GSI Creos Co.) were heat-treated to 623, 803 or 1073 K in vacuo to Materials Science Forum Online: 2004-01-15 ISSN: 1662-9752, Vols. 445-446, pp 331-333 doi:10.4028/www.scientific.net/MSF.445-446.331

Defect study on electron irradiated GaAs by means of positron annihilation

Measurements of the positron lifetime and Doppler-broadened annihilation-radiation have been performed in electron-irradiated GaAs. The positron lifetime at the irradiation induced defects was ∼0.250 ns at 300 K. The defect clustering stage was found to occur at around 520–620 K, and the coarsening and annealing stage is believed to be above 620 K. Similar annealing stages were also observed in GaAs lightly doped with Si (0.2×1018 cm−3). Both the lifetime and the S-parameter in the irradiated GaAs were found to decrease with temperature from 300 K to 100 K, suggesting the coexistence of shallow traps in electron irradiated GaAs.