Positron/positronium annihilation in nanocrystalline silicon thin films

Positron and positronium annihilation investigations were applied to nanocrystalline silicon (nc-Si) thin films, for the first time. The nc-Si thin films with average grain diameters of 3–5 nm show intense blue luminescence at room temperature. The nanometer-sized Si crystallites formed in amorphous Si (a-Si) matrix give rise to this luminescence. Very highS-parameters up to 0.62 were observed in the as-grown a-Si thin film suggesting positronium formation in the a-Si layer. The average lifetime of the positrons in the a-Si was determined to be about 520 ps. TheS-parameters dropped significantly to 0.53 by crystallization of the thin film at 800 °C for 10 seconds, which was almost the same to the value observed in bulk Si (100) substrate. Further crystallization from 60 seconds to 1 hour showed smaller change in theS-parameters than that from the a-Si to 10 seconds. The large change in theS-parameters due to the annealing might be caused by the formation of Si nanocrystallites in a-Si matrix suggesting that positron is a sensitive probe for structural investigations of the nc-Si materials.     

Positron annihilation and phase transitions in n-octadecanol

The positronium formation probability and the mean lifetime in n-octadecanol-1 were observed as a function of temperature. The changes of that probability are discussed in the terms of the spur reaction model, suggesting the residual effect of dielectric constant increase in the rotational phase.     

Positron annihilation lifetime studies of the volume phase transition of polyacrylamides

Discontinuous and continuous volume phase transitions of organic polymer hydrogels, such as polyacrylamide (PAAm) and poly(N-isopropylamide) (PNIPA) gels, uponpH and temperature were studied by the positron annihilation lifetime measurement, which allows the estimation of size, intensity and size distribution of the free volume. Microscopic changes of physical and chemical interactions between gel network and solvent molecules and among conjugated solvent molecules at volume phase transitions of polyacrylamide gels were discussed.     

Positron annihilation studies of chromophore-doped polymers

Doppler-broadening energy spectra and positron annihilation lifetime have been measured as a function of positron implantation energy in pure and chromophore Disperse Red 1 (DR1)-doped poly(methyl methacrylate) (PMMA) polymers. In pure PMMA, the S parameter increases at very short range (<0.02 μm) from the surface to the bulk, while the S parameter of doped PMMA varies with a decrease from a depth of >0.02 μm to about 0.5 μm after an increase, a short distance from the surface. The o-Ps lifetime of the polymers is found to increase from the bulk to the surface, which indicates that the hole size expands near the surface. The o-Ps lifetime distribution becomes broader near the surface. The change of the o-Ps intensity shows the same trend as the change of the S parameter. These results are interpreted as a gradient of DR1 concentration in PMMA, as a function of the depth from the surface to the bulk in the chromophore-doped polymers.     

Positron annihilation spectroscopic studies on Nd-doped ceria

We report a new result on the characterization of Nd-doped ceria, Ce_1−xNd_xO_2−x/2 (x=0.075-0.675) using positron lifetime spectroscopy (LTS) and coincidence Doppler broadening (CDB) measurements. A systematic increase in lifetime that is attributed to formation of Nd-oxygen vacancy associates is seen from x=0.075-0.4 followed by a sharper increase up to x=0.5. The change in profile of lifetime around x=0.4 suggests drastic increase in the concentration of these associates. Discontinuity in lifetime around x=0.5 is ascribed to ordering of oxygen vacancies. Coincidence Doppler broadening measurements indicate reduction in the overlap of positron wavefunction with oxygen core electrons due to trapping of positrons. Low-temperature (50-300 K) lifetime measurements indicate the presence of Rydberg-like positron states associated with Nd³⁺ sites.     

Positron annihilation lifetime studies of free volume in gelatin

The variations of size, intensity, and size distribution of free volumes generated in the network of molecular chains of gelatin at the sol-gel transition were studied by means of the positron annihilation lifetime technique. Although variation in average free-volume radius was not recognized, a variation of free-volume content was observed at the sol-gel transition point of gelatin with an addition of saccharose.     

Positron annihilation and phase transitions in argon intercalated n-nonadecane

Positron annihilation method was applied to study phase transitions in argon intercalated n-nonadecane. Particular phases are easily recognizable due to large differences in ortho-positronium lifetimes. It was found that melting temperature is a non-monotonous function of pressure. In the range of temperatures 301.5–303 K with the rise of pressure the phase sequence rotator – liquid–rigid crystal was observed. After the pressure induced transition to the rigid phase the sample structure became porous due to escape of dissolved argon. The triple point in intercalated samples is located at much lower pressure and temperature than in pure ones. An admixture of alkane with other length of carbon chain increases the ortho-positronium lifetime in rigid phase, indicating Ps location in the interlamellar gaps.     

Positron annihilation and Xe NMR studies of free volume in polymers

The existence and the average size of free volume in bisphenol-A polycarbonate (PC), low-density polyethylene (LDPE), poly (2,6-dimethyl-phenyleneoxide)(PPO), and polytetrafluoroethylene (PTFE) were studied by positron annihilation and ¹²⁹Xe NMR measurements. The ¹²⁹Xe NMR chemical shifts for xenon adsorbed in the polymers indicated that the average pore size of the free volume increased in the following order: PC, LDPE, PPO, and PTFE. This order of the pore size of the free volume agrees well with that estimated from the longest lifetime (τ₃) of ortho-positronium formed in the polymers. The unique correlation that δ⁻¹∝ r is established between the ¹²⁹Xe NMR chemical shift (δ) and the pore size (r), which is deduced from the positron annihilation measurements.     

Positron annihilation lifetime studies of free volume in ovalbumin gels

The dependence upon concentration of sodium chloride of free volume in ovalbumin gels, a main component of an egg white, is studied by the positron annihilation lifetime technique. The average free-volume radius in ovalbumin gels was about 0.27 nm at 298 K, smaller than those of organic polymers such as low-density polyethylene (0.34 nm at 300 K) and polystyrene (0.29 nm at 300 K). These differences suggest that the positronium annihilates in free volumes located close to hydrogen bonds, thus decreasing average free-volume radius. Free-volume content decreased with the increase of NaCl concentration up to 30 mM. At lower concentrations of NaCl, it seems that a correlation exists between microscopic free volume and macroscopic hardness of the ovalbumin.     

Positron annihilation studies of some charge transfer molecular complexes

Positron annihilation lifetimes were measured for some solid charge transfer (CT) molecular complexes of quinoline compounds (2,6-dimethylquinoline, 6-methoxyquinoline, quinoline, 6-methylquinoline, 3-bromoquinoline and 2-chloro-4-methylquinoline) as electron donor and picric acid as an electron acceptor. The infrared spectra (IR) of the solid complexes clearly indicated the formation of the hydrogen-bonding CT-complexes.

The annihilation spectra were analyzed into two lifetime components using PATFIT program. The values of the average and bulk lifetimes divide the complexes into two groups according to the non-bonding ionization potential of the donor (electron donating power) and the molecular weight of the complexes. Also, it is found that the ionization potential of the donors and molecular weight of the complexes have a conspicuous effect on the average and bulk lifetime values. The bulk lifetime values of the complexes are consistent with the formation of stable hydrogen-bonding CT-complexes as inferred from the IR-spectral data.     

Adsorption and desorption of binary mixtures in confined geometry: A pilot positron annihilation study

In confined (nanometer-scale) geometry, the effects of substrate forces and finite size produce a shift of the gas liquid phase boundary from that found in corresponding bulk. The pore transitions also show marked hysteresis. The phase behaviour of a binary gas mixture in confined geometry is likely to depend on the miscibility of the system and the interaction between the substrate and the individual fluid molecules/atoms. Here, we present the results of a pilot positronium annihilation study of the condensation and evaporation of argon-nitrogen mixtures confined in 4 nm diameter cylindrical pores in VYCOR glass.     

Positron annihilation in amino acids

Positron lifetime measurements were performed on eight different amino acids, τ₁, τ₂ and I₂ values were observed at around 0.35 nsec, 0.7–1.2 nsec and 20–30%, respectively. The long-lived component seems to be produced by positronium states trapped in the crystalline lattice or by positronium atoms bound to molecules.     

Positron annihilation spectroscopy for surface and interface studies in nanoscale polymeric films

Positron annihilation spectroscopy (PAS), coupled with a variable mono-energetic positron beam, has been used to investigate surface and interfacial properties in thin polymeric films. Free-volume properties have been measured from ortho-Positronium (o-Ps) lifetime and the S parameter of Doppler broadening of energy spectra from annihilation radiation as a function of the depth and of the temperature in thin polymeric films. Depth profiles of glass transition temperature and nanoscale layered structures in polystyrene (PS) thin films on the Si substrate are presented.     

Positron annihilation in irradiated and unirradiated polyethylenes

Positron lifetimes were measured for four kinds of polyethylene samples and were resolved into four components. The temperature dependence of the two longlived components was examined in detail. In agreement with other results, the longest lived component could be reasonably assigned to ortho-positronium located in amorphous regions. This component was shown to be sensitive to the defects in high-density polyethylene introducted in the course of its production. Both the intensity and the lifetime of the second longest lived component were structure insensitive, i.e., they did not change even on passing through the melting point. This component has been tentatively assigned to a positronium compound state. The effect of gamma-ray irradiation was also examined. Although the intensity (I₄) of the longest lived component was reduced by the irradiation, correlation between I₄ and the free radical concentration was poor, and the reduction in I₄ caused by the irradiation is considered to be due to structure change and not to chemical reasons.     

Positron annihilation in liquids and complex formation

The bubble model, which is conventionally used to fit the observed characteristics of the pickoff component of ortho-Positronium decay in liquids, is subjected in this study to a critical assessment. It is demonstrated that in its present form (namely that of a bubble with a sharp boundary) the model is untenable when confronted jointly with experimental data on the lifetime and angular correlation of the decay gammas. A modified version of the model, free from such shortcomings, is presented. Further, the dramatic turnover in the temperature dependence of the rate for the formation of positronium–acceptor complex in the case of nitro-benzene in different solvents is interpreted as a Kramers’ turnover in the context of the improved Bubble Model.     

Positron annihilation in ionic conductivity polymers

Positron annihilation lifetime spectra and ionic conductivity have been measured for poly(etherurethane)-LiClO₄ as a function of temperature. The effects of Li salt on glas transition free volume and ionic conductivity have been discussed. A correlation between fractional free volume and ionic conductivity was first experimentally confirmed by using the free volume theory.Supported by the National Natural Science Foundation of China.     

Positron annihilation in solid charge-transfer complexes

Positron lifetime and angular correlation measurements have been carried out on 1:1 charge-transfer complexes, on their pure donor and acceptor components and on the 1:1 M mechanical mixtures of these components. Complex formation reduced the intensity of the long-lifetime component of the donor compounds nearly to the low level of the acceptors. The angular correlation curves obtained for the pure acceptor and the complex were practically identical and were substantially broadened as compared to that of the donor.     

Positron annihilation lineshape studies as a supplement method to lifetime measurement

We studied positron annihilation lineshapes by a Ge(Li) γ-ray spectrometer for CCl₄ solution in benzene and two monomer crystals which are polymerized by γ-ray irridation. From the lineshapes we evaluated the para-positronium intensity, and the results agree well with those of lifetime measurements in the cases of CCl₄ solutions in benzene and solid acrylamide. But in solid dimethylitaconate the results from the two different methods have not agreed.