C.V. Falub

Magnetic focusing of an intense slow positron beam for enhanced depth-resolved analysis of thin films and interfaces

The intense reactor-based slow positron beam (POSH) at the Delft research reactor has been coupled to a Two-Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) setup. The design is discussed with a new target chamber for the 2D-ACAR setup based on Monte Carlo simulations of the positron trajectories, beam energy distribution and beam transmission in an increasing magnetic field gradient. Numerical simulations and experiment show that when the slow positron beam with a FWHM of 11.6 mm travels in an increasing axial magnetic field created by a strong NdFeB permanent magnet, the intensity loss is negligible above ∼6 keV and a focusing factor of 5 in diameter is achieved. Monte Carlo simulations and Doppler broadening experiments in the target region show that in this configuration the 2D-ACAR setup can be used to perform depth sensitive studies of defects in thin films with a high resolution. The positron implantation energy can be varied from 0 to 25 keV before entering the non-uniform magnetic field. 2D-ACAR depth-profiling results in He-irradiated Si obtained with the new setup are presented.

Positron beam analysis of structurally ordered porosity in mesoporous silica thin films

Positron beam techniques have been employed to characterise low-k dielectric silica based films, which have two or three dimensional structures of nanometre size pores. Pore fractions vary from 5 to 50%. Positrons implanted in the layer slow down and eventually annihilate with the electrons from the layer. However, in pores of the dielectric films positronium (Ps) is formed before annihilation takes place. The two states of Ps (para-positronium (p-Ps) and ortho-positronium (o-Ps)) are formed with rather different life times of 125 ps and 142 ns, respectively. The behaviour of Ps in the porous material can be described as hot particles with 1 eV initial kinetic energy, which lose their energy by frequent collisions with the atoms of the pore walls. When the pores are interconnected or separated by thin walls allowing permeation of the Ps some of the Ps will effuse from the film into the vacuum. The 2D-ACAR technique enables one to monitor the velocity distribution of the annihilating p-Ps and shows an increasing fraction of surface emitted p-Ps when positrons are implanted closer to the film surface. Measurements of Doppler broadening of 2-gamma annihilation and detection of 3-gamma events (o-Ps) give insight into the frequency of other annihilation events. Combining the results, a complete picture can be obtained of the interactions and transport of the Ps particles in the material. By modelling the Ps behaviour information is obtained on structural parameters of the porous material. A transport model based on multi energy group diffusion of particles describes the results well.

Systematic positron study of hydrophilicity of the internal pore surface in ordered low-k silica thin films

Abstract Non-destructive Doppler Broadening (DB), Positronium fraction ( f-Ps ) and Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR), Positron Beam Analysis (PBA) techniques have been used to study well-ordered mesoporous silica thin films with hydrophobic or hydrophilic character. The DB results, characterized by the S and W parameters, are related to both the open volume and the chemical environment at the positron annihilation site. The f-Ps and 2D-ACAR techniques are very sensitive probes to determine the type of porosity (open or closed in terms of positronium escape) in thin film materials. Samples with varying pore size (well-controlled at 2, 3 and 4.5 nm, or incorporating microporosity in the silica), pore fraction (from 4 to 57%) and extent of hydrophilicity have been studied. In the hydrophilic samples with small pore size the S -parameter increases with the porosity while the Ps -fraction remains almost unchanged. Increasing the pore size leads to smaller changes in S with increasing porosity. However, an abrupt change in the Ps -fraction is observed for samples with porosity higher than 45%, indicating a positronium percolation threshold for samples with 2D ordering structure. On the other hand, for hydrophobic samples with high porosity (57%) the highest S parameter and Ps -fraction were obtained for the three pore sizes studied. 2D-ACAR is used to determine the fraction and velocity of Ps escaping from some of the latter samples exhibiting positronium percolation. The relationship between these observations and porosity will be discussed in terms of branching of the positron annihilation channels inside the mesoporous films.

Depth-selective 2D-ACAR studies on low-k dielectric thin films

Abstract Depth-selective 2D-ACAR investigations on ordered mesoporous silica thin films provide direct evidence that para-positronium (p-Ps) created deep in the films can escape through a network of interconnected pores. The depth dependence of the escape fraction and of the average kinetic energy of non-thermally excited p-Ps is in quantitative agreement with Monte Carlo modeling, assuming classical collisions of p-Ps with the pore walls. The model provides insight in the shape of the angular correlation distributions and their sensitivity to, e.g., the effective wall mass Ms and pore dimensions.

Depth-Selective 2D-ACAR and Coincidence Doppler Investigation of Embedded Au Nanocrystals in MgO

We present a depth-selective 2D-ACAR and two-detector Doppler broadening study on Au nanocrystals in monocrystalline MgO(100), produced in sub-surface layers by ion implantation and subsequent thermal annealing to temperatures beyond the stability range of vacancy clusters in MgO. In contrast to the case of Li nanocrystals, it was found that positrons do not trap inside the Au nanocrystals, but only in defects at the nanocrystal-to-host interface (attached vacancy clusters). This is interpreted in terms of the positron affinity of Au, MgO and the defects. Introduction Metallic nanoprecipitates embedded in crystalline host materials are promising materials for optical applications because of their special linear and non-linear optical properties [1]. Previous positron coincidence Doppler and lifetime studies showed a clear correlation between the presence of vacancy clusters attached to Au nanoclusters in MgO and an important change in surface plasmon frequency [2-4]. In this study, gold nanoclusters in epi-polished MgO(100) single crystals of size 10×10×1 mm 3 were created by means of implantation of 1 MeV Au ions at a dose of 1×10 16 Au ions/cm 2 at room temperature [5]. The implantation was followed by annealing at 1473 K for a period of 22 hrs. The gold nanoclusters were investigated by optical absorption spectroscopy [5], high-resolution X-ray diffraction (XRD) measurements [6] and cross-sectional transmission electron microscopy (X-TEM) [6]. They are present in a layer formed at a depth between ~140 and ~300 nm below the surface. Their size varies from 2-14 nm with a mean size of 4.6 nm. Furthermore, a cubeon-cube orientation relationship between the Au nanoclusters and the MgO host matrix is observed by X-TEM. Contrary to the positron studies in [2-4], we extended the thermal annealing to beyond the stability range of vacancy clusters in MgO. Previous 2D-ACAR studies on Li nanocrystals in MgO [7, 8] have shown that it is possible to probe structural and electronic structure properties of nanocrystals. Therefore depth-selective positron annihilation experiments were carried out, employing S parameter, two-detector Doppler coincidence and 2D-ACAR measurements. Results and discussion Figure 1 shows the S parameter versus the positron implantation energy for an as-grown (reference) MgO(100) sample, and a sample that was Au + implanted and annealed in ambient air at 773 K, 1273 K and 1473 K, respectively. After Au implantation the S parameter increases relative to the low value for as-grown MgO, and a maximum (A) is formed at about 3 keV. This indicates that vacancies are produced under the surface. The S parameter also increases for depths beyond the Rp-range of the Au ions, i.e. >300 nm, indicating that in that range vacancies are produced due to damage by low-mass knock-on ions. After the implanted sample is annealed at 773 K the S parameter increases substantially (peak B in Fig. 1) indicating that vacancies produced by implantation have aggregated into larger clusters. Further annealing in steps of 100 K up to 1273 K causes a reduction of the S parameter showing that part of the vacancies and/or small clusters of vacancies have been removed. At 1273 K the S parameter exhibits two peaks (C and D) positioned at about 3 and 8 keV, respectively. This type of profile has also been observed by Xu et al. [2-4]. Annealing the implanted MgO sample beyond this stage to 1473 K for 22 hours causes a drastic drop in the S parameter, indicating that the vacancies and vacancy clusters in the MgO matrix have been effectively removed. However, the S parameter still exhibits a weak maximum (F) that could be attributed to trapping in Au nanoclusters. Indeed, X-TEM measurements [6] showed the presence of Materials Science Forum Vols. 445-446 (2004) pp. 398-400 online at http://www.scientific.net

Nano-porosity in silica reinforced methyltrimethoxysilane coatings studied by positron beam analysis

Abstract The porosity in particle reinforced sol-gel coatings has been studied. Silica particles (Ludox-TM40) are introduced into methyl silicate coatings to increase the hardness, the elastic modulus and the fracture toughness. The methyl silicate has a relatively low density (about 1.2 g/cm2), while the silica particles are known to be porous. However, the porosity of the silica particles is not accurately known. For model calculations on mechanical properties like the E-modulus this porosity should be known. Positron Beam Analysis (PBA), using the Doppler Broadening (DB) and 2D-Angular Correlation of Annihilation Radiation (2D-ACAR) techniques, was therefore performed for analysis of the porosity. Samples with different weight fractions (0, 20 and 63 wt.%) of silica particles of typically 40 nm in diameter and treated at different curing temperatures (623 and 723 K) were measured. With increasing filler content we observed a decrease in the positron annihilation S-parameter and a broadening of the para-positronium (p-Ps) fraction. By neglecting positron diffusion we can separate porosity in the matrix from that in the particles. This assumption is valid as long as the expected positron diffusion length is short compared to the size of the filler particles, as in the present case. A more detailed description takes into account the local environment of the filler particles affecting their adhesion to the matrix. It is concluded that the density of the silica particles is about 1.4 g/cm2.