S.W.H. Eijt

Formation and stability of rocksalt ZnO nanocrystals in MgO

Coimplantation of Zn and O ions into a single crystalline MgO and subsequent thermal annealing were applied in the synthesis of ZnO nanocrystals. Electron microscopy showed that rocksalt instead of wurtzite ZnO stabilizes for relatively large nanocrystals up to ~15?nm, resulting from its small lattice mismatch with MgO of ~1.7%. The vacancies initially created by implantation induce favorable nanocrystal growth kinetics and are effectively absorbed during the nucleation and growth processes. The optical band edge of the ZnO nanocrystals was detected at ~2.8?eV.

Positron depth profiling of the structural and electronic structure transformations of hydrogenated Mg-based thin films

We report positron depth-profiling studies on the hydrogen sorption behavior and phase evolution of Mg-based thin films. We show that the main changes in the depth profiles resulting from the hydrogenation to the respective metal hydrides are related to a clear broadening in the observed electron momentum densities in both Mg and Mg2Ni films. This shows that positron annihilation methods are capable of monitoring these metal-to-insulator transitions, which form the basis for important applications of these types of films in switchable mirror devices and hydrogen sensors in a depth-sensitive manner. Besides, some of the positrons trap at the boundaries of columnar grains in the otherwise nearly vacancy-free Mg films. The combination of positron annihilation and x-ray diffraction further shows that hydrogen loading at elevated temperatures, in the range of 480–600 K, leads to a clear Pd–Mg alloy formation of the Pd catalyst cap layer. At the highest temperatures, the hydrogenation induces a partial delamination of the ? 5?nm thin capping layer, as sensitively monitored by positron depth profiling of the fraction of ortho-positronium formed at interface with the cap layer. The delamination effectively blocks the hydrogen cycling. In Mg–Si bilayers, we investigated the reactivity upon hydrogen loading and heat treatments near 480 K, which shows that Mg2Si formation is fast relative to MgH2. The combination of positron depth profiling and transmission electron microscopy shows that hydrogenation promotes a complete conversion to Mg2Si for this destabilized metal hydride system, while a partially unreacted, Mg-rich amorphous prelayer remains on top of Mg2Si after a single heat treatment in an inert gas environment. Thin film studies indicate that the difficulty of rehydrogenation of Mg2Si is not primarily the result from slow hydrogen dissociation at surfaces, but is likely hindered by the presence of a barrier for removal of Mg from the readily formed Mg2Si.

Divacancies and the hydrogenation of Mg-Ti films with short range chemical order

We obtained evidence for the partial chemical segregation of as-deposited and hydrogenated Mg1?yTiy films (0 ? y ? 0.30) into nanoscale Ti and Mg domains using positron Doppler-broadening. We exclusively monitor the hydrogenation of Mg domains, owing to the large difference in positron affinity for Mg and Ti. The electron momentum distribution broadens significantly upon transformation to the MgH2 phase over the whole compositional range. This reveals the similarity of the metal-insulator transition for rutile and fluorite MgH2. Positron lifetime studies show the presence of divacancies in the as-deposited and hydrogenated Mg-Ti metal films. In conjunction with the relatively large local lattice relaxations we deduce to be present in fluorite MgH2, these may be responsible for the fast hydrogen sorption kinetics in this MgH2 phase.

Positron beam and Raman analysis of hydrogen plasma treated and annealed Cz-Si

Abstract Positron beam annihilation Doppler-broadening (PADB) and Raman studies were carried out on H-plasma treated and annealed p-type Cz-Si. Samples were treated by a 13.56 MHz H-plasma for 2 h at RT (∼30 °C) and 250 °C, respectively. Annealing was done in air between 100 and 600 °C for annealing times between 10 and 480 min. The Raman spectra of the RT treated samples show no (or only very weak) H 2 -molecule signals, in contrast to the samples H-plasma treated at 250 °C. Raman intensity changes as a function of temperature are observed, which are attributed to the evolution of voids or platelets. The positron annihilation data correlate with these results, since annealing of the RT plasma treated samples reduces the Doppler-broadening S -values with increasing temperature and/or annealing time. Together with an increasing positron diffusion length this suggests that defects acting as positron trapping centers are (partially) annealed out. For the sample plasma treated at 250 °C the following depth dependent behavior of the Doppler parameters is found. Below 100 nm depth the Doppler parameters follow the same trend as those of the RT treated samples. In a zone between 100 and 200 nm depth the S parameter strongly increases after annealing at 600 °C. This is attributed to the formation of positronium (Ps), indicative for the presence of nano-cavities capable of trapping molecular hydrogen in this region.

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.

Surfaces of colloidal PbSe nanocrystals probed by thin-film positron annihilation spectroscopy

Positron annihilation lifetime spectroscopy and positron-electron momentum density (PEMD) studies on multilayers of PbSe nanocrystals (NCs), supported by transmission electron microscopy, show that positrons are strongly trapped at NC surfaces, where they provide insight into the surface composition and electronic structure of PbSe NCs. Our analysis indicates abundant annihilation of positrons with Se electrons at the NC surfaces and with O electrons of the oleic ligands bound to Pb ad-atoms at the NC surfaces, which demonstrates that positrons can be used as a sensitive probe to investigate the surface physics and chemistry of nanocrystals inside multilayers. Ab initio electronic structure calculations provide detailed insight in the valence and semi-core electron contributions to the positron-electron momentum density of PbSe. Both lifetime and PEMD are found to correlate with changes in the particle morphology characteristic of partial ligand removal.

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.

Radial Electron Momentum Densities of Colloidal CdSe Nanocrystals Determined by Positron Beam Analysis

We present depth-resolved positron 2D angular correlation of annihilation radiation (2DACAR) experiments on CdSe quantum dots in the diameter range from 2.5 to 6 nm, deposited as micrometer thin layers. The average radial distribution of the valence electron momentum density (EMD) of CdSe quantum dots has been extracted, which reveals a systematic dependence upon particle size. The quantum confinement related changes and their size scaling observable at the Jones zone momentum of {approx}0.8 a.u. seem to agree with the previous coincidence Doppler study. In addition, the average radial EMD shows an increase in the low-momentum range ( 1.6 a.u.) with respect to that measured on a bulk CdSe single crystal. Possible origins of these are described. First-principles calculations based on the Korringa-Kohn-Rostoker (KKR) method were performed to gain a better insight.