Mohamed Elsayed

Confinement for More Space: A Larger Free Volume and Enhanced Glassy Dynamics of 2-Ethyl-1-hexanol in Nanopores

Broadband dielectric spectroscopy and positron annihilation lifetime spectroscopy are employed to study the molecular dynamics and effective free volume of 2-ethyl-1-hexanol (2E1H) in the bulk state and when confined in unidirectional nanopores with average diameters of 4, 6, and 8 nm. Enhanced α-relaxations with decreasing pore diameters closer to the calorimetric glass-transition temperature (T(g)) correlate with the increase in the effective free volume. This indicates that the glassy dynamics of 2D constrained 2E1H is mainly controlled by density variation.

Identification of As-vacancy complexes in Zn-diffused GaAs

We have used positron annihilation spectroscopy to study the introduction of point defects in Zn-diffused semi-insulating GaAs. The diffusion was performed by annealing the samples for 2 h at 950 °C. The samples were etched in steps of 7 μm. Both Doppler broadening using slow positron beam and lifetime spectroscopy studies were performed after each etching step. Both techniques showed the existence of vacancy-type defects in a layer of about 45 μm. Secondary ion mass spectroscopy measurements illustrated the presence of Zn at high level in the sample almost up to the same depth. Vacancy-like defects as well as shallow positron traps were observed by lifetime measurements. We distinguish two kinds of defects: As vacancy belongs to defect complex, bound to most likely one Zn atom incorporated on Ga sublattice, and negative-ion-type positron traps. Zn acceptors explained the observation of shallow traps. The effect of Zn was evidenced by probing GaAs samples annealed under similar conditions but without Zn t...

Cu diffusion-induced vacancy-like defects in freestanding GaN

Positron annihilation spectroscopy was employed to elucidate the nature and thermal behavior of defects induced by Cu in freestanding GaN crystals. Cu atoms were intentionally introduced in GaN lattice through thermally activated diffusion from an ultrathin Cu capping layer. During isochronal annealing of the obtained Cu-doped GaN in the temperature range of 450-850K, vacancy clusters were found to form, grow and finally vanish. Doppler broadening measurements demonstrate the presence of vacancy-like defects across the 600nm-thick layer below the surface corresponding to the Cu-diffused layer as evidenced by secondary ion mass spectrometry. A more qualitative characterization of these defects was accomplished by positron lifetime measurements. We found that annealing at 450K triggers the formation of divacancies, whereas further increase of the annealing temperature up to 550K leads to the formation of large clusters of about 60 vacancies. Our observations

Glassy dynamics of polymethylphenylsiloxane in one- and two-dimensional nanometric confinement-A comparison

Glassy dynamics of polymethylphenylsiloxane (PMPS) is studied by broadband dielectric spectroscopy in one-dimensional (1D) and two-dimensional (2D) nanometric confinement; the former is realized in thin polymer layers having thicknesses down to 5 nm, and the latter in unidirectional (thickness 50 μm) nanopores with diameters varying between 4 and 8 nm. Based on the dielectric measurements carried out in a broad spectral range at widely varying temperatures, glassy dynamics is analyzed in detail in 1D and in 2D confinements with the following results: (i) the segmental dynamics (dynamic glass transition) of PMPS in 1D confinement down to thicknesses of 5 nm is identical to the bulk in the mean relaxation rate and the width of the relaxation time distribution function; (ii) additionally a well separated surface induced relaxation is observed, being assigned to adsorption and desorption processes of polymer segments with the solid interface; (iii) in 2D confinement with native inner pore walls, the segmental dynamics shows a confinement effect, i.e., the smaller the pores are, the faster the segmental dynamics; on silanization, this dependence on the pore diameter vanishes, but the mean relaxation rate is still faster than in 1D confinement; (iv) in a 2D confinement, a pronounced surface induced relaxation process is found, the strength of which increases with the decreasing pore diameter; it can be fully removed by silanization of the inner pore walls; (v) the surface induced relaxation depends on its spectral position only negligibly on the pore diameter; (vi) comparing 1D and 2D confinements, the segmental dynamics in the latter is by about two orders of magnitude faster. All these findings can be comprehended by considering the density of the polymer; in 1D it is assumed to be the same as in the bulk, hence the dynamic glass transition is not altered; in 2D it is reduced due to a frustration of packaging resulting in a higher free volume, as proven by ortho-positronium annihilation lifetime spectroscopy.

Positron annihilation on defects in silicon irradiated with 15 MeV protons

Microstructure and thermal stability of the radiation defects in n-FZ-Si ([P] ≈ 7 × 10(15) cm(-3)) single crystals have been investigated. The radiation defects have been induced by irradiation with 15 MeV protons and studied by means of both the positron lifetime spectroscopy and low-temperature measurements of the Hall effect. At each step of the isochronal annealing over the temperature range ∼60-700 °C the positron lifetime has been measured for the temperature interval ∼30-300 K, and for samples-satellites the temperature dependences of the charge carriers and mobility have been determined over the range ∼4.2-300 K. It is argued that as-grown impurity centers influence the average positron lifetime by forming shallow (E(b) ≈ 0.013 eV) positron states. The radiation-induced defects were also found to trap positrons into weakly bound (E(b) ≤ 0.01 eV) states. These positron states are observed at cryogenic temperatures during the isochronal annealing up to T(anneal.) = 340 °C. The stages of annealing in the temperature intervals ∼60-180 °C and ∼180-260 °C reflect the disappearance of E-centers and divacancies, respectively. Besides these defects the positrons were found to be localized at deep donor centers hidden in the process of annealing up to the temperature T(anneal.) ≈ 300 °C. The annealing of the deep donors occurs over the temperature range ∼300-650 °C. At these centers positrons are estimated to be bound with energies E(b) ≈ 0.096 and 0.021 eV within the temperature intervals ∼200-270 K and ∼166-66 K, respectively. The positron trapping coefficient from these defects increases from ∼1.1 × 10(16) to ∼6.5 × 10(17) s(-1) over the temperature range ∼266-66 K, thus substantiating a cascade phonon-assisted positron trapping mechanism whose efficiency is described by ≈T(-3) law. It is argued that the value of activation energy of the isochronal annealing E(a) ≈ 0.74-0.59 eV is due to dissociation of the positron traps, which is accompanied by restoration of the electrical activity of the phosphorus atoms. The data suggest that the deep donors involve a phosphorus atom and at least two vacancies. Their energy levels are at least at E > E(c) - 0.24 eV in the investigated material.

Vacancy generation during Cu diffusion in GaAs

Positron lifetime and Doppler broadening spectroscopy were applied for a study of defect properties of semi-insulating GaAs after diffusion of copper. A 30 nm layer of Cu was deposited by evaporation to the undoped GaAs samples. The diffusion of Cu was performed during an annealing step at 1100 °C at different arsenic vapor pressures. The samples were quenched into room temperature water. The initial semi-insulating (SI) undoped GaAs sample shows no positron traps. After annealing, a vacancy-type complex and a shallow positron trap were observed to be efficient positron traps. Due to the Cu contamination during the annealing process, the shallow trap is believed to be the CuGa double acceptor. The nature of the vacancy-type defects could not be determined unambiguously. The concentration of these vacancies shows inverse relationship to the As vapor pressure that refers to the arsenic vacancy as a part of this complex. Moreover, Doppler-coincidence spectroscopy shows clearly that Cu atoms are not bound in ...

Detection of vacancy-like defects during Cu diffusion in GaAs by positron annihilation

The positron annihilation spectroscopy is a method for direct characterization of vacancy-type defects by measuring the positron lifetime. It provides information about open volume and concentration of defects. Such measurements were carried out to study the defect properties of semi-insulating GaAs after copper diffusion. A 30 nm layer of Cu was deposited by evaporation to undoped GaAs samples. The diffusion of Cu was performed during an annealing step at 1100 °C under different arsenic vapor pressures. The samples were quenched into room temperature water. The initial semi-insulating (SI) undoped GaAs sample shows no positron traps in that state. After gentle annealing, a vacancy-type defect complex in addition to shallow positron traps was observed to be an efficient positron trap. After Cu in-diffusion during the annealing process, the shallow positron trap is believed to be the CuGa double acceptor. The exact nature of the vacancy-like defects could not be determined unambiguously. The concentration of these defects exhibits inverse relationship to the arsenic vapor pressure. Thus, the arsenic vacancy is believed to be part of this complex. The temperature-dependent Hall-effect measurements have revealed the presence of an acceptor level at EV + 0.5 eV that is usually attributed to CuGa.

Positron annihilation lifetime spectroscopy at a superconducting electron accelerator

The Helmholtz-Zentrum Dresden-Rossendorf operates a superconducting linear accelerator for electrons with energies up to 35 MeV and average beam currents up to 1.6 mA. The electron beam is employed for production of several secondary beams including X-rays from bremsstrahlung production, neutrons, and positrons. The secondary positron beam after moderation feeds the Monoenergetic Positron Source (MePS) where positron annihilation lifetime (PALS) and positron annihilation Doppler-broadening experiments in materials science are performed in parallel. The adjustable repetition rate of the continuous-wave electron beams allows matching of the pulse separation to the positron lifetime in the sample under study. The energy of the positron beam can be set between 0.5 keV and 20 keV to perform depth resolved defect spectroscopy and porosity studies especially for thin films.

Demonstrating the Influence of Physical Aging on the Functional Properties of Shape-Memory Polymers

Polymers that allow the adjustment of Shape-Memory properties by the variation of physical parameters during programming are advantageous compared with their counterparts requiring synthesis of new material. Here, we explored the influence of hydrolytic (physical) aging on the Shape-Memory properties of the polyetherurethane system Estane, programmed in repeated thermomechanical cycles under torsional load. We were able to demonstrate that physical aging occurred through water adsorption influencing the existing free volume of the samples as well as the functional properties of Estane. Dynamic Mechanical Thermal Analysis determined the glass transition temperatures of dry and hydrolytically aged samples. According to our results, Estane takes up to 3 wt % water for two weeks (at an ambient temperature of θ = 20 °C). The glass transition temperatures of dry samples decreased within this period from 55 to 48 °C as a consequence of a plasticization effect. Next, for both samples, six subsequent thermomechanical cycles under torsional loading conditions were performed. We were able to confirm that hydrolytically aged samples showed higher shape recovery ratios of Rr ≥ 97%, although dry samples revealed better shape fixity values of about 98%. Moreover, it was observed that the shape fixity ratio of both dry and hydrolytically (physically) aged samples remained almost unchanged even after six successive cycles. Besides this, the shape recovery ratio values of the aged samples were nearly unaltered, although the shape recovery values of the dry samples increased from Rr = 81% in the first cycle to 96% at the end of six repeated cycles. Further, the evolution of the free volume as a function of temperature was studied using Positron Annihilation Lifetime Spectroscopy. It was shown that the uptake of two other organic solvents (acetone and ethanol) resulted in much higher specific free volume inside the samples and, consequently, a softening effect was observed. We anticipate that the presented approach will assist in defining design criteria for self-sufficiently moving scaffolds within a knowledge-based development process.

Similarity of Atomic Configurations of Thermally Stable Positron-Sensitive Complexes Produced with 0.9-MeV Electrons and 15-MeV Protons in n–FZ–Si:P Crystals

We observed for the first time the thermally stable point positron-sensitive center of a vacancy type in n–FZ–Si (P) material irradiated at RT by ~ 0.9-MeV electrons. The center that emerges after isochronal annealing at Tanneal.≈ 260 – 280 oC is found to be similar to the vacancy-group-V-atom complex revealed in the same Si material irradiated by 15-MeV protons; the detecting of the centers by the positron trapping is finalized at Tanneal.≥ 520 oC. The annihilation gamma-quanta to be emitted from the positron trap gives rise to a characteristic positron lifetime τ2 (I2 ~ 38–19 %) ≤ 276 – 294 ps which is somewhat longer than the one predicted for unrelaxed single vacancy τV.≈ 254 – 261 ps. Our data suggested a configuration of the complex VopPVop, wherein the atom of phosphorus is tied to a split open vacancy volume 2Vop. It is argued that Vop volume detected by the positron trapping may be formed by extended semi-vacancy, Vs-ext , or by the relaxed inwards vacancy, Vinw , thus resulting in a distorted Vs-extPVs-ext or VinwPVinw configurations.