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Infrared cooled photodetectors must operate at higher temperatures to reduce their size, weight and power consumption (SWaP context). Their stability and image quality are then challenged by extra electrical activity of crystal defects. Knowledge of defect populations is mandatory to improve the material quality of the epitaxial Hg1−xCdxTe (MCT) active layer and the Cd1−xZnxTe (CZT) substrate. Positron annihilation spectroscopy with a slow positron beam was used to study near-surface open-volume defects profiles. Low- and high-momentum fractions (S , W ) were used to characterize the Doppler broadening of the 511 keV electron-positron pair annihilation-line as a function of the positron implantation energy E . The results show that three regions can be identified beneath the surface of the as-grown non-optimized MCT layer. The quasi-linear relationship between the annihilation characteristics in the regions suggests that the defect populations mainly correspond to the same open-volume defect in different concentrations. The probed defect is thought to be related to the mercury vacancy. This hypothesis is discussed in an original way with near-surface elemental profiles using scanning transmission electron spectroscopy combined to energy dispersive x-ray spectroscopy (STEM-EDX). Afterwards, this approach is extended to CZT substrates showing that surface and bulk properties of those fabricated by LYNRED tend to match those that are state-of-the-art. A common open-volume defect is probed, in concentration estimated by Hall effect around 1015 cm−3 and thought to be related to the cadmium vacancy. [ABSTRACT FROM AUTHOR]

Microstructural features of Cu0.1Ni0.8Co0.2Mn1.9O4-based thick films screen-printed on substrate with formed Ag–Pd contacts were investigated using X-ray diffraction, scanning electron microscopy methods. It is established that diffusion processes of elements from contacts area occurs into internal structure of thick films located near grain boundaries. Formed internal nanovoids in thick-film structures were studied by positron annihilation lifetime spectroscopy. The observed positron trapping sites can be defined within two-state positron trapping model at fitting on two components. It is established that positron trapping processes in thick films formed in three and four layers and bulk Cu0.1Ni0.8Co0.2Mn1.9O4 ceramics are similar testifies the same structural type and successful transformation of bulk ceramics into thick films. In some cases, the size of free-volume nanovoids in thick films can rise due to redistribute space of their internal structure at addition glasses and other compounds. Positrons are trapped more strongly in bulk ceramics due to more ramified their grain/pores structure. Studied thick films are characterized by high temperature sensitivity and their properties can be stabilized after thermally induced treatment. [ABSTRACT FROM AUTHOR]

Herein we demonstrate the specifics of using the positron annihilation lifetime spectroscopy (PALS) method for the study of free volume changes in functional ceramic materials. Choosing technological modification of nanostructured MgAl2O4 spinel as an example, we show that for ceramics with well-developed porosity positron annihilation is revealed through two channels: positron trapping channel and ortho-positronium decay. Positron trapping in free-volume defects is described by the second component of spectra and ortho-positronium decay process by single or multiple components, depending on how well porosity is developed and on the experimental configuration. When using proposed positron annihilation lifetime spectroscopy approaches, three components are the most suitable fit in the case of MgAl2O4 ceramics. In the analysis of the second component, it is shown that technological modification (increasing sintering temperature) leads to volume shrinking and decreases the number of defect-related voids. This process is also accompanied by the decrease of the size of nanopores (described by the third component), while the overall number of nanopores is not affected. The approach to the analysis of positron annihilation lifetime spectra presented here can be applied to a wide range of functional nanomaterials with pronounced porosity.

X-ray diffraction and calorimetric investigations were performed for 80GeS2-20Ga2S3, 82GeS2-18Ga2S3 and 84GeS2-16Ga2S3 chalcogenide glasses. It is shown that structure of these glasses is characterized by short range ordering. It is established that Ga2S3 and GeS2 phases can be crystallized in Ge-Ga-S system. Transformation of voids in crystallized (80GeS2-20Ga2S3)100-х(СsCl)x, x = 0; 5; 10; 15 chalcogenide glasses was studied by positron annihilation lifetime spectroscopy. The CsCl content in GeS2-Ga2S3 glassy matrix changed the defect-related component in positron lifetime spectra and confirmed the structural void agglomeration in comparison with the base glass. A larger amount of CsCl in (80GeS2-20Ga2S3)85(СsCl)15 glass resulted in void fragmentation due to loosening of the structure. [ABSTRACT FROM AUTHOR]

Lead nitrate (Pb(NO 3) 2) – filled polyvinyl alcohol (PVA) – polyvinyl pyrrolidone (PVP) composite films were prepared by solution casting method, with filler levels (FL) varying from 2.7 wt% up to 42.7 wt%. Positron annihilation lifetime spectroscopy and measurements of Doppler broadening of annihilation radiation (DBAR) were carried out to monitor the changes in polymer microstructure due to the filler loading. The lineshape parameter, S , increased at lower doping levels, followed by a decrease at higher FL due to the effects of the filler (Pb(NO 3) 2) on the microstructure of PVA-PVP composite films. A significant drop in the intensity (I 3) of the ortho-positronium (o-Ps) component, from 18% down to approximately 2%, is observed on increasing the FL from 16.2 up to 26.7 wt%. This is accompanied by a decrease in the intensity (I 1) of the shortest lifetime component and a compensating increase in intensity (I 2) of the intermediate lifetime component. It indicated the inhibition of positronium (Ps) formation in Pb(NO 3) 2 -filled PVA-PVP composites. • Lead nitrate filled PVA-PVP composite films are studied using PALS and DBAR. • Significant changes are observed in positron parameters due to filler loading. • Intensity of o-Ps component decreases from 18% to 2%, at higher filler levels (FL). • Inhibition of positronium (Ps) formation is observed for FL beyond 19.8 wt%. • Ps trapping rate in the composite decreased significantly at moderate and high FL. [ABSTRACT FROM AUTHOR]

A 4MV Van de Graaff accelerator was used to induce the nuclear reaction 12 C(d,n) 13 N in order to produce an intense beam of positrons. The graphite target was heated so the radioactive 13 N would desorb from the bulk into the vacuum. This radioactive gas is frozen onto a cryogenic freezer where it decays to produce an antiparticle beam of positrons. This high current beam is then guided into a superconducting magnet with field strength up to 7 Tesla where the positrons will be stored in a newly designed Micro-Penning-Malmberg trap. Several source geometries have been experimented on and found a maximum antimatter beam with a positron flux of greater than 0.55 ± 0.03 × 10 6 e + s −1 was achieved. This beam was produced using a solid rare gas moderator composed of krypton (Kr) at a temperature of 25 ± 5 K. Due to geometric restrictions on this set up and other loss mechanisms, 10 7 –10 8 e + s −1 of the total number of positrons are lost. Simulations and preliminary experiments suggest a new geometry, currently under testing, will produce a beam of 10 7 e + s −1 or more. The setup and preliminary results for the new geometry will be discussed as well. [ABSTRACT FROM AUTHOR]

Evolution of free-volume positron trapping defects caused by crystallization process in (80GeS 2 –20Ga 2 S 3 ) 100−х (СsCl) x , 0 ≤ x ≤ 15 chalcogenide-chalcohalide glasses was studied by positron annihilation lifetime technique. It is established that CsCl additives in Ge–Ga–S glassy matrix transform defect-related component spectra, indicating that the agglomeration of free-volume voids occurs in initial and crystallized (80GeS 2 –20Ga 2 S 3 ) 100−х (СsCl) x , 0 ≤ x ≤ 10 glasses. Void fragmentation in (80GeS 2 –20Ga 2 S 3 ) 85 (СsCl) 15 glass can be associated with loosing of their inner structure. Full crystallization in each of these glasses corresponds to the formation of defect-related voids. These trends are confirmed by positron-positronium decomposition algorithm. It is shown, that CsCl additives result in white shift in the visible regions in transmission spectra. The γ-irradiation of 80GeS 2 –20Ga 2 S 3 base glass leads to slight long-wavelength shift of the fundamental optical absorption edge and decreasing of transmission speaks in favor of possible formation of additional defects in glasses and their darkening. [ABSTRACT FROM AUTHOR]

The phenomenon of positron-electron annihilation in lifetime measuring mode is considered as a tool to study nanostructurization in solids possessing mixed positron and positronium (Ps) trapping. Structural inhomogeneities due to guest nanoparticles in such solids are described in terms of substitution trapping in positron- and Ps-related sites within the same host matrix. The developed approach allows estimation of interfacial free-volume voids as being responsible for positron trapping and defect-free bulk positron lifetimes of nanoparticle-modified solids. For the example of arsenic sulfide, As4S4 nanoparticles embedded in polyvinylpyrrolidone (PVP) environment under high-energy ball milling, an alternative algorithm to parameterize these structural imperfections is justified. Interfacial free-volume voids between neighboring nanoparticles filled with loosely packed As4S4 crystallites are considered as the most probable positron trapping sites. The observed variations in mixed positron-Ps trapping modes under nanostructurization are adequately defined with respect to the chemistry of guest nanoparticles. Direct evidence for this algorithm is provided as the basis of experimental three-term decomposed positron lifetime spectra for As4S4-PVP nanocomposites parameterized with respect to different fitting protocols. [ABSTRACT FROM AUTHOR]

The phenomenon of positron-electron annihilation in lifetime measuring mode is considered as a tool to study nanostructurization in solids possessing mixed positron and positronium (Ps) trapping. Structural inhomogeneities due to guest nanoparticles in such solids are described in terms of substitution trapping in positron- and Ps-related sites within the same host matrix. The developed approach allows estimation of interfacial free-volume voids as being responsible for positron trapping and defect-free bulk positron lifetimes of nanoparticle-modified solids. For the example of arsenic sulfide, As4S4 nanoparticles embedded in polyvinylpyrrolidone (PVP) environment under high-energy ball milling, an alternative algorithm to parameterize these structural imperfections is justified. Interfacial free-volume voids between neighboring nanoparticles filled with loosely packed As4S4 crystallites are considered as the most probable positron trapping sites. The observed variations in mixed positron-Ps trapping modes under nanostructurization are adequately defined with respect to the chemistry of guest nanoparticles. Direct evidence for this algorithm is provided as the basis of experimental three-term decomposed positron lifetime spectra for As4S4-PVP nanocomposites parameterized with respect to different fitting protocols. [ABSTRACT FROM AUTHOR]

The free-volume nanostructural transformation in fully crystallized (80GeS2-20Ga2S3)100-x(CsCl)x, x = 0; 5; 10; 15 chalcogenide glasses was studied by positron annihilation lifetime spectroscopy using mixed positron- and positronium-trapping modes. The CsCl content in GeS2-Ga2S3 glassy matrix changed the defect-related component in positron lifetime spectra and confirmed the structural void agglomeration in comparison with the base glass. A larger amount of CsCl in (80GeS2-20Ga2S3)85(CsCl)15 glass resulted in void fragmentation due to loosening of the structure. 'Pure' positron-positronium modes calculated by a decomposition formalism show that a realistic process is connected with the expansion of voids formed as a consequence of CsCl addition and complete crystallization into the inner structure of glasses. The CsCl additive formed new positron-trapping sites in the Ge-Ga-S glassy matrix. The bulk positron lifetime in glasses corresponded to positron trapping occurring in nanocrystalline particles. Interface voids were not particularly large because the characteristic value of 0.27-0.30 ns can be connected with typical free volumes of mono- and di-vacancies. [ABSTRACT FROM AUTHOR]

Structural transformations caused by coarse-grained powdering and fine-grained mechanochemical milling in a dry mode were probed in high-temperature modification of tetra-arsenic tetra-sulfide known as β-AsS. In respect to X-ray diffraction analysis, the characteristic sizes of β-AsS crystallites in these coarse- and fine-grained powdered pellets were 90 and 40 nm, respectively. Positron annihilation lifetime spectroscopy was employed to characterize transformations occurred in free-volume structure of these nanoarsenicals. Experimentally measured positron lifetime spectra were parameterized in respect to three- or two-term fitting procedures and respectively compared with those accumulated for single crystalline realgar α-AsS polymorph. The effect of coarse-grained powdering was found to result in generation of large amount of positron and positronium Ps trapping sites inside arsenicals in addition to existing ones. In fine-grained powdered β-AsS pellets, the positron trapping sites with characteristic free volumes close to bi- and tri-atomic vacancies were evidently dominated. These defects were supposed to originate from grain boundary regions and interfacial free volumes near aggregated β-AsS crystallites. Thus, the cumulative production of different positron traps with lifetimes close to defect-related lifetimes in realgar α-AsS polymorph was detected in fine-grained milled samples. [ABSTRACT FROM AUTHOR]

Abstract: Porous structure evolution in functional humidity-sensitive MgAl2O4 spinel ceramics prepared at different sintering temperatures is studied. It is shown that the increase in sintering temperature from 1100 to 1400°C results in the transformation of the pore size distribution in ceramics. It is established that porous ceramics with tri-modal pore size distribution including open macro-, meso- and micropores up to a few nanometer is humidity sensitive in the 25–95% relative humidity range. It is demonstrated that positron annihilation lifetime spectroscopy supported by four-component fitting procedure is a promising tool for nanostructural characterization of MgAl2O4 spinel-type ceramics. Microstructure of these materials is improved with the increase in sintering temperature; the effect mainly results in the decrease of the additional near grain boundaries phases amount. These phase extractions and free-volume entities in ceramic structure serve as specific trapping centers for positrons penetrating ceramics. It is demonstrated that Tao–Eldrup model can be adequately used for the calculation nanopores size in MgAl2O4 ceramics using o-Ps lifetimes of the third and the fourth components. [Copyright &y& Elsevier]

Herein we demonstrate the specifics of using the positron annihilation lifetime spectroscopy (PALS) method for the study of free volume changes in functional ceramic materials. Choosing technological modification of nanostructured MgAl2O4 spinel as an example, we show that for ceramics with well-developed porosity positron annihilation is revealed through two channels: positron trapping channel and ortho-positronium decay. Positron trapping in free-volume defects is described by the second component of spectra and ortho-positronium decay process by single or multiple components, depending on how well porosity is developed and on the experimental configuration. When using proposed positron annihilation lifetime spectroscopy approaches, three components are the most suitable fit in the case of MgAl2O4 ceramics. In the analysis of the second component, it is shown that technological modification (increasing sintering temperature) leads to volume shrinking and decreases the number of defect-related voids. This process is also accompanied by the decrease of the size of nanopores (described by the third component), while the overall number of nanopores is not affected. The approach to the analysis of positron annihilation lifetime spectra presented here can be applied to a wide range of functional nanomaterials with pronounced porosity. [ABSTRACT FROM AUTHOR]

The advent of intense positron beams makes it possible to perform depth-selective 2D-ACAR (two-dimensional angular correlation of annihilation radiation) studies. The Delft POSH–ACAR setup employs a strong permanent magnet for focusing of the POSH beam on the sample, which leads to a ∼15% spread in implantation energy. The effects of this spread on positron depth-profiling data are discussed, and are shown to be consistent with Doppler experiments on Si(1 0 0) with a subsurface layer of nanocavities. A method is presented to decompose depth-selective 2D-ACAR spectra reliably into their various (layer) components. This is used to reveal strong positron trapping in the nanocavities in Si(1 0 0). [Copyright &y& Elsevier]

Abstract: Sapphire samples, irradiated with swift Kr (245MeV) ions at room temperature in a broad fluence range, were investigated using a continuous and a pulsed positron beam to study the defect structure created by the passage of the ions in depths of a few micrometers. At small doses, monovacancies were identified as dominant defects and positron trapping centres. These monovacancies are assumed to be highly concentrated inside a cylindrical volume around the ion path with an estimated radius of ∼1.5nm. For higher doses a second type of trapping centre emerges. This second class of structural imperfection was associated with the overlap of the individual ion tracks leading to the formation of larger vacancy clusters or voids. [Copyright &y& Elsevier]

Abstract: The accumulation of positrons in a two-dimensional layer of tin embedded in aluminum is examined by Doppler broadening spectroscopy (DBS). For this purpose samples are grown out of high purity materials consisting of a step-shaped layer (0.1–200nm) of tin on a substrate of aluminum and covered by an aluminum layer of constant thickness (200nm). The positron implantation profile is varied by different positron acceleration energies of up to 15keV. The pre-thermal implantation profile of the monoenergetic positron beam is examined since the effect of thermal positron diffusion is less significant at tin layers thicker than 50nm. At thin layers (<50nm), the positrons greatly accumulate either at the aluminum–tin interface or in the tin layer due to its higher positron affinity compared to aluminum. Thus a very high sensitivity of the measurement for low densities of tin is observed. Consequently from the experimental results, a sensitivity threshold for the detection of a low amount of tin in an aluminum matrix with DBS is determined. The DB results are compared to theory by an approximation for pre-thermal implantation in layered materials. [Copyright &y& Elsevier]