Positron Annihilation Spectroscopy Techniques Research Articles

Defectivity of Al:ZnO thin films with different crystalline order probed by Positron Annihilation Spectroscopy

Three Positron Annihilation Spectroscopy (PAS) techniques have been employed to investigate the point defects of Al-doped Zinc Oxide (AZO) thin films grown by Radio Frequency (RF) magnetron sputtering with different substrates and deposition parameters. The films were grown with thickness varying from 100 to 300 nm, and their crystalline quality ranged from single crystalline epitaxial to partially amorphous. We found that the main defect in the crystalline samples is the 3VZn−VO four vacancy complex, with a concentration around 1018−1019 cm−3. In polycrystalline films larger vacancy clusters, within 10%−20% of the total concentration, were detected. These vacancy clusters are inferred to be most likely located at the grain boundaries. In partially amorphous films the concentration of these larger vacancy clusters, located either at grain boundaries or in the amorphous regions of the film, approached even the 40%, and also some sub-nano voids have been observed.

Laser Powder Bed Fusion of Molybdenum and Mo-0.1SiC Studied by Positron Annihilation Lifetime Spectroscopy and Electron Backscatter Diffraction Methods.

Positron annihilation lifetime spectroscopy (PALS) has been used for the first time to investigate the microstructure of additively manufactured molybdenum. Despite the wide applicability of positron annihilation spectroscopy techniques to the defect analysis of metals, they have only been used sparingly to monitor the microstructural evolution of additively manufactured metals. Molybdenum and molybdenum with a dilute addition (0.1 wt%) of nano-sized silicon carbide, prepared via laser powder bed fusion (LPBF) at four different scan speeds: 100, 200, 400, and 800 mm/s, were studied by PALS and compared with electron backscatter diffraction analysis. The aim of this study was to clarify the extent to which PALS can be used to identify microstructural changes resulting from varying LPBF process parameters. Grain sizes and misorientation results do not correlate with positron lifetimes indicating the positrons are sampling regions within the grains. Positron annihilation spectroscopy identified the presence of dislocations and nano-voids not revealed through electron microscopy techniques and correlated with the findings of SiO2 nanoparticles in the samples prepared with silicon carbide. The comparison of results indicates the usefulness of positron techniques to characterize nano-structure in additively manufactured metals due to the significant increase in atomic-level information.

Helium-induced morphology evolution in tungsten under thermal treatment

Surface and near-surface morphology evolution of helium-irradiated tungsten due to thermal cycling up to the ITER-relevant temperature of 1350 K was studied using electron microscopy and positron annihilation spectroscopy techniques. Holes at the surface and bubbles in the near-surface of recrystallized polycrystalline tungsten samples were created by 75 eV helium plasma irradiation with the fluence of 3 × 1023 He m−2 at the surface temperature of 1073 K. Subsequent annealing experiments were combined with a detailed electron microscopy analysis to investigate the shape and density changes of holes and helium bubbles with respect to grain orientation. We show that the initially circular holes and round bubbles became faceted upon heating with 1 K s−1 ramp up to 870 K. Annealing cycles up to 1350 K induced resulted in bubbles removal in the first 5 nm below the surface and surface smoothing. Electron energy loss spectroscopy measurements allowed estimation of helium gas density and pressure inside bubbles. Positron annihilation spectroscopy allowed to investigate the nature of defects and their evolution in the helium-irradiated tungsten with thermal cycling.

Interface effect of Fe and Fe2O3 on the distributions of ion induced defects

The stability of structural materials in extreme nuclear reactor environments—with high temperature, high radiation, and corrosive media—directly affects the lifespan of the reactor. In such extreme environments, an oxide layer on the metal surface acts as a passive layer protecting the metal underneath from corrosion. To predict the irradiation effect on the metal layer in these metal/oxide bilayers, nondestructive depth-resolved positron annihilation lifetime spectroscopy (PALS) and complementary transmission electron microscopy (TEM) were used to investigate small-scale defects created by ion irradiation in an epitaxially grown (100) Fe film capped with a 50 nm Fe2O3 oxide layer. In this study, the evolution of induced vacancies was monitored, from individual vacancy formation at low doses—10−5 dpa—to larger vacancy cluster formation at increasing doses, showing the sensitivity of positron annihilation spectroscopy technique. Furthermore, PALS measurements reveal how the presence of a metal–oxide interface modifies the distribution of point defects induced by irradiation. TEM measurements show that irradiation induced dislocations at the interface is the mechanism behind the redistribution of point defects causing their accumulation close to the interface. This work demonstrates that the passive oxide layers formed during corrosion impact the distribution and accumulation of radiation induced defects in the metal underneath and emphasizes that the synergistic impact of radiation and corrosion will differ from their individual impacts.

Characterizing heavy ions-irradiated Zr/Nb: Structure and mechanical properties

In this work, the radiation responses of Zr/Nb nanostructured metallic multilayers (NMMs) are studied. The nanostructures with different layer thicknesses were deposited on Si (111) substrate by using magnetron sputtering and were subjected to heavy-ion irradiation at room temperature with different fluences. Nanoindentation, XRD, DFT, SIMS, and Variable Energy Positron Annihilation Spectroscopy (VEPAS) techniques were used to study the type and distribution of defects, and strain within the material as well as the changes in the hardness of the structures as a function of damage. Our results suggest that the strain and the irradiation hardening are layer thickness- and damage-dependent while they are independent of the type of irradiated ions. The magnitude of hardening decreases with decreasing individual layer thickness indicating that the number of interfaces has a direct effect on the radiation tolerance enhancement. For thin layers with a periodicity of 27 nm (Zr/Nb27), a transition from hardening to softening occurs at high fluence, and a saturation point is reached in thick layers with a periodicity of 96 nm (Zr/Nb96). The as-deposited thin multilayers presented a significantly higher atomic-scale disorder which increases with ion irradiation compared to the thick multilayers. VEPAS reveals the vacancy defects before and after irradiation that contribute to the presented strain. Based on the findings, thin nanostructured Zr/Nb multilayered structures possess excellent radiation resistance due to the high density of interfaces that act as sinks for radiation-induced point defects.

CHARACTERIZATION OF DEFECTS IN TUNGSTEN SAMPLES INTERESTED IN PFM STUDIES USING POSITRON ANNIHILATION AND PHOTOLUMINESCENCE SPECTROSCOPY

The defect structure of four pure tungsten samples is investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence (PL) and positron annihilation spectroscopy (PAS) techniques before and after annealing. The results of the SEM and XRD analyses show that the grain size is increased up to 43.3[Formula: see text][Formula: see text]m, and their growth in the (2 0 0) orientation occurs at 1673[Formula: see text]K. Also, the smooth of the peaks at 1673[Formula: see text]K in the PL test demonstrates the increase of the size of defects of the sample. Moreover, the results of the PAS indicate a decrease in the concentration of defects and an increase in their size at 1673[Formula: see text]K, which confirms the results obtained from the PL measurement. The collected results from the four examination techniques are in close agreement with each other and reveal that w ith the increase in the annealing temperature of tungsten samples, a gradual coalescence of the defects would happen to lead to an increase in their size and hence a reduction in their number as well as preferred grain growth in the (2 0 0) orientation and virtually perfect recrystallization of the samples at higher temperatures.

Influence of surface activation on the microporosity of PE‐CVD and PE‐ALD SiOx thin films on PDMS

AbstractThe microporosity, structure and permeability of SiOx thin films deposited by microwave plasma‐enhanced chemical vapour deposition (PE‐CVD) and plasma‐enhanced atomic layer deposition (PE‐ALD) on polydimethylsiloxane (PDMS) substrates were investigated by positron annihilation spectroscopy and complementary technique, such as X‐ray photoelectron spectroscopy, infrared spectroscopy, time of flight mass spectroscopy and atomic force microscopy. The SiOx films were deposited onto spin‐coated PDMS substrates, which were previously exposed to an oxygen plasma thus achieving the conversion of the top polymer layer into SiOx. The presence of this oxidised surface near the region led to an overall decrease in micropore density and to a shift towards smaller pore sizes within the deposited SiOx films. A correlation between the oxygen fluence during the oxygen plasma treatment and the microporosity of the PE‐CVD and PE‐ALD SiOx films could be established.

NDT study of precipitation processes in thermally aged Fe-20Cr alloy

Thermal embrittlement of high-chromium ferritic and martensitic steels at intermediate temperatures has been investigated in PM2000 (Fe20Cr) steel exposed to 475 °C by various non-destructive techniques, including magnetic Barkhausen noise (MBN), positron annihilation spectroscopy (PAS) and Mössbauer spectroscopy (MS). These techniques were used in a complementary manner to characterize the phase separation of the Fe-Cr ferrite phase into Fe-rich (α) and Cr-rich ferrite (α') phases, the phenomenon known as 475 °C embrittlement. Together with a reference steel T91, samples were subsequently isothermally annealed up to 750 h. While the MS technique clearly confirmed the formation of bcc Cr-rich precipitates in PM2000 steel with annealing time, the MBN and PAS techniques only indicated the formation of new pinning sites for magnetic domains and trapping sites for positrons in PM2000, up to 500 h annealing time, respectively. This observation was attributed to the precipitation process of Al-rich oxides in the oxide-dispersion strengthened PM2000 steel. This process, competitive to the chromium-rich α' phase precipitation, was found to be dominating the micromagnetic and positron-annihilation characteristics during the early stages of the annealing.

Study of defect production in neutron irradiated Cu, Ni and Pb using positron annihilation spectroscopy (PAS)

Abstract The study of radiation damage by neutrons is important for nuclear reactors and space programs. Doppler broadening is an important technique of Positron Annihilation Spectroscopy (PAS) for the measurement of defect production by neutrons of energy range up to 10 MeV. Doppler broadening in PAS spectra of elements Cu, Ni, and Pb has been carried out using High Purity Germanium (HPGe) detector. A 5.0 Ci Am(Be) neutron source has been used for irradiation of the samples. The available neutron flux at the irradiated position was 3.3 × 105n/cm2/sec. These samples have been irradiated for different time periods ranging from 35 days to 264 days to obtain different neutron fluence. A positron source Na22 of strength 35 mCi was used for PAS. The S and W parameters for Doppler Broadening have been calculated using SP-11 software. Scanning Electron Microscope (SEM) pictures of the samples at different irradiation state have taken for the study of surface morphology of these materials. SEM measurements of the surface of the samples show substantial changes after the irradiation. The electric resistance of these materials, for different periods of neutron irradiation, has also been measured to verify the results. This study shows that the production of defects increases with the neutron dose.

More accurate parameterization of positron implantation depth profiles for the sensitivity range of positron-based characterization techniques

Techniques that employ positron annihilation spectroscopy are powerful tools to investigate defect structures and concentrations in materials. A hindrance to experimental design and the interpretation of results lies in the lack of agreement in the literature concerning the proper form of the positron implantation profile, a function that determines the sensitivity range for all non-slow positron annihilation spectroscopy techniques. Employing the dominant 22Na isotopic source, a positron implantation profile database of 270 common materials is published. The parameters for a novel implantation profile functional form providing superior agreement with simulation are derived. Finally, and most critically, an algorithm is presented and validated, which permits utilization of the published elemental implantation profile parameters to produce the positron implantation profile for any material of interest. This tool provides rapid calculation of the sensitivity range for all positron annihilation techniques, enabling more informed experimental design and more accurate knowledge of the spatial distribution of defects in materials.

Microscopy studies on magnetron sputtered thin films with preferred orientation for positron moderation

Doppler-broadening Spectroscopy (DBS) and positron annihilation lifetime spectroscopy (PALS) are two positron annihilation spectroscopy techniques used for identifying near surface vacancies and lattice dislocations. Positrons manifest a broad energy distribution of about several MeV and in order to generate a low-energy positron beam, the positrons need to pass through a material called “moderator” so that a narrow positron kinetic energy bandwidth can be obtained, without losing too much intensity. The moderator must possess certain properties, among which a large negative positron work function and a large branching ratio to free positrons. The first can be assessed to an intrinsic characteristic of the material, that is usually correlated to the material bandgap, and in order to attain the second, a defect free, epitaxial thin film should be obtained. Due to its 3.4 eV bandgap, GaN represents a very promising candidate for positron moderation and field assisted moderation, that might manage longer average positron diffusion lengths. The aim of the present study is to obtain semi-insulating GaN thin films by magnetron sputtering GaN on different substrates (ZnO, TiO2, Al2O3) with fewer impurities incorporated, which should hold promises of increasing the positron diffusion length in the material. Preliminary depositions made in this study are to be discussed, highlighting both their advantages and limitations derived from their structure characterization.

Pore Structure Analysis of Silica-Gel Solid Nanocomposite Electrolytes with Surface Conduction Enhancement

Solid-state batteries are considered to reach the high energy densities needed for our emerging sustainable energy society. Recently, solid electrolytes (SE) with ion conductivities equalling or even surpassing that of liquid electrolyte solution are already available. However, the rigid nature of the SE powder materials caused poor interface contacts between individual electrolyte particles and with the active electrode particles. To solve this issue, a solid composite electrolytes (SCE) composed of ionic liquid (IL) and Li-salt supported on a porous inorganic matrix have been explored. [1] These composite electrolytes were prepared by sol-gel methods. The low viscous liquid sol-gel precursor can be impregnated into the porous powder electrodes and gelled in the electrodes, thus resulting in good interfacial contacts between the active electrode and the electrolyte. Further drying and curing make the composite electrodes compact, which results in high energy density. However, the ionic conductivity was typically lower than that of pure ionic liquid electrolyte (ILE) used in SCE. This is due to the increase of the IL viscosity in the nano-sized pores, which is known as IL confinement effect. [2] To overcome this problem, our group developed a new type of SCE with water-based sol-gel process, which is named “nano-SCE”. [3] We demonstrated systematic promotion of the Li-ion conductivity by controlling the IL molecules ordering on the silica pore surface with an functional adsorbed ice-water layer. The strong hydrogen bonding between the ice-water and the IL molecules induces the ordering of IL anions and cations on the silica surface. This interfacial interaction weakens the association between the Li-ion and its anion, thus enhancing the ion conductivity well beyond that of pure ILE. Further control of the porous structures will make the conductivity go higher, and thus can be a breakthrough technology to develop the high-conductive SEs. For designing the ideal porous structure and control the synthesis process of nano-SCEs, the silica nanoporous structure must be clarified. However, so far, it is difficult to characterize the silica matrix structure because the silica structure collapsed by the surface tension when the ILE is removed by rinsing with solvents such as ethanol and acetone and after drying both in ambient and vacuum. In this paper, we applied the CO2 supercritical drying method to extract ILE. A careful drying above CO2 super critical point enabled to keep the original structure avoiding the collapse due to the surface tension. The porous silica matrix was successfully obtained without damage or shrinkage. The porous structures of nano-SCEs with different IL/SiO2 material ratio were analysed by SEM, TEM, N2 adsorption/desorption and positron annihilation spectroscopy (PALS) technique. SEM and N2 adsorption/desorption analysis revealed that there are > 10 nm pores distributed, and their size and number were increased with ILE/SiO2 molar ratio. Our measurements confirmed that the IL confinement effect, that is, the conductivity of nano-SCE with smaller pore-size was lower due to the increasing of the viscosity of ILE filled in pores. In contrast, the surface area was decreased with ILE/SiO2 ratio due to the reduction of 5 nm size pores, which is characterized by PALS analysis. From the relationship between surface area and surface enhancement factor (sice-water/sdried) in Fig. 1, the surface conduction was verified to be increased with the surface area, indicating the conduction promotion effect was clearly related with the surface chemistry. A nano-SCE with higher conductivity can be realized by designing the pore structure with high surface area and optimized pore size. [1] J. Le Bideau, J. B. Ducros, P. Soudan and D. Guyomard, Solid-state electrode materials with ionic-liquid properties for energy storage: The lithium solid-state ionic-liquid concept, Adv. Funct. Mater., 2011, 21, 4073–4078. [2] S. Zhang, J. Zhang, Y. Zhang and Y. Deng, Nanoconfined Ionic Liquids, Chem. Rev., 2017, 117, 6755–6833. [3] P. M. Vereecken, X. Chen, K. B. Gandrud, B. Put, A. Sagara, M. Murata, M. Tomiyama, Y. Kaneko, M. Shimada, J. Steele, M. Roeffaers, M. Debucquoy and M. J. Mees, "Mechanism Analysis of Enhanced Li-Ion Conductivity in Mesoporous Silica-Based Solid Nano-Composite Electrolytes," ECS Meeting s, 2018, 470, MA2018-02. Fig. 1 The relationship between surface enhancement factor of nano-SCEs and BET surface area of porous silica matrix. The surface enhancement factor was defined as the ratio of the conductivity with and without functional ice-water layer on the silica surface. Porous silica was obtained by removal of ILE from nano-SCE with ethanol soaking for 36 hours at 40 oC and subsequent drying by CO2 super critical dryer. Figure 1

Location determination of metal nanoparticles relative to a metal-organic framework

Metal nanoparticles (NPs) stabilized by metal-organic frameworks (MOFs) have been intensively studied in recent decades, while investigations on the location of guest metal NPs relative to host MOF particles remain challenging and very rare. In this work, we have developed several characterization techniques, including high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) tomography, hyperpolarized 129Xe NMR spectroscopy and positron annihilation spectroscopy (PAS), which are able to determine the specific location of metal NPs relative to the MOF particle. The fine PdCu NPs confined inside MIL-101 exhibit excellent catalytic activity, absolute selectivity and satisfied recyclability in the aerobic oxidation of benzyl alcohol in pure water. As far as we know, the determination for the location of metal NPs relative to MOF particles and pore structure information of metal NPs/MOF composites by 129Xe NMR and PAS techniques has not yet been reported.

Microstructure evolution of cold-worked Zircaloy-4 under 20 mev Cu ion irradiation

ABSTRACTCold-worked Zircaloy-4, used for the fuel grid of research reactors, was irradiated with 20 MeV Cu ions at different doses varying from 0.007 to 0.15 displacements per atom (dpa) at room temperature. The microstructure evolution was investigated by grazing X-ray diffraction and positron annihilation spectroscopy techniques (PAS). The mechanical properties were studied using nanoindentation. The microstrain, S parameter and nanohardness present similar behaviour. They all increase rapidly during the beginning of irradiation (first stage) and reach saturation value at 0.15 dpa. The experimental results reveal radiation-induced lattice distortion along the (110) direction indicating an anisotropic displacement energy.

Radiation-induced changes of vacancy-type defects in ferroelectric capacitors as revealed by Doppler broadening positron annihilation spectroscopy

Thin film ferroelectric capacitors of composition Pb(Zr0.52Ti0.48)O3 were exposed to Fe3+ radiation (1011 to 1013 ions/cm2), and the change in the defect structure was investigated by Doppler broadening positron annihilation spectroscopy and other characterization techniques. As the radiation fluence increases, a systematic drop of the S parameter of the positron annihilation photopeak is observed and attributed to an increase in the Zr- and Ti-site related vacancies relative to the Pb-sites. The results demonstrate that the radiation has a more significant influence on the Zr- and Ti-sites relative to the Pb-sites. It is also observed that the S parameter of the Mn-doped samples is higher than the undoped counterparts. Coupled with ferroelectricity measurements and X-ray diffraction, the results suggest that the Mn dopant modifies the initial structure of the material and leads to a different functional response.

Fabrication of ZIF-300 membrane and its application for efficient removal of heavy metal ions from wastewater

Membrane technology is an energy-efficient approach for purification of wastewater. In this work, for the first time, a novel pure zeolite imidazolate framework-300 (ZIF-300) MOF membrane was fabricated and applied for removing heavy metal ions from wastewater. The fine structure of ZIF-300 layer was characterized by positron annihilation spectroscopy technique. Cross-flow and long-term operation test was also carried out to investigate the membrane stability. The obtained ZIF-300 membrane exhibited a high and stable performance with the water permeance of 39.2 L/m2 h bar and rejection rate of 99.21% for CuSO4. This high separation performance combined the extraordinary stability demonstrated that the as-prepared pure ZIF-300 MOF membrane could be a potential candidate for the water treatment.

Positron annihilation characteristics and catalytic performances of poly (vinyl alcohol) intercalated montmorillonite supported Pd0 nanoparticles composites

The microstructure of poly (vinyl alcohol)/ montmorillonite (PVA/MMT) matrices and Pd@PVA/MMT catalytic composite was characterized by positron annihilation spectroscopy (PAS) and other techniques, such as high resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), and X-ray photo electron spectroscopy (XPS). XRD and HR-TEM results showed that 20% was the saturation loading percentage of PVA intercalation into MMT. Ortho-positronium (o-Ps) annihilations were mainly distributed in the interlayer spacing of MMT and/or open spaces (free volume) of PVA. Below the saturation loading percentage of PVA (≤ 20%), it was found that the o-Ps annihilation lifetime became shorter after the intercalation of PVA chains into MMT layers. With further addition of PVA, the o-Ps annihilation lifetime became longer after the formation of exfoliated structure. The 20%-PVA/MMT matrix has been selected as the platform to further immobilize Pd specie for preparing novel 0.2%-Pd@20%-PVA/MMT catalytic composite. The effects of Pd immobilization on the microstructure of PVA/MMT matrices were also sensitively detected by PAS. The mean size of the micro-defects (from Tao-Eldrup model) increased from 0.28 nm (20%-PVA/MMT) to 0.29 nm (0.2%-Pd2+@20%-PVA/MMT or 0.2%-Pd0@20%-PVA/MMT), respectively. This increase in the size of the micro-defects of 20%-PVA/MMT matrix after Pd immobilization was mainly due to the replacement of the inter-molecular hydrogen bodings of PVA chains by the interactions formed between the PVA chains and Pd species. The prepared 0.2%-Pd0@20%-PVA/MMT catalytic composite showed excellent catalytic efficiency and recyclability in both Heck-type and Sonogashira-type coupling reactions. The correlations between the microstructure and performance of the 0.2%-Pd0@20%-PVA/MMT composite were thoroughly discussed.

Characterization of dose dependent mechanical properties in helium implanted tungsten

In this work, we utilized spherical nanoindentation to investigate the mechanical property changes of helium (He) implanted tungsten (W) at different damage doses. According to the indentation stress-strain curve, two dose dependent phenomena were revealed: one is an increase in yield stress with increasing damage dose, and the other is enhanced softening following yield. Experimental measurements such as transmission electron microscopy (TEM) and positron annihilation spectroscopy (PAS) techniques, as well as molecular dynamics (MD) simulation were combined to explore the intrinsic mechanism of the dose dependent phenomena. This study proposed an efficient and promising way to measure the damage of the plasma facing materials and established an effective correlation between the mechanical property changes of He implanted W and its microstructure evolutions.

Effects of Y on helium behavior in Y-doped TiH2 films prepared by magnetron sputtering

TiH2 and yttrium-doped TiH2 alloy films with different helium concentrations prepared by magnetron sputtering were investigated using ion beam analysis (IBA), X-ray diffraction (XRD), thermal helium desorption spectrometry (THDS), positron annihilation spectroscopy (PAS) and nanoindentation techniques. It was found that through increasing the concentration of doped Y atoms, strong release peaks of helium will be found at about 820 K. The release at 820 K corresponds to the binding energy of the helium state on grain boundary indicating that the majority of the helium exists in the grain boundaries. In addition, the results of XRD and nanoindentation indicate that the doping of Y in the TiH2 films can improve the hardness and the elastic modulus of the films because of the fine-grain strengthening caused by the decreased size of TiH2 grains.

Identification of defects in the transition metal oxide-doped glass nanocomposite xV2O5–(1-x)(0.05MoO3–0.95ZnO) using positron annihilation spectroscopy and other techniques

A series of transition metal oxide-doped glass nanocomposites of the common nomenclature xV2O5–(1-x)(0.05MoO3–0.95ZnO) for x=0 to 0.9 have been synthesized through the melt quenching process and characterized by X-ray diffraction, Transmission electron microscopy and optical absorption spectroscopic studies. Positron annihilation lifetime and coincidence Doppler broadening measurements are also performed to identify the vacancy type defects present in the samples of the differing V2O5 concentration. The X-ray diffraction patterns showed clear diffraction peaks indicating the presence of nanocrystallites in samples of x=0, 0.1, 0.2 and 0.3 whereas significantly broadened patterns were additionally observed in the case of samples of x=0.5, 0.7 and 0.9 reflecting a dominant amorphous nature. Transmission electron microscopy images indicated the formation of nanocrystallites of varying sizes and morphologies and the high resolution transmission electron microscopy patterns showed well defined fringes, some of them showing discontinuities in samples of larger x. Further, selected area electron diffraction patterns displayed diffuse rings superposed with bright spots, indicating the formation of nanocrystallites dispersed in an amorphous matrix. The optical band gap energy estimated from the UV–Vis absorption measurements decreased with increasing content of V2O5. Positron lifetime measurements indicated the trapping of positrons in the interfacial gaps around the nanocrystallites for lower values of x (i.e., 0 to 0.3) and then in the porous defects within the amorphous matrix of the samples of higher values of x (=0.5 to 0.9). The coincidence Doppler broadening spectroscopic measurements further confirmed the presence of vacancy type defects arising from cation non-stoichiometry and the amorphous character of the largely doped nanocomposites. The studies demonstrated defect characterization as an alternative approach in the investigation of metal oxide nanocomposite formation and evolution during compositional variations and thermal treatments.