Defect-induced effects in nanomaterials

Resume : Mass transfer between the gas phase and oxide materials play an important role in different electrochemical applications (solid oxide and protonic ceramic fuel cells and electrolysis cells, electrochemical exchange reactors etc) and catalytical systems. In this contribution oxygen and hydrogen transfer and surface exchange are considered taking into account defect formation both in the bulk and on the surface of the oxide materials. Parameters of oxygen and hydrogen surface exchange and diffusivities have been determined by means of 16O/18O and H/D isotope exchange method with gas phase equilibration. It was shown that rate-determining stage of the oxygen surface exchange is caused by the defect chemistry of the outermost layer of the oxides based on cobaltites and nikelites of lanthanides. Protonic defects, local ordering-disordering as well as nano-scale domain structure play an important role in surface exchange and diffusivity of hydrogen in protonic conducting oxides based on lanthanum scandate. Hydrogen and methane incorporation into the protonic oxides based on lanthanum scandate has been experimentally and theoretically studied for the first time. [1] M.I. Vlasov, V.M. Zainullina, M.A. Korotin, A.S. Farlenkov, M.V. Ananyev (2019). Effect of proton uptake on the structure of energy levels in the band-gap of Sr-doped LaScO3: diffuse reflectance spectroscopy and coherent potential approximation calculations. Physical Chemistry and Chemical Physics, 21, 7989. [2] E. Tropin, M. Ananyev, N. Porotnikova, A. Khodimchuk, S. Saher, A. Farlenkov, E. Kurumchin, D. Shepel, E. Antipov, S. Istomin, H. Bouwmeester (2019). Oxygen surface exchange and diffusion in Pr1.75Sr0.25Ni0.75Co0.25O4−δ. Physical Chemistry and Chemical Physics, 21, 4779. [3] E.S. Tropin, M.V. Ananyev, A.S. Farlenkov, A.V. Khodimchuk, A.V. Berenov, A.V. Fetisov, V.A. Eremin, A.A. Kolchugin (2018). Surface defect chemistry and oxygen exchange kinetics in La2–xCaxNiO4+δ. Journal of Solid State Chemistry, 262, 199. [4] M.V. Ananyev, A.S. Farlenkov, E.Kh. Kurumchin (2018). Isotopic exchange between hydrogen from the gas phase and proton-conducting oxides: Theory and experiment. International Journal of Hydrogen Energy, 43(29), 13373. [5] M.V. Ananyev, V.A. Eremin, D.S. Tsvetkov, N.M. Porotnikova, A.S. Farlenkov, A.Yu. Zuev, A.V. Fetisov, E.Kh. Kurumchin (2017). Oxygen isotope exchange and diffusion in LnBaCo2O6−δ (Ln=Pr, Sm, Gd) with double perovskite structure. Solid State Ionics, 304, 96. [6] A.S. Farlenkov, A.G. Smolnikov, M.V. Ananyev, A.V. Khodimchuk, A.L. Buzlukov, A.V. Kuzmin, N.M. Porotnikova (2017). Local disorder and water uptake in La1–xSrxScO3–δ. Solid State Ionics, 306, 82.

Resume : In this paper, we observed the fundamental as well as surface point defects based optical transitions in as-deposited cerium oxide thin film with the help of light- electron interaction, assisted by an external electric field. An electric field, acted as a perturbation, was induced across the cerium oxide thin film, which was encapsulated between the gold electrodes. The interaction of electrons with light under electric field revealed the two optical transitions, which were analysed by the Aspnes third derivative model. Based on the results, we observed the fundamental transitions associated with CeO2 thin films. Moreover, the reduced stoichiometry of as deposited CeO2 from Ce+4 to Ce+3 caused the surface point defects as revealed by XPS analysis. This results the Ce4f1 states in the band gap as observed by the electric field assisted optical spectrum.

Resume : The aim of the study is to determine the pathways of oxygen diffusion in cobaltite of lanthanum-strontium, the influence of the surface layer of materials on the kinetics of oxygen exchange. To achieve this, a set of unique methods and approaches was used, such as isotope exchange with gas-phase analysis to study kinetic parameters; X-ray photoelectron spectrometry to study the chemical composition of the near-surface region of dense ceramics (this information will allow discussing the preferred centers of oxygen adsorption on the surface of the material); scanning electron microscopy with energy dispersive analysis to assess the local chemical composition on the surface and study the crystal orientation of grains of polycrystals and single crystals. The combination of methods gives a complete picture of both the chemical composition of the surface and the stages of oxygen exchange. According to the results of the experiment, the stages of the oxygen exchange of the gas phase with oxides and the diffusion paths of oxygen that determine the velocity are determined. An analysis of the results obtained for a single crystal and a polycrystal made it possible to evaluate the effect of grain boundaries in the case of cobaltite lanthanum-strontium. The reporting study was funded by the Russian Federal Property Fund, project No. 20-33-70003

Resume : Shockley-Read-Hall statistics is but one statistic to model the recombination at a defect with two charge states, a single configuration, no excited states and no bound excitons. In reality, most defects contain more than two charge states and/or multiple configurations and/or multiple excited states. Thus, Shockley-Read-Hall is a very particular case of a more general recombination theory that describes the recombination function in all its richness (charge states, excited states, configurations and bound excitons). The problem with Shockley-Read-Hall statistics is that even if one can fit experimental value of the recombination at a certain temperature and dopant density, the result is not generalizable and will lead to an inaccurate prediction at another temperature or dopant density. The question then arises: How does one know what statistics to select in order to describe the recombination function of a defect in a 2D or bulk semiconductor? More precisely: what are the measurements required to ascertain how many charge states, excited states and configurations a defect contains? In this paper we will outline a new methodology for recombination model selection. This methodology is part of an upcoming paper and its rational is documented in our previous papers here https://doi.org/10.1016/j.solmat.2018.07.029 and here https://doi.org/10.1063/1.4906465. It allows for accurate and generalizable prediction of recombination at defects.

Resume : Recently, the use of the organic-inorganic hybrid perovskites as absorbers in solar cells has been demonstrated to provide high efficiency and stability for applications in photovoltaics. The improvement of the cell performance with a power conversion efficiency (PCE) reaching 21% [1] is thought to be linked to their optoelectronic properties of perovskites: strong optical absorption, high carrier mobility and diffusion length. However, from the defect point of view, the complex structure of the materials is prone to formation of defects, which enables charge carrier trapping, and therefore can impact on the long term stability and electrical property of the solar cells. In this work, we have investigated the role of the interface between the electron transport layer (ETL) and the perovskite absorber on the trap parameters determined by Deep Level Transient Spectroscopy technique. The structure of the studied solar cells is ITO/(ETL)/Perovskite/P3HT/Au. In order to examine the effects of the interface on the defect formation, we performed measurements on identical solar cells having ZnO or WO3 nanorod arrays for the ETL. Our results indicate that depending on the nature of the ETL used, specific and additional traps are formed, which affect the PCE of the solar cells. Comparing the two structures, low PCE and high trap density have been obtained in cells using WO3 as ETL. Understanding the structure-property relationships of perovskites may help to improve the design and thus, the performance of hybrid organic-inorganic solar cells. [1] Saliba, M., Matsui, T., Seo, J.Y., Domanski, K., Correa-Baena, J.P., Nazeeruddin, M.K., Zakeeruddin, S.M., Tress, W., Abate, A., Hagfeldt, A., Grätzel, M. Y. Energy and Environmental Science, 9 (2016), 1989–1997.

Resume : The efficiency of solid oxide fuel cells (SOFC) depends critically on materials, e.g. for the cathode where the oxygen reduction reaction (ORR) occurs. Typically, mixed conducting perovskite ABO3 (or perovskite-related) materials are used, doped with Sr to increase oxygen vacancy concentration. The dominating surface terminations are (001) AO and BO2 (relative fractions depending on materials composition, conditions). In this talk, the results of large scale first principles (ab initio) calculations for the two polar (La,Sr)O and MnO2 (001) terminations of (La,Sr)MnO3 cathode materials are discussed[1]. Surface oxygen vacancy and oxygen adsorbate concentrations strongly depend on the average Mn oxidation state, and surface polarity has to be considered. Special attention is paid to the effects of Sr doping on different steps of the ORR. The surface oxygen vacancy concentration for (La,Sr)O is more than 5 orders of magnitude smaller compared to MnO2, which leads to drastically decreased estimated ORR rates. Similar results have recently also been obtained by first principles calculations for (La,Sr)CoO3. Thus, it is predicted [2] for both prototypical SOFC cathode materials that the BO2 termination largely determines the ORR kinetics, although with Sr surface segregation (long-term degradation) its fraction of the total surface area decreases which slows down cathode kinetics. [1] Yu. Mastrikov, R. Merkle, E.A. Kotomin, M.M. Kuklja, J. Maier, J. Mater. Chem. A6, 11929 (2018) [2] E.A. Kotomin, Yu. Mastrikov, R. Merkle, J. Maier, Current Opinion in Electrochemistry, xxx (2020).

Resume : Hafnium oxide nanocrystals (NCs) can be considered as possible candidates for further miniaturization of future resistive random access memories.1, 2 The switching properties of NC assemblies remain under explored due to difficulties in fabricating close packed, long-range ordered NC films with monolayer thickness.3 Here, we demonstrate a novel method to prepare highly ordered assemblies of 6 nm HfO2 NCs, stabilized by oleic acid or trioctylphosphine oxide (TOPO) ligands, via evaporation based self-assembly. X-ray Photoelectron spectroscopy is applied to investigate the oxidation state of near surface HfOx under various conditions. Electrical transport measurements were performed on HfO2 NC devices with micrometer and nanometer sized gaps between the electrodes to determine the resistive switching character of NCs arrays. They enable the observation of cyclic voltammograms with redox reaction peaks, when using micrometer sized gaps and applying changing bias voltages and voltage step sizes. We discuss the electronic properties of these devices in the light of varying contributions of electronic vs ionic transport and highlight the effect on the device stability. We especially focus on the resistive switching behaviour of the NP assemblies which is dependent on the oxygen vacancy formation under the influence of the capping ligands.

Resume : III-nitrides are wide-bandgap semiconductors mainly used in optoelectronics and high power electronics. Currently, blue InGaN/GaN light-emitters are well established with efficiencies up to ~80 %. However, efficiency is known to decrease with increasing emission wavelength due to polarization effects and Auger losses. The introduction of In compositional gradient in InGaN/GaN multi-quantum wells (MQWs) is expected to reduce these effects, and therefore improve the efficiency of these emitters. Swift-heavy ions (SHI) are high energy (tens of MeV) ions that lose their energy mainly by electronic excitation instead of elastic collisions when passing through the material. Thus, SHI irradiation is suggested to be a solution to achieve intermixing in MQWs with a reduced density of lattice defects typically generated by ion irradiation. This work aims to evaluate the role of SHI in InGaN/GaN MQWs. InGaN/GaN MQWs and reference GaN layers grown on sapphire and irradiated with 129Xe SHI (fluence of 2×1012 ions.cm-2) at different energies were studied by optical spectroscopy techniques, such as transmission, µ Raman, photoluminescence (PL) and PL excitation. It is found that Xe SHI irradiation generates crystal damage, evidenced by the activation of GaN phonon DOS and a redshift of the GaN near band edge. Also, the creation of an electronic state, related to InGaN and involved in the excitation of the MQWs green emission, is demonstrated independently on the SHI energy.

Resume : When magnetic or electric ordering in the solid state is too weak to be retained upon removal of the external field, relaxation occurs without remanent magnetization (super-paramagnetism) or polarization (ferroelectric relaxors). Notably, a “ferroelastic relaxor”, i.e. a solid in which ferroelastic domains produce anomalously large elastic compliance in an otherwise stiff material, has, to date, not been reported. Here we report on the observation of elastic domains with relaxor behavior in Sm- or Gd-doped ceria at <5 mol% dopant concentration. The evidence is the following. Alternating electric field (5-30 kV/cm) with f > 50 mHz produces fully reversible electrostrictive contraction in the direction of the field, while for f <50 mHz, the material expands. DC bias, irrespective of direction, causes cubic lattice elongation by ≤ 90 ppm during a few hours. Following DC field removal, induced strain relaxes to its initial state during a few days at 25 °C or a few hours at 100 °C. The same sample can be cycled many times. This behavior, as well as data on the anelastic and electrostrictive response as a function of frequency, is consistent with the material being a “ferroelastic relaxor”. In situ XAS measurements at room temperature under external electric field suggest the existence of elastic nano-domains with a characteristic size of 3/2 unit cells (~0.8nm).

Resume : In this work, we study response of the Al2O3 and MgO single crystals to electronic excitations following swift heavy ion impact. The used ion beam energy (23 MeV I) is below the ion track formation threshold, but grazing incidence irradiation results in formation of long surface tracks in both materials. We study these ion tracks using range of experimental techniques and by means of molecular dynamics simulations. In addition to possible recrystallization during cooling down of the highly excited matter, we have identified properties of molten material that can also play significant role in this process, and may lead to unusual ion track morphologies.

Resume : Nanosized core-shell particles where the shell acts as a tunnel barrier are a promising concept of Sr2FeMoO6-d (SFMO) magnetic tunnel junction devices. In this work, we use the citrate-gel technique for the synthesis of nanosized SFMO powders. SEM images yield a mean particle diameter of about 75 nm. Single-phase SFMO powders were pressed into tablets under a pressure of 4 GPa at 530°C for 1 min. SrMoO4 intergrain tunneling barriers were formed at the surface of the SFMO nanograins by annealing in an Ar flow. After 5 h annealing, XRD reveals the appearance of a small amount of the SrMoO4 phase. The ongoing Mo oxidation was monitored also by XPS. Thereby, an increase of the fraction of Mo6+ ions was obtained from 68% in the as-fabricated sample up to 79% after 5 h annealing. A recovery of the oxidized samples by annealing at 900°C in a 5%H2/Ar flow returns the Mo6+ fraction to the as-fabricated state. In order to exclude any changes in the bulk of the nanograins, magnetization measurements were carried out in the temperature range 4.2-600 K in a magnetic field of B = 0.86 T. All samples were ferrimagnetic with a Curie temperature of about 424 K. The annealing did not show an appreciable change of the magnetic properties giving evidence of a lack of sufficient changes in the grain bulk. The resistivity behavior of composite material Sr2FeMoO6/SrMoO4 up to about 220 K was described by the fluctuation induced tunneling model with a barrier height of ~20 meV, a barrier width of 1.24 nm and a barrier area of 300 nm2. Near room temperature, when the thermal energy kT overcomes the energy barrier, the electrical conductivity changes to a Mott variable range hopping mechanism. This work was supported by the European Union project H2020 - MSCA-RISE-2017-778308- SPINMULTIFILM.

Resume : Compared with traditional metal oxide lithium-ion battery cathodes, nanocarbon-based cathode materials have received much attention for potential application in lithium-ion batteries because of their superior power density and long-term cyclability. However, their lithium-ion storage capacity needs further improvement for practical applications, and the trade-off between capacity and conductivity when oxygen functional groups as lithium-ion storage sites are introduced to the nanocarbon materials needs to be addressed. Here, we report a sequential oxidation-reduction process for the synthesis of single-walled carbon nanotubes for lithium-ion battery cathodes with fast charging, long-term cyclability and high gravimetric capacity. A lithium-ion battery cathode based on highly exfoliated (dbundle<10 nm) and oxygen-functionalized single-walled carbon nanotubes is obtained via the modified Brodie’s method using fuming nitric acid and a mild oxidant (B-SWCNTs). Post treatment including horn sonication and hydrogen thermal reduction developed surface defects and removes the unnecessary C–O groups, resulting in an increase in Li-ion storage capacity. The B-SWCNTs exhibit a high reversible gravimetric capacity of 344 mAh g-1 at 0.1 A g-1 without noticeable capacity fading after 1,000 cycles. Furthermore, it delivers a high gravimetric energy density of 797 Wh kgelectrode-1 at a low gravimetric power density of 300 W kgelectrode-1 and retains its high gravimetric energy density of ~100 Wh kgelectrode-1 at a high gravimetric power of 105 W kgelectrode-1. These results suggest that the highly exfoliated, oxygen-functionalized single-walled carbon nanotubes can be applied to lithium-ion batteries designed for high-rate operations and long cycling.

Resume : An original approach has been presented to model the regularities and parameters of resistive switching based on the kinetic Monte Carlo (kMС) 3D simulation of stochastic migration of oxygen vacancies/ions in metal-oxide memristive devices promising for applications in emerging nonvolatile memory, in-memory and neuromorphic computing systems. The efficiency and flexibility of the approach is demonstrated by the examples of experimentally realized Au/oxide/TiN memristive device structures, in which yttria-stabilized zirconia (ZrO2(Y)) polycrystalline films and amorphous SiOx columnar films obtained by magnetron sputtering are used as the switching oxide material. The proposed approach combines the universal kMС framework for ion migration and relatively simple physics-based methods for taking into account additional factors related to the structure and morphology of specific oxide material, interface phenomena and energetics of electronic processes, and therefore does not require time-consuming calculations and is not critical to the computing power. A comparison with experimental data on electroforming and resistive switching obtained for the corresponding memristive device structures in our group is also presented. The work was supported by the Government of the Russian Federation (contract No. 074-02-2018-330(2)).

Resume : The results of calculating the dose of silicon amorphization by light ions (He+, B+, N+, Si+ and P+) depending on the parameters of the ion implantation (ion energy (50 – 100 keV), ion current density (0.4 – 10 mA/cm2), irradiation temperature (100 – 500 K)) are presented. The dose of amorphization was the radiation dose at which the calculated total concentration of vacancies and divacancies created by ions reached a level equal to 10% of the atomic density of silicon. The generation of vacancies was calculated in the Gaussian approximation taking into account the intensity of their creation in the Kinchin-Pisa approximation depending on the type and energy of the ion. It is also possible to use data from the SRIM program. The kinetics of vacancies and divacancies was calculated in the framework of the diffusion-coagulation model, taking into account the diffusion of vacancies and their reaction with each other with the formation of divacancies. The calculation was carried out on a computer complex created by the authors. The results showed satisfactory agreement with the experimental data available in the literature. The results of the effect of a protective film of silicon dioxide on the dose of amorphization of a silicon substrate are also presented.

Resume : Graphene is a single sheet of carbon atoms forming a honeycomb lattice (HCL). The T3 or dice lattice presents the same structure as HCL with an additional site at the center of each hexagon. The corresponding band structure is similar to that of HCL with an additional flat band crossing the Dirac points. By applying a compressive uniaxial deformation on the dice lattice, the Dirac cones move toward each other until they merge into a single point before a gap opens. This merging of Dirac points signals a topological transition from a semi-metallic phase with two Dirac cones to an insulating phase with a gap. We theoretically study the Klein tunneling effect across this transition. We focus on the effect of the deformation on the transmission across a potential step and on the conductivity. Particularly, for energy equal to half of the step height, we show a transition from a perfect transparency of the junction, known as super Klein tunneling effect [1], to an opposite effect, called anti-super Klein tunneling effect [2], where the step becomes opaque. References [1] D.F. Urban, D. Bercioux, M. Wimmer and W. Hausler, Phys. Rev. B 84, 115136 (2011). [2] Y. Betancur-Ocampo, F. Leyvraz, and T. Stegmann, Nano Lett. 19, (11), 7760-7769 (2019)

Resume : Y3Al5O12, single crystals are known for their interesting properties such as high radiation-resistance, high melting point and high thermal conductivity. Consequently, they are candidates to several technological applications such as fusion energy devices and nuclear applications. Irradiation of single crystal Y3Al5O12 in the reactor (i.e. fast and thermal neutrons and gamma radiation) produces many color centers (such as F-type center, interstitials [1-5] and many others) in the material. Similar defects are also formed by heavy ion irradiation or by fast electrons, while only electronic processes are important in the case of UV, X-ray and low energetic electron irradiation.. In this presentation we report the results of the thermostimulated luminescence measurements, performed between 300 and 720°K of the stored energy in Y3Al5O12 single crystals, irradiated by fast neutrons with fluences of 2.1 x 10\17 or 2.18 x 10\19 n/cm2, or also by 1.8 MeV electrons, or thermochemically reduced. A clear pronounced dose effect was found and analyzed. In particular, four TSL peaks were observed in Y3Al5O12 samples subjected 2.18 x 1019 n/cm2, while in sample subjected 2.16 x 10\17 n/cm2, only three TSL peaks were detected. A comprehensive kinetic analysis of the glow peaks in Y3Al5O12 is performed. As usual, each TSL peak is characterized by the appropriate activation energies, which both crystals are 0.8 – 1.3 eV. The obtained values are compared with the appropriate activation energies for F-type center annealing in neutron- and heavy-ion irradiated Y3Al5O12 as well as also with similar TSL data for Al2O3 and Y3Al5O12.

Resume : In current work, cathodoluminescence properties of the acceptor-doped BaZrO3, namely Ba(Zr0.94Y0.06)O3-δ , have been studied between 78-300 K and compared at the same condition (10 keV electron excitation) with two other materials: well-studied perovskite SrTiO3 (Eg = 3.3 eV) and pure and indium and thallium - doped potassium bromide KBr. In last case, the F color center efficiency was also studied as function of electron dose and impurity doping concentration.

Resume : It has been demonstrated that composites reinforced with Boron, Nitrogen or Carbon elements in the form nanostructures dispersed in a matrix can provide radiation shielding for different range of energies and without the generation of harmful secondary particles. On the other hand, polymers reinforced with carbon nanotubes exhibit electrical response, strongly dependent on the absorbed dosage of radiation. We are proposing to study radiation-shielding nanocomposites that will not have a stand-out performance as barrier materials but will incorporate an inbuilt sensor that can be used for protecting the inside from gamma harsh radiation. The idea is to implement a design that comprises electrical sensing layer, shielding layer made of epoxy/boron nanoparticles composite and, ultimately, a critical irradiation level indicator. This indicator is essentially carbon nanotubes doped epoxy resin plate that exhibits a percolation effect. The un-irradiated plate can be conductive or not, depending on the CNT doping level. If the level is above the threshold, the current is flowing across the plate, but that the conductive path can be destroyed by high enough dosage of radiation. Thus, one has a simple circuit breaker that will immediately signal about critical exposure, in case the protection layer can no longer stop the incoming rays. Experimental and theoretical studies of such structures will be discussed.

Resume : Two-dimensional transition metal dichalcogenides (TMDs) are known to have remarkable optical and electrical properties which makes them be promising materials for optoelectronic and nanoelectronic devices. The performance of TMDs is expected to be easily affected by the defect density. However, defect-related Raman studies of TMDs are rarely done. In the case of tungsten disulfide (WS2), recently, a study of the defect-related Raman scattering like D mode of graphene has been reported. Here, we controlled the density of vacancies in monolayer WS2 and identified the relationship between the amount of vacancy and Raman mode of WS2. The quantifying of the vacancy through D mode of WS2 can be utilized to characterize the quality of monolayer WS2

Resume : Semiconductor metal sulphide based on earth abundant and non-toxic elements have been the subject of numerous studies because of their potential application, particularly in the photovoltaic and optoelectronic fields. Sb2S3 thin films have been deposited at room temperature by thermal evaporation method using Glancing Angle Deposition (GLAD) technique at different incident angles α set as 00°, 20°, 40°, 60°, 75° and 85°. In our work, we studied the structural, morphological and optical properties of these obtained films and discuss the effect of the defect induced in the density of films using different characterization techniques such as X-Ray diffraction, SEM and UV-Vis-NIR spectroscopy. The results have showed that the morphology and anisotropy of the films strongly depended on the deposition angle. Furthermore, SEM micrographs revealed a direct dependence of the morphological features such defects due to the strong shadowing effect produced by the Glancing Angle Deposition (GLAD) technique. These anisotropic effects and their relation to the morphology of samples are discussed.

Resume : Silver nanoparticles is considered as transparent binary oxide. They are applied in different fields of nanoscience and technology. Glancing Angle Deposition (GLAD) method was used to prepare silver oxide thin films for different angles α set as 00°, 20°, 40°, 60°, 75° and 85°. In this study, AgxO thin films were deposited on glass substrates then they were annealed at 300°C for 3 hours in air atmosphere and another annealing in oxygen rich atmosphere at 400°C for 5 hours .The microstructure was measured by using scanning electron microscope (SEM) where different angle deposition effectively influences the surface morphology of the films and the porosity increases with the increase of angles. In addition, the transmittance was measured using a spectrophotometer over the wavelength range of 300–1800 nm where we have low transmittance at 00° and high transmittance at 85°. We were evaluated the crystal structure of silver oxide by X-ray diffraction that show a metallic behavior for 00° and 20°C and the other angles given silver oxide for the samples annealed at 300°C for 3 hours in air atmosphere. For annealing in oxygen rich atmosphere at 400°C for 5 hours, we obtained amorphous phase at 75° and 85°. The band gaps of AgxO thin films are around 3.52 eV and 3.66 eV and all the samples exhibited a direct optical transition. The results have showed that AgxO had a high absorption coefficient (α > 104 cm-1). Thus, we explored also the relationship between the two angles α, angle of deposition, and β, angle of the columns.

Resume : Silicon is universal platform for immobilization of sensing layers for various applications. The possibility of modifying silicon with various metals allows it to be used as the basis for sensor devices. Numerous works describe the method of electrodeposition of noble metals on the silicon surface to form a microrod array. However, these ones have various limitations, including the size of the formed nanoparticles and morphology, the unevenness of the coating thickness, etc. In this work, a galvanic deposition method was used to obtained gold nanotracks on a silicon surface of n- and p-types from a salt of AuCl3 • (H2O) in aqueous solution. The concentration of gold ions in the initial solution ranged from 0.00125 to 0.01 M with the subsequent addition of 5 M HF acid in a 1:1 ratio. The morphological properties of the resulting structures depending on the gold solution temperature, concentration and time of deposition were studied as well as their structural properties. The obtained gold nanotracks demonstrated the potential use for Raman signal amplification on a test analyte Methylene blue (C16H18ClN3S) with a concentration of 10-6 M. Thus, there is a possibility of using gold nanotracks as SERS-active surfaces for sensors. The authors acknowledge the support of the work in frames of H2020 - MSCA - RISE2017 - 778308 - SPINMULTIFILM Project, the Scientific-Technical Program “Technology-SG” [Project No. 3.1.5.1].

Resume : Recently, ion-track porous templates based on silicon and silicon oxide have been used to obtain new functional materials SiO2/Si. For example, noble metal nanostructures were grown in SiO2 pores on a silicon substrate, which were used to enhance the Raman signal to register low concentrations of molecular substances. In the present work, a similar technology was used to obtain nickel structures in the pores of this template. Such studies have already been carried out, but they were not focused on studying the surface morphology of magnetic structures and identifying the prospects for their use. By changing the parameters of electrochemical deposition, nickel structures with fractal morphology were obtained in the pores of the SiO2/Si template. Peculiarities of the preparation of SiO2(Ni)/Si heterostructures are revealed, and the dependences of the structural and magnetic parameters of such systems on the electrochemical deposition of metal into the pores of the SiO2/Si template are determined. Such nickel structures with developed fractal morphology can serve as a solution to the problem of wastewater treatment from magnetic impurities. In addition, solid-state heterostructures SiO2(Ni)/Si where nickel is strongly fixed in the pores of a dielectric SiO2 can potentially act as reusable catalysts for chemical reactions. The authors acknowledge the support of the work in frames of H2020 - MSCA - RISE2017 - 778308 - SPINMULTIFILM Project, the Scientific-Technical Program “Technology-SG” [Project No. 3.1.5.1].

Resume : Recently, transition metal doped nanophosphors have attracted great attention in the field of luminescent materials for advanced solid-state lighting and high-resolution display applications. However, further improving their fluorescence properties especially for nanomaterials still remain a challenge for easy commercialization of these devices. In the present work, highly efficient blue emitting Mn2+ doped BaAl2O4 nanophosphors were successfully synthesized by simple hydrothermal technique. The nanophosphors were then characterized by using the X-ray powder diffraction (XRD), Rietveld refinement of XRD, Field Emission Scanning Electronic Microscopy (FESEM), and Photoluminescence spectroscopy (PL). The as prepared nanophosphors have an average crystallite size of 40-50 nm. The nanophosphors display intense blue emission having two peaks with maxima at around 415 nm and 440 nm under 365 nm near UV excitation which enhances with increasing Mn2+ doping concentration. Color purity of the samples are also enhanced due to the increased doping concentration of the transition metal activator in the BaAl2O4 matrix. Obtained results prove that this nanophosphor is suitable to compete in the rapidly growing field of solid-state lighting and field emission display devices

Resume : The surface-modified materials give the improved dispersion of nanoparticles, and the defect such as etched site is one of the typical factor. The etched hexagonal boron nitride (h-BN) was gained by using cobalt precursor, and the defect caused by catalytic etching is located in N-vacancy. In selective catalytic reduction (SCR) technology, aggregation of the catalyst generally degrades the performance of converting nitrogen oxides (NOX) into harmless gases. To avoid such performance degradation and to achieve a noticeable improvement in SCR performance, even dispersion of the catalyst on the support is required. We synthesized MnOx-CeOx on etched h-BN and studied the dispersion behavior caused by defect in the support. To enhance the effect by surface modification, the etched h-BN flakes were exfoliated by ultra-sonication treatment and were further modified by controlling the surface zeta-potential. Consequently, the specific surface area gradually increased from 41.89 to 67.29 m2/g as the support was modified step by step. The catalytic performance for NOX removal shows 99.8% at a relatively low temperature, ranging between 175 and 225 °C, when the defective h-BN supported catalyst.

Resume : ZnO is one of the strong candidates for the sensors and detectors for the UV emission. Luminescence characteristics of nano-wire and thin-film ZnO was investigated using cathodoluminescence in transmission electron microscopy (TEM-CL). Ultraviolet (UV) emission revealed reduction with time under electron beam, while the visible light did not show degradation of the luminescence intensity. UV luminescence decreased exponentially leaving 30% of intensity after 200 seconds exposure of 200keV electrons, with 12 pA/cm2 current density. The luminescnece intensity was partially recovered when the electron beam was temporarily off. Half-life of the luminescence was measured by turning electron beams ON and OFF for a fixed duration. For the first and second cycles of the electron beam showers, half-life time was measured as 26 seconds. Luminescence degradation from near-band-edge emission (3.29 eV), donor-acceptor-pair transition(3.17 eV), and emission from defects (~2.33 eV) were compared. In-situ luminescence of electrically biased ZnO thin film was characterized using TEM-CL, where localized luminescence was observed.

Resume : One of the main challenges in developing and employing novel 2D materials in the industry is properly probing and efficiently differentiating different few-layered structures. Out of many techniques that may be used to characterize such layered nanomaterials, Raman spectroscopy continues to be one the most reliable in characterizing several of their properties, such as strain, density of defects and number of layers. With this concern, in this work, we show that through the proper use of nearly-resonant Raman spectroscopy, one may unequivocally identify and differentiate layered pristine or doped WS2 structures by probing the emersion of A1g band splitting and essentially count the number of layers using the A1g – A1g* separation value and the related intensity ratio. Also, in this work, we have studied the dependence of this split on the excitation energy and compared its occurrence with ultra-low-wavenumber Raman scattering, Photoluminescence, and Atomic Force Microscopy.

Resume : The efficient solar-driven energy production continues to attract great interest in the last years. Sunlight-driven water splitting is one of the most promising pollution-free strategies for production of hydrogen. Photocatalytic water splitting consists of water decomposition into hydrogen and oxygen by a reaction with photo-generated charge carriers. Band engineering is one of the ways for development of the optimal photocatalytic material with both strong visible light absorption and high charge mobility. Band gap of efficient photocatalyst must be above 1.23 eV, to achieve water splitting, but less than 2.7 eV, to use efficiently a visible light. Moreover, the special conditions exist also for energy positions for boundaries of valence and conduction bands. In this study, we used ab initio (first-principles) calculations, to investigate the structural and electronic properties of SrTiO3 (STO) perovskite photocatalyst (band gap 3.25 eV) and to modify its electronic band structure in desire direction by means of defects and impurities. Unrestricted DFT (open-shell) calculations were performed with the CRYSTAL17 computer code within the linear combination of atomic orbitals (LCAO) approximation with using B1WC advanced hybrid functional. We considered first the bulk STO crystal and its (001) slabs, and then employed a supercell model to simulate point defects (oxygen vacancies and nitrogen substitutional atoms). Our computations demonstrate that creation of such defects indeed makes STO photocatalyst more efficient for sunlight-driven water splitting. The calculated bandgap values of stoichiometric STO and reduced SrTiO(3-c) were compared with the optical bandgaps, determined experimentally by means of diffuse reflectance spectroscopy and Kubelka−Munk method.

Resume : ZnSe nanocrystallites were obtained by chemical deposition in the track temple a-SiO2/Si-n. Solution with Na2SeSO3) as a source of Se ions and ZnCl2 was used for ZnSe deposition. The deposition was carried out at one temperature (750C) for 40, 60 and 90 min. XRD analysis showed formation of ZnSe nanocrystals with sphalerite structure, spatial group F-43m (216). The spectra of photoluminescence and current-voltage characteristics of samples were measured. Computer simulation of ideal and defective ZnSe nanocrystallites was performed.

Resume : The aim of this study is to examine the effect of the formation of surface nickel-silicide films on the profiles of depth distribution of impurity boron atoms in silicon and on the bulk resistivity of Si. Single-crystal p-type Si(111) wafers (KDB-10) with a thickness of ~0.5 μm were used. A NiSi2/Si( 111) film with thickness θ= 50–150 Å was formed by implanting Ni+ ions into Si with annealing, and films with thickness θ= 100–1000 Å were fabricated by solid-phase deposition of Ni atoms with subsequent heating. In vacuum of at least 10–7 Pa. The elemental and chemical composition of the surface was determined by Auger electron spectroscopy (AES), and the depth distribution profiles of impurity atoms were determined by AES in combination with layer-by-layer etching with Ar+ ions with E0= 2 keV incident at an angle of 5°–10° to the surface. All these experiments were performed at room temperature. The obtained experimental data demonstrate that the formation of a silicide film on the surface of Si results in migration of atoms of the intrinsic p-type impurity towards the silicide. The room-temperature bulk resistivity of silicon increases by a factor of 3–4 in the process. This increase in is observed if the depleted layer has a thickness of ~800–1000 Å.

Resume : CdTe nanocrystals were chemically deposited into to a-SiO2/Si -n track template. Chemical deposition of CdTe was carried out using a sulfate solution (1M CdSO4 + 1mM TeO2) at room temperature during 1 hour. X-ray diffraction analysis showed the formation of CdTe nanocrystals with a hexagonal crystal structure (wurtzite). Photoluminescence (PL) was excited by a HeCd laser (λ = 325 nm). The emission bands with maxima 2.25 and 3.3 eV were observed.

Resume : Nanodiamonds are one of the most biocompatible nanoparticles, what opens up wide opportunities for their use in biomedicine. Doping of the nanodiamond cores with impurity atoms can provide them with new properties. This paper presents the results of a study of the recently discovered property of boron-doped nanodiamonds to heat their surroundings under the influence of laser excitation. For the nanodiamonds with the size of 7-11 nm, it was shown on water and hen egg white protein that these biocompatible nanoparticles allow the implementation of both local hyperthermia and thermal ablation modes. Test of the antiproliferative activity and successful approbation of the hyperthermic ability of boron-doped nanodiamonds were carried out in relation to human breast cancer MCF-7 cells with the employment of the MTT method. This study has been performed at the expense of the grant of Russian Science Foundation project No 17-12-01481.

Resume : The ionoluminescence (IL) technique is recognized as one of the most efficient tool for real-time characterization of irradiating insulating materials. The luminescence decay measured with picosecond resolution in LiF crystal during high energy V and Kr ion s irradiation. It may provide very important information about energy dissipation processes and early stages of radiation damage formation in LiF.

Resume : A number of 2D noble metal and transition metal dichalcogenides have been characterized with respect to their optoelectronic properties during the last decade. However, recently, a new representative of this family, PdSe2, has been synthesized with intriguing pentagonal morphology. Two methods were utilized: exfoliation from its bulk crystal and molecular beam epitaxy deposition on the graphene surface [1,2]. Surprisingly, but the experimental band gaps demonstrated a good correlation with theoretical results calculated at the PBE level. Here, using combined density functional and accurate many-body perturbation approaches at the GW-BSE level, we demonstrate that such good agreements were accidental and electronic properties could likely be influenced by the presence of defects in the mechanically exfoliated monolayer and by proximity induced effects caused by the graphene support, thereby resolving inconsistencies between experiment and earlier theory. The predicted fundamental band gaps of single- and bilayer PdSe2 are 2.55 and 1.89 eV with the main contributions from Se 4pz and Pd 4dz2 orbitals to valence band maximum and Pd in-plane orbitals to conduction band minimum. The optical properties calculated including electron-hole interactions reveal the band gap of monolayer PdSe2 significantly decreases due to a large excitonic binding energy of 0.65 eV comparable to that of MoSe2. The calculated absorption spectra of both mono- and bilayer PdSe2 cover a wide region of solar flux demonstrating promising application in solar cells and detectors. The absorption spectra cover a wide region of the solar flux, indicating promising applications for solar cells and detectors.

Resume : Engineering the electronic energy band structure of hybrid nanostructured semiconductor materials through judicious control of their atomic composition is a promising route to increase visible light photoresponse, which is necessary prerequisite for many photocatalytic processes, e.g. photocatalytic water splitting to H2 and O2. In this study we have carried out the first principle calculations to simulate the electronic structure of hybrid ZnS/WS2 nanotubes containing intrinsic sulphur vacancies. Ab initio modelling reported here have been performed within the formalism of hybrid Density Functional Theory and Hartree-Fock method using HSE06 exchange-correlation functional, which is properly adapted and verified relatively to properties of ZnS and WS2 bulk and nanosheets. Pristine ZnS/WS2 nanotubes has been predicted to be attractive for photocatalytic water splitting under influence of solar light. The defect induced levels at the band gap of ZnS/WS2 nanotube containing sulphur vacancy are properly aligned relative to the oxidation and reduction potentials necessary for water splitting under visible light irradiation. The edges of band gap of defective ZnS/WS2 nanotube correspond to the range of visible spectrum. Funding from Latvian Council of Science fundamental and applied research project Nr. LZP-2018/2-0083 is greatly acknowledged.

Resume : Thin films of WO3 were grown by thermal oxidation of different tungsten substrates at 400 °C, under 5 and 590 Torr oxygen pressure. Two of the WO3 films were exposed to helium plasma. Ellipsometric measurements and modelling on obtained samples were performed in order to determine the thicknesses of the films, consolidating the thicknesses (between 20 and 270 nm) estimated previously by FIB cross-section method. Also, the specular reflectance of the WO3 thin films in the 190-2100 nm range has been obtained by spectroscopic ellipsometry, showing a decreasing with WO3 layer thickness, especially at low wavelengths. As a result of the bombardment, we can observe an increase of the modifications/disturbances of the crystalline structure that will be discussed by using the models obtained by the spectroscopic ellipsometry technique.

Resume : β-Ga2O3, is a transparent semiconductor with a wide band gap of ~4.9 eV and a high breakdown voltage of about 8 MV/cm, recognized as a promising material for different electro-optical applications [1]. In this work, the effect of proton irradiation on the Cr3+ luminescence in β-Ga2O3 crystals doped only with Cr and samples co-doped with Cr and Mg was studied. In co-doped samples, the photoluminescence (PL) spectra at room temperature (RT) are dominated by strong narrow lines overlapped on a broad band assigned to the Cr3+ intraionic emission, namely to the 2E→4A2 and 4T2→4A2 transitions, respectively. In these samples, the Cr3+ luminescence is quenched with increasing proton irradiation fluence. On the other hand, the PL spectra of the single doped samples do not present any Cr3+ related emission lines at RT. Interestingly, in these samples the intraionic Cr3+ emission could be activated by the proton irradiation and increases with the fluence. In order to understand this distinct behavior, a study was performed using different complementary characterization techniques, namely, XRD, PL, Raman, PIXE and EPR. The results are discussed considering the influence of the Mg and the defects induced by proton irradiation on the Fermi level position, the crystal field around Cr ions and the mechanisms of energy transfer to Cr3+. [1] Higashiwaki M and Jessen G H 2018 Guest Editorial: The dawn of gallium oxide microelectronics Appl. Phys. Lett. 112

Resume : Lliquid outflows contaminated with radionuclides, either coming from spent reprocessing fuel plants or from accidental cases such as Fukushima Daiichi, have to be treated. Radionuclides soprtion on solid substrate is very attractive due to the immobilization of radiative compounds on a solid phase that can be added in a second step in conditioning matrices such as glass, cement or bitumen. However, since a couple of years alternative processes avoiding the second step are under investigation, especially for volatile radionuclides such as Cesium or Iodine. One of these alternative processes is to consider the substrate used for the decontamination step as a conditioning matrice through quite simple treatment. These last years, the ICSM has developed several kinds of hierarchical porous silica, functionalized in order to selectively uptake radionuclides with for example the selective sorption of Cs towards Na. The benefits of confining radionuclides in pores comes from the fact that these substrates can be densified quite easily (moderate heating, pressure or irradiation). This talk will give you an overview of the synthesis routes developed to design these materials (post functionnalization of existing porous glass beads, emulsion templated monoliths, nanoparticles…) and the first studies undertaken to densify these porous materials and make it promising solutions for « all-in-one » separation/conditioning process.

Resume : Laser-induced breakdown spectroscopy (LIBS) is being developed as a viable tool for fuel retention monitoring in plasma-facing materials. A comparative study of surface modification performed by ablation with ns- and ps-pulse lasers is presented. Ablation crates have been measured using a stylus profiler. Evolution of the craters created by different number or laser pulses has been shown. Ablation rates in vacuum and ambient pressure have been discussed.

Resume : The role of charge traps is of great practical importance for microelectronics and sensor technologies. Gadolinium oxide (Gd2O3) is a wide-gap insulator and is well known as an effective charge trapping material. In this research charge trapping centres in thin Gd2O3 films were investigated. Nanocrystalline 700–900 nm thick Gd2O3 films were deposited on Si/SiO2 substrates by an extraction-pyrolytic method. To study charge trapping, the films were irradiated by weak electrons with energies up to 1.5 keV and current density of approximately 0.1 µA/cm2. For irradiation, a thermocathode source mounted inside a vacuum chamber (10-3 Pa) was used. Immediately after irradiation, photoelectron emission (PE) from the films was measured. The PE was excited by ultraviolet photons of 4–6 eV energy and the dependence of the PE current on the photon energy was recorded. It was found that the intensity of PE current increased with the electron irradiation time. The dependence curve of the PE current versus photon energy had two inflection points. The inflection points can be associated with the depth of charge traps. The maxima in the density of states of these traps are supposedly located at 5.2 eV and 5.7 eV below the vacuum level. The obtained results suggest that the weak-electron-irradiation method, followed by subsequent PE measurements, is a useful tool for characterizing charge traps in wide-gap insulators.

Resume : In this work we review recent results of modelling of formation of trap states and ideas on the optimal way of passivation based on these results. Passivation of quantum dots is a crucial condition determining their optical properties. Dangling bonds on the surface (surface defects) of nanocrystals were commonly considered as the main source of trap states. Recent studies clearly indicate that presence of dangling bonds not always lead to formation of trap states. We also present a new idea on formation of trap states, which considers effect of ground state dipole moment. Results obtained via density functional theory and DFTB approximations indicate the correlation between dipole moment value and formation of deep trap states. Correlation between dipole moment value and deepness of trap states location was demonstrated using different CdS cluster as a model basis and different number of SH groups as passivating agents. Namely, the high values of dipole moment provide higher number of trap states. Rearranging of the same number of SH groups also indicates dipole moment effect on electronic spectra. Application of electrostatic field oriented against dipole moment vector also confirms importance of dipole moment in formation of optical properties of nanocrystals.

Resume : The architectural diversity realized by plasmonic nanostructures is in large part due to seed-mediated colloidal growth modes that are seeded, not only by single-crystal seeds, but by seeds with a well-defined internal defect structure. Multi-twinned seeds have, for example, been used to generate colloids with icosahedral and decahedral structures while seeds with planar defects realize nanoplate geometries. Recently, we demonstrated a nanofabrication route that synergistically combines nanoimprint lithography, directed assembly, and liquid-phase epitaxy to obtain periodic arrays of complex noble metal nanostructures over a square centimeter area. This benchtop process leverages a synthetic strategy in which seed-mediated liquid-phase growth modes are carried out on substrate-immobilized seeds. While the strategy has led to the generation of substrate-based structures with considerable architectural diversity, it is fundamentally limited by the inability to fabricate seeds with the same internal defect structure as those routinely used in colloidal chemistry. Here, we demonstrate a large-area processing route for generating substrate-based Au seeds lined with stacking faults and use them to synthesize arrays of epitaxially aligned Au nanoplates using a plasmon-mediated growth mode. The work advances the possibility of bringing an exciting nanoplate chemistry to the substrate surface and, in doing so, provides the building blocks needed to enable on-chip plasmonic devices.

Resume : Exohedral van der Waals (vdW) hybrids comprising carbon nanotubes (CNT) and fullerene molecules (C60) may be used for engineering of carbon nano-materials with tunable physiochemical properties. As the hybrids preserve the sp2 hybridization of the components, superior electron transport, low percolation threshold, good electron accepting and efficient singlet oxygen sensitizing ability are expected. These hybrids may be useful as electron acceptors and charge carriers in bulk-heterojunction organic photovoltaics (OPV). In the reported study we demonstrate that induced SP3 point defects in CNT (via UV/O3 treatment) can be used for controlling the nano-structure of CNT- C60 hybrids. We describe a two-step process for assembly of CNTs networks and thermal (vacuum) sublimation of fullerenes nanocrystals. Transmission electron microscopy indicates that the hybrids comprise of random 3D scaffolds of individual CNT and fullerite nanocrystals, and Raman analysis reveals their non-covalent nature. While surface migration of the fullerenes on the CNT surface leads to coarsening of the nanocrystals, we find that controlled induction of SP3 point defects in the CNT can trap the initial nano-morphology and preserve the nano-dimensions and structure of the fullerene crystals, also at elevated temperatures and prolonged annealing. The nano-morphology of the treated nanotubes is preserved also when the hybrids are coated by a polymer layer (poly(3-hexylthiophene-2,5-diyl, P3HT), and the polymer-hybrid films show significant quenching of the photoluminescence, indicating that these hybrids could be useful in photovoltaic applications.

Resume : Similar to the BLG structures with periodically arranged holes, the bilayered nanoribbons with holes can be also considered as promising objects for application in the field of nanoelectronics and sensors as new class of quasi 1D carbon nanostructures. As in the case of the graphene nanoribbons charge carriers in bilayered ribbons are also confined in two spatial directions, where the electronic properties can be controlled by changing of the width of the bilayered ribbons [1,2]. Moreover the presence of an array of nanopores could change the electronic properties of bilayered graphene. In the present work one more possible way for electronic structure modification by external electric field was proposed. Based on DFT calculations the transformation of direct band gap to indirect one under the exact value of electric field was predicted. By using wave packet dynamical calculations, we revealed the effect of the holes on the transport properties. We found that the coupling of the electrical current between the two layers of the ribbon is energy dependent giving the possibility to modify the current values in the different layers. The investigation of the stacking of the bilayered ribbons highlights the tunability of the number of the additional energy mini-bands which should be manifested in the current-voltage and optic characteristics of the systems. This work was supported by the ministry of education and science of the Russian Federation increase competitiveness program of NUST "MISiS" (№ K2-2019-016). [1] B. Sahu, H. Min, A. H. MacDonald, and S. K. Banerjee, Phys. Rev. B 78 (2008) 045404 [2] T.-T. Vu, T.-K.-Q. Nguyen, A.-H. Huynh, T.-K.-L. Phan, and V.-T. Tran, Superlattices Microstruct. 102 (2017) 451

Resume : Improving upon the already impressive performance of semiconductors and associated devices is of vital importance to the electronics industry and a key driver for technology innovation. Silicon is a particularly attractive material due to its natural abundance, low toxicity, and unique optoelectronic properties. Nanoscale silicon, such as silicon nanoparticles (SiNPs), exhibits size and surface dependent properties that can be tuned for specific applications. As a result of quantum size effects, SiNPs have a substantially higher optical absorption cross-section than bulk silicon and present the opportunity for increasing the efficiency of photovoltaic devices. However, the photon emission efficiency of SiNPs is often low, owing to the formally forbidden electronic transition across the indirect band gap of silicon. One approach to mitigating this fundamental limitation is the incorporation of tailored impurities into the crystalline lattice. In doing so, new energy levels are introduced, and the impact of the forbidden band gap transition is mitigated, thereby improving the quantum yield and the absorption coefficient. Despite the great promise offered by doping, methods for achieving controlled doping of SiNPs are limited. Ion implantation is the conventional method used in the electronics industry, however, this technique can be costly and results in damage to the crystalline lattice. This presentation will outline our efforts to synthesize well-defined and uniform doped silicon nanoparticles using a post-synthesis doping method to yield materials with tunable optoelectronic properties.

Resume : Native point defects such as oxygen vacancies are known to be a culprit in creating low non-ideal Schottky barriers. This is complicated further in ZnO nanowires when defects concentrate near surface and electrical contact interfaces while interacting with adsorbates creating a range of donors and traps. Nanowires grown with Au nanocatalysts overcome complicated electrical contact fabrication to create intimate epitaxial interfaces with the single crystal ZnO resulting in rectifying electrical transport behavior that is dominated by quantum-mechanical tunneling at the contact edge.[1] The dominance of the interface edge region results in a high sensitivity to the range of defects at the surface and near the Au interface.[2] Here, we discuss comprehensive luminescence spectroscopy, electron microscopy and electrical transport measurements examining the influence of oxygen on the Schottky contacts and show with minor changes to the growth process that high-defect nanowires deviate from the edge-tunneling model producing a range of transport behavior from Ohmic to Schottky. Low-defect nanowires follow the edge-tunneling model and with further oxygen treatment the influence of surface defects is minimized such that high quality Schottky contacts with low leakage current are produced. Through a process of elimination examining the role of oxygen, ZnO polarity and Au structure we identify the major influences on electrical performance. (1) Lord, A. M. et al. Nano Lett. 2015, 15, 4248–4254. (2) Lord, A. M. et al. Nano Lett. 2017, 17, 6626–6636.

Resume : Group III-V and II-VI semiconductor materials are frequently used for opto- and nanoelectronic devices. However, often their performance is limited by the presence of extended defects originating from the lattice misfit between the active device layer and the substrate material used. A viable way to reduce misfit induced strain is by nanopatterning of the surface such that strain can be accommodated by both the substrate and the deposited semiconductor material, leading to larger critical thicknesses for misfit induced defect formation compared to planar films. This so called nanoheteroepitaxy approach is considered in this presentation theoretically (by both molecular static simulations and continuum theory) and experimentally. For the latter, nanopatterning is achieved by employing bottom-up techniques for nanomask formation and reactive ion etching for the transfer of patterns in the third dimension. This combination of techniques will be shown to allow for the simple fabrication of scalable nanostructures on large substrate areas. In particular, hexagonal arrays of vertical nanopillars of SiC and GaAs will be considered, on which III-V semiconductors (GaN and InAs, respectively) are grown by MBE. State-of-the-art HAADF-STEM techniques including differential phase contrast imaging are applied to characterize the growth morphology as well as the defect and strain distribution in overgrown nanopillars.

Resume : Metal nanostructures grown on porous silicon (Me/PSi) have attracted a great attention due to unique plasmonic properties and cost-effective fabrication compatible with basic steps of Si technology. These nanomaterials open disruptive perspectives for a number of applications including ultrasensitive bioanalysis, which is mostly realized through surface enhanced Raman scattering (SERS). Si chips covered with Me/PSi have been widely used as SERS-substrates for detection of substances at concentrations down to single molecule level. Outrageous performances of such nanomaterials have been praised [1] but effect of defects in Si wafer on morphology of PSi and further deposited metal nanostructures have not been reported yet. Here we present a study of reproducibility of morphology, optical properties and SERS-activity of Me/PSi in dependence on defects in parent Si induced by irregular dopant level. The defects were found to cause formation of spiral areas in PSi with porosity deviation up to 30%. This strongly affected SERS-activity of upper metallic nanostructures. What is more a lower porosity was shown to promote detecting small molecules while a higher one was more favorable to be sensitive to macromolecules. This phenomenon was explaned considering electric field distribution and light absorption in SERS-substrates. An original approach to avoid properties’ non-uniformity of SERS-substrates based on PSi was developed. [1] H.V. Bandarenka et al., Materials 2018, 11, 852.

Resume : Carbon nanotubes (CNTs) can be easily functionalized to link them covalently or electrostatically to quantum dots (QDs) to produce nanocomposites. Such materials can exhibit new properties that result either from the individual components (QDs or CNTs) or from the synergetic effects between QDs and CNTs. Those nanocomposites have attracted significant attention in the areas of nanotechnology and nanoengineering such as new catalyst materials [1], counter electrodes for sensitized solar cells [2], optoelectronic devices [3] etc. Thus, as the demand for progress in high energy conversion and energy storage materials increases the development of such nanocomposites in the aforementioned fields is of great interest and therefore CNTs with new or tuned properties may play an important role. Since there is not much known about the synergetic effects between CNTs and QDs and their effect on the optical and electronic properties of such nanocomposites consisting of them we investigate these effects, both experimentally and theoretically with the help of solid-state plane wave Density Functional Theory (DFT) methods. We focus mainly on common carboxylic acid functionalized CNTs that are electrostatically linked to QDs that show potential in the fields of energy conversion (e.g. ZnS, CuInS2 and ZnTe). The main goal of our work is to gather more knowledge about the internal interactions that may be helpful to optimize and fine tune such structures for photo electrochemical applications. [1] S. Banerjee, S.S. Wong, J. Am. Chem. Soc. (2003), 125, 34, 10342-10350 [2] C. V. V. Muralee Gopi et al., Sci Rep 7 (2017), 46519 [3] J. Du et al., J. Phys. Chem. B (2005), 109, 12772–12776

Resume : Interfacial magnetism and interlayer interactions may be modified by ion irradiation [1-3]. That provides a route for the control of magnetism in multilayers, namely in magnetic tunnel junctions (MTJ) [4]. It is, however, necessary to understand the extent of effects produced by the irradiation on the interface-controlled parameters of MTJs. Thus, we irradiated an MTJ with Ar ions at several fluences. The MTJ comprised IrMn/PL/Ru/RL/MgO/FL/Ta/Ru layers, with PL, RL and FL pinned, reference and free layers made of CoFe. Ferromagnetic resonance was used to measure the effective anisotropy of the FL, comprising interfacial perpendicular anisotropy (PMA) and shape anisotropy. The magnetic anisotropy decreased with the fluence, resulting from a stronger decrease of magnetization, due to intermixing of the FL/Ta interface, than of PMA at the MgO/FL interface. The tunnel magnetoresistance (TMR) decreased due to the creation of defects within the MgO barrier. At the higher fluence range, the RL and PL became decoupled and the TMR vanished. We show there is a window of operation for tuning the magnetic anisotropy using ion irradiation, while retaining interlayer coupling and TMR required for MTJ applications. [1] C. Chappert et al., Science 80 (1998) 1919 [2] I.L. Graff et al., J. Appl. Phys. 103 (2008) 033505 [3] V. Höink et al., Appl. Phys. Lett. 86 (2005) 152102 [4] B.M.S. Teixeira et al., Appl. Phys. Lett. 112 (2018) 202403

Resume : In magnetic core-shell nanoparticles, an interesting consequence is the exchange coupling across the core-shell interface that is frequently seen in the form of exchange bias, a horizontal shift in the hysteresis loop accompanied by an increase in coercivity after cooling in a magnetic field[1]. Formation of nanocomposites is an efficient way of fabricating interface by dispersing one magnetic phase into another, leading to enhanced interface effects. In addition to ferromagnetic (FM)/antiferromagnetic (AFM) interfaces, exchange bias and related effects have also been observed in other types of interfaces, e.g. involving ferrimagnet (FI) (e.g. FI/FM or FI/AFM), spin-glass (SG) (e.g. AFM/SG, FM/SG, FI/SG) and AFM/AFM systems. Recently, antiferromagnetic (AFM) nanoparticles have been the subject of renewed attention, due to their surface and interface effects giving rise to exchange anisotropy[2]. In this report, we have investigated the observation of exchange bias in superlattices composed of two AFM nanoparticles namely LaFeO3 (LFO) and NiO. We have observed conventional signatures of exchange bias (Heb) in the form of a shift in the field-cooled hysteresis loop. The magnitude of Heb increases with the cooling magnetic field from 650 Oe to 2050 Oe for cooling field of 10 kOe and 60 kOe, respectively. This shows that the strength of exchange bias effect is tunable by the cooling field. The exchange bias shows strong dependence on temperature and decreases from 2050 Oe at 5 K to 75 Oe at 300 K. The presence of giant vertical shift along magnetization axis is also observed. The vertical shift shows strong dependence on cooling field as well as temperature. From thermoremanent magnetization (TRM), and isothermoremanent (IRM) magnetization measurements, the nanostructures are found to be core shell in nature, consisting of an antiferromagnetic (AFM) core, and a two-dimensional diluted AFM (DAFF) shell [3]. REFERENCES: 1. Nogués, J., et al., Physics Reports, 2005. 422(3): p. 65-117. 2. Winkler, E., et al., . Physical Review B, 2005. 72(13): p. 132409. 3. Benitez, M., et al., Physical Review B, 2011. 83(13): p. 134424.

Resume : We have applied mechanical alloying (MA) to produce soft magnetic nanocomposite material using a mixture of elemental Fe2O3-Zn powders. An optimal milling and sintering conditions to obtain soft magnetic α-Fe/ZnO nanocomposite powders with fine microstructure were investigated by X-ray diffraction, differential scanning calorimeter (DSC) and vibrating sample magnetometer (VSM) measurements. It is found that α-Fe/ZnO nanocomposite powders in which ZnO is dispersed in α-Fe matrix are obtained by MA of Fe2O3 with Zn for 4 hours. The change in magnetization and coercivity also reflects the details of the solid‐state reduction process of hematite by pure metal of Zn during MA. Densification of the MA powders was performed in a spark plasma sintering (SPS) machine at 900-1000°C under 60MPa. Shrinkage change after SPS of MA'ed sample for 5 hours was significant above 300°C and gradually increased with increasing temperature up to 800°C. X-ray diffraction result shows that the average grain size of α-Fe in α-Fe/ZnO nanocomposite sintered at 900°C is in the range of 110nm.

Resume : We consider optical properties of 4 neutral hexacoordinated Si(bzimpy) 2 complexes, containing the 2,6- bis(benzimidazol-2 ′ -yl) pyridine ligand using DFT method. The geometrical parameters of the optimized cluster structures, the electronic absorption spectrum, and the molecular orbitals of the structures under consideration are discussed. The effect of addition of the substituents ligands to the of Si(pincer) 2 compound is considered. We found that succesive addition of substituents shifts HOMO and LUMO to higher values. The charge mobility of the complexes according to Marcus model is also considered. The manipulation may provide desirable electro-optical properties of the materials.

Resume : Aluminium Nitride AlN – nanotubes, nanowires and nanoparticles have been successfully synthesised by using a high temperature, highly non-equilibrium DC arc plasma method and investigated with different spectroscopy methods, including XANES, FTIR, neutron powder diffraction and inelastic neutron scattering. Here we report the results of the cathodoluminescence studies of the AlN nanotubes and nanoparticles, which have been measured between 80 K and room temperature (RT) under electron irradition with 10 keV energy. Low-temperature CL spectra of nanostructured AlN have been compared with those of the commercially available AlN powder. The significant difference between emission spectra of the three investigated samples has been established. Commercial AlN has been found to emit a band peaked at 3.47 eV which is commonly ascribed to oxygen impurities. Emission of the AlN nanoparticles is centered around 3.66 eV while CL spectrum of AlN nanotubes show complex character with at least three peaks at 2.2, 3.0 and 3.5 eV in the photon energy range of 1.8 – 3.8 eV. CL intensity of the nanostructured samples has been found to decrease significantly at RT, most probably due to a combination of non-irradiative relaxations at the surface, electron-phonon interactions and the reabsorption of the emitted light. CL of AlN-nanotube/CsI-scintillator composites has been also studied. Energy transfer via luminescence emission from CsI scintillator to AlN nanotube is demonstrated. Luminescence properties AlN nanotube/polymer composites were also studied and compared with those obtained for AlN nanotubes and nanoparticles.

Resume : The modalities of transformation of metal nanoparticles (NPs) to oxides is strongly dependent on the particle size. It was shown that size-dependent nanoscale Kirkendall effect (NKE) during oxidation of Ni NPs (< 100 nm) enables formation of hollow (single void) or porous (multiple voids) Ni-NiO core-shell particles [1]. While it is generally believed that the voids cause a drop of mechanical strength, the oxidation-induced internal stresses in the particle along with the effect of rigid NiO shell on dislocations nucleation might increase the mechanical strength of partially oxidized Ni NPs. To validate this hypothesis, NiO shell growth on Ni NPs and mechanical properties of oxidized Ni NPs were studied in this work. Single crystal Ni NPs (of 300-500 nm in size) were obtained by solid state dewetting of thin Ni film deposited on a sapphire substrate. The heat treatments in air (200-400°C) were employed for partial oxidation of Ni NPs. The NPs were characterized by atomic force microscopy (AFM) and high-resolution scanning electron microscopy (HRSEM). Mechanical properties of Ni NPs and oxidized Ni-NiO NPs were studied employing uni-axial in-situ micro-compression tests performed with the aid of Hysitron™ picoindenter. The obtained mechanical properties of the partially oxidized NPs were discussed in terms of plasticity mechanisms controlled by heterogeneous nucleation of dislocation loops. [1] J. G. Railsback, A. C. Johnston-Peck, J. Wang, and J. B. Tracy, “Size-dependent nanoscale kirkendall effect during the oxidation of nickel nanoparticles,” ACS Nano, vol. 4, no. 4, pp. 1913–1920, 2010.

Resume : We investigated the oxygen vacancy-induced visible emissions of SrTiO3 (STO) single crystals by using the photoluminescence spectroscopy. We found that the green emission in STO, which should be associated with intermediate states originating from functional ion defects inside the samples, such as oxygen vacancies, showed strong ambient dependence in the annealing process. While high temperature annealing in the O2 atmosphere suppressed the intensity of visible emission, annealing in an O2-free atmosphere, such as N2 or H2, increased it. The broad visible emissions were fitted with three sub-modes, whose intensities showed different evolutions with respect to the ambient environment. Our study demonstrated the systematic development of defect states with the amount of the oxygen vacancies in STO.

Resume : Electron trapping in SiO2 during e-beam deposition of 30–40 nm thick 1% EuF3 doped CaF2 thin films on a Si/SiO2 substrate was studied by the photoelectron emission (PE) technique. Si/SiO2 substrate was an amorphous 1 µm thick SiO2 layer thermally grown on Si wafer. After the e-beam deposition of EuF3:CaF2 nanofilms, PE current from the nanofilms was measured. PE was excited by UV photons of 4–6 eV energy and it was found that the registered photoelectrons were emitted both from the nanofilms and the substrate. The recorded PE spectra had several distinct PE maxima that were associated not with EuF3:CaF2 but with electrons trapped in the defect centres of SiO2 layer. These defect centres presumably existed in the as-fabricated SiO2 layer and were able to trap electrons during the e-beam deposition process of the EuF3:CaF2 nanofilms. The same PE maxima were observed in PE spectra of a bare Si/SiO2 substrate when the substrate was irradiated by weak electrons with energies up to 1.5 keV emitted by a thermocathode source. To confirm that the PE maxima were created only after exposure to accelerated electrons, bare Si/SiO2 substrates were irradiated with 8 keV X-rays and 6 MeV gamma rays and their PE spectra were measured after irradiation. No PE maxima were present in Si/SiO2 spectra after irradiation with photons. The obtained results suggest that the e-beam deposition process of EuF3:CaF2 nanofilms creates charge centers in Si/SiO2 substrate by trapping electrons in SiO2 layer.

Resume : Various efforts have been made to improve oxide thin-film diodes (TFDs) as next-generation devices that have lightweight, transparent, and stable characteristics. Recently, a new type TFD with a metal insulator-oxide semiconductor (MIOS) structure that shows a high rectification ratio, low off current, and stable operation was reported. The electrical conduction of the device follows a space-charge limited current (SCLC) mechanism that relies on the charge carrier injected from the semiconductor-insulator contact area. Therefore, the current-voltage characteristics of the MIOS TFD are precisely controlled by the electrical properties of the oxide semiconductor. However, acute control of the TFD's turn-on voltage has been challenging to date. Herein, we propose a new approach to shift the turn-on voltage of the MIOS TFD. When the amorphous indium-gallium-zinc-oxide (a-IGZO) semiconductor layer was deposited by the radio frequency (RF) sputter, the turn-on voltage of the diode was shifted over 10 V by regulating argon and oxygen gas flow rate. We investigated the relation between electrical characteristics of the diodes and gas flow rates of the RF-sputter. The effect of oxygen vacancy in a-IGZO to the turn-on voltage shift was also verified. Overcoming the precise turn-on voltage shift challenge for the MIOS TFDs will promote the application of TFDs such as a voltage regulator and electrostatic discharge protection circuit.

Resume : Grain boundaries, internal planar defects in polycrystalline materials, play a key role in many macroscopic properties. Interestingly, polycrystalline materials with significant improvements in mechanical strength, corrosion resistance, and radiation-induced segregation resistance can be achieved by increasing the population of special grain boundaries (the coherent twin boundary, low energy boundaries, and/or low index plane grain boundaries). To tailor the material properties related to the nanoscale grain boundary effects, it would definitely require grain boundary character distributions (GBCDs) in the nanocrystalline materials. It was recently demonstrated that the GBCDs in the microcrystalline and nanocrystalline aluminum are strongly correlated [G. S. Rohrer, et al., J Mater Sci (2017) 52:9819-9833], similar to what have been reported in our recent study for coarse-grained and nanocrystalline copper [S. Ratanaphan, et al., J Mater Sci (2017) 52:4070-4085]. Therefore, the distribution of grain boundary populations (or GBCDs) in the polycrystalline aluminum/copper are independent of grain sizes but strongly related to the grain boundary energy distributions (GBEDs), indicating that the steady state distribution of GBCDs might also be achieved in the nanocrystalline materials. It was reported that a grain boundary energy for face-centered cubic (FCC) metals can be obtained by using the closed-form Bulatov–Reed–Kumar (BRK) function [V.V. Bulatov et al. Acta Materialia (2014) 65:161–175]. This function provided a method of extrapolating between known points established by embedded atom method simulations of grain boundary energies in Al, Au, Cu, and Ni. We formulated the distribution function for GBCD in the nanocrystalline aluminum by using a similar scheme proposed in the BRK function. Specifically, the variation of GBCD in aluminum as a function of five-macroscopic parameter was constructed from the scaffolding of the measured grain boundary populations in the lower-dimensional subsets. It was found that while the parameters for the interpolated functions in the microcrystalline and nanocrystalline aluminum were relatively equivalent, the parameters capturing the fraction for Σ3 were significantly different. Therefore, the function for the GBCD reported here in this study could be used to promote the development of advanced grain boundary engineering schemes for nanocrystalline materials.

Resume : Because the surface-to-volume ratio of quasi two-dimensional (2D) materials is extremely high, understanding their surface characteristics is crucial for practically controlling their intrinsic properties and fabricating p-type and n-type layer semiconductors. Van der Waals crystals are expected to have an inert surface because of the absence of dangling bonds. However, the results of this study revealed that the surface of high-quality synthesized molybdenum disulfide (MoS2) is a major n-doping source. The surface electron concentration of MoS2 is nearly four orders of magnitude higher than that of its inner bulk. A substantial thickness-dependent electronic transport in these nanoflakes beyond that of the quantum confinement scale was observed.Scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) measurements confirmed the presence of surface electron accumulation (SEA) in this layer material. The in-situ ARPES measurement indicates that the cleaved MoS2 fresh surface exhibits an intrinsic semiconductor without SEA. The SEA is formed gradually in the MoS2 surface due to desulphurization at room temperature and even at a very low temperature of 85 K. The surface defects created by sulfur vacancies have been confirmed to be the major physical origin of the SEA phenomenon. The study provides an unprecedented insight into transition metal dichalcogenide (TMD) layer materials, which have important implications in material processing, device design, and fundamental research.

Resume : Diamond is unique material for a broad range of technological applications due to its physical and chemical properties. Diamond hosts a wide variety of luminescent defect centers that can act as stable single photon emitters at room temperature or as optically addressable solid-state spin-qubits. It is used in diagnostics and as high power microwave transmission window for plasma heating and stabilization in fusion reactors. In this study, we presented the results of ab initio (first-principles) simulations of vacancies and nitrogen substitutional defects in diamond crystals. Calculations of the atomic, electronic and vibrational properties were performed with the CRYSTAL17 computer code within the linear combination of atomic orbitals (LCAO) approximation. For consideration of defects, we used supercells containing 64 carbon atoms, a few basis sets for C atoms, and performed unrestricted DFT (open-shell) calculations with B1WC and B3LYP advanced hybrid functionals of the density-functional-theory. At the beginning, full geometry optimization and local structural analysis for pristine diamond and systems with defects were performed. Further, for fully-optimized systems and all spin configurations, calculations of harmonic phonon frequencies at the Γ point, infrared (IR) and Raman intensities, IR absorbance spectra, Raman spectra, dielectric functions and loss tangent were performed. It is predicted that in defective diamond additional features in both IR absorbance and Raman spectra as well as in dielectric functions arise in range of 400–1400 cm-1. The calculation results are compared with available experimental data.

Resume : The radiation-resistant binary oxide BeO, is one of the very important materials for applications in fusion reactors. In this work, we analyzed the kinetics of the F-type and V center annealing after neutron irradiation. F-type center kinetics were treated as the bimolecular process with equal concentrations of the complementary F and Oi defects. Such process is controlled by the interstitial oxygen ion mobility, which is much higher than that of the F centers. The F center annealing begins at temperatures 500-700 K, when both F and F+ centers are practically immobile, due to the recombination with mobile Oi defects. It is demonstrated how the shape of the F-annealing curves is determined by two control parameters: Ea and effective pre-exponential factor and strongly depends on irradiation conditions. Special attention is paid to a detailed comparison of diffusion-controlled F center thermal annealing in neutron-irradiated BeO with similar data obtained for for MgO and Al2O3. The results obtained are used for the evaluation of interstitial oxygen migration parameters and are compared with the available ab initio calculations for other oxides. Finally, we also treated the kinetics of F-center annealing in thermochemically-reduced BeO crystals. The obtained activation energy allows to evaluate both the intrinsic F-center migration energy and also the conditions for metal colloid formation in BeO.

Resume : Tungsten oxide-activated LiF crystals are effective scintillation materials. Their disadvantage is the low radiation resistance of the matrix, which leads to degradation of the radiative characteristics of the material. The specific features of radiation defect formation in the MgF2 lattice in comparison with LiF (with a comparable value of the band gap width) determine the existence in these two matrices of a difference in the observed luminescence in the activator glow region from the excitation energy (type). It was possible to synthesize ceramics based on MgF2 activated with tungsten oxide in the field of a powerful radiation flux [1]. Here we report the results of a study of the elemental composition, XRD spectra, and spectral-kinetic characteristics of luminescence in the optical region in tungsten oxide-activated LiF crystals and MgF2-based ceramics. Luminescence was excited by laser radiation at 220 and 450 nm, as well as x-ray and electron radiation at 300 K. A comparative analysis of the results on the influence of the excitation energy (type) on the spectral-kinetic parameters of the observed activator luminescence in the studied LiF and MgF2 materials in the range of 3-1 eV is presented. [1] V. Lisitsyn et.al. Luminescence of the tungsten-activated MgF2 ceramics synthesized under the electron beam, Nuclear Inst. and Methods in Physics Research. 435 (2018) 263–267

Resume : Free-volume defects in the Ge-Ga-S/Se (80GeS2-20Ga2S3 with CsCl and 80GeSe2-20Ga2Se3 after crystallization and gamma-irradiation) glasses were studied using positron annihilation lifetime spectroscopy. It is shown that void fragmentation in (80GeS2-20Ga2S3)85(СsCl)15 glass can be associated with loosing of their inner structure. Full crystallization of these glasses corresponds to the formation of defect-related voids. These trends are confirmed by positron-positronium decomposition algorithm. It is established that changes in defect-related component in the fit of experimental positron lifetime spectra for nanocrystallized glasses testifies in a favor of structural fragmentation of larger free volume entities into smaller ones in 80GeSe2-20Ga2Se3 glass. After irradiation of 80GeS2-20Ga2S3 glasses nanovoids with different size are created as intrinsic structural defects associated with topologically uncoordinated negative-changed centers. These defect centers form additional energy levels both near the bottom of the conduction band and in the vicinity of the valence band, as well as additional intrinsic electric fields.

Resume : Excitation of YAG:Ce phosphors, ceramics, crystals by electron beams, optical, x-ray radiation leads to the appearance of intense luminescence in the visible range of the spectrum. Materials based on YAG:Ce are promising for use as phosphors, scintillators. The synthesis of YAG:Ce is complex, it is carried out from a mixture of oxides at high temperatures, under difficult controlled conditions. Therefore, with all the technologies used to produce them, the materials contain a large number of various defects. This affects the efficiency of luminescence. To date, it has been shown that UV luminescence of YAG:Ce materials in the range of 200-450 nm is associated with the presence of intrinsic lattice defects in the materials. However, little information about the nature associated with intrinsic defect luminescence centers. We report the results of a study of the spectral and kinetic characteristics of the YAG: Ce phosphor ceramics. For research, industrial phosphors and ceramic synthesized in the radiation field were used. Studies were carried out when luminescence was excited by different radiation sources: optical (laser at different wavelengths), x-ray, and high-energy electron flows. All the studied phosphors have in the luminescence spectrum except the main (working) band in the region of 500-750 nm and the band with maxima at 320, 360-380nm. Studies have shown that the ratio between the intensity of UV bands and the band in the visible region of the spectrum depends on the background of the phosphor, the type of excitation. Results of analysis of the shape of bands and the decay kinetics from the method of luminescence excitation.

Resume : Phosphors, ceramics based on YAG:Ce are promising for use as scintillators in light emitting diodes. YAG:Ce phosphors, ceramics are multicomponent systems. High temperature and long duration synthesis of these materials does not allow good reproducibility of the material quality. Therefore, the search and improvement of technologies for their synthesis continue. In addition to the most common methods that employ solid-phase reactions, other techniques are being developed: laser ablation, sol-gel method, hydrothermal, coprecipitation, combustion, etc. One of the options is synthesis of ceramics in the field of high-power radiation fluxes. This paper presents the results of the research in structural and luminescent properties of YAG:Ce ceramics synthesized in a radiation field. The synthesized test samples were two series of ceramics different in composition with a batch content: Al2O3(43%) Y2O3(55%) Ce2O3(2%) (YAG) and Al2O3(40%) Y2O3(52%) Ce2O3(2%) Gd2O3(6%) (YAGG). The structure and luminescent properties of the ceramic samples were studied, and then they were subjected to high-temperature annealing in a vacuum furnace at 1650°C for 8 hours. The resulting ceramics has the structure of yttrium-aluminum garnet (YAG), and it intensively luminesces with the properties characteristic of this ceramics under UV excitation.

Resume : Optical ceramics is a promising material for use as scintillators, luminophores [1-4]. The properties of ceramics are isotropic, products of any shape can be made from ceramics. Therefore, ceramics have advantages over commonly used single-crystal materials [5]. The interest is drawn to optical ceramics based on MgF2. Ceramics based on MgF2, as well as crystals based on LiF, can be promising for use as dosimetric, scintillation materials, phosphores. As activators in lithium fluoride crystals, polyvalent metal ions are introduced by adding metal oxides to the charge: tungsten, titanium, uranium, iron. Obviously, these same activators should be used to introduce ceramics based on MgF2. Here we present the results of a study of the luminescence of ceramics based on MgF2 activated tungsten ions synthesized in an air atmosphere using a powerful electron beam as a heater. Tungsten oxide (WO3) and co-activator in the form of lithium hydroxide (LiOH) with weight concentrations from 0.5 to 3% were added to the charge from MgF2 powder. The temperature in the melting region was sufficient to melt the charge throughout the surface, but non-uniform over the surface. Surfaces of the samples after annealing did not change, the number of fine particles on the surface decreased significantly. When excited by radiation in the range 200 - 300 nm, luminescence is observed in the 340 – 650 nm region. The luminescence spectra have the form of a band with a well-defined emission maximum at 480 nm.

Resume : The positron annihilation lifetime (PAL) spectroscopy method based on the fact that the unstable positron-electron system (positronium Ps) is repelled from ionic cores of atoms and tends to location in open pores. In the case of spinel ceramics, two channels of PAL should be considered – the positron trapping and o-Ps decaying. However, if trapping sites will appear in a vicinity of grain boundaries neighbouring with free-volume pores and extended defects, they can become mutually interconnected resulting in a significant complication of the measured PAL spectra. This occurs provided the input of one of the above annihilation channels will be significantly changed. It is established that lifetimes of the defect components increase with sintering temperature of ceramics. These changes correspond to the increased defects near gnain boundaries of ceramics, nanopore size and smaller amount of nanopores. To apply positron-positronium trapping algorithm it was shown that the chemical-adsorbed water vapor modifies structural defects located at the grain boundaries in a vicinity of pores, this process being accompanied by void fragmentation during water adsorption and agglomeration during water desorption after drying. The physical adsorbed water not modified grain boundaries in oxide MgAl2O4 ceramics located only in nanopores.

Resume : It was reported previously that W18O49 based nanostructures were used for electrocatalysis, energy storage, electrochromic, gas sensing applications. However, the mechanical energy harvesting application of W18O49 nanostructures was not reported to the best of our knowledge. In this work, W18O49 nanorods were synthesized and were found from XPS characterizations that the synthesized W18O49 nanorods have oxygen-deficient sites. Further, the W18O49 nanorods were incorporated in PVDF to form a flexible nanocomposite. FTIR, XRD and Raman spectroscopy was carried out on the nanocomposite films and was found that polar piezoelectric phases have evolved in the nanocomposite owing to the interaction of the W18O49 nanorods with the PVDF. It was also found that the dielectric constant increased and the dielectric loss decreased with the addition of W18O49 nanorods. DSC characterization was employed to study the crystallization kinetics in the W18O49/PVDF nanocomposite films. The W18O49/PVDF nanocomposite generated a peak open-circuit voltage of 6V and a peak short circuit current of 700nA. The W18O49/PVDF nanocomposite could light two commercial blue light-emitting diodes (LEDs) with hand impulse imparting.

Resume : Sr2FeMoO6 (SFMO) films were obtained from the single-phase powders synthesized with using of SrMoO4 and SrFeO3-х precursors. Initially, solution contained 1.0 g of polyvinylpyrrolidone (PVP) with 15 ml ethanol and 2.0 g of PVP, after which the 1.5 g of SFMO powder was added for both samples. Further, solution was mixed in a magnetic stirrer, whereafter its ultrasound treatment was implemented during 20 min. SFMO films were obtained on a polycore substrate from the solution, by using the spin-coating technique. The film was dried during 2 h at 340 K. To improve the microstructure, the obtained films were subjected to heat treatment at 570 K for 1 hour. Investigations of these films were carried out by the ferromagnetic resonance (FMR) method. The measurements were carried out by using of the Brüker Ele Xsys E-500 spectrometer on the microwave frequency 9.45 GHz at temperature 300 К and in the constant magnetic field with induction up to 1.4 Т. Results of studying the field dependences of the films revealed the presence of FMR in the entire angles range of the magnetic field rotation out of plane with a maximum value of the resonance field when it is orthogonal to the film plane. It was shown that with an increase in the concentration of the PVP polymer in the film, the resonance field increases due to the weakening of the dipole – dipole interaction between magnetic grains. This work was supported by the European Union project H2020 –MSCA-RISE-2017-778308- SPINMULTIFILM.

Resume : Sr2FeMoO6 (SFMO) films were obtained from the powder synthesized using of SrMoO4 and SrFeO3-х precursors which were compacted into targets. The SFMO films were fabricated on (001) SrTiO3 (STO) substrates by a pulsed laser deposition (PLD) method. The SFMO polycrystalline target was ablated by using KrF excimer laser to deposit the films on the heated substrate. The research of films surface morphology showed that the surface of films is homogeneous and has a wavelike appearance. It was found that epitaxial monocrystalline (100) films were formed on STO substrate. Magnetic characteristics J(B) exhibit а strong anisotropy. On the remagnetization curves, when the field is perpendicular to the plane of the film, hysteresis is observed, and the width of the loop increases with a decreasing temperature. Investigations of these films were carried out by the ferromagnetic resonance method on the microwave frequency 9.37 GHz in the temperature 300 К and in the magnetic field up to 1.4 Т. Various resonance lines with a complex angular dependence were observed. The nature of the resonances is discussed. This work was supported by the European Union project H2020 –MSCA-RISE-2017-778308- SPINMULTIFILM.

Resume : Oxide-dispersion-strengthened (ODS) ferritic steels have excellent high-temperature creep resistance, which is attributed to the formation of nanoscale oxides in high number density in the ferritic substrate during the manufacturing processes. Defects formation and its impact on the mechanical properties are serious concerns for the application of this category of materials in advanced nuclear reactors, in which structural components have to endure intensive irradiation from high-energy neutrons. Under the in-pile condition, energetic helium ions can be produced via the (n, α)-reaction and are a source of radiation damage. In the present work, defect formation in ODS ferritic steels after helium-ion irradiation was studied. Two kinds of ODS ferritic steels with base compositions of 16Cr-0.1Ti, and 15Cr-4Al-0.6Zr-0.1Ti, respectively, were investigated. Before helium-ion irradiation, observation of both the thin foil and the replica samples with TEM showed that the structures and compositions of the nanoscale oxide particles are different in the two ODS steels. The specimens were irradiated with multiple-energy He ions to 0.4 dpa / 7000 appm_He, and were subsequently thermally annealed at 800℃ for 1 hour in vacuum. Bubbles at nano-meter scale were observed in the specimens, while the distribution, number density, average size of bubbles were significantly different in the two ODS steels. The difference can be correlated with the sink strength of the oxides / matrix interfaces. Hardness of the specimens were tested with nano-indentation technique. Hardening was observed in the helium-implanted specimens before / after the thermal annealing. Contribution of helium bubbles to hardening of the ODS steels is discussed.

Resume : Charge traps and parasitic point defects in wide-gap semiconductors and dielectrics play a key limiting role for capacitor applications, especially as minimization continues through the nanoscale. Despite their ubiquity and decades of research and development, some important and fundamental properties, of even the most commonly applied materials, remain relatively poorly understood. A major limitation is from the available tools for characterizing defects in these materials, and the clarity and reproducibility of their results. In this work an electron irradiation assisted photoelectron emission technique was developed to study charge traps in nanolayered 20–60 nm thick Si3N4 deposited on Si/SiO2 substrate. Sharp photoemission peaks were induced by electron irradiation of Si/SiO2/Si3N4 samples, with characteristic energies revealing microscopic origins of these peaks. Trap energies, originating from SiO2, were observed to shift depending on the electron irradiation dose and the thickness of Si3N4 nanolayers. Improved understanding of the characteristics of these defects can be informative towards the development of high-performance nanocapacitor devices, in addition to furthering understanding of non-volatile memory devices. Acknowledgement: This research was supported by the ERDF project No. 1.1.1.1/16/A/203.

Resume : Self-trapped hole or Vk-center appears in halides as a result of exposure to the high-energy radiation (UV and higher). Formation and migration of these defects have a strong influence on the properties of the materials. Experimental studies suggest that Vk-center in halides can be found in a form of a molecular ion, e.g. F2-. By means of the DFT+U method we investigate stability of such molecular ions in PbF2, PbCl2 and PbBr2 crystals. Other mechanisms of hole localization, like vacancy, are considered as well. Migration of molecular ions and vacancies is modelled by the cNEB method. Structural and electronic properties are calculated and analyzed.

Resume : The self-trapped hole polarons (called also Vk centres) where a hole is shared by two nearest halogens, X2-) are very common color centers created in alkali halides and alkaline-earth halides under all kinds of irradiation (UV light, electrons, gamma rays, neutrons, heavy swift ions). The hole polarons start to migrate and recombine above certain critical temperatures. Their thermally induced decay has been observed by different experimental techniques in almost all alkali halides, as well as in some halides and more complex halides, such as perovskite halides, ammonium halides, halide sodalites etc [1] . In this report, we review and analyse the self-trapped hole center migration temperatures for a series of metal iodides, such as SrI2, CaI2 and CdI2, as a function of halogen-halogen distance in a regular crystalline lattice as well as of halogen-halogen distance in isolated molecular ions. [1] Popov, A. I., Kotomin, E. A., & Maier, J. (2017). Solid State Ionics, 302, 3-6.

Resume : Nanostructures of Aluminium Nitride (AlN) - nanotubes, nanowires and nanoparticles have been successfully synthesised by using a high temperature, highly non-equilibrium DC arc plasma method and investigated with different spectroscopy methods, including XANES, FTIR, neutron powder diffraction and inelastic neutron scattering [1-3]. Here we report the results of the cathodoluminescence studies of the AlN nanotubes and nanoparticles, which have been measured between 80 K and room temperature (RT) under electron irradition with 10 keV energy. Low-temperature CL spectra of nanostructured AlN have been compared with those of the commercially available AlN powder. The significant difference between emission spectra of the three investigated samples has been established. Commercial AlN has been found to emit a band peaked at 3.47 eV which is commonly ascribed to oxygen impurities. Emission of the AlN nanoparticles is centered around 3.66 eV while CL spectrum of AlN nanotubes show complex character with at least three peaks at 2.2, 3.0 and 3.5 eV in the photon energy range of 1.8 - 3.8 eV. CL intensity of the nanostructured samples has been found to decrease significantly at RT, most probably due to a combination of non-irradiative relaxations at the surface, electron-phonon interactions and the reabsorption of the emitted light. CL of AlN-nanotube/CsI-scintillator composites has been also studied. Energy transfer via luminescence emission from CsI scintillator to AlN nanotube is demonstrated. Luminescence properties AlN nanotube/polymer composites were also studied and compared with those obtained for AlN nanotubes and nanoparticles. References [1] Balasubramanian C., et al. Journal of Physics: Condensed Matter 18 (2006): S2095. [2] Bellucci S., Popov A.I. et al. Radiation Measurements 42 (2007): 708-711. [3] Bellucci S., Balasubramanian C., Ivanov A., Popov A., Schober H. J. Neutron Research, 14(2006); 287-291.

Resume : The use of silicon in optoelectronic devices is significantly limited by the low intensity of its luminescence due to indirect energy structure of silicon. One of the possible ways to solve this problem may be to use dislocation-related luminescence, one of the main light-emitting lines of which D1 has luminescence at ~ 1.5 mkm. In this work, the luminescent properties of dislocation structures created in silicon during ion implantation and subsequent annealing is studied. The dependence of the intensity and its temperature dependence for the D1 line upon variation of the initial substrate (type of conductivity and impurity concentration), irradiation regimes (type of ions, energy and dose) and annealing conditions (temperature, duration and annealing atmosphere) is discussed. Particular attention is paid to the effect of boron (both as pristine impurity and additionally implanted) on the luminescent properties of the studied samples. The research is supported by the fellowship of the President of the Russian Federation (SP-1147.2018.3).

Resume : Modern progress in material engineering results in new complex oxide materials with improved ferroelectric and piezoelectric properties. The ability of the ABO3 perovskite structures (e.g., BaTiO3 — BTO) to accept isovalent dopants on both A- and B-sites is the promising way to enhancement of the electromechanical properties of lead-free materials. Our recent calculations show that stoichiometric Ba(1-x)Sr(x)TiO3 (BSTO) and Ba(1-x)Ca(x)TiO3 (BCTO) solid solutions exhibit significantly enhanced piezoelectric properties, in a comparison with a pure BTO [1–3]. However, perovskites reveal often considerable oxygen non-stoichiometry. In this work, we presented the results of first-principles computations and local structural analysis for stoichiometric and non-stoichiometric (oxygen vacancies) BSTO and BCTO perovskite solid solutions. Calculations were performed with the CRYSTAL17 computer code within the linear combination of atomic orbitals (LCAO) approximation, using PBE0 and B1WC advanced hybrid functionals of the density-functional-theory. Different chemical compositions of solid solutions are considered by means of supercell calculations. Both Sr and Ca off-centering within the dodecahedra and the Ti atom displacement within the TiO6 octahedra, as well as change of local atomic structure under stress (compression and tension), are studied. The relation of these structural distortions with electromechanical properties of solid solutions is discussed. [1] L.L. Rusevich, G. Zvejnieks, A. Erba, R. Dovesi, E.A. Kotomin, J. Phys. Chem. A 2017, 121, 9409. [2] L.L. Rusevich, G. Zvejnieks, E.A. Kotomin, M. Maček Kržmanc, A. Meden, Š. Kunej, I.D. Vlaicu, J. Phys. Chem. C 2019, 123, 2031. [3] L.L. Rusevich, G. Zvejnieks, E.A. Kotomin, Solid State Ionics 2019, 337, 76.

Resume : We present mesoscale characterization of carrier transport in W/Mo-doped ms-BiVO4 (BVO) to supplement our earlier study of electron and hole transport in bulk BVO. The mesoscale kinetic Monte Carlo (KMC) approach, supported by first principles-determined electron and hole hopping rates, captures the complex dynamics of electron carriers arising from light absorption and from metal doping. The computations and simulations support the observation that, for the doping level used in experiment, doping atoms do not affect significantly the transport dynamics of the charge carriers compared to stoichiometric BVO. W/Mo doping increases the electron carrier concentration and consequently the electrode conductivity. We used a density functional theory DFT + U method to characterize the electronic structure of doped BVO. Each W and Mo atom brings one more valence electron than a V atom. These excess electrons are mobile. We adopted the theories and methods of our earlier investigations on BVO. The DFT + U theory affords an accurate description of small electron polarons with electrons localized on W or Mo or V, as well as of the hopping barriers from W/Mo-to-V or V-to-V. Calculations on a (331) supercell of W/Mo-doped ms-BVO indicate that the excess electron from W or Mo does not reside on the doping atom. There is a shallow interaction region around the doping atom where the excess electron localized on a V atom is more stable than when localized on the doping atom and slightly more stable than when the excess electron is localized far away from the doping atom. This region is two nearest V atom-wide for W and three nearest atom-wide for Mo. The depths of these stabilization regions are very small, about -0.65 kBT (at 300 K) for W and about -0.73 kBT for Mo. Calculated V to -V hopping barriers in the doped material are little affected within the region and not affected outside the region. Using our recently developed lattice-based kinetic Monte Carlo code adapted to account for doping atoms and their energy stabilization regions, we calculated the diffusivity of electrons for carrier density relevant to experiment as well as the conductivity. The stabilization regions have little effect on the diffusivity compared to the stoichiometric system because of the smallness of the stabilization. The diffusivity is found to decrease slightly with an increasing number of carriers, but the conductivity of a system with electron polarons arising from doping together with light absorption increases compared to that of the un-doped system. This work will set the foundation to study electron transport in gradient (W/Mo)-doped systems and other mixed phase systems.

Resume : Layered transition metal dichalcogenides like MoS2 offers excellent alternative for development of novel opto-electronic devices beyond graphene family. MoS2 has excellent mechanical, thermal and chemical properties and finds applications in transistors, batteries, photodetectors, catalysts and gas sensors. Tailoring the surface electronic properties of MoS2 through defects induced strain is of great importance for its wide-range applicability in flexible electronics. In this work, correlation between defects induced strain and work function of MoS2 has been studied after swift heavy ion (SHI) irradiation. MoS2 is an indirect band gap (1.2 eV) semiconductor in bulk form and becomes direct band gap (1.9 eV) semiconductor when reduced dimensionally to monolayer due to quantum effects at nano-scale. The layered structure of MoS2 crystal has Mo atoms sandwiched between two S atoms. Its in-plane atoms are covalently bonded while out-of-plane atoms are bonded through weak van der Waal force. This plane-dependent nature of bonding makes exfoliation of MoS2 layer possible through weakening of inter-layer van der Waal force by mechanical or chemical exfoliation methods which yields layered MoS2 nanosheets. SHI offers controlled way of introduction of defects in terms of number, density and distribution in the target material and deposits its energy mainly through dense electronic excitation through inelastic collisions. In this work MoS2 nanosheets were exfoliated by sono-chemical exfoliation method in organic solvent DMF. These nanosheets were then irradiated by 100 MeV Silver (Ag) ions at different fluences in a 16 MV Pelletron accelerator. Pristine as well as irradiated MoS2 nanosheets were characterised by X-ray diffraction, FESEM and TEM facilities to investigate the changes in the structural, morphological and crystalline properties. The modifications in the optical and phononic vibrations were analysed by Raman spectroscopy. The change in the surface electronic properties in terms of work function modulation has been explored using scanning Kelvin microscope. The results obtained after these characterizations indicate controlled introduction of defects in the MoS2 nanosheets by SHI irradiation. MoS2 nanosheets and latent tracks are clearly visible in the FESEM images. MoS2 crystal planes and high resolution image of latent tracks are crystal clear in the TEM images. XRD results show complete amorphisation of latent tracks at higher fluence. Raman spectroscopic features of MoS2 phonon modes are clear indication of few layer nanosheets. The defects induced peak in Raman spectroscopy is detected in the irradiated MoS2 and its intensity increases with increasing fluence showing increase in the number of defects. From the shifting of its in-plane (E12g) and out-of-plane (A1g) modes in Raman spectra, the presence of in-plane and out-of-plane tensile strain in the MoS2 nanosheets can be inferred directly. The defects generates systematic introduction of strain, both in-plane and out-of-plane strain, in the MoS2 lattice. In-plane tensile strain increases systematically with the fluence but out-of-plane tensile strain gets relaxed at higher fluences. The difference in the fluence dependent behaviour of in-plane and out-of-plane strain on irradiation induced defects can be explained in terms of nature of Mo-S bond along the basal and the perpendicular plane. Work function measurement in terms of contact potential difference using scanning Kelvin probe microscope demonstrate linear dependence on the in-plane strain. The tensile strain in the basal plane of MoS2 moves the atom farther apart from one another and thus reducing the orbital overlap. In this scenario, the location of valence band maximum moves from K-point to Г-point under crystal stretching and attains more symmetrical shape. This phenomenon lowers the valence bandwidth and Fermi level as well, thus increasing the work function of MoS2. This study provides novel route towards tuning the surface electronic properties of MoS2 nanosheets by strain or defect engineering using SHI irradiation. In addition, it also offers unique opportunity to study the richness of fundamental physics involved at the nano-scale.

Resume : The BaGa2O4 ceramics are materials for secondary electron emission coatings in plasma display panels, for proton ceramic fuel cells, etc. Doping of BaGa2O4 ceramics by rare-earth ions results in modification of their structural properties and results in formation of defects near grain boundaries. In this work we analyzed transformation of defect-related free volumes formed by addition phases near grain boundaries of ceramics. Samples of BaGa2O4 ceramics were obtained by solid-state reaction method from BaCO3 and Ga2O3 components with purity of 99.99%. Powders with 0, 1, 3 and 4 mol.% of Eu2O3 (99.99%) were mixed in an agate mortar for 6 h with further pressing in a steel mold. Prepared pellets were annealed at 1200 ºC for 12 h in air. After that, the annealing of ceramic samples was carried out at 1300 ºC for 4h. It is shown that un-doped BaGa2O4 ceramics contain three phase, samples with 3 and 4 mol.% of Eu2O3 are two-phases (BaGa2O4 and Eu3GaO6 phases), while BaGa2O4 ceramics with 1 mol.% of Eu2O3 contain only one (Ba,Eu)Ga2O4 phase on own structural type. Defect-related free volume and nanopores were investigated by positron annihilation lifetime spectroscopy method using three- and four component fitting procedures. It is established that amount of free volumes correlates with addition phases near grain boundaries.

Resume : The Cu0.1Ni0.8Co0.2Mn1.9O4 ceramics are known materials for negative temperature coefficient thermistors, in-rush current limiters, etc. Typically, structural properties of such materials re studied using different traditional method of structural characterizations. The quantity of the additional defect-related phase and its distribution in bulk and on the surface of ceramics are influenced by temperature-time sintering regimes. It is established that the amount of additional NiO phase in these ceramics extracted during sintering play a decisive role. The process of monolitization from the position of evolution of grain-pore structure was studied in these ceramics using positron annihilation lifetime spectroscopy within two-component fitting procedures. The NiO phase results in transformation of free-volume defects in the inner structure of ceramics. To study free volumes formed by NiO and nanopores in Cu0.1Ni0.8Co0.2Mn1.9O4 ceramics three-component fitting procedures and using positron-positronium trapping algorithm was used this work.

Resume : The possibility of designing composite Gallium-Arsenide (GaAs) nanowires structures via bottom-up methods has attracted significant interest for the engineering of future optoelectronic devices [1]. The crystal structure and the stacking fault density of GaAs nanowires largely determine their optoelectronic properties [2]. Here, we present an investigation of the structural and optical properties of GaAs nanowires. GaAs nanowires were grown by metal-organic vapour phase epitaxy (MOCVD) on GaAs (111)B substrates initially patterned using selected area epitaxy (SAE). Transmission electron microscopy (TEM) shows that GaAs nanowires have a zinc blende (ZB) structure with a varying twin defect density (TDs) along the nanowire’s length. The average local twin defect density (ntdd), computed for a section length of 1 µm along the NW’s axis, decreases from 110 to 13 per micrometer from the base to the tip of the NW. To correlate the GaAs NW crystal structure to its electronic band structure, cathodoluminescence (CL) and time resolved photoluminescence measurements were done at room temperature. The CL intensity increases by 14 times from the bottom to the NW’s tip, while the CL peak position shifted from 897 nm (1.382 eV) to 886 nm (1.399 eV). The CL peaks red-shift along the NWs vertical axis is tentatively attributed to type-II band alignment at the ZB-TD-ZB interfaces. The high surface recombination velocity of GaAs is a major issue for applications in optoelectronic devices. As a result, passivation of the nanowire surface is required for various applications such as lasing. Here, however, we demonstrate that when sufficient nanowire’s length and optimal structural properties are obtained, pure unpassivated GaAs nanowires can achieve low-temperature lasing. This provides insights toward the engineering of GaAs NW for applications in optoelectronic devices. [1] B. Ketterer, M. Heiss, E. Uccelli, J. Arbiol, and A. Fontcuberta I Morral, ‘Untangling the electronic band structure of wurtzite GaAs nanowires by resonant Raman spectroscopy’, ACS Nano, vol. 5, no. 9, pp. 7585–7592, 2011. [2] R. E. Algra et al., ‘Twinning superlattices in indium phosphide nanowires’, Nature, vol. 456, no. 7220, pp. 369–372, 2008.

Resume : Garnet single crystals, in particular, gadolinium gallium garnet Gd3Ga5O12 (GGG), are one of the common devices of quantum electronics. When alloyed with rare-earth ions, they are phosphors and scintillators. Garnet single crystals, in particular, gadolinium gallium garnet Gd3Ga5O12 (GGG), are one of the common devices of quantum electronics. By doping rare-earth ions are promising phosphors and scintillators. GGG single crystals are transparent in a wide range, the band gap is estimated at 5.66 eV, they have a high refractive index, chemical inertness and are characterized by high radiation resistance. To date, the radiative processes and nature of radiation-induced defects has not yet been clarified. This is due to the low efficiency of defect formation in GGG. Here we show studies of the micromechanical properties of GGG single crystals grown by the Czochralski method upon irradiation with ions of energy irradiated with 150 MeV Kr ions at fluences 1013 and 1014 ions / cm2. Comparison of the depth behavior of hardness and calculated energy loss leads to conclusion that ion-induced modifications of structure and hardness are ensured by electronic energy loss. The threshold energy loss from data Fig.1 is about 6-7 keV/nm. The effect at given fluences reaches a plateau at H/H0 about 65 %. At the highest fluence (1013 Kr/cm2) the width of softened zone approaches the thickness of irradiated zone. Amorphization is accompanied by the increase of volume. AFM images show a corresponding surface step.

Resume : Structural imperfections of two dimensional (2D) crystals such as point vacancies, line defects and grain boundaries have considerable impacts on their chemical-physical properties. Here we study the atomic structure and dynamics of defects and grain boundaries (GBs) in monolayer Pd2Se3 using annular dark field scanning transmission electron microscopy (ADF-STEM). The Pd2Se3 monolayers are reproducibly created by thermally induced phase transformation of few-layered PdSe2 films in an in-situ heating holder in the TEM to promote Se loss. A variety of point vacancies, one-dimensional defects, grain boundaries (e.g. 90° GBs in Fig. 1) and defect ring complexes are directly observed in monolayer Pd2Se3, which show a series of dynamics triggered by electron beam. High mobility of vacancies leads to self-healing of point vacancies by migration to the edge and subsequent edge etching under beam irradiation. Specific defects for Pd2Se3 are stabilized by the formation of Se−Se bonds, which can shift in a staggered way to buffer strain, forming a wave-like one-dimensional defect. Bond rotations are observed and play an important role in defect and grain boundary dynamics in Pd2Se3. The GBs form in a meandering pathway and migrate by a sequence of Se−Se bond rotations without large scale vacancy formation. In the GB corners and tilted GBs, other highly symmetric vacancy defects also occur to adapt to the orientation change. These results give atomic level insights into the defects and grain boundaries in Pd2Se3 2D monolayers.

Resume : We review recent experimental data on compression behavior of single crystalline metal nanoparticles produced by solid state dewetting technique and show that they deform elastically up to very high level of stresses approaching the theoretical shear strength of respective material [1-2]. The following catastrophic plastic collapse is characterized by multiple dislocation nucleation events in otherwise pristine nanoparticles. Then, we show that bicrystalline Ag-Au alloy nanoparticles yield plastically at much lower stresses than their single crystalline counterparts. This is related to the heterogeneous nucleation of new dislocations at the grain boundary. Moreover, depositing an ultrathin (15 nm) nanocrystalline overlayer of Au on single crystalline Ag particles of hundreds of nanometers in diameter changes their deformation mode from that characteristic for single crystalline nanoparticles to that of bicrystalline ones, characterized by much lower values of flow stress. Thus, paradoxically, adding more load-bearing material to the particles by coating them with a Au overlayer drastically decreases their strength. This behavior is confirmed by molecular dynamics nanocompression simulations [3, 4]. Indeed, the mechanics of single crystalline core – polycrystalline shell Ag-Au nanoparticles under compression is investigated and compared to single crystalline samples. The results confirm the important role played by dislocations nucleation in the nanocrystalline shell prior to the crystalline core. [1] A. Sharma et al., Nature Commun. 9 (2018) 4102 [2] A. Sharma et al., Adv. Funct. Mater. 29 (2019) 1807554 [3] J. Amodeo and K. Lizoul, Mater. Design 135 (2017) 223-231 [4] A. Goryaeva et al., Phys. Rev. Mat. 3 (2019) 033606

Resume : Materials based on metal oxide nanocrystal networks are essential for many applications ranging from sensors to photovoltaics and photocatalysis. [1,2] In many of these applications electron transport through the nanocrystal network governs the material or device performance and is therefore subject to study and optimization. A sequence of chemical vapor synthesis and thermal annealing in defined gas atmospheres was used to prepare phase-pure anatase TiO2 nanocrystal powders (median particle diameter: 15 nm) featuring clean surfaces. Random networks of these nanocrystals were immobilized from aqueous dispersions onto conducting substrates and are introduced as model systems for electronic conductivity studies. Particle-particle contacts were generated upon thermal annealing while preserving the structural properties of the nanoparticle films. The distribution of electrochemically active electronic states as well as the dependence of the electronic conductivity on the Fermi level position in the semiconductor films was studied in aqueous electrolytes in situ using electrochemical methods. Based on our results we introduce a qualitative model, which highlights the detrimental impact of electron traps located on particle-particle interfaces on the electronic conductivity in random nanoparticle networks. [3] [1] Bai, Y. et al. Chem. Rev. 2014, 114, 10095. [2] Schneider, J. et al. Chem. Rev. 2014, 114, 9919. [3] Rettenmaier, K. et al. ACS Appl. Mater. Interfaces 2019, 11, 39859.

Resume : ZnO nanowires (NWs) are of high interest as building blocks in a vast number of devices and their formation in aqueous solution is typically achieved by the chemical bath deposition (CBD) technique. Most of the efforts have so far been dedicated to optimizing the morphology of un-doped ZnO NWs grown by CBD, but the nature of intrinsic/extrinsic point defects [1] in their center, along with their correlated optical/electrical properties are still unknown, to a large extent, despite its primary importance. In this context, four-point probe resistivity as well as low-temperature cathodoluminescence and Raman scattering measurements are performed to determine the physical properties of ZnO NWs grown by CBD under standard conditions using zinc nitrate and hexamethylenetetramine. It is shown that ZnO NWs exhibit a high electrical conductivity originating from the massive incorporation of hydrogen in different forms [2], which are further investigated by density-functional theory calculations [3]. These findings show the relevance of considering hydrogen as an important defect to control and tune the optical/electrical properties of ZnO NWs grown by CBD for their integration into nanoscale devices. [1] A. Janotti and C.G. Van de Walle, Reports on Progress in Physics 72, 126501 (2000) [2] T. Cossuet et al. The Journal of Physical Chemistry C 122, 22767 (2018) [3] J. Villafuerte et al. “Zinc vacancy – hydrogen complexes as major defects in ZnO nanowires grown by chemical bath deposition” submitted (2020)

Resume : Using planar conductance measurements, we have investigated a set of (p) a-Si:H/(i) a-Si:H/(n) c-Si heterostructures where the thickness of the (i) a-Si:H buffer layer is varied between 2 and 50 nm, well beyond the values used in heterojunction solar cells. From measurements performed at room temperature and using 1D analytical calculations and 2D electrical modeling, we could demonstrate that the deep defect density related to silicon dangling bonds in the (i) a-Si:H layer strongly increases from 1×1017 to 4×1018 cm-3 when the (i) a-Si:H layer thickness is decreased from 50 to 2 nm. This result was interpreted in terms of defect formation and dependence of the defect density upon the position of the Fermi level with respect to the valence band edge. Moreover, quantitative analysis in the framework of the defect-pool model demonstrates that the strong increase of defect density is also promoted by an increase in the width of the valence band tail in the thin (i) a-Si:H layer, suggesting that a very thin layer also suffers from increased disorder. We here extend the measurements and analyses to the temperature dependence of the planar conductance. These confirm the trend observed from room temperature measurements and also reveal features specific to the a-Si:H/c-Si interface. [1] A. Levtchenko et al., Phys. Status Solidi RRL 2019, 1900411

Resume : For typical MgInO UV photodetector, oxygen vacancies play an important role in the oxide material. The number of oxygen vacancies could be as a donor in the thin film. In this work, we investigated the dependence of MgInO photodetector characteristics on the oxygen vacancies concentration ratio by precisely controlling the different oxygen flow in the RF sputter system. It is obvious that by inserting the oxygen flow gas, the oxygen vacancies can be suppressed. Consequently, the sample has higher oxygen vacancies defect show the higher responsivity due to high conductivity of thin film, however, the samples have lower oxygen vacancies defect exhibited a better rejection ratio. Our evaluation results show that the MgInO photodetector exhibited a higher UV-visible rejection ratio and excellent photoresponsivity. Based on this, the MgInO materials system could be applied to a phototransistor, which combines light detection and single magnification in one device, which toward integrated devices.

Resume : Nanoscale ferromagnetic semiconductor (FS) structures are one of the most promising objects for the control over spin interactions by means of different methods, e.g. electrical or optical, and as a result for the creation of new spintronic devices, such spin-LEDs. Usually FS layers grow at relatively low temperatures to avoid phase separations, but nanostructures seem to be formed at higher temperature range. We have elaborated the novel techniques for the MBE growth of (In,Mn)As quantum dots [1-3] at relatively high growth temperatures. (In,Mn)As quantum dots have been grown by molecular beam epitaxy using Mn-atom selective doping. The detailed investigation of the structures obtained demonstrates, that despite relatively high growth temperature, the (In,Mn)As quantum dot structures have a high crystalline quality. Based on (In,Mn)As quantum dots used as active layers, p-i-n diodes were created, which optoelectronic properties were investigated using both optical and electric pumping. References [1] A. Bouravleuv et al., Semiconductors, 47 (2013) 1037-1040. [2] A. Bouravleuv et al., Appl. Phys. Lett., 105 (2014), 232101. [3] A. Bouravleuv et al., Nanotechnology, 27 (2016) 425706.

Resume : Stability and functional properties of nanocrystalline mixed metal oxides are sensitively dependent on defect chemistry and interfaces. Control over defect formation at the nanoscale in vapour phase grown non-equilibrium solids and defect evolution by annealing induced ion diffusion provides efficient means to achieve functional material properties, which significantly differ from their extended solid counterparts. Moreover, understanding the underlying transformation processes opens new pathways to ultimately achieve functional ceramics with tailored grain boundaries and intergranular regions. The present contribution discusses the transformation behaviour of diluted transition metals (Fe, Co) in MgO nanocrystals, which were synthesized by metallocene injection into the Mg combustion flame. Subsequent vacuum annealing of as-synthesized particle powders provides means to control impurity localization and to trigger phase separation in these non-equilibrium solids.[1] By combining structural characterization (XRD, TEM) with X-ray absorption and photoelectron spectroscopy investigations, we tracked valence state and local chemical environment changes of admixed transition metal ions inside the MgO based nanoparticles. Whereas Co-Mg-O nanoparticles exhibit a high thermal stability for Fe-Mg-O nanoparticles we obtained evidence for the formation of impurity-Mg vacancy complexes which ultimately undergo surface migration to enable magnesioferrite nucleation.[2,3] Differences in the behaviour between Co and Fe admixtures will be discussed and rationalized by means of ab initio calculations. [1] A. R. Gheisi et al., Part. Part. Syst. Char. 2017, 34, 1700109. [2] M. Niedermaier et al., J. Phys. Chem. C 2017, 121, 24292. [3] M. Niedermaier et al., J. Phys. Chem. C 2019, 42, 25991

Resume : Introducing foreign chemical species into well-defined composites is an art of defect engineering and materials design. In addition to general doping parameters like concentration and uniformity, the local dopant structures regulated by multi-body interactions may play an even more important role in the apparent physical and chemical properties when the dimension of host materials scales down. Here using 3at% Ce-doped Mn3O4 nanocrystals of <10 nm size as the model system, we will present various atomic configurations of Ce4+ dopant-atoms inside the charge-ordered Mn3O4 matrix, including isolated Ce4+ ions and CeO2 nanoclusters of different orientations, discovered by the electron energy-loss spectroscopic imaging in a scanning transmission electron microscope (STEM-EELS). We found that although Ce4+ ions can be intercalated into the Mn3O4 matrix by preferentially substituting Mn3+ sites and rearranged into nearly coherent CeO2 nanoclusters, the large lattice strain and possible charge unbalance make part of Ce4+ ions deviate from the Mn3+ columns and displaced into interstitial sites. The capability to resolve local dopant arrangements reliably will provide us with better understanding on the doping-induced changes of electronic structures and chemical properties, which in turn serves to the rational materials synthesis. (1) X. Cai, K. Chen, X. Gao, C. Xu, M. Sun, G. Liu, X. Guo, Y. Cai, B. Huang, J. Deng, Z. Liu, A. Tricoli, N. Wang, C. Dwyer and Y. Zhu, Revealing atomic structure and oxidation states of dopants in charge-ordered nanoparticles for migration-promoted oxygen exchange capacity, Chem. Mater. 2019, 31, 5769-5777. (2) X. Cai, X. Gao, G. Liu, Y. Cai, A. Tricoli, N. Wang, C. Dwyer and Y. Zhu, Pinpointing dopant-atoms by STEM-EELS, under preparation, 2020.

Resume : Given the massive shifts facing the future energy-environment paradigm, it is pertinent to evaluate the centrality of energy nanomaterials within the evolving scenario in battery and fuel cells development for next generation. This paper will look into aspects of inducing defects in nano-materials in order to enhance performance of energy storage devices.

Resume : DFT calculations of defects in transition metal oxides often require advanced methods such as hybrid functionals to yield a reasonable description of the electronic structure. When properties of defects at dilute concentrations are desired, the - often incompatible - need for large supercells is added to the above functional requirement. We have recently established self-consistent, site-dependent DFT+U and DFT+U+V as promising approaches to address the challenge of simultaneously meeting these requirements. The minimal added cost of DFT+U(+V) compared to a semi-local functional enables the treatment of large supercells, yet the structural and electronic properties relevant for point-defect calculations agree well with hybrid-functionals. We ascribe this to both the self-consistent determination of the Hubbard parameters that leads to an internal consistency of results, as well as the site dependence, which for localized defect states captures chemical changes on multivalent ions around the defect. We will highlight the performance of the method for oxygen vacancies in the perovskite oxides SrTiO3 and SrMnO3. In the former self-consistent DFT+U+V leads to an electronic structure of oxygen vacancies that agrees well with that of hybrid functionals, which also translates to similar formation energies that agree well with experiment. In SrMnO3 the site dependence of Hubbard U significantly lowers defect formation energies and the critical epitaxial strain for magnetic phase transitions.

Resume : Density functional theory and deep level transient spectroscopy have played inseparable and decisive roles in the identification and understanding of carrier trapping in semiconductors. Important methodological advances were achieved in the last decade, both theoretically and experimentally, for instance in the quantum mechanical description of the interactions between the trapped carriers and the remaining electrons, or in the resolution of concurrent emissions with close emission rates. These allowed junction spectroscopy techniques, supported by insightful first-principles calculations, to come up with detailed electronic-scale models, even when signals were blurred by the presence of random fields like in crystalline alloys. In this paper we explain why the above methods combine so well, and along the way, we will tell a few successful stories, including how we found the workings of major carrier-life-time-limiting traps in SiC power devices, or the recent identification of the defect responsible for the light-induced degradation of the power conversion efficiency of Si solar cells.

Resume : Molecular dynamics simulation and gradient plasticity theory are used to investigate two classical interfaces in nanoscale: grain boundaries (GB) in body-centered cubic Fe bicrystal and Fe/graphene (Gr) interface in composites. Our simulations examine the interactions of dislocation plie-ups with different tilt and twist GBs, showing that the atomic structure at the GB changes during dislocation absorption. The initial GB static energy therefore cannot characterize interface strength, illustrating the need to introduce new mechanical interface energies, as put forth in interfacial gradient plasticity theory. Similar atomistic models were used to study the Gr orientation- and position- dependent strengthening effect in Fe/Gr composites, revealing three possible dislocation-Gr interaction mechanisms: transmission, reflection and gliding. Dislocation transmission can easily occur via Gr’s out-of-plane deformation while reflection is found to be the strongest strengthening mechanism. The detailed understanding of these dislocation-interface interactions may pave a new way to engineering materials in the sub-micron scales.

Resume : Spinel-structured magnesium aluminate (MgAl2O4) possesses high transparency from visible to infrared wavelength range, enhanced strength and high melting temperature, as well as excellent chemical stability and radiation resistance. Combination of these properties makes magnesium aluminate spinel very suitable for a number of technological applications, including elements of fusion and fission reactors, e.g. inert matrices for nuclear fuels, radiofrequency and optical windows for fusion reactors, where spinel exhibits a very high tolerance to irradiation with fast neutrons, due to efficient recombination of primary Frenkel defects – vacancies and interstitials. We have performed the density functional calculations (DFT) to obtain the hole-type defects (V-centres) in magnesium aluminate spinel (MgAl2O4) following the results of recent paramagnetic resonance measurements (EPR) [1]. The hybrid B3LYP functional calculations using large supercells of 448 atoms have demonstrated excellent results for not only bulk properties but also for the V-centres formation in MgAl2O4. Three types of V-centres have been considered and confirmed, namely V1, V2 and V22. The DFT calculations have revealed the atomic relaxation pattern and spin density around the hole-type defects that is suggested as an important complement to the experiments. Moreover, the calculated hyperfine coupling constants (HCCs) have been analyzed and compared with those from the measured EPR spectra. A good correspondence between the calculated and measured HCC values is discussed [2]. 1. A. Lushchik, S. Dolgov, E. Feldbach, A. I. Popov, E. Shablonin, V. Seeman, Nucl. Inst. Methods Phys. Res. B 435 (2018) 31-37. 2. A. Platonenko, D. Gryaznov, E. A. Kotomin, A. Lushchik, V. Seemanb, A. Popov, Nucl. Inst. Methods Phys. Res. B 464 (2020) 60-64.

Resume : Disorder-induced linear magnetoresistance (MR) effect is technologically appealing, especially in the two-dimensional (2D) materials such as graphene, since it offers potential applications in magnetic sensors with nanoscale spatial resolution. Here, we report a room-temperature colossal MR of up to 5,000% at 9 T in terraced single-layer graphene. By laminating single-layer graphene on a terraced substrate, such as TiO2 terminated SrTiO3, we demonstrate a universal one order of magnitude enhancement in the MR compared to conventional single-layer graphene devices. Strikingly, a colossal MR of >1,000% was also achieved in the terraced graphene even at a high carrier density of ~1012 cm-2. Systematic studies of the MR of single-layer graphene on various oxide- and non-oxide-based terraced surfaces demonstrate that the terraced structure is the dominant factor driving the MR enhancement. Our results open a new route for tailoring the physical property of 2D materials by engineering the strain through a terraced substrate.

Resume : Interfaces such as grain boundaries and phase boundaries play a crucial role in the plastic deformation of materials. While the interaction between dislocations and grain boundaries has been studied for decades in simple cases, like low for angle grain boundary or with 2D approaches, realistic features of interfaces have been largely ignored. High angle non-symmetric grain boundaries, phase boundaries with complex intermetallic, segregated solute are among many degrees of freedom that can alter drastically the stabled model for interfacial plasticity. In this presentation, we will explore some possibilities offered by atomistic simulations based on semi-empirical potentials to shade lights on the plasticity at complex interfaces. Examples will include high-angle grain boundaries in FCC metal, complex phase boundary in HCP alloy and crystalline-amorphous interface in cubic-diamond nano-objects.

Resume : Although nanoindentation has been used for more than 40 years for calculating elastic constants (modulus and hardness) of materials at the nanoscale, the test still lacks a concrete theoretical framework. The main problems of the current conceptual framework pertaining nanoindentation include: the one-dimensional consideration of a three-dimensional problem, the calculation of elastic constants after strong local plasticity, and the dependence of the calculated elastic properties on the maximum penetration depth or maximum load. Another problem is that within the current theoretical framework the measurements acquired through the use of instruments of increasing accuracy, are theoretically interpreted by semi-empirical methods involving many assumptions. The proposed framework, in which nanoindentation is considered to be an “inhomogeneous compression” which is due to the tip pyramidal geometry (Berkovich, Vickers), tries to provide solutions to the aforementioned problems. In the proposed framework the effect of the tip geometry is modeled in order to be deducted from the calculation of the modulus of elasticity and hardness. Preliminary results in this direction indicate that the use of gradient theory can actually eliminate the effect of tip geometry by providing values for both the elastic modulus and hardness, independent of the maximum indentation depth or load, i.e. proving that the so-called indentation size effect (ISE) is just an artifact of the specific tip geometry.

Resume : This paper presents a new multiscale computing technology for modeling metal oxide memristive structures with embedded defects used as the basic elements of a resistive memory cell. To study and simulate the patterns and parameters of memristive switching, a multiscale approach is used, in which the first principle quantum-mechanical modeling of the atomic-crystalline structure of metal oxide memrist materials is carried out at the level of the atomic structure, taking into account impurity and artificial structural defects (oxygen vacancies and others). As a result of the calculations, the electronic structure, the density of charge carriers (taking into account their spin polarization), energy zones are determined, data are generated to identify the potentials of interatomic and ion-atom interactions, which are transferred to the next level to model the dynamics of changes in the structural elements of a resistive memory cell using three-dimensional statistical modeling. This work was supported by the Russian Foundation for Basic Research, project No. 19-29-03051 mk.

Resume : Luminescence nanoparticles and nanomaterials (nanophosphors) are relevant in all applications as bulk luminescence materials. However, nanophosphors have found their most notable applications as luminescent markers in molecular biology and medical diagnosis and therapy. These range from non‐invasive in vivo whole‐body diagnosis to in vitro examination of individual organs or cells. It is well known that luminescence properties of nanophosphors suffers from surface defects of nanoparticles. In the current talk unique surface‐dependent properties have been reviewed in popular nanophosphors (Y3Al5O12:Ce; LaPO4:Ce,Tb; YVO4:Eu; ZnWO4; CaF2:Pr,Mn; etc.). The experimental results have been obtained by means of luminescence and VUV excitation spectroscopy technique utilizing synchrotron radiation excitations. The advantages of the experimental methods elaborated in two European synchrotron facilities at DESY (Hamburg, Germany) and MAX IV (Lund, Sweden) will be discussed in details. It is also demonstrated that size-dependent luminescence properties of nanophosphors are significant if the electron thermalization length or the length of free electron pass becomes larger than the size of nanoparticles. Both surface and size-dependent properties play a crucial role in energy transfer processes in nanophosphors.

Resume : Silver nanowires (AgNWs) have drawn significant attention in transparent electrodes and interconnects of integrated circuit in recent years due to their extraordinary chemical stability and high electric conductivity. Generally, failure of the device caused by charge transport has severe influence on the lifetime of devices. This study aims to investigate the influences of planar defects in AgNWs on the electromigration. According to the TEM analysis, the AgNWs obtained from galvanic replacement are with high density of stacking faults, and the stacking faults tend to facet parallel to the {111} plane. In-situ TEM observation of electromigration in single crystal AgNWs reveals that compared to immediately breakdown of defect-free nanowires, defect-rich nanowires undergo longer lifetime due to presence of gradually necking at cathode site. The void formation in electromigration by surface diffusion firstly grows at [111]-Ag and then expand at [202]-Ag. The stacking faults faceting parallel to the {111} plane play an essential role to hinder the atom migration at void expansion stage. The results present the potential possibilities to prolong the lifetime of nanowires instead of adding capping layer or fabricating by complex procedure.

Resume : The talk reports recent progress on the topic of gradient interatomic potentials and their implications. It describes an effort that can generate new types of empirical potentials based on a gradient extension of classical ones used in multiscale simulations. The work is a collaborative effort between the Lab of Mechanics and Materials at the Aristotle University of Thessaloniki, the University of Florida at Gainesville and Liberty University at Lynchburg. The approach is a spring-off of the author’s internal length gradient (ILG) methodology on extending the classical laws of elasticity, plasticity, rheology and electrodynamics to include weakly nonlocal effects in the form of Laplacians of the respective constitutive variables. A most recent extension of the ILG framework has been explored through a generalization of Newton’s gravitational law. This generalization has enabled to revisit, within a simple engineering science treatment, various phenomena and configurations ranging from geological and planetary scales to atomic and elementary particle scales. References [1] E.C. Aifantis, A Concise Review of Gradient Models in Mechanics and Physics, Frontiers in Physics, 2020, 10.3389/fphy.2019.00239 [2] E.C. Aifantis, Internal length gradient (ILG) material mechanics across scales & disciplines, Adv. Appl. Mech. 49, 1-110 (2016) [3] K. E. Aifantis, private communication [4] M. Horstemeyer, private communication

Resume : Silicon optoelectronics is a rapidly developing field of research due to the demand in increased speed of signal transmission in integrated circuits. The main problem of creating such silicon-based circuits is the insufficient luminescence efficiency of cubic silicon (c-Si). To cope with this problem, hexagonal modifications of silicon can be used. In the present work, the possibility of synthesis of light-emitting inclusions of the hexagonal 9R phase (9R-Si) in c-Si substrates by ion irradiation of SiO2/Si system followed by high-temperature annealing has been demonstrated. It is assumed that the relaxation of mechanical stresses arisen upon ion implantation into SiO2 film (provided that the average projected range of ions is smaller than the film thickness) leads to the restructuring of atomic plane packing with the formation of 9R-Si modification during annealing. Such synthesized samples reveal the photoluminescence band at ~ 1235 nm, which corresponds in the photon energy to indirect interband transition in the 9R-Si band structure expected according to our calculation. The influence of 9R-Si synthesis conditions on the luminescent properties is studied and discussed. The work was supported by RFBR (Grant No. 18-32-20168-mol_a_ved).

Resume : Single crystals MgF2 are widely used for optical windows, lenses, polarizers, and as a host material for solid-state lasers, when doped by divalent impurities. Primary radiation defects in ionic solids consist of Frenkel defects-pairs of anion vacancies with trapped electrons (F-type centers) and interstitial ions [1]. Although absorption, EPR and luminescence investigations of F centres in the MgF2 single crystals have already been reported, the appropriate more or less clear understanding of the processes of their thermal annealing is still missing. Recently, using available experimental thermal annealing kinetics for the F centres in MgO and AI2O3, created by electron, neutron and heavy ion irradiations, it was shown that the interstitial migration energy strongly decreases with the radiation fluence [2,3] In this paper, we theoretically analyzed the available experimental kinetics of the F, F2-type center thermal transformation and annealing in a wide temperature range (300-1000 K). Published but not analyzed at that time an optical absorption spectra of MgF2 crystal after neutron irradiation with doses 7 x 10\17nf/cm2 (fast neutron, E>0.1MeV) in the Hydraulic Tube (360K irradiation) and 8 x 10\16nf/cm2 in the Low Temperature Loop (20K) [4] were carefully treated. Spectral peaks corresponding to F and F2 centres with different symmetries (F(D2h), F(C2h), F(C2v)) and metallic colloids were fitted by Gaussian functions. It was shown that the previously proposed phenomenological theory [2,3], which takes into account interstitial diffusion and recombination of interstitials with F3 centers, as well as their mutual sequential transformation with increasing temperature of three types of experimentally observed dimer centers, also perfectly describes a whole set of experimental data of neutron-irradiated MgF2. [1] V.M. Lisitsyn et al., Nucl. Inst. Meth. B. 374 (2016) 24-28. [2] N Kuzovkov et al., Nucl. Inst. Meth. B. 435 (2018) 79-82. [3] E. Kotomin et al., J. Phys. Chem. A. 122 (2017) 28-32. [4] M. Nakagawa et al., Radiat. Eff. Defect. 119 (1991) 663-668.