Defect-induced effects in nanomaterials

Resume : Lanthanum scandate based oxides are promising oxide materials for solid oxide electrochemical devices: protonic ceramic fuel cells (PCFC), electrolysis cells (PCEC) and gas sensors. Generally accepted theory for acceptor doped oxides says that for A(II)B(IV)O3 perovskite substitution of B-site by cation with an effective charge +3 leads to formation of vacancies in oxygen sublattice due to electroneutrality condition. This defect formation is considered as the main mechanism for oxides based on zirconates or cerates of alkali-earth metals. For A(III)B(III)O3 perovskites substitution of A-site by cation with an effective charge +2 causes formation of oxygen vacancies too. Formation of oxygen vacancies as a rule is attributed to a decrease of the local symmetry in close B-cation environment in perovskite structure, since appearance an oxygen vacancy is the reason of existing both 6-coordinated B-cation in the form of structural octahedron, and defected 5-coordinated B-cation. In case of LaScO3, however, we are challenged by the fact that Sc3+ must be 6-coordinated only. Sr-doped LaScO3 oxides possess perovskite-like structure (sp.gr. Pnma) with two nonequivalent positions of oxygen. In strontium-doped lanthanum scandates, in order to satisfy the electroneutrality condition, the substitution of La3+ lanthanum on strontium Sr2+ should lead to a decrease in the oxygen content in the oxide lattice (which is usually associated with the formation of oxygen vacancies according to generally accepted theory) and be accompanied by a decrease in the local symmetry of the Sc3+ ion. It is known that the Sc3+ scandium ion in oxides can be in only one coordination environment equal to 6, which contradicts the formation of oxygen vacancies in its environment, since their appearance should lead to a change in the coordination number of scandium from 6 to 5. Constriction in coordination number 6 and increasing of the symmetry in lanthanum strontium scandates leads us to refuse an idea of oxygen vacancies formation in lanthanum scandate with lanthanum substitution by strontium, and consider a new mechanisms of defect formation, which excludes an existence of an oxygen vacancy in perovskite structure. In this study, we report the novel defect formation mechanism in strontium doped lanthanum scandate, when with strontium doping a release of oxygen due to electroneutrality condition leads to coupling of the structural octahedrons through a joint edge: 2ScO6 – O = Sc2O11. Since strontium doped lanthanum scandate proton-conducting oxides can incorporate hydrogen from H2O-, H2- and CH4-containing gas phase, a new type of defect formation plays an important and determining part in both protonic defects’ formation, protonic and oxygen ionic transport. We submit a description of H2O incorporation taking into account the novel defect formation mechanism including coupling/decoupling of structural octahedron due to H2O release/uptake process.

Resume : We have applied hybrid density functional theory calculations to the analysis of thermoelectric properties of Ir-doped ZnO. The emphasis of this work is on calculating the Seebeck coefficient and electrical conductivity as functions of Fermi level in order to estimate potential of Ir-doped ZnO for p-type conductivity. In particular, the present study was encouraged by previous experimental results showing the Seebeck coefficient's sign change in response to increasing Ir concentration [1]. We describe several possible ZnO-embedded IrOx complexes with varying number of surrounding oxygen atoms and with different Ir oxidation states, including those involving formation of peroxide fragments. All modelled complexes, regardless of Ir oxidation state, demonstrate potential for p-type conduction for the Fermi level, μ[F] in the range 0 < μ[F] < 0.7 eV. Corresponding values of Seebeck coefficient (S) around 90 µV/K as well as the calculated phonon frequencies of peroxide moiety of 810–942 inverse centimeters are in very good agreement with experimental results. In light of this data, we discuss high sensitivity of calculated S(μ[F]) dependencies to the electronic structure. [1] M. Zubkin, R. Kalendarev, J. Gabrusenoks, A. Plaude, A. Zitolo, A. Anspoks, K. Pudzs, K. Vilnis, A. Azens, J. Purans, Thin Solid Films 636, 694 (2017).

Resume : A novel delayed-electroforming process is discovered in defective silicon oxide based ReRAM devices. These devices have simple MIM structure and are CMOS compatible. While most conventional voltage sweep-triggered electroforming occurs at high voltage, this novel electroforming process happens at negligible voltage after pre-stressing with a constant voltage, and allows more energy-efficient switching. A typical delayed-electroforming process consists of three steps: firstly a highly resistive pristine device stressed for hundreds of seconds, then a detection voltage of 10mV is applied across the device to monitor resistance state. The device remains highly resistive (~M?) for minutes after the stress is removed, but abruptly reaches a low resistance state (~100?) under detection voltage. We investigated the effect of charge injection during stressing, and observed this process for devices of different oxide thickness, which indicates that this phenomenon is a common occurrence among various SiOx MIM devices. We also discuss possible causes for this delayed-electroforming.

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 : 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 : In our work, a-SiO2/Si (p – and n- type) structures with an oxide layer were irradiated with 200 MeV Xe ions to a fluence of 108 ions/cm2 at DC-60 cyclotron (Nur-Sultan). Subsequent chemical etching of irradiated SiO2/Si samples was carried out in a 4% aqueous solution of HF with the addition of palladium (m(Pd)=0.025 g) at 18°± 1°C. After that, chemical deposition was carried out using two different solutions: chloride and sulfate. By varying the time and temperature of deposition we obtained nanocrystals of ZnSe2O5 and ZnSeO3. The performed X-ray diffraction analysis (D8 ADVANCE ECO X-ray diffractometer) unambiguously showed formation of ZnSe2O5 nanocrystals in a-SiO2/Si (p – and n- type) templates with an orthorhombic crystal structure and space group Pbcn (60) and ZnSeO3 with an orthorhombic crystal structure and space group Pnma (62).

Resume : Corundum (sapphire, α-Al2O3) is an important radiation-resistant material with potential applications for components of diagnostic windows and breeder blankets. Radiation-induced changes in structural and optical properties of radiation-exposed α-Al2O3 crystalline materials are mainly associated with aluminum and oxygen vacancies. The vacancies in anion sublattice result in the formation of electronic defects after trapping one or two electrons (so called F and F+ -centers). The vast majority of the experiments on point defects in oxides were performed so far using the optical absorption and luminescence, while such rapidly developing techniques as Raman spectroscopy and neutron scattering methods were used only in very rare cases. Note that the optical absorption method does not work in the region of large defect concentrations due to the optical density saturation. Previously it was shown, that Raman scattering techniques proved to be very useful and promising for a better determination of the electronic structure of defects in alkali halides. Here we analyze by means of ab initio calculations the oxygen sublattice defect induced vibrational modes in corundum, as well anion vacancies in KBr. The aim of this study is to find possible “fingerprint” modes for identification of the anion defects, vacancies and interstitials, in various charge states. Using LCAO approach and hybrid B3LYP functional as implemented in the CRYSTAL17 computer code, the structural and vibrational properties are evaluated for each defect, suggesting possible defect-specific modes for vibrational spectroscopy. Obtained results are compared with available experimental data.

Resume : A complex interplay between magnetism, polarization, strain, and stoichiometry is responsible for the functional properties of transition metal oxides. Furthermore, in materials with a cooperative Jahn-Teller distortion, the orbital order can also couple to the defect chemistry and induce novel material properties, as we illustrate through the study of oxygen-deficient LaMnO3 (LMO). At low temperatures, LMO is an insulating A-type antiferromagnet due to the Jahn-Teller distortion that splits the eg orbitals of the high-spin Mn3 ions, resulting in a distorted orthorhombic perovskite structure with alternating long, short, and intermediate Mn-O bond lengths. Our DFT U calculations show that, as a result of these peculiarities, relaxations around an oxygen vacancy (VO) in LMO depend on which type of Mn-O bond is broken, affecting the d-orbital energies and leading to an asymmetric and hence polar localization of the excess electrons with respect to the defect. This behavior is different compared to other manganites, like SrMnO3 (where the two extra electrons reduce the Mn sites adjacent to the vacancy), and can be tuned through strain. Indeed, isostatic and epitaxial strain affect the Mn-O bond lengths and orbital order and can consequently influence the charge localization and polarity.

Resume : Alongside indium gallium zinc oxide (IGZO), many oxides have been investigated for the past 16 years. Whether amorphous or polycrystalline, oxide semiconductors have demonstrated their potential use for large area electronics such as Active Matrix Organic Light Emitting Diodes (AMOLED) and other flexible and transparent electronics. Vacuum and non vacuum processes have been investigated for devices including diodes and thin film transistors (TFTs).Nevertheless, because IGZO use scarce and costly materials, and use many atoms, other more simple semiconductors may be viewed as a replacement. We propose tin oxide in its amorphous phase (a-SnOx). The devices combines abundant atom and simple solution process for TFTs reaching high mobilities. We can obtain the field effect mobility over 50cm2/Vs. a-SnOx demonstrate high performance, low subgap states, and high stability among common stressors making it a candidate for future large area electronics.

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. It takes 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 : Structural modifications of polycrystalline g-Si3N4 spinel sample synthesised via high-pressure method were investigated after irradiation with He+ ions of 150 keV energy and total dose of 10^17 cm^-2. The analysis was based on previous assignment of the PL bands (L. Museur et al., Sci. Rep. 6 (2016) 18523), correlation of their spectral positions and theoretical calculations of the formation energies of cation and anion vacancies in the octahedral and tetrahedral voids. The results indicate strong changes of cathodoluminescence, photoluminescence (PL), PL excitation and Raman spectra. In particular, excitonic PL was significantly inhibited and a new near-IR band appeared with the excitation above the band gap energy of 5.05 eV. This was explained by an effective trapping of photoinduced electrons and holes by charged defects. The spectral shift of PL spectra with the excitation photon energy indicated a heterogeneous nature of the defect sites. The energetic positions of near-IR and visible PL bands correlate, suggesting an interaction with a common cation defect to be an origin. The visible PL of exciton bound to a neutral defect SiVx was red shifted, which was attributed to permutations between empty and occupied octahedral and tetrahedral sites inherent to the spinel structure after collisions with He+ ions. The positively charged cation sites in the spinel structure are compensated by VN''' anion vacancies. The local deformation of the spinel lattice affects PL intensity of the self-trapped exciton at 4.35 eV.

Resume : The CaF2– Si(111) pair has a small lattice mismatch of about 0.6% at room temperature, and the fluorite-like structure of CaF2 is similar to the diamond-like structure of Si. These features allow growing the crystalline pseudomorphic insulator films of CaF2 on Si surfaces instead of SiO2 films which are always amorphous and have a number of intrinsic defects on SiO2 –Si interface. The experiments were made in ultra-high vacuum on the Katun-100 molecular-beam epitaxy (MBE) facility equipped with a CaF2 effusion source having a graphite crucible. In the experiments were made on Si(111) substrates. For all samples, before the growth, the standard procedure of substrates chemical preparation for MBE was used. The aim of experiments was to study the MBE growth of CaF2 films on Si(111) under electron-beam irradiation with the electron energy 20 keV and current density 50 μA/m2. The samples structure and elemental content were studied by reflection high-energy electron diffraction (RHEED) atomic force microscopy (AFM) and Raman spectroscopy. It was found that, during the CaF2 growth on Si, the area exposed to the electron beam undergoes strong modifications, such as in the surface morphology and film chemical composition. With reflection high-energy electron diffraction, atomic force microscopy and Raman spectroscopy, it is shown that the electron beam action leads to the CaSi2 layer synthesis at the interface of the silicon substrate and epitaxial growing CaF2 film. The radiation-induced epitaxial СаSi2 film growth on Si(111) takes place at substrate temperatures above 300 °C. The crystal structures of thin CaSi2 films were found to depend on the deposited СаF2 film thickness: it is the trigonal rhombohedral modification 3R for thin (<20nm) СаF2 films and the trigonal rhombohedral modification 6R for thicker films. This study was supported by the Russian Foundation for Basic Research and by the RosAtom Corporation (Project No. 20-21-00028).

Resume : In the work, we have studied the influence of high-field field injection of electrons on the formation and subsequent evolution of the radiation-induced defects in thin nano-scale dielectric films of MIS devices. One of main effects caused by the irradiation of MIS devices, influencing on efficiency and determining reliability of these, is the changing of the dielectric charge state and the interface resulting in structural change of the materials. We demonstrate that under the concurrent influence by radiation and high-field injection of electrons the accumulation of positive charge in the bulk of the gate dielectric is defined by 1) generation of holes caused by the radiation and high-field ionization, 2) annihilation of a fraction of the trapped charge as a result of interaction with injected electrons, 3) redistribution of local electric fields in the dielectric bulk. We have studied the influence of high-field injection modes on changing of the charge state and defectiveness of MIS structure after the radiation influence. We show that after the radiation influence of MIS structures the high-field injection of electrons can result in erasing of a fraction of the positive charge trapped in the oxide because of its annihilation at interaction with injected electrons. However, the annihilation of positive charge commonly causes an increasing of surface state density. The study of the charge effects, taking place in thermal dioxide films of MIS structures under the concurrent influence of Fowler-Nordheim electron injection and ionizing radiation, is a subject of great interest not only from the perspective of utilization of MOS but besides from the perspective of utilization of radiation MOS sensors.

Resume : Emerging memory classes such as oxide-based resistive, ferroelectric and phase-change random-access memory (OxRAM, FeRAM, PCRAM) are facing more and more interest in the field of memory technology and are discussed as possible successors of flash technology for highly-scaled memory cells. Thereby, radiation hardness [1] is of particular interest, enabling applications in harsh radiation environments, e.g. space. Additionally, there is a scientific interest in the failure mechanisms of the stored memory induced by ion exposure, to better understand the physical background for future improvements. The information storage mechanisms of resistive random-access memory devices are mainly based on the movement of ions, the crystalline structure and, in contrast to flash technology, not directly on charge. In ferroelectric and phase-change memories, where the information storage mechanisms are strongly connected to the crystalline structure (including a missing long-range order) of the active layer, a possibly induced phase change by ionizing irradiation has to be considered. In this study, we compare the effects of heavy ion irradiation on the properties of different emerging memory classes: Phase Change Memory based on germanium-antimony-telluride (GST), OxRAM and FeRAM [2] based on HfO𝑥. We demonstrate the crystallinity changes in these material systems exposed to heavy ion radiation and the relation to defects, as well as the correlation to the electrical properties of full memory devices. Our investigations lead to a better understanding of ion-induced failure mechanisms. This is especially related to oxygen-deficiency in HfOx, the limits of irradiated transistors compared to HfO𝑥-based memory cells and an ion-induced ferroelectric to antiferroelectric phase transition in HfO𝑥-based ferroelectric devices. [1] S. Petzold et al., Heavy Ion Radiation Effects on Hafnium Oxide-Based Resistive Random Access Memory, IEEE Trans. Nucl. Sci. 66, 1715 (2019). [2] T. Kämpfe, T. Vogel et al., Heavy Ion Irradiation Effects on Structural and Ferroelectric Properties of HfO2 Films, 2020 Joint Conference of the IEEE International Frequency Control Symposium and International Symposium on Applications of Ferroelectrics (IFCS-ISAF). IEEE, 1-3 (2020).

Resume : Broad ion irradiation of nanoobjects can considerably change their shape. Examples are ion-beam hammering [1], ion-induced shaping of buried particles [2], and ion-induced viscous flow of nanopillars [3]. Such shape changes are mainly driven by the kinetics of defects generated by binary collisions of ions and recoils. Here we report a new kind of ion-induced structure evolution. Sub-micrometer Sn spheres were irradiated with 30 keV He+ ions in a Helium Ion Microscope (HIM). Above a He+ fluence of 1017/cm², Sn extrusions appeared on the surface of the spheres and were imaged with the HIM. Initially, small, pyramid-like facetted extrusions form at the equator of the tin spheres (north pole pointing to the ion source). Later, each sphere becomes completely covered with tin and appears like a facetted single-crystalline cube. No Sn extrusions were observed for tin spheres with diameters smaller than 100 nm. Transmission electron microscopy and Auger electron spectroscopy studies show that the single crystalline tin spheres are covered with a few-nm-thick SnOx skin and that the extrusions grow epitaxially on the exposed tin surface. A model was developed which assumes that the He+ ions generate 70 Frenkel pairs per ion in the body-centered tetragonal lattice of tin. The implanted helium atoms, interstitials and vacancies are confined by the SnOx skin. Some He atoms will occupy vacancies which will partially inhibit their recombination with interstitials. This results in an increasing pressure of the “interstitial gas”. Furthermore, the He+ ion irradiation will cause erosion of the SnOx skin. The sputter coefficient increases with the angle of incidence, so that openings in the SnOx skin will form in the equator regions first. The interstitials can now escape from the interior of the Sn sphere and form an epitaxial regular Sn lattice on the outside. Computer simulations were performed based on this model. The Frenkel pair generation and the SnOx skin sputtering are simulated with dynamical programs based on the Binary Collision Approximation TRI3DYN [4]. Reaction-diffusion dynamics as well as nucleation and extended defect growth were simulated with a 3D kinetic lattice Monte Carlo program [5] using an RGL-potential for tin. The formation of cavities and their filling with He reproduces the experimentally observed tendency for cube formation. [1] Snoeks et al., Nucl. Instr. Meth B 178 (2001) 62 [2] Schmidt et al., Nucl. Instr. Meth. B 267 (2009) 1345 [3] Xu et al., Semicond. Sci. Technol. 35 (2020) 15021 [4] Möller, Nucl. Instr. Meth. B 322 (2014) 23 [5] Strobel et al., Phys. Rev. B 64 (2001) 245422

Resume : Metallic nanostructures grown on porous silicon (PSi) have attracted great attention due to their prominent plasmonic properties, high stability, and cost-effective fabrication compatible with basic steps of Si technology. These nanomaterials open disruptive perspectives for photovoltaics and biosensing. The last one is mostly realized through surface-enhanced Raman scattering (SERS). Si chips covered with metalized PSi have been widely used as SERS-active substrates, which allow detecting substances at low concentrations down to the single-molecule level. Outrageous performances of such SERS-active nanomaterials have been praised but the effect of defects in Si wafer on the morphology of PSi and further deposited metallic nanostructures have not been considered. These defects lead to poor reproducibility of SERS-signal across the substrate’s surface limiting the reliability of SERS-analysis. Here we present a study of repeatability of morphology, optical properties, and SERS-activity of metallic nanostructures on PSi depending on defects in parent Si induced by irregular dopant level. The defects resulted in the formation of spiral areas in the PSi layer with a porosity deviation up to 30%. This strongly affected SERS-activity of upper metallic nanostructures. This was shown a lower porosity promotes detecting small molecules while a higher porosity is more favorable to find macromolecules. Explanation and discussion of the nature of this phenomenon were given considering simulation of electric field strength distribution and measurement of light absorption in SERS-active substrates. An original approach to avoid non-uniformity in properties of SERS-active substrates based on PSi was developed.

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 : The effect of Au nanoparticle (20 nm in size) decoration on the UV and visible luminescence of ZnO nanorods is studied. ZnO nanorods (100 nm wide, 700 nm long) were synthesized by chemical bath deposition while decoration with small Au nanoparticles (density of 10^10 nanoparticles/cm2) was achieved by immersing the sample in a colloidal solution of nanoparticles. Au decoration has been quantitatively investigated by Scanning Electron Microscopy and Rutherford Backscattering Spectrometry, while the emission properties of the bare and decorated samples have been studied by photo- and cathodo-luminescence. Au decoration of ZnO nanorods causes a significant depletion of free electrons below the surface, leading to a reduction of UV photoluminescence and an increase of visible-UV intensity ratio. The visible emissions are generally ascribed to the presence of surface states and deep levels in ZnO nanostructures. The visible defect components are characterized by blue, green, and orange states with energies at 2.52 eV, 2.23 eV, and 2.03 eV, respectively [1]. The formation of a nano-Schottky junction under the Au nanoparticles decorating ZnO nanorods has been investigated and modeled pointing out a relevant 2D bending of energy bands in the nanorods, leading to a strong electric field beneath the metal-semiconductor interface and parallel to the surface. These data are presented and discussed. [1] E. G. Barbagiovanni, R. Reitano, G. Franzò, V. Strano, A. Terrasi and S. Mirabella, Nanoscale, 2016, DOI:10.1039/c5nr05122c.

Resume : Point defects in crystals have attracted a great attention in recent years due to the possibility of using such centres as the basic elements for quantum computing and quantum telecommunications. AlN is an important material for optoelectronic devices operating in ultraviolet spectral range. The study of defects in AlN is of fundamental importance and is in demand in many fields of application of this material. Optical methods, such as photoluminescence and absorption spectroscopy are effective instruments for investigation of energy structure of defects. For identification of defect luminescence bands it is necessary to consider theoretically the local electron-phonon coupling determining luminescence line shape and its dependence on the temperature. In experiments, luminescence bands with donor-acceptor nature are often observed. Therefore, calculations of the luminescence line shapes of donor-acceptor transitions taking into account lattice relaxations for deep-level defects is necessary for the luminescence bands identification. In this work we have investigated theoretically the energy structure and electron coupling with local lattice vibrations for deep centres in AlN [1]. Using hybrid functional density functional theory, we have calculated local phonon energies, Huang-Rhys parameters, formation energies, charge state thermodynamic transition energies, and luminescence line shapes for different defects in AlN. The defects for luminescence line shape calculations were selected based on their formation energies considering intrinsic defects in AlN, defects and defect complexes containing most common unintentional impurities of carbon, oxygen and silicon. Luminescence line shapes of band to deep centre transitions in AlN have been calculated in dependence on temperature for most abundant defects in AlN. Donor-acceptor luminescence line shapes for shallow donor to deep acceptor and deep donor to deep acceptor transitions have been considered theoretically. Configuration diagrams of oxygen and silicon DX-centres have been calculated, and peak energies of optical transitions of an electron from the DX-centres to deep acceptors have been estimated. Possible assignments of the experimental luminescence bands in AlN based on the calculations have been discussed. [1] I.A. Aleksandrov, K.S. Zhuravlev, Journal of Physics: Condensed Matter, 32, 435501, (2020).

Resume : Lead halide perovskites are well-known for their attractive light emission properties with quantum yield approaching ~95%. Rare-earth ions have been used in the past to alter the photonic properties of semiconductor nanocrystals because their electronic transitions in near-infrared region and can lead to quantum yield beyond 100% due to multi-exciton generation. Doping perovskites with rare-earth ions like Yb3+ can therefore further boost their optical properties. However, the low doping concentrations of Yb3+ have so far restricted the increase in quantum yield. Mechanochemistry is a powerful tool to create defects and achieve higher dopant concentrations. Mechanochemistry also allows the production of such complex materials at a gram scale in a green way, without the use of toxic solvents that could be useful for practical purposes. In this study, we use a custom-built high-energy ball mill to synthesize Yb3+ doped CsPb(Cl0.5Br0.5)3 with higher Yb3+ loading. The effect of milling duration is studied on the incorporation of Yb in perovskite nanocrystals and photophysical properties. We find that by optimal milling, we can incorporate ~5 % Yb3+, giving rise to PL emission at ~980 nm. We will discuss material and photophysical properties obtained from experimental analysis of these materials, such as photoluminescence quantum yield, X-ray diffraction, electron microscopy, to uncover effects that are accessible at high doping levels.

Resume : Group 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. In this work, the effects of SHI irradiation on the luminescence of III-N structures (layers and MQWs) grown on sapphire substrates are studied by optical transmission, µ Raman, photoluminescence (PL), PL excitation (PLE), and time-resolved PL (TRPL). In this study, the influence of different parameters is considered, namely the SHI energy loss and fluence.

Resume : b-Ga2O3 is an emerging wide bandgap semiconductor with promising applications in transparent conducting films for optoelectronic devices, solar-blind ultraviolet photodetectors, photocatalysts and gas sensors [1]. Its wide bandgap and high transparency make b-Ga2O3 is a good host material for optically active centres in the spectral region spanning from the infrared to the ultraviolet. In this context, besides the intrinsic ultraviolet/blue luminescence of undoped b-Ga2O3, Cr-doping has been shown to provide efficient light emission in the red/infrared spectral region. In this work, we present an ionoluminescence study performed with protons and alpha particles with energies from 600 keV to 2.2 MeV of Cr-doped Ga2O3 single crystals co-doped with Si-donors and/or with Mg-acceptors. In particular, along with the quenching of the ultraviolet/blue emission, we observe the enhancement of the Cr3 luminescence yield in Si-doped samples by more than one order of magnitude for a fluence of 1.5E15 protons/cm2 with 2 MeV energy. The observed luminescence by ionoluminescence and photoluminescence at room temperature is dominated by two sharp lines overlapped on a broad band associated, that can be assigned to the 2E -> 4A2 and 4T2 -> 4A2 Cr3 transitions, respectively. On the other hand, in Mg co-doped samples, the yield of the Cr3 emission is high already in the as-grown samples, and no further increase is observed upon irradiation. Additionally, by complementary electrical characterization we present a strong evidence that the pinning of the Fermi level, due to the formation of deep levels, should be the main process responsible for the observed increase of the Cr3 luminescence yield. An annealing study from 500 to 800 ºC of the samples co-doped with Si-donors was also carried out, showing that for temperatures higher than 550 ºC the influence of the irradiation defects on the luminescence properties are removed, and the Cr3 luminescence yields return to their initial values before irradiation. In short, this study contributes to a better understanding of the defect levels that can act as sensitizers for Cr3 luminescence in b-Ga2O3. Furthermore, these results reveal the potential of Cr doped b-Ga2O3 for radiation sensors such as radiation dosimeters, allowing the optical detection and measuring ionizing radiation both in- and ex-situ with high reproducibility and reusability potential. [1] Higashiwaki M and Jessen G H 2018 Guest Editorial: The dawn of gallium oxide microelectronics Appl. Phys. Lett. 112

Resume : The unique structural motifs of nanoparticles (NPs) enable many of their technologically important applications, e.g., in the field of heterogeneous catalysis. X-ray absorption spectroscopy (XAS) is one of a few experimental techniques that allow one to characterize such motifs, and to track their in-situ transformations in small (<3 nm) NPs. Unfortunately, the same reasons that are responsible for the NPs unique functionality (enhanced disorder, heterogeneous environments of metal atoms, low coordinated surface sites and interactions with support and ligands) make XAS data analysis for NPs much more challenging than for their bulk analogues. Here we demonstrate that this problem can be addressed by machine learning methods, which allow us to incorporate into the interpretation of experimental XAS data the insights from theoretical modeling of NPs structure and ab-intio XAS calculations.

Resume : The quest for nanoscale crystals that surpass the performance of a single magnetic core is motivated by the design-concept of controlling the spatial distribution of chemical composition within a single motif [1]. Here, we combine nanochemistry, detailed characterization and Monte Carlo simulations to explore the relation of size and shape on nanoscale surfaces and interfaces emerging in core-shell iron-oxide nanocrystals, while we also discuss those imperfections that assist magnetic properties relevant to nanobiotechnology [2]. Connecting compositional complexity and nanomagnetism allows us to gain insights on how controlled cation vacancy-induced disorder tailors physical properties, including exploitable thermal energy transfer for small-size magnetic nanocarriers. Synchrotron X-ray total scattering experiments corroborate our idea that size-dependent evolution of the metal-cation valence state, produces pinning defects which promote favorable magnetic exchange interactions at subcritical sizes (< 10 nm), and importantly beyond the limitations of finite-size effects alone. We offer a description of atomic interaction effects and knowledge of the different length-scale mechanisms required to facilitate the performance of single-crystal nanoscale particles as functional nanoheaters. [1] J.-H. Lee et al., Nat. Nanotechnol. 6, 418, (2011). [2] A. Lappas et al., Phys. Rev. X 9, 041044 (2019).

Resume : The interest in the novel, often carbonaceous materials with large effective surfaces, high conductivity, stability, is growing due to the downsizing of electrical devices and the demand for low-cost new materials. These materials show great potential applications for electrochemical devices, transistors, and biosensors, or as even as analog materials in laboratory astrophysics. In this work, we present plasma synthesized carbon-based nanomaterials (CBNs) and their analysis. CBNs are produced by low temperature plasma procedures, and examples are given for different processes: for example volume and surface processes, catalyst-free or catalyst driven processes. All these processes result often in conductive nanostructures, like free-standing graphene, vertically aligned graphene nanosheets, thin films, or nanoparticles. These materials are either pure carbons, hydrocarbons, or nitrogen-containing carbons. Nitrogen introduction can be obtained either by growth in the nitrogen-containing atmosphere or plasma post-treatment. These processes finish often in both doped and functionalized materials. A popular technique to simulate plasma-surface interactions is molecular dynamics (MD). MD simulations can provide insight into fundamental mechanisms like the formation of different kinds of bonds. The interplay between doping and functionalization can be advantageous and can result in different functional groups of interest which can be useful e.g. for the synthesis of carbon-based parts of FET biosensors.

Resume : In 2D materials with a high surface-to-volume ratio, such as transition metal dichalcogenides, various post-synthesis approaches to doping have been demonstrated, but doping beyond the solid solubility limit is difficult to achieve, especially in the metal sublattice. Moreover, control over the spatial distribution of dopants remains a challenge. Here, we present a post-growth two-step doping method, which includes annealing followed by deposition of dopants together with Se or S. The Ti, V, Cr, and Fe impurities at W sites are identified by using transmission electron microscopy and electron energy loss spectroscopy. We observe a high density (6.4–15%) of various types of impurity atoms, that are largely confined within nanostripes embedded in otherwise pristine WSe2. With the help of density functional theory calculations, we reveal a novel doping mechanism based on a dislocation climb mechanism in the presence of excess doping atoms. These special 1D heterostructures embedded in 2D planes are promising spatially controlled tailoring of the electronic structure and magnetic properties.

Resume : Carbon dots (CDs), usually defined as carbon nanoparticles with a diameter below 10 nm, are amazing class of fluorescent nanomaterials. This family of materials includes graphene quantum dots (GQDs), zero-dimensional graphene nanoparticles. GQDs with tunable near-infrared (NIR) fluorescence emission promise an excellent bioapplication potential, especially in bioimaging. However, the synthesis of GQDs exhibiting fluorescence emission in near-infrared (NIR) region, in which living systems do not present autofluorescence, is rare and they are synthesized from expensive and even poisonous chemical reagents, using time and energy-consuming approaches and complicated or inefficient purification techniques. Here, we report a facile and fast synthesis of nitrogen-doped GQDs (N-GQDs) emitting from near-ultraviolet to NIR. We reconstruct the different recombination pathways to explain this broadband emission focusing in particular on the NIR band using steady-state and time-resolved photoluminescence measurements. Furthermore, we proved excellent biocompatibility of synthesized N-GQDs by exposure to human vascular smooth muscle cells and demonstrated their potential for in vitro bioimaging applications by confocal fluorescence microscopy.

Resume : Pulse ion implantation of 30 keV MgAl2O4 optical ceramics by Cu2 ions was performed. A comprehensive analysis of the spectroscopic properties of ceramics before and after implantation (XRD, XPS, Raman, ESR, optical absorption, luminescence) was performed. Detected modification spinel matrix luminescent properties as a result of ion bombardment. Besides, it was found that, as a result of pulsed ion implantation, there is a complex relationship between the ion fluence and the concentration of intrinsically optically active centers, due to competing processes of radiation annealing and radiation formation of intrinsic F and F centers. As a result, pulsed copper ion implantation into the surface layer of the ceramic formed «core-shell» nanostructure copper exhibiting surface plasmon resonance. One of the possible channels for the relaxation of energy absorbed by the nanoparticle by the mechanism of localized surface plasmon resonance was observed. It was found that the thermal annealing of implanted ceramics leads to the activation of copper nanoparticles' growth. Heating at temperatures above 200 Celsius initiates the oxidation of copper nanoparticles, presumably by the Valenci-Carter mechanism.

Resume : Confinement of charge carriers in nanoscopic systems has revealed to be an effective strategy to confer ceramic materials unconventional conductive properties by exploiting particle size effects and interfaces characteristics. Strontium titanate (SrTiO3) is a piezoelectric oxide that requires to be doped by acceptor redox species in order to acquire substantial chemical reactivity. In the present work, we show how chemically inert SrTiO3 and donor-doped SrTiO3 ceramics can be turned into active materials by improving the concentration and reactivity of the oxygen defect species using electric-field-assisted treatments [1,2]. The consolidation of hydrothermally prepared perovskite nanoparticles in presence of an electric field arrested the grain growth, retained the specific surface area and enhanced the concentration of Sr vacancies and O─ species. Moreover, the confinement of such ionic defects in mesoscopic particles contributed to a significant improvement of the charge carrier mobility. The effect of these electric-field-induced properties was tested with respect to the total catalytic oxidation of methane. Field-treated materials exhibited more than 95% of methane conversion at 800 °C, with performance over 3 times higher than for the conventionally treated material and other donor-doped perovskites. [1] K. Klauke, B. Kayaalp, M. Biesuz, A. Iannaci, V. M. Sglavo, M. D’Arienzo, S. Lee, S. Jongsu, W. Jung, S. Mascotto, ChemNanoMat 2019, 5, 948–956. [2] B. Kayaalp, K. Klauke, M. Biesuz, A. Iannaci, V. M. Sglavo, M. D’Arienzo, H. Noei, S. Lee, W. Jung, S. Mascotto, J. Phys. Chem. C 2019, 123, 16883–16892.

Resume : Numerous experiments suggest that the capacity decay of Si porous electrodes is related to the significant fracture experienced during the lithiation/de-lithiation process. In this work modeling and surface engineering of nanosized Si is employed to synthesize nanocomposites with enhanced stability. Initially, a multiphysics model is applied to predict the size of Si particles that limit damage formation. The model is experimentally verified against scanning electron microscopy (SEM), which shows for the first-time fracture of Si microparticles after the first cycle. Particles less than 100 nm are predicted to be mechanically stable, and to further increase stability, a facile one step in-situ polymerization process is used to synthesize Si/polydopamine (Si/DPA) nanocomposites, in which ~2 nm of polydopamine (DPA) uniformly coat the surface of the Si. The as-prepared electrodes exhibit higher capacity than previously reported Si/DPA composites: 2000 mAh g-1 at ~700 mA g-1 , with a 66% retention after 100 cycles. A 15% higher capacity retention is observed herein for the Si/DPA nanocomposite electrode compared with the pure Si electrode. The enhanced capacity retention of the nanocomposite electrode can be attributed to the engineered polymeric layer which buffers the expansion upon alloying and enhances adhesion within the nanocomposite electrode.

Resume : Surface is for most semiconductor nanocrystals (NCs) considered a passive or undesired feature that needs to be protected from oxygen to prevent degradation. In semiconductor NCs with purely covalent chemistry, such as silicon, carbon or germanium based NCs, surface species dramatically alter the opto-electronic properties through a direct contribution to the density of states of the NC core. Hence, surface can be an additional powerful tunability tool that can tailor and even enhance the optical properties via (i) strain, (ii) charge transfer and (iii) orbital displacement. In this density functional theory (DFT) study, we test surface tunability of ~2nm silicon NCs (SiNCs), with almost 50 % of constituent atoms residing on the surface, directly covalently linked to a ligand. Silicon is one of the most important materials for industry - used in almost all technologies nowadays, from microelectronics to photovoltaics and light detectors. However, it is lacking efficient light emission due to its inherent indirect bandgap and there is still no Si light source available up to date. Enhancing optical capabilities of silicon is therefore of great interest and importance. We simulate SiNCs with various electronegative ligands (organic, halides) to analyze influence of such ligands onto the radiative rate and general density of states, visualized using the "fuzzy" band-structure approach [1]. We found [2] enhanced radiative rates, but with a lower magnitude than reported before from similar system via tight binding [3,4] and DFT [5]. [1] P. Hapala et al., Phys. Rev. B 87 (2013) 195420; [2] K. Dohnalova, P. Hapala, K. Kusova and I. Infante, Chem. Mater. 32 (2020) 6326; [3] A. N. Poddubny and K. Dohnalova, Phys. Rev B 90 (2014) 245439; [4] K. Dohnalova et al., Light: Science & Applications 2 (2013) e47; [5] K. Kusova et al., Adv. Mater. Interfaces 1 (2014) 1300042.

Resume : When plasmonic nanostructures are arranged in periodic lattices, their scattered light can interact with adjacent nanoparticles via diffraction. Such 1-D, 2-D nanostructures including nanoparticles may be used as building blocks for sophisticated optical devices. There, we describe the method of self-assembly – capillary assisted particle assembly to produce high area and high assembly yield 2-D nanostructures of silver nanoparticles. We observed surface lattice resonances with a high Q factor, which demonstrated the quality of our assembly as well as prooved single-particle positioning accuracy. Our arrays are exposed to air, but that did not destroy the surface lattice resonance phenomenon, so this left space to introduce different materials to the system, e.g. fluorophores for lasing or functionalization of nanoparticles for biosensing applications. Furthermore, our method is not limited to Ag nanoparticles that we had used – self-assembly may be used to pattern a variety of different materials and nanoparticle shapes.

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 : The dilute bismide GaAs1-xBix alloy has experienced an extensive amount of research and represents the emerging class of bismuth-containing group III-V semiconductors. Incorporation of large Bi atoms into the lattice produces perturbations mainly to the valence band states, which leads to the bandgap bowing, as well as large spin-orbit band splitting. The interest in this alloy arises from its potential and already implemented applications in near- to mid-infrared range optoelectronics − lasers, photodetectors, solar cells. The distribution of alloyed atoms in semiconductors often deviates from the expected random distribution and can have significant effects on the properties of the materials. Previous studies of the spontaneous atomic ordering of bismides focused primarily on their atomic and microstructure analysis. The study presented here examines the influence of the CuPt-type ordering on GaAsBi optoelectronic properties, and is the first study that reports the optical anisotropy in GaAsBi alloys. Atomic and bulk-scale structural, chemical, as well as optical study of the dilute GaAsBi alloys was carried out. The optical anisotropy was revealed in samples grown on exact and offcut (001) GaAs substrates via polarized photoluminescence and transmittance spectra, as well as by birefringence and linear dichroism measurements. The observed polarization dependence in all the optical measurements agreed with theoretical predictions for the CuPt ordering. No optical anisotropy was observed in the bismide sample composed of anti-phase domains grown on (001) Ge, in which the anisotropy is not expected. Multiple sample surface and bulk characterization techniques, including SEM, AFM, XRD, and cross-sectional STEM, were employed to exclude other possible sources of structural anisotropies that may cause these optical effects. Atomic-scale HAADF and EDX analysis of Bi atom distribution and XRD suggests that the ubiquitous CuPt ordering is responsible for the optical anisotropy. Further work will be needed to clarify the magnitude of the ordering-induced valence-band splitting in GaAsBi alloys, and to better understand the reasons for such pronounced effects at dilute Bi concentrations. Polarization anisotropy is an important factor to consider for future development of GaAsBi-based lasers and photodetectors. These findings elucidate spontaneous ordering effects in GaAsBi and encourage its further investigations. References: 1. T. Paulauskas et al „GaAs1-xBix growth on Ge: anti-phase domains, ordering, and exciton localization“, Scientific Reports 10, 2002, (2020). 2. T. Paulauskas et al „Polarization dependent photoluminescence and optical anisotropy in CuPtB-ordered dilute GaAs1–xBix alloys“, Journal of Applied Physics 128 (19), (2020). 3. T. Paulauskas et al „Atomic-resolution EDX, HAADF, and EELS study of GaAs1-xBix alloys“, Nanoscale Research Letters 15 (121), (2020).

Resume : GaN is expected to have a high resistance to radiation damage and good stability in extreme environments due to the large displacement energies of the atoms in the crystal lattice and its dynamic annealing properties. For this reason, it is a very interesting material for applications in the field of semiconductor radiation detectors and electronics. In this work we use GaN core-shell p-n junction microwires as building blocks for radiation sensors. In comparison to their thin-film counterparts, the microwires do not only present a superior crystalline quality but may also provide a higher flexibility and compactness. Single wire devices are fabricated through optimized microfabrication steps and electrical characterization is performed under 2 MeV proton and UV radiation. At room temperature, the detectors show a leakage current as low as 1 pA in reverse bias which leads to high signal-to-noise current ratios. At the same time, transient I-V measurements reveal rise and decay times below 25 ms. Besides the electrical signal, the effect of the induced damage by the ionizing irradiation was studied by transient IBIC measurements and IBIC maps. We observed that the electrical signal remains stable even after irradiation with a total proton fluence of 1x10^16 protons/cm^2. On the other hand, irradiation with this fluence led to a small increase of the leakage current, reducing the signal-to-noise current ratio, coupled with a decrease of the forward bias current of the sample, indicating a rise in the resistivity of the device. Finally, we show that the sensor works both in reverse bias and in self-powered mode. Merging this characteristic with the aforementioned flexibility and compactness, this indicates that these devices may be a solution in radiation applications where size limits and low power consumption are key requisites.

Resume : In this study we implement a passivation layer between Cu(In,Ga)Se2 and CdS to perform a fundamental study and to understand the impact of these layers on morphological, structural and optoelectronic properties of CIGS based solar cells. To evaluate the morphological impact of the Al2O3 on CIGS, Raman spectroscopy, X-ray diffraction (XRD), grazing incident X-ray diffraction (GI-XRD) and transmission electron microscopy (TEM) was performed. The structural analysis show that CIGS maintain the same morphological properties. GI-XRD confirmed the non-crystalline nature of Al2O3. The chemical composition is identified by X-ray Photoelectron Spectroscopy (XPS) and TEM-EDS maps. There are evidences of Cu diffusion to the CdS layer when an Al2O3 layer is not present. However, such effect is not visible with the Al2O3 on top of the CIGS, which indicates that the Al2O3 is working as a diffusion blocking layer. TRPL and PL analysis show a very complex behaviour of the recombination channels and are not in agreement, demonstrating that there are several changes when a dielectric layer is placed in between the CIGS and the CdS. Admittance measurements are used to identify equivalent AC circuits and to extract interface defect density (~10^11 cm-3). We also present Muon spin spectroscopy (µSR) results, which suggest that CIGS has a surface-defect region which is reduced when a CdS capping layer is added. However, the introduction of a thin (3nm) Al2O3 layer strongly reduces this effect.

Resume : We analyze the systematic trends in the ABO3 perovskite bulk and (001) surface F-center calculations. For example, we recently performed ab initio calculations for the F-center in the BaTiO3 and SrZrO3 bulk and on the BaO and ZrO2-terminated (001) surface using a supercell model and a hybrid B3PW exchange-correlation functional [1-3]. We find that two Ti atoms nearest to the bulk F-center in BaTiO3 are repulsed, while nearest eight oxygen and four barium atoms relax towards the oxygen vacancy (by 1.06, 0.71 and 0.08% of the lattice constant a0, respectively). The magnitudes of atomic displacements around the F-center located on the BaO-terminated (001) surface in most cases are larger than those around the bulk F-center (0.1, 1.4 and 1.0% of a0). The bulk and BaO-terminated (001) surface F-center bands are located only 0.23 and 0.07 eV under the conduction band bottom, indicating that the F-center is a shallow donor. Our calculations demonstrate considerable increase of the chemical bond covalency between the BaTiO3 bulk F-center and its nearest Ti atoms equal to 0.320e, and even larger increase for BaO-terminated (001) surface F-center and its nearest Ti atom 0.480e, in comparison to relevant Ti-O chemical bond covalency in the perfect BaTiO3 bulk 0.100e. The difference between F-center formation energy in BaTiO3 bulk (10.3 eV) and on its BaO-terminated (001) surface (10.2 eV) trigger the segregation of the F-center from the bulk towards the BaO-terminated (001) surface. We also performed ab initio calculations for BaTiO3/SrTiO3 and PbZrO3/SrZrO3 (001) interfaces [4] as well as ReO3 polar (001) nano-surfaces. For BTO/STO and PZO/SZO heterostructures, we found that the number of interface layers do not influence much the electronic structure of studied structures, while termination of deposited BTO and PZO (001) thin film atop of STO and SZO substrates considerably shift the band edges with respect to the vacuum level and thus reduce the band gap. References: [1] R.I. Eglitis and S. Piskunov, Comput. Condens. Matter 7, 1-6 (2016) [2] M. Sokolov, R.I. Eglitis et al., Int. J. Mod. Phys. B 31, 1750251 (2017) [3] R.I. Eglitis and A.I. Popov, J. Nano-Electron. Phys. 11, 01001 (2019) [4] S. Piskunov and R.I. Eglitis, Solid State Ionics 274, 29-33 (2015)

Resume : Surface enhanced Raman spectroscopy (SERS) is a sensitive and nondestructive technique used for the detection of biological and chemical molecules. Here we outline how semiconductor substrates can be used as platforms for SERS in place of or as a way to further enhance the performance of noble-metal nanomaterials. We demonstrate a universal approach of oxygen incorporation into metal oxide nanowire/metal nanoparticle templates that allows for greater SERS enhancement due to localized surface plasmon resonance excitation, point defect induced optical gap shrinking as well as wettability change of the substrate. We report up to five-fold Raman relative peak intensity enhancement after annealing-induced interstitial oxygen introduction. This approach is applied to defect formation in metal oxide semiconductor nanowires such as ZnO, WO3, TiO2, and NiO and is applicable for a wide variety of probe molecules, making the method suitable for medical, security and environmental applications.

Resume : In many aspects, MoS2 can be considered as prototypical transition metal dichalcogenide (TMD) for which first principal studies are performed to study TMDs. It is well known that defects significantly alter the electronic properties of semiconductors, and thus affect their practical applications. We present a theoretical study of the formation, diffusion and electronic properties of intrinsic points defects (vacancy and interstitial atom) in monolayer and bulk MoS2. All calculations have been performed using a crystal supercell method (the supercell of 96 and 48 atoms was used for bulk and monolayer MoS2 respectively). Quantum Espresso implementation of the DFT was used for the computation of the structure and electronic properties. We used PAW pseudopotentials and PZ LDA and PBE GGA approximations of the exchange-correlation energy. We present the general approach to the calculation of the energy formation of intrinsic point defects in TMDs based on the calculation of the energy formation of Frenkel pair (vacancy interstitial atom) without using chemical potentials. We also, for the first time, present complete calculation of the diffusion coefficient (pre-exponential factor and activation barrier) of intrinsic defect and analyzed the influence of the defect charge states on the diffusion coefficient. So for monolayer MoS2 we have obtained the energy formation of Frenkel pair equal to 12.38 eV and 3.95 eV for Mo and S respectively.

Resume : Graphene and its derivatives have attracted great scientific interest because of their chemical, physical and electrooptical properties. One of them, vertical graphene nanosheets (VGNs) look very promising for future sensor and other applications because of high surface area, unique morphology, optical, chemical, electrical and physical properties. Similarly, to planar graphene, the accurate defect control is crucial for their practical use. Despite intensive recent research performed on planar graphene defectiveness and its relation to the electrooptical properties, the defect analysis of VGNs is still quite modest. In our research, we have tried to contribute to these studies analysing various height VGNs on fused silica substrate produced by microwave plasma enhanced chemical vapour deposition technique. We have studied how the height of VGNs influences their structural, morphological and electrooptical properties and compared them with the features of high quality planar graphene's. The analysis was performed employing XPS, SEM, AFM, Raman scattering spectroscopy, absorption and fluorescence spectroscopy. Defectiveness influence of VGNs on the excited state relaxation dynamics of the film was measured by transient absorption spectroscopy. We demonstrate a clear correlation between the excited state of graphene relaxation dynamics and graphene defect density. We believe that such kind of relation might lead to the effective quality control method of VGNs.

Resume : The effect of 4 MeV electron irradiation on the photoluminescence (PL) properties of Si-doped GaN was investigated. MOCVD grown GaN thin films were irradiated at different doses 1·107, 2.5·107 and 1·108 rad (samples A, B and C, respectively). Photoluminescence (PL) measurements were carried out at room temperature in the 350–650 nm wavelength range using a Perkin-Elmer LS55 PL spectrometer, with an error below 0.5 nm. A source of excitation was light with wavelength λ=315 nm and emittance of 5 μW/cm2. It was obtained that PL spectra from irradiated samples have been changed during long-term period after treatment. The shift positions for near band gap photoluminescence peak as well as its intensity change were detected. After electron irradiation, the shape of PL spectra for samples A-C is almost same. Integral PL intensities of samples B and C have been decrease due to electron irradiation but not dramatically. For Sample A an increase of the integral PL intensity was observed. Small radiation dose (107 rad) results in the increase of integral intensity while larger doses (2.5·107 and 108 rad) results in its decreasing. One can think that that electron irradiation with larger doses results in non-radiative recombination channel magnification. While the treatment with dose of 107 rad, on the contrary, results in its decrease. The last fact is well-known as low doses effect. It is meaning that low doses of external influences result in improvement of useful characteristics (i.e. structural perfection, emission properties, homogeneity and so on).

Resume : Silicon carbide (SiC) is a wide band gap semiconductor suitable for high-voltage, high-power, and high-temperature devices from dc to microwavefrequencies. However, elementary processes of multiplication of electronic excitations under irradiation by synchrotron radiation have not been studied yet in these types of the materials. The aim of the present research is to report on the investigation of multiplication of electronic excitations processes and luminescence in cubic 3C-SiC crystals with theoretically well-studied electronic structure. Another goal of current study is comparative investigation of electronic structure of virgin and neutron irradiated 3C-SiC. The influence of the radiation-induced damage on the electronic structure has a particular interest. For these reason the luminescence and excitation spectra of as grown and neutron 3C-SiC as well as the appropriate reflection spectra were studied under ultraviolet (UV) and vacuum ultraviolet (VUV) synchrotron radiation. Two synchrotron facilities have been utilized for the examination of luminescence and reflection properties: SUPERLUMI station of DORIS III storage ring at DESY (Hamburg, Germany) and FINESTLUMI setup of FinEstBeAMS beamline of MAX IV synchrotron (Lund, Sweden). Results obtained demonstrate that there is clear difference between as grown and neutron-irradiated crystals. Luminescence mechanisms and the appropriate role of neutron-induced defects will be discussed in details.

Resume : Scandium fluoride (ScF3) is a wide-band gap material (Eg≈10eV) belonging to perovskite-type compounds. ScF3 demonstrates strong negative thermal expansion (NTE), which is more pronounced than that of most known NTE materials. Taking into account that luminescence method is very sensitive to lattice structure and symmetry changes the study of intrinsic luminescence properties is extremely important for dipper understanding and analysis of NTE in ScF3. In this study luminescence properties of the single crystal as well as a polycrystalline ScF3 (99.99% Aldrich) have been investigated in wide spectral (1.5-40 eV) as well as temperature (10-300 K) range. Vacuum ultraviolet luminescence excitation spectroscopy technique has been applied. The experiments have been carried out at FINESTLUMI endstation using synchrotron radiation from 1.5 GeV storage ring of MAX IV synchrotron. Emission and excitation spectra as well as their temperature dependencies of ScF3 have been successfully obtained. The origin of luminescence centres will be elucidated and preliminary proposed. The behavior of thermal evolution of excitation spectra in the range of excitonic and/or band-to-band transitions will be discussed in respect of NTE in ScF3. The possibility of non-radiative relaxation of self-trapped exciton in ScF3 with creation of radiation defects will be discussed.

Resume : Sapphire (a-Al2O3) is a technologically important material, in particular, widely used in optical applications such as radiation dosimeters, scintillators and being considered as a promising candidate for windows in fusion reactors. Its optical and mechanical properties are controlled by presence of radiation- (predominantly, neutron) induced defects. In this study, we analyze thermal stability and recombination kinetics of primary anion Frenkel defects - the F-type electronic centers and interstitial oxygen atoms in fast-neutron-irradiated a-Al2O3 single crystals. Theory was developed taking into account neutral and charged Frenkel defect pair formation, defect migration, Coulomb attraction and recombination. Based on ab initio calculations and new theoretical kinetics analysis, we have demonstrated for the first time а co-existence in comparable concentrations of two interstitial types – neutral O atoms and negatively charged O- ions (with attributed absorption energies of 6.5 eV and 5.6 eV, respectively) and have obtained their diffusion parameters, necessary for the prediction of secondary defect-induced reactions and defect stability.

Resume : The smooth (001) surfaces of SrTiO3 (STO) single crystals were investigated by the reflection high-energy electron diffraction (RHEED) method in the temperature range from 5 to 300 K. We have found five structural anomalies depending on temperature [1]. The anti-ferro-distortive phase transition from the cubic structure to tetragonal observed in the STO bulk at 105 K extends on the surface from 70 to 120 K. The found anomalies below 7 K and about 35 K are similar to those in the bulk considered as a crossover between the growth of the ferroelectric atomic displacements with decreasing temperature and quantum-mechanical stabilization of this growth due to the zero-point atomic motion. The other two anomalies are related only to a surface. Differentiation of lattice parameters depending on the depth from a surface revealed their changes, which showed a distinct effect of in-plane contraction on the (001) surface of STO confirmed by the theoretical analysis performed by the DFT method [1,2]. This effect is imposed by the surface symmetry and energetics differences as compared with the bulk. [1] N.V. Krainyukova, V.O. Hamalii, A.V. Peschanskii, А.І. Popov, and E.A. Kotomin, Low temperature structural transformations on the (001) surface of SrTiO3 single crystals, Fiz. Nizk. Temp./Low Temp. Phys. 46, 877-888 (740-750), (2020). [2] V.O. Hamalii, A.V. Peschanskii, А.І. Popov, and N.V. Krainyukova, Intrinsic nanostructures on the (001) surface of strontium titanate at low temperatures, Fiz. Nizk. Temp./Low Temp. Phys. 46, 1377-1385 (1170-1177), (2020)

Resume : Strained SiGe/Ge heterostructures with the (111) surface orientation are attracting attentions owing to their applicability to high mobility nMOS and spintronics devices [1], whereas crystal growth and defect structures of the SiGe/Ge(111) have not been explored sufficiently. Recently we systematically investigated critical thickness of the strained SiGe/Ge-on-Si(111) [2], and found that unusual line-shaped ridge roughness appears on the SiGe surface at the initial stage of strain relaxation. In this study, we clarify an origin of the ridge formation and drastically suppress it by patterning Ge-on-Si substrates as a new approach. In experiments, a Ge layer was grown on a Si(111) substrate with MBE, followed by line & space patterning via photolithography process. Subsequently, a strained SiGe layer was grown on the patterned Ge-on-Si. TEM observations revealed that cracks are generated in the ridge region for the sample without the patterning. By contrast, it was shown that the surface was free from such ridges for the sample with the patterning, which indicates the crack formation can be completely avoided by the patterning. Therefore it can be said that this method can increase critical thickness and highly widen the applicability of the strained SiGe(111) heterostructures to spintronics and high performance electronic devices. This work was partially supported by JSPS KAKENHI (Nos. 16H02333, 19H02175 and 19H05616). [1] K. Hamaya et al., J. Phys. D: Appl. Phys. 51, 393001 (2018). [2] Md. M. Alam et al., Appl. Phys. Express 12, 081005 (2019).

Resume : In recent years, information and communication technologies and associated encryption technology has become more and more important, and the use of the circular polarized light for optical communications has been proposed. In particular, spin LEDs are expected as on-chip circular polarized light generating devices. As a material, we have been focusing on a Ge because it can be monolithically integrated on a Si substrate and can emit lights in telecommunication wavelength bands. Moreover, an employment of a surface orientation of (111) enables epitaxial over-growth of high-quality ferromagnetic materials on the Ge(111), and spin injection from the ferromagnetic materials into the Ge(111) has been realized [1]. Based on these spin-injection technologies, we are aiming to create a spin LED devices with a ferromagnetic electrode. So far, EL emission from Ge has been reported only for Ge with the surface orientation of (100), but EL emission from Ge-on-Si with the (111) orientation has not been explored. In this study, a Ge(111) p-i-n structure is grown on a Si(111) substrate, where both reduction of defects and high-density doping in the epitaxial Ge are issues of great importance to achieve efficient light emission. For the purposes, two-step growth method is employed to reduce such defects and low-temperature growth is partly used for n-type in-situ doping with high-precision. And then, a vertical diode with a mesa structure is fabricated to reduce defect-related leakage currents. As a result, we successfully obtain strong EL emission at room temperature. The sample was grown by using a solid source molecular beam epitaxy (MBE) system. First, an undoped Ge buffer layer of 40 nm was grown on a p-type Si (111) substrate at a temperature of 350℃. Subsequently, a B-doped Ge layer of 500 nm was grown at 700℃, and an undoped Ge layer of 40 nm and a P-doped Ge layer of 300 nm were grown at 300℃. After the growth of the p-i-n structure, a growth of an ultra-thin (UT) Si layer (2 monolayers), P delta doping (2×1014 cm-2) and 7 nm Ge capping were performed to obtain a low resistivity Ohmic contact [2]. An optical active area (125μm×75μm) was mesa-defined by the photo-lithography and following reactive ion etching (RIE) process. As top and back metal contacts, Au and AuGa were deposited, respectively, where the top metal contact area (90μm×40μm) was defined by lift-off process. From the fabricated Ge(111) p-i-n diode, we obtained strong EL emission at room temperature. A clear EL spectrum appeared with an injected current of 220 mA and the EL intensity drastically increased with the injected current increase. We can consider that the EL emission comes from the direct transition owing to the tensile strain induced in the Ge-on-Si. In the future, we can expect the realization of circularly polarized light generating Ge-spin LEDs by replacing metal electrodes with ferromagnetic material ones. This work was supported in part by Grant-in-Aid for Scientific Research (19H02175, 19H05616, 20K21009) from MEXT, Japan. [1] K. Hamaya et al., J. Phys. D: Appl. Phys. 51, 393001 (2018). [2] M. Yamada et al., Appl. Phys. Lett. 107. 132101 (2015).

Resume : Ferroelectric layered perovskites as Bi4Ti3O12 (BTO) are important technological materials due to their physical properties [1]. In particular BTO based structures are promising catalysts for photochemical reactions [2,3]. Therefore, development of physical models that explain the driving mechanisms behind this behavior could further facilitate the development of next generation materials. In this work, we use first principles linear combination of atomic orbitals (LCAO) approach with hybrid density functional theory (DFT) formalism as implemented in CRYSTAL17 code [4] to model BTO in different orthorhombic symmetries, including Fmmm and B2cb. We find that BTO initial geometry within orthorhombic Fmmm symmetry reduces to a tetragonal one, where both in-plane lattice constants coincide. This result qualitatively disagrees with a number or experimental studies that reported Fmmm phase of BTO [5,6]. First principle calculations demonstrate that in order to obtain orthorhombic BTO lattice geometry with different in-plane lattice constants the symmetry should be further reduced, e.g., to B2cb space group. [1] N.A. Benedek et al., Dalton Trans., 44, 10543 (2015). [2] J. Wu, et al., CrystEngComm, 20, 3084 (2018). [3] M. M. Kržmanc, et al., ACS Appl. Mater. Interfaces, 13, 370 (2021). [4] R. Dovesi, et al., CRYSTAL17 User’s Manual, University of Torino, Torino, 2017. [5] B. Aurivillius, Arkiv foer Kemi, 1, 499 (1949). [6] A. V. Knyazev, et al., J. Therm. Anal. Calorim., 122, 747 (2015).

Resume : Recently, scientists are increasingly using the sol-gel technique, which allows a synthesis of the strontium ferromolybdate (SFMO) nanosized powder. The interest in this synthesis method is also due to the fact that with the help of the sol-gel technique, it becomes possible to develop new materials, including the SFMO-(superconductor, semiconductor, dielectric) nanocomposites for a creation of a new spintronic devices generation. However, despite a large number of publications, many issues related to the optimization of the sol-gel technique for the obtaining of nanosized single-phase powders of SFMO double perovskites with magnetic properties required for industry remain unclear, and the obtained experimental data are contradictory. Strontium nitrate, iron nitrate, ammonium molybdate, and high purity citric acid monohydrate in aqueous solution have been used as starting reagents for the synthesis of nanosized SFMO compound by the citrate-gel technique (a kind of the sol-gel technique). The synthesis of SFMO was carried out in several stages, and at the final stage it was annealed at T = 1120 K for 4 h in a continuous flow of a 5%H2/Ar gas mixture. As a result, the single-phase SFMO nanopowders with crystal lattice parameters a = b = 0.55629 nm, c = 0.78936 nm, and antisite defect concentration (occupation of Fe sites by Mo and vice versa) of 6%, and an average grain size of 69 nm were obtained. For a polydisperse powder with a log-normal grain size distribution, the temperature dependences of magnetization in both the field cooled and the zero field cooled mode indicate a two-phase magnetic state of the nanopowder SFMO with a mixture of superparamagnetic and ferrimagnetic particles. The presence of the two-phase magnetic state has also been confirmed by the Mössbauer spectroscopy. It was found that the temperature dependence of the nanopowder magnetization is well approximated by an equation consisting of constants, a Bloch-law spin wave ​term, a higher order spin wave correction, and a superparamagnetic term including the Langevin function. Volume fractions and magnetizations of ferrimagnetic and superparamagnetic particles in SFMO nanopowder were determined. Using the law of approach to the saturation magnetization consisting of an Akulov law term and a superparamagnetic term, the magnetic anisotropy constants of superparamagnetic and ferrimagnetic particles have been calculated based on the experimental data of the field dependences of the SFMO nanopowder magnetization. This work was supported by the European project H2020-MSCA-RISE-2017-778308-SPINMULTIFILM.

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. 20-32-90204) and Grant of the President of Russian Federation (MK-4092.2021.1.2).

Resume : At present stage of scientific development, an extremely urgent task is to introduce the technology of effects of the nanoscale state of matter. A promising area of research is the effects of contact interaction of chemically homogeneous nanoparticles / nanoleers. The quantum mechanical transport effect of the appearance of negative differential resistance (NDR [1]) is known. A resonant tunneling diode (RTD) and a two - layer tunneling transistor (DELTT) are under development, in which the resonant quantum effects of electron penetration through the barrier perform the functions of voltage rectification and, even, signal amplification [2]. The structural elements of such devices have the size of quantum dots of 1-3 nm. However, as the experiment and theoretical calculations show, similar effects can also occur on larger objects. Monodisperse particles in the form of compacts (1grams) obtained using high hydrostatic pressure (P = 300MPa) were brought into contact with the side faces by a mechanical force of the order of 10N. The electrical properties of the resulting linear contact were studied at constant (U ϵ [- 6V- +6V]) and alternating current (excitation signal amplitude I = 150mV, operating frequency range ω ϵ [10 Hz - 2 MHz]) by voltammetry (Potentiostat P30, "ELINS") and impedance spectroscopy (Device Z – 1500, "ELINS") methods. To ensure electrical continuity, the surface of the compressed nanoparticles was hydrated with amospheric moisture for 4 hours (used air humidity range 50-90%). For the first time, the rectifying properties of the contact of hydrated nanoparticles of the same chemical composition (ZrO2 – 3mol%Y2O3) but of different sizes (7.5 nm, 10 and 14 nm) were experimentally established. The dependence of the degree of hydration and amount of dopants were studied. The possibility of obtaining of functional semiconductor heterostructures for electronics and energy of adsorption [3] based on linearly ordered hydrated multi-dimensional oxide nanoparticles of the same chemical composition was shown, An empirical physical model of the established effect was obtained. The study was performed in the scope of the Project H2020/MSCA/RISE/SSHARE number 871284 project and RO-JINR Projects № 268/2020 item 57, 59 and № 269/2020 item 62, 60. [1]https://ko.com.ua/novyj_kvantovyj_diod_imeet_tolshhinu_v_neskolko_atomov_111012. [2] https://radioprog.ru/post/154. [3] A.S. Doroshkevich and al.// Applied Nanoscience. 9(8), 1603-1609 DOI 10.1007/s13204-019-00979-6.

Resume : Two-dimensional materials exhibit new electronic structure properties when the thickness becomes comparable to interatomic spacing. These electronic states are controlled by s and p electrons which are weakly interacting. On the other hand, transition metal oxides (TMO) have strongly interacting d electrons, with narrow bands where the Coulomb repulsion is poorly screened. This gives rise to the wide multiplicity of correlated states occurring in these materials showing almost any electronic ground state of condensed matter. TMO can be metals, insulators, superconductors, ferromagnets, multiferroics and display collective magnetic (spin density waves) and charge (charge density waves) excitations interacting strongly with electrons giving rise to exotic quasiparticle states. In this study, we report on large scale first-principles (ab initio) computer simulations of oxygen vacancies in ultrathin SrTiO3 (STO) and BaTiO3 (BTO) (001) perovskite films. The atomic and electronic properties of this films, as well as formation energy of vacancies were examined within the linear combination of atomic orbitals approximation, with using of the B1WC advanced hybrid exchange-correlation functional of the density-functional-theory (DFT), as implemented in CRYSTAL17 computer code. Various possible spin states of the defective systems were considered by means of unrestricted (open shell) DFT calculations. The systems were studied using a two-dimensional slab model with different number of layers (5–9) and the periodic supercell approach. The defects were considered both inside the films and on the (001) surface of the slabs with TiO2 termination. Dependence of vacancy formation energy on the thickness of slab and position of vacancy in a slab was investigated and compared with results obtained for the bulk STO and BTO. In order to estimate the dependence of defect formation energy on the defects concentration, the calculations were performed for supercells of two different size (2x2 and 3x3). These calculations will be used for prediction of the free-standing this film conductivity at different conditions.

Resume : Trap-assisted photoconductive gain has been observed in organic/hybrid thin films since the early 90’s, providing a route for organic semiconductors to be employed as active layer in highly sensitive UV-vis photodetectors while preserving the advantages of organic materials. [1,2] More recently, we demonstrated direct X-rays photoconversion based on photoconductive gain in flexible organic thin film devices operated below 1V. [3,4] These devices are characterized by an unexpected high X-ray sensitivity that we justified by interpreting the detection mechanism as a photo-modulation of the semiconductor conductivity due to charge accumulation during X-ray exposure. We proposed that, during X-ray irradiation, additional free carriers are generated and accumulate in the organic thin film and to derive a model for such accumulation’s impact on photocurrent we considered the differences in hole and electron carrier transport in organic materials. In fact, the additional electrons and holes generated by the interaction with the X-ray follow a different fate: holes drift along the electric field until they reach the collecting electrode while electrons remain trapped. To guarantee charge neutrality, mobile holes that are collected at the collecting electrode are continuously re-injected from the injecting electrode, i.e. for each electron-hole pair created, more than one hole contributes to the photocurrent, leading ultimately to a photoconductive gain. Crucial for the amplification in this mechanism is the slow recombination dynamics of X-ray generated carriers, resulting here from the presence of deep trap levels which remove free electron carriers from the recombination process. In a very recent work [5] we investigated further the origin of the physical processes and parameters controlling the minority carrier traps that assist the photoconductive gain effect in X-ray detectors based on organic field effect transistors. We demonstrated that by reducing the grain size and increasing the number of grain boundaries, we can increase the density of electron trap states within the material, enhancing the photoconductive gain for the X-ray induced photocurrent. Further, by adding polystyrene to the semiconductor solution, we can reduce the interface hole trap density and consequently the charge carrier mobility is enhanced, as well as the device sensitivity. This study gives an insight on the understanding of the crucial parameters and physical processes that control the X-rays detection performance of organic polycrystalline thin-film semiconductors, fundamental steps to implement real-life applications of direct high-energy radiation detection based on organic thin films. [1] Hiramoto, M. et al., Appl. Phys. Lett. 64, 187–189 (1994). [2] Chen, HY et al., Nature Nanotech 3, 543–547 (2008). [3] L. Basiricò et al., Nat. Commun., vol. 7, 13063, (2016). [4] Ciavatti, A. et al. Appl. Phys. Lett. 111, 183301 (2017). [5] Temiño et al., Nat. Commun., vol. 11, 2136, (2020).

Resume : Photonic devices stand to benefit from the development of chromophores with tunable, precisely controlled spontaneous emission lifetimes. Here we demonstrate a method to continuously tune the radiative emission lifetimes of a class of chromophores by varying the density of electronic states involved in the emission process. In particular, we examined the peculiar composition-dependent electronic structure of copper doped CdZnSe quantum dots. It is shown that the nature and density of electronic states involved with the emission process is a function of copper inclusion level, providing a very direct handle for controlling the spontaneous lifetimes. The spontaneous emission lifetimes are estimated by examining the ratios of emission lifetimes to absolute quantum yields and also measured directly by ultrafast luminescence upconversion experiments. We find excellent agreement between these classes of experiments. This scheme enables us to tune spontaneous emission lifetimes by three orders of magnitude, from ~ 15 ns to over ~ 7 s that is unprecedented in existing lumophores.

Resume : During the synthesis of several semiconductors, the salts present play a crucial role in the crystallization process, while this issue is rarely considered in these studies. Therefore, it is crucial to enlighten the impact of different anionic and cationic species on semiconductor oxides' morpho-structural properties. In this paper, the impact of different halides, respectively their electronegativity (NaX, KX –salts, and HX – as acids) was investigated upon WO3 structure, surface, morphology, and photocatalytic activity (in composite systems with commercial TiO2). Three series of samples were investigated, which were obtained from sodium tungstate dihydrate applying sodium or potassium salts as shaping agents – WO3-NWH-NaX/WO3-NWH-KX, and from ammonium metatungstate hydrate and hydrohalic acids – WO3-AMT-HX. The halide anions' presence did not influence the crystal phase composition – Na and K halide salts led to a 100% hexagonal partial hydrate. However, hydrohalic acids' addition yielded both hexagonal partial hydrate (HPH) and monoclinic (MC) WO3 crystal phases. The presence of W+5 centers were evidenced by Raman spectroscopy and X-ray photoelectron spectroscopy. W+5 and W+6 species affected the band gap values of the NaX and KX series; a higher percentage of W+5, respectively W+6 caused a redshift, while regarding the HX series, it led to a blue shift. Increased electronegativity of the halide anions used during the synthesis has an unfavorable effect upon the composites' photoactivity, while in the case of hydrohalic acids, it had a positive effect. Two composite systems (WO3-AMT-HF+P25 and WO3-AMT-HCl+P25) yielded higher conversion rates (87.7 and 84.6%) than P25 (82.8%). Correlations were found between several Raman peaks ratio, electronegativity, and the composite systems' photocatalytic activity.

Resume : Oxide minerals are natural compounds found in Earth’s crust, some of them, such as cuprite (Cu2O) being often encountered and mined. Synthesized Cu2O nanoparticles are largely researched semiconductors with several application fields: pigment, fungicide, solar cell, catalyst, photocatalyst, gas sensor etc. In the present research the lesser-known cuprite is investigated, to chart the preparation process, the properties and possible applicability of copper (I) oxides mineral counterpart. Cuprite minerals were obtained from the mountain range of Rudabánya, Hungary. As part of the processing, the minerals were crushed with pestle and mortar, submitted to micro grinding in a planetary ball mill and finally were grinded into nanopowders in a wet-type bead mill, using different grinding times, resulting in different size ranges of nanoparticles. Additionally, commercial Cu2O and CuO were purchased (Acros Organics, Sigma-Aldrich), to which the properties of the cuprite minerals were compared. All materials were submitted to thorough investigation of the structural, morphological and optical properties, with the aid of XRD, XPS, Raman spectroscopy, SEM and DRS. Based on powder diffractograms, the raw microgrinded cuprite mineral is majorly composed of Cu2O, with cubic crystal structure typical for cuprite. With the increasing grinding time of the minerals, beside the Cu2O reflections also appear at least two additional reflections on their diffractogram – these belonging to CuO. This structural alteration is corroborated by photoelectron spectroscopic finds: due to high-energy bead milling, cupric ion species appeared on the surface of all nanogrinded cuprites. The success of the milling procedure is proven not only by the broadened Cu2O reflections on the X-ray diffractograms, but also by the SEM micrographs. Raman and diffuse reflectance spectroscopic measurements are in agreement with previous characterization outcomes. CuO having a narrower band gap and being generally more responsive to visible light, than Cu2O, the outcome of the mineral processing can be considered doubly useful: the material was not only reduced to a nanosize more suitable to application, but also its structure was altered to contain possibly more active species. The possible applicability of the μM materials was investigated by adsorption and photocatalytic tests under UV-A light (λmax=365 nm). The used model pollutants in both cases were aqueous solutions of methyl orange, salicylic acid and paracetamol. The preliminary results show various properties when it comes to application in the field of photocatalysis: adsorption phenomena and also possible photocatalytic activity can be observed. These properties are mainly influenced by the preparation process of the nanominerals and the induced defects in their structure eg. the mineral with the longest grinding time, smallest particle size and highest cupric ion content shows adsorption properties in each pollutant case. Acknowledgements. K. Saszet acknowledges the financial support of the Collegium Talentum scholarship provided by the Sapientia Hungariae Foundation.

Resume : Layered halides of alkaline earth metals BaFBr, doped with Eu ions, are used in efficient storage image plate devices for different types of ionizing radiation. Based on the rather high radiation sensitivity of BaFBr, they are interesting for damage creation and energy-storage studies under swift heavy ion irradiation and further development of image plate detectors for ion beam detectors. In this work , we have studied optical absorption spectra of BaFBr single crystals irradiated by 147 MeV Kr ions up to fluences (1010 - 1 1013) ions/cm2 at room temperature. The absorption spectra were measured several times over a fairly long period of time. From the results obtained, it follows that the dependence of the concentration of F centers on the density of absorbed energy passes through a maximum. A decrease in the F (Br -) concentration with an increase in fluence is most likely associated with the aggregation process. Additional investigations of the luminescence of irradiated crystals showed that its intensity depends on the fluence.

Resume : The continuing interest in porous semiconductors, in particular, in porous indium phosphide (InP), is associated with the simplicity of their preparation and the morphological diversity of structures. We have synthetized the porous wires having the ladders-like pattern on the n-InP (111) substrate with the use of electrochemical etching in a water-alcohol solution of hydrofluoric acid. The morphological characteristics of the fabricated microstructures were studied using scanning electron microscopy, while chemical analysis of surface was performed using of the lithium silicon energy dispersive detector (INCA Energy). It was found that the obtained wires have a porous structure. The periodicity of these structures is related to the segregation phenomena that arise during the growth of a InP monocrystal by the Czochralski method. The irregularity of etching of the indium and phosphorus sublattices is connected with the chosen crystallographic orientation of the substrate. The formation of a nanoporous oxide over the surface allows us to consider the obtained structures as ferroelectrics.

Resume : The electrostrictive strain coefficient of RExCe1-xO2-x/2 (RE= Gd or Sm, x ≤ 0.2) undergoes Debye-type relaxation with characteristic relaxation time less than 1 sec. Anelastic behavior of thin films of these materials has been observed during the course of weeks/days, posing the question of the possibility of monitoring electric field- induced dimensional changes on an intermediate time scale ranging from minutes to hours. We were therefore motivated to develop a method for in situ monitoring of electric-field induced strain in ceramic pellets using a laboratory X-ray diffractometer (Rigaku Ultima III, 40 kV, 40 mA, div slit: 1 mm by 5mm). The diffractometer was run in parallel beam mode with a strip detector (Dectris). Diffraction patterns were accumulated every 180 s in “sliding average” mode. For Sm- and Gd- doped ceria, the (422) diffraction peak (2θ=88°) was monitored. A patch of electrolytically-deposited copper on the upper surface of the pellet served as an internal reference ((311)-peak at 2θ=90°). A custom-made BN ceramic sample holder prevented sample heating. The sensitivity of this arrangement to longitudinal strain is better than 20 ppm. Using this setup, we found that the Sm- and Gd- doped ceria ceramics tested expand under the influence of DC bias (approx. 20kV/cm) with a response time of 5-10 hrs . Field-induced changes persist for at least 24 hrs. The origin of this behavior will be discussed elsewhere.

Resume : We report our recent study of the nonlocal dielectric response of a three-dimensional homogenized crystal composed of an InP semiconductor inclusion embedded in a silica glass matrix. The inclusion of InP have the planar ladder-like pattern. Since the conduc-tivity and the plasma frequency of InP depends on temperature, the band structure of a homogenized crystal will undergo significant changes upon cooling and heating. The dependence of the effective averaged plasma frequency on temperature along with the structural anisotropy of the crystal provides additional opportunities for controlling the propagation of electromagnetic modes. The nonlocal permittivity, as the function of frequency and wave vector, is calculat-ed with the use of homogenization theory [1, 2], which is based on the Fourier formal-ism and form-factor division approach. Using the nonlocal effective permittivity, the complex dispersion relation for photonic modes propagating in a periodic semiconduc-tor–dielectric array is described even beyond the long-wavelength limit. In addition, we study the temperature dependence of the nonlocal effective parameters for photonic crystals composed of InP ladders embedded into a silica glass host matrix. In the case of infinitely long ladders, the photonic crystal shows plasma-like behavior with a tempera-ture-dependent effective plasma frequency in the far infra-red range. For the case of isolated InP ladders, the photonic crystal demonstrates a typically dielectric behaviour in the lowest frequency band, whose width can also be tuned by changing the temperature. [1] Cerdán-Ramírez V, Zenteno-Mateo B, Sampedro M P, Palomino-Ovando M A, Flores-Desirena B and Pérez-Rodríguez F. Anisotropy effects in homogenized magnetodielectric photonic crystals. J. Appl. Phys. 106, 103520 (2009). [2] Konovalenko A and Pérez-Rodríguez F. Nonlocal response of tunable photonic metamaterials with semiconductor inclusions. J. Opt. Soc. Am. B 34 2031-40 (2017).

Resume : Black metals are metals with a sponge-like nanoporous surface. Their surface appears black because light enters the pores and does not exit back due to multiple reflections. Black metals can be used as heat-absorbing coatings in thermal detectors or pyroelectric devices. In this research, 500 nm and 1000 nm thick black aluminium (B-Al) films were deposited on 1 mm thick fused silica substrates using pulsed DC magnetron sputtering in a mixed atmosphere of argon and nitrogen of 94% and 6% respectively. The resulting B-Al films had a soft surface which was easily susceptible to scratching. It was found that aluminium nanowires grew from scratches and small abrasions on the film surface when the films were annealed in the vacuum of 10-3 Pa at a substrate temperature of 350 °C. Energy-dispersive X-ray analysis confirmed that the atomic percentage of aluminium was 70–80% both in the film and the nanowires, and the remaining elements were carbon, nitrogen, and oxygen. Longer nanowires grew in the case of a thicker film. Some nanowires had a spiral shape that suggested that their growth was driven by a screw-dislocation mechanism. Scanning electron microscopy did not reveal any visual changes in the shapes of mechanically intact pores after annealing of the B-Al films.

Resume : MgO is as an efficient emitter of exoelectrons. In this research, electron capture in Si/SiO2 substrate during thermally stimulated exoelectron emission (TSEE) measurements of MgO thin films was studied. Si/SiO2 is a substrate commonly used in microelectronics. The substrate consisted of an amorphous 1 µm thick SiO2 layer thermally grown on a Si wafer. Nanocrystalline MgO films were prepared on the substrate by the extraction-pyrolytic method. TSEE from the films was measured and strong TSEE peaks were observed at temperatures of 450 ºC and 525 ºC. Also, photoelectron emission (PE) from the films was measured before and immediately after TSEE measurements. PE was excited by UV photons of 4–6 eV energy and it was found that the registered photoelectrons were emitted both from the film and the substrate. PE spectra recorded after TSEE measurements had several distinct PE maxima that were associated not with MgO but with electrons trapped in the defect centres of the SiO2 layer. We attribute the origin of the defect centres to the SiO2 layer since the same PE maxima were observed in PE spectra of a bare Si/SiO2 substrate after its irradiation with weak electrons with energies up to 1.5 keV emitted by a hot cathode source. Presumably, the defect centres already existed in the as-fabricated SiO2 layer and they could capture exoelectrons emitted by MgO films. To check whether the substrate was charged during TSEE measurements of MgO films, the bare Si/SiO2 substrate was also subjected to TSEE measurements. PE spectra of the bare substrate recorded after its TSEE measurements did not demonstrate any PE maxima. Therefore, our findings suggest that the defect centers that already existed in the SiO2 layer were filled with electrons emitted from MgO film during its TSEE measurements.

Resume : Use of Cr3 ions in the study of the structure in the Mg-Al spinel N. Mironova – Ulmane*, A.I. Popov, G. Krieke, A. Antuzevics, V. Skvortsova, E. Elsts, A.Sarakovskis Institute of Solid State Physics, University of Latvia, Kengaraga Street 8, LV-1063 Riga, Latvia Due to it's a high radiation resistance to radiation damage, magnesium aluminum spinel MgAl2O4 was proposed as a very promising material for RF windows in a fusion reactor. In this work, we present original results on the EPR and optical spectroscopy Cr3 ions in natural and synthetic single crystals (grown by Verneuil method) magnesium aluminium spinel (MgAl2O4) at temperature 10 K. It has been known that cations Mg2 and Al3 in the MgAl2O4 spinels may be differently distributed between the lattice sites. In MgAl2O4 normal spinel unit cell Mg2 occupy 8 tetrahedral (Td) and Al3 16 octahedral (D3d) sites of the unit cell, respectively. The partial occupation of tetrahedral sites by Al3 ions and of octahedral sites by Mg2 is usually referred to as inversion spinel. The chromium ions in the crystal structure of the spinel always occupy octahedral positions. In this position local symmetry of its environment sites is the D3d point symmetry. The EPR and photoluminescence spectra of Cr3 ions are a good indicator of the inversion of the spinel MgAl2O4 lattice. The photoluminescence spectra of Cr3 ions of the natural crystal which we can see efficient red emission R-lines (684.7 and 684.5 nm) due to the 2Eg → 4A2g spin-forbidden transition of Cr3 ions located at the sites with D3d local symmetry. The photoluminescence spectrum of the synthetic single crystal of a stoichiometric spinel MgAl2O4 has lines broader than R - zero-phonon lines in the luminescence spectrum of natural spinel. Besides R- and N-lines which are due to zero-phonon transitions, in the photoluminescence spectra of Cr3 ions wide electronic structures at low temperatures are located at long-wavelength side zero-phonon lines. Dominant contribution in EPR spectra comes from the axial centre, which is formed by Cr3 substitution of Al3 sites in the spinel structure. The line width of the EPR spectra of Cr3 ions in the synthetic stoichiometric single crystals spinel MgAl2O4 is observed to depend on the angle between the direction of the magnetic field and the <111> axis, while for the natural spinel MgAl2O4 this dependence is not observed. The results of studying the effect of heat treatment and radiation on the structure of natural and synthetic stoichiometric MgAl2O4 are discussed.

Resume : Discovery and explanation of surface-enhanced Raman scattering (SERS) phenomena have passed a trail to confident detection of submolar amounts of chemical compounds, which is not possible by ordinary Raman scattering spectroscopy. A variety of SERS-active materials have been fabricated and characterized including colloidal solutions and solid substrates of nanoparticles of noble metals. The latter one is usually presented by an array of metallic nanostructures immobilized on a sculpted host material defining their ordered location. Unique features for SERS-spectroscopy are possessed by grainy metallic coatings on macroporous silicon template that pores have equivalent diameters and the depth varied in a range from 0.5 to 2 microns. The pore geometry provides a concentration of the electromagnetic field in its center due to internal bounces of the excitation light while percolating morphology of metallic film facilitates an additional enhancement between nanograins of metal. However, defects (e.g. uneven pore sizes’ distribution) typical for macroporous silicon, which is fabricated by electrochemical etching (anodization) of silicon wafers, strongly affect the reproducibility of the SERS-signal intensity. In this work, we studied such an effect with SERS-active substrates based on macroporous silicon coated with a layer of silver nanoparticles depending on structural parameters of pores (diameter, depth), the molecular weight of analyte, and regimes of the SERS-measurements (laser wavelength, power density, spot diameter). Structural irregularities of macroporous silicon were found to be critical for reproducibility of SERS-spectra from macromolecules (molecular weight is over 0.5 kDa) resulting in (i) signal deviation up to 25% and (ii) 14% of similar spectra pattern if the laser spot does not overlap or goes much beyond pore boundaries. In contrast, SERS-spectra of the chemical compounds with low molecular weight were not greatly affected by the imperfection of the macroporous template. The observed behavior of the SERS-active substrates was discussed in terms of electric field distribution of the pores under different conditions basing on simulation in COMSOL Multiphysics.

Resume : Aluminum oxynitride (AlON) ceramics reveal an excellent combination of thermomechanical and optical properties in UV, VIS, and IR spectral range. Thus, AlON can be a good alternative to tempered glass and sapphire single crystals and it is also prospective material for high-temperature applications in aerospace engineering and power industry. Lattice defects strongly affect the optical and thermomechanical properties of AlON ceramics and must therefore be understood. In current study we report luminescence properties of nominally pure and europium-doped AlON ceramics prepared by a self-propagating high-temperature synthesis method. Luminescence experiments were carried out under vacuum ultraviolet excitations of synchrotron beam at 1.5 GeV storage ring of MAX IV synchrotron facility (Lund, Sweden). Nominally pure AlON ceramics reveal two broad emission bands in UV spectral range which were attributed to oxygen centers in AlN and oxygen vacancies in Al2O3. The intensity of the last band is strongly dependent on the synthesis parameters. Both emissions are poorly exited in VUV spectral range demonstrating an effective trapping of charge carriers by non-radiative centers, presumably surface loss centers of the ceramics’ grains. The europium-doped AlON ceramics exhibit europium related emission band at about 410 nm. Examining excitation spectra of this emission the multiplication of electronic excitations processes have been successfully observed and analyzed.

Resume : La3Ga5SiO14 (LGS, langasite), La3Ga5,5Ta0,5O14 (LGT, langatate) and Ca3TaGa3Si2O14 (CTGS, catangasite) single crystals belong to the class of functional synthetic materials found their application due to their superior laser, piezoelectric, and electro-optic (EO) properties. The luminescent properties of langasite, langatate and catangasite single crystals have been studied by means of the vacuum ultraviolet excitation spectroscopy utilizing synchrotron radiation from 1.5 GeV storage ring of MAX IV synchrotron facility. The results obtained for the catangasite single crystal are reported for the first time, while the results obtained for langasite and langatate single crystals have been used as references. The catangasite single crystal exhibits two emission bands at 320 nm (3.87 eV) and 445 nm (2.78 eV). Examining excitation spectra in vacuum ultraviolet spectral range, the 320 nm emission band was explained as the emission band of self-trapped exciton in CTGS single crystal. Its atomic structure is discussed. It is also proposed that the 445 nm (2.78 eV) emission in the CTGS is due to the F-centers, which have shown a well-resolved excitation (absorption) band at 5.1 eV (243 nm). The luminescence data for all three compounds are summarized and analyzed.

Resume : We present here an overview of recent results obtained on the modeling and analysis of the thermal annealing of dose-dependent characteristics of radiation damage of several scintillation materials, such as PbWO4, ZnO, BaF2, CaF2, PbF2, Y3Al5O12, Gd3Ga5O12, CsI and KBr. Where possible and where there is sufficient experimental data, we will demonstrate whether the Meyer - Neldel rule for thermal annealing of radiation defects works or not, and we will show whether there is a connection between the migration-recombination properties of defects and the radiation type/fluence. This work has received the funding from Latvian Council of Science via Project LZP-2018/1-0214 “Radiation damage studies in scintillator materials for high-energy physics and medical applications”.

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 : 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 : 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 : Grain boundaries (GBs) are the most significant defects influencing electrical properties of materials. However, the relationship between GB types and the nanoscale electrical behaviour is not fully understood yet, hindering the development and the design of novel electronic materials with a better performance, e.g. conductors, photovoltaics, and thermoelectrics. Here, we provide a systematic study to correlate directly-measured resistivity of well-defined Cu GB segments with their structural characteristics investigating the effect of plane, misorientation and curvature. To this aim Cu thin films are sputtered on c-sapphire substrates and annealed to obtain abnormal grain growth and a variety of [111] tilt GB types, namely ?3, ?7, ?19b, ?21a, ?37c, and low angle GBs in misorientation range of 7°-18°. Then, we create local bi-crystals by focused ion beam (FIB), isolating a single GB. Electron backscatter diffraction (EBSD) is used to identify the GB plane/orientation/inclination, while high resolution transmission electron microscopy (TEM) is exploited to unravel the atomic structures. Subsequently, electrical resistivity measurements are conducted with in-situ Scanning Electron Microscopy (SEM) through four scanning probes using micro-manipulators and a nV sensitive voltage-meter. This methodology enables a direct resistivity measurement for different coincidence site lattice (CSL) and low angle GBs. We found that the resistivity values span for more than one order of magnitude matching the predicted trend by simulations. The GB resistivity can be correlated with excess free volume and energies of the interfaces as found by molecular dynamics simulations (MD). Our results indicate that resistivity is highly dependent on the ?-type of the GB. However, it does not noticeably change for structural variations of GB plane, misorientation deviation from ideal ?-type and inclination within the same ?-type. Nevertheless, GB curvature increases resistivity by 80% due to high density of defects, while large resistivity values are observed for GBs which are not described by a uniform equilibrium atomic structure. Overall, our novel results contribute to a better understanding of the structure-resistivity relation in GBs.

Resume : Redox-based resistive random access memories (ReRAMs) are promising candidates in fields like the memory market and neuromorphic computing. However, the fundamental mechanisms that rule the conduction in these devices are still heavily debated. The present work focuses on studying one model for the conduction, the quantum point contact (QPC) [1], and specifically a single subband approximation (SSA) to this model [2]. With this intent, Pt(20 nm) / Ta(15 nm) / Ta2O5(5 nm) / Pt(20 nm) resistive switching devices were studied, and current-voltage (I-V) curves for both resistance states were obtained. The original QPC model has been found to be hard to apply, as the starting parameters had a strong influence on the fitting results, and the algorithm was not robust. On the other hand, the SSA proved its ability to provide good fits to the data, and to do so better than other conduction mechanisms considered. However, its physical basis is criticized, and it is concluded that, in the devices studied, multiple subbands likely contribute to the conduction, in direct opposition to the assumptions made in such an approximation. A reinterpretation of the SSA’s parameters is proposed, to reconcile the increased performance with a greater physical accuracy. Beyond that, the main challenges and difficulties regarding the application of the QPC to the case of valence change-based ReRAM will be discussed. [1] Miranda, E. and Suñé, J. in 2001 IEEE International Reliability Physics Symposium Proceedings 367 (2001) [2] Miranda, E. A., Walczyk, C., Wenger, C. and Schroeder, T. IEEE Electron Device Lett. 31, 609 (2010)

Resume : Acceptor-doped BaZrO3 is a promising electrolyte for protonic ceramic fuel cells as it combines high bulk proton conductivity with good chemical stability. The protonic conductivity is achieved by dissociative water incorporation into oxygen vacancies formed by acceptors on Zr4 sites. We have investigated the influence of dopants, oxygen vacancies, and protons on the macroscopic elastic and electromechanical properties of acceptor-doped BaZrO3 ceramics. Ceramics of BaZr(1-x)X(x)O(3-x/2 δ)H2δ with X = Al, Ga, Sc, In, Y, Eu and 0.05 < x < 0.2 were prepared by solid state reactive sintering and hydration. Ultrasonic pulsed echo time of flight measurements were used to infer the Young’s and the shear moduli. Both moduli decrease by up to ~20% due to the presence of dopants and oxygen vacancies that cause local lattice distortions [1,2]. Water incorporation into the vacancies decreases the moduli even further. An unexpectedly large electrostriction coefficient (M33 ~10E-15 m2/V2) was detected with a capacitive proximity sensor for all dopants introducing a new class of non-classical electrostrictors. M33 of the hydrated ceramics exhibits a Debye-type relaxation with the relaxation frequency exponentially increasing with the ionic radius closely matching dielectric relaxation measured by impedance spectroscopy. This implies that the protons are associated with the dopants, and the binding strength decreases from Al to Y. [1] M. F. Hoedl, E. Makagon, I. Lubomirsky, R. Merkle, E. A. Kotomin, J. Maier, Acta Mater. (2018) 160, 247 [2] E. Makagon, R. Merkle, J. Maier, I. Lubomirsky, Solid State Ionics (2020) 344, 115130

Resume : An important family of silicon-based substrates used for radio frequency (RF) applications relies on the introduction of a large concentration of traps to prevent the formation of a parasitic conduction layer, detrimental to RF substrate performance. Effective resistivity and harmonic distortion (HD) measurements allow the suitability of the substrate for RF applications to be assessed, but to engineer their values, information on trap properties are required. Those properties are difficult to characterize with conventional capacitance techniques due to the combination of high resistivity and large trap density which induces a Fermi-level pinning. In this work, we used photo-induced current transient spectroscopy to generate a transient current by thermal de-trapping after light-induced trap-filling. The signal was then processed to obtain, over a temperature range, a spectrum depending on the trap energy levels and capture cross-sections. This technique was applied to gold-implanted silicon substrates, thus enabling the detection of four main traps. Quantitative evaluation of the defect concentration was achieved by a fitting of measured and simulated HD results. Finally, information obtained about traps was compared with secondary ion mass spectroscopy measurements and an explanation of the role of each introduced defect on RF behavior was proposed. This work opens the way to the understanding trap behaviour in other RF substrates such as trap-rich silicon-on-insulator substrates.

Resume : Thin film oxide capacitors have been shown to exhibit complex current transients in response to step potentials. These involve a sharp initial increase in current followed by a slower decay, resulting in a distinctive peak in device current. They have been observed in materials such as barium strontium titanate (BST) [1,2] and titanium dioxide [4] capacitors as well as in our own amorphous silicon dioxide devices where they were used to carry out edge detection [5]. It is argued these transients are the result of oxygen vacancy defects drifting under the applied potential and that their timing is an indicator of vacancy mobility. However, such mobility values are derived from a space charge theory which appears to break down when characterised from a relaxation perspective. In this talk we present results which suggest an alternative model based on a combination of field- and charge-driven processes. In addition, we will also comment on the role of light in accelerating the rates of such transients. [8] In previous studies, mobility values were determined from the timing of the peak in current and its proportionality to the applied voltage. [1,2,6,7] Space-charge limited current theory suggests this is the point at which vacancies have traversed across the device and begun collecting at the opposite electrode, resulting in Coloumbic repulsion. Underlying this analysis is an assumption that the transients are defined by a single cause, and that both the rise and fall in conduction are the result of a field-driven redistribution of oxygen vacancies. The vacancies act as donors, forming an n-type region at the negative electrode that modulates electronic conduction. [3] In this way, the two processes may be described with a single state variable – the distribution of vacancies at a given moment. However, the relaxation characteristics of these transients reveal two separate processes. The initial increase in current is volatile on the order of hundreds of milliseconds, whereas the later decrease in current is much more persistent and resets on a scale of tens of minutes. The behaviour seems not to be described by one single process, but instead by two. The timing of the peak is therefore dependent on the rates of both processes, something which is overlooked in the existing literature on this topic. Here, we analyse the current transients by separating them into their two respective processes and attempt to identify their properties separately. From relaxation experiments, combined with the rectifying nature of our devices, we can say with confidence the latter process is a field driven effect, suggesting the drift of charged defects. On the other hand, when using optical measurements, we are able to inject charge across the interface barrier without modulating the applied potential, in turn accelerating that initial increase of device current. This suggests the initial process is instead driven by the injection of charge and not by field. These observations support our hypothesis that transients are the result of two separate processes whose timings are defined not just by field but also the conditions of charge injection. This throws into question the accuracy of mobility values previously determined via current transients, which assume the process to be entirely field driven. [1] S. Zafar, et al. “Oxygen vacancy mobility determined from current measurements in thin Ba0.5Sr0.5TiO3 films,” Appl. Phys. Lett., vol. 73, no. 2, p. 175, Jul. 1998. [2] S. Saha et al. “Transient analysis in Al-doped barium strontium titanate thin films grown by pulsed laser deposition,” J. Appl. Phys., vol. 90, no. 3, pp. 1250–1254, Aug. 2001. [3] R. Meyer, et al. “Oxygen vacancy migration and time-dependent leakage current behavior of Ba0.3Sr0.7TiO3 thin films,” Appl. Phys. Lett., vol. 86, no. 11, p. 112904, Mar. 2005. [4] N. Zhong, et al. “Transient Current Study on Pt/TiO 2- x /Pt Capacitor,” Jpn. J. Appl. Phys., vol. 49, no. 4, p. 04DJ15, Apr. 2010. [5] D. J. Mannion, et al. “Memristor-Based Edge Detection for Spike Encoded Pixels,” Front. Neurosci., vol. 13, p. 1386, Jan. 2020. [6] J. Wang et al. “Oxygen vacancy motion in Er-doped barium strontium titanate thin films,” Appl. Phys. Lett., vol. 89, no. 17, p. 172906, Oct. 2006. [7] S. Zafar, et al. “Measurement of oxygen diffusion in nanometer scale HfO2 gate dielectric films,” Appl. Phys. Lett., vol. 98, no. 15, p. 152903, Apr. 2011.2011. [8] J. Lee, et al. “Charge Transition of Oxygen Vacancies during Resistive Switching in Oxide-Based RRAM,” ACS Appl. Mater. Interfaces, p. acsami.8b18386, Mar. 2019. [9] Y. Li, et al. “Visible-light-accelerated oxygen vacancy migration in strontium titanate,” Sci. Rep., vol. 5, no. 1, p. 14576, Nov. 2015.

Resume : The atomic displacement magnitudes of nearest neighbour atoms around the (001) surface F-center in ABO3 perovskites are considerably larger than the related displacement magnitudes of nearest neighbour atoms around the bulk F-center. In the ABO3 perovskites the electron charge is considerably better localized inside the bulk F-center than in the (001) surface F-center, where the oxygen vacancy charge is more delocalized over the nearest neighbour atoms than in the bulk F-center case. The (001) surface F-center formation energy in the ABO3 perovskites is smaller than the bulk F-center formation energy, which triggers the F-center segregation from the ABO3 perovskite bulk towards its (001) surface. In most cases the (001) surface F-center induced defect level in the ABO3 perovskites is located closer to the (001) surface conduction band bottom than the bulk F-center induced defect level to the bulk conduction band bottom [1-3]. In contrast to the ABO3 perovskite bulk and (001) surface F-centers, the CaF2, BaF2 and SrF2 bulk and surface F-center charge is very well localized inside the fluorine vacancy. The atomic relaxation magnitudes around both bulk and surface F-centers in CaF2, BaF2 and SrF2 crystals, as a rule, are considerably smaller than the relevant atomic displacement magnitudes around bulk and (001) surface F-centers in ABO3 perovskites [4,5]. References: 1. R.I. Eglitis and A.I. Popov, J. Nano-Electron. Phys. 11, 01001 (2019) 2. R.I. Eglitis and S. Piskunov, Comput. Condens. Matter 7, 1-6 (2016) 3. M. Sokolov, R.I. Eglitis et al., Int. J. Mod. Phys. B 31, 1750251 (2017) 4. H. Shi, R.I. Eglitis and G. Borstel, Phys. Rev. B 72, 045109 (2005) 5. H. Shi, R. Jia, R.I. Eglitis, Solid State Ionics 187, 1-7 (2011)

Resume : In the hot topic of the synthesis of highly performing p-type TCOs, copper chromium oxide (CuCrO2) presents promising optical and electrical properties suitable for a wide variety of optoelectronic devices. Nevertheless, this material suffers from relatively high resistivity (ρ > 102 Ω.cm) for stoichiometric thin films at room temperature. Cu vacancies or oxygen interstitial are considered the principal source of p-type doping, with small polaron or band conduction mechanism suggested to interpret the electrical transport mechanism in this material. Furthermore, recent studies reported great electrical performances and adequate transparency for non-stoichiometric CuCrO2 thin films (ρ < 0.01 Ω.cm). In this work, we show that the control of the composition of CuCrO2 thin films allows the synthesis of highly conductive and transparent oxides thin films, by a low-cost and non-vacuum deposition technique. We successfully deposited CuCrO2 thin films out of stoichiometry by Aerosol Assisted CVD at atmospheric pressure. The compositional, structural, and morphological properties were analyzed to understand the effect of the incorporated cationic ratio in the film, Cu/(Cu+Cr), on electrical and optical properties, while addressing the doping mechanism. The incorporated cationic ratio is generally higher than the one in the starting precursor solution, leading to a higher film growth increasing Cu content. The deposition of CuCrO2 out of stoichiometry without any detectable secondary phase is achieved up to a composition of Cu/(Cu+Cr) = 65%. Cu-rich CuCrO2 thin films are characterized by high crystallinity degree, through the formation of large nanocolomnar grains with a strong vertical orientation. The resistivity, the transparency, and the bandgap are reduced with the increase of the Cu/(Cu+Cr) ratio in the film. The greatest electrical and optical properties were found for Cu-rich CuCrO2 thin films with Cu/(Cu+Cr)=65%, resulting in a resistivity of 0.05 Ω.cm, and an average transmittance around 58%, culminating in a Gordon’s Figure of Merit (FoMg) of 2.2 mS. Electrical measurements with temperature under different oxygen partial pressure, together with Raman spectroscopy and XPS revealed a crucial role of oxygen atoms in interstitial sites over the electrical properties, with an enhancement of the conductivity for films annealed in oxygen-rich atmosphere. This is attributed to the creation of defects within the crystal lattice during the thermal treatment. Besides, greater Cu incorporation leads to the synthesis of a composite film formed by Cu2O and CuCrO2. These films present improved carrier mobility and reduced energy gap, with a resistivity around 0.02 Ω.cm, a transmittance of 52%, resulting in a FoMg of 1.4 mS. Finally, the tuning of the optical and electrical performances through a simple chemical approach offers two appealing candidates, Cu-rich CuCrO2 and the composite Cu2O+CuCrO2, for the integration in transparent electronic devices.

Resume : Mixed halide perovskites are promising materials for solar cells, light-emitting diodes and other opto-electronic devices due to their tunable bandgaps. However, under light, these systems show segregation into two phases with different halide compositions (photo-demixing). Interestingly, these two phases tend to remix back in the dark (dark-remixing)1. This phase instability is undesirable when aiming to solar cells with stable output, while it opens opportunities towards new device architectures.2 The observed light induced evolution of different phases involves significant ion transport. Therefore, improved understanding of the underlying defect chemical mechanisms involved in the photo-demixing (and dark remixing) can help both preventing and controlling this effect. While several studies have characterized photo-demixing in 3D mixed halide perovskites, the role of decreasing dimensionality on this process has not yet been fully clarified. In the case of two-dimensional (2D) lead halide perovskites, increased resilience against intrinsic and extrinsic degradation routes has been attributed to the suppression of ion migration in these materials with respect to the 3D counterparts.3,4 It is therefore interesting to probe how the photo-demixing relates to the structure and composition of these mixed halide perovskite systems. Here, we demonstrate for the first time that also 2D mixed halide perovskites show reversible demixing under light. The kinetics of the process is slower for the 2D systems compared with their 3D counterpart. We investigate the evolution of optical and structural properties of 2D perovskite thin films under different light bias conditions, providing information on the thermodynamics and the kinetics of the photo-demixing and dark-remixing processes. These techniques, combined with in-situ conductivity measurements, also allow us to assess the influence of light intensity, thin film composition, substrate, and encapsulation on the ionic properties and phase behavior of these systems. Lastly, we propose a model that describes photo-demixing and dark-remixing in 2D mixed lead halide perovskites based on our previous analysis of the 3D systems.5Our study contributes to defining the thermodynamic picture of 2D mixed halide perovskites, which will aid compositional engineering of optoelectronic devices where photo-demixing is controlled. 1. Hoke ET, Slotcavage DJ, Dohner ER, Bowring AR, Karunadasa HI, McGehee MD. Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chem Sci. 2015;6(1):613-7. 2. Mao W, Hall CR, Bernardi S, Cheng YB, Widmer-Cooper A, Smith TA, et al. Light-induced reversal of ion segregation in mixed-halide perovskites. Nat Mater. 2020. 3. Cho J, DuBose JT, Le ANT, Kamat PV. Suppressed Halide Ion Migration in 2D Lead Halide Perovskites. ACS Materials Letters. 2020. 4. Grancini G, Nazeeruddin MK. Dimensional tailoring of hybrid perovskites for photovoltaics. Nature Reviews Materials. 2019;4(1):4-22. 5. Kim GY, Senocrate A, Wang Y-R, Moia D, Maier J. Photo-Effect on Ion Transport in Mixed Cation and Halide Perovskites and Implications for Photo-Demixing**. Angewandte Chemie International Edition. 2021;60(2):820-6.

Resume : The density functional theory (DFT) calculations are nowadays widely used to investigate defects behavior in materials and, thus, represent an effective complement to experiments. Such calculations can provide us with information inaccessible for experiments and/or suggest an important comparison. It is very much due to improved methods available in modern DFT computer codes (e.g. the hybrid density functionals). We present here several cases when the DFT calculations were particularly useful. The DFT calculations of EPR parameters and atomic and electronic structure were performed for hole-type defects in MgAl2O4. The main focus was placed on the calculation of the so-called V-centers with the single hole trapped on a regular oxygen ion. The calculated hyperfine coupling constants were very consistent with the experimental data, but the atomic and electronic structure established and the thermodynamic stability of such defects confirmed on the basis of calculated formation energies [1]. Another example considered the superoxide defect in -Al2O3. The DFT calculations demonstrated the interstitial oxygen dumbbell could exist in the two configurations, symmetric and asymmetric [2]. However, the relaxation pattern around the hole-type center in the asymmetric configuration shows a good agreement with that found from the measured EPR measurements. In addition, the calculated formation energies confirmed the asymmetric configuration being a ground state one. We emphasize important role of bond lengths, spin density and atomic charges for such calculations to get deeper insight into defects behavior. In addition, the calculated formation energies will be discussed for oxygen vacancies in ABO3 perovskite materials with transition metals (e.g., Fe, Ti) and emphasis on the comparison with experimental data. [1] A. Platonenko et al., Nucl. Instr. Meths. Phys. Res. B 464 (2020) 60. [2] V. Seeman et al., Scientific Report 10 (2020) 15852. [3] D. Gryaznov et al. J. Mater. Chem. A 4 (2016) 13093. [4] D. Gryaznov et al. J. Phys. Chem. C 117 (2013) 13776.

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 the observation of elastic domains with relaxor behavior in Sm- or Gd-doped ceria ceramics 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 non-alternating electric field, 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 ~1.5 unit cells (~0.8nm).

Resume : Semiconductor quantum dots (QDs) that exhibit size-dependent optical properties have proven to be suitable materials for many prospective applications in the area of nanotechnology. As the demand for smaller, faster and more efficient electronic devices increases, QDs with tunable properties play an important role in the miniaturization process of nanoengineering (including new catalyst materials, sensitization of solar cells, optoelectronic devices and biomedical applications). Among them ternary chalcogenides like CuInS2 are presently considered as promising alternatives to the good performing, but highly toxic, Cd and Pb based QDs. The most common synthesis strategy to yield CuInS2 nanoparticles involves 1-dodecanethiol (DDT), which serves as solvent, stabilizing ligand and sulfur source. The usage of this ligand is a double-edged sword, since due to its strong bonding it produces high-quality nanocrystals but at the same time leads to a challenging exchangeability with other ligands. Since DDT hasn’t any functional groups, this can limit the fields of application through compatibility issues, which is a major drawback since ligand exchange is often mandatory (e.g. enhance the coupling between QDs and electrodes or substrates in solar cells and aqueous phase transfer for biomedical applications). Our work therefore aims to surmount this problem and consequently obtain a broader field of application by overcoming existing compatibility issues. We present a synthesis route that allows for universal surface modification with thiol-containing molecules. These molecules with various terminal groups were used to functionalize the nanoparticles and stabilize them in different media, making any surface charge and polarity accessible. The success of the ligand exchange was confirmed by FT-IR spectroscopy and pH dependent zeta potential and DLS size measurements. It is further shown that this method is transferable and could be used to functionalize other QDs that crystalize in the zincblende structure (or in derivate of it, like chalcopyrite).

Resume : Bandgap engineering in semiconductor nanostructures is usually achieved by varying the growth conditions or by imposing high strain levels. Here, we report post-growth bandgap engineering in nanowires by mere exposition to low-energy ionized hydrogen gas. The GaAs/GaAsN core/multishell nanowires in this study contain only 0.7%, 2% and 3% of N, which is typical for dilute nitrides [1]. At such low concentrations, the N atoms behave as strongly localized and perturbing lattice defects and give rise to a number of counterintuitive effects. Among others, they create a red-shift of the GaAsN bandgap as high as 0.35 eV when increasing N from 0 to 3%. With μ-Raman and μ-photoluminescence measurements on single wires, we demonstrate that these defects can be engineered on demand by controlled hydrogenation. The H atoms are incorporated in N-H complexes and annihilate the electronic perturbation effect of the N atoms [2]. Thereby, we can effectively tune the GaAsN bandgap up to the value of pure GaAs. This giant tuning is accompanied by a photoluminescence signal increase of more than one order of magnitude. Our low energy hydrogenation approach creates new, fast and effective possibilities for tuning the optical properties of nanowires at the nanoscale and forming novel site-controlled quantum dots and quantum rings. [1] M Yukimune et al 2019 Nanotech. 30 244002 [2] F Biccari et al 2018 Adv. Mater. 30, 1705450

Resume : Gallium oxide is one of the most promising materials of future electronics due to its wide bandgap (4.4-4.9 eV) and other advantages. Its properties strongly depend on impurity-defect composition that is sensitive to technological parameters. In this work, the properties of β-Ga2O3 layers obtained by magnetron sputtering and modified by ion implantation are studied. Among the advantages of these methods is their non-equilibrium nature promising for obtaining p-type Ga2O3. Ion implantation also allows the controlled introduction of point defects and impurity atoms to a given depth, which determine the electrical and optical parameters of Ga2O3. The dependence of chemical composition, structure, electrical and optical properties on the deposition and ion-modification conditions is analyzed. Ion implantation is also used for the formation of Ga2O3 nanocrystals in Al2O3 film deposited on Si substrate. The possibility of creating effective UV-range photodetectors is shown for the samples with ion-synthesized Ga2O3 nanocrystals. The solutions proposed here are intended to significantly expand the range of applications of gallium oxide in advanced electronic and optoelectronic devices. The work was supported by RFBR (19-57-80011), Department of Science and Technology (DST/IMRCD/BRICS/Pilot Call 3/ Ga2O3 /2019) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq – Brazil). N.A.A. acknowledges the support of the President of the Russian Federation fellowship.

Resume : The deposition of a thin elemental Al layer on the clean surface of bulk insulating SrTiO3 results in the formation of surface of oxygen vacancies on the oxide. As a result a two dimensional electron system (2DES) with remarkable electronic properties is obtained. The electronic structure of such 2DES has been shown to be directly related with the spin-to-charge conversion process that takes place in devices based on such system [1]. In this work we will present measurements of the electronic structure of the 2DES by means of angle resolved photoelectron spectroscopy (ARPES) together with self-consistent tight binding supercell calculations that are in good agreement with our measurements. Moreover we will show how we can tailor the electronic carrier density by controlling the Al layer thickness and thus the amount of oxygen vacancies induced in the surface. We demonstrate that it is possible to tune the electronic density in Al/STO to match that of the celebrated LAO/STO system and perform a comparative study of the electronic structure in both systems. In conclusion, we show how defects such as oxygen vacancies can be used as an effective tuning knob to tailor a desired electronic structure. [1] D.C. Vaz, et al. Nature Materials 18, 1187 (2019). We acknowledge financial support from Comunidad de Madrid (Atracción de Talento grant No. 2018-T1/IND-10521) and by MICINN PID2019-105238GA-I00.

Resume : We discuss defect-related free volumes in raw functional nanomaterials such as chalcogenide GeSe2-Ga2Se3 and GeS2-Ga2S3-CsCl glasses or oxide Cu0.4Co0.4Ni0.4Mn1.8O4 MgAl2O4 ceramics which are caused by different technological and post-technological modifications of these compounds. Positron annihilation lifetime spectroscopy (PALS) method within different approaches is used as a main tool for investigations. In the case of 80GeSe2-20Ga2Se3 glasses it is shown that crystallization process during annealing at 380°C for 25 and 50 h indicates specific fragmentation of larger free-volume nanovoids into a greater number of smaller ones. For GeS2-Ga2S3-CsCl glasses it is established that the addition of CsCl transforms free volume due to the void agglomeration in glasses with 10 % mol. of CsCl. In Cu0.1Ni0.8Co0.2Mn1.9O4 ceramics with 8 % of NiO phase addition, positron trapping defects near grain boundaries are formed. Extended defects in nanoporous MgAl2O4 ceramics were characterized by PALS before and after water-immersion treatment. It is shown that in both cases the same type of positron traps prevails, but positron trapping in defects near grain boundaries occurs more efficiently in the water-moistened ceramics. The water vapor modifies defects in ceramics located near grain boundaries and this process is accompanied by the void fragmentation at water adsorption with further void agglomeration at water desorption after drying.

Resume : Single?walled carbon nanotubes (SWCNTs) are a 1D nanomaterial that shows fluorescence in the near?infrared (NIR, >800?nm). In the past, covalent chemistry was less explored to functionalize SWCNTs as it impairs NIR emission. However, certain sp3 defects (quantum defects) in the carbon lattice have emerged that preserve NIR fluorescence and even introduce a new, red?shifted emission peak. Here, we report on quantum defects, introduced using light?driven diazonium chemistry, that serve as anchor points for peptides and proteins. We show that maleimide anchors allow conjugation of cysteine?containing proteins such as a GFP?binding nanobody. In addition, an Fmoc?protected phenylalanine defect serves as a starting point for conjugation of visible fluorophores to create multicolor SWCNTs and in situ peptide synthesis directly on the nanotube. Therefore, these quantum defects are a versatile platform to tailor both the nanotube's photophysical properties as well as their surface chemistry.

Resume : The lack of efficient group-IV light emitting materials is still a bottleneck for large scale, low cost CMOS-compatible data transmission technologies. The main disadvantage in CMOS-compatible group-IV materials, like Si and Ge, is that their band gap is indirect, leading to very inefficient light emission requiring phonon-assisted electron-hole recombination. In this work we use first-principles calculations to show that the introduction a split-[110]-interstitial defect can be used to enhance the direct band gap emission in Ge. The Ge-Ge split-[110]-interstitial defect, the most common interstitial defect in Ge, affects the electronic structure of Ge in two ways that improve light emission. Firstly, it hybridises the conduction band minimum states at the L point with the conduction band state at the zone centre, enhancing direct electron-hole recombination with the zone-centre valence band maximum. Secondly, it introduces two new states at the valence and conduction bands with Delta character, with its wavefunction pinned at the interstitial defect, offering a new direct transition for light emission. We explore how to further enhance the emission from these two recombination paths by changing the majority carrier population to saturate non-radiative paths, and by introducing Sn-Ge split-[110] interstitials. Finally, we explore quantum confinement to shift the wavelength to key spectral regions for datacom and sensing applications.

Resume : A model device based on an epitaxial stack combination of titanium nitride (111) and monoclinic hafnia (11-1) was grown onto a c-cut Al2O3-substrate to target the role of grain boundaries in resistive switching. The texture transfer results in 120 degree in-plane rotated m-HfO2 grains, and thus, in a defined subset of allowed grain boundary orientations of high symmetry. These engineered grain boundaries thread the whole dielectric layer, thereby providing predefined breakdown paths for electroforming-free resistive random access memory devices. Combining X-ray diffraction and scanning transmission electron microscopy (STEM) based localized automated crystal orientation mapping (ACOM), a nanoscale picture of crystal growth and grain boundary orientation was obtained. High-resolution STEM revealed low-energy grain boundaries with facing (-1-1-2) and (-121) surfaces. The uniform distribution of forming voltages below 2 V – within the operation regime – and the stable switching voltages indicate reduced intra- and device-to-device variation in grain boundary engineered hafnium oxide based random access memory devices [1]. [1] S. Petzold et al., Adv. Electron. Mater. 5, 1900484 (2019).

Resume : Partial differential equations (PDE) have been widely used to reproduce patterns in nature, and to give an insight on the mechanism underlying pattern formation. Despite a number of PDE models have been proposed, they rely on pre-request knowledge of physical laws and symmetries, and one has difficulties to develop a model to reproduce a given desired pattern. Here we present a method that estimates the PDE models reproducing given patterns based on the “order parameters”, that is, features considering symmetries of patterns. Our method successfully estimates parameters in a model as well as the best model to make the target pattern from the viewpoint of the inverse problems, especially Bayesian model selection. We apply our method two-dimensional and three-dimensional nontrivial patterns, namely dodecagonal quasi-crystals, double gyroid, and Frank-Kasper structures reproduced by using a family of phase-field crystal models. Our method not only estimates the parameters to reproduce these patterns but also gives an insight on the appropriate number of length scales in the model.

Resume : Optical properties of binary lead halides, PbX2 (X=F, Cl, Br) make these materials suitable for optoelectronic devices and radiation detectors. These properties of PbX2 strongly depend on experimentally confirmed presence of self-trapped charge carriers. The hybrid DFT computational modelling was performed for the first time for small radius polarons - self-trapped electrons (STEL) and holes (STH) in PbX2 crystals widely used as scintillators. The atomic, electronic structure, spin and charge distributions and formation energies for both types of polarons were predicted for orthorhombic and STEL for the cubic PbF2, STH structure identified in a controversial case of PbCl2. The positions of STEL/STH energy levels in the band structure were determined. We also confirmed and analyzed in detail experimentally suggested models for other STEL/STH cases in PbX2.

Resume : The extreme downscaling of logic devices has required optimization of the device architectures and material performance. Miniaturization bottlenecks have emerged with the evolution of technology nodes and have included mobility degradation within the channel and enhanced contact resistances. The latter, caused by the shrinkage of the metal/semiconductor contact areas, has become one of the most critical contributors to MOSFET parasitics [1]. The introduction of Si1-xGex source/drain (S/D) stressors within Si pMOS devices has been successfully shown to improve device characteristics. Thanks to its larger lattice parameter compared to Si, pseudomorphic Si1-xGex allows the transfer of compressive strain to the channel, thereby enhancing the hole mobility. An additional benefit of using Si1-xGex is on the contact resistance, as it enables the Schottky barrier height to be lowered [2] and higher active B doping concentrations within the region adjacent to the metal contact to be realised [3]. However, the smaller covalent radius of B compared to Si and Ge results in a reduction of the Si1-xGex lattice parameter, thereby reducing the amount of compressive strain transferred to the channel [4]. Targeting higher Ge contents may compensate this, but it does increase the risk for unwanted strain relaxation and formation of dislocations. Despite difficulties in precisely quantifying its impact, material relaxation is expected to reduce the strain transfer efficiency and may affect the electrical properties of the material. In this contribution, we have investigated the impact of strain relaxation on the electrical and contact properties of B-doped Si1-xGex layers pseudomorphically grown on 300 mm Si(001) wafers. Three different Ge concentrations (x = 0.25, 0.50 and 0.65) were considered. The specific contact resistivity of subsequently fabricated Ti / Si1-xGex contacts were evaluated using the multiring circular transmission line method [5]. Both the S/D layer and contact resistivity were found to increase with increasing Si1-xGex layer thickness. This is, however, not caused by the (small) increase in electrical bandgap induced by (partial) layer relaxation. Instead, we found that B incorporation during epitaxial growth is affected by the initiation of layer relaxation and decreases with higher degrees of strain relaxation. Besides, to assess the impact of different strain conditions on the boron incorporation, we compared the electrical properties and composition profiles of Si1-xGex:B layers grown on different Si1-yGey strain relaxed buffers (SRBs) i.e. y = 0, 0.25, 0.50 and 0.70. [1] P. Raghavan, et al., 2015 IEEE CICC, pp. 1-5. [2] C-N. Ni, et al., (VLSI-TSA), pp. 1-2 (2016). [3] R. Loo, et al., ECS J. Solid State Sci. Technol., 6(1), P14–P20 (2017). [4] K. W. Shin, et al., Jpn. J. Appl. Phys., 57(6), pp. 065504.1-065504.4 (2018). [5] H. Yu, et al., IEEE Electron Device Letters, 36(6), pp. 600-602 (2015).