Light-emitting diodes are budget-friendly solid-state light sources that find use in the very diverse areas: from lighting to data display devices. The history of the discovery of the electroluminescence phenomenon and development of the technology of inorganic (LED) and organic light-emitting diodes (OLED) is presented. Today, there has been a widespread transition to light-emitting diode sources based on inorganic semiconductors, ensuring significant energy savings. However, OLED technologies continue to undergo intensive development. Advantages of OLED over other light sources and areas of their practical use are considered. We discussed the fundamental principles of manufacturing organic light sources with a white emission spectrum, based on low-molecular-weight and polymeric compounds, and considered various approaches for developing polymeric materials with a white emission spectrum. Prospects of the development of the market of OLED light sources are evaluated.

The article represents a translation of Section 6.2 of the monograph authored by A.S. Burlov, V.G. Vlasenko, D.A. Garnovskiy, A.I. Uraev, E.I. Maltsev, D.A. Lypenko, and A.V. Vannikov, Electroluminescent Organic Light-Emitting Diodes Based on Metal Coordination Compounds, Rostov-on-Don: Yuzhn. Fed. Univ., 2015; ISBN 978-5-9275-1469-4 (see Additional Information), dedicated to reviewing the photoluminescence and electroluminescence properties of metal complexes based on (Zn, Cd, Al, Sc, Be, B) with azoles (2-(2-hydroxyphenyl)benzothiazoles, 2-(2-hydroxyphenyl)benzoxazoles, 2-(2-hydroxyphenyl)benzimidazoles, azaindoles, oxadiazoles, and their derivatives) as ligands. Modifying such ligands by introducing electron-donating substituents into various positions of the phenyl fragments leads to significant changes in their electroluminescence characteristics. Metal complex compounds with ligands of this type are widely used for creating OLED devices emitting white light.

This review article explores and analyzes published data on the photoluminescence and electroluminescence characteristics of rare earth element (REE) complex compounds, which are promising materials for creating highly efficient OLED devices. Distinguishing features of such compounds include their narrow emission bands, Stokes shift, long lifetime, and high quantum yields, enabling high resolution and color purity in constructed OLED devices. Using REE complexes as emissive layers in light-emitting diodes ensures significant color saturation and high device efficiency.

The article represents a translated, revised, and updated excerpt from the book Electroluminescent Organic Light-Emitting Diodes Based on Metal Coordination Compounds, Rostov-on-Don: Yuzhn. Fed. Univ., 2015; ISBN 978-5-9275-1469-4 (see Additional Information). The main technological stages for manufacturing of laboratory samples for electroluminescence are described in general terms. The most popular laboratory techniques for the formation of thin semiconducting organic films, such as solution spin coating and vacuum thermal deposition, are considered in more detail. Some methodological approaches used in our laboratory are outlined. Measurements of the polymer layer thickness by the interference method and by atomic force microscopy are considered in detail. For the interference method, the principal sources of systematic errors are examined. Concerning atomic force microscopy (AFM), two techniques for measuring thickness are considered: the express technique (macro-needle scratching) and scratching with an AFM probe in contact mode. Systematic errors associated with the first technique are determined, followed by recommendations regarding its potential application. The last section highlights the necessary adjustment for calibrating thickness sensors during film deposition if the calibration is conducted based on macro-needle scratching results.

The article represents a translated, revised, and updated Section 6.3 of the monograph by A.S. Burlov, V.G. Vlasenko, D.A. Garnovskiy, A.I. Uraev, E.I. Maltsev, D.A. Lypenko, and A.V. Vannikov, Electroluminescent Organic Light-Emitting Diodes Based on Metal Coordination Compounds, Rostov-on-Don: Yuzhn. Fed. Univ., 2015; ISBN 978-5-9275-1469-4 (see Additional Information). This section is dedicated to reviewing the photoluminescence and electroluminescence properties of metal complexes based on bi- and polydentate ligands of the azomethine type (Schiff bases). Many metal complexes of Schiff bases find wide use as luminescent materials. One of the applications of these compounds is the use as emissive layers or dopants for manufacturing OLED. The photo- and electroluminescence properties of a series of the metal complexes with the ligands based on the derivatives of hydroxybenzaldehyde, 2-(N-tosylamino)benzaldehyde, 1‑phenyl-3-methyl-4-formylpyrazol-5-one, and some others are reviewed. The systematic categorization of these compounds extends beyond their photophysical properties to consider the electroluminescence characteristics observed in OLED devices employing them as the basis. By modifying known azomethine-type ligand systems, one can obtain metal complex compounds with exceptional thermal stability, solubility, and quantum efficiency. These characteristics enable their use as luminescent materials in highly efficient OLED devices.

The review is devoted to analysis of photo- and electroluminescent properties of metal complexes (Al, Be, Mg, Ga, In, Zn, and B) based on substituted quinoline derivatives. Literature data shows that changing the substituents in quinoline ligands and the metal complexing agent can affect the color and intensity of photoluminescence in the corresponding metal complexes. OLED devices fabricated based on these complexes have exhibited high brightness and efficient electroluminescence across a broad spectral range from red to blue. Furthermore, these compounds possess excellent electron-transport properties, enabling their utilization as conductive layers in OLEDs.

This review summarizes the results of the synthesis, photoluminescence, and electroluminescence of the main classes of cyclometallated iridium(III) complex compounds. The primary focus of the review is on heteroleptic iridium(III) complexes, which contain two equivalent C^N ligands and an additional auxiliary ligand. Numerous examples demonstrate that the selection and modification of the ligand systems in iridium(III) complexes, using various donor or acceptor groups, alter significantly their spectral characteristics, often achieving record levels of brightness and emission efficiency for OLED devices manufactured based on them.

The article represents a translated, revised, and updated excerpt from the book Electroluminescent Organic Light-Emitting Diodes Based on Metal Coordination Compounds, Rostov-on-Don: Yuzhn. Fed. Univ., 2015; ISBN 978-5-9275-1469-4 (see Additional Information). The measurement of performance characteristics of organic light-emitting diodes (OLEDs), including current–voltage, luminance–voltage, and spectral characteristics is described in detail in relaton to the laboratory measuring facility existing at the Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences. A detailed description of this setup, including the essential minimum set of instruments required for characterizing OLEDs, can serve as a guide for researchers embarking on experiments in the field of electroluminescence. The second part of the article describes the principal limiting factors for OLED efficiency.

The article discusses the results of the synthesis of spinel NiCo2O4 by a new method of hydrothermolysis, as well as methods of precipitation of oxalates and ultrasonic aerosol pyrolysis from aqueous solutions of chlorides and nitrates of nickel and cobalt. The preparation conditions for each synthesis method were selected, the morphology of the obtained samples was considered, the porous structure of the material was studied, and the mesoporosity of the powders was estimated. Single-phase spinel NiCo2O4 was obtained only by precipitation of oxalates; two other methods of synthesis, due to higher temperatures of the processes, lead to the appearance of an impurity of nickel–cobalt double oxide and a decrease in the nickel content in the spinel composition. The nature of the porosity of the materials obtained and the morphology depend on the synthesis method: the mesoporous structure in ultrasonic spray pyrolysis, macroporous structure in hydrothermolysis. The sample obtained by the oxalate precipitation method has the maximum specific surface area of 65.40 ± 0.33 m2/g and a pore volume of 0.409 cm3/g. The samples after hydrothermolysis have the minimum specific surface area of 10.22 ± 0.06 m2/g and a porosity of 0.055 cm3/g. The powders synthesized by ultrasonic spray pyrolysis have intermediate values of these characteristics between the other two methods, 41.68 ± 0.13 m2/g and 0.144 m3/g, respectively.

Colloidal-chemical transformations accompanying the thermal degradation of a homogeneous aqueous solution of nickel(II) ammoniac complexes are investigated by thermodynamic and kinetic methods. A competitive growth mechanism of β-Ni(OH)2 nano- and microcrystals in the solution bulk and on the solution–solid interface is proved. The weight growth rate for each of the kinetic routes is controlled by the kinetics of the first-order homogeneous reaction of $${\text{Ni}}\left( {{\text{N}}{{{\text{H}}}_{{\text{3}}}}} \right)_{6}^{{2 + }}$$ degradation. The reason for the competitive weight gain of the sol and β-Ni(OH)2 film lies in the different activation energies for $${\text{Ni}}\left( {{\text{N}}{{{\text{H}}}_{{\text{3}}}}} \right)_{6}^{{2 + }}$$ conversion to sol microcrystals (131.0 ± 27.0 kJ/mol) and to the film (94.0 ± 24.0 kJ/mol). This gives rise to the existence of two different temperature areas where sol or film growth is preferable. At 70–75°C, the sol and film growth rates are equal. An interpretation of the temperature-dependent colloidal growth mechanism of Ni(OH)2 crystals is proposed. The influence of the morphology and thickness of β-Ni(OH)2 films on their optical band gap width, photocatalytic and electrical activities is determined.

The effects of the glycine content (ϕ) and annealing temperature on the phase composition and morphology of NiCo2O4 spinel samples manufactured by solution combustion synthesis (SCS) reactions were studied. With small amounts of glycine in the precursor, the major phase in the thus-prepared samples was nickel cobalt oxide; when ϕ ≥ 0.7 metallic nickel was in addition formed in the samples, and it was only at ϕ ≥ 1.4 that NixCo3 – xO4 spinels, where x < 1, started to form. The highest-nickel sample (Ni0.72Co2.22O4) was prepared at ϕ = 2.2. An increase in annealing temperature from 400 to 700°С was accompanied by a decrease in nickel amount in the NixCo3 – xO4 spinel crystal structure to x = 0.07–0.13 and an increase in nickel amount in double oxide samples.

Solution-combustion synthesis of Ni x Co 3 – x O 4 ( x < 1) spinels from aqueous solutions of nickel and cobalt nitrates using citric acid–glycine mixtures as a dual fuel was investigated by XRD and SEM. A decrease in combustion temperature at minimal amounts of glycine (φ gl = 0.5) and citric acid (φ citr = 0.2–0.3) afforded for the synthesis of NiCo 2 O 4 spinel after heat treatment of as-synthesized powders at 400°C for 20 h. An increase in the fuel content of starting solutions (for Σφ = φ gl + φ gl > 0.9) led to an increase in reaction temperature and the formation (after annealing at 400°C) of non-stoichiometric spinels Ni x Co 3 – x O 4 ( x < 1) with an admixture of double oxides Ni y Co 1 – y O ( y = 0.71–0.77).

Nanostructured hollow Co3O4 spheres are synthesized by ultrasonic spray pyrolysis. Nanocrystals forming the sphere structure have different sizes and packing density, depending on the synthesis conditions. The samples are studied by X-ray diffraction, scanning electron microscopy, low-temperature nitrogen adsorption, vacuum pycnometry, and Raman and X-ray photoelectron spectroscopies. The field and temperature dependences of magnetization show that the magnetic properties and the Néel temperature of Co3O4 hollow spheres are determined by the size and packing density of nanocrystals.

Phase transformations of bovine serum albumin (BSA) are investigated using the Brilluoin light scattering in the temperature range from 300 to 380 K in solutions with concentrations of 50 and 5 mg/mL. As the reference sequence, the sequence of phase transformations in a concentrated (100 mg/mL) BSA solution are used for analyzing the results. It is shown that the sequence of BSA phase transformations is modified upon a decrease in the concentration. For example, in the vicinity of denaturation, the region with fibrilform protein aggregates disappears, and the properties of gel-type high-temperature phase change.

The temperature dependences of the veloсity of hypersound ( V ( T )) in solutions with various concentrations of guanidine hydrochloride (GndHCl) are studied by Brillouin–Mandelstam light scattering spectroscopy in the temperature range from 263 to 353 K. It is shown that the temperature dependence of the sound velocity has a pronounced maximum at low concentrations of GndHCl. An increase in the concentration of GndHCl in a solution is accompanied by a shift of the maximum to the low-temperature region and an increase in the absolute values of the sound velocity, which means a decrease in the adiabatic compressibility over the entire range of studied temperatures. The temperature dependence of the adiabatic compressibility is constructed at low concentrations of GndHCl. The contribution of relaxation processes to the temperature behavior of the attenuation of hypersound in solutions of GndHCl is determined. It is shown that their behavior is activated thermally and described by the Arrhenius law. The values of the activation energy of relaxation processes are calculated. Possible mechanisms that underlie the observed phenomena are discussed.

Temperature dependences of hypersound velocity V(T) in solutions with different guanidine hydrochloride (GdnHCl) concentrations have been studied by Brillouin light scattering at temperatures from 263 to 353 K. It has been shown that for low GdnHCl concentrations the V(T) dependence has a pronounced maximum. An increase in the GdnHCl concentration in the solution is accompanied by a shift of the maximum towards lower temperatures and also an increase in the absolute value of hypersound velocity and, hence, a decrease in the adiabatic compressibility in the entire temperature range studied. The temperature dependence of adiabatic compressibility at low concentrations of GndHCl is ploted. The contribution of relaxation processes to the temperature behavior of hypersound attenuation in GndHCl solutions is determined. It is shown that their behavior is thermally activated and described by Arrhenius ' law. The values of activation energy of relaxation processes are calculated. Possible mechanisms underlying the effects observed are considered.

Vanadium oxides V2O5 and V2O3 have been synthesized by ultrasonic spray pyrolysis in the form of nanostructured spherical agglomerates with an average diameter of 0.5–1.5 μm. By changing the synthesis conditions, the vanadium oxidation state and microspheres surface morphology can be varied. The microspheres of V2O5 are formed during aerobic synthesis, while V2O3 microspheres are produced under an atmosphere of argon. An increase in the concentration of the initial solution leads to an increase in both size of V2O5 nanoparticles and the diameters of the V2O5 microspheres. Long-term storage of V2O3 in air results in morphological degradation of the microspheres.

Single-phase hollow bismuth ferrite microspheres ∼1 µm in diameter were synthesized by ultrasonic spray pyrolysis in a stream of air or argon. All the samples exhibited photocatalytic activity exceeding that of the standard TiO2 catalyst under UV irradiation. The samples synthesized in an argon stream exhibit catalytic activity under irradiation with a blue light. A comprehensive study showed that the appearance of the catalytic activity under the action of visible light was not associated with the powder particle morphology, oxygen defectiveness, and a change in the band gap width. An X-ray photoelectron spectroscopic study revealed decreased content of adsorbed oxygen on the surface of BiFeO3 synthesized in an argon stream. Possible mechanisms responsible for the appearance of the photocatalytic activity in the visible range are discussed.

Single-phase powders of bismuth ferrite have been synthesized for the first time by ultrasonic spray pyrolysis from nitrate solution in the presence of tartaric acid. The produced powders consist of hollow spherical agglomerates with a mean diameter of -1 mu m. The influence of tartaric acid concentration on the synthesis temperature and morphology of hollow spheres was established. The agglomerate shells have open and closed pores. The mean values of the shell thickness and the ratios of closed and open pores were estimated and their dependence on the concentration of tartaric acid was found. It was shown that the magnetic properties are determined not only by the size of nanoparticles composing the agglomerates, but also by the morphology of hollow sphere shells. (C) 2018 Elsevier B.V. All rights reserved.