Epitaxial VO2 films on r-sapphire substrates with ideal composition, dense structure and superior functional properties were obtained by water-assisted MOCVD from vanadyl hexafluoroacetylacetonate ...followed by the annealing under optimized conditions. The obtained films exhibit both intense optical switch properties in IR region and unique MIT characteristics: change in resistance by 4.6 orders in the temperature range 40–80 °C, hysteresis width less 0.7 °C and transition width 1.5 °C. The proposed synthetic approach opens new opportunities to fabricate highly sensitive and ultrafast switches for optical, electronic and biomedical devices.
•Highly oriented (100)VO2 films grown on r-Al2O3 at 350 °C using MOCVD based on reaction between VO(hfa)2 and water vapors.•Manipulation with MIT of VO2 by annealing under controlling oxygen pressure in ranges of 15–120 min and 550–650 °C.•Record MIT parameters for VO2 films: change in resistance by 4.6 hysteresis width less 0.7 °C and transition width 1.5 °C.•Ultra-sharp electrical and optical MIT (IR reflectance) for epitaxial VO2 films.•The new model of VO2 recrystallization at temperatures below 600 °C based on melting of VxO2x+1 impurities.
Vanadium dioxide is widely known for its metal-insulator transition (MIT), in which drastic changes in resistivity and IR-transparency occur. This makes VO
2
thin films promising materials for ...high-frequency optoelectronic devices. To get the most MIT sharpness, thin films should not contain impurities of hyper-oxygen or hypo-oxygen phases arising during VO
2
synthesis. To ascertain the conditions of single-phase VO
2
existence, the equilibrium boundaries of VO
2
with neighboring phases were determined using the electromotive force method (EMF) with a solid electrolyte ZrO
2
(Y
2
O
3
). Our data for the high-oxygen boundary of VO
2
existence in equilibrium with the V
6
O
13
phase agree with the only data known in the literature. We established that VO
2
is, in equilibrium with the V
9
O
17
phase at the low-oxygen boundary, which forms V
8
O
15
under further reduction. The temperature of the peritectoid decomposition of V
9
O
17
is established, and the corresponding corrections to the phase diagram of the vanadium-oxygen system are introduced. The Gibbs energies for V
9
O
17
, V
8
O
15
, and V
6
O
13
formation reactions are calculated. It is also shown that the IR reflectance of VO
2
films brought to equilibrium at the high-oxygen boundary is much greater than that of films equilibrated at the low-oxygen boundary.
Highlights
Equilibrium boundaries of VO
2
phase stability were studied by the EMF method.
Low-oxygen boundary revealed more complex equilibriums than previously assumed.
Thermodynamic data about equilibriums of V
8
O
15
, V
9
O
17
, VO
2
, V
6
O
13
were obtained.
New information was added to the phase diagram of the vanadium-oxygen system.
Influence of nonstoicometry on MIT was shown by IR experiments with thin VO
2
films.
The hydrothermal method is the most effective approach for the synthesis of VO
2
metastable polymorphs with unique powder crystallite morphology. In this work, we expanded the capabilities of this ...method, directing it to the growth of oriented crystallites in self-organized systems on single crystal substrates. According to our investigations, a large variety of 3D structures of vanadium dioxide can be obtained using one single crystal substrate r-sapphire by fine tuning of synthesis parameters. The orientation growth of six-pointed vanadium dioxide crystallites fits into an epitaxial growth model describing unit cell relations between the VO
2
(M
1
) film and r-sapphire substrate. We describe the process of VO
2
(M
1
) phase stabilization in the films and the changes of resistivity and terahertz transparency of the films based on the metal-insulator transition (MIT).
The hydrothermal method is an effective approach for the synthesis of VO
2
films with unique crystallites morphology and sharp electrical and optical switch properties.
Industrial 2G HTS wires optimized for low-temperature application are studied using XRD. We aim at understanding the origin of noticeable variation of the lift factor LF = J c (20K, 8T)/ J c (77 K, ...sf) with the final goal of narrowing down the statistical scatter of the performance in production wires. Two approaches are used - (1) the direct characterization of Y123 defect state through analysis of diffuse scattering effects and (2) quantification of YBCO nanostructure (contents of Y 2 O 3 precipitates, their size, strain state of Y 2 O 3 precipitates and Y123 matrix). Only weak correlations between the parameters extracted from XRD patterns and J c (20K, 8T) have been found. A rather complex strain state of Y123 with out-of-plane tension, anisotropic in-plane compression, and (likely) overall contraction is observed. Further plans are discussed.
An efficient synthesis of 4,7‐dibromo1,2,5thiadiazolo3,4‐dpyridazine is reported. For the first time, palladium‐catalysed cross‐coupling reactions of a dihalo derivative was found to be a powerful ...tool for the selective formation of various mono‐ and diarylated derivatives of strongly electron‐accepting heterocycles. Suzuki–Miyaura coupling can be successfully employed for the preparation of mono‐arylated derivatives, whereas Stille coupling is useful for both mono‐ and diaryl(hetaryl)ated heterocycles. The cyclic voltammogram showed that 4,7‐dibromo1,2,5thiadiazolo3,4‐dpyridazine can be easily oxidized to form a stable anion radical. The calculated values of ELUMO confirmed that 1,2,5thiadiazolo3,4‐dpyridazine is one of the strongest electron–acceptor systems.
The 4,7‐dibromo derivative of highly electron‐deficient 1,2,5thiadiazolo3,4‐dpyridazine was synthesized. Conditions for the selective formation of both mono‐ and di‐aryl(hetaryl)ated 1,2,5thiadiazolo3,4‐dpyridazines by palladium‐catalysed cross‐coupling reactions were found.
Magnetic films of NiFe permalloy and CoNiFe ternary alloy are used in products of nanoelectronics and microelectronics and in densely packed magnetic memory. NiFe and CoNiFe coatings decrease ...corrosion and wear in electrical devices; they are also used in electrocatalytic materials. Large stresses in magnetic films lead to deformations and the malfunctioning of devices due to peeling of the film from the silicon substrate. In this study, to clarify the nature of the phenomena occurring during the electrochemical deposition of NiFe and CoNiFe films of various thicknesses, which lead to mechanical stresses, investigation of their parameters is carried out. It is demonstrated that measurement of the bending of wafers consisting of Si, SiO
2
, Si
3
N
4
, NiCr, and Ni after the deposition of CoNiFe or NiFe layers on each of them allows determination of the mechanical stresses in the films. It is established that the beinding of the wafers with Si
3
N
4
is negative and greater than that with SiO
2
. The bending after depositing NiCr and Ni layers is negative. The bending of silicon wafers with CoNiFe ternary-alloy films has a maximum value of 180 μm at a film thickness of 12 μm, and that with NiFe films has a maximum value of 150 μm at a film thickness of 15 μm. The bending after depositing NiFe and CoNiFe films is positive. No peeling of the films is observed. The difference in the deformation signs of Ni and CoNiFe or NiFe films and in the direct dependence of the silicon-wafer bending on the thickness of NiFe and CoNiFe films makes it possible to relate the mechanical stresses to hydrogenation (hydrogen embrittlement) and hydrogen release after the process. It is determined that the magnetization of NiFe films is lower than that of triple CoNiFe films. The latter are promising for use in magnetic-field converters.
A theoretical and experimental study of the dependence of the magnetoresistance for two spin-tunnel junctions (STJs) of ellipsoidal shape has been made. The one-sided homogeneous magnetization ...reversal mode of an ellipsoidal STJ with different aspect ratios has been experimentally selected. Despite the reverse inhomogeneous remagnetization, this selection has allowed for the calculation of the magnetic parameters of these elements by developing the Stoner-Wohlfarth theory.
This study documents ferruginous accumulations in hypogene karst cavities in the Eocene limestones of central-eastern Crimean Piedmont, including unusual quartz-goethite tubes in Tavrida Cave, and ...aims to reveal their origin and significance to the problem of iron source for the adjacent Kerch-Taman iron ore province. We employed speleological, petrographic, mineralogical, and geochemical (including REE analysis) methods. Ferruginous materials dominated by goethite precipitated in open cavities, although metasomatic replacement of the host rock also occurred. Although the formation of conduits/cavities enclosing ferruginous materials clearly predated the main phase of hypogene speleogenesis in Tavrida Cave, both speleogenetic phases were controlled by the same cross-formational flow paths. Geochemical characteristics of the studied materials in all sites are largely similar, pointing to their genetic affinity. REEY patterns in most of the samples are remarkably similar to those found in iron formations worldwide, for which significant hydrothermal contribution is established. We show that these accumulations had been formed from acidic, reduced, hydrothermal fluids rising from a deep source and intruding oxidizing environment of the karstified Eocene aquifer, where goethite had been precipitated at the boundary of the iron reduction/oxidation zone. This mechanism is consistent with the hypogene origin of host karst conduits. These findings have implications to the origin of iron ores in the adjacent Kerch-Taman province, where deposits are closely associated with mud volcanoes operating there since at least the Middle Miocene. Although the marine sedimentary (hydrogenetic) origin of these deposits is universally accepted, the source of iron is a matter of debate. Revealing of the deep fluid source of iron for the ferruginous accumulations in hypogene caves in Piedmont suggests that the deep fluid system associated with mud volcanism in the Kerch-Taman region could have been a significant, if not the major, supplier of iron for ore deposits in the Cimmerian basin.
•Unusual quartz-goethite accumulations are found in hypogene karst cavities in Crimean Piedmont.•REEY and trace element geochemistry of ferruginous materials in four sites is similar, pointing to their genetic affinity.•Fe was brought by acidic, reduced, low-temperature, hydrothermal fluids rising from a deep source.•Goethite has precipitated where these fluids mixed with oxidizing groundwater of the stratiform Eocene aquifer.•We suggest the deep origin of Fe for iron ore deposits accumulated in the Cimmerian basin to form the Kerch-Taman province.•The deep origin of Fe for iron ore deposits accumulated in the Cimmerian basin to form the Kerch-Taman province is suggested.
A series of low‐symmetry ZnII, MgII, and metal‐free porphyrazine derivatives with one 1,2,5‐chalcogenadiazole ring (with S, Se or Te) and three pyrazines bearing tert‐butylsulfanyls were synthesized. ...The absorption maxima of ZnII complexes lay at 660, 674, and 707 nm for S, Se, and Te, respectively, indicating increasing contribution of the 1,2,5‐chalcogenadiazole rings to the π‐electronic system. Photophysical studies revealed that introduction of Se as a chalcogen and ZnII as a central metal causes an increase in the singlet oxygen quantum yield, reaching a value of 0.81, whereas the combination of S and MgII gives only 0.39. This takes place on the account of the fluorescence quantum yields that differ significantly from 0.072 (ZnII+Se) to 0.51 (MgII+S). These results demonstrated the two‐dimensional heavy atom effect combining the role of the chalcogen (first dimension) as well as the central metal (second dimension). Attempts to prepare oxygen‐containing isologues led to the formation of an unexpected side product.
The effect of the introduction of different chalcogendiazole rings into porphyrazine analogues is discussed from the point of view of the Q‐band position, quantum yields of fluorescence emission, and singlet oxygen production. The successful synthesis of the target low‐symmetry zinc, magnesium, and metal‐free macrocycles is described. The formation of an unexpected product is also noted.
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