Alexey B. Tarasov

Enhanced Resonant Tunneling in Symmetric 2D Semiconductor Vertical Heterostructure Transistors.

Tunneling transistors with negative differential resistance have widespread appeal for both digital and analog electronics. However, most attempts to demonstrate resonant tunneling devices, including graphene-insulator-graphene structures, have resulted in low peak-to-valley ratios, limiting their application. We theoretically demonstrate that vertical heterostructures consisting of two identical monolayer 2D transition-metal dichalcogenide semiconductor electrodes and a hexagonal boron nitride barrier result in a peak-to-valley ratio several orders of magnitude higher than the best that can be achieved using graphene electrodes. The peak-to-valley ratio is large even at coherence lengths on the order of a few nanometers, making these devices appealing for nanoscale electronics.

Facile preparation of nitrogen-doped nanostructured titania microspheres by a new method of Thermally Assisted Reactions in Aqueous Sprays

N-doped nanocrystalline titania microspheres with controlled visible light absorption were obtained by a new route utilizing a heterogeneous hydrolysis of TiCl4 vapor with reactive droplets of aqueous aerosols, followed by instant calcination of the products in a preheated flow reactor. Thus prepared separate microspheres with a diameter of 0.5–3 microns were found to consist of anatase or rutile nanocrystals, depending on the parameters of the synthesis. Doped titania samples with different nitrogen contents were synthesized using water solutions with various concentrations of urea in the hydrolyzing aerosols. N− species were found to be a major form of nitrogen impurity. A plausible mechanism for the incorporation of nitrogen into titanium dioxide was proposed. The photocatalytic activity of the obtained powders under UV-visible or visible illumination was found to have a complex dependence on the calcination temperature and the titania doping level.

Synthesis, structure, and properties of vanadium pentoxide nanotubes

Vanadium oxide nanotubes are synthesized by the hydrothermal method with the use of polycrystalline vanadium oxide V2O5 and 1-hexadecylamine as a structural template. The structure of the vanadium oxide nanotubes is investigated using small-angle X-ray diffraction and transmission electron microscopy. It is demonstrated that the structure of the vanadium oxide nanotubes is characterized by a combination of fragments with different interlayer distances associated with the twisting of oxide layers in the form of “nanorolls.” The thermal stability, morphology, and surface properties of the nanotubes, as well as the role of the organic template in the formation of their structure, are discussed.

Light-induced reactivity of gold and hybrid perovskite as a new possible degradation mechanism in perovskite solar cells

We suggest a new degradation mechanism of commonly used gold electrodes in hybrid perovskite solar cells (PSCs) originating from chemical interaction between gold and highly reactive iodine-containing byproducts formed in the course of perovskite decomposition. Intensive UV-irradiation of perovskite would typically lead to the release of volatile I2 and CH3NH3I (MAI) resulting in the formation of recently reported highly reactive polyiodide melts (RPMs) with the general formula MAI–nI2. These RPMs react aggressively with metallic gold at room temperature causing the formation of [AuI2]− and [AuI4]− complexes and, consequently, precipitation on the gold interface of a new (MA)2Au2I6 phase with a tetragonal symmetry. The high rate and depth of this reaction renders gold an easy target for attack under these particular conditions despite its notorious chemical inertness, thus making gold unsuitable for widespread use in iodine-based perovskite solar cells; other cheaper and more stable materials are needed as a better choice for further development in this area.

A new formation strategy of hybrid perovskites via room temperature reactive polyiodide melts

Here we introduce a new solvent-free preparation method for hybrid metal halide perovskites involving the direct reaction of metallic lead with polyiodide melts. We discovered new reactive polyiodide melts (RPMs) that can be prepared simply by adding elemental iodine to halide salts of the organic A cations of common hybrid perovskites, e.g. methylammonium iodide (MAI) and formamidinium iodide (FAI), and their corresponding bromide salts MABr and FABr. For MAI/I2 ratios ranging from 1 : 1 to 1 : 3 they form room temperature ionic liquids containing polyiodide anions and organic counterions. We find that metallic lead can be converted within a few seconds into pure or mixed cation/anion large–grain perovskite films of high electronic quality by a reaction with the RPM. The melts can dissolve also lead derivatives, opening up a realm of opportunities for future development of self-flux growth, liquid phase epitaxy and crystallization of perovskites for solar cell applications.

Solution Combustion Synthesis of Copper Nanopowders: The Fuel Effect

ABSTRACT Pure copper nanoparticles have previously been successfully produced by different combustion methods, but most of them require the use of an inert atmosphere (N2, Ar) during the synthesis process or the usage of addition post reducing of metal oxides. In this article, novel modification of solution combustion synthesis technique for one-step metallic Cu nanoparticles preparation was developed. The main unique feature of our approach is the use of microwave-assisted foam preparation. Also, the effect of different types of fuels (urea, citric acid, glycine, and hexamethylenetetramine) on the combustion process and characteristics of resultant solid products was investigated. It was shown that the combination of microwave-assisted foam preparation and use of hexamethylenetetramine as a fuel allows producing pure metallic Cu nanoparticles (~67 nm) by one-step solution combustion synthesis under normal air atmosphere without any post reduction.

Formamidinium iodide: crystal structure and phase transitions

Crystal structure, thermal behaviour and phase transitions of formamidinium iodide were studied by DTG, DSC, powder diffraction and X-ray crystallography.

Highly efficient photocatalytic conversion of solar energy to hydrogen by WO3/BiVO4 core–shell heterojunction nanorods

Photocatalytic splitting of water under solar light has proved itself to be a promising approach toward the utilization of solar energy and the generation of environmentally friendly fuel in a form of hydrogen. In this work, we demonstrate highly efficient solar-to-hydrogen conversion efficiency of 7.7% by photovoltaic–photoelectrochemical (PV–PEC) device based on hybrid MAPbI3 perovskite PV cell and WO3/BiVO4 core–shell nanorods PEC cell tandem that utilizes spectral splitting approach. Although BiVO4 is characterized by intrinsically high recombination rate of photogenerated carriers, this is not an issue for WO3/BiVO4 core–shell nanorods, where highly conductive WO3 cores are combined with extremely thin absorber BiVO4 shell layer. Since the BiVO4 layer is thinner than the characteristic carrier diffusion length, the photogenerated charge carriers are separated at the WO3/BiVO4 heterojunction before their recombination. Also, such architecture provides sufficient optical thickness even for extremely thin BiVO4 layer due to efficient light trapping in the core–shell WO3/BiVO4 nanorods with high aspect ratio. We also demonstrate that the concept of fill factor can be used to compare I–V characteristics of different photoanodes regarding their optimization for PV/PEC tandem devices.