A new class of two-dimensional nanomaterials, MXenes, which are carbides/nitrides/carbonitrides of transition and refractory metals, has been critically analyzed. Since the synthesis of the first family member in 2011 by Yury Gogotsi and colleagues, MXenes have quickly become attractive for a variety of research fields due to their exceptional properties. Despite the fact that this new family of 2D materials was discovered only about ten years ago, the number of scientific publications related to MXene almost doubles every year. Thus, in 2021 alone, more than 2000 papers are expected to be published, which indicates the relevance and prospects of MXenes. The current paper critically analyzes the structural features, properties, and methods of synthesis of MXenes based on recent available research data. We demonstrate the recent trends of MXene applications in various fields, such as environmental pollution removal and water desalination, energy storage and harvesting, quantum dots, sensors, electrodes, and optical devices. We focus on the most important medical applications: photo-thermal cancer therapy, diagnostics, and antibacterial treatment. The first results on obtaining and studying the structure of high-entropy MXenes are also presented.

One area of constant interest in many fields of industry is development of functional multilayer coatings that possess excellent performance characteristics. That is why in our brief review the results of studies of structure and properties of multilayer structures based on binary nitrides of transition or refractory metals obtained by various physical-vapor deposition (PVD) techniques are presented. The influence of substrate temperature, substrate bias voltage, bilayer thickness and interface boundaries on the structure of coatings and their properties, such as hardness, plasticity, wear and corrosion resistance, are discussed in detail. This review may be useful for students and growing community of researchers interested in the synthesis-structure-properties relationship in multilayer coatings based on metal nitrides.

a b s t r a c tThe multicomponent nitride coatings from TiZrNbAlYCr high entropy alloy (HEA) were fabricated using the vacuum-arc method. The effect of nitrogen pressure on the crystallite size, elemental and phase composition of (TiZrNbAlYCr)N coatings was investigated. A bias voltage applied to the substrate during the deposition process was À200 V. The partial nitrogen pressure was 0.05 Pa, 0.27 Pa, and 0.5 Pa. Bodycentered cubic (BCC) lattice with crystallites of 15 nm in size was formed at the lowest pressure. An increase in the pressure led to the formation of the two-phase structure: BCC phase with crystallite size of 15 nm and face-centered cubic (FCC) phase with crystallite size of about 3.5 nm. The same two-phase state was found in coatings fabricated at 0.5 Pa, while the mean crystallite size was 7 nm. The maximum hardness of the deposited coatings was about 47 GPa.

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New conductive materials for tissue engineering are needed for the development of regenerative strategies for nervous, muscular, and heart tissues. Polycaprolactone (PCL) is used to obtain biocompatible and biodegradable nanofiber scaffolds by electrospinning. MXenes, a large class of biocompatible 2D nanomaterials, can make polymer scaffolds conductive and hydrophilic. However, an understanding of how their physical properties affect potential biomedical applications is still lacking. We immobilized Ti 3 C 2 T x MXene in several layers on the electrospun PCL membranes and used positron annihilation analysis combined with other techniques to elucidate the defect structure and porosity of nanofiber scaffolds. The polymer base was characterized by the presence of nanopores. The MXene surface layers had abundant vacancies at temperatures of 305− 355 K, and a voltage resonance at 8 × 10 4 Hz with the relaxation time of 6.5 × 10 6 s was found in the 20−355 K temperature interval. The appearance of a long-lived component of the positron lifetime was observed, which was dependent on the annealing temperature. The study of conductivity of the composite scaffolds in a wide temperature range, including its inductive and capacity components, showed the possibility of the use of MXene-coated PCL membranes as conductive biomaterials. The electronic structure of MXene and the defects formed in its layers were correlated with the biological properties of the scaffolds in vitro and in bacterial adhesion tests. Double and triple MXene coatings formed an appropriate environment for cell attachment and proliferation with mild antibacterial effects. A combination of structural, chemical, electrical, and biological properties of the PCL−MXene composite demonstrated its advantage over the existing conductive scaffolds for tissue engineering.