A review has been published in Physics-Uspekhi (with 423 references ) devoted to the current state of research of atomically thin films. The structure and properties of atomically thin monoelemental films, such as 2D Fe, Au, Li, as well as Si, Ge, B etc., are described in detail. Two-dimensional films of metallic compounds like FeO, CuO, ZnO and FeC, CoC and CuC are considered. The main approaches to the stabilization of monoatomic films inside pores or between layers of other 2D materials are presented, and the exfoliation mechanism of ionic-covalent films with a polar surface into weakly bounded monolayers is described.
Paper have published in Physics-Uspekhi 64, 1, 28 (2021) The isolation and subsequent detailed study of graphene have shown its significant prospects and potential for use in a wide range of technologies, such as composite materials, low-dimensional catalysts, touch screens, conductive ink, electronic paper, and organic light-emitting diodes. In 10-20 years, the introduction of graphene-based transistors and other logic units is expected. The main obstacles to the wide use of graphene in electronics are the requirements of high quality of the synthesized atomic structure and the absence of a bandgap. The latter is a fundamental problem, to which no universal solution has been found yet. Each of the approaches proposed, e.g., functionalization, the introduction of defects into the structure, or graphene splitting into separate ribbons, has its drawbacks. Indeed, the chemical adsorption of atoms on graphene leads to a change in the hybridization of carbon atoms from sp2 to sp3 accompanied by destruction of the p-system responsible for graphene conductivity. The proposed partial functionalization by forming hydrated or fluorinated regions (in a limiting case, separate periodically arranged chains of hydrogen) solves the problem, because a bandgap opens between the regions due to the dimensional effect. However, in spite of experimental confirmation of the effect, this technique requires adsorption with atomic precision, which at present is hardly a solvable problem. Using monolayers of different compositions can be an alternative way, especially as pioneering study has demonstrated the flexibility of the micromechanical exfoliation approach for obtaining planar two-dimensional structures from any crystals with weakly bound layers. The resulting materials have a planar structure as thin as one or a few atomic layers, whereas the lateral size can exceed a few micrometers. The atomic-size thickness, quantum dimensional effects, and high anisotropy of the optical properties of two-dimensional nanomaterials, as well as prospects of their application, have continuously attracted great interest of the world scientific community. Studies on finding new quasi-2D films turned out to be so efficient that up to 2020 they numbered a few hundred. This led to a paradoxical situation of a lack of resources in the scientific community sufficient for their thorough investigation. Moreover, the latest theoretical studies predict the possibility of five to six thousand more compounds in a quasi-2D state, which ultimately makes 2D materials one of the most vast and poorly studied areas of up-to-date materials science. The aim of the present review is to inform a wide range of readers about the current state of materials science in the field of noncarbon 2D structures. However, because of the great number of discovered and predicted two-dimensional crystals mentioned above, the discussion is focused on atomically thin films only. This gives rise to a certain difficulty in classifying the compounds described; for example, whether the films of phosphorene or silicene are atomically thin or not? Indeed, both structures have a thickness of a single atom and differ only by the degree of lattice corrugation. Therefore, a criterion should be introduced, according to which the present review will consider only those compounds with the corrugation degree (the out-of-plane displacement of atoms D) much smaller than the lattice parameter of the crystal a, i.e., D<<a. Notably, this criterion excludes phosphorene from consideration, but requires describing silicene and other allied materials. The review consists of three main sections. Section 2 considers single-layer films consisting exclusively of metal atoms (Fe, Au, etc.) and the possibility of their stabilization using other two-dimensional materials (mainly graphene). Section 3 presents a detailed description of various two-dimensional metal compounds, in particular, oxides and carbides of transition metals. Section 4 is devoted to films consisting of elements of groups 13-16 of the periodic table: silicene, borophene, and allied materials.
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