Modeling the influence of structure modification of low-size ZnO, β-C3N4, InSe, and single-layer boron on their physical properties : dissertation for the degree of candidate of physical and mathematical sciences : 01.04.07

Low-dimensional materials (two-dimensional, one-
dimensional and zero-dimensional systems) are at the peak of research in the field of materials science, physics and chemistry. These materials are already finding their first application in various industries such as electronics, energy (batteries, solar panels), chemical technology (catalysis). It also discusses many potential applications for low-dimensional materials ranging from the purification and desalination of water and up to use in medicine (drug delivery). The optical properties of low-dimensional systems are also intensively studied by modern science. The main areas of application of the optical properties of low-dimensional systems under consideration are photochemistry and luminescence. In prototypes of photochemical catalysts, low-dimensional semiconductors generate an electron-hole pair, which is then used for the electrochemical reaction occurring on the metal part of the hybrid system. In the luminescence region, low-dimensional semiconductors emit a photon due to the recombination of an electron-hole pair induced by an electric field. If the adsorption of molecules changes the luminescent properties of the material, then such a material can be used as a sensor. A systematic study, through modeling from first principles, of the effect of the modification of low-dimensional systems on their electronic structure and optical properties is the subject of a dissertation.
One of the main features of low-dimensional systems is the significant contribution of the surface to all the properties of the material. If the electrical, optical and magnetic properties of three-dimensional systems can be considered by the model of an infinite periodic system, while neglecting the contribution of the surface and the environment. In low-dimensional systems the contribution from the surface and the influence of doping of near-surface regions, the shape and defects of the surface, and changes in the surface structure after modification should be taken into account and the contribution from the adsorption of molecules on the surface is inevitable. To study these effects, the most prominent representatives of various classes of low-dimensional materials are considered: ZnO, β-C3N4, InSe and
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borophene. Zinc oxide usually forms large nanoparticles with a structure identical
or close to the structure of the array and therefore a small contribution of the surface to physical properties. Carbon nitride forms nanoparticles with an atomic structure close to the array, but with a large contribution from the surface due to the small size of the nanoparticles. Indium selenide (InSe) represents a system of layers connected by weak non-covalent bonds (such systems are called “van der Waals” in modern literature)[1-3], which are easily melted to form a single-layer membrane (that is, that is, we can say that the material consists of one surface). A single-layer boron membrane (borophene), the various allotropes of which have an atomic structure different from boron crystals, and which, unlike the other systems studied, is a conductor. Such a choice of objects of study allows us to study the relationships between the physical and chemical properties of nanomaterials systematically, since the study covers the most common morphological types of low-dimensional materials-large and small nanoparticles, van der Waals systems, conductors and semiconductors.
The main physical property of the materials under study is a change in which with changes in the atomic structure we will investigate will be their photoactivity. There are two main ways to increase the efficiency of photoactive materials. The first method is doping, the second is chemical modification of the surface. Both of these approaches were studied in the course of the work performed. Zinc oxide was chosen as an object for studying the effect of doping on the optical properties of low- dimensional systems, which is considered as a promising material for multiple applications, such as photoelectronics and photochemistry. [4-7] Beryllium was chosen as a dopant for ZnO, which, unlike transition metal impurities, is not prone to clustering in the zinc oxide matrix. [8] Due to the low dimensionality and the large contribution of the surface in nanosystems, it is not always possible to draw a clear line between doping and surface modification. Therefore, most of our work is devoted to this topic. One of the phases of carbon nitride (β-C3N4) was chosen to study the effect of surface formation, its chemical modification and photoactive defects. Unlike layered materials such as indium selenide (InSe) or materials with a

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chemically neutral surface, such as ZnO, the surface of β-C3N4 contains chemically
active centers. These centers were identified and simulated for their effect on the optical properties of carbon nitride before and after modification by means of hydrogen, oxygen and fluorine. Another way to manipulate optical properties is to mechanically distort the crystal structure. The most famous example of this phenomenon is mechanoluminescence. In low-dimensional systems, such a mechanical action is often unavoidable due to distortion of two-dimensional membranes under the influence of temperature or substrate. To study this phenomenon, we chose one of the most promising materials for nanoelectronics and photochemistry – indium selenide (InSe). This material combines the high mobility of charge carriers, suitable optical properties with the flexibility of single layers. The effect of distortions of a single-layer membrane on the adsorption of molecules and optical properties was considered in our work. Also, using the example of this material, another phenomenon unique to nanosystems was studied – molecular doping, which is realized through the exchange of charge between the surface and molecules adsorbed on it. For a single-layer boron membrane, it was shown that after inevitable oxidation, it turns from a metal into a semiconductor whose optical properties are sensitive to the adsorption of molecules. In other words, another method of manipulating the optical properties of nanomaterials can be implemented in this system.
Another physical property that will be investigated in our work is the so-called d0 magnetism. Many nanomaterials not containing transition metal ions exhibit paramagnetism, and often ferromagnetism in the absence of transition metal ions. As shown in many works devoted to this problem, such magnetism is unstable from a chemical point of view. The search for materials with d0 magnetism stable to chemical influences is an important task not only from an applied, but also from a scientific point of view. As a result of studies carried out in the course of the work, various variants of the implementation of chemically stable d0 magnetism on the modified surface of carbon nitride, in oxidized boron membranes, and in non- covalently modified indium selenide were shown.

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Thus, the fundamental scientific problem is the lack of systematic knowledge
about the relationship between the features of the atomic structure of a surface, its chemical stability and the optical and magnetic properties of materials, which complicates further progress in the development of new nanomaterials for optical and magnetic applications. The result of this state of affairs is the absence of a clear protocol for modeling nanosystems, which, on the one hand, leads to the fact that modeling of nanosystems sometimes turns out to be redundant and does not provide new information, but more often than not it is insufficient when some of the important properties are unexplored. Our work is a step towards the creation of a protocol guiding the systematic theoretical study of the physical properties of nanomaterials.
The degree of development of the research topic. With the development of faster computing capability and sophisticated computer programs, materials simulation is a very important method for scientists and engineers, ideally and theoretically, the different size of materials can be modeled from first principles. It is well known that almost all matter is made up of materials, so the design and optimization of materials are eternal problems, however, the emerging nanotechnology which changes the distribution and arrangement of atoms to obtain different properties of materials has brought significant changes in the design of materials [9]. Nowadays the nanotechnology has been widely used in energy, medical, aerospace and other fields. [10,11] Computer modeling of nanomaterials can describe the correlation between the material’s microstructure and its macroscopic properties quantitatively. The research is optimizing the design of the material structure through modeling in nanoscale, then calculate the energy of nanomaterials to assist researching on nanomaterials’ structure and properties.
Nanotechnology is becoming one of the main driving forces for the economic development of countries around the world. It is widely used in the fields of information, energy, environmental protection, biotechnology and medicine, various industries, national defense, etc., which leads to new technological changes, promotes the transformation and modernization of traditional industries, and forms

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a new industry based on nanotechnology. In the field of information, nanodevices,
which play a key role in next-generation microcircuits, display and memory devices, which play a key role in the competitiveness of the future information industry. In the field of energy, nanotechnology can be used in new efficient and alternative energy sources and storage (such as lithium-ion batteries, solar cells, fuel cells, hydrogen) and key technologies for efficient use of energy. [12, 13] In the field of environmental protection, nanotechnology can be used to control water, air and soil pollution that were previously difficult to control. [14, 15] In biomedicine, nanotechnology can be used to develop technologies for early diagnosis of diseases. Rapid, low-cost testing of major diseases such as AIDS, hepatitis, tissue and organ repair and low-toxic high-efficiency treatment technologies. [16-18]
Currently, there are many software products for modeling the structure and properties of existing and hypothetical materials. Materials Studio is a relative mature modeling and simulation software for nanomaterials. Nanomaterials can be modeled and their physical properties can be calculated with this software. Recently some researchers use GPU (Graphics Processing Unit) to accelerate the modeling and simulation for nanomaterials. [19] This work is a simulation of nanosystems and a description of their resistance to the environment. Zinc oxide has a polar surface that can form a wide range of nanostructures, in the one-dimensional oxide nano- systems, it is one of the most promising materials for fabrication optoelectronic devices, the nanostructure of zinc oxide has high catalytic efficiency and strong adsorption ability. The electronic structures and optical properties of beryllium doped zinc oxide have been calculated using this software, we recognized that doping can improve the efficiency of photoactive materials, beryllium doped zinc oxide can be used in ultraviolet photoelectric equipment. For the modelling of carbonitride, we use density functional theory-based methods realized in the plane- wave pseudopotential approach in the Cambridge Sequential Total Energy Package codes. In the early research, under the local density approximation by using first pseudopotential band method, the theory predicts that the hardness of C3N4 can be comparable to diamond, after the theoretical prediction, the experimenters have the

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opportunity to use various methods for the directed synthesis of this new covalent
compound of high hardness in the laboratory. Various approaches have been developed, such as synthesis of chemical vapor deposition [20,21], synthesis of vapor deposition [22,23], synthesis at high temperature and high pressure [24]. But in the preparation of a carbon-nitrogen film, an amorphous film is obtained in most cases, and it is difficult to obtain a film of a single crystal phase. Due to the serious loss of nitrogen content, it’s difficult to obtain the ideal stoichiometry of carbon and nitrogen by using high temperature and high pressure method. So the study of carbon properties is more inclined to computational simulation. Without conducting expensive measurements, modeling the spectrum of electron states of carbonitride makes it possible to obtain important information about the electronic and optical properties, and focus on more specific field of practical application, it can be used in photocatalysts, fuel cell electrodes, lighting equipment, chemical sensors, and other devices. [25] Two-dimensional materials beyond graphene is emerging area of current material sciences. Boron monolayers is the one from this class. we not only demonstrate that borophene (similarly to phosphorene) is unstable at ambient conditions but provide comprehensive study of the physical properties of oxidized borophene sheets and suggest possible applications in the areas of solar energy, sensors, coating and spintronic. Indium Selenide discussed as the one of the most prospective two dimensional materials, we vary not only the adsorbents but also the size of supercell and especially the modes of the optimization, we also check the influence of in-plane and out-of-plane distortions of the substrate, interaction of InSe with environment at some narrow range of conditions and even slight change of these conditions could provide significant change in chemical properties. Two- dimensional materials have ultrathin thickness, the high surface area provides a large number of reactive sites, which makes them efficient adsorbents for gas molecules, these materials efficient in catalysis, sensing, solar energy conversion and storage technologies. [26-29]
Purpose and objectives of the work. The purpose of the thesis is a comprehensive study of the atomic structure of new materials for optics, electronics

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and photoelectronics based on oxide and low-dimensional systems and the formation
of a systematic description of the relationship between the morphology of the material, its chemical stability and the effect of modification in various ways (doping, creating defects, surface oxidation, etc.) on its electronic structure and optical properties. Another goal of the work is to develop a general approach to an adequate description of the physical and chemical properties of nanomaterials with different chemical compositions and morphologies. To achieve the goal of the work, the following tasks were solved:
1. Obtaining information on the relationship between the atomic structure and the optical properties of zinc oxide doped with beryllium (Znx-1BexO) for various dopant concentrations.
2. Modeling the atomic structure of carbon nitride (β-C3N4) for a system with intrinsic defects (vacancies), surfaces, and nanoclusters. Study of the chemical stability of the surface and identification of the mechanisms of the influence of defects and their chemical passivation on the optical properties of carbon nitride.
3. A systematic study of the step-by-step oxidation process of allotropes of two- dimensional single-layer boron. Analysis of chemical stability and study of the electronic structure of BxOy films.
4. Investigation of the effect of distortions of the crystal lattice of a single-layer indium selenide (InSe) membrane on the optical properties and the adsorption characteristics of molecules on its surface. Establishing the relationship between the adsorption properties and flexibility of a single-layer InSe.
5. Study of the formation of chemically stable magnetic centers with the participation of oxidized defects on the surface of β-C3N4 and BxOy films.
Scientific novelty:
1. For the first time, a systematic study was made of the effect of a stepwise increase in the content of beryllium in zinc oxide on its electronic structure and optical properties.
2. The atomic structure of the β-C3N4 surface and its defects, as well as the atomic structure of the nanoclusters of this compound, were modeled for the first

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time. The effect of disordering in the atomic structure of β-C3N4 on the formation of
optical properties is revealed.
3. For the first time, a systematic study of the interactions of two-dimensional
boron membranes with the environment was performed.
4. For the first time, chemically stable magnetic centers have been identified in
low-demensional materials that do not contain transition metals .
5. For the first time, the role of various methods of modifying the atomic structure in the formation of the adsorption characteristics of molecules on the
surface has been established.
6. For the first time, strategies have been developed for modeling the chemical
stability of free two-dimensional systems, two-dimensional systems on a substrate and the surface of three-dimensional systems.
7. For the first time, a theoretical assessment of the influence of spatial distortions of the InSe membrane on its electronic structure, optical and chemical properties has been made.
Theoretical and practical significance of the work:
1. The results obtained expand the fundamental understanding of the relationship between the atomic structure and the optical properties of pure and chemically modified low-dimensional systems.
2. The developed approach for assessing chemical stability provides the basis for further theoretical studies in the field of low-dimensional systems. The theoretical calculation protocol developed for InSe can be further used to simulate the electronic structure of similar flexible low-dimensional systems.
3. New methods are proposed to increase the efficiency of photoactive systems.
4. New stable materials for photonics, photochemistry and sensors are proposed.
5. The detected chemically stable magnetic centers on oxidized defects in β- C3N4 and BxOy films are scientifically interesting as magnetic centers in nonmolecular materials without transition element atoms. The results can be used for further development of magnetic materials without transition elements.

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Methodology and research methods. Density functional theory is the most
propagated approach in first principles calculations of realistic systems. This method plays an important role in condensed matter theory and material sciences. The electronic structure determines the basic properties of matter such as electric, magnetic, thermal and mechanical.
In adiabatic approximation we exclude dense small size nuclei from consideration and reduce multi-atomic system to multi-electron system. In order to discuss these systems, we make further reduction of multi-electron problem to single-electron by considering of the motion of electron in the field of others. For this approach Hartree-Fock method was developed. The main disadvantage of approximation is ignoring of the spin correlation energy between antiparallel electrons, while density functional theory considering the correlation energy of electrons within exchange-correlation therm. Density functional theory established on Hohenberg-Kohn Theorem, its core idea is to use the density of particles to reflect properties of the ground states of molecules, atoms and solids, so that the corresponding electronic structure and total energy can be obtained. However, Hohenberg-Kohn theorem cannot solve because of the difficulty of the interaction term in kinetic energy functional, so Kohn-Sham equation is proposed.
Calculations were performed within the framework of the density functional theory by using the plane-wave pseudopotential approach in the Cambridge Sequential Total Energy Package codes. We used the generalized gradient approximation of Perdew-Burke-Ernzerhof scheme to describe the exchange- correlation potential. For all systems under study, the following procedure was followed: building a model, choosing parameters for optimization, calculating and analyzing properties.
Thesis to defend:
1.The varying of the optical properties of BexZn1-xO is related to the different impurity concentration inducing the changes in the lattice parameter.
2.The main contribution to the change in the optical properties of β-C3N4 is made by the deviation of the atomic structure from ideal. The minimum disordering

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with a standard deviation of 0.3 Å atoms from stoichiometric positions is sufficient
to change the absorption spectra of β-C3N4.
3.Modification of defects on the β-C3N4 surface lead to a significant change in
the energy gap between the valence and conduction bands.
4.The two-dimensional boron monolayer (borophene) is a chemically unstable
material and, regardless of the initial configuration, will oxidize at room temperature until an amorphous BxOy film forms. Oxidation of borophene leads to the transformation of its electronic structure from metal to semiconductor, which makes it a promising material for solar energy, and the influence of adsorption of molecules on the electronic structure of oxidized borophene makes it possible to use it as a sensor.
5.The calculated adsorption energies of various gases on the surface of a single- layer indium selenide (InSe) depend on whether optimization has been made only of atomic positions (which corresponds to a monolayer on a substrate) or of atomic positions and lattice parameters (which corresponds to the case of a free membrane). The obtained results remove the contradictions between the experimental data and previous calculations.
6.Oxidation of borophene, as well as defects on the surface of β-C3N4, can lead to the appearance of a magnetic moment in these structures due to the presence of broken bonds. The passivation of these broken bonds is difficult, which makes d0 magnetism chemically stable in these systems.
The degree of reliability of the work results is determined by the use of modern certified computer programs for molecular dynamics and quantum chemical modeling. The results obtained during the work correspond to the known literature data.
Approbation of work
The main results of the dissertation were presented and discussed at 7 international conferences, congresses, symposia.
Scanning Probe Microscopy (Yekaterinburg, 2017), Master class from Springer Nature magazine – Publishing Academy (Yekaterinburg, 2017), Scanning Probe

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Microscopy (SPM-2018) (Yekaterinburg, 2018), XVII International Feofilov
Symposium on Spectroscopy of Crystals Doped with Rare Earth and Transition Metal Ions (Yekaterinburg, 2018), XIX All-Russian Workshop on the Problems of Condensed Matter Physics (SPFCS-19) in Memory of Tankeev A.P. (Yekaterinburg, 2018), Sino-Russian ASRTU Conference Alternative Energy: Materials, Technologies and Devices (Yekaterinburg, 2018), The Sixth International Young Researchers’ Conference Physics. Technologies. Innovation. (Yekaterinburg, 2019).
Personal contribution of the author. The purpose of the work was formulated by the supervisor. The selection of research objects and the formulation of problems were carried out by the supervisor and scientific consultant D.V. Boukhvalov in cooperation with the author of the thesis.
The author has carried out the whole complex of calculations, including the choice of the appropriate mode, pseudopotentials and approximations, building models and visualization. The author took a decisive part in the preparation of scientific publications and reports at conferences.
Discussion and analysis of the results obtained were carried out with the participation of Ph.D. Boukhvalov D.W.
Publications. On the topic of the dissertation work, the author published 7 articles indexed in the international databases WoS, Scopus and included in the list of the Higher Attestation Commission, 2 theses of reports at international conferences.
The structure and scope of the dissertation. The dissertation consists of an introduction, 6 chapters, a conclusion and a list of references. The volume of the dissertation is 143 pages, including 55 figures, 7 tables and a bibliographic list of 239 items.