Проведено исследование влияния введения примесных атомов азота и бора на электронные и оптические свойства углеродных диаманов с типом упаковки АА. Расчеты в теории функционала плотности показали, что внедрение примесных атомов высокой концентрации 6.3 и 12.5 % (от числа атомов углерода) в структуру диамана практически не влияет на постоянную решетки, вызывая при этом значительное изменение ширины запрещенной зоны. Внедрение атома азота увеличивает ширину запрещенной зоны диамана на 0.97 эВ, внедрение атома бора уменьшает запрещенную зону на 0.94 эВ, а одновременное включение примесных атомов азота и бора уменьшает запрещенную зону на 0.82 эВ. Отмеченные особенности структурных и электронных свойств позволяют рассчитывать на успешное использование диаманов, допированных бором и азотом, для синтеза латеральных гетероструктур, которые могут быть использованы для создания приборов наноэлектроники. Полученные в работе спектры комбинационного рассеяния и ИК-спектры окажутся полезными при идентификации легированных диаманов, поскольку атомы бора и азота внутри кристаллической решетки диамана обладают характерными колебательными модами.

Методом теории функционала электронной плотности исследовано влияние фторирования бор-нитридного фуллерена B12N12 на его активность по отношению к молекуле фавипиравира, являющейся лекарством против вируса COVID-19. Рассмотрено два типа фторирования фуллерена: внешнее допирование с образованием структуры B12N12F2 и эндоэдральное допирование с образованием комплекса F–@B12N12. Показано, что фторированные кластеры могут присоединять фавипиравир по тому же механизму, что и исходный фуллерен. Установлено, что взаимодействие лекарства с эндоэдральным комплексом оказывается слишком слабым, в то время как внешнее допирование фтором обеспечивает усиление энергии связи между кластером и лекарством.

The effect of fluorination of boron nitride fullerene B12N12 on its activity towards the favipiravir molecule (a drug against the COVID-19 virus) is studied by the density functional theory. Two types of fullerene fluorination are considered: external doping with the formation of the B12N12F2 structure and endohedral doping with the formation of the F–@B12N12 complex. It is shown that fluorinated clusters can attach favipiravir by the same mechanism as initial fullerene. It is found that the interaction between the drug and the endohedral complex is too weak, while external doping by fluorine increases the binding energy between the cluster and the drug.

By means of the density functional theory, using various exchange-correlation functionals (B3LYP, PBE, TPSS), the stable structure of azidofullerenes with the chemical composition C60N60 was predicted for the first time. Structural, energy, electronic and vibrational characteristics are studied in detail, and their comparison with the similar properties of the experimentally synthesized molecules of fullerene C-60, hexaazidobenzene C-6(N-3)(6), and tetraazidomethane C(N-3)(4) is carried out. The HOMOLUMO gap for the C(60)N(60)azidofullerene is equal to 0.25 eV for the PBE functional and 1.06 eV for the B3LYP functional. The IR spectrum of the C60N60 molecule contains peaks at frequencies corresponding to the symmetric and asymmetric valence vibrations of the azide group, and to the radial vibration of carbon atoms in the C-60 fullerene. The azidofullerene molecule is capable of storing a large amount of energy approximate to 6.0 kJ / g. Azidofullerene C60N60 can be a reasonable candidate for the high energy density material.

Stability to the formation of vacancies in the bulk of a structure and the possibility of a stable surface have been examined for the first time with density functional theory for high energy density solid atomic nitrogen phases, whose dynamical stability at normal pressure is theoretically predicted. It has been shown that phases with of the P6̅2c and Pccn crystal symmetries are unstable to the formation of vacancies at atmospheric pressure. The R3̅ and P 2 1 phases are stable with respect to the formation of vacancies, but the surface of such structures introduces instability inducing their transition from a metastable atomic solid phase to a molecular one. The gauche phase of nitrogen with the I 2 1 3 crystal symmetry is stable with respect to the considered structural perturbations and is the most promising for experimental synthesis at atmospheric pressure.

The effect of spacer layers on electron transport through two-barrier nanostructures was studied using the numerical solution of the time-dependent Schrodinger–Poisson equations with exact discrete open boundary conditions. The formulation of the problem took into account both the active region consisting of a quantum well and barriers, as well as the presence of highly doped contact layers and spacer layers. The use of the time formulation of the problem avoids the divergence of the numerical solution, which is usually observed when solving a stationary system of the Schrodinger–Poisson equations at small sizes of spacer layers. It is shown that an increase in the thickness of the emitter spacer leads to a decrease in the peak current through the resonant tunneling nanostructures. This is due to the charge accumulation effects, which, in particular, lead to a change in the potential in an additional quantum well formed in the emitter spacer region when a constant electric field is applied. The valley current also decreases as the thickness of the emitter spacer increases. The peak current and valley current are weakly dependent on the thickness of the collector spacer. The collector spacer thickness has a strong effect on the applied peak and valley voltages. The above features are valid for all three different resonant tunneling nanostructures considered in this study. For the RTD structures based on Al0.3Ga0.7As/GaAs, the optimized peak current value Ipmax = 5.6 × 109 A/m2 and the corresponding applied voltage Vp = 0.44 V. For the RTD structures based on AlAs/In0.8Ga0.2As, Ipmax = 14.5 × 109 A/m2 (Vp = 0.54 V); for RTD structures based on AlAs/In0.53Ga0.47As, Ipmax = 45.5 × 109 A/m2 (Vp = 1.75 V).

В рамках теории функционала плотности для энергонасыщенных твердых атомарных фаз азота, которые предсказываются теорией динамически устойчивыми при нормальном давлении, впервые исследованы вопросы устойчивости к формированию вакансий в объеме структуры и возможность реализации стабильной поверхности. Показано, что фазы с симметриями кристаллической решетки P 62c и Pccn являются неустойчивыми к формированию вакансий при нормальном давлении. Фазы R3 и P 21 устойчивык формированию вакансий, однако поверхность таких структур вносит неустойчивость, вызывающую их переход из твердого атомарного метастабильного состояния в молекулярное. Гош фаза азота с симметрией кристаллической решетки I 213 является устойчивой к рассмотренным структурным возмущениями является наиболее перспективной для экспериментального синтеза при нормальном давлении.

При нормальных условиях атомы азота образуют двухатомные молекулы с сильной тройной ковалентной связью (Eb ∼ 229 ккал/моль [1]). Азотные кластеры и кристаллические структуры состоят из одинарных (Eb ∼ 38,4 ккал/моль [1]) и/или двойных (Eb ∼ 100 ккал/моль [1]) связей между атомами. Для азота характерно, что энергия тройной связи больше, чем сумма энергий трех одинарных связей, что нехарактерно для многих других элементов. Это позволяет накапливать большое количество энергии при создании азотсодержащих структур с одинарными связями между атомами азота. Молекулярная фаза азота при нормальных условиях более термодинамически стабильна по сравнению с кластерами азота и кристаллическими структурами, поэтому немолекулярные структуры имеют тенденцию распадаться на двухатомные молекулы с выделением большого количества энергии. Этот процесс происходит без образования загрязняющих соединений, поскольку молекулы N2 являются основным компонентом атмосферного воздуха. Поэтому различные немолекулярные азотные структуры рассматриваются как экологически чистые материалы с высокой плотностью энергии (HEDM — high energy density materials). Теоретические оценки эффективности немолекулярного азота в качестве HEDM, в несколько раз выше, чем у других высокоэнергетических веществ [1, 2].

Within the framework of the density functional theory, the resistance of solid atomic hydrogen with an I4(1)/amd crystal structure to the formation of point defects, such as a vacancy, an interstitial site, and a double vacancy, was studied. At the pressures below 265 GPa, the equilibrium vacancy concentration per atom reaches a critical value, even near the temperature of liquid helium. Thus, the destruction of the metastable atomic state occurs at higher pressures than predicted by the calculations based on the phonon spectra. For atomic hydrogen at room temperature, pressures exceeding 280 GPa are required. The elastic stability of the I4(1)/amd atomic phase in the pressure range from 250-600 GPa was investigated. It is shown that there are regions of instability of the I4(1)/amd atomic hydrogen phase, where a transition to the structure with FDDD symmetry occurs. (C) 2021 Elsevier B.V. All rights reserved.

Within the framework of the density functional theory, the possibility of the formation of single-bonded solid atomic nitrogen phases as a result of adiabatic compression of molecular and cluster nitrogen structures at zero temperature has been studied. It has been demonstrated that nitrogen clusters N8(C2v)-B, which are theoretically predicted as one of the promising candidates for high energy density materials, can transform under compression into a solid atomic phase with crystal lattice symmetry P21. The P21 phase is dynamically stable under decompression to zero pressure. It is shown that the ε-N2 molecular phase transforms under compression into a solid atomic phase with R3̄c symmetry, and retains a vibrationally stable crystal structure when the pressure is reduced to 30 GPa, transforming into a stable cluster form at lower pressures. The atoms in the P21 and R3̄c solid atomic phases are linked by single bonds; therefore, these structures can store a large amount of energy ≈1.4 eV per atom. A detailed comparison of the properties of new P21 and R3̄c solid atomic phases with other nitrogen crystal structures that are dynamically stable at low pressures has been carried out.

A new solid atomic phase of nitrogen, which is dynamically stable at pressures above 20 GPa, has been predicted within the density functional theory. This phase has a low symmetry of the crystal lattice $$P\bar {1}$$ and exhibits electronic properties unique for nitrogen crystal structures in the low-pressure region, which are characteristic of semimetals. The structural, energy, mechanical, and electronic properties of this phase are calculated and compared with similar characteristics of the gauche phase of nitrogen.

Using density functional methods, the results of the analysis of traditional adsorbents and adsorbents based on nanosized particles capable of trapping 1,4-dioxane and 2-methyl-1,3-dioxolane molecules in milk are presented. We considered the following interacting compounds: 1,4-dioxane — primary amine, 1,4-dioxane — secondary amine, 1,4-dioxane — fullerene C 20 , 1,4-dioxane — a fragment of the structure of activated carbon, 2-methyl-1,3-dioxolane — primary amine, 2-methyl-1,3-dioxolane — secondary amine, 2-methyl-1,3-dioxolane — fullerene C 20 , 2-methyl-1,3-dioxolane — a fragment of the structure of activated carbon. We determined the optimal configurations of the corresponding interacting structures, estimated their binding energies and chemical potentials. The highest binding energy was obtained for 1,4-dioxane adsorbed on C 20 fullerene. At the same time, the energy gaps between the occupied HOMO and unoccupied LUMO molecular states were calculated, which makes it possible to characterize the reactivity and stability of molecules. Compounds of 1,4-dioxane and 2-methyl-1,3-dioxolane with amines have rather large gaps HOMO-LUMO. Using the concept of the electronic localization function, we found that a covalent bond is formed between 1,4-dioxane and C 20 fullerene with a sufficiently high degree of electron localization in the bond region. In other cases, the value of the localization function indicates the absence of a chemical bond between the compounds. The proposed study gives recommendations on the adsorption of 1,4-dioxane and 2-methyl-1,3-dioxolane for further solid-phase microextraction, which will allow them to be found in milk by gas chromatography using a flame ionization detector.

The structure, stability, and interlayer heat transfer of Stone–Wales bilayer graphene have been studied within a nonorthogonal tight binding model. The most stable configuration has been identified among several metastable isomers differing in the mutual arrangement of the layers. It has been established that the structure under consideration is characterized by a stronger interlayer interaction than bilayer graphene, but its stiffness in the vertical direction is 17% smaller. The heat transfer between two layers of Stone–Wales graphene, one of which is initially cooled to 0 K and the second is heated to 77−7000 K, has been studied by the molecular dynamics method. The strain dependence of the interlayer heat transfer of the bilayer structure under study has been determined. It has been shown that the intensity of interlayer heat transfer strongly depends on the temperature and strain. Features of the interlayer interaction in Stone–Wales bilayer graphene that are atypical of usual bilayer graphene have been revealed and explained.

We studied the hydrogen adsorption on the surface of a covalently bonded bilayer borophene-graphene heterostructure decorated with Pt, Ni, Ag, and Cu atoms. Due to its structure, the borophene-graphene bilayer combines borophene activity with the mechanical stability of graphene. Based on the density functional theory calculations, we determined the energies and preferred adsorption sites of these metal atoms on the heterostructure's borophene surface. Since boron atoms in different positions can have different reactivities with respect to metal atoms, we considered seven possible adsorption positions. According to our calculations, all three metals adsorb in the top position above the boron atom and demonstrate catalytic activity. Among the metals considered, copper had the best characteristics. Copper-decorated heterostructure possesses a feasible near-zero overpotential for hydrogen evolution reaction. However, the borophene-graphene bilayer decorated with copper is unstable with respect to compression. Small deformations lead to irreversible structural changes in the system. Thus, compression cannot be used as an effective mechanism for additional potential reduction.

A new crystalline phase of nitrogen that has the P-62c symmetry of the crystal lattice and is stable at zero external pressure has been revealed in ab initio calculations. Its structural, mechanical, electronic, and phononic properties at various pressures have been studied. The results obtained have been compared with the known nitrogen phases stable at low pressures.

AlGaAs MHEMT transistors are studied in the microwave frequency range with a gate length of 0.15 μm. It is found that the discrepancy between the experimental and theoretically calculated, within the model, S -parameters does not exceed 0.5% in the frequency range from 1 to 30 GHz. The static characteristics of the device are satisfactorily described by the indicated model in the voltage range of the runoff up to 2.5 V. For the analysis of the noise characteristics, the Fukui model is used. It is found that the influence of the parasitic drain capacitance and the values of the drain and source inductances do not significantly affect the noise characteristics of the transistor, and an increase in the parasitic capacitance and a decrease in the parasitic gate inductance can lead to a significant reduction in the high-frequency noise figure.

The possibility of the formation of A15 germanium hydride is investigated theoretically by the density functional method. It is shown that A15 germanium hydride Ge2H6 is stable in a certain pressure range of about 125 GPa. The results of calculations of the structural, phonon and electronic properties, energy characteristics of the normal phase are presented. The critical temperature of the superconducting transition is estimated.

The nonorthogonal tight-binding potential is augmented by long-range terms needed for a correct description of the interlayer interaction in bilayer graphene. The molecular dynamics method is used to study the heat transfer between two distorted graphene layers, one of which is initially cooled down to 0 K, and the second one is heated up to 77−7000 K. The characteristic time of the heat transfer depending on the initial temperature of the heated layer and the distortion of the layers is determined. It is demonstrated that both factors significantly affect the intensity of interlayer heat transfer. It is found that, during the characteristic time of temperature equalization, thermally induced defects of various types, including melting, separation of the layers, and tangential shear of the heated layer, can appear in the system. It is shown that the formation of thermally induced defects can result in more than an order of magnitude increase in the rate of interlayer heat transfer.