Benzene Adsorption on C<Subscript>24</Subscript>, Si@C<Subscript>24</Subscript>, Si-Doped C<Subscript>24</Subscript>, and C<Subscript>20</Subscript> Fullerenes

The absorption feasibility of benzene molecule in the C₂₄, Si@C₂₄, Si-doped C₂₄, and C₂₀ fullerenes has been studied based on calculated electronic properties of these fullerenes using Density functional Theory (DFT). It is found that energy of benzene adsorption on C₂₄, Si@C₂₄, and Si-doped C₂₄ fullerenes were in range of –2.93 and –51.19 kJ/mol with little changes in their electronic structure. The results demonstrated that the C₂₄, Si@C₂₄, and Si-doped C₂₄ fullerenes cannot be employed as a chemical adsorbent or sensor for benzene. Silicon doping cannot significantly modify both the electronic properties and benzene adsorption energy of C₂₄ fullerene. On the other hand, C₂₀ fullerene exhibits a high sensitivity, so that the energy gap of the fullerene is changed almost 89.19% after the adsorption process. We concluded that the C₂₀ fullerene can be employed as a reliable material for benzene detection.     

Aluminum nitride nanotubes

An AlN nanotube (AlNNT) was theoretically predicted in 2003. In comparison with the carbon nanotubes, the AlNNTs are wide-band-gap nanostructures with high reactivity, high thermal stability and sharp electronic sensitivity toward some chemicals. The B3LYP predicts an HOMO–LUMO gap of 3.74–4.27 eV for zigzag AlNNTs, while the experimental bad gap of bulk AlN is about 6.28 eV. The lowest strain energy of AlNNTs relative to its AlN nanosheet compared to the nanosheets of carbon and BN nanotubes with an equivalent diameter suggests the feasibility of AlNNT synthesis from its nanosheet. Theoretical methods predict a Young’s Modulus of about 453 GPa for AlNNTs that is smaller than that of carbon (1 TPa), BN (870 GPa) and GaN (796 GPa) nanotubes. In 2003, the faceted single-crystalline hexagonal AlNNTs were synthesized and extensively explored by means of density functional theory calculations. Several works have suggested different potential applications for AlNNTs including chemical sensors, hydrogen storage, gas adsorbent, and electron field emitter. This review is a comprehensive study on the latest achievements in the structural analyses, synthesis, and property evaluations based on the computational methods on the AlNNTs in the light of the development of nanotubes.     

Electronic structure and quadrupole interactions in triple molybdates Li<Subscript>2</Subscript>M<Subscript>3</Subscript>Al(MoO<Subscript>4</Subscript>)<Subscript>4</Subscript>, M = Cs,Rb

Within the density functional theory the electronic structure of triple molybdates Li₂M₃Al(MoO₄)₄, where M = Cs, Rb, is studied for the first time. It is found that all molybdates studied belong to wide band insulators with a band gap of ~4 eV. Quadrupole frequencies and asymmetry parameters of the electric field gradient near magnetic ⁷Li, ²⁷Al, ⁸⁷Rb, and ¹³³Cs nuclei are calculated and experimental NMR spectra are interpreted.     

Structure,energy, and spin characteristics of La@C<Subscript>60</Subscript>-Lu@C<Subscript>60</Subscript> lanthanide endofullerenes

Calculations of the equilibrium geometric and electronic structure of lanthanide endofullerenes are presented. Two types of the Ln@C₆₀ structure are found. For endofullerenes of the first type (La@C₆₀-Dy@C₆₀), the stable position of the lanthanide atom is achieved at a distance of 0.67R from the center of fullerene (R is the fullerene radius); in endofullerenes of the second type (Ho@C₆₀-Lu@C₆₀), the character of the interaction between the lanthanide atom and fullerene changes because of the transfer of unpaired electrons from the corresponding atom to fullerene. It is found that in endofullerenes of the second type, metal atom mobility increases, and two minima of the potential energy appear, which corresponds to the Ln position in the center and at a distance of 0.5R from the center. Based on the obtained spin density distribution for La@C₆₀-Lu@C₆₀ endofullerenes, we conclude that there is spin leakage.     

Electron energy structure and X-ray spectra of wide-band GaN, AlN, and AlN-GaN semiconductors

The electronic energy structure of GaN, AlN, and AlGaN crystals with the wurzite structure is calculated by the local coherent potential method using the cluster version of the MT-approximation within the framework of the multiple scattering theory. The calculated densities of electron states are compared with XPS spectra of gallium and aluminum, AlL _{II, III} XES, and also with K-spectra of gallium and AlL _{II, III} XAFS absorption. The comparison of the electronic structure of Al_xGa_1?x N crystals and binary GaN and AlN and the interpretation of their features are performed. The concentration dependence of the width of the upper subband of the valence band and the band gap in Al_xGa_1?x N (x = 0, 0.25, 0.5, 0.75, 1) crystals on the content of aluminum is studied and its non-linear character shown.     

First-principles study of the pressure effects on the structural and electronic properties of crystalline organic azide C<Subscript>10</Subscript>H<Subscript>8</Subscript>N<Subscript>6</Subscript>O<Subscript>4</Subscript>

Within the density functional theory with regard to the dispersion interaction the crystal structure parameters of organic C₁₀H₈N₆O₄ azide are determined. The pressure effect in the range 0-20 GPa on its structural and electronic properties is studied. Parameters of the equation of state in the Vinet and Birch–Murnaghan models are determined. Within the quasi-particle method (G ₀ W ₀) the energy band structure is calculated. It is shown that the hydrostatic pressure of 20 GPa results in the approach of planes of C₁₀H₈N₆O₄ molecules and their shift relative to each other. This is accompanied by a broadening of the upper valence bands and a decrease in the band gap from 5.07 eV to 3.97 eV.     

Adsorption Mechanisms of 6-Chloro-3-Hydroxy-2-Pyrazinecarboxamide on Pristine,Si- and Al-Doped C<Subscript>60</Subscript> Fullerenes: A DFT Study

Fullerenes have been of research interest and they have been particularly studied for their possible applications as drug delivery vehicles. In the present research, the optimized molecular geometries, electronic properties and the possible interaction mechanisms between C₆₀, Si- or Al-doped C₆₀ and 6-chloro-3-hydroxy-2-pyrazinecarboxamide were investigated using quantum mechanical calculations. The calculated binding energies to the Si- and Al-doped fullerenes suggest that doping of fullerene nanocage enhances the interaction mechanism and alters the chemical and electronic properties. The results and parameters found in this research reveal further insight into drug delivery systems.     

DFT based insights into reactivity descriptors of encapsulated B<Subscript>24</Subscript>N<Subscript>24</Subscript> nanocages

The purpose of this study is to probe the DFT based chemical reactivity parameter, electrophilicity index as a possible molecular engineering of endohedral BN-nanocages. The structure and electronic properties of endohedral boron nitride nanocages have been investigated as a function of alkali atom inside the nanocage using density functional theory. We have calculated and analyzed basic characteristic related to the reactive behavior, such as HOMO–LUMO band gap, chemical hardness, chemical potential, vertical electron affinity, and vertical ionization potential, as well as the global electrophilicity index, ω(I, A) of the encapsulated B₂₄N₂₄ nanocages. We also investigated the MQZVP basis set effect on total electronic energy of the clusters.     

Molecular structures and thermochemistry of the derivatives of C<Subscript>24</Subscript> fullerene by attaching a variety of chemical groups

A series of exohedrally functionalized derivatives of the D ₆-symmetrical C₂₄ fullerene, with attached -CH₂OH, -CONH₂, -COOH, and -COH chemical groups, have been investigated by using density-functional theory approach at the UB3LYP/6-31G(d) level. According to the calculated results, the C₂₄(COOH) is the most stable structure, with −73.58 kcal mol⁻¹ value for the functionalization reaction energy and 3.16 eV for the dissociation energy, while C₂₄(CONH₂) displays the largest dipole moment (3.09 D). It was also found that the HOMO-LUMO energy gaps, the vertical ionization potentials (VIP), and vertical electron affinities (VEA) of these functionalized derivatives are similar to those of the more stable C₂₄ fullerene. Moreover, their corresponding HOMO and LUMO orbitals are mainly associated with the surface of the cage. Also, the vibrational frequencies of these derivatives are discussed. It was concluded that it would be possible to produce novel species for bio-medical applications by attaching selected chemical groups.     

Electron interactions in the (η<Superscript>2</Superscript>-C<Subscript>60</Subscript>)Pd[P(Ph<Subscript>2</Subscript>)C<Subscript>5</Subscript>H<Subscript>4</Subscript>]<Subscript>2</Subscript>Fe complex

The electronic structure of the (η²-C₆₀)Pd[P(Ph₂)C₅H₄]₂Fe complex was calculated by the “hybrid” B3LYP method. Comparison of the experimental X-ray emission C-Kα spectrum and theoretical spectrum of the compound demonstrated that the electron interactions between the C₆₀ core, palladium atom, and organometallic fragment are described correctly in the framework of the quantum chemical method used. The electronic structure of the organometallic fullerene complex can be presented as a set of blocks of orbitals corresponding to different types of chemical bond. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2640–2644, December, 2005.     

Modeling of half-Heusler crystals with the chalcopyrite structure: Li<Subscript>2</Subscript>MgZn<Emphasis Type="Italic">X</Emphasis><Subscript>2</Subscript> (X = N,P, As,Sb)

A model of Li₂MgZnX ₂ half-Heusler compounds with the chalcopyrite structure is considered. The electronic structure is studied from first principles, showing that Li₂MgZnX ₂ are direct-gap crystals, except for pseudo-direct-gap Li₂MgZnP₂, with a band gap of 2.7 eV, 2.2 eV, 3.3 eV, and 2.5 eV for X = N, P, As, and Sb, respectively. The band structure and chemical bonding in the model crystals are found to be similar to those in LiMgX and LiZnX half-Heusler crystals. Total electron density and deformation electron density distributions are obtained. It is found that Mg–X and Zn–X ionic-covalent bonds are stronger than Li–X ionic bonds in Li₂MgZnX ₂ crystals, which allows Li atoms to move in the space between MgX ₄ and ZnX ₄ cation tetrahedra.     

Resonant emission of UO<Subscript>2</Subscript>, U<Subscript>3</Subscript>O<Subscript>8</Subscript>, and UO<Subscript>2+<Emphasis Type="Italic">x</Emphasis></Subscript> valence electrons under SR excitation near the O<Subscript>4,5</Subscript>(U) absorption edge

The structure of the resonant electron emission (REE) spectra of UO₂ (REE appears under the excitation with synchrotron radiation near the O_4,5(U) absorption edge at ∼100 eV and ∼110 eV) is studied with regard to the X-ray O_4,5(U) absorption spectrum of UO₂ and a quantitative scheme of molecular orbitals based on the X-ray electron spectroscopy data and the results of a relativistic calculation of the electronic structure of UO₂. The structure of the REE spectra of U₃O₈ and UO_2+x is studied for comparison, and the effect of the uranium chemical environment in oxides on it is found. The appearance of such a structure reflects the processes of excitation and decay involving the U5d and electrons of the outer valence MOs (OVMOs, from 0 to ∼13 eV) and inner valence MOs (IVMOs, from ∼13 eV to ∼35 eV) of the studied oxides. It is noted that REE spectra show the partial density of states of U6p and U5f electrons. Based on the structure of REE spectra, it is revealed that U5f electrons directly participate (without losing the f nature) in the chemical bonding of uranium oxides and are delocalized within CMOs (in the middle of the band), which results in the enhancement of the intensity of the REE spectra of CMO electrons during resonances. The U6d electrons are found to be localized near the bottom of the outer valence band and are observed in the REE spectra of the studied oxides as a characteristic maximum at 10.8 eV. It is confirmed that U6p electrons are effectively involved in the formation of IVMOs, which leads to the appearance of the structure in the region of IVMO electron energies during resonances. This structure depends on the chemical environment of uranium in the considered oxides.     

Boron nitride nanotube (BNNT) as a sensor of hydroperoxyl radical (HO<Subscript>2</Subscript>): A DFT study

In this present study, the adsorption behavior of HO₂ radical on the exterior surface of (5, 0) zigzag boron nitride nanotube (BNNT) has been investigated. The electronic structures and geometries of studied complexes were calculated at B3LYP-D3/6-31++G (d, p) computational level. The value of adsorption energy for the most stable configuration (A) is obtained ?0.68 eV, indicating physisorption process. Meaningful change of HOMO–LUMO gap after adsorption confirming BNNT can be introduced as a promising sensor for sensing of HO₂ radical.     

Quantitative analysis of compression isotherms of fullerene C<Subscript>60</Subscript> Langmuir layers

The structure of fullerence C₆₀ Langmuir layers formed from a solution in cyclohexane has been determined from their compression isotherms. The layer structure, in particular, the size of formed aggregates, has been shown to depend on the initial surface concentration of fullerene. A quantitative model of the Langmuir layer has been proposed. Degrees of water surface coverage with fullerene (the initial one and that corresponding to the onset of one-phase state formation), intermolecular distances of fullerene in aquaaggregates, aggregation numbers, and distances between aquaaggregates have been found to play the role of the model parameters. Constants characterizing the structure of a stable monolayer have been determined on the basis of the formulated model.     

Electronic and Chemical Characterization of Aluminum–Nitrogen (AlN) Substituted Fullerenes: C58AlN to C24Al12N12

Density functional theory calculations (B3LYP/6-311G*) are applied to devise a series of AlN-substituted C₆₀ fullerenes, avoiding weak homonuclear Al–Al and N–N bonds. The substitutional structures, energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, ionization potentials, binding energies, as well as dipole moments have been systematically investigated. The band gap (HOMO–LUMO gap) is larger for all the AlN-substituted fullerenes than C₆₀. The properties of heterofullerenes, especially, the HOMO–LUMO strongly depend on the number of AlN units. Natural charge analyses indicate that doping of fullerene with AlN units exerts electronic environment diversity to the cage. High charge transfer on the surfaces of our heterofullerenes provokes more studies on their possible application for hydrogen storage.     

A structural analysis of ultrathin barrier (In)AlN/GaN heterostructures for GaN-based high-frequency power electronics

Metal–organic chemical vapor deposition (MOCVD) is one of the best growth methods for GaN-based materials as well-known. GaN-based materials with very quality are grown the MOCVD, so we used this growth technique to grow InAlN/GaN and AlN/GaN heterostructures in this study. The structural and surface properties of ultrathin barrier AlN/GaN and InAlN/GaN heterostructures are studied by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. Screw, edge, and total dislocation densities for the grown samples have been calculated by using XRD results. The lowest dislocation density is found to be 1.69 × 10⁸ cm⁻² for Sample B with a lattice-matched In_0.17Al_0.83N barrier. The crystal quality of the studied samples is determined using (002) symmetric and (102) asymmetric diffractions of the GaN material. In terms of the surface roughness, although reference sample has a lower value as 0.27 nm of root mean square values (RMS), Sample A with 4-nm AlN barrier layer exhibits the highest rough surface as 1.52 nm of RMS. The structural quality of the studied samples is significantly affected by the barrier layer thickness. The obtained structural properties of the samples are very important for potential applications like high-electron mobility transistors (HEMTs).     

Effect of Nitrogen and Phosphorus Dopants on the Electronic Properties of ZrO<Subscript>2</Subscript> Nanotubes

The electronic structure of hexagonal ZrO₂ nanotubes, pure and doped with N and P atoms, has been calculated by the linearized augmented cylindrical wave method. The calculated band structures and densities of states demonstrate that the substitution of nitrogen or phosphorus for a part of the oxygen atoms leads to a decrease in the optical gap from 4 to 1.95 and 1.9 eV, which makes such nanotubes candidate materials for creation of electrodes for electrochemical photolysis of water.     

Thiozonation and thiozonolysis of triatomic sulfur (S<Subscript>3</Subscript>) on the C<Subscript>70</Subscript> fullerene: a DFT study

Density functional theory calculations have been carried out to investigate the [2?+?x] x?=?1, 2, and 3 cycloaddition reactions (paths A, B, and C) of triatomic sulfur (S₃) with the C₇₀ fullerene in terms of geometry, energies, and electronic structures. The thiozonation (S₃) on the hexagon–hexagon and hexagon–pentagon bonds of the C₇₀ fullerene through 1,3-dipolar reaction, i.e., [2?+?3] cycloaddition, is generally exothermic, while through the chelotrope additions, i.e., [2?+?1] cycloaddition, are endothermic. The results indicate that the 1,3-dipolar cycloaddition is the most preferable path. Having more negative values of reaction energies E_r together with the lower barrier heights, thiozonation of the hexagon–hexagon bonds is thermodynamically and kinetically more favorable than hexagon–pentagon ones. Moreover, the addition of thiozone to the hexagon–hexagon bonds near the pole area of the C₇₀ leads to more negative reaction energies. Therefore, it is established that the arrangement and position of C=C bonds play an important role in the thiozonation of C₇₀ fullerene. Thiozonolysis of triatomic sulfur (S₃) indicates that S–S bond cleavage has not occurred, instead a sulfur bridge over a C–C bond or a four-membered ring of 1,2-dithietane-1-sulfide is preferred to be formed.     

The influence of magnetic ordering on the electronic energy structure of CuFeS<Subscript>2</Subscript>

The modified LAPW+lo method with Wien2k software package was used to calculate the electronic energy structure of CuFeS₂ as a component of chalcopyrite. CuFeS₂ was found to be a conductor in the absence of antiferromagnetic ordering. Antiferromagnetic ordering in (001) layers leads to appearance of an energy gap and transforms CuFeS₂ into a semiconductor. In the GGA+U approximation, E g ≈ 0.75 eV was achieved for U = 6 eV, which is close to experimental value.