DFT investigation on the structures and stabilities of exohedral derivatives for D<Subscript>3</Subscript> C<Subscript>32</Subscript> fullerene: C<Subscript>32</Subscript>X<Subscript>n</Subscript> (X = H and Cl)

A systematic investigation of D₃ C₃₂ fullerene and its derivatives C₃₂X_n (X = H and Cl) has been performed using B3LYP/6-31G(d) method based on the density functional theory. The geometry structures, reaction energies, relative stabilities, and electronic properties have been studied. By investigating the possible C₃₂X_n (X = H and Cl) molecules, C₃₂H₂ and C₃₂Cl₂ behave more thermodynamically accessible with respect to other derivatives. The frontier molecular orbitals and electronic density of states calculations of C₃₂X₂ system indicate that H and Cl passivation have less contribution to the electronic structures, but significantly improve the stability of D₃ C₃₂ fullerene. Finally, the ¹³C NMR chemical shifts of C₃₂H₂ and C₃₂Cl₂ have been simulated to provide helpful information for further experiment identification.     

Impact of a physiological medium on the aggregation state of C<Subscript>60</Subscript> and C<Subscript>70</Subscript> fullerenes

The C₆₀ and C₇₀ fullerene-cluster size distribution in aqueous solutions and a physiological medium is studied via dynamic light scattering. The initial aqueous solutions of fullerenes obtained via different methods are found to contain clusters with a characteristic size of about 100 nm. The additional aggregation of fullerenes is observed after their transfer into a physiological medium (0.9% NaCl) and is established to depend on the preparation method. The cluster-size distribution in a fullerene–pectic-acid mixture is found to vary in water and a physiological medium. The results reveal the need for additional studies of the structure and properties of C₆₀ and C₇₀ molecules, as well as their complexes with medicines, in a physiological medium for medical applications.     

Exotic ferromagnetism in the two-dimensional quantum material C<Subscript>3</Subscript>N

The search for and study of exotic quantum states in novel low-dimensional quantum materials have triggered extensive research in recent years. Here, we systematically study the electronic and magnetic structures in the newly discovered two-dimensional quantum material C₃N within the framework of density functional theory. The calculations demonstrate that C₃N is an indirect-band semiconductor with an energy gap of 0.38 eV, which is in good agreement with experimental observations. Interestingly, we find van Hove singularities located at energies near the Fermi level, which is half that of graphene. Thus, the Fermi energy easily approaches that of the singularities, driving the system to ferromagnetism, under charge carrier injection, such as electric field gating or hydrogen doping. These findings not only demonstrate that the emergence of magnetism stems from the itinerant electron mechanism rather than the effects of local magnetic impurities, but also open a new avenue to designing field-effect transistor devices for possible realization of an insulator–ferromagnet transition by tuning an external electric field.     

Modeling of the structure and electronic structure of condensed phases of small fullerenes C<Subscript>28</Subscript> and Zn@C<Subscript>28</Subscript>

A comparative analysis of the stability factors and electronic structure of two possible crystalline forms of small fullerene C₂₈ and endohedral fullerene Zn@C₂₈ with diamond and lonsdaleite structures is performed using a cluster model. Atoms of elements that, when placed inside C₂₈ cages, have no significant effect on the stability of free small-fullerene molecules are shown to be able to dramatically change the electronic properties and reactivity of the C₂₈ skeleton and to be favorable for forming small-fullerene crystalline modifications, which are covalent crystals. In contrast, if the presence of foreign atoms inside C₂₈ cages stabilizes the isolated nanoparticles, then molecular crystals (such as C₆₀ fullerites) are formed due to weak van der Waals forces.     

Electronic structure,magnetic properties,and stability of the binary and ternary carbides (Fe,Cr)<Subscript>3</Subscript>C and (Fe,Cr)<Subscript>7</Subscript>C<Subscript>3</Subscript>

The structural, electronic, and magnetic properties of the binary and ternary carbides (Fe,Cr)₃C and (Fe,Cr)₇C₃ have been investigated within the ab initio density functional theory. The crystal structure of the binary carbides has been optimized and the preferred positions for replacement of chromium or iron impurities in the corresponding carbides have been determined. The changes in the electronic structure and magnetic properties have been investigated, the formation energies of the ternary carbides as functions of the impurity concentrations have been calculated, and conclusions have been drawn regarding the influence of the impurity on the stability of the carbides under investigation.     

Encapsulation of small fullerenes into nitrogenated holey nanotubes: a density functional theory study

Encapsulation of fullerene into nanotubes based on a C₂N sheet, known as nitrogenated holey graphene, was investigated using density functional theory. The structural and electronic properties of these carbon hybrid materials, consisting of nitrogenated holey nanotubes and a small C₂₀ fullerene, were studied. The formation energies showed that encapsulation of the fullerene into the nitrogenated holey nanotube is an exothermic process. To characterise the electronic properties, the electronic band structure and density of states of armchair and zigzag nitrogenated holey nanotubes were calculated. Filling these nanotubes with the C₂₀ fullerene resulted in a p-type semiconducting character. The energy band gap of the nitrogenated holey nanotubes decreased with fullerene encapsulation. The results are indicative of the possibility of band gap engineering by encapsulation of small fullerenes into nitrogenated holey nanotubes.     

Collision dynamics of He@C<Subscript>60</Subscript>+He@C<Subscript>60</Subscript> at low energies

A semi-empirical molecular dynamics model is developed. The central collisions of C₆₀+C₆₀ and He@C₆₀+He@C₆₀ at different incident energies are investigated based on this model. It is found that the dimer structures have been produced at proper incident energies and these fullerene dimers could be formed by a self-assembly of C₆₀ fullerene and He@C₆₀. The He atom has a significant effect at higher incident energy and this embedded He atom can enhance the stability of the dimer structure.     

Comparative study on the structure and properties of small C<Subscript>n</Subscript> and NaC<Subscript>n</Subscript> clusters

The structures, binding energies, and electronic properties of C_n and NaC_n (n=2–12) clusters have been systematically investigated using density functional theory (DFT). A number of previously undiscovered isomers of NaC_n clusters are reported, including fan-like, linear and three-dimensional structures. Moreover, NaC_n clusters with even n are found to be more stable than those with odd n, in contrast with the case of C_n clusters.     

Structure and electronic properties of Hn@C20 molecule

Density functional theory has been employed to optimize the structure of endohedral doped C₂₀ fullerene. We have also investigated electronic properties. We have found that C₂₀ cage can accommodate up to 8 hydrogen atoms. Some hydrogen atoms get chemisorbed on the inner surface of C₂₀ cage and form C-H bond. Structural deformation is found to increase with increase in H-atoms. From the analysis of electronic properties, we observe that due to endohedral doping of hydrogen atoms inside C₂₀, H-atoms acquire net negative charge by accepting electrons and fullerene molecules acquire positive charge by donating electrons to H-atoms. For endohedral complexes where H₃ triangular molecule formation takes place, the nature of net charge transfer changes, i.e. fractional electronic charge is transferred from H-atoms to fullerene. C₂₀ doped with odd number of H-atoms should be more reactive compared to the even number case. Most of the present results are similar to those of endohedral C₆₀.     

Thermally stimulated desorption of C<Subscript>60</Subscript> and C<Subscript>70</Subscript> fullerenes from rigid-chain polyimide films

Polyimide-fullerene composite thin coatings are investigated using thermal desorption mass spectrometry in the temperature range 20–800°C. It is found that, at temperatures below the temperature of decom-position of the polymer matrix, thermally stimulated desorption of fullerene molecules is limited by the diffusion of fullerene molecules in the matrix. The diffusion coefficients and activation energies of diffusion of C₆₀ and C₇₀ fullerene molecules are determined from the experimental data on thermally stimulated desorption in the framework of several approaches. It is revealed that the diffusion of C₇₀ molecules in the polyimide matrix is more hindered than the diffusion of C₆₀ molecules in the same matrix.     

Structural and electronic properties of endohedral phosphorus fullerene P@C60: an off-centre displacement of P inside the cage

Single P-doped endohedral P@C₆₀ is investigated via semiempirical and first-principles calculations. Unlike the encased N atom, which is situated on the centre of the C₆₀ cage and not covalently bound to the carbon atoms of the fullerene cage, static geometric optimization shows that the encased P atom occupies an off-centre position and is bound to the carbon atoms of the fullerene cage. The electronic ground state of the doped system is the spin quarter state, with spin density distribution significantly compressed by the cage.     

First-principles study of structural evolution of medium-sized Si<Subscript>N</Subscript> clusters (41 N 50) within stuffed fullerene cages

The stuffed fullerene structures of medium-sized Si_N ( ) clusters have been systemically studied using the all-electron density functional theory with gradient correction. For each cluster size, fullerene cages with different topologies and filled by different number of atoms were constructed and optimized to find the lowest-energy structure. The core atoms filled in the fullerene cages tend to form cage-like structures that resemble structural character of bulk diamond fragments. The size-dependent physical properties such as binding energies, electronic gaps, and ionization potentials have been discussed. Si₄₅ exhibits relatively higher stability that can be associated with the low chemical reactivity observed by experiment.     

Electronic interactions in two-dimensional polymers of the C60 fullerene

The electronic structure of tetragonal and rhombohedral polymers of the C₆₀ fullerene is investigated using x-ray emission spectroscopy. It is found that, compared to the C₆₀ molecular crystals, the formation of intermolecular covalent bonds in two-dimensional layers of the C₆₀ fullerene polymers leads to a broadening of the maxima in the CK _α x-ray emission spectra, a decrease in the density of high-energy states, and an increase in the width of the valence band of the polymer. The experimental data are interpreted by analyzing the results of the calculations performed within the density functional theory for the C₆₀ fullerene cage forming eight and twelve covalent bonds. It is shown that the electronic interactions between C₆₀ molecules in the polymerized layers are provided by two types of molecular orbitals located at energies 0.5–3.0 and ∼5.0 eV below the energy of the Fermi level.     

Effect of heating of the electronic subsystem on the thermal stability of the C<Subscript>60</Subscript> and C<Subscript>20</Subscript> fullerenes and (C<Subscript>20</Subscript>)<Subscript>2</Subscript> cluster molecule

The effect of heating of the electronic subsystem on the thermal stability of C₆₀ and C₂₀ fullerenes and a (C₂₀)₂ cluster molecule is investigated theoretically. It is demonstrated that the excitation of electrons to upper energy levels in accordance with the Fermi-Dirac distribution function does not lead to a substantial change in the activation energy E _a for decay of the C₂₀ fullerene. The stability of the C₆₀ fullerene and the (C₂₀)₂ cluster molecule likewise does not change radically. However, the inclusion of corrections associated with the finite sizes of the heat bath leads to the activation energy E _a which is in better agreement with the calculated height of the potential barrier preventing the cluster decay.     

First-principles calculations of SO<Subscript>2</Subscript> sensing with Si nanowires

High chemical reactivity and large surface-to-volume ratio have recently led to growinginterest in the employment of silicon nanowires (SiNWs) in sensing applications forchemical species detection. The working principle of SiNWs sensors resides in thepossibility to induce modifications in their electronic properties via molecularinteraction. A detailed analysis of the interaction of Si with molecular compounds is thenrequired to design and optimize NW-based sensors. Here we study the mechanisms ofadsorption on SiNWs of SO₂, an air pollutant with pernicious effects on humans.First-principles density-functional calculations are performed to calculate the electronicstructure of a SO₂molecule adsorbed at a silicon surface in case of undoped substrate and in presence ofsubstitutional subsurface and deep boron impurities. Comparing the results with the caseof NO₂ adsorption –a similar molecule that, nonetheless has a very different interaction with a Si surface –,we show the specific traits of SO₂ interaction: formation of localized states in theband-gap and absence of reactivation of pre-existing and passivated sub-surfaceimpurities. A connection between the modifications in the system electronic structure andthe strength of the molecular interaction is discussed.     

密度泛函理论计算内掺氢分子富勒烯H2＠C60及其二聚体的几何结构和电子结构

采用密度泛函理论中的广义梯度近似(generalized gradient approximation,简称GGA),对内掺氢分子富勒烯H₂＠C₆₀及其二聚体的几何结构和电子结构进行了计算研究.发现无论是在H₂＠C₆₀单体,还是在其二聚体中,氢倾向以分子形式存在于碳笼中心处,且在室温下氢分子可以做自由旋转.电子结构分析表明,氢分子掺入到C₆₀和C₁₂₀中,仅对距离费米能级以下-8eV至-5eV能级处有一定的贡献,其他能级的分布和能隙几乎没有变化. 关键词： 几何结构 电子结构 密度泛函     

Stuffed fullerene structures for medium-sized silicon clusters

The structural properties of medium-sized silicon clusters (Si₄₀, Si₄₅ and Si₅₀) have been studied using an unbiased global genetic algorithm search incorporated with a tight-binding model, followed by gradient-corrected density functional calculations. Stuffed fullerene cages are obtained as energetically favorite structures. The stuffing/cage ratio (m/n for Si_m@Si_n) can be understood by a space filling picture. The present results, along with our recent works on Si_N (N=27-39) clusters [20], suggest that stuffed fullerene cages are the preferred structural growth pattern of medium-sized silicon clusters.     

Distorted cage structures of Si36 cluster

Using full-potential linear-muffin-tin-orbital molecular-dynamics (FP-LMTO-MD) method, we have performed calculations on the fullerene cage structures and the binding energies of Si₃₆ cluster. It is found that their atomic arrangement tends towards tetrahedral geometry. The distorted structures are very stable. In addition, we have also investigated a stacked structure by tricapped trigonal prisms (TTP). The stacked structure is slightly more stable than the distorted fullerene cages. Their electronic states suggest that they present different electric properties.     

Stepwise computational synthesis of fullerene C<Subscript>60</Subscript> derivatives. Fluorinated fullerenes C<Subscript>60</Subscript>F<Subscript>2<Emphasis Type="Italic">k</Emphasis></Subscript>

Reactions of fullerene C₆₀ with atomic fluorine are studied by the unrestricted broken spin symmetry Hartree-Fock (UBS HF) approach implemented in semiempirical codes based on the AMI technique. The calculations are focused on a successive addition of a fluorine atom to the fullerene cage following the indication of the highest chemical susceptibility of the cage atom, which is calculated at each step. The proposed computational synthesis is based on the effectively unpaired-electron concept of the chemical susceptibility of fullerene atoms. The obtained results are analyzed from the standpoints of energy, symmetry, and the composition abundance. A good fitting of the data to experimental findings proves a creative role of the suggested synthetic methodology.     

Bucky-corn: van der Waals composite of carbon nanotube coated by fullerenes

ABSTRACT

Can a C₆₀ layer cover a surface of single-wall carbon nanotube (SWCNT) forming an exohedral pure-carbon hybrid with only van der Waals interactions? The aim of the present paper is to address this question and to demonstrate that the fullerene shell layer in such a bucky-corn structure can be stable. Theoretical study of the structure, stability and electronic properties of bucky-corn hybrids is reported for the shell of C₆₀ and C₇₀ molecules on an individual SWCNT, C₆₀ dimers on an individual SWCNT as well as C₆₀ molecules on SWNT bundles. The geometry and total energies of the bucky-corn hybrids were calculated by the molecular dynamics method, while the density functional theory method was used to simulate the electronic band structures.