Formation of nanooctahedra in molybdenum disulfide and molybdenum diselenide using pulsed laser vaporization

Pulsed laser vaporization has been used to produce nanooctahedra of MoS2 and MoSe2. The nanooctahedra primarily form in two- or three-layer nested octahedra, although nesting up to five layers has been observed. Tilting the TEM sample stage and mapping how the images of single particles transformed provided the evidence to verify their octahedral geometry. Analysis of 30 two- and three-layered octahedra showed that their outer edge lengths clustered at approximately 3.8 nm and approximately 5.1 nm, respectively. This discreet sizing and the high symmetry of these closed nanooctahedra represent the closest inorganic analogy yet to the carbon fullerenes. The geometrical implications for forming octahedra from these layered compounds are investigated by considering different atomic arrangements assuming either trigonal prismatic or octahedral coordination around the Mo atom and yields two possible configurations for the actual structure of the nanooctahedra. A preliminary survey of pulsed laser vaporization of other layered metal chalcogenides shows that these dichalcogenides differ in their tendency to form small closed layered fullerene-like structures. These materials can be ranked from highest tendency to lowest as follows: NbSe2, WS2, WSe2, SnS2, TaS2, GaS, ReS2, and MoTe2.     

Graphite-incorporated MoS2 nanotubes: a new coaxial binary system

Graphite-filled MoS2 nanotubes were synthesized by pyrolizing propylene inside MoS2 nanotubes prepared by a template-assisted technique. The large coaxial nanotubes were constituted of graphite sheets inserted between the MoS2 layers, forming the outer part, and coaxial multiwall carbon nanotubes intercalated with MoS2 inside. High-resolution electron microscopy (HREM) and electron energy loss spectroscopy techniques along with molecular dynamics simulation and quantum mechanical calculations were used to characterize the samples. The one-dimensional structures exhibit diverse morphologies such as long straight and twisted nanotubes with several structural irregularities. The interplanar spacing between the MoS2 layers was found to increase from 6.3 to 7.4 A due to intercalation with carbon. Simulated HREM images revealed the presence of mechanical strains in the carbon-intercalated MoS2 layers as the reason for obtaining these twisted nanostructures. The mechanism of formation of carbon-intercalated MoS2 tubular structures and their stability and electronic properties are discussed. Our results open up the possibility of using MoS2 nanotubes as templates for the synthesis of new one-dimensional binary-phase systems.     

Stability of hollow spheroidal silicon clusters Sin and SinHn

The structures of Si_n and Si_nH_n fullerenes with 20 ≶n ≶60 are calculated in the MINDO/3 approximation using the Monte Carlo technique for geometry optimization. The calculations show that spheroidal silicon clusters consisting of more than 36 atoms are stable and the bond energy increases with their size. This increase is not noticed for compact clusters calculated as an alternative. For n ≥40-50, the latter have lower bond energies compared to fullerenes. The geometry optimization of the tetrahedral cluster Si₄₅ results in a structure close to spheroidal, which gains in bond energy. The addition of hydrogen atoms to small deformed fullerenes and their geometry optimization make it possible to obtain stable spheroidal structures Si_nH_n whose bond energy is greater than that of alternative compact silicon hydride clusters. When the size of spheroidal clusters Si_nH_n increases, i. e., when n > 36, the hydrogen elimination barriers decrease abruptly; the Si_nH_n diamond structure of the cluster is more advantageous when n ≥50.     

Stability, dynamics, and lubrication of MoS2 platelets and nanotubes

A model is introduced to investigate structure, stability, dynamics, and properties of MoS(2). The tribological behavior of the material is obtained from the autocorrelation function, ACF, of the forces, using the Green-Kubo equation, and by the classical Amontons' laws. In the idealized system, i.e. without defects, junctions, vacancies, asperities, and impurities, both models find a superlubrication regime, in agreement with some experiments. In nanotubes, NTs, friction is an order of magnitude lower than in the layered systems. The calculations also show that there is a substantial stabilization, per atom, for the formation of multiwall NTs with at least four walls.     

Simulating the thermal behavior and fragmentation mechanisms of exohedral and substitutional silicon-doped C60

Structures, thermal behavior, and fragmentation mechanisms of exohedral and substitutional silicon-doped C(60) containing 1-12 Si atoms are investigated by extensive molecular-dynamics simulations. A nonorthogonal tight-binding model is used to mimic the interatomic interactions in the doped fullerenes. Beginning from the minimum-energy structures, the temperature of the doped fullerenes is slowly increased until fragmentation takes place. A correlation can be established between the exohedral and substitutional structures and the corresponding fragmentation mechanisms and fragmentation temperatures. Exohedral C(60)Si(m) fullerenes fragment into two homonuclear pieces, the Si(m) cluster and the C(60) fullerene that remains intact. In contrast, the substitutional C(60-m)Si(m) heterofullerenes undergo structural transformations, including the partial unraveling of the cage, prior to fragmentation. Then, ejection of atoms or small molecules takes place from the distorted structures. The slow heating rate used, combined with long simulation runs, allows us to determine the fragmentation temperature of exohedral and substitutional Si-doped fullerenes as a function of the number of silicon atoms. Substitutional Si-doped fullerenes exhibit much higher fragmentation temperatures (1000-1500 K higher) than the exohedral fullerenes. This can be understood from the different bonding of the Si atoms in both structures.     

Experimental and theoretical studies on carbon-nitrogen clusters C2nN7-

C(2n)N7(-) cluster ions are produced by laser ablating on the K(3)[Fe(CN)6] sample. DFT calculations have been performed for these cluster anions. Various isomeric structures of these clusters are optimized and their energies are compared to find the most stable isomers. The most stable structure for C8N7(-) is similar to that of adenine by theoretical calculation, which is in agreement with the collision-induced dissociation (CID) experimental results. With the increasing even numbers of C atoms from 8 to 16, the N atoms in the double-ring structure are gradually substituted by C atoms from the six-membered ring to the five-membered ring. All these C(2n)N7(-) (n = 3-9) clusters exhibit planar aromatic characters. The energy difference and incremental binding energy analyses show that C(2n)N7(-) (n = 4-8) clusters are more stable than C6N7(-) and C18N7(-), which are consistent with the observed mass spectrum.     

Identification of defect sites on MgO(100) thin films by decoration with Pd atoms and studying CO adsorption properties.

CO adsorption on Pd atoms deposited on MgO(100) thin films has been studied by means of thermal desorption (TDS) and Fourier transform infrared (FTIR) spectroscopies. CO desorbs from the adsorbed Pd atoms at a temperature of about 250 K, which corresponds to a binding energy, E(b), of about 0.7 +/- 0.1 eV. FTIR spectra suggest that at saturation two different sites for CO adsorption exist on a single Pd atom. The vibrational frequency of the most stable, singly adsorbed CO molecule is 2055 cm(-)(1). Density functional cluster model calculations have been used to model possible defect sites at the MgO surface where the Pd atoms are likely to be adsorbed. CO/Pd complexes located at regular or low-coordinated O anions of the surface exhibit considerably stronger binding energies, E(b) = 2-2.5 eV, and larger vibrational shifts than were observed in the experiment. CO/Pd complexes located at oxygen vacancies (F or F(+) centers) are characterized by much smaller binding energies, E(b) = 0.5 +/- 0.2 or 0.7 +/- 0.2 eV, which are in agreement with the experimental value. CO/Pd complexes located at the paramagnetic F(+) centers show vibrational frequencies in closest agreement with the experimental data. These comparisons therefore suggest that the Pd atoms are mainly adsorbed at oxygen vacancies.     

ANALYSIS OF FRAGMENT INTERACTION AND ASSIGNMENT OF ELECTRONIC SPECTRUM FOR COMPLEX ION[S2MoS2FeCl2]^2—

<正> The electronic structures of complex ion [S2MoS2FeCl2]2- (1) and its fragments MoS42- (2) and FcCl2(3) have been calculated base on the LCAO-HFS method with restricted open shell or closed shell. The interaction between the fragments 3 and 2 and the formation of complex ion 1 have been discussed. It was found that the Fe(Ⅱ) donated electrons to the Mo(Ⅵ)and accepted electrons from the sulphur ligand and that the stability of complex ion 1 is contributed from both direct and indirect interactions through the bridging sulphur atoms. In addtion, the electron transition energies of complex ion 1 were calculated and its electronic absorptions were assigned. It was shown that the calculated wavelengths of the absorption bands are in agreement with the observed ones.     

Fullerene-acene chemistry: single-crystal X-ray structures for a [60]fullerene-pentacene monoadduct and a cis-bis[60]fullerene adduct of 6,13-diphenylpentacene

[structure: see text]. Crystal structures of the cis-bis[60]fullerene adduct of 6,13-diphenylpentacene (with the closest carbon atoms on adjacent fullerenes only 3.065(8) angstroms apart) and the [60]fullerene-pentacene monoadduct are in remarkably close agreement with the MM2 calculated structures.     

Advances in the synthesis of inorganic nanotubes and fullerene-like nanoparticles

In analogy to graphite, nanoparticles of inorganic compounds with lamellar two-dimensional structure, such as MoS(2), are not stable against folding, and can adopt nanotubular and fullerene-like structures, nicknamed inorganic fullerenes or IF. Various applications for such nanomaterials were proposed. For instance, IF-WS(2) nanoparticles were shown to have beneficial effects as solid lubricants and as part of tribological surfaces. Further applications of IF for high-tensile-strength fibers, hydrogen storage, rechargeable batteries, catalysis, and in nanotechnology are being contemplated. This Minireview highlights some of the latest developments in the synthesis of inorganic nanotubes and fullerene-like structures. Some structural aspects and properties of IF, which are distinct from the bulk materials, are briefly discussed.     

Strain-dependent electronic and magnetic properties of MoS(2) monolayer, bilayer, nanoribbons and nanotubes

We investigate the strain-dependent electronic and magnetic properties of two-dimensional (2D) monolayer and bilayer MoS(2), as well as 1D MoS(2) nanoribbons and nanotubes using first-principles calculations. For 2D monolayer MoS(2) subjected to isotropic or uniaxial tensile strain, the direct band gap of MoS(2) changes to an indirect gap that decreases monotonically with increasing strain; while under the compressive strain, the original direct band gap is enlarged first, followed by gap reduction when the strain is beyond -2%. The effect of isotropic strain is even stronger than that of uniaxial strain. For bilayer MoS(2) subjected to isotropic tensile strain, its indirect gap reduces monotonically to zero at strain about 6%; while under the isotropic compressive strain, its indirect gap increases first and then reduces and turns into direct gap when the strain is beyond -4%. For strained 1D metallic zigzag MoS(2) nanoribbons, the net magnetic moment increases slightly with axial strain from about -5% to 5%, but drops to zero when the compressive strain is beyond -5% or increases with a power law beyond 5%. For 1D armchair MoS(2) nanotubes, tensile or compressive axial strain reduces or enlarges the band gap linearly, and the gap can be fully closed for nanotubes with relatively small diameter or under large tensile strain. For zigzag MoS(2) nanotubes, the strain effect becomes nonlinear and the tensile strain can reduce the band gap, whereas compressive strain can initially enlarge the band gap and then decrease it. The strain induced change in projected orbitals energy of Mo and the coupling between the Mo atom d orbital and the S atom p orbital are analyzed to explain the strong strain effect on the band gap and magnetic properties.     

Inorganic nanotubes and fullerene-like materials

Following the discovery of fullerenes and carbon nanotubes, it was shown that nanoparticles of inorganic layered compounds, like MoS2, are unstable in the planar form and they form closed cage structures with polyhedral or nanotubular shapes. Various issues on the structure, synthesis, and properties of such inorganic fullerene-like structures are reviewed, together with some possible applications.     

Structures,stabilities, and IR and 13C-NMR spectra of dihedral fullerenes: A density functional theory study

Dihedral fullerenes are thermodynamically stable molecules with D nd or D nh symmetry.Based on experimental findings,two series of dihedral fullerenes with five-fold(C5) and six-fold(C6) symmetry have been studied using density functional theory(DFT).The DFT calculations showed that for both series the stabilities increased with increasing fullerene size.Structural analyses indicated that the stabilities are related to specific local geometries.In the case of the more abundant C5 series,the presence of approximately planar pentagons and hexagons on the top bowl favors their formation.That is to say,those fullerenes with small dihedral angles within the polygons are readily formed,because planar hexagons lead to strengthened conjugation which lowers average bonding energies(ABE) and increases thermodynamic stabilities.Non-planar hexagons at equatorial positions in tube-shaped fullerenes have an adverse effect on the conjugation and inhibit their formation.Calculations also demonstrated that fullerenes in the two series,including C 50(D 5h),C 60(D 6h),C 80(D 5d),C 96(D 6d),C 110(D 5h),and C 120(D 5d),have thermodynamically stable triplet structures with strong conjugation.The calculated IR and 13 C NMR spectra of the fullerenes show some similarities and regular trends due to their homogenous structures.The electronic structures indicate that short double bonds in hexagons with high electron occupancies are readily attacked by electrophilic agents and can also be coordinated by transition metals.Mechanistic discussions suggested that C 2 additions and C 2 losses constitute reversible processes at high temperature and C 2 additions in pentagonal fusions are crucial to the kinetics of the curvature of structures.C 3 additions lead to the formation of large fullerenes of other types.     

Structural and electronic trends among group 15 polyhedral fullerenes

We have investigated the structural and electronic characteristics of tetrahedral, octahedral, and icosahedral fullerenes composed of group 15 elements phosphorus, arsenic, antimony, and bismuth. Systematic quantum chemical studies at the DFT and MP2 levels of theory were performed to obtain periodic trends for the structural principles, stabilities, and electronic properties of the elemental nanostructures. Calibration calculations for polyhedral clusters with up to 20 atoms showed the applied theoretical approaches to be in good agreement with high-level CCSD(T)/cc-pVTZ results. By studying fullerenes up to P₈₈₈, As₅₄₀, Sb₆₂₀, and Bi₆₂₀, we found their structures and stabilities to converge smoothly toward their experimental bulk counterparts. The diameters of the largest studied cages were 4.8, 3.7, 4.8, and 5.1?nm for the P, As, Sb, and Bi fullerenes, respectively. Comparisons with the experimentally known allotropes of the studied elements suggest the predicted polyhedral cages to be thermodynamically stable. All studied group 15 polyhedral fullerenes were found to be semiconducting, and density of states analysis illustrated clear periodic trends in their electronic structure. Relativistic effects become increasingly important when moving from P to Bi and taking the spin?Corbit effects into account by using a two-component procedure had a significant positive effect on the relative stability of bismuth clusters.     

An ab initio MO study on the structures and electronic states of hydrogen-bonded O3- HF and SO2-HF complexes

Ab initio molecular orbital (MO) calculations have been carried out for base-hydrogen fluoride (HF) complexes (base = O3 and SO2) in order to elucidate the structures and energetics of the complexes. The ab initio calculations were performed up to the QCISD(T)/6-311++G(d,p) level of theory. In both complexes, hydrogen-bonded structures where the hydrogen of HF orients toward one of the oxygen atoms of bases were obtained as stable forms. The calculations showed that cis and trans isomers exist in both complexes. All calculations for the SO2-HF complex indicated that the cis form is more stable in energy than the trans form. On the other hand, in O3-HF complexes, the stable structures are changed by the ab initio levels of theory used, and the energies of the cis and trans forms are close to each other. From the most sophisticated calculations (QCISD(T)/6-311++G(d,p)//QCISD/6-311+G(d) level), it was predicted that the complex formation energies for cis SO2-HF, trans SO2-HF, cis O3-HF, and trans O3-HF are 6.1, 5.7, 3.4, and 3.6 kcal/mol, respectively, indicating that the binding energy of HF to SO2 is larger than that of O3. The harmonic vibrational frequencies calculated for cis O3-HF and cis SO2-HF complexes were in good agreement with the experimental values measured by Andrews et al. Also, the calculated rotation constants for cis SO2-HF agreed with the experiment.     

TATB晶体结构的周期性密度泛函理论研究

对TATB晶体进行DFT-B3LYP/6-31G~(* *)周期性计算研究，求得其能带能带结 构和电子结构。探讨了结构-性能关系，从带隙约为4.1eV扒知TATB晶体的导电性处 于半导体和绝缘体之间，计算所得升华热为136.25kJ·mol~(-1)， 与实验值良好 相符，从原子间距和Mülliken集居分析，发现TATB晶体中同一层分子之间存在氢 键，而不同层之间距离较大，作用较弱，TATB分子中硝基氧带较多负电荷而氨基氢 带较多正电荷，这使TATB很难成为电子受体和给体，故化学上很稳定，考察晶体中 点电荷静电势，发现其在(001)面上的投影呈均匀分布，而在(100)和(010)面上的 揣影则有明显界面，表明同层分子间电子呈高度离域，异层之间相互作用极小，这 可解释TATB晶体沿c轴鼓胀以及受热循环后长大的各向异性和不可复原性等实验事 实。     

基于密度泛函理论分析团簇ConMoS(n=1~5)电子性质、光学性质及磁性

为了从理论层面深入探究团簇Co_nMoS （n=1~5）的电子性质、光学性质及磁性,弄清其内在关联,依据拓扑学原理和密度泛函理论,在B3LYP/def2-TZVP量子化学水平和多个自旋多重度下对该团簇进行结构优化并分析。结果表明：团簇Co_nMoS共有21种稳定构型;通过对NPA （自然布居分析,natural population analysis）电荷、静电势、亲电指数、电离势、光学电负性和折射率等分析得出,金属原子有高概率失去电子,非金属原子相对更容易得到电子,团簇Co₅MoS中的构型5a在最稳定构型中有高的得失电子能力、反应活性和折射率,Co和Mo原子易发生亲核反应,S原子易发生亲电反应;对该团簇自旋布居数、原子磁矩、轨道磁矩和态密度分析发现,该团簇磁性主要由Co原子的d轨道提供,且团簇Co₃MoS表现出了比其它尺寸团簇更为稳定和优异的磁性。最终得出团簇Co₃MoS在磁性方面有较好的表现且构型5a在活性和光学领域有一定的潜力。     

M4 @Si28 (M=Al,Ga): metal-encapsulated tetrahedral silicon fullerene

It is known that silicon fullerenes cannot maintain perfect cage structures like carbon fullerenes. Previous density-functional theory calculations have shown that even with encapsulated species, nearly all endohedral silicon fullerenes exhibit highly puckered cage structures in comparison with their carbon counterparts. In this work, we present theoretical evidences that the tetrahedral fullerene cage Si(28) can be fully stabilized by encapsulating a tetrahedral metallic cluster (Al(4) or Ga(4)). To our knowledge, this is the first predicted endohedral silicon fullerene that can retain perfectly the same cage structure (without puckering) as the carbon fullerene counterpart (T(d)-C(28) fullerene). Density-functional theory calculations also suggest that the two endohedral metallosilicon fullerenes T(d)-M(4)@Si(28) (M=Al and Ga) can be chemically stable because both clusters have a large highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap ( approximately 0.9 eV), strong spherical aromaticity (nucleus-independent chemical shift value of -36 and -44), and large binding and embedding energies.     

基于密度泛函理论分析团簇ConMoS(n=1~5)电子性质、光学性质及磁性

为了从理论层面深入探究团簇 Co_nMoS(n=1~5)的电子性质、光学性质及磁性,弄清其内在关联,依据拓扑学原理和密度泛函理论,在B3LYP/def2-TZVP量子化学水平和多个自旋多重度下对该团簇进行结构优化并分析。结果表明：团簇ConMoS共有21种稳定构型;通过对NPA(自然布居分析,natural population analysis)电荷、静电势、亲电指数、电离势、光学电负性和折射率等分析得出,金属原子有高概率失去电子,非金属原子相对更容易得到电子,团簇Co₅MoS中的构型5a在最稳定构型中有高的得失电子能力、反应活性和折射率,Co和Mo原子易发生亲核反应,S原子易发生亲电反应;对该团簇自旋布居数、原子磁矩、轨道磁矩和态密度分析发现,该团簇磁性主要由Co原子的d轨道提供,且团簇Co₃MoS表现出了比其它尺寸团簇更为稳定和优异的磁性。最终得出团簇Co₃MoS在磁性方面有较好的表现且构型5a在活性和光学领域有一定的潜力。     

Structure and stability of oxygen adsorption on Si(n) (n = 5-10) clusters

The structures, binding energies, and electronic properties of one oxygen atom (O) and two oxygen atoms (2O) adsorption on silicon clusters Si(n) with n ranging from 5 to 10 are studied systematically by ab initio calculations. Twelve stable structures are obtained, two of which are in agreement with those reported in previous literature and the others are new structures that have not been proposed before. Further investigations on the fragmentations of Si(n)O and Si(n)O2 (n = 5-10) clusters indicate that the pathways Si(n)O --> Si(n-1) + SiO and Si(n)O2 --> Si(n-2) + Si2O2 are most favorable from thermodynamic viewpoint. Among the studied silicon oxide clusters, Si8O, Si9O, Si5O2 and Si8O2 correspond to large adsorption energies of silicon clusters with respect to O or 2O, while Si8O, with the smallest dissociation energy, has a tendency to separate into Si7 + SiO. Using the recently developed quasi-atomic minimal-basis-orbital method, we have also calculated the unsaturated valences of the neutral Si(n) clusters. Our calculation results show that the Si atoms which have the largest unsaturated valences are more attractive to O atom. Placing O atom right around the Si atoms with the largest unsaturated valences usually leads to stable structures of the silicon oxide clusters.