Single chirality extraction of single-wall carbon nanotubes for the encapsulation of organic molecules

The hollow inner spaces of single-wall carbon nanotubes (SWCNTs) can confine various types of molecules. Many remarkable phenomena have been observed inside SWCNTs while encapsulating organic molecules (peapods). However, a mixed electronic structure state of the surrounding SWCNTs has impeded a detailed understanding of the physical/chemical properties of peapods and their device applications. We present a single-chirality purification method for SWCNTs that can encapsulate organic molecules. A single-chiral state of (11,10) SWCNTs with a diameter of 1.44 nm, which is large enough for molecular encapsulation, was obtained after a two-step purification method: metal-semiconductor sorting and cesium-chloride sorting. The encapsulation of C(60) to the (11,10) SWCNTs was also succeeded, promising a route toward single-chirality peapod devices.     

豆荚型纳米材料C60@SWNTs的制备和表征

通过气相扩散的方法将C60填入单壁碳纳米管（SWNTs）,制备了豆荚型纳米材料C60@SWNTs,并利用高分辨电子显微镜（HRTEM）和拉曼光谱（Raman spectra）对其进行了表征.结果均证明C60以较高的比例填充入单壁碳纳米管中.HRTEM结果表明,填入单壁碳纳米管的C60之间的距离与面心立方C60晶体中C60之间的距离有细微的差别,说明C60分子与SWNTs间存在弱的范德华相互作用.此外,还观察到在电子束的照射下,C60在SWNT中两两聚合的现象.     

Theoretical investigation of C56 fullerene isomers and related compounds

All the 924 classical isomers of fullerene C(56) have been investigated by PM3, and some most stable isomers are refined with HCTH/3-21G and B3LYP6-31G(d) methods. D(2):003 with the least number of adjacent pentagons is predicted to be the most stable isomer at B3LYP/6-31G(d) level, while C(s):022 and C(2):049 possess nearly degenerate energies with relative energies of 0.03 and 3.90 kcal/mol, respectively. However, as to dianionic C(56)(2-) fullerene, C(2v):011 is predicted to be the most stable isomer. Investigations also show that the encapsulation of Ca atom in C(56) fullerene is exothermic and the metallofullerenes Ca@C(56) can be described as Ca(2+)@C(56)(2-). The computed relative stabilities show that the D(2):003 behaves more thermodynamically stable than other isomers in a wide temperature interval, and C(2v):011 should also be an important component. The electronic isomerization of C(56) (C(2v):011) and C(50) (D(5h):002) indicates that this phenomenon might be rather general in fullerenes and causes different properties, thus bringing about new possible applications of fullerenes. The static second-order hyperpolarizabilities of the three most stable isomers are slightly larger than that of C(60).     

On the structure and relative stability of C50 fullerenes

The complete set of 271 classical fullerene isomers of C50 has been studied by full geometry optimizations at the SAM1, PM3, AM1, and MNDO quantum-chemical levels, and their lower energy structures have also been partially computed at the ab initio levels of theory. A D(5h) species, with the least number of pentagon adjacency, is predicted by all semiempirical methods and the HF/4-31G calculations as the lowest energy structure, but the B3LYP/6-31G* geometry optimizations favor a D3 structure (with the largest HOMO-LUMO gap and the second least number of adjacent pentagons) energetically lower (-2 kcal/mol) than the D(5h) isomer. To clarify the relative stabilities at elevated temperatures, the entropy contributions are taken into account on the basis of the Gibbs energy at the HF/4-31G level for the first time. The computed relative-stability interchanges show that the D3 isomer behaves more thermodynamically stable than the D(5h) species within a wide temperature interval related to fullerene formation. According to a newly reported experimental observation, the structural/energetic properties and relative stabilities of both critical isomers (D(5h) and D3) are analyzed along with the experimentally identified decachlorofullerene C50Cl10 of D(5h) symmetry. Some features of higher symmetry C50 nanotube-type isomers are also discussed.     

Spectroelectrochemistry of carbon nanostructures.

This review is focused on charge-transfer reactions at carbon nanotubes and fullerenes. The spectroelectrochemistry of fullerenes deals with the spin states of fullerenes, the role of mono-anions and the reactivity of higher charged states in C60. The optical (Vis-NIR) spectroelectrochemistry of single-walled carbon nanotubes (SWNTs) follows changes in the allowed optical transitions among the Van Hove singularities. The Raman spectroelectrochemistry of SWNT benefits from strong resonance enhancement of the Raman scattering. Here, both semiconducting and metallic SWNTs are analyzed using the radial breathing mode (RBM) and G-modes as well as the second order (D, G') and intermediate frequency modes. Raman spectroelectrochemistry of SWNT allows the addressing of index-identified tubes and even single isolated nanotubes. Optical and Raman spectroelectrochemistry of fullerene peapods, C60@SWNT and C70@SWNT indicates effective shielding of the intratubular fullerene (peas). The most striking effect in the spectroelectrochemistry of peapods is the so-called "anodic Raman enhancement" of intratubular C60. Double-walled carbon nanotubes (DWNTs) give a specific spectroscopic response in Vis-NIR spectroelectrochemistry for the inner and the outer tube. They are better distinguishable by Raman spectroelectrochemistry which allows a precise tracing of the specific doping response of outer/inner tubes.     

Electronic structures of single-walled carbon nanotubes encapsulating ellipsoidal C70

The molecular orientation of ellipsoidal C(70) in single-walled carbon nanotubes (SWCNTs) depends on the tube diameter (d(t)). Photoluminescence (PL) studies reveal that the fullerene encapsulation effects on the optical transition energy of SWCNTs are significantly different for C(70) and C(60) at d(t) = 1.405-1.431 nm. This indicates that the transition from the "lying" alignment to the "standing" alignment occurs at d(t) ≈ 1.41 nm and the electronic states of SWCNTs are very sensitive to the interspacing between the encapsulated molecules and the SWCNTs. The present findings suggest that the electronic structure of SWCNTs is tunable not only by alternating the encapsulated molecules but also by controlling their molecular orientations, thus paving the way for development of novel SWCNT-based devices.     

Structure and Electronic Properties of Fullerenes C52q+: Is C522+ an Exception to the Pentagon Adjacency Penalty Rule?

The structure, vibrational spectra and electronic properties of the neutral, singly and doubly charged C52 fullerenes were studied by means of the Hartree-Fock method and density functional theory. Different isomers were considered, in particular those with the lowest possible number (five or six) of adjacent pentagons, and an isomer with a four-atom ring. For neutral and singly charged species, the most stable isomer is that with the lowest number of adjacent pentagons, namely five. However, for C(52)2+, the most stable structure has six adjacent pentagons. This finding, which contradicts the pentagon adjacency penalty rule, is a consequence of complete filling of the HOMO pi shell and the near-perfect sphericity of the most stable isomer. The simulated vibrational spectra show important differences in the positions and intensities of the vibrations for the different isomers.     

C40异构体的结构和稳定性的理论研究

利用Gaussian98程序,采用密度泛函(DFT)方法中的B3LYP,选用6-31G基组对富勒烯(Fullerene)C40的6种异构体[D5d,Td,D2h,C3v,D2(Ⅰ),D2(Ⅱ)]进行了几何构型优化,其中,对于Td对称性的C40由于易发生Jahn-Teller畸变,则降低其对称性为D2d,再进行优化.对它们的平衡几何和电子结构进行了比较具体的分析,同时,根据计算得到的总能量推断出这6种异构体的稳定性顺序是D2(Ⅰ)>D5d>Td>C3v>D2h>D2(Ⅱ).     

Comparison of geometric, electronic, and vibrational properties for isomers of small fullerenes C20-C36

We employ the self-consistent-charge density-functional tight-binding (SCC-DFTB) method for computing geometric, electronic, and vibrational properties for various topological isomers of small fullerenes. We consider all 35 five- and six-member rings containing isomers of small fullerenes, C20, C24, C26, C28, C30, C32, C34, and C36, as first part of a larger effort to catalog CC distance distributions, valence CCC angle distributions, electronic densities of states (DOSs), vibrational densities of states (VDOSs), and infrared (IR) and Raman spectra for fullerenes C20-C180. Common features among the fullerenes are identified and properties characteristic for each specific fullerene isomer are discussed.     

Expanding the number of stable isomeric structures of the C80 cage: a new fullerene Dy3N@C80

The production, isolation, and spectroscopic characterization of a new Dy3N@C80 cluster fullerene that exhibits three isomers (1-3) is reported for the first time. In addition, the third isomer (3) forms a completely new C80 cage structure that has not been reported in any endohedral fullerenes so far. The isomeric structures of the Dy3N@C80 cluster fullerene were analyzed by studying HPLC retention behavior, laser desorption time-of-flight (LD-TOF) mass spectrometry, and UV-Vis-NIR and FTIR spectroscopy. The three isomers of Dy3N@C80 were all large band-gap (1.51, 1.33, and 1.31 eV for 1-3, respectively) materials, and could be classified as very stable fullerenes. According to results of FTIR spectroscopy, the Dy3N@C80 (I) (1) was assigned to the fullerene cage C80:7 (I(h)), whereas Dy3N@C80 (II) (2) had the cage structure of C80:6 (D(5h)). The most probable cage structure of Dy3N@C80 (III) (3) was proposed to be C80:1 (D(5d)). The significant differences between Dy3N@C80 and other reported M3N@C80 (M = Sc, Y, Gd, Tb, Ho, Er, Tm) cluster fullerenes are discussed in detail, and the strong influence of the metal on the nitride cluster fullerene formation is concluded.     

Valence of D5h C50 fullerene

The energetic and electronic properties of D5h C50 before and after passivation by H or Cl are investigated using first-principle computational method of density functuional theory with generalized gradient approximation and local density approximation functionals. The results show that H or Cl addition can lead to energetic stabilization. Additions also increase the highest occupied molecular orbit-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of C50 fullerides and make them chemically more stable. In the series of C50H2m (m = 0 approximately 7), the Saturn-shaped D5h C50H10 has the largest HOMO-LUMO gap, which suggests that such a structure of C50H10 is a "magic-number" stable one of C50 adducts, and ten is a pseudovalence or effective valence of C50 fullerene pseudoatom. This point also is supported by the energetic properties of C50H2m series such as binding energies, etc. A minimal energy reaction pathway is constructed to get C50H10 and C50H14. Some useful experience for determining the favorable addition sites was summarized. A simple steric method is developed to predict the effective valences of classical fullerenes.     

Derivatives and dimers of C50-D5h and C50-D3: a comparison of two closely related but quite differently behaving fullerenes

A density functional theory study on the reactivity of the energetically most stable C(50) isomers, C(50)-D(5h) and C(50)-D(3), is presented. We explore the reactivity of both fullerenes towards the addition of hydrogen and halogen atoms and towards the formation of dimers. The addition patterns of the derivatives C(50)X(2n) (X = H, F, Cl) that are preferably formed after the saturation of the most reactive sites are investigated as well. The study reveals that while the results for C(50)-D(5h) are in agreement with simple empirical rules, C(50)-D(3) does not show the expected behaviour.     

Selective host-guest interaction of single-walled carbon nanotubes with functionalised fullerenes

Exohedrally functionalised fullerenes have been inserted in single-walled carbon nanotubes (SWNTs) with the aid of supercritical carbon dioxide to form peapods; C(61)(COOEt)(2) are encapsulated in SWNTs in high yield, whereas C(61)(COOH)(2) aggregate via hydrogen bonding to form a supramolecular complex, which sterically hinders encapsulation and causes it to adhere to the exterior surface of the SWNTs.     

Quantum-Chemical Simulation of New Hybrid Nanostructures: Small Fullerenes C20 and C28 in Single-Walled Boron-Nitrogen Nanotubes

A quantum-chemical simulation of new hybrid nanostructures consisting of regular chains of the small fullerenes C₂₀ and C₂₈ encapsulated into the bulk of achiral zigzag single-walled boron-nitrogen nanotubes [(C₂₀,C₂₈)@BN-NT]. The electronic properties and the nature of interatomic bonds in these nanostructures are analyzed as a function of the fullerene and the distances between fullerenes in the chain and between fullerenes and tube walls. The electronic characteristics of hybrid nanostructures are compared with those of "isolated" fullerenes and nanotubes, and (C20,C28) + BN-NT structures simulating fullerene adsorption on tube surface as the initial stage of (C₂₀,C₂₈)@BN-NT formation.     

Trifluoromethyl derivatives of insoluble small-HOMO-LUMO-gap hollow higher fullerenes. NMR and DFT structure elucidation of C2-(C74-D3h)(CF3)12, Cs-(C76-Td(2))(CF3)12, C2-(C78-D3h(5))(CF3)12, Cs-(C80-C2v(5))(CF3)12, and C2-(C82-C2(5))(CF3)12

Reaction of a mixture of insoluble higher fullerenes with CF3I at 500 degrees C produced a single abundant isomer of C74(CF3)12, C76(CF3)12, and C80(CF3)12, two abundant isomers of C78(CF3)12 and C82(CF3)12, and an indeterminant number of isomers of C84(CF3)12. Using a combination of 19F NMR spectroscopy, DFT calculations, and the structures and spectra of previously reported fullerene(CF3)n compounds, the most-probable structures of six of the seven isolated compounds were determined to be specific isomers of C2-(C74-D3h)(CF3)12, Cs-(C76-Td(2))(CF3)12), C2-(C78-D3h(5))(CF3)12), Cs-(C80-C2v(5))(CF3)12), C2-(C82-C2(5))(CF3)12), and C2-(C82-C2(3))(CF3)12) containing ribbons and/or loops of edge-sharing para-C6(CF3)2 hexagons. The seventh isolated compound is a C1 isomer of C78(CF3)12 containing two such ribbons. This set of compounds represents only the second reported isolable compound with the hollow C74-D3h cage and the first experimental evidence for the existence of the hollow fullerenes C76-Td(2), C78-D3h(5), C80-C2v(5), and C82-C2(5) in arc-discharge soots.     

Theoretical Study of the Molecular Electrostatic Potential of C_(50)

1 INTRODUCTION All fullerenes made so far obey the isolated pentagon (IPR)[1], which governs the stability of fullerenes comprising hexagons and exact 12 pen- tagons. Smaller fullerenes, which violate the IPR, are predicted to have high instability and especially difficult isolation due to their condensed pentagons and increased strain. The production of smaller fullerene C36 has been reported[2]. However, the definite characterization of a C36-based solid is in doubt. Moreover, variou…     

Adsorption of atomic hydrogen on single-walled carbon nanotubes

We have investigated atomic and electronic structures of hydrogen-chemisorbed single-walled carbon nanotubes (SWCNTs) by density functional calculations. We have searched for relative stability of various hydrogen adsorption geometries with coverage. The hydrogenated SWCNTs are stable with coverage of H/C, theta >/= 0.3. The circular cross sections of nanotubes are transformed to polygonal shapes with different symmetries upon hydrogen adsorption. We find that the band gap in carbon nanotubes can be engineered by varying hydrogen coverage, independent of the metallicity of carbon nanotubes. This is explained by the degree of sp(3) hybridization.     

异质富勒烯C58P2的理论研究

用ＭＮＤＯ半径能方法研究了异质富勒烯Ｃ５８Ｐ２各异构体的结构和相对稳定性。结果表明,１,４－位取代的异构体最稳定,异构体的稳定性随杂原子同距离的增加而降低。Ｃ５５８Ｐ２分子和稳定性比全碳富勒烯Ｃ６０低,但仍具有相当的稳定性,是潜在的合成目标。杂原子磷的掺杂改善了富勒烯的氧化还原性能。考察了各异构体的整体硬度和绝对电负性,产根据绝对电负性的差异预测了富勒烯和异质富勒烯间形成聚合物时的电子转移。     

C50(D5h)衍生物-异质富勒烯C48P2的分子行为理论研究

用半经验的AM1和MNDO方法以及密度泛函B3LYP/3-21G方法对C₅₀(D_5h)的衍生物C₄₈P₂的所有可能的异构体进行了系统的理论研究. 优化了稳定构型, 计算了生成热、HOMO-LUMO能级差、NICS值、红外光谱及电子光谱, 并与C₄₈X₂(X=B, N)的分子行为进行了比较. 计算结果表明: (1) C₄₈P₂的最稳定异构体是异构体C₄₈P₂-78, 该异构体对应于赤道位置六元环内的1,4-取代产物; (2) 决定C₄₈P₂异构体稳定性的主要因素是碳笼的张力, 而稳定性和芳香性之间没有明显的相关性; (3) 相对较稳定的C₄₈P₂异构体的HOMO-LUMO能级差比C₅₀的HOMO-LUMO能级差大; (4) 计算出的红外光谱以及电子光谱可以供实验参考; 计算出的NICS 值也可以用来表征C₄₈P₂各异构体. (5) C₄₈P₂与C₄₈X₂(X=B, N)具有相同的取代选择性规律及稳定性决定因素, 并且相对较稳定的异构体均具有比C₅₀本体大的HOMO-LUMO能级差.