Santiago Melchor

CoNTub: an algorithm for connecting two arbitrary carbon nanotubes.

We have developed the first computer program for determining the coordinates of heterojunctions between two arbitrary carbon nanotubes. This software implements the topological algebra based on the concept of strip, a continuous subset of carbon rings containing all the topological defects (nonhexagonal carbon rings). The user easily generates any heterojunction by merely introducing the indices (i,j) and length of the two nanotubes to be connected. The resulting structure is immediately visualized and can be exported in the protein-data-bank (PDB) format. Two classes of heterojunctions are distinguished depending on whether a cone between the connected nanotubes is required. This method is applicable to all kinds of two nanotube heterojunctions, including Dunlaps knees and others related. In addition, this program also generates single- and multiwalled carbon nanotubes (SWNT and MWNT). This application has been implemented as a Java applet, and it is freely available at the following web address: http://www.ugr.es/local/gmdm/java/contub/contub.html

Reversible attachment of platinum alloy nanoparticles to nonfunctionalized carbon nanotubes.

The formation of monodisperse, tunable sized, alloyed nanoparticles of Ni, Co, or Fe with Pt and pure Pt nanoparticles attached to carbon nanotubes has been investigated. Following homogeneous nucleation, nanoparticles attach directly to nonfunctionalized single-walled and multi-walled carbon nanotubes during nanoparticle synthesis as a function of ligand nature and the nanoparticle work function. These ligands not only provide a way to tune the chemical composition, size, and shape of the nanoparticles but also control a strong reversible interaction with carbon nanotubes and permit controlling the nanoparticle coverage. Raman spectroscopy reveals that the sp(2) hybridization of the carbon lattice is not modified by the attachment. In order to better understand the interaction between the directly attached nanoparticles and the nonfunctionalized carbon nanotubes, we employed first-principles calculations on model systems of small Pt clusters and both zigzag and armchair single-walled carbon nanotubes. The detailed comprehension of such systems is of major importance since they find applications in catalysis and energy storage.

Clar-Kekule structuring in armchair carbon nanotubes.

Geometrical patterns on armchair nanotubes and their dependence on length (up to 10 nm) have been studied using first-principles methods. The results indicate that finite nanotubes do not show a uniform bond structure. The previous structural classification of armchair nanotubes in Clar, Kekule, and incomplete-Clar types becomes unified with lengthening, not in a bond-uniform structure, as PBC models report, but into an alternated sequence of Clar and Kekule domains in all cases, with possible mechanical and electronic consequences.

CoNTub v2.0 - Algorithms for Constructing C3-Symmetric Models of Three-Nanotube Junctions

Here, a method is described for easily building three-carbon nanotube junctions. It allows the geometry to be found and bond connectivity of C(3) symmetric nanotube junctions to be established. Such junctions may present a variable degree of pyramidalization and are composed of three identical carbon nanotubes with arbitrary chirality. From the indices of the target nanotube, applying the formulas of strip algebra, the possible positions of the six defects (heptagonal rings) needed can be found. Given the multiple possibilities that arise for a specific pair of indices, the relation between the macroscopic geometry (interbranch angles, junction size, and pyramidalization) and each specific solution is found. To automate the construction of these structures, we implemented this algorithm with CoNTub software, version 2.0, which is available at ( http://www.ugr.es/local/gmdm/contub2 ). In addition, a classification of three-nanotube junctions, 3TJ, in seven types based on the location of defects has been proposed, i.e. 3TJ(0:0:6), 3TJ(0:1:5), 3TJ(0:2:4), 3TJ(0:3:3), 3TJ(1:1:4), 3TJ(1:2:3), and 3TJ(2:2:2) types.

Evidence of an Unexpectedly Long C−C Bond (>2.7 Å) in 1,3-Metalladiyne Complexes [Cp2MCCR]2 (M = Ti, Zr): QTAIM and ELF Analyses

Topological analyses of the electron density using the quantum theory of atoms in molecules (QTAIM) and electron localization function (ELF) have been carried out, at the B3LYP/DGVZVP and MP2/DGVZVP theoretical levels, on different 1,3-metalladiyne cyclic compounds [Cp2M(CCR)]2, (M = Ti, Zr; R = F, CH3, H, SiH3). The QTAIM results indicate the presence of an extraordinarily long C-C bond (in a 2.7-3.0 A range) connecting the CCR moieties, contrary to the common geometrical assumption of a M-M bond in similar metallacycles. The existence of this C-C bond is also supported by the distinct consequences on the reaction profiles for the Ti and Zr complexes, the CC oxidative coupling reactions being favored only for the Ti complexes. Moreover, the consequences of this bonding in the coupling/cleavage reactions of these metallacyclic complexes are reported and analyzed, revealing the transcendence of these long-range bonds in the overall behavior of these compounds.