Sergio B. Legoas

Molecular-dynamics simulations of carbon nanotubes as gigahertz oscillators.

Recently, Zheng and Jiang [Phys. Rev. Lett. 88, 045503 (2002)]] have proposed that multiwalled carbon nanotubes could be the basis for a new generation of nano-oscillators in the several gigahertz range. In this Letter, we present the first molecular dynamics simulation for these systems. Different nanotube types were considered in order to verify the reliability of such devices as gigahertz oscillators. Our results show that these nano-oscillators are dynamically stable when the radii difference values between inner and outer tubes are of approximately 3.4 A. Frequencies as large as 38 GHz were observed, and the calculated force values are in good agreement with recent experimental investigations.

Gigahertz nanomechanical oscillators based on carbon nanotubes

We report molecular dynamics studies of carbon nanotubes as mechanical gigahertz oscillators. Our results show that different oscillatory regimes exist but that sustained oscillations are possible only when the radii difference values of the inner and outer tubes are . Frequencies as large as 87?GHz were obtained. Calculated force and frequency values are in good agreement with estimated data from recent experimental investigations.

New families of carbon nanotubes based on graphyne motifs

Electronic properties of proposed new families of carbon single walled nanotubes are investigated. These nanotubes, called graphynes, result from the elongation of covalent interconnections of graphite-based nanotubes by the introduction of yne groups. Analogous to ordinary nanotubes, armchair, zigzag and chiral graphyne nanotubes are possible. Tight-binding and ab initio density functional methods were used to predict the electronic properties of these unusual nanotubes. Of the three graphyne nanotube families analysed here, two provide metallic behaviour for armchair tubes and either metallic or semiconducting behaviour for zigzag nanotubes. For the other graphyne nanotube family investigated a diameter and chirality independent bandgap is predicted and a bandgap modulation study by structural distortions has been carried out for small longitudinal tube deformations. Interestingly, while the bandgap is insensitive to structure, the stress-induced bandgap changes can strongly depend both on the nanotube type and whether the strain is tensile or compressive.

Origin of Anomalously Long Interatomic Distances in Suspended Gold Chains

The discovery of long bonds in gold atom chains has represented a challenge for physical interpretation. In fact, interatomic distances frequently attain 3.0-3.6 A values, and distances as large as 5.0 A may be occasionally observed. Here we studied gold chains by transmission electron microscopy and performed theoretical calculations using cluster ab initio density functional formalism. We show that the insertion of two carbon atoms is required to account for the longest bonds, while distances above 3 A may be due to a mixture of clean and one C atom contaminated bonds.

Lock-and-key effect in the surface diffusion of large organic molecules probed by STM

F. Sato, S. B. Legoas, R. Otero, F. Hümmelink, P. Thostrup, E. Lægsgaard, I. Stensgaard, F. Besenbacher, and D. S. Galvão Instituto de F́ısica “Gleb Wataghin”, Universidade Estadual de Campinas, C.P. 6165, 13083-970 Campinas SP, Brazil Departamento de F́ısica, Universidade Federal de Roraima, 69304-000 Boa Vista RR, Brazil and Interdisciplinary Nanoscience Center (iNANO) and Departament of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark (Dated: February 1, 2008)

Graphene to fluorographene and fluorographane: a theoretical study

We report here a fully reactive molecular dynamics study on the structural and dynamical aspects of the fluorination of graphene membranes (fluorographene). Our results show that fluorination tends to produce defective areas on the graphene membranes with significant distortions of carbon-carbon bonds. Depending on the amount of incorporated fluorine atoms, large membrane holes were observed due to carbon atom losses. These results may explain the broad distribution of the structural lattice parameter values experimentally observed. We have also investigated the effects of mixing hydrogen and fluorine atoms on the graphene functionalization. Our results show that, when in small amounts, the presence of hydrogen atoms produces a significant decrease in the rate of fluorine incorporation onto the membrane. On the other hand, when fluorine is the minority element, it produces a significant catalytic effect on the rate of hydrogen incorporation. We have also observed the spontaneous formation of new hybrid structures with different stable configurations (chair-like, zigzag-like and boat-like) which we named fluorographane.

Chaotic signature in the motion of coupled carbon nanotube oscillators

The motion of coupled oscillators based on multiwalled carbon nanotubes is studied using rigid-body dynamics simulations. The results show the existence of chaotic and regular behaviours for a given total energy, indicating the manifestation of chaos in nanoscaled mechanical systems based on carbon nanotube oscillators. Different regular motions are observed for different total energies, and they can be obtained by appropriately choosing the initial conditions. This possibility can allow the construction of multi-functional nano-devices based on multiwalled carbon nanotube oscillators.

Ordered phases of encapsulated diamondoids into carbon nanotubes

Diamondoids are hydrogen-terminated nanosized diamond fragments that are present in petroleum crude oil at low concentrations. These fragments are found as oligomers of the smallest diamondoid, adamantane (C(10)H(16)). Due to their small size, diamondoids can be encapsulated into carbon nanotubes to form linear arrangements. We have investigated the encapsulation of diamondoids into single walled carbon nanotubes with diameters between 1.0 and 2.2 nm using fully atomistic simulations. We performed classical molecular dynamics and energy minimizations calculations to determine the most stable configurations. We observed molecular ordered phases (e.g. double, triple, 4- and 5-stranded helices) for the encapsulation of adamantane, diamantane, and dihydroxy diamantane. Our results also indicate that the functionalization of diamantane with hydroxyl groups can lead to an improvement on the molecular packing factor when compared to non-functionalized compounds. Comparisons to hard-sphere models revealed differences, especially when more asymmetrical diamondoids were considered. For larger diamondoids (i.e., adamantane tetramers), we have not observed long-range ordering but only a tendency to form incomplete helical structures. Our calculations predict that thermally stable (at least up to room temperature) complex ordered phases of diamondoids can be formed through encapsulation into carbon nanotubes.

Temperature effects on the occurrence of long interatomic distances in atomic chains formed from stretched gold nanowires

The origin of long interatomic distances in suspended gold atomic chains formed from stretched nanowires remains the object of debate despite the large amount of theoretical and experimental work. Here, we report new atomic resolution electron microscopy observations acquired at room and liquid-nitrogen temperatures and theoretical results from ab initio quantum molecular dynamics on chain formation and stability. These new data are suggestive that the long distances are due to contamination by carbon atoms originating from the decomposition of adsorbed hydrocarbon molecules.

Carbon nanotube with square cross-section: an ab initio investigation.

Recently, Lagos et al. [Nat. Nanotechnol. 4, 149 (2009)] reported the discovery of the smallest possible silver square cross-section nanotube. A natural question is whether similar carbon nanotubes can exist. In this work we report ab initio results for the structural, stability, and electronic properties for such hypothetical structures. Our results show that stable (or at least metastable) structures are possible with metallic properties. They also show that these structures can be obtained by a direct interconversion from SWNT(2,2). Large finite cubanelike oligomers, topologically related to these new tubes, were also investigated.