Reversible actuation of α-borophene nanoscrolls.

In this work, we proposed and investigated the structural and electronic properties of boron-based nanoscrolls (armchair and zigzag) using the DFTB+ method. We also investigated the electroactuation process (injecting and removing charges). A giant electroactuation was observed, but the results show relevant differences between the borophene and carbon nanoscrolls. The molecular dynamics simulations showed that the scrolls are thermally and structurally stable for a large range of temperatures (up to 600 K), and the electroactuation process can be easily tuned and can be entirely reversible for some configurations.

Evaluation of the Reax Force-Field for Studying the Collision of an Energetic Proton with the DNA.

The early DNA damage induced by ionizing radiation depends on how ionizing particles transfer energy to this molecule and the surrounding medium, mostly water. In preliminary studies, we found that the energy transferred by a 4 keV proton to a cytosine-guanine base pair in a classical simulation collision using the ReaxFF potential is much smaller than that obtained by a quantum calculation using time-dependent density functional theory (TDDFT). We observed that there are two main reasons for that: no accurate force-field for this situation and problems while dealing with the proton charge during the collision. Here, we only focus on the interaction potential. We calibrated the van der Waals energy term of the ReaxFF potential using TDDFT calculations and a genetic algorithm, specifically for the interaction of a proton with the DNA constituent atoms (carbon, hydrogen, phosphorus, nitrogen, and oxygen). We obtained a significant improvement in the interaction potential and, consequently, in the scattering angle of the proton colliding with the target atoms in question. However, we conclude that despite the improvement for the force-field and scattering angle, the classical charge equilibration method should also be improved to properly describe the proton-DNA collision process.

Graphenylene-based nanoribbons for novel molecular electronic devices.

In the last decade, graphene has been frequently cited as one of the most promising materials for nanoelectronics. However, despite its outstanding mechanical and electronic properties, its use in the production of real nanoelectronic devices usually imposes some practical difficulties. This happens mainly due to the fact that, in its pristine form, graphene is a gapless material. We investigate theoretically the possibility of obtaining rectifying nanodevices using another carbon based two dimensional material, namely the graphenylene. This material has the advantage of being an intrinsic semiconductor, posing as a promising material for nanoelectronics. By doping graphenylene, one could obtain 2-dimensional p-n junctions, which can be useful for the construction of low dimensional electronic devices. We propose 2-dimensional diodes in which a clear rectification effect was demonstrated, with a conducting threshold of approximately 1.5 eV in direct bias and current blocking with opposite bias. During these investigations were found specific configurations that could result in devices with Zener-like behavior. Also, one unexpected effect was identified, which was the existence of transmission dips in electronic conductance plots. This result is discussed as a related feature to what was found in graphene nanoribbon systems under external magnetic fields, even though the external field was not a necessary ingredient to obtain such effect in the present case.

Theoretical study of collision dynamics of fullerenes on graphenylene and porous graphene membranes.

A comparative study regarding the behavior of graphene, porous graphene and graphenylene monolayers under high energy impact is reported. Our results were obtained using a computational model constructed to perform investigations of the dynamics of high velocity fullerenes colliding with free standing sheets of those materials. We employed fully reactive molecular dynamics simulations in which the interatomic interactions were described using ReaxFF force field. During the simulations, free standing monolayers of the investigated materials were submitted to collision with a C60 fullerene molecule at impact angles within the range 0°≤θ≤75°. We considered kinetic energies in the range 0eV≤Ek≤1500eV, that corresponds to a projectile velocity v in the range 0Å/fs≤v≤0.2Å/fs. Also, the failure dynamics of each one of the 2-dimensional materials is described in a comparative analysis in which relevant differences and unique features observed in the mechanical stress dissipation processes are highlighted. Finally, performing hundreds of simulations we were able to map many possible scenarios for these collisions and to construct diagrams that elucidate, for each one of the materials, the possible behaviors under the action of a highly energetic C60 projectile as a function of energy and incident angle.

Giant and Tunable Anisotropy of Nanoscale Friction in Graphene.

The nanoscale friction between an atomic force microscopy tip and graphene is investigated using friction force microscopy (FFM). During the tip movement, friction forces are observed to increase and then saturate in a highly anisotropic manner. As a result, the friction forces in graphene are highly dependent on the scanning direction: under some conditions, the energy dissipated along the armchair direction can be 80% higher than along the zigzag direction. In comparison, for highly-oriented pyrolitic graphite (HOPG), the friction anisotropy between armchair and zigzag directions is only 15%. This giant friction anisotropy in graphene results from anisotropies in the amplitudes of flexural deformations of the graphene sheet driven by the tip movement, not present in HOPG. The effect can be seen as a novel manifestation of the classical phenomenon of Euler buckling at the nanoscale, which provides the non-linear ingredients that amplify friction anisotropy. Simulations based on a novel version of the 2D Tomlinson model (modified to include the effects of flexural deformations), as well as fully atomistic molecular dynamics simulations and first-principles density-functional theory (DFT) calculations, are able to reproduce and explain the experimental observations.

Burning Graphene Layer-by-Layer.

Graphene, in single layer or multi-layer forms, holds great promise for future electronics and high-temperature applications. Resistance to oxidation, an important property for high-temperature applications, has not yet been extensively investigated. Controlled thinning of multi-layer graphene (MLG), e.g., by plasma or laser processing is another challenge, since the existing methods produce non-uniform thinning or introduce undesirable defects in the basal plane. We report here that heating to extremely high temperatures (exceeding 2000 K) and controllable layer-by-layer burning (thinning) can be achieved by low-power laser processing of suspended high-quality MLG in air in "cold-wall" reactor configuration. In contrast, localized laser heating of supported samples results in non-uniform graphene burning at much higher rates. Fully atomistic molecular dynamics simulations were also performed to reveal details of oxidation mechanisms leading to uniform layer-by-layer graphene gasification. The extraordinary resistance of MLG to oxidation paves the way to novel high-temperature applications as continuum light source or scaffolding material.

Enhanced Mechanical Stability of Gold Nanotips through Carbon Nanocone Encapsulation.

Gold is a noble metal that, in comparison with silver and copper, has the advantage of corrosion resistance. Despite its high conductivity, chemical stability and biocompatibility, gold exhibits high plasticity, which limits its applications in some nanodevices. Here, we report an experimental and theoretical study on how to attain enhanced mechanical stability of gold nanotips. The gold tips were fabricated by chemical etching and further encapsulated with carbon nanocones via nanomanipulation. Atomic force microscopy experiments were carried out to test their mechanical stability. Molecular dynamics simulations show that the encapsulated nanocone changes the strain release mechanisms at the nanoscale by blocking gold atomic sliding, redistributing the strain along the whole nanostructure. The carbon nanocones are conducting and can induce magnetism, thus opening new avenues on the exploitation of transport, mechanical and magnetic properties of gold covered by sp(2) carbon at the nanoscale.

Fullerenes generated from porous structures.

A class of macromolecules based on the architecture of the well-known fullerenes is theoretically investigated. The building blocks used to geometrically construct these molecules are the two dimensional structures: porous graphene and biphenylene-carbon. Density functional-based tight binding methods as well as reactive molecular dynamics methods are applied to study the electronic and structural properties of these molecules. Our calculations predict that these structures can be stable up to temperatures of 2500 K. The atomization energies of carbon structures are predicted to be in the range of 0.45 eV per atom to 12.11 eV per atom (values relative to the C60 fullerene), while the hexagonal boron nitride analogues have atomization energies between -0.17 eV per atom and 12.01 eV per atom (compared to the B12N12 fullerene). Due to their high porosity, these structures may be good candidates for gas storage and/or molecular encapsulation.

Bending of layer-by-layer films driven by an external magnetic field.

We report on optimized architectures containing layer-by-layer (LbL) films of natural rubber latex (NRL), carboxymethyl-chitosan (CMC) and magnetite (Fe3O4) nanoparticles (MNPs) deposited on flexible substrates, which could be easily bent by an external magnetic field. The mechanical response depended on the number of deposited layers and was explained semi-quantitatively with a fully atomistic model, where the LbL film was represented as superposing layers of hexagonal graphene-like atomic arrangements deposited on a stiffer substrate. The bending with no direct current or voltage being applied to a supramolecular structure containing biocompatible and antimicrobial materials represents a proof-of-principle experiment that is promising for tissue engineering applications in biomedicine.