Traian Dumitrică

Breakdown of continuum mechanics for nanometre-wavelength rippling of graphene

It is known that graphene exhibits natural ripples with characteristic lengths of around 10 nm. But when it is stretched across nanometre-scale trenches that form in a reconstructed copper surface, it develops even tighter corrugations that cannot be explained by continuum theory.

Wavelike rippling in multiwalled carbon nanotubes under pure bending

Objective molecular dynamics is used to systematically investigate elastic bending in carbon nanotubes up to 4.2 nm in diameter. A contrasting behavior is revealed: While single-wall tubes buckle in a gradual way, with a clear intermediate regime before they fully buckle, multiwalled tubes with closed cores exhibit a rate- and size-independent direct transition to an unusual wavelike mode with a 1 nm characteristic length. This rippling mode has a nearly-linear bending response and causes a ∼35% reduction in the stiffness of the thickest multiwalled tubes.

Elasticity of ideal single-walled carbon nanotubes via symmetry-adapted tight-binding objective modeling

The elastic response for a large catalog of carbon nanotubes subjected to axial and torsional strain is derived from atomistic calculations that rely on an accurate tight-binding description of the covalent binding. The application of the computationally expensive quantum treatment is possible due to the simplification in the number of atoms introduced by accounting for the helical and angular symmetries exhibited by the elastically deformed nanotubes. The elasticity of nanotubes larger than ∼1.25nm in diameter can be represented with an isotropic elastic continuum.

Stability of polycrystalline and wurtzite Si nanowires via symmetry-adapted tight-binding objective molecular dynamics

The stability of the most promising ground state candidate Si nanowires with less than 10 nm in diameter is comparatively studied with objective molecular dynamics coupled with nonorthogonal tight-binding and classical potential models. The computationally expensive tight-binding treatment becomes tractable due to the substantial simplifications introduced by the presented symmetry-adapted scheme. It indicates that the achiral polycrystalline of fivefold symmetry and the wurtzite wires of threefold symmetry are the most favorable quasi-one-dimensional Si arrangements. Quantitative differences with the classical model description are noted over the whole diameter range. Using a Wulff energy decomposition approach it is revealed that these differences are caused by the inability of the classical potential to accurately describe the interaction of Si atoms on surfaces and strained morphologies.

Effective-tensional-strain-driven bandgap modulations in helical graphene nanoribbons.

Despite the practical importance of graphene nanoribbons, little is known about their electronic structure other than in the idealized fl at-form presupposition. To describe the strain stored in helical nanoribbons and fractional carbon nanotubes, we supplement the standard elasticity concepts with an effective tensional strain. Using π -orbital tight binding (TB) and objective molecular dynamics (MD) coupled with density functional theory (DFT) TB, we formulate an effective-strain theory describing how twisting couples to the fundamental bandgap. In spite of the open and closed edges, as well as the inverse Poynting effect, from the effective-strain perspective, the twist-induced bandgap modulations appear strikingly similar to those exhibited by seamless carbon nanotubes in tension. In all three nanostructures, the bandgap modulations are driven by the effective tensional strain. Graphene nanoribbons (GNRs) and fractional carbon nanotubes (FCNTs), hexagonal atomic structures shaped into strips with nanometer-size widths with open and closed edges, are mechanically robust and electrically conducting and thus are well suited for use in nano-electromechanical systems (NEMS). The recent progress in preparation technologies, [ 1–3 ]

Dislocation onset and nearly axial glide in carbon nanotubes under torsion

The torsional plastic response of single-walled carbon nanotubes is studied with tight-binding objective molecular dynamics. In contrast with plasticity under elongation and bending, a torsionally deformed carbon nanotube can slip along a nearly axial helical path, which introduces a distinct (+1,-1) change in wrapping indexes. The low energy realization occurs without loss in mass via nucleation of a 5-7-7-5 dislocation dipole, followed by glide of 5-7 kinks. The possibility of nearly axial glide is supported by the obtained dependence of the plasticity onset on chirality and handedness and by the presented calculations showing the energetic advantage of the slip path and of the initial glide steps.

Distinct Element Method Modeling of Carbon Nanotube Bundles With Intertube Sliding and Dissipation

The recently developed distinct element method for mesoscale modeling of carbon nanotubes is extended to account for energy dissipation and then applied to characterize the constitutive behavior of crystalline carbon nanotube bundles subjected to simple tension and to simple shear loadings. It is shown that if these structures are sufficiently long and thick, then they become representative volume elements. The predicted initial stiffness and strength of the representative volumes are in agreement with reported experimental data. The simulations demonstrate that energy dissipation plays a central role in the mechanical response and deformation kinematics of carbon nanotube bundles.

Electrically active screw dislocations in helical ZnO and Si nanowires and nanotubes.

While the presence of axial screw dislocations in helical nanowires and nanotubes is known to be due to the growth process, their effect on the electronic properties remains unexplored. Relying on objective molecular dynamics simulations coupled to density functional tight-binding models for ZnO and Si, and supporting density functional theory calculations, we predict significant screw-dislocation-induced band gap modifications in both materials. The effect originates in the highly distorted cores and should be present at radii larger than those considered in our simulations (maximum ∼2 nm) as well as in other materials. The observed band gap dependences on the size of the Burgers vector and wall thickness could motivate new strategies for growing, via the screw dislocation mechanism, stable nanostructures with desired band gaps.

Synthesis, electromechanical characterization, and applications of graphene nanostructures

The emerging field of graphene brings together scientists and engineers as the discovered fundamental properties and effects encountered in this new material can be rapidly exploited for practical applications. There is potential for a two-dimensional graphene-based technology and recent works have already demonstrated the utility of graphene in building nanoelectromechanical systems, complex electronic circuits, photodetectors and ultrafast lasers. The state-of-the-art of substrate-suported graphene growth, and the current fundamental understanding of the electromechanical properties of graphene and graphene nanoribbons, represent important knowledge for developing new applications.

Chiral graphene nanoribbons: Objective molecular dynamics simulations and phase-transition modeling

There is a growing need to understand the stability of quasi-one-dimensional one-layer-thick graphene nanoribbons. Objective molecular dynamics based on density-functional tight-binding models are used to investigate the stability against torsional deformations of nanoribbons with bare, F-, and OH-decorated armchair edges. The prevalence of chiral nanoribbons, including homochiral ones, prompted the construction of a simple phenomenological model inspired from the Landau phase transition theory. Our model is based on atomistic data and gives the structural parameters of the nanoribbon as a function of its edge chemistry and axial strain.