Emiliano Cadelano

Gap opening in graphene by shear strain

We exploit the concept of strain-induced band-structure engineering in graphene through the calculation of its electronic properties under uniaxial, shear, and combined uniaxial-shear deformations. We show that by combining shear deformations to uniaxial strains it is possible modulate the graphene energy-gap value from zero up to 0.9 eV. Interestingly enough, the use of a shear component allows for a gap opening at moderate absolute deformation, safely smaller than the graphene failure strain.

Elastic properties of hydrogenated graphene

There exist three conformers of hydrogenated graphene, referred to as chair-, boat-, or washboard-graphane. These systems have a perfect two-dimensional periodicity mapped onto the graphene scaffold but they are characterized by a sp 3 orbital hybridization, have different crystal symmetry, and otherwise behave upon loading. By first-principles calculations we determine their structural and phonon properties, as well as we establish their relative stability. Through continuum elasticity we define a simulation protocol addressed to measure by a computer experiment their linear and nonlinear elastic moduli and we actually compute them by first principles. We argue that all graphane conformers respond to any arbitrarily oriented extension with a much smaller lateral contraction than the one calculated for graphene. Furthermore, we provide evidence that boat-graphane has a small and negative Poisson ratio along the armchair and zigzag principal directions of the carbon honeycomb lattice axially auxetic elastic behavior. Moreover, we show that chair-graphane admits both softening and hardening hyperelasticity, depending on the direction of applied load.

Atomistic simulations of the sliding friction of graphene flakes

AbstractUsing a tight-binding atomistic simulation, we simulate the recent atomic-force microscopy experiments probing the slipperiness of graphene flakes made slide against a graphite surface. Compared to previous theoretical models, where the flake was assumed to be geometrically perfect and rigid, while the substrate is represented by a static periodic potential, our fully-atomistic model includes quantum mechanics with the chemistry of bond breaking and bond formation, and the flexibility of the flake. These realistic features, include in particular the crucial role of the flake rotation in determining the static friction, in qualitative agreement with experimental observations.

Elastic fields and moduli in defected graphene

By means of tight-binding atomistic simulations we study a family of native defects in graphene which have recently been detected experimentally. Their formation energy is found to be as large as several electronvolts, consistent with the empirical evidence of high crystalline quality in most graphene samples. Defects, especially if associated with bond reconstructions, induce sizable deformation and stress fields with a spatial distribution closely related to their actual symmetry. The description of such fields proposed here is believed to be useful for the unambiguous characterization of images obtained by electron microscopy. We also argue that they define the basin of mutual interaction between two nearby defects. Finally, we provide evidence that defects differently affect the linear elastic moduli of monolayer graphene. In general, both the Young modulus and the Poisson ratio are decreased, but their dependence upon the defect surface density is remarkably more pronounced for vacancy-like than for number-like defects.

Folded Graphene Membranes: Mapping Curvature at the Nanoscale

While the unique elastic properties of monolayer graphene have been extensively investigated, less knowledge has been developed so far on folded graphene. Nevertheless, it has been recently suggested that fold-induced curvature (without in-plane strain) could possibly affect the local chemical and electron transport properties of graphene, envisaging a material-by-design approach where tailored membranes are used in enhanced nanoresonators or nanoelectromechanical devices. In this work we propose a novel method combining apparent strain analysis from high-resolution transmission electron microscopy (HREM) images and theoretical modeling based on continuum elasticity theory and tight-binding atomistic simulations to map and measure the nanoscale curvature of graphene folds and wrinkles. If enough contrast and resolution in HREM images are obtained, this method can be successfully applied to provide a complete nanoscale geometrical and physical picture of 3D structure of various wrinkle and fold configurations.

Effect of hydrogen coverage on the Young's modulus of graphene

We blend together continuum elasticity and first principles calculations to measure by a computer experiment the Young modulus of hydrogenated graphene. We provide evidence that hydrogenation generally leads to a much smaller longitudinal extension upon loading than in pristine graphene. Furthermore, the Young modulus is found to depend upon the loading direction for some specific conformers, characterized by an anisotropic linear elastic behavior.

Interplay between bending and stretching in carbon nanoribbons

Received 3 March 2010; published 12 April 2010We investigate the bending properties of carbon nanoribbons by combining continuum elasticity theory andtight-binding atomistic simulations. First, we develop a complete analysis of a given bended configurationthrough continuum mechanics. Then, we provide by tight-binding calculations the value of the bending rigidityin good agreement with recent literature. We discuss the emergence of a stretching field induced by the fullatomic-scale relaxation of the nanoribbon architecture. We further prove that such an in-plane strain field canbe decomposed into a first contribution due to the actual bending of the sheet and a second one due to edgeeffects.DOI: 10.1103/PhysRevB.81.144105 PACS number s : 62.25. g, 46.70.Hg, 62.20.D

Electronic and Optical properties of cadmium fluoride: the role of many-body effects

Electronic excitations and optical spectra of

Electronic structure of fluorides: general trends for ground and excited state properties

CdF_{2}

One- and two-particle effects in the electronic and optical spectra of barium fluoride.

are calculated up to ultraviolet employing state-of-the-art techniques based on density functional theory and many-body perturbation theory. The GW scheme proposed by Hedin has been used for the electronic self-energy to calculate single-particle excitation properties as energy bands and densities of states. For optical properties many-body effects, treated within the Bethe-Salpeter equation framework, turn out to be crucial. A bound exciton located about 1 eV below the quasiparticle gap is predicted. Within the present scheme the optical absorption spectra and other optical functions show an excellent agreement with experimental data. Moreover, we tested different schemes to obtain the best agreement with experimental data. Among the several schemes, we suggest a self-consistent quasiparticle energy scheme.