arXiv subjects

Luciano Colombo

Publications and source records attributed to Luciano Colombo.

33 records · Page 2Linked to original sources

arXiv2012

The effect of the hydrogen coverage on the Young 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.

cond-mat.mtrl-sci↗

arXiv2012

Theory of single-molecule experiments in the overstretching force regime

We present a statistical mechanics analysis of the finite-size elasticity of biopolymers, consisting of domains which can exhibit transitions between more than one stable state at large applied force. The constant-force (Gibbs) and constant-displacement (Helmholtz) formulations of single molecule stretching experiments are shown to converge in the thermodynamic limit. Monte Carlo simulations of continuous three dimensional polymers of variable length are carried out, based on this formulation. We demonstrate that the experimental force-extension curves for short and long chain polymers are described by a unique universal model, despite the differences in chemistry and rate-dependence of transition forces.

physics.bio-ph↗

arXiv2011

Order-disorder phase change in embedded Si nano-particles

We investigated the relative stability of the amorphous vs crystalline nanoparticles of size ranging between 0.8 and 1.8 nm. We found that, at variance from bulk systems, at low T small nanoparticles are amorphous and they undergo to an amorphous-to-crystalline phase transition at high T. On the contrary, large nanoparticles recover the bulk-like behavior: crystalline at low T and amorphous at high T. We also investigated the structure of crystalline nanoparticles, providing evidence that they are formed by an ordered core surrounded by a disordered periphery. Furthermore, we also provide evidence that the details of the structure of the crystalline core depend on the size of the nanoparticle

cond-mat.mes-hall↗

arXiv2010

Enlightening the atomistic mechanisms driving self-diffusion of amorphous Si during annealing

We have analyzed the atomic rearrangements underlying self-diffusion in amorphous Si during annealing using tight-binding molecular dynamics simulations. Two types of amorphous samples with different structural features were used to analyze the influence of coordination defects. We have identified several types of atomic rearrangement mechanisms, and we have obtained an effective migration energy of around 1 eV. We found similar migration energies for both types of samples, but higher diffusivities in the one with a higher initial percentage of coordination defects.

cond-mat.mtrl-sci↗

arXiv2010

Effect of hydrogen on the microstructure evolution of nanocrystalline silicon

We investigated the effect of dissolved hydrogen on the microstructure evolution of nanocrystalline silicon. Through molecular dynamics simulations we characterize the local and overall structural features of several hydrogenated samples by the quantitative calculation of their crystallinity. We prove that hydrogen incorporation is detrimental for the recrystallization and accumulates at grain boundaries, thus increasing their effective area. We further observe the formation of silicon hydrogen complexes at very high hydrogen concentration.

cond-mat.mes-hall↗

arXiv2010

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 extention 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.

cond-mat.mes-hall↗

arXiv2010

Reply to "Comment on Gap opening in graphene by shear strain''

In reply to the Comment by Ramasubramaniam regarding our article [Phys. Rev. B {\bf 81}, 241412(R) (2010)] we clarify that our results are indeed valid provided that out-of-plane atomic relaxations are inhibited, as it may occur in the technologically relevant case of supported graphene sheets. We argue that the Comment, while overall interesting, is addressed to the different case of a free standing graphene monolayer and, therefore, does not invalidate our conclusions. We also remark that the rippled configuration discussed in the Comment is most likely affected by major size effects and that, contrary to what suggested, our conclusions are not in contrast with the recent work by Pereira {\it et al.} [Phys. Rev. B {\bf 80}, 045401 (2009)].

cond-mat.mes-hall↗

arXiv2010

Self assembling of Poly(3-hexylthiophene) (P3HT)

We study the assembling of P3HT chains in vacuo by means of a combination of first principles density functional theory and model potential molecular dynamics. We find that, in the absence of any external constraints, the pi-pi interchain interaction between thiophenes is the major driving force for the assembling. Single chains stack in a staggered geometry giving rise to the formation of two-dimensional hydrophobic foils. These, in turn, assemble into a zigzag bulk polymer structure in agreement with experimental findings. Finally, in the presence of some external constraint (like e.g. a substrate), when the alignment of single chains is favored instead of the stacking, a different bulk structure is possible where thiophene rings are aligned.

cond-mat.soft↗

arXiv2010

Atomistic investigation of the poly(3-hexylthiophene) adhesion on nanostructured titania

We study the adhesion of poly(3-hexylthiophene) on nanostructured titania surface in vacuo by means of model potential molecular dynamics. We generate large scale atomistic models of nanostructured titania surfaces (consisting of spherical nanocaps on top of a (110) rutile surface) and we study the adhesion of an oligothiophene as a function of their local curvature and roughness. In the limit of a perfect planar surface, the maximum adhesion energy is calculated to be as large as 0:6 eV per monomer, and it corresponds to the oligothiophene oriented along the [$\bar{1}10$] direction of the surface. Deformations of the polymer are observed due the incommensurability between the titania and the polymer lattice parameters. When the surface is nanostructured, the adhesion of the polymer is affected by the local morphology and a nonmonotonic dependence on the surface curvature is observed. The atomistic results are explained by a simple um model that includes the strain energy of the polymer and its electrostatic interaction with the local surface charge.

cond-mat.mtrl-sci↗

arXiv2010

A lattice model describing scale effects in nonlinear elasticity of nano-inhomogeneities

We present a procedure to map the constitutive laws of elasticity (both in the linear and nonlinear regime) onto a discrete atomic lattice and we apply the resulting elastic lattice model to investigate the strain field within an embedded nano-inhomogeneity. We prove that its elastic behavior at the nanoscale is governed by relevant atomistic effects. In particular, we demonstrate that such effects on the linear and nonlinear elastic properties are described by the same scaling exponent, in a large range of elastic contrast between the matrix and the nano-inhomogeneity. This suggests that the linear and nonlinear elastic behaviors of the composite system belong to the same universality class (at least within the nanometer length scale here investigated).

cond-mat.mes-hall↗

arXiv2010

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.

cond-mat.mes-hall↗

arXiv2010

Interplay between bending and stretching in carbon nanoribbons

We investigate the bending properties of carbon nanoribbons by combining continuum elasticity theory and tight-binding atomistic simulations. First, we develop a complete analysis of a given bended configuration through continuum mechanics. Then, we provide by tight-binding calculations the value of the bending rigidity in good agreement with recent literature. We discuss the emergence of a stretching field induced by the full atomic-scale relaxation of the nanoribbon architecture. We further prove that such an in-plane strain field can be decomposed into a first contribution due to the actual bending of the sheet and a second one due to edge effects.

cond-mat.mes-hall↗

arXiv2010

Nonlinear elasticity of monolayer graphene

By combining continuum elasticity theory and tight-binding atomistic simulations, we work out the constitutive nonlinear stress-strain relation for graphene stretching elasticity and we calculate all the corresponding nonlinear elastic moduli. Present results represent a robust picture on elastic behavior of one-atom thick carbon sheets and provide the proper interpretation of recent experiments. In particular, we discuss the physical meaning of the effective nonlinear elastic modulus there introduced and we predict its value in good agreement with available data. Finally, a hyperelastic softening behavior is observed and discussed, so determining the failure properties of graphene.

cond-mat.mes-hall↗

arXiv2009

Atomistic simulations of the sliding friction of graphene flakes

Using 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.

cond-mat.mes-hall↗

arXiv2001

Role of defects in the electronic properties of amorphous/crystalline Si interface

The mechanism determining the band alignment of the amorphous/crystalline Si heterostructures is addressed with direct atomistic simulations of the interface performed using a hierarchical combination of various computational schemes ranging from classical model-potential molecular dynamics to ab-initio methods. We found that in coordination defect-free samples the band alignment is almost vanishing and independent on interface details. In defect-rich samples, instead, the band alignment is sizeably different with respect to the defect-free case, but, remarkably, almost independent on the concentration of defects. We rationalize these findings within the theory of semiconductor interfaces.

cond-mat.mtrl-sci↗