Nanomechanics of few-layer materials: do individual layers slide upon folding?

Folds naturally appear on nanometrically thin materials, also called "2D materials", after exfoliation, eventually creating folded edges across the resulting flakes. We investigate the adhesion and flexural properties of single-layered and multilayered 2D materials upon folding in the present work. This is accomplished by measuring and modeling mechanical properties of folded edges, which allows for the experimental determination of the bending stiffness (κ) of multilayered 2D materials as a function of the number of layers (n). In the case of talc, we obtain κ ∝ n 3 for n ≥ 5, indicating no interlayer sliding upon folding, at least in this thickness range. In contrast, tip-enhanced Raman spectroscopy measurements on edges in folded graphene flakes, 14 layers thick, show no significant strain. This indicates that layers in graphene flakes, up to 5 nm thick, can still slip to relieve stress, showing the richness of the effect in 2D systems. The obtained interlayer adhesion energy for graphene (0.25 N/m) and talc (0.62 N/m) is in good agreement with recent experimental results and theoretical predictions. The obtained value for the adhesion energy of graphene on a silicon substrate is also in agreement with previous results.

Development and Characterization of Titanium Dioxide Ceramic Substrates with High Dielectric Permittivities.

Titanium dioxide substrates have been synthesized by means of solid-state reactions with sintering temperatures varying from 1150 °C up to 1350 °C. X-ray diffraction and scanning electron microscopy (SEM) where employed to investigate the crystal structure, grain size and porosity of the resulting samples. The obtained ceramics are tetragonal (rutile phase) with average grain sizes varying from 2.94 µm up to 5.81 µm. The average grain size of samples increases with increasing temperature, while the porosity decreases. The effect of microstructure on the dielectric properties has been also studied. The reduction of porosity of samples significantly improves the dielectric parameters (relative dielectric permittivity and loss tangent) in comparison to those of commercial substrates, indicating that the obtained ceramic substrates could be useful in the miniaturization of telecommunication devices.

Soliton instability and fold formation in laterally compressed graphene.

We investigate-through simulations and analytical calculations-the consequences of uniaxial lateral compression applied to the upper layer of multilayer graphene. The simulations of compressed graphene show that strains larger than 2.8% induce soliton-like deformations that further develop into large, mobile folds. Such folds were indeed experimentally observed in graphene and other solid lubricants two-dimensional (2D) materials. Interestingly, in the soliton-fold regime, the shear stress decreases with the strain s, initially as s(-2/3) and rapidly going to zero. Such instability is consistent with the recently observed negative dynamic compressibility of 2D materials. We also predict that the curvatures of the soliton-folds are given by r(c) = δ√(ß/2α) where 1 ≤ δ ≤ 2 and ß and α are respectively related to the layer bending modulus and to the interlayer binding energy of the material. This finding might allow experimental estimates of the ß/α ratio of 2D materials from fold morphology.

Tunable band gap of boron nitride interfaces under uniaxial pressure.

In this work we show, by means of a density functional theory formalism, that the interaction between hydrogen terminated boron nitride surfaces gives rise to a metallic interface with free carriers of opposite sign at each surface. A band gap can be induced by decreasing the surface separation. The size of the band gap changes continuously from zero up to 4.4 eV with decreasing separation, which is understood in terms of the interaction between surface states. Due to the high thermal conductivity of cubic boron nitride and the coupling between band gap and applied pressure, such tunable band gap interfaces may be used in highly stable electronic and electromechanical devices. In addition, the spatial separation of charge carriers at the interface may lead to photovoltaic applications.

Boron nitride nanotubes as templates for half-metal nanowires.

We investigate by means of a GGA + U implementation of density functional theory the electronic and structural properties of magnetic nanotubes composed of an iron oxide monolayer and (n,0) boron nitride (BN) nanotubes, with n ranging from 6 to 14. The formation energy per FeO molecule of FeO covered tubes is smaller than the formation energy of small FeO nanoparticles, which suggests that the FeO molecules may cover the BN nanotubes rather than aggregating locally. Both GGA (PBE) and Van der Waals functionals predict an optimal FeO-BN interlayer distance of 2.94 Å. Depending on the diameter of the tube, novel electronic properties for the FeO covered BN nanotubes were found. They can be semiconductors, intrinsic half-metals or semi-half-metals that can become half-metals if charged with either electrons or holes. Such results are important in the spintronics context.