We report a first-principles study of edge-reconstructed, few-layered graphene nanoribbons. We find that the nanoribbon stability increases linearly with increasing width and decreases linearly with increasing number of layers (from three to six layers). Specifically, we find that a three-layer 1.3 nm wide ribbon is energetically more stable than the C60 fullerene, and that a 1.8 nm wide ribbon is more stable than a (10,0) carbon nanotube. The morphologies of the reconstructed edges are characterized by the presence of five-, six-, and sevenfold rings, with sp3 and sp2 bonds at the reconstructed edges. The electronic structure of the few-layered nanoribbons with reconstructed edges can be metallic or semiconducting, with band gaps oscillating between 0 and 0.28 eV as a function of ribbon width.
The electrical transport properties of a four-layered hydrogen-terminated cubic boron nitride sub-nanometer film in contact with gold electrodes are investigated via density functional calculations. The sample exhibits asymmetric metallic surfaces, a fundamental feature that triggers the system to behave like a typical p–n junction diode for voltage bias in the interval −0.2 ≤ V ≤ 0.2, where a rectification ratio up to 62 is verified. Further, in the wider region −0.3 ≤ V ≤ 0.3, negative differential resistance with a peak-to-valley ratio of 10 is observed. The qualitative behavior of the I–V characteristics is described in terms of the hydrogenated cBN film equilibrium electronic structure. Such a film shows metallic surfaces due to surface electronic states at a fraction of eV above and below the Fermi level of the N–H terminated and B–H terminated surfaces, respectively, with a wide bulk-band gap characteristic of BN materials. Such a mechanism is supported by transmission coefficient calculations, with the Landauer–Büttiker formula governing the I–V characteristics.
In this work hybrids of titanium manopartides and polyaniline are obtained by pulsed electrodeposition at different pH (1.5, 3.9 and 5.9) and characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, cyclic voltammetry, ultraviolet-visible, and Raman spectroscopies. We found that films deposited at pH 5.9 with nanoparticles incorporation are composed of emeraldine meanwhile films without nanoparticles are composed of pernigraniline. As a result, films deposited with nanoparticles incorporation present conductivity 6 times higher than that of films deposited. without nanoparticles. Films deposited at pH 3.9 with or without nanoparticles incorporation are both made of pernigraniline. Even though films with nanoparticles incorporation still present higher conductivity. To explain such a result, we performed first-principles calculations on polyaniline/TiO2 interface. The calculations predict a metallic polyaniline/TiO2 interface in spite of polyaniline and TiO2 being semiconductors. At pH 1.5, the presence of nanoparticles has negligible effect on films characteristics. We believe that at low pH (pH 1.5) H atoms tend to bind TiO2 surface resulting in positively charged nanoparticles, which are further screened by SO4-2 anions. Such a screening layer prevents the physical contact between nanoparticles and polyaniline monomers diminishing the effects of nanopartide presence.
We propose an effective model for solute separation from fluids through reverse osmosis based on core-softened potentials. Such potentials have been used to investigate anomalous fluids in several situations under a great variety of approaches. Due to their simplicity, computational simulations become faster and mathematical treatments are possible. Our model aims to mimic water desalination through nano-membranes through reverse osmosis, for which we have found reasonable qualitative results when confronted against all-atoms simulations found in the literature. The purpose of this work is not to replace any fully atomistic simulation at this stage, but instead to pave the first steps towards coarse-grained models for water desalination processes. This may help to approach problems in larger scales, in size and time, and perhaps make analytical theories more viable. (C) 2016 Elsevier B.V. All rights reserved.
We present a theoretical study of the vibrational spectrum, in the G band region, of laterally hydrogenated single wall carbon nanotubes through molecular dynamics simulations. We find that bilateral hydrogenation which can be induced by hydrogenation under lateral strain causes permanent oval deformations on the nanotubes and induces the splitting of vibrational states in the G-band region. We propose that such splitting can be used as a Raman fingerprint for detecting nanotubes that have been permanently modified due to bilateral hydrogenation. In particular, our results may help to clarify the recent findings of Araujo and collaborators [Nano Lett. 12, 4110 (2012)1 which have found permanent modifications in the Raman G peaks of locally compressed carbon nanotubes. We have also developed an analytical model for the proposed phenomenon that reproduces the splitting observed in the simulations. (C) 2015 Elsevier Ltd. All rights reserved.
Understanding layer interplay is the key to utilizing layered heterostructures formed by the stacking of different two-dimensional materials for device applications. Boron nitride has been demonstrated to be an ideal substrate on which to build graphene devices with improved mobilities. Here we present studies on the morphology and optical response of annealed few-layer hexagonal boron nitride flakes deposited on a silicon substrate that reveal the formation of linear wrinkles along well-defined crystallographic directions. The wrinkles formed a network of primarily threefold and occasionally fourfold origami-type junctions throughout the sample, and all threefold junctions and wrinkles formed along the armchair crystallographic direction. Furthermore, molecular dynamics simulations yielded, through spontaneous symmetry breaking, wrinkle junction morphologies that are consistent with both the experimental results and the proposed origami-folding model. Our findings indicate that this morphology may be a general feature of several two-dimensional materials under proper stress-strain conditions, resulting in direct consequences in device strain engineering.
A Monte-Carlo-based simulated annealing process combined with ab initio calculations is employed to investigate electronic and structural properties of boron nitride (BN)-doped graphene, in a wide doping range. We find that, for a given BN doping concentration, the doping-induced band gap can vary over an order of magnitude depending on the placement of the B and N atoms. We propose an analytical tight-binding model that reproduces the dependence of the band gap on both the concentration and the morphology obtained in the ab initio calculations and provides an upper bound for the band gap at a given BN concentration. We also predict that the dependence of the band gap with applied tensile stress should be strong, nonmonotonic, and anisotropic, within the range of strain values attainable experimentally.
A new experimental procedure was used to analyse the corrosion behaviour of hot-dip Zn coated steel based on the association of the scanning vibrating electrode technique (SVET) and uniaxial tensile strain applied up to 3.1% in 0.01 M NaCl solutions without release of the elastic strain during the test. The nucleation of localized corrosion sites on the coating occurs at very low strain values around the yield point and the maximum current densities increase continuously with the increase of the applied strain. The nucleation of localized corrosion on the Zn surface was favoured by the rupture of the passive film in contact with the solution by the action of slip steps and fine intergranular cracks, rather than by the exposure of the steel substrate. Straining of the Zn coating immersed in the solution was comparatively a much more aggressive condition than straining the sample in air before the corrosion tests. (C) 2015 Elsevier Ltd. All rights reserved.
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) = delta root beta/2 alpha, where 1 <= delta <= 2, and beta and alpha 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 beta/alpha a ratio of 2D materials from fold morphology.
We apply first-principles methods to investigate the electronic properties of semiconductor carbon nanotubes deposited on hydrogen-terminated diamond surfaces. We found that the band gap of the diamond-nanotube system can be continuously varied (from 0 to 0.8 eV) through the application of an external electric field. The metal semiconductor transition occurs for values of field between 0.03 and 0.035 V/angstrom, which could make viable the production of stable field-effect devices at nanometric scale. We also found that at zero value of electric field the nanotube electrical behavior can be modified by changing the diamond surface termination, which may be useful for producing ohmic metal semiconductor contacts. On the other hand, at zero values of field, tubes deposited on nitrogen-doped diamond are metallic regardless of the surface termination. Thus, nitrogen doping could be useful in situations in which the atomistic details of the diamond surface are difficult to control.
This work presents a study of corrosion resistance and cell viability of carbon films on bare and nitrided Ti-6Al-4V. Films deposited on bare alloy significantly improve the corrosion resistance. Unexpectedly, films deposited on nitrided alloy present delamination and cracking after 16 days. We associate, film failure with the presence of pores combined with a weak film/substrate interaction that allows diffusion of ions at the interface. We found that films tend to diminish the osteoblastic cell viability and the observed variations on film roughness do not improve cell viability. (C) 2014 Elsevier Ltd. All rights reserved.
A hybrid structure that presents phases of three extended allotropes of carbon, nanotube, graphene, and diamond, is proposed in this work. According to our first-principles calculations, such structure can be made energetically stable through the application of pressures of the order of 100 kbar to alternate graphene nanotube layers, which were recently synthesized in large-area films. The existence of sp(3) dangling bonds in the hybrid structure gives rise to an exceptionally large density of states near the Fermi level, leading to a ferromagnetic ground state.
This work presents a comparative wear, corrosion and wear-corrosion (the last one in a simulated physiological solution) study of graphite-like a-C:H (GLCH) films deposited on bare and nitrided Ti6Al4V alloy. Films, deposited by r.f. PACVD, presented low porosity and promoted high corrosion resistance. The friction coefficient of the films was very low with appreciable wear resistance at room conditions. However, due to the simultaneous action of both load and the corrosive environment in wear-corrosion tests a marked reduction in the coating lifetime was observed. Unexpectedly, films deposited on the nitrided alloy presented a lifetime at least ten times shorter than that of films on bare alloy. We explain such a result in terms of film/substrate interaction. The weak GLCH/nitrided alloy interaction facilitates fluid penetration between the film and the substrate which leads to a fast film delamination. Such an interpretation is supported by force curve measurements, which show that the interaction between GLCH and nitrided alloy is four times weaker than that between GLCH and bare alloy. (C) 2012 Elsevier B.V. All rights reserved.
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.
We report a novel mechanical response of few-layer graphene, h-BN, and MoS2 to the simultaneous compression and shear by an atomic force microscope (AFM) tip. The response is characterized by the vertical expansion of these two-dimensional (2D) layered materials upon compression. Such effect is proportional to the applied load, leading to vertical strain values (opposite to the applied force) of up to 150%. The effect is null in the absence of shear, increases with tip velocity, and is anisotropic. It also has similar magnitudes in these solid lubricant materials (few-layer graphene, h-BN, and MoS2), but it is absent in single-layer graphene and in few-layer mica and Bi2Se3. We propose a physical mechanism for the effect where the combined compressive and shear stresses from the tip induce dynamical wrinkling on the upper material layers, leading to the observed flake thickening. The new effect (and, therefore, the proposed wrinkling) is reversible in the three materials where it is observed.
In this work, an atomic force microscope (AFM) is combined with a confocal Raman spectroscopy setup to follow in situ the evolution of the G-band feature of isolated single-wall carbon nanotubes (SWNTs) under transverse deformation. The SWNTs are pressed by a gold AFM tip against the substrate where they are sitting. From eight deformed SWNTs, five exhibit an overall decrease in the Raman signal intensity, while three exhibit vibrational changes related to the circumferential symmetry breaking. Our results reveal chirality dependent effects, which are averaged out in SWNT bundle measurements, including a previously elusive mode symmetry breaking that is here explored using molecular dynamics calculations.
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 angstrom. 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.
A first-principles formalism is employed to investigate the interaction of iron oxide (FeO) with a boron nitride (BN) nanotube. The stable structure of the FeO-nanotube has Fe atoms binding N atoms, with bond length of roughly similar to 2.1 angstrom, and binding between O and B atoms, with bond length of 1.55 angstrom. In case of small FeO concentrations, the total magnetic moment is (4 mu(Bohr)) times the number of Fe atoms in the unit cell, and it is energetically favorable to FeO units to aggregate rather than randomly bind to the tube. As a larger FeO concentration case, we study a BN nanotube fully covered by a single layer of FeO. We found that such a structure has a square FeO lattice with Fe-O bond length of 2.11 angstrom, similar to that of FeO bulk, and total magnetic moment of 3.94 mu(Bohr)/Fe atom. Consistent with experimental results, the FeO covered nanotube is a semi-half-metal which can become a half-metal if a small change in the Fermi level is induced. Such a structure may be important in the spintronics context.
We performed an ab initio study of molecular-doped periodic assemblies of ligand-stabilized Au nanoparticles. We found that the most stable dopant positions are near the nanoparticle surfaces, away from the center of interstitial positions. The dopants provide an effective screening mechanism, strongly reducing the nanoparticles charging energies. We also found a linear dependence of the Fermi level with dopant concentration, consistent with recent experiments, up to a critical concentration. For larger concentrations, a new regime is predicted. These features are well reproduced by a simple, analytical model for the material.
A new class of boron nitride fullerenes, which are stoichiometric and presents homopolar bonds, is proposed in this work. A combined first-principles/elastic-model approach predicts that stoichiometric fullerenes with more than 1000 atoms which present homopolar bonds are energetically more stable than those without homopolar bonds. The HOMO-LUMO gap of stoichiometric fullerenes with homopolar bonds is 1.7 eV smaller than that of fullerenes without homopolar bonds, which may lead to distinct optical and electrochemical properties. The distribution of B-B and N-N in those new fullerenes gives rise to an electric dipole moment which could make possible to separate them from apolar fullerenes. (C) 2010 Elsevier B.V. All rights reserved.
A first-principles formalism is employed to investigate the effects of size and structure on the electronic and electrochemical properties of Au nanoparticles with diameters between 0.8 and 2.0 nm. We find that the behavior of the ionization potentials (IPs) and the electron affinities (EAs) as a function of cluster size can be separated into many-body and single-electron contributions. The many-body part is only (and continuously) dependent on particle size, and can be very well described in terms of the capacitance of classical spherical conductors for clusters with more the 55 atoms. For smaller clusters, molecule-like features lead the capacitance and fundamental gap to differ systematically from those of a classical conductor with decreasing size. The single-electron part fluctuates with particle structure. Upon calculating the neutral chemical potential mu(0) = (IP + EA)/2, the many-body contributions cancel out, resulting in fluctuations of mu(0) around the bulk Au work function, consistent with experimental results. The values of IP and EA changes upon functionalization with thiolated molecules, and the magnitude of the observed changes does not depend on the length of the alkane chain. The functionalization can also lead to a transition from metallic to non-metallic behavior in small nanoparticles, which is consistent with experimental observations.
We perform first-principles investigations of two-dimensional, triangular lattices of Au-38 nanoparticles deposited on a graphene layer. We find that lattices of thiolate-covered nanoparticles cause electronic structure modifications in graphene such as minigaps, charge transfer, and new Dirac points, but graphene remains metallic. In contrast, for a moderate coverage of nanoparticles (approximate to 0.2 nm(-2)), a lattice of bare (noncovered) Au nanoparticles may induce periodic deformations on the graphene layer leading to the opening of a band gap of a few tens of meV at the Dirac point, in such a way that a properly charged system might become a semiconductor.
We perform first-principles calculations for the interaction of the transition metal atoms Fe, Co, and W, as well as the FeO molecule, with the boron nitride fullerene B36N36. The stable structure of the atom-fullerene complexes may have the dopant atom either at the center of the cage or making covalent bonds with the fullerene wall, with similar total energies. We also find that the FeO molecule has a binding energy with the fullerene 2.5 eV larger than those of the transition metal atoms, and that it produces larger distortions in the cage. The electronic structure changes upon doping with the presence of several gap states. No magnetic moment is induced on the BN cage and, in general, the hybrid structures have the same magnetic moments as the isolated dopants.
We investigate, by means of first-principles calculations, structural and transport properties of junctions made of symmetric dithiolated molecules placed between Au electrodes. As the electrodes are pulled apart, we find that it becomes energetically favorable that Au atoms migrate to positions between the electrode surface and thiol terminations, with junction structures alternating between symmetric and asymmetric. As a result, the calculated IV curves alternate between rectifying and nonrectifying behaviors as the electrodes are pulled apart, which is consistent with recent experimental results.
We make use of first-principles calculations to study the effects of functionalization and compression on the electronic properties of 2D lattices of Au nanoparticles. We consider Au-38 particles capped by methylthiol molecules and possibly functionalized by the dithiolated conjugated molecules benzenedimethanethiol and benzenedicarbothialdehyde. We find that the nonfunctionalized lattices are insulating, with negligible band dispersions even for a compression of 20% of the lattice constant. Distinct behaviors of the dispersion of the lowest conduction band as a function of compression are predicted for functionalized lattices: The band dispersion of the benzenedimethanethiol-functionalized lattice increases considerably with compression, while that of the benzenedicarbothialdehyde-functionalized lattice decreases.
We perform first-principles calculations to study the relative stability of boron nitride fullerenes B(n)N(m). We consider fullerenes with octahedral, icosahedral and tubular shapes, in a size range with (n + m) between 20 and 288. A scaling analysis allows the total energy comparisons to extend to larger sizes. We find that tubular-shaped fullerenes are as stable as octahedral-shaped ones for (n + m) <= 80. We also find that, in N-rich conditions, icosahedral fullerenes with line defects composed of N-N bonds may be as stable as octahedral fullerenes. (c) 2006 Elsevier B.V. All rights reserved.
We investigate, by means of first-principles calculations, the possible formation of Au-covered carbon fullerenes. Coverages between 32 and 92 Au atoms over C-60 were considered. Among those, the most stable structure is Au92C60, which retains the I-h symmetry of C-60 after geometry optimization. Au92C60 has a small formation energy per Au atom (0.65 eV), which is in fact smaller than those of the Au-38 and Au-55 gold clusters, which were already observed. Au92C60 can be further stabilized by an overlayer of methylthiol molecules, which reduces the formation energy per gold atom to a value smaller than that of the bulk phase of gold.