Modulation of spin and charge currents through functionalized 2D diamond devices.

In this study, we explore the potential of functionalized two-dimensional (2D) diamond for spin-dependent electronic devices using first-principles calculations. Specifically, we investigate functionalizations with either hydroxyl (-OH) or fluorine (-F) groups. In the case of an isolated layer, we observe that the quantity and distribution of (-OH) or (-F) on the 2D diamond surface significantly influence thesp2/sp3ratio of the carbon atoms in the layer. As the coverage is reduced, both the band gap and magnetic moment decrease. When the 2D diamond is placed between gold contacts and functionalized with (-OH), it results in a device with lower resistance compared to the (-F) functionalization. We predict that the maximum current achieved in the device increases with decreasing (-OH) surface coverage, while the opposite behavior occurs for (-F). Additionally, the surface coverage alone can alter the direction of current rectification in (-F) functionalized 2D diamonds. For all studied systems, a single spin component contributes to the total current for certain values of applied bias, indicating a spin filter behavior.

Universal deformation pathways and flexural hardening of nanoscale 2D-material standing folds.

In the present work, we use atomic force microscopy nanomanipulation of 2D-material standing folds to investigate their mechanical deformation. Using graphene, h-BN and talc nanoscale wrinkles as testbeds, universal force-strain pathways are clearly uncovered and well-accounted for by an analytical model. Such universality further enables the investigation of each fold bending stiffness κ as a function of its characteristic height h 0. We observe a more than tenfold increase of κ as h 0 increases in the 10-100 nm range, with power-law behaviors of κ versus h 0 with exponents larger than unity for the three materials. This implies anomalous scaling of the mechanical responses of nano-objects made from these materials.

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.

Dynamic negative compressibility of few-layer graphene, h-BN, and MoS2.

We report a novel mechanical response of few-layer graphene, h-BN, and MoS(2) 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 MoS(2)), but it is absent in single-layer graphene and in few-layer mica and Bi(2)Se(3). 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.

Flow through Deformed Carbon Nanotubes Predicted by Rigid and Flexible Water Models.

In this study, using nonequilibrium molecular dynamics simulation, the flow of water in deformed carbon nanotubes is studied for two water models TIP4P/2005 and simple point charge/FH (SPC/FH). The results demonstrated a nonuniform dependence of the flow on the tube deformation and the flexibility imposed on the water molecules, leading to an unexpected increase in the flow in some cases. The effects of the tube diameter and pressure gradient are investigated to explain the abnormal flow behavior with different degrees of structural deformation.

Controlling the electrical behavior of semiconducting carbon nanotubes via tube contact.

The electromechanical behavior of single-walled carbon nanotubes (SWNTs) in contact with different materials is investigated by scanning probe microscopy. An anomalous diamond/semiconducting nanotube behavior is observed, which is consistent with ab initio calculations: the formation of a broken-gap heterojunction between semiconducting SWNTs and a hydrogenated diamond surface results in a metallic response for such SWNTs.

Doped assemblies of gold nanoparticles: structural and electronic properties.

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.

First-principles investigation of electrochemical properties of gold nanoparticles.

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 micro(0) = (IP+EA)/2, the many-body contributions cancel out, resulting in fluctuations of micro(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.

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.

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.

Compression-Induced Modification of Boron Nitride Layers: A Conductive Two-Dimensional BN Compound.

The ability to create materials with improved properties upon transformation processes applied to conventional materials is the keystone of materials science. Here, hexagonal boron nitride (h-BN), a large-band-gap insulator, is transformed into a conductive two-dimensional (2D) material- bonitrol-that is stable at ambient conditions. The process, which requires compression of at least two h-BN layers and hydroxyl ions, is characterized via scanning probe microscopy experiments and ab initio calculations. This material and its creation mechanism represent an additional strategy for the transformation of known 2D materials into artificial advanced materials with exceptional properties.

Apparent Softening of Wet Graphene Membranes on a Microfluidic Platform.

Graphene is regarded as the toughest two-dimensional material (highest in-plane elastic properties) and, as a consequence, it has been employed/proposed as an ultrathin membrane in a myriad of microfluidic devices. Yet, an experimental investigation of eventual variations on the apparent elastic properties of a suspended graphene membrane in contact with air or water is still missing. In this work, the mechanical response of suspended monolayer graphene membranes on a microfluidic platform is investigated via scanning probe microscopy experiments. A high elastic modulus is measured for the membrane when the platform is filled with air, as expected. However, a significant apparent softening of graphene is observed when water fills the microfluidic system. Through molecular dynamics simulations and a phenomenological model, we associate such softening to a water-induced uncrumpling process of the suspended graphene membrane. This result may bring substantial modifications on the design and operation of microfluidic devices which exploit pressure application on graphene membranes.

Transport Properties of Hydrogenated Cubic Boron Nitride Nanofilms with Gold Electrodes from Density Functional Theory.

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.

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.

Iron oxide doped boron nitride nanotubes: structural and magnetic properties.

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 ~2.1 Å, and binding between O and B atoms, with bond length of 1.55 Å. In case of small FeO concentrations, the total magnetic moment is (4 µ(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 Å, similar to that of FeO bulk, and total magnetic moment of 3.94 µ(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.

Electron states in a lattice of Au nanoparticles: the role of strain and functionalization.

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 Au38 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.