Unlocking the Potential of Nanoribbon-Based SbS/SbSe van-der-Waals Heterostructure for Solar-Energy-Conversion and Optoelectronics Applications.

High-performance nanosized optoelectronic devices based on van der Waals (vdW) heterostructures have significant potential for use in a variety of applications. However, the investigation of nanoribbon-based vdW heterostructures are still mostly unexplored. In this study, based on first-principles calculations, we demonstrate that a SbS/SbSe vdW heterostructure, which is formed by isostructural nanoribbons of stibnite (SbS) and antimonselite (SbSe), possesses a direct band gap with a typical type-II band alignment, which is suitable for optoelectronics and solar energy conversion.

CO adsorption on MgO thin-films: formation and interaction of surface charged defects.

Two-dimensional (2D) materials formed by thin-films of metal oxides that grow on metal supports are commonly used in heterogeneous catalysis and multilayer electronic devices. Despite extensive research on these systems, the effects of charged defects at supported oxides on surface processes are still not clear. In this work, we perform spin-polarized density-functional theory (DFT) calculations to investigate formation and interaction of charged magnesium and oxygen vacancies, and Al dopants on MgO(001)/Ag(001) surface.

Improved Performance of Organic Light-Emitting Transistors Enabled by Polyurethane Gate Dielectric.

Organic light-emitting transistors (OLETs) are multifunctional optoelectronic devices that combine in a single structure the advantages of organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs). However, low charge mobility and high threshold voltage are critical hurdles to practical OLET implementation. This work reports on the improvements obtained by using polyurethane films as a dielectric layer material in place of the standard poly(methyl methacrylate) (PMMA) in OLET devices.

Stability and Rupture of an Ultrathin Ionic Wire.

Using a combination of in situ high-resolution transmission electron microscopy and density functional theory, we report the formation and rupture of ZrO_{2} atomic ionic wires. Near rupture, under tensile stress, the system favors the spontaneous formation of oxygen vacancies, a critical step in the formation of the monatomic bridge. In this length scale, vacancies provide ductilelike behavior, an unexpected mechanical behavior for ionic systems.

Structural Metastability and Fermi Surface Topology of SrAlSi.

SrAlSi crystallizes into either a semimetallic, CaAlSi-type, α phase or a superconducting, BaZnP-type, β phase. We explore possible transformations by employing pressure- and temperature-dependent free-energy calculations, vibrational spectral calculations, and room-temperature synchrotron powder X-ray diffraction (PXRD) measurements up to 14 GPa using a diamond anvil cell. Our theoretical and empirical analyses together with all reported baric and thermal events on both phases allow us to construct a preliminary - diagram of transformations.

Reorganization Energy upon Controlled Intermolecular Charge-Transfer Reactions in Monolithically Integrated Nanodevices.

Intermolecular electron-transfer reactions are key processes in physics, chemistry, and biology. The electron-transfer rates depend primarily on the system reorganization energy, that is, the energetic cost to rearrange each reactant and its surrounding environment when a charge is transferred. Despite the evident impact of electron-transfer reactions on charge-carrier hopping, well-controlled electronic transport measurements using monolithically integrated electrochemical devices have not successfully measured the reorganization energies to this date.

Franckeite as an Exfoliable Naturally Occurring Topological Insulator.

Franckeite is a natural superlattice composed of two alternating layers of different composition which has shown potential for optoelectronic applications. In part, the interest in franckeite lies in its layered nature which makes it easy to exfoliate into very thin heterostructures. Not surprisingly, its chemical composition and lattice structure are so complex that franckeite has escaped screening protocols and high-throughput searches of materials with nontrivial topological properties.

Oxygen effects on the electronic transport in stanene.

In this work, we study theoretically the structural, electronic and transport properties of oxidized stanene using a combination of density functional theory (DFT), quantum molecular dynamics and the Landauer-Büttiker theory for the ballistic transport. Our results clearly show that oxygen adsorb onto stanene surface in both molecular or atomic forms, thus causing considerable modifications to its electronic structure and transport properties. Nevertheless, our quantum conductance calculations reveal that, in spite of oxidation, stanene still remains a good conductor that might be applied as field effect transistors, gas sensors and other devices.

Structural and magnetic properties of a defective graphene buffer layer grown on SiC(0001): a DFT study.

Understanding the role of defects in the magnetic properties of the graphene buffer layer (BL) grown on substrates should be important to provide hints for manufacturing future graphene-based spintronic devices in a controlled fashion. Herein, density functional theory was applied to assess the structure and magnetic properties of defective BL on 6H-SiC(0001). Particularly, we conducted a thorough study of one and two vacancies and Stone-Wales defects in the BL.

Pressure dependence of room-temperature structural properties of CaAlSi.

We investigated the pressure dependence of the crystal structure of CaAlSiby means ofcalculations and room-temperature synchrotron x-ray powder diffraction.calculations reproduce satisfactorily the experimentally observed pressure-dependent structural evolution up to 3 GPa where the title system remains in the trigonalP3¯m1phase. In the pressure range 3-8 GPa, pressure evolution of the calculated in-plane lattice parameters is steeper than the observed.

Flat bands and gaps in twisted double bilayer graphene.

We present electronic structure calculations of twisted double bilayer graphene (TDBG): a tetralayer graphene structure composed of two AB-stacked graphene bilayers with a relative rotation angle between them. Using first-principles calculations, we find that TDBG is semiconducting with a band gap that depends on the twist angle, that can be tuned by an external electric field. The gap is consistent with TDBG symmetry and its magnitude is related to surface effects, driving electron transfer from outer to inner layers.

Spiro-Carbon: A Metallic Carbon Allotrope Predicted from First Principles Calculations.

A structurally stable microporous metallic carbon allotrope, poly(spiro[2.2]penta-1,4-diyne) or, for short, spiro-carbon, with I4 /amd (D ) symmetry is predicted by first-principles calculations using density functional theory (DFT). The calculations of electronic, vibrational, and structural properties show that spiro-carbon has lower relative energy than other elusive carbon allotropes such as T-Carbon and 1-diamondyne (Y-Carbon).

Layer breathing and shear modes in multilayer graphene: a DFT-vdW study.

In this work, we study structural and vibrational properties of multilayer graphene using density-functional theory with van der Waals (vdW) functionals. Initially, we analyze how different vdW functionals compare by evaluating the lattice parameters, elastic constants and vibrational frequencies of low energy optical modes of graphite. Our results indicate that the vdW-DF1-optB88 functional has the best overall performance on the description of vibrational properties.

Linear magnetoresistivity in layered semimetallic CaAlSi.

According to an earlier Abrikosov model, a positive, nonsaturating, linear magnetoresistivity (LMR) is expected in clean, low-carrier-density metals when measured at very low temperatures and under very high magnetic fields. Recently, a vast class of materials were shown to exhibit extraordinary high LMR but at conditions that deviate sharply from the above-mentioned Abrikosov-type conditions. Such deviations are often considered within either classical Parish-Littlewood scenario of random-conductivity network or within a quantum scenario of small-effective mass or low carriers at tiny pockets neighboring the Fermi surface.

Giant and Tunable Anisotropy of Nanoscale Friction in Graphene.

The nanoscale friction between an atomic force microscopy tip and graphene is investigated using friction force microscopy (FFM). During the tip movement, friction forces are observed to increase and then saturate in a highly anisotropic manner. As a result, the friction forces in graphene are highly dependent on the scanning direction: under some conditions, the energy dissipated along the armchair direction can be 80% higher than along the zigzag direction.

Effects of edge magnetism on the Kohn anomalies of zigzag graphene nanoribbons.

The effects of edge magnetism on the Kohn anomaly (KA) of the G-band phonons of zigzag graphene nanoribbons (ZGNRs) are studied using a combination of the tight-binding and mean-field Hubbard models. We show that the opening of an energy gap, induced by magnetic ordering, significantly changes the KA effects, particularly for narrow ribbons in which the gap is larger than the phonon energy. Therefore, the G-band phonon frequency and lifetime are altered for a magnetically-ordered edge state with respect to an unpolarized edge state.

Electronic and structural properties of vacancies and hydrogen adsorbates on trilayer graphene.

Using ab initio calculations, we study the electronic and structural properties of vacancies and hydrogen adsorbates on trilayer graphene. Those defects are found to share similar low-energy electronic features, since they both remove a p(z) electron from the honeycomb lattice and induce a defect level near the Fermi energy. However, a vacancy also leaves unpaired σ electrons on the lattice, which lead to important structural differences and also contribute to magnetism.

Chemical analysis and molecular models for calcium-oxygen-carbon interactions in black carbon found in fertile Amazonian anthrosoils.

Carbon particles containing mineral matter promote soil fertility, helping it to overcome the rather unfavorable climate conditions of the humid tropics. Intriguing examples are the Amazonian Dark Earths, anthropogenic soils also known as "Terra Preta de Índio'' (TPI), in which chemical recalcitrance and stable carbon with millenary mean residence times have been observed. Recently, the presence of calcium and oxygen within TPI-carbon nanoparticles at the nano- and mesoscale ranges has been demonstrated.

Magnetic response of zigzag nanoribbons under electric fields.

Spin excitations in zigzag graphene nanoribbons are studied when the system is subjected to an electric field in the transversal direction. The magnetic properties and the lifetime of the spin excitations are systematically investigated and compared using a tight-binding electron-electron model treated by a mean-field Hubbard model. The effects of electron-hole asymmetry introduced by next-nearest neighbor hopping are also investigated.

Probing the electronic properties of ternary A MB ( = 1: A = Ca, Sr; M = Rh, Ir and = 3: A = Ca, Sr; M = Rh) phases: observation of superconductivity.

We follow the evolution of the electronic properties of the titled homologous series when as well as the atomic type of A and M are varied where for = 1, A = Ca, Sr and M = Rh, Ir while for = 3, A = Ca, Sr and M = Rh. The crystal structure of = 1 members is known to be CaRhB-type (), while that of = 3 is CaRhB-type (); the latter can be visualized as a stacking of structural fragments from AMB (6/) and AMB. The metallic properties of the = 1 and 3 members are distinctly different: on the one hand, the = 1 members are characterized by a linear coefficient of the electronic specific heat ≈ 3 mJ mol K, a Debye temperature ≈ 300 K, a normal conductivity down to 2 K and a relatively strong linear magnetoresistivity for fields up to 150 kOe.

An explicit formula for optical oscillator strength of excitons in semiconducting single-walled carbon nanotubes: family behavior.

The sensitive structural dependence of the optical properties of single-walled carbon nanotubes, which are dominated by excitons and tunable by changing diameter and chirality, makes them excellent candidates for optical devices. Because of strong many-electron interaction effects, the detailed dependence of the optical oscillator strength f(s) of excitons on nanotube diameter d, chiral angle θ, and electronic subband index P (the so-called family behavior), however, has been unclear. In this study, based on results from an extended Hubbard Hamiltonian with parameters derived from ab initio GW plus Bethe-Salpeter equation (GW-BSE) calculations, we have obtained an explicit formula for the family behavior of the oscillator strengths of excitons in semiconducting single-walled carbon nanotubes (SWCNTs), incorporating environmental screening.

An atlas of carbon nanotube optical transitions.

Electron-electron interactions are significantly enhanced in one-dimensional systems, and single-walled carbon nanotubes provide a unique opportunity for studying such interactions and the related many-body effects in one dimension. However, single-walled nanotubes can have a wide range of diameters and hundreds of different structures, each defined by its chiral index (n,m), where n and m are integers that can have values from zero up to 30 or more. Moreover, one-third of these structures are metals and two-thirds are semiconductors, and they display optical resonances at many different frequencies.

Quantifying defects in graphene via Raman spectroscopy at different excitation energies.

We present a Raman study of Ar(+)-bombarded graphene samples with increasing ion doses. This allows us to have a controlled, increasing, amount of defects. We find that the ratio between the D and G peak intensities, for a given defect density, strongly depends on the laser excitation energy.