Monolayer honeycomb structures of group IV elements and III-V binary compounds

Using first-principles plane wave calculations, we investigate two dimensional honeycomb structure of Group IV elements and their binary compounds, as well as the compounds of Group III-V elements. Based on structure optimization and phonon mode calculations, we determine that 22 different honeycomb materials are stable and correspond to local minima on the Born-Oppenheimer surface. We also find that all the binary compounds containing one of the first row elements, B, C or N have planar stable structures. On the other hand, in the honeycomb structures of Si, Ge and other binary compounds the alternating atoms of hexagons are buckled, since the stability is maintained by puckering. For those honeycomb materials which were found stable, we calculated optimized structures, cohesive energies, phonon modes, electronic band structures, effective cation and anion charges, and some elastic constants. The band gaps calculated within Density Functional Theory using Local Density Approximation are corrected by GW0 method. Si and Ge in honeycomb structure are semimetal and have linear band crossing at the Fermi level which attributes massless Fermion character to charge carriers as in graphene. However, all binary compounds are found to be semiconductor with band gaps depending on the constituent atoms. We present a method to reveal elastic constants of 2D honeycomb structures from the strain energy and calculate the Poisson's ratio as well as in-plane stiffness values. Preliminary results show that the nearly lattice matched heterostructures of ...Comment: 12 Pages, 7 Figures, 1 Table; http://link.aps.org/doi/10.1103/PhysRevB.80.15545

A First-Principles Study of Zinc Oxide Honeycomb Structures

We present a first-principles study of the atomic, electronic, and magnetic properties of two-dimensional (2D), single and bilayer ZnO in honeycomb structure and its armchair and zigzag nanoribbons. In order to reveal the dimensionality effects, our study includes also bulk ZnO in wurtzite, zincblende, and hexagonal structures. The stability of 2D ZnO, its nanoribbons and flakes are analyzed by phonon frequency, as well as by finite temperature ab initio molecular-dynamics calculations. 2D ZnO in honeycomb structure and its armchair nanoribbons are nonmagnetic semiconductors but acquire net magnetic moment upon the creation of zinc-vacancy defect. Zigzag ZnO nanoribbons are ferromagnetic metals with spins localized at the oxygen atoms at the edges and have high spin polarization at the Fermi level. However, they change to nonmagnetic metal upon termination of their edges with hydrogen atoms. From the phonon calculations, the fourth acoustical mode specified as twisting mode is also revealed for armchair nanoribbon. Under tensile stress the nanoribbons are deformed elastically maintaining honeycomblike structure but yield at high strains. Beyond yielding point honeycomblike structure undergo a structural change and deform plastically by forming large polygons. The variation in the electronic and magnetic properties of these nanoribbons have been examined under strain. It appears that plastically deformed nanoribbons may offer a new class of materials with diverse properties.Comment: http://prb.aps.org/abstract/PRB/v80/i23/e23511

Dielectric properties measurements of brown and white adipose tissue in rats from 0.5 to 10 GHz

Brown adipose tissue (BAT) plays an important role in whole body metabolism and with appropriate stimulus could potentially mediate weight gain and insulin sensitivity. Although imaging techniques are available to detect subsurface BAT, there are currently no viable methods for continuous acquisition of BAT energy expenditure. Microwave (MW) radiometry is an emerging technology that allows the quantification of tissue temperature variations at depths of several centimeters. Such temperature differentials may be correlated with variations in metabolic rate, thus providing a quantitative approach to monitor BAT metabolism. In order to optimize MW radiometry, numerical and experimental phantoms with accurate dielectric properties are required to develop and calibrate radiometric sensors. Thus, we present for the first time, the characterization of relative permittivity and electrical conductivity of brown (BAT) and white (WAT) adipose tissues in rats across the MW range 0.5-10GHz. Measurements were carried out in situ and post mortem in six female rats of approximately 200g. A Cole-Cole model was used to fit the experimental data into a parametric model that describes the variation of dielectric properties as a function of frequency. Measurements confirm that the dielectric properties of BAT (εr = 14.0-19.4, σ = 0.3-3.3S/m) are significantly higher than those of WAT (εr = 9.1-11.9, σ = 0.1-1.9S/m), in accordance with the higher water content of BAT

Adiponectin and Cardiac Hypertrophy in Acromegaly

Background. Adiponectin is an adipocytes-derived hormone which has been shown to possess insulin-sensitizing, antiatherogenic, and anti-inflammatory properties. In acromegaly, the data on adiponectin is contradictory. The relationship between adiponectin levels and cardiac parameters has not been studied.Objectives. The aim of this study was to find out how adiponectin levels were affected in acromegalic patients and the relationship between adiponectin levels and cardiac parameters.Material and Methods. We included 30 subjects (15 male, 15 female), diagnosed with acromegaly and 30 healthy (10 male, 20 female) subjects. Serum glucose, insulin, GH, IGF-1 and adiponectin levels were obtained and the insulin resistance of the subjects was calculated. Echocardiographic studies of the subjects were performed.Results. We determined that adiponectin levels were significantly higher in the acromegalic group than the control group. In the acromegalic group, there was no statistically significant relation between serum adiponectin and growth hormone (GH), or insulin-like growth factor-1 (IGF-1) levels (p = 0.3, p = 0.1). We demonstrated that cardiac function and structure are affected by acromegaly. IVST, PWT, LVMI, E/A ratio, DT, ET, IVRT, VPR, and LVESV values were increased and the results were statistically significant. In the acromegalic group, adiponectin levels were positively related with left ventricle mass index (LVMI) but this correlation was found to be statistically weak (p = 0.03). In our study, there was a positive correlation between VAI and LVM. We also could not find any correlation between VAI and adiponectin levels.Conclusions. Although insulin resistance and high insulin levels occur in active acromegaly patients, adiponectin levels were higher in our study as a consequence of GH lowering therapies. Our study showed that adiponectin levels may be an indicator of the cardiac involvement acromegaly. However, the usage of serum adiponectin levels in acromegalic patients as an indicator of cardiac involvement should be supported with other, wide, multi-centered studies

First-principles study of two- and one-dimensional honeycomb structures of boron nitride

This paper presents a systematic study of two- and one-dimensional honeycomb structures of boron nitride (BN) using first-principles plane-wave method. In order to reveal dimensionality effects, a brief study of all allotropic forms of three-dimensional (3D) BN crystals and truly one-dimensional atomic BN chains are also included. Two-dimensional (2D) graphenelike BN is a wide band-gap semiconductor with ionic bonding through significant charge transfer from B to N. Phonon-dispersion curves demonstrate the stability of 2D BN flakes. Quasi-one-dimensional (1D) armchair BN nanoribbons are nonmagnetic semiconductors with edge states. Upon passivation of B and N with hydrogen atoms these edge states disappear and the band gap increases. Bare zigzag BN nanoribbons are metallic but become a ferromagnetic semiconductor when both their edges are passivated with hydrogen. However, their magnetic ground state, electronic band structure, and band gap are found to be strongly dependent on whether B or N edge of the ribbon is saturated with hydrogen. Vacancy defects in armchair and zigzag nanoribbons affect also the magnetic state and electronic structure. Harmonic, anharmonic, and plastic regions are deduced in the variation in the total energy of armchair and zigzag nanoribbons as a function of strain. The calculated force constants display a Hookian behavior. In the plastic region the nanoribbon is stretched, whereby the honeycomb structure of hexagons change into different polygons through sequential structural transformations. In order to reveal dimensionality effects these properties are contrasted with those of various 3D BN crystals and 1D BN atomic chain. © 2009 The American Physical Society

First-principles approach to monitoring the band gap and magnetic state of a graphene nanoribbon via its vacancies

Using first-principles plane-wave calculations we predict that electronic and magnetic properties of graphene nanoribbons can be modified by the defect-induced itinerant states. Structure optimization gives rise to significant reconstruction of atomic structure, which is in good agreement with transmission electron microscope images. The band gaps of armchair nanoribbons can be modified by hydrogen-saturated holes. The band-gap changes depend on the width of the ribbon as well as on the position of the hole relative to the edges of the ribbon. Defects due to periodically repeating vacancy or divacancies induce metallization as well as magnetization in nonmagnetic semiconducting nanoribbons due to the spin polarization of local defect states. Antiferromagnetic ground state of semiconducting zigzag ribbons can change to ferrimagnetic state upon creation of vacancy defects, which reconstruct and interact with edge states. Even more remarkable is that all these effects of vacancy defects are found to depend on their geometry and position relative to the edges. It is shown that these effects can, in fact, be realized without really creating defects. © 2008 The American Physical Society

First-principles study of defects and adatoms in silicon carbide honeycomb structures

We present a study of mechanical, electronic and magnetic properties of two-dimensional (2D), monolayer of silicon carbide (SiC) in honeycomb structure and its quasi-one-dimensional (quasi-1D) armchair nanoribbons using first-principles plane-wave method. In order to reveal dimensionality effects, a brief study of three-dimensional (3D) bulk and 1D atomic chain of SiC are also included. Calculated bond-lengths, cohesive energies, charge transfers and band gaps display a clear dimensionality effect. The stability analysis based on the calculation of phonon frequencies indicates that 2D SiC monolayer is stable in planar geometry. We found that 2D SiC monolayer in honeycomb structure and its bare and hydrogen passivated nanoribbons are ionic, nonmagnetic, wide band gap semiconductors. The band gap is further increased upon self-energy corrections. The mechanical properties are investigated using the strain energy calculations. The effect of various vacancy defects, adatoms, and substitutional impurities on electronic and magnetic properties in 2D SiC monolayer and in its armchair nanoribbons is also investigated. Some of these vacancy defects and impurities, which are found to influence physical properties and attain magnetic moments, can be used to functionalize SiC honeycomb structures. © 2010 The American Physical Society

Electronic and magnetic properties of 3d transition-metal atom adsorbed graphene and graphene nanoribbons

In this paper, we theoretically studied the electronic and magnetic properties of graphene and graphene nanoribbons functionalized by 3d transition-metal (TM) atoms. The binding energies and electronic and magnetic properties were investigated for the cases where TM atoms adsorbed to a single side and double sides of graphene. We found that 3d TM atoms can be adsorbed on graphene with binding energies ranging between 0.10 and 1.95 eV depending on their species and coverage density. Upon TM atom adsorption, graphene becomes a magnetic metal. TM atoms can also be adsorbed on graphene nanoribbons with armchair edge shapes (AGNR's). Binding of TM atoms to the edge hexagons of AGNR yields the minimum energy state for all TM atom species examined in this work and in all ribbon widths under consideration. Depending on the ribbon width and adsorbed TM atom species, AGNR, which is a nonmagnetic semiconductor, can either be a metal or a semiconductor with ferromagnetic or antiferromagnetic spin alignment. Interestingly, Fe or Ti adsorption makes certain AGNR's half-metallic with a 100% spin polarization at the Fermi level. Present results indicate that the properties of graphene and graphene nanoribbons can be strongly modified through the adsorption of 3d TM atoms. © 2008 The American Physical Society

A comparative study of lattice dynamics of three- and two-dimensional MoS2

This paper presents a comparative study of the lattice dynamics of three-dimensional layered MoS2 and two-dimensional single layer MoS2 based on the density functional theory. A comprehensive analysis of energetics and optimized structure parameters is performed using different methods. It is found that the van der Waals attraction between layers of three-dimensional (3D) layered MoS2 is weak but is essential to hold the layers together with the equilibrium interlayer spacing. Cohesive energy, phonon dispersion curves, and corresponding density of states and related properties, such as Born-effective charges, dielectric constants, Raman and infrared active modes are calculated for 3D layered as well as 2D single layer MoS2 using their optimized structures. These calculated values are compared with the experimental data to reveal interesting dimensionality effects. The absence of a weak interlayer interaction in 2D single layer MoS2 results in the softening of some of Raman active modes. © 2011 American Chemical Society