Interactions of selected organic molecules with a blue phosphorene monolayer: self-assembly, solvent effect, enhanced binding and fixation through coadsorbed gold clusters.

In this paper we investigate the interaction between a pristine blue phosphorene monolayer and selected organic molecules like amino acids and nucleic acid bases. These molecules are bound to the substrate by a weak van der Waals interaction leading to their physisorption. When isolated, they tend to orient themselves parallel to the surface and are located in flat minima with very low libration frequencies; thus the electronic structures of the substrate and physisorbed molecules are not affected except for relative shifts. Even though the regular self-assembly of these molecules on the pristine blue phosphorene cannot be realized under this weak interaction, only their irregular coating of the substrate can occur due to increased intermolecular coupling. In a solvent like water, the weak binding energy is further decreased. Gold adatoms and gold clusters can form strong chemical bonds with pristine blue phosphorene and modify its electronic and magnetic state depending on the coverage. While full coverage of a blue phosphorene monolayer by gold adatoms leads to instabilities followed by clustering, relatively lower coverage can attribute very interesting magnetic and electronic states, like a spin gapless semiconductor. When bound to the gold clusters already adsorbed on the blue phosphorene monolayer, amino acid and nucleic acid base molecules form relatively strong chemical bonds and hence can be fixed to the surface; they are reoriented to gain self-assembly character and the whole system acquires new functionalities.

Computed tomography depiction of normal inguinal lymph nodes in children.

BACKGROUND: The aim of the study was to establish computed tomography (CT) characteristics, distribution and provide normative data about size of normal inguinal lymph nodes in a paediatric population. MATERIALS AND METHODS: Four hundred eighty-one otherwise healthy children (147 girls, mean age: 8.87, range 0-17 years) underwent pelvic CT in the setting of high-energy trauma were included in the study. Both axial and coronal 1.25-mm reconstructions were evaluated for the presence, location (deep or superficial), number, presence of fat attenuation, and shape of the lymph nodes, short-axis diameter of the biggest lymph node for each of right and left inguinal regions. RESULTS: A total of 7556 lymph nodes were detected in 481 subjects (the mean count of superficial and deep inguinal lymph nodes was 13.35 [range 6-23] and 2.36 [range 0-7] per subject, respectively): 15% (1135/7556) deep located, 85% (6421/7556) superficially located, 86.6% (6547/7556) with fat attenuation, 99.2% (7496/7556) oval in shape, 0.8% (60/7556) spherical. The short-axis diameter of the lymph nodes increased with age. Pearson's correlation coefficient for superficial and deep lymph nodes in boys and girls, respectively: 0.538 (p < 0.001), 0.504 (p < 0.001), 0.452 (p < 0.001) and 0.268 (p < 0.001). The mean maximum short-axis diameters in different age groups and gender varied between 6.33 ± 0.85 mm and 8.68 ± 1.33 mm for superficial, 3.62 ± 1.16 mm and 5.83 ± 1.05 mm for deep inguinal lymph nodes. CONCLUSIONS: Inguinal lymph nodes were multiple, commonly contained fat, and were oval in shape. The data determined about inguinal lymph node size in different paediatric age groups may be applicable as normative data in daily clinical CT evaluation practice.

Stable, one-dimensional suspended and supported monatomic chains of pnictogens: a metal-insulator framework.

Group-VA elements P, As, Sb, and Bi can construct free-standing, stable zigzag monatomic chain structures, which show unusual properties. They are normally semimetals with bands crossing at the Fermi level, but a very narrow gap opens due to spin-orbit coupling. They attain one quantum of conductance under a small bias potential; Bi, being an exception, attains two quanta of conductance. Finite size chains are magnetic semiconductors; their magnetic moments and the order of spin states show an even-odd disparity depending on the number of chain atoms. Variations of the HOMO-LUMO band gaps depending on the spin polarization and the size of the finite chains offer critical tunability. In the periodic, zigzag compound chains, a small band gap opens at the Fermi level. The mysterious zigzag geometry, cohesion, stability and band order of all these chains are well-explained by a simple bond model. When placed on the parent or other monolayers like graphene, h-BN and GaSe, these chains become weakly bound and construct a 1D metallic channel. The artificial grids or networks of these metallic chains on the insulating substrates can constitute metal-insulator frameworks of desired geometry. The zigzag phosphorene chain, having the highest stability, remains stable even at full coverage of adsorbates like H and OH, whereas other chains dissociate. While P-chains can be synthesized on GaSe and graphene substrates, phosphorene nanoribbons can transform into suspended chains under excessive tensile strain. Additionally, we showed that As, Sb, and Bi zigzag chains are weakly bound to their parent monolayers and remain stable. Nitrogen monatomic chains, on the other hand, are prone to instability. The diverse properties unveiled in this study based on the density functional method offer tunability through electric fields and strain.

Structure dependent optoelectronic properties of monolayer antimonene, bismuthene and their binary compound.

Two-dimensional (2D) antimonene, bismuthene, and their binary compound 2D BiSb possess high spin-orbit coupling (SOC) and potential topological insulator properties upon engineering their structural and chemical properties. Based on many-body first-principles calculations, we show that these materials can exhibit isotropic or anisotropic optoelectronic properties depending on their geometry, i.e. buckled (hb) or asymmetrical washboard (aw) phases. SOC significantly alters their optoelectronic properties, which is predominantly evident in 2D bismuthene. hb-antimonene absorbs light in the visible and partially in the ultraviolet regimes, while the absorption band edge for aw-antimonene, hb- and aw-bismuthene is set at the infrared region, absorption being spread as a broadband optical response through the spectral range. An exciton binding with 0.18 eV energy is detected for hb-bismuthene. Due to their broadband optical response, antimonene, bismuthene, and their binary compound offer possibilities towards applications as 2D materials in solar cells, light-emitting devices, photodetectors and light modulation.

Onset of vertical bonds in new GaN multilayers: beyond van der Waals solids.

A suspended single layer of GaN in a honeycomb structure is stable in a planar geometry. By stacking these GaN layers one can construct bilayers or multilayers, even new three-dimensional (3D) periodic structures. In this study, we clarified how the planar layers are buckled with the onset of vertical Ga-N bonds. Among the four stable phases of bilayer GaN, only one of them maintains the planar geometry, which is bound by weak van der Waals interactions. For the remaining three phases, the layers are buckled with the onset of weak vertical bonds, and attain total energies slightly lower than that of the planar geometry. Structural phase changes, as well as direct-indirect band transitions take place under strain and electrostatic charging. The vertical bonds become shorter in multilayers, and eventually attain the bulk value. Among the stable phases of 3D periodic GaN, only one with a graphite-like structure behaves as a layered, van der Waals solid; whereby others are 3D uniform crystals beyond the van der Waals solid.

Diagnosis of intracranial calcification and hemorrhage in pediatric patients: Comparison of quantitative susceptibility mapping and phase images of susceptibility-weighted imaging.

PURPOSE: To prospectively compare the diagnostic capabilities of quantitative susceptibility mapping (QSM) with those of phase images of susceptibility-weighted imaging (SWI) in the detection and differentiation of intracranial calcification and hemorrhage in pediatric patients. METHOD: Sixteen pediatric patients (9 girls, 7 boys) with a mean age of 9.4±6.3 (SD) years (range, 6 days-15 years) were included. Fifty-nine calcifications and 31 hemorrhages were detected. Sensitivities and specificities of the two magnetic resonance (MR) imaging techniques were calculated and compared using McNemar test. RESULTS: QSM had a sensitivity of 84.7% and specificity of 100% for the detection of calcification. SWI phase images had a sensitivity of 49.1% and specificity of 100%. For the detection of hemorrhage, QSM had a sensitivity of 90.3% and a specificity of 98.3% whereas SWI phase images yielded a sensitivity of 64.5% and specificity of 96.6%. Overall, QSM displayed significantly better sensitivity than SWI phase images in identification of calcification and hemorrhage (P<0.05). CONCLUSION: QSM is more reliable than SWI phase images in the identification of intracranial calcification and hemorrhage in pediatric patients using MR imaging.

Stable single-layer honeycomblike structure of silica.

Silica or SiO(2), the main constituent of Earth's rocks has several 3D complex crystalline and amorphous phases, but it does not have a graphitelike layered structure in 3D. Our theoretical analysis and numerical calculations from the first principles predict a single-layer honeycomblike allotrope, hα silica, which can be viewed to be derived from the oxidation of silicene and it has intriguing atomic structure with reentrant bond angles in hexagons. It is a wide band gap semiconductor, which attains remarkable electromechanical properties showing geometrical changes under an external electric field. In particular, it is an auxetic metamaterial with a negative Poisson's ratio and has a high piezoelectric coefficient. While it can form stable bilayer and multilayer structures, its nanoribbons can show metallic or semiconducting behavior depending on their chirality. Coverage of dangling Si orbitals by foreign adatoms can attribute new functionalities to hα silica. In particular, Si(2)O(5), where Si atoms are saturated by oxygen atoms from top and bottom sides alternatingly can undergo a structural transformation to make silicatene, another stable, single layer structure of silica.

Enhanced reduction of graphene oxide by means of charging and electric fields applied to hydroxyl groups.

We present a first-principles study of the effects of charging and perpendicular electric fields on hydroxyl groups, both of which mediate the reduction of graphene oxide through the formation of H2O and H2O2. Starting with an investigation of the interaction between the hydroxyl groups and graphene, we determine the equilibrium binding geometry, binding energy, and the diffusion path with a minimum energy barrier and show that those equilibrium properties are strongly affected by external agents. While co-adsorbed H and O form bound OH, co-adsorbed H and OH in close proximity form H2O with almost no energy barrier. When negatively charged or subjected to a perpendicular electric field, the energy barrier between two OH co-adsorbed in close proximity is weakened or totally suppressed, forming an oxygen atom strongly bound at the bridge site, together with a water molecule. The water molecule by itself is very weakly bound to graphene and is prone to desorb from the surface, leading to the reduction of graphene oxide. It is therefore demonstrated that the reduction of graphene oxide is promoted to a large extent by negative charging or an applied perpendicular electric field, through the formation of weakly bound water molecules from hydroxyl groups.

Effects of charging and perpendicular electric field on the properties of silicene and germanene.

Using first-principles density functional theory calculations, we showed that electronic and magnetic properties of bare and Ti adatom adsorbed single-layer silicene and germanene, which are charged or subjected to a perpendicular electric field, can be modified to attain new functionalities. In particular, when subjected to a perpendicular electric field, buckled atoms have the symmetry between their planes broken, opening a gap at the Dirac points. The occupation of 3d orbitals of the adsorbed Ti atom changes with charging or applied electric field, inducing significant changes in magnetic moment. We predict neutral silicene uniformly covered by Ti atoms to become a half-metal at a specific value of coverage and hence allow the transport of electrons in one spin direction, but block the opposite direction. These calculated properties, however, exhibit a dependence on the size of the vacuum spacing between periodically repeating silicene and germanene layers, if they are treated using a plane wave basis set within periodic boundary conditions. We clarified the cause of this spurious dependence and show that it can be eliminated by the use of a local orbital basis set.

Effects of charging and electric field on graphene functionalized with titanium.

Titanium atoms are adsorbed to graphene with a significant binding energy and render diverse functionalities to it. Carrying out first-principles calculations, we investigated the effects of charging and static electric field on the physical and chemical properties of graphene covered by Ti adatoms. When uniformly Ti covered graphene is charged positively, its antiferromagnetic ground state changes to ferromagnetic metal and attains a permanent magnetic moment. Static electric field applied perpendicularly causes charge transfer between Ti and graphene, and can induce metal-insulator transition. While each Ti adatom adsorbed to graphene atom can hold four hydrogen molecules with a weak binding, these molecules can be released by charging or applying electric field perpendicularly. Hence, it is demonstrated that charging and applied static electric field induce quasi-continuous and side specific modifications in the charge distribution and potential energy of adatoms absorbed to single-layer nanostructures, resulting in fundamentally crucial effects on their physical and chemical properties.

Frictional figures of merit for single layered nanostructures.

We determine the frictional figures of merit for a pair of layered honeycomb nanostructures, such as graphane, fluorographene, MoS2 and WO2 moving over each other, by carrying out ab initio calculations of interlayer interaction under constant loading force. Using the Prandtl-Tomlinson model we derive the critical stiffness required to avoid stick-slip behavior. We show that these layered structures have low critical stiffness even under high loading forces due to their charged surfaces repelling each other. The intrinsic stiffness of these materials exceeds critical stiffness and thereby the materials avoid the stick-slip regime and attain nearly dissipationless continuous sliding. Remarkably, tungsten dioxide displays a much better performance relative to others and heralds a potential superlubricant. The absence of mechanical instabilities leading to conservative lateral forces is also confirmed directly by the simulations of sliding layers.

Two- and one-dimensional honeycomb structures of silicon and germanium.

First-principles calculations of structure optimization, phonon modes, and finite temperature molecular dynamics predict that silicon and germanium can have stable, two-dimensional, low-buckled, honeycomb structures. Similar to graphene, these puckered structures are ambipolar and their charge carriers can behave like a massless Dirac fermion due to their pi and pi(*) bands which are crossed linearly at the Fermi level. In addition to these fundamental properties, bare and hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and magnetic properties, which are size and orientation dependent. These properties offer interesting alternatives for the engineering of diverse nanodevices.

Half-metallic silicon nanowires: first-principles calculations.

From first-principles calculations, we predict that specific transition metal (TM) atom-adsorbed silicon nanowires have a half-metallic ground state. They are insulators for one spin direction, but show metallic properties for the opposite spin direction. At high coverage of TM atoms, ferromagnetic silicon nanowires become metallic for both spin directions with high magnetic moment and may have also significant spin polarization at the Fermi level. The spin-dependent electronic properties can be engineered by changing the type of adsorbed TM atoms, as well as the diameter of the nanowire. Present results are not only of scientific interest, but also can initiate new research on spintronic applications of silicon nanowires.

Transition-metal-ethylene complexes as high-capacity hydrogen-storage media.

From first-principles calculations, we predict that a single ethylene molecule can form a stable complex with two transition metals (TM) such as Ti. The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules, reaching to gravimetric storage capacity of approximately 14 wt %. Dimerization, polymerizations, and incorporation of the TM-ethylene complexes in nanoporous carbon materials are also discussed. Our results are quite remarkable and open a new approach to high-capacity hydrogen-storage materials discovery.

Size-dependent alternation of magnetoresistive properties in atomic chains.

Spin-polarized electronic and transport properties of carbon atomic chains are investigated when they are capped with magnetic transition-metal (TM) atoms like Cr or Co. The magnetic ground state of the TM-C(n)-TM chains alternates between the ferromagnetic (F) and antiferromagnetic (AF) spin configurations as a function of n. In view of the nanoscale spintronic device applications the desirable AF state is obtained for only even-n chains with Cr; conversely only odd-n chains with Co have AF ground states. When connected to appropriate metallic electrodes these atomic chains display a strong spin-valve effect. Analysis of structural, electronic, and magnetic properties of these atomic chains, as well as the indirect exchange coupling of the TM atoms through non-magnetic carbon atoms are presented.

Titanium-decorated carbon nanotubes as a potential high-capacity hydrogen storage medium.

We report a first-principles study, which demonstrates that a single Ti atom coated on a single-walled nanotube (SWNT) binds up to four hydrogen molecules. The first H2 adsorption is dissociative with no energy barrier while the other three adsorptions are molecular with significantly elongated H-H bonds. At high Ti coverage we show that a SWNT can strongly adsorb up to 8 wt % hydrogen. These results advance our fundamental understanding of dissociative adsorption of hydrogen in nanostructures and suggest new routes to better storage and catalyst materials.

Atomic and electronic structure of carbon strings.

This paper presents an extensive study of various string and tubular structures formed by carbon atomic chains. Our study is based on first-principles pseudopotential plane wave and finite-temperature ab initio molecular dynamics calculations. Infinite- and finite-length carbon chains exhibit unusual mechanical and electronic properties such as large cohesive energy, axial strength, high conductance, and overall structural stability even at high temperatures. They are suitable for structural and chemical functionalizations. Owing to their flexibility and reactivity they can form linear chain, ring, helix, two-dimensional rectangular and honeycomb grids, three-dimensional cubic networks, and tubular structures. Metal-semiconductor heterostructures and various quantum structures, such as multiple quantum wells and double-barrier resonant tunnelling structures, can be formed from the junctions of metallic carbon and semiconducting BN linear chains. Analysis of atomic and electronic structures of these periodic, finite, and doped structures reveals fundamentally and technologically interesting features, such as structural instabilities and chiral currents. The double covalent bonding of carbon atoms depicted through self-consistent charge density analysis underlies the chemical, mechanical, and electronic properties.

Chiral single-wall gold nanotubes.

Based on first-principles calculations we show that gold atoms can form both freestanding and tip-suspended chiral single-wall nanotubes composed of helical atomic strands. The freestanding, infinite (5,5) tube is found to be energetically the most favorable. While energetically less favorable, the experimentally observed (5,3) tube stretching between two tips corresponds to a local minimum in the string tension. Similarly, the (4,3) tube is predicted as a favorable structure yet to be observed experimentally. Analysis of band structure, charge density, and quantum ballistic conductance suggests that the current on these wires is less chiral than expected, and there is no direct correlation between the numbers of conduction channels and helical strands.

Ab-initio electron transport calculations of carbon based string structures.

First-principles calculations show that monatomic strings of carbon have high cohesive energy and axial strength, and exhibit stability even at high temperatures. Because of their flexibility and reactivity, carbon chains are suitable for structural and chemical functionalizations; they also form stable ring, helix, grid, and network structures. Analysis of electronic conductance of various infinite, finite, and doped string structures reveal fundamental and technologically interesting features. Changes in doping and geometry give rise to dramatic variations in conductance. In even-numbered linear chains, strain induces a substantial decrease of conductance. The double covalent bonding of carbon atoms underlies their unusual chemical, mechanical, and transport properties.

Tunable adsorption on carbon nanotubes.

We investigated the adsorption of a single atom, hydrogen and aluminum, on single-wall carbon nanotubes from first principles. The adsorption is exothermic, and the associated binding energy varies inversely as the radius of the zigzag tube. We found that the adsorption of a single atom and related properties can be modified continuously and reversibly by the external radial deformation. The binding energy on the high curvature site of the deformed tube increases with increasing radial deformation. The effects of curvature and radial deformation depend on the chirality of the tube.