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Search Results: 1 - 10 of 59577 matches for " David Tománek "
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Formation and Stability of Cellular Carbon Foam Structures:An {\em Ab Initio} Study
Zhen Zhu,David Tománek
Physics , 2012, DOI: 10.1103/PhysRevLett.109.135501
Abstract: We use ab initio density functional calculations to study the formation and structural as well as thermal stability of cellular foam-like carbon nanostructures. These systems with a mixed $sp^2/sp^3$ bonding character may be viewed as bundles of carbon nanotubes fused to a rigid contiguous 3D honeycomb structure that can be compressed more easily by reducing the symmetry of the honeycombs. The foam may accommodate the same type of defects as graphene, and its surface may be be stabilized by terminating caps. We postulate that the foam may form under non-equilibrium conditions near grain boundaries of a carbon-saturated metal surface.
Phase coexistence and metal-insulator transition in few-layer phosphorene: A computational study
Jie Guan,Zhen Zhu,David Tománek
Physics , 2014, DOI: 10.1103/PhysRevLett.113.046804
Abstract: Based on {\em ab initio} density functional calculations, we propose $\gamma$-P and $\delta$-P as two additional stable structural phases of layered phosphorus besides the layered $\alpha$-P (black) and $\beta$-P (blue) phosphorus allotropes. Monolayers of some of these allotropes have a wide band gap, whereas others, including $\gamma$-P, show a metal-insulator transition caused by in-layer strain or changing the number of layers. An unforeseen benefit is the possibility to connect different structural phases at no energy cost. This becomes particularly valuable in assembling heterostructures with well-defined metallic and semiconducting regions in one contiguous layer.
Designing electrical contacts to MoS$_2$ monolayers: A computational study
Igor Popov,Gotthard Seifert,David Tománek
Physics , 2012, DOI: 10.1103/PhysRevLett.108.156802
Abstract: Studying the reason, why single-layer molybdenum disulfide (MoS$_2$) appears to fall short of its promising potential in flexible nanoelectronics, we found that the nature of contacts plays a more important role than the semiconductor itself. In order to understand the nature of MoS$_2$/metal contacts, we performed ab initio density functional theory calculations for the geometry, bonding and electronic structure of the contact region. We found that the most common contact metal (Au) is rather inefficient for electron injection into single-layer MoS$_2$ and propose Ti as a representative example of suitable alternative electrode materials.
High stability of faceted nanotubes and fullerenes of multi-phase layered phosphorus: A computational study
Jie Guan,Zhen Zhu,David Tománek
Physics , 2014, DOI: 10.1103/PhysRevLett.113.226801
Abstract: We present a paradigm in constructing very stable, faceted nanotube and fullerene structures by laterally joining nanoribbons or patches of different planar phosphorene phases. Our ab initio density functional calculations indicate that these phases may form very stable, non-planar joints. Unlike fullerenes and nanotubes obtained by deforming a single-phase planar monolayer at substantial energy penalty, we find faceted fullerenes and nanotubes to be nearly as stable as the planar single-phase monolayers. The resulting rich variety of polymorphs allows to tune the electronic properties of phosphorene nanotubes (PNTs) and fullerenes not only by the chiral index, but also by the combination of different phosphorene phases. In selected PNTs, a metal-insulator transition may be induced by strain or changing the number of walls.
Tiling Phosphorene
Jie Guan,Zhen Zhu,David Tománek
Physics , 2014,
Abstract: We present a scheme to categorize the structure of different layered phosphorene allotropes by mapping their non-planar atomic structure onto a two-color 2D triangular tiling pattern. In the buckled structure of a phosphorene monolayer, we assign atoms in "top" positions to dark tiles and atoms in "bottom" positions to light tiles. Optimum $sp^3$ bonding is maintained throughout the structure when each triangular tile is surrounded by the same number $N$ of like-colored tiles, with $0{\le}N{\le}2$. Our ab initio density functional calculations indicate that both the relative stability and electronic properties depend primarily on the structural index $N$. The proposed mapping approach may also be applied to phosphorene structures with non-hexagonal rings and 2D quasicrystals with no translational symmetry, which we predict to be nearly as stable as the hexagonal network.
Computational study of the thermal conductivity in defective carbon nanostructures
Zacharias G. Fthenakis,David Tománek
Physics , 2012, DOI: 10.1103/PhysRevB.86.125418
Abstract: We use non-equilibrium molecular dynamics simulations to study the adverse role of defects including isotopic impurities on the thermal conductivity of carbon nanotubes, graphene and graphene nanoribbons. We find that even in structurally perfect nanotubes and graphene, isotopic impurities reduce thermal conductivity by up to one half by decreasing the phonon mean free path. An even larger thermal conductivity reduction, with the same physical origin, occurs in presence of structural defects including vacancies and edges in narrow graphene nanoribbons. Our calculations reconcile results of former studies, which differed by up to an order of magnitude, by identifying limitations of various computational approaches.
Topologically protected conduction state at carbon foam surfaces:An ab-initio study
Zhen Zhu,Zacharias G. Fthenakis,Jie Guan,David Tománek
Physics , 2014, DOI: 10.1103/PhysRevLett.112.026803
Abstract: We report results of \textit{ab initio} electronic structure and quantum conductance calculations indicating the emergence of conduction at the surface of semiconducting carbon foams. Occurrence of new conduction states is intimately linked to the topology of the surface and not limited to foams of elemental carbon. Our interpretation based on rehybridization theory indicates that conduction in the foam derives from first- and second-neighbor interactions between $p_\|$ orbitals lying in the surface plane, which are related to $p_\perp$ orbitals of graphene. The topologically protected conducting state occurs on bare and hydrogen-terminated foam surfaces and is thus unrelated to dangling bonds. Our results for carbon foam indicate that the conductance behavior may be further significantly modified by surface patterning.
Direct observation of optically induced transient structures in graphite using ultrafast electron crystallography
Ramani K. Raman,Yoshie Murooka,Chong-Yu Ruan,Teng Yang,Savas Berber,David Tománek
Physics , 2008, DOI: 10.1103/PhysRevLett.101.077401
Abstract: We use ultrafast electron crystallography to study structural changes induced in graphite by a femtosecond laser pulse. At moderate fluences of ~< 21mJ/cm^2, lattice vibrations are observed to thermalize on a time scale of ~8ps. At higher fluences approaching the damage threshold, lattice vibration amplitudes saturate. Following a marked initial contraction, graphite is driven nonthermally into a transient state with sp^3-like character, forming interlayer bonds. Using ab initio density functional calculations, we trace the governing mechanism back to electronic structure changes following the photo-excitation.
Formation and Properties of Selenium Double-Helices inside Double-Wall Carbon Nanotubes: Experiment and Theory
Toshihiko Fujimori,Renato Batista dos Santos,Takuya Hayashi,Morinobu Endo,Katsumi Kaneko,David Tománek
Physics , 2013,
Abstract: We report the production of covalently bonded selenium double-helices within the narrow cavity inside double-wall carbon nanotubes. The double-helix structure, characterized by high-resolution transmission electron microscopy and X-ray diffraction, is completely different from the bulk atomic arrangement and may be considered a new structural phase of Se. Supporting ab initio calculations indicate that the observed encapsulated Se double-helices are radially compressed and have formed from free Se atoms or short chains contained inside carbon nanotubes. The calculated electronic structure of Se double-helices is very different from the bulk system, indicating the possibility to develop a new branch of Se chemistry.
Simulated scanning tunneling microscopy images of few-layer-phosphorus capped by graphene and hexagonal boron nitride monolayers
Pablo Rivero,Cedric M. Horvath,Zhen Zhu,Jie Guan,David Tománek,Salvador Barraza-Lopez
Physics , 2014, DOI: 10.1103/PhysRevB.91.115413
Abstract: Elemental phosphorous is believed to have several stable allotropes that are energetically nearly degenerate, but chemically reactive. To prevent chemical degradation under ambient conditions, these structures may be capped by monolayers of hexagonal boron nitride ({\em h}-BN) or graphene. We perform {\em ab initio} density functional calculations to simulate scanning tunneling microscopy (STM) images of different layered allotropes of phosphorus and study the effect of capping layers on these images. We find that protective monolayers of insulating {\em h}-BN allow to distinguish between the different structural phases of phosphorus underneath, even though the images are filtered through only nitrogen atoms that appear transparent. No such distinction is possible for phosphorus films capped by semimetallic graphene that masks the underlying structure. Our results suggest that the real-space imaging capability of STM is not hindered by selected capping layers that protect phosphorus surfaces.
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