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27 results on '"Carlsson, Johan M."'

2. Destroyed quantum Hall effect in graphene with [0001] tilt grain boundaries

Author

Bergvall, Anders, Carlsson, Johan M., and Lofwander, Tomas

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

The reason why the half-integer quantum Hall effect (QHE) is suppressed in graphene grown by chemical vapor deposition (CVD) is unclear. We propose that it might be connected to extended defects in the material and present results for the quantum Hall effect in graphene with [0001] tilt grain boundaries connecting opposite sides of Hall bar devices. Such grain boundaries contain 5-7 ring complexes that host defect states that hybridize to form bands with varying degree of metallicity depending on grain boundary defect density. In a magnetic field, edge states on opposite sides of the Hall bar can be connected by the defect states along the grain boundary. This destroys Hall resistance quantization and leads to non-zero longitudinal resistance. Anderson disorder can partly recover quantization, where current instead flows along returning paths along the grain boundary depending on defect density in the grain boundary and on disorder strength. Since grain sizes in graphene made by chemical vapor deposition are usually small, this may help explain why the quantum Hall effect is usually poorly developed in devices made of this material., Comment: 5 pages, 4 figures

Published
2014
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8. Atomistic Modeling of Charge Transport across a Carbon Nanotube–Polyethylene Junction

Author

Penazzi, Gabriele, Carlsson, Johan M., Diedrich, Christian, Olf, Günter, Pecchia, Alessandro, and Frauenheim, Thomas

Abstract

The conduction mechanism in carbon nanotube (CNT) polymer nanocomposites is complex, and there has been a considerable amount of work invested in understanding the role of the distribution of the CNTs in the composite and how it influences the conductivity. However, less interest has been devoted to the electron transport across a single CNT–polymer–CNT junction. We present a first atomistic study of the electron transmission through a CNT–polyethylene–CNT junction. The morphology of the junction is described using classical molecular dynamics simulations, and transport properties are calculated within density functional tight binding method. The electron transmission depends noticeably on the CNT–CNT separation and on the consequent polymer wrapping. At CNT–CNT distances shorter than 6 Å, the polyethylene molecules do not penetrate in the space between the CNTs. In this near contact regime, the electron transmission proceeds via direct tunneling between the two CNTs across a vacuum region without relevant contribution from the surrounding polymer. For distances larger than 6 Å, the PE molecules enter into the junction region. The frontier orbitals of the PE molecules in the junction provide localized states, which can couple to the CNT metallic states. This resonance tail increases the electron transmission probability between the CNTs across the junction by several orders of magnitude, thus lowering the effective barrier. The gradual interpenetration of the polymer is resembled in transmission fluctuations. An averaging of the transmission in energy and time along MD trajectories allows a quantitative estimation of the junction resistance and tunneling barrier.

Published
2024
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25. A metal-free polymeric photocatalyst for hydrogen production from water under visible light.

Author

Xinchen Wang, Maeda, Kazuhiko, Thomas, Arne, Takanabe, Kazuhiro, Gang Xin, Carlsson, Johan M., Domen, Kazunari, and Antonietti, Markus

Subjects
POLYMERS, SOLAR energy, RENEWABLE energy sources, CRYSTALS, ELECTRONIC equipment, PHOTOSYNTHESIS, WATER use
Abstract

The production of hydrogen from water using a catalyst and solar energy is an ideal future energy source, independent of fossil reserves. For an economical use of water and solar energy, catalysts that are sufficiently efficient, stable, inexpensive and capable of harvesting light are required. Here, we show that an abundant material, polymeric carbon nitride, can produce hydrogen from water under visible-light irradiation in the presence of a sacrificial donor. Contrary to other conducting polymer semiconductors, carbon nitride is chemically and thermally stable and does not rely on complicated device manufacturing. The results represent an important first step towards photosynthesis in general where artificial conjugated polymer semiconductors can be used as energy transducers. [ABSTRACT FROM AUTHOR]

Published
2009
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26. Graphene: Buckle or break.

Author

Carlsson, Johan M.

Subjects
*GRAPHITE, *CRYSTAL lattices, *LATTICE theory, *CRYSTALS, *MONTE Carlo method
Abstract

The article reports on several theoretical studies about the existence of two-dimensional crystals through the isolation of free-standing graphene sheets. A study inferred that two-dimensional crystals are impossible to exist because the periodic order of the atoms cannot be maintained. However, a study using Monte Carlo simulations has confirmed that the sheets can be supported by the formation of finite-sized ripples.

Published
2007
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27. A metal-free polymeric photocatalyst for hydrogen production from water under visible light.

Author

Wang X, Maeda K, Thomas A, Takanabe K, Xin G, Carlsson JM, Domen K, and Antonietti M

Abstract

The production of hydrogen from water using a catalyst and solar energy is an ideal future energy source, independent of fossil reserves. For an economical use of water and solar energy, catalysts that are sufficiently efficient, stable, inexpensive and capable of harvesting light are required. Here, we show that an abundant material, polymeric carbon nitride, can produce hydrogen from water under visible-light irradiation in the presence of a sacrificial donor. Contrary to other conducting polymer semiconductors, carbon nitride is chemically and thermally stable and does not rely on complicated device manufacturing. The results represent an important first step towards photosynthesis in general where artificial conjugated polymer semiconductors can be used as energy transducers.

Published
2009
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