265 research outputs found
Knock-on damage in bilayer graphene: Indications for a catalytic pathway
We study by high-resolution transmission electron microscopy the structural response of bilayer graphene to electron irradiation with energies below the knock-on damage threshold of graphene. We observe that one type of divacancy, which we refer to as the butterfly defect, is formed for radiation energies and doses for which no vacancies are formed in clean monolayer graphene. By using first principles calculations based on density-functional theory, we analyze two possible causes related with the presence of a second layer that could explain the observed phenomenon: an increase of the defect stability or a catalytic effect during its creation. For the former, the obtained formation energies of the defect in monolayer and bilayer systems show that the change in stability is negligible. For the latter, ab initio molecular dynamics simulations indicate that the threshold energy for direct expulsion does not decrease in bilayer graphene as compared with monolayer graphene, and we demonstrate the possibility of creating divacancies through catalyzed intermediate states below this threshold energy. The estimated cross section agrees with what is observed experimentally. Therefore, we show the possibility of a catalytic pathway for creating vacancies under electron radiation below the expulsion threshold energy. © 2013 American Physical Society
Atomic carbon chains as spin-transmitters: an \textit{Ab initio} transport study
An atomic carbon chain joining two graphene flakes was recently realized in a
ground-breaking experiment by Jin {\it et al.}, Phys. Rev. Lett. {\bf 102},
205501 (2009). We present {\it ab initio} results for the electron transport
properties of such chains and demonstrate complete spin-polarization of the
transmission in large energy ranges. The effect is due to the spin-polarized
zig-zag edge terminating each graphene flake causing a spin-splitting of the
graphene bands, and the chain states. Transmission occurs when the
graphene -states resonate with similar states in the strongly hybridized
edges and chain. This effect should in general hold for any -conjugated
molecules bridging the zig-zag edges of graphene electrodes. The polarization
of the transmission can be controlled by chemically or mechanically modifying
the molecule, or by applying an electrical gate
Chiral templating of self-assembling nanostructures by circularly polarized light
PMCID: PMC4387888.-- et al.The high optical and chemical activity of nanoparticles (NPs) signifies the possibility of converting the spin angular momenta of photons into structural changes in matter. Here, we demonstrate that illumination of dispersions of racemic CdTe NPs with right- (left-)handed circularly polarized light (CPL) induces the formation of right- (left-)handed twisted nanoribbons with an enantiomeric excess exceeding 30%, which is â 1/410 times higher than that of typical CPL-induced reactions. Linearly polarized light or dark conditions led instead to straight nanoribbons. CPL templating of NP assemblies is based on the enantio-selective photoactivation of chiral NPs and clusters, followed by their photooxidation and self-assembly into nanoribbons with specific helicity as a result of chirality-sensitive interactions between the NPs. The ability of NPs to retain the polarization information of incident photons should open pathways for the synthesis of chiral photonic materials and allow a better understanding of the origins of biomolecular homochirality.This material is based on work partially supported by the Center for Solar and Thermal
Energy Conversion, an Energy Frontier Research Center funded by the US Department
of Energy, Office of Science, Office of Basic Energy Sciences under award number
#DE-SC0000957, and by ARO MURI W911NF-12-1-0407 ‘Coherent Effects in Hybrid
Nanostructures for Lineshape Engineering of Electromagnetic Media’ (N.A.K. and S.L.).
We acknowledge support from the NSF under grant ECS-0601345; CBET 0933384; CBET 0932823; and CBET 1036672. Financial support from the Robert A. Welch Foundation (C-1664) is also acknowledged (S.L.). Support from the NIH grant GM085043 (P.Z.) is gratefully acknowledged. The work of P.K. was supported by the NSF DMR grant No. 1309765 and by the ACS PRF grant No. 53062-ND6.Peer Reviewe
An accurate measurement of electron beam induced displacement cross sections for single-layer graphene
We present an accurate measurement and a quantitative analysis of
electron-beam induced displacements of carbon atoms in single-layer graphene.
We directly measure the atomic displacement ("knock-on") cross section by
counting the lost atoms as a function of the electron beam energy and applied
dose. Further, we separate knock-on damage (originating from the collision of
the beam electrons with the nucleus of the target atom) from other radiation
damage mechanisms (e.g. ionization damage or chemical etching) by the
comparison of ordinary (12C) and heavy (13C) graphene. Our analysis shows that
a static lattice approximation is not sufficient to describe knock-on damage in
this material, while a very good agreement between calculated and experimental
cross sections is obtained if lattice vibrations are taken into account.Comment: 10 pages including supplementary inf
Formation of buckminsterfullerene (C60) in interstellar space
Buckminsterfullerene (C60) was recently confirmed to be the largest molecule
identified in space. However, it remains unclear how, and where this molecule
is formed. It is generally believed that C60 is formed from the build up of
small carbonaceous compounds, in the hot and dense envelopes of evolved stars.
Analyzing infrared observations, obtained by Spitzer and Herschel, we found
that C60 is efficiently formed in the tenuous and cold environment of an
interstellar cloud illuminated by strong ultraviolet (UV) radiation fields.
This implies that another formation pathway, efficient at low densities, must
exist. Based on recent laboratory and theoretical studies, we argue that
Polycyclic Aromatic Hydrocarbons are converted into graphene, and subsequently
C60, under UV irradiation from massive stars. This shows that alternative -
top-down - routes are key to understanding the organic inventory in space.Comment: 22 pages, 4 figures, 3 sup. figures, 1 sup. vide
Structure-Sensitive Mechanism of Nanographene Failure
The response of a nanographene sheet to external stresses is considered in
terms of a mechanochemical reaction. The quantum chemical realization of the
approach is based on a coordinate-of-reaction concept for the purpose of
introducing a mechanochemical internal coordinate (MIC) that specifies a
deformational mode. The related force of response is calculated as the energy
gradient along the MIC, while the atomic configuration is optimized over all of
the other coordinates under the MIC constant-pitch elongation. The approach is
applied to the benzene molecule and (5, 5) nanographene. A drastic anisotropy
in the microscopic behavior of both objects under elongation along a MIC has
been observed when the MIC is oriented either along or normally to the C-C
bonds chain. Both the anisotropy and high stiffness of the nanographene
originate at the response of the benzenoid unit to stress.Comment: 19 pages, 7 figures 1 tabl
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