46 research outputs found
Electron beam generation and structure of defects in carbon and boron nitride nanotubes
The nature and role of defects is of primary importance to understand the physical properties of C and BN single walled nanotubes. Transmission electron microscopy (TEM) is a well known powerful tool to study the structure of defects in materials. However, in the case of SWNTs, the electron irradiation of the TEM may knock out atoms. This effect may alter the native structure of the tube, and has also been proposed as a potential tool for nanoengineering of nanotubular structures. Here we develop a theoretical description of the irradiation mechanism. First, the anisotropy of the emission energy threshold is obtained via density functional based calculations. Then, we numerically derive the total Mott cross section for different emission sites of carbon and boron nitride nanotubes with different chiralities. Using a dedicated STEM microscope with experimental conditions optimised on the basis of derived cross-sections, we are able to control the generation of defects in nanotubular systems. Either point or line defects can be obtained with a spatial resolution of a few nanometers. The structure, energetics and electronics of point and line defects in BN systems have been investigated. Stability of mono- and di- vacancy defects in hexagonal boron nitride layers is investigated, and their activation energies and reaction paths for diffusion have been derived using the nudged elastic band method (NEB) combined with density functional based techniques. We demonstrate that the appearance of extended linear defects under electron irradiation is more favorable than a random distribution of point defects and this is due to the existence of preferential sites for atom emission in the presence of pre-existing defects, rather than thermal vacancy nucleation and migration
Evidence for anisotropic dielectric properties of monoclinic hafnia using high-resolution TEM valence electron energy-loss spectroscopy and ab initio time-dependent density-functional theory
The effect of nanocrystal orientation on the energy loss spectra of
monoclinic hafnia (m-HfO) is measured by high resolution transmission
electron microscopy (HRTEM) and valence energy loss spectroscopy (VEELS) on
high quality samples. For the same momentum-transfer directions, the dielectric
properties are also calculated ab initio by time-dependent density-functional
theory (TDDFT). Experiments and simulations evidence anisotropy in the
dielectric properties of m-HfO, most notably with the direction-dependent
oscillator strength of the main bulk plasmon. The anisotropic nature of
m-HfO may contribute to the differences among VEELS spectra reported in
literature. The good agreement between the complex dielectric permittivity
extracted from VEELS with nanometer spatial resolution, TDDFT modeling, and
past literature demonstrates that the present HRTEM-VEELS device-oriented
methodology is a possible solution to the difficult nanocharacterization
challenges given in the International Technology Roadmap for Semiconductors.Comment: 5 pages, 3 figure
A comparative study of density functional and density functional tight binding calculations of defects in graphene
The density functional tight binding approach (DFTB) is well adapted for the
study of point and line defects in graphene based systems. After briefly
reviewing the use of DFTB in this area, we present a comparative study of
defect structures, energies and dynamics between DFTB results obtained using
the dftb+ code, and density functional results using the localised Gaussian
orbital code, AIMPRO. DFTB accurately reproduces structures and energies for a
range of point defect structures such as vacancies and Stone-Wales defects in
graphene, as well as various unfunctionalised and hydroxylated graphene sheet
edges. Migration barriers for the vacancy and Stone-Wales defect formation
barriers are accurately reproduced using a nudged elastic band approach.
Finally we explore the potential for dynamic defect simulations using DFTB,
taking as an example electron irradiation damage in graphene
Graphene edge structures: Folding, scrolling, tubing, rippling and twisting
Conventional three-dimensional crystal lattices are terminated by surfaces,
which can demonstrate complex rebonding and rehybridisation, localised strain
and dislocation formation. Two dimensional crystal lattices, of which graphene
is the archetype, are terminated by lines. The additional available dimension
at such interfaces opens up a range of new topological interface possibilities.
We show that graphene sheet edges can adopt a range of topological distortions
depending on their nature. Rehybridisation, local bond reordering, chemical
functionalisation with bulky, charged, or multi-functional groups can lead to
edge buckling to relieve strain, folding, rolling and even tube formation. We
discuss the topological possibilities at a 2D graphene edge, and under what
circumstances we expect different edge topologies to occur. Density functional
calculations are used to explore in more depth different graphene edge types.Comment: Additional figure in published versio
Excitons and stacking order in h-BN
The strong excitonic emission at 5.75 eV of hexagonal boron nitride (h-BN)
makes this material one of the most promising candidate for light emitting
devices in the far ultraviolet (UV). However, single excitons occur only in
perfect monocrystals that are extremely hard to synthesize, while regular h-BN
samples present a complex emission spectrum with several additional peaks. The
microscopic origin of these additional emissions has not yet been understood.
In this work we address this problem using an experimental and theoretical
approach that combines nanometric resolved cathodoluminescence, high resolution
transmission electron microscopy and state of the art theoretical spectroscopy
methods. We demonstrate that emission spectra are strongly inhomogeneus within
individual flakes and that additional excitons occur at structural
deformations, such as faceted plane folds, that lead to local changes of the
h-BN stacking order
Revisiting graphene oxide chemistry via spatially-resolved electron energy loss spectroscopy
The type and distribution of oxygen functional groups in graphene oxide and
reduced graphene oxide remain still a subject of great debate. Local analytic techniques
are required to access the chemistry of these materials at a nanometric scale. Electron
energy loss spectroscopy in a scanning transmission electron microscope can provide the
suitable resolution, but GO and RGO are extremely sensitive to electron irradiation.
In this work we employ a dedicated experimental set-up to reduce electron illumina-
tion below damage limit. GO oxygen maps obtained at a few nanometres scale show
separated domains with diferent oxidation levels. The C/O ratio varies from about 4:1 to 1:1, the latter corresponding to a complete functionalization of the graphene flakes.
In RGO the residual oxygen concentrates mostly in regions few tens nanometres wide. Specific energy-loss near-edge structures are observed for diferent oxidation levels. By combining these findings with first principles simulations we propose a model for the highly oxidized domains where graphene is fully functionalized by hydroxyl groups forming a 2D-sp3 carbon network analogous to that of graphane.AT, AZ and OS acknowledge support from the Agence Nationale de la Recherche (ANR), program of future investment TEMPOS-CHROMATEM (No. ANR-10-EQPX-50). The work has also received funding from the European Union in Seventh Framework Programme (No. FP7/2007 -2013) under Grant Agreement No. n312483 (ESTEEM2). AMB and WKM are grateful for Financial support from the Spanish Ministry MINECO and the European Regional development Fund (project ENE2013-48816-C5-5-R) and from the Regional Government of Aragon and the European Social Fund (DGA-ESF-T66 Grupo Consolidado).
The authors are grateful to P. Launois, S. Rouziere and C.P. Ewels for useful discussion.Peer reviewe