56 research outputs found
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
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Alpha ' formation kinetics and radiation induced segregation in neutron irradiated 14YWT nanostructured ferritic alloys
Nanostructured ferritic alloys are considered as candidates for structural components in advanced nuclear reactors due to a high density of nano-oxides (NOs) and ultrafne grain sizes. However, bimodal grain size distribution results in inhomogeneous NO distribution, or vice versa. Here, we report that density of NOs in small grains (2µm) before and after irradiation. After 6dpa neutron irradiation at 385–430°C, α′ precipitation has been observed in these alloys; however, their size and number densities vary considerably in small and large grains. In this study, we have investigated the precipitation kinetics of α′ particles based on the sink density, using both transmission electron microscopy and kinetic Monte Carlo simulations. It has been found that in the presence of a low sink density, α′ particles form and grow faster due to the existence of a larger defect density in the matrix. On the other hand, while α′ particles form far away from the sink interface when the sink size is small, Cr starts to segregate at the sink interface with the increase in the sink size. Additionally, grain boundary characteristics are found to determine the radiation-induced segregation of Cr
Atomic resolution mapping of localized phonon modes at grain boundaries
Phonon scattering at grain boundaries (GBs) is significant in controlling
nanoscale device thermal conductivity. However, GBs could also act as
waveguides for selected modes. To measure localized GB phonon modes, meV energy
resolution is needed with sub-nm spatial resolution. Using monochromated
electron energy loss spectroscopy (EELS) in the scanning transmission electron
microscope (STEM) we have mapped the 60 meV optic mode across GBs in silicon at
atomic resolution and compared it to calculated phonon densities of states
(DOS). The intensity is strongly reduced at GBs characterised by the presence
of five- and seven-fold rings where bond angles differ from the bulk. The
excellent agreement between theory and experiment strongly supports the
existence of localized phonon modes and thus of GBs acting as waveguides
Atomic Configuration of Nitrogen Doped Single-Walled Carbon Nanotubes
Having access to the chemical environment at the atomic level of a dopant in
a nanostructure is crucial for the understanding of its properties. We have
performed atomically-resolved electron energy-loss spectroscopy to detect
individual nitrogen dopants in single-walled carbon nanotubes and compared with
first principles calculations. We demonstrate that nitrogen doping occurs as
single atoms in different bonding configurations: graphitic-like and
pyrrolic-like substitutional nitrogen neighbouring local lattice distortion
such as Stone-Thrower-Wales defects. The stability under the electron beam of
these nanotubes has been studied in two extreme cases of nitrogen incorporation
content and configuration. These findings provide key information for the
applications of these nanostructures.Comment: 25 pages, 13 figure
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