172 research outputs found
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
Effects of epitaxial strain on the growth mechanism of YBa2Cu3O7-x thin films in [YBa2Cu3O7-x / PrBa2Cu3O7-x] superlattices
We report on the growth mechanism of YBa2Cu3O7-x (YBCO). Our study is based
on the analysis of ultrathin, YBa2Cu3O7-x layers in c-axis oriented YBa2Cu3O7-x
/ PrBa2Cu3O7-x superlattices. We have found that the release of epitaxial
strain in very thin YBCO layers triggers a change in the dimensionality of the
growth mode. Ultrathin, epitaxially strained, YBCO layers with thickness below
3 unit cells grow in a block by block two dimensional mode coherent over large
lateral distances. Meanwhile, when thickness increases, and the strain relaxes,
layer growth turns into three dimensional, resulting in rougher layers and
interfaces.Comment: 10 pages + 9 figures, accepted in Phys. Rev.
Graphene re-knits its holes
Nano-holes, etched under an electron beam at room temperature in singlelayer
graphene sheets as a result of their interaction with metalimpurities, are
shown to heal spontaneously by filling up with either non-hexagon,
graphene-like, or perfect hexagon 2D structures. Scanning transmission electron
microscopy was employed to capture the healing process and study atom-by-atom
the re-grown structure. A combination of these nano-scale etching and
re-knitting processes could lead to new graphene tailoring approaches.Comment: 11 pages, 4 figure
Position and momentum mapping of vibrations in graphene nanostructures in the electron microscope
Propagating atomic vibrational waves, phonons, rule important thermal,
mechanical, optoelectronic and transport characteristics of materials. Thus the
knowledge of phonon dispersion, namely the dependence of vibrational energy on
momentum is a key ingredient to understand and optimize the material's
behavior. However, despite its scientific importance in the last decade, the
phonon dispersion of a freestanding monolayer of two dimensional (2D) materials
such as graphene and its local variations has still remained elusive because of
experimental limitations of vibrational spectroscopy. Even though electron
energy loss spectroscopy (EELS) in transmission has recently been shown to
probe the local vibrational charge responses, these studies are yet limited to
polar materials like boron nitride or oxides, in which huge signals induced by
strong dipole moments are present. On the other hand, measurements on graphene
performed by inelastic x-ray (neutron) scattering spectroscopy or EELS in
reflection do not have any spatial resolution and require large microcrystals.
Here we provide a new pathway to determine the phonon dispersions down to the
scale of an individual freestanding graphene monolayer by mapping the distinct
vibration modes for a large momentum transfer. The measured scattering
intensities are accurately reproduced and interpreted with density functional
perturbation theory (DFPT). Additionally, a nanometre-scale mapping of selected
momentum (q) resolved vibration modes using graphene nanoribbon structures has
enabled us to spatially disentangle bulk, edge and surface vibrations
Atomic-resolution spectroscopic imaging of ensembles of nanocatalyst particles across the life of a fuel cell
The thousandfold increase in data-collection speed enabled by
aberration-corrected optics allows us to overcome an electron microscopy
paradox - how to obtain atomic-resolution chemical structure in individual
nanoparticles, yet record a statistically significant sample from an
inhomogeneous population. This allowed us to map hundreds of Pt-Co
nanoparticles to show atomic-scale elemental distributions across different
stages of the catalyst aging in a proton-exchange-membrane fuel cell, and
relate Pt-shell thickness to treatment, particle size, surface orientation, and
ordering.Comment: 28 pages, 5 figures, accepted, nano letter
Cross-sectional imaging of individual layers and buried interfaces of graphene-based heterostructures and superlattices
By stacking various two-dimensional (2D) atomic crystals [1] on top of each
other, it is possible to create multilayer heterostructures and devices with
designed electronic properties [2-5]. However, various adsorbates become
trapped between layers during their assembly, and this not only affects the
resulting quality but also prevents the formation of a true artificial layered
crystal upheld by van der Waals interaction, creating instead a laminate glued
together by contamination. Transmission electron microscopy (TEM) has shown
that graphene and boron nitride monolayers, the two best characterized 2D
crystals, are densely covered with hydrocarbons (even after thermal annealing
in high vacuum) and exhibit only small clean patches suitable for atomic
resolution imaging [6-10]. This observation seems detrimental for any realistic
prospect of creating van der Waals materials and heterostructures with
atomically sharp interfaces. Here we employ cross sectional TEM to take a side
view of several graphene-boron nitride heterostructures. We find that the
trapped hydrocarbons segregate into isolated pockets, leaving the interfaces
atomically clean. Moreover, we observe a clear correlation between interface
roughness and the electronic quality of encapsulated graphene. This work proves
the concept of heterostructures assembled with atomic layer precision and
provides their first TEM images
Dielectric response of pentagonal defects in multilayer graphene nano-cones
The dielectric response of pentagonal defects in multilayer graphene nano-cones has been studied by electron energy loss spectroscopy and ab initio simulations. At the cone apex, a strong modification of the dielectric response is observed below the energy of the π plasmon resonance. This is attributed to π → π* interband transitions induced by topology-specific resonant π bonding states as well as π*–σ* hybridization. It is concluded that pentagonal defects strongly affect the local electronic structure in such a way that multi-walled graphene nano-cones should show great promise as field emitters
Deciphering chemical order/disorder and material properties at the single-atom level
Correlating 3D arrangements of atoms and defects with material properties and
functionality forms the core of several scientific disciplines. Here, we
determined the 3D coordinates of 6,569 iron and 16,627 platinum atoms in a
model iron-platinum nanoparticle system to correlate 3D atomic arrangements and
chemical order/disorder with material properties at the single-atom level. We
identified rich structural variety and chemical order/disorder including 3D
atomic composition, grain boundaries, anti-phase boundaries, anti-site point
defects and swap defects. We show for the first time that experimentally
measured 3D atomic coordinates and chemical species with 22 pm precision can be
used as direct input for first-principles calculations of material properties
such as atomic magnetic moments and local magnetocrystalline anisotropy. This
work not only opens the door to determining 3D atomic arrangements and chemical
order/disorder of a wide range of nanostructured materials with high precision,
but also will transform our understanding of structure-property relationships
at the most fundamental level.Comment: 21 pages, 4 figure
Crops that feed the world 6. Past successes and future challenges to the role played by maize in global food security
Time-dependent forces between a swift electron and a small nanoparticle within the dipole approximation
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