15 research outputs found
Imaging of isotope diffusion using atomic-scale vibrational spectroscopy
The spatial resolutions of even the most sensitive isotope analysis
techniques based on light or ion probes are limited to a few hundred
nanometres. Although vibration spectroscopy using electron probes has achieved
higher spatial resolution, the detection of isotopes at the atomic level has
been challenging so far. Here we show the unambiguous isotopic imaging of 12C
carbon atoms embedded in 13C graphene and the monitoring of their
self-diffusion via atomic level vibrational spectroscopy. We first grow a
domain of 12C carbon atoms in a preexisting crack of 13C graphene, which is
then annealed at 600C for several hours. Using scanning transmission electron
microscopy electron energy loss spectroscopy, we obtain an isotope map that
confirms the segregation of 12C atoms that diffused rapidly. The map also
indicates that the graphene layer becomes isotopically homogeneous over 100
nanometre regions after 2 hours. Our results demonstrate the high mobility of
carbon atoms during growth and annealing via selfdiffusion. This imaging
technique can provide a fundamental methodology for nanoisotope engineering and
monitoring, which will aid in the creation of isotope labels and tracing at the
nanoscale
Excitonic effects in energy loss spectra of freestanding graphene
In this work we perform electron energy-loss spectroscopy (EELS) of
freestanding graphene with high energy and momentum resolution to disentangle
the quasielastic scattering from the excitation gap of Dirac electrons close to
the optical limit. We show the importance of many-body effects on electronic
excitations at finite transferred momentum by comparing measured EELS with ab
initio calculations at increasing levels of theory. Quasi-particle corrections
and excitonic effects are addressed within the GW approximation and
Bethe-Salpeter equation, respectively. Both effects are essential in the
description of the EEL spectra to obtain a quantitative agreement with
experiments, with the position, dispersion, and shape of both the excitation
gap and the plasmon being significantly affected by excitonic effects
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
Structural Distortions and Charge Density Waves in Iodine Chains Encapsulated inside Carbon Nanotubes
Atomic chains are perfect systems for getting fundamental insights into the electron dynamics and coupling between the electronic and ionic degrees of freedom in one-dimensional metals. Depending on the band filling, they can exhibit Peierls instabilities (or charge density waves), where equally spaced chain of atoms with partially filled band is inherently unstable, exhibiting spontaneous distortion of the lattice that further leads to metal-insulator transition in the system. Here, using high-resolution scanning transmission electron microscopy, we directly image the atomic structures of a chain of iodine atoms confined inside carbon nanotubes. In addition to long equidistant chains, the ones consisting of iodine dimers and trimers were also observed, as well as transitions between them. First-principles calculations reproduce the experimentally observed bond lengths and lattice constants, showing that the ionic movement is largely unconstrained in the longitudinal direction, while naturally confined by thenanotube in the lateral directions. Moreover, the trimerized chain bears the hallmarks of a charge density wave. The transition is driven by changes in the charge transfer between the chain and the nanotube and is enabled by the charge compensation and additional screening provided by the nanotube.Peer reviewe