474 research outputs found
Thermometry of Silicon Nanoparticles
Current thermometry techniques lack the spatial resolution required to see
the temperature gradients in typical, highly-scaled modern transistors. As a
step toward addressing this problem, we have measured the temperature
dependence of the volume plasmon energy in silicon nanoparticles from room
temperature to 1250C, using a chip-style heating sample holder in a
scanning transmission electron microscope (STEM) equipped with electron energy
loss spectroscopy (EELS). The plasmon energy changes as expected for an
electron gas subject to the thermal expansion of silicon. Reversing this
reasoning, we find that measurements of the plasmon energy provide an
independent measure of the nanoparticle temperature consistent with that of the
heater chip's macroscopic heater/thermometer to within the 5\% accuracy of the
chip thermometer's calibration. Thus silicon has the potential to provide its
own, high-spatial-resolution thermometric readout signal via measurements of
its volume plasmon energy. Furthermore, nanoparticles in general can serve as
convenient nanothermometers for \emph{in situ} electron microscopy experiments.Comment: 6 pages, 3 figure
Electron Transport Driven by Nonequilibrium Magnetic Textures
Spin-polarized electron transport driven by inhomogeneous magnetic dynamics
is discussed in the limit of a large exchange coupling. Electron spins rigidly
following the time-dependent magnetic profile experience spin-dependent
fictitious electric and magnetic fields. We show that the electric field
acquires important corrections due to spin dephasing, when one relaxes the
spin-projection approximation. Furthermore, spin-flip scattering between the
spin bands needs to be taken into account in order to calculate voltages and
spin accumulations induced by the magnetic dynamics. A phenomenological
approach based on the Onsager reciprocity principle is developed, which allows
us to capture the effect of spin dephasing and make a connection to the well
studied problem of current-driven magnetic dynamics. A number of results that
recently appeared in the literature are related and generalized.Comment: 4 pages, 1 figur
Tree-level electron-photon interactions in graphene
Graphene's low-energy electronic excitations obey a 2+1 dimensional Dirac
Hamiltonian. After extending this Hamiltonian to include interactions with a
quantized electromagnetic field, we calculate the amplitude associated with the
simplest, tree-level Feynman diagram: the vertex connecting a photon with two
electrons. This amplitude leads to analytic expressions for the 3D angular
dependence of photon emission, the photon-mediated electron-hole recombination
rate, and corrections to graphene's opacity and dynamic
conductivity for situations away from thermal equilibrium, as
would occur in a graphene laser. We find that Ohmic dissipation in perfect
graphene can be attributed to spontaneous emission.Comment: 5 pages, 3 figure
Electron tomography at 2.4 {\AA} resolution
Transmission electron microscopy (TEM) is a powerful imaging tool that has
found broad application in materials science, nanoscience and biology(1-3).
With the introduction of aberration-corrected electron lenses, both the spatial
resolution and image quality in TEM have been significantly improved(4,5) and
resolution below 0.5 {\AA} has been demonstrated(6). To reveal the 3D structure
of thin samples, electron tomography is the method of choice(7-11), with
resolutions of ~1 nm^3 currently achievable(10,11). Recently, discrete
tomography has been used to generate a 3D atomic reconstruction of a silver
nanoparticle 2-3 nm in diameter(12), but this statistical method assumes prior
knowledge of the particle's lattice structure and requires that the atoms fit
rigidly on that lattice. Here we report the experimental demonstration of a
general electron tomography method that achieves atomic scale resolution
without initial assumptions about the sample structure. By combining a novel
projection alignment and tomographic reconstruction method with scanning
transmission electron microscopy, we have determined the 3D structure of a ~10
nm gold nanoparticle at 2.4 {\AA} resolution. While we cannot definitively
locate all of the atoms inside the nanoparticle, individual atoms are observed
in some regions of the particle and several grains are identified at three
dimensions. The 3D surface morphology and internal lattice structure revealed
are consistent with a distorted icosahedral multiply-twinned particle. We
anticipate that this general method can be applied not only to determine the 3D
structure of nanomaterials at atomic scale resolution(13-15), but also to
improve the spatial resolution and image quality in other tomography
fields(7,9,16-20).Comment: 27 pages, 17 figure
Polarized light emission from individual incandescent carbon nanotubes
We fabricate nanoscale lamps which have a filament consisting of a single
multiwalled carbon nanotube. After determining the nanotube geometry with a
transmission electron microscope, we use Joule heating to bring the filament to
incandescence, with peak temperatures in excess of 2000 K. We image the thermal
light in both polarizations simultaneously as a function of wavelength and
input electrical power. The observed degree of polarization is typically of the
order of 75%, a magnitude predicted by a Mie model of the filament that assigns
graphene's optical conductance to each nanotube wall.Comment: 5 pages, 4 figure
STEM EBIC Mapping of the Metal-Insulator Transition in Thin-film NbO 2
This article has been published in a revised form in Microscopy and Microanalysis https://doi.org/10.1017/S1431927617007802. This version is free to view and download for private research and study only. Not for re-distribution, re-sale or use in derivative works. © copyright holder
Dark-field transmission electron microscopy and the Debye-Waller factor of graphene
Graphene's structure bears on both the material's electronic properties and
fundamental questions about long range order in two-dimensional crystals. We
present an analytic calculation of selected area electron diffraction from
multi-layer graphene and compare it with data from samples prepared by chemical
vapor deposition and mechanical exfoliation. A single layer scatters only 0.5%
of the incident electrons, so this kinematical calculation can be considered
reliable for five or fewer layers. Dark-field transmission electron micrographs
of multi-layer graphene illustrate how knowledge of the diffraction peak
intensities can be applied for rapid mapping of thickness, stacking, and grain
boundaries. The diffraction peak intensities also depend on the mean-square
displacement of atoms from their ideal lattice locations, which is
parameterized by a Debye-Waller factor. We measure the Debye-Waller factor of a
suspended monolayer of exfoliated graphene and find a result consistent with an
estimate based on the Debye model. For laboratory-scale graphene samples,
finite size effects are sufficient to stabilize the graphene lattice against
melting, indicating that ripples in the third dimension are not necessary.Comment: 10 pages, 4 figure
Dark-field transmission electron microscopy and the Debye-Waller factor of graphene
Graphene\u27s structure bears on both the material\u27s electronic properties and fundamental questions about long-range order in two-dimensional crystals. We present an analytic calculation of selected area electron diffraction from multilayer graphene and compare it with data from samples prepared by chemical vapor deposition and mechanical exfoliation. A single layer scatters only 0.5% of the incident electrons, so this kinematical calculation can be considered reliable for five or fewer layers. Dark-field transmission electron micrographs of multilayer graphene illustrate how knowledge of the diffraction peak intensities can be applied for rapid mapping of thickness, stacking, and grain boundaries. The diffraction peak intensities also depend on the mean-square displacement of atoms from their ideal lattice locations, which is parameterized by a Debye-Waller factor. We measure the Debye-Waller factor of a suspended monolayer of exfoliated graphene and find a result consistent with an estimate based on the Debye model. For laboratory-scale graphene samples, finite size effects are sufficient to stabilize the graphene lattice against melting, indicating that ripples in the third dimension are not necessary
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