53,514 research outputs found
A comparative study of the field emission properties of aligned carbon nanostructures films, from carbon nanotubes to diamond
International audienceThe electron field emission properties of different graphitic and diamond-like nanostructures films are compared. They are prepared in the same CVD chamber on SiO{2}/Si(100) and Si(100) flat surfaces, respectively. These nanostructures are thoroughly characterized by scanning electron emission (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Films of dense aligned carbon nanotubes by far display the lowest threshold fields around few V/μ m and the largest emission currents. Carbon nanofibers, with platelet arrangement of the graphitic planes parallel to the substrate, exhibit higher emission thresholds around 10 V/μ m. Diamond nanostructures, either modified through ammonia incorporation within the gas phase or not, exhibit the largest emission threshold around 25 V/μ m. The high enhancement factors, deduced from the Fowler-Nordheim plots, can explain the low emission thresholds whereas limitations to the electron transport ever occur through different processes (i) surface modifications of the surface, as the transformation of the SiO{2} barrier layer into SiN{x} in the presence of ammonia evidenced by XPS; (ii) different orientation of the graphitic basal planes relative to the direction of electron transport (carbon nanofiber) and (iii) presence of a graphitic nest at the interface of the carbon nanostructure and the substrate, observed when catalyst is deposited through mild evaporation
Secondary electron emission from freely supported nanowires
We present secondary electron (SE) emission results from freely supported carbon/silicon nitride (Si3N4) hybrid nanowires using scanning electron microscopy. We found that, contrary to bulk materials, the SE emission from insulating or electrically isolated metallic nanowires is strongly suppressed by the penetrating beam. A mechanism of the SE suppression by the positive specimen charging is proposed, which is based on a total emission yield calculation using the Monte Carlo technique. This finding provides an important basis for studying low-energy electron emission from nanostructures under a penetrating electron beam
One-dimensional carbon nanostructures for terahertz electron-beam radiation
One-dimensional carbon nanostructures such as nanotubes and nanoribbons can feature near-ballistic electronic transport over micron-scale distances even at room temperature. As a result, these materials provide a uniquely suited solid-state platform for radiation mechanisms that so far have been the exclusive domain of electron beams in vacuum. Here we consider the generation of terahertz light based on two such mechanisms, namely, the emission of cyclotronlike radiation in a sinusoidally corrugated nanowire (where periodic angular motion is produced by the mechanical corrugation rather than an externally applied magnetic field), and the Smith-Purcell effect in a rectilinear nanowire over a dielectric grating. In both cases, the radiation properties of the individual charge carriers are investigated via full-wave electrodynamic simulations, including dephasing effects caused by carrier collisions. The overall light output is then computed with a standard model of charge transport for two particularly suitable types of carbon nanostructures, i.e., zigzag graphene nanoribbons and armchair single-wall nanotubes. Relatively sharp emission peaks at geometrically tunable terahertz frequencies are obtained in each case. The corresponding output powers are experimentally accessible even with individual nanowires, and can be scaled to technologically significant levels using array configurations. These radiation mechanisms therefore represent a promising paradigm for light emission in condensed matter, which may find important applications in nanoelectronics and terahertz photonics.DMR-1308659/National Science Foundationhttp://ultra.bu.edu/papers/Tantiwanichapan-2016-PRB-CNT-THz.pd
Photoemission electron microscopy of localized surface plasmons in silver nanostructures at telecommunication wavelengths
We image the field enhancement at Ag nanostructures using femtosecond laser
pulses with a center wavelength of 1.55 micrometer. Imaging is based on
non-linear photoemission observed in a photoemission electron microscope
(PEEM). The images are directly compared to ultra violet PEEM and scanning
electron microscopy (SEM) imaging of the same structures. Further, we have
carried out atomic scale scanning tunneling microscopy (STM) on the same type
of Ag nanostructures and on the Au substrate. Measuring the photoelectron
spectrum from individual Ag particles shows a larger contribution from higher
order photoemission process above the work function threshold than would be
predicted by a fully perturbative model, consistent with recent results using
shorter wavelengths. Investigating a wide selection of both Ag nanoparticles
and nanowires, field enhancement is observed from 30% of the Ag nanoparticles
and from none of the nanowires. No laser-induced damage is observed of the
nanostructures neither during the PEEM experiments nor in subsequent SEM
analysis. By direct comparison of SEM and PEEM images of the same
nanostructures, we can conclude that the field enhancement is independent of
the average nanostructure size and shape. Instead, we propose that the
variations in observed field enhancement could originate from the wedge
interface between the substrate and particles electrically connected to the
substrate
All-optical spatio-temporal control of electron emission from SiO2 nanospheres with femtosecond two-color laser fields
Field localization by nanostructures illuminated with laser pulses of well-defined waveform enables spatio-temporal tailoring of the near-fields for sub-cycle control of electron dynamics at the nanoscale. Here, we apply intense linearly-polarized two-color laser pulses for all-optical control of the highest energy electron emission from SiO2 nanoparticles. For the size regime where light propagation effects become important, we demonstrate the possibility to control the preferential emission angle of a considerable fraction of the fastest electrons by varying the relative phase of the two-color field. Trajectory based semi-classical simulations show that for the investigated nanoparticle size range the directional steering can be attributed to the two-color effect on the electron trajectories, while the accompanied modification of the spatial distribution of the ionization rate on the nanoparticle surface has only a minor effect
Exciton lifetime and emission polarization dispersion in strongly in-plane asymmetric nanostructures
We present experimental and theoretical investigation of exciton
recombination dynamics and the related polarization of emission in highly
in-plane asymmetric nanostructures. Considering general asymmetry- and
size-driven effects, we illustrate them with a detailed analysis of
InAs/AlGaInAs/InP elongated quantum dots. These offer a widely varied
confinement characteristics tuned by size and geometry that are tailored during
the growth process, which leads to emission in the application-relevant
spectral range of 1.25-1.65 {\mu}m. By exploring the interplay of the very
shallow hole confining potential and widely varying structural asymmetry, we
show that a transition from the strong through intermediate to even weak
confinement regime is possible in nanostructures of this kind. This has a
significant impact on exciton recombination dynamics and the polarization of
emission, which are shown to depend not only on details of the calculated
excitonic states but also on excitation conditions in the photoluminescence
experiments. We estimate the impact of the latter and propose a way to
determine the intrinsic polarization-dependent exciton light-matter coupling
based on kinetic characteristics.Comment: 11 pages, 8 figure
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