2,344 research outputs found
Femtosecond time-resolved photoemission as a probe of electronic transport in single wall carbon nanotubes
We have performed the first time-domain measurements of the electron-electron (e-e) and electron-phonon (e-ph) dynamics in single-wall carbon nanotube samples (bucky paper) using time-resolved two-photon photoemission. In these room temperature experiments the absorption of a visible femtosecond pump pulse creates a non-equilibrium electron distribution whose evolution in time can be probed by a second UV-pulse. The decay of the excited electron distribution is characterized by a fast channel on the subpicosecond time-scale—associated with thermalization of the non-equilibrium distribution—and a slower channel which can be attributed to e-ph interaction. Once thermalized the electron distribution cools down to the lattice temperature as determined by the electron-phonon coupling constant g which was found to be 1×1015 Wm−3 K−1
Electron-Phonon Interaction in Single-Wall Carbon Nanotubes: A Time Domain Study
We investigate the electron-phonon (e-ph) interaction in single-wall carbon nanotube samples at room temperature using femtosecond time-resolved photoemission. By probing electrons from the vicinity of the Fermi level we are able to study the e-ph interaction in the metallic nanotube species only. The observed electron dynamics can be used to calculate e-ph scattering matrix elements for two likely scattering scenarios: forward scattering from twistons and backscattering by longitudinal acoustic phonons. The corresponding matrix elements reveal an intrinsically weak e-ph interaction approximately 50% smaller than predicted by tight-binding calculations
Coupled opto-electronic simulation of organic bulk-heterojunction solar cells: parameter extraction and sensitivity analysis
A general problem arising in computer simulations is the number of material
and device parameters, which have to be determined by dedicated experiments and
simulation-based parameter extraction. In this study we analyze measurements of
the short-circuit current dependence on the active layer thickness and
current-voltage curves in poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid
methyl ester (P3HT:PCBM) based solar cells. We have identified a set of
parameter values including dissociation parameters that describe the
experimental data. The overall agreement of our model with experiment is good,
however a discrepancy in the thickness dependence of the current-voltage curve
questions the influence of the electric field in the dissociation process. In
addition transient simulations are analyzed which show that a measurement of
the turn-off photocurrent can be useful for estimating charge carrier
mobilities.Comment: 10 pages, 12 figures, 2 tables, Accepted for publication in Journal
of Applied Physic
Observations of the Io plasma torus
The short wavelength spectrography on the IUE satellite was used to obtain spectra of the plasma torus near the orbit of Io about Jupiter. Three exposures of about 8 hours each taken in March and May 1979 show emission features due to SII, SIII, and OIII. The absence of features at other wavelengths permits upper limits to be other species in the torus
Observations of polar aurora on Jupiter
North-south spatial maps of Jupiter were obtained with the SWP camera in IUE observations of 10 December 1978, 19 May 1979, and 7 June 1979. Bright auroral emissions were detected from the north and south polar regions at H Ly alpha (1216 A) and in the H2 Lyman bands (1250-1608 A) on 19 May 1979; yet no enhanced polar emission was detected on the other days. The relationship between the IUE observing geometry and the geometry of the Jovian magnetosphere is discussed
Temperature Dependence of Electron to Lattice Energy-Transfer in Single-Wall Carbon Nanotube Bundles
The electron-phonon coupling strength in single-wall carbon nanotube (SWNT)
bundles has been studied directly in the time-domain by femtosecond
time-resolved photoelectron spectroscopy. We have measured the dependence of
H(T_e,T_l), the rate of energy-transfer between the electronic system and the
lattice as a function of electron and lattice temperatures T_e and T_l. The
experiments are consistent with a T^5 dependence of H on the electron- and
lattice-temperatures, respectively. The results can be related to the e-ph mass
enhancement parameter lambda. The experimentally obtained value for
lambda/theta_D^2, where theta_D is the Debye temperature, suggests that e-ph
scattering times at the Fermi level of SWNT bundles can be exceptionally long,
exceeding 1.5 ps at room temperature.Comment: 5 pages, 4 figures, submitted to the Journal of Nanoscience and
Nanotechnologiy, special issue on nanotube
Electronic structure and dynamics of optically excited single-wall carbon nanotubes
We have studied the electronic structure and charge-carrier dynamics of
individual single-wall carbon nanotubes (SWNTs) and nanotube ropes using
optical and electron-spectroscopic techniques. The electronic structure of
semiconducting SWNTs in the band-gap region is analyzed using near-infrared
absorption spectroscopy. A semi-empirical expression for
transition energies, based on tight-binding calculations is found to give
striking agreement with experimental data. Time-resolved PL from dispersed
SWNT-micelles shows a decay with a time constant of about 15 ps. Using
time-resolved photoemission we also find that the electron-phonon ({\it e-ph})
coupling in metallic tubes is characterized by a very small {\it e-ph}
mass-enhancement of 0.0004. Ultrafast electron-electron scattering of
photo-excited carriers in nanotube ropes is finally found to lead to internal
thermalization of the electronic system within about 200 fs.Comment: 10 pages, 10 figures, submitted to Applied Physics
Spatial imaging of UV emission from Jupiter and Saturn
Spatial imaging with the IUE is accomplished both by moving one of the apertures in a series of exposures and within the large aperture in a single exposure. The image of the field of view subtended by the large aperture is focussed directly onto the detector camera face at each wavelength; since the spatial resolution of the instrument is 5 to 6 arc sec and the aperture extends 23.0 by 10.3 arc sec, imaging both parallel and perpendicular to dispersion is possible in a single exposure. The correction for the sensitivity variation along the slit at 1216 A is obtained from exposures of diffuse geocoronal H Ly alpha emission. The relative size of the aperture superimposed on the apparent discs of Jupiter and Saturn in typical observation is illustrated. By moving the planet image 10 to 20 arc sec along the major axis of the aperture (which is constrained to point roughly north-south) maps of the discs of these planets are obtained with 6 arc sec spatial resolution
Anisotropy of quasiparticle lifetimes and the role of disorder in graphite from ultrafast time-resolved photoemission spectroscopy
Femtosecond time-resolved photoemission of photoexcited electrons in highly oriented pyrolytic graphite (HOPG) provides strong evidence for anisotropies of quasiparticle (QP) lifetimes. Indicative of such anisotropies is a pronounced anomaly in the energy dependence of QP lifetimes between 1.1 and 1.5 eV—the vicinity of a saddle point in the graphite band structure. This is supported by recent ab initio calculations and a comparison with experiments on defect-enriched HOPG which reveal that disorder, e.g., defects or phonons, increases electron energy relaxation rates
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