193 research outputs found
Impact Excitation by Hot Carriers in Carbon Nanotubes
We investigate theoretically the efficiency of intra-molecular hot carrier
induced impact ionization and excitation processes in carbon nanotubes. The
electron confinement and reduced screening lead to drastically enhanced
excitation efficiencies over those in bulk materials. Strong excitonic coupling
favors neutral excitations over ionization, while the impact mechanism
populates a different set of states than that produced by photoexcitation. The
excitation rate is strongly affected by optical phonon excitation and a simple
scaling of the rate with the field strength and optical phonon temperature is
obtained.Comment: 5 pages 4 figure
Drain Voltage Scaling in Carbon Nanotube Transistors
While decreasing the oxide thickness in carbon nanotube field-effect
transistors (CNFETs) improves the turn-on behavior, we demonstrate that this
also requires scaling the range of the drain voltage. This scaling is needed to
avoid an exponential increase in Off-current with drain voltage, due to
modulation of the Schottky barriers at both the source and drain contact. We
illustrate this with results for bottom-gated ambipolar CNFETs with oxides of 2
and 5 nm, and give an explicit scaling rule for the drain voltage. Above the
drain voltage limit, the Off-current becomes large and has equal electron and
hole contributions. This allows the recently reported light emission from
appropriately biased CNFETs.Comment: 4 pages, 4 EPS figure, to appear in Appl. Phys. Lett. (issue of 15
Sept 2003
Exciton-phonon effects in carbon nanotube optical absorption
We find that the optical properties of carbon nanotubes reflect remarkably
strong effects of exciton-phonon coupling. Tight-binding calculations show that
a significant fraction of the spectral weight of the absorption peak is
transferred to a distinct exciton+phonon sideband, which is peaked at around
200 meV above the main absorption peak. This sideband provides a distinctive
signature of the excitonic character of the optical transition. The
exciton-phonon coupling is reflected in a dynamical structural distortion,
which contributes a binding energy of up to 100 meV. The distortion is
surprisingly long-ranged, and is strongly dependent on chirality.Comment: 5 pages, 3 figure
Band Structure and Quantum Conductance of Nanostructures from Maximally-Localized Wannier Functions: The Case of Functionalized Carbon Nanotubes
We have combined large-scale, -point electronic-structure
calculations with the maximally-localized Wannier functions approach to
calculate efficiently the band structure and the quantum conductance of complex
systems containing thousands of atoms while maintaining full first-principles
accuracy. We have applied this approach to study covalent functionalizations in
metallic single-walled carbon nanotubes. We find that the band structure around
the Fermi energy is much less dependent on the chemical nature of the ligands
than on the functionalization pattern disrupting the conjugation
network. Common aryl functionalizations are more stable when paired with
saturating hydrogens; even when paired, they still act as strong scattering
centers that degrade the ballistic conductance of the nanotubes already at low
degrees of coverage.Comment: To be published in Phys. Rev. Let
The role of contacts in graphene transistors: A scanning photocurrent study
A near-field scanning optical microscope is used to locally induce
photocurrent in a graphene transistor with high spatial resolution. By
analyzing the spatially resolved photo-response, we find that in the n-type
conduction regime a p-n-p structure forms along the graphene device due to the
doping of the graphene by the metal contacts. The modification of the
electronic structure is not limited only underneath the metal electrodes, but
extends 0.2-0.3 um into the graphene channel. The asymmetric conduction
behavior of electrons and holes that is commonly observed in graphene
transistors is discussed in light of the potential profiles obtained from this
photocurrent imaging approach. Furthermore, we show that photocurrent imaging
can be used to probe single- / multi-layer graphene interfaces
Carbon Nanotubes as Schottky Barrier Transistors
We show that carbon nanotube transistors operate as unconventional "Schottky
barrier transistors", in which transistor action occurs primarily by varying
the contact resistance rather than the channel conductance. Transistor
characteristics are calculated for both idealized and realistic geometries, and
scaling behavior is demonstrated. Our results explain a variety of experimental
observations, including the quite different effects of doping and adsorbed
gases. The electrode geometry is shown to be crucial for good device
performance.Comment: 4 pages, 5 figures, appears in Physical Review Letter
Competition between magnetic field dependent band structure and coherent backscattering in multiwall carbon nanotubes
Magnetotransport measurements in large diameter multiwall carbon nanotubes
(20-40 nm) demonstrate the competition of a magnetic-field dependent
bandstructure and Altshuler-Aronov-Spivak oscillations. By means of an
efficient capacitive coupling to a backgate electrode, the magnetoconductance
oscillations are explored as a function of Fermi level shift. Changing the
magnetic field orientation with respect to the tube axis and by ensemble
averaging, allows to identify the contributions of different Aharonov-Bohm
phases. The results are in qualitative agreement with numerical calculations of
the band structure and the conductance.Comment: 4 figures, 5 page
Electron-phonon effects and transport in carbon nanotubes
We calculate the electron-phonon scattering and binding in semiconducting
carbon nanotubes, within a tight binding model. The mobility is derived using a
multi-band Boltzmann treatment. At high fields, the dominant scattering is
inter-band scattering by LO phonons corresponding to the corners K of the
graphene Brillouin zone. The drift velocity saturates at approximately half the
graphene Fermi velocity. The calculated mobility as a function of temperature,
electric field, and nanotube chirality are well reproduced by a simple
interpolation formula. Polaronic binding give a band-gap renormalization of ~70
meV, an order of magnitude larger than expected. Coherence lengths can be quite
long but are strongly energy dependent.Comment: 5 pages and 4 figure
- …