1,511 research outputs found
Radiative Lifetime of Excitons in Carbon Nanotubes
We calculate the radiative lifetime and energy bandstructure of excitons in
semiconducting carbon nanotubes, within a tight-binding approach. In the limit
of rapid interband thermalization, the radiative decay rate is maximized at
intermediate temperatures, decreasing at low temperature because the
lowest-energy excitons are optically forbidden. The intrinsic phonons cannot
scatter excitons between optically active and forbidden bands, so
sample-dependent extrinsic effects that break the symmetries can play a central
role. We calculate the diameter-dependent energy splittings between singlet and
triplet excitons of different symmetries, and the resulting dependence of
radiative lifetime on temperature and tube diameter.Comment: 4 pages, 3 figure
Mobility in semiconducting carbon nanotubes at finite carrier density
Carbon nanotube field-effect transistors operate over a wide range of
electron or hole density, controlled by the gate voltage. Here we calculate the
mobility in semiconducting nanotubes as a function of carrier density and
electric field, for different tube diameters and temperature. The low-field
mobility is a non-monotonic function of carrier density, and varies by as much
as a factor of 4 at room temperature. At low density, with increasing field the
drift velocity reaches a maximum and then exhibits negative differential
mobility, due to the non-parabolicity of the bandstructure. At a critical
density 0.35-0.5 electrons/nm, the drift velocity saturates at
around one third of the Fermi velocity. Above , the velocity increases
with field strength with no apparent saturation.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
Fermi-level alignment at metal-carbon nanotube interfaces: application to scanning tunneling spectroscopy
At any metal-carbon nanotube interface there is charge transfer and the
induced interfacial field determines the position of the carbon nanotube band
structure relative to the metal Fermi-level. In the case of a single-wall
carbon nanotube (SWNT) supported on a gold substrate, we show that the charge
transfers induce a local electrostatic potential perturbation which gives rise
to the observed Fermi-level shift in scanning tunneling spectroscopy (STS)
measurements. We also discuss the relevance of this study to recent experiments
on carbon nanotube transistors and argue that the Fermi-level alignment will be
different for carbon nanotube transistors with low resistance and high
resistance contacts.Comment: 4 pages, 3 ps figures, minor corrections, accepted by Phys. Rev. Let
Valence Force Model for Phonons in Graphene and Carbon Nanotubes
Many calculations require a simple classical model for the interactions
between sp^2-bonded carbon atoms, as in graphene or carbon nanotubes. Here we
present a new valence force model to describe these interactions. The
calculated phonon spectrum of graphene and the nanotube breathing-mode energy
agree well with experimental measurements and with ab initio calculations. The
model does not assume an underlying lattice, so it can also be directly applied
to distorted structures. The characteristics and limitations of the model are
discussed.Comment: 4 pages, 3 figure
Device modeling of long-channel nanotube electro-optical emitter
We present a simple analytic model of nanotube electro-optical emitters,
along with improved experimental measurements using PMMA-passivated devices
with reduced hysteresis. Both the ambipolar electrical characteristics and the
motion of the infrared-emission spot are well described. The model indicates
that the electric field is strongly enhanced at the emission spot, and that
device performance can be greatly improved by the use of thinner gate oxides
Self-organization of steps in growth of strained films on vicinal substrates
Journal ArticleComputer simulations show that if stress is present, steps on a vicinal surface can self-organize into a regular array of step bunches. Such self-organization can provide templates for subsequent fabrication of "quantum wire" nanostructures. The size and spacing of the bunches can be controlled independently. We analyze the requirements for optimal ordering
Large Scale Electronic Structure Calculations with Multigrid Acceleration
We have developed a set of techniques for performing large scale ab initio
calculations using multigrid accelerations and a real-space grid as a basis.
The multigrid methods permit efficient calculations on ill-conditioned systems
with long length scales or high energy cutoffs. The technique has been applied
to systems containing up to 100 atoms, including a highly elongated diamond
cell, an isolated C molecule, and a 32-atom cell of GaN with the Ga
d-states in valence. The method is well suited for implementation on both
vector and massively parallel architectures.Comment: 4 pages, 1 postscript figur
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
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