2,715 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
Stable unidimensional arrays of coherent strained islands
We investigate the equilibrium properties of arrays of coherent strained
islands in heteroepitaxial thin films of bidimensional materials. The model we
use takes into account only three essential ingredients : surface energies,
elastic energies of the film and of the substrate and interaction energies
between islands via the substrate. Using numerical simulations for a simple
Lennard-Jones solid, we can assess the validity of the analytical expressions
used to describe each of these contributions. A simple analytical expression is
obtained for the total energy of the system. Minimizing this energy, we show
that arrays of coherent islands can exist as stable configurations. Even in
this simple approach, the quantitative results turn out to be very sensitive to
some details of the surface energy.Comment: 24 pages, 7 figures. to be published in Surface Scienc
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
Efficient and reliable method for the simulation of scanning tunneling images and spectra with local basis sets
Based on Bardeen's perturbative approach to tunneling, we have found an
expression for the current between tip and sample, which can be efficiently
coded in order to perform fast ab initio simulations of STM images. Under the
observation that the potential between the electrodes should be nearly flat at
typical tunnel gaps, we have addressed the difficulty in the computation of the
tunneling matrix elements by considering a vacuum region of constant potential
delimited by two surfaces (each of them close to tip and sample respectively),
then propagating tip and sample wave functions by means of the vacuum Green's
function, to finally obtain a closed form in terms of convolutions. The current
is then computed for every tip-sample relative position and for every bias
voltage in one shot. The electronic structure of tip and sample is calculated
at the same footing, within density functional theory, and independently. This
allows us to carry out multiple simulations for a given surface with a database
of different tips. We have applied this method to the Si(111)-(7x7) and
Ge(111)-c(2x8) surfaces. Topographies and spectroscopic data, showing a very
good agreement with experiments, are presented.Comment: 10 pages, 11 figure
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
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