83 research outputs found
Transport in Nanotubes: Effect of Remote Impurity Scattering
Theory of the remote Coulomb impurity scattering in single--wall carbon
nanotubes is developed within one--electron approximation. Boltzmann equation
is solved within drift--diffusion model to obtain the tube conductivity. The
conductivity depends on the type of the nanotube bandstructure (metal or
semiconductor) and on the electron Fermi level. We found exponential dependence
of the conductivity on the Fermi energy due to the Coulomb scattering rate has
a strong dependence on the momentum transfer. We calculate intra-- and
inter--subband scattering rates and present general expressions for the
conductivity. Numerical results, as well as obtained analytical expressions,
show that the degenerately doped semiconductor tubes may have very high
mobility unless the doping level becomes too high and the inter--subband
transitions impede the electron transport.Comment: 13 pages, 4 figure
Large radius exciton in single-walled carbon nanotubes
The spectrum of large radius exciton in an individual semiconducting
single-walled carbon nanotube (SWCNT) is described within the framework of
elementary potential model, in which exciton is modeled as bound state of two
oppositely charged quasi-particles confined on the tube surface. Due to the
parity of the interaction potential the exciton states split into the odd and
even series. It is shown that for the bare and screened Coulomb electron-hole
(e-h) potentials the binding energy of even excitons in the ground state well
exceeds the energy gap. The factors preventing the collapse of single-electron
states in isolated semiconducting SWCNTs are discussed.Comment: 14 pages, 1 figure, 5 table
Photoconductivity of biased graphene
Graphene is a promising candidate for optoelectronic applications such as
photodetectors, terahertz imagers, and plasmonic devices. The origin of
photoresponse in graphene junctions has been studied extensively and is
attributed to either thermoelectric or photovoltaic effects. In addition, hot
carrier transport and carrier multiplication are thought to play an important
role. Here we report the intrinsic photoresponse in biased but otherwise
homogeneous graphene. In this classic photoconductivity experiment, the
thermoelectric effects are insignificant. Instead, the photovoltaic and a
photo-induced bolometric effect dominate the photoresponse due to hot
photocarrier generation and subsequent lattice heating through electron-phonon
cooling channels respectively. The measured photocurrent displays polarity
reversal as it alternates between these two mechanisms in a backgate voltage
sweep. Our analysis yields elevated electron and phonon temperatures, with the
former an order higher than the latter, confirming that hot electrons drive the
photovoltaic response of homogeneous graphene near the Dirac point
Thermal infrared emission reveals the Dirac point movement in biased graphene
Graphene is a 2-dimensional material with high carrier mobility and thermal
conductivity, suitable for high-speed electronics. Conduction and valence bands
touch at the Dirac point. The absorptivity of single-layer graphene is 2.3%,
nearly independent of wavelength. Here we investigate the thermal radiation
from biased graphene transistors. We find that the emission spectrum of
single-layer graphene follows that of a grey body with constant emissivity (1.6
\pm 0.8)%. Most importantly, we can extract the temperature distribution in the
ambipolar graphene channel, as confirmed by Stokes/anti-Stokes measurements.
The biased graphene exhibits a temperature maximum whose location can be
controlled by the gate voltage. We show that this peak in temperature reveals
the spatial location of the minimum in carrier density, i.e. the Dirac point.Comment: Accepted in principle at Nature Nanotechnolog
Helicity and broken symmetry of DNA-nanotube hybrids
We study breaking of the “supersymmetry" of an intrinsically achiral armchair carbon nanotube by means of a
helical perturbation. Lowering of the symmetry results in the appearance of a non-zero effective mass for nanotube low-energy
excitations, which otherwise are massless Dirac fermions. Other important consequences of the symmetry breaking are opening of
gaps in the energy spectrum and shifting of the Fermi points, which we classify according to their functional dependence on the
nanotube and helix parameters. Within each class the gaps are proportional to the inverse of the nanotube radius, and appear to
be sensitive to the exact position of the helix in a unit cell. These results are of immediate importance for the study of
DNA-nanotube complexes, and can be verified by means of optical/electron spectroscopy or tunneling microscopy
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