438 research outputs found
Chemical doping of individual semiconducting carbon-nanotube ropes
We report the effects of potassium doping on the conductance of individual semiconducting single-walled carbon nanotube ropes. We are able to control the level of doping by reversibly intercalating and de-intercalating potassium. Potassium doping changes the carriers in the ropes from holes to electrons. Typical values for the carrier density are found to be ∼100–1000 electrons/μm. The effective mobility for the electrons is μeff∼20–60 cm2 V-1 s-1, a value similar to that reported for the hole effective mobility in nanotubes [R. Martel et al., Appl. Phys. Lett. 73, 2447 (1998)]
Scanning Tunneling Spectroscopic Studies of the Effects of Dielectrics and Metallic Substrates on the Local Electronic Characteristics of Graphene
Atomically resolved imaging and spectroscopic characteristics of
graphene grown by chemical vapor deposition (CVD) on copper
foils are investigated and compared with those of mechanical
exfoliated graphene on SiO_2. For exfoliated graphene, the local
spectral deviations from ideal behavior may be attributed to strain
induced by the SiO_2 substrate. For CVD grown graphene, the
lattice structure appears strongly distorted by the underlying
copper, with regions in direct contact with copper showing nearly
square lattices whereas suspended regions from thermal relaxation
exhibiting nearly honeycomb or hexagonal lattice structures. The
electronic density of states (DOS) correlates closely with the
atomic arrangements of carbon, showing excess zero-bias
tunneling conductance and nearly energy-independent DOS for
strongly distorted graphene, in contrast to the linearly dispersive
DOS for suspended graphene. These results suggest that graphene
can interact strongly with both metallic and dielectric materials in
close proximity, leading to non-negligible modifications to the
electronic properties
Localization, Coulomb interactions and electrical heating in single-wall carbon nanotubes/polymer composites
Low field and high field transport properties of carbon nanotubes/polymer
composites are investigated for different tube fractions. Above the percolation
threshold f_c=0.33%, transport is due to hopping of localized charge carriers
with a localization length xi=10-30 nm. Coulomb interactions associated with a
soft gap Delta_CG=2.5 meV are present at low temperature close to f_c. We argue
that it originates from the Coulomb charging energy effect which is partly
screened by adjacent bundles. The high field conductivity is described within
an electrical heating scheme. All the results suggest that using composites
close to the percolation threshold may be a way to access intrinsic properties
of the nanotubes by experiments at a macroscopic scale.Comment: 4 pages, 5 figures, Submitted to Phys. Rev.
Disorder, pseudospins, and backscattering in carbon nanotubes
We address the effects of disorder on the conducting properties of metal and
semiconducting carbon nanotubes. Experimentally, the mean free path is found to
be much larger in metallic tubes than in doped semiconducting tubes. We show
that this result can be understood theoretically if the disorder potential is
long-ranged. The effects of a pseudospin index that describes the internal
sublattice structure of the states lead to a suppression of scattering in
metallic tubes, but not in semiconducting tubes. This conclusion is supported
by tight-binding calculations.Comment: four page
Non-volatile molecular memory elements based on ambipolar nanotube field effect transistors
We have fabricated air-stable n-type, ambipolar carbon nanotube field effect
transistors (CNFETs), and used them in nanoscale memory cells. N-type
transistors are achieved by annealing of nanotubes in hydrogen gas and
contacting them by cobalt electrodes. Scanning gate microscopy reveals that the
bulk response of these devices is similar to gold-contacted p-CNFETs,
confirming that Schottky barrier formation at the contact interface determines
accessibility of electron and hole transport regimes. The transfer
characteristics and Coulomb Blockade (CB) spectroscopy in ambipolar devices
show strongly enhanced gate coupling, most likely due to reduction of defect
density at the silicon/silicon-dioxide interface during hydrogen anneal. The CB
data in the ``on''-state indicates that these CNFETs are nearly ballistic
conductors at high electrostatic doping. Due to their nanoscale capacitance,
CNFETs are extremely sensitive to presence of individual charge around the
channel. We demonstrate that this property can be harnessed to construct data
storage elements that operate at the few-electron level.Comment: 6 pages text, 3 figures and 1 table of content graphic; available as
NanoLetters ASAP article on the we
High-Field Electrical Transport in Single-Wall Carbon Nanotubes
Using low-resistance electrical contacts, we have measured the intrinsic
high-field transport properties of metallic single-wall carbon nanotubes.
Individual nanotubes appear to be able to carry currents with a density
exceeding 10^9 A/cm^2. As the bias voltage is increased, the conductance drops
dramatically due to scattering of electrons. We show that the current-voltage
characteristics can be explained by considering optical or zone-boundary phonon
emission as the dominant scattering mechanism at high field.Comment: 4 pages, 3 eps figure
Evidence for Strain-Induced Local Conductance Modulations in Single-Layer Graphene on SiO_2
Graphene has emerged as an electronic material that is promising for device applications and for studying two-dimensional electron gases with relativistic dispersion near two Dirac points. Nonetheless, deviations from Dirac-like spectroscopy have been widely reported with varying interpretations. Here we show evidence for strain-induced spatial modulations in the local conductance of single-layer graphene on SiO_2 substrates from scanning tunneling microscopic (STM) studies. We find that strained graphene exhibits parabolic, U-shaped conductance vs bias voltage spectra rather than the V-shaped spectra expected for Dirac fermions, whereas V-shaped spectra are recovered in regions of relaxed graphene. Strain maps derived from the STM studies further reveal direct correlation with the local tunneling conductance. These results are attributed to a strain-induced frequency increase in the out-of-plane phonon mode that mediates the low-energy inelastic charge tunneling into graphene
Kondo physics in carbon nanotubes
The connection of electrical leads to wire-like molecules is a logical step
in the development of molecular electronics, but also allows studies of
fundamental physics. For example, metallic carbon nanotubes are quantum wires
that have been found to act as one-dimensional quantum dots, Luttinger-liquids,
proximity-induced superconductors and ballistic and diffusive one-dimensional
metals. Here we report that electrically-contacted single-wall nanotubes can
serve as powerful probes of Kondo physics, demonstrating the universality of
the Kondo effect. Arising in the prototypical case from the interaction between
a localized impurity magnetic moment and delocalized electrons in a metallic
host, the Kondo effect has been used to explain enhanced low-temperature
scattering from magnetic impurities in metals, and also occurs in transport
through semiconductor quantum dots. The far higher tunability of dots (in our
case, nanotubes) compared with atomic impurities renders new classes of
Kondo-like effects accessible. Our nanotube devices differ from previous
systems in which Kondo effects have been observed, in that they are
one-dimensional quantum dots with three-dimensional metal (gold) reservoirs.
This allows us to observe Kondo resonances for very large electron number (N)
in the dot, and approaching the unitary limit (where the transmission reaches
its maximum possible value). Moreover, we detect a previously unobserved Kondo
effect, occurring for even values of N in a magnetic field.Comment: 7 pages, pdf onl
Scanned Probe Microscopy of Electronic Transport in Carbon Nanotubes
We use electrostatic force microscopy and scanned gate microscopy to probe
the conducting properties of carbon nanotubes at room temperature. Multi-walled
carbon nanotubes are shown to be diffusive conductors, while metallic
single-walled carbon nanotubes are ballistic conductors over micron lengths.
Semiconducting single-walled carbon nanotubes are shown to have a series of
large barriers to conduction along their length. These measurements are also
used to probe the contact resistance and locate breaks in carbon nanotube
circuits.Comment: 4 page
Quantized Adiabatic Charge Transport in a Carbon Nanotube
The coupling of a metallic Carbon nanotube to a surface acoustic wave (SAW)
is proposed as a vehicle to realize quantized adiabatic charge transport in a
Luttinger liquid system. We demonstrate that electron backscattering by a
periodic SAW potential, which results in miniband formation, can be achieved at
energies near the Fermi level. Electron interaction, treated in a Luttinger
liquid framework, is shown to enhance minigaps and thereby improve current
quantization. Quantized SAW induced current, as a function of electron density,
changes sign at half-filling.Comment: 5 pages, 2 figure
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