794 research outputs found
10 to 50 nm Long Quasi Ballistic Carbon Nanotube Devices Obtained Without Complex Lithography
A simple method combining photolithography and shadow (or angle) evaporation
is developed to fabricate single-walled carbon nanotube (SWCNT) devices with
tube lengths L~10-50 nm between metal contacts. Large numbers of such short
devices are obtained without the need of complex tools such as electron beam
lithography. Metallic SWCNTs with lengths ~ 10 nm, near the mean free path
(mfp) of lop~15 nm for optical phonon scattering, exhibit near-ballistic
transport at high biases and can carry unprecedented 100 mA currents per tube.
Semiconducting SWCNT field-effect transistors (FETs) with ~ 50 nm channel
lengths are routinely produced to achieve quasi-ballistic operations for
molecular transistors. The results demonstrate highly length-scaled and
high-performance interconnects and transistors realized with SWCNTs.Comment: PNAS, in pres
Monte Carlo study of coaxially gated CNTFETs: capacitive effects and dynamic performance
Carbon Nanotube (CNT) appears as a promising candidate to shrink field-effect
transistors (FET) to the nanometer scale. Extensive experimental works have
been performed recently to develop the appropriate technology and to explore DC
characteristics of carbon nanotube field effect transistor (CNTFET). In this
work, we present results of Monte Carlo simulation of a coaxially gated CNTFET
including electron-phonon scattering. Our purpose is to present the intrinsic
transport properties of such material through the evaluation of electron
mean-free-path. To highlight the potential of high performance level of CNTFET,
we then perform a study of DC characteristics and of the impact of capacitive
effects. Finally, we compare the performance of CNTFET with that of Si nanowire
MOSFET.Comment: 15 pages, 14 figures, final version to be published in C. R. Acad.
Sci. Pari
Mechanism of Ambipolar Field-Effect Carrier Injections in One-Dimensional Mott Insulators
To clarify the mechanism of recently reported, ambipolar carrier injections
into quasi-one-dimensional Mott insulators on which field-effect transistors
are fabricated, we employ the one-dimensional Hubbard model attached to a
tight-binding model for source and drain electrodes. To take account of the
formation of Schottky barriers, we add scalar and vector potentials, which
satisfy the Poisson equation with boundary values depending on the drain
voltage, the gate bias, and the work-function difference. The current-voltage
characteristics are obtained by solving the time-dependent Schr\"odinger
equation in the unrestricted Hartree-Fock approximation. Its validity is
discussed with the help of the Lanczos method applied to small systems. We find
generally ambipolar carrier injections in Mott insulators even if the work
function of the crystal is quite different from that of the electrodes. They
result from balancing the correlation effect with the barrier effect. For the
gate-bias polarity with higher Schottky barriers, the correlation effect is
weakened accordingly, owing to collective transport in the one-dimensional
correlated electron systems.Comment: 21 pages, 10 figures, to appear in J. Phys. Soc. Jp
Unexpected Scaling of the Performance of Carbon Nanotube Transistors
We show that carbon nanotube transistors exhibit scaling that is
qualitatively different than conventional transistors. The performance depends
in an unexpected way on both the thickness and the dielectric constant of the
gate oxide. Experimental measurements and theoretical calculations provide a
consistent understanding of the scaling, which reflects the very different
device physics of a Schottky barrier transistor with a quasi-one-dimensional
channel contacting a sharp edge. A simple analytic model gives explicit scaling
expressions for key device parameters such as subthreshold slope, turn-on
voltage, and transconductance.Comment: 4 pages, 4 figure
Ground-state energy of the electron liquid in ultrathin wires
The ground-state energy and the density correlation function of the electron
liquid in a thin one-dimensional wire are computed. The calculation is based on
an approximate mapping of the problem with a realistic Coulomb interaction law
onto exactly solvable models of mathematical physics. This approach becomes
asymptotically exact in the limit of small wire radius but remains numerically
accurate even for modestly thin wires.Comment: (v3) Replaced with the published version. 4 pages, 1 figur
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Si photocathode with Ag-supported dendritic Cu catalyst for CO2 reduction
Si photocathodes integrated with Ag-supported dendritic Cu catalysts are used to perform light-driven reduction of CO2 to C2 and C3 products in aqueous solution. A back illumination geometry with an n-type Si absorber was used to permit the use of absorbing metallic catalysts. Selective carrier collection was accomplished by a p+ implantation on the illumination side and an n+ implantation followed by atomic layer deposition of TiO2 on the electrolyte site. The Ag-supported dendritic Cu CO2 reduction catalyst was formed by evaporation of Ag followed by high-rate electrodeposition of Cu to form a high surface area structure. Under simulated 1 sun illumination in 0.1 M CsHCO3 saturated with CO2, the photovoltage generated by the Si (∼600 mV) enables C2 and C3 products to be produced at -0.4 vs. RHE. Texturing of both sides of the Si increases the light-limited current density, due to reduced reflection on the illumination side, and also deceases the onset potential. Under simulated diurnal illumination conditions photocathodes maintain over 60% faradaic efficiency to hydrocarbon and oxygenate products (mainly ethylene, ethanol, propanol) for several days. After 10 days of testing, contamination from the counter electrode is observed, which causes an increase in hydrogen production. This effect is mitigated by a regeneration procedure which restores the original catalyst selectivity. A tandem, self-powered CO2 reduction device was formed by coupling a Si photocathode with two series-connected semitransparent CH3NH3PbI3 perovskite solar cells, achieving an efficiency for the conversion of sunlight to hydrocarbons and oxygenates of 1.5% (3.5% for all products)
Microwave Transport in Metallic Single-Walled Carbon Nanotubes
The dynamical conductance of electrically contacted single-walled carbon
nanotubes is measured from dc to 10 GHz as a function of source-drain voltage
in both the low-field and high-field limits. The ac conductance of the nanotube
itself is found to be equal to the dc conductance over the frequency range
studied for tubes in both the ballistic and diffusive limit. This clearly
demonstrates that nanotubes can carry high-frequency currents at least as well
as dc currents over a wide range of operating conditions. Although a detailed
theoretical explanation is still lacking, we present a phenomenological model
of the ac impedance of a carbon nanotube in the presence of scattering that is
consistent with these results.Comment: Added reference
Carbon Nanotube Field-Effect Transistors With Integrated Ohmic Contacts and High-k Gate Dielectrics
High performance enhancement mode semiconducting carbon nanotube field-effect
transistors (CNTFETs) are obtained by combining ohmic metal-tube contacts, high
dielectric constant HfO2 films as gate insulators, and electrostatically doped
nanotube segments as source/drain electrodes. The combination of these elements
affords high ON currents, subthreshold swings of ~ 70-80 mV/decade, and allows
for low OFF currents and suppressed ambipolar conduction. The doped source and
drain approach resembles that of MOSFETs and can impart excellent OFF states to
nanotube FETs under aggressive vertical scaling. This presents an important
advantage over devices with metal source/drain, or devices commonly referred to
as Schottky barrier FETs
Scaling analysis of Schottky barriers at metal-embedded semiconducting carbon nanotube interfaces
We present an atomistic self-consistent tight-binding study of the electronic
and transport properties of metal-semiconducting carbon nanotube interfaces as
a function of the nanotube channel length when the end of the nanotube wire is
buried inside the electrodes. We show that the lineup of the nanotube band
structure relative to the metal Fermi-level depends strongly on the metal work
function but weakly on the details of the interface. We analyze the
length-dependent transport characteristics, which predicts a transition from
tunneling to thermally-activated transport with increasing nanotube channel
length.Comment: To appear in Phys.Rev.B Rapid Communications. Color figures available
in PRB online versio
Suppressing Diffusion-Mediated Exciton Annihilation in 2D Semiconductors Using the Dielectric Environment
Atomically thin semiconductors such as monolayer MoS2 and WS2 exhibit
nonlinear exciton-exciton annihilation at notably low excitation densities
(below ~10 excitons/um2 in MoS2). Here, we show that the density threshold at
which annihilation occurs can be tuned by changing the underlying substrate.
When the supporting substrate is changed from SiO2 to Al2O3 or SrTiO3, the rate
constant for second-order exciton-exciton annihilation, k_XX [cm2/s], is
reduced by one or two orders of magnitude, respectively. Using transient
photoluminescence microscopy, we measure the effective room-temperature exciton
diffusion coefficient in chemical-treated MoS2 to be D = 0.06 +/- 0.01 cm2/s,
corresponding to a diffusion length of LD = 350 nm for an exciton lifetime of
{\tau} = 20 ns, which is independent of the substrate. These results, together
with numerical simulations, suggest that the effective exciton-exciton
annihilation radius monotonically decreases with increasing refractive index of
the underlying substrate. Exciton-exciton annihilation limits the overall
efficiency of 2D semiconductor devices operating at high exciton densities; the
ability to tune these interactions via the dielectric environment is an
important step toward more efficient optoelectronic technologies featuring
atomically thin materials
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