839 research outputs found
Etching-dependent reproducible memory switching in vertical SiO2 structures
Vertical structures of SiO sandwiched between a top tungsten electrode
and conducting non-metal substrate were fabricated by dry and wet etching
methods. Both structures exhibit similar voltage-controlled memory behaviors,
in which short voltage pulses (1 s) can switch the devices between high-
and low-impedance states. Through the comparison of current-voltage
characteristics in structures made by different methods, filamentary conduction
at the etched oxide edges is most consistent with the results, providing
insights into similar behaviors in metal/SiO/metal systems. High ON/OFF ratios
of over 10 were demonstrated.Comment: 6 pages, 3 figures + 2 suppl. figure
Three-terminal devices to examine single molecule conductance switching
We report electronic transport measurements of single-molecule transistor
devices incorporating bipyridyl-dinitro oligophenylene-ethynylene dithiol
(BPDN-DT), a molecule known to exhibit conductance switching in other
measurement configurations. We observe hysteretic conductance switching in 8%
of devices with measurable currents, and find that dependence of the switching
properties on gate voltage is rare when compared to other single-molecule
transistor devices. This suggests that polaron formation is unlikely to be
responsible for switching in these devices. We discuss this and alternative
switching mechanisms.Comment: 5 pages, 4 figures. Supporting material available upon reques
Controlling charge injection in organic field-effect transistors using self-assembled monolayers
We have studied charge injection across the metal/organic semiconductor
interface in bottom-contact poly(3-hexylthiophene) (P3HT) field-effect
transistors, with Au source and drain electrodes modified by self-assembled
monolayers (SAMs) prior to active polymer deposition. By using the SAM to
engineer the effective Au work function, we markedly affect the charge
injection process. We systematically examine the contact resistivity and
intrinsic channel mobility, and show that chemically increasing the injecting
electrode work function significantly improves hole injection relative to
untreated Au electrodes.Comment: 5 pages, 2 figures. Supplementary information available upon reques
Impact of edge shape on the functionalities of graphene-based single-molecule electronics devices
We present an ab-initio analysis of the impact of edge shape and
graphene-molecule anchor coupling on the electronic and transport
functionalities of graphene-based molecular electronics devices. We analyze how
Fano-like resonances, spin filtering and negative differential resistance
effects may or may not arise by modifying suitably the edge shapes and the
terminating groups of simple organic molecules. We show that the spin filtering
effect is a consequence of the magnetic behavior of zigzag-terminated edges,
which is enhanced by furnishing these with a wedge shape. The negative
differential resistance effect is originated by the presence of two degenerate
electronic states localized at each of the atoms coupling the molecule to
graphene which are strongly affected by a bias voltage. The effect could thus
be tailored by a suitable choice of the molecule and contact atoms if edge
shape could be controlled with atomic precision.Comment: 11 pages, 20 figure
Kondo resonances and anomalous gate dependence of electronic conduction in single-molecule transistors
We report Kondo resonances in the conduction of single-molecule transistors
based on transition metal coordination complexes. We find Kondo temperatures in
excess of 50 K, comparable to those in purely metallic systems. The observed
gate dependence of the Kondo temperature is inconsistent with observations in
semiconductor quantum dots and a simple single-dot-level model. We discuss
possible explanations of this effect, in light of electronic structure
calculations.Comment: 5 pages, four figures. Supplementary material at
http://www.ruf.rice.edu/~natelson/publications.htm
Universal Scaling of Nonequilibrium Transport in the Kondo Regime of Single Molecule Devices
Scaling laws and universality are often associated with systems exhibiting
emergent phenomena possessing a characteristic energy scale. We report
nonequilibrium transport measurements on two different types of single-molecule
transistor (SMT) devices in the Kondo regime. The conductance at low bias and
temperature adheres to a scaling function characterized by two parameters. This
result, analogous to that reported recently in semiconductor dots with Kondo
temperatures two orders of magnitude lower, demonstrates the universality of
this scaling form. We compare the extracted values of the scaling coefficients
to previous experimental and theoretical results.Comment: 4.5 pages, 3 figure
Inelastic electron tunneling via molecular vibrations in single-molecule transistors
In single-molecule transistors, we observe inelastic cotunneling features
that correspond energetically to vibrational excitations of the molecule, as
determined by Raman and infrared spectroscopy. This is a form of inelastic
electron tunneling spectroscopy of single molecules, with the transistor
geometry allowing in-situ tuning of the electronic states via a gate electrode.
The vibrational features shift and change shape as the electronic levels are
tuned near resonance, indicating significant modification of the vibrational
states. When the molecule contains an unpaired electron, we also observe
vibrational satellite features around the Kondo resonance.Comment: 5 pages, 4 figures. Supplementary information available upon reques
Bi-stable tunneling current through a molecular quantum dot
An exact solution is presented for tunneling through a negative-U d-fold
degenerate molecular quantum dot weakly coupled to electrical leads. The tunnel
current exhibits hysteresis if the level degeneracy of the negative-U dot is
larger than two (d>2). Switching occurs in the voltage range V1 < V < V2 as a
result of attractive electron correlations in the molecule, which open up a new
conducting channel when the voltage is above the threshold bias voltage V2.
Once this current has been established, the extra channel remains open as the
voltage is reduced down to the lower threshold voltage V1. Possible
realizations of the bi-stable molecular quantum dots are fullerenes, especially
C60, and mixed-valence compounds.Comment: 5 pages, 1 figure. (v2) Figure updated to compare the current
hysteresis for degeneracies d=4 and d>>1 of the level in the dot, minor
corrections in the text. To appear in Phys. Rev.
Stepwise Quenching of Exciton Fluorescence in Carbon Nanotubes by Single Molecule Reactions
Single-molecule chemical reactions with individual single-walled carbon
nanotubes were observed through near-infrared photoluminescence microscopy. The
emission intensity within distinct submicrometer segments of single nanotubes
changes in discrete steps after exposure to acid, base, or diazonium reactants.
The steps are uncorrelated in space and time, and reflect the quenching of
mobile excitons at localized sites of reversible or irreversible chemical
attack. Analysis of step amplitudes reveals an exciton diffusional range of
about 90 nanometers, independent of nanotube structure. Each exciton visits
approximately 104 atomic sites during its lifetime, providing highly efficient
sensing of local chemical and physical perturbations
Circular-Polarization Dependent Cyclotron Resonance in Large-Area Graphene in Ultrahigh Magnetic Fields
Using ultrahigh magnetic fields up to 170 T and polarized midinfrared
radiation with tunable wavelengths from 9.22 to 10.67 um, we studied cyclotron
resonance in large-area graphene grown by chemical vapor deposition.
Circular-polarization dependent studies reveal strong p-type doping for
as-grown graphene, and the dependence of the cyclotron resonance on radiation
wavelength allows for a determination of the Fermi energy. Thermal annealing
shifts the Fermi energy to near the Dirac point, resulting in the simultaneous
appearance of hole and electron cyclotron resonance in the magnetic quantum
limit, even though the sample is still p-type, due to graphene's linear
dispersion and unique Landau level structure. These high-field studies
therefore allow for a clear identification of cyclotron resonance features in
large-area, low-mobility graphene samples.Comment: 9 pages, 3 figure
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