249 research outputs found
Transport Studies of Isolated Molecular Wires in Self-Assembled Monolayer Devices
We have fabricated a variety of novel molecular diodes based on
self-assembled-monolayers (SAM) of solid-state mixture of molecular wires (1,4
benzene-dimethane-thiol), and molecular insulator spacers (1-pentanethiol) with
different concentration ratios r of wires/spacers, which were sandwiched
between two gold (Au) electrodes. We introduce two new methods borrowed from
Surface Science to (i) confirm the connectivity between the
benzene-dimethane-thiol molecules with the upper Au electrode, and (ii) count
the number of isolated molecular wires in the devices.
The electrical transport properties of the SAM diodes were studied at
different temperatures via the conductance and differential conductance
spectra. We found that a potential barrier caused by the spatial connectivity
gap between the pentanethiol molecules and the upper Au electrode dominates the
transport properties of the pure pentanethiol SAM diode (r = 0). The transport
properties of molecular diodes with low r-values are dominated by the
conductance of the isolated benzene-dimethane-thiol molecules in the device. We
found that the temperature dependence of the molecular diodes is much weaker
than that of the pure pentanethiol device indicating the importance of the
benzene-dimethane-thiol simultaneous bonding to the two Au electrodes that
facilitate electrical transport. From the differential conductance spectra we
also found that the energy difference, Delta between the Au electrode
Fermi-level and the benzene-dimethane-thiol HOMO (or LUMO) level is ~1.5 eV;
whereas it is ~2.5 eV for the pentanethiol molecule. The weak temperature
dependent transport that we obtained for the SSM diodes reflects the weak
temperature dependence of Delta.Comment: 38 p 8 Fi
Comment on Frequency response and origin of the spin-1/2 photolumines-cence-detected magnetic resonance in a pi-conjugated polymer
In a recent paper Segal et al. [1] attempted to explain the dynamics of spin
1/2 photoluminescence detected magnetic resonance (PLDMR) in films of a
pi-conjugated polymer, namely a soluble de-rivative of poly(phenylene-vinylene)
[MEH-PPV] using a model (dubbed TPQ), in which the PLDMR is due to spin
dependent triplet-polaron interactions that reduce the polarons density and
consequent quenching of singlet excitons. We studied the full PLDMR and
photoinduced ab-sorption (PA) dynamics of MEH-PPV films as a function of
microwave power at various tempera-tures. We show, firstly, that the TPQ model
is incompatible with the full frequency dependent spin 1/2 PLDMR response;
secondly, it is not in agreement with the spin-1 PLDMR temperature dependence;
thirdly, it predicts a much shorter triplet exciton lifetime than that obtained
experimentally; and fourthly, that is in contradiction with the temperature
dependencies of spin 1/2 PLDMR and triplet exciton PA. In contrast, an
alternative model, namely the spin dependent re-combination of polarons, is
capable of explaining the whole body of experimental results, and in particular
the PLDMR dynamics.Comment: 7 pages, 3 figure
Ultrafast response of surface electromagnetic waves in an aluminum film perforated with subwavelength hole arrays
The ultrafast dynamics of surface electromagnetic waves photogenerated on
aluminum film perforated with subwavelength holes array was studied in the
visible spectral range by the technique of transient photomodulation with 100
fs time resolution. We observed a pronounced blueshift of the resonant
transmission band that reveals the important role of plasma attenuation in the
optical response of nanohole arrays. The blueshift is inconsistent with
plasmonic mechanism of extraordinary transmission and points to the crucial
role of interference in the formation of transmission bands. The transient
photomodulation spectra were successfully modeled within the Boltzmann equation
approach for the electron-phonon relaxation dynamics, involving non-equilibrium
hot electrons and quasi-equilibrium phonons.Comment: 4 pages, 3 figure
Organic spin-valves: physics and applications
Journal ArticleSpin-valve devices of organic semiconductors in the vertical configuration using a variety of exotic and regular ferromagnetic electrodes were fabricated and studied as a function of applied magnetic field, temperature and applied bias voltage. These devices show that spin polarized carriers can be injected from ferromagnetic electrodes into organic semiconductors and diffuse without loss of spin polarization for distances of the order of 100 nm at low temperatures
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