100 research outputs found
Relaxation dynamics in covalently bonded organic monolayers on silicon
We study the dynamic electrical response of a silicon-molecular
monolayer-metal junctions and we observe two contributions in the admittance
spectroscopy data. These contributions are related to dipolar relaxation and
molecular organization in the monolayer in one hand, and the presence of
defects at the silicon/molecule interface in the other hand. We propose a small
signal equivalent circuit suitable for the simulations of these molecular
devices in commercial device simulators. Our results concern monolayers of
alkyl chains considered as a model system but can be extended to other
molecular monolayers. These results open door to a better control and
optimization of molecular devices.Comment: 1 pdf file including text, figures and tables. Phys. Rev. B, in pres
Decoherence-free creation of atom-atom entanglement in cavity via fractional adiabatic passage
We propose a robust and decoherence insensitive scheme to generate
controllable entangled states of two three-level atoms interacting with an
optical cavity and a laser beam. Losses due to atomic spontaneous transitions
and to cavity decay are efficiently suppressed by employing fractional
adiabatic passage and appropriately designed atom-field couplings. In this
scheme the two atoms traverse the cavity-mode and the laser beam in opposite
directions as opposed to other entanglement schemes in which the atoms are
required to have fixed locations inside a cavity. We also show that the
coherence of a traveling atom can be transferred to the other one without
populating the cavity-mode.Comment: 4 pages, 5 figures, Submitted to Phys. Re
Optical gratings induced by field-free alignment of molecules
We analyze the alignment of molecules generated by a pair of crossed
ultra-short pump pulses of different polarizations by a technique based on the
induced time-dependent gratings. Parallel polarizations yield an intensity
grating, while perpendicular polarizations induce a polarization grating. We
show that both configurations can be interpreted at moderate intensity as an
alignment induced by a single polarized pump pulse. The advantage of the
perpendicular polarizations is to give a signal of alignment that is free from
the plasma contribution. Experiments on femtosecond transient gratings with
aligned molecules were performed in CO2 at room temperature in a static cell
and at 30 K in a molecular expansion jet.Comment: 10 pages, 5 figures, submitted to PR
An artificial spiking synapse made of molecules and nanoparticles
Molecule-based devices are envisioned to complement silicon devices by providing new functions or already existing functions at a simpler process level and at a lower cost by virtue of their self-organization capabilities, moreover, they are not bound to von Neuman architecture and this may open the way to other architectural paradigms. Here we demonstrate a device made of conjugated molecules and metal nanoparticles (NPs) which behaves as a spiking synapse suitable for integration in neural network architectures. We demonstrate that this device exhibits the main behavior of a biological synapse. These results open the way to rate coding utilization of the NOMFET in perceptron and Hopfield networks. We can also envision the NOMFET as a building block of neuroelectronics for interfacing neurons or neuronal logic devices made from patterned neuronal cultures with solid-state devices and circuits
High Conductance Ratio in Molecular Optical Switching of Functionalized Nanoparticle Self-Assembled Nanodevices
Self-assembled functionalized nano particles are at the focus of a number of
potential applications, in particular for molecular scale electronics devices.
Here we perform experiments of self-assembly of 10 nm Au nano particles (NPs),
functionalized by a dense layer of azobenzene-bithiophene (AzBT) molecules,
with the aim of building a light-switchable device with memristive properties.
We fabricate planar nanodevices consisting of NP self-assembled network
(NPSANs) contacted by nanoelectrodes separated by interelectrode gaps ranging
from 30 to 100 nm. We demonstrate the light-induced reversible switching of the
electrical conductance in these AzBT NPSANs with a record on/off conductance
ratio up to 620, an average value of ca. 30 and with 85% of the devices having
a ratio above 10. Molecular dynamics simulation of the structure and dynamics
of the interface between molecular monolayers chemisorbed on the nano particle
surface are performed and compared to the experimental findings. The properties
of the contact interface are shown to be strongly correlated to the molecular
conformation which in the case of AzBT molecules, can reversibly switched
between a cis and a trans form by means of light irradiations of well-defined
wavelength. Molecular dynamics simulations provide a microscopic explanation
for the experimental observation of the reduction of the on/off current ratio
between the two isomers, compared to experiments performed on flat
self-assembled monolayers contacted by a conducting cAFM tip.Comment: pdf files : publication and supporting informatio
Molecular Signature of Polyoxometalates in Electron Transport of Silicon-based Molecular Junctions
Polyoxometalates (POMs) are unconventional electro-active molecules with a
great potential for applications in molecular memories, providing efficient
processing steps onto electrodes are available. The synthesis of the
organic-inorganic polyoxometalate hybrids
[PMO(Sn(CH)CC(CH)N)] (M = Mo,
W) endowed with a remote diazonium function is reported together with their
covalent immobilization onto hydrogenated n-Si(100) substrates. Electron
transport measurements through the resulting densely-packed monolayers
contacted with a mercury drop as a top electrode confirms their homogeneity.
Adjustment of the current-voltage curves with the Simmons equation gives a mean
tunnel energy barrier of 1.8 eV and 1.6 eV, for the Silicon-Molecules-Metal
(SMM) junctions based on the polyoxotungstates (M = W) and polyoxomolybdates (M
= Mo), respectively. This follows the trend observed in the electrochemical
properties of POMs in solution, the polyoxomolybdates being easier to reduce
than the polyoxotungstates, in agreement with lowest unoccupied molecular
orbitals (LUMOs) of lower energy. The molecular signature of the POMs is thus
clearly identifiable in the solid-state electrical properties and the unmatched
diversity of POM molecular and electronic structures should offer a great
modularity
Influence of Molecular Organization on the Electrical Characteristics of {\pi}-conjugated Self-assembled Monolayers
Two new thiol compounds with {\sigma}-{\pi}-{\sigma} structure were
synthesized and self-assembled on gold substrates. The morphology and the
structural characterization of SAMs assessed by infrared spectroscopy, contact
angle, XPS, electrochemistry and scanning tunneling microscopy (STM) show the
formation of monolayers. SAMs with a terthiophene (3TSH) core as conjugated
system are much better organized compared to those with a naphthalene
carbodiimide (NaphSH) core as demonstrated by the cyclic voltammetry and STM
studies. The surface concentration of 3TSH and NaphSH is respectively three and
six times lower than ordered SAMs of pure alkyl chains. A large number of I/V
characteristics have been studied either by STS measurements on gold substrates
or by C-AFM on gold nanodots. Transition Voltage Spectroscopy (TVS) was used to
clearly identify the transport in these partially organized monolayers. The
chemical nature of the conjugated system, donor for 3TSH and acceptor for
NaphSH, involves an opposite rectification associated to the asymmetrical
coupling of the molecular orbitals and the electrodes. The conductance
histograms show that the 3TSH junctions are less dispersed than those of NaphSH
junctions. This is explained by a better control of the molecular organization
in the molecular junctions.Comment: Full paper with supporting informatio
Lactobacillus rhamnosus GG encapsulation by spray-drying: milk proteins clotting control to produce innovative matrices
A well-known probiotic strain, L.\ua0rhamnosus GG, was encapsulated by spray-drying in milk water-insoluble matrices upon reconstitution in hot water by exploiting and controlling the clotting reaction of milk proteins during the process. The feed solution, composed of probiotic bacteria and milk proteins, was or not subjected to the action of chymosin, a proteolytic enzyme. To optimize microencapsulation efficiency, different outlet air temperatures were tested (55, 70 and 85\ua0°C). After spray-drying, small microparticles were recovered for further characterization. All drying conditions led to excellent bacterial survival rates
Evaluation of three parasite lactate dehydrogenase-based rapid diagnostic tests for the diagnosis of falciparum and vivax malaria
BACKGROUND: In areas where non-falciparum malaria is common rapid diagnostic tests (RDTs) capable of distinguishing malaria species reliably are needed. Such tests are often based on the detection of parasite lactate dehydrogenase (pLDH). METHODS: In Dawei, southern Myanmar, three pLDH based RDTs (CareStart Malaria pLDH (Pan), CareStart Malaria pLDH (Pan, Pf) and OptiMAL-IT)were evaluated in patients presenting with clinically suspected malaria. Each RDT was read independently by two readers. A subset of patients with microscopically confirmed malaria had their RDTs repeated on days 2, 7 and then weekly until negative. At the end of the study, samples of study batches were sent for heat stability testing. RESULTS: Between August and November 2007, 1004 patients aged between 1 and 93 years were enrolled in the study. Slide microscopy (the reference standard) diagnosed 213 Plasmodium vivax (Pv) monoinfections, 98 Plasmodium falciparum (Pf) mono-infections and no malaria in 650 cases. The sensitivities (sens) and specificities (spec), of the RDTs for the detection of malaria were- CareStart Malaria pLDH (Pan) test: sens 89.1% [CI95 84.2-92.6], spec 97.6% [CI95 96.5-98.4]. OptiMal-IT: Pf+/- other species detection: sens 95.2% [CI95 87.5-98.2], spec 94.7% [CI95 93.3-95.8]; non-Pf detection alone: sens 89.6% [CI95 83.6-93.6], spec 96.5% [CI95 94.8-97.7]. CareStart Malaria pLDH (Pan, Pf): Pf+/- other species: sens 93.5% [CI95 85.4-97.3], spec 97.4% [95.9-98.3]; non-Pf: sens 78.5% [CI95 71.1-84.4], spec 97.8% [CI95 96.3-98.7]. Inter-observer agreement was excellent for all tests (kappa > 0.9). The median time for the RDTs to become negative was two days for the CareStart Malaria tests and seven days for OptiMAL-IT. Tests were heat stable up to 90 days except for OptiMAL-IT (Pf specific pLDH stable to day 20 at 35 degrees C). CONCLUSION: None of the pLDH-based RDTs evaluated was able to detect non-falciparum malaria with high sensitivity, particularly at low parasitaemias. OptiMAL-IT performed best overall and would perform best in an area of high malaria prevalence among screened fever cases. However, heat stability was unacceptable and the number of steps to perform this test is a significant drawback in the field. A reliable, heat-stable, highly sensitive RDT, capable of diagnosing all Plasmodium species has yet to be identified
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