242 research outputs found
The heterogeneous OH oxidation of palmitic acid in single component and internally mixed aerosol particles: vaporization, secondary chemistry, and the role of particle phase
International audienceWe studied the OH oxidation of submicron aerosol particles consisting of pure palmitic acid (PA) or thin (near monolayer) coatings of PA on aqueous and effloresced inorganic salt particles. Experiments were performed as a function of particle size and OH exposure using a continuous-flow photochemical reaction chamber coupled to a chemical ionization mass spectrometer (CIMS) system, for detection of gas and particle-bound organics, and a DMA/CPC for monitoring particle size distributions. The loss rate of PA observed for pure PA aerosols and PA on crystalline NaCl aerosols indicates that the OH oxidation of PA at the gas-aerosol interface is efficient. The pure PA oxidation data are well represented by a model consisting of four main processes: 1) surface-only reactions between OH and palmitic acid, 2) secondary reactions between palmitic acid and OH oxidation products, 3) volatilization of condensed-phase mass, and 4) a surface renewal process. Using this model we infer a value of ?OH between 0.8 and 1. The oxidation of palmitic acid in thin film coatings of salt particles is also efficient, though the inferred ?OH is lower, ranging from ~0.3 (+0.1/?0.05) for coatings on solid NaCl and ~0.05 (±0.01) on aqueous NaCl particles. These results, together with simultaneous data on particle size change and volatilized oxidation products, provide support for the ideas that oxidative aging of aliphatic organic aerosol is a source of small oxidized volatile organic compounds (OVOCs), and that OH oxidation may initiate secondary condensed-phase reactions
Electrical Switching Dynamics in Circular and Rectangular Ge2Sb2Te5 Nanopillar Phase Change Memory Devices
We have measured the critical phase change conditions induced by electrical
pulses in Ge2Sb2Te5 nanopillar phase change memory devices by constructing a
comprehensive resistance map as a function of pulse parameters (width,
amplitude and trailing edge). Our measurements reveal that the heating scheme
and the details of the contact geometry play the dominant role in determining
the final phase composition of the device such that a non-uniform heating
scheme promotes partial amorphization/crystallization for a wide range of pulse
parameters enabling multiple resistance levels for data storage applications.
Furthermore we find that fluctuations in the snap-back voltage and set/reset
resistances in repeated switching experiments are related to the details of the
current distribution such that a uniform current injection geometry (i.e.
circular contact) favors more reproducible switching parameters. This shows
that possible geometrical defects in nanoscale phase change memory devices may
play an essential role in the performance of the smallest possible devices
through modification of the exact current distribution in the active
chalcogenide layer. We present a three-dimensional finite element model of the
electro-thermal physics to provide insights into the underlying physical
mechanisms of the switching dynamics as well as to quantitatively account for
the scaling behaviour of the switching currents in both circular and
rectangular contact geometries. The calculated temporal evolution of the heat
distribution within the pulse duration shows distinct features in rectangular
contacts providing evidence for locally hot spots at the sharp corners of the
current injection site due to current crowding effects leading to the observed
behaviour
First principles theory of inelastic currents in a scanning tunneling microscope
A first principles theory of inelastic tunneling between a model probe tip
and an atom adsorbed on a surface is presented, extending the elastic tunneling
theory of Tersoff and Hamann. The inelastic current is proportional to the
change in the local density of states at the center of the tip due to the
addition of the adsorbate. We use the theory to investigate the vibrational
heating of an adsorbate below an STM tip. We calculate the desorption rate of H
from Si(100)-H(21) as function of the sample bias and tunnel current,
and find excellent agreement with recent experimental data.Comment: 5 pages, RevTeX, epsf file
STM induced hydrogen desorption via a hole resonance
We report STM-induced desorption of H from Si(100)-H(2) at negative
sample bias. The desorption rate exhibits a power-law dependence on current and
a maximum desorption rate at -7 V. The desorption is explained by vibrational
heating of H due to inelastic scattering of tunneling holes with the Si-H
5 hole resonance. The dependence of desorption rate on current and bias
is analyzed using a novel approach for calculating inelastic scattering, which
includes the effect of the electric field between tip and sample. We show that
the maximum desorption rate at -7 V is due to a maximum fraction of
inelastically scattered electrons at the onset of the field emission regime.Comment: 4 pages, 4 figures. To appear in Phys. Rev. Let
Measurement of the conductance of a hydrogen molecule
Recent years have shown steady progress in research towards molecular
electronics [1,2], where molecules have been investigated as switches [3-5],
diodes [6], and electronic mixers [7]. In much of the previous work a Scanning
Tunnelling Microscope was employed to address an individual molecule. As this
arrangement does not provide long-term stability, more recently
metal-molecule-metal links have been made using break junction devices [8-10].
However, it has been difficult to establish unambiguously that a single
molecule forms the contact [11]. Here, we show that a single H2 molecule can
form a stable bridge between Pt electrodes. In contrast to results for other
organic molecules, the bridge has a nearly perfect conductance of one quantum
unit, carried by a single channel. The H2-bridge provides a simple test system
and a fundamental step towards understanding transport properties of
single-molecule devices.Comment: 6 pages, 4 figure
Trapping and aerogelation of nanoparticles in negative gravity hydrocarbon flames
We report the experimental realization of continuous carbon aerogel production using a flame aerosol reactor by operating it in negative gravity (−g; up-side-down configuration). Buoyancy opposes the fuel and air flow forces in −g, which eliminates convectional outflow of nanoparticles from the flame and traps them in a distinctive non-tipping, flicker-free, cylindrical flame body, where they grow to millimeter-size aerogel particles and gravitationally fall out. Computational fluid dynamics simulations show that a closed-loop recirculation zone is set up in −g flames, which reduces the time to gel for nanoparticles by ≈10[superscript 6] s, compared to positive gravity (upward rising) flames. Our results open up new possibilities of one-step gas-phase synthesis of a wide variety of aerogels on an industrial scale
Dissipative Van der Waals interaction between a small particle and a metal surface
We use a general theory of the fluctuating electromagnetic field to calculate
the friction force acting on a small neutral particle, e.g., a physisorbed
molecule, or a nanoscale object with arbitrary dispersive and absorptive
dielectric properties, moving near a metal surface. We consider the dependence
of the electromagnetic friction on the temperature , the separation , and
discuss the role of screening, non-local and retardation effects. We find that
for high resistivity materials, the dissipative van der Waals interaction can
be an important mechanism of vibrational energy relaxation of physisorbed
molecules, and friction for microscopic solids. Several controversial topics
related to electromagnetic dissipative shear stress is considered. The problem
of local heating of the surface by an STM tip is also briefly commented on.Comment: 11 pages, No figure
Vibrational Excitations in Weakly Coupled Single-Molecule Junctions: A Computational Analysis
In bulk systems, molecules are routinely identified by their vibrational
spectrum using Raman or infrared spectroscopy. In recent years, vibrational
excitation lines have been observed in low-temperature conductance measurements
on single molecule junctions and they can provide a similar means of
identification. We present a method to efficiently calculate these excitation
lines in weakly coupled, gateable single-molecule junctions, using a
combination of ab initio density functional theory and rate equations. Our
method takes transitions from excited to excited vibrational state into account
by evaluating the Franck-Condon factors for an arbitrary number of vibrational
quanta, and is therefore able to predict qualitatively different behaviour from
calculations limited to transitions from ground state to excited vibrational
state. We find that the vibrational spectrum is sensitive to the molecular
contact geometry and the charge state, and that it is generally necessary to
take more than one vibrational quantum into account. Quantitative comparison to
previously reported measurements on pi-conjugated molecules reveals that our
method is able to characterize the vibrational excitations and can be used to
identify single molecules in a junction. The method is computationally feasible
on commodity hardware.Comment: 9 pages, 7 figure
Optical Detection of a Single Nuclear Spin
We propose a method to optically detect the spin state of a 31-P nucleus
embedded in a 28-Si matrix. The nuclear-electron hyperfine splitting of the
31-P neutral-donor ground state can be resolved via a direct frequency
discrimination measurement of the 31-P bound exciton photoluminescence using
single photon detectors. The measurement time is expected to be shorter than
the lifetime of the nuclear spin at 4 K and 10 T.Comment: 4 pages, 3 figure
Quantum transport through STM-lifted single PTCDA molecules
Using a scanning tunneling microscope we have measured the quantum
conductance through a PTCDA molecule for different configurations of the
tip-molecule-surface junction. A peculiar conductance resonance arises at the
Fermi level for certain tip to surface distances. We have relaxed the molecular
junction coordinates and calculated transport by means of the Landauer/Keldysh
approach. The zero bias transmission calculated for fixed tip positions in
lateral dimensions but different tip substrate distances show a clear shift and
sharpening of the molecular chemisorption level on increasing the STM-surface
distance, in agreement with experiment.Comment: accepted for publication in Applied Physics
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