3,697 research outputs found
Charge-transfer photodissociation of adsorbed molecules via electron image states
The 248nm and 193nm photodissociation of submonolayer quantities of CHBr
and CHI adsorbed on thin layers of n-hexane indicate that the dissociation
is caused by dissociative electron attachment from sub-vacuum level
photoelectrons created in the copper substrate. The characteristics of this
photodissociation-- translation energy distributions and coverage dependences
show that the dissociation is mediated by an image potential state which
temporarily traps the photoelectrons near the n-hexane--vacuum interface, and
then the charge transfers from this image state to the affinity level of a
co-adsorbed halomethane which then dissociates.Comment: submitted to Journal of Chemical Physic
Cation mono- and co-doped anatase TiO nanotubes: An {\em ab initio} investigation of electronic and optical properties
The structural, electronic, and optical properties of metal (Si, Ge, Sn, and
Pb) mono- and co-doped anatase TiO nanotubes are investigated, in order
to elucidate their potential for photocatalytic applications. It is found that
Si doped TiO nanotubes are more stable than those doped with Ge, Sn, or
Pb. All dopants lower the band gap, except the (Ge, Sn) co-doped structure, the
decrease depending on the concentration and the type of dopant.
Correspondingly, a redshift in the optical properties for all kinds of dopings
is obtained. Even though a Pb mono- and co-doped TiO nanotube has the
lowest band gap, these systems are not suitable for water splitting, due to the
location of the conduction band edges, in contrast to Si, Ge, and Sn mono-doped
TiO nanotubes. On the other hand, co-doping of TiO does not improve
its photocatalytic properties. Our findings are consistent with recent
experiments which show an enhancement of light absorption for Si and Sn doped
TiO nanotubes.Comment: revised and updated, 23 pages (preprint style), 7 figures, 5 table
Detection mechanism for ferroelectric domain boundaries with lateral force microscopy
The contrast mechanism for the visualization of ferroelectric domain
boundaries with lateral force microscopy is generally assumed to be caused by
mechanical deformation of the sample due to the converse piezoelectric effect.
We show, however, that electrostatic interactions between the charged tip and
the electric fields arising from the surface polarization charges dominate the
contrast mechanism. This explanation is sustained by quantitative analysis of
the measured forces as well as by comparative measurements on different
materials
Pneumatic capillary gun for ballistic delivery of microparticles
A pneumatic gun for ballistic delivery of microparticles to soft targets is
proposed and demonstrated. The particles are accelerated by a high speed flow
of Helium in a capillary tube. Vacuum suction applied to a concentric, larger
diameter tube is used to divert substantially all of the flow of Helium from
the gun nozzle, thereby preventing the gas from hitting and damaging the
target. Speed of ejection of micron-sized gold particles from the gun nozzle,
and their depth of penetration into agarose gels are reported.Comment: 7 pages, 3 figure
Nonlocal van der Waals density functional: The simpler the better
We devise a nonlocal correlation energy functional that describes the entire
range of dispersion interactions in a seamless fashion using only the electron
density as input. The new functional is considerably simpler than its
predecessors of a similar type. The functional has a tractable and robust
analytic form that lends itself to efficient self-consistent implementation.
When paired with an appropriate exchange functional, our nonlocal correlation
model yields accurate interaction energies of weakly-bound complexes, not only
near the energy minima but also far from equilibrium. Our model exhibits an
outstanding precision at predicting equilibrium intermonomer separations in van
der Waals complexes. It also gives accurate covalent bond lengths and
atomization energies. Hence the functional proposed in this work is a
computationally inexpensive electronic structure tool of broad applicability
Lifetime and polarization of the radiative decay of excitons, biexcitons and trions in CdSe nanocrystal quantum dots
Using the pseudopotential configuration-interaction method, we calculate the intrinsic lifetime and polarization of the radiative decay of single excitons (X), positive and negative trions (X+ and X−), and biexcitons (XX) in CdSe nanocrystal quantum dots. We investigate the effects of the inclusion of increasingly more complex many-body treatments, starting from the single-particle approach and culminating with the configuration-interaction scheme. Our configuration-interaction results for the size dependence of the single-exciton radiative lifetime at room temperature are in excellent agreement with recent experimental data. We also find the following. (i) Whereas the polarization of the bright exciton emission is always perpendicular to the hexagonal c axis, the polarization of the dark exciton switches from perpendicular to parallel to the hexagonal c axis in large dots, in agreement with experiment. (ii) The ratio of the radiative lifetimes of mono- and biexcitons (X):(XX) is ~1:1 in large dots (R=19.2 Å). This ratio increases with decreasing nanocrystal size, approaching 2 in small dots (R=10.3 Å). (iii) The calculated ratio (X+):(X−) between positive and negative trion lifetimes is close to 2 for all dot sizes considered
Mechanism for Spontaneous Growth of Nanopillar Arrays in Ultrathin Films Subject to a Thermal Gradient
Several groups have reported spontaneous formation of periodic pillar-like
arrays in molten polymer nanofilms confined within closely spaced substrates
maintained at different temperatures. These formations have been attributed to
a radiation pressure instability caused by acoustic phonons. In this work, we
demonstrate how variations in the thermocapillary stress along the nanofilm
interface can produce significant periodic protrusions in any viscous film no
matter how small the initial transverse thermal gradient. The linear stability
analysis of the interface evolution equation explores an extreme limit of
B\'{e}nard-Marangoni flow peculiar to films of nanoscale dimensions in which
hydrostatic forces are altogether absent and deformation amplitudes are small
in comparison to the pillar spacing. Finite element simulations of the full
nonlinear equation are also used to examine the array pitch and growth rates
beyond the linear regime. Inspection of the Lyapunov free energy as a function
of time confirms that in contrast to typical cellular instabilities in
macroscopically thick films, pillar-like elongations are energetically
preferred in nanofilms. Provided there occurs no dewetting during film
deformation, it is shown that fluid elongations continue to grow until contact
with the cooler substrate is achieved. Identification of the mechanism
responsible for this phenomenon may facilitate fabrication of extended arrays
for nanoscale optical, photonic and biological applications.Comment: 20 pages, 9 figure
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