440 research outputs found
Electrically Driven Light Emission from Individual CdSe Nanowires
We report electroluminescence (EL) measurements carried out on three-terminal
devices incorporating individual n-type CdSe nanowires. Simultaneous optical
and electrical measurements reveal that EL occurs near the contact between the
nanowire and a positively biased electrode or drain. The surface potential
profile, obtained by using Kelvin probe microscopy, shows an abrupt potential
drop near the position of the EL spot, while the band profile obtained from
scanning photocurrent microscopy indicates the existence of an n-type Schottky
barrier at the interface. These observations indicate that light emission
occurs through a hole leakage or an inelastic scattering induced by the rapid
potential drop at the nanowire-electrode interface.Comment: 12 pages, 4 figure
Growth and Transport Properties of Complementary Germanium Nanowire Field Effect Transistors
n- and p-type Ge nanowires were synthesized by a multistep process in which axial elongation, via vapor–liquid–solid (VLS) growth, and doping were accomplished in separate chemical vapor deposition steps. Intrinsic, single-crystal, Ge nanowires prepared by Au nanocluster-mediated VLS growth were surface-doped in situ using diborane or phosphine, and then radial growth of an epitaxial Ge shell was used to cap the dopant layer. Field-effect transistors prepared from these Ge nanowires exhibited on currents and transconductances up to 850 µA/µm and 4.9 µA/V, respectively, with device yields of \u3e85%
An Ab Initio Approach to the Solar Coronal Heating Problem
We present an ab initio approach to the solar coronal heating problem by
modelling a small part of the solar corona in a computational box using a 3D
MHD code including realistic physics. The observed solar granular velocity
pattern and its amplitude and vorticity power spectra, as reproduced by a
weighted Voronoi tessellation method, are used as a boundary condition that
generates a Poynting flux in the presence of a magnetic field. The initial
magnetic field is a potential extrapolation of a SOHO/MDI high resolution
magnetogram, and a standard stratified atmosphere is used as a thermal initial
condition. Except for the chromospheric temperature structure, which is kept
fixed, the initial conditions are quickly forgotten because the included
Spitzer conductivity and radiative cooling function have typical timescales
much shorter than the time span of the simulation. After a short initial start
up period, the magnetic field is able to dissipate 3-4 10^6 ergs cm^{-2} s^{-1}
in a highly intermittent corona, maintaining an average temperature of K, at coronal density values for which emulated images of the Transition
Region And Coronal Explorer(TRACE) 171 and 195 pass bands reproduce observed
photon count rates.Comment: 12 pages, 14 figures. Submitted to Ap
Temperature dependent fluorescence in disordered Frenkel chains: interplay of equilibration and local band-edge level structure
We model the optical dynamics in linear Frenkel exciton systems governed by
scattering on static disorder and lattice vibrations, and calculate the
temperature dependent fluorescence spectrum and lifetime. The fluorescence
Stokes shift shows a nonmonotonic behavior with temperature, which derives from
the interplay of the local band-edge level structure and thermal equilibration.
The model yields excellent fits to experiments performed on linear dye
aggregates.Comment: 4 pages, 3 figure
Many-Body Effects on Tunneling of Electrons in Magnetic-Field-Induced Quasi One-Dimensional Electron Systems in Semiconductor Nanowhiskers
Effects of the electron-electron interaction on tunneling in a semiconductor
nanowhisker are studied in a magnetic quantum limit. We consider the system
with which bulk and edge states coexist. In bulk states, the temperature
dependence of the transmission probability is qualitatively similar to that of
a one-dimensional electron system. We investigate contributions of edge states
on transmission probability in bulk states. Those contributions can be
neglected within our approximation which takes into account only most divergent
terms at low temperatures.Comment: 9 pages, 6 figure
Heat conduction in a 1D harmonic chain with three dimensional vibrations
We study vibrational energy transport in a quasi 1-D harmonic chain with both
longitudinal and transverse vibrations. We demonstrate via both numerical
simulation and theoretic analysis that for 1-D atomic chain connected by 3D
harmonic springs, the coefficient of heat conduction changes it continuously
with its lattice constant, indicating the qualitative difference from the
corresponding 1-D case where the coefficient is independent of the lattice
constant.Comment: 4 pages, 4 figure
Atomistic simulations of self-trapped exciton formation in silicon nanostructures: The transition from quantum dots to nanowires
Using an approximate time-dependent density functional theory method, we
calculate the absorption and luminescence spectra for hydrogen passivated
silicon nanoscale structures with large aspect ratio. The effect of electron
confinement in axial and radial directions is systematically investigated.
Excited state relaxation leads to significant Stokes shifts for short nanorods
with lengths less than 2 nm, but has little effect on the luminescence
intensity. The formation of self-trapped excitons is likewise observed for
short nanostructures only; longer wires exhibit fully delocalized excitons with
neglible geometrical distortion at the excited state minimum.Comment: 10 pages, 4 figure
Profiles of heating in turbulent coronal magnetic loops
Context: The location of coronal heating in magnetic loops has been the
subject of a long-lasting controversy: does it occur mostly at the loop
footpoints, at the top, is it random, or is the average profile uniform?
Aims: We try to address this question in model loops with MHD turbulence and
a profile of density and/or magnetic field along the loop.
Methods: We use the ShellAtm MHD turbulent heating model described in Buchlin
& Velli (2006), with a static mass density stratification obtained by the
HydRad model (Bradshaw & Mason 2003). This assumes the absence of any flow or
heat conduction subsequent to the dynamic heating.
Results: The average profile of heating is quasi-uniform, unless there is an
expansion of the flux tube (non-uniform axial magnetic field) or the variation
of the kinetic and magnetic diffusion coefficients with temperature is taken
into account: in the first case the heating is enhanced at footpoints, whereas
in the second case it is enhanced where the dominant diffusion coefficient is
enhanced.
Conclusions: These simulations shed light on the consequences on heating
profiles of the complex interactions between physical effects involved in a
non-uniform turbulent coronal loop.Comment: 9 pages, 8 figure
Structure of nanoparticles embedded in micellar polycrystals
We investigate by scattering techniques the structure of water-based soft
composite materials comprising a crystal made of Pluronic block-copolymer
micelles arranged in a face-centered cubic lattice and a small amount (at most
2% by volume) of silica nanoparticles, of size comparable to that of the
micelles. The copolymer is thermosensitive: it is hydrophilic and fully
dissolved in water at low temperature (T ~ 0{\deg}C), and self-assembles into
micelles at room temperature, where the block-copolymer is amphiphilic. We use
contrast matching small-angle neuron scattering experiments to probe
independently the structure of the nanoparticles and that of the polymer. We
find that the nanoparticles do not perturb the crystalline order. In addition,
a structure peak is measured for the silica nanoparticles dispersed in the
polycrystalline samples. This implies that the samples are spatially
heterogeneous and comprise, without macroscopic phase separation, silica-poor
and silica-rich regions. We show that the nanoparticle concentration in the
silica-rich regions is about tenfold the average concentration. These regions
are grain boundaries between crystallites, where nanoparticles concentrate, as
shown by static light scattering and by light microscopy imaging of the
samples. We show that the temperature rate at which the sample is prepared
strongly influence the segregation of the nanoparticles in the
grain-boundaries.Comment: accepted for publication in Langmui
- …