286 research outputs found
Three-dimensional confinement in the conduction band structure of InP
Includes bibliographical references.Strong quantum confinement in InP is observed to significantly reduce the separation between the direct and indirect conduction band states. The effects of three-dimensional confinement are investigated by tailoring the initial separation between conduction band states using quantum dots (QDs) of different sizes and hydrostatic pressure. Analyses of the QD emission spectra show that the X1c states are lowest in energy at pressures of ~6 GPa, much lower than in the bulk. The transition to the X1c states can be explained by either a sequence of Γ-L and L-X crossings, or by the crossover between strongly coupled Γ and X states.The work at CSU was supported by the National Science Foundation, and that at NREL by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division
A pseudopotential study of electron-hole excitations in colloidal, free-standing InAs quantum dots
Excitonic spectra are calculated for free-standing, surface passivated InAs
quantum dots using atomic pseudopotentials for the single-particle states and
screened Coulomb interactions for the two-body terms. We present an analysis of
the single particle states involved in each excitation in terms of their
angular momenta and Bloch-wave parentage. We find that (i) in agreement with
other pseudopotential studies of CdSe and InP quantum dots, but in contrast to
k.p calculations, dot states wavefunction exhibit strong odd-even angular
momentum envelope function mixing (e.g. with ) and large
valence-conduction coupling. (ii) While the pseudopotential approach produced
very good agreement with experiment for free-standing, colloidal CdSe and InP
dots, and for self-assembled (GaAs-embedded) InAs dots, here the predicted
spectrum does {\em not} agree well with the measured (ensemble average over dot
sizes) spectra. (1) Our calculated excitonic gap is larger than the PL measure
one, and (2) while the spacing between the lowest excitons is reproduced, the
spacings between higher excitons is not fit well. Discrepancy (1) could result
from surface states emission. As for (2), agreement is improved when account is
taken of the finite size distribution in the experimental data. (iii) We find
that the single particle gap scales as (not ), that the
screened (unscreened) electron-hole Coulomb interaction scales as
(), and that the eccitonic gap sclaes as . These scaling
laws are different from those expected from simple models.Comment: 12 postscript figure
An accurate description of quantum size effects in InP nanocrystallites over a wide range of sizes
We obtain an effective parametrization of the bulk electronic structure of
InP within the Tight Binding scheme. Using these parameters, we calculate the
electronic structure of InP clusters with the size ranging upto 7.5 nm. The
calculated variations in the electronic structure as a function of the cluster
size is found to be in excellent agreement with experimental results over the
entire range of sizes, establishing the effectiveness and transferability of
the obtained parameter strengths.Comment: 9 pages, 3 figures, pdf file available at
http://sscu.iisc.ernet.in/~sampan/publications.htm
Electron and Hole Transfer from Indium Phosphide Quantum Dots
Electron-and hole-transfer reactions are studied in colloidal InP quantum dots (QDs). Photoluminescence quenching and time-resolved transient absorption (TA) measurements are utilized to examine hole transfer from photoexcited InP QDs to the hole acceptor N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) and electron transfer to nanocrystalline titanium dioxide (TiO 2 ) films. Core-confined holes are effectively quenched by TMPD, resulting in a new ∼4-ps component in the TA decay. It is found that electron transfer to TiO 2 is primarily mediated through surface-localized states on the InP QDs
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Quantum Dot Solar Cells: High Efficiency through Multiple Exciton Generation
Impact ionization is a process in which absorbed photons in semiconductors that are at least twice the bandgap can produce multiple electron-hole pairs. For single-bandgap photovoltaic devices, this effect produces greatly enhanced theoretical thermodynamic conversion efficiencies that range from 45-85%, depending upon solar concentration, the cell temperature, and the number of electron-hole pairs produced per photon. For quantum dots (QDs), electron-hole pairs exist as excitons. We have observed astoundingly efficient multiple exciton generation (MEG) in QDs of PbSe (bulk Eg = 0.28 eV), ranging in diameter from 3.9 to 5.7nm (Eg = 0.73, 0.82, and 0.91 eV, respectively). The effective masses of electron and holes are about equal in PbSe, and the onset for efficient MEG occurs at about three times the QD HOMO-LUMO transition (its ''bandgap''). The quantum yield rises quickly after the onset and reaches 300% at 4 x Eg (3.64 eV) for the smallest QD; this means that every QD in the sample produces three electron-hole pairs/photon
Modeling H2 formation in the turbulent ISM: Solenoidal versus compressive turbulent forcing
We present results from high-resolution three-dimensional simulations of the
turbulent interstellar medium that study the influence of the nature of the
turbulence on the formation of molecular hydrogen. We have examined both
solenoidal (divergence-free) and compressive (curl-free) turbulent driving, and
show that compressive driving leads to faster H2 formation, owing to the higher
peak densities produced in the gas. The difference in the H2 formation rate can
be as much as an order of magnitude at early times, but declines at later times
as the highest density regions become fully molecular and stop contributing to
the total H2 formation rate. We have also used our results to test a simple
prescription suggested by Gnedin et al. (2009) for modeling the influence of
unresolved density fluctuations on the H2 formation rate in large-scale
simulations of the ISM. We find that this approach works well when the H2
fraction is small, but breaks down once the highest density gas becomes fully
molecular.Comment: 13 pages, 8 figures, accepted for publication in MNRA
LISA observations of massive black hole mergers: event rates and issues in waveform modelling
The observability of gravitational waves from supermassive and
intermediate-mass black holes by the forecoming Laser Interferometer Space
Antenna (LISA), and the physics we can learn from the observations, will depend
on two basic factors: the event rates for massive black hole mergers occurring
in the LISA best sensitivity window, and our theoretical knowledge of the
gravitational waveforms. We first provide a concise review of the literature on
LISA event rates for massive black hole mergers, as predicted by different
formation scenarios. Then we discuss what (in our view) are the most urgent
issues to address in terms of waveform modelling. For massive black hole binary
inspiral these include spin precession, eccentricity, the effect of high-order
Post-Newtonian terms in the amplitude and phase, and an accurate prediction of
the transition from inspiral to plunge. For black hole ringdown, numerical
relativity will ultimately be required to determine the relative quasinormal
mode excitation, and to reduce the dimensionality of the template space in
matched filtering.Comment: 14 pages, 2 figures. Added section with conclusions and outlook.
Matches version to appear in the proceedings of 10th Annual Gravitational
Wave Data Analysis Workshop (GWDAW 10), Brownsville, Texas, 14-17 Dec 200
3D characterization of CdSe nanoparticles attached to carbon nanotubes
The crystallographic structure of CdSe nanoparticles attached to carbon
nanotubes has been elucidated by means of high resolution transmission electron
microscopy and high angle annular dark field scanning transmission electron
microscopy tomography. CdSe rod-like nanoparticles, grown in solution together
with carbon nanotubes, undergo a morphological transformation and become
attached to the carbon surface. Electron tomography reveals that the
nanoparticles are hexagonal-based with the (001) planes epitaxially matched to
the outer graphene layer.Comment: 7 pages, 8 figure
Prediction of a Strain Induced Conduction Band Minimum in Embedded Quantum Dots
Free standing InP quantum dots have previously been theoretically and
experimentally shown to have a direct band gap across a large range of
experimentally accessible sizes. We demonstrate that when these dots are
embedded coherently within a GaP barrier material, the effects of quantum
confinement in conjunction with coherent strain suggest there will be a
critical diameter of dot (60A), above which the dot is direct, type I, and
below which it is indirect, type II. However, the strain in the system acts to
produce another conduction state with an even lower energy, in which electrons
are localized in small pockets at the interface between the InP dot and the GaP
barrier. Since this conduction state is GaP X_1c-derived and the highest
occupied valence state is InP, Gamma-derived, the fundamental transition is
predicted to be indirect in both real and reciprocal space (``type II'') for
all dot sizes. This effect is peculiar to the strained dot, and is absent in
the free-standing dot
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