496 research outputs found
Theory of resonance energy transfer involving nanocrystals: the role of high multipoles
A theory for the fluorescence resonance energy transfer (FRET) between a pair
of semiconducting nanocrystal quantum dots is developed. Two types of
donor-acceptor couplings for the FRET rate are described: dipole-dipole (d-d)
and the dipole-quadrupole (d-q) coupling. The theory builds on a simple
effective mass model which is used to relate the FRET rate to measureable
quantities such as the nanocrystal size, fundamental gap, effective mass,
exciton radius and dielectric constant. We discuss the relative contribution to
the FRET rate of the different multipole terms, the role of strong to weak
confinement limits, and the effects of nanocrystal siz-es.Comment: 12 pages, 7 figure
Analytical Continuation Approaches to Electronic Transport: The Resonant Level Model
The analytical continuation average spectrum method (ASM) and maximum entropy
(MaxEnt) method are applied to the dynamic response of a noninteracting
resonant level model within the framework of the Kubo formula for electric
conductivity. The frequency dependent conductivity is inferred from the
imaginary time current-current correlation function for a wide range of
temperatures, gate voltages and spectral densities representing the leads, and
compared with exact results. We find that the MaxEnt provides more accurate
results compared to the ASM over the full spectral range.Comment: 6 pages, 5 figure
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Giant Light-Emission Enhancement in Lead Halide Perovskites by Surface Oxygen Passivation.
Surface condition plays an important role in the optical performance of semiconductor materials. As new types of semiconductors, the emerging metal-halide perovskites are promising for next-generation optoelectronic devices. We discover significantly improved light-emission efficiencies in lead halide perovskites due to surface oxygen passivation. The enhancement manifests close to 3 orders of magnitude as the perovskite dimensions decrease to the nanoscale, improving external quantum efficiencies from <0.02% to over 12%. Along with about a 4-fold increase in spontaneous carrier recombination lifetimes, we show that oxygen exposure enhances light emission by reducing the nonradiative recombination channel. Supported by X-ray surface characterization and theoretical modeling, we propose that excess lead atoms on the perovskite surface create deep-level trap states that can be passivated by oxygen adsorption
Shake-induced order in nanosphere systems
Self-assembled patterns obtained from a drying nanosphere suspension are
investigated by computer simulations and simple experiments. Motivated by the
earlier experimental results of Sasaki and Hane and Schope, we confirm that
more ordered triangular lattice structures can be obtained whenever a moderate
intensity random shaking is applied on the drying system. Computer simulations
are realized on an improved version of a recently elaborated
Burridge-Knopoff-type model. Experiments are made following the setup of Sasaki
and Hane, using ultrasonic radiation as source for controlled shaking.Comment: 7 pages, 10 figure
Tight-Binding model for semiconductor nanostructures
An empirical tight-binding (TB) model is applied to the
investigation of electronic states in semiconductor quantum dots. A basis set
of three -orbitals at the anions and one -orbital at the cations is
chosen. Matrix elements up to the second nearest neighbors and the spin-orbit
coupling are included in our TB-model. The parametrization is chosen so that
the effective masses, the spin-orbit-splitting and the gap energy of the bulk
CdSe and ZnSe are reproduced. Within this reduced TB-basis the
valence (p-) bands are excellently reproduced and the conduction (s-) band is
well reproduced close to the -point, i.e. near to the band gap. In
terms of this model much larger systems can be described than within a (more
realistic) -basis. The quantum dot is modelled by using the (bulk)
TB-parameters for the particular material at those sites occupied by atoms of
this material. Within this TB-model we study pyramidal-shaped CdSe quantum dots
embedded in a ZnSe matrix and free spherical CdSe quantum dots (nanocrystals).
Strain-effects are included by using an appropriate model strain field. Within
the TB-model, the strain-effects can be artifically switched off to investigate
the infuence of strain on the bound electronic states and, in particular, their
spatial orientation. The theoretical results for spherical nanocrystals are
compared with data from tunneling spectroscopy and optical experiments.
Furthermore the influence of the spin-orbit coupling is investigated
Metastability in pressure-induced structural transformations of CdSe/ZnS core/shell nanocrystals
The kinetics and thermodynamics of structural transformations under pressure
depend strongly on particle size due to the influence of surface free energy.
By suitable design of surface structure, composition, and passivation it is
possible, in principle, to prepare nanocrystals in structures inaccessible to
bulk materials. However, few realizations of such extreme size-dependent
behavior exist. Here we show with molecular dynamics computer simulation that
in a model of CdSe/ZnS core/shell nanocrystals the core high pressure structure
can be made metastable under ambient conditions by tuning the thickness of the
shell. In nanocrystals with thick shells, we furthermore observe a wurtzite to
NiAs transformation, which does not occur in the pure bulk materials. These
phenomena are linked to a fundamental change in the atomistic transformation
mechanism from heterogenous nucleation at the surface to homogenous nucleation
in the crystal core. Our results suggest a new route towards expanding the
range of available nanoscale materials
Detecting, distinguishing, and spatiotemporally tracking photogenerated charge and heat at the nanoscale
Since dissipative processes are ubiquitous in semiconductors, characterizing
how electronic and thermal energy transduce and transport at the nanoscale is
vital for understanding and leveraging their fundamental properties. For
example, in low-dimensional transition metal dichalcogenides (TMDCs), excess
heat generation upon photoexcitation is difficult to avoid since even with
modest injected exciton densities, exciton-exciton annihilation still occurs.
Both heat and photoexcited electronic species imprint transient changes in the
optical response of a semiconductor, yet the unique signatures of each are
difficult to disentangle in typical spectra due to overlapping resonances. In
response, we employ stroboscopic optical scattering microscopy (stroboSCAT) to
simultaneously map both heat and exciton populations in few-layer \ch{MoS2} on
relevant nanometer and picosecond length- and time scales and with 100-mK
temperature sensitivity. We discern excitonic contributions to the signal from
heat by combining observations close to and far from exciton resonances,
characterizing photoinduced dynamics for each. Our approach is general and can
be applied to any electronic material, including thermoelectrics, where heat
and electronic observables spatially interplay, and lays the groundwork for
direct and quantitative discernment of different types of coexisting energy
without recourse to complex models or underlying assumptions.Comment: 22 pages, 4 figures, SI included as ancilliary fil
Approximation Algorithms for the Job Interval Selection Problem and Related Scheduling Problems
Strong coupling theory for driven tunneling and vibrational relaxation
We investigate on a unified basis tunneling and vibrational relaxation in
driven dissipative multistable systems described by their N lowest lying
unperturbed levels. By use of the discrete variable representation we derive a
set of coupled non-Markovian master equations. We present analytical treatments
that describe the dynamics in the regime of strong system-bath coupling. Our
findings are corroborated by ``ab-initio'' real-time path integral
calculations.Comment: 4 LaTeX pages including 3 figure
The survival and function of IL-10-producing regulatory B cells are negatively controlled by SLAMF5
B cells have essential functions in multiple sclerosis and in its mouse model, experimental autoimmune encephalomyelitis, both as drivers and suppressors of the disease. The suppressive effects are driven by a regulatory B cell (Breg) population that functions, primarily but not exclusively, via the production of IL-10. However, the mechanisms modulating IL-10-producing Breg abundance are poorly understood. Here we identify SLAMF5 for controlling IL-10+ Breg maintenance and function. In EAE, the deficiency of SLAMF5 in B cells causes accumulation of IL10+ Bregs in the central nervous system and periphery. Blocking SLAMF5 in vitro induces both human and mouse IL-10-producing Breg cells and increases their survival with a concomitant increase of a transcription factor, c-Maf. Finally, in vivo SLAMF5 blocking in EAE elevates IL-10+ Breg levels and ameliorates disease severity. Our results suggest that SLAMF5 is a negative moderator of IL-10+ Breg cells, and may serve as a therapeutic target in MS and other autoimmune diseases
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