10 research outputs found
Colloidal CdSe Quantum Rings
Semiconductor quantum rings are of
great fundamental interest because
their non-trivial topology creates novel physical properties. At the
same time, toroidal topology is difficult to achieve for colloidal
nanocrystals and epitaxially grown semiconductor nanostructures. In
this work, we introduce the synthesis of luminescent colloidal CdSe
nanorings and nanostructures with double and triple toroidal topology.
The nanorings form during controlled etching and rearrangement of
two-dimensional nanoplatelets. We discuss a possible mechanism of
the transformation of nanoplatelets into nanorings and potential utility
of colloidal nanorings for magneto-optical (e.g., Aharonov–Bohm
effect) and other applications
Auger-Limited Carrier Recombination and Relaxation in CdSe Colloidal Quantum Wells
Using time-resolved
photoluminescence spectroscopy, we show that
two-exciton Auger recombination dominates carrier recombination and
cooling dynamics in CdSe nanoplatelets, or colloidal quantum wells.
The electron–hole recombination rate depends only on the number
of electron–hole pairs present in each nanoplatelet, and is
consistent with a two-exciton recombination process over a wide range
of exciton densities. The carrier relaxation rate within the conduction
and valence bands also depends only on the number of electron–hole
pairs present, apart from an initial rapid decay, and is consistent
with the cooling rate being limited by reheating due to Auger recombination
processes. These Auger-limited recombination and relaxation dynamics
are qualitatively different from the carrier dynamics in either colloidal
quantum dots or epitaxial quantum wells
Surface-Area-Dependent Electron Transfer Between Isoenergetic 2D Quantum Wells and a Molecular Acceptor
We report measurements
of electron transfer rates for four isoenergetic
donor–acceptor pairs comprising a molecular electron acceptor,
methylviologen (MV), and morphology-controlled colloidal semiconductor
nanoparticles of CdSe. The four nanoparticles include a spherical
quantum dot (QD) and three differing lateral areas of 4-monolayer-thick
nanoplatelets (NPLs), each with a 2.42 eV energy gap. As such, the
measurements, performed via ultrafast photoluminescence, relate the
dependence of charge transfer rate on the spatial extent of the initial
electron–hole pair wave function explicitly, which we show
for the first time to be related to surface area in this regime that
is intermediate between homogeneous and heterogeneous charge transfer
as well as 2D to 0D electron transfer. The observed nonlinear dependence
of rate with surface area is attributed to exciton delocalization
within each structure, which we show via temperature-dependent absorption
measurements remains constant
Nonmonotonic Dependence of Auger Recombination Rate on Shell Thickness for CdSe/CdS Core/Shell Nanoplatelets
Nonradiative Auger
recombination limits the efficiency with which
colloidal semiconductor nanocrystals can emit light when they are
subjected to strong excitation, with important implications for the
application of the nanocrystals in light-emitting diodes and lasers.
This has motivated attempts to engineer the structure of the nanocrystals
to minimize Auger rates. Here, we study Auger recombination rates
in CdSe/CdS core/shell nanoplatelets, or colloidal quantum wells.
Using time-resolved photoluminescence measurements, we show that the
rate of biexcitonic Auger recombination has a nonmonotonic dependence
on the shell thickness, initially decreasing, reaching a minimum for
shells with thickness of 2–4 monolayers, and then increasing
with further increases in the shell thickness. This nonmonotonic behavior
has not been observed previously for biexcitonic recombination in
quantum dots, most likely due to inhomogeneous broadening that is
not present for the nanoplatelets
Assessment of Anisotropic Semiconductor Nanorod and Nanoplatelet Heterostructures with Polarized Emission for Liquid Crystal Display Technology
Semiconductor nanorods
can emit linear-polarized light at efficiencies
over 80%. Polarization of light in these systems, confirmed through
single-rod spectroscopy, can be explained on the basis of the anisotropy
of the transition dipole moment and dielectric confinement effects.
Here we report emission polarization in macroscopic semiconductor–polymer
composite films containing CdSe/CdS nanorods and colloidal CdSe nanoplatelets.
Anisotropic nanocrystals dispersed in polymer films of poly butyl-<i>co</i>-isobutyl methacrylate (PBiBMA) can be stretched mechanically
in order to obtain unidirectionally aligned arrays. A high degree
of alignment, corresponding to an orientation factor of 0.87, was
achieved and large areas demonstrated polarized emission, with the
contrast ratio <i>I</i><sub>∥</sub>/<i>I</i><sub>⊥</sub> = 5.6, making these films viable candidates for
use in liquid crystal display (LCD) devices. To some surprise, we
observed significant optical anisotropy and emission polarization
for 2D CdSe nanoplatelets with the electronic structure of quantum
wells. The aligned nanorod arrays serve as optical funnels, absorbing
unpolarized light and re-emitting light from deep-green to red with
quantum efficiencies over 90% and high degree of linear polarization.
Our results conclusively demonstrate the benefits of anisotropic nanostructures
for LCD backlighting. The polymer films with aligned CdSe/CdS dot-in-rod
and rod-in-rod nanostructures show more than 2-fold enhancement of
brightness compared to the emitter layers with randomly oriented nanostructures.
This effect can be explained as the combination of linearly polarized
luminescence and directional emission from individual nanostructures
Elevated Temperature Photophysical Properties and Morphological Stability of CdSe and CdSe/CdS Nanoplatelets
Elevated temperature
optoelectronic performance of semiconductor
nanomaterials remains an important issue for applications. Here we
examine 2D CdSe nanoplatelets (NPs) and CdS/CdSe/CdS shell/core/shell
sandwich NPs at temperatures ranging from 300 to 700 K using static
and transient spectroscopies as well as in situ transmission electron
microscopy. NPs exhibit reversible changes in PL intensity, spectral
position, and emission line width with temperature elevation up to
∼500 K, losing a factor of ∼8 to 10 in PL intensity
at 400 K relative to ambient. Temperature elevation above ∼500
K yields thickness-dependent, irreversible degradation in optical
properties. Electron microscopy relates stability of the core-only
NP morphology up to 555 and 600 K for the four and five monolayer
NPs, respectively, followed by sintering and evaporation at still
higher temperatures. Reversible PL loss, based on differences in decay
dynamics between time-resolved photoluminescence and transient absorption,
results primarily from hole trapping in both NPs and sandwich NPs
Red, Yellow, Green, and Blue Amplified Spontaneous Emission and Lasing Using Colloidal CdSe Nanoplatelets
There have been multiple demonstrations of amplified spontaneous emission (ASE) and lasing using colloidal semiconductor nanocrystals. However, it has been proven difficult to achieve low thresholds suitable for practical use of nanocrystals as gain media. Low-threshold blue ASE and lasing from nanocrystals is an even more challenging task. Here, we show that colloidal nanoplatelets (NPLs) with electronic structure of quantum wells can produce ASE in the red, yellow, green, and blue regions of the visible spectrum with low thresholds and high gains. In particular, for blue-emitting NPLs, the ASE threshold is 50 μJ/cm<sup>2</sup>, lower than any reported value for nanocrystals. We then demonstrate red, yellow, green, and blue lasing using NPLs with different thicknesses. We find that the lateral size of NPLs does not show any strong effect on the Auger recombination rates and, correspondingly, on the ASE threshold or gain saturation. This observation highlights the qualitative difference of multiexciton dynamics in CdSe NPLs and other quantum-confined CdSe materials, such as quantum dots and rods. Our measurements of the gain bandwidth and gain lifetime further support the prospects of colloidal NPLs as solution-processed optical gain materials
Anisotropic Photoluminescence from Isotropic Optical Transition Dipoles in Semiconductor Nanoplatelets
Many important light-matter
coupling and energy-transfer processes
depend critically on the dimensionality and orientation of optical
transition dipoles in emitters. We investigate individual quasi-two-dimensional
nanoplatelets (NPLs) using higher-order laser scanning microscopy
and find that absorption dipoles in NPLs are isotropic in three dimensions
at the excitation wavelength. Correlated polarization studies of the
NPLs reveal that their emission polarization is strongly dependent
on the aspect ratio of the lateral dimensions. Our simulations reveal
that this emission anisotropy can be readily explained by the electric
field renormalization effect caused by the dielectric contrast between
the NPLs and the surrounding medium, and we conclude that emission
dipoles in NPLs are isotropic in the plane of the NPLs. Our study
presents an approach for disentangling the effects of dipole degeneracy
and electric field renormalization on emission anisotropy and can
be adapted for studying the intrinsic optical transition dipoles of
various nanostructures
Low-Threshold Stimulated Emission Using Colloidal Quantum Wells
The use of colloidal semiconductor
nanocrystals for optical amplification
and lasing has been limited by the need for high input power densities.
Here we show that colloidal nanoplatelets produce amplified spontaneous
emission with thresholds as low as 6 μJ/cm<sup>2</sup> and gain
as high as 600 cm<sup>–1</sup>, both a significant improvement
over colloidal nanocrystals; in addition, gain saturation occurs at
pump fluences 2 orders of magnitude higher than the threshold. We
attribute this exceptional performance to large optical cross-sections,
slow Auger recombination rates, and narrow ensemble emission line
widths
Size-Dependent Biexciton Quantum Yields and Carrier Dynamics of Quasi-Two-Dimensional Core/Shell Nanoplatelets
Quasi-two-dimensional
nanoplatelets (NPLs) possess fundamentally
different excitonic properties from zero-dimensional quantum dots.
We study lateral size-dependent photon emission statistics and carrier
dynamics of individual NPLs using second-order photon correlation
(g<sup>(2)</sup>(Ï„)) spectroscopy and photoluminescence (PL)
intensity-dependent lifetime analysis. Room-temperature radiative
lifetimes of NPLs can be derived from maximum PL intensity periods
in PL time traces. It first decreases with NPL lateral size and then
stays constant, deviating from the electric dipole approximation.
Analysis of the PL time traces further reveals that the single exciton
quantum yield in NPLs decreases with NPL lateral size and increases
with protecting shell thickness, indicating the importance of surface
passivation on NPL emission quality. Second-order photon correlation
(g<sup>(2)</sup>(Ï„)) studies of single NPLs show that the biexciton
quantum yield is strongly dependent on the lateral size and single
exciton quantum yield of the NPLs. In large NPLs with unity single
exciton quantum yield, the corresponding biexciton quantum yield can
reach unity. These findings reveal that by careful growth control
and core–shell material engineering, NPLs can be of great potential
for light amplification and integrated quantum photonic applications