22 research outputs found
Core/Shell Colloidal Semiconductor Nanoplatelets
We have recently synthesized atomically flat semiconductor
colloidal
nanoplatelets with quasi 2D geometry. Here, we show that core/shell
nanoplatelets can be obtained with a 2D geometry that is conserved.
The epitaxial growth of the shell semiconductor is performed at room
temperature. We report the detailed synthesis of CdSe/CdS and CdSe/CdZnS
structures with different shell thicknesses. The shell growth is characterized
both spectroscopically and structurally. In particular, the core/shell
structure appears very clearly on high-resolution, high-angle annular
dark-field transmission electron microscope images, thanks to the
difference of atomic density between the core and the shell. When
the nanoplatelets stand on their edge, we can precisely count the
number of atomic planes forming the core and the shell. This provides
a direct measurement, with atomic precision, of the core nanoplatelets
thickness. The constraints exerted by the shell growth on the core
is analyzed using global phase analysis. The core/shell nanoplatelets
we obtained have narrow emission spectra with full-width at half-maximum
close to 20 nm, and quantum yield that can reach 60%
Selective Electrophoretic Deposition of CdSe Nanoplatelets
In the fields of nanoparticle synthesis
and application, the control
of the particle size, shape and composition is crucial. The tuning
of these different parameters can be performed during the synthesis,
but often, additional selection steps to improve the purity of a given
nanoparticlesā population are necessary. These additional postsynthesis
selection steps, that can include size selective precipitation, ultracentrifugation
or liquid chromatography, are usually long and not well suited for
a large quantity of materials. Here, we demonstrate that electrophoresis
performed directly in organic solvent can be used to select and/or
separate semiconductor nanoparticles according to their size and shape.
In particular, we show that 2D nanoplatelets (NPL) can be very efficiently
separated from spherical nanoparticles as the side product obtained
during the NPL synthesis. The selectivity of the electrophoretic deposition
we observe is mostly related to the nanoparticle surface charge. We
show that centimeter scale, uniform film of nanoplatelets can be obtained
even on nonconducting substrates. Compared to other methods this technique
is fast, easy to implement and scalable, and should find various uses
both in the fields of the nanoparticle synthesis and their applications
Self-Assembly of CdSe Nanoplatelets into Giant Micrometer-Scale Needles Emitting Polarized Light
We report on the self-assembly of
colloidal CdSe nanoplatelets
into micrometers long anisotropic needle-like superparticles (SPs),
which are formed in solution upon addition of an antisolvent to a
stable colloidal dispersion. Optical fluorescence microscopy, transmission
electron microscopy, and small-angle X-ray scattering provide detailed
structural characterization and show that each particle is composed
of 10<sup>6</sup> nanoplatelets organized in highly aligned columns.
Within the SPs, the nanoplatelets are stacked on each other to maximize
the contact surface between the ligands. When deposited on a substrate,
the planes of the platelets are oriented perpendicularly to its surface
and the SPs exhibit polarized emission properties
Spectroscopy of Single CdSe Nanoplatelets
We collect and resolve spectrally and temporally the photoluminescence of single CdSe nanoplatelets. The emission intensity of single nanoplatelets at room temperature shows ON and OFF periods with a usual blinking statistics, while at 20 K, their emission intensity can be extremely stable in time. At room temperature, the emission spectra of single nanoplatelets are similar to ensemble measurements with a full width at half-maximum of 40 meV. At 20 K, we obtain a resolution-limited spectral line width (<0.4 meV). The fluorescence lifetime of single nanoplatelets decreases when the temperature decreases to reach 200 ps at 20 K. This lifetime shortening is concomitant with an increase of the nanoplateletsā emission intensity
Electrolyte-Gated Colloidal Nanoplatelets-Based Phototransistor and Its Use for Bicolor Detection
Colloidal nanocrystals are appealing
candidates for low cost optoelectronic
applications because they can combine the advantages of both organic
materials, such as their easy processing, and the excellent performance
of inorganic systems. Here, we report the use of two-dimensional colloidal
nanoplatelets for photodetection. We show that the nanoplatelets photoresponse
can be enhanced by two to three orders of magnitude when they are
incorporated in an all solid electrolyte-gated phototransistor. We
extend this technique to build the first colloidal quantum dot-based
bicolor detector with a response switchable between the visible and
near-IR
Type-II CdSe/CdTe Core/Crown Semiconductor Nanoplatelets
We have synthesized
atomically flat CdSe/CdTe core/crown nanoplatelets
(NPLs) with thicknesses of 3, 4, and 5 monolayers with fine control
of the crown lateral dimensions. In these type-II NPLs, the charges
separate spatially, and the electron wave function is localized in
the CdSe core while the hole wave function is confined in the CdTe
crown. The excitonās recombination occurs across the heterointerface,
and as a result of their spatially indirect band gap, an important
emission red shift up to the near-infrared region (730 nm) is observed
with long fluorescence lifetimes that range from 30 to 860 ns, depending
on the type of interface between the core and the crown. These type-II
NPLs have a high quantum yield of 50% that can be further improved
to 70% with a gradient interface. We have characterized these novel
CdSe/CdTe core/crown NPLs using UVāvis, emission, and excitation
spectroscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy,
and high-resolution transmission electron microscopy
Phonon Line Emission Revealed by Self-Assembly of Colloidal Nanoplatelets
We show that colloidal nanoplatelets can self-assemble to form a 1D superlattice. When self-assembled, an additional emission line appears in the photoluminescence spectrum at low temperatures. This emission line is a collective effect, greatly enhanced when the NPLs are self-assembled. It is attributed to the longitudinal optical (LO) phonon replica of the band-edge exciton, and its presence in self-assembled nanoplatelets is explained using a model based on an efficient photons reabsorption between neighboring nanoplatelets. The presence of phonon replica at low temperatures in ensemble measurements suggests the possibility to design a laser, based on self-assembled nanoplatelets
Fast, Efficient, and Stable Conjugation of Multiple DNA Strands on Colloidal Quantum Dots
A novel method for covalent conjugation
of DNA to polymer coated
quantum dots (QDs) is investigated in detail. This method is fast
and efficient: up to 12 DNA strands can be covalently conjugated per
QD in optimized reaction conditions. The QD-DNA conjugates can be
purified using size exclusion chromatography and the QDs retain high
quantum yield and excellent stability after DNA coupling. We explored
single-stranded and double-stranded DNA coupling, as well as various
lengths. We show that the DNA coupling is most efficient for short
(15 mer) single-stranded DNA. The DNA coupling has been performed
on QDs emitting at four different wavelengths, as well as on gold
nanoparticles, suggesting that this technique can be generalized to
a wide range of nanoparticles
Recombination Dynamics of Band Edge Excitons in Quasi-Two-Dimensional CdSe Nanoplatelets
We report a time-resolved study of
the photoluminescence of CdSe
colloidal nanoplatelets with two different thicknesses. By studying
the exciton recombination dynamics we assess the exciton fine structure
in these systems. The splitting between bright and dark excitons is
enhanced compared to epitaxial quantum well structures as result of
dielectric confinement. Despite of strong variations in the absolute
magnitude, by comparison with literature data we find a relatively
slightly varying brightādark exciton lifetime ratio in very
different CdSe-based colloidal nanostructures, regardless of growth
technique and of core and shell properties such as materials, dimensions,
etc. This finding points to a universal mechanism in the dark exciton
recombination
Synthesis of Zinc and Lead Chalcogenide Core and Core/Shell Nanoplatelets Using Sequential Cation Exchange Reactions
We present the synthesis of a novel
type of chalcogenide nanoplatelets.
Starting from CdS core or CdSe/CdS core/shell nanoplatelets, we use
sequential cation exchange to copper and then to either zinc or lead
to obtain ZnS and PbS core or ZnSe/ZnS and PbSe/PbS core/shell structures.
The procedure preserves well the 2D geometry of the nanoplatelets,
provided that they are more than 6 monolayers (ā¼1.8 nm) thick.
The core/shell structure is also well conserved during the cation
exchange as verified by TEM images. The nanoplatelets exchanged with
Zn crystallize in a zinc blende structure, like the initial Cd-based
material, whereas when Pb is used, the final nanoplatelets have a
rock-salt crystal structure. We explored the copper cation exchange
process using energy dispersive X-ray spectrometry with 1 nm resolution
on nanoplatelets standing on their edges, and we show that copper
ions diffuse uniformly from the outside of the nanoplatelet to the
inside during the exchange