19 research outputs found
Colloidal Atomic Layer Deposition (c-ALD) using Self-Limiting Reactions at Nanocrystal Surface Coupled to Phase Transfer between Polar and Nonpolar Media
Atomic layer deposition (ALD) is widely used for gas-phase
deposition
of high-quality dielectric, semiconducting, or metallic films on various
substrates. In this contribution we propose the concept of colloidal
ALD (c-ALD) for synthesis of colloidal nanostructures. During the
c-ALD process, either nanoparticles or molecular precursors are sequentially
transferred between polar and nonpolar phases to prevent accumulation
of unreacted precursors and byproducts in the reaction mixture. We
show that binding of inorganic ligands (e.g., S<sup>2ā</sup>) to the nanocrystal surface can be used as a half-reaction in c-ALD
process. The utility of this approach has been demonstrated by growing
CdS layers on colloidal CdSe nanocrystals, nanoplatelets, and CdS
nanorods. The CdS/CdSe/CdS nanoplatelets represent a new example of
colloidal nanoheterostructures with mixed confinement regimes for
electrons and holes. In these materials holes are confined to a thin
(ā¼1.8 nm) two-dimensional CdSe quantum well, while the electron
confinement can be gradually relaxed in all three dimensions by growing
epitaxial CdS layers on both sides of the quantum well. The relaxation
of the electron confinement energy caused a shift of the emission
band from 510 to 665 nm with unusually small inhomogeneous broadening
of the emission spectra
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
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
Carrier Cooling in Colloidal Quantum Wells
It has recently become possible to chemically synthesize
atomically
flat semiconductor nanoplatelets with monolayer-precision control
over the platelet thickness. It has been suggested that these platelets
are quantum wells; that is, carriers in these platelets are confined
in one dimension but are free to move in the other two dimensions.
Here, we report time-resolved photoluminescence and transient-absorption
measurements of carrier relaxation that confirm the quantum-well nature
of these nanomaterials. Excitation of the nanoplatelets by an intense
laser pulse results in the formation of a high-temperature carrier
population that cools back down to ambient temperature on the time
scale of several picoseconds. The rapid carrier cooling indicates
that the platelets are well-suited for optoelectronic applications
such as lasers and modulators
Low Voltage, Hysteresis Free, and High Mobility Transistors from All-Inorganic Colloidal Nanocrystals
High-mobility solution-processed all-inorganic solid
state nanocrystal
(NC) transistors with low operation voltage and near-zero hysteresis
are demonstrated using high-capacitance ZrO<sub><i>x</i></sub> and hydroxyl-free Cytop gate dielectric materials. The use
of inorganic capping ligands (In<sub>2</sub>Se<sub>4</sub><sup>2ā</sup> and S<sup>2ā</sup>) allowed us to achieve high electron mobility
in the arrays of solution-processed CdSe nanocrystals. We also studied
the hysteresis behavior and switching speed of NC-based field effect
devices. Collectively, these analyses helped to understand the charge
transport and trapping mechanisms in all-inorganic NCs arrays. Finally,
we have examined the rapid thermal annealing as an approach toward
high-performance solution-processed NCs-based devices and demonstrated
transistor operation with mobility above 30 cm<sup>2</sup>/(V s) without
compromising low operation voltage and hysteresis
Particle-Level Engineering of Thermal Conductivity in Matrix-Embedded Semiconductor Nanocrystals
Known manipulations of semiconductor thermal transport
properties
rely upon higher-order material organization. Here, using time-resolved
optical signatures of phonon transport, we demonstrate a ābottom-upā
means of controlling thermal outflow in matrix-embedded semiconductor
nanocrystals. Growth of an electronically noninteracting ZnS shell
on a CdSe core modifies thermalization times by an amount proportional
to the overall particle radius. Using this approach, we obtain changes
in effective thermal conductivity of up to 5Ć for a nearly constant
energy gap
Two-Dimensional Growth of CdSe Nanocrystals, from Nanoplatelets to Nanosheets
We report the continuous lateral extension of cadmium
selenide
nanoplatelets into nanosheets using continuous injection of precursors
at high temperature. We show that we can obtain CdSe nanosheets with
lateral dimensions up to 700 nm and a well-defined thickness that
can be tuned with atomic precision. When the nanosheetsā lateral
size increases, they roll on themselves to form nanoscrolls that can
unroll upon surface modification. The final geometry of the nanosheets
can be tuned to different morphologies using precursors that favor
the growth of specific crystal facets. We provide a detailed study
of the CdSe nanosheets growth and its optimization for three different
thicknesses
Real-Time in Situ Probing of High-Temperature Quantum Dots Solution Synthesis
Understanding the formation mechanism
of colloidal nanocrystals is of paramount importance in order to design
new nanostructures and synthesize them in a predictive fashion. However,
reliable data on the pathways leading from molecular precursors to
nanocrystals are not available yet. We used synchrotron-based time-resolved <i>in situ</i> small and wide-angle X-ray scattering to experimentally
monitor the formation of CdSe quantum dots synthesized in solution
through the heating up of precursors in octadecene at 240 Ā°C.
Our experiment yields a complete movie of the structure of the solution
from the self-assembly of the precursors to the formation of the quantum
dots. We show that the initial cadmium precursor lamellar structure
melts into small micelles at 100 Ā°C and that the first CdSe nuclei
appear at 218.7 Ā°C. The size distributions and concentration
in nanocrystals are measured in a quantitative fashion as a function
of time. We show that a short nucleation burst lasting 30 s is followed
by a slow decrease of nanoparticle concentration. The rate-limiting
process of the quantum dot formation is found to be the thermal activation
of selenium