12 research outputs found
Highly Luminescent Water-Dispersible NIR-Emitting Wurtzite CuInS<sub>2</sub>/ZnS Core/Shell Colloidal Quantum Dots
Copper indium sulfide
(CIS) quantum dots (QDs) are attractive as
labels for biomedical imaging, since they have large absorption coefficients
across a broad spectral range, size- and composition-tunable photoluminescence
from the visible to the near-infrared, and low toxicity. However,
the application of NIR-emitting CIS QDs is still hindered by large
size and shape dispersions and low photoluminescence quantum yields
(PLQYs). In this work, we develop an efficient pathway to synthesize
highly luminescent NIR-emitting wurtzite CIS/ZnS QDs, starting from
template Cu<sub>2ā<i>x</i></sub>S nanocrystals (NCs),
which are converted by topotactic partial Cu<sup>+</sup> for In<sup>3+</sup> exchange into CIS NCs. These NCs are subsequently used as
cores for the overgrowth of ZnS shells (ā¤1 nm thick). The CIS/ZnS
core/shell QDs exhibit PL tunability from the first to the second
NIR window (750ā1100 nm), with PLQYs ranging from 75% (at 820
nm) to 25% (at 1050 nm), and can be readily transferred to water upon
exchange of the native ligands for mercaptoundecanoic acid. The resulting
water-dispersible CIS/ZnS QDs possess good colloidal stability over
at least 6 months and PLQYs ranging from 39% (at 820 nm) to 6% (at
1050 nm). These PLQYs are superior to those of commonly available
water-soluble NIR-fluorophores (dyes and QDs), making the hydrophilic
CIS/ZnS QDs developed in this work promising candidates for further
application as NIR emitters in bioimaging. The hydrophobic CIS/ZnS
QDs obtained immediately after the ZnS shelling are also attractive
as fluorophores in luminescent solar concentrators
Unravelling the Size and Temperature Dependence of Exciton Lifetimes in Colloidal ZnSe Quantum Dots
We
report on the temperature dependence of the band-edge photoluminescence
decay of organically capped colloidal ZnSe quantum dots (QDs) in the
size range from 4.0 to 7.5 nm. A similar trend is observed for all
investigated sizes: the decay time is short (ā¼5 ns) above 20
K and increases sharply below 20 K, eventually reaching a constant
value (270ā400 ns) at sufficiently low temperatures (<4
K). The temperature regime in which the decrease of lifetime occurs
depends on the QD size and is lower for larger QDs. This behavior
can be modeled by a Boltzmann distribution between a lower long-lived
and a higher short-lived exciton states, with an energy separation
ranging from 3.3 Ā± 0.2 to 1.5 Ā± 0.1 meV in the 4.0 Ā±
0.3 to 7.5 Ā± 0.5 nm size range. We show that this energy separation
is consistent with coupling of the lowest exciton state to a confined
acoustic phonon
Formation of Colloidal Copper Indium Sulfide Nanosheets by Two-Dimensional Self-Organization
Colloidal 2D semiconductor nanosheets
(NSs) are an interesting
new class of materials due to their unique properties. However, synthesis
of these NSs is challenging, and synthesis procedures for materials
other than the well-known Pb- and Cd-chalcogenides are still underdeveloped.
In this paper, we present a new approach to make copper indium sulfide
(CIS) NSs and study their structural and optical properties. The CIS
NSs form via self-organization and oriented attachment of 2.5 nm chalcopyrite
CuInS<sub>2</sub> nanocrystals (NCs), yielding triangular- and hexagonal-shaped
NSs with a thickness of ā¼3 nm and lateral dimensions ranging
from 20 to 1000 nm. The self-organization is induced by fast cation
extraction, leading to attractive dipolar interactions between the
NCs. Primary amines play a crucial role in the formation of the CIS
NSs, both by forming <i>in situ</i> the cation extracting
agent, and by preventing the attachment of NCs to the top and bottom
facets of the NSs. Moreover, DFT calculations reveal that the amines
are essential to stabilize the covellite crystal structure of the
product CIS NSs. The NSs are indium-deficient and the off-stoichiometry
gives rise to a plasmon resonance in the NIR spectral window
Room-Temperature Interconversion Between Ultrathin CdTe Magic-Size Nanowires Induced by Ligand Shell Dynamics
The formation mechanisms of colloidal magic-size semiconductor
nanostructures have remained obscure. Herein, we report the room temperature
synthesis of three species of ultrathin CdTe magic-size nanowires
(MSNWs) with diameters of 0.7 Ā± 0.1 nm, 0.9 Ā± 0.2 nm, and
1.1 Ā± 0.2 nm, and lowest energy exciton transitions at 373, 418,
and 450 nm, respectively. The MSNWs are obtained from Cd(oleate)2 and TOP-Te, provided diphenylphosphine and a primary alkylamine
(RNH2) are present at sufficiently high concentrations,
and exhibit sequential, discontinuous growth. The population of each
MSNW species is entirely determined by the RNH2 concentration
[RNH2] so that single species are only obtained at specific
concentrations, while mixtures are obtained at concentrations intermediate
between the specific ones. Moreover, the MSNWs remain responsive to
[RNH2], interconverting from thinner to thicker upon [RNH2] decrease and from thicker to thinner upon [RNH2] increase. Our results allow us to propose a mechanism for the formation
and interconversion of CdTe MSNWs and demonstrate that primary alkylamines
play crucial roles in all four elementary kinetic steps (viz., monomer
formation, nucleation, growth in length, and interconversion between
species), thus being the decisive element in the creation of a reaction
pathway that leads exclusively to CdTe MSNWs. The insights provided
by our work thus contribute toward unravelling the mechanisms behind
the formation of shape-controlled and atomically precise magic-size
semiconductor nanostructures
Tailoring ZnSeāCdSe Colloidal Quantum Dots <i>via</i> Cation Exchange: From Core/Shell to Alloy Nanocrystals
We report a study of Zn<sup>2+</sup> by Cd<sup>2+</sup> cation exchange (CE) in colloidal ZnSe nanocrystals (NCs). Our results reveal that CE in ZnSe NCs is a thermally activated isotropic process. The CE efficiency (<i>i.e</i>., fraction of Cd<sup>2+</sup> ions originally in solution, Cd<sub>sol</sub>, that is incorporated in the ZnSe NC) increases with temperature and depends also on the Cd<sub>sol</sub>/ZnSe ratio. Interestingly, the reaction temperature can be used as a sensitive parameter to tailor both the composition and the elemental distribution profile of the product (Zn,Cd)Se NCs. At 150 Ā°C ZnSe/CdSe core/shell hetero-NCs (HNCs) are obtained, while higher temperatures (200 and 220 Ā°C) produce (Zn<sub>1ā<i>x</i></sub>Cd<sub><i>x</i></sub>)Se gradient alloy NCs, with increasingly smoother gradients as the temperature increases, until homogeneous alloy NCs are obtained at <i>T</i> ā„ 240 Ā°C. Remarkably, sequential heating (150 Ā°C followed by 220 Ā°C) leads to ZnSe/CdSe core/shell HNCs with thicker shells, rather than (Zn<sub>1ā<i>x</i></sub>Cd<sub><i>x</i></sub>)Se gradient alloy NCs. Thermal treatment at 250 Ā°C converts the ZnSe/CdSe core/shell HNCs into (Zn<sub>1ā<i>x</i></sub>Cd<sub><i>x</i></sub>)Se homogeneous alloy NCs, while preserving the NC shape. A mechanism for the cation exchange in ZnSe NCs is proposed, in which fast CE takes place at the NC surface, and is followed by relatively slower thermally activated solid-state cation diffusion, which is mediated by Frenkel defects. The findings presented here demonstrate that cation exchange in colloidal ZnSe NCs provides a very sensitive tool to tailor the nature and localization regime of the electron and hole wave functions and the optoelectronic properties of colloidal ZnSeāCdSe NCs
Anisotropic 2D Cu<sub>2ā<i>x</i></sub>Se Nanocrystals from Dodecaneselenol and Their Conversion to CdSe and CuInSe<sub>2</sub> Nanoparticles
We
present the synthesis of colloidal anisotropic Cu<sub>2ā<i>x</i></sub>Se nanocrystals (NCs) with excellent size and shape
control, using the unexplored phosphine-free selenium precursor 1-dodecaneselenol
(DDSe). This precursor forms lamellar complexes with CuĀ(I) that enable
tailoring the NC morphology from 0D polyhedral to highly anisotropic
2D shapes. The Cu<sub>2ā<i>x</i></sub>Se NCs are
subsequently used as templates in postsynthetic cation exchange reactions,
through which they are successfully converted to CdSe and CuInSe<sub>2</sub> quantum dots, nanoplatelets, and ultrathin nanosheets. The
shape of the template hexagonal nanoplatelets is preserved during
the cation exchange reaction, despite a substantial reorganization
of the anionic sublattice, which leads to conversion of the tetragonal
umangite crystal structure of the parent Cu<sub>2ā<i>x</i></sub>Se NCs into hexagonal wurtzite CdSe and CuInSe<sub>2</sub>,
accompanied by a change of both the thickness and the lateral dimensions
of the nanoplatelets. The crystallographic transformation and reconstruction
of the product NCs are attributed to a combination of the unit cell
dimensionalities of the parent and product crystal phases and an internal
ripening process. This work provides novel tools for the rational
design of shape-controlled colloidal anisotropic Cu<sub>2ā<i>x</i></sub>Se NCs, which, besides their promising optoelectronic
properties, also constitute a new family of cation exchange templates
for the synthesis of shape-controlled NCs of wurtzite CdSe, CuInSe<sub>2</sub>, and other metal selenides that cannot be attained through
direct synthesis approaches. Moreover, the insights provided here
are likely applicable also to the direct synthesis of shape-controlled
NCs of other metal selenides, since DDSe may be able to form lamellar
complexes with several other metals
Interplay between Surface Chemistry, Precursor Reactivity, and Temperature Determines Outcome of ZnS Shelling Reactions on CuInS<sub>2</sub> Nanocrystals
ZnS
shelling of IāIIIāVI<sub>2</sub> nanocrystals
(NCs) invariably leads to blue-shifts in both the absorption and photoluminescence
spectra. These observations imply that the outcome of ZnS shelling
reactions on IāIIIāVI<sub>2</sub> colloidal NCs results
from a complex interplay between several processes taking place in
solution, at the surface of, and within the seed NC. However, a fundamental
understanding of the factors determining the balance between these
different processes is still lacking. In this work, we address this
need by investigating the impact of precursor reactivity, reaction
temperature, and surface chemistry (due to the washing procedure)
on the outcome of ZnS shelling reactions on CuInS<sub>2</sub> NCs
using a seeded growth approach. We demonstrate that low reaction temperatures
(150 Ā°C) favor etching, cation exchange, and alloying regardless
of the precursors used. Heteroepitaxial shell overgrowth becomes the
dominant process only if reactive S- and Zn-precursors (S-ODE/OLAM
and ZnI<sub>2</sub>) and high reaction temperatures (210 Ā°C)
are used, although a certain degree of heterointerfacial alloying
still occurs. Remarkably, the presence of residual acetate at the
surface of CIS seed NCs washed with ethanol is shown to facilitate
heteroepitaxial shell overgrowth, yielding for the first time CIS/ZnS
core/shell NCs displaying red-shifted absorption spectra, in agreement
with the spectral shifts expected for a type-I band alignment. The
insights provided by this work pave the way toward the design of improved
synthesis strategies to CIS/ZnS core/shell and alloy NCs with tailored
elemental distribution profiles, allowing precise tuning of the optoelectronic
properties of the resulting materials
Self-Assembly of Colloidal Hexagonal Bipyramid- and Bifrustum-Shaped ZnS Nanocrystals into Two-Dimensional Superstructures
We present a combined experimental,
theoretical, and simulation
study on the self-assembly of colloidal hexagonal bipyramid- and hexagonal
bifrustum-shaped ZnS nanocrystals (NCs) into two-dimensional superlattices.
The simulated NC superstructures are in good agreement with the experimental
ones. This shows that the self-assembly process is primarily driven
by minimization of the interfacial free-energies and maximization
of the packing density. Our study shows that a small truncation of
the hexagonal bipyramids is sufficient to change the symmetry of the
resulting superlattice from hexagonal to tetragonal, highlighting
the crucial importance of precise shape control in the fabrication
of functional metamaterials by self-assembly of colloidal NCs
Luminescent CuInS<sub>2</sub> Quantum Dots by Partial Cation Exchange in Cu<sub>2ā<i>x</i></sub>S Nanocrystals
Here,
we show successful partial cation exchange reactions in Cu<sub>2ā<i>x</i></sub>S nanocrystals (NCs) yielding luminescent
CuInS<sub>2</sub> (CIS) NCs. Our approach of mild reaction conditions
ensures slow Cu extraction rates, which results in a balance with
the slow In incorporation rate. With this method, we obtain CIS NCs
with photoluminescence (PL) far in the near-infrared (NIR), which
cannot be directly synthesized by currently available synthesis protocols.
We discuss the factors that favor partial, self-limited cation exchange
from Cu<sub>2ā<i>x</i></sub>S to CIS NCs, rather
than complete cation exchange to In<sub>2</sub>S<sub>3</sub>. The
product CIS NCs have the wurtzite crystal structure, which is understood
in terms of conservation of the hexagonal close packing of the anionic
sublattice of the parent NCs into the product NCs. These results are
an important step toward the design of CIS NCs with sizes and shapes
that are not attainable by direct synthesis protocols and may thus
impact a number of potential applications
Highly Luminescent (Zn,Cd)Te/CdSe Colloidal Heteronanowires with Tunable ElectronāHole Overlap
We report the synthesis of ultranarrow (Zn,Cd)ĀTe/CdSe
colloidal
heteronanowires, using ZnTe magic size clusters as seeds. The wire
formation starts with a partial Zn for Cd cation exchange, followed
by self-organization into segmented heteronanowires. Further growth
occurs by inclusion of CdSe. The heteronanowires emit in the 530 to
760 nm range with high quantum yields. The electronāhole overlap
decreases with increasing CdSe volume fraction, allowing the optical
properties to be controlled by adjusting the heteronanowire composition