18 research outputs found
Do Binary Supracrystals Enhance the Crystal Stability?
We
study the oxygen thermal stability of two binary systems. The
larger particles are magnetic amorphous Co (7.2 nm) or Fe<sub>3</sub>O<sub>4</sub> (7.5 nm) nanocrystals, whereas the smaller ones (3.7
nm) are Au nanocrystals. The nanocrystal ordering as well as the choice
of the magnetic nanoparticles very much influence the stability of
the binary system. A perfect crystalline structure is obtained with
the Fe<sub>3</sub>O<sub>4</sub>/Au binary supracrystals. For the Co/Au
binary system, oxidation of Co results in the chemical transformation
from Co to CoO, where the size of the amorphous Co nanoparticles increases
from 7.2 to 9.8 nm in diameter. During the volume expansion of the
Co nanoparticles, Au nanoparticles within the binary assemblies coalesce
and are at the origin of the instability of the binary nanoparticle
supracrystals. On the other hand, for the Fe<sub>3</sub>O<sub>4</sub>/Au binary system, the oxidation of Fe<sub>3</sub>O<sub>4</sub> to
Îł-Fe<sub>2</sub>O<sub>3</sub> does not lead to a size change
of the nanoparticles, which maintains the stability of the binary
nanoparticle supracrystals. A similar behavior is observed for an
AlB<sub>2</sub>-type CoâAg binary system: The crystalline structure
is maintained, whereas in disordered assemblies, coalescence of Ag
nanocrystals is observed
A Generalized Electrochemical Aggregative Growth Mechanism
The early stages of nanocrystal nucleation and growth are still
an active field of research and remain unrevealed. In this work, by
the combination of aberration-corrected transmission electron microscopy
(TEM) and electrochemical characterization of the electrodeposition
of different metals, we provide a complete reformulation of the VolmerâWeber
3D island growth mechanism, which has always been accepted to explain
the early stages of metal electrodeposition and thin-film growth on
low-energy substrates. We have developed a Generalized Electrochemical
Aggregative Growth Mechanism which mimics the atomistic processes
during the early stages of thin-film growth, by incorporating nanoclusters
as building blocks. We discuss the influence of new processes such
as nanocluster self-limiting growth, surface diffusion, aggregation,
and coalescence on the growth mechanism and morphology of the resulting
nanostructures. Self-limiting growth mechanisms hinder nanocluster
growth and favor coalescence driven growth. The size of the primary
nanoclusters is independent of the applied potential and deposition
time. The balance between nucleation, nanocluster surface diffusion,
and coalescence depends on the material and the overpotential, and
influences strongly the morphology of the deposits. A small extent
of coalescence leads to ultraporous dendritic structures, large surface
coverage, and small particle size. Contrarily, full recrystallization
leads to larger hemispherical monocrystalline islands and smaller
particle density. The mechanism we propose represents a scientific
breakthrough from the fundamental point of view and indicates that
achieving the right balance between nucleation, self-limiting growth,
cluster surface diffusion, and coalescence is essential and opens
new, exciting possibilities to build up enhanced supported nanostructures
using nanoclusters as building blocks
The Role of Nanocluster Aggregation, Coalescence, and Recrystallization in the Electrochemical Deposition of Platinum Nanostructures
By
using an optimized characterization approach that combines aberration-corrected
transmission electron microscopy, electron tomography, and in situ
ultrasmall angle X-ray scattering (USAXS), we show that the early
stages of Pt electrochemical growth on carbon substrates may be affected
by the aggregation, self-alignment, and partial coalescence of nanoclusters
of <i>d</i> â 2 nm. The morphology of the resulting
nanostructures depends on the degree of coalescence and recrystallization
of nanocluster aggregates, which in turn depends on the electrodeposition
potential. At low overpotentials, a self-limiting growth mechanism
may block the epitaxial growth of primary nanoclusters and results
in loose dendritic aggregates. At more negative potentials, the extent
of nanocluster coalescence and recrystallization is larger and further
growth by atomic incorporation may be allowed. On one hand, this suggests
a revision of the VolmerâWeber island growth mechanism. Whereas
this theory has traditionally assumed direct attachment as the only
growth mechanism, it is suggested that nanocluster self-limiting growth,
aggregation, and coalescence should also be taken into account during
the early stages of nanoscale electrodeposition. On the other hand,
depending on the deposition potential, ultrahigh porosities can be
achieved, turning electrodeposition in an ideal process for highly
active electrocatalyst production without the need of using high surface
area carbon supports
Supracrystalline Colloidal Eggs: Epitaxial Growth and Freestanding Three-Dimensional Supracrystals in Nanoscaled Colloidosomes
The
concept of template-confined chemical reactions allows the synthesis
of complex molecules that would hardly be producible through conventional
method. This idea was developed to produce high quality nanocrystals
more than 20 years ago. However, template-mediated assembly of colloidal
nanocrystals is still at an elementary level, not only because of
the limited templates suitable for colloidal assemblies, but also
because of the poor control over the assembly of nanocrystals within
a confined space. Here, we report the design of a new system called âsupracrystalline
colloidal eggsâ formed by controlled assembly of nanocrystals
into complex colloidal supracrystals through superlattice-matched
epitaxial overgrowth along the existing colloidosomes. Then, with
this concept, we extend the supracrystalline growth to lattice-mismatched
binary nanocrystal superlattices, in order to reach anisotropic superlattice
growths, yielding freestanding binary nanocrystal supracrystals that
could not be produced previously
High-Yield Seeded Growth of Monodisperse Pentatwinned Gold Nanoparticles through Thermally Induced Seed Twinning
We show that thermal
treatment of small Au seeds results in extensive
twinning and a subsequent drastic improvement in the yield (>85%)
of formation of pentaÂtwinned nanoparticles (NPs), with preselected
morphology (nanorods, bipyramids, and decahedra) and aspect ratio.
The âqualityâ of the seeds thus defines the yield of
the obtained NPs, which in the case of nanorods avoids the need for
additives such as Ag<sup>+</sup> ions. This modified seeded growth
method also improves reproducibility, as the seeds can be stored for
extended periods of time without compromising the quality of the final
NPs. Additionally, minor modification of the seeds with Pd allows
their localization within the final particles, which opens new avenues
toward mechanistic studies. Together, these results represent a paradigm
shift in anisotropic gold NP synthesis
Composite Supraparticles with Tunable Light Emission
Robust luminophores
emitting light with broadly tunable colors
are desirable in many applications such as light-emitting diode (LED)-based
lighting, displays, integrated optoelectronics and biology. Nanocrystalline
quantum dots with multicolor emission, from core- and shell-localized
excitons, as well as solid layers of mixed quantum dots that emit
different colors have been proposed. Here, we report on colloidal
supraparticles that are composed of three types of CdÂ(Se,ZnS) core/(Cd,Zn)ÂS
shell nanocrystals with emission in the red, green, and blue. The
emission of the supraparticles can be varied from pure to composite
colors over the entire visible region and fine-tuned into variable
shades of white light by mixing the nanocrystals in controlled proportions.
Our approach results in supraparticles with sizes spanning the colloidal
domain and beyond that combine versatility and processability with
a broad, stable, and tunable emission, promising applications in lighting
devices and biological research
Shape Control of Colloidal Cu<sub>2â<i>x</i></sub>S Polyhedral Nanocrystals by Tuning the Nucleation Rates
Synthesis
protocols for colloidal nanocrystals (NCs) with narrow
size and shape distributions are of particular interest for the successful
implementation of these nanocrystals into devices. Moreover, the preparation
of NCs with well-defined crystal phases is of key importance. In this
work, we show that SnÂ(IV)-thiolate complexes formed in situ strongly
influence the nucleation and growth rates of colloidal Cu<sub>2â<i>x</i></sub>S polyhedral NCs, thereby dictating their final size,
shape, and crystal structure. This allowed us to successfully synthesize
hexagonal bifrustums and hexagonal bipyramid NCs with low-chalcocite
crystal structure, and hexagonal nanoplatelets with various thicknesses
and aspect ratios with the djurleite crystal structure, by solely
varying the concentration of SnÂ(IV)-additives (namely, SnBr<sub>4</sub>) in the reaction medium. Solution and solid-state <sup>119</sup>Sn NMR measurements show that SnBr<sub>4</sub> is converted in situ
to SnÂ(IV)âthiolate complexes, which increase the Cu<sub>2â<i>x</i></sub>S nucleation barrier without affecting the precursor
conversion rates. This influences both the nucleation and growth rates
in a concentration-dependent fashion and leads to a better separation
between nucleation and growth. Our approach of tuning the nucleation
and growth rates with in situ-generated Snâthiolate complexes
might have a more general impact due to the availability of various
metalâthiolate complexes, possibly resulting in polyhedral
NCs of a wide variety of metalâsulfide compositions
Galvanic Replacement Coupled to Seeded Growth as a Route for Shape-Controlled Synthesis of Plasmonic Nanorattles
Shape-controlled
synthesis of metal nanoparticles (NPs) requires
mechanistic understanding toward the development of modern nanoscience
and nanotechnology. We demonstrate here an unconventional shape transformation
of Au@Ag coreâshell NPs (nanorods and nanocubes) into octahedral
nanorattles via room-temperature galvanic replacement coupled with
seeded growth. The corresponding morphological and chemical transformations
were investigated in three dimensions, using state-of-the-art X-ray
energy-dispersive spectroscopy (XEDS) tomography. The addition of
a reducing agent (ascorbic acid) plays a key role in this unconventional
mechanistic path, in which galvanic replacement is found to dominate
initially when the shell is made of Ag, while seeded growth suppresses
transmetalation when a composition of Au:Ag (âŒ60:40) is reached
in the shell, as revealed by quantitative XEDS tomography. This work
not only opens new avenues toward the shape control of hollow NPs
beyond the morphology of sacrificial templates, but also expands our
understanding of chemical transformations in nanoscale galvanic replacement
reactions. The XEDS electron tomography study presented here can be
generally applied to investigate a wide range of nanoscale morphological
and chemical transformations
Self-Organization of Highly Symmetric Nanoassemblies: A Matter of Competition
The properties and applications of metallic nanoparticles are inseparably connected not only to their detailed morphology and composition but also to their structural configuration and mutual interactions. As a result, the assemblies often have superior properties as compared to individual nanoparticles. Although it has been reported that nanoparticles can form highly symmetric clusters, if the configuration can be predicted as a function of the synthesis parameters, more targeted and accurate synthesis will be possible. We present here a theoretical model that accurately predicts the structure and configuration of self-assembled gold nanoclusters. The validity of the model is verified using quantitative experimental data extracted from electron tomography 3D reconstructions of different assemblies. The present theoretical model is generic and can in principle be used for different types of nanoparticles, providing a very wide window of potential applications
Collective Plasmonic Properties in Few-Layer Gold Nanorod Supercrystals
Gold nanorod supercrystals have been
widely employed for the detection
of relevant bioanalytes with detection limits ranging from nano- to
picomolar levels, confirming the promising nature of these structures
for biosensing. Even though a relationship between the height of the
supercrystal (i.e., the number of stacked nanorod layers) and the
enhancement factor has been proposed, no systematic study has been
reported. In order to tackle this problem, we prepared gold nanorod
supercrystals with varying numbers of stacked layers and analyzed
them extensively by atomic force microscopy, electron microscopy and
surface enhanced Raman scattering. The experimental results were compared
to numerical simulations performed on real-size supercrystals composed
of thousands of nanorod building blocks. Analysis of the hot spot
distribution in the simulated supercrystals showed the presence of
standing waves that were distributed at different depths, depending
on the number of layers in each supercrystal. On the basis of these
theoretical results, we interpreted the experimental data in terms
of analyte penetration into the topmost layer only, which indicates
that diffusion to the interior of the supercrystals would be crucial
if the complete field enhancement produced by the stacked nanorods
is to be exploited. We propose that our conclusions will be of high
relevance in the design of next generation plasmonic devices