223,642 research outputs found
Correlative Microscopy of Morphology and Luminescence of Cu porphyrin aggregates
Transfer of energy and information through molecule aggregates requires as
one important building block anisotropic, cable-like structures. Knowledge on
the spatial correlation of luminescence and morphology represents a
prerequisite in the understanding of internal processes and will be important
for architecting suitable landscapes. In this context we study the morphology,
fluorescence and phosphorescence of molecule aggregate structures on surfaces
in a spatially correlative way. We consider as two morphologies, lengthy
strands and isotropic islands. It turns out that phosphorescence is quite
strong compared to fluorescence and the spatial variation of the observed
intensities is largely in line with the amount of dye. However in proportion,
the strands exhibit more fluorescence than the isotropic islands suggesting
weaker non-radiative channels. The ratio fluorescence to phosphorescence
appears to be correlated with the degree of aggregation or internal order. The
heights at which luminescence saturates is explained in the context of
attenuation and emission multireflection, inside the dye. This is supported by
correlative photoemission electron microscopy which is more sensitive to the
surface region. The lengthy structures exhibit a pronounced polarization
dependence of the luminescence with a relative dichroism up to about 60%,
revealing substantial perpendicular orientation preference of the molecules
with respect to the substrate and parallel with respect to the strands
SiH Aggregates: From Simple Building Blocks to Highly Magnetic Functionalized Materials
Density-functional theory based global geometry optimization is used to
scrutinize the possibility of using endohedrally-doped hydrogenated Si clusters
as building blocks for constructing highly magnetic materials. In contrast to
the known clathrate-type facet-sharing, the clusters exhibit a predisposition
to aggregation through double Si-Si bridge bonds. For the prototypical
CrSiH cluster we show that reducing the degree of hydrogenation
may be used to control the number of reactive sites to which other cages can be
attached, while still preserving the structural integrity of the building block
itself. This leads to a toolbox of CrSiH monomers with
different number of double "docking sites", that allows building network
architectures of any morphology. For (CrSiH) dimer and
[CrSiH](CrSiH) trimer structures we
illustrate that such aggregates conserve the high spin moments of the dopant
atoms and are therefore most attractive candidates for cluster-assembled
materials with unique magnetic properties. The study suggests that the
structural completion of the individual endohedral cages within the
doubly-bridge bonded structures and the high thermodynamic stability of the
obtained aggregates are crucial for potential synthetic polymerization routes
controlled dehydrogenation
Beam-like topologically interlocked structures with hierarchical interlocking
Topologically interlocked materials and structures, which are assemblies of
unbonded interlocking building blocks, are a promising concept for versatile
structural applications. They have been shown to display exceptional mechanical
properties including outstanding combinations of stiffness, strength, and
toughness, beyond those achievable with common engineering materials. Recent
work established the theoretical upper limit for the strength and toughness of
beam-like topologically interlocked structures. However, this theoretical limit
is only achievable for structures with unrealistically high friction
coefficients and, therefore, it remains unknown if it is achievable in actual
structures. Here, we propose, inspired by biological systems, a hierarchical
approach for topological interlocking which overcomes these limitations and
provides a path toward optimized mechanical performance. We consider beam-like
topologically interlocked structures with geometrically designed surface
morphologies, which increases the effective frictional strength of the
interfaces, and hence enables us to achieve the theoretical limit with
realistic friction coefficients. Using numerical simulations, we examine the
effect of sinusoidal surface morphology with controllable amplitude and
wavelength on the maximum load-carrying capacity of the structure. Our study
discusses various effects of architecturing the surface morphology of beam-like
topological interlocked structures, and most notably, it demonstrates its
ability to significantly enhance the structure's mechanical performance
Self Assembled Clusters of Spheres Related to Spherical Codes
We consider the thermodynamically driven self-assembly of spheres onto the
surface of a central sphere. This assembly process forms self-limiting, or
terminal, anisotropic clusters (N-clusters) with well defined structures. We
use Brownian dynamics to model the assembly of N-clusters varying in size from
two to twelve outer spheres, and free energy calculations to predict the
expected cluster sizes and shapes as a function of temperature and inner
particle diameter. We show that the arrangements of outer spheres at finite
temperatures are related to spherical codes, an ideal mathematical sequence of
points corresponding to densest possible sphere packings. We demonstrate that
temperature and the ratio of the diameters of the inner and outer spheres
dictate cluster morphology and dynamics. We find that some N-clusters exhibit
collective particle rearrangements, and these collective modes are unique to a
given cluster size N. We present a surprising result for the equilibrium
structure of a 5-cluster, which prefers an asymmetric square pyramid
arrangement over a more symmetric arrangement. Our results suggest a promising
way to assemble anisotropic building blocks from constituent colloidal spheres.Comment: 15 pages, 10 figure
Dispersity effects in polymer self-assemblies : a matter of hierarchical control
Advanced applications of polymeric self-assembled structures require a stringent degree of control over such aspects as functionality location, morphology and size of the resulting assemblies. A loss of control in the polymeric building blocks of these assemblies can have drastic effects upon the final morphology or function of these structures. Gaining precise control over various aspects of the polymers, such as chain lengths and architecture, blocking efficiency and compositional distribution is a challenge and, hence, measuring the intrinsic mass and size dispersity within these areas is an important aspect of such control. It is of great importance that a good handle on how to improve control and accurately measure it is achieved. Additionally dispersity of the final structure can also play a large part in the suitability for a desired application. In this Tutorial Review, we aim to highlight the different aspects of dispersity that are often overlooked and the effect that a lack of control can have on both the polymer and the final assembled structure
Morphology and conduction properties of graphite-filled immiscible PVDF/PPgMA blends
Graphite was dispersed in immiscible polyvinylidene "uoride/maleated polypropylene (PVDF/PPgMA) blends to
improve electrical and thermal conductive properties by building a double-percolation structure. The morphology
of PVDF/PPgMA blends was !rst investigated for several compositions by selective solvent extraction, scanning
electron microscopy, and dynamic mechanical thermal analysis. Blends of PVDF and PPgMA were prepared in different
relative fractions, and a PVDF/PPgMA ratio of 7/3 showed a well-co-continuous structure. From this blend, the
morphology and properties of composites with different concentrations of graphite were investigated to prepare
double-percolated structures. Graphite was observed to selectively localize in the PPgMA phase. The electrical and
thermal conductive properties of graphite-containing blends were measured, showing enhanced conductivity for
the double-percolation structures compared with single-polymer composites containing the same graphite loadings
- …